Elisa Velasco Fernandez - Academia.edu (original) (raw)

Papers by Elisa Velasco Fernandez

The intermediate band solar cell (IBSC) is a solar cell that, in order to increase its efficiency... more The intermediate band solar cell (IBSC) is a solar cell that, in order to increase its efficiency over that of single gap solar cells, takes advantage of the absorption of below-bandgap energy photons by means of an intermediate band (IB) located in the semiconductor bandgap. For this process to improve the solar cell performance, the belowbandgap photon absorption has to be effective and the IB cannot limit the open-circuit voltage of the cell. In this paper we provide a guide to the new researcher interested in the idea in order he can quickly become familiar with the concept and updated with the most relevant experimental results.

Intermediate band solar cells (IBSCs) are a new kind of devices capable of surpassing the Shockle... more Intermediate band solar cells (IBSCs) are a new kind of devices capable of surpassing the Shockley Queisser efficiency limit for conventional solar cells. This novel technology requires the use of a new type of material named intermediate band (IB) material which makes a better use of the solar spectrum thanks to the existence of a collection of electronic levels within the band gap of the semiconductor. Quantum Dots (QDs) remain as a feasible technology to implement IB materials. InAs/GaAs QD-IBSCs were manufactured in order to test the validity of the concept, although their real size and shape are far from the optimum. This causes extra electron levels to appear within the nanostructure confining potential, degrading the performance of the device. In this paper, the effect of these extra levels will be studied through a multiple level IBSC model based on the detailed balance, but modified so a term accounting for the non-radioactive recombination (NRR) is also included. The model...

In order to surpass the efficiency limit of single gap solar cells, intermediate band solar cells... more In order to surpass the efficiency limit of single gap solar cells, intermediate band solar cells (IBSC) have to fulfill two requirements: the production of extra photocurrent by absorption of sub-bandgap photons in electronic transitions involving the intermediate band (IB) and the preservation of a high output voltage, not limited by the existence of this band. This work presents experimental evidence of the production of electron-hole pairs by absorption of two sub-bandgap photons in IBSC prototypes fabricated with InAs/GaAs QD material. The experiments were carried out at low temperatures using a specifically designed modulated photocurrent measurement set-up with two light beams. The results are analysed with the help of a simple equivalent circuit model. This analysis is also used to highlight the relevance of the two-photon mechanism demonstrated in the experiment. It is discussed that, although the absorption of sub-bandgap photons in one of the IB transitions and subsequent...

In this paper we present an optical characterization for quantum dot intermediate band solar cell... more In this paper we present an optical characterization for quantum dot intermediate band solar cells (QDIBSCs). The cells were developed by growing a stack of ten InAs/GaAs QDs layers between p and n doped GaAs conventional emitters. Electroluminescence, EL, photoreflectance, PR, and transmission electron microscopy, TEM, were applied to the samples in order to test and characterize them optically. The results, derived from the application of the different techniques, showed a good correlation. TEM images revealed a very good structural quality of the QDs, which seem to evolve in shape-strain from the bottom to the top of the stack. Corresponding to the quality observed by TEM, strong signals from EL and PR resolved unambiguously the energy band diagram of the QDIBSCs. By fitting PR data we were able to indentify the coexistence of bands and discrete energy levels coming from the IB material. The PR data evidenced also a strong electric field over the dots, attributed to the space cha...

The use of half-filled intermediate band materials has been proposed as a means to implement sola... more The use of half-filled intermediate band materials has been proposed as a means to implement solar cells with efficiency exceeding that of single gap solar cells. An intermediate band can be regarded, at first, as a mere collection of energy levels within the semiconductor bandgap. However, its recombination properties are expected to be different from those traditionally attributed to deep levels. Hence, while deep centers behave mainly as non-radiative recombination centers, the IB is expected to exhibit negligible non-radiative recombination. It is the purpose of this work to study these phenomena by exploiting computational models based on ab-initio calculations.

