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Publications/Abstracts IBC

Abstracts of IBC Publications

Abstract: According to previous electronic structure calculations, substituting in CuGaS2 chalcopyrite some Ga atoms by Ti or Cr gives an intermediate band material that could lead to new photovoltaic cells of higher efficiency.
New DFT calculations are now carried out to check the thermodynamic viability of such substituted structures. Total energy, disorder entropy and vibration (phonon) contributions are combined to obtain the free energy of formation of those materials from the closest known stable compounds. Except for low substitution levels and high temperatures, the substituted structures appear to be less stable than the simpler compounds, but to a extent smaller than Mn-doped GaAs, which can however be experimentally obtained. The solubility is computed to be lower for Ti than for Cr; the latter shows also some tendency to clustering. Hydrothermal synthesis of Cu(Ga,Cr)S2 has been undertaken, and characterization results suggest that the desired intermediate band electronic structure has been obtained.

Abstract: Semiconductor quantum dots may be used in so-called third-generation solar cells that have the potential to greatly increase the photon conversion efficiency via two effects: (1) the production of mul tiple excitons from a single photon of sufficient energy and (2) the formation of intermediate bands in the bandgap that use sub-bandgap photons to form separable electron–hole pairs. This is possible because quantization of energy levels in quantum dots produces the following effects: enhanced Auger processes and Coulomb coupling between charge carriers; elimination of the requirement to conserve crystal momentum; slowed hot electron–hole pair (exciton) cooling; multiple exciton generation; and formation of minibands (delocalized electronic states) in quantum dot arrays. For exciton multiplication, very high quantum yields of 300–700% for exciton formation in PbSe, PbS, PbTe, and CdSe quantum dots have been reported at photon energies about 4–8 times the HOMO–LUMO transition energy (quantum dot bandgap), respectively, indicating the formation of 3–7 excitons/photon, depending upon the photon energy. For intermediate-band solar cells, quantum dots are used to create the intermediate bands from the confined electron states in the conduction band. By means of the intermediate band, it is possible to absorb below-bandgap energy photons. This is predicted to produce solar cells with enhanced photocurrent without voltage degradation.

Abstract: The characteristics of intermediate band solar cells containing 10, 20, and 50 InAs quantum dot (QD) layers embedded in otherwise “standard” Al,GaAs solar cell structures have been compared.
The short-circuit current densities of the cells decreased and the quantum efficiencies of the devices showed a concomitant reduction in the minority carrier lifetime in the p emitters with increasing number of QD layers. Dislocations threading up from the QDs toward the surface of the cells, and revealed by bright field scanning transmission electron microscopy, are the most likely cause of the deterioration in the electrical performance of the cells.

Abstract: The optical properties of a novel potential high efficiency photovoltaic material have been studied. This material is based on a chalcopyrite-type semiconductor (CuGaS2) with some Ga atom substituted by Ti and is characterized by the formation of an isolated transition-metal band between the valence band and the conduction band. We present a study in which ab-initio density functional theory calculations within the generalized gradient approximation are carried out to determine the optical reflectivity and absorption coefficient of the materials of interest. Calculations for the host semiconductor are in good agreement with experimental results within the limitations of the approach. We find, as desired, that because of the intermediate band, the new Ti-substituted material would be able to absorb photons of energy lower than the band gap of the host chalcopyrite. We also analyze the partial contributions to the main peaks of its spectrum.

Abstract: Novel materials for high efficiency photovoltaic solar cell have been study. Their vibrational and optical properties are the main objective of this work. The materials of interest are based on a chalcopyrite-type semiconductor (CuGaS2) with some Ga atoms substituted by a transition metal atom. The insertion of the metal gives rise to the formation of an isolated band between the valence band and the conduction band of the host semiconductor. Here we present a frozen phonon ab-initio density functional theory calculations of the vibrational properties of the host chalcopyrite and the substituted alloys. This study can be used to calculate the phonon contribution to the free energy balance of formation and assess their thermodynamic viability. We have also carried out a study of the optoelectronic properties of these materials such as optical reflectivity and absorption coefficient. We demonstrate that the intermediate band increases the absorption of the material in the main region of the solar spectrum, as desired for photovoltaic applications.

