Symposium ES15—Fundamental Understanding of the Multifaceted Optoelectronic Properties of Halide Perovskites

Power conversion efficiency (PCE) of lead halide perovskite solar cell (over 23%) has surpassed those of CIGS and CdTe, approaching the top value of crystalline Si cell. Our group has been able to achieve PCE over 21% by low cost ambient fabrication. However, high PCE of single-cell enabled by lead halide-based perovskite absorbers are now being saturated, taking the Shockley Queisser (SQ) limit of open-circuit voltage (V_OC) (ca.1.32V) into account. Tandem cell making, which can further increases PCE up to 28% or more, leads to higher material and process cost and will raise a question if performance/cost ratio can be accepted in industry. Therefore, a smart way is to create a single cell which has high PCE comparable with that of GaAs (>28%) by reducing bandgap energy to <1.4 eV without accompaniment of increase in V_OC loss. This possibility will be in a family of metal halide perovskite out of those depending on use of lead. In addition to such efficiency issue, high performance of organo lead halide materials is not compatible with robust high stability required for practical use. Ensuring the intrinsic thermal stability (desirably >200^oC) of the perovskites is a key issue before industrialization. In addition, toxicity of lead-based perovskites are going to become the most formidable challenges for real use (commercialization), in particular, for applications to IoT society, which is one of the most promising field of perovskite photovoltaic device in terms of high voltage output even under weak illumination. These thoughts urge us to concentrate our next research of perovskite photovoltaics (PV) more on development of non-lead high efficiency absorbers. Sn perovskite is still a strong candidate because Sn(II) has been found to be stabilized against ambient air by metal doping method (such as Ge). Regarding Bi-based perovskites, we found AgBi₂I₇ as a promising all-inorganic absorber having high thermal and moisture stability. Stability also highly depends on the property of charge transport materials (CTMs), especially, the kind of hole transporter. Spiro-OMeTAD does not work at high temperature while P3HT, for example, is thermally stable. In our collaboration with JAXA, P3HT-based perovskite devices showed robust stability by exposure to high (100^oC) and low (-80^oC) temperatures and also to high energy particle radiations (iScience, 2018, 2, 148). Selection of CTMs is another important key in combination with non-lead perovskite materials. In conclusion, next direction of perovskite PV should be to enhance PV performance of non-lead all-inorganic semiconductor materials by extended compositional engineering, in parallel with developing thermally stable CTMs. Our on-going studies on non-lead perovskite materials in our group will be introduced in the talk.

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 23.3% in 2018. According to Web of Science, publications on PSC increase exponentially since 2012 and total number of publications reaches already over 10,000 as of October 2018, which is indicative of a paradigm shift in photovoltaics. Although small area cell exhibited superb efficiency surpassing the performance of thin film technologies, scale-up technology is required toward commercialization. In addition, further higher efficiency toward Shockley–Queisser limit is required in parallel. In this talk, Large-area coating technology is introduced using perovskite cluster embedded coating solution, followed by brief introduction on history of perovskite solar cell. Bi-facial stamping method was developed for not only scale-up technique but also interface modification and low-temperature phase stabilization. For higher efficiency, managing recombination is critical. Methodology reducing recombination is developed via interface and bulk engineering. Current-voltage hysteresis is also discussed because hysteresis is related to the stability of perovskite solar cell. Ion migration is now visualized and confirmed to correlate with hysteresis.

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 band gaps and relatively benign defects and grain boundaries for systems based on Group 14 metals (e.g., Ge, Sn and Pb) [2]. Indeed, these materials have enabled unprecedented rapid improvement in perovskite photovoltaic performance to levels above 20% power conversion efficiency and with open circuit voltages above 1V for a single junction photovoltaic (PV) device [3]. This talk will provide an historical perspective on foundational work related to the organic-inorganic perovskite semiconductors, including discussion of crystal structure flexibility [4,5], semiconducting properties, film deposition approaches and electronic device applications of the three-dimensional and lower-dimensional perovskite structures. 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] W.-J. Yin, T. Shi, Y. Yan, Adv. Mater. 26, 4653 (2014).
[3] W. S. Yang et. al., Science 356, 1376 (2017).
[4] B. Saparov and D. B. Mitzi, Chemical Reviews 116, 4558 (2016).
[5] D. B. Mitzi, Prog. Inorg. Chem. 48, 1 (1999).

Metal halide perovskites are exciting materials for a range of optoelectronic devices. One of their most tantalizing features is the potential for tunable emission with high luminescence yields. Such properties are promising for reaching the radiative efficiency limits in single and multi-junction solar cells as well as color-tunable light-emitting diodes. However, there are a number of challenges in attaining high luminescence yields and color stability across a range of bandgaps.

Here, I will present a selection of our group’s ongoing work to understand the origin of non-radiative losses in a range of halide perovskite films, crystals and device systems, and how we can use this information to push materials and devices towards their efficiency limits. We use a selection of nano- and micro-scale imaging techniques including photoluminescence, photo-emission and nano-X-Ray-Diffraction microscopy to visualise the impact of defects and strain on local charge carrier recombination. We also employ passivation techniques designed to remove these spatially heterogeneous losses, which we demonstrate on small and large bandgap bulk 3D perovskites and 2D confined perovskite nano-platelets. Finally, we show that these approaches ultimately lead to improved solar cell and LED performance and bandgap stability.

In recent years, the organic-inorganic hybrid perovskite has gained an increasing research interest in academia for applications in thin film solar cells, due to rapidly increased efficiency (from 3.8 to 23.3% within a decade) [1], high absorption coefficient [2], low-cost fabrication process, and material availability [3]. Among the hybrid perovskites, MAPbI₃ (CH₃NH₃PbI₃) based solar cells have shown high power conversion efficiencies but with several obstacles such as thermal instability, hysteresis loss at room temperature. Therefore, the commercialization of these solar cells is still a challenge. Understanding and resolving these issues necessitate the investigation of the sample at the atomic scale to determine the underlying fundamental processes.
Here, we present the growth and experimental characterization of thin MAPbI₃ films on Au (111) under ultra-high vacuum conditions (UHV=1x10^-10 Torr). The thin films were prepared by vacuum evaporation of the precursor molecules MAI and PbI₂ with a thickness of a few monolayers (approx. 4 nm). We characterize the sample with scanning tunneling microscopy (STM), and X-ray photoelectron spectroscopy (XPS), obtaining information about the atomic structure, and chemical composition. For the electronic properties analysis, we used ultraviolet photoemission spectroscopy (UPS), and inverse photoemission spectroscopy (IPES). Our study will provide the basis for further understanding ion incorporation and stability at the atomic scale.

Reference
Kojima A¹, Teshima K, Shirai Y, Miyasaka, J. Am. Chem. Soc., 2009, 131 (17), pp 6050–6051 (2009).
Wan-Jian Yin, Tingting Shi, Yanfa Yan, Adv. Mater., 26: 4653–4658 (2014).
Stefano Razza, Sergio Castro-Hermosa, Aldo Di Carlo, and Thomas M. Brown, APL Materials 4, 091508 (2016).

Hybrid organic-inorganic perovskites possess a diverse set of properties that make them excellent materials for a variety of applications. New perovskite compounds with complex structures and novel properties promise to further expand the applicability of this class of materials, but may require new processing approaches to be synthesized. Issues of solubility and degradation in the synthesis of complex perovskites can be addressed via resonant infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE). RIR-MAPLE is a physical vapor deposition technique that has been shown to deposit perovskite materials in a manner that preserves the integrity of the perovskite components and maintains the composition of the target source. [1, 2] Because RIR-MAPLE uses lower concentration solutions to form frozen targets (compared to conventional approaches) and has different deposition schemes available to deliver precursors to the substrate, [3] it is uniquely positioned to act as a framework for studying complex perovskites with limited solubility.

Using CH₃NH₃PbI₃ (methylammonium lead triiodide, or MAPbI) as an initial model system, the effects of RIR-MAPLE solution concentration and deposition scheme were studied. Because MAPbI can easily be spin-cast and its properties have been widely studied, it acted as a reference to compare any effects of RIR-MAPLE deposition on material properties.

Baseline (21.7 mM) concentrations of CH₃NH₃I (methylammonium iodide, or MAI) and PbI₂ were increased by a factor of 1.5 times to yield solutions that had high inorganic, high organic, and high overall component concentrations. Reference spin-casting solutions were created by using 1.4 and 1.6 M concentrations of MAI and PbI₂ to create the same relative non-stoichiometric conditions as in the RIR-MAPLE solutions. Also, a small amount (~20 µL) of monoethylene glycol (MEG) was added to additional spin-cast solutions to observe possible effects on material properties because this chemical serves as a co-matrix to resonantly absorb laser energy during RIR-MAPLE deposition.

RIR-MAPLE deposition schemes based upon multiple source solutions within a partitioned target cup were adapted for MAPbI to drastically alter how perovskite precursors are delivered to the chosen substrate. Substrate temperatures were controlled across all deposition schemes to observe any effects on how precursors react in these schemes.

The crystallographic properties of all films were studied using X-ray diffraction (XRD) and synchrotron grazing-incidence wide-angle X-ray scattering (GIWAXS). Film morphology was studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The optoelectronic properties of the films were determined by photoluminescence and UV-Visible absorbance spectroscopies.

This work is important to establish fundamental growth mechanisms of perovskite thin films deposited by RIR-MAPLE and expands upon previous foundational studies. By demonstrating versatility in solution compositions and deposition schemes, RIR-MAPLE is positioned favorably to address the requirements to synthesize complex perovskite materials systems.

This research used beamline 11-BM of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704.

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

References:
[1] Barraza, E.T., Dunlap-Shohl, W.A., et al. J. Electron. Mater. 47 (2018)
[2] Dunlap-Shohl, W.A., Barraza, E.T., et al. ACS Energy Lett. 3 (2018)
[3] Ge, W., et al. Org. Electron. 22 (2015)

Organic-inorganic perovskite solar cells are currently under the spotlight. Despite numerous advantages, their poor stability hinders commercialization of perovskite-based devices. To increase perovskite stability various strategies have been envisaged [1]. Mixing different halides (I, Br, Cl) has been shown both experimentally and theoretically to have a strong impact on the device performance and stability [2-5]. However, the stabilizing effect of the halides critically depends on their distribution in the mixed compound, a topic that is currently under intense debate [6-8]. A fundamental understanding remains largely elusive regarding the correlation between the structure of the mixed-perovskites and their electronic properties at the atomic level.
In this work, combining scanning tunneling microscopy (STM), density functional theory (DFT) and UV/X-ray photoelectron spectroscopy (UPS/XPS), we reveal the exact location of I and Cl anions in the mixed CH₃NH₃PbBr_3-yI_y and CH₃NH₃PbBr_3-zCl_z perovskite lattices. Additionally, we demonstrate the impact of halide-incorporation on the material electronic properties and stability. Furthermore, we determine the ideal Cl-incorporation ratio for stability increase without detrimental bandgap modification. The increased material stability induced by chlorine incorporation is verified by performing photoelectron spectroscopy on a device architecture. Our findings provide an important direction for the fabrication of stable perovskite devices.
References:
[1] Noh, J. H.; Im, S. H.; Heo, J. H.; Mandal, T. N.; Seok, S. I., Chemical Management for Colorful, Efficient, and Stable Inorganic–Organic Hybrid Nanostructured Solar Cells. Nano Letters 2013, 13 (4), 1764-1769.
[2] Liu, J.; Prezhdo, O.V. J. Phys. Chem. Lett. 2015, 6 (22), 4463.
[3] Quarti, C.; Mosconi, E.; Umari, P.; De Angelis, F. Inorg. Chem. 2017, 56, 74.
[4] Colella, S.; Mosconi, E.; Pellegrino, G.; Alberti, A.; Guerra, V.L.P.; Masi, S.; Listorti, A.; Rizzo, A.; Condorelli, G.G.; De Angelis, F.; Gigli, G. J. Phys. Chem. Lett. 2014, 5 (20), 3532.
[5] Yu, H.; Wang, F.; Xie, F.; Li, W.; Chen, J.; Zhao, N. Adv. Funct. Mater. 2014, 24, 7102.
[6] Ye, M.; Hong, X.; Zhang, F.; Liu, X. J. Mater. Chem. A 2016, 4, 6755.
[7] Zhang, T.; Yang, M.; Benson, E.E.; Li, Z.; Lagemaat, J.; Luther, J.M.; Yan, Y; Zhu, K; Zhao, Y. Chem. Commun. 2015, 51, 7820.
[8] Luo, S.; Daoud, W.A. Materials 2016, 9, 123.

Incorporation of inorganic cations has been recently demonstrated a technique to enhance the efficiency and lifetime in perovskite solar cells. However, the understanding and correlation of ultrafast photophysics in relation with opto-electrical properties to the device performance are still lacking. Here, we systematically analysis the role of inorganic cations on the photophysics using photoluminescence techniques and correlate with photovoltaic properties. The lack of the integration of Rb⁺ with organic cations MAFA (MA, methylammonium, and FA, formamidinium,) leads to higher defect density and trap-assisted monomolecular recombination in thin films. While, the incorporation of Cs⁺, increases the perovskite grain size and shows longer charge carrier lifetime by mitigating the defects to enhance the power conversion efficiency (PCE); indicating the better incorporation of Cs⁺ to MAFA. While the concomitant presence of Rb⁺ with Cs⁺, delay the non-radiative losses by suppressing the defect density significantly in quaternary-cation based perovskite system (RbCsMAFA) compared to double-cation MAFA system. Lower defects density and a more balanced charge carrier diffusion length results the PCEs over 19% in quaternary-cation perovskites though photoluminescence quantum yield (PLQY) is comparable. While the concomitant incorporation of Rb+/Cs⁺ is the key to suppress the defects and charge carrier recombination for efficient photovoltaics, solitary integration of Rb⁺ with MAFA is still a challenge.

Due to their unique opto-electronic properties, metal halide perovskites are of great interest as solar energy material. At present, non-radiative losses prevent solar cell efficiencies to reach their theoretical maximum. In this work we suppressed the non-radiative decay in methylammonium lead iodide(MAPbI₃) layers by exposing them to a light soaking treatment under ambient conditions. First of all, this treatment leads to an increase of the PL quantum efficiency from < 1% to 50%. Additionally, photo-induced time-resolved microwave conductivity (TRMC) data demonstrate that while the mobility and the trap density remain constant, the light soaking treatment reduces the non-radiative band-to-band recombination between electrons and holes. We attribute the enhancement in effective carrier lifetimes to an increased fraction of radiative recombination, leading to enhanced recycling of carriers.¹ These light soaking studies were extended to mixed cation, mixed halide perovskites, (FA_0.79MA_0.16Cs_0.05) Pb (I_1-xBr_x)₃ which are known to undergo phase separation under continuous illumination. For x < 0.5 we find that on light soaking in a nitrogen environment, the charge carrier lifetime increases, while for x > 0.5 the lifetime shortens. By analysing the TRMC traces, we propose that for x < 0.5 light soaking leads to a reduction of charge trapping in shallow states. Next, we investigated how additives such as K⁺, Cs⁺ and Rb⁺ affect the mobility and decay kinetics of photo-induced excess carriers in metal halide perovskites.^2,3 The above results help to provide a framework for which cation and halide composition the best performance and stability can be expected.

¹Brenes, R.; Guo, D.; et al. Metal Halide Perovskite Polycrystalline Films Exhibiting Properties of Single Crystals. Joule 2017,1, 155-167.
²Hu, Y.; et al. Perovskite Solar Cells: Understanding the Role of Cesium and Rubidium Additives in Perovskite Solar Cells: Trap States, Charge Transport, and Recombination. Advanced Energy Materials 2018, 8, 1870073.
³ Abdi-Jalebi, M.; et al. Maximizing and stabilizing luminescence from halide perovskites with potassium passivation. Nature 2018, 555, 497.

