Lattice distortion inducing exciton splitting and coherent quantum beating in CsPbI3 perovskite quantum dots

Anisotropic exchange splitting in semiconductor quantum dots results in bright-exciton fine-structure splitting important for quantum information processing. Direct measurement of fine-structure splitting usually requires single/few quantum dots at liquid-helium temperature because of its sensitivity to quantum dot size and shape, whereas measuring and controlling fine-structure splitting at an ensemble level seem to be impossible unless all the dots are made to be nearly identical. Here we report strong bright-exciton fine-structure splitting up to 1.6 meV in solution-processed CsPbI₃ perovskite quantum dots, manifested as quantum beats in ensemble-level transient absorption at liquid-nitrogen to room temperature. The splitting is robust to quantum dot size and shape heterogeneity, and increases with decreasing temperature, pointing towards a mechanism associated with orthorhombic distortion of the perovskite lattice. Effective-mass-approximation calculations reveal an intrinsic 'fine-structure gap' that agrees well with the observed fine-structure splitting. This gap stems from an avoided crossing of bright excitons confined in orthorhombically distorted quantum dots that are bounded by the pseudocubic {100} family of planes.

Impact of Strain Relaxation on 2D Ruddlesden-Popper Perovskite Solar Cells

Although the photovoltaic performance of perovskite solar cells (PSCs) has reached the commercial standards, the unsatisfactory stability limits their further application. Hydrophobic interface and encapsulation can block the damage of water and oxygen, while the instability induced by intrinsic residual strain remains inevitable. Here, the residual strain in a two-dimensional (2D) Ruddlesden-Popper (RP) perovskite film is investigated by X-ray diffraction and atomic force microscopy. It's found that the spacer cations contribute to the residual strain even though they are not in the inorganic cages. Benefited from strain relaxation, the film quality is improved, leading to suppressed recombination, promoted charge transport and enhanced efficiency. More significantly, the strain-released devices maintain 86 % of the initial efficiency after being kept in air with 85 % relative humidity (RH) for 1080 h, 82 % under maximum power point (MPP) tracking at 50 °C for 804 h and 86 % after continuous heating at 85 °C for 1080 h.

Polishing the Lead-Poor Surface for Efficient Inverted CsPbI3 Perovskite Solar Cells

Triiodide cesium lead perovskite (CsPbI₃ ) has promising prospects in the development of efficient and stable photovoltaics in both single-junction and tandem structures. However, achieving inverted devices that provide good stability and are compatible to tandem devices remains a challenge, and the deep insights are still not understood. This study finds that the surface components of CsPbI₃ are intrinsically lead-poor and the relevant traps are of p-type with localized states. These deep-energy-level p traps induce inferior transfer or electrons and serious nonradiative recombination at the CsPbI₃ /PCBM interface, leading to the considerable open-circuit voltage (V_oc ) loss and reduction of fill factor (FF). Compared to molecular passivation, polishing treatment with 1,4-butanediamine can eliminate the nonstoichiometric components and root these intrinsically lead-poor traps for superior electron transfer. The polishing treatment significantly improves the FF and V_oc of the inverted CsPbI₃ photovoltaics, creating an efficiency promotion from 12.64% to 19.84%. Moreover, 95% of the initial efficiency of the optimized devices is maintained after the output operation for 1000 h.

Cesium Lead Iodide Perovskites: Optically Active Crystal Phase Stability to Surface Engineering

Among perovskites, the research on cesium lead iodides (CsPbI₃) has attracted a large research community, owing to their all-inorganic nature and promising solar cell performance. Typically, the CsPbI₃ solar cell devices are prepared at various heterojunctions, and working at fluctuating temperatures raises questions on the material stability-related performance of such devices. The fundamental studies reveal that their poor stability is due to a lower side deviation from Goldschmidt's tolerance factor, causing weak chemical interactions within the crystal lattice. In the case of organic-inorganic hybrid perovskites, where their stability is related to the inherent chemical nature of the organic cations, which cannot be manipulated to improve the stability drastically whereas the stability of CsPbI₃ is related to surface and lattice engineering. Thus, the challenges posed by CsPbI₃ could be overcome by engineering the surface and inside the CsPbI₃ crystal lattice. A few solutions have been proposed, including controlled crystal sizes, surface modifications, and lattice engineering. Various research groups have been working on these aspects and had accumulated a rich understanding of these materials. In this review, at first, we survey the fundamental aspects of CsPbI₃ polymorphs structure, highlighting the superiority of CsPbI₃ over other halide systems, stability, the factors (temperature, polarity, and size influence) leading to their phase transformations, and electronic band structure along with the important property of the defect tolerance nature. Fortunately, the factors stabilizing the most effective phases are achieved through a size reduction and the efficient surface passivation on the delicate CsPbI₃ nanocrystal surfaces. In the following section, we have provided the up-to-date surface passivating methods to suppress the non-radiative process for near-unity photoluminescence quantum yield, while maintaining their optically active phases, especially through molecular links (ligands, polymers, zwitterions, polymers) and inorganic halides. We have also provided recent advances to the efficient synthetic protocols for optically active CsPbI₃ NC phases to use readily for solar cell applications. The nanocrystal purification techniques are challenging and had a significant effect on the device performances. In part, we summarized the CsPbI₃-related solar cell device performances with respect to the device fabrication methods. At the end, we provide a brief outlook on the view of surface and lattice engineering in CsPbI₃ NCs for advancing the enhanced stability which is crucial for superior optical and light applications.

Interplay of Kinetic and Thermodynamic Reaction Control Explains Incorporation of Dimethylammonium Iodide into CsPbI3

CsPbI₃ is a promising material for optoelectronics owing to its thermal robustness and favorable bandgap. However, its fabrication is challenging because its photoactive phase is thermodynamically unstable at room temperature. Adding dimethylammonium (DMA) alleviates this instability and is currently understood to result in the formation of DMA _x Cs_1-x PbI₃ perovskite solid solutions. Here, we use NMR of the ¹³³Cs and ¹³C local structural probes to show that these solid solutions are not thermodynamically stable, and their synthesis under thermodynamic control leads to a segregated mixture of yellow one-dimensional DMAPbI₃ phase and δ-CsPbI₃. We show that mixed-cation DMA _x Cs_1-x PbI₃ perovskite phases only form when they are kinetically trapped by rapid antisolvent-induced crystallization. We explore the energetics of DMA incorporation into CsPbI₃ using first-principles calculations and molecular dynamics simulations and find that this process is energetically unfavorable. Our results provide a complete atomic-level picture of the mechanism of DMA-induced stabilization of the black perovskite phase of CsPbI₃ and shed new light on this deceptively simple material.

