Hybrid perovskites hold tremendous promise for next-generation solar cells, more than any other recently developed low-cost active PV material. However, stresses are generated in perovskite films during processing and magnified in service by environmental effects such as thermal cycling, resulting in the evolution of defects and propagation of damage for fragile materials. Unfortunately, perovskite layers are exceptionally fragile as measured by their fracture energy—moreso than OPVs by an order of magnitude and c-Si or CIGS solar cells by two orders of magnitude. Surprisingly, despite the significance of film stresses for device stability, the origin and value of stresses in perovskite films has been largely overlooked.
We report on the stress values of perovskite films, which are tensile and can exceed 50 MPa in magnitude, a value which is comparable to the yield stress of copper. These stress values provide a significant driving force for fracture, contributing to perovskite instability and reducing device performance. We show a direct correlation between processing temperature and film stress, a result of the extremely high thermal expansion coefficient of perovskites. No evidence of stress relaxation is observed, and the film stress is purely elastic due to the thermal expansion mismatch between the perovskite and substrate. Methods for reducing stress in perovskite films are presented, which include using lower processing temperatures and substrates with high thermal expansion coefficients. From these guidelines, we show improved environmental stability and demonstrate the feasibility of reliable perovskite devices on flexible substrates resistant to temperatures up to 85°C without encapsulation.