InGaP Solar Cells Research Articles

InGaP-based InP quantum dot solar cells with extended optical absorption range

In the quest for an efficient optical absorption of broad-band solar irradiation, intermediate-band solar cells composed of wide-bandgap semiconductors have attracted attention. In the present study, we developed and investigated the performance of wide-bandgap InGaP-based InP quantum dot (QD) solar cells. The solar cells were fabricated by solid-source molecular beam epitaxy, and their optical absorption range was found to be up to ∼850 nm, which is larger than the ∼680 nm optical absorption range of the host InGaP solar cells. Through the measurements of the voltage-dependent quantum efficiency, the photocarriers generated in the InGaP host were determined to be captured into the InP QDs, rather than expelled from the solar cells. The findings of this study highlight the need for the development of an optimized structure of intermediate-band solar cells to mitigate the capture of the photocarriers.

Electrical performance of the InGaP solar cell irradiated with low energy electron beams

AbstractThe investigation of the radiation degradation characteristics of InGaP space solar cells is important. In order to understand the mechanism of the degradation by radiation the samples of the InGaP solar cell were irradiated in vacuum and at ambient temperature with electron beams from a Cockcroft‐Walton type accelerator at Osaka Prefecture University. The threshold energies for recoil were obtained by theoretical calculation. The energies and the fluences of the electron beams were from 60 to 400 keV and from 3×1014 to 3×1016 cm‐2, respectively. The light‐current‐voltage measurements were performed. The degradation of Isc caused by the defects related to the phosphorus atoms was observed and the degradation was suppressed by irradiation at an energy higher than the threshold energy for recoiling Indium atoms. At an energy of 60 keV, where the recoil does not occur, the Voc was degraded. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Novel Non‐radiative Exciton Harvesting Scheme Yields a 15% Efficiency Improvement in High‐Efficiency III–V Solar Cells

High‐efficiency III–V solar cells typically incorporate an indirect wide‐bandgap semiconductor as a passivation layer to limit surface recombination at higher photon energies. The poor extraction efficiency of the carriers photogenerated in this window layer limits the performance of the devices in the high‐energy region of the spectrum. To address this problem, a resonance energy transfer (RET)‐mediated luminescent down‐shifting (LDS) layer is engineered by depositing an epilayer of colloidal quantum dots (QDs) on an InGaP solar cell. In this configuration, while the QDs act as a standard LDS layer, excitons are also funneled from the window layer to the QD epilayer using near‐field RET. The luminescence energy of the QDs is tuned below the bandgap of the window layer and the emitted light is absorbed in the p–n junction, where carriers are generated and efficiently extracted. The overall performance of the solar cell is found to be significantly improved after hybridization, with a large 14.6% relative and 2% absolute enhancement of the photon conversion efficiency.

Er(3+)/Yb(3+) upconverters for InGaP solar cells under concentrated broadband illumination.

The inability of solar cell materials to convert all incident photon energy into electrical current, provides a fundamental limit to the solar cell efficiency; the so called Shockley-Queisser (SQ) limit. A process termed upconversion provides a pathway to convert otherwise unabsorbed low energy photons passing through the solar cell into higher energy photons, which subsequently can be redirected back to the solar cell. The combination of a semi-transparent InGaP solar cell with lanthanide upconverters, consisting of ytterbium and erbium ions doped in three different host materials (Gd2O2S, Y2O3 and NaYF4) is investigated. Using sub-band gap light of wavelength range 890 nm to 1045 nm with a total accumulated power density of 2.7 kW m(-2), a distinct photocurrent was measured in the solar cell when the upconverters were applied whereas a zero current was measured without upconverter. Furthermore, a time delay between excitation and emission was observed for all upconverter systems which can be explained by energy transfer upconversion. Also, a quadratic dependence on the illumination intensity was observed for the NaYF4 and Y2O3 host material upconverters. The Gd2O2S host material upconverter deviated from the quadratic illumination intensity dependence towards linear behaviour, which can be attributed to saturation effects occurring at higher illumination power densities.

