AlGaAs Solar Cells Research Articles

Low‐Temperature Photoluminescence Investigation of Carbon‐Doped AlGaAs Grown by MOVPE on Vicinal Substrates

AlGaAs is promising absorber material for multi‐junction solar cells (MJSC) for its tunable bandgap to match various bandgap combination designs. However, the path to achieving high‐quality AlGaAs for solar cell application remains to be determined. In this study, we grow and characterize carbon‐doped p‐Al0.375Ga0.625As on exact and vicinal GaAs(0 0 1) substrates, using metal‐organic vapor phase epitaxy (MOVPE). Low‐temperature photoluminescence (PL) measurement is performed in the range from 6 K to 300 K. The Arrhenius plot of PL intensities reveals an increasing trend for non‐radiative recombination in conjunction with increasing miscut angle. The presence of atomic ordering that inversely correlates to the miscut angle is proposed to explain the PL linewidth and peak energy dependency on substrate orientation. The conflicting tendency of non‐radiative recombination and ordering with miscut angle are further validated using solar cell devices. A slightly vicinal substrate is recommended for AlGaAs solar cells according to our observation.

Optoelectronic optimization of graded-bandgap thin-film AlGaAs solar cells. Part II: optimal antireflection front-surface texturing.

In Part I [Appl. Opt.59, 1018 (2020)APOPAI0003-693510.1364/AO.381246], we used a coupled optoelectronic model to optimize a thin-film AlGaAs solar cell with a graded-bandgap photon-absorbing layer and a periodically corrugated Ag backreflector combined with localized ohmic Pd-Ge-Au backcontacts, because both strategies help to improve the performance of AlGaAs solar cells. However, the results in Part I were affected by a normalization error, which came to light when we replaced the hybridizable discontinuous Galerkin scheme for electrical computation by the faster finite-difference scheme. Therefore, we re-optimized the solar cells containing an n-AlGaAs photon-absorbing layer with either a (i) homogeneous, (ii) linearly graded, or (iii) nonlinearly graded bandgap. Another way to improve the power conversion efficiency is by using a surface antireflection texturing on the wavelength scale, so we also optimized four different types of 1D periodic surface texturing: (i) rectangular, (ii) convex hemi-elliptical, (iii) triangular, and (iv) concave hemi-elliptical. Our new results show that the optimal nonlinear bandgap grading enhances the efficiency by as much as 3.31% when the n-AlGaAs layer is 400nm thick and 1.14% when that layer is 2000nm thick. A hundredfold concentration of sunlight can enhance the efficiency by a factor of 11.6%. Periodic texturing of the front surface on the scale of 0.5-2 free-space wavelengths provides a small relative enhancement in efficiency over the AlGaAs solar cells with a planar front surface; however, the enhancement is lower when the n-AlGaAs layer is thicker.

Al0.35Ga0.65As/InGaP heterojunction solar cell based on temperature-graded growth

The p-Al0.35Ga0.65As/n-InGaP heterojunction solar cells are promising competitors compared with conventional InGaP or AlGaAs solar cells as the heterogeneous combination can overcome the demerits of each material. However, InGaP has an optimized growth temperature much lower than that of AlGaAs in metal-organic vapor phase epitaxy (MOVPE) growth. Therefore, a challenge arises from oxygen contamination at the hetero-interface during temperature adjustment for obtaining MOVPE-grown high-quality p-AlGaAs/n-InGaP heterojunction. Here we report that a temperature-graded layer (TGL) of AlGaAs can solve the issue of growth temperature mismatch, and significantly improve the voltage performance of AlGaAs/InGaP solar cells. Absolute electroluminescence measurement and atomic force microscopy confirmed a smooth interface with less non-radiative recombination after TGL was applied. The temperature-graded growth is verified able to improve the AlGaAs/InGaP interface quality and provides a scalable method for AlGaAs-based heterogeneous growth.

Enhancing intermediate band solar cell performances through quantum engineering of dot states by droplet epitaxy

AbstractWe report the effect of the quantum dot aspect ratio on the sub‐gap absorption properties of GaAs/AlGaAs quantum dot intermediate band solar cells. We have grown AlGaAs solar cells containing GaAs quantum dots made by droplet epitaxy. This technique allows the realization of strain‐free nanostructures with lattice matched materials, enabling the possibility to tune the size, shape, and aspect ratio to engineer the optical and electrical properties of devices. Intermediate band solar cells have been grown with different dot aspect ratio, thus tuning the energy levels of the intermediate band. Here, we show how it is possible to tune the sub‐gap absorption spectrum and the extraction of charge carriers from the intermediate band states by simply changing the aspect ratio of the dots. The tradeoff between thermal and optical extraction is in fact fundamental for the correct functioning of the intermediate band solar cells. The combination of the two effects makes the photonic extraction mechanism from the quantum dots increasingly dominant at room temperature, allowing for a reduction of the open circuit voltage of only 14 mV, compared to the reference cell.

