Natalie P. Holmes

Effect of template on the formation of phase-inversed molecularly imprinted polymer thin films: an assessment

A range of molecularly imprinted polymer films (MIPFs) consisting of acrylonitrile (AN) as a matrix monomer, methacrylamide (MAM) as a functional monomer (96 : 4) for 2,4-dinitrotoluene (DNT) and 2,4,6-trinitrotoluene (TNT) were prepared using a phase inversion method with post polymerisation introduction of the template. The films exhibited no rebinding activity following template removal, prompting speculation of a collapse in the pore structure of the materials. Addition of extremely low concentrations of cross-linker (0.01% ethylene glycol dimethacrylate (EDGMA)) successfully stabilised the macroscopic structure of the films. The same approach was then applied to the preparation of conventionally imprinted films. The average molecular weight of the polymer chains in the MIPFs was found to decrease with increasing template concentration, confirming results from previous studies identifying TNT as a chain transfer agent. Failure to observe rebinding in the films in either the liquid or gas phases suggested that template removal precipitates irreversible changes in microscopic pore structure of the MIPFs. Thermogravimetric analysis (TGA) showed a proportionate loss in mass of the template from the non-extracted MIPFs, confirming the successful encapsulation of the template. Differential scanning calorimetry (DSC) measurements revealed distinct differences between the non-extracted films and the non-imprinted control polymer that become more exaggerated with the presence of increasing amounts of template in the MIPF formulation. Following template removal, the MIPF thermograms closely resembled the non-imprinted control, suggesting that template removal promotes changes in the MIPF microstructure consistent with the loss of template cavities and the formation of a more homogeneous structure.

Solution processable interface materials for nanoparticulate organic photovoltaic devices

Nanoparticulate zinc oxide can be prepared at low temperatures from solution processable zinc acetylacetonate. The use of this material as a cathode interfacial layer in nanoparticulate organic photovoltaic devices results in comparable performances to those based on reactive calcium layers. Importantly, the enhanced degradation stability and full solution processability make zinc oxide a more desirable material for the fabrication of large area printed devices.

Modelling transport in nanoparticle organic solar cells using Monte Carlo methods

Water-based nanoparticle (NP) organic solar cells eliminate the need for harmful organic solvents during deposition. However, the core-shell NP structure should limit charge extraction resulting in poor performance. Here, we use dynamic Monte Carlo modelling to show that for optimised NP structures the core-shell character does not severely limit performance. Simulations further reveal that small NPs are more susceptible to extensive phase segregation, which diminishes charge carrier percolation pathways from the cores to the electrodes and thus inhibits charge extraction. Simulated performance behaviour was used to propose an explanation for the experimentally observed change in performance due to annealing.

Optimisation of purification techniques for the preparation of large-volume aqueous solar nanoparticle inks for organic photovoltaics

In this study we have optimised the preparation conditions for large-volume nanoparticle inks, based on poly(3-hexylthiophene) (P3HT):indene-C60 multiadducts (ICxA), through two purification processes: centrifugal and crossflow ultrafiltration. The impact of purification is twofold: firstly, removal of excess sodium dodecyl sulfate (SDS) surfactant from the ink and, secondly, concentration of the photoactive components in the ink. The removal of SDS was studied in detail both by a UV–vis spectroscopy-based method and by surface tension measurements of the nanoparticle ink filtrate; revealing that centrifugal ultrafiltration removed SDS at a higher rate than crossflow ultrafiltration even though a similar filter was applied in both cases (10,000 Da M w cut-off). The influence of SDS concentration on the aqueous solar nanoparticle (ASNP) inks was investigated by monitoring the surface morphology/topography of the ASNP films using atomic force microscopy (AFM) and scanning electron microscopy (SEM) and photovoltaic device performance as a function of ultrafiltration (decreasing SDS content). The surface morphology/topography showed, as expected, a decreased number of SDS crystallites on the surface of the ASNP film with increased ultrafiltration steps. The device performance revealed distinct peaks in efficiency with ultrafiltration: centrifuge purified inks reached a maximum efficiency at a dilution factor of 7.8 × 104, while crossflow purified inks did not reach a maximum efficiency until a dilution factor of 6.1 × 109. This difference was ascribed to the different wetting properties of the prepared inks and was further corroborated by surface tension measurements of the ASNP inks which revealed that the peak efficiencies for both methods occurred for similar surface tension values of 48.1 and 48.8 mN m−1. This work demonstrates that addressing the surface tension of large-volume ASNP inks is key to the reproducible fabrication of nanoparticle photovoltaic devices.