Self-doped polymer with fluorinated phenylene as hole transport layer for efficient polymer solar cells

Abstract

pH neutral conjugated polyelectrolytes (CPEs) hole-transporting layer (HTL) materials are promising candidates to replace poly(3,4-ethylenedioxythiophene):(styrenesulfonate) (PEDOT:PSS) for achieving high performance and stable polymer solar cells (PSCs). However, compared to a plenty of CPEs developed as efficient electron-transporting layer (ETL), only numbered CPEs can be used as HTLs, mainly due to the low conductivity and mismatched energy levels of CPEs with the HOMO (highest occupied molecular orbital) level of donor. In this work, two new conductive CPEs with neutral pH, namely PCPDPhSO3K and PCPDffPhSO3K, based on benzene and 1,4-difluorobenzene units were designed and synthesized. Due to the strong p-type self-doping property of the CPEs, high conductivity and homogeneous conductive network were achieved in CPEs. CPEs was also found to effectively improve the work function (WF) of ITO to diminish the energy barrier between active layer and anode. Such improved conductivity and better energy alignment resulted in a simultaneous enhancement of short-circuit current density (Jsc), open-circuit voltage (Voc) and fill factor (FF) in CPEs modified devices. The single-junction PSC based on PCPDffPhSO3K HTL showed a power conversion efficiency (PCE) of 9.50%, which has a significant improvement in contrast to the PEDOT:PSS-based device (PCE = 8.60%). These results demonstrated that CPEs developed as efficient HTLs can be realized by precise backbone engineering for highly performance PSCs.