Deciphering Halogen Competition in Organometallic Halide Perovskite Growth

Free Halogen Substitution of Chiral Hybrid Metal Halides for Activating the Linear and Nonlinear Chiroptical Properties

Halogen substitution has been proven as an effective approach to the band gap engineering and optoelectronic modulation of organic-inorganic hybrid metal halide (OIHMH) materials. Various high-performance mixed halide OIHMH film materials have been primarily obtained through the substitution of coordinated halogens in their inorganic octahedra. Herein, we propose a new strategy of substitution of free halogen outside the inorganic octahedra for constructing mixed halide OIHMH single crystals with chiral structures, resulting in a boost of their linear and nonlinear chiroptical properties. The substitution from DMA4[InCl6]Cl (DMA = dimethylammonium) to DMA4[InCl6]Br crystals through a facile antisolvent vaporization method produces centimeter-scale single crystals with high thermal stability along with high quantum yield photoluminescence, conspicuous circularly polarized luminescence, and greatly enhanced second harmonic generation (SHG). In particular, the obtained DMA4[InCl6]Br single crystal features an intrinsic chiral structure, exhibiting a significant SHG circular dichroism (SHG-CD) response with a highest reported anisotropy factor (gSHG-CD) of 1.56 among chiral OIHMH materials. The enhancements in both linear and nonlinear chiroptical properties are directly attributed to the modulation of octahedral distortion. The mixed halide OIHMH single crystals obtained by free halogen substitution confine the introduced halogens within free halogen sites of the lattice, thereby ensuring the stability of compositions and properties. The successful employment of such a free halogen substitution approach may broaden the horizon of the regulation of structures and the optoelectronic properties of the OIHMH materials.

Reaction Dynamics of C(NH2)3SnI3 Formation from Vacuum-Deposited C(NH2)3I and SnI2 Bilayer Thin Films Investigated by In Situ Infrared Multiple-Angle Incidence-Resolved Spectroscopy

Understanding the formation process of organic-inorganic halide perovskite (OIHP) thin films is important for the fabrication of high-quality thin films, which, in turn, are crucial for achieving high-performance devices. To address this challenge, we developed an in situ system of infrared multiple-angle incidence-resolved spectroscopy (IR-MAIRS) combined with a vacuum deposition system. "Orientation-free" isotropic spectra constructed from IR-MAIRS spectra enable us to perform quantitative analysis of the formation process of C(NH2)3SnI3 (GASnI3) thin films from unreacted C(NH2)3I (guanidine hydroiodide (GAI))/SnI2 bilayer structures predeposited in a vacuum. The analysis of the dependence of the GASnI3 formation rate on the reaction temperature using the Avrami model has revealed that a diffusion-controlled reaction process of GAI and SnI2 governs the formation kinetics. The present study points to the usefulness of in situ IR-MAIRS analysis in understanding the growth mechanisms of vacuum-deposited OIHP thin films and hence the potential to accelerate the development of vacuum processes for the fabrication of high-quality OIHP thin films.

Influence of Halides on the Interactions of Ammonium Acids with Metal Halide Perovskites

Additive engineering is a common strategy to improve the performance and stability of metal halide perovskite through the modulation of crystallization kinetics and passivation of surface defects. However, much of this work has lacked a systematic approach necessary to understand how the functionality and molecular structure of the additives influence perovskite performance and stability. This paper describes the inclusion of low concentrations of 5-aminovaleric acid (5-AVA) and its ammonium acid derivatives, 5-ammoniumvaleric acid iodide (5-AVAI) and 5-ammoniumvaleric acid chloride (5-AVACl), into the precursor inks for methylammonium lead triiodide (MAPbI₃) perovskite and highlights the important role of halides in affecting the interactions of additives with perovskite and film properties. The film quality, as determined by X-ray diffraction (XRD) and photoluminescence (PL) spectrophotometry, is shown to improve with the inclusion of all additives, but an increase in annealing time from 5 to 30 min is necessary. We observe an increase in grain size and a decrease in film roughness with the incorporation of 5-AVAI and 5-AVACl with scanning electron microscopy (SEM) and atomic force microscopy (AFM). Critically, X-ray photoelectron spectroscopy (XPS) measurements and density functional theory (DFT) calculations show that 5-AVAI and 5-AVACl preferentially interact with MAPbI₃ surfaces via the ammonium functional group, while 5-AVA will interact with either amino or carboxylic acid functional groups. Charge localization analysis shows the surprising result that HCl dissociates from 5-AVACl in vacuum, resulting in the decomposition of the ammonium acid to 5-AVA. We show that device repeatability is improved with the inclusion of all additives and that 5-AVACl increases the power conversion efficiency of devices from 17.61 ± 1.07 to 18.07 ± 0.42%. Finally, we show stability improvements for unencapsulated devices exposed to 50% relative humidity, with devices incorporating 5-AVAI and 5-AVACl exhibiting the greatest improvements.

