Inverted Metamorphic Multijunction Research Articles

Polymer nanocomposite for protecting photovoltaic cells from solar ultraviolet in space

Abstract Polymer nanocomposite coatings of solar photovoltaic cells that absorb solar ultraviolet (UV) radiation and convert it into visible and near-infrared (NIR) light can increase the operational lifetime and the energy efficiency of the cells. We report a polymer nanocomposite spectrum converting layer (SCL) made of colorless polyimide CORIN impregnated with the nanoparticles (NPs) of fluoride NaYF4 doped with three-valent ions of Europium at a molar concentration of 60%. The NPs were the nanocrystals (179 ± 35 nm in size) in thermally stable hexagonal beta-phase. The visible-NIR photoluminescence quantum yield of the nano-powder was ∼69%. The SCLs were applied using the open-air multi-beam multi-target pulsed laser deposition method to silicon heterojunction (SHJ), copper-indium-gallium-selenide (CIGS), and inverted metamorphic multijunction (IMM) solar cells. The cells were exposed to UV radiation from a 365 nm light emitting diode. The I–V characteristics of the cells were measured with a solar simulator using AM0 filter. The proposed SCLs improved the UV stability of all three types of the cells: the power degradation of SHJs and IMMs cells was stopped or slightly reversed and the degradation rate of CIGSs decreased by ∼25%. The proposed SCLs have great commercial potential, especially for applications to space power.

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.

Inverted metamorphic AlGaInAs/GaInAs tandem thermophotovoltaic cell designed for thermal energy grid storage application

We demonstrate an inverted metamorphic multijunction (IMM) photovoltaic cell comprising lattice-mismatched 1.2 eV AlGaInAs and 1.0 eV GaInAs junctions optimized for high-temperature thermophotovoltaic (TPV) applications. This device differs from traditional IMM solar cells because the mismatched junctions are grown at a single lattice constant. This architecture enables removal of the compositionally graded buffer that otherwise filters light from the junctions below and absorbs sub-bandgap light via free-carrier absorption. Sub-bandgap absorption dramatically reduces the efficiency of TPV systems using high reflectivity cells to enable band edge spectrum filtering. Three components required development to enable this device: (1) a lattice-mismatched 1.2 eV AlGaInAs junction, (2) a metamorphic contact layer grown after the graded buffer, and (3) a transparent tunnel junction that sits in front of the 1.0 eV GaInAs junction. Growth conditions that minimize oxygen defect incorporation maximize AlGaInAs cell quality, enabling a 0.41 V bandgap open circuit voltage offset at 22 mA/cm2 under AM1.5D. A mismatched GaInAs:Se layer is developed as a low resistance contact. Lastly, we develop a GaAsSb:C/GaInP:Se tunnel junction suitable for high-power densities with more transparency than the GaAsSb:C/GaInAs:Se structure used in past IMM cells. We characterize the tandem device under a high-intensity spectrum that approximates the emission from a 2150 °C blackbody radiator and deduce a projected ideal TPV efficiency of 39.9% at ∼30% of the blackbody irradiance and 36% ideal TPV efficiency under the full 118 W/cm2 irradiance. Improvements to the back-surface reflectivity and series resistance are expected to increase the ideal TPV efficiency well above 40%.

Internal Resistive Barriers Related to Zinc Diffusion During the Growth of Inverted Metamorphic Multijunction Solar Cells

Majority carrier barriers at heterointerfaces are a common source of non-linear resistance that hinders concentrator solar cell performance. The source of a particular barrier is often unclear in a multijunction device with numerous heterointerfaces. We demonstrate Zn-dopant diffusion during inverted metamorphic multijunction (IMM) device growth to be one key cause of internal barrier formation. Using an inverted GaAs/GaAs tandem solar cell with a high temperature annealing layer grown in between each subcell, we simulate the annealing conditions of a multijunction growth in a simplified structure. Through analysis of the device by secondary ion mass spectrometry (SIMS) and electrochemical capacitance–voltage profiling, we show that annealing causes Zn to diffuse out of the top cell Ga0.5In0.5P back surface field (BSF) and accumulate in the GaAs base. Through equilibrium band modeling, we show that the resultant doping profile forms an energetic barrier to hole flow in the valence band, which correlates with fill factor losses in the current–voltage curves measured under concentration. When we, instead, employ a C-doped Al0.2Ga0.8As BSF layer in the top cell, we do not observe evidence of a heterojunction barrier. We attribute this difference to the reduced diffusivity of carbon, confirmed by SIMS, as well as more favorable valence band offsets between GaAs and Al0.2Ga0.8As. Finally, we compare 5-junction IMM cells with Al0.2Ga0.8As:C and Ga0.5In0.5P:Zn BSF layers in the GaAs third junction, respectively, and show a significantly improved device performance under concentration when Al0.2Ga0.8As:C is employed. We demonstrate the importance of designing annealing tolerance into multijunction structures that are subjected to extended annealing during growth.

