Multijunction Solar Cells With Graded Band Gap
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Solution Overview
Problem
Current solar cells for space missions face challenges in maintaining high energy conversion efficiency and radiation resistance over their service life due to complex design specifications and unpredictable interactions with space environmental factors, which complicates the optimization of material parameters like band gap and doping levels.
Innovation Solution
A multijunction solar cell design featuring a graded band gap in at least one subcell, where the band gap decreases from the top surface to the junction, enhancing current collection and increasing power output, and incorporating a metamorphic layer to maintain lattice constant throughout the thickness, thereby improving radiation hardness and efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If a multijunction solar cell uses multiple photovoltaic regions with different band gap energies to achieve spectral splitting, then energy conversion efficiency is improved, but device complexity increases
Solution Approach 1:
The solar cell is divided into multiple photovoltaic regions (subcells) stacked in series, each with a different band gap energy. The first subcell has a band gap of 1.8-2.0 eV, the second has 1.4-1.6 eV, and the third has 1.0-1.2 eV. This segmentation allows each region to capture different portions of the solar spectrum, achieving spectral splitting and high energy conversion efficiency while maintaining a structured, manageable design.
Solution Approach 2:
Each photovoltaic region is assigned a specific local quality in the form of a tailored band gap energy optimized for its position in the stack. The top subcell uses higher band gap materials (InGaAlP) to capture high-energy photons, while lower subcells use progressively lower band gap materials (GaInAs, InGaAs) to capture lower-energy photons. This local optimization of material properties maximizes overall energy conversion efficiency.
2Use of energy by moving object
If the solar cell is designed for maximum efficiency under AM0 illumination, then energy conversion efficiency is improved, but adaptability to different space environmental conditions deteriorates
Solution Approach 1:
The solar cell design incorporates adjustable material composition parameters within specified ranges rather than fixed values. The band gap energies are defined as ranges (first subcell: 1.8-2.0 eV, second: 1.4-1.6 eV, third: 1.0-1.2 eV) allowing optimization for different spectral conditions. This parameter flexibility enables the cell to maintain high efficiency across varying space environmental conditions while being optimized for AM0 illumination.
3Ease of manufacture
If the band gap is uniform throughout the active layer, then manufacturing simplicity is maintained, but current collection efficiency deteriorates
Solution Approach 1:
The active layer incorporates a graded band gap structure where the band gap energy varies spatially from the front surface to the back surface. The band gap is highest at the front surface and progressively decreases toward the back surface, creating optimal conditions for photon absorption and carrier collection at each depth. This local variation in band gap energy significantly improves current collection efficiency compared to a uniform band gap 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 graded band gap design increases the short circuit current and overall efficiency of the solar cell, while the metamorphic layer enhances radiation resistance, addressing the challenges of maintaining performance over the satellite's service life in space environments.
Implementation Method 1
Each subcell is designed to convert photons over different spectral or wavelength bands to electrical current
Implementation Method 2
at least one of which has a graded band gap throughout at least a portion of the thickness of its active layer
Implementation Method 3
incorporating a metamorphic layer to maintain lattice constant throughout the thickness, thereby improving radiation hardness and efficiency
Data Source
Figure 1A~1B
Figure 2
Figure 3A
AI summary
A multijunction solar cell including an upper first solar subcell having a first band gap and positioned for receiving an incoming light beam; a second solar subcell disposed below and adjacent to and lattice matched with said upper first solar subcell, and having a second band gap smaller than said first band gap; wherein at least one of the solar cells has a graded band gap throughout its thickness.