High Performance Inverted RbCsFAPbI3 Perovskite Solar Cells Based on Interface Engineering and Defects Passivation

Lead halide-based perovskites solar cells (PSCs) are intriguing candidates for photovoltaic technology due to their high efficiency, low cost, and simple fabrication processes. Currently, PSCs with efficiencies of >25% are mainly based on methylammonium (MA)-free and bromide (Br) free, formamide lead iodide (FAPbI3)-based perovskites, because MA is thermally instable due to its volatile nature and Br incorporation will induce blue shift in the absorption spectrum. Therefore, MA-free, Br-free formamidine-based perovskites are drawing huge research attention in recent years. The hole transporting layer (HTL) is crucial in fabricating highly efficient and stable inverted p-i-n structured PSCs by enhancing charge extraction, lowering interfacial recombination, and altering band alignment, etc. Here, this work employs a NiO_x /PTAA bi-layer HTL combined with GuHCl (guanidinium hydrochloride) additive engineering and PEAI (phenylethylammonium iodide) passivation strategy to optimize the charge carrier dynamics and tune defects chemistry in the MA-free, Br-free RbCsFAPbI3-based perovskite absorber, which boosts the device efficiency up to 22.78%. Additionally, the device retains 95% of its initial performance under continuous 1 sun equivalent LED light illumination at 45 °C for up to 500 h.

Critical Influence of Organic A'-Site Ligand Structure on 2D Perovskite Crystallization

Organic A'-site ligand structure plays a crucial role in the crystal growth of 2D perovskites, but the underlying mechanism has not been adequately understood. This problem is tackled by studying the influence of two isomeric A'-site ligands, linear-shaped n-butylammonium (n-BA⁺ ) and branched iso-butylammonium (iso-BA⁺ ), on 2D perovskites from precursor to device, with a combination of in situ grazing-incidence wide-angle X-ray scattering and density functional theory. It is found that branched iso-BA⁺ , due to the lower aggregation enthalpies, tends to form large-size clusters in the precursor solution, which can act as pre-nucleation sites to expedite the crystallization of vertically oriented 2D perovskites. Furthermore, iso-BA⁺ is less likely to be incorporated into the MAPbI₃ lattice than n-BA⁺ , suppressing the formation of unwanted multi-oriented perovskites. These findings well explain the better device performance of 2D perovskite solar cells based on iso-BA⁺ and elucidate the fundamental mechanism of ligand structural impact on 2D perovskite crystallization.

Multifunction Sandwich Structure Based on Diffusible 2-Chloroethylamine for High-Efficiency and Stable Tin-Lead Mixed Perovskite Solar Cells

Low-bandgap tin-lead mixed perovskites (PVKs) are necessary for all-perovskite tandem solar cells. This work proposes a multifunctional sandwich structure with 2-chloroethylamine (CEA) as the top and bottom interface layer and perovskite as the core layer. The sandwich structured CEA allows large ClCH₂CH₂NH₃⁺ and small Cl^- to diffuse into the crystal lattice and grain boundaries of perovskites, endowing an excellent antioxidation property by forming Sn(0), coordinating with SnI₂, and controlling the perovskite crystallization process. Moreover, the energy level alignment at the interface of the perovskite and transport layer becomes more matched. As a result, the CEA-modified champion device acquires a power conversion efficiency of 18.13% with an open-circuit voltage of 0.82 V and a short-circuit current density of 30.06 mA cm^-2. Meanwhile, the environmental stability of CEA-modified devices is substantially enhanced. This work introduces a new strategy for improving the performance and stability of tin-lead mixed perovskite solar cells.

The Chemical Design in High-Performance Lead Halide Perovskite: Additive vs Dopant?

Metal halide perovskite solar cells (PSCs) have attracted enormous attention as one of the most promising candidates for next-generation photovoltaics in the past few years. During the development of PSCs, various chemicals have been added to improve film quality and device performance. However, there are still debates about whether these chemicals are additives as removed from the final film or dopants incorporated into the crystal lattice. It is important to clarify whether these added chemicals are additives or dopants when designed for high-quality perovskite films' fabrications. Herein, we summarized several commonly used chemicals for hybrid and all-inorganic perovskites, such as MACl, DMAI, MAAc, and alkali metal cations. The underlying mechanism and their roles during the formation of perovskite films were discussed. In the end, we proposed some conclusive important factors to clarify additives and dopants, which would be helpful for the further chemical design for improving high-performance perovskite devices.

