Quantum Dot Solar Cells

We studied time-resolved carrier recombination in InAs/GaAs quantum dot (QD) solar cells. The electric field in a p-i-n diode structure spatially separates photoexcited carriers in QDs, strongly affecting the conversion efficiency of intermediate-band solar cells. The radiative decay lifetime is dramatically reduced in a strong electric field (193 kV/cm) by efficient recombination due to strong carrier localization in each QD and significant tunneling-assisted electron escape. Conversely, an electric field of the order of 10 kV/cm maintains electronic coupling in the stacked QDs and diminishes tunneling-assisted electron escape.

Quantum Dot (QD) Solar Cells have emerged as promising candidates for next-generation photovoltaic technologies due to their tunable bandgaps, solution processability, and potential for low-cost fabrication. However, their photovoltaic performance is often limited by factors such as low light absorption and charge carrier recombination. Plasmonic nanostructures, which exploit the resonant oscillation of conduction electrons in metallic nanoparticles, offer a viable strategy to enhance light absorption and improve charge carrier dynamics in QD solar cells. This paper reviews the integration of plasmonic nanostructures with QD solar cells, examining the mechanisms by which plasmonics can enhance photovoltaic performance. We discuss various types of plasmonic materials, their configurations within solar cell architectures, and the resultant improvements in efficiency. Additionally, the challenges and future prospects of plasmonic-enhanced QD solar cells are explored.

We investigated the effects of doping on the photovoltaic efficiency in a GaAs reference cell, and in undoped, n-doped, and p-doped InAs/GaAs quantum-dot (QD) solar cells. We found that the photovoltaic efficiency of the undoped QD solar cell is almost the same as that of the reference cell. However, the efficiency improves monotonically with increasing inter-dot ndoping, while p-doping deteriorates the photovoltaic conversion. We observed a 50 % increase in photovoltaic efficiency in the device n-doped to provide approximately six electrons per dot as compared with the undoped QD cell. In this QD solar cell, the short circuit current density increases to 24.30 mA/cm 2 compared with 15.07 mA/cm 2 in the undoped QD solar cell without deterioration of the open circuit voltage. To identify the physical mechanisms that provide this improvement, we investigated the spectral characteristics of the photovoltaic response and photoluminescence of our QD solar cells. We found that the electron capture into QDs is substantially faster than the hole capture, which leads to an accumulation of electrons in QDs. The electrons trapped in dots enhance IR transitions. The built-in-dot electron charge together with charged dopants outside the dots creates potential barriers, which suppress the fast electron capture processes and at the larger scale form a potential profile which precludes degradation of the open circuit voltage. All of these factors lead to the enhanced harvesting of IR energy and a radical improvement of the QD solar cell efficiency. Higher efficiencies are anticipated with further increase of doping level and at higher radiation intensity. This makes the QD solar cells promising candidates for use with concentrators of solar radiation.

We report In0.4Ga0.6As quantum dot (QD) solar cells with In0.2Ga0.8As cap layers, which extends the photoabsorption spectra toward a wavelength longer than those of In0.4Ga0.6As QD solar cells without cap layers. Well-aligned 50-stack In0.4Ga0.6As QD structures with In0.2Ga0.8As cap layers can be grown without using a strain balancing technique. The photoluminescence wavelength of ten-stack In0.4Ga0.6As QDs with an In0.2Ga0.8As cap layer becomes longer, as a result of the reduced strain in the QDs achieved by using the cap layer. The cell characteristics of multistacked In0.4Ga0.6As QD solar cells are improved by employing In0.2Ga0.8As cap layers.

