Halogenated polystyrene derivatives passivate and prevent volatilization in inverted perovskite solar cells

Abstract

Preventing the volatilization of organic cations at high temperatures is necessary for structural stability and long-term operation of perovskite photovoltaic cells (with organolead halide perovskites). At the same time, effective bulk and interface passivation of the perovskite layer, along with prevention of ion migration effects, is also required for achieving high efficiency and stability. Here we show through systematic study with polystyrene and halogenated polystyrene derivatives, namely, PS–X (X = Br, Cl and F) and poly(pentafluorostyrene), that a single polymeric species can effectively achieve not just bulk and interface passivation but also prevent the volatilization of the methylammonium organic cation. Through the systematic modulation in the polymer structure, we find that the key aspects for achieving this are the dipolar nature of the polymer species along with its diffuse charge distribution pattern and a high (+) quadrupole moment. Using high temperature and time-dependent X-ray diffraction, we show that volatilization of the methylammonium species is suppressed by at least 20 °C, thereby maintaining the structural stability of perovskite thin films for more than >720 h at high temperatures of 85 °C. This results in thermally stable perovskite solar cells retaining 95% of their initial efficiency after 2000 h under ISOS D-2 testing conditions. Additionally, the combined interface and bulk passivation using poly(pentafluorostyrene) leads to efficiency and V_oc of as high as 24% and 1.17 V. It also greatly improves storage and operational stability of the devices, retaining 97%, and 95% of initial efficiency after 3500 h and 500 h under ISOS D-1 and ISOS L-1 testing conditions respectively.