Halide perovskite nanocrystals demonstrate intriguing optical properties such as near unity quantum yields, fast radiative recombination and wide emission tunability. Importantly, not only their size plays a role, but also the dimensionality, as previously shown for 2D nanoplatelets (NPls), 1D nanowires and 0D quantum Dots (QDs). Particularly the NPls provide an excellent platform for understanding the fundamental properties of perovskites as their thickness can be tuned with monolayer-controlled precision. Due to a reduced screening effect, not only do they exhibit extremely high exciton binding energies but also strongly enhanced carrier cooling rates. Using transient absorption and photoluminescence spectroscopy, we investigate the thickness-dependent properties of these nanoplatelets. In this presentation we present our insights into carrier relaxation, exciton-exciton annihilation, exciton diffusion in single NCs and NC films as well as energy transfer between NPls of varying thickness.

Hot carrier relaxation and transfer in the archetypal CH₃NH₃PbI₃ is investigated using ultrafast pump-push-probe spectroscopy. Excited state absorption of thermalized carriers from the perovskite band edge to higher states is stimulated by using a push pulse, with carrier relaxation times consistent with the literature. Our results reveal evidential charge transfer to bathophenanthroline (bphen), an organic acceptor with energy levels 1 eV above the perovskite band edges. This ultrafast sub-ps charge transfer is only realized after overcoming the interfacial barrier between the perovskite and bphen. Utilising the push pulse with supporting theoretical analysis suggests that the broad photo-induced absorption band in the visible region unambiguously arises due to the promotion of excited carriers to higher excited states. Understanding the ultrafast charge transfer is crucial for the development of efficient, functional hot carrier solar cells and optoelectronic devices.

Among the aspects of Halide Perovskites, HaPs, which make them such fascinating materials the different time-scales of the dynamics of interconnected processes stand out. Short-time behavior (< sec) is determined by electronic charge carrier dynamics, while longer time effects are typically due to atom/ion dynamics, characteristic of halide perovskites, such as ion / defect movement, self-healing and others [1-3]. These slower processes are likely interdependent and hitherto not (well) elucidated, both as phenomena per se, and in terms of effects on the performance of HaP-based devices.
Our recent work [2] provides clear evidence for self-healing, i.e., under certain conditions damage in optical properties can be reversed and, in particular, the status quo ante [3] can be re-established completely or mostly, in several types of Br-based HaP single crystals, as measured by 2-photon confocal microscopy. We demonstrated qualitatively but unequivocally that the effect is an intrinsic property of the materials, as it was measured in the bulk of single crystals, with typical times of minutes to hours.
Here we report on the products of decomposition and possible chemical pathways that can lead to self-healing, paying particular attention to kinetics of the phenomena. We include now quantitative results of our experiments following the degradation / healing process kinetics in situ and of the energy-dependent damage threshold.
We also show results for the more intensely studied Methylammonium, MAPbI₃.
Furthermore we measure, analyze and explain the crucial differences between the light-induced damaging and recovery mechanisms in the bulk and at the surface under different atmospheres (i.e. air, N₂, O₂, CH₃NH₂, CH₃COOH).
Finally, we measure and analyze the temperature dependence of the healing process, as it provides information about the energy of formation of the material from their binary halide constituents. This follows and compares to the reported positive enthalpy of formation of the Br and I MAPb perovskites [4-5], viz. their entropic stabilization.
We will put our results in the broader perspective of other HaP physico-chemical properties and performances of HaP-based device characteristics.
References
[1] G. Kim et al. “Large tunable photoeffect on ion conduction in halide perovskites and implications for photodecomposition”, Nat. Mat. 2018
[2] D.R. Ceratti et al.,“Self-Healing Inside APbBr₃ Halide Perovskite Crystals”,Adv. Mat. 1706273, 2018.
[3] W. Nie et al., “ Light-activated photocurrent degradation and self-healing in perovskite solar cells” Nat. Commun., vol. 7, 2016.
[4] E.Tenuta et al.,“Thermodynamic origin of instability in hybrid halide perovskites”, Sci.Rep. 6, 37654, 2016.
[5] G. P. Nagabhushana et al. “Direct calorimetric verification of thermodynamic instability of lead halide hybrid perovskites”. 113(28): 7717–7721 PNAS, 2016

The constituents of hybrid organic-inorganic lead halide perovskite materials have significantly evolved since the first photovoltaic devices made out of the standard methylammonium lead triiodide (MAPI, CH₃NH₃PbI₃) to the latest developments relying on mixed cations, mixed anions perovskite systems and arrangements of 2D-3D layers. Indeed, it appears that mixed-composition perovskites and complex structures including a low-dimensional perovskite layer on top of a bulk three-dimensional perovskite film can perform significantly better than standard systems, via both a better short-circuit current, and a larger open-circuit voltage. These parameters, commonly used to assess the performances of photovoltaic devices, directly relate to more fundamental properties of the material: the bandgap of the absorber, determined by its electronic structure, and the ratio between radiative and non-radiative charge carrier recombination quantum yields, which depends on more complex dynamical phenomena and scattering processes.
Here, we use a combination of ultrafast spectroscopy techniques to scrutinize the carrier dynamics in mixed-cations, mixed-halide lead perovskite thin films. Our results evidence the formation of charge transfer excitons (CTE) astride the boundaries of domains of various halide compositions. A global analysis of photoinduced transient Stark signals shows that CTE evolve gradually from Br-rich to I-rich domains over tens to hundreds of picoseconds. Rather than constituting recombination centers, boundaries between domains of various halide compositions appear then to favor charge carrier separation by driving photogenerated holes along channels of decreasing bromide content.
The ultrafst dynamics of photoinduced Stark signals observed in transient absorption spectra of 2D-3D layered perovskites perpared by use of various long-chain organic cations allowed to evidence charge transfer between domains of different dimensionality taking place in competition with energy transfer. These findings show that vectorial charge separation takes place at the interface, which is at the origin of the improved efficiency of solar cells based on structured materials embodying the low dimensional layer, when compared to pristine 3D perovskite.

The Shockley-Queisser limit for solar cell efficiency of ~ 33% can be overcome if hot carriers can be harvested before they thermalize. Recently, carrier cooling time up to 100 picoseconds was observed in hybrid organic-inorganic lead halide perovskites, but it is unclear whether these long-lived hot carriers can migrate long distance for efficient collection. We report direct visualization of hot carrier migration in CH₃NH₃PbI₃ thin films by ultrafast transient absorption microscopy, demonstrating three distinct transport regimes. Quasi-ballistic transport was observed to correlate with excess kinetic energy; resulting in up to 230 nanometers transport distance in 300 fs that could overcome grain boundaries. The nonequilibrium transport persisted over tens of picoseconds and ~ 600 nanometers before reaching the diffusive transport limit. These results suggest potential applications of hot carrier devices based on hybrid perovskites.

Thermodynamic calculations revealed that single junction solar cell conversion efficiencies can exceed the Shockley-Queisser limits and reach around 66% under 1-sun illumination if the excess energy of hot photogenerated carriers is utilized before they cool down to the lattice temperature (i.e., hot-carrier solar cells). Organic–inorganic lead halide perovskite semiconductors have recently emerged as the leading contender in low-cost high-performance solar cells. The key for the realization of hot-carrier (HC) solar cell include the slow hot-carrier cooling and effective extraction of hot-carrier energies which requires fast hot-carrier injection into charge collection layer before hot-carrier cooling down to the lattice temperature. Another related approach that leverages slow HC cooling for efficient energy extraction of HCs is multiple exciton generation (MEG). MEG or carrier multiplication (CM) is a process that generates more than one electron-hole pair from the absorption of one high-energy photon (with at least twice the bandgap energy, Eg), which can boost the photovoltaic efficiencies to ~44%. Perovskite NCs with their novel slow hot-carrier cooling are therefore also highly promising candidates for MEG.
In this talk, firstly I will present our recent observations that the weakly quantum confined MAPbBr₃ nanocrystals have up to ~ 2 orders slower hot-carrier cooling times and around 4 times larger hot-carrier temperatures than their bulk-film counterpart.^[1] This is attributed to their intrinsic phonon bottleneck and Auger-heating effects at low and high carrier densities, respectively. Importantly, we demonstrate efficient room temperature hot-electrons extraction (up to about 83%) by an energy-selective electron acceptor layer within ~1 ps from surface-treated perovskite nanocrystal very thin films (~30 nm). These new insights would allow the development of extremely thin absorber and concentrator-type hot-carrier perovskite solar cells. In the second part, I will show our most recent works on the efficient MEG (up to ~75% in slope efficiency) with low MEG thresholds (down to ~2.25Eg) in intermediate-confined colloidal FAPbI₃ NCs.^[2] Efficient MEG occurs via inverse Auger process within 90 fs, afforded by the slow cooling of energetic hot carriers. These insights may lead to the realization of next generation of solar cells and efficient optoelectronic devices.

References:
[1] Li, M. et al. “Slow cooling and highly efficient extraction of hot carriers in colloidal perovskite nanocrystals” Nat. Commun. 8, 14350 (2017).
[2] Li, M. et al. “Low threshold and efficient multiple exciton generation in halide perovskite nanocrystals” Nat. Commun. 9, 4197 (2018).

Solution-processed organometallic perovskite based solar cells have emerged as a promising thin-film photovoltaic technology.¹ The presentation will review recent optical spectroscopy and diffraction results on monocrystals of halide perovskites, colloidal nanocrystals or thin-films.² 2D multilayered phases, composed of perovskites multilayers sandwiched between two layers of large organic cations,³ have recently demonstrated improved solar cells photostability under standard illumination as well as humidity resistance.⁴ In this case, intrinsic quantum and dielectric carrier confinements are afforded by the organic inner barriers, which lead to a stable Wannier exciton at room temperature. However, solar cells or LED device efficiencies are related to internal exciton dissociation through low energy states, as shown from the investigation of both thin films and small exfoliated single crystals.⁴ References: ¹ W. Nie et al, Nature Comm. 2016 ; W. Nie et al, Adv. Mat. 2017 ; H. Tsai et al, Adv. Ener. Mat. 2017 ; H. Tsai et al, Science. 2018; ² H.H. Fang et al, Nature Comm. 2017; K. Appavoo et al, Phys. Rev. B 2017 ; Q. Shen et al, Appl. Phys. Lett. 2017 ; M. Fu et al, Nanoletters 2017 ; M. Fu et al, Nat. Comm. 2018; ³ C. Stoumpos et al, Chem 2017; C. M. M. Soe et al, Adv. Ener. Mat.; C. M. M. Soe et al, J. Am. Chem. Soc. 2017 ; L. Mao et al, J. Am. Chem. Soc. 2017 ; M. Smith et al, Chem. Sci. 2017 ; L. Mao et al, J. Am. Chem. Soc. 2018; ⁴ H. Tsai et al, Nature 2016; J. C. Blancon et al, Science 2017 ; J. C. Blancon et al, Nat. Comm. 2018 ; H. Tsai et al, Adv. Mat. 2018; INSA Rennes, Institut FOTON, UMR 6082, CNRS, Rennes, France

In just a few years, perovskites solar cells (PSCs) have emerged as one of the most promising solar cell technologies. So far, the PCEs of PSCs above 22% have been reported, which is rivalling values achievable with crystalline silicon solar cells. In spite of the fast growth in the photovoltaic performance, PSCs are severely limited in their large scale applications due to instability issues. These include intrinsic instabilities of the metal halide perovskites, environmental instabilities, and device operation related instability.

In this work, the first issue, i.e. intrinsic instability of the metal halide perovskite is studied by combing chemical bonding analysis of DFT electronic structure calculations and experimental degradation study using Ultraviolet–visible spectroscopy. A comprehensive set of chemical bonding analysis of DFT calculations are done for AMX₃ (A=Cs, MA, FA, M=Pb, Sn, X=I, Br, Cl) perovskites. Bond order, net charge, steric repulsion, and Crystal Orbital Hamilton Population (COHP) analysis reveal the relation of covalent, ionic, steric interactions, as well as bonding/antibonding characters with their stability, respectively. As a part of this work, Ultraviolet–visible spectroscopy of AMX₃ perovskites film degradation during incremental heating were carried out to probe their thermal stability. A systematic comparison of the theoretical analysis and experimental data points to the most important factors responsible for the trends in the thermal stability of AMX₃ perovskites.

Interestingly, this sets of chemical bonding analysis also shows promise in explaining the structural instability of metal halide perovskites, i.e. transition of black phase (3D structure) to yellow phase (2D structure). The results provide important insights in strategies for stabilizing metal halide perovskites by tuning their composition.

An issue of critical importance in lead-based halide perovskites, much sought-after class of semiconductors in photovoltaics (PV) research, is defects; “deep” defect levels can prove catastrophic for PV performance by causing non-radiative charge carrier recombination^[1], whereas impurity induced energy levels in the band gap could lead to increased absorption of sub-gap photons which can enhance efficiencies^[2]. While experimental detection of defects is non-trivial and identification of the origin of defect states is usually impossible, density functional theory (DFT) calculations have been widely applied to accurately predict defect formation energies and transition levels^[3]. In this work, we use DFT to study various intrinsic and extrinsic point defects in mixed bromide/chloride (MAPbBr_3-yCl_y) and iodide/bromide (MAPbI_3-yBr_y) perovskites (MA = methylammonium) with varying compositions (y = 0, 0.75, 1.5, 2.25, 3). We observe that in MAPbI₃, MAPbBr₃ and all I-Br alloyed perovskites, vacancy defects are the dominant intrinsic defects and create shallow transition levels (i.e., energy levels close to the valence or conduction band edges), while higher energy defects create deeper levels; this is in good agreement with the computational literature^[1,4]. The equilibrium Fermi level changes from inside the valence band (very p-type conductivity) to mid-gap (intrinsic conductivity) on going from I/Br-rich to Pb-rich chemical potential conditions. In MAPbBr_3-yCl_y perovskites, it is seen that vacancy defects are again the lowest energy defects and the equilibrium Fermi level follows the same trend as in MAPbBr₃, but halogen vacancies (V_Br and V_Cl) create deeper levels in the band gap the higher the value of y, indicating that in mixed Br/Cl or pure Cl perovskites, there is a danger of non-radiative recombination of carriers owing to deep defect levels. In order to study the influence of extrinsic point defects on the optoelectronic properties as determined by dominant intrinsic defects, we further performed high-throughput density functional theory computations to study all elements from periods II to VI of the periodic table as substituents at the Pb-site in MAPbBr₃, MAPbBr_1.5Cl_1.5 and MAPbCl₃. Our results revealed that several transition metals like Sc, Y, Zr, Nb and Hf create lower energy substitutional defects than the dominant intrinsic defects in the three perovskites, and shift the equilibrium Fermi level towards the conduction band maximum, thus making the semiconductor conductivity more n-type. These substituents can not only help overcome the adverse effect of deep lying intrinsic defects, but their mid-gap energy provide an opportunity for sub-gap absorption which can potentially realize intermediate band solar cells^[2]. Lastly, we performed correlation analysis on the computational data and discovered that the electronic and structural properties obtained from a much cheaper unit cell calculation can be used to reliably predict formation energies and transition levels of substitutional defects in halide perovskites. Such models can lead to accelerated prediction of impurity levels and allow efficient materials design of defect-tolerant perovskites as well as perovskites with suitably placed defect levels.

REFERENCES
[1] Y. Yan et al., Springer IP. 79-105 (2016).
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[3] Freysoldt et al., Rev. Mod. Phys. 86, 253 (2014).
[4] T. Shi et al., Appl. Phys. Lett. 106, 103902 (2015).

The high performance of recently emerged lead halide perovskite-based photovoltaic devices has been attributed to remarkable carrier properties in this kind of material: long carrier diffusion length, long carrier lifetime, and low electron-hole recombination rate. The charge carrier trapping at defects on surfaces or grain boundaries is detrimental for the performance of perovskite solar cells. In practice, it is one of the main limiting factors for carrier lifetime. In my talk, it will discuss about surface defects responsible for carrier trapping based on comprehensive first-principles investigations and it is proposed that PbI₂-rich condition is preferred to MAI-rich one, while intermediate condition has possibility to be the best choice [1]. On the other hand, in the continuous quest for better performing materials in photovoltaics and in view of their usage in optoelectronic devices, theoretical studies based on density functional and many-body perturbation theories on the electronic and optical properties of the mixed-valence Cs₂Au₂I₆ fully inorganic double perovskite [2] and Ruddlesden−Popper organic−inorganic halide perovskites [3] will be discussed.

