Solvent-Assisted Crystallization of an α-Fe2O3 Electron Transport Layer for Efficient and Stable Perovskite Solar Cells Featuring Negligible Hysteresis

Inorganic-organic metal halide perovskite solar cells (PSCs) show power conversion efficiency values approaching those of state-of-the-art silicon solar cells. In a quest to find suitable charge transport materials in PSCs, hematite (α-Fe₂O₃) has emerged as a potential electron transport layer (ETL) in n-i-p planar PSCs due to its low cost, UV light stability, and nontoxicity. Yet, the performance of α-Fe₂O₃-based PSCs is far lower than that of state-of-the-art PSCs owing to the poor quality of the α-Fe₂O₃ ETL. In this work, solvent-assisted crystallization of α-Fe₂O₃ ETLs was carried out to examine the impact of solvents on the optoelectronic properties of α-Fe₂O₃ thin films. Among the various solvents used in this study (deionized water, ethanol, iso-propanol, and iso-butanol), optimized ethanol-based α-Fe₂O₃ ETLs lead to champion device performance with a power conversion efficiency of 13% with a reduced hysteresis index of 0.04 in an n-i-p-configured PSC. The PSC also exhibited superior long-term inert and ambient stabilities compared to a reference device made using a SnO₂ ETL. Through a series of experiments spanning structural, morphological, and optoelectronic properties of the various α-Fe₂O₃ thin films and their devices, we provide insights into the reasons for the improved photovoltaic performance. It is noted that the formation of a pinhole-free compact morphology of ETLs facilitates crack-free surface coverage of the perovskite film atop an α-Fe₂O₃ ETL, reduces interfacial recombination, and enhances charge transfer efficiency. This work opens up the route toward novel ETLs for the development of efficient and photo-stable PSCs.

In Situ GISAXS Observation and Large Area Homogeneity Study of Slot-Die Printed PS-b-P4VP and PS-b-P4VP/FeCl3 Thin Films

Mesoporous hematite (α-Fe₂O₃) thin films with high surface-to-volume ratios show great potential as photoelectrodes or electrochemical electrodes in energy conversion and storage. In the present work, with the assistance of an up-scalable slot-die coating technique, locally highly ordered α-Fe₂O₃ thin films are successfully printed based on the amphiphilic diblock copolymer poly(styrene-b-4-vinylpyridine) (PS-b-P4VP) as a structure-directing agent. Pure PS-b-P4VP films are printed under the same conditions for comparison. The micellization of the diblock copolymer in solution, the film formation process of the printed thin films, the homogeneity of the dry films in the lateral and vertical direction as well as the morphological and compositional information on the calcined hybrid PS-b-P4VP/FeCl₃ thin film are investigated. Because of convection during the solvent evaporation process, a similar dimple-type structure of vertically aligned cylindrical PS domains in a P4VP matrix developed for both printed PS-b-P4VP and hybrid PS-b-P4VP/FeCl₃ thin films. The coordination effect between the Fe³⁺ ions and the vinylpyridine groups significantly affects the attachment ability of the P4VP chains to the silicon substrate. Accordingly, distinct feature sizes and homogeneity in the lateral direction, as well as the thicknesses in the perpendicular direction, are demonstrated in the two printed films. By removing the polymer template from the hybrid PS-b-P4VP/FeCl₃ film at high temperature, a locally highly ordered mesoporous α-Fe₂O₃ film is obtained. Thus, a facile and up-scalable printing technique is presented for producing homogeneous mesoporous α-Fe₂O₃ thin films.

Enhanced charge extraction with all-carbon electrodes for inorganic CsPbBr3 perovskite solar cells

All-carbon electrodes are made by optimizing MWCNT and CB for inorganic CsPbBr₃ PSCs, achieving a maximized 7.62% efficiency and improved stability.