Solar cells are renewable energy sources that convert sunlight into electricity. Kesterite thin-film solar cells are a type of solar cell that consists of copper, zinc, tin, sulfur, and selenium. These cells are advantageous because they are made up of abundant earth elements, which can help resolve concentrated resources or instability in supply and demand.
Challenges in Kesterite Thin-Film Solar Cells
One of the challenges of kesterite thin-film solar cells is their low efficiency, which needs to be overcome for the expansion of the market for these cells. Defects in the light-absorbing layer of a solar cell can cause a loss of recombination in carriers and a change in the energy absorption band, leading to a shorter diffusion length and carrier lifetime. Identifying the defect or defect cluster that affects these properties is critical.
Identifying Defective Energy Levels in Kesterite Thin-Film Solar Cells
A research team from DGIST and Incheon National University identified the defective energy level in the absorption layer that deteriorated thin-film solar cell properties. Based on the admittance spectroscopy of kesterite thin-film solar cells, the team found that defects and defect clusters were more easily formed in the area with a large elemental variation in the light absorption layer. A defect with a deep energy level of over 150 meV drastically deteriorated the properties of kesterite thin-film solar cells.
The researchers found that carrier diffusion length decreased as the defect energy level deepened on the surface of the light-absorbing layer, leading to decreased electric current properties. The main reason for the deteriorated properties was the large amplitude of the band gap fluctuation arising from the defect that caused greater carrier recombination and a shorter carrier lifetime, reducing voltage and electric current properties.
This study is significant as it suggests that it is crucial to prevent the formation of defects with deep energy levels at the interface of the light absorber layer to improve the efficiency of kesterite thin-film solar cells. It also proposed the range of deep main defect energy levels of over 150 meV from experiment results and identified the specific types of defects. This method is expected to provide directions in research for understanding defect properties and improving efficiency not only for kesterite thin-film solar cells but also other types of solar cells.
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