Symposium ET05 : Fundamental Aspects of Halide Perovskite (Opto)electronics and Beyond

Perovskites are the wonder compounds of materials science, with examples of dielectrics, semiconductors, metals, and superconductors. This talk will address the chemical and physical properties that make halide perovskites unique. Following six years of intensive research, there has been a number of breakthroughs in understanding, but many challenges and opportunities remain.

These organic-inorganic semiconductors satisfy the optoelectronic criteria for an active photovoltaic layer, i.e. spectral response in the visible range combined with light electron and hole effective masses. In addition, they are structurally and compositionally flexible with large dielectric constants, and the ability to alloy on each of the lattice sites. To understand the success of methylammonium lead iodide photovoltaics, we have been applying materials theory and simulation across multiple length scales [1-5].

I will discuss issues ranging from disorder associated with molecular rotations and tilting of the inorganic network, to macroscopic polarisation arising from charged defect formation and diffusion. A number of unsolved mysteries will be outlined including self-healing effects, apparent ferroelectricity, light-enhanced ion transport, and ultimately, the origin of their high performance in optoelectronic devices.

This research has been supported by the Royal Society and a wide collaboration network with contributions from current and former group members including Federico Brivio, Keith Butler, Jarvist Frost, Jonathan Skelton, Katrine Svane, Ruoxi Yang, Lucy Whalley, Youngkwang Jung, Jacob Wilson, and Samantha Hood.

1. “Atomistic origins of high-performance in hybrid halide perovskite solar cells” Nano Lett., 14, 2584 (2014)

2. “Dynamics of methylammonium ions in hybrid organic–inorganic perovskite solar cell” Nature Comm. 6, 7124 (2015)

3. "Direct observation of dynamic symmetry breaking above room temperature in methylammonium lead iodide perovskite" ACS Energy Lett. 1, 880 (2016)

4. "Slow cooling of hot polarons in halide perovskite solar cells" ACS Energy Lett. 2, 2647 (2017)

5. "Local strain heterogeneity influences the optoelectronic properties of halide perovskites" arXiv 1803.01192 (2018)

Two-dimensional (2D) metal halide perovskites have become highly promising semiconductors with a high degree of structural flexibility and tunable optoelectronic properties. They have a general formula of (A’)₂(A)_n-1M_nX_3n+1, where A = Cs⁺, CH₃NH₃⁺ (MA), HC(NH₂)₂⁺ (FA), M = Ge²⁺, Sn²⁺, Pb²⁺ and X = Cl^-, Br^-, I^-, are the perovskite components and A’⁺ = RNH₃ is an organic spacer. There are three kinds of 2D organic inorganic hybrid perovskites so far: Ruddelsden-Popper, Cation-ordered and Jacobson-Dion. These kinds vary from one another in ways the inorganic slabs stack and the way the spacer cations interact with the inorganic slabs. Generally, 2D perovskites form from solution via the bottom-up self-assembly of individual, semiconducting perovskite sheets having an adjustable slab thickness of up to few nanometers, separated by insulating bulky organic molecules. As a result, they behave as natural multiple quantum wells (QWs) with the semiconducting perovskite layers representing the wells and the insulating organic spacers representing the barriers. The width of the barrier is fixed and depends only on the length of the A’ cation, while the width of the well can be adjusted by varying the thickness of perovskite slabs, which is defined by the n variable in (A’)₂(A)_n-1M_nX_3n+1. It is critical to understand the thermodynamic and chemical limitations of the maximum layer thickness that can be sandwiched between the organic bilayers while retaining the structural integrity of the 2D perovskite. The so-called “hollow” perovskites are a new form which lies between the conventional 3D and 2D perovskites. While the overall network is 3D there massive vacancies in it that give the materials different properties. The chemical and structural aspects of these materials will be presented and devices made from them will be described.

Halide Perovskites, HaPs, may be mostly normal (inorganic) semiconductors and, indeed, we should be careful to describe them with concepts from organic electronics. HOWEVER, it is remarkable that a material with over-all high quality optoelectronic properties can result from fast, low temperature, solution preparation. This suggests that there may also be issues with using some concepts from “classical” semiconductors. Now we can ask if minor revisions will do or if major ones are needed (rejection is not an option; HaPs exist…).

For that we need to define and understand what remains special about HaPs and what is/are the reason(s) for what, if anything, remains special. Being able to do so may help answer the nagging question if this is all because Pb is so unique, or if we can generalize to find other materials like these. I will consider the general question and, as things look now, 5.5 months before the talk, consider the relevance of mechanical, in conjunction with other properties, to challenge what we think we know about defects in HaPs, note a confusing semantic issue and more, to arrive at an answer, to guide us in future work.

Work done with Gary Hodes (Weizmann), with input from many others, whom I will credit in my talk.

The perovskite crystal structure hosts a wealth of intriguing properties, and the renaissance of interest in halide (and hybrid organic-inorganic) perovskites (HOIPs) has further broadened the palette of exciting physical phenomena. Breakthroughs in HOIP synthesis, characterization, and solar cell design have led to remarkable increases in reported photovoltaic efficiency.

However, the observed long carrier lifetime and PV performance have eluded comprehensive physical justification. The hybrid perovskites serve as an enigmatic crossroads of physics. Concepts from crystalline band theory, molecular physics, liquids, and phase transitions have been applied with some success, but the observations of HOIPs make it clear that none of these conceptual frameworks completely fits. In this talk, recent theoretical progress in understanding HOIPs will be reviewed and integrated with experimental findings. The large amplitude motions of HOIPs will be highlighted, including ionic diffusion, anharmonic phonons, and dynamic incipient order on various length and time scales. The intricate relationships between correlated structural fluctuations, polar order, and excited charge carrier dynamics will also be discussed.

The combination of properties halide perovskites (HaPs) possesses (e.g., high absorption coefficient, low effective mass, low exciton binding energy, low carrier recombination lifetimes, etc.) should (and does) allow high-performing optoelectronic devices. However, there is one fundamental property that does not fit the expected prognosis coming from the superior material’s properties – its carrier mobility.
When comparing mobility values of HaPs (~1-100 cm²V^-1s^-1) with those polar semiconductors, such as GaAs or CdTe (~10³-10⁵ cm²V^-1s^-1),¹ a significant difference is revealed. Mobility temperature dependence, which points on the scattering mechanism, is found in HaPs, GaAs, CdTe and other polar semiconductors to be similar and suggest scattering by polar optical phonon. Low defect density, as found for HaPs (~10¹⁰ cm^-3 in single crystals) as well as for other high-quality polar semiconductors (e.g., GaAs and CdTe), make scattering by defects insignificant (at temperatures > ~100 K). Therefore, the origin of charge scattering and, thus, their mobility, probably relate to more intrinsic properties of these polar semiconductors.
So what makes the mobility in HaP to be so different from that in other polar semiconductors? Correlation of experimentally-derived physical values from semiconductors, including HaPs, reveals a clear correlation between the mechanical properties (or what is called a ‘deformation potential’) and the charge mobility. In fact, the softer the material, the lower its mobility. In my talk, I will explain the origins for such relation, which suggest that in a soft material such as HaPs mobility will never reach values as high as in GaAs.


1. Brenner, T. M., Egger, D. A., Rappe, A. M., Kronik, L., Hodes, G. & Cahen, D. Are Mobilities in Hybrid Organic-Inorganic Halide Perovskites Actually ‘High’? J. Phys. Chem. Lett. 6, 4754–4757 (2015).

The dynamic motion, within the lattice of single crystal CH₃NH₃PbBr₃(001) hybrid halide perovskite, was investigated using powder and single crystal x-ray diffraction, and x-ray photoemission spectroscopy. Single crystal x-ray diffraction studies indicate the methylammonium adopts multiple orientations within the crystal, at room temperature, evidence of a soft and disordered methylammonium sublattice within a stiff and ordered PbBr₃ matrix lattice. Temperature dependent x-ray photoemission spectroscopy for bromine and lead core level peaks near the cubic to tetragonal I phase transition tend to support the characterization of methylammonium as a lattice polar liquid within the CH₃NH₃PbBr₃ crystal. The Br 3d_5/2 core level Debye-Waller factor plots exhibited a temperature dependence indicative of an effective Debye temperature of 160±61K, while the Pb 4f_7/2 core-level Debye-Waller factor plots show little temperature dependence, indicative of a very stiff lattice along the <001> direction. MAPbBr₃ satisfies the criteria for a lattice polar liquid and does not meet the criteria required for a ferroelectric material.

Metal halide perovskite thin films have achieved remarkable performance in optoelectronic devices, but suffer from spatial heterogeneity in their electronic properties. To achieve higher device performance and reliability needed for wide-spread commercial deployment, spatial heterogeneity of optoelectronic properties in the perovskite thin film needs to be understood and controlled. Clear identification of the causes underlying this heterogeneity, most importantly the spatial heterogeneity in charge trapping behavior, has remained elusive. Here, a multimodal imaging approach consisting of photoluminescence, optical transmission, and atomic force microscopy is utilized to separate electronic heterogeneity from morphology variations in perovskite thin films. By comparing highly oriented and randomly oriented polycrystalline perovskite thin film samples, we reveal that disorders in the crystallographic orientation of the grains play a dominant role in determining charge trapping and electronic heterogeneity. This work also demonstrates a polycrystalline thin film with uniform charge trapping behavior by minimizing crystallographic orientation disorder. These results suggest that single crystals may not be required for perovskite thin film based optoelectronic devices to reach their full potential.

Perovskite stability is of the core importance and difficulty in current research and application of perovskite solar cells. Nevertheless, over the past century, the formability and stability of perovskite still relied on simplified factor based on human knowledge, such as the commonly used tolerance factor t. Instead of t, we proposed a new factor (μ+t)^η, where μ and η are the octahedral factor and the atomic packing fraction respectively. As a stability descriptor (μ+t)^η is able to predict the relative stability among any two perovskites with the accuracy ~90%, much better than ~70% of t [1,2].

We further combined machine learning (ML) with first-principles density functional calculations, proposed a strategy to calculate the decomposition energies, considered to be closely related to thermodynamic stability, of 354 kinds halide perovskites, established the machine learning relationship between decomposition energy and compositional ionic radius and investigated the stability of 14190 halide double perovskites. The ML model, which was trained based on the theoretical data, has been validated by experimental results of a series of rare earth metal halide perovskites (up to ~10³ kinds), performs much better than descriptors such as tolerance factor t and (μ+t)^η and further provides elemental and concentration suggestion for improving the stability of mixed perovskite [3].

[1] Qingde Sun and Wan-Jian Yin*, JACS 139, 14905-14908 (2017)
[2] Fazel Shojaei, Wan-Jian Yin*, JPCC (2018).
[3] Zhenzhu Li, Qichen Xu, Qingde Sun, Zhufeng Hou, Wan-Jian Yin*, preprint arXiv:1803.06042.

The development of organic-inorganic lead halide perovskites with very large efficiency requires us to understand the operation of the solar cell. This class of semiconductors presents remarkable bulk electronic and optical properties, but the contacts to the device are a key aspect of the operation and show important dynamic interactions. We describe the results of analysis of kinetic phenomena using frequency modulated techniques. First with impedance spectroscopy we provide an interpretation of capacitances as a function of frequency both in dark and under light, and we discuss the meaning of resistances and how they are primarily related to the operation of contacts in many cases. The capacitance reveals a very large charge accumulation at the electron contact, which has a great impact in the cell measurements, both in photovoltage decays, recombination, and hysteresis. We also shows the identification of the impedance of ionic diffusion by measuring single crystal samples. Working in samples with lateral contacts, we can identify the effect of ionic drift on changes of photoluminescence, by the creation of recombination centers in deffects of the structure. We also address new methods of characterization of the optical response by means of light modulated spectroscopy. The IMPS is able to provide important influence on the measured photocurrent. We describe important insights to the measurement of EQE in frequency modulated conditions, which shows that the quantum efficiency can be variable at very low frequencies.

References
Lopez-Varo, P.; Jiménez-Tejada, J. A.; García-Rosell, M.; Ravishankar, S.; Garcia-Belmonte, G.; Bisquert, J.; Almora, O. Device Physics of Hybrid Perovskite Solar cells: Theory and Experiment, Adv. Energy Mater. 2018, 1702772.
Peng, W.; Aranda, C.; Bakr, O. M.; Garcia-Belmonte, G.; Bisquert, J.; Guerrero, A. Quantification of Ionic Diffusion in Lead Halide Perovskite Single Crystals, ACS Energy Lett. 2018.
Li, C.; Guerrero, A.; Zhong, Y.; Gräser, A.; Luna, C. A. M.; Köhler, J.; Bisquert, J.; Hildner, R.; Hüttner, S. Real-Time Observation of Iodide Ion Migration in Methylammonium Lead Halide Perovskites, Small 2017, 1701711.
Ravishankar, S.; Aranda, C.; Boix, P. P.; Anta, J. A.; Bisquert, J.; Garcia-Belmonte, G. Effects of Frequency Dependence of the External Quantum Efficiency of Perovskite Solar Cells, J. Phys. Chem. Lett. 2018, 3099-3104.

Further breakthroughs in perovskite solar cells require advances in new compositions and underpinning materials science. Indeed, a greater fundamental understanding of perovskite materials requires atomic-scale characterization of their underlying structural, defect and transport behaviour. In this context, combined modelling-experimental work is now a powerful approach for investigating these properties at the atomistic level. This presentation will describe such studies on hybrid perovskites [1-3] in two related areas: (i) the defect and ion transport properties of methylammonium lead iodide (MAPbI₃) in relation to atmospheric effects and degradation; (ii) the molecular cation dynamics and octahedral distortion of halide perovskites with mixed A-cations (MA, FA, Cs) in relation to their improved stability and photovoltaic performance.

[1] N. Aristidou et al., Nature Commun., 8, 15218 (2017); C. Eames et al., Nature Commun., 6, 7497 (2015)
[2] R. Brenes et al., Adv. Mater., 30, 1706208 (2018); R. Brenes et al., Joule, 1, 155 (2017).
[3] D. Ghosh et al, ACS Energy Lett., 2, 2424 (2017)

Perovskite semiconductors are a new class of semiconductors, significantly different from organic and also inorganic semiconductors. We have applied thermally stimulated current measurements to study the polarization processes of the organic cation in MAPbI₃ thin films across the orthorhombic to tetragonal phase transition. The nature of the cation polarization within the orthorhombic phase was found to be highly repeatable, with a separation of 20 K between polarization and depolarization processes, and was investigated with respect to its extrinsic polarizability by external electric fields. Our results show that the polarization of organic cations is correlated with a sudden improvement in solar cell performance and has impact on the working mechanisms of perovskite solar cells.

Methylammonium lead iodide (MAPI) is the archetype material of the class of halide perovskites that are currently in the focus of photovoltaic research because of high conversion efficiencies. MAPI exhibits some anomalies properties such as a huge apparent low frequency dielectric constant and severe hysteretic current-voltage behavior. In order to investigate the key features of its performance, not only electronic, but also ionic transport properties need to be considered [1,2]. Here we report on huge photo effects on ion conductivity in MAPI. To measure and separate both transport contributions not only in the dark, but also under illumination, we carried out a variety of tailored electrochemical studies. In this way, an enhancement of ionic conductivity by as much as two orders of magnitude in MAPI by light illumination can be unambiguously demonstrated [3]. The mechanism of ion conduction under light is proposed to rely on a translation of a great fraction of the generated holes into iodine vacancies which are the ionic charge carriers. Localized holes correspond to neutral iodine which can -owing to site and poly-anionic stabilization- occupy interstitial sites leaving vacant iodine sites that are causing the ionic conductivity. This process is reversible under homogeneous conditions. If, however, iodine is irreversibly removed under illumination, photo-decomposition occurs. This unexpected finding does not only give rise to a decomposition path for metal halide perovskites, it also allows one to tune ion transport by light.

References
[1] T. -Y. Yang, G. Gregori, N. Pellet, M. Grätzel, J. Maier, Angew. Chemie Int. Ed. 54, 7905–7910 (2015).
[2] A. Senocrate, I. Moudrakovski, G. Y. Kim, T. –Y. Yang, G. Gregori, M. Grätzel, J. Maier, Angew. Chemie Int. Ed. 56, 1-6 (2017).
[3] G. Y. Kim, A. Senocrate, T. –Y. Yang, G. Gregori, M. Grätzel, J. Maier, Nature Mater. 17, 445-449 (2018).

Perovsktie solar cells have entered a stage that serious consideration of their feasibility of commercialization with their fast increasing efficiency and stability. Nevertheless, the understanding of these materials has not keep the pace with the efficiency enhancement. In this talk, I will present our recent progress in understanding the materials and physics of polycrystalline perovskite solar cells. The charge recombination in the defective films will be analyzed, and passivation techniques which involves of different types of molecular structures will be presented. Starting from proposing the first passivation concept using fullerene early in 2014, we continue to strive for new passivation molecules to improve the passivation effect in terms of the amount and types of defects that can be passivated. I will also discuss the nature of the defects and passivation in perovskites which is very different from traditional semiconductors such as silicon.

Solar cells based on halide perovskites show efficiencies close to highly-optimized silicon solar cells. However, ions migrating in these perovskites lead to device degradation and complicate the characterization of perovskite solar cells. We recently showed that transient ion-drift is a powerful method to quantify activation energy, concentration, and diffusion coefficient of mobile ions in perovskite solar cells. [1] By studying methylammonium lead triiodide (MAPbI3) we could identify three migrating ion species which we attribute to the migration of iodide (I-) and methylammonium (MA+). We found that both MA+ and I- ions migrate at room temperature, but at very different timescales (seconds and milliseconds respectively). These results suggest that the migration of MA+ ions is the major factor influencing current-voltage hysteresis in perovskite solar cells.

Recently it was shown that introducing potassium into triple-cation perovskites passivates surfaces and stabilizes luminescence without compromising charge transport or extraction. [2] This has been attributed to the mitigation of both non-radiative losses and ion migration in perovskite films. In these triple-cation perovskites with potassium passivation, we find that the activation energy of mobile anions is not influenced by potassium passivation, but that the concentration decreases and the diffusion coefficient increases with increasing potassium passivation. We furthermore find that injected charge carriers influence both the activation energy and the diffusion coefficient for mobile anions. This quantification of mobile ions in triple-cation perovskites will lead to a better understanding of ion migration and the influence of passivating agents on that migration.

References
[1] M. H. Futscher et al. arXiv:1801.08519 (2018)
[2] M. Abdi-Jalebi et al. Nature 555, 497 (2018)

A pertinent investigation of the exceptional photoelectrochemical properties of halide perovskites has to consider the significant ion transport present in these materials.^[1,2] Such transport gives rise to bulk and boundary polarization phenomena during operation, and it is also relevant for the degradation kinetics of halide perovskites materials and related photovoltaic devices.^[3,4] In this contribution we analyze the nature of the ionic conductivity in methylammonium lead iodide, the archetypal hybrid halide perovskite, by means of various electrical, electrochemical and nuclear magnetic techniques.^[5] Under equilibrium conditions, iodine vacancies are unambiguously shown to be the dominant ionic carriers, while electron holes dominate the electronic conductivity at high (and excess electrons at low) iodine activities. The contributions of methylammonium and lead ions, instead, are small (upper limits are given). As a follow-up, we discuss the changes of the various charge carrier concentrations as a function of the decisive control parameters (stoichiometry and doping content).^[6] Based on these equilibrium considerations, we can also discuss the charge carrier chemistry under illumination where not only electronic, but also ionic conductivity are largely enhanced.^[7]


References
[1] Z. Xiao et al., Nat. Mater. 2015, 14, 193.
[2] T.-Y. Yang et al., Angew. Chemie 2015, 54, 7905.
[3] Y. Cheng et al., J. Mater. Chem. A 2016, 4, 12748.
[4] J. Carrillo et al., Adv. Energy Mater. 2016, 1502246.
[5] A. Senocrate et al., Angew. Chemie 2017, 56, 7755.
[6] A. Senocrate et al., Solid State Ion. 2018, 321, 69.
[7] G. Y. Kim et al., Nature Mater. 2018, 17, 445.

Organic-inorganic hybrid perovskite solar cells (PSCs) are promising low-cost photovoltaic technology owing to the high energy conversion efficiency. However, the device stability has a large gap to the ideal level for future application.
Here I would like to introduce our recent approaches in achieving highly stable PSCs. We proposed a strategy to control the diffusion of ions/charges/molecules by developing nano-carbon electron transporting layer in p-i-n structure perovskite solar cells. It successfully enabled better stability because the diffusion of iodide within the device was hindered before it induced corrosion of the metal electrode while the diffusion of electrons was improved. We further control the ions/charges/molecules diffusion within n-i-p structure perovskite solar cells. The ions diffusion was reduced to decrease the charge trap states when the device was aging. This helped to obtain device with excellent stability and high efficiency. The device performance was certified by a public test center with the record of certified stabilized power output.

[1] E. B. Bi; H. Chen; F. X. Xie; Y. Z. Wu; W. Chen; Y. J. Su; A. Islam; M. Gratzel*; X. D. Yang*; L. Y. Han*. Nature Communications 2017, 8: 7.
[2] Y. B. Wang; Y. F. Yue; X. D. Yang*; L. Y. Han, Advanced Energy Materials 2018, Doi.org/10.1002/aenm.201800249.
[3] Y. B. Wang, T. H. Wu, W. T. Tang, E. B. Bi, H. Chen, X. D. Yang*, L. Y. Han*, 2018, submitted.

Organic-inorganic halide ABX₃ (A: methylammonium—MA, formamidinium—FA, Cs; B: Pb, Sn; X: I, Br) perovskites currently serve as absorber materials in highly efficient solar cells. Preparation of films which are highly crystalline; defect free; stable against heat, light, and moisture; and with desired optoelectronic properties are present challenges. Cationic and anionic alloying / doping has been shown to improve phase stability, increase grain size and uniformity, and reduce sub-bandgap absorption and recombination due to defects to improve solar cell performance. A combination of photothermal deflection spectroscopy, spectroscopic ellipsometry, and unpolarized transmittance measurements of solution processed perovskite thin films prepared with mixed cation/anion composition are used to study the impact of these variations on sub-bandgap absorption due to defects. In particular, Urbach energies from sub-band gap absorption are correlated with structural, electrical, above band gap optical properties, and device performance. As an example of A-cation substitution, replacing 0.4 FA with MA in x = 0.15 FASn_0.6Pb_0.4I_1-xBr_x reduces Urbach energies from 53 to 26 meV and is accompanied by increased grain size, reduced defect density, and improved electronic properties. For anion substitution, I to Br ratios in FA_0.6MA_0.4Sn_0.6Pb_0.4I_1-xBr_x (x ≤ 0.15) show materials prepared with x ≤ 0.04 having the lower Urbach energy than higher Br content. In FA_0.8Cs_0.2PbI_1-xBr_x, Urbach energies are reduced from lead thiocyanate (Pb(SCN)₂) treatment and solvent annealing preparation. Links will be established between the particular cation/anion composition configurations currently used in solar cell device grade perovskite films, structural and electrical properties, and the associated sub-bandgap absorption characteristics. Further expansion of these studies will help to more fundamentally understand the defect states present in particular compositions and to identify practical pathways to improvements in stability and electronic quality for experimentally produced perovskite thin films used as solar cell absorbers.

Operational lifetime is one of the main challenges for perovskite solar cells towards commercialization [1]. Other than the stability of perovskite materials themselves [2], the operational lifetime issue may also come from several other factors, including the interface with perovskite and the electrodes being used [3-5]. Interface engineering is expected to not only improve the performance, but also to increase the operational lifetime of the devices. In this work, we modify electron transport layer (ETL) surface with an interfacial layer to prevent interaction between ETL and CH₃NH₃PbI₃ [6]. This strategy indeed leads to significantly improved operational lifetime of our devices. We show that it is necessary to consider not only the band alignment at the interface, but also interface chemical interactions between the thin interface layer and the perovskite film.

[1] L. Qiu, L.K. Ono, Y.B. Qi*, Mater. Today Energy 7 (2018) 169.
[2] S. Wang, Y. Jiang, E.J. Juarez-Perez, L.K. Ono, Y.B. Qi*, Nat. Energy 2 (2016) 16195.
[3] Z. Hawash, L.K. Ono, S.R. Raga, M.V. Lee, Y.B. Qi*, Chem. Mater. 27 (2015) 562.
[4] L.K. Ono, S.R. Raga, M. Remeika, A.J. Winchester, A. Gabe, Y.B. Qi*, J. Mater. Chem. A 3 (2015) 15451.
[5] Y. Kato, L.K. Ono, M.V. Lee, S. Wang, S.R. Raga, Y.B. Qi*, Adv. Mater. Interfaces 2 (2015) 1500195.
[6] L. Qiu, L.K. Ono, Y. Jiang, M.R. Leyden, S.R. Raga, S. Wang, Y.B. Qi*, J. Phys. Chem B 122 (2018) 511.

Hybrid perovskite thin films have many vacancies at surface and interface during the film formation, which degrade the stability and photovoltaic performance. Passivation via post-treatment can enhance the film quality, but present methods are slightly destructive to three-dimensional perovskite (3DP) due to the solvent effect, which hinders fabrication reproducibility. Herein, we demonstrate that 4-fluoroaniline (FAL) is an effective antisolvent candidate for surface/interface passivation and thus nondestructive during the fabrication. Density functional theory (DFT) calculation reveals that the antisolvent and non-destructive properties are attributed to the conjugated amine in aromatic ring. Hot vapor assisted colloidal process (HVACP) is employed for the post-treatment. The molecular passivation yields an ultrathin protection layer with hydrophobic fluorine tail and thus enhances the stability and optoelectronic properties. FAL perovskite solar cell (PSC) delivers 20.48% power conversion efficiency (PCE) in the ambient condition. Micro-photoluminescence reveals that passivation activates dark defective state at the surface and interface, delivering the impact picture of boundary on the local carriers. Different from previous passivation reagents, FAL is suitable for post-treating various perovskites and healing the defects via a simple process, without destructiveness to pristine perovskite. Therefore, this work demonstrates a generic nondestructive chemical approach for improving the performance and stability of PSC.

Perovskites have recently attained great attention owing to its excellent electronic and optical properties. The optoelectronic properties can be further tailored according to the needs of applications. It is demonstrated that perovskites photovoltaic devices with a theoretical energy conversion efficiency limit of 31% can be realized if large grain size with uniform intra-grain and inter-grain crystallinity are ensured [1-2]. However, achieving a good crystallinity along with larger grain using the low-temperature solution processed organic-inorganic halide polycrystalline perovskites (MAPBI₃) remains a challenging task. Herein, a uniform and large grain size of max approx. 3 µm is demonstrated for two steps solution process of MAPbI₃ perovskites at 100°C using a simple annealing method. The annealing time duration varies from 1 hour to 24 hours long, while the grain size gradually increases with the prolongation of the annealing time. However, the grain size remains almost constant when the annealing time longer than 12 hours. Furthermore, a distinctly larger grain size of max. about 6 µm is also exhibited for the same perovskite film deposited on a PbI₂ seed layer prepared by chemical vapor deposition method. An optical measurement is being carried on with some initial promising results to characterize the photocurrent loss and recombination center on the grain boundaries, intra-grain, and inter-grain scale of perovskites using femtosecond laser of confocal microscope. The investigations on such microcrystalline perovskites with large grains are highly essential for conducting electronic and optical measurements to enhance the performance of perovskite devices.

Acknowledgement: This work was financially supported by a grant from Shenzhen Municipal Science and Technology projects (Grant No. 201605313001154) and The Hong Kong PhD Fellowship Scheme supported by the Hong Kong RGC.

Reference:
[1] Sibel Y. Leblebici et al., Facet-dependent photovoltaic efficiency variations in single grains of hybrid halide perovskite, Nature Energy 1, 16093 (2016).
[2] DOE/Lawrence Berkeley National Laboratory. (2016, July 4). Discovery could dramatically boost efficiency of perovskite solar cells: Nanoscale images yield surprise that could push efficiency to 31 percent. ScienceDaily. Retrieved May 16, 2018 from www.sciencedaily.com/releases/2016/07/160704145730.htm

Conventionally, in preparing stable colloidal quantum dots (QDs), long hydrocarbon chains capping ligands such as n-octylamine (OA) are indispensable. However, the commonly used long hydrocarbon capping ligands are always insulator and block efficient carrier transport between QDs, leading to inferior performance of light-emitting diode (LED), and other optoelectronic devices. Until now, ligand exchange, reducing ligand chain length and ligand density control are adopted to increase the film conductivity of QDs, but still didn’t break the dilemma of the trade-off between the film conductivity and colloidal stability of QDs. In this work, for the first time, we overcame this dilemma, successfully synthesized methylamine lead bromide (MAPbBr₃) QDs with an unsaturated conjugated alkyl-amine, 3-Phenyl-2-propen-1-amine (PPA), as ligand. Owing to the denser electron cloud overlapping and delocalization effect of conjugated molecules, PPA effectively improves the conductivity of QDs film without compromising its colloidal stability. With analogous quantum photoluminescence yield and stability of OA-capped MAPbBr₃ QDs, films of PPA-capped MAPbBr₃ QDs show high electronic conductivity, which carrier mobility is nearly increased 22 times of that of OA-capped MAPbBr₃ QDs films. As an example of application in LEDs, the QD-LED based on PPA-capped MAPbBr₃ QDs exhibited a maximum luminance of 9052 cd m^-2 and a maximum current efficiency of 9.08 cd A^-1, which is 8 times of that of QD-LED based on OA-capped MAPbBr3 QDs (1.14 cd A^-1). This work provides critical solution for the poor conductivity of QDs in applications of energy-related devices
DOI：10.1002/anie.201801780

Multicomponent quasi-two-dimensional perovskites (Q-2DPs) have efficient luminescence and improved stability, which are highly desirable for light emitting diode (LED) and perovskite solar cell (PSC). However, the lack of radiative recombination at room temperature is still not well understood and the performance of PSC is not good enough as well. The open-circuit voltage (V_oc) is even lower than that of 3D PSC with narrower band gap. In this work, we study the energy transfer of excitons between their multiple components by time-resolved photoluminescence (TRPL) and find that charge transfer from high energy states to low energy state is greatly suppressed at elevated temperature due to increasing trap-mediated recombination. This may reveal the bottleneck of luminescence at room temperature in Q-2DPs, leading to large photovoltage loss in PSC. Therefore, we develop a p-i-n bulk heterojunction (BHJ) structure to reduce the nonradiative recombination. We obtain high V_oc of 1.18 V for (PMA)₂MA_N-1Pb_NI_3NCl (N = 5) in PSC, much higher than the planar counterparts. The enhanced efficiency is attributed to the improved exciton dissociation via BHJ interface. Our results provide an important step towards high V_oc and stable 2D PSCs, which could be used for tandem solar cell and colorful photovoltaic windows.

Hybrid organic-inorganic perovskite material has emerged as one of the most promising semiconducting materials for optoelectronic applications. The power conversion efficiency (PCE) of the three-dimensional (3D) perovskite based solar cell has achieved 22%. With a higher flexibility in structural engineering, the two-dimensional (2D) perovskite not only allows for property tuning in a broader sense, but also demonstrates higher stability in devices compared to the 3D perovskite. Here, we present the first complete series of the Dion-Jacobson phases in the halide perovskite family, incorporating the 3-(aminomethyl)piperidinium (3AMP) or 4-(aminomethyl)piperidinium (4AMP) as spacing cations. The general formula for the DJ perovskite is A′A_n–1Pb_nI_3n+1 (A′ = 3AMP or 4AMP, A = methylammonium (MA)). Compared with the Ruddlesden−Popper (RP) phases, the DJ perovskite only has one sheet of organic cations in between the layers, resulting in much closer interlayer distance (~4Å). The inorganic layers in the DJ phases are stacking perfectly on top of each other with no displacement. With a slight modification on the organic cation (3AMP vs. 4AMP), the optical properties are heavily influenced by the distortion of the inorganic layers, as the 3AMP series (less distorted) has narrower band gaps than the 4AMP series (more distorted). We further demonstrate their difference in solar cell devices, as the (3AMP)(MA)₃Pb₄I₁₃ has the best PCE of 7.3%, much higher than the corresponding (4AMP)(MA)₃Pb₄I₁₃. With compositional engineering on the existing system, we optimize the device performance of (3AMP)(MA_0.75FA_0.25)₃Pb₄I₁₃ (FA = formamidinium) to 12.0%. The new DJ system highlights the crucial role of functional organic cations in the 2D hybrid perovskite, where they influence the overall property of the material by interactions with the inorganic framework, which ultimately affect the device performance.

Recent years have witnessed rapid development of halide perovskite as a new class of optical-gain media for lasing applications. Driven by the rapidly increased research on organic-inorganic perovskite CH₃NH₃PbX₃ (X = Cl, Br, I) (Science, 356, 1376 (2017)), inorganic perovskite CsPbX₃ has also attracted high attention because it shows increased air-stability (Nat. Commun. 8, 15640 (2017), Nat. Photon. 11, 108 (2017)). For achieving amplification spontaneous emission (ASE) with low threshold from perovskite thin films, ultra-compact grains and smooth morphology are prerequisites. Meanwhile, the prepared perovskite thin films have good photo and environmental stability under ambient conditions is also paramount. However, the solution-processed thin films generally difficult to control perovskite crystallization and film quality due to low solubility of the cesium bromide (CsBr) precursor, result in inevitable large pinholes and poor surface coverage(Nat. Commun. 6, 8056 (2015)). Such defects may result in reduced optical confinement effect and poor ASE from the perovskite films, seriously affecting their lasing performance.
To solve these issues, herein, we demonstrate that by dual source thermal evaporation which enables the attainment of nearly pinhole-free thin films, inorganic perovskite (such as CsPbX₃ or CsSnX₃) films exhibit enhanced crystallization, improved photoluminescence (PL) uniformity and intensity, and long-term reliability. The ASE with improved emission intensity and reduced threshold from evaporated thin film were demonstrated in our previous works (J. Phys. Chem. C, 28, 121(2017); PSS RRL, 5, 12, (2018)). Notably, the unannealed CsPbBr₃ thin films fabricated by thermal evaporation exhibit ultralow ASE threshold of ~ 3.3 μJ/cm², enhanced crystallization, improved surface morphology and gain coefficient above 300 cm^-1. Stable ASE intensity without degradation for at least 7 hours is observed under continuous excitation under ambient conditions. Meanwhile, a Fabry-Perot (F-P) cavity laser based on unannealed CsPbBr₃ thin film, featuring ultralow threshold and directional output is also realized. Our works advocate that the perovskite thin films possess excellent film morphology and excellent long-term stability which prove to be critical to enhance ASE and lasing performance, as well as highlights the feasibility of evaporated CsPbBr₃ thin films as practical optical gain media for the further light-emitting applications.

Organohalide lead hybrid perovskites (HPs) have become the benchmark, state-of-the-art materials in third generation, perovskite solar cell devices, achieving a power conversion efficiency of over 22%. Yet, the underlying photo-physical properties of HPs are still under debate. Here we use density functional theory within the generalized gradient approximation with a Hubbard correction (GGA+U) to study structural properties, band structures, charge carrier dynamics and electron-phonon coupling in HPs with different compositions. Our preliminary DFT+U simulations reveal the formation of light-induced self-trapped hole polarons in HPs with different halides, which may have profound implications on charge transport, recombination, and experimentally observed device instability under illumination. Moreover, we argue that polaron induced loss of inversion symmetry and enhanced Rashba splitting might be responsible for our recent experimentally observed room-temperature ultrafast photocurrent and free-space terahertz emission generation from unbiased CH3NH3PbI3 benchmark HPs. Polarization dependence of the observed photoresponse is consistent with the Bulk Photovoltaic Effect caused by a combination of injection and shift currents. Ballistic by nature, these photocurrents may enable next generation perovskite solar cells with efficiency that can theoretically exceed the Shockley–Queisser limit. We also developed a computational method that allows estimating the polaron size, while minimizing self-interaction errors, as well as the overall computational requirement of each calculation.

