Low Band‐Gap Conjugated Polymers with Strong Interchain Aggregation and Very High Hole Mobility Towards Highly Efficient Thick‐Film Polymer Solar Cells

DOI: 10.1002/adma.201305092 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 Dand A-units. [ 1–8 ] So far, single junction BHJ PSCs with power conversion effi ciencies (PCEs) up to 9.2% have been realized. [ 9–23 ] However, such high PCEs were typically achieved with thin active layers of ∼100 nm. High effi ciency 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. [ 1 ] For large area printing, toleration of large thickness variations of the active layer would be another major concern toward good quality or yield control. [ 24,25 ] In principle, high hole mobility of a LBG polymer donor can accelerate hole transport in the active layer of a PSC and decreases recombination of holes and electrons, from which high PCE can be realized with a thick active layer. The advantages of fl uorinated heterocycles in the constructions of highly effi cient LBG D-A conjugated polymers possessing lower-lying highest occupied molecular orbital (HOMO) levels for PSCs with higher open-circuit voltages ( V oc ) have been proposed. [ 10,11,26–28 ] Especially, the utilizations of fl uorinated benzothiadiazoles (BTs) [ 19,23,26,29–39 ] as the A-units for polymer donors, including mono and difl uorinated ones, have attracted increasing attention, and PCEs up to 7.9% have been demonstrated. Theoretical analysis indicates that 4,7-dithienyl5,6-difl uorobenzothiadiazole will signifi cantly increase the planarity of the structure and decrease torsional disorder, in comparison to that of 4,7-dithienylbenzothiadiazole. [ 40 ] Decreased bimolecular recombinations and enlarged face-on orientations could be found for 4,7-dithienyl-5,6-difl uorobenzothiadiazolebased conjugated polymers in active layer blends for PSCs. [ 37 ]