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Results for “multijunction solar cell”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Tunnel junctions for multijunction solar cells

Tunnel junctions for multijunction solar cells are provided. According to an aspect of the invention, a tunnel junction includes a first layer including p-type AlGaAs, a second layer including n-type GaAs, wherein the second layer is a quantum well, and a third layer including n-type AlGaAs or n-type lattice matched AlGaInP. The quantum well can be GaAs or AlxGaAs with x being more than about 40%, and lattice matched GaInAsNSb in the Eg range of from about 0.8 to about 1.4 eV.

Jain, Nikhil↗
DOE OSTI2022

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 Al2O3 films. Further, 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.

14 SOLAR ENERGY↗
DOE OSTI2022

Record Efficiency Multijunction Solar Cells with Strain-Balanced Quantum Well Superlattices

The absorption edge of a GaAs solar cell can be tuned to longer wavelengths with strain-balanced quantum wells, as an alternative to metamorphic epitaxy. For one-sun photovoltaic applications, an optimum bandgap for both single and multijunction solar cells is near the broad water absorption peak at 930 nm and can be accessed by a Ga0.9In0.1As alloy. This alloy is strained with respect to the GaAs substrate, and so only very thin ~10 nm layers can be grown without relaxation via dislocation formation. However, solar cells require significant thickness for complete absorption of the incoming solar spectrum, requiring many such layers. We have designed quantum well superlattices with up to 300 sets of carefully tuned strain-balanced GaInAs/GaAsP pairs by limiting composition modulation and material degradation. We have incorporated quantum well solar cells into multijunction devices to achieve record 32.9% two-junction and 39.5% three-junction solar cells under standard one-sun illumination. We will describe the physics of the quantum well devices and the growth conditions that lead to sharp interfaces and good material quality, as well as the characterization of the efficiencies.

GaAs↗
DOE OSTI2022

How Should Researchers Measure Perovskite-Based Monolithic Multijunction Solar Cells’ Performance? A Calibration Lab's Perspective

Perovskite multijunctions (PVSK MJs) have made remarkable progress with monolithic PVSK/PVSK tandems surpassing the efficiency of single-junction (1 J) PVSK cells and PVSK/Si cells reported to exceed the 30% efficiency mark. These efficiencies are reported at standard test conditions (STC), established by the photovoltaic (PV) community to facilitate comparison between devices and technologies. Herein, it is discussed why an accurate STC performance measurement for a MJ is more complicated than for a 1 J cell and the special aspects to be considered when measuring the current-voltage characteristics and the performance of PVSK-containing MJ cells are emphasized. It is discussed why a spectrally adjustable solar simulator is needed and the sequence of accurate performance measurement, namely measurement of the spectral response of each junction, adjustment of the spectrum, and appropriate protocols for measuring the power output of the device at STC, is presented. For all these, common errors and pitfalls that sometime lead to misleading interpretation of the results are presented, and the methods to evaluate the accuracy of the data when a spectrally adjustable solar simulator is not available are recommended. Finally, first step is taken toward recommending performance measurement approaches when high throughput is required as will eventually be in a production line.

14 SOLAR ENERGY↗
DOE OSTI2021

Proton-Radiation Tolerant All-Perovskite Multijunction Solar Cells

Radiation-resistant but cost-efficient, flexible, and ultralight solar sheets with high specific power (W g -1 ) are the "holy grail" of the new space revolution, powering private space exploration, low-cost missions, and future habitats on Moon and Mars. Herein, this study investigates an all-perovskite tandem photovoltaic (PV) technology that uses an ultrathin active layer (1.56 micrometers) but offers high power conversion efficiency, and discusses its potential for high-specific-power applications. This study demonstrates that all-perovskite tandems possess a high tolerance to the harsh radiation environment in space. The tests under 68 MeV proton irradiation show negligible degradation (<6%) at a dose of 10 13 p + cm -2 where even commercially available radiation-hardened space PV degrade >22%. Using high spatial resolution photoluminescence (PL) microscopy, it is revealed that defect clusters in GaAs are responsible for the degradation of current space-PV. By contrast, negligible reduction in PL of the individual perovskite subcells even after the highest dose studied is observed. Studying the intensity-dependent PL of bare low-gap and high-gap perovskite absorbers, it is shown that the V OC , fill factor, and efficiency potentials remain identically high after irradiation. Radiation damage of all-perovskite tandems thus has a fundamentally different origin to traditional space PV.

