High-efficiency GaAs Solar Cells Research Articles

Using electron channeling contrast imaging to inform and improve the growth of high-efficiency GaAs solar cells on nanopatterned GaAs substrates

Patterned substrates provide opportunities for reducing the cost of high-efficiency III-V devices by incorporating mechanically weak layers beneficial for substrate reuse (e.g. by spalling). In this work, the functionality of electron channeling contrast imaging (ECCI) as a tool to efficiently understand and mitigate defect formation is exemplified by developing a process in which high-quality III-V material can be grown on nanopatterned GaAs substrates. Reactive ion etching used in the patterning process was found to damage the GaAs substrate surface, leading to the formation of stacking faults in the epitaxial material as observed by ECCI. Etching the patterned substrates in a 1 NH4OH: 1 H2O2: 50 DI H2O solution for 10 s prior to growth removed the substrate surface damage and stacking faults were no longer present. Growth of solar cell device structures initially produced samples with many macroscale flaws creating shunts in the devices, which complicated the assessment of material quality by device measurements. However, ECCI revealed that the epitaxial material surrounding macroscale flaws was free from any crystallographic defects such as stacking faults and threading dislocations. With this knowledge, we focused on refining the patterning process to eliminate the macroscale flaws. Solar cells were then grown on the improved nanopatterned substrates and exhibited device structures with defect densities less than 5 × 105 cm−2 and average conversion efficiency of 24.8%, nearly identical to devices grown on unpatterned epi-ready substrates (25.0%).

Development of High-Efficiency GaAs Solar Cells Grown on Nanopatterned GaAs Substrates

Development of High-Efficiency GaAs Solar Cells Grown on Nanopatterned GaAs Substrates

Variation in the Photocurrent Response Due to Different Emissive States in Methylammonium Lead Bromide Perovskites

Thin films and crystals of methylammonium lead bromide (MAPbBr3) perovskites have strong photoluminescence (PL). Previous studies have shown that the emission arises from different states. However, the role of these states in the performance of a solar cell has not been reported. We have used photocurrent and photoluminescence microscopies (PCM and PLM) to investigate the correlation between the photocurrent (PC) and the PL behavior in the different regions of MAPbBr3 thin film solar cells. Our results show that the PC and the PL responses from the different regions in the thin film show poor correlation compared to the correlation between those of a high efficiency GaAs solar cell. Furthermore, we establish a relationship between the different emissive states and the PC and the PL responses. Out of the two emissive states at 2.34 and 2.28 eV that have been reported, only the state at 2.34 eV has a dominant contribution to the PC. Our results suggest that the emission at 2.28 eV is related to traps, which...

Effect of low-V/III-ratio metalorganic vapor-phase epitaxy on GaAs solar cells

Single-junction GaAs solar cells were grown by metalorganic vapor-phase epitaxy (MOVPE) at various input V/III ratios. All growth parameters other than V/III ratio were carefully controlled for an accurate comparison. Nearly identical cell performance characteristics including short-circuit current density (Jsc) and open-circuit voltage (Voc) indicate that cell performance is independent of V/III ratio. To determine the relationship between the electrically measured Voc and V/III ratio in a more precise manner, photoluminescence (PL) was applied as a potent optical measurement tool, which does not depend on device processing and contacting issues. We also evaluated the projected cell performance under low-concentration sunlight by electroluminescence (EL) analysis. Similarly to electrical measurement, optical measurement showed no obvious degradation owing to a low V/III ratio. This study strongly demonstrates that low-cost high-efficiency GaAs solar cells can be realized by MOVPE using a low V/III ratio.

Origami Solar-Tracking Concentrator Array for Planar Photovoltaics

Solar-tracking concentrators can potentially lead to low-cost photovoltaic modules that minimize the use of costly semiconductor materials by improving optical collection and coupling. However, solar concentrators and accompanying trackers have proven to be expensive, bulky, and heavy, thereby resulting in increased balance-of-system costs. Here we demonstrate a lightweight and low-profile, and potentially low-cost planar solar-tracking concentrator based on the ancient Japanese art of origami. The tightly packed hexagonal concentrator and tracker arrays are fabricated by cutting and folding thin reflecting sheets that capture and direct concentrated light onto a small, high-efficiency GaAs solar cell. The tracker enables single-axis solar tracking via a simple one-dimensional translational motion of an actuator with minimal energy expense (∼2.9 J/m2/day). Further, we demonstrate stable operation over 10 000 cycles. The solar concentrated cell achieves a 450% increase in diurnal energy output compared wit...

