Two-junction Cell Research Articles

Enhanced optical and electrical properties of narrow-bandgap cerium borate glasses and nanostructured glass-ceramics: influence of V2O5-incorporation

Improvement of optical and electrical properties of borate glasses is a great demand for upgrading optoelectronic devices. Additionally, the transformation of borate glasses to nanostructured glass-ceramics endows them with outstanding beneficial properties to widen their application range. Herein, novel borate glass compositions are synthesized by the facile melt quenching method. The influences of V2O5-incorporation and nanocrystallization on the structural, optical, and electrical properties of the investigated cerium borate glasses are studied. The heat-treatment process promotes the precipitation of CeO2 and VCeO4 nanocrystals in the glass network. Linear and high dependence of the band gap of the as-prepared glasses on the V2O5 concentration is observed. A narrow band gap (about 1.6), which lies in the range of ideal bandgaps for two-junction photovoltaic cell applications, is achieved. Strong absorption in the visible and/or UV regions is attained for the studied samples, which is beneficial to shield laser emissions (up to 794 nm) and/or protect against the hazard of UV radiation. High DC conductivity (about 3 × 10−6 S m−1) is approved for the glass sample containing V2O5 content of 10 mol%. The investigated glasses and nanostructured glass-ceramics are recommended as promising candidates for UV-protection Sunglasses, laser safety, visible light irradiation photocatalyst, and next-generation semiconductors.

Very Large Scale Integration of Josephson-Junction-Based Superconductor Random Access Memories

Arrays of vortex transitional (VT) memory cells with functional density up to 1 Mbit/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> have been designed, fabricated, and successfully demonstrated. This progress is due to recent advances in design optimization and in superconductor electronics fabrication achieved at MIT Lincoln Laboratory. As a starting point, we developed a demo array of VT cells for the 100-μA/μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> MIT LL fabrication process SFQ5ee with eight niobium layers. The studied two-junction memory cell with a two-junction nondestructive readout occupied 168 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , resulting in an over 0.5 Mbit/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> functional density. Then, we reduced the cell area down to 99 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (corresponding to over 0.9 Mbit/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> functional density) by utilizing self-shunted Josephson junctions (JJs) with critical current density JC of 600 μA/μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and eliminating shunt resistors. The fabricated high-JC memory cells were fully operational and possessed wide read/write current margins, quite close to the theoretically predicted values. We discuss approaches to further increasing the integration scale of superconductor memory and logic circuits: 1) miniaturization of superconducting transformers by using soft magnetic materials; and 2) reduction of JJ area by using planar high-JC junctions similar to variable thickness bridges.

Highly efficient Ag-alloyed Cu(In,Ga)Se2 solar cells with wide bandgaps and their application to chalcopyrite-based tandem solar cells

In this work, Ag alloying effects in wide-bandgap Cu(In,Ga)Se2 (CIGS) solar cells are investigated, and their application in tandem solar cells is attempted. With a three-stage process, highly efficient Ag-alloyed CIGS (ACIGS) solar cells with Eg ≥ 1.4 eV are fabricated; the benefits of Ag alloying are discussed with a focus on the control of structural/electronic disorders. The ACIGS solar cells are found to exhibit particularly improved device performance in specific compositions; in general, Ag alloying helps to enhance VOC in wide-bandgap CIGS solar cells. With wide-bandgap ACIGS cells as a top subcell, various kinds of chalcopyrite-based tandem (two-junction and two-terminal) cells are demonstrated. Prospects and limitations of chalcopyrite-based tandem cells are investigated by examining beam-splitting tandem, mechanically stacked tandem, and monolithic tandem cells.

Absorption enhancement of GaInP nanowires by tailoring transparent shell thicknesses and its application in III-V nanowire/Si film two-junction solar cells.

