Effects of ITO based back contacts on Cu(In,Ga)Se2 thin films, solar cells, and mini-modules relevant for semi-transparent building integrated photovoltaics

• Published in: Solar energy materials and solar cells 2023-03, Vol.251, p.112169, Article 112169
• Main Authors: Hölscher, Torsten, Placidi, Marcel, Becerril-Romero, Ignacio, Fonoll-Rubio, Robert, Izquierdo-Roca, Victor, Thomere, Angélica, Bailo, Eduard, Schneider, Thomas, Kempa, Heiko, Scheer, Roland, Pérez-Rodríguez, Alejandro
• Format: Article
• Language: English
• ISSN: 0927-0248; 1879-3398
• DOI: 10.1016/j.solmat.2022.112169

This study presents the results of the development of semi-transparent Cu(In,Ga)Se2 (CIGSe) mini-modules for the application in building integrated photovoltaics (BIPV). Applying in-situ X-ray diffraction in real-time during CIGSe growth we find that the bulk of indium-tin-oxide (ITO), acting as the transparent back contact, is chemically stable in CIGSe processing. CIGSe layers grown on reactively sputtered ITO (Ar/O2 flux ratio = 60:1) or on ITO annealed in ambient air have a proportionally higher (220/204) orientation compared to CIGSe layers grown on as fabricated ITO sputtered solely by Ar. However, independent from the fabrication and annealing state of the ITO back contact, after CIGSe deposition at high substrate temperatures ≥600 °C accumulation of Ga at the CIGSe/ITO back contact interface combined with reduced solar cell efficiency is observed. This Ga accumulation visible in elemental depth profiles is attributed to the formation of gallium-oxide (GaOx). Applying a very thin (≈10–30 nm) functional molybdenum layer in between CIGSe and the ITO back contact inhibits the formation of GaOx. Based on this Mo/ITO back contact configuration semi-transparent 10 × 10 cm2 mini-modules with 14 cells interconnected in series have been fabricated. Module parameters resulted in a fill factor of 63% and >12% in efficiency. The solar active coverage of the modules amounts to ≈70%, and the average visible transmittance (in the range 380–780 nm) of the transparent sections was 27.6% (9.6% for the total area of the device). Optimisation of the Mo/ITO contact allows increasing this transparency to values > 50%. Long-term outdoor testing of a semi-transparent module prototype reveals no degradation in electric output power for 3 months, demonstrating the device stability under changing climatic conditions. •Investigation of Cu(In,Ga)Se2 growth on ITO back contacts with in-situ XRD.•Enhanced (220/204) texture of Cu(In,Ga)Se2 layers deposited on air-annealed and reactive Ar/O2 sputtered ITO back contacts.•Analysis method for identifying GaOx layers in GDOES depth profiles at the ITO/CIGSe back contact interface.•Development of 10x10 cm2 semi-transparent Cu(In,Ga)Se2 mini-module prototype with ITO based back contact.•Demonstration of long term stability of module prototype under outdoor conditions.