III-V Multijunction Solar Cells Research Articles

Self-consistent electrical parameter extraction from bias dependent spectral response measurements of III-V multi-junction solar cells

Spectral response of multi-junction solar cell is complicated because of the interplay between external measurement conditions such as bias light intensity, monochromatic light intensity, bias voltage, and intrinsic electrical properties of series interconnected subcells. In this paper, we report an experimental study on the bias voltage-dependent spectral response (SR) for multi-junction solar cell. A self-consistent iteration loop was developed from a nonlinear least square Powell hybrid algorithm that was used for curve fitting the experimental SR versus bias voltage data of each subcell. We demonstrated for the first time that this approach enabled us to derive the electrical parameters such as dark saturation currents (J0), shunt resistance (Rsh), series resistance (Rs), and spectra response (Jphoto) for each subcell of a Ga0.99In0.01As/Ge dual junction solar cell with stable convergence. The accuracies of the fitting results were confirmed by the agreement between the J–V curves calculated on the basis of these parameters and the experimental J–V curve of multi-junction solar cell measured under AM1.5 and 1 sun condition. Copyright © 2013 John Wiley & Sons, Ltd.

Pt 금속 박막을 이용한 InAlP층의 텍스쳐 구조 형성 및 반사율 측정

태양전지의 표면에 텍스쳐 구조를 형성하면 빛의 반사율을 줄일 수 있으므로 태양전지의 효율을 증가시킬 수 있다. 또한 표면의 텍스쳐 구조는 넓은 파장대역에서 빛의 반사를 줄여주기 때문에 다중접합 III-V화합물 태양전지에 아주 유용하다. 본 연구에서는 얇은 Pt층을 식각 마스크로 사용하여 다중접합 III-V태양전지의 window층으로 사용되는 InAlP층에 다양한 텍스쳐 구조를 형성하고 반사율을 측정하였다. 습식식각에 의해 나노미터 크기로 형성된 피라미드 꼴 텍스쳐 구조는 <TEX>$0.3{\sim}1.5{\mu}m$</TEX>의 넓은 파장영역에서 빛의 반사율을 13.7%까지 감소시켰다. Textured surface has been fabricated to reduce the light reflectance from the solar cells. The textured surface is very suitable for the multi-junction III-V solar cells because it can decrease the light reflectance over a large wavelength range. In this study, we have generated a textured structure on InAlP which is used for the window layer of the multi-junction III-V solar cells. Ultra-thin Pt layer (0.7 nm) has been used for wet etching mask. An array of nanosized pyramid shape formed on InAlP surface dramatically reduces the light reflectance up to 13.7% over a large wavelength range (i.e., <TEX>$0.3{\sim}1.5{\mu}m$</TEX>).

Indoor and Outdoor Comparison of CPV III–V Multijunction Solar Cells

Concentrator III-V multijunction solar cells from seven vendors have been characterized via indoor flash test, outdoor on-sun test, and spectral response. Most vendors have achieved similar efficiency in both indoor flash test and on-sun test. The relative difference among different cell vendors changes in two tests because of their different spectral responses and temperature coefficients. The spectral sensitivity of III-V multijunction cells leads to performance variation in the field that is not always well predicted by traditional indoor flash test or instantaneous on-sun tests. The results for different epitaxial designs and limitations of the test methodologies are presented. The comparative results lead to future design modifications for maximum energy output.

Measuring IV Curves and Subcell Photocurrents in the Presence of Luminescent Coupling

High-quality, direct-bandgap solar cells emit significant luminescence at their band edge when forced to operate in forward bias, thereby creating a possible source of photocurrent in lower bandgap junctions of a multijunction cell. We study the effects of luminescent coupling on the measurement of the subcell photocurrents for a series-connected III-V multijunction solar cell. We describe a technique that uses a set of light-emitting diodes (LEDs) and a Xenon-lamp white-light source to accurately determine the subcell photocurrents under a reference spectrum, taking the luminescent coupling current into account. The technique quantifies the luminescent coupling efficiencies and compensates for any spectral overlap between the LEDs and the other junctions. Since quantum efficiency curves are used in the adjustment of the simulator spectrum, we also show how to correct those curves to remove the effects of luminescent coupling.

