The overall efficiency of organic photovoltaic devices has made impressive progress in recent years, but often active materials now require complex, multi-step synthesis, potentially limiting their application. We report approaches to prepare building blocks in just one or two steps, allowing the preparation of conjugated polymers of low synthetic complexity which can be readily upscaled. I will highlight how this approach can be used to readily build libraries of conjugated polymers to more rapidly identify promising materials for application in OPV and OECT devices. Finally approaches to introduce additional functionality via post-polymerisation backbone modification will be discussed.
Carboranes are polyhedral clusters with unusual molecular bonding and electronic properties. The icosahedral C2B10H12 carborane exists as either the para (p), meta (m), or ortho (o) isomer, with an increasing polarity and electron-withdrawing nature as the carbon atoms are positioned closer to each other. Here we examine the effects of the inclusion of different caborane isomers into a range of linear and non-linear aromatic systems and polymers, focusing on their photoluminescence properties.
One of the key areas of study in organic photovoltaics is the development of so-called 'non-fullerene acceptors' (NFAs), which enjoy several benefits over older, fullerene-based acceptors, such as low cost, high absorptivity, and tuneability. A recent report Fei et. al. demonstrated conversion efficiencies of 13% in donor-acceptor blends comprising a fluorinated derivative of the common donor PBDB-T and an alkylated derivative of the ITIC (C8-ITIC) acceptor species. Understanding the underlying dynamics of this material is therefore important for the rational design of new NFAs.
In order to understand the photophysical processes in C8-ITIC, we performed ultrafast transient absorption studies of four donor-acceptor blends, containing various combinations of C8-ITIC, PFBDB-T, and their unmodified predecessors. Long-lived excitons form at the acceptor regardless of the excitation frequency, suggestive of rapid energy transfer from the donor to the acceptor. Exciton decay at early times was more rapid in C8-ITIC compared to non-alkylated ITIC. A distinct change in exciton decay characteristics was observed at longer timescales in tandem with spectral drift in the acceptor’s excitonic peak. We use global analysis and a broader array of ultrafast spectroscopic techniques to elucidate the identity and mechanism behind this feature. Our results will help to shed light on the efficiency of this material and aid the development of more efficient and effective non-fullerene acceptors.
We have used a femtosecond-resolved spectroscopic technique based on the Stark effect (electromodulated
differential absorption) in order to investigate free charge generation and charge drift in solar cell devices of
neat conjugated polymer pBTTT and in its 1:1 (by weight) blend with PCBM. In the latter, the fullerene
molecules intercalate between the polymer side-chains, yielding a co-crystal phase. Our results show that free
charge generation in both materials is ultrafast and strongly dependent on the applied reverse bias. Charge
drift to the electrodes (under strong reverse bias) occurs with comparable dynamics on the 1.2 ns time scale
for neat pBTTT and the blend, and is probably dominated by hole transport within/between polymer chains.
We have fabricated high-mobility ambipolar polymer transistors and have integrated multiple transistors to demonstrate
their implementation into CMOS-like logic circuitry. The performance of a selenophene-based polymer semiconductor
PSeDPPBT is initially screened using standard long-channel field-effect transistors. The polymer exhibits high and
balanced hole and electron mobilities of ∼ 0.5 cm2/Vs and ∼ 1.0 cm2/Vs, respectively. Next, exploiting the beneficial
electronic properties of PSeDPPBT, we have fabricated ambipolar inverters, ring oscillators and logic NOR gates.
Ambipolar inverters are shown to exhibit voltage inversion with proper noise margins and no voltage loss over multiple
stages. The potential speed of ambipolar logic is demonstrated by the realization of ambipolar ring oscillators with
unprecedented performance. The feasibility to perform logic operations is demonstrated by the fabrication of ambipolar
NOR gates. The combined results, (i) no loss in voltage over multiple inverters, (ii) the unprecedented speed, and (iii) the
accomplishment of a functionally complete logic operation, demonstrate the feasibility of ambipolar logic as a reliable
substitute for complementary-based logic in order to realize cost-efficient electronics.
We focus upon the role of interfacial energetics and morphology in influencing the separation of CT states into dissociated charge carriers. In particular, we undertake transient optical studies of films comprising regioregular poly(3-hexylthiophene) (P3HT) blended with a series of perylene-3,4:9,10-tetracarboxydiimide (PDI) fullerene electron acceptors. For the PDI film series, we observe a close correlation between the PDI electron affinity and the efficiency of charge separation. This correlation is discussed in the context of studies of charge photogeneration for other organic donor/acceptor blend films, including other polymers, blend compositions, and the widely used electron phenyl-C61-butyric acid methyl ester(PCBM). Furthermore, we compare the charge recombination dynamics observed in films comprising P3HT blended with three fullerene derivatives: PCBM and two alternative pyrazolinofullerenes. Transient absorption data indicate that replacement of PCBM with either of the pyrazolinofullerene derivatives results in a transition from nongeminate to monomolecular (geminate) recombination dynamics. We show that this transition cannot be explained by a difference in interfacial energetics. However, this transition does correlate with nanomorphology data that indicate that both pyrazolinofullerenes yield a much finer phase segregation with correspondingly smaller domain sizes than observed with PCBM. Our results therefore provide clear evidence of the role of nanomorphology in determining the nature of recombination dynamics in such donor/acceptor blends.
We investigate charge injection and transport in organic field-effect transistors fabricated by using poly(2,5-bis(3-tetradecylthiophene-2-yl)thieno[3,2-b]thiophene) (pBTTT-C14) or poly(3-hexylthiophene) (P3HT) as the active polymer
layer. We show that in high mobility devices where the channel resistances are low compared to the contact resistances,
the device performance can be dominated by the metal/organic semiconductor (OSC) contacts. However, in sets of
devices where the channel resistance is dominant over the contacts (usually the lower mobility devices), we see
pronounced field dependence in the saturation regime mobilities consistent with a Poole-Frenkel model of charge
transport within the channel. The field-dependent mobility in short-channel devices produces nonlinear output current-voltage
characteristics which can be modeled consistently in the Poole-Frenkel framework.
