FA-based Perovskite Research Articles

Controlling Intermediate Phase Formation to Enhance Photovoltaic Performance of Inverted FA-Based Perovskite Solar Cells

Controlling Intermediate Phase Formation to Enhance Photovoltaic Performance of Inverted FA-Based Perovskite Solar Cells

Bridging-Solvent Strategy for Quasi-Single-Crystal Perovskite Films and Stable Solar Cells.

Controllable fabrication of formamidinium (FA)-based perovskite solar cells (PSCs) with both high efficiency and long-term stability is the key to their further commercialization. However, the diversity of PbI2 complexes and perovskite compositions usually leads to light sensitive PbI2 residues and phase impurities in the film, which can accelerate the device degradation. Here, the crystallization kinetics of FA-based perovskite films are studied and a bridging-solvent strategy is proposed to modulate the reaction kinetics between PbI2 and ammonium salts by prohibiting the formation of undesired intermediates. N-methylpyrrolidone (NMP) solvent is introduced into the PbI2 precursor solution to obtain stable and homogeneous PbI2-NMP complex films. The strong interaction between NMP and formamidinium iodide (FAI) molecules promotes the conversion from PbI2-NMP into (001)-oriented quasi-single-crystal perovskite films with negligible impurities, long carrier lifetime of 1.5µs and a large grain size of 3µm. The optimized PSCs exhibit a high power conversion efficiency of 24.1%, as well as superior shelf stability which maintains 95% initial efficiency after storage in air for 1200h (T95=1200h), and operating stability with T96=300h under continuous working at the maximum power point. This work offers a simple and reproducible method for fabricating phase-pure and uniaxially oriented perovskite films.

Equally Efficient Perovskite Solar Cells and Modules Fabricated via N-Ethyl-2-Pyrrolidone Optimized Vacuum-Flash.

Efficiency reduction in perovskite solar cells (PSCs) during the magnification procedure significantly hampers commercialization. Vacuum-flash (VF) has emerged as a promising method to fabricate PSCs with consistent efficiency across scales. However, the slower solvent removal rate of VF compared to the anti-solvent method leads to perovskite films with buried defects. Thus, this work employs low-toxic Lewis base ligand solvent N-ethyl-2-pyrrolidone (NEP) to improve the nucleation process of perovskite films. NEP, with a mechanism similar to that of N-methyl-2-pyrrolidone in FA-based perovskite formation, enhances the solvent removal speed owing to its lower coordination ability. Based on this strategy, p-i-n PSCs with an optimized interface attain a power conversion efficiency (PCE) of 24.19% on an area of 0.08 cm2. The same nucleation process enables perovskite solar modules (PSMs) to achieve a certified PCE of 23.28% on an aperture area of 22.96 cm2, with a high geometric fill factor of 97%, ensuring nearly identical active area PCE (24%) in PSMs as in PSCs. This strategy highlights the potential of NEP as a ligand solvent choice for the commercialization of PSCs.

Efficiency and stability enhancement in FA-based perovskite solar cells using controlled reduction of graphene oxide interlayer – An experimental investigation

Reduced graphene oxide (rGO) as an interlayer and interface engineer, due to its conductivity and hydrophobicity, can be coated on top of perovskite layers to increase the photovoltaic performance and stability of perovskite solar cells. Herein, we employed reduced graphene oxide as an interlayer. By controlling the reduction level of rGO, its potential for boosting perovskite solar cells' efficiency was further increased. Results showed that increasing the reduction degree of rGO to an optimal level increased its conductivity and hole mobility, which induced charge transfer at perovskite/hole transport layer and increased the open-circuit voltage of solar cells. Our rGO reduction controlling strategy enables FAPbI3-based solar cells to record a PCE of 22.29%. In addition, increasing reduction degree increased hydrophobicity behavior of rGO. It increased perovskite solar cell resistance against humidity and heating degradation. The developed method in current study can push up perovskite solar cells to industrialization.

