Symposium EN09 : Advances in the Fundamental Science of Halide Perovskite Optoelectronics

Halide perovskites such as CH3NH3PbI3 have recently emerged as promising materials for low-cost, high-efficiency solar cells. The efficiency of perovskite-based solar cells has increased rapidly, from 3.8% in 2009 to more than 23% recently by modifying material compositions and engineering cell architectures and defect properties. The emergence of high efficiency perovskite solar cells can be attributed to the intrinsic properties that distinguish them from conventional semiconducting solar cell absorber materials. However, despite the enormous progress of the perovskites in solar cell applications, challenges are still standing in their way to large-scale commercial applications, including their poor long-term stability, which could be partially attributed to the intrinsic thermodynamic instability of CH3NH3PbI3 and related materials, and the toxicity of Pb, currently used in halide perovskite based solar cells with high power conversion efficiencies. Recently, various approaches have been proposed to overcome these bottlenecks, including defect control, alloying, as well as atomic transmutation. In this talk, I will discuss some of our recent theoretical investigations on ordered and disordered halide perovskites to understand their material properties and provide theoretical insights and possible solutions to the usage of halide perovskites for solar cell and other optoelectronic applications.

In this talk, we will discuss (1) electronic properties of MA-Pb-halide perovskites with at different halide compositions with various defects and impurities, obtained using first principles density functional theory calculations and machine learning; (2) dynamics of lattice thermal equilibration, especially between the organic and inorganic sublattices, from ab initio molecular dynamics simulations.

Halide perovskite materials are at the forefront of scientific and technological interest in the last decade, due to their extraordinary physical properties and their indisputable performance in solar cells, and in other opto-electronic applications. The most studied halide perovskite has the chemical formula APbX₃ (A = methylammonium, formamidinium, Cs; X= I, Cl, Br) prepared as bulk single crystals, thin films or nanostructures. Those were characterized by their long carrier diffusion length, unity emission quantum efficiency, tolerance for defects, anharmonicity, reduced elastic stiffness, strong carrier-phonon coupling and polaron formation. However, spin properties were investigated only to a lesser extent, albeit, they may be of a paramount importance in the control of the optical properties of halide perovskites, thus demands further exploration. Most recent years studies uncovered the occurrence of a Rashba effect, related to the existence of a an exotic spin-orbit field which split both valence and conduction band in k-space into two valleys, symmetrically spaces away from a Brillouin point with opposing spin helicity, hence dictating spin selective recombination.^{1, 2} Other studies explored the g-factor of photo-generated carriers, as well as the carrier spin relaxation time (T1 ~ 1nsec) and dephasing time (T2 ~ 80 psec).^{3, 4}

The current work reports magneto-optical measurements of MAPbBr₃ high quality single crystal where the observation brought about for the first time, a strong evidence for the existence of an Overhauser effect, viz., occurrence of coupling between a photo-generated carrier' spins and nuclei spins, the last related to the existence of neutral absence isotopes. A photo-generated carrier applies a local field on nuclear spins via hyperfine interactions and induces their mutual alignment. Then, the effective magnetic field created by the nuclei acts back on a carrier, affecting its polarization and the helicity of a recombination emission. The effective nuclear field may enhance or suppress a Zeeman and Rashba fields. The current work exploited the counter balance between the Rashba, Overhauser and a Zeeman field. A follow up of the exciton emission intensity or its circular polarization versus strength of an external magnetic field, revealed a special trend corroborating the occurrence of an Overhauser field < 0.2 Tesla. Furthermore, the Rashba field is pronounced as a non-Zeeman behavior, as well as by occurrence of unformal ordering of exciton fine structure. The study discovered a dominant route causing de-coherence of carrier's spin, with significance impact on the perovskites use in quantum information or memory devices.

1.Isarov, M. et al. Rashba Effect in a Single Colloidal CsPbBr₃ Perovskite Nanocrystal Detected by Magneto-Optical Measurements. Nano Lett. 17, 5020, (2017); 2.Becker, M. A. et al. Bright triplet excitons in caesium lead halide perovskites. Nature 553, 189, (2018); 3.Odenthal, P. et al. Spin-polarized exciton quantum beating in hybrid organic–inorganic perovskites. Nature Physics 13, 894, (2017); 4.Zhang, C. et al. Magnetic field effects in hybrid perovskite devices. Nature Physics 11, 427 (2015).

While octahedral tilting or B atom displacements as a single repeated structural motif are well known in perovskites, we find that removing the standard restriction to such a minimal unit cell size in total energy structural optimization, leads in some perovskites to the formation of a ‘polymorphous network’, manifesting a distribution of different tilt angles and different B-atom displacement in different octahedra. Such systems cannot be described crystallographically by a small, repeated unit cell, and were generally described as thermally disordered compounds, modelled by high temperature molecular dynamics via time-dependent dynamic motions with entropy-driven disorder. What is different about the polymorphous network for the cubic halide perovskite FASnI₃ (where FA=CH(NH₃)₂) is that the distribution of local motifs emerges already from the (density functional) minimization of the static, T=0 internal energy of a large supercell, constrained to have the global cubic lattice vectors. This a-thermal distribution represents a correlated set of displacements and is very different from the time-dependent uncorrelated entropic thermal disorder calculated by MD, or from the single sharp monomorphous values of these deformation parameters . This distribution of local motifs is also different from the periodically repeated ordered double-potential well models that address anharmonic polar fluctuations. The existence of such a polymorphous distribution is easy to miss using standard energy minimization protocols (such as those based on following gradients to the nearest local minimum) but is revealed once one initially applies a random atomic displacement ("nudge") off the cubic sites and explores lower symmetries in the minimization process.
These polymorphous networks represent a different paradigm in understanding cubic halide and oxide perovskites Our finding suggests that the widely discussed single formula unit cubic Pm-3m structure of halide perovskites does not really exist, except as a macroscopically averaged fictitious structural model. Because X-ray diffraction has a rather long coherence length, such polymorphous systems were often fit in structure refinement models by a macroscopically averaged (“fictitious monomorphous”) cubic Pm-3m unit cells. Significantly, compared to the monomorphous assumption, we show here that the cubic polymorphous network (i) fits much better the observed Pair Distribution Function (PDF), which probes the local environment. (ii) a significantly lowering of calculated total energies, by ~70-150 meV/f.u., and (iii) up to 300% larger band gaps, on account of the reduced level repulsion afforded by larger octahedral tilting and rotations. Thus, (iv) the band gap renormalization energy (~200 meV) is now closer to experiment relative to the values computed with respect to the band gap of the monomorphous model (390-640 meV). (v) Use of the polymorphous structure leads to the reversal of the predicted sign of the mixing enthalpies of the solid solutions from negative (ordering-like; not seen experimentally) to positive (experimentally observed phase-separating), in better agreement with observations. (vi) Remarkably, despite the existence of a distribution of motifs, the calculated band structure (unfolded to the primitive Brillion zone from the supercell) shows sharp band edge states and a correspondingly fast rise of the absorption spectrum, and leads to a broad and slow rising absorption tail. Finally, relative to the monomorphous case, (vii) polymorphous networks have a much larger (by ~50%) calculated dielectric constant, where the ionic contribution now dominates the electronic contribution as expected from near ferroelectrics. The polymorphous approximant could thus serve as a useful practical structure to use with standard band structure approaches to predict properties, replacing the fictitious monomorphous structures .In collaboration with Xingang Zhao,Zhi Wei and Gustavo Dalpian.

Halide perovskites (HaPs) are highly promising materials for a range of optoelectronic devices. HaPs are also very interesting scientifically because of the unusual structural dynamics that occur in the material. These include the appearance of complex structural disorder and sizable nuclear anharmonic effects already at room temperature, which challenge our basic understanding of coupling between nuclear vibrations and optoelectronic properties in a semiconductor.

In this talk, I will present our recent explorations of the consequences of the unusual structural phenomena in HaPs for their optoelectronic properties. Theoretical calculations based on density functional theory, molecular dynamics, and tight-binding modeling will be used to examine the impact of structural dynamics on pertinent device-relevant observables. Consequences of the structural dynamics and anharmonicity in HaPs will be discussed for the charge-carrier mobility, Urbach energy, and defect energetics. It will be shown that the impact of the unusual structural dynamics on the optoelectronic properties of HaPs cannot be neglected when understanding these materials microscopically and designing new functional compounds.

In recent years, the interest in halide perovskites rose at a rapid pace due to their tremendous success in the field of photovoltaic while other fields, like light emitting diodes, show great potential as well. One intriguing property of this material class is the wide tunability of the band-gap that can be induced by changing the perovskite composition. While changes in band gap are regularly reported, it is unclear how the respective conduction and valence band positions change and what the underlying origins of these changes are. Knowing the band positions is however crucial for device design, i.e. ensuring efficient charge transport across the various interfaces.
In this talk, I will discuss recent findings regarding the variations in ionization energy and electron affinity, covering the complete library of lead and tin based halide perovskite systems. [1] Using a combination of photoelectron spectroscopy, density functional theory, and a tight binding model we are able to reliably extract the relevant energy level positions. Furthermore, we are able to explain the origin of these changes based on changes in hybridization strength, atomic level positions, and lattice distortion.

References
[1] S. Tao, Ines Schmidt, G. Brocks, J. Jiang, I. Tranca, K. Meerholz, and S. Olthof , Absolute energy level positions in tin- and lead-based halide perovskites, Nature Communications 10, 2560 (2019)

Lead-based hybrid organic-inorganic perovskite photovoltaic materials have been the focus of much research in recent years because to their low fabrication cost and extremely high photoconversion efficiencies¹. Inspired by the rapid rise in efficiencies of lead based perovksite solar cells and by the environmental concern of the heavy metal content, lead-free alternatives are attracting increasing attention. Perovskite-inspired materials with 0D and 2D crystallographic structure with the formula A₃B₂X₉, where A is the monovalent cation (methyl ammonium MA, formamidinium FA, Rb, or Cs), B is the metal cation (Sb and/or Bi), and X is the halide anion (I, Cl and/or Br), have recently attracted the attention of the researchers looking for the next lead-free material. Recently, dual-site alloys Cs₃(Bi_1-xSb_x)₂(I_1-yBr_y)₉exhibiting a transition between 0D to 2D crystal structures and a non-linear bandgap tunability, have been synthesized². The discovery of non-linear bandgap behavior (bandgap bowing) in this series has opened up a new pathway to achieve lead-free all-inorganic perovskites for multi-junction solar cells. However, compared to their lead-based counterparts³, the origins of the bandgap bowing in these systems are not yet understood. In this study, we employ detailed first-principles DFT calculations to model Cs₃(Bi,Sb)₂I₉, Cs₃(Bi,Sb)₂Br₉, Cs₃Sb₂(I,Br)₉, Cs₃Bi₂(I,Br)₉ systems to better understand the pathways to reduced bandgaps in dual-site alloys, as well as to systematically decouple the structural and chemical origins to the bandgap bowing. Our results suggests that bandgap bowing is caused by mixing at the B-site and that the non-linearity in the bandgap exists regardless of the crystallographic dimensionality of the alloy. With better understanding of the bandgap behavior we aim to tailor these lead-free perovksite-inspired mixed systems to achieve optimum optoelectronic properties.

References:
1. National Renewable Energy Laboratory, “NREL Efficieny Chart.” [Online]., https://www.nrel.gov/pv/cell-efficiency.html.
2. S. Sun, N. T. P. Hartono, Z. D. Ren, F. Oviedo, A. M. Buscemi, M. Layurova, D. X. Chen, T. Ogunfunmi, J. Thapa, S. Ramasamy, C. Settens, B. L. DeCost, A. G. Kusne, Z. Liu, S. I. P. Tian, I. M. Peters, J. P. Correa-Baena and T. Buonassisi, Joule, , DOI:10.1016/j.joule.2019.05.014.
3. A. Goyal, S. McKechnie, D. Pashov, W. Tumas, M. Van Schilfgaarde and V. Stevanović, Chem. Mater., 2018, 30, 3920–3928.

Metal halide perovskites have been widely studied for light-emitting applications (LEDs) [1-2], as well as photovoltaics [3-4] showing drastic increase in efficiencies due to their unique opto-electronic properties. The control of non-radiative losses in the active perovskite layer is still one of limiting factors that diminishes the efficiency of devices below their theoretical performance limits. Recently, it has been shown that lattice strain present in halide perovskites correlated with non-radiative electron-hole recombination [5]. To prove the concept, we systematically investigate how volumetric strain affects the atomic and electronic structure, charged native point defect concentration, and luminescence properties by combining state-of-the-art simulations and experiment. Our first-principles density functional theory (DFT) calculationsincluding relativistic effects show enhanced Rashba splitting and Schottky vacancy defect disorder under compressive pressure, while experiments of perovskite LEDs under hydrostatic pressure display a pronounced decrease in photoluminescence peak position and intensity under compressive pressure. The combination of theory and experiment provide a unified model of pressure-induced recombination in these soft semiconductors.

[1] R. F. Service, Science, 2019, 364, 918
[2]S. A. Veldhuis, P. P. Boix, N. Yantara, M. Li, T. C. Sum, N. Mathews, and S. G. Mhaisalkar, Adv. Mater., 2016, 28, 6804–6834.
[3] L. D. Whalley, J. M. Frost, Y.-K. Jung, and A. Walsh, J. Chem. Phys., 2017, 146, 220901.
[4]P. Gao, M. Grätzel, and M. K. Nazeeruddin, Energy Environ. Sci., 2014, 7, 2448–2463.
[5]T. W. Jones, A. Osherov, M. Alsari, M. Sponseller, B. C. Duck, Y.-K. Jung, C. Settens, F. Niroui, R. Brenes, C. V. Stan, Y. Li, M. Abdi-Jalebi, N. Tamura, J. E. Macdonald, M. Burghammer, R. H. Friend, V. Bulović, A. Walsh, G. J. Wilson, S. Lilliu, and S. D. Stranks, Energy Environ. Sci., 2019, 12, 596.

In the last decade perovskite nanocrystals (NCs) burst into the consciousness of the scientific community as a new class of semiconductors and became the material class at the forefront of research efforts. Particularly, the topic of doping arose in recent years, involving doping or alloying by magnetic ions, which induced changes in the optical and magnetic properties.
In this work a novel dynamic cation exchange strategy driven by a simultaneous anion exchange was implemented¹ to incorporate Ni²⁺ ions into CsPbBr₃ perovskite nanocrystals² at room temperature and ambient conditions. Ni is especially interesting as a dopant, as it combines an electron spin of S=1, a lack of nuclear spins (with the exception of ⁶¹Ni with a low natural abundancy of < 1%) and relatively weak spin-orbit coupling compared to Pb. Magnetic dopants in semiconducting materials induce spin-exchange interactions with the host carrier's spins, mostly leading to a giant magnetization within the host lattice. The lack of spin-orbit and nuclear spin coupling of the Ni dopants should preserve this giant magnetization and consequently the carrier's spin coherency for a length of time. Long spin coherence time is the holy-grail of spin-based devices.
The present work describes a thorough investigation of the doping mechanism into perovskite nanocrystals and characterization of the produced structures and composition, using electron microscopy and spectroscopic techniques. The doping of CsPbBr₃ NCs with Ni²⁺ ions was carried out using ion exchange procedures, involving post-treatment with either NiCl₂, PbCl₂, PbBr₂ or NiBr₂ precursors at room temperature. The reaction with NiCl₂ was utilized for the anion-cation co-exchange in order to achieve a uniform Ni incorporation into the lattice, and the use of all other reagents was dedicated to control experiments. The results indicate the essential need for co-exchange of cation and anion, enabling integration of Ni²⁺ ions with a concentration from < 1% to about 12%. The observations revealed a uniform distribution of the Ni ions across the nanocrystals. Moreover, the nanocrystals exhibit improved luminescence quantum yields beyond those of the non-doped CsPbCl₃ and CsPbBr₃. The observations were corroborated by a theoretical density functional theory calculation, confirming that the exchange of Ni is energetically favorable.

1. Chen, D.; Zhou, S.; Fang, G.; Chen, X.; Zhong, J., Fast Room-Temperature Cation Exchange Synthesis of Mn-Doped CsPbCl₃ Nanocrystals Driven by Dynamic Halogen Exchange. ACS Applied Materials & Interfaces 2018, 10 (46), 39872-39878.
2. Yong, Z.-J.; Guo, S.-Q.; Ma, J.-P.; Zhang, J.-Y.; Li, Z.-Y.; Chen, Y.-M.; Zhang, B.-B.; Zhou, Y.; Shu, J.; Gu, J.-L.; Zheng, L.-R.; Bakr, O. M.; Sun, H.-T., Doping-Enhanced Short-Range Order of Perovskite Nanocrystals for Near-Unity Violet Luminescence Quantum Yield. Journal of the American Chemical Society 2018, 140 (31), 9942-9951.

In this talk we describe the role of interfaces, surface defects, and strain on non-radiative recombination losses in hybrid perovskite semiconductors. Using combinations of scanning-probe, electron backscatter diffraction (EBSD), and time-resolved photoluminescence microscopy and spectroscopy, we explore the microscopic origins of non-radiative recombination in both archetypal methylammonium lead tri-iodide (MAPI), and more advanced mixed-cation perovskites. Using EBSD, we image local grain orientation and strain, and correlate surface strain with increased non-radiative recombination in MAPI. We explore the interface between the perovskite and various common charge extraction layers, combining experiment and simulation to show that surface recombination at the charge extraction layers is a limitation in current perovskite solar cell architectures. Finally, we explore alternative passivation and extraction layers to propose ways to more routinely approach the theoretical voltage limits in perovskite photovoltaic using scalable process chemistry.

The excellent photovoltaic performance of halide perovskites goes along with a high photoluminescence yield (PL) that makes them suitable for a wide range of photonic devices and various optoelectronic applications, such as photodetectors, lasers and light emitting diodes.
However, due to the complexity of the materials and the related devices, often traditional macroscopic characterisation tools are not able to unveil the physical processes underlying the working principles of the solar cells, especially the transport properties. Indeed, electronic as well as ionic transport are still under investigation. Here we quantify electrons, holes and ions transport and investigate the mutual impact of both transport mechanisms.
To do so, we used innovative characterisation methods that combine hyperspectral imaging system (HI) and a time-resolved fluorescence imaging (TR-FLIM) set-ups. We employed these optical techniques to study lateral transport of charge carriers in hybrid perovskites and probe a large variety of transport parameters (diffusion length, mobility, lifetimes…) which are often cross-linked. Photonic transport is also observed. Besides, measuring the local variations of time resolved luminescence in the nanosecond scale under lateral electric bias allows a direct access to charge carrier collection and transport properties for each kind of charge carrier –electrons and holes-. At long time scale such experiment allows to probe the ionic transport, especially associated with iodine species. It is confirmed when looking at spectrally resolved images where we can monitor ionic transport a part of the electronic transport and see the impact of the ions migration on the material properties. More precisely, the iodine seems to affect the band structure and locally modify the bandgap; it also degrades the local quasi Fermi level splitting (i.e. the internal bias).
The novel experimental approach, can thus image and quantify the transport mechanisms occurring within the material, providing new insights on these complex transport phenomena.

[1] Delamarre et al. Applied Physics Letters 100, 131108 (2012)
[2] Bercegol et al. J. Phys. Chem. C, 122 (41), 23345 (2018)
[3] Bercegole et al. Nature Comm. 10 1586 (2019)

Hybrid organic-inorganic perovskite photovoltaic materials have been the focus of much research in recent years due in part to their extremely high photoconversion efficiencies[1] and also their ability to be synthesized by solution processing. These materials crystallize in the perovskite, AMX₃ crystal structure where A is the monovalent cation (methyl ammonium, MA, formamidinium, FA, Rb, and/or Cs), M is the metal cation (Pb²⁺), and X is the halide anion (I⁻, Cl⁻, and/or Br⁻). As the field has progressed, formulations of halide perovskite solar cells (PSC) have evolved from the simple MAPbI₃ to more complex alloys, e.g. RbCsMAFA-perovskite.[2] With these more complicated formulations understanding the operation and stability of these materials becomes more challenging. For example, it is important to understand the geometric constraints and so-called “tolerance factors” when formulating new mixed phases in order to target stable structures.[3] Additionally, the composition and phase homogeneity across the film can also influence device performance.

In this study we present results using operando X-ray characterization techniques previously developed by our groups coupled with standard laboratory device measurements.[4] Using this operando technique we are able to monitor the crystal structure by X-ray diffraction (XRD) of the PSC in full device stacks during operation. Here, we explore the phase stability of mixed A-site PSCs of the form XPbI₃ where X = FA, Cs, and/or MA. Simultaneous JV and XRD measurements on devices are measured in both inert and humid (~50% relative humidity) conditions, at room temperature for up to 15 hours. Here we observe degradation pathways for these mixed A-site formulations which result in phase segregated films in some formulations and greater stability in others. To explain the experimentally observed mixing and de-mixing in these systems, we present a hypothesized framework developed using complementary first-principles calculations of mixed A-site halide perovskites. This is then validated using in-situ X-ray diffraction and spatially resolved time of flight secondary ion mass spectrometry. With greater understanding of the structural stability of these alloys we aim to inform well-tuned alloy formulations targeting long term stability in devices.

References
[1] National Renewable Energy Laboratory, “NREL Efficieny Chart.” [Online]. Available: http://www.nrel.gov/ncpv/images/efficiency_ chart.jpg.
[2] M. Saliba et al., Science (2016), 354, 206–209.
[3] Z. Li, et al., Chem. Mater. (2016), 28, 284–292.
[4] L. T. Schelhas et al., ACS Energy Lett. (2016), 1, 1007–1012.

The hybrid metal halide perovskite, CH₃NH₃PbI₃ (herein MAPbI₃) has three crystal phases, orthorhombic, tetragonal, and cubic, with phase transformations at 163K and 327K. While these crystal phases represent the average bulk structure, it is well established that MAPbI₃, and in general the metal halide perovskite family, exhibits large dynamical fluctuations that are believed to play an important role in their optoelectronic and other properties [1-4]. More specifically, in its average cubic phase, MAPbI₃ has been predicted to have tetragonal domains that are both small and dynamic [2-3]. Scattering experiments have observed phase coexistence but did not have the energy resolution necessary to determine if the disorder is static or dynamic. [4-6].
We investigate the hypothesis of dynamic tetragonal domains in cubic MAPbI₃ with high-resolution inelastic neutron scattering from a deuterated single crystal. Measurements of the critical scattering at the tetragonal R-point through the tetragonal-cubic phase transition were performed with thermal and cold neutron triple-axis spectrometers. R-point scattering (in both Q and energy transfer E) persists above the 327K tetragonal-cubic phase transition with decreasing peak amplitude and increasing linewidth. This suggests the presence of small, dynamic, tetragonal domains within the average cubic structure. These results will be compared with scattering from the “central peak” observed in soft mode transitions of other perovskites.[7]
We (MFT, NJW) gratefully acknowledge full support from the Center for Hybrid Organic Inorganic Semiconductors for Energy (CHOISE), an Energy Frontier Research Center funded by the Office of Basic Energy Sciences, Office of Science within the U.S. Department of Energy through contract number DE-AC36-08G028308.
References
[1] A. Gold-Parker, P.M. Gehring, J.M. Skelton, I.C. Smith, D. Parshall, J.M. Frost, H.I. Karunadasa, A. Walsh, MF Toney, Proc Nation Acad Sci 115, 11905–11910 (2018).
[2] C Quarti, E Mosconi, JM Ball, V D’Innocenzo, C Tao, S Pathak, HJ Snaith, A Petrozza, F De Angelis, F. Energy Environ. Sci. 9, 155−163 (2016).
[3] L.D. Whalley, J.M. Frost, Y.-K. Jung, A. Walsh, J Chem. Phys. 146, 220901 (2017).
[4] AN Beecher, OE Semonin, JM Skelton, J.M Frost, MW Terban, H Zhai, A Alatas, JS Owen, A Walsh, SJ Billinge, ACS Energy Lett. 880. (2016).
[5] R. Comin, M.K. Crawford, A.H. Said, N. Herron, W.E. Guise, X. Wang, P.S. Whitfield, A. Jain, X. Gong, A.J.H. McGaughey, E.H. Sargent, PRB, 95, 094301 (2016).
[6] P.S. Whitfield, N. Herron, W.E. Guise, K. Page, Y.Q. Cheng, I. Milas, M.K. Crawford, Sci. Rep. 6, 35685, (2016).
[7] M. Songvilay, M. Bari, Z.-G. Ye, G. Xu, P.M. Gehring, W.D. Ratcliff, K. Schmalzl, F. Bourdarot, B. Roessli, C. Stock, PRM, 2, 123601, (2018).

Impedance spectroscopy is a non-destructive characterization technique that has been used for the characterization of perovskite solar cells since the beginning of this technology. Impedance spectroscopy is a characterization method in the frequency domain that allows to decouple physical processes with different characteristic times at the working conditions i.e. under illumination and applied bias. However, different models and interpretation have been employed to explain the impedance measurements. Despite the huge potentiality of this technique for the characterization of perovskite solar cell a complete model of impedance for this kind of cells applicable in all the conditions and configuration has been elusive for the moment. Undoubtedly, the combined action of electron and holes and ions in perovskite solar cell is at the base of the complex behavior observed in this kind of devices. In this presentation, we compare the well know dye sensitized solar cells with the perovskite solar cells highlighting similarities and differences, and studying the evolution from the former to the later. In addition the interest of impedance characterization of different types of perovskite optoelectronic devices is discussed.

Photovoltaic devices based on halide perovskites with outstanding optoelectronic properties have exhibited tremendous progress in performance. To understand the origin of these properties comprehensively, detailed knowledge on the underlying electronic band structure is required. Here, we present complementary results from low-energy electron diffraction (LEED), angle-resolved photoelectron spectroscopy (ARPES), and density functional theory (DFT) calculations for CH3NH3PbBr3 and CH3NH3PbI3 single crystals. For both, sharp LEED patterns corresponding to the (001) surfaces of CH3NH3PbBr3 and CH3NH3PbI3 were observed together with well-resolved, dispersive valence band (VB) features. Noteworthy, the LEED patterns of CH3NH3PbI3 reveal a coexistence of the cubic and tetragonal phase at the sample surface already at room temperature. From ARPES, we determined a bandwidth of ~1.0 eV and of ~1.3 eV along the X-R direction in CH3NH3PbI3 and CH3NH3PbBr3 single crystals, respectively. In good agreement with results from DFT calculations, the hole effective-mass, mh*, with values as small as 0.18 m0 (CH3NH3PbI3) and 0.25 m0 (CH3NH3PbBr3) were found. Furthermore, the fundamental differences between linear and logarithmic methods in determining the VB onset are discussed and addressed, revealing the fact that logarithmic method is more preferable to determine the VB onset for perovskite systems.[1,2] In addition, surface photovoltage effect (SPV), which has so far hardly been discussed in this context, is found to play significant effect in photoemission studies. We provide evidence that due to the presence of surface band bending, not only visible light but also UV light, as used in ultraviolet photoemission spectroscopy (UPS), induces substantial SPV (over 700 meV) in four modern perovskites. Given such effect, the conflicting energy levels as probed by UPS can be better rationalized.[3] Within the framework laid out here, the consistency of relating the energy level alignment in perovskite-based photovoltaic and optoelectronic devices with their functional parameters is substantially enhanced.


[1] Endres et al., J. Phys. Chem. Lett. 2016, 7, 2722
[2] Zu et al., J. Phys. Chem. Lett. 2019, 10, 601
[3] Zu et al., ACS Appl. Mater. Interfaces, 10.1021/acsami.9b05293.

Metal halide perovskites present ideal properties for both light absorbing and light emitting devices. Despite their promising optoelectronic properties and the progress made over the last decade, perovskites still display instabilities when exposed to environmental stresses such as moisture, oxygen, bias, temperature, and light [1]. Here, we present our ongoing study of the fundamental science surrounding this dynamic behavior, typically detrimental for optoelectronics. Our first focus is the impact of humidity cycling on charge carrier recombination through the use of in situ micro-photoluminescence (micro-PL) [2, 3]. Here, we consider a set of four benchmark compositions based on Cs_xFA_1-xPb(I_yBr_1-y) known for their superior stability and suitability for tandem photovoltaics, where Cs varies between 10-17% and Br within 17-38%. Each of the films is exposed to identical relative humidity (rH) treatments (<5%, 15%, 35%, 55%, and <5% rH) while the luminescence is simultaneously monitored to understand the changes to the optical response. While all samples considered exhibit an increase in overall PL emission upon reaching 35% rH, their luminescence response bifurcates at 55% rH, when the 17%-Br samples sustain their previous enhancement but those containing 38%-Br display a marked reduction in light emission. After returning to a low humidity ambient, all compositions present PL hysteresis dependent on the Br/Cs ratio. The irreversibility of the luminescence after the humidity cycle is due to surface-limited degradation leading to the formation of additional trap states. Next, we evaluate the water-driven recombination dynamics in the prototypical MAPbI₃ and MAPbBr₃ compositions, capturing light emission every ~1 s as the films undergo multiple humidity cycles. Our results indicate that water modifies I-containing material quickly, with abrupt increases in emission when the film is first exposed to water (~30% rH), and as the film returns to a low moisture environment. Contrastingly, the Br samples are less affected by water, and require high humidity (>50% rH) for a modest luminescence enhancement. Given the dynamic behavior of the I-based films, we investigated the impact of excitation wavelength on light emission. Surprisingly, the energy of the incident photons is anticorrelated with the area contained within the rH-PL loop. Based on their absorption depth, shorter wavelength photons create electron hole pairs more likely to nonradiatively recombine at the new, near-surface defects created through our controlled moisture-induced degradation. Inspired by our wavelength-dependent PL measurements, we also measured the voltage dynamics across dark-light-dark cycles for both the MAPbI₃ and MAPbBr₃ compositions through the use of a Kelvin probe and a wavelength tunable light source [4]. While the I-containing film shows a wavelength-independent trend in voltage decay, the Br-containing sample exhibits a 10x slower wavelength-dependent voltage relaxation that displays an exponential trend across the range of 405-605 nm. The slower voltage process is associated with the MAPbBr₃ lattice constant that increases resistance to ion migration back to equilibrium under dark conditions relative to MAPbI₃. The methods employed above can be extended to include the other environmental stressors (temperature and bias) and are material independent, easily applied to other critical compositions, such as lead-free variants. Finally, we will discuss the use of machine learning to understand the impact of the environmental parameters on perovskites recovery [1].

[1] J. M. Howard, et al., Joule, 3, 325– Invited Perspective, FRONT COVER
[2] J. M. Howard, et al., Journal of Physical Chemistry Letter, 9, 3463 (2018)
[3] J. M. Howard, et al., in preparation.
[4] E. M. Tennyson, J. M. Howard, et al., in Review.

CH₃NH₃PbI₃ perovskite layers on compact TiO₂ have been investigated using nitrogen and carbon K-edge x-ray emission (XES) and absorption (XAS) spectroscopy. These techniques, and their combination in Resonant Inelastic X-ray Scattering (RIXS), probe the organic component of the metal-organic compounds and can be used to examine the local partial density of occupied and unoccupied states. The measured spectra are strongly influenced by ultrafast (femtosecond-scale) proton dynamic effects, as modelled using ab initio molecular dynamics combined with density functional theory calculations of the electronic structure. The dynamic effects at the organic component are very similar to what has been observed in comparable molecules in solid, liquid, and gas phase experiments [1-3], and can be well understood by comparing and contrasting with reference materials. The spectral signature and related dynamic effects are influenced by the interaction of the organic and inorganic components of the material.
Experimentally, it is also shown that the organic portion of the perovskite is quickly and strongly influenced by radiation damage, with soft x-rays from a third-generation synchrotron light source undulator causing significant degradation of the material on the time scale of a few seconds. The degradation mechanisms will be discussed in terms of their influence on experimental studies, as well as in terms of the general decomposition of metal-organic perovskite materials.
[1] M. Blum, M. Odelius, L. Weinhardt, S. Pookpanratana, M. Bär, Y. Zhang, O. Fuchs, W. Yang, E. Umbach, and C. Heske, J. Phys. Chem. B 116, 13757−13764 (2012).
[2] F. Meyer, M. Blum, A. Benkert, D. Hauschild, S. Nagarajan, R. G. Wilks, J. Andersson, W. Yang, M. Zharnikov, M. Bär, C. Heske, F. Reinert, and L. Weinhardt, J. Phys. Chem. B 118, 13142−13150 (2014).
[3] L. Weinhardt, E. Ertan, M. Iannuzzi, M. Weigand, O. Fuchs, M. Bär, M. Blum, J. D. Denlinger, W. Yang, E. Umbach, M. Odelius, and C. Heske, Phys. Chem. Chem. Phys. 17, 27145—27153 (2015).

Hybrid organic-inorganic halide perovskites, such as methylammonium lead iodide (CH₃NH₃PbI₃) have emerged as potential hot-carrier (HC) solar cell materials. Despite rapid progress in device performances, the feasibility of HC injection in hybrid perovskites before carrier cooling is not fully understood. One experimental challenge is the difficulty in using optical methods to independently measure electron vs hole dynamics. We have shown that extreme ultraviolet (XUV) spectroscopy in the 40 to 80 eV energy range measures Iodine 4d→5p transitions and provides separate and distinct signals for valence band and conduction band dynamics. We use this technique to measure the rate of electron and hole injection into charge collection layers such as TiO₂, NiO and Co octaethylporphyrin after visible light excitation of the perovskite layer. The intense 3p→3d absorption, also in the XUV energy range, provides a unique spectroscopic fingerprint of the transition metal electronic structure with distinct peak changes corresponding to photoinduced oxidation state changes. The tabletop instrument uses high-harmonic generation to produce 15 femtosecond XUV probe pulses, enabling us to observe both carrier cooling and charge injection on the femtosecond to picosecond time scales.

Leveraging their structural and chemical flexibility, halide perovskites have shown significant optoelectronic performance and stability improvements over the last several years with addition of alkaline metals (e.g. Cs, Rb and K) to the major organic A-site constituents (methylammonium and formamidinium). More recently, several studies have shown improved film quality and solar cell performance with addition of cations targeting B-site incorporation. However, the spatial distribution and actual incorporation of B-site cations has yet to be firmly established. To understand the distribution of these potential dopants and their local effects on optoelectronic properties, we use a combination of X-ray nanofluorescence to probe the elemental composition and nano-diffraction to identify changes in the local crystal structure. We pair these with correlative microscopic evaluations of optoelectronic quality via photoluminescence mapping and E-beam induced current measurements. We thus develop relationships between local chemistry, grain size, crystal quality, and optoelectronic enhancement to identify the impacts of a range of metal cations. This new information on the distribution of cations targeting B-site incorporation rationalizes the wider composition space available to tune the properties of halide perovskite materials.

We have become interested in the addition of thallium to halide perovskites, both in A-site and B-site substitutions. Cs₂AgTlBr₆ (CATB) double perovskite for example offers the lowest band gap in the halide perovskite family of materials with a direct band gap of 0.95 eV in single crystals. Additionally, thallium halides have applications in radiation detectors due to their high mobility and high average atomic number. Controlling the Tl oxidation state is one of the challenges facing the synthesis of these nanocrystals. For instance, we have found that Tl³⁺ readily reduces to Tl⁺ under most reaction conditions. Here, we report progress on thallium bromide nanocrystal chemistry and have developed chemical routes to cubic CsCl-type TlBr and trigonal dolomite-type Tl₂AgBr₃ nanocrystals. TlBr nanocrystal synthesis involves hot injection of trimethylsilyl bromide (TMSBr) into a thallium (III) acetate solution in octadecene, oleic acid, and oleylamine at 70 ^oC. Tl₂AgBr₃ nanocrystals are synthesized using the same reaction conditions with a both thallium (III) acetate and silver (I) acetate in the precursor solution. The nanocrystals are uniform with diameters between 10 and 15 nm. The Tl₂AgBr₃ nanocrystals exhibit an indirect band gap of 3.1 eV and a direct band gap of 3.7 eV. This is the first report of Tl₂AgBr₃ nanoparticle synthesis and the first characterization of the band gap of Tl₂AgBr₃. TlBr has a direct band gap of 3.05 eV, consistent with bulk band gap of TlBr in literature. We were able to assemble face centered cubic (FCC) superlattices of the TlBr and Tl₂AgBr₃ nanocrystals. The size of the TlBr nanocrystals could be varied by changing the reaction temperature, with smaller (~5 + 3 nm) TlBr nanocrystals formed at higher reaction temperatures (110 ^oC).

