Facile synthesis of 1-(2,6-diisopropylphenyl)-2,5-di(2-thienyl)pyrrole- based narrow band gap small molecules for solar cell applications

In this study, 1-(2,6-diisopropylphenyl)-2,5-di(2-thienyl)pyrrole-based two new low band gap organic small molecules, TPT-SM1 and TPT-SM2, were prepared and their optical, electrical and photovoltaic properties were investigated. TPT-SM1 was synthesized by simply linking the electron rich 1-(2,6- diisopropylphenyl)-2,5-di(2-thienyl)pyrrole and electron deficient tricyanovinyl (-C(CN)C(CN)₂) units. On the other hand, TPT-SM2 was prepared by attaching the electron donating non planar triphenyl amine and electron accepting tricyanovinyl groups respectively on both side of 1-(2,6- diisopropylphenyl)-2,5-di(2-thienyl)pyrrole unit. Interestingly, the absorption band of TPT-SM1 and TPT-SM2 was found to be located exactly at the maximum solar flux region of the solar spectrum and their strong absorption band appears at the region of 470-800 nm and 490-900 nm, respectively, in film state. The optical band gap of TPT-SM1 and TPT-SM2 was calculated to be 1.58 eV and 1.42 eV, respectively, and the electrochemical studies revealed that the HOMO energy level was located at around -5.15 eV. The solution processed small molecule organic solar cells (SMOSCs) prepared by using TPT-SM1 and TPT-SM2 as an electron donor and PC₇₁BM as an electron acceptor at 1:2 wt% donor:acceptor blend ratio showed maximum power conversion efficiency (PCE) of 1.19% and 1.68%, respectively.