Symposium EN15 : Novel Materials Physics of Perovskite Semiconductors

Three-(3D) and two-dimensional (2D) layered halide perovskites are highly promising candidates for optoelectronic applications, and this has sparked new investigations of these materials from the synthetic, physicochemical and applications point of view. The 3D versions of these compounds adopt the three-dimensional ABX₃ perovskite structure, which consists of a network of corner-sharing BX₆ octahedra, where the B atom is a divalent metal cation (typically Ge²⁺, Sn²⁺ or Pb²⁺) and X is a monovalent anion (typically Cl⁻, Br⁻, I⁻); the A cation is selected to balance the total charge and it can be a Cs⁺ or a small molecular species. Such perovskites afford several important features including excellent optical properties that are tunable by controlling the chemical compositions, they exhibit ambipolar charge transport with high mobilities. Another class of materials gaining significance are the two-dimensional (2D) perovskites -a blend of perovskites with layered crystal structure- (Ruddlesden-Popper type) offer a greater synthetic versatility and allow for more specialized device implementation due to the directional nature of the crystal structure. A remarkable advantage of the 2D perovskites is that their functionality can be easily tuned by incorporating a wide array of organic cations into the 2D framework, in contrast to the 3D analogues which have limited scope for structural engineering. We also present the new homologous series, (C(NH₂)₃)(CH₃NH₃)_nPb_nI_3n+1 (n = 1, 2, 3), of layered 2D perovskites which is different from Ruddlesden-Popper type. Structural characterization by single-crystal X-ray diffraction reveals that these compounds adopt an unprecedented structure type which is stabilized by the alternating ordering of the guanidinium and methylammonium cations in the interlayer space (ACI). The these 2D perovskites combine structural characteristics from both Dion-Jacobson (DJ) and Ruddlesden-Popper (RP) structure archetypes. Compared to the more common Ruddlesden-Popper (RP) 2D perovskites, the perovskites we describe here have a different stacking motif and adopt a higher crystal symmetry.

In past several years, halide perovskites have been reshaping researchers’ understanding on designing and developing high-performance optoelectronic/electro-optical semiconductors. The mechanical softness of the halide perovskites has broad implications on their versatile properties. In this report, we present our recent understanding and observations of the roles of softness in several aspects – incommensurate epitaxy of the thin film crystals, dimensionality and strain engineering, and hidden carrier dynamics. It shows that compared to hard semiconductors, new paradigms of engineering halide perovskite's physical properties may emerge.

We report novel two-dimensional lead halide perovskite structures templated by a unique conjugated aromatic dication, N,N-dimethylphenylene-p-diammonium (DPDA). The asymmetrically substituted primary and tertiary ammoniums in DPDA facilitates the formation of two-dimensional network (2DN) perovskite structures incorporating a conjugated dication between the MX₄^2- (M = Sn, Pb,; X = Br, I) layers. These 2DN structures of (DPDA)PbI₄ and (DPDA)PbBr₄ were characterized by single crystal X-ray diffraction, showing uniquely low distortions in the Pb-X-Pb bond angle for 2D perovskites. The Pb-I-Pb bond angle is very close to ideal (180°) for a 2DN lead iodide perovskite which can be attributed to the rigid diammonium DPDA’s ability to insert into the PbX₆^2- octahedral pockets. Optical characterization of (DPDA)PbI₄ shows an excitonic absorption peak at 2.29 eV (541 nm), which is red-shifted compared to similar 2DN lead iodide structures. Temperature dependent photoluminescence of both compounds reveal both a self-trapped exciton and free exciton emission feature. The reduced exciton absorption energy and emission properties are attributed to the dication induced structural order of the inorganic PbX₄^2- layers. The 2D tin perovskite analog (DPDA)SnI₄ showed similar structural distortions and bonding. To test the difference the choice of cation makes in 2D perovskites, quasi-2D (Ruddelson-Popper) perovskite structures were formed in thin-film solar cells using this new DPDA dication.

CsPb₂Br₅ is a 2D perovskite where single layer Cs acts as a spacer, monolayer PbBr₆ octahedrons are edge shared. Although CsPb₂Br₅ was synthesized more than 10 years ago^{[1, 2]}, only recently it began to draw a lot of attention. Despite extensive research over past two years, its fundamental bandgap and basic electronic and optical properties still remain controversial: is this a wide bandgap indirect semiconductor or a direct bandgap material with strong green luminescence at ~520 nm? Zhang et al. reported that the attachment of CsPb₂Br₅ dots to CsPbBr₃ nanocrystals can enhance external quantum efficiency of CsPbBr₃ light-emitting diode (LED) by 50%^[3]. Wang et al. later demonstrated a nearly 90% quantum efficiency with CsPb₂Br₅ nanocrystals. Using ion exchange with I and Cl, they further expended emission wavelength from UV to red^[4]. But these claims of high luminescence have met with skeptics. Li et al. observed no photoluminescence, in agreement with their calculation result of indirect wide bandgap semiconductor^[5]. Since then, the controversy remains, some group still report strong visible bandgap and strong photoluminescence and even demonstrate lasing action in CsPb₂Br₅ microplates^{[6, 7]}, while others reported the opposite^[8]. In this talk, I will discuss the reason for different observations and present our result of the bandgap investigation of CsPb₂Br₅.

References:
[1] I. Y. Kuznetsova, I. S. Kovaleva, V. A. Fedorov, Russ. J. Inorg. Chem. 2001, 46, 1730.
[2] M. Rodova, J. Brozek, K. Knizek, K. Nitsch, J. Therm. Anal. Calorim. 2003, 71, 667.
[3] X. L. Zhang, B. Xu, J. B. Zhang, Y. Gao, Y. J. Zheng, K. Wang, X. W. Sun, Adv. Funct. Mater. 2016, 26, 4595.
[4] K. H. Wang, L. Wu, L. Li, H. B. Yao, H. S. Qian, S. H. Yu, Angew. Chem.-Int. Edit. 2016, 55, 8328.
[5] G. P. Li, H. Wang, Z. F. Zhu, Y. J. Chang, T. Zhang, Z. H. Song, Y. Jiang, Chem. Commun. 2016, 52, 11296.
[6] X. S. Tang, Z. P. Hu, W. Yuan, W. Hu, H. B. Shao, D. J. Han, J. F. Zheng, J. Y. Hao, Z. G. Zang, J. Du, Y. X. Leng, L. Fang, M. Zhou, Adv. Opt. Mater. 2017, 5.
[7] W. S. Longfei Ruan, Aifei Wang, Aishuang Xiang, Zhengtao Deng, The Journal of Physical Chemistry Letters 2017, DOI: 10.1021/acs.jpclett.7b01657.
[8] M. D. B. Ibrahim Dursun, Bekir Turedi, Badriah, A. S. Alamer, Jun Yin, Issam Gereige,, O. F. M. Ahmed Alsaggaf, Mohamed Eddaoudi,, a. O. M. Bakr, ChemSusChem 2017, ChemSusChem 10.1002/cssc.201701131.

The structure of a material, light and electrical field are fundamental ingredients for any opto-electronic semiconducting device. In state-of-the-art semiconductors like Silicon and Gallium Arsenide, the crystal structural parameters such as bond length, crystallinity, vacancy and strains are invariant to external stimuli such as electromagnetic radiation and electric fields. However, in sharp contrast, hybrid perovskites exhibit a strong propensity for undergoing structural modifications with light and electric fields. While this raise challenge for elucidating the exact mechanisms of device operation but also offer now opportunity to discover new functionalities. Therefore, a basic principle on the interplay between structure, light and electrical field with in-situ correlated measurement is critical for understanding optoelectronic transport and determine the design principles for operation of perovskite based devices.
In my talk, using correlated in-situ structure and transport measurements, I will focus on understanding these complex effects arising from the interaction between structure, light and field during perovskite cell operation in both 3D and 2D systems. Briefly, in a 3D perovskite system, we discover that continuous light illumination leads to a uniform lattice expansion in hybrid perovskite thin-films, which is critical for obtaining high-efficiency photovoltaic devices. Measurements reveal that light-induced lattice expansion significantly benefits the performance of a mixed-cation pure-halide planar device, boosting the power conversion efficiency from 18.5% to 20.5%. This is a direct consequence of lattice strain relaxation and increase in the crystallite size that dramatically suppresses the interface non-radiative recombination, resulting in enhanced photovoltage and photocurrent collection near low field. In 2D solution-processed quantum wells, on the other hand, optical field generates coulomb bound electron-hole pairs due to the unique low dimension structure of the materials. Furthermore, the multi-stacked quantum wells present potential barriers that block the efficient separation of electron-hole pairs. Both of those properties are unfavorable for photovoltaic cell operation. Here we elucidate the critical role of field-assisted charge carrier separation that overcomes these bottlenecks leading to the efficient photocurrent collection.
As a summary, our studies demonstrate the key factors that should be accounted for in the design of optoelectronic devices using hybrid perovskite materials in both 3D and 2D systems.

Quantum and dielectric confinement effects in Ruddlesden-Popper 2D hybrid perovskites create excitons with a binding energy exceeding 150 meV. We exploit the large exciton binding energy to study exciton and carrier dynamics as well as electron−phonon coupling (EPC) in hybrid perovskites using absorption and photoluminescence (PL) spectroscopies. At temperatures <75 K, we resolve splitting of the excitonic absorption and PL into multiple regularly spaced resonances every 40−46 meV, consistent with EPC to phonons located on the organic cation. We also resolve resonances with a 14 meV spacing, in accord with coupling to phonons with mixed organic and inorganic character. These assignments are supported by density-functional theory calculations. Hot exciton PL and time-resolved PL measurements show that vibrational relaxation occurs on a picosecond time scale competitive with that for PL. At temperatures >75 K, excitonic absorption and PL exhibit homogeneous broad- ening. While absorption remains homogeneous, PL becomes inhomogeneous at temperatures <75K, which we speculate is caused by the formation and subsequent dynamics of a polaronic exciton. We also explore how changes to the cation affect EPC and vibrational relaxation.

In the last 6 years, hybrid halide perovskites have emerged as a highly efficient class of solar absorbers, with efficiencies reaching 22.4 %, quickly surpassing other 3rd generation devices.[1] The highest performing hybrid perovskite is the cubic CH₃NH₃PbI₃ (MAPI), which is made from earth-abundant elements and can be easily solution processed, dramatically reducing manufacturing costs. Unfortunately, chemical stability is a major concern for these materials and much effort has been devoted to increasing the stability of MAPI based devices.[2]

Recently, partial substitution of iodine with the pseudohalide ion, SCN^–, to form the layered (CH₃NH₃)₂Pb(SCN)₂I₂ (MAPSI), has been suggested as novel route to increase stability whilst retaining high efficiencies. In this work, we explain why MAPSI can still possess an ideal electronic structure for light absorption, despite the loss in connectivity when moving from a cubic to a layered structure.[3] We also explain, for the first time, why MAPSI is more stable than MAPI. Lastly, we demonstrate that MAPSI can act as a parent structure for a related family of materials whose optoelectronic properties can be fine-tuned for use in photovoltaic applications.[4] We further screen this extended family for their defect properties and suggest solar cell architectures likely to result in efficient devices.

References

[1] M. Grätzel, Nat. Mater. 13, 838–842 (2014)
[2] A. M. Ganose, C. N. Savory and D. O. Scanlon, Chem. Commun. 53, 20–44 (2017)
[3] A. M. Ganose, C. N. Savory and D. O. Scanlon, J. Phys. Chem. Lett. 6, 4594–4598 (2015)
[4] A. M. Ganose, C. N. Savory and D. O. Scanlon, J. Mater. Chem A 5, 7845–7853 (2017)

Organic-inorganic hybrid materials have been broadly investigated owing to their various properties used in optoelectronics and solar cell [1].
Here, we report synthesis of two new organic-inorganic hybrid materials which are obtained by slow evaporation at room temperature using the same organic cation for two different molar ratio. These two compounds are characterized by X-ray diffraction, infrared and Raman spectroscopy, optical absorption and photoluminescence measurements.
Additionally, we demonstrate that these compounds can be used in organic solar cells. In fact, the energy band gap of these materials was found to be closed to that used in interfacial layers [2] of some organic solar cells.
By optimizing optical, electrical, and morphological properties of these new wide bandgap materials, bulk heterojunction solar cells with conversion efficiency exceeding 9.5 % are obtained in normal device structures with all-solution-processed interlayers in normal device structure. More importantly, the morphology and especially the surface roughness of these hybrid layers is crucial to obtain hole blocking behavior leading to fill factor up to 72 %.

[1] Vitalii Yu. Kotov et al., New J. Chem., 2016, 40, 10041--10047
[2] Sadok Ben Dkhil et al., Adv. Energy Mater. 2014, 4, 1400805
[3] S. Ben Dkhil et al.. Towards high-temperature stability of PTB7-based bulk heterojunction solar cells: impact of fullerene size and solvent additive, Adv. Energ. Mater. 2016, 1601486
[4] S. Ben Dkhil et al. Square-Centimeter-Sized High-Efficiency Polymer Solar Cells: How the Processing Atmosphere and Film Quality Influence Performance at Large Scale, Adv. Energ. Mater. (2016), 1600290/1-10.

The success of perovskite photovoltaics is underpinned by their favourable optoelectronic properties, notably their combination of high charge-carrier mobilities with low rates of charge-carrier recombination^[1]. Sub-bandgap trap states in hybrid lead halide perovskites can act as nonradiative recombination centres, leading to shorter charge-carrier lifetimes and limiting the open-circuit voltage (V_oc) in perovskite solar cells^[2]. It is therefore essential to understand the nature and energy scale of these trap states for the development and optimization of technology based on these materials.
In this study^[3] we investigated the influence of sub-bandgap trap states on charge-carrier recombination through an analysis of the low-temperature photoluminescence (PL) of FAPbI_3, a perovskite material used in some of the most efficient and stable perovskite solar cells ^[4]. We observed a power-law time dependence in the emission intensity and an additional low-energy emission peak that exhibits an anomalous relative Stokes shift. Using a rate-equation model and a Monte Carlo simulation, we revealed that both phenomena arise from an exponential trap-density tail with characteristic energy scale of ≈3 meV. Since charge-carrier recombination from sites deep within the tail causes emission with energy downshifted by up to several tens of meV, such phenomena may in part be responsible for V_oc losses commonly observed in these materials. We propose that the origin of the band-tail states in FAPbI3 may lie in the rotational freedom of the polar organic cation. These results underline the suitability of viewing hybrid perovskites as classic semiconductors, whose electronic bandstructure picture is moderated by a modest degree of energetic disorder.
[1] C. Wehrenfennig, G. E. Eperon, M. B. Johnston, H. J. Snaith, L. M. Herz, Adv. Mater. 2014, 26, 1584.
[2] A. Baumann, S. Väth, P. Rieder, M. C. Heiber, K. Tvingstedt, V. Dyakonov, J. Phys. Chem. Lett. 2015, 6, 2350.
[3] A. D. Wright, R. L. Milot, G. E. Eperon, H. J. Snaith, M. B. Johnston, L. M. Herz, Adv. Funct. Mater. 2017, 27, 1700860.
[4] W. S. Yang, J. H. Noh, N. J. Jeon, Y. C. Kim, S. Ryu, J. Seo, S. Il Seok, Science 2015, 348, 1234.

Since the first report on the solid-state perovskite solar cell with power conversion efficiency (PCE) of 9.7% and 500 h-stability in 2012 by our group, perovskite photovoltaics have received great attention and as a result highest certified PCE of 22.1% was reported in 2017. It is believed that perovskite solar cell is promising next-generation photovoltaics due to superb performance and very low cost. High efficiency has been achieved by spin coating procedure, which is however limited to small area. For commercially available and scale-up considerations together, a large-area coating technology is required. Here, we report on non-heating large-area coating technology for high efficiency perovskite solar cells. We successfully coated perovskite films with dimension up to twenty by twenty square centimeters at room temperature on both glass substrate and flexible polymer substrate. The room-temperature coated perovskite film showed highly crystalline and (110)-oriented structure. The average PCE of about 17-18% was achieved by non-heating large-area coating method. Experimental details and opto-electronic properties will be discussed.

To be advised---pending

Over the last few years metal halide perovskites have risen to become a very promising PV material, captivating the research community and expanding into significant industrial effort. In the most efficient devices, which now exceed 22% solar to electrical power conversion efficiency, the perovskite is present as a solid polycrystaline absorber layer sandwiched between n and p-type charge collection contacts. Beyond efficiency, long-term stability is of critical importance. Within the thin film device there exist up to six discrete material layers, in addition to the perovskite absorber layer, all of which could degrade under operation, or induce degradations to their neighbouring layers. In this talk, I will present the key steps and key challenges which we have overcome on the path from mesostructured hybrid solar cells which would only last a few hours under continuous operation, to robust thin-film perovskite solar cells capable of surviving for 1000’s of hours under rigorous stressing. I will specifically present recent work on more stable charge extraction layers and perovskite absorber layers, which deliver “non-degrading” solar cells, stressed under elevated temperature full spectrum light soaking. I will finish by presenting a brief summary the efforts by Oxford PV Ltd which have enabled the perovskite technology surpass the International Electrotechnical Commission stability tests.

