Gelation of the electron transport layer to improve the thermal stability of efficient perovskite solar cells

Abstract

Halide perovskite solar cells have shown superior power conversion efficiency (PCE); however, long-term stability remains a challenge for their commercialization. The electron transport layer (ETL) and its interface with high electron transport ability, less charge recombination, and defect passivation capability are crucial for enhancing device stability. Tin oxide (SnO₂), a promising ETL used in optoelectronic devices, suffers from intrinsic defects such as surface hydroxyls and a non-uniform morphology, which can negatively affect the device performance and shorten the device lifetime. In this study, we report a simple and effective method that employs ammonium oxalate (AMO) as an additive to induce gelation of SnO₂ colloid solution. We found that gelation of the SnO₂-ETL provides an effectively uniform surface morphology and prevents moisture infiltration, even under high humidity conditions. Moreover, the gelation of the ETL passivates interfacial defects (such as surface hydroxyls and oxygen vacancies) and enhances charge extraction from the perovskite layer. Additionally, the ammonium group promotes higher film crystallization, and thereby reduces charge recombination. As a result, optimized PSCs based on the gelated ETL achieved an improved PCE of up to 21.40%. The resulting PSCs without encapsulation also exhibited excellent moisture and thermal stability. Accordingly, this work suggests that employing a gelated ETL is a promising strategy to address the thermal instability of planar-PSCs.