Relation between charge carrier mobility and lifetime in organic photovoltaics

The relationship between charge carrier lifetime and mobility in a bulk heterojunction based organic solar cell, utilizing diketopyrrolopyrole- naphthalene co-polymer and PC71BM in the photoactive blend layer, is investigated using the photoinduced charge extraction by linearly increasing voltage technique. Light intensity, delay time, and temperature dependent experiments are used to quantify the charge carrier mobility and density as well as the temperature dependence of both. From the saturation of photoinduced current at high laser intensities, it is shown that Langevin-type bimolecular recombination is present in the studied system. The charge carrier lifetime, especially in Langevin systems, is discussed to be an ambiguous and unreliable parameter to determine the performance of organic solar cells, because of the dependence of charge carrier lifetime on charge carrier density, mobility, and type of recombination. It is revealed that the relation between charge mobility (μ) and lifetime (τ) is inversely proportional, where the μτ product is independent of temperature. The results indicate that in photovoltaic systems with Langevin type bimolecular recombination, the strategies to increase the charge lifetime might not be beneficial because of an accompanying reduction in charge carrier mobility. Instead, the focus on non-Langevin mechanisms of recombination is crucial, because this allows an increase in the charge extraction rate by improving the carrier lifetime, density, and mobility simultaneously. © 2013 AIP Publishing LLC.

Scanning electron microscopy with energy dispersive spectroscopy and Raman and infrared spectroscopic study of tilleyite Ca5Si2O7(CO3)2-Y

The mineral tilleyite-Y, a carbonate-silicate of calcium, has been studied by scanning electron microscopy with chemical analysis using energy dispersive spectroscopy (EDX) and Raman and infrared spectroscopy. Multiple carbonate stretching modes are observed and support the concept of non-equivalent carbonate units in the tilleyite structure. Multiple Raman and infrared bands in the OH stretching region are observed, proving the existence of water in different molecular environments in the structure of tilleyite. Vibrational spectroscopy offers new information on the mineral tilleyite.

Enhancing current density using vertically oriented organic photovoltaics

We report a new organic photovoltaics (OPV) design, a wrapped OPV, which can circumvent both challenges of short exciton diffusion length [1], and low charge carrier mobility [2] of organic semiconductors by orienting the OPV vertically, to capture; manage; guide and use all incident photons and therefore, generate higher current. Resonant light, on being transmitted into a wrapped OPV, makes multiple passes through the photoactive layer and is absorbed completely, thus achieving benefits of thick photoactive layer while maintaining its ultra-thin thickness requirement. The current density generated from a wrapped OPV is twice than that generated by a similar OPV with flat orientation.

Flexible Ag electrode for use in organic photovoltaics

High efficiency organic photovoltaic cells discussed in literature are normally restricted to devices fabricated on glass substrates. This is a consequence of the extreme brittleness and inflexibility of the commonly used transparent conductive oxide electrode, indium tin oxide (ITO). This shortcoming of ITO along with other concerns such as increasing scarcity of indium, migration of indium to organic layer, etc. makes it imperative to move away from ITO. Here we demonstrate a highly flexible Ag electrode that possesses low sheet resistances even in ultra-thin layers. It retains its conductivity under severe bending stresses where ITO fails completely. A P3HT:PCBM blend organic solar cell fabricated on this highly flexible electrode gives an efficiency of 2.3%.

New microscale vertically oriented organic photovoltaics cells

We initially look at the changing energy environment and how that can have a dramatic change on the potential of alternative energies, in particular those of organic photovoltaicvs (OPV) cells. In looking at OPV's we also address the aspects of where we are with the current art and why we may not be getting the best from our materials. In doing so, we propose the idea of changing how we build organic photovoltaics by addressing the best method to contain light within the devices. Our initial effort is in addressing how these microscale optical concentrators work in the form of optical fibers in terms of absorption. We have derived a mathematical method which takes account of the input angle of light to achieve optimum absorption. However, in doing so we also address the complex issue how the changing refractive indices in a multilayer device can alter how we input the light. We have found that by knowing the materials refractive index our model takes into account the incident plane, meridonal plane, cross sectional are and path length to ensure optical angular input. Secondly, we also address the practicalities of making such vertical structures the greater issue of changing light intensity incident on a solar cell and how that aspects alters how we view the performance of organic solar cells.

Optimized multilayer indium-free electrodes for organic photovoltaics

Flexible multilayer electrodes that combine high transparency, high conductivity, and efficient charge extraction have been deposited, characterised and used as the anode in organic solar cells. The anode consists of an AZO/Ag/AZO stack plus a very thin oxide interlayer whose ionization potential is fine-tuned by manipulating its gap state density to optimise charge transfer with the bulk heterojunction active layer consisting of poly(n-3- hexylthiophene-2,5-diyl) and phenyl-C61-butyric acid methyl ester (P3HT:BC61BM). The deposition method for the stack was compatible with the low temperatures required for polymer substrates. Optimisation of the electrode stack was achieved by modelling the optical and electrical properties of the device and a power conversion efficiency of 2.9% under AM1.5 illumination compared to 3.0% with an ITO-only anode and 3.5% for an ITO:PEDOT electrode. Dark I-V reverse bias characteristics indicate very low densities of occupied buffer states close to the HOMO level of the hole conductor, despite observed ionization potential being high enough. Their elimination should raise efficiency to that with ITO:PEDOT.