Chlorides, other Halides, and Pseudo-Halides as Additives for the Fabrication of Efficient and Stable Perovskite Solar Cells

Perovskite solar cells (PSCs) are attracting a tremendous attention from the scientific community due to their excellent power conversion efficiency, low cost, and great promise for the future of solar energy. The best PSCs have already achieved a certified power conversion efficiency (PCE) of 25.5 % after an unprecedented rapid performance rise. However, high requirements with respect to large area, high-efficiency devices, and stability are still the challenges. Major efforts, especially for achieving a high degree of chemical control, have been made to reach these targets. The use of halide additives has played a critical role in improving the efficiency and stability. The present paper reviews the important breakthroughs in PSC technologies made by using halide additives, especially chloride, and pseudo-halide additives for the preparation of the perovskite layers, other layers, and interfaces of the devices. These additives help perovskite (PVK) crystallization and layer morphology control, grain boundary reduction, bulk and interface defects passivation, and so on. Normally, these halide additives play different roles depending on their categories and their location. Herein, recent progresses made due to additives employment in every possible layer of PSCs are reviewed, with focus on chloride, other halides, and pseudo-halides as additives in PVK films, halide additives in carrier transport layers, and at PVK-contact interfaces. Finally, an outlook of engineering of these additives in PSC progress is given.

An in-situ defect passivation through a green anti-solvent approach for high-efficiency and stable perovskite solar cells

Surface and grain boundary defects in halide perovskite solar cells are highly detrimental, reducing efficiencies and stabilities. Widespread halide anion and organic cation defects usually aggravate ion diffusion and material degradation on the surfaces and at the grain boundaries of perovskite films. In this study, we employ an in-situ green method utilizing nontoxic cetyltrimethylammonium chloride (CTAC) and isopropanol (IPA) as anti-solvents to effectively passivate both surface and grain boundary defects in hybrid perovskites. Anion vacancies can be readily passivated by the chloride group due to its high electronegativity, and cation defects can be synchronously passivated by the more stable cetyltrimethylammonium group. The results show that the charge trap density was significantly reduced, while the carrier recombination lifetime was markedly extended. As a result, the power conversion efficiency of the cell can reach 23.4% with this in-situ green method. In addition, the device retains 85% of its original power conversion efficiency after 600 h of operation under illumination, showing that the stability of perovskite solar cells is improved with this in-situ passivation strategy. This work may provide a green and effective route to improve both the stability and efficiency of perovskite solar cells.

Beneficial effects of cesium acetate in the sequential deposition method for perovskite solar cells

The cesium cation (Cs+) is widely used as a dopant for highly efficient and stable formamidinium lead tri-halide perovskite (FAPbX3, X = I, Br, Cl) solar cells. Herein, we introduce a small amount of cesium acetate (CsAc) that can effectively stabilize FAMAPbI3 under thermal- and light illumination-stress. We show that incorporated Cs+ leads to relaxation of strain in the perovskite layer, and that Ac- forms a strong intermediate phase with PbI2, which can help the intercalation of the PbI2 film with Cs+ and cation halide (FAI, MAI, MACl) in the sequential deposition process. The addition of CsAc reduces the trap density in the resulting perovskite layers and extends their carrier lifetime. The CsAc-modified perovskite solar cells show less hysteresis phenomena and enhanced operational and thermal stability in ambient conditions. Our findings provide insight into how dopants and synthesis precursors play an important role in efficient and stable perovskite solar cells.

Origin, Influence, and Countermeasures of Defects in Perovskite Solar Cells

Defects are considered to be one of the most significant factors that compromise the power conversion efficiencies and long-term stability of perovskite solar cells. Therefore, it is urgent to have a profound understanding of their formation and influence mechanism, so as to take corresponding measures to suppress or even completely eliminate their adverse effects on device performance. Herein, the possible origins of the defects in metal halide perovskite films and their impacts on the device performance are analyzed, and then various methods to reduce defect density are introduced in detail. Starting from the internal and interfacial aspects of the metal halide perovskite films, several ways to improve device performance and long-term stability including additive engineering, surface passivation, and other physical treatments (annealing engineering), etc., are further elaborated. Finally, the further understanding of defects and the development trend of passivation strategies are prospected.

