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Publications/Abstracts MBC

Abstracts of MBC Publications

Abstract: Cyclometalated ruthenium complexes of [Ru(C^N^N) (N^N^N)] configuration are a promising new class of molecular sensitizers for dye-sensitized solar cells, as a result of their broad and red-shifted visible absorption in comparison to the analogous [Ru(N^N^N)2] type coordinative complexes.

Abstract: In order to estimate the performance of solar cells with downshifters under realistic irradiation conditions we used spectral distributions as they may be found outdoors. The spectral distributions were generated on a minutely basis by means of the spectrum simulation model SEDES2, using minutely measured data for global, direct, and diffuse irradiation from a Dutch meteorological station. Hourly aggregated spectra for a number of typical days (clear summer day, cloudy summer day, clear winter day, cloudy winter day) were used in modelling the output of the solar cell with and without downshifter. It was found that the simulated short current enhancement, which varies between about 7 and 23%, is linearly related with the average photon energy of the spectra.

Abstract: Spectral down converters have been proposed as a good means to enhance the efficiency of underlying solar cells. Spectral down converters shift the incident solar spectrum towards the red, where most types of solar cells have an external quantum efficiency that is higher than in the blue part of the spectrum. Both organic fluorescent molecules and inorganic semiconductor nanocrystals have been shown to increase solar cell efficiency by up to 10% using the standard AM1.5 solar spectrum.
Calculations of performance enhancement under non-AM1.5 conditions have shown that even larger increases are possible [1]. These calculations were based on existing designs of multicrystalline silicon solar cells, and the down converter properties (QD concentration and size) were optimized.
In this contribution, we focus on the simulation of the outdoor performance of solar cells with spectral down converters, i.e, multicrystalline silicon solar cells with semiconductor nanocrystals as converter material [2]. Daily and annual performance of these devices can be simulated using spectra that vary over the course of the day. However, spectral data on a daily basis is not readily available, and we therefore have employed a spectrum simulation model that is able to simulate clear and also cloudy days: SEDES2 [3]. Through the availability of global, direct, and diffuse irradiation data on a minutely basis, we were able to model a full year of spectra for the Netherlands [4]. These spectra are aggregated to yield hourly spectra, which are subsequently used in modeling the solar cell output. In our contribution we will show the varying performance of spectral converters for a number of typical days, such as a clear summer day, a cloudy summer day, a clear winter day, and a cloudy winter day.
It will be shown that the performance of cells with spectral converters is better in regions with a higher diffuse-to-direct irradiance ratio (North-West Europe) than in regions with low diffuse-todirect irradiance ratio (e.g., Spain, Southern USA)

Abstract: The purpose of this work is twofold: to reduce the spectral losses of state of the art solar cells by converting the spectrum of the incoming light to match the bandgap of the solar cell, and to reduce the expensive solar cell area by concentration of the sunlight using cheap polymer materials doped with luminescent dyes or particles. Both purposes can be combined in the luminescent concentrator (LC).

Abstract: The cross-link chemistry of CdTe quantum dots (QDs) in solution is studied for different types of aliphatic (flexible) and aromatic (rigid) dithiol linker molecules. A remarkable difference in the cross-linking efficiency is observed: the rigid dithiols are shown to form aggregates at much lower concentrations. Qualitative and quantitative information on the formation of aggregates is obtained from cryogenic transmission electron microscopy (cryo-TEM) images and photoluminescence decay measurements. The luminescence decay curves are analyzed with a model for energy transfer to neighboring QDs in aggregates. The analysis shows that the cross-linking efficiency is 4 times higher for the rigid dithiols than for the flexible dithiols. The difference is attributed to the formation of loops for the flexible dithiols by attaching with both thiol groups to the same nanocrystal surface (preventing cross-linking), whereas the rigid aromatic dithiols cannot form loops and the second thiol group is oriented away from the surface (enabling cross-linking). The difference in conformation between flexible and rigid dithiols is confirmed by studies on the red-shift in the optical absorption spectra due to capping exchange of amines by monothiols or dithiols and by molecular simulations.