Enhanced stability of FA-based perovskite: Rare-earth metal compound EuBr2 doping

It is highly desirable to enhance the long-term stability of perovskite solar cells (PSCs) so that this class of photovoltaic cells can be effectively used for the commercialization purposes. In this contribution, attempts have been made to use the two-step sequential method to dope EuBr2 into FAMAPbI3 perovskite to promote the stability. It is shown that the device durability at 85 °C in air with RH of 20%–40% is improved substantially, and simultaneously the champion device efficiency of 23.04% is achieved. The enhancement in stability is attributed to two points: (i) EuBr2 doping effectively inhibits the decomposition and α–δ phase transition of perovskite under ambient environment, and (ii) EuBr2 aggregates in the oxidized format of Eu(BrO3)3 at perovskite grain boundaries and surface, hampering humidity erosion and mitigates degradation through coordination with H2O.

Unlocking the Ambient Temperature Effect on FA-Based Perovskites Crystallization by In Situ Optical Method.

Multiple cation-composited perovskites are demonstrated as a promising approach to improving the performance and stability of perovskite solar cells (PSCs). However, recipes developed for fabricating high-performance perovskites in laboratories are always not transferable in large-scale production, as perovskite crystallization is highly sensitive to processing conditions. Here, using an in situ optical method, the ambient temperature effect on the crystallization process in multiple cation-composited perovskites is investigated. It is found that the typical solvent-coordinated intermediate phase in methylammonium lead iodide (MAPbI3) is absent in formamidinium lead iodide (FAPbI3), and nucleation is almost completed in FAPbI3 right after spin-coating. Interestingly, it is found that there is noticeable nuclei aggregation in Formamidinium (FA)-based perovskites even during the spin-coating process, which is usually only observed during the annealing in MAPbI3. Such aggregation is further promoted at a higher ambient temperature or in higher FA content. Instead of the general belief of stress release-induced crack formation, it is proposed that the origin of the cracks in FA-based perovskites is due to the aggregation-induced solute depletion effect. This work reveals the limiting factors for achieving high-quality FA-based perovskite films and helps to unlock the existing narrow processing window for future large-scale production.

Perovskite grain fusion strategy via controlled methylamine gas release for efficient and stable formamide-based perovskite solar cells

The effectiveness of post-treatment on formamide (FA)-based perovskites by applying methylammonium (MA) has not been reported so far, due to the formation of fatal δ-phase. Herein, we select a series of carboxyl compounds to form their MA salts, utilizing the volatile nature of MA+, so as to control the release of MA gas via an MA-gas-releasing secondary grain growth (MGR-SGG) technique, stabilizing FA-based perovskite while inhibiting the formation of photo-inactive δ-phase. After such a treatment, perovskite grains fuse into larger ones, forming perovskites with better crystallinity, while the remaining compounds, including the carboxyl molecules and the undecomposed MA salts left on the perovskite surface, present passivation effect. The synergetic effects are tuned by varying the carboxyl acid backbones, and the moderate OPEM exhibits suitable MA gas release rate and strong interaction with perovskite, therefore sufficiently diminish defect density, contributing to increased open-circuit voltages by more than 20 mV, and enhanced maximum power conversion efficiency from 21.3% (untreated cells) to 22.4% (MGR-SGG treated cells), along with promoted device stability.

Highly Stable and Efficient Formamidinium-Based 2D Ruddlesden-Popper Perovskite Solar Cells via Lattice Manipulation.

Formamidinium (FA)-based 2D perovskites have emerged as highly promising candidates in solar cells. However, the insertion of 2D spacer cations into the perovskite lattice concomitantly introduces microstrain and unfavorable orientations that hinder efficiency and stability. In this study, by finely tuning the FA-based 2D perovskite lattice through spacer cation engineering, a stable lattice structure with balanced distortion, microstrain relaxation, and reduced carrier-lattice interactions is achieved. These advancements effectively stabilize the inherently soft lattice against light and thermal-aging stress. To reduce the photocurrent loss induced by undesired crystal texture, a polarity-matched molecular-type selenourea (SENA) additive is further employed to modulate the crystallization kinetics. The introduction of the SENA significantly inhibits the disordered crystallization induced by spacer cations and drives the templated growth of the quantum well structure with a vertical orientation. This controlled crystallization process effectively reduces crystal defects and enhances charge separation. Ultimately, the optimized FA-based perovskite photovoltaic devices achieve a remarkable power conversion efficiency (PCE) of 20.03% (certified steady-state efficiency of 19.30%), setting a new record for low-n 2D perovskite solar cells. Furthermore, the devices exhibit less than 1% efficiency degradation after operating at maximum power point for 1000 h and maintain excellent stability after thermal aging and cycles of cold-warm shock, respectively.

