A symmetric benzoselenadiazole based D–A–D small molecule for solution processed bulk-heterojunction organic solar cells

Benzoselenadiazole as the central acceptor unit with terminal donor unit of n-hexylbithiophene was chosen to design the symmetric donor-acceptor-donor (D–A–D) configuration based small organic chromophore, 4,7-bis(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]selenadiazole, RTh-BSe-ThR, for the fabrication of solution-processed bulk-heterojunction organic solar cells (BHJ-OSCs). RTh-BSe-ThR chromophore showed assuring solubility in common organic solvents and exhibited wider absorption in the visible region with an optical band gap (E_g ^opt) of ∼1.87 eV. Benzoselenadiazole unit with two n-hexylbithiophene units exhibited an excellent electrochemical behavior with the highest occupied molecular orbital (HOMO) of −5.38 eV and the lowest unoccupied molecular orbital (LUMO) of −3.51 eV. The fabricated BHJ-OSCs with RTh-BSe-ThR:PC₆₁BM (1:3, w/w) blend thin film displayed a power conversion efficiency (PCE) of ∼3.46% and a high value of short circuit current density (J_SC) of ∼11.20 mA/cm². The enhancement in the photovoltaic parameters might be attributed to the significant improvements in the interfacial area of bulk heterojunction which might enhance the light harvesting property and the shunt resistance of the blend thin films.