Brianna Conrad

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.

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.

Current matched GaAsP/SiGe tandem device on Si over 20% efficiency under indoor measurement

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.

Spectral response of steady-state photoluminescence from GaAs1-xPx layers grown on a SiGe/Si system

Measuring the spectral response of photoluminescence (SRPL) in solar cells has recently attracted attention as it can be used as a contactless relative measure of external quantum efficiency (EQE) prior to full device fabrication. However, this technique requires that the monitored PL spectrum originates mainly from a region in the solar cell with uniformly distributed majority carriers. For a stack of thin films with a similar material composition, the slightly different emission spectrum from each layer may lead to the superposition of several luminescence peaks. This letter presents the measurement of the SRPL from GaAsP tandem solar cells and outlines a method for separating the individual layer contributions. Good agreement between measured SRPL and EQE at short wavelengths has been achieved, and the deviations at longer wavelengths have been analyzed. This study also reveals unexpected bandgap narrowing resulting from a variable material composition within the active region.

Improved GaAsP/SiGe tandem on silicon outdoors and under concentration

Tandem solar cells made from III-V layers grown on silicon have the potential for high efficiencies at relatively low cost. Recent work has produced a GaAsP/SiGe tandem with over 20% efficiency based on indoor measurements. In this work, improvements are made to this device and outdoor performance is reported for the first time. Light trapping and substrate thinning techniques developed on a SiGe single junction are adapted to the demands of tandem cell processing. Devices are fabricated with front metalization optimized for low and high concentration applications, with potential efficiencies over 25%.

Spatially resolved EL and PL coupling of a dual junction solar cell

Tandem solar cell design has been leading the solar cell efficiency table. The high radiative recombination of the III-V materials employed in tandem structure becomes an important aspect to consider on the design stage. It is well known that the reabsorption of such process affects the efficiency of each cell and thus the overall system. A measurement technique of analyzing such effect is presented in this paper, limited to the luminescent coupling in a dual-junction solar cell system. The material presented here provides a unique opportunity to get a deeper understanding on the amount as well as the relation between the two luminescent coupling phenomena.

Performance improvement for epitaxially grown SiGe on Si solar cell using a compositionally graded SiGe base

Silicon germanium (SiGe) is a material with high mobility and relatively low bandgap making it an attractive candidate for the bottom subcell in a III-V tandem solar cell grown on silicon (Si) substrate. This paper reports on the performance improvement of an epitaxially grown SiGe on Si solar cell by growing a higher Ge composition SiGe layer in the base. The purpose of growing a higher Ge composition SiGe layer in the base is to improve the light absorption. The first iteration of this structure was an Si0.18Ge0.82 solar cell fabricated with a 1 μm thick Si0.12Ge0.88 layer in the base. This solar cell had a lower efficiency compared with the reference solar cell without the Si0.12Ge0.88 layer. One of the main reasons for the lower efficiency is believed to be the high threading dislocation density (TDD) caused by the abrupt change of lattice constant between Si0.18Ge0.82 and Si0.12Ge0.88 in the base. In order to reduce the TDD, the second iteration of the structure was fabricated with a compositionally ...

Short ciruit current improvement of SiGe solar cell in a gallium arsenide phosphide - silicon germanium dual junction solar cell on Si substrate

Light trapping capability is analyzed for the SiGe solar cell in the GaAsP-SiGe tandem device. One-dimensional model predicts an achievable Jsc of 17.46 mA/cm2 below the top cell, with the electrical parameter from quantum efficiency fitting. Experimentally, a Jsc of 15.59 mA/cm2 was achieved from the Si.lsGe.s 2 cell, with textured Si02 back surface reflector and a single layer SiN, anti-reflection coating. A step-by-step path to reach the 21 mA/cm2 Jsc is provided.

Optical and electrical analysis of graded buffer layers in III–V/SiGe on silicon tandem solar cells

A graded buffer layer is needed in order to grow III-V/SiGe on silicon substrate because of the lattice mismatch. In this work, optical absorption in the graded buffer layer is quantified by measuring the transmittances. We demonstrate a 7 μm graded buffer layer with Ge composition from 0% to 82% is equivalent to a 2 μm SiGe film with 82% Ge composition in terms of optical absorption. A SiGe solar cell was fabricated and characterized. Quantum efficiency measurements show that around 1 mA/cm2 short circuit current is generated from the graded buffer layer in the fabricated solar cell.