Multi-quantum Well Solar Cells Research Articles

Nanostructure-Based CZGS/CZGSe Multiple Quantum Wells Could Sense Wider Solar Spectrum to Enhance Light-Harvesting Efficiency

Nanostructures in kesterite solar cells are incorporated for performance enhancement. As high recombination limits the efficiency of kesterite-based CZTS/Se solar cell, substituting Ge for Sn as Cu-Zn-Ge-S/Se (CZGS/Se) increases optical characteristics and solar cell overall performance. This research analyses MQWSC (multi-quantum-well solar cell) employing Cu-Zn-Ge-S as a barrier material and Cu-Zn-Ge-Se as a well material. Introducing quantum wells as (2, 10, 20, 30, 40, 50, 70, 100 QWs) in CZGS kesterite groups highlights the novelty of device. To enhance the performance its optic, electric, and structural characteristics are widely researched across nanostructures. An examination of the carrier dynamics of the device is carried out by varying various parameters. By introducing various loss mechanisms, including deep defect and shallow defect, the device’s worst-case operation is also evaluated. A remarkable efficiency of 29% without defect and 20% with defective states obtained from the proposed structure, which signify new route to facilitate the performance of PV (photovoltic) cells for the next generation.

Degradation in Efficiency of InGaN/GaN Multiquantum Well Solar Cells With Rising Temperature

In this article, the authors theoretically and experimentally investigate the mechanism of degradation of InGaN/GaN multi-quantum well (MQW) solar cells. InGaN/GaN MQW solar cells with chips of size <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1\times1$ </tex-math></inline-formula> mm2 were fabricated and characterized within a range of temperature of 50 °C–250 °C to determine the effects of dislocations on their efficiency. The internal intensity of X-ray diffractions was used to construct a 2-D physical model of the density of edge dislocations in the InGaN/GaN MQW that was then simulated in Silvaco. The results show that the relative error in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\eta $ </tex-math></inline-formula> between the simulation and the experiment was less than 5% over the examined range of temperature. This shows that an intensified number of nonradiative recombinations owing to high temperature contribute to edge dislocations that eventually degrade the performance of photoelectric devices. The work here provides a theoretical and experimental basis for the growth of high-quality InGaN materials to fabricate highly efficient solar cells.

Quantum efficiency of InGaN–GaN multi-quantum well solar cells: Experimental characterization and modeling

InGaN-based multi-quantum well (MQW) solar cells are promising devices for photovoltaics (e.g., for tandem solar cells and concentrator systems), space applications, and wireless power transfer. In order to improve the efficiency of these devices, the factors limiting their efficiency and stability must be investigated in detail. Due to the complexity of a MQW structure, compared with a simple pn junction, modeling the spectral response of these solar cells is not straightforward, and ad hoc methodologies must be implemented. In this paper, we propose a model, based on material parameters and closed-formula equations, that describes the shape of the quantum efficiency of InGaN/GaN MQW solar cells, by taking into account the layer thickness, the temperature dependence of the absorption coefficient, and quantum confinement effects. We demonstrate (i) that the proposed model can effectively reproduce the spectral response of the cells; in addition, (ii) we prove that the bulk p-GaN layer absorbs radiation, but the carriers photogenerated in this region do not significantly contribute to device current. Finally, we show that (iii) by increasing the temperature, there is a redshift of the absorption edge due to bandgap narrowing, which can be described by Varshni law and is taken into account by the model, and a lowering in the extraction efficiency due to the increase in recombination (mostly Shockley–Read–Hall) inside the quantum wells, which is also visible by decreasing light intensity.

Photon recycling characteristics of InGaAs/GaAsP multiple quantum well solar cells incorporating a spectrally selective filter and distributed Bragg reflector.

Photon management plays a vital role in the power conversion efficiency of III-V semiconductor solar cells. However, the photon recycling characteristics of GaAs-based multi-quantum-well (MQW) solar cells employed different optical designs had yet been fully explored. In this work, we investigate the impact of the spectrally selective filter (SSF) and distributed Bragg reflector (DBR) on the photovoltaic characteristics of single-junction, strain-balanced In0.1Ga0.9As/ GaAs0.85P0.15 MQW solar cells. Specifically, the SSFs with cutoff wavelengths of 880, 910, and 940 nm are designed and implemented on MQW solar cells with and without the incorporation of a rear DBR. Photon confinement in the vertical direction is verified based on the characterizations of reflectance, electroluminescence, and external quantum efficiency. We show that the photon confinement reduces the saturation current density, up to 26 times and 3 times for the 880 nm SSF-MQW and SSF-MQW-DBR devices, respectively, compared to that of the 940 nm devices. Furthermore, by comparing the SSF-MQW-DBR solar cells under simulated one-sun and concentrated illumination conditions, the open-circuit voltage exhibits a maximal net increase for the 910 nm SSF due to tradeoff between the short-circuit and saturation current density. The proposed SSF design may offer a viable approach to boost the performance of GaAs-based MQW solar cells.

