Roger C. Hiorns

A brief guide to polymer nomenclature (IUPAC Technical Report)

The universal adoption of an agreed nomenclature has never been more important for the description of chemical structures in publishing and online searching. The International Union of Pure and Applied Chemistry (IUPAC) and Chemical Abstracts Service (CAS) make similar recommendations. The main points are shown here with references to original documents. Further details can be found in the IUPAC Purple Book.

A universal route to improving conjugated macromolecule photostability

This predictive study demonstrates that the introduction of aromatic-oxy-alkyl links surprisingly makes materials more resistant to photo-oxidative degradation by reducing hydrogen abstraction. This revelation makes it possible, for the first time, to design a toolbox of substituents for soluble, photostable conjugated materials.

Enhanced thermal stability of organic solar cells by using photolinkable end-capped polythiophenes

The use of poly(3-hexylthiophene) (P3HT) end-capped with anthracene (P3HT-A) in a blend with [6,6]-phenyl C61 butyric acid methyl ester (PCBM) is demonstrated to physically stabilize bulk-heterojunction photovoltaic solar cells. Bulk heterojunction-based devices are known to undergo phase separation of donor and acceptor materials during operation resulting in the formation of large (μm scale) PCBM crystals that dramatically decrease the photovoltaic characteristics of the cell. By way of a facile UV-curing step, the P3HT-A chain most likely reacts with PCBM fullerene via a [2+2] cyclo-addition to stabilize the blend. Photovoltaic devices based on P3HT-A and PCBM have been optimised in terms of thermal annealing to obtain initial devices to determine the UV-curing protocols. UV-exposure was found to improve device stability while simultaneously having a minimal effect on device efficiency. Optical microscopy demonstrates that the few reactions of P3HT-A with PCBM are efficient enough to prevent the formation of micro-sized PCBM crystals responsible for the failure of solar cells.

THE FIRST EXAMPLE OF A POLY(ETHYLENE OXIDE)-POLY(METHYLPHENYLSILANE) AMPHIPHILIC BLOCK COPOLYMER : VESICLE FORMATION IN WATER

A new amphiphilic multiblock copolymer of polymethylphenylsilane and poly(ethylene oxide) has been synthesised and demonstrated to form well-defined aggregates in water.

A tentative theory for conjugated rod-coil multi-block copolymer assembly and the initial characterisation by atomic force microscopy and small angle neutron scattering of poly(polymethylphenylsilane-block-polyisoprene)

Polydisperse (Mw/Mn = 2.4) rod-coil, multi-block poly(polymethylphenylsilane-block-polyisoprene) (PMPS-b-PI), containing semiconductor segments, was characterised using atomic force microscopy (AFM) and neutron scattering. Despite high polydispersities (Mw/Mn (PMPS) = 1.64; Mw/Mn (PI) = 1.34) resulting in disorder, solid-state PMPS-b-PI was indicated to form a regular, modulated morphology with average domain repeat unit of 18.8 nm at the surface (AFM) and 18.6nm in the bulk (neutron). The observed regularity was tentatively proposed to result from a lamella-like morphology preferred by rod-coil, multi-block copolymers and the helical, segmental geometry of PMPS. This tentative theory was corroborated by good agreement between expected and found domain sizes, which indicated that copolymers did not, to a recognisable extent, fold and reverse direction.

Increased thermal stabilization of polymer photovoltaic cells with oligomeric PCBM

The first oligomerisation of phenyl-C61-butyric acid methyl ester (PCBM) using a facile atom transfer radical addition polymerization (ATRAP) and its exploitation for organic photovoltaic devices is described. Oligo{(phenyl-C61-butyric acid methyl ester)-alt-[1,4-bis(bromomethyl)-2,5-bis(octyloxy)benzene]} (OPCBMMB) shows opto-electronic properties equivalent to those of PCBM but has a higher glass transition temperature. When mixed with various band gap semiconducting polymers, OPCBMMB delivers performances similar to PCBM but with an enhanced stabilization of the bulk heterojunction in photovoltaic devices on plastic substrates under thermal stress, regardless of the degree of crystallinity of the polymer and without changing opto-electronic properties.

