Multijunction Solar Cell Tunnel Diode for Low-Absorption Current Flow
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Solution Overview
Problem
Existing multijunction solar cells face challenges in achieving high transparency and efficiency due to undesirable incorporation of n-type dopants and absorption losses in tunnel diodes, particularly with As-containing layers, which reduce light reception by underlying subcells.
Innovation Solution
Incorporating a thin As-containing intermediate layer between the n+ and p+ layers of the tunnel diode, designed to be thinner than both, and using InAlP for the n+ layer with a high indium content to form a direct semiconductor, reduces dopant cross-contamination and absorption losses, enhancing the transparency and peak current density of the multijunction solar cell.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If As-containing layers are used in tunnel diodes, then the tunnel current is increased, but absorption losses occur that reduce light reception by underlying subcells
Solution Approach 1:
The patent applies local quality by creating a thin intermediate layer (5-20 nm) of As-containing material specifically at the tunnel diode interface between n+ and p+ layers. This localized As-containing region provides the necessary tunnel current enhancement only where needed for charge carrier transport, while the bulk of the structure remains free of As to minimize light absorption losses. The spatially selective placement of As resolves the contradiction between needing As for electrical performance and avoiding As for optical performance.
Solution Approach 2:
The thin As-containing intermediate layer acts as a mediator between the n+ InAlP layer and the p+ layer. It facilitates efficient charge carrier tunneling and reduces dopant cross-contamination between the n+ and p+ regions, thereby maintaining high tunnel current without requiring thick As-containing layers that would cause significant light absorption. The intermediate layer mediates the interaction between contrasting material requirements.
2Power
If n-type dopants are incorporated into the p+ layer of the tunnel diode, then the tunnel current is enhanced, but dopant cross-contamination occurs that degrades device performance
Solution Approach 1:
The As-containing intermediate layer serves as a barrier that prevents n-type dopants from the n+ InAlP layer from diffusing into the p+ layer during fabrication and operation. This intermediary layer maintains sharp dopant junctions and prevents cross-contamination while still enabling high tunnel current through its unique electronic structure. The intermediate layer physically and electronically separates the doped regions.
Solution Approach 2:
The tunnel diode structure employs a composite material approach by combining InAlP (n+ layer), As-containing intermediate layer, and III-V material (p+ layer). Each material is selected for its specific properties: InAlP for high electron concentration, As-containing layer for preventing dopant diffusion and enabling tunneling, and III-V material for the p+ region. The composite structure achieves both high tunnel current and dopant isolation.
3Reliability
If thick As-containing layers are used in tunnel diodes, then the transparency to light is improved, but absorption losses increase that reduce efficiency
Solution Approach 1:
The patent changes the critical parameter of layer thickness to resolve the contradiction. By reducing the As-containing intermediate layer thickness to 5-20 nm (much thinner than conventional As layers), the structure maintains sufficient transparency to allow light transmission while minimizing absorption losses. This parameter optimization enables the layer to be thin enough for light transmission but thick enough to provide the necessary electrical functions.
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 solution achieves high transparency and increased peak current density, thereby improving the efficiency of the multijunction solar cell by minimizing absorption losses and maintaining optimal light reception across subcells.
Implementation Method 1
an intermediate layer is disposed between the n+ layer and the p+ layer, the intermediate layer being thinner than the n+ layer and the p+ layer in each case
Implementation Method 2
The n+ layer of the first tunnel diode comprises InAlP; the p+ layer of the first tunnel diode comprises an As-containing III-V material
Data Source
AI summary
A stacked monolithic upright metamorphic multijunction solar cell, comprising at least one first subcell having a first band gap, a first lattice constant and being made up of germanium by more than 50%, a second subcell, which is disposed above the first subcell and has a second band gap and a second lattice constant, a metamorphic buffer disposed between the first subcell and the second subcell, including a sequence of at least three layers having lattice constants which increase from layer to layer in the direction of the second subcell, and a first tunnel diode, which is situated between the metamorphic buffer and the second subcell and which has an n+ layer and a p+ layer, the second band gap being larger than the first band gap, the n+ layer of the first tunnel diode comprising InAlP, the p+ layer of the first tunnel diode comprising an As-containing III-V material.