Replacing 2,1,3-benzothiadiazole with 2,1,3-naphthothiadiazole in PCDTBT: towards a low bandgap polymer with deep HOMO energy level

With the rising interest in using the medium bandgap polymer, poly(2,7-carbazole-alt-4,7-dithienyl-2,1,3-benzothiadiazole) (PCDTBT) with deep HOMO energy level for polymer solar cells (PSCs), we have developed an analogous polymer with a lower bandgap, namely, poly(2,7-carbazole-alt-4,7-dithienyl-2,1,3-naphthothiadiazole) (PCDTNT) by replacing 2,1,3-benzothiadiazole (BT) with 2,1,3-naphthothiadiazole (NT) in PCDTBT. Its optical, electrochemical, and photovoltaic properties are fully characterized in comparison with PCDTBT. Clearly, the λ_max position of PCDTNT is significantly red-shifted by ∼30 nm, corresponding to a lower optical bandgap (1.71 eV) from the absorption edge of the thin film than that of PCDTBT (1.88 eV). A bulk-heterojunction (BHJ) PSC that incorporated PCDTNT with the low-lying HOMO energy level as a p-type material delivers a higher V_OC value of 0.81 V and a power conversion efficiency (PCE) value of 1.31%.