Biaxially extended side chain engineering of benzodithiophene based conjugated polymers and their applications in polymer solar cells
Abstract Herein, three benzodithiophene (BDT) based conjugated polymers, PBDTT FBT, P3TBDT FBT, and PTBOT FBT, with gradually increased biaxially extended side chain conjugation are synthesized. The increase in the side chain conjugated plane is revealed to enhance the p p* 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 non fullerene 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.