Abstract: Luminescent solar concentrators (LSCs) concentrate light in addition to reducing spectral losses, and simply consist of transparent polymer sheets doped with luminescent species [1]. Sunlight incident on the top surface is first absorbed by the luminescent species and is then re-radiated, ideally with high luminescence quantum efficiency (Qe), such that a fraction of the emitted light is trapped in the sheet and can be converted by a solar cell at the edge. Advantages over conventional geometric concentrators include that (i) the LSC reduces thermalisation losses and heat dissipation in an attached solar cell by converting the incident spectrum to improve the match with the absorption spectrum of the cell, (ii) both direct and diffuse radiation can be collected, (iii) expensive solar tracking is not required and, (iv) LSCs are ideally to suited to building integration via façades or active windows. However, the development of this promising concentrator was initially limited by the performance of the luminescent dyes available. Particular problems were their poor stability under solar irradiation and the large re-absorption losses owing to significant overlap of the absorption and emission.
We are currently evaluating the performance of both quantum dots (QDs) and organic dyes as the luminescent species in the LSC. The important characteristics of organic dyes are that they: (i) can provide extremely high Qes (near unity), (ii) are available in a wide range of colours and, (iii) new molecular species are now available with better re-absorption properties that may also provide the necessary UV stability. QDs have advantages over dyes in that: (i) their absorption spectra are far broader, extending into the UV, (ii), their absorption properties may be tuned simply by the choice of nanocrystal size, and (iii) they are inherently more stable than organic dyes. Moreover, (iv) there is a further advantage in that the red-shift between absorption and luminescence is quantitatively related to the spread of QD sizes, which may be determined during the growth process, providing an additional strategy for minimising losses due to re-absorption [2]. However, as yet QDs can only provide reasonable Qes (Qe > 0.8 have been reported [3] for core-shell QDs).
We have developed thermodynamic models for planar luminescent concentrators [4,5], modules [6] and stacks [7] by applying a detailed balance argument to relate the absorbed light to the spontaneous emission using self-consistent three-dimensional (3D) fluxes. Comparison with measurements on small test slabs [4,5], modules [6] and stacks [7] show that our 3D flux models can predict both the room temperature red-shift and the total flux escaping each surface, providing a tool for optimisation of the LSC. More recently, we have developed a simplified linearised model [8] in order to facilitate calculations on practical sized devices. For an idealised mirrored 40×10×0.5cm LSC doped with QDs matched to an InGaP cell we calculate that 78% of the luminescence will be lost through the top surface. Recently, pioneering use has been made of wavelengthselective cholesteric liquid crystal coatings applied to the top surface in order to reduce these losses [9]. These focal conic cholesteric coatings are transparent to incoming light but reflect the emitted light. Experimental results [9] suggest that a factor of two increase in light output can be achieved by tuning the bandwidth of the coating. We will extend our thermodynamic approach to include these effects by modifying the boundary conditions at the top surface using the measured reflectivities of the liquid crystal coatings. The extended model will be verified by comparison with measurements on test LSCs with appropriate top surface coatings, and, will allow us to investigate quantitatively the properties of the top surface coating, luminescent species, doping density, transparent matrix, and geometry that will maximise the efficiency of the LSC.

