Optimization of solution-processed oligothiophene:fullerene based organic solar cells by using solvent additives

The optimization of solution-processed organic bulk-heterojunction solar cells with the acceptor-substituted quinquethiophene DCV5T-Bu-4 as donor in conjunction with PC61BM as acceptor is described. Power conversion efficiencies up to 3.0% and external quantum efficiencies up to 40% were obtained through the use of 1-chloronaphthalene as solvent additive in the fabrication of the photovoltaic devices. Furthermore, atomic force microscopy investigations of the photoactive layer gave insight into the distribution of donor and acceptor within the blend. The unique combination of solubility and thermal stability of DCV5T-Bu-4 also allows for fabrication of organic solar cells by vacuum deposition. Thus, we were able to perform a rare comparison of the device characteristics of the solution-processed DCV5T-Bu-4:PC61BM solar cell with its vacuum-processed DCV5T-Bu-4:C-60 counterpart. Interestingly in this case, the efficiencies of the small-molecule organic solar cells prepared by using solution techniques are approaching those fabricated by using vacuum technology. This result is significant as vacuum-processed devices typically display much better performances in photovoltaic cells. Keywords

Friction and material transfer in micro-scale sliding contact between aluminium alloy and steel

A ball-on-flat reciprocating micro-tribometer has been used to measure the friction coefficient between aluminium alloy strip and a steel ball. A relatively small ball and correspondingly low contact load is used to give a contact width of the order of 100μm, closer to asperity contact widths than generally found for this type of test. The effects of load, initial strip surface roughness, lubricants and boundary additives are investigated. It is found that the friction coefficient is significantly reduced by the addition of a lubricant. Observations of the wear tracks and ball surface show that the material transfer from aluminium to the ball is reduced in the presence of the lubricant. The initial friction coefficient is further reduced by the addition of a boundary additive, but the friction coefficient after 8 cycles is unchanged. Copyright © 2004 by Springer Science+Business Media, Inc.

Friction of sliding surfaces carrying adsorbed lubricant layers

In concentrated contacts the behaviour of lubricants is much modified by the high local pressures: changes can arise both from molecular ordering within the very thin film lubricant layers present at the interface as well as from the deposition on the component surfaces of more solid-like polymeric boundary layers. These 'third bodies' separating the solid surfaces may have rheological or mechanical properties very different from those observed in the bulk. Classical elasto-hydrodynamic theory considers the entrapped lubricant to exhibit a piezo-viscous behaviour while the conventional picture of more solid boundary lubricant layers views their shear strength r as being linearly dependent on local pressure p, so that T = TO + ap where TO and a are constants. If TO is relatively small, then the coefficient of friction \i = T Ip ~ a and so Amonton's laws are recovered. However, the properties of adsorbed or deposited surface films, or indeed other third bodies such as debris layers, may be more complex than this. A preliminary study has looked quantitatively at the influence of the pressure dependence of the shear strength of any surface layer on the overall friction coefficient of a contact which is made up of an array of asperities whose height varies in a Gaussian manner. Individual contact points may be elastic or plastic. The analysis results in plots of coefficient of friction versus the service or load parameter PIH&NRa where P is the nominal pressure on the contact, HS the hardness of the deforming surface, N the asperity density, R the mean radius of curvature of the asperities, and a is the standard deviation of their height distribution. In principle, any variation oft withp can be incorporated into the model; however, in this initial study we have used data on colloidal suspensions from the group at the Ecole Centrale de Lyon as well as examining the effect of functional relationships of somewhat greater complexity than a simple linear form. Results of the analysis indicate that variations in fj. are possible as the load is varied which depend on the statistical spread of behaviour at individual asperity contacts. The value of this analysis is that it attempts to combine the behaviour of films on the molecular scale with the topography of real engineering surfaces and so give an indication of the effects at the full-size or macro-scale that can be achieved by chemical or molecular surface engineering.

Extent of oxidation of hydrocarbons desorbing from the lubricant oil layer in spark-ignition engines

The extent of oxidation of hydrocarbons desorbing from the oil layer has been measured directly in a hydrogen-fueled, spark-ignited engine in which the lubricant oil was doped with a single component hydrocarbon. The amount of hydrocarbon desorbed and oxidized could be measured simultaneously as the dopant was only source of carbon-containing species. The fraction oxidized was strongly dependent on engine load, hydrogen fuel-air ratio and dopant chemical reactivity, but only modestly dependent on spark timing and nitrogen dilution levels below 20 percent. Fast FID measurements at the cylinder exit showed that the surviving hydrocarbons emerge late in the exhaust stroke. © Copyright 1996 Society of Automotive Engineers, Inc.