Surface functionalization of nanoscale materials has significant impacts on their properties due to their large surface-to-volume ratio. In this work, we studied temperature dependent crystal phase transitions in CH₃NH₃PbBr₃ Perovskite quantum dots (PQDs) ligated with octylaminebromide (P-OABr) and 3-aminopropyl triethoxysilane (P-APTES), using a framework of static and dynamic spectroscopy. P-OABr undergoes the expected structural phase transition from tetragonal to orthorhombic phase at ~ 140 K, established by the emergence of a higher energy band at 2.64 eV in the photoluminescence (PL) spectrum, while no phase transition was observed in the case of P-APTES. Such phase stabilization is a result of variation in their respective surface energies, an important contributing factor to the Gibbs free energy for nanomaterials. On further investigating the consequences of this altered crystal phase diagram using time-resolved PL, excitation power dependent PL and Raman microscopy over a range of 300 – 20 K, we observe significant differences in recombination rates and charge carrier types between P-APTES and P-OABr. Our findings highlight how aspects of PQD phase stabilization are linked to nanoscale morphology and the surface energy manipulation of the crystal phase diagram, providing critical insights into the synthesis of stable perovskite crystals for solar energy conversion and other applications such as light emitting diodes.

This work was supported by NASA MIRO grant No. NNX15AQ01A

(1) Sarang, S.; Bonabi Naghadeh, S.; Luo, B.; Kumar, P.; Betady, E.; Tung, V.; Scheibner, M.; Zhang, J. Z.; Ghosh, S. Stabilization of the Cubic Crystalline Phase in Organometal Halide Perovskite Quantum Dots via Surface Energy Manipulation. J. Phys. Chem. Lett. 2017, 8 (21), 5378–5384.

The opportunity of lasing from organolead halide perovskite materials has recently attracted extensive attention in order to realize electrically driven lasers. So far, for devices with planar structure, most reports focus on CH₃NH₃PbI₃(MAPbI₃) films, which are unstable when in operation due to phase transitions and elemental redistribution. Herein, we demonstrate highly stable amplified spontaneous emission (ASE) with ultralow threshold from formamidinium-based perovskite CH(NH₂)₂PbI₃(FAPbI₃) films. ASE from MABr-stabilized FAPbI₃ films was also achieved, with an ultralow threshold of about 1.6 μJ/cm². More importantly, upon continuous operation under pulsed laser for several hours, the ASE intensity in the MAPbI₃ film decreased to 9% of the initial value, while it was maintained above 90% in the FAPbI₃ film. The low trap density, smooth film morphology, high thermal stability, and the excitonic emission in nature of the FAPbI₃ film are expected to contribute to its low lasing threshold and high stability, demonstrating a strong potential for applications in continuouswave pumped lasers and electrically driven lasers.

Quasi two-dimensional (2D) layered organic-inorganic perovskite materials (e.g. (BA)₂(MA)_n-1Pb_nI_3n+1; BA = butylamine; MA = methylamine), have recently attracted a wide attention due to their superior moisture stability as compared with three-dimensional counterparts. Inevitably, hydrophobic yet insulating long-chained organic cations improve the stability at the cost of hindering charge transport, leading to the unsatisfied performance of subsequently fabricated devices. Here, we reported the synthesis of Quasi-2D (iBA)₂(MA)_n-1Pb_nI_3n+1 perovskites, where the relatively pure phase (iBA)₂PbI₄ and (iBA)₂MA₃Pb₄I₁₃ films can be obtained. Because of the shorter branched-chain of iBA as compared with that of its linear equivalent (n-butylamine, BA), the resulting (iBA)₂(MA)_n-1Pb_nI_3n+1 perovskites exhibit much enhanced photodetection properties without sacrificing their excellent stability. Through hot-casting, the optimized (iBA)₂(MA)_n-1Pb_nI_3n+1 perovskite films with n=4 give the significantly improved crystallinity, demonstrating the high responsivity of 117.09 mA/W, large on-off ratio of 4.0×10² and fast response speed (rise and decay time of 16 ms and 15 ms, respectively). These figure-of-merits are comparable or even better than those of state-of-the-art Quasi-2D perovskites-based photodetectors reported to date. Our work not only paves a practical way for future perovskite photodetector fabrication via modulation of their intrinsic material properties, but also provides a direction for further performance enhancement of other perovskite optoelectronics.

Owing to the wide absorption range and high thermal stability, formamidinium (FA) based lead iodide perovskites have recently emerged as the most promising light-absorber materials for photovoltaics. However, they suffer from fast degradation to undesirable non-perovskite polymorphs in the ambient atmosphere, which retards the real-world application of FA-based PSCs. Herein, a new strategy of in situ grain encapsulation is demonstrated to address this issue. This strategy is realized by co-addition of tetraethylorthosilicate (TEOS) and H₂O into the perovskite precursor solution. The hydrolysis of TEOS produces silica oligomers in the precursor solution. Driven by the perovskite crystallization, silica fills in the grain boundaries and covers the surfaces of perovskite grains, forming encapsulating layers at the “grain”-scale. Silica protects perovskite grains from the ambient air, thus improving the stability. Furthermore, SiO₂ passivates defects and reduces the recombination, enhancing the photovoltaic performance. Using the in situ grain encapsulation strategy, FA-based PSCs can exhibit a power conversion efficiency of 19.5% and keep stable for 1000 h in the ambient atmosphere.

Zinc oxide (ZnO) substrates of varying morphologies, including single crystalline (SC), micro-structured (MS) and nanostructured (NS) substrates, are interfaced with pure (CH₃NH₃PbI₃)and mixed (CH₃NH₃PbI_3-xCl_x)halide hybrid perovskite thin films. The perovskite/ZnO interfaces are characterized by means of electron microscopy correlated with charge transfer properties that are probed by temperature, power and time-resolved photoluminescence (PL) spectroscopy. SC-ZnO acts as an effective electron extraction layer as evidenced by PL quenching, reduced exciton density and recombination lifetime in the perovskite thin film. On the other hand, MS-ZnO is observed to result in a mild increase of the PL intensity of the perovskite film at room temperature, and NS-ZnO further results in PL intensity enhancement by up to a factor of thirty thousand and increase of recombination rates by 50%. These trends vary with temperature, and our results demonstrate the critical role played by morphology of the underlying substrates in charge dissociation and extraction in perovskite thin films. We conclude that while SC-ZnO can be implemented as an electron extraction layer in photovoltaic devices, MS- and NS- ZnO can be incorporated as scaffold in optical devices that require high quantum yield.
This work was supported by NASA MIRO grant No. NNX15AQ01A.

Halide perovskites have remarkable optoelectronic properties which enabled their success as solar cells, with device efficiencies quickly rising above 22%. Perovskites have the formula AMX₃, where A is a large cation occupying voids between a corner-connected framework of MX₆ octahedra. Much of this work is based on the organic-inorganic CH₃NH₃PbI₃ and HC(NH₂)₂PbI₃, however these hybrid perovskites are limited by stability issues which have led researchers to pursue all-inorganic compositions. The tolerance factor of the 3D structure limits the number of such compounds, prompting the extension of the perovskite structure to other main group metals such as Bi³⁺ and Sb³⁺ which retain the ns² lone pair that plays an important role in the halide perovskite electronic structure.

One structure family that maintains a corner-connected MX₆ octahedral framework with trivalent M³⁺ are the defect perovskites A₃M₂X₉. We have been investigating the optoelectronic properties of the iodide defect perovskites A₃M₂I₉ (A = Cs, Rb; M = Sb, Bi), finding that these four compounds have strong electron-phonon coupling that results in self-trapped exciton photoluminescence (PL). They also show promise as semiconductor radiation detectors, with each responding to alpha particle irradiation. However, the A₃M₂I₉ are not entirely isostructural. The archetypical structure is a trigonal 2D bilayer that can be viewed as a (111) slicing of the AMX₃ structure, caused by an ordered vacancy on every third M site. In contrast, Cs₃Bi₂I₉ has a 0D dimer structure of isolated Bi₂I₉ bioctahedra, while Cs₃Sb₂I₉ can form either of these structures depending on preparation method. Even the 2D Rb₃M₂I₉ compounds have distorted octahedra which lower the symmetry to monoclinic. To evaluate the size requirements that govern structure in this family, we modified these 2D and 0D structures using mixed halides.

We found that addition of Cl in a 1:2 ratio with I induced the trigonal 2D structure from the 0D Cs₃Bi₂I₉, resulting in the novel 2D defect perovskite Cs₃Bi₂I₆Cl₃ with I atoms on the terminal sites while the Cl atoms take the bridging sites that bind the octahedral bilayers. Cs₃Bi₂I₆Cl₃ shows a bandgap of 2.04 eV which is remarkably similar to 2.06 eV bandgap of the 0D parent compound Cs₃Bi₂I₉. Beyond inducing the 0D to 2D transformation, the addition of Cl also converts the monoclinic Rb₃Bi₂I₉ to the higher-symmetry trigonal 2D structure, forming the new compound Rb₃Bi₂I₆Cl₃ with a band gap of 2.01 eV. Furthermore, the PL of Cs₃Bi₂I₆Cl₃ is nearly identical to that of Cs₃Sb₂I₉, showing that the self-trapped exciton emission is retained even though the inter-octahedral connections consist of Bi-Cl bonding. This remarkable finding imparts a greater flexibility to the defect perovskites, as similar optoelectronic properties can be obtained in higher-dimensional structures. These results will guide tuning of these compounds towards functional inorganic perovskites.

In the colloidal synthesis of inorganic perovskite materials, cesium oleate (CsOL) is the most commonly used Cs precursor. This precursor is synthesized by combining oleic acid with either cesium carbonate (Cs₂CO₃) or cesium acetate (CsOAc), and the long alkyl chain of CsOL is expected to impart solubility in the non-polar solvents used in synthesis, similar to Pb(oleate)₂. However, despite its ubiquitous use in literature, CsOL has been observed to be insoluble at room temperature and leads to surprisingly inconsistent results in perovskite nanocrystal synthesis depending on the Cs salt from which the precursor is derived. We show that under the conditions used in most reports, the amount of oleic acid (OA) added, while stoichiometrically sufficient, still leads to incomplete conversion of the Cs salts to CsOL. This results in a mixture of Cs sources being present during the reaction, causing decreased homogeneity and reproducibility. When a proper Cs:OA ratio is used, complete conversion is readily obtained even under mild conditions, resulting in a precursor solution that is soluble at room temperature and yields identical synthetic results regardless of the initial Cs source. Further, ¹H NMR of solutions prepared using varying Cs:OA ratios confirms that the maximum ratio of Cs:OA obtainable in solution, with any excess Cs present in the precipitate. The super-stoichiometric ratio observed is attributed to the monovalent nature of the Cs cation which leads to a permanent dipole moment in Cs oleate, reducing solubility compared to commonly used divalent oleate complexes such as Pb(oleate)₂. Dynamic light scattering reveals that the addition of Cs yields a reverse micelle-like structure with a diameter of 3.1 nm, consistent with the excess oleic acid complexing with the CsOL in the nonpolar solvent. Careful control of the ligand ratio yields a fully soluble precursor, which is shown to enable facile, reproducible, and scalable synthesis via the slow addition of precursors, with the notable ability to control particle morphology through injection rate.

Recently, organometal halide perovskites as promising candidates for light-emitting diodes have been studied greatly. However, for light-emitting application, the low photoluminescence quantum yield (PLQY) of OHPs film is critical to hinder the efficiency improvement of OHP film based light emitting diodes (PeLEDs). Although OHPs exhibit high photoluminescence quantum yield (PLQY>90%) in solution for nanocrystals or quantum dots, PLQY of OHPs is rather low in film state, which is a fatal limit for the high performance of PeLEDs.
To improve the PLQY of perovskite film, low dimensional perovskites were proposed to achieve high PLQY perovskite film. Nevertheless, the long-chain alkyl halides are indispensable to form quasi-2D structure, in which the insulative ligands will restrict the conduction of perovskite films and hamper the movement of carriers. In addition, partial substitution of A-site (ABX3) cations is an effective method to stabilize the crystal lattice and improve the PLQY. It is reported that Cs introduction to perovskite leads to improved PLQY and better stability of device, for example, device efficiency increased form 2.76cd/A to 10.09cd/A. While CsBr exhibits low solubility in solvent which leads to the difficulty in preparation for Cs-doping provskite film, to pursue other better suitable A-site substitute cations is desirable for the higher performance of PeLEDs.
Owing to RbBr exhibits higher solubility than CsBr in solvent, we select RbBr as dopant to incorporate into FAPbBr3 perovskite film. The optimal PeLEDs with 7% Rb incorporation exhibit a maximum reported luminance of 65353cd/m2, EQE of 7.17% and a highest efficiency of 24.22cd/A, the maximum luminance and current efficiency is enhanced by ~10 fold and ~5 fold compared to the controlled device, respectively. The enhanced device performance can be attributed to Rb ion replaces the site of FA+ partially, and distorts the crystal structure of FAPbBr3, thus affects the photoelectronic properties of FAPbBr3, in turn, improves the PLQY of perovskite films and decreases trap density of films. In addition, a little bit of Rb incorporation will not restrict the conduction of perovskite films. The Rb incorporation provides a new way to tune optical spectra, increase PLQY of perovskite film, and enhance the performance and stability of device.

DOI: 10.1021/acsami.8b00079

Low-dimensional halide perovskites have recently attracted intense interest as alternatives to the three-dimensional (3D) perovskites because of their greater tunability and higher environmental stability. Herein, we present the new homologous two-dimensional (2D) 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. Multilayer (n >1) 2D perovskites incorporating diammonium cations were successfully synthesized by solid state grinding method for m = 4 and 6, and by solution method for m=7 - 9. Structural characterization by single-crystal X-ray diffraction for the m = 8 and m = 9 series (n=1 - 4) reveals that these compounds adopt the space group of Cc for even n members and Pc for odd n members. The optical bandgaps are 2.15 eV for two-layer (n=2), 2.01 eV for three-layer (n=3) and 1.90 eV for four-layer (n=4), all behaving as direct bandgap semiconductor, also confirmed by DFT calculations. The materials exhibit excellent solution processability and casting the materials of n=3 into thin-films was successfully accomplished. The films show a clear tendency for the higher-m members to have preferred orientation on the substrate, with m = 8 exhibiting almost perfect vertical layer orientation and m = 9 displaying both vertical and parallel layer orientation, as confirmed by GIWAXS measurements. The vertical layer orientation for the (NH₃C₈H₁₆NH₃)(CH₃NH₃)₂Pb₃I₁₀ member results in the best thermal, light and air stability within this series, thus showing excellent potential for solar cell applications.

Sensitized triplet-triplet annihilation upconversion (TTA-UC) is a non-linear process where delayed fluorescence is generated through multiple diffusion-controlled triplet-triplet energy transfer steps.^1,2,3 TTA-UC can be applied in photovoltaics, photocatalysis and biological imaging. Here we sensitize organic triplet excited states using perovskite nanocrystals, which are modified by molecular ligands. Subsequently triplet energy is transferred to free organic molecules, where the TTA-UC occurs.⁴ In comparison to the sensitization with phosphorescent molecules, this system stands out due to low energy loss as well as strong and broad absorption band. We use ultrafast transient absorption and fluorescence spectroscopies to study the nature and the time scales of the energy transfer steps. Through magnetic field effect, we reveal how the Zeeman splitting of TT pairs influences the TTA in this system. This work provides a fundamental understanding of the hybrid system between perovskite nanocrystals and triplet organic molecules and inspires its application in optoelectronic devices.

References.
1. Phys. Rev. Lett., 2006, 97, 143903.
2. Coord. Chem. Rev., 2010, 254, 2560–2573.
3. J. Am. Chem. Soc., 2013, 135 (51), pp 19056–19059
4. Chem. Commun., 2017, 53, 8261—8264.

We have investigated the temperature-dependence of light-induced electron spin resonance (LESR) and photocurrents in perovskite solar cells. The perovskite layer was (FAPbI₃)_0.85(MAPbBr₃)_0.15. PCBM and doped-PTAA (poly-triarylamine) layer was used as an electron transpot layer (ETL) and hole transport layer (HTL), respectively.
The temperature-dependence of LESR suggested the existence of an interfacial thermal barrier and charge accumulation between the perovskite and the HTL. The temperature-dependence of the photocurrents reflected that most of the photocurrents are temperature-independent and ESR silent. However, part of the currents expericence thermal barrier, passing through the interfacial states, and also are detected by ESR.

Hybrid organic-inorganic halide perovskite solar cells attract enormous attentions due to their excellent photovoltaic properties which can compete with that of the crystalline silicon solar cells. However, there are some issues in perovskite solar cell that need to be fully understood and solved before its commercialization. These include the operation mechanism, device stability, environment impact, and so on. In this study, charge extraction characteristics at the perovskite/TiO₂ interface is studied by interface engineering. So that self-assembled monolayer of phosphonic acids with different chain length (spacer group) and functional (tail) group were adsorbed on mp-TiO₂ to modulate the surface property and interfacial energy barrier to investigate the effect of surface modification on charge extraction and transport from the perovskite to the mp-TiO₂ layer and thus the electrode. Two series of SAM-forming molecules with alkyl or phenyl as the spacer and various polar group as terminal substituents were prepared to tune the energy alignment and tunnelling distance at the mp-TiO₂ and perovskite interface. The work function of the SAM-modified mp-TiO₂ varied from -3.80 eV to -4.87eV, with that of the pristine mp-TiO₂ at -4.19 eV. A correlation of charge mobility, charge extraction and transport with respect to the energy alignment of mp-TiO₂ was attempted based on the systematic study. The study serves as a guide to engineer ETL interfaces with simple SAMs to improve the charge extraction, series resistance, and carrier balance. In this study, a maximum PCE of ~16.09% with insignificant hysteresis was fabricated, which is 17% higher than the standard device.

Organic-inorganic halide perovskites have been attracting attention as materials for light-emitting diodes (LEDs) due to unique emission properties, which include high color purity with a very narrow emission and tuning the emission wavelength easily through the adjustment of the halide components, covering the entire range of visible light. However, one roadblock to LED application is the low exciton binding energies, which results in low luminous efficiencies. One of the proposed strategies to enlarge the exciton binding energy is reduction of the dimensionality of the perovskite crystal structure. Here, we report highly luminescent thin films of formamidinium (FA) lead bromide perovskite (photoluminescence quantum yield, PLQY = 35.7%) via the formation of low dimensional architectures. A mixture of 3D FAPbBr₃ and a new type of 2D perovskite FA₂PbBr₄ intercalated with dimethyl sulfoxide (DMSO) was formed by controlling the molar ratio of FABr/PbBr₂ in the precursor solutions and annealing condition. A type I band alignment between the lower bandgap 3D FAPbBr₃ and the higher bandgap 2D FA₂PbBr₄ was supported by density functional theory (DFT) calculations, which results in exciton confinement in the 3D phase and a substantial increase in PLQY. The composite films also exhibited excellent air and thermal stability. Details of analysis will be presented and discussed.

Hybrid halide perovskites (HOIHPs) have been extensively studied in recent years due to their potential use as light-harvester in photovoltaic devices. While the efficiencies of such devices pose no limitation to commercial applications, the severe lack of stability of the materials remains an important issue to be overcome. Indeed, HOIHPs are known to easily degrade under moderate thermal stress^[1] or upon oxygen^[2] and/or light exposure.^[3] Notably, recent calorimetric studies even suggested that some HOIHPs (MAPbI₃ in particular and MAPbBr₃ to a minor extent) could be thermodynamically unstable,^[4] in agreement with DFT calculations.^[5] In contrast, other studies indicate the materials to be intrinsically stable.^[1,6] Obviously these questions need to be unambiguously answered. In this contribution, we discuss the underlying thermodynamics of HOIPs, both intrinsically (i.e., with respect to temperature) and extrinsically (against oxygen exposure and/or light). Intrinsically, we find the materials to be stable (albeit, in the case of MAPbI₃, only slightly) under standard conditions, and we also can assess the most favorable degradation path upon heating. Extrinsically, our considerations reveal a large tendency towards degradation of HOIHPs in the presence of oxygen, especially under real conditions. Notably, light itself can activate a relevant photodecomposition pathway.^[7] We discuss these issues on a quantitative level, in conjunction with experimental observations of the degradation phenomena.

References
[1] B. Brunetti et al., Sci. Rep. 2016, 6, 31896.
[2] N. Aristidou et al., Angew. Chemie 2015, 54, 8208.
[3] Y. Li et al., J. Phys. Chem. C. 2017, 121, 3904.
[4] G. P. Nagabhushana et al., Proc. Natl. Acad. Sci. U. S. A. 2016, 113, 7717.
[5] E. Tenuta et al., Sci. Rep. 2016, 6, 37654.
[6] I. L. Ivanov et al., J. Chem. Thermodyn. 2018, 116, 253.
[7] G. Y. Kim et al., Nature Mater. 2018, 17, 445.

Over the past five years, the organolead trihalide perovskites such as CH₃NH₃PbI₃ (MAPbI₃) have gained significant research interests with a rapid rise of power conversion efficiency of up to 22%. Integrating MAPbI₃ with the two dimensional transition metal dichalcogenides such as MoS₂ has the potential of achieving enhanced photosensitivity and broadened absorption bandwidth. In this work, we have integrated MAPbI₃ polycrystalline thin film with few-layer MoS₂, and demonstrated up to two orders of magnitude enhancement of the photoresponse in the heterostructure compared with the single layer MAPbI₃ and MoS₂. We mechanically exfoliated few-layer MoS₂ flake and transferred it onto the SiO₂/Si substrate. The sample was fabricated into 2 point device, on top of which we spincoated uniformly 500 nm MAPbI₃ film. Between the two parallel Au electrodes, half of the area contains the MoS₂-MAPbI₃ hybrid structure, while the other half contains only single layer MAPbI₃. We performed high-resolution photocurrent mapping within those two channels, and observed that the MoS₂-MAPbI₃ region had up to two orders of magnitude higher photocurrent than single layer MAPbI₃. The enhancement is attributed to the charge transfer between these two materials due to band alignment, which facilitated photo-carrier separation. The MoS₂-MAPbI₃ hybrid device showed faster transient photoresponse of 200-300 μs, which makes it promising for constructing high performance photo-detectors.

*This work was supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under Award No. DE-SC0016153, NSF Grant No. OIA-1538893, and NSF Nebraska Materials Research Science and Engineering Center (MRSEC) Grant No. DMR-1420645.

We report on the material preparation and emission properties of pure CsPbCl₃ and Pr³⁺ doped CsPbCl₃ perovskite crystals for possible applications in multi-wavelength photonics and nuclear radiation detection. The material properties of lead halide perovskites continue to be of great current for optoelectronic applications such as solar cells, light emitting diodes, lasers, and radiation detectors. In this work, we prepared pure and Pr³⁺ doped CsPbCl₃ bulk crystals in an effort to further extend its functionality. The investigated materials were synthesized from purified precursors of PbCl₂, CsCl, and PrCl₃ followed by melt growth in a vertical Bridgman station. Purification steps included directional freezing and multi-pass unidirectional translation in a zone-melting system. The pure CsPbCl₃ crystal exhibited a light yellow color, whereas the Pr: CsPbCl₃ crystal was light green indicating the incorporation of Pr³⁺ ions into the host lattice. Under UV optical pumping (340 nm) the pure CsPbCl₃ samples showed bandedge related emission centered at ~410 nm and a broad defect-related emission band extending to ~600 nm. The Pr: CsPbCl₃ samples exhibited a bright red emission under resonant intra-4f excitation of Pr³⁺ ions at 450 nm, with strongest lines peaking at 492 nm, 621 nm, 647 nm, and 732 nm. Furthermore, the Pr: CsPbCl₃ sample also exhibited IR emission bands centered at ~1.6 µm, ~2.5 µm and ~4.5 µm under optical pumping at ~1500 nm. The observation of IR emission at room temperature reflects on the low maximum phonon energy of chloride crystals (< 250 cm^-1), which reduced non-radiative decay though multiphonon relaxations of Pr³⁺ ions. More details of the materials preparation, emission properties of pure and Pr³⁺ doped CsPbCl₃ crystals as well as initial studies for nuclear radiation detection will be presented at the conference.

Hybrid organic-inorganic lead halide perovskites such as MAPbI₃ or FAPbI₃ (MA=CH₃NH₃⁺ and FA=CH(NH₂)₂⁺) and their mixed cation analogues represent one of the most promising alternatives to conventional Si based solar photovoltaics. Intriguing aspects include tunable band gap depending on composition, large optical absorption coefficients, and certified power conversion efficiencies (PCEs) that now exceed 22%. Additionally, composition-based tunable band gap makes these systems ideal candidates for multi-junction solar cells.
Unfortunately, there has been relatively little work done to explore perovskites compositional cation uniformity. Variation in cation stoichiometry may represent an important limiting factor for these devices as bandgaps – and by immediate extension open circuit voltages – are exquisitely sensitive to local cation composition. Although reports already exist, suggesting cation compositional heterogeneity across perovskite films, this is the first direct study of this phenomenon.
Here, we provide the first direct evidence for intrafilm cation heterogeneities within mixed cation FA_xMA_1-xPbI₃ films using a spatially-resolved, super-resolution infrared photothermal heterodyne imaging (IR-PHI) technique. We establish that these films exhibit large compositional spatial heterogeneities with cation distributions varying on the order of ~20%, with some areas exceed stoichiometric differences that exceed thrice the expected ideal stoichiometry.
The impact of these cation heterogeneities is further corroborated by emission measurements showing intrafilm emission energies differing by over 30meV directly correlated to local stoichiometry. These measurements thus reveal, for the first time, cation stoichiometric heterogeneities and their direct impact on local photovoltaic response-determining optical properties of mixed cation perovskites.

Interfacial studies and band alignment engineering on electron transport layer (ETL) play a key role for fabrication of high performance perovskite solar cells (PSCs). Here, we inserted an amorphous layer of SnO₂ (a-SnO₂) between the TiO₂ ETL and the perovskite absorber and studied on the charge transport properties of the device. The double-layer structure of TiO₂ compact layer (c-TiO₂) and a-SnO₂ ETL leads to modification of interface energetics, resulting in improved charge collection and decreased carrier recombination in PSCs. The optimized device based on a-SnO₂/c-TiO₂ ETL shows a maximum power conversion efficiency (PCE) of 21.4% as compared to 19.33% for c-TiO₂ based device. Moreover, the modified device demonstrates a maximum open circuit voltage (V_oc) of 1.223 V with 387 mV loss in potential, which is among the highest value reported for PSCs. In addition, the optimized PSC depicts a negligible hysteresis, and stabilized performance measured under continuous light (AM 1.5G) and UV light illumination. The stability results show that the device on c-TiO₂/a-SnO₂ retains about 91% of its initial PCE value after 500 h light illumination, which is higher than pure c-TiO₂ (67%) based devices. Interestingly, using a-SnO₂/c-TiO₂ ETL the PCE loss was only 10% of initial value under continuous UV light illumination after 30 h, which is higher than that of c-TiO₂ based device (28% PCE loss).

Lead Bromide-based perovskites (HaPs) are of interest as parent composition for wide bandgap ( > 1.75 eV) absorbers for low-cost solar spectrum splitting to boost solar-to-electrical energy conversion efficiency/area by adding them to c-Si or CIGS PV cells, and for photoelectrochemical solar fuel synthesis. Deep in-gap electronic states in solar cell absorbers serve as recombination centers and are detrimental for the cell’s photovoltaic performance, especially the open circuit voltage (V_oc) of the cell.
However, since several studies showed that the concentration of defects in the HaPs is relatively low ( < 10¹⁶ cm^-3), there is a general difficulty to observe deep in-gap states by direct optical absorption of sub-band gap photons in the HaPs.
Here we use modulated Surface PhotoVoltage (SPV) measurements, a non-destructive contactless technique that relies on direct sub-band gap photon absorption, with exponentially higher sensitivity than photocurrent-based measurements. SPV can show the existence of in-gap states and their relative positions with respect to the band edges both within the bulk semiconductor band gap, as well as at the interfaces of the HaP with the Electron Transport Layer (ETL) or the Hole Transport Layer (HTL).
For the first time, we reveal that three different, deep, defect levels exist in the bulk of the mix- Cation Lead Tribromide layers. Two of the levels are close to midgap, and the third lies ~0.8 eV below the Conduction Band Maximum (CBM). We also find that aging in inert atmosphere, as well as light soaking, increases the defect concentration. Furthermore, by performing photoconductivity measurements over a wide excitation intensity range, we show that under steady-state 1-sun equivalent conditions, these sub-band gap defect levels play an active role as non-radiative recombination centers.

Quickly following the development of perovskite solar cells, the field of perovskite light-emitting diodes (LEDs) has proceeded rapidly, with some devices already showing comparable efficiencies with those of the state-of-the-art organic LEDs. Perovskite LEDs offer tunable emission wavelength from visible to near-infrared; however, in certain spectral range the device performance is highly unstable. Taking red-emitting perovskite LEDs as an example, since the devices often rely on a mixed bromide-iodide perovskite for light emission, the typical light-induced phase segregation behavior of the material system results in a rapid red-shift of the electroluminescence (EL) peak, leading to a color change of the LEDs. In this presentation, we will introduce our recent endeavors on composition engineering and optical microspectroscopy to address the aforementioned stability problem of perovskite LEDs. We show that by simultaneously introducing lead thiocyanate and organoiodine additives in the perovskite precursors one can obtain a stable perovskite phase with high photoluminescence quantum yield (PLQE). The corresponding LEDs exhibit stable spectral emission and good performances (e.g., irradiance of 400 W sr^-1m^-2, maximum external quantum efficiency (EQE) of 14.5% and maintained at 12.0% at a high current density of 500 mA/cm²), despite the fact that perovskite film is discontinuous. The material strategy has been examined in the wavelength range from 630nm to 780nm. To better understand the morphology-performance correlation, we apply charge-modulation microspectroscopy (CMM) to investigate the electric-field inhomogeneity in the perovskite film, the impact of electron- (or hole-) transport layer on the LED EL properties, as well as the spatially resolved degradation processes in the electrode-covered and –uncovered regions. The findings provide some guidelines to the design of perovskite LED structures.

Organic-inorganic metal halide hybrids, consisting of a great variety of inorganic metal halide anions and organic cations, are an emerging class of functional crystalline materials with exceptional structural tunability. By choosing appropriate organic and inorganic components, the crystallographic structures can be finely engineered with the inorganic metal halides forming three-dimensional (3D) networks, two-dimensional (2D) planar or corrugated layers, one-dimensional (1D) chains or tubes, and zero-dimensional (0D) structures. The structural versatility of this class of materials suggests there is a vast parameter space to explore novel crystal structures exhibiting properties obeying non-obvious trends.
In this talk, I will present our recent efforts in developing and studying new classes of 0D metal halide hybrids. By choosing appropriate organic and inorganic components, the emitting species in 0D structure can be tuned from MX₆ octahedra to MX₅ pyramids, MX₄ seesaw structure, and metal halide clusters. Due to the structural reorganization and efficient intersystem crossing on the excited states, highly luminescent broadband emissions with large Stokes shift have been achieved for these 0D metal halide hybrids. Our findings show the molecule nature of small metal halide species in the 0D structure and allow us to relate the emission from either structure reorganization or localized excitons in metal halides to molecular phosphorescence. The application of these 0D materials as down converter in optically pumped white light emitting diodes will also be discussed.

The metal halide perovskite solar cells (PSCs) has become one of the most popular types of photovoltaic devices in past 6 years. High power- conversion- efficiency (PCE), panchromatic light absorptions and long carrier diffusion length, etc, makes the PSCs significantly competitive among the thin film solar cells. However, optimal function and performance of the PSCs usually require high quality of a perovskite film with full coverage, low roughness, big grain size and proper thickness, etc. Numerous methods have been applied to make high quality perovskite film, among which, the solution- based spin casting is one of the most vastly used methods. Although different groups have succeeded in making perovskite film using the spin casting, specific conditions such as solvent type, thermal treatment etc, varies largely. Lack of rational comparisons and instructive guidance on the deposition condition selection result in huge difficulty in performing repeatable experiments. Herein, we compare the effect of solvents, co-solvents and their ratios on the quality of the perovskite thin films. DMF and/or GBL are used as major solvents, while DMSO is applied as the co-solvent. Several ratios of DMF/GBL to DMSO from 10:0, 9:1 to 7:3 have been tried to prepare the perovskite precursor solutions. Interestingly, we observe significant differences on crystallinity, morphology and thickness of the thin film made by different types and combination of solvents. For instance, 1) From the XRD data, perovskite deposited from DMF based solution exhibits 10 times stronger crystallinity as compared with samples made from GBL. 2) SEM images indicate that DMF based solution guarantees compact and 2 times thicker films than the GBL ones, while the surface of DMF made layer shows much higher roughness. 3) Reducing the amount of the DMSO in the solution can promote the smoothness of the surface but result in the decrease of the film thickness. 4) Crystals with smaller grain size are observed in samples made from solution without DMSO, while excessive of DMSO content leads to vacancies inside the films. 5) Films deposited from the combined solvents of 90% DMF and 10% DMSO presents best compactness, thickness, smoothness and crystallinity among all other solvent combinations, showing 1.5-2 times longer charge carrier lifetime and superb PCE with the best device exceeding 17%. We attribute the better quality of DMF deposited layers to higher PbI₂ solubility and lower boiling point of the DMF, as compared with GBL. The addition of DMSO slows down the crystal growth rate of perovskite by forming MAI-DMSO-PbI₂ intermediate, guaranteeing larger crystal grain size. While excessive DMSO, which does not participate in the formation of the intermediates, remains in the supersaturated film (before annealing) and causes rupture and vacancies in the film during the vigorous annealing process. (This work was supported by the Morin Foundation Trust and the NSF, Inspire program #1344267)

Organic-inorganic lead halide perovskites (mainly CH3NH3PbI3) are being extensively studied because their excellent photovoltaic properties, such as suitable bandgap, high optical absorption and long carrier lifetime. To explain the large recombination time, the hypothesis that the formation of ferroelectric domains can separate the diffusion pathways of electrons and holes, has been proposed. We have found that a two-dimensional hole confinement in CH3NH3PbI3 is possible under room temperature conditions.
In this work, we explore the possibility of increasing their photovoltaic efficiency through additional sub-bandgap absorption. This would result in the creation of extra electron-hole pairs and in an increase in photocurrent without a decrease in open-circuit voltage. We assess the formation of a new band in the gap as well as its effect on the absorption features of hybrid halide perovskites CH3NH3PbI3 (MAPI).This approach has been widely studied to improve the efficiency of common semiconductors with photovoltaic performance such as GaP, CuGaS2, thiospinels, SnS2 or Si. We report here the electronic structure of new CH3NH3PbI3 perovskite derivatives, in which narrow band is obtained by replacing Pb atoms by Cr atoms. To deal with the bandgap underestimation problem of common DFT methods, quasiparticle calculations have been applied via the GW approximation. The investigation of the electronic structure of new CH3NH3PbI3 perovskites suggests that the presence of point defects play an important role in the coupling of two low energy photons to achieve a higher energy electron excitation (like in the Z-scheme of photosynthesis), which would maximize the photovoltaic performance.

G. García, P. Palacios, et al. Scientific Reports, 2018, 8,2511

Lead halide perovskites have applications in the fields of photovoltaics, light emitting diodes, and lasers. Their success results from their high absorption coefficients, low exciton binding energy, long charge carrier diffusion lengths, and high photoluminescence quantum yields. In order to optimize these devices to their full potential, it is important to understand their fundamental photophysical properties and how processing conditions affect their optical and electronic properties. Currently in the literature, the majority of time-resolved optical studies on perovskite nanocrystals utilize time-resolved photoluminescence to determine the excited state lifetimes. However, the nonradiative recombination processes should be further investigated due to the fact that they can have drastically different charge carrier recombination rates. Here, steady state and time-resolved optical spectroscopy is used to study the photophysical properties of hybrid organic-inorganic methylammonium lead bromide nanocrystals. Time-resolved spectroscopy measurements show that the band edge bleach decay dynamics are longer compared to the photoluminescence decay. These results indicate that dark carrier recombination is the primary mechanism leading to the long lived excited state lifetime of perovskite nanocrystals. Additionally, the effect of the precursor starting material purity on the excited state lifetimes is investigated.

We measure the inter-diffusion of methylammonium (MA) and formamidinium (FA) cations using lateral heterojunctions formed between MAPbI₃ and FAPbI₃. We confirm the creation of the gradient using UV-Vis and steady-state photoluminescence (PL) measurements demonstrating a bandgap of ~1.58 eV for the MAPbI₃ portion and ~1.49 eV for the FAPbI₃ portion of the film. We confirm that there is no change in the film morphology and crystallinity as evidenced by SEM and XRD, respectively. Using PL line scans across the lateral gradient, we image the inter-diffusion of MA and FA as a function of position and time. Using Fick’s Diffusion equations to fit the PL line scans, we determine the MA-FA inter-diffusion coefficient to be ~2x10^-8 cm²s^-1. We also monitor the inter-diffusion of cations spatially and temporally using in-situ widefield and confocal PL imaging to correlate cation motion with grain morphology. We observe the regions with low intensity undergo a faster exchange from MAPbI₃ to FAPbI₃. Furthermore, from confocal scans, we observe regional variations in MA-FA exchange. We show that regions with low PL intensity demonstrate a higher final FA concentration than regions with high PL intensity. This shows that the MA-FA exchange is faster in regions with higher defect-concentration suggesting a connection between non-radiative decay and ion exchange.