Energy band engineering of InGaN/GaN multi-quantum-well solar cells via AlGaN electron- and hole-blocking layers

In this paper, we perform a comprehensive study on energy band engineering of InGaN multi-quantum-well (MQW) solar cells using AlGaN electron- and hole-blocking layers. InGaN MQW solar cells with AlGaN layers were grown by metalorganic chemical vapor deposition, and high crystal quality was confirmed by high resolution X-ray diffraction measurements. Time-resolved photoluminescence results showed that the carrier lifetime on the solar cells with AlGaN layers increased by more than 40% compared to that on the reference samples, indicating greatly improved carrier collections. The illuminated current-density (J–V) measurements further confirmed that the short-circuit current density (Jsc) of the solar cells also benefited from the AlGaN layer design and increased 46%. At room temperature, the InGaN solar cells with AlGaN layers showed much higher power conversion efficiency (PCE), by up to two-fold, compared to reference devices. At high temperatures, these solar cells with AlGaN layers also delivered superior photovoltaic (PV) performance such as PCE, Jsc, and fill factor than the reference devices. These results indicate that band engineering with AlGaN layers in the InGaN MQW solar cell structures can effectively enhance the carrier collection process and is a promising design for high efficiency InGaN solar cells for both room temperature and high temperature PV applications.

Design optimization of multi quantum well solar cells

This study provides the optimized picture of multi quantum well (MQW) solar cell using Brennan method and Schrodinger equation. MQW solar cell is optimized by varying some parameters such as Indium composition of the ‘well’ and ‘barrier’; band gap difference of ‘well’ and ‘barrier’; number of quantum wells; the thickness of ‘well’ and ‘barrier’. The photovoltaic parameters of optimized GaN/InGaN 12MQW solar cell are short circuit current density of ∼2.10 mA/cm2, open circuit voltage of ∼0.85 V, fill factor of ∼0.86 and conversion efficiency of ∼8.92%.

Quantitative optical measurement of chemical potentials in intermediate band solar cells

Having shown some demonstration of two photon absorption and multiband emission processes in quantum dots (QD), multiquantum wells (MQW), and highly mismatched alloys, intermediate band solar cells are currently the subject of numerous studies. To better understand the underlying mechanisms, our objective is to experimentally probe the multiband operation of this device. We used photoluminescence recorded with a calibrated hyperspectral imager which provides spectrally resolved images with a spatial resolution of 2 μm and spectral resolution of 2 nm on proof of concept QD and MQW solar cells samples. Device emission can be described with the generalized Planck’s law from which the quasi-Fermi level splitting of the three bands can be determined. The advantage of the technique is that it can be used to investigate the intermediate band material without the need to make contacts or a full device structure. We also discuss the usefulness of a dual-beam method.

Photovoltaic response of InGaN/GaN multi-quantum well solar cells enhanced by inserting thin GaN cap layers

Structural properties and photovoltaic response of InGaN/GaN multi-quantum well (MQW) solar cells with low temperature grown GaN cap (LT-cap) layers were investigated. It is found that inserting a thin LT-cap layer (around 0.75nm) between each InGaN quantum well (QW) and GaN quantum barrier (QB) can remarkably improve photovoltaic response of InGaN/GaN MQW solar cells. This is attributed to the increased optical absorption of InGaN QWs due to higher indium content and thicker InGaN QWs thickness when LT-cap layers are added. However, if the LT-cap layer is too thick, the localization effect is enhanced and the defect density of InGaN QW layers will increase too much. Both of them will result in a reduced photovoltaic response of InGaN/GaN MQW solar cells.

