Bis(4-methylthio)phenyl)amine-based hole transport materials for highly-efficient perovskite solar cells: insight into the carrier ultrafast dynamics and interfacial transport

Hole transport layers(HTLs) play a significant role in the performance of perovskite solar cells. A new class of linear smallmolecules based on bis(4-methylthio)phenyl)amine as an end group, carbon, oxygen and sulfur as the center atoms for the center unit(denoted as MT-based small-molecule), respectively, have been applied as HTL, and two of them presented the efficiency over 20% in the planar inverted perovskite solar cells(PSCs), which demonstrated a significant improvement in comparison with the widely used HTL, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(known as PEDOT:PSS), in the planar inverted architecture. The ultrafast carrier dynamics show that the excited hot carrier cooling process of MT-based small-molecule HTL samples is faster than that of PEDOT:PSS samples. The kinetic analysis of photo-bleaching peaks of femtosecond transient absorption spectra reveals that the traps at the interface between MT-based small-molecule HTLs and MAPbI_3 can be filled much quicker than that at PEDOT/MAPbI_3 interfaces. Moreover, the hole injection time from MAPbI_3 to MT-based small-molecule HTLs is around 10 times quicker than that to PEDOT:PSS. Such quick trap filling and hole extraction bring a significant enhancement in photovoltaic performances. These findings uncover the carrier transport mechanisms and illuminate a promising approach for the design of new HTLs for highly-efficient perovskite solar cells.