Hybrid inorganic-organic perovskites, due to their optoelectronic properties leading to very high-power conversion efficiencies, have shown great potential as absorbers in solar cells. Long-range diffusion of charge carriers was shown to be an important property contributing to a higher extraction of the photogenerated carriers. Here, we develop a streak-camera based time-resolved imaging technique and use it to directly probe carrier diffusion and recombination kinetics in hybrid perovskites. We study MAPbI₃ obtained via different fabrication routes, FAPbI_3, FA_0.85MA_0.15Pb(I_0.85Br_0.15)₃ and MAPbBr_3. Small diffusion coefficients of ~0.9 × 10^-5 and ~2.1 × 10^-5 cm²s^-1 are found in the films showing the highest device efficiencies, i.e., in FA_0.85MA_0.15Pb(I_0.85Br_0.15)₃ and MAPbI₃ (acetonitrile processed), respectively. In contrast, much higher values are found in MAPbI₃ and FAPbI₃ films, i.e. 4.9 × 10^-3 cm²s^-1 and 2.7 × 10^-3 cm²s^-1, respectively. We find that in addition to diffusion, the interplay between the monomolecular and bimolecular recombination processes is a critical factor determining device efficiency.

Organic-inorganic metal halide perovskites have emerged as attractive materials for solar cells with power-conversion efficiencies now exceeding 21%. We discuss the fundamental photophysical processes that have enabled these materials to be such efficient light-harvesters and charge collectors.

As photovoltaic power conversion efficiencies of single-junction cells approach the Shockley-Queisser limit, the recombination and mobility of charge-carriers will be limited only by intrinsic properties. We demonstrate that at the intrinsic limit, the mobility of charge-carriers is predominantly governed by interaction of carriers with optical vibrations of the lead halide lattice (Fröhlich interaction)^[1]. Therefore, predictions of maximum attainable mobilities can be made from easily accessible parameters, such as LO phonon frequencies and limits of the dielectric function.^[2]

In the absence of trap-mediated charge recombination, bi-molecular (band-to-band) recombination will dominate the charge-carrier losses near the Shockley-Queisser limit. We show that in methylammonium lead triiodide perovskite, such processes can be fully explained as the inverse of absorption,^[3]and exhibit a dynamic that is heavily influences by photon reabsorption inside the material.^[4]

Finally, we examine the prospect of such highly performing hybrid lead iodide perovskites in solar concentrator environments.^[5]We demonstrate that in the absence of degradation, perovskite solar cells can fundamentally exhibit appreciably higher energy-conversion efficiencies under solar concentration, where they should be able to exceed the Shockley-Queisser limit and exhibit strongly elevated open-circuit voltages.

[1] Wright, A. D.; Verdi, C.; Milot, R. L.; Eperon, G. E.; Pérez-Osorio, M. A.; Snaith, H. J.; Giustino, F.; Johnston, M. B.; Herz, L. M. Nature Communications 2016, 7, 11755
[2] Herz, L. M. ACS Energy Lett. 2017, 2, 1539
[3] Davies, C. L.; Filip, M. R.; Patel, J. B.; Crothers, T. W.; Verdi, C.; Wright, A. D.; Milot, R. L.; Giustino, F.; Johnston, M. B.; L. M. Herz, Nature Communications 2018, 9, 293
[4] Crothers, T. W.; Milot, R. L.; Patel, J. B.; Parrott, E. S.; Schlipf, J.; Müller-Buschbaum, P.; Johnston, M. B.; Herz,L. M. Nano Lett. 2017, 17, 5782
[5] Lin, Q.; Wang, Z.; Snaith, H. J.; Johnston, M. B.; L. M. Herz, Advanced Science 2018, 5 1700792

Hybrid perovskites exhibit pronounced momentum splitting at band edges due to a strong Rashba spin-orbit coupling effect. This effect was invoked by a number of groups to explain the high efficiency of hybrid perovskite solar cells. It was argued that the Rashba-induced splitting effectively suppresses the radiative recombination by mismatched spins and momenta between photoexcited electrons and holes. In the present study, we perform first-principles calculations to explicitly examine the impact of the Rashba effect on the radiative recombination coefficient in the archetypal hybrid perovskite, CH₃NH₃PbI₃. We demonstrate that the band extrema have consistent spin orientation and the momentum mismatch affects the radiative recombination coefficient by less than a factor of two.The computed radiative recombination coefficients are as high as in typical direct-gap semiconductors. Our insights establish a solid basis for accurate modeling of hybrid perovskites.

Recently, 2-dimensional (2D) Ruddlesden-Popper lead-halide based perovskite layered systems are attracting attention. Compared to their 3D counterpart, 2D systems offer greater tunability and stability, making them candidates for high-performance optoelectronic applications. Here we report the charge carrier recombination rate and spin depolarization times in single crystals of 2D perovskites PEA₂PbI₄ (MAPbI₃)_n−1 (PEA, phenethylammonium; MA, methylammonium; n = 1, 2, 3, 4). Layer thickness dependent charge carrier recombination rates were observed with the fastest rates for n = 1 due to the large exciton binding energy. Interestingly the slowest recombination rates occurred for the n = 2 sample and not n = 4. Room temperature spin-decoherence times also show a nonmonotonic layer thickness dependence with an increasing spin-coherence lifetime with increasing layer thickness from n = 1 to n = 4, followed by a decrease in lifetime from n = 4 to n = ∞. The longest decoherence time of ~7 ps is observed in the n = 4 sample. Our results are consistent with two contributions; Rashba-splitting increases the spin-coherence time going from the n = ∞ to the layered systems, while phonon-scattering which increases for smaller layers decreases the spin-coherence time. The interplay between these two factors contributes to the layer thickness dependent spin-coherence lifetimes. To correlate we monitored the LO and TO phonon frequency and phonon linewidth. For thinner layers the phonon frequencies decrease and broaden substantially indicating a large electron-phonon coupling.

In a very short time span, the power conversion efficiency (PCE) of metal halide perovskite solar cells (PSCs) has reached 23%, a massive improvement for solution-processed photovoltaic devices. Towards this end, both surface and bulk recombination of photogenerated charge carriers in the perovskite absorber layer are the major efficiency-limiting factors. [1] In this respect, controlling the growth and crystallization of the perovskite thin film is crucial for the performance of the perovskite solar cells as the crystal growth dynamics are very susceptible to the processing conditions [2]. Interestingly, some studies report a beneficial effect from water inclusion during processing, while others claim an adverse effect. Moreover, the effect of humidity and light exposure on the performance of PSCs is still debated. In this study, we fabricated, using a two-step protocol, MAPbI₃ perovskite solar cells with SnO₂ electron transport layers to study how the device performance and photophysics change upon exposure to humidity and light. Reference devices, not exposed to humidity and light, exhibit 18.4 % PCE with 22.5 mA/cm² short-circuit current density (J_sc) and 1.12 V open circuit voltage (V_oc). After exposing the perovskite absorber layer to 55% relative humidity under 1-sun illumination prior to completing the device fabrication, a reduction in V_oc to 1.09 V was observed. We study the influence of excess lead iodide (PbI₂), which is commonly believed to be passivating the PSCs, on the humidity resistance of MAPbI₃ devices. While the V_oc value of samples with excess-PbI₂ was lowered to 1.10V, we did not observe any change in J_sc. Since V_oc losses can be attributed to non-radiative recombination, we performed time-resolved photoluminescence (TR-PL) spectroscopy before and after humid air exposure of perovskite thin films under 1-sun illumination. We will discuss in detail how the exposure to humid air under illumination alters the surface structure of the perovskite samples, affects the device performance, and the photophysical processes.
References
[1] Ye Yang, Mengjin Yang, David T. Moore, Yong Yan, Elisa M. Miller, Kai Zhu and Matthew C. Beard, Nature Energy 2017, 2, 16207.
[2] Esma Ugur, Arif D. Sheikh, Rahim Munir, Jafar I. Khan, Dounya Barrit, Aram Amassian, and Frédéric Laquai, ACS Energy Lett. 2017, 2, 1960–1968.

A major puzzle from recent studies on LHPs is that optoelectronic performances suggest nearly perfect semiconductors despite the unavoidable presence of defects from room temperature and solution processing. Here we explain the essential physics in this class of materials based on their disordered phonon dynamics and dielectric functions. We show that the dielectric function of a hybrid organic-inorganic lead halide perovskite (LHP) possesses combined characteristics of a polar liquid and a ferroelectric material. The latter response in the THz region may lead to dynamic and local ordering of polar nano domains by an extra electron or hole, resulting a quasiparticle which we call a ferroelectric large polaron. Compared to a conventional large polaron, the collective nature of polarization in a ferroelectric large polaron may give rise to order(s)-of-magnitude larger reduction in the Coulomb potential and introduce potential barriers to charge carrier scattering. The ferroelectric large polaron may explain the defect tolerance, low recombination rates, and slow cooling of charge carriersin lead halide perovskites, as well as providing a design principle for high performance semiconductors from nano, molecular, and hybrid materials.

Solar cells based on lead halide perovskites have recently emerged showing a tremendous increase of power-conversion efficiency which exceeded 22%. In this contribution, the device physics of perovskite solar cells is addressed. The focus is on recombination of charge carriers because this process is ultimately limiting the efficiency. Furthermore, the performance and changes thereof during light-soaking and operation under real weather conditions are addressed.

The origin of the open-circuit voltage is discussed based on the reciprocity relation between electroluminescence and photovoltaic quantum efficiency.¹ Sharp absorption onset and high radiative recombination yield due to an extraordinary defect tolerance are identified as reasons for the outstanding optoelectronic properties of perovskites. Furthermore, the role of defect and surface recombination are addressed by employing a detailed analysis of the diode ideality factor.² Upon deliberately introducing defects in a controlled way it is found that the defect tolerance does not span to any kind of extrinsic defect.

The current-voltage curve of perovskite solar cells yields different results dependent on the initial voltage and scan rate of the voltage sweep. The resulting hysteresis is related to recombination as well.³ These results are explained based on the mixed ionic and electronic conductivity of the material, where displaced ions change interface and defect recombination rates. Reversible effects are observed on timescales of hours and their origins distinguished from irreversible degradation.⁴ The interplay of all these processes is analyzed for long-term operation under real weather conditions, where a better low-light performance and a low temperature coefficient result in relatively higher energy yields compared to a silicon solar cell.

An outlook is given on strategies aiming for a further improvement of open-circuit voltage and performance of perovskite solar cells toward their thermodynamic limit.


References
1. Tress, W. et al. Adv. Energy Mater. 5, 140812 (2015).
2. Tress, W. et al. Energy Environ. Sci. 11, 151–165 (2018).
3. Tress, W. et al. Energy Environ. Sci. 8, 995–1004 (2015).
4. Domanski, K. et al. Nat. Energy 3, 61–67 (2018).

Hybrid organic-inorganic perovskite semiconductors have recently emerged as high performance thin-film photovoltaic materials. Their combination of good optoelectronic properties and low-cost synthesis processes had led to an unprecedented rate of development in perovskite-based solar cell devices over several years. Despite this development, however, there are still ongoing efforts to reduce unwanted non-radiative carrier loss and further push solar conversion efficiencies towards theoretical limits. One of the phenomenological observations highlighting the efficiency limits is the non-uniform radiative emission seen in photoluminescence (PL) microscopy, suggesting that there is an underlying nanoscale variation in the carrier trapping centers in solution processed perovskite films.
Here, we utilize time resolved photoemission electron microscopy (TR-PEEM) to view directly the nanoscale electronic variation and its effect on photo-excited carriers in mixed cation perovskite films. We investigate regions with different PL efficiency and find an increased number of nanoscale trap centers in low PL efficiency regions. We show that these traps are due to occupied mid gap states and probe the corresponding ultrafast hole trapping dynamics at these nanoscale locations. Our work gives a direct view at the nanoscale distribution of trap centers in perovskite materials and their connection to macroscale carrier recombination.

In intensity-modulated photovoltage spectroscopy (IMVS) and impedance spectroscopy (IS) experiments of perovskite solar cells (PSCs), two features with different power dependences are observed to coexist corresponding to two relaxation routes. The relaxation rate of the slower feature is independent of light power while that of the faster one is proportional to the light power. Similar power dependence is observed in PSCs with various hole transport layers. Apparently, understanding the slower process will be helpful in optimizing the electric hysteresis in PCSs. In addition, because previous report assigns the faster process to the recombination of free carriers, understanding the faster feature will be helpful in minimizing the photocarrier loss in PSCs.
We notice that the recombination mechanisms involving only free carriers (e.g., band-to-band recombination) yield sublinear power dependence. Therefore at least two kinds of carrier species should be considered to explain the linear power dependence of the faster feature. Consequently, the next question is what this second carrier species other than free carriers is. We think it could be either accumulated carriers involved in the surface polarization model or the mobile ions/vacancies. In order to convincingly assign it, various photoelectric measurements are performed to provide additional information to these carriers. Our results indicate that these carriers should 1) be photogenerated with fixed lifetime (regardless of light power) so that its number is proportional to light power, 2) readily migrate along the electrode/perovskite surface so that it could uniformly distribute the device area (0.26 cm²), 3) be thermally activated before it could recombine with the free carriers, 4) be directly relavant to the slower IMVS or IS feature.

Metal-halide perovskites have demonstrated exceptional optoelectronic properties for next generation photovoltaics and light-emitting diodes. Recently, cation substitution has been reported to generate luminescence very efficiently, yet the underlying photo-physics are poorly understood. Here, we study the origin of this increased brightness by combining transient absorption and photoluminescence (PL) spectroscopy to track charge carrier dynamics in perovskite thin films. Unexpectedly, we find the recombination behavior to change from the previously-reported second to a first order regime dynamically within tens of nanoseconds after excitation, in line with fluence-dependent PLQE measurements. In temperature-dependent PL we find a redshift of the luminescence with decreasing temperature, directly mapping localized shallow traps. Supported by DFT calculations and transistor measurements we propose that energetic disorder in the distribution of electronic states leads to spatial accumulation of charges, creating n- and p-type doped regions, explaining the PLQE observations. Our results indicate that strong luminescence can be achieved in mixed-cation perovskites even at low carrier densities and thereby provides a roadmap for highly efficient LEDs.

Organic-inorganic perovskites enable a combination of useful organic and inorganic properties within a single molecular-scale composite and have attracted substantial interest for use within organic-inorganic electronic devices [1], in part due to the high carrier mobilities, long minority carrier lifetimes, tunable structures/band gaps, relatively benign defects and grain boundaries, and facile processing for systems based on Group 14 metals (e.g., Ge, Sn and Pb). Most recently, these materials have enabled unprecedented rapid improvement in performance within single junction photovoltaic (PV) devices, from an initial demonstration in 2009 [2] to levels with >20% power conversion efficiency and open circuit voltages >1V [3]. This talk will provide an historical perspective on foundational work related to the organic-inorganic perovskite semiconductors, including discussion of crystal structure flexibility, semiconducting properties, film deposition approaches and electronic device applications of the three-dimensional and lower-dimensional perovskite structures [4,5]. Recent trends in the field, as they relate to application in photovoltaics and related devices, will also be coupled into this discussion.

[1] D. B. Mitzi, K. Chondroudis, C. Kagan, IBM J. Res. Develop. 45, 29 (2001).
[2] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 131, 6050 (2009).
[2] W. S. Yang et. al., Science 356, 1376 (2017).
[4] D. B. Mitzi, Prog. Inorg. Chem. 48, 1 (1999).
[5] B. Saparov and D. B. Mitzi, Chemical Reviews 116, 4558 (2016).

Lead halide perovskite absorbers have achieved high photovoltaic performance exceeding the efficiency of CIGS and CdTe and their long term stability against heat, moisture, and light are being improved by compositional engineering of perovskite and surrounding carrier transport materials. For industrial applications, thermal stability of perovskites and carrier transport materials is a critical issue in comparison with thermally highly strong inorganic solar cell (Si, CdTe, etc.). Metal oxide electron transport layers (ETLs) generally have advantage in higher thermal stability than organic ETLs. We have been working with TiO₂ ETL-based multi-cation perovskite cells, which yielded efficiency over 21% by ambient air solution processes.¹ Light intensity dependence of Voc shows ideality factor low enough (n<1.4) for the perovskite solar cell to work as a high voltage power source even under weak light. Such merit meets a requirement in solar cell application to space satellite missions, which needs high photovoltaic performance even under very weak sunlight (Mars and Jupiter). We have examined the durability of perovskite solar cells which have thermally stable compositions comprising FA-based perovskites, TiO₂ ETL, and P3HT as hole transport layer (HTL). These cells exhibit good stability against thermal impact between temperature range between －80^oC and +100^oC. We also confirmed very poor thermal stability of spiro-OMeTAD as a reference HTL. On exposure to high energy electron and proton radiations as accelerated conditions simulating long term space irradiations, the perovskite cells demonstrated high stability and tolerance, which are superior to those of Si and GaAs solar cells.² Space applications also require fabrication of lightweight flexible devices. Thin film substrate-based perovskite solar cells were fabricated by low-temperature multilayer coating methods using amorphous TiO_x as ETL, which yield efficiency up to 18%. Future perspectives of industrialization of perovskite photovoltaic devices will be discussed focusing on the durable composition of perovskite devices and advantage of lightweight thin film device.

References
[1] T. Singh, T. Miyasaka, et al. Adv. Func. Mat., 2018, DOI: 10.1002/adfm.201706287.
[2] Y. Miyazawa, T. Miyasaka, et al. iScience, 2018, 2, 148-155.

Since the first report on the high efficiency, stable solid-state perovskite solar cell (PSC) in 2012 by our group, following two seed works on perovskite-sensitized liquid junction solar cells in 2009 and 2011, PSC demonstrated its power conversion efficiency (PCE) of 22.7% in 2017. According to Web of Science, publications on PSC increase exponentially since 2012 and total number of publications reaches about 9,000 as of May, 2018, which is indicative of a paradigm shift in photovoltaics. Although high photovoltaic performance was achieved, current-voltage hysteresis has been issued because it is related to stability of PSC. In this talk, methodologies to remove hysteresis are described. Interlayers at heterojunction are found to play important role in reducing hysteresis and improving stability. Manipulation of Frenkel defect is a universal approach toward hysteresis-free PSC. We have discovered novel methods for large-area perovskite coating and phase transformation in perovskite, which will be discussed in detail. In addition to photovoltaics, perovskite can be used to other applications. Long charge diffusion length and high energy stability of organic-inorganic halide perovskite are suitable for low-dose, high resolution X-ray imaging. We demonstrated X-ray image using millimeter thick perovskite film based on multicrystalline perovskite crystal with single-crystal-like optoelectronic properties. For heading toward Shockley–Queisser limit, research direction in PSC is proposed in this talk.

In our work on perovskite solar cells (PSC) we have achieved efficiencies above 20% with a mixed composition of iodide/bromide and methyl ammonium/formamidinium [1]. For cells larger than 1 cm² we have obtained 19.6% [2], replacing the anti-solvent step in the perovskite film formation with a vacuum flash treatment. With the use of SnO₂ compact underlayers as electron acceptor contacts we have constructed planar perovskite solar cells with a hysteresis free efficiency above 20% [3]. The cation mixing strategy has been developed further by including the Cs in a so-called ‘triple cation’ composition, i.e. Cs/FA/Ma as well as Rb in a quadruple cation mixture. Larger grains grown in a monolithic manner are observed and for example reproducibility and device stability are improved [4]. At the meeting we will discuss our follow up works [5] and present our champion data; up to 22% efficiency with an external electroluminescence of 4%, and an outstanding open-circuit voltage of 1.24 V at a band gap of 1.63 eV entailing one of the smallest loss-in-potential of 0.39 V ever measured for any solar cell material. Furthermore, we will report promising stability at 85 ^oC for 500 h under full solar illumination and maximum power point tracking (during which 95% of the initial performance was retained). Recently, we have also commented on the standardization of PSC aging tests [6].
Keywords: Perovskite, composition, stability
References
[1] Bi et al., Science Advance, DOI: 10.1126/sciadv.1501170
[2] X. Li et al., Science, DOI:10.1126/science.aaf8060
[3] Correa et al., Energy & Envir. Sci., DOI:10.1039/C5EE02608C
[4] M. Saliba et al., Energy & Envir. Sci., 2016, DOI: 10.1039/C5EE03874J
[5] M. Saliba et al., Science 10.1126/science.aah5557 (2016)
[6] Domanski et al. Nature Energy 01 January 2018. DOI: 10.1038/s41560-017-0060-5

Perovskite solar cells (PSCs) have become a competitive photovoltaic (PV) technology with rapid progress of efficiencies reaching to about 23%. Uniquely, PSCs have the highest efficiencies when they are solution processed, so one can envision solar cells printed in a similar manner and scale as newspapers. In addition, the bandgap tunability through perovskite composition engineering can enable high-efficiency multijunction devices, including perovskite/perovskite, perovskite/silicon, or perovskite/thin-film absorber (e.g., CIGS). Thus, PSCs are suited to helping address the challenge of terawatt-scale, PV-based electricity production that can power the future world. In this talk, I will discuss our recent progress in two areas: (1) scalable fabrication of high-efficiency, large-area perovskite solar cells and modules; (2) development of perovskite-based tandem devices. I will discuss our recent studies toward better control of film formation across the device stack at large scales by improving the precursor chemistry to better match the processing methods. The precursor chemistry and growth conditions affect significantly the physical and optoelectronic properties of perovskites. The challenges associated with perovskite solar module fabrication will be discussed. I will show the impact of interconnections on the performance of perovskite solar modules fabricated by scalable depositions. Toward perovskite-based tandem device development, I will discuss our recent effort on improving the optoelectronic properties of wide-bandgap as well as low-bandgap perovskite absorbers through solution chemistry engineering. Challenges and progress on perovskite-based tandem devices will also be discussed. These results demonstrate a promising path towards commercialization of the perovskite photovoltaic technology.

CsPbX₃ (X = Cl, Br, I) perovskite quantum dots (QDs) have attracted many attentions because of their excellent photoluminescence (PL) properties such as high quantum yields, narrow PL peak widths, and emission color tunability by elemental composition of the halide ions. CsPbBr₃ QD, which exhibit highly-pure green emission under blue and UV light irradiation, is a good candidate for applications to wide-color gamut displays. However, photodegradation of the CsPbBr₃ QD under excitation is a significant problem to be solved for their practical use. Herein, we found recovery of photodegraded CsPbBr₃ QD after continuous blue light excitation. In this work, the photobleaching and recovery phenomena of the CsPbBr₃ QD are investigated by evaluation of their optical properties.
CsPbBr₃ QD was synthesized by a conventional hot-injection method. Cs₂CO₃ was added into a mixture of 1-octadecene and oleic acid. The mixture was dried at 120 °C for 1 h, and then dissolved at 150 °C under Ar. Next, a mixture of 1-octadecene and PbBr₂ was vacuum-dried at 120 °C for 1 h, and then purged with Ar. Oleylamine and oleic acid were added to the solution. After complete dissolution of PbBr₂, the solution was heated to 180 °C. The solution of cesium oleate kept at 100 °C was injected into the PbBr₂ solution, and, 5 s later, the mixture was cooled in an ice-water bath. Synthesized CsPbBr₃ QD was precipitated by addition of tert-butyl alcohol, and then collected by centrifugation. After vacuum-drying for 1 day, yellow sample was obtained. For photobleaching test, the dried sample in a sample holder was placed on a plane light source of 468-nm blue LEDs with ~50 W m⁻² at room temperature; its absorption and PL spectra were measured at designated duration during and after 72-h irradiation using UV-vis and fluorescence spectrometers.
According to transmission electron microscopy, the obtained sample was cubic particles with an average size of 8 nm. CsPbBr₃ with cubic structure was verified from the X-ray diffraction pattern. A narrow PL peak was observed at ~520 nm under 468-nm excitation. During the continuous blue light irradiation, the sample color changed from yellow to black. Actually, its absorbance at 700 nm, in which CsPbBr₃ QD had no light absorption, increased as the irradiation duration prolonged. The PL intensity decreased to 20% of the initial intensity after 72-h irradiation, revealing photodegradation of CsPbBr₃ QD. During storage in the dark at room temperature after 72-h irradiation, body color of the degraded sample returned to yellow from black. This phenomenon was consistent with a change in absorption spectra. At the same time, recovery of PL intensity was observed. After storage for 18 days, the final PL intensity reached up to 90% to the initial intensity. Such recovery of degraded CsPbBr₃ QD gives us a hint to solve the photobleaching problem. In the presentation, the observed photobleaching and recovery will be discussed with further analyses.

Grain boundaries play a key role in the performance of thin-film optoelectronic devices, yet their effects in halide perovskite materials are still not understood. The biggest factor limiting progress is the inability to identify grain boundaries. Non-crystallographic techniques can misidentify grain boundaries, leading to conflicting literature reports about their influence; however, the gold standard – electron backscatter diffraction (EBSD) – destroys halide perovskite thin films. We solve this problem by using a solid-state EBSD detector with 6,000 times higher sensitivity than the traditional phosphor screen and camera. Correlating true grain size with photoluminescence lifetime, carrier diffusion length and mobility, shows that grain boundaries are not benign but have a recombination velocity of 1670 cm/s, comparable to that of crystalline silicon. We also observe amorphous grain boundaries that give rise to locally brighter photoluminescence intensity and longer lifetimes. This anomalous grain boundary character offers a possible explanation for the mysteriously long lifetime and record efficiency achieved in small-grained halide perovskite thin films. It also suggests a new approach for passivating grain boundaries to lead to even better performance in optoelectronic devices.

Hybrid organic-inorganic halide perovskites have emerged as a new family of optoelectronic materials because of their wide tunability in structural and optoelectronic properties via varying the compositions in A, B and X sites of the formula ABX₃. While the primary structural and optoelectronic properties of hybrid organic-inorganic halide perovskites are dominated by transient metal divalent cations in the B-site and halide anions in the X-site, recent research has shown the impact of A-site monovalent cations on phase and device stability and the additional tunablity in structural and optoelectronic properties. In this work, we focused on the investigation of A-site cations on the structural and optoelectronic properties of perovskites with a formula A_x(MA_0.17FA_0.83)_1-xPb_1-ySn_y(I_0.83Br_0.17)_3, where A are Cs⁺, Rb⁺, and guanidinium (GA⁺), or Cs⁺-GA⁺ and Rb⁺-GA⁺ with x = 0 – 0.2, and y = 0 – 1.0. Because of the smaller ionic radii of Cs⁺ (1.8 Å) and Rb⁺ (1.52 Å) than methylammonium (MA⁺, 2.16 Å), formamidinium (FA⁺, 2.53 Å) and GA⁺ (2.78 Å), adding Cs⁺ and Rb⁺ pushes the Goldschmidt tolerance factor into the cubic perovskite phase regime for each Sn composition while adding GA⁺ leads to an opposite direction. We prepared densely-packed, pinhole-free perovskite films with one-step solution process with mixed solvents plus anti-solvent wash method followed by thermal annealing. X-ray diffraction (XRD) patterns show pure cubic phase for all composition perovskites. The lattice parameter of these perovskites show non-linear lattice parameter versus Sn composition for each A composition with alloyed Pb-Sn having larger lattice parameters than those of pure Pb or Sn perovskites. However, a linear decrease or increase of lattice parameter versus A composition with each fixed Sn composition. The bandgaps deduced from the UV-Vis absorption spectra edges show that the bandgaps decrease with the increasing of Sn to the minima around 75% Sn and then increased to pure Sn. In Pb-rich perovskites, adding Cs⁺ or Rb⁺ increases the bandgaps for each fixed Sn composition, while in Sn-rich perovskites, it causes the decrease of bandgaps. The mechanism of A-site cations on the structural and optoelectronic properties of Pb-Sn and pure Sn perovskites will be discussed based on their impacts on B-X orbital overlap, BX₆ octahedral tilting, and strain tolerance. In addition, we also fabricated solar cells with the p-i-n structure and achieved a record maximum PCE of 9.61% for a low band gap (1.26 eV) perovskite of Cs_0.10(MA_0.17FA_0.83)_0.9Pb_0.25Sn_0.75(I_0.83Br_0.17)₃. Moreover, this 75% Sn device can retain 80% of initial PCE after 30 days storage in inert condition followed by over 100 hours in ambient condition. Overall, this study demonstrated the impact of A-site cations on structural and optoelectronic properties of Pb-Sn and pure Sn double halide perovskites and provides a route to enhance phase and device stability for high Sn or pure Sn perovskite solar cells.

Electrode polarization is a universal phenomenon taking place at the interface between a metallic electrode and an ionic/electronic semiconductor which needs to be studied in detail for organometallic halide perovskite (OMHP) devices. In general, interfaces between hybrid perovskite and electrode or charge transport layers are gaining more attention as studies showed that interfaces can significantly control the operation of hybrid perovskite devices as well as long term performance stability. Previously, the increase of dielectric permittivity in low frequencies <100 kHz in thin films of OMHP was attributed to the ionic migration and accumulation at the interface and space charge polarization enhanced along grain boundaries^1,2. It was suggested that long range ion diffusion under external field would be the governing mechanism at low frequencies. In order to gain insight on the origin of dielectric permittivity at low frequency regime here, we study single crystals of MAPbI₃ excluding polarization enhanced through grain boundaries effect. This study is done using broadband dielectric spectroscopy (BDS) in dark condition by varying temperatures and external biases. Generally, there are three sources contributing to the conduction in OMHPs including ions, electrons and holes and polarons. So far, the atomistic origin of slow dynamic process in OMHPs was explained by the transport of ionic species and the drift and/or diffusion of polarons was totally overlooked. In our ac conductivity measurements, we observed that the conductivity decreases with decreasing frequency and temperatures. The accumulation of ions at the interface usually block the carriers from conduction and conductivity is expected to drop at this region. In this study we found that at low frequencies, there is a plateau in ac conductivity. This conductivity is only droped if a large bias is applied. The polaronic nature of charge carriers in OMHPs has been demonstrated but the transport, migration and accumulation of polarons at the interfaces was not explored. It was suggested that small polarons which transfer through hopping mechanism may not directly contribute in the overall carrier mobility unlike large polarons but they can form charge accumulations states³. Here, we conclude that ion migration and accumulation is not solely responsible for giant dielectric constant at low frequency in OMHPs but also there is an effective role from polarons. Indeed, polarons can migrate and accumulate at the interface first. This study opens a way for better understanding of a key aspect in the operation of highly efficient OMHP devices.
References:
1. Yang, T. Y. et al. Angewandte Chemie 2015, 127 (27), 8016.
2. Senocrate, A. et a. Angewandte Chemie International Edition 2017, 56 (27), 7755.
3. Devreese J.T. Polarons in Ionic Crystals and Polar Semiconductors. Springer US: 1984.

Methylammonium lead bromide (MAPbBr₃) has grown in prominence as an attractive photovoltaic (PV) absorber owing to its higher stability compared to MAPbI₃, and its desirable electronic, absorption and defect properties which can be further tailored by composition engineering. Inspired by recent work on partial substitution of Pb in MAPbBr₃ by Cobalt to yield additional energy states within the semiconductor band gap leading to intermediate band photovoltaics (IBPVs) ^[1,2], we explore the possibility of substituting Pb by various other elements selected from across the periodic table in a high-throughput fashion. Using state-of-the-art density functional theory (DFT) computations, we study the crystalline and electronic structure as well as the energetics of 1/8th substitution of Pb in a MAPbBr₃ supercell by all cationic elements from periods 2, 3, 4, 5 and 6. Both the density of states and the calculated charge transition levels are used to probe the energy states created by the substituent atom in the electronic structure of MAPbBr₃, revealing several substituents that create mid-gap states while retaining the parent band gap (~ 2 eV at the PBE level of theory). Formation energies of substituent defects are calculated in various charged states ^[3,4] and compared with the energetics of dominant intrinsic defects, established as the vacancy defects V_MA (dominant acceptor) and V_Br (dominant donor) ^[4]. It is observed that depending on the chemical potential of relevant species, transition metals Zr, Hf, Nb and Sc, and group V element Sb create low formation energy defects that compensate for the dominant intrinsic defects and shift the equilibrium Fermi level closer to the conduction band minimum, making the semiconductor conductivity more n-type. The relative stability of these substitution defects coupled with the fact that they produce mid-gap energy levels not only makes them promising candidates for IBPVs but raises the possibility of these impurities creating deep trap states in the MAPbBr₃ band gap that can cause harmful non-radiative charge carrier recombination and reduce the PV efficiencies. Further, many of the promising substituents thus identified were experimentally synthesized and characterized, and the measured absorption coefficients compared favorably with the computed spectra. Lastly, machine learning techniques were applied on the high-throughput computational data to yield simple predictive models for the substituent transition levels as a function of structural and electronic features derived from a significantly cheaper unit cell calculation on a completely Pb-substituted hybrid perovskite.

REFERENCES

[1] M.D. Sampson et al., J. Mater. Chem. A. 5, 3578 (2017).
[2] A. Luque et al., Nat. Photonics. 6, 146–152 (2012).
[3] Freysoldt et al., Rev. Mod. Phys. 86, 253 (2014).
[4] Y. Yan et al., Springer IP. 79-105 (2016).

Mixed halide hybrid perovskites are highly efficient semiconductors with promising applications in optoelectronics. Defects and disorder dictate the properties of this material. Structural disorder can be defined as local variations in the crystal structure, whereas chemical disorder refers to local inhomogeneities in material composition. Both types of disorder are highly dynamic and are likely responsible for the favourable (e.g. long carrier lifetimes [1]) and unfavourable (e.g. the Hoke effect [2]) properties observed in this material. In the rapidly maturing field of hybrid perovskite optoelectronics, where the hunt is now on for the best performing material composition, little attention is given to the fundamental role of disorder. Here we aim to elucidate the relevant time- and length- scales for both types of dynamic disorder in this material, with the aim of gaining a better understanding of their relation to the optoelectronic properties. In particular, we use a combination of structural and thermodynamic techniques to investigate transitions in the prototypical mixed halide hybrid perovskite CH₃NH₃Pb(Br_xI_1-x)₃. We use temperature dependent powder X-ray diffraction and heat capacity measurements under illumination to quantify the nature of— and energy associated with light-induced order/disorder transitions.

References
[1] Y. Chen, H. T. Yi, X. Wu, R. Haroldson, Y. N. Gartstein, Y. I. Rodionov, K. S. Tikhonov, A. Zakhidov, X. -Y. Zhu & V. Podzorov. (2016). Extended carrier lifetimes and diffusion in hybrid perovskites revealed by Hall effect and photoconductivity measurements. Nature Communications, 7, 12253.
[2] Hoke, E. T., Slotcavage, D. J., Dohner, E. R., Bowring, A. R., Karunadasa, H. I., & McGehee, M. D. (2015). Reversible photo-induced trap formation in mixed-halide hybrid perovskites for photovoltaics. Chemical Science, 6(1), 613-617.

Perovskite Solar Cells (PSCs) have already attracted considerable attention attributed to its intriguing properties, showing a tremendous jump in efficiency from 3.8%¹ by Kojima et al. (2009) to 22.1%² by Yang et al. (2017). A lot of research has been done on the efficiency improvement of PSCs by optimization of the film morphology at the interfaces in the device. Analyzing these interfaces by various characterization techniques including electrochemical impedance spectroscopy (EIS) in a solid state active device geometry³ is not only difficult to decipher but sometime is misleading as well since there may be multiple processes occurring at similar time scale at multiple interfaces.
To simplify the analysis of the perovskite films EIS should be carried out in perovskite-liquid electrolyte interface. Recently, Li et. al. have measured the flat band potential, charge carrier density and type of charge carrier accumulation at the perovskite-liquid interface from Mott-Schottky plot for spin coated and spray coated films of methylammonium lead tri-iodide (MAPbI₃) perovskite.⁴
In this work, MAPbI₃ perovskite has been synthesized by one step spin coating of lead acetate-trihydrate and methylammonium iodide precursor on pre-heated substrate. Significant difference in film morphology has been observed as the substrate temperature (T_subt) was varied from room temperature to 120 °C prior to spin coating. Nucleation and growth mechanism is revisited to find out the optimum T_subt for fabricating uniform perovskite films and is attributed to the fast homogeneous nucleation followed by delayed growth. We confirmed that if the transformation temperature is just below the equilibrium melting point of the material, the nucleation is more homogeneous and hence more uniform and compact films are formed. EIS measurement at perovskite-liquid electrolyte interface reveals the impact of film morphology on the anomalous diffusion behaviour observed at low frequency regime and on the open circuit voltage (V_OC) of the device. More uniform and compact films show less degree of diffusion across the interface as compare to the rough ones. Also the V_OC (1.018 V) is highest for the device fabricated by more uniform and compact film (T_subt = 100 °C) among all the devices followed by the one fabricated at T_subt = 120 °C (0.963 V). This reveals that film morphology is an important factor in deciding the kinetics at the interface and V_OC and hence the performance of the device.
1. Kojima, A. et al. J. Am. Chem. Soc. 131, 6050–6051 (2009).
2. Yang, W. S. et al. Science. 356, 1376–1379 (2017).
3. Bag, M. et al. J. Am. Chem. Soc. 137, 13130–13137 (2015).
4. Li, Z. et al. Chem. Commun. 53, 2467–2470 (2017).

