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-b']dithiophene (BDT) and 4,6-bis(furan-2-yl)-2,5-dioctylpyrrolo [3,4-c]pyrrole-1,3(2H,5H)-dione (fPPD, furan pi-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 pi-bridged PPD) units. This study demonstrates that the pi-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 (E-g), deeper HOMO level, and higher crystallinity, but lower carrier mobility (mu(h)) and relatively poor power conversion efficiency (PCE) compared to P(BDT-tPPD). For P(BDT-fPPD), the E-g, HOMO, mu(h), and PCE were determined as 2.20 eV, -5.44 eV, 1.19 x 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 x 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.

Automated summary

The new polymer P(BDT-fPPD) showed distinct properties compared to P(BDT-tPPD), with higher bandgap energy, deeper HOMO level, and higher crystallinity but lower carrier mobility and power conversion efficiency. The differences were attributed to the differences in backbone curvature between the two polymers, impacting their overall performance in organic solar cells.