The compatibility of methylammonium and formamidinium in mixed cation perovskite: the optoelectronic and stability properties

The methylammonium (MA) and formamidinium (FA) are the most commonly used organic cations in perovskite solar cells (PSCs), whereas the impact of size and polarity differences between these two on the photovoltaic performances has been rarely revealed. Herein, we systematically investigated the phase distribution, optoelectronic and stability properties of FA-MA mixed perovskites. To identify the phase homogeneity, depth-dependent grazing-incidence wide-angle x-ray scattering measurements were employed, which demonstrates that the mixed cation perovskite possesses a FA-rich phase on the film surface and the bottom is comprised of MA-rich phase. Additionally, upon long-time illumination, a new PL peak is appeared at 778 nm, representing the generation of MA-rich phase induced by ion migration. It is worth noting that the phase splitting and inhomogeneous phase distribution would not bring any obvious detrimental effects to the photovoltaic performances and stability properties. Through judiciously tuning the cation proportion in pure-iodide perovskite, the additive-free PSCs achieve an efficiency as high as 20.7%. Furthermore, the PSCs with a broad range of FA/MA ratios show improved humidity/thermal/light stability despite the phase inhomogeneity. Therefore, the work shows that the MA and FA cations have a high compatibility in perovskite structure and the precise ratio control can further improve the performances.

Perovskite Crystallization Dynamics during Spin-Casting: An In Situ Wide-Angle X-ray Scattering Study

In situ wide-angle X-ray scattering (WAXS) has been measured during the spin coating process used to make the precursor films required for the formation of thin films of perovskite. A customized hollow axis spin coater was developed to permit the scattered X-rays to be collected in transmission geometry during the deposition process. Spin coating is the technique most commonly used in laboratories to make thin perovskite films. The dynamics of spin-casting MAPbI_3-x Cl _x and FAPbI_3-x Cl _x films have been investigated and compared to investigate the differences between the dynamics of MAPbI_3-x Cl _x and FAPbI_3-x Cl _x film formation. In particular, we focus on the crystallization dynamics of the precursor film formation. When casting MAPbI_3-x Cl _x , we observed relatively fast 1D crystallization of the intermediate product MA₂PbI₃Cl. There was an absence of the desired perovskite phase formed directly; it only appeared after an annealing step that converted the MA₂PbI₃Cl to MAPbI₃. In contrast, slower crystallization via a 3D precursor was observed for FAPbI_3-x Cl _x film formation compared to MAPbI_3-x Cl _x . Another important finding was that some FAPbI_3-x Cl _x perovskite was generated directly during spin-casting before annealing. These findings indicate that there are significant differences between the crystallization pathways for these two perovskite materials. These are likely to explain the differences in the lifetimes of the resulting perovskite solar cell devices produced using FA and MA cations.

Local Structure and Dynamics in Methylammonium, Formamidinium, and Cesium Tin(II) Mixed-Halide Perovskites from 119Sn Solid-State NMR

Organic–inorganic tin(II) halide perovskites have emerged as promising alternatives to lead halide perovskites in optoelectronic applications. While they suffer from considerably poorer performance and stability in comparison to their lead analogues, their performance improvements have so far largely been driven by trial and error efforts due to a critical lack of methods to probe their atomic-level microstructure. Here, we identify the challenges and devise a ¹¹⁹Sn solid-state NMR protocol for the determination of the local structure of mixed-cation and mixed-halide tin(II) halide perovskites as well as their degradation products and related phases. We establish that the longitudinal relaxation of ¹¹⁹Sn can span 6 orders of magnitude in this class of compounds, which makes judicious choice of experimental NMR parameters essential for the reliable detection of various phases. We show that Cl/Br and I/Br mixed-halide perovskites form solid alloys in any ratio, while only limited mixing is possible for I/Cl compositions. We elucidate the degradation pathways of Cs-, MA-, and FA-based tin(II) halides and show that degradation leads to highly disordered, qualitatively similar products, regardless of the A-site cation and halide. We detect the presence of metallic tin among the degradation products, which we suggest could contribute to the previously reported high conductivities in tin(II) halide perovskites. ¹¹⁹Sn NMR chemical shifts are a sensitive probe of the halide coordination environment as well as of the A-site cation composition. Finally, we use variable-temperature multifield relaxation measurements to quantify ion dynamics in MASnBr₃ and establish activation energies for motion and show that this motion leads to spontaneous halide homogenization at room temperature whenever two different pure-halide perovskites are put in physical contact.

