Influence Of Film Morphology Research Articles

Influence of Film Morphology on Transient Photocurrent Pulse Shape in Organic Thin Films: A Monte Carlo Study

The influence of film morphology on the broadening of the time-of-flighttransient photo-current pulse is investigated using Monte Carlo simulation.Simulation of the time-of-flight transient photo-current pulse shape is carriedout for homogeneous and inhomogeneous organic thin films by varying theoverall energetic disorder. In homogeneous system, the value of the tailbroadening parameter (W) of the photocurrent pulse is found to decrease upondecreasing the energetic disorder, which can be attributed to the variation inthe non-thermal field assisted diffusion. Interestingly, in the case of inhomo-geneous system, upon decreasing the overall energetic disorder of the systemthe value of W initially attains a maximum value before it starts decreasing.This observation is explained in terms of the morphology dependent carrierdiffusion. This study asserts the importance of the influence of the morphologydependent carrier diffusion on the charge transport in disordered systems andthe related experimental measurements.

Influence of film morphology and crystallinity on charge carrier concentration-dependent hole mobility in hexamethyldisilazane treated Pentacene bottom contact devices

The effect of substrate temperature and deposition rate on the film morphology, crystallinity, and electronic properties of Pentacene transistors treated with hexamethyldisilazane (HMDS) is studied. The gate bias dependence of mobility is used to estimate the width of the density of states and thereby quantify the disorder of the highest occupied molecular orbital. A low deposition rate and the substrate held at room temperature are shown to be the optimal conditions for good mobility (0.20 cm2 V−1 s−1) and low electronic disorder (50 ± 10 meV). X-ray diffraction measurements are performed to quantify the ratio of the two crystalline phases (thin-film phase and bulk phase) present in the film. The crystalline phases, rather than grain size, plays a significant role in determining the charge carrier mobility. Film deposition with the substrate at room temperature leads to low electronic disorder as the film is composed of one crystalline phase (thin-film phase), while high substrate temperature makes the film increasingly polymorphic, leading to increased electronic disorder (up to 230 meV). A high deposition rate leads to poor morphology of Pentacene near the source/drain electrode edge, thereby leading to increased contact resistance and electronic disorder. Hence, a low growth rate at room temperature is required for HMDS treated substrates to induce good crystalline properties of the film in the channel region, which results in enhanced electronic properties of the transistors.

Reaction Kinetics and Influence of Film Morphology on the Oxidation of Propene Thin Films by O(3P) Atomic Oxygen

We present results detailing the oxidative reactivity of condensed propene thin films, with particular attention to epoxide product formation because of its importance in the industrial production ...

Factors Affecting the Power Conversion Efficiency in ZnO DSSCs: Nanowire vs. Nanoparticles

A comparative assessment of nanowire versus nanoparticle-based ZnO dye-sensitized solar cells (DSSCs) is conducted to investigate the main parameters that affect device performance. Towards this aim, the influence of film morphology, dye adsorption, electron recombination and sensitizer pH on the power conversion efficiency (PCE) of the DSSCs is examined. Nanoparticle-based DSSCs with PCEs of up to 6.2% are developed and their main characteristics are examined. The efficiency of corresponding devices based on nanowire arrays (NW) is considerably lower (0.63%) by comparison, mainly due to low light harvesting ability of ZnO nanowire films. The dye loading of nanowire films is found to be approximately an order of magnitude lower than that of nanoparticle-based ones, regardless of their internal surface area. Inefficient anchoring of dye molecules on the semiconductor surface due to repelling electrostatic forces is identified as the main reason for this low dye loading. We propose a method of modifying the sensitizer solution by altering its pH, thereby enhancing dye adsorption. We report an increase in the PCE of nanowire DSSCs from 0.63% to 1.84% as a direct result of using such a modified dye solution.

Facile chemical bath deposition method for interconnected nanofibrous polythiophene thin films and their use for highly efficient room temperature NO2 sensor application

Interconnected nanofibrous polythiophene (INPTh) film was deposited on the glass substrate through a simple chemical bath deposition method. The influence of monomer concentration on INPTh film properties as well as on NO2 sensing properties of the film was studied. The morphological and structural studies were carried out using FTIR spectroscopy, FE-SEM microscope, and AFM analysis. The FTIR spectra confirmed the formation of PTH structure. The interconnected nanofibrous surface morphology was observed in FE-SEM images. The roughness of the film and thickness (225nm–442nm) was found to increase with monomer concentration up to 0.5M, after that, both decreased at 0.6M monomer concentration. The highest selectivity of PTh thin film towards NO2 was observed than the other gases like H2S, SO2, NH3, CO and LPG. The influence of film morphology and thickness on gas sensing properties was observed, which was varied with monomer concentration. The film deposited at 0.5M monomer concentration showed the highest NO2 gas response of 47.58% at room temperature. Present results revealed that monomer concentration was also one of the deposition parameters for tuning the morphological as well as gas sensing properties of the chemical bath deposited PTh film.

