Top Junction Research Articles

Multijunction a-Si:H/c-Si solar cells with vertically-aligned architecture based on silicon nanowires

The new design for full silicon multi-junction solar cells, which consists of combination of silicon nanowires (SiNWs) and conventional silicon-based hetero-junctions, is proposed in the paper. A top junction is based on SiNWs covered by a-Si:H p-i-n structures while bottom junction is based on amorphous/crystalline silicon (a-Si:H/c-Si) hetero-junctions. Vertically-aligned design of top p-i-n junction is a way to enhance absorption in a-Si:H without rising its thickness and therefore it allows one to increase short circuit current. This is a key issue for current match with bottom a-Si:H/c-Si heterojunction. Silicon wires of submicron diameters and micron length are formed by etching. A cost effective technology to form the periodic SiNWs on the silicon front surface was developed based on dry plasma etching using submicron size polystyrene spheres as a mask. A significant photocurrent increase was experimentally demonstrated for a-Si:H p-i-n top sub-cells deposited on SiNWs. While bottom a–Si:H/c–Si heterojunction sub-cells formed on the surface with SiNWs demonstrate strong light trapping effect, which leads to enhanced external quantum efficiency in the long wavelength region compared to planar reference cell.

31% European InGaP/GaAs/InGaAs Solar Cells for Space Application

We report a triple junction InGaP/GaAs/InGaNAs solar cell with efficiency of ~31% at AM0, 25 °C fabricated using a combined molecular beam epitaxy (MBE) and metal-organic chemical vapour deposition (MOCVD) processes. The prototype cells comprise of InGaNAs (Indium Gallium Nitride Arsenide) bottom junction grown on a GaAs (Gallium Arsenide) substrate by MBE and middle and top junctions deposited by MOCVD. Repeatable cell characteristics and uniform efficiency pattern over 4-inch wafers were obtained. Combining the advantages offered by MBE and MOCVD opens a new perspective for fabrication of high-efficiency space tandem solar cells with three or more junctions. Results of radiation resistance of the sub-cells are also presented and critically evaluated to achieve high efficiency in EOL conditions.

The GaAs/GaAs/Si solar cell – Towards current matching in an integrated two terminal tandem

Non-concentrating tandem solar cells offer the potential to increase conversion efficiencies beyond 30%. Of particular interest are configurations with a silicon bottom cell – to leverage existing manufacturing infrastructure – and III-V compound semiconductor top cells to combine high efficiencies with long-term stability and experience in applications. In this work we investigate the GaAs/GaAs/Si triple-junction architecture. GaAs and Si form a non-ideal bandgap combination, which poses a challenge to the current matching requirement. As band-to-band absorption in GaAs is almost two thirds of that in Si, absorption can be balanced by replacing the GaAs top junction with a GaAs/GaAs double junction. This opens up a possibility for an integrated two terminal solar cell for the GaAs-Si material system. In this study, we numerically evaluate the efficiency and energy yield potential of the GaAs/GaAs/Si triple-junction architecture. We find that, with state-of-the-art material quality, the GaAs/GaAs/Si architecture has the potential to achieve 33.0% efficiency, and harvesting efficiencies between 31.4% and 32.1%. We fabricated a GaAs/GaAs/Si four-terminal tandem solar cell with a mathematically combined efficiency of 20.4%.

Effect of Mo capping layers thickness on the perpendicular magnetic anisotropy in MgO/CoFeB based top magnetic tunnel junction structure**Project supported by the National Fundamental Research Program of China (Grant No. 2011CB921804) and Beijing Key Subject Foundation of Condensed Matter Physics, China (Grant No. 0114023).

A detailed study of the magnetic characterizations of the top structure MgO/CoFeB/Mo is presented. The samples show strong perpendicular magnetic anisotropy (PMA) when the thickness of CoFeB is 0.9 nm and 1.1 nm. The saturation magnetic moment and interface anisotropy constant are 1566 emu/cm3 and 3.75 erg/cm2, respectively. The magnetic dead layer (MDL) is about 0.23 nm in this system. Furthermore, strong capping layer thickness dependence is also observed. The strong PMA of 1.1 nm CoFeB only exists in a Mo cap layer thickness window of 1.2–2 nm. To maintain PMA, the metal layer could not be too thin or thick in these multilayers. The oxidation and diffusion of the metal capping layer should be respectively responsibility for the degradation of PMA in these thin or thick metal capping layer samples.

