Relationship between the energy levels and the photovoltaic properties of oligothiophenes

A series of linear π-conjugated oligothiophenes, α,α'- dihexylquinquethiophene (DH5T), 2,5-bis(5'-hexyl-2,2'-bithiophene-5-yl)thieno[3, 2-b]thiophene (DH5TT), and α, α'-dihexylheptathiophene (DH7T), has been synthesized using the Suzuki coupling reaction. The optical and electrochemical properties of oligothiophenes were easily tuned by controlling the thiophene number. The UV-vis absorption and photoluminescence (PL) spectra are gradually red-shifted on going from DH5T and DH5TT to DH7T due to the increase in α-conjugation length. The energy band gap decreased as the oligothiophene length increased. The optical band gaps of DH5T, DH5TT, and DH7T occur at 2. 39, 2.25, and 2.01 eV, respectively. Bulk heterojunction organic photovoltaic cells (OPVs) fabricated from oligomers showed the power conversion efficiency of 0.45-0.8% under AM 1.5 (100 mW/cm²). Among them, DH5T showed the best OPV performance of an open circuit voltage (VOC) of 0.51 V, short-circuit current (J_SC) of 4.25 mA/cm², and fill factor (FF) of 0.37, resulting in the power conversion efficiency of 0.80%. Moreover, the relationship between conjugation length and photovoltaic properties was systematically investigated in terms of the energy band gap and open circuit voltage (V_OC).