InGaP/GaAs tandem solar cells fabricated using solid-source molecular beam epitaxy

We report high-quality InGaP solar cells and their application to InGaP/GaAs tandem solar cells fabricated using solid-source molecular beam epitaxy (MBE). The short-circuit current density of InGaP solar cells increases as the absorption layer thickness increases. The highest efficiency of 12.8% with an open circuit voltage (Voc) of 1.32 V was obtained without anti-reflection coating. The performance of InGaP/GaAs tandem solar cells is considerably changed by changing the carrier concentration in a tunnel junction between the top InGaP and bottom GaAs cells. A high efficiency of 21.1% was obtained with a high Voc of 2.3 V, which indicates that high-performance InGaP/GaAs tandem solar cells can be fabricated using solid-source MBE.

Enhanced ultraviolet responses in thin-film InGaP solar cells by down-shifting

Layers of poly(methyl methacrylate) doped with the Eu complex Eu(DPEPO)(hfac)3 (EuDH) provide a means for down-shifting incident ultraviolet (UV) light into the visible range, with beneficial effects on the performance of solar cells, as demonstrated with thin-film InGaP devices formed by epitaxial liftoff. Experimental and computational results establish important aspects of gain and loss mechanisms in the UV range. Measurements show that InGaP cells with coatings of EuDH doped PMMA exhibit enhanced currents (8.68 mA cm(-2)) and power conversion efficiencies (9.48%), both due to increased responses at wavelengths between 300-360 nm.

InGaP solar cells fabricated using solid-source molecular beam epitaxy

We report the growth conditions for InGaP epitaxial films and the characteristics of InGaP solar cells on a GaAs (100) substrate fabricated using solid-source molecular beam epitaxy. Photoluminescence and X-ray diffraction measurements indicate that a growth temperature of 480°C is suitable for realizing high quality InGaP epitaxial growth with solid-source molecular beam epitaxy. The open circuit voltage of InGaP solar cells grown at 1.0μm/h is higher than that of the cells grown at 0.5μm/h. The highest conversion efficiency is obtained for an InGaP solar cell grown at 480°C and a growth rate of 1.0μm/h.

Dependence of Biasing Voltage and Illumination Power on the Built-in Electric Field of InGaP Solar Cells

Dependence of Biasing Voltage and Illumination Power on the Built-in Electric Field of InGaP Solar Cells

Dependence of Biasing Voltage and Illumination Power on the Built-in Electric Field of InGaP Solar Cells

The electroreflectance spectra of InGaP solar cells at various biasing voltages and illumination levels were studied. The Franz–Keldysh oscillations were observed and the electric field at the junction of solar cells can be extracted. The measured electric field decreases with increasing biasing voltage and illumination level. The theoretical electric fields as a function of the biasing voltage and the photocurrent are calculated on the basis of the depletion approximation in the junction theory and the photovoltaic effect, respectively.

Metamorphic GaAsP and InGaP Solar Cells on GaAs

We have investigated wide-bandgap, metamorphic GaAs <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-</sub> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> P <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> and In <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</i> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-</sub> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</i> P solar cells on GaAs as potential subcell materials for future 4-6 junction devices. We identified and characterized morphological defects in tensile GaAs <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-</sub> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> P <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> graded buffers that lead to a local reduction in carrier collection and a global increase in threading dislocation density (TDD). Through adjustments to the graded buffer structure, we minimized the formation of morphological defects and, hence, obtained TDDs ≈ 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> for films with lattice mismatch ≤1.2%. Metamorphic In <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</i> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-</sub> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</i> P solar cells were grown on these optimized GaAs <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-</sub> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> P <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> graded buffers with bandgaps ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Eg</i> ) as high as 2.07 eV and open-circuit voltages ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Voc</i> ) as large as 1.49 V. Such high bandgap materials will be necessary to serve as the top subcell in future 4-6 junction devices. We have also shown that the relaxed GaAs <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-</sub> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> P <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> itself could act as an efficient lower subcell in a multijunction device. GaAs <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.66</sub> P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.34</sub> single-junction solar cells with <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Eg</i> = 1.83 eV were fabricated with <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oc</sub> = 1.28 V. Taken together, we have demonstrated that GaAs <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-</sub> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> P <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> graded buffers are an appropriate platform for low-TDD, metamorphic GaAs <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-</sub> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> P <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> and In <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</i> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-</sub> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</i> P solar cells, covering a wide bandgap range.

