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Contrary to early motivation, the majority of aluminium ion batteries developed to date do not utilise multivalent ion storage; rather, these batteries rely on monovalent complex ions for their main redox reaction. This limitation is somewhat frustrating because the innate advantages of metallic aluminium such as its low cost and high air stability cannot be fully taken advantage of. Here, we report a tetradiketone macrocycle as an aluminium ion battery cathode material that reversibly reacts with divalent (AlCl2+) ions and consequently achieves a high specific capacity of 350 mAh g−1 along with a lifetime of 8000 cycles. The preferred storage of divalent ions over their competing monovalent counterparts can be explained by the relatively unstable discharge state when using monovalent AlCl2+ ions, which exert a moderate resonance effect to stabilise the structure. This study opens an avenue to realise truly multivalent aluminium ion batteries based on organic active materials, by tuning the relative stability of discharged states with carrier ions of different valence states., Aluminium ion batteries have been developed based on the storage of monovalent complex ions, impairing their original motivation of storing multivalent ions. Here, the authors demonstrate the divalent ion storage of tetradiketone macrocycles by tuning the relative stability of discharged states.

This article is part of the Organic Bioelectronics special issue. Electronic doping in organic materials has remained an elusive concept for several decades. It drew considerable attention in the early days in the quest for organic materials with high electrical conductivity, paving the way for the pioneering work on pristine organic semiconductors (OSCs) and their eventual use in a plethora of applications. Despite this early trend, however, recent strides in the field of organic electronics have been made hand in hand with the development and use of dopants to the point that are now ubiquitous. Here, we give an overview of all important advances in the area of doping of organic semiconductors and their applications. We first review the relevant literature with particular focus on the physical processes involved, discussing established mechanisms but also newly proposed theories. We then continue with a comprehensive summary of the most widely studied dopants to date, placing particular emphasis on the chemical strategies toward the synthesis of molecules with improved functionality. The processing routes toward doped organic films and the important doping–processing–nanostructure relationships, are also discussed. We conclude the review by highlighting how doping can enhance the operating characteristics of various organic devices. This work was from King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) under award nos. OSR-2018-CARF/CCF-3079 and OSR-2019- CRG8-4095.3. F.A. acknowledges the support from The Wilkinson Charitable Foundation. C.M. acknowledges financial support from the Swedish Research Council (grant no. 2018- 03824) and the Knut and Alice Wallenberg Foundation through a Wallenberg Academy Fellowship Prolongation grant.

We report the synthesis of a novel polythiophene-based host-guest copolymer incorporating a Pt-porphyrin complex (TTP-Pt) into the backbone for efficient singlet to triplet polymer exciton sensitization. We elucidated the exciton dynamics in thin films of the material by means of Transient Absorption Spectrosopcy (TAS) on multiple time scales and investigated the mechanism of triplet exciton formation. During sensitization, singlet exciton diffusion is followed by exciton transfer from the polymer backbone to the complex where it undergoes intersystem crossing to the triplet state of the complex. We directly monitored the triplet exciton back transfer from the Pt-porphyrin to the polymer and found that 60% of the complex triplet excitons were transferred with a time constant of 1087 ps. We propose an equilibrium between polymer and porphyrin triplet states as a result of the low triplet diffusion length in the polymer backbone and hence an increased local triplet population resulting in increased triplet-triplet annihilation. This novel system has significant implications for the design of novel materials for triplet sensitized solar cells and upconversion layers.

In this article we discuss the synthesis of four new low band-gap co-polymers based on the diketopyrrolopyrrole (DPP) and benzotriazole (BTZ) monomer unit., In this article we discuss the synthesis of four new low band-gap co-polymers based on the diketopyrrolopyrrole (DPP) and benzotriazole (BTZ) monomer unit. We demonstrate that the BTZ unit allows for additional solubilizing side-chains on the co-monomer and show that the introduction of a linear side-chain on the DPP-unit leads to an increase in thin-film order and charge-carrier mobility if a sufficiently solubilizing, branched, side chain is attached to the BTZ. We compare two different synthetic routes, direct arylation and Suzuki-polycondensation, by a direct comparison of polymers obtained via the two routes and show that direct arylation produces polymers with lower electrical performance which we attribute to a higher density of chain Furthermore we demonstrate that a polymer utilizing this design motif and synthesized via Suzuki-polycondensation ((l-C18)-DPP-(b-C17)-BTZ) exhibits exceptionally high and near balanced average electron and hole mobilities >2 cm2 V–1 s–1 which are among the highest, robustly extracted mobility values reported for DPP copolymers in a top-gate configuration to date. Our results demonstrate clearly that linear side chain substitution of the DPP unit together with co-monomers that allow for the use of sufficiently long or branched solubilizing side chains can be an attractive design motif for solution processable, high mobility DPP copolymers.

Doping is one of the most important methods to control charge carrier concentration in semiconductors. Ideally, the introduction of dopants should not perturb the ordered microstructure of the semiconducting host. In some systems, such as modulation-doped inorganic semiconductors or molecular charge transfer crystals, this can be achieved by spatially separating the dopants from the charge transport pathways. However, in conducting polymers, dopants tend to be randomly distributed within the conjugated polymer, and as a result the transport properties are strongly affected by the resulting structural and electronic disorder. Here, we show that in the highly ordered lamellar microstructure of a regioregular thiophene-based conjugated polymer, a small-molecule p-type dopant can be incorporated by solid state diffusion into the layers of solubilizing side chains without disrupting the conjugated layers. In contrast to more disordered systems, this allows us to observe coherent, free-electron-like charge transport properties, including a nearly ideal Hall effect in a wide temperature range, a positive magnetoconductance due to weak localization and the Pauli paramagnetic spin susceptibility.

We reveal some of the key mechanisms during charge generation in polymer:fullerene blends exploiting our well-defined understanding of the microstructures obtained in pBTTT:PCBM systems via processing with fatty acid methyl ester additives. Based on ultrafast transient absorption, electro-absorption, and fluorescence up-conversion spectroscopy, we find that exciton diffusion through relatively phase-pure polymer or fullerene domains limits the rate of electron and hole transfer, while prompt charge separation occurs in regions where the polymer and fullerene are molecularly intermixed (such as the co-crystal phase where fullerenes intercalate between polymer chains in pBTTT:PCBM). We moreover confirm the importance of neat domains, which are essential to prevent geminate recombination of bound electron–hole pairs. Most interestingly, using an electro-absorption (Stark effect) signature, we directly visualize the migration of holes from intermixed to neat regions, which occurs on the subpicosecond time scale. This ultrafast transport is likely sustained by high local mobility (possibly along chains extending from the co-crystal phase to neat regions) and by an energy cascade driving the holes toward the neat domains.

Poster presented by James H Bannock and Thomas W Phillips at 2nd SCI/RSC Symposium on Continuous Processing and Flow Chemistry, Novartis, Horsham, UK on 24th September 2013.

We investigate the mobility of poly(3-hexylthiophene) (P3HT) over a carrier-density range from 1015 to 1020?cm-3. Hole-only diodes were used for densities below 1016?cm-3 and field-effect transistors were used for carrier densities higher than 1018?cm-3. To fill the gap, intermediate densities were probed using chemically doped Schottky diodes and transistors. Combining of the mobilities in doped and undoped devices experimentally establishes the full relation of the mobility over the whole carrier-density range.

