The significance of bilayer window (CdS:O/CdS) on the performance of CdTe thin film solar cells

• Published in: Optical materials 2024-09, Vol.155, p.115816, Article 115816
• Main Authors: Das, N.K., Farhad, S.F.U., Gupta, A.K. Sen, Ahamed, E.M.K. Ikball, Matin, M.A., Amin, N.
• Format: Article
• Language: English
• Subjects: CdS:O/CdS bilayer; CdSxTe1-x; CdTe solar cell; CSS; SCAPS-1D; Window layer
• ISSN: 0925-3467
• DOI: 10.1016/j.optmat.2024.115816

Ultrathin and compact CdS:O/CdS bilayer window in CdTe solar cells has higher potential for enhanced p-type doping, reduced Te diffusion, and improved power conversion efficiency (PCE) compared to traditional single-layer CdS or CdS:O windows. In this effort, we used a two-gun RF sputtering equipment to deposit compact CdS:O (∼50 nm)/CdS(∼50 nm) bilayers onto the borosilicate and ITO-coated glass substrates at low temperature (≤250 °C). Intriguingly CdS:O/CdS bilayers retain its hexagonal {0002} preferential orientation but with around 10.58 % optical bandgap (Eg) increment compared to CdS (Eg ≈ 2.32 eV) of equal thickness (∼100 nm). Close-spaced sublimation (CSS) technique was used to deposit the CdTe thin film onto the CdS and CdS:O/CdS layers at 650 °C. The effect of CdS and CdS:O/CdS layers on the microstructural and morphological features of CdCl2-treated CdTe films were critically analyzed in revealing a promising {0001}/{111}-like partial-epitaxy presumably due to the preferential cubic CdTe growth along the orientation. The CdTe films grown over a CdS:O/CdS layer exhibited smaller average crystallite and grain sizes than those grown over a single CdS layer. An essential finding was the formation of an optimal intermix layer of CdSxTe1-x, particularly evident in the CdTe films grown on the CdS:O/CdS bilayer. To comprehensively explore the effect of CdS:O/CdS window layers on PCE, complete CdTe solar cells were fabricated using three different window layers: CdS, CdS:O, and CdS:O/CdS, which are all considered as rudimentary as no layer optimization was performed. Notably, CdTe solar cell that incorporates the CdS:O/CdS bilayer window exhibited the most promising performance among all the tested rudimentary cells and corroborated the numerically simulated findings conducted by the SCAPS-1D simulator. •A two gun-RF sputtered CdS:O/CdS bilayer coating was integrated atop the CdTe coating pre-deposited by closed space sublimation (CSS).•The hexagonal CdS:O/CdS bilayer induced an {0001}/{111}-like partial epitaxy with the cubic CdTe.•The role of CdS:O/CdS coating in the formation of CdSxTe1-x intermix layer.•The new bilayer showed favorable PCE in comparison with individual CdS and CdS:O layers.•The study used SCAPS-1D simulations to corroborate the experimental findings.