Biaxially-extended side-chain engineering of benzodithiophene-based conjugated polymers and their applications in polymer solar cells

Herein, three benzodithiophene (BDT)-based conjugated polymers, PBDIT-EBT, P3TBDT-FBT, and PTBOT-FBT, with gradually increased biaxially-extended side-chain conjugation are synthesized. The increase in the sidechain conjugated plane is revealed to enhance the pi-pi* transition and intramolecular charge transfer (ICT) of the polymer to result in improved absorption at near-ultraviolet region. Besides, the electron-withdrawing ability of the large side-chain conjugation effectively downshifts the energy levels of the derived polymer. Through providing complementary absorption spectra and suitable cascade energy levels, PTBOT-FBT with the largest side-chain conjugation is demonstrated as an effective third photoactive component for both fullerene and nonfullerene-acceptor (NFA) binary bulk-heterojunction (BHJ) systems, which enables a respective improvement of 2.5% and 11.8% in PCE of the studied fullerene- and NFA-based ternary solar cell devices. Moreover, it is found that the ternary NFA-based BHJ film exhibits a more favorable morphology after adding PTBOT-FBT as compared to the fullerene-based film, suggesting the higher tolerance of the polymers with bulky sidechains in the NFA system.