All-Polymer Solar Cells: Impact of the Length of the Branched Alkyl Side Chains on the Polymer Acceptors on the Interchain Packing and Electronic Properties in Amorphous Blends
TH Wang and V Coropceanu and JL Bredas, CHEMISTRY OF MATERIALS, 31, 6239-6248 (2019).
All-polymer solar cells are attracting increasing attention because polymer acceptors present specific advantages, especially over fullerene derivatives. The length of the branched alkyl side chains on the polymer acceptors has been reported to have different impacts in the crystalline versus amorphous domains of donor/acceptor blends. Given that amorphous domains are difficult to characterize experimentally, here, molecular dynamics simulations are combined with density functional theory calculations to examine at the molecular scale the role that the side- chain length plays on the interchain packing and electronic properties. As representative examples, blends of the poly4,8-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-bthiophene)-2-carboxylate-2-6-diyl) (PCE10) donor with the poly(thieno3,4-c1-dione-alt-3,4-difluorothiophene) (PTPD2FT) acceptor are discussed; two different branched side chains on the thieno3,4cpyrrole-4,6-dione (TPD) moieties of the acceptor polymer are considered, that is, 2-hexyldecyl and 2-decyltetradecyl. Increasing the side-chain length is found to decrease the PTPD2FT backbone planarity and importantly to bring a higher extent of interactions between the side chains and their own TPD moieties, which plays a critical role in determining the interchain packings. The nature of these packings are then correlated with the following: (i) the electron-transfer rates between neighboring PTPD2FT chains; (ii) the energetic distribution of the interfacial charge-transfer states; and (iii) the nonradiative recombination processes from the charge-transfer states to the ground state and the associated voltage losses. Overall, our findings point to a higher electron mobility and a lower nonradiative voltage loss in the PCE10/PTPD2FT blends where the polymer acceptor has shorter side chains.
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