GaAsP/SiGe dual-junction solar cells have been epitaxially grown on silicon substrates which have the potential of achieving 1-sun tandem efficiencies of 40%. With the addition of light trapping this lattice-matched two-terminal structure can be current-matched and facilitates high performance from the III–V top cell while maintaining the cost advantages of silicon solar cells. The SiGe graded buffer allows for lattice matching of the top and bottom cell while providing a low dislocation interface between the silicon substrate and the device layers. This two-terminal dual-junction structure design is presented and demonstrates a 10.4% relative improvement in JSC and a 1.7% absolute improvement in efficiency over previous best devices. These initial structures have reached an efficiency of 18.9% under 1-sun. The devices suffer from high series resistance and exhibit reduced fill factors. A near term pathway to efficiencies approaching 25% is described.

Quaternary materials lattice-matched to InP have been examined using variable angle spectroscopic ellipsometry (VASE). The optical constants of InGaAsP and InAlGaAs, lattice matched to InP with bandgaps of 1.2eV, have been determined. The native oxide layer of the materials was simulated using a weighted Bruggeman effective medium approximation (EMA) approach comprising the known optical constants of the native oxides of the end-point materials. The extracted optical constants from VASE measurements are compared with interpolated values from literature data, in order to assess the effectiveness of such an approach for quaternary alloys. Additionally, the absorption coefficients of InGaAsP and InAlGaAs are compared, the results of which can be used to evaluate the effectiveness of both material systems in high efficiency multijunction (MJ) photovoltaic (PV) devices.

Transfer-printing is a key enabling technology for the realization of ultra-high-efficiency, mechanically stacked III–V solar cells with low cost. In this paper, we present the latest results for microscale CPV cells grown on GaAs and InP substrates for ultra-high-efficiency, four-terminal, mechanically stacked architectures. We describe the latest findings from a combination of modeling, growth, characterization and processing of tunnel junctions, single junction and multijunction solar cells, with the ultimate goal of using transfer-printing to produce the first solar cell with 50% conversion efficiency.

Inverted metamorphic multijunction solar cells have shown high solar conversion efficiencies and utilized InGaP based metamorphic (MM) buffers to change the lattice constant using compositional graded buffer layers while minimizing dislocation density in the final material layers. In this study, optimization of InGaP metamorphic buffers was done by systematically exploring key metalorganic vapor phase epitaxy (MOVPE) growth conditions and the MM buffer epitaxial stack structure. To optimize MOVPE growth parameters, growth temperature and V/III ratio were varied during the growth of a standard MM buffer test structure and the final InGaP buffer layer was characterized by photoluminescence, X-ray reciprocal space maps, atomic force microscope, cathodoluminesence, and ex situ bow measurements. The in situ measurement of wafer curvature was also monitored during MM buffer layer growth. Evaluation of material characterization data provided optimized growth conditions for the InGaP based MM buffer. The second part of this study evaluated the actual layer thickness and number of compositional graded steps in a MM buffer. Our results showed that in situ deflectometer measurements of the wafer curvature of the MM buffer layer can be correlated to ex situ determined strain relaxation of the final buffer layer of the MM buffer. Process optimization tests showed a growth temperature of 580°C with a V/III ratio of 37 provided for the best surface roughness, highest PL intensity and also allowed for low dislocation defect density of the final buffer layer. Using the optimized growth conditions, further optimization of the step grade layers showed that a 350nm thick grade layer for a six step layer MM buffer for a final buffer composition targeted for In0.8Ga0.2P provided the best surface roughness and 100% final buffer relaxation.

Transfer-printing is an important, commercial technology for manufacturing state of the art CPV modules, and has emerged recently as a key enabling technology for the realization of ultra-high-efficiency, mechanically stacked III-V solar cells with low cost. This paper presents the latest results for microscale CPV cells grown on GaAs, InP and GaSb substrates for ultra-high-efficiency, four-terminal, mechanically stacked architectures. The latest findings from a combination of modeling, growth, processing and characterization of single and multijunction solar cells are described, and the roadmap to the long-term goal of using transfer-printing to produce the first solar cell with 50% conversion efficiency is outlined.

With its wide bandgap and good diode performance, GaAsP is an excellent candidate for the top cell in a silicon-based multijunction tandem device. Even though the material is not lattice matched to silicon, inclusion of a graded SiGe buffer between the GaAsP layer and the Si substrate has previously been demonstrated to enable lattice matching. The SiGe layer may then serve as a high-quality current-matched bottom cell to form a tandem dual-junction structure. This paper describes the design, fabrication, analysis, and improvement of the GaAsP top solar cell in a three-terminal GaAsP/SiGe tandem solar cell on a silicon substrate. Uncertified GaAsP top cell efficiencies have been improved from 8.4% to 18.4% with bandgap voltage offsets (W-oc) of 0.48 and 0.31 V under concentration factors of 1 and 20 x, respectively. This progress is made by improved III-V material quality, reduced series resistance, and an addition of antireflection coating. Improving the optics, material quality, and fill factor (FF) should further improve the efficiency of the GaAsP top cell in this tandem structure grown on an Si substrate.

GaAsP solar cells have been grown on Si substrates facilitated by a SiGe graded buffer layer. Materials-based characterization demonstrates threading dislocation densities (TDD) as low as 8.0×106cm-2 via cathodoluminescence in III–V layers from dual-junction solar cells. The difference in material quality and device performance between lattice-match conditions at room temperature and growth temperature are quantified. These improvements are primarily realized through the use of an in-situ optical stress sensor in order to evaluate lattice-mismatch during MOCVD growth. Thus, due to improved material quality, window layer design, and contact resistance, we have achieved GaAsP/SiGe tandem performance with an AM1.5G open-circuit voltage of 1.458V, a top subcell external quantum efficiency-extracted short-circuit current density of 13.8mA/cm2 (no AR), and a fill factor of 82.8%.

