Electron-transporting small molecule/o-xylene hybrid additives to boost the performance of simplified inverted polymer solar cells

Electron-transporting small molecule bathophenanthroline (Bphen) together with o-xylene has been used as hybrid additives to improve the performance of simplified inverted polymer solar cells employing ITO alone as cathode and photoactive layer based on polymer [[2,6′-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b] dithiophene] [3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7-Th) and (He et al., Nat. Photon. 9:174, 2015)-phenyl C71-butyric acid methyl ester (PC71BM). Because the Bphen additive could spontaneously aggregate onto the indium tin oxide (ITO) cathode during spin coating, the mixed solution containing PTB7-Th, PC71BM, and Bphen, thereby forming a thin cathode-modifying layer, the ternary blend PTB7-Th:PC71BM:Bphen functioned similar to a layered polyethylenimine ethoxylated (PEIE, a conventional cathode-modifying material)/binary blend PTB7-Th:PC71BM combination. The o-xylene additive was confirmed to enrich the distribution of PTB7-Th donor towards the bottom area of photoactive layer, thereby benefiting the charge collection in inverted structure. With the o-xylene optimization, the PTB7-Th:PC71BM:Bphen blend thin film showed a power conversion efficiency (PCE) of 5.87% in simplified inverted structure of ITO/photoactive layer/MoO3/Ag, much higher than that (4.21%) of the PTB7-Th:PC71BM blend thin film. The current research can be considered as a useful attempt towards fabricating low-cost photovoltaic devices without marked loss in PCE.