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.     

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.     

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.     

High-performance counter electrode based on nitrogen-doped porous carbon nanoribbons for quantum dot-sensitized solar cells

Nitrogen-doped porous carbon nanoribbons (NPCNs) are facilely prepared by carbonization of polypyrrole (PPy) nanotubes followed by a chemical activation process. NPCN counter electrodes are subsequently fabricated by depositing NPCNs onto Ti mesh for quantum dot-sensitized solar cells (QDSCs). Electrochemical tests are carried out to evaluate the electrocatalytic performance of obtained NPCN electrode. The data of electrochemical tests suggest that the NPCN electrode has a superior electrocatalytic ability towards polysulfide (S₂²⁻/S²⁻) electrolyte regeneration reaction and displays a high stability in polysulfide electrolyte. The excellent electrocatalytic performance of NPCN electrode can be ascribed to their large surface area, 2D porous nanoribbon morphology, and nitrogen atom doping, which provides abundant electrocatalytic active sites and facilitates the electrolyte diffusion. Consequently, a power conversion efficiency of 3.27% is obtained by using NPCN electrode as the counter electrode for QDSC. This efficiency is close to the QDSC assembled with commonly used PbS electrode (4.0%).     

Novel materials for high-efficiency III–V multi-junction solar cells

As a result of developing wide bandgap InGaP double hetero structure tunnel junction for sub-cell interconnection, InGaAs middle cell lattice-matched to Ge substrate, and InGaP-Ge heteroface structure bottom cell, we have demonstrated 38.9% efficiency at 489-suns AM1.5 with InGaP/InGaP/Ge 3-junction solar cells by in-house measurements. In addition, as a result of developing a non-imaging Fresnel lens as primary optics, a glass-rod kaleidoscope homogenizer as secondary optics and heat conductive concentrator solar cell modules, we have demonstrated 28.9% efficiency with 550-suns concentrator cell modules with an area of 5445 cm². In order to realize 40% and 50% efficiency, new approaches for novel materials and structures are being studied. We have obtained the following results: (1) improvements of lattice-mismatched InGaP/InGaAs/Ge 3-junction solar cell property as a result of dislocation density reduction by using thermal cycle annealing, (2) high quality (In)GaAsN material for 4- and 5-junction applications by chemical beam epitaxy, (3) 11.27% efficiency InGaAsN single-junction cells, (4) 18.27% efficiency InGaAs/GaAs potentially modulated quantum well cells, and (5) 7.65% efficiency InAs quantum dot cells.     

A numerical study of Bi-periodic binary diffraction gratings for solar cell applications

In this paper, a numerical study is made of simple bi-periodic binary diffraction gratings for solar cell applications. The gratings consist of hexagonal arrays of elliptical towers and wells etched directly into the solar cell substrate. The gratings are applied to two distinct solar cell technologies: a quantum dot intermediate band solar cell (QD-IBSC) and a crystalline silicon solar cell (SSC). In each case, the expected photocurrent increase due to the presence of the grating is calculated assuming AM1.5D illumination. For each technology, the grating period, well/tower depth and well/tower radii are optimised to maximise the photocurrent. The optimum parameters are presented. Results are presented for QD-IBSCs with a range of quantum dot layers and for SSCs with a range of thicknesses. For the QD-IBSC, it is found that the optimised grating leads to an absorption enhancement above that calculated for an ideally Lambertian scatterer for cells with less than 70 quantum dot layers. In a QD-IBSC with 50 quantum dot layers equipped with the optimum grating, the weak intermediate band to conduction band transition absorbs roughly half the photons in the corresponding sub-range of the AM1.5D spectrum. For the SSC, it is found that the optimised grating leads to an absorption enhancement above that calculated for an ideally Lambertian scatterer for cells with thicknesses of 10 μm or greater. A 20 μm thick SSC equipped with the optimised grating leads to an absorption enhancement above that of a 200 μm thick SSC equipped with a planar back reflector.     