Two terthiophenes bearing core fluorinated thienyl units have been synthesised as potential semiconductor materials for organic field-effect transistors. Polymerisation of these compounds has been achieved using conventional iron(III) chloride oxidative coupling methods and by electrochemical oxidation. Characterisation of the fluorinated materials has been achieved by absorption spectroscopy and cyclic voltammetry. A soluble hexyl-functionalised polymer (poly8b) was used in an OFET device; hole mobilities were measured up to 3 × 10−3 cm2 · V−1 · s−1, and the device had an on/off ratio of 105 and a turn-on voltage of +4 V.

One of the key challenges for organic photovoltaics (OPV) is to achieve high device efficiencies with photoactive layers thick enough for scalable printing methodologies, which typically require thicknesses of 300 nm. For laboratory scale spin coated devices, however, the optimum device is typically limited by a photoactive layer thickness of 80 – 100 nm. The reasons of the limited effective thickness remains debated, and the choice of material combinations that can enable efficient thick devices is still very limited. So far, most research on this issue has focused on the drop off of device fill factor FF with thicker photoactive layers, which has mainly been attributed to bimolecular recombination losses during transport. Drop off of device short circuit currents Jsc with increasing thickness has been less studied, and normally considered to result from the same recombination / transport limitation limiting FF. Herein we focus upon the thickness dependence of Jsc for four representative blends exhibiting very different thickness dependencies. We analyses several possible causes of the drop of Jsc with thickness, and conclude that the key determinant is the density of intraband tail states in the photoactive blend, a factor not previously considered to be important in limiting device thickness. We find a correlation between the tail state density and Jsc vs thickness, and provide both experimental and modelling analyses which indicate that such behaviour can result from space charge layer effects generated in the presence of tail states. Our study employs four series of organic donor:acceptor bulk heterojunction solar cells with which exhibit maximal photoactive layer thicknesses for efficient photocurrent generation and device performance ranging from 80 to 530 nm. These blends and devices are analyzed by as correct photocurrent data, transient photovoltage and charge extraction of carrier kinetics and densities, ambient photoemission spectroscopies of material energetics, kelvin probe measurements of work function, space charge limited current measurements of material doping, Mott-Schottky analyses of apparent doping density and by device simulations. Two methodologies are employed to quantify bimolecular recombination losses at short circuit, both methodologies conclude that these losses are too small to explain the loss of Jsc for thicker devices. Two further methodologies are employed to estimate the space charge layer depth for the devices studied, from which we conclude that the drop off of Jsc­ with photoactive layer thickness correlates with space charge layer width, with efficient photocurrent generation requiring the material space charge layer width to be greater than the photoactive layer thickness. Analyses of the dark doping density of the absorber layer by work function and SCLC measurements indicate this space charge layer does not result from unintentional material. doping. Instead, we observe that the formation of this space charge layer is correlated with the presence of tail (shallow trap) states in the absorber layer. Device simulation studies quantitatively support this conclusion, indicating that the presence of tail states can, under irradiation, generate a space charge layer which screens the built field in the device for thick absorber layers. The results and conclusions reported herein helps explain the widely reported observation that many bulk heterojunctions which perform well when spin coated as thin absorber layers, do not perform well when printed with thicker absorber layers. We further demonstrate a simple strategy to address this issue, with a simple polymer purification to remove low molecular weight fractions shown to reduce tail state density and enable the fabrication of efficient devices with thicker photoactive layer thicknesses.

The molecular-level arrangement of the donor and acceptor (i.e. the local morphology) in organic solar cells governs charge separation and charge transport via its effect on the mobility of charges. However, the nanometre-scale mobility in such systems, which can be measured using terahertz (THz) spectroscopy, has been little investigated at relevant low excitation densities, due to extremely weak signals. Here, we study the mobility over short distances and at ultrashort times using time-resolved optical-pump-THz-probe (OPTP) spectroscopy on pBTTT:PCBM blends. This complements our previous results obtained with transient absorption (TA) and electro-modulated differential absorption (EDA) techniques. In the pBTTT:PCBM system, the co-existence of fullerene-/polymer-rich (‘neat’) and co-crystalline (‘intermixed’) regions can be controlled through choice of composition (weight ratio of the two components, use of processing additive). We demonstrate high short-range mobilities that help charges separate, and we show how this mobility of photogenerated charges develops in time, in particular as the charges move between different phase regions of the blend. By reducing the pump fluence below the threshold for nonlinear recombination mechanisms, we access these properties at solar cell operating conditions. Overall, we explain the necessity of different local phases through their influence on charge lifetime and mobility.

Organic electronics focuses on developing next-generation electronic devices, including organic photovoltaics, organic field-effect transistors and organic photodetectors, which rely heavily on the development of organic semiconductor materials, owing to their unique advantages like mechanical flexibility and stretchability, low processing temperature, low fabrication cost. This thesis focuses on the synthesis, characterization and application of novel organic semiconductor materials. Benzothiadiazole, as one of the most widely used electronwithdrawing units, has attracted great interest and extensive effort in the design of smallmolecular or polymerized semiconductor materials. Most of the approaches to tune the energetics of benzothiadiazole focuses on changing the heteroatom in the thiadiazole ring or functionalizing the 5 and/or 6 position of the benzo ring. Here in my work, we annulated an additional ring (2-(1,3-dithiol-2-ylidene)malonitrile, DTYM) in the 4,5-positions of the benzo ring for the first time, and coupled with indacenodithiophene core to get two small-molecular non-fullerene acceptors (BTSCN-IDT, FBTSCN-IDT). The photovoltaic performance of both acceptors was evaluated in organic photovoltaics when blended with donor polymer PM6, and the fluorinated acceptor FBTSCN-IDT delivered a comparatively better power conversion efficiency of 13.7% under dim light condition, which can be ascribed to the more red-shifted absorption and more coplanar molecular backbone. In addition, we annulated the same additional ring to the 5,6-positions of the benzothiadiazole to allow polymerization through 4,7-positions with four different donor units and received four polymer semiconductors (P1- P4). The effect of introducing DTYM group on benzothiadiazole was evaluated and found that the incorporation of DTYM group leads to a more twisted backbone but an enhanced dipole moment. As a result, all polymers presented n-type semiconductor behaviour in organic fieldeffect transistors, with the polymer P3 showing the highest saturation electron mobility up to 5.2 × 10-3 cm2 V-1 s-1 among them. However, most of the reported organic semiconductor materials have long conjugated molecular backbone which may induce chemical instability, low solvent solubility and high fabrication cost from the complicated synthesis. On this basis, two fully non-fused nonfullerene acceptors (BTIC-1, BTIC-2) were synthesized and applied in organic photovoltaics and photodetectors. Both acceptors share the same backbone but have the same alkoxy group on different positions to form varying number of intramolecular conformational locks to tune the molecular structure. Theoretical calculation and optical spectrum indicate that BTIC-2 presents a more planar conformation and thus a more red-shifted absorption up to 900 nm. As a result, BTIC-2 outperforms BTIC-1 in photodetector performance, with a low dark current of 2.4 × 10-7 A cm-2, leading to a high responsivity of 0.357 A W-1 and a specific detectivity of 1.7 × 1011 Jones at 828 nm under the reverse bias of -2 V. Lastly, we are also interested in developing novel fused-ring building blocks for constructing semiconductor materials. Two novel core molecules were designed and synthesized by bridging benzothiadiazole unit and adjacent thiophene units using bridging atom oxygen instead of the commonly used nitrogen atom, as oxygen is more electronegative than nitrogen which may lead to lower-lying energy levels and thus high open-circuit voltage. Open Access