Versatile Defect Passivation Methods for Metal Halide Perovskite Materials and their Application to Light-Emitting Devices

Metal halide perovskites (MHPs) have emerged as promising emitters because of their excellent optoelectronic properties, including high photoluminescence quantum yields (PLQYs), wide-range color tunability, and high color purity. However, a fundamental limitation of MHPs is their low exciton binding energy, which results in a low radiative recombination rate and the dependence of PLQY on the excitation intensity. Under the operating conditions of light-emitting diodes (LEDs), the injected current densities are typically lower than the trap density, leading to a low actual PLQY. Moreover, the defects not only initiate the decomposition of MHPs caused by extrinsic factors, but also intrinsically stimulate ion migration across the interface and lead to the corrosion of electrodes due to interaction between those electrodes, even under inert conditions. The passivation of defects has proven to be effective for mitigating the effects of defects in MHPs. Herein, the origins and theoretical calculations of the defect tolerance in MHPs and the impact of defects on both the performance and stability of perovskite LEDs are reviewed. The passivation methods and materials for MHP bulk films and nanocrystals are discussed in detail. Based on the currently reported advances, specific requirements and future research directions for display applications are suggested.

Microstructural Evolution of Hybrid Perovskites Promoted by Chlorine and its Impact on the Performance of Solar Cell

The role of Cl in halide hybrid perovskites CH₃NH₃PbI₃(Cl) (MAPbI₃(Cl)) on the augmentation of grain size is still unclear although many reports have referred to these phenomena. Herein, we synthesized MAPbI₃(Cl) perovskite films by using excess MACl-containing precursors, which exhibited approximately an order of magnitude larger grain size with higher <110>-preferred orientation compared with that from stoichiometric precursors. Comprehensive mechanisms for the large grain evolution by Cl incorporation were elucidated in detail by correlating the changes in grain orientation, distribution of grain size, and the remaining Cl in the perovskite during thermal annealing. In the presence of Cl, <110>- and <001>-oriented grains grew faster than other grains at the initial stage of annealing. Further annealing led to the dissipation of Cl, resulting in the shrinkage of <001> grains while <110> grains continuously grew, as analyzed by x-ray rocking curve and diffraction. As a result of reduced grain boundaries and enhanced <110> texture, the trap density of perovskite solar cells diminished by ~10% by incorporating MACl in the precursor, resulting in a fill factor more than 80%.

Thermal and illumination effects on a PbI2 nanoplate and its transformation to CH3NH3PbI3 perovskite

The vapor transformation of crystalline PbI₂ nanoplates into CH₃NH₃PbI₃ under annealing and illumination condition was systematically investigated in nanoscale, and the detail pathway of structural transformation and mechanism are discussed.

The formation of a functional pentacene/CH3NH3PbI3-xClx perovskite interface: optical gating and field-induced charge retention

We fabricated a functional pentacene/CH₃NH₃PbI_3-xCl_x perovskite interface where optical gating and field assisted charge retention occur. Using a pentacene/perovskite field effect transistor (FET) test platform, we investigated the interfacial charge transfer associated with optical gating through threshold voltage measurements under illumination. Importantly, bistable electrical conduction in pentacene/perovskite FET devices was achieved as a result of field-induced charge retention at the interface and the origin is discussed to be associated with interfacial charging at the pentacene/perovskite interface. Interfacial contact modification associated with ion migration and other possible effects in the perovskite layer plays a crucial role in forming a functional interface involving organic semiconducting materials.

In Situ Real-Time Study of the Dynamic Formation and Conversion Processes of Metal Halide Perovskite Films

Metal halide perovskite solar cells (PSCs) have advanced to the forefront of solution-processed photovoltaic techniques and made stunning progress in power conversion efficiency (PCE). Further improvements in device performances rely on perfecting the structure and morphology of perovskite films. However, undesirable defects such as pinholes and grain boundaries are often created in film preparations due to lack of knowledge of the precise reaction mechanism. Here, in situ grazing-incidence X-ray diffraction (GI-XRD) investigations are performed, facilitated by other techniques, on the formation of the widely adopted MAPbI₃ (MA = methylammonium) perovskite films from their intermediate adduct (IA) phases. The influences of solvent vapor atmospheres on MAPbI₃ films are also systematically investigated, where the dynamic conversion processes between different phases are visualized in real time. Further in situ GI-XRD and infrared spectroscopy measurements reveal that the IA phases contain both N,N-dimethylformamide and dimethyl sulfoxide (DMSO) as coordinating molecules. By tuning the DMSO concentration in perovskite precursors, the ideal perovskite film is formed and the best PCE is achieved for the planar MAPbI₃ -based PSCs. These findings highlight the role of IA phases and the effect of solvent atmospheres on the quality of perovskite films, providing direct insights into their growth mechanism.

