Insights into the Morphological Instability of Bulk Heterojunction PTB7-Th/PCBM Solar Cells upon High-Temperature Aging
Journal
ACS Applied Materials and Interfaces
Journal Volume
9
Journal Issue
17
Pages
14808-14816
Date Issued
2017
Author(s)
Abstract
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. © 2017 American Chemical Society.
Subjects
bulk heterojunction; morphology evolution; polymer solar cells; thermal stability; X-ray scattering
SDGs
Other Subjects
Butyric acid; Carboxylation; Composite films; High resolution transmission electron microscopy; Hole mobility; Light transmission; Molecules; Morphology; Polymer solar cells; Solar cells; Solar power generation; Thermal stress; Thermodynamic stability; Transmission electron microscopy; X ray scattering; Bulk heterojunction; Bulk heterojunction (BHJ); High temperature aging; Morphological instability; Morphological stability; Morphology evolution; Poly (3-hexylthiophene); Power conversion efficiencies; Heterojunctions
Publisher
American Chemical Society
Type
journal article