Modulation above pump beam energy in photoreflectance

Photoreflectance is used for the characterisation of semiconductor samples, usually by sweeping the monochromatized probe beam within the energy range comprised between the highest value set up by the pump beam and the lowest absorption threshold of the sample. There is, however, no fundamental upper limit for the probe beam other than the limited spectral content of the source and the responsivity of the detector. As long as the modulation mechanism behind photoreflectance does affect the complete electronic structure of the material under study, sweeping the probe beam towards higher energies from that of the pump source is equally effective in order to probe high-energy critical points. This fact, up to now largely overseen, is shown experimentally in this work. E1 and E0 + Δ0 critical points of bulk GaAs are unambiguously resolved using pump light of lower energy. This type of upstream modulation may widen further applications of the technique

Introducción histórica a la física del caos

Si conociéramos exactamente las leyes de la naturaleza y la situación del universo en el momento inicial, podríamos predecir exactamente la situación de ese mismo universo en un instante posterior. Pero incluso si las leyes naturales no tuvieran secretos para nosotros, sólo podríamos conocer la situación inicial aproximadamente. Si eso nos permitiera predecir la situación posterior con la misma precisión, es todo lo que necesitamos y deberíamos decir que el fenómeno se ha previsto, que está gobernado por leyes. Pero no es siempre así: puede suceder que pequeñas diferencias en las condiciones iniciales produzcan grandes diferencias en el fenómeno posterior. Un pequeño error al principio produciría un enorme error al final. La predicción se vuelve imposible y estamos ante el fenómeno fortuito

Cu(In,Ga)Se2 absorber thinning and the homo-interface model: Influence of Mo back contact and 3-stage process on device characteristics

Thinning the absorber layer is one of the possibilities envisaged to further decrease the production costs of Cu(In,Ga)Se2 (CIGSe) thin films solar cell technology. In the present study, the electronic transport in submicron CIGSe-based devices has been investigated and compared to that of standard devices. It is observed that when the absorber is around 0.5 μm-thick, tunnelling enhanced interface recombination dominates, which harms cells energy conversion efficiency. It is also shown that by varying either the properties of the Mo back contact or the characteristics of 3-stage growth processing, one can shift the dominating recombination mechanism from interface to space charge region and thereby improve the cells efficiency. Discussions on these experimental facts led to the conclusions that 3-stage process implies the formation of a CIGSe/CIGSe homo-interface, whose location as well as properties rule the device operation; its influence is enhanced in submicron CIGSe based solar cells

Física del caos

¿Qué es la física del caos? Anteriormente hemos encuadrado esta teoría dentro de los grandes paradigmas en la historia de la ciencia. En este capítulo nos adentramos en sus fundamentos. Dentro de los llamados sistemas dinámicos, veremos qué relación hay entre lo que se llama sensibilidad a las condiciones iniciales y la posibilidad de predecir el futuro de un sistema o predecibilidad. Enlazando con las ideas inherentes a la teoría del caos desde el punto de vista científico llegaremos, por fin, al efecto mariposa. Más tarde abordaremos las ideas más relevantes de la teoría para la predicción meteorológica

Intermediate bands and non radiative recombination

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

Study of GaAs(Ti) thin films as candidates for IB solar cells manufacturing

Thin films of GaAs(Ti) have been deposited by sputtering on glass and n_GaAs substrates under different process conditions. Optical characteristics of these samples have been analyzed to study the potential of this material in intermediate Band solar cell manufacturin

Predicted photoreflectance signatures on QD selective contacts for hot carrier solar cells

The CO2 emission of our present energy transformation processes, based mainly on burning fossil fuels, is possibly the main cause of global climatic change. The photovoltaic conversion of solar energy is a clean way of producing which for sustainability should (and most probably will) become a major source of electricity. The sun is a huge resource but relatively diluted and it is reasonable to expect that only high efficiency extraction can be cost effective for mass exploitation. New concepts are neccessary such as hot carrier solar cells

Evaluation of the efficiency potential of intermediate band solar cells based on thin-film chalcopyrite materials

This paper discusses the potential of the intermediate band solar cell _IBSC_ concept to improve the efficiency of thin-film chalcopyrite solar cells. The results show that solar cells based on CuGaS2, with a radiative limiting efficiency of 46.7%, exhibit the highest potential. A simple method for the identification of transition elements that when incorporated in CuGaS2 could possibly introduce an intermediate band is also described. The IBSC concept is also applied under the assumptions that thin-film solar cells are not to be operated under concentrated light and that a non-negligible contribution of nonradiative recombination exists

Optoelectronic Characterisation of Intermediate Band Solar Cells by Photoreflectance Comparison to Other Advanced Architectures.

The fabrication and design of novel materials and devices for advanced photovoltaics, like the intermediate-band solar cell (IBSC), requires the use of specific characterization tools providing information about their optoelectronic properties. We have tested the suitability of photoreflectance for the characterization of IBSC prototypes based on quantum dots and compared the results obtained with those predicted by the theory. Nonidealities in operative devices have been identified and detailed information has been obtained about the electronic structure of the materials. We have compared PR spectra of IBSCs with those obtained from alternative device architectures, namely a triple-junction solar cell and a multi-quantum well structure. Some general conclusions are drawn demonstrating the potential of the technique