Deciphering 2D/3D perovskite heterojunctions from microscopic perspective

Abstract

Perovskite solar cells (PSCs) that are composed of two-dimensional/three-dimensional (2D/3D) perovskite in structure combine the durability of 2D perovskites with the efficient movement of charge carries in 3D perovskites. This innovative approach improves the performance of PSCs in the fast-growing field of photovoltaic research. However, the transportation of charge carriers across the boundaries between 2D and 3D perovskite is impeded by the irregular distribution of the distinct fragments of 2D perovskite. This raises inquiries regarding the function of 2D perovskite in 2D/3D PSCs. Hence, this thesis intends to investigate the management of the 2D perovskite distribution and orientation and examine its influence on 3D grains. Additionally, electron microscopy will be used to provide feedback and verify the results obtained. This thesis initially explores the impact of halide ions on structural issues and optoelectronic characteristics of 2D/3D perovskite heterojunctions. By manipulating the halide ion composition, including the incorporation of chloride (Cl-) and bromide (Br-) ions, the goal we have is to improve the dispersion, durability, and efficiency of perovskite films. Our approach involves varying concentrations of formamidinium chloride (FACl) in PEAI solutions and substituting phenethylammonium iodine (PEAl) with phenethylammonium bromine (PEABr) to assess their effects on 2D perovskite layer formation. The results demonstrate that chloride addition leads to more uniform 2D perovskite coverage and improved stability of the 2D/3D PSCs. However, while PEABr influences the ion exchange dynamics, achieving consistent layer thickness and orientation remains challenging. These findings highlight the critical role of halide ions in optimizing the microstructure and enhancing the optoelectronic performance of perovskite materials. This study provides valuable insights into the potential of tailored halide incorporation for advancing perovskite solar cell technologies, paving the way for the promotion of high-efficiency and durable PSCs. The effectiveness of 2D surface passivation has been demonstrated in achieving cutting-edge perovskite optoelectronics, particularly in the field of solar cells, and the microstructural and phase heterogeneities of 2D perovskite passivators can strongly influence their effective roles. But synthesis of co-homogenized, stable microstructure and phase in such passivators remains a challenge because of uncontrolled surface reactions. Herein we leverage a [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) molecular interlayer to optimize the interaction between a 2D passivator and 3D perovskite, leading to a uniform, purer-phase 2D perovskite capping layer. This PCBM interlayer further mitigates the detrimental grain-boundary etching encountered in conventional approaches, creating a molecular passivation directly onto the perovskite surface. The inverted PSCs made as such feature a laminate-structured perovskite heterointerface at the electron-extracting side, which contributes to improved charge energetics and film stability, owing to regulated band alignment and laminate-layer protection, respectively. Power conversion efficiencies up to 26% are achieved, together with enhanced device stabilities under ISOS-standardized protocols, showing T90 lifetimes over 1,000 h in both the damp-heat test (85 °C, 85% relative humidity) and maximum-power-point tracking under one-sun illumination. Lattice-resolved insights are provided to link the microstructure to device performance, shedding light on the significance of passivator-microstructure uniformity and reliability on the performance of perovskite optoelectronics.