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InGaN/GaN Multiple Quantum Well Solar Cells with Good Open-Circuit Voltage and Concentrator Action
Abstract
The photovoltaic properties of large-chip-size (2.5× 2.5
mm2) InGaN/GaN multiple quantum well (MQW) solar cells grown
by metal organic chemical vapor deposition were studied under
concentrated AM1.5G sun irradiation. We demonstrate a high open-circuit
voltage of 2.31 V for blue-light-emitting InGaN/GaN MQW solar cells
under 1 sun. The higher open-circuit voltage is mainly ascribed to the
extremely low reversed saturation current density of approximately
10-19 mA/cm2. The open-circuit voltage and
short-circuit current density were found to increase as sunlight
intensity increases, with a peak value of 2.50 V observed at 190 suns,
showing a great potential for concentrator applications.
... The power conversion efficiency (ƞ) has an increase of ~20% when the concentration increase from 1 to 333 suns corresponding to an enhancement of 19% in Voc, while FF remains almost constant. Additionally, the optimized quantum well solar cells show a relatively high Voc of 2.31 V [5], displaying an equivalent band-gap (2.76 eV from electroluminescence test) to Voc difference (Eg/q-Voc) of 0.45 V. A discussion behind this high Voc is given based on evaluation of materials and device properties in the dark case. ...
We report photovoltaic properties and concentration action of InGaN/GaN multi-quantum well (MQW) solar cells on c-plane sapphire substrate grown by metal-organic vapor phase epitaxy (MOVPE). The fabricated QW solar cells show a comparatively high open-circuit voltage of 2.06 at one sun and good concentration properties. The open circuit voltage (Voc) keeps increasing logarithmically with concentration ratio until 60 suns. The peak Voc of InGaN/GaN MQW solar cells, which have a predominant peak energy of 2.7 eV from electroluminescence (EL) measurements, is found to be 2.45 V when the concentration ratio reaches 333x. It was found that after an optimization of InGaN/GaN MQWs, the Voc can reach 2.31 V, displaying a bandgap to Voc difference (Eg/q-Voc) of 0.45 V. The obtained Eg/q-Voc is comparable to that of single-crystalline silicon or GaAs solar cells. The higher open-circuit voltage is mainly ascribed to extremely very low reversed saturation current density of around 10-19 mA/cm2, which could be attributed to the good crystal quality evidenced by TEM and HRXRD measurements. In addition, we give some discussions upon dependences of conversion efficiency and fill factor on concentration ratio for the fabricated MQW solar cells.
We proposed a new approach of enhancing the quantum efficiency of InGaN/GaN multiple quantum well (MQW) solar cells using resonant cavity in this study. The resonant-cavity-enhanced (RCE) structure is designed with a top mirror and bottom distributed Bragg reflector (DBR). The origin of the enhancement in quantum efficiency was the resonance-induced increase of the optical field, which caused more photons to be absorbed in the InGaN absorption layers. We simulated the photovoltaic performance of InGaN/GaN MQW solar cells by comparing RCE-type and conventional solar cells. The best structural parameters are estimated to be 600 nm for cavity length, 10% for the reflectivity of top mirror and as high as possible for the reflectivity of bottom DBR (in simulation we used 99%). As a result, the RCE-type InGaN/GaN MQW solar cell shows an enhancement in short-circuit current density by 2.12 times and conversion efficiency by 2.13 times, as compared to those of a conventional InGaN/GaN MQW solar cell.
We demonstrate high quantum efficiency InGaN/GaN multiple quantum well (QW) solar cells with spectral response extending out to 520 nm. Increasing the number of QWs in the active region did not reduce the carrier collection efficiency for devices with 10, 20, and 30 QWs. Solar cells with 30 QWs and an intentionally roughened p-GaN surface exhibited a peak external quantum efficiency (EQE) of 70.9% at 390 nm, an EQE of 39.0% at 450 nm, an open circuit voltage of 1.93 V, and a short circuit current density of 2.53 mA/cm2 under 1.2 suns AM1.5G equivalent illumination.