Numerical Analysis for Radiation Resistant InGaP Solar Cell

Numerical analyses are carried out to optimize radiation resistant of InGaP solar cell under the effect of low energy protons. The radiation degradation parameters of InGaP used to investigate the effect of cell configuration, base thickness, junction depth, base carrier concentration, and AlInP window layer. Numerical analyses for both cells structure n–p and p–n shows that, the n–p structure is more radiation resistant in a solar cell with shallow junction (0.05 µm), thin base thickness (0.4 µm) and low base carrier concentration (1 ×1016 cm-3), while p–n cell structure is relatively radiation resistant in deep junction solar cell (0.1 µm) with thin base thickness and low base carrier concentration. The formation of AlInP window layer increases the radiation resistant of p–n cell structure, while no significant change in the radiation resistant of n–p cell structure was observed due to the formation of AlInP.

Efficiency Enhancement in Single-Junction InGaP Solar Cells by Using Self-Assembled Nanospheres

A single-junction InGaP solar cell with polystyrene (PS) nanospheres on the surface has been developed. The self-assembly of PS nanospheres is one of the simplest and the fastest methods with which to build a 2-D closely packed periodic structure. Due to the scattering of the PS nanospheres, the light path length can be increased when compared to the InGaP solar cells without PS nanospheres on the top. An increase short-circuit current from 8.93 to 10.31 mA is improved when a single-junction InGaP solar cell is coated with PS nanospheres. The conversion efficiency measured can also be improved from 8.79% to 9.71%. The single-junction InGaP solar cell with PS nanospheres was achieved.

Simulation of Temperature Characteristics of InGaP/InGaAs/Ge Triple-Junction Solar Cell under Concentrated Light

Using an equivalent circuit model, the temperature characteristics of an InGaP/InGaAs/Ge triple-junction solar cell under concentrated light conditions were analyzed in detail. The current–voltage (I–V) characteristics of the single-junction solar cells (InGaP, InGaAs, and Ge solar cells) were measured at various temperatures. From the dark I–V characteristics of each single-junction solar cell, the diode parameters and temperature exponents were extracted. The extracted diode parameters and temperature exponents were applied to the equivalent circuit model for the triple-junction solar cell, and the solar-cell performance was calculated. There was good agreement between the measured and calculated I–V characteristics of the triple-junction solar cell at various temperatures under concentrated light conditions.

Simulation of Temperature Characteristics of InGaP/InGaAs/Ge Triple-Junction Solar Cell under Concentrated Light

Using an equivalent circuit model, the temperature characteristics of an InGaP/InGaAs/Ge triple-junction solar cell under concentrated light conditions were analyzed in detail. The current–voltage (I–V) characteristics of the single-junction solar cells (InGaP, InGaAs, and Ge solar cells) were measured at various temperatures. From the dark I–V characteristics of each single-junction solar cell, the diode parameters and temperature exponents were extracted. The extracted diode parameters and temperature exponents were applied to the equivalent circuit model for the triple-junction solar cell, and the solar-cell performance was calculated. There was good agreement between the measured and calculated I–V characteristics of the triple-junction solar cell at various temperatures under concentrated light conditions.

Metamorphic GaAsP buffers for growth of wide-bandgap InGaP solar cells

GaAs x P 1 − x graded buffers were grown via solid source molecular beam epitaxy (MBE) to enable the fabrication of wide-bandgap InyGa1−yP solar cells. Tensile-strained GaAsxP1−x buffers grown on GaAs using unoptimized conditions exhibited asymmetric strain relaxation along with formation of faceted trenches, 100–300 nm deep, running parallel to the [01¯1] direction. We engineered a 6 μm thick grading structure to minimize the faceted trench density and achieve symmetric strain relaxation while maintaining a threading dislocation density of ≤106 cm−2. In comparison, compressively-strained graded GaAsxP1−x buffers on GaP showed nearly-complete strain relaxation of the top layers and no evidence of trenches but possessed threading dislocation densities that were one order of magnitude higher. We subsequently grew and fabricated wide-bandgap InyGa1−yP solar cells on our GaAsxP1−x buffers. Transmission electron microscopy measurements gave no indication of CuPt ordering. We obtained open circuit voltage as high as 1.42 V for In0.39Ga0.61P with a bandgap of 2.0 eV. Our results indicate MBE-grown InyGa1−yP is a promising material for the top junction of a future multijunction solar cell.

