FAPbI3 Films Research Articles

Exploring the Limits: Degradation Behavior of Lead Halide Perovskite Films under Exposure to Ultrahigh Doses of γ Rays of Up to 10 MGy

Herein, we show that thin films of MAPbI3, FAPbI3, (CsMA)PbI3, and (CsMAFA)PbI3, where MA and FA are methylammonium and formamidinium cations, respectively, tolerate ultrahigh doses of γ rays approaching 10 MGy without significant changes in their absorption spectra. However, among the studied materials, FAPbI3 was the only one that did not form metallic lead due to its extreme radiation hardness. Infrared near-field optical microscopy revealed the radiation-induced depletion of organic cations from the grains of MAPbI3 and their accumulation at the grain boundaries, whereas FAPbI3 on the contrary lost FA cations from the grain boundaries. The multication (CsMAFA)PbI3 perovskite underwent a facile phase segregation to domains enriched with MA and FA cations, which is a principally new radiation-induced degradation pathway. Our findings suggest that the radiation hardness of the rationally designed perovskite semiconductors could go far beyond the impressive threshold of 10 MGy we set herein for FAPbI3 films, which opens many exciting opportunities for practical implementation of these materials.

Modeling Monte Carlo simulation on photon regeneration effects of perovskite FAPbI3 for photovoltaic applications.

Controlling the photon regeneration effects in FAPbI3 films is a noteworthy approach to improve the photovoltaic (PV) efficiency of FAPbI3-based solar cells. However, the lack of systematic study on the relationship between photon regeneration effects and PV efficiency in the experimental process makes it difficult to control the photon regeneration effects effectively. In this work, we combine the Monte Carlo sampling method and the polar coordinate calculation method to design a new algorithm for a detailed simulation of the main processes of photon regeneration effects affecting the PV efficiency in a model based on an n-i-p type FAPbI3 perovskite solar cell (PSC). The algorithm is validated to be used to compare the power-conversion efficiency (PCE) of different PSCs to filter out the PSC structure with the highest PCE or to determine the range of material parameter values corresponding to the highest PCE. This work opens up new ideas to effectively control the photon regeneration effects in PSCs to improve the device PV efficiency.

Intercalation of Ammonium Cationic Ligands Enabled Grain Surface Passivation in Sequential‐Deposited Perovskite Solar Cells

Solution‐processed formamidinium lead iodide (FAPbI3) perovskite typically contains a high number of ionic defects that are intrinsically formed during film formation. To reduce the defects, postsynthetic surface passivation treatment is widely practiced. However, the practicality of the surface passivation approach is limited by the poor coverage and incomplete adsorption of passivators into the defective sites. Unprecedentedly, the use of 4‐(trifluoromethyl)benzylammonium iodide (CF3BZAI) is demonstrated as a novel passivator additive for sequentially deposited perovskite films. Due to its unique molecular structure and trifluoromethyl (–CF3) moiety, CF3BZAI is expected to have enhanced adsorption with defect sites during the film formation. Owing to grain surface passivation, the CF3BZAI‐intercalated FAPbI3 (target) film has enhanced morphology and crystallinity as well as significantly fewer defects than the normal FAPbI3 film. Interestingly, the intercalation of CF3BZAI passivators does not lead to the formation of a low‐dimensional perovskite phase in FAPbI3 films. The best perovskite solar cell (PSC) device based on the target film achieves a maximum efficiency of ≈22.4%, which is much higher than the efficiency (≈20.7%) of the normal device. CF3BZAI‐assisted grain surface passivation is a facile yet effective strategy to enhance the performance and stability of FAPbI3‐based PSCs.

