Current OLED displays rely on a circularly polarised (CP) filter to enhance contrast by trapping ambient light inside the display. However, this means that 50% of the randomly polarised light emitted from each OLED pixel never leaves the screen, halving display efficiency and operational lifetime. One simple route to fabricate CP-emitting OLEDs is to use electroluminescent (EL) polymer – small molecule blends. Our approach is to pair a chiral small molecule with a non-chiral device optimised polymer, which allows for CP-dependent applications while retaining much of the performance properties of the original polymer. Previously circularly polarised polymer emission has been achieved via thick cholesteric stacks of liquid crystalline polymers, where linearly polarised light becomes circularly polarised. Here we show that it is possible to control whether cholesteric packing or chiral dipole dominates emission using film thickness; remarkably this allows us to change the handedness of the CP EL emission in the same materials system. We compare how the chemical structure of the non-chiral polymer and post-deposition processing impacts the chiroptical response of the resulting device, in an effort to provide a set of design rules for future high performance CP-OLEDs. We demonstrate a liquid-crystalline light emitting polymer with a record high induced absorption dissymmetry factor, which additionally shows no change in device characteristics (no trapping, etc) in the blends, as well as strong CP-PL and EL emission.
We report solution processed highly photosensitive thin film transistors (TFTs) based on poly(9,9-dioctylfluorene-cobithiophene)
(F8T2) as an active photoconducting material. Bottom gate contact coplanar device structure on Si wafer
transistors was used. The photosensitivity of the drain photocurrent was investigated for different F8T2 annealing
temperatures and illumination irradiances. Transistors annealed at 280oC show the highest drain current, approximately 8
times higher than the as-spincoated device at room temperature with a gate voltage of -40V. However, the field effect
mobilities in the saturation regime for all devices at different annealing temperatures are in the same order of ~10-4
cm2/Vs. The field effect mobilities of the transistors were not affected by illumination, but the drain photocurrent of the
transistor was significantly increased and the threshold voltage was shifted towards zero bias voltage when the polymer
absorbs photons. The measured maximum responsivity was ~18.5 A/W for an LED light source with a peak wavelength
of 465 nm and 19 nm bandwidth at 5 μW/cm2 light intensity. This is so far the highest reported for F8T2
phototransistors. The characteristics of transistors dominated by the photoconductive effect (turn-off) as well as the
photovoltaic effect (turn-on) against a wide range of illumination intensities are reported.
We report preliminary studies of the nature of hole injection from poly(3,4-ethylenedioxythiophene)/polystyrenesulphonic acid (PEDOT:PSS) into three commercial conjugated light emitting polymers (LEPs). Sumation's LUMATION Green 1300, LUMATION Blue, and Merck's SuperYellow LEPs are studied in combination with interlayers of poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl)-diphenylamine) (TFB), and poly[9,9-dioctylfluorene-co-(bis-N,N'-(3-carboxyphenyl)-bis-N,N'-phenylbenzidine)] (BFA). Despite the highest occupied molecular orbitals (HOMOs) of the interlayers being close to that of PEDOT:PSS and the LEP, different interlayers have different effects on hole injection and OLED device performance. We use dark injection transient current method to show that interfacial morphology changes results in modulation of hole trap densities that in turn affect hole injection. Depending on the interlayer/LEP combination partial penetration of interlayer into the LEP layer may also occur resulting in additional changes in the bulk transport properties of the LEP. Our results show that it is not the interfacial energy level alignment but the physical morphology changes at the interface which are important for varying hole injection into the device. A combination of either improved or reduced hole injection due to variations in physical contact, intermixing and trapping at the interlayer/LEP boundary dominate device performance.
Here we report results of time-of-flight (ToF) measurements on blends of different ratios of poly(9,9-dioctylfluorene-cobis-
N,N'-(4-methoxylphenyl)-bis-N,N'-phenyl-1,4-phenylenediamine) (PFMO) and the structurally similar poly(9,9-
dioctylfluorene-co-N-(4-methoxyphenyl)diphenylamine) (TFMO). It is shown that the hole mobility can be tuned over
three orders of magnitude with a mobility minimum at 10% PFMO and 90% TFMO. We also use Raman microscopy to
demonstrate that the blends do not phase separate within the one micron resolution of our experiment.
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.
Semiconducting polymers can be used in light-emitting-diodes (LEDs), photovoltaics (PVs), and field-effect-transistors (FETs). In all of these devices charge carrier transport is a major issue, the mobility being directly related to device performance. In LEDs and PVs, charge transport occurs vertically through a bulk semiconducting polymer film. This bulk mobility is determined by the average interchain hopping distance a, the polaron relaxation energy λ, the level of energetic and spatial disorder σ and Σ, the presence of charge traps and different structural phases. In FETs, charge transport occurs horizontally along the interface between the semiconducting polymer film and the insulating material. The FET mobility is also determined by the above parameters but these may be different from the bulk. Also, there are additional factors such as surface features which have to be circumnavigated, specific interface trap states, and the high charge carrier densities effectively filling all the deep sites. Here we present results looking at the difference between the bulk mobility, as measured by time-of-flight (TOF) photocurrent, versus the FET mobility, as measured by the FET transfer characteristics. Three different polyfluorene copolymers are investigated. In all three materials, the room temperature hole TOF bulk mobility was found to be greater than the FET mobility. This indicates that models based on deep site filling due to the high FET carrier densities cannot be correct. Temperature measurements also show that the level of energetic disorder σ in the FETs is the same or less than that in the bulk, as is the polaron relaxation energy λ or thermal activation energy of any deep traps. The results instead indicate that it is the average interchain hopping distance which is greater at the insulator-semiconductor interface in FETs than in the bulk films, and it is this which is responsible for the difference in mobility.
