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Air-Stable Inverted Organic Solar Cells with Pentacene Anode Buffer Layer

A small-molecular organic solar cell with an inverted structure of indium–tin oxide (ITO)/bathocuproine (BCP)/fullerene (C60)/copper phthalocyanine (CuPc)/pentacene/Ag is reported. Although the Ag electrode usually acts as the cathode because of its low work function, the Ag electrode appropriately works as the anode by inserting a pentacene thin layer between Ag and the active layer. The unencapsulated device shows a power conversion efficiency of 0.28% under 100 mW/cm2 AM1.5G simulated illumination, and a lifetime of 700 h. The lifetime is 700 times longer than that of a conventional device with a structure of ITO/CuPc/C60/BCP/Ag. The long lifetime is attributed to the inverted structure in which the top electrode acts as the anode because this structure could effectively protect C60 from the diffusion of oxygen. By substituting naphthalene-1,4,5,8-tetracarboxylic dianhydride for BCP as a cathode buffer, the power conversion efficiency and the lifetime are improved to 0.40% and more than 1200 h, respectively.