III-V Multijunction Solar Cells Research Articles

Sequential fabrication of multiple Ge nanomembranes from a single wafer: Towards sustainable recycling of Ge substrates

Germanium (Ge) substrates are employed for III-V multijunction solar cells (MJSCs). However, due to limited availability and environmental impact, Ge is a critical raw material. Detachable Ge nanomembranes (NMs) are gaining attention to reduce Ge consumption through substrate reuse. The Porous germanium Efficient Epitaxial LayEr Release (PEELER) process enables Ge NM detachment and substrate reuse, reducing costs and Ge material consumption. In this work, we demonstrate the sequential production of three NMs from one parent substrate using the PEELER process, yielding monocrystalline NMs, with a surface suitable for further epitaxial growth of III-V layers (RMS < 3.2 nm). By avoiding the costly Chemical Mechanical Polishing (CMP), our approach enables a more economical and sustainable production of high-efficiency Ge-based solar cells, compared to the conventional production process.

Electric Orbit Raising Radiation-Induced Coverglass Darkening and Its Impact on III-V Multijunction Solar Cell Performance

Electric Orbit Raising Radiation-Induced Coverglass Darkening and Its Impact on III-V Multijunction Solar Cell Performance

Multi-junction solar cell measurements at ultra-high irradiances for different temperatures and spectra

This paper reports experimental results on the impact of the temperature and spectral conditions on the behaviour of III-V multi-junction solar cells, employed in concentrator photovoltaic modules, operating under ultra-high concentration (UHC) conditions. The electrical parameters of the I–V curves of a 3-junction solar cell were measured using an experimental set-up based on a multi-flash indoor solar simulator, which allows adjustment of the cell operating temperature up to 85°C and reaching irradiance concentration levels up to 2200 suns under controlled spectral conditions. The measurements presented are unprecedented, as there are no previous publications that combine different spectral and temperature-controlled experimental conditions in the UHC range, thus extending the range of studies reported in works carried out at lower concentrations.

Triple junction solar cell with Cu In1-x Gex Se as absorbents layers

The design of multi-junction solar cells is guided by both the theoretical optimum bandgap combinations as well as the realistic limitations to materials with these bandgaps. Nowadays, triple-junction III-V multi-junction solar cells are commonly used as GaAs, InGaAs; InGaP ... In this work, we are interested in studying triple junctions based on thin-film solar cells Cu(In1-xGax) Se2, CuInSe2, and CuGaSe2 quaternaries using Silvaco ATLAS software. Incorporating Cu(In0.34Ga0.66) Se2 as an absorber in the middle sub-cell increased the open-circuit voltage by 0.72 V. The highest cell efficiency is 20.89 % (Voc = 2.33 V, Jsc = 9.97 mA/cm2, FF = 89.62%). This triple-junction solar cell demonstrates the potential and limitations of future improvements when voltage and current are considered.

Low-Cost Passivated Al Front Contacts for III-V/Ge Multijunction Solar Cells

Improving the performances and reducing costs of III-V multijunction solar cells are crucial in aerospatial energy systems and in terrestrial concentrator modules. We attempted to achieve both objectives by implementing non-ohmic metal/semiconductor interface contacts on the front surface of III-V/Ge triple-junction solar cells. We demonstrate the feasibility of this concept for this type of solar cell by a simple evaporation of Al only either on the GaAs contact layer or the AlInP window. The best results were obtained when sulfur passivation by (NH4)2Sx was conducted on the GaAs contact layer. This allowed for a reduction in reverse saturation dark current density by one order of magnitude and a slight increase in Voc of almost 20 mV under 1 sun illumination relative to a reference device with Pd/Ge/Ti/Pd ohmic contacts. However, poor performances were observed at first under concentrated sunlight. Further annealing the solar cells with Al front metallization resulted in the reduction of Voc to the same level as the reference solar cell but allowed for good performances under high illumination. Indeed, an efficiency over 34% was observed at 500 suns light intensity both for Al and Pd/Ge/Ti/Pd contacted solar cells.