Abstract: Luminescent concentrators absorb solar radiation and transform it into light of more suitable wavelengths, which is then concentrated on small area solar cells. Usually they consist of polymer plates containing luminescing substances. Optimisation of the luminescent concentrator concerning efficiency and long-term stability requires among others very low absorption and scattering losses in the polymer matrix, decrease of losses through the escape cone, use of extrem stable luminescent substances and exploitation of the largest possible part of the spectrum by combination of several dyes with low reabsorption losses.
The poster addresses the first three of those factors. Concentrator plates were produced by polymerisation of acrylic monomers in self-made glass cuvettes using thermal as well as UV polymerisation processes. The influence of parameters such as sealing material, the condition of glass surface, temperature regime and the initiator used is demonstrated. The polymerisations were performed in ovens with circulating air or in a water bath. Best results (minimum absorption/scattering losses, broadest spectral region with high transparency) were obtained for distilled methylmethacrylate (MMA) polymerised in the water bath.
Better use of the solar spectrum and decreasing loss of luminescent light can, in principle, be reached by use of more than 1 dye establishing a radiation-less energy transfer from dye 1 to dye 2. Measurements on PMMA layers containing a type of quantum dots and Pyrromethen580 showed increased luminescence under certain conditions, but the results are still not conclusive. For long drying times of layers containing the quantum dots plus the dye degradation of Pyrromethen580 is found.
The unsufficient stability of organic dyes with respect to light has been one limiting factor for the concept of the luminescence concentrator in the past. Since quantum dots are intrinsically much more stable against light and, additionally, have a very large shift between absorption and luminescence wavelengths, i. e. low reabsorption losses, there have been efforts to include them into transparent polymers. We present results of first experiments to produce polymer plates containing quantum dots and/or dyes without performing a polymerisation process. This may be of interest as the production of highly transparent plates with quantum dots included is difficult. The casting procedure used is described, and the distribution of the luminescing substances is shown by microscope pictures of cross-sections. The intensity of the luminescence at the edges, which is rather large, is compared with that of plates with homogeneously distributed luminescing substance.

Abstract: In the light of recent advances in the stability of organic dyes and the efficiency of quantum dots, there has been a resurgence of interest in luminescent solar concentrators (LSCs) as a key photovoltaic component. We will report on the fabrication and characterisation of composite LSCs which consist of thin films containing the luminescent species deposited on a transparent substrate. The luminescent species investigated were a Pyrromethene 580 dye and a core-shell quantum dot (QD). The characterisation was carried out using short-circuit current as well as photoluminescence measurements. The spectral response was studied under both broadband and monochromatic radiation. The measurements and characterisation methods were found to be consistent. A thermodynamic model was applied and quantum efficiencies (QEs) of the QDs extracted. Within errors, the results were consistent with the manufacturer’s specifications of 40% to 60%. The dye based LSCs provided above-unity concentration ratios in spite of a relatively small spectral absorption range under broadband illumination.

Abstract: There has recently been a revival of interest in the Luminescent Solar Concentrator (LSC) which was originally proposed 30 years ago this year [1]. The LSC consists of a transparent plate or plates doped with luminescent centres which absorb the incident radiation and re-radiate isotropically. A significant fraction of the light is trapped, due to total internal reflection, inside the plate and wave-guided to photovoltaic cells at the edges. The cells ideally have band-gaps optimised for the luminescent wavelength.
The renewed interest results from a number of factors including: the availability of photo-stable organic dyes with high luminescent quantum efficiency (QE), new dyes which function in the red region of the spectrum and polymer matrices with low background absorption. In addition, novel luminescent centres such as core-shell quantum dots have been proposed and there are nowadays higher efficiency cells available with more appropriate band-gaps [2]. There is also a growing interest in building integrated photovoltaics. The LSC is particularly suited to this application as it is relatively inexpensive, does not require tracking and works in both diffuse and direct sunlight.
We will report new results on LSCs fabricated with both dye and quantum dot dopants using highly transparent polymers as host matrices. A selection of coumarin and perylene dyes as well as CdSe/ZnS core-shell quantum dots have been studied. We have developed a thermodynamic model [3] which has proved extremely useful in characterising the LSC. The model, which is based on detailed-balance and the three-dimensional radiative transfer processes occurring within the concentrator, takes the measured absorption of the dopant and predicts the spectral variation and intensity of the luminescence [3]. The plates are also characterised using a short circuit current measurement technique [4] in which a solar cell of known spectral response scans one edge of uniformly illuminated LSC. The thermodynamic model can then be used to determine the QE of the dopant in the host medium, which can compared with its value in solution.
We will present results on dye and quantum dot LSCs in single plates, stacked plates and composite configurations and also an LSC designed for short-wavelength applications. We will show that dyes and quantum dots can be incorporated into polymers with QEs comparable with those in solution. We will use a version of the thermodynamic model which can be extended to plates of relatively large area to predict the performance practical-sized LSCs might ultimately achieve, in terms of concentration ratio and overall system efficiency.

