Functionalization of perovskite thin films with moisture-tolerant molecules

<i>D</i> ^{6 <i>h</i>} Symmetric Radical Donor–Acceptor Nanographene Modulated Interfacial Carrier Transfer for High-Performance Perovskite Solar Cells

Hybrid organic-inorganic perovskites (HOIPs) have received a lot of research attention over the past decade, related to the rapid increase in the power conversion efficiency of perovskite solar cells. The materials used for solar cells are mainly three-dimensional (3D) HOIPs, with a general formula of ABX3 with A being a small monovalent organic cation, B a divalent metal ion, and X a halide anion. More recently, the related material class of 2D HOIPs, with a general formula of (A*)2BX4, is receiving increased attention by combining a generally enhanced material stability compared to 3D HOIPs with a much higher degree of compositional flexibility. 2D HOIPs can accommodate bulkier organic cations (A*) with a conjugated organic core. Depending on the relative alignment between the frontier energy levels of the organic core and the inorganic framework, energy/charge transfer between the components of the hybrid is possible. We built in a carbazole derivative as the organic cation into a 2D HOIP. Through electron paramagnetic resonance experiments combined with computational calculations, we show that excitons generated in the inorganic layer undergo charge transfer at the organic−inorganic interface, resulting in a positive polaron delocalized over several carbazole moieties. In another material system, we incorporate an organic charge-transfer complex (CTC) with a pyrene derivative as the donor and TCNQ as the acceptor into the organic layer of a 2D HOIP. Based on time-resolved spectroscopy, we show that holes are transferred to the inorganic layer upon excitation of the CTC while electrons stay localized on TCNQ acceptor molecules.

Abstractengineering has emerged as a promising approach to improve the stability and power conversion efficiency of perovskite solar cells (PSCs) by regulating the crystallization or defects. Conventional methods typically focus on a single functional group, leading to a deficiency in simultaneously addressing the above mentioned two aspects. Here, an innovative approach using (methylsulfonyl)phenyl)prop‐2‐en‐1‐amine hydroiodide (MSPPAI) is presented to concurrently and effectively modulate perovskite crystallization and defect passivation. The unique structure of MSPPAI, combining a rigid conjugated structure with multisite anchoring groups (─NH2 and ─SO2─), enables precise regulation through strong interaction with perovskite components. This interaction promotes the preferred (100) orientation of perovskite crystals, enhances the grain size, and thus improves the film quality. Meanwhile, the conjugated structure and approximate coplanarity further facilitate ordered crystallization and directional growth. Furthermore, by preventing volatile loss and coordinating with residual Pb2+, MSPPAI could effectively stabilize grain boundaries and surfaces to reduce defects and prevent degradation. Utilizing these mechanisms, the corresponding MSPPAI based devices achieves an efficiency of 25.54% and exhibits excellent stability that maintains 93% of its initial efficiency even after 1600 h under humid conditions. This molecular design strategy presents a novel approach for improving the efficiency and stability of PSCs.

A perovskite solar cell owing very high stabilities and power conversion efficiencies

In recent years, it has been established that solar cells based on organic-inorganic hybrid perovskite materials have substantial potential for the development of highly efficient photovoltaic devices and offers robust opportunities for research to the scientific community and industry. Power conversion efficiency (PCE) of perovskite solar cells (PSCs) have already surpassed 24.8 % within a decade, which is now third highest efficiency among single-junction photovoltaic materials. This report briefly introduces hybrid-halide perovskite materials, their structural properties, various possible device architectures and a comparative study of photo-voltaic performance. For commercialization, high stability of devices is must and here we have thoroughly discussed possible degradation mechanisms of PSCs, that is, moisture; oxygen; heat; structural stability; UV-light effect; defect-states, ion-migration and various approaches to passivity based upon recent reports. A proper encapsulation with optimized chemical composition (enhanced interaction between organic/inorganic cation and BX6 octahedron) PSCs could possess superior stability for long-run while maintaining optoelectronic properties.

