III-V Solar Cells Research Articles

Retraction: "Water splitting system with a 13.1% solar to hydrogen conversion efficiency using 3-junction III-V solar cells with pulse plated Pt black catalyst"

Retraction The authors hereby retract the above article. This paper reports on a highly efficient system for water splitting using a three-junction solar cell with a platinum black catalyst under simulated sunlight. After this manuscript was accepted, it was found that there was an error in the conditions for electroplating the platinum black catalyst on the solar cell. Specifically, there was an error in the settings for the pulse power supply used when plating platinum black. Although the pulse current, pulse width, and cycle were set correctly, the base current value was the same as the pulse current, which was found in a separate experiment using the same equipment. The authors checked the current output under the conditions described in this manuscript using an oscilloscope, and it was confirmed that it was a direct current, not a pulsed current. The measurement results for hydrogen production under simulated sunlight irradiation presented in the paper were correct, and the conclusions regarding hydrogen production were also correct. However, there was an error in the plating conditions for platinum black described in the paper, and the title and content of the discussion also needed to be revised. Based on the above, the authors reached the consensus that the above paper should be retracted and that the authors will submit it again as a new paper with no mistakes in the Japanese Journal of Applied Physics.

In Situ Smoothing of Facets on Spalled GaAs(100) Substrates during OMVPE Growth of III-V Epilayers, Solar Cells, and Other Devices: The Impact of Surface Impurities/Dopants.

One possible pathway toward reducing the cost of III-V solar cells is to remove them from their growth substrate by spalling fracture, and then reuse the substrate for the growth of multiple cells. Here we consider the growth of III-V cells on spalled GaAs(100) substrates, which typically have faceted surfaces after spalling. To facilitate the growth of high-quality cells, these faceted surfaces should be smoothed prior to cell growth. In this study, we show that these surfaces can be smoothed during organometallic vapor-phase epitaxy growth, but the choice of epilayer material and modification of the various surfaces by impurities/dopants greatly impacts whether or not the surface becomes smooth, and how rapidly the smoothing occurs. Representative examples are presented along with a discussion of the underlying growth processes. Although this work was motivated by solar cell growth, the methods are generally applicable to the growth of any III-V device on a nonplanar substrate.

Metasurface absorber based single junction thin film solar cell exceeding 30% efficiency.

In this article, we report, as per our knowledge, for the first time, a thin film single junction solar cell with a metasurface absorber layer directly incorporated. We have used an interconnected dual inverted split ring resonator pattern in the InAsP absorber layer. The structure eliminated patterns of conventional metals, such as silver, aluminum, and gold, from the active layer, a common drawback in conventional solar absorbers, hindering their direct integration into solar cells. Optical simulation results show a peak ideal short circuit current density of 76.23mA/cm2 for the meta-absorber structure under solar illumination. This current is the highest among previously reported absorbers based on Group IV materials and III-V compounds, overcoming the low solar absorption of such metasurfaces. The final proposed solar cell structure combines this meta-absorber layer with traditional efficiency enhancement methods namely anti-reflecting coating, textured back reflector, and transparent top electrode. This novel single junction structure shows a solar absorption efficiency of 97.86% and a power conversion efficiency of 30.87%, the highest for III-V solar cells. Our device proves the ability of metasurface absorber layers to produce high-efficiency solar cells and is expected to pave the way for integrating novel meta-devices into state-of-the-art photovoltaic devices, aiding the global transition towards clean energy sources.

Gallium nanoparticles as antireflection structures on III-V solar cells

The integration of nanostructures on top of solar cells has been previously demonstrated as an effective method to increase the collection efficiency by coupling to sunlight. In this work, this approach is implemented by using core-shell gallium nanoparticles (Ga-NPs) as functional light scatterers on III-V solar cells, investigating how the Ga-NPs affect their photovoltaic performance. The effect is studied in GaAs and in GaAsSbN superlattice-based solar cells with different bandgaps for a wide range of Ga-NPs sizes. After NP size optimization, average improvements in the short-circuit current around 18 % are obtained in both types of solar cells with these nanostructures. The underlying physical mechanism is studied by performing optical measurements and simulations. It is found that the Ga-NPs can be treated as an antireflection coating due to both their geometry and the refractive index in the cases when their localized surface plasmon resonances do not spectrally overlap with the solar cell absorption range. Lastly, a facile method to attenuate the plasmonic effect of the Ga-NPs it is also shown as a proof of concept. In this method, an annealing treatment at low temperature is performed causing an improvement of the photovoltaic performance. This work demonstrates that Ga-NPs can successfully enhance the efficiency of III-V solar cells in a very controllable manner and paves the way for the design of new functional nanostructures.

