Property modulation of benzodithiophene-based polymers via the incorporation of a covalently bonded novel 2,1,3-benzothiadiazole-1,2,4-oxadiazole derivative in their main chain for polymer solar cells

Abstract

Two new electron accepting monomers (BBOB and BOB) containing two serially connected different electron deficient units, such as 2,1,3-benzothiadiazole and 1,2,4-oxadiazole, were prepared and copolymerized with electron-rich benzodithiophene (BDT) derivative to afford polymers P(BDT-BBOB) and P(BDT-BOB), respectively. The optical band gaps of P(BDT-BBOB) and P(BDT-BOB) are calculated to be 2.32 eV and 1.99 eV, respectively, and their highest occupied molecular energy levels are determined to be −5.31 eV and −5.27 eV, respectively. Each of the newly synthesized polymers, i.e.P(BDT-BBOB) and P(BDT-BOB), is used as an electron donor, along with PC₆₁BM as an electron acceptor, in the preparation of polymer solar cells (PSCs). The PSCs made with the configuration of ITO/PEDOT:PSS/P(BDT-BBOB) or P(BDT-BOB):PC₆₁BM (1 : 2 wt%)/LiF/Al gave a maximum power conversion efficiency (PCE) of 1.76% and 2.46%, respectively, and the device performance was further improved to 3.31% and 4.21%, respectively, by simply treating the photoactive layer of PSCs with isopropyl alcohol. Overall, the opto-electrical and photovoltaic properties of the two polymers are found to be quite dependent on the configuration of the covalently bonded 2,1,3-benzothiadiazole and 1,2,4-oxadiazole units incorporated in the polymer main chain.