Fully-inorganic CsPbX₃ (X=Cl, Br, I) perovskite quantum dots (PQDs) are receiving much attention as a photoactive semiconductor in various optoelectronic devices because of their excellent opto-electrical and photophysical features resulting from the intrinsic properties of lead halide PQDs such as size- and composition-tunable optical bandgap, high absorption coefficient, narrow emission, high photoluminescence quantum yield, and high charge-carrier mobility. Recently, CsPbI₃ perovskite quantum dot (CsPbI₃-PQD) solar cells have shown the best device performance in the field of colloidal quantum dot (CQD) solar cells, achieved by methyl acetate (MeOAc) and formamidinium iodide treatments for enhanced electronic coupling within CsPbI₃-PQD solids. MeOAc treatment for CsPbI₃-PQDs generates acetate ions, which substitute for the oleates (act as insulators) on the surface of CsPbI₃-PQDs, as a result of MeOAc hydrolysis. However, the by-products, which are formed during the hydrolysis of MeOAc, induce the cubic-phase lattice distortion, nanocrystal aggregation and undesired surface traps.
Herein, we demonstrate a new surface management strategy for CsPbI₃-PQDs, which removes the oleates more effectively without any side effects. In this strategy, a solution of sodium acetate (NaOAc) in MeOAc directly releases acetate ions minimizing the hydrolysis of MeOAc. This approach improves the electronic coupling in CsPbI₃-PQD solids, by preservation of their nanocrystal size and minimization of surface trap states. As a result, the NaOAc-treated CsPbI₃-PQD solar cells showed a power conversion efficiency (PCE) of 13.3% (higher than a 12.4% of a lead nitrate-treated control device) by only the development of surface management of CsPbI₃-PQDs without the aid of antireflective coating layer. In addition, this performance is comparable to a PCE 13.4% of the previously reported best performance of CsPbI₃-PQD solar cells, which use an antireflective coating layer.

Two-dimensional (2D) layered organohalide perovskites form natural “multiple quantum wells” have emerged as more intrinsically stable materials for solar cells than conventional three-dimensional (3D) halide perovskites (such as MAPbI3, CsPbI3, et al). This is because long organic spacers are inserted between perovskite slabs which can stabilize the structure. The organic ligands are bonded to the perovskite sheet with hydrogen bonds and to each other with van der Waals interactions. It has been demonstrated that the diversity of spacer organic cations greatly impacts the tunability of 2D organohalide perovskites in structural dimensionality as well as optoelectronic properties. Initial work has shown that 2D layered perovskite based solar cell is able to work stably over 46 days in the air with high humanity, however, it has a relatively low power conversion efficiency (PCE) comparing to that of its 3D counterpart. The major reason is attributed to poor charge transport property of the insulating organic spacers. Although the recent finding suggests that the undesired charge loss in the 2D perovskite structure could be overcome by tuning the crystalline orientation, the underlying crystallization kinetics mechanism is largely unexplored. It thus requires enormous work to establish a fundamental understanding of crystallization and factors related to this process, which eventually contributes to the improvement of carrier transport in the 2D perovskite thin films and photovoltaic devices. In our study, synchrotron based in-situ GIXRD technique was used to study crystallization dynamics of 2D perovskite films during fabrication and learn how the length of the organic spacers (PMA, PEA, PPA, PBA) influence the orientation of crystallites. We investigated a fundamental understanding of 2D perovskite crystallization during spin-coating and thermal annealing, which will develop as a guideline for the fabrication of the highly crystallized, oriented, and stable 2D perovskite films.

2D metal-halide perovskites are self-assembled quantum well structures with excellent optoelectronic properties and strongly-bound excitonic states. In these systems, strong spin-orbit coupling and the presence of crystal lattice asymmetry leads to a Rashba type band splitting. We show that the crystal lattice space group and symmetry in layered metal-halide perovskites can be controlled by halogen para-substituents. We report the synthesis and ultrafast electronic state dynamics of layered benzylammonium lead iodide perovskites (4-XC₆H₄CH₂NH₃)₂PbI₄ with X = H, F, Cl and Br. We will present x-ray diffraction and second-harmonic generation experiments from which we extract information on the symmetry of the crystal structure. We demonstrate that the fabricated materials are non-centrosymmetric 2D Ruddlesden-Popper metal-halide perovskites with layer number n=1. Using circularly polarised broadband transient absorption spectroscopy (CTA) we track the spin-polarised populations of the optically active exciton species on ultrafast timescales and present how the modification of the 2D perovskite crystal structure leads to a significant enhancement of the exciton spin lifetime.

Inorganic perovskite quantum dots (IPQDs) have demonstrated remarkable success as a more energy-efficient alternative to well-studied metal chalcogenide QDs, and are attracting attention from the energy and chemical industries for a wide range of applications including photovoltaic devices, LED displays, and solar-enabled organic synthesis (photocatalysis). The facile bandgap tunability at room temperature through anion exchange reactions, high photoluminescence quantum yield, and relatively high defect tolerance, differentiate IPQDs from the other colloidal semiconductor nanocrystals. Despite the groundbreaking advancements of IPQDs in the field, their unique ionic nature (different than metal chalcogenide QDs) require new colloidal synthesis routes and surface chemistries. Conventionally, batch synthesis methods are utilized to synthesize, screen, and optimize solution-processed QDs. However, the massive reaction parameter space associated with IPQDs, in combination with the inherent mass and heat transfer challenges of batch methods, necessitate the utilization of material- and time-efficient synthesis methods for fundamental and applied studies of IQPDs. In this work, we develop and utilize a modular microfluidic platform for accelerated in-situ studies of IPQDs anion exchange reactions with minimum reagent consumption. The developed flow synthesis platform enables precise process control of halide exchange reactions, isolating reaction kinetics from precursor mixing rates in a gas-liquid segmented flow system. Utilizing the modular microfluidic strategy, we study in detail the effects of halide composition, ligand ratios, and halide salt source across reaction (residence) times ranging from 0.5 to 90 s. Capitalizing on the wealth of the systematic studies of anion exchange reaction, enabled by the developed time- and material-efficient strategy, we postulate a three-stage reaction mechanism for the homogeneous IPQDs halide exchange reaction. We complement our in situ findings of IPQDs anion exchange reactions with off-line material characterization techniques, gleaning new insights on the overall understanding of the anion exchange reaction network. The results of our kinetic studies of IPQDs halide exchange reactions in combination with the versatility and performance efficiency of the developed modular microfluidic platform, enable on-demand synthesis of IPQDs with a desired bandgap (composition) for targeted applications in optoelectronics and energy technologies.

Nowadays the study of MAPbX (X=Halide I, Cl or Br) perovskites’ properties is common in the race for having high efficiency and stable solar cells based on these promising materials. The poor stability in the environment of solar cells made with MAPbI₃ has received great attention from researchers, and, on the other hand, there is also a necessity to find lead-free alternatives. One of the alternatives explored to solve the stability problem proposes to replace the organic cation MA for a hydrophobic one; on this regard, phosphonium salts possess this characteristic and they have been used as ionic liquids, so they also have high ionic conductivity as well. In parallel, bismuth and antimony compounds have been studied to replace the lead in the perovskite structure; in most cases, these lead-free solar cells display lower efficiencies, nonetheless showing improved stability at ambient and working conditions. Moreover, the properties of these lead-free perovskites make them excellent candidates to be used in tandem solar cells or other kind of optoelectronic applications, such as light emitting diodes. Conversely, these new materials, in many cases, are not reported in the literature, nor in the materials data bases, hence it is required to perform thorough studies to measure and understand their characteristics and properties. In this work, single crystals of BiI₃-tetraethylphosphonium iodide were obtained by the supersaturation method; this is a new, stable and highly luminescent hybrid perovskite. These (EtP)₃Bi₂I₉ single crystals have displayed thermochromic properties and the single crystal XRD studies have shown that they possess a tetragonal crystalline structure with a space group I42-d and unit cell dimensions of a = b = 27.42 Å and c = 13.72 Å. Based on the results obtained for this new perovskite, tetraethylphosphonium iodide proves to be a promising candidate as an alternative organic cation in the solution of the instability problem. Finally, optical characterization results of the (EtP)₃Bi₂I₉ perovskite show it can be useful in many optoelectronic applications, such as light emitting diodes, photodetectors and tandem solar cells.

Two-dimensional (2D) halide perovskites exhibit excellent potential for optoelectronics because of their outstanding physical properties and structural diversity. Although the optical properties are mainly decided by the inorganic parts, the structures are significantly influenced by the shape and size of the spacer cations. Despite the large amount of recent new 2D perovskite reports, there is no existing rule that can predict the structures of the perovskites before the synthetic experiments. To achieve this, we carefully design our experiments by changing one factor of the organic cation at a time to assess the role each may play. We start with primary diamines NH₂C_mH_2mNH₂ (m = 4−9), using both solution and solid-state grinding method, we get the series (NH₃C_mH_2mNH₃)(CH₃NH₃)_n-1Pb_nI_3n+1 (m = 4−9 / n = 1−4), where m represents the carbon-chain number and n equals layer-thickness number. Even when the carbon-chain length is as short as m = 4, the structures are still closer to the Ruddlesden−Popper (RP) phase. If the primary diamines are replaced by cyclic diamines, the uncommon Dion-Jacobson (DJ) phase can be achieved. Furthermore, when aromatic diamines are utilized instead of the aliphatic ones, fine tuning of the structures can provide better insight for what kind of cations can template the rare DJ phase. Fabrication of photovoltaic devices using these materials shows promising solar cell performances. By using smaller 3-aminopyrrolidinium cation, more unique (110)-oriented perovskites can be synthesized. The highly distorted structures give rise to white-light emission at room temperature. By rational design of the structures for 2D perovskites, we may achieve the optimal properties ideal for their optoelectronic applications

Organometallic halide perovskites (OMHPs) have recently undergone a remarkable development as highly efficient optoelectronic materials for a variety of applications particularly solar cells, light emitting diodes and photodetectors. To fully use the potential of OMHPs, several important challenges must be overcome. One of these challenges is the understanding and control of their defect structures. The available data from multiple research studies suggest that trap-assisted recombination exists in OMHPs despite their large carrier lifetimes, which makes these materials highly attractive, and this has resulted in the perovskite boom over the last decade. Conventional spectroscopy faces serious obstacles in OMHPs due to their low defect concentrations and capture cross section, and therefore the electronic structure of such semiconductors remains poorly understood. Currently, such understanding is limited, restricting the power conversion efficiencies of OMHP solar cells from reaching their Shockley–Queisser limit. In more mature semiconductors like Si, the knowledge of defects is one of the major factors in successful technological implementation. This knowledge and its control can make a paradigm shift in the development of OMHP devices.
The detection of shallow and deep traps is extremely challenging due to non-radiative nature of electrical transitions which cannot be studied by photoluminesce measurements. Moreover, as a result of the low activation energy, shallow traps cannot be detected by thermal emission methods. Here, we report on a deep level and shallow levels defects parameters and their effect on the free charge transport properties of the single crystalline methylammonium lead bromide (MAPbBr₃) perovskite for the first time. The transport properties of single crystal MAPbBr₃ are investigated by Time of Flight (ToF) measurements at several biases. These measurements are further fitted by Monte Carlo simulation allowing us the detection of parameters of energy states in the band gap. The analysis of charge carrier transport by ToF and MC simulation explain the long hole carrier lifetime and memory effect in MAPbBr₃ devices. Here, our studies provide strong evidence for deep levels in OMHPs and open a richer picture of the role and properties of shallow levels in MAPbBr₃ single crystals as a system model for the first time. The presented electrical and charge transport properties are crucial for informing the processing conditions towards the elimination of these defects as was done for classical inorganic semiconductors. The deeper knowledge of the electronic structure of OMHPs could open further opportunities in the development of more feasible technologies.

Chiral materials are of particular interest and have a wide range of potential applications in life science, material science, spintronic and optoelectronic devices. Two-dimensional (2D) hybrid organic-inorganic lead halide perovskites have attracted increasing attention. Incorporating the chiral organic ligands into the layered lead-iodide frameworks have been suggested for achieving the chirality transfer in 2D perovskites, so that the pure 2D perovskites with strong chirality exhibit CD. Meanwhile, 2D perovskites that possess the merits of both chiral materials and perovskites can realize circularly polarized light (CPL) emission and detection and thus would be promising materials for many imaging applications including biomedical, optical and spintronic devices. Nevertheless, the study of chiral pure 2D perovskites is still in its infancy. Here we report on the strong CPL emission and sensitive CPL detection in the visible-wavelength range in pure chiral (R-/S-MBA)₂PbI₄ (MBA = C₆H₅C₂H₄NH₃) 2D perovskites, which are successfully synthesized with a needle shape and millimeter size by incorporating the chiral molecules. The chiral 2D perovskites (R-MBA)₂PbI₄ and (S-MBA)₂PbI₄ exhibit an average degree of circularly polarized photoluminescence (PL) of 9.6% and 10.1% at 77 K, respectively, and a maximum degree of the circularly polarized PL of 17.6% is achieved in (S-MBA)₂PbI₄. The degree of circularly polarized PL dramatically decreases with increasing temperature, implying that the lattice distortion induced by the incorporated chiral molecules and/or temperature-dependent spin flipping might be the origin for the observed chirality. Finally, CPL detection has been achieved with decent performance in our chiral 2D perovskite microplate/MoS₂ heterostructural devices. The high degree of the circularly polarized PL and excellent CPL detection together with the layered nature of pure chiral 2D perovskites enables them to be a class of very promising materials for developing and exploring spin associated electronic devices based on the chiral 2D perovskites.

Organo-metal-halide perovskites have gained tremendous attention as potential materials for photovoltaics, demonstrating efficiencies approaching the best silicon solar cells. Many approaches have been adopted to manipulate perovskite formation including anti-solvent processing, compressed-gas treatment, and post-deposition thermal annealing, where films can be deposited using different coating techniques such as spin-coating or blade-coating. Understanding the role of processing strategies on crystallization pathways is of crucial importance as crystallization strongly affects the perovskite film microstructure, its stability, and devices performance. Moreover, crystallization pathways become more complicated for perovskites with a mixed stoichiometric mixture such as Cs_xFA_1−xPbI₃ due to the thermodynamic and kinetic competition to form secondary phases. Herein, using time-resolved x-ray scattering, we investigate the film formation of Cs-FA-containing perovskites with stoichiometry [Cs_0.15FA_0.85PbI₃] in situ during spin coating, blade coating, and the subsequent post-deposition thermal annealing, while different processing approaches such as anti-solvent [chlorobenzene (CB)] and compressed-gas (N₂) treatments were applied during film casting.
We show how different processing routes affect the competition between the formation of the non-perovskite δ-phase and perovskite α-phase during film formation. When either anti-solvent or compressed-gas treatment is used, both δ-phase and α-phase are induced during casting, with the δ-phase more dominant in the as-cast film. However, each approach works with different mechanisms; while anti-solvent induces immediate crystallization from the bulk wet film, applying compressed N₂ works by depleting volatiles from the top-surface leading to surface-induced crystallization that occurs after reaching supersaturation. When neither treatment is applied, the as-cast film is mostly amorphous with little non-perovskite δ-phase formation. In addition, we show the evolution of phases during thermal annealing. Our results reveal that, for the non-treated films, crystallization of perovskite α-phase occurs predominantly from amorphous phase rather than δ-phase to α-phase transformation during thermal annealing. However, direct phase transformation of δ-phase to α-phase is more dominant for CB- and N₂- treated films, while perovskite crystallization occurs from an initially ordered film compared to non-treated films. Furthermore, our findings reveal that using blade-coating promotes completely different crystallization pathways than spin-coating, while solvothermal direct crystallization of perovskite α-phase occurs predominantly, leading to overcoming the non-perovskite δ-phase formation without the need for post-deposition thermal annealing. Our work highlights the importance of real-time investigation of film formation which can provide an in-depth understanding of the mechanisms of perovskite formation and help to establish processing-microstructure-functionality relationships.

Solution-processable perovskite nanocrystals (NCs) are gaining increasing interest in the field of photovoltaics because of their enhanced stability compared to their thin-film counterparts. However, charge transfer dynamics in perovskite NC based light-harvesting systems are not well understood. By applying femtosecond differential transmission (DT) spectroscopy we investigate the photoinduced charge transfer from inorganic perovskite CsPbBr₃ NCs to the fullerene derivative phenyl-C61-butyric acid methyl ester (PCBM) for two fundamentally different architectures, namely layer-by-layer heterostructures as well as blend structures. Varying the thickness of the NC layer on top of the PCBM in the layer-by-layer heterostructure, an exciton diffusion length of 290 ± 28 nm for CsPbBr₃ NC is extracted. The diffusion process is followed by an ultrafast exciton dissociation (within 200 fs) at the CsPbBr₃ NC/PCBM interface. In blend structures an overall faster charge transfer process is observed. Furthermore, photoconductivity measurements on a blend structure-based photodetector reveal an effective charge extraction from the active layer resulting in a high photosensitivity. DT measurements on this blend structure including adjacent electron- or hole-transport layers give insight into the extraction process and suggest a certain degree of phase segregation, which assists the charge collection.

Recently, hybrid perovskite materials have emerged as attractive alternatives for realizing cost-effective efficient perovskite solar cells. To date, impressive efficiency has been realized from the state-of-the-art solar cells through generic interface engineering and film morphological manipulation of perovskite active-layer in macroscopic scale. To further boost the efficiency of perovskite solar cells, microscopically tuning optoelectronic properties of hybrid perovskite materials represents a promising direction. In this study, we report efficient perovskite solar cells by a novel hybrid perovskites material that is incorporated with heterovalent neodymium cations (Nd³⁺). As compared with pristine hybrid perovskite materials, Nd³⁺-doped hybrid perovskite materials possess superior film quality with highly reduced trap-states, significantly enlarged charge carrier lifetimes, dramatically enhanced and balanced charge carrier mobilities. As a result, planar heterojunction perovskite solar cells by Nd³⁺-doped hybrid perovskite materials exhibit highly reproducible power conversion efficiency of 21.15% and significantly suppressed photocurrent hysteresis. These findings open a new window of tuning the optoelectronic properties of hybrid perovskite materials and boosting the device performance of perovskite solar cells.

Solar energy represents one of the most important renewable energy sources. It can be converted to electricity and chemical fuels by photovoltaics and photoelectrochemical devices. The efficiency of solar energy conversion is governed by ultrafast carrier dynamics at solar energy conversion material interfaces. To understand such interfaces and develop next generation of solar energy conversion devices, ultrafast spectroscopic probes are needed. By developing transient reflectance spectroscopy to monitor the surface carriers that are within ~ 10 nm of the surface emerging solar cell materials, crucial parameters such as surface recombination and carrier diffusion are determined. Our results indicate the formation of 2D layered structure on Sn based perovskite solar cell surfaces that could effectively reduce surface recombination by 10-fold and increase carrier diffusion length to 2.5 µm.

The power conversion efficiency of light harvesting devices is limited by the rapid thermalization of charge carriers that are photoexcited with energies above the bandgap of the absorbing material. As these so-called ‘hot’ carriers are difficult to collect, their cooling places an upper bound on the available photon energy that a given solar cell may utilize. Recently, studies on hot carriers in methylammonium lead iodide (MAPbI₃) perovskite have noted that they cooldown at an appreciably slower rate than carriers in other photovoltaic materials. However, the relaxation rate is still rapid in absolute terms, and hence, their capture and collection (before relaxing to the band edge) have not been shown so far. Here we demonstrate and explain the efficiency of hot carrier extraction from MAPbI₃ using TiO₂ and Spiro-OMeTAD as an electron transporting layer (ETL) and hole transporting layer (HTL), respectively, via real-time observation of the carrier dynamics with femtosecond transient absorption spectroscopy and supported by density functional theory (DFT) calculation. Time-resolved experiments establish that a quasi-equilibrium distribution of the hot carriers is directly populated upon excess-energy excitation of the pristine perovskite. This quasi-equilibrium distribution of hot carriers while not appreciably affected by the presence of TiO₂, is virtually absent in the presence of Spiro-OMeTAD, which is indicative of efficient hot hole extraction at the interface of MAPbI₃. DFT calculations predict that deep energy levels of MAPbI₃ exhibit electronically delocalized character, causing a strong overlap with the localized charge of the valence band of Spiro-OMeTAD lying on the surface of MAPbI₃. Consequently, the hot holes could be easily extracted from the deep-energy levels of MAPbI₃ by the HTL. These findings reveal the origins of efficient hot hole extraction in perovskites and offer a practical blueprint for optimizing interlayers of perovskite solar cells in order to enable hot carrier utilization.

With their excellent optoelectronic properties, the practical application of single-crystalline organolead halide perovskite materials is limited by the lack of a method to prepare high quality perovskite single crystals in large dimension. Herein, we report our development of a low temperature-gradient crystallization (LTGC) method for high-quality CH₃NH₃PbBr₃ (MAPbBr₃) perovskite single crystals with lateral dimension as large as two inches. The theoretical analysis suggests that a small temperature gradient should be used to restrain the growth condition, particularly the solution concentration, within the optimal single-crystal-growth (OSCG) zone. The solubility curve as a function of temperature reveals a sharp turning point at 60 °C, across which the first-order solubility derivative (dC/dT) shows very different behaviors: below this temperature, the dC/dT changes dramatically as the temperature increases, while above this temperature, the dC/dT enters a plateau where further temperature change has little effect on the derivative, meaning that one can attain both a substantial crystal growth rate and crystallization yield below this temperature. Utilizing this discovery, a MAPbBr₃ single crystal as large as 47 × 41 × 14 mm is obtained with high quality via the LTGC method. The single crystal exhibits the best optoelectronic quality among all MAPbBr₃ materials reported in the literature, including the best trap state density, mobility, carrier lifetime, and diffusion length. These superior optoelectronic properties are further transferred into a high-performance planar photodetector. The device exhibits high operational stability, high external quantum efficiency (13,453%), excellent detectivity as high as 8 × 10¹³ Jones, and a fast response speed as quick as 15.8 µs.

Organic-inorganic halide perovskite (OIHP) materials beyond the classic methylammonium lead iodide (MAPI) are actively researched for use in perovskite solar cells (PSC) and perovskite-based light emitting diodes (PLED). Specifically, the use of other inorganic cations such for formamidimium (FA) has been reported to be advantageous, in particular in PLED. While the minimum of the conduction band (CBM) and the maximum of the valence band (VBM) of OIHPs are typically dominated by states due to the inorganic sub-lattice (such as PbI), it was recently suggested (PCCP 2019, 21, 8161) based on cluster calculatons of FA-doped MAPI that FA-centered states may be close to the CBM (with fractions of an eV). This could have significant effect on electronic properties, as such states could be occupied (forming neutral FA species) by photoexcitation (in PSC) or electron injection (in PLED).
Here, we compute ab initio the properties of bulk and nanoclusters of FAPI and compare them to those of FA-doped MAPI. We consider the effects of van der Waals interactions and of the choice of the functional (GGA or hybrid) on the bandstructure. We observe FA-centered states in proximity to the CBM and a strong nanosizing effect whereby FA-centered states may fall through the CBM of the PbI sub-lattice in small nanoparticles. The results suggest that one indeed should consider effects of FA-centered states in OIHP using formidimium either as a dopant or as majority cation (FAPI).

An ideal solar cell contact collects either the electron or hole with no energy loss while perfectly rejecting the other carrier. Nonideal charge transfer at the interface between absorber and contact can severely limit the efficiency of solution-processed solar cells like perovskites. Interfacial layers (IFLs) such as 2,2’,7,7’-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9’-spirobifluorene (spiro-OMeTAD) are frequently used to help improve charge transfer and therefore efficiency. However, the field lacks systematic methods of studying how IFLs impact efficiency due to a few key challenges. These include separating interface phenomena from bulk processes, separate quantification of contact selectivity and interfacial recombination, and measuring the effects of operando solar cell function on charge transfer. Our work seeks to address these challenges to answer lingering questions about IFLs. For example, there are mixed reports concerning whether the selectivity or recombination of spiro-OMeTAD-modified contacts is more influential in determining solar cell characteristics (primarily V_oc). It is well known that the addition of Li-TFSI (lithium bis(trifluoromethane)sulfonimide, a common additive) facilitates the oxidation of spiro-OMeTAD in air, effectively p-doping the film and shifting the Fermi level down. This shift would intuitively increase the hole selectivity of the contact and should therefore increase the V_oc. However, the V_oc of perovskite cells decreases with increased Li-TFSI concentration. Because increased interfacial recombination theoretically decreases V_oc, recombination could be more influential in determining V_oc than the selectivity. However, there are no quantitative reports of the selectivity or recombination of spiro-OMeTAD-modified contacts, which this study seeks to address. To do so we use the silicon interdigitated back-contact solar cell as a tool to experimentally measure the selectivity and recombination. Additionally, we use numerical simulation to extract the corresponding exchange current densities (J_os) as charge transfer parameters for both electrons (J_on) and holes (J_op). We find that spiro-OMeTAD IFLs both with and without 25 mol% Li-TFSI strongly affect both the selectivity (defined as J_on/J_op) and recombination (defined as (J_onJ_op)^1/2) of gold contacts. Neat spiro-OMeTAD causes gold to become two orders of magnitude less hole selective while the addition of Li-TFSI causes it to become three orders of magnitude more hole selective (a five order of magnitude increase from neat). Neat spiro-OMeTAD causes the recombination to go down by six orders of magnitude while the presence of Li-TFSI causes it to go down by five orders of magnitude (an order of magnitude increase compared to neat). Recall that reported perovskite V_ocs go down with increasing Li-TFSI concentration; a decrease in hole selectivity or an increase in recombination would cause this to occur. We find that both the hole selectivity and recombination increase upon addition of Li-TFSI indicating that the recombination change dominates decreases in V_oc. We emphasize that only by measuring both selectivity and recombination can we definitively come to this conclusion. We also observe that when the solar cell using spiro-OMeTAD-modified gold as the hole selective contact is operated in the power quadrant, the recombination decreases and the hole selectivity increases. From these results we conclude that changes to the selectivity and recombination properties of spiro-OMeTAD-modified contacts contribute to hysteretic behavior in perovskite solar cells. Our methods of studying charge transfer at spiro-OMeTAD interfacial layer-modified solar cell contacts shed light on the complex interplay between contact selectivity, recombination, and solar cell parameters. These insights will help the field engineer targeted interfacial layers for improved perovskite and other solution-processed solar cell efficiency.

Herein, we present a new family of bilayered lead bromide perovskites, incorporating long chain aliphatic alkylammonium cations as the organic spacer between the inorganic sublattices with formula (C_mH_2m+1NH₃)₂(CH₃NH₃)Pb₂Br₇ (m = 6−8). Structural characterization by single-crystal X-ray diffraction demonstrates uniaxial expansion of the stacking axis of the studied perovskites, as the length of the alkylammonium cation increases. Differential scanning calorimetry was used to study the reversible phase transitions of the reported bilayered 2D perovskites. The semiconducting properties of the materials were examined by UV-Vis absorption and photoluminescence (PL) emission measurements. The studied compounds have a similar absorption spectra that are typical of layered perovskites, with an absorption edge around 2.85 eV and an excitonic peak above the bandgap. PL at room temperature displays a strong narrow emission, close to the energy value of the excitonic peak in the absorption spectra. The band structure of the materials was calculated based on density functional theory (DFT), which affirms that the materials are direct band gap semiconductors. Film fabrication demonstrates that the layered perovskites can successfully be cast into thin films through the hotcasting technique. Switching spectroscopy- piezoresponse force microscopy (SSPFM) on thin films shows polarization switching, indicating ferroelectric behavior along with the non-centrosymmetric structures of the materials. Thus, tuning the spacer cation of the 2D hybrid lead bromide perovskite structure by incorporating long-chain alkylamines, provides a platform to combine ferroelectricity with the excellent optical properties and stability of layered perovskites for fundamental and applied studies.

There is an increasing interest in two-dimensional (2D) Ruddlesden-Popper perovskites for solar harvesting as a result of their superior chemical stability as compared to their bulk counterparts.^1,2 Both purely 2D and blends of 2D/3D phases have already been successfully employed in solar cells with reported efficiencies of >18% and >22%, respectively.^3,4 With this increasing technological relevance of 2D perovskites, it is essential to understand the physical processes that govern their opto-electronic performance. Particularly, the reduced dimensionality in 2D perovskites results in excitonic excited states, dramatically modifying the dynamics of charge collection. While charge carrier dynamics in bulk systems is increasingly well understood, a detailed understanding about the spatial dynamics of excitons in 2D perovskites is lacking.⁵
Here, we present the direct measurement of the intrinsic diffusivities and diffusion lengths of excitons in single crystalline 2D perovskites using time-resolved microscopy.⁶ This technique allows us to follow the temporal evolution of a diffraction limited exciton population with sub-nanosecond resolution, revealing the spatial and temporal exciton dynamics.⁷ Using the versatility of perovskite materials, we study the influence of the organic spacer, the halide and cation composition, as well as the dimensionality (n = 1 and 2) on the diffusion dynamics of excitons in 2D perovskites. We find that changes in these parameters can lead to differences in diffusion lengths of up to two orders of magnitude. Our results provide important insights into the spatial exciton dynamics in 2D perovskites and yield clear design rules for more efficient 2D perovskite solar cells and light emitting devices.

1. Krishna, A., Gottis, S., Nazeeruddin, M. K. & Sauvage, F. Mixed Dimensional 2D/3D Hybrid Perovskite Absorbers: The Future of Perovskite Solar Cells? Adv. Funct. Mater. 29, 1806482 (2019).
2. Ortiz-Cervantes, C., Carmona-Monroy, P. & Solis-Ibarra, D. Two-Dimensional Halide Perovskites in Solar Cells: 2D or not 2D? ChemSusChem 12, 1560 (2019).
3. Yang, R. et al. Oriented Quasi-2D Perovskites for High Performance Optoelectronic Devices. Adv. Mater. 30, 1804771 (2018).
4. Liu, Y. et al. Ultrahydrophobic 3D/2D fluoroarene bilayer-based water-resistant perovskite solar cells with efficiencies exceeding 22%. Sci. Adv. 5, eaaw2543 (2019).
5. Straus, D. B. & Kagan, C. R. Electrons, Excitons, and Phonons in Two-Dimensional Hybrid Perovskites: Connecting Structural, Optical, and Electronic Properties. J. Phys. Chem. Lett. 9, 1434 (2018).
6. Michael, S. & Prins, F. Direct Measurement of Exciton Diffusion in Two-Dimensional Ruddlesden-Popper Perovskites. In preparation.
7. Akselrod, G. M. et al. Visualization of exciton transport in ordered and disordered molecular solids. Nat. Commun. 5, 3646 (2014).

Two-dimensional perovskites are an emerging new class of materials with potential application in a broad range of opto-electronic devices. These materials are formed by layers of inorganic metal-halide octahedrals separated by large organic cations. The organic cations improve the stability and give a large freedom to tune the opto-electronic properties. However, the organic cations used so far act only as a dielectric non-conductive layer that contributes to the large exciton binding energy of these materials. This large binding energy limits their application in opto-electronic devices in which efficient charge carrier separation is required (such as solar cells, photo-detectors and photo-catalists). In order to achieve charge separation, we consider the replacement of non-functional organic cations by a strongly electron accepting moiety.

We have explored the introduction of functional organic chromophores theoretically by density functional theory calculations and show that it is possible to introduce conjugated molecules that have a significant effect on the electronic structure. Strong electron acceptors or donors lead to conduction band or valence band edges that are localized on the organic part of the materials. This could lead to enhanced charge separation.

To test this idea experimentally we have introduced strongly electron accepting perylene diimide (PDI) molecules in 2D CsPbBr3 nanoplatelets as a model system for 2D perovskites. We show by ultrafast transient absorption and fluorescence that photoexcitation of these perovskite nanoplatelet:PDI conjugates leads to fast quenching of the fluorescence, fast decay of the bleach of the nanoplatelets and the appearance of a photoinduced absorption feature specific for the the PDI anion. This latter feature unequivocally shows that the fluorescence quenching is due to electron transfer to the PDI molecules. Finally, we show by time-resolved microwave conductivity (TRMC) measurements that this charge transfer leads to long-lived hole conduction (tens of microseconds) in the two-dimensional perovskite nanoplatelets. This opens up a new synergistic approach, where the properties of two-dimensional perovskites can be tuned for specific device applications by introduction of strong functional dyes in the organic component of the material.

Halide perovskites are generating enormous excitement for their use in high-performance yet inexpensive optoelectronic applications. Nevertheless, a number of fundamental questions about these materials still remain and need to be answered to push devices to their theoretical performance limits. For example, we still know very little about the specific nature of the defects leading to trap states, carrier recombination pathways or anisotropies of carrier diffusion.

In this talk, I will present a number of new techniques we are developing to try to address these open questions in 2D and 3D halide perovskite semiconductors. These techniques focus on understanding charge carrier behavior, including recombination, trapping and diffusion, and how these properties link to chemical and material properties. I will present a high-resolution luminescence microscopy technique employing two-photon excitation to allow us to visualize and time-resolve carrier diffusion in three-dimensions, revealing anisotropic and depth-dependent carrier diffusion properties. Furthermore, we link the local luminescence properties to high-resolution crystallographic and chemical properties using synchrotron nano-probe X-Ray beamlines and low-dose scanning electron diffraction measurements. Through these measurements, we start to reveal the nature of the defects associated with local non-radiative power losses and heterogeneous diffusion and provide guidelines about how we can ultimately eliminate these unwanted loss pathways and homogenize carrier diffusion.

Organic and inorganic metal halide perovskites have emerged in recent years as revolutionary semiconductor materials for lighting and energy harvesting applications. Many of their facinating properties originate from the fact thet metal-halide perovskites should be classed as crystalline liquids rather than hard semiconductor materials. In this talk I will discuss excitons and polaron in perovskites.

I will start by describing the exciton fine structure splitting in semiconductors, which reflects the underlying symmetry of the crystal and quantum confinement. Since the latter factor strongly enhances the exchange interaction, most work has focused on nanostructures. Here, we report on the first observation of the bright excitone structure splitting in a bulk semiconductor crystal, where the impact of quantum confinement can be specially excluded, giving access to the intrinsic properties of the material. Detailed investigation of the exciton photoluminescence and reflection spectra of a bulk methylammonium lead tribromide single crystal reveals a zero magnetic field splitting as large as 200 µeV. The observed splitting can be understood in the exciton picture combined with symmetry considerations.

Next I will discuss the formation of the polaron, which is the results of the exciton – phonon coupling. The polaron formation is made in evidence by observation of the exciton effective mass enhancement. In the end I will describe some optical properties of a very new system: hollow perovskites, which exhibit optical properties of the excitons in 3D and quantum confinement regime.

The open-circuit voltage (V_OC) is currently considered to be the main limitation on the path to approach the radiative (Shockley-Queisser) efficiency limit in hybrid perovskite-type solar cells. The V_OC is determined by recombination processes in the solar cell including bulk, interface and/or contact recombination.[1] Under ideal conditions the open-circuit voltage approaches the internal quasi-Fermi level splitting (QFLS), which may be estimated by measuring the external photoluminescence quantum yield (PLQY) and the absorption properties.[2] The PLQY and quasi-Fermi level splitting can also be estimated through time-resolved photoluminescence (TRPL) measurements, if the radiative recombination constants and photon recycling are taken into account properly.[3] A survey of the literature shows that the reported PLQY values for given open-circuit voltages sometimes vary by orders of magnitude which is difficult to reconcile from a theoretical perspective. In this contribution, we show that careful consideration of the above points leads to a consistent picture of the interrelation between the QFLS, PLQY and TRPL lifetime. Finally we discuss possible sources of error in the analysis and how to cross-check data.

[1] M. Stolterfoht C. M. Wolff, J. A. Márquez, S. Zhang, C. J. Hages, D. Rothhardt, S. Albrecht, P. L. Burn, P. Meredith, T. Unold and D. Neher, Nature Energy 3 (2018) 847.
[2] Z. Liu, L. Kruckemeier, B. Krogmeier, B. Klingebiel, J. A. Marquez, S. Levcenko, S. Oz, S. Mathur, U. Rau, T. Unold, and T. Kirchartz., ACS Energy Lett. 4 (19) 110.
[3] F. Staub H. Hempel, J.-C. Hebig, J. Mock, U. W. Paetzold, U. Rau, T. Unold, and T. Kirchartz, Phys. Rev. Appl. 6 (2016) 044017.

Grain boundaries (GBs) have been established to play a vital role in determining the power conversion efficiency of organic-inorganic halide perovskite (OIHP) thin film solar cells. As a special category of GBs, ferroelastic twin boundaries (TBs) are recently discovered to exist in both CH₃NH₃PbI₃thin films and single crystals (SCs), however their impact on the carrier transport and recombination in OIHP materials remains unexplored. Here, using the scanning photocurrent microscopy, we have demonstrated the uniform distribution of electric field across the TBs in OIHP SCs as long as the influence of the crystal surface is well suppressed through surface passivation, suggesting that the TBs have little influence on the carrier transport across them. Also, the photoluminescence (PL) imaging and the spatial-resolved PL intensity and lifetime results confirm the electronic benign nature of the TBs, in strong contrast to regular GBs that will block the carrier transport and also behave as the non-radiative recombination centers. Our observations reveal the fundamentally different role of TBs and GBs played in carrier transport of OIHP materials, which will provide insights into the performance improvement of the OIHP based optoelectronic devices.

Metal-halide perovskites have emerged as excellent solution-processable semiconductors for optoelectronic applications such as solar cells and light-emitting devices. Substitution of the monovalent cations has advanced luminescence yields and device efficiencies. Here, we control the cation alloying to push optoelectronic performance through alteration of the charge carrier dynamics in mixed-halide perovskites. In contrast to single-halide perovskites, we find high luminescence yields for photo-excited carrier densities far below solar illumination conditions. Using time-resolved spectroscopy we show that the charge-carrier recombination regime changes from second to first order within the first tens of nanoseconds after excitation in these films. Supported by temperature-dependent microscale-mapping of the optical bandgap and chemical composition, electrically-gated transport measurements and first-principles calculations, we demonstrate that spatially-varying energetic disorder in the electronic states causes local charge accumulation, creating photo-doped regions, which unearths a strategy for efficient light emission at low charge-injection in solar cells and LEDs.