Optoelectronic optimization of graded-bandgap thin-film AlGaAs solar cells.

An optoelectronic optimization was carried out for an $ {{\rm Al}_\xi }{{\rm Ga}_{1 - \xi }}{\rm As} $AlξGa1-ξAs (AlGaAs) solar cell containing (i) an $ n $n-AlGaAs absorber layer with a graded bandgap and (ii) a periodically corrugated Ag backreflector combined with localized ohmic Pd-Ge-Au backcontacts. The bandgap of the absorber layer was varied either sinusoidally or linearly. An efficiency of 33.1% with the 2000-nm-thick $ n $n-AlGaAs absorber layer is predicted with linearly graded bandgap along with silver backreflector and localized ohmic backcontacts, in comparison to 27.4% efficiency obtained with homogeneous bandgap and a continuous ohmic backcontact. Sinusoidal grading of the bandgap is predicted to enhance the maximum efficiency to 34.5%. Thus, grading the bandgap of the absorber layer, along with a periodically corrugated Ag backreflector and localized ohmic Pd-Ge-Au backcontacts, can help realize ultrathin and high-efficient AlGaAs solar cells for terrestrial applications.

Enhanced Current Collection in 1.7 eV GaInAsP Solar Cells Grown on GaAs by Metalorganic Vapor Phase Epitaxy

Quaternary GaInAsP solar cells with a bandgap of ∼1.7 eV offer an attractive Al-free alternative to AlGaAs solar cells for integration in next generation of III–V multijunction solar cells with five or more junctions. Development of a high quality 1.7 eV solar cell is also highly sought for III–V/Si tandem solar cells. In this work, we systematically investigate the impact of varying base thicknesses and doping concentrations on the carrier collection and performance of 1.7 eV GaInAsP solar cells. The photoresponse of these cells is found to be very sensitive to p-type zinc doping concentration in the base layer. Prototype 1.7 eV GaInAsP n-i-p solar cell designs are demonstrated that leverage enhanced depletion width as an effective method to achieve peak quantum efficiency exceeding 90%. We also show the importance of optimal i-layer thickness as a critical parameter to reduce the drop in fill-factor (FF) due to field-aided collection. Furthermore, we demonstrate substantial improvement in the cell performance when the GaInAsP base layer is grown at 650 °C instead of 600 °C. The best GaInAsP solar cell ( Eg ∼ 1.65 eV) in this study achieved J SC of 21.1 mA/cm2, V OC of 1.18 V, FF of 83.8%, and an efficiency of 20.8 ± 1% under AM1.5D spectrum (21.5 ± 1% under AM1.5G spectrum). These results highlight the potential of Al-free GaInAsP solar cells for integration in the next generation of III–V multijunction solar cells.

Growth of InGaAsP solar cells and their application to triple-junction top cells used in smart stack multijunction solar cells

The authors report on high-quality InGaAsP (1.61–1.65 eV) solar cells grown on a GaAs substrate; their study is the first to grow these using solid-source molecular beam epitaxy (SS-MBE). A temperature of 430 °C was found to be suitable for the growth of the InGaAsP solar cells. The properties of these InGaAsP solar cells were found to be better than those of AlGaAs solar cells that had the same bandgap energy, and it was found to be suitable for use as the second cell in a triple-junction top cell used in a smart stack multijunction solar cell. The authors also developed an InGaP/InGaAsP/GaAs solar cell and found that it had an impressive open-circuit voltage of 3.16 V. This result indicates that high-performance InGaP/InGaAsP/GaAs triple-junction solar cells can be fabricated using SS-MBE.

A substrate removal processing method for III–V solar cells compatible with low-temperature characterization

In this work, we present a substrate removal procedure for solar cells compatible with direct attachment of the epitaxy to a holder through a thermally and electrically conductive interface. In our case this procedure was motivated by the need to develop a processing technique compatible with low-temperature characterization of the devices. The method is based on the use of indium to bond a thin-film epitaxial structure to a silicon support. The adequate properties of indium, namely, low tensile strength and good thermal and electrical conductivity, allow characterizing the devices at very low temperatures without causing strain-induced degradation in the samples. Following this method, we have fabricated and characterized thin-film (1.74µm) AlGaAs solar cells with and without a layer of InAs quantum dots. We show the adequacy of our method to measure at low temperatures by means of measuring the photocurrent or quantum efficiency of the devices at different temperatures, ranging from 300K to 20K.