To enable the ultimate commercialization of emerging Pb halide perovskite (ABX₃) solar cell technology, extensive efforts have been undertaken to overcome their toxicity and air-instability issues, by identifying analogous nontoxic or low-toxicity and air-stable halide perovskite-based absorbers. Substituting Pb by a combination of monovalent and trivalent cations to form A₂B(I)B(III)X₆ halide double perovskites has been considered as an attractive approach for achieving this goal. So far, theoretical studies have predicted many halide double perovskites as promising absorbers, primarily because of their suitable optoelectronic properties. Some halide double perovskites have been synthesized, but have not produced efficient solar cells. Besides optoelectronic properties, defect properties of the absorbers must be appropriate also for producing efficient solar cells. A promising absorber must exhibit a suitable semiconductivity with sufficiently low majority carrier density of e.g. <10¹⁷ cm⁻³ and have a low density of deep level defects since both the free carriers and the defects enhance recombination of photo-excited carriers. In this paper, we systhemcaitlly report the defect properties of the prospective halide double perovskite semiconductors in comparison with their Pb analogues. We also suggest the optimal synthesis conditions for suitable conductivity and low deep defects, which are important for efficient solar cells.

Organic-inorganic perovskite solar cells (PSCs) have shown enormous success in the past decade, increasing in power conversion efficiency from ~4% in 2009 to >22%. One of the critical properties that has been attributed to this success is “defect tolerance”: in organic-inorganic perovskites, the majority of point defects with low formation energy lie within or near the conduction or valance band. Defects with deep states, which act as electronic traps, are expected to be much less common due to their high formation energies. We demonstrate that, despite the preference for shallow defects, point defects play an integral and role in materials properties and PSC device performance.
We first study the role of point defects on nanoscale luminescence properties of inorganic-organic perovskites by using cathodoluminescence in scanning transmission electron microscope (STEM). By correlating local luminescence properties with compositional variations using STEM, we demonstrate that iodide segregation induced by electron beam is correlated with a spatially-localized high-energy emission. Similar high-energy emission has been observed in photoluminescence (PL) measurements for films made in the presence of excess methyl ammonium iodide, demonstrating that the observed defect segregation is relevant to practical device design.
Next, we study the effects of directional point defect segregation under an applied electric field on current extraction from PSCs. Specifically, we use electron beam induced current measurements in a scanning electron microscope to measure the inhomogeneity in current extraction before and after forward biasing the device. These measurements point to preferential defect migration at extended defects and allow us identify low frequency capacitive elements related to compensation of charge defect segregation under applied biasing.
Finally, we directly track the migration of deep defects in PSCs through intensity dependent PL mapping of laterally biased perovskite films. Using Monte Carlo simulations of defect drift and diffusion to model these time dependent luminescence maps, we provide evidence for four deep level defects in these films, extract their mobilities, and demonstrate their significant impact on PL intensity. Particularly, removal of defect states by mild voltage biasing results in over an order of magnitude increase in luminescence. Overall, our work demonstrates the ways in which deep and shallow defects play a critical role in PSCs and suggest that, despite their “defect tolerance,” the ultimate stability and performance of PSCs will be dependent on either minimizing the presence of point defects in these materials or inhibiting defect migration.

Metal halide perovskite materials with the formula APbX₃ (A is a cation; X is a halogen) are leading the pack of solution-processed semiconductors for solar cells and optoelectronics. A key to the success of these materials is their hallmark long radiative charge carrier lifetimes, which allows carriers to be collected as useful charges before recombining. Herein, we grew high-quality single crystal perovskites of A= methylammonium, formamidinium, and Cs, and X=I and Br; and used time-resolved photoluminescence (at low excitation fluence upon two-photon excitation), together with temperature dependent optical and structural characterization techniques to study the radiative charge carrier lifetimes and their relationship to crystal structure and composition. We uncover compositions with long carrier lifetimes (a few microseconds) and others with ultralong lifetimes (tens of microseconds). We combine experiment and modeling to showcase the differing roles played by the cation units and halides in modulating the carrier lifetimes and the defect formation energies of the perovskites.

Our work paves the way for rationally tailoring perovskite materials properties for optoelectronics through compositional design and selection.

Hybrid organic-inorganic halide perovskites, particularly the methylammonium lead trihalide perovskites (MAPbX₃; MA=CH₃NH₃⁺ , X=Br⁻ or I⁻), have gained extensive interest for optoelectronic, photovoltaic and thermoelectric applications recently. In the hybrid perovskites, the A-site species of the conventional inorganic perovskite, such as Cs⁺ in CsPbI₃, is replaced by an organic molecule (e.g. CH₃NH₃⁺). The introduction of a molecular species reduces the sublattice symmetry and introduces local internal vibrational modes. Recently the dynamics of methylammonium ions have been recognized to be of importance for the thermal and electrical properties, but the detailed structure-property-process relationships are still lacking. In this work, we combine ab initio lattice dynamics and molecular dynamics simulations to connect both the harmonic spectra and anharmonic interactions of vibrational modes, as well as the correlated disorder between the molecular orientation and the vibrational modes of the underlying octahedral network, to the phase transition and thermal properties of bulk MAPbI₃. By comparing to inorganic perovskites (CsPbI₃), we first show that the ultra-low lattice conductivity currently under debate arises primarily from the strong anharmonicity, although the low-group velocity cannot be neglected. Based on the resolved interactions between individual modes, we show that the internal modes of the organic molecules interact strongly with the soft modes residing on the inorganic framework at the M and R high-symmetric points, which facilitates the phase transitions between orthorhombic, tetragonal and cubic phases. Furthermore, spectral energy density analysis demonstrates the spectral broadening of vibrational modes due to the presence of orientational disorder. This study will also be of interest to the thermal stability and moisture resistance of hybrid perovskites.

Methylammonium lead iodide perovskite materials have been shown to be efficient in photovoltaic devices. The current fabrication process has not been perfected, leaving defects such as site vacancies, which can trap charge and have a detrimental effect on photogenerated charge carriers. Here, focus is placed on the effect a Pb site vacancy has on the morphology and charge carrier dynamics following photoexcitation. Excited state electronic structures are often found in open shell configurations with a single unpaired electron in the conduction band. To accurately describe unpaired electrons, spin-polarized calculations are performed on both neutral and charged vacancy systems. This work presents spin-polarized ground state electronic structures, non-radiative rates of charge carrier relaxation, and introduces an extension to a novel procedure to compute photoluminescence spectra for open shell models. Electronic structure calculations are done in VASP with the PBE functional and plane wave basis set and the charge carrier dynamics are computed using Redfield theory within the reduced density matrix formalism. Early results show the vacancy of the Pb ion introducing a new energy state within the unblemished material band gap region. This additional unoccupied state is expected to increase the non-radiative relaxation lifetime of the excited electron, allowing for a longer lifetime of the charge carrier and increased opportunity for secondary relaxation mechanisms or collection to take place.

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 review our understanding in the identification 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. Then, I will discuss the correlation between the presence/formation of defects and the observed semiconductor instabilities under photo-excitation.

Owing to their easy synthesis, tunable electronic properties and large absorption coefficients, lead halide hybrid perovskite semiconductors have emerged as attractive candidates for photovoltaic applications [1,2]. Intrinsic point defects, surface states, grain boundaries and external substituents play an important role in these materials in determining their solar cell efficiencies [3,4]. Here, we considered MAPbBr_3-yCl_y perovskites (where MA = methylammonium and y = {0, 0.75, 1.5, 2.25, 3}) as parent semiconductors and used first principles computations to study intrinsic point defects, namely vacancy, self-interstitial and anti-site. In MAPbBr_3-yCl_y perovskites, while the lattice constant decreases as y goes from 0 to 3, the band gap increases from ~ 2 eV for y = 0 to ~ 2.5 eV for y = 3 [1]. For defect calculations, the supercell approach was implemented in a density functional theory (DFT) framework to create 2x2x2 cells with one defect per 8 units (or 96 atoms) of MAPbBr_3-yCl_y, achieving a satisfactorily dilute defect concentration [5]. By studying the defects in various charged states, charge transition levels were calculated for each and it was seen that while vacancy defects and cation/cation anti-site defects create shallow levels (i.e., close to the valence or conduction band), cation/anion anti-site defects create deeper levels in the band gap. The latter could act as non-radiative recombination centers, proving to be detrimental to solar cell performance [3,4]. Further, for different points in the calculated chemical ranges for MAPbBr_3-yCl_y perovskite stability, the formation energy of each defect was estimated as a function of the fermi level, as it changes from valence to conduction band. The equilibrium fermi levels as determined by dominant acceptor-like and donor-like defects for each perovskite were seen to shift to the right with increasing Cl content, meaning the conductivity of MAPbBr_3-yCl_y becomes more n-type with increasing y. While Br vacancy showed a low formation energy in Br-rich MAPbBr_3-yCl_y perovskites owing to strong anti-bonding between Pb s and Br p orbitals, Cl vacancy showed a higher formation energy in perovskites with larger values of y. Lastly, the equilibrium growth conditions necessary for creating different intrinsic defects were determined; this helps us understand the electrical properties of the perovskite, and also paves a path for the study of suitable external substituent defects that can compensate for dominant intrinsic defects and thus change the properties.


REFERENCES

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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. The high performance of these semiconductors has been attributed to their tolerance to defects, however, the exact nature, their quantification, and the effect of charge-carrier traps on material performance is still unclear. To understand the effect of charge-carrier traps we compare differently prepared perovskite devices, where we measure the trap distribution using deep-level transient spectroscopy and admittance spectroscopy. We show a correlation between fabrication conditions and the variety and the density of charge-carrier traps. 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.

Hybrid organohalide perovskites have emerged as one class of most promising semiconductor materials for the next-generation optoelectronics because of their exceptional properties, though the demand of lead-free perovskites is still a great challenge. Here we show that, by using high-throughput first-principles electronic structure calculations, we have generated the quantum data of about 2000 organic-inorganic double perovskites and successfully uncovered about 40 promising lead-free candidate materials with promising materials properties for photovoltaic applications. These research findings provide a wide platform to perform further experimental studies to realize lead-free perovskite solar cells.

Hybrid halide perovskites AM^IIX₃ (A = amine or alkali metal cation; M^II = divalent cation; X = Cl, Br and I) have emerged as potentially useful light-absorbing materials for solar cell applications.^1,2 However, stability issues and the toxicity of Pb have led to a search for Pb-free alternatives. One of the ways of achieving this is to form a halide double perovskite A₂M^IM^IIIX₆ in which M^I and M^III are monovalent and trivalent cations. Compared with single perovskites, double perovskties have a broader chemical diversity because both M^I and M^III sites can be modified. Recent efforts have focused on Bi and In based double perovskites: Cs₂AgBiBr₆, Cs₂AgBiCl₆, Cs₂AgInBr₆, (CH₃NH₃)₂KBiCl₆, (CH₃NH₃)₂TlBiBr₆, (CH₃NH₃)₂AgBiBr₆ and (CH₃NH₃)₂AgSbI₆.^3–9 In this paper we show that it is also possible to incorporate rare earth elements (Y and Gd) into the perovskite framework, forming (CH₃NH₃)₂KYCl₆ and (CH₃NH₃)₂KGdCl₆.¹⁰ Both perovskites possess large direct band gaps and exhibit a rhombohedral to face centered cubic phase transition at high temperature. Our work expands the scope of hybrid perovskites to rare-earth containing materials, enabling the possibility of future applications in solid-state lighting and magnetism.
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(7) Deng, Z.; Wei, F.; Sun, S.; Kieslich, G.; Cheetham, A. K.; Bristowe, P. D. J. Mater. Chem. A 2016, 4 (31), 12025.
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The APbX₃ perovskites (A = monovalent cation, X = halide) exhibit remarkable properties as solar-cell absorbers. However, these materials have intrinsic instabilities and even the origin of the materials' superior photophysical properties is still under debate. Finding both structural and functional analogs of a material is important for understanding which design principals must be reproduced in an analog. I will discuss our attempts at capturing the photophysical properties of APbX₃ perovskites with new compositions and new inorganic architectures.

The ABX₃ perovskites constrain the B-site metals to divalent cations, limiting the number of analogs we can synthesize. We recently showed that the family of A₂BB'X₆ double perovskites, which can accommodate a much greater range of metals, have promising optical properties as absorbers. Armed with this substitutional flexibility, we have explored alternative metals that can be incorporated into the perovskite lattice. Studying the electronic differences between the lead perovskites and lead-free double perovskites have allowed us to understand how to tune double perovskites to better absorb sunlight. I will share our understanding of these materials, including the basis for dramatic reduction in bandgaps induced through dilute impurity alloying. I will also present new materials developed in my group as light absorbers, which deviate from the typical perovskite lattice.

The recent development of organic halide perovskites such as CH₃NH₃PbI₃ has led to a revolution in PV research. The power conversion efficiency (PCE) of solar cells made of this type of materials has witnessed an unprecedented rate of increase, from an initial PCE of 3.8% in 2009 to above 22% in 2016 [1]. However, its instability and toxicity have led to major concerns on its viability as a mainstream PV absorber material. Nevertheless, the progress on halide perovskites has inspired us to search for novel semiconductor materials that can inherit the excellent optical and electronic properties of halides, while avoiding their severe limitations. As opposed to their oxide and halide siblings, chalcogenide perovskites with the chemical formula of ABX₃, where A represents an alkaline earth element such as Ca, Sr and Ba; B represents a group IVB element such as Ti, Zr and Hf, and X is S and Se, received little attention. Recently, our theoretical investigation found that the chalcogenide perovskites can be a direct band gap semiconductor with strong light absorption and good carrier transport, with potential of defect tolerance [2]. In this work, we present our results on the synthesis and characterization of chalcogenide perovskites, in particular BaZrS₃, by high temperature sulfurization of their oxide counterparts [3]. Their crystal structures were identified by XRD and composition by EDX. UV-vis measurements confirmed that they are semiconductors with band gap value of 1.7-1.8 eV, consistent with theoretical predictions. High pressure Raman studies show that the perovskite phase is stable against volume compression, with the band gap value decrease with increasing pressure [4]. This suggests that by alloying cations with smaller radius such as Ti, it is possible to reduce the band gap. Subsequent experiments show that a moderate Ti incorporation reduces the band gap to 1.5 eV. Finally, we show our preliminary results on the growth of chalcogenide perovskite thin films.

[1] The NREL Research Cell Efficiency Records (http://www.nrel.gov/ncpv/).
[2] Y. Y. Sun et al. “Chalcogenides perovskites for photovoltaics”, Nano Lett. 15, 581 (2015).
[3] S. Pereraa et al. “Chalcogenide perovskites – an emerging class of ionic semiconductors”, Nano Energy 22, 129 (2016).
[4] N. Gross et al. “Stability and band gap tuning of the chalcogenide perovskite BaZrS₃ in Raman and optical investigations at high pressure”, Phys. Rev. Appl., accepted.

Lead-organohalide perovskites have been the subject of extensive research since the turn of the decade due to their promising electronic properties and affordable synthesis process. However, thermodynamic instability and lead toxicity issues have stifled their path to commercialization. Recently, Bismuth-based halide double perovskites have emerged as promising candidates for solar cell absorber materials. With Bi³⁺ being isoelectronic to Pb²⁺, these Bi-halide double perovskites, such as Cs₂AgBiBr₆, show comparable electronic properties to the lead-organohalide perovskites alleviating issues related to lead-toxicity. However, these halide double perovskites have indirect band gaps that are larger than the ideal 1.6 – 2.0 eV range required to efficiently use the solar spectrum. They are also observed to degrade over a period of weeks on exposure to ambient air and light.

In this work, we use the vast composition-space of all-inorganic double perovskite oxides containing bismuth to identify promising solar cell absorber materials. By screening through 100’s of hypothetical A'A"BBiO₆ double perovskites using regression analysis and high-throughput density-functional theory (DFT) calculations, we predict a stable family of compounds with a general formula of A'A"TeBiO₆, where A' is an alkali metal cation, such as Na, K, Rb and Cs, and A" is an alkaline-earth metal cation such as Mg, Ca, Sr and Ba. We predict an indirect band gap of 1.94 eV and 1.99 eV for KBaTeBiO₆ and RbBaTeBiO₆, respectively, which is comparable to the best performing Bi-halide double perovskite Cs₂AgBiBr₆ (2.06 eV). The effective mass of holes and electrons in case of KBaTeBiO₆ is 0.25m_e and 0.28m_e, respectively, which is comparable to the mass of holes and electrons (0.14m_e and 0.37m_e, respectively) in Cs₂AgBiBr₆. We have successfully synthesized KBaTeBiO₆ using wet-chemistry synthesis confirming its stability. Preliminary UV-vis measurements show an indirect band gap of 1.7 eV for the synthesized compound. In addition to these results, we will discuss the effect of composition on the electronic structure of these oxide double perovskites, including the role of defects. Our work demonstrates that inorganic Bi-based double perovskite oxides are promising benign alternatives to lead-halide perovskites for photovoltaic applications. It also highlights the combination of data-analytics and DFT calculations as a powerful approach to accelerate the discovery of promising materials.

