Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV18)
Publication date: 21st February 2018
Advances in the development of non-fullerene electron acceptors such as indaceno[1,2-b:5,6-b']dithiophene (IDT)-based small molecules have made fullerene-free organic solar cells to a competitive alternative to conventional fullerene-based organic photovoltaics with efficiencies approaching 13%.[1] Despite the recent progress and enormous potential of non-fullerene organic solar cells (NF-OSCs), the influence of the polymers molecular weight on the performance and structure-property relationship of polymer:small molecule bulk-heterojunction systems has not been studied in detail so far, even though this could play a key role to further improve the efficiency.
For this purpose, we systematically investigated the effect of the polymers molecular weight on the photovoltaic performance, film morphology, charge carrier mobility and non-geminate recombination dynamics of NF-OSCs using the low bandgap polymer PTB7-Th (with molecular weights in the range of 50-300 kDa) and the small molecule O-IDTBR (O = n-octyl) as donor and acceptor species, respectively. Inverted bulk-heterojunction solar cells with an open-circuit voltage of about 1 V, a short-circuit current density of ca. 15.2 mA cm−2, a fill factor over 60%, and a power conversion efficiency of 10% were obtained using high molecular weight polymers. The photovoltaic performance, however, was significantly influenced by the different molecular weight of PTB7-Th exhibiting an optimum at 200 kDa. This can be correlated with the improved field-effect hole mobility (ca. 1.1 × 10−2 cm2 V−1 s−1) and the lower activation energy of charge transport (25 meV) of PTB7-Th. Further considerable contributions originate from the favorable film morphology and phase separation, the higher degree of crystallinity (π−π stacking of polymer backbone), the higher charge carrier concentration and lifetime (4.5 µs) as well as the lower non-geminate recombination rate constant.
[1] Zhao et al., J. Am. Chem. Soc. 2017, 139, 7148-7151.
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