Abstract: The Luminescent Solar Concentrator (LSC) consists of a transparent matrix material (usually a flat plate) with solar cells connected to one or more sides. The transparent matrix contains luminescent molecules or particles such as, e.g., organic dyes or quantum dots. Part of the light emitted by the luminescent particles is guided towards the solar cells by total internal reflection, the plate functioning as a waveguide.
About 25% of the light emitted by the dye however is emitted within the optical escape cone of the matrix material and is lost. A way to overcome this loss is by application of wavelength selective mirrors. One way is to apply selectively-reflective chiral nematic (cholesteric) liquid crystal (LC) layer(s). These layers have the property that they allow the excitation wavelength to enter the plate, but reflect the light emitted by the dye trapping it in the waveguide. We present both simulations and experimental results.

Abstract: The annual performance of a multi-crystalline silicon cell (mc-Si) and an amorphous silicon cell (a-Si) is calculated using modelled spectra in combination with the well-known solar cell one-diode model. Two different sets of modelled minutely spectra are utilized for modelling cell performance: 1) Simulated spectral data, using measured irradiation data from KNMI (Royal Netherlands Meteorological Institute) and the SEDES2 spectral model, 2) Scaled AM1.5 spectra using global tilt irradiance. The modelled energy performance derived from each set of spectra is compared and a mismatch factor (MMF) is determined to quantify the amount of the spectral effects. Both the modelled solar cell performance and calculated MMF are then graphed against global irradiance, air mass, and sky clearness index for every month. The results show that spectral effects are larger for a-Si than for mc-Si, as was expected. Detailed minutely data shows MMF to vary between 0.66 and 1.77 for a-Si and between 0.74 and 1.11 for mc-Si solar cells. From the annual yield based on modelled and scaled AM1.5 spectra, it is concluded that a-Si is up to 8% more effective than mc-Si per installed Wp. The annual spectral effect was found to be −3% for a-Si and −1.7% for mc-Si. This indicates that in general models, which assume the AM1.5 spectrum, overestimate the energy yield.

Abstract: We present detailed investigations on the optical properties of PbSe nanocrystals. The absorption spectra of monodisperse, quasi-spherical nanocrystals exhibit sharp features as a result of distinct optical transitions. To study the size-dependence, absorption spectra of nanocrystals ranging form 3.4 nm to 10.9 nm in diameter are analysed and a total of 11 distinct optical transitions is identified. The assignment of the various optical transitions is discussed and compared to theoretically calculated transition energies. By plotting all transitions as a function of nanocrystal size we find that the energy changes as E D-1.5 for the lowest energy transitions. The transition energy extrapolates to ~0.3 eV for infinite crystal size, in agreement with the bandgap of bulk PbSe at the L-point in the Brillouin zone. In addition, high energy transitions are observed which extrapolate to 1.6 eV for infinite crystal size, which is in good agreement with the bulk bandgap of PbSe at the Σ-point in the Brillouin zone. Tight binding calculations confirm that the high-energy transitions originate from the Σ-point in the Brillouin zone.
The Σ-character of the high energy transitions may be of importance to explain the mechanism behind Multiple Exciton Generation in PbSe nanocrystals.

