Graded CIGS Absorber Layer for High Efficiency Solar Cells
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Solution Overview
Problem
Existing copper-indium-gallium-selenide (CIGS) solar cells face challenges in achieving high efficiency due to difficulties in controlling sodium diffusion and maintaining adhesion between the absorber layer and electrodes, particularly on non-glass substrates, leading to suboptimal minority carrier lifetime and efficiency.
Innovation Solution
A solar cell design with a graded composition CIGS p-type semiconductor absorber layer, comprising a sodium-containing molybdenum layer and multiple sublayers with varying copper and gallium ratios, optimized for efficient sodium diffusion and adhesion, achieving a minority carrier lifetime below 2 nanoseconds and conversion efficiency above 13.4 percent.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If sodium diffusion is controlled to improve open circuit voltage, then conversion efficiency improves, but adhesion between absorber layer and electrode deteriorates
Solution Approach 1:
The patent applies local quality by creating a graded composition CIGS absorber layer where the copper and gallium ratios vary through the thickness of the layer. The bottom sublayer has a composition optimized for adhesion to the molybdenum electrode, while upper sublayers have compositions optimized for light absorption and charge carrier generation. This spatial variation in composition allows simultaneous optimization of adhesion and conversion efficiency.
Solution Approach 2:
The absorber layer is segmented into multiple sublayers with different compositions. The graded composition structure divides the absorber layer into regions with varying Cu/(Ga+In) ratios, allowing each sublayer to perform its specific function optimally - the bottom sublayer for adhesion and the upper sublayers for photovoltaic conversion.
2Productivity
If minority carrier lifetime is reduced to below 2 nanoseconds, then conversion efficiency increases, but adhesion and sodium distribution become more difficult to control
Solution Approach 1:
The patent applies preliminary action by pre-doping the molybdenum electrode with sodium before depositing the CIGS absorber layer. This ensures that sodium is already present and properly distributed in the electrode and interface region before absorber layer formation, facilitating controlled sodium diffusion during subsequent processing steps and enabling precise control of the final sodium distribution in the completed device.
3Reliability
If graded composition is implemented to improve adhesion, then open circuit voltage increases, but device complexity increases
Solution Approach 1:
The patent applies parameter changes by systematically varying the copper and gallium ratios through the thickness of the absorber layer. The Cu/(Ga+In) ratio is changed from higher values in the bottom sublayer to lower values in the upper sublayers, creating a graded composition that improves adhesion and open circuit voltage while maintaining a relatively simple overall device structure.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution enhances the open circuit voltage and conversion efficiency of CIGS solar cells, achieving efficiencies up to 15.7-18.1 percent with a minority carrier lifetime of less than 2 nanoseconds, while improving adhesion and sodium distribution, even on non-glass substrates.
Implementation Method 1
The sodium diffuses from the first transition metal layer into the p-type semiconductor absorber layer during the step of depositing the p-type semiconductor absorber layer
Implementation Method 2
copper-indium-gallium-selenide solar cells
Data Source
AI summary
A solar cell containing a plurality of CIGS absorber sublayers has a conversion efficiency of at least 13.4 percent and a minority carrier lifetime below 2 nanoseconds. The sublayers may have a different composition from each other.