Assessing the Role of Polymer Molecular Weight for High-Performance Indacenodithiophene-Based Fullerene-Free Organic Solar Cells
Sebastian F. Hoefler a, Thomas Rath a, Mathias Hobisch a, Nadiia Pastukhova b, Egon Pavlica b, Gvido Bratina b, Dorothea Scheunemann c, Sebastian Wilken c, Gregor Trimmel a
a Institute for Chemistry and Technology of Materials (ICTM), Graz University of Technology, Stremayrgasse 9, 8010 Graz, Austria
b Laboratory of Organic Matter Physics, University of Nova Gorica, Vipavska 11c, 5270 Ajdovščina, Slovenia
c Department of Physics, Energy and Semiconductor Research Laboratory, Carl von Ossietzky University of Oldenburg, Carl-von-Ossietzky-Straße 9-11, 26129 Oldenburg, Germany
International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV18)
Benidorm, Spain, 2018 May 28th - 31st
Organizers: Emilio Palomares and Rene Janssen
Poster, Sebastian F. Hoefler, 127
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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