Isolation of III-V/Ge Multijunction Solar Cells by Wet Etching

Microfabrication cycles of III-V multijunction solar cells include several technological steps and end with a wafer dicing step to separate individual cells. This step introduces damage at lateral facets of the junctions that act as charge trapping centers, potentially causing performance and reliability issues, which become even more important with today’s trend of cell size reduction. In this paper we propose a process of wet etching of microtrenches that allows electrical isolation of individual solar cells with no damage to the sidewalls. Etching with bromine-methanol, the solution that is typically used for nonselective etching of III-V compounds, results in the formation of unwanted holes on the semiconductor surfaces. We investigate the origin of holes formation and discuss methods to overcome this effect. We present an implementation of the isolation step into a solar cell fabrication process flow. This improved fabrication process opens the way for improved die strength, yield, and reliability.

Non-linear luminescent coupling in series-connected multijunction solar cells

The assumption of superposition or linearity of photocurrent with solar flux is widespread for calculations and measurements of solar cells. The well-known effect of luminescent coupling in multijunction solar cells has also been assumed to be linear with excess current. Here we show significant non-linearities in luminescent coupling in III-V multijunction solar cells and propose a simple model based on competition between radiative and nonradiative processes in the luminescent junction to explain these non-linearities. We demonstrate a technique for accurately measuring the junction photocurrents under a specified reference spectrum, that accounts for and quantifies luminescent coupling effects.

Advanced III-V Multijunction Solar Cells Fabricated by Semiconductor Wafer Bonding

not Available.

Analysis and Prediction of Energy Production in Concentrating Photovoltaic (CPV) Installations

A method for the prediction of Energy Production (EP) in Concentrating Photovoltaic (CPV) installations is examined in this study. It presents a new method that predicts EP by using Global Horizontal Irradiation (GHI) and the Photovoltaic Geographical Information System (PVGIS) database, instead of Direct Normal Irradiation (DNI) data, which are rarely recorded at most locations. EP at four Spanish CPV installations is analyzed: two are based on silicon solar cells and the other two on multi-junction III-V solar cells. The real EP is compared with the predicted EP. Two methods for EP prediction are presented. In the first preliminary method, a monthly Performance Ratio (PR) is used as an arbitrary constant value (75%) and an estimation of the DNI. The DNI estimation is obtained from GHI measurements and the PVGIS database. In the second method, a lineal model is proposed for the first time in this paper to obtain the predicted EP from the estimated DNI. This lineal model is the regression line that correlates the real monthly EP and the estimated DNI in 2009. This new method implies that the monthly PR is variable. Using the new method, the difference between the predicted and the real EP values is less than 2% for the annual EP and is in the range of 5.6%–16.1% for the monthly EP. The method that uses the variable monthly PR allows the prediction of the EP with reasonable accuracy. It is therefore possible to predict the CPV EP for any location, using only widely available GHI data and the PVGIS database.

Ultra-High Efficiency Photovoltaic Cells for Large Scale Solar Power Generation

The primary targets of our project are to drastically improve the photovoltaic conversion efficiency and to develop new energy storage and delivery technologies. Our approach to obtain an efficiency over 40% starts from the improvement of III-V multi-junction solar cells by introducing a novel material for each cell realizing an ideal combination of bandgaps and lattice-matching. Further improvement incorporates quantum structures such as stacked quantum wells and quantum dots, which allow higher degree of freedom in the design of the bandgap and the lattice strain. Highly controlled arrangement of either quantum dots or quantum wells permits the coupling of the wavefunctions, and thus forms intermediate bands in the bandgap of a host material, which allows multiple photon absorption theoretically leading to a conversion efficiency exceeding 50%. In addition to such improvements, microfabrication technology for the integrated high-efficiency cells and the development of novel material systems that realizes high efficiency and low cost at the same time are investigated.

