High Efficiency Bifacial Cu(In,Ga)Se₂ Solar Cells and Its Application to Transparent Modules

The bifacial Cu(In,Ga)Se₂ solar cell holds significant promise for future energy applications such as buildings, mobility, and public infrastructure due to its high efficiency, long lifetime, and versatility. By replacing the conventional opaque Mo back electrode with a transparent conducting oxide (TCO) film, it can enhance photovoltaic power generation from additional light incident on the rear side while also improving the processability of laser scribing for modularization, enabling a transparent module structure. However, the interface presents challenges in forming ohmic contacts and has a high recombination rate at the interface, which degrades the photovoltaic efficiency.
In this study, we utilized ITO thin films as the back electrode and investigated ohmic contact formation as well as interface passivation capabilities. Several methods were explored to create an ohmic ITO/CIGS interface, and their passivation abilities were compared, achieving over 21% cell efficiency for front light incidence while also obtaining a rear-incident photocurrent of at least 50% relative to the front. The introduction of an ultrathin Mo layer into the ITO/CIGS interface was effective in forming the ohmic contact but degraded the passivation at the interface. Optimizing the ITO layer itself proved to be more effective, not only for improving ohmic contact but also for enhancing interface passivation, compared to the Mo layer approach. For the ITO/CIGS samples with confirmed bifacial performance, we are currently conducting interface structure analysis, electrical resistance measurements, and interface defect analysis, aiming to compare these results with bifacial performance and further discuss the relationship between ITO thin film characteristics and the electrical properties of the interface (transport, passivation). Additionally, we will present research findings on interface structures and transparent electrode materials, beyond ITO and Mo, that can simultaneously improve contact resistance and passivation capabilities.
The P3 laser scribing process, used to isolate the front electrode in monolithically integrated modules, can negatively affect the CIGS/CdS interface due to laser-induced heating, reducing photovoltaic efficiency. The bifacial structure resolves this issue by allowing the laser to be directed from the substrate side, preventing direct heating at the interface. Using the bifacial CIGS solar cell and the laser scribing process, we successfully demonstrated a bifacial CIGS solar module with 18% efficiency (10x10 cm²), one of the top performances globally. Substrate-incident laser scribing also enables the creation of apertures for light transmission by selectively removing the film structure without compromising electrical integrity. While the laser scribing process caused minimal damage for standard module applications, further optimization was necessary to reduce this damage for transparent module applications. We thoroughly investigated the origin of the laser-induced damage and, as a result, achieved a 12% efficiency in a transparent CIGS module with a 30% transmission aperture, where the efficiency loss was kept below 2%.
Our approach to transforming CIGS solar cells into transparent modules encountered several challenges, including laser scribing-related damage, light transmission loss, and stability issues. We plan to present solutions to these challenges based on detailed analyses of each issue. As demonstrated in our research, the excellent efficiency and transparency of bifacial CIGS solar cells make them a promising technology for future applications, such as building windows.