Search

GaAs is often used as a multijunction subcell due to its high material quality on GaAs substrates, despite having a non-optimal bandgap. The bandgap can be beneficially reduced using many layers of thin, strain-balanced GaInAs in a superlattice or quantum well device, but achieving excellent carrier collection without increased recombination has proven challenging. Here, we develop and demonstrate high performance, optically thick GaInAs/GaAsP strain-balanced solar cells. Excellent material quality is achieved in thick superlattices by using growth conditions that limit progressive thickness and composition fluctuations. Bandgap-voltage offsets as low as 0.31 V are shown in superlattice cells using thin, highly strained GaP barriers. Optically thick superlattice cells with over 2500 nm of total GaInAs in the depletion region are developed, enabling 3.8 mA/cm2 of extra photocurrent beyond the GaAs band edge under the AM0 space spectrum. Optimized superlattice solar cells are incorporated into two-junction devices that achieve 29.2% efficiency under the AM0 space spectrum due to their improved bandgap combination and high subcell voltages. [ABSTRACT FROM AUTHOR]

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

High‐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. [ABSTRACT FROM AUTHOR]

The optical and electrical properties of many III-V alloys change with the degree of CuPt atomic ordering, which is very sensitive to growth conditions. The bulk ordered alloy is elongated along the normal to the ordered planes, and is asymmetrically strained when coherent to a cubic substrate. Here, we demonstrate in situ measurement of the anisotropic strain due to ordering using two-dimensional wafer curvature. The measurement is sensitive to bulk anisotropies, and so is complementary to other in situ measurements that are sensitive to surface anisotropies. Using ab initio calculations, we determine a maximum strain anisotropy of 0.27% between [110] and [110] when perfectly ordered single-variant GaInP2 is coherent to a (001) cubic substrate. We relate the in situ measurement of strain anisotropy on various GaInP2 samples to ex situ measurements of the order parameter to validate the measurement and confirm the capability to predict material properties. The measurement monitors change in ordering during growth, useful for quickly determining the growth condition dependence of ordering or monitoring order-disorder transitions. More generally, this measurement technique could, in principle, be used to monitor phase changes in any epitaxial system for which the strain anisotropy of the two phases differs. [ABSTRACT FROM AUTHOR]

By using photovoltaic cells under high-intensity laser illumination, much higher photoconversion efficiencies are obtained than under the solar spectrum. We demonstrate a monocrystalline Si based minimodule to convert laser light into electricity using edge-illuminated Si "minicells" based on polysilicon on silicon oxide passivating contacts. Ten fully metallized devices were stacked in series and illuminated from the edge, creating a high-voltage, low-current minimodule that mitigates resistive and Auger-Meitner losses. The minimodule shows a high open-circuit voltage of >5 V when tested under 1-Sun illumination and >7 V at higher illumination intensities when tested with a flash simulator. The fill factor in these minimodules is limited by shunting at low illuminations but reaches a maximum of ∼78% for higher illumination intensities >40 Suns (current density >1200 mA/cm2), indicating that the minimodules are not limited by series resistance up to 40 Suns. Under 1000 nm monochromatic light, we measure efficiencies >40% at > 40 Suns equivalent illumination, showing the potential of passivated contact Si cells for laser power conversion. • c- Si infrared laser power converters utilizing poly- Si/SiO x passivating contacts. • "Minicell" devices are fabricated with simple processing and combined into high-voltage, low-current multijunction. • Modules achieve efficiency >40% for infrared illumination >40 Suns. • Low series resistance is achieved despite only mechanical contacting of individual minicells within module. • Quokka3 simulations predict behavior and inform pathways for improvement. [ABSTRACT FROM AUTHOR]

The growth of quaternary Ga 0.68 In 0.32 As 0.35 P 0.65 by metal-organic vapor phase epitaxy is very sensitive to growth conditions because the composition is within a miscibility gap. In this investigation, we fabricated 1 µm-thick lattice-matched GaInAsP films grown on GaAs(001) for application to solar cells. The growth temperature and substrate miscut are varied in order to characterize the effect of the surface diffusion of adatoms on the material quality of alloys. Transmission electron microscopy and two-dimensional in-situ multi-beam optical stress determine that growth temperatures of 650 °C and below enhance the formation of the CuPt B atomic ordering and suppress material decomposition, which is found to occur at the growth surface. The root-mean-square (RMS) roughness is reduced from 33.6 nm for 750 °C to 1.62 nm for 650 °C, determined by atomic force microscopy. Initial investigations show that the RMS roughness can be further reduced using increased miscut angle, and substrates miscut toward (111)A, leading to an RMS roughness of 0.56 nm for the sample grown at 600 °C on GaAs miscut 6° toward (111)A. Using these conditions, we fabricate an inverted hetero-junction 1.62 eV Ga 0.68 In 0.32 As 0.35 P 0.65 solar cell without an anti-reflection coating with a short-circuit current density, open-circuit voltage, fill factor, and efficiency of 12.23 mA/cm 2 , 1.12 V, 86.18%, and 11.80%, respectively. [ABSTRACT FROM AUTHOR]