References
[1] H. Uratani and K. Yamashita, J. Phys. Chem. Lett., 8, 742−746 (2017).
[2] G. Giorgi, K. Yamashita and M. Palummo, J. Mater. Chem. C, DOI: 10.1039/c8tc03496f (2018)
[3] G. Giorgi, K. Yamashita and M. Palummo, J. Phys. Chem. Lett., DOI: 10.1021/acs.jpclett.8b02653 (2018)

The Rashba effect has served as a model for the combined effects of spin-orbit coupling and inversion symmetry breaking in the halide perovskites. Simply interpreted, it is the splitting of spin components of parabolic energy bands, along directions perpendicular to a polar axis. However, the dynamical polar fluctuations of the halide perovskites suggest that they do not conform exactly to this simple model, because of the lack of a singular static polar axis. Computational simulations of the Rashba effect in the halide perovskites have often relied on either time-averaged or highly-selective crystal structures, and truncations of simulation cells, which do not accurately reflect the actual dynamics of halide perovskites.

In this work, we present a new model for spin-orbit effects in halide perovskites, treating the fluctuating crystal lattice as a disordered, spin-coupled medium. This model is described by spin transmission amplitudes and spin-flip amplitudes through this disordered medium. We parameterize our model on large-scale molecular dynamics trajectories, which captures the anharmonic, large-amplitude lattice fluctuations. Electronic structure calculations on these large simulation cells are aided by a tight-binding framework based on first-principles density functional theory calculations. We apply our model to carrier dynamics in halide perovskites, describing carrier mobility, relaxation, recombination, spin transport, and clarifying the role of spin-orbit coupling in these processes. Predictions of the temperature dependence of the carrier dynamics are given by this model, and compared with experimental results. We discuss the extent to which these effects on the optoelectronic properties may be interpreted as local and short-lived Rashba effects.

Flash Infrared Annealing (FIRA) results in pinhole free layers with micrometer size crystalline domains. The fast annealing times, and comparable solar cell efficiency compared to the traditional antisolvent fabricated perovskites make FIRA a highly promising method for the scaleup of perovskite solar cells. In this work we investigate how the Flash Infrared Annealing affects the crystal growth of MAPbI₃ and its dependence on the substrates used. We measure the grain size, crystal structure and orientation using Electron Back-Scattered Diffraction (EBSD). We find a highly oriented cubic structure for perovskite annealed by FIRA and a consistent crystal rotation within perovskite grains. Besides, we study how the structural properties of the resulting films affect its photophysics. Combining photoluminescence lifetime and spectral maps we show how the growth method affects the steady-state and dynamic optical properties of the resulting films. Our findings directly relate structural properties to the photophysics of lead halide perovskites.

Organic-inorganic metal halide perovskites have attracted much interest and shown great promise in recent years due to their compatibility with cheap solution processing, ease of fabrication, and enhanced power conversion efficiencies. Currently, perovskite solar cells are transitioning from small area devices to large area devices that are compatible with manufacturing. Blade-coating is a promising deposition technique because it is a low cost, environmentally friendly, and easily translated to roll-to-roll processing. Most current work has focused on fabricating high efficiency perovskite solar cells using the single step-based blade coating. However, the most recent 23.3% record efficiency device was fabricated using the two-step spin-coating method. In this study, we investigate the possibility of fabricating large area perovskite solar cells by sequentially depositing PbI₂ and mixed organic iodides consisting of FAI and MABr (where FA is formamidinium, and MA is methylammonium) using the blade coating technique. The blade-coated bi-layer precursor films are annealed under different conditions to examine conversion to the perovskite phases. This study will focus on understanding the formation and growth behaviors of perovskites formed in a two-step blade-coating process when the pre-deposited PbI₂ films are reacted with organic iodide solution with various concentrations of mixed FA/MA cations. Comprehensive understating of the nucleation and growth behavior of perovskites during the intercalation process will provide insights to improve control of the film quality and allow device performance for devices beyond the simple MAPbI₃ system to be improved.

Hybrid halide perovskites have been attracting extensive interest as next generation photovoltaic technologies. Large bandgap metal halide perovskites such as MAPbBr₃ offer a range of applications in multijunction solar cells, electrochemical energy storage, electrocatalysis, and LEDs, as well as providing an interesting platform for comparison with the more commonly studied MAPbI₃, and mixed-halide perovskites.

It was shown that substituting Br for I increases the chemical stability making MAPbBr₃ much more stable under environmental conditions when compared to MAPbI₃. We study this increase in stability under environmental conditions and show that it stems from a change in ion migration properties when going from MAPbI₃ to MAPbBr₃. Furthermore, we investigate the evolution of this behavior in different MAPbBr₃ solar cells as a function of the grain size of the active perovskite film. Grain boundaries change ion migration by providing alternative pathways for the ions to migrate, thereby affecting crucial properties of the process such as activation energy and diffusion coefficient. In our work, we use Transient Ion Drift to quantify the activation energies, diffusion coefficients, and concentrations of the mobile ions, and show the link between grain size, ion migration, and stability.

The nature of trap defects such as the under-coordinated ions at the perovskite surface and grain boundaries are always trapping the free electrons or holes by the electrostatic force and speeding up the ion migration via the defect vacancy channels, which is significantly limiting the charge extraction efficiency and devices long-term stability in perovskite solar cells. In this work, we induced an interface electric field which supplied by a molecular dipole to the interlayer of electron transfer layer (ETL) and perovskite. We employed Kelvin probe force microscope (KPFM) and Femtosecond transient absorption (fs-TA) to systematic study the states of charge distribution and transport properties in the perovskite after add the extra interface electric field. The results show a strong gradient electron accumulation at the applied interface and forming a homo-junction perovskite which supplied an extra built-in electric field (E_in), and finally resulting in faster interface charge transportation. Furthermore, the ion migration can efficiently be suppressed at the applied interface and thus reduce the disorder of energy level. Based on these results, we fabricated the PC₆₁BM based n-i-p architecture perovskite solar cell and achieving a PCE of 20.14% with high V_oc of 1.14V, which is the record efficiency of PC₆₁BM based n-i-p devices. These devices also show higher stability compared to the standard cells.

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.
Here, first I will present our results on the role of structural and point defects in determining the nature and dynamic of photo-carriers in metal-halide perovskites. Then, I will discuss our understanding of key parameters which must be taken into consideration in order to evaluate the suscettibility of the perovkite crystals (2D and 3D) to the formation of defects, allowing one to proceed through a predictive synthetic procedure. Finally, I will show the correlation between the presence/formation of defects and the observed semiconductor instabilities. 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.

The all-inorganic cesium-lead-bromide perovskite (CsPbBr₃) has attracted the attention of the photovoltaics community owing to its intrinsic thermal stability and high tolerance to humidity and light-induced effects [1–3]. Nevertheless, the coexistence of additional ternary phases (Cs₄PbBr₆ and CsPb₂Br₅) in thin-films and single crystals [4], and its influence in the optoelectronic properties of the material is still a matter of debate and research [5]. Recent studies have shown that CsPb₂Br₅ effectively passivates CsPbBr₃ and reduces non-radiative recombination, which results in photodetectors and solar cells with outstanding performance [6,7]. Lei et al. reported enhancement of the photoluminescence and power conversion efficiency by controlled PbBr₂ excess and high-temperature annealing of solar cells, synthesized by vacuum-thermal coevaporation of the CsPbBr₃ absorber [3], while Luchkin et al. measured a significant current increase in coevaporated CsPbBr₃ films after moderate-temperature thermal aging [8]. However, the mechanisms by which the optical and electrical properties are enhanced remain unclear.

In the present contribution, coevaporated CsPbBr₃ films with excess PbBr₂ on glass and Si substrates were studied. We probed the composition and optical properties of the surface and cross-section of the films by energy-dispersive X-ray (EDX) and cathodoluminescence (CL) spectroscopy, as well as by photoluminescence (PL) microscopy in a correlative analysis approach. The samples were investigated as-deposited and after (post-deposition) annealing at 70°C for 30 min in a N₂ atmosphere. The EDX and CL cross-sectional analysis reveals that the temperature treatment increases the thickness of the CsPbBr₃ layer in the film on Si substrate, while on the glass substrate the change is negligible. We estimate from absolute PL hyperspectral imaging an external PL quantum yield (PLQY) of 0.04% and quasi-Fermi-level splitting (QFLS) values exceeding 1.8 eV under one-sun equivalent conditions. Regardless of the thicker CsPbBr₃ layer, we do not observe significant variations on the PLQY or QFLS after annealing. We discuss the effects of the substrate in the phase transformation and the influence of the phases on the optoelectronic properties of the films.

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Organic-inorganic hybrid perovskite solar cells show the promises as the next-generation photovoltaic technology. The efficiency has quickly increased from 3.8% [1] to 23.2% since 2009. [2] The morphology and crystal quality of perovskite films are the critical factors affecting solar cell efficiency. Here we introduce a new organic ammonium additive (2-NAM), exhibiting strong Lewis acid-base interaction with perovskite. [3] 2-NAM is expected to retard the kinetics for crystal growth instead of generating multiple nucleation points, finally resulting in larger crystal grain sizes. In addition, 2-NAM effectively passivates defect formed by the uncoordinated Pb atom. As result, the number of defects decreases almost three times. After introducing 2-NAM, efficiency increases from the 17.1 ± 0.8% to 18.6 ± 0.9% for 0.1 cm² cell and 15.5 ± 0.5% to 16.5 ± 0.6% for 1.0 cm² cell, respectively. Besides the improved efficiency, the stability is enhanced.

Hybrid inorganic-organic perovskite solar cells have attracted much attention because of their superior materials properties, inexpensive fabrication methods, and their rapidly increasing solar conversion efficiency from 3.8 % to more than 22 % within 10 years. To further improve the solar conversion efficiency, all aspects of the device should be comprehensively analyzed at the fundamental level [1]. One problem not completely answered is whether the grain boundaries in halide perovskites are beneficial or detrimental and which atomistic feature makes the grain boundaries have such properties. Density functional theory (DFT) calculation studies claim that grain boundaries do not have gap states [2,3], whereas another study claimed that the lifetime can be reduced based on the molecular dynamics simulations [4]. Our investigation of extended defects in other materials (e.g. CdTe) shows that grain boundaries can be non-stoichiometric depending on the growth conditions [5,6], indicating that grain boundaries in halide perovskites are likely non-stoichiometric due to segregation of intrinsic or extrinsic defects. In this presentation, we aim to discuss the stability and electrical properties of grain boundaries in halide perovskites in comparison to CdTe and provide a clue to passivate grain boundaries in general.

[1] Ji-Sang Park, Sunghyun Kim, Zijuan Xie, and Aron Walsh, Point defect engineering in thin-film solar cells, Nature Reviews Materials 3, 194–210 (2018).
[2] W.-J. Yin, H. Chen, T. Shi, S.-H. Wei, and Y. Yan, Origin of High Electronic Quality in Structurally Disordered CH3NH3PbI3 and the Passivation Effect of Cl and O at Grain Boundaries, Adv. Electronic Mater. 1, 1500044 (2015).
[3] Y. Guo, Q. Wang, and W. A. Saidi, Structural Stabilities and Electronic Properties of High-Angle Grain Boundaries in Perovskite Cesium Lead Halides, J. Phys. Chem. C 121, 1715 (2017).
[4] R. Long, J. Liu, and O. V. Prezhdo, Unravelling the Effects of Grain Boundary and Chemical Doping on Electron-Hole Recombination in CH3NH3PbI3 Perovskite by Time-Domain Atomistic Simulation, Journal of the American Chemical Society 138, 3884 (2016).
[5] J.-S. Park, J. Kang, J.-H. Yang, W. Metzger, and S.-H. Wei, Stability and electronic structure of the low-Σ grain boundaries in CdTe: a density functional study, New J. Phys. 17, 013027 (2015).
[6] J.-S. Park, J.-H. Yang, T. Barnes, and S.-H. Wei, Effect of intermixing at CdS/CdTe interface on defect properties, Appl. Phys. Lett. 109, 042105 (2016).

Colloidal inorganic perovskite nanocrystals (PNCs) are solution-processable functional materials whose emission can be easily tuned via both size and composition.1 Their exciting optical properties such as the large absorption cross-section and high photoluminescence quantum yield (PLQY) make them ideal candidates for a broad range of photonics and optoelectronics applications.2 In this work, we present an overview of the exceptionally efficient exciton transport mediated by Förster Resonant Energy Transfer (FRET) in perovskite systems of increasing dimensionality. With a specifically designated optical setup, we directly measure the spatial extent of exciton hopping in a controlled two-dimensional assembly of 0D PNCs, which provides a flat energy landscape with minimal geometric disorder.3 Steady-state and time-resolved PL microscopy, together with physical modeling of exciton transport, shows an exciton diffusion length of 200 nm and diffusivity as high as 0.5 cm2/s, which greatly exceed the values reported for FRET-mediated exciton diffusion in chalcogen-based quantum dot solids, and, importantly, matches the optical absorption depth. We further explore the exciton diffusion paradigm in 1D perovskite nanowires and 2D nanosheets, where we image the diffusion across the whole system, with diffusion lengths larger than 1μm. In addition to the exciton diffusion mapping, a significant portion of this work has been dedicated to the optimization of the substrate and the sample passivation. Specifically, we show that with a thermal-based atomic layer deposition process we are able to apply a ~3nm-thick transparent ceramic coating (aluminum oxide) which ensure optical stability over a four month period, thus overcoming the instability issue which often hinders the actual integration of perovskite materials in optoelectronics devices. Our investigation therefore provides the foundation for employing FRET-mediated exciton diffusion in nanostructured perovskites, while also demonstrating practical guidelines to use these bright emitters in optoelectronic devices beyond proof of principle.
References
[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. 15, 6 (2015).
[2] M. V. Kovalenko, L. Protesescu and M. I. Bodnarchuk, Science, 358, 6364 (2017).
[3] G. M. Akselrod, P. B. Deotare, N. J. Thompson, J. Lee, W. A. Tisdale, M. A. Baldo, V. M. Menon and V.
Bulović, Nat. Commun. 5, 3646 (2014).

Layered hybrid perovskites halides are under intense investigation for electronic, optoelectronic, dielectric and photovoltaic applications due to advantages in stability, diversity, tunability and solution synthesizability. We reveal, through first-principle calculations how structural disordering and inversion symmetry breaking adapt as functions of composition and topological dimensionality of the octahedral net (controlled by the number of perovskite layers n) in a prototypical series of hybrid Ruddlesden-Popper halides PEA ₂PbX₄+(n-1)MAPbX₃ (PEA= phenylethylammonium, MA = methylammonium, X=Cl, Br and I). We provide mechanistic interpretation of these phenomena as the complex interplay of Pb-X covalent bonding, hydrogen bonding between ammonium head of PEA and apical halides, and van der Waals interactions between the aromatic tails of PEA. Our study provides rational routes for the design and manipulation of these materials towards white-light emission, Rashba-Dresselhaus spin splitting, and ferroelectricity. This work was supported by the Singapore Berkeley Research Initiative for Sustainable Energy(SinBeRISE) Program.

Solar cells based on Methyl-Ammonium Lead Triiodide (MAPbI3) perovskite have gained attention due to their remarkable progress in performance efficiency during recent years [1]. However, it has been hampered to put the material on the market due to their device stability under exposure to moisture [2] which is one of the major obstacle toward outdoor application of photovoltaic devices. A comprehensive study on degradation mechanism initiated with water molecules is thus essential for practical realization of MAPbI3 based solar cells. Using first-principles calculation based on the density-functional theory, we here focus on and investigate ion-migration dynamics in a MAPbI3 intercalated with water and their enhancement by the influence of grain boundary (GB). The nudged elastic band (NEB) method is employed to find the barrier potential and corresponding optimal minimum energy path (MEP) of migrated ions. We find that one of H ions of a water molecule segregated into a GB is dissociated, migrated along the GB, and attracted by an N atom in the MAPbI3 with comparatively lower potential barrier (~0.27eV) [3], following the H-ion release from an ammonium. Such migration of H depends upon the subsequent changes of charge states of surrounding atoms [4]. Additionally, a vacant space around the N atom plays as a cage for the diffused H atom. The water intercalation greatly reduces the barrier potential for an H-ion motion in the GB interior of MAPbI3 which can be liable to initiate the degradation of crystallinity of the perovskite. The iso-surface of electronic charge distribution at HOMO-LUMO and partial density of state (PDOS) of the ruling N atom reveal the mentioned phenomenon. More importantly, the antisymmetric GB structure is prominent for faster molecular attractions due to their weak activation energies.
*Work supported by the JSPS KAKENHI Grant Number 18H01708
References:
[1] W. Ming, D. Yang, T. Li, L. Zhang, and M.H. Du, Adv. Sci. 5, 1700662 (2018).
[2] Y.-H. Kye, C.-J. Yu, U.-G. Jong, Y. Chen, and A. Walsh, J. Phys. Chem. Lett. 9, 2196 (2018).
[3] K. L. Kohlstedt, W. S. Williams, and J. B. Woodhouse, J. Appl. Phys. 41, 4476 (1970).
[4] J.C Bourgoin, J.W Corbett. Phys. Lett. A 2, 38 (1972).

