Research on intermediate band solar cells and development of experimental techniques for their characterization under concentrated illumination

Abstract This work is a contribution to the research and development of the intermediate band solar cell (IBSC), a high efficiency photovoltaic concept that features the advantages of both low and high bandgap solar cells. The resemblance with a low bandgap solar cell comes from the fact that the IBSC hosts an electronic energy band -the intermediate band (IB)- within the semiconductor bandgap. This IB allows the collection of sub-bandgap energy photons by means of two-step photon absorption processes, from the valence band (VB) to the IB and from there to the conduction band (CB). The exploitation of these low energy photons implies a more efficient use of the solar spectrum. The resemblance of the IBSC with a high bandgap solar cell is related to the preservation of the voltage: the open-circuit voltage (VOC) of an IBSC is not limited by any of the sub-bandgaps (involving the IB), but only by the fundamental bandgap (defined from the VB to the CB). Nevertheless, the presence of the IB allows new paths for electronic recombination and the performance of the IBSC is degraded at 1 sun operation conditions. A theoretical argument is presented regarding the need for the use of concentrated illumination in order to circumvent the degradation of the voltage derived from the increase in the recombi¬nation. This theory is supported by the experimental verification carried out with our novel characterization technique consisting of the acquisition of photogenerated current (IL)-VOC pairs under low temperature and concentrated light. Besides, at this stage of the IBSC research, several new IB materials are being engineered and our novel character¬ization tool can be very useful to provide feedback on their capability to perform as real IBSCs, verifying or disregarding the fulfillment of the “voltage preservation” principle. An analytical model has also been developed to assess the potential of quantum-dot (QD)-IBSCs. It is based on the calculation of band alignment of III-V alloyed heterojunc-tions, the estimation of the confined energy levels in a QD and the calculation of the de¬tailed balance efficiency. Several potentially useful QD materials have been identified, such as InAs/AlxGa1-xAs, InAs/GaxIn1-xP, InAs1-yNy/AlAsxSb1-x or InAs1-zNz/Alx[GayIn1-y]1-xP. Finally, a model for the analysis of the series resistance of a concentrator solar cell has also been developed to design and fabricate IBSCs adapted to 1,000 suns. Resumen Este trabajo contribuye a la investigación y al desarrollo de la célula solar de banda intermedia (IBSC), un concepto fotovoltaico de alta eficiencia que auna las ventajas de una célula solar de bajo y de alto gap. La IBSC se parece a una célula solar de bajo gap (o banda prohibida) en que la IBSC alberga una banda de energía -la banda intermedia (IB)-en el seno de la banda prohibida. Esta IB permite colectar fotones de energía inferior a la banda prohibida por medio de procesos de absorción de fotones en dos pasos, de la banda de valencia (VB) a la IB y de allí a la banda de conducción (CB). El aprovechamiento de estos fotones de baja energía conlleva un empleo más eficiente del espectro solar. La semejanza antre la IBSC y una célula solar de alto gap está relacionada con la preservación del voltaje: la tensión de circuito abierto (Vbc) de una IBSC no está limitada por ninguna de las fracciones en las que la IB divide a la banda prohibida, sino que está únicamente limitada por el ancho de banda fundamental del semiconductor (definido entre VB y CB). No obstante, la presencia de la IB posibilita nuevos caminos de recombinación electrónica, lo cual degrada el rendimiento de la IBSC a 1 sol. Este trabajo argumenta de forma teórica la necesidad de emplear luz concentrada para evitar compensar el aumento de la recom¬binación de la IBSC y evitar la degradación del voltage. Lo anterior se ha verificado experimentalmente por medio de nuestra novedosa técnica de caracterización consistente en la adquisicin de pares de corriente fotogenerada (IL)-VOG en concentración y a baja temperatura. En esta etapa de la investigación, se están desarrollando nuevos materiales de IB y nuestra herramienta de caracterizacin está siendo empleada para realimentar el proceso de fabricación, comprobando si los materiales tienen capacidad para operar como verdaderas IBSCs por medio de la verificación del principio de preservación del voltaje. También se ha desarrollado un modelo analítico para evaluar el potencial de IBSCs de puntos cuánticos. Dicho modelo está basado en el cálculo del alineamiento de bandas de energía en heterouniones de aleaciones de materiales III-V, en la estimación de la energía de los niveles confinados en un QD y en el cálculo de la eficiencia de balance detallado. Este modelo ha permitido identificar varios materiales de QDs potencialmente útiles como InAs/AlxGai_xAs, InAs/GaxIni_xP, InAsi_yNy/AlAsxSbi_x ó InAsi_zNz/Alx[GayIni_y]i_xP. Finalmente, también se ha desarrollado un modelado teórico para el análisis de la resistencia serie de una célula solar de concentración. Gracias a dicho modelo se han diseñado y fabricado IBSCs adaptadas a 1.000 soles.

