Synthesis and characterization of boron-doped Si quantum dots for all-Si quantum dot tandem solar cells

Multiple layers of Si quantum dots (QDs) in SiO₂ with a narrow size distribution were synthesized by a co-sputtering technique. Structural, electrical and optical properties of Si QD/SiO₂ multilayer films with various boron (B) concentrations introduced during the sputtering process were studied. X-ray photoelectron spectroscopy (XPS) revealed B-B/B-Si bonding, which suggests possible boron inclusion in the nanocrystals. The addition of boron was observed to suppress Si crystallization, though the boron concentration was found to have little effect on the QD size. Reductions in film resistivity were observed with the increase in boron concentration, which is believed to be a consequence of an increase in carrier concentration. This is supported by a large decrease in the activation energy accompanying the drop in resistivity, consistent with the Fermi energy moving towards the valence bands. The photoluminescence (PL) intensity was found to decrease with increase in boron concentration.     

Colloidal quantum dot solar cells

In recent years colloidal quantum dots solar cells have been the subject of extensive research. A promising alternative to existing silicon solar cells, quantum dot solar cells are among the candidates for next generation photovoltaic devices. Colloidal quantum dots are attractive in photovoltaics research due to their solution processability which is useful for their integration into various solar cells. Here, we review the recent progresses in various quantum dot solar cells which are prepared from colloidal quantum dots. We discuss the preparation methods, working concepts, advantages and disadvantages of different device architectures. Major topics discussed in this review include integration of colloidal quantum dots in: Schottky solar cells, depleted heterojunction solar cells, extremely thin absorber solar cells, hybrid organic-inorganic solar cells, bulk heterojunction solar cells and quantum dot sensitized solar cells. The review is organized according to the working principle and the architecture of photovoltaic devices.     

Linker-free quantum dot sensitized TiO2 photoelectrochemical cells

Aerosol based techniques were used to characterize and deposit quantum dots (QDs). Using an electrospray-assisted characterization technique, the mobility diameter of CdSe QDs was successfully measured in real-time. The electrospray technique was also used to deposit CdSe QDs onto nanostructured TiO₂ films. Compared to conventional methodologies such as dip coating with linker-containing molecules or chemical bath deposition, an electrospray system enabled uniform deposition of QDs over the nanostructured TiO₂ surface in a short processing time. As-deposited films were annealed to enhance binding between the QDs and TiO₂ surface. These QD-decorated TiO₂ films were used in photoelectrochemical cells, for which the photoenergy conversion efficiencies were tested. Optimization of the deposition time of the QDs resulted in increased efficiencies. Multiple layers of QDs caused a decrease in energy-conversion efficiency, likely due to inhibition of the transportation of photogenerated electrons into the TiO₂ structure. The energy-conversion efficiency trends were supported by time-resolved photoluminescence decay data.     

A theoretical approach on the strain-induced dislocation effects in the quantum dot solar cells

The numbers of the quantum dot layers that can be embedded in the active region of the quantum dot intermediate band solar cells affects on the photocurrent and also can produce strain-induced dislocations in the cell. To enhance the absorption of the low energy photons in the system, the number of the quantum dot layers needs to be increased, but in this way, dislocations and defects of the cell non-radiative recombination will also increase. In this paper, the characteristics of intermediate band solar cells containing 10, 20, and 50 InAs quantum dot layers embedded in the active region of the cells have been considered and compared. There are an optimum number of quantum dot layers for significant absorption of low energy photons. Furthermore, for a cell with 10 QD layers, the current–voltage characteristics and internal quantum efficiency have been investigated for different values of minority carriers recombination lifetimes (or diffusion lengths) and electron filling factors. Electron filling factor, gives a design constraints for the size of the quantum dots and distance between the layers. The results showed that the perfect cells need to be considered from two aspects; first, from the optimum number of the quantum dot layers to control the strain-induced dislocations that produce non-radiative recombinations and reduce the photocurrent and second, the dots spacing and size that need to be justified for wavefunction penetration into barrier region that reduces the non-radiative recombinations.     

Multi-stacked quantum dot solar cells fabricated by intermittent deposition of InGaAs

We report GaAs-based quantum dot (QD) solar cells fabricated by the intermittent deposition of InGaAs using molecular beam epitaxy. We obtained a highly stacked and well-aligned InGaAs QD structure of over 30 layers without using a strain compensation technique by the intermittent deposition of InGaAs layers. Moreover, there was no degradation in crystal quality. The external quantum efficiency of multi-stacked InGaAs QD solar cells extends the photo-absorption spectra toward a wavelength longer than the GaAs band gap, and the quantum efficiency increases as the number of stacking layers increases. The performance of the QD solar cells indicates that the novel InGaAs QDs facilitate the fabrication of highly stacked QD layers that are suitable for solar cell devices requiring thick QD layers for sufficient light absorption.     