Broadband quasi-omnidirectional antireflection AlGaInP window for III-V multi-junction solar cells through thermally dewetted Au nanotemplate

Al(Ga)InP subwavelength structures (SWS) were fabricated and optimized through thermally dewetted Au nanotemplate and ICP pattern-transfer. When λ< 900 nm, most AlGaInP nanostructures exhibit the reflectivity of less than 2% and insensitive to the incident angle up to 45°. When λ extends to 1800 nm, the reflectivity of less than 5% over 0°-45° is achieved in the optimized nanostructure, which benefits III-V multi-junction solar cells to improve their efficiency. Moreover, not only is such cost-effective nano-fabrication process completely compatible with the other processing of III-V solar cells, but their defined disordered SWS benefit the antireflection performance over broadband and wide view according to the comparison between the measurement and simulation results from AlGaInP SWS.

Real-Time Structural Analysis of Compositionally Graded InGaAs/GaAs(0 0 1) Layers

Compositionally step-graded InGaAs/GaAs(0 0 1) buffers with overshooting (OS) layers were evaluated by several characterization techniques for higher efficiency metamorphic III-V multijunction solar cells. By high-resolution X-ray diffraction, we found that fully relaxed or tensile strained top layers can be obtained by choosing appropriate OS layer thickness. Moreover, from real-time structural analysis using in situ X-ray reciprocal space mapping (in situ RSM), it was proved that the top layer is almost strained to the OS layers, and it is independent of the thicknesses of the OS layers. Dislocations in the vicinity of the OS layers were observed by transmission electron microscopy, and the validity of results of in situ RSM was confirmed from the viewpoint of misfit dislocation behavior. Finally, by photoluminescence measurements, we showed that tensile strained top layers may be suitable for the improvement of minority-carrier lifetime.

(Invited) Si, SiGe, Ge, and III-V Semiconductor Nanomembranes and Nanowires Enabled by SiGe Epitaxy

SiGe epitaxy in conjuction with selective etching lends itself as an excellent method for manufacturing of nanomembranes and nanowires for a variety of semiconductor materials and nano-geometries. Early device application of this technique has been demonstrated for the device architecture of Silicon-On-Nothing (SON) MOSFET and localized SON MOSFET allowing accommodation of a SOI-type MOSFET on a bulk Si wafer. In the meantime, this approach has been successfully extended to multichannel and multi-nanowire devices. Together with the availability of thin Ge layers (e.g. by virtue of Ge condensation), SiGe epitaxy opens up the possibility of a heterogenous co-integration of high efficiency III-V multijunction solar cells, III-V complementary logic, thermovoltaic device structures, lasers, modulators and detectors with Si-based electronics collocated on the same Si substrate. SiGeC epitaxial layers offer an additional engineering window for lattice mismatch and stress control, as well for achieving desirable selectivities of the required selective etches. Furthermore, the nanomembranes and nanowires made available by SiGe epitaxy and selective etches on ubiquitous Si substrates are excellent vehicles for exploration of the differences between the material properties of 3D (bulk)) and 2D (nanomembranes) and 1D (nanowire) systems.

