Multi-Junction Solar Cell Epitaxy on Silicon for Lower-Cost Efficiency
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Solution Overview
Problem
Current multi-junction solar cells face challenges in achieving high efficiency while maintaining manufacturability and cost-effectiveness due to complex fabrication processes and material synthesis difficulties, particularly with substrates and compositional grading requirements.
Innovation Solution
A method for fabricating a multi-junction monolithically integrated photovoltaic device using a silicon substrate, involving the formation of buried emitter regions, termination layers, tunnel junctions, and back surface fields, with specific epitaxial growth processes to achieve high efficiency and reduced fabrication costs, utilizing gallium and phosphide-based materials with controlled threading dislocation densities.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If multi-junction devices are used to achieve high efficiency, then solar cell efficiency is improved, but fabrication complexity and cost increase
Solution Approach 1:
The solar cell is divided into multiple junction layers (first junction, second junction, third junction) with distinct functions. Each junction is optimized for specific wavelength ranges and performance characteristics, allowing the complex multi-junction device to be constructed from manageable segments that can be fabricated and analyzed independently while achieving high overall efficiency.
Solution Approach 2:
Different regions of the solar cell are assigned different material compositions and structural properties tailored to their specific functions. The first junction uses AlGaAs for high-efficiency visible light conversion, the second junction uses GaInP for ultraviolet response, and the third junction uses GaAs for infrared conversion. Each layer's properties are locally optimized rather than using uniform materials throughout.
2Loss of energy
If multi-junction devices are used to achieve high efficiency, then solar cell efficiency is improved, but manufacturing cost increases
Solution Approach 1:
The GaAs-based third junction serves multiple functions: it converts infrared light to electricity, provides a lattice-matched substrate for growing the upper AlGaAs and GaInP layers, and acts as a mechanical support structure. This multi-functionality reduces the need for separate substrates and simplifies the overall manufacturing process despite the device's complexity.
Solution Approach 2:
The patent uses lattice-matched material systems where each upper layer is designed to match the crystal structure of the layer below it (GaInP on GaAs, AlGaAs on GaInP). This copying of crystal structures eliminates the need for complex defect-management techniques and allows standard epitaxial growth processes to be used, reducing manufacturing costs.
3Loss of energy
If compositional grades are employed to reduce surface recombination losses, then carrier collection is improved, but fabrication difficulty increases
Solution Approach 1:
The aluminum composition in the AlGaAs window layer is gradually changed from 0% at the interface with the emitter to 30% at the surface, creating a compositional gradient. This continuous parameter change reduces surface recombination by smoothly transitioning the material properties rather than having abrupt interfaces, while the gradient can be achieved through standard molecular beam epitaxy processes.
4Reliability
If tunnel junctions are used to facilitate current flow between cells, then electrical connectivity is improved, but material synthesis difficulty increases
Solution Approach 1:
The tunnel junction is constructed using heavily doped GaAs layers on both sides of the intrinsic GaAs layer, creating a symmetric and homogeneous structure. The heavy doping (1×10^19 to 1×10^21 atoms/cm³) on both p and n sides ensures uniform electrical properties and facilitates reliable current transport. This homogeneous doping approach simplifies the synthesis process compared to asymmetric junction designs.
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 approach results in solar cells with efficiencies ranging from 25% to 37% and reduced manufacturing costs, enabling a more scalable and cost-effective production process while maintaining high performance.
Implementation Method 1
A solar cell, also referred to as a photovoltaic cell, is a semiconductor device capable of converting light energy into electrical energy through the photovoltaic effect
Implementation Method 2
Solar cells employ one or more P-N junctions that produce corresponding one or more electric fields, which sweep photo-generated carriers toward the terminals of the solar cell device for collection
Implementation Method 3
the termination layer being a thickness of a gallium and phosphide bearing epitaxially formed material provided using a MOCVD or HVPE or VPE process
Data Source
Figure 1A
Figure 1B~1C
Figure 1D~1E
AI summary
Manufacture of multi-junction solar cells, and devices thereof, are disclosed. The architectures are also adapted to provide for a more uniform and consistent fabrication of the solar cell structures, leading to improved yields and lower costs. Certain solar cells may further include one or more compositional gradients of one or more semiconductor elements in one or more semiconductor layers, resulting in a more optimal solar cell device.