Fast Charge Transfer and High Stability via Hybridization of Hygroscopic Cu-BTC Metal-Organic Framework Nanocrystals with a Light-Absorbing Layer for Perovskite Solar Cells

Perovskite solar cells (PSCs) have great potential as an efficient solar energy harvesting system due to their outstanding optoelectronic properties, but the charge accumulation and recombination, as well as the moisture-induced degradation of the light-absorbing perovskite layers, remain great bottlenecks in practical applications for future technology. As a solution to this challenge, here we report a strategy to realize moisture-stable PSCs allowing fast charge transfer that, in turn, leads to high power conversion efficiency (PCE). Hybridization of hygroscopic copper(II) benzene-1,3,5-tricarboxylate metal-organic frameworks (Cu-BTC MOFs) with a light-absorbing perovskite layer for PSCs, where a moderate level of moisture attracted by Cu-BTC MOFs during the synthesis step, leads to enhanced perovskite crystallization. Besides, the perovskite-MOF hybrid facilitates the transfer of photoexcited electrons from the perovskite to TiO₂ by providing additional channels for electron extraction. This enables a high PCE of 20.5% in a triple-cation perovskite-MOF device with negligible hysteresis compared to reference devices. Moreover, the perovskite-MOF hybrid exhibits high stability in ambient air under dark conditions over a long period (up to 22 months), while the unmodified counterpart quickly decomposes into PbI₂. Consequently, this work provides a promising clue to realizeing fast charge transfer and high stability for high-performance PSCs.

Hole transport free carbon-based high thermal stability CsPbI1.2Br1.8 solar cells with an amorphous InGaZnO4 electron transport layer

Due to their low cost, tunable band gap and excellent thermostability, all-inorganic halide perovskites CsPbX₃ (X = Br, I) have become a kind of promising photovoltaic material. However, compared to the organic-inorganic hybrid perovskite solar cells, the performance of CsPbX₃ solar cells still needs to be improved. In this work, for the first time, we applied the sol-gel derived amorphous InGaZnO₄ film as electron transport layers (ETLs) in CsPbX₃-based devices. In these devices, the carbon electrode deposited by screen printing replaced the unstable hole transport layer and the expensive metal electrode to obtain hole transport free carbon-based devices, which significantly simplifies the preparation process and reduces the production cost. With the application of amorphous InGaZnO₄ films, devices show a relatively high power conversion efficiency (9.07%) and excellent thermal stability. Compared with the reported CsPbX₃ devices using SnO₂ or TiO₂ ETLs, the performance of amorphous InGaZnO₄ based devices has been significantly improved. This work provides a promising route to prepare highly thermally stable all-inorganic perovskite solar cells using a-IGZO films.

Tailoring Defects Regulation in Air-Fabricated CsPbI3 for Efficient Inverted All-Inorganic Perovskite Solar Cells with Voc of 1.225 V

Air fabrication of CsPbI₃ perovskite photovoltaics has been attractive and fast-moving owing to its compatibility to low-cost and up-scalable fabrication. However, due to the inevitable erosions, undesirable traps are formed in air-fabricated CsPbI₃ crystals and seriously hinder photovoltaic performance with poor reproduction. Here, 3, 5-difluorobenzoic acid hydrazide (FBJ) is incorporated as trap regulation against external erosions in air-fabricated CsPbI₃. Theoretical simulations reveal that FBJ molecules feature stronger absorbance on CsPbI₃ than water, which can regulate trap formations for water erosions. In addition, FBJ with solid bonding interaction to CsPbI₃ can enlarge formation energy of various defects during crystallization and further suppress traps. Moreover, profiling to reductive hydrazine groups, FBJ inhibits traps for oxidation erosions. Consequently, a champion efficiency of 19.27% with an impressive V_oc of 1.225 V is realized with the inverted CsPbI₃ devices. Moreover, the optimized devices present superior stability and contain 97.4% after operating at 60 °C for 600 h.

Stabilized High-Membered and Phase-Pure 2D All Inorganic Ruddlesden-Popper Halide Perovskites Nanocrystals as Photocatalysts for the CO2 Reduction Reaction

In contrast to the 2D organic-inorganic hybrid Ruddlesden-Popper halide perovskites (RPP), a new class of 2D all inorganic RPP (IRPP) has been recently proposed by substituting the organic spacers with an optimal inorganic alternative of cesium cations (Cs⁺ ). Nevertheless, the synthesis of high-membered 2D IRPPs (n > 1) has been a very challenging task because the Cs⁺ need to act as both spacers and A-site cations simultaneously. This work presents the successful synthesis of stable phase-pure high-membered 2D IRPPs of Cs_n+1 Pb_n Br_3n+1 nanosheets (NSs) with n = 3 and 4 by employing the strategy of using additional strong binding bidentate ligands. The structures of the 2D IRPPs (n = 3 and 4) NSs are confirmed by powder X-ray diffraction and high-resolution aberration-corrected scanning transmission electron microscope measurements. These 2D IRPPs NSs exhibit a strong quantum confinement effect with tunable absorption and emission in the visible light range by varying their n values, attributed to their inherent 2D quantum-well structure. The superior structural and optical stability of the phase-pure high-membered 2D IRPPs make them a promising candidate as photocatalysts in CO₂ reduction reactions with outstanding photocatalytic performance and long-term stability.

Anion-Exchange Driven Phase Transition in CsPbI3 Nanowires for Fabricating Epitaxial Perovskite Heterojunctions

Anion-exchange in halide perovskites provides a unique pathway of bandgap engineering for fabricating heterojunctions in low-cost photovoltaics and optoelectronics. However, it remains challenging to achieve robust and sharp perovskite heterojunctions, due to the spontaneous anion interdiffusion across the heterojunction in 3D perovskites. Here, it is shown that the anionic behavior in 1D perovskites is fundamentally different, that the anion exchange can readily drive an indirect-to-direct bandgap phase transition in CsPbI₃ nanowires (NWs) and greatly lower the phase transition temperature. In addition, the heterojunction created by phase transition is epitaxial in nature, and its chemical composition can be precisely controlled upon postannealing. Further study of the phase transition dynamics reveals a threshold-dominating anion exchange mechanism in these 1D NWs rather than the gradient-dominating mechanism in 3D systems. The results provide important insights into the ionic behavior in halide perovskites, which is beneficial for applications in solar cells, light-emitting diodes (LEDs), and other semiconductor devices.