Facile synthesis of poly(3-hexylthiophene)- block -poly(ethylene oxide) copolymers via Steglich esterification

Poly(ethylene oxide) has been coupled to poly(3-hexylthiophene) using esterification to produce pure diblock copolymers, highly relevant for use in organic electronic devices. The new synthetic route described herein uses a metal-free coupling step, for the first time, to afford well-defined polymers in high yields following facile purification.

Suppression of Thermally Induced Fullerene Aggregation in Polyfullerene-Based Multiacceptor Organic Solar Cells

A novel main-chain polyfullerene, poly[fullerene-alt-2,5-bis(octyloxy)terephthalaldehyde] (PPC4), is investigated for its hypothesized superior morphological stability as an electron-accepting material in organic photovoltaics relative to the widely used fullerene phenyl-C61-butyric acid methyl ester (PCBM). When mixed with poly(3-hexylthiophene-2,5-diyl) (P3HT), PPC4 affords low-charge-generation yields because of poor intermixing within the blend. The adoption of a multiacceptor system, by introducing PCBM into the P3HT:polyfullerene blend, was found to lead to a 3-fold enhancement in charge generation, affording power conversion efficiencies very close to that of the prototypical P3HT:PCBM binary control. Upon thermal stressing and in contrast to the P3HT:PCBM binary, photovoltaic devices based on the multiacceptor system demonstrated significantly improved stability, outperforming the control because of suppression of the PCBM migration and aggregation processes responsible for rapid device failure. We rationalize the influence of the fullerene miscibility and its implications on the device performance in terms of a thermodynamic model based on Flory-Huggins solution theory. Finally, the potential universal applicability of this approach for thermal stabilization of organic solar cells is demonstrated, utilizing an alternative low-band-gap polymer-donor system.

Transport mechanisms in 8-tris-hydroxyquinoline aluminium (Alq3) electronic layers: a study by photodipolar absorption

This paper describes the role of traps in the electronic conductivity of tris(8-hydroxyquinoline)aluminium (Alq3) in a conventional sandwich structure with indium tin oxide and aluminium electrodes. New results obtained by photodipolar absorption techniques and impedance spectroscopy are presented. The former method acts as a probe to highlight the role of traps. It is shown that optical pumping of electrons to trap levels gives a clear increase in dielectric absorption due to the reorientation of dipoles associated with trapped charges. The trap depth is estimated to be around E t = 0.19 eV, a value in good agreement with theoretical calculations and thermoluminescence measurements. The latter method permits a representation of the sample in terms of a circuit composed of a parallel capacitor (C p) and resistor (R p) both in series with a resistor R s ≈ 50 Ω located on the anode side. A logarithmic plot of R p as a function of the dc bias voltage gives a linear law that is recognized, for the first time, to be a consequence of a trapped charge limited (TCL) current. The linearity can be improved by the introduction of a field-dependent mobility.

Designing intrinsically photostable low band gap polymers: a smart tool combining EPR spectroscopy and DFT calculations

A rapid and efficient method to identify the weak points of the complex chemical structure of low band gap (LBG) polymers, designed for efficient solar cells, when submitted to light exposure is reported. This tool combines Electron Paramagnetic Resonance (EPR) using the ‘spin trapping method’ coupled with density functional theory modelling (DFT). First, the nature of the short life-time radicals formed during the early-stages of photo-degradation processes are determined by a spin-trapping technique. Two kinds of short life-time radical (R˙ and R′O˙) are formed after ‘short-duration’ illumination in an inert atmosphere and in ambient air, respectively. Second, simulation allows the identification of the chemical structures of these radicals revealing the most probable photochemical process, namely homolytical scission between the Si atom of the conjugated skeleton and its pendent side-chains. Finally, DFT calculations confirm the homolytical cleavage observed by EPR, as well as the presence of a group that is highly susceptible to photooxidative attack. Therefore, the synergetic coupling of a spin trapping method with DFT calculations is shown to be a rapid and efficient method for providing unprecedented information on photochemical mechanisms. This approach will allow the design of LBG polymers without the need to trial the material within actual solar cell devices, an often long and costly screening procedure.