Crecimiento de compuestos II-VI mediante la técnica MOCVD: Aplicación al crecimiento de CdTe; HgTe y Hg1-xCdxTe.

RESUMEN La tesis está centrada en el estudio del crecimiento de materiales II-VI mediante la técnica de depósito de capas delgadas MOCVD (MetalOrganic Vapour Phase Deposition). En concreto se ha estudiado el crecimiento de los compuestos CdTe y HgTe, así como de su aleación ternaria Hg1-xCdxTe. El empleo de estos materiales nos ha permitido profundizar en las posibilidades de la técnica ya que cada uno de ello se ha crecido mediante una configuración diferente. El crecimiento de CdTe se ha realizado sobre tres substratos de naturaleza muy diferente: vidrio, GaAs y GaS. Adicionalmente sobre el primero hemos estudiado la influencia de un tratamiento térmico post-crecimiento en el mismo reactor MOCVD, obteniendo muestras con mayor calidad y tamaño de grano que las muestras sin tratar, lo que sin duda es beneficioso de cara a su utilización en el desarrollo de dispositivos fotovoltaicos. En el crecimiento de CdTe sobre GaAs(100) hemos obtenido de manera reproducible capas con orientación (111) ó (100). Un estudio sistemático de la morfología de las muestras en función del tiempo de crecimiento, es decir del grosor, nos ha permitido detectar dos mecanismos de crecimiento para el CdTe(111), uno para tiempos inferiores a 100 s y otro para tiempos superiores. Por lo que respecta al crecimiento de CdTe(100) sobre GaAs(100), hemos obtenido muestras libres de los defectos superficiales conocidos como "hillocks", que son observados sistemáticamente por otros autores. El crecimiento de CdTe sobre GaS(001), nos ha permitido poder relacionar la orientación de este substrato con la de la capa de CdTe crecida sobre él mediante análisis de texturas por difracción de rayos X. En el crecimiento de HgTe, se ha utilizado una doble entrada de precursores ya que en una de ellas se sitúa el baño de Hg que se utiliza como material fuente. Hemos comprobado, con ayuda de la simulación numérica, la influencia de la doble entrada en el comportamiento hidrodinámico del proceso, contrastando las predicciones de dicha simulación con los resultados experimentales y obteniendo un buen acuerdo. El crecimiento de CdTe y HgTe, nos ha permitido abordar el crecimiento de su ternario mediante el método de interdifusión de capas múltiples, obteniendo muestras de Hg1-xCdxTe(100) con distintas composiciones x, que han sido sistemáticamente caracterizadas mediante diversas técnicas. ____________________________________________________________________________________________________The key issue of the thesis is the crystalline growth of thin layers of II-VI semiconductor compounds by MOCVD (Metal Organic Vapour Phase Deposition). It has been studied the growth of CdTe, HgTe and Hg1-xCdxTe. The study of these materials has allowed to investigate the possibilities of the technique due to the different configuration employed to growth each material. The growth of CdTe has been achieved on three different substrates: glass, GaAs, and GaS. In the growth on glass the influence of the post-growth treatment in the same MOCVD reactor has been studied, obtaining samples with better quality and grain size than the samples without treatment, this fact is of great interest in the development of photovoltaic devices. A systematic investigation of the morphology for samples grown with different times, and consequently with different thickness, has allow to detect two growth mechanism for the CdTe (111), the first one for low growth times (< 100 s) and the another one for higher growth times. In the growth of CdTe on GaAs(100), we have obtain hillocks free samples, these defects are usually observed for other author. For the growth of CdTe on GaS(100) the relationship between the orientation of layer and the substrate has been established by X ray texture analysis. In the growth of HgTe a double entry to the reactor has been used, with the help of the numerical simulation, its effect has been analyzed and correlated with the experimental results. Finally the growth of CdTe and HgTe has permitted to growth the ternary allow Hg1-xCdxTe. The interdiffused multilayer process has been employed, obtaing samples of Hg1-xCdxTe with different composition x. These samples have been systematically analyzed with different techniques

Enhanced operational stability through interfacial modification by active encapsulation of perovskite solar cells

Encapsulates are, in general, the passive components of any photovoltaic device that provides the required shielding from the externally stimulated degradation. Here we provide comprehensive physical insight depicting a rather non-trivial active nature, in contrast to the supposedly passive, atomic layer deposition (ALD) grown Al2O3 encapsulate layer on the hybrid perovskite [(FA0.83MA0.17)0.95Cs0.05PbI2.5Br0.5] photovoltaic device having the configuration: glass/FTO/SnO2/perovskite/spiro-OMeTAD/Au/(±) Al2O3. By combining various electrical characterization techniques, our experimental observations indicate that the ALD chemistry produces considerable enhancement of the electronic conductivity of the spiro-OMeTAD hole transport medium (HTM), resulting in electronic modification of the perovskite/HTM interface. Subsequently, the modified interface provides better hole extraction and lesser ionic accumulation at the interface, resulting in a significant lowering of the burn-in decay and nearly unchanged charge transport parameters explicitly under the course of continuous operation. Unlike the unencapsulated device, the modified electronic structure in the Al2O3 coated device is essentially the principal reason for better performance stability. Data presented in this communication suggest that the ionic accumulation at the spiro-OMeTAD/perovskite interface triggers the device degradation in the uncoated devices, which is eventually followed by material degradation, which can be avoided by active encapsulation

