The thiophene, thieno[3,2-b]thiophene, selenophene, and furan moieties have been widely used as π-bridges to tune TPD-based polymers’ photophysical and photovoltaic properties.[83-87]Facchetti et al . reported a series of polymers TPD-1-3, and explored the effects of alkyl side chain length and fluorine atom substitutions on physical properties of TPD-based polymers.[88] The TPD-1-3 exhibited good solubilities, high hole transport, and matched frontier energy levels with IT-F4. The efficiency of the devices decreases as the chain length increase, and the efficiency of 11.7 %, 11.8 % and 12.1 % were obtained for TPD-1, TPD-2 and TPD-3-based PSCs by using o-xylene solvent. The presence of the F atom on the BDT deepens the HOMO energy level, resulting in TPD-3F: IT-4F based OSC with a PCE of 13.8 % and a high V OC 0.91eV. However, by matching with non-fullerene acceptor Y6, the TPD-3 exhibited an PCE of 15.2 %, while that for TPD-3F is 11.4 % probably due to the mismatch of HOMO energy level.[89] Recently, Min et al . designed a TPD-based polymer donor PTTB-F, which showed an decent PCE of 18.06 % in binary OSCs by blending with L8-BO.[77] By employing thieno[3,2-b]thiophene as the π-bridge, Liang et al. synthesized TPD-based polymers PBDTT-6ttTPD and PMOT16.[47] By blending with IDIC, the PMOT16-based OSCs exhibited a PCE of 10.04 %. Later, they synthesized polymers PMTT56, PMOT39 and PEHTT by introducing different side chains in BDT unit.[92] By matching with IT-2F, PMTT56, PMOT39 and PEHTT based OSCs achieved the highest PCEs of 12.6 %, 10.5 % and 10.8 %, respectively. It should be noted that the high PCEs were obtained by using non-halogenated solvent toluene. By adding PC71BM as the second acceptor, the PCE of PMTT56-based ternary devices was further enhanced to 13.2 %. Later, Kim et al. developed three polymers PBDT-H, PBDT-F and PBDT-Cl based on thienothiophene π-bridgedN -octylthieno[3,4-c]pyrrole-4,6-dione (8ttTPD) and benzo[1,2-b :4,5-b’ ] dithiophene (BDT) units.[90] They found that the incorporation of highly planar structured 8ttTPD unit could improve crystallinity and hole mobilities of the BDT-based polymers. Besides, the introduction of the halogen atoms on BDT unit could tune the crystallinity and energy levels. Synergistic effects of incorporated 8ttTPD unit and halogen atoms could significantly facilitate the charge transporting properties and charge recombination process, which is stemmed from the enhanced crystallinity and hole mobility of the polymers. Therefore, the PBDT-Cl:Y6-based OSCs achieved the highest PCE of 15.63 %, which out-performs the 11.84 % for PBDT‐H:Y6 and 14.86 % for PBDT‐F:Y6. To investigate the effect of molecular weight on the photovoltaic performances, Hwang et al. [91] synthesized three batches of PBDTT-8ttTPD with different molecular weights. The higher molecular weight could improve the ordering of polymer packing, π–π stacking distance, absorption coefficient, and nanomorphology of the blend films. The batch with highest molecular weight demonstrates the highest PCE of 11.05 %, which is much higher than 8.27 % and 5.34 % for the batches with the medium and lowest molecular weight.
By introducing the third unit (D2 or A2) into the D-A copolymer, the random terpolymer strategy have been proved to be an efficient method to fine-tune the frontier energy levels, optical properties, and film-forming properties of the polymer.[83,96,97]The TPD unit was also broadly used to in terpolymers. Chen and coworkers designed and synthesized a D–A1–D–A2-type terpolymer (PM6-TPD-5  %) via random copolymerization.[93] The addition of TPD results in polymers with lower HOMO, wider light absorption, optimal molecular packing, and more ideal morphology of blend film. PM6-TPD-5 %: Y6-based binary organic solar cells showed an encouraging PCE of 16.3  %, which out-performs the 14.8 % for PM6-TPD-10  %: Y6 and 15.6 % for PM6: Y6. Besides, a PCE of 17.1  % was obtained in PM6: PM6-TPD-5  %: Y6 based ternary OSCs. Cao et al. synthesized a set of random terpolymers PTAZ-TPD10-Cn , which are composed of an electron-rich unit BDT, and two electron-deficient units of TAZ and TPD.[94] The PCE of the N2200 and ITIC-based devices decreases monotonically when shortening the side chain of PTAZ-TPD10-Cn from decyl to hexyl. However, the PCEs of PTAZ-TPD10-Cn :PC61BM-based OSCs increase monotonically. Toppare and coworkers synthesized two random copolymers P1 and P2 by using benzodithiophene (BDT) and thiophenes as donor moiety, TPD as acceptor, and selenophene as π-bridging unit.[95] Compared with polymer P2, P1 containing alkyl thiophene have a larger molecular weight and better solubility, resulting in a higher PCE of 7.94 % for P1: ITIC which is much higher than 1.96 % for P2: ITIC.