Single-junction GaAs Solar Cells Research Articles

Modeling lattice-matched InP-based multijunction solar cells

Currently, the multijunction solar cell structure gives the highest efficiency for photovoltaic solar cells. In this study, tandem, triple, and quadruple junction multijunction solar cell structures that include an InP subcell on InP substrate were investigated. A model that can calculate both the voltage-current characteristics and external quantum efficiency was demonstrated. The model gave fitted results with the experimental data for a single junction GaAs solar cell and a tandem solar cell that provides efficiency higher than 30%. The model was used to optimize the lattice-matched AlAsSb/InP tandem, AlAsSb/InP/InGaAsP triple, and AlAsSb/InP/InGaAsP/GaAsSb quadruple junction solar cell designs and the highest efficiencies obtained for these cells were 29.9%, 36.69%, and 42.79%, respectively, under AM1.5 spectrum without any sun concentration. This lattice-matched quadruple junction structure gives the opportunity of having high efficiency without wafer bonding.

Upright and Inverted Single-Junction GaAs Solar Cells Grown by Hydride Vapor Phase Epitaxy

Hydride vapor phase epitaxy (HVPE) is a low-cost alternative to conventional metal-organic vapor phase epitaxy (MOVPE) growth of III-V solar cells. In this work, we show continued improvement of the performance of HVPE-grown single-junction GaAs solar cells. We show over an order of magnitude improvement in the interface recombination velocity between GaAs and GaInP layers through the elimination of growth interrupts, leading to increased short-circuit current density and open-circuit voltage compared with cells with interrupts. One-sun conversion efficiencies as high as 20.6% were achieved with this improved growth process. Solar cells grown in an inverted configuration that were removed from the substrate showed nearly identical performance to on-wafer cells, demonstrating the viability of HVPE to be used together with conventional wafer reuse techniques for further cost reduction. These devices utilized multiple heterointerfaces, showing the potential of HVPE for the growth of complex and high-quality III-V devices.

InGaAs quantum well-dots based GaAs subcell with enhanced photocurrent for multijunction GaInP/GaAs/Ge solar cells

MOCVD-grown GaAs single-junction solar cells (SC) with quantum well-dots (QWD) were fabricated and tested. The QWD were formed by the deposition of In0.4Ga0.6As layers separated with GaAs spacers. A remarkable improvement of photocurrent was achieved and the reduction of open-circuit voltage was partly suppressed by decreasing the spacers’ growth rate as well as increasing their thickness up to 40 nm. Based on the experimentally obtained characteristics of these single-junction SCs we estimated that using QWD media in the middle (GaAs-based) subcell can provide 1 abs. %, increasing the efficiency of the triple-junction GaInP/GaAs/Ge SCs.

Novel double-stage high-concentrated solar hybrid photovoltaic/thermal (PV/T) collector with nonimaging optics and GaAs solar cells reflector

A novel double stage high-concentration hybrid solar photovoltaic thermal (PV/T) collector using nonimaging optics and world record thin film single-junction gallium arsenide (GaAs) solar cells has been developed. We present a detailed design and simulation of the system, experimental setup, prototype, system performance, and economic analysis. The system uses a parabolic trough (primary concentrator) to focus sunlight towards a secondary nonimaging compound parabolic concentrator (CPC) to simultaneously generate electricity from single junction GaAs solar cells, as well as high temperature dispatchable heat. This study is novel in that (a) the solar cells inside the vacuum tube act as spectrally selective mirrors for lower energy photons to maximize the system exergy, and (b) secondary concentrator allows the thermal component to reach a concentration ratio ∼60×, which is significantly higher than conventional PV/T concentration ratios. The maximum outlet temperature reached was 365°C, and on average the thermal efficiency of the experiment was around 37%. The maximum electrical efficiency was around 8%. The total system electricity generation is around 25% of incoming DNI, by assuming the high temperature stream is used to power a steam turbine. The installed system cost per unit of parabolic trough aperture area is $283.10per m2.