Lattice matched and current matched GaAsP/SiGe tandem solar cell on Si has the potential of 40% efficiency. This paper describes our design, fabrication and improvement of this tandem solar cell. This tandem device has achieved efficiencies of 20.6% and 20.2% under 1X and 2.2X, respectively. Current matching between top cell and bottom cell is realized by manipulating the bottom cell active area and lamp spectrum during JV measurements. Improving the optics and cell structure should lead to current matching in this tandem device and produce an efficiency over 25% and over 30% under 1X and 20X standard AM1.5G, respectively.

Single-junction GaAs-based solar cell structures are grown by metalorganic chemical vapor deposition system at the growth rates of 14 μm/hr and 56 μm/hr. The X-ray diffraction study reveals that the crystal quality of the structures with varying the growth rates is comparable. From the external quantum efficiency spectra, it is observed that different behaviors exist in the short wavelengths (<; 450 nm) and the long wavelengths (> 500 nm) as the growth rate increases. The short-circuit current densities of the standard and fast grown cells are comparable. However, the open-circuit voltage of the fast grown cell is lower by above 40 mV as a result of the reduced minority carrier lifetime in the base layer, which is estimated by PC1D simulation.

GaAsP/SiGe dual-junction solar cells have been grown on silicon substrates which have the potential of achieving tandem efficiencies of 40%. This lattice-matched structure facilitates high performance from the III-V top cell while maintaining the cost advantages of silicon solar cells. The SiGe graded buffer allows for lattice matching of the top and bottom cell while providing a low dislocation interface between the silicon substrate and the device layers. Initial structures have reached an efficiency of 18.9%. Near term improvements to 25.0% under AM1.5G will be described.

Single junction GaAs solar cells grown by MOCVD are fabricated over a range of growth rates targeting up to 56 μm/hr in order to evaluate the effect on photovoltaic device performance. MOCVD recipe conditions are provided. Dopant incorporation efficiency is found to increase at high growth rates, potentially due to reduced Zn desorption as the time required to deposit a monolayer of GaAs is reduced. Device results are characterized by light and dark-IV as well as external quantum efficiency and verified against bulk minority carrier lifetime data from time-resolved photoluminescence. High growth rate solar cells degrade less than 4% relative to baseline devices with Voc and Jsc losses of 1% and 3%, respectively. The comparison suggests that both bulk Shockley Read Hall (SRH) lifetime and surface recombination velocity (SRV) are affected by growth rate and contribute to a reduction in performance.

A double layer ARC for a GaAsP/SiGe tandem cell on Si is designed with a transfer matrix model. The importance of considering window thickness and material to be variable parameters in both design optimization and robustness investigation is demonstrated. In this process, optical constants of GaAs.84P.16, Ga.59In.41P, and Al.65In.35P are measured and used to estimate non-zero collection probability in the window layer. Experimental deposition of the ARC verifies the model and achieves a Spectral Weighted Reflectance of 1.9 %. Further modeling will better define the collection probability and suggest additional strategies for device efficiency improvement.

InP based solar cells utilize tunnel junctions consisting of highly doped n and p type InGaAs layers. Tellurium doped bulk InGaAs was grown by MOCVD on InP substrates and optimized for highest doping level as a function of MOCVD growth conditions. In addition the material was optimized for surface morphology and crystal quality. Temperature, V-III ratio, and strain growth parameters have been explored based on constant Te flux.

This work discusses Hall effect measurements by the van der Pauw (vdP) method of thin gallium arsenide phosphide (GaAs.84P.16) grown on silicon (Si) substrates with graded silicon germanium (SiyGe1-y) working as a buffer layer. The material characterized is used as the top cell of a recently demonstrated III-V/SiGe on Si tandem cell. We report for the first time data of the Hall mobility of the material obtained through measurement of samples with different doping levels and thicknesses and at various temperatures. This data is then used for predicting device performance using a 1-D diode model. The technique can be applied as a material screening test to ensure optimal device performance.

Time-resolved photoluminescence is an established technique for characterizing carrier lifetimes in semiconductors, but the dependence of lifetime on excitation fluence has been only qualitatively investigated. We develop a quantitative approach for fitting fluence-dependent PL decay data to a Shockely-Read-Hall model of carrier recombination in order to extract the trap state density. We demonstrate this approach by investigating growth rate-dependent trap densities in gallium arsenide-indium gallium phosphide double heterostructures. The techniques developed here can be applied for rapid, non-destructive quantification of trap state densities in a variety of materials.

We have investigated the microstructure and device characteristics of GaAs0.82P0.18 solar cells grown on Si0.20Ge0.80/Si graded buffers. Anti-phase domains (APDs) were largely self-annihilated within the In0.39Ga0.61P initiation layer although a low density of APDs was found to propagate to the surface. A combination of techniques was used to show that the GaAs0.82P0.18 cells have a threading dislocation density of 1.2 ± 0.2 × 107 cm−2. Despite these extended defects, the devices exhibited high open-circuit voltages of 1.10–1.12 V. These results indicate that cascading a GaAs0.82P0.18 top cell with a lower-bandgap Si0.20Ge0.80 cell is a promising approach for high-efficiency dual-junction devices on low-cost Si substrates.