Multijunction Photovoltaic Devices Research Articles

The Progress of Interface Design in Perovskite‐Based Solar Cells

Organic–inorganic halide perovskite has received extensive attention as a light harvester for next‐generation low‐cost and high‐performance photovoltaics. Its superior optoelectronic properties are attractive among most thin film absorber materials, such as an extremely high absorption coefficient, optimal band gap, ambipolar carrier transport property, and high defects tolerance. However, it requires suitable electrodes and carrier transport materials to fulfill efficient photovoltaic process within an entire device. Thus, the interfaces along the device play a crucial role in determining device photovoltaic performance. Here, the progress of understanding interfaces in perovskite photovoltaics is reviewed from the perspective of processing chemistry and photophysics of carriers, which are the key parameters for the corresponding device photovoltaic behavior. This study is mainly focused on the relevant working mechanism, interface design fundamentals, and the resulting carrier dynamic control over the entire architecture. The study of the interfaces with appropriate materials design provides a fundamental understanding of the photocarrier behavior, including separation, transportation, and collection. The accumulative efforts will contribute to the construction of high‐efficiency perovskite‐based single junction and multijunction photovoltaic devices. It also affects other properties of perovskite solar cells, including J–V hysteresis phenomenon, and long‐term stability. Suggestions with respect to required improvements and future research directions are provided based on the current field of available literature.

Development of High-Bandgap AlGaInP Solar Cells Grown by Organometallic Vapor-Phase Epitaxy

AlGaInP solar cells with bandgaps between 1.9 and 2.2 eV are investigated for use in next-generation multijunction photovoltaic devices. This quaternary alloy is of great importance to the development of III–V solar cells with five or more junctions and for cells optimized for operation at elevated temperatures because of the high bandgaps required in these designs. In this work, we explore the conditions for the organometallic vapor-phase epitaxy growth of AlGaInP and study their effects on cell performance. Initial efforts focused on developing ∼2.0-eV AlGaInP solar cells with a nominal aluminum composition of 12%. Under the direct spectrum at 1000 W/m2 (AM1.5D), the best of these samples had an open-circuit voltage of 1.59 V, a bandgap-voltage offset of 440 mV, a fill factor of 88.0%, and an efficiency of 14.8%. We then varied the aluminum composition of the alloy from 0% to 24% and were able to tune the bandgap of the AlGaInP layers from ∼1.9 to ∼2.2 eV. While the samples with a higher aluminum composition exhibited a reduced quantum efficiency and increased bandgap-voltage offset, the bandgap-voltage offset remained at 500 mV or less, up to a bandgap of ∼2.1 eV.

Quantum Dot Luminescent Concentrator Cavity Exhibiting 30-fold Concentration

Luminescent solar concentrators doped with CdSe/CdS quantum dots provide a potentially low-cost and high-performance alternative to costly high-band-gap III–V semiconductor materials to serve as a top junction in multijunction photovoltaic devices for efficient utilization of blue photons. In this study, a photonic mirror was coupled with such a luminescent waveguide to form an optical cavity where emitted luminescence was trapped omnidirectionally. By mitigating escape cone and scattering losses, 82% of luminesced photons travel the length of the waveguide, creating a concentration ratio of 30.3 for blue photons in a waveguide with a geometric gain of 61. Further, we study the photon transport inside the luminescent waveguide, showing unimpeded photon collection across the entire length of the waveguide.

Light Emitting Diodes in the Experimental Practice for the Characterization of Novel Photovoltaics

ABSTRACTLight Emitting Diodes (LEDs) have recently gained importance in the experimental practice of photovoltaic (PV) devices. LEDs have already been proposed as the alternative to conventional xenon or halogen based solar simulators. Multi-junction PV devices use coloured LEDs in experimental tools as well: LEDs can transform a conventional solar simulator in a spectrally adjustable simulator for spectral characterization of multi-junction modules. Other useful applications include evaluating the dependence of the electrical parameters on the average photon energy and spectral responsivity measurements of multi-junction PV devices.

Structural dependences of localization and recombination of photogenerated carriers in the top GaInP Subcells of GaInP/GaAs double-junction tandem solar cells.

