Reducing photovoltage loss at anode contact of methylammonium-free inverted perovskite solar cells by conjugated polyelectrolyte doping
The efficiency of perovskite solar cells (PSCs) developed rapidly in recent years, but the stability still lagged behind. Ion migration effect, especially from the small methylammonium (MA) cations, is a main factor for stability issues and MA-free perovskite is one appreciated pathway to suppress ion migration. In this work, we reveal there is a much lower valence band maximum (VBM) of -5.8 eV for the most studied MA-free perovskite of FA0.83Cs0.17PbI2.7Br0.3, which is much different from traditional perovskite and the huge energy level mismatch between perovskite and hole transport layer (HTL) is a main factor to limit the device performance of MA-free PSCs. It is found doping with conjugated polyelectrolyte of poly[(9,9-bis(3’-((N,N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)](PFN-Br) in perovskite can significantly promote anode contact and result in better device performance as well as stability of FA0.83Cs0.17PbI2.7Br0.3 based MA-free PSCs in inverted planar structure. The PFN-Br arise the energy position of VBM of perovskite and results in well-matched energy levels between 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 the corresponding interface recombination is effectively suppressed. As a result, a 60mV increased VOC is achieved, which promotes champion device efficiency to 20.32%. At the same time, the efficient device displays a significant stability under continuous illumination and bias at MPP conditions, which could remain 80% of its initial power conversion efficiency (PCE) under continuous operation under one sun illumination over 500 hours.