Co-existence of Radiative and Non-Radiative Surface Plasmon Resonance Modes: Power Balance and Influence of Film Morphology

The power balance and conditions of co-existing radiative and non-radiative surface plasmon resonance modes have been exploited at the metal film-water interface. Angular (AIM) and wavelength (WIM) interrogation sensing conditions were applied. Polycrystalline, void-free metal layers were sputter deposited, composed of nm-sized crystalline grains. Radiative decay of propagating SPs, with emission into the upper half-space, has been observed for 50-nm thin Au and Ag-layers on BK7 glass and polymeric substrates in the AIM. The associated thermo-optical power budget has been quantified by calorimetric heat flow recordings, comprising a micro-fluidic set-up. For the Au-water interface, at least 88 ± 4 % of the SP-absorbed optical power dissipates by phonon excitation, followed by heat transfer from the metal film into the adjacent liquid phase. Only a small fraction of 2.4 ± 0.6 % dissipates through radiative emission. In presence of a rough interface and WIM conditions, radiative SP-decay persists by emission into the lower half-space. The luminescence type feature most likely originates from radiative excitation and decay of localized SPs, coupled to propagating SPs within the film volume. The spectral characteristic is in accord with earlier reported experimental observations by others and numerical simulations.

Inkjet-printed gold nanoparticle chemiresistors: Influence of film morphology and ionic strength on the detection of organics dissolved in aqueous solution

The influence of film morphology on the performance of inkjet-printed gold nanoparticle chemiresistors has been investigated. Nanoparticles deposited from a single-solvent system resulted in a “coffee ring”-like structure with most of the materials deposited at the edge. It was shown that the uniformity of the film could be improved if the nanoparticles were deposited from a mixture of solvents comprising N-methyl-2-pyrrolidone and water. Electrical conductivity measurements showed that both “coffee ring” and “flat” films were qualitatively similar suggesting that the films have similar nanoscale structures. To form the functional chemiresistor device, the 4-(dimethylamino)pyridine coating on the nanoparticle was exchanged with 1-hexanethiol to provide a hydrophobic sensing layer. The performance of 1-hexanethiol coated gold nanoparticle chemiresistors to small organic molecules, toluene, dichloromethane and ethanol dissolved in 1 M KCl in regard to changes in impedance and response times was unaffected by the film morphology. For larger hydrocarbons such as octane, the rate of uptake of the analyte into the film was significantly faster when the flatter nanoparticle film was used as opposed to the “coffee ring” film which has a thicker edge. Furthermore, the presence of potassium and chloride ions in the solution media does not significantly affect the impedance of the nanoparticle film at 1 Hz (<2% variation in film impedance over more than four orders of magnitude change in ionic strength). However, the ionic strength of the media affected the partitioning of the analyte into the hydrophobic nanoparticle film. The response of the sensor was found to increase with an increased salt concentration due to a salting-out of the analyte from the solution.

Impedance measurements at thin polyaniline films – the influence of film morphology

AC impedance measurements of thin 100-nm polyaniline films electrosynthesized in various acidic electrolyte solutions known to support different film morphologies are reported. By carefully defining experimental parameters obtained data could be satisfactorily explained invoking a simple equivalent circuit assuming a double layer capacity assigned to the polymer/solution interface, a pseudocapacity related to the internal redox capacity of the film, a film and a solution resistance and could be related to film morphologies. The data can be related to structural/morphological features of PANI films in close agreement with generally accepted changes of PANI films as a function of the degree of oxidation and type of anion present during electropolymerization.

Film morphology effects on the electrical and optical properties of bulk heterojunction organic solar cells based on MEH-PPV/C 60 composite

The influence of film morphology on the electrical behaviour of an MEH-PPV/C 60 organic solar cells has been investigated. The dissociation of photogenerated charge pairs in composites of buckminsterfullerenes (C 60) in a conjugated polymer matrix (MEH-PPV) forming dispersed heterojunctions was studied at low C 60 acceptor concentrations to separate electron transfer from charge transport effects. The motivation of this study was to analyse the strong dependence of organic solar cell efficiencies on the morphology of the composite. Two effects controlling film morphology have been investigated; the first one being the influence of the fullerene concentration and the second one is the effect of the organic solvent used to deposit the photoactive layer. The sample morphology was studied using atomic force microscopy (AFM). Photoluminescence (PL) experiments and current–voltage ( I– V) measurements were performed on the deposited photovoltaic film to investigate the influence of dispersion on the charge transfer process between MEH-PPV and C 60. An attempt to explain all the results will be presented.