The interest in perovskite solar cells is growing rapidly due to their versatile applicability for energy harvesting systems. In a short period of time, devices already reached efficiencies up to 22%, making them comparable to established thin-film solar cells like Cu(In,Ga)Se₂ or CdTe. The quantity of publications dealing with planar and rigid solar cells is growing tremendously; however, fibrous solar cells have not been in focus yet. Since the 1D structure provides a greater flexibility in comparison to planar systems, applications ranging from e-textiles/wearables to lightweight applications are feasible. Here, the single step fabrication of phase-pure organic-inorganic lead halide perovskite fibers by inert electrospinning technique is presented. Morphological, as well as optical/photonic properties have been studied and demonstrate first comprehensive data on electrospun organic-inorganic hybrid materials. Substitution of the absorbing layer in planar heterojunction solar cells with perovskite fibers resulted in a photoelectric response under simulated sunlight conditions. These flexible 1D hybrid perovskite fibers are potential elements for flexible optoelectronics and mark a starting point towards competitive fibrous solar cells.

The photovoltaic performance of world-leading Organic-inorganic halide perovskite (OHP) solar cells remains limited by defective electronic states, which introduce non-radiative recombination pathways for charge carriers. In OHP thin films, it is emerging that surface defects are the most prevalent and thus have the largest impact on luminescence and device efficiency. We have recently shown that the addition of potassium halides can increase luminescence yields substantially but the direct impact on carrier traps has not yet been elucidated.

Here, we employ a state-of-the-art photoemission electron microscopy (PEEM) setup to map local surface defect states on triple cation, mixed-halide perovskite ((CsFAMA)Pb(I_0.85Br_0.15)₃) films with < 30 nm spatial resolution. Our results show that chemical passivation by potassium doping reduces the density of surface defects on the films. Confocal photoluminescence maps of fiducially marked sites show a clear anti-correlation between areas of high photoluminescence intensity, and regions of high trap densities based on the photoemission fingerprints.

Finally, we examine how increasing levels of potassium doping affect the local carrier trapping dynamics by integrating PEEM with time-resolved pump-probe spectroscopy. This enables us to monitor the rate and intensity of photoexcited hole trapping into these intra-band surface states. When teamed with the spatial resolution of PEEM beyond the diffraction limit, such time-resolved measurements provide a uniquely powerful tool for characterising defect states, both in local regions and across larger regions of films.

We have probed the temperature dependence of methylammonium lead iodide (CH₃NH₃PbI₃) absorption spectra. We extract the Urbach energy as the reciprocal value of the slope of the absorption at the band edge plotted in logarithmic scale. Its value depends on the material disorder and generally correlates well with the loss in the open-circuit voltage (V_OC) of optimized cells, compared to their bandgap [1]. When cooling CH₃NH₃PbI₃ film we find a strong decrease in their Urbach energy and a slow decrease of their optical band gap energy.
From the theoretical Urbach energy temperature dependence we obtain an average energy of electronicaly active phonon states of 110 ± 20 cm^-1, which implicates that the dynamic disorder of CH₃NH₃PbI₃ is mainly caused by cage vibrations [2]. This gives further evidence that the density of active static defects in perovskites is very low in comparison to other materials used for solar cells, including bulk monocrystal semiconductors.
From comparing the photoconductivity and emission-based absorption spectroscopies, we find that the CH₃NH₃PbI₃ band structure is slightly indirect. This may be caused by spin-orbital coupling, the so-called Rashba splitting effect. Our results prove the direct nature (non-phonon assisted) of both absorption and emission in CH₃NH₃PbI₃. The experimentally observed long photoluminescence decay time is given by small overlap between free electrons and holes in the k space.
Finally, we found a strong correlation between the V_OC deficiency, compared to the bandgap, of finalized solar cells and Urbach energies measured by PL spectroscopy. These results will help to establish more refined practical efficiency limits of perovskite solar cells by taking into account the Urbach energy, to be compared to the Shockley-Queisser limit, which only considers the bandgap.

[1] S. De Wolf et al.: J. Phys. Chem. Lett. 5 (2014) 1035.
[2] M. Ledinský et al.: J. Phys. Chem. Lett. 6 (2015) 401.

Lead halide perovskite have been dominating the optoelectronic field for last several years. Particularly, due to the superior optical properties, colloidal nanocrystals of CsPbX₃ are finding great opportunities in quantum dot light emitting diodes (QLEDs). Green and red nanocrystals are progressing rapidly in QLEDs with efficiencies 12.9 % and 6.8% respectively.^{1, 2} Blue nanocrystals, however, are still lagging far behind red and green owing to their poor PLQYs. The best efficiency reported on these blue perovskite QLEDs is only 0.07 %.³ Our group, recently discovered that in addition to the PLQYs, the transport layers are limiting the performance of the LEDs. By employing a combination of HTL and a buffer layer (TFB:PFI), we achieved ~7 fold increase in the efficiency to 0.5%.⁴ Further, we improved the quality of nanocrystals by doping with Mn, which eventually lead to a factor of 4x enhancement in the EQE (2.1%) with a FWHM of 17 nm, meeting the standards of pure NTSC blue coordinates. We then used down converting green and red nanocrystals to construct all perovskite white LEDs for the first time. In this presentation, I will discuss the evolution of blue nanocrystals in LEDs and the impact of Mn doping on their PLQY and device performance.
References
1.F. Yan, J. Xing, G. Xing, L. Quan, S. T. Tan, J. Zhao, R. Su, L. Zhang, S. Chen, Y. Zhao, A. Huan, E. H. Sargent, Q. Xiong, H. V. Demir. Nano Letters., Article ASAP, 2018. DOI: 10.1021/acs.nanolett.8b00789.
2. X. Zhang, C. Sun, Y. Zhang, H. Wu, C. Ji, Y. Chuai, P. Wang, S. Wen, C. Zhang, W. W. Yu, J. Phys. Chem. Lett. 2016, 7, 4602.
3. E.-P. Yao, Z. Yang, L. Meng, P. Sun, S. Dong, Y. Yang, Y. Yang, Adv. Mater. 2017, 29, 1606859.
4. M. K. Gangishetty, S. Hou, Q. Quan, and D. N. Congreve,* Adv. Mater, DOI: 10.1002/adma.201706226.

Hybrid organic-inorganic perovskites (HOIPs) formed with organoammonium iodide and lead iodide precursor solutions are remarkable absorbing layers for photovoltaic (PV) devices. HOIPs with mixed cations, specifically those comprising combinations of formamidinium, methylammonium (CH₃NH₃⁺) and cesium cations, have resulted in devices with record-setting efficiencies. Troublingly, recent reports have shown that CH₃NH₃⁺ is consumed through undesired side reactions with processing solvents, resulting in active layers with compositions that stray from the intended cation stoichiometry and structural variations that depend on how the CH₃NH₃⁺-containing precursor solutions are processed. Thus, a greater understanding of the complex precursor-solvent chemistries is requisite for precise stoichiometric control over HOIP thin films, which critically impacts their structural development and dictates the performance and stability of PV devices comprising such layers.

Our studies elucidate the chemistries that take place between CH₃NH₃⁺ and commonly-used processing solvents, N,N-dimethylformamide (DMF) and dimethylsulfoxide (DMSO). A series of experiments involving sequential addition of precursors to DMF/DMSO solvent mixtures before and after thermal annealing of the precursor solutions suggest that reaction of CH₃NH₃⁺ with DMSO produces dimethylammonium, (CH₃)₂NH₂⁺ during solution annealing. X-ray diffraction on model HOIP films formed with precursor solutions in which CH₃NH₃I is intentionally and stoichiometrically replaced with (CH₃)₂NH₂I confirms that substitution of CH₃NH₃⁺ with (CH₃)₂NH₂⁺ alters the perovskite structure. When CH₃NH₃I is the sole organoammonium iodide in the precursor solution, the formation of (CH₃)₂NH₂⁺ and its subsequent incorporation in thin films produces a cubic-phase film instead of the conventionally accessed tetragonal-phase CH₃NH₃PbI₃. Interestingly, devices that incorporate such cubic-phase films exhibit enhanced stability towards humidity compared with devices with the more conventionally accessed tetragonal CH₃NH₃PbI₃. Our study emphasizes the importance of precise control over solution chemistries; the by-product(s) of which, when incorporated in the solid state, can result in structural differences that can ultimately impact macroscopic properties and performance.

Halide perovskites are inexpensive and easily processable next generation semiconductors. We here demonstrate perovskite solid-state confinement in nanoporous oxide matrices as a general strategy to control the size of the nanocrystallites (<10 nm) in the strong quantum size effect region. Photoluminescence tuning between near infrared and ultraviolet is achieved by manipulating the size of perovskite crystals through confinement in nanoporous alumina (npAAO) or silicon (npSi) scaffolds [1].
Our novel method of nanocrystalline perovskites preparation within a porous oxide matrix results in device-relevant structure that requires no colloidal stabilization. Low-voltage LEDs with narrow, blue-shifted emission fabricated with perovskite nanocrystallites confined within npAAO thin films support the general concept for next-generation photonic devices. The template-controlled size of the perovskite crystals is quantified in npSi with microfocus high-energy X-ray depth profiling in transmission geometry, verifying the growth of perovskite nanocrystals throughout the entire thickness of the nanoporous films. We study in detail exciton recombination, exciton-phonon interactions and energy trap states in confined and bulk semiconductor films using low temperature photoluminescence spectroscopy down to 3.8 Kelvin.
Further areas of application include photon detectors, (polarized) electroluminescent devices, single-photon sources and metasurfaces. Future developments will include increasing the efficiency of the LEDs, exploring their applications in flexible devices and in depth study of the fundamental properties of the confined structures.

[1] S. Demchyshyn, J. Roemer, H. Groiss, H. Heilbrunner, C. Ulbricht, D. Apaydin, A. Boehm, U. Ruett, F. Bertram, G. Hesser, M. Scharber, N. S. Sariciftci, B. Nickel, S. Bauer, E. D. Glowacki and M. Kaltenbrunner, “Confining Metal-Halide Perovskites in Nanoporous Thin Films”, Science Advances 3 (8), e1700738 (2017).

Since the advent of organic/inorganic metal halide perovskites and their expanding application in low cost solution-phase processing of high efficiency optoelectronics, a wide expanse of colloidal synthesis techniques have been developed. However, due to the inherent limitations of batch screening approaches, these studies struggle to effectively characterize large parameter spaces and thereby develop a complete understanding of the fundamental nucleation and growth pathways of perovskite quantum dots (QDs). Recent works in colloidal QD growth characterization have implemented rapid microfluidic screening strategies. However, similar to their flask-based predecessors, these flow studies have not accounted for the effect of reactant mixing rates which are known to significantly influence growth pathways in the controlled synthesis of perovskite nanocrystals.

Herein, we present a systematic study of the mass transfer-tuned synthesis for three different cesium-lead-tribromide perovskite nanocrystal reaction strategies using an intelligent microfluidic screening technology. The microfluidic characterization platform consists of modular heated units equipped with a unique in-situ spectral monitoring probe (UV-Vis absorption and fluorescence spectroscopy) which may translate along the 27 cm tubular microreactor reaching 68 sampling ports. Complete platform automation enables high-efficiency collection of inline photoluminescence and absorption spectra spanning four orders of magnitude in residence time (i.e. growth time), from 100ms to 17 min. The portability of the sampling probe allows us to decouple fluid velocity-controlled mass transfer from reaction time.

This microfluidic approach enables rapid discovery, screening, and optimization of colloidal QDs with desired optoelectronic properties via high-throughput screening (>10,000 experimental conditions) of the accessible synthesis parameter space. Utilizing this developed intelligent microfluidic platform, we systematically studied the effect of early mixing timescales on the QD nanocrystal size and size distribution. Varying the average fluid velocity and slug size tunes the degree of mixing within droplets containing the cesium-lead-tribromide precursors, resulting in perovskite nanocrystals with different optoelectronic properties.

As research on perovskites has rapidly developed in recent years, various research groups have made great efforts on improving the process and characteristics of perovskite light-emitting devices (PeLEDs). Perovskite has advantages of low material cost and simple process. However, it also has technical issues like short life-time and low stability. In this study, we demonstrated large area PeLEDs that are all-solution processed under ambient condition using organic-inorganic hybrid perovskite compounds as next generation light sources. In this work, an electron transport layer (ETL) was optimized to improve the electrical characteristics, to reduce the operating bias voltage and, to improve the optical characteristics. To improve the electron injection at the interface between the cathode and the perovskite layer, a thin layer of polyethyleneimine (PEI) was spin-coated on the perovskite layer prior to the deposition of the cathode. The PEI could reduce the work function of the cathode; hence, the electron injection barrier was significantly reduced. The electron transport properties of PEI were further improved by simply doping some elements. Solution processable n-type semiconducting materials (Cs₂CO₃, Alq₃, and CsF) were selected as n-type dopant. Also, thin layer of polymethylenemethacrylate (PMMA) was spin-coated on the perovskite layer to reduce the pin-holes in the perovskite layers. As fabricated PeLEDs based on these solution processable materials showed extremely low turn on voltage and high maximum luminance. Also, a silicone encapsulation was used to prevent the device from degradation by moisture and oxygen. The silicone encapsulation materials for commercial LEDs were used. Liquid silicone was coated on the as-fabricated PeLEDs to passivate the devices. Based on our results, the coated liquid silicone did not affect the performance of the PeLEDs. The lifetime of the PeLEDs encapsulated with commercial silicone was good performance which was comparable to the glass-lid encapsulation. Since the silicone was flexible, these encapsulation materials could be used in flexible PeLEDs. The luminescence characteristics of green region were confirmed by using a perovskite compound of MAPbBr₃ which is attracting attention as a next generation light source, and its value as a next generation light-emitting devices was confirmed by applying it to a large area flexible device.

We investigate the impact of excess PbI₂ in the precursor solution on the structural and optical properties of thin films of the model hybrid perovskite methylammonium lead iodide (MAPbI₃). We find that excess of PbI₂ in the precursor solution results in crystalline PbI₂ in the final thin film that is located at the grain boundaries. From UPS we find that this crystalline PbI₂ phase has no direct impact on the electronic structure of MAPbI₃. In contrast to that, temperature dependent absorption measurements indicate a systematic change in the temperature dependence of the exciton binding energy in the perovskite. We also observe a decrease in the critical temperature and a concomitant smearing out of the tetragonal – orthorhombic phase transition as a function of excess PbI₂. Our results thus help to better understand the exact role of PbI₂ in the perovskite layer and pave the way for a more tailored design of perovskite solar cell.

The emerging class of hybrid perovskite systems has attracted immense attention recently due to their uniquely interesting properties which has led to solar cell architectures with very high conversion efficiency. These materials have also been explored for other optoelectronic device applications such as lasing, photo-sensors, light emitting devices (LED). Lately, this class of systems is being further explored for futuristic optoelectronic devices by employing small molecular engineering. Such molecular manipulations have been shown to control the dimensionality and associated photo-physics of these systems such as the electron-hole interactions and excitonic effects. In particular, one sub-group of such hybrid perovskites has exhibited broadband emitting properties which are of great interest as potential materials for white light emitting diodes (LEDs).
The discussion of broadband luminescence in the literature has focused on the identification of the specific contributions to such emission (e.g. bound excitons, self-trapped excitons, STE) and their possible connection to the specific structural features and organic/inorganic components in the system. In this work we report the observation of broadband emission in 1D ribbon system of (H₃N-CH₂-CH₂-NH₃)₈(Pb₄Br₁₈)●Br₆ as well as 2D corrugated system of (H₃N-CH₂-CH₂-NH₃)(Pb₂Cl₆), the common amine used being ethylene diamine (En), which is the smallest amine not used thus far in this context. We have employed several techniques such as single crystal x-ray diffraction (SC-XRD), steady state photoluminescence (PL), UV-absorbance, diffused reflectance spectroscopy (DRS), time resolved photoluminescence (TRPL) and Raman spectroscopy to study and compare the structural and optical properties of the two materials as well as their respectively diammonium salts. Our studies have brought out structure-specific contributions and interplay of the molecular and STE contributions. The primary absorption process appears to be driven by the molecular component while the STE appears to be centered on the inorganic component which may involve the intrinsically heightened polarization at the organic-inorganic interface. Distortions can be important for localization but do not appear uniquely control this phenomenon.

An open counter [1] is a unique detector that can operate in air at atmospheric pressure to detect and count a small number of low-energy photoelectrons. Therefore, photoemission yield spectroscopy in air (PYSA) can be performed by employing an open counter as the detector. Recently, PYSA measurement was performed at various temperatures [2-4], and its applicability to the analysis of temperature dependence on the change of work function was demonstrated [3, 4]. In these cases, PYSA measurement was performed as follows. UV light emitted from a deuterium lamp was monochromatized using a grating monochromator, which was then focused on the sample surface. The number of photoelectrons emitted from the sample surface was counted using an open counter. During the measurement, the sample temperature was controlled using a small heater connected to a temperature controller. This method was considered effective for the comparison of the electronic structures of materials that exhibit different properties at high temperatures. Temperature effects of CH₃NH₃PbI₃ perovskite solar cells having simple planar architecture were reported [5]. According to this report, the obvious changes in the crystal structure which seriously affects the performance of the solar cells were found. Therefore, PYSA was applied to analyze the change in the electronic structure of CH₃NH₃PbI₃ at various temperatures. We discussed the change in the threshold energy of photoemission, which corresponds to the first-ionization potentials, at around the phase transition temperature of the perovskite.

[1] H. Kirihata and M. Uda, Rev. Sci. Instrum. 52 (1981) 68
[2] D. Yamashita, A. Ishizaki, and T. Yamamoto, Anal. Sci. 30 (2014) 575
[3] D. Yamashita, A. Ishizaki, and T. Yamamoto, Mater. Trans. 56 (2015) 1445
[4] D. Yamashita and A. Ishizaki, Appl. Surf. Sci. 363 (2016) 240
[5] L. Cojocaru, S. Uchida, Y. Sanehira, V. Gonzalez-Pedro, J. Bisquert, J. Nakazaki, T. Kubo, and H. Segawa, Chem. Lett. 44 (2015) 1557

Although metal halide perovskites (MHPs) have emerged as exceptional materials for optoelectronics, their charge transport properties remain under scientific debate. One of the reasons for this is that most charge transport measurements are conducted on polycrystalline thin films, which adds complexity to the system and makes it difficult to interpret the results in an unambiguous way. In this work, we concentrated on surface-guided planar nanowires of CsPbBr₃, single crystals with a 1D nature, which serve as a simplified model system for charge transport measurements. The surface-guided growth of MHP nanowires on sapphire results in ordered and well-defined arrays, which can be easily integrated into functional devices. We studied the charge transport in these arrays as well as in individual CsPbBr₃ nanowires, all having uniform crystallographic orientation and well-defined facets. We fabricated the first field-effect transistor on a single nanowire of MHPs and measured charge transport characteristics such as field-effect mobility and charge carrier concentration. We also observed intriguing time-dependent electrical behavior in dark and under illumination, related to the dynamic nature of these soft semiconductors. Surface-guided growth of MHP nanowires enables fast, simple and efficient fabrication of multiple devices in parallel manner for fundamental research and optoelectronic applications.

Halide perovskites are a promising new class of semiconducting materials for a wide variety of optoelectronic applications, such as photovoltaics, light emitting diodes, lasers and optical sensing. They have received widespread attention due to their many attractive synthetic and photophysical properties, namely; solution processability, high tunability, long charge carrier lifetimes and high charge carrier mobilities. Photoluminescence spectroscopy is a powerful tool for the photophysical investigation and materials optimisation of halide perovskites. Using photoluminescence spectroscopy we have investigated the photophysics of a range of halide perovskite photovoltaic absorbers and light emitters.

Methyl ammonium lead iodide (MAPI) is one of the most efficient and widely investigated perovskite absorbers for photovoltaic cells. Using time-resolved photoluminescence, the charge carrier lifetime of MAPI was measured and was found to increase with the thermal annealing duration of the perovskite layer. In addition, temperature dependent photoluminescence spectroscopy was used to monitor the change in the perovskite bandgap with temperature and determine the orthorhombic to tetragonal and tetragonal to cubic phase transition temperatures.

Photoluminescence spectroscopy is particularly well suited for the study of perovskite materials for light emitting applications. Two dimensional perovskites are a promising material for the creation of single component white light source. We investigated the two dimensional white light emitter, α-(DMEN)PbBr₄, using steady state photoluminescence to determine the chromaticity coordinates of the emission and time-resolved photoluminescence to probe the excited state lifetimes.

Two-dimensional lead halide perovskites (2D LHPs) are solution-processed semiconductor quantum wells with great promise for optoelectronic applications. Their properties are highly tunable,. and can be modified by changing the quantum-well thickness as well as their chemical constituents. Since 2D LHPs have high exciton binding energies, understanding exciton dynamics is important for designing novel 2D LHP devices. Here, we present a study based on temperature-dependent photoluminescence to elucidate exciton-phonon interactions in these materials. Our study is based on iodide perovskites with varied quantum-well thicknesses, cations, and organic spacer lengths. The results show that tuning these parameters affects photoluminescence properties as well as phase transitions. This study provides a fundamental understanding of 2D LHP photophysics, necessary for developing novel 2D LHP optoelectronics.

Recently we have revealed that poly-crystalline thin-films of hybrid lead-halide perovskites emit broadband electromagnetic radiation within the terahertz (THz) frequency window [1]. This radiation mainly arises from ultrafast electron-hole separation due to different diffusivities of photo-generated carriers (i.e., photo-Dember effect). A transient photocurrent with a rise-time shorter than the period of an optical phonon mode can coherently drive the corresponding phonon. By means of this, longitudinal optical (LO) phonons can be coherently launched via ultrafast photo-excitation and can be detected via measurement of the associated THz emission [1, 2]. In a polaron picture, a carrier alters the equilibrium position of ions within a polar semiconductor and this effectively induces a potential well for the carrier causing its “self-trapping” around the displaced ions [3]. Polaron formation proceeds through long range carrier – LO phonon coupling [4] and was lately evoked to account for the surprising opto-electronic properties of the hybrid perovskites [5]. Here we show that we can time-resolve polaron formation dynamics via monitoring of the emitted THz radiation from the coherent LO phonon mode.
In MAPbI₃, we observe a strong emission peak at 1.15 THz corresponding to the lowest energy LO phonon mode of the inorganic sub-lattice. By time- and frequency-resolving this LO phonon-associated emission, we observe an intriguing dependence on the excitation photon energy. When we excite the perovskite at its band-edge (770 nm), the emission at the LO mode arises instantaneously. However, when a well-above bandgap excitation (400 nm) is used, the emission at the LO phonon mode emerges with a finite time delay (~300 fs). This suggests that hot-carriers cannot form polarons due to their excess kinetic energy. Therefore, initial carrier cooling dominates over the polaron formation. Furthermore, we observe a dynamic softening of the LO mode within the first ps suggesting that electronic charge alters the stiffness constant of the ionic bonds within the material as predicted in other polaronic systems [3].

[1] B. Guzelturk et al. Adv. Mater. 30, 1704737 (2018)
[2] T. Dekorsky et al. Phys. Rev. Lett. 74, 738 (1995)
[3] D. Emin, Camridge University Press, New York (2012)
[4] K. Miyata et al. Sci. Adv. e1701469 (2017)
[5] X. Y. Zhu and V. Podrozov J. Phys. Chem. Lett. 6, 4758 (2015)

Lead halide perovskites are promising materials for a range of applications owing to their structural uniqueness and optoelectronic properties. Understanding the relationship between the atomic/superstructures and the associated properties of perovskite materials is vital to fully utilize their potential performances. We present the detailed pressure processing of CsPbBr3 perovskite nanocube superlattices (NC-SLs) for the first time. By using diamond anvil cell combined with in situ synchrotron-based small/wide angle X-ray scattering (SAXS and WAXS) and photoluminescence (PL) probes (shown A in the Figure), the NC-SL transformations are correlated at both atomic and superlattice levels with the PL transition through a pressure cycle of 0 ↔ 17.5 GPa. In-situ SAXS and WAXS measurements monitored its mechanical and structural changes showing that the NC-SL went through multiple phase transitions both at both atomic and superlattice levels. After the pressure process, the individual CsPbBr3 NCs fused into two-dimensional nanoplatelets (NPLs) with a uniform thickness (~10.1 nm). The pressure-synthesized perovskite NPLs exhibited a pure single cubic crystal structure, a 1.6-fold enhanced photoluminescence quantum yield, and a longer emission lifetime than the starting NCs. These results suggest that pressure processing can provide a novel approach for the quick conversion of lead halide perovskites into structures with enhanced properties.

Instability issues associated with lead halide perovskite materials and solar cells have captured a significant amount of research attention. My group at OIST investigates these materials to obtain deepened understanding of degradation mechanisms. In this talk, I will present our findings on degradation of lead halide perovskites and the strategies to overcome some of the instability issues.

Semiconducting metal-halide perovskites present various types of chemical interactions which give them a characteristic fluctuating structure sensitive to the operating conditions of the device, to which they adjust. This makes the control of structure-properties relationship, especially at interfaces where the device realizes its function, the crucial step in order to control devices operation. In particular, given their simple processability at relatively low temperature, one can expect an intrinsic level of structural/chemical disorder of the semiconductor which results in the formation of defects.
Metal halide perovskites have been successfully applied as optically active layers in photovoltaic and optoelectronic devices. The high efficiency of such research-scale devices hold the promise for an imminent application of perovskites in large scale energy production and lighting solutions. Intrinsic thermal and photo-physical instability of these materials pose, however, a challenge for further developments in device scale up and long-term reliability.
Instabilities are manifested as light-induced ion migration and segregation, eventually leading to material degradation under prolonged exposure to light. Understanding, controlling and eventually blocking such material instabilities are fundamental steps towards large scale exploitation of perovskite in optoelectronic devices. By combining photoluminescence measurements under controlled conditions with ab initio simulations we identify photo-instabilities related to competing light-induced formation and annihilation of trap states, disclosing their characteristic length and time scales and the factors responsible for both processes. We show that short range/short time defect annihilation can prevail over defect formation, happening on longer scales, when effectively blocking undercoordinated surface sites, which act as a defect reservoir. By an effective surface passivation strategy we are thus able to stabilize the perovskite layer towards such photo-induced instabilities, leading to improved optoelectronic material quality and enhanced photo-stability in a working solar cell. The proposed strategy represents a simple solution towards longer stability perovskite thin films that could be easily implemented in large scale manufacturing.

Perovskites have emerged as low-cost, high-efficiency photovoltaics with certified efficiencies of 22.1% approaching already established technologies. The perovskites used for solar cells have an ABX₃ structure where the cation A is methylammonium (MA), formamidinium (FA), or cesium (Cs); B is Pb; and X is Br or I. Unfortunately, single-cation perovskites often suffer from phase, temperature or humidity instabilities. This is noteworthy for CsPbX₃ and FAPbX₃ which are stable at room temperature as a photoinactive “yellow phase” instead of the more desired photoactive “black phase” that is only stable at higher temperatures. Moreover, apart from phase stability, operating perovskite solar cells (PSCs) at elevated temperatures is required for passing industrial norms.
Recently, double-cation perovskites (using MA, FA or Cs, FA) were shown to have a stable “black phase” at room temperature. These perovskites also exhibit unexpected, novel properties. For example, Cs/FA mixtures suppress halide segregation enabling band gaps for perovskite/silicon or perovskite/perovskite tandems.(1) In general, adding more components increases entropy that can stabilize unstable materials. Here, we take the mixing approach further to investigate triple cation (with Cs, MA, FA) perovskites resulting in improved reproducibility and stability.(2) We then use multiple cation engineering to integrate the seemingly too small rubidium (Rb) (that never shows a black phase as a single-cation perovskite) to study novel multication perovskites.(3)
One composition containing Rb, Cs, MA and FA resulted in a stabilized efficiency of 21.6% and an electroluminescence of 3.8%. The V_oc of 1.24 V at a band gap of 1.63 eV leads to a very small loss-in-potential of 0.39 V, one of the lowest measured on any PV material indicating the almost recombination-free nature of the novel compound. Polymer-coated cells maintained 95% of their initial performance at 85°C for 500 hours under full illumination and maximum power point tracking. This is a crucial step towards industrialisation of perovskite solar cells.

Lastly, to explore the theme of multicomponent perovskites further, molecular cations were revaluated using a globularity factor. Using the multication strategy, we studied an ethylammonium-containing compound that yielded an open-circuit voltage of 1.59 V.(4)

The last part elaborates on a roadmap on how to extend the multication to multicomponent engineering providing a series of new compounds that are highly relevant candidates for the coming years.

(1) McMeekin et al. Science (2016)
(2) Saliba et al., Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency. EES (2016)
(3) Saliba et al., Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance. Science (2016)
(4) Gholipour, ...,Saliba, Globularity Selected Large Molecules for a New Generation of Multication Perovskites, Advanced Materials (2017)

As a new semiconductor material, methyl ammonium lead iodide (CH₃NH₃PbI₃) perovskite has attracted much attention in recent years owing to its outstanding photoelectric properties. Although CH₃NH₃PbI₃ perovskite has achieved tremendous progress in photovoltaic devices, a deeper understanding of its intrinsic physical properties is still of great interest. The anisotropy of properties is an important feature of semiconductor materials, while limited relevant work has been reported regarding the anisotropy in CH₃NH₃PbI₃ perovskite. In this work, we report the fast growth of high-quality CH₃NH₃PbI₃ single crystal by using a non-seeded solution growth method. When exposed the CH₃NH₃PbI₃ single crystal to moisture, it was found that the (001) facet exhibited more sensitive to water molecules and showed faster erosion rate compared with the (100) facet and (112) facet. We then developed a top-down strategy to prepare CH₃NH₃PbI₃ single-crystalline thin films of tens of micrometers in thickness with different orientations, which provided the possibility of directly studying the anisotropy of carrier transport in CH₃NH₃PbI₃ single crystals. Vertical-structured FET devices based on CH₃NH₃PbI₃ single-crystalline thin films with different orientations were fabricated. Compared with the [100] and [112] orientation, it was found that carriers have lower mobility along the [001] orientation than along the [100] and [112] orientation. The origin of the anisotropy in moisture erosion and carrier transport was elucidated from the perspective of crystal structure. The reveal of the orientation-dependent moisture erosion and carrier transport in perovskite CH₃NH₃PbI₃ single crystal may deepen the understanding of physical properties of the material and guide the design of stable and high-performance optoelectronic devices in the future.

In organic-inorganic hybrid perovskite solar cells, optimization of hole transport materials (HTMs) is important for enhancing solar power conversion efficiency and improving stability [1,2]. At OIST, a team of researchers in the Energy Materials and Surface Sciences Unit has been making concerted efforts to study 2,2’,7,7’-tetrakis[N,N-di-(4-methoxyphenyl)amino]-9,9’-spirobifluorene (spiro-MeOTAD), which is the most widely used HTM in perovskite solar cells [2-8]. In this talk, we will present our latest understanding of fundamental interactions between Li-bis(trifluoromethanesulfonyl)-imide (LiTFSI), 4-tert-butylpyridine (t-BP) and spiro-MeOTAD. Also, we will show how gas exposure (e.g., exposure to O₂, H₂O, N₂) influences electronic structures and conductivity of such HTM films. In addition, we will propose further strategies to improve perovskite solar cell performance and stability.

[1] L.K. Ono and Y.B. Qi, Research progress on organic–inorganic halide perovskite materials and solar cells. J. Phys. D. Appl. Phys. 51 (2018) 093001.
[2] Z. Hawash, L.K. Ono, and Y.B. Qi, Recent Advances in Spiro-MeOTAD Hole Transport Material and Its Applications in Organic-inorganic Halide Perovskite Solar Cells. Adv. Mater. Interfaces 5 (2018) 1700623.
[3] L.K. Ono, Z. Hawash, E.J. Juarez-Perez, L. Qiu, Y. Jiang, and Y.B. Qi, The influence of secondary solvents on the morphology of spiro-MeOTAD hole transport layer for lead halide perovskite solar cells. (2018) submitted.
[4] E.J. Juarez-Perez, M.R. Leyden, S. Wang, L.K. Ono, Z. Hawash, and Y.B. Qi, Role of the Dopants on the Morphological and Transport Properties of Spiro-MeOTAD Hole Transport Layer. Chem. Mater. 28 (2016) 5702-5709.
[5] Z. Hawash, L.K. Ono, and Y.B. Qi, Moisture and Oxygen Enhance Conductivity of LiTFSI-Doped Spiro-MeOTAD Hole Transport Layer in Perovskite Solar Cells. Adv. Mater. Interfaces 3 (2016) 1600117.
[6] Z. Hawash, L.K. Ono, S.R. Raga, M.V. Lee, and Y.B. Qi, Air-Exposure Induced Dopant Redistribution and Energy Level Shifts in Spin-Coated Spiro-MeOTAD Films. Chem. Mater. 27 (2015) 562-569.
[7] L.K. Ono⁺, S.R. Raga⁺, M. Remeika, A.J. Winchester, A. Gabe, and Y.B. Qi, Pinhole-Free Hole Transport Layers Significantly Improve the Stability of MAPbI₃-Based Perovskite Solar Cells Under Operating Conditions. J. Mater. Chem. A 3 (2015) 15451-15456. (⁺These authors contributed equally)
[8] Y. Kato, L.K. Ono, M.V. Lee, S.H. Wang, S.R. Raga, and Y.B. Qi, Silver Iodide Formation in Methyl Ammonium Lead Iodide Perovskite Solar Cells with Silver Top Electrodes. Adv. Mater. Interfaces 2 (2015) 1500195.

Emerging organolead-halide-perovskite-based materials and devices have recently received tremendous attention due to their extraordinary photonic and optoelectronic performance and potential low production costs. Many physical and chemical properties beyond light harvests have erupted. Despite the rapid development of perovskite materials and devices, the impact of environmental factors on different stages of materials synthesis and device fabrication still remains unclear. Moisture or humidity is widely recognized as one of the major lethal factors in perovskite devices degrade or decompose after fabrication. However, recent reports have shown that moisture could be crucial to obtain high-performance perovskite films in a suitable moisture of ~35% relative humidity. Although a few attempts have been made to investigate the morphological evolution of organohalide perovskite films, a comprehensive understanding of the environmental influence in the process of thermal annealing is not achieved yet, as it is critically important to further improve the optoelectronic device performance. By taking advantage of synchrotron-based grazing incident wide-angle x-ray scattering (GIWAXS), we in situ monitor the transformation of organohalide perovskites from precursors to final films upon thermal annealing at different relative humidity. in situ GIWAXS reveals the formation of crystalline perovskite materials over relevant time and moisture scales to decipher the effect of humidity on both phase transition and crystal structures during annealing. These in situ measurements demonstrate that moderate humidity accelerates the formation of organohalide perovskites and improves the orientation of the film, but more than 50% realtive humidity retards the formation of perovskites and destructs their crystal structures. Furthermore, the highly orientated films obtained at optimized relative humidity are observed which could be attributed to the hydration of precursors. These findings clearly elucidate the influence of moisture environment in annealing process of organohalide perovskites and, in turn, allow us to correlate the improved performance of organohalide perovskite matrials and devices to structural features in terms of environmental effects.

I will discuss advances - including doping strategies and reduced-dimensional perovskites - in increasing the stability of the perovskite active layer.