Advanced Device Design for Photoelectrochemical Water Splitting Derived By a Detailed Balance Approach

Solar fuel generation by direct photoelectrolysis of water is a pathway towards energy storage and transportation fuel demands in a sustainable energy economy. High band gap semiconductor materials may drive the water splitting reaction, but cannot use substantial parts of the solar spectrum, while common photovoltaic (PV) absorbers fail to provide sufficient threshold voltage for photoelectrochemical (PEC) conversion. Tandem absorber structures overcome this trade-off featuring promising efficiencies [1]. Tandem cell implementation by epitaxial III-V growth has enabled landmark solar-to-hydrogen (STH) conversion efficiencies [2]. In contrast to the maximum power metric in PV, efficient STH conversion requires maximizing the photocurrent versus a load voltage consisting of the thermodynamic potential of water splitting (1.23V) and various voltage loss mechanisms accumulating throughout the photoelectrochemical cell, often summarized in a generalized overvoltage term for modelling approaches. We discovered that sunlight absorption during illumination through the aqueous PEC electrolyte flawed both efficiency and band gap predictions in earlier theoretical studies and derived PEC tandem device guidelines defining an interdependence between allowable overvoltage loss and electrolyte transmission length [1]. Even though optimum band gap combinations for STH conversion have been discussed for decades, state-of-the-art inverted metamorphic multijunction (IMM) III-V growth and processing strategies developed in the context of high-efficiency PV [3] finally enabled us to tailor water splitting device structures according to maximum performance limits [Fig. 1] and validate our prediction. A first step was to relax the harsh current limitation imposed by the bottom absorber of the classical GaInP/GaAs PEC tandem [2] by replacing it with a metamorphic GaInAs with a band gap of approximately 1.2 eV [Fig. 1(a)]. Further reductions in overvoltage loss and electrolyte layer thickness enable further reductions of top and bottom junctions to unlock higher STH limit efficiency [Fig. 1(b)] that could be implemented by the integration of a 1.7 eV absorber on Si. Advanced PEC measurement protocols [4] drastically reduce performance estimation errors induced by the spectral mismatch of laboratory light sources and light transmission through epoxy used for device area confinement. Valid PEC results of our tailored devices as well as of analogue PV device characterization provide us with important feedback for our detailed balance model, where we start to unravel overvoltage loss into its fundamental components to obtain advanced guidance for future technical development.

High‐efficiency indium gallium nitride/Si tandem photovoltaic solar cells modeling using indium gallium nitride semibulk material: monolithic integration versus 4‐terminal tandem cells

AbstractIn this work, we present a double‐junction solar cell with a crystalline silicon solar cell as a bottom junction and an indium gallium nitride‐based semibulk‐structured solar cell as a top junction. Using SILVACO Atlas and taking into account polarization effects in III‐N materials, we have shown that 50% of indium is needed to ensure the current matching between the top cell and the bottom cell in 2‐terminal configuration. Such high indium composition is technologically challenging to grow. Thus, we have also modeled a 4‐terminal configuration showing the same performance (i.e. conversion efficiency close to 29%) where only 25% of indium content is needed. Copyright © 2016 John Wiley & Sons, Ltd.

High‐efficiency GaInP/GaAs/GaInNAs solar cells grown by combined MBE‐MOCVD technique

AbstractTriple‐junction GaInP/GaAs/GaInNAs solar cells with conversion efficiency of ~29% at AM0 are demonstrated using a combination of molecular beam epitaxy (MBE) and metal‐organic chemical vapor deposition (MOCVD) processes. The bottom junction made of GaInNAs was first grown on a GaAs substrate by MBE and then transferred to an MOCVD system for subsequent overgrowth of the two top junctions. The process produced repeatable cell characteristics and uniform efficiency pattern over 4‐inch wafers. Combining the advantages offered by MBE and MOCVD opens a new perspective for fabrication of high‐efficiency tandem solar cells with three or more junctions. Copyright © 2016 John Wiley & Sons, Ltd.