Abstract: In this work we present standard and beyond Density Functional Theory calculations for metal doped chalcogenide compounds as derivatives of CuGaS2 chalcopyrite and MgIn2S4 spinel. The purpose of the work is to develop a material which can be used to create a more efficient photovoltaic solar cell. This material must have a partially filled band inside the band-gap of the appropriate semiconductor as the aforementioned systems.
For chalcopyrite alloy materials, we have previously made ab-initio calculations using the Density Functional formalism in the Generalized Gradient approach for different metal substituents as the Ti, V, Cr, and Mn. For the Ti and Chromium cases, the appearing of an intermediate band seems to be promising so we have made more calculations using two more advanced methods where we corrected the correlation and exchange effects. For spinel alloy materials we first presented standard density functional calculations with Vanadium as susbtituent.

Abstract: With a 63.2% theoretical efficiency limit, the intermediate band solar cell (IBSC) is a new photovoltaic device proposed to overcome the 40.7% efficiency limit of conventional single gap solar cells. Quantum dot technology can be used to take the IBSC concept into practice. In this respect, the results of experiments carried out recently to characterize IBSC solar cells containing different numbers of InAs quantum dot layers as well as the theoretical models used to describe and analyze the related experimental data are summarized here. Electroluminescence and quantum efficiency measurements confirm that the main operating conditions for IBSCs are complied with in structures with a low number of QD layers. These conditions include the production of photocurrent from absorption of below band gap energy photons and the formation of distinctive quasi-Fermi levels associated with each electronic band (i.e., the conduction, valence, and intermediate bands).

Abstract: This paper presents and studies the operation of a tandem of intermediate band solar cells (IBSC).This approach is the one considered feasible in order to effectively increase the number of gaps involved in the operation of this type of solar cell in the sense of making a better use of the solar spectrum. This is so because it can preserve the fact that the intermediate band has to be half-filled with electrons in order to serve to the purpose of both receiving electrons from the valence band as to supply them to the conduction band. The limiting efficiency of the series connected tandem of two IBSC is found to be 72.5 % under maximum concentration, close to the limit of a six-junction tandem solar cell but with the potential advantage of requiring only one tunnel junction.

Abstract: For concentration solar cells, the resistance produced by the metal grid can be, if is not properly designed, the main degradation factor for the cell performance. We present a method for the design of metal grid patterns for concentration solar cells. The method proposed is based on minimising shadow factor for a given working solar concentration. The method analyses the power dissipation, by Joule effect, for the three main contributors to series resistance, which are, the emitter layer resistance, the metal-semiconductor contact resistance and the metallization resistance. Two circular symmetry metal grids patterns are studied, analysing the influence in the presence or not of metal rings in them. The results summarise that the presence of metal rings leads to a better compromise for the series resistance and shadow factor trade off for concentration solar cells.

Abstract: Absorption measurements in the far infrared have been carried out in order to characterize the optical transitions of a new photovoltaic device known as the Intermediate Band Solar Cell (IBSC). These solar cells possess energy levels within the gap of a conventional semiconductor, the so-called intermediate band (IB), which are intended to increase their conversion efficiencies over the limiting value of single gap solar cells. The IB allows two photons of energy lower than the semiconductor bandgap to be absorbed by means of a transition between the valence band (VB) to the intermediate band (IB) and then to the conduction band (CB). As a result, a net electron-hole pair is created in the VB and CB. The existence of this two-photon absorption process has already been shown experimentally in our previous work, proving also that the subsequent production of photocurrent is feasible. However, the detection of photon absorption due to transitions from the IB to the CB has proved difficult sinceit is weak. It is, however, necessary to allow the IBSC to exceed the limiting efficiency of single gap solar cells without violating the second law of thermodynamics. In this work, we have studied the IB-CB transition experimentally by applying Fourier Transform infrared (FTIR) techniques.

