title: Insights into the Morphological Instability of Bulk Heterojunction PTB7-Th/PCBM Solar Cells upon High-Temperature Aging.
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.
authors.affiliation: Centre for Condensed Matter Sciences, National Taiwan University , Taipei 10617, Taiwan.|Institute of Polymer Science and Engineering, National Taiwan University , Taipei 10617, Taiwan.
authors.forename: Yen-Ju
authors.initials: YJ
authors.lastname: Hsieh
authors.affiliation: Institute of Nuclear Energy Research , Longtan, Taoyuan 32546, Taiwan.
authors.forename: Yu-Ching
authors.initials: YC
authors.lastname: Huang
authors.affiliation: Centre for Condensed Matter Sciences, National Taiwan University , Taipei 10617, Taiwan.|Institute of Polymer Science and Engineering, National Taiwan University , Taipei 10617, Taiwan.
authors.forename: Wei-Shin
authors.initials: WS
authors.lastname: Liu
authors.affiliation: Centre for Condensed Matter Sciences, National Taiwan University , Taipei 10617, Taiwan.|Materials Science Division, Argonne National Laboratory , 9700 Cass Avenue, Lemont, Illinois 60439, United States.
authors.forename: Yu-An
authors.initials: YA
authors.lastname: Su
authors.affiliation: Institute of Nuclear Energy Research , Longtan, Taoyuan 32546, Taiwan.|Department of Materials Science and Engineering, National Taiwan University , Taipei 10617, Taiwan.
authors.forename: Cheng-Si
authors.initials: CS
authors.lastname: Tsao
authors.affiliation: Institute of Organic and Polymeric Materials, National Taipei University of Technology , Taipei 10608, Taiwan.
authors.forename: Syang-Peng
authors.initials: SP
authors.lastname: Rwei
authors.affiliation: Centre for Condensed Matter Sciences, National Taiwan University , Taipei 10617, Taiwan.|Institute of Polymer Science and Engineering, National Taiwan University , Taipei 10617, Taiwan.
authors.forename: Leeyih
authors.identifier: http://orcid.org/0000-0002-6368-0141
authors.initials: L
authors.lastname: Wang
keywords: X-ray scattering; bulk heterojunction; morphology evolution; polymer solar cells; thermal stability