Interactions between water and triacylglycerols may explain faster aging rates in stored germplasm at low temperatures

The purpose of this research is to explore the extent and significance of possible interacting factors on the viability of stored germplasm. Our work begins with characterizing the kinetics of TAG and water phase changes in peanut (Arachis hypogaea) and Papaya (Carica papaya) seeds equilibrated to different water contents and stored at temperatures between -5 and -80°C. Water and TAG phase was measured using a Perkin Elmer Differential Scanning Calorimeter. Cytoplasm ultra-structure was visualized without chemical fixatives using low temperature scanning electron microscopy (cryo-SEM) performed with a Zeiss DSN 960 scanning microscope equipped with a Cryotrans CT-1500 cold plate (Oxford, UK).

Research on wide-bandgap intermediate band solar cells and development of experimental techniques for their characterization

El trabajo que ha dado lugar a esta Tesis Doctoral se enmarca en la invesitagación en células solares de banda intermedia (IBSCs, por sus siglas en inglés). Se trata de un nuevo concepto de célula solar que ofrece la posibilidad de alcanzar altas eficiencias de conversión fotovoltaica. Hasta ahora, se han demostrado de manera experimental los fundamentos de operación de las IBSCs; sin embargo, esto tan sólo has sido posible en condicines de baja temperatura. El concepto de banda intermedia (IB, por sus siglas en inglés) exige que haya desacoplamiento térmico entre la IB y las bandas de valencia y conducción (VB and CB, respectivamente, por sus siglas en inglés). Los materiales de IB actuales presentan un acoplamiento térmico demasiado fuerte entre la IB y una de las otras dos bandas, lo cual impide el correcto funcionamiento de las IBSCs a temperatura ambiente. En el caso particular de las IBSCs fabricadas con puntos cuánticos (QDs, por sus siglas en inglés) de InAs/GaAs - a día de hoy, la tecnología de IBSC más estudiada - , se produce un rápido intercambio de portadores entre la IB y la CB, por dos motivos: (1) una banda prohibida estrecha (< 0.2 eV) entre la IB y la CB, E^, y (2) la existencia de niveles electrónicos entre ellas. El motivo (1) implica, a su vez, que la máxima eficiencia alcanzable en estos dispositivos es inferior al límite teórico de la IBSC ideal, en la cual E^ = 0.71 eV. En este contexto, nuestro trabajo se centra en el estudio de IBSCs de alto gap (o banda prohibida) fabricadsas con QDs, o lo que es lo mismo, QD-IBSCs de alto gap. Hemos fabricado e investigado experimentalmente los primeros prototipos de QD-IBSC en los que se utiliza AlGaAs o InGaP para albergar QDs de InAs. En ellos demostramos une distribución de gaps mejorada con respecto al caso de InAs/GaAs. En concreto, hemos medido valores de E^ mayores que 0.4 eV. En los prototipos de InAs/AlGaAs, este incremento de E^ viene acompaado de un incremento, en más de 100 meV, de la energía de activación del escape térmico. Además, nuestros dispositivos de InAs/AlGaAs demuestran conversión a la alza de tensión; es decir, la producción de una tensión de circuito abierto mayor que la energía de los fotones (dividida por la carga del electrón) de un haz monocromático incidente, así como la preservación del voltaje a temperaura ambiente bajo iluminación de luz blanca concentrada. Asimismo, analizamos el potencial para detección infrarroja de los materiales de IB. Presentamos un nuevo concepto de fotodetector de infrarrojos, basado en la IB, que hemos llamado: fotodetector de infrarrojos activado ópticamente (OTIP, por sus siglas en inglés). Nuestro novedoso dispositivo se basa en un nuevo pricipio físico que permite que la detección de luz infrarroja sea conmutable (ON y OFF) mediante iluminación externa. Hemos fabricado un OTIP basado en QDs de InAs/AlGaAs con el que demostramos fotodetección, bajo incidencia normal, en el rango 2-6/xm, activada ópticamente por un diodoe emisor de luz de 590 nm. El estudio teórico del mecanismo de detección asistido por la IB en el OTIP nos lleva a poner en cuestión la asunción de quasi-niveles de Fermi planos en la zona de carga del espacio de una célula solar. Apoyados por simuaciones a nivel de dispositivo, demostramos y explicamos por qué esta asunción no es válida en condiciones de corto-circuito e iluminación. También llevamos a cabo estudios experimentales en QD-IBSCs de InAs/AlGaAs con la finalidad de ampliar el conocimiento sobre algunos aspectos de estos dispositivos que no han sido tratados aun. En particular, analizamos el impacto que tiene el uso de capas de disminución de campo (FDLs, por sus siglas en inglés), demostrando su eficiencia para evitar el escape por túnel de portadores desde el QD al material anfitrión. Analizamos la relación existente entre el escape por túnel y la preservación del voltaje, y proponemos las medidas de eficiencia cuántica en función de la tensión