A thin-film silicon/silicon hetero-junction hybrid solar cell for photoelectrochemical water-reduction applications

A hybrid tandem solar cell consisting of a thin-film, nanocrystalline silicon top junction and a silicon heterojunction bottom junction is proposed as a supporting solar cell for photoelectrochemical applications. Tunneling recombination junction engineering is shown to be an important consideration in designing this type of solar cell. The best hybrid cell produced has a spectral utilization of 30.6mAcm−2 a JSC of 14mAcm−2, a VOC of 1.1V, a fill factor of 0.67 and thus an efficiency of 10.3%. A high solar-to-hydrogen efficiency of 7.9% can be predicted when using the hybrid cell in conjunction with current a-SiC photocathode technology.

Analysis of luminescent coupling effects innseries-connected multijunction solar cells

In this work, the luminescent coupling (LC) effects on photocurrent-matched and photocurrent-mismatched multijunction solar cells are investigated from fundamental physical theories and modeled by spice circuit simulations. It is demonstrated that the voltage increase of a photocurrent-matched cell is constant in the voltage range from maximum power point to open-circuit point, and this increase depends on the number of junctions and LC efficiency. Through the LC effects, it is shown that at the operation point of a photocurrent-matched double-junction (2J) cell, the photons from radiative recombination (which is the dominated recombination mechanism) of the top junction, is not wasted, but instead couples downward to the bottom junction, doubles the second junction's recombination current density, and leads to a significant voltage increase of = 17.8 mV. The same physics extended to a photocurrent-matched triple-junction (3J) cell with an increase of = 46 mV and n-junction cell with an increase of . Furthermore, it is shown that the theoretical prediction matches the circuit simulations exactly, and this tens of millivolts enhancement in voltage increases with increasing the number of junctions, consequently leads to a greater improvement in cell performance.

Assessment of Rear-Surface Processing Strategies for III–V on Si Multijunction Solar Cells Based on Numerical Simulations

The manufacturing of high-efficiency III–V on Si multijunction solar cells needs the development of hybrid, i.e., adapted to both families of materials, solar cell processing techniques, able to extract the full photovoltaic potential of both the subcells. This fact especially impacts the processing of the silicon rear surface of the tandem, which cannot receive treatments commonly used in the single-junction Si solar cell industry [Al-back surface field (BSF), thermal SiO2, and so on], since these would result in an excessive thermal load that would deteriorate the III–V upper layers (top cell, tunnel junction, and buffer layer). However, the Si bottom cell requires an advanced design with good rear passivation, a good ohmic contact, and good carrier selectivity, so that its contribution to the efficiency of the tandem is maximized. Accordingly, in this paper, several low-temperature compatible rear-surface passivation techniques for the Si bottom subcell in a monolithic III–V/Si tandem solar cell are explored. In particular, aluminum BSFs, passivated emitter and rear cell (PERC)-like architecture, passivated emitter and rear locally diffused (PERL)-like architecture formed with low thermal loads, and heterojunction with intrinsic thin layer (HIT)-like processes are assessed using numerical simulations, and a comparison of the Si bottom cell performance for the mentioned alternatives in a GaAsP/Si dual-junction solar cell is presented.

Four-Junction Wafer-Bonded Concentrator Solar Cells

The highest solar cell conversion efficiencies are achieved with four-junction devices under concentrated sunlight illumination. Different cell architectures are under development, all targeting an ideal bandgap combination close to 1.9, 1.4, 1.0, and 0.7 eV. Wafer bonding is used in this work to combine materials with a significant lattice mismatch. Three cell architectures are presented using the same two top junctions of GaInP/GaAs but different infrared absorbers based on Germanium, GaSb, or GaInAs on InP. The modeled efficiency potential at 500 suns is in the range of 49–54% for all three devices, but the highest efficiency is expected for the InP-based cell. An efficiency of 46% at 508 suns was already measured by AIST in Japan for a GaInP/GaAs//GaInAsP/GaInAs solar cell and represents the highest independently confirmed efficiency today. Solar cells on Ge and GaSb are in the development phase at Fraunhofer ISE, and the first demonstration of functional devices is presented in this paper.

