Highly efficient polymer solar cells by step-by-step optimizing donor molecular packing and acceptor redistribution

BM as the active layer were improved from 6.74% to 8.75% by employing 80 min for self-assembly and 20 s of methanol soaking. The PCE was improved even further to 9.72% by inserting a PFN interfacial layer. The performance improvement was mainly attributed to the optimized PffBT4T-2OD molecular packing during the self-assembly process, ideal vertical phase separation driven by methanol soaking and efficient charge collection by insertion of a PFN interfacial layer. The molecular packing and vertical phase separation were characterized by grazing incidence X-ray diffraction (GIXD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), respectively. The experimental results solidly supported the effectiveness of the step-by-step optimization strategy.