This work was supported by a Ralph E. Powe Junior Faculty Enhancement Award from Oak Ridge Associated Universities to R.M. This work used computational resources of the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by the National Science Foundation grant number ACI-1053575.

Perovskite solar cells (PSCs) based on lead-containing hybrid organic-inorganic halide perovskites have demonstrated great promise as a new photovoltaic technology for the clean-energy future. However, the toxicity and the phase stability of these perovskites are of significant concern. In this context, we have theoretically predicted and experimentally validated a new group of all-inorganic halide perovskites based on the environmentally-friendly and earth-abundant element, titanium (Ti(IV)), which possess ideal optical bandgaps for solar cells. Further, we have developed low-temperature facile methods for fabricating high-quality Ti-based halide perovskite thin films. The physical properties of the as-prepared Ti-based halide perovskite thin films are measured, suggesting their superior potential in PSC applications. The formation mechanisms of these Ti-based halide perovskites and device physics of Ti-based PSCs are also studied extensively, the understanding from which provides guidance for further boosting the performance of Ti-halide-based PSCs.

With its promising electronic properties, Cs₂SnI₆ achieved a power conversion efficiency (PCE) of ~6% when used as a charge regenerator with a ruthenium complex sensitizer. However, its performance as a light absorber in lead-free perovskite solar cells is poor, despite its appropriate bandgap of ~1.5 eV. Indeed, no in-depth research into this deficiency has been carried out to date, and so a clear understanding of the charge transfer kinetics of Cs2SnI6 is required to better understand this issue.
We studied the charge transfer mechanism of perovskite Cs₂SnI₆ and clarified the function of its surface state in photo-conversion devices. From the cyclic voltammetry, we found that the faradaic reactions of iodine species induce charge transfer through a surface state in the bandgap of Cs₂SnI₆, which mainly occurs at around +0.9 V vs. normal hydrogen electrode (NHE). Also, its surface state charging is confirmed by Mott-Schottky measurement. Furthermore, it has been further confirmed that the charge transfer mainly occurs through the mid-gap of Cs₂SnI₆ using two types of photovoltaic devices with Cs₂SnI₆ as either a regenerator or a light absorber. The performance of Cs₂SnI₆ as the regenerator system is strongly dependent on the highest occupied molecular orbital of organic dyes and its role as the light absorber system depends on the conduction band of TiO₂. Consequently, Cs₂SnI₆ shows efficient charge transfer with a thermodynamically favorable charge acceptor level. Our results confirm the importance of surface state engineering in future designs of Cs₂SnI₆ lead-free perovskite devices.

The sheer number of potential combinations of component species poses a challenging obstacle when optimizing functional materials. The huge combinatorial search space renders an exhaustive search useless and forces us to narrow down the selection of candidate solutions taken into consideration, even when using an advanced methods for the design of experiments. To overcome these limitations, we employ novel ideas from machine learning-based optimization.

In this talk, we present a novel search method that uses techniques from transfer-learning to drastically reduce the cost of the optimization process.
We present an algorithm to leverage cheap, inaccurate approximations of the actual objective. For example, these approximations, also called information sources, may be provided by Density Functional Theory (DFT) calculations at lower levels of theory or from classical semi-empirical force field simulations. These sources are typically not only noisy but also inherently biased due to the limitation of the underlying internal models. Note that this setting goes significantly beyond the notion of multi-fidelity, since information sources cannot be expected to form a hierarchy; for example, a MD simulation may be more accurate for the temperature for which it was calibrated. Therefore, the algorithm ‘learns’ the relationship of the approximations and the objective. The decision regarding which candidate solution to explore next, and what information source to use, maximizes the expected value of information about the unknown optimum. Related theoretical results guarantee that the algorithm is consistent, i.e., it will obtain a near-optimal solution.

To demonstrate the value of this approach, we implement such an algorithm to search for a metal halide perovskite composition which maximizes the binding energy to the solvent, computed via ab initio DFT calculations. Here a “composition” for the candidate material is determined by choosing a combination out of three halides (X) and one out of three cations (A), comprising our metal halide perovskite monomer (PbX₃A), and one out of sixteen blends of solvents. Ab initio calculations of lower theory provide cheap approximations. Finding the solution to the optimal mixed halide and solvent blends to use in metal halide perovskites is currently of great interest to the experimentally driven community, whose only recourse at the moment is largely trial-and-error or driven by chemical intuition.

While photovoltaic (PV) devices based on three-dimensional (3D) perovskites, (Cs, MA, FA)Pb(I, Br)₃ (MA=methylammonium, FA=formamidinium), have attracted substantial recent interest, because of the unprecedented rise in power conversion efficiency to values above 20%, the perovskite family offers profound structural and electronic flexibility beyond these 3D systems [1]. A key theme for the current talk will center around how the organic cation either directly or indirectly impacts the photophysical and thermal properties of the given semiconductor. In one recent example, acene-based organic cations template single-layer <100>-oriented perovskites, in which the details of the organic cation and the hydrogen bonding interaction between the lead(II) halide (PbX₄^2- X=Cl, Br, I) perovskite layers and the organic cations fine tune the position of the band edge and luminescence [2]. More complex organic cations (e.g., oligothiophenes) lead to hybrids in which the conduction and valence bands are comprised of states that principally derive from inorganic and organic components, respectively, opening the possibility of creating electronic properties akin to quantum well systems. In another example, specific choice of organic cation in the layered lead(II) halide perovskites lowers the melting temperature below the decomposition point, ultimately creating hybrid perovskites that can be melt-processed into thin-film form [3] and providing a promising avenue to solvent-free film deposition of functional perovskite semiconductors. As seen by these and other examples, the independent and diverse tunability of the organic and inorganic structural components is expected to provide (and is already providing) many exciting opportunities beyond the 3D hybrid perovskite systems.

[1] B. Saparov and D. B. Mitzi, Chemical Reviews 116, 4558 (2016).
[2] K. Du, Q. Tu, X. Zhang, Q. Han, J. Liu, S. Zauscher, D. B. Mitzi, Inorg. Chem. 56, 9291 (2017).
[3] T. Li, W. A. Dunlap-Shohl, Q. Han, D. B. Mitzi, Chem. Mater. 29, 6200 (2017).

As the low-cost and high efficient solar cell absorbers, halide hybrid perovskites increasingly catch researchers attention. However, the instability of the materials is still not solved. Based on our latest research, the ionization energy of A site molecule can be a very important factor which determines the thermodynamical stability of hybrid halide perovskites [1]. Meanwhile, the size of the molecule determines the stable phase at room temperature and the bandgap. Here we present a new three-membered ring radical (CH₂)₂NH₂ which demonstrates a low ionization energy, that implies a good stability, and owns a reasonable size that generates suitable bandgap for photovoltaics. We use density functional theory (DFT) method to research the three-membered cyclic cation based perovskite’s structural, stability, electronic properties.

The choice of the exchange–correlation functional is important for DFT to accurately predict the correct polymorphism transition order of halide hybrid perovskites. It can shed light on predicting the room temperature favorable phase. From the experimental point of view, CH₃NH₃PbI₃ prefers the orthorhombic phase as the low-temperature structure. However, the hexagonal phase demonstrates the lowest total energy at 0 K when the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional is used, which contradicts to the experimental observation. We found that when the van der Waals correction vdW(D3) is added to the PBE, the order of polymorphism of CH₃NH₃PbI₃ is consistent with the experimental data. We suggest the PBE+vdW(D3) method should be a prefered choice for structural and total energy calculations of halide hybrid perovskites.

It is found from the calculations that the ionization energy of the three-membered radical (CH₂)₂NH₂ is lower than both CH₃NH₃ and HC(NH₂)₂. The lower ionization energy of the organic cation, the more stable the halide hybrid perovskites will be. Then, we use PBE+vdW(D3) method to estimate the stability of (CH₂)₂NH₂PbI₃, and we found that the reaction enthalpy of (CH₂)₂NH₂PbI₃ is lower than other mainstream perovskites (CH₃NH₃PbI₃, HC(NH₂)₂PbI₃). It is reported that compared with CH₃NH₃PbI₃, CH₃NH₃PbCl₃ is more stable at ambient condition. Our calculated result demonstrates that the stability of (CH₂)₂NH₂PbI₃ is even better than CH₃NH₃PbCl₃.

We use G0W method to calculate the bandgap of cubic (CH₂)₂NH₂PbI₃ which is 1.49 eV that is 0.09 eV higher than the calculated value of HC(NH₂)₂PbI₃, but 0.17 eV lower than CH₃NH₃PbI₃. We also found that the energy difference between low-temperature and high-temperature phases correlates with the high-temperature transition point. Based on this finding, we conclude that (CH₂)₂NH₂PbI₃ will be stable at cubic phase above approximately 190 K.

[1] C Zheng, and O Rubel. "Ionization energy as a stability criterion for halide perovskites." J. Phys. Chem. C, 2017, 121 (22), pp 11977–11984

Ternary sulfides and selenides in the distorted-perovskite structure (“chalcogenide perovskites”) are predicted by theory to be semiconductors with band gap in the visible-to-infrared and may be useful for optical, electronic, and energy conversion technologies. Here we use computational thermodynamics to predict the pressure-temperature phase diagrams for select chalcogenide perovskites. We highlight the windows of thermodynamic equilibrium between solid chalcogenide perovskites and the vapor phase at high temperature and very low pressure. These results can guide adsorption-limited growth of ternary chalcogenides by molecular beam epitaxy (MBE).

Lead free perovskites, especially Bi-based perovskites, are excellent candidates for replacing Pb-based perovskite absorbers in the large scale commercialization of perovskite solar cells due to their higher stability but much less toxicity. However, before their practical application, it is of such importance to capture the fate (relaxation, trapping, recombination and etc.) of their photoexcited charger carriers for helping optimize the photovoltaic performance. By employing the ultrafast laser initialized time-resolved X-ray absorption near edge structure (TR-XANES) spectroscopy, which directly interrogates the orbitals of atoms forming the materials, we have characterized the ~ 4 nm nanocrystals of a representative lead free Bi-based perovskite. Through examining the Bi L3 edge, we have found that most of photoexcited electrons are freely wandering (delocalized) in the conduction band. By contrast, the results probed at Br K edge uncover that the photogenerated holes are strongly localized at Br 4p orbitals in the valence band, forming strong exciton-polarons alive for µs. Our time-resolved element-specific study of lead-free perovskites for the first time will advance the understanding of the photophysical behavior of photogenerated charge carriers in solar cell materials.

This research used resources of the Advanced Photon Source and the Center for Nanoscale Materials, U.S. Department of Energy (DOE) Office of Science User Facilities operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

Hybrid metal-halide perovskite materials show great promise for photovoltaic devices, with power conversion efficiencies (PCE) having recently exceeded 22%. Moreover, the fabrications methods such as thermal evaporation, commonly used currently in industry, can be utilised to fabricate perovskite thin films.[1-2] However, hybrid metal-halide perovskite photovoltaic devices have been affected by anomalous hysteresis, whereby the current-voltage (J-V) characteristics are dependent upon both scan rate and direction.[3] To understand the cause of the anomalous hysteresis, we use optimised co-evaporated perovskite solar cells and then characterise them with high resolution microscopy and current-voltage scans.[4] We demonstrate using transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) that under identical growth conditions the interface affects perovskite morphology and crystallinity. We further correlate this with electron diffraction patterns of the perovskite at the interface to find that the devices that are hysteretic, incorporate amorphous regions of perovskite at the interface. However, when the perovskite is grown on top of an organic layer, such as PCBM, the interface consists of only crystalline perovskite. Not only does this lead to a hysteresis free device, it also leads to the device exhibiting a stabilised power output which is comparable to the measured solar cell efficiency from the J-V characterisation. We then utilise this optimised device structure to track and characterise the temperature-dependence of key current-voltage parameters. The results show that devices exhibit respectable open circuit voltages and short circuit current densities from 295 K to 200 K. However, below 200 K the device shows over 5 orders of magnitude decrease in short circuit current density. Fourier transform photocurrent spectroscopy (FTPS) was then employed to understand and explore spectral features associated with the MAPbI₃ based solar cells.

References
[1] J. B. Patel, R. L. Milot, A. D. Wright, L. M. Herz, M. B. Johnston, J. Phys. Chem. Lett. 2016, 7 (1), 96
[2] M. Liu, M. B. Johnston, H. J. Snaith, Nature 2013, 501 (7467), 395.
[3] H. J. Snaith, A. Abate, J. M. Ball, G. E. Eperon, T. Leijtens, N. K. Noel, S. D. Stranks, J. T.-W. Wang, K. Wojciechowski, W. Zhang, J. Phys. Chem. Lett. 2014, 5, 1511
[4] J. B. Patel, J. Wong-Leung, S. Van Reenen, N. Sakai, J. T. W. Wang, E. S. Parrott, M. Liu, H. J. Snaith, L. M. Herz, M. B. Johnston, Adv. Electron. Mater. 2017, 3, 1600470

In recent years, perovskite solar cells have been attracting enormous attention from academia as well as industry. To fabricate high performance perovskite solar cells, it is of paramount importance to obtain a better understanding of fundamental aspects with the help of surface science and advanced characterization.¹ My group at OIST is making concerted efforts to investigate these materials and devices and to develop innovative strategies to improve their performance. In this talk, I will introduce our research progress on the improvement of perovskite solar cell stability, reproducibility and performance.

References
[1] L. K. Ono, Y. B. Qi*, J. Phys. Chem. Lett. 7, 4764 (2016).

As a derivative of organic-inorganic halide perovskites (e.g. CH₃NH₃PbI₃), van der Waals Ruddlesdon-Popper perovskites (C₄H₉NH₃)₂PbX₄ became popular for their self-assembled electronic quantum well structure and huge exciton binding energy. However, electron phonon coupling, a fundamental process that limits various optoelectronic performances, remains obscure, partly due to the lack of systematic studies on high-quality single crystalline samples free from exfoliation/transfer. In this work, we performed temperature-dependent photoluminescence (PL) studies on single crystalline (C₄H₉NH₃)₂PbI₄ flakes grown by CVD on Si, SiO₂/Si or muscovite mica with thickness of about 15-100 nm. Epitaxial relation were observed between perovskite flakes and mica/Si substrates. Due to the substrate effect therein, the structural phase transition at around 240 K were hindered and room-temperature phase were stabilized till liquid nitrogen temperature. By analyzing the temperature-dependent PL line width (FWHM), we showed that the Pb-I longitudinal optical (LO) phonon couples strongly to excitons via Fröhlich mechanism and the coupling strength decreased significantly as flake thickness was reduced. This trend was explained by the LO phonon confinement in out-of-plane direction of perovskite flakes. In contrast, the PL line width of three-dimensional hybrid perovskites CH₃NH₃PbBr₃ increased significantly as flake thickness was reduced to several unit cells. This discrepancy was attributed to the lack of highly polar interface in layered van der Waals perovskites, which in turn reduces interface electron-phonon scattering. Our work demonstrates the controllability in electron-phonon scattering of ultrathin layered van der Waals perovskites and their potential in various optoelectronic applications.

The methyl ammonium halide family of perovskite materials has revolutionized solar energy conversion, generating power conversion efficiencies exceeding 22%. In addition to promise for solar energy conversion, there is novel materials physics of these perovskite materials. One example are the existence and interplay of excitons and charge carriers are which are key fundamental properties for emerging photovoltaic applications. Despite intense studies it is still challenging to yield information on intrinsic transport parameters and their fs dynamics, such as carrier mobility, density and the initial dynamic pathways of exciton formation and decay. There have been very limited research efforts to provide simultaneously local microstructure measurement and fs carrier dynamics in the same perovskite samples. Here we perform a comprehensive, spatial–temporal spectroscopy characterization of the methyl ammonium perovskite samples using laser-scanning confocal microscopy and ultrafast terahertz spectroscopy- which demonstrate a powerful and versatile approach to fully characterize response functions of excitons and charge carriers, and intrinsic transport properties. Our results show photo-luminescence quenching and lifetime variations due to the impact of local heterogeneity. The samples also show multiple sharp quantum transitions from excitonic Rydberg states at 10.1 meV and 12.1 meV assigned to 1s-2p and 1s-3p internal transitions, characterizing weakly bound excitons with binding energy ~13.5 meV. Transient carrier and exciton populations are precisely determined using ultrafast terahertz conductivity, which is superior to conventional linear optical measurements.
To obtain fundamental insights into excitonic states, we computed the electronic band structure, carrier effective masses, and optical properties using ab-initio density functional theory. Simulations with spin-orbit coupling were necessary to provide reasonable effective masses and exciton binding energies. Simulations give a direct band gap of 1.58 eV, and a dielectric constant ~18, indicating high dielectric screening of internal quantum states leaving excitons weakly bound. As found in other calculations, heavy electrons and light holes combine to provide effective masses.
The excitonic/carrier dynamics and complementary spatial–temporal spectroscopy methods demonstrated shine new light on fundamental perovskite materials physics that have clear implications towards their applications in photovoltaics and optoelectronics.

