Graded Buffer Layer for Lattice Mismatch in Multijunction Solar Cells
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Solution Overview
Problem
Multijunction solar cells face limitations in efficiency and cost-effectiveness due to the need for nearly perfect lattice matching of inserted subcells and the poor quality of GaInAsN alloy materials, which are lattice-matched to GaAs or Ge layers, leading to degradation in overall solar cell efficiency.
Innovation Solution
The use of graded buffer layers to accommodate lattice mismatches between layers, allowing for epitaxial relationships and optimal bandgap selection, thereby relaxing the requirement for lattice matching and minimizing internal stresses, while maintaining efficient electron movement and photoconversion performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If lattice-matched GaInAsN alloy is used to achieve perfect lattice matching between layers, then epitaxial relationship is maintained, but material quality deteriorates and overall solar cell efficiency degrades
Solution Approach 1:
A graded buffer layer is introduced as an intermediary between the GaAs substrate and the GaInAsN alloy layer. This buffer layer has a composition that gradually transitions from pure GaAs at the substrate interface to GaInAsN at the alloy interface, acting as a mediator that accommodates the lattice mismatch without requiring perfect lattice matching of the final GaInAsN layer, thus maintaining material quality while achieving epitaxial relationship.
Solution Approach 2:
The composition parameter of the buffer layer is gradually changed from pure GaAs to GaInAsN through controlled grading. This parameter change allows the lattice constant to transition smoothly, accommodating the lattice mismatch between substrate and final layer without compromising material quality, thereby resolving the contradiction between lattice matching precision and material quality.
2Reliability
If conventional window layer is used to reduce minority-carrier recombination, then surface passivation is improved, but light transmission to lower cell layers is reduced
Solution Approach 1:
The window layer is designed with local quality optimization by using a graded composition structure. The buffer layer has varying composition and optical properties through its thickness, being more transparent near the light-entry surface and more passivating near the emitter interface. This local quality variation allows simultaneous achievement of good light transmission and effective surface passivation.
Solution Approach 2:
The graded buffer layer functions as a composite material with continuously varying composition from GaAs to GaInAsN. This composite structure combines the transparency of GaAs with the passivation properties of GaInAsN, achieving both light transmission and minority-carrier recombination reduction in a single integrated layer.
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 performance and efficiency of solar cells with improved lattice matching and epitaxial relationships, allowing for the selection of optimal bandgaps for photoconversion, resulting in increased efficiency and reduced costs compared to conventional solar cells.
Implementation Method 1
the graded buffer layer has a buffer-layer lattice parameter that increases or decreases as needed with increasing distance from the first layer toward the second layer
Implementation Method 2
allowing for epitaxial relationships and optimal bandgap selection
Implementation Method 3
incident light energy, and specifically solar energy, is converted to electrical energy through the photovoltaic effect
Implementation Method 4
the buffer layer must have optical properties which allow most of the light that can be used by the subcells beneath the BSF to be transmitted by the BSF
Data Source
AI summary
A solar cell includes a first layer having a first-layer lattice parameter, a second layer having a second-layer lattice parameter different from the first-layer lattice parameter, wherein the second layer includes a photoactive second-layer material; and a third layer having a third-layer lattice parameter different from the second-layer lattice parameter, wherein the third layer includes a photoactive third-layer material. A transparent buffer layer extends between and contacts the second layer and the third layer and has a buffer-layer lattice parameter that varies with increasing distance from the second layer toward the third layer, so as to lattice match to the second layer and to the third layer. There may be additional subcell layers and buffer layers in the solar cell.