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.