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Bifacial thin film solar cells based on copper indium gallium diselenide or CIGS can collect solar energy from both their front and their rear side – and thus potentially yield more solar electricity than their conventional counterparts. So far, however, their fabrication has led to only modest energy conversion efficiencies. An Empa team has now developed a new low-temperature production process resulting in record efficiencies of 19.8% for front and 10.9% for rear illumination.
Moreover, they also produced the first bifacial perovskite-CIGS tandem solar cell, as recently published in Nature Energy (“Efficiency boost of bifacial Cu(In,Ga)Se2 thin-film solar cells for flexible and tandem applications with silver-assisted low-temperature process”), opening up the possibility of even higher energy yields in future.
The idea is as straightforward as it is simple: If I can collect both direct sunlight as well as its reflection via the rear end of my solar cell, this should increase the yield of energy the cell produces. Potential applications are, for instance, building-integrated photovoltaics, agrivoltaics – the simultaneous use of areas of land for both photovoltaic power generation and agriculture – and vertically or high-tilt installed solar modules on high-altitude grounds. Enter the bifacial solar cell. According to the International Technology Roadmap of Photovoltaics, bifacial solar cells could capture a market share of 70% of the overall photovoltaics market by 2030.
Although bifacial solar cells based on silicon wafers are already on the market, thin film solar cells have so far lagged behind. This is, at least in part, due to the rather low efficiency of bifacial CIGS thin film solar cells caused by a critical bottleneck problem: For any bifacial solar cell to be able to collect reflected sunlight at the rear side, an optically transparent electrical contact is a prerequisite. This is achieved by using a transparent conductive oxide (TCO) that replaces the opaque back contact in conventional – i.e. mono-facial – solar cells made of molybdenum.
The new method of making mixed halide-perovskites results in solar cells with improved stability and performance. The new method results in better control over perovskite crystallization rates. This means the crystal structure is more ordered, in part due to researchers understanding and taking advantage of the faster crystallization of bromide relative to iodide.
The result is a material with fewer defects and less halide migration and thus less segregation of the bromide and iodide. This in turn means uniform mixing of bromide and iodide across the material, which allows the material to absorb light evenly. The end result is that solar cells made using the new method will perform better under real-world conditions.
Typical halide perovskite solution deposition uses an anti-solvent drip procedure to initiate crystallization of the halide film. The standard anti-solvent method for producing bromide-iodide mixed halide perovskite films often leads to excessive defect formation (e.g., bromide vacancies) owing to the rapid crystallization of bromide vs. iodide-perovskite phases. Simulations show that halide migration is enhanced in the presence of a large population of halide vacancies. This limits the stability of bromide-iodide mixed halide perovskites under light and heat.
In comparison to the anti-solvent approach, the gentler gas-quench method better controls crystallization, first producing a bromide-rich surface layer that then induces top-down columnar growth to form a gradient structure with less bromide in the bulk than in the surface region. The anti-solvent method does not produce such a gradient structure.
In this study, researchers from the National Renewable Energy Laboratory, the University of Toledo, and the University of Colorado Boulder demonstrated that the gas-quench method also produces fewer bromide vacancies and results in materials with a higher quality opto-electronic performance. Solar cells made using the gas-quench method retain desirable light absorption properties and provide enhanced performance in the form of a higher charge carrier mobility, higher open circuit voltage, and enhanced stability.
Scientists have taken a major step toward a circular carbon economy by developing a long-lasting, economical catalyst that recycles greenhouse gases into ingredients that can be used in fuel, hydrogen gas, and other chemicals. The results could be revolutionary in the effort to reverse global warming, according to the researchers.
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