Low resistance and highly transparent ITO–Ag–ITO multilayer electrode using surface plasmon resonance of Ag layer for bulk-heterojunction organic solar cells

We comprehensively investigated the electrical, optical, structural, mechanical, interfacial, and surface properties of ITO–Ag–ITO (IAI) multilayer electrodes grown on glass substrates by linear facing target sputtering (LFTS) for bulk-heterojunction organic solar cells (OSCs). Although the single ITO electrode with a thickness of 150 nm showed a fairly high sheet resistance of 34 Ω/square, the IAI multilayer electrode exhibited a very low sheet resistance of 4.4 Ω/square due to the low resistivity of the inserted Ag layer. Without using a substrate heating or post-annealing process, we were able to obtain an IAI multilayer electrode with a low sheet resistance, comparable to that of a crystalline ITO electrode, using the room-temperature LFTS process. In addition, the surface plasmon resonance (SPR) and antireflection of the optimized Ag layer significantly increased the optical transmittance of the IAI multilayer. It was found that the optimization of the thickness of the Ag layer is very important to obtain transparent IAI multilayer electrodes, because the SPR effect is critically affected by the Ag morphology. Moreover, the OSC fabricated on the optimized IAI electrode with an Ag thickness of 16 nm showed a higher power conversion efficiency (3.25%) compared to that prepared on the amorphous ITO electrode (2.35%), due to its low sheet resistance and high optical transmittance at 400–600 nm, which corresponds to the absorption wavelength of the organic active layer. This indicates that IAI multilayer electrodes grown by LFTS are promising transparent conducting electrodes for OSCs or flexible OSCs due to their very low resistivity and high optical transmittance.