Organic photovoltaics using thin gold film as an alternative anode to indium tin oxide

Indium Tin Oxide (ITO) is the most commonly used anode as a transparent electrode and more recently as an anode for organic photovoltaics (OPVs). However, there are significant drawbacks in using ITO which include high material costs, mechanical instability including brittleness and poor electrical properties which limit its use in low-cost flexible devices. We present initial results of poly(3-hexylthiophene): phenyl-C61-butyric acid methyl ester OPVs showing that an efficiency of 1.9% (short-circuit current 7.01 mA/cm2, open-circuit voltage 0.55 V, fill factor 0.49) can be attained using an ultra thin film of gold coated glass as the device anode. The initial I-V characteristics demonstrate that using high work function metals when the thin film is kept ultra thin can be used as a replacement to ITO due to their greater stability and better morphological control.

P-n junction organic photovoltaics fabricated by all solution processing

We have prepared p-n junction organic photovoltaic cells using an all solution processing method with poly(3-hexylthiophene) (P3HT) as the donor and phenyl-C 61-butyric acid methyl ester (PCBM) as the acceptor. Interdigitated donor/acceptor interface morphology was observed in the device processed with the lowest boiling point solvent for PCBM used in this study. The influences of different solvents on donor/acceptor morphology and respective device performance were investigated simultaneously. The best device obtained had characteristically rough interface morphology with a peak to valley value ∼15 nm. The device displayed a power conversion efficiency of 1.78%, an open circuit voltage (V oc) 0.44 V, a short circuit current density (J sc) 9.4 mA/cm 2 and a fill factor 43%.

Scanning electron microscopy study of worn Al-Si alloy surfaces

A pin-on-disc machine was used to wear Al-Si alloy pins under dry conditions. Unmodified and modified binary alloys and commercial multi-component alloys were tested. The surfaces of the worn alloys were examined by scanning electron microscopy to identify distinct topographical features to aid elucidation of the mechanisms of wear.

Scanning electron microscopy studies of wear in LM13 and LM13-graphite particulate composite

Examination of the structure of worn surfaces has shown that the wear of LM13 and LM13-graphite particulate composite is controlled by the nature and extent of subsurface deformation. The addition of graphite influences the wear characteristics by affecting the plastically deformed zone. The possible mechanisms of wear are discussed.

Assessment of conjunctival goblet cell density using laser scanning confocal microscopy versus impression cytology

Purpose To determine the association between conjunctival goblet cell density (GCD) assessed using in vivo laser scanning confocal microscopy and conjunctival impression cytology in a healthy population. Methods Ninety (90) healthy participants undertook a validated 5-item dry eye questionnaire, non-invasive tear film break-up time measurement, ocular surface fluorescein staining and phenol red thread test. These tests where undertaken to diagnose and exclude participants with dry eye. The nasal bulbar conjunctiva was imaged using laser scanning confocal microscopy (LSCM). Conjunctival impression cytology (CIC) was performed in the same region a few minutes later. Conjunctival goblet cell density was calculated as cells/mm2. Results There was a strong positive correlation of conjunctival GCD between LSCM and CIC (ρ = 0.66). Conjunctival goblet cell density was 475 ± 41 cells/mm2 and 466 ± 51 cells/mm2 measured by LSCM and CIC, respectively. Conclusions The strong association between in vivo and in vitro cellular analysis for measuring conjunctival GCD suggests that the more invasive CIC can be replaced by the less invasive LSCM in research and clinical practice.

Three-dimensional reconstruction of black tiger prawn (Penaeus monodon) spermatozoa using serial block-face scanning electron microscopy

Serial Block-Face Scanning Electron Microscopy (SBF-SEM) was used in this study to examine the ultrastructural morphology of Penaeus monodon spermatozoa. SBF-SEM provided a large dataset of sequential electron-microscopic-level images that facilitated comprehensive ultrastructural observations and three-dimensional reconstructions of the sperm cell. Reconstruction divulged a nuclear region of the spermatophoral spermatozoon filled with decondensed chromatin but with two apparent levels of packaging density. In addition, the nuclear region contained, not only numerous filamentous chromatin elements with dense microregions, but also large centrally gathered granular masses. Analysis of the sperm cytoplasm revealed the presence of degenerated mitochondria and membrane-less dense granules. A large electron-lucent vesicle and "arch-like" structures were apparent in the subacrosomal area, and an acrosomal core was found in the acrosomal vesicle. The spermatozoal spike arose from the inner membrane of the acrosomal vesicle, which was slightly bulbous in the middle region of the acrosomal vesicle, but then extended distally into a broad dense plate and to a sharp point proximally. This study has demonstrated that SBF-SEM is a powerful technique for the 3D ultrastructural reconstruction of prawn spermatozoa, that will no doubt be informative for further studies of sperm assessment, reproductive pathology and the spermiocladistics of penaeid prawns, and other decapod crustaceans. J. Morphol., 2016. (c) 2016 Wiley Periodicals, Inc.

Atomically resolved scanning tunneling microscopy on perovskite manganite single crystals

Atomically resolved scanning tunneling microscopy was conducted on cleaved single crystals of the cubic perovskite Pr0.68Pb0.32MnO3.Several different surface configurations could be resolved including a frequent square arrangement with atomic distances in excellent agreement to the bulk lattice constant of the cubic structure. We also observed stripe formation and a surface reconstruction. The latter is likely related to a polar rare earth-oxygen terminated surface. (C) 2010 American Institute of Physics.