Supported by U.S. DOE Office of Science, Basic Energy Sciences, Materials Science and Engineering Division.

The optical and transport properties of lead-halide perovskites (LHPs) have been used as a basis for new solar cell technologies showing record improvements in efficiencies. In the search for the microscopic origins of this success, many recent studies suggest that structurally dynamic effects are active already at room temperature and standard operating conditions and may affect device performance and/or stability. Here, we explore this issue using first-principles calculations based on density functional theory. In particular, we focus on ion migration and dynamic distortions.

Solution processable hybrid organic-inorganic perovskites (HOIPs) such as methylammonium lead iodide (MAPbI3) represent a research forefront owing to prospects of enhanced performance in solar energy conversion, solid-state lighting, and information processing applications. In contrast to traditional inorganic semiconductors, the interactions between the organic and inorganic sub-lattices in HOIPs likely give rise to their extraordinary optoelectronic properties, such as long carrier lifetimes (possibly via a direct-indirect bandgap character) and hot carrier protection. Better understanding of the fate of non-equilibrium phonons is hence crucial for the further improvement of HOIP-based technologies. Direct probing of lattice temperature can provide insights into the mechanisms and timescales of electron-phonon and phonon-phonon interactions. Here we employ infrared transient absorption spectroscopy to investigate lattice heating in MAPbI3 and formamidinium lead iodide (FAPbI3). The strong temperature sensitivity of the absorbance of organic vibration modes permit probing of lattice thermalization with tens-of-fs time resolution. We observe long thermal equilibration time (hundreds-of-picosecond) that is one to two orders-of-magnitude slower than those observed for fully inorganic semiconductors. The slow thermal equilibration in HOIPs arise from a widely separated (weakly overlapping) phonon density of states (PDOS), which can impact the electronic and heat transport properties of HOIPs, and provide insights to manipulate such properties for the broader class of organic-inorganic hybrid materials.

Organometal halide perovskites (OMHPs) have rapidly emerged as promising low-cost semiconductors for numerous applications, including photovoltaic cells and light emitting diodes. The energy of the valence band maximum, conduction band minimum, and transport gap are important parameters for fundamentally understanding the electronic properties of these materials and for successfully integrating these OMHPs into electronic devices. However, currently there is a large variation in literature reported valence and conduction band energies for almost all OMHPs, including the most widely studied OMHP, methylammonium lead iodide, where reported values for the valence band maximum range from 5.4 to 6.4 eV. These variations may potentially be attributed to differences in perovskite processing method, surface contamination, surface degradation, sample damage during the photoelectron spectroscopy measurement, or the method used to define the valence band maximum. In this talk I will present the use of a low-energy (10.2 eV) photon source and a low-energy (4-5 eV) detector for ultraviolet and inverse photoelectron spectroscopy measurements, respectively, of damage-prone materials. These low-energy photoelectron spectroscopy systems are shown to nearly eliminate sample damage and are used to probe the energetics of OMHPs as a function of both processing and composition. We find that the ionization energy and electron affinity for the same nominal material can vary based on the processing method and precursor used to produce the perovskite.

Organometal halide perovskite-based solar cells have stormed the world of photovoltaics, demonstrating power conversion efficiencies of over 20% within less than a decade of their existence. The band gap of these perovskites has been shown to be variable based on their phase, as well as the extent of compression or extension of the perovskite cage. [1] The understanding and control of perovskite structure is therefore key to solar cell performance.

Here, we present a systematic study on ABX₃ perovskites using density functional theory, evaluating the accuracy of methodologies for band gap prediction. We demonstrate the change in electronic properties with changing lattice parameters and halogen substitutions in ABX₃ perovskites. Moreover, we highlight the effects of A-X bond lengths and octahedral tilting on the electronic properties of these perovskites. By developing a systematic description of key structure-property relationships for these materials, we provide mechanistic insight and hope to contribute to future materials design principles.

References
[1] Huang, L. Y., and Lambrecht, W. R. Electronic band structure, phonons, and exciton binding energies of halide perovskites CsSnCl₃, CsSnBr₃, and CsSnI₃. Phys. Rev. B, 88(16), 165203, 2013.

All-inorganic halide perovskite have received an enormous amount of attention because of unique properties, including facile deposition processing due to high solubility in polar organic solvents and tunability of band gap energy through manipulation of the halide composition, which is useful in solution-based fabrication of photonic and optoelectronic thin-film devices. To understand the intrinsic optoelectrical properties of inorganic perovskite, it is necessary to obtain high-quality inorganic perovskite samples, with a single phase, high purity, and a macroscopic size, known as crystals. Pure CsPbBr₃ and Cs₄PbBr₆ single crystals were separately grown by AVC (anti-solvent vapor-assisted crystallization process) with dimensions of millimeters. We found the correlation between photocurrent generation and PL (photoluminescence) in perovskite crystals. The CsPbBr₃ crystals, which have 3D perovskite structure, showed a sensitive steady-state photoresponse and a poor PL signal. Contrastively, the Cs₄PbBr₆ crystals, which have 0D perovskite structure, exhibited more than 1 order magnitude higher PL intensity than CsPbBr₃, which generated an ultralow photocurrent under illumination. Photocurrent and PL have a negative relationship, which is ascribed to the recombination or dissociative process of excitons due to the difference of exciton binding energy. We attribute their contrasting opto-electrical characteristics to a difference of exciton binding energy, induced by coordination geometry of [PbBr₆]^4- octahedron sublattice. Our works indicate the importance of the crystal structure of perovskite materials in understanding their superior optoelectrical performance.

Hybrid lead-halide perovskites have risen to become promising materials for use in thin-film photovoltaic (PV) devices, with efficiencies in excess of 22% achieved after just a few years research¹. However, further development of these devices has been hampered by the lack of stability of the materials, especially when compared with those of incumbent technologies. It has recently been found that chemical site-substitution at one or more of the A, B or X sites in the perovskite’s ABX₃ structure can be used to tune both the physical properties and the stability of the materials^2,3. Results reported here focus on the degradation routes and kinetics of thin films of methlyammonium (MA) / formamidinium (FA) lead iodide perovskites from the solid solution MA_1−xFA_xPbI₃, 0 ≤ x ≤ 1 in low humidity conditions.⁴ The degradation route favoured for MA-rich phases is decomposition to PbI₂ and precursor organic cation salts, whereas FA-rich phases quickly transform to the non-perovskite hexagonal δ-phase. However, kinetic analysis of PbI₂ formation, measured, using powder X-ray diffraction, shows an exponential decay in the rate of formation up to x = 0.6, for which negligible amounts of PbI₂ was produced even after 10 days exposure to the atmosphere. The observed increase in stability was supported by ab initio simulations of the decomposition reaction energies. This combined experimental – computational study provides a greater understanding of favoured degradation routes in hybrid lead-halide perovskites, leading to likely improvements in the long-term stability of perovskites solar cells.

¹ M. A. Green et al., Prog. Photovolt. Res. Appl. 2017, 25, 3-13
² M. T. Klug et al., Energy Environ. Sci. 2017, 10, 236-246
³ C. C. Stoumpos et al., Inorg. Chem. 2013, 52, 9019-9038
⁴ B. Charles, J. Dillon, O.J. Weber, M. S. Islam and M.T. Weller J. Mat. Chem. A, 2017, DOI: 10.1039/C7TA08617B

The facile chemical transformation of halide perovskites via ion exchange has been attributed to the characteristic of "soft" crystal lattice that enables ions to readily migrate in the lattice. Kinetic studies of such process could provide mechanistic insights on the ion migration dynamics, yet from the experimental aspect little effort has been made. Herein, we present a general method to in situ visualize and monitor the kinetics of cation or anion interdiffusion in a specifically designed perovskite heterostructure via spatially resolved photoluminescence measurement. A CsPbCl₃/CsPbBr₃ heterostructure was fabricated by stacking single-crystal CsPbCl₃ microplates on top of single-crystal CsPbBr₃ nanowires to study chlorine-bromine interdiffusion behavior. Time-dependent confocal photoluminescence microscopy and energy-dispersive X-ray spectroscopy showed the solid-state anion interdiffusion can readily occur, leading to the formation of halide concentration gradients along the nanowire. Quantitative analysis of composition profiles across the heterojunction using Fick's law allowed us, for the first time, to extract interdiffusion coefficients of the couple and an activation energy of 0.44±0.02 eV for ion diffusion from temperature-dependent studies. Moreover, comparative studies on MAPbI₃/CsPbBr₃ heterostructure revealed limited extent of iodine-bromine interdiffusion likely due to the complex phase diagram of mixed alloys of CsPb(Br,I)₃. In contrast to the relatively mobile anions, A-site cation interdiffusion across the interface in MAPbBr₃/CsPbBr₃ junction was barely observed at room temperature. Our results present an important step for developing a model system to investigate the kinetics of the solid-state ion migration. Moreover, the gained insights can provide guidelines for rationally designing perovskite heterostructures that could potentially give rise to new intriguing properties for both fundamental studies and technological applications.

Metal halide perovskites (MHPs) and organometal halide perovskites (OMHPs) display enormous potential as next generation materials for optoelectronics, such as photovoltaics, lasers, and light emitting diodes. MHPs and OMHPs cover a diverse range of chemical compositions; however, the current highest performing (O)MHPs in photovoltaics include lead, a well-known toxic metal. Recent studies show that lead’s group 14 neighbor, tin, is also suitable for use in photovoltaics as a low-cost and environmentally friendly lead alternative. Like their lead counterparts, tin perovskites can have a range of chemical compositions consisting of numerous A and X site ions. Early investigations have primarily examined the influence of A and X site ions on the photovoltaic performance and optical properties of these tin-based perovskites. However, the continual development of tin (O)MHPs necessitates accurate determination of their electronic structures. Ultraviolet photoelectron spectroscopy (UPS) and inverse photoelectron spectroscopy (IPES) are complimentary techniques that provide a direct means for probing occupied and unoccupied electronic states in semiconducting materials. Here we present a novel VUV photon source for UPS measurements, a low-energy IPES system, and the application of these systems to probe the valence and conduction band energies of tin based (O)MHPs as a function of composition. Our VUV photon source utilizes H Lyman-α emission with narrow linewidth, broad tunable intensity, and lower background compared to typical UPS sources. This, in conjunction with our newly developed low energy IPES system, allows us to perform measurements without risk of sample degradation and gain a complete picture of material and interfacial energetics.

In recent years, the interest in hybrid organic - inorganic perovskites rose at a rapid pace due to their tremendous success in the field of photovoltaics. In addition to the thin film properties of the active layer, the performance of optoelectronic devices strongly depends on the appropriate energetic alignment between the active- and adjacent layers. In order to choose adequate transport materials for the increasingly complex hybrid perovskite compositions in a non-trial-and-error fashion, it is important to understand how the induced changes in band gap relate to shifts in the valence and/or conduction band.
In this poster, I will present recent findings regarding measurements of the electronic structure of various hybrid perovskites using photoelectron spectroscopy. Hereby, we cove the whole range of lead and tin based perovskite systems, therefore varying the halogens (Cl, Br, I) as well as cations (Cs, MA, FA). We investigate how the changes in composition influence the ionization energy as well as electron affinity and compare these results with DFT calculations.

Organic-inorganic metal halide (OIMH) perovskite absorbers such as methyl ammonium lead iodide (CH₃NH₃PbI₃, MAPbI₃) have been used to prepare high efficiency solar cells when they are paired with appropriate electron and hole transport materials (ETMs and HTMs). The power conversion efficiency (PCE) of OIMH perovskites solar cells has improved to more than 22% [1]. The tremendous progress in improving PCE is mainly due to a unique combination of the intrinsic optical and electrical properties of OIMH perovskites, such as large absorption coefficient, long carrier diffusion lengths, desirable optical band gaps, and fast carrier collection rates, and the existence of cost effective and facile preparation methods. Among several deposition methods, the single-step solution deposition coupled with the anti-solvent dropping technique has been widely used in the fabrication of high efficiency perovskite solar cells [2]. Although, this method produces a dense, uniform, and conformal perovskite film, the size of grains is extremely small (200 nm). The small grains form a large number of grain boundaries that increases the density of charge carrier traps resulting in higher non-radiative recombination of carriers. By fabricating larger grains with a high degree of crystallinity, the non-radiative recombination in perovskite films can be reduced and device performance improved.
Here, we adapt a technique demonstrated for the two-step deposition process [3] to enhance the grain size and crystallinity by adding small amounts of metal cations (Cd²⁺, Zn²⁺, and Fe²⁺) during the single-step solution deposition process. This metal-additive-assisted single-step deposition fabrication process resulted in grains that are ~1 μm in diameter, which is an increase of a factor of 5. In addition to the improved grain size, the degree of crystallinity of the grains is also improved with the metal cation inclusion, as determined by the X-ray spectroscopy analysis. These results suggest that the process may be an effective and facile method to fabricate high efficiency perovskite photovoltaic devices.

[1] NREL, "Solar Cell Efficiency Chart". [http://www.nrel.gov/ncpv/images/efficiency_chart.jpg] accessed: October 2017.
[2] Song, Z.; Watthage, S. C.; Phillips, A. B.; Heben, M. J. “Pathways toward High-Performance Perovskite Solar Cells: Review of Recent Advances in Organo-Metal Halide Perovskites for Photovoltaic Applications.” J. Photon. Energy. 2016, 6, 022001.
[3] S. C. Watthage, Z. Song, N. Shrestha, A. B. Phillips, G. K. Liyanage, P. J. Roland, R. J. Ellingson, and M. J. Heben, "Enhanced Grain Size, Photoluminescence, and Photoconversion Efficiency with Cadmium Addition During the Two-Step Growth of CH₃NH₃PbI₃," ACS Applied Materials & Interfaces, 2016.

The bismuth ferrite BiFeO₃ is promising as magnetoelectric material because both the ferroelectric and antiferromagnetic orders coexist in this material at room temperature. Doping of BiFeO₃ or the substitution of bismuth by other chemical elements can modify its physical properties. The goal of this paper is to determine the influence of the bismuth substitution by gadolinium and neodymium on crystal lattice dynamics at room temperature.

The substituted solid solutions of BiFeO₃ of the types Bi_1-xGd_xFeO₃ and Bi_1-xNd_xFeO₃ with the atomic part of the substitutive element x up to 0.20 were synthesized by means of the solid-state reaction method using powders of oxides Bi₂O₃, Nd₂O₃, Gd₂O₃, and Fe₂O₃ of pure grade quality. The X-ray diffraction method was applied using the diffractometer Dron-3 on Cu K_α radiation. The experimental data were collected during scanning repeated ten times in the 2Θ range from 20° to 90° at the scanning speed of 10°/6 min. The infrared reflection spectra of the samples in tablet powder mixtures were recorded with VERTEX 80v FT-IR spectrometer (Bruker). As standards for the recording of the absolute values of the reflection coefficient R, an aluminum mirror with R = 97% and a single crystal silicon wafer were used.

The X-ray patterns show the displacement of the positions of reflexes of BiFeO₃ with the substitution of bismuth by gadolinium and neodymium. This confirms the formation of the solid solutions Bi_1-xGd_xFeO₃ and Bi_1-xNd_xFeO₃. The lattice constants of the formed solid solutions decrease with the increase of the content of the substitutive element Gd. This may be explained by the lower ionic radii of Gd³⁺ with respect to Bi³⁺. The small inclusion of the impurity phases of Bi₂₅FeO₃₉ and Bi₂Fe₄O₉ in addition to the main phase of BiFeO₃ is also indicated. Nevertheless the small presence of the impurity phases allows to make conclusions regarding physical properties of these materials, including their crystal lattice dynamics.

Two extremums at 18.2 μm (strong extremum) and 22.5 μm (rather weak extremum) on the infrared reflection spectra of the solid solutions Bi_1-xGd_xFeO₃ and Bi_1-xNd_xFeO₃ were discovered. The extremum at 18.2 μm is caused by the bending of the mechanical vibrations of the Fe – O bonds and the extremum at 22.5 μm is caused by the stretching of the mechanical vibrations of the Fe – O bonds. The increase of the atomic part of the substitutive element x up to 0.20 leads to the growth of the reflectivity and the absorption coefficients of the solid solutions Bi_1-xGd_xFeO₃ and Bi_1-xNd_xFeO₃ in comparison with BiFeO₃. The displacement of the absorption band maximum into the side of the spectrum with longer wavelengths with the increase of the content of the substitutive elements was also found. Such behavior of the spectra can be explained by decreasing elastic constants of the solid solutions Bi_1-xGd_xFeO₃ and Bi_1-xNd_xFeO₃ occurring when the new substitution-induced bonds Gd – O and Nd – O are formed.