In high-efficiency GaInP/GaAs double-junction tandem solar cells, GaInP layers play a central role in determining the performance of the solar cells. Therefore, gaining a deeper understanding of the optoelectronic processes in GaInP layers is crucial for improving the energy conversion efficiency of GaInP-based photovoltaic devices. In this work, we firmly show strong dependences of localization and recombination of photogenerated carriers in the top GaInP subcells in the GaInP/GaAs double-junction tandem solar cells on the substrate misorientation angle with excitation intensity- and temperature-dependent photoluminescence (PL). The entire solar cell structures including GaInP layers were grown with metalorganic chemical vapor deposition on GaAs substrates with misorientation angles of 2° (denoted as Sample 2°) and 7° (Sample 7°) off (100) toward (111)B. The PL spectral features of the two top GaInP subcells, as well as their excitation-power and temperature dependences exhibit remarkable variation on the misorientation angle. In Sample 2°, the dominant localization mechanism and luminescence channels are due to the energy potential minima caused by highly ordered atomic domains; In Sample 7°, the main localization and radiative recombination of photogenerated carriers occur in the atomically disordered regions. Our results reveal a more precise picture on the localization and recombination mechanisms of photogenerated carriers in the top GaInP subcells, which could be the crucial factors in controlling the optoelectronic efficiency of the GaInP-based multijunction photovoltaic devices.

Design of antireflective nanostructures and optical coatings for next-generation multijunction photovoltaic devices.

The successful development of multijunction photovoltaic devices with four or more subcells has placed additional importance on the design of high-quality broadband antireflection coatings. Antireflective nanostructures have shown promise for reducing reflection loss compared to the best thin-film interference coatings. However, material constraints make nanostructures difficult to integrate without introducing additional absorption or electrical losses. In this work, we compare the performance of various nanostructure configurations with that of an optimized multilayer antireflection coating. Transmission into a four-junction solar cell is computed for each antireflective design, and the corresponding cell efficiency is calculated. We find that the best performance is achieved with a hybrid configuration that combines nanostructures with a multilayer thin-film optical coating. This approach increases transmitted power into the top subcell by 1.3% over an optimal thin-film coating, corresponding to an increase of approximately 0.8% in the modeled cell efficiency.

Ultrabroadband and Wide-Angle Hybrid Antireflection Coatings With Nanostructures

Ultrabroadband and wide-angle antireflection coatings (ARCs) are essential to realizing efficiency gains for state-of-the-art multijunction photovoltaic devices. In this study, we examine a novel design that integrates a nanostructured antireflection layer with a multilayer ARC. Using optical models, we find that this hybrid approach can reduce reflected AM1.5D power by 10-50 W/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> over a wide angular range compared to conventional thin-film ARCs. A detailed balance model correlates this to an improvement in absolute cell efficiency of 1-2%. Three different ARC designs are fabricated on indium gallium phosphide, and reflectance is measured to show the benefit of this hybrid approach.

Ellipsometric studies of AlxGa1−xAs0.5Sb0.5 (0.0 ≤ x ≤ 0.6) alloys lattice-matched to InP(100)

We report on the optical properties of AlxGa1−xAs0.5Sb0.5 (0.0 ≤ x ≤ 0.6) alloys grown by metal-organic vapor phase epitaxy on InP(100) substrates for InP-based multi-junction photovoltaic device applications. Spectroscopic ellipsometry is used to determine the complex dielectric function ε = ε1 + iε2, complex refractive index N = n + ik, normal-incidence reflectivity R, and absorption coefficients α from 0.73 to 6.45 eV at room temperature. The measured spectra exhibit four above-bandgap critical point (CP) structures E1, E1 + Δ1, E0′, and E2, and the CP energies are accurately obtained from the standard lineshape analysis. The fundamental bandgap E0 CP energies are estimated from the multilayer modeling of ellipsometric data. Dependence of the CP energies on composition x is discussed.

CuIn(Se1−xTex)2 solar cells with tunable narrow-bandgap for bottom cell application in multijunction photovoltaic devices

Narrow-bandgap CuIn(Se1-x,Tex)2–based solar cells for bottom cell application in multijunction photovoltaic devices were developed and characterized. Compact and well crystallized 2μm films are obtained by single-stage coevaporation process on Mo-coated soda lime glass. Narrow bandgaps ranging from 0.98eV to 0.88eV were measured depending on the Te content of the films. The solar cells were characterized by current density versus voltage under standard AM 1.5 illumination and under a wider bandgap CIGSe/SLG filter. The same methodology was applied for the spectral response analysis, and the beneficial effect of the narrow bandgap in the multijunction configuration was remarkable in the low energy spectral region. Efficiencies higher than 7% under AM 1.5 illumination, and close to 2% in the multijunction-like configuration, were achieved.

