Single-junction GaAs Solar Cells Research Articles

Impact of using dual back surface field layers of different materials on GaAs single junction solar cell performance

This research aspires to investigate the impact of employing dual BSF layers on the performance of single junction GaAs solar cells using the Silvaco TCAD simulator. A layer of GaAs, InGaP, and InAlGaP has been implemented as a second BSF layer on top of the original BSF layer of the n-InGaP/n-GaAs/p-GaAs/p-InAlGaP structured solar cell. The results show that using GaAs as a second BSF layer has increased the carrier’s recombination and degraded the cell efficiency due to its lower energy bandgap, which creates a potential well that lessens the number of photogenerated carriers flowing through the conduction band toward electrodes. However, adding InGaP and InAlGaP as a second BSF layer decreases the recombination rate and generates a broad electric field region leading to extra photogenerated carriers drifting through the cell, which increases the efficiency from 29.42% to 29.81% for the case of using InGaP and 30.33% for the case of using InAlGaP. Furthermore, increasing the thickness and doping of the second BSF layer reduces the carriers’ recombination at the boundaries of this layer, which implies efficiency enhancement.

Ultra-Thin GaAs Single-Junction Solar Cells for Self-Powered Skin-Compatible Electrocardiogram Sensors.

GaAs thin-film solar cells have high efficiency, reliability, and operational stability, making them a promising solution for self-powered skin-conformal biosensors. However, inherent device thickness limits suitability for such applications, making them uncomfortable and unreliable in flexural environments. Therefore, reducing the flexural rigidity becomes crucial for integration with skin-compatible electronic devices. Herein, this study demonstrated a novel one-step surface modification bonding methodology, allowing a streamlined transfer process of ultra-thin (2.3µm thick) GaAs solar cells on flexible polymer substrates. This reproducible technique enables strong bonding between dissimilar materials (GaAs-polydimethylsiloxane, PDMS) without high external pressures and temperatures. The fabricated solar cell showed exceptional performance with an open-circuit voltage of 1.018V, short-circuit current density of 20.641mAcm-2, fill factor of 79.83%, and power conversion efficiency of 16.77%. To prove the concept, the solar cell is integrated with a skin-compatible organic electrochemical transistor (OECT). Competitive electrical outputs of GaAs solar cells enabled high current levels of OECT under subtle light intensities lower than 50mWcm-2, which demonstrates a self-powered electrocardiogram sensor with low noise (signal-to-noise ratio of 32.68dB). Overall, this study presents a promising solution for the development of free-form and comfortable device structures that can continuously power wearable devices and biosensors.

Review on ultrahigh growth rate GaAs solar cells by metalorganic vapor-phase epitaxy

The aim of this review paper is to summarize a decade of research focused on enhancing metalorganic vapor-phase epitaxy (MOVPE) growth rates of GaAs, driven by the imperative for most cost-effective and energy-efficient III–V compounds’ production. While MOVPE is renowned for producing high-quality devices, it has been constrained by production cost. For example, MOVPE was traditionally thought to have moderate growth rates that limit the throughput of the cost-intensive reactors. Recent research endeavors, however, have demonstrated ultrafast growth rates, exceeding 280 μm/h, with a remarkable group III precursor utilization efficiency of over 50%. It is worth noting that even with increased growth rates, the surface quality remains unaffected in terms of roughness and morphology. Nonetheless, optoelectronic properties, such as minority carrier lifetime, deteriorate for both p- and n-doped materials under constant growth conditions. This is attributed to an increase in the defect density of arsenic antisites, particularly EL2 and HM1 defects, as revealed by deep-level transient spectroscopy investigations. Some of these losses can be mitigated by optimizing growth conditions, such as elevating the temperature and reducing the V/III ratio. The latter not only restores some of the material quality but also increases the growth rate and reduces precursor consumption. Still, fully recovering the original reference lifetimes remains a challenge. Solar cell results indicate that structures with predominantly n-type absorbers are less affected by reduced minority carrier lifetimes. A remarkable 24.5% efficiency was achieved in a GaAs single-junction solar cell grown at 120 μm/h, representing less than 1 min of growth time for the absorber layers.