In this paper, InGaN/GaN multiple quantum well solar cells (MQWSCs) with an In content of 0.15 are fabricated and studied. The short-circuit density, fill factor and open-circuit voltage (Voc) of the device are 0.7 mA/cm2, 0.40 and 2.22 V, respectively. The results exhibit a significant enhancement of Voc compared with those of InGaN-based hetero and homojunction cells. This enhancement indicates that the InGaN/GaN MQWSC offers an effective way for increasing Voc of an In-rich InxGa1-xN solar cell. The device exhibits an external quantum efficiency (EQE) of 36% (7%) at 388 nm (430 nm). The photovoltaic performance of the device can be improved by optimizing the structure of the InGaN/GaN multiple quantum well.
We investigated the photovoltaic performance of InGaN/GaN multiple quantum well (MQW) solar cells by comparing vertical-type and conventional lateral-type solar cells. We found that both bottom reflector and front surface texturing of vertical-type InGaN/GaN MQW solar cells enhanced light absorption by 45%, leading to an enhancement of the short circuit current density (JSC) by 1.6 times, compared to that of a lateral-type structure. For the vertical-type InGaN/GaN solar cell, Ag was used for bottom reflectors and pyramid textured surfaces were formed by KOH etching after a lift-off process, whereas lateral-type structures were fabricated on sapphire substrates having smooth surfaces. As a result, the vertical InGaN/GaN MQW solar cells showed a high fill factor of 80.0% and conversion efficiency of 2.3%; in contrast, the conventional lateral structure produced a fill factor of 77.6% and a conversion efficiency of 1.4%.
The optical properties of wurtzite-structured InN grown on sapphire substrates by molecular-beam epitaxy have been characterized by optical absorption, photoluminescence, and photomodulated reflectance techniques. These three characterization techniques show an energy gap for InN between 0.7 and 0.8 eV, much lower than the commonly accepted value of 1.9 eV. The photoluminescence peak energy is found to be sensitive to the free-electron concentration of the sample. The peak energy exhibits very weak hydrostatic pressure dependence, and a small, anomalous blueshift with increasing temperature.
We present the growth, fabrication, and photovoltaic characteristics of In(x) Ga(1-x)N/GaN(x similar to 0.35) multiple quantum well solar cells for concentrator applications. The open circuit voltage, short circuit current density, and solar-energy-to-electricity conversion efficiency were found to increase under concentrated sunlight. The overall efficiency increases from 2.95% to 3.03% when solar concentration increases from 1 to 30 suns and could be enhanced by further improving the material quality. (C) 2010 American Institute of Physics. [doi:10.1063/1.3481424]
We report on the unique thermal properties of In0.28Ga0.72N/GaN multiple quantum well solar cells. The devices exhibited an external quantum efficiency of 69% (26%) at 390 nm (460 nm), an open circuit voltage of 2.04 V, a fill factor of 63%, a short circuit current density of 2 mA/cm2, and a peak output power of 2.63 mW/cm2 at room temperature under 1-sun AM1.5G illumination. Thermal measurements showed that the peak output power increased with temperature up to 2.73 mW/cm2 at 70 °C, signifying the potential of III-nitride solar cells for concentrator photovoltaic applications.
High-efficiency multijunction or tandem solar cells based on group III–V semiconductor alloys are applied in a rapidly expanding range of space and terrestrial programs. Resistance to high-energy radiation damage is an essential feature of such cells as they power most satellites, including those used for communications, defense, and scientific research. Recently we have shown that the energy gap of In1−xGaxN alloys potentially can be continuously varied from 0.7 to 3.4 eV, providing a full-solar-spectrum material system for multijunction solar cells. We find that the optical and electronic properties of these alloys exhibit a much higher resistance to high-energy (2 MeV) proton irradiation than the standard currently used photovoltaic materials such as GaAs and GaInP, and therefore offer great potential for radiation-hard high-efficiency solar cells for space applications. The observed insensitivity of the semiconductor characteristics to the radiation damage is explained by the location of the band edges relative to the average dangling bond defect energy represented by the Fermi level stabilization energy in In1−xGaxN alloys. © 2003 American Institute of Physics.
