A research team from the University of Santiago de Compostela and Ghent University has published a study on improving the efficiency of ultrathin CIGS (u-CIGS) solar cells. Using TCAD simulation tools, the study optimized rear-passivation parameters, including cell pitch, opening width, and interface trap density. Results showed that absorber thickness and doping concentration significantly influenced performance. However, traps at the u-CIGS/Al₂O₃ interface caused efficiency losses by increasing carrier recombination. Introducing a negative fixed charge density in the rear passivation layer reduced this recombination through field-effect passivation. The optimized device achieved 15% efficiency with a 1.5 µm cell pitch, 300 nm opening width, and a fixed bandgap. The findings improve understanding of rear-passivation effects on u-CIGS solar cell efficiency.