Efficiency enhancement and equivalent circuit analysis of polymer solar cells with a CF₄-plasma-treated ITO anode surface

The effects of CF₄ plasma treatment on indium-tin-oxide (ITO) surfaces in polymer solar cells are reported. The polymer solar cell consisted of indium-tin-oxide (ITO), poly (styrene sulfonate)-doped poly(3,4-ethylene dioxythiophene) (PEDOT: PSS, 26 nm), a mixture of poly(2-methoxy-5-(2′- ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV) and [6,6]-phenyl-C ₆₁-butyric acid methyl ester (PCBM) (70 nm), LiF (1 nm) and Al (80 nm), which served, respectively, as an anode, a holetransport layer, a photo-active layer, a cathode interfacial layer and a cathode. The device with CF₄ plasma treatment showed an improved power conversion efficiency under AM 1.5 solar illumination (100 mW/cm²). The admittance analysis showed that treating the ITO surface with CF₄ plasma could improve the performance of the device because the contact resistance was decreased. This suggested that CF₄ treatment of ITO can enhance hole transport across the polymer/ITO interface due to the increased work function of the F-containing ITO surface.