Influence of Formamidine Formate Doping on Performance and Stability of FAPbI3-Based Perovskite Solar Cells

Formamidine lead iodide (FAPbI3) perovskite material is very suitable for solar photovoltaic devices because of its ideal low band gap, theoretically high efficiency, and wide range of solar spectral absorption, coupled with its good thermal stability. A two-step spin coating method could control the crystallization process of formamidine lead iodide perovskite films better, resulting in more easily repeatable high-quality films. However, it is still difficult to avoid the formation of halide I-vacancy during the preparation of films, which will affect device performance and stability. In this paper, we added small molecular formamidine formate (FAHCOO) into the PbI2 precursor solution. Due to the high binding energy between HCOO− and I-vacancy, film defects caused by I-vacancies could be passivated. A molecular exchange process could be introduced in the two-step method with the addition of FAHCOO. The exchange process could delay the crystallization process in perovskite films and make them transform more fully; thus, ultimately improving the crystallization quality of the films. In addition, by adding FAHCOO to the PbI2 precursor solution, a small number of FAPbI3 can be pre-generated as templates. These templates could induce the growth of specific crystal planes of FAPbI3 in the second step reaction; thereby, improving the crystallinity of FAPbI3 films. The FAPbI3 of devices with optimized FAHCOO show a champion power conversion efficiency (PCE) of 19.04%, apparently higher than that of the controlled devices without FAHCOO (16.69%). For working stability tests under AM 1.5G illumination in an air environment, PSCs with FAHCOO showed nearly 100% of their initial efficiency after a 4100 s tracking test, while the original control device dropped to about 94%.

Phase-dependent memristive behaviors in FAPbI3-based memristors

In this paper, memristors with a structure of Ag/FAPbI3/FTO were fabricated by using FAPbI3 perovskite as the active layer. The FAPbI3 films were synthesized with a solution-based method and different annealing temperatures resulted in different phases, i.e., δ-phase and α-phase. The devices based on FAPbI3 films with these two phases were characterized by voltage sweeps with different ranges and polarity. It was found that devices based on δ-FAPbI3 films resulted from annealing at 80 °C exhibited good memristive behaviors, i.e., the device resistance (in terms of current) changes continuously according to the charge flowing through the device. In particular, under the stimulation of positive voltage sweeps, the device current gradually increased, and in contrast, negative voltages reduced the current accordingly with the number of applied sweeps. The device response in terms of changes in current showed good consistency with the number of applied voltage sweeps, even for situations with combination of positive and negative voltage sweeps. It was further found that relatively large voltages with a threshold of ~0.4 V can be used to induce memristive response in δ-FAPbI3 based devices. The memristive effect is related to Ag electrodes, which are expected to gradually form bridge-like conductive filaments (CFs) between top and bottom electrodes. The repeated positive voltages applied to device induce gradual formation of CFs while negative voltages tend to disrupt CFs also in a gradual manner. As a result, memristive behavior was realized in Ag/δ-FAPbI3/FTO structured memristors, which can be used for neuromorphic device applications (e.g., electronic synapses).

Stable α‐FAPbI3 in Inverted Perovskite Solar Cells with Efficiency Exceeding 22% via a Self‐Passivation Strategy

AbstractFormamidinium lead iodide (FAPbI3) has endowed power conversion efficiencies (PCEs) up to 25.5% in regular‐structured perovskite solar cells (PSCs) because of its optimal bandgap and enhanced thermal stability. However, the performance of FAPbI3‐based inverted‐structured PSCs is unsatisfactory. Herein, four kinds of commonly used hole transport materials (HTMs) are selected, including PEDOT:PSS, PTAA, NiOx, and MeO‐2PACz, to study their impact on the methylamine chloride (MACl)‐assisted one‐step deposition of FAPbI3 films. It is found that MeO‐2PACz is the optimal substrate for stabilizing black‐phase FAPbI3 and the corresponding inverted‐structured PSCs show the best photovoltaic performance. Nonetheless, the PCE is restricted by low open‐circuit voltage (VOC) due to non‐radiative recombination caused by MACl residues. Therefore, homologous PbI2 in situ passivation is implemented to passivate defects at grain boundaries. The addition of excess PbI2 in precursor solution remarkably decreases charge trap densities and elongates carrier lifetimes. As a result, the optimized device achieves an impressive PCE of 22.13%, which is the highest efficiency of FAPbI3 based on inverted‐structured PSCs. Moreover, the best device exhibits free hysteresis and excellent long‐term stability, maintaining 92% of the initial PCEs after 800 h aging under ambient conditions.

