A Dopant-Free Polymeric Hole-Transporting Material Enabled High Fill Factor Over 81% for Highly Efficient Perovskite Solar Cells

Although perovskite solar cells (PVSCs) have achieved rapid progress in the past few years, most of the high-performance device results are based on the doped small molecule hole-transporting material (HTM), spiro-OMeTAD, which affects their long-term stability. In addition, some defects from under-coordinated Pb atoms on the surface of perovskite films can also result in nonradiative recombination to affect device performance. To alleviate these problems, a dopant-free HTM based on a donor-acceptor polymer, PBT1-C, synthesized from the copolymerization between the benzodithiophene and 1,3-bis(4-(2-ethylhexyl)thiophen-2-yl)-5,7-bis(2-alkyl)benzo[1,2-c:4,5-c ']dithiophene-4,8-dione units is introduced. PBT1-C not only possesses excellent hole mobility, but is also able to passivate the surface traps of the perovskite films. The derived PVSC shows a high power conversion efficiency of 19.06% with a very high fill factor of 81.22%, which is the highest reported for dopant-free polymeric HTMs. The results from photoluminescence and trap density of states measurements validate that PBT1-C can effectively passivate both surface and grain boundary traps of the perovskite.