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Tunable Polaron Distortions Control the Extent of Halide Demixing in Lead Halide Perovskites
Photoinduced phase separation in mixed halide perovskites emerges from their electro-mechanical properties and high ionic conductivities, resulting in photoinduced I$^-$ rich charge carrier traps that diminish photovoltaic performance. Whether photoinduced phase separation stems from the polycrystalline microstructure or is an intrinsic material property has been an open question. We investigate the nanoscale photoinduced behavior of single-crystal mixed Br$^-$/I$^-$ methylammonium (MA + ) lead halide perovskite (MAPb(Br x I 1- x ) 3 ) nanoplates, eliminating effects from extended structural defects. Even in these nanoplates, we find that phase separation occurs, resulting in I$^-$rich clusters that are nucleated stochastically and stabilized by polarons. Upon lowering the electron-phonon coupling strength by partially exchanging MA + for Cs + , a phase-separated steady state is not reached, nevertheless transient I$^-$ clustering still occurs. Our results, supported by multiscale modeling, demonstrate that photoinduced phase separation is an intrinsic property of mixed halide perovskites, the extent and dynamics of which depends on the electron-phonon coupling strength.
Halide exchange and surface modification of metal halide perovskite nanocrystals with alkyltrichlorosilanes
Metal halide perovskite nanocrystals have recently emerged as promising materials for light emitting displays and lasing applications due to their narrow emission wavelengths, high photoluminescence quantum yields, and readily adjustable emission wavelengths. For these metal halide perovskite nanocrystals to be useful in commercial applications, their stability must be increased and the photoluminescence quantum yields of the iodide (red emitting) and chloride (blue emitting) containing derivatives must also be increased. The photoluminescence quantum yields of blue emitting CsPbCl 3 nanoparticles lag behind those of green emitting CsPbBr 3 nanoparticles, with maximum photoluminescence quantum yields of 1–10% previously reported for CsPbCl 3 as compared to 80–100% for CsPbBr 3 . Herein, we show that alkyltrichlorosilanes (R-SiCl 3 ) can be used as Cl-sources for rapid anion exchange with host CsPbBr 3 nanocrystals. This anion exchange reaction is advantageous in that it can be performed at room temperature and results in highly dispersible nanoparticles coated with siloxane shells. CsPbCl 3 nanoparticles produced through Cl-exchange with R-SiCl 3 show significantly improved long-term stability and high photoluminescence quantum yields of up to 12%. Finally, these siloxane coated nanocrystals are even stable in the presence of water, whereas CsPbCl 3 nanoparticles synthesized through other routes rapidly degrade in the presence of water.
Cohesive Energy-Lattice Constant and Bulk Modulus-Lattice Constant Relationships: Alkali Halides, Ag Halides, Tl Halides
In this note we present two expressions relating the cohesive energy, E(sub coh), and the zero pressure isothermal bulk modulus, B(sub 0), of the alkali halides. Ag halides and TI halides, with the nearest neighbor distances, d(sub nn). First, we show that the product E(sub coh)d(sub 0) within families of halide crystals with common crystal structure is to a good approximation constant, with maximum rms deviation of plus or minus 2%. Secondly, we demonstrate that within families of halide crystals with a common cation and common crystal structure the product B(sub 0)d(sup 3.5)(sub nn) is a good approximation constant, with maximum rms deviation of plus or minus 1.36%.