FAPbI3 Perovskite Solar Cells: From Film Morphology Regulation to Device Optimization

Organic–inorganic hybrid perovskite solar cells (PSCs) have attracted great attentions due to their rapid increase of power conversion efficiency (PCE). Although the highest PCE of PSCs (25.7%) has been achieved via using formamidinium lead iodide (FAPbI3) with a suitable bandgap, there is still a lack of systematic analysis on FAPbI3‐based PSCs toward high stability and high efficiency. Herein, the progress in FAPbI3 films and achievements in their high‐efficiency and long‐term stability PSCs are comprehensively reviewed. First, the progress from the aspects of morphology, defect, dimension, and strain for FAPbI3 film optimization is summarized and then the development of FAPbI3 PSCs in both efficiency and stability is discussed. Then, the methods to improve the FAPbI3 film quality by morphology control, defect passivation, dimensional regulation, and strain engineering, as well as strategies to optimize the device structure and interface layers, which are critical to promote device stability and efficiency, are evaluated. Finally, the outlook and strategies for realizing commercialized FAPbI3 PSCs with high efficiency and long lifetime are discussed.

Influence of Photon Pump Fluence on Charge Carriers in FAPbI3 and Manganite Perovskites

FAPbI3 and FA(Mn:Pb)I3 perovskite films were prepared and evaluated through steady and transient absorption spectroscopy. According to the analysis using Elliot’s model, there were no considerable differences except for the absorption intensity between FAPbI3 and FA(Mn:Pb)I3 perovskite films: the value of the optical gap (Eg) and the position of exciton resonance (E0) were the same. The femtosecond transient absorption showed biexponential relaxation properties of the charge carriers, suggesting that biexcitons are more easily generated in FA(Mn:Pb)I3 than FAPbI3 perovskite. The generation of biexcitons in FA(Mn:Pb)I3 was also confirmed by the photon pump fluence dependence. Moreover, we were able to estimate the average number of absorbed photons directly from the photon pump power dependence without needing any further experimental measurements such as photoluminescence. Our findings may offer a new way of understanding photoinduced carrier dynamics in perovskite manganites.

Stabilizing black-phase FAPbI3in humid air with secondary ammoniums

Three secondary ammoniums (SAs) were applied to stabilize α-FAPbI3under high humidity. Substituent-dependent distributions and functions of SAs in FAPbI3films were investigated. A photovoltaic efficiency of 23.38% with high stability was achieved.

Synergetic Co‐Modulation of Crystallization and Co‐Passivation of Defects for FAPbI3 Perovskite Solar Cells

AbstractEnhancing crystallinity, passivating the grain boundary and interfacial defects have been validated to be critical for improving the power conversion efficiency (PCE) and stability of perovskite solar cells (PSCs). Herein, a synergetic co‐modulation and co‐passivation strategy is proposed to simultaneously enhance crystallinity and passivate the grain boundary and surface defects of FAPbI3 based PSCs. The 4‐fluoro‐phenethylammonium iodide (4‐F‐PEAI) added in precursor solution and poly (9‐vinylcarbazole) (PVK) added in antisolvent can jointly modulate the crystallization of FAPbI3 films. The 4‐F‐PEAI‐derived 2D perovskite, which is spontaneously formed at the grain boundaries of FAPbI3, can passivate the defects effectively. In the meantime, PVK left on top of a FAPbI3 layer can passivate the surface defects and meanwhile function as an interfacial barrier layer between FAPbI3 and hole transport layer (HTL) to mitigate the detrimental interfacial charge recombination. With the holistic benefit of the enhanced crystallinity, reduced defects and trap sites, and mitigated non‐radiative recombination and suppressed ion migration, the encouraging PCEs up to 21.6% is achieved for the resulting modified PSCs. Additionally, this strategy endows the device with notably enhanced operational stability under continuous exposure to illumination, with more than 84% of the initial PCE being maintained after continuous illumination for 800 h.