Recently, all-inorganic perovskites CsPbX₃ (X = Cl, Br, I) nanowires (NW)s have recently demonstrated potential applications in integrated photonics devices such as lasers and photodetectors due to their unique physical and chemical characteristics. Considering the perovskite based integration application, large-scale growth or assembly of perovskite nanowires with horizontal alignment on surfaces is highly desiable. Here, we demonstrate the controlled growth of in-plane directional perovskite CsPbBr₃ NWs, induced by graphoepitaxial effect on annealed M-plane sapphire substrates ^[1]. High-performance photodetectors constructed on these individual NWs exhibit excellent photoresponse with an ultrahigh responsivity of 4400 A/W and fast response speed of 252 μs. Furthermore, we achieved wavelength-tunable CsPbX₃ nanowire laser arrays at room temperature with quite low lasing thresholds and high quality factors based on the directional growth approach ^[2]. Meanwhile, we studied the exciton−photon coupling effect of these perovskite nanowire cavities under the excitation of a pulsed laser, from which highly atomic composition dependent Rabi splitting of ∼210 ± 13, 146 ± 9, and 103 ± 5 meV for the CsPbCl₃, CsPbBr₃, and CsPbI₃ are obtained at room temperature. In addition, by using a novel temperature difference triggers growth strategy, high quality CsPbX₃ nanowire arrays with the integration of the merits of the liquid- and gas-phase methods was successfully synthesized, these nanowire arrays show excellent stability and good optoelectronic properties at room temperature ^[3]. This work presents an important step toward scalable growth of high-quality perovskite NWs, which will provide promising opportunities in constructing integrated nanophotonic and optoelectronic systems.

References:
[1] M. Shoaib, W. Zheng, A. Pan*, et al., J. Am. Chem. Soc, 2017, 139, 15592.
[2] X. Wang, M. Shoaib, A. Pan*, et al. ACS Nano, 2018, 12, 6170.
[3] Y. Wang, A. Pan, T. Zhai*, et al., Small, 2018, 14, 1803010

Hybrid organic-inorganic perovskite solar cell (PSC) has been paid much attention due to its rapid progress of power conversion efficiency exceeding 22%. Many methods for preparing the perovskite films have been proposed, such as one-step deposition, sequential deposition, vapor-assisted deposition, and so on. Among them, the sequential deposition, typically synthesized by the immersion of PbI₂ into the MAI solution to form the MAPbI₃ perovskite film, shows the promise because of its better control over the surface morphology and crystallization kinetics. However, the incomplete conversion during sequential deposition, resulting in the residue of PbI₂ left, becomes the problem and thereby influences the device performance reproducibly. In this regard, we introduce a vacuum treatment to treat the samples at the different stages of the sequential deposition to investigate the level of PbI₂ left. The SEM and XRD are used to identify the morphology, the crystallization and the conversion of the perovskite film during the sequential deposition. Our result indicates that the as-prepared PbI₂ forms a compact morphology after the vacuum treatment, leading to more amount of PbI₂ left within the resulting perovskite film. On the other hand, the vacuum treatment is applied for the PbI₂ film right after the MAI immersion, showing a less amount of PbI₂ left within the resulting perovskite film. The UV-Vis and EIS are carried out to analyze the optical property and charge transport property, respectively, of perovskite films with different level of PbI₂ residue. J-V characteristic of the perovskite solar cells with the different level of PbI₂ residue is tested under AM 1.5G illumination. More details of the conversion efficiency of perovskite solar cells, affected by vacuum treatment at the different stages of sequential deposition, will be discussed in the presentation.

Doping of semiconductors allows tunability of charge carriers and subsequently electronic properties necessary for the development of many technologies. However, controlled doping in lead-halide perovskite semiconductors has proven to be difficult. Lower dimensional perovskites such as nanocrystals, with their high surface area to volume ratio, offer an opportunity for electronic doping via molecular charge transfer. In our work, we explore the tunability of the electronic properties of perovskite nanocrystal films using physically adsorbed molecular dopants. Incorporation of the dopant molecules into CsPbI₃ nanocrystal thin films is confirmed via infrared and photoelectron spectroscopies. We discover pre-treated CsPbI₃ nanocrystal films to be slightly p-type in behavior. Incorporating an electron-accepting dopant increases conductivity while an electron-donating molecule results in lower conductivity, appearing to compensate the p-type nanocrystal arrays. Time-resolved spectroscopic measurements reveal time scales on the order of Auger-mediated recombination in the presence of excess electrons or holes. Transport measurements demonstrate that both the local and long-range hole mobility is improved by doping of the nanocrystal arrays using an electron-accepting molecule. The improved photo-excited hole mobility in p-type arrays lead to an enhancement in photo-transistors.

Hybrid organic-inorganic perovskite solar cell (PSC) has been paid much attention due to its rapid progress of power conversion efficiency exceeding 22%. Many methods for preparing the perovskite films have been proposed, such as one-step deposition, sequential deposition, vapor-assisted deposition, and so on. Among them, the sequential deposition, typically synthesized by the immersion of PbI₂ into the MAI solution to form the MAPbI₃ perovskite film, shows the promise because of its better control over the surface morphology and crystallization kinetics. However, the incomplete conversion during sequential deposition, resulting in the residue of PbI₂ left, becomes the problem and thereby influences the device performance reproducibly. In this regard, we introduce a vacuum treatment to treat the samples at the different stages of the sequential deposition to investigate the level of PbI₂ left. The SEM and XRD are used to identify the morphology, the crystallization and the conversion of the perovskite film during the sequential deposition. Our result indicates that the as-prepared PbI₂ forms a compact morphology after the vacuum treatment, leading to more amount of PbI₂ left within the resulting perovskite film. On the other hand, the vacuum treatment is applied for the PbI₂ film right after the MAI immersion, showing a less amount of PbI₂ left within the resulting perovskite film. The UV-Vis and EIS are carried out to analyze the optical property and charge transport property, respectively, of perovskite films with different level of PbI₂ residue. J-V characteristic of the perovskite solar cells with the different level of PbI₂ residue is tested under AM 1.5G illumination. More details of the conversion efficiency of perovskite solar cells, affected by vacuum treatment at the different stages of sequential deposition, will be discussed in the presentation.

Outstanding power conversion efficiency of organo-metal halide perovskite solar cell loses its open-circuit voltage (Voc) and stability in ambient environment. Understanding the underlying mechanisms of instability and Voc loss are urgently needed to resolve the issues. Ionic defects (anion and cation vacancies) in perovskite, especially in mixed cation and halide perovskite, are regarded as the major factor to deteriorate the stability and efficiency. Suppress ionic defects can directly retard both trapped-assist recombination of charge carriers and the diffusion of gas molecules (i.e. moisture, oxygen) in perovskite and therefore improve the Voc and stability of device.
Herein, we report a general strategy of post-treatment using Lewis adduct to passivate ionic vacancies in crystalline perovskite film to improve its device performance and stability. The Lewis adducts of organo ammonium salt were investigated. The systematical characterizations of passivated films reveal the electronic disorder and trapped density are successfully reduced after Lewis adduct post-treatment. A more than 50 mV increase in Voc has been achieved due to the less non-radiative recombination of charge carriers as compared with the device without passivation. In term of device stability, less than 1% power conversion efficiency was observed when the device was irradiated 10 min continuously and the 300-second maximum power point was tracked simultaneously in the ambient. Compared with conventional passivation approaches which are carried out at the crystallization step of perovskite film and are difficult to control, this strategy can directly passivate the ionic defects after the film is crystallized. That gives an avenue to the ultrafast or one-step large-scale coating process to obtain a high-quality perovskite film with ease which has potential to become a commercial viable process.

Recently, all-inorganic perovskites CsPbX₃ (X = Cl, Br, I) nanowires (NW)s have recently demonstrated potential applications in integrated photonics devices such as lasers and photodetectors due to their unique physical and chemical characteristics. Considering the perovskite based integration application, large-scale growth or assembly of perovskite nanowires with horizontal alignment on surfaces is highly desiable. Here, we demonstrate the controlled growth of in-plane directional perovskite CsPbBr₃ NWs, induced by graphoepitaxial effect on annealed M-plane sapphire substrates ^[1]. High-performance photodetectors constructed on these individual NWs exhibit excellent photoresponse with an ultrahigh responsivity of 4400 A/W and fast response speed of 252 μs. Furthermore, we achieved wavelength-tunable CsPbX₃ nanowire laser arrays at room temperature with quite low lasing thresholds and high quality factors based on the directional growth approach ^[2]. Meanwhile, we studied the exciton−photon coupling effect of these perovskite nanowire cavities under the excitation of a pulsed laser, from which highly atomic composition dependent Rabi splitting of ∼210 ± 13, 146 ± 9, and 103 ± 5 meV for the CsPbCl₃, CsPbBr₃, and CsPbI₃ are obtained at room temperature. In addition, by using a novel temperature difference triggers growth strategy, high quality CsPbX₃ nanowire arrays with the integration of the merits of the liquid- and gas-phase methods was successfully synthesized, these nanowire arrays show excellent stability and good optoelectronic properties at room temperature ^[3]. This work presents an important step toward scalable growth of high-quality perovskite NWs, which will provide promising opportunities in constructing integrated nanophotonic and optoelectronic systems.

References:
[1] M. Shoaib, W. Zheng, A. Pan*, et al., J. Am. Chem. Soc, 2017, 139, 15592.
[2] X. Wang, M. Shoaib, A. Pan*, et al. ACS Nano, 2018, 12, 6170.
[3] Y. Wang, A. Pan, T. Zhai*, et al., Small, 2018, 14, 1803010

Solar cells based on Methyl-Ammonium Lead Triiodide (MAPbI3) perovskite have gained attention due to their remarkable progress in performance efficiency during recent years [1]. However, it has been hampered to put the material on the market due to their device stability under exposure to moisture [2] which is one of the major obstacle toward outdoor application of photovoltaic devices. A comprehensive study on degradation mechanism initiated with water molecules is thus essential for practical realization of MAPbI3 based solar cells. Using first-principles calculation based on the density-functional theory, we here focus on and investigate ion-migration dynamics in a MAPbI3 intercalated with water and their enhancement by the influence of grain boundary (GB). The nudged elastic band (NEB) method is employed to find the barrier potential and corresponding optimal minimum energy path (MEP) of migrated ions. We find that one of H ions of a water molecule segregated into a GB is dissociated, migrated along the GB, and attracted by an N atom in the MAPbI3 with comparatively lower potential barrier (~0.27eV) [3], following the H-ion release from an ammonium. Such migration of H depends upon the subsequent changes of charge states of surrounding atoms [4]. Additionally, a vacant space around the N atom plays as a cage for the diffused H atom. The water intercalation greatly reduces the barrier potential for an H-ion motion in the GB interior of MAPbI3 which can be liable to initiate the degradation of crystallinity of the perovskite. The iso-surface of electronic charge distribution at HOMO-LUMO and partial density of state (PDOS) of the ruling N atom reveal the mentioned phenomenon. More importantly, the antisymmetric GB structure is prominent for faster molecular attractions due to their weak activation energies.
*Work supported by the JSPS KAKENHI Grant Number 18H01708
References:
[1] W. Ming, D. Yang, T. Li, L. Zhang, and M.H. Du, Adv. Sci. 5, 1700662 (2018).
[2] Y.-H. Kye, C.-J. Yu, U.-G. Jong, Y. Chen, and A. Walsh, J. Phys. Chem. Lett. 9, 2196 (2018).
[3] K. L. Kohlstedt, W. S. Williams, and J. B. Woodhouse, J. Appl. Phys. 41, 4476 (1970).
[4] J.C Bourgoin, J.W Corbett. Phys. Lett. A 2, 38 (1972).

The nature of trap defects such as the under-coordinated ions at the perovskite surface and grain boundaries are always trapping the free electrons or holes by the electrostatic force and speeding up the ion migration via the defect vacancy channels, which is significantly limiting the charge extraction efficiency and devices long-term stability in perovskite solar cells. In this work, we induced an interface electric field which supplied by a molecular dipole to the interlayer of electron transfer layer (ETL) and perovskite. We employed Kelvin probe force microscope (KPFM) and Femtosecond transient absorption (fs-TA) to systematic study the states of charge distribution and transport properties in the perovskite after add the extra interface electric field. The results show a strong gradient electron accumulation at the applied interface and forming a homo-junction perovskite which supplied an extra built-in electric field (E_in), and finally resulting in faster interface charge transportation. Furthermore, the ion migration can efficiently be suppressed at the applied interface and thus reduce the disorder of energy level. Based on these results, we fabricated the PC₆₁BM based n-i-p architecture perovskite solar cell and achieving a PCE of 20.14% with high V_oc of 1.14V, which is the record efficiency of PC₆₁BM based n-i-p devices. These devices also show higher stability compared to the standard cells.

Hybrid halide perovskites have been attracting extensive interest as next generation photovoltaic technologies. Large bandgap metal halide perovskites such as MAPbBr₃ offer a range of applications in multijunction solar cells, electrochemical energy storage, electrocatalysis, and LEDs, as well as providing an interesting platform for comparison with the more commonly studied MAPbI₃, and mixed-halide perovskites.

It was shown that substituting Br for I increases the chemical stability making MAPbBr₃ much more stable under environmental conditions when compared to MAPbI₃. We study this increase in stability under environmental conditions and show that it stems from a change in ion migration properties when going from MAPbI₃ to MAPbBr₃. Furthermore, we investigate the evolution of this behavior in different MAPbBr₃ solar cells as a function of the grain size of the active perovskite film. Grain boundaries change ion migration by providing alternative pathways for the ions to migrate, thereby affecting crucial properties of the process such as activation energy and diffusion coefficient. In our work, we use Transient Ion Drift to quantify the activation energies, diffusion coefficients, and concentrations of the mobile ions, and show the link between grain size, ion migration, and stability.

Organic-inorganic metal halide perovskites have attracted much interest and shown great promise in recent years due to their compatibility with cheap solution processing, ease of fabrication, and enhanced power conversion efficiencies. Currently, perovskite solar cells are transitioning from small area devices to large area devices that are compatible with manufacturing. Blade-coating is a promising deposition technique because it is a low cost, environmentally friendly, and easily translated to roll-to-roll processing. Most current work has focused on fabricating high efficiency perovskite solar cells using the single step-based blade coating. However, the most recent 23.3% record efficiency device was fabricated using the two-step spin-coating method. In this study, we investigate the possibility of fabricating large area perovskite solar cells by sequentially depositing PbI₂ and mixed organic iodides consisting of FAI and MABr (where FA is formamidinium, and MA is methylammonium) using the blade coating technique. The blade-coated bi-layer precursor films are annealed under different conditions to examine conversion to the perovskite phases. This study will focus on understanding the formation and growth behaviors of perovskites formed in a two-step blade-coating process when the pre-deposited PbI₂ films are reacted with organic iodide solution with various concentrations of mixed FA/MA cations. Comprehensive understating of the nucleation and growth behavior of perovskites during the intercalation process will provide insights to improve control of the film quality and allow device performance for devices beyond the simple MAPbI₃ system to be improved.