Open-Circuit Voltage in AlGaAs Solar Cells With Embedded GaNAs Quantum Wells of Varying Confinement Depth

We investigate the photovoltaic properties of AlGaAs solar cells with embedded GaNAs quantum wells (QWs) with N concentrations in the range of 0–3.1%, for which the QW confinement energy can be tuned by adjusting the N concentration. We systematically study the dependence of open-circuit voltage $V_{{{\rm OC}}}$ in relation to the lowest band-to-band transition energy. In samples with low N concentrations (shallow QW confinement), $V_{{\rm{OC}}}$ degrades and is limited by the lowest transition energy in the solar cell, i.e., the QW transition. With increasing N concentration, N > 0.5% (deep QW confinement), $V_{{\rm{OC}}}$ does not degrade further and is no longer limited by the QW transition energy. The highest N sample exhibits a remarkably small offset between the lowest transition energy and the achieved $V_{{\rm{OC}}}$ of 0.23 V, which is beyond the detailed balance limit of standard solar cells. $V_{{\rm{OC}}}$ dependence is explained by analyzing the current–voltage (I–V) characteristics under different illumination conditions, from which information about the balance of escape and recombination rates of carriers from the QWs is extracted. In the deeply confined QWs, tunneling and thermal carrier escape is completely suppressed, allowing the recovery of $V_{{{\rm OC}}}$ .

Bias voltage dependence of two-step photocurrent in GaAs/AlGaAs quantum well solar cells

We investigated photoresponses of AlGaAs solar cells in which coupled GaAs quantum wells were embedded in the i-region of p-i-n diodes; we studied how the bias voltage Vb affects the normal photocurrent I generated by the visible light and a “two-step” photocurrent ΔI generated by the absorption of visible and infrared photons. We found that as Vb exceeds −0.2 V, ΔI rises and peaks at 0.6 V, while the normal photocurrent I falls to about half of its saturated level. These findings are discussed in terms of a rate equation model to show that ΔI is mainly determined by the balance of escape and recombination of photogenerated carriers.

Numerical simulation of front graded and fully graded AlGaAs/GaAs solar cell

One of the promising methods to enhance the performance of third generation solar cells is to use compositionally graded layers. The aim of this paper is to study the effect of fully graded AlGaAs solar cells using numerical simulation with SCAPS 1D. The gradient is simultaneously in the composition and in doping concentration. To attain this goal, we have optimized the standard p–i–n device in first step. Then, we have optimized the usual front graded device simulation of the fully graded device was done and the most important observation is that the open circuit-voltage (Voc) is much higher than Voc of device with uniform band gap where the open circuit voltage density decrease inconsiderately. We showed that a grading strategy with fully graded device can indeed lead to a more favorable trade-off between Jsc and Voc than can be obtained with uniform AlGaAs device.

Investigation of InGaP/(In)AlGaAs/GaAs triple-junction top cells for smart stacked multijunction solar cells grown using molecular beam epitaxy

We report high-quality InGaP/(In)AlGaAs/GaAs triple-junction solar cells fabricated using solid-source molecular beam epitaxy (MBE) for the first time. The triple-junction cells can be used as top cells for smart stacked multijunction solar cells. A growth temperature of 480 °C was found to be suitable for an (In)AlGaAs second cell to obtain high-quality tunnel junctions. The properties of AlGaAs solar cells were better than those of InAlGaAs solar cells when a second cell was grown at 480 °C. The high-quality InGaP/AlGaAs/GaAs solar cell had an impressive open-circuit voltage of 3.1 V. This result indicates that high-performance InGaP/AlGaAs/GaAs triple-junction solar cells can be fabricated using solid-source MBE.

Special Issue Dedication: James J. Coleman

This special issue of IEEE Journal of Quantum Electronics on Optoelectronic Device Integration is dedicated to Professor James J. Coleman on the occasion of his 60th Birthday. Dr. J.J. Coleman (S'73-M'76-SM'80-F'92) received the B.S., M.S., and Ph.D. degrees in electrical engineering from the University of Illinois at Urbana-Champaign, Urbana. He studied MOCVD-grown heteroface AlGaAs solar cells, lowthreshold single mode AlGaAs-GaAs double heterostructure lasers, and quantum well heterostructure laser devices at Rockwell International, Anaheim, CA. A brief review of Dr. Coleman's professional contributions is presented.

실내조명 응용을 위한 투명 집광 렌즈를 이용한 태양전지 효율 향상

Improvements in characteristics of a single junction AlGaAs/GaAs solar cell are observed by capping a PMMA lens on it. In order to show the effect of the lens, characteristics of a single junction AlGaAs/GaAs solar cell before and after the lens formation are compared under the one-sun illumination condition (). Characteristics of the solar cell under very weak illumination condition (about 1200 lux) is also measured with the lighting of a fluorescent desk lamp. About 5% of cell efficiency is improved after the capping of PMMA lens on the single junction AlGaAs solar cell and of electrical power was generated with the lighting of a desk lamp.

