Quantum Dot Intermediate Band Solar Cells Research Articles

Theoretical Investigation and Improvement of Characteristics of InAs/GaAs Quantum Dot Intermediate Band Solar Cells by Optimizing Quantum Dot Dimensions

Quantum dot (QD)-based solar cells have been the focus of extensive research. One of the critical challenges in this field is optimizing the size and placement of QDs within the cells to enhance light absorption and overall efficiency. This paper theoretically investigates InAs/GaAs QD intermediate band solar cells (QD-IBSC) employing cylindrical QDs. The goal is to explore factors affecting light absorption and efficiency in QD-IBSC, such as the positioning of QDs, their dimensions, and the spacing (pitch) between the centers of adjacent dots. Achieving optimal values to enhance cell efficiency involves modifying and optimizing these QD parameters. This study involves an analysis of more than 500 frequency points to optimize parameters and evaluate efficiency under three distinct conditions: output power optimization, short-circuit current optimization, and generation rate optimization. The results indicate that optimizing the short-circuit current leads to the highest efficiency compared to the other conditions. Under optimized conditions, the efficiency and current density increase to 34.3% and 38.42 mA/cm2, respectively, representing a remarkable improvement of 15% and 22% compared to the reference cell.

Panning ideal In(Ga)As(P)/InGaP quantum dot structures for intermediate band solar cells

The In(Ga)As(P)/InGaP quantum dot (QD)system has been investigated for QD intermediate band solar cells. In order to obtain optical transition energies compatible with the ideal ones required for the device record performance, namely: 1.95 eV, 1.24 eV and 0.7 eV, first disordered InGaP with a bandgap of 1.91 eV as the barrier material has been obtained. Then InAs QDs nucleated at 490 °C, 1.32 MLs thick, covered by a 3–4 nm GaAs capping layer and In-flushed provided radiation emission in the energy interval between 1.15 eV and 1.35 eV, fully compatible with the ideal 1.24 eV. The 4 nm capped structures have been proven to exhibit a stronger PL emission at 1.24 eV. Nominal InAs QDs suffer a pronounced incorporation of Ga, a consequence of In/Ga intermixing at their capping layer and barrier interfaces. An As/P intermixing also takes place at the quantum dot—barrier interface. The encouraging results herald further improvement of QD intermediate band solar cell’s performance.

Design and simulation of type-I graphene/Si quantum dot superlattice for intermediate-band solar cell applications

Recent experiments suggest graphene-based materials as candidates for use in future electronic and optoelectronic devices. In this study, we propose a new multilayer quantum dot (QD) superlattice (SL) structure with graphene as the core and silicon (Si) as the shell of QD. The Slater–Koster tight-binding method based on Bloch theory is exploited to investigate the band structure and energy states of the graphene/Si QD. Results reveal that the graphene/Si QD is a type-I QD and the ground state is 0.6 eV above the valance band. The results also suggest that the graphene/Si QD can be potentially used to create a sub-bandgap in all Si-based intermediate-band solar cells (IBSC). The energy level hybridization in a SL of graphene/Si QDs is investigated and it is observed that the mini-band formation is under the influence of inter-dot spacing among QDs. To evaluate the impact of the graphene/Si QD SL on the performance of Si-based solar cells, we design an IBSC based on the graphene/Si QD (QDIBSC) and calculate its short-circuit current density (Jsc) and carrier generation rate (G) using the 2D finite difference time domain (FDTD) method. In comparison with the standard Si-based solar cell which records Jsc = 16.9067 mA/cm2 and G = 1.48943 × 1028 m−3⋅s−1, the graphene/Si QD IBSC with 2 layers of QDs presents Jsc = 36.4193 mA/cm2 and G = 7.94192 × 1028 m−3⋅s−1, offering considerable improvement. Finally, the effects of the number of QD layers (L) and the height of QD (H) on the performance of the graphene/Si QD IBSC are discussed.Graphical abstract

Enhanced current generation in quantum-dot intermediate band solar cells through optimizing the position of quantum dot layers

We present a numerical study using a self-consistent drift-diffusion approach to characterize the effect of light trapping on the quantum efficiency (QE) of a thin-film quantum dot intermediate band solar cell (QD-IBSC). In the concept of the IBSCs, the photons with energies lower than the bandgap of the host material can be absorbed in QDs due to the presence of the IB, leading to high photocurrent and conversion efficiency. However, in general, the quality of the QD structure is degraded due to the accumulated strain when the number of QD layers stacked vertically is increased, which is required to increase light absorption. In the present work, we adopted a Fabry-Pérot (FP) resonance structure to achieve high absorption with fewer QD layers. We demonstrate numerically, in a InAs/GaAs-based QDSC structure, that an increase in the QE by 8 times at maximum is achievable by tuning the position of the QD layers inserted.