The high efficiency limit of the intermediate band solar cell (IBSC) corresponds to the case of u... more The high efficiency limit of the intermediate band solar cell (IBSC) corresponds to the case of using as intermediate band (IB) host material a semiconductor with gap in the range of 2 eV. Traditional photovoltaic materials, such as Si and GaAs, are not appropriate to produce IB devices because their gaps are too narrow. To overcome this problem, we propose the implementation of a multi-junction device consisting of an IBSC combined with a single gap cell. We calculate the efficiency limits using the detailed balance model and conclude that they are very high (> 60% under maximum concentration) for any fundamental bandgap from 0.7 to 3.6 eV in the IBSC inserted in the tandem. In particular, the two-terminal tandem of a GaAs-based IBSC current matched to an optimized AlGaAs top cell has an efficiency limit as high as 64%.

The intermediate band solar cell (IBSC), the multiple exciton generation solar cell (MEGSC) and t... more The intermediate band solar cell (IBSC), the multiple exciton generation solar cell (MEGSC) and the hot carrier solar cell (HCSC) are three novel concepts in photovoltaics which aim to achieve high efficiency devices. In this paper we assess to what extent their physical principles of operation have been experimentally verified. It is found that there is experimental evidence supporting the underlying theory for all three.

Clinical microbiology and infection : the official publication of the European Society of Clinical Microbiology and Infectious Diseases, Jan 27, 2017

The aim of this study was to determine the predictive capacity of response at treatment week (TW)... more The aim of this study was to determine the predictive capacity of response at treatment week (TW) 4 for the achievement of sustained virologic response 12 weeks after the scheduled end of therapy date (SVR12) to treatment against hepatitis C virus (HCV) genotype (GT) 3-infection with all-oral direct-acting antiviral (DAA)-based regimens. From a prospective multicohort study, HCV GT3-infected patients who completed a course of currently recommended DAA-based therapy at 33 Spanish hospitals and who had reached SVR12 evaluation timepoint were selected. TW4 HCV-RNA levels were categorized in target not detected (TND), below the lower limit of quantitation (LLOQTD) and ≥LLOQ. A total of 123 patients were included, 86 (70%) subjects received sofosbuvir/daclatasvir+/-ribavirin, 27 (22%) sofosbuvir/ledipasvir/ribavirin and 10 (8.1%) sofosbuvir/ribavirin, respectively. One-hundred and fourteen out of 123 (92.7%) patients presented SVR12 in an on-treatment approach, while 9/123 (7.3%) subject...

Actas Del I Coloquio Sobre Geografia De La Rioja Geografia Fisica Vol 2 1985 Isbn 84 00 05945 X Pags 127 138, 1985

Miradas a La Investigacion Arquitectonica Construccion Gestion Tecnologia I Congreso Internacional Sobre Investigacion En Construccion Y Tecnologia Arquitectonicas 11 13 Jun 2014 Escuela Tecnica Superior De Arquitectura De Madrid, 2014

La investigación que aquí se presenta trata de establecer el potencial de las herramientas simpli... more La investigación que aquí se presenta trata de establecer el potencial de las herramientas simplificadas de certificación energética de edificios existentes, desarrolladas como fruto de la Directiva 2002/91/CE y su trasposición parcial en el RD 235/2013, para obtener mejoras en la calificación energética del edificio residencial, a partir de la incorporación de mejoras de diseño pasivo.

De la misma manera que se ha hecho para los buques Ro-Pax se incluye un gráfico en el que se obse... more De la misma manera que se ha hecho para los buques Ro-Pax se incluye un gráfico en el que se observa el aumento del tamaño de los buques dentro de una misma naviera salvo excepciones.

Thin Solid Films, 2008

We have demonstrated recently that two below bandgap energy photons can lead to the creation of o... more We have demonstrated recently that two below bandgap energy photons can lead to the creation of one electron-hole pair in a quantum-dot intermediate band solar cell (QD-IBSC). To be effective, the devices used in the experiments were designed to a) half-fill the intermediate band with electrons; b) to allocate the quantum dots in a flat-band potential region, and c) to prevent tunnelling from the n emitter into the intermediate band. QD-IBSCs have also shown degradation in their open-circuit voltage when compared with their counterparts without quantum dots. This loss is due to the presence of the intermediate band (IB) together with the incapacity of the quantum dots to absorb sufficient below bandgap light as to contribute significantly to the photogenerated current. It is predicted, nevertheless, that this voltage loss will diminish if concentration light is used leading to devices with efficiency higher than single gap solar cells. A circuit model that includes additional recombination levels to the ones introduced by the IB is described to support this discussion.