Optical design of ZnO-based antireflective layers for enhanced GaAs solar cell performance

A series of hierarchical ZnO-based antireflection coatings with different nanostructures (nanowires and nanosheets) is prepared hydrothermally, followed by means of RF sputtering of MgF2 layers for coaxial nanostructures. Structural analysis showed that both ZnO had a highly preferred orientation along the 〈0001〉 direction with a highly crystalline MgF2 shell coated uniformly. However, a small amount of Al was present in nanosheets, originating from Al diffusion from the Al seed layer, resulting in an increase of the optical bandgap. Compared with the nanosheet-based antireflection coatings, the nanowire-based ones exhibited a significantly lower reflectance (∼2%) in ultraviolet and visible light wavelength regions. In particular, they showed perfect light absorption at wavelength less than approximately 400 nm. However, a GaAs single junction solar cell with nanosheet-based antireflection coatings showed the largest enhancement (43.9%) in power conversion efficiency. These results show that the increase of the optical bandgap of the nanosheets by the incorporation of Al atoms allows more photons enter the active region of the solar cell, improving the performance.

Reflective Barrier Optimization in Ultrathin Single-Junction GaAs Solar Cell

This paper proposes a theoretical analysis of electronic transport in ultrathin (220 nm) single-junction GaAs solar cell. Using an in-house electronic quantum transport model, we shed light on two detrimental phenomena, namely the “back-diffusion” and the “contact-to-contact diffusion.” While the back-diffusion degrades both the short-circuit current and the fill factor, the contact-to-contact diffusion reduces the open-circuit voltage. The so-called window and back-surface-field barriers used to reflect minority carriers away from contacts reduce these two detrimental phenomena. In a second part, we then show a synthesis of performance optimization of window/GaAs/back-surface-field heterojunctions varying thicknesses, materials, and material composition profiles. Our results conclude that the $\mathrm{\text{Al}_{0.4}\text{Ga}_{0.6}\text{As}(10\; nm)/\text{GaAs}/\text{In}_{0.49}\text{Ga}_{0.51}P(10\; nm)}$ structure provides the best output power.

Integration of subwavelength optical nanostructures for improved antireflection performance of mechanically flexible GaAs solar cells fabricated by epitaxial lift-off

We demonstrate the integration of subwavelength moth-eye and Al2O3 nanoisland structures fabricated on polymer packaging sheets and the surface of conventional Al2O3/TiO2 bilayer antireflection coatings, respectively, with epitaxial lift-off single-junction GaAs solar cells. The mechanically flexible cell structure with the integrated optical nanostructures shows substantially improved photovoltaic performance under various incident angles and bending radii compared to devices without such structures: the increase in short-circuit current density arising from integration of these nanostructures ranges from 9% at normal incidence to 52% at 80° incidence; and the reduction in short-circuit current density under moderate bending decreases from 9.7% to 6.7%.

Fabrication and analysis of thin-film GaAs solar cell on flexible thermoplastic substrate using a low-pressure cold-welding

We present flexible thin-film GaAs solar cells fabricated on thermoplastic substrates by a low-pressure cold-welding and epitaxial lift-off process. The use of polyethylene terephthalate (PET) film as a flexible substrate enables cold welding (130 °C) of gold layers between a thin-film GaAs solar cell and PET film with very low mechanical pressure (0.4 MPa), due to their thermoplastic properties. The feasibility of the proposed technique was demonstrated by fabricating GaAs single junction solar cells (without antireflection coating) on PET film, having an efficiency of 13.2%. The fabricated solar cell also showed a stable performance after 2000 cycles of bending.