Organic conjugated polymer semiconductor materials have grasped the attention of many researchers, particularly in the last 10 years, for their use in electronic devices. In order to engineer materials for specific applications, it is essential to consider the polymer design and chemical structures. With thoughtful polymer design, it is possible to modify and manipulate physical and optoelectronic properties to optimise semiconductor materials for intended applications. Post-polymerisation techniques have been widely explored as a useful tool to modify polymers with desired properties or specialised functionality. Nucleophilic aromatic substitution (SNAr) has been identified as a useful reaction to facilitate the modification of conjugated polymers without introducing unwanted defects within the polymer backbone. In this thesis, SNAr was used as technique to introduce conjugated polymer sidechains and various other functional groups along conjugated polymer backbones. Polymers were designed and synthesised for use in electronic devices such as; organic thin-film transistors (OTFTs) and organic solar cell (OSC) devices, where physical and optoelectronic properties were evaluated for all polymers. The work presented (Chapter 2-6) looks at polymers designed with a monofluorinated benzothiadiazole as a repeat unit within the backbone of a conjugated polymer. A range of nucleophiles were substituted onto the polymer backbones via SNAr, directly onto the monofluorinated benzothiadiazole repeat unit, highlighting the versatility of this post-polymerisation modification method. In chapters 2 and 3, this method was used to develop graft co-polymers with an n-type conjugated polymer backbone and p-type conjugated polymer sidechains. In chapter 3, single-component graft co-polymer structures were shown to provide a comparatively similar solar cell device performance to the blended polymers, however, with higher recorded Voc values. Further exploration of the versatility of post-polymerisation modification requires the development of new conjugated backbone polymers that can readily undergo modification. Hence, in Chapter 4, the direct modification of two novel n-type conjugated polymer backbones via SNAr was demonstrated, including the quantitative incorporation of substituents along the polymer backbone. Unsubstituted polymers achieved an electron mobility of 0.16 cm2 V-1 s-1 in OTFT devices. After substitution, mobilities reduced by around one order of magnitude. Further developments looked at azide functionalised conjugated polymers, where polymer thin films were successfully thermally cross-linked or photo-patterned (high azide content polymer). Finally in Chapter 5, polymers were modified with increasing anionic content, for use as interlayers between active polymer layers and electrodes within OSCs, for example. Increasing the anionic content did not largely affect the electronic properties of the polymer, however, did modify the work function of indium tin oxide (ITO). Open Access

This thesis reports on the development of electron deficient (n-type) small molecule and polymer semiconductors. Firstly, the synthesis of a new electron deficient 4,5,6-trifluoro-2,1,3-benzothiadiazole (TFBT) end group is presented. Coupling of TFBT to an electron rich indacenodithiophene (IDT) core, through direct arylation conditions, affords a TFBT IDT material which performs as a poorly ambipolar semiconductor in organic field effect transistor (OFET) devices. A range of related TFBT-based materials with expanded IDT or cyclopentadithiophene (CDT) cores were prepared and characterised. A six-fold nucleophilic aromatic substitution reaction with cyanide was developed and applied to all fluorinated materials. This one step modification resulted in the formation of 2,1,3-benzothiadiazole-4,5,6-tricarbonitrile (TCNBT) end group. This modification dramatically changed structural, optoelectronic and semiconducting properties compared to their fluorinated counterparts. This highlights the importance of strong π-acceptors, like cyano groups, in influencing electron accepting properties, compared to inductively withdrawing fluorine atoms. TCNBT-based semiconductors were utilised in a range of applications such as organic field effect transistors (OFETs) and organic photodetectors (OPDs), demonstrating good stability and high electron mobility. The strong electron accepting properties of the TCNBT end group resulted in low band gap materials that strongly absorb in the NIR range and were utilised to afford semi transparent electronic devices. A range of electron deficient donor-acceptor (D-A) type polymers were also synthesised, based on benzothiadiazole, functionalised with a mixture of cyano, nitro and fluoro groups. This study highlights the importance of molecular engineering and how small structural modifications have a great impact on the nature of the resulting semiconductor. More specifically, the effect of fluorine atoms and their influence on backbone planarity is shown to affect charge transport properties. On the other hand, cyano groups cause backbone twisting that is not easily overcome in the solid state and in turn disrupts charge transport in the polymer backbone. When applied to OFET and organic photovoltaic (OPV) devices, these polymers performed differently to the small molecules presented in this thesis, with fully cyanated polymers showing lower electron mobilities compared to the ones containing fluorine atoms. This demonstrates that the impact of cyano groups have a different effect in polymeric compared to small molecule systems. Open Access

Remarkable advances have been seen in the area of organic photovoltaics (OPVs), with the power conversion efficiencies (PCEs) doubling in the past four year and exceeding over 18%. The commercial potential of OPV technology has been demonstrated, partly due to the fast development of new electron accepting materials. Although fullerene derivatives possess excellent electron mobility and affinity, they have some intrinsic limitations, including weak absorption in the visible region, elevated synthetic difficulty in tuning the electronic/optical properties, and poor morphology stability. Developing new electron acceptors to further improve the performance (including efficiency and stability) of OPV cells is necessary. This thesis focuses on the synthesis and charsterisation of novel electron accepting materials. Initially star-shaped oligomers were developed, however precise modulation of the molecular structure was needed to achieve proper energetics and high electron mobility. The unfavorable morphology, blended with electron donating materials, appeared to be another reason that limited the device performance. By combination of a benzodithiophene core and four diketopyrrolopyrrole arms, an oligomer was developed, which exhibited better p-type (electron donating) behavior in both OPV and organic field-effect transistor devices. Perylene diimide tetramers were developed to construct new electron acceptors and the broad absorption spectrum shown in ring-fused molecule was first explained by time-dependent density function theory (TD-DFT) calculations. In addition, linear ladder-type molecules were explored. Via a facile aromatic extension strategy, cyclopentadithienothiophene was prepared as a bent small molecule acceptor, which demonstrated promising device performance with high short-circuit current. Fully alkylated dithienocyclopentacarbazole flanked by electronwithdrawing nonfluorinated or fluorinated 1,1-dicyanomethylene-3-indanone were developed to investigate the effect of side chains and fluorination. Compared to the electron acceptor with bulky arylalkyl chains, alkyl side chains were found to enhance molecular packing and crystallinity and fluorination led to a more optimal nanoscale blend morphology and high performance. Finally, two wide-bandgap acceptors were developed for front-cell materials in tandem solar cells. High PCEs were achieved and in excellent agreement with theoretical simulations. Open Access

The use of organic semiconductors (OSCs) for electronic applications is of interest due to their myriad desirable properties, including being lightweight, flexible and low-cost. This work focuses primarily on the synthesis and characterisation of novel molecular OSCs for field-effect transistor (OFET) applications. While the majority of high-performing OFETs are comprised of p-type OSCs, the performance of n-type OSCs still lags behind. New n-type OSCs are eagerly sought for use in complementary circuits and organic photovoltaics (OPVs), where balanced charge transport between the n-type and p-type components is crucial. In Chapter 2, core fluorination is used as a tool to modify the n-type characteristics and device performance of an indacenodithiophene (IDT)-based molecular OSC – 2,7-dihexyl-IDT-di(C(CN)2). Fluorination is shown to effectively depress the lowest-unoccupied molecular orbital (LUMO) energy level as well as cause a surprising improvement in solubility. OFET devices fabricated via solution-processing of the OSC thin-film display high mobility in the absence of non-idealities, in addition to good retention of performance in ambient conditions. The following chapters (Chapters 3 – 5) then seek to modify this fluorinated core and investigate the effects on electronic, physical and device performance. In Chapter 3 we perform alkyl side-chain engineering, including modifications to the side-chain length, branching and regiochemistry. The nature and position of the alkyl side-chains are shown to have a significant impact on the physical properties – particularly processability and charge transfer characteristics – as well as device performance. In Chapter 4, the use of an alternative electron-withdrawing group (EWG) – N-cyanoimine – is explored in an attempt to avoid several issues associated with dicyanomethylene. Chapter 5 focuses on further modifications to the fluorinated core, including i) end-functionalisation via direct heteroarylation coupling and ii) substitution of the fluorine groups via nucleophilic aromatic substitution. Finally, in Chapter 6 a route to a novel class of nonacyclic IDT-based ladder-type molecular is outlined. Electronic and physical properties are explored to examine the applicability of these materials as OSCs for OFETs. Open Access