Efficient wide-bandgap perovskite photovoltaics will enable tandem solar cells when successfully combined with low-bandgap absorbers such as crystalline silicon. However, wide-bandgap perovskite solar cells (PSCs) today exhibit performance far inferior to that of sub-1.6 eV bandgap PSCs. Their tendency to form a high density of deep-level trap states underpins this limitation. Here we show that healing the deep traps in mixed cation-halide wide-bandgap perovskites – in effect, increasing the defect tolerance via cation engineering – could enable further performance improvements in PSCs. We achieve a stabilized power conversion efficiency (PCE) of 20.7% for 1.65-eV-bandgap PSCs by incorporating a small concentration of dipolar cation additive. The devices exhibit a high open-circuit voltage (V_oc) of 1.22 V and a fill factor (FF) that exceeds 80%. We also achieve a PCE of 19.3% for 1.1 cm² large-area devices with a V_oc of 1.24 V. We achieved commensurable improvements in 1.74-eV-bandgap PSCs, where we obtained a stabilized PCE of 19.1% together with a high V_oc of 1.25 V and FF of over 80%. From density functional theory calculations, we find that the presence and reorientation of the dipolar cation in mixed perovskites can heal the defects that introduce deep trap states. Our findings shed light on defect healing in perovskite materials and pave the way to further increasing the efficiency of perovskite-enabled tandem photovoltaic devices.

Organic-inorganic mixed halide perovskite (MAPbX₃: MA=CH₃NH₃⁺, X =Cl^-, Br^- , or I^-) are extensively used as absorbing materials for solar cell due to its broad absorption range and long charge carrier diffusion length. Despite its high photovoltaic efficiency, their poor stability remains a major challenge for high performance device and their commercialization.
Perovskite films based on CH₃NH₃PbI_3-xCl_x undergo rapid degradation when exposed to oxygen and moisture. To overcome this problem, many researchers have been studied about the degradation mechanism of perovskite films. However, these results were not simultaneously obtained with structural and chemical properties of perovskite films. Thus it is not provided complete degradation mechanism of perovskite films.
In this work, we investigated the degradation mechanism of perovskite films by performing simultaneous measurement of the structural and chemical informations using the photo thermal induced resonance technique combined with atomic force microscope. Finally, this study will contribute to understanding of the mechanism of the degradation process of perovskite films and enhance the stability of perovskite optoelectronics field.

Mixed-cation lead mixed-halide perovskite solar absorbers such as (FA/Cs)Pb(I/Br)₃ exhibit remarkable and tunable optoelectronic properties that make them attractive for next generation solar cell technologies [1]. In practice such materials are always polycrystalline however the role of grain boundaries (GBs) remains poorly understood and a subject of much speculation. For example, separate experimental studies suggest GBs in MAPbI₃ can be both provide an efficient separation of electrons and hole as well as be responsible for increased non-radiative recombination [2,3]. There have been relatively few theoretical predictions of GBs and these have thus far only focussed on MAPbI₃ and associated intrinsic defects [4,5].

In this talk, I will present our recent theoretical predictions on the electronic properties of two types of GB in (FA/Cs)Pb(I/Br)₃. We employ density functional theory methods similar to those we have previously applied to model extended defects in a range of other materials including CZTS, TiO₂ and Fe₃O₄ [6-8]. We find that for the Σ3 (111) GB there is strong segregation of Br to the GB (but not Cs) and it remains electrically benign with no preferential electron or hole trapping. However, for the Σ3 (112) GB there is strong segregation of both Cs and Br and this is associated with the introduction of shallow electron trap states at the GB. We note that this may be beneficial for efficient electron/hole separation since there is no tendency for holes to trap at the GB [9]. These results highlight the point that not all GB types are equivalent and so one needs to consider a range of GBs to build a statistical picture. Importantly, mixed perovskite absorbers present the possibility of much wider compositional variations (even without intrinsic defects) and so understanding GB properties is key to understanding the behaviour of real polycrystalline materials.

[1] D.P. McMeekin et al, Science 351, 151 (2016)
[2] J.S. Yun et al, J. Phys. Chem. Lett. 6, 875 (2015)
[3] D.W. De Quilettes et al, Science 348, 683 (2015)
[4] W. Shan et al, J. Phys. Chem. Lett. 8, 5935 (2017)
[5] W-J. Yin et al, Adv. Mater. 26, 4653 (2014)
[6] B. Mendis, K.P. McKenna et al, J. Mater. Chem. A 6, 189 (2018)
[7] K.P. McKenna et al, Nat. Commun. 5, 5740 (2014)
[8] S. Wallace and K.P. McKenna, Adv. Mater. Inter. 1, 1400078 (2014)
[9] K. P. McKenna, manuscript in preparation

Chemical stabilities of hybrid perovskite materials demand further improvement towards long-term and large-scale photovoltaic applications. Herein, we report enhanced chemical stability of CH₃NH₃PbI₃ by doping divalent anion Se^2- in the form of PbSe in precursor solutions to enhance the hydrogen-bonding-like interactions between organic cation and inorganic framework, as evidenced by redshifted N-H stretch from Fourier-transform infrared spectra. As a result, in 100% humidity at 40 °C, the 10% w/w PbSe-doped CH₃NH₃PbI₃ films exhibited >140-fold stability improvement over pristine CH₃NH₃PbI₃ films. Whilst perovskite structure of the PbSe-doped CH₃NH₃PbI₃ films reserved, a top efficiency of 10.4% with 70% retention after 700 hours aging in ambient air was achieved on an unencapsulated 10% w/w PbSe:CH₃NH₃PbI₃-based cell. Significantly, the incorporated Se^2- effectively suppressed iodine diffusion in solar cell, leading to enhanced chemical stability of the silver electrodes. Successful doping of divalent Se^2- in perovskite lattice is further confirmed by enlarged lattice spacing from X-ray diffraction patterns, asymmetric photoluminescence peaks with shoulder electronic states, and concomitantly increased electrical conductivity of 5%, 10% w/w PbSe:CH₃NH₃PbI₃ thin films. This work could advance the fundamental understanding of degradation and stabilization of hybrid perovskites in both material and device settings.

Solution-processed thin-film perovskite solar cells (PSCs), where the record efficiency has rocketed from ~4% to ~23% — comparable to commercial silicon-based solar cells — in just nine years, offer unprecedented promise of low-cost, high-efficiency renewable electricity generation. Organic-inorganic halide perovskites (OIHPs) at the heart of PSCs have unique structures, which entail rotating organic cations inside inorganic cages, imparting them with desirable optical and electronic properties. To exploit these properties for PSCs application, the reliable deposition of high-quality OIHP thin films over large areas is critically important. The microstructures and grain-boundary networks in the resulting polycrystalline OIHP thin films are equally important as they control the PSC performance and stability. Fundamental phenomena pertaining to synthesis, crystallization, coarsening, microstructural evolution, and grain-boundary engineering involved in the processing of OIHP thin films for PSCs will be discussed with specific examples. Additionally, the discovery of Pb-free, Ti-based all-inorganic halide perovskites will be presented, together with the demonstration of viable PSCs based on these new materials. The overall goal of our research is to have deterministic control over scalable processing of tailored halide perovskite thin films with desired compositions, microstructures, and grain-boundary networks for large-area, high-efficiency, and stable PSCs.

Over the past decade, organic-inorganic halide perovskite semiconductors have attracted substantial research attention for application in optoelectronic devices. The field is moving towards more and more complex chemical compositions enabling dramatically improved device performances. Those improvements however, were mostly achieved through empirical optimization of processing conditions. Due to the fast and complex chemical reactions of hybrid metal halide perovskites, significant variations in material properties and device performances are observed from previous reports. Therefore, in depth understanding of the fundamental film formation processes and relevant synthesis parameters is critical for the control of the final film properties and to achieve reproducible, high performance devices.
Here, we establish mechanistic insights into the film formation process of metal halide perovskites by developing complementary in-situ characterization techniques including synchrotron diffraction, optical imaging and photoluminescence spectroscopy. These in-situ characterization techniques complement each other in providing a holistic picture of the relation between phase, optical response and morphology evolution. With our newly designed systems, we are able to characterize and monitor fast reaction/formation of perovskite thin films immediately after the spin-coating process which is commonly used in lab research.
As a model system, we studied methylammonium lead iodide (CH₃NH₃PbI₃) as one of the most studied compounds among organic-inorganic halide perovskites. At relevant time scales, we demonstrate how the precursor chemistry influences phase formation, crystallization kinetics, film morphology, and optical response. We find that the perovskite film morphology is directly related to the structure of the intermediate phase and by tuning the precursor chemistry, we are able to tune the film morphology. The precursor chemistry plays the main role in the perovskite film formation and prevents the formation of needle-like morphology. In addition, using the Cl chemistry, the disappearance and re-appearance of perovskite phases was observed over the course of crystallization which is distinctly different from other non-Cl chemistries. The combination of state-of-art in-situ characterization techniques could pave the way towards assessing the roles of synthesis and processing designs very efficiently thus, enables mechanistic insights and control of the film properties for high efficiency devices.

Photovoltaic devices based on hybrid organic-inorganic perovskite absorbers have reached outstanding performance over the past few years, surpassing power conversion efficiency of over 22%. This talk we discuss the progress at the National Renewable Energy Lab (NREL) on the challenges in hybrid perovskite solar cells (HPSCs) and stability of HPSC devices and materials. This talk will highlight work at NREL to develop understand and enhance stability of HPSCs. Discussion will focus on efforts to more carefully understand the implications of process on stability and efficiency in the HPSC devices. Connections to aspects of material formation and processing for high-volume manufacturing will also be made. In the case of stability, an examination of different perovskite active layers their formation and resulting interfacial electronic structure with contacts in the HPSCs stack will be presented. Work at NREL indicate interface formation of the active layer with different carrier transport materials has direct implications for performance and it evolution over time in the resulting devices will be built upon. Results extending these results to additional active layers and associated interfaces studies in which photoemission, time resolved spectroscopy, structural studies and device level studies are combined indicates the importance of both processing and impacts of interface electronics and carrier dynamics. Results on the extension of existing stable architectures to mini-module devices will be presented along with performance data for these systems.

Organic-inorganic hybrid perovskites are novel photovoltaic materials with high power conversion efficiency over 22% and low-cost solution processability. However, scaling up of perovskite fabrication remains a challenge due to the complex fluid dynamics within perovskite precursor solution when drying. Here, we show the fliud flow pattern observed by in-situ microscopy and report that surfactants additives can dramatically suppress the flow. The surfactant additives enabled the deposition of uniform, full-coverage perovskite film at a coating rate of 180 meter per hour and resulted in stabilized module efficiencies of 15.3% and 14.6% measured at aperture areas of 33.0 and 57.2 square centimeter, respectively. The result indicates that surfactants could be a kind of general additives in perovskite inks for improving perovskite film quality in scalable solution coating methods.

Two dimensional metal halide perovskites have demonstrated exceptional stability and performance in various optoelectronic applications. A key to higher performance is to align the insulating organic layers vertical to the substrates to avoid inhibition of charge transport. Several fabrication routes have been presented in literature but there is not yet a general strategy to orient 2D perovskite sheets vertically to the electrodes, due to lack of understanding of the crystallization process. In our research, based on our previous discovery that vertically oriented 2D perovskite crystallization can occur at liquid-air interface (Nature Communications 9, 1336 (2018)), we found that a low supersaturation during 2D perovskite crystallization is crucial for a strong degree of crystallographic orientation, confirmed by our in-situ grazing incidence X-ray diffraction measurement on thin films formed from different supersaturation environments. This understanding leads to a general strategy to fabricate vertically oriented 2D perovskite thin films through rational selection of solution formulation, organic spacers and processing conditions. It also allows control of the degree of orientation from complete random to complete vertical orientation. With this strategy we demonstrate vertically oriented thin films with various organic spacers and devices with high performance and long term stability.

Organic-inorganic lead-based halide hybrid perovskites (PVK) have attracted tremendous attention in recent years because of their remarkable optoelectronic properties and their unprecedented potential in inexpensive photovoltaic (PV) applications. Despite the rapid progress in the PV sector, however, use of the technology in areas such as microelectronics, and particularly thin-film transistors (TFTs), has been plagued by relatively low carrier mobility, high threshold voltages and moderate channel current ON/OFF ratio. In this work, density gradient ultracentrifugation technique was used to first sort polychiral semiconducting single-walled carbon nanotubes (s-SWCNT), and secondly to incorporate them into mixed-cation perovskite (MA_1-xFA_x)Pb(I_1-xBr_x)₃channel layers to produce TFTs with significantly enhanced performance characteristics. Optimised transistors are shown to combine low voltage operation (-1 V) with high carrier mobility (32.25 cm²/Vs) and exceptionally high channel current ON/OFF ratio (10⁷). The low threshold voltage and low sub-threshold slope(225 mV/dec) of the PVK/s-SWCNT-based TFTs enable the fabrication of high-performance inverters (NOT gates) and memristive transistors (memtransistors). Furthermore, temperature-dependent charge transport measurements reveal that the PVK/SWCNT-based TFTs undergo a change in the transport mechanism from ambipolar, in the high temperature range (300 – 160 K) to strictly unipolar at low temperatures (160 – 80 K), revealing the interplay of carrier injection, trapping and emission in such mixed-dimensional channels. The activation energy of holes was estimated to be 99.1 meV, highlighting the important role of defects in metal-halide perovskite materials. The combination of processing versatility, high charge carrier mobility, low-voltage operation and high current ON/OFF ratio highlight the potential of mixed-dimensional PVK/SWCNT systems for application in large-area microelectronics.

Perovskite compounds exhibit exquisite electronic features for photovoltaic applications. Incorporation of those materials into solid state solar cells allowed the recording of very high power conversion efficiencies (PCEs) above 22%, which are comparable to the current commercial available materials^[1]. However in order for perovskite compounds to reach eventually the market, some severe limitations have to be addressed. Those are not other than their inherent environmental instability and their composition of toxic elements (e.g. Pb)^[2]. In this work we address both those important issues by the discovery of a new family of 3D perovskites, namely “hollow” perovskites, with chemical formula (A)_1-x(en)_x(M)_1-0.7x(I)_3-0.4x, (A = methylammonium (MA), formamidinium (FA); M = Sn, Pb, en = ethylenediammonium)^[3-4]. Incorporation of en cations in the 3D perovskite structure leads to massive M and I vacancies in the 3D [MX₃] framework, thus the term hollow. By adjusting the percentage of en in the structure we were able to fine tune the optical properties of the corresponding materials, maintaining at the same time the desired 3D structure dimensionality. These hollow perovskites exhibit significantly blue-shifted direct band gaps over a very wide energy range, from 1.25-1.51 eV for Sn-based perovskites and from 1.53-2.1 eV for the Pb-based analogues. DFT calculations revealed that as metal halide fragments are eliminated from the perovskite structure, the bands themselves become less disperse (i.e. narrower) due to the reduced lengths of fragments with M-I overlap. A most important outcome from this synthetic strategy is the superior enhancement of the air stability of the corresponding materials. The Sn based (MA)_0.6(en)_0.4(Sn)_0.72(I)_2.84 perovskite is stable in air for at least 9 days, while the (FA)_0.61(en)_0.39(Pb)_0.727(I)_2.844 perovskite is stable in air for more than 300 days. Both lifetimes are the highest reported for a 3D ASnX₃ and a 3D FAPbI₃ phase respectively. This family of perovskite compounds poses as a new platform of promising light absorbers that can be utilized in single junction or tandem solar cells.

References
[1] Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D., Prog. Photovoltaics Res. Appl. 2016, 24, 905.
[2] Stranks, S. D.; Snaith, H. J., Nat. Nanotech. 2015, 10, 391.
[3] Ke, W.; Stoumpos, C. C.; Zhu, M.; Mao, L.; Spanopoulos, I.; Liu, J.; Kontsevoi, O. Y.; Chen, M.; Sarma, D.; Zhang, Y.; Wasielewski, M. R.; Kanatzidis, M. G., Sci. Adv. 2017, 3, e1701293
[4] Spanopoulos, I.; Ke, W.; Stoumpos, C. C.; Schueller, E. C.; Kontsevoi, O. Y.; Seshadri, R.; Kanatzidis, M. G., J. Am. Chem. Soc. 2018, 140, 5728.

Lead halide perovskite (CsPbX₃, X = Cl, Br, or I) quantum dots (QDs) have recently attracted considerable interest for light-emitting device (LED) applications such as thin film displays and solid-state lighting, owing to electroluminescence emission with narrow full width at half maximum (FWHM), tunable color properties by anion exchange method, and ease of solution processability (1,2). The optical properties of perovskite QDs as ionic nanocrystals are greatly affected by highly polar washing solvent due to occurring cation- and anion-defects (3). In this work, we achieved low driving voltage and high efficiency perovskite QD-LED using Gel permeation chromatography (GPC) with nonpolar solvent to remove excess ligand such as oleic acid (OA), oleylamine (OAM) and synthesize solvent 1-octadecene (ODE). We confirmed completely remove these impurities by GPC in contrast to conventional reprecipitation process. In addition, we demonstrated the effect of interfacial layer between the hole transport layer and the perovskite QDs using alkyl ammonium salts containing the Br anion, oleylamine bromide (OAM-Br) to passivate cation- and anion-defects. The LED based on OAM-Br interfacial layer exhibited higher efficiency compare to the LED without OAM-Br layer due to the suppression of interfacial cation- and anion-defects.
Reference: (1) L. Protesescu, S. Yakunin, M. I. Bodnarchuk, F. Krieg, R.Caputo, C. H. Hendon, R. X. Yang, A. Walsh, and M. V. Kovalenko, Nano Lett., 2015, 15, 3692. (2) J. D. Roo, M. Ibanez, P. Geiregat, G. Nedelcu, W. Walravens, J. Maes, J. C. Martins, I. V. Driessche, M. V. Kovalenko, and Z. Hens, Adv. Mater. 2016, 28, 8718. (3) T. Chiba, K. Hoshi, Y-J. Pu, Y. Takeda, Y. Hayashi, S. Ohisa, S. Kawata, and J. Kido, ACS Appl. Mater. Interfaces, 2017, 9, 18054.

Metal halide perovskites are excellent optoelectronic materials that are quite suitable for thin-film solar cells, light-emitting diodes (LEDs), and lasers. This is because they exhibit strong optical absorption, high luminescence efficiencies, long carrier lifetimes, long diffusion lengths, and low densities of nonradiative recombination centers [1]. Recently, unique properties of halide perovskites have been discovered, e.g., photon recycling, ion migration, and local rearrangement of molecular dipoles [2-4]. These properties are expected to lead to the development of new-type optoelectronic devices.
One of the most important properties of halide perovskites is that they crystallize in low-temperature solution processes. In particular, it is remarkable that large-size single crystals of halide perovskites can be grown in low-temperature solution exhibiting high optical transparency and exceptionally low surface light scattering. These optical properties are beneficial for novel optical transmittance applications in the broad spectral range from visible to infrared.
In this work, we studied the photocarrier-induced refractive index change in organic-inorganic hybrid perovskites. The crystal used in this study was a methylammonium lead bromide (MAPbBr3) single crystal. To measure the photorefractive properties of the halide perovskite, we developed an interferometric system that is synchronized with photoexcitation. The photorefractive phase shift was measured by detecting the phase shifts of the transmitted light through the perovskite single crystal mounted in the interferometric system. From the time-resolved measurements and excitation-pulse fluence dependence of the photorefractive phase shifts, we clarified that photogenerated carriers cause extremely large and long-lived changes in the refractive index. This result shows that the perovskite single crystal works as an optical phase shifter. Moreover, we demonstrated that infrared laser light are tunable to any desired polarization configuration by employing the photorefractive phase shift. Our demonstrations provide new pathways to develop optical devices with organic-inorganic hybrid perovskites leading to variable wave plates, optical switches, and phase modulators.
Part of this work was supported by JST-CREST (JPMJCR16N3) and JSPS KAKENHI (18K13481).

[1] Y. Kanemitsu, J. Mater. Chem. C 5 3427 (2017).
[2] T. Yamada et al., Phys. Rev. Applied 7 014001 (2017).
[3] D. W. deQuilettes et al., Nat. Commun. 7, 11683 (2016).
[4] Y. Chen et al., Nat. Commun. 7, 12253 (2016).
[5] H. Tahara et al., Adv. Opt. Mater. 6, 1701366 (2018).

Resistive random access memory (ReRAM) based on organohalide perovskite (OHP) shows excellent properties compared to conventional metal oxide resistive memory such as low operating voltage, high on/off ratio, and fast switching speed in several experimental demonstrations. Nevertheless, this technology is still in the premature stage. The design strategy of the active material and the device architecture must be established for the OHP ReRAM to be promoted to the stage of the commercial development. Thus, a device simulation based on finite element method is necessary to investigate the optimal device architecture on constituent materials and geometry. In this work, we will present a stochastic finite element simulation of conductive filament formation to perform reverse engineering on the material parameters of the OHP, in other words, to estimate which material parameters critically affect switching characteristics. In addition, the effects of device geometry and the nature of OHP/metal contacts were also studied.

In spite of the unprecedented advance of MAPbI₃-based perovskite solar cell, there are still remaining issues to be resolved for its industrial applications. Especially, hysteresis in current-voltage (I-V) curve is one of big challenges since it can limit its potential large scale application by causing abnormal efficiency drop. Various studies have proposed different physical origins such as ferroelectric polarization, charge trapping/detrapping and ion migration, none of study has clearly explained the microscopic origin of the hysteretic behavior in MAPbI₃-based perovskite solar cell. In this study, thus we have employed first-principles density functional theory calculations to identify the atomistic origin of hysteresis in MAPbI₃. Our study has found that excess electrons or holes can stabilize two different Iodine Frenkel defect structures in MAPbI₃. Since excess charge carriers can be easily accumulated near the electrode interface, the formation of two different Iodine Frenkel defects is inevitable and reversely it can act as a charge trap in MAPbI₃-based solar cell. Thus, the hysteretic behavior of I-V curve is a result of charge trapping and detrapping during the formation of two types of Iodine Frenkel defects near electrodes. Based on our understanding, we have suggested several possible ways to suppressed the hysteresis in MAPbI3-based solar cell.

In the past decade, organic-inorganic halide perovskite solar cells (PVSCs) have achieved outstanding progress in power conversion efficiency. However, solution-processed polycrystalline perovskite thin films possess a significant number of defects. Accurately probing the defect states distribution and understanding the origins of the defects are critical for further improving the device performance. Thermal admittance spectroscopy (AS) is a powerful characterization technique to determine the energetic distribution of trap states for different inorganic (e.g., CdTe, Cu(In, Ga)Se₂, etc.) and organic thin film solar cells, and has recently been used to characterize PVSCs. Interestingly, some unique trap signatures were observed in the AS measurements of PVSCs, but their origin has yet to be determined.
Here, we perform AS measurements on the PVSCs in the n-i-p planer structure to determine the impact of electron and hole transporting layers (ETL and HTL) on the trap states. We measured PVSCs with and without the SnO₂/C₆₀-SAM ETL and the spiro-OMeTAD HTL as well as devices with different HTL thicknesses, doping densities, and materials. The AS measurements at different DC bias voltages are employed to identify whether the trap features are originated from the bulk or interface defects. Our results show that defect states are less dependent on the ETL but strongly affected by the HTL. Additionally, we compared the admittance spectra of the n-i-p and p-i-n devices with the same absorber but different ETLs/HTLs. The devices exhibit different capacitance signatures, indicating that defect states probed by AS are affected by the ETLs/HTLs. More importantly, we identify that the trap states that were originally attributed to perovskite absorber layer by previous studies may likely be originated from the spiro-OMeTAD HTL. Our approach provides insights on the defect state measurement using AS and the limitations of this method.

Lead halide perovskites (ABX₃) have been attractive materials for various applications in optoelectronics such as solar cells and light emitting diodes (LEDs). The quantum efficiency of green and red perovskite LEDs has reached over 10% within the last few years.^1,2 The composition of the perovskite has been shown to play a crucial role on their optical properties. Recently, several groups including ours, found that doping the B cation site with Mn²⁺ can significantly improve photoluminescence quantum yields of perovskite quantum dots.³ In addition to the Mn²⁺, many other B cations have been suggested as potential dopants, however, only a few other cations have been studied in perovskites thus far.⁴ In this work, we have explored a variety of cations with different atomic sizes and various oxidation states from +1 to +3 as dopants in perovskite. We have grown bulk perovskite crystals using these dopants and studied the changes in crystal structure, and optical properties. In a poster, I will discuss the impact of the dopants, both the type of the dopant and its concentration on the optical properties of perovskites. Our finding will eventually lay-down a platform for high performance, Pb-less perovskite light emitting diodes.
References
1. Yang, X. et al. Efficient green light-emitting diodes based on quasi-two-dimensional composition and phase engineered perovskite with surface passivation. Nat. Commun. 9, 570 (2018).
2. Xiao, Z. et al. Efficient perovskite light-emitting diodes featuring nanometre-sized crystallites. Nat. Photonics 11, 108 (2017).
3. Parobek, D. et al. Exciton-to-Dopant Energy Transfer in Mn-Doped Cesium Lead Halide Perovskite Nanocrystals. Nano Lett. 16, 7376–7380 (2016).
4. Swarnkar, A., Mir, W. J. & Nag, A. Can B-Site Doping or Alloying Improve Thermal- and Phase-Stability of All-Inorganic CsPbX3 (X = Cl, Br, I) Perovskites? ACS Energy Lett. 3, 286–289 (2018).

Organic-inorganic lead halide perovskites have emerged as remarkable photovoltaic (PV) absorber materials in recent years. To date, polycrystalline thin-film perovskite photovoltaic devices have reached power conversion efficiencies reaching 22.7 %.¹ However, the involvement of a toxic element of lead and long-term instability are still the main issues in the large-scale commercial application of perovskites.

With excellent long-term durability and moisture tolerance, the possibility of flexible tuning of electronic properties, and coupled to low cost synthesis routes, the two dimensional (2D) hybrid halide perovskites have recently attracted much attention, and have been successfully used as light absorbers in efficient photovoltaic devices.^2,3 2D hybrid halides perovskites which feature long chain alkylammonium cations are generally suggested for light emitting diode applications, dues to their strong photoluminescence (PL),⁴ but none have been applied thus far for photovoltaic applications. Additionally, the previous study has shown the luminescence quenching in the Pb-based analogues, (AEQT)PbX₄ (X = Cl, Br, I), the energy transfer and charge separation between organic and inorganic components of the structures are still not fully understood.^4,5

Herein, we investigate the geometrical, electronic and optical properties of the semiconducting 2D perovskites (AEQT)BX₄ (B = Pb, Sn; X = Cl, Br, I), using relativistic hybrid density functional theory calculations. We demonstrate that unlike the traditional 2D perovskites, the choice of the organic ammonium cation has a considerable effect on the carrier transport properties, and the energy transfer between the organic and inorganic components is symmetry-disallowed. The electronic structures of the series are flexibly tailored by different halides and metal cations, with band gaps from 2.06 to 2.68 eV. Unique energy level alignments greatly hinder the electron-hole recombination in (AEQT)PbCl₄, (AEQT)PbBr₄, (AEQT)PbI₄ and (AEQT)SnBr₄, and thereby enhance the PL efficiencies. With a moderate fundamental band gap (2.06 eV) and strong direct valence band to conduction band transition, (AEQT)SnI₄ is the only composition that shows intense and broad optical absorption, and as expect displays a high spectroscopic limited maximum efficiency (SLME) of 21.9 %. Our results indicate the (AEQT)SnI₄ is a promising class of stable and efficient light-absorbing materials for photovoltaics.

1. https://www.nrel.gov/pv/assets/images/efficiency-chart.png. (Access 12^th June 2018)
2. A. M. Ganose et al., Chem. Commun., 2017, 53, 20.
3. I. C. Smith et al., Angew. Chem. Int. Ed., 2014, 53, 11232.
4. Mitzi, D. B. et al., Inorg. Chem. 1999, 38, 6246.
5. Chondroudis, K. et al., Chem. Mater. 1999, 11, 3028.

In conventional organic light emitting diode (OLED), aluminium quinolone (Alq₃) has been widely used as an electron transporting layer (ETL) in the past. And in the perovskite light emitting diode (pLED) deivice, 2,2′,2"-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) has been generally used as an ETL because it has better ETL property based on the higher electron mobility than Alq3 in the device. However, TPBi has disadvantage in terms of device life time compared to the Alq₃ because of the inferior thermal property. Therefore, we designed and synthesized new organic electron transporting materials for perovskite light emitting diode (pLED) which includes thiazolo[5,4-b]pyridine and benzo[h]quinolone moieties, respectively. We report the pLED device performance having new two ETL compounds as well as TPBi. The device configuration is ITO/PEDOT:PSS-PSS:Na/perovskite of phenylethylammonium-formamidiniumPbBr₃/trioctylphosphine oxide/ETL/LiF/Al. When new ETL materials were used, it showed high luminance efficiency of more than 10 Cd/A in green PLED device.

Organic-inorganic hybrid semiconductors with a perovskite lattice have recently attracted great attention due to their extraordinary performance in photovoltaics and excellent light-emission properties.^[1,2] Among them, unique layered structures exist building up from atomically thick quantum wells, each of which has one layer of inorganic octahedra that is sandwiched by organic layers of long chain ligands. As a result, couple electron-hole pairs (excitons) experience strong dielectric confinement, leading to tightly bound excitons with strong emission even up to room temperature.^[3]

In this work, pressure induced changes in the optical emission of two-dimensional perovskite crystals ((PEA)₂PbI₄), has been systematically studied at room temperature by several optical techniques, including photoluminescence (PL), time-resolved PL and Raman spectroscopy, all as a function of pressure. The crystals are pressurized inside a diamond anvil cell (DAC). At the pressure range of 0 to 3.5 GPa, the photoluminescence, originally at spectral position of ~2.3 eV, continuously shift to lower energies (red shift), and exhibits an ultra-wide tunable energy range of up to 350 meV spanning nearly the whole visible spectrum. Moreover, this energy tunability is fully reversible. In the used pressure range, the intensity of the emitted light is almost constant, while the PL lifetime keeps decreasing when increasing pressure, which implies that the efficiency of radiative recombination of excitons might be enhanced by pressure. First-principles simulations and X-ray diffraction (XRD) by synchrotron radiation both indicate that strong anisotropic compression along in-plane and out-plane directions should account for the red-shift of the band gap. Such a large optical tunability and constant emission quality within a relatively applied low pressure, has the potential to expand the applications of two-dimensional lead halide perovskite crystals in photonic and optoelectronic devices.


[1] H. Tsai, W. Nie, J.-C. Blancon, C. C. Stoumpos, R. Asadpour, B. Harutyunyan, A. J. Neukirch, R. Verduzco, J. J. Crochet, S. Tretiak, L. Pedesseau, J. Even, M. A. Alam, G. Gupta, J. Lou, P. M. Ajayan, M. J. Bedzyk, M. G. Kanatzidis, and A. D. Mohite, Nature 536, 312 (2016).
[2] H. Zhu, Y. Fu, F. Meng, X. Wu, Z. Gong, Q. Ding, M. V. Gustafsson, M. T. Trinh, S. Jin, and X. Y. Zhu, Nat. Mater. 14, 636 (2015).
[3] M. D. Smith, L. Pedesseau, M. Kepenekian, I. C. Smith, C. Katan, J. Even, and H. I. Karunadasa, Chem. Sci. 8, 1960 (2017).

While tremendous efforts have been made for developing thin organic lead halide perovskite films suitable for a variety of potential photoelectric applications such as solar cells, field-effect transistors, and photodetectors, only a few works have focused on the micro or nanopatterning of perovskite films which is one of the most critical issues for large area and uniform micro or nanoarrays of perovskite-based devices. At present, patterning is only feasible at microscale, and at sub-100nm scale lithography of thin perovskite films has not yet been reported. Here, we demonstrate perovskite patterning at sub-100 nm scale without losing structural integrity and optical properties of the perovskite by an easy and simple method using self-assembly of the block copolymers. In order to make perovskite selectively segregated into one block, we utilized diblock copolymer, polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) consisting of non-interacting PS block and interacting Lewis base polymer P2VP block. By controlling the volume fraction (f_P2VP) and the amount of perovskite, the effective volume fraction (f_{eff, P2VP}) can be determined and various self-assembled nanostructures such as spheres, cylinders, lamellae could be successfully formed. The domain size of a perovskite crystal in the pattern could also be controlled by simply changing the molecular weight of the block copolymers. Our simple but highly controllable, nanoscale patterning of perovskites by self-assembly of block copolymers is promising and will enable applications of perovskite to highly integrated optoelectronic nanoscale devices.

Methylammonium lead halide perovskites have been applied to versatile applications owing to their attractive optoelectronic properties such as long carrier diffusion length, am-bipolar conductivity, broad color-tunability exchanging by halide ions. These potentials for applications with high performance are based on perovskite electro-optical features and they are expected to be used as a next generation and solution processable semiconductor materials¹. Recently, bright luminescence from methylammonium lead tri-bromide (MAPbBr₃) perovskite nanocrystals (PeNCs) has been reported owing to the development of methods for preparing these PeNCs. Especially, MAPbBr₃ PeNCs were prepared by ligand-assisted reprecipitation² (LARP) inspired by the reprecipitation method for preparing organic and nano/micro crystals. LARP can provide uniform MAPbBr₃ PeNCs with narrow emission, which applies for successful implementation of PeNCs into LED.
Herein, we propose Ostwald ripening as a size-tunable technique for MAPbBr₃ PeNCs using LARP. In a typical Ostwald ripening process, large crystals absorb solute from small ones in dispersion, as a result, large crystals grow bigger, and small crystals shrink. MAPbBr₃ PeNCs could be size-controlled from several tens of nanometer size to 4 nm through shrinking of MAPbBr₃ PeNCs and their PL peaks were consequently blue shifted from 514 nm to 457 nm. This suggests that Ostwald ripening can be expected to be an effective method for preparing PeNCs in the low nanometer size range.

References
1) B. R. Sutherland, et al., Nature, 2016, 10, 295-302.
2) F. Zhang, et al., ACS Nano, 2015, 9, 4533-4542.

Semiconductor colloidal nanocrystals (NCs) are efficient fluorescence emitters, whose emission wavelengths can be tuned by varying their sizes and chemical compositions. Recently, lead halide NCs with perovskite lattices have opened access to the deep-blue and green regions of the electromagnetic spectrum [1], where traditional II-VI nanocrystals such as the prototypical CdSe, undergo rapid degradation.

In this work, we investigate the optical properties of all-inorganic CsPbX₃ (X = Cl, Br, I, Cl_aBr_3-a and Br_aI_3-a) perovskite nanocrystals. Specifically, we focus on their ability to convert low-energy into high-energy photons in a so-called upconversion photoluminescence (UCPL) process. Overall, all the investigated nanocrystals exhibit robust and efficient UCPL, characterized as a function of temperature, excitation energy and laser power. The UCPL phenomenon takes place in two distinguishable ways: (i) multiple-photon absorption and (ii) one-photon with the subsequent lattice vibrational (phonon) absorption. The latter mechanism demands an energy of up to 200 meV from the thermal bath, equivalently to the total energy of about ten optical phonons of CsPbX₃ compounds. However, after the first few consecutive absorption steps, the finite optical phonon population in the nanocrystals would become deficient, therefore decreasing the UCPL efficiency. We argue that the annihilation of multiple low-energy phonons creates a high-energy vibration, which repopulates the optical phonon bath and consequently increases the light upconversion probability [2]. This energy recycling mechanism is particularly strong in semiconductors having low thermal conductivity, such as lead halide perovskites [3]. Our work explains for the outstanding laser cooling effect in these materials [4] and reveals the potential of high-quality CsPbX₃ nanocrystals for several applications such as bioimaging, photovoltaic light-energy harvesting and optical refrigerators.

References
[1] L. Protesescu et. al., Nano Lett. 15 (6), 3692 (2015).
[2] T. H. Do, A. Granados del Aguila et al., to be submitted.
[3] R. Heiderhoff et al., J. Phys. Chem. C, 121 (51), 28306 (2017).
[4] S. T. Ha et. al., Nat. Photon. 10, 115 (2015).

Research on lead halide semiconductors with perovskite lattices is a rapidly growing field in nanoscience and semiconductor physics. They have great potential for low-cost yet efficient solar cells and light-emitting devices [1,2]. Their optical properties can be tuned by tailoring the chemical composition and/or the nanostructure spatial dimension with very high precision and high quality. For example, the perovskite semiconductors can grow in stable layered structures, which comprise alternatively stacked layers of lead-halide octahedra and long-chain organic molecules. The inorganic framework, sandwiched between two organic layers, forms an atomically thin quantum well with macroscopic continuity.