(Invited) Potential and Activities of III-V/Si Tandem Solar Cells

III-V compound/Si tandem solar cells are expected to have great potential of space and terrestrial application because of high efficiency, light weight and low cost potential. However, problems to be solved are high density of dislocations and large residual strain in III-V compound on Si in the case of hetero epitaxy of III-V compound materials on Si substrates. There are other approaches such as epitaxial lift-off and direct bonding technologies in order to prevent such problems. This paper presents efficiency potential of III-V/Si tandem solar cells with efficiencies of more than 35% under 1-sun AM1.5G and effects of dislocation upon solar cell properties of III-V-on-Si single-junction solar cells and III-V/Si tandem solar cells. Because III-V/Si system has large misfit and difference in thermal expansion coefficient, generation of misfit and thermal stress induced dislocations affect on solar cell properties. Effects of dislocations upon minority-carrier lifetime are derived from considering one dimensional transport of minority carriers to dislocations. In order to realize high efficiency solar cells with similar efficiency by homo-epitaxially grown solar cells, low density dislocation of less than 3x105 cm-2 is necessary. The paper also reviews approaches on reduction in dislocation density in III-V compound films on Si and improvements in efficiencies of III-V compound single-junction solar cells on Si substrates and III-V/Si tandem solar cells. Especially, effective dislocation density reduction in III-V compound films on Si substrates by thermal cycle annealing (TCA) and insertion of strained-layer super-lattices (SLS) in our study are demonstrated. As a result of combination of TCA process and SLS insertion, high quality GaAs films with a dislocation density of 1-2x106 cm-2 have been realized. Reduction in dislocation density of GaAs-on-Si layers due to TCA and SLS insertion have been explained by coalescence of dislocations under high temperature and high stress condition and dislocation movement towards edge and interface by mismatch stress, respectively. In addition, effectiveness of thicker-layer growth, selective area growth, low-temperature growth for hetero epitaxy of III-V compound films on Si substrates are also reviewed. We have demonstrated 20% efficiency (under 1-sun of AM1.5G) with hetero-epitaxially grown GaAs single-junction solar cells on Si. By employing high quality GaAs films on Si, heteroface GaAs thin films solar cells with an Al0.8Gao.2As window layer were fabricated on Si substrates. The thickness of Si substrates was 200mm. The n-GaAs base layer (carrier concentration of 5x1017 cm-3) with a thickness of 1.3-1.5mm was grown on an Al0.6Ga0.4As/GaAs super lattice (SL) layer (20nm/100nm x 5pairs), In0.1Ga0.9 As/GaAs SLS (10nm/10nm x 5 pairs), and thermal cycle annealed GaAs buffer layer (TCA:9000C x 5 times, 2mm thick) which were grown on Si substrates. The thickness and carrier concentration of p-GaAs emitter layer were 0.5 mm and 2x1018 cm-3, respectively. The total epitaxial layer thickness was about 3.5-5 mm. A Si3N4 layer was used as an anti-reflection film. Ti/Ag was used as electrode metal. The cell area was 1 cm2. The total-area conversion efficiency of 18.3% at AM0 and 20.0% at AM1.5G were the highest ever reported for GaAs-on-Si single-junction solar cells. Such high-efficiency GaAs-on-Si cells have been attained by combining TCA, SLS and SL buffer layers. As a result of such a combination, high efficiency GaAs-on-Si solar cells have been realized using high quality GaAs films with a dislocation density of 3.7x106 cm-2. The first space flight of forty-eight 2 cm x 2 cm GaAs-on-Si solar cells with an average AM0 total-area efficiency of 16.9% have been carried out using the Engineering Test Satellite No. 6 (ETS-VI). The GaAs-on-Si solar cells have been demonstrated to be more radiation-resistant on space than GaAs-on-GaAs solar cells and 50, 100 and 200-mm-thick Si space solar cells. These results show that the III-V/Si-based multi-junction solar cells have great potential for space applications. Ohio State University has also demonstrated 17.1% (AM0) efficiency with GaAs single-junction solar cells by using Ge buffer layer. Nagoya Inst. Tech. has achieved 22.1% with GaAs/Si tandem solar cell by hetero epitaxy. Most recently, Fraunhofer ISE has demonstrated 27.9 % efficiency (under 48.3-suns of AM1.5D) InGaP/GaAs/Si 3-junction solar cells by using epitaxial lift-off and direct bonding. Future prospects are also discussed. Further improvement in efficiency for GaAs-on-Si single-junction solar cells and III-V/Si multi-junction solar cells can be obtained by reducing dislocation densities to less than 3x105 cm-2.