We report the growth of InGaN thick films and InGaN/GaN double heterostructures by molecular beam epitaxy at the substrate temperatures 700–800 °C, which is optimal for the growth of GaN. X-ray diffraction and optical absorption studies show phase separation of InN for InxGal−xN thick films with x>0.3. On the other hand, InxGal−xN/GaN double heterostructures show no evidence of phase separation within the detection capabilities of our methods. These observations were accounted for using Stringfellow’s model on phase separation, which gives a critical temperature for miscibility of the GaN–InN system equal to 2457 K.© 1997 American Institute of Physics.
A photovoltaic conversion efficiency of 40.8% at 326 suns concentration is demonstrated in a monolithically grown, triple-junction III-V solar cell structure in which each active junction is composed of an alloy with a different lattice constant chosen to maximize the theoretical efficiency. The semiconductor structure was grown by organometallic vapor phase epitaxy in an inverted configuration with a 1.83 eV Ga{sub .51}In{sub .49}P top junction lattice-matched to the GaAs substrate, a metamorphic 1.34 eV In{sub .04}Ga{sub .96}As middle junction, and a metamorphic 0.89 eV In{sub .37}Ga{sub .63}As bottom junction. The two metamorphic junctions contained approximately 1 x 10{sup 5} cm{sup -2} and 2-3 x 10{sup 6} cm{sup -2} threading dislocations, respectively.
We report on the fabrication and photovoltaic characteristics of InGaN solar cells by exploiting InGaN/GaN multiple quantum wells (MQWs) with In contents exceeding 0.3, attempting to alleviate to a certain degree the phase separation issue and demonstrate solar cell operation at wavelengths longer than previous attainments (>420 nm). The fabricated solar cells based on In(0.3)Ga(0.7)N/GaN MQWs exhibit an open circuit voltage of about 2 V, fill factor of about 60%, and an external efficiency of 40% (10%) at 420 nm (450 nm).
High-quality p- Ga N /i- In <sub>0.1</sub> Ga <sub>0.9</sub> N /n- Ga N heterojunctional epilayers are grown on (0001)-oriented sapphire substrates by metal organic chemical vapor deposition. The Pendellösung fringes around the InGaN peak in high-resolution x-ray diffraction (HRXRD) confirm a sharp interface between InGaN and GaN films. The corresponding HRXRD and photoluminescence measurements demonstrate that there is no observable phase separation. The improvement in crystal quality yields high-performance photovoltaic cells with open-circuit voltage of around 2.1 eV and fill factor up to 81% under standard AM 1.5 condition. The dark current-voltage measurements show very large shunt resistance, implying an insignificant leakage current in the devices and therefore achieving the high fill factor in the illuminated case.
The absorption coefficient for a 0.4-μm-thick GaN layer grown on a polished sapphire substrate was determined from transmission measurements at room temperature. A strong, well defined exciton peak for the A and B excitons was obtained. The A, B, and C excitonic features are clearly defined at 77 K. At room temperature, an energy gap Eg=3.452±0.001 eV and an exciton binding energy ExA,B=20.4±0.5 meV for the A and B excitons and ExC=23.5±0.5 meV for the C exciton were determined by analysis of the absorption coefficient. From this measured absorption coefficient, together with the detailed balance approach of van Roosbroek and Shockley, the radiative constant B=1.1×10−8 cm3/s was obtained.
InGaN/sapphire-based photovoltaic (PV) cells with blue-band GaN/InGaN multiple-quantum-well absorption layers grown on patterned sapphire substrates were characterized under high concentrations up to 150-sun AM1.5G testing conditions. When the concentration ratio increased from 1 to 150 suns, the open-circuit voltage of the PV cells increased from 2.28 to 2.50 V. The peak power conversion efficiency (PCE) occurred at the 100-sun conditions, where the PV cells maintained the fill factor as high as 0.70 and exhibited a PCE of 2.23%. The results showed great potential of InGaN alloys for future high concentration photovoltaic applications.
Although the fill factor of a solar cell is a useful parameter in characterizing the cell performance, it cannot be expressed explicitly in terms of other cell parameters. The accuracy of previously published and new analytical approximations for this factor are compared and the most useful expressions are summarized, including the effects of temperature, ideality factor and series and shunt parasitic resistances.
