Abstract
A series of copolymers, based on benzo[1,2-b:4,5-b′]dithiophene (BDT) as the electron donor and 2,1,3-benzothiadiazole (BT)/diketopyrrolo[3,4-c]pyrrole (DPP) as the electron acceptors, were synthesized for highly efficient polymer solar cells. By changing the BT/DPP ratio in the conjugated backbone, the absorption, energy levels, molecular aggregation and carrier mobility could be finely tuned. With increased DPP content, the absorption range was extended to the longer wavelength region with narrower bandgaps. The highest occupied molecular orbital (HOMO) levels were also raised up and the molecular aggregation was enhanced. The balance of these factors would afford a remarkable device performance enhancement. Polymer P3 with BT:DPP = 0.7:0.3 (molar ratio) exhibited the highest power conversion efficiency (PCE) of 9.01%, with open circuit voltage (
V
oc
= 0.73 V, short current density (
J
sc
= 18.45 mA·cm
−2
, and fill factor (FF) = 66.9%. The PCE value was improved by 48.7% compared to P1 and by 117.6% compared to P7, respectively, indicating a great potential in photovoltaic application.