Effect of electron irradiation fluence on the output parameters of GaAs solar cell

GaAs solar cells, as other semiconductor devices, suffer from degradations of their electrical and physical properties under particle irradiations (electrons and protons) in space environment. In this work, numerical simulation is used to model the effect of 1 MeV electron irradiation on the performance degradation of a single junction GaAs solar cell, and to identify the defects responsible for the degradation of the output parameters of GaAs solar cells. We found that the GaAs solar cell is sensitive to 1016 cm−2 electron irradiation fluence, and the electron defects E3 and E4, and the hole defect H4 created by 1 MeV electron irradiation, are the most responsible for the degradation of short-circuit current (Jsc). These defects also alter the open-circuit voltage (Voc). Simulation results also reveal that electron trap defect E2 has a significant influence on the solar cell performance when it is combined with the other defects.

Non-Lambertian Reflective Back Scattering and Its Impact on Device Performance of Ultrathin GaAs Single-Junction Solar Cells

This paper studies non-Lambertian scattering and its impacts on the optical properties and device performance of the ultrathin GaAs single-junction solar cell with a reflective back scattering layer. The Phong distribution is used to quantify the scattering effectiveness of the textured back surface, as well as its impacts on device absorptance, emittance, photon extraction and recycling factor, short-circuit current density $(J_{{\rm sc}})$ , external quantum efficiency (EQE), and power conversion efficiency. Both a general GaAs cell design and the ultrathin cell design are carefully investigated. A Phong exponent m of ∼12 is determined by fitting both simulated $J_{{\rm sc}}$ and EQE to their experimental values, with a more accurate averaged reflectivity of the textured Al0.52In 0.48P/Au interface taken into account. Additionally, the measured open-circuit voltage ( $V_{{\rm oc}}$ ) is lower than the best achievable value due to the nonradiative recombination in the device, and a limited lifetime of ∼130 ns is determined by fitting the simulated and measured $V_{{\rm oc}}$ ; a specific series resistivity of 1.2 Ω·cm2 is determined to account for the 77.8% fill factor.

Fabrication and characterization of single junction GaAs solar cell epitaxially grown on Si substrate

In this paper, we present GaAs solar cells on Si substrate by direct epitaxial growth of III–V layers on Si substrate. Fabricated solar cells have shown relatively high energy conversion efficiency of 11.17% without anti reflection coating. By analyzing external quantum efficiency, dark I–V characteristics, and photo luminescence spectra, we have found that possible defect state near the band edge strongly impact on the performance of GaAs solar cell on Si and termination of these defects will further improve the performance of GaAs solar cell directly grown on Si substrates.

Effect of the circle-grid electrodes on concentrated GaAs solar cell efficiency

In this study, we investigate the effect of the shading factor of the front grid pattern on concentrated solar cell efficiency, taking the trade-off between the series resistance of the electrodes and the amount of incident light into consideration. We examine the thermal effect with regard to five different circle-grid electrode patterns of the front contact. The front contacts with different grid patterns affect the characteristics of light-concentrated-type GaAs single-junction solar cells. The device parameters analyzed include the open-circuit voltage (Voc), short-circuit current (Isc), fill factor (FF) and conversion efficiency (η). The results of our study show that for a concentration ratio greater than 60x with AM1.5G, the device with a shading factor of 7.1% has the best cell efficiency of 27.05%, due to the smaller current crowding at the center spot. The results indicate that the conversion efficiency of solar cells can be improved by establishing a compromise between the shading effect and the series resistance effect.