Conventional synthetic methods often give poor control over reaction conditions especially when applied to large quantity synthesis. Flow chemistry has been reported to provide a more con- trolled environment for synthesis and is particularly beneficial for the synthesis of functional ma- terials with properties that are sensitive to the reaction conditions. This thesis describes the development of several flow systems for the synthesis of electronically functional materials. An integrated flow system was developed for the preparation of the non-fullerene acceptor FBR is described. In addition to translating the two reaction steps to flow, the system also carries out in-line separation and solvent change in between the steps. In contrast to conventional batch conditions, the flow process dramatically reduced the reaction time from >12 hours down to 10 – 20 minutes. A separation procedure after the first step was able to isolate THF from aqueous base and simultaneously swapped the solvent from THF to toluene in preparation for carrying out the second reaction step. A complete flow system for synthesising P3HT polymer was also reported, including both monomer activation step and polymerisation. The system was able to produce P3HT polymer with high molecular weight (50.3 kDa) and low dispersity (1.20). The effect of changing the Grignard to monomer ratio on the polymer properties was investigated in detail. Additionally, in-line NMR spectroscopy analysis was employed in the flow system to provide near real-time analysis of the monomer activation step. Finally, the post-polymerisation functionalisation of the conjugated polymer F8BT-F with 4-methoxylthiophenol group was trans- lated to a flow reactor. The substitution reaction time was cut down to 30 minutes from >12 hours in batch. Open Access

Soluble processable organic semiconductors offer several advantages over their traditional inorganic counterparts. They can be processed by a variety of large-area techniques, are lightweight, robust and flexible. Furthermore, their properties can be readily tuned by modifications to the chemical structure of the semiconductor. This influences both the fundamental optoelectronic properties, such as band gap and energy levels, as well as the physical properties such as molecular aggregation and thin-film morphology. This work is focussed upon the potential of 1,1’-binaphthyl (BN) derivatives as building blocks in the development of new materials for organic photovoltaics (OPV) applications. Due to steric interaction between the protons in the 8,8’ position, BN is inherently twisted with a torsional angle of about 90°. Incorporation of BN into organic semiconducting materials was therefore expected to lead to non-planar materials with suppressed crystallisation in the solid state. This strategy was explored for novel acceptor materials for organic solar cells, in which large scale crystallisation can lead to detrimental device performance and stability. Two classes of BN cored semiconductors were synthesised, containing either diketopyrrolopyrrole or rhodanine end-groups with varying alkyl sidechains. Their optoelectronic performance is reported, along with preliminary device data. Functionalised BN derivatives also exhibit atropisomerism, and their incorporation into organic semiconducting polymers and small molecules was also explored as an approach to afford chirally active, electronically functional materials. Open Access

The potential of organic electronic devices, over the past few years, has been proven tremendous. Flexible and transparent electronic devices will soon enter the market, but there are still some challenging aspects requiring further improvements. A generic approach to enhance the charge transport properties of the organic semiconductors is by introducing molecular additives. In particular for thin lm transistors, doping the semiconductor layer leads to higher charge carrier mobility, as well as improving several parameters of the device. The work in this thesis focuses on identifying suitable molecular additives that can dope organic semiconductors for thin lm transistors. Chapter 5 presents the application of tris(penta uorophenyl)borane B(C6F5)3 as an e ective p-type dopant. B(C6F5)3 was applied in several small molecule:polymer blends and the improvement in the charge transport properties, as measured from organic thin lm transistors (OTFTs), was remarkable. In particular the blend diF-TESADT:PTAA and the C8-BTBT:C16-IDT-BT showed hole mobility of 8 and 11 cm2/Vs, respectively. OTFT parameters, such as the threshold voltage and the contact resistance were signi cantly reduced with the addition of molecular additives. The impact of the dopant addition in the blends was studied by AFM and XRD, which revealed that long-range crystallinity was induced in certain systems. Chapter 6 presents the DMBI-BDZC as a new n-type dopant. For transistors integration into circuits balanced charge transport is required and thus extensive optimisation is still required for n-type materials. DMBI-BDZC was used to dope a small molecule (NDI3HU-DTYM2) and a polymeric (N2200) semiconductor. In both systems free electron generation was observed at an optimum dopant concentration, as measured by electron paramagnetic resonance (EPR). OTFTs with enhanced charge carrier mobility and reduced contact resistance was achieved with the dopant addition. By further analysis of the device characteristics, reduction in both activation energy and density of traps states was observed in the doped devices. The morphology studies of the organic semiconducting thin lms revealed that the dopant addition is not disrupting the structure. Furthermore, the reduction in activation energy at low voltages is in accordance with the trap density calculations as performed from the OTFTs. In chapter 7 a single organic layer light emitting transistor (OLET) is demonstrated. The key behind this device structure was the identi cation of the polymer PDITTT. This polymer combines high charge transport properties (1 cm2/Vs) as well as having green light emission. The properties of the polymer allowed the fabrication of a single layer organic semiconducting OLET. Blending the polymer with the small molecule C8-BTBT resulted in enhanced charge transport OTFTs. OTFTs and OLETs were fabricated from both the PDITTT and C8-BTBT:PDITTT. Open Access

This thesis focuses on the development of high-performance phototransistors based on several solution-processed semiconductors. After giving an outline of the relevant theory and experimental methods, the thesis is divided into three experimental chapters. The first chapter presents the development of an ambipolar organic phototransistor based on [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). The balanced electron and hole transport characteristics are achieved by fine-tuning of the injection electrode work function using a self-assembled monolayer. The ambipolar PC61BM phototransistor exhibits promising photoresponse in the UVA region, with maximum photosensitivity and responsivity of 9×103 and 3×103 A/W respectively. In addition, the phototransistor shows fast temporal response with switching times of

Carboranes, C2B10H12, are bulky, icosahedral cluster compounds, with unique molecular bonding and electronic properties. Since their discovery in the middle of the 19th century, they have been studied and used in a wide range of chemical fields, with the aims of exploiting their excellent thermal and chemical stability, electron-withdrawing capability, and, more recently, their ability to act as modifiers and promotors of photoluminescence (PL). The effects of carboranes on PL have now been widely reported in the literature, but questions remain about the underlying processes dictating these behaviours. This work is an attempt to answer some of those questions. In this thesis, the three structural isomers of carborane (ortho, meta and para) have been incorporated into a range of linear and non-linear aromatic systems, forming highly sterically strained structures. This phenomenon correlates with reduced carborane rotation, as determined by DFT and NMR investigations. All carboranes studied are fluorescent, showing local emission (LE) from the aromatic moiety, however the ortho-carborane compounds also demonstrate multi-emissive behaviour, as the carborane introduces a delocalised emissive species, associated with intramolecular charge-transfer (ICT) and twisted-ICT (TICT) states, the latter of which occurs when the carboranyl C-C bond is perpendicular to the aromatic plane. By introducing phenyl groups to ortho-carborane, it was shown that the ground state could be tuned to favour TICT formation, through favourable π-π interactions between the aromatic substituents ‘locking’ the C-C bond in the perpendicular arrangement. Chrysene and phenanthrene-based ortho-carborane compounds were found to exhibit a peculiar, concentration-dependent, low-energy emissive peak. Through detailed temperature-dependent optical studies, these emissive excited states were assigned to excimers, the first time such species, involving a carborane in a charge-transfer state, has been observed. In aggregated states, formed in bad solvents, the PL intensity for the meta and para-carboranes is reduced, but for the ortho-carborane compounds the intensity increases enormously, as a result of aggregation-induced emission (AIE). AIE is determined to be as a result of TICT emissive species, where the excited state is localised inside ortho-carborane, reducing non-radiative decay routes and boosting PL quantum yield. The aggregates were directly imaged with a combination of scanning and transmission electron microscopy (SEM and TEM respectively) and were found to form spherical nanoparticles, with evidence of crystallinity. Open Access

