Improved Carrier Transport in Perovskite Solar Cells Probed by Femtosecond Transient Absorption Spectroscopy

CH 3 NH 3 PbI 3 perovskite thin films have been deposited on glass/indium tin oxide (ITO)/Hole Transport Layer (HTL) substrates, utilizing two different materials as the HTLs.In the first configuration the super hydrophilic polymer poly (3,4-ethylenedioxythiophene)-poly (styrenesulfonate), known as PEDOT:PSS, was employed as the HTL material, while on the second case the non-wetting poly (triarylamine) semiconductor polymer, known as PTAA was used.It was found that when PTAA is used as HTL material the averaged power conversion efficiency (PCE) of the perovskite solar cells remarkably increases from 12.60% to 15.67%.In order to explore the mechanism behind this enhancement, the aforementioned perovskite/HTL arrangements were investigated by time resolved transient absorption spectroscopy (TAS) performed under inert conditions.By means of TAS, the charge transfer, carrier trapping and holes injection dynamics from the photo-excited perovskite layers to the HTL can be directly monitored via the characteristic bleaching profile of the perovskite at ~750 nm.TAS studies revealed faster relaxation times and decay dynamics when the PTAA polymer is employed, that potentially account for the enhanced PCE observed.The TAS results are correlated with the structure and crystalline quality of the corresponding perovskite films, investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), atomic force microscopy (AFM), microphotoluminescence (μPL) and transmittance spectroscopy.It is concluded that TAS is a benchmark technique for the understanding of the carrier transport mechanisms in PSCs and constitutes a figure-of-merit tool towards their efficiency improvement.