Demux devices based on A-SiC:H

In this paper we present results about the functioning of a multilayered a-SiC:H heterostructure as a device for wavelength-division demultiplexing of optical signals. The device is composed of two stacked p-i-n photodiodes, both optimized for the selective collection of photogenerated carriers. Band gap engineering was used to adjust the photogeneration and recombination rates profiles of the intrinsic absorber regions of each photodiode to short and long wavelength absorption and carrier collection in the visible spectrum. The photocurrent signal using different input optical channels was analyzed at reverse and forward bias and under steady state illumination. This photocurrent is used as an input for a demux algorithm based on the voltage controlled sensitivity of the device. The device functioning is explained with results obtained by numerical simulation of the device, which permit an insight to the internal electric configuration of the double heterojunction.These results address the explanation of the device functioning in the frequency domain to a wavelength tunable photocapacitance due to the accumulation of space charge localized at the internal junction. The existence of a direct relation between the experimentally observed capacitive effects of the double diode and the quality of the semiconductor materials used to form the internal junction is highlighted.

Effet de la microstructure sur les propriétés excitoniques des polymères semi-conducteurs semi-cristallins

Les polymères semi-conducteurs semicristallins sont utilisés au sein de diodes électroluminescentes, transistors ou dispositifs photovoltaïques organiques. Ces matériaux peuvent être traités à partir de solutions ou directement à partir de leur état solide et forment des agrégats moléculaires dont la morphologie dicte en grande partie leurs propriétés optoélectroniques. Le poly(3-hexylthiophène) est un des polymères semi-conducteurs les plus étudiés. Lorsque le poids moléculaire (Mw) des chaînes est inférieur à 50 kg/mol, la microstructure est polycristalline et composée de chaînes formant des empilements-π. Lorsque Mw>50 kg/mol, la morphologie est semicristalline et composée de domaines cristallins imbriquées dans une matrice de chaînes amorphes. À partir de techniques de spectroscopie en continu et ultrarapide et appuyé de modèles théoriques, nous démontrons que la cohérence spatiale des excitons dans ce matériau est légèrement anisotrope et dépend de Mw. Ceci nous permet d’approfondir la compréhension de la relation intime entre le couplage inter et intramoléculaire sur la forme spectrale en absorption et photoluminescence. De plus, nous démontrons que les excitations photogénérées directement aux interfaces entre les domaines cristallins et les régions amorphes génèrent des paires de polarons liés qui se recombinent par effet tunnel sur des échelles de temps supérieures à 10ns. Le taux de photoluminescence à long temps de vie provenant de ces paires de charges dépend aussi de Mw et varie entre ∼10% et ∼40% pour les faibles et hauts poids moléculaires respectivement. Nous fournissons un modèle permettant d’expliquer le processus de photogénération des paires de polarons et nous élucidons le rôle de la microstructure sur la dynamique de séparation et recombinaison de ces espèces.

Investigations on Photoconductivity and Electrical Switiching in Selected Chalcogenide Glasses