Generalization of the two-dimensional optical analysis of cylindrical concentrators

A two-dimensional optical analysis of cylindrical concentrators valid for any incidence angle of the solar rays is described. Unlike previous two-dimensional studies, it takes into account: (a) the angle κ defined by the solar rays and a plane perpendicular to the focal line and (b) the variations of the image width as a function of κ.An equation relating κ to solar coordinates has been obtained. The curves of κ as a function of time for several dates and three orientations of the concentrator are presented.The analysis is applied in detail to the cylindrical-parabolic concentrator and to the fixed-mirror solar concentrator, both with flat receivers. The local concentration factor and its mean value for different values of κ are obtained. Using these results and taking into account the useful range of κ, criteria are given to select the concentrator orientation and the receiver width.     

Design and fabrication of low concentrating second generation PRIDE concentrator

Prototype first generation Photovoltaic Facades of Reduced Costs Incorporating Devices with Optically Concentrating Elements (PRIDE) technology incorporating 3 and 9 mm wide single crystal silicon solar cells showed excellent power output compared to a similar non-concentrating system when it was characterized both indoors using a flash and continuous solar simulator. However, durability and instability of the dielectric material occurred in long-term characterisation when the concentrator was made by using casting technology. For large scale manufacturing process, durability, and to reduce the weight of the concentrator, second generation PRIDE design incorporated 6 mm wide “Saturn” solar cells at the absorber of dielectric concentrators. Injection moulding was used to manufacture 3 kWp of such PV concentrator module for building façade integration in Europe. Special design techniques and cost implications are implemented in this paper. A randomly selected PV concentrator was characterised at outdoors from twenty-four (≈3 kWp) 2nd-G PRIDE manufactured concentrators. The initial PV concentrators achieved a power ratio of 2.01 when compared to a similar non-concentrating system. The solar to electrical conversion efficiency achieved for the PV panel was 10.2% when characterised outdoors. In large scale manufacturing process, cost reduction of 40% is achievable using this concentrator manufacturing technology.     

Multi-junction III–V solar cells: current status and future potential

Our recent R&D activities of III–V compound multi-junction (MJ) solar cells are presented. Conversion efficiency of InGaP/InGaAs/Ge has been improved up to 31–32% (AM1.5) as a result of technologies development such as double hetero-wide band-gap tunnel junction, InGaP–Ge hetero-face structure bottom cell, and precise lattice-matching of InGaAs middle cell to Ge substrate by adding indium into the conventional GaAs layer. For concentrator applications, grid structure has been designed in order to reduce the energy loss due to series resistance, and world-record efficiency InGaP/InGaAs/Ge 3-junction concentrator solar cell with an efficiency of 37.4% (AM1.5G, 200-suns) has been fabricated. In addition, we have also demonstrated high-efficiency and large-area (7000 cm²) concentrator InGaP/InGaAs/Ge 3-junction solar cell modules of an outdoor efficiency of 27% as a result of developing high-efficiency InGaP/InGaAs/Ge 3-junction cells, low optical loss Fresnel lens and homogenizers, and designing high thermal conductivity modules.Future prospects are also presented. We have proposed concentrator III–V compound MJ solar cells as the 3rd generation solar cells in addition to 1st generation crystalline Si solar cells and 2nd generation thin-film solar cells. We are now developing low-cost and high output power concentrator MJ solar cell modules with an output power of 400 W/m² for terrestrial applications.     

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.     

Optimization of stacking high-density quantum dot molecules for photovoltaic effect

Using a modified molecular beam epitaxial (MBE) process called thin-capping-and-regrowth technique we grew quantum dot molecule (QDM) structures having high dot volume density (greater than 10¹² cm^?3) and thus suitable as an active layer for effective photovoltaic energy conversion. Stacking of QDMs is time consuming and may introduce defects; therefore, optimization of stack number of QDMs is important in terms of achieving high-performance and low-cost solar cells. Samples with 1, 3, 5, 7 and 10 stacks of high-density QDMs are grown, fabricated into solar cells and characterized optically and electrically. It is found that the solar cell performance initially improves with the number of stacks, yet deteriorates once the number exceeds 5 stacks. We attribute the deterioration to defects and these put the optimized number of QDM stacks in our structure to 3–5.     

