Abstract
We report a pronounced thickness effect of bulk heterojunction (BHJ) layers on the performance and stability of inverted polymer solar cells with the BHJ layers of poly[(4,8-bis(5-(octylthio)thiophen-2-yl)benzo[1,2-b:4,5-b']-dithiophene-co-3-fluorothieno[3,4- b]thiophene-2-carboxylate] (PBDT-TS1) and [6,6]-phenyl-C-71-butyric acid methyl ester (PC71BM). The thickness of the BHJ layers was varied from 40 to 120 nm by changing solution concentrations and spin coating speeds. The results showed that the film thickness considerably affected the performance and stability of devices. The power conversion efficiency reached ca. 9% at the thickness of 80 nm by the optimized nanoscale phase separation between donor and acceptor components. However, the devices with 120 nm-thick BHJ layers showed better device stability under continuous illumination with a simulated solar light due to the well-maintained surface morphology and nanostructure in addition to the improved morphological volume stability.