Outdoor Performance of Perovskite Photovoltaic Technology

In the case of emerging photovoltaic technologies such as perovskite, most published works have focused on laboratory-scale cells, indoor conditions and no international standards have been fully established and adopted. Accordingly, this chapter shows a brief introduction on the standards and evaluation methods for perovskite solar minimodules under natural sunlight conditions. Therefore, we propose evaluating the outdoor performance in terms of power, following the international standard IEC 61853–1 to obtain the performance according to the power rating conditions. After some rigorous experimental evaluations, results shown that the maximum power (Pmax) evolution for the analyzed minimodules could be correlated with one of the three patterns commonly described for degradation processes in the literature, named convex, linear, and concave. These patterns were used to estimate the degradation rate and lifetime (T80). Moreover, ideality factor (nID) was estimated from the open-circuit voltage (Voc) dependence on irradiance and ambient temperature (outdoor data) to provide physical insight into the recombination mechanism dominating the performance during the exposure. In this context, it was observed that the three different degradation patterns identified for Pmax can also be identified by nID. Finally, based on the linear relationship between T80 and the time to first reach nID = 2 (TnID2), is demonstrated that nID analysis could offer important complementary information with important implications for this technology outdoor development, due that the changes in nID could be correlated with the recombination mechanisms and degradation processes occurring in the device

Electron injection and scaffold effects in perovskite solar cells

In spite of the impressive efficiencies reported for perovskite solar cells (PSCs), key aspects of their working principles, such as electron injection at the contacts or the suitability of the utilization of a specific scaffold layer, are not yet fully understood. Increasingly complex scaffolds attained by the sequential deposition of TiO2 and SiO2 mesoporous layers onto transparent conducting substrates are used to perform a systematic characterization of both the injection process at the electron selective contact and the scaffold effect in PSCs. By forcing multiple electron injection processes at a controlled sequence of perovskite–TiO2 interfaces before extraction, interfacial injection effects are magnified and hence characterized in detail. An anomalous injection behavior is observed, the fingerprint of which is the presence of significant inductive loops in the impedance spectra with a magnitude that correlates with the number of interfaces in the scaffold. Analysis of the resistive and capacitive behavior of the impedance spectra indicates that the scaffolds could hinder ion migration, with positive consequences such as lowering the recombination rate and implications for the current–potential curve hysteresis. Our results suggest that an appropriate balance between these advantageous effects and the unavoidable charge transport resistive losses introduced by the scaffolds will help in the optimization of PSC performance.Unión Europea 7PM / 2007-2013Unión Europea ERC 307081 (POLIGHT)Ministerio de Economía y Competitividad de España MAT2014-54852-RMinisterio de Economía y Competitividad de España MAT2015-70611-ER

Optical amplification in hollow-core negative-curvature fibers doped with perovskite CsPbBr3 nanocrystals

We report a hollow-core negative-curvature fiber (HC-NCF) optical signal amplifier fabricated by the filling of the air microchannels of the fiber with all-inorganic CsPbBr3 perovskite nanocrystals (PNCs). The optimum fabrication conditions were found to enhance the optical gain, up to +3 dB in the best device. Experimental results were approximately reproduced by a gain assisted mechanism based on the nonlinear optical properties of the PNCs, indicating that signal regeneration can be achieved under low pump powers, much below the threshold of stimulated emission. The results can pave the road for new functionalities of the HC-NCF with PNCs, such as optical amplification, nonlinear frequency conversion and gas sensors

Enhanced nanoscopy of individual CsPbBr3 perovskite nanocrystals using dielectric sub-micrometric antennas