Seed-Assisted Growth of Methylammonium-Free Perovskite for Efficient Inverted Perovskite Solar Cells

The traditional way to stabilize α-phase formamidinium lead triiodide (FAPbI₃ ) perovskite often involves considerable additions of methylammonium (MA) and bromide into the perovskite lattice, leading to an enlarged bandgap and reduced thermal stability. This work shows a seed-assisted growth strategy to induce a bottom-up crystallization of MA-free perovskite, by introducing a small amount of α-CsPbBr₃ /DMSO (5%) as seeds into the pristine FAPbI₃ system. During the initial crystalization period, the typical hexagonal α-FAPbI₃ crystals (containing α-CsPbBr₃ seeds) are directly formed even at ambient temperature, as observed by laser scanning confocal microscopy. It indicates that these seeds can promote the formation and stabilization of α-FAPbI₃ below the thermodynamic phase-transition temperature. After annealing not beyond 100 °C, CsPbBr₃ seeds homogeneously diffused into the entire perovskite layer via an ions exchange process. This work demonstrates an efficiency of 22% with hysteresis-free inverted perovskite solar cells (PSCs), one of the highest performances for MA-free inverted PSCs. Despite absented passivation processes, open-circuit voltage is improved by 100 millivolts compared to the control devices with the same stoichiometry, and long-term operational stability retained 92% under continuous full sun illumination. Going MA-free and low-temperature processes are a new insight for compatibility with tandems or flexible PSCs.

The Universal Growth of Ultrathin Perovskite Single Crystals

Perovskites have engaged significant attention owing to rich species and remarkable physical properties as well as optoelectronic applications. Compared to bulk counterparts, ultrathin perovskites exhibit more available compositions due to the breaking of bulk lattice limitation. Coupled with crystal lattice relaxation and quantum confinement, infinite intriguing properties of ultrathin perovskites deserve to be explored. Developing ultrathin perovskites with alterable composition and structure is a necessity to fully explore this versatile family. Herein, a universal strategy is conceived via constructing oriented solvent microenvironment induced by the interfacial electric field originated from the charge separation between solid and liquid phases, which is conducive to controlling the precursor distribution and makes crystals preferentially nucleate and grow in the preferentially lateral mode. From layered to nonlayered, organic to inorganic, and toxic to low-toxic lead-free perovskite, a full-range synthesis is achieved of ultrathin perovskites. This work opens up opportunities both for ultrathin perovskite exploration through compositional engineering and for device miniaturization in energy conversion applications.

α-BaF2 Nanoparticle Substrate-Enabled γ-CsPbI3 Heteroepitaxial Growth for Efficient and Bright Deep-Red Light-Emitting Diodes

All-inorganic CsPbI₃ perovskite is attractive for deep-red light-emitting diodes (LEDs) because of its excellent carrier mobility, high color purity, and solution processability. However, the high phase transition energy barrier of optically active CsPbI₃ black phase hinders the fabrication of efficient and bright LEDs. Here, we report a novel α-BaF₂ nanoparticle substrate-promoted solution-processable heteroepitaxial growth to overcome this hindrance and obtain high-quality optically active γ-CsPbI₃ thin films, achieving efficient and bright deep-red LEDs. We unravel that the highly exposed planes on the α-BaF₂ nanoparticle-based heteroepitaxial growth substrate have a 99.5% lattice matching degree with the (110) planes of γ-CsPbI₃. This ultrahigh lattice matching degree initiates solution-processed interfacial strain-free epitaxial growth of low-defect and highly oriented γ-CsPbI₃ thin films on the substrate. The obtained γ-CsPbI₃ thin films are uniform, smooth, and highly luminescent, based on which we fabricate efficient and bright deep-red LEDs with a high peak external quantum efficiency of 14.1% and a record luminance of 1325 cd m^-2.

CsPbI3-Based Phase-Stable 2D Ruddlesden-Popper Perovskites for Efficient Solar Cells

Inorganic CsPbI₃ perovskite has shown great promise in highly stable perovskite solar cells due to the lack of volatile organic components. However, the inferior phase stability in ambient conditions resulted from the very small Cs⁺, limiting their practical applications. Here, CsPbI₃-based 2D Ruddlesden-Popper (RP) perovskites were developed using two thiophene-based aromatic spacers, namely, 2-thiophenemethylamine hydroiodide (ThMA) and 2-thiopheneformamidine hydroiodide (ThFA), which significantly improved the phase stability by releasing the large inner stress of black-phase CsPbI₃. The optimized ThFA-based 2D RP perovskite (n = 5, ThFA-Cs) device achieves a record efficiency of 16.00%. Importantly, the ThFA-Cs devices could maintain an average of 98% of their initial efficiencies after being stored in N₂ at room temperature for 3000 h and 92% of their initial value at 80 °C for 960 h. This work provides a new perspective for exploration of the phase-stable CsPbI₃-based perovskite with reduced dimensions for high-performance solar cells.

Controlling the Phase Transition in CsPbI3 Nanowires

Cesium lead iodide (CsPbI₃) is a promising semiconductor with a suitable band gap for optoelectronic devices. CsPbI₃ has a metastable perovskite phase that undergoes a phase transition into an unfavorable nonperovskite phase in an ambient environment. This phase transition changes the optoelectronic properties of CsPbI₃ and hinders its potential for device applications. Therefore, it is of central importance to understand the kinetics of such instability and develop strategies to control and stabilize the perovskite phase. Here, we use ultralong CsPbI₃ nanowires as a model platform to investigate the phase transition kinetics. Our results depict the role of environmental stressors (moisture and temperature) in controlling the phase transition dynamics of CsPbI₃, which can serve as guiding principles for future phase transition studies and the design of related photovoltaics. Furthermore, we demonstrate the controllability of phase propagation on individual nanowires by varying the moisture level and temperature.

Strain-induced bandgap engineering in CsGeX3 (X = I, Br or Cl) perovskites: insights from first-principles calculations

Based on density functional theory and following first-principles methods, this paper investigated the electronic structures, densities of states, effective masses of electrons and holes, and optical properties of CsGeX₃ (X = I, Br or Cl) perovskites under triaxial strains of -4% to 4%. The calculated results show that the tuning range of the bandgaps of the CsGeI₃, CsGeBr₃, and CsGeCl₃ perovskites are 1.16 eV, 1.64 eV, and 1.63 eV, respectively. This result shows that the bandgap of the CsGeX₃ perovskite is tuned over the entire visible spectrum by applying strain. Also, it is found that the change of the bandgap is caused by the change of the Ge-X long bond. Besides, the optimal bandgaps of CsGeI₃ and CsGeBr₃ can be achieved by applying compressive strains, providing theoretical support for adjusting the bandgaps of CsGeX₃ perovskites. The effective masses of electrons and holes of CsGeX₃ perovskites decrease gradually with the strains changing from 4% to -4%, which is conducive to the transmission of electrons and holes. In addition, the optical properties of CsGeX₃ perovskites change from redshifted to blueshifted under different strains.