Enhanced Current Generation in Quantum-Dot Intermediate Band Solar Cells Through Optimizing the Position of Quantum Dot Layers

Enhanced Current Generation in Quantum-Dot Intermediate Band Solar Cells Through Optimizing the Position of Quantum Dot Layers

Demonstration of a GaSb/GaAs Quantum Dot Intermediate Band Solar Cell Operating at Maximum Power Point.

Intermediate band solar cells (IBSCs) promise high efficiencies while maintaining a low device structural complexity. A high efficiency can be obtained by harvesting below-band-gap photons, thus increasing the current, while at the same time preserving a high voltage. Here, we provide experimental proof that below-band-gap photons can be used to produce nonzero electrical work in an IBSC without compromising the voltage. For this, we manufacture a GaSb/GaAs quantum-dot IBSC. We use light biasing and make our cell operate at the maximum power point at 9K. We measure the photocurrent response to absorption of photons with an energy of less than 1.15eV while the cell is operating at 1.15V. We also show that this result implies the existence of three quasi-Fermi levels linked to the three electronic bands in our device, as demanded by the IBSC theory to preserve the output voltage of the cell.

Optimized Operation of Quantum-Dot Intermediate-Band Solar Cells Deduced from Electronic Transport Modeling

So far physics of quantum electronic transport has not tackled the problems raised by quantum dot intermediate-band solar cells. Our study shows that this physics imposes design rules for the inter-subband transition. We developed an analytical model that correctly treats, from a quantum point-of-view, the trade-off between the absorption, the recombination and the electronic transport occurring in this transition. Our results clearly indicate that it is essential to control the transit rate between the excited state of the quantum dot and the embedding semiconductor. For that, we propose to assume the dot in a tunnel-shell whose main characteristics can be obtained by a simple analytical formula. Moreover, we show that in a realistic case, the energy transition only needs to be larger than 0.28 eV to obtain a quasi Fermi-level splitting. This quite small value designates the quantum dot solar cell as a serious candidate to be an efficient intermediate-band solar cell. This work gives a framework to design efficient inter-subband transitions and then opens new opportunities for quantum dot intermediate-band solar cells.

Projected Performance of InGaAs/GaAs Quantum Dot Solar Cells: Effects of Cap and Passivation Layers

In this work, the effects of Al x Ga1– x As cap and passivation (such as SiO2, Si3N4, and HfO2) layers on the performance of InGaAs/GaAs-based quantum dot intermediate band solar cells (QDIBSCs) have been studied. The low surface recombination rate of ~103 per cm3s is achieved by optimizing the composition, $x =0.40$ , and thickness (200 nm) of the Al x Ga1– x As cap layer. The optical reflectance is also evaluated for devices with different passivation. The solar cell with Si3N4 shows the lowest reflectance of 10.53%. The photogeneration rate has been enhanced at the quantum dot region because of the improvement of the photocurrent provides by both cap and passivation layers. There is also an increment found in the average external quantum efficiency of 39.56% as compared to that of the bared conventional QDIBSC. As a result, the solar cell, with both Al0.40Ga0.60As cap and Si3N4 passivation layers, shows the conversion efficiency of 27.8%, which is higher than that of 21.6% for conventional In0.53Ga0.47AS/GaAs-based QDIBSC. These results indicate that GaAs-based QDIBSCs with both Al0.40Ga0.60As cap and Si3N4 passivation layers are promising for next-generation photovoltaic applications.