Time-of-flight (TOF) photocurrent measurements have been used to study charge transport in films of regioregular
poly(3-hexylthiophene) (P3HT). Devices in which the P3HT film had been deposited directly onto an indium tin oxide
(ITO) electrode produced high dark currents as a result of hole injection into P3HT from ITO. Photocurrent transients in
such devices were disperse. It was found however, that these dark currents could be significantly reduced by inserting a
dense TiO2 layer between the ITO and the polymer film. The resulting devices gave non-dispersive transients with hole
and electron mobilities in the range of 1 - 2 10-4 cm2 V-1 s-1 at room temperature. The mobility values were observed to
be almost independent of film thickness over the range of 350 nm to 4.3 μm. Temperature dependence studies showed a
weak dependence on temperature with a low energetic disorder parameter according to analysis using the Gaussian
Disorder Model (GDM) of 71 meV.
A common strategy to improve the electrical performance of organic field effect transistors is to optimize the charge carrier mobility of the semiconducting thin film. Polymer semiconductor transport properties have shown a dependence on the chain length, due principally to the strong influence of molecular weight on the thin film microstructure. In this work, we report on a study of the influence of increasing molecular weight of poly(2,5-bis(3-docecylthiophen-2-yl)thieno[3,2-b]thiophenes) (pBTTT-C12) on the polymer bulk thermal properties, thin film microstructure and the electrical performance of thin film field effect transistor devices. Clear differences can be observed within a number average molecular weight range of 8,000 - 18,000 Dalton. A Liquid crystalline phase was only observed at the highest molecular weight, different thin film morphology was observed within the molecular weight range, and the field effect mobility was shown to increase with increasing molecular weight.
Displays based on polymer light emitting diodes are attractive due to their emissive nature, their wide viewing angles and the ability of electroluminescent conjugated polymers to be solution processable at room temperature and pressure. It is difficult, however, to deposit separate red, green and blue (RGB) pixels and to maximize
performance by making the devices multi-layered. Here we present recent results on a semiconducting conjugated reactive-mesogen OLED material which is solution processable, can be potentially cured and patterned by photolithography and used in multi-layer devices. This material consists of a conjugated pentathiophene core with reactive endgroups. Spectroscopy, calorimetry and microscopy show that it forms crystalline, aggregate, liquid-crystalline and isotropic phases at a range of different temperatures. The material is deposited by spincoating from solution. Low density doping with a cationic photointiator and exposure to a specific UV wavelength to avoid damage to the conjugated core leads to cross-linking into an insoluble network. Current-voltage-luminousity and spectral measurements in standard OLED device structures show the effect of cross-linking on the transport and injection properties of the material. Quenching of fluorescence and electroluminescence is discussed. Insertion of lower-energy gap, fluorescent small molecules can potentially be used to tune the emission to any desired colour but material limitations to this technique due to dopant removal during the washing procedure were observed.
The interface between the semiconducting polymer and the gate dielectric is one of the most critical regions of a polymeric thin film transistor. For polymeric TFTs, it is difficult to disaggregate the contributions of the electronic structure of the semiconductor and that of the dielectric because, in part, the microstructure of thin films of semiconducting polymers is strongly affected by the chemical functionality at the surface of the dielectric. We have developed a lamination technique that can be used to transfer semiconducting films formed on surfaces that yield films
with high mobility to other dielectrics. We have studied films of semiconducting polymers, such as poly[5,5'-bis(3-dodecyl-2-thienyl)-2,2'-bithiophene] and poly(3-hexylthiophene) using this method. The effects of self-assembled monolayers (SAMs) formed on inorganic dielectrics on device performance are discussed. Our results suggest that mobility is mainly controlled by the structure of the semiconducting film and that the threshold voltage of TFTs may be modified through the use of SAMs.
The development of p-type semiconducting polymers demonstrating good stability under ambient operation is of importance for the development of low cost, printed electronics. We present here the synthesis and full characterisation of two soluble terthiophene polymers, and examine the effect of introducing a fused aromatic heterocycle, thieno[2,3-b]thiophene, into a terthiophene polymer backbone. This heterocycle contains a cross-conjugated central double bond, and its inclusion was shown to have a marked influence on the optical, thermal and electrical properties of the terthiophene polymer. Transistors were fabricated from both polymers, and the operation and storage lifetime under ambient operation was compared.
This work describes the development of solution processable liquid crystalline semiconductors and their applications in field-effect transistors. The relationship between liquid crystal molecular structure, its corresponding phase behaviour and electrical performance is examined. Molecular design methodology is employed to control the liquid crystalline morphology. The thermal, optical and electrical behaviour of these materials is characterised and X-ray diffraction scattering technique is used to reveal details of morphology and molecular orientation.
New polymerisable liquid crystalline organic semiconductors based on small molecule "reactive mesogens" are reported. These molecules, comprising p-conjugated cores with reactive endgroups, were designed, synthesised then solution processed into thin films. The mesogenic morphology was attained thermally and fixed through a post fabrication photopolymerisation reaction. Thermal, optical and electrical properties of these thin films were characterised to reveal details of morphology and molecular orientation. Both bulk (time-of-flight) and surface (field-effect) charge carrier mobilities were measured. The relationship between molecular structure, corresponding macrostructure processability and charge mobility is discussed. Fabrication and characterisation of field-effect transistors based on reactive mesogens are presented and discussed.
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