Stable and High-Efficiency Perovskite Solar Cells Using Effective Additive Ytterbium Fluoride.

With better light utilization, larger tolerance factor, and higher power conversion efficiency (PCE), the HC(NH2 )2 + (FA)-based perovskite is proven superior to the popular CH3 NH3 + (MA)- and Cs-based halide perovskites in solar cell applications. Unfortunately, limited by intrinsic defects within the FA-based perovskite films, the perovskite films can be easily transformed into a yellow δ-phase at room temperature in the fabrication process, a troublesome challenge for its further development. Here, ytterbium fluoride (YbF3 ) is introduced into the perovskite precursor for three objectives. First of all, the partial substitution of Yb3+ for Pb2+ in the perovskite lattice increases the tolerance factor of the perovskite lattice and facilitates the formation of the α phase. Second, YbF3 and DMSO in the solvent form a Lewis acid complex YbF3 ·DMSO, which can passivate the perovskite film, reduce defects, and improve device stability. Consequently, the YbF3 modified Perovskite solar cell exhibits a champion conversion efficiency of 24.53% and still maintains 90% of its initial efficiency after 60 days of air exposure under 30% relative humidity.

Orientated crystallization of FA-based perovskite via hydrogen-bonded polymer network for efficient and stable solar cells

Incorporating mixed ion is a frequently used strategy to stabilize black-phase formamidinum lead iodide perovskite for high-efficiency solar cells. However, these devices commonly suffer from photoinduced phase segregation and humidity instability. Herein, we find that the underlying reason is that the mixed halide perovskites generally fail to grow into homogenous and high-crystalline film, due to the multiple pathways of crystal nucleation originating from various intermediate phases in the film-forming process. Therefore, we design a multifunctional fluorinated additive, which restrains the complicated intermediate phases and promotes orientated crystallization of α-phase of perovskite. Furthermore, the additives in-situ polymerize during the perovskite film formation and form a hydrogen-bonded network to stabilize α-phase. Remarkably, the polymerized additives endow a strongly hydrophobic effect to the bare perovskite film against liquid water for 5 min. The unencapsulated devices achieve 24.10% efficiency and maintain >95% of the initial efficiency for 1000 h under continuous sunlight soaking and for 2000 h at air ambient of ~50% humid, respectively.

A residual strain regulation strategy based on quantum dots for efficient perovskite solar cells

A simple strategy is developed to simultaneously release tensile strain and passivate defects in an FA-based perovskite using CsPbI3 QDs, achieving a PCE of 23.3%.

Elucidating degradation mechanisms of mixed cation formamidinium-based perovskite solar cells under device operation conditions

Organic cation-based metal halide perovskites suffer from volatile material decomposition, and the approach to ensure stability under device operation conditions is still far from commercialization. Formamidinium (FA)-based perovskite solar cells (PeSCs) have been widely used, showing efficient power conversion efficiencies (PCEs). Despite the high PCE, FA-based perovskites lack stability, but the related degradation mechanisms are quite indefinite. In this study, we compared two different systems by introduction of inorganic and organic cations to increase FA-based perovskite stability. To relate the device operational stability, we exposed the mixed cation perovskite films under 1 sun illumination at 60 °C in ambient air and investigated the difference in the phase transition of inorganic-mixed and organic-mixed FA perovskites. Morphological and structural characterizations were collected to confirm that the type of the secondary cation plays a major role in determining whether perovskite degradation is triggered at the grain boundary or perovskite grain surfaces. Although the PCEs of the two device systems are similar, the degradation mechanisms varied significantly depending on the chemical properties of perovskite components.

Crystallization control of air-processed wide-bandgap perovskite for carbon-based perovskite solar cells with 17.69% efficiency

The preparation of high-efficiency perovskite solar cells (PSCs) in the ambient air environment has been challenging, especially for methylamine-free formamidine (FA)-based PSCs. One of the essential reasons is the slow nucleation rate of perovskite in a high-humidity air environment, leading to the uncontrolled growth of the intermediate phase and poor crystalline quality of the film. Here, we focus on the preparation of wide-bandgap FA-based perovskite films in the ambient air environment. A simple component engineering strategy is proposed to regulate the crystallization process of perovskite film. The nucleation process of perovskite can be significantly promoted by modulating the interaction of components in the precursor solution. The uncontrollable growth of the one-dimensional intermediate phase is suppressed, ultimately resulting in the improvement of the crystallinity of perovskite films. The defect state density of the film is significantly reduced, and the non-radiative recombination processes are effectively suppressed. The efficiency of the hole transport layer (HTL)-free carbon-based PSCs (C-PSCs) reached 17.69 %, which is the highest efficiency of HTL-free C-PSCs prepared with wide-bandgap perovskite (>1.6 eV).

