Multiple Quantum Wells Cell Research Articles

GaTlAs Quantum Well Solar Cells for Sub-band Gap Absorption

ABSTRACTDespite the improvements seen in efficiency of GaAs cells over the years, there remains room for improvement for it to approach the theoretical single junction limit posited by Shockley and Quiesser decades ago. One of the more pursued options is the growth of quantum wells within the structure of GaAs to enhance its photon absorption below its bandgap. Multiple Quantum Wells (MQW) have been an ongoing topic of research and discussion for the scientific community with structures like InGaAs/GaAs and InGaP/GaAs quantum wells producing promising results that could potentially improve overall energy conversion. Here, we used WEIN2K, a commercial density functional theory package, to study the ternary compound Ga1-xTlxAs and determine its electronic properties. Using these results combined with experimental confirmation we extend these properties to simulate its application to form a MQW GaAs/ Ga1-xTlxAs solar cell. Ga1-xTlxAs is a tunable ternary compound, with its bandgap being strongly dependent on the concentration of Tl present. Concentrations of Tl as low as 7% can reduce the bandgap of Ga1-xTlxAs to roughly 1.30 eV from GaAs’s 1.45 eV at room temperature with as little as a 1.7% increase in lattice constant. The change in bandgap, accompanied by the relatively small change in lattice constant makes Ga1-xTlxAs a strong candidate for a MQW cell with little to no strain balancing required within the structure to minimize unwanted defects that impede charge collection within the device. Our GaAs photodiode with TlGaAs MQWs shows an expanded absorption band and improved conversion efficiency over the standard GaAs photovoltaic cell with dilute concentrations of Tl incorporated into the compound.

An InGaN-Based Solar Cell Including Dual InGaN/GaN Multiple Quantum Wells

An InGaN/GaN solar cell including a dual multiple quantum wells (MQWs) structure is investigated. It shows an obvious advantage over the conventional InGaN/GaN cell, which only contains a single MQWs structure. Because the short current density ( ${J} _{\mathrm { {sc}}})$ increases, the 1 sun power conversion efficiency significantly improves from 0.62% (single MQWs cell) to 1.02% (dual MQWs cell). From the measurement of EQE and PL spectra, the enhancement of effective photoelectric response within the solar spectrum mainly contributes to the high device performance, due to the introduced upper MQWs of higher In content.

Effect of Barrier Thickness on Carrier Transport Inside Multiple Quantum Well Solar Cells Under High-Concentration Illumination

Carrier transport inside InGaAs/GaAs/GaAsP multiple quantum well (MQW) solar cells was discussed under high-concentrated sunlight illumination up to 338 suns. Current–voltage (I–V) characteristic curves for a GaAs reference cell and MQW cells with GaAsP barrier thickness of 2, 4, and 6 nm were investigated under dark and high-concentration illumination. Carrier collection efficiency (CCE) was estimated by net photocurrent, which is the difference between illuminated current and dark current density at each bias voltage normalized by the value at the saturated point. For the 2-nm barrier, CCE exhibited almost no degradation compared with the GaAs reference cell. On the other hand, CCE for the 6-nm barrier degraded with forward biases as the sunlight concentration ratio increased. The degradation in CCE under a high-concentration ratio is shown to be the result of carrier accumulation in quantum wells. Thin barriers seemed to eliminate such accumulation with the help of the carrier tunneling effect through the barriers.

Absorption threshold extended to 1.15 eV using InGaAs/GaAsP quantum wells for over‐50%‐efficient lattice‐matched quad‐junction solar cells

