Thick‐Film High‐Performance Bulk‐Heterojunction Solar Cells Retaining 90% PCEs of the Optimized Thin Film Cells

Abstract

Most optimized, high performance, bulk‐heterojunction (BHJ) polymer solar cells have an active layer thickness of about 100 nm. The thin active layer is unfavorable for optical absorption and film coating. This contribution employs a ternary cell to address this problem. In this paper, thick BHJ cells can be fabricated that retain 90% power conversion efficiencies (PCEs) of the optimized thin‐film cells. The BHJs under investigations are PTB7:PC71BM, PTB7‐Th:PC71BM, and P3HT:PCBM. Into these BHJs, a ternary component, p‐DTS(fbtth2)2 (DTS) is introduced. Without DTS, the binary thick‐film devices (≈200, ≈200, and ≈400 nm) have PCEs of 6.3%, 7.4%, and 3.2%. With DTS, the corresponding BHJs have markedly improved PCEs of 7.6%, 8.3%, and 3.9%, respectively. The results are more than 90% the PCEs of the optimized binary BHJs. The origins of the improvement are investigated. Addition of the ternary component DTS enhances hole mobility and reduces trap states. Both observations are well correlated with improved fill factors of the ternary BHJ cells. Photothermal deflection spectroscopy and 1H nuclear magnetic resonance are used to trace the electronic and the nanoscale interactions of the DTS with the polymer and fullerene. The results suggest the DTS behaves as a conducting bridge in between two neighboring polymer segments.