Dual-junction Solar Cells Research Articles

Material and device analysis of SiGe solar cell in a GaAsP–SiGe dual junction solar cell on Si substrate

Low bandgap Si(1−x)Ge(x) solar cells are designed, fabricated, characterized and analyzed for the purpose of acting as the bottom cell in a GaAsP–SiGe tandem solar cell. The development of the SiGe cell under a GaAsP cell can lead to a 34% relative increase in efficiency over that of a silicon solar cell. This work focuses on making a SiGe bottom cell that can generate 21mA/cm2 of short circuit current (Jsc) at 1Sun from photons beyond 780nm and with a 450mV band gap–voltage offset (Woc) under 20Suns illumination. Numerical and analytical methods are introduced to demonstrate the SiGe solar cells׳ performance limits and to examine the trade-offs among I–V performance, cell structure and material composition. Material compositions are confirmed with energy-dispersive X-ray spectroscopy (EDS) and electrochemical capacitance voltage profiling (ECV). First principles analysis shows that a Si.15Ge.85 cell with a 5µm base can produce the expected current and an efficiency of 8.8% under 20Suns. By increasing base doping, we achieve a Woc of 435mV under 20Suns with a Si.18Ge.82 cell. The best 1Sun short circuit current measured from a Si.12Ge.88 cell at wavelengths beyond 780nm without anti-reflection coating, back surface reflector, or back texturing is 12.9mA/cm2. Adding a back surface field to the structure will lead to a higher Voc and lower Woc. Implementing light trapping with a higher base angle of pyramids can lead to the target Jsc of 21mA/cm2.

Design and Modeling of Metamorphic Dual-Junction InGaP/GaAs Solar Cells on Si Substrate for Concentrated Photovoltaic Application

We have investigated the concentrated photovoltaic performance of metamorphic monolithic InGaP/GaAs dual-junction (2-J) solar cells on Si substrate under AM1.5d spectrum using finite-element analysis. The current-matching condition between each subcell was realized for threading dislocation density varying from 10 5 to 10 7 cm -2 , emanating from the mismatch between GaAs and Si substrate. Through comprehensive cell design and by mitigating the losses due to shadowing effect and series resistance, we present an optimal cell design for harnessing the maximum potential of 2-J InGaP/GaAs cell integrated on Si substrate for concentrated photovoltaics. The optimization of front grid spacing and sheet resistance of the window layer were the key design parameters taken into consideration for extending the peak performance toward higher concentrations. Finally, we present an optimized 2-J InGaP/GaAs cell design on Si, which exhibited a theoretical conversion efficiency of 33.11% at 600 suns at a realistic TDD of 10 6 cm -2 , indicating a promising future for integrating III-V cell technology on Si for low-cost concentrated photovoltaics.

Design of Dual-Junction Three-Terminal CdTe/InGaAs Solar Cells

In this paper we report the modeling-based design of a CdTe (1.5 eV) on InGaAs (0.74 eV) dual-junction three-terminal solar cell. The device was optimized for thickness of active layers and for doping levels. Efficiency of 26.6% was predicted for the tandem photovoltaic (PV) device under standard testing conditions (STC). We investigated tandem PV device performance at different temperatures and found the device had a total efficiency temperature coefficient of approximately −0.15%/°C. We also investigated device suitability for concentrated photovoltaic (CPV) applications in addition to its conventional PV applications. The device proved a good candidate for CPV applications under 1000 suns.

Analyses of Interfaces in Wafer-Bonded Tandem Solar Cells by Aberration-Corrected STEM and EELS

High-quality III-V multi junction solar cells on Si can be fabricated by surface-activated wafer bonding at room temperature. In this way the formation of lattice-mismatch induced defects in the active solar cell layers can be reduced or completely avoided. In this approach, a 10 μm thin GaInP/GaAs dual junction solar cell is grown lattice-matched on GaAs and transferred to Si by direct wafer bonding (Figure 2).

MOCVD-Grown GaP/Si Subcells for Integrated III–V/Si Multijunction Photovoltaics

Enabled by a heteroepitaxial nucleation process that yields GaP-on-Si integration free of heterovalent-related defects, GaP/active-Si junctions were grown by metalorganic chemical vapor deposition. n-type Si emitter layers were grown on p-type (1 0 0)-oriented Si substrates, followed by the growth of n-type GaP window layers, to form fully active subcell structures compatible with integration into monolithic III-V/Si multijunction solar cells. Fabricated test devices yield good preliminary performance characteristics and demonstrate great promise for the epitaxial subcell approach. Comparison of different emitter layer thicknesses, combined with descriptive device modeling, reveals insight into recombination dynamics at the GaP/Si interface and provides design guidance for future device optimization. Additional test structures consisting of GaP/active-Si subcell substrates with subsequently grown GaAs <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-y</sub> step-graded buffers and GaAs <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.75</sub> P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.25</sub> terminal layers were produced to simulate the optical response of the GaP/Si junction within a theoretically ideal dual-junction solar cell.

