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-C-butyric acid methyl ester (PTB7-Th/PCBM) 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)/PCBM composite film. The multilength-scale evolution of the morphology of PTB7-Th/PCBM film from the scattering profiles of grazing incidence small-angle and wide-angle X-ray scattering indicates the PCBM molecules spatially confine the self-organization of polymer chains into large domains during cast drying and upon thermal activation. Moreover, some PCBM 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 PCBM in the active layer with the bis-adduct of PCBM (bis-PCBM) exhibits robust morphology against thermal stress. Accordingly, the PTB7-Th/PCBM:bis-PCBM (8 wt %) device has an extremely stable power conversion efficiency.
Forming nano-scale interpenetrating networks in electron donor/acceptor blends via phaseseparation tuning and maintaining their optimal morphology are of paramount importance for the favorable performance and operational lifetime of bulk heterojunction (BHJ) polymer solar cells. In this work, two fulleropyrrolidine derivatives that bear π-conjugated terthiophene (3T) and two hexyl (H) or ethylhexyl (EH) chains, abbreviated as 3T-H-C 60 and 3T-EH-C 60 , are separately used as a compatibilizer of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM). The effect of the loading of such an agent on the photovoltaic properties of solar devices and the morphological stability of the P3HT/PCBM blend was examined. The substitution of PCBM with a tiny amount of 3T-H-C 60 or 3T-EH-C 60 apparently enhances the short-circuit current density, reflecting the formation of small PCBM clusters within the P3HT matrix. More importantly, optical microscopy, transmission electron microscopy and photoluminence measurements verify that the presence of 1 wt% 3T-H-C 60 or 3 wt% 3T-EH-C 60 effectively hinders the thermal motion of PCBM molecules to form micro-scale clusters, leading to an extremely stable morphology even under long-term exposure to elevated temperatures. Accordingly, the power conversion efficiencies of P3HT/PCBM:3T-H-C 60 and P3HT/PCBM:3T-EH-C 60 devices remain very steady during prolonged aging at 130 °C. This work provides valuable guidelines for the design of an effective compatibilizer for thermally stable polymer/fullerene BHJ solar cells.Recently, several efforts have been made to establish a rational strategy for improving the environmental stability of solar devices by stabilizing the bulk-heterojunction (BHJ) film morphology.Sivula et al. applied a small amount of dihexyl-functionalized thiophene as a co-monomer to reduce the regioregularity of P3HT from >96% to 91%, to suppress the crystallization-induced phase separation and enhance the thermal stability of BHJ devices 15 . Kim et al. copolymerized 2-bromo-3-(6bromohexyl)thiophene and 2-bromo-3-hexylthiophene to prepare bromine-functionalized P3HT, which can be cross-linked upon irradiation under UV light, restricting the thermal motion of fullerene acceptors and the consequent formation of large clusters, preserving long-term performance 16 . Bertho et Terthiophene-C 60 dyad can be used as promising compatibilizer to suppress the thermal motion of PCBM inside the P3HT matrix, leading to an extremely robust morphology.
Thermal stability is a bottleneck toward commercialization of polymer solar cells (PSCs). The effect of PCBM aggregation on a multilength scale on the bulk-heterojunction (BHJ) structure, performance, and thermal stability of PSCs is studied here by grazing-incidence small- and wide-angle X-ray scattering. The evolution of hierarchical BHJ structures of a blend film tuned by regioregularity of polymers from the as-cast state to the thermally unstable state is systematically investigated. The thermal stability of PSCs with high polymer regioregularity values can be improved because of the good mutual interaction between polymer crystallites and fullerene aggregates. The insights obtained from this study provide an approach to manipulate the film structure on a multilength scale and to enhance the thermal stability of P3HT-based PSCs.
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