Dominant recombination mechanism in perovskite solar cells: A theoretical study

Abstract

Different recombination losses have been shown to play a role in the photocurrent and photovoltage loss of lead halide perovskite solar cells. While it is believed that trap density is extremely small in high-efficiency perovskite cells, trap-assisted recombination cannot be ruled out due to the existence of tail states, for instance at the grain boundaries. In this study, the optical and electrical performance of a perovskite solar cell of the structure: ITO/PEDOT:PSS/CH3NH3PbI3/PC61BM/LiF/Al was modelled using numerical drift-diffusion simulations with embedded optical modelling via the transfer matrix method. The total recombination was calculated as the sum of bimolecular recombination and recombination via tail states. The influence of charge carrier mobility on photovoltaic parameters (short-circuit current, fill factor, open-circuit voltage, and power conversion efficiency) of perovskite solar cells was studied. Next, the impact of Urbach energy on the recombination process was investigated by simulation of bimolecular and tail state recombination rates as a function of voltage for various values of Urbach energy. It was shown that by increasing the Urbach energy, the bimolecular recombination rate decreases, but the tail state recombination rate increases at all voltages. Furthermore, the slope of the open-circuit voltage-light intensity curve, the light ideality factor, demonstrates the dominant recombination at different Urbach energy levels. It was found that the slope increases from 1.04 at EU = 25 meV to1.65 at EU = 150 meV. It can be concluded that the bimolecular recombination is the dominant recombination at low EU, while the tail state recombination overcomes the bimolecular recombination at high EU.