Abstract
Tunnel Junctions, as addressed in this review, are conductive, optically transparent semiconductor layers used to join different semiconductor materials in order to increase overall device efficiency. The first monolithic multi-junction solar cell was grown in 1980 at NCSU and utilized an AlGaAs/AlGaAs tunnel junction. In the last 4 decades both the development and analysis of tunnel junction structures and their application to multi-junction solar cells has resulted in significant performance gains. In this review we will first make note of significant studies of III-V tunnel junction materials and performance, then discuss their incorporation into cells and modeling of their characteristics. A Recent study implicating thermally activated compensation of highly doped semiconductors by native defects rather than dopant diffusion in tunnel junction thermal degradation will be discussed. AlGaAs/InGaP tunnel junctions, showing both high current capability and high transparency (high bandgap), are the current standard for space applications. Of significant note is a variant of this structure containing a quantum well interface showing the best performance to date. This has been studied by several groups and will be discussed at length in order to show a path to future improvements.
Highlights
This review will be a discussion of both development and analysis of tunnel junction structures and their application to multi-junction solar cells
This interlayer is of quantum well thickness at around 50 Å or less and has a twofold effect on the tunnel junction characteristics: it serves to reduce the carry-over of tellurium into the AlGaAs layer and the quantum well energy level produces increased tunneling current
The first proof of concept monolithic tandem cell used an AlGaAs/AlGaAs tunnel junction due to factors involved in liquid phase epitaxy, significantly the high thickness required more transparency
Summary
This review will be a discussion of both development and analysis of tunnel junction structures and their application to multi-junction solar cells. The highest efficiency devices incorporate multiple solar cells in a vertically connected stack for peak efficiency at various wavelengths within the solar spectrum These multi-junction devices require a transparent and conductive layer to join them, most commonly in the form of tunneling junctions. The first tandem cell to achieve higher efficiency than any single cell was described in 1990 at the Solar Energy Research Institute ( NREL) [3] This cell consisted of an InGaP-GaAs tandem cell structure utilizing a GaAs/GaAs tunnel junction. The dopants used in this second structure consisted of carbon doping for the p-type side (very common in modern structures) and selenium on the n-type side (tellurium is more commonly used today) This tunnel junction showed the advantages of newer growth methods by utilizing metal–organic chemical vapor deposition (MOCVD) to grow considerably thinner layers resulting in lower optical absorption in the tunnel junction and reduced series resistance.
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