Quantum engineering of intrinsic losses in the diluted nitride InAsN quantum dot intermediate band solar cell

Intrinsic losses, including below-band gap, thermalization, mismatch angle, Carnot, and emission, are investigated in a structure of quantum dot intermediate band solar cells (QD-IBSCs). Owing to the excellent irradiance resistance of III-nitride material and reduction of Auger recombination processes, diluted nitride of InAsN is considered as quantum dot (QD) material. Then, AlPSb is considered as the barrier material according to the principles of intermediate band operation and the band structures of InAsN and AlPSb in which phosphorous molar fraction is optimized by the minimizing of total intrinsic losses. The engineering of QD size and period offers a way to engineer the confined energy levels within the QDs and ultimately the intrinsic losses. This was carried out with the help of a finite element model in the context of a three-dimensional Schrodinger equation created by means of MATLAB software and tight binding method, resulting in the minimum intrinsic loss of 55.18% for the InAs0.995N0.005 / AlP0.7 Sb0.3 QD-IBSC at 1-sun concentration.

A hot-carrier assisted InAs/AlGaAs quantum-dot intermediate-band solar cell

The intermediate-band assisted hot-carrier solar cell (IB-HCSC) concept has been proposed in order to assist the extraction of hot carriers from an absorber that has an intermediate band (IB). In this study, we used a heterostructure based on ten layers of In(Ga)As quantum dots (QDs) embedded in an Al0.2Ga0.8As single-junction solar cell designed for an intermediate band solar cell (IBSC). QD-IBSCs have proven to be limited by the thermal escape of photo-carriers from QDs at room temperature. In such cases, fundamental improvements in conversion efficiency are only possible if carriers in the IB are not in thermal equilibrium with either conduction or valence bands. Additionally, the IB-HCSC concept provides a high-efficiency limit and enables us to work with all relaxation mechanisms (thermalization, carrier-carrier scattering, and thermal radiation). Under high irradiation, we confirmed the emergence of a hot carrier population in the QDs, limited mostly by thermionic emission, which assists the IBSC by providing a thermoelectric gain in voltage without hindering the possibility of sequential two-photon absorption. Absolute intensity calibrated photoluminescence spectroscopy indicated that the triggering mechanism happens when the QD ensemble is estimated to have a high carrier concentration that behaves as a metal-like IB. Experimental results suggested that the hot carrier effect also occurred in other solar cells based on quantum heterostructures and directions for improvements of hot-carrier assisted QD-IBSCs are proposed for further efficiency gain in optimized device architectures.

Adiabatic two-step photoexcitation effects in intermediate-band solar cells with quantum dot-in-well structure

We studied the dynamics of electrons generated by two-step photoexcitation in an intermediate-band solar cell (IBSC) comprising InAs/GaAs/Al0.3Ga0.7As dot-in-well (DWELL) structure using time-resolved photocurrent (TRPC) measurement. The examined IBSC exhibited considerably slower photocurrent decay than a conventional InAs/GaAs quantum dot IBSC, which is due to the extraordinarily long-lived electrons in the DWELL. In order to retrieve the electron lifetime from the decay profile, we developed a model reproducing the observed decay and performed parameter fitting. The fitting results indicate that the electron lifetime in the DWELL is approximately 30 μs. In the two-colour excitation TRPC measurement, we found that an additional infrared (IR) light accelerates the photocurrent decay while the photocurrent increases by approximately 3%, because the additional IR light causes two-step photoexcitation of electrons in the DWELLs towards the conduction band. Furthermore, we demonstrated that the open-circuit voltage increases with increasing of the contribution of the second IR excitation process.

Guided-mode resonance gratings for enhanced mid-infrared absorption in quantum dot intermediate-band solar cells

Achieving strong absorption of low-energy photons is one of the key issues to demonstrate quantum dot solar cells working in the intermediate band regime at practical concentration factors and operating temperatures. Guided-mode resonance effects may enable large enhancement of quantum dot intraband optical transitions. We propose quantum dot thin-film cells designed to have significant field waveguiding in the quantum dot stack region and patterned at the rear-side with a sub-wavelength diffraction grating. Remarkable increase of the optical path length at mid-infrared wavelengths is shown owing to guided-mode resonances. Design guidelines are presented for energy and strength of the second-photon absorption for III-V quantum dots, such as InAs/GaAs and GaSb/GaAs, whose intraband and intersubband transitions roughly extends over the 2 - 8 µm range. The proposed design can also be applied to quantum dot infrared detectors. Angle-selectivity is discussed in view of applications in concentrator photovoltaic systems and infrared imaging systems.

