Application of ray-tracing to interpreting beam prints from fibre laser cutting

2D ray-tracing has been used to investigate the influence of inclined cutting front and the cutting edge on the exiting, surplus, laser beam while cutting 6 mm mild steel with a fibre laser, wavelength = 1070 nm. The ray-tracing model, which assumed a Gaussian beam, has investigated the distribution of this surplus beam as a function of kerf wall inclination angle. Multiple reflections are shown to broaden the spatial distribution of the surplus beam. Dross and kerf wall roughness also broaden the spatial distribution of the surplus beam. Comparison of an experimentally obtained beam print from the surplus beam for 1.07 µm fibre laser cutting of 6 mm stainless steel with modelled results allows inclination of the cut front to be estimated. The experimentally obtained beam print has features consistent with roughness/dross induced beam broadening

Increasing organic solar cell efficiency with polymer interlayers

We demonstrate how organic solar cell efficiency can be increased by introducing a pure polymer interlayer between the PEDOT:PSS layer and the polymer: fullerene blend. We observe an increase in device efficiency with three different material systems over a number of devices. Using both electrical characterization and numerical modeling we show that the increase in efficiency is caused by optical absorption in the pure polymer layer and hence efficient charge separation at the polymer bulkheterojunction interface

A general mechanism for controlling thin film structures in all-conjugated block copolymer : fullerene blends

Block copolymers have the potential to self-assemble into thermodynamically stable nanostructures that are desirable for plastic electronic materials with prolonged lifetimes. Fulfillment of this potential requires the simultaneous optimisation of the spatial organisation and phase behaviour of heterogeneous thin films at the nanoscale. We demonstrate the controlled assembly of an all-conjugated diblock copolymer blended with fullerene. The crystallinity, nanophase separated morphology, and microscopic features are characterised for blends of poly(3-hexylthiophene-block-3-(2-ethylhexyl) thiophene) (P3HT-b-P3EHT) and phenyl-C61-butyric acid methyl ester (PCBM), with PCBM fractions varying from 0–65 wt%. We find that PCBM induces the P3HT block to crystallise, causing nanophase separation of the block copolymer. Resulting nanostructures range from ordered (lamellae) to disordered, depending on the amount of PCBM. We identify the key design parameters and propose a general mechanism for controlling thin film structure and crystallinity during the processing of semicrystalline block copolymers

In Situ Visualization and Quantification of Electrical Self-Heating in Conjugated Polymer Diodes Using Raman Spectroscopy

Self-heating in organic electronics can lead to anomalous electrical performance and even accelerated degradation. However, in the case of disordered organic semiconductors, self-heating effects are difficult to quantify using electrical techniques alone due to complex transport properties. Therefore, more direct methods are needed to monitor the impact of self-heating on device performance. Here, self-heating in poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta [2,1-b;3,4-b′] dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT) diodes is visualized using Raman spectroscopy, and thermal effects due to self-heating are quantified by exploiting temperature-dependent shifts in the polymer vibrational modes. The temperature increases due to self-heating are quantified by correlating the Raman shifts observed in electrically biased diodes with temperature-dependent Raman measurements. Temperature elevations up to 75 K are demonstrated in the PCPDTBT diodes at moderate power of about 2.6–3.3 W cm−2. Numerical modeling rationalizes the significant role of Joule and recombination heating on the diode current–voltage characteristics. This work demonstrates a facile approach for in situ monitoring of self-heating in organic semiconductors for a range of applications, from fundamental transport studies to thermal management in devices

Fullerene-free organic solar cells with an efficiency of 3.7% based on a low-cost geometrically planar perylene diimide monomer

The aggregate-induced limitation for high power-conversion efficiencies (PCEs) of perylene-diimide (PDI): polymer solar cells can be circumvented when two simple rules are respected; the aggregate size of PDI remains short enough and the omnipresent PDI aggregates are electronically interconnected. Following these guidelines, a PCE of 3.7% is delivered by using the solution-processable, planar PDI monomer of N,N’-bis(1-ethylpropyl)-perylene-3,4,9,10-tetracarboxylic diimide as the electron acceptor mixed with the low-energy gap polymeric donor poly[(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b;4,5-b’]dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiophene))-2,6-diyl] (PBDTTT-CT). The PBDTTT-CT:PDI composite absorbs strongly the light in the region of 400 nm–800 nm and after adding a small amount of 1,8-diiodooctane (DIO) efficient photocurrent generation is achieved. Space-charge limited dark current and transient photovoltage measurements suggest that the use of the DIO component optimizes the electron/hole carrier mobility ratio, suppresses the non-geminate recombination losses and improves the charge extraction efficiency