Effects of leakage current on the short circuit current in the dual-junction solar cells

The characteristics of short circuit current (I\(_\mathrm{sc})\) in dual-junction GaInP/GaAs solar cells have been investigated, and the experimental results show that the photo current of GaInP top cell is higher than that of GaAs bottom cell and the I\(_\mathrm{sc}\) of the device is usually not limited by GaAs bottom cell if a leakage current occurs. The current versus voltage (I–V) curves of the solar cell were simulated with subcells based on single diode model and the I\(_\mathrm{sc}\) shows a dependence on the leaky subcells, i.e., the I\(_\mathrm{sc}\) is between the photo current of GaAs and that of GaInP subcells in the case of a leaky GaAs subcell, it is equal to the photo current of GaAs subcell in the case of a leaky GaInP subcell, and it is lower than the photo current of GaInP subcell when both GaAs and GaInP subcells are leaky.

Numerical analysis of p-GaAs/n-GaAs tunnel junction employing InAs intermediate layer for high concentrated photovoltaic applications

We report the numerical analysis of p-GaAs/n-GaAs tunnel junction employing InAs as the intermediate layer at the junction. Incorporation of this intermediate layer introduces an intermediate level at the junction bandgap leading to enhanced tunneling of carriers. By doing so, we obtain a two order of enhancement in the tunnel current. Furthermore, the performance of GaInP/GaAs dual-junction solar cells using the TJ with InAs intermediate layer is calculated under concentrated suns condition and it shows a conversion efficiency exceeding 30% under 1800 suns condition.

Comparison of Direct Growth and Wafer Bonding for the Fabrication of GaInP/GaAs Dual-Junction Solar Cells on Silicon

Two different process technologies were investigated for the fabrication of high-efficiency GaInP/GaAs dual-junction solar cells on silicon: direct epitaxial growth and layer transfer combined with semiconductor wafer bonding. The intention of this research is to combine the advantages of high efficiencies in III-V tandem solar cells with the low cost of silicon. Direct epitaxial growth of a GaInP/GaAs dual-junction solar cell on a GaAsyP1-y buffer on silicon yielded a 1-sun efficiency of 16.4% (AM1.5g). Threading dislocations that result from the 4% lattice grading are still the main limitation to the device performance. In contrast, similar devices fabricated by semiconductor wafer bonding on n-type inactive Si reached efficiencies of 26.0% (AM1.5g) for a 4-cm2 solar cell device.

Performance evaluation of multi-junction solar cell with a new InGaP tunnel junction

In this paper the performance of InGaP/GaAs dual junction solar cell with InGaP/InGaP tunnel junction has been investigated using various single layer and double layer anti-reflection coatings like Al2O3, TiO2, ITO, Si3N4, ITO + Al2O3 and TiO2 + Al2O3. In order to determine the best anti-reflection coating, reflectivity and absorptivity graphs of each ARC is simulated and compared. The simulation of the multi-junction solar cell has been carried through computational numerical modelling TCAD tool ATLAS. The electric field developed across each layer along with photogeneration rates is determined and the simulation results are validated with published experimental data. The multi-junction solar cell model is implemented with optimised InGaP/GaAs dual junction cell having Al2O3 and TiO2 as double layer anti-reflection coating with effective InGaP/InGaP tunnel junction. A maximum conversion efficiency of 39.9724% is obtained under AM1.5G illumination.

Semi-transparent Si-rich SixC1−x p–i–n photovoltaic solar cell grown by hydrogen-free PECVD

