Abstract
Improving the performances of photovoltaic (PV) devices by suppressing non-radiative energy losses through the passivation of surface defects and enhancing perovskite solar cells stability to the level of standard PV materials represents one critical challenge in the field of perovskite solar cells. We report the beneficial features of introducing a tetrapropylammonium quaternary ammonium (TPA +) cation that combines two key functionalities, namely surface passivation of CH3NH3PbI3 nanocrystals through strong ionic/electrostatic interaction with the surface and bulk passivation via formation of type I heterostructure which acts as a barrier for recombination and reduces water ingress. As a result, non-encapsulated perovskite films with only 2 mol % of TPA + reached power conversion efficiencies over 17% under spectral irradiance corresponding to air mass (A.M.) 1.5G conditions. Furthermore, these films retain more than 85% of the initial 2 performance for over 1500 hours under ambient conditions, including a relative humidity of ~55±5%. The stability of these films is also significantly enhanced at 60 °C thermal stress or under 85% humidity storage. Perovskite thin films before and after the TPA + incorporation are characterized by X-ray diffraction, solid-state NMR spectroscopy, optical absorption spectroscopy and SEM imaging.