Researchers from the Visvesvaraya National Institute of Technology in India have proposed a new copper indium gallium diselenide (CIGS) solar cell structure using tungsten disulfide (WS2) as the back surface field (BSF) layer.
BSF layers consist of a higher doped region at the rear surface of the solar cell and are commonly used to increase a device's voltage. WS2 – an extremely slick, dry film lubricant coating that functions in harsh conditions – can be used in solar cells as an electron transport layer or buffer layer, and nanosheets and nanoparticles prepared from electronic WS2 powders are commonly applied to nanoelectronics, optoelectronics, gas-sensing devices, hydrogen evolution reactions and energy storage devices.
“The novelty of this research work is that for the first time through simulation work, in the history of CIGS solar cells, a 25.70 % efficiency has been obtained at 200 nm CIGS absorber layer thickness and 50 nm back surface field layer thickness,” the research's corresponding author, Sushama M. Giripunje, told pv magazine.
He also explained that the thickness minimization of CIGS cells is aimed at reducing the consumption of rare earth materials such as indium and gallium, as well as their cost per watt. “This impact will be evident in the mass production scale of CIGS solar cells,” he added. “This kind of structure is not reported earlier to the best of the authors' knowledge.”
The scientists used the SCAPS-1D solar cell capacitance software, developed by the University of Ghent, to simulate the novel cell configuration. They assumed the cell relied on a back contact layer made of nickel (Ni), a PEDOT:PSS layer, a CIGS absorber, a buffer layer made of WS2, a window layer based on zinc oxide (ZnO), and a front electrode made with aluminum (Al).
“The BSF layer, located at the cell’s rear surface, can reduce the recombination of charge carriers, such as electrons and holes, which in turn decreases the loss of carriers,” the group said. “Another factor contribution is reflectance; here, part of the incident light may be reflected into the cell by the BSF layer in its capacity as a reflective layer. Improving the light’s path within the cell enhances absorption and increases current production.”
The scientists simulated the performance of the device under standard illumination conditions and found it achieved a power conversion efficiency of 25.7%, an open-circuit voltage of 0.81 V, a short-circuit current density of 39.33 mA/cm2, and a fill factor of 79.89%.
“At a defect density of 10 cm3 for the CIGS absorber layer, the device gives the best output, and from series and shunt resistance variation, it has been confirmed that a low value of the series resistance and higher shunt resistance is favorable for better performance,” they further explained.
The novel cell concept was presented in the paper “Designing and simulating of new highly efficient ultra-thin CIGS solar cell device structure: Plan to minimize cost per watt price,” which was recently published in the Journal of Physics and Chemistry of Solids.
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