A semi-transparent silicon-rich (Si-rich) silicon carbide (SixC1−x) based thin-film p–i–n junction photovoltaic solar cell is demonstrated, which is synthesized by hydrogen-free plasma enhanced chemical vapor deposition (PECVD) with the conversion efficiency optimized by detuning plasma power and intrinsic layer thickness. The hydrogen-free PECVD process effectively decelerates the decomposition of hydrogen from the Si-rich SixC1−x films so as to facilitate the passivation of surface dangling bonds. When synthesizing at a fluence ratio of R = [CH4]/[SiH4]+[CH4] = 60%, the optical bandgap of Si-rich SixC1−x film is greatly red-shifted to 1.54 eV due to the enhancement of Si content. By reducing the RF plasma power to 20 W, the Si-rich SixC1−x enlarges its above-bandgap (λ= 0.4–0.8 μm) optical absorption up to 104–105 cm−1, which is even one order of magnitude larger than that of bulk crystalline Si. The incomplete dissociation of CH4 and SiH4 under low-power PECVD growth also helps to suppress the defects and improve the electrical properties of Si-rich SixC1−x. By doping with the PH3 and B2H6 fluence ratios of 5.8% and 2.1%, the resistivities of n-type and p-type Si-rich SixC1−x films are decreased to 0.87 and 0.12 Ω cm, respectively. Thinning the intrinsic Si-rich SixC1−x layer to 25 nm further promotes the conversion efficiency of the Si-rich SixC1−x based p–i–n photovoltaic solar cell. After annealing at 650 °C, the open-circuit voltage (Voc) and short-circuit photocurrent density (Jsc) of the annealed Si-rich SixC1−x and amorphous Si based dual-junction thin-film photovoltaic solar cell are increased to 0.78 V and 19.1 mA cm−2, respectively, leading to a filling factor of 35% and a conversion efficiency of up to 5.24%.

Analytical Model of Multi-junction Solar Cell

Multi-junction solar cells (MJSCs) are a current trend in the field of solar cells and form the backbone of concentrated photovoltaic systems. They are an attractive option because of their high efficiency, better power production and cost effectiveness. The aim of this paper is to present a general mathematical model of MJSC, suitable for computer simulation. This model investigates cell characterization curves including current density and power curves as a function of voltage for different concentration levels and number of junctions. The effect of varying material properties of junctions and tunneling layers is also analyzed. Two different types of MJSCs have been tested on the model, including InGaP–GaAs dual-junction solar cell with tunneling layer of InGaP and InGaP–GaAs–Ge triple-junction solar cell with tunneling layers of GaAs. Paper also presents the simulation results which are in agreement with practical conclusions.

Influence of PH3 exposure on silicon substrate morphology in the MOVPE growth of III–V on silicon multijunction solar cells

Dual-junction solar cells formed by a GaAsP or GaInP top cell and a silicon bottom cell seem to be attractive candidates to materialize the long sought-for integration of III–V materials on silicon for photovoltaic applications. One of the first issues to be considered in the development of this structure will be the strategy to create the silicon emitter of the bottom subcell. In this study, we explore the possibility of forming the silicon emitter by phosphorus diffusion (i.e. exposing the wafer to PH3 in a MOVPE reactor) and still obtain good surface morphologies to achieve a successful III–V heteroepitaxy as occurs in conventional III–V on germanium solar cell technology. Consequently, we explore the parameter space (PH3 partial pressure, time and temperature) that is needed to create optimized emitter designs and assess the impact of such treatments on surface morphology using atomic force microscopy. Although a strong degradation of surface morphology caused by prolonged exposure of silicon to PH3 is corroborated, it is also shown that subsequent anneals under H2 can recover silicon surface morphology and minimize its RMS roughness and the presence of pits and spikes.

A GaAs/GaInP dual junction solar cell grown by molecular beam epitaxy

We report the recent result of GaAs/GaInP dual-junction solar cells grown by all solid-state molecular-beam-epitaxy (MBE). The device structure consists of a GaIn0.48P homojunction grown epitaxially upon a GaAs homojunction, with an interconnected GaAs tunnel junction. A photovoltaic conversion efficiency of 27% under the AM1.5 globe light intensity is realized for a GaAs/GaInP dual-junction solar cell, while the efficiencies of 26% and 16.6% are reached for a GaAs bottom cell and a GaInP top cell, respectively. The energy loss mechanism of our GaAs/GaInP tandem dual-junction solar cells is discussed. It is demonstrated that the MBE-grown phosphide-containing III—V compound semiconductor solar cell is very promising for achieving high energy conversion efficiency.

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.

Understanding phosphorus diffusion into silicon in a MOVPE environment for III–V on silicon solar cells

Dual-junction solar cells formed by a GaAsP or GaInP top cell and a silicon bottom cell seem to be attractive candidates to materialize the long sought-for integration of III–V materials on silicon for photovoltaic applications. When manufacturing a multi-junction solar cell on silicon, one of the first processes to be addressed is the development of the bottom subcell and, in particular, the formation of its emitter. In this study, we analyze, both experimentally and by simulations, the formation of the emitter as a result of phosphorus diffusion that takes place during the first stages of the epitaxial growth of the solar cell. Different conditions for the Metal-Organic Vapor Phase Epitaxy (MOVPE) process have been evaluated to understand the impact of each parameter, namely, temperature, phosphine partial pressure, time exposure and memory effects in the final diffusion profiles obtained. A model based on SSupremIV process simulator has been developed and validated against experimental profiles measured by ECV and SIMS to calculate P diffusion profiles in silicon formed in a MOVPE environment taking in consideration all these factors.

