Number Of Quantum Dot Layers Research Articles

Efficiency enhancement of solar cells using multi-layer interdiffused InGaAs/ GaAs quantum dots: A numerical approach

In this work, a mathematical model has been developed to analyze the effect of the number of quantum dot layers on the performance of solar cells. We have developed an analytical expression to obtain the spectral response and conversion efficiency of p-i-n solar cells with multi-layer interdiffused InGaAs quantum dots embedded in the GaAs matrix. Each layer consists of lens-shaped quantum dots having a non-homogenous composition due to interdiffusion. The comparative analysis of as-grown and interdiffused dots shows that neglecting the effect of interdiffusion leads to an overestimation of the photocurrent density of a solar cell. According to this study, including QDs layers increases the EQE toward longer wavelengths, increasing the short circuit density. However, a trade-off is observed between the photocurrent and open circuit voltage on increasing the number of quantum dot layers. In the proposed model, we have incorporated the impact of interdiffusion to predict the photocurrent density and open-circuit voltage of realistic-shaped quantum dot-based solar cells. In addition, the effect of spacer layer thickness on photocurrent density has also been discussed. The computed values were reasonably consistent with the reported experimental data.

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

Modeling of Relative Intensity Noise in QD-VCSEL

In this paper, the dynamics and statics properties of a 1.3 m InAs/InGaAs Quantum-Dot (QD) vertical-cavity surface-emitting laser (VCSEL) have been investigated using a self-consistent coupled rate-thermal equations model. Results demonstrate that self-heating caused by current transition through the Bragg mirrors and active area of the laser produces temperature rise and deteriorates the performance of the laser where it is most apparent at higher temperatures. Although the minimum value of Relative intensity noise (RIN) and the turn-on delay and maximum value of the output power occurs at the same bias current, the maximum achievable 3-dB modulation bandwidth is obtained at lower bias currents. In addition, the larger aperture size and the greater number of QD layers modify the self-heating and enhance the efficiency and output characteristics of the laser where the rolled-off of the laser happens at higher bias currents.

Quantum-Dot Based Vertical External-Cavity Surface-Emitting Lasers With High Efficiency

We demonstrate an optically pumped vertical external-cavity surface-emitting laser (VECSEL) based on quantum dots (QDs) with a high optical gain. The photoluminescence (PL) emission of the QDs at the operating wavelength of 1030 nm has been optimized towards a high integrated intensity by using a dot density in the range of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> and a narrow PL linewidth of around 30 meV due to an improved size homogeneity. Additionally, post-growth rapid thermal annealing at 650 °C for 45 s was found to reduce the non-radiative recombination centers and further improve the output power efficiency. During this study, the number of QD layers, the heat sink temperature and the heat spreader material were analyzed. The best results with a record slope-efficiency of 35% and a maximum continuous-wave output power of 5.7 W could be achieved with an RTA treated QD based VECSEL containing 24 QD layers in the gain section. The laser was processed with a solder-based flip-chip bonding technique on a diamond heat sink and temperature stabilized at 4 °C. Furthermore, an excellent symmetric beam profile with M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> = 1.12 was achieved.

The study of voltage loss reasons in GaAs solar cells with embedded InGaAs quantum dots

In the work the effect of the number of In0.8Ga0.2As stacked quantum dots (QDs) embedded in a single-junction GaAs solar cell (SC) on its photoelectric characteristics has been studied. A series of GaAs SC structures, which differed by a number of embedded QD layers, were grown using metalorganic vapor phase epitaxy. By analyzing the electroluminescence spectra and the current–voltage characteristics of the obtained structures the main reason for the voltage loss in SC with QDs has been established which is an increase in recombination through the QD levels with an increase the number of QD layers.

Degradation of 1.3 μm InAs Quantum-Dot Laser Diodes: Impact of Dislocation Density and Number of Quantum Dot Layers

This paper investigates the impact of dislocation density and active layer structure on the degradation mechanisms of 1.3 μm InAs Quantum Dot (QD) lasers for silicon photonics. We analyzed the optical behavior of two sets of samples, having different dislocation densities and different number of quantum dot layers in the active region. The samples were subjected to a short-term step-stress experiment and to long-term constant current operation in order to investigate the dominant degradation processes. The results indicate that: (i) the temperature stability is much higher in the devices grown on native substrate, thanks to the lower defect density; (ii) the roll-off current is considerably higher for the devices with higher number of layers, due to the lower density of carriers in the QDs; (iii) in nominal ground-state operating regime, the degradation rate is limited by the density of dislocations, that may serve as preferential paths for the diffusion of non-radiative recombination centers; (iv) at extreme injection levels and operating temperatures, the devices exhibit a blue shift of the spectral emission; possible explanations for this process are discussed in the paper.