Spatial Inhomogeneity of Methylammonium Lead-Mixed Halide Perovskite Examined by Space- and Time-Resolved Microwave Conductivity

Reducing the spatial inhomogeneity of solution-processed, multicrystalline methylammonium lead iodide (MAPbI₃) perovskite is of great importance for improving its power conversion efficiency, suppressing point-to-point deviations, and delaying degradation during operation. Various techniques, such as conducting-mode atomic force microscopy and photoluminescence mapping, have been applied for this intriguing class of materials, revealing nonuniform electronic properties on the nanometer-to-micrometer scale. Here, we designed a new space- and time-resolved microwave conductivity system that enables mapping of the transient photoconductivity with resolution greater than ∼45 μm. We examined the effects of the precursor concentration of MAPbI₃ and the mixing of halides (I^- and Br^-) on the charge carrier dynamics, crystal size, and inhomogeneity of the films. The optoelectronic inhomogeneity of MAPbI₃ and MAPb(I_1-x Br _x )₃ on the sub-millimeter and millimeter scales shows a general correlation with their crystallite sizes, whereas the precursor concentration and halide mixing affect the inhomogeneity in a different way, providing a basis for uniform processing of a multicrystalline perovskite film.

Photoinduced Bulk Polarization and Its Effects on Photovoltaic Actions in Perovskite Solar Cells

This article reports an experimental demonstration of photoinduced bulk polarization in hysteresis-free methylammonium (MA) lead-halide perovskite solar cells [ITO/PEDOT:PSS/perovskite/PCBM/PEI/Ag]. An anomalous capacitance-voltage (CV) signal is observed as a broad "shoulder" in the depletion region from -0.5 to +0.5 V under photoexcitation based on CV measurements where a dc bias is gradually scanned to continuously drift mobile ions in order to detect local polarization under a low alternating bias (50 mV, 5 kHz). Essentially, gradually scanning the dc bias and applying a low alternating bias can separately generate continuously drifting ions and a bulk CV signal from local polarization under photoexcitation. Particularly, when the device efficiency is improved from 12.41% to 18.19% upon chlorine incorporation, this anomalous CV signal can be enhanced by a factor of 3. This anomalous CV signal can be assigned as the signature of photoinduced bulk polarization by distinguishing from surface polarization associated with interfacial charge accumulation. Meanwhile, replacing easy-rotational MA⁺ with difficult-rotational formamidinium (FA⁺) cations largely minimizes such anomalous CV signal, suggesting that photoinduced bulk polarization relies on the orientational freedom of dipolar organic cations. Furthermore, a Kelvin probe force microscopy study shows that chlorine incorporation can suppress the density of charged defects and thus enhances photoinduced bulk polarization due to the reduced screening effect from charged defects. A bias-dependent photoluminescence study indicates that increasing bulk polarization can suppress carrier recombination by decreasing charge capture probability through the Coulombic screening effect. Clearly, our studies provide an insightful understanding of photoinduced bulk polarization and its effects on photovoltaic actions in perovskite solar cells.

Intermediate Phase Intermolecular Exchange Triggered Defect Elimination in CH3NH3PbI3 toward Room-Temperature Fabrication of Efficient Perovskite Solar Cells

The solvent-engineered one-step spin-coating method has been widely used to produce full-coverage CH₃NH₃PbI₃ films for perovskite solar cells by forming an intermediate phase. However, the resultant CH₃NH₃PbI₃ films usually contain numerous structural and compositional defects mainly resulting from the fast crystallization of the intermediate phase as well as the escape of CH₃NH₃I species induced by the inevitably thermal annealing recipe. Herein, a facile room-temperature intermolecular exchange route is proposed to enable conversion of the intermediate phase into uniform and ultra-flat CH₃NH₃PbI₃ films. It can effectively inhibit the formation of structural and compositional defects in the resultant films, and even repair their inherent defects. As a result, the efficiency of perovskite solar cells can be boosted to 19.45% with a stabilized value of 18.55%, which is much higher than that from the ones fabricated by thermal annealing. This study suggests a facile and low-cost route to room-temperature fabrication of highly efficient perovskite solar cells including flexible ones.