Variable temperature (1.7–400 K) and field (0–30 T) Hall measurements were employed to study carrier transport phenomena in p-type GaN (C) and n-type InN and InGaN. All three materials show metallic conduction which in the p-GaN is due to compensated carbon-related acceptors and in the InN and InGaN due to intrinsic shallow donors. The InGaN shows a negative magnetoresistance over the complete investigated temperature range.
High-performance GaInN-based solar cells with high open-circuit voltage, high short-circuit current density, and good fill factor have been obtained using a combination of two different GaInN superlattice structures. The GaInN barrier thicknesses (3 and 0.6 nm) in both superlattice structures were optimized, resulting in a thick GaInN-based active layer with a low pit density in the device. The conversion efficiency is approximately 2.5% under a solar simulator of air mass 1.5G and an irradiation intensity of 155 mW/cm2.
We fabricated and characterized a nonpolar a-plane nitride-based solar cell on an r-plane sapphire substrate. The maximum external quantum efficiency of the solar cell reached 62% at a wavelength of approximately 400 nm. The open-circuit voltage, the short-circuit current density, and the fill factor of the solar cell were 0.9 V, 4.8 mA/cm2, and 57%, respectively. A conversion efficiency of 1.6% was obtained from the solar cell under a solar simulator of air mass 1.5 G and an irradiation intensity of 155 mW/cm2 at room temperature.
We experimentally demonstrate the III-V nitrides as a high-performance photovoltaic material with open-circuit voltages up to 2.4 V and internal quantum efficiencies as high as 60%. GaN and high-band gap InGaN solar cells are designed by modifying PC1D software, grown by standard commercial metal-organic chemical vapor deposition, fabricated into devices of variable sizes and contact configurations, and characterized for material quality and performance. The material is primarily characterized by x-ray diffraction and photoluminescence to understand the implications of crystalline imperfections on photovoltaic performance. Two major challenges facing the III-V nitride photovoltaic technology are phase separation within the material and high-contact resistances.
A metamorphic Ga0.35In0.65P/Ga0.83In0.17As/Ge triple-junction solar cell is shown to provide current-matching of all three subcells and thus composes a device structure with virtually ideal band gap combination. We demonstrate that the key for the realization of this device is the improvement of material quality of the lattice-mismatched layers as well as the development of a highly relaxed Ga1−yInyAs buffer structure between the Ge substrate and the middle cell. This allows the metamorphic growth with low dislocation densities below 106 cm−2. The performance of the approach has been demonstrated by a conversion efficiency of 41.1% at 454 suns (454 kW/m2, AM1.5d ASTM G173–03).
We report on III-nitride photovoltaic cells with external quantum efficiency as high as 63%. InxGa1−xN/GaN p-i-n double heterojunction solar cells are grown by metal-organic chemical vapor deposition on (0001) sapphire substrates with xIn = 12%. A reciprocal space map of the epitaxial structure showed that the InGaN was coherently strained to the GaN buffer. The solar cells have a fill factor of 75%, short circuit current density of 4.2 mA/cm2, and open circuit voltage of 1.81 V under concentrated AM0 illumination. It was observed that the external quantum efficiency can be improved by optimizing the top contact grid.
A super-thin AlN layer is inserted between the intrinsic InGaN and p-InGaN in the InGaN solar cell structure to improve the photovoltaic property. The dark current is markedly decreased by more than two orders of magnitude and the short-circuit current density is increased from 0.77 mA/cm2 to 1.25 mA/cm2, leading to a doubled conversion efficiency compared to the conventional structure. Electrical transport analysis reveals that the forward electrical property is greatly improved in the range of open circuit voltage and the leakage current mechanism changes from defect related Poole-Frenkel emission to interface tunneling emission. The improvement on the electrical and photovoltaic properties is ascribed to insertion of the AlN interlayer, which not only provides a barrier to reduce tunneling for electrons, but also suppresses the nonradiative recombination.
The authors have observed that for InxGa1−xN epitaxial layers grown on bulk GaN substrates exhibit slip on the basal plane, when in the presence of free surfaces that intercept the heterointerface and for indium compositions x ≥ 0.07. This leads to almost complete relaxation of the local misfit strain by generation of radial-shape dislocation half loops. For x ≥ 0.17, generation of straight misfit dislocations by glide on the secondary 〈113〉 {112} slip system is observed, in addition to the radial-shape half loops at surface pits. These two mechanisms act independently with no observed interaction between them, leading to the conclusion that slip on the basal plane occurs first during the growth process. The secondary slip system is activated later and involves a significantly higher critical stress energy.