Influence of Sb accumulation on the inter-band and inter-subband transitions of InAs/GaAs1-x Sbx sub-mono layer (SML) quantum dot heterostructures

Abstract The possibilities of employing InAs sub-mono layer (SML) quantum dots (QD) as long wavelength sources and infrared photodetectors have been explored in this work. To accomplish this, various heterostructures based on InAs SML QDs capped with GaAs1-xSbx were studied. The effects of Sb accumulation on the top of QDs were explored and their influences on the optical properties were duly explained. Two opposing phenomena, viz. Reduction in conduction band offset and increase in hydrostatic strain, with higher concentration of Sb accumulation were observed. This had a net effect of red-shift in the photoluminescence emission. In addition, the number of QD layers was varied from 2 to 8 layers. The SML QD structure with optimum number of layers facilitated photoluminescence wavelength close to 1300 nm, which is suitable for telecommunication applications. In addition, the possibilities of these SML QDs to be used as photodetectors in the long wavelength infrared (LWIR) and far infrared (FIR) regions have been portrayed.

InAs/InP quantum dot VECSEL emitting at 1.5 μm

A high-power InAs quantum dot (QD) vertical-external-cavity surface-emitting laser emitting at 1.5 μm is reported. The active region employs 20 layers of high-density Stranski–Krastanow InAs quantum dots on an InP substrate. The QD density and emission wavelength were independently adjusted by employing a double-cap growth sequence. Optimization of the spacer layer thickness and strain compensation rendered possible nucleation of a relatively high number of QD layers per antinode of the electromagnetic standing wave, which in turn enabled a high output power continuous wave operation of about 2.2 W. The operation wavelength could be tuned over 60 nm, taking advantage of the broadband gain characteristic of QD media.

Wide-wavelength-range control of photoluminescence polarization in closely stacked InAs/GaAs quantum dots

We developed a method of growing closely stacked InAs/GaAs quantum dots (QDs) to control the photoluminescence (PL) polarization characteristics in a wide wavelength range. The emission wavelength of the closely stacked QDs redshifted with decreasing substrate temperature during stacking growth, while the PL polarization characteristic was controlled by the GaAs spacer layer thickness and the number of QD layers. A unified rule for the optimum GaAs spacer layer thickness that both enhances the transverse magnetic (TM)-polarized component and achieves a high PL intensity for all growth temperatures was revealed. 30-layer stacked QDs with the optimum spacer layer thickness grown at substrate temperatures from 430 to 480 °C exhibited TM-enhanced polarization characteristics in the 1.15–1.3 μm band. Moreover, we studied the one-dimensional electronic states in the closely stacked QDs with the optimized GaAs spacer layer thickness by time-resolved PL.

Understanding the Bandwidth Limitations in Monolithic 1.3 μm InAs/GaAs Quantum Dot Lasers on Silicon

In this paper, we present measurements and simulations of the small-signal modulation response of monolithic continuous-wave 1.3 μ m InAs/GaAs quantum dot (QD) narrow ridge-waveguide lasers on a silicon substrate. The 2.5 mm-long lasers investigated demonstrate 3 dB modulation bandwidths of 1.6 GHz, D -factors of 0.3 GHz/mA1/2, modulation current efficiencies of 0.4 GHz/mA1/2, and K -factors of 2.4 ns and 3.7 ns. Since the devices under test are not designed for high-speed operation due to their long length and hence long photon lifetime, the modulation response curves are used as a fitting template for numerical simulations with spatiotemporal resolution to gain insight into the underlying laser physics. The obtained parameter set is used to unveil the true potential of the laser material in an optimized device geometry by modeling the small-signal response at different cavity lengths, mirror reflectivities, and for different numbers of QD layers. The simulations predict a maximum 3 dB modulation bandwidth of 5 GHz to 7 GHz for a 0.75 mm-long cavity with 99% and 60% high-reflection coatings and ten QD layers. Modeling the impact of dislocations on the dynamic performance qualitatively reveals that enhanced non-radiative recombination in the wetting layer leaves the modulation bandwidth of QD lasers on silicon almost unaffected, while dislocation-induced optical loss does not pose a problem, as long as sufficient gain is provided by the QD active region.