InGaAs/GaAsP strain balanced multi-quantum wires grown on misoriented GaAs substrates for high efficiency solar cells

Quantum wires (QWRs) form naturally when growing strain balanced InGaAs/GaAsP multi-quantum wells (MQW) on GaAs [100] 6° misoriented substrates under the usual growth conditions. The presence of wires instead of wells could have several unexpected consequences for the performance of the MQW solar cells, both positive and negative, that need to be assessed to achieve high conversion efficiencies. In this letter, we study QWR properties from the point of view of their performance as solar cells by means of transmission electron microscopy, time resolved photoluminescence and external quantum efficiency (EQE) using polarised light. We find that these QWRs have longer lifetimes than nominally identical QWs grown on exact [100] GaAs substrates, of up to 1 μs, at any level of illumination. We attribute this effect to an asymmetric carrier escape from the nanostructures leading to a strong 1D-photo-charging, keeping electrons confined along the wire and holes in the barriers. In principle, these extended lifetimes could be exploited to enhance carrier collection and reduce dark current losses. Light absorption by these QWRs is 1.6 times weaker than QWs, as revealed by EQE measurements, which emphasises the need for more layers of nanostructures or the use light trapping techniques. Contrary to what we expected, QWR show very low absorption anisotropy, only 3.5%, which was the main drawback a priori of this nanostructure. We attribute this to a reduced lateral confinement inside the wires. These results encourage further study and optimization of QWRs for high efficiency solar cells.

1-eV InGaAsN/GaAs quantum well structure for high efficiency solar application grown by MOVPE

In this article, we have demonstrated 1-eV energy band gap In0.22GaAsNy/GaAs multi-quantum well (MQW) solar cells grown by a metal organic vapor phase epitaxy (MOVPE) system. With nitrogen incorporation, the equivalent band gap energy of quantum well structure will decrease and reach near 1eV. The structures of In0.22GaAsNy/GaAs MQWs were inserted into an intrinsic absorption layer for absorbing the incident light in the longer wavelength region. From the measurement results, the absorption band edge of In0.22GaAsNy/GaAs MQW solar cells is extended over 1300nm. On the other hand, in order to improve the device performance pre-deteriorated by nitrogen incorporation, a hybrid structure which consists of In0.22GaAs/GaAs and In0.22GaAsN0.043/GaAs quantum wells is adopted; with this hybrid quantum well structure as the absorption layer, consequently the short circuit current is enhanced from 10.85 to 15.29mA/cm2.

High Concentration InGaN/GaN Multi-Quantum Well Solar Cells with a Peak Open-Circuit Voltage of 2.45 V

We report InGaN/GaN multi-quantum well (MQW) solar cells with a comparatively high open-circuit voltage and good concentration properties. The open circuit voltage (Voc) keeps increasing logarithmically with concentration ratio until 60 suns. The peak Voc of InGaN/GaN MQW solar cells, which has a predominant peak wavelength of 456 nm from electroluminescence measurements, is found to be 2.45 V when the concentration ratio reaches 333×. Furthermore, the dependence of conversion efficiency and fill factor on concentration ratio are analyzed.

Self-annihilation of electron-irradiation-induced defects in InAsxP1−x∕InP multiquantum well solar cells

Irradiation by 1MeV electron of InAsxP1−x∕InP multiquantum well (MQW) solar cells results in the appearance of a dominant electron-emitting level E1, in the upper half of the band gap. The level E1, however, exhibits an interesting behavior as it was found to completely anneal out with storage at room temperature with no change in other majority carrier levels. No any other defect has been observed in electron-irradiated n-type InAsxP1−x∕InP MQW solar cell structures, which contrasts to conventional n-InP solar cell counterparts (structures without QWs). The study reveals that introduction of QWs in the intrinsic region of p-i-n structures enhances significantly the device radiation tolerance well beyond what has been thus far reported for conventional Si or III-V semiconductor-based solar cells.

Effect of well number on the performance of quantum-well solar cells

The effect of increasing the number of quantum wells in a strain-compensated, multiquantum-well solar cell is investigated. It is found that as the well number is increased, dark current level close to the operating point rises linearly. Short-circuit current in the AM0 spectrum also rises linearly with the inclusion of more quantum wells. This allows the cell to maintain a constant open-circuit voltage irrespective of the number of wells grown. This is anticipated to have advantages when the cell is used as a replacement for the GaAs junction in the existing generation of tandem and triple-junction cells since current levels can be matched to the upper junction without detriment to the voltage performance. This result allows us to predict a tandem cell AM0 efficiency of 23.8% based on the 50-well cell.