Perovskite solar cells continue to gain momentum in research and gain in efficiency, and multijunction perovskite solar cells offer a roadmap to performance levels well beyond that possible with single junction silicon or other thin-film technologies. Operational stability is an area which is critical for any PV technology, and even more so for perovskites, which are dissimilar to
conventional semiconductors in a number of ways. I will present different approaches we have adopted to improving the efficiency, and fundamental
stability of the perovskite absorber materials and devices, and highlight degradations which can occur in the bulk of the perovskite absorber materials and induced by the charge extraction heterojunction. I will give further insight into what factors influence stability, and how to mitigate degradation. Beyond stability, I will highlight how moving from a single absorber layer, to a multijunction cell should lead to much higher efficiencies, and I will show experimental realisation
of progress along such a road map, including record perovskite-on-silicon PV cells. Beyond research, I will highlight our progress towards manufacturing scale-up through the technology company, Oxford PV Ltd., and key challenges which need to be overcome to deliver an industrialised perovskite PV technology.

State-of-the-art perovskite materials demonstrate photoluminescence quantum efficiencies (PLQE) above 90% due to low non-radiative recombination rates and unparalleled defect tolerance. The optoelectronic properties that have allowed perovskites to emerge as a leading active layer material in high efficiency thin film photovoltaics (PVs) – high absorption coefficient, small Stokes shift, high PLQE, solution processability, chemical tunability – simultaneously situate perovskites to function superbly as a coherent quantum material. In this work, we explore perovskites as a platform for strong light-matter coupling to sustain all-optical operations.Though light is weakly interacting, it is possible to form interacting quasi-particles, called exciton-polaritons, that have characteristics of both light and matter. Traditionally, polaritons have been studied at cryogenic temperatures in all-inorganic semiconducting materials (e.g. GaAs heterostructures). In this work, we study the room-temperature formation of exciton-polaritons with large Rabi splittings in semiconductor microcavities, using solution-processed 2D perovskites as self-assembled quasi-quantum well structures for the active layer. Polariton formation is probed by angle resolved reflectivity and photoluminescence measurements through a k-space imaging setup. The polariton-polariton interaction strength is explored by the confinement of photo-generated exciton-polaritons via modifications to the potential environment of the microcavity, presenting opportunities to study room-temperature Bose-Einstein condensation. The realization of facilely-fabricated room-temperature exciton-polaritons has the potential to revolutionize a wide range of devices, from PVs to low-threshold lasers to all-optical switches.

An unusual combination of low exciton binding energy and strong optical absorption renders hybrid organic-inorganic perovskites interesting candidates for efficient absorber materials in next-generation solar cells. This interesting behavior triggered numerous studies to better understand excitonic effects, e.g. in MAPbI₃. In experiment, exciton binding energies in this material range from as high as 62 meV to as low as 2 meV. At the same time, a line shape of the absorption edge similar to that of GaAs was reported, with no clear excitonic peak and a binding energy potentially less than 10 meV at room temperature. In contrast, cutting-edge first-principles simulations based on the Bethe-Salpeter framework seem to consistently overestimate exciton binding energies.

In this work, we use density-functional and many-body perturbation theory to study atomic geometries, electronic structure, and optical properties of MAPbI₃ and find good agreement with earlier results and experiment. Electronic band structures and gaps are predicted using Hedin’s GW approximation, including the spin-orbit interaction. We account for free electrons within our first-principles theoretical spectroscopy and specifically study their impact on the electron-hole interaction. This allows us to conclude that additional screening due to free electrons can explain consistently smaller exciton binding energies, compared to those in the material without free electrons. Furthermore, we also observe that the absorption line shape in MAPbI₃ with free electrons strongly resembles that of the spectrum without free electrons up to high free-electron concentrations. Interestingly, our simulations show that this unexpected behavior can be explained by formation of Mahan excitons. These dominate the absorption edge and we show that this makes this material robust against free-electron induced changes of the optical absorption that are observed in other semiconductors.

Perovskite solar cells (PSCs) have set themselves apart from their dye-sensitized (DSSC) and organic (OSC) predecessors, with impressive efficiencies at 24%. Despite the key role that charge transport layers (CTLs) have played in this success, researchers are still debating the importance of parameters that govern charge transfer at the PSC interfaces, namely: (i) the interfacial energy offset, ΔE between the perovskite and CTL; (ii) CTL structure; and (iii) intrinsic perovskite properties.
This talk will detail our progress in developing a complete picture of electron/hole transfer at PSC interfaces using a combination of time-resolved spectroscopy and device work.^1,2 Specifically, we will first outline our observation of highly efficient (>75%), nanosecond charge transfer to CTLs at remarkably low values of ΔE (<0.1eV).
We will also show that careful selection of CTLs with electron-donating functional groups can slow interfacial recombination by passivating surface traps, as has been previously observed with Lewis bases, inert polymers and other molecules.^3-5 Remarkably, the resulting coordination between the perovskite and CTL produces a 3-fold enhancement in charge transfer yield. Finally, we will discuss the impact of such passivation on device stability.^6,7
This novel insight into the charge separation mechanism in PSCs goes some way to explaining the success of triarylamines (e.g. PTAA, PTPD) as HTLs in such systems and has important implications for the design of next-generation CTLs.

1. R. J. E. Westbrook et al, J. Phys. Chem. C 2018, 122, 1326−1332
2. R J. E. Westbrook, W. Xu et al, Manuscript in Preparation
3. N. K. Noel et al, ACS Nano, 2014, 8(10), 9815
4. L. Zuo et al, Sci Adv., 2017, 3, e1700106
5. W. Xu et al, Nat. Photonics, 2019, 13, 418
6. Aristadou et al, Nat. Commun., 2017, 8, 15218
7. X. Bu et al, Solar RRL, 2019, 3, 1800282

Although significant progresses have been made toward to optoelectronics application including solar cells, large color gamut LEDs, photodetectors, and X-ray detectors, the fundamental understanding of ultrafast carrier dynamics of organic-inorganic perovskite materials remains unclear. The ultrafast dynamics, which reveals some novel physical phenomena such as hot carrier cooling, phonon bottle-neck effect was widely studied by ultrafast optical spectroscopies, which include pump-probe transient absorption (transmission, reflection, time-resolved THz, optical Kerr effect, and the most popular time-resolved photoluminescence(TRPL). However, it remains a challenge to study the perovskite optoelectronic devices in-situ in an ultrafast fashion.
In this talk, we use an ultrafast photocurrent spectroscopy with sub-25 picosecond time resolution to reveal the evolution of ultrafast carrier dynamics from sub-25 ps to microsecond in-situ perovskite solar cells and photoconductors. We address the basic questions of carrier photogeneration, recombination, transport, trapping, in addition to directly extracting carrier mobility, lifetime, and the property of trap states such as density, energy level, and capture cross-section.

This year, we are celebrating 10 years of perovskite solar cell (PSC). Since our first discovery of PSC in 2009, enormous efforts have been put into different aspects of PSCs and the progress has been incredibly fast on all fronts. While preparing a comprehensive review¹ on the background, on-going R&Ds, and future direction of PSC research recently, we realized that, although efficiency level has gone beyond 24%, PSCs face serious challenges of practical stability and durability required for industrialization. Although compositional engineering of perovskites by mixing different cations and anions, using modulator molecules and mixing 2D and 3D structures have doubtlessly improved the stability of perovskites against heat and moisture, use of organic moieties still remain a challenge to improve the stability further. Intrinsic stability of the perovskite crystal structure and robust properties of carrier transport materials are going to be the keys to the long term durability of the device. In this respect, use of all-inorganic perovskite materials and combination with dopant-free carrier transport materials are highly desired. We have conducted some work in this direction which includes stabilization of CsPbI₃ black phase ² and use of dopant-free hole transport materials (HTMs). Dopant-free HTMs combined with all-inorganic perovskites have yielded sufficiently high efficiency of 15%. Intrinsic thermal stability of the device was improved without using diffusible dopants. In our collaboration with JAXA, stability of perovskites was investigated in space environment for satellite applications. Here, organic cations in perovskite such as methyl ammonium are instable under exposure to vacuum and high temperatures >100^oC. Perovskite materials demonstrate its high stability against exposure to high energy particle radiations (proton and electron beams) up to dose of 10¹⁵ particles/cm² due to use of thin absorber films that can avoid accumulation of particles and also exhibit defect tolerant nature. ³ The lecture will also introduce our current efforts in making PSCs based on both lead and lead-free perovskites, and future perspectives of perovskite photovoltaics.
A. K. Jena, A.，Kulkarni, T. Miyasaka, Chem. Rev. 2019, 119, 3036–3103.
A. K. Jena, A. Kulkarni, Y. Sanehira, M. Ikegami, and T. Miyasaka, Chem. Mater. 2018, 30, 6668-6674.
Y. Miyazawa, M. Ikegami, H.-W. Chen, T. Ohshima, M. Imaizumi, K. Hirose, T. Miyasaka, iScience 2018, 2, 148-155.

Halide Perovskites (HaPs) present a remarkable case of just stable (against decompositon into binaries) compounds that can function as active component for PV, light-emission and radiation detection , all demanding functions, over time periods that seem incompatible with their free enrgy of formation (again, from the binaries). Actually, the Achilles heel of the materials is their surface, true to Pauli's famous dictum, because of the law of mass-action: as long as the system remains a hermetically close one, it can withstand the onslaught of electronic carriers and photons. Add to that that it are to a large extent the interfaces that make the devices function (again following a famous dictum, this time from Kroemer) and.. those are made with surfaces. Thus, what might appear esoteric materials chemistry and physics issues, are highly relevant for device design. In principle, apart from the remarkable self-healing ability (known to some exent also for CIGS) this is not new as nearly every (inorganic) semiconductor material that we use today went through the process of taming its surfaces to get control over interfaces made with them. In this talk I will, depending on developments in the half year between abstract writing and presentation, present data on the self-healing and (de)stabilization processes of HaPs and put their behaviour in perspective with respect to some other semiconductors, to arrive at insights that should be useful to device design and building.

Surfaces of metal halide perovskites (MHP) present an interesting set of questions and challenges that are only beginning to be addressed, namely the existence, origin, density and energy of electronic surface gap states. MHP surfaces are prone to ion diffusion and chemical reactions, and to degradation under various environmental conditions and probing tools, all of which likely induce some density of electronic gap states. In sufficient density, these states affect the electronic structure and optoelectronic performance of all MHP interfaces and devices. There is currently no consensus on potential profile and occurence of band bending in perovskite films, from substrate to surface. Available data point to a strong dependence of these profile on the nature and surface potential of the substrate on which the films are fabricated, and on the perovskite processing conditions. Limits in Fermi level excursion have been observed and also point to some low density of (surface or bulk) gap states.[1] To address some of these issues, several groups have started to use organic molecular dopants (reductants and oxidants), as “gentle” probes of the electronic occupation and density of surface states, and as modifier agents of these states. [2,3] This talk describes our recent investigations of the interaction between molecular reductants (mostly [RuCp*Mes]₂) and oxidants (Mo(tfd)₃ and derivatives) and surfaces of 3D MHPs CsPbBr₃ and FA_xMA_1-xPb(I_yBr_1-y)₃, and the 2D Ruddlesden-Popper phase BA₂PbI₄. Using a combination of electron spectroscopies and contact potential measurements, we determine the sign and magnitude of surface photovoltage occurring at these free surfaces, and the extent of surface energy level shifts resulting from charge exchange with the molecular dopants. We also show evidence obtained via photoemission spectroscopy of a density of (surface) filled states above the valence band maximum of FAPbBr₃ following electron bombardment, and of MAPbI₃, likely due to the presence of DMSO in the precursor solution. The role of surface doping in changing the occupation of these states will be discussed.

[1] Zohar et al., ACS Energy Lett. 4, 1 (2019)
[2] Zu et al., ACS Appl. Mat. & Interf., 9, 41456 (2017)
[3] Perry et al., Adv. Electron. Mater. 4, 1800087 (2018)

The surface of hybrid perovskites plays a crucial role in the performance and stability of optoelectronic devices, as it strongly influences the recombination rate of excited charge carriers. Recently, it has been reported that molecular ligands such as benzylamine are capable of significantly reducing the surface trap state density in thin films. Here I will report on the mechanism that governs the surface passivation of hybrid perovskites by benzylamine. To this end, we developed a versatile approach to investigate the influence of benzylamine passivation on the well-defined crystal surface of freshly cleaved methylammonium lead tribromide single crystals. We show that benzylamine is capable of permanently passivating surface trap states in these single crystals, resulting in enhanced photoluminescence intensities and charge carrier lifetimes. Additionally, we show that exposure of the perovskite surface to benzylamine leads to replacement of the methylammonium cations by benzylammonium, thereby creating a thermodynamically more stable two-dimensional perovskite (BA)2PbBr4 on the surface of the 3D crystal. This conversion from a 3D to 2D perovskite drives an anisotropic etching of the crystal surface, with the {100} planes being most prone to etching.

The fields of photovoltaics, photodetection and light emission have seen tremendous activity in recent years with the advent of hybrid organic-inorganic perovskites. Yet, there have been far fewer reports on the photo-responsive transfer property of perovskite materials.¹ The lateral and interfacial transport requirements of transistors make them particularly vulnerable to surface contamination and defects rife in polycrystalline films and bulk single crystals. Here, we demonstrate a spatially-confined inverse temperature crystallization strategy which synthesizes micrometre-thin single crystals of organometal trihalide perovskite with sub-nanometer surface roughness and very low surface contamination. The photo-responsive transfer property of perovskite materials were investigated and record, room-temperature field-effect mobility in both p and n channel devices were reported, with 10⁴ to 10⁵ on-off ratio and low turn-on voltages.² This work paves the way for integrating hybrid perovskite crystals into printed, flexible and transparent electronics.

Reference:
1. Y.-H. Lin, P. Pattanasattayavong, T. D. Anthopoulos, Advanced Materials 2017, 29, 1702838.
2. W. Yu, F. Li, L. Yu, M. R. Niazi, Y. Zou, D. Corzo, A. Basu, C. Ma, S. Dey, M. L. Tietze, U. Buttner, X. Wang, Z. Wang, M. N. Hedhili, C. Guo, T. Wu, A. Amassian, Nature Communications 2018, 9, 5354.

Organic-inorganic hybrid perovskites ABX₃ have emerged as revolutionary solar absorbers with power conversion efficiencies rapidly increasing from 3.8% in 2009 to 24.2%. However, perovskite solar cells suffer from poor long-term stability, especially the moisture. Hydrophobic cations such as aliphatic alkylammonium and aromatic alkylammonium have been used to improve the resistivity to moisture, leading to the formation of the quasi 2-dimensional perovskites (A₁)₂(A₂)_n-1B_nX_3n+1 (where A₁ is large-size bulky organic cation and A₂ is Cs, methylammonium or formamidinium).
Different from their 3D counterparts, previous reports have shown that the fabricated quasi-2D perovskite films are actually a mixture of multiple perovskite phases with different n values instead of a homogeneous perovskite phase with an identical n value. These multiple phases are arranged spontaneously along the direction perpendicular to the substrate from small-n to large-n (from substrate side to the top surface of films). Vertical phase distribution plays an important role in the quasi-2D perovskite solar cells. So far, the driving force and how to tailor the vertical distribution of layer number (n) have been not discussed.
In this work, we report that the vertical distribution of n in the quasi-2D perovskite films deposited on PEDOT:PSS is different from that on glass substrate. The vertical distribution of n could be explained by the sedimentation equilibrium because of the colloidal feature of the perovskite precursors. But the addition of acid will change the precursors from the colloid to solution that therefore changes the vertical distribution. A self-assembly layer of 4-bromobenzenediazonium tetrafluoroborate covalently anchored onto the PEDOT:PSS induces the appearance of desired vertical distribution. The quasi-2D perovskite solar cells (n = 4) with the surface anchoring layer display a higher open-circuit voltage of 1.11 V and a higher efficiency of 13.74% comparing to the reference quasi-2D perovskite cells. The enhanced performance is attributed to the faster charge transport and lower trap densities.

Hybrid organic-inorganic perovskites with appealing opto-electronic properties have attracted significant interest for photovoltaic application. The use of chloride (Cl⁻)-containing precursor species to induce improved perovskite thin-film microstructure and improved optoelectronic properties is well-established. However, the mechanism for the formation of perovskite films with highly textured, micron-sized grains in the presence of Cl⁻ remains elusive. Using synchrotron-based, in situ two-dimensional grazing incidence wide-angle X-ray scattering (GIWAXS) complemented by scanning electron microscopy (SEM) imaging, we present an oriented attachment mechanism via mineral bridge formation for the microstructural evolution of chloride-derived perovskite films. We have identified the crucial role of the oriented chlorine-containing intermediate phase as the mineral bridge, which templates the reorientation of primary, nanoscale perovskite grains followed by fusion into a uniaxial oriented monolithic grain. The resulting perovskite films exhibit micron-sized grains with preferential orientation of the tetragonal (110) direction perpendicular to the substrate with improved solar cell efficiency. Further device physics analysis also shows that highly textured perovskite films exhibit improved charge transport across the interface between the perovskite (110) crystal facet and charge transporting layers (CTLs). Our findings pave the way for rational microstructural design of chloride-derived perovskite films and highlight control of perovskite grain orientation for further exploration of the relationship between perovskite thin film microstructure and photovoltaic properties.

Perovskite solar cells (PSCs) have attracted considerable attention due to their outstanding power conversion efficiency (PCE) as high as 24.2%. A conventional PSC consists of electrode, hole transport material (HTM), electron transport material (ETM), and a conductive substrate. HTMs, among them are of importance component for high performance PSCs due to their efficient hole extraction, transportation, and blocking the electron transfer to the electrode. The major concern for HTMs is the materials used, especially spiro-OMeTAD which may be prone to detradation. Therefore, developing novel HTMs to replace spiro-OMeTAD is required to enhance the stability and efficiency of PSCs. Here, we present DFT calculations combined with the Marcus theory to investigate the electronic and charge transport properties of porphyrin-based HTMs and device performance containing these materials. Based on the calculated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy level of reference molecules, we screened molecules which can be suitable alternatives to spiro-OMeTAD. Among them, tetraphenylporphyrin (TPP) shows the most promising properties which are relatively low HOMO and high LUMO energy level that facilitate comparably high open-circuit voltage and low reorganization energy and high transfer integral ensuring high hole mobility. TPP was synthesized and utilized as a HTM in solar cell device. Interestingly, the PCE of TPP was shown to be comparable to that of spiro-OMeTAD. Here we proposed new strategy to develop novel organic small molecules as HTMs by DFT calculation for stable and efficient PSCs and showed TPP may become one of promising alternatives to spiro-OMeTAD.

In this study, the effect of the monovalent cation on monomolecular and bimolecular recombination rate constants in halide perovskites (HaPs) was investigated. Based on ultrafast optical measurements on HaP thin films with consistent morphology and trap site densities, our results reveal a key insight: the monovalent cation plays a different role in monomolecular and bimolecular recombination mechanisms; indicating that this remarkably customizable optoelectronic material may be more tunable than previously thought.

Lead halide perovskites (LHPs) continue to rise with promise for efficient optoelectronic applications including photovoltaics. A wide variety of deposition techniques and treatments have been developed for LHPs with the goal of improving crystallization and thus efficiency and stability. Of particular interest to us is the exposure of perovskite films to methylamine gas. Such treatment has been shown to liquefy the film and dramatically decrease grain boundary defects in the treated film, especially at elevated temperatures. To better understand this reaction, we applied in-situ UV-visible spectrophotometry in controlled conditions of temperature and methylamine gas pressure to obtain a quantitative phase diagram highlighting solid, solid intermediate, and liquid phases. In this presentation, we discuss the impact of forcing crystallization along paths through this phase diagram in a unique system with no foreign solvent. Additionally we stress the importance of the liquid-substrate interaction as evidenced by contact angles measured between liquid perovskite and various substrates. Lastly, we extend these findings and methods to the two-dimensional system of butylammonium lead iodide and demonstrate controllable partial conversion. These insights may lead to more rational tuning of crystal quality, grain size, and hybrid 3D/2D properties.

The success of halide perovskites has often been attributed to their defect tolerance. While there have been a number of studies showing that intrinsic defects in organic-inorganic hybrid halide perovskites tend to exhibit shallow defect levels only, whether such a conclusion can be extended to pure inorganic perovskite semiconductors is still an open question. In particular, in the past several months, pure inorganic perovskite solar cells have been demonstrated to achieve appealing efficiency (~17%). In this work, we systematically investigate the intrinsic defects in representative halide perovskites. We will present the results obtained with hybrid density functional including spin-orbital coupling (SOC) effect using large supercells with an aim of concluding the existence of defect tolerance based on the state-of-the-art computational technique. Several possible origins of the defect tolerance, including the SOC, the unusual bonding nature of the band edge states and the competition between ionicity and covalency will be discussed.

Thanks to numerous studies focusing on enhancing performances of organic-inorganic halide perovskite (OIHP)-based photoelectronic devices, the impressive improvements have been reported. Representatively, since the emergence of OIHP-based photovoltaics in 2012, the powerconversion efficiency (PCE) of OIHP-based photovoltaics exceeds 24% nowadays. As strategies to enhance performance, compositional, interfacial engineering and developing novel deposition methods are utilized. However, as growth in the PCE has stagnated by 20%, another method is required for tuning intrinsic properties of OIHPs to ultimately overcome the hurdles, which block the way toward commercialization.
In this work, we suggest pressure-induced crystallization process as a technique for improving performance of OIHP-based photoelectronic devices by engineering intrinsic properties of OIHPs. As high-quality OIHP films can be fabricated with annealing anti-solvent washed intermediate-state, we exerted pressure during crystallization process with nanostructured polymer molds. By pressing OIHP films during annealing, nanostructures of OIHP films are controlled. By pressing the pre-crystallized film with intaglio nanohole-arrayed polyurethaneacrylate (PUA) molds, nanopillar structure emerges on the surface of OIHP films. Because of the aligned surficial nanostructure of OIHP films, light scattering effect was observed on the surface of OIHP films, which are induced by photonic crystal effect. Also, nanopillar arrays were clearly observed with scanning electron microscopic (SEM) and atomic force microscopic (AFM) images.
With nanopillar-structured OHIP films, we developed 2-terminal photoconductors to confirm improved photoelectronic characteristics after the process. Firstly, we measured current-voltage (I-V) characteristics of devices employing OIHP films, which exhibited increased photocurrent and decreased dark current and responsivity and detectivity are enhanced by an order. Furthermore, motivated by the fact that pressure-induced crystallization process contributes to improve OIHP film quality, we analyzed trap-density and electrical properties with conductive AFM (c-AFM) and revealed that trap-density of film was reduced and electrical conductivity was significantly enhanced.
In conclusion, we fabricated a nanopillar structure on the surface of OIHP film through applying simple soft imprinting technology and confirmed the advantages of the process by investigating photophysical properties of films. Considering that the process we suggested can be conducted with low cost and high reproducibility, the pressure-induced crystallization process can be a promising solution that can overcoming recent stagnation in terms of OIHP-based device performance.

X-ray imaging is an important technique used for medical imaging and the nondestructive inspection of industrial products. Highly sensitive X-ray detectors which enable X-ray imaging at low dose are strongly required to reduce the risk of cancer. Lead halide perovskites (APbX₃; A = CH₃NH₃⁺, Cs⁺, X = Br^–, I^–) are promising candidates for X-ray detectors due to their excellent physical properties including high absorption coefficients, high carrier mobility, and long carrier diffusion length. Y. C. Kim et al.[1] achieved CH₃NH₃PbI₃-based X-ray imaging devices with the high sensitivity of up to 11 μC mGy_air^–1 cm^–2 which is at least ten times as high as the sensitivities achieved with currently used amorphous selenium or thallium-doped cesium iodide detectors. W. Wei et al.[2] reported Si-integrated CH₃NH₃PbBr₃ single crystals for highly sensitive X-ray imaging with the sensitivity of 25 μC mGy_air^–1 cm^–2. The biggest problem of organic-inorganic hybrid perovskites, which is partly composed of an organic cation such as CH₃NH₃⁺, is instability to heat and moisture. To solve this problem, replacing the organic cation by Cs⁺ has been proposed[3]. For X-ray imaging, fabrication of thick CsPbBr₃ films over a large area is required. However, previous methods such as spin-coating are not applicable to the fabrication of CsPbBr₃ films with the thickness of more than 10 μm.
Therefore, we are working on the fabrication of thick CsPbBr₃ films by using a mist deposition method. This method is good at large-area film fabrication[4]. In addition, this method seems to have potentials for the increase of film thickness by repeating fabrication process because a precursor solution is atomized and the mist is injected to the pre-heated underlying film, resulting in fast solvent-evaporation and the growth of the film without remarkable dissolution damages.
In this conference, we report the fabrication of thick CsPbBr₃ films with the thickness of 100 μm. The influence of substrate temperature and deposition rate on the orientation of CsPbBr₃ films were mainly investigated. At optimized fabrication conditions, the obtained CsPbBr₃ thick films were composed of columnar crystals and had high carrier mobility of 28 cm² V^–1 s^–1, which is comparable to that for single crystals.

[1] Y. C. Kim et al., Nature 550, 87 (2017) [2] W. Wei et al., Nature Photonics 11, 315 (2017) [3] M. Kulbak et al., J. Phys. Chem. Lett. 7, 167 (2016) [4] T. Ikenoue et al., Thin Solid Films 520, 1978 (2012)

The energy level alignment that occurs at the interfaces in planar-hetero structured perovskite photovoltaic devices strongly influences the charge transport across the interface, and thus plays a crucial role in overall device performance. To directly observe the energy level alignment requires pristine homogeneous surfaces that are free of contamination including adventitious carbon. Coevaporation offers the ability grow perovskite thin films insitu, and the method involves thermally evaporating the perovskite precursors such as PbI2 and CH3NH3I. Some early reports have shown that the perovskite film formation and stoichiometry are problematic at ultralow coverages when the film is just starting to form, often times it was reported that there was excess PbI2 and a deficiency in CH3NH3I. Using photoemission spectroscopy, we investigated the perovskite precursors PbI2 and CH3NH3I on gold and highly oriented pyrolytic graphite (HOPG) surfaces. Results show that the nature of the surface and the deposition conditions can strongly influence the film formation. Excessive iodine observed in the initial evaporation stages appears to be substrate dependent, and this may influence the overall energy level alignment.

The ability to be processed in solution, provides Hybrid Organic-Inorganic Perovskite (HOIP)-based solar cells a unique cost advantage over existing technologies for materials like silicon, chalcogenides or III-Vs. However, while the morphology of the final crystalline thin film is known to be heavily influenced by solvent selection, these effects remain poorly understood. Further, experimental routes to study the nucleation and early stage growth of these materials are limited in resolution and scope. In contrast, molecular simulation models and techniques have a significant role to play in this endeavour by providing exquisitely detailed thermodynamic and kinetic information that help uncover how solvent-solute interactions influence the formation of moieties that will ultimately nucleate and grow into thin films. These moieties chiefly include haloplumbate complexes (PbX₂ - PbX₆^4-, where X = Cl, Br, or I). Experimental metrics like the Gutmann Donor Number (DN) have previously been used to predict the formation of these complexes in solution. It has been suggested that higher DN solvent environments limit the formation of higher order haloplumbate complexes (PbX₄^2--PbX₆^4-) and delay the nucleation of the perovskite crystal from solution, leading to desirable final film properties. Now, utilizing Density Functional Theory (DFT), we have suggested a DN cut-off value as a solvation threshold to guide the selection of solvents for HOIPs processing. This cut-off predicts whether the formation and distribution of lower order haloplumbates (PbX-PbX₃^-) are preferred in certain solvent environments over undesirable, higher order haloplumbate complexes (PbX₄^2--PbX₆^4-). We identified this solvation threshold by comparing the enthalpic preferences of halide complexation to Pb²⁺ center against the surrounding solvent molecules. Our results identified a range of solvent environments, based on their DN values, that favour solvent coordination to the Pb²⁺ center over halide ions (Cl-, Br-, and I-), limiting the rate of nucleation and Pb-X coordination in these solutions. It was revealed that higher DN solvents are required to outcompete halides for complexation to Pb²⁺ in chloride rich solutions than in solutions rich with bromide and iodide ions, following an electronegativity trend. This predictive model can be used to guide the selection of solvents for specific lead halide environments (PbCl₂, PbBr₂, or PbI₂) to optimize for an improved final film morphology.

Two-dimensional inorganic-organic (2D-IO) hybrid systems are Ruddlesden-Popper phase-like materials which can be tailor-synthesized to achieve desired and tunable thermoelectric, optoelectronic and multiferroic properties. The lead-free inorganic-organic hybrids are the demand of the day for real life applications. Much importantly, these 2D-IO hybrids are easy to synthesize and have shown excellent structural and chemical stability against heat, humidity, temperature and external environment conditions as compared to other inorganic-organic hybrid counterparts. We have investigated tunability in the structural and optical properties of solution processed lead-free 2D-IO hybrids, the (C₁₂H₂₅NH₃)₂Cu(Cl_1-xBr_x)₄ and (C₆H₉C₂H₄NH₃)₂Cu(Cl_1-x Br _x)₄ systems. (C₁₂H₂₅NH₃)₂Cu(Cl_1-xBr_x)₄ are found to be more stable than (C₆H₉C₂H₄NH₃)₂Cu(Cl_1-x Br _x)₄ counterparts. Also, Br substitution at the Cl atomic sites weakens the crystalline phase purity as well as material stability. (C₁₂H₂₅NH₃)₂Cu(Cl_1-xBr_x)₄ systems show uniform crystalline phase for all the compositions 0 ≤ x ≤ 1, whereas (C₆H₉C₂H₄NH₃)₂Cu(Cl_1-x Br _x)₄ exhibit binary crystalline phase for the composition 0 < x ≤ 1 through the splitting of (00l) diffraction peaks. The optical properties are primarily determined by the metal-halogen network of the 2D-IO hybrids. Raman modes are related to the buckling of distorted CuX₆ (X = Cl or Br) octahedral in both of the 2D-IO systems. The Raman vibration frequencies show red-shift by the replacement of higher electronegative Cl atoms with lower electronegative Br atoms due to the reduced bonding strength of Cu-X vibrations. The vibrations related to asymmetric (1582 cm^-1) and symmetric (1490 cm^-1) deformation of NH₃-X bands also systematically redshift towards lower wavenumbers with the increasing Br/Cl ratio in both types of systems. The optical absorption of 2D-IO systems is nearly independent of the organic part and show a systematic shift in the optical features including the band-edge as a function of increasing Br-content.

Hybrid organic-inorganic perovskites (HOIPs) possess the possibilities of enabling photoinduced dielectric polarization. Photoinduced dielectric polarization plays an important role on suppressing the charge recombination, facilitating charge transport, and even polarizing excited states, which are critically important to the development of optoelectronic functionalities in HOIPs. However, it is difficult to realize whether photoinduced polarization contains an electrical polarization within the dipolar polarization regime in HOIPs due to lacking the mechanism of coupling dipolar and electric polarization. This issue originates from mobile ions in HOIPs, which results in the difficulty of detecting dielectric polarization. Here, we use capacitance-voltage (C-V) measurement to probe the photoinduced dipolar polarization and then confirm it by the magneto-capacitance measurement. The capacitance-voltage (C-V) detect the dipolar polarization by applying a low alternating bias of 50 mV while the mobile ions are continuously drifted by gradually scanning the bias from -0.1 V to 1.5 V. In this manner, continuously drifted ions cannot respond to the low alternating bias, enabling the detection of photoinduced electrical polarization in dipolar polarization regime. Then, to confirm the photoinduced dipolar polarization, magneto-capacitance was used to solely detect the dipolar polarization at 1 MHz in HOIPs (MA_xFA_(1-x)PbI₃, x in the range of 0-0.75) under photoexcitation. Magneto-capacitance measurement is a signature tool to exclude the effects of mobile ions (i.e. surface polarization) as mobile ions do not respond to magnetic fields. It was found that the photoexcitation can substantially increase the magneto-capacitance amplitude, confirming that the photoexcitation indeed induces dipolar polarization in HOIPs. In summary, our studies provide a fundamental understanding of photoinduced dipolar polarization effects on the optoelectronic functionalities of HOIPs.

Two-dimensional highly luminescent CsPb₂Br₅ has been considered as a promising candidate for optoelectronic applications in recent years due to its stability at various ambient conditions. CsPb₂Br₅ can be synthesized [1,2] as a green photo-luminescence (PL) emissive material as well as PL-inactive one. This is in part the reason for the controversial results on the band gap and origin of PL reported by different groups [3]. In this work, we present the results of a complex approach to resolve the PL puzzle using the same-spot Raman-PL probe at ambient and high pressure environments. Our study reveals that CsPbBr₃ nanocrystals are the origin of the green emission in CsPb₂Br₅ [4]. The Raman-PL spectra under hydrostatic pressure have ruled out the alternative theory of defect states such as Br vacancies. The pressure-dependent absorption shows that the bandgap of CsPb₂Br₅ is 0.3-0.4 eV higher than those typically reported. It remains nearly constant with pressure up to 2 GPa in accordance with our DFT calculations [4-6]. We further prove that CsPbBr_3-x X_x (X = Cl or I) is also responsible for the PL of CsPb₂Br_5-xX_x [4]. Our results open up new opportunities to understand and develop highly efficient inorganic lead halide optoelectronic devices.
[1] Z. Zhang, Y. Zhu, W. Wang, W. Zheng, R. Lin, F. Huang, J. Mater. Chem. C 2018, 6, 446.
[2] K. H. Wang, L. Wu, L. Li, H. B. Yao, H. S. Qian, S. H. Yu, Angew. Chem.-Int. Edit. 2016, 55, 8328.
[3] J. Bao, V. G. Hadjiev, Nano-Micro Lett. 2019, 11, 26.
[4] C. Wang, Y, Wang, et al., Adv. Mater. 2019, accepted.
[5] I. Dursun, M. D. Bastiani, B. Turedi, B. Alamer, A. Shkurenko, J. Yin, I. Gereige, A. Alsaggaf, O. F. Mohammed, M. Eddaoudi, O. M. Bakr, ChemSusChem 2017, 10, 3746.
[6] T. X. Zhang, Z. H. Chen, Y. M. Shi, Q. H. Xu, Nanoscale 2019, 11, 3186.

Hybrid lead halide perovskites are emerging semiconductors which have demonstrated extraordinary performance on photovoltaic and light-emitting devices. Meanwhile, the potential of perovskites for spin-related optoelectronics has just begun to raise up due to the strong spin-orbital coupling (SOC) for efficient spin manipulation by using optical methods and relatively long spin coherence lifetime for spin-related optoelectronic properties to occur. However, traditional investigations on spintronic devices based on semiconductor/ferromagnet heterojunctions mainly focus on the spin injection and spin transport. The understanding of the magnetism properties at the perovskite/ferromagnet interface remains unclear. By using the depth-sensitive polarized neutron reflectometry studies, we have found that a circularly polarized photoexcitation induces a magnetization into the surface of perovskite (MAPbBr3) film underneath of ferromagnetic Co layer at room temperature. On contrast, a linearly polarized light does not generate any detectable magnetization within the perovskite surface in the MAPbBr3/Co sample during the polarized neutron reflectometry measurement. This observation provides an evidence to show optically induced magnetization on the perovskite surface in contact with Co surface through spin-polarized excited states. Furthermore, the perovskite/Co interface demonstrates a magneto-capacitance phenomenon, indicating that the electrical polarization on perovskite surface is coupled with magnetic polarization on the Co surface. The observed magnetization indicates that circularly polarized light-generated spin states in hybrid perovskite layer can directly interact with ferromagnet Co through electric-magnetic coupling, leading to an optically induced magnetization.

Organic inorganic hybrid perovskites are superior candidates towards EL pumped lasing device, based on their excellent optical gain property and high dielectric constant. However, it is rather challenging due to the low carrier density under electrical operation condition, which requires extremely low threshold for realizing coherent interaction of excited states. Here, we found that, when nanometer-size small grains are used to passivate micrometer-size large grains in perovskite (MAPbBr₃) films, the light-emitting states essentially become linearly polarized in the direction of applied electrical field during EL operation. The EL output from the large-grain component in the mixed large/small grains slowly become linearly polarized with the time constant up to 10 seconds at constant bias in electrical field direction in the ITO/PEDOT:PSS/ MAPbBr₃/Bphen:PMMA/LiF(0.7 nm)/Ag device. This observation indicates that the light-emitting states in large-grain component with low bandgap gradually become electrically polarized during EL operation. Simultaneously, the EL intensity is progressively increasing with similar time scale at constant bias, showing a self-passivation of grain boundary defects occurring at the interfaces between large and small grains within mixed large/small grains under the influence of electric field. Combining these two phenomena leads to a hypothesis that the self-passivation of grain boundary defects provides the necessary condition to enable the electrically induced polarization of light-emitting states in large-grain component located in mixed large/small grains. This hypothesis is verified by the critical observation: when the grain boundary defects are further decreased by continuously decreasing the size of small grains shown by photoluminescence studies, the electrically induced polarization of light-emitting states in large-grain component is largely increased from 19 % to 29 % in the MAPbBr₃ LEDs prepared with mixed large/small grains. Clearly, mixing large/small grains presents an important method to realize electrically induced polarization of light-emitting states in high-performance MAPbBr₃ LEDs.