Abstract: The intermediate band solar cell concept can be implemented in practice by means of quantum dots (QDs), but a number of challenges must be solved in order to make progress. This paper describes some of them: a) the problem of having the quantum dots embedded in the space charge region, b) the identification of the energy levels involved in the QD system and c) the weak absorption provided by the dots. Regarding the first, the inclusion of semiconductor dumping field layers sandwiching the region containing the stack of QDs is suggested as a way to drive the QDs into a flat band potential region. Concerning “b”, the intermediate band is found to be separated from the conduction band by only 0.2 eV, far from the optimum value. Finally, the weak light absorption provided by the dots is discussed as a factor, together with the low intermediate band to conduction band bandgap that prevents a significant quasi Fermi level split between the IB and the CB under normal illumination conditions.

Abstract: It is known that solar cells with high current, based on materials of low band gap, present low voltages and viceversa. Intermediate band (IB) solar cells have been proposed as a means of increasing the current of solar cells without a substantial decrease of the voltage. The present status of the research on this topic is presented. IB materials and the related solar cells have been manufactured using quantum dots. The basic principles of the IB solar cell have been proved on the basis of these test structures. An accurate modelling of the behaviour of the IB-QD solar cells is presented. Bulk intermediate band materials are also sought using very precise band calculations. Thermodynamic stability of recommended compounds is considered. The extent to which this might lead to non-radiative recombination is also assessed.

Abstract: There is a practical interest in developing semiconductors with levels situated within their band gap while preventing the non-radiative recombination that these levels promote. In this paper, the physical causes of this non-radiative recombination are analyzed and the increase in the density of the impurities responsible for the mid-gap levels to the point of forming bands is suggested as the means of suppressing the recombination. Simple models supporting this recommendation and helping in its quantification are presented.

Abstract: A photovoltaic device based on an intermediate electronic band located within the otherwise conventional band gap of a semiconductor, the so-called intermediate band solar cell IBSC , has been proposed for a better utilization of the solar spectrum. Experimental IBSC devices have been engineered using quantum dot technology, but their practical implementation results in a departure of key underpinning theoretical principles, assumed to describe the operation of the IBSC, away from the ideal. Two principles which are only partially fulfilled are that i the intermediate band should be half filled with electrons and ii the region containing the quantum dots should not be located fully within the junction depletion region. A model to describe the operation of the devices under these nonidealized conditions is presented and is used to interpret experimental results for IBSCs with ten layers of quantum dots. Values for the electron and hole lifetimes, associated with recombination from the conduction band to the intermediate band and from the intermediate band to the valence band 0.5 and 40 ps, respectively are thus obtained.

Abstract: Several alloy semiconductors containing a transition metal atom of the type MGa4X3 with X=As or P and M=Sc or Ti have been found previously to present an isolated partially filled intermediate band within the usual band gap of the host semiconductors and have been proposed as highly efficient photovoltaic materials. In this paper, we carry out an ab initio investigation of band structures and electronic properties for the more chemically stable TixGa1−xP compound as a candidate for isolated intermediate band formation. We have calculated the electronic structures using self-consistent density functional theory method in both local density approximation and generalized gradient approximation GGA approaches and compared the GGA results with those obtained with the exact exchange method that we have implemented in the code SIESTA. Using spinpolarized localized wave functions to represent the valence electrons states and nonlocal pseudopotentials for the core electrons, we have also studied in detail the TixGa1−xP compounds at different dilution levels of the Ti transition metal atom x=3.125%, 6.25%, 12.5%, 25.0% and for two cubic and tetragonal different crystal cells. Results at the different dilutions show in all cases a fully spin-polarized structure and, except for the case of immediate Ti neighbors, indicate a rather small spin coupling between Ti atoms and confirm the presence of the isolated narrow partially filled intermediate band for this compound. They also show the higher suitability of isotropic crystal structures for obtaining in these materials the intermediate band with the desired small band width.