Herein, we study the optical properties of high-quality two-dimensional (PEA)₂PbI₄ crystals. Four intrinsic optical transitions are resolved in the luminescence spectrum, originating from the radiative recombination of coupled electron-hole pairs (excitons) [3]. Intriguingly, the light emission is dominated by two excitons, each of which is split into two linearly polarized and orthogonal states. Their energy splitting is in the range of ~1-2 meV, which is much larger than that of perovskite nanocrystals [4]. The highly anisotropic excitons may be resulted from the spin-orbit coupling and the exceptionally strong electron-hole exchange interaction. Moreover, at the low-energy side of the two doublets, we observe a feature arising from the recombination of a bright exciton. This line shows a circular dichroism, which can be induced either by circularly polarized excitation or by external magnetic field. This work is in line with the previous literature, where multiple excitonic features were resolved in bulk-like lead halide perovskites [5]. The observed energy landscape cannot be fully explained by available theoretical schemes for this material family. Our findings provide essential parameters for establishing a complete physical picture that governs the outstanding optical properties of two-dimensional layered perovskite semiconductors.

References
[1] A. Kojima et al., J. Am. Chem. Soc., 131, 6050 (2009).
[2] Z. K. Tan et al., Nat. Nanotechnol., 9, 687 (2014).
[3] T. H. Do, A. Granados del Aguila et al., to be submitted.
[4] C. Yin et al., Phys. Rev. Lett., 119 (2), 026401 (2017).
[5] T. H. Do, A. Granados del Aguila et al., Phys. Rev. B, 96, 075308 (2017).

Low-cost hybrid organic-inorganic perovskites such as methylammonium lead iodide(MAPbI₃) have been developed intensively due to those high absorption coefficient and easily tunable band gap, and easily fabricated by various deposition method such as solution-based process. However, solution-processed device using hybrid organic-inorganic perovskite is unstable and sensitive to water and ambient moisture. Therefore, the device stability is one of most important properties to achieve high performance hybrid organic-inorganic perovskite based devices.
In this paper, the electrical characteristics and stability of phototransistors using MAPbI₃ active layer were investigated. MAPbI₃ thin-film fabricated by chemical vapor deposition (CVD) with MAI and PbI₂ sources, and its physical, chemical, optical properties were compared with the solution-processed MAPbI₃ thin-film. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) revealed that solution-processed MAPbI₃ thin-film was easily transformed (from MAPbI₃ to PbI₂) after 7 days later when exposed to air. However, CVD-grown MAPbI₃ thin-film showed great air stability maintaining its chemical composition and crystalinity above 1 month. Futhermore, atomic force microscope (AFM) and scanning electron microscope (SEM) indicated that CVD-grown MAPbI₃ thin-film has large grain size and small surface roughness as compared to solution-processed MAPbI₃. MAPbI₃ phototransistor exhibits lnsulating behavior in transfer characteristics, but ambipolar properties were observed when the visible light was irradiated. Finally, electrical stability such as negative bias stress (NBS), positive bias stress (PBS) of MAPbI₃ phototransistors were examined for the optoelectronic application.

Recently, colloidal lead halide perovskite nanoplatelets have emerged as promising semiconductor materials due to their exciting properties such as tunability, facile processability, and bright emission with high color purity. In particular, their quantum-and-dielectric-confined nature makes colloidal lead bromide perovskite nanoplatelets a favorable candidate for the next-generation deep-blue-emitting (437nm) material platform. However, for light-emitting applications, poor photostability is one of the critical challenges that those nanoplatelets face. When exposed to UV excitation, they either suffer from photobleaching or transform into thicker–more bulk-like–structures with red-shifted emission.
In this study, we systematically investigate the factors that affect the photostability of the deep-blue-emitting perovskite nanoplatelets by monitoring the photoluminescence and absorption spectra over time. We find that freshness of the prepared precursor solutions for ligand-assisted reprecipitation is critical to obtain better stability with high photoluminescence quantum yield of perovskite nanoplatelets. Also, photobleaching is found to be the sign of intrinsic instability in nanoplatelets while transformation represents an extrinsic instability. Importantly, moisture is identified as the key extrinsic factor responsible for the transformation of nanoplatelets into more bulk-like structures. Furthermore, we observe that the substitution of the organic cation from formamidinium to methylammonium and addition of excess ligands significantly enhances the intrinsic stability of perovskite nanoplatelets. Lastly, we demonstrate that the dropcasted film of methylammonium-based nanoplatelets with excess ligands is impressively stable under ambient conditions and does not transform even under intense UV in the presence of moisture, as a result of improved intrinsic stability. This study expands our understanding of the factors that affect perovskite nanoplatelet photostability and opens up new possibilities for fabrication of stable perovskite-nanoplatelet-based optoelectronic devices with enhanced stability.

All-inorganic cesium lead halide perovskites quantum dots (QDs), CsPbX₃ (X = Cl, Br, I), have recently attracted much attention for use in light emitting devices (LEDs), given their high colour purity and narrow full width at half maximum (FWHM) over the entire visible wavelength range as well as their low cost solution processing (1, 2). Here, we demonstrate anion-exchange red perovskite QDs CsPb(Br/I)₃ from pristine CsPbBr₃ using ammonium iodine salts, long alkyl based oleylammonium iodide (OAMI) and aryl based aniline hydoroiodide (AnHI), for use in highly efficient LEDs. The ester solvent ethyl acetate, which has a low dielectric constant, is used as poor solvent in reprecipitation process to remove impurities and prevent surface defects in the perovskite QDs. The anion exchange CsPb(Br/I)₃ films exhibit a strong red shift in their of photoluminescence (PL) spectrum from the green emission at 508 nm in the case of the pristine QDs to one in the deep-red region at 649 nm owing to the replacement of Br anions by I anions in the perovskite QDs. The anion exchange CsPb(Br/I)₃ film based on OAMI shows a high surface coverage ratio and is free of pinholes, whereas that of AnHI based CsPb(Br/I)₃ exhibits a slightly rough surface owing to a reduction in the surface ligand, which results in the aggregation of the QDs. LEDs formed using the anion-exchange CsPb(Br/I)₃ based on OAMI show an remarkable high EQE of more than 20% as well as high color purity, with the Commission Internationale de l'Eclairage (CIE) at (0.72, 0.28), which completely cover BT.2020 color gamut. Similarly, the LEDs formed using the QDs based on AnHI show a peak EQE of 14.1% and CIE coordinates of (0.71, 0.28). Further, they exhibit longer operational stability as compared to that of LEDs formed using the OAMI based CsPb(Br/I)₃.

Reference:
(1) L. Protesescu et al., Nano Lett., 2015, 15, 3692.
(2) T. Chiba et al., ACS Appl. Mater. Interfaces, 2017, 9, 18054.

Thanks to the efforts of many research groups worldwide in halide perovskite (HaP) research, >22 % efficient small photovoltaic (PV) devices have been reported [1]. Such devices, based on HaPs with mostly iodide as halide, can show a remarkably low voltage loss (E_G-V_OC) of ~ 0.4 V. Unfortunately, up to now, for the higher bandgap HaPs, where the halide is only Br, the highest reported V_OC still leaves (E_G-V_OC) ~ 0.75 V [2]. This large loss severely detracts from the potential of HaPs that have enough Br substitution for I, for optimal use in the higher E_G-based cell for tandem configurations or spectral splitting systems. The origin of this increased loss remains an open question.
There are 3 main issues to be checked to understand what limits the V_OC from getting closer to the theoretical maximum value for pure APbBr₃-based PV cells (~2.0 V).
–1- what is the absorber’s in-gap density of states and what range of energies have these states in the material’s band gap;
-2- as -1-, for interfacial in-gap states @ the HaP-hole/-electron transport layer interface;
-3- what is the mismatch in energy level alignment of the cell’s layers, to see if this can explain the significant energetic losses?
Here we used (FM_0.85MA_0.1Cs_0.05)PbBr₃ (from hereon mixed-cation) as the photo-absorber. Previously we showed that this mixed-cation HaP has relatively long carrier diffusion lengths and low carrier density compared to single cation Br-based HaPs, more resembling the I-based HaPs, with which the small (E_G-V_OC) cells can be made.[1]
By using optoelectronic measurements such as Contact Potential Difference, Surface Photovoltage, Electroluminescence and Capacitance Voltage on mixed cation Br-based HaP stand-alone films, as well in different device configurations, we can now provide insights into the origins of the large (E_G-V_OC) for the BR-based HaP-based PV cells and suggest how these losses may be minimized.

Several discussions with Dr. Davide Ceratti (WIS) and Prof. Antoine Kahn (Princeton U) are gratefully acknowledged.

Reference
[1] M. Kulbak, I. Levine, E. Barak Kulbak, S. Gupta, A. Zohar, I. Balberg, G. Hodes, D. Cahen, Adv. Energy Mater.2018, 1800398.

Organic-inorganic lead halide hybrid perovskites have been receiving considerable attention for applications to optoelectronic devices such as photovoltaic cells and light emitting diodes because of their excellent electronic properties including strong absorption coefficient and long carrier lifetime [1]. The structure of hybrid perovskites can be tuned in various manners and classified by dimensionalities into three-, two- and one-dimensional (3D, 2D, and 1D) perovskites. Compared with 3D and 2D-layered crystals, the limited number of studies have been done for 1D crystals, while they are expected to show high and fast optical nonlinear effects owing to the enhancement of electron density of states and electron-electron interaction. Among the lead halides 1D perovskites, we have found that bromide compounds show some advantages for applications: stability in air and broadband emission of light. Here we report the structure of 1D lead bromide perovskites and structural dependence on their optical properties. We prepared three different structures (triple-chain, face, and side shared) by inserting various aromatic organic compounds. We observed the yellow, green, and white light luminescence depending on the structures of lead bromide perovskites. Among them, triple-chain lead bromide perovskite crystal showed extremely broadband white light photoluminescence similar to that of core-shell quantum wires structure [2]. The Stokes shift of the triple-chain lead bromide crystals examined in the present experimental was much larger than that reported previously for core-shell 1D wires. The results open possibility of new application of 1D bromide perovskites to optoelectronic devices on the basis of chemical modification.

[1] Y. Liu et al. Adv.Sci.2018, 5, 1700471.
[2] Z. Yuan et al. Nat. Commun., 2017, 8, 14051.

Two-dimensional perovskites are emerging for their light emitting properties and applications in lasing, metamaterials and nanophotonic devices.^1,2 The correlation between structure and optical properties is essential to improve the device performance and functionality and might lead to perovskite films with adjustable optical properties dynamically tunable by external stimuli. Phase transitions in Ruddlesden-Popper perovskites such as (BA)₂(MA)_n-1[Pb_nI_3n+1] (BA = butylammonium; MA = methylammonium),^3,4 in combination with the high flexibility of the lead halide lattice, can allow to modulate the optoelectronic properties in a controlled and reversible way. However, a thorough correlation between the thermal-induced structural changes and the optical properties is not yet established.
Here,⁵ we expose BA₂PbI₄ (n=1) and BA₂MAPb₂I₇ (n=2) to a wide temperature range (300-77K) and characterize two new low temperature phases of BA₂MAPb₂I₇. We combine spectroscopic and structural characterization with ab initio calculations to study their characteristic phase transitions and rationalize the structural changes affecting the optical properties and leading to a sharp thermo-optical response of their luminescence. Volume contraction at low temperature causes the increase of out-of-plane tilt of PbI₆ octahedra resulting in a sharp bandgap blue-shift. On top of this, the dimensionality further impacts on the thermal evolution of the volume expansion and tilt system of the perovskite, reversing in BA₂PbI₄ the trend of the continuous thermal shift of the band-gap typically observed in MAPbI₃.
Our results stress the importance of the structure-function relationship and show that heat-mode inter-conversion of the crystal packing can be exploited to design stimulus-responsive materials. Since the β→α phase transitions of these alkylammonium-based perovskites occur close to room temperature, they can be easily exploited to switch the structure-optical properties of the material.

[1] B. Gholipour et al, Adv. Mater. (2017), 29, 1604268
[2] B. R. Sutherland et al, Nat. Photonics (2016) 10, 295
[3] D. Cortecchia et al, Chem. Mater. (2017) 29, 10088
[4] C. C. Stoumpos et al, Chem. Mater. (2016) 28, 2852
[5] D. Cortecchia et al, to be submitted

Organometallic halide perovskite (PSCs) has recently emerged as promising cost-effective and highly efficient nanostructured solar cells. The organometallic halide perovskite such as CH₃NH₃PbI₃ has poor thermal and air stability. To avoid this problem, there are many researchers who used the three-dimensional-type perovskite crystal structure with mixed cation such as cesium (Cs) and methyl amine (MA). The mixed cation perovskite has reported high thermal stability with improvement of the power conversion efficiency (PCE). The inorganic cesium lead iodide perovskite (CsPbI₃) has been reported to have highly efficient optical and electrical properties. It should also be noted that the reported CsPbI₃ perovskite solar cells with highly efficiency has Cs₄PbI₆ in X-ray diffraction (XRD) measurement. In this study, we replaced MA with Cs and used vacuum deposition method to yield efficient inorganic planar heterojunction CsPbI₃ solar cells. The CsPbI₃ perovskite layer was formed by vacuum deposition of lead iodide (PbI₂) and cesium iodide (CsI). The perovskite stoichiometry depends on the ratio of CsI and PbI₂. The ratio of the codeposition rates of PbI₂ and CsI was adjusted to PbI₂/CsI molar ratio. The CsPbI₃ film of PbI₂/CsI molar ratio of 1was deposited for obtained ideal stoichiometry of CsPbI₃. In this case, a power conversion efficiency of 5.71% was obtained with a short-circuit current density (J_sc) of 13.50 mA/cm², an open-circuit voltage (V_oc) of 0.55 V, and a fill factor (FF) of 0.51. This ideal film has not only CsPbI₃ but also Cs₄PbI₆ was observed by XRD measurement. The Cs₄PbI₆ was considered to constitution of 3CsI and CsPbI₃. To obtain the pure CsPbI₃ perovskite film, we controlled the PbI₂/CsI molar ratio of 1 to 3. The pure CsPbI₃ perovskite film has J_sc of 13.50 mA/cm², V_oc of 0.55 V, and FF of 0.51, leading to PCE of 3.77%. These results indicated the effect of Cs₄PbI₆ to improve the solar cell performance of V_oc and FF in CsPbI₃ based perovskite solar cells.

The discovery of hybrid halide perovskites materials heralded a new era in optoelectronic technologies, with an unprecedented rise to above 20% in the efficiencies of photovoltaic devices in just a few years. However, several crucial issues, such as stability and toxicity, still need to be tackled before an industrial-scale use. Although the presence of Pb in archytypical perovskites is unlikely to be a barrier or pose any considerable environmental risk for use in PV electricity generation, Pb may prevent use in certain applications such as bio electronics and consumer electronics products. Therefore, several effort has been put in the research of a lead-free, stable all inorganic compound with improved thermal stability.

Recently, a new class of candidate photovoltaic materials – the double halide perovskites with chemical formula A₂BB'X₆ (A, B=monovalent cations, B’=trivalent cation, X=halogen) – has been identified as a possible alternative to the better-known APbX₃ perovskites. In these materials, the B/B’ cations (average valence 2+) substitute for lead and are fully ordered in a double perovskite superlattice. Of particular interest is Cs₂AgBiBr₆, which does not contain toxic elements, and is highly stable both structurally and chemically. Several studies exploring its implementation in devices have started to appear in the literature, yet very little is known about the relationship between the crystal structure and the opto-electronic properties of this double perovskite compound. Hence, there is an urgent need for a detailed understanding of these materials at a more fundamental level.

We have investigated the temperature-dependent structural behaviour of Cs₂AgBiBr₆ using heat capacity measurements, X-ray powder and single-crystal diffraction and neutron powder diffraction and discovered that this compound undergoes a low-temperature structural phase transition (T_S~122 K) from cubic to tetragonal. The crystal structures of both high- and low-temperature phases were refined based on our diffraction data. The temperature dependence of the exciton energy in proximity to the direct band gap was determined using reflectivity measurements. We found a direct, linear relationship between the tetragonal strain and the exciton energy, demonstrating that the latter is controlled by the BBr₆ octahedral rotation. Meanwhile time-resolved photoluminescence measurements indicated a qualitative change in the charge-carrier recombination mechanisms at a temperature that correlates well with the phase transition. Further absorption and photoluminescence spectral measurements probing the temperature dependence of the indirect bandgap transition will also be reported.

The bismuth based perovskites are low-toxic and air stable materials with promising photo-absorber properties. The detailed studies on these materials are important for further development of solar cells. In this work, we have fabricated the Bi-based perovskite materials (Cs₃Bi₂I₉, CsBi₃I₁₀) by solution process and investigated the crystal growth and optoelectronic properties of those materials. The XRD patterns of those materials suggest a hexagonal crystalline phase having different dominating diffraction peaks. The crystal quality is found to be affected by stoichiometric and growth temperature. The opto-physical properties show a bandgap of ~2.1 eV for Cs₃Bi₂I₉ thin film whereas it is quite smaller for the CsBi₃I₁₀ thin film (~1.75 eV). A device efficiency of over 1% is achieved for Bi-based perovskite device. Although the Bi-based perovskites are stable under air ambient, the poor film morphology and mixed crystalline phase are speculated to be deleterious for getting efficient solar cell device.

Recently, electrochemical impedance spectroscopy (EIS) on perovskite solar cells (PSC) has gained a lot of interest in the literature. The time constants are on a favorable timescale to enable high quality EIS measurements and the cells have become stable enough to allow for reliable and reproducible measurement results, making EIS a most promising tool for characterization and diagnosis of PSC. However, there are still elusive phenomena and the impedance response is not fully understood. Basically, two main processes are identified but there is no general agreement on their physical origin. Negative loops that appear in the medium and low frequency ranges further complicate a straight interpretation of the impedance response of PSC.

Here, we present a case study of a PSC based on a new compound material (MA_0.6FA_0.4PbI_2.8Br_0.2) that does not show a phase transition for relevant ambient conditions and has proven to exhibit better thermal stability than standard MAPbI₃. Measurements were conducted on samples based on either of these materials. Fresh samples as well as samples that have been subjected to a temperature treatment beyond the MAPbI₃ phase transition temperature of 328 K were used. All cells were characterized by EIS and intensity modulated photocurrent/-voltage spectroscopy (IMPS/IMVS). IMPS is usually only measured under short circuit conditions (SCC) and IMVS only under open circuit conditions (OCC). In contrast to this practice, we have measured EIS, IMPS and IMVS at various potentials including OCC and at different light intensities. We show that the three measurement techniques form a triplet where one of them can be calculated with good accuracy if the other two are available.

As a result of this study, we will show how impedance measurement under open circuit conditions (OCC) can help to distinguish between different mechanisms but are less sensitive to device degradation. Measurements at a voltage range between 0 Volts and OCC can help to quantify degradation, which is represented by a decrease in the impedance at voltages below the maximum power point (MPP) and by an increase beyond the MPP.

The similarity of EIS and IMVS results has been discussed in the literature. We will argue and demonstrate how a comparison of EIS and IMVS can help to distinguish between interface effects and the behavior of the bulk photoactive layers. This contribution will be complemented by a few practical guidelines for reliable photoelectrochemical impedance measurements, as this comprehensive study was performed on some of the most stable PSC for which a degradation of less than 20% after 500 hours has been demonstrated, even at an elevated temperature of 85°C.

Recently, observations of twin domain in methylammonium lead triiodide (MAPbI₃) have drawn significant attention. However, whether this twin domain is ferroelectric and/or ferroelastic remains unclear. In addition, previous investigations were limited to the ferroic properties of this twin domain, whereas, the chemical behavior which can correlate with either ferroelectricity or ferroelasticity, has rarely been studied. In this work, we unveil the correlation of ferroelastic domains and chemical variation in the MAPbI₃ twin domains using multiple functional imaging techniques. We unambiguously show the mechanical origin of piezoelectric-like contrast by using multiple advanced piezoresponse force microscopy techniques, suggesting the non-ferroelectricity of this twin domain. The combination of helium ion microscopy secondary ion mass spectrometry (HIM-SIMS) and nanoscale infrared spectroscopy (Nano-IR) indicates the ion segregation correlating with the twin domain. Emission excited by polarized light reveals large-scale ordering of crystallographic orientation/chemical make-up correlating with twin domain and its alternative interaction with light. Moreover, density functional theory (DFT) calculations provide a picture describing the interaction of elastic strain, chemical segregation, and ferroelasticity. This work unveils a new structural-chemical interplay in MAPbI₃, providing a new line of interpreting and understanding the ferroic, chemical, and optoelectronic behaviors of related HOIPs.

All-inorganic cesium lead halide perovskites CsPbX₃ (X = Cl, Br, I) have attracted much attention for their excellent optoelectronic properties. Since the nanocrystals (NCs) possess superior luminescent properties such as high photoluminescence quantum yields (PLQYs) at room temperature, these materials have been intensively studied for application in optoelectronic devices such as light emitting diodes and lasers [1]. Our previous studies have clarified that trions generated in perovskite NCs contribute to the nonradiative recombination and blinking phenomenon, leading to the reduction in the PLQYs [2]. However, the trion generation and recombination processes have not been fully understood yet. Controlling trion generation leads to the improvement in the PLQYs of these materials and is essential for achieving high-performance perovskite-based optoelectronic devices. Recently, it was reported that a surface treatment to the NCs improves the luminescence efficiency [3]. This previous study indicates that the surface state of these NCs may have great impact on their trion generation mechanisms [4].
In this work, we investigated the trion generation dynamics by modifying the surface state of cesium lead halide perovskite CsPbBr₃ NCs. Femtosecond transient absorption (TA) spectroscopy was employed to investigate the ultrafast carrier dynamics in these NCs. The sample used in this experiment was a solution of the NCs dispersed in octane. We used sodium thiocyanate (NaSCN) as a capping (surface-modifying) ligand and prepared both surface-treated and untreated samples. We analyzed the excitation power dependence of the differential TA signals for each sample, and compared the extracted components of excitons, trions, and biexcitons between the surface-treated and untreated samples. We clarified that the generation of trions are clearly suppressed by the surface treatment as observed the difference in excitation power dependence, while exciton and biexciton components exhibit the same. Furthermore, we found that two independent trion generation pathways exist by analyzing the excitation power dependence of trion components. We discuss the trion generation dynamics in CsPbBr₃ NCs from the viewpoints of extrinsic surface traps and intrinsic Auger recombination.
Part of this work was supported by JST-CREST (JPMJCR16N3).

[1] Protesescu L. et al., Nano Lett. 2015, 15, 3692.
[2] Yarita N. et al., J. Phys. Chem. Lett. 2017, 8, 1413.
[3] Koscher B. A. et al., J. Am. Chem. Soc. 2017, 139, 6566.
[4] Yarita N. et al., J. Phys. Chem. Lett. 2017, 8, 6041.

Mn-doped all-inorganic perovskite quantum dots (QDs) provide prominent applications in the fields of low-cost light source or LEDs, because of their remarkable properties including dual-color emission and reduced lead content, as well as high photoluminescence quantum yields (PLQYs) and high stability. The real dual-color emission with two strong emission peaks in individual crystals is particularly promising in the application of white LEDs. However, the existing approaches for synthesis of all-inorganic perovskite QDs with real dual-color emission and high Mn substitution ratio usually require hash conditions, such as high temperature and nitrogen protection, which is a major hurdler for the practical manufacturing. Here we present a gram-scale approach to synthesize both CsPb_xMn_1−xCl₃ and CsPb_1-xMn_xCl_yBr_3-y QDs at 100°C in the air with high Mn substitution ratio, up to 55.64% atomically. The as-prepared CsPb_1-xMn_xCl_yBr_3-y QDs exhibit high PLQYs of 62.41% and dual-color emission with two strong emission peaks around at 400 - 460 nm and 600 nm, respectively. Furthermore, the unique advantage of the optical emission and high PLQYs properties of the CsPb_xMn_1−xCl₃ QDs has been demonstrated as invisible ink for encryption application and polymer composites. Our gram-scale synthesis approach for Mn-doped all-inorganic perovskite QDs may boost the future research and practical application of QDs-based white LED, spintronics, and molecular barcoding.

Solution process is the most widely used method to prepare perovskite absorbers for high performance solar cells due to its ease of fabrication and low capital cost. However, an insufficient level of reproducibility of the solution process is often a concern. Complex precursor solution chemistry is likely one of the main reasons for the reproducibility issue. Here we report the effects of triple cation lead mixed-halide perovskite precursor solution aging on the quality of the resulting films and the device performance. Our study revealed that precursor solution aging has a great influence on the colloidal size distribution of the solution, which then affects the phase purity of the films and device performance. We determined the optimum aging hours that led to the best device efficiency along with the highest reproducibility. Dynamic light scattering revealed the formation of micron-sized colloidal intermediates in the solution when aged longer than the optimum hours and further analysis along with X-ray diffraction measurements suggested there were two chemical origins of the large aggregates, FA-based and Cs-based complexes.

Lead-halide perovskite semiconductors MAPbX₃ (MA = CH₃NH₃, X = I, Br, and Cl) attract attention as a new class of photonic device materials [1]. The energy conversion efficiency of MAPbI₃ thin-film solar cells has been improved rapidly and is now reaching to 22.7%. The sharp optical absorption edge and high-efficient band-to-band light emission of perovskites are the key factors for such high conversion efficiency. Due to these superior optical properties, a unique phenomenon of repeated photon emission and reabsorption, so-called photon recycling, appears remarkably in perovskite single crystals [2-4].
A wide bandgap perovskite MAPbCl₃ is attractive for optical devices in blue spectral region. Particularly, for laser and optical switch applications, a nonlinear optical response of the material also becomes more crucial. However the nonlinear optical responses in MAPbCl₃ are less explored. With this in mind, we conducted the measurements to obtain the nonlinear refractive index and nonlinear absorption coefficient for the wide wavelength range. Thin-film perovskite samples are usually influenced by the polycrystalline grain structure, making it difficult to measure intrinsic properties [1]. Thus, we employed perovskite single crystals in order to eliminate such influences and determine the nonlinear optical coefficients.
The single crystal samples used in this study were prepared by antisolvent vapor diffusion method. The nonlinear refractive index and nonlinear absorption coefficient were determined by performing Z-Scan method with variable incident laser intensity. Moreover, the wavelength dependence of the nonlinear optical coefficient was measured by changing the incident laser wavelength. Furthermore, we measured two-photon photoluminescence excitation (PLE) spectrum of MAPbCl₃. The wavelength dependence of the nonlinear absorption coefficient can also be obtained from the two-photon PLE spectrum. We observed the trend that the nonlinear absorption coefficient increases at shorter wavelengths [5]. Our results contribute to understanding the nonlinear optical properties of the lead-halide perovskites and lead to advanced optical applications.
Part of this work was supported by JST-CREST (JPMJCR16N3).

[1] Y. Kanemitsu, J. Mater. Chem. C 5, 3427−3437 (2017).
[2] Y. Yamada et al., J. Am. Chem. Soc. 137, 10456−10459 (2015).
[3] T. Yamada et al., Adv. Electron. Mater. 2, 1500290 (2016).
[4] T. Yamada et al., Phys. Rev. Applied. 7, 014001 (2017).
[5] T. Yamada et al., Phys. Rev. Lett. 120, 057404 (2018).

Transition metal perovskite chalcogenides are promising materials for photovoltaic applications with excellent optoelectronic properties, stability, and rich tunability. Recent experimental studies have revealed their promising potential, including band gap tunability all the way from 2.1 eV to 1.3 eV. We report in depth optical and X-ray spectroscopic study of Ruddlesden-Popper series of the perovskite sulfide, Ba_n+1Zr_nS_3n+1b(n≥1). The polycrystalline samples were synthesized with an iodine catalyzed solid state reaction, and single crystals were grown with salt flux methods. Structural, chemical, and thermogravimetric studies establish that these materials have good optoelectronic properties and stability. High external luminescence efficiency, up to 0.15%, is obtained via quantitative photoluminescence measurements. An effective, bulk minority carrier lifetime longer than 65 ns and very low surface recombination are determined from time-resolved photoluminescence measurements. X-ray absorption spectroscopy illustrates the role of Zr-S covalent bonding in determining the electronic structure.

References:
[1] S. Niu, D. Sarkar, K. Williams, Y. Zhou, Y. Li, E. Bianco, H. Huyan, S. B. Cronin, M. E. McConney, R. Haiges, R. Jaramillo, D. J. Singh, W. A. Tisdale, R. Kapadia, J. Ravichandran, Optimal Bandgap in a 2D Ruddlesden–Popper Perovskite Chalcogenide for Single-Junction Solar Cells. Chem. Mater. 2018, 30, 4882.
[2] S. Niu, H. Huyan, Y. Liu, M. Yeung, K. Ye, L. Blankemeier, T. Orvis, D. Sarkar, D. J. Singh, R. Kapadia, J. Ravichandran, Bandgap Control via Structural and Chemical Tuning of Transition Metal Perovskite Chalcogenides. Adv. Mater. 2017, 29, 1604733.

Although lead-halide perovskite (LHP) solar cells have reached 22.7% efficiency to date, they still face stability issues. Recent studies have suggested that shifting to lower dimensional (LD) perovskite structures may extend the cell’s environmental stability. However, these LD perovskite structures tend to have low photocurrents and solar cell performance. It has been suggested that their culprit is not only lifetime, but also their charge-carrier mobility. To understand this, we conduct a detailed study using PbI₂-based LD perovskites, which are synthesized by intentionally introducing a pre-defined amount of large A-site cations to force the structure to split into a layered compound. We measure the LD perovskite device performance, and characterize the absorber using THz spectroscopy and time-resolved microwave photoconductivity to understand the mobility. Finally, we relate the mobility results with the structures and device performances.

Solution-processed mixed halide perovskites (e.g. MAPb(I_1-xBr_x)₃) are excellent materials for multi-junction solar cells due to their ideal characteristics, which include large optical absorption coefficients, long carrier diffusion lengths, long-lived carrier lifetimes, and tunable bandgaps. Unfortunately, light-induced halide phase segregation has prevented their effective integration into working devices. We present kinetic Monte Carlo simulations and complementary optical experiments which show that during illumination, halide migration is directed by the energetics of charge carriers. The nucleation of a low-bandgap, I-rich domain emerges as a mechanism to trap charge carriers and reduce their energy. Furthermore, migration rates in stoichiometric and halide-deficient MAPb(I_1-xBr_x)₃ thin films are dictated by halide vacancy hopping barriers and are modulated by the number of available vacancies. An established photosegregation excitation intensity threshold is independent of the number of vacancies and instead depends critically on parameters such as carrier diffusion length, lifetime and bandgap tunability. Superb agreement with experimental nucleation kinetics and optics validates the model and prompts its application to fundamental, experimentally-immutable aspects of photosegregation. By simulating nucleation with varying ionic mobilities, we determine how domain formation is influenced by species dependent I^- vs. Br^- diffusion rates and asymmetric hopping in tetragonal vs. cubic symmetry. The simulations further suggest that near ubiquitous emission energies, which converge on that for MAPb(I_0.8Br_0.2)₃ (i.e. x~0.2) following photosegregation, arise from the existence of kinetically trapped Br^- within nucleated I-rich domains surrounded by a pure I^- barrier. These simulations ultimately reveal that the ideal characteristics of mixed halide hybrid perovskites, specifically their large carrier diffusion lengths, are responsible for inducing photosegregation. The study thus sheds new light on important parameters that define photoinduced halide phase segregation in mixed halide hybrid perovskites and presents opportunities for ultimately controlling as well as managing the phenomenon.

Metal halide perovskites are ionic in nature and its properties can be tuned through the exchange between halide ions. For example, by tailoring the ratio of Cl:Br and Br:I it is possible to modulate the absorption and emission properties of metal halide perovskites across the entire visible region. However, the ease of halide ion exchange property poses a problem to create a tandem structure with layers of metal halide perovskites of different compositions. In order to keep the lead halide perovskite nanocrystals intact without undergoing exchange of halide ions and retain the original band structure one needs to suppress the halide ion migration across the nanocrystals. We have now successfully achieved this task by capping CsPbBr₃ and CsPbI₃ nanocrystals with PbSO₄-Oleate. The linear assembly of the nanocrystals that resemble that of a peapod structure inhibits the exchange of anions. Absorption measurements show that the nanocrystal assemblies maintain their identity as either CsPbBr₃ or CsPbI₃, for several days. Furthermore, we have electrophoretically deposited these assemblies as hierarchical structures on electrode surfaces and employ them in light emitting devices. The effectiveness of PbSO₄-Oleate capping of lead halide perovskite nanocrystals offers new opportunities to overcome the challenges of halide ion exchange and aid towards the tandem design of perovskite light harvesting assemblies.

Lead-based halide perovskites have demonstrated superior optoelectronic properties since the emergence of perovskite solar cells (PSCs). However, the toxicity and phase stability of lead halide perovskite brings inevitable concerns with the practical application for the solar panel. Based on the environmental friendly element of Titanium (Ti), we have predicted a series of Ti vacancy-ordered double perovskite compounds, Cs₂TiI₆, Rb₂TiI₆, K₂TiI₆ and In₂TiI₆, which possess optimal bandgap and suitable absorption. Here, we successfully synthesis the Titanium-based materials which indicates plausible photovoltaic applications because of the proper band-gap range. Furthermore, we prepared the pinhole-free titanium-based thin film by the evaporation method. The electrons/holes diffusion length of such thin films with proper crystallographic textures illustrates indicated the possible construction of planer solar cell. Thus, this work provides new direction in the design and development of high performance Ti-based thin-film PSCs of the future.

The outstanding photophysical properties of APbX₃ perovskites (A = organic/inorganic monocation, X = halide) for optoelectronic applications has prompted a vigorous search for analogs. Indeed, finding structural and functional analogs is a time-tested approach for both better understanding the current champion material as well as for paving the way for second-generation materials. In this regard, there has been intense recent interest in the photophysical properties of halide double perovskites, where Pb²⁺ sites are replaced by two different metals that yield an average charge of 2+. However, despite this compositional versatility, all double perovskites with different metals have so far featured large bandgaps of ca. 2 eV or more. Although oxide perovskites feature both high-bandgap insulators and metals, such diversity has not yet been seen in their halide analogs. I will present work from our labs aimed at understanding how to manipulate the electronic structure of halide double perovskites through substitution chemistry. I will describe new design rules that have enabled the synthesis of halide double perovskites with unprecendented small (direct) bandgaps. We expect this work to aid in considerably expanding the electronic portfolio of halide perovskites for fundamental studies and applications in technology.

Lead-free perovskite solar cell is one of the research issues on perovskite solar cells. We have focused on Sn perovskite material. One of these Sn-related solar cells is SnPb mixed metal perovskite solar cell. We discuss how to enhance the efficiency from the view point of less trap densities in hetero-interfaces and the bulk layer. SnF2(DMSO)2 doping to the perovskite layer and the introduction of spike band structure in the cell gave SnPb perovskite solar cells with 19% efficiency. The results lead us to developing Pb-free Sn-related perovskite solar cells. Pb of SnPb perovskite layer was replaced by Ge. A theoretical study showed that it is possible to prepare a SnGe mixed metal perovskite material which absorbs the sunlight. In this study, a new type of SnGe mixed metal perovskite solar cells are reported with enhanced efficiency and stability. XRD spectra showed that the structure is perovskite. The structure of GeSn perovskite was discussed from the view point of the band structure, XPS analysis, and the Urbach energy. Most of the Ge atoms are at the hetero-interface of the perovskite/PEDOT-PSS as well as at the interface of the pervskite/C60. They passivate the surface of the Sn perovskite (so-called graded structure). For SnGe(0)-PVK device, where SnGe(X) stands for SnGe perovskite with X% Ge content, the PCE was 3.31 %. Upon doping with 5 wt% of Ge, the overall efficiency was enhanced to 4.48 %. With the Ge content more than 10wt%, all the photovoltaic parameters decreased significantly which resulted in an efficiency as low as 0.80 % for SnGe(0.2)-PVK device. After optimization, 7.89% of SnGe(5)-PVK device is reported. In addition, the stability of the device in air without encapsulation has been improved significantly with the Ge doping. 80 % of efficiency was kept after doping with Ge (5%) from its original performance. However, only 10 % of the efficiency was retained for non-doped sample SnGe(0). This work provides a platform for further research on lead-free Sn-Ge based perovskite solar cells.

