The US National Renewable Energy Laboratory (NREL) and First Solar have used cracked film lithography (CFL) to build a bifacial cadmium telluride solar cell with a power density of 20.3 mW cm−2. They claim that the cell has a higher bifacial power density than any polycrystalline absorber currently produced at scale.
“Common passivation layers such as Al2O3 and other oxides, are highly resistive, must be applied directly to the CdTe surface, and require high temperature CdCl2 treatment after they are deposited,” they said, noting that they used CFL to solve this problem. “CFL is an inexpensive method for patterning metal microlattices with ideal properties for high-resistance applications such as CdTe.”
Researcher Chris Muzzillo said CFLs are particularly suitable for CdTe cells because of their low cost. “We found that compared to photolithography, CFL has lower material and equipment costs for patterning metal meshes in III-V photovoltaics,” he said. pv magazine.
Muzzillo and his team described their findings in the paper “Cracked Film Lithography with CuGaOx Buffers for Bifacial CdTe Photovoltaics”, published recently Nano-micro small. In all CFL processes, metal is grown directly on the surface of the semiconductor, resulting in high-quality connections with low contact resistance and improved durability.
In the proposed cell configuration CuGaOx/CFL grid samples showed superior passivation for reduced gold (Au) coverage, which in turn led to higher open-circuit voltage in the solar cell. They also helped improve the device’s fill factor from 70.8% to 73.3%.
“We estimate that the previous CdTe bifacial record of 1 solar front and back efficiency of 12.5% and 7.6% produces 16.3 mW cm−2 1 sun in front + 0.5 sun behind,” the research team said. “On the other hand, our best CFL grid produces 20.3 mW cm−2 1 sun in front + 0.5 sun behind – a 25% jump over the record bifacial CdTe power density, and with 1 sun in front + 0.1 sun behind, our cell’s power density is 39% higher.”
Muzzilo said commercialization of the cell is being considered.
“Compared to bifacial silicon, the back efficiency of bifacial CdTe is still relatively low,” he concluded. “We continue to work to improve front and back efficiency with the goal of demonstrating high enough additional power density to justify the process complexity of CFL-patterned bifacial CdTe.”