Application of high-density daylight for indoor illumination

A fiber optic solar concentrator system with a simple parabolic profile is designed and fabricated to deliver a stream of high-density solar flux into the interior of a building for indoor illumination. The system consists of a small dish concentrator (30 cm in diameter), an optical fiber cable and a diffuser at the end. A series of tests are performed using a goniophotometer and spectrometer to investigate the photometric characteristics of the system in terms of luminous intensity distribution and spectral radiance, which are used in link with the actual photometric measurements for a lightless mock-up space of 4.9 m × 4.9 m × 2.6 m. The illuminance on the working plane was continuously monitored at 25 points by using photometric sensors. Measurements clearly indicate the photometric characteristics of the present system where a constant level of indoor illumination is observed depending on the sky clearness.     

A new trough solar concentrator and its performance analysis

The operation principle and design method of a new trough solar concentrator is presented in this paper. Some important design parameters about the concentrator are analyzed and optimized. Their magnitude ranges are given. Some characteristic parameters about the concentrator are compared with that of the conventional parabolic trough solar concentrator. The factors having influence on the performance of the unit are discussed. It is indicated through the analysis that the new trough solar concentrator can actualize reflection focusing for the sun light using multiple curved surface compound method. It also has the advantages of improving the work performance and environment of high-temperature solar absorber and enhancing the configuration intensity of the reflection surface.     

V-trough concentrator on a photovoltaic full tracking system in a hot desert climate

A V-trough concentrator with a two-axis tracker system to increase the performance of photovoltaics was designed by the authors and installed on the roof-top of the building of the National Research Institute of Astronomy and Geophysics at Helwan in South Cairo. The V-trough concentrator system comprises two flat mirrors with dimensions 50 cm × 18 cm. They are fixed with the reflecting surfaces facing each other with a separation of about 11 cm, on a wooden table of 50 cm axis length. A sample of polycrystalline and amorphous silicon solar cells were fixed into the system, and similar solar cells of each type were fixed separate to the system, to estimate the electrical gain. The measurements were performed daily at different air masses for one complete year. The temperature of the solar cells in and out of the system were measured for comparison. Also, measurements for beam and global solar radiation and other meteorological conditions were recorded. The optical losses of the system were analyzed and details of collectable energy calculations are presented. The energy gain from the isolated contribution of the V-trough concentrators is also evaluated.     

Phosphorus-doped silicon quantum dots for all-silicon quantum dot tandem solar cells

Doping of Si quantum dots is important in the field of Si quantum dots-based solar cells. Structural, optical and electrical properties of Si QDs formed as multilayers in a SiO₂ matrix with various phosphorus (P) concentrations introduced during the sputtering process were investigated for its potential application in all-silicon quantum dot tandem solar cells. The formation of Si quantum dots was confirmed by transmission electron microscopy. The addition of phosphorus was observed to modify Si crystallization, though the phosphorus concentration was found to have little effect on quantum dot size. Secondary ion mass spectroscopy results indicate minimal phosphorus diffusion from Si QDs layers to adjacent SiO₂ layers during high-temperature annealing. Resistivity is significantly decreased by phosphorus doping. Resistivity of slightly phosphorus-doped (0.1 at% P) films is seven orders of magnitude lower than that of intrinsic films. Dark resistivity and activation energy measurements indicate the existence of an optimal phosphorus concentration. The photoluminescence intensity increases with the phosphorus concentration, indicating a tendency towards radiative recombination in the doped films. These results can provide optimal condition for future Si quantum dots-based solar cells.     

Efficient polymer nanocrystal hybrid solar cells by improved nanocrystal composition

We demonstrate the importance of the nanocrystal surface treatment and the inorganic composition for hybrid solar cells. Mixtures of CdSe nanorods and CdSe quantum dots integrated in hybrid solar cells together with the conjugated polymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) perform better than nanorod and quantum dot only based devices. In addition larger sized quantum dots show a similar improvement after integration in respective solar cells. Power conversion efficiency values exceeding 3% are observed. A first result on the shelf lifetime of such a device is highlighted.     