Hybrid Perovskite solar cells have been the subject of intense interest due to significant optoelectronic properties (high absorbance, long diffusion length, etc.). An accelerated competition is taking place to obtain highest performance in single junction and tandem solar cells with efficiencies up to 20 %. Nevertheless, huge scientific problem remains be solved that directly impact performance and future commercialization, namely the stability under external agents as light, oxygen, humidity and heat.

The organic ions play an important role in the perovskite stability, which make them sensitive to moisture; Some studies showed the perovskite 2D are more resistant to humidity conditions in contrast to 3D. However, a decrement in density of carriers photogenerated is obtained where the vertical orientation is wishing to get good performance in solar cells. We explore other strategies to extract carriers mainly doping 2D perovskite BA₂MA₃PbI₄I₁₃ with single walled carbon nanotubes and graphene. Experimental measurements to explain how structural changes (domain distributions, band gap positions) can modify optoelectronics properties as photoluminescence, conductivity, Seebeck coefficient, I-V curves and absorbance will be discuss.


References
[1] Alexander Z. Chen et al. Origin of vertical orientation in two-dimensional metal halide perovskites and its effect on photovoltaic performance. Nature Comm 9. 1336, 2018.
[2] Daniel B. Strauss et al. Electrons, Excitons, and Phonons in Two-Dimensional Hybrid Perovskites: Connecting Structural, Optical, and Electronic Properties. J. Phys. Chem. Lett., 9, 1434-1447, 2018.

Halide perovskites, especially layered quasi-2D perovskites, offer a number of advantages to creating bright and efficient light-emitting applications. Their combination of excellent charge carrier mobility and low density of recombination centers have enabled their rapid ascent in electroluminescent devices. To bring perovskite LEDs to commercialization, a remaining issue of stability needs to be addressed. In this presentation, we pinpointed the chief cause of the dramatic degradation of halide perovskites in light-emitting diodes (LEDs). We studied photogenerated charges accumulating at exposed perovskite facets activate the physisorbed oxygen, converting it into reactive superoxide that triggers perovskite degradation.
We thus aimed at developing a strategy to protect the perovskite facets. At an applied level, we achieve perovskite films that exhibit a near-perfect passivation, attested to by their photoluminescence quantum yields (PLQYs) that closely approach 100%. These films are stable under continuous illumination in ambient conditions over hundred hours. In addition, we report orders of magnitude improvement in device operating stability relative to the best-performing prior perovskite reports.

The success of halide perovskites as optoelectronic materials has spurred immense interest in these remarkable compounds. The perovskite structure has formula AMX₃ and consists of corner-connected MX₆ (M²⁺ a metal cation, X^– a halide) octahedra with large A⁺ cations (Cs⁺, CH₃NH₃⁺, CH(NH₂)₂⁺ = FA⁺) in the voids. This framework is quite flexible, enabling substitution on all three sites to form a variety of compositions. Halide substitution tunes the band gap across the entire visible range, exemplified in the CsPbX₃ nanocrystal series, while A-site mixing has stabilized structures such as Cs_1-xFA_xPbI₃ with unstable end members. However, compositions of optoelectronic interest remain limited to the main group M²⁺ cations Pb and Sn.
To expand halide perovskites to the trivalent main group metals M³⁺, researchers have turned to the double perovskites, which use alternating M⁺ and M³⁺ cations to fully substitute the M-site and form the elpasolite structure with formula A₂M⁺M³⁺X₆. The first example was Cs₂AgBiBr₆, with a reasonable 2.2 eV gap. Many experimental and theoretical works followed, but it quickly became apparent that only specific pairings of M⁺ and M³⁺ form 3D double perovskites with bandgaps below 3 eV. Double perovskites utilizing alkali metals as M⁺ have long been studied as scintillators, but their bandgaps lie outside the visible spectrum where optoelectronic materials are sought. This narrows the field of M⁺ to Ag, Cu, Au, Tl, and In, but these have issues of their own. Tl⁺ halides are extremely toxic, while Au⁺ halides are light sensitive and prefer linear coordination. Cu⁺ is unstable in octahedral coordination, while In⁺ reduces both Sb³⁺ and Bi³⁺, seemingly leaving Ag⁺ as the only option.
We wondered whether the In⁺ cation could coexist with In³⁺ without reducing In³⁺ and maintain an octahedral perovskite framework. The only inorganic halides with In⁺ are mixed-valent binaries with non-octahedral In⁺, but we hypothesized that Cs⁺ would be large enough to stabilize the In⁺X₆ octahedron. Accordingly, the solid-state reaction of CsX, InX, and InX₃ yielded the mixed-valent double perovskites CsInX₃ (X = Cl, Br), with bandgaps of 2.20 eV for the bromide and 2.97 eV for CsInCl₃, similar to the Cs₂AgBiX₆ analogues. The CsInX₃ structures have significant disorder at the In⁺ site due to lone pair expression, characterized by large x-ray diffraction thermal parameters. CsInCl₃ in particular can form two structures, one which is a 3D perovskite and another in which an In⁺Cl₆ octahedron rotates 45° to better accommodate the larger In⁺ ion, similar to the case of the mixed valent CsTlCl₃. The phase behavior is studied by synchrotron x-ray diffraction, showing evolution to a cubic structure at high temperature. DFT calculations show indirect band gaps, and as a result there is no photoluminescence at room temperature. The CsInX₃ are a unique addition to the inorganic double perovskites, with CsInBr₃ just the second bromide after Cs₂AgBiBr₆.

Metal halide perovskites are generating enormous interest for their use in optoelectronic devices including photovoltaics and light-emitting diodes. One of their most remarkable properties is their apparent defect tolerance – films can be produced using relatively crude processing methods yet they still exhibit very good device performance. Calculations have suggested that this is at least partly because many defects cause only shallow trap states which may not be catastrophic for device performance (unlike deeper trap states). Nevertheless, there is still substantial non-radiative losses suggesting defects are not entirely benign and they still must be understood and addressed before devices can approaches their performance limits.

Here, I will cover our ongoing work focusing on defects and their impact on non-radiative losses, as well as their mitigation through passivation treatments. I will present recent results in which we use multimodal approaches to determine relationships between local chemistry, structural and luminescence properties in perovskite thin films using synchrotron nano X-Ray Diffraction (n-XRD) and nano X-Ray fluorescence (n-XRF) measurements, as well as confocal and wide-field luminescence imaging. We reveal an intimate connection between strain and non-radiative decay, revealing these strain-related defects as a primary origin of non-radiative losses. I will also outline the action of passivation treatments, such as chemical and light-induced treatments, on relieving these strain patterns.

The work provides a platform for designing new and more effective passivation post-treatments or film fabrication methods, which will push devices ever closer to their efficiency limits.

To date, the main limitation toward hybrid perovskites’ implementation into commercial light-absorbing and light-emitting devices is this material lack of stability upon exposure to: humidity, oxygen, temperature, light, and bias [1, 2]. Thus, understanding and controlling the driving forces for perovskites’ degradation and the possible pathways for recovery are imperative for the development of reliable devices. We resolve the influence of each abovementioned parameter onto the perovskites’ electrical and optical responses from the nano- to the macroscale. We demonstrate, in real-time, the dynamic open-circuit voltage of perovskite solar cells by novel scanning probe microscopy methods [3, 4], resulting from light-induced ion migration [5]. We determine the effect of chemical composition of the photoluminescence hysteresis of Cs-triple cation perovskites, as a function of humidity cycles [6]. Additionally, we identify a fully reversible voltage response within grains for Cs-triple cations perovskites upon exposure to 1-sun illumination. These measurements show the importance of correlating the local, nanoscale behavior with macroscopic electrical and optical responses. Finally, we will present a machine learning approach to track device performance [1], including a route to prevent material degradation, and to material recovery.


[1] Joule, invited perspective – to appear (2018)
[3] ACS Energy Letters 2, 1825 (2017)
[2] Adv. Energy Materials 5, 1501142 (2015)
[4] ACS Energy Letters 2, 2761 (2017)
[5] Nano Letters 17, 2554 (2017)
[6] J. Phys. Chem. Letters 9, 3463 (2018)

The twin domain in methylammonium lead triiodide (MAPbI₃) has drawn extensive research efforts, starting the discussion on its ferroic nature. Given the coupling of defect chemistry and ionic states with ferroelectricity/ferroelasticity, the research efforts should be extended to its chemical behavior. Our earlier investigations revealed the ion segregation correlating to the ferroelastic domain contrast. To follow up, we systematically investigate the chemical evolution of the ion migration in the MAPbI₃ twin domains in this work. Using Band Excitation Contact Kelvin Probe Force Microscopy (BE-cKPFM), we reveal the absence of ferroelectric polarization switching in this material, as our data indicate an underlying electrochemical effect regarding ion migration. This disproves the ferroelectric origin of the previously observed butterfly and hysteresis loops in Switching Spectroscopy piezoelectric force microscopy (SS-PFM). In addition, Band Excitation Scanning Kelvin Probe Force Microscopy (BE-scKPFM) measurement, which was utilized to study the electrochemical activities in the twin domain, indicates that the difference in ion migration and/or surface charging effect in the adjacent domains. This result implies a different ionic conductivity and a variation of electronic properties in adjacent domains. Combining Band Excitation PFM (BE-PFM), nanoscale infrared spectroscopy (Nano IR), and scanning probe microscope (SEM), we clarify the correlation between ionic diffusion, electronic properties, and chemical segregation. We reveal that the methylammonium segregation leads to a decrease of electronic conductivity and an increase of ionic conductivity. Overall, this work provides new insights into understanding the role of the twin domain in photovoltaic action.

Hybrid organic metal halide Ruddlesden-Popper (R-P) phases have recently been the subject of intense research efforts due to their good power conversion efficiencies in photovoltaics and controllable emission for light emitting diodes, while possessing better environmental stability compared to their three-dimensional counterparts. The thin film structures of these layered perovskites are still poorly understood relative to the bulk. In this study, we use optical spectroscopy, X-ray scattering, and transmission electron microscopy to characterize the structures of these thin films of (C₄H₉NH₃)₂(CH₃NH₃)₂Pb₃I₁₀ and (C₄H₉NH₃)₂(CH₃NH₃)₃Pb₄I₁₃ on the meso- and nanoscales. Previous studies suggest a preferential orientation of the Pb-I sheets in spin coated films perpendicular to the substrate, so that the layer stacking direction is in the plane of the film. By measuring in-plane and off-specular X-ray diffraction with grazing-incident wide-angle X-ray scattering (GIWAXS), we observe that some expected peaks along the stacking direction of the Pb-I sheets are missing, indicating disorder in perovskite layer stacking. Because the diffraction patterns represent a bulk, average structure, we used transmission electron microscopy (TEM) to explore film structure on a local scale and find that these films consist of small crystalline grains in an amorphous matrix, contradictory to previous reports suggesting single-crystalline quality thin films. When using known crystal structures to index these SAED patterns, we see that the thin films comprise not only the targeted R-P phase, but also regions with lower and higher Pb-I sheet thickness (i.e. phase impurities). This phase intergrowth creates structural defects that interrupt layer stacking and is the cause broadening of in-plane diffraction peaks, causing them to be absent from previous GIWAXS measurements. Finally, because these films produce efficient photovoltaics despite this high degree of structure disorder, we measured the absorption coefficient using photothermal deflection spectroscopy (PDS) and find Urbach energies of 32 meV for the R-P phases compared to 19 meV for methylammonium lead iodide. Despite the structural defects, the R-P films appear to maintain a low degree of electronic disorder suggesting that the Pb-I regions are electronically isolated from each other.

Solar cells with hybrid inorganic-organic lead halide based perovskite absorber layers have achieved remarkably high photovoltaic device performances in a relatively short amount of time when compared to comparable efficiency devices based upon other semiconductor absorbers. This perovskite family offers a wide range of tunable opto-electronic properties obtained via alloying of either / both the organic and inorganic components, and, even in their polycrystalline form, these perovskites are generally quite electronically forgiving semiconductors. However, as the material composition is manipulated, film stability and any increase in defect concentration remain issues. Here we will provide an overview of the optical properties, in the form of the complex index of refraction or complex dielectric function spectra, as well as the physical origin for the features present in those optical properties for perovskite layers with considerations toward composition, sample handling / environmental exposure, and incorporation in devices. Proper measurement and analysis yielding accurate values of the complex optical properties over the ultraviolet to millimeter wavelength range and complementary techniques sensitive to different levels of optical absorption enables tracking of the bandgap and higher energy critical point transitions, electrical transport properties manifested as free carrier absorption, sub-bandgap absorption due to defects, and film degradation or decomposition upon atmospheric exposure as deduced from in-situ / ex-situ spectroscopic ellipsometry, optical Hall effect, and photothermal deflection spectroscopy. Once acquired, these optical properties serve as input for external quantum efficiency simulations of photovoltaic device performance. Features observed optically will be correlated with full functioning device performance, illustrating how accurate optical property measurements over different wavelength ranges and levels of absorption provide insight into device functionality.

Recently, organometal halide perovskite solar cells (PSCs) have received great attention. The power conversion efficiency (PCE) of PSCs have shown a dramatic increase and certified PCEs adopting mixed organic cations and halide anions have reached over 22%. The PCE is considerably affected by photovoltaic property of each component of a PSC. In spite of the significance in the crystallographic information, however, microstructural observation for crystal structure analysis of the perovskite layer has not been actively conducted. Until now it is widely believed that each phase of the organometal halide perovskite solely exists with orthorhombic phase < 165K < tetragonal phase < 327K < cubic phase. Nevertheless we newly observed that the tetragonal and cubic phases coexist at room temperature in the conventional MAPbI₃ thin film device.

Furthermore, surprisingly, superlattices composed of mixture of tetragonal and cubic planes without any compositional change was also found. Formation of the superlattice is achieved by only intrinsic structural transition without artificial modifications and, therefore, most phenomena concerned with the structural superlattice are expected to spontaneously and automatically occur in context with situation. The organometal halide perovskite self-adjusts their microstructural configuration and self-organizes buffer layers inside crystal or at hetero-interface by introducing the self-assembled superlattices. We believe, this report will be a vital cornerstone to bring the PCEs of the organometal halide perovskite solar cells one step closer to theoretical maximum point and redefine possibility of the organometal halide perov-skite as promising materials for not only solar cell but also various application.

Despite the comparable power conversion efficiency between polycrystalline Si and perovskite solar cells (PSC), the commercialization of PSCs is inhibited by their low stability. A recent study demonstrated that the commonly used spin-casting and annealing for perovskite thin film create strain in the material and lead to decreased stability. Other publications have observed an association between lattice constant and band gap in halide perovskites ABX₃ (A= methylammonium, formamidinium, cesium; B=lead; X=Bromide, Iodide, chloride), suggesting their band gap can be tuned through the manipulation of strain in the material. However, the knowledge of strain distribution and its local effect is lacking for making highly stable PSCs or stress-sensitive optoelectronics. To study the local effect of strain, we map CsPbBr₃ crystals with nanoscale X-ray Diffraction Microscopy (nanoXRD) with approximately 100-nm spatial resolution and 0.1% sensitivity in strain detection. By exploiting the thermal expansion coefficient mismatches, a strain gradient is created in a CsPbBr₃ crystal at the edge of a platinum pad on quartz substrate and characterized by nanoXRD. We find that the observed compressive strain reduces the stability of the material, which proves that strain should be avoided for highly stable halide perovskites. We also characterize the same crystal with ex-situ photoluminescence (PL) mapping and observe PL peak shifts red at compressively strained locations in the crystal. This demonstrates the band gap narrowing by substrate-induced strain locally, which should be brought into attention for the fabrication process of halide perovskites on substrates. On the other hand, the strain effect on band gap shows potential of stress-sensitive optoelectronic applications with halide perovskites.

The microstructure of absorber layers is pivotally important for all thin-film solar technologies. Despite its unprecedented performance development in recent years, little is known about the microstructure of metal-halide perovskites and its effect on the macroscopic device performance. Yet, recent publications have frequently called attention to the urgent need of spatially resolved microstructure characterization techniques in order to correlate the microscopic structure with macroscopic device properties.
In this work, we report on the spatial investigation of methylammonium lead iodide (MAPbI3) grain properties by electron backscattered diffraction (EBSD) with high resolution. We resolve diffraction pattern ambiguities that are related to the close-to-cubic perovskite unit cell, and develop a comprehensive three-dimensional picture of the crystal orientation. We identify predominant orientation directions and observe orientation cross-talk between neighboring grains. The local crystal information correlates with ferroelectric and electronic properties that we probe with piezo-response force microscopy (PFM) and kelvin probe force microscopy (KPFM) measurements. If the ferroelectric polarization influences the charge carrier recombination and transport, as was predicted by simulations, then the orientation and shape of polarized domains within grains would directly influence the device performance. In turn, this renders engineering of the grain orientation and size a pivotal parameter for the optimization of perovskite solar cells which is not yet commonly investigated in most perovskite solar cell studies. These tools are indispensable for the future relation of the microscopic structure to the optoelectronic properties of perovskite devices as they allow to monitor device optimization and to understand fundamental processes of perovskite solar cells. Therefore, we expect EBSD and PFM to become the most often employed characterization techniques in the future for the correlation of microscopic structure and macroscopic device performance. Their strong correlation allows to draw conclusions about the microstructure from ferroelectric features and, likewise, to derive the ferroelectric polarization from crystallographic observations. Understanding the microstructure would not least be the key to future ab-initio engineering of new (non-toxic) and highly efficient perovskite solar cells.

References
[1] H. Röhm, T. Leonhard, M.J. Hoffmann, A. Colsmann, Energy Environ. Sci. (2017), 10, 950-955.
[2] D. Rossi, A. Pecchia, M. Auf der Maur, T. Leonhard, H. Röhm, M.J. Hoffmann, A. Colsmann, A. Di Carlo, Nano Energy (2018), 48, 20-26.
[3] T. Leonhard, A. Schulz, H. Röhm, S. Wagner, F. Altermann, W. Rheinheimer, M.J. Hoffmann, A. Colsmann, submitted.

Perovskite solar cells (PSCs) have attracted considerable attention in recent years due to their rapidly increasing power conversion efficiency (PCE), which currently exceeds 22%. Given that most high efficiency PSCs are made of polycrystalline films, an important feature is their grain size distribution, because grain boundaries can limit carrier diffusion and serve as nonradiative recombination sites, thereby reducing the PCE. Due to the formation of grain boundary grooves during perovskite film growth, many techniques for estimating grain size rely on surface morphology characterization using scanning electron microscopy (SEM) or atomic force microscopy (AFM). However, not all grain boundaries exhibit clear topographical features.
Here, we report the direct identification through photoluminescence (PL) microscopy of grain boundaries in formamidinium lead iodide (FAPbI3) thin film samples that cannot be observed by either SEM or AFM. We demonstrate that these “invisible” boundaries impede carrier diffusion and limit radiative recombinations events. Optical characterization is further supported by electron backscatter diffraction (EBSD) measurements that are used to confirm crystal orientation differences across these otherwise imperceptible boundaries. Then, we present PL lifetime measurements that show how these "invisible" boundaries serve as nonradiative recombination sites, thereby decreasing carrier lifetimes, and likely reducing PCEs.
In order to quantify the carrier diffusion resistance in different types of grain boundaries, we developed a carrier density probing method. Using a high-speed intensified emCCD camera, we are able to measure the 2D PL intensity distribution with nanosecond resolution. Comparing the PL distribution with excitation near and far away from the grain boundary, we can see the effect of grain boundaries in blocking carriers. A carrier diffusion and recombination model is used to fit the observed PL evolution to extract the diffusion and recombination coefficients. Together with a mapping of steady state PL intensity distribution and the carrier density probing method, we are able to estimate the resistivity of carrier diffusion across grain boundaries.
Finally we will discuss the impact of these boundaries as well as this new characterization method for the synthesis and analysis of thin-film perovskite solar cells.

We mapped ion transport in hybrid organic–inorganic perovskite solar cells under illumination using impedance spectroscopy (IS) as a function of applied bias and device degradation. We observe evidence of mass (ion) diffusion and extrapolate conductivities (∼10^–7 S cm^–1) and diffusion coefficients (∼10^–7 cm² s^–1) for the mobile ionic species at varying applied biases. We show that ions respond to low applied forward bias in a predictable manner, characterized by an increased double layer capacitance at the hole-transporting (HTM) and electron-transporting material (ETM) interfaces. This is presumably due to ion accumulation and electronic charge pinning or screening effects under external biasing. Unexpectedly, at high forward biases, we found that there is a capacitive discharge in the double layer resulting in ion redistribution in the bulk. Furthermore, we show that double-layer capacitance as a result of ion accumulation significantly impacts the electronic properties of the device and thus device performance. Lastly, we show that as the device degrades there is an overall depletion of capacitive effects coupled with increased ion mobility.

Formamidinium lead iodide (FAPbI₃) based perovskites have attracted a great deal of interest as light absorbers in solar cells due to their superior thermal stability and more suitable band gap compared to perovskites based on methylammonium (MA). However, preferential formation of a photo-inactive δ-FAPbI₃ phase at room temperature has been a major impediment. The need for prolonged heat-treatments at temperatures >150 ⁰C to obtain the desirable α-FAPbI₃ perovskite phase often leads to film degradation. Heat treatment also produces fine-grained films with high grain-boundary density, which is detrimental to PSCs performance and stability. In this context, we have discovered a new phenomenon, where fine-grained (∼175 nm) δ-FAPbI₃ thin films transform rapidly to phase-pure α-FAPbI₃ perovskite thin films with ultra-large grain size exceeding ∼5 μm. The large-grained nature of the films is confirmed using appropriate materials characterization techniques. The improved kinetics of transformation is explained by studying the phase and morphological evolutions during film-solvent interaction. The nature of phase nucleation and growth is studied through in-situ microscopy techniques. In-situ X ray diffraction and solvent polarity effects on the transformation rate are also studied to corroborate the proposed mechanism. Devices with high efficiency are fabricated with these ultra-large grained films and are characterized. The use of this novel approach to achieve 5-µm grains by a brief low-temperature treatment enables a promising path toward achieving ‘single-crystal’ films with superior optoelectronic properties and chemical stability.

Within the past few years, metal halide perovskites have been attracting significant interest due to their successful application to optoelectronic devices. These materials have been used in lasers, photodetectors, and most commonly, in photovoltaic devices and light emitting diodes. Despite the cheap and simple fabrication methods by which these materials are deposited, high quality perovskite films can be readily fabricated, and the power conversion efficiencies of lead halide perovskite solar cells are now approaching certified values of 23%. However, perovskite-based devices are yet to achieve their full potential. One of the major hindrances to achieving this potential is an incomplete understanding of perovskite surfaces and interfaces. Deficiencies at these interfaces may be responsible for the largest losses in perovskite based optoelectronic devices; hindering charge extraction, increasing non-radiative recombination rates and hysteresis, and significantly increasing the voltage loss in perovskite photovoltaics. We propose surface doping the perovskite material as a means to combat these interface deficiencies. Herein, we will discuss doping of the perovskite material at various interfaces using well-established charge-transfer dopants. We show the doping of the perovskite material through both solid-state NMR and surface characterisation techniques, and further characterise the material through photoluminescence measurements, showing a reduction in the non-radiative recombination of the material. Using this surface doped material, we show photovoltaic devices with reduced hysteresis, low voltage losses, and steady-state power conversion efficiencies in excess of 20%.

Exciting reports about single layer cesium lead halide perovskite LEDs have shown that conventional electron and hole injection layers are not necessarily needed. A sandwiched device structure consisting of a perovskite-polymer composite film between two electrodes can emit bright light at low operating and threshold voltages (~2.0 volts). We studied the role of grain size, polymers inclusion, morphology, emission changes, and electrode material in CsPbBr3 based LED. These simple single layer devices are easier and cheaper to fabricate as compared to their multilayered counterparts and removes the possible reaction routes between perovskite and the charge injection layers. We also investigate underlying mechanisms of how ion migration within the perovskite layer forms a p-i-n junction which allows efficient charge injection directly from the electrodes, independently of their work function, while still having efficient radiative recombination.

Perovskite solar cells have drawn great attention in the past years. Besides the high absorption and long carrier diffusion length of the material, perovskite solar cells have demonstrated potential for a promising alternative to conventional silicon solar cells due to their lower fabrication costs and reported power conversion efficiencies reaching 22.1%.

Plasmonic absorption enhancement has been extensively utilized to improve the performance of various solar cell technologies. Here, we report on a simulation study to further boost the efficiency of perovskite solar cells by embedding plasmonic concentrators in the back metal contacts. Specifically, three dimensional finite-difference time-domain (FDTD) simulations are performed on perovskite solar cells, consisting of perovskite films with varying thickness on top of flat or corrugated gold electrodes with varying light trapping geometries (nanodisk or nanohole arrays). The calculated electric fields in the simulation volume enable the decoupling of the absorption within the perovskite and gold films, respectively, which allows for the calculation of the cell power conversion efficiency (PCE) as a function of relevant design parameters. By systematically leveraging the geometry dimensions, the optimal nanostructure designs are obtained. The results show that 100nm-thick perovskite films on top of corrugated gold electrodes can exhibit up to 52% PCE increase compared to their flat counterparts (from 19.2% for a flat cell to 29.2% for an optimized nano-corrugated cell). Moreover, we show that a 150nm-thick perovskite film cell with opportunely corrugated back metal contacts can exhibit a PCE value (31.3%) comparable to that of a 400nm-thick bulk-like cell (31.6%).

These findings may pave the way for plasmon-enhanced high-performance thin-film perovskite solar cells fabricated via scalable methods such as nano-imprint lithography.

The new generation of photovoltaic materials is driven by the unprecedented rapid performance improvement of hybrid organic-inorganic perovskites solar cells (PSCs). Fully inorganic CsPbI₃ perovskites have also demonstrated their great potential for photovoltaics in world record quantum dot solar cells and as an ideal material candidate for tandem devices with a band gap close to 1.8 eV. In this study we explore the composition-dependent phase stability of CsPbI₃ films by combinatorial studies. We show that coevaporated CsPbI₃ can be synthesised in a stable perovskite phase under Cs rich conditions without the need of a post deposition annealing treatment. We investigate the compositional dependence of the electronic properties by THz spectroscopy and quantitative hyperspectral photoluminescence imaging. Compositional regions with considerably large carrier mobilities and a high photoluminescence quantum yield are identified. From these measurements a maximum quasi-Fermi level splitting of 1.27 eV and charge carrier diffusion lengths of around 3 microns are estimated, indicating this material’s potential for high efficiency solar cells. We find that the charge carrier mobility decreases with increasing Cs content corresponding to a lowering of the electronic dimensionality as the Cs content is increased in the films. First planar pin-type solar cells have been fabricated showing stabilised efficiencies exceeding 11%. Based on the comparison of the optoelectronic characterisation performed in the CsPbI₃ films and the solar cells properties of the corresponding devices we conclude that the devices are strongly limited by interface recombination. Optimisation of the device architecture with better matched extraction layers exhibiting less interface recombination could be expected to lead to 18% efficient inorganic perovskites solar cells.

With the massive interest in halide perovskites there has been a wide range of research into combinations of these materials. MAPbI₃, however, still demonstrates very desirable properties. The long term stability of MAPbI₃ remains a significant challenge to any true commercial uptake. The degradation can be significantly affected by many factors but remains unclear [16]. Particular attention has focussed on the effect of non-stoichiometry of the material [19] which may have a significant role in the stability.

Simulations can provide a very valuable tool for studying the defect behaviour, stability and degradation of these perovskite materials due their ability to design and explore exact conditions and examine the local atomic behaviour. Although very powerful, ab initio simulations are limited to modelling relatively small cells due to their computational demands. Classical mechanics provides a powerful method to reach larger sizes and far more configurations.

We use our recently published forcefield [3] to study defects in the MAPbI₃ system. We observe that defects are encouraged by non-stoichiometry in the system. We examine the local and long range structural effects of the defects with a particular focus on dynamic behaviour in the ions (rotation/vibrations) and tilt of the octahedra. From this we are able to comment on the influence in general on degradation. We couple these results to experimental probing of degradation in non-stoichiometric samples.

[1] T.A. Berhe, W.-N. Su, C.-H. Chen, C.-J. Pan, J-H. Cheng, H.-M Chen, M.-C Tsai, L.-Y. Chen., A.A. Dubaleb, B.-J. Hwang, Energy Environ. Sci. 2016, 9, 323
[2] Z. Song, S.C. Watthage, A.B. Phillips, B.L. Tompkins, R.J. Ellingson, M.J. Heben, Chem. Mater., 2015, 27, 4612
[3] C.M. Handley, C.L. Freeman, PCCP 2017, 19, 2322

Organometal halide perovskites are emerging as promising photovoltaic (PV) materials due to their strong absorbance throughout the visible region, relatively high charge-carrier mobilities, and power conversion efficiencies (PCE) that are on par with polycrystalline silicon. As these materials progress towards commercial applications, understanding the factors limiting the PCE and stability is becoming essential. Interfaces and grain boundaries are some of the most influential aspects for both device performance and stability, as these are prime areas for defect formation and charge recombination. One means of improving both device performance and stability is to passivate these surfaces and grain boundaries by applying surface ligands. Although multiple surface treatments and additives have been applied to perovskite PVs, there is a lack of understanding of how these molecules are interacting with the perovskites. In this study, we show how the binding group influences photoluminescence (PL) and photovoltaic (PV) properties and stability for a series of surface ligands consisting of varying binding groups. These include ligands with both one and two potential binding groups, including zwitterionic molecules. We expected that zwitterionic ligands would display higher binding affinities and potentially improved trap state passivation, as the negatively charged part of the zwitterionic ligands can bind with positively charged undercoordinated Pb²⁺ ions of the perovskite film, while the positively charged group can fill in A-site vacancies. We find that zwitterions indeed lead to higher photoluminescence quantum yields (PLQYs) than monofunctional ligands.

In the perovskite solar cells, interfaces are crucial for efficient photovoltaic performance as they are responsible for both the injection and the transport of the charge carriers. Using ab-inito molecular dynamics simulations and density functional theory calculations, we have systematically determined the structural, electronic, and transport properties at a model CH₃NH₃PbI₃/phenyl-C₆₁-butyric acid methyl ester (PCBM) interface. The CH₃NH₃PbI₃ is the photoactive layer while PCBM is the electron transport layer. We have observed that PCBM prefers to attach to the perovskite surface via ester moiety of PCBM molecule. Further, from the analysis of interatomic distances on several interface models, we found that the bonding at the interface and the stability of interface is sensitive to the chemical composition at the surface i.e. CH₃NH₃I vs. PbI₂ surface terminations of the perovskite. Different preferences in bonding at the interface leads to the change in electronic and transport properties across two chemical terminations. Finally, we have shown that the stability of the interface can be increased by the introduction of certain types of defects at the perovskite surface, which may result in the better coverage of PCBM on the perovskite surface. However, such kind of defects could deteriorate the photovoltaic performance due to an increase in the potential barrier for the transport of charge across the interface.

Formamidinium lead halides (FAPbX₃, X=Cl, Br, I) perovskite materials have recently drawn an increased amount of attention owing to their superior optoelectronic properties and enhanced material stability as compared to their methylammonium-based (MA-based) analogues. Herein, we report a study of pressure-induced structural and optical evolution of FAPbI₃ hybrid organicinorganic perovskite nanocrystals (NCs) using a synchrotron-based X-ray scattering technique coupled with in situ absorption and photoluminescence (PL) spectroscopies. As a result of their unique structural stability and soft nature, FAPbI₃ NCs exhibit a wide range of bandgap tunability (1.44 eV – 2.17 eV) as a function of pressure (0 – 13.4 GPa). The study presented here not only provides an efficient and chemically orthogonal means to controllably engineer the bandgap of FAPbI₃ NCs using pressure, but more importantly sheds light on how to strategically design the bandgaps of FA-based hybrid organic-inorganic perovskites for various optoelectronic applications.

Organic-inorganic halide perovskite solar cells (PSCs) have drawn a great deal of attention in the photovoltaic research community due to their high efficiency over 22% and simple manufacturing process.¹ A high quality perovskite film is critical to obtain PSCs with both superior performance and high reproducibility, especially when constructing large area polycrystalline films in ambient condition.^2-5 Motivated by the common wisdom that thicker films are easier to process and therefore offer better controllability in large area production of optoelectronics device when compared with thinner ones. Here, we employ a simple perovskite formation process to fabricate perovskite films with thickness over 1 μm in ambient condition. By careful control of the perovskite component and formation process, the resultant perovskite films exhibit full coverage and excellent crystallinity with low surface roughness and low thickness variation. The high quality of the films is further evidenced by significantly enhanced photoluminescence lifetime, slower charge recombination constant, much lower intrinsic trap density, higher conductivity and much longer carrier diffusion length to achieve the balance of device efficiency and film thickness. These results suggest that we obtain high quality thick perovskite thin films with this simple method. Our resultant mesoscopic PSCs with active area of 0.1 cm² achieve an average power conversion efficiency (PCE) of 19.1% (with small PCE standard deviation) and a stabilized efficiency approaching 19%. Moreover, our 5 cm × 5 cm PSC module device with active size of 12 cm² also deliver a PCE of 15.3%, strongly suggesting that our method is compatible with further upscaling. In addition, the resultant un-encapsulated small size PSCs exhibit an excellent T80 lifetime exceeding 1600 h under continuous light illumination with maximum power point tracking in dry N₂ environment.
References
¹ W. S. Yang, B.-W. Park, E. H. Jung, N. J. Jeon, Y. C. Kim, D. U. Lee, S. S. Shin, J. Seo, E. K. Kim, J. H. Noh, S. I. Seok, Science 356, 1376 (2017).
² Y. Deng, E. Peng, Y. Shao, Z. Xiao, Q. Dong, J. Huang, Energy Environ. Sci. 8, 1544 (2015).
³ Z. Liu, J. Hu, H. Jiao, L. Li, G. Zheng, Y. Chen, Y. Huang, Q. Zhang, C. Shen, Q. Chen, H. Zhou, Advanced Materials 29, 1606774 (2017).
⁴ Y. Jiang, M. R. Leyden, L. Qiu, S. Wang, L. K. Ono, Z. Wu, E. J. Juarez-Perez, Y. B. Qi, Adv. Funct. Mater. 1703835 (2017)
⁵ Z. Wu, S. R. Raga, E. J. Juarez-Perez, X. Yao, Y. Jiang, L. K. Ono, Z. Ning, H. Tian, Y. B. Qi, Adv. Mater. 1703670 (2017).

Inverted planar heterojunction perovskite solar cells (PSCs) have attracted attention because of the low-temperature, versatility of energy-band engineering, and a simplified device structure[1, 2]. However, their low power conversion efficiencies (PCEs) are still inferior to the regular PSCs with a mesoporous TiO2 structure[3, 4], mainly due to low open-circuit voltages. To address this issue, we demonstrate a solution-processed secondary growth (SSG) technique that could be used to tune the nature of mixed-cation mixed-halide perovskite films. The resulting films exhibit more n-type in nature and establish a wider bandgap layer close to the surface of the perovskite, along with the reduction in unwanted centers. By the use of perovskite film treated with the SSG-G growth, we ultimately achieved a comparable Voc (1.21 V) to regular structure PSCs without compromising the short-circuit current density and fill factor, leading to substantial increases in PCEs for inverted planar heterojunction PSCs[5]. Such high PCEs of 21.51% have been the best result for inverted planar heterojunction PSCs. Our approach will also be broadly applicable to other perovskite-based optoelectronic devices.
Reference:
[1] D. Luo, L. Zhao, J. Wu, Q. Hu, Y. Zhang, Z. Xu, Y. Liu, T. Liu, K. Chen, W. Yang, W. Zhang, R. Zhu, Q. Gong, Adv. Mater., 2017, 29, 1604758.
[2] Y. Wu, X. Yang, W. Chen, Y. Yue, M. Cai, F. Xie, E. Bi, A. Islam, L. Han, Nat. Energy 2016, 1, 16148.
[3] M. Saliba, T. Matsui, K. Domanski, J. Y. Seo, A. Ummadisingu, S. M. Zakeeruddin, J. P. Correa-Baena, W. R. Tress, A. Abate, A. Hagfeldt, M.Grätzel, Science 2016 354, 206.
[4] W. S. Yang, B. W. Park, E. H. Jung, N. J. Jeon, Y. C. Kim, D. U. Lee, S. S. Shin, J. Seo, E. K. Kim, J. H. Noh, S. I. Seok, Science 2017, 356, 1376-1379.
[5] D. Luo, W. Yang, Z. Wang, A. Sadhanala, Q. Hu, R. Su, R. Shivanna, G. F. Trindade, J. F. Watts, Z. Xu, T. Liu, K. Chen, F. Ye, P. Wu, L. Zhao, Jiang Wu, Y. Tu, Y. Zhang, X. Yang, W. Zhang, R. H. Friend, Q. Gong, H. J. Snaith, R. Zhu, Science 2018, 360, 1442.