Abstract: Density Functional Theory (DFT) calculations at the GGA level have been carried out for Ti-substituted chalcopyrite-type CuGaS2, as it might constitute an intermediate band material of the kind that has been proposed to lead to enhanced efficiency photovoltaic cells. According to these calculations an intermediate band appears when Ti substitutes Ga at a 25% level in this structure, resulting in a magnetic halfmetallic compound. This intermediate band slightly overlaps the conduction band and, when a higher accuracy calculation approach like the introduction of a Hubbard type empirical correction is used (GGA + U method), it splits leaving a filled narrow band, well isolated inside the band gap. Considering the nanocrystalline form in which these chalcopyrite-type compounds are used in solar cells, an assessment of the effects of a small crystal size in this system have been carried out with a slab model. In this calculation a decreased bandgap width is observed, which can be as a result of surface termination effects.

Abstract: Results of quantum calculations in M-doped chalcopyrite Cu4MGa3S8 (with M=Ti, V, Cr or Mn) are evaluated. The purpose of this work is the quest of a compound which possesses an isolated narrow partiallyfilled electronic band sited into the host semiconductor bandgap. The aforementioned material could be useful for designing novel solar cells with very high efficiency. Density Functional Theory calculations in the spinpolarized GGA approach have been carried out in all cases for obtain band dispersion diagrams and densities electronic states. For the systems having Cr and Ti as dopants, where the results reveal promising features, an advanced DFT+U formalism has been used to ascertain their properties with higher certainty. Finally, after having reasoned that Cu4TiGa3S8 has the desired features, a prediction of its energetic feasibility has been formulated.

Abstract: We present intermediate-band solar cells manufactured using quantum dot technology that show for the first time the production of photocurrent when two sub band-gap energy photons are absorbed simultaneously. One photon produces an optical transition from the intermediate-band to the conduction band while the second pumps an electron from the valence band to the intermediate-band. The detection of this two photon absorption process is essential to verify the principles of operation of the intermediate-band solar cell. The phenomenon is the cornerstone physical principle that ultimately allows the production of photocurrent in a solar cell by below band gap photon absorption, without degradation of its output voltage.

Abstract: The metallic intermediate band solar cell (MIBCELL) concept has been researched for the last three years within the 5th Framework Program of the European Commission. This research pursued an ambitious objective: to put this concept into practice and thus approach a long term goal of electricity produced by a cell at a cost of less than 0.5€/Wp. As a result of this work, intermediate band (IB) material candidates have been identified and MIBCELL prototypes have been produced using quantum dot technology. Low cost manufacturing approaches, based on the use of nanoporous materials have also been researched. The quantum dot approach has provided experimental evidence of the performance of the cells under MIBCELL operating principles. The theory underlying the MIBCELL concept is also now better understood.
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Abstract: A new kind of photovoltaic material, Intermediate Band Material (MIB), has been proposed in previous works. This new photovoltaic materials is principally characterized by an intermediate band, isolated from the valence and conduction bands of the host semiconductors and is partially occupied. Therefore, this material can absorb photons with a lower energy than the bandgap energy of the original host semiconductor, and thus increasing significantly the limiting efficiency of conventional solar cells. However, although the operation of this solar cell has been proposed, it is necessary to use some method to be able to propose a material with this property. In this work, a theoretical study of the electronic and optoelectronic properties using quantum mechanics calculations is presented.
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Abstract: The purpose of this work is to study the so-called phonon bottleneck effect —the reduction in the efficiency of electron de-excitation via the emission of phonon(s)— and the photon absorption fundamentals in the quantum dot intermediate-band solar cell, which aims to exploit the absorption of sub-bandgap photons without degrading the cell voltage. This could be achieved by means of a self-ordered quantum dot superlattice that would create an electron intermediate band within the gap of the host material, which is sandwiched between two n and p conventional semiconductor emitters. When operating, the phonon bottleneck effect would keep the electron gasses in the conduction and intermediate bands completely separated and three electron gasses are allowed. The proper design of the QD superlattice aims to take advantage of the phonon bottleneck effect to become the radiative recombination dominant on the one hand, and to achieve strong photon absorption on the other.
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Abstract: The intermediate band solar cell (IBSC) has the potential of achieving 63.2 % efficiency under maximum concentrated sunlight. This efficiency relies on a material with three bands: a valence band, an intermediate metallic band and a conduction band. In order to achieve high efficiencies the Fermi level of the intermediate band (IB) must be well within the intermediate band. This ensures both a supply of electrons capable of photon induced transition to the conduction band as well as a large population of holes that allow electrons to transition from the valence band to the intermediate band. The use of quantum dot technology may be used to implement an IBSC by positioning the quantum dots close enough and periodically enough so that their wave functions couple to create an intermediate band. The effective half-filling of the IB could then be achieved by introducing some doping. However, the introduction of dopants, modifies the former energy spectra with respect to an undoped quantum dot. This paper explores the energy spectrum of the quantum dot system when an impurity is introduced in the center of a quantum dot. Results show that the inclusion of this impurity allows both a proper IBSC band structure and a Fermi level positioning.
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Abstract: The objective of this work is to present a novel monochromal power illumination system based on the combination of several emitting diodes arranged in a hemispherical cavity, and the use of a pulsed current source. A detailed methodology of design and optimization of the optical system, by taking into account both the electro-optical properties of the emitting diodes and the geometry of the cavity, is described. The performance of two illumination devices has been tested through the measurement of the Isc-Voc curve of some concentrator GaAs solar cells. The current density measured in some devices with our monochromatic source is comparable to that obtained with a concentration level of 150 to 200 suns AM1.5D.