Investigation of the open-circuit voltage in wide-bandgap InGaP-host InP quantum dot intermediate-band solar cells

To analyze the open-circuit voltage (Voc) in intermediate-band solar cells, we investigated the current–voltage characteristics in wide-bandgap InGaP-based InP quantum dot (QD) solar cells. From the temperature dependence of the current–voltage curves, we show that the Voc in InP QD solar cells increases with decreasing temperature. We use a simple diode model to extract Voc at the zero-temperature limit, V0, and the temperature coefficient C of the solar cells. Our results show that, while the C of InP QD solar cells is slightly larger than that of the reference InGaP solar cells, V0 significantly decreases and coincides with the bandgap energy of the InP QDs rather than that of the InGaP host. This V0 indicates that the Voc reduction in the InP QD solar cells is primarily caused by the breaking of the Fermi energy separation between the QDs and the host semiconductor in intermediate-band solar cells, rather than by enhanced carrier recombination.

Projected Performance of an InxGa1-xAs Quantum Dot Intermediate Band Solar Cell

Quantum Dot Intermediate Band Solar Cell (QDIBSC) is one of the emerging technologies in the solar photovoltaic arena, which has immense potential to be demonstrated as a high efficiency device. For a QDIBSC to surpass the efficiency of a single junction cell, optimization of design is required. In this work, a QDIBSC model based on In0.53Ga0.47As quantum dots has been designed and evaluated with respect to dot size and spacing. The impact of carrier lifetime on short-circuit current and open-circuit voltage is studied. The conversion efficiency has been enhanced from 27.1% to 32.62% as compared to a conventional single junction cell.

Influence of built-in charge on photogeneration and recombination processes in InAs/GaAs quantum dot solar cells

Selective doping of quantum dots is often used to improve efficiency of intermediate band solar cells (IBSC) due to IR harvesting and built-in-dot charge. To investigate the effects of the built-in-dot charge on recombination processes and device performance InAs/GaAs quantum dot IBSCs with direct Si doping in the quantum dots are fabricated, and the I–V characteristics and transients of the open circuit voltage and short circuit current are measured. The decay times of both the open circuit voltage and the short circuit current increase as the concentration of n-type doping increases in the quantum dots. The observed increase in the charge carrier lifetime is attributed to suppressed recombination of electron–hole pairs through the states of quantum dots and shrinking the depletion layer. This is supported by measurements of both photovoltage and photoluminescence spectra.

Three-Bandgap Absolute Quantum Efficiency in GaSb/GaAs Quantum Dot Intermediate Band Solar Cells

In this work, we study type-II GaSb/GaAs quantum-dot intermediate band solar cells (IBSCs) by means of quantum efficiency (QE) measurements. We are able, for the first time, to measure an absolute QE which clearly reveals the three characteristic bandgaps of an IBSC; E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">G</sub> , E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</sub> , and E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</sub> , for which we found the values 1.52, 1.02, and 0.49 eV, respectively, at 9 K. Under monochromatic illumination, QE at the energies E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</sub> and E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</sub> is 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> and 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-8</sup> , respectively. These low values are explained by the lack of efficient mechanisms of completing the second sub-bandgap transition when only monochromatic illumination is used. The addition of a secondary light source (E = 1.32 eV) during the measurements produces an increase in the measured QE at E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</sub> of almost three orders of magnitude.

The Effects of Interdot Spacing and Dot Size on the Performance of InGaAs/GaAs QDIBSC

In0.53Ga0.47As/GaAs-based quantum dot intermediate band solar cells (QDIBSCs) have been designed and optimized for the next generation photovoltaic technology. The wave behavior of charge carriers inside the dot and their barrier have been analyzed with different dot sizes and interdot spacing. The device characteristics such as short circuit current density, Jsc, open circuit voltage, Voc, and conversion efficiency, η, have been evaluated. Based on the behavior of electron wave function, it is found that varying the dot spacing leads to a change in the IB width and in the density of states, whereas varying the size of dots leads to a formation of a second IB. For a fixed dot spacing, two ranges of dot sizes vary the number of IBs in In0.53Ga0.47As/GaAs QDIBSC. Smaller dots of a size ranging from 2 nm to 5 nm form a single IB while larger dots of a size ranging from 6 nm to 9 nm can produce 2 IBs. The efficiency of 2 IBs close to 1 IB suggests that formation of multiple IBs can possibly enhance the device efficiency.