Analysis of InAs/GaAs quantum dot solar cells using Suns-Voc measurements

The performance of InAs/GaAs quantum dot solar cells was investigated up to an optical concentration of 500-suns. A high temperature spacer layer between successive layers of quantum dots was used to reduce the degradation in the open circuit voltage relative to a control device without quantum dots. This improvement is explained using optical data while structural imaging of quantum dot stacks confirm that the devices are not limited by strain. The evolution of the open circuit voltage as a function of number of suns concentration was observed to be nearly ideal when compared with a high performance single junction GaAs solar cell. Analysis of Suns-Voc measurements reveal diode ideality factors as low as 1.16 which is indicative of a low concentration of defects in the devices.

Performance Evaluation and Prediction of Single-Junction and Triple-Junction GaAs Solar Cells Induced by Electron and Proton Irradiations

Space-graded single-junction (SJ) and triple-junction (TJ) GaAs solar cells, produced by MOCVD, are evaluated through different energy electron- and proton-irradiations to compare radiation effects on these solar cells. Mean degradations of the short circuit current, open circuit voltage and maximum power are presented and analyzed. Compared to the radiation data of the single-junction GaAs cell, the triple-junction GaAs cell has a superior radiation-hardness performance at the same electron or proton energy and fluence. Degradations at different electron or proton energies have been correlated with displacement damage dose. Monte Carlo calculations were completed to analyze displacement damage dose deposited in the solar cell active regions by space radiation environments. The performance degradations of both solar cells in space were predicted. This study provides reference data for the design of these GaAs solar arrays in the typical space radiation environments to ensure the security and reliability of on-orbit spacecrafts.

Ultra-thin GaAs single-junction solar cells integrated with a reflective back scattering layer

This paper reports the proposal, design, and demonstration of ultra-thin GaAs single-junction solar cells integrated with a reflective back scattering layer to optimize light management and minimize non-radiative recombination. According to our recently developed semi-analytical model, this design offers one of the highest potential achievable efficiencies for GaAs solar cells possessing typical non-radiative recombination rates found among commercially available III-V arsenide and phosphide materials. The structure of the demonstrated solar cells consists of an In0.49Ga0.51P/GaAs/In0.49Ga0.51P double-heterostructure PN junction with an ultra-thin 300 nm thick GaAs absorber, combined with a 5 μm thick Al0.52In0.48P layer with a textured as-grown surface coated with Au used as a reflective back scattering layer. The final devices were fabricated using a substrate-removal and flip-chip bonding process. Solar cells with a top metal contact coverage of 9.7%, and a MgF2/ZnS anti-reflective coating demonstrated open-circuit voltages (Voc) up to 1.00 V, short-circuit current densities (Jsc) up to 24.5 mA/cm2, and power conversion efficiencies up to 19.1%; demonstrating the feasibility of this design approach. If a commonly used 2% metal grid coverage is assumed, the anticipated Jsc and conversion efficiency of these devices are expected to reach 26.6 mA/cm2 and 20.7%, respectively.

Efficiency improvement of III–V GaAs solar cells using biomimetic TiO2 subwavelength structures with wide-angle and broadband antireflection properties

The power conversion efficiency (PCE) improvement of III–V gallium arsenide (GaAs) single-junction (SJ) solar cells with titanium dioxide (TiO2) subwavelength structures (SWSs) as antireflection coatings (ARCs) is reported. TiO2 SWSs are fabricated by dry etching using dotted gold nanopatterns prepared with glancing angle deposition and thermal dewetting processes. Their optical reflection properties are studied in the wavelength range of 350–900nm at incident light angles (θi) of 3–80°, together with the theoretical prediction by a rigorous coupled-wave analysis. For SWSs on the 50nm-thick TiO2 layer/GaAs substrate, a lower solar weighted reflectance (SWR) of ~6.2% is obtained compared to that (i.e., SWR ~9.5%) of the TiO2 single-layer ARC (SLARC). By employing the TiO2 SWSs into the III–V GaAs SJ solar cell, the increased short circuit current density (Jsc) of 24.82mA/cm2 is achieved, thus leading to an improved PCE of 19.66% at θi=0° (i.e., Jsc=18.38 and 23.65mA/cm2 and PCE=14.74 and 18.98% for the solar cells with bare surface and TiO2 SLARC, respectively). For incident angle-dependent device characteristics, the solar cell with the TiO2 SWSs also exhibits superior performance over a wide θi range of 20–80°. Additionally, after the encapsulation, its Jsc and PCE values are slightly increased to 26.15mA/cm2 and 20.73%, respectively. For the stability, there are no significant variations in Jsc and PCE after about 50 days.

