Thin-Film III–V Solar Cells Using Epitaxial Lift-Off

Abstract

Epitaxial lift-off is used to create thin-film III-V solar cells without sacrificing the GaAs wafer. It is based on selective etching of an AlAs release layer between the wafer and the cell structure using an HF solution. The wafer can be reused for subsequent deposition runs thereby reducing the cost of the cells. The thin-film cell can be transferred to any new carrier, e.g. glass, plastic, silicon or metal foil. Although epitaxial lift-off was first demonstrated in 1978, it took until the 1990s to make significant progress in understanding the process and devising new ways to increase the etch rate. The first single-junction epitaxial lift-off cells were made in 1996. Thin-film cells offer new cell applications based on their flexibility, low weight and possibility to deposit the cell structure in reverse order. Today the world record for single-junction cells is held by a thin-film GaAs cell, who's performance is partly based on the increased photonrecycling factor in cells with a backcontact acting as a mirror. Also state of the art tandem and inverted metamorphic thin-film cells have been demonstrated.

Improvements in ultra‐light and flexible epitaxial lift‐off GaInP/GaAs/GaInAs solar cells for space applications

“…The inverted epitaxial structure is metallized, with the metal serving as back‐contact, back reflector and support layer for the ultra‐thin solar cells. The epitaxial structure is nondestructively separated from the substrate by an epitaxial lift‐off (ELO) process, whereby a HF solution selectively etches the AlAs release layer whilst the resulting crevice is bent open to avoid mass transfer limitations and thus maintain a constant high etch rate 13,14 . In this way, full 4″ films are released from the wafer.…”

“…The epitaxial structure is nondestructively separated from the substrate by an epitaxial lift-off (ELO) process, whereby a HF solution selectively etches the AlAs release layer whilst the resulting crevice is bent open to avoid mass transfer limitations and thus maintain a constant high etch rate. 13,14 In this way, full 4 00 films are released F I G U R E 1 (A) Schematic of the IMM triple junction structure in growth direction; the three subcells are connected by tunnel diodes (TD). The last grown (GaInAs) cell with a bandgap of $1.0 eV serves as the bottom subcell in the final configuration.…”

Ultra-lightweight and flexible inverted metamorphic four junction solar cells for space applications

“…After the growth, the substrate is removed in a non-destructive manner by applying the Epitaxial Lift-Off (ELO) process that was used to manufacture the IMM3J solar cells [11]. The ELO process separates the IMM layer structure from the substrate by selectively etching away a thin intermediate AlAs release layer, without damaging the IMM layer structure nor the substrate [12,13]. This enables multiple reuses of the substrate which results in considerable savings in costs and precious raw materials.…”

Thin-film Solar Cells with MgF2/Ag back mirror patterning for improved near-IR reflectance

“…Producing thin-film solar cells by epitaxial lift-off (ELO) allows the reuse of the wafer lowering the production costs of high-efficiency III-V devices [1]. ELO solar cells further benefit from photon recycling because the lifted thin-films are commonly mounted on a metal foil that works not only as a mechanical support and back electrical contact but also as a mirror to the incident light [2].…”