European Materials Research Society Conference Symposium: Advanced Inorganic Materials and Concepts For Photovoltaics, 2011

The intermediate band solar cell (IBSC) is a photovoltaic device with a theoretical conversion ef... more The intermediate band solar cell (IBSC) is a photovoltaic device with a theoretical conversion efficiency limit of 63.2%. In recent years many attempts have been made to fabricate an intermediate band material which behaves as the theory states. One characteristic feature of an IBSC is its luminescence spectrum. In this work the temperature dependence of the photoluminescence (PL) and electroluminescence (EL) spectra of InAs/GaAs QD-IBSCs together with their reference cell have been studied. It is shown that EL measurements provide more reliable information about the behaviour of the IB material inside the IBSC structure than PL measurements. At low temperatures, the EL spectra are consistent with the quasi-Fermi level splits described by the IBSC model, whereas at room temperature they are not. This result is in agreement with previously reported analysis of the quantum efficiency of the solar cells.

Revista de la Escuela de Medicina Legal, 2012

Hiperostosis frontal interna E. Ruiz-Tagle Fernández 13 Fecha de recepción del artículo Febrero d... more Hiperostosis frontal interna E. Ruiz-Tagle Fernández 13 Fecha de recepción del artículo Febrero de 2012 Fecha de aceptación del artículo Abril de 2012

Nano letters, Jan 14, 2015

We demonstrate a new class of semiconductor device: the optically triggered infrared photodetecto... more We demonstrate a new class of semiconductor device: the optically triggered infrared photodetector (OTIP). This photodetector is based on a new physical principle that allows the detection of infrared light to be switched ON and OFF by means of an external light. Our experimental device, fabricated using InAs/AlGaAs quantum-dot technology, demonstrates normal incidence infrared detection in the 2-6 μm range. The detection is optically triggered by a 590 nm light-emitting diode. Furthermore, the detection gain is achieved in our device without an increase of the noise level. The novel characteristics of OTIPs open up new possibilities for third generation infrared imaging systems ( Rogalski , A. ; Antoszewski , J. ; Faraone , L. J. Appl. Phys. 2009 , 105 ( 9 ), 091101 ).

Thin Solid Films, 2009

The feasibility of implementing the intermediate-band (IB) concept into a relevant thin-film tech... more The feasibility of implementing the intermediate-band (IB) concept into a relevant thin-film technology has been assessed. Compounds belonging to the group of I-III-VI2 chalcopyrites, currently used as absorbers in the leading thin-film technology, appear as promising candidates for the realization of IB-devices. In this paper we first analyze the expected performance of such a thin-film intermedíate band solar cell (TF-IBSC) by considering different levéis of idealization. In the second part of the paper some issues relevant for the practical realization of IBs in chalcopyrites are discussed and impurities acting as potential IB-precursors in the chalcopyrite sulfide host identified.

Solar Energy Materials and Solar Cells, 2012

With the purpose of assessing the absorption coefficients of quantum dot solar cells, symmetry co... more With the purpose of assessing the absorption coefficients of quantum dot solar cells, symmetry considerations are introduced into a Hamiltonian whose eigenvalues are empirical. In this way, the proper transformation from the Hamiltonian's diagonalized form to the form that relates it with rpoint exact solutions through k.p envelope functions is built accounting for symmetry. Forbidden transitions are thus determined reducing the calculation burden and permitting a thoughtful discussion of the possible options for this transformation. The agreement of this model with the measured external quantum efficiency of a prototype solar cell is found to be excellent.