como una herramienta útil para evaluar la limitación del voltaje relacionada con el túnel en QD-IBSCs. Además, realizamos medidas de luminiscencia en función de la temperatura en muestras de InAs/GaAs y verificamos que los resltados obtenidos están en coherencia con la separación de los quasi-niveles de Fermi de la IB y la CB a baja temperatura. Con objeto de contribuir a la capacidad de fabricación y caracterización del Instituto de Energía Solar de la Universidad Politécnica de Madrid (IES-UPM), hemos participado en la instalación y puesta en marcha de un reactor de epitaxia de haz molecular (MBE, por sus siglas en inglés) y el desarrollo de un equipo de caracterización de foto y electroluminiscencia. Utilizando dicho reactor MBE, hemos crecido, y posteriormente caracterizado, la primera QD-IBSC enteramente fabricada en el IES-UPM. ABSTRACT The constituent work of this Thesis is framed in the research on intermediate band solar cells (IBSCs). This concept offers the possibility of achieving devices with high photovoltaic-conversion efficiency. Up to now, the fundamentals of operation of IBSCs have been demonstrated experimentally; however, this has only been possible at low temperatures. The intermediate band (IB) concept demands thermal decoupling between the IB and the valence and conduction bands. Stateof- the-art IB materials exhibit a too strong thermal coupling between the IB and one of the other two bands, which prevents the proper operation of IBSCs at room temperature. In the particular case of InAs/GaAs quantum-dot (QD) IBSCs - as of today, the most widely studied IBSC technology - , there exist fast thermal carrier exchange between the IB and the conduction band (CB), for two reasons: (1) a narrow (< 0.2 eV) energy gap between the IB and the CB, EL, and (2) the existence of multiple electronic levels between them. Reason (1) also implies that maximum achievable efficiency is below the theoretical limit for the ideal IBSC, in which EL = 0.71 eV. In this context, our work focuses on the study of wide-bandgap QD-IBSCs. We have fabricated and experimentally investigated the first QD-IBSC prototypes in which AlGaAs or InGaP is the host material for the InAs QDs. We demonstrate an improved bandgap distribution, compared to the InAs/GaAs case, in our wide-bandgap devices. In particular, we have measured values of EL higher than 0.4 eV. In the case of the AlGaAs prototypes, the increase in EL comes with an increase of more than 100 meV of the activation energy of the thermal carrier escape. In addition, in our InAs/AlGaAs devices, we demonstrate voltage up-conversion; i. e., the production of an open-circuit voltage larger than the photon energy (divided by the electron charge) of the incident monochromatic beam, and the achievement of voltage preservation at room temperature under concentrated white-light illumination. We also analyze the potential of an IB material for infrared detection. We present a IB-based new concept of infrared photodetector that we have called the optically triggered infrared photodetector (OTIP). Our novel device 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. We have fabricated an OTIP based on InAs/AlGaAs QDs with which we demonstrate normal incidence photodetection in the 2-6 /xm range optically triggered by a 590 nm light-emitting diode. The theoretical study of the IB-assisted detection mechanism in the OTIP leads us to questioning the assumption of flat quasi-Fermi levels in the space-charge region of a solar cell. Based on device simulations, we prove and explain why this assumption is not valid under short-circuit and illumination conditions. We perform new experimental studies on InAs/GaAs QD-IBSC prototypes in order to gain knowledge on yet unexplored aspects of the performance of these devices. Specifically, we analyze the impact of the use of field-damping layers, and demonstrate this technique to be efficient for avoiding tunnel carrier escape from the QDs to the host material. We analyze the relationship between tunnel escape and voltage preservation, and propose voltage-dependent quantum efficiency measurements as an useful technique for assessing the tunneling-related limitation to the voltage preservation of QD-IBSC prototypes. Moreover, we perform temperature-dependent luminescence studies on InAs/GaAs samples and verify that the results are consistent with a split of the quasi-Fermi levels for the CB and the IB at low temperature. In order to contribute to the fabrication and characterization capabilities of the Solar Energy Institute of the Universidad Polite´cnica de Madrid (IES-UPM), we have participated in the installation and start-up of an molecular beam epitaxy (MBE) reactor and the development of a photo and electroluminescence characterization set-up. Using the MBE reactor, we have manufactured and characterized the first QD-IBSC fully fabricated at the IES-UPM.