Characterization of acoustic Lamb wave propagation in polycrystalline diamond films by laser ultrasonics

The propagation of acoustic Lamb waves in free standing chemical vapor deposited polycrystalline diamond has been studied using a laser ultrasonic technique. The influence of film morphology, quality, and thickness on the waves has been assessed. Acoustic waves with high velocities in the range 8700–12 200 ms−1 were observed; the lowest values were recorded for films with the lowest crystal quality and highest nondiamond content. High quality films with differing crystal textures or thickness show little variation. The influence of temperature on the dispersion characteristics of Lamb wave propagating in a 50 mm diam polycrystalline diamond wafer were also investigated. Little variation was apparent across the range studied (30–250 °C). Material parameters extracted from the dispersion chracteristics of the acoustic signal together with scanning electron microscopy studies suggested that void, microcrack, and grain boundary density most influences the propagation of low frequency Lamb waves in free standing CVD diamond films.

Influence of film morphology on the vibrational spectra of dioctyl substituted polyfluorene (PFO)

Raman spectroscopy has been performed on samples of dioctyl substituted polyfluorene (PFO) with four morphologies: (i) as spin-coated, (ii) crystalline, (iii) glassy, (iv) prepared so as to contain a fraction of material with an intra-chain ordered conformation. The different morphologies were thermally induced in films spin-coated from solutions of PFO in tetrahydrofuran, chloroform and cyclohexane on aluminium, silicon and quartz substrates. There are no differences in the Raman spectra measurement for the glassy and spin-coated samples. The spectra of the samples with the intra-chain ordered state and in the crystalline phase show shifts in positions and linewidths of the Raman peaks. The vibrational modes are identified by reference to published spectra for both model compounds and related phenylene-based polymers.

Performance of diamond-based photoconductive devices in the UV range

A detailed investigation of diamond-based UV planar photoconductors with interdigitated contacts is presented, analyzing in particular the influence of film morphology and texturing on device performance. Photoluminescence and photocurrent spectroscopies reveal a strong correlation between structural defects and detector selectivity, whereas impurities affect photocurrent yield mainly in highly oriented materials. Responsivity measurements show that diamond-based devices give the highest UV-visible discrimination in comparison to UV-enhanced Si- and GaN-based sensors. For λ ≤225 nm, an almost linear photocurrent-light intensity relationship is observed, although, in a few cases, a superlinear dependence on illumination intensity is found. A theoretical model, capable of describing the observed field dependendence of photocurrent gain is presented, giving minority carriers lifetime and mobility in the range 0.1–1 ns and 10–300 cm 2 V −1 s −1 , respectively.

The Influence of Film Morphology on the Photovoltaic Effect in α-Sexithienyl Thin Film Devices

ABSTRACTWe report measurements made on the photovoltaic effect in devices fabricated with thin films of α-sexithienyl (αT6) as the active layer. aT6 was sublimed onto glass/ITO substrates (as the positive contact), and the substrate temperature was varied between room temperature and 156°C during sublimation to control the film morphology. As the substrate temperature increases, the average size of the αT6 grains increases, from ˜200nm at 22° C to ˜1500nm at 150°C, and order within the grains increases. Aluminium metal was evaporated onto the αT6 as the top (negative) contact.The dark current-voltage (I-V) curve is strongly rectifying, whereas under illumination it becomes almost symmetrical about ˜0.7V on the voltage axis, which represents the maximum open-circuit voltage. This behaviour indicates that the efficiency of charge separation is strongly bias dependent, and that charges are separated by the bulk field rather than at, for example, a Schottky barrier. The spectral response of the photocurrent is independent of the film morphology, which is surprising since the environment in which excitons recombine radiatively in αT6 is known to depend on the morphology of the film. The spectral response is consistent with previous measurements on photoconductivity, indicating that the efficiency of charge separation increases with increasing photon energy. There is also a significant photocurrent at energies well below the absorption edge of αT6, suggesting an exponential distribution of defect states extending at least 0.6eV into the optical gap, which may be a distribution of X-traps below the absorption edge. As the substrate temperature during growth increases from 28C to 150C the efficiency of charge separation decreases by an order of magnitude, suggesting that exciton dissocation is assisted by traps or defects which are present in smaller numbers in the more ordered films.