The versatility of vapor phase epitaxy enables control of various semiconductor morphologies that can meet the geometrical requirements for a designated application as well as for the understanding of intrinsic physical properties. Here, by utilizing van der Waals and ionic epitaxy, we grow halide perovksite into both high quality nanostructures (flake and nanowire) and thin film through which some intrinsic and previously unnoticed photo physics and carrier dynamics can be revealed. The unique geometry in halide perovksite nanowire network provides us with a good platform to understand the photon absorption, transport and waveguiding effect along the wire direction. Photo induced ion migration, previously a disturbing phenomenon, can also be taken advantage of in this system to facilitate the fabrication of halide perovskite double heterojunctions. On the other hand, the single crystal high quality thin film by high temperature growth enables a minimization of defect density that can fully reveal its intrinsic optical properties. Surprisingly, we found very significant hot photoluminescence in pure inorganic CsPbBr₃ which is previously believed to be non-existing. As a step further, we identified a possible photo Dember effect and Gigahertz emission through an anomalous photoluminescence decay curve that results from a discrepancy of electron/hole mobility. Our study on both nanostructured and thin film halide perovskite provides knowledge on both the fundamental physics and the possible underlying application ideas in this emerging material family.

Lead halide perovskites have been demonstrated as high performance materials in solar cells and light-emitting devices. These materials are characterized by coherent charge transport expected from crystalline semiconductors, but phonon dynamics typical of liquids. I will discuss how such “crystal-liquid” duality leads to the protection of charge carriers in a process which is called solvation in chemistry or large polaron formation in physics. As a result, the Coulomb potential is much screened on the picosecond time scale by a subset of longitudinal optical phonons, thus reducing the scattering of a charge carrier with charged defects, with other charge carriers, and with the remaining phonon bath. Such ultrafast charge carrier protection can account for the exceptional defect tolerance, moderate charge carrier mobility, and low radiative recombination rates. Large polaron formation, along with the liquid or phonon-glass character, may also explain the dramatic reduction in hot carrier cooling rates. Based on lessons we have learned from lead halide perovskites, I propose soft, polar, and dynamic disorder as design principles for defect tolerant semiconductors from nano, molecular, and hybrid materials.
H. Zhu, K. Miyata, Y. Fu, J. Wang, P. P. Joshi, D. Niesner, K. W. Williams, S. Jin, X.-Y. Zhu, “Screening in crystalline liquids protects energetic carriers in hybrid perovskites,” Science, 2016, 353, 1409-1413
K. Miyata, D. Meggiolaro, M. T. Trinh, P. P. Joshi, E. Mosconi, S. Jones, F. De Angelis, X.-Y. Zhu, “Large polarons in lead halide perovskites,” Science Adv. 2017, 3, e1701217.
K. Miyata, T. L. Atallah, X.-Y. Zhu, “Lead halide perovskites: crystal-liquid duality, phonon glass electron crystal, and large polaron formation,” Science Adv. 2017, 3, e1701469.

To be announced

With the efficiency of perovskite photovoltaic cells rapidly improving, the Shockley-Queisser limit¹ is not too far from reach. On approaching this limit, charge-carrier extraction will be limited only by radiative bimolecular recombination of electrons with holes.² However, the fundamental physics behind this process, and its link with material stoichiometry, is still not understood. Here we show that bimolecular charge-carrier recombination in methylammonium lead triiodide perovskite can be fully explained as the inverse process of absorption. By carefully considering for contributions to the absorption from bound excitons and unbound electron-hole continuum states, with support from GW ab initio calculations³, we are able to determine bimolecular radiative recombination rate constants from absorption spectra. We show that the sharpening of photon, electron and hole distribution functions increases the bimolecular radiative rate by roughly an order of magnitude as the temperature is lowered from room temperature to 50K. Our findings thus give a fundamental understanding of the electronic processes in these hybrid perovskites, which will allow for guided exploration of alternative stoichiometries with desirable photovoltaic properties.

References
¹W. Shockley and H. J. Queisser, J. Appl. Phys. 32, 510–519 (1961).
²M. B Johnston and L. M. Herz, Acc. Chem. Res. 49, 146–154 (2016).
³M. Filip et al. Phys. Rev. B 90, 245145 (2014)

Understanding charge dynamics in semiconductors for solar cells is critical to their identification, development and potential commercial implementation. Optical spectroscopic techniques such as photoluminescence spectroscopy, transient absorption spectroscopy, time-resolved microwave conductivity and terahertz spectroscopy are traditionally employed to elucidate optoelectronic properties in these systems. Yet many such techniques involve high-power optical sources and photo-generated carrier densities many orders of magnitude higher than present under typical solar cell operating conditions. An alternative approach is to study carrier dynamics using steady-state illumination rather than with high-fluence pulsed optical sources. In this presentation I describe a simple and versatile contactless technique to study the photoconductivity of a semiconductor, as function of incident optical power-density, at optical power densities ≤ 1 sun. By employing thin solution-processed films of the highly-studied, high-performance hybrid-halide perovskite methylammonium lead iodide ((MA)PbI3), I evaluate a proxy for mobility-lifetime product, with a strong dependence on incident optical power densities even below 1 mW/cm2.

Photo-induced processes play key roles in photovoltaic and photo-catalytic applications of halide perovskites and other nanoscale materials. They require understanding of the material’s dynamical response to the photo-excitation on atomic and nanometer scales. Our non- adiabatic molecular dynamics techniques,¹ implemented within time-dependent density functional theory,^2-4 allow us to model such non-equilibrium response in real time. The talk will focus on photo-initiated energy and charge transfer, relaxation and recombination in hybrid organic-inorganic perovskites. Focusing on realistic aspects of perovskite structure,⁵ we demonstrate that strong interaction at the perovskite/TiO₂ interface facilitates ultrafast charge separation,⁶ how dopants can be used to both decrease and increase charge recombination,^7-9 that grain boundaries constitute a major reason for charge losses,⁹ that moderate humidity increases charge lifetime, while high humidity accelerates losses,¹⁰ that hole trapping by iodine interstitial, surprisingly, extends carrier lifetime,¹¹ that collective nature of dipole motions inhibits nonradiative relaxation,¹² that organic cation orientation has a strong effect on inorganic ion diffusion and current-voltage hysteresis,¹³ and that the experimentally observed dual (hot/cold) fluorescence originates from two types of perovskites substructures. Our simulations provide a unifying description of quantum dynamics on the nanoscale, characterize the rates and branching ratios of competing processes, resolve debated issues, and generate theoretical guidelines for development of novel systems for solar energy utilization.

1. Wang, L. J.; Akimov, A.; Prezhdo, O. V. J. Phys. Chem. Lett. 2016, 7, (11), 2100- 2112.
2. Akimov, A. V.; Prezhdo, O. V. J. Chem. Theory Comput. 2013, 9, (11), 4959-4972.
3. Akimov, A. V.; Prezhdo, O. V. J. Chem. Theory Comput. 2014, 10, (2), 789-804.
4. Pal, S.; Trivedi, D. J.; Akimov, A. V.; Aradi, B.; Frauenheim, T.; Prezhdo, O. V. J. Chem. Theory Comput. 2016, 12, (4), 1436-1448.
5. Jankowska, J.; Long, R.; Prezhdo, O. V. ACS Energy Lett. 2017, 2, (7), 1588-1597.
6. Long, R.; Fang, W. H.; Prezhdo, O. V. J. Phys. Chem. C 2017, 121, (7), 3797-3806.
7. Liu, J.; Prezhdo, O. V. J. Phys. Chem. Lett. 2015, 6, (22), 4463-4469.
8. Long, R.; Prezhdo, O. V. ACS Nano 2015, 9, (11), 11143-11155.
9. Long, R.; Liu, J.; Prezhdo, O. V. J. Am. Chem. Soc. 2016, 138, (11), 3884-3890.
10. Long, R.; Fang, W. H.; Prezhdo, O. V. J. Phys. Chem. Lett. 2016, 7, (16), 3215-3222.
11. Li, W.; Liu, J.; Bai, F. Q.; Zhang, H. X.; Prezhdo, O. V. ACS Energy Lett. 2017, 2, (6), 1270-1278.
12. Jankowska, J.; Prezhdo, O. V. J. Phys. Chem. Lett. 2017, 8, (4), 812-818.
13. Tong, C. J.; Geng, W.; Prezhdo, O. V.; Liu, L. M. ACS Energy Lett. 2017, 2, (9), 1997-2004.

Inorganic cesium lead halide, CsPbX₃ (X = Cl, Br, I), perovskite nanocrystals (NCs) have recently driven a paradigm shift in light-harvesting and light-emitting technologies due to their tunable optical bandgaps, bright photoluminescence (PL), and higher stabilities than methylammonium-based perovskites. Whereas research efforts in the literature has primarily focused on the synthesis and surface properties of these materials, to successfully implement CsPbX₃ in applications it is imperative to understand their carrier dynamic processes, which are controlled by their size, shape, and composition. These processes, which include carrier generation, carrier cooling, and carrier recombination by radiative and nonradiative pathways, ultimately determine the efficiency of optoelectronic technologies. In this work we show the impact of morphology and composition on the carrier dynamics of CsPbBr₃ NCs. We compared the ultrafast dynamics of CsPbBr₃ bulk films, nanocubes, and nanowires with transient absorption spectroscopy (TAS) and time-resolved photoluminescence (TrPL) and observed the fastest exciton decay lifetime of the films relative to the nanocubes and nanowires. In order to study the impact of composition on the relaxation dynamics of CsPbX₃ NCs, we partially substituted bromide ions with chloride ions via anion exchange reactions and examined their ultrafast dynamics with TAS and TrPL. We observed that incorporation of chloride ions increased the rates of carrier cooling and carrier recombination through Auger and trap states.

Lead halide perovskites exhibit surprisingly long charge carrier lifetimes for a direct bandgap semiconductor. Recently, we found evidence for a slightly indirect bandgap in MAPbI₃ by using pressure to change photoelectric properties. This indirect bandgap is induced by Rashba splitting, and could be one reason for the long lifetimes.
The Rashba splitting requires an electric field across the lead atom, which could originate from the organic cation methylammonium. We recently investigated MAPbI₃, which has a methylammonium (MA) cation with a dipole moment of 2.3 Debye inside the lead iodide cage. We use FAPbI₃ (Dipole moment of formamidinium (FA) = 0.2 Debye), CsPbI₃ (Cesium(Cs) has no dipole moment) to compare materials with the same cage structure but different cation dipole moments. We study photoluminescence and time resolved photoluminescence under pressure to track the changes in optoelectronic behavior of different lead iodine perovskite derivatives.
We find that the cation has a large influence on the magnitude of the Rashba splitting. MAPbI₃ with large dipole moment has a clear side peak in the PL spectrum. FAPbI₃ with its smaller dipole moment has an asymmetric PL shape, but no distinct side peak and CsPbI₃ with no cation dipole moment has a perfectly symmetric PL peak. We quantify the asymmetry of the photoluminescence spectrum, and the lifetime of each material to understand the influence of the cation on lead halide based semiconductor perovskites. Our results lead to a structural understanding of the Rashba-split bandgap of metal halide perovskites, which is crucial for designing novel perovskite materials.

Drift-diffusion model is an indispensable modeling tool to understand the carrier dynamics (transport, recombination, and collection) and simulate practical-efficiency of solar cells (SCs) through taking into account various carrier recombination losses existing in multilayered device structures. Exploring the way to predict and approach the SC efficiency limit by using the drift-diffusion model will enable us to gain more physical insights and design guidelines for emerging photovoltaics, particularly perovskite solar cells. Our work finds out that two procedures are the prerequisites for predicting and approaching the SC efficiency limit. Firstly, the intrinsic radiative recombination needs to be corrected after adopting optical designs which will significantly affect the open-circuit voltage at its Shockley–Queisser limit. Through considering detailed balance between emission and absorption of semiconductor materials at the thermal equilibrium, and the Boltzmann statistics at the non-equilibrium, we offer a different approach to derive the accurate expression of intrinsic radiative recombination with the optical corrections for semiconductor materials. The new expression captures light trapping of the absorbed photons and angular restriction of the emitted photons simultaneously, which are ignored in the traditional Roosbroeck-Shockley expression. Secondly, the contact characteristics of the electrodes need to be carefully engineered to eliminate the charge accumulation and surface recombination at the electrodes. The selective contact or blocking layer incorporated nonselective contact that inhibits the surface recombination at the electrode is another important prerequisite. With the two procedures, the accurate prediction of efficiency limit and precise evaluation of efficiency degradation for perovskite solar cells are attainable by the drift-diffusion model. Our work is fundamentally and practically important to mathematical modeling and physical understanding of solar cells [1].

[1] X. Ren, Z.S. Wang, W.E.I. Sha*, W.C.H. Choy*, ACS Photonics, 2017, 4 (4), pp 934–942

Perovskite solar cells (PSCs) emerging as the most promising next-generation photovoltaic devices have been received great attention. In PSC device, admittedly, Spiro-OMeTAD is the most widely used hole-transporting material (HTM). However, the pristine Spiro-OMeTAD suffers from low hole mobility and conductivity, which requires chemical dopants (Li-TFSI and tBP) to increase conductivity thereby improving power conversion efficiency (PCE). Discouragingly, such dopants not only induce deleterious effects on stability but also significantly affect the hysteresis of PSCs. In this study, F4-TCNQ was introduced into Spiro-OMeTAD as an alternative dopant to replace the widely used Li-TFSI and tBP. By optimizing the doping concentration of F4-TCNQ, the PSC based on 1.5 mol% F4-TCNQ doped Spiro-OMeTAD exhibited the best PCE, which reached about 91% of the PSC based on the Spiro-OMeTAD doped by state-of-the-art Li-TFSI and tBP. Moreover, the PSC based on F4-TCNQ doped Spiro-OMeTAD showed lower hysteresis and better stability. This work not only offers a promising dopant for Spiro-OMeTAD, but also provides a viable approach to address the challenges of hysteresis and instability.

Perovskites are emerging as a promising photovoltaic technology for space applications. Not only can they be produced at dramatically lower cost than established space solar technologies, they can in fact achieve higher specific power (power output per mass > 20 W/g). We have recently found that they exhibit remarkable radiation resistance, suggesting that unshielded, ultralight perovskite solar cells could enable a 10x or higher breakthrough in specific power of space solar panels. We have fabricated cells and are experimentally evaluating several critical aspects of space operation: radiation resistance, stability under sunlight in vacuum, UV resistance, and extended thermal cycling.
We fabricated our cells on quartz superstrates, and achieved typical AM0 efficiencies of 13–16%. The device architecture adopted for our study is ITO/NiO/ MAPbI3/PCBM/Ag. Radiation testing has been performed with 1 MeV electrons and with 30 KeV, 50 KeV, and 350 KeV protons. To prevent the superstrate from shielding the cells, we irradiate the cells from the back side, through thin Ag contacts. Monte Carlo simulations predict that the 1MeV electrons and 350 KeV protons largely penetrate the entire perovskite structure. The 30 KeV and 50 KeV protons stop largely in the NiO and the ITO layers, respectively. Cell performance was measured before and after the radiation exposure. The 1 MeV electrons did not significantly damage the cells at fluence of 10¹³, 10¹⁴, and 10¹⁵ cm^-3, and the remaining power at a fluence of 10¹⁶ e^-/cm² was 90 %. By comparison, this electron fluence in a III-V GaAs solar cell is reported to degrade efficiency by 50 %. The perovskite solar cells under 30, 50 and 350 KeV protons at a fluence of 10¹² p⁺/cm² maintained the open circuit voltage and short circuit current, while the fill factor degraded from 70%, to 37%, 42%, and 55%, respectively. However, after annealing the solar cells in a vacuum chamber at 90 ^oC for 3 days, the performance was completely recovered in all three cases, suggesting that the degradation in orbit would likely be negligible. We further tested the cells at the most damaging energy (30 KeV) at a higher fluence of 10¹³ p⁺/cm². All efficiency parameters (Isc, Voc and FF) were degraded, resulting in a remaining efficiency of 2 %. However, annealing under vacuum restored the efficiency of the solar cells to 85 % of the initial value. In order to evaluate the light- and UV-stability of perovskite solar cells operating in space, we have loaded both irradiated and non-irradiated cells into a vacuum chamber where they can be continuously illuminated with simulated AM0 sunlight, or higher intensity UV light. A vacuum feedthrough permits us to periodically measure the cells’ efficiency parameters. We plan to monitor cell performance vs. time, over a range of anticipated operating temperatures (40–80°C), and under different steady-state load conditions (near maximum power point, open circuit, or short circuit).

Metal oxides are desirable photoanodes for photoelectrochemical (PEC) splitting of water due to their high stability in aqueous electrolytes and resistance to oxidation. However, most metal oxides have bandgaps that are too large for efficiently absorbing sunlight. In this talk, we show that oxide double perovskites exhibit unusual flexibility for bandgap engineering, due to the multiple choices of metal cations. Taking barium bismuth niobate double perovskite as a case study, density-functional theory calculation (DFT) suggests that the bandgap can be effectively narrowed if the oxide is made Bi-rich and Nb-poor, giving a composition of Ba₂Bi_1+xNb_1-xO₆. The excess Bi atoms possess 5+ charge states to maintain overall charge neutrality within the material. As a result, the conduction band minimum is derived from the lower-energy unoccupied Bi 6s states, leading to effective bandgap reduction. Material synthesis confirms the predictions from DFT calculation. We find that with x = 0.4, the Ba₂Bi_1.4Nb_0.6O₆ double perovskite oxide produces pure single phase thin films with a narrow nearly-direct bandgap of about 1.64 eV. We further show that photoanodes made of Ba₂Bi_1.4Nb_0.6O₆ thin films exhibit promising PEC activity and stability. Our results suggest a new strategy for bandgap engineering of metal oxides towards highly active photoanodes for efficient water-splitting applications.