Nitrate Transport and Not Photoinhibition Limits Growth of the Freshwater Cyanobacterium Synechococcus Species PCC 6301 at Low Temperature1

The effect of low temperature on cell growth, photosynthesis, photoinhibition, and nitrate assimilation was examined in the cyanobacterium Synechococcus sp. PCC 6301 to determine the factor that limits growth. Synechococcus sp. PCC 6301 grew exponentially between 20°C and 38°C, the growth rate decreased with decreasing temperature, and growth ceased at 15°C. The rate of photosynthetic oxygen evolution decreased more slowly with temperature than the growth rate, and more than 20% of the activity at 38°C remained at 15°C. Oxygen evolution was rapidly inactivated at high light intensity (3 mE m−2 s−1) at 15°C. Little or no loss of oxygen evolution was observed under the normal light intensity (250 μE m−2 s−1) for growth at 15°C. The decrease in the rate of nitrate consumption by cells as a function of temperature was similar to the decrease in the growth rate. Cells could not actively take up nitrate or nitrite at 15°C, although nitrate reductase and nitrite reductase were still active. These data demonstrate that growth at low temperature is not limited by a decrease in the rate of photosynthetic electron transport or by photoinhibition, but that inactivation of the nitrate/nitrite transporter limits growth at low temperature.

Interaction of Osmotic Stress, Temperature, and Abscisic Acid in the Regulation of Gene Expression in Arabidopsis

The impact of simultaneous environmental stresses on plants and how they respond to combined stresses compared with single stresses is largely unclear. By using a transgene (RD29A-LUC) consisting of the firefly luciferase coding sequence (LUC) driven by the stress-responsive RD29A promoter, we investigated the interactive effects of temperature, osmotic stress, and the phytohormone abscisic acid (ABA) in the regulation of gene expression in Arabidopsis seedlings. Results indicated that both positive and negative interactions exist among the studied stress factors in regulating gene expression. At a normal growth temperature (22°C), osmotic stress and ABA act synergistically to induce the transgene expression. Low temperature inhibits the response to osmotic stress or to combined treatment of osmotic stress and ABA, whereas low temperature and ABA treatments are additive in inducing transgene expression. Although high temperature alone does not activate the transgene, it significantly amplifies the effects of ABA and osmotic stress. The effect of multiple stresses in the regulation of RD29A-LUC expression in signal transduction mutants was also studied. The results are discussed in the context of cold and osmotic stress signal transduction pathways.

Expression of Small Heat-Shock Proteins at Low Temperatures1 : A Possible Role in Protecting against Chilling Injuries

We previously reported that short exposure of tomato (Lycopersicon esculentum L.) fruits to high temperature protects them from chilling injury. To study the involvement of heat-shock proteins (HSPs) in the acquisition of low-temperature tolerance, we cloned two heat-shock-induced genes that are also expressed at low temperatures. The cloned cDNAs belong to the small HSP group. Sequence analyses of the clones showed perfect homology to the tomato-ripening gene tom66 and to the tomato chloroplastic HSP21 gene tom111. The expression of both genes was induced by high temperature in fruits, flowers, leaves, and stems, but not by low or ambient temperatures or by other stresses such as drought and anaerobic conditions. When the heated fruits were transferred to low temperature, tom66 and tom111 mRNA levels first decreased but were then reinduced. Induction was not observed in nonheated fruits at low temperature. Immunodetection of tom111-encoded protein indicated that this protein is present at low temperatures in the heated fruits. The results of this study show that the expression of tom66 and tom111 is correlated with protection against some, but not all, symptoms of chilling injury.

Bathorhodopsin structure in the room-temperature rhodopsin photosequence: picosecond time-resolved coherent anti-Stokes Raman scattering.

Structural changes in the retinal chromophore during the formation of the bathorhodopsin intermediate (bathoRT) in the room-temperature rhodopsin (RhRT) photosequence (i.e., vision) are examined using picosecond time-resolved coherent anti-Stokes Raman scattering. Specifically, the retinal structure assignable to bathoRT following 8-ps excitation of RhRT is measured via vibrational Raman spectroscopy at a 200-ps time delay where the only intermediate present is bathoRT. Significant differences are observed between the C=C stretching frequencies of the retinal chromophore at low temperature where bathorhodopsin is stabilized and at room temperature where bathorhodopsin is a transient species in the RhRT photosequence. These vibrational data are discussed in terms of the formation of bathoRT, an important step in the energy storage/transduction mechanism of RhRT.