Conversion efficiency improvement of ELO GaAs solar cell, deposited on water soluble sacrificial buffer

We demonstrate the improvement of power conversion efficiency of an epitaxially-lifted-off single junction GaAs solar cell deposited on a water-soluble sacrificial buffer architecture by introducing an additional germanium (Ge) interlayer between GaAs and the fluoride buffer. The epitaxial lift-off (ELO) technique has been extensively used to separate III-V device layers from their single crystal GaAs substrates. However, conventional ELO requires the use of concentrated hydrofluoric acid (HF) for extended times to etch out the sacrificial layer, subsequently degrading the surface roughness of the parent wafer. As a result, the wafer has to undergo expensive and intensive chemical mechanical polishing (CMP) processes, costing about 25 % of a pristine 6-inch GaAs substrate. In our previous work, we demonstrated a method to eliminating the need for CMP post-processing by using water-assisted ELO (H2O-ELO). A water-soluble, 3-layer buffer architecture was developed using alkaline earth compounds. However, devices suffered low performance due to a high defect density in the GaAs active layers. A Ge interlayer was introduced to provide a more favorable surface energy for GaAs growth, which leads to an improvement of GaAs crystal quality. The Ge deposition conditions were optimized to achieve a high-quality Ge layer on triple-layer fluoride buffer. Single junction GaAs devices fabricated on Ge/(Ca,Sr)F2/BaF2/(Ca,Sr)F2 showed improvement of solar cell performance parameters featuring increases in Voc by 23.7 %, and fill factor (F.F.) by 4.9 %, which results in an overall improvement of power conversion efficiency from 10.3 % to 12.69 %.

Epitaxial lift-off process for GaAs solar cells controlled by InGaAs internal sacrificial stressor layers and a PMMA surface stressor

Epitaxial lift-off (ELO) techniques enable the development of thin-film III–V solar cell devices that are flexible and lightweight. To this end, we report an ELO process employing an internal sacrificial stressor layer (ISSL) and a surface polymer (PMMA) stressor layer. The combined action enhances the lateral etching rate and promotes a more controllable release process. The ISSL consists of quantum well-like GaInAs heterostructures that enable an accurate control of the stress required to enhance the lateral etching of the sacrificial layer, and hence the release of the thin film. More specifically, the use of the ISSL results in about 5-fold faster etch rate of the AlAs sacrificial layer. The ISSL layers can be etched away after the lift-off. Likewise, the PMMA surface stressor, which serves also as a sacrificial intermediate transfer layer, can be easily removed. The proof-of-concept device demonstration of the enhanced ELO technique was made by fabricating single-junction GaAs solar cells. The solar cell performance was evaluated under AM1.5d illumination and by external quantum efficiency measurements. Modelling based analysis shows that although the GaAs solar cell would require improvement of the front contact, yet the novel release process was successfully validated. • Controlled epitaxial lift-off (i-ELO) with internally strained double sacrificial layer (ISSL) and surface stressor (PMMA). • Proof-of-concept GaAs thin-film solar cell fabricated via i-ELO. • Study of lateral etching dynamics with respect to etchant concentration, radius of curvature, and thickness of the thin-film. • Controlled lift-off process without degrading the solar cell structure.

Robust Design of High-Performance Optoelectronic Chalcogenide Crystals from High-Throughput Computation.

High-performance functional materials are the cornerstones of the continuous advance of modern science and technology, but the development of new materials is still challenging. Here, we propose a robust design strategy for novel crystalline solids based on group-theory classification and high-throughput computation, as demonstrated by the successful identification of new optoelectronic semiconductors. First, by means of theoretical group analysis and composition engineering, we obtained 78 prototypical crystal structures and built a computational materials database containing 21,060 ternary chalcogenide compounds. Our high-throughput screening of the coordination characteristics, phase stability, and electronic structures provided 97 candidate semiconductors, including 93 completely new compounds. Among them, 22 crystals with excellent dynamical and thermal stability are predicted to show high photovoltaic conversion efficiency (>30%), comparable to the currently most efficient single-junction GaAs solar cell, owing to their optimal electronic properties and outstanding optical absorption. This discovery of new chalcogenide crystals offers excellent candidates for optoelectronic applications and suggests that our design strategy is a promising way to search for unknown high-performance functional materials.