Multi quantum well multijunction solar cell for space applications

Standard InGaP-GaAs-Ge cell is modified by incorporating pseudomorphic multi-quantum wells (MQW) of InGaAs to increase the photocurrent of the limiting GaAs subcell. The approach as demonstrated here enables the multijunction device to be made with all sub-cells optimized, rather than with a deliberately degraded top cell, as is now the case. A modelling of the performance of the proposed device indicates possibilities for achieving practical efficiencies in excess of 35 percent under typical space sunlight illumination. The current/voltage and spectral quantum efficiency characteristics of GaAs MQW solar cells grown by chemical beam epitaxy are found to be consistent with model predictions, showing both an increase of the cell operating wavelength range and a superior current output. The device is predicted as being capable to achieve end of life efficiency of about 30 percent at radiation doses equivalent of 10 years operation in Geo-stationary orbits (1 MeV electron radiation fluences in excess of 1015cm–2). The projected radiation tolerance of the proposed device exceed significantly those of the existing conventional space solar cell technologies, making it particularly suitable for use in long duration space missions. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Performance degradation of GaAs multi quantum well solar Cells after proton irradiation

Abstract The performance degradation of GaAs multi quantum well solar cells (MQWSCs) has been studied using 2 MeV proton irradiation with fluences from 3×109 to 2x1013 p/cm2. The results have shown that the spectral response of MQWSCs clearly reduces in the short wavelength side after the proton irradiation, and the experimentally determined absolute ΔV ∝ loss was found to be a good match to a model which describes the absolute ΔV ∝. The cells are seen to display good irradiation tolerance for the degradation rate of the Pmax under the irradiation with a high fluence of 3x1011 p/cm2 being only 25%.

Potentially modulated multi-quantum wells for high-efficiency solar cell applications

Abstract The expected high performance of multi-quantum well (MQW) solar cells depends not only on the material quality ensuring minimum non-radiative recombination losses at the heterointerfaces, but also on the escape rate of photogenerated carriers out of QWs into the “base” region having minimum radiative losses within QWs. For this, we propose and investigate potentially modulated MQW solar cell structures, for which the MQW structure is modified from a series of conventional square-shaped QWs to “step-like” potentially modulated QWs. We have studied “two-step” and “three-step” MQWs introduced into the intrinsic region of GaAs p–i–n solar cells. Compared with a square-shaped MQW cell, a three-step modulated MQW cell having the same average potential well depth showed photoluminescence characteristics with a smaller thermal activation energy and higher average carrier temperature. The dark current characteristics also indicated that photogenerated carriers escape out of QWs more efficiently in this type of structure.

Optical properties of potentially modulated multi-quantum well solar cell structures

High performance of multi-quantum well (MQW) solar cells depends not only on the material quality ensuring minimum non-radiative recombination losses expected at the heterointerfaces, but also on the escape rate of photogenerated carriers out of QWs with minimum radiative losses within QWs. We investigated potentially modulated MQW solar cell structures, for which the MQW structure is modified from a series of conventional square-shaped QWs to “step-like” potentially modulated QWs. We fabricated “two-step” and “three-step” MQWs introduced into the intrinsic region of GaAs p–i–n solar cells by atomic H-assisted molecular beam epitaxy (H-MBE). Compared with a square-shaped MQW cell, a three-step modulated MQW cell having the same average potential well depth showed photoluminescence characteristics with a smaller thermal activation energy and higher average carrier temperature and indicated that photogenerated carriers escape out of QWs more efficiently in this type of structure.

Numerical prediction of InGaAs/GaAs MQW solar cell characteristics under concentrated sunlight

The characteristics of InGaAs/GaAs multi-quantum well (MQW) solar cells under concentrated sunlight were analyzed by self-consistent numerical calculation and were compared with GaAs p-i-n cells that have the same structure except for the QWs. Prior to the calculations, the absorption coefficients of the InGaAs MQW cell were determined by fitting the calculated quantum efficiencies to experimental ones. In the simulation, mainly the influences of concentrated sunlight and temperature were investigated. Results of the simulation showed that the rate of the conversion efficiency and open-circuit voltage ( V oc) of MQW cells increased faster than that of p-i-n cells as the light intensity increased, while the rate of efficiency of the MQW cell decreased a little slower than that of the p-i-n cell with the temperature increase. MQW cells compensate for the disadvantage of lower efficiency and V oc than p-i-n cells in a concentrated sunlight system.