Simulation of electrical conductivity in a pi-conjugated polymeric conductor with infrared light

Irradn. with IR light is found to stimulate the elec. cond. of a film of an org. polymeric conductor [poly(3,4-ethylenedioxythiopene) with polystyrene sulfonate]. The change in cond. is found to be linear in the intensity of the irradn. (4-400 mW/cm2). Both frequency and time domain measurements reveal that the change in resistance induced by irradn., relaxes according to DR(t) ~ (1/t)0.6, with t as the time after excitation. As a possible mechanism for this relaxation, the authors model the diffusion of heat from the polymer film to the supporting glass substrate. By assuming that the change in resistance is linear with the raise in temp. caused by the IR irradn., one predicts a DR(t) ~ (1/t)0.5 dependence. The similarity between the model and exptl. behavior is taken as an indication that the relaxation is limited by heat transport from the polymer film and that the thermalization of the charge carriers occurs on a shorter time scale. Elec. characterization is complemented with optical measurements. These show IR-induced transient absorption of the polymer film with practically the same relaxation behavior as the change in resistance. This suggests that the optical transients are also due to thermal excitations. In the sub-ps time domain, measurements of the change in optical transmission (DT/T) induced by the IR pulse show a very short-lived component with a lifetime close to the instrumental resoln. (.apprx.500 fs). The rapid response is followed by a slow component that decays according to (DT/T)(t) ~ (1/t)0.65. This is interpreted in terms of cooling of the excited charge carriers limited by heat transport, indicating that the thermalization of the carriers occurs on the sub-ps time scal

The interfaces of poly(p-phenylene vinylene) and fullerene derivatives with Al, LiF, and Al/LiF studied by secondary ion mass spectroscopy and x-ray photoelectron spectroscopy: Formation of AlF3 disproved

Two mutually exclusive mechanisms have been proposed to explain the improved electron injection by the insertion of a LiF layer between the metal cathode and the active organic layer of organic photoelectronic devices: the dipole and the doping mechanism. The possibility of the doping mechanism was studied by investigating the interface of poly[2-methoxy-5-(3',7'dimethyl-octyloxyl)-1,4-phenylenevinylene] (MDMO-PPV) or 1-(3-(methoxycarbonyl)propyl)-1-phenyl[6,6]C-61 (PCBM) with Al, LiF, or Al/LiF. In this mechanism, Li dopes the organic layer, after liberation via the reaction Al+3LiF-->AlF3+3Li. If this reaction takes place, AlF3 should be detectable at the surface. However, SIMS measurements showed that AlF3 is not present at the Al/LiF/MDMO-PPV and Al/LiF/PCBM interfaces. This is evidence that the proposed reaction does not occur. Other evidence that the doping mechanism cannot be the general mechanism to explain the enhanced electron injection comes from the presence of LiF on both organic surfaces. XPS measurements indicate that there is a reaction of Al with the carboxylic oxygen of PCBM, and that a LiF layer between PCBM and Al prevents this reaction. (C) 2002 American Institute of Physic

Real-space measurement of the potential distribution inside organic semiconductors

We demonstrate that the soft nature of organic semiconductors can be exploited to directly measure the potential distribution inside such an organic layer by scanning-tunneling microscope (STM) based spectroscopy. Keeping the STM feedback system active while reducing the tip-sample bias forces the tip to penetrate the organic layer. From an analysis of the injection and bulk transport processes it follows that the tip height versus bias trace obtained in this way directly reflects the potential distribution in the organic layer

Non-linearity in the I-V characteristic of poly(3,4-ethylenedioxythiophene) :poly(styrenesulfonic acid) (PEDOT:PSS) due to Joule heating

Deviations from Ohm's law are reported for elec. conduction in thin films of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS). A bridge method is used with two independent currents: a pump' current flowing through mainly one resistive part and a small probe' current. Changes in resistance are independent of the probe current, but linearly proportional to the elec. power delivered by the pump current. The change in resistance is interpreted in terms of Joule heating and a const. relating the relative change in resistance to the heat prodn. is extd.: as=8*10-8 ?s cm3/