Effect of Mn Doping on Multilayer PbS Quantum Dots Sensitized Solar Cell

The effect of Mn-ion doping on the device performance of multilayer PbS quantum dot (QD) sensitized solar cells is investigated. Undoped as well as the doped PbS QDs, with different doping concentration, are synthesized using simple chemical methods on poly-vinyl alcohol matrix. QDs are characterized using ultraviolet visible spectroscopy, x-ray diffraction analysis, high-resolution transmission electron microscopy, energy dispersive x-ray, and photoluminescence spectroscopy. The QDs were introduced as sensitizer in ZnO-based solar cells in single as well as multiple layers. The current density vs. voltage characteristics are obtained for different Mn doping concentrations as well as for multiple numbers of QD layers, under artificial illumination. Enhanced photo-conversion efficiency was observed in multiple layered doped PbS QDs sensitized solar cell compared to undoped QDs sensitized solar cell.

Room-Temperature Group-IV LED Based on Defect-Enhanced Ge Quantum Dots

As recently demonstrated, defect-enhanced Ge quantum dots (Ge-DEQDs) in a crystalline Si matrix can be employed as CMOS-compatible gain material in optically pumped lasers. Due to the stability of their optical properties up to temperatures beyond 300 K, the Ge-DEQD system is a highly promising candidate for the realization of an electrically pumped group-IV laser source for integration in a monolithic optoelectronic platform fit for room-temperature operation. We report on the realization of light-emitting diodes based on Ge-DEQDs operating at telecom wavelengths and above room temperature. The DEQD electroluminescence characteristics were studied spectrally resolved as a function of driving current and device temperature. The experimental results show that the excellent optical properties of Ge-DEQDs are maintained under electrical pumping at high current densities and at device temperatures of at least 100 °C. Furthermore, the emission intensity scales with the number of quantum dot layers embedded int...

SnS4(4-), SbS4(3-), and AsS3(3-) Metal Chalcogenide Surface Ligands: Couplings to Quantum Dots, Electron Transfers, and All-Inorganic Multilayered Quantum Dot Sensitized Solar Cells.

Three inorganic capping ligands (ICLs) for quantum dots (QDs), SnS4(4-), SbS4(3-) and AsS3(3-), were synthesized and the energy levels determined. Proximity between the ICL LUMO and QD conduction level governed the electronic couplings such as absorption shift upon ligand exchange, and electron transfer rate to TiO2. QD-sensitized solar cells were fabricated, using the ICL-QDs and also using QD multilayers layer-by-layer assembled by bridging coordinations, and studied as a function of the ICL ligand and the number of QD layers.

Experimental demonstration of the effect of field damping layers in quantum-dot intermediate band solar cells

Intermediate band solar cells must demonstrate the principle of voltage preservation in order to achieve high conversion efficiencies. Tunnel escape of carriers has proved deleterious for this purpose in quantum dot intermediate band solar cells. In previous works, thick spacers between quantum dot layers were demonstrated as a means of reducing tunnel escape, but this approach is unrealistic if a large number of quantum dot layers have to be grown. In this work we report experimental proof that the use of field damping layers is equally effective at reducing tunnel carrier escape, by reducing the potential drop in the QD-stack and the associated electric field. Moreover, we demonstrate that the fact that tunnel carrier escape takes place under short-circuit conditions does not imply that voltage preservation cannot be achieved. We describe a theory that relates the evolution of the tunnel escape to bias voltage and the preservation of the voltage in an IBSC. Temperature and voltage-dependent quantum efficiency measurements, temperature dependent open-circuit voltage measurements and calculations of the internal electric field in IBSCs serve as the basis of the proposed theory.

Photoluminescence study of the effect of strain compensation on InAs/AlAsSb quantum dots

We investigate stacked structures of InAs/AlAsSb/InP quantum dots using temperature- and power-dependent photoluminescence. The band gap of InAs/AlAsSb QDs is 0.73eV at room temperature, which is close to the ideal case for intermediate band solar cells. As the number of quantum dot layers is increased, the photoluminescence undergoes a blue-shift due to the effects of accumulated compressive strain. This PL red shift can be counteracted using thin layers of AlAs to compensate the strain. We also derive thermal activation energies for this exotic quantum dot system.

