Multi-Junction Solar Cell Emitter Superlattice for Lower Sheet Resistance
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Solution Overview
Problem
Multi-junction solar cells with InAlGaP upper cells face high sheet resistance leading to increased series resistive losses and reduced efficiency, particularly in concentrator applications, due to the need for reduced metal finger spacing which increases shadowing.
Innovation Solution
Incorporation of a superlattice structure in the emitter layer of the top subcell, composed of alternating thin semiconductor layers with varying band gap energies, to enhance transverse conductivity without significantly reducing the open circuit terminal voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If InAlGaP upper cell is used to increase band gap and open circuit terminal voltage, then Voc increases to values greater than 1.4 V, but sheet resistance increases approximately five times compared to InGaP upper cell
Solution Approach 1:
The patent applies parameter changes by modifying the emitter composition from pure InAlGaP to an InGaP/InAlGaP graded structure. The InGaP layer has lower sheet resistance while the InAlGaP layer provides higher Voc, and the grading between them reduces discontinuities. This compositional parameter change resolves the contradiction by achieving both low sheet resistance and high open circuit voltage.
Solution Approach 2:
The patent uses composite materials by creating a heterostructure combining InGaP and InAlGaP layers in the emitter. The InGaP layer contributes to lower sheet resistance while the InAlGaP layer contributes to higher band gap and Voc. The composite structure leverages the advantages of both materials to resolve the contradiction between low sheet resistance and high open circuit terminal voltage.
2Loss of energy
If metal finger spacing is reduced to decrease series resistive losses at higher currents, then series resistance decreases, but shadowing increases reducing overall solar cell efficiency
Solution Approach 1:
The patent changes the electrical parameter of the emitter by introducing the InGaP/InAlGaP graded structure, which reduces sheet resistance. This parameter change allows for reduced series resistive losses without requiring reduced metal finger spacing, thereby avoiding increased shadowing while still achieving low series resistance at higher currents.
3Reliability
If heterojunction cell design with InGaP emitter and InAlGaP base is used, then transverse conductivity is improved, but substantial voltage loss of approximately 160 mV occurs compared to exclusively InAlGaP cell
Solution Approach 1:
The patent inverts the conventional heterojunction approach by placing the InGaP layer with lower sheet resistance at the emitter position (top) and the InAlGaP layer with higher Voc at the base position (bottom). This inverted structure allows the low-resistance InGaP to provide excellent transverse conductivity at the contact surface while the high-Voc InAlGaP provides the voltage benefit, avoiding the 160 mV loss of the conventional configuration.
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 superlattice structure significantly reduces sheet resistance while maintaining open circuit terminal voltage and short-circuit current, enhancing overall solar cell efficiency with minimal voltage loss.
Implementation Method 1
the emitter of the top subcell includes a superlattice or is made of a superlattice structure
Implementation Method 2
Multi-junction solar cells generally comprise three or more subcells with different band gaps that are matched to one another
Data Source
AI summary
A stacked multi-junction solar cell comprising a stack composed of a bottom subcell, at least one middle subcell, and a top subcell, wherein each subcell has an emitter and a base at least the top subcell is made of a III-V semiconductor material or includes a III-V semiconductor material, and the emitter of the top subcell includes a superlattice.