Winning high performance materials is the most important challenges in modern 3D printing technology. The excellent material properties and low-cost production of organic-inorganic metal halide perovskites make them promising building blocks for fully integrated optoelectronics devices^1,2. The practical realization of perovskite devices necessitates a high-precision control over the shape, composition and crystallinity. Many clever nanofabrication methods^3-5 have been devised to shape perovskites, however, it is still limited to in-plane and low aspect ratio with simple forms. To satisfy the demands for cutting-edge optoelectronics^6,7 with freeform circuitry and high integration density, we developed a nano-precision three-dimensional (3D) printing for organic-inorganic metal halide perovskites. The 3D printing method uses a femtoliter ink meniscus to guide evaporation-induced crystallization in mid-air, fabricating freestanding 3D perovskite nanostructures with a preferred crystal orientation. Stretching the ink meniscus with pulling process enables on-demand control of the nanostructure's diameter and hollowness, leading to an unprecedented tubular-solid transition. By varying the pulling direction, we successfully demonstrated a layer-by-layer stacking of perovskite nanostructures with programmed shapes and positions. In this talk, we will present our results and discuss the prospects of our work for potential applications in customized, freeform optoelectronics.

References:
1. Kojima, A., Teshima, K., Shirai, Y., Miyasaka, T. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. J. Am. Chem. Soc. 131, 6050-6051 (2009).
2. Green, M. A., Ho-Baillie, A., Snaith, H. J. The emergence of perovskite solar cells. Nat. Photon. 8, 506-514 (2014).
3. Oener, S. Z., Khoram, P., Brittman, S., Mann, S. A., Zhang, Q., Fan, Z., Boettcher, S. W., Garnett, E. C. Perovskite nanowire extrusion. Nano Lett. 17, 6557-6563 (2017).
4. Liu, P., He, X., Ren, J., Liao, Q., Yao, J., Fu, H. Organic−inorganic hybrid perovskite nanowire laser arrays. ACS Nano 11, 5766-5773 (2017).
5. Mao, J., Sha, W. E. I., Zhang, H., Ren, X., Zhuang, J., Roy, V. A. L., Wong, K. S., Choy, W. C. H. Novel direct nanopatterning approach to fabricate periodically nanostructured perovskite for optoelectronic applications. Adv. Funct. Mater. 27, 1606525 (2017).
6. Park, S. H., Su, R., Jeong, J., Guo, S.-Z., Qiu, K., Joung, D., Meng, F., McAlpine, M. C. 3D printed polymer photodetectors. Adv. Mater. 30, 1803980 (2018).
7. Kong, Y. L., Tamargo, I. A., Kim, H., Johnson, B. N., Gupta, M. K., Koh, T.-W., Chin, H.-A., Steingart, D. A., Rand, B. P., McAlpine, M. C. 3D printed quantum dot light-emitting diodes. Nano Lett. 14, 7017-7023 (2014).

Dilute magnetic semiconductors (DMS) constitute a material class which combines semiconducting properties with long-range magnetic order by introducing a substantial amount of magnetic dopants with unpaired spins to an otherwise non-magnetic host semiconductor. Fully inorganic DMS have been known for decades and advanced material processing techniques have enabled control over spin injection and the control of magnetism by electric fields and currents, yet only at cryogenic temperatures.
Due to their outstanding optoelectronic properties and high defect tolerance, organo-metal halide perovskites provide an ideal system for efficient magnetic doping. We induce long-range magnetic order in lead-halide perovskite by partially substituting lead with manganese and copper, using simple solution and solid state processing techniques. We perform temperature and magnetic field dependent magnetometry to characterise the magnetic ordering mechanisms of the material system. Polarisation-dependent, low temperature magneto-photoluminescence measurements reveal coupling between localised magnetic impurities and optically excited charge carriers, towards optical control of spin-states.

Organo-lead halide perovskite is promising candidate material class for various optoelectronic applications including solar cells due to excellent charge transport properties and defect tolerance. Although its record high efficiency is reaching 24.2%, understanding of its photo-physical properties and operating mechanism of solar cell still needs further investigations. In particular, abnormal built-in potential across illuminated MAPbI₃ perovskite surface has been reported by a number of studies, and ion migration and lattice poling have been suggested as possible origins. However, investigations on polycrystalline perovskite thin-films often possess limitations on correlating internal charge separation mechanism with lattice structure due to the high degree of structural defects as well as an uncontrollable crystal orientation.
In this study, single-crystal perovskite is investigated to provide single domain which enable facet-dependent characterization of the charge behavior such as change in electrical polarization. We scrutinize the aspect of facets of the single-crystal perovskite by employing Kelvin probe force microscopy. Simultaneous changes in surface potential in response to illumination and transient pulsing are measured and analyzed quantitatively. On (100) and (112) plane, light induced poling effect inversely affects potential as fermi level is shifted under light. We also quantify the relation between the magnitude of the poling and angle of MA-I bond from the surface measured. Absence of electrical polarization effect perpendicular to (110) plane manifests MA-I bonds are realigned when light is shed upon. Finally, growth of MaPbI₃ (110) plane on crystalline substrate is suggested to enhance the structural stability against illumination as well as optoelectronic properties.

A fundamental understanding of the nucleation and growth processes involved in synthesis of lead halide perovskite nanocrystals (NCs) is unrealized, due to the rapidity of the associated reaction kinetics¹. Accordingly, an ability to probe such processes on timescales below 100 ms is required. Microfluidic systems integrating real-time optical detection modules enable such observations, and thus are preferred over traditional flask-based environments². Indeed, microfluidic reactors have been used for the controlled synthesis of a variety of nanoscale materials, allowing the fast and efficient exploration of the lead halide perovskite NCs reaction parameter space³. More specifically, droplet-based microfluidic systems offer many advantages over classical flask-based synthesis methods through the precise control of reagent concentrations, reaction times and temperatures.
Herein, we describe an optofluidic platform for the real-time monitoring of the fast nucleation and growth kinetics associated with the formation of formamidinium lead iodide perovskite (FAPbI₃) NCs on μs - ms timescales. This platform integrates in-line photoluminescence (PL) and time-correlated single photon counting (TCSPC) modules, for extraction of intensity, spectral and fluorescence decaytime information during nucleation and growth. The high temporal resolution of the platform enables facile monitoring of early time processes, with deadtimes as small as 3 ms. Real-time assessment of NC fluorescence decaytime kinetics at early times during nucleation provides additional contrast to conventional time-integrated fluorescence emission measurements. The experimental set-up consists of two parts: a microfluidic platform for fast droplet generation and precise temperature control, and an integrated optical detection system, allowing both optically-sectioned TCSPC and PL measurements. The optical properties of the early time FAPbI₃ NCs were investigated at room temperature by conducting time-dependent analyses between 3 and 280 ms. As the reaction progresses, we observe that the PL peak shifts to the red, with the emission intensity increasing dramatically over the first 300 ms. This confirms that the kinetics governing NC nucleation and growth are indeed very fast, and thus access to early reaction times is critical. The evolution of fluorescence decays with reaction time indicates equally fast reaction kinetics, with a progression towards longer lifetimes. We also investigated the effect of reaction temperatures (up to 60°C) on the PL spectra and decays associated with early-time NCs, whilst keeping all the other experimental conditions constant. An increase in temperature resulted in a faster and further red shift in the photoluminescence maximum, corresponding to larger NCs being formed at higher temperatures. Remarkably, data show for the first time the observation of transient FAPbI₃ NCs species emitting at 558 nm, well below the reported value (790nm) for fully-grown FAPbI₃ NCs.
In summary, we report a novel platform for early-stage, in-line characterization of the nucleation and growth of FAPbI₃ NCs using droplet-based microfluidics. We believe that this platform will allow for a deeper understanding of the fast reaction kinetics associated with perovskite NC synthesis, and for superior engineering of the optical and electronic properties of these materials.

References

1. Protesescu, L. et al. Nano Letters 2015 15 (6), 3692-3696
2. Maceiczyk, R. M. et al. Curr. Opin. Chem. Eng. 2015, 8, 29−35.
3. Lignos, I. et al. Nano Letters 2016 16 (3), 1869-1877

Beyond the admirable photovoltaic properties, the unique opto-electrical properties of organic-inorganic halide perovskite (OIHP) combined with their relatively low-cost production, have made this class of materials a great candidate in photodetectors¹, LEDs², and radiation sensors³. The remarkable progress has driven accelerated efforts to further improve the performance and stability of these materials. However, the phenomenological applications have greatly outpaced the fundamental understanding of the physical processes which govern the material’s electronic behavior. Key among those are at the electrode interface^{4, 5}. Exploiting the coupled electronic and ionic properties of these materials requires exquisite control over the electrode contact region. Development of strategies for realizing reliable contacts have been key to the success of semiconductor device fabrication, whereas this challenge is magnified for materials with mixed ionic and electronic carriers like OIHPs. In addition, we have shown that OIHPs consist of volatile elements like halides, which can react with the metallic electrode leading to the interfacial doping⁶. Charge injection at electrode-OIHPs interface may induce interfacial trapped states and recombination regions leading to unfavorable effect on charge collection efficiency,^{7, 8} induce electrochemical reactions and result in interfacial degradation^{4, 8}. Research has shown that the onset of electrochemical reactions and degradation in OIHP devices under long term operating conditions is at the electrode interface⁵. Recently, we demonstrated that the triple phase boundary between electrode, bulk, and environment can affect the charge transport properties of MAPbBr₃ devices⁹.
The lack of understanding stems largely from the lack of appropriate tools to capture the electrochemical dynamics on the length scales of the local inhomogeneities and time scales over which the coupled dynamics take place. We implemented time resolved Kelvin probe force microscopy (tr-KPFM) to explore the spatial and temporal charge dynamics at MAPbBr₃ devices^{10, 11}. Here, the temporal dynamics of the electric field and charge distribution at the electrode interface is visualized by tr-KPFM mapping. The results demonstrate an interplay of several phenomena, including charge injection, recombination and ion migration, leading to an unbalanced charge dynamic in MAPbBr₃-Au interface under forward and reverse biases, explaining the origin of the current-voltage hysteresis in these devices. We contrast the bias assisted charge dynamics under both illuminated and dark conditions, providing a comprehensive picture of overall carrier dynamics and interface properties in MAPbBr₃ with lateral Au electrodes. Remarkably, illumination leads to formation of a wider space charge region due to accumulation of negative charges (electrons and halide ions) at the positive electrode, which can effectively screen the external electric field leading to lower charge collection efficiency. The results suggest that the choice of contact or interfacial engineering can control the performance of OIHP devices without requiring modification of the material’s bulk properties.
1. M. Ahmadi et al., Adv. Mater. 29 (41), 1605242 (2017).
2. Y. Meng, M. Ahmadi et al., Org. Electron., 64, 47 (2019).
3. E. Lukosi, T. Smith, J. Tisdale, D. Hamm, C. Seal, B. Hu and M. Ahmadi, Nucl. Instr. Meth. Phys. Res. A, 927, 401 (2019).
4. H. Wang et al., Energy Environ. Sci., (2019).
5. P. Schulz et al., Chem. Rev., 119 (5), 3349 (2019).
6. T. Wu, R. Mukherjee, M. Ahmadi et al., J. Am. Chem. Soc., 139 (48), 17285 (2017).
7. J. T. Tisdale, E. Muckley, M. Ahmadi et al., Adv. Mater. Interfaces 5 (18), 1800476 (2018).
8. S. T. Birkhold et al., ACS Energy Lett., 3 (6), 1279 (2018).
9. M. Ahmadi et al., ACS Appl. Mater. Interfaces, 11 (16), 14722 (2019).
10. L. Collins, M. Ahmadi et al., Nanotechnology 29 (44), 445703 (2018).
11. L. Collins, M. Ahmadi et al., ACS Nano 11 (9), 8717 (2017).

The interface between CsPbBr₃ and CuI is studied using density functional theory and band offset calculations to explore the most likely atomic configuration and charge redistribution as a function of the number of atomic layers. The surface and interface energies for the (100) surfaces are calculated for the two terminations in CsPbBr₃ (CsBr and PbBr₂ termination) and CuI (Cu and I termination) using the bond cleaving method; eight unique interfaces exist between CsPbBr₃ and CuI. PbBr₂ on Cu termination with Br bonded to Cu and PbBr₂ on I termination with Pb bonded to I are found to be the low energy interfaces. The potential barrier between the layers decreases as the system increases from 7 to 9 atomic layers showing a type I band offset with a gradient pointing from CuI into CsPbBr₃. This indicates the potential for thin film CuI as a charge transport layer for CsPbBr₃ active layer devices.

Inorganic perovskite quantum dots (QDs), especially CsPbX3 (X=Cl, Br, I), have been intensively studied in solar cells, LEDs and photodetectors due to their high photoluminescence (PL) quantum yield, strong absorption cross section, optical tunability and improved stability [1, 2]. Phototransistors, which are fabricated via hybridizing perovskite QDs, have been reported to have high responsivity and detectivity [3, 4]. However, perovskite QDs passivated with oleic acid (OA) ligands make it difficult to have efficient charge transport from QDs to the channel material in a phototransistor, which results in large hysteresis and low signal to noise ratio (SNR). In this work, charge transfer at the interface of CsPbBr3 QDs and Indium gallium zinc oxide (IGZO) was studied based on the platform of QD/IGZO phototransistors. By systematically controlling the displacement of OA in the CsPbBr3 film mediated by solvent polarity engineering, the performance of QD/IGZO phototransistors were remarkably improved. The responsivity of the QD/IGZO phototransistors were enhanced to 8 times larger than the reference samples. The SNR was also boosted from 1 magnitude to more than 6 magnitude. These performance improvements indicate the charge transfer enhancement at the CsPbBr3/IGZO interface, which was further confirmed through the stationary and time resolved PL analysis associated with exciton quenching dynamics.

[1] Protesescu, Loredana, et al. "Nanocrystals of cesium lead halide perovskites (CsPbX3, X= Cl, Br, and I): novel optoelectronic materials showing bright emission with wide color gamut." Nano letters 15.6 (2015): 3692-3696.
[2] Stranks, Samuel D., et al. "Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber." Science 342.6156 (2013): 341-344.
[3] Zou, Chen, et al. "A Highly Sensitive UV–vis–NIR All Inorganic Perovskite Quantum Dot Phototransistor Based on a Layered Heterojunction." Advanced Optical Materials 6.14 (2018): 1800324.
[4] Liu, Xiang, et al. "Dual-gate phototransistor with perovskite quantum dots-PMMA photosensing nanocomposite insulator." IEEE Electron Device Letters 38.9 (2017): 1270-1273.

We report here effect of interlayer spacing in 2-dimensional (2D) layered perovskites (LPs) of [C₆H₅(CH₂)_nNH₃]₂PbI₄ (anilinium (An) for n = 0, benzylammonium (BzA) for n = 1 and phenylethylammonium (PEA) for n = 2) on resistive switching performance. Interlayer spacing was increased from 6.98 Å to 13.29 Å for (An)₂PbI₄, 14.20 Å for (BzA)₂PbI₄ and 15.92 Å for (PEA)₂PbI₄ as chain length was increased as revealed by X-ray diffraction (XRD), where monolayer of organic cation is intercalated between inorganic PbI₄^2- layers. Memristor devices were fabricated with the structure of Ag/ PMMA (polymetylmethacrylate)/ LP/ Pt. All these samples showed bipolar switching behavior, where the devices underwent the abrupt SET process near +0.2 V and the gradual RESET process within -0.5 V. Devices were operated by ohmic conduction in low resistance state (LRS, ON state) and hopping conduction in high resistance state (HRS, OFF state). ON/OFF ratio, which means the ratio of LRS (ON state) to HRS (OFF state), was increased from 10⁶ to 10⁸ as interlayer spacing was enlarged due to gradual increase in resistance at HRS, associated with insulating properties enhanced with interlayer. Although endurance and retention were slightly improved from 1.3×10² cycles and 2×10³ s for (An)₂PbI₄ device to 2.2×10² cycles and 5.5×10³ s for (PEA)₂PbI₄ device, this negligible increment was related to intralayer filament formation pathway.

Organic–inorganic halide perovskites have become the “next big thing” in emerging semiconductor materials, with their unprecedented rapid development and successful application in high-performance photovoltaics. Yet, their inherent instabilities, notably in the presence of moisture, remain a crucial challenge for these materials. This has focused the interest of the scientific community on perovskites derivatives such as 2D perovskites. These materials are significantly more stable and possess a higher tunability of their properties significantly expanding the field of their application from energy harvesting through LEDs to single material white light emitters. The variety of possible ways to tune the optical properties of 2D perovskites is their huge advantage, while at the same time, the mutual dependence between different tuning parameters hinder our fundamental understanding of their properties. In this work we attempt to address this issue for (C_nH_2n+1NH₃)₂PbI₄ (with n=4,6,8,10,12) by means of optical spectroscopy in magnetic field up to 67T. Our experimental results, supported by DFT calculations, clearly demonstrate that the reduced mass of the exciton increases by around 30% in the low temperature phase. This is reflected by a 2-3 fold decrease of the diamagnetic coefficient. Our studies show that the effective mass which is essential parameter for optolectronic device operation can be tuned by the variation of organic spacers and/or moderate cooling achievable using Peltier coolers. In addition, we show that often observed complex features of absorption and transmission spectra track each other in magnetic field providing strong evidence for the phonon replica nature of the observed side bands.

Ion migration has been one of the most important effects to be considered for organo-lead halide perovskite optoelectronic devices since it can significantly affect the long-term device stability and performance [1]. On the other hand, the mobile nature of the ionic defects in perovskite materials has opened up possibility for implementing new device functionalities such as neuromorphic and memory devices. Especially, resistive memory devices based on organo-metal halide perovskite materials have recently shown outstanding performances; a low-voltage operation enabled by an ease of ion migration and a high ON/OFF ratio which are essential for realizing low-power consumption memory. In this presentation, we report unipolar resistive memory devices in a cross-point array architecture made by using a non-halide lead source to deposit perovskite films via a simple single-step spin-coating method [2]. Our perovskite memory devices achieved a high ON/OFF ratio up to 10⁸with a relatively low operation, a large endurance, and long retention times. In addition, we discuss a potential resistive switching mechanism for our perovskite memory devices which exhibit a unique unipolar (non-polar) resistive switching unlike the previously reported perovskite memory devices. Furthermore, a direct demonstration of one-diode-one-resistor scheme using our cross-point perovskite memory devices achieved a selective operation of memory cells connected via external diodes. These results, combined with a high-yield device fabrication based on solution-process demonstrated here, will contribute towards developing low-cost and high-density practical perovskite memory devices.

Reference
1. C. Eames et al. Nat. Commun. 6:7497 (2015)
2. K. Kang et al. Adv. Mater. 31, 1804841 (2019).

Lead halide perovskites have emerged as promising new semiconductor materials for high-efficiency photovoltaics, light-emitting applications and quantum optical technologies. Their luminescence properties are governed by the formation and radiative recombination of bound electron-hole pairs known as excitons, whose bright or dark character of the ground state remains unknown and debated [1, 2]. While symmetry analysis predicts a singlet non-emissive ground exciton topped with a bright exciton triplet, it has been predicted that the Rashba effect may reverse the bright and dark level ordering.
Spectroscopically resolved emission from single lead halide perovskite nanocrystals at cryogenic temperatures provides unique insight into physical processes that occur within these materials. At low temperatures the emission spectra collapse to narrow lines revealing a rich spectroscopic landscape and unexpected properties, completely hidden at the ensemble level and in bulk materials.
In this talk, I will discuss how magneto-photoluminescence spectroscopy provides a direct spectroscopic signature of the dark exciton emission of single lead halide perovskite nanocrystals [3]. The dark singlet is located several millielectronvolts below the bright triplet, in fair agreement with an estimation of the long-range electron hole exchange interaction. Nevertheless, these perovskites display an intense luminescence because of an extremely reduced bright-to-dark phonon-assisted relaxation [4]. Resonant photoluminescence excitation spectroscopy allows the determination of the optical coherence lifetimes in these nanocrystals and to assess their suitability as sources of indistinguishable single photons [5].
References:
[1] M. Fu, P. Tamarat, J. Even, A. L. Rogach, and B. Lounis, “Neutral and Charged Exciton Fine Structure in Single Lead Halide Perovskite Nanocrystals Revealed by Magneto-optical Spectroscopy,” Nano Lett., vol. 17, no. 5, pp. 2895–2901, Apr. 2017.
[2] G. Nedelcu, A. Shabaev, T. Stöferle, R. F. Mahrt, M. V. Kovalenko, D. J. Norris, G. Rainò, and A. L. Efros, “Bright triplet excitons in caesium lead halide perovskites,” Nature, vol. 553, no. 7687, pp. 189–193, Jan. 2018.
[3] P. Tamarat, M. I. Bodnarchuk, J.-B. Trebbia, R. Erni, M. V. Kovalenko, J. Even, and B. Lounis, “The ground exciton state of formamidinium lead bromide perovskite nanocrystals is a singlet dark state,” Nat. Mater., pp. 1–9, May 2019.
[4] P. Tamarat, J.-B. Trebbia, M. I. Bodnarchuk, M. V. Kovalenko, J. Even, and B. Lounis, “Unraveling exciton-phonon coupling in individual FAPbI3 nanocrystals emitting near-infrared single photons.,” Nat. Commun., vol. 9, no. 1, p. 3318, Aug. 2018.
[5] L. Hou et al., to be submitted (2019)

Organic-inorganic metal halide perovskites have emerged as attractive materials for solar cells with power-conversion efficiencies now exceeding 23%. As these devices are approaching the Shockley-Queisser limit, bimolecular (band-to-band) recombination will dominate the charge-carrier losses, withtrap-mediated charge recombination becoming less prominent.

We show that in methylammonium lead triiodide perovskite, bimolecular recombination can be fully explained as the inverse of absorption,^[1] and exhibits a dynamic that is heavily influenced by photon reabsorption inside the material.^[2,3] Such photon recycling is shown to slow charge losses from thin hybrid perovskite films, depending on light out-coupling.^[2] Interestingly, for thin films comprising a quasi-two-dimensional (2D) perovskite region interfaced with a 3D MAPbI₃ perovskite layer the blue-shifted emission originating from quasi-2D regions overlaps significantly with the absorption spectrum of the 3D perovskite, allowing for highly effective “heterogeneous photon recycling”. We show that this combination fully compensates for the adverse effects of electronic confinement, yielding quasi-2D perovskites with highly efficient charge transporting properties.^[3]

In addition, we investigate optoelectronic properties of mixed tin-lead iodide and mixed iodide-bromide lead perovskites. We show how band-gap bowing in tin-lead perovskitesis compatible with a mechanism arising from bond bending to accommodate the random placement of unevenly sized lead and tin ions.^[4] While tin-rich compositions exhibit fast, mono-exponential recombination that is almost temperature-independent, in accordance with high levels of electrical doping,^[4,5] lead-rich compositions show slower, stretched-exponential charge-carrier recombination that is strongly temperature-dependent, in accordance with a multiphonon assisted process. Finally, in the context of silicon-perovskite tandem cells, we discuss the mechanisms underlying detrimental halide segregation in mixed iodide-bromide lead perovskites with desirable electronic band gaps near 1.75eV.^[6]

[1] C. L. Davies, M. R. Filip, J. B. Patel, T. W. Crothers, C. Verdi, A. D. Wright, R. L. Milot, F. Giustino, M. B. Johnston, L. M. Herz, Nature Communications 9, 293 (2018)
[2] T. W. Crothers, R. L. Milot, J. B. Patel, E. S. Parrott, J. Schlipf, P. Müller-Buschbaum, M. B. Johnston, L. M. Herz,Nano Lett. 17, 5782 (2017)
[3] S. G. Motti, T. Crothers, R. Yang, Y. Cao, R. Li, M. B. Johnston, J. Wang, and L. M. Herz, Nano Lett. 19, doi.org/10.1021/acs.nanolett.9b01242(2019)
[4] R. L. Milot, M. T. Klug, C. L. Davies, Z. Wang, H. Kraus, H. J. Snaith, M. B. Johnston, and L. M. Herz, Adv. Mater. 30, 1804506 (2018).
[5] E. S. Parrott, T. Green, R. L. Milot, M. B. Johnston, H. J. Snaith, and L. M. Herz, Advanced Functional Materials 28, 1802803 (2018).
[6] A. Knight, A. D. Wright, J. B. Patel, D. McMeekin, H. J. Snaith, M. B. Johnston, and L. M. Herz, ACS Energy Lett. 4, 75 (2019).

Organo-lead halide perovskites are excellent candidates for applications in light emitting diodes (LEDs). Methylammonium lead bromide (MAPbBr₃) is the most investigated perovskite for visible perovskite LEDs, but its use is still limited by a low photoluminescence quantum yield (PLQY). Efforts to increase the PLQY of this material mainly consist in fine-tuning the morphology and reducing the grain size of the polycrystalline perovskite to the micro- and nanoscale. However, the origins of the increased PLQY with reduced crystal size remain unclear. Here we elucidate the physical processes underlying the light emission of MAPbBr₃ thin films using time-resolved spectroscopy.¹ A systematic correlation between the enhanced PL properties and the reduction of the crystal size, achieved by using different additives during solution-processing, is observed. The co-existence of free carriers and excitons at low excitation densities is shown in as-cast polycrystalline MAPbBr₃ (crystal size of the order of few μm), while only excitons are present at high excitation densities. Using the Burstein-Moss and Saha models, important quantities such as the exciton binding energy, the reduced exciton effective mass and the trap density are estimated. We then explain the increased PLQY upon crystal size reduction by the presence of a bright exclusively exitonic population even at low excitation densities, together with reduced surface trapping thanks to passivation by the additives.

¹N. Droseros , G. Longo, J. C. Brauer, M. Sessolo , H. J. Bolink , N. Banerji. ACS Energy Lett. (2018), 3, 1458

Metal-halide perovskites show excellent properties for photovoltaic and optoelectronic applications, with power conversion efficiencies of solar cell and LEDS now exceeding 20%. This is unexpected, because these polycrystalline, solution-processed materials are likely to contain a significant density of defects compared to melt-grown semiconductors. Yet, typical effects from defects, such as strong absorption below the bandgap, low open circuit voltage in devices and dominant non-radiative recombination were not observed. In this contribution, we investigate thin films of metal-halide perovskites CH3NH3PbX3 (X = Br,I) with multidimensional optical spectroscopy to resolve the dynamics of band and defect states on ultrafast timescales. We report an unexpected coupling between the band transitions and a continuum of sub-bandgap states, which we report to extend at least 350 meV below the band edge. We explain the comparatively large bleach signal of these dark sub-bandgap states, compared to the levels detected in linear absorption, with oscillator strength borrowing from the band-edge transition. Our results imply that, upon valence to conduction band excitation, the sub-gap states are instantaneously bleached by the presence of charges in the band for the duration of the carrier lifetime and conversely, that almost dark sub-bandgap states can be populated by light excitation. Our findings provide fundamental insights into the photophysical origin of the exceptional defect tolerance of hybrid perovskites materials.

Semiconductor quantum wells and superlattices, which are usually fabricated through metal-organic chemical vapor deposition or molecular beam epitaxy, are key building blocks in modern optoelectronics. The ability to simultaneously realize defect-free epitaxial growth and to individually fine-tune the chemical composition and band structure of each layer is essential for achieving the desired performance. Such structures are challenging to realize using organic or hybrid materials because of the difficulty of controlling the materials growth. In this talk, I will present a molecular approach to the synthesis of high-quality organic-inorganic hybrid perovskite quantum wells through incorporating widely tunable organic semiconducting building blocks. By introducing sterically tailored groups into the molecular motif, the strong self-aggregation of the conjugated organic molecules can be suppressed, and single crystalline organic-perovskite hybrid quantum wells (down to one mono-layer thick) and superlattices can be easily obtained via one-step solution-processing. Energy transfer and charge transfer between adjacent organic and inorganic layers are extremely fast and efficient, owing to the atomically-flat interface and ultra-small interlayer distance. The 2D hybrid perovskite superlattices are surprisingly stable, due to the protection of the bulky hydrophobic organic groups. The molecularly engineered 2D semiconductors are promising candidates for use in next-generation nanoelectronics, optoelectronics, and photonics.

In colloidal nanocrystal form, lead halide perovskites possess high photoluminescence quantum yields, while in bulk single-crystal form they exhibit long charge-carrier diffusion lengths. However, without proper strategies to diminish crystal surface defects and manage surface quality, the desired characteristics of perovskites cannot be effectively exploited for photovoltaic and optoelectronic devices. Here I discuss novel strategies to passivate the surface defects and improve the surface quality of perovskite nanocrystals and bulk single-crystals, enabling the fabrication of efficient devices. We demonstrate the passivation of CsPbX₃-type nanocrystals with molecular ligands and metal dopants leading to stable near-unity quantum yield emitters, as well as efficient blue and red light-emitting diodes (LEDs). We also show the importance of designing crystal growth conditions, such as solvent, temperature, and substrate in order to grow bulk single-crystals with low-defect densities and good surface quality. Depending on the composition, MAPbX₃-type single crystals grown (tens of microns thick) under optimal conditions were used to realize: a) very sensitive visible-blind UV-photodetectors with nanosecond response time; and b) single-crystal solar cells with >21% power conversion efficiency. Unlike thin film polycrystalline solar cells, efficient cells with a grain-free single-crystal absorber are an ideal unobstructed system for investigating the device physics and chemistry of perovskites.

Halide perovskites exhibit remarkable properties as solar-cell absorbers, featuring both direct bandgaps suitable for sunlight absorption and long-lived charge carriers beneficial for charge extraction. However, the intrinsic instabilities and high toxicity of these water-soluble lead salts may impede the commercialization of this technology. Notably, even the origin of the materials' superior photophysical properties remains unclear, underscoring the importance of synthesizing and studying functional analogs of the lead perovskites. However, most stable materials studied as analogs have displayed weak sunlight absorption and short carrier lifetimes.

We recently introduced halide double perovskites, which can accommodate a much greater range of metals, as solar absorbers. Armed with this substitutional flexibility, we have explored alternative metals that can be incorporated into the perovskite lattice. Studying the electronic differences between the lead perovskites and lead-free double perovskites has shown us how to synthetically tune double perovskites to efficiently absorb sunlight. I will share our understanding of how to manipulate the symmetry and energy of the bandgap transition in these materials through i) dilute impurity alloying, ii) stoichiometric metal substitution, and iii) dimensional reduction. Our recent studies have led us to double perovskites with very similar properties to the lead perovskites, with small bandgaps and long-lived carriers. I will further present a pen-and-paper method for both understanding and predicting halide double perovskite band structures based on orbital symmetry arguments.

Hybrid organic-inorganic metal halide perovskites have demonstrated remarkable properties for thin-film solar absorbers and optoelectronics, including their ease of preparation, compositional tunability, defect tolerance and high charge-carrier mobilities. The promise of these materials has also motivated the development of modified bulk and nanoscale perovskites, including reduced-dimensional perovskites (2D, 1D) and quantum dots (QDs), with emergent optoelectronic properties. Perovskite QDs, in addition to a tunable band gap and high photoluminescence quantum yield, are of interest due to unique physical and structural phenomena such as the stabilization of metastable crystallographic phases. Luther et al. reported the stabilization of the high temperature, low-band gap cubic perovskite phase of CsPbI₃ at room temperature in small QD particles via colloidal synthesis (Swarnkar et al. Science 2016, 354, 6308, 92). In addition, alloying of these CsPbI₃ QDs with FAPbI₃ (yielding Cs_1–xFA_xPbI₃) by a simple cation exchange approach allows for access to the full compositional range (i.e. x = 0–1), unlike thin-film fabrication and direct synthesis of bulk Cs_1–xFA_xPbI₃ (Hazarika et al. ACS Nano 2018, 12, 10327).
Herein, we report on structural studies of Cs_1–xFA_xPbI₃ QD films by synchrotron X-ray techniques. 15-nm Cs_1–xFA_xPbI₃ QDs (x = 0, 0.5) were deposited from colloidal solution, by both drop- and spin-casting methods, and investigated by grazing incidence small-angle and wide-angle X-ray scattering (GISAXS/GIWAXS). GIWAXS patterns indicate coherent particle ordering on the substrate for single-layer spin-coated films, while the subsequent ligand exchange and particle overcoating (2–5 layers) leads to reorganization and isotropic ordering of the particles. In addition, drop-casting and slow evaporation of the solvent from the colloidal solution also results in isotropic ordering. Interestingly, the diffraction patterns show evidence of distortion from the originally reported cubic perovskite phase, with significant tetragonal (Cs_0.5FA_0.5PbI₃) and orthorhombic (CsPbI₃) character. The distortion and phase contributions across the full range of alloyed Cs_1–xFA_xPbI₃ QDs will be reported and discussed.

Hybrid perovskite nanoparticles (PeNPs) have significant potential to be used in perovskite light-emitting diodes (PeLEDs) because of their high photoluminescence quantum efficiency and, facile color tunability and synthesis. However, the highest electroluminescence effiencies of PeLEDs based on hybrid PeNPs are still much lower than those of PeLEDs based on hybrid perovskite bulk films and all-inorganic PeNPs. Here, we suggest a strategy to improve the electroluminescence efficiency of PeLEDs based on hybrid PeNPs. We passivate the defect states of PeNPs by introducing large organic cation into the formamidinium lead bromide nanocrystals. Based on this high quality PeNPs, we were able to fabricate efficient PeLEDs. Out work provides a promising way to improve luminescent efficiency of the PeLEDs based on hybrid PeNPs.

Colloidal organic/inorganic lead halide perovskite nanocrystals (NCs) are considered promising blue and green narrow-band emitters for the next-generation light-emitting diodes. High photoluminescence efficiencies are attained in these materials without epitaxial overcoating of the NC surfaces for electronic passivation of the surface states [1]. The major practical bottleneck of these materials relates to their labile surface chemistry. In particular, typically used ling chain capping ligands are problematic due to their dynamic and loose binding as well as their highly insulating nature. We have recently rationalized the typical observation of a degraded luminescence upon aging or the luminescence recovery upon post-synthesis surface treatments using a simple surface-structure model, supported by DFT calculations [2]. Healing of the surface trap states requires restoration of all damaged PbX₆ octahedra and establishing a stable outer ligand shell. Restoration of such a structure, seen as an increase in the luminescence quantum efficiency to 90-100% and improvement in the overall robustness of CsPbBr₃ NCs, was attained using a facile post-synthetic treatment with a PbBr₂+DDAB (didodecyldimethylammonium bromimde) mixture. In our most recent work [3], we have used DDAB as a sole ligand directly in the synthesis of perovskite NCs. We then used such NCs in LEDs and demonstrate high external quantum efficiencies of up to 3.6% in blue region (460nm) and 10% in the green region (520 nm).
1. M. V. Kovalenko, L Protesescu, M. I. Bodnarchuk. Science 2017, 358, 745-750
2. M. I. Bodnarchuk,S. C. Boehme, S. ten Brinck, C. Bernasconi, Y. Shynkarenko, F. Krieg, R. Widmer, B. Aechlimann, D. Günther, M. V. Kovalenko, I. Infante. ACS Energy Letters 2018, 4, 63–74
3. Y. Shynkarenko, M. Bodnarchuk et al. submitted

Hybrid perovskites with the ability to control spin, charge, and light could establish a new semiconductor technology that is especially useful for optoelectronic devices. While CH₃NH₃PbI₃ (methylammonium lead triiodide, or MAPbI) easily can be solution-processed, the same is not true for hybrid perovskites comprising larger, more complex organic molecules that have incompatible solubility with metal halides. Alternatively, vapor-phase deposition of organic precursors can introduce degradation and make stoichiometric deposition with inorganic precursors more difficult. However, resonant infrared, matrix-assisted pulsed laser evaporation (RIR-MAPLE), a versatile thin-film deposition technique that features aspects of both solution-based and vapor-phase deposition, enables a wide variety of hybrid perovskite thin films that can be difficult to achieve otherwise.

RIR-MAPLE is a variation of pulsed laser deposition that uses a low-energy laser (2.94μm Er:YAG) to vaporize a frozen target matrix (polyalcohol) by resonant absorption in hydroxyl bonds. Hybrid perovskite precursors are dissolved in the target (comprising a mixture of the polyalcohol matrix and polar solvent), released by the matrix vaporization, and gently transferred to the substrate intact. This technique preserves the organic precursor material that is responsible for unique optical properties and enables stoichiometric growth based on the target composition. In addition, by removing solvent via dynamic vacuum throughout the deposition, RIR-MAPLE prevents significant incorporation of solvent in the film. Furthermore, RIR-MAPLE uses very small amounts of each precursor to deposit thin films with thicknesses comparable to other established methods. Thus, the gentle transfer of precursor materials and greater control of precursor delivery have enabled the growth of bulk (3D) [1,2] and layered (2D) [3] hybrid perovskites, including those with larger organic molecules.

This talk will review the development of RIR-MAPLE growth of hybrid perovskite thin films, demonstrating application to 3D MAPbI and 2D hybrid perovskites (such as oligothiophene- and phenethylammonium-based metal halide perovskites). The research focus is to understand the formation of perovskite films based on precursor delivery from the RIR-MAPLE target. Materials characterization (including X-ray diffraction, atomic force microscopy, scanning electron microscopy, grazing-incidence wide-angle X-ray scattering, and UV-visible absorbance/photoluminescence spectroscopy) will be reported for a variety of RIR-MAPLE growth conditions and material systems.