In 2009, organic-inorganic hybrid perovskite was first used as the light-absorbing material for solar cells. The rapidly increased efficiency, simple preparation process, and low cost have aroused widespread concern. The last five years have witnessed the increase of the power conversion efficiency in the organic-inorganic hybrid perovskite solar cells from 3.8% to 19.3%. At present, most researches focus on how to improve the photoelectric conversion efficiency rather than the stability. With the improvement of the power conversion efficiency, people have realized that the long-term stability is also one of the key issues in practical applications.The present preliminary researches indicate that there are two main factors connected with the stability. One is the stability of the perovskite materials, including thermal stability and humidity stability; the other is the stability of solar devices, mainly related to the design and optimization of devices' structure. To solve the problems of stability of perovskite materials, the main point is its crystal structure. Based on the tolerance factor related to the stability of the perovskite lattice structure, choosing a more suitable size of the moiety can reduce its sensitivity to humidity and improve its stability. To design the device structure, we should try to select a hydrophobic material to protect the perovskite materials from being affected by the surrounding environment. Researches have so far showed that by optimizing the design of the solar cell structure via combining the elements utilized and the bonding interface work, the stability of the hybrid perovskites solar cell is supposed to be entirely solved, and this will determine the practical process of hybrid perovskite photovoltaic materials. However, by the moment, the study on stability of perovskite solar cells is far from being sufficient.

Solar cells incorporating organic inorganic metal halide perovskites as the absorber material have achieved power conversion efficiencies of more than 25% after only a decade of research. The extremely rapid improvement in efficiency of perovskite solar cells compared to pre-established absorber materials, such as silicon, cadmium telluride, or gallium arsenide, is mainly due to their cheap and easy low-temperature, solution-processing preparation techniques such as spin coating. To further improve the power conversion efficiency of perovskite solar cells, it is still necessary to develop a fundamental understanding of the device physics. The focus of this work lies, therefore, on investigating the energy band diagram of perovskite solar cells, both in the dark and under illumination at open circuit conditions, predominantly using photoelectron spectroscopy (PES). Two different architectures are investigated and compared: i) the classical architecture where the perovskite absorber is deposited onto the electron extraction layer and ii) the inverted architecture where the perovskite is deposited onto the hole extraction layer. Initial experiments showed that perovskite absorbers are extremely light-sensitive, meaning that even small intensities of background light, like the visible light emitted from the X ray source, can induce a photovoltage which will significantly affect the PES measurements by shifting all spectra to higher or lower binding energies. To shield the sample from the visible light emitted by the X ray source, the setup of the X ray photoelectron spectroscopy (XPS) system used in this work was improved by installing an aluminum window in between the sample and the X ray source. In the next step, a comparative study of several different perovskite absorbers on n type SnO2 (classical) and p type NiOx substrates (inverted architecture) was performed. It was proven that the underlying substrate has no effect on the doping level of the perovskite absorbers, as it has previously been proposed in the literature. The perovskite absorbers are always measured to be n doped. It is suggested that the literature reported substrate effect originates from background light during the measurement. This leads to an unnoticed photovoltage resulting in a binding energy shift of all spectra, which leads to the determination of incorrect doping levels. For both architectures, the majority of the photovoltage and hence the open-circuit voltage of the full device is identified at the n-type perovskite | p-type hole extraction layer interface. The interfaces between the perovskite and the respective hole extraction layer (classical: spiro MeOTAD and inverted: NiOx) were then investigated in detail. For the perovskite | spiro MeOTAD interface a classical step-by-step interface experiment was performed. Since spiro MeOTAD films used in perovskite solar cell devices are usually doped with LiTFSI, at first a vacuum deposition process of LiTFSI doped spiro MeOTAD through co-evaporation of both materials was developed. The interface characterization proved that a band bending occurs in the dark, which changes to a flat band situation under illumination, corresponding to a surface photovoltage. For the inverted architecture, the perovskite | NiOx interface was investigated using the tapered cross-section PES method, which demonstrated the presence of a band bending in the dark as well. Finally, the results from the photovoltage measurements and the detailed interface characterizations were combined to derive complete energy band diagrams for both architectures under dark and illuminated open-circuit conditions.

Owing to their superior thermal stability, metal halide inorganic perovskite materials continue to attract interest for photovoltaics applications. The highest reported power conversion efficiency (PCE) for solar cells based on inorganic perovskites is over 20 %. As this PCE corresponds to 73 % of the theoretical limit, there remains more room for further improving the device PCEs than for improving organic-inorganic hybrid perovskite solar cells (PSCs). The main loss is in the photovoltage, which is limited by interfaces in terms of non-radiative recombination caused by traps and energy-level mismatch. Furthermore, inefficient charge extraction at interfacial contacts reduces the photocurrent and fill factor. This Minireview summarizes the recent developments in the fundamental understanding of how the interfaces and interfacial layers influence the performance of solar cells based on inorganic perovskite absorbers. An outlook for the development of highly efficient and stable inorganic PSCs from the interface point of view is also given.