Enhancing the Phase Stability of Formamidinium Lead Triiodide by Addition of Calcium Chloride

Formamidinium lead triiodide (FAPbI3) with stable black perovskite phase and without significantly changing its bandgap is demonstrated by the incorporation of CaCl2. More incorporated CaCl2 in FAPbI3 creates a desirable crystal structure, satisfying the Goldschmidt tolerance factor, and thereby stabilizing black perovskite phase under ambient conditions. However, more incorporated CaCl2 in FAPbI3 is likely to degrade the perovskite due to its hygroscopic characteristics. In addition, the incorporated CaCl2 in FAPbI3 solution reduces solubility, causing the formation of smaller crystal grains and pinholes due to quick precipitation during the non-solvent dripping crystallization process. Accordingly, 5 % CaCl2 incorporated FAPbI3 films have the best phase stability and solar cell performance with 18.0 % of power conversion efficiency at 1 sun condition.

Dip coated SnO2 film as electron transport layer for low temperature processed planar perovskite solar cells

Perovskite solar cells (PSCs) made their mark in the photovoltaic research community due to their accelerating efficiencies, bandgap tunability, ease of fabrication, choice of substrates, etc. The electron transport layer (ETL) plays a decisive role in determining the performance and scalability of PSCs. SnO2 is a potential candidate for ETL in PSCs due to its high transmittance, low sintering temperatures, and suitable deep conduction and valence band positions that allows efficient electron extraction at the interfaces. In this work, we developed a simple, solution-processable low-temperature deposition of SnO2 on fluorine-doped tin oxide (FTO) glass substrates employing dip-coating technique for the first time. The number of dipping cycles are optimized to attain uniform coverage of the SnO2 layer on FTO. Formamidinium lead tri-iodide (FAPbI3) perovskite is deposited onto the dip-coated SnO2 electrode by vapor assisted solution process. XRD reveals the successful SnO2 phase formation by a low-temperature dip-coating method and FAPbI3 perovskite film formation by vapor assisted solution process with an unremarkable presence of the photo inactive δ phase. The surface morphology of the FAPbI3 films is smooth with an average grain size of 220 nm. A preliminary study on the planar device's photovoltaic performance using CuSCN as hole transport material (HTM) and Au as back contact exhibited a power conversion efficiency of 3.2% using four deposition cycles of SnO2 by dip-coating as the electron transport layer. This study demonstrates the utilization of dip-coating as a cost-effective method to deposit ETL that paves the way for PSCs' scale-up.

Moisture-Resistant FAPbI3 Perovskite Solar Cell with 22.25 % Power Conversion Efficiency through Pentafluorobenzyl Phosphonic Acid Passivation.

Perovskite solar cells (PSCs) have shown great promise for photovoltaic applications, owing to their low-cost assembly, exceptional performance, and low-temperature solution processing. However, the advancement of PSCs towards commercialization requires improvements in efficiency and long-term stability. The surface and grain boundaries of perovskite layer, as well as interfaces, are critical factors in determining the performance of the assembled cells. Defects, which are mainly located at perovskite surfaces, can trigger hysteresis, carrier recombination, and degradation, which diminish the power conversion efficiencies (PCEs) of the resultant cells. This study concerns the stabilization of the α-FAPbI3 perovskite phase without negatively affecting the spectral features by using 2,3,4,5,6-pentafluorobenzyl phosphonic acid (PFBPA) as a passivation agent. Accordingly, high-quality PSCs are attained with an improved PCE of 22.25 % and respectable cell parameters compared to the pristine cells without the passivation layer. The thin PFBPA passivation layer effectively protects the perovskite layer from moisture, resulting in better long-term stability for unsealed PSCs, which maintain >90 % of the original efficiency under different humidity levels (40-75 %) after 600 h. PFBPA passivation is found to have a considerable impact in obtaining high-quality and stable FAPbI3 films to benefit both the efficiency and the stability of PSCs.

