Search

By applying various experimental techniques, e.g., transient absorption in a strong electric field and delayed field collection of charge carriers, it has been proven that in neat conjugated polymers singlet excitations can dissociate into pairs of free charge carriers in a strong electric field. Random walk theory has been developed to treat this process analytically. At variance of conventional 3D Onsager theory it is assumed that an exciton with finite lifetime can first transfer endothermically an electron to an adjacent site, thereby generating a charge transfer state whose energy is above the energy of that of the initial exciton. In a second step the latter can fully dissociate in accordance with Onsager΄s concept Brownian motion. The results indicate that, depending on the energy required for the first jump, the first jump contributes significantly to the field dependence of the dissociation yield. Disorder weakens the temperature dependence of the yield dramatically and precludes extracting information on the exciton binding energy from it. The chapter will also address the problem of photogeneration in donor–acceptor blends used in organic photovoltaic cells and emphasize the crucial role of geminate recombination of the electron–hole pair at the internal interface. [ABSTRACT FROM AUTHOR]

Abstract: Persistent and transient hole-burning (HB) at 4.2K have been applied to study the intrinsic properties of electronic excitations of a ladder type π-conjugated poly(para-phenylene) in solution. Narrow spectral holes less than 1meV wide have been detected. The dependencies of the HB efficiency on the burning dose and wavelength and doping the samples by electron scavenger are interpreted in terms of a photoreaction related to the so-called two-level-systems, specific low-energy excitations characteristic for disordered materials. At transient HB an additional hole broadening was observed which stems from the triplet energy transfer under conditions of lack of correlation of site energies of the singlet and triplet states of chromophores. [Copyright &y& Elsevier]

Abstract: The luminescence of a ladder-type methyl-poly(para-phenylene) (MeLPPP) doped with platinum–porphyrin dye PtOEP covering the concentration 10−3–5% by weight has been measured employing cw and transient techniques. Upon excitation into the range of absorption of the host, strong phosphorescence of the dopant is observed. Possible ways of populating the dopant triplet state are considered. [Copyright &y& Elsevier]

The method of transient hole-burning using a triplet “bottle neck” is applied to study the intrinsic properties of electronic excitations on isolated chains of a ladder-type π-conjugated poly(para-phenylene) in solutions. Transient spectral changes of absorption comprising holes and anti-holes were studied. The dependence of the hole area on the burning power reveals a pronounced saturation at a relative level of ca. 10% of the total absorbance, independent on the concentration of solutions. This saturation effect is rationalized in terms of strong exciton annihilation arising when two excitations are simultaneously present on one chain. The saturated depth of holes and spectral location of anti-holes are a sensor of the size of the conjugated segments of the polymer chains. The experimental results support the view that the extent of the wave function of singlet excitations is restricted by severe topological defects of the chain rather than by a relatively weak energetic disorder imposed by an environment. [Copyright &y& Elsevier]

The luminescence of a ladder-type methyl-poly(para-phenylene) (MeLPPP) doped by platinum-porphyrin dye PtOEP covering the concentration 10−3 to 5% by weight has been measured employing cw and transient techniques. Upon excitating into the range of absorption of the host strong phosphorescence of the dopant is observed. Possible ways of populating of the dopant triplet state are considered. It is shown that the main channel is singlet–singlet energy transfer among chromophor groups of the polymer followed by Fo¨rster-type transfer to the guest and subsequent intersystem crossing. [Copyright &y& Elsevier]

Transient photocurrents in a 150 nm thick film of a copolymer of a phenyl-substituted poly-phenylenevinylene were observed upon excitation by a 10 ns laser flash. The signal is due to a superposition of time-dependent generation of mobile holes and their motion. The essential quantity considered was the number of holes transported in either the neat sample or a film doped with trinitrofluorene as a function of electric field, temperature, photon dose at different photon energies, and polarity of the irradiated electrode. Based upon complementary spectroscopic evidence it has been argued that the rate limiting step for charge generation is the dissociation of geminate pairs forming an electron at an either unintentional or intentional electron acceptor and hole at an adjacent chain. The field and temperature dependence can be fitted by a recent theory by Arkhipov et al. that differs from the conventional Onsager description. [Copyright &y& Elsevier]