Flash Infrared Annealing (FIRA) results in pinhole free layers with micrometer size crystalline domains. The fast annealing times, and comparable solar cell efficiency compared to the traditional antisolvent fabricated perovskites make FIRA a highly promising method for the scaleup of perovskite solar cells. In this work we investigate how the Flash Infrared Annealing affects the crystal growth of MAPbI₃ and its dependence on the substrates used. We measure the grain size, crystal structure and orientation using Electron Back-Scattered Diffraction (EBSD). We find a highly oriented cubic structure for perovskite annealed by FIRA and a consistent crystal rotation within perovskite grains. Besides, we study how the structural properties of the resulting films affect its photophysics. Combining photoluminescence lifetime and spectral maps we show how the growth method affects the steady-state and dynamic optical properties of the resulting films. Our findings directly relate structural properties to the photophysics of lead halide perovskites.

Doping of semiconductors allows tunability of charge carriers and subsequently electronic properties necessary for the development of many technologies. However, controlled doping in lead-halide perovskite semiconductors has proven to be difficult. Lower dimensional perovskites such as nanocrystals, with their high surface area to volume ratio, offer an opportunity for electronic doping via molecular charge transfer. In our work, we explore the tunability of the electronic properties of perovskite nanocrystal films using physically adsorbed molecular dopants. Incorporation of the dopant molecules into CsPbI₃ nanocrystal thin films is confirmed via infrared and photoelectron spectroscopies. We discover pre-treated CsPbI₃ nanocrystal films to be slightly p-type in behavior. Incorporating an electron-accepting dopant increases conductivity while an electron-donating molecule results in lower conductivity, appearing to compensate the p-type nanocrystal arrays. Time-resolved spectroscopic measurements reveal time scales on the order of Auger-mediated recombination in the presence of excess electrons or holes. Transport measurements demonstrate that both the local and long-range hole mobility is improved by doping of the nanocrystal arrays using an electron-accepting molecule. The improved photo-excited hole mobility in p-type arrays lead to an enhancement in photo-transistors.

Ultrafast time-resolved terahertz spectroscopy (TRTS) allows for the direct probing of charge carriers and quasi-particles in semiconductors. The sensitivity of the technique to both the carrier mobility and its density can help elucidate the mechanisms of their temporal evolution. The use of gas photonics provides short, ultra-broadband THz pulses and, thus, offers an improved time-resolution.
Here, we present a study of the early steps of the charge carrier dynamics in lead halide perovskite thin films from the point of view of the THz mobility. Taking advantage of a 200 fs time-resolution, we were able to temporally follow the cooling of hot carriers through the observation of the change of their mobility. This change is understood as resulting from a modification of the carrier effective mass.1
Our results are compatible with a hot carrier cooling mechanism implying LO phonon emission. This is subjected to a phonon bottleneck,2 and competes with a dynamic screening process, which time-constant was identified as being due to polaron formation.3 While the screened hot carriers take longer to cool down,4 the dynamic screening process does not produce a change in mobility when only cold carriers are involved.
Measurements applied to perovskite samples of various compositions are compared to elucidate the role of cations and anions in both processes.
1. Beard, M. C. et al. Phys. Rev. B 62, 15764–15777 (2000).
2. Fu, J. et al. Nat. Commun. 8, 1300 (2017).
3. Miyata, K. et al. Sci. Adv. 3, e1701217 (2017).
4. Joshi, P. P. et al. Adv. Mater. 1803054 (2019).

Pseudo-Halide have started to become a new stream of research because of their capability of not only replacing the scarce options of standard halides in perovskites (Cl, Br and I), but also because of the unique and useful optolectronic properties therein. This work will desribe the versatality of using a organi pseudo halide based salt to fabricate various perovskites and demonstrate their resulting - interesting and unique photo-physical and optoelectronic properties.

In this talk we describe the role of interfaces and surface defects on non-radiative recombination losses in hybrid perovskite semiconductors. Using combinations of microscopy and time-resolved photoluminescence spectroscopy we show that not only are perovskite surfaces sources of significant non-radiative loss, but that chemical passivation of surface states can lead to near-ideal semiconductor properties, achieving over 90% PL internal quantum efficiency and quasi-Fermi level splittings that exceed 96% of the Shockley-Queisser limit under illumination. We next explore the interface between the perovskite and various charge extraction layers, combining experiment and simulation to show that surface recombination at the charge extraction layers is a limitation in current perovskite solar cell architectures.

My talk will focus on the means and developments to analyze and tailor interfaces in halide perovskite (HaP) based semiconductor devices to gain control over the electronic properties at the nanoscale, as interfacial design routes determine the electronic coupling between the perovskite absorber and adjacent charge extraction and transport layers. On the one hand, the device characteristics can be affected by the alignment of the frontier molecular orbitals of an organic charge transport layers (CTL) with the electronic transport level in the perovskite. On the other hand, the doping type of the substrate underneath can template the doping type of subsequently deposited HaP films. In our studies we elucidated these mechanisms by examining a selection of organic, oxide and carbon nanotube charge transport layers adjacent to the perovskite film [1].
In my talk, I will highlight the use of ultraviolet and X-ray photoemission spectroscopy (UPS/XPS) as well as inverse photoemission spectroscopy (IPES) to determine the surface energetics and electronic energy level alignment at the MHP/CTL interface while at the same time tracking the interface chemistry. This approach enables us to explain band offset in the perovskite layer by either chemical interactions or by changes in the electrostatic potential. The results not only suggest guidelines on how to integrate charge extraction layers into perovskite photovoltaic devices but also explain more generally to what extent the electronic structure of the perovskite is subject to extrinsic perturbations [2].
I will conclude my talk by further demonstrate the impact of surface treatment and interfacial design routes to the achievement of record power conversion efficiencies in HaP-based quantum dot solar cells [3]. Therein, our approach is driven by a targeted ligand exchange chemistry that results in subsequent anion and cation exchange reactions at the quantum dot surface [4].

[1] P. Schulz ACS Energy Lett. 2018, 3, 1287
[2] J.A. Christians, P. Schulz, J.S. Tinkham, T.H. Schloemer, S.P. Harvey, B.J. Tremolet de Villiers, A. Sellinger, J J. Berry, J.M. Luther, Nature Energy 2018, 3, 68
[3] E. M. Sanehira, A. R. Marshall, J. A. Christians, S. P. Harvey, P. N. Ciesielski, L. M. Wheeler, P. Schulz, L. Y. Lin, M. C. Beard, J. M. Luther, Sci. Adv. 2017, 3, eaao4204
[4] L.M. Wheeler, E.M. Sanehira, A.R. Marshall, P. Schulz, M. Suri, N.C. Anderson, J.A. Christians, D. Nordlund, D. Sokaras, T. Kroll, S.P. Harvey, J.J. Berry, L.Y. Lin, J.M. Luther J. Am. Chem. Soc. 2018, 140, 10504

Increasing attention has been placed over the past few years on 2D Ruddlesden-Popper phase (RPP) metal halides, perceived as interesting photovoltaic materials in their own right, and also as potentially efficient capping materials to improve the long-term stability of 3D metal halide perovskite solar cells. Energy level alignment at 2D/3D heterojunctions and stability of the 2D materials are therefore of considerable interest. We focus here on the electronic structure, surface potential and response to light irradiation of films of the 2D RPP metal halides, BA₂PbI₄ (n=1) and BA₂CsPb₂I₇ (n=2). Direct and inverse photoemission spectroscopy coupled with theoretical computation of the DOS are used to determine the materials electron affinity (EA)/ionization energy (IE), equal to 3.1/5.8 eV for BA₂PbI₄ [1] and 3.3/5.7 eV for BA₂CsPb₂I₇. A full identification of the various contribution to the valence and conduction band DOS is made. We investigate the surface potential of these films via Kelvin probe force microscopy and show reproducible surface photovoltage under supra-band gap light irradiation, corresponding to a reduction of the downward band bending. Long term degradation of the work function is not observed, in contrast to previous observations on 3D perovskite surfaces. Finally, a composition analysis of the n=2 BA₂CsPb₂I₇, using XPS, shows that the surface is not stoichiometric and resembles the composition of RPPs with larger n. We propose that the n>2 phases at the surface create a type I interface with the n=2 phase. This information together with the IE and EA as determined by UPS and IPES, respectively, outline the energy landscape of this material.

[1] Scott Silver, Jun Yin, Hong Li, Jean-Luc Brédas, Antoine Kahn, Adv. Energ. Mat. 1703468 (2018)

In metal halide perovskite-based solar cells, optimization of hole transport materials (HTMs) is important for enhancing solar power conversion efficiency and 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 [2-9]. In this talk, we will present our latest understanding of fundamental interactions between Li-bis(trifluoromethanesulfonyl)-imide (LiTFSI), 4-tert-butylpyridine (t-BP), spiro-MeOTAD and perovskites. Also, we will show how gas exposure (e.g., exposure to O₂, H₂O, N₂) influences electronic energy level alignments and conductivity of such HTM films that is closely associated with performances and stability in perovskite-based solar cells.

[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 a spiro-MeOTAD hole transport layer for lead halide perovskite solar cells. J. Phys. D: Appl. Phys. 51 (2018) 294001.
[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.
[9] L.K. Ono, P. Schulz, J.J. Endres, G.O. Nikiforov, Y. Kato, A. Kahn, and Y.B. Qi, Air-Exposure-Induced Gas-Molecule Incorporation into Spiro-MeOTAD Films. J. Phys. Chem. Lett. 5 (2014) 1374-1379.

Perovskite-based solar cells have been a popular research topic for the better part of the last decade due to their rapidly increasing power conversion efficiencies¹ and the ease of their fabrication². TiO₂ is frequently used as an electron transport material (ETM) in perovskite-based solar cells because of its low cost, low toxicity, versatility in sizes and structures, and chemical stability.^3-4 Despite the successes of perovskites in solar cells, there are still aspects about their operation that are not fully understood. One example is the role of ligand groups acting as a bridge between the perovskite and ETM. This computational research aims to explore the effects that these ligand groups, which are adsorbed to a polyoxotitanate (TiO₂ for brevity) nanocluster, have on the photoexcited charge carrier dynamics within a perovskite-based solar cell. Density functional theory is used to find the ground state properties of the perovskite/TiO₂ solar cell, as well as compute the molecular dynamics trajectory, where non-adiabatic electron-phonon couplings are calculated “on-the-fly.” These couplings are then used to calculate the Redfield tensor which is ultimately used to compute the photoexcited charge carrier dynamics. It is found that the ligand groups have can alter the band gap, band alignment, charge transfer characteristics, accessible excitation pathways, and excited charge carrier lifetimes.


Resources

1. Nie, W. Y.; Tsai, H. H.; Asadpour, R.; Blancon, J. C.; Neukirch, A. J.; Gupta, G.; Crochet, J. J.; Chhowalla, M.; Tretiak, S.; Alam, M. A.; Wang, H. L.; Mohite, A. D., High-efficiency solution-processed perovskite solar cells with millimeter-scale grains. Science 2015, 347 (6221), 522-525.

2. Loi, M. A.; Hummelen, J. C., HYBRID SOLAR CELLS Perovskites under the Sun. Nat. Mater. 2013, 12 (12), 1087-1089.

3. Bao, J. H.; Gundlach, L.; Yu, Z. H.; Benedict, J. B.; Snoeberger, R. C.; Batista, V. S.; Coppens, P.; Piotrowiak, P., Hot Hole Hopping in a Polyoxotitanate Cluster Terminated with Catechol Electron Donors. J. Phys. Chem. C 2016, 120 (36), 20006-20015.

4. Negre, C. F. A.; Young, K. J.; Oviedo, M. B.; Allen, L. J.; Sanchez, C. G.; Jarzembska, K. N.; Benedict, J. B.; Crabtree, R. H.; Coppens, P.; Brudvig, G. W.; Batista, V. S., Photoelectrochemical Hole Injection Revealed in Polyoxotitanate Nanocrystals Functionalized with Organic Adsorbates. J. Am. Chem. Soc. 2014, 136 (46), 16420-16429.

Highly stable halide perovskite solar cells employ semiconductor oxides as electron transport materials. Defects in these oxides, such as oxygen vacancies (O_vac), act as recombination centres and, under air and UV light, reduce the stability of the solar cell. Under the same conditions, the PbZrTiO₃ ferroelectric oxide employs O_vac for the creation of defect-dipoles responsible for photo-carrier separation and current transport, evading device degradation. We report the application of PbZrTiO₃ as the electron extraction material in triple cation halide perovskite solar cells. The application of a bias voltage (poling) up to 2 V, under UV light, is a critical step to induce charge transport in the ferroelectric oxide. Champion cells result in power conversion efficiencies of ~ 11 % after poling. Stability analysis, carried out at 1-sun AM 1.5 G, including UV light in air for unencapsulated devices, shows negligible degradation for hours in comparison with reference solar cells applying SnO₂ which degrades in only a few minutes. Our experiments indicate the effect of ferroelectricity from the PZT, however alternative conducting mechanisms affected by the accumulation of charges or the migration of ions (or the combination of them) can also be present. Our results demonstrate, for the first time, the application of a ferroelectric oxide as an electron extraction material in efficient and stable PSCs. These findings are also a step forward the development of next generation ferroelectric oxide-based electronic and optoelectronic devices.
1. Pérez-Tomas, A.; Xia, H.; Wang, Z.; Kim, H.-S.; Shirley, I.; Turren-Cruz, S.-H.; Morales-Melgares, A.; Saliba, B.; Tanenbaum, D.; Saliba, M.; Zakeeruddin, S. M.; Gratzel, M.; Hagfeldt, A.; Lira-Cantu, M., PbZrTiO3 Ferroelectric Oxide as electron extraction material in Halide Perovskite Solar Cells. Sustainable Energy & Fuels 2018, Accepted.
2. Reyna, Y.; Perez-Tomas, A.; Mingorance, A.; Lira-Cantu, M., Stability of Molecular Devices: Halide Perovskite Solar Cells. In Molecular Devices for Solar Energy Conversion and Storage, Tian, H.; Boschloo, G.; Hagfeldt, A., Eds. Springer-Verlag Berlin: Berlin, 2018; pp 477-531.
3. Pérez-Tomás, A.; Lima, A.; Billon, Q.; Shirley, I.; Catalan, G.; Lira-Cantú, M., The Solaristor concept. https://en.wikipedia.org/wiki/Solaristor. Wikipedia 2018.
4. Mingorance, A.; Xie, H.; Kim, H.-S.; Wang, Z.; Balsells, M.; Morales-Melgares, A.; Domingo, N.; Kazuteru, N.; Tress, W.; Fraxedas, J.; Vlachopoulos, N.; Hagfeldt, A.; Lira-Cantu, M., Interfacial Engineering of Metal Oxides for Highly Stable Halide Perovskite Solar Cells. Advanced Materials Interfaces 2018, 0 (0), 1800367.
5. Hagfeldt, A.; Lira-Cantu, M., Recent concepts and future opportunities for oxides in solar cells. Applied Surface Science 2018, In Press.
6. Perez-Tomas, A.; Mingorance, A.; Reyna, Y.; Lira-Cantu, M., Metal Oxides in Photovoltaics: All-Oxide, Ferroic, and Perovskite Solar Cells. In The Future of Semiconductor Oxides in Next Generation Solar Cells, 1st ed.; Lira-Cantu, M., Ed. Elsevier Singapur: 2017; p 566.
7. Lira-Cantú, M., Perovskite solar cells: Stability lies at interfaces. Nature Energy 2017, 2 (7), 17115.
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Understanding the energy transfer mechanism across hybrid interfaces which combine both inorganic and organic semiconductors is crucial in the advancement of optoelectronic devices. The exact transfer mechanism between inorganic and organic materials remains obscure, particularly where there is a large amount of structural inhomogeneity within the material.
Recent advances have shown that both lead sulfide (PbS) quantum dots, as well as CsPbX₃quantum dots are capable of efficient triplet energy transfer at such organic-inorganic hybrid interfaces. [1,2,3]
Combining scanning probe microscopies with ensemble time-resolved optical spectroscopy, we investigate the underlying mechanism of energy transfer at hybrid interfaces comprised of lead halide perovskites and organic semiconductors. By understanding interplay between the morphology and the band alignment arising from the perovskite composition, as well as the mechanism of the energy transfer at the hybrid interface, we can tailor our active materials to increase device performance by optimizing this energy transfer process.

