Improving the efficiency of polymer solar cells via a treatment of methanol : water on the active layers

Abstract

Significant improvement in the power conversion efficiency (PCE) of polymer solar cells (PSCs) has been observed when the active layer was treated with a mixture of methanol and water (M : W). For the PSCs based on ITO/PEDOT:PSS/PCDTBT:PC₇₁BM/Al, the PCE was 6.58% when the surface of the PCDTBT:PC₇₁BM film was treated with pure methanol. However M : W (6 : 1) treatment on the PCDTBT:PC₇₁BM film could further improve the PCE to 7.44% which is even higher than the PCE values of the PSCs with LiF (6.00%) or PFN (6.88%) as a cathode interlayer. Similarly in the PSCs based on ITO/PEDOT:PSS/PTB7:PC₇₁BM/LiF/Al, M : W (6 : 1) treatment on the surface of the PTB7:PC₇₁BM film can increase the PCE to 8.47% while the PCE values of the PSCs with methanol treatment and without any treatment on the PTB7:PC₇₁BM film are 8.14% and 7.41%, respectively. Combined contact angle measurements and X-ray photoemission spectroscopy depth profiling demonstrated that the M : W treatments resulted in a more favorable redistribution of the polymer and PC₇₁BM in the active layer because evaporation of the solvents drove PC₇₁BM to migrate from the interior of the blend film to the top (air) surface.