Exploring the Influence of TCO Thickness and Ag Addition on Performance of CIGS Thin Film Solar Cells for Bifacial and Tandem Configurations
Authors: Ali Amanat , Jeon Dong-Hwan , Hoang Van-Quy , Lee Jaebaek , Hwang Dae-Kue , Kang Jin-Kyu , Sung Shi-Joon , Kim Dae-Hwan 
Published: December 2024

Cu (In, Ga) Se2 (CIGS) thin-film solar cells have attracted a lot of interest in the field of renewable energy sources because of their beneficial features, which include low production costs, excellent efficiency, and suitability for tandem, semitransparent (or bifacial), and flexible cell applications. The advancement of transparent-conducting oxide (TCO) back contacts for CIGS solar cells shows potential for several applications but their low power conversion efficiency is still an issue. The spontaneous formation of GaOx at the interface between the TCO and CIGS layers has impeded the efficient extraction of photocarriers and promoted the generation of charge recombination events near this interface. To enhance their performance, this work focuses on improving the connection of CIGS onto In2O3:Sn (ITO) substrates for application in bifacial semi-transparent CIGS solar cells. Initially, layers of ITO (Ar/O2 flux ratio = 20:0) ranging in thickness from 80 to 320 nm were deposited on glass substrates. The results demonstrated that the optical bandgap Eg of the ITO layer varied as a function of its thickness, which was explained by the Burstein-Moss (B-M) shift phenomena. Compared to the 80 nm-thick ITO layer, the band gap of the 320 nm-thick ITO layer increased by 0.029 eV. The open-circuit voltage (Voc) of the corresponding ITO/CIGS solar cells experienced a rise of 0.023 V, attributed to the reduction in the Schottky barrier at the ITO/CIGS interface. The 200 nm ITO thickness was found to be the ideal thickness. It shows better performance than other ITO thicknesses, with lower series resistance and shunt conductance. Moreover, the study implemented an advanced supply of silver-promoted low-temperature (T453oC). This procedure resulted in enhanced quality of crystallinity within the absorber. Furthermore, our results indicate that the addition of Ag can enlarge the size of grains of CIGS. It also identifies Ag as an effective strategy for boosting the efficiency of CIGS solar cells. To compare the performance of Mo and TCO-based devices, CIGS solar cells were fabricated on both Mo and TCO back contacts. ITO back contacts were employed in CIGS devices, and their performance was comparable to that of traditional CIGS solar cells fabricated with a Mo back metal electrode. Without any post-deposition treatment or anti-reflective coatings, the resultant devices obtained a power conversion efficiency of 16.86% with Mo back contact and 15.97% with ITO back contact under simulated AM 1.5 illumination. These outcomes demonstrate the success of our strategy and have important origination for fabricating more efficient solar cells and a cleaner, more environmentally friendly energy future.
Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Ministry of Science and ICT (No. 2023R1A2C1007386) and by the DGIST R&D programs of the Ministry of Science and ICT (24-ET-01).