Developing Low-Cost Wide Bandgap Polymer Donors Based on 2-Cyanopyrazine for Efficient Organic Solar Cells with Negative HOMO Offsets

A minimal energy offset that can still achieve fast and efficient charge separation between the donor and acceptor is crucial to attaining excellent power conversion efficiency in organic solar cells. Here, two wide bandgap polymers (PBDTCP-F and PBDTCP-Cl) are prepared based on 2-cyanopyrazine as an acceptor unit, in combination with a fluorinated or chlorinated BDT donor unit. The designed bithiophene cyano-substituted pyrazine monomer can be easily synthesized in only three steps. These two polymers displayed deeper HOMO energy levels and a more coplanar backbone. Interestingly, when blended with IT-4F acceptor, negative HOMO offsets are observed from −0.02 eV of PBDTCP-F to −0.06 eV of PBDTCP-Cl, resulting in Voc above 0.95 V for both devices. Compared to PBDTCP-Cl, OSCs based on PBDTCP-F with a significantly smaller negative HOMO offset and higher CCL value exhibit a decent PCE of 9.56% due to its suitable driving force and phase separation. More importantly, the device can still achieve a PCE of nearly 10% even with a negative HOMO offset. This study presents an economically viable electron-deficient moiety aimed at designing high-performance polymer donors and emphasizes the importance of coordination energy offsets.