Quantum Dot Spectral Tuning of Multijunction III-V Solar Cells

AbstractImproving the production of photocurrent in the middle junction of a InGaP/Ga(In)As/Ge triple-junction solar cells (TJSC) can improve the overall conversion efficiency of cell. One possible method to improve the middle junction photocurrent is inserting a quantum dot (QD) superlattice (SL) into the stack. It has been predicted that QD-SL enhanced TJSCs have an efficiency ceiling of 47% under a one-sun AM0 illumination spectrum. Additionally, QD array enhanced GaAs cells have the added benefit of possible intermediate band effects, anisotropic absorption and enhanced radiation tolerance. Embedding InAs quantum dots (QDs) in a single junction GaAs solar cell can increase sub-GaAs bandgap photocurrent generation. This method has been shown to improve the short circuit current density (Jsc) of single junction cells under simulated 1 sun air mass zero (AM0) illumination. However, the increase in strain due to the InAs QD self-assembly may cause defects that reduce the minority carrier lifetime resulting in losses in the cell open circuit voltage (Voc) on the order of 300-500 mV. The introduction of strain compensation (SC) layers into the superlattice (SL) structure of a QD solar cell has previously been shown to improve the device performance, including the partial recovery of Voc. Strain compensation can be used effectively to balance the residual strain, impede dislocation formation, and improve the solar cell characteristics. The effect of GaP strain compensation on the solar cell electrical and material properties was investigated. High resolution X-ray diffraction (HRXRD) scans along the symmetric (004) Bragg peak show weak intensity and wide FWHM at the zero order SL peak (SL0) for non-SC samples. Optimum SC thickness was theoretically determined using a zero in plane stress method and experimentally verified using HRXRD. Optimal strain compensation was then used to increase the QD SL stacking from 5x to 10x and 20x. Use of SC resulted in shifting of the SL0 peak toward the substrate peak as well as reduced FWHM and improved SL peak definition. Kinematical diffraction modeling of the QD structures using numerical simulation indicated this peak shift resulted from reduced overall strain in the SL stack up to 5ML of SC. The material quality improvement in the SC QD solar cells was manifested in an improved spectral response and Jsc. The optoelectronic results for GaAs solar cells with QD SL’s demonstrate a strong dependence on GaP SC layer thickness. In addition, comparison of multi junction (MJ) solar cells which incorporate the SC QD SL’s demonstrate the utility of additional sub-GaAs bandgap current contribution as a tool for additional current-matching optimization in MJ solar cells.

Basic Concepts and Interfacial Aspects of High-Efficiency III-V Multijunction Solar Cells

Among various types of solar cells, MOVPE-grown triple-junction III-V compound semiconductors are today's most efficient photovoltaic devices with conversion efficiencies exceeding 40%. A next-generation multijunction cell with four or more junctions and optimized band gaps is expected to break the present record efficiency surpassing the 50% mark. High band gap material combinations that are lattice matched to GaAs are already well established, but the required low band gap combinations containing a band gap around 1eV are still to be improved. For this purpose, we have developed a low band gap tandem (two-junction) solar cell lattice matched to InP. For the top and bottom subcells InGaAsP (Eg = 1.03 eV) and InGaAs (Eg = 0.73 eV) were utilized, respectively. A new interband tunnel junction was used to connect the subcells, including thin and highly doped layers of n-type InGaAs and p-type GaAsSb. The delicate MOVPE preparation of critical interfaces was monitored with in-situ reflectance anisotropy spectroscopy (RAS). After a contamination-free transfer, the RAS signals were then benchmarked in ultrahigh vacuum (UHV) with surface science techniques like low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). XPS measurements revealed that the sharpest InGaAs/GaAsSb interface was achieved when the GaAsSb layer in the tunnel junction of the solar cell was grown on III-rich (2×4)- or (4×2)-reconstructed InGaAs(100) surfaces. The improved interface preparation had a positive impact on the overall performance of the tandem cell, where slightly higher efficiencies were observed for the cells with the III-rich-prepared tunnel junction interfaces.

