Highly stable bulk heterojunction organic solar cells based on asymmetric benzoselenadiazole-oriented organic chromophores

A new asymmetrically designed D-A-D organic chromophore 4-(7-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]selenadiazol-4-yl)-N,N-diphenylaniline (RSeT) with benzoselenadiazole acceptor unit has been synthesized using two different electron donor materials of triphenylamine (TPA) and n-hexyl bithiophene unit. Attachment of nonplanar electron-donating TPA unit subsequently heightened the optoelectronic properties and altered the solubility in organic solvents. RSeT of the optical bandgap of ~1.95 eV with the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels of −5.31 and −3.36 eV was applied as an electron donor material for stable bulk heterojunction organic solar cells. In this work, two different buffer layers of poly (3, 4-ethylenedioxythiophene):polystyrene sulfonate acid (PEDOT:PSS) and compact titanium oxide (c-TiO₂) were used and their influences on the device performance were investigated. Buffer layer of c-TiO₂ with RSeT:PC₆₁BM (1:3, w/w) active layer achieved a high open-circuit voltage (V_oc) of ~0.985 V, short-circuit current density (J_sc) of ~11.26 mA/cm² and improved fill factor of ~0.62 which resulted in a high power conversion efficiency (PCE) of ~6.88%. After 30 days, interestingly, RSeT:PC₆₁BM (1:3, w/w) devices with a c-TiO₂ buffer layer demonstrated good stability with the PCE value remaining at 85% of its initial value.