Natural Amino Acid Enables Scalable Fabrication of High-Performance Flexible Perovskite Solar Cells and Modules with Areas over 300 cm2.

Upscaling large-area formamidinium (FA)-based perovskite solar cells (PSCs) has been considered as one of the most promising routes for the commercial applications of this rising photovoltaics technology. Here, a natural amino acid, phenylalanine (Phe), is introduced to regulate the nucleation and crystal growth process of the large-scale coating of FA-based perovskite films. Better film coverage and larger grain sizes are observed after adding Phe. Moreover, it is found that Phe can effectively passivate defects within perovskite films and suppress the nonradiative recombination due to the strong interaction with under-coordinated Pb2+ ions in the perovskite films. Rigid PSCs based on the blade-coated perovskite films containing Phe obtain a champion efficiency of 21.95%. The corresponding unencapsulated devices also exhibit excellent ambient stability, retaining 95% of their initial efficiencies after storage in the glovebox at 20°C for 1000h. Further, the strategy is applied to fabricate flexible PSCs and modules on polyethylene terephthalate/indium doped tin oxide substrates via slot-die coating. Phe modified flexible devices achieve outstanding efficiencies of 20.21%, 12.1%, and 11.2% with aperture areas of 0.10, 185, and 333cm2 , respectively. The strategy here has paved a promising way for the large-scale production of flexible PSCs.

Highly efficient inverted planar solar cell using formamidinium-based quasi-two dimensional perovskites

Formamidinium (FA)-based quasi-two dimensional (Q-2D) perovskite normally has better environmental stability than methylammonium (MA)-based one, however, it is difficult to form larger n (n > 2) phases of layered FA-based perovskite due to their larger formation energies, resulting in insufficient absorption of sunlight and then poor performance of its solar cell. Here we partially replace FA with MA to form (BA)2(FA1-xMAx)3Pb4I13 mixture, which contains enriched larger n (n = 3, 4, and larger) phases, where BA is butylammonium. Particularly, the (BA)2(FA0.8MA0.2)3Pb4I13 perovskite film show the best stability under ambient conditions. Furthermore, using inverted planar heterojunction structure, the photovoltaic device based on (BA)2(FA0.8MA0.2)3Pb4I13 demonstrates power conversion efficiency of 14.2 %, which is among the highest efficiencies for the FA-rich Q-2D perovskites with n ≤ 4. This work highlights that FA-rich Q-2D perovskites have great promise for optoelectronic devices.

A Universal Strategy of Intermolecular Exchange to Stabilize α-FAPbI3 and Manage Crystal Orientation for High-Performance Humid-Air-Processed Perovskite Solar Cells.

Preparation of high-performance perovskite solar cells without strict environmental control is an inevitable trend of commercialization. Humidity is considered the main factor hindering perovskite performance. Formamidine (FA)-based perovskites suffer from the instability of photoactive black α-FAPbI3, especially in humid air, and numerous defects in the surface and bulk of perovskite films limit their performance. In this work, long-chain n-heptylamine (nHA) is introduced via antisolvent engineering into an FA-based perovskite film. nHA removes the negative intermediate adduct and promotes the formation of α-FAPbI3 at room temperature in humid air via intermolecular exchange behavior. Moreover, the existence of nHA in the final perovskite film also reduces the defects and suppresses ion migration. The champion device delivers a power conversion efficiency (PCE) of 23.7% (certificated 22.76%) with negligible hysteresis, and the fabricated devices exhibit superior reproductivity. The device stability is also enhanced, maintaining 95% of its initial PCE after 1500 h in ambient air. Moreover, the PCE has no attenuation at the maximum power point under continuous 1-sun light soaking for 500 h. The universality of this method is also demonstrated by other perovskite compositions, including methylamine lead iodine (MAPbI3 ) and FAx MA1- x PbI3 in humid air.

