Epitaxial Lift-off Solar Cells Research Articles

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

AbstractA thin, lightweight, flexible solar cell is developed that maximizes the power‐to‐mass ratio under AM0 illumination and has a competitive efficiency after typical high energy electron irradiation. The inverted metamorphic triple junction (IMM3J) solar cells with Ga0.51In0.49P/GaAs/Ga0.73In0.27As subcells are grown on GaAs substrates and have a total epitaxy thickness of about 10 μm. After epitaxial growth, the inverted layer stack is metallized, with the metal serving as back‐contact, back reflector and support layer for the ultra‐thin solar cells before the GaAs substrate is separated by an epitaxial lift‐off (ELO) process. The nondestructive nature of the ELO process makes multiple reuses of the GaAs substrate possible. The solar cell structure is optimized for maximum EOL efficiency, that is, after 1‐MeV electron irradiation with a fluence of 1 × 1015 cm−2, by means of simulations that include the irradiation induced defects in the various semiconductor alloys. Assuming realistic charge carrier lifetime in the materials, we predict a near‐term efficiency potential for the IMM3J ELO of 30.9% under AM0 illumination before and 26.7% after irradiation. Several IMM3J ELO solar cells with an area of approximately 20 cm2 from different development stages were tested under AM0 illumination. The newest solar cells (generation III) with a mass density of only 13.2 mg/cm2 reach conversion efficiencies of 30.2% at 25°C. The resulting power‐to‐mass ratio of 3.0 W/g for the bare solar cell is one of the highest published ratios. After irradiation, a conversion efficiency of 25.4% was measured for “generation II” devices under AM0 illumination, which corresponds to a power‐to‐mass ratio of 2.6 W/g. IMM3J ELO solar cells from “generation I” were also tested for mechanical stability as “de‐risking” test of this new cell technology. No degradation of the cell performance was found after dipping the cell in liquid N2 and then heating up to 25°C for five times, despite of strong deformation of the flexible cell during the temperature cycle.

Omnidirectional Current Enhancement From Laminated Moth-Eye Textured Polymer Packaging for Large-Area, Flexible III-V Solar Modules

Epitaxial lift-off (ELO) processes have allowed for cheaper development of mechanically flexible, ultra-thin, and high-efficiency III-V solar cells. ELO solar cells are natural candidates for applications where solar cells must conform to curved surfaces and provide high efficiency and high specific power generation (W/kg). Such examples include power generation for unmanned aerial vehicles, electric vehicles, and portable electrical power. However, when considering these mobile solar applications, large variations in angle of incidence (AOI) that inevitably occur can greatly decrease overall system efficiency due to significant Fresnel reflections. In this article, we demonstrate the integration of moth-eye antireflection nanostructures on the polymer packaging layer of ELO solar cell arrays using a low-cost, colloidal self-assembly process. The moth-eye structures mitigate Fresnel reflections and increase photocurrent generation over all measured angles of incidence relative to ELO solar cell arrays with traditional untextured polymer packaging. The nanostructures survive a commercial lamination procedure, an important criterion that must be met to ensure the feasibility of integration into commercial processing. Outdoor solar characterization measurements are performed and, under direct optical illumination, moth-eye textured solar cell arrays show a maximum I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sc</sub> enhancement of ~58% at 79° AOI relative to traditional untextured polymer packaged solar cell arrays, and when exposed to both direct and diffuse optical illumination a maximum I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sc</sub> enhancement of ~23% at 79° AOI is observed.

Effects of copper diffusion in gallium arsenide solar cells for space applications

High efficiency, thin-film Epitaxial Lift-Off (ELO) III–V solar cells offer excellent characteristics for implementation in flexible solar panels for space applications. However, the current thin-film ELO solar cell design generally includes a copper handling and support foil. Copper diffusion has a potentially detrimental effect on the device performance and the challenging environment provided by space (high temperatures, electron and proton irradiation) might induce diffusion. It is shown that heat treatments induce copper diffusion. The open-circuit voltage (Voc) is the most affected solar cell parameter. The decrease in Voc can be explained by enhanced non-radiative recombination via Cu trap levels in the middle of the band gap. The decrease in Voc is found to be dependent on junction depth. In all Cu cells annealed at T≥300°C signs of Cu diffusion are present, which implies that a barrier layer inhibiting Cu diffusion is necessary. Electron radiation damage was found to have no influence on Cu diffusion.