Scanning Transmission Electron Microscopy Investigations of an Efficiency Enhanced Annealed Cu(In1-xGax)Se2 Solar Cells with Sputtered Zn(O,S) Buffer Layer

Cu(In1-xGax)Se2 (CIGS) is a direct band gap semiconductor widely used in photovoltaic (PV) energy conversion devices due to its high sunlight absorbance and high temperature stability. A conventional CIGS solar cell is presented as a stack of glass/Mo/CIGS/CdS/i-ZnO/ZnO:Al. The highest efficiencies are typically obtained with a CdS buffer layer deposited by chemical bath deposition (CBD). However, CdS CBD at industrial scale is material inefficient and environmentally unfriendly, as it generates large amounts of toxic/carcinogenic waste. Therefore, a transition to Cd-free buffer layers deposited by a dry vacuum process is mandatory for sustainable CIGS PV in-line production. ZnO1-xSx (ZnO0.75S0.25) via sputtering is an alternative to the CdS CBD in CIGS solar cells, providing a negative conduction band offset [1]. Recently, a significant efficiency enhancement was reported after an annealing treatment of the complete solar cell stack with the sputtered buffer ZnO0.75S0.25 [2]. Among the possible causes of this enhancement inter-diffusion occurring at the absorber/buffer layer interface could play a major role. In the following, we investigate the interface of a similar CIGS solar cell before and after 200°C annealing using advanced scanning transmission electron microscopy (STEM) techniques. Our first results, obtained by high resolution STEM (HR-STEM) and energy dispersive X-ray spectroscopy (EDX), demonstrate the absence of any inter-diffusion or intermixing layer at the absorber/buffer layer interface. Interestingly, we systematically observe the presence of stacking faults in CIGS in close proximity to the absorber/buffer layer interface, independently from the annealing process. HR-STEM imaging reveals an order occurring between ZnO0.75S0.25 crystals and the CIGS crystals, where an epitaxial relationship arises subsequent to the 200°C annealing. This change at the CIGS/buffer interface could result in a lower density of interface defects, which in turn would explain the efficiency enhancement observed in the annealed solar cell stack. [1] C. Platzer-Björkman et al, Journal of Applied Physics, 100 (2006) 044506-1 – 044506-9. [2] M. Zutter et al, Phys. Status Solidi RRL, 13 (2019) 1900145-1 - 1900145-8