FULLSPECTRUM: a new PV wave making more efficient use of the solar spectrum

The project FULLSPECTRUM — an Integrated Project (IP) in the terminology of the European Commission — pursues a better exploitation of the FULL solar SPECTRUM by (1) further developing concepts already scientifically proven but not yet developed and (2) by trying to prove new ones in the search for a breakthrough in photovoltaic (PV) technology. More specific objectives are the development of: (a) III–V multijunction cells (MJC), (b) solar thermo-photovoltaic (TPV) converters, (c) intermediate band (IB) materials and cells (IBC), (d) molecular-based concepts (MBC) for full PV utilisation of the solar spectrum and (e) manufacturing technologies (MFG) for novel concepts including assembling. MJC technology towards 40% efficiency will be developed using lower cost substrates and high light concentration (up or above 1000 suns). TPV is a concept with a theoretically high efficiency limit because the entire energy of all the photons is used in the heating process and because the non-used photons can be fed back to the emitter, therefore helping in keeping it hot. In the IBC approach, sub-bandgap photons are exploited by means of an IB. Specific IB materials will be sought by direct synthesis suggested by material-band calculations and using nanotechnology in quantum dot (QD) IBCs. In the development of the MBC, topics such as the development of two-photon dye cells and the development of a static global (direct and diffuse) light concentrator by means of luminescent multicolour dyes and QDs, with the radiation confined by photonic crystals, will be particularly addressed. MFG include optoelectronic assembling techniques and coupling of light to cells with new-optic miniconcentrators.     

Experimenting with concentrated sunlight using the DLR solar furnace

The high flux solar furnace that is operated by the Deutsche Forschungsanstalt für Luft- und Raumfahrt (DLR) at Cologne was inaugurated in June 1994 and we are now able to look back onto one year of successful operation. The solar furnace project was founded by the government of the State Northrhine Westfalia within the Study Group AG Solar. The optical design is a two-stage off-axis configuration which uses a flat 52 m² heliostat and a concentrator composed of 147 spherical mirror facets. The heliostat redirects the solar light onto the concentrator which focuses the beam out of the optical axis of the system into the laboratory building. At high insolation levels (>800 W/m²) it is possible to collect a total power of 20 kW with peak flux densities of 4 MW/m². Sixteen different experiment campaigns were carried out during this first year of operation. The main research fields for these experiments were material science, component development and solar chemistry. The furnace also has its own research program leading to develop sofisticated measurement techniques like remote infrared temperature sensing and flux mapping. Another future goal to be realized within the next five years is the improvement of the performance of the furnace itself.     

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.     

Development and applications of a two-dimensional optical analysis of non-perfect cylindrical concentrators

The two-dimensional optical analysis of cylindrical concentrators valid for any incidence angle of the solar rays described in [1] is extended to non-perfect concentrators. To this end, it is assumed that the normal to each differential element of the specular surface departs from its correct position by an angle , the possible values of which follow a gaussian distribution of mean value and standard deviation σ.The main purpose of the present paper is to determine the intensity distribution at the receiver plane of the mentioned concentrators. The mathematical treatment is given explicitly and applied to the cylindrical-parabolic and the cylindrical fixed faceted-mirror solar concentrators. Both are considered to have flat receivers and to be located in the E-W direction. Concentration factors and geometrical losses are calculated for typical examples of both concentrator types, for several values of the receiver width and different times of the year and hours of the day. This information is required for a subsequent appropriate selection of the receiver width. Results for = 0 and values of σ within 0–6 mrad range are shown.Changes in the mean concentration factors and in the total geometrical losses due to parallel displacements of the receiver plane as referred to its correct position, are analysed as well. Furthermore, losses due to the shadow projected by the receiver onto the concentrator are given; for the fixed faceted-mirror concentrator this is done as a function of the angle of incidence of the radiation.The mentioned analyses are of special importance when deciding the precision to which a given concentrator will be constructed. In this context, it is convenient to take into account that the final precision requirements will come from a compromise between cost and solar energy conversion efficiency.