Conflicted effects of a solvent additive on PTB7:PC₇₁BM bulk heterojunction solar cells

Recently, polymer-fullerene based bulk heterojunction (BHJ) solar cells, which contain blends of poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7) and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM), have been widely studied due to exhibiting high power conversion efficiency (PCE) and well-defined nanomorphology. Because of the short exciton diffusion pathway (less than 10 nm) in organic thin films, the optimization of PTB7:PC₇₁BM BHJ with optimized morphology is very important between the donor and acceptor. In order to increase nanoscale phase separation, the chemical additives of 1,8-diiodooctane (DIO) have been used in PTB7:PC₇₁BM blend systems. However, the mechanism studies of DIO in BHJ solar cells and its effectiveness on device stability are unclear. In this study, we fabricated polymer solar cells (PSCs) based on PTB7:PC₇₁BM BHJ with various DIO concentrations to investigate not only correlation between device performances and different morphologies, but also the influence of additives on device stabilities. Positive effects of DIO, which were induced by efficient charge separation in BHJ at optimized blending ratio, are proved by the results of time-resolved photoluminescence (TRPL), and negative effects of DIO on a device stability have been investigated according to the ISOS-D-1 protocol.