A non-absorbing transparent shell is proposed to be coated on the outer surface of the core photoactive GaInP nanowire array (NWA) of the III-V nanowire (NW)/Si film two-junction solar cell. Interestingly, the diluted (at the filling ratio of 0.25) GaInP NWA with core / transparent shell structure can absorb more light than that in bare denser (at the filling ratio of 0.5) NWA. This allows for less source material consumption during the fabrication of III-V NWA/Si film two-junction cell. Meanwhile, the condition of current matching between the top III-V NWA and Si film sub cell can be easily fulfilled by tailoring the coating thickness of the transparent coating. Beyond the advantages on light absorption, the surface passivation effects introduced by the addition of some transparent dielectric coatings can reduce the surface recombination rate at the top NWA sub cell surface. This facilitates the effective extraction of photo-generated carriers and enhances output stability of the top NWA sub cell. From electrical simulation, a power conversion efficiency of 29.9% can be obtained at the optimized coating geometry.

Analysis of Multijunction Solar Cell Current–Voltage Characteristics in the Presence of Luminescent Coupling

Luminescent coupling in multijunction solar cells is the phenomenon in which a junction in forward bias radiates photons that are absorbed in and converted to photocurrent by the junction beneath the radiating one. This effect can be significant in modern high-efficiency multijunction cells. We have previously developed a combined measurement and analytical approach to characterize the short-circuit current including the effects of nonlinear coupling in terms of measurable parameters η and φ that describe the coupling strength and linearity, respectively. Here, we develop an analytical model for the full current-voltage characteristic V (J) of a multijunction cell in the presence of luminescent coupling, in terms of the η and φ parameters. We compare the model with the measured V (J) parameters of GaInP/GaAs two-junction cells that exhibit differing degrees of luminescent coupling, and show that the model well describes the measurements. We then use the model to explore the consequences of luminescent coupling on the operating parameters of an idealized two-junction cell as a function of the top-junction thickness, focusing on the open-circuit voltage, fill factor, and efficiency. The results demonstrate that the strong luminescent coupling can significantly alter the dependence of cell efficiency on junction thickness, and that consequently the well-known optical-thinning design rules must be modified.

Effects of Internal Luminescence and Internal Optics on $V_{\bf oc}$ and $J_{\bf sc}$ of III--V Solar Cells

For solar cells dominated by radiative recombination, the performance can be significantly enhanced by improving the internal optics. We demonstrate a detailed model for solar cells that calculates the external luminescent efficiency and discuss the relationship between the external and internal luminescence. The model accounts for wavelength-dependent optical properties in each layer, parasitic optical and electrical losses, multiple reflections within the cell, and assumes isotropic internal emission. For single-junction cells, the calculation leads to Voc, and for multijunction cells, the calculation leads to the Voc of each junction as well as the luminescent coupling constant. In both cases, the effects of the optics are most prominent in cells with high internal radiative efficiency. Exploiting good material quality and high luminescent coupling, we demonstrate a two-junction nonconcentrator cell with a conversion efficiency of (31.1 ± 0.9)% under the global spectrum.

Spectral-splitting concentrator photovoltaic modules based on AlGaAs/GaAs/GaSb and GaInP/InGaAs(P) solar cells

A concentrator photovoltaic module with sunlight spectral splitting by Fresnel lens and dichroic filters is developed. The photoelectric conversion efficiency of such a module is estimated at a level of 49.4% when three single-junction cells are used and may reach 48.5–50.6% when a tandem two-junction cell is combined with narrow-band cells. Single-junction AlGaAs, GaAs, GaSb, and InGa(P)As solar sells are fabricated by zinc diffusion from the vapor phase into an n-type epitaxial layer. GaInP/GaAs cascade solar cells are prepared by MOS hydride epitaxy. The overall efficiency of the three single-junction solar cells developed for the spectral-splitting module is 38.1% (AM1.5D) at concentration ratio K c = 200x. The combination of the solar cells with the cascade structure demonstrates an efficiency of 37.9% at concentrations of 400–800 suns. The parameters of the spectral-splitting photovoltaic module are measured. The photovoltaic efficiency of this module reaches 24.7% in the case of three single-junction cells and 27.9% when the two-junction and single-junction cells are combined.