Elucidating the role of anionic chemistry towards high-rate intermediate-temperature Na-metal halide batteries
Abstract not provided
Two-Dimensional Halide Perovskites Incorporating Straight Chain Symmetric Diammonium Ions, (NH 3 C m H 2 m NH 3 )(CH 3 NH 3 ) n -1 Pb n I 3 n +1 ( m = 4–9; n = 1–4)
We report that low-dimensional halide perovskites have recently attracted intense interest as alternatives to the three-dimensional (3D) perovskites because of their greater tunability and higher environmental stability. Herein, we present the new homologous 2D series (NH 3 C m H 2m NH 3 )(CH 3 NH 3 ) n-1 Pb n I 3n+1 (m = 4–9; n = 1–4), where m represents the carbon-chain number and n equals layer-thickness number. Multilayer (n > 1) 2D perovskites incorporating diammonium cations were successfully synthesized by the solid-state grinding method for m = 4 and 6 and by the solution method for m = 7–9. Structural characterization by single-crystal X-ray diffraction for the m = 8 and m = 9 series (n = 1–4) reveals that these compounds adopt the Cc space group for even n members and Pc for odd n members. The optical bandgaps are 2.15 eV for two-layer (n = 2), 2.01 eV for three-layer (n = 3), and 1.90 eV for four-layer (n = 4). The materials exhibit excellent solution processability, and casting thin-films of the n = 3 members was successfully accomplished. The films show a clear tendency for the higher-m members to have preferred orientation on the glass substrate, with m = 8 exhibiting almost perfect vertical layer orientation and m = 9 displaying both vertical and parallel layer orientation, as confirmed by grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements. Lastly, the vertical layer orientation for the (NH 3 C 8 H 16 NH 3 )(CH 3 NH 3 ) 2 Pb 3 I 10 member results in the best thermal, light, and air stability within this series, thus showing excellent potential for solar cell applications.
Intrinsic Halide Immiscibility in 2D Mixed-Halide Ruddlesden–Popper Perovskites
Halide alloying of 3D and 2D Ruddlesden–Popper lead halide perovskites (RPPs) allows their bandgaps to be fine-tuned for optoelectronic applications. Many studies of mixed-halide RPPs assume that halide mixing yields homogeneous alloys like in 3D lead halide perovskites, with halide segregation only occurring under perturbations like light or heat. Here we carefully investigate the mixed I/Br phases in solution grown microplates of three representative n = 1 and n = 2 RPP phases to reveal heterogeneous halide microscale domains – even in absence of photoinduced phase separation. Such halide immiscibility is revealed by X-ray diffraction and heterogeneity is confirmed by secondary ion mass spectrometry (SIMS) imaging, however, comparison with photoluminescence (PL) imaging results show how PL information alone can falsely imply homogeneous alloy formation in heterogeneous RPPs. Finally, we then demonstrate the use of spectral imaging as an alternative high-throughput tool for accurate halide phase characterization in mixed-halide RPPs.
Bandgap calculations and trends of organometal halide perovskites
Energy production from the Sun requires a stable efficient light absorber. Promising candidates in this respect are organometal perovskites (ABX{sub 3}), which have been intensely investigated during the last years. Here, we have performed electronic structure calculations of 240 perovskites composed of Cs, CH{sub 3}NH{sub 3}, and HC(NH{sub 2}){sub 2} as A-cation, Sn and Pb as B-ion, and a combination of Cl, Br, and I as anions. The calculated gaps span over a region from 0.5 to 5.0 eV. In addition, the trends over bandgaps have been investigated: the bandgap increases with an increase of the electronegativities of the constituent species, while it reduces with an increase of the lattice constants of the system.
Photoinduced Phase Segregation in Mixed Halide Perovskites: Thermodynamic and Kinetic Aspects of Cl–Br Segregation
Not provided.
Distinguishing Models for Mixed Halide Lead Perovskite Photosegregation via Terminal Halide Stoichiometry
Photoinduced halide segregation in mixed halide hybrid perovskites [i.e., APb(I 1–x Br x ) 3 ] represents an intrinsic instability that impedes their commercialization. Resolving this issue requires developing a microscopic understanding of the phenomenon. Key to this is distinguishing existing models of halide photosegregation by comparing their corresponding predictions to experiment. In this work, we test the temperature dependency of predicted perovskite terminal stoichiometries (x terminal ), following photosegregation, in single and double cation mixed halide perovskites. Our results show a general temperature invariance of x terminal . This largely supports the idea that band gap difference between parent and halide-segregated perovskite phases drives photosegregation. Beyond this, careful examination of temperature- and halide composition-dependent emission energies suggests an alternative explanation for the enhanced photostability of mixed cation/mixed halide perovskites, linked to band gap energy differences between parent and phase-segregated perovskite phases. Together, our results make inroads in clarifying the microscopic origin of mixed halide perovskite photosegregation and pave the way toward approaches that stabilize their use in applications.
