(Invited) Fabrication and Applications of High-Efficiency, Lightweight, Multi-Junction Solar Cells by Epitaxial Liftoff

Abstract

MicroLink Devices has developed an epitaxial liftoff (ELO) technology for removing thin device layers from the surface of a semiconductor wafer using a flexible metallic sheet to provide structural support. The wafer is left intact and available for use in a subsequent growth cycle. This technology has been applied to high-efficiency, multi-junction solar cells. Removal of the substrate allows the devices to achieve very high specific power of approximately 1000W/kg. For large area devices, the cost of the substrate can be a significant component of the overall device manufacturing cost. By utilizing ELO technology to reuse the substrate for multiple growth cycles, the manufacturing cost of the device can be dramatically reduced. A flexible solar cell fabricated using ELO is shown in Figure 1. Lightweight, flexible, high-efficiency solar cells have a broad range of applications in both space and terrestrial markets. MicroLink has demonstrated the use of these devices to extend the flight time of electric unmanned aerial vehicles (UAVs). The lightweight and flexible solar cells can be embedded in the wings of the vehicle, adding only minimal weight and conforming to the flight surfaces to avoid introducing addition drag. Figure 2 contains an image of a Raven UAV with MicroLink’s lightweight flexible solar cells integrated into the surface of the wings. The solar array contributes an additional 40W of power to the aircraft under typical operating conditions leading to a 62% increase in flight time. MicroLink has also demonstrated a Mobile Solar Power (MSP) panel designed for use by soldiers in the field which is capable of recharging a standard issue BB-2590 battery. This panel is shown in Figure 3 lying on top of a conventional CIGS panel. MicroLink’s MSP panel dramatically reduces the size of the panel, allowing it to be used on-the-go. This panel has demonstrated a specific power of 117W/kg, more than double the specific power of CIGS based panels (typically 43W/kg). By reducing the weight and cost of high-efficiency photovoltaic devices, MicroLink’s novel approach to device fabrication allows for a wide array of previously impractical applications of solar power. Figure 1