Owing to their superior thermal stability, metal halide inorganic perovskite materials continue to attract interest for photovoltaics applications. The highest reported power conversion efficiency (PCE) for solar cells based on inorganic perovskites is over 20 %. As this PCE corresponds to 73 % of the theoretical limit, there remains more room for further improving the device PCEs than for improving organic–inorganic hybrid perovskite solar cells (PSCs). The main loss is in the photovoltage, which is limited by interfaces in terms of non‐radiative recombination caused by traps and energy‐level mismatch. Furthermore, inefficient charge extraction at interfacial contacts reduces the photocurrent and fill factor. This Minireview summarizes the recent developments in the fundamental understanding of how the interfaces and interfacial layers influence the performance of solar cells based on inorganic perovskite absorbers. An outlook for the development of highly efficient and stable inorganic PSCs from the interface point of view is also given.

Over the past few years, the power conversion efficiency of perovskite solar cells have shown a tremendous progress from 3.8% in 2009 to 23.3% in 2018. Perovskites have exhibited excellent advantages in photovoltaic devices and other promising optoelectronic devices owing to their exceptional material properties, including direct and tunable bandgaps, strong light absorption, high electron/hole mobilities, long charge carrier lifetimes and diffusion lengths. The outstanding performance of perovskite solar cells is closely related with the deposition techniques and material composition of perovskite films. The preparation process of perovskite film is crucial for obtaining high efficiency devices, and it usually requires to fabricate a high coverage, compact and uniform perovskite layer. At present, the preparation technology of perovskite absorption layer mainly includes one-step processing, two-step processing, dual-source thermal evaporation processing, vapor-assisted solution processing and some scalable processing methods, and there are many reports and summaries about this work. However, perovskites still have some shortcomings such as insufficient light absorption range, poor long-term stability, the lead toxicity, which need to be overcome to realize higher power conversion efficiency and further product application. Compositional control engineering of perovskite materials becomes one of the effective ways to solve the above problems, but the summary of the research in this area is still lacking. In this review, we summarize the recent progress on the perovskite materials with different component systems, including organic-inorganic lead halide perovskite, all-inorganic lead halide perovskite, low-lead perovskite and lead-free perovskite. We also discuss some representative material compositions and the research on their corresponding preparation methods, the optimization of device structure and the effects on the device performance. Moreover, we compare and summarize the advantages and disadvantages of perovskite materials with different component systems. The purpose is to provide ideas on how to improve the efficiency and stability of perovskite solar cells through compositional controlling, and finally realize commercial application.

The modified multi-step thermal annealing process for highly efficient MAPbI3-based perovskite solar cells

Efficient and stable MAPbI3 perovskite solar cells achieved via chlorobenzene/perylene mixed anti-solvent

Nb2C MXenes modified SnO2 as high quality electron transfer layer for efficient and stability perovskite solar cells

Organic–inorganic hybrid lead halide perovskites have gained significant attention as light-harvesting materials in thin-film photovoltaics due to their exceptional optoelectronic properties and simple fabrication process. The power conversion efficiency of perovskite solar cells (PSCs) has surged beyond 25% in a short time span. Their transition to commercial market is a “work in progress” due to limited long-term operational stability and the persisting environmental concern due to the presence of lead. Comprehensive investigations on the interplay of material composition and interfacial effects on the device performance of PSCs based on methylammonium lead iodide have shown the crucial role of an A-site cation in incipient deterioration of the material through external stimuli (moisture, light, oxygen, or heat). Consequently, a partial or complete replacement of A-site cations by up to four isoelectronic substituents has resulted in many new perovskite compositions. The correlations between the chemical composition and the optoelectronic properties are, however, not always easy to determine. A-site cation management is governed by stability and charge neutrality of the lattice, and the choices include Cs+-cations and organic cations such as CH3NH3+ or CH(NH2)2+ and combinations thereof. Since the size of the cations is an important structural parameter, an adequate compositional engineering of the A-site could effectively optimize the stability by reducing non-radiative defect sites and enhancing carrier lifetimes. This Perspective reflects on the experimental strategies for A-site cation management and their direct impact on the stability and device performance. It also highlights the opportunities and challenges for further research and industrial commercialization of PSCs.

High-efficiency perovskite photovoltaic modules achieved via cesium doping