Carrier extraction from GaSb quantum rings in GaAs solar cells using direct laser excitation

The authors report on the analysis of the hole escape mechanisms from type-II GaSb quantum rings (QRs) embedded within the active region of a GaAs single junction solar cell. When the solar cell is excited by using a 1064 nm infrared laser with excitation energy lower than the bandgap of the GaAs matrix, photogenerated electron-hole pairs are created directly within the GaSb QRs. The QR photocurrent exhibits a linear dependence on the excitation intensity over several decades. The thermal activation energy was found to be weakly dependent on the incident light level and increased by only a few meV over several orders of excitation intensity. The magnitude of the relative absorption in the author's QRs when directly probed by using a 1064 nm laser with an incident power density of approximate to 2.6 W cm(-2) is found to be approximate to 1.4 x 10(-4) per layer. The thermal escape rate of the holes was calculated and found to be approximate to 10(11) to 10(12) s(-1), which is much faster than the radiative recombination rate 10(9) s(-1). This behaviour is promising for concentrator solar cell development and has the potential to increase solar cell efficiency under a strong solar concentration.

Influences of Au nanoparticle density on the performance of GaAs solar cells

The influence of the Au nanoparticle density on the performance of single-junction GaAs solar cells has been investigated. The single-junction GaAs solar cells are grown on (100) GaAs substrates by using a low-pressure metalorganic chemical vapor deposition (MOCVD) system. Au nanoparticles are dispersed on the surfaces of the cells by micro-pipetted casting and the density of the dispersed nanoparticles varies from 4.7 × 109 to 4.3 × 1010 cm−2. The conversion efficiencies of the GaAs solar cells are observed to depend strongly on the Au nanoparticle densities. For Au nanoparticle densities lower than 2.8 × 1010 cm−2, the conversion efficiency improvement of the GaAs solar cells mainly originates from the antireflection coating effect. In contrast, for the sample with a Au nanoparticle density of 2.8 × 1010 cm−2, the antireflection effect decreases due to the shadow effect and the local field enhancement effect due to the Au nanoparticles starts to be effective. The GaAs solar cell with a Au nanoparticle density of 1.9 × 1010 cm−2 shows a maximum conversion enhancement compared to that of the reference sample.

Carrier extraction behaviour in type II GaSb/GaAs quantum ring solar cells

The introduction of quantum dot (QD) or quantum ring (QR) nanostructures into GaAs single-junction solar cells has shown enhanced photo-response above the GaAs absorption edge, because of sub-bandgap photon absorption. However, to further improve solar cell performance a better understanding of the mechanisms of photogenerated carrier extraction from QDs and QRs is needed. In this work we have used a direct excitation technique to study type II GaSb/GaAs quantum ring solar cells using a 1064 nm infrared laser, which enables us to excite electron–hole pairs directly within the GaSb QRs without exciting the GaAs host material. Temperature and laser intensity dependence of the current–voltage characteristics revealed that the thermionic emission process produced the dominant contribution to the photocurrent and accounts for 98.9% of total photocurrent at 0 V and 300 K. Although the tunnelling process gives only a low contribution to the photocurrent, an enhancement of the tunnelling current was clearly observed when an external electric field was applied.