Progress on Perovskite Materials and Solar Cells with Mixed Cations and Halide Anions

Organic-inorganic halide perovskite materials (e.g., MAPbI₃, FAPbI₃, etc.; where MA = CH₃NH₃⁺, FA = CH(NH₂)₂⁺) have been studied intensively for photovoltaic applications. Major concerns for the commercialization of perovskite photovoltaic technology to take off include lead toxicity, long-term stability, hysteresis, and optimal bandgap. Therefore, there is still need for further exploration of alternative candidates. Elemental composition engineering of MAPbI₃ and FAPbI₃ has been proposed to address the above concerns. Among the best six certified power conversion efficiencies reported by National Renewable Energy Laboratory on perovskite-based solar cells, five are based on mixed perovskites (e.g., MAPbI_1-xBr_x, FA_0.85MA_0.15PbI_2.55Br_0.45, Cs_0.1FA_0.75MA_0.15PbI_2.49Br_0.51). In this paper, we review the recent progress on the synthesis and fundamental aspects of mixed cation and halide perovskites correlating with device performance, long-term stability, and hysteresis. In the outlook, we outline the future research directions based on the reported results as well as related topics that warrant further investigation.

Bromine substitution improves excited-state dynamics in mesoporous mixed halide perovskite films

In this study, ultrafast transient absorption spectroscopy (TAS) is utilized to examine the excited-state dynamics in methylammonium lead iodide/bromide (MAPb(I_1-xBr_x)₃) perovskites as a function of bromide content. TAS spectral behavior reveals characteristic lifetimes for thermalization, recombination, and charge carrier injection of MAPb(I_1-xBr_x)₃ from x = 0 to 0.3 infiltrated in mesoporous titania films. Carrier recombination and charge injection lifetimes demonstrated a discernable increase with Br content likely because high carrier populations are supported by the higher density of vacant electronic states in mixed-halide perovskites due to the increased capacity of the conduction band. However, we observe for the first time that carrier thermalization lifetimes significantly decrease with increasing Br. This suggests that the shift in crystal structure from tetragonal towards pseudocubic accelerates carrier cooling, resulting in the relief of the hot phonon bottleneck. Furthermore, the stabilized MAPb(I_1-xBr_x)₃ samples exhibit a lower Burstein-Moss shift of 0.07-0.08 eV compared to pure MAPbI₃ (0.12 eV). Our results provide evidence that Br inclusion contributes to a broadening of the parabolic conduction band and to improvement in electron-phonon coupling and phonon propagation in the lattice.

A PbI2-xClx seed layer for obtaining efficient planar-heterojunction perovskite solar cells via an interdiffusion process

Despite the previous reports on the fabrication of CH₃NH₃PbI_3-xCl_x films via sequential deposition, the positioning and formation of PbI₂ in MAPbI_3-xCl_x perovskite films made from the seed layer containing PbI₂ and PbCl₂ in different ratios have not yet been addressed. In this study, the PbI₂ content in a perovskite absorber layer is controlled by changing the PbCl₂ ratio in a PbI_2-xCl_x seed layer. The addition of PbCl₂ in the seed layer facilitates PbI₂ generation and affects the morphology of the perovskite film. By integrating a perovskite absorber via the PbI_2-xCl_x seed-layer into a solar cell, we investigated the effects of the correlation between the chlorine and PbI₂ contents on the device performance through intensity-modulated photocurrent spectroscopy and intensity-modulated photovoltage spectroscopy. Elemental depth profiling analyses confirm that not only was the formed PbI₂ preferentially located near the metal-oxide layer, but residual chlorine was adsorbed at the TiO₂ layer. Our findings demonstrate that the geometric features of the formed PbI₂ affected the perovskite solar cells according to the chlorine content, likely because of the elemental gradient induced by annealing. The PbI_2-xCl_x-derived planar-heterojunction perovskite solar cells exhibited maximum power-conversion efficiencies of 17.56% at reverse scan and 17.21% at forward scan, suppressed current density-voltage hysteresis, and good performance distributions.

Origin of Reversible Photoinduced Phase Separation in Hybrid Perovskites

The distinct physical properties of hybrid organic-inorganic materials can lead to unexpected nonequilibrium phenomena that are difficult to characterize due to the broad range of length and time scales involved. For instance, mixed halide hybrid perovskites are promising materials for optoelectronics, yet bulk measurements suggest the halides reversibly phase separate upon photoexcitation. By combining nanoscale imaging and multiscale modeling, we find that the nature of halide demixing in these materials is distinct from macroscopic phase separation. We propose that the localized strain induced by a single photoexcited charge interacting with the soft, ionic lattice is sufficient to promote halide phase separation and nucleate a light-stabilized, low-bandgap, ∼8 nm iodide-rich cluster. The limited extent of this polaron is essential to promote demixing because by contrast bulk strain would simply be relaxed. Photoinduced phase separation is therefore a consequence of the unique electromechanical properties of this hybrid class of materials. Exploiting photoinduced phase separation and other nonequilibrium phenomena in hybrid materials more generally could expand applications in sensing, switching, memory, and energy storage.

