Eliminating Charge Accumulation via Interfacial Dipole for Efficient and Stable Perovskite Solar Cells

Impact of Dipole Effect on Perovskite Solar Cells

Interface modification and bulk doping are two major strategies to improve the photovoltaic performance of perovskite solar cells (PSCs). Dipolar molecules are highly favored due to their unique dipolarity. This review discusses the basic concepts and characteristics of dipoles. In addition, the role of dipoles in PSCs and the corresponding conventional characterization methods for dipoles are introduced. Then, we systematically summarize the latest progress in achieving efficient and stable PSCs in dipole materials at several key interfaces. Finally, we look forward to the future application directions of dipole molecules in PSCs, aiming at providing deep insight and inspiration for developing efficient and stable PSCs.

Effect of Single-Crystal TiO2/Perovskite Band Alignment on the Kinetics of Electron Extraction

The kinetics of electron extraction at the electron transfer layer/perovskite interface strongly affects the efficiency of a perovskite solar cell. By combining transient absorption and time-resolved photoluminescence spectroscopy, the electron extraction process between FA0.83Cs0.17Pb(I0.83Br0.17)3 and TiO2 single crystals with different orientations of (100), (110), and (111) were probed from subpicosecond to several hundred nanoseconds. It was revealed that the band alignment between the constituents influenced the relative electron extraction process. TiO2(100) showed the fastest overall and hot electron transfer, owing to the largest conduction band and Fermi level offset compared to FA0.83Cs0.17Pb(I0.83Br0.17)3. It was found that an early electron accumulation in these systems can have an influence on the following electron extraction on the several nanosecond time scale. Furthermore, the existence of a potential barrier at the TiO2/perovskite interface was also revealed by performing excitation fluence-dependent measurements.

Synergy of 3D and 2D Perovskites for Durable, Efficient Solar Cells and Beyond

Three-dimensional (3D) organic-inorganic lead halide perovskites have emerged in the past few years as a promising material for low-cost, high-efficiency optoelectronic devices. Spurred by this recent interest, several subclasses of halide perovskites such as two-dimensional (2D) halide perovskites have begun to play a significant role in advancing the fundamental understanding of the structural, chemical, and physical properties of halide perovskites, which are technologically relevant. While the chemistry of these 2D materials is similar to that of the 3D halide perovskites, their layered structure with a hybrid organic-inorganic interface induces new emergent properties that can significantly or sometimes subtly be important. Synergistic properties can be realized in systems that combine different materials exhibiting different dimensionalities by exploiting their intrinsic compatibility. In many cases, the weaknesses of each material can be alleviated in heteroarchitectures. For example, 3D-2D halide perovskites can demonstrate novel behavior that neither material would be capable of separately. This review describes how the structural differences between 3D halide perovskites and 2D halide perovskites give rise to their disparate materials properties, discusses strategies for realizing mixed-dimensional systems of various architectures through solution-processing techniques, and presents a comprehensive outlook for the use of 3D-2D systems in solar cells. Finally, we investigate applications of 3D-2D systems beyond photovoltaics and offer our perspective on mixed-dimensional perovskite systems as semiconductor materials with unrivaled tunability, efficiency, and technologically relevant durability.

Recent Progress in Interfacial Dipole Engineering for Perovskite Solar Cells

Design and modification of interfaces have been the main strategies in developing perovskite solar cells (PSCs). Among the interfacial treatments, dipole molecules have emerged as a practical approach to improve the efficiency and stability of PSCs due to their unique and versatile abilities to control the interfacial properties. Despite extensive applications in conventional semiconductors, working principles and design of interfacial dipoles in the performance/stability enhancement of PSCs are lacking an insightful elucidation. In this review, we first discuss the fundamental properties of electric dipoles and the specific roles of interfacial dipoles in PSCs. Then we systematically summarize the recent progress of dipole materials in several key interfaces to achieve efficient and stable PSCs. In addition to such discussions, we also dive into reliable analytical techniques to support the characterization of interfacial dipoles in PSCs. Finally, we highlight future directions and potential avenues for research in the development of dipolar materials through tailored molecular designs. Our review sheds light on the importance of continued efforts in this exciting emerging field, which holds great potential for the development of high-performance and stable PSCs as commercially demanded.