[1] Wu, Congreve, Wilson et al. Nature Photonics10, 31–34 (2016)
[2] Nienhaus et al., ACS Nano, 11, 7848–7857 (2017)
[3] Mase et al. Chem. Commun., 53, 8261-8264 (2017)

Surface and interface passivation are of central importance for high efficiency perovskite devices. Despite the enormous efforts that have been undertaken in the last years to improve the power conversion efficiency of the devices, the surface properties are still not very well understood. Moreover, the polycrystallinity of the material leads to lateral fluctuations of the surface properties on the nanometer scale, which makes the analysis difficult and challenging. Scanning probe microscopy (SPM) techniques are ideal to gain a much deeper understanding of the surfaces and interfaces due to their high spatial resolution.
In this contribution, we present our results obtained on hybrid perovskites studied with scanning tunneling microscopy (STM) and spectroscopy (STS) combined with Kelvin Probe Force Microscopy (KPFM). All our studies have been carried out under ultra-high vacuum (UHV) conditions on clean absorbers that have not been exposed to air.
In a first part, we show why ultra-high vacuum is indispensable for an accurate analysis of the nanometer scale properties. We compare KPFM measurements before and after air exposure in our UHV KPFM setup and compare the results to measurements under ambient conditions. In particular, we discuss the changes of the facet-dependent contrast of the grains, which is smeared out and reduced after air exposure, and we discuss the observed changes of the work function at the grain boundaries. Furthermore, we compare our results to the available literature.
In the second part, we discuss in detail the nanometer scale variations of coevaporated methylammonium lead iodide absorbers with minority carrier lifetimes exceeding one microsecond.In particular, we investigate the lateral variations of the workfunction on these absorbers layers and discuss the implications for the resulting solar cell devices. We will link our results from SPM to the specific growth conditions we used during synthesis.
Finally, we compare the results to another thin film technology, namely Cu(In,Ga)Se₂. This comparison is essential to understand why hybrid organic inorganic perovskites outperform Cu(In,Ga)Se₂ in terms of minority carrier lifetime and exceptionally high open-circuit voltages. In this part we use STS measurements to compare the local density of states of the perovskite and of the Cu(In,Ga)Se₂ surfaces. It becomes evident from our measurements that the defect density at the surface is much lower for the perovskites, which underlines why these semiconductors perform so exceptionally well as absorber layers in solar cells.

CsPbI₃, a promising candidate for photovoltaic and light-emitting diode applications, is structurally unstable and transforms into an orthorhombic phase at room temperature. CsPbI₃ in nanocrystal form, on the other hand, has displayed better thermal stability than in the bulk, and the stability depends on the size of the nanocrystal. Yet, not many studies have shed light on the underlying mechanisms.
Using first-principles calculations, we demonstrate phase stability changes as a function of varying crystal size, which result from the competition between short-range bond formation and long-range electrostatic energies. Comparing these simulations with experiment, we show that this mechanism is analogous to the application of negative pressure, explaining the correlation between high-symmetry phase stability and nanostructuring.
These results suggest that bulk phase CsPbI₃ can be stabilized in a negative pressure environment with the application of hydrostatic tensile strain. Using density functional theory, we explore negative pressure induced phase transitions, and their consequent optoelectronic properties.

Halide Perovskites have a remarkably low defect density, especially if we consider the quick and “chimie douce” way of preparation of films as well as of most single crystals. The densities are all deduced from the common (all indirect) measurements for charge or neutral defects. In this talk I will show how this behaviour reflects a quite fundamental property of these materials, with a rather simple basis. The talk will combine experimental results from several sources, including our own, for thermodynamic, optical, and electrical data. It is plausible that our conclusions can be generalized to help look for other ultra-low defect density materials.
Thanks to David Egger (Regensburg), Omer Yaffe and Leeor Kronik (Weizmann Inst.)

Under solar irradiation, most photons absorbed in methyl ammonium lead iodide perovskite (MAPbI₃) possess ~0.4 eV of excess energy above the 1.65 eV band gap (BG), and characterizing the important stages of free carrier generation and relaxation dynamics requires ultrafast spectroscopic investigation. Carrier cooling and recombination in hybrid perovskites occurs in ps and ns timescales respectively. Probing exciton dissociation however requires extreme (sub-10 fs) time resolution. The significance of optic phonon coupling to free carriers in these materials (polarons) has recently been the subject of lively debate. To date only a few time domain measurements have reported impulsive phonon activity, and those reports have not converged in terms of the active phonon frequencies or coupling strengths. Here, we present high S/N sub-10 fs pump-probe spectroscopy of MAPbI₃ films to record ultrafast exciton dissociation, which has not been reported previously.
High above band gap excitation in MAPbI₃ provides an opportunity to observe free carrier generation followed by cooling dynamics. Transient transmission (TT) spectra of MAPbI₃ films after photoexcitation reveal a number of features which are common to previous pump-probe studies of this system. First, a photoinduced bleach at the optical BG which appears instantly and grows to its full magnitude during carrier cooling, is attributed to hot carrier induced screening of the exciton transition and state filling. Another is an absorption feature rising below the band edge assigned to band gap renormalization and shifting of exciton transition. This feature also appears immediately after excitation, but decays over the carrier cooling stage. The third is a slow rising broad induced absorption feature in the inter-band region. Using the extreme time resolution here, a sharp red shift of exciton transition is observed at early trimes. Surprisingly, the photoinduced band bleach derived by extracting a band integral over the whole probing range, appears abruptly with a delay of 20 fs after the pump excitation. This delay in rising of band integral bleach is assigned to exciton dissociation or the breakup of localized e-h pair. To the best of our knowledge this is the first recording of this phase of free carrier formation following above BG photoexcitation in this material, or in bulk semiconductors. Observing this phase hinges on the ultrashort pump pulses ability to generate such localized carrier pairs even high above the BG due to coherent excitation of a broad band of “k” states. Finally, the delayed ~0.4 ps rise of the band integral is compatible with carrier cooling.
The pump-probe spectra also reveal weak periodic modulations. The residual modulations at different probe wavelengths were extracted by subtracting the transient signal. The Fourier analysis of the residual modulation detects predominantly two of active phonon modes: low frequency mode ~110 cm^-1 assigned to the Pb-I stretching and high frequency mode ~240 cm^-1 assigned to the torsions of methylammonium cation (Fig. 2d). The amplitudes of these spectral modulations were used to estimate the electron-phonon coupling strength, assuming a displaced harmonic oscillator model. The estimated coupling strengths (Huang-Rhys parameter, S: 0.02-0.04) for the MAPbI₃ films are well within the weak coupling regime, which is compatible with free carriers existing as large polarons in these materials. These observed phonon modes may not be responsible for perovskite’s moderate carrier mobility, which requires intermediate electron-phonon coupling.
In conclusion, sub 10fs above band gap pump-probe data resuggest that, instantly generated localized hot excitons dissociate to free carriers within ~20 fs. Faint spectral modulations in the transient signals are assigned to coherent phonons. These modulations are assigned to longitudinal optical (LO) phonons weakly coupled to the electronic transition.

Hybrid organic-inorganic semiconductors feature complex lattice dynamics due to the "softness" arising from non-covalent bonds between molecular moieties and the inorganic network, and due to the ionic character of the crystal. Such complex lattice motion has profound consequences on the fundamental character of primary photoexcitations with respect to purely covalent semiconductor crystals. In this work, we establish that this dynamic structural complexity gives rise to the coexistence of diverse excitonic resonances in a prototypical two-dimensional lead iodide perovskite, each with a distinct degree of polaronic character. By means of high-resolution resonant impulsive stimulated Raman spectroscopy, we address the coupling of both charge carriers and excitons to low-frequency optical phonons (those with frequency ≤ 50 cm⁻¹). Resonant photoexcitation results in vibrational wavepacket dynamics that evolve along different configurational coordinates for distinct excitons and photocarriers. Employing density functional theory calculations, we assign the observed coherent vibrational modes to various phonons involving motion in the lead-iodide layers. We thus conclude that different excitons induce specific lattice reorganizations that are distinct from those involving charge carriers, which are signatures of polaronic binding. Our conclusions provide a novel perspective of the energetic/configurational landscape involving globally neutral primary photoexcitations in a broad class of emerging hybrid semiconductor materials.

Organic-inorganic hybrid perovskites such as methylammonium lead iodide (CH₃NH₃PbI₃) are solution-processable semiconductors for high-efficiency solar cells and light-emitting devices owing to their long carrier lifetime and diffusion length. Determining whether the polar organic cations with strong dynamic disorder benefit the optoelectronic properties of CH₃NH₃PbI₃ has been challenging. Herein, via transient absorption measurements employing an infrared pump tuned to a methylammonium N-H stretching vibration, we observe vibrational energy transfer from the selectively excited organic mode to the entire crystal lattice in nanosecond timescale. The observed transient electronic signatures, during the period of thermal-nonequilibrium when the induced thermal motions are mostly concentrated on the organic sublattice, reveal that the induced motions of the organic cations do not apparently alter absorption or photoluminescence response of CH₃NH₃PbI₃, beyond thermal effects. Our results suggest that the attractive optoelectronic properties of CH₃NH₃PbI₃ mainly derive from the inorganic lead-halide framework.

CsPbBr₃ is a promising type of light‐emitting halide perovskite with inorganic composition and desirable thermal stability. The luminescence efficiency of pristine CsPbBr₃ thin films, however, appears to be limited. In this work, light emitting diodes based on CsPbBr₃|Cs₄PbBr₆ composites are demonstrated. Both quantum efficiency and emission brightness are improved significantly compared with similar devices constructed using pure CsPbBr₃. The high brightness can be attributed to the enhanced radiative recombination from CsPbBr₃ crystallites confined in the Cs₄PbBr₆ host matrix. The unfavorable charge transport property of Cs₄PbBr₆ can be circumvented by optimizing the ratio between the host and the guest components and the total thickness of the composite thin films. The inorganic composition of the emitting layer also leads to improved device stability under the condition of continuous operation.

Layered 2D halide perovskites, with their alternating organic and inorganic atomic layers that form self-assembled multi-quantum wells (MQW), have been shown to possess robust light-matter coupling, owing to quantum and dielectric confinement effect. Within their periodic structures lie a hotbed of robust photophysical phenomena, rising from the coherent four-way interplays between exciton, spin, phonon, and photon. Herein, we explicate these intricate dynamics via transient absorption spectroscopy. Few to be highlighted here are: (i) the robust tunable spin-selective optical Stark effect, which are few times stronger than in conventional inorganic semiconductors; (ii) ultrafast carrier thermalization and spin relaxation via exchange interaction; and (iii) strong coherent exciton-photon coupling. Origin of transient spectral features and exciton relaxation pathways are also revealed from detailed phenomenological modelling of the transient dynamics. Importantly, our work unravels the understanding of complex optical spin and quasi-particle interactions in these layered 2D halide perovskites, which are the key to exploit their full potential.

The extended carrier lifetime in hybrid halide perovskites was attributed to a quasi-indirect band gap that arises due to Rashba splitting in both conduction and valence band edges. We will present results for an effective relativistic band structure of (CH₃NH₃)PbI₃with focus on the dispersion of electronic states near the band edges of (CH₃NH₃)PbI₃affected by thermal structural fluctuations [1]. We establish a relation between the magnitude of Rashba splitting and the deviation of Pb-atom from its centrosymmetric site in the PbI₆octahedron. In order for the splitting energy to reach the thermal energy of 26 meV (room temperature), the displacement should be of the order 0.3 Ang, which is far above the static displacements of 0.1 Ang for Pb-atoms in the tetragonal phase of (CH₃NH₃)PbI₃.

The significant dynamic enhancement of the Rashba splitting observed at earlier simulation times (less than 2 ps) later weakens and becomes less than the thermal energy despite the average displacement of Pb-atoms remaining large (0.37 Ang). It is randomization of Pb-displacement vectors and associated cancelation of the net effective magnetic field acting on electrons at the conduction band edge is responsible for reduction of the Rashba splitting.

The lattice dynamics also leads to deterioration of Bloch character [2] for states in the valence band, which leads to subsequent localization of holes. A bipolar mobility of charge carriers in (CH₃NH₃)PbI₃is therefore affected. These results call into question the quasi-indirect band gap as a reason for the long carrier lifetime observed in (CH₃NH₃)PbI₃at room temperature. An alternative mechanism involves dynamic localization of holes and their reduced overlap with electrons in reciprocal space.

[1] C. Zheng, S. Yu, and O. Rubel, ArXiv:1810.00275 [Cond-Mat.Mtrl-Sci] (2018).
[2] O. Rubel, A. Bokhanchuk, S. J. Ahmed, and E. Assmann, Phys. Rev. B 90, 115202 (2014).

In recent years, there have been extensive studies made on optical and electronic properties of lead halide perovskite semiconductors, MAPbX₃ (MA = CH₃NH₃, X = I, Br, and Cl) from the fundamental physics viewpoint and from the interest in the application to functional photonic devices [1]. These direct-gap semiconductors exhibit sharp absorption spectra with very small Urbach energies, strong light emission with no essential Stokes shift, and long carrier diffusion lengths, leading to high energy conversion efficiencies of solar cells. Besides solar cell applications, MAPbX₃ perovskites are considered as materials for light-emitting diodes, lasers, optical modulators, and nonlinear optical crystals [2-4]. Especially, it is very important to elucidate their nonlinear optical response for future device applications. Among them, a wide-gap semiconductor MAPbCl₃, has attracted attention as a photonic device material in the blue spectral region [2].
In this work, we clarify the nonlinear refractive index and nonlinear absorption coefficient of MAPbCl₃ single crystals and their wavelength dependence. The thin film sample of MAPbCl₃ has a grain structure and strong light scattering occurs in the blue spectral region, being an obstacle for measuring their essential optical characteristics. In order to eliminate the influence of light scattering, a large bulk single crystal was used. Third-order nonlinear optical coefficients were determined by the Z-scan method. We determined the nonlinear refractive index from the close aperture measurement and the nonlinear absorption coefficient from the open aperture measurement at various excitation wavelengths. Nonlinear optical responses can be explained by a simple two band model. Our measurements clearly shows that MAPbCl₃ perovskites are simple direct-gap semiconductors and the nonlinear optical coefficients are comparable to GaAs single crystals [5].
Part of this work was supported by JST-CREST (JPMJCR16N3)
[1] Y. Kanemitsu and T. Handa, Jpn. J. Appl. Phys. 57, 090101 (2018).
[2] T. Yamada et al., Phys. Rev. Lett. 120, 057404 (2018).
[3] M. Nagai et al., Phys. Rev. Lett. 121, 145506 (2018).
[4] H. Tahara et al., Adv. Optical Mater. 6, 1701366 (2018).
[5] R. W. Boyd, Nonlinear Optics (Academic, 2003).

Organic-inorganic hybrid perovskite semiconductors are collecting much attention as a new class of optical device materials. In particular, CH₃NH₃PbI₃ (MAPbI₃) shows highly efficient photoluminescence (PL) with no Stoke shift even at room temperature due to the band-to-band transition. Many unique optical phenomena based on efficient PL with no Stokes shift, such as photon recycling and radiative cooling, have been discussed [1,2]. Photon recycling is caused by the repeated light emission and reabsorption processes. The laser cooling of semiconductors is induced by anti-Stokes PL (AS-PL) via phonon absorption. One important factor for these phenomena in semiconductors is a high external quantum efficiency (EQE). However, laser cooling has not been achieved even for GaAs/GaInP quantum well with EQE of 99.5% [3]. Therefore, it is important to clarify the material properties and experimental conditions required for laser cooling. Specifically, it is necessary to clarify the physics of AS-PL and its reabsorption in the perovskites by employing both optically thin and thick samples.
In this study, we prepared optically thin film and thick single crystal of MAPbI₃, and investigated their AS-PL characteristics [4]. Using PL excitation (PLE) spectroscopy, we determined the excitation-energy dependence of the AS-PL and the Stokes PL (S-PL) by dividing the PL spectrum into anti-Stokes and Stokes parts. We obtained the up-conversion gain spectrum defined as the intensity difference between the AS-PLE and S-PLE spectra. The gain spectra showed a broad spectral shape and its maximum point was located below the balanced point where the anti-Stokes PLE and Stokes PLE intensities become equal. The broad shape of the up-conversion gain spectrum suggests that unique phonon dynamics, such as large anharmonicity and polaron formation, play an important role of efficient up-conversion process in the perovskites.
Part of this work was supported by JST-CREST (JPMJCR16N3) and JSPS Research Fellowships for Young Scientists (17J07890).
[1] Y. Yamada et al., J. Am. Chem. Soc. 137, 10456–10459 (2015).
[2] S.-T. Ha et al., Nat. Photonics 10, 115-121 (2016).
[3] D. A. Bender et al., Appl. Phys. Lett. 102, 252102 (2013).
[4] T. Yamada et al., submitted for publication.