Simulation study of GaAsP/Si tandem solar cells

A model, adapted from the Shockley–Queisser detailed balance model to tandem solar cells with a monolithically grown GaAsxP1−x top junction on a Si bottom junction, has been developed. Updated data have been used for the absorption spectrums. Two surface geometries, flat and ideally textured, have been investigated. As an important improvement over existing models, the effects of threading-dislocations-related Shockley–Read–Hall recombinations in the GaAsxP1−x cell, due to the lattice mismatch between the GaAsxP1−x epilayers and the Si substrate, have been taken into consideration. Auger recombinations in the Si bottom cell and luminescent coupling between the cells have also been considered. For a dislocation-free 2-μm-thick top cell, maximal theoretical efficiencies of 41.6% and 39.1% have been calculated for a textured and a flat surface, respectively. For threading dislocation (TD) densities below 104cm−2, the impact of TDs in the GaAsxP1−x layers on the solar cell performances is very limited. With TD densities over 105cm−2, the top cell open-circuit voltage is reduced, hence the overall efficiency. For TD densities over 4×106cm−2, as the diffusion length of minority carriers in the base gets smaller than the base thickness, the short-circuit current in the top GaAsxP1−x cell is also reduced, resulting in a decrease in the optimal top cell bandgap. Using non-ideal EQEs and surface recombination rates from published experimental data, the long-term efficiency potential of the investigated technology has been estimated to be ~35.1% for an ideally textured GaAsxP1−x/Si tandem cell with a TD density of 105cm−2 (~33.0% with a flat surface).

Effect of Deposition Conditions and Annealing Temperature on Tunnel Magnetoresistance and the Structure of MgO-Based Double-Barrier Magnetic Tunnel Junctions

Tunnel magnetoresistance (TMR) was measured in CoFeB–MgO-based double-barrier magnetic tunnel junctions. The variation of sputtering power density is used to control the relative B content of the middle electrode. MR ratios in both top and bottom junctions are suppressed with respect to the single-barrier case. While the bottom junction shows a saturation of the MR as a function of high-temperature annealing for all sputtering power densities, the top junction exhibits an increase in MR as a function of annealing temperature with higher values for higher sputtering power density. The suppression of high MR is attributed to a lack of strong crystallization in the middle electrode, which is confirmed by cross-sectional transmission electron microscopy. Slight crystallization of the middle electrode is achieved at the highest sputtering power density despite the fact that boron diffusion is suppressed due to the adjacent MgO tunnel barriers. Optimal deposition and postannealing conditions resulted in MR values of 140% and 80% for the top and bottom junctions, respectively.

Enhanced conversion efficiency in wide-bandgap GaNP solar cells

In this work, we demonstrate ∼2.05 eV dilute nitride GaNP solar cells on GaP substrates for potential use as the top junction in dual-junction integrated cells on Si. By adding a small amount of N into indirect-bandgap GaP, GaNP has several extremely important attributes: a direct-bandgap that is also tunable, and easily attained lattice-match with Si. Our best GaNP solar cell ([N] ∼ 1.8%, Eg ∼ 2.05 eV) achieves an efficiency of 7.9%, even in the absence of a window layer. This GaNP solar cell's efficiency is 3× higher than the most efficient GaP solar cell to date and higher than other solar cells with similar direct bandgap (InGaP, GaAsP). Through a systematic study of the structural, electrical, and optical properties of the device, efficient broadband optical absorption and enhanced solar cell performance are demonstrated.

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.

Progress Towards a 30% Efficient GaInP/Si Tandem Solar Cell

The performance of dual-junction solar cells with a Si bottom cell has been investigated both theoretically and experimentally. Simulations show that adding a top junction with an energy bandgap of 1.6 -1.9eV to a standard silicon solar cell enables efficiencies over 38%. Currently, top junctions of GaInP (1.8eV) are the most promising as they can achieve 1-sun efficiencies of 20.8% [1]. We fabricated mechanically stacked, four terminal GaInP/Si tandem solar cells using a transparent adhesive between the subcells. These tandem devices achieved an efficiency of 27% under AM1.5g spectral conditions. Higher efficiencies can be achieved by using an improved Si-bottom cell and by optimizing the dual-junction device for long-wavelength light and luminescent coupling between the two junctions.