Highly Efficient and Stable CsPbTh3 (Th = I, Br, Cl) Perovskite Solar Cells by Combinational Passivation Strategy

The distorted lead iodide octahedra of all-inorganic perovskite based on triple halide-mixed CsPb(I_2.85 Br_0.149 Cl_0.001 ) framework have made a tremendous breakthrough in its black phase stability and photovoltaic efficiency. However, their performance still suffers from severe ion migration, trap-induced nonradiative recombination, and black phase instability due to lower tolerance factor and high total energy. Here, a combinational passivation strategy to suppress ion migration and reduce traps both on the surface and in the bulk of the CsPhTh₃ perovskite film is developed, resulting in improved power conversion efficiency (PCE) to as high as 19.37%. The involvement of guanidinium (GA) into the CsPhTh₃ perovskite bulk film and glycocyamine (GCA) passivation on the perovskite surface and grain boundary synergistically enlarge the tolerance factor and suppress the trap state density. In addition, the acetate anion as a nucleating agent significantly improves the thermodynamic stability of GA-doped CsPbTh₃ film through the slight distortion of PbI₆ octahedra. The decreased nonradiative recombination loss translates to a high fill factor of 82.1% and open-circuit voltage (V_OC ) of 1.17 V. Furthermore, bare CsPbTh₃ perovskite solar cells without any encapsulation retain 80% of its initial PCE value after being stored for one month under ambient conditions.

Ionic Liquid Treatment for Highest-Efficiency Ambient Printed Stable All-Inorganic CsPbI3 Perovskite Solar Cells

All-inorganic cesium lead triiodide (CsPbI₃ ) perovskite is well known for its unparalleled stability at high temperatures up to 500 °C and under oxidative chemical stresses. However, upscaling solar cells via ambient printing suffers from imperfect crystal quality and defects caused by uncontrollable crystallization. Here, the incorporation of a low concentration of novel ionic liquid is reported as being promising for managing defects in CsPbI₃ films, interfacial energy alignment, and device stability of solar cells fabricated via ambient blade-coating. Both theoretical simulations and experimental measurements reveal that the ionic liquid successfully regulates the perovskite thin-film growth to decrease perovskite grain boundaries, strongly coordinates with the undercoordinated Pb²⁺ to passivate iodide vacancy defects, aligns the interface to decrease the energy barrier at the electron-transporting layer, and relaxes the lattice strain to promote phase stability. Consequently, ambient printed CsPbI₃ solar cells with power conversion efficiency as high as 20.01% under 1 sun illumination (100 mW cm^-2 ) and 37.24% under indoor light illumination (1000 lux, 365 µW cm^-2 ) are achieved; both are the highest for printed all-inorganic cells for corresponding applications. Furthermore, the bare cells show an impressive long-term ambient stability with only ≈5% PCE degradation after 1000 h aging under ambient conditions.

Stabilization of Metastable Halide Perovskite Lattices in the 2D Limit

Metal halide perovskites constitute a new class of semiconductors that are structurally tailorable, exhibiting rich structural polymorphs. In this perspective, the polymorphism in lead halide perovskites is described-a material system currently used for high-performance photovoltaics and optoelectronics. Strategies for stabilizing the metastable perovskite polymorphs based on crystal size reduction and surface functionalization are critically reviewed. Focus is on an unprecedented stabilization of metastable perovskite lattices in the 2D limit (e.g., with a thickness down to a few unit cells) due to the dominance of surface effects. This stabilization allows the incorporation of various A-cations that deemed oversized for 3D perovskites into the 2D perovskite lattices, which bring new insights on the relationships between the crystal structures and optoelectronic properties and lead to emergent ferroelectricity in halide perovskites. A comprehensive understanding is provided on how the A-cations influence the structural, optoelectronic, and ferroelectric properties, with an emphasis on the second order Jahn-Teller distortion caused by the oversized A-cations. Finally, future perspectives on new structure exploration and studies of fundamental photophysical properties using stabilized perovskite lattices are provided.

Additive-Free, Low-Temperature Crystallization of Stable α-FAPbI3 Perovskite

Formamidinium lead triiodide (FAPbI₃ ) is attractive for photovoltaic devices due to its optimal bandgap at around 1.45 eV and improved thermal stability compared with methylammonium-based perovskites. Crystallization of phase-pure α-FAPbI₃ conventionally requires high-temperature thermal annealing at 150 °C whilst the obtained α-FAPbI₃ is metastable at room temperature. Here, aerosol-assisted crystallization (AAC) is reported, which converts yellow δ-FAPbI₃ into black α-FAPbI₃ at only 100 °C using precursor solutions containing only lead iodide and formamidinium iodide with no chemical additives. The obtained α-FAPbI₃ exhibits remarkably enhanced stability compared to the 150 °C annealed counterparts, in combination with improvements in film crystallinity and photoluminescence yield. Using X-ray diffraction, X-ray scattering, and density functional theory simulation, it is identified that relaxation of residual tensile strains, achieved through the lower annealing temperature and post-crystallization crystal growth during AAC, is the key factor that facilitates the formation of phase-stable α-FAPbI₃ . This overcomes the strain-induced lattice expansion that is known to cause the metastability of α-FAPbI₃ . Accordingly, pure FAPbI₃ p-i-n solar cells are reported, facilitated by the low-temperature (≤100 °C) AAC processing, which demonstrates increases of both power conversion efficiency and operational stability compared to devices fabricated using 150 °C annealed films.

Tunable morphology and highly stable α-CsPbI3 Nano-bricks for photoelectric devices

CsPbI₃ with suitable bandgap (∼1.73 eV) and excellent photoelectric performance are considered promising candidates for new-generation photovoltaic and photoelectric devices. However, the phase instability of CsPbI₃ hinders its application in photoelectric devices. In this study, cubic phase CsPbI₃ nano-bricks with high stability are prepared on Si substrates by catalyst-freehigh-pressure pulsedlaser deposition.Theeffectsofthetarget-substrate distance on the morphological, structural and photoluminescence propertiesofCsPbI₃ nano-bricks are investigated. CsPbI₃ nano-bricks exhibits excellent long-term stability for 12 months, which can be attributed to the smaller lattice mismatch between Si (100) and CsPbI₃ (100). Furthermore, the photodetector based on CsPbI₃/n-Si heterojunction exhibited obvious spectral response in the red band of 710 nm. This work provides a novel idea for the preparation of photoelectric devices with cubic CsPbI₃.