Solar Energy Materials and Solar Cells, 2012

The intermediate band solar cell (IBSC) is based on a novel photovoltaic concept and has a limiti... more The intermediate band solar cell (IBSC) is based on a novel photovoltaic concept and has a limiting efficiency of 63.2%, which compares favorably with the 40.7% efficiency of a conventional, single junction solar cell. It is characterized by a material hosting a collection of energy levels within its bandgap, allowing the cell to exploit photons with sub-bandgap energies in a two-step absorption process, thus improving the utilization of the solar spectrum. However, these intermediate levels are often regarded as an inherent source of supplementary recombination, although this harmful effect can in theory be counteracted by the use of concentrated light. We present here a novel, low-temperature characterization technique using concentrated light that reveals how the initially enhanced recombination in the IBSC is reduced so that its open-circuit voltage is completely recovered and reaches that of a conventional solar cell.

Progress in Photovoltaics: Research and Applications, 2012

ABSTRACTIn this paper, we present calculations of the absorption coefficient for transitions betw... more ABSTRACTIn this paper, we present calculations of the absorption coefficient for transitions between the bound states of quantum dots grown within a semiconductor and the extended states of the conduction band. For completeness, transitions among bound states are also presented. In the separation of variables, single band k·p model is used in which most elements may be expressed analytically. The analytical formulae are collected in the appendix of this paper. It is concluded that the transitions are strong enough to provide a quick path to the conduction band for electrons pumped from the valence to the intermediate band. Copyright © 2012 John Wiley & Sons, Ltd.

Journal of Materials Chemistry, 2012

An intermediate band solar cell is a novel photovoltaic device with the potential to exceed the e... more An intermediate band solar cell is a novel photovoltaic device with the potential to exceed the efficiency of single gap solar cells. In the last few years, several prototypes of these cells, based on different technologies, have been reported. Since these devices do not yet perform ideally, it is sometimes difficult to determine to what extent they operate as actual intermediate band solar cells. In this article we provide the essential guidelines to interpret conventional experimental results (current-voltage plots, quantum efficiency, etc.) associated with their characterization. A correct interpretation of these results is essential in order not to mislead the research efforts directed towards the improvement of the efficiency of these devices.

The intermediate band solar cell (IBSC) is a solar cell that, in order to increase its efficiency... more The intermediate band solar cell (IBSC) is a solar cell that, in order to increase its efficiency over that of single gap solar cells, takes advantage of the absorption of below-bandgap energy photons by means of an intermediate band (IB) located in the semiconductor bandgap. For this process to improve the solar cell performance, the belowbandgap photon absorption has to be effective and the IB cannot limit the open-circuit voltage of the cell. In this paper we provide a guide to the new researcher interested in the idea in order he can quickly become familiar with the concept and updated with the most relevant experimental results.

Intermediate band solar cells (IBSCs) are a new kind of devices capable of surpassing the Shockle... more Intermediate band solar cells (IBSCs) are a new kind of devices capable of surpassing the Shockley Queisser efficiency limit for conventional solar cells. This novel technology requires the use of a new type of material named intermediate band (IB) material which makes a better use of the solar spectrum thanks to the existence of a collection of electronic levels within the band gap of the semiconductor. Quantum Dots (QDs) remain as a feasible technology to implement IB materials. InAs/GaAs QD-IBSCs were manufactured in order to test the validity of the concept, although their real size and shape are far from the optimum. This causes extra electron levels to appear within the nanostructure confining potential, degrading the performance of the device. In this paper, the effect of these extra levels will be studied through a multiple level IBSC model based on the detailed balance, but modified so a term accounting for the non-radioactive recombination (NRR) is also included. The model...

In order to surpass the efficiency limit of single gap solar cells, intermediate band solar cells... more In order to surpass the efficiency limit of single gap solar cells, intermediate band solar cells (IBSC) have to fulfill two requirements: the production of extra photocurrent by absorption of sub-bandgap photons in electronic transitions involving the intermediate band (IB) and the preservation of a high output voltage, not limited by the existence of this band. This work presents experimental evidence of the production of electron-hole pairs by absorption of two sub-bandgap photons in IBSC prototypes fabricated with InAs/GaAs QD material. The experiments were carried out at low temperatures using a specifically designed modulated photocurrent measurement set-up with two light beams. The results are analysed with the help of a simple equivalent circuit model. This analysis is also used to highlight the relevance of the two-photon mechanism demonstrated in the experiment. It is discussed that, although the absorption of sub-bandgap photons in one of the IB transitions and subsequent...