Optimized short-circuit current mismatch in multi-junction solar cells

Multi-junction photovoltaic devices include two or more component sub-cells which are electrically interconnected in series. At any power point, the current output of the total device is limited by the sub-cell with the smallest current density. Therefore, the maximum efficiency is reached when the sub-cells have equal current densities at their respective maximum power points. In this case the sub-cells are so called “power matched”. We report an experimental procedure in which the current–voltage characteristics of tandem solar cells can be measured under various irradiance spectra, i.e. under various short-circuit current matching conditions. This permits the probing of the optimized short circuit current mismatch, where the sub-cells are power matched, which is essential to define design rules for the tandem stack. The method applies well to devices where one of the sub-cells is metastable. We show that, in the case of thin-film silicon tandem cells, the optimum mismatch changes significantly after light induced degradation. Consequently, the degradation factor of such devices is shown to depend not only on material quality but also on the initial short circuit current matching. This experiment also provides relative quantification of the fill factors of each sub-cell. Our example suggests that a high bottom cell deposition rate can be detrimental to the fill factor of the top cell in the case of thin-film silicon tandem cells deposited in superstrate configuration.

Chalcogenisation of Cu–Sb metallic precursors into Cu3Sb(SexS1−x)3

Cu3SbS3 is a novel chalcogenide semiconductor with p-type conductivity and an energy bandgap of 1.84eV. By incorporating selenium into this material to form Cu3Sb(SexS1−x)3 where x=Se/(Se+S), the energy bandgap can be altered to be in the range 1.38–1.84eV for 0<x<0.49. The energy bandgap can hence be adjusted to be near the optimum for making the absorber layer for use in single and multi-junction photovoltaic solar cell devices. In this paper these materials were prepared using a two-stage process that involved magnetron sputtering of the Cu–Sb precursor layers followed by conversion to Cu3SbS3 by annealing in the presence of elemental sulphur and to Cu3Sb(SxSe1−x)3 by annealing in the presence of a mixture of sulphur and selenium. The films synthesised were characterised using scanning electron microscopy, energy dispersive x-ray analysis, x-ray diffraction, secondary ion mass spectroscopy and photo-electrochemical measurements. When the Cu3SbS3 was formed on glass substrates it had a cubic crystal structure whereas when it was formed on Mo-coated glass it had the monoclinic crystal structure. Likewise the layers of Cu3Sb(S,Se)3 formed on Mo-coated glass also had the monoclinic crystal structure. Spectral response curves were recorded over the spectral range 400–1400nm for semiconductor—electrolyte junctions. Photovoltaic solar cell devices were made using p-type Cu3Sb(SxSe1−x)3 as the absorber layer and n-type CdS as the buffer layer. The photovoltaic effect was observed in these devices.

Development of indium-rich InGaN epilayers for integrated tandem solar cells

ABSTRACTInGaN epilayers have been investigated for use in photovoltaic solar cells for the past years. At present, almost all photovoltaic device structures reported have exhibited very low short circuit currents and thus very low solar conversion efficiency. This phenomenon has been attributed to point and extended defect chemistry in InGaN epilayers (e.g. vacancies, misfit dislocations, and V-defects), as well as to spinodal decomposition of the strained InGaN wurtzite lattice system. These defects become more dominant for higher indium concentration InGaN epilayers needed for multijunction photovoltaic device structures. In this work, we will report on the growth and characterization of indium-rich InGaN epilayers that have been grown by novel MOCVD growth technology, including the growth at superatmospheric reactor pressures.

To Bias or Not to Bias? An “How-To” Guide for Spectral Response Measurements of Thin Film Multi-Junction Photovoltaic Modules

ABSTRACTCurrent-matching between junctions in a multi-junction photovoltaic device is fundamental for its performance prediction: the measurement of the spectral response of each junction provides a valuable information to optimize the device performance. Various aspects make spectral response measurements of such devices particularly challenging. In this work these aspects are analysed theoretically and guidelines to avoid their impact in the experimental practice are given.

