• Energy Research

Arsenic alloying is observed for epitaxial layers nominally intended to be In0.75Ga0.25N. Voids form beneath their interfaces with GaAs substrates, acting as sources of Ga + As out-diffusion into the growing epilayers. As a result, heteroepitaxial single-phase quaternary InxGa1-xAsyN1-y, films are formed with x similar to 0.55 and 0.05 menor que y menor que 0,10. While an undoped epilayer retains the wurtzite structure, a Mn-doped sample showed randomly spaced dopant segregations, which, together with a slightly higher As concentration, led to a transformation from the hexagonal to the twinned cubic phase.

The insertion of quantum dots in a host material produces band offsets which are greatly dependent on the field of strains brought about by this insertion. Based on the Empiric KP Hamiltonian model, the energy spectrum of the quantum dot/host system is easily calculated and a relationship between the conduction and valence band offsets is determined by the energy at which the lowest peak of the sub-bandgap quantum efficiency of an intermediate band solar cell is situated; therefore knowledge of the valence band offset leads to knowledge of both offsets. The calculated sub-bandgap quantum efficiency due to the quantum dot is rather insensitive to the value of the valence band offset. However, the calculated quantum efficiency of the wetting layer, modeled as a quantum well, is sensitive to the valence band offset and a fitting with the measured value is possible resulting in a determination of both offsets in the finished solar cell with its final field of strains. The method is applied to an intermediate-band solar cell prototype made with InAs quantum dots in GaAs.

Current prototypes of quantum-dot intermediate band solar cells suffer from voltage reduction due to the existence of carrier thermal escape. An enlarged sub-bandgap EL would not only minimize this problem, but would also lead to a bandgap distribution that exploits more efficiently the solar spectrum. In this work we demonstrate InAs/InGaP QD-IBSC prototypes with the following bandgap distribution: EG = 1.88 eV, EH = 1.26 eV and EL > 0.4 eV. We have measured, for the first time in this material, both the interband and intraband transitions by means of photocurrent experiments. The activation energy of the carrier thermal escape in our devices has also been measured. It is found that its value, compared to InAs/GaAs-based prototypes, does not follow the increase in EL. The benefits of using thin AlGaAs barriers before and after the quantum-dot layers are analyzed.

Abstract The quantum dot intermediate band solar cell has the potential for very high conversion efficiency. However, the cells manufactured so far show efficiencies below the expectations mainly because the sub-bandgap photocurrent associated to the quantum dots is too low and because of a substantial reduction of the voltage. We present a new Hamiltonian for the use with the k·p method with low computing power demands. With it, we show here the fundamentals that explain the low light absorption coefficient and, consequently, the low photocurrent observed. We also prove that the bandgap of the host material, GaAs in our case, is reduced by the introduction of the quantum dots, which also explains the voltage reduction. The model is justified by the agreement with internal quantum efficiency measurements. It opens the path for improvement and suggests changes for increasing the photocurrent and for the compensation of the voltage reduction.

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.

AbstractA model of the increase in sales of photovoltaic modules is presented, based on coupling the learning curve and the demand elasticity equations. The model, solved for constant demand elasticity, shows good agreement with past sales data, better than any exponential growth curve. We have used this model, properly refined, to extrapolate the market evolution for the next half century, and we have compared the results with the goals of the RIGES scenario for CO2 abatement, presented to the Rio Conference of 1992. The model predicts a very fast growth that will slow down when a certain level of annual sales is reached. It is uncertain if the size of the photovoltaics industry will reach a level with a major environmental impact. New technological findings may be the fastest way to reach the environmental goals, but changes of the present marketing scheme or electricity price increases may also lead to the desired environmental impact in the first half of the century.Se presenta un modelo basado en el acoplamiento de las ecuaciones de la curva de aprendizaje y la curva de elasticidad de la demanda. El modelo, resuelto para elasticidad de la demanda constante, muestra un buen acuerdo con las ventas pasadas, mejor que cualquier exponencial. Hemos usado este modelo, refinado adecuadamente, para extrapolar la evolución del mercado al próximo medio siglo y hemos comparado los resultados con los objetivos del escenario RIGES para reducción del CO2 presentados en la cumbre de Río de 1992. El modelo predice un crecimiento muy rápido que se ralentizará cuando se alcance un cierto nivel de ventas anuales. Aunque este negocio ha de llegar a ser muy grande, no es seguro si alcanzará un impacto ambiental significativo. Nuevos hallazgos tecnológicos pueden constituir el medio más rápido para alcanzar los objetivos ambientales, pero cambios en el actual modo de comercialización, o incrementos del precio de la electricidad pueden también conducir a alcanzar los impactos ambientales significativos deseados en la primera mitad del siglo. Copyright © 2001 John Wiley & Sons, Ltd.

AbstractA procedure for extracting the internal reflectivities and the mean tilt angle of light rays inside silicon wafers from reflectance measurements is presented. the procedure, based on a simple model of light confinement, is applied to silicon wafers textured with random pyramids. the results obtained are shown to be in good agreement with ray‐tracing calculations and theoretical estimates.Se presenta un método para extraer, a partir de medidas de reflectancia, los valores de las reflectividades internas y del ángulo medio de inclinación de los rayos de luz en el interior de células solares. Este método está basado en un modelo sencillo del confinamiento interno de la luz y se aplica aqui a obleas de silicio con textura piramidal aleatoria. Los resultados obtenidos están de acuerdo con los calculados mediante trazado de rayos y con las estimaciones teóricas.

Intermediate band solar cells (IBSCs) fabricated to date from In(Ga)As/GaAs quantum dot arrays (QD-IBSC) exhibit a quantum efficiency (QE) that extends to below bandgap energies. However, the production of sub-bandgap photocurrent relies often on the thermal and/or tunneling escape of carriers from the QDs, which is incompatible with preservation of the output voltage. In this work, we test the effectiveness of introducing a thick GaAs spacer in addition to an InAlGaAs strain relief layer (SRL) over the QDs to reduce carrier escape. From an analysis of the QE at different temperatures, it is concluded that escape via tunneling can be completely blocked under short-circuit conditions, and that carriers confined in QDs with an InAlGaAs SRL exhibit a thermal escape activation energy over 100 meV larger than in the case of InAs QDs capped only with GaAs.