Composition-Morphology Correlation in PTB7-Th/PC₇₁BM Blend Films for Organic Solar Cells

From a morphological perspective, the understanding of the influence of the [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) content on the morphology of the active layer is not complete in organic solar cells (OSCs) with bulk heterojunction (BHJ) configuration based on the low-bandgap polymer poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2- b;4,5- b']dithiophene-2,6-diyl- alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4- b]thiophene-)-2-carboxylate-2-6-diyl] (PTB7-Th). In this work, we obtain the highest power conversion efficiency (PCE) of 10.5% for BHJ organic solar cells (OSCs) with a PTB7-Th/PC₇₁BM weight ratio of 1:1.5. To understand the differences in PCEs caused by the PC₇₁BM content, we investigate the morphology of PTB7-Th/PC₇₁BM blend films in detail by determining the domain sizes, the polymer crystal structure, optical properties, and vertical composition as a function of the PC₇₁BM concentration. The surface morphology is examined with atomic force microscopy, and the inner film morphology is probed with grazing incidence small-angle X-ray scattering. The PTB7-Th crystal structure is characterized with grazing incidence wide-angle X-ray scattering and UV/vis spectroscopy. X-ray reflectivity is employed to yield information about the film vertical composition. The results show that in PTB7-Th/PC₇₁BM blend films, the increase of PC₇₁BM content leads to an enhanced microphase separation and a decreased polymer crystallinity. Moreover, a high PC₇₁BM concentration is found to decrease the polymer domain sizes and crystal sizes and to promote polymer conjugation length and formation of fullerene-rich and/or polymer-rich layers. The differences in photovoltaic performance are well explained by these findings.