Films Of Regioregular Poly Research Articles

Impact of low energy ion beams on the properties of rr-P3HT films

Two types of ions (fluorine and titanium) are implanted into films of regio-regular poly(3-hexylthiophene-2,5-diyl) (rr-P3HT) spin-coated on glass substrates with subsequent annealing in argon atmosphere to modify their electrical properties and structure. The ion energy and fluence were within 0.2–40 keV and 1013–1015 cm−2 respectively. The dc resistance enhances after the intensive ion beam treatment while the ac impedance decreases. Ti ion implantation with 40 keV energy and 1014 cm−2 fluence induces decrease of the ac impedance by almost two orders of magnitude and appearance of the molecular hydrogen features in 1H NMR spectrum. The UV–VIS spectra of the films are blue shifted after their exposal to the ion beams, which correlates with the presence of oxygen. The ratio of the oxygen to carbon peak intensities (ξ=O1s/C1s) in the XPS spectra is proposed as a measure for the local partial disturbance of the film. EPR spectra demonstrate formation of the paramagnetic states with g factor <2, which is accompanied with the down-field shift of the NMR spectrum. The ion beams are found to have no significant etching effect as per results of the film thickness measurements and AFM images.

Matter of minutes degradation of poly(3-hexylthiophene) under illumination in air

Films of regioregular poly(3-hexylthiophene) [P3HT] were monitored for degradation by means of in situ fluorescence quenching measurements. The films were found to be stable under UV or VIS energy illumination in vacuo and stable to the air when kept in the dark. Under illumination in air however, degradation products were formed immediately as seen by the fluorescence quenching. The quantum yield for defect product formation was calculated to be around 3 × 10−6 per incident photon of above band-gap energy, a relatively rapid reaction that could create over 1016 defects per cm3 in optically thin films of P3HT in less than a second under sunlight, or in only a matter of minutes under room lighting. The degradation pathway is thought to involve attack of ground state molecular oxygen on the P3HT excited state. The reaction itself is self-inhibiting, as the P3HT excited state is thought to be effectively quenched by the increasing formation of defects, leading to slower and slower defect formation until the film enters a ‘quasi-stable’ state where no further degradation by this mechanism will occur. The upper limit to the number of defects to be found in films of P3HT formed by this mechanism is estimated to be around 2 × 1017 cm−3.

Infrared and Raman spectroscopy of organic thin films used for electronic devices

We present Raman imaging and photoexcitation infrared studies of organic semiconductor films, which are used in organic light-emitting diodes, thin-film transistors, and photovoltaic solar cells. Raman images of the ratio (R) of intensity of the 1596-cm−1 band (b3g) to that of the 1533-cm−1 band (ag) were used for characterizing the 1.55-nm and the 1.44-nm phases in pentacene thin films. The Raman spectrum of N,N′-di-1-naphthaleyl-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPD) film formed on a MoO3-deposited indium–tin-oxide (ITO)/glass substrate indicated that the cation of NPD is formed upon oxidation by MoO3. The Raman image of the sample showed the heterogeneous distribution of the cation of NPD. Photoinduced infrared absorption from a film of regioregular poly(3-octylthiophene) (P3OT) and [6,6]-phenyl-C61-butyric acid methyl ether (PCBM) blend was measured by difference infrared spectroscopy. The temperature dependence of photoinduced infrared absorption due to positive polarons showed the existence of two kinds of carrier traps.

Effect of Gate Voltage in Organic Phototransistors Based on Polythiophene/Fullerene Bulk Heterojunction Nanolayers

Here we report the effect of gate voltage on the performance of organic phototransistors made using bulk heterojunction films of regioregular poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM). To understand the illumination effect, a monochromatic light (520 nm) corresponded to the maximum absorption of present P3HT:PCBM blend film was employed. Results showed that the present device followed a p-type field-effect transistor in the dark and under illumination though the saturation trend of output curves became weakened under illumination. The responsivity exhibited a two-stage trend as a function of gate voltage. The maximum responsivity reached 0.13 A/W under illumination of green light at 81μW/cm2.

Molecular Ordering and 2D Conductivity in Ultrathin Poly(3-hexylthiophene)/Gold Nanoparticle Composite Films

This paper reports the first comparison of the structure and electrical conductivity properties of spin cast (SC) and Langmuir-Schaeffer (LS) films of regioregular poly(3-hexylthiophene) (P3HT). In addition, the effect of incorporating highly monodisperse Au nanoparticles (NPs), with a core diameter of approximately 5 nm, into SC and LS P3HT films is described. A detailed picture of molecular organization in the films has been obtained using ultraviolet-visible absorption spectroscopy, atomic force microscopy, field-emission scanning electron microscopy, X-ray diffraction, and X-ray reflectivity. Film morphology was correlated with pseudo-two-dimensional conductivity measured using scanning electrochemical microscopy, with P3HT in the semiconducting regime. It was found that SC films, which were slightly thicker than those formed with the LS technique, exhibited greater organization. This resulted in an order of magnitude higher lateral conductivity for the SC films. Inclusion of Au NPs (50 wt %) into both SC and LS films resulted in the formation of uniform and relatively flat (rms roughness approximately 1 nm) composite films. Surprisingly, the addition of NPs did not disrupt the characteristic crystal structure found for the native P3HT films. The effect of Au NPs on film lateral conductivity was found to be determined by the distribution of Au NPs within the polymer, which varied significantly between SC and LS films. Whereas Au NPs aggregated into hexagonally packed clusters in SC films, NPs in LS films were predominantly uniformly distributed between the lamella bilayer. It was found that, while the inclusion of Au NPs caused the lateral conductivity to decrease in SC films, in LS films, the lateral conductivity increased by a factor of 2.

Moving Redox Fronts in Conjugated Polymers Studies from Lateral Electrochemistry in Polythiophenes

The propagation speed of the front of electrochemical conversion, from semiconductor to highly doped polymer, in films of regioregular poly(3-hexylthiophene) spin cast on insulating substrates was analyzed. Propagation of the p-doped zone in polymer electrochromic devices was imaged simultaneously with recording of electrochemical data. The current is proportional to the propagation speed and has a Tafel-like behavior when taking the resistive drop in the film into account. The resistivity in the film, which gradually lowers the propagation speed, was used for determination of the conductivity of the p-doped polymer. By combining these values with the doping charge injected into the film during front migration we estimated the hole carrier mobility for different doping levels. © 2004 The Electrochemical Society. All rights reserved.

Recombination of electronic excitations in regioregular poly(3-dodecylthiophene)

We use optical spectroscopy to study the recombination kinetics of long lived electronic excitations in a thin film of regioregular poly(3-dodecylthiophene). We have measured the steady-state photo-induced absorption spectrum, as well as the time-resolved decay of the transient photo-induced absorption in the ms time domain as a function of temperature. The transient decay of these absorptions seems to follow two hyperbolic decay components. We discuss one possible model that could give rise to such decay components.