We gratefully acknowledge support from the Center for Hybrid Organic Inorganic Semiconductors for Energy (CHOISE), an Energy Frontier Research Center funded by the Office of Basic Energy Sciences, Office of Science within the US Department of Energy.

References:
[1] Barraza, E.T., Dunlap-Shohl, W.A., et al., J. Electron. Mater., 47, p.917 (2018).
[2] Dunlap-Shohl, W.A., Barraza, E.T., et al., ACS Energy Lett., 3, p.270 (2018).
[3] W. A. Dunlap-Shohl, E. T. Barraza, et al., Mater. Horizons, DOI: 10.1039/C9MH00366E (2019).

Over the last decade, the research on perovskite solar cells has been intensively pursued, not only for their many technological applications especially in photonics and optoelectronics but also for their very interesting photophysical properties. The fabrication strategies, which lead to the realization of record power-conversion efficiencies exceeding 24%, involve solution-based deposition approaches. Given the inherent complexity of solution-based bottom-up approaches, it has been extremely challenging to scale-up the promising deposition strategies, owing to the issues associated with uncontrolled growth of the perovskite structures. Therefore, to achieve a good reproducibility and to extend the existing knowledge for the applications of perovskite materials, it is of key importance to gain a fundamental understanding of the mechanism of growth and formation of perovskite structures. Moreover, the enticing optoelectronic properties of perovskites are found to be subservient to the growth, and formation of their structures. Fundamentally, the texture of perovskite films depends on the nucleation and growth and to study these processes, x-ray diffraction (XRD) is undoubtedly the technique of choice. Towards this end, we have investigated the formation of various solution processed perovskite systems using in situ Synchrotron-based x-ray scattering techniques, including grazing incidence wide angle x-ray scattering, grazing incidence x-ray diffraction, and X-ray reflectivity. In my presentation, the fundamental insights gained through structural characterization and their correlation with the efficiency of devices will be discussed.

References
1) Alessandro Greco, A. Hinderhofer*, M. Ibrahim. Dar*, et al. Kinetics of Ion-Exchange Reactions in Hybrid Organic-Inorganic Perovskite Thin Films Studied by in Situ Real-Time X-ray Scattering. The Journal of Physical Chemistry Letters, 2018, 9, 6750-6754. DOI:10.1021/acs.jpclett.8b02916
2) Y. Liu, S.Akin, .., M. Ibrahim Dar*, M. Grätzel; Ultra-Hydrophobic 3D/2D Fluoroarene Bilayer-Based Water-Resistant Perovskite Solar Cells with Efficiencies Exceeding 22%. Science Advances, 2019, 5, eaaw2543, DOI: 10.1126/sciadv.aaw2543
3) M. Ibrahim. Dar*, A. Hinderhofer, G. Jacopin, V. Belova, N. Arora, S. M. Zakeeruddin, F. Schreiber and M. Grätzel. Function Follows Form: Correlation between the Growth and Local Emission of Perovskite Structures and the Performance of Solar Cells. Advanced Functional Materials, 2017, 27, 1701433. DOI: 10.1002/adfm.201701433
4) N. Arora,† M. Ibrahim Dar,†* A. Hinderhofer, N. Pellet, F. Schreiber, S. M. Zakeeruddin, M. Grätzel Perovskite solar cells with CuSCN hole extraction layers yield stabilized efficiencies greater than 20%. Science 2017, 358, 768-771. DOI: 10.1126/science.aam5655

Although hybrid halide perovskite solar cells (PSCs) have recently reached record efficiency among thin film photovoltaic technologies, stability of these devices remains a pressing problem for commercialization. Lamination processes represent an attractive means of fabricating PSCs due to their self-encapsulating nature and compatibility with high-throughput manufacturing methods. These techniques often involve high temperature and pressure, which represents an underexplored region of perovskite processing parameter space. In this work, we investigate the behavior of the archetypal halide perovskite, methylammonium lead iodide (MAPbI₃), under elevated temperatures and pressures. We also characterize the interactions of MAPbI₃ with the commonly used electron and hole transport layers (ETL and HTL) SnO₂ and NiO_x, and find that the latter is particularly susceptible to detrimental interactions at temperatures not far above those commonly used in ordinary perovskite film deposition techniques, with deleterious effects on device performance. SnO₂ can also evince reactions with the perovskite precursor methylammonium iodide, but is more robust than NiO_x. Applying the above knowledge, we investigate a laminated bifacial device fabrication strategy that mitigates intrinsic and interface-related threats to the perovskite absorber, and report that such devices can reach power conversion efficiencies of >12%. These results not only advance the state of the art in laminated PSCs, but also reveal heretofore unknown interactions in commonplace device architectures that should be taken into account when developing device fabrication schemes.

Hybrid organic-inorganic perovskites (HOIPs) formed from organoammonium iodide and lead iodide precursor solutions are promising materials for photovoltaic applications. While lead polyiodide and lead-solvent complexes formed in solution are intermediates for HOIP crystallization, the influence of solvent choice upon the formation of such intermediates is not well understood. In this talk, I will highlight two examples in which solution chemistries can drastically impact solid-state structural development, and consequently, performance of solar cells that incorporate these films. In the first example, I will show how the formation of lead polyiodides in the precursor solutions is correlated with the basicity of the processing solvent (quantified by Gutmann’s donor number, D_N). Solvents with low D_N exhibit a strong propensity to form lead polyhalides. We infer that such solvents interact weakly with the lead salt precursor. These solvents favor the precipitation of HOIP single crystals from solution. Conversely, high-D_Nsolvents suppress the formation of lead polyiodides, indicative of strong lead-solvent coordination. Such solvents support the formation of stable precursor solutions for HOIP thin-film processing and may be added in fractional quantities to tune the basicity of the processing solvent. The tunability introduced by high-D_Nadditives provides finer control over perovskite crystallization, post-deposition processability, and the morphology of HOIP active layers for photovoltaic applications. In the second example, I will highlight undesirable side reactions that take place between DMSO, a common Lewis base additive for perovskite processing, with perovskite precursors. We identified two distinct reaction pathways by which dimethylammonium and ammonium are produced as a result of reactions between DMSO and methyammonium cation; contrary to previous reports in the literature, these reactions need not be catalyzed by the presence of formic acid. The presence of these impurities alter the stoichiometry of the precursor solution, and when incorporated in the solid state alters the perovskite structure and optoelectronic properties.

Since 2009, organic inorganic hybrid perovskite materials have appeared as a game changer among the thin films technologies for photovoltaic (PV) purposes. The absorber displays valuable features such as a direct tunable band gap, a high absorption coefficient, good carrier diffusion lengths and an ambipolar conductivity. This accounts for the impressive breakthroughs experienced with this technology which achieved 24.2% power conversion efficiency in 2019 [1].
Yet, the classic architecture for perovskite solar cells (PSCs) consisting of a bilayer of compact and mesoporous TiO₂ (electron transport layer – ETL), perovskite (absorber) and Spiro-OMeTAD (hole transport layer) suffers from various instabilities and from hysteretic behaviors in the J-V characteristic. The defects at the ETL/perovskite interface have been pointed out as a major cause of these features, and a lot of effort is put into interface engineering to sort this out [2]. A common way to work on this issue is to change the nature of the extraction layer, using novel organic (C₆₀-derivatives …) or inorganic (ZnO, SnO₂ …) ETLs. Another complementary approach is to stack interfacial layers with thickness no more than a few nanometers to passivate the defects and tune energy levels at the interface.
Here, we propose a cell architecture using metal oxides as ETL coated with self-assembled monolayers (SAMs) deposited by solution process. The deposition is performed on TiO₂ and ZnO substrates, deposited by spray pyrolysis and atomic layer deposition respectively: after careful cleaning, the substrates are immersed in a solution containing the organic molecules where the formation of the SAM occurs. The functionalized film is obtained after rinsing. Various molecules which differ by their anchoring group (phosphonic acid, carboxylic acid), spacer (alkyl chain, benzyl) or functionalization group (-NH₂, -halide …) are considered. The choice of the acid function influences the grafting of the molecule onto the metal oxide. The Lewis base is used to passivate interfacial defects by curing under-coordinated lead at the interface. Substitution groups on the molecule allow to tune its dipole moment ranging from negative to positive values along the anchoring group-functionalizing group axis. Once the molecule is grafted, it modifies the oxide work function, allowing for a control on band alignment between the oxide and the perovskite. On the one hand, contact angle, x-ray photoemission spectroscopy, Fourier transform infrared spectroscopy and Kelvin probe measurements are used to assess the deposition of the molecular layers and to probe the quality of the films along with their chemical and optoelectrical properties. On the other hand, the effect of the surface modification on the growth of the overlaying perovskite and on the interface are extracted from scanning electron microscopy. The use of wide field time-resolved and spectrally resolved photoluminescence imaging also gives access to precious information regarding the quality of the perovskite films grown on top of these novel layers. The extraction abilities of such architectures are assessed using an appropriate model taking into account the drift-diffusion equation under various fluxes and interfaces with different properties. The integration of such interfacial layers in full devices will also be discussed.
References
[1] https://www.nrel.gov/pv/assets/pdfs/best-research-cell-efficiencies-190416.pdf, accessed 16/05/2019.
[2] Aydin, Erkan, Michele Bastiani, et Stefaan Wolf. « Defect and Contact Passivation for Perovskite Solar Cells ». Adv. Mat., 1900428 (2019)

Mixed-halide mixed-cation perovskites are fascinating photovoltaic absorbers behind perovskite solar cells with high power conversion efficiencies (PCE) > 20% and keep increasing. Chemical compositions of those perovskite precursors are complex and typically developed via empirical trials. This calls for a better understanding of the underlying reasons for the improved properties of perovskite films. Here, we do so by systematically investigating the in-situ solidification (tracking phase evolutions) of various perovskite precursors via Grazing Incidence Wide-Angle X-Ray Scattering (GIWAXS). In particular, we discovered the inherent phase segregations (e.g., MA/I-rich and FA/Br-rich phases), which is directly linked to limited efficiency and reproducibility of the resulting solar cells, occurs during the formation of the perovskite films. The presence of both Cs⁺ and Rb⁺ suppresses this segregation, promotes the direct formation of the photoactive 3C (also known as alpha phase) perovskite phase, and results in high-quality films and efficient solar cells.
Moreover, we also found that halide and cation engineering leads to a systematic widening of the anti-solvent processing window. This window widens from seconds, in case of single cation/halide systems (e.g., MAPbI₃, FAPbI₃, FAPbBr₃), to several minutes for mixed-cation mixed-halide systems. This behavior is closely related to the phase transformation of the disordered sol-gel state into one or more crystalline byproducts.
Beyond explaining the root causes of successfully empirical perovskite compositions, our study points to a new direction in the development of perovskite formulations that can further stabilize the sol-gel state and promote its controlled and direct conversion to the desirable perovskite phase and microstructure. This is critical in achieving better performing, reproducible, cost-efficient and scalable manufacturing of hybrid perovskite solar cells.

References:
Dang, H. X., et al., Multi-Cation Synergy Suppresses Phase Segregation in Mixed-Halide Perovskites. Joule 2019.
Wang, K. et al., Kinetic Stabilization of the Sol-Gel State in Perovskites Enables Facile Processing of High-Efficiency Solar Cells. Advanced Materials. In press.

Highly sensitive photodetection with large amplified photocurrent (gain) has been generally achieved by photoelectron emission or avalanche effect in inorganic photodetectors such as Si and GaAs, which needs a sufficiently strong external electric field (~100 V). Here, we demonstrate a high-gain and low-voltage photodetector with an organic-inorganic hybrid structure composed of perovskite nanoparticles as visible light absorber embedded at the interface between an organic compound coordinating Europium (Eu-terpy complex) and TiO₂ mesoporous film. The devise achieves significantly high incident photon to current efficiency of 290,000% (gain value of 2900) with the highest level of responsivity up to 1289 A/W even under low applied bias -0.5 V and low irradiation light (< 1 mW/cm²), which is more than four orders of magnitude larger than those of inorganic photodetectors. Such high performance of the detector is caused by photomultiplication phenomena at the specific interface composed of the perovskite nanoparticles and Eu complex molecular layers. The perovskite nanoparticles are excited by visible light irradiation, and their electrons transfer to the conduction band of TiO₂ under the applied reverse bias, whereas the holes are trapped at the interface between perovskite and Eu complex layers. The trapped holes are accumulated at the interface between perovskite and Eu-terpy complex, which results in buildup of a high electric field at the interface. Finally, a large external tunneling injection of electrons occurs from the Ag electrode, and the incident photon to current efficiency of the device exceeds 10⁵% even under low applied bias -0.5 V. As found here, the devices made with thick perovskite films show slow response to light and therefore, the nanoparticle-based structure or significantly thin layer (<5 nm) of perovskite absorber is critical for achieving high responsivity and large amplified photocurrent.

Perovskite solar cells (PSCs) have attracted enormous attention due to their potential for low-cost fabrication photovoltaic devices on flexible or rigid substrates. While many approaches have been used to control perovskite formation, thermal annealing has been a typical procedure for the state-of-the-art perovskite solar cells, giving rise to additional costs and challenges for applying perovskites in flexible and/or tandem photovoltaics. Recently, thiocyanate containing additives, such as MASCN, have been shown to be candidates for highly efficient room-temperature perovskite processing. [1] Nevertheless, the mechanisms of perovskite formation and crystallization pathways involved in MASCN-additive-processing approach are unclear. Using time-resolved grazing incidence wide-angle x-ray scattering (GIWAXS), we investigate the perovskite formation in situ during spin coating and the subsequent drying process, aiming at revealing the mechanisms of additive-assisted-perovskite-formation.

Time-resolved monitoring of the perovskite thin film formation process reveals the formation of precursor phases on the route of perovskite formation, whereas perovskite formation is dominated by a sol-gel process. Our findings reveal that the nature of the precursor phase and their formation/dissociation dynamics have an impact on the extent of nucleation and growth of perovskite phase affecting the microstructure of the perovskite film. We show that a DMSO-precursor phase is obtained in the as-cast film for the MASCN-free films, whereas an additional (presently unidentified) phase is obtained when adding MASCN to the precursor solution. The latter precursor phase is less stable than the DMSO precursor phase and dissociates shortly upon applying N₂ flow on the film leading to fast room-temperature conversion to perovskite. Moreover, MASCN aids in the dissociation of DMSO-precursor phase which decays faster with the presence of MASCN in the wet film. The combination of two precursors with fast and slow decay rates may contribute to the formation of micron-sized perovskite crystals, through seeding perovskite nuclei combined with the slow growth of the perovskite phase. Understanding the mechanism of room-temperature-additive-processing will pave the way for more facile control of perovskite formation, while the use of N2 flow provides the suitability for forming perovskite directly on roll-to-roll processing at room temperature without the need for subsequent separate steps.
[1] Q. Han et.al, Energy Environ. Sci., 2017, 10, 2365—2371.

We report pulsed terahertz (THz) emission from solution-processed metal halide perovskite films. The obtained THz electric field from MAPbI3 is just one order of magnitude lower than p-InAs, one of the most efficient semiconductor THz emitters, while for FAPbI3, it is only 5 times lower. The pulsed THz emission is enabled by a unique combination of efficient charge separation, high carrier mobilities, and more importantly the presence of surface defects. The mechanism of generation was identified by investigating the dependence of the THz electric field amplitude on surface defect densities, excess charge carriers, excitation intensity and energy, temperature and external electric field. We also show for the first time THz emission from a curved surface, which is not possible for any crystalline semiconductor and paves the way to focus high-intensity sources. These results represent a possible new direction for perovskite optoelectronics, and for THz emission spectroscopy as a complementary tool in investigating surface defects on metal halide perovskites, of fundamental importance in the optimization of solar cells and light-emitting diodes.

Perovskite solar cells are an important photovoltaic technology with high efficiencies exceeding 20% due to their optimal band gap, large absorption coefficient, and high charge mobilities. One of these challenges is the understanding and control of their defect structures because perovskite compounds are relatively soft ionic crystals and ions are migrating in the crystals relatively low activation energy.
In this study, we have investigated the effect of low energy ion irradiation to both all inorganic and organic-inorganic lead halide perovskite films on the structure, morophology optoelectronic properties and photovoltaic properties systematically. Several kinds of halogen and hydrogen ion beam irradiations on the perovskite thin film was performed by using a tandem type ion accelerator with changing acceleration voltages and irradiation time.
Detailed experimental results will be reported on the conference.

References:
Su, T.-S. et al..Sci. Rep. 5, 16098; 10.1038/ srep 16098 (2015).
Kavan, L., O’Regan, B., Kay, A. & Grätzel, M. J. Electroanal. Chem. 346, 291–307 (1993).

We used an ion selective membrane in conjunction with electrochemical impedance spectroscopy (EIS) to measure ion transport in methylammonium lead triiodide (MAPbI₃) powder. We placed a methylammonium selective layer in between the active material and the electrode in EIS studies and found evidence of ion transport with a millisecond (ms) time constant under continuous illumination. These values are consistent with reported values of ionic conduction in thin-film perovskite solar cells. Electrospray ionization mass spectrometry (ESI-MS) revealed direct chemical evidence of methylammonium diffusion into the ion selective layer. We found no experimental evidence indicating the mobility of lead ions or protons, suggesting that methylammonium is the only mobile cation under illumination in MAPbI₃.

One of the greatest interests of nanotechnology is the growth of semiconducting materials based on self-assembly of inorganic nanostructures. A facile technique consists to induce a simple cooperative interaction between nanoscale constituents and obtain larger assemblies. In this work, we will report the formation of three-dimensional nanostructures formed from perovskite CsPbBr₃ and CsPbI₃ quantum dots (QDs) by droplet evaporation method after deposited on planar oxide substrates. The colloidal solutions of CsPbBr₃ and CsPbI₃ QDs were synthesized via hot injection method. The UV-vis absorption spectrum of the QDs shows optical band gap energy of 2.4 eV and 2.1 eV for CsPbBr₃ and CsPbI₃, respectively. XRD measurements revealed that both CsPbBr₃ and CsPbI₃ QDs have an orthorhombic crystal phase at room temperature. The nanostructured oxide surface of both ZnO and TiO₂ were obtained by thermal oxidation process at high temperature 250 °C and 1020°C, respectively. The toluene solution containing monodisperse QDs was deposited (dropwise fashion) on the surface of both nanostructured ZnO and TiO₂ rutile substrates and evaporated in air at room temperature. On TiO₂ surface, different morphologies from nanocubes (edge length ~293 nm) when decorated with CsPbBr₃ NCs to microbelts (thickness ~500 nm and length ~20um) with CsPbI₃ QDs have been observed by controlling the quantum dots concentration. On the ZnO surface it was formed nanoparalelepiped (edge length 320 nm) and broom-like nanostructures (212 nm) of CsPbBr₃. We discuss our results by considering that the driven forces of quantum dots coalescence during solvent evaporation are lateral capillary meniscus inducing mass transport at the colloidal solution/oxide substrate interface.

There has been a recent surge of studies on organic-inorganic hybrid perovskites (OIHPs) as promising candidates for photovoltaic materials due to their high photovoltaic performance and low synthesis costs. Nevertheless, their vulnerability to moisture, light, and heat is preventing them from being used in practice. It was suggested that the stability issues can be partially solved by using two-dimensional (2D) OIHPs. However, unanswered questions on dynamical structural fluctuations of 2D OIHPs make it difficult to predict other physical properties theoretically.
In this study, we explore energetically favored and entropically stabilized local structures of 2D OIHPs using ab-initio calculations. Furthermore, we provide simulated core-level spectroscopic properties as a means to validate their atomic-scale structure, and explore additional electronic/optical properties of 2D OIHPs depending on their structural details and dynamics at finite temperature.

Methylammonium lead iodide (MAPI) is currently in the focus of photovoltaic research not only because of high conversion efficiencies but also because of various intriguing physical-chemical properties. Here we show that equilibrium space charge effects at MAPI/TiO₂ and MAPI/Al₂O₃ interfaces are formed as a consequence of ionic excess charges, which is a totally novel view point in the photovoltaic community and of great relevance for the performance. For the analysis, we will apply a generalized picture that considers the equilibrium distribution of both ionic and electronic carriers [1-3]. These are attributed to a positive ionic excess charge at the interface, most probably due to Pb²⁺ adsorption. We give clear conductivity experiments on MAPI-Al₂O₃ and MAPI-TiO₂ composites as well as on MAPI thin-films on Al₂O₃ substrates with different thickness [4]. There is a positive ionic interfacial charge that leads to the equilibrium space charge potential. Zeta potential and surface analysis measurements identify Pb²⁺ adsorption at the oxide interface as reason for the space charge potential of above 0.5 V. The so-formed field leads to a local depletion of holes and iodine vacancies, but an accumulation of conduction band electrons. This work provides not only a novel view on such interfaces, but may enable a novel approach of interface engineering as well as a better understanding of the behavior in mesoporous systems.

Perovskite semiconductors including films and crystals are emerging materials that have wide ranging applications in the field of optoelectronics due to unique properties such as solution-processing, flexibility on substrates, and ambient environment processing. Although the ultrafast photophysics dynamics of carriers has been studied using ultrafast optical spectroscopies for the past few years, understanding its charge carrier dynamics in-situ devices has been a main challenge. Upon photon excitation, the photogenerated carriers carry out recombination, trapping, and transport in the range of several picoseconds to nanoseconds. Therefore, traditional photoconductivity measurement approaches such as time-of-light photoconductivity, photo charge extraction by linearly increasing voltage (CELIV) have temporal limitations towards understanding the carrier transport dynamics due to their low time resolution of a couple of nanoseconds.
In this work, we use ultrafast photocurrent spectroscopy (with sub-30 picosecond time resolution) to address the fundamental photophysics dynamics in-situ perovskite thin-films and crystal photoconductive devices, such as carrier photogeneration, recombination, transport, trapping and de-trapping. We investigate the photocurrent dependence with temperature, electrical field, and intensity. In addition to extracting the carrier mobility, lifetime and trapping level, we map out the evolution of carrier dynamics in the temporal range from sub-30 picoseconds to 1 microsecond.

Smart photovoltaic windows are a green technology that allow for adjustable transparency and electrical power generation from solar energy. Halide perovskites have been observed to exhibit thermochromic behavior which, if modified, can be utilized in this technology. Cesium tin iodide/ bromide serves as a non-toxic potential candidate for this device as the intrinsic materials are heavily doped with Sn⁴⁺ which can boost current density under relatively small voltage. In this work, both the thermochromic and electrochromic phenomenon in cesium tin iodide/ bromide thin films are being explored. The thin films are synthesized from solution using a doctor blade method onto an ITO/glass slide and are capped with polydimethylsiloxane to prevent oxidation. Thermochromic behavior is verified by directly heating the film and observing a visible change in color, denoting a phase transformation. The film and all phases are characterized using X-ray diffraction. The electrochromic phenomenon caused by joule heating is studied as a function of applied bias and current density. The understanding of the thermochromic and electrochromic phenomenon in the halide perovskite thin film will allow for the development of devices in which the performance and stability can be measured.

The non-trivial coupling of ferroelectricity and photovoltaic properties makes ferroelectric semiconductors an interesting material system for both fundamental study and technological application. Ferroelectric organic-inorganic halide perovskite has low band gap, high charge carrier mobility, and large polarization density, and hence is a good candidate for studying photo-ferroelectricity. By solution method, we successfully prepare one ferroelectric halide perovskite, (Cyclohexanemethylamine)₂PbBr₄, which is also epitaxially grown on mica substrate by chemical vapor deposition method. We apply X-ray diffraction to characterize the crystal structure and steady state photoluminescence to probe the optical properties. Second harmonic generation is used to prove the inversion symmetry breaking, and PE loop and switchable photo diode effect are characterized by transport studies. The work suggests a new class of thin film photo-ferroelectric materials which could be useful for future energy and memory devices.

Single crystalline perovskites exhibit high optical absorption, long carrier lifetime, large carrier mobility, low trap-state-density and high environmental stability. As large single-crystalline silicon wafers have revolutionized many industries ranging from solar cells to household electronics including computers and cell phones, it is envisioned that the availability of large perovskite single-crystalline will revolutionize its broad applications in photovoltaics, lasers, photodetectors, LEDs, etc. Recently, by fine-tuning the crystal nucleation and growth process, a low temperature gradient crystallization (LTGC)method is developed to grow high-quality perovskite CH₃NH₃PbBr₃ single crystals with high carrier mobility of 81 ± 5 cm² V^-1 s^-1, long carrier lifetime of 899 ± 127 ns and ultralow trap state density of 6.2 ± 2.7 × 10⁹ cm^-3. In fact, they are better than perovskite single crystals reported in prior work: their application in photosensors gives superior detectivity as high as ~10¹³ Jones as well as remarkable water resistance and long-term environmental stability. Meanwhile, the response time is as small as 40 μs, ~3 orders of magnitude faster than their thin film devices. Furthermore, a large-area (~1300 mm²) imaging assembly composed of a 729-pixel sensor array is designed and constructed, showing excellent imaging capability thanks to its superior quality and uniformity. This work opens a new possibility to use the large high-quality perovskite single-crystal-based devices for more advanced imaging sensors. Even though the minimum width of each electrode is only 40 μm in the present example, limited by the linewidth of the shadow mask used in our lab for evaporation. However, common fabrication techniques, such as the nanoimprint lithography and electron beam exposure used in the present day fab, will offer much higher resolution in electrode fabrication.

We report the temperature-dependent photoluminescence of multi-layered 2D lead iodide perovskites (2D LHPs) with varying quantum-well thicknesses (n = 2-4), A-site cations (methylammonium and formamidinium), and organic spacers (butylammonium, hexylammonium, and phenethylammonium). In general, new features of broad emission emerge at low temperature, and linewidth broadening as well as shifts in peak positions were observed in all samples. In particular, we found that broad emission from self-trapped excitons and trap states can coexist, although they were primarily found in bromide/chloride and iodide 2D LHPs respectively. Moreover, increasing quantum-well thicknesses leads to the suppression of the broad emission. We then analyzed the broad emission with out-of-plane distortion of Pb-(µ-I)-Pb angle and electron-phonon coupling. The electron-phonon interactions obtained from temperature-dependent linewidths and peak shifts show that both optical phonon energies and coupling efficiencies decrease with increasing quantum-well thicknesses. This finding supports the proposed behaviors of self-trapped excitons, and explains why the broad emission is mainly observed in thin 2D LHPs.

Two-dimensional semiconductor nanocrystals with a few unit-cell thicknesses have attracted much attention as highly desirable materials for photonic applications such as in lasing due to their narrow linewidth, low Auger decay probability and potential of significantly increasing the transition dipole strength upon cooling. In particular, the enhancement of the transition dipole at low temperature via giant oscillator strength transition (GOST) effect is unique to 2-dimensional semiconductor structures that relates to the coherent expansion of the exciton wavefunction in space. In this work, we investigated 2-dimensional CsPbBr₃ nanocrystals’ capability to exhibit enhanced transition dipole by systematically studying the temperature dependent absorption and time-resolved fluorescence spectra of CsPbBr₃ nanoribbons and nanoplatelets of varying thickness. To obtain reliable spectroscopic data, we first developed the synthetic procedure producing CsPbBr₃ nanoribbons with high ensemble uniformity of the size and morphology, which exhibit high quantum yield (> 60%). These nanoribbons were stable under laser excitation in vacuum environment exhibiting stable and well-defined fluorescence spectra determined by thickness. From temperature dependent absorption and time resolved fluorescence emission measurements, we observe the GOST effect in nanoribbons and nanoplatelets, manifesting a larger absorption cross-section as well as decreased fluorescence lifetimes. For example, in 3 unit cell-thick nanoribbons the fluorescence lifetime decreases from ~ 3 ns at 300 K to below 10 ps at 60 K without changing the fluorescence quantum yield significantly. This behavior is similar to that observed in 2nm thick CdSe platelets where the fluorescence lifetime reaches 150 ps at similar temperatures. These results indicate that few unit cell-thick lead halide perovskite 2-dimensional structures can be superior to their II-VI counterparts where large oscillator strengths are desirable.

Because 2D organic inorganic hybrid perovskites (OIHP) exhibit desirable optoelectrical properties such as high absorption coefficient, strong quantum confinement effect and highly tunable bandgap, 2D OIHP is considered as promising semiconductors for next generation optoelectronic devices. Additionally, many researches have reported that incorporation of chiral organic cation into the 2D OIHP lattices triggers another intriguing property of OIHP, which is defined as different optical response to left-handed and right-handed light. However, to increase the possibility to develop workable devices such as chiral light emitting diode (LED), wavelength tunability of chiroptical response should be guaranteed. In this study, to obtain wide wavelength tunability of chiroptical phenomenon, chemical composition engineering of 2D OIHP is carried out. Varying the mixing ratio of iodide and bromide anion in (S- or R-C₆H₅CH₂(CH₃)NH₃)₂PbI_4(1-x)Br_4x basically modifies band gap of chiral OIHP, leading to a shift of circular dichroism (CD) signal from 495 nm to 474 nm. It is also found that abrupt crystalline structure transition occurs and CD signal disappears when the 2D organic inorganic hybrid perovskite is transformed to bromide-determinant phase. To obtain material having CD at the wavelength range lower than 474 nm, S- or R-C₁₂H₇CH₂(CH₃)NH₃ with larger spacer group can be adopted. Thereby CD signal can further blue-shift to around 375 nm. Circularly polarized photoluminescence (PL) from our chiral 2D perovskite over a broad wavelength range at 70 K is also confirmed. Such a wide wavelength tunability of chiroptical property will advance the realization of color tunable circularly polarized light-emitting OIHPs.

Metal trihalide perovskites are rapidly redefining the landscape of solid-state semiconductors utilized as active medium in photovoltaics and in light generation. Within this materials space, organic-inorganic hybrid formamidinium lead bromide (FAPbBr₃) has arisen as a promising candidate for efferent light emitting devices, due to its capacity for sharp and bright green light emissions (530 nm). Herein we have applied a facile single-step ligand-mediated method for phase-controlled synthesis of FAPbBr₃ cube- and rod-shaped nanocrystals (NCs), starting from different ratios of precursor agents. Examining their structural and optoelectronic properties – using a combination of synchrotron X-ray diffraction, X-ray spectroscopy, scanning electron microscopy and steady-state and time-resolved photoluminescence (PL) – we reveal the two NC types to fundamentally differ. While the cube-shaped NCs exhibit properties aligning with that of bulk FAPbBr₃, the nanorods exhibit a two-phase microstructure and the co-existence of both a typical cubic perovskite structure alongside the formation of a new low-symmetry monoclinic phase (P2/m). Further, the two-phase nanorods display a bright dual PL emission (peaks centered near 490 nm and 530 nm) and complex luminescence dynamics, properties characteristic of quasi-2D perovskites. The two phase nanorods generation can be assigned to the proton exchange in the presence of excess of FA⁺ during the synthesis.

Halide perovskites have attracted great attention due to their excellent photovoltaic performance. Many growth methods have been developed to prepare these compounds with different morphologies. The identification of their crystal growth mechanism is crucial for understanding their fundamental chemical and physical properties. In this work, we have observed the formation of CH₃NH₃PbI₃ microcuboids and CsPbI₃ microwires via solvothermal method using lead iodide (PbI₂), methylamine (CH₃NH₂) or cesium acetate (CsOAc) as precursors and hydroiodic acid (HI) in isopropyl alcohol solution. We have systematically changed the solution temperature and PbI₂ concentration during the synthesis process. X-Ray diffraction shows that both CH₃NH₃PbI₃ and CsPbI₃ have high crystallinity belonging to the tetragonal (I4/mcm space group) and orthorhombic (Pnma space group) crystal phase, respectively. We have observed that the CH₃NH₃PbI₃ morphology is synthesis parameter dependent changing from small irregular particles to very regular polyhedral shape. We have also observed the presence of hollow spaces and/or hopper-type morphology at high temperature and concentration. CsPbI₃ perovskites present microwire morphology which also depends on synthesis parameter. UV-Vis absorption measurement indicated a band gap energy around 1.5 eV and 2.7 eV for CH₃NH₃PbI₃ and CsPbI₃ samples, respectively. We observed that the optical band gap energy for both compounds is synthesis parameter independent. We discuss the change in the morphology based on Wulff construction.

The optimization of perovskite (typically ABX₃) for photovoltaic (PV) performance through mixed cations (A = Cs, methylammonium (MA), and formamidinum (FA)) and halides (X = Br and I) has led to engineered materials comprising six (A_jB_kC_1-j-kPbX_zY_3-z) or more components. The role each of these components plays in increasing the PV performance is not yet well understood.

Here, we use transient photoluminescence microscopy to study the diffusion of carriers in single crystals and thin films for a variety of perovskite compositions (MAPbI₃, CsFAPbBrI, and CsMAFAPbBrI) in order to understand the effects of composition. Unexpectedly, the diffusion of carriers in single crystals was observed to be independent of the perovskite composition. When cast into thin films, however, the same materials showed a marked difference in diffusion constants and lengths. Along with angle-dependent and energy dispersive x-ray data, we determined that the limited transport in some compositions was caused by inhomogeneous crystallization at the grain boundaries. Those compositions containing methylammonium had consistent mixed-halide content across grains, while non-MA-containing compositions showed a gradient in the halide mixture, with higher Br content—and thus a larger bandgap—at the surface of film grains. Such a potential barrier inhibits the movement of carriers from grain to grain and thus their ability to reach electrical contacts.

This work reveals that compositional tuning of perovskites for PV performance has more to do with controlling grain boundary gradients than increasing transport within the bulk material. While most high-performance PV cells contain MA, we show it may be possible to produce high performance PV cells from desirable non-MA perovskites. Theoretical composition yields show current crystallization schemes favor narrow gradients in MA-containing materials, though solvent systems could be engineered to produce homogenous grains in films without the need for MA incorporation.

Ordered arrays of stable, single-crystal cesium lead bromide (CsPbBr₃) nanowires can be used to study fundamental properties of halide perovskites in a well-defined and simple one-dimensional model system. Specifically, we use multiple spectroscopy techniques and correlate them with state-of-the-art electron microscopy methods, to reveal, with atomic resolution, large and continuous lattice rotations due to heteroepitaxial stress and lattice relaxation. We show that the lattice distortions give rise to bandgap modulations that are the dominant contributors to an anomalous size-dependent emission spectral shift well beyond the quantum confinement regime. Understanding the lattice behavior in strained perovskite and its effect on the optoelectronic properties of these dynamic materials, from the atomic scale up, is essential to evaluate their performance limits and fundamentals of charge carrier dynamics.

Perovskite solar cells (PSCs) have emerged as a very promising solar cell technology due to the rapid increase of the power conversion efficiency (PCE). Further improvement in the device efficiency is limited by understanding the device working mechanism that still under debate. Herein, we propose and demonstrate how the interfaces at perovskite/charge selective layer (CSL) play major role. We have employed capacitance/impedance based techniques to understand the AC response, combined with the application of DC potential, and its implications in charge re-distribution in an operating device upon relatively low temperatures. We show that the lateral interfaces can be identified directly through an accurate determining of the equivalent circuit model which fits the capacitance/impedance spectra perfectly at different conditions. Resistance and/or capacitance circuit elements are time independent that cannot describe the large capacitance at low frequencies. However, introducing a time dependent component is necessary to describe the observed transient current behavior that is responsible for the hysteresis in the current–voltage curves. We perform current–voltage and capacitance–frequency simulations to confirm the role of time dependent component. This study provides an important progress in characterizing the electronic and ionic defect properties of perovskite devices.

In an effort to explore lead-free perovskite semiconductors, we recently reported [1] all-inorganic CsSnBr₃ to have excellent semiconducting properties and thermal stability that are comparable or superior to those of CH₃NH₃PbI₃. However, tin-based perovskites are prone to oxidation and formation of defect states. In this work, CsSnBr₃ crystals with shiny and crack-free surfaces were grown by the Bridgman method, and the CsSnBr₃ crystal boules were cut and polished into plates. Hall measurements revealed p-type carrier concentrations of 10¹²–10¹⁴ cm^-3 and carrier mobilities around 2×10¹ cm² V⁻¹ s⁻¹, indicating low defect densities in the CsSnBr₃ crystals. We further fabricated a 1-mm-thick CsSnBr₃ plate into a photodetector with the device structure Al / TQB / CsSnBr₃ / Au, where TQB is an organic electron transport material. Without an external bias, the photodetector is responsive to the entire visible spectral range, reaching a peak quantum efficiency of 7% at 700 nm. [2] This functional CsSnBr₃ photodetector clarifies undue concerns about bulk defects in this material, and suggests its broader applications in X-ray photodetectors, solar cells, and other optoelectronic devices.
References
Li, B.; Long, R.; Xia, Y., Mi, Q. Angew. Chem. Int. Ed. 2018, 57, 13154–13158.
Li, B.; Long, R.; Yao, Q.; Zhu, Z.; Mi, Q. J. Phys. Chem. Lett. 2019, doi: 10.1021/acs.jpclett.9b01405.

