This Small Business Innovation Research Phase I project develops a method of lifting-off an epitaxially grown high-efficiency (> 30%) triple-junction III-V solar cell from a Ge or GaAs wafer onto a polyimide substrate. A high-coefficient of thermal expansion (CTE) polyimide wafer is used to induce a crack that propagates parallel to the surface at the epi/wafer interface, due to the mismatch between the coefficients of thermal expansion, as the wafer is cooled down below room temperature. The lift-off happens in a fraction of a second and no expensive ion implantation or slow chemical etching of a sacrificial layer is needed. The epi-layer is attached to a low-cost flexible polyimide substrate having a thickness between 25 and 100 microns which serves as the permanent carrier of the solar cell. Inverted and non-inverted cells can be lifted off using this technique. The base substrate can be reused to grow another epi-layer and the cycle repeated. For these devices, the cost of substrate materials is about 40% of the cost of the finished cell. This process will result in savings of raw materials and grinding and etching costs, up to a total savings of 30% of the cost of the cell.
The broader impact/commercial potential of this project will be to develop a method to transfer epitaxially-grown device layers from semiconductor wafers to inexpensive polymeric substrates to create high-performance flexible circuits. The application of this technology goes beyond photovoltaics (PV). Space-grade as well as terrestrial high-efficiency PV cells and modules can be made lighter, flexible, and less expensive when this process is integrated into the manufacturing sequence of established suppliers. Additionally, when the thinned cell is transferred to a metallic substrate, the reduced thickness improves performance of terrestrial concentrating PV systems. Once this method is demonstrated, we will leverage our relationships with major high-efficiency solar cell manufacturers to negotiate licensing of the technology for their applications.
There is a need for lightweight and flexible III-V solar cells and modules with efficiency > 20% that are cost-effective for commercial and military space and terrestrial applications. It is desired to achieve a specific power ratio of 1000 W/Kg and reduce the cost to $ 50/W. OptiCOMP has developed dry epitaxial lift-off (DELO) technology by driving a controlled crack at the epi/wafer interface and transferring the epi-layer wafer scale to a flexible polyimide substrate and reusing the GaAs wafer to grow another epi-layer multiple times. The polyimide substrate has a thickness between 50 and 100 micron which yields a specific power ratio of 1000 W/Kg. The crack is driven purely by the thermal stresses due to the mismatch in CTE between GaAs and polyimide without the necessity for any external mechanical force or tool to aid crack propagation. This allows the use of automated roll-to-roll processes which reduce the cost of the cell considerably. There are several potential NASA and DoD programs that can benefit from our technology, such as, NASA's Global Precipitation Measurement (GPM) spacecraft, which will be powered by multi-junction solar cells, and DARPA's Fast Access Spacecraft Testbed (FAST) program, which will provide a cost-effective means for spacecraft to travel to the outer solar system, as well as the Very High Efficiency Solar Cells (VHESC) program at DARPA, which aims to develop ultra-high efficiency 10 cm2 solar cells for soldiers and Marines. Efficient and inexpensive conversion of solar energy into electrical energy is one of society’s most pressing technological challenges. Solving this problem for the soldier will enable commercialization for the general public, such as cell phones and laptops. This also has application for the lift-off of thin Si non-PV circuit from SOI wafer. The flexible substrate market for display applications is growing very rapidly. According to a report by IHS Electronics & Media, the flexible substrate market is forecast to grow to $ 500 million by 2020 from just $ 2.5 million in 2013. The vast majority of this market (> 95%) uses plastic substrates instead of thin glass or metal foil. The savings are not only monetary. This has huge implications on natural resources. Gallium is the rarest component of the photovoltaic compounds. All the newly developed solar cell materials, such as CIGS, contain Gallium. As the substrate must currently be ground away for the epitaxial lift-off, a significant portion of the world's current Ga production from mining goes to waste. This creates one of the most expensive toxic wastes because GaAs contains arsenic, which is toxic. Thus, cost is not the only issue but also availability of the material. Therefore, OptiCOMP's technology will solve the bottleneck in the Ga supply chain and will have broad impact on society and the ecology.
