Later, the effect of alkyl chains on optical and photovoltaic properties was further studied.[101]The GIWAXS test results indicate that as the alkyl side chain length decreases from C56 to C48, the crystal surface orientation and molecular order increases significantly. As a result, PBDT-BiTPD-C48 with the shortest alkyl side chains achieved the best photovoltaic performance with the PCE of14.1 %, while that for other two polymers (C52 and C56) are 11.1 % and 7.8 %, respectively. Cai et al. [102] synthesized three polymers 2HD/C8, C8/2BO/C6, and 2BO/C6/C6 by introducing side alkyl chains into different positions. Introducing alkyl side chains to the thiophene (T) motifs in C8/2BO/C6 or to the BDT motifs in 2BO/C6/C6 decrease the crystallinity and face-on crystallite orientation, resulting in serious charge recombination loss and decreased FF andJ SC in BHJ devices. Hwang et al. [31] synthesized two biTPD-based polymers PBDT-biTPD(BO) and PBDT-biTPD(HD)with different lengths of the alkyl side chain. Due to the strong electron-withdrawing strength of biTPD, both polymers show deeper HOMO and LUMO energy levels than those for TPD-based polymers. By blending with IT-4F, PBDT-biTPD(BO) and PBDT-biTPD(HD)-based OSCs showed the PCE of 9.49 % and 9.32 %, respectively.
3.3. TzBI-Based Polymers.
Scheme 3 Synthetic Route to TzBI
The imide-functionalized benzotriazole (TzBI) unit was firstly reported by Huang et al [51]. The Diels-Alder reaction of 1, 3-di-2-thienothiophene [3,4-c][1,2,5] thiadiazole-2-S (IV) (1) with dimethyl acetylenedicarboxylate yield 4,7-Di(thien-2-yl)-2,1,3-benzothiadiazole-5,6-Dicarboxylate (2). Followed by alkaline hydrolysis, anhydride and imide formation, N-alkyl-4,7-di(thien-2-yl)-2,1,3- benzothiadiazole-5,6-dicarboxylic imide (5) was prepared accordingly. The reduction reaction of compound 5 by iron powder, the cyclization reaction with excess NaNO2 and condensation reaction with amine give the compound 4,8-di(thien-2-yl)-6-octyl-2-octyl-5H- pyrrolo[3,4-f ]benzotriazole-5,7(6H )-dione (TzBI) successfully (Scheme 3). Presented in Figure 8 are the chemical structures of some TzBI-based polymers and their corresponding photophysical properties and photovoltaic performance are summarized in Table 4. They synthesized TzBI-based WBG polymer PTZBIBDT by coupling with BDT unit.[51] The PTZBIBDT:PC71BM OSCs exhibited a PCE of 8.63 %. By blending with a high molecular weight polymer acceptor N2200, the PTzBI:N2200-based all-PSC devices exhibited a high PCE of 9.16 %. It should be noted that this efficiency was obtained from the OSCs that was prepared from the environmentally friendly solvent MeTHF.[103] They also synthesized a wide-bandgap polymer PTzBI-DT by coupling with DTBDT unit.[104]Devices based on PTzBI:ITIC and PTzBI-DT:ITIC achieved PCEs of 10.24 % and 9.43 %, respectively.
Incorporating appropriate side chains into the TzBI groups can affect the optoelectronic properties of polymers. Huang et al. [105] synthesized a polymer PTzBI-O, by replacing the alkyl side chain in TzBI with a polar substituent. The addition of oxygen atom enhanced the electron-withdrawing ability of theTzBI, resulting in a red-shifted absorption and strong preaggregation of PTzBI-O. A PCE of 7.9 % was achieved for PTzBI-O:N2200 based all-PSC devices. They further synthesized a copolymers PTzBI-Si that has a siloxane-terminated side-chain in TzBI.[106]Because the siloxane-functionalized side chains endow PTzBI-Si with excellent solubility in the green-solvent MeTHF, the photovoltaic performance of PTzBI-Si:N2200-based OSCs processed from green-solvent MeTHF was improved to 10.24 %. The increased the molecular weight of PTzBI-Si could optimize the morphology of the active layer,[107] leading to a PCE of 11.5 % in PTzBI-Si:N2200-based devices.