Atomic-Scale Imaging and Nano-Scale Mapping of Cubic α-CsPbI3 Perovskite Nanocrystals for Inverted Perovskite Solar Cells

Colloidal synthesized cubic α-CsPbI₃ perovskite nanocrystals having a smaller lattice constant (a = 6.2315 Å) compared to the standard structure, and nanoscale mapping of their surfaces are reported to achieve superior photovoltaic performance under 45-55% humidity conditions. Atomic scale transmission electron microscopic images have been utilized to probe the precise arrangement of Cs, Pb, and I atoms in a unit cell of α-CsPbI₃ NCs, which is well supported by the VESTA structure. Theoretical calculation using density functional theory of our experimental structure reveals the realization of direct band to band transition with a lower band gap, a higher absorption coefficient, and stronger covalent bonding between the Pb and I atoms in the [PbI₆]^4- octahedral, as compared to reported standard structure. Nanoscale surface mapping using Kelvin probe force microscopy yielding contact potential difference (CPD) and conductive atomic force microscopy for current mapping have been employed on α-CsPbI₃ NCs films deposited on different DMSO doped PEDOT:PSS layers. The difference of CPD value under dark and light illumination suggests that the hole injection strongly depends on the interfaces with PEDOT:PSS layer. The carrier transport through grain interiors and grain boundaries in α-CsPbI₃ probed by the single-point c-AFM measurements reveal the excellent photosensitivity under the light conditions. Finally, inverted perovskite solar cells, employing α-CsPbI₃ NCs film as an absorber layer and PEDOT:PSS layer as a hole transport layer, have been optimized to achieve the highest power conversion efficiency of 10.6%, showing their potential for future earth abundant, low cost, and air stable inverted perovskite photovoltaic devices.

Black Phase of Inorganic Perovskite Stabilized with Carboxyimidazolium Iodide for Stable and Efficient Inverted Perovskite Solar Cells

As all-inorganic perovskite (CsPbI_3-xBr_x) is prone to phase transition from the α phase (black phase) to the δ phase (yellow phase) in a humid environment or under heating, improving the phase stability of all-inorganic perovskite of the black phase is one of the urgent problems to solve. Herein, 1,2-dimethyl-3-acetylimidazolium iodide (DMAII) is spin-coated onto the surface of CsPbI_3-xBr_x perovskite for use in p-i-n perovskite solar cells (PSCs). We find that the DMAII coating has two effects on the CsPbI_3-xBr_x perovskite film: surface passivation and phase stabilization of perovskite. Traps in the CsPbI_3-xBr_x perovskite film can be reduced significantly by DMAII passivation, resulting in enhanced hole extraction and suppressed charge recombination. Consequently, the power conversion efficiency (PCE) is improved from 10.81 to 13.14%. Moreover, the DMAII coating can significantly inhibit the phase transition from the α phase to the δ phase in a humid environment or under heating, as characterized by the X-ray diffraction pattern, UV-vis absorption spectrum, and film color. After exposing the CsPbI_3-xBr_x perovskite films to a humid atmosphere (relative humidity = 40-60%) for 6 h, the PCE decreases dramatically to only 0.12% of the initial PCE for the PSC without the DMAII coating, while the PCE maintains 80% of the initial PCE for the PSC with the DMAII coating. In addition, when the PSC devices are heated at 120 °C for 4 h, the control PSC shows a 96% decrease in PCE, while the PCE decay is only 9% for the DMAII-coated PSC. These findings indicate that carboxyl-substituted imidazolium iodide is a kind of promising material to not only passivate traps but also stabilize the black phase of all-inorganic perovskite.

In Situ Low-Temperature Growth and Superior Luminescent Property of Well-Aligned, High-Quality Cubic CsPbBr3 Micrometer-Scale Single Crystal Arrays on Transparent Conductive Substrates

Direct assembly of high-quality single-crystal perovskite microarrays on transparent conductive substrates and carrier injection layers is vital to realize high-performance optoelectronic devices. Although cubic-phase CsPbBr₃ is considered to have a higher structural and optical quality than the orthorhombic one, obtaining a well-aligned assembly directly on the aforementioned substrates is still challenging. Here we developed a solvent-assisted crystallization strategy with the assistance of surface modifiers, through which the in situ low-temperature growth of well-aligned cubic single-crystal CsPbBr₃ microarray with a preferential out-of-plane [100] orientation is achieved for the first time on commercial transparent conductive substrates. As compared with the control orthorhombic samples, the cubic CsPbBr₃ single crystals possess superior properties including a higher photoluminescence internal quantum efficiency, fewer defect states, a weaker scattering by phonons, and an appearance of lasing. The results presented here can pave the way for future design and applications of optoelectronic devices based on perovskite microarrays.

A comparative study of the mechanical stability, electronic, optical and photocatalytic properties of CsPbX3 (X = Cl, Br, I) by DFT calculations for optoelectronic applications

Organic free Cs-based perovskite materials are potential candidates for electronic and optoelectronic applications. A systematic comparative study of the mechanical, electronic, optical, and photocatalytic properties of CsPbX₃ (X = Cl, Br, I) was conducted using density functional theory to compare the applicability of these materials in optoelectronic, photocatalytic, and photovoltaic (PV) devices. We calculated structural and elastic properties to determine the better agreement of damage-tolerance and electronic and optical responses for suitable device applications. Optimized lattice parameters and elastic constants showed excellent agreement with the experimental data whereas some properties were found to be much better than other theoretical reports. CsPbBr₃ is thermodynamically more stable and more ductile compared to the other two perovskites. The hydrostatic pressure dependent mechanical stability showed that CsPbCl₃ and CsPbBr₃ sustained stability under low applied pressure, whereas the stability of CsPbI₃ was very high. The electronic band gap calculations showed that CsPbCl₃, CsPbBr_3, and CsPbI₃ are suitable for green, orange, and red emissions of optical spectra owing to the proper electronic band gaps. CsPbI₃ can be shown as the best photocatalyst for the hydrogen evolution reaction and CsPbBr₃ is the most stable photocatalyst due to its nearly balanced oxidation and reduction potentials, but CaPbCl₃ is better for O₂ production. The density of states and other optical properties have been reported in this study. Thus, our findings would be beneficial for experimental studies and can open a new window for efficient electronic, optoelectronic, and hydrogen production along with the biodegradation of polluted and waste materials.

Organic free Cs-based perovskite materials are potential candidates for electronic and optoelectronic applications.

Phase evolution of all-inorganic perovskite nanowires during its growth from quantum dots

All-inorganic lead-halide perovskites have emerged as an exciting material owing to their excellent optoelectronic properties and high stability over hybrid organometallic perovskites. Nanowires of these materials, in particular, have shown great promise for optoelectronic applications due to their high optical absorption coefficient and low defect state density. However, the synthesis of the most promising alpha-Cesium lead iodide (α-CsPbI₃) nanowires is challenging as it is metastable and spontaneously converts to a non-perovskiteδ-phase. The hot-injection method is one of the most facile, well-controlled, and commonly used approaches for synthesizing CsPbX₃nanostructures. But the exact mechanism of growing these nanowires in this technique is not clear. Here, we show that the hot-injection method produces photoactive phases of quantum dots (QDs) and nanowires of CsPbBr_3,and QDs of CsPbI₃, but CsPbI₃nanowires are grown in their non-perovskiteδ-phase. Monitoring the nanowire growth during the hot-injection technique and through detailed characterization, we establish that CsPbI₃nanowires are formed in the non-perovskite phase from the beginning rather than transforming after its growth from perovskite to a non-perovskite phase. We have discussed a possible mechanism of how non-perovskite nanowires of CsPbI₃grow at the expense of photoactive perovskite QDs. Our findings will help to synthesize nanostructures of all-inorganic perovskites with desired phases, which is essential for successful technological applications.