Metamorphic front- and rear-junction 1.7 eV GaInP solar cells with high open-circuit voltage

1.7 eV absorber materials are important in high-performance III-V multi-junction solar cells. Here, we show that rapid thermal annealing significantly improves the carrier lifetimes in lightly doped metamorphic 1.7 eV Ga0.37In0.63P (MM GaInP) grown on GaAs by molecular beam epitaxy. A low threading dislocation density of ∼6 × 105 cm−2 was achieved through the use of an InxGa1-xAs graded buffer. Annealing enables minority carrier lifetimes in 1.7 eV p- and n-GaInP of 3.9 and 28 ns, respectively, both of which are higher than annealed lattice-matched 1.9 eV Ga0.51In0.49P (LM GaInP) grown in the same chamber. With the benefit of annealing, MM 1.7 eV GaInP front-junction solar cells show a high peak internal quantum efficiency of 94.3% and an open-circuit voltage (VOC) of 1.17 V. We also demonstrate the first MM 1.7 eV GaInP rear-heterojunction cells with high fill factor and VOC of 85% and 1.21 V, respectively, by taking advantage of long carrier lifetime while eliminating majority carrier blocking. The high VOC values for the 1.7 eV GaInP cells presented in this work indicate promising tolerance to threading dislocations in MM GaInP.

Electrical passivation of III-V multijunction solar cells with luminescent coupling effect

Perimeter recombination is one of the causes identified for having a nonuniform luminescent coupling effect in III-V multijunction solar cells. A potential solution could be the electrical passivation of the multijunction solar cell perimeter. To test this hypothesis, electrical passivation of InGaP/GaAs/Ge 3-junction solar cells was done by the atomic layer deposition of thin Al 2 O 3 films. Perimeter passivation of a 3-junction solar cell relatively increased current collection measured at luminescent coupling and direct subcell excitation by as much as 11.4% and 29.8%, respectively. Meanwhile, the current homogeneity improved relatively by as much as 7.9% after electrical passivation. At 1-sun global standard illumination (AM 1.5G), 0.2% absolute conversion efficiency increase was achieved. Therefore, electrical passivation of fully working III-V multijunction solar cells is a non-invasive, post-treatment process that can recover losses due to surface defects caused by oxidation. • Post-treatment passivation for fully working multijunction solar cell. • Larger conversion efficiencies in multijunction solar cells with smaller active areas. • Spatial current uniformity improvement after perimeter passivation. • Reduction of shunt leakage current in a subcell made of indirect bandgap material. • Alleviate lateral resistance effects by electrical passivation.

Epitaxy and characterization of InP/InGaAs tandem solar cells grown by MOVPE on InP and Si substrates

The integration of III-V multi-junction solar cells on Si substrates is currently one of the most promising possibilities to combine high photovoltaic performance with a reduction of the manufacturing costs. In this work, we propose a prospective study for the realization of an InP/InGaAs tandem solar cell lattice-matched to InP on a commercially available Si template by direct MOVPE growth. The InP top cell and the InGaAs bottom cell were firstly separately grown and optimized using InP substrates, which exhibited conversion efficiencies of 13.5% and 11.4%, respectively. The two devices were then combined in a tandem device by introducing an intermediate InP/AlInAs lattice-matched tunnel junction, showing an efficiency of 18.4%. As an intermediate step towards the realization of the tandem device on Si, the InP and InGaAs single junction solar cells were grown on top of a commercial InP/GaP/Si template. This transitional stage enabled to isolate and evaluate the effects of the growth of III-V on Si on the photovoltaic performance through the comparison with the aforementioned devices on InP. Each cell was electrically characterized by external quantum efficiency and dark and illuminated current-voltage under solar simulator. The material quality was also analyzed by means of X-ray diffraction, Atomic-Force Microscopy, Transmission Electron and Scanning Electron Microscopy. The III-V on Si devices showed efficiencies of 3.6% and 2.0% for the InP and InGaAs solar cells, respectively.