Magnetodielectric Response from Spin-Orbital Interaction Occurring at Interface of Ferromagnetic Co and Organometal Halide Perovskite Layers via Rashba Effect

The spin on a ferromagnetic Co surface can interact with the asymmetric orbital on an organometal halide perovskite surface, leading to an anisotropic magnetodielectric effect. This study presents an opportunity to integrate ferromagnetic and semiconducting properties through the Rasbha effect for achieving spin-dependent electronic functionalities based on thin-film design.

Correlation of annealing time with crystal structure, composition, and electronic properties of CH3NH3PbI3−xClx mixed-halide perovskite films

Perovskite films composition evolves from a phase separation into uniform single phase with high preferred crystal orientation upon annealing.

Hybrid Perovskite Thin-Film Photovoltaics: In Situ Diagnostics and Importance of the Precursor Solvate Phases

Solution-processed hybrid perovskite semiconductors attract a great deal of attention, but little is known about their formation process. The one-step spin-coating process of perovskites is investigated in situ, revealing that thin-film formation is mediated by solid-state precursor solvates and their nature. The stability of these intermediate phases directly impacts the quality and reproducibility of thermally converted perovskite films and their photovoltaic performance.

Low-Pressure Vapor-Assisted Solution Process for Thiocyanate-Based Pseudohalide Perovskite Solar Cells

In this report, we fabricated thiocyanate-based perovskite solar cells with low-pressure vapor-assisted solution process (LP-VASP) method. Photovoltaic performances are evaluated with detailed materials characterizations. Scanning electron microscopy images show that SCN-based perovskite films fabricated using LP-VASP have long-range uniform morphology and large grain sizes up to 1 μm. The XRD and Raman spectra were employed to observe the characteristic peaks for both SCN-based and pure CH₃ NH₃ PbI₃ perovskite. We observed that the Pb(SCN)₂ film transformed to PbI₂ before the formation of perovskite film. X-ray photoemission spectra (XPS) show that only a small amount of S remained in the film. Using LP-VASP method, we fabricated SCN-based perovskite solar cells and achieved a power conversion efficiency of 12.72 %. It is worth noting that the price of Pb(SCN)₂ is only 4 % of PbI₂ . These results demonstrate that pseudo-halide perovskites are promising materials for fabricating low-cost perovskite solar cells.

Insight into the CH3NH3PbI3/C interface in hole-conductor-free mesoscopic perovskite solar cells

Perovskite solar cells (PSCs) with hole-conductor-free mesoscopic architecture have shown superb stability and great potential in practical application. The printable carbon counter electrodes take full responsibility of extracting holes from the active CH3NH3PbI3 absorbers. However, an in depth study of the CH3NH3PbI3/C interface properties, such as the structural formation process and the effect of interfacial conditions on hole extraction, is still lacking. Herein, we present, for the first time, an insight into the spatial confinement induced CH3NH3PbI3/C interface formation by in situ photoluminescence observations during the crystallization process of CH3NH3PbI3. The derived reaction kinetics allows a quantitative description of the perovskite formation process. In addition, we found that the interfacial contact between carbon and perovskite was dominant for hole extraction efficiency and associated with the photovoltaic parameter of short circuit current density (JSC). Consequently, we conducted a solvent vapor assisted process of PbI2 diffusion to carefully control the CH3NH3PbI3/C interface with less unreacted PbI2 barrier. The improvement of interface conditions thereby contributes to a high hole extraction proved by the charge extraction resistance and PL lifetime change, resulting in the increased JSC valve.

Low temperature synthesis of hierarchical TiO2 nanostructures for high performance perovskite solar cells by pulsed laser deposition

High aspect-ratio TiO₂ nanostructures directly assembled with pulsed laser deposition could improve interfacial contact for superior perovskite photovoltaic cells.

Interplay of structural and compositional effects on carrier recombination in mixed-halide perovskites

We investigate the effect of grain structure and bromide content on charge transport in methylammonium lead iodide/bromide perovskites by probing the steady-state and time-resolved photoluminescence of planar films with distinct morphologies.