(Invited) New Approaches to Photoelectrochemical Cascade Reactions and Protection of Photoelectrodes

A half-century of research has positioned direct photoelectrochemical (PEC) fuel generation as a promising technology that still requires substantial development. Work on the hydrogen evolution reaction (HER) has provided a strong basis for realizing the generation of more complex fuels through the carbon dioxide reduction reaction (CO2RR), but multiple challenges remain. The CO2RR requires larger driving forces and integration of multiple catalysts in order to selectively generate multi-carbonproducts, while still presenting many unsolved issues from HER, such stabilizing photoelectrodes under operation. In this talk I will discuss advances in the integration of multiple catalytic microenvironments in a single photoelectrochemical device, and progress toward the stabilization of photoelectrodes for fuel generation.First, I will highlight the adaptation of three-terminal tandem (3TT) photovoltaic technology to photoelectrochemical applications. In our 3TT devices, a two-junction III-V solar cell device has an additional contact, enabling two unique catalyst sites operating at different voltages under the same illumination. Idealized circuit modeling shows the promise of these 3TT devices compared to traditional two-terminal, two-junction devices, particularly with respect to spectral tolerance. We have adapted the structure of 3TT photovoltaics to function as PEC devices and developed multiple catalysts for integration into the two solution contact sites. We demonstrate progress toward light-driven cascade catalysis for multi-carbon CO2RR products.Second, I will highlight our approaches to protective schemes for photoelectrodes: one which is independent of semiconductor chemistry, and one which is driven by that chemistry. In the first approach, transparent conductive encapsulants (TCEs) are demonstrated as photoabsorber-agnostic protective layers. Unlike many protection schemes, these TCEs can be applied to semiconductors post-processing and without substantial modification, providing facile and robust protection. We have characterized the electrochemical performance of TCEs and demonstrate their integration with multiple semiconductors for the reduction of methyl viologen as a proxy for PEC fuel formation. In the second approach, we use the knowledge generated over fifty years of photoelectrode research to develop a new material, ZnTiN2, which can be directly integrated with established semiconductors and which will degrade under PEC conditions. We leverage this degradation to create protective layers which may be self-healing, and demonstrate rapid refinement of ZnTiN2 optoelectronic properties enabling integration with other semiconductors in tandem configurations.

(Invited) Solar Flow Batteries: Integrated Photoelectrochemical Solar Energy Conversion and Redox Flow Battery Systems

Due to the intermittent nature of sunlight, practical solar energy utilization systems demand both efficient solar energy conversion and inexpensive large scale energy storage. We have developed hybrid solar-charged storage devices called solar flow batteries (SFBs) that integrate photoelectrochemical solar cells with redox flow batteries (RFBs). In these devices, photoexcited carriers collected at the semiconductor-liquid electrolyte interface convert the redox couples to charge up the RFB without external electric bias; which can be discharged to generate the electricity when needed. By matching various silicon solar cells and tandem III-V solar cells (Chem 2018, 4, 2644) with various organic redox couples and optimizing the SFB device design, we improved their round trip solar-to-output electricity efficiency (SOEE) and eventually achieved a record SOEE of 20% with a 500-hour lifetime using high performance tandem perovskite/silicon solar cells (Nature Mater. 2020, 19, 1326). The design principles and the quantitative analysis model for voltage matching solar cells with RFBs (Acc. Chem. Res. 2020, 53, 2611) light up the pathways for achieving even better performance and stability yet maintaining a low cost, which would make these SFBs practical for standalone solar energy conversion and storage systems in remote off-grid locations.

The Intermediate Connection of Subcells in Si-based Tandem Solar Cells.