14 SOLAR ENERGY↗
DOE OSTI2021

Multijunction solar cells with graded buffer Bragg reflectors

Distributed Bragg reflectors are incorporated into the compositionally graded buffers of metamorphic solar cells, adding functionality to the buffer without adding cost. The reflection aids in collection in subcells that are optically thin due to low diffusion length, high bulk recombination, radiation hardness, partially-absorbing quantum structures, or simply for cost savings. Performance enhancements are demonstrated in GaAs subcells with QWs, which is beneficial when GaAs is not the ideal bandgap.

France, Ryan Matthew↗
DOE OSTI2022

Progress in three-terminal heterojunction bipolar transistor solar cells

Conventional solar cells, including multijunction solar cells, are based on pn junctions as building blocks. In contrast, the three-terminal heterojunction bipolar transistor solar cell (3T-HBTSC) explores the use of a bipolar transistor structure to build a solar cell. The limiting efficiency of this transistor structure equals that of a double-junction solar cell. However, since the 3T-HBTSC does not require tunnel junctions, its minimal structure has only three semiconductor layers, while the minimal structure of a double junction solar cell has six. This work reviews the operation principles of this solar cell and the steps carried out towards its practical implementation. Experimental results on a GaInP/GaAs HBTSC prototype with bottom interdigitated contacts are presented.

14 SOLAR ENERGY↗
DOE OSTI2023

High Efficiency Multijunction Devices: Solar Cells, Thermophotovoltaics, LEDs

High-efficiency III-V multijunction solar cells require multiple alloys with optimal bandgaps and excellent material quality. However, many useful alloys are lattice-mismatched to available substrates, and require strain-engineering tricks to enable efficient use. To this extent, we have developed low-dislocation density metamorphic material and optically-thick stress-balanced superlattices, enabling access to a wide variety of III-V alloys. Combined with transparent tunnel junctions, these components allow optimizing multijunction devices for a variety of applications. We have designed multijunction cells for the G173G-terrestrial and AM0-space spectra with record 39.5% and 34.2% efficiency, respectively, and also thermophotovoltaic cells for blackbody spectra that reach over 40% TPV-efficiency. Finally, using these PV components, we demonstrate a multijunction LED with high quantum efficiency.

high-efficiency↗
DOE OSTI2022

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

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. Additionally, 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.

14 SOLAR ENERGY↗
DOE OSTI2020

Development of Lattice-Mismatched GaInAsP for Radiation Hardness

We develop lattice-mismatched GaInAsP as an alternative alloy to pure As-based alloys currently used in III-V multijunction solar cells. Increasing the alloy phosphorous and indium content while maintaining an optimal bandgap may allow high efficiency multijunction devices with increased radiation hardness. Here, 1.0-eV GaInAsP is developed and implemented into single and multijunction solar cell devices. The lattice-mismatched GaInAsP must be grown strain free, and the subcell thickness must be maintained below the thickness where surface-driven phase separation occurs. As observed in transmission electron microscopy and cathodoluminescence imaging, phase separation strengthens in the GaInAsP layer and leads to interfacial defect formation when the cell thickness is too great. We show single junction 1.0-eV Ga 0.5 In 0.5 As 0.7 P 0.3 with excellent carrier collection and a bandgap-voltage offset of 0.40 V. This material quality approaches that of 1.0-eV Ga 0.7 In 0.3 As used in inverted metamorphic multijunction devices, but has increased phosphorus content and consequently is expected to have a higher radiation resistance. We incorporate the 1.0-eV GaInAsP subcell into a 3-junction inverted metamorphic solar cell to test the performance of the subcell in a multijunction. No additional loss is observed upon integration into a multijunction cell: both the carrier collection and voltage of the GaInAsP subcell are unchanged from single junction devices. While further materials development and radiation testing is still required, these preliminary results indicate that lattice-mismatched GaInAsP can be effectively used in multijunction solar cells to replace radiation-soft materials.

