Solvent Annealing Control of Bulk Heterojunction Organic Solar Cells with 6.6% Efficiency Based on a Benzodithiophene Donor Core and Dicyano Acceptor Units

A novel semiconductor organic molecule, denoted as VC89, having A-D-D1-D-A structure, was synthesized and all relevant physical and chemical features for its application in solar cells were investigated. The structure comprises 2-ethylhexoxy substituted BDT (donor D1 unit) as a core and a dicyano acceptor unit (DCV) as the terminal acceptor group (A) linked through cyclopentadithiophene (CDT) (donor D) moiety. The BHJ OSC VC89:PC₇₁BM (1:2), processed with chloroform (CF) as solvent, showed an overall power conversion efficiency (PCE) of 4.63% with short circuit current J_SC = 9.28 mA/cm², open circuit voltage V_OC = 0.96 V, and fill factor (FF) = 0.52. When the active layer was processed using DIO as a solvent additive (3% v/v in CF), the corresponding solar cell showed a PCE of 6.05% with J_SC = 10.96 mA/cm², V_OC = 0.92, and FF = 0.60. The PCE was further improved to 6.66% with J_SC = 11.68 mA/cm², V_OC = 0.92, and FF = 0.62, when the DIO/CF (3% v/v)-processed active layer was treated with THF vapors (solvent vapor annealing, SVA). The increase in PCE was due to the enhancement in both the J_SC and FF due to the use of the dicyano groups as electron acceptor units. On one hand, J_SC is determined by the enhancement of the film light absorbance, which is reflected in a better IPCE and better charge collection. On the other hand, we show herein that the use of solvent annealing after treatment with chemical additives also leads to better nanomorphologies that substantially improve the solar cell efficiency.