Martin Diaz

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.

Wide Band Gap Gallium Phosphide Solar Cells

Gallium phosphide (GaP), with its wide band gap of 2.26 eV, is a good candidate for the top junction solar cell in a multijunction solar cell system. Here, we design, fabricate, characterize, and analyze GaP solar cells. Liquid phase epitaxy is used to grow the semiconductor layers. Four generations of GaP solar cells are developed and fabricated with each solar cell structure being designed and improved based on the first principles analyses of the predecessor solar cells. Quantum efficiency and current-voltage measurements are used to analyze the solar cell performance and to develop predictive models. We create a GaP solar cell with an efficiency of 2.42% under AM 1.5G one sun illumination.

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.

Improving GaP Solar Cell Performance by Passivating the Surface Using AlxGa1-xP Epi-Layer

A good candidate for the top junction cell in a multi-junction solar cell system is the GaP solar cell because of its proper wide band gap. Here, for the first time, we passivate the front surface of these GaP solar cells with an AlGaP layer. To study the passivation effect of this layer, we design a novel growth procedure via liquid phase epitaxy. X-Ray diffraction results show that the resulting passivation epitaxial layer is of good quality. Integrated quantum efficiency measurements show an 18% increase in current due to the AlGaP. The current-voltage measurements indicate that with this AlGaP surface passivation layer, the GaP solar cells efficiency is 2.90%. This is an improvement over previously reported results for GaP solar cells.

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.

Wide band gap Gallium Phosphide solar cells for multi-junction solar cell system

Gallium Phosphide (GaP) solar cells have been designed, fabricated, characterized and analyzed as candidates for the top junction solar cell in a multi-junction solar cell system. Liquid phase epitaxy (LPE) has been used as the growth method for the epitaxial layers. Open circuit voltage (Voc) of 1.535V has been achieved under one sun illumination from the outdoor test. Quantum efficiency (QE) measurements were used in characterizing our solar cell devices. The QE analysis results show that the high front surface recombination velocity and the low diffusion length in the n-type epi-layer region are the two major limitations for the low Voc and short circuit current density (Jsc). An improved structure has been designed based on our current experimental results.

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.

Quantum efficiency model driven design for wide band gap gallium phosphide solar cells

The wide band gap of GaP (2.26eV) makes it a very good candidate for the top junction solar cell in a multi-junction solar cell system. A wide band gap solar cell can increase the efficiency of the system by absorbing and converting the high energy photons more efficiently. Quantum efficiency (QE) is a powerful tool in analyzing the solar cells performance by identifying the recombination from different regions of a solar cell. In this work, a QE model has been developed in curve fitting the measured QE curve to further analyze the solar cell and improve the solar cell design. With the continuous improved designs based on the QE analysis results, the best reported efficiency of a GaP solar cell has been achieved. This solar cell has an open circuit voltage (Voc) of 1.55V, short circuit current density (Jsc) of 1.97mA/cm2, fill factor (FF) of 79.4% and efficiency of 2.42% compared to the previous best reported efficiency of 1.17%.