We demonstrate an efficient, simple, and low-cost approach for enhanced nanoscopy in individual green emitting perovskite (CsPbBr3) nanocrystals via TiO2 dielectric nanoantenna. The observed three- to five-fold emission enhancement is attributed to near-field effects and emission steering promoted by the coupling between the perovskite nanocrystals and the dielectric sub-micrometric antennas. The dark-field scattering configuration is then exploited for surface-enhanced absorption measurements, showing a large increase in detection sensitivity, leading to the detection of individual nanocrystals. Due to the broadband spectral response of the Mie sub-micrometric antennas, the method can be easily extended to electronic transitions in other spectral regions, paving the way for absorption nanoscopy of many different quantum emitters from organic molecules to quantum dots.We demonstrate an efficient, simple, and low-cost approach for enhanced nanoscopy in individual green emitting perovskite (CsPbBr3) nanocrystals via TiO2 dielectric nanoantenna. The observed three- to five-fold emission enhancement is attributed to near-field effects and emission steering promoted by the coupling between the perovskite nanocrystals and the dielectric sub-micrometric antennas. The dark-field scattering configuration is then exploited for surface-enhanced absorption measurements, showing a large increase in detection sensitivity, leading to the detection of individual nanocrystals. Due to the broadband spectral response of the Mie sub-micrometric antennas, the method can be easily extended to electronic transitions in other spectral regions, paving the way for absorption nanoscopy of many different quantum emitters from organic molecules to quantum dots

Revealing giant exciton fine-structure splitting in 2D perovskites using van der Waals passivation

The study of two-dimensional (2D) van der Waals materials has been an active field of research in the development of new optoelectronics and photonic applications over the last decade. Organic-inorganic layered perovskites are currently some of the most promising 2D van der Waals materials, due to their exceptional optical brightness and enhanced excitonic effects. However, low crystal quality and spectral diffusion usually broaden the exciton linewidth, obscuring the fine structure of the exciton in conventional photoluminescence experiments. Here, we propose a mechanical approach for reducing the effect of spectral diffusion by means of hBN-capping on layered perovskites with different thicknesses, revealing the exciton fine structure. We used a stochastic model to link the reduction of the spectral linewidth with the population of active charge fluctuation centres present in the organic spacer taking part in the dynamical Stark shift. Active fluctuation centres are reduced by a factor of 3.7 to 7.1 when we include hBN-capping according to our direct spectral measurements. This rate is in good agreement with the analysis of the overlap between the squared perovskite lattice and the hexagonal hBN lattice. Van der Waals forces between both lattices cause the partial clamping of the perovskite organic spacer molecules, and hence, the amplitude of the dynamical Stark shift characteristic of the spectral diffusion effect is reduced. Our work provides an easy and low-cost solution to the problem of accessing important fine-structure excitonic state information, along with an explanation of the important carrier dynamics present in the organic spacer that affect the quality of the optical emission

Improvement of Photovoltaic Performance of Colloidal Quantum Dot Solar Cells Using Organic Small Molecule as Hole-Selective Layer

A novel organic small molecule bis-triphenylamine with spiro(fluorene-9,9′-xanthene) as the conjugated system, named BTPA-4, is successfully synthesized and employed as the hole-selective layer (HSL) in colloidal quantum dots solar cells (CQDSCs). The introduction of BTPA-4 layer can significantly prolong effective carrier lifetime (τeff), increase charge recombination resistance (Rrec), and thus diminish the interfacial charge recombination at the PbS-QDs/Au electrode interface. The effect of BTPA-4 as HSL in the device performance is especially significant for the open-circuit voltage (Voc) and power conversion efficiency (PCE), with a ∼ 10% and 15% enhancement respectively, comparing with those of device without the HSL. Furthermore, the PbS CQDSCs with BTPA-4 possessed a noticeably stable property for over 100 days of storage under ambient atmosphere

The violent youth of bright and massive cluster galaxies and their maturation over 7 billion years

In this study, we investigate the formation and evolution mechanisms of the brightest cluster galaxies (BCGs) over cosmic time. At high redshift (z ∼ 0.9), we selected BCGs and most massive cluster galaxies (MMCGs) from the Cl1604 supercluster and compared them to low-redshift (z ∼ 0.1) counterparts drawn from the MCXC meta-catalogue, supplemented by Sloan Digital Sky Survey imaging and spectroscopy. We observed striking differences in the morphological, colour, spectral, and stellar mass properties of the BCGs/MMCGs in the two samples. High-redshift BCGs/MMCGs were, in many cases, star-forming, late-type galaxies, with blue broad-band colours, properties largely absent amongst the low-redshift BCGs/MMCGs. The stellar mass of BCGs was found to increase by an average factor of 2.51 ± 0.71 from z ∼ 0.9 to z ∼ 0.1. Through this and other comparisons, we conclude that a combination of major merging (mainly wet or mixed) and in situ star formation are the main mechanisms which build stellar mass in BCGs/MMCGs. The stellar mass growth of the BCGs/MMCGs also appears to grow in lockstep with both the stellar baryonic and total mass of the cluster. Additionally, BCGs/MMCGs were found to grow in size, on average, a factor of ∼3, while their average Sérsic index increased by ∼0.45 from z ∼ 0.9 to z ∼ 0.1, also supporting a scenario involving major merging, though some adiabatic expansion is required. These observational results are compared to both models and simulations to further explore the implications on processes which shape and evolve BCGs/MMCGs over the past ∼7 Gyr