Low band-gap (LBG) D-A conjugated polymers incorporating electron-donating (D) and electron-accepting (A) units on the backbone have become the most important polymer donors in bulk-heterojunction (BHJ) polymer solar cells (PSCs) because of tunable band gaps and energy levels via intramolecular charge transfer (ICT) effect of the D- and A-units. So far, single junction BHJ PSCs with power conversion efficiencies (PCEs) up to 9.2% have been realized. However, such high PCEs were typically achieved with thin active layers of ~100 nm. High efficiency PSCs with thicker active layers would be not only more compatible to solution printing technology, a probable commercialization processing for PSCs, but also more effective in the utilization of incident light. For large area printing, toleration of large thickness variations of the active layer would be another major concern toward good quality or yield control. In principle, high hole mobility of a LBG polymer donor can accelerate hole transport in the active layer of a PSC and decrease recombination of holes and electrons, from which high PCE can be realized with a thick active layer.
We have synthesized a series of LBG D-A conjugated polymers comprising oligothiophenes as the D-units and 5,6-difluorobenzothiadiazole (FBT), dithienobenzothiadiazole (DTfBT), and dithienobenzooxadiazole (DTfBO) as the A-units. The polymers could show very strong interchain aggregation in solutions at room temperature (RT), whose absorption spectra in RT solution were almost comparable to that of corresponding thin solid film. The aggregates of the polymers in solutions could be separated up heating, supplying enough solution-processing condition for high quality films. The strong interchain aggregation abilities of the polymers resulted in high hole mobility. For example, field-effect hole mobilities based on neat films could be up to 1.92 cm2/(V s), only based on annealing at a mild condition of 100 °C. The polymers also displayed high space charge limited current (SCLC) hole mobilities. BHJ PSCs were fabricated with varied thickness of the active layers. Delightedly, the BHJ PSCs based on the polymer donors exhibited maximum PCEs with thick active layers of 200 nm or beyond. The PCE variations were relatively small for different thickness of active layers. For a FBT-based polymer, BHJ PSCs with active layer thickness variations from 100 to 440 nm could show PCEs all over 6.5%, whose maximum PCE of 7.64% was achieved with 230 nm thick active layer. For another FBT-based polymer in BHJ PSCs with active layer thickness variations from 100 to 355 nm, the PCEs were all over 6.7% and the maximum PCE of 7.78% was achieved with 200 nm thick active layer. Our results suggest that LBG D-A conjugated polymers with strong interchain aggregation abilities would be benefit to realize high hole mobility, from which highly efficient thick-film BHJ PSCs could be achieved.