Drastic improvement in the power conversion efficiency (PCE) of polymer-based solar cells (PSCs) has been made in the last decade, mainly by the devleopment of new donor–acceptor (D–A) semiconducting polymers. One of the largest issues in improving PCEs in PSCs is to realize the high short-circuit current (Jsc) and high open-circuit voltage (Voc) at the same time. This is interpreted as that to create a semiconducting polymer with both a narrower band gap and deeper HOMO energy level. However, it is well-known that these two parameters are always trade-off. Whereas the narrow band gap can increase the light harvesting property and thus Jsc, it inevitably gives rise to a shallower HOMO energy level, resulting in a low Voc, or to a deeper LUMO energy level, which diminishes the energy offset of LUMOs between the polymer and fullerene, in turn preventing the efficient charge separation at the polymer/fullerene interface. In this regard, it is a daughting challenge for synthetic chemists to create new semiconducting polymers that can overcome this trade-off issue. Recently, we reported on a series of naphtho[1,2-c:5,6-c′]bis[1,2,5]thiadiazole (NTz)-based D–A copolymers, which possess narrow band gaps of 1.5–1.7 eV and showed PCEs over 8% in solar cells.[1-3] However, the Voc of their cells were limited to 0.7–0.8 V, due to the moderately deep HOMO energy levels of ~5.2 eV. Naphtho[1,2-c:5,6-c′]bis[1,2,5]oxadiazole (NOz), a new fused heterocycle as an oxygen analogue of NTz, can deepen both the HOMO and LUMO energy levels while maintaining the band gap as compared to NTz. This is likely as a result of stronger electron negativity of oxygen than sulfur. Thus, the incorporation of NOz into the semiconducting polymer is expected to realize both high Jsc and Voc when it is used as the p-type material for PSCs. Here, we synthesized a D–A copolymer consisting of oligothiophene and NOz. The NOz copolymer was found to have a narrow band gap of 1.52 eV and a deep HOMO energy level of 5.36 eV. The solar cells with an inverted structure using the NOz copolymer and PCBMs demonstrated very high Voc of ~1 V despite having such a narrow band gap, indicating that the photon energy loss is significantly reduced, and high PCEs of up to 8.9%. [1] I. Osaka, M. Shimawaki, H. Mori, I. Doi, E. Miyazaki, T. Koganezawa, K. Takimiya, J. Am. Chem. Soc. 2012, 134, 3498–3507. [2] I. Osaka, T. Abe, M. Shimawaki, T. Koganezawa, K. Takimiya, ACS Macro Lett., 2012, 1, 437–440. [3]
I. Osaka, T. Kakara, N. Takemura, T. Koganezawa, K. Takimiya, J. Am. Chem. Soc. 2013, 135, 8834–8837.