A luminescent down-shifting and moth-eyed anti-reflective film for highly efficient photovoltaic devices.

Adhesive polydimethylsiloxane (PDMS) films were developed to increase the performance of photovoltaic devices. The films combined two separate features of moth-eye patterns to reduce the reflection of incident light at the film surface and luminescent down-shifting (LDS) CdZnS/ZnS-core/shell quantum dots (QDs) to convert ultraviolet (UV) radiation into visible light at 445 nm. The films were both flexible and self-adhesive, easily attachable to any surface of a solar cell module. By simply attaching the developed films on high-efficiency GaAs solar cells, the short circuit current density and power conversion efficiency of the solar cells increased to 33.8 mA cm(-2) and 28.7%, by 1.1 mA cm(-2) and 0.9 percentage points in absolute values, respectively. We showed that the enhancement of the GaAs solar cells was attributed to both the anti-reflection (AR) properties of the moth-eye patterns and the LDS of QDs using a scattering matrix method and external quantum efficiency measurements. The developed films are versatile in application for solar cells, and expected to aid in overcoming limits of material absorption and device structures.

100-period, 1.23-eV bandgap InGaAs/GaAsP quantum wells for high-efficiency GaAs solar cells: toward current-matched Ge-based tandem cells

Major challenges for InGaAs/GaAsP multiple quantum well (MQW) solar cells include both the difficulty in designing suitable structures and, because of the strain-balancing requirement, growing high-quality crystals. The present paper proposes a comprehensive design principle for MQWs that overcomes the trade-off between light absorption and carrier transport that is based, in particular, on a systematical investigation of GaAsP barrier effects on carrier dynamics that occur for various barrier widths and heights. The fundamental strategies related to structure optimization are as follows: (i) acknowledging that InGaAs wells should be thinner and deeper for a given bandgap to achieve both a higher absorption coefficient for 1e-1hh transitions and a lower compressive strain accumulation; (ii) understanding that GaAs interlayers with thicknesses of just a few nanometers effectively extend the absorption edge without additional compressive strain and suppress lattice relaxation during growth; and (iii) understanding that GaAsP barriers should be thinner than 3 nm to facilitate tunneling transport and that their phosphorus content should be minimized while avoiding detrimental lattice relaxation. After structural optimization of 1.23-eV bandgap quantum wells, a cell with 100-period In0.30GaAs(3.5 nm)/GaAs(2.7 nm)/GaAsP0.40(3.0 nm) MQWs exhibited significantly improved performance, showing 16.2% AM 1.5 efficiency without an anti-reflection coating, and a 70% internal quantum efficiency beyond the GaAs band edge. When compared with the GaAs control cell, the optimized cell showed an absolute enhancement in AM 1.5 efficiency, and 1.22 times higher efficiency with 38% current enhancement with an AM 1.5 cut-off using a 665-nm long-pass filter, thus indicating the strong potential of MQW cells in Ge-based 3-J tandem devices. Copyright © 2013 John Wiley & Sons, Ltd.

Numerical Modeling of GaAs Solar Cell Performances

The process of modeling photovoltaic devices is a tedious task in that it depends heavily on several intrinsic and extrinsic properties of the material. In this paper, numerical solutions are obtained using the Personal Computer 1 Dimension (PC1D) software package in order to improve solar cells performance. The analysis deals with high efficiency GaAs solar cells, in order to search the technological parameters leading to optimal performances of the cells, the effects of the doping level and the thicknesses of the base and emitter layers were also investigated. The optimal fill factor and the conversion efficiency that were obtained are 86.76 % and 25.8 % respectively. DOI: http://dx.doi.org/10.5755/j01.eee.19.8.5392

Solar powered micrometeorite sensors using indoor ambient light for the International Space Station

Abstract Sensors for detecting micrometeorite impact locations and magnitudes as well as pressure vessel leaks have been under investigation for some time by the NASA Langley Research Center and other related entities. NASA has been investigating the use of the Distribution Impact Detection System (DIDS) for use on the International Space Station (ISS). However, the DIDS currently requires thionyl chloride lithium batteries which pose explosion and toxicity hazards, and replacing batteries is tedious and utilizes scarce man-hours. Carrying replacement batteries into space is also expensive. To hardwire new sensing devices into the ISS while in orbit would be time consuming. To overcome this problem, high efficiency GaAs solar cells have been studied under low light conditions comparable to those found inside the ISS. The cells were also studied for temperature dependence. Solar concentrators were investigated for possible use with ambient lighting. The power generated by the cells was stored in a large 300 F supercapacitor. A DC to DC boost regulator was modified to produce an output voltage of 3.55 V that is required by the DIDS. The successful operation of the DIDS with ambient light power, supercapacitor energy storage, and boost regulation was demonstrated.