One of the many interesting optical properties of semiconducting nanocrystals is their ability to perform luminescence up-conversion via a one photon route. In this, a carrier is excited by an incident photon with energy less than the material band gap. The excited carrier subsequently absorbs energy from multiple phonons before emitting the combined thermal and optical energy as a single, higher energy photon. If a system demonstrates one photon up-conversion with enough efficiency, thermal energy may be emitted as up-converted photons faster than it is replaced through thermalization losses. In this regime, the material demonstrates optically driven cooling. As one photon up-conversion is apparently ubiquitous in semiconducting nanocrystals, it is believed that mid-gap surface trap states may play a role in the excitation mechanism. Interestingly, however, this phenomenon has recently been demonstrated in CsPbBr₃ nanocrystals with an external quantum efficiency greater than 70%. This high efficiency seems at odds with previous studies, as perovskite nanocrystals are known to generally lack mid-gap states.

This presentation will discuss our recent progress in examining one photon up-conversion in all-inorganic perovskite nanocrystals, especially regarding the potential role of an intermediate state in the excitation process, as well as the optimization of the up-conversion via plasmonic enhancement in systems coupled to metal nanoparticles. Specifically, we have shown that a facile surface treatment increases the efficiency of one photon up-conversion in CsPbBr₃ nanocrystals, even as it increases the traditional down-converted photoluminescence quantum yield to essentially unity indicating a lack of mid-gap trap states. This increase in up-conversion efficiency, despite the lack of mid-gap states, suggests that such states act as loss pathways rather than acting as intermediate states necessary to the excitation mechanism. Additionally, we show that through the presence of a localized surface plasmon, e.g. by depositing gold nanoparticles, the observed up-conversion blue shifts, indicating more efficient use of thermal energy to drive optical emission. These results suggest another mechanism may be the key to understanding and optimizing one photon up-conversion, one that does not involve discrete mid-gap states. The efficiency of this up-conversion mechanism in CsPbBr₃ makes this material a good candidate for optically driven cooling. As such, we will also discuss initial thermometry studies of cooling power in perovskite nanocrystal films.

Methylammonium lead halide perovskites have attracted significant interest for their desirable electronic properties. Nanocrystals formed from these materials have shown great promise for use in LEDs and lasers due to their high fluorescence quantum yield and the tunability of their emission throughout the visible spectrum. The rapid kinetics of common nanocrystal syntheses such as hot-injection – combined with the inherent instability of the partially capped nucleation centers that are nascent nanocrystals – precludes the use of time-resolved spectroscopy to measure the evolution of excited state dynamics during the formation of fully passivated nanocrystals. The ability to measure electronic structure and dynamics during this process would deepen our understanding of the relationship between synthetic parameters and the resulting functional electronic properties. Here we use an ambient, room temperature ligand-mediated nanocrystal synthesis with reaction timescales more favorable for study via transient absorption (TA) spectroscopy. By coupling this synthesis with transient absorption spectroscopy, we are able to provide the first ever characterization of excited state dynamics in unstable, immature perovskite nanocrystals as they grow and their surfaces are capped with passivating ligands. We observe a previously unreported signature in the TA spectrum of immature nanocrystals that is not present in fully formed nanocrystals. This work helps elucidate the role the surface of these materials plays in excited state dynamics, as well as providing new insight into the evolution of exciton dynamics during perovskite nanocrystal growth and formation. Future measurements utilizing a single-shot transient absorption (ssTA) spectrometer that is capable of measuring entire transients in a few seconds will enable the spectroscopic observation of nanocrystal syntheses in real-time.

The observation of a ground optically forbidden “dark” exciton state in semiconductor nanocrystals was first reported in the seminal paper of Nirmal et al. in 1995. ¹ Later research in nanowires, nanorods, and nanoplatelets has shown that the ground exciton state in all these semiconductor structures is a dark exciton, leading us to believe that the ground exciton must be dark. Because dark excitons release photons slowly, hindering emission, semiconductor nanostructures that disobey this rule have been sought. However, despite considerable experimental and theoretical efforts, no semiconductors have been identified in which the lowest exciton is bright. Three years ago however cesium lead halide perovskite (CsPbX3, with X = Cl, Br or I ) nanocrystals were grown, which without too much effort, demonstrated very bright photoluminescence (PL) with quantum yield 50-90% at room temperature. This bright emission was traced to a very short radiative decay time. The nanocrystals emit light about 20 and 1,000 times faster than any other semiconductor nanocrystal at room and cryogenic temperatures, respectively. The increase of the decay time with temperature is inconsistent with a dark ground state exciton suggesting that in these nanocrystals the ground exciton state is bright. We use an effective-mass model and group theory to demonstrate the possibility of such a ground bright state existing, which can occur when the strong spin–orbit coupling in the conduction band of perovskites is combined with the Rashba effect.² We then apply our model to CsPbX3 nanocrystals, and measure size- and composition- dependent fluorescence at the single-nanocrystal level. The bright triplet character of the lowest exciton explains the anomalous photon-emission rates of these materials. The existence of this bright triplet exciton is further confirmed by analysis of the fine structure in low-temperature fluorescence spectra. More generally, our results provide criteria for identifying other semiconductors that exhibit bright excitons, with potential implications for optoelectronic devices.
¹Nirmal, M. et al. “Observation of the “dark exciton” in CdSe quantum dots.” Phys. Rev. Lett. 75, 3728–3731 (1995).
² Becker, M. A. “Bright triplet excitons in caesium lead halide perovskites,” Nature, 553, 189-193 (2018)

Fullerenes are very efficient electron acceptors commonly used in conjunction with molecules allowing for charge separation in organic photovoltaic devices. In this work, we investigate the optically induced charge transfer from inorganic perovskite CsPbBr₃ nanocrystals (NCs) to the fullerene derivative phenyl-C61-butyric acid methyl ester (PCBM) in layer-by-layer as well as in blend structures. We observe a significant quenching of CsPbBr₃ NCs photoluminescence. In the differential transmission (DT) transients of the CsPbBr₃ NCs/PCBM composite, an ultrafast decay is observed, which is referred to the charge transfer process from CsPbBr₃ NCs to PCBM. A subsequent decay occurs on a longer timescale, which is referred to the recombination of separated electrons and holes at CsPbBr₃ NCs/PCBM interfaces. Varying the thickness of the NC layer on top of the PCBM in the layer-by-layer heterostructure, an exciton diffusion length of 290 ± 28 nm for CsPbBr₃ NC is extracted. The diffusion process is followed by an ultrafast exciton dissociation (within 200 fs) at the CsPbBr₃ NC/PCBM interface. In blend structures an overall faster charge transfer process is observed. For practical applications, the subsequent collection of separated electrons and holes from the composite is critical. Therefore, we have attached hole- and electron-collecting layers to the blend structures. DT transients on such more device-oriented structures give further insight into the complete charge separation process as it occurs in corresponding photodetectors.

Low dimensional halide perovskites have attracted enormous attentions due to their superior optical and electronic properties. Very recently, intense research has been directed toward the fabrication of the low dimensional perovskite materials for laser applications, the performance of such lasers is highly dependent on the quality of the material and cavity, which makes their fabrication challenging. Herein, we demonstrate that cesium lead halide perovskite nanorods fabricated via vapor methods, which can also serve as gain media and effective cavities for tunable lasers^[1,2]. Meanwhile, we have also achieved single-mode laser based on cesium lead halide perovskite submicron sphere at room temperature^[3]. Furthermore, we achieved epitaxial growth of single-crystal Cesium Lead Bromide Perovskite film on Metal Oxide Perovskite (SrTiO3)^[4] and developed a novel vapor-solution method for preparation of high-quality perovskite films on flexible substrates^[5], both perovskite based structures show high-performance in photodetection among visible light spectrum. Moreover, the photodetection band can be extended to near-infrared range through designing novel perovskite–erbium silicate nanosheet hybrid structures, which exhibit remarkable spectral response at ≈1.54 µm^[6]. Considering the perovskite based integration application, we demonstrated the controlled growth of in-plane directional perovskite CsPbBr₃ nanowires on annealed M-plane sapphire substrates, and the guided nanowire exhibit excellent photoresponse properties a very fast response speed ^[7]. In addition, we achieved high qulity laser arrays based on the in-plane directional CsPbX₃perovskite nanowires, which also show composition-dependent strong exciton-photon coupling^[8].Unfortunately, the devices based on perovskite nanostructures still need improvement due to their environmental instability and device fabrication process incompatibility. To address these problems, we report electroluminescence (EL) devices fabricated by directly growing CsPbBr₃ nanoplates on prepatterned indium tin oxide (ITO) electrodes via a vapor-phase deposition^[9]. Besides,we demonstrate the visualizing carrier transport of these all-inorganic CsPbBr₃ peroviskite nanoplates by electric field modulated photoluminescence imaging and abtained the carrier mobility of about 28 cm²V^-1S^-1[10] . Importantly, up to now, realizing the controlled growth of two dimensional perovskite at sub-10 nm scale remains a great challenge, which has largely limit the development and applications of two dimensional perovskites. We employed the atomic crystal semiconductor such as TMDc (eg. WS₂, MoS₂) as substrate materials, and obtained uniform TMDc/Perovskite hybrid heterostructures, with the thickness can be well controlled at sub-10 nm scale. These results suggest that the novel low dimensional perovskite nanostructures may open up new opportunities for various applications in integrated electronics, optoelectronics, and quantum electronics.

References:
[1] H. Zhou, X. Wang, A. Pan*, et al. ACS Nano, 2017, 11, 1189.
[2] X. Wang, X. Wang, A. Pan*, et al. Nano Res., 2017, 10, 3385.
[3] B. Tang, A. Pan*, L. Zhang, et al. ACS Nano, 2017, 11, 10681.
[4] J. Chen, A. Pan, S. Jin*, et al. J. Am. Chem. Soc., 2017, 139, l13525.
[5] W. Hu, W. Huang, A. Pan*, et al. Adv. Mater., 2017, 29, 1703256.
[6] X. Zhang, X. Wang, A. Pan*, et al. Adv. Mater., 2017, 29, 1604431.
[7] M. Shoaib, A. Pan*, et al. J. Am. Chem. Soc., 2017, 139, 15592.
[8] X. Wang, M. Shoaib, A. Pan*, et al. ACS Nano,2018, 12, 6170.
[9] X. Hu, X. Wang, A. Pan*, et al. ACS Nano, 2017, 11, 9869.
[10] X. Hu, X. Wang, A. Pan*, et al. Nano Lett., 2018, 18, 3024.

Understanding the nature of defects in organic-inorganic hybrid perovskites (OHPs) is one of the key challenges to improving perovskite-based devices. Surface defects are of special interest because they exist at interfaces in the final device. The concept of ion migration has been used to explain the anomalous current-voltage hysteresis phenomenon observed in hybrid organic-inorganic perovskites (OHPs), and has also been implicated as potential cause for long-term material degradation. Theoretical studies have predicted vacancy assisted migration may be a possible mechanism,¹ but the dynamics that occur at the atomic scale are still not well clarified experimentally. Scanning tunneling microscopy (STM) offers the ability to probe the surface of OHPs with atomic resolution, including resolving individual vacancy defects.^2-4 Scanning the same area multiple times allows for observation of dynamic events. Here, ion migration to and from the surface was imaged at the atomic scale, and DFT calculations indicate that a step-wise mechanism for migration of MABr vacancies is energetically favorable, shedding light on the mechanism by which vacancy-assisted ion migration occurs. It is also shown that the presence of surface vacancies affects the local work function, which has implications for charge transfer between layers in a device.

References
¹ J. Azpiroz, E. Mosconi, J. Bisquert and F. De Angelis, Eng. Environ. Sci. 8, 2118 (2015).
² L. She, M. Liu, D. Zhong, ACS Nano. 10, 1126 (2016).
² R. Ohmann, L.K. Ono, H.S. Kim, H. Lin, M.V. Lee, Y. Li, N.G. Park, Y.B. Qi, J. Am. Chem. Soc. 137, 16049 (2015).
⁴ Y. Liu, K. Palotas, X. Yuan, T. Hou, H. Lin, Y. Li and S.T. Lee, ACS Nano. 11 2060 (2017).

Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) comprise a promising and versatile class of solution-processable materials with outstanding optoelectronic properties. By exploiting chemical, structural and quantum degrees of freedom, researchers have demonstrated luminescent devices with continuously tunable band-gap energies and desirable narrow excitonic luminescence. However, numerous studies reveal undesirable sideband features, leading to “intrinsic white-light emission” that is detrimental to color purity. Insofar as 2D HOIPs are recognized as “natural” quantum-well semiconductor structures, optical properties have, to date, been treated within a framework developed for conventional semiconductor (e.g., III-V) materials. Namely, light-matter interactions are assumed to be completely described by the lowest-order term of the multipolar Hamiltonian, i.e., the electric dipole (ED) term. However, HOIPs exhibit a number of unconventional structural and electronic features; giant spin-orbit coupling, the significant presence of p-orbital contributions to electronic states at both conduction- and valence-band extrema, and a number of possible static and dynamic symmetry breaking mechanisms evince the possibility of unconventional optoelectronic properties that are neglected in the ED approximation. Here, using energy-momentum spectroscopies, we identify a bright optical transition in 2D lead-halide HOIPs that exhibits striking polarization- and momentum-dependent radiation patterns at an energy 90 meV below the primary exciton emission. Combined experimental, electromagnetic and quantum mechanical analyses indicate that this transition is intrinsic in origin and arises from an unconventionally fast magnetic dipole (MD) transition from an excitonic excited state. These results suggest the presence of odd-parity ground state electronic symmetries that have not yet been established, and urge deeper theoretical inspection of the nature of the excited states in these materials. In addition to being of great fundamental interest, this demonstration of multipolar transitions may be crucial for resolving open questions regarding the previously observed bounty of complex optical signatures and anomalous low-energy sideband emissions in 2D HOIPs.

The high optoelectronic quality of Halide Perovskite, HaP semiconductors is remarkable, given their low-temperature solution preparation. Up to now, there was little direct experimental evidence for significant trap densities within their band gap, from optical and/or electronic measurements. However, by using variable low-energy UV-photo emission spectroscopy (VE-UPS) we could detect in-gap states in (FA,MA,Cs)PbBr₃ perovskite film around the mid gap and estimate the defect densities. We find that the observed defects not only reduce the PL quantum efficiency, but also can dictate other electronic transport properties under illumination, such as the structure of the electronic junction and the charge carrier diffusion lengths. Customized Electron Beam-Induced Current (EBIC) measurements, with and without illumination, as function of bias, allow to map and track changes in the junction structure (space charge region) and diffusion lengths. We found changes in the EBIC profile shape that can be explained by a reduction of minority carrier diffusion length with increasing illumination intensity in this type of HaP. In addition, we find that, under reverse bias, the position of the junction remains the same, close to the ETL. This observation suggests that even if ions are mobile, they are not dictating the junction configuration and, hence, ion migration is not the main cause for the reduced device performance. By applying a two-level defect model, we can explain our experimental results and predict the cell`s performance as function of illumination in (FA,MA,Cs)PbBr₃ -based perovskite solar cells.

All inorganic Cs-Pb-X based perovskite systems have recently attracted renewed research interest due to the various stoichiometries that can be crystallized from the same precursors. 3D (CsPbBr₃), 2D (CsPb₂Br₅) and 0D (Cs₄PbBr₆) perovskite derivatives can crystallize from CsBr and PbBr₂ precursors depending on the reaction parameters. Cs₄PbBr₆ has been found to be especially interesting due to its strong and stable green photoluminescence in powders, thin films, single crystals and nanocrystals forms. However, it was reported, in 1893^[1], that Cs₄PbX₆ powders are colorless, irrespective of the halide in the structure, and the reported bandgaps are all in the UV region due to the negligible electronic couplings between [PbX₆]⁴⁻ octahedra. Hence, the origin of the green PL has been debated over and it has been proposed to be either due to intrinsic defects within the Cs₄PbBr₆^[2] or because of contamination with CsPbBr₃ nanocrystals^[3].

