Plasmonic Back Reflectors: A Small Molecule Non‐fullerene Electron Acceptor for Organic Solar Cells

Organic bulk heterojunction photovoltaic devices predominantly use the fullerene derivatives C60]PCBM and C70]PCBM as the electron accepting component. This report presents a new organic electron accepting small molecule 2‐{7‐(9,9‐di‐n‐propyl‐9H‐fluoren‐2‐yl)benzoc]1,2,5]thiadiazol‐4‐yl}methylene]malononitrile (K12) for organic solar cell applications. It can be processed by evaporation under vacuum or by solution processing to give amorphous thin films and can be annealed at a modest temperature to give films with much greater order and enhanced charge transport properties. The molecule can efficiently quench the photoluminescence of the donor polymer poly(3‐n‐hexylthiophene‐2,5‐diyl) (P3HT) and time resolved microwave conductivity measurements show that mobile charges are generated indicating that a truly charge separated state is formed. The power conversion efficiencies of the photovoltaic devices are found to depend strongly on the acceptor packing. Optimized K12:P3HT bulk heterojunction devices have efficiencies of 0.73±0.01% under AM1.5G simulated sunlight. The efficiencies of the devices are limited by the level of crystallinity and nanoscale morphology that was achievable in the blend with P3HT.