(Invited) Inverted Metamorphic Multijunction III-Vs for Photo-Electrochemical Hydrogen Production Systems: Challenges in Absorber Stabilization and Device Scale-up

While III-V semiconductors have achieved the highest photo-electrochemical solar-to-hydrogen conversion efficiencies, they are remarkably unstable during operation in a harsh electrolyte[1]. The first half of this talk will focus on the degradation mechanism of inverted metamorphic multijunction (IMM) III-V cells and surface modification strategies aimed at protecting them from photocorrosion. We applied noble metal catalysts, oxide coatings by atomic layer deposition, and MoS2 in an effort to protect the GaInP2 surface that was in contact with acidic electrolyte. We also grew thin epitaxial capping layers from III-V alloys that should be more intrinsically stable than GaInP2. The ability of the various modifications to protect the IMM’s surface was evaluated by operating at each electrode at short circuit for extended periods of time. The second part of this talk will identify the challenges encountered while scaling the IMM III-V absorber areas of from ~0.15 cm2 up to 16 cm2 and incorporating them in a photoreactor capable of generating 3 standard liters of hydrogen in 8 hours under natural sunlight. To successfully scale photo-electrochemical water-splitting technologies from bench to demonstration size requires addressing predictable and unpredictable complications. Despite using Comsol multiphysics to model our photoreactor and identify suitable specifications for a prototype, several practical issues were uncovered during testing that led to multiple iterations of photoreactor design between the initial and final generation. Several bottlenecks that ranged from counter electrode composition and orientation to bubble removal needed redress in order to meet our performance targets. Ultimately, the demonstration-scale system was able to generate nearly twice the target volume of hydrogen in an 8-hour outdoor trial. [1] NATURE ENERGY 2, 17028 (2017)

Building a Six-Junction Inverted Metamorphic Concentrator Solar Cell

We propose practical six-junction (6J) inverted metamorphic multijunction (IMM) concentrator solar cell designs with the potential to exceed 50% efficiency using moderately high quality junction materials. We demonstrate the top three junctions and their monolithic integration lattice matched to GaAs using 2.1-eV AlGaInP, 1.7-eV AlGaAs or GaInAsP, and 1.4-eV GaAs with external radiative efficiencies >0.1%. We demonstrate tunnel junctions with peak tunneling current >400 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> that are transparent to <;2.1-eV light. We compare the bottom three GaInAs(p) junctions with bandgaps of 1.2, 1.0, and 0.7 eV grown on InP and transparent metamorphic grades with low dislocation densities. The solution to an integration challenge resulting from Zn diffusion in the GaAs junction is illustrated in a five-junction IMM. Excellent 1-sun performance is demonstrated in a complete 6J IMM device with V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> = 5.15 V, and a promising pathway toward >50% efficiency at high concentrations is presented.

Next Generation Radiation Hard IMM Space Solar Cells

High efficiency space solar cells based on the GaInP2 /InGaAs/Ge triple junction lattice matched device with 1 sun, AM0 efficiencies of approximately 30% are commercially available from several companies. Multiple approaches have been suggested for significantly surpassing the 30% efficiency level, but achieving this in a commercially viable cell has not been easy. We review those approaches, pointing out the advantages and challenges of each. SolAero has been focused on developing the inverted metamorphic multi-junction (IMM) solar cell, and has recently demonstrated a radiation hard version. We present the most recent performance data, including the response to particle radiation. The IMM cell can be used in a number of rigid or flexible configurations, and considerable effort has been focused on cell packaging and panel integration. The IMM device is ready to start qualification to the ECSS and AIAA standards.

