Elimination of grain surface concavities for improved perovskite thin-film interfaces

The surface of individual grains of metal halide perovskite films can determine the properties of heterointerfaces at the microscale and the performance of the resultant solar cells. However, the geometric characteristics of grain surfaces have rarely been investigated. Here we elaborate on the existence of grain surface concavities (GSCs) and their effects on the charge-extracting, chemical and thermomechanical properties of buried perovskite heterointerfaces. The evolution of GSCs is triggered by grain-coalescence-induced biaxial tensile strain and thermal-coarsening-induced grain-boundary grooving. As such, GSCs are tailorable by regulating the grain growth kinetics. As a proof of concept, we used tridecafluorohexane-1-sulfonic acid potassium to alleviate biaxial tensile strain and grain-boundary grooving by molecular functionalization, thus forming non-concave grain micro-surfaces. The resultant perovskite solar cells demonstrate enhanced power conversion efficiency and elevated power conversion efficiency retention under ISOS-standardized thermal cycling (300 cycles), damp heat (660 h) and maximum power point tracking (1,290 h) tests. This work sheds light on micro-surface engineering to improve the durability and performance of perovskite solar cells and optoelectronics.