Organic semiconductors as a promising class of optoelectronic materials have attracted significant interest because of their fascinating physical–chemical properties, simple fabrication scheme to afford wearable or flexible devices, much wider scopes for easy production, and shaping and tuning of semiconductor properties compared to inorganic materials, such as silicon (Si).^1,2 At present, a number of organic semiconductor classes based on small molecules and polymers have been discovered and widely used for next-generation organic and hybrid optoelectronics, such as organic thin film transistors, organic photodiodes, organic solar cells, and organic light-emitting diodes (OLEDs).^{3 – 5}

SnO₂ has attracted much attention due to its low-temperature synthesis (ca. 140 °C), high electron mobility, and low-cost manufacturing. However, lattice mismatch and oxygen vacancies at the SnO₂/CsPbI_3−xBr_x interface generally lead to undesirable nonradiative recombination in optoelectronic devices. The traditional TiO₂ used as the electron transport layer (ETL) for all-inorganic perovskite solar cells (PSCs) requires high-temperature sintering and crystallization, which are not suitable for the promising flexible PSCs and tandem solar cells, raising concerns about surface defects and device uniformity. To address these challenges, we present a bilayer ETL consisting of a SnO₂ layer using electron beam evaporation and a TiO₂ layer through the hydrothermal method, resulting in an enhanced performance of the perovskite solar cell. The bilayer device exhibits an improved power conversion efficiency of 11.48% compared to the single-layer device (8.09%). The average fill factor of the bilayer electron transport layer is approximately 15% higher compared to the single-layer electron transport layer. Through a systematic investigation of the use of ETL for CsPb_3−xBr_x PSCs on optical and electronic properties, we demonstrate that the SnO₂/TiO₂ is an efficient bilayer ETL for PSCs as it significantly enhances the charge extraction capability, suppresses carrier recombination at the ETL/perovskite interface, facilitates efficient photogenerated carrier separation and transport, and provides high current density and reduced hysteresis.

Electron transport layer (ETL) plays an undeniable role in improving the performance of n-i-p planar perovskite solar cells (PSCs). Titanium dioxide (TiO₂) is known as a promising ETL material for perovskite solar cell. In this work, the effect of annealing temperature on optical, electrical, and surface morphology of the electron-beam (EB)-evaporated TiO₂ ETL, and consequently on the performance of perovskite solar cell, was investigated. It was found that annealing treatment at an optimized temperature of 480 °C considerably improved the surface smoothness, density of grain boundaries, and carrier mobility of TiO₂ film, which resulted in nearly 10-fold improvement in power conversion efficiency (11.16%) in comparison with the unannealed device (1.08%). The improvement in performance of the optimized PSC is attributed to the acceleration of charge carrier extraction, as well as suppression of the recombination at the ETL/Perovskite interface.

All-inorganic CsPbI_3−xBr_x perovskite solar cells (PSCs) are becoming increasingly mature due to their excellent optoelectronic properties. However, because of the poor environmental stability of the perovskite material, the device is susceptibly decomposed when exposed to moisture, high temperature, and high illumination. Therefore, a critical task is to address the problem of poor long-term stability in the environment, which serves as a significant obstacle impeding the commercialization of perovskite solar cells. This article introduces the incorporation of PEO into all-inorganic CsPbI_3−xBr_x perovskites with an advantageous thermal stability. PEO acts as a passivating agent near the grain boundary, and its high viscosity characteristics effectively improve the film-forming properties, leading to a substantial reduction in defects and to improving the surface uniformity. In addition, the grain boundaries that serve as water and oxygen penetration channels are filled, resulting in a substantial improvement in device stability. With 7.5 mg/mL PEO doping into CsPbI_3−xBr_x, the unencapsulated device maintained its original power conversion efficiency of 98% after being placed in a dark environment of 40% humidity and 25 °C for 10 days. Using PEO effectively enhanced the performance of the devices, with the highest PCE reaching 10.95%, significantly improving environmental stability.