Photocatalytic oxidation of propene in gas phase at low concentration by optimized TiO2 nanoparticles

In the present study, nanocrystalline titanium dioxide (TiO2) was prepared by sol–gel method at low temperature from titanium tetraisopropoxide (TTIP) and characterized by different techniques (gas adsorption, XRD, TEM and FTIR). Variables of the synthesis, such as the hydrolyzing agent (acetic acid or isopropanol) and calcination temperatures (300–800 °C), were analyzed to get uniform size TiO2 nanoparticles. The effect that these two variables have on the structure of the resultant TiO2 nanoparticles and on their photocatalytic activity is investigated. The photocatalytic activities of TiO2 nanoparticles were evaluated for propene oxidation at low concentration (100 ppmv) under two different kinds of UV light (UV-A ∼ 365 nm and UV-C ∼ 257.7 nm) and compared with Degussa TiO2 P-25, used as reference sample. The results show that both hydrolyzing agents allow to prepare TiO2 nanoparticles and that the hydrolyzing agent influences the crystalline structure and its change with the thermal treatments. Interestingly, the prepared TiO2 nanoparticles possess anatase phase with small crystalline size, high surface area and higher photocatalytic activity for propene oxidation than commercial TiO2 (Degussa P-25) under UV-light. Curiously, these prepared TiO2 nanoparticles are more active with the 365 nm source than with the 257.7 nm UV-light, which is a remarkable advantage from an application point of view. Additionally, the obtained results are particularly good when acetic acid is the hydrolyzing agent at both wavelengths used, possibly due to the high crystallinity, low anatase phase size and high surface oxygen groups’ content in the nanoparticles prepared with it, in comparison to those prepared using isopropanol.

Computational modeling of intermediate temperature proton exchange membrane (PEM) fuel cells

A two-phase three-dimensional computational model of an intermediate temperature (120--190°C) proton exchange membrane (PEM) fuel cell is presented. This represents the first attempt to model PEM fuel cells employing intermediate temperature membranes, in this case, phosphoric acid doped polybenzimidazole (PBI). To date, mathematical modeling of PEM fuel cells has been restricted to low temperature operation, especially to those employing Nafion ® membranes; while research on PBI as an intermediate temperature membrane has been solely at the experimental level. This work is an advancement in the state of the art of both these fields of research. With a growing trend toward higher temperature operation of PEM fuel cells, mathematical modeling of such systems is necessary to help hasten the development of the technology and highlight areas where research should be focused.^ This mathematical model accounted for all the major transport and polarization processes occurring inside the fuel cell, including the two phase phenomenon of gas dissolution in the polymer electrolyte. Results were presented for polarization performance, flux distributions, concentration variations in both the gaseous and aqueous phases, and temperature variations for various heat management strategies. The model predictions matched well with published experimental data, and were self-consistent.^ The major finding of this research was that, due to the transport limitations imposed by the use of phosphoric acid as a doping agent, namely low solubility and diffusivity of dissolved gases and anion adsorption onto catalyst sites, the catalyst utilization is very low (∼1--2%). Significant cost savings were predicted with the use of advanced catalyst deposition techniques that would greatly reduce the eventual thickness of the catalyst layer, and subsequently improve catalyst utilization. The model also predicted that an increase in power output in the order of 50% is expected if alternative doping agents to phosphoric acid can be found, which afford better transport properties of dissolved gases, reduced anion adsorption onto catalyst sites, and which maintain stability and conductive properties at elevated temperatures.^

Design of a low power - high temperature heated ceramic sensor to detect halogen gases

The design, construction and optimization of a low power-high temperature heated ceramic sensor to detect leaking of halogen gases in refrigeration systems are presented. The manufacturing process was done with microelectronic assembly and the Low Temperature Cofire Ceramic (LTCC) technique. Four basic sensor materials were fabricated and tested: Li2SiO3, Na2SiO3, K2SiO3, and CaSiO 3. The evaluation of the sensor material, sensor size, operating temperature, bias voltage, electrodes size, firing temperature, gas flow, and sensor life was done. All sensors responded to the gas showing stability and reproducibility. Before exposing the sensor to the gas, the sensor was modeled like a resistor in series and the calculations obtained were in agreement with the experimental values. The sensor response to the gas was divided in surface diffusion and bulk diffusion; both were analyzed showing agreement between the calculations and the experimental values. The sensor with 51.5%CaSiO3 + 48.5%Li 2SiO3 shows the best results, including a stable current and response to the gas. ^