In this thesis, we present the results of our investigations on the photoconducting and electrical switching properties of selected chalcogenide glass systems. We have used XRD and X-ray photoelectron spectroscopy (XPS) analysis for confinuing the amorphous nature of these materials and for confirming their constituents respectively.Photoconductivity is the enhancement in electrical conductivity of materials brought about by the motion of charge carriers excited by absorbed radiation. The phenomenon involves absorption, photogeneration, recombination and transport processes and it gives good insight into the density of states in the energy gap of solids due to the presence of impurities and lattice defects. Photoconductivity measurements lead to the determination of such important parameters as quantum efficiency, photosensiti\'ity, spectral sensitivity and carrier lifetime. Extensive research work on photoconducting properties of amorphous semiconductors has resulted in the development of a variety of very sensitive photodetectors. Photoconductors are finding newer and newer uses eyery day. CdS, CdSe. Sb2S3, Se, ZnO etc, are typical photoconducting materials which are used in devices like vidicons, light amplifiers, xerography equipment etc.Electrical switching is another interesting and important property possessed by several Te based chalcogenides. Switching is the rapid and reversible transition between a highly resistive OFF state, driven by an external electric field and characterized by a threshold voltage, and a low resistivity ON state, Switching can be either threshold type or memory type. The phenomenon of switching could find applications in areas like infonnation storage, electrical power control etc. Investigations on electrical switching in chalcogenide glasses help in understanding the mechanism of switching which is necessary to select and modify materials for specific switching applications.Analysis of XRD pattern gives no further infonuation about amorphous materials than revealing their disordered structure whereas x-ray photoelectron spectroscopy,XPS) provides information about the different constituents present in the material. Also it gives binding energies (b.e.) of an element in different compounds and hence b.e. shift from the elemental form.Our investigations have been concentrated on the bulk glasses, Ge-In-Se, Ge-Bi-Se and As-Sb-Se for photoconductivity measurements and In-Te for electrical switching. The photoconducting properties of Ge-Sb-Se thin films prepared by sputtering technique have also been studied. The bulk glasses for the present investigations are prepared by the melt quenching technique and are annealed for half an hour at temperatures just below their respective glass transition temperatures. The dependence of photoconducting propenies on composition and temperature are investigated in each system. The electrical switching characteristics of In-Te system are also studied with different compositions and by varying the temperature.

Substitutional Impurities in PPV Crystals: An Intrinsic Donor-Acceptor System for High V(oc) Photovoltaic Devices

Defects are usually present in organic polymer films and are commonly invoked to explain the low efficiency obtained in organic-based optoelectronic devices. We propose that controlled insertion of substitutional impurities may, on the contrary, tune the optoelectronic properties of the underivatized organic material and, in the case studied here, maximize the efficiency of a solar cell. We investigate a specific oxygen-impurity substitution, the keto-defect -(CH(2)-C=O)- in underivatized crystalline poly(p-phenylenevinylene) (PPV), and its impact on the electronic structure of the bulk film, through a combined classical (force-field) and quantum mechanical (DFT) approach. We find defect states which suggest a spontaneous electron hole separation typical of a donor acceptor interface, optimal for photovoltaic devices. Furthermore, the inclusion of oxygen impurities does not introduce defect states in the gap and thus, contrary to standard donor-acceptor systems, should preserve the intrinsic high open circuit voltage (V(oc)) that may be extracted from PPV-based devices.

Quenching of Photoactivity in Phthalocyanine Copper(II)-Titanate Nanotube Hybrid Systems

Titanate nanotubes (TiNTs) were obtained by hydrothermal treatment of anatase powder in aqueous NaOH solution and then modified with 2,9,16,23-tertracarboxyl phthalocyanine copper(H) (CuPc). This hybrid organic inorganic nanoscopic system was characterized by X-ray diffraction, microscopy, and spectroscopy. Transmission electron microscopy (TEM) images of pure and modified TiNTs revealed multiwall structures with an average outer diameter of 9 nm and a length of several hundred nanometers. The tubular morphology of the TiNTs was covered with CuPc-film. The amount of CuPc adsorbed onto the TiNTs was quantified by electron paramagnetic resonance (EPR). Using the same technique and spin-trapping methodology, the photogeneration of reactive oxygen species (ROS) from the TiNTs was systematically investigated. A drastic quenching of photoactivity was observed in the CuPc/TiNT hybrid system. Electron transfer from excited CuPc states to the TiNT conduction band followed by electron recombination may be the cause of this quenching.

Photoconductivity excitation spectrum for stretch-oriented PPV

We have obtained the photoconductivity (PC) excitation spectrum for a stretch-oriented poly(paraphenylene vinylene) film over a wide spectral range (up to 5 eV). The measurements were performed in the surface cell configuration with the electric field parallel or perpendicular to the stretch direction. Although the sample had a stretch ratio of similar to 4, the dark conductivity and the steady-state photoconductivity were both about 40 and 20 times higher with the electric field parallel to the average chain direction, respectively. However, the shape of the PC excitation spectrum was independent of field direction and did not show a significant rise in the ultraviolet, as is usually observed for measurements in the photodiode configuration. The implications of these results to the charge photogeneration mechanism in conjugated polymers are discussed.

