Optimized Charge Transport Channel Enables Thick-Film All-Small-Molecule Organic Solar Cells

Using large-scale industrial production, such as roll-to-roll (R2R) solution printing technology, to achieve lab-to-manufacturing translation should be an inevitable process for the commercialization of organic solar cells (OSCs). Optimizing the charge transport channel to improve the charge carrier transport and suppress the charge carrier recombination in the thick active layer plays a key role in constructing efficient thick-film OSCs. Therefore, exploring the design methods of efficient and thickness-insensitive photovoltaic materials is of great importance for fabricating thick-film OSCs. In this work, we fabricated a series of all-small-molecule organic solar cells (ASM-OSCs) with varied active layer thicknesses using PC71BM as the acceptor and DRTB-T-C2 (C2) and DRTB-T-C4 (C4) with different molecular orientations as donors. Although donor compounds possess very similar absorption properties and energy levels due to the identical conjugated structures, there are considerable differences in the thickness tolerability of ASM-OSCs. With the film thickness increasing from 100 to 300 nm, the power conversion efficiencies (PCEs) of C4:PC71BM-based OSCs slightly decreased from 7.45 to 7.09%, while the PCEs of C2:PC71BM-based devices dramatically reduced from 6.99 to 5.61%. Higher face-on ratios of the C4:PC71BM blend film give an optimized charge transport channel as supported by the increased and balanced hole and electron mobilities and reduced charge recombination in the thick active layer, and an optimal PCE of 7.71% is achieved in C4:PC71BM-based devices with an active layer thickness of 230 nm. This work demonstrates that modulating the molecular orientation of photovoltaic materials via subtle side-chain tuning is a simple and effective method to optimize charge transport channels and thus attain thick-film OSCs. To the best of our knowledge, PCEs over 7% are the highest values so far observed from a thick-film ASM-OSC using a wide-band-gap (2.0 eV) small-molecule donor, making it potential in constructing highly efficient multijunction devices.