Plastic infrared detectors based on poly(3,4-ethylenedioxythiophene): poly(styrene sulfonic acid)

Illumination of films of the p-conjugated polymeric conductor poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) with infrared radiation results in a transient enhancement of its electrical conductivity. This phenomenon can be employed to detect infrared light. Recent experimental evidence is reviewed indicating that the transient increment in the conductivity can be understood in terms of a local heating effect. Thermalization of the carriers, a process which can be studied by optical techniques, is found to proceed very rapidly

Design and synthesis of processible functional copolymers

Novel tailor-made AB, ABAC, and ABAA type heteroaromatic copolymers, in which B represents an N-alkylpyrrole unit, were synthesized via Pd-catalyzed Stille polymerization and characterized by a variety of analytical techniques. MALDI-TOF mass spectrometry allowed the detailed characterization of polymer weight and determination of end groups. By varying the nature of the repeating monomer units and their sequence, copolymers with promising properties for photovoltaic devices could be obtained

The interfaces of poly(p-phenylene vinylene) and fullerene derivatives with Al, LiF, and Al/LiF studied by secondary ion mass spectroscopy and x-ray photoelectron spectroscopy: Formation of AlF3 disproved

Two mutually exclusive mechanisms have been proposed to explain the improved electron injection by the insertion of a LiF layer between the metal cathode and the active organic layer of organic photoelectronic devices: the dipole and the doping mechanism. The possibility of the doping mechanism was studied by investigating the interface of poly[2-methoxy-5-(3',7'dimethyl-octyloxyl)-1,4-phenylenevinylene] (MDMO-PPV) or 1-(3-(methoxycarbonyl)propyl)-1-phenyl[6,6]C-61 (PCBM) with Al, LiF, or Al/LiF. In this mechanism, Li dopes the organic layer, after liberation via the reaction Al+3LiF-->AlF3+3Li. If this reaction takes place, AlF3 should be detectable at the surface. However, SIMS measurements showed that AlF3 is not present at the Al/LiF/MDMO-PPV and Al/LiF/PCBM interfaces. This is evidence that the proposed reaction does not occur. Other evidence that the doping mechanism cannot be the general mechanism to explain the enhanced electron injection comes from the presence of LiF on both organic surfaces. XPS measurements indicate that there is a reaction of Al with the carboxylic oxygen of PCBM, and that a LiF layer between PCBM and Al prevents this reaction. (C) 2002 American Institute of Physic

Morphology and thermal stability of the active layer in poly(p-phenylenevinylene)/methanofullerene plastic photovoltaic devices

The morphology of composite thin films consisting of a conjugated polymer (poly[2-methoxy-5-(3‘,7‘-dimethyloctyloxy)-1,4-phenylenevinylene], MDMO-PPV) and methanofullerene ([6,6]-phenyl C61 butyric acid methyl ester, PCBM), which are used as the active layer in polymer photovoltaic devices, has been extensively studied using transmission electron microscopy (TEM) and selected-area electron diffraction (SAED). Composite MDMO-PPV:PCBM films have been prepared with PCBM concentrations varying from 20 to 90 wt %. PCBM-rich clusters are clearly observed in TEM bright-field mode when the PCBM concentration is increased to ca. 75 wt % in the composite film. The SAED analysis shows that these clusters consist of many PCBM nanocrystals with random crystallographic orientations. Furthermore, we show that these nanocrystals are also present in the homogeneous matrix at PCBM concentrations below 75 wt %. Annealing of the blend films has been performed at temperatures between 60 and 130 °C for different times. In all cases, but especially when the annealing temperature is above the glass transition temperature of MDMO-PPV (80 °C), PCBM molecules show high diffusion mobility, resulting in accelerated phase segregation and in the formation of large PCBM single crystals in the film. The observed phase segregation, even at temperatures as low as 60 °C, indicates that the thermal stability of MDMO-PPV:PCBM films will likely limit the long-term performance of solar cells based on these materials