Multiple roles of negative thermal expansion material for high-performance fully-air processed perovskite solar cells

For fully-air-processed perovskite solar cells (PSCs), high-quality perovskite films and improved intrinsic stability are critical for high photovoltaic performance. However, strain, as a critical factor of perovskite instability, not only harms the efficiency of PSCs but also accelerates the degradation of the PSCs devices. In this report, we introduce a novel negative thermal expansion (NTE) material into perovskite film for the first time, growing a high-quality perovskite films with high crystallinity, large grain size and low defect density. More importantly, effective residual strain relaxation of perovskite films is achieved, the thermal expansion compensation properties of NTE materials reduces the overall thermal expansion coefficient for perovskite films. As a result, the fully-air processed PSC achieves a champion power conversion efficiency (PCE) of 22.40 %. Simultaneously, the unencapsulated device retains 93.78 % of its original PCE after storage in ambient air after 90 days. Furthermore, these devices also show excellent thermal stability and remarkable light stability. The present study not only provides insights into the multi role of NTE materials, but also provides a new path for the application of NTE materials in the photovoltaic field.