Self-stability of un-encapsulated polycrystalline MAPbI3 solar cells via the formation of chemical bonds between C60 molecules and MA cations

The day-dependent photovoltaic performance of P3CT-Na based CH 3 NH 3 PbI 3 (MAPbI 3 ) solar cells can be improved by adding small molecules (urea, PCBM and C 60 ) into the MAPbI 3 crystal thin films to passivate the defects at the grain boundaries. The experimental results show that the passivation mechanism of the different additives does not only depend on the type of additives but also be related to the chemical interaction between additives and MAPbI 3 crystals. Atomic-force microscopic images, day-dependent photoluminescence (PL) spectra, and temperature-dependent PL shows that the chemical bonds between C 60 molecules and MA cations can be formed and thereby stabilizing the MAPbI 3 crystal thin films. In other words, the stable and high-efficiency P3CT-Na based MAPbI 3 solar cells can be realized when the surface defects of MAPbI 3 crystal grains are passivated with the MA-C 60 -MA cations. Our results provide a way to increase the device stability of the solution-processed polycrystalline MAPbI 3 thin film based solar cells. • MA-C 60 -MA cations are formed, which resulted in stable perovksite solar cells. • Migrations of C 60 and PCBM molecules were observed by using PL spectra. • The highest PCE increases from 16.28% to 19.27% on the 2nd day. • The PCE of un-encapsulated solar cells increases from 15.49% to 15.86% within 21 days.