Conjugated Polymers Based on Difluorobenzoxadiazole toward Practical Application of Polymer Solar Cells

To advance polymer solar cells (PSCs) toward real-world applications, it is crucial to develop materials that are compatible with a low-cost large-scale manufacturing technology. In this context, a practically useful polymer should fulfill several critical requirements: the capability to provide high power conversion efficiencies (PCEs) via low-cost fabrication using environmentally friendly solvents under mild thermal conditions, resulting in an active layer that is thick enough to minimize defects in large-area films. Here, the development of new photovoltaic polymers is reported through rational molecular design to meet these requirements. Benzodithiophene (BDT)-difluorobenzoxa-diazole (ffBX)-2-decyltetradecyl (DT), a wide-bandgap polymer based on ffBX and BDT emerges as the first example that fulfills the qualifications. When blended with a low-cost acceptor (C60-fullerene derivative), BDT-ffBX-DT produces a PCE of 9.4% at active layer thickness over 250 nm. BDT-ffBX-DT devices can be fabricated from nonhalogenated solvents at low processing temperature. The success of BDT-ffBX-DT originates from its appropriate electronic structure and charge transport characteristics, in combination with a favorable face-on orientation of the polymer backbone in blends, and the ability to form proper phase separation morphology with a fibrillar bicontinuous interpenetrating network in bulk-heterojunction films. With these characteristics, BDT-ffBX-DT represents a meaningful step toward future everyday applications of polymer solar cells.