This thesis investigates a range of dinuclear zinc(II), copper(I) bis-imine, and copper(I) NHC complexes as emitters within OLEDs. The syntheses and photophysical properties of seven dinuclear zinc(II) disalphen macrocycles are described in Chapter 2. Five are weakly luminescent in solution (λem = 531 – 597 nm, PLQE < 1 %), moderately luminescent in the solid state (λem = 569 – 630 nm, PLQE = 1 – 5 %), and display aggregation induced enhanced emission (AIEE), however, are unsuitable candidates for OLEDs due to their low PLQE values. The syntheses and photophysical properties of eight dinuclear zinc(II) bis-salphens and one mononuclear zinc(II) salphen complex are described in Chapter 3. The complexes are moderately luminescent in solution (λem = 531 – 700 nm, PLQE = 2 – 27 %), and show weaker emission in the solid state (λem = 584 – 625 nm, PLQE < 1 – 4 %). Significant colour tuning (green to red emission) is reported across the dinuclear complexes. Two dinuclear zinc(II) bis-salphens are used as emitters in three OLED devices (up to 1.1 cd/A and 0.35 lm/W). A study of the solid state emission of copper(I) bis-imines and copper(I) NHCs is presented in Chapter 4. The emission from three copper(I) NHCs is further investigated (λem = 441 – 509 nm, PLQE = 3 – 45 %), and one displays bright blue emission (λem = 441 nm, PLQE = 45 %), and is trialled as an emitter in four OLED devices. A synthetic and photophysical investigation of several novel organic compounds and dinuclear zinc(II) complexes is presented in Chapter 5, however, the dinuclear complexes are not isolated. Four mononuclear zinc(II) complexes, five diimines, and one organic compound are presented. The organic compound has highly efficient, deep blue emission in solution (λem = 388 – 456 nm, PLQE = 83 %), and is further investigated as an emitter for OLEDs. Open Access

A major attraction of polymeric semiconductors is that their optoelectronic properties can be tuned by controlling local microstructure, such as chain arrangement and conformation, in principle allowing functionality to be tailored to specific device applications. This local microstructure is influenced by intrinsic factors, such as chemical structure and molecular weight,but also by film formation conditions, such as solvent and casting temperature. Here, the effect of blending conjugated with insulating polymers, an additional tool to control structural and optoelectronic properties, is extensively explored. Focusing predominantly on widely studied poly(3-hexylthiophene-2,5-diyl) (P3HT) as a model system, the addition of a polar insulator poly(ethylene oxide) (PEO) is shown to affect both the nature of aggregation (in terms of excitonic coupling), its temperature dependence and evolution during film formation. Spatially restricting evaporation of a P3HT:PEO blend leads to periodic variation in aggregation type, attributed to self-induced compositional changes, demonstrating that insulators can be used to achieve a measure of spatial control. The origin of a temperature dependent reduction in effective vibronic peak spacing of photoluminescence spectra P3HT:PEO is investigated by considering self-absorption, coupling to insulator vibration and additional Raman modes, and correlated with increased non-radiative decay. Finally, the two chemical functionalities, namely poly(thiophene) and oligo(ethylene oxide), are combined as a graft polymer, a block co-polymer, and blends with both, demonstrating that matching side chain and insulator polarity can promote translational order, aggregation and planarity. Together, this work demonstrates how the addition of insulating to conjugated polymers influences local microstructure and hence enables control of optoelectronic characteristics, with potential applications to a wide range of conjugated polymers and devices. Open Access

Besides being lightweight, flexible and cheap, one of the major attractions of plastic electronics is that they can be chemically modified to cater a broad range of applications. By changing the chemical structures of the molecule, the physical properties and in turn the device performance of the materials can be tuned accordingly. Though there are limited systematic ways to precisely determine the behaviour of an organic semiconductor based on its structure, chemists have learned to adjust the properties of a material based on well-understood functional groups or building blocks. This thesis demonstrates four applications of strategic designs of novel structures and the modifications of the existing materials. By simply changing the branch point of an alkyl solubilising group on naphthalene diimide – bithiophene (NDI-T2) polymer can result in a 5-fold increase in its OFET performance. By using a new type of acetonitrile alkylation reaction, a highly solubilising group – super-branch was synthesised. With the super-branch, we were able to synthesise a NDI polymer with six unsubstituted thiophenes as the donor. Through the use of random co-polymerisation with a short chain NDI, we have successfully improved the super-branch polymer’s electron mobility by 3 orders of magnitude to 0.11 cm2 / Vs, comparable to N2200. The post-polymerisation modification on PEGylated polymer was realised by a nucleophilic aromatic substitution to replace a fluoro group on the polymer. A series of fluorinated benzotriazole polymers was also successfully synthesised to offer a relatively high hole mobility of 0.50 cm2 / Vs. Open Access

Organic semiconductors have several advantages over their inorganic counterparts. For example, the solution processability allows for the fabrication of flexible, lightweight and low-cost semiconducting devices. However, much work is needed to compete with the high-performance of inorganics. The first half of this work focuses primarily on nucleophilic aromatic substitution (SNAr) reactions with molecules and polymers containing the fluorinated 2,1,3-benzothiadiazole (BT) unit. Chapter 2 explores the effect the type of heteroatom on the BT unit (N, O and S) of a carbazole-based polymer has on the optical properties and organic photovoltaic (OPV) performance. The following chapter focuses on post-polymerisation SNAr reactions, in which thiols were found to displace fluorine groups on a variety of polymers, including polymers containing fluorinated benzotriazole (BTz) and thienothiophene (TT) units instead of BT. The reaction was then taken a step further, by substituting fluorine atoms on P(F8fBT) with a diverse range of end-functionalised thiols, thioacetates and alcohols. It was also found that the amount of thiol substitution could be finely tuned, which led to the development of multifunctionalised polymers. The second half of this thesis explores the possible applications of the BT SNAr reaction towards semiconducting polymer nanoparticles (SPN) and OPVs. In Chapter 4, SPNs from polymers functionalised with azide and carboxylic groups were synthesised and the surface was found to be reactive to strained-alkynes and amines, respectively. In the final chapter, a trimethoxysilane-functionalised OPV polymer was synthesised and was found to increase the thermal stability of devices via cross-linking, with no additives required. Open Access

