Inverted Metamorphic Multijunction Solar Cell with Graded Buffer Layers
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Solution Overview
Problem
Existing fabrication processes for inverted metamorphic multijunction solar cells using III-V compound semiconductor materials are not adequate to produce commercially viable and energy-efficient solar cells, due to practical difficulties in material choice and fabrication steps.
Innovation Solution
The development of a multijunction solar cell structure with four subcells, where high band gap subcells are grown epitaxially on a substrate, followed by lower band gap subcells, and a surrogate substrate is used to support the lattice-mismatched lower subcell, with metamorphic layers grown using vapor deposition methods to control lattice constants and electrical properties, and surfactant-assisted growth to minimize threading dislocations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If lattice-mismatched lower band gap subcells are grown to achieve spectral splitting and improved efficiency, then energy conversion efficiency is improved, but threading dislocations increase reducing cell reliability
Solution Approach 1:
A metamorphic buffer layer is introduced as an intermediary between the lattice-matched high band gap subcell and the lattice-mismatched lower band gap subcell. This buffer layer has a graded composition that gradually transitions the lattice constant, reducing misfit dislocations while enabling the growth of low band gap materials for improved spectral splitting and energy conversion efficiency.
Solution Approach 2:
The lattice constant and composition of the buffer layer are gradually changed through controlled deposition of alloy layers with varying stoichiometry. This parameter gradient allows the structure to accommodate lattice mismatch between subcells while minimizing threading dislocation formation, thereby maintaining both high efficiency and reliability.
2Productivity
If multiple metamorphic layers with different compositions are used to achieve four junction spectral splitting, then energy efficiency is improved, but device complexity and fabrication difficulty increase
Solution Approach 1:
The solar cell is divided into four distinct junctions or subcells, each with a specific band gap energy tailored to absorb different portions of the solar spectrum. This segmentation enables spectral splitting across multiple energy ranges, achieving high overall efficiency while maintaining a systematic and manufacturable structure.
Solution Approach 2:
Each subcell and buffer layer is engineered with specific local compositional properties (band gap, lattice constant, thickness) optimized for its particular function in the stack. This local quality control allows the complex multi-junction device to be constructed from modular units with well-defined characteristics, facilitating fabrication and quality control.
3Manufacturing precision
If surfactant-assisted vapor deposition is used to minimize threading dislocations, then manufacturing precision is improved, but ease of manufacture decreases
Solution Approach 1:
A surfactant layer is introduced as an intermediary during the vapor deposition process. This surfactant modifies the growth interface to reduce surface energy and promote smoother layer formation, thereby minimizing threading dislocation nucleation while maintaining compatibility with existing vapor deposition manufacturing equipment.
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 enables the production of a four-junction inverted metamorphic solar cell with improved efficiency, as demonstrated by increased short circuit current and fill factor, achieving an efficiency of 32.2% with enhanced radiation resistance and spectral splitting capabilities.
Implementation Method 1
a first metamorphic buffer layer grown over the first high band gap subcell and having a graded composition so as to reduce misfit dislocations
Implementation Method 2
metamorphic layers grown using vapor deposition methods
Implementation Method 3
surfactant-assisted growth to minimize threading dislocations
Implementation Method 4
high band gap subcells are grown epitaxially on a substrate, followed by lower band gap subcells... achieving an efficiency of 32.2%
Data Source
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AI summary
A multijunction solar cell including an upper first solar subcell having a first band gap; a second solar subcell adjacent to the first solar subcell and having a second band gap smaller than the first band gap; a first graded interlayer adjacent to the second solar subcell; the first graded interlayer having a third band gap greater than the second band gap; and a third solar subcell adjacent to the first graded interlayer, the third subcell having a fourth band gap smaller than the second band gap such that the third subcell is lattice mismatched with respect to the second subcell. A second graded interlayer is provided adjacent to the third solar subcell; the second graded interlayer having a fifth band gap greater than the fourth band gap; and a lower fourth solar subcell is provided adjacent to the second graded interlayer, the lower fourth subcell having a sixth band gap smaller than the fourth band gap such that the fourth subcell is lattice mismatched with respect to the third subcell.