This paper presents a new hole transportation layer (HTL) combined with PEDOT:PSS and Copper doped Nickel Oxide (Cu:NiO_x) in Perovskite-based solar cells (SCs). Thin films of Cu:NiO_x are prepared on Fluorine doped Tin Oxide (FTO) glass substrates using a solution containing Nickel acetate tetrahydrate, 2-Methoxyethanol and Monoethanolamine (MEA) and Copper acetate monohydrate. After that PEDOTPSS are prepared on Cu:NiO_x forming the composite hole transportation layer. The organic-inorganic CH₃NH₃PbI₃ active layer derived from Pb-acetate based solution, [6,6]-phenyl C₆₁-butyric acid methyl ester, and Al electrode are subsequently deposited, forming the inverted planar SC. Results indicate that PCE values higher than 10% can be realized using this low-temperature anneal process. Effects of anneal temperature and solution concentration on the properties of the SC are elucidated. The stability of the SC based on PEDOT/NiO_x composite HTL are explored and compared to the stability of SC using single PEDOT HTL.

CsPbBr₃ is a direct-gap semiconductor where optical absorption takes place across the fundamental bandgap, but this all-inorganic halide perovskite typically exhibits above-bandgap emission when excited over an energy level, lying above the conduction-band minimum. We probe this bandgap anomaly using wavelength-dependent multiphoton absorption spectroscopy and find that the fundamental gap is strictly two-photon forbidden, rendering it three-photon absorption (3PA) active. Instead, two-photon absorption (2PA) commences when the two-photon energy is resonant with the optical gap, associated with the level causing the anomaly. We determine absolute nonlinear optical dispersion over this 3PA−2PA region, which can be explained by two-band models in terms of the optical gap. The polarization dependence of 3PA and 2PA is also measured and explained by the relevant selection rules. CsPbBr₃ is highly luminescent under multiphoton absorption at room temperature with marked polarization and wavelength dependence at the 3PA−2PA crossover and therefore has potential for nonlinear optical applications.

Titanium dioxide (TiO₂) metal oxides are currently the most efficient scaffolds for electron transport layers in perovskite solar cells (PCSs). The efficiencies of TiO₂ based perovskite solar cells has now exceeded 22% and they can remain stable for over 400 days with negligible hysteresis.^[1] In addition, carbon nanotubes (CNTs) have played multifunctional roles in a range of PV cells because of their fascinating properties.^[2] In particular, their ability to reduce hysteresis, and improve the stability of TiO₂-based PSCs. Our previous work developed a TiO₂ nanofiber-based photoelectrode and its performance in a PSC was optimized by tuning the type and amount of CNTs loaded into the mesoporous TiO₂.^[3] Among the different types of CNTs incorporated into TiO₂, single- walled CNTs (SWCNTs) proved to be superior to other types and this was attributed to their highly conductive properties, and higher defect tolerance. Herein, we build on our previous work by studying the origin of performance enhancement in such systems. In doing so, electron injection, and the acquisition of single photons using Transient Absorption Stereomicroscopy (TAS) and Time-Correlated Single Photon Counting (TCSPC) has been investigated. Our studies show that highly conductive SWCNTs incorporated in TiO₂ photoelectrodes provided a fast electron transfer within the photoelectrode, and decrease the number of trap states compared to control TiO₂. On the basis of our theoretical calculations, the improved open-circuit voltage (Voc) of the cells can be attributed to a shift in energy level of the photoelectrodes after the introduction of SWCNTs. Furthermore, our highest performing photoelectrodes have generated a power conversion efficiency of over 19%, which is amongst the highest ever achieved for methyl-ammonium lead iodide perovskites.

References
[1] W. S. Yang, B.-W. Park, E. H. Jung, N. J. Jeon, Y. C. Kim, D. U. Lee, S. S. Shin, J. Seo, E. K. Kim, J. H. Noh, et al., Science 2017, 356, 1376–1379.
[2] T. J. Macdonald, D. D. Tune, M. R. Dewi, C. T. Gibson, J. G. Shapter, T. Nann, ChemSusChem 2015, 8, 3396–3400.
[3] M. Batmunkh, T. J. Macdonald, C. J. Shearer, M. Bat Erdene, Y. Wang, M. J. Biggs, I. P. Parkin, T. Nann, J. G. Shapter, Adv. Sci. 2017, 4, DOI 10.1002/advs.201600504.

The radiative generation and recombination of charge carriers in semiconductors control both photovoltaic and LED operation. Understanding of these processes in hybrid perovskites has advanced,^1–3 but remains incomplete.

Using femtosecond transient absorption (TA) and photoluminescence (PL) experiments on the formation of emissive states at early times after photoexcitation. We find that the PL signal rises over 2 picoseconds while initially hot photo-generated carriers cool to the band edge. This shows that PL of hot carriers is slower than that of cold carriers, as expected from strongly-allowed radiative transitions. We conclude that electrons and holes show strong overlap in momentum space, despite the potential presence of a small band offset that we model to arise from a Rashba effect.^4,5 We find that photon recycling processes further affect externally measured radiative recombination rates in hybrid perovskites. Taking into account photon recycling, we connect the externally measured radiative efficiencies with the actual internal values, and derive internal PLQEs exceeding 80%.

Fundamentally, radiative rates are controlled by the time-scales of carrier-carrier interactions, which we study with two-dimensional electronic spectroscopy with sub-10fs resolution. We report the dependence of carrier scattering rates on excess energy and carrier density and extract carrier thermalization times from 8 to 85 fs. These values allow for mobilities of up to at carrier densities lower than , and limit the electronic coherence times in lead-halide hybrid perovskites. I will discuss how the extracted characteristic carrier thermalization times are relevant for the application of hybrid perovskites in hot carrier photovoltaics.

(1) Johnston, M. B.; Herz, L. M. Hybrid Perovskites for Photovoltaics: Charge-Carrier Recombination, Diffusion, and Radiative Efficiencies. Acc. Chem. Res. 2016, 49, 146–154.
(2) Saba, M.; Cadelano, M.; Marongiu, D.; Chen, F.; Sarritzu, V.; Sestu, N.; Figus, C.; Aresti, M.; Piras, R.; Geddo Lehmann, A.; et al. Correlated Electron–hole Plasma in Organometal Perovskites. Nat. Commun. 2014, 5, 5049.
(3) Staub, F.; Hempel, H.; Hebig, J.-C.; Mock, J.; Paetzold, U. W.; Rau, U.; Unold, T.; Kirchartz, T. Beyond Bulk Lifetimes: Insights into Lead Halide Perovskite Films from Time-Resolved Photoluminescence. Phys. Rev. Appl. 2016, 6, 44017.
(4) Niesner, D.; Wilhelm, M.; Levchuk, I.; Osvet, A.; Shrestha, S.; Batentschuk, M.; Brabec, C.; Fauster, T. Giant Rashba Splitting in CH3NH3PbBr3 Organic-Inorganic Perovskite. Phys. Rev. Lett. 2016, 117, 1–6.
(5) Isarov, M.; Tan, L. Z.; Bodnarchuk, M. I.; Kovalenko, M. V; Rappe, A. M.; Lifshitz, E. Rashba Effect in a Single Colloidal CsPbBr 3 Perovskite Nanocrystal Detected by Magneto-Optical Measurements. Nano Lett. 2017, 17, 5020–5026.

The confluence of band gap modulation with the wide absorption range and spontaneous dissociation of excitons of hybrid organic-inorganic metal halide perovskites has led to the rapid evolution of efficient perovskite solar cells. The low temperature (<100 K) time-integrated photoluminescence of the methylammonium based perovskites CH₃NH₃PbI₃ and CH₃NH₃PbBr₃ revealed two well defined emission peaks, whereas formamidinium based perovskite, CH(NH₂)₂PbBr₃ showed a single emission peak throughout the entire temperature range. Furthermore, the photoluminescence (PL) of perovskites irrespective of their composition, exhibited a continuous blueshift by raising the temperature from 15 K to 300 K. Density functional theory and classical molecular dynamics simulations allowed us to assign the additional PL peak observed in methylamonium based perovskites at low temperature to the presence of molecularly disordered orthorhombic phase, and also rationalizes that the unconventional blue shift of the PL peaks with the temperature is due to the thermal expansion of the lattice. In my presentation, I will discuss the temperature-dependence of band gap and decay dynamics of photoluminescence in various perovskites using time-integrated and time-resolved photoluminescence spectroscopy.

References:

Dar, M. I. et al. submitted.
Dar, M. I. et al. Sci. Adv. 2016, 2, e1601156.
Dar, M. I. et al. Adv. Funct. Mater. 2017, 27, 1701433.

Over the last years, methylammonium lead iodide (MAPbI₃) solar cells have shown an unprecedented progress towards power conversion efficiencies beyond 20%. The light harvesting perovskite thin-films can be deposited from solution, paving the way for a low-cost fabrication. However, the toxic and water-soluble lead compounds may be one of the great obstacles to overcome on their way to market maturity. The quest for alternative, non-toxic light harvesters is partly hampered by a lack of fundamental understanding of the MAPbI₃ properties and energy conversion mechanisms.
As part of this process, the scientific community controversially discusses the importance of ferroic properties for the exceptional performance of MAPbI₃ light-harvesting layers. Some simulations have predicted ferroelectricity in MAPbI₃ with alternating polarized domains affecting the charge carrier transport. Any experimental evidence towards ferroelectricity would therefore provide helpful guidance for the quest to find non-toxic MAPbI₃ replacements.
Therefore, we performed comprehensive AFM study including Piezoresponse Force Microscopy (PFM), photo-conductive AFM (pc-AFM) under illumination and Kelvin Probe Force Microscopy (KPFM). We explore the ferroelectric properties of MAPbI₃ perovskites by vertical and horizontal PFM imaging and detected domains of alternating polarization. High-resolution and spatially resolved pc-AFM images also revealed alternating charge carrier extraction patterns which confirmed the local vertical polarization components of the ferroelectric domains. The correlation of the sample properties with atomic force and kelvin probe force micrographs evidenced the ferroelectric nature of the domains.
[H. Röhm, T. Leonhard, M.J. Hoffmann, A. Colsmann, Energy Environ. Sci. 2017, 10, 950-955]

Halide perovskites AMX₃ have shown tremendous promise for a variety of applications due to their excellent charge transport characteristics, with solar cells based on the flagship material CH₃NH₃PbI₃ attaining efficiencies above 22%. One application which has recently attracted interest is the use of perovskites as semiconductor hard radiation detectors. The first steps toward hard radiation detection have recently been achieved in CH₃NH₃PbBr₃:Cl and HC(NH₂)₂PbI₃. However, the hybrid perovskites have stability issues which have yet to be resolved, and inorganic perovskites such as CsSnBr₃ or CsSnI₃ have not yet reached the same level of success, with the possible exception of CsPbBr₃. Less explored are the defect perovskites, which have compositions A₂MX₆ or A₃M₂X₉ to allow for the substitution of tetravalent or trivalent metals M in the archetypical AMX₃ formula and subsequent charge balancing with ordered vacancies. Recently, exploration of new compositions has included the double perovskites A₂M’M’’’X₆, in which a +1 and +3 metal M’ and M’’’ form an ordered structure that is a doubling of the AMX₃ unit cell.
Among the known perovskite derivatives, the iodide defect perovskites A₃M₂I₉ (A = Cs⁺, Rb⁺; M = Sb³⁺, Bi³⁺) are of particular interest. They possess the ns² lone pair that has been a crucial component of the remarkable properties of Pb-based perovskites, and are 2D and 0D perovskite derivatives similarly based on MI₆ octahedra with greater stability than the hybrid perovskite compositions. The defect perovskites are suitable candidates for room-temperature radiation detection due to their wide bandgaps (1.9-2.2 eV), high densities, and resistivities above 10⁹ Ωcm.
Here we report on the radiation response of single crystals of the defect perovskites A₃M₂I₉ (A = Cs⁺, Rb⁺; M = Sb³⁺, Bi³⁺). All four materials respond to ²⁴¹Am 5.5 MeV alpha-particles in both hole and electron collection configurations, permitting evaluation of the charge transport for each charge carrier. The ²⁴¹Am response spectra for electrons is used to estimate the electron mobility-lifetime products, which range from 4 x 10^-6 cm²V^-1 for Rb₃Sb₂I₉ to 5 x 10^-5 cm²V^-1 in Cs₃Bi₂I₉. Hole responses yield similar values ranging from 3 x 10^-6 cm²V^-1 in Rb₃Sb₂I₉ to 3 x 10^-5 cm²V^-1 for Cs₃Bi₂I₉. The rise time of the response pulse in each material is used to estimate the mobility, which are below 10 cm²V^-1s^-1 for all four defect perovskites. The achievement of alpha particle response is remarkable given the relatively low mobilities measured here, and implies that the lifetime of charge carriers should be quite high. These results are compared with recent photoluminescence measurements indicating potential self-trapping of charge carriers, which would simultaneously reduce the mobility and enhance the lifetimes of charge carriers.

Sensitive X-ray detection is crucial for medical diagnosis, industrial inspection and scientific research. The recently emerged hybrid lead halide perovskites have demonstrated low-cost fabrication and outstanding performance for direct X-ray detection whereas they all contain toxic Pb in a soluble form. Here, we report lead-free, stable and sensitive X-ray detectors using solution processed double perovskite Cs₂AgBiBr₆ single crystals and cold isostatically pressed Cs₂AgBiBr₆ wafers. For single crystals, we largely eliminated Ag⁺/Bi³⁺ disordering and improved the crystal resistivity through thermal annealing and surface treatment, resulting in a detector with minimum detectable dose rate down to 59.7 nGy_air s^-1, comparable to the latest record of 0.036 μGy_air s^-1 using CH₃NH₃PbBr₃ single crystals. For wafers, we significantly suppressed the ionic migration and stabilized the dark current by in-situ forming two dimensional BiOBr, resulting in a detector with high voltage resistance. Our findings inform a new generation of highly efficient and low-cost X-ray detectors based on perovskite single crystals and wafers.


Reference:
[1] W. C. Pan, J. Tang et. al. Cs₂AgBiBr₆ single-crystal X-ray detectors with a low detection limit. Nature Photonics 11.11 (2017): 726

Perovskites based on sodium bismuth titanate doped with barium titanate – Na0.5Bi0.5TiO3-xBaTiO3 (abbreviated NBT-BT) are considered the most promising lead-free candidate materials to substitute Pb(Zr_1-xTi_x)O₃ (PZT) materials in devices designed to respect standards and environmental laws. Taking into account the toxicity of lead-based systems, there is an urgent need to develop environmental friendly materials and numerous lead-free piezoelectric materials are under investigation in worldwide spread laboratories for replacing PZT in future devices. However, it is difficult to match or surpass the applicability of PZT in devices such as ferroelectric access memories (FeRAMs), piezoelectric ultrasonic transducers or pyroelectric infrared (IR) sensors, due to very high dielectric, piezoelectric and pyroelectric properties coupled with a high phase transition temperature.
Structural and polar transformations in NBT-BT are more complicated than in other perovskite solid solutions, also due to the strong disorder of the A-sites occupied by Na+, Bi3+ or Ba2+ ions, with different valence, mass and ionic radius. NBT transforms successively, from the high temperature cubic paraelectric into tetragonal antiferroelectric (or ferroelectric) and further into a rhombohedral ferroelectric phase. In solid solution with BT, the ground ferroelectric phase changes from rhombohedral R3c to tetragonal ferroelectric P4mm, at the so-called morphotropic phase boundary (MPB) (x ≈ 0.06-0.07). However, despite the fact that ferroelectric materials with MPB have enhanced ferroelectric and piezoelectric properties, it is difficult to transpose them in thin films since MPB is limited to a small composition range. The goal of this study was to investigate the optical, structural, dielectric, pyroelectric and ferroelectric properties of NBT-BT thin films obtained by pulsed laser deposition as a function of composition, from pure NBT across and beyond morphotropic phase boundary (MPB) (x=0, 0.04, 0.06, 0.08). Dielectric and ferroelectric measurements were performed using an impedance analyzer HP 4294A and RT 66A Ferroelectric Test System. XRD, SEM, HR-TEM and AFM techniques have been used for morphologic and structural characterizations of NBT-BT films. Pyroelectric properties were investigated with a Woollam Variable Angle Spectroscopic Ellipsometer (VASE) system under different temperature conditions.