One-step synthesis of FA1-xGAxPbI3 perovskites thin film with enhanced stability of alpha (α) phase

In this work, formamidinium lead iodide (FAPbI3) with different Guanidinium (GA) content (2.5%, 5%, 10% and 20%) were successfully deposited by a spin coating method. The influence of GA incorporation into the FA1-xGAxPbI3 films was studied. The crystal structures, surface morphology and optical properties have been characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM), Atomic force microscopy (AFM), Transmission electron microscopy (TEM), photoluminescence and UV–Visible spectrometer, respectively. The crystalline structure of the undesirable δ and desirable α-phase were found in the orientation of (010) and (110) planes at diffraction angles 2 θ = 11.5° and 13.8°, respectively. It is observed from the XRD results that the GA content plays an important role in the formation of α/δ phases and stabilization of the α-FAPbI3 phase. Herein, the 10% GA is an adequate amount for FA1-xGAxPbI3 lead to a homogenous, stable α-phase, rough surface, large in grain size and formation of pinhole-free perovskite film. The optical analysis revealed that by increasing the GA content decrease the band gape to lower energy from 1.75 to 1.41 eV. Furthermore, after 15 days the stability of 10% GA was found that remained excellent in a ~60% humid environment. The obtained results indicated that the incorporation of GA into FAPbI3 films makes it promising candidates for photovoltaic application.

FAPbI3 perovskite thin film having α/δ phase junction and its light harvesting performance in solar cell

In this study, FAPbI3 perovskite thin films having α/δ phase junction have been prepared by the two-step solution method at short annealing time. In first stage, the structural, morphological and optical properties of the PbI2 and FAPbI3 films have been investigated. The XRD, SEM and UV–Visible analyses have shown that the FAPbI3 film quality has improved with increasing annealing temperature and the film annealed at 170 °C have bandgap of 1.49 eV and high covering capability with average grain size of ~ 61 nm. The transition from PbI2 to FAPbI3 form has enhanced with increasing annealing temperature. Moreover, the α/δ phase junction form crystalline depend annealing temperature has been observed in FAPbI3 films and the α/δ phase junction has enhanced with increasing annealing temperature. In second stage, performance of the FAPbI3 films have been investigated as active layer in solar cell having structure of Glass/ITOPEDOT:PSS(~ 50 nm)/FAPbI3(~ 400 nm)/PC61BM(~ 70 nm)/Al(~ 100 nm). The best solar cell has performance parameters; Jsc is 18.12 mA/cm2, Voc is 0.74 V, FF is 59% and PCE is 7.91%. The results showed that presence of dominant δ phase in FAPbI3-based solar cells has caused S-sharped J–V character.

Stabilizing Formamidinium Lead Iodide Perovskite by Sulfonyl‐Functionalized Phenethylammonium Salt via Crystallization Control and Surface Passivation

Bulky organic ammonium cations have been widely used to stabilize lead halide perovskites via surface passivation or dimensionality modulation. Herein, a sulfonyl fluoride‐functionalized phenethylammonium salt (SF‐PEA) is reported as a bifunctional additive to stabilize the formamidinium lead iodide (FAPbI3) perovskite. The sulfonyl group is found to interact with PbI2 in the precursor and slow down the crystallization of FAPbI3 during thermal annealing, leading to improved crystalline quality and decreased structural defects. After annealing, the spontaneous assembly of SF‐PEA on the crystal surface of FAPbI3 not only passivates the surface defects, but also protects the perovskite from phase transition that is caused by strain or moisture invasion. The resulting FAPbI3 films are extremely stable, which can maintain their black phase for more than 3 months in air with 40–50% relative humidity, much better than pristine and unsubstituted phenethylammonium (PEA)‐based samples. Because of the greatly improved phase stability and crystallization quality, a champion power conversion efficiency (PCE) of 21.25% (certified PCE of 20.70%) is achieved in planar n–i–p structured solar cells, which is the highest one among “methylammonium‐free” FAPbI3 perovskite photovoltaics.

