Chris Ebert

High-efficiency thin-film InGaP/InGaAs/Ge tandem solar cells enabled by controlled spalling technology

In this letter, we demonstrate the effectiveness of the controlled spalling technology for producing high-efficiency (28.7%) thin-film InGaP/(In)GaAs/Ge tandem solar cells. The controlled spalling technique was employed to separate the as-grown solar cell structure from the host Ge wafer followed by its transfer to an arbitrary Si support substrate. The structural and electrical properties of the thin-film tandem cells were examined and compared against those on the original bulk Ge substrate. The comparison of the electrical data suggests the equivalency in cell parameters for both the thin-film (spalled) and bulk (non-spalled) cells, confirming that the controlled spalling technology does maintain the integrity of all layers in such an elaborate solar cell structure.

Analysis of tandem III–V/SiGe devices grown on Si

This paper introduces the modeling developed to assess the potential of a III-V/SiGe tandem device. Demonstration of value will be executed via materials and solar cell device models. III-V top cell candidates are evaluated and a high-value composition is identified. Initial windowless GaAsP solar cells demonstrate a bandgap-voltage offset of 0.58.

Single-junction GaAsP solar cells grown on SiGe graded buffers on Si

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.

Quantification of trap state densities in GaAs heterostructures grown at varying rates using intensity-dependent time resolved photoluminescence

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.

Material and Device Improvement of GaAsP Top Solar Cells for GaAsP/SiGe Tandem Solar Cells Grown on Si Substrates

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 (Woc) of 0.48 and 0.31 V under concentration factors of 1 and 20 ×, 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.

GaInP window layers for GaAsP on SiGe/Si single and dual-junction solar cells

GaAsP solar cells have been grown on Si substrates facilitated by a SiGe graded buffer layer. Here, single-junction p+/n GaAsP and tandem n+/p GaAsP/SiGe solar cells are reported with an interest in improving efficiency by evaluation of the III-V device passivation layers and pathways to their optimization. Solar cells with varying window thicknesses are reported for both structures and assist in directing focus of future research. The GaAsP/SiGe on Si tandem solar cell demonstrates a result towards AM1.5G 20.8% AR-corrected efficiency.

Dual-junction GaAsP/SiGe on silicon tandem solar cells

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.

Double layer antireflection coating and window optimization for GaAsP/SiGe tandem on Si

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.

Analysis of gaas photovoltaic device losses at high MOCVD growth rates

Gallium arsenide material has been deposited via metal organic chemical vapor deposition (MOCVD) at growth rates varying between 14 μm/hr and 56 μm/hr. Photovoltaic device results indicate a 6-7% relative decrease in efficiency between 14 and 56 μm/hr GaAs solar cells, due to a reduction in short-circuit current and open-circuit voltage. By simulating the experimental characterization data, it is established that performance losses are associated with rear surface recombination velocity and Shockley-Read-Hall lifetime. The relative impact of these loss mechanisms will be quantified and conclude with discussions on their mitigation.

Analysis of GaAs solar cells at High MOCVD growth rates

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.