Design of AlxGa1−xAs/GaAs/InyGa1−yAs triple junction solar cells with anti-reflective coating

Multi-junction solar cells (SC) made from III–V compound semiconductors are still in the development phase. Here, we perform calculations for multi-junction cells: AlxGa1−xAs top junction, GaAs middle junction and InyGa1−yAs bottom junction (all of these materials with band-gaps between 2.1 and 0.8eV) in order to obtain the optimal band gap and thickness for each junction under the AM1.5 solar radiation spectrum. The ideal photo-current density is around 15.5mA/cm2. In order to reduce the natural reflectivity, an anti-reflective coating (ARC) was chosen, based on a MgF2/ZnS double layer, allowing for a significant increase of the current density with respect to a cell without it. Calculations of external quantum efficiency (QE) were also performed for the three cases mentioned above: ideal one, taking into account the total reflection and with the ARC double layer. Finally, when more realistic calculations are done, taking into account the carrier recombination at each sub-cell, and the light reflection for a tandem cell with the designed ARC on top, the expected conversion efficiency (η), under the AM 1.5 spectrum (without concentration), was determined to be around 38.5%, making this an attractive III–V compound tandem cell to be investigated in the near future.

Light-management potential of dual-function n-SiOx in the top junction of micromorph solar cells with different front electrodes

The optical performance of micromorph solar cells employing mixed-phase n-SiOx as advanced n-layer in the top junction has been systematically investigated with efficient state of the art ZnO front electrodes characterized by different scattering properties. The layer is applied with the dual function of n-layer and intermediate reflector. Enhanced top cell current complemented by significant dependence on the front texture has been observed. With smoother (locally flat-like) front electrode, interference effects are present with adverse impact on the spectral response of the bottom cell. Moving to rougher substrates these effects are strongly attenuated thus fully disclosing the strength of the present scheme, which is the simultaneous enhancement of both top cell current and total current. Quantitatively the gain is impacted by the front electrode morphology. Guidelines for a cross-optimization of texture and dual-function n-SiOx properties/thickness are supplied in order to take full advantage from this design.

Luminescent emission of multi-junction InGaP/InGaAs/Ge PV cells under high intensity irradiation

Combined electroluminescence (EL) and photoluminescence (PL) measurements were conducted in order to investigate the presence of luminescent emission of InGaP/InGaAs/Ge at different operating conditions of the tandem. Luminescent emission from cell samples was observed at different sun concentrations, voltage biases and temperatures. A high intensity pulsed solar simulator was used to photoexcite the device which exhibited strong radiative recombination from both the top InGaP and middle InGaAs junctions. Luminescent emission from the device was investigated under a range of voltage biases and was clearly observed at the maximum power point voltage of the sample under test indicating its presence during typical operating conditions of the solar cells. Investigation of the emission was also performed at relatively high temperatures (up to 60°C) in order to mimic the outdoor operating conditions of a solar cell device. Luminescence was detected at high temperatures indicating that significant radiative recombination is present at even higher temperatures. Outdoor measurements under actual solar spectrum demonstrated the presence of luminescent emission in agreement with indoor testing. The significant amount of radiative recombination at the band-gap edges of the top junctions observed in our measurements gives evidence that optical coupling to the lower ones may occur. Finally, excitation power dependent PL was performed using monochromatic laser sources in order to investigate the impact of externally induced photocurrents of different intensity upon the radiative signal of each junction.

Photoluminescence analysis of coupling effects: The impact of shunt resistance and temperature

In multi-junction devices, due to the series connection of junctions, recombination current from the top junctions can be directed to the bottom ones affecting their electrical characteristics. Recently, luminescence coupling effects during External Quantum Efficiency (EQE) measurements at very intense light bias conditions indicated high recombination current flowing towards the bottom junctions of the cells. In an attempt to find the magnitude of coupling current as well as the factors affecting the optical interactions between junctions, excitation and voltage dependent Photoluminescence (PL) measurements of GaInP/GaInAs/Ge have been carried out. An investigation using junctions with different shunt resistances has been conducted to identify the impact of shunts on the coupling current. Furthermore the impact of temperature on the coupling current has been considered. Our results show that a maximum of 2.3% of the recombination current of the top junction is converted to coupling current in the middle junction depending on the devices under examination. The coupling efficiency depends on the shunt resistance of the top junctions as well as on the temperature. Furthermore a physical model of the current limiting junction was built taking into consideration the impact of local ohmic shunts in the solar cell device and used to validate the experimental data taken.