Pronounced Cosolvent Effects in Polymer:Polymer Bulk Heterojunction Solar Cells with Sulfur-Rich Electron-Donating and Imide-Containing Electron-Accepting Polymers

The performance of solar cells with a polymer:polymer bulk heterojunction (BHJ) structure, consisting of poly­[4,8-bis­(5-(2-ethylhexyl)­thiophen-2-yl)­benzo­[1,2-b:4,5-<i>b</i>′]­dithiophene-<i>alt</i>-3-fluorothieno­[3,4-<i>b</i>]­thiophene-2-carboxylate] (PTB7-Th) donor and poly­[[<i>N</i>,<i>N</i>′-bis­(2-octyldodecyl)-naphthalene-1,4,5,8-bis­(dicarboximide)-2,6-diyl]-<i>alt</i>-5,5′-(2,2′-bithiophene)] (P­(NDI2OD-T2)) acceptor polymers, was investigated as a function of cosolvent (<i>p</i>-xylene:chlorobenzene (pXL:CB)) composition ratio. A remarkable efficiency improvement (∼38%) was achieved by spin-coating the photoactive blend layer from pXL:CB = 80:20 (volume) rather than pXL alone, but the efficiency then decreased when the CB content increased further to pXL:CB = 60:40. The improved efficiency was correlated with a particular PTB7-Th:P­(NDI2OD-T2) donor–acceptor blend nanostructure, evidenced by a fiber-like surface morphology, a red-shifted optical absorption, and enhanced PL quenching. Further device optimization for pXL:CB = 80:20 films yielded a power conversion efficiency of ∼5.4%. However, these devices showed very poor stability (∼15 min for a 50% reduction in initial efficiency), owing specifically to degradation of the PTB7-Th donor-component. Replacing PTB7-Th with a more stable donor polymer will be essential for any application potential to be realized