Organic-Inorganic Perovskites (OIPs) have demonstrated bandgap tunability over a 400-800nm range, making them attractive for optoelectronic applications. This bandgap may be tuned by substituting the halides of the OIP, which has prompted much research into the phase stability of mixed-halide OIPs. A miscibility gap was predicted for mixed I-Br OIPs, consistent with the spontaneous phase separation observed both at equal I:Br composition and under illumination, for a wider range of alloys.

This work investigates phase stability and halide migration in single-halide and mixed-halide OIPs by examining changes in the interface profiles of OIP-OIP thin film lateral heterostructures. The effects of heat, light, and defect concentration in the OIP thin films on halide migration (or lack thereof) are systematically investigated. Methylammonium (MA)PbI₃-MAPbBr₃, MAPb(I_0.88Br_0.12)₃-MAPbBr₃ and MAPb(I_0.3Br_0.7)₃-MAPbBr₃ heterostructures were synthesized by halide exchange and their interface profiles after exposure to heat/light were characterized using Energy-Dispersive X-ray analysis, optical microscopy, and confocal microscopy. None of the interface profiles changed after extreme and lengthy exposure to heat. The profile of the MAPb (I_0.3Br_0.7)₃-MAPbBr₃ interface did not change after lengthy exposure to light (1 Sun). The roles of defect concentration and interface morphology are explored as explanations for this unexpected behavior. A comparison of our results on the phase stability at lateral heterojunctions to the phase stability of alloy thin films will be presented.

We have employed a combination of first principles method and evolutionary algorithm (USPEX) to predict the crystal structure of layered hybrid organic-inorganic perovskites. We generated different fit structures by substituting the organic cations and halides. Furthermore, we predict the most stable configuration from the different polymorphs initially generated using very stringent relaxation conditions. We found stable structures for organic molecules possessing various bonding characters as well as molecular shapes. The reliability of this approach is demonstrated on a number of known structures and we found a strong correlation between the properties of the system and the bonding situation. Our investigation shows that this approach has significant potential for the study of hybrid perovskites as memristor and for solar applications.

All-inorganic cesium lead halide perovskite nanocrystals (IP-NCs) are nowadays extensively studied because of their outstanding optoelectronic properties. Being of a cubic shape and typically featuring relatively narrow size distribution, CsPbX₃ (X = Cl, Br, I) nanocrystals are the ideal starting material for the development of homogeneous thin films as required for photovoltaic and optoelectronic applications. Recent experiments reveal the spontaneous merging of drop-casted CsPbBr₃ nanocrystals, which is promoted by room humidity and can be accelerated by mild temperature treatments. This fusion can be arrested by electron beam irradiation, which induces new C=C bonds between the surface ligands making the IP-NCs more stable. Here, we make use of atom-resolved annular dark-field imaging microscopy and valence electron energy loss spectroscopy in a state-of-the-art low-voltage monochromatic scanning transmission electron microscope, to investigate the aggregation between individual nanocrystals at the atomic level. We show that the merging process preserves the elemental composition and electronic structure of CsPbBr₃, and takes place between nanocrystals of different size and orientation. In particular, we reveal seamless stitching for aligned nanocrystals, similar as reported in the past for graphene flakes. Since the crystallographic alignment occurs naturally in drop-casted layers of CsPbX₃ nanocrystals, our findings constitute the essential first step towards the development of large-area nanosheets with bandgap energies predesigned by the nanocrystal choice.

The current world demand for electricity has grown in the last decades, which has led to a boost in photovoltaic energy research. In the last few years a new type of solar cell, perovskite solar cell (PSC), has gained interest in photovoltaic community due to its high efficiency and low cost. Nowadays the most promising perovskite is the CH₃NH₃PbI₃ that presents incredible efficiencies as high as 22.1%. However, it faces a problem concerning the commercial use due to its lead content and stability. In this way, new lead-free materials have been studied in the last few years. One of it is the lead-free perovskite-like material (CH₃NH₃)₃Bi₂I₉ that can act as the absorption material in solar cells and has a better stability than CH₃NH₃PbI₃. The precursor of this material is the bismuth triiodide (BiI₃), a semiconductor material suitable as well for photovoltaic applications due to its optical bandgap of 1.67 eV [1, 2]. Usually BiI₃ is made by solution processes [3] and in this work we compared BiI₃ thin films obtained by this route with thermal evaporated BiI₃ thin films. We used a spin coated solution of BiI₃ in DMF:DMSO to make the solution process films and evaporation of BiI₃ powder in a vacuum chamber with base pressure of 2 x 10^-5 Torr to make thermal evaporated bismuth triiodide thin films. We estimated an indirect bandgap of 1.71 eV and 1.74 eV for bismuth triiodide thin films grown by thermal evaporation through ultraviolet-visible spectroscopy and photoluminescence measurements, respectively. We compared the morphology and crystallinity of BiI₃ thin films grown by thermal evaporation and solution processes through scanning electron microscope images and x-ray diffraction measurements. The results indicate that the thermal evaporated thin films are smoother than the spin coating ones, which suggests that the bismuth triiodide thin films obtained by physical routes can be a better candidate for photovoltaic applications than the bismuth triiodide thin films obtained by solution methods.
Acknowledgements: CNPq, Capes, Fapesp, Lamult Unicamp.
References:
[1] Riley E. Brandt et al. J. Phys. Chem. Lett., 6, 4297−4302 (2015)
[2] Nikolas J. Podraza et al. Journal of Applied Physics, 114, 033110 (2013).
[3] Umar H. Hamdeh. Chem. Mater. 28, 6567−6574 (2016).

Recently, halide perovskite materials, which have good photoelectric properties due to direct transition, high density of state of conduction band and intra-atomic transition, have been actively studied as photoelectric materials because of easy band tuning, low cost raw materials, low temperature process and flexibility. A complete band gap tuning characteristic by mixed halide perovskite was attractive enough to lead to investigations about tandem solar cell and LED as well as single-junction solar cell. Despite of many unprecedented results, there are still remained the critical problem such as environmental issues and long term instability. Among the various strategies to solve these problems, we focused on the development of a new composition and conducted the following research as first step. Herein, we report a phase diagram of MAPb(Br_1-x-yCl_xI_y)₃ with bandgap of each composition, 0≤x≤1 and 0≤y≤1. Optical absorption, photoluminescence, and crystallographic structure of MAPb(Br_1-x-yCl_xI_y)₃ are investigated in the forms of bulk powder which were synthesized via a simple solid-state reaction process. As the results, the ternary phase diagram having the vertex of MAPbI₃, MAPbBr₃ and MAPbCl₃ was obtained with the maps of lattice constant, energy bandgap, and photoluminescence intensity. It was found that a certain bandgap value, in the range of 1.55 - 2.9 eV, can be built up from various combinatorial compositions. These interesting results could be understood based on the first principles calculation, from the view point of smooth orbital mixing

Cesium tin iodide (CsSnI₃) perovskite is a promising candidate material for the photovoltaic application owing to its more ideal bandgap, higher thermal stability, and reduced toxicity compared with the state-of-the-art lead-based hybrid perovskites. However, CsSnI₃ suffers from the issue of extreme sensitivity to the ambient atmosphere and the difficulty in manipulating its defect properties. This makes developing efficient stable CsSnI₃-based perovskite solar cells (PSCs) a siginificant challenge. In this regard, we have rationally engineered the microstructure/composition of solution-processed CsSnI₃ thin films by employing a variety of approaches. These approaches include: i) tailoring the nucleation and grain growth of CsSnI₃; ii) modulating the tin-vacancy concentration in the film; iii) alloying of CsSnI₃ with other perovskites; and iv) functionalizing the grain boundaries confocally in CsSnI₃ thin films. Extensive materials characterization has been performed to demonstrate the validity of these approaches. A combination of these efforts by us have led to CsSnI₃-based perovskites solar cells with much improved efficiency and stability. Guidelines for further enhancing the performance of CsSnI₃ based PSCs are also provided.

Hybrid organic-inorganic halide perovskites (with chemical formula of AMX₃, where A represents a small monovalent organic molecule, M is a divalent group-IVA cation and X is a halogen anion) with the prototype material of CH₃NH₃PbI₃ have recently attracted intense interest as low-cost and high-performance photovoltaic absorbers. Despite the high power conversion efficiency exceeding 22% achieved by their solar cells, two key issues — the poor device stabilities associated with their intrinsic material instability and the toxicity due to water soluble Pb²⁺ — need to be resolved before large-scale commercialization. We used photovoltaic-functionality-directed materials screening approach to rationally design via first-principles calculations Pb-free halide perovskites. Screening criteria involve thermodynamic and crystallographic stability, as well as solar band gaps, light carrier effective masses, reasonable exciton binding, etc. We considered both single atomic substitutions in AMX₃ normal perovskites (altering A, M and X individually)[1] as well as double substitution of 2M into B+C pair in A₂BCX₆ double-perovskites [2,3]. Chemical trends in phase stabilities and optoelectronic properties are discussed with some promising cases comparable to CH₃NH₃PbI₃. Several of our proposed compounds have been verified by experimental synthesis. Meanwhile, our joint theory-experiment study indicates that highly-oriented Sn-based two-dimensional perovskites are promising Pb-free solar absorbers, showing power conversion efficiencies up to 5.94% (without the requirement of further device structure engineering) and more importantly high device stability.[4] We will also present our recent progress on design of layered Sb/Bi-based halide perovskites with promisingly enhanced solar cell performance.

Reference:
[1] Dongwen Yang, Jian Lv, Xingang Zhao, Qiaoling Xu, Yuhao Fu, Yiqiang Zhan, Alex Zunger*, and Lijun Zhang*, Chem. Mater. 29, 524 (2017).
[2] Xin-Gang Zhao, Jihui Yang, Yuhao Fu, Dongwen Yang, Qiaoling Xu, Liping Yu, Su-Huai Wei*, and Lijun Zhang*, J. Am. Chem. Soc.139, 2630 (2017).
[3] Xin-Gang Zhao, Dongwen Yang, Yuanhui Sun, Tianshu Li, Lijun Zhang*, Liping Yu, and Alex Zunger, J. Am. Chem. Soc. 139, 6718 (2017).
[4] Yuqin Liao，Hefei Liu，Wenjia Zhou，Dongwen Yang, Yuequn Shang，Zhifang Shi，Binghan Li, Xianyuan Jiang, Lijun Zhang*, Li Na Quan, Rafael Quintero-Bermudez, Brandon R. Sutherland, Qixi Mi, Edward H. Sargent, and Zhijun Ning*, J. Am. Chem. Soc. 139, 6693 (2017).

The performance of hybrid organic-inorganic perovskites (HOIP) films greatly depends on their quality. As HOIP films are usually prepared from solutions of its precursors, it is very important to study the evolution process from precursors to HOIPs. But the investigation on the growth of perovskite thin films from precursor solution is limited by their polycrystallinity. Study on single crystals can help gain a better understanding. In this work, methylammonium lead triiodide (MAPbI₃) single crystals grown from precursor solutions with different methylammonium iodide (MAI): lead iodide (PbI₂) ratios were investigated. We observed a V-shaped dependence of the onset temperature of MAPbI₃ crystallization on the MAI: PbI₂ ratio. When the MAI: PbI₂ ratio is less than 1.7, the crystallization onset temperature decreases with the increasing MAI: PbI₂ ratio. However, it increases with the increasing MAI: PbI₂ ratio at higher MAI: PbI₂ ratio. These are attributed to the effects of MAI on the supersaturation concentration of precursors and the interfacial energy of the crystal growth. At low MAI: PbI₂ ratio, more MAI can lead to the supersaturation of the precursors at a lower temperature. At high MAI: PbI₂ ratio, the crystal growing plans change from (100)-plane-dominated to (001)-plane-dominated. The latter has higher interfacial energy than the former. Hence, the interfacial energy between the crystal growing plans and the precursor solution increases with the increasing MAI solution, leading to higher crystallization onset temperature.

'Organic-Inorganic lead halide perovskite solar cells have emerged as one of the most promising thin film photovoltaic technology. However, stability of these solar cells over prolonged solar irradiation is a major concern and has not been thoroughly investigated. In this work, using lead-acetate as the source material, we have fabricated devices with the architecture, glass/ITO/PEDOT:PSS/MAPbI₃/PCBM/Ag and obtained efficiency of about 13%. We investigate the effect of adding excess lead to the active layer on the photo-induced degradation of efficiency. This work demonstrates that 5% excess Pb in devices is the optimal concentration for devices in regards to efficiency and stability and that these devices are able to retain more than 50% of the initial efficiency after 1 hour of prolonged exposure. Characterization of samples before and after illumination reveals a decrease in charge carrier lifetimes and minor changes in crystallinity and morphology as observed by X-ray diffraction and scanning electron microscopy, respectively. These findings are attributed to the formation of PbI₂ precipitates which nucleate at grain boundaries of the perovskite material. For the optimum amount of PbI₂ in the grain boundary, there is an alteration of the band structure at the active layer and charge transport layer interface. For the optimum concentration of 5 mol% excess Pb, the device retains the initial efficiency under prolonged illumination.

Metal halide perovskites (MHPs) are revolutionizing the solar cell research field - the record power conversion efficiency of MHPs based solar cells has reached 22%, which rivals that of silicon solar cells. This represents the highest efficiency among all solution processable materials and the fastest rate of efficiency improvement in the history of all photovoltaic materials. Based on this trend, MHPs have been called the “next big thing in photovoltaics” and worldwide research efforts have grown explosively.
Despite the impressive solar cell performance demonstrations, the microscopic mechanisms of the high performance are poorly understood, precluding more rational progress toward further increase in efficiency. In this talk, I will present our work that employed a combination of temperature dependent neutron scattering, X-ray diffraction and time resolved optical spectroscopy to study the role of organic cations in MHPs in determining the optoelectronic properties relevant for photovoltaic performance. Our findings show that the rotation of organic cations provides a major contribution to long charge carrier lifetime responsible for the high solar cell efficiency. Implications of our results in achieving improved performance with MHP solar cells will be discussed.

Organic-inorganic halide perovskite solar cells have demonstrated high power conversion efficiencies in recent years, yet operational instability and lack of durability are the major concerns preventing their commercialization at large scale. Previous studies reported a variety of factors that cause the degradation of perovskite materials and devices, including moisture, heat, oxygen, light illumination, and electric filed. However, to mitigate the stability issues, the fundamental understanding of physicochemical processes and degradation kinetics of perovskite materials are needed. Here, by coupling a temperature programmed desorption (TPD) system with a spectrally selective light source, we measure in-situ mass spectrometry of the evolved gas species during the thermal- and photo-induced degradation of organic-inorganic halide perovskites. We show that the volatile species such as ammonium (NH₃), aminocarbyne fragments (CNH₂), hydrogen (H₂), and iodine/hydrogen iodide (I/HI) are released from methylammonium lead iodide (MAPbI₃) or formamidinium lead iodide (FAPbI₃) at different rates under 1 Sun of simulated solar illumination. Among them, I/HI vapors severely deteriorate perovskite films due to irreversible chemical chain reactions. The incorporation of Cs⁺ effectively suppresses the photoelectrosynthesis of I/HI gases and thus improves the photostability of the mixed-cation perovskites. Additionally, we investigate the dependence of photodegradation kinetics on incident photon energy by applying UV filters with different cutoff wavelengths to the light source. The results explain the interplay between the UV photons and photostability of perovskite films and elucidate the degradation mechanisms. Our findings suggest that compositional engineering of perovskite materials and UV filtering can prevent the photodegradation of the organic components and thus increase operational and long-term stability of organic-inorganic halide perovskite solar cells.

Hybrid organic-inorganic metal halides (HOIMH) are a large family of hybrid materials that consist of 3D, 2D, 1D, or 0D anionic metal halide framework and organic cations. The greater structural flexibility of the lower-dimensional HOIMH allows the incorporation of various inorganic structures (such as corner-, edge-, face-shared octahedra, tetrahedra, etc.) and organic cations [with different sizes and different type of bonding (σ or π bonding)]. Electronic structure calculations of a series of low-dimensional HOIMH show that the large compositional and structural space can be explored to engineer the band alignment at the interface. Excitons can be localized at either the organic or the inorganic component for specific optical applications (such as lighting and radiation detection).

The 0D HOIMH, in which the metal-halide anions are spatially separated by organic or inorganic cations, exhibit strong exciton self-trapping, which may lead to efficient exciton emission. Photoluminescence quantum efficiency (PLQE) higher than 90% at room temperature has been reported for several 0D HOIMH, including (C₄N₂H₁₄X)₄SnX₆ (X = Br, I). On the other hand, inorganic Cs₄PbBr₆ exhibits UV emission, which is quenched at T > 100 K, and strong green emission, whose origin is still under debate. Hybrid density functional calculations on (C₄N₂H₁₄X)₄SnX₆ (X = Br, I) and Cs₄PbBr₆ show large exciton binding energies and exothermic exciton trapping at deep halogen vacancy levels inside the band gap. The calculated excitation and emission energies of excitons are in good agreement with experimental values. Our results suggest that suppressing exciton migration and the subsequent energy loss at defects are critical for efficient luminescence. The thermally-activated exciton migration should be limited by the large exciton binding energy and the weak coupling between inorganic metal-halide octahedra, which are the luminescent centers. However, fast exciton migration may occur through the resonant exchange of the excitation energy provided that the conditions of the wavefunction overlap between luminescent centers and the spectral overlap between excitation and emission are both satisfied. These conditions can be prevented by incorporating large molecular cations in 0D HOIMH, which suppress the electronic coupling between luminescent centers and allow more extended exciton relaxation and consequently large Stokes shift that prevents the spectral overlap. Our results explain the high PLQE observed in (C₄N₂H₁₄X)₄SnX₆ (X = Br, I) and the thermal quenching of luminescence in Cs₄PbBr₆. The frequently observed green luminescence in Cs₄PbBr₆ is likely the result of exciton emission from CsPbBr₃ inclusions within the bulk of Cs₄PbBr₆.