In this paper we present an optical characterization for quantum dot intermediate band solar cell... more In this paper we present an optical characterization for quantum dot intermediate band solar cells (QDIBSCs). The cells were developed by growing a stack of ten InAs/GaAs QDs layers between p and n doped GaAs conventional emitters. Electroluminescence, EL, photoreflectance, PR, and transmission electron microscopy, TEM, were applied to the samples in order to test and characterize them optically. The results, derived from the application of the different techniques, showed a good correlation. TEM images revealed a very good structural quality of the QDs, which seem to evolve in shape-strain from the bottom to the top of the stack. Corresponding to the quality observed by TEM, strong signals from EL and PR resolved unambiguously the energy band diagram of the QDIBSCs. By fitting PR data we were able to indentify the coexistence of bands and discrete energy levels coming from the IB material. The PR data evidenced also a strong electric field over the dots, attributed to the space cha...

The use of half-filled intermediate band materials has been proposed as a means to implement sola... more The use of half-filled intermediate band materials has been proposed as a means to implement solar cells with efficiency exceeding that of single gap solar cells. An intermediate band can be regarded, at first, as a mere collection of energy levels within the semiconductor bandgap. However, its recombination properties are expected to be different from those traditionally attributed to deep levels. Hence, while deep centers behave mainly as non-radiative recombination centers, the IB is expected to exhibit negligible non-radiative recombination. It is the purpose of this work to study these phenomena by exploiting computational models based on ab-initio calculations.

The high efficiency limit of the intermediate band solar cell (IBSC) corresponds to the case of u... more The high efficiency limit of the intermediate band solar cell (IBSC) corresponds to the case of using as intermediate band (IB) host material a semiconductor with gap in the range of 2 eV. Traditional photovoltaic materials, such as Si and GaAs, are not appropriate to produce IB devices because their gaps are too narrow. To overcome this problem, we propose the implementation of a multi-junction device consisting of an IBSC combined with a single gap cell. We calculate the efficiency limits using the detailed balance model and conclude that they are very high (> 60% under maximum concentration) for any fundamental bandgap from 0.7 to 3.6 eV in the IBSC inserted in the tandem. In particular, the two-terminal tandem of a GaAs-based IBSC current matched to an optimized AlGaAs top cell has an efficiency limit as high as 64%.

The intermediate band solar cell (IBSC), the multiple exciton generation solar cell (MEGSC) and t... more The intermediate band solar cell (IBSC), the multiple exciton generation solar cell (MEGSC) and the hot carrier solar cell (HCSC) are three novel concepts in photovoltaics which aim to achieve high efficiency devices. In this paper we assess to what extent their physical principles of operation have been experimentally verified. It is found that there is experimental evidence supporting the underlying theory for all three.

Clinical microbiology and infection : the official publication of the European Society of Clinical Microbiology and Infectious Diseases, Jan 27, 2017

The aim of this study was to determine the predictive capacity of response at treatment week (TW)... more The aim of this study was to determine the predictive capacity of response at treatment week (TW) 4 for the achievement of sustained virologic response 12 weeks after the scheduled end of therapy date (SVR12) to treatment against hepatitis C virus (HCV) genotype (GT) 3-infection with all-oral direct-acting antiviral (DAA)-based regimens. From a prospective multicohort study, HCV GT3-infected patients who completed a course of currently recommended DAA-based therapy at 33 Spanish hospitals and who had reached SVR12 evaluation timepoint were selected. TW4 HCV-RNA levels were categorized in target not detected (TND), below the lower limit of quantitation (LLOQTD) and ≥LLOQ. A total of 123 patients were included, 86 (70%) subjects received sofosbuvir/daclatasvir+/-ribavirin, 27 (22%) sofosbuvir/ledipasvir/ribavirin and 10 (8.1%) sofosbuvir/ribavirin, respectively. One-hundred and fourteen out of 123 (92.7%) patients presented SVR12 in an on-treatment approach, while 9/123 (7.3%) subject...