Deep-Red Perovskite Light-Emitting Diodes Based on One-Step-Formed γ-CsPbI3 Cuboid Crystallites

Inorganic CsPbI₃ perovskite with high chemical stability is attractive for efficient deep-red perovskite light-emitting diodes (PeLEDs) with high color purity. Compared to PeLEDs based on ex-situ-synthesized CsPbI₃ nanocrystals/quantum dots suffering from low conductivity and efficiency droop under high current densities, in situ deposited 3D CsPbI₃ films from precursor solutions can maintain high conductivity but show high trap density. Here, it is demonstrated that introducing diammonium iodide can increase the size of colloids in the precursor solution, retard the phase-transition rate, and passivate trap states of the in-situ-formed cuboid crystallites. The PeLED based on the one-step-formed 3D CsPbI₃ cuboid crystallite films shows a peak external quantum efficiency (EQE) value up to 15.03% because of the high conductivity and reduced trap states. Furthermore, this one-step method also has a wide processing window, which is attractive for flow-line production of large-area PeLED modules. The fabrication of a 9 cm² PeLED that exhibits a peak EQE of 10.30% is successfully demonstrated.

Remarkable Black-Phase Robustness of CsPbI3 Nanocrystals Sealed in Solid SiO2 /AlOx Sub-Micron Particles

This work combines the high-temperature sintering method and atomic layer deposition (ALD) technique, and yields SiO₂ /AlO_x -sealed γ-CsPbI₃ nanocrystals (NCs). The black-phase CsPbI₃ NCs, scattered and encapsulated firmly in solid SiO₂ sub-micron particles, maintain in black phases against water soaking, ultraviolet irradiation, and heating, exhibiting remarkable phase stability. A new phase-transition route, from γ via β to α phase without transferring into δ phase, has been discovered upon temperature increasing. The phase stability is ascribed to the high pressure exerted by the rigid SiO₂ encapsulations, and its condensed amorphous structures that prevent the permeation of H₂ O molecules. Nanoscale coating of Al₂ O₃ thin films, which are deposited on the surface of the CsPbI₃ -SiO₂ by ALD, enhances the protection against O₂ infiltration, greatly elevating the high-temperature stability of CsPbI₃ NCs sealed inside, as the samples remain bright after 1-h annealing in air at 400 °C. These fabrication and encapsulation techniques effectively prevent the formation of δ-CsPbI₃ under harsh environment, bringing the high-pressure preservation of black-phase CsPbI₃ from laboratory to industry toward potential applications in both photovoltaic and fluorescent areas.

A Systematic Review of Metal Halide Perovskite Crystallization and Film Formation Mechanism Unveiled by In Situ GIWAXS

Metal halide perovskites are of fundamental interest in the research of modern thin-film optoelectronic devices, owing to their widely tunable optoelectronic properties and solution processability. To obtain high-quality perovskite films and ultimately high-performance perovskite devices, it is crucial to understand the film formation mechanisms, which, however, remains a great challenge, due to the complexity of perovskite composition, dimensionality, and processing conditions. Nevertheless, the state-of-the-art in situ grazing-incidence wide-angle X-ray scattering (GIWAXS) technique enables one to bridge the complex information with device performance by revealing the crystallization pathways during the perovskite film formation process. In this review, the authors illustrate how to obtain and understand in situ GIWAXS data, summarize and assess recent results of in situ GIWAXS studies on versatile perovskite photovoltaic systems, aiming at elucidating the distinct features and common ground of film formation mechanisms, and shedding light on future opportunities of employing in situ GIWAXS to study the fundamental working mechanisms of highly efficient and stable perovskite solar cells toward mass production.

Efficient and Stable CsPbI3 Inorganic Perovskite Photovoltaics Enabled by Crystal Secondary Growth

Defect-triggered phase degradation is generally considered as the main issue that causes phase instability and limited device performance for CsPbI₃ inorganic perovskites. Here, a defect compensation in CsPbI₃ perovskite through crystal secondary growth of inorganic perovskites is demonstrated, and highly efficient inorganic photovoltaics are realized. This secondary growth is achieved by a solid-state reaction between a bromine salt and defective CsPbI₃ perovskite. Upon solid-state reaction, the Br^- ions can diffuse over the entire CsPbI₃ perovskite layer to heal the undercoordinated Pb²⁺ and conduct certain solid-state I/Br ion exchange reaction, while the organic cations can potentially heal the Cs⁺ cation vacancies through coupling with [PbI₆ ]^4- octahedra. The carrier dynamics confirm that this crystal secondary growth can realize defect compensation in CsPbI₃ . The as-achieved defect-compensated CsPbI₃ not only improves the charge dynamics but also enhances the photoactive phase stability. Finally, the CsPbI₃ -based solar cell delivers 20.04% efficiency with excellent operational stability. Overall, this work proposes a novel concept of defect compensation in inorganic perovskites through crystal secondary growth induced by solid-state reaction that is promising for various optoelectronic applications.

Molten-Salt-Assisted CsPbI3 Perovskite Crystallization for Nearly 20%-Efficiency Solar Cells

Dynamic manipulation of crystallization is pivotal to the quality of polycrystalline films. A molten-salt-assisted crystallization (MSAC) strategy is presented to improve grain growth of the all-inorganic perovskite films. Compared with the traditional solvent annealing, MSAC enables more intensive mass transfer by means of convection and diffusion, which is beneficial to the interaction among the precursor colloids and to inducing in-plane growth of perovskite grains, resulting in the formation of high-quality perovskite films with suppressed pinhole and crack formation. Additionally, the introduction of molten salt alters the intermediate phases, and thus changes the crystallization pathways by reducing the energy barrier to produce films with desired optical and electrical properties. As a result, the MSAC strategy endows the devices with champion steady-state output efficiency of 19.83% and open-circuit voltage (V_oc ) as high as 1.2 V, among the highest for this type of solar cell, thanks to its effectively reduced V_oc deficit.

Liquid-phase sintering of lead halide perovskites and metal-organic framework glasses

[Figure: see text].