Transmission electron microscopy study of GaInNAs(Sb) thin films grown by atomic hydrogen-assisted molecular beam epitaxy

The quaternary GaInNAs is a promising material system for use in next generation multijunction photovoltaic devices. We have investigated the effect of introducing antimony on the growth by using transmission electron microscopy and energy dispersive x-ray (EDX) spectroscopy. Two-dimensional growth was observed in GaInNAs films with striation features associated with compositional fluctuation and nanometer scale elemental segregation on the growth front. On the contrary, GaInNAsSb films exhibit uniform contrast throughout. EDX profile indicates uniform compositional distribution, as antimony atoms suppress the surface mobilites of adatoms resulting in a lower probability to generate the favored bonds, such as Ga-N and In-As.

Numerical modeling of SiH 4 discharge for Si thin film deposition for thin film transistor and solar cells

Amorphous and microcrystalline silicon thin films are used in solar cells as a multi-junction photovoltaic device. Plasma enhanced chemical vapor deposition is used and high deposition rate of a few nm/s is required while keeping film quality. SiH 4 is used as a precursor diluted with H 2. Electron impact processes give complex interdependent plasma chemical reactions. Many researchers suggest keeping high H/SiH x ratio is important. Numerical modeling of this process for capacitively coupled plasma and inductively coupled plasma is done to investigate which process parameters are playing key roles in determining it. A full set of 67 volume reactions and reduced set are used. Under most of conditions, CCP shows 100 times higher H/SiH 3 ratio over ICP case due to its spatially localized two electron temperature distribution. Multi hollow cathode type CCP is also modeled as a 2 × 2 hole array. For Ar, the discharge is well localized at the neck of the hole at a few Torr of gas pressure. H 2 and SiH 4 + H 2 needed higher gas pressure and power density to get a multi hole localized density profile. H/SiH 3 was calculated to be about 1/10.

A review of polymer multijunction solar cells

Polymer solar cells are one of the most promising prospects for widespread renewable energy due to their low cost, light weight, and mechanical flexibility. However, to date, low efficiencies (7.9%) of these devices inhibit their application. New materials and device designs are needed to increase the efficiency and make this technology available for large-scale applications. A polymer multijunction solar cell made of two or more subcells in series, parallel, or other special connections offers a potential solution to the losses in the current polymer single-junction solar cells. In this article, the recent developments in polymer multijunction photovoltaic materials, cell structures, and device modelling are reviewed. In addition, the current challenges that need to be addressed to achieve siginificantly higher efficiency are discussed.

Schottky-quantum dot photovoltaics for efficient infrared power conversion

Planar Schottky photovoltaic devices were prepared from solution-processed PbS nanocrystal quantum dot films with aluminum and indium tin oxide contacts. These devices exhibited up to 4.2% infrared power conversion efficiency, which is a threefold improvement over previous results. Solar power conversion efficiency reached 1.8%. The simple, optimized architecture allows for direct implementation in multijunction photovoltaic device configurations.

The design and optimization of advanced multijunction solar cells using the Silvaco ATLAS software package

In this paper, the design and optimization of advanced multijunction photovoltaic devices, utilizing a newly introduced modeling technique (J. Sol. Energy Mater. Sol. Cells, submitted for publication), is demonstrated. In our opinion, the introduction of this modeling technique to the photovoltaic community will prove to be of great importance in aiding the design and optimization of advanced solar cells. A model of an InGaP/GaAs/InGaNAs/Ge four-junction solar cell is prepared and is fully simulated. The major stages of the process are explained and the simulation results are compared to published theoretical and experimental data. An example of cell parameters optimization is also presented. The flexibility of the proposed methodology is demonstrated. The methodology and design considerations for this process are explained.

III–N–V semiconductors for solar photovoltaic applications

III–N–V semiconductors are promising materials for use in next-generation multijunction solar cells because these materials can be lattice matched to substrates such as GaAs, Ge and Si, with a range of bandgaps that are complementary to those of other III–V semiconductors. Several potentially high-efficiency multijunction photovoltaic device designs using III–N–V materials are discussed. The main roadblock to the development of these solar cell devices is poor minority-carrier transport in the III–N–V materials. The present understanding of the material properties of GaInNAs lattice matched to GaAs and GaNPAs lattice matched to Si is reviewed.