High-efficiency III-V Solar Cells Research Articles

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.

A Brief Review of High Efficiency III-V Solar Cells for Space Application

The demands for space solar cells are continuously increasing with the rapid development of space technologies and complex space missions. The space solar cells are facing more critical challenges than before: higher conversion efficiency and better radiation resistance. Being the main power supply in spacecrafts, III-V multijunction solar cells are the main focus for space application nowadays due to their high efficiency and super radiation resistance. In multijunction solar cell structure, the key to obtaining high crystal quality and increase cell efficiency is satisfying the lattice matching and bandgap matching conditions. New materials and new structures of high efficiency multijunction solar cell structures are continuously coming out with low-cost, lightweight, flexible, and high power-to-mass ratio features in recent years. In addition to the efficiency and other properties, radiation resistance is another sole criterion for space solar cells, therefore the radiation effects of solar cells and the radiation damage mechanism have both been widely studied fields for space solar cells over the last few decades. This review briefly summarized the research progress of III-V multijunction solar cells in recent years. Different types of cell structures, research results and radiation effects of these solar cell structures under different irradiation conditions are presented. Two main solar cell radiation damage evaluation models—the equivalent fluence method and displacement damage dose method—are introduced.

Direct growth of GaAs solar cells on Si substrate via mesoporous Si buffer

Due to Silicon (Si) material abundance and lower cost, integration of high efficiency III-V solar cells on Si substrates is of major importance for future solar energy harvesting devices. In this paper, we report on the growth optimization with a detailed characterization of epitaxial growth of crystalline GaAs on porous silicon layers (PSL), and demonstration of single-junction GaAs solar cell on PSL performances. GaAs deposition is performed on engineered porous Si surfaces with different growth temperatures. One and two-steps growth (TSG) were also investigated. X-ray diffraction demonstrated almost one order of magnitude lower threading dislocation density (TDD) of 2× 108 cm-2 for TSG process of GaAs on PSL compared to the one-step growth. Atomic Force Microscopy and Scanning Electron Microscopy showed that a reduction of growth temperature leads to surface morphology improvement. A single junction GaAs solar cell heterostructure grown by TSG and fabricated atop the porous layer, demonstrated higher open-circuit voltage (Voc) and fill factor (FF) when compared to an identical structure grown on crystalline Si (c-Si).

Close-spaced vapor transport reactor for III-V growth using HCl as the transport agent

Low-cost methods of III-V deposition are an important component of making high-efficiency III-V solar cells cost-competitive for terrestrial applications. Here, we report the design and testing of a close-spaced vapor transport system for the growth of epitaxial GaAs films using solid precursors and HCl as the transport agent. Previous work on a related system demonstrated growth of high-quality GaAs using H2O as the transport agent, but the use of H2O generates oxide-related defects and limits material compatibility. The new design also enables independent handling of source and substrate material. The effect of process conditions on growth rate, surface morphology, and substrate-orientation-dependent growth is discussed. We demonstrate successful doping of Si and Zn with average transport efficiencies of approximately 30% and 20%, respectively, based on secondary-ion-mass-spectrometry data. Room-temperature hall mobilities approached those achieved for GaAs grown by metal-organic vapor phase epitaxy and water-based close-spaced vapor transport, 2210–3400 cm2 V−1 s−1 for n-GaAs and 70–110 cm2 V−1 s−1 for p-GaAs depending on dopant concentration. Initial results on doping and cross-contamination suggest this system should be capable of homoepitaxial growth of p-n junctions.

Defect Formation and Strain Relaxation in graded GaPAs/GaAs, GaNAs/GaAs and GaInNAs/Ge Buffer Systems for high-efficiency Solar Cells

Transmission electron microscopy of cross-section specimens and high-resolution X-ray diffraction analyses have been applied to investigate the formation of defects and the relaxation of layer strain in step-graded GaPxAs1−x and GaNyAs1−y buffer layer systems grown by metal-organic vapour phase epitaxy on GaAs (001) substrates with 6° miscut towards (111)A. The investigations have been complemented by characterization of the layer surfaces employing optical microscopy. The comparison of the different buffer concepts reveals characteristic differences in the formation of defects and in the relaxation of tensile layer strain. For GaPAs layers dislocations and microtwins form, releasing the major part of the tensile misfit strain. In contrast, for GaNAs dislocations and microtwins are largely absent, at least in the upper part of the buffer structure, and microcracks are generated. Consequently, during subsequent growth of layers with tensile strain, strain relaxation and defect formation can be effectively hindered by introducing intermediate GaNyAs1−y layers with concentrations y > 2 % into a GaAs1−xPx buffer structure [1]. A similar concept can be used for layer systems with compressive strain, however, modified by using layers of differing alloy composition. The use of dilute nitride layers appears to offer a new concept for engineering defect distributions and layer strain in lattice-mismatched compound semiconductor layer structures. Such concepts are of particular interest not only but especially also for applications in high-efficiency III-V solar cells.