Highly efficient flexible perovskite solar cells and their photo-stability

We report a simple method to fabricate highly efficient and robust flexible perovskite solar cells (F-PSCs) along with their photo-stability. The solar cells were prepared on indium tin oxide (ITO) coated polyethylene terephthalate substrates. The solar cells incorporated methylammonium lead iodide chloride (CH₃NH₃PbI_3−xCl_x) perovskite as the light absorber, tin oxide (SnO₂) as the electron transport layer (ETL), Spiro-MeOTAD as the hole transport layer and Ag as the top electrode. This structure exhibited a very high external quantum efficiency (EQE) of ~80% at 500 nm and a power conversion efficiency (PCE) of 12.7%  ±  0.6% (active area 9.6 mm²) with multiple bending cycles showing no effect on the performance, even after 100 bending cycles. To evaluate their lifetime in the dark and under actual operating conditions, a systematic study was performed on the unsealed devices. The solar cells showed a halflife (T ₅₀) of about 340 h when stored in the dark in an ambient environment, but they degraded very rapidly in direct sunlight. The main reason behind their degradation was found to be the rapid degradation in their short circuit current density (J _sc). The same device design was processed to fabricate large area flexible solar modules with an area of 5  ×  5 cm² (active area 15.84 cm²) and the solar modules exhibited PCE of 5.6%  ±  0.2%. Substantial loss of PCE in the modules compared to small area devices has been attributed to the larger series resistance of the longer ITO electrodes used in the modules. (paper)