Performance analysis of GaAsBi/InGaAs heterostructure for III-V multi-junction solar cells

This study analyses the dilute GaAsBi as a candidate for the nominally 1.0 eV bandgap subcell in III-V multi-junction solar cells. The GaAsBi/InGaAs heterojunction solar cells are grown by molecular beam epitaxy pseudomorphically on thin InGaAs buffer layers to reduce GaAsBi lattice-mismatch with a GaAs substrate. We demonstrate a power conversion efficiency of 6.2%, one of the highest reported in GaAsBi-based solar cells thus far. The experimental photovoltaic and quantum efficiency measurements of the solar cells with intrinsic and n-type doped GaAsBi are modelled numerically to determine non-radiative recombination effects as well as optimal layer thickness and doping concentration to improve the performance. This work advances the GaAsBi-based device design and shows that current-matching conditions for a 3-junction inverted-growth solar cell could be achieved using a 1.0 eV GaAsBi bottom junction. • GaAsBi-based solar cells are considered as a candidate for the nominally 1.0 eV bandgap subcell in III-V multi-junction photovoltaics. • A heterojunction GaAsBi/InGaAs solar cells are grown pseudomorphically by MBE on thin InGaAs buffer layers. • We demonstrate so far the highest achieved power conversion efficiency of 6.2% in GaAsBi-based solar cells. • Numerical modelling of the J-V and quantum efficiency allows for the elucidation of charge carrier lifetimes and optimal device polarity. • The optimal doping concentration and GaAsBi absorber layer thickness are determined to improve the cells' V oc and J sc .

III-V material growth on electrochemically porosified Ge substrates

III-V semiconductor materials for high-efficiency multi-junction solar cells are often grown on germanium (Ge) substrates. However, apart from being considered as a rare element, Ge substrates are one of the major cost shares of a III-V multi-junction solar cell. To reduce costs and material consumption, we aim at re-usable porosified Ge substrates. Prior to the growth, the porous layers are subjected to an annealing procedure to close the wafer surface and to form a predetermined breaking area some microns below the surface. Later, the III-V epitaxial layers are mechanically lifted at the porous layer, so the substrate can be reused. Here, we demonstrate the III-V epitaxy material quality by growing Al0.5Ga0.49In0.01As/Ga0.99In0.01As double heterostructures on porous Ge substrates and characterize them in detail to understand how the porous layers affect the structural and opto-electronic properties of theIII-Vcompounds compared to a reference grown on germanium “epi-ready” substrates. We find no significant influence of the porous Ge substrate on the layer’s composition, thickness or roughness. However, cathodoluminescence measurements reveal a defect density of 4.5 × 105 cm−2 in comparison with 6.8 × 104 cm−2 for the reference case. Those defects were identified as threading dislocations by electron channeling contrast imaging. The lifetime of minority carriers measured by time resolved photoluminescence shows no difference in the low injection regime between both samples either, indicating a high quality opto-electronic material deposited on porous Ge. These first promising results indicate a path for both: reducing costs of III-V multi-junction solar cells and a reduced germanium consumption.

Experimental optical and structural properties of ZnS, MgF2, Ta2O5, Al2O3 and TiO2 deposited by electron beam evaporation for optimum anti-reflective coating designs

