Monolayer passivation of the transparent electrode in organic solar cells

The surface of transparent indium-tin oxide (ITO) electrode is electronically modified by a chemically bonded monolayer of organic molecules or a single layer of adsorbed acid ions. The electrode surface was examined with Kelvin probe force microscopy in vacuum. Treatments with phosphoric acid and periodic acid both lead to an increase in work function of about 0.7 eV with a good homogeneity. For both chemical monolayer and acid treatments the substrates show a uniform work function different from untreated ITO as expected for an uninterrupted film. We show that these treatments lead to an improvement of the phthalocyanine/C60 solar cell performance compared to cells on untreated ITO. Monochromatic external quantum efficiencies of up to 36% and power conversion efficiencies of up to 1.7% were obtained.