Tandem solar cells are rationally designed and fabricated by stacking multiple subcells to achieve power conversion efficiency well above the Shockley-Queisser (SQ) limit. There is a large selection pool for the subcell candidates, such as Si, III-V, Kesterite, Perovskite, and organic solar cells. A series of different combinations of these subcells have been successfully demonstrated in practical tandem solar cell devices. However, there has not been a systematic summary of how to connect subcells in a tandem solar cell using a practical, cost-effective, and efficiency-beneficial fashion. In this work, the connection manners of subcells within a tandem cell are classified into three main categories, performing sequential growth, using the physical connection, and applying an intermediate layer, focusing on systematical description of intermediate layers using different materials. The advantages and disadvantages of these connection methods and their applicability to tandem cell types are further elaborated using two typical example models, III-V/Si and Perovskite inclusive tandem cell devices. Eventually, this work can provide useful guidance on how to carry out a suitable intermediate connection in the design of tandem solar cells depending on the selected subcells and device structure.

Pulsed laser ejection of single-crystalline III-V solar cells from GaAs substrates

The best III-V solar cells start out as single-crystalline multilayers on GaAs substrates. Separating these multilayers from their growth substrate enables higher performance and wafer reuse, which are both critical for terrestrial III-V solar cell viability. Here, we remove rigidly bonded, 16 mm2 × 3.5 μm thick devices from a GaAs substrate using an unfocused Nd:YAG laser pulse. The pulse is absorbed by a low-band-gap, lattice-matched layer below the device, driving an ablation event that ejected the crystalline multilayer from the substrate. Minutes of selective wet-chemical etching and device finishing yield a 0.1 cm2 device with a 17.4% power conversion efficiency and open-circuit voltage of 1.07 V, using AM1.5 direct with no anti-reflection coating. We show that the performance is comparable to similar cells produced via conventional processes. We discuss unique process characteristics, such as the potential to separate wafer-sized solar cells per laser pulse.

Influence of outdoor conditions on PV module performance – an overview

The utilization of renewable sources of energy is of significant interest today. This is particularly the case due to the growing interest in addressing global warming, carbon footprint and the associated challenges for the environment. In this context, the enhanced use of solar panels is relevant and timely. With a view to understand and appreciate the fundamentals of the workings of the solar panels and the influence of the outdoor weather-related parameters on their operational characteristics, a study is presented in this paper. A detailed procedure for performance measurement of PV modules in outdoor conditions is reported. Improvement in the precision of outdoor performance measurements of photovoltaic (PV) modules is investigated for a wide range of outdoor conditions. A comparative performance evaluation of the currently available PV modules under the influence of humidity, irradiance and particle radiation is presented. PV parameters show strong dependence on these outdoor conditions. The instability in solar cell modules when reacting with water or under high humidity inhibits the high performance of solar cell modules. Irradiation results depict that the silicon-based PV modules show a decreasing trend of power conversion efficiency with increasing solar irradiance. The efficiency increases with increased solar irradiance for CdTe, GaAs and CIGS solar cells in the irradiance range of 200 to 1000 W•m-2. Tandem and multi-junction solar cells exhibit a high-power conversion efficiency when the solar irradiance increases from 0 - 70 suns. Perovskite solar cells have better particle radiation tolerance than silicon, III-V and CIGS solar cells. The shading problem is discussed briefly for solar cell modules. This study is aimed to provide valuable and comparable information on the degradation performance of solar cells as function of humidity, irradiance and particle radiation, and serves as the basis for future development.