A MBE-grown high-efficiency GaAs solar cell with a directly deposited aluminum front contact

A novel metallization scheme for GaAs p-n solar cells grown by molecular beam epitaxy (MBE) is described. A p/sup +/-GaAs contact layer was grown on top of the AlGaAs window layer, followed by a pure aluminum layer. This MBE-grown aluminum layer serves both as metallization and as self-aligned mask for selective etching down to the AlGaAs window. The solar cell showed an efficiency of about 20%; the I-V characteristics revealed that negligible series resistance was present in the structure. To test the novel p-contact a second sample was grown. Using a transmission-line model (TLM) structure, a metal-semiconductor contact resistance of 1.5*10/sup -2/ Omega -cm/sup 2/ was found. >

GaAs/Ge heterojunction grown by metal-organic chemical vapor deposition and its application to high efficiency photovoltaic devices

We have studied the heteroepitaxial growth of GaAs on Ge substrates by metal-organic chemical vapor deposition (MOCVD). Different growth conditions and substrate orientations were employed to examine the properties of GaAs grown upon Ge substrates, and in particular the GaAs/Ge interface. The interface properties were found to strongly depend on growth conditions. By small changes in the growth temperature, the GaAs/ Ge interface was altered from active to passive. Only a narrow temperature window (600 to 630° C) for the initial GaAs layer growth gave the passive-Ge junction together with good surface morphology. Accordingly, a high efficiency (19%, AMO) GaAs solar cell was grown by atmospheric pressure MOCVD on a Ge substrate without any junction in the Ge.

Effects of metalorganic chemical vapor deposition growth conditions on the GaAs/Ge solar cell properties

We have studied the effects of metalorganic chemical vapor deposition (MOCVD) growth conditions on the properties of GaAs solar cells grown upon Ge substrates, and in particular the GaAs/Ge interface. The interface properties were found to strongly depend on growth conditions. By small changes in the growth temperature, the GaAs/Ge interface was altered from active to passive. Only a narrow temperature window (600–630 °C) for the initial GaAs layer growth gave the passive-Ge junction together with good surface morphology. Accordingly, a high efficiency (19%, AM0) GaAs solar cell was grown by atmospheric pressure MOCVD on a Ge substrate without any junction in the Ge.

7579. High-efficiency GaAs solar cells grown by molecular-beam epitaxy: M R Melloch et al,J Vac Sci Technol,B8, 1990, 379–383

7579. High-efficiency GaAs solar cells grown by molecular-beam epitaxy: M R Melloch et al,J Vac Sci Technol,B8, 1990, 379–383

The effects of trap-induced lifetime variations on the design and performance of high-efficiency GaAs solar cells

The authors investigate the impact of Shockley-Read-Hall (SRH) lifetime behavior directly on GaAs solar cell performance and design optimization for various lifetime-limiting defects, characterized by trap position, cross section, and trap density. It is demonstrated theoretically and experimentally, for a specific cell design, how far GaAs cell efficiency can be from the optimum efficiency if certain types of bulk defects limit the lifetime and are not properly accounted for. A realistic situation is considered where cell designers/manufacturers know the bulk lifetime at some nominal carrier concentrations but are unaware of the trap characteristics or defect responsible for this lifetime. An improved GaAs p/n heteroface cell design is proposed. This design utilizes a high-low junction and a thin base, with AlGaAs back-surface passivation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Reduction of bulk and surface recombination in GaAs for improved solar cell performance

A technique for suppressing recombination centers and increasing minority-carrier diffusion lengths in bulk n-type GaAs is described. It is shown that such pretreatment of the material leads to substantially improved long-wavelength photocurrent collection in Zn-diffused solar cells fabricated directly in the bulk substrates. Passivation of the front surface of the cells with ammonium sulfide improves their short-wavelength photoresponse. These results suggest the possibility of realizing high-efficiency GaAs solar cells fabricated without the use of costly epitaxial technology. >

High-efficiency GaAs solar cells grown by molecular-beam epitaxy

Previously, solar cells fabricated from molecular-beam epitaxually (MBE)-grown material have been inferior in performance to those fabricated from metalorganic chemical vapor deposited (MOCVD) material. We have obtained 1-sun air mass (AM) 1.5 efficiencies of 23.8% for 0.25 cm2 GaAs solar cells fabricated on MBE-grown material. This is the first solar cell fabricated on MBE material which is of comparable performance to solar cells fabricated on MOCVD material. Details of the MBE system preparation and film growth procedure along with a detailed evaluation of the solar cells will be presented.

