Transparent Electrode Technologies for Organic Solar Cells and Perovskite Solar Cells
Authors: Kim Han-Ki 
Published: May 2020

The importance of transparent electrode technology for solar cells and perovskite solar cells will be introduced. First, we introduce the roll-to-roll sputtering technique and multilayer electrode for flexible solar cells. We fabricated flexible inverted solar cell (IOSC) modules (10 cm× 10 cm) on roll-to-roll (RTR) sputtered ITO/Ag/ITO multilayer cathodes. By using a pilot-scale RTR sputtering system equipped with mid-range frequency power for dual ITO targets and direct current power for the Ag target, we were able to continuously deposit a high-quality ITO/Ag/ITO multilayer on PET substrate with a width of 700 mm and length of 20,000 mm as a function of Ag thickness. To optimize the electrical and optical properties of the ITO/Ag/ITO multilayer, the thickness of Ag layer was varied by controlling of the DC power applied on Ag targets during the RTR sputtering process. At the Ag thickness of 12 nm, the ITO/Ag/ITO multilayer had a very low sheet resistance of 3.03 Ohm/square and high transmittance of 88.17%, which are better values than those of amorphous ITO film. Various bending test results showed that the high failure strain of the Ag inter layer led to good flexibility of the multilayer films. A strip-type ITO/Ag/ITO cathode was successfully patterned using a RTR wet etching process. Successful operation of flexible IOSC modules on RTR sputtered ITO/Ag/ITO cathodes indicate that the RTR sputtering technique is a promising coating process for fabrication of high-quality transparent and flexible cathodes and can advance the commercialization of cost-efficient flexible IOSCs. Second, we fabricated high-performance flexible CH3NH3PbI3 (MAPbI3) perovskite solar cells with a power conversion efficiency of 16.8% on transparent and flexible ITO electrode prepared by specially designed ion plating system. The ion plated ITO films on PET substrate showed a lower sheet resistance and higher optical transmittance than conventional sputtered ITO films because the high-energy input into a growing ITO film led to densification of the growing film during ion plating process. At optimal coating condition, the ion-plated ITO film with a thickness of 100 nm showed a sheet resistance of 15.75 Ohm/square and an optical transmittance of 84%, which are better than those of sputtered ITO film. Outer and inner bending tests demonstrated that the mechanical flexibility of the ion plated ITO film was superior to that of the conventional sputtered ITO film due to better adhesion between ion plated ITO and PET substrate. Flexible perovskite solar cells with the structure of Au/PTAA/MAPbI3/ZnO/ITO/PET showed a higher power conversion efficiency of 16.8% than sputtered ITO-based flexible perovskite solar cells (15.4%) due to lower sheet resistance and higher optical transmittance. Finally, we introduce a linear facing target sputtering technique to realize semi-transparent perovskite solar cells. By effective confinement of high density plasma between ITO targets, we can deposit plasma damage free IZTO electrode on layer. This indicates that linear facing target sputtering is a promising plasma damage free sputtering technique to deposit transparent electrode on perovskite active layer for high efficient semi-transparent perovskite solar cells.