Quantum dot solar concentrator: Device optimisation using spectroscopic techniques

The quantum dot solar concentrator (QDSC) is a novel non-tracking solar concentrator comprising quantum dots (QDs) seeded in materials such as plastics and glasses, that concentrates both direct and diffuse solar energy on attached photovoltaic cells. Spectroscopic measurements have been undertaken for a range of different quantum dot (QD) types and transparent host materials. High transparency in the matrix material and QDs with high quantum efficiency are essential for an efficient QDSC. An optimum matrix material for a QDSC has been determined based on absorption characteristics and an optimum commercially available QD type has been chosen using steady-state absorption, photoluminescence and photoluminescence excitation spectroscopy of QDs in solution and solid matrices.     

Delta doping and positioning effects of type II GaSb quantum dots in GaAs solar cell

GaSb quantum dot (QD) solar cell structures were grown by molecular beam epitaxy on GaAs substrates. We investigate the reduction in open-circuit voltage and study the influence of the location of QD layers and their delta doping within the solar cell. Devices with 5 layers of delta-doped QDs placed in the intrinsic, n- and p-regions of a GaAs solar cell are experimentally investigated, and the deduced values of J_sc, V_oc, fill factor, efficiency (η) are compared. A trade-off is needed to minimize the V_oc degradation while maximizing the short circuit current density (J_sc) enhancement due to sub-bandgap absorption. The voltage recovery is attributed to the removal of the QDs from the high-field region which reduces SRH recombination. The devices with p- or n-doped QDs placed in the flat band potential (p- or n-region) show a recovery in J_sc and V_oc compared to devices with delta-doped QDs placed in the depletion region. However, there is less photocurrent arising from the absorption of sub-band gap photons. Furthermore, the long wavelength photoresponse of the n-doped QDs placed in the n-region shows a slight improvement compared to the control cell. The approach of placing QDs in the n-region of the solar cell instead of the depletion region is a possible route towards increasing the conversion efficiency of QD solar cells.     

Improving the optical efficiency and concentration of a single-plate quantum dot solar concentrator using near infra-red emitting quantum dots

Low luminescent quantum yields and large overlap between quantum dot (QD) emission and absorption spectra of present commercially-available visible-emitting QDs have led to low optical efficiencies for single-plate quantum dot solar concentrators (QDSCs). It is shown that using near infra-red (NIR) emitting QDs, re-absorption of QD emitted photons can be reduced greatly, thereby diminishing escape cone losses thus improving optical efficiencies and concentration ratios. Using Monte-Carlo ray-trace modelling, escape cone losses are quantified for different types of QD. A minimum 25% escape cone loss would be expected for a plate with refractive index of 1.5 containing QDs with no spectral overlap. It is shown that escape cone losses account for ∼57% of incident photons absorbed in QDSCs containing commercially-available visible-emitting QDs.     

Spin-coating silicon-quantum-dot ink to improve solar cell efficiency

The optical and electrical properties of quantum dots (QDs) have generated great interest in the use of QDs for photovoltaics. The optical coupling of QDs with currently manufactured solar cells is one of the most promising photovoltaic applications of QDs. Here we demonstrate that by spin-coating Si-QD ink at the solar cell surface the efficiency of screen-printed Si solar cells may be improved. The enhancement of solar cell efficiency results from the porous Si-QD film induced increase of light absorption. The solar cell efficiency may be further improved as the efficiency of down-shifting short-wavelength light to red light by Si-QDs increases.     

Quantum dot solar concentrators: Electrical conversion efficiencies and comparative concentrating factors of fabricated devices

A novel, non-tracking concentrator is described, which uses nano-scale quantum dot technology to render the concept of a fluorescent dye solar concentrator (FSC) a practical proposition. The quantum dot solar concentrator (QDSC) comprises quantum dots (QDs) seeded in materials such as plastics and glasses that are suitable for incorporation into building façades. Photovoltaic (PV) cells attached to the edges convert direct and diffuse solar energy collected into electricity for use in the building. Small scale QDSC devices were fabricated. Devices have been characterised to determine current, voltage and power readings. Electrical conversion efficiencies, fill factors and comparative concentrating factors are reported.     