Abstract: One of the major loss mechanisms in state of the art photovoltaic cells is spectral loss resulting from inefficient use of ultraviolet photons and the lack of absorption of infrared photons by the solar cell.
For a Si solar cell e.g., spectral losses alone result in over 55% loss of the energy of the solar spectrum. Converting the spectrum of the incoming light such that it has a better match with the absorption spectrum of the solar cell can reduce spectral losses, especially in the case of a small absorption band, such as for dye sensitized solar cells and polymer solar cells. In this paper it is shown that the ultraviolet response of a multi crystalline silicon solar cell and polymer solar cell can be enhanced by application of a polymer coating doped with a luminescent dye. An increase in the power conversion efficiency is obtained for coatings with luminescent dyes with an absorption onset < 450 nm. Coatings with luminescent dyes that absorb at higher wavelengths give rise to lower power conversion efficiencies. When applied to a dye sensitized solar cell, a decrease in the cell performance as observed.

Abstract: Luminescent concentrator (LC) plates with different dyes were combined with standard multicrystalline silicon solar cells. External quantum efficiency measurements were performed, showing an increase in electrical current of the silicon cell (under AM1.5, 1 sun conditions, at normal incidence) compared to a bare cell. The influence of dye concentration and plate dimensions are addressed. The best results show a 1.7 times increase in the current from the LC/silicon cell compared to the silicon cell alone. To broaden the absorption spectrum of the LC, a second dye was incorporated in the LC plates. This results in a relative increase in current of 5-8% with respect to the one dye LC, giving. Using a ray-tracing model, transmission, reflection and external quantum efficiency spectra were simulated and compared with the measured spectra. The simulations deliver the luminescent quantum efficiencies of the two dyes as well as the background absorption by the polymer host. It is found that the luminescent quantum efficiency of the red emitting dye is 87%, which is one of the major loss factors in the measured LC. Using ray-tracing simulations it is predicted that increasing the luminescent quantum efficiency to 98% would substantially reduce this loss, resulting in an increase in overall power conversion efficiency of the LC from 1.8 to 2.6%.

Abstract: A ray-tracing simulation has been developed for Luminescent Solar Concentrators. By fitting to measurements on the devices, parameters such as the quantum efficiency of the dyes employed can be determined. Once a complete description of the device is available it be comes clear where the losses originate from and directions for the improvement of the devices can be given.

Abstract: Quantum Dot Solar Concentrators (QDCs) have been fabricated by the incorporation of quantum dots into highly transparent polymer host materials. UV polymerisation techniques were found to reduce the quantum dot quantum efficiency in comparison with thermally polymerised samples. The sample plates were characterised using photocurrent techniques in individual and stacked configurations. Due to the increase in absorption, stacking the QDC plates results in a 16% increase in photocurrent. This configuration could also reduce thermalisation losses when coupled to solar cells with appropriate band gaps.

Abstract: The inclusion of quantum dots in a plastic layer on top of solar cells increases their performance under standard and non-standard illumination conditions. The quantum dots effectively modify the incident spectrum such that a better match is obtained between the incident spectrum and the spectral response of the solar cells. As solar cell designs have improved over the past years, new optimized converter layers may have to be defined. Investigating new and future designs of multicrystalline silicon solar cells has revealed that the beneficial effect of deploying converter layers is less pronounced, as the new and future designs show an improved blue response. Nevertheless, a relative short circuit current increase of 7.5% is found for optimized converter layers. Combined optimization of solar cell and converter showed that particularly reduction of the front side recombination velocity can result in further performance improvement.

Abstract: NO ABSTRACT.

Abstract: Stable dispersions of molecularlike aggregates of CdTe quantum dots are prepared by chemical cross-linking. Cryo-TEM images confirm the presence of cross linked quantum dots and show that the size of the small aggregates can be controlled by the amount of cross-linker added. Optical measurements reveal two types of interdot interactions within these quantum-dot molecules: exciton energy transfer and electronic coupling. Quantitative information on the energy transfer rates in quantum-dot molecules is obtained by photoluminescence lifetime measurements. The degree of electronic coupling is dependent on the size of the quantum dots, which is supported by quantum mechanical calculations.

