Potassium ion as kinetic controller in ionic double layer for hysteresis-free perovskite solar cell

Since ion migration and interaction with the external contacts has been regarded as one of origins for photocurrent density (J)-voltage (V) hysteresis and phase segregation in perovskite solar cells (PSCs) under operational condition, control of ionic movement in organic-inorganic halide perovskite presents a big challenge for achieving hysteresis-free and stable PSCs. As a universal method, potassium doping proposed to bulk perovskite film to minimize or eliminate the hysteresis[1]. Here, we report direct observation of moderately retarded ion migration in K+-doped (FAPbI3)0.875(CsPbBr3)0.125 perovskite by in-situ photoluminescence (PL) imaging. However, more impressive is the effect on the kinetics for generation of the ionic double layer at the vicinity of the contacts as it is reduced by two orders of magnitude in the time scale when devices are doped with K+ as detected by impedance spectroscopy. A significantly reduced hysteresis in K+-doped perovskite is responsible for more prolonged stability exhibiting ~96% of initial power conversion efficiency (PCE) after 22 days than relatively short-lived perovskite undoped with K+ ion. This work highlights the clear correlation of ion migration and a fast generation of the double layer close to the contacts with severe hysteresis and long-term instability in PSCs and the importance of K+ ion in reducing the kinetics affecting the ionic attachment to the contact surface.