Fullerene (C₆₀)-modulated surface evolution in CH₃NH₃PbI₃ and its role in controlling the performance of inverted perovskite solar cells

We report here the effect of fullerene (C₆₀) incorporation on the growth of CH₃NH₃PbI₃ perovskite crystals and the effect on photovoltaic performance of perovskite solar cells (PSCs) prepared in inverse geometry. Incorporation of C₆₀ induced the growth of larger gains and compact thin film of perovskite with reduced defects, which led to its enhanced photovoltaic performance. Apart from that, C₆₀ also participates in transportation and collection of photo-generated electrons. The optimum incorporation of C₆₀ resulted in an impressive improvement in the power conversion efficiency (PCE) of champion PSC from 9.2 to 12.8%. Moreover, the C₆₀-doped PSCs exhibited improved air stability compared to undoped devices. The enhanced PCE in C₆₀-doped PSCs is a result of enhanced optical absorption and separation of photo-generated charge and their transportation in the active layer. Since the size of C₆₀ molecules is of the order of nm, they easily get filled into the perovskite voids and facilitate another percolation path ways for charge carriers to transport and suppress the recombination losses via passivating the recombination centres in perovskite layers. The compact perovskite layer with larger grains led to reduced inter-granular grain boundaries with reduced defects, which restricts the fast diffusion of moisture into active layer and resulted in improved stability in device performance.