Actas Del I Coloquio Sobre Geografia De La Rioja Geografia Fisica Vol 2 1985 Isbn 84 00 05945 X Pags 127 138, 1985

Miradas a La Investigacion Arquitectonica Construccion Gestion Tecnologia I Congreso Internacional Sobre Investigacion En Construccion Y Tecnologia Arquitectonicas 11 13 Jun 2014 Escuela Tecnica Superior De Arquitectura De Madrid, 2014

La investigación que aquí se presenta trata de establecer el potencial de las herramientas simpli... more La investigación que aquí se presenta trata de establecer el potencial de las herramientas simplificadas de certificación energética de edificios existentes, desarrolladas como fruto de la Directiva 2002/91/CE y su trasposición parcial en el RD 235/2013, para obtener mejoras en la calificación energética del edificio residencial, a partir de la incorporación de mejoras de diseño pasivo.

De la misma manera que se ha hecho para los buques Ro-Pax se incluye un gráfico en el que se obse... more De la misma manera que se ha hecho para los buques Ro-Pax se incluye un gráfico en el que se observa el aumento del tamaño de los buques dentro de una misma naviera salvo excepciones.

Thin Solid Films, 2008

We have demonstrated recently that two below bandgap energy photons can lead to the creation of o... more We have demonstrated recently that two below bandgap energy photons can lead to the creation of one electron-hole pair in a quantum-dot intermediate band solar cell (QD-IBSC). To be effective, the devices used in the experiments were designed to a) half-fill the intermediate band with electrons; b) to allocate the quantum dots in a flat-band potential region, and c) to prevent tunnelling from the n emitter into the intermediate band. QD-IBSCs have also shown degradation in their open-circuit voltage when compared with their counterparts without quantum dots. This loss is due to the presence of the intermediate band (IB) together with the incapacity of the quantum dots to absorb sufficient below bandgap light as to contribute significantly to the photogenerated current. It is predicted, nevertheless, that this voltage loss will diminish if concentration light is used leading to devices with efficiency higher than single gap solar cells. A circuit model that includes additional recombination levels to the ones introduced by the IB is described to support this discussion.

European Materials Research Society Conference Symposium: Advanced Inorganic Materials and Concepts For Photovoltaics, 2011

The intermediate band solar cell (IBSC) is a photovoltaic device with a theoretical conversion ef... more The intermediate band solar cell (IBSC) is a photovoltaic device with a theoretical conversion efficiency limit of 63.2%. In recent years many attempts have been made to fabricate an intermediate band material which behaves as the theory states. One characteristic feature of an IBSC is its luminescence spectrum. In this work the temperature dependence of the photoluminescence (PL) and electroluminescence (EL) spectra of InAs/GaAs QD-IBSCs together with their reference cell have been studied. It is shown that EL measurements provide more reliable information about the behaviour of the IB material inside the IBSC structure than PL measurements. At low temperatures, the EL spectra are consistent with the quasi-Fermi level splits described by the IBSC model, whereas at room temperature they are not. This result is in agreement with previously reported analysis of the quantum efficiency of the solar cells.

Revista de la Escuela de Medicina Legal, 2012

Hiperostosis frontal interna E. Ruiz-Tagle Fernández 13 Fecha de recepción del artículo Febrero d... more Hiperostosis frontal interna E. Ruiz-Tagle Fernández 13 Fecha de recepción del artículo Febrero de 2012 Fecha de aceptación del artículo Abril de 2012

Nano letters, Jan 14, 2015

We demonstrate a new class of semiconductor device: the optically triggered infrared photodetecto... more We demonstrate a new class of semiconductor device: the optically triggered infrared photodetector (OTIP). This photodetector is based on a new physical principle that allows the detection of infrared light to be switched ON and OFF by means of an external light. Our experimental device, fabricated using InAs/AlGaAs quantum-dot technology, demonstrates normal incidence infrared detection in the 2-6 μm range. The detection is optically triggered by a 590 nm light-emitting diode. Furthermore, the detection gain is achieved in our device without an increase of the noise level. The novel characteristics of OTIPs open up new possibilities for third generation infrared imaging systems ( Rogalski , A. ; Antoszewski , J. ; Faraone , L. J. Appl. Phys. 2009 , 105 ( 9 ), 091101 ).