Theoretical conversion efficiency of a two-junction III-V nanowire on Si solar cell

The continuity and Poisson equations are solved numerically to obtain J-V characteristics and photoconversion efficiency of a two-junction solar cell. The cell consists of a top junction comprised of nanowires with bandgap of 1.7 eV grown on a bottom junction comprised of a Si substrate. The lattice relaxation possible in nanowires permits lattice-mismatched III-V material growth on Si, thereby achieving the optimum bandgaps in a two-junction cell. The model indicates a limiting efficiency of 42.3% under a concentration of 500 Suns (AM1.5 D spectrum). This limiting efficiency is similar to that calculated for the planar lattice-matched triple-junction Ge/InGaAs/InGaP cell. Methods of fabricating the nanowire/Si cell are discussed including requirements for nanowire sidewall surface passivation. The model indicated that passivation of the nanowire sidewall surfaces that produces a surface recombination velocity of 3000 cm·s−1 and surface trap density of 1012 cm−2 should be sufficient to yield high efficiency solar cells.

Next-generation multijunction solar cells: The promise of II-VI materials

High concentration photovoltaic (HCPV) systems offer the highest photovoltaic (PV) conversion efficiencies. Also, as production is beginning to ramp up, HCPV is becoming cost competitive with thin-film poly-CdTe and crystalline Si systems in high solar insolation regions. High solar concentrations, X ∼ 500, are used to increase cell efficiencies and greatly reduce the cell area per unit of incident solar radiation, thereby greatly reducing the cell cost per watt. The monolithic three-junction (3J) solar cells presently used in HCPV systems typically consist of two epitaxial III-V homojunctions, such as GaInP and GaInAs, grown on an active Ge substrate by metal-organic chemical vapor deposition (MOCVD). The III-V bandgaps are chosen to match the currents generated in each junction and minimize the energy lost to thermalization of the electron-hole pairs generated, subject to the constraint of approximate lattice matching. We propose using cells consisting of one or more CdTe-based II-VI homojunctions grown on large-area active Si substrates by high-throughput MBE or a less expensive high-vacuum deposition technique as an alternative to III-V based multijunction cells grown by MOCVD. The bandgap of Si is more optimal than that of Ge for two-junction (2J) or 3J cells, and lattice mismatches affect the efficiencies of such cells only slightly, which allows greater freedom in the choice of bandgaps, and thus the potential for higher efficiencies. Also, such cells could be manufactured at a much lower cost due to the larger area, much lower cost and superior mechanical properties of Si substrates as compared to Ge substrates. The much lower cell cost also would enable medium concentration PV systems that would require more cell area, but with simplified, less expensive tracking and optics, resulting in lower overall system costs. Promising initial results from material-property measurements and single-junction and 2J CdZnTe/Si cell characterization results are given. Both the promise of the proposed technology and the challenges it faces are discussed.

Proposed monolithic triple-junction solar cell structures with the potential for ultrahigh efficiencies using II–VI alloys and silicon substrates

The efficiencies of monolithic single-crystal II–VI and III–V, two-junction and three-junction solar cells are calculated. The structures consist of II–VI or III–V homojunctions grown on an active-junction substrate (silicon for II–VI and germanium for III–V) and of inverted three-junction II–VI or III–V structures, with the band gaps chosen to maximize the efficiencies. Our calculations for the II–VI cells give theoretical efficiencies up to 44% under 1 sun and 50% under 500 suns, ∼3% absolute higher than for the III–V cells. Maximum obtainable laboratory and production-line efficiencies for multijunction II–VI cells are predicted. Preliminary laboratory results for II–VI two-junction cells are also presented.