New results on bound excitons in quantum wells

The formation of bound excitons (BE) is investigated for a GaAs/GaAlAs multiple quantum well (QW) system. The photoluminescence (PL) spectra are analysed as a function of the excitation energy. It was found that the carriers photogeneration, either in the barrier or directly in the well, do not play an important role on the BE formation. We conclude that defects localized at interfaces are ionized by of capture charges which in turn bound the free exciton (FE).

Feedback-controlled running holograms in strongly absorbing photorefractive materials

We propose a mathematical model for the movement in absorbing materials of photorefractive holograms under feedback constraints. We use this model to analyze the speed of a fringe-locked running hologram in photorefractive sillenite crystals that usually exhibit a strong absorption effect. Fringe-locked experiments permit us to compute the quantum efficiency for the photogeneration of charge carriers in photorefractive crystals if the effect of bulk absorption and the effective value of the externally applied field are adequately taken into consideration. A Bi12TiO20 sample was measured with the 532-nm laser wavelength, and a quantum efficiency of φ = 0.37 was obtained. Disregarding absorption leads to large errors in φ. © 2000 Optical Society of America.

Neue konjugierte Polyarylene mit Leiterstruktur

Zusammenfassung: Im Rahmen der Arbeit wird über die Darstellung neuartiger, konjugierter Polyarylene mit Leiterstruktur berichtet. Über eine zweistufige Synthese wurde ein ethylenüberbrücktes Leiterpolymer (LPDP) hergestellt. Dabei wurde das Vorläuferpolymer mit Samarium(II)jodid unter milden Bedingungen zum Leiterpolymeren cyclisiert. Erste Untersuchungen zeigen, daß LPDP im Gegensatz zum 'gewinkelten Polyacen' eine sehr vielversprechende Elektrolumineszenz-Eigenschaft besitzt. Durch den Einbau chiraler Alkylsubstituenten in entsprechenden meta-Phenylen-Analoga der Leiterpolymere vom Polyacen-Typ wurde versucht, eine Vorzugsdrehrichtung der helikalen Leiterpolymere im Laufe der polymeranalogen Cyclisierung zu induzieren. Es zeigt sich, daß für eines der chiralen Derivate ein CD-Effekt im Bereich der Absorption der helikalen Polyacen-Hauptkette auch auf molekularer Ebene auftritt. Weiterhin wird die erfolgreiche Synthese eines neuen, heteroaromatischen Leiterpolymeren, LPPPT, beschrieben, welches alternierend aus 1,4-Phenylen- und 2,5-Thienylen-Einheiten aufgebaut ist. Eine LED in der Konfiguration ITO/LPPPT/Al zeigt eine orange Lichtemission. Die Quanteneffizienz der freien Ladungsträgerbildung für LPPPT wurde in Experimenten zur Ladungstraegergeneration im elektrischen Feld (Coronaentladung) zu ca. 1 % bei 10E7 V/m bestimmt. Letztlich wurde die Synthese eines neuen, heteroaromatischen Leiterpolymeren mit Carbazol-Einheiten, LPPPC, in der Hauptkette beschrieben, das alternierend aus 1,4-Phenylen- und Carbazol-3,6-diyl-Einheiten aufgebaut ist. Untersuchungen am LPPPC ergaben, daß das Polymer gute Lochleitereigenschaften besitzt, daneben weisen dünne Filme von LPPPC auch photovoltaische Eigenschaften auf.

