Ziggy Pulwin

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.

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.

Effect of Growth Temperature on GaAs Solar Cells at High MOCVD Growth Rates

Increasing epitaxial growth rate is an important path toward III-V solar cell cost reductions; however, photovoltaic device performance has been shown to degrade with increasing growth rate. In this study, gallium arsenide (GaAs) material has been deposited via metal-organic chemical vapor deposition (MOCVD) at growth rates varying between 14 and 60 μm/h. Deep-level transient spectroscopy is utilized to elucidate an exponential rise in EL2 trap density as a function of growth rate when all other growth conditions are held constant. Evidence is provided that this EL2 defect is responsible for limiting the Shockley-Read-Hall (SRH) lifetime of very high growth rate solar cells. The effect of growth temperature on devices at high growth rate is subsequently investigated as a strategy to reduce trap density and improve solar cell performance. From this investigation, EL2 trap density is suppressed, and single-junction on-substrate GaAs solar cells grown at 60 μm/h are reported with 1.01 V 1-sun open-circuit voltage and 23.8% AM1.5G efficiency.

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.

Fast growth rate GaAs and InGaP for MOCVD grown triple junction solar cells

Triple junction solar cells (TJSC) are the highest efficiency solar cells available today and are utilized in space and concentrator photovoltaic terrestrial applications. These cells are manufactured using metalorganic chemical vapor deposition (MOCVD) in large scale commercial reactors. Since TJSC process time is largely driven by the growth of the (In)GaAs middle cell and InGaP top cell, increasing the MOCVD growth rate can reduce the process time and increase reactor throughput. In this paper, we discuss a materials characterization comparison of (In)GaAs and InGaP grown at conventional growth rate and faster growth rates. Our results show similar material characteristics of (In)GaAs grown at ∼ 66% higher growth rate as measured by photoluminescence, x-ray, AFM surface roughness, and background doping and for InGaP grown at ∼ 57% higher growth rate for photoluminescence and sheet resistivity uniformity. These higher growth rates incorporated into a MOCVD growth process can lead to ∼ 20% reduction in process time and lower the cost of TJSC manufacture.

Optimization of tellurium doped InGaAs grown by MOCVD for solar cell tunnel junctions application

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.

Optimization of MOCVD grown MQW structures for triple junction solar cells

Triple junction solar cells (TJSC) are the main components of concentrator photovoltaic (CPV) systems. Metalorganic chemical vapor deposition (MOCVD) is the manufacturing process of choice used to produce state of the art solar cells. An improvement in cell efficiency through improvement in cell design lowers the overall cost of solar electricity and stimulates widespread adoption of solar systems. Recently, the introduction of multiquantum well (MQW) structures into TJSC devices has been shown to improve short circuit currents and improve cell efficiency. Optimization of the MOCVD growth process for MQW layers using In.09Ga.91As wells and GaAs0.9P0.1 barriers is important for best cell performance. Since these layers are strained, the need to strain balance the MQW stack is important to reduce defects causing SRH trap/defect assisted recombination. The use of in-situ wafer curvature measurements allow for evaluation of strain compensation at growth temperature. Growth temperature also plays a role in growth of MQW layers with higher temperatures causing interdiffusion of wells and barriers. Optimum growth of MQW layers show sharp x-ray rocking curves, PL spectra with narrow line width, and TEM images with abrupt well/barrier interfaces. In this work, we discuss MOCVD growth optimization leading to high material quality of the MQW structures with up to 50 periods. Material characterization of optimized MQW structures with x-ray, PL, SIMS and TEM are discussed. To demonstrate the optimized growth conditions, a solar cell was fabricated with a MQW and external quantum efficiency was measured showing the absorption to higher wavelength than bulk InGaAs. Triple junction solar cells with MQWs are thus a proven way to improve cell efficiency and cell currents.

Optimization of InGaAs metamorphic buffers for triple junction solar cells

Triple junction solar cells (TJSC) using metamorphic buffers have been reported with over 40% efficiencies and offer higher theoretical efficiency than solar cells lattice matched to germanium. Metamorphic (MM) buffers composed of step graded InGaAs layers allow for alloys of InGaAs and InGaP to be grown with different band gap energies enabling higher efficiency solar cells. In this paper, we initially discuss MOCVD growth experiments to optimize the material properties of metamorphic InGaAs buffer layers grown at ∼ 14 μm/hour, namely; growth temperature and overshoot layer composition. InGaAs buffer layers were evaluated using x-ray reciprocal space mapping (RSM) to determine strain relaxation of the buffer layers, atomic forces microscopy (AFM) to optimize surface morphology and photoluminescence (PL) to assess material quality. Selected growth conditions were also evaluated by transmission electron microscopy (TEM) and cathodoluminescence (CL). After these layers were optimized for growth temperature and overshoot layer, we also evaluated layer thickness and the number of step grades in the MM buffer using the same characterization techniques. Our results show that growth of the step grade InGaAs layers at 620 °C and using an overshoot later of approximately 12% Indium in InGaAs provide for the optimum surface morphology and strain relaxation of > 90% for a final buffer layer InGaAs composition with ∼ 9% indium mole fraction. Cross section TEM images of the MM buffer also show dislocations starting at the step grade and being terminated at the layer interfaces. No threading dislocations were observed in the uppermost part of the epitaxial layer stack with TEM. CL data on MM buffer structures reveal the presence of crosshatching associated with misfit dislocations and threading dislocations densities were reduced by an order of magnitude to ∼105/cm2 for the optimized growth conditions. Our data also shows that seven layer grades between lattice matched InGaAs and the final layer of the grade provide for the best buffer material characteristics. This materials study demonstrates that MOCVD metamorphic buffers can be produced to enable optimization of the band gaps for the middle and top cells leading to enhanced solar cell performance of triple junction solar cells.

Analysis of high growth rate MOCVD structures by solar cell device measurements

Metal organic chemical vapor deposition (MOCVD) tools are integral to many technologies, including the growth of high-efficiency multijunction III-V solar cells. Veeco MOCVD has recently developed new tool designs that allow increased MOCVD growth rates that could drastically increase solar cell throughput and reduce manufacturing costs. It is important, however, to understand the trade-offs between increased throughput and decreased material quality and device performance. We fabricate multijunction III-V solar cells from materials grown by both standard and fast growth rate techniques. We analyze the open circuit voltage, short circuit current, fill factor, efficiency, and ideality factor of both types of devices. Comparison of these devices reveals that the existing fast growth protocols result in solar cells with similar performance to standard growth cells. The results suggest that increased growth rates can enable higher throughput fabrication of solar cells without significant performance sacrifices.