In situ studies of spin-coated polymer films

Spin-coating is a facile, straightforward, solution processing technique for the production of uniform thin films, which has been utilised for a wide range of applications including organic electronic and photonic devices, sensors, membranes and optical coatings. Many of the applications of spin-coated polymer films utilise the propensity of polymers to self-assemble, resulting in the formation of well-ordered, intricate morphologies that evolve towards thermodynamic equilibrium. Understanding the interplay between processes such as phase-separation and crystallisation, which control the final morphology, has therefore been the topic of intense theoretical and experimental studies in the field of polymer science. In particular the potential of spun-cast organic electronic devices to help alleviate the world's dependence upon fossil fuels in terms of energy generation and energy efficiencies has driven the need for greater control over the final morphology, in order to produce more efficient devices. I have developed the technique of stroboscopic microscopy, which facilitates direct imaging during spin-coating, allowing us to directly observe, in real-time, the processes of self-assembly at the microscale. I have advanced the technique to operate in three different modes, which allow observations of topography, composition and crystallisation. This thesis presents the direct observations of self-assembly processes that occur during the spin-coating of model polymer systems [polystyrene:poly(methyl methacrylate) and polystyrene:poly(ethylene glycol)], systems relevant to organic electronics [polystyrene:poly 9,9’-dioctlyflourene] and polystyrene colloidal dispersions. A number of key parameters have been investigated including the effects of; rotation rate, composition, polydispersity and the interplay between crystallisation and phase separation. The observation that of have been made may be utilised to either, rationally design processing conditions that will allow targeted morphologies to be attained or information obtained in real-time may be used to direct and control self-assembly processes in order to achieve desired morphologies

Does 1,8-diiodooctane affect the aggregation state of PC71BM in solution?

1,8-Diiodooctane (DIO) is an additive used in the processing of organic photovoltaics and has previously been reported, on the basis of small-angle X-ray scattering (SAXS) measurements, to deflocculate nano-aggregates of [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) in chlorobenzene. We have critically re-examined this finding in a series of scattering measurements using both X-rays and neutrons. With SAXS, we find that the form of the background solvent scattering is influenced by the presence of DIO, that there is substantial attenuation of the X-rays by the background solvent and that there appears to be beam-induced aggregation. All three factors call into question the suitability of SAXS for measurements on these samples. By contrast, small-angle neutron scattering (SANS) measurements, performed at concentrations of 15 mg ml−1 up to and including 40 mg ml−1, show no difference in the aggregation state for PC71BM in chlorobenzene with and without 3% DIO; we find PC71BM to be molecularly dissolved in all solvent cases. In situ film thinning measurements of spin-coated PC71BM solution with the DIO additive dry much slower. Optical imaging shows that the fullerene films possess enhanced molecular mobility in the presence of DIO and it is this which, we conclude, improves the nanomorphology and consequently solar cell performance. We propose that any compatible high boiling solvent would be expected to show the same behaviour

Ligand-Directed Self-Assembly of Organic-Semiconductor/Quantum-Dot Blend Films Enables Efficient Triplet Exciton-Photon Conversion

Blends comprising organic semiconductors and inorganic quantum dots (QDs) are relevant for many optoelectronic applications and devices. However, the individual components in organic-QD blends have a strong tendency to aggregate and phase-separate during film processing, compromising both their structural and electronic properties. Here, we demonstrate a QD surface engineering approach using electronically active, highly soluble semiconductor ligands that are matched to the organic semiconductor host material to achieve well-dispersed inorganic–organic blend films, as characterized by X-ray and neutron scattering, and electron microscopies. This approach preserves the electronic properties of the organic and QD phases and also creates an optimized interface between them. We exemplify this in two emerging applications, singlet-fission-based photon multiplication (SF-PM) and triplet–triplet annihilation-based photon upconversion (TTA-UC). Steady-state and time-resolved optical spectroscopy shows that triplet excitons can be transferred with near unity efficiently across the organic–inorganic interface, while the organic films maintain efficient SF (190% yield) in the organic phase. By changing the relative energy between organic and inorganic components, yellow upconverted emission is observed upon 790 nm NIR excitation. Overall, we provide a highly versatile approach to overcome longstanding challenges in the blending of organic semiconductors with QDs that have relevance for many optical and optoelectronic applications

Triplet-Charge Annihilation in a Small Molecule Donor: Acceptor Blend as a Major Loss Mechanism in Organic Photovoltaics