Recent Criterion on Stability Enhancement of Perovskite Solar Cells

Perovskite solar cells (PSCs) have captured the attention of the global energy research community in recent years by showing an exponential augmentation in their performance and stability. The supremacy of the light-harvesting efficiency and wider band gap of perovskite sensitizers have led to these devices being compared with the most outstanding rival silicon-based solar cells. Nevertheless, there are some issues such as their poor lifetime stability, considerable J–V hysteresis, and the toxicity of the conventional constituent materials which restrict their prevalence in the marketplace. The poor stability of PSCs with regard to humidity, UV radiation, oxygen and heat especially limits their industrial application. This review focuses on the in-depth studies of different direct and indirect parameters of PSC device instability. The mechanism for device degradation for several parameters and the complementary materials showing promising results are systematically analyzed. The main objective of this work is to review the effectual strategies of enhancing the stability of PSCs. Several important factors such as material engineering, novel device structure design, hole-transporting materials (HTMs), electron-transporting materials (ETMs), electrode materials preparation, and encapsulation methods that need to be taken care of in order to improve the stability of PSCs are discussed extensively. Conclusively, this review discusses some opportunities for the commercialization of PSCs with high efficiency and stability.

Development and mechanism of a fluorescent probe for a Mn(ii) ionic complex capable of recognizing chloroform vapor molecules

The monoclinic [PPh3(Me)]2[MnBr4] complex readily develops ion-dipole interactions with chloroform vapor molecules, causing reversible structural transitions and fluorescence changes.

Achieving High-Performance Perovskite Photovoltaic by Morphology Engineering of Low-Temperature Processed Zn-Doped TiO2 Electron Transport Layer

Perovskite solar cells (PSCs) have become one of the most promising renewable energy converting devices. However, in order to reach a sufficiently high power conversion efficiency (PCE), the PSCs typically require a high-temperature sintering process to prepare mesostructured TiO₂ as an efficient electron transport layer (ETL), which prohibits the PSCs from commercialization in the future. This work investigates a low-temperature synthesis of TiO₂ nanocrystals and introduces a two-fluid spray coating process to produce a nanostructured ETL for the following deposition of perovskite layer. The temperature during the whole deposition process can be maintained under 150 °C. Compared to the typical planar TiO₂ layer, the perovskite layer fabricated on a nanostructured TiO₂ layer shows uniform compactness, preferred orientation, and high crystallinity, leading to reproducible and promising device performance. The detail mechanisms are revealed by the contact angle test, morphology characterization, grazing incident wide angle X-Ray scattering measurement, and space charge limited currents analysis. Finally, optimized device performance can be achieved through adequate Zn doping in the TiO₂ layer, demonstrating an average PCE of 19.87% with champion PCE of 21.36%. The efficiency can maintain over 80% of its original value after 3000 h storage in ambient atmosphere. This study suggests a promising approach to offer high-efficiency PSCs using the low-temperature process.

α-CsPbI3 Bilayers via One-Step Deposition for Efficient and Stable All-Inorganic Perovskite Solar Cells

The emerging inorganic CsPbI₃ perovskites are promising wide-bandgap materials for application in tandem solar cells, but they tend to transit from a black α phase to a yellow δ phase in ambient conditions. Herein, a gradient grain-sized (GGS) CsPbI₃ bilayer is developed to stabilize the α phase via a single-step film deposition process. The spontaneously upward migration of (adamantan-1-yl)methanammonium (ADMA) based on the hot-casting technique causes self-assembly of the hierarchical morphology for the perovskite layers. Due to the strong steric effect of the surficial ADMA cation, a self-assembly tiny grain-sized CsPbI₃ layer is in situ formed at the surface site, which exhibits notably enhanced phase stability by its high surface energy. Meanwhile, a large grain-sized CsPbI₃ layer is obtained at the bottom site with high charge mobility and low trap density of states, which benefits from the regulated growth rates by the interaction between ADMA and perovskites. The perovskite solar cell (PSC) based on the GGS CsPbI₃ bilayer shows an efficiency of 15.5% and operates stably for 1000 h under ambient conditions. This work confirms that redistributing the surface energy of perovskite films is a facile strategy to stabilize metastable PSCs without the cost of efficiency loss.