14 SOLAR ENERGY↗
DOE OSTI2025

Charge carrier extraction and recombination effects in GaInAs/GaAsP multi-quantum well solar cells

The carrier extraction and transport mechanisms as well as the relative contributions of radiative and non-radiative recombination processes are investigated in high-quality strain-balanced GaInAs/GaAsP multi-quantum well solar cells recently implemented in record efficiency multijunction solar cells. A comprehensive suite of complementary characterization techniques including temperature- and suns-dependent photoluminescence and photovoltaic measurements are employed to analyze thermal escape and tunneling rates, which demonstrate the need to move beyond simple drift-diffusion models of p–n junctions. This study examines the processes that best characterize the operation of these devices across varying temperatures using a simple two-diode model, incorporating multiple transport protocols, and provides insights into the performance-limiting processes and pathways for their optimization.

14 SOLAR ENERGY↗
DOE OSTI2021

Lamination of transparent conductive adhesives for tandem solar cell applications

A transparent conductive adhesive (TCA) interlayer for mechanically stacked multijunction solar cells provides adhesive strength and electrical connection between sub-cells when stacked. The fabrication of TCA sheets from an ethyl-vinyl acetate (EVA) matrix and silver coated compliant conductive microspheres, similar to industry-standard EVA encapsulant sheets, compatible with a standard lamination process was developed. Blade coating TCA sheets using 3D printed blades and testing of vertical conduction through the interlayer with a TCA conductivity jig allowed for reproducible measurements and low-cost fabrication. The supplemental information provides design files to enable other researchers to produce their own blades and testing jigs. We identified 0.5 mm as an optimal blade height for producing lab-scale TCA sheets and 120 °C at 3 psi for 10 min as the optimal vacuum lamination process to minimize resistance and possible damage to samples. For TCA sheets with coverages between 0.75% and 9%, the series resistance was approximately 0.2 O cm2 which should result in minimal losses for one-sun photovoltaic applications.

14 SOLAR ENERGY↗
DOE OSTI2022

Triple-Junction III-V Solar Cells with 39.5% AM1.5G and 34.2% AM0 Efficiencies

III-V multijunction solar cells have the highest efficiencies of any PV technology. These devices are currently the dominant space PV technology, and are being investigated for a wide range of other applications where efficiency is a key metric. In this presentation, we describe a 3-junction cell with the highest 1-sun efficiency to date: 39.5% under the global spectrum, which is higher than previous 6-junction devices. When tuned for the space spectrum, the cell achieves 34.2% beginning-of-life efficiency, without consideration for radiation hardness. These efficiency improvements are enabled by combining advances in device architectures and extended-defect control strategies. This latest 3-junction device includes a high-performance front-junction GaInP top cell, an optically-thick strain-balanced quantum well middle cell, and a lattice-mismatched GaInAs bottom cell with record performance despite dislocations. These advances not only allow high efficiency photovoltaics for both space and terrestrial PV applications, but also help enable new applications of III-V materials.

GaInAs↗
DOE OSTI2025

Laser Ablation for Low-Cost Multijunction III-V Solar Cell Mesa Isolations

Eliminating photolithography from solar cell processing is a significant opportunity for cost reduction for III-V solar cells. In this work, we test femtosecond laser ablation and scribing as an alternative to contact photolithography and wet chemical etching for mesa isolation, when processing multijunction cells. We demonstrate that upright multijunction solar cells isolated by using the laser as a scribe to cleave through the substrate had virtually no performance loss when compared to a baseline cell processed with photolithography. By contrast, cells isolated by laser ablating through the active layers have performance losses that cannot be fully eliminated with post-processing etches. This demonstration of photolithography-free mesa isolation with no performance losses is promising for less expensive III-V manufacturing.