Theoretical Optimization of Base Doping Concentration for Radiation Resistance of InGaP Subcells of InGaP/GaAs/Ge Based on Minority-Carrier Lifetime

One of the fundamental objectives for research and development of space solar cells is to improve their radiation resistance. InGaP solar cells with low base carrier concentrations under low-energy proton irradiations have shown high radiation resistances. In this study, an analytical model for low-energy proton radiation damage to InGaP subcells based on a fundamental approach for radiative and nonradiative recombinations has been proposed. The radiation resistance of InGaP subcells as a function of base carrier concentration has been analyzed by using the radiative recombination lifetime and damage coefficient K for the minority-carrier lifetime of InGaP. Numerical analysis shows that an InGaP solar cell with a lower base carrier concentration is more radiation-resistant. Satisfactory agreements between analytical and experimental results have been obtained, and these results show the validity of the analytical procedure. The damage coefficients for minority-carrier diffusion length and carrier removal rate with low-energy proton irradiations have been observed to be dependent on carrier concentration through this study. As physical mechanisms behind the difference observed between the radiation-resistant properties of various base doping concentrations, two mechanisms, namely, the effect of a depletion layer as a carrier collection layer and generation of the impurity-related complex defects due to low-energy protons stopping within the active region, have been proposed.

Efficiency improvement of single-junction InGaP solar cells fabricated by a novel micro-hole array surface texture process

In this study, single-junction InGaP solar cells fabricated by a novel micro-hole array surface texture process are presented. The characteristics of the single-junction InGaP solar cells with and without the micro-hole array surface texture are studied. An increase of 10.4% in short-circuit current is found when a single-junction InGaP solar cell is fabricated by the micro-hole array surface texture process. The conversion efficiency measured under one-sun air mass 1.5 global illumination at room temperature can also be improved from 13.8% to 15.9% when the size of the micro-holes is 5.3 µm and the period of micro-hole array is designed to 5 µm.

A new approach to determine accurately minority-carrier lifetime

Abstract Electron or proton irradiations introduce recombination centers, which tend to affect solar cell parameters by reducing the minority-carrier lifetime (MCLT). Because this MCLT plays a fundamental role in the performance degradation of solar cells, in this work we present a new approach that allows us to get accurate values of MCLT. The relationship between MCLT in p -region and n -region both before and after irradiation has been determined by the new method. The validity and accuracy of this approach are justified by the fact that the degradation parameters that fit the experimental data are the same for both short-circuit current and the open-circuit voltages. This method is applied to the p + / n -InGaP solar cell under 1 MeV electron irradiation.

Equivalent Model for InGaP-Based Solar Cell under High Concentration

In this study the current–voltage (I–V) characteristics of a monocrystalline InGaP solar cell have been investigated. The experimental examination is carried out under a high concentration of light. The variations of the two reverse saturation currents are consistent with the physical significance of both the diffusion and the space-charge generation-recombination terms through their exponential variations. The simulation results clearly demonstrated that the solar cell is described with reasonable accuracy by a two-diode equivalent model that simulates the effects of the double-exponential dark current–voltage characteristics on the open-circuit voltage, fill factor, and conversion efficiency of the solar cell at a high concentration. The theoretical results are in good agreement with the experimental observations.

The potential of III‐V semiconductors as terrestrial photovoltaic devices

AbstractIII‐V semiconductors, GaAs and in particular InGaP, are used in many different electronic applications, such as high power and high frequency devices, laser diodes and high brightness LED. Their direct bandgap and high reliability make them ideal candidates for the realisation of high efficiency solar cells: in the past years they have been successfully used as power sources for satellites in space, where they are able to produce electricity from sunlight with an overall efficiency of around 30%. Nowadays, the use of arsenides and phosphides as photovoltaic (PV) devices is confined only to space applications since their price is much higher than conventional Si flat panel modules, the leading PV market technology. But with the introduction of multijunction solar cells capable of operating in high concentration solar light, the area and, therefore, the cost of these cells can be reduced and will eventually find an application and market also on Earth. This article will review the situation of semiconductor solar cell materials, focusing on Si, GaAs, InGaP and multijunction solar cells and will discuss future trends and possibilities of bringing III‐V technology from space to Earth. Copyright © 2006 John Wiley &amp; Sons, Ltd.