Effect of donor groups on the photovoltaic performance of benzothiadiazole-core substituted small molecules for fullerene-based bulk heterojunction organic solar cells

Herein, two symmetric donor-π-acceptor-π-donor-type π-conjugated small molecule donors (BTM and BTP), based on a benzothiadiazole central acceptor core unit and end-capped with electron-donating triphenylamine and 3,5-dimethoxyphenyl)vinyl, are synthesized via a multistep coupling reaction. The structural, thermal, optical, and electrochemical properties of the small molecules are systematically analyzed; subsequently, the molecules are applied as electron donor materials for bulk heterojunction organic solar cell fabrication. Compared with BTM, BTP exhibits a strong absorption spectrum with a precisely tuned optical bandgap (approximately 1.87 eV), desired morphology, and improved interfacial properties. The solar devices fabricated with BTP:PC₇₁BM (1:2.5, w/w) exhibit a higher power conversion efficiency (PCE) of approximately 7.08% with an enhanced current density (J_SC) of approximately 12.90 mA/cm², high open-circuit voltage (V_OC) of approximately 0.989 V, and fill factor (FF) of approximately 55.5% compared with BTM-based devices (approximately 4.86%). Notably, the high V_OC achieved for the BTP:PC₇₁BM (1:2.5, w/w) devices is a result of the deep-lying HOMO energy level and low LUMO energy offset (approximately 0.51 eV) between the BTP donor and PC₇₁BM acceptor. The high hydrophobicity and stable surface morphology of BTP:PC₇₁BM (1:2.5, w/w) exhibits a high room temperature stability approximately > 94% and retain 6.70% of PCE of its original PCE (7.08%) after 288 hr.