In this study, we explore all different reaction parameters governing the formation of Cs₄PbBr₆ and CsPbBr₃ and the origin of the green PL in the Cs₄PbBr₆ phase. Solvent, anti-solvent, absolute concentration and relative ratios between precursors can all influence the phase of the perovskite powders. The nature and size of the polybromoplumbate complexes formed in solution also plays a critical role in determining the phase of the formed perovskite powders. Importantly, ¹³³Cs and ²⁰⁷Pb solid-state nuclear magnetic resonance (ssNMR) were carried out for the first time on 0D Cs₄PbBr₆ and it suggests that the 3D CsPbBr₃ impurities are not the source of the green PL. However, high-resolution transmission microscopy detects 3D-like domains of widely varying sizes in both bulk and nanocrystalline Cs₄PbBr₆ samples. Finally, we propose a 2D Cs₂PbBr₄ impurity to be responsible for the controversial optical properties of the wide bandgap 0D Cs₄PbBr₆.

[1] W. A. P. Wheeler, “Uber die Casium- und Kalium-Blei halogenide,” Zeitschrift für Anorg. Chemie, vol. 3, no. 1, pp. 195–210, 1893.
[2] J. Yin et al., “Point Defects and Green Emission in Zero-Dimensional Perovskites,” J. Phys. Chem. Lett., vol. 9, no. 18, pp. 5490–5495, 2018.
[3] X. Chen et al., “Centimeter-Sized Cs₄PbBr₆ Crystals with Embedded CsPbBr₃ Nanocrystals Showing Superior Photoluminescence : Nonstoichiometry Induced Transformation and Light-Emitting Applications,” Adv. Funct. Mater., vol. 1706567, pp. 1–7, 2018.

Various modulation spectroscopic methods have been developed and employed in the PV research; impedance, capacitance, admittance, light-intensity modulation, to name a few. Each of them is built on its own physical picture, whose basic notion has long been tested and the validity confirmed, at least in the inorganic semiconductor research. However, once these techniques are applied to the perovskite PVs, we have encountered difficulties in interpreting the data in a consistent manner. It is now clear that the culprit is the ion motion, which is as influential as electron motion in some cases. We will try to reinterpret the conflicting results from the point of view of the ion motion.
In the modulation spectroscopy, one usually modulates the applied potential or the carrier density. The response would be the extracted current or the voltage across the device. As the word “spectroscopy” implies, one measures the response as a function of the modulation frequency in the linear regime, i.e., with the small enough modulation depth so that nonlinearity does not enter*. The out-of-phase response normally carries the important information because it reflects the carrier dynamics in the device. In the traditional inorganic PVs, only electrons are the charge carriers. Despite this simplicity, it is hard to distinguish various mechanisms, notable examples being (1) charge trapping-detrapping processes and (2) Maxwell-Wagner polarization. The response becomes multiplicatively more complex in the presence of ions, because the (3) ionic motion not only cause displacement current but also changes the internal electrostatic potential landscape affecting the electron distribution. The three processes enumerated above all fall in similar frequency ranges** so that it is hard to distinguish them.
We have performed various modulation spectroscopy experiments and numerical simulations. Model calculations of the three processes above reveal the signatures one should look for in the data peculiar to each process. Therefore, it is of essential importance to analyze the same set of data in detail and in various ways. We will review our effort in analyzing the data to identify the processes inside the perovskite PVs. The responses as a function of frequency, temperature, illumination intensity and wavelength are combined to arrive at an overall semi-quantitative picture. We will present exemplifying results.

* Although this aspect is not often discussed in the literature, one needs to pay attention to what extent a particular experiment perturbs the system. It is important when data from different types of modulation experiments are compared.
** It is not clear if this is accidental or if there is underlying physics, i.e., correlated ion-electron motion mediated by defects.

A key debate involving mixed-cation lead mixed-halide perovskite thin films relates to the effects of process conditions on film morphology and local performance of perovskite solar cells. The halide distribution in these thin films has been shown to be of great importance for optoelectronic properties and device performances [1]. In particular, in mixed Br/I perovskites with high Br content, the thin films exhibit multiple emission peaks, suggesting segregation of halides [2].
To understand the underlying mechanisms, we map the spatial distribution of the ions in these complex multi-element halide lead perovskite thin films by synchrotron-based nanoprobe X-ray fluorescence (n-XRF). To get more insights into the growth mechanism and the resulting distribution of the halides and alkali ions, we fabricate films with different thicknesses and use scanning probe techniques as well as time-resolved optical spectroscopy.
These insights into the interplay between composition, microstructure, and macroscopic properties pave the way to improved performance in this rapidly growing family of multinary lead-halide materials for solar cell applications.

[1] D. P. McMeekin et al., A mixed-cation lead mixed-halide perovskite absorber for tandem solar cells. Science 351, 151-155 (2016).
[2] E. T. Hoke et al., Reversible photo-induced trap formation in mixed-halide hybrid perovskites for photovoltaics. Chemical Science 6, 613-617 (2015).

Solar cells based on organic-inorganic halide perovskites have entered the research field of photovoltaics by storm, already reaching efficiencies close to highly optimized silicon solar cells. In this context ion migration has been shown to drastically influence perovskites properties at various timescales. To understand the effect of ion migration we compare differently prepared perovskite devices, where we measure activation energies, concentrations and diffusion coefficients of mobile ions using transient ion drift and admittance spectroscopy. We show a correlation between fabrication conditions and ion migration. Our results shed light on the strong influence on the exact fabrication conditions, and will help to develop more robust protocols for the fabrication of highly efficient perovskite devices.

Mixed-halide perovskites are strong candidates for tandem solar cells and light-emitting devices because they exhibit bandgap tunability over the entire visible spectrum, which is easily accomplished via halide mixing. The most efficient solar cells employ a mixture of halides (typically bromide and iodide) and cations to stabilize the perovskite phase while obtaining the desired bandgap. A limitation of these materials is that defect-mediated halide transport has drastic effects on the optoelectronic properties of the perovskite layer. Halide migration is thought to contribute to hysteresis during solar cell device testing, and mixed bromide-iodide perovskites phase separate into I-rich and Br-rich domains upon light exposure. While much progress has been made towards understanding halide movement in response to an electric field, less is known regarding halide movement down a concentration gradient. In particular, interdiffusion of Br⁻ and I⁻ in a methylammonium-lead-halide matrix (MAPb(Br_xI_1−x)₃) – and the roles played by their respective vacancies V_Br and V_I – have not been extensively explored. To probe these phenomena, we fabricated perovskite-perovskite heterostructures from MAPb(Br_xI_1−x)₃ thin films. We then used changes in the interface profile composition to extract the Br-I interdiffusion constant as well as to examine vacancy accumulation and phase stability.

A challenge in studying halide diffusion has been to obtain interdiffusion constants in conditions relevant to optoelectronic devices, such as in device-quality thin films and at device-relevant temperatures. It is also preferable to study interdiffusion in a purely methylammonium-lead-halide matrix, since the nature of the matrix may affect the interdiffusion constant. Using area-selective halide substitution enabled easy fabrication of MAPb(Br_xI_1−x)₃ – MAPbBr₃ heterostructures from device-quality MAPb(Br_xI_1−x)₃. Optical microscopy proved to be a practical, quasi-real-time and non-destructive way to track interfacial composition, as interface profiles obtained via optical microscopy were found to be quantitatively equivalent to those obtained via Energy Dispersive X-Ray (EDX) analysis. Strong interface stability confirmed low interdiffusion constant values (D_i ≈ 7 x 10^-12 cm²/s at 100°C and D_i ≤ 10^-13 cm²/s at 70°C), which we take to be upper bounds of the interdiffusion constant. Br-vacancy accumulation along the interface indicated faster diffusion of Br⁻ than I⁻. More broadly, the lack of major interdiffusion suggested that the MAPb(Br_xI_1−x)₃ miscibility gap extends to higher temperatures than previously thought, which is promising for perovskite-perovskite heterostructure devices. We anticipate that the insights gained regarding halide and defect diffusion will enhance understanding of how defects affect optoelectronic properties, and will thus enable design of more efficient optoelectronic devices.

Semiconductor perovskites of methylammonium (MA) lead halide (MAPbX₃, X = Br, Cl, I or mixtures thereof) have been drawing great research attention due to their superior performance in solar cells with a rapidly-rising photo-conversion efficiency towards that of the crystalline silicon counterparts. The main obstacle for commercial advancement of these organolead halide solar cells lies in the instability and degradation of light-harvesting MAPbX₃ films. Especially bothersome is the ion migration effect, which is believed to be responsible for the slow conductivity response, anomalous current-voltage hysteresis, giant dielectric constant and switchable photocurrent in the MAPbI₃ perovskites. Meanwhile, ion migration is manifested in mixed-halide MAPbBr_xI_3-x (0 < x < 3) perovskites as a light-induced segregation of iodide- and bromide-rich domains that can revert back to the original phase in the dark. The iodide-rich domains in the MAPbBr_xI_3-x perovskites would cause a red shift in the photoluminescence (PL) that has been similarly observed in the CsPbBr_xI_3-x perovskites, confirming a universal role played by the migration of halide ions in the light-induced phase segregation process. In addition to the bulk mixed-halide perovskites mentioned above, the phase segregation effect has also been observed in the CsPbBr_xI_3-x nanocrystals (NCs) albeit from the electroluminescence measurements. Given the nanoscale size of the iodide-rich domain that are preferentially located along the grain boundaries of the bulk mixed-halide perovskites, the CsPbBr_xI_3-x NCs with a normally defective surface could provide a succinct understanding of the phase segregation process from a bottom-up point of view.

Here we focus on mixed-halide CsPbBr_1.2I_1.8 NCs to investigate the phase segregation effect at both the ensemble-film and the single-particle levels. With laser excitation, the ensemble film of CsPbBr_1.2I_1.8 NCs demonstrates a blue shift from ~630 to ~520 nm in the PL that can revert back in the dark, which can be attributed to the migration of iodide ions out of and back to the excitation volume. For an isolated single CsPbBr_1.2I_1.8 NC, the PL is also blue shifted upon laser excitation but never returns back in the dark, signifying the necessary existence of nearby NCs to channel the migration of iodide ions. Interestingly, the blue-shifted PL can also be induced when the CsPbBr_1.2I_1.8 NCs are electrically biased in the dark without the injection of excited-state charge carriers. This strongly suggests that it is the local electric field to break the iodide bonds that triggers the ion migration process in photo-excited CsPbBr_1.2I_1.8 NCs. While the enlarged energy bandgap is mainly caused by bromide enrichment in the mixed-halide CsPbBr_1.2I_1.8 NCs, we additionally show that lattice distortion by the migration of iodide ions could also make a minor contribution, as verified by the observation of a blue shift in the PL as large as 20 nm from the single-halide CsPbI₃ NCs upon laser excitation.

Except the blue-shifted PL, we believe that the ion migration mechanism proposed above could also be employed to describe the phase segregation processes observed in all the other mixed-halide perovskite materials. A spatial size of tens of nanometers are normally required for the formation of a iodide-rich domain in bulk films of the mixed-halide perovskites, which is obviously difficult to be accommodated in any single CsPbBr_1.2I_1.8 NC to trap the photo-excited excitons, so that they would recombine with a photon energy dictated by the bandgap of the bromide-rich domains.

Due to the highly deformable and polar nature of the metal halide framework, hybrid perovskites are very prone to lattice relaxation, which causes self-trapping of the elementary excitations into phonon-dressed localized states. In low-dimensional perovskites (e.g. 2D structures), where Coulomb interactions are enhanced by reduced dielectric screening and quantum confinement effects, the formation of self-trapped excitons (small polarons) manifests itself in apparent radiative recombination effects. Ab-initio calculations have indicated that the excess charge is spatially confined to one crystal unit cell or less, inducing local distortions of the inorganic layers. Photoexcitation in 2D perovskites gives rise to photoinduced lattice deformations associated to polaronic exciton states, with a characteristic fine structure in the absorption line-shapes. On the other hand, for charge carriers in 3D perovskites, the lattice distortion leads to the formation of charged polaron states that, due to the long-range electron phonon interaction typical of ionic crystals, may extend over several lattice sites (large polarons). Large polarons can display band-like coherent transport with substantial mobility (>1 cm²/Vs), that falls with increasing temperature. In this presentation, we will provide an overview of our recent theoretical and experimental studies on small and large polaron generation and relaxation dynamics in 2D and 3D perovskites, with particular focus on: i. Ultrafast formation of self-trapped electrons and holes at specific inorganic lattice sites of (EDBE)PbX₄ (X=Cl, Br), which is responsible for broadband white-light emission; ii. Determination of the actual geometrical pattern associated to large self-trapped polaron states in MAPbI₃, and their influence on charge carrier effective mass and mobility; iii. Implications of polaron formation and transport in perovskite-based devices, such as X-ray scintillators, light-emitting diodes (LEDs), light-emitting field-effect transistors (LEFETs), and solar cells.

Perovskites exhibit excellent opto-electronic properties – apart from their rather moderate charge carrier mobilities. These low mobilities have been attributed to the formation of large polarons and dynamic disorder due to low energetic phonons. However, the prevalent Hellwarth-Feynman theory of polarons still overestimates the mobility by an order of magnitude and the limiting mechanism is still unclear.
Here, we investigate the mismatch between polaron theory and observe mobilities by varying cation composition of perovskites. To this end, the charge carrier mobility and the optical phonon spectrum of XPbI₃ perovskite thin films with different Cs, MA and FA composition are measured by time-resolved terahertz spectroscopy.
The main phonon modes are observed for all samples at ca. 0.9 THz (30 cm^-1) and ca. 1.9 THz (63 cm^-1) and exhibit only minor shifts with cation composition. This observation supports the assignment of these main optical phonon modes to I-Pb vibrations on the anion lattice. The cation composition only slightly distorts the unit cell with minor impact on the I-Pb bond.
The measured charge carrier mobilities are in the range of 40-90 cm²/Vs which further supports similar polaron formation for different cation compositions. The difference can be attributed to variations in the ionic screening with cation composition and to the micro structure of the samples.
Finally, we identify several reasons for the difference between measured mobilities and estimation by the Hellwarth-Feynman theory.
First, we find higher mobilities for lower photo-excited charge carrier concentrations. The measured mobility on MAPbI₃ at low injection is 81 cm²/cm which is 2-3 times larger than typical literature values. This value is relatively close to the mobility of 230 cm²/Vs which we estimated by the Hellwarth-Feynman theory.
Second, the observed phonon modes are mostly of transversal character, which are not considered in the mobility estimation by the Hellwarth-Feynman theory. The potential impact of such phonon modes on the mobility of halide perovskites will be discussed.

The presence of surface and grain boundary defects in organic–inorganic halide perovskite films is detrimental to both the performance and operational stability of perovskite solar cells (PSCs). Compositional engineering and surface treatment of perovskites are effective approaches to passivate these defects. Here, we study the effect of chloride additives on the bulk and surface defects of mixed-cation and halide PSCs. We found that using an anti-solvent technique, the perovskite film is divided into two separated layers, i.e., a bottom layer with large grains and a thin capping layer with small grains. The addition of formamidinium chloride (FACl) into the precursor solution removes the small-grained perovskite capping layer and suppresses the formation of bulk and surface defects, providing a perovskite film with remarkably enhanced crystallinity and large grain size up to over 1 μm. Time-resolved photoluminescence measurements show longer lifetimes for perovskite films modified by FACl and subsequently passivated by 1-Adamantylamine hydrochloride (ADAHCl) as compared to the reference sample. These treatments improve the film quality and enable a high PCE of 21.2% and V_oc of 1152 mV with negligible hysteresis. Importantly, molecular engineering by applying chloride (Cl) additives greatly enhances the stability of the PSCs, which show only 5% degradation after aging for 90 days. Additionally, we have demonstrated the effect of Cl additives on ideality factors of our devices as well as lower recombination using impedance spectroscopy.