Type-II Tunnel Junctions for Multijunction Solar Cells
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Solution Overview
Problem
Current tunnel junctions in multijunction solar cells, particularly those with InP lattice constant, face challenges in achieving high peak tunneling current with low optical and electrical losses, which limits their efficiency and performance at higher solar intensities.
Innovation Solution
A high bandgap, type-II tunnel junction is introduced, comprising a p-doped AlGaInAs tunnel layer and a n-doped InP tunnel layer, grown using Metal Organic Vapor Phase Epitaxial (MOVPE) reactor, which enhances tunneling current and optical transparency, reducing parasitic losses and material complexity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional homojunction tunnel diodes are used, then electrical connection between subcells is achieved, but optical transparency is poor and fabrication complexity increases
Solution Approach 1:
The patent employs a heterojunction structure combining AlGaInAs and InP materials with different bandgaps to form a type-II tunnel junction. This composite material approach enables simultaneous achievement of low electrical resistance and high optical transparency, resolving the contradiction between electrical connection reliability and optical energy loss.
Solution Approach 2:
The patent modifies the bandgap parameters by selecting AlGaInAs with higher bandgap and InP with lower bandgap to create a type-II alignment. This parameter change enables the tunnel junction to be transparent to photons with energy below the AlGaInAs bandgap while maintaining effective tunneling current, thus improving optical transparency without sacrificing electrical connection.
2Productivity
If highly doped AlGaInAs homojunction is used, then tunneling current is achieved, but optical losses increase and fabrication difficulty increases
Solution Approach 1:
The patent divides the tunnel junction into two distinct doped layers: a heavily p-doped AlGaInAs layer and a heavily n-doped InP layer. This segmentation allows each layer to be optimized independently for its specific function while simplifying the overall fabrication process compared to complex homojunction structures.
Solution Approach 2:
The patent introduces an intermediate InP layer between the p-doped AlGaInAs and the underlying structure. This intermediary layer facilitates lattice matching and reduces fabrication complexity while maintaining the high tunneling current performance through proper band alignment.
3Power
If multiple subcells with different bandgaps are stacked, then power output is increased, but lattice matching requirements increase complexity
Solution Approach 1:
The patent maintains lattice constant homogeneity by selecting AlGaInAs and InP materials that are both lattice-matched to the InP substrate. This homogeneous lattice structure throughout the multijunction device eliminates dislocation formation and simplifies the lattice matching requirements while enabling high power output through multiple subcells.
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 high bandgap type-II tunnel junction increases peak tunneling current, improves material growth processes, and achieves higher optical transparency, leading to enhanced power conversion efficiency and reduced costs, enabling solar cells to operate effectively at higher solar intensities.
Implementation Method 1
tunnel junctions (or tunnel diodes)... must exhibit a high peak current density
Implementation Method 2
A solar cell is a device that is capable of converting sunlight energy to electricity by photovoltaic effect
Implementation Method 3
grown using Metal Organic Vapor Phase Epitaxial (MOVPE) reactor
Data Source
AI summary
A type-II tunnel junction is disclosed that includes a p-doped AlGaInAs tunnel layer and a n-doped InP tunnel layer. Solar cells are further disclosed that incorporate the high bandgap type-II tunnel junction between photovoltaic subcells.