Synthesis of modified benzothiadiazole-thiophene-cored acceptor and carbazole/indolocarbazole alternating conjugated polymers and their photovoltaic applications

Abstract

Two low band gap (LBG) copolymers PCz-DTBTT and PICz-DTBTT, featuring a modified 2,1,3-benzothiadiazole–thiophene-cored unit 2,5-di(4-(4-octylthiophen-2-yl)-2,1,3-benzothiadiazol-7-yl)thiophene (DTBTT) as acceptor and electron-rich building blocks carbazole (Cz) or indolo[3,2-b]carbazole (ICz) as donor, were synthesized via Suzuki crossing-coupling method. Both copolymers show good solubility in common organic solvents and thermal stabilities. The optical study exhibits a broad absorption in the range of 300–700 nm with the optical band gap (\(E_{g}^{\text{opt}}\)) of about 1.78 eV, while the electrochemical cyclic voltammetry (CV) measurement shows the relatively deeper highest occupied molecular orbital (HOMO) energy level (about −5.45 eV). Photovoltaic devices based on the blends of PCz-DTBTT/PC71BM (w:w; 1:2) and PICz-DTBTT/PC71BM (w:w; 1:2) with devices configuration as ITO/PEDOT:PSS/polymer:PC71BM/Ca/Al, show the power conversion efficiency (PCE) up to 2.07 and 2.21 %, with the open circuit voltage (Voc) of 0.75 and 0.80 V, short circuit current density (Jsc) of 6.47 and 6.40 mA cm−2, fill factor (FF) of 42.6 and 43.2 % under an AM1.5 simulator (100 mW cm−2), respectively. It is apparent that DTBTT-based devices exhibit the broad response range, covering from 300 to 720 nm.