Ten-layer InAs/GaAs quantum dot (QD) solar cells exhibiting enhanced short circuit current (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sc</sub> ) and open circuit voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oc</sub> ) comparable to a control GaAs p-i-n solar cell are reported. One-sun J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sc</sub> is enhanced by 3.5% compared to that of the GaAs control, while the open circuit voltage is maintained at 994 mV. Results were achieved using optimized InAs QD coverage and a modified strain balancing technique, resulting in a high QD density (3.6×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> ), uniform QD size (4 × 16 nm) and low residual strain (103 ppm). The suppression of larger, coalesced InAs islands is shown to reduce non-radiative recombination in the depletion region, reducing dark current densities. The resulting enhanced open circuit voltage is a promising result for the future of InAs QD-enhanced GaAs solar cells. The extrapolation of short circuit current density enhancement with increasing quantum dot layers is evaluated here and potential efficiencies exceed baseline GaAs values. In the final work, a twenty-period QD will be grown. The QD growth improvement used here will be applied in this case assuming a linear enhancement in J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sc</sub> and a continued suppression of voltage degradation typical of these devices. The efficiency value of this device may show the potential to exceed that of a comparable baseline GaAs cell. In order to further evaluate carrier dynamics, temperature dependent PL will be used to extract activation energies with particular focus on barrier layer material. In addition, the choice of depth placement in the i-region has been shown to give varied device results in QW solar cells. This will also be in focus for QD solar cells, as these devices continue to be studied and optimized for photovoltaic energy conversion.

Progress in device engineering and surface passivation strategies has led to steady progress in colloidal quantum dot (QD) solar cells. Bulk homojunction (BHJ) device architecture has several advantages over the conventional planar junction in developing QD solar cells. Herein, surface ligand chemistry is utilized to control the doping type and dispersibility of oppositely doped PbS QDs to develop BHJ solar cells. Thiocyanate and thiol ligand combination is introduced to develop p‐PbS QD ink, which is blended with halide‐passivated n‐PbS QDs to build BHJ solar cells. It is shown that BHJ solar cells are benefited from high energy offset and higher hole mobility. This leads to the superior carrier extraction from a thicker active layer without compromising fill factor and open circuit voltage. Power conversion efficiency has reached 10.7% in 530 nm‐thick BHJ solar cells, a 24% improvement over the best performing planar solar cells. With the help of the 1D solar cell capacitance simulator, it is shown that a 15% efficient QD solar cell can be realized by further improving the hole mobility above 0.1 cm2 V−1 s−1.

Multi-stacked GaSb/GaAs quantum dot (QD) solar cells are theoretically studied by two-dimensional simulation with the implementation of Poisson’s equation, continuity equations, drift-diffusion transport model and thermionic emission model. Such physical models are numerically solved by finite element method. QD layers are inserted into the i-region of the GaAs p-in structure. Each QD layer is separated by a GaAs spacer layer. Effects of spacer layer thickness and number of QD layers on the solar cell characteristics are investigated. Spacer layer thickness also determines on the distribution of hole wave functions which are altered by strain and impact absorption properties. Thinner spacer layer results in higher strain which shifts the absorption edge towards shorter wavelengths and is responsible for open-circuit voltage enhancement, while thicker spacer layer increases the Shockley-Read-Hall (SRH) recombination that degrades both open-circuit voltage and fill factor. Increasing the number of QD layers leads to higher short-circuit current density and lowers both open-circuit voltage and fill factor which are derived from higher GaSb amount and SRH recombination. The simulation results suggest that the highest efficiency of 25.82% is achieved by using the 40-stacked QD layers with 15-nm spacer layer.