Modeling and simulation of solar cells quantum well based on SiGe/Si

In recent years, the development of quantum well solar cells QWSCs (Quantum Well Solar Cells) has generated a great deal of interest. These configurations have shown good promise to optimize the low conversion efficiency of conventional solar cells because of the high rate of absorption losses present in them. In this work, we are interested in modeling and simulation of two different structures of solar cells, a simple solar cell based on silicon Si and a quantum well solar cell SiGe/Si. When a solar cell is compared to 80 quantum well layers of Si_0.8Ge_0.2with a pin solar cell based on Si. The short circuit current J_sc increases from 23.55 to 37.48 mA/cm² with a relative increase of 59.15% found. In addition, the limit of the absorption band of the lower energy photons extends from 1100 nm to 2000 nm.     

CdSe quantum dot-single wall carbon nanotube complexes for polymeric solar cells

The development of lightweight, flexible polymeric solar cells which utilize nanostructured materials has been investigated. Incorporation of quantum dots (QDs) and single wall carbon nanotubes (SWNTs) into a poly(3-octylthiophene)-(P3OT) composite, has been shown to facilitate exciton dissociation and carrier transport in a properly structured device. Optimization towards an ideal electron acceptor for polymeric solar cells that exhibits high electron affinity and high electrical conductivity has been proposed in the form of QD-SWNT complexes. Specifically, the synthesis of CdSe-aminoethanethiol-SWNT complexes has been performed, with confirmation by microscopy (SEM, TEM, and AFM) and spectroscopy (FT-IR and optical absorption). Polymer composites containing these complexes in P3OT have been used to fabricate solar cells which show limited efficiency due to recombination and surface effects, but an open-circuit voltage (V_OC) of 0.75 V. However, evaluation of the optical absorption spectra for these nanomaterial-polymeric composites has shown a marked enhancement in the ability to capture the available irradiance of the air mass zero (AM0) spectrum.     

Antireflecting–passivating dielectric films on crystalline silicon solar cells for space applications

Antireflecting–passivating TiO₂–SiO₂ double layers on crystalline silicon (Si) were optimized and characterized for space solar cells applications. In the numeric optimization, the MgF₂–glass–adhesive–TiO₂–SiO₂–Si structure was considered. In order to fabricate the TiO₂–SiO₂ double layer, titanium films were deposited on Si wafers in a vacuum chamber, and then, the sample was annealed in oxygen at high temperatures. Glasses with evaporated MgF₂ thin films were bonded to the TiO₂–SiO₂–Si samples so as to obtain the complete structure. A gain of up to 23.5% in the maximum power is demonstrated for simulated c-Si solar cells using the optimized structure. Characterization of the TiO₂–SiO₂–Si structure using transmission electron microscopy (TEM) and X-ray reflectivity (XRR) as well as optical characterization are presented.     

Quantum dot integration in heterostructure solar cells

InAs self-assembled quantum dots (SA-QDs) were incorporated into GaAlAs/GaAs heterostructure for solar cell applications. The structure was fabricated by molecular beam epitaxy on p-GaAs substrate. After the growth of GaAs buffer layer, multi-stacked InAs QDs were grown by self-assembly with a slow growth rate of 0.01 ML/s, which provides high dot quality and large dot size. Then, the structure was capped with n-GaAs and wide band gap n-GaAlAs was introduced. One, two or three stacks of QDs were sandwiched in the p–n heterojunction. The contribution of QDs in solar cell hetero-structure is the quantized nature and a high density of quantized states. I–V characterization was conducted in the dark and under AM1 illumination with 100 mW/cm² light power density to confirm the solar cell performance. Photocurrent from the QDs was confirmed by spectral response measurement using a filtered light source (1.1-μm wavelength) and a tungsten halogen lamp with monochromator with standard lock-in technique. These experimental results indicate that QDs could be an effective part of solar cell heterostructure. A typical I–V characteristic of this yet-to-be-optimized solar cell, with an active area of 7.25 mm², shows an open circuit voltage V_oc of 0.7 V, a short circuit current I_sc of 3.7 mA, and a fill factor FF of 0.69, leading to an efficiency η of 24.6% (active area).     

Statistical theory of multiple exciton generation in quantum dot solar cells

The high efficiency of quantum dot solar cells is determined by the so-called multiple exciton generation (MEG) effect in quantum dots when a single photon is absorbed. The present work proposes a statistical approach to the process of simultaneous generation of many excitons in quantum dots when a high-energy photon is absorbed. This approach is based on the statistical Fermi approach to simultaneous generation of elementary particles (nucleons and π-mesons) in the nucleon-nucleon collision. Analysis of the results shows that the exciton quantum yields calculated using the statistical approach agree rather well with the experimental data.     