MgF2, ZnS, Ta2O5, Al2O3 and TiO2 are some of the most common dielectric materials actually used in solar cells as anti-reflective coatings. For such purpose, they have been deposited by electron beam evaporation, which is one of the preferred technique in industrial settings. In this work we aim to understand the relationship between their optical and physical properties and deposition parameters and thicknesses, being spectroscopic ellipsometry the main technique used for such issue. MgF2 and ZnS have demonstrated rather good optical quality except for the thinnest samples. For Ta2O5, O2 flow during growth has been revealed as critical parameter for ensuring the proper composition of the layer. Al2O3 samples showed a very important amount of Al hydroxide, which is detrimental of the optical quality of these layers. TiO2 samples have also contamination problems, in this case due to oxygen species and Ti3+ oxide presence. For all these oxides, optical properties are clearly influenced by layer thickness. The layer optical parameters n and k have been determined as a function of wavelength (300–1800 nm) and validated against experimental reflectance and transmittance data. Finally, the impact that the proper knowledge of n and k values have when designing broad-band anti-reflective coatings for III-V multijunction solar cells is quantified in terms of short-circuit current relative increase.

Delta-Doping for Enhanced III-V Tunnel Junction Performance

We show that delta-doping boosts the performance of tunnel junctions (TJs) used as interconnects in III-V multijunction solar cells by orders of magnitude. The peak current of our baseline TJ design consisting of p-GaAs&#x002F;n-GaAs surrounded by <formula><tex>$\text{Ga}_{0.51} \text{In}_{0.49}\text{P}$</tex></formula> clads is improved by a factor of &#x223C;5 &#x00D7; <formula><tex>$10^5$</tex></formula>. The relative benefits of delta-doping are even stronger in TJs based on wider-bandgap materials with reduced optical absorption. Importantly, we find that delta-doped TJs can survive the thermal loads that would be encountered during growth of additional subcells. Delta doping is a simple and versatile method to improve TJ performance that can be implemented by virtually any epitaxial growth method.

Triple-junction solar cells with 39.5% terrestrial and 34.2% space efficiency enabled by thick quantum well superlattices

<h2>Summary</h2> Multijunction solar cell design is guided by both the theoretical optimal bandgap combination as well as the realistic limitations to materials with these bandgaps. For instance, triple-junction III-V multijunction solar cells commonly use GaAs as a middle cell because of its near-perfect material quality, despite its bandgap being higher than optimal for the global spectrum. Here, we modify the middle cell bandgap using thick GaInAs/GaAsP strain-balanced quantum well (QW) solar cells with excellent voltage and absorption. These high-performance QWs are incorporated into a triple-junction inverted metamorphic multijunction device consisting of a GaInP top cell, GaInAs/GaAsP QW middle cell, and lattice-mismatched GaInAs bottom cell, each of which has been highly optimized. We demonstrate triple-junction efficiencies of 39.5% and 34.2% under the AM1.5 global and AM0 space spectra, respectively, and the global efficiency is higher than previous record six-junction devices.

The Diffusion Effect of Copper on the Flexible GaInP/GaAs Solar Cells

Electroplating copper is widely used in the fabrication of flexible III-V multijunction solar cell because of its excellent characteristics of lightness, flexibility and conductivity of thin Cu, which can be directly applied as the flexible support and the external electrode. In this letter, we studied the diffusion effect of copper in the flexible GaInP/GaAs solar cells under high temperature annealing, which plays an essential role in achieving a promising ohmic contact between contact layer and metal. However, the degradation in the performance of flexible cell was observed when the annealing procedure was employed at the last step of the device fabricating process. The secondary ion mass spectrometry and electroluminescence results have indicated that the degradation of the solar cell performance after high-temperature annealing is attributed to the excess copper diffusion into active region of the solar cell. High temperature diffusion of copper can be solved by prior-annealing to form ohmic contact.