Advances in High-Efficiency III-V Multijunction Solar Cells

The high efficiency of multijunction concentrator cells has the potential to revolutionize the cost structure of photovoltaic electricity generation. Advances in the design of metamorphic subcells to reduce carrier recombination and increase voltage, wide-band-gap tunnel junctions capable of operating at high concentration, metamorphic buffers to transition from the substrate lattice constant to that of the epitaxial subcells, concentrator cell AR coating and grid design, and integration into 3-junction cells with current-matched subcells under the terrestrial spectrum have resulted in new heights in solar cell performance. A metamorphic Ga0.44In0.56P/Ga0.92In0.08As/ Ge 3-junction solar cell from this research has reached a record 40.7% efficiency at 240 suns, under the standard reporting spectrum for terrestrial concentrator cells (AM1.5 direct, low-AOD, 24.0 W/cm2, 25∘C), and experimental lattice-matched 3-junction cells have now also achieved over 40% efficiency, with 40.1% measured at 135 suns. This metamorphic 3-junction device is the first solar cell to reach over 40% in efficiency, and has the highest solar conversion efficiency for any type of photovoltaic cell developed to date. Solar cells with more junctions offer the potential for still higher efficiencies to be reached. Four-junction cells limited by radiative recombination can reach over 58% in principle, and practical 4-junction cell efficiencies over 46% are possible with the right combination of band gaps, taking into account series resistance and gridline shadowing. Many of the optimum band gaps for maximum energy conversion can be accessed with metamorphic semiconductor materials. The lower current in cells with 4 or more junctions, resulting in lower I2R resistive power loss, is a particularly significant advantage in concentrator PV systems. Prototype 4-junction terrestrial concentrator cells have been grown by metal-organic vapor-phase epitaxy, with preliminary measured efficiency of 35.7% under the AM1.5 direct terrestrial solar spectrum at 256 suns.

Progress in crystalline multijunction and thin-film photovoltaics

Photovoltaics are important in terrestrial applications such as remote power and renewable energy but are the primary source of electrical power for space systems. For space applications, priorities are high conversion efficiency and resistance to radiation-induced degradation. The emphasis is on III-V multijunction solar cells and comparatively lower efficiency but flexible, lightweight thin-film solar cells. Triple-junction III-V solar cells have reached conversion efficiencies of 30% at air mass zero (AM0). New lattice mismatch techniques and nitride materials hold promise for further efficiency increases. Thin-film solar cells generally have less than 15% efficiency but greater radiation resistance, lower cost, and lower mass.

III-V Multi-Junction Materials and Solar Cells on Engineered SiGe/Si Substrates

ABSTRACTThe monolithic integration of high efficiency III-V compound solar cell materials and devices with lower-cost, robust and scaleable Si substrates has been a driving force in photovoltaics (PV) basic research for decades. Recent advances in controlling mismatch-induced defects that result from structural and chemical differences between III-V solar cell materials and Si using a combination of SiGe interlayers and monolayer-scale control of III-V/IV interfaces, have led to a series of fundamental advances at the material and device levels, which establish that the great potential of III-V/Si PV is within reach. These include demonstrations of GaAs epitaxial layers on Si that are anti-phase domain-free with verified dislocation densities at or below 1×106 cm−2 and negligible interface diffusion, minority carrier lifetimes for GaAs on Si in excess of 10 ns, single junction GaAs-based solar cells on Si with open circuit voltages (Voc) in excess of 980 mV, efficiencies beyond 18%, and area-independent PV characteristics up to at least 4 cm2. These advances are attributed in large part to the use of a novel “engineered Si substrate” based on compositionally-graded SiGe buffers such that a high-quality, low defect density, relaxed, “virtual” Ge substrate could be developed that can support lattice-matched III-V epitaxy and thus merge III-V technology based on the GaAs (or Ge) lattice constant with Si wafers. This paper focuses on recent results that extend this work to the first demonstration of high performance III-V dual junction solar cells on SiGe/Si. Open circuit voltages in excess of 2 V at one-sun have been obtained for the conventionally “lattice-matched” In0.49Ga0.51P/GaAs dual junction cells on inactive, engineered SiGe/Si; to our knowledge is the first demonstration of &gt; 2V solar power generation on a Si wafer. Comparisons with identical cells on GaAs substrates reveal that the Voc on engineered Si retains more than 94% of its homoepitaxial value, and that at present both DJ/GaAs and DJ/SiGe/Si cells are similarly limited by current mismatch in these early cells, and not fundamental defect factors associated with the engineered Si substrates.