Modeling of InGaN/Si tandem solar cells

We investigate theoretically the characteristics of monolithic InGaN/Si two-junction series-connected solar cells using the air mass 1.5 global irradiance spectrum. The addition of an InGaN junction is found to produce significant increases in the energy conversion efficiency of the solar cell over that of one-junction Si cells. Even when Si is not of high quality, such two-junction cells could achieve efficiencies high enough to be practically feasible. We also show that further, though smaller, improvements of the efficiency can be achieved by adding another junction to form an InGaN/InGaN/Si three-junction cell.

Direct-bonded GaAs∕InGaAs tandem solar cell

A direct-bonded GaAs∕InGaAs solar cell is demonstrated. The direct-bonded interconnect between subcells of this two-junction cell enables monolithic interconnection without threading dislocations and planar defects that typically arise during lattice-mismatched epitaxial heterostructure growth. The bonded interface is a metal-free n+GaAs∕n+InP tunnel junction. The tandem cell open-circuit voltage is approximately the sum of the subcell open-circuit voltages. The internal quantum efficiency is 0.8 for the GaAs subcell compared to 0.9 for an unbonded GaAs subcell near the band gap energy and is 0.7 for both of the InGaAs subcell and an unbonded InGaAs subcell, with bonded and unbonded subcells similar in spectral response.

Band-gap engineering withHfSxSe2−x

The band gaps of HiS 2 (1.96 eV) and HfSe 2 (1.13 eV) cover the range of ideal band gaps for photovoltaic applications which is for a single-junction cell about 1.45 eV and for a two-junction cell about 1.1 eV and 1.8 eV.Additionally, in favor of applications the transition-metal dichalcogenides have a strong tendency to grow as thin films due to their quasi-two-dimensional character. Employing the chemical vapor transport method, single crystals of this ternary semiconductor were grown with nine different compositions. They were characterized in detail by low-energy electron diffraction, Laue diffraction. and energy dispersive x-ray analysis. In addition the absorption at room temperature was measured, revealing an absorption coefficient α(hν) that is remarkably high for an indirect semiconductor. The determined band gaps of HfS x Se 2 - x show a nearly linear dependence on x, which is perfect for band-gap engineering. Furthermore, in the gap an exponential behavior of α(hν) is observed, which could be interpreted as an Urbach tail.

Modeled performance of polycrystalline thin‐film tandem solar cells

AbstractThe optimum bandgaps for two‐terminal monolithic multijunction devices have typically been studied under a fixed set of environmental conditions, using ideal device models. This study examines the effects of a realistic dark‐current for typical state‐of‐the‐art thin‐film polycrystalline cells on the optimum energy gap for a series‐connected two‐junction cell. The optimum energy gaps are compared for a series‐connected tandem cell under standard reference conditions and the energy produced for five different reference days where the temperature, spectral irradiance and total irradiance varied. The optimum bandgaps were found to be 1.72 ± 0.02 eV for the top cell and 1.14 ± 0.02 eV for the bottom cell. Published in 2002 by John Wiley &amp; Sons, Ltd.

Characteristics of GaAs-based concentrator cells

GaAs-based cells, including GaAs single-junction cells, AlGaAs/GaAs two-junction cells, and InGaP/GaAs two-junction cells grown on GaAs substrates by metal-organic chemical vapor deposition (MOCVD) are examined in various levels of concentration and backside cooling temperature. All types of cells have shown boost of efficiency in low and medium ranges of concentration. The cell efficiencies obtained are 31.5% at 20-suns of AM1.5 for InGaP/GaAs tandem cell, and 29.2% at 7-suns of AM1.5 for AlGaAs/GaAs tandem cell, respectively. The GaAs single-junction cell is also examined as the reference. A new equivalent circuit model reveals that increase of apparent leakage current is responsible for a rapid efficiency drop in the high-concentration region. It is possible to improve it by reducing contact resistance and using uniform concentrated illumination.