Thin Solid Films, 2009

The feasibility of implementing the intermediate-band (IB) concept into a relevant thin-film tech... more The feasibility of implementing the intermediate-band (IB) concept into a relevant thin-film technology has been assessed. Compounds belonging to the group of I-III-VI2 chalcopyrites, currently used as absorbers in the leading thin-film technology, appear as promising candidates for the realization of IB-devices. In this paper we first analyze the expected performance of such a thin-film intermedíate band solar cell (TF-IBSC) by considering different levéis of idealization. In the second part of the paper some issues relevant for the practical realization of IBs in chalcopyrites are discussed and impurities acting as potential IB-precursors in the chalcopyrite sulfide host identified.

Solar Energy Materials and Solar Cells, 2012

With the purpose of assessing the absorption coefficients of quantum dot solar cells, symmetry co... more With the purpose of assessing the absorption coefficients of quantum dot solar cells, symmetry considerations are introduced into a Hamiltonian whose eigenvalues are empirical. In this way, the proper transformation from the Hamiltonian's diagonalized form to the form that relates it with rpoint exact solutions through k.p envelope functions is built accounting for symmetry. Forbidden transitions are thus determined reducing the calculation burden and permitting a thoughtful discussion of the possible options for this transformation. The agreement of this model with the measured external quantum efficiency of a prototype solar cell is found to be excellent.

Solar Energy Materials and Solar Cells, 2012

The intermediate band solar cell (IBSC) is based on a novel photovoltaic concept and has a limiti... more The intermediate band solar cell (IBSC) is based on a novel photovoltaic concept and has a limiting efficiency of 63.2%, which compares favorably with the 40.7% efficiency of a conventional, single junction solar cell. It is characterized by a material hosting a collection of energy levels within its bandgap, allowing the cell to exploit photons with sub-bandgap energies in a two-step absorption process, thus improving the utilization of the solar spectrum. However, these intermediate levels are often regarded as an inherent source of supplementary recombination, although this harmful effect can in theory be counteracted by the use of concentrated light. We present here a novel, low-temperature characterization technique using concentrated light that reveals how the initially enhanced recombination in the IBSC is reduced so that its open-circuit voltage is completely recovered and reaches that of a conventional solar cell.

Progress in Photovoltaics: Research and Applications, 2012

ABSTRACTIn this paper, we present calculations of the absorption coefficient for transitions betw... more ABSTRACTIn this paper, we present calculations of the absorption coefficient for transitions between the bound states of quantum dots grown within a semiconductor and the extended states of the conduction band. For completeness, transitions among bound states are also presented. In the separation of variables, single band k·p model is used in which most elements may be expressed analytically. The analytical formulae are collected in the appendix of this paper. It is concluded that the transitions are strong enough to provide a quick path to the conduction band for electrons pumped from the valence to the intermediate band. Copyright © 2012 John Wiley & Sons, Ltd.

Journal of Materials Chemistry, 2012

An intermediate band solar cell is a novel photovoltaic device with the potential to exceed the e... more An intermediate band solar cell is a novel photovoltaic device with the potential to exceed the efficiency of single gap solar cells. In the last few years, several prototypes of these cells, based on different technologies, have been reported. Since these devices do not yet perform ideally, it is sometimes difficult to determine to what extent they operate as actual intermediate band solar cells. In this article we provide the essential guidelines to interpret conventional experimental results (current-voltage plots, quantum efficiency, etc.) associated with their characterization. A correct interpretation of these results is essential in order not to mislead the research efforts directed towards the improvement of the efficiency of these devices.