Inverted metamorphic Ga0.3In0.7As photovoltaic converters with sub‐0.60 eV bandgaps grown on InP and GaAs are presented. Threading dislocation densities are 1.3 ± 0.6 × 106 and 8.9 ± 1.7 × 106 cm−2 on InP and GaAs, respectively. The devices generate open‐circuit voltages of 0.386 and 0.383 V, respectively, under irradiance producing a short‐circuit current density of ≈10 A cm−2, yielding bandgap‐voltage offsets of 0.20 and 0.21 V. Power and broadband reflectance measurements are used to estimate thermophotovoltaic (TPV) efficiency. The InP‐based cell is estimated to yield 1.09 W cm−2 at 1100 °C versus 0.92 W cm−2 for the GaAs‐based cell, with efficiencies of 16.8 versus 9.2%. The efficiencies of both devices are limited by sub‐bandgap absorption, with power weighted sub‐bandgap reflectances of 81% and 58%, respectively, the majority of which is assumed to occur in the graded buffers. The 1100 °C TPV efficiencies are estimated to increase to 24.0% and 20.7% in structures with the graded buffer removed, if previously demonstrated reflectance is achieved. These devices also have application to laser power conversion in the 2.0–2.3 µm atmospheric window. Peak laser power converter efficiencies of 36.8% and 32.5% are estimated under 2.0 µm irradiances of 1.86 and 2.81 W cm−2, respectively. [ABSTRACT FROM AUTHOR]

Ordering-induced effects on dislocations in metallic alloys have been extensively studied due to their importance in technology applications. We demonstrate that dislocation behavior in ordered III-V semiconductor alloys can be drastically different. This is because ordering in bulk metallic alloys is generally stable, whereas the surface-stabilized group-III sublattice ordering of a III-V alloy is only metastable in the bulk. Here, we show that dislocation glide can release some of the energy stored by ordering of III-V alloys, enhancing the glide of any dislocation which cuts through the ordered layers to create an antiphase boundary in the ordering pattern. This leads to an experimentally observed glide-plane switch which is unique to ordered III-V alloys. Implications for other unique strain-relaxation processes in III-V ordered alloys are also discussed. [ABSTRACT FROM AUTHOR]

Antiphase-domain (APD) free GaP films were grown on Si(100) substrates prepared by annealing under dilute AsH3 in situ in an MOCVD reactor. LEED and AES surface analysis of Si(100) surfaces prepared by this treatment show that AsH3 etching quickly removes O and C contaminants at a relatively low temperature (690-740 °C), and creates a single-domain "A-type" As/Si surface reconstruction. The resulting GaP epilayers grown at the same temperature are APD-free, and could thereby serve as templates for direct growth of III-V semiconductors on Si. This single chamber process has a low thermal budget, and can enable heteroepitaxial integration of III-Vs and Si at an industrial scale. [ABSTRACT FROM AUTHOR]

WideÐband gap metal halide perovskites are promising semiconductors to pair with silicon in tandem solar cells to pursue the goal of achieving power conversion efficiency (PCE) greater than 30% at low cost. However, wideÐband gap perovskite solar cells have been fundamentally limited by photoinduced phase segregation and low open-circuit voltage. We report efficient 1.67Ðelectron volt wideÐband gap perovskite top cells using triple-halide alloys (chlorine, bromine, iodine) to tailor the band gap and stabilize the semiconductor under illumination. We show a factor of 2 increase in photocarrier lifetime and charge-carrier mobility that resulted from enhancing the solubility of chlorine by replacing some of the iodine with bromine to shrink the lattice parameter. We observed a suppression of light-induced phase segregation in films even at 100-sun illumination intensity and less than 4% degradation in semitransparent top cells after 1000 hours of maximum power point (MPP) operation at 60°C. By integrating these top cells with silicon bottom cells, we achieved a PCE of 27% in two-terminal monolithic tandems with an area of 1 square centimeter. [ABSTRACT FROM AUTHOR]