Efficiency Enhancement of GaAs Single-Junction Solar Cell by Nanotextured Window Layer

In order to improve efficiency of flexible III-V semiconductor multi-junction solar cells, it is important to enhance the current density for efficiency improvement and to attain an even efficiency of solar cells on a curved surface. In this study, the nanotextured InAlP window layer of a GaAs single-junction solar cell was employed to suppress reflectance in broad range. The nanotextured surface affects the reflectance suppression with the broad spectrum of wavelength, which causes it to increase the current density and efficiency of the GaAs single-junction solar cell and alleviate the efficiency drop at the high incident angle of the light source. Those results show the potential of the effectively suppressed reflectance of multi-junction solar cells and even performance of solar cells attached on a curved surface.

Trapezoidal grid fingers to reduce shadowing loss and improve short circuit current

Traditional metallic contacts on solar cells can cover a substantial portion of the device area, resulting in shadowing of light that diminishes the amount of current collected. In this paper, we developed trapezoidal electron-beam evaporated Ti/Al front grid fingers on GaAs single-junction solar cells to decrease the optical shadowing and increase the photogenerated current. We observe a high fill factor of 87% at 1-Sun light intensity and we observe no degradation on the open-circuit voltage. The short-circuit current density is found to increase by 0.4 mA/cm2 when compared to previously demonstrated electroplated Ni/Au front grid fingers, resulting in an efficiency improvement of ~0.8% absolute.

The Separation Improvement in Transferring GaAs Solar Cell on Copper Tungsten

In this study, a fast-epitaxial lift-off (ELO) method was introduced and applied to transfer gallium arsenide (GaAs) single junction (SJ) solar cells to copper tungsten (CuW) substrates. In the ELO process, the time of separating solar cell epitaxial layers (epilayers) from the host substrate would influence the performance of devices. Therefore, external and internal trenches were designed to shorten the separation time. Besides, the usage of a stirrer was also used to accelerate the process. It should be noted that ELO separation time depends on chip size, rather than the size of the substrate. It only cost about 4 h to separate a 4 × 4 mm2 chip from the host substrate in our ELO etchant. After the ELO process, the efficiency of the ELO solar cell can reach 15.81% better than 15.27% of the Ref-cell grown on GaAs substrate.

Potential high efficiency of GaAs solar cell with heterojunction carrier selective contact layers

Advanced configurations of gallium arsenide (GaAs) solar cells are proposed, using hydrogenated amorphous silicon (a-Si: H) and zinc oxide (ZnO) layers as passivation and carrier selective contacts (CSC), as alternatives to the conventional epitaxial GaInP CSC layers. The cell operation is simulated based on the AFORS-HET simulation program. The results show that with a wide-gap ZnO window layer, single-junction GaAs solar cells can reach a higher short-circuit current density (Jsc) and open-circuit voltage (Voc) than that obtained with conventional GaInP CSC layers without affecting the fill factor (FF). Notably, wide-gap and n-type doping ZnO layers show great potential as electron selective contacts in GaAs solar cells with high barriers at the valence band for blocking hole-transport. An efficiency of 30% is reached for a single-junction GaAs cell with a front ZnO electron selective contact. A 4-terminal GaAs/c-Si tandem solar cell configuration with an estimated conversion performance of 35.16% is proposed. These results indicate enormous potential for the development of low-cost and high-efficiency GaAs-based solar cells in the future.

High Efficiency Inverted GaAs and GaInP/GaAs Solar Cells With Strain‐Balanced GaInAs/GaAsP Quantum Wells

AbstractHigh‐efficiency solar cells are essential for high‐density terrestrial applications, as well as space and potentially vehicle applications. The optimum bandgap for the terrestrial spectrum lies beyond the absorption range of a traditional dual junction GaInP/GaAs cell, with the bottom GaAs cell having higher bandgap energy than necessary. Lower energy bandgaps can be achieved with multiple quantum wells (QWs), but such a pathway requires advanced management of the epitaxial growth conditions in order to be useful. Strain‐balanced GaAsP/GaInAs QWs are incorporated into a single junction GaAs solar cell and a dual junction GaInP/GaAs solar cell, leading to 27.2% efficiency in the single junction device and a one‐sun record 32.9% efficiency in the tandem device. Good carrier collection and low non‐radiative recombination are observed in the cells with up to 80 QWs. The GaAs cells employ a rear‐heterojunction architecture to boost the open‐circuit voltage to over 1.04 V in the quantum well device, despite the large number of QWs.