Growth and Fabrication of GaAs Thin-Film Solar Cells on a Si Substrate via Hetero Epitaxial Lift-Off

We demonstrate, for the first time, GaAs thin film solar cells epitaxially grown on a Si substrate using a metal wafer bonding and epitaxial lift-off process. A relatively thin 2.1 μm GaAs buffer layer was first grown on Si as a virtual substrate, and a threading dislocation density of 1.8 × 107 cm−2 was achieved via two In0.1Ga0.9As strained insertion layers and 6× thermal cycle annealing. An inverted p-on-n GaAs solar cell structure grown on the GaAs/Si virtual substrate showed homogenous photoluminescence peak intensities throughout the 2″ wafer. We show a 10.6% efficient GaAs thin film solar cell without anti-reflection coatings and compare it to nominally identical upright structure solar cells grown on GaAs and Si. This work paves the way for large-scale and low-cost wafer-bonded III-V multi-junction solar cells.

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.

Passivated, Highly Reflecting, Laser Contacted Ge Rear Side for III-V Multi-Junction Solar Cells

This article describes the successful integration of a passivated, highly reflecting Ge rear side into a III-V multijunction solar cell. The use of lowly doped Ge and the new rear side leads to the aimed increase in Ge cell current up to 1.6 mA.cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> , demonstrated by external quantum efficiency and I-V measurements. For the contact formation, two different types of laser processes were conducted and evaluated-the laser fired contact route and the PassDop route. In both cases, the formation of a local back surface field preserves the passivation of the contact points. A laser pitch and laser power variation leads to a good performing back contact. The passivation effect is proven experimentally and is qualitatively accessed with cell simulations.

Bidirectional planar-displacement waveguide tracker for high-concentration photovoltaics.

A bidirectional planar-displacement waveguide tracker was devised to replace the traditional two-axis tracking system for high-concentration photovoltaics, with improved module thickness, optical field uniformity, and current matching. The concentrating magnification reaches 725 times, and the sun tracking angle is more than 170°, which is equivalent to 11.3 tracking hours per day. The module thickness is only 6.16 cm. This design enabled us to place the module flat on the ground, in which swing was not required. This will greatly improve the mechanical strength and the lifetime of the module and solve the development dilemma faced by III-V multijunction solar cells.

Perovskite Solar Cells for Space Applications: Progress and Challenges.

Metal halide perovskites have aroused burgeoning interest in the field of photovoltaics owing to their versatile optoelectronic properties. The outstanding power conversion efficiency, high specific power (i.e., power to weight ratio), compatibility with flexible substrates, and excellent radiation resistance of perovskite solar cells (PSCs) enable them to be a promising candidate for next-generation space photovoltaic technology. Nevertheless, compared with other practical space photovoltaics, such as silicon and III-V multi-junction compound solar cells, the research on PSCs for space applications is just in the infancy stage. Therefore, there are considerable interests in further strengthening relevant research from the perspective of both mechanism and technology. Consequently, the approaches used for and the consequences of PSCs for space applications are reviewed. This review provides an overview of recent progress in PSCs for space applications in terms of performance evolution and mechanism exploration of perovskite films and devices under space extreme environments.

A Brief Review of High Efficiency III-V Solar Cells for Space Application

The demands for space solar cells are continuously increasing with the rapid development of space technologies and complex space missions. The space solar cells are facing more critical challenges than before: higher conversion efficiency and better radiation resistance. Being the main power supply in spacecrafts, III-V multijunction solar cells are the main focus for space application nowadays due to their high efficiency and super radiation resistance. In multijunction solar cell structure, the key to obtaining high crystal quality and increase cell efficiency is satisfying the lattice matching and bandgap matching conditions. New materials and new structures of high efficiency multijunction solar cell structures are continuously coming out with low-cost, lightweight, flexible, and high power-to-mass ratio features in recent years. In addition to the efficiency and other properties, radiation resistance is another sole criterion for space solar cells, therefore the radiation effects of solar cells and the radiation damage mechanism have both been widely studied fields for space solar cells over the last few decades. This review briefly summarized the research progress of III-V multijunction solar cells in recent years. Different types of cell structures, research results and radiation effects of these solar cell structures under different irradiation conditions are presented. Two main solar cell radiation damage evaluation models—the equivalent fluence method and displacement damage dose method—are introduced.