• ECCI is used to identify stacking fault pyramids in GaP on Si nucleation layers. • Stacking fault pyramids act as blocking sites for the glide of dislocations. • The pulsed GaP growth initiation process influences the formation of stacking fault pyramids. • A reduction of the stacking fault pyramid density from 6 × 107 cm−2 to 4 × 105 cm−2 is achieved. Thin gallium phosphide layers were deposited on (0 0 1) Silicon surfaces via organometallic vapor phase epitaxy and characterized by electron channeling contrast imaging (ECCI). Stacking fault pyramids at a density of up to 6 × 107 cm−2 were identified in the GaP nucleation layer by varying the diffraction conditions. We show that these defects originate at the GaP/Si interface and propagate on all four {1 1 1} planes. We observed that such stacking fault pyramids interact with the gliding of misfit dislocations during lattice-mismatched growth and enhance the threading dislocation density. The initial pulsing of TEGa and TBP during the nucleation of GaP on Silicon has been found to strongly influence the formation of those pyramidal defects. Changing the number of pulse cycles allowed us to lower their density by two orders of magnitude from 6 × 107 cm−2 to 4 × 105 cm−2. [ABSTRACT FROM AUTHOR]

We developed a monolithic CdTe-PbS tandem solar cell architecture in which both the CdTe and PbS absorber layers are solution-processed from nanocrystal inks. Due to their tunable nature, PbS quantum dots (QDs), with a controllable band gap between 0.4 and ∼1.6 eV, are a promising candidate for a bottom absorber layer in tandem photovoltaics. In the detailed balance limit, the ideal configuration of a CdTe (Eg = 1.5 eV)-PbS tandem structure assumes infinite thickness of the absorber layers and requires the PbS band gap to be 0.75 eV to theoretically achieve a power conversion efficiency (PCE) of 45%. However, modeling shows that by allowing the thickness of the CdTe layer to vary, a tandem with efficiency over 40% is achievable using bottom cell band gaps ranging from 0.68 and 1.16 eV. In a first step toward developing this technology, we explore CdTe-PbS tandem devices by developing a ZnTe-ZnO tunnel junction, which appropriately combines the two subcells in series. We examine the basic characteristics of the solar cells as a function of layer thickness and bottom-cell band gap and demonstrate open-circuit voltages in excess of 1.1 V with matched short circuit current density of 10 mA/cm2 in prototype devices. [ABSTRACT FROM AUTHOR]

Direct growth of GaP on Si enables the integration of III–V and Si optoelectronic devices for a wide variety of applications, and has therefore been the subject of much research for many decades. Most of this effort has been directed toward overcoming the two main technical challenges: 1) removing atmospheric oxygen and carbon contamination and 2) establishing the conditions needed for “APD-free” III–V epitaxy ( i.e ., without antiphase domains). We have developed an OMVPE process for APD-free GaP growth on Si which overcomes both challenges by using AsH 3 to clean and prepare the Si surface in situ at a relatively low temperature. This process is based upon a very brief “AsH 3 -cleaning” step which simultaneously removes atmospheric contamination (thereby eliminating the need for Si regrowth) and creates a single-domain As-terminated Si surface. Here we discuss the key process steps using results from a suite of analysis tools. [ABSTRACT FROM AUTHOR]

Photovoltaic conversion efficiencies of 32.6 ± 1.4% under the AM1.5 G173 global spectrum, and 35.5% ± 1.2% at 38-suns concentration under the direct spectrum, are demonstrated for a monolithic, dual-junction 1.7/1.1 eV solar cell. The tandem cell consists of a 1.7 eV GaInAsP top-junction grown lattice-matched to a GaAs substrate, followed by a metamorphic 1.1 eV GaInAs junction grown on a transparent, compositionally graded metamorphic AlGaInAs buffer. This bandgap combination is much closer to the dual-junction optimum and offers headroom for absolute 3% improvement in efficiency, in comparison to the incumbent lattice-matched GaInP/GaAs (∼1.86/1.41 eV) solar cells. The challenge of growing a high-quality 1.7 eV GaInAsP solar cell is the propensity for phase separation in the GaInAsP alloy. The challenge of lattice-mismatched GaInAs solar cell growth is that it requires minimizing the residual dislocation density during the growth of a transparent compositionally graded buffer to enable efficient metamorphic tandem cell integration. Transmission electron microscopy reveals relatively weak composition fluctuation present in the 1.7 eV GaInAsP alloy, attained through growth control. The threading dislocation density of the GaInAs junction is ∼1 × 106 cm−2, as determined from cathodoluminescence measurements, highlighting the quality of the graded buffer. These material advances have enabled the performance of both junctions to reach over 80% of their Shockley-Queisser limiting efficiencies, with both the subcells demonstrating a bandgap-voltage offset, WOC (=Eg/q-VOC), of ∼0.39 V. [ABSTRACT FROM AUTHOR]