AbstractBandgap engineering of strain‐balanced InGaAs/GaAsP multiple quantum wells (MQWs) allows high‐quality materials with an absorption edge beyond GaAs to be epitaxially grown in Ge/GaAs‐based multijunction solar cells. We demonstrate MQW solar cells with effective bandgaps ranging from 1.31 eV to as low as 1.15 eV. The bandgap‐voltage‐offset of MQWs is found to be independent of effective bandgaps and superior to a bulk reference by approximately 0.1 V. This implies the merit of high photovoltage as compared with bulk cells with the same bandgap in addition to their widely bandgap‐tunable property. Towards the realization of fully lattice‐matched quad‐junction devices, we demonstrate a 70‐period, 1.15‐eV bandgap MQW cell as a promising material in 0.66/1.15/1.51/1.99‐eV quad‐junction cells, whose practical efficiency has a potential to achieve over 50%. With such a large period number of MQWs, the reverse‐biased external quantum efficiency reaches an average of over 60% in the spectral region corresponding to a 1.15‐eV subcell; this is achieved with only a‐few‐percent drop at short‐circuit condition. The device presented here reaches the target open‐circuit voltage and over 75% of the current density required for realizing a 1.15‐eV subcell in a 50%‐efficient quad‐junction solar cell. We believe that future devices which exploit light‐trapping structures and enhanced carrier extraction will be able to reach the desired target. Copyright © 2015 John Wiley & Sons, Ltd.

The Effect of 3-MeV Proton Irradiation on the Performance of InGaN/GaN MQWs Solar Cells

The effect of 3-MeV proton irradiation on InGaN/GaN multiple quantum wells (MQWs) solar cells was investigated. The irradiation degradation of the photoluminescence characteristics of the InGaN film was also measured, indicating its potential to be used in space solar cells. With the proton fluence increasing from 0 to 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> /cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , the conversion efficiency (η) of the InGaN/GaN MQWs cell declined from 0.61% to 0.14%.

100-period, 1.23-eV bandgap InGaAs/GaAsP quantum wells for high-efficiency GaAs solar cells: toward current-matched Ge-based tandem cells

Major challenges for InGaAs/GaAsP multiple quantum well (MQW) solar cells include both the difficulty in designing suitable structures and, because of the strain-balancing requirement, growing high-quality crystals. The present paper proposes a comprehensive design principle for MQWs that overcomes the trade-off between light absorption and carrier transport that is based, in particular, on a systematical investigation of GaAsP barrier effects on carrier dynamics that occur for various barrier widths and heights. The fundamental strategies related to structure optimization are as follows: (i) acknowledging that InGaAs wells should be thinner and deeper for a given bandgap to achieve both a higher absorption coefficient for 1e-1hh transitions and a lower compressive strain accumulation; (ii) understanding that GaAs interlayers with thicknesses of just a few nanometers effectively extend the absorption edge without additional compressive strain and suppress lattice relaxation during growth; and (iii) understanding that GaAsP barriers should be thinner than 3 nm to facilitate tunneling transport and that their phosphorus content should be minimized while avoiding detrimental lattice relaxation. After structural optimization of 1.23-eV bandgap quantum wells, a cell with 100-period In0.30GaAs(3.5 nm)/GaAs(2.7 nm)/GaAsP0.40(3.0 nm) MQWs exhibited significantly improved performance, showing 16.2% AM 1.5 efficiency without an anti-reflection coating, and a 70% internal quantum efficiency beyond the GaAs band edge. When compared with the GaAs control cell, the optimized cell showed an absolute enhancement in AM 1.5 efficiency, and 1.22 times higher efficiency with 38% current enhancement with an AM 1.5 cut-off using a 665-nm long-pass filter, thus indicating the strong potential of MQW cells in Ge-based 3-J tandem devices. Copyright © 2013 John Wiley & Sons, Ltd.

Open-circuit voltage characteristics of InP-based quantum well solar cells

Open-circuit voltage characteristics of InP-based multiple-quantum-well (MQW) solar cells are studied experimentally and theoretically. Experimental results are presented for spectral response and terminal characteristics of In0.53Ga0.47As/InP MQW cells and compared with results from a corresponding bulk p-i-n control cell. Open-circuit voltages measured for these cells, and for other InGaAs/InP and InAsP/InP MQW cells reported in the literature, are then analyzed using a simple ideal theory for MQW cells which attributes open-circuit voltage reductions to increased radiative recombination in the quantum wells. The large (0.19–0.33 V) measured open-circuit voltage reductions which accompany introduction of the quantum wells are shown to agree with predictions from the ideal theory to within ∼0.03 V on average. Finally, implications of this work for the design of efficient InP-based MQW solar cells are discussed.