14 SOLAR ENERGY↗
DOE OSTI2025

Local halide heterogeneity drives surface wrinkling in mixed-halide wide-bandgap perovskites

Compositional heterogeneity in wide-bandgap (1.8 - 2.1 eV) mixed-halide perovskites is a key bottleneck in the processing of high-quality solution-processed thin films and prevents their application in efficient multijunction solar cells. Notably, mixed-cation (formamidinium-methylammonium) wide-bandgap perovskite films are prone to form micrometer-scale wrinkles which can interfere with the smooth surfaces ideal for multijunction devices. Here, we study the formation dynamics of wrinkled mixed-halide perovskite films and its impact on the local composition and optoelectronic properties. We use in situ X-ray scattering during perovskite film formation to show that crystallization of bromide-rich perovskites precedes that of mixed-halide phases in wrinkled films cast using an antisolvent-based process. Using nanoscopic X­-ray fluorescence and hyperspectral photoluminescence imaging, we also demonstrate the formation of iodide- and bromide-rich phases in the wrinkled domains. This intrinsic spatial halide segregation results in an increased local bandgap variation and Urbach energy. Morphological disorder and compositional heterogeneity also aggravate the formation of sub-bandgap electronic defects, reducing photostability and accelerating light-induced segregation of iodide and bromide ions in thin films and solar cells.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Sputtered p-Type Cu x Zn 1– x S Back Contact to CdTe Solar Cells

As thin-film cadmium telluride (CdTe) solar cells gain prominence, one particular challenge is optimizing contacts and their interfaces to transfer charge without losses in efficiency. Back contact recombination is still significant and will prevent CdTe solar technology from reaching its full potential in device efficiency, and transparent back contacts have not been developed for bifacial solar technology or multijunction solar cells. To address these challenges, here we investigate sputtered Cu x Zn 1– x S as a p-type semi-transparent back contact material to thin-film polycrystalline CdTe solar cells at Cu concentrations x = 0.30, 0.45, and 0.60. This material is selected for its high hole conductivity (160–2120 S cm –1 ), wide optical band gap (2.25–2.75 eV), and variable ionization potential (approximately 6–7 eV) that can be aligned to that of CdTe. We report that without device optimization, CdTe solar cells with these Cu x Zn 1– x S back contacts perform as well as control cells with standard ZnTe:Cu back contacts. We observe no reduction in external quantum efficiency, low contact barrier heights of approximately 0.3 eV, and carrier lifetimes on par with those of baseline CdTe. These cells are relatively stable over one year in air, with V OC and efficiency of the x = 0.30 cell decreasing by only 1 and 3%, respectively. Using scanning electron microscopy and scanning transmission electron microscopy to investigate the Cu x Zn 1– x S/CdTe interface, we demonstrate that the Cu x Zn 1– x S layer segregates into a bilayer of Cu-Te-S and Zn-Cd-S, and thermodynamic reaction calculations support these findings. Despite its bilayer formation, the back contact still functions well. This investigation explains some of the physical mechanisms governing the device stack, inspires future work to understand interfacial chemistry and charge transfer, and elicits optimization to achieve higher-efficiency CdTe cells.

14 SOLAR ENERGY↗
DOE OSTI2020

Choose Your Own Adventure: Fabrication of Monolithic All-Perovskite Tandem Photovoltaics

Metal halide perovskites (MHPs) have transfixed the photovoltaic (PV) community due to their outstanding and tunable optoelectronic properties coupled to demonstrations of high-power conversion efficiencies (PCE) at a range of bandgaps. This has motivated the field to push perovskites to reach the highest possible performance. One way to increase the efficiency is by fabricating multijunction solar cells, which can split the solar spectrum, reducing thermalization loss. Low-cost all-perovskite tandems have a real chance to soon exceed 30% PCE, which could transform the PV industry. Achieving this goal requires the identification of perovskite sub-cells that are both highly efficient and can be effectively integrated. In this review, it is discussed how to navigate the multiple-choice adventure in choosing between the myriad of options and considerations present when deciding what perovskite materials, contact layers, and processing tools to use. Some of the potential fabrication pitfalls often encountered in MHP based tandem PVs are highlighted, so that they can hopefully be avoided in the future.

14 SOLAR ENERGY↗