Insights into the Morphological Instability of Bulk Heterojunction PTB7-Th/PCBM Solar Cells upon High-Temperature Aging

The impact of the morphological stability of the donor/acceptor mixture under thermal stress on the photovoltaic properties of bulk heterojunction (BHJ) solar cells based on the 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]/phenyl-C₆₁-butyric acid methyl ester (PTB7-Th/PC₆₁BM) blend is extensively investigated. Both optical microscopy and transmission electron microscopy micrographs show that long-term high-temperature aging stimulates the formation of microscale clusters, the size of which, however, is about 1 order of magnitude smaller than those observed in thermally annealed poly(3-hexylthiophene)/PC₆₁BM composite film. The multilength-scale evolution of the morphology of PTB7-Th/PC₆₁BM film from the scattering profiles of grazing incidence small-angle and wide-angle X-ray scattering indicates the PC₆₁BM molecules spatially confine the self-organization of polymer chains into large domains during cast drying and upon thermal activation. Moreover, some PC₆₁BM molecules accumulate into ∼30-40 nm clusters, the number of which increases with heating time. Therefore, the hole mobility in the active layer decays much more rapidly than the electron mobility, leading to unbalanced charge transport and degraded cell performance. Importantly, the three-component blend that is formed by replacing a small amount of PC₆₁BM in the active layer with the bis-adduct of PC₆₁BM (bis-PC₆₁BM) exhibits robust morphology against thermal stress. Accordingly, the PTB7-Th/PC₆₁BM:bis-PC₆₁BM (8 wt %) device has an extremely stable power conversion efficiency.