Inverted Metamorphic Multijunction Semiconductors for Exceptionally High Photoelectrolysis Efficiencies: Materials Development and Measurement Challenges

In order to economically generate renewable hydrogen fuel from solar energy using semiconductor-based devices, the U.S. Department of Energy Fuel Cells Technology Office has established technical targets of over 20% solar-to-hydrogen (STH) efficiency with several thousand hours of stability under operating conditions. Our goal is to improve STH efficiency from just over 10% to over 20% via novel tandem semiconductor materials and configurations. Our primary focus is to develop inverted metamorphic multijunction (IMM) III-V semiconductors that have bandgaps optimized for water splitting. The IMM devices are grown by metal organic chemical vapor deposition using a GaAs substrate and use a transparent step-graded buffer layer to allow off-lattice growth. We used IMM growth to achieve a high-quality, lattice-mismatched lower bandgap (1.2 eV) InGaAs bottom cell that captures a larger fraction of the solar spectrum. This 1.8/1.2 eV bandgap combination of the GaInP2/InGaAs configuration is able to generate higher photocurrents, and thus efficiencies, than the 1.8/1.4 eV bandgap combination of the lattice-matched GaInP2/GaAs configuration. We will also discuss measurement challenges in appraising solar-to-hydrogen efficiency, some of which are specific to tandem absorbers. We have recently discovered several systematic errors in measurements made by our group and others that consistently overrate efficiency. Using more stringent measurement standards, we have confirmed over 15% STH on our most advanced IMM devices. Our focus in the next year will be on photon management strategies at the semiconductor/electrolyte interface. Capturing a significant portion of the ~25% of photons lost to reflection at this interface should allow the realization of devices that exceed 20% STH efficiency.

Advanced Device Design for Photoelectrochemical Water Splitting Derived By a Detailed Balance Approach

Solar fuel generation by direct photoelectrolysis of water is a pathway towards energy storage and transportation fuel demands in a sustainable energy economy. High band gap semiconductor materials may drive the water splitting reaction, but cannot use substantial parts of the solar spectrum, while common photovoltaic (PV) absorbers fail to provide sufficient threshold voltage for photoelectrochemical (PEC) conversion. Tandem absorber structures overcome this trade-off featuring promising efficiencies [1]. Tandem cell implementation by epitaxial III-V growth has enabled landmark solar-to-hydrogen (STH) conversion efficiencies [2]. In contrast to the maximum power metric in PV, efficient STH conversion requires maximizing the photocurrent versus a load voltage consisting of the thermodynamic potential of water splitting (1.23V) and various voltage loss mechanisms accumulating throughout the photoelectrochemical cell, often summarized in a generalized overvoltage term for modelling approaches. We discovered that sunlight absorption during illumination through the aqueous PEC electrolyte flawed both efficiency and band gap predictions in earlier theoretical studies and derived PEC tandem device guidelines defining an interdependence between allowable overvoltage loss and electrolyte transmission length [1]. Even though optimum band gap combinations for STH conversion have been discussed for decades, state-of-the-art inverted metamorphic multijunction (IMM) III-V growth and processing strategies developed in the context of high-efficiency PV [3] finally enabled us to tailor water splitting device structures according to maximum performance limits [Fig. 1] and validate our prediction. A first step was to relax the harsh current limitation imposed by the bottom absorber of the classical GaInP/GaAs PEC tandem [2] by replacing it with a metamorphic GaInAs with a band gap of approximately 1.2 eV [Fig. 1(a)]. Further reductions in overvoltage loss and electrolyte layer thickness enable further reductions of top and bottom junctions to unlock higher STH limit efficiency [Fig. 1(b)] that could be implemented by the integration of a 1.7 eV absorber on Si. Advanced PEC measurement protocols [4] drastically reduce performance estimation errors induced by the spectral mismatch of laboratory light sources and light transmission through epoxy used for device area confinement. Valid PEC results of our tailored devices as well as of analogue PV device characterization provide us with important feedback for our detailed balance model, where we start to unravel overvoltage loss into its fundamental components to obtain advanced guidance for future technical development.

Progress toward an advanced four-subcell inverted metamorphic multi-junction (IMM) solar cell

Progress toward an advanced four-subcell inverted metamorphic multi-junction (IMM) solar cell

Development of Silver-Carbon-Nanotube Metal Matrix Composites for Metal Contacts on Space Photovoltaic Cells