Ultra-High-Speed Growth of GaAs Solar Cells by Triple-Chamber Hydride Vapor Phase Epitaxy

In photovoltaic (PV) power generation, highly efficient III-V solar cells are promising for emerging mobile applications, such as vehicle-integrated PVs. Although hydride vapor phase epitaxy (HVPE) has received attention due to its lower fabrication costs, realization of high throughput performance while maintaining solar-cell characteristics using this growth method is essential. In this study, the effect of atmospheric-pressure triple-chamber HVPE growth conditions on GaAs solar-cell properties were carefully investigated in conjunction with defect analysis using deep-level transient spectroscopy (DLTS). Based on the analysis on GaAs reaction processes, the suppression of arsine thermal cracking in the HVPE hot-wall reactor was important to achieve fast GaAs growth using a low input V/III ratio. Moreover, the DLTS results revealed that the reduced input V/III ratio was effective in suppressing the generation of EL2 traps, which is a common GaAs midgap complex defect involving arsenic antisites. Although the EL2 trap density increased with the growth rate, the performance of GaAs solar cells that were grown under reduced arsine thermal cracking exhibited almost no considerable cell parameter deterioration at a growth rate of up to 297 μm/h. Consequently, a conversion efficiency of 24% with a high open-circuit voltage of 1.04 V was achieved for the cells that were grown at 200 μm/h.

CuI as a Hole-Selective Contact for GaAs Solar Cells

Carrier-selective contacts have emerged as a promising architecture for solar cell fabrication. In this report, the first hole-selective III-V semiconductor solar cell is demonstrated using copper iodide (CuI) on i-GaAs. Surface passivation quality of GaAs is found to be essential for open-circuit voltage (VOC), with good correlation between photoluminescence properties of the GaAs layer and the VOC. Passivation with <10 nm thick In0.49Ga0.51P layers is shown to provide an over 300 mV improvement. Oxygen-rich CuI is formed by natural oxidation in the atmosphere, and the increased oxygen content of ∼10% is validated by energy-dispersive X-ray measurements. The oxygen incorporation is shown to improve hole selectivity and thus solar conversion efficiency. Ultraviolet photoelectron spectroscopy indicates a high work function of ∼6 eV for the oxygen-rich CuI. With optimized GaAs surface passivation and oxygen-rich CuI, a VOC of nearly 1 V and a solar conversion efficiency of 13.4% are achieved. The solar cell structure includes only undoped GaAs, a surface passivation layer, and non-epitaxial CuI contact and is therefore very promising to various low-cost crystal growth methods. The results have a significant impact on III-V solar cell fabrication and costs as it (i) enables fully carrier-selective architectures, (ii) reduces cell fabrication complexity, and (iii) is suitable for layers grown by low-cost crystal growth techniques.

III-V SOLAR CELLS

In this review article solar cells based on III-V materials are addressed, starting by a brief description of their operation principle, including key materials’ issues. Subsequently, the different types of III-V solar cells are presented, together with their state-of-the-art performance. Various approaches to reduce their costs are then discussed, and an outlook of the research in this field concludes the paper.

Revealing fundamentals of charge extraction in photovoltaic devices through potentiostatic photoluminescence imaging

Revealing fundamentals of charge extraction in photovoltaic devices through potentiostatic photoluminescence imaging

Carrier Density and Thickness Optimization of InxGa1-xN Layer by Scaps-1D Simulation for High Efficiency III-V Solar CelL

In this study, the indium gallium nitride (InxGa1-xN) p-n junction solar cells were optimized to achieve the highest conversion efficiency. The InxGa1-xN p-n junction solar cells with the whole indium mole fraction (0 £ x £ 1) were simulated using SCAPS-1D software. Optimization of the p- and n-InxGa1-xN layer's thickness and carrier density were also carried out. The thickness and carrier density of each layer was varied from 0.01 to 1.50 µm and 1015 to 1020 cm-3. The simulation results showed that the highest conversion efficiency of 23.11% was achieved with x = 0.6. The thickness (carrier density) of the p- and n-layers for this In0.6Ga0.4N p-n junction solar cell are 0.01 (1020) and 1.50 μm (1019 cm-3), respectively. Simulation results also showed that the conversion efficiency is more sensitive to the variations of layer's thickness and carrier density of the top p-InxGa1-xN layer than the bottom n-InxGa1-xN layer. Besides that, the results also demonstrated that thinner p-InxGa1-xN layer with higher carrier density offers better conversion efficiency.

Heterosubstrate Illumination Effects in III-V Solar Cells.