Commercial deployment of perovskite solar cells (PSCs) requires extending their service lifetimes from months to years.¹ Grain boundaries exacerbate perovskite material degradation through several different mechanisms. The degradation rate of PSCs due to water exposure is linearly dependent on perovskite crystal size.² Similarly, grain size and uniformity are correlated to extent of degradation by light and oxygen.³ Additionally, smaller grain sizes and reduced crystallinity increase ion migration.⁴

To mitigate these issues, the research community is attempting to grow thin (~200 nm), wafer-scale (~10 cm x 10 cm), perovskite crystals. Chen et al. recently grew small monocrystalline perovskite films (on the scale of mm x mm x nm) by drawing precursor solution up between two substrates via capillary action and heating to crystallize.⁵ This unusual precursor deposition method allows the absorber layer to be the last step in device fabrication, which increases the number of possible materials for charge transport layers, contacts, and substrates. Lee et al. recently fabricated monocrystalline perovskite films (cm x µm x nm) by intensely heating concentrated precursor solution at the deposition point, which resulted in instant crystallization.⁶

We demonstrate the growth of large (cm x cm x nm) monocrystalline perovskite films by combining the capillary precursor deposition method with intense local heating. We will report the effects of precursor concentration, temperature, substrate roughness, and substrate separation distance on morphology and crystallinity. Film surface area and crystallinity are assessed via confocal optical microscopy and x-ray diffraction, respectively, while film thickness is determined by atomic force microscopy. Film homogeneity is further characterized via submicron-scale photoluminescence and Raman mapping. These studies will improve understanding and control of perovskite film growth, and help enable commercial deployment of PSCs.

References
1. Chang, N.L.; Ho-Baillie, A.W.Y.; Vak, D.; Gao, M.; Green, M.A.; Egan, R.J. Sol. Energy Mater. Sol. Cells 2018, 174, 314-324.
2. Wang, Q.; Chen, B.; Liu, Y.; Deng, Y.; Bai, Y.; Dong, Q.; Huang, J. Energy Environ. Sci. 2017, 10, 516-522.
3. Sun, Q.; Fassl, P.; Becker-Koch, D.; Bausch, A.; Rivkin, B.; Bai, S.; Hopkinson, P.E.; Snaith, H.J.; Vanyzof, Y. Adv. Energy Mater. 2017, 7, No. 1700977.
4. Hu, M.; Bi, C.; Yuan, Y.; Bai, Y.; Huang, J. Adv. Sci. 2016, 3, No. 1500301.
5. Chen, Y.-X.; Ge, Q.-Q.; Shi, Y.; Liu, J.; Xue, D.-J.; Ma, J.-Y.; Ding, J.; Yan, H.-J.; Hu, J.-S.; Wan, L.-J. J. Am. Chem. Soc. 2016, 138, 16196-16199.
6. Lee, L.; Baek, J.; Park, K.S.; Lee, Y.-E.; Shrestha, N.K.; Sung, M.M. Nat. Commun. 2017, 8, No. 15882.

The highest power conversion efficiency (PCE) of perovskite solar cells (PSCs) employed the regular (n-i-p) architecture which has PCE of over 22%, but this is achieved by a thick layer of mesoporous TiO₂ which require high-temperature post-deposition annealing. In this work, we employed the inverted planar (n-i-p) perovskite solar cell architecture using a low-temperature annealing process. The hole transport layer is poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT: PSS); methylammonium iodide (MAI) and dehydrated lead acetate were the precursor materials for the perovskite layer and phenyl –C61-butyric acid methyl ester (PCBM) as the electron transport layer. In this work, we vary the concentrations of the methylammonium iodide and dehydrated lead acetate and we also investigated the effect of post-deposition annealing temperature of the perovskite active layer on the photovoltaic performance. We obtained power conversion efficiency (PCE) over 12% at annealing temperatures lower than 100^oC.

Methylammonium lead iodide exhibits spectacular photovoltaic performance with solar cell efficiencies over 19%. However, there remains significant controversy over the existence of ferroelectricity and its impact on the photovoltaic response in these materials. In this work, we definitively confirm ferroelectricity in methylammonium lead iodide using single crystals. This study employs d₃₃ Berlincourt piezoelectric measurements, band excitation piezoresponse force microscopy with concurrent contact Kelvin probe force microscopy, and temperature dependent Rayleigh analysis. Large signal poling greater than 16 V/cm induced permanent macroscopic ferroelectric domains (up to 40 µm wide, and mm in length), which show preferential stabilization over a period of weeks and a distinguishable domain specific electronic response. The impact of the ferroelectric domains on the opto-electronic response was characterized through X-ray photoemission spectroscopy, scanning microwave impedance microscopy and electric force microscopy. The ability to control the ferroelectric response provides routes to increase both device stability and improve photovoltaic performance through domain engineering.

Lead halide perovskite semiconductors have been studied intensively with respect to application as active layers in solar cells and light-emitting diodes, because of their excellent optoelectronic properties. Particularly, the solution-based synthesis method that became available for this material class, offers a cost-effective production of photonic devices with high-performance owing to a direct band-gap with strong radiative band-to-band recombination, a high crystal quality with extremely low defect/trap densities, and photon recycling [1-3]. In addition to solution-based growth of thin films and single crystals, colloidal synthesis enables preparation of novel perovskite nanocrystals. Their experimentally verified high photoluminescence quantum yields and wavelength-tunable luminescence that covers the entire visible spectrum make them promising candidates for light-emitting diodes, lasers, and single photon sources. A detailed knowledge about the exciton dynamics in single nanocrystals is essential to design such new devices. Here, we summarize optical properties of organic-inorganic hybrid perovskite FAPbBr3 and all-inorganic halide perovskite CsPbBr3 nanocrystals. Their exciton dynamics were studied by femtosecond transient-absorption and single-dot spectroscopy. From the simultaneous measurement of the second-order photon correlation g(2) and photoluminescence-decay curves of single nanocrystals [4], we evaluated the luminescence quantum yields of biexcitons. The exciton and charged-exciton lifetimes were determined as a function of the absorption cross-section of the single nanocrystals. We discuss the characteristic decay dynamics of excitons, charged excitons, and biexcitons in lead halide perovskite nanocrystals [5,6].
The author would like to thank many colleagues and his students for their contributions and discussions. Part of this work was supported by JST-CREST (Grant No. JPMJCR16N3).

[1] Y. Yamada et al., J. Am. Chem. Soc. 136, 11610 (2014).
[2] Y. Yamada et al., J. Am. Chem. Soc. 137, 10456 (2015).
[3] T. Yamada et al., Phys. Rev. Applied 7, 014001 (2017).
[4] N. Hiroshige et al., Phys. Rev. B 95, 245307 (2017).
[5] N. Yarita et al., J. Phys. Chem. Lett. 8, 1413 (2017).
[6] H.-C. Wang et al., Angew. Chem. Int. Ed. 56, 13650 (2017).

The astonishing performance of lead-halide perovskites in optoelectronic devices is due to a unique combination of highly efficient generation, transport and collection of charge carriers. The initial stage of charge generation, i.e. exciton dissociation, has attracted a huge interest. Experimental estimates of the exciton binding energy (Eb) in MAPbI3 lie in the 6-25 meV range, implying an efficient exciton dissociation at room temperature. Despite its pivotal importance, the physical mechanisms beyond efficient charge separation in lead-halide perovskites are still largely unknown and no quantitative models of how the various dielectric contributions (electronic, vibrational, pseudo-rotational) affect Eb in lead-halide perovskites have been reported.
Here, we investigate the factors which influence the exciton binding energy in a series of metal-halide perovskites using accurate first-principles calculations coupled to ab initio molecular dynamics simulations. By introducing a novel computational approach to self-consistently incorporate the effect of phonon screening into a full relativistic Bethe Salpeter equation framework, we effectively simulate the electronic and vibrational contributions affecting the exciton binding energy for the parent MAPbI3 compound. By calculating Eb for a few snapshots along an ab initio MD trajectory we verify that dynamic disorder does not significantly affect the exciton binding energy. We also use a phenomenological model to approximately account for exciton screening due to MA cations rotation, finding that this contribution is irrelevant for models delivering realistic values of the static dielectric constant. We show, on the other hand, that Eb is strongly modulated by screening from low-frequency phonons, which accounts for a factor 2 reduction in the converged Eb value. Our phonon-screened, full relativistic estimate of Eb for MAPbI3 amounts to 15 meV, in agreement with current experimental estimates. Based on this background, we then explore how different material combinations (e.g. Pb -> Pb:Sn-> Sn; I -> Br; MA -> FA -> Cs) can be optimized to tune the value of Eb.

Organometal halide perovskite semiconductors have emerged as promising candidates for
optoelectronic applications because of the outstanding charge carrier transport properties,
achieved with low-temperature synthesis. Here, we present highly sensitive sub-bandgap
external quantum efficiency (EQE) measurements of Au/spiro-OMeTAD/
CH₃NH₃Pb(I_1−xBr_x)₃ /TiO₂ /FTO/glass photovoltaic devices. The room-temperature spectra
show exponential band tails with a sharp onset characterized by low Urbach energies (E_u)
over the full halide composition space. All CH₃NH₃Pb(I_1−xBr_x)₃ compositions, with
corresponding bandgaps of 1.6 ≤ E_g ≤ 2.3 eV, exhibit Urbach energies in the range of 15 -23
meV, lower than those for most semiconductors with similar bandgaps (especially with E_g >
1.9 eV). Intentional aging of CH₃NH₃Pb(I_1−xBr_x)₃ for up to 2300 h, reveals no change in E_u,
despite the appearance of the PbI₂ phase due to decomposition, and confirms a high degree of
crystal ordering. Moreover, sub-bandgap EQE measurements reveal an extended band of subbandgap electronic states that can be fit with one or two point defects for pure CH₃NH₃PbI₃ or
mixed CH₃NH₃Pb(I_1−xBr_x)₃ compositions, respectively. The study presents experimental
evidence of defect states close to midgap that could impact photocarrier recombination and
energy conversion efficiency in higher bandgap CH₃NH₃Pb(I_1−xBr_x)₃ alloys. The combination
of sub-bandgap EQE and photoluminescence-based studies provides a future path toward
assessing the roles of synthesis and processing on efficiency-limiting recombination centers in
the material.

Perovskite nanocrystals have emerged as an interesting material for light-emitting and other optoelectronic applications. Excitons are known to play an important role in determining the optical properties of these nanocrystals and their energetic levels as well as quantization properties have been extensively explored. Despite this, there are still many aspects of perovskites that are still not well known, e.g. the homogeneous and inhomogeneous linewidths of the energetic transitions, quantities that cannot be directly extracted by linear absorption optical spectroscopy on nanocrystal ensembles. Here, we present temperature-dependent absorption and four-wave mixing (FWM) experiments on thick methylammonium lead iodide (MAPI) perovskite nanoplatelets exhibiting bulk-like absorption and emission spectra. Dephasing times T₂ of excitons are determined to lie in the range of several hundreds of femtoseconds at low temperatures. This value enables us to distinguish between the homogeneous and inhomogeneous contribution to the total broadening of the excitonic transitions. These turn out to be predominantly inhomogeneously broadened at low temperatures and homogeneously broadened at room temperature. Furthermore, we find excitonic quantum beats, which allow for the determination of the exciton binding energy and we extract E_B = 25±2 meV in the low temperature regime, in good agreement with other reports.

Spin-orbit coupling and the resulting spin splittings in the band structure of lead halide perovskites were proposed to strongly influence the optical and transport properties of these novel materials. Calculations predict that Rashba-type spin splittings strongly enhance carrier lifetimes and electron-acoustic-phonon scattering. While spin-orbit coupling is a necessary consequence of the contribution of the Pb 6p orbitals to the conduction bands, few studies are available that determine the actual strength of the resulting Rashba splitting.
The most direct method to probe the electronic structure of a solid is angle-resolved photoelectron spectroscopy (ARPES). The technique is surface-sensitive with an information depth in the nm range and thus requires high-quality crystalline surfaces. ARPES measurements on (CH₃NH₃)PbBr₃ single crystals cleaved in ultrahigh vacuum show circular valence-band maxima centered around the high-symmetry points, characteristic for a Rashba system. This interpretation is supported by the circular dichroism found in laser-based ARPES experiments. The extracted Rashba parameters are amongst the largest reported to date, with a Rashba energy of 160 meV (240 meV) in the low-temperature orthorhombic (room-temperature cubic) phase. These values are significantly larger than the ones found in calculations for the bulk material. The difference can be attributed to the breaking of the bulk crystal symmetry at the surface, resulting in additional fields which increase the Rashba splitting. The symmetry-breaking goes in hand with a preferential orientation of the long axis of the unit cell along the surface normal in the orthorhombic phase. The possibility of a Rashba splitting in the bulk material is investigated using photoluminescence and photocurrent spectroscopies. Both measurements find two optical transitions close to the band gap, consistent with a direct-indirect character of the band gap. The splitting between the two is larger in the low-temperature orthorhombic phase than in the room-temperature cubic one. For the lower-energy transitions, photocurrents induced in orthorhombic (CH₃NH₃)PbI₃ by circularly polarized light depend on the helicity of the photons. This phenomenon is known as the circular photogalvanic effect. It implies a band structure with spin-splittings, in which spin currents are optically induced.
The strong Rashba splitting at the surface of (CH₃NH₃)PbBr₃ and the possibility to optically induce spin currents in (CH₃NH₃)PbI₃ make lead halide perovskites candidates for opto-spintronics applications. The findings also point out the importance of spin effects in understanding the exceptional photophysics in this class of materials.

Perovskite halides display complex thermal transport properties due to the existence of lattice instabilities and phase transitions inherent to the perovskite structure, as well as disparate phonon frequencies in the case of hybrid organic-inorganic perovskite halides. In this talk, we will discuss our work on using first principles modeling to predict phonon renormalization, phonon-phonon interaction, and electron-phonon equilibration in perovskite halides, in conjunction with ultrafast spectroscopy measurements.

References:
1. P. Guo, Y. Xia, J. Gong, C. C. Stoumpos, K. M. McCall, G. C. B. Alexander, H. Zhou, J. B. Ketterson, M. G. Kanatzidis, T. Xu, M. K. Y. Chan, R. D. Schaller, “Polar Domains in Metal-Halide Perovskites Uncovered by Acoustic Phonon Anomalies,” American Chemical Society Energy Letters, 2017. DOI:10.1021/acsenergylett.7b00790.
2. A. Y. Chang, Y.-J. Cho, K.-C. Chen, C.-W. Chen, A. Kinaci, M. K. Y. Chan, H.-W. Lin, R. D. Schaller, “Evidence of Slow Organic-to-Inorganic Sub-Lattice Thermalization in Methylammonium Lead Halide Perovskites,” Advanced Energy Materials, 6, 1600422 (2016). DOI:10.1002/aenm.201600422.

Dresselhaus-Rashba semiconductors with a strong spin-orbit interaction are actively studied for applications in spintronic devices [1]. In semiconductors that lack spatial inversion symmetry, bulk spin-orbit coupling becomes odd in the electron's momentum, which gives rise to a splitting of the spin sub-bands in energy. Application of this effect to solar cells can lead to the development of new high-efficiency materials. Such materials are expected to combine benefits of direct and indirect band gap semiconductors, namely a very long lifetime of optical excitations and the strong optical absorption.

It is believed that an exceptionally long lifetime of optical excitations in hybrid halide perovskites is associated with the presence of a Dresselhaus-Rashba splitting at the band edges [2,3]. In this presentation, I will analyze the structural origin and chemical trends of the Dresselhaus-Rashba effect in perovskites, robustness of the splitting with respect to a dynamic disorder, as well as consequences of the spin texture for the radiative recombination of optical excitations.

[1] A. Manchon, H. C. Koo, J. Nitta, S. M. Frolov, and R. A. Duine, Nat. Mater. 14, 871 (2015).
[2] O. Rubel and A. Bokhanchuk, arXiv 1508.03612 (2015).
[3] D. Niesner, M. Wilhelm, I. Levchuk, A. Osvet, S. Shrestha, M. Batentschuk, C. Brabec, and T. Fauster, Phys. Rev. Lett. 117, 1 (2016).