Numerical analysis for high-efficiency GaAs solar cells fabricated on Si substrates

This paper describes some recent developments in GaAs thin-film solar cells fabricated on Si substrates by metalorganic chemical vapor deposition and numerically analyzes them.GaAs solar cells with efficiency of more than 18% are successfully fabricated on Si substrates by reducing the dislocation density. Photovoltaic properties of GaAs/Si cells are analyzed by considering the effect of nonuniform dislocation distribution on recombination properties of GaAs thin films on Si substrates. Numerical analysis shows that the effect of majority-carrier trapping must be considered. High efficiency GaAs solar cells with total-area efficiency of over 20% on Si substrates can be realized if dislocation density can be reduced to less than 5×105 cm−2.

Tunneling effects in the current-voltage characteristics of high-efficiency GaAs solar cells

We present evidence that tunneling via states in the forbidden gap is the dominant source of excess current in the dark current-voltage (I-V) characteristics of high-efficiency DMCVD grown Al x Ga 1- x As/GaAs ( x ⩾ 0.85) solar cells. The dark forward and reverse I-V measurements were made on several solar cells, for the first time, at temperatures between 193 and 301 K. Low-voltage reverse-bias I-V data of a number of cells give a thermal activation energy for excess current of ∼0.026 ± 0.005 eV, which corresponds to the carbon impurity in GaAs. However, other energy levels between 0.02 eV and 0.04 eV were observed in some cells which may correspond to impurity levels introduced by Cu, Si, Ge or Cd. The forward-bias excess current is mainly due to carrier tunneling between localized levels created in the space-charge layer by impurities such as carbon, which are incorporated during the solar cell growth process. A model is suggested to explain the results.

High-efficiency GaAs solar cells

An updated review of the state of the art in the development of GaAs solar cells is provided, with emphasis on AlGaAs-GaAs cells suitable for space applications. A set of theoretically derived characteristics is given for this type of solar cell. Comparison of measured performance with theory shows excellent agreement. Data on the effects of radiation damage (high-energy electrons, protons, and neutrons) is also integrated into a form useful for evaluation purposes. Techniques for fabricating (AlGa)As-GaAs solar cells in quantities large enough for practical applications are discussed and are shown to have been demonstrated. The possibility of extending these techniques to the fabrication of very thin low-weight cells for space applications is also considered. Finally, the results obtained to date in the development of GaAs solar cells for applications requiring concentrated sunlight are reviewed, for terrestrial as well as for space applications. As a milestone toward the practical application of AlGaAs-GaAs solar cells in space systems, a brief account is provided on the development status of small experimental AlGaAs-GaAs solar-cell panels for specific space flights.

High-Efficiency, Thin-Film GaAs Solar Cells

The present research is directed toward demonstrating the feasibility of producing high-efficiency GaAs solar cells with high power-to-weight ratio by organo-metallic chemical vapor deposition (OM-CVD) growth of thin epi-layers on suitable substrates. Antireflection-coated, metal-oxide-semiconductor (AMOS), GaAs solar cells grown on bulk polycrystalline Ge substrates were initially studied, with the best efficiency achieved being about 9 percent AM1 (7 percent AM0). Subsequently, a new direct deposition method for fabricating ultra-thin top layer, epitaxial n+ /p shallow homojunction solar cells on (100) GaAs substrates (without anodic thinning) was developed by means of which large area (1 cm2) cells were produced with about 19 percent AM1 (15 percent AM0) conversion efficiency. An AM1 conversion efficiency of about 18 percent (14 percent AM0), or about 17 percent (13 percent AM0) with 5 percent grid coverage, was achieved for a single-crystal, GaAs, n+ /p cell grown by OM-CVD on a Ge wafer. These achievements led to the fabrication, for the first time, of thin GaAs epi-layers OM-CVD grown with good crystallographic quality, using a (100) Si-substrate on which a thin Ge epi-interlayer was first deposited by CVD from GeH4 and processed for improved surface morphology.