Reference cell based current vs voltage (IV) measurements assume that the effect of an illumination spectrum on a solar cell's performance can be fully captured by the multiplication of the spectrum with the device's spectral response and subsequent integration. This is based on a fundamental understanding that sub-bandgap light will be minimally absorbed, if at all, in the active layers of the solar cell and therefore not contribute to the power generation. In this work we show a novel phenomenon in which illumination of the substrate is required for good performance in solar cells with III-V active layers and germanium substrates, despite negligible contribution to the short circuit current or open circuit voltage, or increase of generated power beyond that expected from the III-V junction. Discovered in the course of characterizing cells for low-light conditions, we confirm the observation with additional IV and electroluminescence measurements and modeling that reproduces experimental results. This phenomenon has implications for device characterization under non-standard light sources, the development of solar cells for conditions lacking long-wavelength light such as indoor photovoltaics under light-emitting diode illumination, and the prediction of device performance under spectra that differ from the test conditions.

Modeling free-carrier absorption in ultrathin III-V solar cells with light management.

Ultrathin III-V solar cells with proper light management have become more attractive than their optically thick counterparts as they are less expensive and lightweight, can maintain photon absorption, and have high radiation tolerance for space-related applications. Comprehensive optical modeling efforts have provided pathways to improve device efficiency in ultrathin GaAs solar cells with light trapping structures. Usually, the absorption mechanism known as free-carrier absorption (FCA) is ignored in these models due to the ultrathin layers and the direct bandgap of GaAs. This manuscript reports the significance of considering FCA as a parasitic loss caused by the optical enhancement in highly doped non-active layers between the ultrathin solar cell and backside light trapping structures. We model FCA based on Drude theory in a p-type AlGaAs layer behind ultrathin GaAs solar cells with a planar mirror and cylindrical gratings. Our results show that, depending on the AlGaAs thickness and doping concentration, free carriers will absorb transmitted photons and reduce the backside reflectance, degrading the current and voltage output from ideal conditions. One example shows that for a 300 nm-thick GaAs solar cell, the Ag mirror's peak reflectance decreases nearly 12% when the AlGaAs back layer is 800 nm-thick at a doping concentration of 4x1019 cm-3. Notably, the cylindrical grating designs with 38.5%, 46.5%, and 64.9% AlGaAs coverage resulted in an absolute efficiency reduction of 0.6%, 1.8%, and 2.9% at a doping concentration of 4x1019 cm-3, respectively. This novel study demonstrates that FCA in non-active layers must be properly addressed in the device design to progress the efficiency of ultrathin III-V solar cells with light trapping structures.

Flexible multijunction solar cells embedded inside smart dust modules for outdoor applications to Smart Grids

The functioning of self-sustaining smart dust modules plays a vital role in the development of the smart electric grid. In this work, we first devise a flexible triple-junction III-V solar cell embedded inside a smart dust module suitable for outdoor applications. These flexible solar cells are demonstrated to have a bending radius of over 5 cm and exhibit a conversion efficiency of around 25% under air mass 1.5G (1 sun) conditions. Under normal incidence of sunlight at the same conditions, our cell with its small active area of 0.4 cm2 can generate around 10.1 mW of electrical power. This is sufficient to meet the direct current power consumption (∼5.4 mW) requirements of our dust module, which includes a temperature/moisture sensor, a 3-axis linear accelerometer, and a Bluetooth chip. In comparison to a silicon-based flexible solar cell, our demonstrated III-V solar cell is smaller, requiring approximately 40% of the area to produce the same electrical power output. Our dust module is controlled using a self-developed Android application installed on a smartphone and can sustain continuous-wave operation for data collection and wireless transmission even when the incident angle of the sunlight reaches 75°. Pulse-mode operation is still possible even in the case of nearly 90° illuminations (for example, at sunset). Overall, the measurement results for this flexible solar cell are promising, allowing for further reduction in the size of a self-sustaining smart dust module with improved reliability. These advantages could facilitate development of the next generation of energy saving smart electric grids.