Abstract: Photon Recycling can be briefly described as a process involving the reabsorption of photons emitted in radiative recombination process. It affects the operation and design of direct-gap solar cells, as it has been theoretically and experimentally proved in different ways. Many of its effects can be described in terms of an effective radiative coefficient B which becomes reduced, non-constant along the device and dependent on the working conditions and the structure of the cell. In this paper, we deepen knowledge in the non-linear dependence of photon recycling on the wavelength and the intensity of the light exciting the device. To verify such dependences, dark I-V and illumination Isc-Voc curves of some GaAs solar cells, measured under high intensity monochromatic light, are compared. Experimental results show that photon recycling introduces noticeable deviations between dark and illuminated curves at different wavelengths. These deviations vary at different voltage values, and illumination curves can overpass or underpass dark I-V curve at different points.
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Abstract: This work introduces the five lines of research that the FULLSPECTRUM project is pursuing. Sponsored by the European Commission under the Sixth Framework Programme, FULLSPECTRUM aims to make better use of the solar spectrum than conventional single-gap cells currently do. The aforementioned lines of research are: 1) multi-junction solar cells, 2) solar thermophotovoltaic converters, 3) intermediate band materials and cells, 4) molecular-based concepts, and 5) novel, non-imaging optic techniques for sunlight concentration and assembling procedures, as well as normative related work. Some of the photovoltaic concepts involved are completely novel requiring a profound, basic scientific research and innovative technological approach. Others, such as multi-junction cells, have already been proven scientifically and probably just need further technological development. This work summarises the efforts that FULLSPECTRUM will be making during the next five years towards a more efficient generation of electricity and at a lower and competitive cost.

Abstract: A general model to describe the operation of intermediate band solar cells ~IBSCs!, incorporating a significant number of physical effects such as radiative coupling between bands, and impact ionization and Auger recombination mechanisms, is presented in equivalent circuit form. The model is applied to IBSC prototypes fabricated from InAs quantum dots structures to determine the value of the circuit elements involved. The analysis shows evidence of splitting between the conduction and intermediate band quasi-Fermi levels, one of the fundamental working hypotheses on which operation of the IBSC depends. The model is also used to discuss the limitations and potential of this type of cell.

Abstract: Intermediate band solar cells are characterized by the existence of a collection of energy levels in the middle of the otherwise conventional semiconductor band gap. According to the standard Shockley-Read-Hall recombination theory, the states corresponding to these energy levels behave as nonradiative recombination centers and, therefore, are detrimental to solar cell performance. Nevertheless, the theory of the intermediate band solar cells predicts an enhancement of the solar cell efficiency well above the limiting efficiency of single gap solar cells (63.2% vs. 40.7%) when these levels exist. This paper clarifies the reasons.