Abstract: Luminescent solar concentrators have advantages over geometric concentrators in that tracking is unnecessary and both direct and diffuse radiation can be collected. We have developed self-consistent 3D thermodynamic models for planar concentrators, modules and stacks but, evaluating the resulting integral equations is computationally intensive. With a view to developing computationally tractable models for such systems that can be applied to practical sized devices, we have developed an optimal, self-consistent linearisation of the depth dependence of the chemical potential for a single planar concentrator that results in only analytic expressions. This linearised 3D flux model is validated by comparison with the results of our original 3D flux model. The results for test concentrators containing both quantum dots and organic dyes as the luminescent species show excellent agreement with experiment.

Abstract: Planar converters containing quantum dots as wavelength-shifting moieties on top of multi-crystalline and amorphous silicon solar cells were studied. The highly efficient quantum dots shift by means of absorption and re-emission, the wavelengths where the spectral response of the solar cell is low to wavelengths where the spectral response is high, in order to improve the conversion efficiency of the solar cell. It was calculated that quantum dots with an emission at 603 nm increase the multi-crystalline solar cell short-circuit current by nearly 10%. Simulation results for planar converters on hydrogenated amorphous silicon solar cells show no beneficial effects, due to the high spectral response at low wavelength. Experimental results on multi-crystalline silicon solar cell however do not confirm these findings. (PDF full version)

Abstract: The luminescent properties of core-shell quantum dots (QDs) are being exploited in an unconventional solar concentrator module which promises to reduce the cost of photovoltaic electricity. Luminescent solar 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 present experimental and theoretical results with a novel system in which the dyes are replaced by quantum dots. We have developed self-consistent thermodynamic models for planar concentrators and modules and find that these threedimensional flux models show excellent agreement with experiment. (PDF full version)

Abstract: Molecular based concepts offer the potential of low materials and processing costs in photovoltaics, which is especially interesting if high efficiencies can be obtained. To accomplish high efficiencies a better utilisation of the solar spectrum is of high importance. The concept of two photon absorption in dye sensitized solar cells and full spectrum aspects of luminescent flat plates concentrators are discussed in this paper. The two photon dye cell can be compared to a tandem solar cell on the molecular level. The luminescent concentrator offers the potential to employ full spectrum utilization in combination with static concentration of direct and diffuse light.

Abstract: A planar converter containing quantum dots as wavelength-shifting moieties on top of a solar cell were studied. The highly efficient quantum dots are to shift the wavelengths where the spectral response of the solar cell is low to wavelengths where the spectral response is high in order to improve the conversion efficiency of the solar cell. It was calculated that quantum dots with an emission at 603 nm increase the multicrystalline solar cell short-circuit current by nearly 10%. Simulation results for planar converters on hydrogenated amorphous silicon solar cells show no beneficial effects, due to the high spectral response at low wavelength.

Abstract: Planar converters containing quantum dots as wavelength-shifting moieties on top of a multicrystalline silicon and an amorphous silicon solar cell were studied. The highly efficient quantum dots are to shift the wavelengths where the spectral response of the solar cell is low to wavelengths where the spectral response is high, in order to improve the conversion efficiency of the solar cell. It was calculated that quantum dots with an emission at 603 nm increase the multi-crystalline solar cell short-circuit current by nearly 10%. Simulation results for planar converters on hydrogenated amorphous silicon solar cells show no beneficial effects, due to the high spectral response at low wavelength.

Abstract: The luminescent properties of core-shell quantum dots (QDs) are being exploited in an unconventional solar concentrator which promises to reduce the cost of photovoltaic electricity. Luminescent solar 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 present experimental and theoretical results with a novel concentrator in which the dyes are replaced by quantum dots. We have developed a self-consistent thermodynamic model for planar concentrators and find that this three-dimensional flux model shows excellent agreement with experiment.