Improving the Photostability of Small-Molecule-Based Organic Photovoltaics by Providing a Charge Percolation Pathway of Crystalline Conjugated Polymer.

Abstract

Photostability of small-molecule (SM)-based organic photovoltaics (SM-OPVs) is greatly improved by utilizing a ternary photo-active layer incorporating a small amount of a conjugated polymer (CP). Semi-crystalline poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (PPDT2FBT) and amorphous poly[(2,5-bis(2-decyltetradecyloxy)phenylene)-alt-(5,6-dicyano-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (PPDT2CNBT) with similar chemical structures were used for preparing SM:fullerene:CP ternary photo-active layers. The power conversion efficiency (PCE) of the ternary device with PPDT2FBT (Ternary-F) was higher than those of the ternary device with PPDT2CNBT (Ternary-CN) and a binary SM-OPV device (Binary) by 15% and 17%, respectively. The photostability of the SM-OPV was considerably improved by the addition of the crystalline CP, PPDT2FBT. Ternary-F retained 76% of its initial PCE after 1500 h of light soaking, whereas Ternary-CN and Binary retained only 38% and 17% of their initial PCEs, respectively. The electrical and morphological analyses of the SM-OPV devices revealed that the addition of the semi-crystalline CP led to the formation of percolation pathways for charge transport without disturbing the optimized bulk heterojunction morphology. The CP also suppressed trap-assisted recombination and enhanced the hole mobility in Ternary-F. The percolation pathways enabled the hole mobility of Ternary-F to remain constant during the light-soaking test. The photostability of Ternary-CN did not improve because the addition of the amorphous CP inhibited the formation of ordered SM domains.