The study of voltage loss reasons in GaAs solar cells with embedded InGaAs quantum dots

In the work the effect of the number of In0.8Ga0.2As stacked quantum dots (QDs) embedded in a single-junction GaAs solar cell (SC) on its photoelectric characteristics has been studied. A series of GaAs SC structures, which differed by a number of embedded QD layers, were grown using metalorganic vapor phase epitaxy. By analyzing the electroluminescence spectra and the current–voltage characteristics of the obtained structures the main reason for the voltage loss in SC with QDs has been established which is an increase in recombination through the QD levels with an increase the number of QD layers.

Epitaxial Lift-Off of Single-Junction GaAs Solar Cells Grown Via Hydride Vapor Phase Epitaxy

Hydride vapor phase epitaxy (HVPE) is attracting attention as a technology for reducing the epitaxial cost of III–V solar cells. To further reduce manufacturing costs, it is effective to combine HVPE with substrate-reuse technology using epitaxial lift-off (ELO). However, ELO of solar cells grown via HVPE has not yet been demonstrated. Although Al(Ga)As is typically used as the release layer for the ELO process, there is a difficulty in growing Al-containing materials on HVPE because aluminum monochloride reacts with the reactor. Here, we present the growth of AlAs via HVPE using aluminum trichloride generated by the reaction between Al metal and HCl gas at a temperature of 500 °C. The AlAs layer grown via HVPE exhibits sufficient crystal quality to achieve the ELO process. We obtained a conversion efficiency of 21.63% for a HVPE-grown GaAs single-junction solar cell peeled off from a GaAs substrate. This performance is almost similar to that of HVPE-grown solar cells on GaAs substrates without ELO.

Patterning Metal Grids for GaAs Solar Cells with Cracked Film Lithography: Quantifying the Cost/Performance Tradeoff.

We introduce cracked film lithography (CFL) as a way to reduce the cost of III-V photovoltaics (PV). We spin-coat nanoparticle suspensions onto GaAs thin-film device stacks. The suspensions dry in seconds, forming crack networks that we use as templates through which to electroplate the solar cells' front metal grids. For the first time, we show that heating the crack template allows it to flow and refill cracks, which decreases crack footprint and improves final grid transmittance. We demonstrate 24.7%-efficient single-junction GaAs solar cells using vacuum-free CFL grids. These devices are only 1.7% (absolute) less efficient than the baseline grids patterned by photolithography with the loss mostly resulting from the reduced transparency of the CFL pattern. Additional optimization could decrease this difference. Initial cost modeling suggests that CFL is more scalable than photolithography: In particular, CFL's lower materials and equipment costs could greatly reduce the levelized cost of electricity of III-V PV at scale, a potential step toward terrestrial deployment.

Wide-gap ZnO layer as electron-selective front contact for single-junction GaAs solar cells

Herein, a single-junction GaAs solar cell using a wide-gap n-type doped ZnO layer as a front electron-selective contact is proposed. An atomic layer deposition approach was used to deposit a thin ZnO layer on a GaAs substrate, which demonstrated excellent surface passivation properties by significantly reducing the native oxide statuses of the Ga–O and As–O bonds at the GaAs surface. Additionally, n-type doped ZnO layer acted as a front electron-selective contact layer that blocked hole transport and promoted electron collection at the front side. A wide-gap ZnO window layer was shown to enhance cell absorption in the short wavelength range and increase built-in potential, and was proven to be a suitable alternative to the conventional AlGaAs front window layers used in highly efficient single-junction GaAs cells.

Direct growth of GaAs solar cells on Si substrate via mesoporous Si buffer

Due to Silicon (Si) material abundance and lower cost, integration of high efficiency III-V solar cells on Si substrates is of major importance for future solar energy harvesting devices. In this paper, we report on the growth optimization with a detailed characterization of epitaxial growth of crystalline GaAs on porous silicon layers (PSL), and demonstration of single-junction GaAs solar cell on PSL performances. GaAs deposition is performed on engineered porous Si surfaces with different growth temperatures. One and two-steps growth (TSG) were also investigated. X-ray diffraction demonstrated almost one order of magnitude lower threading dislocation density (TDD) of 2× 108 cm-2 for TSG process of GaAs on PSL compared to the one-step growth. Atomic Force Microscopy and Scanning Electron Microscopy showed that a reduction of growth temperature leads to surface morphology improvement. A single junction GaAs solar cell heterostructure grown by TSG and fabricated atop the porous layer, demonstrated higher open-circuit voltage (Voc) and fill factor (FF) when compared to an identical structure grown on crystalline Si (c-Si).

