Reducing photovoltage loss at the anode contact of methylammonium-free inverted perovskite solar cells by conjugated polyelectrolyte doping

The efficiency of perovskite solar cells (PSCs) has developed rapidly in recent years, but the stability has still lagged behind. The ion migration effect, especially from small methylammonium (MA) cations, is a main factor for stability issues, and the use of MA-free perovskites is one potential pathway to suppress ion migration. In this work, we reveal that there is a much lower valence band maximum (VBM) of -5.8 eV for the most studied MA-free perovskite FA(0.83)Cs(0.17)PbI(2.7)Br(0.3), which is much different from that of traditional perovskites, and the huge energy level mismatch between perovskites and the hole transport layer (HTL) is a major factor in limiting the device performance of MA-free PSCs. It was found that doping the conjugated polyelectrolyte poly[(9,9-bis(3 '-((N,N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN-Br) into the perovskite can significantly promote anode contact and result in better device performance as well as stability of FA(0.83)Cs(0.17)PbI(2.7)Br(0.3) based MA-free PSCs in an inverted planar structure. PFN-Br increases the energy position of the VBM of perovskites and results in well-matched energy levels between the perovskite and HTL of poly[bis(4-phenyl) (2,4,6-trimethylphenyl) amine (PTAA). Carrier extraction and transportation are highly encouraged at the surface of PTAA/perovskite, and corresponding interface recombination is effectively suppressed. As a result, a 60 mV increase in V-OC is achieved, and this promotes an excellent device efficiency of 20.32%. At the same time, the efficient device displays significant stability under continuous illumination and bias under MPP conditions, and could maintain 80% of its initial power conversion efficiency (PCE) under continuous operation under one sun illumination over 500 hours.