Ligand-Free Processable Perovskite Semiconductor Ink

Traditional covalent semiconductors require complex processing methods for device fabrication due to their high cohesive energies. Here, we develop a stable, ligand-free perovskite semiconductor ink that can be used to make patterned semiconductor-based optoelectronics in one step. The perovskite ink is formed via the dissolution of crystals of vacancy-ordered double perovskite Cs₂TeX₆ (X = Cl^-, Br^-, I^-) in polar aprotic solvents, leading to the stabilization of isolated [TeX₆]^2- octahedral anions and free Cs⁺ cations without the presence of ligands. The stabilization of the fundamental perovskite ionic octahedral building blocks in solution creates multifunctional inks with the ability to reversibly transform between the liquid ink and the solid-state perovskite crystalline system in air within minutes. These easily processable inks can be patterned onto various materials via dropcasting, spraying or painting, and stamping, highlighting the crucial role of solvated octahedral complexes toward the rapid formation of phase-pure perovskite structures in ambient conditions.

Strain analysis and engineering in halide perovskite photovoltaics

Halide perovskites are a compelling candidate for the next generation of clean-energy-harvesting technologies owing to their low cost, facile fabrication and outstanding semiconductor properties. However, photovoltaic device efficiencies are still below practical limits and long-term stability challenges hinder their practical application. Current evidence suggests that strain in halide perovskites is a key factor in dictating device efficiency and stability. Here we outline the fundamentals of strain within halide perovskites relevant to photovoltaic applications and rationalize approaches to characterize the phenomenon. We examine recent breakthroughs in eliminating the adverse impacts of strain, enhancing both device efficiencies and operational stabilities. Finally, we discuss further challenges and outline future research directions for placing stress and strain studies at the forefront of halide perovskite research. An extensive understanding of strain in halide perovskites is needed, which would allow effective strain management and drive further enhancements in efficiencies and stabilities of perovskite photovoltaics.

Highly Stable Inorganic Lead Halide Perovskite toward Efficient Photovoltaics

ConspectusOwing to the remarkable progress achieved over the past decade with research efforts from the perspectives of material synthesis, device configuration, and theoretical investigation, metal halide perovskites have emerged as a revolutionary class of light-absorbing semiconductors. The perovskite photovoltaic devices have demonstrated an impressive increase in power conversion efficiency. For single-junction perovskite solar cells, the value has risen from the initial one-digit maximum to the state-of-art record of 25.5%. Among various chemical and structural variations of perovskites, inorganic lead halides possess a more favorable operational stability and hold greater potential for perovskite/silicon tandem photovoltaics' top cells. At the initial stage of exploring all-inorganic perovskites for optoelectronic applications, many concepts, technical routes, and modification strategies were directly adopted from research on the more-developed field of organic-inorganic hybrid perovskite (OIHP). However, as understandings on inorganic perovskite deepen with research experience gained from both experimental and theoretical progression, it has been found that the difference between the asymmetric, volatile organic cations and the spherical, stable inorganic cations can lead to drastic changes on overall material properties and the subsequent device performances. In detail, the disparities reflect the crystalline and phase profiles of the material, the fabrication and passivation rationales of perovskite thin films, and the photophysics in the assembled optoelectronic devices. Therefore, the discussions of all-inorganic perovskites have their own exclusivity and are worthy of a specialized scrutinization.Here in this Account, the latest progress on the stabilization of inorganic lead halide perovskites for efficient photovoltaic applications is highlighted. A library of chemical compositions will be discussed with a focus on notable works about CsPbI₃, which possesses a more favorable bandgap as a tandem to commercialized silicon solar cells. To underscore the influence of the crystal phase on the stability of inorganic perovskites, fundamental investigations regarding the chemical and physical properties, including experimental and theoretical studies, will be summarized as a means of phase control. The stability of inorganic lead halide perovskites can also be improved by the strategic introduction of external components to the light-absorbing layer, such as the incorporation of inorganic halides, organic cations, OIHPs with low dimension, etc. These strategies can synergistically improve the stability and efficiency of the fabricated devices from the perspectives of compositional tuning, dimensional engineering, surface termination, and low-dimension capping. On the basis of a careful examination and an analysis of works achieved in these categories from our group and others, we will then discuss some promising approaches toward achieving more stable and efficient photovoltaics using inorganic lead halide perovskites.

Organic Matrix Assisted Low-temperature Crystallization of Black Phase Inorganic Perovskites

All-inorganic perovskites have attracted increasing attention for applications in perovskite solar cells (PSCs) and optoelectronics, including light-emitting devices (LEDs). Cesium lead halide perovskites with tunable I/Br ratios and a band gap aligning with the sunlight region are promising candidates for PSCs. Although impressive progress has been made to improve device efficiency from the initial 2.9 % with low phase stability to over 20 % with high stability, there are still questions regarding the perovskite crystal growth mechanism, especially at low temperatures. In this Minireview, we summarize recent developments in using an organic matrix, including the addition and use of organic ions, polymers, and solvent molecules, for the crystallization of black phase inorganic perovskites at temperatures lower than the phase transition point. We also discuss possible mechanisms for this low-temperature crystallization and their effect on the stability of black phase perovskites. We conclude with an outlook and perspective for further fabrication of large-scale inorganic perovskites for optoelectronic applications.

A novel and facile synthesis strategy for highly stable cesium lead halide nanowires

As promising low-dimensional semiconductor materials, cesium lead halide (CsPbX₃, X = Cl, Br, I) perovskite-like nanowires (NWs) can be widely applied to the field of semiconductor devices and integrated optoelectronics. Therefore, developing a facile and efficient synthesis method of cesium lead halide perovskite-like NWs can bring both fundamental and practical impacts to the field of optoelectronics. Here, we developed a synthesis strategy of all-inorganic cesium lead halide CsPbI₃ perovskite-like NWs under catalyst-free, solution-phase, and low-temperature conditions. The synthesis strategy was designed such that no inert gas is required and thus enables the synthesis to be carried out in air, which significantly reduces temperature, steps, time, and cost required for the reaction. The as-synthesized NWs were 7 μm in length and 80–100 nm in diameter with ideal morphology. Most of the CsPbI₃ NWs were crystallized in orthorhombic phases that were arranged orderly with a uniform growth direction. In addition, the CsPbI₃ NWs showed a photoluminescence peak near 610 nm and the fluorescence lifetime was 7.34 ns. The photoluminescence mechanism of CsPbI₃ NWs involves the self-trapping behaviour in the radiative recombination process. The composition of CsPbI₃ NWs is highly related to the synthesis temperature. The facile synthesis strategy has opened up a novel path for the synthesis of perovskite-like NWs, laying the foundation for the application of nano-optoelectronic devices, fluorescent anti-counterfeiting, and fluorescent composite materials.

Under air ambient, low temperature, and short reaction time conditions, a facile operation step is proposed to synthesize CsPbI₃ nanowires.