Determination of the origin of the series resistance through electroluminescence measurements of GaAs and Al xGa 1− xAs solar cells and LEDs

A simple method to determine the origin of the total series resistance in GaAs and AlGaAs solar cells and LEDs is given. It allows to discriminate between series resistance effects due to the emitter resistivity and parasitic resistance effects mainly due to the metal–semiconductor contact and the metal of the grid. The method consists of fitting the current–voltage and light intensity–current characteristics. The former is affected by the total series resistance and the latter just by the series resistance due to non uniform current distribution.

Photovoltaic properties of an AlxGa1−xAs solar cell (x=0–0.22) grown on Si substrate by metalorganic chemical vapor deposition and thermal cycle annealing

The effects of a thermal cycle annealing (TCA) process on the defects in GaAs and AlxGa1−xAs solar cells on Si substrates are described in this paper. The defect density is reduced and the solar cell efficiency is improved by TCA. The defect density and the solar cell efficiency are evaluated in detail with respect to TCA temperature and Al composition. The problems involved in the fabrication of a high efficiency AlGaAs solar cell on a Si substrate are discussed.

Experimental analysis of the efficiency of heterostructure GaAsAlGaAs solar cells

Some years ago Multiple Quantum Wells (MQW) solar cells were introduced as an alternative to obtain high efficiencies. Based on the simple Shockley diode model, the short-circuit current could be increased without loss in the open-circuit voltage. Applying the Detailed Balance Theory, including radiative recombination in the i-region, leads to less optimistic predictions of the limiting efficiency of MQW cells. We present an experimental study in order to compare the efficiency of MQW solar cells with heterostructure cells with graded Al compositions and single bandgap solar cells. Compared to the homojunction AlGaAs cell, an increase of the short-circuit current is observed by the incorporation of GaAs in the i-region. However, the open-circuit voltage is reduced by the implementation of GaAs, due to an increase of the non-radiative recombination current. To estimate the maximum possible open-circuit voltage, the radiative recombination current is determined by measuring the light emission as a function of the applied voltage. From this experiment we conclude that the maximum possible open-circuit voltage of all the heterostructure cells is considerably lower than the homogeneous AlGaAs cell and close to the value of the GaAs cell, showing the relation between the open-circuit voltage and the smallest bandgap in the cell. The measured curves can be well predicted by calculations based on the Detailed Balance Theory. We find no principal advantage of MQW cells over cells with graded composition or single bandgap cells.

High efficiency AlGaAs/Si monolithic tandem solar cell grown by metalorganic chemical vapor deposition

The improvements of the AlGaAs solar cell grown on the Si substrate and the AlGaAs/Si tandem solar cell by metalorganic chemical vapor deposition have been investigated. The active-area conversion efficiency of the Al0.1Ga0.9As solar cell on the Si substrate as high as 12.9% has been obtained by improving the growth sequence and adopting an Al compositionally graded band emitter layer. A high efficiency monolithic AlGaAs/Si tandem solar cell with the active-area conversion efficiency of 19.9% and 20.6% (AM0 and 1 sun at 27 °C) under two-terminal and four-terminal configurations, respectively, is demonstrated.

AlGaAs solar cells grown by MBE for high-efficiency tandem cells

AlGaAs solar cells for AlGaAs/GaAs tandem cell applications have been investigated. The performance of the AlGaAs cell grown by a molecular beam epitaxy (MBE) technique is believed to have strong dependence on growth temperature. The conversion efficiency, electrical properties, and the crystal quality of AlGaAs cells MBE grown at different temperatures have been characterized and correlations between each characteristic are discussed. The conversion efficiency is maximized at a growth temperature of 660°C. This agrees well with the results that the bulk recombination current is the lowest at this temperature. The structure of an AlGaAs cell in the AlGaAs/GaAs tandem configuration is optimized through numerical simulation using the parameters obtained from the AlGaAs and GaAs single junction cells. The efficiency for tandem cells is shown to be the highest when the thickness of the AlGaAs cell was chosen so that the incident light near the absorption edge might be shared with the bottom GaAs cell.

Time-resolved photoluminescence characterization of GaAs and AlGaAs on Si substrate grown by MOCVD

Abstract GaAs layers and AlGaAs solar cells grown on Si substrates have been characterized by time-resolved photoluminescence (TRP). The effects of the growth temperature, the strained layer superlattice (SLS), and the thermal cycle annealing (TCA) on the TRP characteristics are discussed. The minority carrier lifetime is increased with increasing growth temperature and by using SLS + TCA. The longest minority carrier lifetime of GaAs on Si obtained in this study is 0.50 ns.