Various solar cells and materials have great potential as future generation solar cells and materials including crystalline Si. Si tandem solar cells such as III-V/Si tandem, II-VI/Si tandem, Chalcopyrite/Si tandem and Perovskite/Si tandem solar cells are expected to have great potential of space and terrestrial application because of high efficiency, light weight and low cost potential. Quantum and Nano structures such as MQWs ‘(Multi Quantum Wells) and QDs (Quantum Dots) are also expected as high-efficiency solar cells. However, there are some problems to be solved. Because point defects in crystalline Si, high density of dislocations and interface recombination in Si tandem solar cell materials and interface recombination in MQWs and QDs affect on solar cell properties, understanding and controlling those defects are very important for realizing higher efficiency. This paper presents analytical results for high efficiency potential of single crystalline Si solar cells, Si tandem solar cells, and MQW and QD solar cells. Although 25.6% efficiency has been attained with hetero-junction and back contact structure and by using bulk minority-carrier lifetime of around 10 msec, high efficiency of more than 27.5% is expected to be realized by increasing bulk lifetime of more than 100 msec Si. Really, high bulk lifetime of around 100 msec has been obtained by the MCZ (Magnetic-field-applied Czochralski) method and decreasing carbon concentration to less than 1014 cm-3 in Si. Understanding behavior of point defects such as Cs-Oi and so forth in Si and decreasing those defects density are suggested for realizing higher bulk lifetime and higher efficiency Si solar cells of more than 27.5%. High efficiency potential of Si tandem solar cells such as III-V/Si tandem, II-VI/Si tandem, Chalcopyrite/Si tandem and Perovskite/Si tandem solar cells is also analyzed by using measure of external radiative efficiency: 43% for III-V/Si 3-J(Junction) tandem, 38% for III-V/Si 2-J tandem, 33% for II-V/Si 2-J tandem, 32% for Chalcopyrite/Si tandem, and 32% for Perovskite/Si tandem solar cells. Effects of dislocations upon minority-carrier lifetime in III-V/Si tandem solar cells are also presented. Our recent result for mechanically stacked InGaP/GaAs//Si 3-junction tandem solar cell with efficiency of 27.4% is shown. Although MQW and QD solar cells are expected as high efficiency solar cells, one of big problems is higher interface recombination near MQWs and QDs because of their lattice mismatching. Lattice mismatch (Δa/a0) dependence of interface recombination velocity (SI) in MQW and QD solar cells is analyzed. Open-circuit voltage drop by interface recombination in MQWs and QDs and their efficiency potential is discussed.

In order to enhance understanding of the short circuit improvement in InAs/GaAs quantum dot (QD) solar cells, the thermally assisted and tunneling mechanisms of carrier escape from the QD quantum confinement are investigated. The dependence of voltage biased spectral responsivity for QD solar cells at room temperature is studied to analyze carrier extraction through tunneling. Photoexcited carrier confinement and escape were also studied by means of temperature dependent spectral response (TDSR) and temperature dependent photoluminescence (TDPL). Energy required to move a carrier from the ground state to the first excited state, thermal activation energy (E a ), in a quantum dot is calculated from TDPL to be 114 meV. It is found that at room temperature carrier escape from the quantum dot confinement is affected by both thermal assisted escape and tunneling while at low temperature tunneling is the dominant in carrier escape from both wetting layer and QDs. For all temperature ranges, carrier exchange between ground states and excited states and carrier escape from ground states (GS) is first thermal escape to excited states (ES) then tunneling

In this letter, the capture/emission time constant ( $\tau$ ), which depends on electronic coupling between the materials and structural/energetic disorder at the donor–acceptor interface, is considered in the model of hybrid organic/inorganic quantum dot (QD) solar cells as a factor that is responsible for degradation in solar cell parameters via an undesirable s-shape in the current–voltage (I–V) characteristic. In the model, charge transport is described by a drift-diffusion approximation and a set of coupled rate equations in transport layers and QDs, respectively. The analysis has revealed that the magnitude of accumulated photo-generated electrons at the QD interface and imbalanced carrier extraction from the active region is the key factors for the appearance of s-kink in the I–V characteristic.

An InAs/GaAs quantum dot (QD) solar cell was compared to a similar bulk GaAs p-i-n structure. A 5% increase of the photocurrent was measured in the QD solar cell under Global Air Mass 1.5 condition, accompanied by a smaller value of the open circuit voltage (0.765 V) as compared with the reference GaAs device (0.922 V). The enhanced Urbach tail absorption in QD device greatly contributes to the photocurrent measured below the bandgap. This tail extension also explains the decrease in open circuit voltage in the device and makes it difficult to realize the concept of intermediate band solar cell at room temperature.

