Understanding the Interplay of Transport-Morphology-Performance in PBDB-T Based Polymer Solar Cells: Article No. 1900524

Polymer-polymer blends have been reported to exhibit exceptional thermal and ambient stability. However, power conversion efficiencies (PCEs) from devices using polymeric acceptors have been recorded to be significantly lower than those based on conjugated molecular acceptors. Here, two organic nonfullerene bulk heterojunction (BHJ) blends ITIC:PBDB-T and N2200:PBDB-T, together with their fullerene counterpart, PCBM:PBDB-T, are adopted to understand the effect of electron acceptors on device performance. We comprehensively investigate free charge carrier properties using time-resolved microwave conductivity (TRMC) measurements. The nonfullerene devices show an improved PCE of 10.06% and 6.65% in the ITIC and N2200 based cells, respectively. In comparison, the PCBM: PBDB-T based devices yielded a PCE of 5.88%. The optimal N2200: PBDB-T produced the highest TRMC mobility, longest lifetime, and greatest free carrier diffusion length. We found that such phenomena can be associated with the unfavorable morphology of the all-polymer BHJ microstructure. In contrast, the solar cell using either the PCBM or ITIC acceptors display a more balanced donor and acceptor phase separation, leading to more efficient free carrier separation and transport in an operating device. Sacrificing efficiency for superior stability, we show that the improved structure in all-polymer blend could deliver a more stable morphology under thermal stress.