The use of conjugated polymers as the active layer in transistor and photovoltaic devices offers many advantages over inorganic materials, such as the potential for low cost, large-scale, solution processed, flexible devices. There has been considerable progress in this area in recent years due to innovative material design, development of new and exciting device architectures, and a deeper understanding of the underlying operating principles, and morphological requirements for optimal performance. Nevertheless, new materials with even higher performance and stable device characteristics are currently needed if commercialisation is to be achieved. This thesis focuses on the design and synthesis of a range of ring fused conjugated polymers for use in organic electronics. Ring fused polymers are particularly promising for this application since they offer the potential to help planarise the backbone, enhance intermolecular π-π interactions, and thus improve charge transport. In the first chapter a series of five polymers based on a five-membered, dithienogermolodithiophene (DTTG) monomer (an extended version of dithienogermole) is reported. Through careful consideration of the choice of acceptor co-monomer and device optimisation, a peak saturated hole mobility of 0.22 cm2/Vs was obtained in devices spin-coated from a non-chlorinated solvent mixture, which at the time of writing was the highest reported charge carrier mobility of any germanium based polymer. The second chapter focuses on the synthesis of a novel bisthieno[2’,3’:4,5]thieno[3,2-c:2’,3’-e]azepine-4,6(5H)-dione (BTTA) monomer in which two thieno[3,2-b]thiophene units are bridged by an azepine-2,7-dione ring. The choice of co-monomer on the optoelectronic and surface morphology of the polymers are reported, using a combination of experimental techniques. The photovoltaic and electrical performance of the polymers investigated in inverted BHJ OPV devices and TG-BC field effect transistors was high, with peak values of 6.78% and 0.027 cm2/Vs observed under optimised conditions. The third chapter explores the use of side-chain engineering to improve the performance of one of the most promising ring fused electron accepting materials; poly[4,9(benzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetrone)-alt-5,5’(2,2’-bithiophene)] (pNDI-BT). In particular, the use of alkyl side-chain branching position as an effective strategy for improving charge carrier mobility was demonstrated, and for the first time, the influence of the branch-point on the performance of all-polymer OPV devices was examined, with a gradual decrease in power conversion efficiency observed as the branch-point was moved further from the polymer backbone. The final chapter builds on this work, and uses a range of fluorinated and non-fluorinated, thiophene and selenophene containing monomers, to further improve the electrical performance of a series of NDI based polymers. Open Access

The development of new semiconducting conjugated polymers has been an important aspect of plastic electronics research since the early days of the field, as it allows the optoelectronic properties to be tuned for various applications. Dithienometalloles, which consist of a 2,2’-bithiophene fused with a bridging heteroatom, have been an important class of monomers for some time. The heteroatom serves an important role, both in holding the bithiophene unit rigidly coplanar and also in tuning the energetics of the system. In addition, these moieties often possess alkyl chains located on the bridging heteroatom that serve to confer solubility and processability to the final polymers. In this thesis we report the synthesis and properties of a number of dithienometallolecontaining polymers, in which the necessary solubilising groups have been moved from the bridging heteroatom to the adjacent thiophene backbone of the dithienometallole. This enabled the preparation of novel, soluble arsole-containing conjugated polymers. These dithienoarsole containing materials were used in both organic field effect transistors and organic photovoltaic devices, showing promising performances as well as significantly improved air stability over their phosphorus-containing analogues. The effect of changing the bridging heteroatom in dithienometalloles was also studied by comparing the optoelectronic properties and device performances of polymers with carbonyl, silicon, germanium, and nitrogen bridging groups. Finally, dithienopyrrole-based sensors were developed that showed substantial colour changes on exposure to basic solutions, as well as potential for use as readily dopable materials for electronic applications. Open access

This thesis reports the development of a droplet-based flow synthesis of silver nanowires. Using traditional batch methods of production it can be difficult to achieve the consistent reaction conditions needed to obtain nanocrystals with the desired properties. Microfluidic reactors offer superior control over reaction conditions and enable the scalable and continuous production of consistent and monodisperse nanoparticles. Silver nanowires are typically synthesised using the polyol reaction, where silver nitrate is reduced in hot ethylene glycol. Previous reports describing the synthesis of silver nanowires have come to contradictory conclusions about the conditions needed for growth. In this thesis a hot-injection polyol synthesis was adapted to a straightforward heat-up procedure for the production of silver nanowires. The success of the reaction was found to be dependent on the batch of ethylene glycol used. The modified synthesis enabled the development of a flow process using a polytetrafluoroethylene tubing-based droplet-based flow reactor. The reactor produced consistent silver nanowires over an eight hour period with no sign of reactor fouling. The reactor was used to investigate the effect of varying the reaction temperature, residence time, and concentration of iron nitrate and sodium chloride additives on the length of the silver nanowires produced. This thesis finishes with the development of an inline liquid-liquid separator based on the selective wetting and permeation of a porous capillary by one of the liquids. Efficient separation of aqueous-organic, aqueous-fluorous, and organic-fluorous flows was achieved over a wide range of flow rates. The separator was successfully applied to the inline aqueous-organic extraction of the pH indicator 2,6-dichloroindophenol. The organic extract and aqueous raffinate were separated using a porous capillary. UV-visible absorption spectroscopy showed the concentration of indicator in the aqueous raffinate to be less than one percent of its original value, confirming the efficacy of the extraction and separation process. Open Access

Next-generation electronic devices which are cheap, lightweight and flexible could be realised through the use of solution processable organic polymer and small molecule semiconductors. Unlike inorganic semiconductors such as silicon, soluble organic semiconductors could be processed using traditional high through-put printing techniques such as roll-to-roll processing and ink-jet printing, which would dramatically reduce manufacturing costs. Whilst organic semiconductors are not expected to be as high performance as inorganic semiconductors, improvements in performance are still required before commercialisation is possible. One way to help improve performance is to exploit the chemical versatility of organic materials. Many different structures can be synthesised through chemical modification, allowing the optoelectronic properties (such as the optical band gap and energy levels) and physical properties (such as solid state structure) of the material to be tuned. Materials can therefore be chemically designed to optimise their performance in organic electronic devices. This work is focused on exploring the relationship between chemical structure, material properties and device performance, through the design and synthesis of new materials for organic field-effect transistors (OFET) and organic photovoltaics (OPV). The majority of the new materials synthesised in this thesis are new donor-acceptor polymers (Chapters 2-6), in which an electron donating monomer and electron accepting monomer are co-polymerised. Whilst there is a vast wealth of different donor monomer structures available, there has been less focus on the synthesis of new electron accepting monomers. In this work the common electron acceptor monomer 2,1,3-benzothiadiazole (BT) is chemically modified to either increase the solubility (Chapter 2) or increase the electron accepting strength (Chapters 3 and 4). Increasing the strength of the electron accepting unit in donor-acceptor polymers was found to induce N-type (electron conducting) behaviour in OFET devices (Chapter 3) or improve OPV performance by reducing the optical band gap and increasing light absorption (Chapter 4). Power conversion efficiencies of ~6.5% in OPV devices were achieved. In chapter 6 a novel BT based acceptor monomer is designed to maximise polymer backbone planarity which resulted in promising hole mobilities of up to 0.5 cm2/Vs when tested in OFET devices. In Chapter 5 the strength of the common electron accepting unit benzo[d][1,2,3]thiadiazole (BTz) is also increased through chemical modification. Similarly to Chapter 4, we find that increasing the strength of the electron accepting unit of the donor-acceptor polymer improves the OPV performance through increased light absorption, resulting in efficiencies of ~6.5% in OPV devices. Finally in Chapter 7, new electron rich conjugated small molecules are synthesised and the optoelectronic properties investigated. Open Access