Capping vertically aligned InGaAs/GaAs(Sb) quantum dots with a AlGaAsSb spacer layer in intermediate‐band solar cell devices

AbstractThis study demonstrated the feasibility of fabricating a highly stacked vertically aligned InGaAs/GaAs(Sb) quantum dot (QD) structure with an AlGaAsSb spacer layer for improving the device performance of QD intermediate‐band solar cell (QD‐IBSC) devices. The power‐dependent photoluminescence measurements of the proposed structure revealed a blueshift in the QD ground‐state emissions when the excitation power was increased, indicating the formation of an intermediate band inside the QD structure. Capping the InGaAs QDs with a GaAsSb layer prevented the QDs from collapsing because there was less In–Ga intermixing between the QDs and GaAsSb layer. In addition to maintaining the QD structure, the carrier lifetime was extended by tuning the energy band alignment of the InGaAs/GaAsSb QD structure. Inserting the AlGaAsSb layer into the spacer layer increased the band gap, which in turn increased the open‐circuit voltage of the QD‐IBSC. The QD‐IBSC in this work shows an extension of external‐quantum efficiency by up to 1200 nm (compared with a GaAs reference cell) through the absorption by QDs and increased the open‐circuit voltage from 0.67 to 0.70 V by adopting the AlGaAsSb spacer layer. These results confirm that adopting a columnar InGaAs/GaAs(Sb) QD structure with a AlGaAsSb spacer layer can enhance the performance of QD‐IBSC devices. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Si-Doped InAs/GaAs Quantum-Dot Solar Cell With AlAs Cap Layers

One of the requirements for strong subbandgap photon absorption in the quantum-dot intermediate-band solar cell (QD-IBSC) is the partial filling of the intermediate band. Studies have shown that the partial filling of the intermediate band can be achieved by introducing Si doping to the QDs. However, the existence of too many Si dopants leads to the formation of point defects and, hence, a reduction of photocurrent. In this study, the effect of Si doping on InAs/GaAs QD solar cells with AlAs cap layers is studied. The AlAs cap layers prevent the formation of the wetting layer during QD growth and reduce the Si doping density needed to achieve QD state filling. Furthermore, the passivation of defect states in the QD with moderate Si doping is demonstrated, which leads to an enhancement of the carrier lifetime in the QDs and, hence, the open-circuit voltage.

Performance Evaluation of Quantum-Dot Intermediate-Band Solar Cells

This work aims to evaluate quantum-dot intermediate-band solar cells (QDIBSCs) from two points of view. The first focuses on study of the dynamic characteristics of a model built on investigation of the effect of optical generation and recombination lifetimes on the induced electron density, induced photocurrent, and thus power conversion efficiency. In this phase, the main factors affecting emission and absorption of charge carriers are discussed, being found to depend on QDIBSC parameters such as the quantum dot (QD) size, barrier width, and doping density in the intrinsic region. Also, the solar wavelength response is determined. The relations between the photocurrent, efficiency, and model parameters such as the recombination lifetime and incident solar wavelengths are studied. The maximum values of photocurrent and efficiency obtained are 350 mA/cm2 and 76%, respectively. The second part is devoted to determination of the allowable number of intermediate bands (IBs) for a certain material composition (InAs0.9N0.1/GaAs0.98Sb0.02), representing a research derivation. The bandgap and subbandgap energies, and corresponding response wavelengths are determined. Their optimum values to achieve the maximum efficiency (about 74% at maximum solar concentration) are given. The main contribution distinguishing this work is the ability to apply the general condition for the QD intrinsic region to the research derivation. This opens the way to experimental achievement of the different types of IBs in order to obtain the required efficiency.