Dimension-Controlled Growth of Antimony-Based Perovskite-like Halides for Lead-Free and Semitransparent Photovoltaics

Antimony (Sb) has been identified as a promising candidate for replacing toxic lead (Pb) in perovskite materials because Sb-based perovskite-like halides exhibit not only intrinsic thermodynamic stability but also a unique set of intriguing optoelectronic characteristics. However, Sb-based perovskite-like halides still suffer from poor film morphology and uncontrollable halide constituents, which result from the disorder of the growth process. Herein, we propose a simple strategy to facilitate heterogeneous nucleation and control the dimension transformation by introducing bis(trifluoromethane)sulfonimide lithium (LiTFSI), which produces high-quality two-dimensional MA₃Sb₂I_9-xCl_x films. As the spacer molecule among Sb-based pyramidal clusters, LiTFSI plays a role in forming a zero-dimensional intermediate phase and retarding crystallization. The slower dimension transformation well stabilizes the band gap of perovskite-like films with a fixed Cl/I ratio (∼7:2) and avoids random "x" values in MA₃Sb₂I_9-xCl_x films prepared from the conventional method. Based on this method, Sb-based perovskite-like solar cells (PLSCs) achieve the highest recorded power conversion efficiency (PCE) of 3.34% and retain 90% of the initial PCE after being stored under ambient conditions for over 1400 h. More importantly, semitransparent Sb-based PLSCs with PCEs from 2.62 to 3.06% and average visible transparencies from 42 to 23% are successfully obtained, which indicates the great potential of the emerging Pb-free halide semiconductor for broad photovoltaic applications.

MACl-Induced Intermediate Engineering for High-Performance Mixed-Cation Perovskite Solar Cells

Recently, mixed-cation perovskites have been extensively used for high-performance solar cells. Nevertheless, the mixed-cation perovskite based on formamidinium methylammonium lead tri-iodide (FA_xMA_1-xPbI₃) fabricated through the existing methods often suffers from phase stability and trap density. Herein, we demonstrate a facile intermediate engineering approach to improve the quality of the mixed-cation perovskite based on FA_xMA_1-xPbI₃. Varying concentrations of methylammonium chloride (MACl) are used to treat the FA-MA-PbI₃-solvent intermediate. It is noted that MACl has a strong impact on the crystallization kinetics and charge carrier dynamics as well as the defect density of the obtained perovskite. The mixed-cation perovskite treated with 20 mg mL^-1 MACl yields a large grain size, highly uniform morphology, and better crystalline stability. Subsequently, the device with an acquired high-quality mixed-cation perovskite shows a high efficiency of 20.40%, which is obviously higher than that obtained from the traditional nontreated method. Moreover, the device prepared through the developed method could retain over 85% of the initial efficiency after 860 h at room temperature.

In Situ Passivation on Rear Perovskite Interface for Efficient and Stable Perovskite Solar Cells

Despite the rocketing rise in power conversion efficiencies (PCEs), the performance of perovskite solar cells (PSCs) is still limited by the carrier transfer loss at the interface between perovskite (PVSK) absorbers and charge transporting layers. Here, we propose a novel in situ passivation strategy by using [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) to improve the charge dynamics at the rear PVSK/CTL interface in the n-i-p structure device. A pre-deposited PCBM-doped PbI₂ layer is redissolved during PVSK deposition in our routine, establishing a bottom-up PCBM gradient that is facile for charge extraction. Meanwhile, the surface defects are in situ-passivated via PCBM-PVSK interaction, which substantially suppresses the trap-assisted recombination at the rear interface. Due to the synergistic effect of charge-extraction promotion and trap passivation, the fabricated PSCs deliver a champion PCE of 20.10% with attenuated hysteresis and improved long-term stability, much higher than the 18.39% of the reference devices. Our work demonstrates a promising interfacial engineering strategy for further improving the performance of PSCs.