Galvanic-Cell-Based Synthesis and Photovoltaic Performance of ZnO-CdS Core-Shell Nanorod Arrays for Quantum Dots Sensitized Solar Cells

Zinc oxide (ZnO) is recognized as one of the most attractive metal oxides because of its direct wide band gap (3.37 eV) and large exciton binding energy (60 meV), which make it promising for various applications in solar cells, gas sensors, photocatalysis and so on. Here, we report a facile synthesis to grow well-aligned ZnO nanorod arrays on SnO2: F (FTO) glass substrates without the ZnO seed layer using a Galvanic-cell-based method at low temperature (&lt;100°C). CdS quantum dot thin films were then deposited on the nanorod arrays in turn by an effective successive ionic layer adsorption and reaction (SILAR) process to form a ZnO/CdS core-shell structure electrode. Structural, morphological and optical properties of the ZnO/CdS nanorod heterojunctions were investigated. The results indicate that CdS quantum dot thin films were uniformly deposited on the ZnO nanorods and the thickness of the CdS shell can be controlled by varying the number of the adsorption and reaction cycles. The number of quantum dots layers affects on photovoltaic performance of the ZnO/CdS core-shell nanorod arrays has been investigated as photoanodes in quantum dots sensitized solar cells.

The Dependence of Multijunction Solar Cell Performance on the Number of Quantum Dot Layers

The performance improvements of adding InAs quantum dots (QDs) in the middle subcell of a lattice matched triple-junction InGaP/InGaAs/Ge photovoltaic device are studied using the simulated external quantum efficiency, photocurrent, open circuit voltage, fill factor, and efficiency under standard testing conditions. The QDs and wetting layer are modeled using an effective medium consisting of trap states for the former and low confinement potentials for the latter. Although the efficiency stabilizes for more than 100 layers of QDs for the structure studied, the efficiency achieves an absolute efficiency of 31.1% under one sun illumination for 140 layers of QDs. This corresponds to a relative increase of 1.3% compared with a control structure with no QD layers. The performance of the device depends intricately on the magnitude of the confinement potentials representing the wetting layer.

Transient behaviour of quantum-dot saturable absorber mirrors at varying excitation fluence

We present results from studying the carrier dynamics in self-assembled InAs/GaAs quantum-dot saturable absorbers intended for mode-locking of solid-state lasers. Four samples are examined, featuring controlled variations in the resonance condition of the electric field inside the absorber, the number of quantum-dot (QD) layers and the thickness of the GaAs barriers between these QD layers. Pump-probe experiments are conducted at a wide range of excitation fluences and reveal a fast relaxation component of the initial bleaching at low excitation fluences, while a slowly relaxing induced transparency becomes dominant at higher excitation fluences. Time-resolved photoluminescence measurements reveal a large and slowly relaxing induced transparency due to a capture of excess carriers from the barrier bands into the QDs and a slow radiative recombination there. The resonance condition as well as the thickness of the barriers between the QD layers can be used to control the relaxation behaviour. The fastest response is obtained in a structure with an increased number of QD layers at each individual anti-node of the electric field, which is attributed to the appearance of efficient non-radiative recombination channels and capture centres. These centres are probably related to dislocations and other defects appearing in thick QD stacks.

High Speed 1.55 μm InAs/InGaAlAs/InP Quantum Dot Lasers

We report static and dynamic characteristics of InAs/InP quantum dot (QD) lasers emitting near 1.55 μm. The gain section was optimized for a high speed operation using a unique spatially resolved model. The measured modulation capability dependence on structural parameters (barrier width and the number of QD layers) is consistent with the model predictions. Short cavity lasers with a modal gain of more than 10 cm-1 per dot layer exhibit a small signal modulation bandwidth above 9 GHz and large signal modulation at up to 22 Gb/s with an on/off ratio of 3 dB.

Carrier Dynamics and Modulation Capabilities of 1.55-$\mu$m Quantum-Dot Lasers

We present a spatially resolved model for multilayer quantum-dot (QD) lasers. The detailed model incorporates the homogeneous and inhomogeneous gain broadenings. Using this model, we examined effects of various structural parameters on the modulation response. Modulation bandwidth is increased with shortening the distance from the contacts to the active area, shortening the distance between QD layers, and with increasing the number of QD layers. The number of QD layers should be carefully chosen in order to prevent a transport bottleneck. Asymmetric positioning of the QD layers with respect to the waveguide core was found to improve the modulation bandwidth.