Synthesis and photovoltaic properties of heteroaromatic low-band gap oligomers for bulk heterojunction solar cells

Two new low band gap oligomers, TPyTDzQ-P1 and TPyTDzQ-P2, comprised of electron rich 1-(2,6-diisopropylphenyl)-2,5-di(2-thienyl)pyrrole or 1-(p-octylphenyl)-2,5-di(2-thienyl)pyrrole (TPyT) and electron deficient 6,7-di-2-methylpropane-4,9-bis-(thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline (DzQ) units were synthesized using the Suzuki polycondensation reaction. The oligomers were found to harvest the solar flux from the ultraviolet (300 nm) to near infrared (1200 nm) regions. The optical bad gaps of the two oligomers were calculated to be 1.08 eV and 1.10 eV, respectively, while the electrochemical band gaps were calculated to be 1.17 eV and 1.19 eV, respectively. The two oligomers were used as electron donor materials in bulk heterojunction (BHJ) solar cells. The solution processable bulk heterojunction solar cells with ITO/PEDOT:PSS/TPyTDzQ-P1 or TPyTDzQ-P2:PC70BM/TiOx/Al configuration showed a maximum power conversion efficiency (PCE) of 0.43% with a short-circuit current density (Jsc) of 3.41 mA/cm2, open-circuit voltage (Voc) of 0.39 V and fill factor (FF) of 32%.