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Light scattering and organic electronics research groupResearch |
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CURRENT RESEARCH PROJECTS 1. Interface-controlled polymer-dielectric interfaces in field-effect transistors (FETs) and diodes 2. Light scattering studies from push-pull-type donor-acceptor polymer-based FETs 3. Modulation spectroscopy - triplet states and charge transfer complexes 4. High pressure optical spectroscopy 1. Interface-controlled polymer-dielectric interfaces We are pursuing several strategies for improving the polymer-dielectric interface in FETs and metal-insulator-semiconductor (MIS) diodes. The goal is to enhance the charge carrier mobilities, reduce interface trap density, and thus improve FET/MIS device performance. Matrix-assisted pulsed laser evaporation (MAPLE), a derivative of pulsed laser deposition, is an alternative method of depositing polymer and biomaterial films that allows homogenous film coverage of high molecular weight organic materials for a layer-by-layer growth without any laser induced damage. We have recently shown the MAPLE technique to be a viable alternative for fabricating device quality organic MIS diodes and FETs, and overcoming the solvent selectivity, inherent swelling, and dissolution problems associated with polymer dielectrics. PFB, a fluorene co-polymer, and poly3-(hexylthiophene) (P3HT) were deposited using MAPLE in FETs and MIS structures. Current-voltage characteristics of MAPLE grown FETs without any surface modification show a better performance compared to spincoated FETs. Capacitance-voltage and conductance-voltage investigations of the MIS structures show that loss as well as accumulation capacitance and time constant dispersions are smaller in the MAPLE grown film than in the spincoated film. The FETs show a consistently better on/off ratio for the MAPLE grown PFB films compared to the spincoated films. MAPLE grown films provide an added advantage of patterning the active layer with minimum surface modification requirements of the dielectric-polymer interface. This opens up potential applications of the MAPLE technique in nanostructured organic devices.
Getting away from oxide dielectrics in organic FETs is not only cost-effective but has tremendous advantages for improving carrier mobility and stability of devices. Current efforts focus on processing of polymer dielectrics in organic FETs and MIS structures. We are investigating the effects of solvents on the properties of organic dielectrics, and consequently, on the electrical characteristics. 2. Light scattering studies from push-pull-type donor-acceptor polymer-based FETsThe design of systems incorporating both donor and acceptor chromophores are of particular interest for applications in bipolar FETs and solar cells. Oligomers and polymers based on donor-acceptor (D-A) moieties have a high degree of tunability of their electronic and optical properties as the relative contribution of the electron-rich and -poor components may be controlled. Diketopyrrolopyrrole (DPP) containing copolymers have recently gained a lot of interest in organic optoelectronics for applications in FETs and solar cells. In collaboration with a synthetic chemistry group, we are working with a host of D-A polymers in FETs. Raman scattering/imaging studies of DPP-based FETs show almost no change in the Raman spectrum after the devices are allowed to operate at a gate vol tage (in the saturation region), indicating that the devices suffer minimal damage due to the metal-polymer contact, unlike other fluorene-based copolymers and pentacene-based FETs. The figure below shows the Raman spectrum of a DPP film from a FET structure; the intensity profile of the 1200 cm -1 peak is seen. The overall intensity is enhanced near the Au pad and the channel area. No significant changes are seen in the Raman images indicating the polymer does not undergo any structural changes upon biasing. This is is contrast to pentacene FETs, where changes are seen in the 1165 cm -1 Raman peak before and after biasing. Surface-enhanced Raman effect on the polymer and the dielectric layer is also being probed close to the Au conacts in FET geometries due to the metal evaporation on the polymer layer.
3. Modulation spectroscopy - triplet states and charge transfer complexes We are currently exploring triplet-enhanced conjugated polymers for applications in FETs and solar cells. Photo-induced absorption (PIA) is used to estimate the diffusion length of triplet excitons. In a recent work we show that adding a quencher to the triplet-enhanced polymer quenches the triplet-triplet absorption signature, providing a mechanism for estimating the diffusion length of triplet excitons. Experiments on two different molecular weights of the polymer sample show that the diffusion length of triplet excitons is in the few µm range and depends upon the size of the p-conjugated system . Another area that we are working on is charge transfer complex (CTC) states in polymer blends for photovoltaics. Using modified photocurrent measurement techniques, we are studying CTC states in various donor-acceptor-donor polymer:fullerene blends.
4. High pressure optical spectroscopy Application of hydrostatic pressure increases the intermolecular interactions and changes the molecular geometry without a change in the chemical make-up of compounds. At the simplest level higher intermolecular interactions in conjugated polymers result in a decrease of the luminescence quantum yield, which is clearly seen as one goes from a solution state to the solid-state. A common feature seen in all conjugated polymers is a red-shift of the PL spectrum with increasing pressure (shown in the figure below), indicative of an increasing degree of conjugation. More recently we have focused our efforts on the structure-property relationship in polyfluorenes (PF), a class of blue-emitting polymers. Di-octyl substituted PF (PF8), in particular, has different backbone conformations, which may be induced by thermal/solvent treatment. The as-is polymer has some fraction of the planar Cbeta conformation (the backbone torsional angle ~180°) ; hence, the PL is always dominated by this low-energy phase. Thermal annealing of PF8 eliminates the low-energy Cbeta conformation making the polymer more non-planar (Calpha /Cgamma ). The figure below shows the position of the 0-1 PL peak in as-is and thermally annealed PF8 samples. The inset shows the calculated energy gap of a fluorene dimer as a function of pressure for three different backbone conformations. Since the annealed sample is mainly in the Calpha and Cgamma conformation, pressure produces two effects: increased effective conjugation similar to the as-is sample and planarization of the backbone driving it toward a Cbeta conformation. Our recent efforts entail high pressure optical studies of polymer heterojunctions that have applications in organic solar cells, mainly to understand the interfaces effects and obtain the band-edge offsets under enhanced interactions. Details are provided in publication # 73.
Check publications (#66 and #69) for more details.
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