Scalable fabrication of high-quality crystalline and stable FAPbI3 thin films by combining doctor-blade coating and the cation exchange reaction.

Formamidinium lead iodide (FAPbI3) is one of the most extensively studied perovskite materials due to its narrow band gap and high absorption coefficient, which makes it highly suitable for optoelectronic applications. Deposition of a solution containing lead iodide (PbI2) and formamidinium iodide (FAI) or sequential deposition of PbI2 and FAI usually leads to the formation of films with a poor morphology and an unstable crystal structure that readily crystallize into two different polymorphs: the photoinactive yellow phase and the photoactive black phase. In this work, 2D 2-phenylethylammonium lead iodide (PEA2PbI4) thin films are deposited by a scalable doctor-blade coating technique and used as a growth template for the high-quality 3D FAPbI3 perovskite thin films which are obtained by organic cation exchange. We report the structural, morphological and optical properties of these converted 3D FAPbI3 perovskite films which we compare to the directly deposited 3D FAPbI3 films. The converted FAPbI3 thin films are compact, smooth, and highly oriented and exhibit better structural stability in comparison with the directly deposited 3D films. These results not only underscore the importance of the employed deposition techniques in fabricating highly crystalline and stable perovskite thin films but also provide a strategy to easily obtain very compact perovskite layers using doctor-blade coating.

Direct Observation and Quantitative Analysis of Mobile Frenkel Defects in Metal Halide Perovskites Using Scanning Kelvin Probe Microscopy

Ion migration is seen as a primary stability concern of halide perovskite-based photovoltaic and optoelectronic devices. Here, we provide experimental studies of long-distance, reversible ion migration in methylammonium lead iodide (MAPbI3) and formamidinium lead iodide (FAPbI3) films. We use time-resolved scanning Kelvin probe microscopy on insulator-coated lateral electrodes to probe the dynamic redistribution of charged Frenkel defects over micrometer distances after application of an electric field. We combine these dynamic measurements with drift–diffusion simulations that yield self-consistent pictures of the sign, distribution, mobility, and activation energy of the associated, mobile Frenkel defects. This comprehensive approach is applied to study the impact of an organic cation on ionic mobility in metal halide perovskites, which we find to be significantly reduced in the case of FAPbI3 films compared to MAPbI3 films.

Tuning Molecular Interactions for Highly Reproducible and Efficient Formamidinium Perovskite Solar Cells via Adduct Approach.

The Lewis acid-base adduct approach has been widely used to form uniform perovskite films, which has provided a methodological base for the development of high-performance perovskite solar cells. However, its incompatibility with formamidinium (FA)-based perovskites has impeded further enhancement of photovoltaic performance and stability. Here, we report an efficient and reproducible method to fabricate highly uniform FAPbI3 films via the adduct approach. Replacement of the typical Lewis base dimethyl sulfoxide (DMSO) with N-methyl-2-pyrrolidone (NMP) enabled the formation of a stable intermediate adduct phase, which can be converted into a uniform and pinhole-free FAPbI3 film. Infrared and computational analyses revealed a stronger interaction between NMP with the FA cation than DMSO, which facilitates the formation of a stable FAI·PbI2·NMP adduct. On the basis of the molecular interactions with different Lewis bases, we proposed criteria for selecting the Lewis bases. Owed to the high film quality, perovskite solar cells with the highest PCE over 20% (stabilized PCE of 19.34%) and average PCE of 18.83 ± 0.73% were demonstrated.

High performance ambient-air-stable FAPbI3 perovskite solar cells with molecule-passivated Ruddlesden–Popper/3D heterostructured film

We report a Ruddlesden–Popper/3D heterostructure combined with molecule passivation within α-phase FAPbI3 films for high-performance and ambient-air-stable solar cells.