Figure 9 (a) Chemical structures of the donor polymers (b) Normalized ultraviolet–visible absorption of the donor polymers in dilute chloroform solution and as neat films. (c) Energy level diagram of thin films obtained from cyclic voltammetry measurements. Reproduced with permission.[108] Copyright 2018, Springer Nature Limited.
By fine-tuning the number of carbon atoms connected to the branched alkyl group in TzBI unit, they synthesized a series of polymer, termed P2F-EH, P2F-EHp and P2F-EO, respectively.[108] As branching point was moved away conjugated core, the HOMO energy level of the polymers is consistently lowered (P2F-EH, -5.36 eV; P2F-EHp, -5.38 eV; P2F-EO, -5.41 eV). The energy levels of the materials are illustrated in Figure 9. The P2F-EHp has the best photovoltaic performance (7.28 %), which is higher than 6.13 % for P2F-EH: ITIC and 6.73 % for P2F-EO: ITIC. By blending with IT-2F, the P2F-EHp:IT-2F device exhibited an optimized PCE of 12.11 %. By paring with BTPTT-4F and finely optimizing the morphology of the blend film, P2F-EHp: BTPTT-4F displayed a PEC over 16 %.[109]
The polymer PTzBI-S and PTzBI-Ph with an alkylthiothienyl or alkylphenyl chain on the BDT unit were also synthesized.[110]The PTzBI-S:ITIC and PTzBI-Ph:ITIC showed PCEs of 9.12 % and 10.19 %, respectively. Zhang et al. [50] synthesized polymer PBDTS-TzBI that have a PCE of 9.04 % with a highV OC of 0.94 V without using any processing additive when blending with ITIC. Negash et al. [111] synthesized PBDTTSi-TzBI and PBDTTS-TzBI by replacing the alkyl side chain with the alkyl silyl or alkylthio side chains in BDT. A PCE of 9.6 % was obtained for PBDTTS-TzBI:ITIC based devices.
Li et al. [112] synthesized three TzBI-based polymers, namely, PHT-EHp, PFT-EHp and PClT-EHp. Owing to the strong electron-withdrawing ability of both imide group and halogen atoms, the polymers substitutions exhibited slight blue-shift absorption and deepened HOMO levels (-5.60 eV, -5.64 eV, and -5.71 eV, respectively). By blending with ITIC, the PCl-EHp showed a high PCE of 15.8 % that is higher than 7.7 % for PHT-EHp and 15.4 % for PFT-EHp. Additonally, the PClT-EHp:Y6DT-based binary devices demonstrated an efficiency 16.4 %. To explore the impact of the fluorine substitution points on thiophene π-bridge on photovoltaic performance, Huang et al. [113] synthesized PTzBI-p F and PTzBI-d F. The PTzBI-d F has a more planar molecular conformation, narrower bandgap, and higher hole mobility. The PTzBI-d F: Y6-based device has an optimized PCE of 16.8 %, which is much higher than 1.4 % for PTzBI-p F: Y6. Using L8-BO as the second acceptor,[114] the PTzBI-d F:L8-BO:Y6 ternary devices exhibited a promising PCE of 18.26 % and excellent long-term thermal stability under 85 °C.