Thermal Unequilibrium of PdSn Intermetallic Nanocatalysts: From In Situ Tailored Synthesis to Unexpected Hydrogenation Selectivity

Effective control on chemoselectivity in the catalytic hydrogenation of C=O over C=C bonds is uncommon with Pd-based catalysts because of the favored adsorption of C=C bonds on Pd surface. Here we report a unique orthorhombic PdSn intermetallic phase with unprecedented chemoselectivity toward C=O hydrogenation. We observed the formation and metastability of this PdSn phase in situ. During a natural cooling process, the PdSn nanoparticles readily revert to the favored Pd₃ Sn₂ phase. Instead, using a thermal quenching method, we prepared a pure-phase PdSn nanocatalyst. PdSn shows an >96 % selectivity toward hydrogenating C=O bonds of various α,β-unsaturated aldehydes, highest in reported Pd-based catalysts. Further study suggests that efficient quenching prevents the reversion from PdSn- to Pd₃ Sn₂ -structured surface, the key to the desired catalytic performance. Density functional theory calculations and analysis of reaction kinetics provide an explanation for the observed high selectivity.

Strain in perovskite solar cells: origins, impacts and regulation

Metal halide perovskite solar cells (PSCs) have seen an extremely rapid rise in power conversion efficiencies in the past few years. However, the commercialization of this class of emerging materials still faces serious challenges, one of which is the instability against external stimuli such as moisture, heat and irradiation. Much focus has deservedly been placed on understanding the different origins of intrinsic instability and thereby enhancing their stability. Among these, tensile strain in perovskite films is an important source of instability that cannot be overcome using conventionally extrinsic stabilization approaches such as encapsulation. Here we review recent progress in the understanding of the origin of strain in perovskites as well as its corresponding characterization methods, and their impacts on the physical properties of perovskites and the performance of PSCs including efficiency and stability. We then summarize the latest advances in strain-regulation strategies that improve the intrinsic stability of perovskites and photovoltaic devices. Finally, we provide a perspective on how to make further progress in stable and high-efficiency PSCs via strain engineering.

Can Vacuum Deposition Apply to Bismuth-Doped γ-CsPbI3 Perovskite? Revealing the Role of Bi3+ in the Formation of Black Phase

The B-site doped CsPbI₃ has been demonstrated to be very promising for photovoltaics owing to its low black phase transition temperature. Though B-site doped black-CsPbI₃ perovskites have been successfully achieved by solution-processing, it is unclear whether these systems are available by other methods such as vacuum deposition. In this work, heterovalent doped CsPb_1-xBi_xI₃ is targeted. To incorporate Bi³⁺ into the final film via vacuum deposition, the solid solution precursor Pb_1-xBi_xI₂ (0.01 ≤ x ≤ 0.04) is developed. However, these coevaporated films not only are dominated by another hexagonal perovskite phase but also fail to decrease the black phase transition temperature. The role of Bi³⁺ in the formation of the black phase is further studied by solution methods with different types of precursors. It is demonstrated that the key factor in the low-temperature black phase transition is small grain size, as well as the colloid size within the precursor solution, rather than simple substitution of Pb²⁺ with Bi³⁺.

All-Evaporated, All-Inorganic CsPbI3 Perovskite-Based Devices for Broad-Band Photodetector and Solar Cell Applications

Following the rapid increase of organic metal halide perovskites toward commercial application in thin-film solar cells, inorganic alternatives attracted great interest with their potential of longer device lifetime due to the stability improvement under increased temperatures and moisture ingress. Among them, cesium lead iodide (CsPbI₃) has gained significant attention due to similar electronic and optical properties to methylammonium lead iodide (MAPbI₃), with a band gap of 1.7 eV, high absorption coefficient, and large diffusion length, while also offering the advantage of being completely inorganic, providing a higher thermal stability and preventing material degradation. On a device level, however, it seems also essential to replace organic transport layers by inorganic counterparts to further prevent degradation. In addition, devices are mostly fabricated by spin coating, limiting their reproducibility and scalability; in this case, exploring all-evaporated devices allows us to improve the quality of the layers and to increase their reproducibility. In this work, we focus on the deposition of CsPbI₃ by CsI and PbI₂ co-evaporation. We fabricate devices with an all-inorganic, all-evaporated structure, employing NiO and TiO₂ as transport layers, and evaluate these devices for both photodetector and solar cell applications. As a photodetector, low leakage current, high external quantum efficiency (EQE) and detectivity, and fast rise and decay times were obtained, while as a solar cell, acceptable efficiencies were achieved. These all-inorganic, all-evaporated devices represent one step forward toward higher stability and reproducibility while enabling large area compatibility and easier integration with other circuitry and, in future, the possible commercialization of perovskite-based technology.

Trojans That Flip the Black Phase: Impurity-Driven Stabilization and Spontaneous Strain Suppression in γ-CsPbI3 Perovskite

The technological progress and widespread adoption of all-organic CsPbI₃ perovskite devices is hampered by its thermodynamic instability at room temperature. Because of its inherent tolerance toward deep trap formation, there has been no shortage to exploring which dopants can improve the phase stability. While the relative size of the dopant is important, an assessment of the literature suggests that its relative size and impact on crystal volume do not always reveal what will beneficially shift the phase transition temperature. In this perspective, we analyze the changes in crystal symmetry of CsPbI₃ perovskite as it transforms from a thermodynamically stable high-temperature cubic (α) structure into its distorted low-temperature tetragonal (β) and unstable orthorhombic (γ) perovskite structures. Quantified assessment of the symmetry-adapted strains which are introduced due to changes in temperature and composition show that the stability of γ-CsPbI₃ is best rationalized from the point of view of crystal symmetry. In particular, improved thermal-phase stability is directly traced to the suppression of spontaneous strain formation and increased crystal symmetry at room temperature.

Recent Progress on Perovskite Surfaces and Interfaces in Optoelectronic Devices

Surfaces and heterojunction interfaces, where defects and energy levels dictate charge-carrier dynamics in optoelectronic devices, are critical for unlocking the full potential of perovskite semiconductors. In this progress report, chemical structures of perovskite surfaces are discussed and basic physical rules for the band alignment are summarized at various perovskite interfaces. Common perovskite surfaces are typically decorated by various compositional and structural defects such as residual surface reactants, discrete nanoclusters, reactions by products, vacancies, interstitials, antisites, etc. Some of these surface species induce deep-level defect states in the forbidden band forming very harmful charge-carrier traps and affect negatively the interface band alignments for achieving optimal device performance. Herein, an overview of research progresses on surface and interface engineering is provided to minimize deep-level defect states. The reviewed subjects include selection of interface and substrate buffer layers for growing better crystals, materials and processing methods for surface passivation, the surface catalyst for microstructure transformations, organic semiconductors for charge extraction or injection, heterojunctions with wide bandgap perovskites or nanocrystals for mitigating defects, and electrode interlayer for preventing interdiffusion and reactions. These surface and interface engineering strategies are shown to be critical in boosting device performance for both solar cells and light-emitting diodes.