Time-of-flight photocurrent (TOF) measurements on the blue emitting electroluminescent polymer poly(9,9-dioctylfluorene) (PFO) show that the room temperature hole mobility can vary from 10-2 to 5x10-5 cm2/Vs depending on how the polymer films are prepared. It also undergoes irreversible increases when the samples are annealed. These results can be related to PFOs complex phase behavior and show the importance of understanding and controlling the polymer film structure for device applications. We also present new TOF measurements on the green emitting electroluminescent polymer poly(9,9-dioctylfluorene-co-benzothiadiazole) (BT). Previous TOF measurements have shown that BT exhibits dispersive electron transport and that holes are very heavily trapped, no hole transport signal being measurable using this technique. The new TOF measurements on a recently synthesized batch of BT show less dispersive electron transport with a mobility of 10-3 cm2/Vs as well as non-dispersive hole transport with a mobility of 2 x 10-3 cm2/Vs. This new batch therefore exhibits the highly desirable property of both good electron and hole transport in the same electroluminescent polymer. It is proposed that this is due to a change in the molecular weight and/or polydispersity of the material and indicates the importance of further development of relatively well known materials. TOF measurements of the variation of the hole and electron mobility with temperature are examined within the framework of the Gaussian disorder model.
Polymer field-effect-transistors (FETs) have been proposed for use in display driver circuitry, information storage and processing, and identity tags. To maximize performance in polymer FETs it is important to have a high carrier mobility. A major issue in this area is how the FET mobility, as measured by analysis of source-drain current-voltage characteristics, relates to the bulk mobility, as measured by a technique such as the time-of-flight (TOF) photocurrent
or space-charge-limited current (SCLC) methods. Here we report comparative FET, TOF and SCLC measurements of polyfluorene copolymer devices. Poly (9,9-dioctylfluorene-co-bithiophene) (F8T2) and poly (9,9-dioctylfluorene-co-bis-N, N’-(4 butylphenyl)-bis-N,N’-phenyl-1,4-phenylenediamine) (PFB) were used as the active material. Polymer FETs
were fabricated in two different structures. The first involved a silicon substrate with a thermally grown oxide dielectric onto which Au source-drain electrodes were deposited. The polymer was then spin-coated on top and annealed to improve the chain packing. The second involved spin-coating the polymer onto a glass substrate followed
by the thermal evaporation of top Au source-drain contacts. A polymer insulator was then spin-coated followed by a top Au gate electrode. TOF and SCLC measurements were conducted on diode structures consisting of a polymer layer (of order 1 μm and 100nm respectively) sandwiched between indium-tin-oxide (ITO) or ITO coated with poly (ethylenedioxythiophene)/polystyrene sulphonic acid (PEDOT/PSS) on glass bottom electrode and a Au or Al top
electrode. Trapping effects, and the difference between bulk and surface packing can describe the differences of mobilities derived from the three methods. It is interesting to note that they are all within a similar order.
Poly(9,9-dicotylfluorene)(PFO) exhibits very good, non- dispersive hole transport but very poor electron transport. To achieve the maximum efficiency in a PFO light emitting diode it is important to balance the electron and hole currents. Here we report three schemes to achieve this in single layer devices. Firstly, by using different treatments to change the work function of the indium tin oxide anode contact, the hole current can be varied by up to 4 orders of magnitude, thus allowing it to be adjusted to the same level as the electron current. Secondly, the hole mobility can be decreased by doping PFO with a hole trapping, emissive material. Upon the addition of 5% by weight of a red-emitting tetraphenylporphyrin, hole transport in PFO becomes as highly dispersive as electron transport, resulting in a decrease in the current for a given applied bias but an increase in the electroluminescent efficiency. Thirdly, the electron mobility can be increased by doping PFO with an emissive, electron transporting material. The electroluminescent polyfluorene copolymer poly(9,9-dioctylfluorene-co-benzothiadiazole (BT) exhibits strong but dispersive electron transport. PFO devices doped with BT show very high efficiencies, high peak brightnesses and very low turn on voltages.
