Influence of thiophene and furan π–bridge on the properties of poly(benzodithiophene-alt-bis(π–bridge)pyrrolopyrrole-1,3-dione) for organic solar cell applications

A new polymer, P(BDT-fPPD), which incorporates 4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-bʹ]dithiophene (BDT) and 4,6-bis(furan-2-yl)-2,5-dioctylpyrrolo [3,4-c]pyrrole-1,3(2H,5H)-dione (fPPD, furan π–bridged PPD) units, was prepared. The optoelectrical, crystalline, charge transport, backbone curvature, and photovoltaic properties of P(BDT-fPPD) were thoroughly studied. P(BDT-fPPD) was also compared to the polymer P(BDT-tPPD), comprising BDT and 4,6-bis(thiophen-2-yl)-2,5-dioctylpyrrolo [3,4-c]pyrrole-1,3(2H,5H)-dione (tPPD, thiophene π–bridged PPD) units. This study demonstrates that the π–bridges, such as thiophene and furan, attached between the BDT and pyrrolo [3,4-c]pyrrole-1,3(2H,5H)-dione (PPD) units greatly altered the properties of the resulting polymers. In particular, the backbone curvature of P(BDT-fPPD) was significantly different from that of P(BDT-tPPD), which resulted in P(BDT-fPPD) having a higher bandgap energy (Eg), deeper HOMO level, and higher crystallinity, but lower carrier mobility (μh) and relatively poor power conversion efficiency (PCE) compared to P(BDT-tPPD). For P(BDT-fPPD), the Eg, HOMO, μh, and PCE were determined as 2.20 eV, −5.44 eV, 1.19 × 10−5 cm−2 V−1 s−1, and 2.62%, respectively. The corresponding values for P(BDT-tPPD) were 2.11 eV, −5.39 eV, 2.95 × 10−4 cm−2 V−1 s−1 and 5.29%, respectively. Interestingly, the inclusion of a small amount of P(BDT-tPPD) in the PTB7-Th:PC70BM blend enhanced the PCE of the resulting ternary organic solar cells (OSCs), whereas the insertion of P(BDT-fPPD) lowered the PCE of the ternary OSCs. The lower mobility and PCE obtained for P(BDT-fPPD) are mainly attributed to its poor blending with PC70BM. [Display omitted] •New polymer namely P(BDT-fPPD) was prepared.•The properties of P(BDT-fPPD) and P(BDT-tPPD) were compared.•P(BDT-fPPD) and P(BDT-tPPD) showed s-type and linear curvature backbones.•P(BDT-tPPD) provided higher PCE in binary and ternary OSCs than P(BDT-fPPD).•The higher crystallinity of P(BDT-fPPD) provoked aggregation and offer low PCE.