Their unique optical and electronic properties make metal halide perovskites very attractive for photovoltaic and other optoelectronic applications. Meanwhile, the power conversion efficiency (PCE) of metal halide perovskite solar cells (PSCs) has reached more than 24%. Despite this impressive progress for solution-processed photovoltaic devices, performances of the PSCs are still limited by the open-circuit voltage (V_oc) and fill factor (FF) which are both controlled by the charge carrier dynamics. Thus, understanding the intrinsic limitations of these materials and the working mechanism of solar cells is vital to push the device efficiencies closer to the radiative Shockley-Queisser limit. Time-resolved optical spectroscopy of perovskite thin films and perovskite/charge transport layer interfaces allows identifying the photophysical processes in the absence of electric fields. On the other hand, bias-dependent photoluminescence on devices allows distinguishing between two competing processes: field-induced spatial charge separation and nonradiative charge recombination. Here, we study how the photoluminescence intensity and radiative decay in n-i-p PSC devices change when applying an external bias. We performed bias-dependent time-resolved photoluminescence (TR-PL) measurements at room-temperature in vacuo to reduce environmentally-induced and temperature-dependent photophysical changes during the PL measurements. We observe a gradual decrease of the PL intensity at V > V_oc. On the other hand, for V < V_oc no change of the PL is observed compared to the unbiased device. We will discuss how interface engineering using 2D perovskites with different cations alters charge carrier dynamics of PSCs under external bias.

The presentation will describe some of the recent experimental and theoretical results for various classes of layered halide perovskites. Basic symmetry analysis of their electronic states will be proposed introducing simplified layered structures and compared to the case of 3D bulk materials and colloidal quantum dots (QD). The exciton fine structure, electron-phonon interactions , quantum and dielectric confinement effects will be further analyzed. The presentation will adress the ongoing debate about the nature of the exciton ground state in perovskite QD.

Over the past few years, organic-inorganic halide 3D perovskites were found to present remarkable optoelectronic properties. A great attention has been paid to perovskites thin films, as an ideal building block for PV and LED devices. On the other hand, the study of single crystals has proven necessary to unveil some of the intrinsic properties of these semiconductors [1,2].
While the 2D-layered perovskites, forming self-assembled natural quantum well structures where the perovskite layer is separated by large organic cations, have been known for decades, they are attracting growing interest, for their strong photoluminescence properties, their chemical versatility and their low sensitivity to external degradation mechanisms due to UV light and moisture. Due to the quantum and dielectric confinements, 2D-layered perovskites exhibit high oscillator strengths and robust excitons, making them particularly relevant for light-emitting devices, and in the past, several works have been done to demonstrate the strong coupling regime at room temperature between excitons and photon modes in vertical microcavities [3,4] or in the distributed feedback geometry [5], or between excitons and plasmons [6]. More recently, 2D perovskites have proved to be interesting also for photovoltaics with efficiencies around 15 % and a better stability than their 3D counterpart [7]. Nevertheless, many of the fundamental photophysics properties of these 2D-layered perovskites, in particular the recombination dynamics of the excitons, remain to understood.
Pure-phase monocrystalline thin films of Ruddlesden-Popper phenylethylammonium-based 2D-layered perovskites of formula (C₆H₅C₂H₄NH₃)₂(CH₃NH₃)_n-1Pb_nX_3n+1 are produced using the “Anti-solvent Vapor assisted Capping Crystalllization” method [8]. The exciton recombination dynamics of the phases n = 1, 2, 3, 4 is investigated using time-resolved micro-photoluminescence (PL) in a large range of power excitation. The PL dynamics is highly dependant of the fluence due to the competing effect of traps saturation and exciton-exciton annihilation. At low fluence, emission mechanism is excitonic, dominated by defect-assisted recombination. At high fluence, the dynamics is dominated by exciton-exciton annihilation, the order of magnitude of the annihilation rate is evaluated.

References:
[1] D. Shi et al. Science 347, Issue 6221, 519-522 (2015).
[2] H. Diab et al. J. Phys. Chem. Lett., 7 (24), 5093–5100 (2016)
[3] A. Bréhier et al, Appl. Phys. Lett. 89, 171110 (2006)
[4] H. S. Nguyen et al, Appl. Phys. Lett. 104, 081103 (2014).
[5] T. Fujita et al, Phys. Rev. B57, 12428 (1998).
[6] C. Symonds et al, New J. Phys. 10 (2008) 065017
[7] H. Tsai, Nature, Volume 536, Issue 7616, 312-316 (2016).
[8] F. Lédée et al, Cryst. Eng. Comm. 19, 2598 – 2602(2017).

Acknowledgements:
The project leading to this application has received funding from the European Union’s Horizon 2020 programme, through a FET Open research and innovation action under grant agreement No 687008, and from Agence Nationale de la Recherche within the project EMIPERO.

The unique hybrid nature of 2D Ruddlesden-Popper (R-P) perovskites has bestowed upon them not only tunability of their electronic properties but also high-performance electronic devices with improved environmental stability as compared to their 3D analogs. However, there is limited information about their inherent heat, light and air stability, and how different parameters such the inorganic layer number and length of organic spacer molecule affect stability. To gain deeper understanding on the matter we have expanded the family of 2D R-P perovskites, by utilizing pentylamine (PA)₂(MA)_n-1Pb_nI_3n+1 (n = 1-6, PA = CH₃(CH₂)₄NH₃⁺, C5) and hexylamine (HA)₂(MA)_n-1Pb_nI_3n+1 (n = 1-4, HA = CH₃(CH₂)₅NH₃⁺, C6) as the organic spacer molecules between the inorganic slabs, creating two new series of layered materials in single crystal form, for up to n = 6 and 4 layers, respectively.[1] The increase in the length of the organic spacer molecules does not affect their optical properties, however it has a pronounced effect on the air, heat and light stability of the fabricated thin films. We fabricated films on various substrates, and performed extensive environmental stability tests, evaluating their air, heat and light stability, both with and without encapsulation. Multiparameter, invaluable information was extracted from these studies, which showed that for the same number of layers the PA based materials, exhibited improved heat, light and air stability (e.g. stable for 450 days in air), as compared to BA, HA and 3D analogues. Furthermore, we verified for the first time that hybrid halide perovskites are inherently heat and light stable in the absence of moisture, a most critical finding for their potential commercialization. Lastly, evaluation of the out of plane mechanical properties of the corresponding materials showed that their soft and flexible nature can be compared to the current commercially available polymer substrates (e.g. PMMA), rendering them suitable for fabricating flexible and wearable electronic devices, expanding their utilization beyond photovoltaic applications.


References
[1] Spanopoulos, I.; Hadar, I.; Ke, W.; Tu, Q.; Chen, M.; Tsai, H.; He, Y.; Shekhawat, G.; Dravid, V. P.; Wasielewski, M. R.; Mohite, A. D.; Stoumpos, C. C.; Kanatzidis, M. G., J. Am. Chem. Soc. 2019, 141, 5518.

While the stimulated emission of light was postulated by Albert Einstein over 100 years ago, its nonlinear counterpart has only been observed in a handful of experiments so far. We recently discovered that nonlinear coherent amplification of an ultraviolet femtosecond laser pulse can occur in a piece of optically excited sapphire [1]. The nonlinear, two-photon stimulated emission holds promises for laser technologies, microscopy and laser-spectroscopy as it is inherently nonlinear, providing temporal/spatial focusing and control over selection rules.

Here, we discuss our recent experimental efforts to expand the nonlinear stimulated emission from the ultraviolet into the infrared regime. To that extent, we utilize novel lead-halide layered (2D) and bulk (3D) metal-halide perovskite semiconductors, which demonstrated promising properties for photovoltaic and optoelectronic devices [2,3]. With band gap energies in the visible spectrum and long carrier lifetimes, they are an ideal sample system to explore two-photon stimulated emission in an ultrafast pump-probe experiment.

We will also discuss, how we utilize the special selection rules in nonlinear absorption and emission to reveal population and carrier dynamics of dark states in 2D and 3D perovskites. By performing ultrafast linear- and circular polarized pump-probe measurements in the single and two-photon regime, we resolve the dynamics of bright and dark states separately.

[1] Thomas Winkler et al. Nature Physics volume 14, pages 74–79 (2018)
[2] Michael B. Price et al. Nature Communications volume 6, 8420 (2015)
[3] Johannes M. Richter et al. Nature Communications volume 8, 376 (2017)

Understanding the electronic structure of metal halide perovskite (MHP) surfaces and interfaces is of considerable interest for the development of MHP based devices. A number of techniques, including ultraviolet and X-ray photoemission spectroscopy (UPS, XPS), contact potential difference (CPD) measurements, and Kelvin probe force microscopy (KPFM), have been applied to gather information on these surfaces and interfaces.^[1-2] However, standard photoemission spectroscopy, such as UPS and XPS, only probes the top few nm of the film and cannot distinguish between flat bands and band bending occurring over several tens of nanometers. Furthermore, photoemission can induce non-equilibrium conditions of surface photovoltage (SPV) and unwanted band-flattening, providing distorted information on the equilibrium position of the Fermi level. Therefore, contactless, non-invasive, and non-destructive, Kelvin probe (KP) based CPD measurements serve as an important complementary technique to sort out some of these issues.^[3] CPD measurements in the dark and under illumination (SPV measurements) provide valuable information on work function changes caused by the generation of electron-hole pairs near the surface/interface and on charging and discharging of surface/interface defect states upon illumination. Yet, CPD measurements can also be skewed and lead to erroneous results owing to changes in chemical composition and electronic structure of the surface under illumination.^[4-5] Obtaining accurate SPV signals can therefore be challenging when working on perovskite surfaces, which are sensitive to, and degrade under, irradiation. Care must be taken to distinguish SPV signal and reversible band flattening as a result of photo-excitation, which occurs over short time scales (μs), from the long-term changes in surface work function due to surface reorganization or changes in stoichiometry (irreversible, or slowly reversible over hours), e.g., when the perovskite film surface undergoes degradation and decomposition during the CPD or SPV measurement. We present here a vacuum-based study^[6] of the surface potential and response to illumination of two different types of perovskite films, methylammonium lead bromide (MAPbBr₃) and the 2D Ruddlesden−Popper phase butylammonium lead iodide (BA₂PbI₄, n = 1), using KP-based CPD and SPV measurements. We show that supra-band gap illumination of both MAPbBr₃and BA₂PbI₄leads to halide loss from the surface of the material, accompanied by a contamination-induced modification of the KP work function.^[6] If undetected, this can lead to misinterpretations of the MHP surface potential. Repetitive calibration of the tip work function with HOPG is necessary to ensure reliable results. In contrast to MAPbBr₃, BA₂PbI₄exhibits a significant SPV corresponding to a partial flattening of an upward surface band bending. Our results illustrate the effectiveness of the Kelvin probe-based technique in providing complementary information on the energetics of perovskite surfaces and the necessity to monitor the work function of the probe to avoid erroneous conclusions when working on these materials.

[1] Schulz, P. et al. Energy Environ. Sci. 2014, 7, 1377−1381.
[2] Miller, E. M. et al. Phys. Chem. Chem. Phys. 2014, 16, 22122−22130.
[3] Barnea-Nehoshtan, L.et al., J. Phys. Chem. Lett. 2014, 5, 2408−2413.
[4] Zu, F. S. et al. Adv. Opt. Mater. 2017, 5, 1700139.
[5] Li, Y. et al. J. Phys. Chem. C 2015, 119, 23996−24002.
[6] Zhang, F. et al. J. Phys. Chem. Lett. 2019, 10, 890–896.

The spatial localization of charge carriers to promote the formation of bound excitons and concomitantly enhance radiative recombination has long been a goal for luminescent semiconductors. Zero-dimensional materials structurally impose carrier localization and result in the formation of so-called self-trapped excitons. We present fully inorganic, perovskite-derived zero-dimensional Sn^II material Cs₄SnBr₆ and a general series of Cs_4-xA_xSn(Br_1-yI_y)₆ (A=Rb, K; x≤1, y≤1) that exhibit room-temperature broadband photoluminescence centered with quantum yields (QYs) of 10-20 % [1]. The emission peak of these materials ranges from 500 nm to 620 nm. We also present the synthesis, the structure as well as electronic and optical properties of a family of hybrid one- and two-dimensional tin (II) bromide compounds comprising guanidinium [G, C(NH₂)₃⁺] and mixed cesium-guanidinium cations: G₂SnBr₄, CsGSnBr_4, and Cs₂GSn₂Br₇ [2]. G₂SnBr₄ has a one-dimensional structure that consists of chains of corner-shared [SnBr₅]^2- square pyramids and G cations situated in-between the chains. G₂SnBr₄ is a luminescent phase with a broad emission band resulting from trapped excitonic states. In addition, we will discuss similar antimony-based compounds [3], which benefit from higher chemical robustness as compared to 0D Sn halides.
We will also present several unconventional applications of such metal halide luminophores, which harness molecular-like, moderately fast and single exponential radiative recombination kinetics. In particular, we show that the strong temperature dependence of the photoluminescence of low-dimensional tin-halides is ideally suited for remote thermography and thermal imaging [4]. The lifetimes values can be varied over several orders of magnitude by adjusting the temperature (up to 20 ns °C^-1) and the sensitive range spans up to one hundred centigrade, and is tunable from -100 to 110 degC going from [C(NH₂)₃]₂SnBr₄ to Cs₄SnBr₆ and (C₄N₂H₁₄I)₄SnI₆. Through the implementation of cost-effective hardware for fluorescence lifetime imaging, based on time-of-flight technology, these thermoluminophores have been used to record thermographic videos with high spatial and thermal resolution.


References:
1. B.M. Benin et al. Angew. Chem. 2018, 57, 11329–11333
2. O. Nazarenko et al.., 2019, 31, 2121–2129
3. V. Morad et al. J. Am. Chem. Soc. 2019, in print
4. S. Yakunin et al. Nature Materials 2019, in print

Pulsed Laser Deposition (PLD) has offered unique options for the development of complex materials thin film growth, allowing stoichiometric transfer and multi-compound deposition independent of the relative volatility of the elements, and ultimate control of interfaces. In the field of complex oxides, PLD opened the way to high-Tc superconducting films requiring stoichiometric transfer of multiple (4-5) cations. In this presentation we discuss the rather unexplored but huge potential of PLD for the controlled formation and study of multication-multihalide perovskite thin films. More specifically we present the unique single-source deposition of multicompound halide perovskites in-vacuum. We demonstrate the deposition of a variety of compounds from Cs(Pb/Sn)(X)₃, X=I, Br, targets fabricated by uniaxial and isostatic pressing of CsI, CsBr, PbI₂, SnI₂, PbBr₂ powders. The isostatic pressing method presents the advantage that no solvents are involved during the process and produces a compact dense target, as required for PLD deposition. We will discuss the effect of milling and isostatic pressing parameters for the fabrication of targets with perovskite phase and their effect on the final perovskite thin film formation. The effect of PLD deposition parameters, including laser energy, fluence and frequency, as well as the system background pressure, will be discussed and linked to the structural – optoelectronic properties of the films. This work represents an important step forward in the development of controlled growth and future scalability of halide perovskites for efficient optoelectronic devices.

Organic-inorganic hybrid perovskites allow for creating a wide variety of structures: from a 3D structure using small organic building blocks to essentially 2D layered structures using larger organic building blocks. This opens an avenue towards a quite new class of organic-inorganic nano-composites in which the inorganic perovskite sheet acts as a template for the self-assembly of organic chromophores confined between the sheets of the inorganic layer. Consequently, an organization can be obtained for the organic chromophores that resemble the order observed in a single crystal. The number of inorganic sheets can easily be tuned by changing the stoichiometry of the organic small and large building blocks. In this way, a fluent transition of electro-optical properties can be achieved of the inorganic part from confined 2D structures to strongly delocalized quasi-3D structures. We will discuss the structures obtained so far.
The use of carbazole ammonium salts in 2D hybrid perovskites leads to materials for solar cells with enhanced photoconductivity and stronger resistance toward moisture yielding solar cells with strongly enhanced stability compared to the 3D MAPI perovskite material [1].
Also, the use of pyrene ammonium salts to synthesize 2D hybrid perovskites has been explored and initial results on the structure and optoelectronic properties will be discussed. Transitions between 1D and 2D structures were observed by specific modification of the stoichiometry of the components and also in function of temperature [2].
In combination with the introduction of extra secondary interactions in the organic layer, a material is obtained with an exceptionally low bandgap. This was realized by intercalating strong electron acceptor molecules (e.g. TCNQ and TCNB) in the organic layer. The pyrene chromophore acting as a donor together with TCNQ leads to the formation of an organic charge transfer complex between the inorganic sheets [3, 4].
In more recent work we are exploring mono tethered oligo-thiophene ammonium salts as building blocks for low dimensional hybrid perovskites. In this way, a new group of truly organic-inorganic hybrid materials is disclosed with possibly new applications for thin film electronics.

[1] R. Herckens, W. T. M. Van Gompel, W. Song, M. C. Gélvez-Rueda, A. Maufort, B. Ruttens, J. D'Haen, F. Grozema, T. Aernouts, L. Lutsen, D. Vanderzande, Journal of Materials Chemistry A (2018) 6 (45), 22899-22908
[2] W. T. M. Van Gompel, R. Herckens, K. Van Hecke, B. Ruttens, J. D’Haen, L. Lutsen and D. Vanderzande, ChemNanoMat (2019), 5, 323–327.
[3] W. T. M. Van Gompel, R. Herckens, K. Van Hecke, B. Ruttens, J. D'Haen, L. Lutsen, D. Vanderzande, Chem. Commun., 2019, 55, 2481-2484.
[4] N. Marchal, W. T. M. Van Gompel, M. C. Gélvez-Rueda, K. Vandewal, K. Van Hecke, H.-G. Boyen, B. Conings, R. Herckens, L. Lutsen, C. Quarti, F. C. Grozema, D. Vanderzande, D. Beljonne, Chemistry of Materials, under revision.

Over the last seven years we have witnessed the rise of lead-halide perovskites for optoelectronic applications such as photovoltaics, sensors and light-emitting diodes. Well before that, oxide perovskites have been extensively investigated and are today pivotal in many technological applications. Yet, a rational connection between these two important classes of materials is missing. In this talk, we will employ a computational design strategy to explore this missing link and demonstrate that for each halide perovskite there are several lookalike oxide perovskites with similar optoelectronic properties. As a proof of concept, we will also report on the synthesis of Ba₂AgIO₆, the oxide analog of Cs₂InAgCl₆.
We will begin by showcasing recent efforts towards new materials that do not contain Pb, for which computational design approaches from first-principles have been extensively successful and revealed another class of compounds; the so-called halide double perovskites. Five inorganic crystals have been since synthesized and characterized; Cs₂BiAgCl₆, Cs₂BiAgBr₆, Cs₂SbAgCl₆, Cs₂SbAgBr₆,and Cs₂InAgCl₆. Among these, Cs₂BiAgBr₆ has the narrower indirect band gap of 1.9 eV, and Cs₂InAgCl₆ is the only direct band gap semiconductor, yet with a large gap of 3.3 eV. All of them exhibit low carrier effective masses and consequently, are prominent candidates for opto-electronic applications such as photovoltaics, light-emitting devices, sensors, and photo-catalysts.¹ We will outline the computational design strategy that lead to the synthesis of these compounds, and particularly focus on the insights we can get from first-principles calculations in order to facilitate the synthesis, improve their opto-electronic properties and the in-silico identification of compounds with properties that are similar to the lead-halide perovskites². This rational design approach allows us to further develop a universal analogy concept that can be used to identify analogs between oxide and halide perovskites. This new concept of analogs led us to identify a new oxide double perovskite semiconductor, Ba₂AgIO₆, which exhibits an electronic band structure remarkably similar to that of our recently discovered halide double perovskite Cs₂AgInCl₆, but with a band gap in the visible range at 1.9 eV. We report the successful synthesis of Ba₂AgIO₆ by solution processes and we perform crystallographic and optical characterization. We show that Ba₂AgIO₆ and Cs₂AgInCl₆ are both analogs of the well-known transparent conductor BaSnO₃, but the significantly lower band-gap of Ba₂AgIO₆ makes this new compound much more promising for oxide-based optoelectronics and for novel monolithic halide/oxide devices.³

[1] Patent WO 2017/037448 Al (2015); G. Volonakis et al., JPCL 7 1254 (2016); G. Volonakis et al., JPCL 8 772 (2017); G. Volonakis, and F. Giustino APL 112 243901 (2018)
[2] G. Volonakis et al., JPCL 8 3917 (2017)
[3] Patent Application 19386013.7 (2019); G. Volonakis et al., JPCL 10 1722 (2019)

Over the past ten years, a steep rise in the efficiency of metal halide perovskite solar cells has been observed. With the technological giant leap comes an impressive number of synthetic routes that lead to crystalline thin films used in devices. In the case of MAPbI₃ (MA = CH₃NH₃), using chlorinated precursors has attracted high interest as the morphological, structural and electronic properties of the resulting layers are improved when using PbCl₂ as a lead source. The absence of chlorine in the final perovskite layer has been explained by the large ionic radii difference between chlorine and iodine, making alloy formation impossible. However, halide exchange between MAPbI₃ and MAPbCl₃ has already been observed when exposing MAPbI₃ (MAPbCl₃) to a MACl (MAI) solution, without any alloying and despite the large lattice parameter mismatch (6.275 Å and 5.66 Å for MAPbI₃ and MAPbCl₃ respectively, in the cubic phase).
Here, the structure and microstructure of MAPbI₃ thin layers synthesized from PbCl₂ and MAI precursors and optimized for devices in terms of efficiency and aging are studied. X-ray diffraction (XRD) measurement at room temperature on these layers surprisingly reveal the presence of MAPbCl₃ phase in the film and a double texturation along [hh0] and [00l] MAPI directions. By in-situ XRD study of the annealing process, we unravel the role of MAPbCl₃ as a crystallization intermediate for MAPbI₃ thin film. Three stages are observed: for the first one, MAPbCl₃ and MAPbI₃ coexist in the layer, the intensity of the former decreasing in favor of the latter, and the MAPbI₃ cell being under an important and increasing compressive strain (up to 0.55%). The second stage is characterized by the formation of PbI₂, MAPbI₃ degradation phase, as a mechanism to partially relax the strain. During the last stage, MAPbCl₃ disappears and MAPbI₃ strain is suddenly released, with no further evolution of degradation phase. When completely removing the chlorinated phase from the layer through careful annealing, PbI₂ cannot be avoided but the double texture is no longer observed at room temperature. This two-fold texture is the signature of twin domains present in the MAPbI₃ crystallites. Considering the ferroelastic behavior of MAPbI₃ which supposes that the response to an external strain is a structural perturbation, the twin domains find their origin in the strain caused by the coexistence of MAPbI₃ and MAPbCl₃ in the layer. To gain further insight into the correlation between the strain levels and the twin domains, we studied the behavior upon temperature of the two population of crystallites in layers presenting different levels of strain.
In summary, we identified one crystallization path of MAPbI₃ thin layers through ionic exchange and we characterized the strain in the layer induced by this process. We report here the XRD signature of the ferroelastic character of MAPbI₃ that have been documented only by local probes so far. Through in-situ and temperature dependent measurements, we unraveled the direct correlation between strain and twin domains.

The measurement of ionic and electronic conductivities and their dependencies on the control parameters (stoichiometry, doping level, temperature) are discussed in terms of defect diagrams. The major stoichiometric parameter is the iodine partial pressure that has a significant influence on the properties. As far as the dopants are concerned, extrinsic (Na, oxygen) and native frozen defects are considered [1,2].
While these results show far-reaching similarities to typical oxide perovskites, the finding that under illumination not only the electronic but also the ionic conductivity is greatly enhanced, is surprising. The relevance of this effect on photo-degradation but also on photo-demixing in perovskite mixtures is set out [3].
In addition to bulk properties, interfacial effects are investigated in the light of mixed conductivity. It is shown that the space charge effects at MAPI/Al₂O₃ and MAPI/TiO₂ interfaces are determined by ionic excess charges rather than electronically dominated. Such phenomena are well studied in the case of mixed conductivity and ionically conducting solids (nanoionics) [4], but provide a paradigm change in understanding and probably also in designing relevant photoactive interfaces.

References
[1] T.-Y. Yang, G. Gregori, N. Pellet, M. Grätzel, and J. Maier, Angew. Chem. Int. Ed. 54, (2015) 7905
[2] A. Senocrate and J. Maier, J. Am. Chem. Soc., (Perspective) 141, (2019) 8382
[3] G. Y. Kim, A. Senocrate, T.-Y. Yang, G. Gregori, M. Grätzel, and J. Maier, Nat. Mat. 17, (2018) 445
[4] J. Maier, Nat. Mat. 4, (2005) 805

The large amount of ionic bonds in the perovskite and the distorted lattice structure make the material posssess a large number of mobile ions, and the migration of these ions is considered to be the main cause of device instability. The effect of ionic transport on the physical properties of CH₃NH₃PbI₃ perovskite films was investigated by using a in-situ fluorescence imaging system under light and electric field. It was found that prolonged ion migration can cause irreversible changes in perovskites with a large number of defect states near the electrodes, indicated by quenched fluorescence. With higher humidity (> 30%RH), ion migration is enhanced, even lead to morphology change of the film. It indicates that water molecules facilitate the ion migration in perovskites, which may explain the moisture-related burn-in degradation in perovskite solar cells. To quantitatively study the ion migration in perovskites and the effect of composition, moisture and light, perovskites were tested under different light intensity and temperatures using a cryogenic system. The electronic conductivity and the ionic conductivity are extracted via Galvanostatic and IV scanning measurement. The temperature-dependent relationship of the conductivity is summarized and analyzed. Further, the electronic and ionic transport properties of the material can be obtained by using Arrenhenius equation fitting, e.g. energy level of defects, carrier scattering mode and activation energy of ion migration. The corresponding results are as follows: light and water molecules can reduce the activation energy of ion migration by several times, and the substitution of organic cations by inorganic cations (Cs⁺) can greatly inhibit ion migration, especially under light; we demonstrated a solution-processed CsPbI₂Br solar cell can exhibit tremendous improvement on long-term operational stability under continuous steady-state operation, showing 1500 h operational stability under continuous light illumination at MPP tracking. The excellent long-term stability of CsPbI₂Br indictes the importance Cs⁺ in the inhibition of ion migration in perovskite solar cells.
Ion migration behavior in mixed cation hybrid perovskite film is systemically investigated. Phase segregation and ion migration is identified in CsFAMA system by a combination of in situ photoluminescence scan and Galvanostatic measurement. Near the anode/cathode side, visible PL red/blue shift can be observed under synergetic effect of light and electric field. The Phase segregation was found to be correlated with the halide migration in the film. To combine the high efficiency of CsFAMA and the high stability of inorganic CsPbX₃，we put forward a new structure to construct a CsPbBr₃-clusters passivated perovskite film. In this structure, CsPbBr₃ forms as cluster, suppresses ion migration and passivates CsFAMA grains. CsFAMA films with 3% mol of CsPbBr₃ additive demonstrate enhanced PL lifetime, decreased defect state density, larger activation energy of ion migration, and suppressed phase segregation. As a result, the passivated PSCs exhibit around 20% stabilized power conversion efficiency without ‘burn-in’ exponential decay, with a champion open circuit voltage of 1.2 V for 1.62 eV bandgap perovksite. Furthermore, the target device achieves significantly extended long-term operational stability by remaining 90% of the initial efficiency after 500 h continuous operation under maximum power point (MPP) and light illumination. More importantly, under both continuous full-sun illumination, MPP operation and thermal stress (65 °C), the passivated planar type PSCs using TiO₂ as electron transport layer and Spriro-OMeTAD as hole transport layer demonstrate a 125 h device lifetime (T₈₀).

While an ITO/MAPbI₃/Au or ITO/FAPbI₃/Au device appears uncomplicated, the degradation mechanisms are non-trivial to fully elucidate. In this study, we identify defect mediated, thermal and voltage-induced degradation reactions between the acid salts methylammonium and formamidinium at the ITO/perovskite interface that are exacerbated by O₂-plasma treatments relative to UV-ozone. Thermally induced reactions catalyzed by the perovskite/Au interface are also discussed. Moreover, we systematically deconstruct electrically stressed devices to characterize cathodic and anodic electrochemical reactions at both the ITO and Au electrodes within a voltage window of -1.5 to 1.5 V. For example, the acidic etching of ITO can be cathodically induced near room temperature influencing impurities, electronic properties, and stability. At the Au electrode, exceeding a threshold voltage can anodically oxidize iodide to produce a small amount of triiodide which serves as an excellent lixiviant to facilitate the dissolution of Au as AuI or [AuI₂]^- species, transport these species through the perovskite layer, and electroplate neutral Au at the cathode. Our results illustrate that, for a single device, a variety of degradation pathways are possible with the dominant mechanism highly dependent on the specific temperature and bias regime as well as detailing how device stability can be affected by processing, reverse bias, and bias history. Finally, we believe our observations and approach to be pedagogical for experimental design aiming to uncover degradation mechanisms in more highly engineered perovskite devices.

Clarifying the connection between the mixed ionic-electronic conducting properties and the observed huge capacitive/inductive behavior and current-voltage hysteresis in hybrid perovskites based devices is critical to the advance of the field. Here, we discuss the observation that changes in electronic charge transfer processes, such as recombination, at interfaces of hybrid perovskite solar cells due to mobile ion redistribution can give rise to apparent capacitive behavior.[1,2] This capacitive effect differs from the chemical capacitance, which accounts for bulk polarization effects, and traditionally describes the capacitive behavior of mixed conductors.[3,4] It is also not simply associated with an accumulation or depletion of charges in the active layer, and cannot therefore be adequately described with a conventional capacitor. We identify the bipolar transistor as a most appropriate circuit element to reproduce analytically the observation of ionically-gated electron transfer at interfaces. We include transistors into an equivalent circuit model that couples ionic and electronic carrier dynamics[1]. We propose that the huge capacitive and/or inductive behavior[5] observed for thin film perovskite solar cells is best described by electronic currents that are in fact an amplified version of the ionic current. The resulting model yields good fits to the experimental impedance spectra and can reproduce large perturbation transient measurements such as hysteretic current-voltage characteristics.[6] Since the model emphasizes the importance of ionic transport in the perovskite layer and of the space-charge behavior at interfaces in these devices, we perform an electrode material and thickness dependence analysis of perovskite devices. Our results help elucidating the space charge situation at perovskite/metal interfaces and clarify the relative contributions of chemical capacitance and of interfacial ionically-gated electronic charge transfer to the capacitive behavior of hybrid perovskite based devices.

References

[1] D. Moia, I. Gelmetti, P. Calado, W. Fisher, M. Stringer, O. Game, Y. Hu, P. Docampo, D. Lidzey, E. Palomares, J. Nelson, P. R. F. BarnesEnergy Environ. Sci. (2019), 12, 1296–1308.
[2] Pockett, A.; Eperon, G. E.; Sakai, N.; Snaith, H. J.; Peter, L. M.; Cameron, P. J. Phys. Chem. Chem. Phys. 2017, 19 (8), 5959–5970.
[3] A. Senocrate, I. Moudrakovski, G. Y. Kim, T.-Y. Yang, G. Gregori, M. Grätzel, J. Maier, Angew. Chem. Int. Ed. Engl. (2017), 56 (27), 7755–7759.
[4] J. Jamnik, J. Maier, Phys. Chem. Chem. Phys. (2001), 3 (9), 1668–1678.
[5] O. Almora,K. T. Cho, S. Aghazada, I. Zimmermann, G. J. Matt, C. J. Brabec, M. K.Nazeeruddin, G. Garcia-Belmonte, Nano Energy (2018), 48, 63–72.
[6] H. J. Snaith, A. Abate, J. M. Ball, G. E. Eperon, T. Leijtens, N. Noel, S. D. Stranks,J. T. W. Wang, J. Phys. Chem. Lett. (2014), 5 (9), 1511–1515.

Perovskite solar cells (PSCs) have become a competitive photovoltaic (PV) technology with rapid progress of efficiencies reaching above 24% for single-junction devices. The bandgap tunability through perovskite composition engineering is attractive for developing ultrahigh-efficiency tandem solar cells, including perovskite/perovskite, perovskite/silicon, or perovskite/thin-film absorber (e.g., CIGS). In this talk, I will discuss our recent progress on suppressing defects in perovskites covering both the wide-bandgap (~1.7–1.8 eV) and low-bandgap (~1.2–1.3 eV) perovskite compositions. For wide-bandgap PSCs, the challenge is to reduce the voltage deficit, which requires strategies for suppressing defect density. For low-bandgap PSCs based on Sn-Pb mixed perovskite films, key challenges lie at both the material and device levels. The high defect density associated with oxidation of Sn²⁺ to Sn⁴⁺ and formation of Sn vacancies significantly limits carrier lifetime and charge collection. I will discuss these challenges and show our recent progress on the general approaches to reduce defect density in perovskites for both low- and wide-bandgap perovskite films. The precursor chemistry and growth conditions affect significantly the physical and optoelectronic properties of perovskites. Applications of perovskite absorbers in highly efficient tandem devices will be discussed.

Demand for high-density memory semiconductors, which are essential to build large capacity for artificial intelligence and the Internet of Things, has been increasing day by day. PbI₂ based resistive switching memories have been proposed as one of next generation memories because of low operating voltage, high on/off ratio and possibility of multi-level storage as nonvolatile memory. We report here resistive switching memory characteristics of imidazolium lead iodide depending on molar ratio of PbI₂ to imidazolium iodide (ImI). It also proved experimentally and computationally to show the optimum performance for structural reason at any ratio composition. the stoichiometric composition results in hexagonal structure of (ImI)PbI₃ which has molar ratio of PbI₂:ImI = 1:1 analyzed by X-ray diffraction. The structure forms one-dimensional face-sharing [PbI₃^-] chain. Bipolar resistive switching characteristics having forming process are observed regardless of mixing ratio at low SET/RESET operating voltage of 0.2V and -0.2V, respectively. ON/OFF ratio is increased from 10⁶ to 10⁹ as ImI content is increased due to the increased HRS associated with pronounced insulating characteristics by ImI. Whereas, the stoichiometric (ImI)PbI₃ exhibits 5 times longer endurance (10³) and one order of magnitude longer retention time (10⁴ s) as compared to other compositions. Multilevel data storage also appeared by changing the compliance current. Conduction mechanism is investigated I-V measurement depending on temperature. Low resistance state (LRS) and high resistance state (HRS) are associated with Ohmic conduction and Schottky conduction, respectively. Density functional theory (DFT) calculation shows that defect formation energy of iodine vacancy is estimated to be low indicating that (ImI)PbI₃ has enough concentration of iodide vacancy for the filament formation and Further energy barrier calculations reveal that iodide migration preferentially occurs along 1-dimensional [234] crystallographic direction rather than interlayer [130] direction with lower energy. A good performance of (ImI)PbI₃-based memristor is thus related to the low defect formation energy of iodide vacancy and the preferential growth of the filament along 1-dimensional chain. These results suggest that (ImI)PbI₃-based memristor is potential candidate for next-generation memory application.

Lead-halide perovskites have emerged as a new class of high-performance semiconductors with applications in solar cells, solid-state lighting, and photocatalysis. Some of their excellent properties, including high charge carrier mobility and defect-tolerance, can be attributed to the presence of the heavy-metal lead with Pb 6s² states at the band edge. However, lead-halide perovskites suffer from stability issues due to the generally weaker metal-halide bonds when compared to metal nitrides and oxides, which are more robust. On the other hand, metal oxides and nitrides typically have large band gaps that make them unattractive for solar energy harvesting and visible-light optoelectronics.
In this work, we will present new, stable quaternary oxynitrides, designed using first-principles density-functional-theory calculations, with predicted band gaps ranging from 1.6 to 3.3 eV, spanning the entire visible spectrum, that can act as alternatives to lead-halide perovskites. These semiconductors were predicted from a high-throughput computational search of the expansive ABO_3-xN_x composition space, where A and B are inorganic cations, with B being a p-block metal with its s-states at either the valence or conduction band edge of the resulting compound. Several of these oxynitrides are on the convex hull of competing phases, indicating thermodynamic stability against decomposition, while others are slightly metastable. Their band structures feature a very disperse conduction band, similar to many transparent conducting oxides. The smaller electronegativity of nitrogen with respect to oxygen reduces the band gap substantially compared to oxides. In addition, some of these new compounds have a ferroelectric polarization as high as 16 μC/cm². The spontaneous polarization in such semiconductors can be expected to efficiently separate electrons and holes, leading to improved efficiency in solar cells.
The combination of a narrow, tunable band gap, high electron mobility, and ferroelectric polarization in one material would allow for several exciting applications. For example, the ferroelectric BiFeO₃ is known to show the bulk photovoltaic effect, but with poor power conversion efficiency and low current density due to a combination of wide band gap and low electron mobility. The new polar oxynitrides, presented here, are expected to overcome these challenges.