Organometallic complexes play a crucial role in Organic Electronics and due to their interesting optoelectronic properties they have been widely applied in organic light-emitting diodes and organophotovoltaic devices. Organometallic complexes are often used in the form of small molecules and their exact orientation in the bulk material can have a major effect on device performance. In this work, different approaches of covalently incorporating metal-porphyrin complexes into polymers were explored with the aim of producing homogeneous donor-acceptor type polymers which allow for the elimination of phase separation effects within the active material of the respective devices. A set of design rules was developed for the creation of porphyrin-polymer complexes and their application as host-guest systems in the triplet-sensitisation of semi-conducting polymers was studied, determining the optimal functional groups as well as matching absorption and emission properties. To this end, a synthesis pathway was developed that enabled the creation of metal porphyrins as well as their specific functionalisation for use in different co-polymerisation reactions. Platinum (II), palladium (II) and zinc (II) porphyrins were synthesised and successfully incorporated into a series of semi-conducting polymers: regiorandom and regioregular poly(3-hexylthiophene) (P3HT), poly(bis(decylthiophene)phenylene) (TTP) and poly(para-phenylenevinylene) (PPV). Successful co-polymerisation was reported for all polymers and the electronic interaction between the polymer hosts and porphyrin guests was analysed. The successful creation of triplet-sensitised polymers was shown and the full triplet formation pathway was mapped by spectroscopic means, creating a novel tool for the generation and analysis of polymer triplet excitons. Finally, computational methods were used for the accurate description of polymer triplet excited states and their usefulness in the prediction of excited state properties was shown. Polymer models were designed, computed and compared to experimental data and the use of computational methods as a widely applicable and precise tool for determining polymer excited states was discussed, further substantiating the design rules developed throughout this work. Open Access

In this work, we explore the uses of fluorination in the context of π-conjugated polymers. Using extensively studied model systems such as poly(3-hexylthiophene) (P3HT) and poly(2,5-bis(3- alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT), we are able to probe the influence of thiophene fluorination on the electronic structure and thin-film morphology, two defining aspects of organic semiconductor performance in electronic devices such as organic field effect transistors (OFET). Experimental data obtained from Ultraviolet photoelectron, UV-Visible and Raman spectroscopy along with differential scanning calorimetry, atomic force microscopy and X-ray diffraction suggest that fluorination leads to a stabilisation of the frontier molecular orbital levels and increased backbone planarity. The latter manifest itself as an enhanced propensity to aggregate and much higher melting point. Hole carrier mobilities in OFET devices are considerably enhanced in the backbone-fluorinated polymers, an observation attributed to the increased planarity of the polymer backbone. Computational calculations also predict an increased backbone rigidity and a strong preference for the coplanar arrangement of fluorinated thiophene rings. The origins of such planarisation are thought to be intramolecular in nature, likely from sulfur-fluorine inter-ring interactions as evidenced by S-F short contacts observed in the single crystal X-ray structure of a bithiophene monomer precursor. By synthesising well-defined diblock and alternating copolymers, backbone fluorination is then explored as a potential tool to control phase separation and dielectric constants, two key factors involved in the context of organic photovoltaic (OPV) devices. The utility of fluorine atoms as a good leaving-group in nucleophilic aromatic displacements on electron-poor aromatic systems is then used to produce a P3HT derivative end-capped with pentafluorobenzene. This polymer can then be reacted with good nucleophiles in mild conditions. As a proof of concept, three functional moieties are tethered to the P3HT derivative in this manner, opening the door towards sensing applications and hybrid organic/inorganic semiconductor interface engineering. Open Access

Spray deposition is a promising technique for the solution processing of plastic electronic devices due to its potential for high-speed, large-scale device fabrication using low capital cost equipment. Most recent reports of spray-deposited electronic devices have used ultrasonic systems for film deposition to minimise surface roughness, but such systems suffer from low materials throughput, and are consequently ill-suited to industrial manufacturing. Gas-driven spray-heads enable higher throughput materials delivery but can result in an unacceptably rough film due to the large, broadly distributed droplet sizes that are often generated and the hard-impact nature of the deposition process. This thesis describes a new automated gas-driven spray coater for the controlled deposition of a broad variety of solution processed electronic materials. It is specifically suited to the deposition of polymer films such as poly(3-hexylthiophene) (P3HT), overcoming many of the usual disadvantages of conventional gas-driven spray coaters. Key features of the system include: a novel, high performance 3D-printed spray-head for the generation of ultrafine sprays; full three-dimensional position and velocity control of the spray head; integrated temperature control; and independent control of solution and gas flow rates. To determine the optimum solution composition for spray-deposition of P3HT films, the aggregation and gelation dynamics of various P3HT/o-xylene solutions were first investigated by static light scattering. On the basis of these measurements, we conclude that dilute solutions (with polymer concentration

In this work we investigate the charge transport, injection and optical properties of several series of fluorene based copolymers. These properties are key areas for polymer semiconductor research studies that are intended to address the challenges of achieving electrically pumped polymer lasing. In general, high charge carrier mobility and luminescence efficiency have been found to be mutually exclusive, however, optimising both together, as well as demonstrating high optical gain will be a significant step to offset losses associated with the mechanisms for lasing. A further requirement is that it is necessary to achieve efficient charge injection in the particular materials that are found to have both of these properties optimised. We concentrate our investigation on fluorene-based copolymers, which have been amongst the earliest classes of materials of interest for polymer lasers (including studies in optically pumped polymer lasers), and since it has previously been shown that high hole mobility and luminescence efficiency may be achieved in fluorene based copolymers with simple alterations to side chain configurations. However, ohmic hole injection into poly(9,9-dioctylfluorene) (PFO) has been difficult to achieve because of its deep-lying highest occupied molecular orbital (HOMO) level of ~5.8eV below the vacuum level. Therefore, in our first experimental study, we revisit the charge transport properties of PFO based on recent developments in the use of transition metal oxides as hole injecting electrodes. With the ability to achieve ohmic injection in PFO, we are able to use Dark Injection Space Charge Limited Current (DI-SCLC) and field effect transistor (FET) techniques, both of which require ohmic injecton for charge transport measurements, and make comparisons with measurements from Time-of-Flight (ToF) photocurrent experiments. Together, these techniques span several orders of magnitude in charge carrier density, which provides an additional perspective on charge transport and the presence of charge traps. The subsequent chapters introduce three series of fluorene-based copolymers. The first series of blue emitting polymers has a resemblance to familiar fluorene based polymers with hexyloxyphenyl substituents instead of linear alkyl side chains and the series contains copolymers with a phenoxazine (BPPX) unit derived from more familiar triarylamine structures. Optical measurements show these polymers are free of morphologies such as the beta-phase and we obtain a hole mobility of ~10-3 cm2/Vs, photoluminescence quantum efficiency (PLQE) of 65.7% and optical gain of 41cm-1 for poly(9,9-di(4-hexyloxyphenyl)fluorene). Transient DI-SCLC measurements are used to show differences in charge trapping behaviour in the series. The final chapter of experimental results is based on novel poly(indenofluorene-fluorene) and poly(indenopyrazine-fluorene) copolymers. In the poly(indenofluorene-fluorene) series we consider the role of increased backbone rigidity compared to polyfluorene and the variation of side chain length and structure on charge transport and optical properties. Copolymers in this series yield a time-of-flight hole mobility of ~10-2 cm2/Vs, PLQE of 61.5% and optical gain of 43cm-1. Finally, we show that optical gain is possible in the indenopyrazine-fluorene series, which has been designed with the aim of achieving balanced electron and hole mobility. Open Access