Gravity-Assisted Heat Pipe With Strong Marangoni Fluid for Waste Heat Management of Single and Dual-Junction Solar Cells

This study investigates the cooling of single and multijunction solar cells with an inclined, gravity-assisted heat pipe, containing a 0.05 M 2-propanol/water mixture that exhibits strong concentration Marangoni effects. Heat pipe solar collector system thermal behavior was investigated theoretically and semi-empirically through experimentation of varying input heat loads from attached strip-heaters to simulate waste heat generation of single-junction monocrystalline silicon (Si), and dual-junction GaInP/GaAs photovoltaic (PV) solar cells. Several liquid charge ratios were investigated to determine an optimal working fluid volume that reduces the evaporator superheat while enhancing the vaporization transport heat flux. Results showed that a 45% liquid charge, with a critical heat flux of 114.8 W/cm2, was capable of achieving the lowest superheat levels, with a system inclination of 37 deg. Solar cell semiconductor theory was used to evaluate the effects of increasing temperature and solar concentration on cell performance. Results showed that a combined PV/heat pipe system had a 1.7% higher electrical efficiency, with a concentration ratio 132 suns higher than the stand-alone system. The dual-junction system also exhibited enhanced performance at elevated system temperatures with a 2.1% greater electrical efficiency, at an operational concentration level 560 suns higher than a stand-alone system.

Luminescent Coupling in GaAs/GaInNAsSb Multijunction Solar Cells

Multijunction subcell short-circuit currents are typically calculated by integrating the product of the external quantum efficiency and the input solar irradiance. The actual subcell current can be much greater than the result of this calculation when luminescence from a high bandgap subcell couples into a lower bandgap subcell. A model is developed, based on detailed balance, which quantifies luminescent coupling current under different spectral conditions. Steady-state laser and pulsed flash measurements on GaAs/GaInNAsSb dual-junction solar cells demonstrate a luminescent coupling factor of ~35%, compared with the theoretical maximum value of ~48%. The deviation from maximum is due to nonradiative current in the GaAs subcell.

Modelling of Dual-Junction Solar Cells including Tunnel Junction

Monolithically stacked multijunction solar cells based on III–V semiconductors materials are the state-of-art of approach for high efficiency photovoltaic energy conversion, in particular for space applications. The individual subcells of the multi-junction structure are interconnected via tunnel diodes which must be optically transparent and connect the component cells with a minimum electrical resistance. The quality of these diodes determines the output performance of the solar cell. The purpose of this work is to contribute to the investigation of the tunnel electrical resistance of such a multi-junction cell through the analysis of the current-voltage (J-V) characteristics under illumination. Our approach is based on an equivalent circuit model of a diode for each subcell. We examine the effect of tunnel resistance on the performance of a multi-junction cell using minimization of the least squares technique.

A high efficiency dual-junction solar cell implemented as a nanowire array

In this work, we present an innovative design of a dual-junction nanowire array solar cell. Using a dual-diameter nanowire structure, the solar spectrum is separated and absorbed in the core wire and the shell wire with respect to the wavelength. This solar cell provides high optical absorptivity over the entire spectrum due to an electromagnetic concentration effect. Microscopic simulations were performed in a three-dimensional setup, and the optical properties of the structure were evaluated by solving Maxwell's equations. The Shockley-Queisser method was employed to calculate the current-voltage relationship of the dual-junction structure. Proper design of the geometrical and material parameters leads to an efficiency of 39.1%.

InGaP/GaAs Dual-Junction Solar Cell with AlGaAs/GaAs Tunnel Diode Grown on 10° off Misoriented GaAs Substrate

InGaP/GaAs dual-junction solar cells with different tunnel diodes (TDs) grown on misoriented GaAs substrates are investigated. It is demonstrated that the solar cells with P++-AlGaAs/N++-GaAs TDs grown on 10° off GaAs substrates not only show a higher external quantum efficiency (EQE) but also generate a higher peak current density (Jpeak) at higher concentration ratios (185×) than the solar cells with P++-GaAs/N++-InGaP TDs grown on 6° off GaAs substrates. Furthermore, the cell design with P++-AlGaAs/N++-GaAs TDs grown on 10° off GaAs substrates does not generate a disordered InGaP epitaxial layer during material growth, and thus shows superior current–voltage characteristics.