Synthesis of two benzo[1,2-b:3,4-b′]dithiophene-based conjugated polymers with different side chains and their applications in photovoltaic devices

Two D–A conjugated polymers (named PBDTC6DBT and PBDTC10DBT) with benzo[1,2-b:3,4-b′]dithiophene (BDT) as donor and 4,7-dithiophen-2,1,3-benzothiadiazole (DBT) as acceptor, were designed and synthesized. PBDTC6DBT and PBDTC10DBT have hexyl and decyl as side chains in the DBT units, respectively. The polymers were characterized by spectroscopic (NMR and UV–vis) methods, GPC, DSC and cyclic voltammetry. In order to study the photovoltaic properties of the two conjugated polymers, photovoltaic devices with the configuration of ITO/poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT:PSS)/polymer:[6,6]-phenyl C61-butyric acid methyl ester (PC61BM)/LiF/Al, were fabricated, in which PBDTC6DBT or PBDTC10DBT acted as electron donor in active layers. According to the experimental data, when the blend ratio of PBDTC6DBT and PC61BM reached 1:2, the best photovoltaic properties among the PBDTC6DBT-based devices were obtained, and the open-circuit voltage, the short-circuit current density and the power conversion efficiency were measured to be 0.63 V, 6.18 mA/cm2 and 1.86%, respectively. As to PBDTC10DBT-based devices, the maximum of power conversion efficiency reached 0.99% with the blend ratio of PBDTC10DBT and PC61BM being 1:2. PBDTC10DBT and PBDTC6DBT were both promising donor candidates in the application of polymer solar cells.