Molecular versus polymeric hole transporting materials for perovskite solar cell application

The p-type semiconducting organic layer used in perovskite solar cells (PSCs) for the hole extraction and transport is a key component of the device to achieve high performance. Organic hole transporting materials (HTMs) can be essentially divided into two subclasses depending on their molecular weight: molecular and polymeric. In the present paper, we fully characterize and compare the electrical response of PSCs prepared with the benchmark molecular Spiro-OMeTAD HTM and the conducting polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) selected as a low-cost and efficient polymer HTM. For both compounds, cells have been prepared without and with doping. Doping the molecular HTM with bis(trifluoromethane)sulfoimide (LiTFSI) and 4-tert-butylpyridine (tBP) gives rise to a dramatic power conversion efficiency increase from 5.1% to 17.7%. Impedance spectroscopy investigation of the cells shows that the performance of undoped Spiro-OMeTAD cells is limited by the HTM layer resistance and by the strong charge recombination occurring at the HTM/perovskite interface. In the case of P3HT-based devices, the cell performance is increased by doping to a much less extent. Using LiTFSI and tBP additives leads to the best performance. In impedance spectra, they eliminate the intermediate frequency inductive loop. The additives limit the charge recombination at the interface, reduce the interfacial defects and favor the hole transfer from the perovskite to the HTM layer. Importantly, we show that the P3HT conductivity is not the main limiting parameter of the cell efficiency because of the intrinsic conducting properties of the thiophene chain and that increasing the interchain order does not produce a significant enhancement of the PSC efficiency.