Structural Optimization and Temperature-Dependent Electrical Characterization of GaAs Single-Junction Solar Cells

This paper reports the structural optimization and the temperature-dependent electrical characterization of GaAs single-junction solar cells (SJSCs) based on the p+-n-n+ junction structure. First, the effects of the p+-Al0.9Ga0.1As window layer were investigated using illuminated current density-voltage (LJ−V) and photoreflectance measurements to determine the optimal structure of the SJSC. In addition, the thickness and the doping concentration of different layers, such as the p+-GaAs emitter and the n-GaAs base layers, were optimized, leading to the maximum efficiency (η = 21.53%). These results show that the device performance can be improved further when compared to the world record (η = 28.8%). Second, the capacitance-voltage (C−V) and the dark J−V characteristics of a GaAs SJSC with the optimal structure were investigated over the temperature range from 100 to 300 K. As the temperature was increased, both the junction capacitance at zero voltage (Cj0) and the carrier concentration in the active region (Nd) increased while the depletion width (W) and the built-in potential (Vbi) decreased due to purely thermal effects. Among the SC parameters obtained from the dark J−V curves, the ideality factor (n) decreased from 4.38 to 1.70 and the reverse saturation current density (J0) increased from 6.05 × 10 −12 to 4.26 × 10 −10 A/cm2 with increasing temperature. These values at 300 K were similar to those simulated theoretically and reported previously for high-quality GaAs junction diodes.

GaAs solar cells grown on intentionally contaminated GaAs substrates

III-V materials such as GaAs and GaInP have some of the best electronic and optical properties of any semiconductor materials, but deposition of these materials relies on high-quality single crystal GaAs substrates for their superior performance. Unfortunately, the cost of these substrates makes these high-efficiency devices only accessible in high-value or niche markets. Here, we explore the effect of growing bulk GaAs crystals with lower purity input materials in order to reduce their cost. We observe that intentional impurities added to the melt before the crystal growth occurs segregate to the top of the boule. Single junction GaAs solar cells grown on substrates made from contaminated boules showed no performance degradation compared to a high-purity control substrate. These results suggest that lower purity Ga and As source materials can be used during crystal growth to reduce the cost of substrates.

Influences of PbS Quantum Dot Layers on Power Conversion Efficiency of Single Junction GaAs Solar Cells

PbS quantum dots were coated on the surface of single junction GaAs solar cells by a drop coating method. The thickness of PbS quantum dot layer was controlled through changing the number of coating layers. The influences of PbS coating layers on characteristics of GaAs single junction solar cells were investigated through I-V characteristic measurements, optical reflectance spectra, and quantum efficiencies. The results indicated that, the short-circuit current can be improved up to 15% with two PbS coating layers. Other parameters such as Voc and FF are hardly affected by the number of PbS coating layers. Based on the results of optical reflectance spectra and quantum efficiencies, the enhancement in the short-circuit current can be attributed to the surface antireflection layer and the ability to transfer high energy photon-generated charge carriers.

Spectral analysis of the electroluminescence and photoresponse of heterostructures with InGaAs quantum objects

In this work, physical and optical properties of In0.8Ga0.2As quantum dots (QDs) embedded in the structure of a single-junction GaAs solar cell (SC) grown by MOVPE technique were investigated using spectral characteristics of external quantum yield (EQE) and electroluminescence (EL). It has been found that, in characterizing QD physical parameters, simplified model of a thin stressed quantum well can be applied to a wetting layer (WL). It has been demonstrated that the EL spectra allows determining the absorption energy of photons in WL and QDs more accurately compared to the EQE spectra. Energies of “Ee-Ehh” and “Ee-Elh” transitions in WL have been determined and were 1.325 eV and 1.388 eV respectively. The calculated values of the WL thickness (5.63 Å) and In composition of QDs (xIn = 80%) coincide with the technological parameters used in the epitaxial growth of the device.