Organic photovoltaics (OPV) are close to reaching a landmark 20% device efficiency. One of the proposed reasons that OPVs have yet to attain this milestone is their propensity toward triplet formation. Herein, a small molecule donor, DRCN5T, is studied using a variety of morphology and spectroscopy techniques, and blended with both fullerene and non-fullerene acceptors. Specifically, grazing incidence wide-angle X-ray scattering and transient absorption, Raman, and electron paramagnetic resonance spectroscopies are focused on. It is shown that despite DRCN5T's ability to achieve OPV efficiencies of over 10%, it generates an unusually high population of triplets. These triplets are primarily formed in amorphous regions via back recombination from a charge transfer state, and also undergo triplet-charge annihilation. As such, triplets have a dual role in DRCN5T device efficiency suppression: they both hinder free charge carrier formation and annihilate those free charges that do form. Using microsecond transient absorption spectroscopy under oxygen conditions, this triplet-charge annihilation (TCA) is directly observed as a general phenomenon in a variety of DRCN5T: fullerene and non-fullerene blends. Since TCA is usually inferred rather than directly observed, it is demonstrated that this technique is a reliable method to establish the presence of TCA

Insights into the influence of solvent polarity on the crystallization of poly(ethylene oxide) spin-coated thin films via <i>in situ</i> grazing incidence wide-angle X-ray scattering

Controlling polymer thin-film morphology and crystallinity is crucial for a wide range of applications, particularly in thin-film organic electronic devices. In this work, the crystallization behavior of a model polymer, poly(ethylene oxide) (PEO), during spin-coating is studied. PEO films were spun-cast from solvents possessing different polarities (chloroform, THF, and methanol) and probed via in situ grazing incidence wide-angle X-ray scattering. The crystallization behavior was found to follow the solvent polarity order (where chloroform &lt;THF &lt;methanol) rather than the solubility order (where THF &gt; chloroform &gt; methanol). When spun-cast from nonpolar chloroform, crystallization largely followed Avrami kinetics, resulting in the formation of morphologies comprising large spherulites. PEO solutions cast from more polar solvents (THF and methanol) do not form well-defined highly crystalline morphologies and are largely amorphous with the presence of small crystalline regions. The difference in morphological development of PEO spun-cast from polar solvents is attributed to clustering phenomena that inhibit polymer crystallization. This work highlights the importance of considering individual components of polymer solubility, rather than simple total solubility, when designing processing routes for the generation of morphologies with optimum crystallinities or morphologies.</p

Determination of Solvent–Polymer and Polymer–Polymer Flory–Huggins Interaction Parameters for Poly(3-hexylthiophene) via Solvent Vapor Swelling

We report the use of solvent vapor swelling of ultrathin polymer films to determine Flory–Huggins solvent–polymer and polymer–polymer interaction parameters (χ_<i>i–j</i>) for poly­(3-hexylthiophene) (P3HT) and polystyrene (PS) over a wide solvent composition range. From the calculated interaction parameters, we constructed a polymer/polymer/solvent phase diagram that was validated experimentally. χ_{tetrahydrofuran–P3HT} (1.04 ± 0.04) and χ_{CHCl₃–P3HT} (0.99 ± 0.01) were determined through swelling of ultrathin P3HT films. Similar experiments using PS films gave χ_{tetrahydrofuran–PS} = 0.41 ± 0.02 and χ_CHCl₃–PS = 0.39 ± 0.01, consistent with literature values. As expected, these χ_<i>i–j</i> parameters indicated that P3HT is less compatible than PS with either solvent. From δ_PS (17.9 ± 0.2 MPa^1/2) and δ_P3HT (14.8 ± 0.2 MPa^1/2), determined through regular solution theory, we calculated χ_PS–P3HT = 0.48 ± 0.06 at 23 °C. The resulting phase diagram was validated by solution-based transmission measurements of PS/P3HT blends in <i>o</i>-xylene. Although we focused on PS/P3HT blends in this work, this approach is easily adaptable to other polymer/polymer combinations of interest

The Relationship between Charge Density and Polyelectrolyte Brush Profile Using Simultaneous Neutron Reflectivity and In Situ Attenuated Total Internal Reflection FTIR

We report on a novel experimental study of a pH-responsive polyelectrolyte brush at the silicon/D₂O interface. A poly­[2-(diethylamino)­ethyl methacrylate] brush was grown on a large silicon crystal which acted as both a substrate for a neutron reflectivity solid/liquid experiment but also as an FTIR-ATR spectroscopy crystal. This arrangement has allowed for both neutron reflectivities and FTIR spectroscopic information to be measured in parallel. The chosen polybase brush shows strong IR bands which can be assigned to the N–D⁺ stretch, D₂O, and a carbonyl group. From such FTIR data, we are able to closely monitor the degree of protonation along the polymer chain as well as revealing information concerning the D₂O concentration at the interface. The neutron reflectivity data allows us to determine the physical brush profile normal to the solid/liquid interface along with the corresponding degree of hydration. This combined approach makes it possible to quantify the charge on a polymer brush alongside the morphology adopted by the polymer chains