Efficiency limits for silicon/perovskite tandem solar cells: a theoretical model

The primary goal of this work is related to the extension of an analytic electro-optical model. It will be used to describe single-junction crystalline silicon solar cells and a silicon/perovskite tandem solar cell in the presence of light-trapping in order to calculate efficiency limits for such a device. In particular, our tandem system is composed by crystalline silicon and a perovskite structure material: metilammoniumleadtriiodide (MALI). Perovskite are among the most convenient materials for photovoltaics thanks to their reduced cost and increasing efficiencies. Solar cell efficiencies of devices using these materials increased from 3.8% in 2009 to a certified 20.1% in 2014 making this the fastest-advancing solar technology to date. Moreover, texturization increases the amount of light which can be absorbed through an active layer. Using Green’s formalism it is possible to calculate the photogeneration rate of a single-layer structure with Lambertian light trapping analytically. In this work we go further: we study the optical coupling between the two cells in our tandem system in order to calculate the photogeneration rate of the whole structure. We also model the electronic part of such a device by considering the perovskite top cell as an ideal diode and solving the drift-diffusion equation with appropriate boundary conditions for the silicon bottom cell. We have a four terminal structure, so our tandem system is totally unconstrained. Then we calculate the efficiency limits of our tandem including several recombination mechanisms such as Auger, SRH and surface recombination. We focus also on the dependence of the results on the band gap of the perovskite and we calculare an optimal band gap to optimize the tandem efficiency. The whole work has been continuously supported by a numerical validation of out analytic model against Silvaco ATLAS which solves drift-diffusion equations using a finite elements method. Our goal is to develop a simpler and cheaper, but accurate model to study such devices.

A simple light-driven transmembrane proton pump.

Light-induced lipophilic porphyrin/aqueous acceptor charge separation across a single lipid-water interface can pump protons across the lipid bilayer when the hydrophobic weak acids, carbonylcyanide m-chlorophenylhydrazone and its p-trifluoromethoxyphenyl analogue, are present. These compounds act as proton carriers across lipid bilayers. In their symmetric presence across the bilayer, the positive currents and voltages produced by the photogeneration of porphyrin cations are replaced by larger negative currents and voltages. The maximum negative current and voltage occur at the pH of maximum dark conductance. The reversed larger current and voltage show a positive ionic charge transport in the same direction as the electron transfer. This transport can form an ion concentration gradient. The movement of protons is verified by an unusual D2O isotope effect that increases the negative ionic current by 2- to 3-fold. These effects suggest that an interfacial pK shift of the weak acid caused by the local electric field of photoformed porphyrin cations/acceptor anions functions as the driving force. The estimated pumping efficiency is 10-30%. Time-resolved results show that proton pumping across the bilayer occurs on the millisecond time scale, similar to that of biological pumps. This light-driven proteinless pump offers a simple model for a prebiological energy transducer.

Photoconductivity and electrical properties of diamond films grown by MPCVD

The electrical and photoconductive features of as-grown microwave-plasma-assisted chemical-vapour deposition (MPCVD) diamond films are studied in correlation with magnetic results obtained from electron paramagnetic resonance (EPR). Also, the morphology is analysed by atomic force microscopy (AFM) showing [111] crystals with a good uniformity of the deposit. The photoresponse as well the current-voltage features observed show an efficient photogeneration of carriers while the optoelectronic characteristics of the metal-diamond junction have an ideality factor of 1.6 together with a rectification ratio of about 10(4) at +/-2.5 V. The nature of the mechanisms responsible for the conduction is discussed. (C) 1998 Elsevier Science S.A.

Photoconductivity and electrical properties of diamond films grown by MPCVD

The electrical and photoconductive features of as-grown microwave-plasma-assisted chemical-vapour deposition (MPCVD) diamond films are studied in correlation with magnetic results obtained from electron paramagnetic resonance (EPR). Also, the morphology is analysed by atomic force microscopy (AFM) showing [111] crystals with a good uniformity of the deposit. The photoresponse as well the current-voltage features observed show an efficient photogeneration of carriers while the optoelectronic characteristics of the metal-diamond junction have an ideality factor of 1.6 together with a rectification ratio of about 10(4) at +/-2.5 V. The nature of the mechanisms responsible for the conduction is discussed. (C) 1998 Elsevier Science S.A.