Correlation between the performance of organic bulk-heterojunction solar cells and the molecular structures of alcohol solvents

Abstract

Morphology control is an important issue for boosting the performance of organic bulk-heterojunction (BHJ) solar cells. In this study, we investigated the correlation between alcohol solvents and the morphologies 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-C-70-butyric acid methyl ester (PC70BM)-based organic solar cells by spin-casting the alcohol onto the active layers. We found that the morphologies strongly depended on the structure of the alcohol [alkyl chain length and hydroxyl (OH) group position]. Ethanol or 2-propanol showed the highest performance among the alcohols considered here. Atomic force microscopy images and absorption spectra demonstrated that the alcohols affected the morphologies of PC70BM rather than those of PTB7. The morphologies of PC70BM were dependent on the solubilities of the alcohols to the active layers and the hydrogen-bonding strengths between the PC70BM and alcohol molecules. Our results indicate that the use of alcohols for solvent annealing is a simple and efficient method for developing high-performance organic BHJ solar cells. (c) 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 44367.