High external quantum efficiency (EQE) p-i-n heterojunction solar cells grown by NH3-based molecular beam epitaxy are presented. EQE values including optical losses are greater than 50% with fill-factors over 72% when illuminated with a 1 sun AM0 spectrum. Optical absorption measurements in conjunction with EQE measurements indicate an internal quantum efficiency greater than 90% for the InGaN absorbing layer. By adjusting the thickness of the top p-type GaN window contact layer, it is shown that the short-wavelength (<365 nm) quantum efficiency is limited by the minority carrier diffusion length in highly Mg-doped p-GaN.
The InGaN alloy system offers a unique opportunity to develop high efficiency multi-junction solar cells. In this study, single junction solar cells made of Inx Ga1–xN are successfully developed, with x = 0, 0.2, and 0.3. The materials are grown on sapphire substrates by MBE, consisting of a Si-doped InGaN layer, an intrinsic layer and an Mg-doped InGaN layer on the top. The I –V curves indicate that the cell made of all-GaN has low series resistance (0.12 Ω cm2) and insignificant parasitic leakage. Contact resistances of p and n contacts are 2.9 × 10–2 Ω cm2 and 2.0 × 10–3 Ω cm2, respectively. Upon illumination by a 200 mW/cm2, 325 nm laser, Voc is measured at 2.5 V with a fill factor of 61%. Clear photo-responses are also observed in both InGaN cells with 0.2 and 0.3 Indium content when illuminated by outdoor sunlight. But it is difficult to determine the solar performance due to the large leakage current, which may be caused by the material defects. A thicker buffer layer or GaN template can be applied to the future growth process to reduce the defect density of InGaN films. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
A model based on the overall energy balance is used to calculate the critical thickness for an InGaN epitaxial layer on a GaN substrate. The critical thickness values as a function of the indium content are found to be lower than values predicted by models proposed by Fischer or People and Bean. We also used the energy balance model to estimate the effect of the hexagonal symmetry of wurtzite materials on the critical thickness; this results in reduction of the critical thickness by as much as 20% of its corresponding isotropic value. From the small amount of experimental data available we conclude that the energy balance model is more appropriate for describing the critical thickness of the InGaN/GaN material system than the models developed by Fischer or People and Bean.
The concept of ultrafast light modulator for wavelength similar to 1.5 mu m controlled by fundamental harmonic of a Ti:Al2O3 laser has been demonstrated. The possibility of realization of this concept was experimentally confirmed. We have demonstrated strong spectrally wide nonlinear response in multilayer heterostructures based on GaAs/(AlGa)(x)O-y with the relaxation time of 1-3 ps in the spectral range where both materials have negligible absorption. (C) 2010 American Institute of Physics. [doi:10.1063/1.3475415]
We reported on the InGaN p-i-n homojunction solar cells with In contents of 0.02 and 0.12. Under the same illumination of Xe lamp, In<sub>0.02</sub> Ga<sub>0.98</sub>N cell exhibited a fill factor (FF) of 67.8%, a higher open-circuit voltage (V<sub>oc</sub>) of 2.15 V while In<sub>0.12</sub>Ga<sub>0.88</sub>N cell showed a FF of 64.8% and a lower V<sub>oc</sub> of 1.35 V. The measurements of atomic force microscopy (AFM) images and leakage currents revealed that V-shaped defects, which are known to cause the increase in reverse current, were the main factor dominating the V<sub>oc</sub> of In<sub>0.12</sub>Ga<sub>0.88</sub>N cells. Short-circuit current versus open-circuit voltage (J<sub>sc</sub> - V<sub>oc</sub>) curves were consistent with the characteristics expected from the current-voltage equation. Relative external quantum efficiencies (EQEs) were also measured and showed strong dependence on V-shaped defects. These results indicate that defects in bulk InGaN has a negative impact on the photovoltaic performances of solar cells based on III-nitrides.