Electron injection in dye-sensitized solar cells (DSSC) has been extensively studied in the present thesis. Many transient and steady-state experimental techniques were utilized: ultrafast transient absorption spectroscopy in the visible, near-IR and mid-IR regions; time-correlated single photon counting; incident photon to current conversion measurement; temperature dependence of luminescence and device efficiency; determination of energy conversion efficiency parameters. All these methods were collated and analysed as a whole, providing a very detailed picture of the electron injection. It was found that in devices utilizing the most efficient Ru-bipyridyl dyes and commercially viable, performance optimized iodine/triiodide electrolytes, electron injection kinetics are multiphasic, with significant components in the femtosecond, tens of picoseconds and in nanosecond regimes. The broad non-exponential kinetics gave rise to non-100% efficient injection. A fraction of the dyes manage to decay giving rise to luminescence. All of these observations were fully quantified and validated by performing measurements on different systems. The huge length of the delay lines in TAS measurements and very broad probe wavelengths used in the current work gave a very detailed picture of the injection process. Along the way, some very basic experiments were done, which added confidence to the results obtained in the thesis. In addition, interesting phenomena occurring in the ultrafast timescales were found and spectra of transient species such as the dye excited states were recorded. Finally, relevance of the current work to the practical aspects of the solar cells was described and future work discussed. Open Access

This thesis reports the development of droplet flow reactors for the preparation of semiconducting polymers, focussing specifically on poly(3-hexylthiophene) (P3HT) pre- pared by Grignard metathesis polymerisation, and random copolymers of P3HT with poly(3-hexylselenophene) (P3HS). In contrast to conventional continuous flow reactors (where the flow consists of a single solvent phase), droplet reactors incorporate an immiscible ’carrier fluid’ that divides the solvent phase into a stream of near-identical microlitre-sized droplets which then pass through the reactor sequentially. In this way, the solvent phase is confined to a well-defined volume that no longer makes contact with the surrounding reactor architecture, and thereby overcomes blockage problems associated with preparation of polymers in continuous flow reactors. To overcome the limited solubility of the Ni(dppp)Cl2 catalyst widely used in Grignard metathesis polymerisation, a more soluble derivative, Ni(dppp)Br2, has been prepared. This catalyst allows P3HT to be synthesised with accurate control over a wide range of molecular weights, even at high throughput (up to 1 kg/day). A scalable ’tube-in-shell’ reactor was developed to provide improved thermal uniformity along the flow reac- tor, enabling the preparation of high molecular weight P3HT with Mw up to 190 kg/mol (PDI 1.5) and lower weight polymers with narrow polydispersity (PDI 1.1). To facilitate multistep chemistries in droplets, a new three-phase methodology is presented that enables the controlled addition of reagents downstream of the original droplet gener- ation. With a view to applying P3HT in electronic devices, an investigation into the influ- ence of polymer purity on the efficiency of P3HT:fullerene bulk heterojunction devices is reported. Polymer purity is shown to have a strong influence over polymer solubility and final device efficiency. With the purest polymers, efficiencies of up to 5.0 % and 7.0 % were obtained with PC60BM and IC60BA, respectively (PC60BM: phenyl-C61-butyric acid methyl ester, IC60BA: indene-C60 bis-adduct). The thesis finishes with the development of a new style of liquid-liquid separator, a key tool for post-processing droplet flows and enabling sequential operations. Open Access

Organic semiconductors have emerged as potential replacements for inorganic materials based on several promising advantages, such as their low-cost, high-throughput fabrication and tolerance for flexible substrates. While these attributes ensure that the use of organic semiconductors is appealing, further improvements in electronic performance and stability are still required. This work focuses on the design, synthesis and analysis of a novel range of organic semiconductors incorporating carborane units. Carboranes possess unusual properties which, in principle, make them highly attractive species to incorporate into optoelectronically active materials, although they have been largely untested as components of organic electronic devices. An uncommon three-dimensional delocalised bonding motif infers upon carboranes extreme thermal and chemical stability and a strong electron-withdrawing nature, traits that, if correctly exploited, could potentially lead to organic semiconductors with improved stabilities and optimised electronic properties. This thesis reports a number of approaches to the incorporation of carborane into semiconducting polymers. In the first, ortho-carborane is directly fused to the aromatic backbone via both carbon atoms. The novel difunctionalisation of benzocarborane, and its subsequent incorporation into two previously unreported polymer systems, is reported. Copolymerisation with electron-rich and electron-poor comonomers established that the incorporation of benzocarborane did not prevent electronic delocalisation along the polymer backbone, and that it acted as a mildly electron withdrawing comonomer. P-type behaviour in organic field effect transistors was observed. Based on these results, a second-generation monomer unit was synthesised, incorporating two fused thienyl rings and facilitating observable improvements in molecular ordering and charge-transporting behaviour. The second approach focuses on the attachment of carborane to the conjugated polymer backbone via the functionalisation of one of its carbon atoms, either directly or via a vinylene spacer. Hence carborane functionalised benzodithiophene monomers were prepared via a novel method of carborane C-arylation, before subsequent polymerisation with a range of electron accepting comonomers. The resultant polymers demonstrate OFET performance higher than that reported for non-carborane containing equivalents and exhibit promising solar cell performance. The final chapter details the attachment of various carboranes onto two polythiophene systems via a vinylene linker. Through careful consideration of carborane and vinyl bond stereochemistries, and pendant group spacing it was observed that optical, material, OFET and OPV properties of the resultant polymers could be controlled. Open Access

The first part of this thesis is devoted to the synthesis, purification and application of a series of indenopyrazine based polymers for their use in optoelectronic devices. The work focuses on the effect of alternating chain length on the degree of polymerisation, optical and charge carrier properties The next section studies the copolymerisation of indenopyrazine with dithiophene benzothiadiazole acceptor units and the effect substituents have on the planarity of the polymer backbone and the delocalisation of molecular orbitals and solar cell performance. The final part of this thesis investigates indenopyrazine based polymers as charge carrier materials in p-type transistors and probes the effect p-dopants have on the energy levels, charge carrier mobility and optical properties. The second part is the synthesis of the novel alkylidene-indenopyrazine monomer and probes the introduction of enforced planarity and finally the synthesis towards the novel the semiconductor indenopyrazinedithiophene. Open Access

The nanoscale morphology of the active layer in organic photovoltaic devices is critical to their performance. Blend morphologies can be manipulated by using a variety of additives, co-solvents and by thermal annealing. However, one crucial issue for any organic photovoltaic device is operational lifetime, which is largely dictated by changes in the active layer morphology. Such changes may be due to reordering of the polymer material, or diffusion and aggregation of fullerene derivatives to a more thermodynamically stable state. This reordered structure can potentially have detrimental effect by losses in interfacial surface area and an increase in exciton recombination. Many novel polymers are designed without this consideration and may be susceptible to this type of degradation. Herein, we explore a variety of approaches to stabilise the blend morphology, principally by the incorporation of various reaction functionalities onto the polymer backbone. The three main areas which have been investigated are; varying of acceptor strengths, chain end modifications with various crosslinking chemistries and thermally cleavable leaving groups. Firstly, poly(dithienogermole-benzothiadiazole) analogues are synthesised to investigate how changes to the acceptor strength of the D-A co-polymers will affect the bulk heterojunction morphology and polymer performance. Then the synthesis and functionalisation of benzotriazole in poly(cyclopentadithiophene-benzotriazole) with alkyl bromine crosslinking groups and thienopyrroledione in poly(dithienogermole-thienopyrroledione), with both alkyl bromide & alkyl oxetane crosslinking groups was investigated. Further studies will investigate the effects on material and optoelectronic properties as well as device performance in OPV and suitability of additives with crosslinking. Lastly, we explore the incorporation of thermal cleavable BOC groups on thienopyrroledione, in poly(cyclopentadithiophene-thienopyrroledione), the deprotection of Boc in polymeric materials and its effects on material and optoelectronic properties. Open Access