Photovoltaic conversion efficiency in copper-phthalocyanine/ perylenetetracarboxylic acid benzimidazole heterojunction solar cells

Summary form only given. Recently relatively high photovoltaic energy conversion efficiencies of about 1% in heterojunction-type organic solar cells have been reported. No report, however, has appeared that evidenced the possibility of increasing the efficiency exceeding 2%. The origins which prevent further increase of the efficiency in organic heterojunction solar cells have been ascribed to low photon collection efficiency, low quantum efficiency of charge carder generation, high resistivity of organic semiconductors, and so on. Very little quantitative discussion examining the possible origins of depressing the efficiency has been given, except for some earlier analyses on metal-organic semiconductor Schottky-type solar cells. In this report, we will show one of the possible ways to analyze the conversion efficiency of heterojunction solar cells quantitatively using our experimental data on copper phthalocyanine/perylenetetracarboxylic acid benzimidazole haterojunction cells. Our conclusion is that low photon collection efficiency and low open-circuit voltage are the major origins of low conversion efficiency in organic haterojunction-type solar cells. When proper combinations of heterojunciton dyes are carefully selected and thicknesses of organic layers are thin enough, neither quantum efficiency of charge separation nor resistivity of dye layers causes lowering of conversion efficiency.