Halogen Redox Shuttle Explains Voltage-Induced Halide Redistribution in Mixed-Halide Perovskite Devices
Voltage-induced halide segregation greatly limits the optoelectronic applications of mixed-halide perovskite devices, but a mechanistic explanation behind this phenomenon remains unclear. In this work, we use electron microscopy and elemental mapping to directly measure the halide redistribution in mixed-halide perovskite solar cells with quasi-ion-impermeable contact layers under different bias polarities to find iodide and bromide accumulation at the cathode and anode, respectively. This is consistent with a mechanism based on preferential iodide oxidation at the anode, leading to unbalanced I$^{+}_{i}$, I$^{-}_{X}$, and Br$^{-}_{X}$ fluxes. Importantly, switching the anode from "inert" Au to "active" Ag prevents segregation because Ag oxidation precludes the oxidation of lattice iodide, which suggests employing redox-active additives as a general strategy to suppress halide segregation. Finally, these results show that halide perovskite devices operate as solid-state electrochemical cells when threshold voltages are exceeded, providing fresh insight to understand the impacts of voltage bias on halide perovskite devices.
Photoinduced Halide Segregation in Ruddlesden–Popper 2D Mixed Halide Perovskite Films
2D lead halide perovskites, which exhibit bandgap tunability and increased chemical stability, have been found to be useful for designing optoelectronic devices. Reducing dimensionality with decreasing number of layers (n = 10–1) also imparts resistance to light-induced ion migration as seen from the halide ion segregation and dark recovery in mixed halide (Br:I = 50:50) perovskite films. The light-induced halide ion segregation efficiency, as determined from difference absorbance spectra, decreases from 20% to <1% as the dimensionality is decreased for 2D perovskite film from n = 10 to 1. The segregation rate constant (ksegregation), which decreases from 5.9 × 10 –3 s –1 (n = 10) to 3.6 × 10 –4 s –1 (n = 1), correlates well with nearly an order of magnitude decrease observed in charge-carrier lifetime (τ average = 233 ps for n = 10 vs τ avg = 27 ps for n = 1). The tightly bound excitons in 2D perovskites make charge separation less probable, which in turn decreases the halide mobility and resulting phase segregation. Furthermore, the importance of controlling the dimensionality of the 2D architecture in suppressing halide ion mobility is discussed.
Iodine Electrochemistry Dictates Voltage‐Induced Halide Segregation Thresholds in Mixed‐Halide Perovskite Devices
Abstract Owing to straightforward stoichiometry–bandgap tunability, mixed‐halide perovskites are ideal for many optoelectronic devices. However, unwanted halide segregation under operational conditions, including light illumination and voltage bias, restricts practical use. Additionally, the origin of voltage‐induced halide segregation is still unclear. Herein, a systematic voltage threshold study in mixed bromide/iodide perovskite devices is performed and leads to observation of three distinct voltage thresholds corresponding to the doping of the hole transport material (0.7 ± 0.1 V), halide segregation (0.95 ± 0.05 V), and degradation (1.15 ± 0.05 V) for an optically stable mixed‐halide perovskite composition with a low bromide content (10%). These empirical threshold voltages are minimally affected by composition until very Br‐rich compositions, which reveals the dominant role of iodide/triiodide/iodine electrochemistry in voltage‐induced Br/I phase separation and transport layer doping reactions in halide perovskite devices.