Despite the remarkable performance of hybrid organic-inorganic perovskites (HOIPs) in solar cell, light emission, and photodetector, it remains to require further advances in fundamental understandings of HOIPs photophysics. Recently, the discovery of ferroic twin domains in HOIPs has initiated contentious discussion on the ferroic nature of HOIPs. Given the interconnected nature of defect chemistry, ionic states, and ferroic properties, the effects of ferroic domains on the optoelectronic properties of HOIPs can no longer be ignored. Ferroelectric polarization—which has yet to be unambiguously established despite considerable effects to do so—is thought to facilitate the dissociation of photoinduced electron-hole pairs, benefitting photovoltaic action. Alternate to ferroelectricity, ferroelasticity was also proposed in these materials. Here, we systematically studied the piezoelectric response of multiple CH₃NH₃PbI₃ thin films that exhibit twin domains and have different crystallographic planes parallel to the substrate. In doing so, we demonstrate that the piezoelectric contrast between twin domains along studied orientations is below 1 pm/V. Therefore, the ferroelectricity (if there is) along these orientations is < 1 pm/V. By applying electric biases, we find that the domain changes are governed by ion redistribution and structural deformation under electric biases. This indicates that the biases induced domain evolution in these materials is different from classical ferroelectric materials, where the ferroelectric polarization switching is the major driver. The ion redistribution and structural variation point to the potential effects of twin domains on solar cell operation. In addition, we for the first time reveal the photoluminescence (PL) variations in CH₃NH₃PbI₃ twin domains, indicating the different optoelectronic properties between domains. Density functional theory (DFT) simulation further indicates that the photogenerated electrons and holes show preferential distributions in the ferroelastic twin domains due to strain and chemical inhomogeneity between domains. In turn, this preferential charge distribution results in different lattice strain, alternating the behaviors of the ferroelastic twin domains in CH₃NH₃PbI_3. In-situ X-ray diffraction and band excitation piezoresponse force microscopy (BE-PFM) measurements present experimental evidence of the interaction between charge carriers and ferroelastic twin domains. This work provides an in-depth understanding of the effects of twin domains on optoelectronic properties of HOIPs, which are helpful for further improving the optoelectronic performance of HOIPs.

Organic-inorganic halide perovskites (OIHPs) are strong candidates for high-performance low-cost solar energy, light emission and detection applications^_1-3. Despite extensive research, they remain far from well understood, particularly the role of phonons and organic molecule dynamics on the optoelectronic properties. It is known that there is a strong coupling between the dynamics of cation off-centering/orientation in the cage of octahedral and the inorganic framework tilting^{4, 5} but a full accounting of how this impacts thermal transport and the long-charge carrier lifetimes that help enable high power conversion efficiencies⁶ is still elusive. A hot-phonon bottleneck effect has been shown to significantly extend the cooling time of hot charge carriers in OIHPs, which thermalize first through carrier-optic-phonon scattering, followed by optic phonon decay to acoustic phonons, and finally to the thermal conduction. To better understand the fundamental physics of these processes, it is informative to adjust the lattice dynamics independent of electronics by changing isotopes. In this work, we make use of the large 2:1 mass ratio of deuterium to hydrogen to alter the hydrogen-related dynamical modes of the organic molecules. Even though deuteration does not change the overall mass density of the lattice significantly, we find using neutron scattering that deuteration results in a large 20-50% softening in the longitudinal acoustic (LA) phonons near zone boundaries⁷. This softening of the LA phonon occurs as the liberation modes of NH₃ and CH₃ soften and push down on the zone boundary LA phonons via mode anticrossing. This anticrossing behavior demonstrates that there is strong coupling between these hydrogen-controlled molecular modes and the LA phonons. The observed zone boundary softening of the LA phonons causes a significant fraction of the LA-phonon density of states to have a reduced group velocity⁸ which would tend to lower the LA-phonon contribution to thermal conductivity. However, such large changes in the phonon dispersion curves can also change the phase space for the allowed three-phonon scattering processes controlling thermal conductivity⁹, making thermal transport difficult to predict from a simple inspection of the dispersion curves. Remarkably, we also find that this results in a 50% suppression in the already ultralow thermal conductivity¹⁰ owing to a decrease in the propagation velocities of the LA phonons caused by the phonon softening. Finally, we use first-principles calculations to show that light- or X-ray-induced lattice expansions/distortions^{11, 12} associated with improved performance produce relatively small changes to these phonons, indicating that previous x-ray measurements⁵ are not strongly biased by the probe and that these phonon properties are likely retained under operating conditions. Our findings highlight the importance of phonon-molecular mode interactions in organometallic halide perovskites and suggest a route to enhance hot-carrier properties by tuning the hot-phonon bottleneck.

1. Ahmadi, M., T. Wu, and B. Hu, Adv. Mater., 2017. 29(41): p. 1605242.
2. Lukosi, E., Ahmadi, M. et al., Nucl. Instr. Meth. Phys. Res. A, 2019. 927: p. 401-406.
3. Meng, Y., Ahmadi, M. et al., Organic Electronics, 2019. 64: p. 47-53.
4. Brivio, F., et al., Phys. Rev. B, 2015. 92(14): p. 144308.
5. Beecher, A.N., et al., ACS Energy Lett., 2016. 1(4): p. 880-887.
6. Tong, J., et al., Science, 2019. 364(6439): p. 475-479.
7. Manley, M.E., Ahmadi, M. et al., submitted to ACS Energy Lett., 2019.
8. Manley, M.E., et al., Nat. Comm., 2018. 9(1): p. 1823.
9. Manley, M.E., et al., Nat. Comm., 2019. 10(1): p. 1928.
10. Pisoni, A., et al., J. Phys. Chem. Lett., 2014. 5(14): p. 2488-2492.
11. Tsai, H., et al., Science, 2018. 360(6384): p. 67-70.
12. Batignani, G., et al., Nat. Comm., 2018. 9(1): p. 1971.

In recent years, the perovskite solar cells (PSCs) based on printable triple mesoscopic structure develop rapidly showing a certified power conversion efficiency (PCE) of 15.6% for the lab-scale device, 10.4% for 100 cm² and an AM 1.5 sunlight stability of over 10000 h.
The mesoporous ZrO₂ (mp-ZrO₂) layer plays an important role in separating the mesoporous TiO₂ (mp-TiO₂) and the carbon layer, whose thickness closely relates to the open-circuit voltage (V_OC) and PCE. However, thicker mp-ZrO₂ layer needs more perovskite solution to fill in and causes problems in the cost and reproducibility of devices.
To reduce the thickness of the mp-ZrO₂ layer, two directions can be taken into account. One is to select a more insulating material to replace ZrO₂. The other one is to modify the TiO₂ with proper materials to decrease the recombination between the TiO₂ and the carbon layer. Herein, we choose Al₂O₃ as a modification material on TiO₂. Through simple spraying pyrolysis method, a thin layer of Al₂O₃ is deposited on the surface of the TiO₂ layer. The insert of Al₂O₃ does not change the pore-filling situation or the crystallization of perovskite in the mesoporous scaffold, but accelerates the injection of electrons and reduces the recombination of light-induced carriers. For the thickness of ZrO₂ layer decreases with the help of Al₂O₃, the usage of solvent and PbI₂ is also reduced, which could protect the environment and are benefit for human's health. The modified TiO₂ makes the device both more cost-efficiently and environment-friendly. We support modifying TiO₂ with Al₂O₃ through spraying pyrolysis method improves the performance of devices in all aspects and reduces the usage of raw materials, which would make sense in the commercialization of printable mesoscopic PSCs.

Designing hybrid perovskite optoelectronics which are simultaneously robust, scalable, and highly efficient relies on characterizing their intrinsic charge transport phenomena. We present investigations into picosecond carrier dynamics of sub-micron thick halide perovskite films in the terahertz (THz, or far-infrared) regime. Terahertz emission spectroscopy (TES) is employed to discern the influence of perovskite quantum confinement and electron/hole transport layers on photocurrent generation. Notably, we observe THz emission from a range of 2D halide perovskites. Emission was observed down to “n=1” perovskite layer architectures like butylammonium lead iodide (BA₂PbI₄). These results suggest significant photo-Dember charge separation within 2D perovskite layers, brought upon by a difference in excited electron-hole mobilities. Using this response, we can interrogate carrier dynamics within the 2D perovskite planes and near interfaces with typical charge transport layers like PCBM, Spiro-OMeTAD, TiO₂, etc. Comparing material parameters extracted from systematically varied THz emission spectra allows us to further analyze the relationship between photoexcited carrier transport and local perovskite environment. This work offers non-traditional insights to help guide the rapid development of perovskite solar cells, and strengthens our foundational knowledge of hybrid perovskite materials. Cohesively optimizing perovskite potential through diverse scientific frontiers is necessary to achieve the revolutionary energy technology of our future.

Despite notable achievements of perovskite solar cells have been made for efficiency improvement by optimizing the device structure, and film morphology and structure, the fundamental photophysics of perovskite solar cell, especially the photoexcited carrier dynamic process including relaxation, transport, recombination, and trapping remains unclear. Most studies on solar cell physics were carried out with steady-state measurements, while its ultrafast carrier dynamics has not been demonstrated so far.
Aim to elucidate ultrafast carrier dynamics in-situ solar cells with more than 20 % power conversion efficiency, in this talk, we present the carrier dynamics in-situ organic–inorganic halide perovskite solar cell using ultrafast photocurrent spectroscopy with sub-40 picosecond time. With comprehensive investigation on temperature-dependent, voltage-dependent and laser power-dependent photocurrent of this solar cell, we elucidate the photoexcited carrier relaxation and recombination process from sub-40 picosecond to microsecond. This study establishes the novel characterization approach for in-situ solar cells, in addition to addressing the fundamental desirable questions including carrier lifetime, mobility, trap density, and carrier transport mechanisms.

Gamma-ray spectroscopy that quantifies the gamma-ray energies is a critical technology used in many disciplines such as astrophysics, nuclear material detection and medical diagnosis. Recently, organic–inorganic lead halide perovskite materials have received intense interest as solid state gamma ray detectors owing to their intrinsic properties such as high effective atomic number, low defect density and excellent transport property. Specifically, chlorine-doped methylammonium lead tribromide (MAPbBr_3-xCl_x) and methylammonium lead triiodide (MAPbI₃) single crystals have shown energy resolutions of about 10% for various gamma ray energies from 60 keV (²⁴¹Am) to 662 keV (¹³⁷Cs). [1, 2] While those demonstrations are highly promising, the device operational mechanism of the hybrid perovskite detectors are greatly under explored, and the electrical pulses generated from the gamma-ray photon are not well understood which are essential for further detector optimization and construction of gamma-ray spectra.
Here we report the working mechanism of solid-state gamma-ray detectors made from MAPbBr_3-xCl_x single crystals. Specifically, we build devices with high or low work function contacts that collect only p-type (holes) or n-type carriers (electrons). Detailed device characteristics reveal that the main source of the dark noise in MAPbBr_3-xCl_x crystals originates from the thermally injected electrons from the impurity to the conduction band edge. As a result, only p-type device can observe clear gamma-ray induced electrical pulses from several radioactive sources with different amplitudes that correspond to gamma-ray photons at various energies. In addition, we discover the unusually long rise time (100 micron second) of pulses at room temperature can be greatly reduced at lower temperature. This suggests the long rise time arises from the slow structural dynamics under bias, such as interfacial ion migration. Our study reveals the operational mechanism and paves the way for material optimizations to improve the performance of hybrid perovskite-based gamma-ray detector.

1. Wei, H., DeSantis, D., Wei, W., Deng, Y., Guo, D., Savenije, T. J., Cao, L., Huang, J. Dopant compensation in alloyed CH₃NH₃PbBr_3-xCl_x perovskite single crystals for gamma-ray spectroscopy. Nat. Mater. 16, 826-833 (2017)
2. He, Y., Ke, W., Alexander,G. C. B., McCall, K. M., Chica, D. G., Liu, Z., Hadar, I., Stoumpos, C. C., Wessels, B. W.,Kanatzidis, M. G. Resolving the Energy of γ-Ray Photons with MAPbI3 Single Crystals. ACS Photon. 5, 4132-4138 (2018)

Metal-organic halide perovskites, with the chemical formula CH₃NH₃PbX₃ (X = I, Br, Cl), benefit from unique material properties that include continuous band-gap tunability, high carrier mobility, low non-radiative carrier recombination rates, and simple deposition approaches.^1-3 Thanks to these advantages, perovskite thin-film devices are promising candidates for photovoltaic and light-emitting technologies.⁴ To facilitate the integration of perovskite thin films into such optoelectronic device architectures, a need exist for simple and efficient fabrication methods.
Here we present a template-stripping technique to produce ultra-flat and flexible perovskite thin films.⁵ Spin-coated CH₃NH₃PbBr₃ perovskite thin films can be mechanically cleaved from a silicon template using an epoxy adhesive to expose ultra-flat perovskite surfaces with root-mean-square roughness down to 2.7 nm. The flatness and flexibility of these films enable new processing strategies based on dry interfacing. To demonstrate this, we interface perovskite thin films with plasmonic hole arrays. Through the interaction of the perovskite film with surface plasmon polaritons at the perovskite-metal interface, we are able to obtain a six-fold enhancement of the outcoupling of the perovskite film’s emission in the out-of-plane direction. In addition, by using prepatterned silicon templates, we show that our template-stripping method can produce structured perovskite surfaces with micrometer-sized structures.
Based on these demonstrations, we envision that our template-stripping approach can be utilized for the straightforward integration of perovskite materials into optoelectronic devices.

References
[1] L. M. Herz, Annu. Rev. Phys. Chem. 2016, 67, 65–89.
[2] M. R. Filip et al., Nat. Commun. 2014, 5, 5757.
[3] H. Huang et al., ACS Energy Lett. 2017, 2, 2071–2083.
[4] S. Stranks et al., Nat. Nanotechnol. 2015, 10, 391–402.
[5] A. C. Hernández Oendra et al., in preparation.

Photovoltaic devices based on Lead Halide Perovskites (LHPs) have drawn significant attention due to their outstanding performance in optoelectronic devices. Despite being processable from solution at low temperature, perovskite solar cells have demonstrated certified power conversion efficiencies (PCE's) in excess of those of polycrystalline silicon. However, LHPs exhibit poor stability under ambient environmental conditions and hence restrict commercialization for a long-term use. As the material's electronic properties (mobility and carrier lifetime in particular) are directly related to the device performance, developing an extensive understanding of the stability of optoelectronic properties is crucial for future applications.

Several previous studies have investigated the stability of LHP based devices in air. However almost all studies of electronic properties are carried out via electronic devices, and hence include the implicit encapsulation electrodes and transport layers provide.

In this work, we present electronic properties of solution processed hybride halide perovskite thin films studied via Time Resolved Microwave Conductivity (TRMC) under two different conditions: (1) Extended exposure to elevated temperatures and (2) Extented exposure to ambient (air) conditions. TRMC is a technique enabling direct evaluation of a material's mobility without any contacts, and in the absence of interfacial effects. This study enables us to make unambiguous statements on material stability in these compounds, which could be highly informative for LHP-based optoelectronic applications.

Recently, two-dimensional (2D) Ruddlesden−Popper (RP) perovskites have attracted broad attention for the excellent moisture resistance and tailorable composition.^[1] However, compared with the 3D perovskite, the quantum confinement effect restrains interphase exciton splitting and charge transport owing to the incorporation of an insulating organic spacer layer. Meanwhile, randomly oriented 2D octahedral sheets inside thin film greatly hinder the charge carrier transfer along the vertical direction of the photovoltaic device. Phase engineering in 2D RP perovskites may offer a promising avenue to these obstacles.^[2] Our recent studies have revealed that spontaneously generated 3D phases within 2D perovskite grain boundary not only promote the exciton splitting at the interface of 2D phases but also provide an additional channel for charge transport in device. These 3D phases can be introduced by two different approaches¾either the molecular design of organic spacer or the optimization of perovskite thin film processing.
Firstly, we employed the shorter branched isobutylamine (iso-BA) to replace linear n-butylamine (n-BA) and generate two aliphatic systems of (n-BA)₂(MA)₃Pb₄I₁₃ and (iso-BA)₂(MA)₃Pb₄I₁₃ 2D perovskites (n = 4). It was found that the iso-BA 2D perovskite exhibit remarkably enhanced optical absorption and crystallinity along with preferably aligned orientation than n-BA analogue.^[3] The resulting solar cells exhibit high PCEs of 10.63% and 8.82% with hot-casting and room-temperature processing, respectively. Transient transport studies establish the formation of 3D phases in iso-2D perovskite, which in turn accelerates charge/energy transfer and reduces charge accumulation caused by the unbalanced transfer rate, thereby significantly increasing the PCE.^[4]
Second, among aromatic organic spacer based 2D perovskite system, we introduced S-bearing thiophene-2-ethylamine (TEA) to displace phenylethylamine (PEA).^[5] The intensive interaction between S and Pb atom not only stabilizes 2D inorganic lattices but also disrupts their long-range ordering which distorts the lattices and creates the nucleation sites for the formation of 3D phases. As a result, the best cell based on (TEA)₂(MA)₃Pb₄I₁₃ yield a prominent stable PCE of 11.2% without any photocurrent hysteresis. Importantly, the unencapsulated device display excellent ambient stability by preserving 80% efficiency after 270 h storage in air with 60 ± 5% relative humidity at 25 °C.
Third, by utilization of NH₄Cl additive and DMSO solvent, we demonstrate their favorable synergistic effects on 2D (PEA)₂(MA)₃Pb₄I₁₃ perovskites fabricated at room temperature, which enable to passivate trap states within 3D phase (NH₄Cl) and promote the 2D/3D inter-phase charge transfer (DMSO).^[6] The optimal cell delivers an impressive PCE (n = 4) of 13.41% with eliminated hysteresis.
These findings highlight the importance of 3D phase engineering to modulate the charge carrier dynamics and improve optoelectronic properties of 2D RP perovskites, which offer an effective approach toward commercialized perovskite solar cells with high efficiency and excellent stability.

Reference
[1] Y. Chen, Y. Sun, J. Peng, K. Zheng, Z. Liang, Adv. Mater. 2018, 30, 1703487.
[2] Y. Chen, S. Yu, Y. Sun, Z. Liang, J. Phys. Chem. Lett. 2018, 9, 2627−2631.
[3] Y. Chen, Y. Sun, J. Peng, W. Zhang, X. Su, K. Zheng, T. Pullerits, Z. Liang, Adv. Energy Mater. 2017, 7, 1700162.
[4] K. Zheng, Y. Chen, Y. Sun, J. Chen, P. Chábera, R. Schaller, M. J. Al-Marri, S. E Canton, Z. Liang, T. Pullerits, J. Mater. Chem. A 2018, 6, 6244−6250.
[5] Y. Yan, S. Yu, A. Honarfar, T. Pullerits, K. Zheng, Z. Liang, Adv. Sci. 2019, 1900548.
[6] S. Yu, Y. Yan, Y. Chen, P. Chábera, K. Zheng, Z. Liang, J. Mater. Chem. A 2019, 7, 2015−2021.

In recent years, SnO2, which is an electron-selective layer (ESL) of a planar-type perovskite solar cell (PSC), has attracted attention as a promising substance. While SnO2 has many advantages, SnO2 films contain surface defects that can degrade electronic properties by trapping charge carriers. Herein, we fabricate n-i-p planar-type PSCs with ethylene diamine tetraacetic acid (EDTA)-blended SnO2 (E-SnO2) ESL and investigate the effect of selecting a mixture of solvent, on the solar-cell performance is discussed. We gain better crystallinity, uniform morphologies, higher carrier mobility and enhanced interfacial quality of PSCs with solvent-mixed E-SnO2 ESL compared to traditional SnO2 ESL. The electrical properties of PSCs based on solvent-mixed E-SnO2 ESL using Kelvin probe force microscopy (KPFM) and a conductive atomic force microscope (c-AFM) are examined and the work function and local current of the surface are measured. We also investigate the potential distribution of the cross-sectional area of PSCs. By using KPFM, we can compare the band structure and explain the carrier transport mechanism between the solvent-mixed E-SnO2 ESL and the PSC interface. These photovoltaic performances, optical and electronical properties offer the potential to improve device operation by optimizing solvent-mixed E-SnO2 ESL quality.

Heterostructure consist of organolead halide perovskite and 2D transition metal dichalcogenides (TMDs) was reported to exploit as a high-performance photodetector and a stable perovskite solar cell. TMDs assisted the effective carrier transfer in the interfaces with its high charge mobility and facilitated well aligned band structure for providing the advanced device design. In that point, fabrication and investigation of the charge transport mechanism and the band alignment in various heterostructures is significant for understanding the interfacial properties between the halide-dependent perovskites and TMDs. We fabricated the heterostructure with exfoliated MoS₂ and WS₂ with organolead lead halide perovskites, and conductive atomic force microscopy was used thereafter to display the electrical contacts of the heterostructure. Photocurrent also enhanced in the heterostructures by enhanced charge transport in the interface. Furthermore, Kelvin probe force microscopy was conducted to demonstrate the band alignment in the different type of heterojunction. We propose the efficient heterostructure from the charge transport mechanism and the band structure in the various junctions depending on the halide-dependent perovskites and TMDs.

Organic-inorganic hybrid perovskites (OIHPs ), i.e., such as methylammonium lead iodide perovskites (CH₃NH₃PbI₃) have demonstrated distinguished optoelectronic properties for a variety of applications including photovoltaics (PV), light emitting diodes (LED), and photodetectors, etc. To this end, solution-based crystallization of OIHPs has been widely employed to prepare perovskite thin films desired for the device fabrication process. attracted interests due to their enormous potential in photovoltaics (PV) application, such as Solar cell, light emitting diodes (LED), and photodetectors. Most applications are based on the polycrystalline thin film which synthesized via dissolution & recrystallization in organic solvent. However, it has limitations such as irregular morphologies with randomly oriented grains, which cause large grain boundaries which deteriorate carrier lifetime and diffusion length. Despite the versatility of solution-based synthesis, however, the as-prepared perovskite films exhibit relatively irregular surface roughness implying the existence of randomly-oriented crystal domains and large density of grain boundaries, which ultimately reduce the device performance and lifetime. Moreover, the device fabrication based on the typical “top-down” lithography remains difficult to apply due to the general instability of OHIPs to the solvents involved in the process. However, it has limitations such as irregular morphologies with randomly oriented grains, which cause large grain boundaries which deteriorate carrier lifetime and diffusion length.
In this study, we report the CVD patterning of lead iodide (PbI₂) thin films on a micro-scale pre-patterned Si substrate by P(NBOC-r-GMA) polymer, followed by conversion into CH₃NH₃PbI₃ using methylammonium iodide (CH₃NH₃I) vapor. Detailed analysis of PbI₂ film growth on the polymer patterns suggests that the surface energy difference between polymer and Si promotes the selective deposition of PbI₂ precursor onto hydrophilic Si surface with a uniform thickness of about 100 nm. We observe the formation of [0001]-oriented PbI₂ nanoplatelets are preferred on Si surface, and the selectivity and surface coverage of patterned PbI₂ thin films can be controlled via modulation of growth temperature. The patterned PbI₂ films maintain their morphology after perovskite conversion by virtue of vapor-solid intercalation. We also demonstrate photodetector arrays based on the patterned CH₃NH₃PbI₃ crystalline thin films and confirm the potential for the various optoelectronic applications. Our results highlight the advantage of CVD patterning of perovskite materials in large scale production for various optoelectronic applications.

Quasi-2D Ruddlesden-Poper perovskites have demonstrated interesting optical properties in down-conversion regime. In our work, we found the infrared-to-visible up-conversion photoluminescence (PL) peaked at 521 nm occurring in quasi-2D perovskite [(PEA)₂(MA)₄Pb₅Br₁₆ with n = 5] films by using infrared CW 980 nm laser beam to excite the gap states at room temperature. With increasing the CW laser intensity, the up-conversion PL intensity is almost quadratically increased with the power dependence factor of 1.7. This presents an evidence to show that the up-conversion PL is essentially a two-photon process occurring through the gap states. Furthermore, the two-photon up-conversion PL shows a strong dependence of n value in the 2D perovskites. As lowering the n value, the up-conversion PL signal is dramatically decreased, becoming negligible when the n value is lower than 3. Simultaneously, the gap states are non-detectable in optical absorption when n < 3. This verifies that the gap states are indeed responsible for generating the two-photon up-conversion PL. Moreover, it was found that the two-photon up-conversion PL shows an appreciable dependence of magnetic field through spin mixing mechanism. This provides direct evidence that the gap states are essentially spatially extended states to enhance two-photon up-conversion PL in 2D perovskites.

CH₃NH₃PbI₃ (MAPI) nanocrystals with tetragonal crystal structure and cuboidal shape terminated by (110) and (002) facets and double perovskite Cs₂AgBiBr₆ nanocrystals with cubic double perovksite crystal structure and cuboidal shape terminated by {001} facets were synthesized. Oleylamine and oleic acid are used in the synthesis of both nanocrystals and their interactions with the nanocrystal surfaces were studied and compared using ¹H nuclear magnetic resonance (¹H NMR) spectroscopy and nuclear Overhauser effect (NOESY) spectroscopy. Oleylamine and oleic acid both cap the MAPI nanocrystals, while only oleylamine bonds to Cs₂AgBiBr₆. In the synthesis of Cs₂AgBiBr₆ nanocrystals, oleic acid promotes the ionic metathesis reaction, but does not serve as a capping ligand. It could be substituted with diisooctylphosphinic acid to produce nanocrystals with similar size, cuboidal shape, uniformity, cubic double perovskite crystal structure, and optical properties. Superlattices of MAPI and Cs₂AgBiBr₆ nanocrystals were assembled and studied with grazing incidence small- and wide-angle X-ray scattering (GISAXS and GIWAXS) with in situ heating. MAPI nanocrystals undergo a tetragonal-to-cubic phase transition at 55-60^oC similar to bulk films and begin to degrade at 90^oC, when PbI₂ begins to form. Both hexagonal and rhombohedral phases of PbI₂ are observed as thermal degradation products. In situ photoluminescence (PL) shows that the emission energy increases with increasing temperature and decreases in intensity. In contrast, the PL peak wavelength of Cs₂AgBiBr₆ nanocrystals does not change with increasing temperature. Also, GISAXS and GIWAXS showed that Cs₂AgBiBr₆ nanocrystals are more stable, up to 250^oC. At this temperature, the nanocrystals sinter, but the Cs₂AgBiBr₆ crystal orientation remains the same on the substrate.

As a zero-dimensional all-inorganic perovskite with excellent optical properties, Cs₄PbBr₆ has attracted considerable attention as well as intensive debates, especially two opposing mechanisms of its highly efficient green photoluminescence (PL): embedded CsPbBr₃ nanocrystals versus intrinsic Br vacancy states. Here, we provide sensitive but noninvasive methods that can identify CsPbBr₃ nanocrystal inclusions in Cs₄PbBr₆ as dominant green PL source in this compound. We first synthesized both green PL emissive and non-emissive Cs₄PbBr₆, obtained the complete Raman spectrum of Cs₄PbBr₆ and assigned all Raman bands based on density functional theory simulations. We then used correlated Raman-PL as a passive structure-property method to identify the difference between emissive and non-emissive Cs₄PbBr₆ and revealed the existence of CsPbBr₃ nanocrystals in emissive Cs₄PbBr₆. We finally employed a diamond anvil cell to probe the response of luminescence centers to hydrostatic pressure and excluded the effects of Br vacancies. The resolution of this long-lasting controversy paves the way for device applications of low-dimensional perovskites, and our comprehensive optical technique integrating structure-property with dynamic response can be applied to other materials to understand their luminescence centers.

Halide perovskites (HaPs) are intriguing optoelectronic materials and promising candidates for efficient solar-cell devices. In particular, HaPs exhibit small Urbach energies and sharp optical absorption edges allowing for efficient collection of sunlight in thin-film photovoltaic devices. On the other hand, HaPs also exhibit complex nuclear dynamics and structural effects, including anharmonicity and disorder, which is unusual for efficient optoelectronic materials. Moreover, since small Urbach energies indicate a low amount of disorder, the aforementioned optoelectronic properties are difficult to rationalize. Using first-principles methods, we study the disorder potential induced for electronic states in various paradigmatic HaPs. To account for the complex nuclear motions at elevated temperatures, we used molecular dynamics simulations based on density functional theory. With this approach, we take into account anharmonicity in the lattice dynamics to all orders in the Taylor expansion of the crystal potential. We find that correlations in the disorder potential are dynamically confined to atomic distances, and that the massive nuclear motions of A-site and X-site ions dynamically shortens them. This length-scale of the correlations in the disorder is similar to the one reported for usual inorganic semiconductors, and we show that the dynamic shortening of the disorder leads to favorable distributions of band edge energies. We conclude that sharp optical absorption edges and small Urbach energies, which are highly desired properties of any solar absorber material, are enabled by this dynamic mechanism.

Formamidinium lead halide perovskites have attracted wide attention as photoelectronic conversion materials due to the high photoelectronic conversion efficiency (PCE), low cost and simple synthetic process. The structural, electronic and optical properties of mixed formamidinium lead halide perovskites FAPbI_xCl_3－x (FA＝ NH₂CH=NH₂⁺ , x＝0～3) have been investigated by the first-principles theory. Our results show that FA cations lie along [001] direction in the trigonal FAPbX₃ (X＝Cl, Br, I). However, the direction is slightly shifted owing to the distortion of PbX₆ (X＝Cl, I) octahedrons in the mixed FAPbI_xCl_3－x. The Pb—I bond distances (0.315～0.334 nm) are larger than Pb—Cl bond distances (0.282～0.302 nm). With the increase of I/Cl ratio, the lattice parameters and volumes of FAPbI_xCl_3－x increase. The FA cations play a crucial role in balancing the crystal structure, but they do not participate into the process of frontier orbital transition directly. They just play the role of charge donors to contribute ca. 0.76 e to PbI3 framework. FAPbI_xCl_3－x are direct band-gap semiconductors, with the direct bandgap nature at Z (0, 0, 0.5) symmetry point. The valence band maximum (VBM) is composed of antibonding orbitals of I 5p (Cl 3p) and a few Pb 6s orbitals, and the conduction band minimum (CBM) is composed of Pb 6p orbital. There exists a combined covalent and ionic bonding mechanism between Pb and I (Cl) ions. As the I/Cl ratio increases, the band gaps decrease and the absorption spectra are red shifted. FAPbI₃ has an ideal band gap of 1.53 eV. It exhibits the superior absorption spectrum especially in the range of 300 nm to 500 nm, which elucidates that FAPbI3 has great potential as the photoelectronic conversion material. Our results could provide theoretical guidance for the experimental design and synthesis of perovskite solar cells.

Finding renewable energy sources is of paramount importance to meet the increasing global energy demand whilst minimizing the impact on the environment. The research community has focused on solar energy as it is endlessly available, and have ranked the methylammonium lead iodide (MAPbI₃) as the most promising candidate amongst perovskite solar cells. Despite its high efficiency, the MAPbI₃ is sensitive to humidity, light, and temperature, its instability affects primarily on the crystalline structure and eventually leads to degradation. Three crystalline structures are known for this material, orthorhombic, tetragonal, and cubic which exist in different temperatures. Here we report on several processes detected from laser excitation of MAPbI₃ single crystal at ambient conditions. A phase transition from tetragonal to cubic phase was induced by excitation of over 15 mW laser power. The phases were characterized by LF-Raman and PL, taken simultaneously with the increase of exciting laser power and the spectral changes were assigned to the structural differences. In addition, Raman stimulation of iodine vapors signal was observed, those vapors were generated from the core of the focus wherein the highest temperature lead to degradation. The stimulated Raman phenomenon was enabled due to the unique properties of the MAPbI₃ single crystal and revealed viability to use this material for additional applications in other research fields.

Mixed halide perovskites containing a compositional mixture of formamidinium (FA+) and methylammonium (MA+) ions are the actual standard for obtaining record-efficiency perovskite solar cells. Although the compositional tuning that brings to optimal performance of the devices has been largely established, little is understood on the role of even small quantities of the MA+ ions in boosting the efficiencies of primarily FA+-based hybrid perovskite solar cells. In this study, we use Car-Parrinello molecular dynamics in large supercells containing different ratios of FA+ and MA+ ions. Our analysis shows that the introduction of MA+ cations in the perovskite crystal reduces the orientational disorder of the FA+ cations and leads to a lowering of the vibrational intensity of the inorganic lattice. The charge asymmetry of the MA+ cation within the inorganic cage plays a key role for such ordering process, as it accelerates the reorganization of the organic molecules towards energetically more favorable configurations. Such structural/vibrational optimization facilitates the transport of the photogenerated carries and, in conjunction with the higher thermal stability of FA+-based perovskites, it should be at the origin of the enhanced photovoltaic properties of mixed FA+-MA+ perovskites.

Electrochemical reactivity at the hybrid perovskite interfaces remains one of the most complex phenomena affecting their applications, both via stability of hybrid perovskite structures and via enabling novel functionalities such as sensing [1, 2]. However, exploration of these phenomena is severely limited by the intrinsic complexity of the hybrid perovskite electrochemistry [3, 4], presence of multiple mobile ionic species, and possible role of the environmental species at the triple-phase junctions [5]. Here we report the in-situ studies of electrochemistry of hybrid perovskite-metal interface via time- and voltage- resolved time of flight secondary ion mass spectrometry (ToF-SIMS) measurements of lateral perovskite heterostructures. ToF-SIMS data allows visualizing lateral chemical composition along the surface and its time evolution with light and bias. The machine learning workflow combining Hough transform and non-negative matrix factorization is developed to extract the salient features of associated chemical changes. These data are further compared to the time-resolved Kelvin Probe Force Microscopy measurements. Furthermore, the non-negative tensor decomposition is used to separate the time- and voltage-dynamics in the multidimensional data sets. Our in-situ characterizations provide a comprehensive information on the chemical nature of moving species, ion accumulation, and interfacial electrochemical reactions.

1. Wang, H., et al., Kinetic and material properties of interfaces governing slow response and long timescale phenomena in perovskite solar cells. Energy & Environmental Science, 2019. 12: p. 2054-2079
2. Schulz, P., D. Cahen, and A. Kahn, Halide Perovskites: Is It All about the Interfaces? Chemical Reviews, 2019. 119(5): p. 3349-3417.
3. Wang, J., et al., Investigation of Electrode Electrochemical Reactions in CH3NH3PbBr3 Perovskite Single-Crystal Field-Effect Transistors. 2019. 31(35): p. 1902618.
4. Pospisil, J., et al., Reversible Formation of Gold Halides in Single-Crystal Hybrid-Perovskite/Au Interface upon Biasing and Effect on Electronic Carrier Injection. 2019. 29(32): p. 1900881.
5. Ahmadi, M., et al., Environmental Gating and Galvanic Effects in Single Crystals of Organic–Inorganic Halide Perovskites. ACS Applied Materials & Interfaces, 2019. 11(16): p. 14722-14733.

We recently undertook a rigorous treatment of the interaction of a charged cantilever with an electrically conductive sample having significant sample impedance. The results were surprising. Starting from an electromechanical model of the cantilever-sample interaction, we used Lagrangian mechanics to derive coupled equations of motion for the cantilever position and charge [1]. Using this approach we derived equations describing the cantilever frequency shift and friction observed in Kelvin-probe and electric-force microscopy experiments. A key player in our theory is the transfer function describing the voltage drop across the tip-sample gap; this transfer function depends on the tip-sample capacitance and the complex sample impedance (i.e., sample capacitance and resistance). Our central new finding is that the cantilever frequency and dissipation measure the real and imaginary parts, respectively, of this transfer function. We show how scanned-probe, broadband local dielectric spectroscopy measurements enable us characterize the local sample impedance. Our new treatment enables us to clearly understand, for the first time, how both sample capacitance and resistance contribute to the frequency shift and dissipation observed in electrical scanned probe microscope experiments. We have used these insights to study light-induced conductivity and capacitance in a series of 3D perovskite [2,3] and 2D [4] perovskite and organic photovoltaic films as a function of time, temperature, and substrate. Many, though not all, of the perovskite samples show a conductivity which rises promptly upon illumination and recovers slowly in the dark. This recovery shows an activated temperature dependence with a large activation consistent with ion or vacancy motion. These findings are generally consistent with the Kim-Maier picture that light creates vacancies [5].