Cross-sector storage and modeling needed for deep decarbonization

Noah Kittner is an assistant professor in energy in the Department of Environmental Sciences and Engineering, Gillings School of Global Public Health and in the Environment, Ecology, and Energy Program at the University of North Carolina at Chapel Hill. His research examines the role of energy storage in the transition to low-carbon energy systems and deep decarbonization. Sergio Castellanos is an assistant professor in the Department of Civil, Architectural and Environmental Engineering at the University of Texas at Austin, where he leads the Rapid, Equitable & Sustainable Energy Transitions (RESET) Lab focusing on equitable pathways for decarbonized energy systems. Patricia Hidalgo-Gonzalez is an assistant professor in the Jacobs School of Engineering and an affiliate member in the Center for Energy Research at the University of California, San Diego. She is the director of the Renewable Energy and Advanced Mathematics Laboratory. She is one of the academic co-leads of the IEEE Power & Energy Society Task Force "Data-Driven Controls for Distributed Systems." Her research focuses on high penetration of renewable energy using optimization, control theory, and machine learning with safety guarantees. Daniel M Kammen is the James and Katherine Lau Distinguished Chair in Sustainability, and Chair of the Energy and Resources Group, Professor in the Goldman School of Public Policy, and Professor of Nuclear Engineering at the University of California, Berkeley. He has served as Chief Technical Specialist for Renewable Energy and Energy Efficiency at the World Bank, and Science Envoy in the US Department of State. His work is focused on the science and policy of decarbonization at scales from homes to villages to large utility-scale systems. He has been a Coordinating Lead Author for the IPCC since 1999. Sarah Kurtz is a Professor at the University of California Merced. She spent more than 30 years at the National Renewable Energy Laboratory studying multi-junction III-V solar cells, reliability of photovoltaic modules and systems, and other solar-related projects. She is now leading a study of the value of long-duration storage, funded by the California Energy Commission.

Engineering the reciprocal space for ultrathin GaAs solar cells

III-V solar cells dominate the high efficiency charts, but with significantly higher cost than other solar cells. Ultrathin III-V solar cells can exhibit lower production costs and immunity to short carrier diffusion lengths caused by radiation damage, dislocations, or native defects. Nevertheless, solving the incomplete optical absorption of sub-micron layers presents a challenge for light-trapping structures. Simple photonic crystals have high diffractive efficiencies, which are excellent for narrow-band applications. Random structures offer a broadband response instead but suffer from low diffraction efficiencies. Quasirandom (hyperuniform) structures lie in between, providing high diffractive efficiency over a target wavelength range, which is broader than simple photonic crystals, but narrower than a random structure. In this work, we present a design method to evolve a simple photonic crystal into a quasirandom structure by modifying the spatial-Fourier space in a controlled manner. We apply these structures to an ultrathin GaAs solar cell of only 100 nm. We predict a photocurrent for the tested quasirandom structure of 25.3 mA/cm2, while a planar structure would be limited to 16.1 mA/cm2. The modified spatial-Fourier space in the quasirandom structure increases the amount of resonances, with a progression from discrete number of peaks to a continuum in the absorption. The enhancement in photocurrent is stable under angle variations because of this continuum. We also explore the robustness against changes in the real-space distribution of the quasirandom structures using different numerical seeds, simulating variations in a self-assembly method. We observe a standard deviation of the quasirandom structures of only 0.3 mA/cm2. The approach presented here can be applied to guide future optimizations and experimental design and fabrication of new classes of photonic crystals for GaAs and other ultrathin solar cells.

Morphological study of polycrystalline indium phosphide using metalorganic chemical vapor deposition

III–V semiconductor solar cells have demonstrated the highest power conversion efficiencies to date. However, the cost of III-V solar cells has historically been too high to be practical outside of specialty applications. This stems from the cost of raw materials, need for a lattice-matched substrate for single-crystal growth, and complex epitaxial growth processes. To address these challenges, here, we explore the direct non-epitaxial growth of thin poly-crystalline films of III-Vs on metal substrates by using metalorganic chemical vapor deposition. This method minimizes the amount of raw material used while utilizing a low-cost substrate. Specifically, we focus on InP which is known to have a low surface recombination velocity of carriers, thereby, making it an ideal candidate for efficient poly-crystalline cells where surface/interface properties at the grain boundaries are critical. The grown InP films are 1-3 lm thick and are composed of micron-sized grains that generally extend from the surface to the Mo substrate. They exhibit similar photoluminescence peak widths and