Wide-Bandgap III-V Materials for Multijunction Solar Cell Applications

My proposal aims to address cost and efficiency issues associated with solar technology for space applications as well as its potential for positive global environmental impacts. The goal of this proposal aligns well with NASA's objectives outlined in the STR TABS for Space Power & Energy Storage (TA03). Three individual but highly complementary research projects are proposed that will culminate in an integration of all three ideas into two demonstrative triple junction solar cells. For space applications, high specific power is a key requirement for power generation. While some types of solar technology, such as Si single junction, organic, and dye-sensitized solar cells, achieve relatively low cost, they are quite limited in efficiency. Multijunction solar technology can achieve a much larger efficiencies and specific power by stacking solar cells consisting of different bandgap materials to collect a broad portion of the solar spectrum. Multijunction cells are typically made from III-V materials, which are quite expensive relative to alternatives, such as Si and organics. To make this technology more competitive for both space and terrestrial applications, we must lower the cost per watt, which can be achieved either by reducing cost or improving efficiency. The first two projects aim to improve efficiency through the introduction of a wide bandgap top junction to better collect the higher energy photons in the solar spectrum. The two materials proposed to study here are AlxIn1-xP (x=0.36-0.42, hereafter referred to as AlInP) with bandgaps near 2.34 eV and InyGa1-yP (y=0.25-0.37) with bandgaps around 2.15 eV. Although AlInP pn-junctions have not been previously studied for photovoltaic applications, this material boasts the largest direct bandgap of the III-arsenide/phosphides, making it a highly attractive top junction candidate. Conversely, InxGa1-xP has already been studied quite extensively and used in space applications, though only for the x=0.49 composition, as it is lattice matched to commercially available GaAs substrates. Metamorphic InxGa1-xP with x=0.36-0.42 remains largely unstudied, as it is not lattice matched to common substrate which necessitates complex graded buffers to achieve these compositions with minimal strain-induced crystalline defects that can harm solar cell performance. Recent advancements in graded buffer growth techniques have enabled investigation of this promising material. These compositions are of particular interest for their larger bandgaps, and direct absorption processes, similar to AlInP. This proposal seeks to study and optimize the growth conditions and device structures of AlInP and metamorphic InGaP solar cells. The third project focuses on reducing cost through the integration of III-V cells onto Si for its low cost and established manufacturing infrastructure. Previous studies on GaAs0.7P0.3 solar cells grown on GaP/Si templates indicate challenges with extended crystalline defect nucleation upon relaxation of GaP on Si. Here we propose to study the evolution of these defects through GaP solar cell growth on both GaP/Si templates and native GaP substrates. The characterization of solar cell performance and material quality will enable a direct comparison between growth on the two substrates to help determine how these defects nucleate and evolve throughout growth. All samples will be grown via molecular beam epitaxy. Devices will be characterized for both solar cell performance as well as material quality, the first of which will be determined with lighted and dark current-voltage and internal quantum efficiency measurements. Microstructural characterization carried out with Hall effect measurements, photoluminescence, electron beam-induced current, and atomic force, Nomarski, scanning, and transmission electron microscopies. More »