REFERENCES

[1] Dwyer, R. P.; Harrell, L. E. & Marohn, J. A. Lagrangian and impedance spectroscopy treatments of electric force microscopy. Phys. Rev. Appl., 2019, 11: 064020, doi: 10.1103/PhysRevApplied.11.064020

[2] Tirmzi, A. M.; Dwyer, R. P.; Hanrath, T. & Marohn, J. A. Coupled Slow and Fast Charge Dynamics in Cesium Lead Bromide Perovskite. ACS Energy Lett., 2017, 2:488-496, doi: 10.1021/acsenergylett.6b00722

[3] Tirmzi, A. M.; Christians, J. A.; Dwyer, R. P.; Moore, D. T.; Marohn, J. A. Substrate-Dependent Photoconductivity Dynamics in a High-Efficiency Hybrid Perovskite Alloy. J. Phys. Chem. C 2019, 123(6): 3402-3415, doi: 10.1021/acs.jpcc.8b1178320

[4] Giridharagopal, R.; Precht, J.T.; Jariwala, S.; Collins, L.; Jesse, S.; Kalinin, S. V. & Ginger, D. S. Time-Resolved Electrical Scanning Probe Microscopy of Layered Perovskites Reveals Spatial Variations in Photoinduced Ionic and Electronic Carrier Motion. ACS Nano 2019, 13(3):2812-2821, doi: 10.1021/acsnano.8b08390

[5] Kim, G. Y.; Senocrate, A.; Yang, T.-Y.; Gregori, G.; Grätzel, M. & Maier, J. Large Tunable Photoeffect on Ion Conduction in Halide Perovskites and Implications for Photodecomposition. Nat. Mater., 2018, 17: 445-449, doi: 10.1038/s41563-018-0038-0

Lead halide perovskites APbX₃ [A = Cs⁺, CH₃NH₃⁺ (MA⁺), and CH(NH₂)₂⁺; X = I⁻, Br⁻, and Cl⁻] have attracted attention both from the viewpoint of fundamental physics and device applications such as solar cells and light-emitting devices because of their unique optoelectronic properties. To clarify the origin of the advanced features of this class of materials, we have so far studied the photoresponses of lead halide perovskite thin films, single crystals, and perovskite-based devices, revealing unique carrier recombination dynamics.^1-5) In addition, the recent researches have suggested the polaron formation is the key mechanism for the unique nature of halide perovskites. However, the impact of electron-phonon interaction on the band-edge optical properties are still under discussion.
Magneto-reflectance spectroscopy is one of the most powerful tools to derive the band-edge optical parameters of semiconductors including exciton binding energy, reduced mass, effective dielectric constant, etc. However, previous magneto-optical studies on lead halide perovskites have estimated the exciton binding energies by employing extremely strong pulsed magnetic fields exceeding 100 T. This approach was difficult for the accurate determination of exciton binding energy at zero magnetic field. Moreover, previous theoretical works have suggest that special attention is needed for the excitonic properties under strong magnetic field in the case of strong exciton-phonon interaction. Therefore, the measurements with high-quality crystals under weak magnetic fields are essential for an accurate estimation.
In this work, we performed high-sensitivity measurements using circular dichroism of reflectance at low temperatures (1.5 K) under relatively weak magnetic fields (< 7T), where the cyclotron energy is sufficiently small compared with the LO phonon energy and exciton binding energy. We successfully observed the higher-order exciton states (1s, 2s, and 3s) of a MAPbBr₃ single crystal, which enabled us to determine exciton binding energy accurately. The estimated exciton binding energy is much smaller than that of previous work under strong field. In the presentation, we will discuss the discrepancy between the data of low- and strong magnetic field in conjunction with the strong exciton-phonon interaction. Also, the results of different halide perovskites will be compared.
Part of this work was supported by JST-CREST (Grant No. JPMJCR16N3) and JSPS KAKENHI (19K03683).

References 1) Y. Yamada, et al., Appl. Phys. Express 7, 032302 (2014). 2) Y. Yamada, et al., J. Am. Chem. Soc. 136, 11610 (2014). 3) Y. Yamada, et al., J. Am. Chem. Soc. 137, 10456 (2015). 4) Y. Yamada, et al., Bull. Chem. Soc. Jpn. 90, 1129 (2017). 5) Y. Yamada, et al., J. Phys. Chem Lett. 8, 5798 (2017).

Metal-halide perovskites show many interesting properties important for its use in optoelectronic devices, such as solar cells. One of the prime characteristics is their very low deep defect density, leading to relatively low non-radiative losses. This is directly translates into a high density of photo-generated carriers in these films resulting in a very high open-circuit voltage (V_OC).¹ However, although excellent power conversion efficiencies (PCEs) have been reported (>24%), the remaining V_OC losses are an open challenge to maximize the PCE. To reveal the correlation between electronic quality and the perovskite solar cells we report here on a set of spectroscopic measurements.

First, we probed the temperature dependence of the absorption spectra of methylammonium lead iodide and bromide (CH₃NH₃PbI₃/Br₃), both for films and bulk crystals. We extracted the Urbach energy, E₀, as the reciprocal value of the slope of the absorption at the band edge, when plotted in a logarithmic scale. Its value depends on the material disorder and generally correlates well with the loss in V_OC of optimized cells, compared to their bandgap.^2,3 We found a strong decrease in E₀ upon cooling, accompanied by a slow decrease in their optical band gap energy. Values measured for films and bulk crystals were highly comparable, indicating bulk-like properties of perovskite films.

Next, from the theoretical E₀ temperature dependence, we obtained the average energy of electronically active phonon states to be about 100 cm^-1, which implicates that the dynamic disorder of halide perovskite films and single crystals is mainly caused by cage vibrations.⁴ We were able to calculate the density of active static defects in perovskites from the temperature dependence as well. Its value is very low for all characterized materials in comparison with other materials used for solar cells, including bulk monocrystal semiconductors like GaAs or crystalline silicon. Thin methylammonium lead iodide films doped by Cs and/or Rb reveal an even lower deep defect density, which is in agreement with the overall superior properties of finalized cell based on these materials.

Finally, we found a strong correlation between the V_OC deficiency of finalized solar cells and E₀ measured by PL spectroscopy. We will show these results in combination with measurements of the quantum yield PL efficiency and correlate these with non-radiative V_OC losses in the solar cell. These results will help to establish more practical efficiency limits of perovskite solar cells by taking into account the E₀, not only the Shockley-Queisser radiative based limit, which considers just the bandgap of active material.
References
[1] Oriol Arteaga(1) Liu, Z.; Krückemeier, L.; Krogmeier, B.; Klingebiel, B.; Márquez, J. A.; Levcenko, S.; Öz, S.; Mathur, S.; Rau, U.; Unold, T.; et al. Open-Circuit Voltages Exceeding 1.26 V in Planar Methylammonium Lead Iodide Perovskite Solar Cells. ACS Energy Lett. 2019, 4 (1), 110–117. https://doi.org/10.1021/acsenergylett.8b01906.
[2] De Wolf, S.; Holovsky, J.; Moon, S.-J.; Loeper, P.; Niesen, B.; Ledinsky, M.; Haug, F.-J.; Yum, J.-H.; Ballif, C. Organometallic Halide Perovskites: Sharp Optical Absorption Edge and Its Relation to Photovoltaic Performance. J. Phys. Chem. Lett. 2014, 5 (6), 1035–1039. https://doi.org/10.1021/jz500279b.
[3] Ledinsky, M.; Schönfeldová, T.; Holovský, J.; Aydin, E.; Hájková, Z.; Landová, L.; Neyková, N.; Fejfar, A.; De Wolf, S. Temperature Dependence of the Urbach Energy in Lead Iodide Perovskites. J. Phys. Chem. Lett. 2019, 10 (6), 1368–1373. https://doi.org/10.1021/acs.jpclett.9b00138.
[4] Ledinský, M.; Löper, P.; Niesen, B.; Holovský, J.; Moon, S.-J.; Yum, J.-H.; De Wolf, S.; Fejfar, A.; Ballif, C. Raman Spectroscopy of Organic–Inorganic Halide Perovskites. J. Phys. Chem. Lett. 2015, 6 (3), 401–406. https://doi.org/10.1021/jz5026323

The demand for large area high-energy radiation detection systems combining high sensitivity and low-cost fabrication, has pushed the research in the last ten years to develop and design both novel materials and device geometries. Despite organic semiconductors have attracted a great attention, their low atomic number strongly limits the high-energy radiation absorption and, blending the organic solution with high Z nanoparticles is necessary to maximize their radiation absorption. Hybrid organic-inorganic perovskites have been recently proposed as alternative materials for X- and γ-photon direct detection, thanks to their high Z atoms, combined with high charge mobility.
In this work we report on thin film X-ray detectors made of solution processed Cesium-containing triple cation perovskite, namely Cs0.05(MA0.17FA0.78)Pb(I0.8Br0.2)3 (CsFAMA), where cesium (Cs) has added to mixed organic cations (methylammonium (MA) and formamidinium CH3(NH2)2 (FA)) and mixed halides (I and Br). We demonstrate how X-ray detectors based on solution processed CsFAMA film possess a high sensitivity, with values up to 80 µC mGy-1 cm-3 in short-circuit conditions, and up to 380 µC Gy-1 cm-2 when operated under low (i.e. 4V) bias conditions: two orders of magnitude higher than previously reported perovskite thin films and comparable to perovskite single-crystal at 50V operating bias [1] . We performed radiation hardness tests and verified that the detectors are still properly working after receiving a total dose of 10 Gy in few minutes. Indeed, the combination of thin films and long carrier diffusion length allows in perovskites the efficient collection of photogenerated charges, even in presence of defects or radiation-induced traps, resulting in a higher radiation tolerance than thicker films and single crystals. Finally, we characterized perovskites thin films with enhanced stability in air. Remarkably, the electrical performance of the final detectors is still unaffected after 50 days of storage in air, with degradation of X-ray sensitivity limited to 15%.
In the light of the above discussed state-of-the-art, we reckon that thin film perovskite devices: i) surpass the state-of-the-art performance of inorganic large area detectors (a-Se and poly-CZT); ii) can overcome the scalability limitation of thick layers and single crystals; iii) allow to envisage battery-operated wearable detectors thanks to their low voltage operation. Ourstudies confirm the great potential of perovskite thin films devices (e.g. solar cells and X-ray detectors) for space application, where light-weight, large area, mechanical flexibility and radiation hardness are crucial features.

[1] L. Basiricò et al. Advanced Functional Materials (2019) in press

The field of metal halide perovskites is moving towards more and more complex compositions enabling improved device performance and stability. Most of the improvements however, were achieved through empirical optimization of processing conditions. Fast and complex chemical reactions lead to significant variations in material properties. Precise control over device performance requires a better understanding and active control over synthetic parameters, thus in situ monitoring of evolving properties can help identify synthesis and structure-property relationships.

We study metal halide perovskite synthesis via multi-modal in situ characterization combining synchrotron diffraction, photoluminescence, infrared and optical imaging to shed light on evolving material properties. Using this approach, we are able to identify different formation paths determined by the nature of the chemical precursor, including the precursor phase transition to perovskite phase and direct perovskite formation from molecular building blocks. Moreover, we find that final film morphology is determined in the first seconds of annealing and can be related to crystalline precursor phases. This new multi-modal platform enables simultaneous optical as well as structural characterization already during spin coat deposition of multicomponent precursors and antisolvents, thus allows new insights into the complex dynamics related to perovskite formation.

In semiconductor physics, the dielectric response, charge carrier mobility and other electronic material properties at finite temperatures, are always treated within the framework of the harmonic approximation. This approach is very successful in capturing the properties of tetrahedrally bonded semiconductors such as silicon and GaAs.

In my talk, I will show that halide perovskites are fundamentally different due to their strongly anharmonic lattice dynamics. Large amplitude, local polar fluctuations induced by lattice anharmonicity localize the electronic states and enhance the screening of electric charge within the material. In other words, in some aspects, halide perovskites behave more like a liquid than a crystalline solid. I will also discuss the implications of these findings on other families of semiconductors such as organic and rock-salt semiconductors.

Metal halide perovskite solar cell research keep progressing on various fronts. My group at OIST uses surface science and advanced characterization to obtain fundamental understanding about perovskite materials and solar cells. The research focus in the field has been shifting from power conversion efficiencies to stability and upscalability. In this talk, I will present our research progress on degradation mechanisms of perovskite materials, development of strategies to improve stability of perovskite materials and devices, upscalable fabrication of perovskite solar cells and modules, and surface science understanding of perovskite materials and stability.

Interfacial energy level alignment between the constituent layers of a solar cell is critical in order to avoid photogenerated charge carrier losses and hence to achieve high device efficiency¹.
In this presentation we focus on the integrated perovskite/bulk heterojunction solar cell (referred to here as the integrated cell), which applies a small bandgap organic semiconductor based bulk heterojunction (BHJ) blend on top of a larger bandgap perovskite layer in order to harvest both the visible and near-infrared part of the solar spectrum^2,3. We reach a high power conversion efficiency (PCE) of 17.7%, however this is lower than the 19.2% (calculated based on the increase in short circuit current), which is due to decreased fill factor in the integrated cell compared to the reference perovskite device. We show that the origin of such losses is the presence of a 250 meV energetic barrier at the perovskite/BHJ interface leading to undesired charge carrier accumulation and recombination. A possible strategy to overcome this limitation is to shift the energy levels of either the perovskite or the BHJ layer. We will demonstrate how this can be achieved by small compositional change of the perovskite layer.
We will show two promising ways to tune the valence and conduction band edges of the widely used methylammonium lead iodide (MAPI) photoactive layer. First, by incorporating tin and bromide ions into MAPI, shifting the valence band edge between -5.4 and -5.0 eV and the conduction band edge between -3.8 and -3.5 eV is achieved. Applying only 15% tin and 30% bromide leads to a perovskite layer with the same bandgap (1.6 eV) as MAPI but with more than 200 meV shallower energy levels.
Second, we will show that that a similar shift (100-200 meV) towards shallower energy levels can also be achieved by the simple incorporation of only 12.5% bromide into MAPI. Despite the increased bandgap (1.65 eV) the best single-junction devices with such mixed halide system can reach PCE of 19.2% in single-junction devices. Surface photovoltage and energy level measurements show that the origin of such improvement is the shallower Fermi level in the developed mixed halide perovskite, which changes the interfacial band bending and by that helps hole extraction in the device. We will show that both of the developed perovskite layers with energy levels shifted shallower by around 200 meV compared to MAPI can be applied in the integrated perovskite/bulk heterojunction solar cell in order to improve on their efficiency.
Our results give an invaluable tool to increase the performance of perovskite based integrated devices through improved interfacial energy level alignment, which tool can also be applied in any perovskite multi-junction devices.

References:
1. Wang, S., Sakurai, T., Wen, W. & Qi, Y. Energy Level Alignment at Interfaces in Metal Halide Perovskite Solar Cells. Adv. Mater. Interfaces 1800260, 1–30 (2018).
2. Kim, J. et al. High-Performance Integrated Perovskite and Organic Solar Cells with Enhanced Fill Factors and Near-Infrared Harvesting. Adv. Mater. 28, 3159–3165 (2016).
3. Gao, K. et al. Highly Efficient Porphyrin-Based OPV/Perovskite Hybrid Solar Cells with Extended Photoresponse and High Fill Factor. Adv. Mater. 29, (2017).

Lead halide perovskite (LHP) have received a lot of attention over the past decade, notably due to the unprecedented evolution of LHP solar cells leading to efficiency up to more than 20% in only a few years. Recently, a new approach based on colloidal perovskite nanocrystals (PNC) of CsPbI₃ led to great improvements for nanocrystal-based solar cells[1]. A tremendous amount of work has been done over the past few years to better understand the electronic/excitonic structure, optoelectronic properties and degradation mechanisms and new surface chemistry tools are being developed[2-4]. However, there is still a lack of a clear, unified picture of the different mechanisms that limit the efficiency of the devices, whether at the material level or at the interfaces in device, or on how to address these limitations. There is also room for improvement of the transport properties of PNC films through more tailored surface chemistry treatments. A more standardized approach of film preparation and characterization could lead to better understanding the limitations of PNC devices and improving their overall efficiency.
At Institut des Nanosciences de Paris, we recently synthesized formamidinium (FA) lead iodide (FAPbI₃) nanocrystals, which have the smallest band gap of the LHP nanocrystal family (around 1.6 eV), ideal for photovoltaic applications and which show better stability than their Cesium-based counterparts. Characterizations using X-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS), X-ray photoemission (XPS) and transport measurements on spin-coated films will be presented. These measurements allowed us to deduce the crystalline structure and inorganic stoichiometry of these nanocrystals, in agreement with literature. We also report on a ligand exchange procedure, using a process previously developed for CsPbI₃ [5], to enhance carrier transport in films. Valence band photoemission (UPS) combined with Inverse photoemission (IPES) measurements have been conducted in collaboration with a team at Politecnico Milano, suggesting that FAPI is a near intrinsic semiconductor with a weak n-type character. We are also developing a new route of film deposition that consists in spraying the nanocrystals in vacuum. We will present preliminary characterizations of the obtained films showing good agreement with spin-coated ones. This new deposition technique would allow for unprecedented in-situ measurements on these PNC as for instance studying the interfaces in devices or films under stresses such as radiation, temperature, oxygen or humidity.

[1] L. Protesescu et al., Nano Lett.,2015,15,6,3692-3696.
[2] L. Protesescu et al., ACS Nano,2017,11,3,3119-3134.
[3] Q. A. Akkerman et al.,Nature Materials, 2018, 17, 394-405.
[4] P. Tamarat et al., Nature Materials, 2019.
[5] E.M. Sahenira et al., Science Adv., 2017,3,10.

Double perovskite crystals such as Cs₂AgBiBr₆ have appeared as a potential answer to inherent instability and toxicity of classic hybrid organics-inorganic led-halide perovskites. In their structure Pb²⁺ cation is substituted by pair of cations with formal oxidation states +1 and +3 forming so-called “double perovskite” with general formula A⁺¹₂B⁺¹B⁺³X₆. Band structure calculations show that many of the double perovskite compounds have a band gap in the range promising for solar cell applications and some of the possible compounds have already been synthesized corroborating theoretical predictions. So far Cs₂AgBiBr₆ has been the most intensively studied representative of double perovskite family. Theoretical calculations predict that the fundamental band gap of Cs₂AgBiBr₆ has an indirect nature but surprisingly the photoluminescence emission can be easily observed from this compound.
In this work we show that overall picture of the emission form Cs₂AgBiBr₆ might be far more complicated than indirect band to band emission. Using a combination of photoluminescence (PL), photoluminescence excitation (PLE) and absorption studies we show that the PL emission in Cs₂AgBiBr₆ is dominated by a strong electron-phonon interaction. Our experimental results indicate that the emission from this material is related to a color center (or self-trapped exciton) rather than a band-to-band transition. The resonant excitation of this center results in a strong enhancement of PL emission demonstrating the competition between the nonradiative recombination paths related to the indirect bandgap and color center emission. Crucially, we show that the significant PL broadening together with the large Stokes shift between emission and PLE maximum can be well explained using Franck-Condon model indicating a strong electron-phonon coupling characterized by a relatively large Huang-Rhys factor ~12. In addition, the nontrivial nature of the PL emission in Cs₂AgBiBr₆ is revealed by magnetic field studies. Surprisingly, the PL exhibits an unexpected red shift and decrease of its intensity with increasing magnetic field. Such behavior is extremely atypical for semiconductors, where usually a blue shift and increase of intensity of the emission line is observed at high magnetic fields as a consequence of the squeezing of the exciton wave function by magnetic field. Moreover, the PL shift in magnetic field depends on temperature and systematically decreases as the temperature increase from 4K to 100K. These results suggest that there remain many open questions concerning the exact nature of PL emission in these fascinating compounds.

Metal halide perovskite (MHP) solar cells have garnered significant attention over the past six years for their high efficiency and low processing costs. However, their relatively poor environmental stability remains as one of the preeminent challenges impeding their wide-spread proliferation. One increasingly popular method for improving the stability of these materials and devices is to cap the 3D MHP layer with a layer of a higher stability 2D Ruddlesden-Popper phase perovskite.^[1,2] These 2D-capped devices have demonstrated success in improving device lifetimes with minimal reduction in efficiency, yet the energy level alignment at these 2D/3D interfaces has not been measured directly and remains largely unknown. In this study, we fabricate an interface between the 3D methylammonium lead iodide (MAPbI₃) and the 2D butylammonium lead iodide (BA₂PbI₄) using a butylamine post-treatment technique investigated in the literature^[3] and probe the ionization energy and electron affinity of the resulting film as well as the valence and conduction band offsets at the interface using ultraviolet and inverse photoelectron spectroscopy, respectively. The resulting energy diagram indicates a type I heterojunction with little if any charge transfer between the two layers. The BA₂PbI₄^[4] layer acts as a small barrier for the extraction of electrons and a larger barrier for holes. Design guidelines for optimizing the 2D/3D interface in future work will be discussed.

References:
[1] A. Krishna, S. Gottis, M. K. Nazeeruddin, F. Sauvage, Adv. Funct. Mater. 2019, 29, 1806482.
[2] F. U. Kosasih, C. Ducati, ChemSusChem 2018, 11, 4193.
[3] Y. Lin, Y. Bai, Y. Fang, Z. Chen, S. Yang, X. Zheng, S. Tang, Y. Liu, J. Zhao, J. Huang, J. Phys. Chem. Lett. 2018, 9, 654.
[4] S. Silver, J. Yin, H. Li, J.-L. L. Brédas, A. Kahn, Adv. Energy Mater. 2018, 8, 1703468.

Hybrid perovskite solar cells based on methylammonium lead triiodide (MAPbI₃) are of tremendous scientific and technological interest, with power conversion efficiencies already beyond 22%. Typically, the MAPbI₃ thin films for planar solar cells are polycrystalline, with average grain sizes ranging from 100 nm to a few microns. Due to the correspondingly high interfacial area, it is thus imperative to characterize and understand the role of grain boundaries in the photo-generated carrier transport for MAPbI₃ thin films. As with many photovoltaics, these interfaces may prove critical to the future design and optimization of high efficiency and reliability MAPbI₃ devices. Accordingly, we demonstrate the first fully-3D photo-generated carrier mapping throughout polycrystalline MAPbI₃ thin films via Tomographic Atomic Force Microscopy (TAFM). These results unambiguously identify for conventionally prepared polycrystalline MAPbI₃ that the grain boundaries act as highly interconnected 2D conductive channels, including photoconductivities up to 4 times greater than the adjacent bulk. The conclusions are also supported by co-located I/V, I/intensity, surface potential, and topographic mapping.

Thermal management in hybrid and all-inorganic halide perovskite devices is expected to be of outstanding importance for the performance and reliability of solar cells, light-emitting diodes, and lasers, because both lifetime and performance are influenced by temperature or temperature gradients. Aside from hybrid halide perovskites, where the A-site cation is organic, the interest in all-inorganic perovskites is continuously increasing, as they provide a higher stability compared to their hybrid analogues. As thermal conductivities, thermal diffusivities, and heat capacities of these perovskites are typically low, their experimental assessment is extremely challenging [1-3].

Here, local thermal transport studies on all-inorganic caesium lead-halide based perovskite thin films, with Cl, Br as halides (X), will be presented. Re-crystallized CsPbX₃ and CsPb₂X₅ layers were realized by the application of a planar hot pressing (PHP), previously developed by us [4,5]. Highly 100 oriented films are confirmed by X-ray diffraction (XRD) and electron backscattered diffraction (EBSD) analysis. Furthermore, we evidence a coexistence of grains consisting of the 3D phase (CsPbX₃) and the 2D phase (CsPb₂X₅) depending on the preparation conditions of pristine layers [6]. Extended quantitative thermal conductivity measurements without the need of elaborate sample preparation are performed applying an advanced 3ω-technique in the frequency domain using our Scanning Near-field Thermal Microscope [3]. We simultaneously map the topography, thermal conductivity (e.g. CsPbBr₃ and CsPb₂Br₅: 0.43 W/mK and 0.33 W/mK, respectively), thermal diffusivity (both 0.3 mm²/s), and the volumetric thermal capacity (1.3 J/(cm³K) and 1.1 J/(cm³K)) with a high spatial resolution of typically 100 nm to understand the thermal properties in dependence of the dimensionality of the perovskite material. For comparison and as a reference, their single crystalline analogues, which are grown by anti-solvent vapour crystallisation, have been studied. In addition to heat transport analyses in dependence of the halide and dimensionality, the influence of temperature-depended phase-transitions (e.g. 3D CsPbBr₃: orthorhombic - tetragonal - cubic) on the thermal properties are analysed the behaviour near the phase-transition is discussed in detail.

Our results demonstrate, that thermal management in all-inorganic halide perovskite devices will be a serious challenge and requires particular attention.

[1] Lee, W. et al., PNAS 2017, 114 (33), 8693.
[2] Kovalsky, A. et al., J. Phys. Chem. C 2017, 121, 3228.
[3] Haeger, T. et al., J. Phys. Chem. Lett. 2019, 10, 3019
[4] Pourdavoud, N. et al., Adv. Mater. 2017, 29, 1605003.
[5] Pourdavoud, N. et al., Adv. Mater. Technol. 2018, 1700253
[6] Pourdavoud, N. et al., Adv. Mater. (submitted).

Organic-inorganic hybrid two-dimensional (2D) perovskites (n≤5) have recently attracted significant attention due to their promising stability and optoelectronic properties. Normally, 2D perovskites contain a mono cation (e.g., methylammonium (MA⁺) or formamidinium (FA⁺)). Here, we report for the first time on fabricating 2D perovskites (n=5) with mixed cations of MA⁺, FA⁺, and cesium (Cs⁺). The use of these triple cations leads to the formation of a smooth, compact surface morphology with larger grain size and fewer grain boundaries compared to the conventional MA-based counterpart. The resulting perovskite also exhibits longer carrier lifetime and higher conductivity in triple-cation 2D perovskite solar cells (PSCs). The power conversion efficiency (PCE) of 2D PSCs with triple cations was enhanced by more than 80% (from 7.80% to 14.23%) compared to PSCs fabricated with a mono cation; the PCE is also higher than that of PSCs based on binary-cation (MA⁺-FA⁺ or MA⁺-Cs⁺) 2D structures.

Single junction perovskite solar cells have now achieved efficiencies exceeding 24% and silicon-perovskite tandem solar cells are outperforming single junction silicon devices. The path to achieve the last remaining efficiency percentiles to reach the Shockley-Queisser limit for single junction devices requires to understand the main limiting processes that hinder device performance.

One of the crucial parameters that control device performance is the photogenerated-carrier separation. Efficient charge separation can be achieved by understanding and controlling the electric potential distribution. In perovskite solar cells, the electric potential profile is controlled by mobile halide vacancies that migrate and accumulate at the contacts in response to an applied potential. The potential distribution is dependent on the vacancy concentration. Therefore, it is crucial to quantify the mobile vacancy concentration to understand the electric potential distribution in a perovskite device.

In this talk, we will first show how to measure and calculate the vacancy concentration and provide concentration values for a range of perovskite compositions and contacts. We will then briefly show how to easily simulate the band diagram of one’s own perovskite solar cell with an online-open-access platform we developed.¹ We will conclude this talk by discussing some important examples showing the role of the illumination side, the mobile ion concentration and recombination rate in controlling the performance of perovskite solar cells.

¹https://lucabertoluzzi.shinyapps.io/band_diagram_PSC/

Metal halide perovskite materials exhibit exceptional performance characteristics for low-cost optoelectronic applications. Though widely considered defect tolerant materials, perovskites still exhibit a sizeable density of deep sub-gap non-radiative trap states, which create local variations in photoluminescence [1] that fundamentally limit device performance. These trap states have also been associated with light-induced halide segregation in mixed halide perovskite compositions [2] and local strain [3], both of which can detrimentally impact device stability [4]. Understanding the nature of these traps will be critical to ultimately eliminate losses and yield devices operating at their theoretical performance limits with optimal stability. We recently used local photoemission electron microscopy (PEEM) to directly visualize the deep trap sites, revealing spatially heterogeneous nanoscale clusters.

In this talk we detail a low-dose, high resolution and multi modal approach to determine 1) the physical origins of these deep trap states 2) their effect on the electronic properties of (Cs_0.05FA_0.78MA_0.17)Pb(I_0.83Br_0.17)₃ thin films and 3) their association with sub-grain crystallographic boundaries. By combining scanning electron and synchrotron X-Ray microscopy techniques with PEEM measurements we reveal that there are distinct structural and compositional fingerprints associated with the generation of these trap sites. In addition, our scanning electron diffraction measurements achieve a spatial resolution of 4nm with an accumulated electron dose of only ~6 e/Ų (over an order of magnitude lower than established tolerable dose limits for metal halide perovskites). We will also explore how this combination of high-resolution and low accumulated dose provides new insights into the pristine crystallography of these materials on the nanoscale; thus helping to answer ongoing open questions such as ‘what truly defines a grain?’ and ‘are grain boundaries beneficial or detrimental to performance’?
References
1. D. W. de Quilettes et al., Impact of microstructure on local carrier lifetime in perovskite solar cells. Science. 348, 683–686 (2015).
2. A. J. Knight et al., Electronic Traps and Phase Segregation in Lead Mixed-Halide Perovskite. ACS Energy Lett. 4, 75–84 (2019).
3. T. W. Jones et al., Lattice strain causes non-radiative losses in halide perovskites. Energy Environ. Sci. 12, 596–606 (2019).
4. J. Zhao et al., Strained hybrid perovskite thin films and their impact on the intrinsic stability of perovskite solar cells. Science Advances. 3, eaao5616 (2017).

Metal-halide perovskites undergo multiple temperature-dependent polymorph transitions, generally with the smallest bandgap phases thermodynamically favored only at elevated temperatures. We present nanoconfinement as a strategy to shift the thermodynamics of polymorph transitions in order to stabilize high-performance metal-halide perovskites against temperature-induced polymorph transitions and humidity-induced degradation. Specifically, when crystal sizes are reduced to the sub-micron length scale, the surface free energy contribution to the total Gibbs free energy of the crystals becomes increasingly important. By exploiting the dependence of the surface free energy on the symmetry of the crystal structure, it is possible to shift polymorph transitions to lower temperatures under nanoconfinement compared to the bulk. For methylammonium lead iodide (MAPbI₃) crystals confined in anodized aluminum oxide (AAO) templates with pore sizes ranging from 20 – 250 nm, the cubic polymorph was found to be stable at temperatures above 200 K, compared to 330 K for the bulk. Similarly, nanoconfined γ-CsPbI₃ formed at 370 K in AAO templates, compared to 448 K in the bulk phase and was stable to temperatures as low at 4 K once formed. Such stability afforded the extraction of phonon energies of CsPbI₃ from temperature-dependent photoluminescence spectra. Furthermore, these nanoconfined crystals exhibit excellent stability against humidity-induced degradation, with no change in their X-ray diffraction patterns over a period of at least two years upon storage in air.

Lewis base solvents, such as dimethylsulfoxide (DMSO), that strongly coordinate with Pb²⁺ are commonly added to hybrid organic-inorganic perovskite (HOIP) precursor solutions to improve thin-film morphology and subsequent photovoltaic (PV) performance. Although nearly all previous studies of precursor solution chemistry have focused on oxygen-donor solvents as Lewis base additives, we expect “softer,” sulfur-donor solvents to coordinate more strongly with Pb²⁺, a borderline-soft Lewis acid. To confirm this hypothesis, we performed extended X-ray absorption fine structure (EXAFS) spectroscopy at the Pb L_III absorption edge to probe the coordination environment of Pb²⁺ in solutions comprising mixtures of S- and O-donor solvents. Of the S- and O-donor solvent pairs examined, the S-donor solvent consistently outcompetes its O-donor structural analog for coordination sites around Pb²⁺. In the specific comparison between N-methyl-2-pyrrolidone thione (NMPT) and N-methylpyrrolidone (NMP), density-functional theory calculations indicate that NMPT coordination with Pb²⁺ is favored by 10 kcal/mol relative to NMP coordination with Pb²⁺. Similar to the addition of DMSO to precursor solutions, the incorporation of NMPT in fractional quantities provides morphological control of the perovskite film. Unlike DMSO, however, we found NMPT to be stable against undesired transmethylation or disproportionation reactions with methylammonium [1]. Consequentially, films processed with NMPT exhibit no sub-gap electronic states near the valence band edge, in contrast with films prepared using DMSO. We thus believe NMPT merits serious consideration as a solution additive for producing stable HOIP precursor solutions and high-quality HOIP thin films.

[1] Hamill, J. C.; Sorli, J. C.; Pelczer, I.; Schwartz, J. and Loo, Y. -L. Acid-catalyzed reactions activate DMSO as a reagent in perovskite precursor inks. Chemistry of Materials 2019, 31, 2114-2120.

Metal-Halide perovskite-based solar cells have shown a remarkable rise in performance in less than a decade with power conversion efficiencies now exceeding 23%. Recent literature provides a constant stream of new recipes and processing techniques, leading to improved power conversion efficiencies although often with limited detailed understanding of the mechanism for performance improvement. In particular, additive engineering and stoichiometric variations are widely studied to optimize the performances of the solar cells.^[1,2] However, the underlying impact of additives and stoichiometry on transport and recombination properties are only little understood. In this study, we take advantage of a newly developed advanced characterization technique using a highly sensitive Hall-effect measurement performed at variable light intensity.^[3,4] This new carrier-resolved photo-Hall technique gives access to both majority and minority carriers properties (n, p, µ_n and µ_p). In addition, carrier recombination lifetime τ and diffusion length L_d can be extracted with this technique using light intensities comparable to 1 sun. Importantly, all parameters can be extracted using the same sample and at the same light intensity level. We use the new technique to explore the impact of excess PbI₂ in MAPbI₃ precursor solution (MA: methylammonium), which has been suggested to improve solar cell efficiency through defect passivation at the grain boundaries.^[5–7] Samples with similar grain sizes are compared to follow the evolution of transport and recombination properties of the perovskite film with and without the addition of excess PbI₂. We show that excess PbI₂ has a negligible impact on carrier density, conductivity and mobility, while it significantly improves the lifetime by one order of magnitude (from ~1 ns to ~20 ns). Combined with SEM images, these results suggest that extra PbI₂ effectively passivates deep traps at the MAPbI₃ grain boundaries. Therefore, this study offers an in-depth analysis of the MAPbI₃ properties using an important new characterization technique and supports the passivation role of extra PbI₂.

[1] E. Aydin, M. Bastiani, S. Wolf, Adv. Mater. 2019, 1900428, 1900428.
[2] Q. Han, Y. Bai, K. Z. Du, T. Li, D. Ji, Y. Zhou, C. Cao, D. Shin, J. Ding, A. D. Franklin, J. T. Glass, J. Hu, M. J. Therien, J. Liu, D. B. Mitzi, Energy Environ. Sci. 2017, 10, 2365.
[3] O. Gunawan, Y. Virgus, K. F. Tai, Appl. Phys. Lett. 2015, 106, 062407.
[4] O. Gunawan, S. R. Pae, D. M. Bishop, Y. S. Lee, N. J. Jeon, J. H. Noh, X. Shao, T. Todorov, B. David, B. Shin, arXiv Prepr. arXiv1802.07910 2018.
[5] Q. Chen, H. Zhou, T. Song, S. Luo, Z. Hong, H. Duan, L. Dou, Y. Liu, Y. Yang, Nano Lett. 2014, 14, 4158.
[6] L. Wang, C. McCleese, A. Kovalsky, Y. Zhao, C. Burda, J. Am. Chem. Soc. 2014, 136, 12205.
[7] T. Zhang, N. Guo, G. Li, X. Qian, Y. Zhao, Nano Energy 2016, 26, 50.

Hybrid perovskites have emerged as excellent semiconductors enabling efficient opto-electronic devices such as solar cells, light emitting diodes, lasers as well as radiation detectors with performances approaching state-of-the-art. Apart from the intrinsic properties of the semiconductor, interfaces are critical to make a superior device. Hence, understanding the nature of the interface between metal contacts and perovskite is essential to further advance device performance, especially in low power detection applications (i.e. single photon detection mode) where a clean interface with minimized dark recombination is required.

Here, we use scanning photocurrent microscopy (SPCM) on lateral methylammonium lead triiodide (MAPbI) thin film device with commonly used high work function metal and low work function metal contacts to investigate perovskite-metal interfaces. By comparing the spatially resolved photocurrent maps of devices with Au (high work-function metal) to Ti (low work-function metal), we find that a Schottky barrier exists in both cases and the barrier is higher for the Ti/perovskite junction resulting in a lower photocurrent. Our results also suggest that the MAPbI thin films used in this study have a surface work function higher than Au indicating the presence of doping or band bending near the surface. From the decay of the photocurrent profile near the metal contacts, we estimate charge carrier diffusion length to be 9 ± 2 µm. Using this knowledge, we successfully demonstrate a single crystal MAPbI gamma ray detector from which sharp gamma-ray induced pulses are observed. This is benefitted from the clean interface built with high work function metal that blocks the dark current and extracts photo-generated carriers. Our study indicates that the interface plays a significant role especially in solid state detector operating at low flux photon counting mode.

Halide perovskites have been extensively studied and show promise for a variety of optoelectronic application, e.g., satellites, energy conversion technologies, medical imaging, and in particular, photocathode materials for x-ray Free Electron Lasers (x-FEL's) and photo-detectors. A thin layer of cesium (Cs) on pure inorganic perovskites, particularly CsPbBr₃ and CsPbI₃, has been shown to significantly improve their photocathode performance [1], primarily because the Cs coating significantly reduces the work function from ~4 eV to between 1.8 - 2.3 eV, depending on surface termination, crystal face, and degree of sub-monolayer coverage. In the present work, we shall: (i) combine findings from Density Functional Theory (DFT) with a phenomenological model developed by Gyftopoulos and Levine to predict the reduction in work function in relation to Cs coverage. (ii) describe the variation in work function Φ as a function of submonolayer coverage θ, (iii) explore the relationship between emitted charge and work function variation using a thermal-field-photoemission model for which the transmission probability is evaluated using a novel approach to finding the Gamow tunneling factor for Schottky barriers, and (iv) demonstrate the performance of the new thermal-field and photoemission methods of evaluating current density compared to exact transfer matrix approaches (TMA) for the evaluation of transmission probability (methods which are applicable to photocathodes as well as thermal-field emitters in general).

[1] "Cesium coated halide perovskites as new photocathode material"; S.G. Lewis, D. Ghosh, K. L. Jensen, D. Finkenstadt, A. Shabaev, S. Lambrakos, F. Liu, W. Nie, L. Zhou, J.J. Crochet, N. Moody, A.D. Mohite, S. Tretiak, and AJ Neukirch (submitted to ACS Chemistry of Materials)