A new approach to modelling quantum dot concentrators

Luminescent collectors have advantages over geometric concentrators in that tracking is unnecessary and both direct and diffuse radiation can be collected. However, development has been limited by the performance of luminescent dyes. We have recently proposed a novel concentrator in which the dyes are replaced by quantum dots (QDs). Advantages over dyes include that the absorption threshold can be tuned by choice of dot diameter, and that the red shift between absorption and luminescence is related to the spread of dot sizes. In this paper we discuss how we have developed a self-consistent thermodynamic model for planar concentrators which allows for re-absorption by the QDs.     

Fabrication and full characterization of state-of-the-art quantum dot luminescent solar concentrators

The fabrication and full characterization of luminescent solar concentrators (LSCs) comprising CdSe core/multishell quantum dots (QDs) is reported. TEM analysis shows that the QDs are well dispersed in the acrylic medium while maintaining a high quantum yield of 45%, resulting in highly transparent and luminescent polymer plates. A detailed optical analysis of the QD-LSCs including absorption, emission, and time-resolved fluorescence measurements is presented. Both silicon and GaAs solar cells attached to the side of the QD-LSCs are used to measure the external quantum efficiency and power conversion efficiency (2.8%) of the devices. Stability tests show only a minor decrease of 4% in photocurrent upon an equivalent of three months outdoor illumination. The optical data are used as input for a ray-trace model that is shown to describe the properties of the QD-LSCs well. The model was then used to extrapolate the properties of the small test devices to predict the power conversion efficiency of a 50×50 cm² module with a variety of different solar cells. The work described here gives a detailed insight into the promise of QD-based LSCs.     

Crystalline silicon thin-film solar cells on ZrSiO4 ceramic substrates

The development of a low-cost substrate is one of the major technological challenges for crystalline Si thin-film solar cells. Zirconium silicate (ZrSiO₄) ceramics is a material which can meet the demanding physical requirements as well as the cost goals. Thin microcrystalline Si films were deposited by atmospheric pressure CVD on ZrSiO₄-based ceramic substrates coated with barrier layers. The Si film was transferred into a multicrystalline grain structure by zone-melting recrystallization (ZMR). Film growth was analyzed in situ and correlated with substrate and barrier layer properties. Thin-film solar cells were fabricated from selected coarse-grained films. The best solar cell achieved an efficiency of 8.3% with a short circuit current density of 26.7 mA/cm². The effective diffusion length obtained from internal quantum efficiency measurements was about 25 μm.     

CdSe quantum dots sensitized nanoporous hematite for photoelectrochemical generation of hydrogen

A novel system of CdSe quantum dots (QDs) sensitized porous hematite (α-Fe₂O₃) films has been investigated as a potential photoelectrode for hydrogen generation via photoelectrochemical (PEC) splitting of water. Before sensitization, nanoporous hematite thin films were prepared by spray pyrolysis. Characterizations for crystalline phase formation, crystallite size, absorption spectra, and flatband potential were carried out to analyze PEC data. Loading time of sensitizer to hematite thin films was found to be crucial in affecting its PEC properties. Film having sensitizer loading time as 42 h exhibited best photocurrent density of 550 μA cm⁻² at 1.0 V versus SCE. Current study, for the first time, explores the possibility of using low band gap QDs sensitization on a low band gap film, hematite in PEC splitting of water.     

Tuning photocurrent response through size control of CdTe quantum dots sensitized solar cells

The photovoltaic performance of CdTe quantum dots (QDs) sensitized solar cells (QDSSCs) as a function of tuning the band gap of CdTe QDs size is studied. The tuning of band gap was carried out through controlling the size of QDs. Presynthesized CdTe QDs of radii from 2.1 nm to 2.5 nm) were deposited by direct adsorption (DA) technique onto a layer of TiO₂ nanoparticles (NPs) to serve as sensitizers for the solar cells. The characteristic parameters of the assembled QDSSCs were measured under AM 1.5 sun illuminations. The values of current density (J_sc) and overall efficiency (η) increase with decreasing CdTe QDs size, since the lowest unoccupied molecular orbital (LUMO) levels shifts closer to vacuum level, which causes an increase in the driving force. Consequently the electrons’ injections to the conduction band (CB) of TiO₂ NPs become faster. The maximum values of J_sc (1105 μA/cm²) and η (0.190%) were obtained for the smallest CdTe QDs size (2.10 nm). The open circuit voltages (V_oc) varies slightly with the size of the CdTe QDs, however it is only dictated by the CB level of TiO₂ NPs and the VB of the electrolyte. Furthermore, the photocurrent response of the assembled cells to ON–OFF cycles of the illumination indicates the prompt generation of anodic current.