Abstract: The purpose of this paper is to present to the Russian scientific community a research program that, with the cooperation of one of its institutional members, the Ioffe Physicotechnical Institute, has been presented and sponsored by the European Commission (EC) in order to provide a long-term basis for the development of the photovoltaic conversion of solar energy. This program constitutes what in the EC is known as an integrated project: it is called FULLSPECTRUM for short, as in the title of this paper, and involves 19 research centers in eight different countries with a grant of 8.4 million euros for five years.

Abstract: A half-metallic isolated band in the band-gap of GaAs and GaP semiconductors has been found for Ti and Sc transition metal impurities and proposed as highly-efficient photovoltaic materials. In this paper, we have investigated by first principle calculations, the spin polarized and non-polarized dispersion band structures and lattice constants of Ga3As4Ti and Ga4As3Ti alloy semiconductor compounds. We have carried out a comparative study of these compounds in order to identify the basic features of the isolated intermediate band formation in the semiconductor band-gap. We use an ab-initio fully self-consistent density functional theory method in the local density approximation (LDA), with norm-conserving, non-local pseudopotentials for core electrons. To assess the results, we first determined the electronic properties of GaAs and compared them with the experimental results. We find that spin wave functions of the polarized GanAsmTi compounds noticeably modify the nature and properties of the intermediate band that have already shown in the corresponding paramagnetic compounds.

Abstract: Intermediate Band Material (MIB) has been proposed in previous works as a new kind of photovoltaic materials. These materials are principally characterized by an intermediate partially occupied band, isolated from the valence and conduction bands of the host semiconductor. This material have a theoretical efficiency greater than the conventional solar cells because of can absorb photons having lower energy than the bandgap of the original host semiconductor. However, although the operation of this solar cell has been described, is necessary to use some method to be able to propose some material having this properties. In this work, a theoretical study of the electronic and optoelectronic properties using quantum mechanics calculations is presented.

Abstract: We have calculated and predicted the phonon dispersion and phonon density of states of GanAsmTi and GanPmTi half-metallic semiconductor compounds using an eight-atom simple cubic cell structure. We use the ab-initio density functional frozen phonon method in the LDA and GGA approximation. We have first determined the phonon spectra of GaAs and GaP and found very good agreement with experimental results. We find that i) in all GanAsmTi and GanPmTi cases studied, the acoustic modes could be easily identified with the phonon modes of GaAs and GaP respectively, and ii) additional Ti high frequency optical modes appear well separated in all the Brillouin zone for Ga3As4Ti and Ga3P4Ti semiconductor compounds.

Abstract: A new kind of photovoltaic material, Intermediate Band Material (IMB), has been proposed in previous works. This new photovoltaic materials is principally characterized by an intermediate band, isolated from the valence and conduction bands of the host semiconductors and is partially occupied. Therefore, this material can absorb photons with a lower energy than the bandgap energy of the original host semiconductor, and thus increasing significantly the limiting efficiency of conventional solar cells. However, although the operation of this solar cell has been proposed, it is necessary to use some method to be able to propose a material with this property. In this work, a theoretical study of the electronic and optoelectronic properties using quantum mechanics calculations is presented.

Abstract: 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 scienti?cally proven but not yet developed and (2) by trying to prove new ones in the search for a breakthrough in photovoltaic (PV) technology. More speci?c 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% ef?ciency will be developed using lower cost substrates and high light concentration (up or above 1000 suns). TPV is a concept with a theoretically high ef?ciency limit because the entire energy ofall 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 ofan IB. Speci?c IB materials will be sought by direct synthesis suggested by material-band calculations and using nanotechnology in quantum dot (QD) IBCs. In the development ofthe MBC, topics such as the development oftwo-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 con?ned by photonic crystals, will be particularly addressed. MFG include optoelectronic assembling techniques and coupling oflight to cells with new-optic miniconcentrators.