Physics and technologies of superhigh-efficiency tandem solar cells

The present status of superhigh-efficiency tandem solar cells has been reviewed and the key issues for realizing superhigh-efficiency have been discussed. The mechanical, stacked, three-junction cells of monolithically grown InGaP/GaAs two-junction cells and InGaAs cells have reached the highest efficiency (attainable in Japan) of 33.3% at 1-sun AM 1.5. Future prospects for realizing superhigh-efficiency and low-cost tandem solar cells are discussed.

High efficiency GaAs-based multi-junction solar cells grown by metalorganic vapor phase epitaxy

A 25.2% efficiency measured under 1-sun, air mass 0 (AM0) illumination has been achieved in a three-junction AlGaAs( E g = 1.93 eV)/GaAs/InGaAsP( E g = 0.95 eV) cascade solar cell. The cascade structure grown by atmospheric pressure metalorganic vapor phase epitaxy (MOVPE) consists of a monolithic, series-connected AlGaAs/GaAs cell mechanically stacked on an InGaAsP single junction cell. The efficiency obtained in this cascade structure represents the highest 1-sun AM0 efficiency achieved in any solar cell operating in a two-terminal configuration. The performance of this triple-junction cell indicates that the GaAs component cell is the current-limiting cell in the cascade structure. To evaluate the monolithic AlGaAs/GaAs two-junction cells for space applications, radiation experiments were conducted. The cells were exposed to 1 MeV electron radiation at progressively higher fluences. Results of these experiments indicate that the two-junction cells degrade to about 70% of their initial efficiencies after exposure of a total fluence of 10 16 electrons/cm 2. Quantitative results of electron radiation effects on the cell performance are addressed and discussed in this paper.

Stability of P-I-N Amorphous Silicon Solar Cells with Boron-Doped and Undoped I-Layers

AbstractBoron-doping the i-layer in p-i-n amorphous silicon solar cells improves the device performance when the density of impurities in the undoped i-layer material is high (&lt; 1020 cm-3). While this technique can boost the initial device efficiencies for poor quality i-layer material, our devices degrade faster than devices made with undoped, low impurity i-layer material. We have measured the degradation of photovoltaic parameters as a function of continuous AM1 exposure time for devices with and without B-doped i-layers. For single junction p-i-n solar cells with comparable initial conversion efficiencies (&lt; 7%, area &lt; 1cm2) we find that our devices containing i-layers deposited from gas mixtures containing 2–3 ppm diborane degrade faster than devices containing undoped i-layers. Similar effects are observed when two-junction stacked cells with B-doped i-layers are compared to two-junction stacked cells with undoped i-layers.

Influence of band gap span on cascade solar cell efficiency

A multijunction cascade solar cell modeling program is applied to isolattice constant structures in AlGaAsSb. The “peak” efficiency is determined for two- and three-active-junction cells for lattice constants ranging from 5.70 to 5.85 Å. In iso-lattice constant AlGaAsSb structures the highest efficiency is obtained by using a two-junction cascade cell. Cascade cells with three or more active junctions give a lower efficiency. The lower peak efficiency values of three-junction AlGaAsSb cascade cells compared with those of the two-junction cells are caused by a less-than-required difference between direct band gap energies of the bottom and top cells. These results may be explained on the basis of intrinsic losses arising from absorbed photon energy utilization and transparency losses.

Mutual phase locking in multijunction Josephson arrays

Mutual phase locking of Josephson oscillations in a two-junction cell and multijunction arrays is analysed. The locking is due to ac currents of Josephson frequency ω, which flow through some special coupling circuit. For the most important case of almost identical junctions, the locking frequency range is shown to be proportional to the imaginary part of the complex conductivity Y(ω) of the coupling circuit. The power and linewidth of coherent oscillations, as well as the coherence stability with respect to the junction intrinsic noise and parameter spread, have been calculated. The maximum locking range and, hence, the maximum parameter tolerances are shown to take place in the closed-loop-type (ring) structures with long-range junction interactions.