Dry-epitaxial lift-off for high efficiency solar cells

There is a need for lightweight and flexible solar cells and arrays with efficiency > 30% and specific power > 1000 W/kg for commercial and military terrestrial and aerial (UAV) applications and for space missions, that are cost-effective. Inverted metamorphic triple-junction (IMM3J) solar cells achieve the highest efficiencies, but cost > $ 250/W. This high cost is split among the substrate (40%), epitaxial growth (30%) and front side processing including metallization (30%). It is desired to reduce the cost to $ 50/W. This is especially important for space missions exceeding 10 kW, for short-term missions to low earth orbit, and for picosatellites such as cubesats. Epitaxial lift-off (ELO) is used to transfer the epi-layer to a flexible substrate and reuse the Ge or GaAs wafer to grow another epi-layer. A dry epitaxial lift-off (DELO) process is presented by driving a controlled crack at the epi/wafer interface and transferring wafer scale to a polyimide substrate permanent carrier of the fragile solar cell, thus eliminating the need for detaching the substrate. 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 process yields single atomic plane cleavage which reduces the need for post lift-off polishing, and allows multiple reuses of the substrate up to 10 times. This allows the use of automated roll-to-roll processing. This combined with cheaper metallization using aerosol jet 3D printing and additive manufacturing techniques allows reducing the cost down to $ 50/W. This will open up terrestrial applications of III-V solar cells.