Comparative investigation of transparent ITO/Ag/ITO and ITO/Cu/ITO electrodes grown by dual-target DC sputtering for organic photovoltaics

We investigated the characteristics of transparent indium tin oxide (ITO)/Ag/ITO (IAI) and ITO/Cu/ITO (ICI) multilayer electrodes grown by continuous dual-target dc sputtering for bulk heterojunction organic solar cells (OSCs). The IAI and ICI multilayer electrodes show a significant reduction in their sheet resistance and resistivity with increasing thickness of the Ag and Cu layers, respectively, despite the very small thickness of ITO (80 nm). However, the IAI electrode exhibits a much higher optical transmittance in the visible wavelength region under optimized conditions due to the more effective surface plasmon resonance of the Ag layer than that of the Cu layer. The Auger electron spectroscopy depth profile results for the IAI and ICI electrodes show that there is no severe interfacial reaction between the metal (Ag or Cu) layers and the ITO layers due to the high formation enthalpy of the Ag-O and Cu-O phases at room temperature. Moreover, the OSC fabricated on the IAI electrode shows a higher power conversion efficiency (3.26%) than the OSC prepared on the ICI electrode (2.78%) due to its high optical transmittance in the region of 400-600 nm corresponding to the absorption wavelength range of the organic active layer. This indicates that the IAI multilayer electrode is a promising transparent conducting electrode for OSCs or flexible OSCs due to its very low resistivity and high optical transmittance in the 400-600 nm range.