Stress-Balanced Quantum Well Solar Cell Junction
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Solution Overview
Problem
The challenge in constructing multiple quantum well photovoltaic junctions for solar cells is the lattice mismatch between the substrate and the quantum well materials, leading to defects and reduced efficiency due to strain-induced recombination of carriers and shifts in the absorption edge.
Innovation Solution
A photovoltaic junction with an intrinsic region comprising a multiple quantum well stack where tensile stress in some quantum wells is balanced by compressive stress in others, using diluted nitride and nitrogen-free materials to maintain elastostatic equilibrium and reduce strain differentials, allowing for deeper quantum wells and extended absorption wavelengths.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If deeper quantum wells are used to extend the wavelength region of photons which can be absorbed, then the absorption capability is improved, but the lattice mismatch increases leading to more dislocations and defects
Solution Approach 1:
The patent employs a composite quantum well structure combining GaInAsP and GaInAs materials with different lattice constants. The GaInAsP quantum wells provide tensile strain while GaInAs quantum wells provide compressive strain, creating a stress-balanced composite system that extends absorption to longer wavelengths without excessive lattice mismatch defects
Solution Approach 2:
The patent balances the tensile stress from GaInAsP quantum wells with compressive stress from GaInAs quantum wells. This stress compensation approach counteracts the harmful effects of lattice mismatch, allowing deeper quantum wells to be grown without generating excessive dislocations while maintaining extended wavelength absorption
2Adaptability or versatility
If quantum wells with larger lattice mismatches are used to extend absorption to longer wavelengths, then the wavelength range is improved, but the stress differential between barrier and quantum well layers increases
Solution Approach 1:
The patent creates a composite quantum well system where GaInAsP wells (providing tensile stress) are interleaved with GaInAs wells (providing compressive stress). This composite arrangement balances the overall stress in the structure, enabling extended wavelength absorption while maintaining manageable stress differentials
Solution Approach 2:
The patent applies different material compositions and strain characteristics to different regions of the quantum well stack. By locally varying the quantum well material (GaInAsP vs GaInAs) and their respective strain states, the patent achieves both extended wavelength absorption and balanced stress distribution throughout the 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
This approach enhances the internal quantum efficiency and absorption coefficient of solar cells by minimizing strain-induced defects and maintaining the absorption edge at longer wavelengths, improving the overall efficiency and material quality of the photovoltaic junction.
Implementation Method 1
The quantum wells have a smaller band gap than the surrounding barriers and bulk material, enabling the device to absorb photons of longer wavelength. A large volume of experimental work has also demonstrated that the absorption coefficients per unit thickness of material in the quantum wells can be several times larger than the bulk
Implementation Method 2
A photovoltaic cell usually comprises two or more layers of semiconductor material from which charge carriers are liberated by incident light, such as sunlight. One or more semiconductor junctions between the layers operate to separate the liberated charge carriers which then move to electrodes thereby providing electrical power
Data Source
AI summary
A photovoltaic junction for a solar cell is provided. The photovoltaic junction has an intrinsic region comprising a multiple quantum well stack formed from a series of quantum wells separated by barriers, in which the tensile stress in some of the quantum wells is partly or completely balanced by compressive stress in the others of the quantum wells. The overall elastostatic equilibrium of the multiple quantum well stack may be ensured by engineering the structural and optical properties of the quantum wells only, with the barriers having the same lattice constant as the materials used in the oppositely doped semiconductor regions of the junction, or equivalently as the actual lattice size of the junction or intrinsic region, or the bulk or effective lattice size of the substrate. Alternatively, the barriers may contribute to the stress balance.