If conduction in a polymer light emitting diode is by trap- free, space-charge-limited current (TFSCLC), the injecting contact must be ohmic. Such an ohmic contact is, by definition, the perfect injecting electrode. The absolute injection efficiency of any specific contact can therefore be quantified by the ratio of the actual current density, J, to that of the ideal TFSCLC current density, JTFSCLC. In this work we report on the injection efficiency of positive carriers from pretreated indium tin oxide (ITO) and metal electrodes into poly(9,9-dioctylfluorene) (PFO) and two representative copolymers. The ITO was either washed only in solvents, exposed to an oxygen plasma or coated in a film of poly(ethylenedioxythiophene)/polystyrenesulphonic acid blend. Time-of-flight photocurrent measurements were used to find the positive carrier mobility, (mu) p, so that JTFSCLC could be calculated. Dark injection current transient measurements were used to confirm the presence or absence of TFSCLC conduction. Steady-state, current density- voltage measurements were then used to compare the actual current to JTFSCLC, thus allowing the absolute injection efficiency, J/J(subscript TFSCLC, to be calculated.
We report electroabsorption studies of electric fields in organic light emitting diodes made form substituted poly(para phenylene vinylene) derivatives and solar cells made form zinc phthalocyanine (ZnPc) and perylenetetracarboxylic diimide (PTCDI). The electric field in LEDs is not proportional to the applied bias due to the development of an internal electric field during operation that opposes the applied bias. This counter field is weaker for devices measured in vacuum than for those measured in an ambient atmosphere and is no longer apparent for devices that were prepared and tested under an inert atmosphere. We also observed that the built-in potential increased with operating time. The combination of these two processes leads to an increase in the turn-on voltage of organic LEDs with increasing operating time. We have detected an electric field at the electrode/organic LEDs with increasing operating time. We have detected an electric field at the electrode/organic interface of organic solar cells which is insensitive to the external DC bias. The interface field has a different spectral signature from that of the bulk of the two layers and is attributed to charged transfer-induced dipoles. Rectifying behavior due to the formation of a pn junction under illumination is observed in bilayer solar cells, but not single layer devices made from ZnPc or PTCDI.
Charge trapping will have a strong effect on the performance of organic light emitting diodes. Here different models for such trapping in disordered organic semiconductors are presented. The benefits of different transient experimental techniques are explored. Results are presented for electroluminescent polymer diodes which are fully depleted or contain a depletion region type Schottky barrier. The transient behavior can be explained by a single energy trap site emptying to a Gaussian distribution of transport sites.
The polymer crystals of the soluble polydiacetylene 4BCMU are thermochromic. Four distinct phases have been identified: the low temperature, highly ordered, crystalline Blue Phase I; the high temperature, semi-disordered mixed phase consisting of the crystalline Red Phase II and liquid- crystalline Yellow Phase II; the low temperature, highly ordered, crystalline Red Phase III; and the highly disordered, amorphous Yellow Melt Phase. The structure of the crystals and the conformation of the backbone and sidegroups are reported. The structure of the phases in the solutions and solution-caste films are also identified.
The current-voltage characteristics of ITO/polymer film/Al or Au devices of poly(phenylene vinylene) (PPV) and a dialkoxy PPV copolymer can be fitted at high applied bias to a power law of the form J equals KVm where m increases with decreasing temperature, log(K) is proportional to m, and K is proportional to d-(alpha m) where d is the film thickness and (alpha) is a constant. (alpha) 2 and 1 for the Al and Au cathode devices respectively. Different single carrier space charge limited conduction (SCLC) theories, including either an exponential trap distribution or a hopping transport field and temperature dependent mobility, are used to try and explain this behavior. Both models are in good agreement with the general experimental results, but can also be criticized on a number of specific issues.Mixed SCLC models and the effect of dispersive transport are also explored. It is concluded that carrier mobility and trap measurements are required to distinguish between these models. To this end, initial trap measurements of ITO/PPV/Al devices using deep level transient spectroscopy (DLTS) are reported. Very deep positive carrier transport with emptying times > 4 minutes have been detected. The non-exponential DLTS transients have been successfully modeled on an isoelectronic trap level emptying to a Gaussian distribution of transport states, with a trap depth and density of 0.8eV and 4 by 1016 cm-3 respectively.
We report studies focusing on the nature of trap states present in single layer ITO/polymer/metal devices of poly(p- phenylene vinylene) and its soluble derivative poly(2,5- dialkoxy-p-phenylene vinylene). In the high applied bias regime the IV characteristics from 11 to 290K can be successfully modeled by space charge limited current (SCLC) theory with an exponential trap distribution, giving a trap density of between 1018 and 4 X 1017 cm-3 and a characteristic energy Et of 0.15 eV. Measured conductance transients of PPV are non-exponential and follow a power-law relationship with time whose decay rate decreases with decreasing temperature. This can be directly related to the emptying of the trap distribution deduced from the SCLC analysis. Due to variations in structure, conformation and environment, the polymer LUMO and HOMO density of states form a Gaussian distribution of chain energy sites. The sites involved in carrier transport are those towards the center of the distribution. The deep sites in the tail of the distribution in the carrier energy gap are the observed traps for both positive and negative carriers. The same deep sites dominate the photo- and electroluminescence emission spectra. The model implies that the emissive material in organic light emitting diodes should be made as structurally disordered as possible.
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