All-inorganic perovskite cesium lead halide quantum dots (QDs) have been widely investigated as promising materials for optoelectronic application, because of its outstanding photoluminescence (PL) properties and benefits from quantum effects. Although QDs with full-spectra visible emission have been synthesized for years, the PL quantum yield (PLQY) of pure blue emitting QDs still stays at low level in contrast to their green or red emitting counterparts. Herein, we obtained core-shell structured cubic CsPbBr₃@amorphous CsPbBr_x (A-CsPbBr_x) perovskite QDs via a facile hot injection method and centrifugation process. The core/shell structure QDs showed a record pure blue emission PLQY of 84%, which is much higher than that of blue emitting cubic CsPbBr₃ QDs and CsPbBrxCl_3-x QDs. Furthermore, a blue emitting QDs-assisted-LED with bright pure blue emission was prepared and illustrated the core-shell QDs' promising prospect in optoelectrical application. Recently, another work shows the transformation of the crystal structure and remarkably high crystallinity of the γ-crystals, leading to the record blue PLQY of 91% at 480 nm.

Despite the demonstrated high efficiency of perovskite solar cells and light emitting devices, the understanding of the intrinsic stability of perovskites is far from complete. In this work, using an ultrasensitive, exfoliated 2D perovskite single-crystal sheet/graphene heterostructure device, we reveal spontaneous iodide loss as an important degradation pathway of 2D perovskite single crystals, which acts as a donor and n-dopes the 2D perovskite semiconductor, generating positively charged iodide vacancies.
First, single crystals of 2D perovskites are exfoliated to a thin sheet of ~55 nm, and a 2D perovskite/graphene heterostructure-based field-effect transistor is employed to monitor the 2D perovskite degradation. We show that the 2D perovskite thin crystal sheet is intrinsically unstable, leading to a continuous shift of the Fermi level of graphene with respect of the Dirac point position observed in I_d vs. V_g curves. Moreover, the degradation is accelerated by light illumination. The mechanism underlying this effect is the spontaneous loss of iodide, which is confirmed by cross-sectional scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy (EDS), ultraviolet photoelectron spectroscopy (UPS), and Kelvin probe measurements. The release of iodide generates iodide vacancies, which induce an n-type doping of the perovskite, shifting the Fermi level of both perovskite and graphene up. Furthermore, we show that a graphene cover on the perovskite thin crystal sheet can improve perovskite stability by preventing iodide loss. An ultra-stable phototransistor based on the graphene/2D perovskite/graphene structure is demonstrated, which shows almost no degradation over 75 days.

Halide perovskite families have been widely studied for photovoltaics [1-2] due to their unique opto-electronic properties, but recently, they are being studied for light-emitting applications [3]. Beyond regular perovskites based on a corner-sharing octahedral network, Cs₄PbBr₆, which is called as zero-dimensional perovskite, is getting attention because of its highly-efficient green light luminescence [4]. The reported band gap of Cs₄PbBr₆ of 3.9 eV is in the ultraviolet (UV) region, thus green light emission in this material cannot be due to its intrinsic band gap. The origin of the green light emission has not clarified and two hypotheses have been suggested to explain this phenomenon: (i) CsPbBr₃ phase impurities within the material and (ii) luminescent defect states within the band gap of Cs₄PbBr₆. We investigate the chemical and physical properties of the Cs₄PbBr₆ based on first-principles simulations, including a thorough analysis within the modern theory of defect formation [5], from which we predict the origin of the green light emission.

[1] L. D. Whalley, J. M. Frost, Y.-K. Jung, and A. Walsh, J. Chem. Phys., 2017, 146, 220901.
[2] P. Gao, M. Grätzel, and M. K. Nazeeruddin, Energy Environ. Sci., 2014, 7, 2448–2463.
[3] 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.
[4] M. I. Saidaminov, J. Almutlaq, S. Sarmah, I. Dursun, A. A. Zhumekenov, R. Begum, J. Pan, N. Cho, O. F. Mohammed, and O. M. Bakr, ACS Energy Lett., 2016, 1, 840-845.
[5] A. Walsh and A. Zunger, Nature. Mater. 2017, 16, 964

Multiple exciton generation (MEG) is a process that addresses the conversion of highly energetic photons by using their excess energy to promote additional electrons to the conduction band. However, this process requires that the energy of the exciting photon is at least twice the band-gap of the semiconductor, which is hardly achievable with visible light. In our studies we have shown that it is possible to use multiple low-energy photons to efficiently generate multiple excitons in metal halide perovskite nanocrystals. Additionally, high-order absorption processes, deviating from the expected two-photon absorption, can be observed at very specific energetic positions. We could associate these specific energies to multiples of the semiconductor band-gap energy. In such a way, we have developed a method by means of which we were able to describe this effect as a resonance behaviour between multi-photon absorption (MPA) and MEG, which has never been seen before.

Interestingly, the resonance effect between MPA and MEG could only be observed once the perovskites were assembled in an orderly fashion, forming supercrystals. This phenomenon can be attributed to the electronic coupling given by the close-packed arrangement of the individual nanocrystals that reduces the incidence of Auger recombination, which represents the main opponent of the survival of multiple excitons. When the supercrystal geometry is kept, the chemical composition of the perovskites can be varied to tune the energetic position of the MPA-MEG resonances. We have in fact demonstrated that the same effect can be obtained for different halides (X) in CsPbX₃ perovskites.


<!--[if !supportLists]-->1) <!--[endif]-->Aurora Manzi, Yu Tong, Julius Feucht, En-Ping Yao, Lakshminarayana Polavarapu, Alexander S Urban, Jochen Feldmann, Nature communications, 1518 (2018)

Currently lead based all-inorganic cesium lead halide perovskite nanocrystals (NCs), have been shown great potential for high-performance light-emitting diodes (LEDs) and solar cells owing to their excellent optical properties and inexpensive synthesis process. Here, we successfully doped hetrovalent Bi³⁺ ion into the lattices of CsPbBr₃ perovskite NCs through a hot-injection method. The Bi³⁺ cation has been chosen as the dopant by reason of its similar ionic radius to preserve the integrity of perovskite structure. We achieved maximum 6.93% Bi doping which is considerably higher as compared to those achieved in previous reports via hot injection method.
It was found that by increasing the Bi³⁺ ion concentration, the morphology of CsPbBr₃ NCs changed from cubic to hexagon and precisely tune the band structure and photoluminescence (PL) of host CsPbBr₃ NCs by inducing the trap states within the band gap. Time-resolved photoluminescence (TRPL) spectroscopy revealed that Bi³⁺ ion doping significantly enhance the lifetime of charge carriers. This work indicates that Bi doped CsPbBr₃ NCs occurs better optical properties than pure CsPbBr₃ NCs and can be a promising material for high -performance perovskite LEDs.

An intellectual explosion in the ever-expanding world of nanoscience has led to the development of facile synthetic protocols for a high level of material control at the nanoscale. The culmination of several decades of research has led to the ability to routinely synthesize specific types of cadmium chalcogenide quantum dots with quantum efficiencies exceeding 95%. However, enabled by a facile synthesis and driven forward by excellent optoelectronic properties, the all-inorganic perovskite quantum dots (CsPbX₃; X = Cl, Br, I) have attracted considerable attention among academic researchers for a relatively nascent field. This is a material that performs with up to 90% photoluminescent quantum yield following the synthesis, quite encouraging, but accessing materials with the highest possible quality is crucial to fully realize the potential of the material. This leads to the interesting question: What prevents the quantum efficiency from truly being unity? The same property that enables easy access of the material, namely a low temperature synthesis, also opens the door for a number of very real problems. While it is generally accepted that the lead halide perovskites are positioned with an unusually high defect tolerance, they are not defect impervious, and are particularly susceptible to the pernicious shallow electronic states that result from the lead-rich synthetic conditions and labile surface halides. We have been working to carefully understand the role that the lead-rich surface conditions of CsPbX₃ quantum dots have on the optoelectronic properties of the material, conditions that result in multi-exponential excited state lifetimes and sub-optimal quantum efficiencies. Through careful manipulation of the surface structure we are able to alleviate the deleterious lead-rich surface, changing the hard to describe multi-exponential excited state lifetimes to single-exponential lifetimes typical of two-level systems, an unusual property for a nanocrystal ensemble. For example, through our studies we have found that no more than 50 extra surface leads are responsible for reducing the photoluminescence quantum yield of a CsPbBr₃ nanoparticle to 85%, these lead atoms represent a small but detrimental population of surface lead atoms. A careful manipulation and removal of harmful atoms while maintaining the integrity of the nanoparticle as a whole is crucial in being able to access the highest quality nanomaterials. To this end, tailoring approaches to manage atoms in specific locations or with specific energies is necessary to continue to improve material performance and understand how to best utilize the materials moving forward.

Perovskite quantum dots (QDs) as a new type of colloidal nanocrystals have gained significant attention for both fundamental research and commercial applications owing to their appealing optoelectronic properties and excellent chemical processability. For their wide range of potential applications, synthesizing colloidal QDs with high crystal quality is of crucial importance. However, like most common QD systems, those reported perovskite QDs still suffer from a certain density of trapping defects, giving rise to detrimental non-radiative recombination centers and thus quenching luminescence. In this work, we developed a trioctylphosphine (TOP)-based route which yields phase-stable monodisperse CsPbI₃ QDs with the best-so-far quantum efficiency up to 100%, signifying the achievement of almost complete elimination of the trapping defects. Ultrafast kinetic analysis with time-resolved transient absorption spectroscopy evidences the negligible electron or hole trapping pathways in our QDs, which explains such a high quantum efficiency. Solar cells based on these high-quality perovskite QDs exhibit power conversion efficiency of 9%, showing great promise for practical application.

The fabrication and growth of both thin films and single crystals of metal halide perovskites, are typically done using highly polar solvents such as dimethyl sulfoxide, dimethylformamide and gamma-butyrolactone. Notably, these solvents all contain the functional group R=O. Conversely, the synthesis of perovskite nanocrystals employs a combination of non-polar solvents, and weaker aliphatic amines and carboxylic acids, which act as the ligands. It is well known that the ammonium capped nanoparticles are inherently unstable due to the lability of the ligands, resulting in the formation of defects on the nanoparticle surface. To date, there have been few attempts to impart the know-how which has been developed in macro scale syntheses to nanoparticle syntheses.

Here, we explore the application of a phosphine oxide (R₃P=O) to synthesise cesium lead bromide perovskite nanocrystals, replacing the labile amine ligand and mirroring the R=O group required in the macroscale synthesis. The synthetic route we present provides us with exceptionally high yields of very monodisperse particles, which are not only highly emissive, but can also be synthesised in air, obviating the need for laborious solvent degassing, and complicated oxygen and moisture free techniques.

Through a comparison of both synthetic routes, we observe improved stability of the R₃P=O nanocrystals to washing. Additionally, we are able to expand this synthetic protocol to a variety of other anions and cations, as well as being able to replace the carboxylic acid with either phosphonic or phosphinic acids , demonstrating the robustness and versatility of this technique. By using nuclear magnetic resonance and x-ray photoelectron spectroscopy we are able to investigate the surface of our nanoparticles and determine the cause of increased stability and show the importance of phosphine oxide in the synthesis of perovskite nanocrystals.

In contrast to bulk perovskite films, recent reports have shown that in perovskite nanocrystals the excitonic absorption onset and photoluminescence (PL) peak exhibit a blue shift due to quantum confinement when their size in at least one dimension approaches the exciton Bohr radius of the respective perovskite composition.[1] For decreasing thickness of organic-inorganic perovskite nanoplatelets – separated by centrifugation – increasing exciton binding energies and decreasing PL decay times have been observed.[2] Recently, we have developed a new method which enables the direct synthesis of quantum-confined inorganic CsPbBr₃ nanoplatelets of uniform thickness with an atomic layer precision. These nanoplatelets are characterized by a highly efficient light-emission in the blue spectral range. Here, transient absorption spectroscopy is applied to CsPbBr₃ nanoplatelets of different thickness varying from two to six monolayers to gain additional insight into confinement effects on the dominant exciton dynamics in these systems. A special focus is put on the fast exciton-exciton annihilation process, which plays an increasingly important role for higher excitation powers.

[1] J. A. Sichert, Y. Tong, N. Mutz, M. Vollmer, S. Fischer, K. Z. Milowska, R. G. Cortadella, B. Nickel, C. Cardenas-Daw, J. K. Stolarczyk, A. S. Urban, J. Feldmann, Nano letters 2015, 15(10), 6521-6527.

[2] V. A. Hintermayr, A. F. Richter, F. Ehrat, M. Döblinger, W. Vanderlinden, J. A. Sichert, Y. Tong, L. Polavarapu, A. S. Urban, J. Feldmann, Advanced Materials 2016, 28(43), 9478-9485.

Lead halide perovskites APbX₃ [A = methylammonium (MA), formamidinium (FA), and Cs, X=I, Br, and Cl] has excellent properties suitable for optical devices. The halide perovskites have been extensively studied on single crystals and thin films. Moreover, the synthesis of high-quality nanocrystals has been developed in recent years [1]. Since excitons are strongly confined within nanocrystals, optical responses depends on the size of the nanocrystals. One advantage of nanocrystals is that a spatially confined exciton enhances the radiative recombination rate, and then nanocrystals exhibit high photoluminescence quantum yields (PLQY_S) even at room temperature. On the contrary, such spatial confinement works negatively for multiple excitons leading to a decrease in PLQYs as the non-radiative Auger recombination rate of multiple excitons is enhanced [2, 3]. Therefore, it is necessary to elucidate the impact of spatial confinement on the radiative processes for nanocrystal device applications. In particular, size-dependent PL dynamics of nanocrystals should be elucidated.
In this study, we report on the size-dependent PL dynamics of perovskite nanocrystals revealed by single dot spectroscopy. The samples used in this study were two types of perovskite nanocrystals with different A site cations (FAPbBr₃ and CsPbBr₃), which were prepared by hot injection method. Time-resolved and spectrally resolved PL measurements were performed for individual single nanocrystals. Single-dot spectroscopy enables us to analyze optical properties for each individual nanocrystal that cannot be obtained in ensemble nanocrystal measurements. From the obtained results, we clarified that those nanocrystals with larger absorption cross sections exhibit longer exciton lifetimes. Since this tendency was observed with both FAPbBr₃ and CsPbBr₃ nanocrystals, it was suggested that spatial confinement is the dominant factor determining the exciton lifetime. Furthermore, since the trion (charged exciton) generation is a major factor lowering PLQYs, the size dependence of trion generation probability is analyzed from the proportion of on-state and off-state in PL blinking behavior. These nanocrystal size dependences of PL dynamics for excitons, trions and multiple excitons provide important insights for improving the performance of nanocrystal-based devices.
Part of this work was supported by JST-CREST (JPMJCR16N3).

[1] L. Protesescu et al., Nano Lett. 2015, 15, 3692.
[2] J. A. Castaneda et al., ACS Nano 2016, 10, 8603.
[3] N. Yarita et al., J. Phys. Chem. Lett. 2017, 8, 1413.

Hybrid organic-inorganic lead halide perovskites have revolutionized optoelectronic applications, including solar cells, light-emitting diodes, and photo/radiation detectors. Despite the rapid advances in applications, fundamental understanding of these remarkable materials has just begun. While most applications use thin films that contain multiple grains, high-quality single crystals that contain minimal defects are pertinent to understanding the fundamental properties, hence the limit of attainable optoelectronic performance. However, depositing electrical contacts on fragile single crystals can often alter them, e.g. by inducing doping. Here we present two contactless techniques, far-infrared (FIR) reflectance and surface photovoltage (SPV) measurements, to probe lattice dynamics and photogenerated carrier dynamics, respectively, on high-quality methylammonium lead bromide (MAPbBr₃) single crystals. FIR reflectance shows three coherent infrared-active phonon modes between 40 to 200 cm^–1 that result in reststrahlen bands with much higher peak reflectance than has been previously reported for single crystal MAPbBr₃. The phonon mode strength and damping are comparable to what is commonly observed in classical oxide perovskite single crystals, suggesting phonon coherency across macroscopic scale and that the crystal likely contains few grain boundaries or stacking faults. However, these crystals can still contain point defects, which contribute to SPV signals. By performing SPV measurements over different spectral ranges, we are able to separate the effects of surface and bulk defects on the recombination dynamics of photogenerated charged carriers. We further apply SPV measurements to obtain the minority carrier (electron) diffusion length for the MAPbBr₃ crystal. This study demonstrates that both FIR reflectance and SPV measurements provide useful electromagnetic response information on the halide perovskite single crystal properties.

In the pursuit of efficient tandem solar cells, high band-gap photovoltaic absorbers are required for top cells. Hybrid perovskites are promising candidates, as their band gaps can be widely tuned via compositional adjustments. Specifically, perovskites of the form FA_1-xCs_xPb(I_1-yBr_y)₃ have shown high device efficiencies as well as high band gaps suitable for tandem applications [1]. In this family of perovskites, we have observed a tetragonal-cubic phase transition that is coincident with a change in the temperature coefficient of the band gap and also occurs within solar cell operating temperatures. It has been suggested [2] that the phase transition may impact light induced phase segregation, which has been identified as a major concern that lowers operating voltages in mixed cation perovskite solar cells [1]. Thus, it is key to understand the phase behavior of this family of perovskites, as well as the impact of structural changes on electronic properties.

We explore here a wide compositional space of FA_1-xCs_xPb(I_1-yBr_y)₃ thin films with temperature-dependent synchrotron X-ray diffraction. At room temperature, many of these compositions exhibit tetragonal peaks that we have indexed to the P4/mbm space group, corresponding to concerted octahedral tilting about the c axis of the perovskite crystal structure. This is distinct from tetragonal MAPbI₃, which has alternating tilt directions about the c axis corresponding to the I4/mcm space group. We compute the structure factors of a number of Bragg peaks that allow us to model the average octahedral tilt angle as a function of the temperature. Within the FA_1-xCs_xPb(I_1-yBr_y)₃ family, we show how varying the composition affects the phase transition temperature and we present a room temperature phase diagram. Additionally, we show that the temperature coefficient of the band gap presents a discontinuity at the phase transition temperature. This effect is related to band gap tuning observed in mixed Pb/Sn perovskites, which share the same tetragonal space group [3]. This work establishes a phase diagram that may help interpret photostability and will enable better prediction of band gaps in this family of hybrid perovskites.

References

[1] K.A. Bush et al. Compositional Engineering for Efficient Wide Band Gap Perovskites with Improved Stability to Photo-induced Phase Segregation, ACS Energy Lett. 3, 428-435 (2018).
[2] A. J. Barker et al. Defect-Assisted Photoinduced Halide Segregation in Mixed-Halide Perovskite Thin Films, ACS Energy Lett. 2, 1416-1424 (2017).
[3] R. Prasanna et al. Band Gap Tuning via Lattice Contraction and Octahedral Tilting in Perovskite Materials for Photovoltaics, J. Am. Chem Soc. 139, 32, 11117-11124 (2017).

Lead-halide perovskites are a family of semiconductor materials with excellent optoelectronic properties ideally suited for next-generation photovoltaic and light-emitting applications. Particularly, inorganic perovskites CsPbX₃ are drawing increasing research interests because of their better stability. There are rich structural phase transitions in the inorganic perovskites owing to their soft and dynamical ionic lattice. However, the fundamental understandings of intrinsic phase transition mechanism are still elusive. In this talk, I will focus on the systematical study of the phase transition between a non-perovskite and a perovskite phase using single-crystal CsPbI₃ nanowires platform. The non-perovskite phase with a large bandgap and poor photoactivity can be thermal-driven transformed to a meta-stable perovskite phase with a decreasing bandgap and excellent optoelectronic properties. Moisture introduces vacancy in the crystal lattice and lowers the kinetic barrier from perovskite phase to non-perovskite phase. We further realize robust thermochromic solar cells for smart photovoltaic window applications via stable, controllable and reversible phase transition. Another feature of the soft ionic lattice is facile ion migration. Anion exchange chemistry was demonstrated in CsPbX₃ nanostructures with high PLQY throughout the exchange reaction. Via developing a novel localized anion exchange, we demonstrate spatially resolved multi-color CsPbX₃ nanowire heterojunctions. These perovskite heterojunctions show tunable photoluminescence over the entire visible spectrum with high resolution down to 500 nm, which represent key building blocks for high-resolution displays. Moreover, the intrinsic solid-solid anion exchange dynamics can be resolved in these perovskite hetero-junction nanowires through non-destructive optical methods. These fundamental understandings can offer guidelines for engineering the perovskite materials with novel functional devices.

Colloidal lead halide perovskite nanocrystals (NCs) have recently emerged as versatile photonic sources. Their processing and luminescent properties are largely governed by the lability of their surface structure, that is the surface regions of the NC core and ligand shell. In this study and using CsPbBr₃ NCs as a model system,[1] we model the nanocrystal surface structure and its effect on the emergence of the trap states using density functional theory. We then rationalize the typically observed effects of the surface treatment and aging on the luminescent characteristics. We illustrate and discuss the utility of common elemental analysis methods, namely inductively-coupled plasma – optical emission spectrometry and X-ray photoelectron spectroscopy, for elucidating the chemical changes induced by surface treatments. We propose a strategy for healing the surface trapping states and for improving the colloidal stability by the combined treatment with didodecyldimethyl ammonium bromide and lead bromide and validate this approach experimentally. This simple procedure results in robust colloids, which are both highly pure and exhibit high photoluminescence quantum yields of up to 95-100%.

[1] manuscript submitted

The recently re-emerged two-dimensional organic-inorganic hybrid perovskites (2DHPs) are solution processable semiconductors exhibiting strong quantum and dielectric confinement effects, as well as excellent luminescence properties. However, even some of the most fundamental optical properties, especially the complex refractive index (RI), currently are unknown for 2DHP single crystals. As for any optoelectronic materials, refractive index is crucially important since: 1) The real part (n) encodes the phase velocity of light, which, e.g., underpins the resonant condition of a laser cavity, and 2) the imaginary part (k) dictates absorption, which is key for optimizing the thickness of a solar cell. Herein, using a newly-developed dielectric-coating based technique, we determine the complex, anisotropic RI for 2DHP single crystals with various perovskite-layer thicknesses and different types of organic spacers. We found 2DHPs that are only one or two perovskite layers in thickness exhibit negative permittivity due to strong in-plane exciton resonances, but positive permittivity owing to substantially weaker out-of-plane exciton resonances. The extreme excitonic anisotropy in the periodic 2DHPs leads to a natural hyperbolic dispersion in the visible range, which has been previously achieved only with artificial metamaterials. Our observation opens new possibilities in enhanced emission & absorption properties, nonlinear optics, and enhanced light-matter interactions.

Hybrid halide perovskites, extensively used in the field of optoelectronics, are a class of materials extremely promising for their excellent properties combined with the mild synthetic protocols. Mainly used in solar cells¹ and light emitting diodes,^2,perovskites are formed from solution by self-assembly of precursors. The resulting soft material is often unable to express its ideal potential, due to unsuitable morphology or elevated density of electronic defects as a consequence of the low-temperature processing. Based on our previous findings on the interaction between perovskite precursors and addives,^3-4 here we explore the use of a tailored biopolymer, starch, as templating agent for the growth of formamidinium (FA)- and methylammonium (MA)-based tri-iodide perovskite films. The presence of the macromolecule brings the enormous technological advantage of allowing the deposition of the perovskite layer with a one-step method, avoiding solvent dripping or two-steps method. Furthermore, it allows a fine tuning of the solution viscosity (making the solution suitable for different large area deposition techniques), of the perovskite grain size and of the layer thickness, by simply adjusting the polymer:perovskite relative concentration. Our approach was validated by outstanding performances in both light emitting (LED) and photovoltaic (PV) devices with outstanding results. We obtained an inverted, planar, mild temperature processed solar cell with a 17% efficiency and a LED characterized by an EQE of ~5% and, most important, among the highest reported radiances for NIR PeLEDs. (i.e. 206.7 W/sr*m² obtained at very high currents; about 1000 mA/cm²).
The micro and nano-structural changes and their influence on optoelectronic properties were studied as a function of starch concentration, confirming that the device performances are strongly related to the perovskite film structure. The overall picture that emerged is that the presence of starch influences i) the interface properties within the device stack, ii) the crystallographic scenario of the active material, iii) the dielectric landscape surrounding the perovskite crystals, allowing, for the LED device, a minimization of Auger losses at high current regimes and for an overall enhancement of the perovskite light emission and the device capability in sustaining high carriers densities.


(1) Green, M. A.; Ho-Baillie, A. ACS Energy Lett.2017,2(4), 822.
(2) Colella, S.; Mazzeo, M.; Rizzo, A.; Gigli, G.; Listorti, A. J. Phys. Chem. Lett.2016, 7(21), 4322.Green, M. A.; Ho-Baillie, A. ACS Energy Lett.2017,2(4), 822.
(3) Masi, S.; Rizzo, A.; Munir, R.; Listorti, A.; Giuri, A.; Esposito Corcione, C.; Treat, N. D.; Gigli, G.; Amassian, A.; Stingelin, N.; Colella, S. Adv. Energy Mater.2017, 7, 1602600
(4) Masi, S.; Aiello, F.; Listorti, A.; Balzano, F.; Altamura, D.; Giannini, C.; Caliandro, R.; Uccello-Barretta, G.; Rizzo, A.; Colella, S. Chem. Sci.2018, 9(12).

Two dimensional Ruddlesden-Popper perovskite (2D RPP) plays a key role in the current widespread investigations into the potential uses in optoelectronics because of the quantum well superlattices-like structure. This unique structure makes 2D RPP have higher moisture stability, and stronger exciton binding energy than 3D perovskite. However, the complexity in the wet-chemical processes make the synthesis of high purity homologous 2D perovskites still a big issue. In addition, the fundamental mechanism of the molecular origin of the phase transition and unusual blueshift of the emission in 2D RPP with different layered perovskite compounds is still lacking. In this direction, we grew a high purity millimeter-sized single crystal 2D RPP (BA₂MA_n-1Pb_nBr_3n+1 and BA₂MA_n-1Pb_nI_3n+1, n=1, 2, 3) which cover whole visible light range by using slow evaporation at a constant-temperature (SECT) solution growth. Via the XRD, spectral mapping, and fluorescence lifetime mapping measurements, the large sized 2D RPP crystals show well crystallinity and highly phase purity. Furthermore, due to the pyramid step-liked structure, we obtain cavity-free lasing behavior with around 3.7 uJ/cm² low threshold from these homologous large sized crystals. Also, with high pure phase crystal, we can identify the carrier dynamics before and after phase transition directly without defect state induced effect. Our result demonstrates that solution growth homologous organic−inorganic hybrid 2D perovskite single crystals open up a new window as a promising candidate for optoelectronic devices. (This work has been published in Nano Lett. 2018, 18, 3221−3228)

Perovskite-based optoelectronic devices have gained significant attention due to their remarkable performance and low processing cost, particularly for solar cells. However, for perovskite light-emitting diodes (LEDs), non-radiative charge carrier recombination has limited electroluminescence (EL) efficiency. Here we demonstrate perovskite-polymer bulk heterostructure LEDs exhibiting record-high external quantum efficiencies (EQEs) exceeding 20%, comparable with the best solution-processed OLEDs and quantum-dot LEDs. This performance is achieved with an emissive layer comprising quasi-2D and 3D perovskites and an insulating polymer.

Transient optical spectroscopy reveals that photogenerated excitations at the quasi-2D perovskite component migrate to lower-energy sites within 1 ps. The dominant component of the photoluminescence (PL) is primarily bimolecular and is characteristic of the 3D regions. From the near-unity PL quantum efficiencies and transient kinetics of the emissive layer with/without charge-transport contacts, we find non-radiative recombination pathways to be effectively eliminated. Light outcoupling from planar LEDs, as used in OLED displays, generally limits EQE to 20-30%, and we model our reported EL efficiency of over 20% in the forward direction to indicate the internal quantum efficiency (IQE) to be close to 100%. Together with the low drive voltages needed to achieve useful photon fluxes (2-3 V for 0.1-1 mA/cm2), these results establish that perovskite-based LEDs have significant potential for light-emission applications. In general, the ideal luminescence performance we observed is of critical importance for realizing perovskite optoelectronics operating near the radiative limits.

We also present some related results from our group and collaborating groups, on luminescent perovskites for LEDs and photovoltaics.

We report a cost-effect approach for improving the performance of perovskite solar cells by using novel high-mobility 2-dimentional materials. The efficiency of perovskite solar cells can be substantially enhanced when 2-dimensional material flakes are coated on the perovskite grain boundaries. The flakes can conduct hole currents efficiently from grain boundaries to hole transport layers in the devices and lead to efficiency enhancements that increase with increasing hole mobilities of the flakes. The results indicate that perovskite grain boundaries are electrically benign and even favorable for photovoltaic performance if accumulated charges around them are conducted out by high mobility hole transport materials.

The hybrid organic lead halide perovskites based on mixed-cations and -anions have been considered as an outstanding composition to achieve high efficiency with long-term stability of perovskite solar cells (PSCs). In this study, the effect of light on the crystal structure of perovskites employing triple cations and double halides was investigated and correlated to the charge-transfer properties. Under the continuous photon excitation, a gradual increase in photoluminescence was observed from the perovskite film, coupled with a minute red-shift. Furthermore, the continuous light soaking induced a gradual peak shift toward lower angle in the X-ray diffraction pattern of the perovskite, strongly suggesting a photo-induced structural response. Notably, a lattice expansion occurred in a certain preferred orientation, which exerted an influenced on the crystal symmetry. The trap density of a complete PSC near maximum power point (mpp) was measured under continuous light, showing a gradual decrease. An increase in the photocurrent at mpp was readily observed as a consequence of the reduced trap density. The beneficial charge-transfer properties of PSC under illumination were addressed by relating to the photo-induced structural response of the perovskite.

Isopropanol solution based methylammonium lead triiodide (MAPbI₃) is studied during the crystallization process. The crystal growth starts in an unstable suspension far from equilibrium by forming different dendritic patterns and terminates with aggregation of stable cubic crystalline grains into fractal clusters. Using transmission electron microscopy (TEM), the time evolution of a newly mixed suspension was studied over a period of two weeks at room temperature and a sequence of the morphological changes was observed. The crystallization process started with single dendritic growth exhibiting branches at 90 degrees angles to one another. After 4 hours, a multi-dendritic growth pattern and a transformation into small crystalline quantum dots were observed. After a week, clusters of crystal grains were formed into a fractal pattern and these patterns appear to be stable also during the second week. Electron and x-ray diffraction revealed the crystallinity of the quantum dots and the clusters of micrometer-sized crystals. Scanning transmission electron microscope (STEM) together with energy dispersive X-ray spectroscopy (EDS) showed that newly formed large grains, from a one hour old solution, displayed a core-shell structure with higher percentage of Pb atoms as compared to iodine at the surface. In, the inner core of the grains the percentage of iodine was slightly higher. The electron diffraction (ED) scan over the newly formed grains revealed a polycrystalline surface whereas the inner part had a single crystal pattern. The same solution, now one-week-old, contained grains with only single crystal patterns in the ED scan and showed no core-shell character or polycrystalline surface. The measured percentage of iodine atoms compared to lead was 2:1 throughout the cross section, which is a quantitative value within the measurement. It can be concluded from these measurements that the suspension approaches higher crystallinity of the perovskite grains in an irreversible process, where the perovskite grains are insoluble in isopropanol. The perovskite material has also been characterized with scanning electron microscopy (SEM) and photoluminescence (PL) mapping where both techniques showed a very porous crystalline material. The PL mapping revealed two peaks at 730 and 760 nm for a thin film spin coated from a newly mixed solution, while a film deposited from a one week old solution showed three peaks, the last one at 830 nm. Because of the high crystallinity, it is suggested that all three peaks are due to band-to-band transitions and not due to localized states. These data will be analyzed further; however, the results contain information of the content of quantum dots versus larger crystals, as well as displaying emission intensity variations at different positions of the grains. The purpose with this project is to understand these phenomena of crystal growth. A new mesoporous perovskite material has been designed for optoelectronic purposes.

The ongoing development of halide perovskite (HaP) based optoelectronics has revealed that the numerous interfaces in the device play a crucial role for device functionality, efficiency and stability.¹ Importantly, many critical material and interfacial properties are still poorly understood and difficult to assess, a deficiency that often limits efforts to improve the performance.² I will present our most recent results exploring the mechanisms by which organic semiconductor films, transition metal oxides, and carbon nanotube (CNT) interlayers enable or suppress charge transfer to an adjacent HaP semiconductor film.
We use photoemission, X-ray absorption, and transient optical spectroscopy to probe the energy level alignment and exchange of charge carriers between a set of various lead halide based perovskite films and different charge transport layers. Therein, we show that ground state charge transfer between perovskite and a CNT layer can lead to band bending in the transport layer beneficial for charge extraction.³ Hence, in one example an integrated thin CNT interlayer facilitates rapid charge carrier collection and improves the performance of respective solar cells.⁴ In contrast to this observation, band bending induced in the perovskite film by the charge transport material such as high work function oxides or interface defects induced by the oxide formation process can be detrimental for charge carrier collection and thus impede the functionality of respective devices.⁵
After examining the recent key developments in chemical and electronic structure characterization of HaPs, I will iterate the next steps towards interlayer tailoring and analysis.

[1] P. Schulz, ACS Energy Lett. 3 (2018) 1287–1293
[2] R. L. Z. Hoye, P. Schulz, L. T. Schelhas, A. M. Holder, K. H. Stone, J. D. Perkins, D. Vigil-Fowler, S. Siol, D. O. Scanlon, A. Zakutayev, A. Walsh, I. C. Smith, B. C. Melot, R. C. Kurchin, Y. Wang, J. Shi, F. C. Marques, J. J. Berry, W. Tumas, S. Lany, V. Stevanović, M. F. Toney, T. Buonassisi, Chem. Mater. 29 (2017) 1964–1988
[3] P. Schulz, A.-M. Dowgiallo, M. Yang, K. Zhu, J. L. Blackburn, J. J. Berry, J. Phys. Chem. Lett. 7 (2016) 418-425
[4] R. Ihly, A.-M. Dowgiallo, M. Yang, P. Schulz, N. J. Stanton, O. G. Reid, A. J. Ferguson, K. Zhu, J. J. Berry, J. L. Blackburn, Energy Environ. Sci. 9 (2016) 1439-1449
[5] P. Schulz, J.O. Tiepelt, J. A. Christians, I.Levine, E. Edri, E.M. Sanehira, G. Hodes, D. Cahen, A. Kahn, ACS Appl. Mater. Interfaces 46 (2016) 31491-31499.

Hybrid organic-inorganic perovskite thin films or crystals are usually prepared from the perovskite precursor solution. The crystallization of perovskite is affected by the supersaturation of the perovskite precursors in solution and the interfacial energy between the perovskite and solvent. Here, I will present some of our methods to prepare high-quality perovskite thin films and crystals by varying the interfacial energy between perovskite and solvent. The high-quality peorvskite thn films lead to highly efficient perovskite solar cells.

Photon recycling is critical for achieving record-high open-circuit voltages (V_OC) and power conversion efficiencies (PCE) in single-junction GaAs solar cells. This phenomenon can be observed only in materials with extraordinarily high photoluminescence quantum efficiency (PLQE) and small escape cones. Photon recycling has been observed in perovskite solar cells but has not significantly improved device performance thus far. With recent advances in perovskite material quality, photon recycling could produce practical gains in V_OC and PCE. However, the achievable gains in today’s materials remain uncertain. Here we investigate photon recycling in the most emissive (>90% internal PLQE) perovskite films reported to date—humidity-passivated CH₃NH₃PbI₃, phosphine-oxide-passivated CH₃NH₃PbI₃, and KI-passivated triple-cation perovskites (Cs_0.06FA_0.79MA_0.15Pb(I_0.85Br_0.15)₃). Using experimentally measured rate constants and material parameters, we develop a rigorous framework for quantifying (the extent of) photon recycling in thin-film samples and predicting the associated V_OC and PCE gains. This analysis clarifies the opportunity for photon recycling to push the real-world performance of perovskite solar cells toward theoretical limits.