Cost Reduction of IMM Solar Cells by Recycling Substrates Using Wet Chemical Etching

This program focuses on reducing the cost of substrate reclaim for high-efficiency solar cells fabricated via an epitaxial lift-off (ELO) process, while increasing the number of reuse cycles possible for a given substrate. We will introduce a new multi-layer etch-stop structure into inverted metamorphic (IMM) triple-junction solar cells grown on 100-mm GaAs substrates. The etch-stop structure will be grown between the original GaAs substrate and the ELO release layer, thereby becoming the effective substrate surface after the ELO process. The purpose of the etch-stop structure is to prevent pits and surface damage incurred during ELO from damaging the original GaAs surface. The standard method of reclaiming the GaAs substrate after ELO is to employ chemo-mechanical polishing (CMP) to remove the defect-ridden GaAs surface and chemically polish the underlying GaAs to a yield surface that is suitable for successive epitaxial growth. This process works effectively but reduces resultant substrate thickness and causes some wafer damage itself, which then requires further polishing. These factors accumulate over time, practically limiting the number of reclaim cycles to 5–10 for a given substrate. With the incorporation of the proposed multi-layer etch-stop structure, the defects are isolated in the etch-stop structure, which can be dissolved by successive selective wet chemical etches to produce the original pristine GaAs surface with its original thickness. All mechanical polishing is eliminated in this proposed work, ensuring a constant substrate thickness through repeated substrate reclaim cycles and also drastically reducing the estimated cost of the recycling process to <$1 per substrate.

Thin, flexible, high efficiency ELO IMM solar cells, currently under development by MicroLink, are attractive as replacements for the solar cells in existing power arrays, for next-generation spacecraft designs, and for solar electric power (SEP) systems. One of the advantages of ELO cells is that they are less costly than the current generation of Ge-based triple-junction solar cells because the substrate on which the solar cell is grown can be reused multiple times: ultimately the effective cost of the substrate approaches the cost of repolishing after ELO. This advantage will be further enhanced by the proposed substrate reuse technology development: the cost of repolishing the substrate may drop from tens of dollars to less than one dollar which will result in a substantial reduction in the cost of the ELO cell. Major commercial manufacturers of spacecraft solar panels have shown substantial interest in MicroLink's solar cells and in lightweight, flexible photovoltaic modules that contain these cells. The flexible, conformal nature of the cells also makes them suitable for integration with spacesuits and other structures, such as remote fuel generation facilities or habitats. Finally, lightweight, conformal ELO solar cell arrays are useful for reducing the weight and increasing the operational range of unmanned aerial vehicles (UAVs).

Thin, flexible, high efficiency ELO IMM solar cells are also well suited for commercial and military satellites and spacecraft, either as replacements for existing solar cells in conventional array designs, or in novel, high power arrays. The cells are attractive for electrical power generation in unmanned aerial vehicles (UAVs), which has been demonstrated to allow substantial endurance enhancements. The cells are also useful for terrestrial power generation in high value military applications, such as off-grid power generation and battery charging. Finally, the cells are attractive for the terrestrial concentrator photovoltaics (CPV) market. In all cases, the proposed substrate reuse technology development will substantially reduce the cost of ELO solar cells and arrays, thereby making the cells more commercially attractive.

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