Giant Apparent Optical Circular Dichroism in Thin Films of Bismuth-Based Hybrid Organic-Inorganic Metal Halide Semiconductor Through Preferred Orientation
Introducing chirality into organic/inorganic hybrid materials can impart chiroptical properties such as circular dichroism. The ability to tune chiroptical properties in self-assembled materials can have important implications for spintronic and optoelectronic applications. Here, a chiral organic cation, (R/S)-4-methoxy-a-methylbenzylammonium, is incorporated to synthesize the bismuth-based hybrid organic-inorganic metal halide semiconductor, (R/S-MeOMePMA)BiI4. Thin films of this Bi-based compound demonstrate large chiroptical responses, with circular dichroism anisotropy (gCD) values up to ˜0.1, close to the highest value observed in another chiral metal-halide semiconductor, (R-MBA2CuCl4). Detailed investigation reveals that this large gCD in (R/S-MeOMePMA)BiI4 is caused by the apparent CD effect. Careful selection of deposition conditions and the concomitant thin-film orientation enables the control of gCD, with maximum value observed when its thin film has a well-crystallized preferred (001) orientation parallel to the substrate. The results support a growing body of evidence that low symmetry plays an important role in achieving unusually large gCD in these chiral metal-halide materials and provides design rules for achieving large chiroptical response via morphology control.
Equations of state of anhydrous AlF 3 and AlI 3 : Modeling of extreme condition halide chemistry
Abstract not provided
Reducing Surface Recombination Velocity of Methylammonium-Free Mixed-Cation Mixed-Halide Perovskites via Surface Passivation
We control surface recombination in the mixed-cation, mixed-halide perovskite, FA0.83Cs0.17Pb(I0.85Br0.15)3, by passivating nonradiative defects with the polymerizable Lewis base (3-aminopropyl)trimethoxysilane (APTMS). We demonstrate average minority carrier lifetimes >4 µs, nearly single exponential monomolecular photoluminescence decays, and high external photoluminescence quantum efficiencies (>20%, corresponding to ~97% of the maximum theoretical quasi-Fermi-level splitting) at low excitation fluence. We confirm both the composition and valence band edge position of the FA0.83Cs0.17Pb(I0.85Br0.15)3 perovskite using multi-institutional, cross-validated, X-ray photoelectron spectroscopy and UV photoelectron spectroscopy measurements. We extend the APTMS surface passivation to higher bandgap double-cation (FA and Cs) compositions (1.7, 1.75, and 1.8 eV) as well as the widely used triple-cation (FA, MA, and Cs) composition. Finally, we demonstrate that the average surface recombination velocity decreases from ~1000 to ~10 cm/s post APTMS passivation for FA0.83Cs0.17Pb(I0.85Br0.15)3. Our results demonstrate that surface-mediated recombination is the primary nonradiative loss pathway in many methylammonium (MA)-free mixed-cation mixed-halide films with a range of different bandgaps, which is a problem observed for a wide range of perovskite active layers and reactive electrical contacts. Our study also provides insights to develop passivating molecules that help reduce surface recombination in MA-free mixed-cation mixed-halide films and indicates that surface passivation and contact engineering will enable near-theoretical device efficiencies with these materials.
Impact of Surfaces on Photoinduced Halide Segregation in Mixed-Halide Perovskites
Photoinduced halide segregation currently limits the perovskite chemistries available for use in high-bandgap semiconductors needed for tandem solar cells. Here, we study the impact of post-deposition surface modifications on photoinduced halide segregation in methylammonium lead mixed-halide perovskites. By coating a perovskite surface with the electron-donating ligand trioctylphosphine oxide (TOPO), we are able to both reduce nonradiative recombination and dramatically slow the onset of halide segregation in CH 3 NH 3 PbI 2 Br films. This result, coupled with the observation that the rate of halide segregation can be tuned by varying the selective contact, provides a direct link between surface modifications and photoinduced trap formation. On the basis of these observations, we discuss possible mechanisms for photoinduced halide segregation supported by drift-diffusion simulations. In conclusion, this work suggests that improved understanding and control of perovskite surfaces provides a pathway toward stable and high-performance wide-bandgap perovskites for the next generation of tandem solar cells.