Recent Advances on the Strategies to Stabilize the α-Phase of Formamidinium Based Perovskite Materials

Perovskite solar cells (PSC) are considered promising next generation photovoltaic devices due to their low cost and high-power conversion efficiency (PCE). The perovskite material in the photovoltaic devices plays the fundamental role for the unique performances of PSC. Formamidinium based perovskite materials have become a hot-topic for research due to their excellent characteristics, such as a lower band gap (1.48 V), broader light absorption, and better thermal stability compared to methylammonium based perovskite materials. There are four phases of perovskite materials, named the cubic α-phase, tetragonal β-phase, orthorhombic γ-phase, and δ-phase (yellow). Many research focus on the transition of α-phase and δ-phase. α-Phase FA-based perovskite is very useful for photovoltaic application. However, the phase stability of α-phase FA-based perovskite materials is quite poor. It transforms into its useless δ-phase at room temperature. This instability will lead the degradation of PCE and the other optoelectronic properties. For the practical application of PSC, it is urgent to understand more about the mechanism of this transformation and boost the stability of α-Phase FA-based perovskite materials. This review describes the strategies developed in the past several years, such as mixed cations, anion exchange, dimensions controlling, and surface engineering. These discussions present a perspective on the stability of α-phase of FA-based perovskite materials and the coming challenges in this field.

Dual-functional passivators for highly efficient and hydrophobic FA-based perovskite solar cells

Ionic defects and humidity instability are the main limitations to realize the commercialization of FA-based perovskite solar cells. To overcome these issues, here, we report a dual-functional molecular 2-Fluorothiophenol (FTP) to achieve high-quality FA-based perovskites with simultaneously efficient defects passivation and improved hydrophobicity. It is revealed that SH¯ groups of FTP induce highly oriented crystallization toward (0 0 1) crystal plane of cubic-phase Cs0.05FA0.95PbI3 perovskite structure, thus improving the charge carrier transport. Moreover, a strong Lewis acid-base interaction between the SH¯ group and Pb2+ in perovskite is of great benefit to passivate the surface and grain boundary defects, with additional merit for improving humidity stability of the FA-based perovskite films in conjunction with the hydrophobic group of FTP. The introduction of FTP boosts the efficiency from 20.30% to 22.66% for 0.04 cm2 solar cells and from 16.86% to 20.01% for 1.0 cm2 devices. More importantly, the solar cells demonstrate excellent humidity stability, which can retain 90% of their original efficiency after being kept for 80 days under ambient air conditions.

Inhibition of PbI2-induced defects by doping MABr for high-performance perovskite solar cells.

The performance of FAPbI3-based perovskite solar cells (PSCs) has been drastically improved with a photoelectric conversion efficiency (PCE) of 25.7%, showing attractive application potential. To achieve higher efficiency and longer-term stability of PSCs, a large number of passivation methods are used to inhibit Pb and I defects of FAPbI3 films. In this study, we developed a facile method to suppress the residual PbI2 of perovskite films and the related defects efficiently by a two-step spin-coating process, which prominently improves the carrier lifetime of α-FAPbI3 films (from 459 ns to 1346 ns). In addition, the morphology and crystallization characteristics of the films were also significantly improved, and the grains grew up to 1 μm. The efficiency of FAPbI3-based PSCs fabricated by this method reached 22.64%.

Development of formamidinium lead iodide-based perovskite solar cells: efficiency and stability.

Perovskite materials have been particularly eye-catching by virtue of their excellent properties such as high light absorption coefficient, long carrier lifetime, low exciton binding energy and ambipolar transmission (perovskites have the characteristics of transporting both electrons and holes). Limited by the wider band gap (1.55 eV), worse thermal stability and more defect states, the first widely used methylammonium lead iodide has been gradually replaced by formamidinium lead iodide (FAPbI3) with a narrower band gap of 1.48 eV and better thermal stability. However, FAPbI3 is stabilized as the yellow non-perovskite active phase at low temperatures, and the required black phase (α-FAPbI3) can only be obtained at high temperatures. In this perspective, we summarize the current efforts to stabilize α-FAPbI3, and propose that pure α-FAPbI3 is an ideal material for single-junction cells, and a triple-layer mesoporous architecture could help to stabilize pure α-FAPbI3. Furthermore, reducing the band gap and using tandem solar cells may ulteriorly approach the Shockley–Queisser limit efficiency. We also make a prospect that the enhancement of industrial applications as well as the lifetime of devices may help achieve commercialization of PSCs in the future.