Near infrared (NIR) PbS quantum dots (QDs) have attracted significant research interest in solar cell applications as they offer several advantages, such as tunable band gaps, capability of absorbing NIR photons, low cost solution processability and high potential for multiple exciton generation. Nonetheless, reports on solar cells based on NIR PbS/CdS core-shell QDs, which are in general more stable and better passivated than PbS QDs and thus more promising for solar cell applications, remain very rare. Herein we report high efficiency bulk heterojunction QD solar cells involving hydrothermally grown TiO2 nanorod arrays and PbS/CdS core-shell QDs processed in air (except for a device thermal annealing step) with a photoresponse extended to wavelengths >1200 nm and with a power conversion efficiency (PCE) as high as 4.43%. This efficiency was achieved by introducing a thin, sputter-deposited, uniform TiO2 seed layer to improve the interface between the TiO2 nanorod arrays and the front electrode, by optimizing TiO2 nanorod length and by conducting QD annealing treatment to enhance charge carrier transport. It was found that the effect of the seed layer became more obvious when the TiO2 nanorods were longer. Although photocurrent did not change much, both open circuit voltage and fill factor clearly changed with TiO2 nanorod length. This was mainly attributed to the variation of charge transport and recombination processes, as evidenced by series and shunt resistance studies. The optimal PCE was obtained at the nanorod length of ∼450 nm. Annealing is shown to further increase the PCE by ∼18%, because of the improvement of charge carrier transport in the devices as evidenced by considerably increased photocurrent. Our results clearly demonstrate the potential of the PbS/CdS core-shell QDs for the achievement of high PCE, solution processable and NIR responsive QD solar cells.

Quantum dot (QD) solar cells are proposed as high-efficiency solar cells. However, their reported conversion efficiencies have been lower than half of the ideal value. To improve their efficiency, the optimization of their cell structure in terms of various parameters, e.g., dot size, interdot distance, type of materials, and QD/bulk interface structure, is necessary. In this paper, we focused on the most important factor for the improvement in the conversion efficiency of Si/SiC type QD solar cells and investigated the effect of the atomistic structure of the QD/bulk interface on carrier transfer by tight-binding simulation. We constructed models of Si/SiC systems and analyzed the effect of QD/bulk interface defects on their electronic structure and carrier transfer properties. It was suggested that electrons trapped at the QD/bulk interface and the type of SiC crystal structure affect electron transfer.

Quantum dot semiconductors have gain great attraction for the development of high efficiency solar cells due to remarkable optoelectronic properties such as tunable bandgap, multiple exciton generation (MEG) and high extinction coefficient. Despite quantum dot solar cells having theoretical power conversion efficiency of about 66%, actual maximum efficiency is only 16.6%. So, it is important to further understand the relations between properties, elements composition, material structures and synthesis methods, as well as devices architectures in order to propose strategies addressed to close the gap between actual and theoretical power conversion efficiencies. This article exposes the advances related to materials and methods of synthesis, and their impact in quantum dot properties. It also introduces some recent quantum dot solar cells designs.

In order to enhance absorption at infrared range for GaAs–based solar cell, we have grown multi–stacked InAs/InGaAs/GaAs quantum dots (QDs) heterostructure in the active region by Solid–Source Molecular Beam Epitaxy (SS–MBE). Two different families of dots were observed in the photoluminescence (PL) spectra. Temperature–dependent study was carried out at 10–240 K temperature range. Distinctive, asymmetric shape located in the high energy for the PL spectra of the QD solar cell sample can be deconvoluted in two sub–bands. From the temperature–dependent PL measurement, the two sub–bands are associated with the ground–state emission from the two families of InAs dots with different size. Besides, the spectral response of multi–stacked InAs/InGaAs QD solar cells extends the photo–absorption spectra towards a wavelength longer than the GaAs bandgap of 1280 nm. However, the QDs solar cell shows an enhanced short–circuits current density of 7.8 mA/cm² compared to the GaAs reference cell. The performance of the QD solar cells indicates that the InAs/InGaAs/GaAs QD heterostructures facilitate the fabrication of highly stacked QD layers that are suitable for solar cells devices requiring thick QD layers for sufficient light absorption.