Design and chemical engineering of carbazole-based donor small molecules for organic solar cell applications

A serious of carbazole-based acceptor-donor-acceptor (A-D-A) type small molecules have been designed, synthesized and characterized for efficient bulk heterojunction organic solar cell (BHJ OSCs) applications. The synthesized molecules have an electron-rich carbazole unit acting as the electron donor (D) and an electron deficient phenylcyanovinylene unit acting as an electron acceptor to assembled A-D-A type conjugated small molecules (FHCS, NHCS, FBCS, and NBCS). The effect of chemical engineering in designed small molecules through fluorination, nitration and introducing alkyl chain has been studied in view of their optical, physio-chemical and photovoltaic properties. The NHCS and NBCS show broader absorption as compared to the FHCS and FBHS due to the strong electron withdrawing ability of the nitro groups. Interestingly, it was found that the alkyl chain did not affect the thermal and photophysical properties of these small molecules, but it has good impact on the energy level and photovoltaic properties. The FHCS and NHCS molecules showed a deep-lying HOMO energy level of − 5.21 eV, as compared to the FBCS and NBCS. NHCS and NBCS molecules showed low band gaps of 1.81 and 1.77 eV, respectively. The organic photovoltaic device fabricated with the configuration of ITO/PEDOT:PSS/donor:PC61BM/LiF/Al, exhibited the best device performance for the NBCS molecules with: the short circuit current (JSC) of 4.35 mA cm, open circuit voltage (VOC) of 0.75 V, fill factor (FF) of 30.07, and power conversion efficiency of ~ 1%.