The advanced solar cells used in space vehicles today are rapidly moving towards thin-film-based inverted metamorphic multijunction (IMM) solar cells mounted on flexible substrates. However, the IMM cells are more prone to cracking than state-of-the-art triple junction cells. The cell cracking can lead to metal contact failure on IMM cells, compromising the power generation. To mitigate the power loss and increase the lifetime of IMM cells, silver metal films imbedded with carbon nanotubes (CNTs), otherwise known as metal matrix composites, have been developed and investigated for the reinforced mechanical strength against stress-induced cracking. We have primarily focused on (1) surface functionalization of CNTs to make their surface more hydrophilic and wetting to metals, (2) optimization of a cyanide-free electrochemical deposition of silver, (3) electrochemical deposition, drop casting and nanospreader technique to control the composite microstructure, and (4) mechanical and electrical characterization of the composite films. We observe that carboxylation of CNTs produces a stable, homogeneous suspension of negatively charged CNTs at pH > 6. Lustrous-mirror-finish silver films are also successfully deposited, using a commercial cyanide-free silver-plating solution with precise control of current density. Currently, one of the microstructures being explored is a silver-carbon-nanotube layer-by-layer structure, where the surface coverage of CNTs is an important parameter that directly affects the CNT packing fraction and metal intercalation through the CNT network. We quantify the CNT surface coverage as a function of different deposition variables by digitally analyzing scanning electron microscopy images. In this presentation, we will further discuss how this surface coverage correlates to the mechanical and electrical properties of the MMC films. We characterize the mechanical properties, using nanoindentation and strain failure tests. The initial nanoindentation analysis reveals that the composite film has a lower elastic modulus (10 GPa) than pure silver (73 GPa), which is contrary to our initial prediction given the high elastic modulus of CNTs (1000 GPa). The lower elastic modulus is attributed to the electroplating process of silver, in which hydrogen is incorporated and trapped within the composite. Our finite element analysis also corroborates this speculation, where the elastic modulus near 10 GPa is predicted with approximately 4% void fraction. While the composite elastic modulus is lower than that of pure silver, the strain failure tests show that carbon nanotubes can bridge 20 to 50-μm-wide microcracks, maintaining electrical conductivity of the composite

Pushing Inverted Metamorphic Multijunction Solar Cells Toward Higher Efficiency at Realistic Operating Conditions

A unique aspect of the inverted metamorphic multijunction (IMM) solar cell is the bandgap tunability of each junction, creating extremely flexible device designs. The optimal structure has subcell photocurrents that are matched for a given spectrum. However, the subcell photocurrents depend on the cell operating temperature, and therefore, the bandgaps need to be optimized for a certain range of operating conditions. In addition, imperfect material quality results in a loss of voltage and current that depends on the cell bandgap and thickness. In this case, an iterative process of multijunction design and subcell characterization is necessary to determine the optimal design. We compare two different three-junction devices to demonstrate the effect of bandgap selection and lattice-mismatched material quality on device performance at different temperatures. The triple-junction (3J)-IMM design with two lattice-mismatched junctions of perfect material quality (2MMJ) is theoretically optimal at room temperature but experimentally performs similarly to a simpler design with one mismatched junction (1MMJ) at higher temperature because of material quality tradeoffs and the temperature dependence of the designs. Significant progress in the growth, processing, and measurement has led to a 1MMJ design with (42.6 ± 2.1)% peak efficiency at 327 suns and (40.9 ± 2.0)% efficiency at 1093 suns under the direct spectrum.

Lattice-Matched Solar Cells With 40% Average Efficiency in Pilot Production and a Roadmap to 50%

A commercial lattice-matched InGaP/InGaAs/Ge solar cell has reached an average efficiency of 40% at 500 kW/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The design changes that lead to this result are discussed. These data are complemented with a presentation of the latest new solar cell development results from the laboratory. Inverted metamorphic multijunction (IMM) solar cells have been prototyped with 42.4% efficiency at 325 suns for concentrator applications and 33.6% efficiency at 1 sun AM0 for space applications. Six subcell devices are now under development. These results are used, along with other experimental data and other industrial constraints, as input to a computer model to predict what practical efficiency might be achievable with this device approach. The computer model suggests that 45% and 50% efficiencies are technologically feasible with a three-junction and five-junction device, respectively, at an irradiance of 500 kW/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and 25 °C using known materials, device architectures, and manufacturing methods.

Experimental Results From Performance Improvement and Radiation Hardening of Inverted Metamorphic Multijunction Solar Cells

This paper discusses results from continued development of inverted metamorphic multijunction (IMM) solar cells with air mass zero (AM0) conversion efficiencies greater than 34%. An experimental best four-junction IMM (IMM4J) design is presented. In an effort to improve IMM performance in space radiation environments, 1-MeV electron irradiation studies are conducted on the individual IMM4J subcells. These data are used to engineer an IMM4J structure with beginning of life AM0 conversion efficiency of approximately 34% and an end of life (EOL) remaining factor greater than 82%, where EOL is defined as performance after exposure to 1-MeV electron irradiation at 1E15 e/cm $^2$ fluence. Next-generation IMM designs are explored and an avenue toward AM0 conversion efficiencies of greater than 35% is presented.