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By employing random walk an analytic theory for the dissociation of singlet excitons in a random organic solid, for instance, a conjugated polymer, has been developed. At variance of conventional three-dimensional 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 of 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. [ABSTRACT FROM AUTHOR]

The delayed luminescence and phosphorescence of ladder-type methyl-poly(para-phenylene) (MeLPPP) doped with benzil at a concentration of 20% by weight has been measured. The introduction of benzil leads to a dramatic reduction of the polymer singlet emission. At the same time, a new band with maximum at 611 nm appears, corresponding to the phosphorescence of MeLPPP. The phosphorescence decay on the short time scale is close to an exponential law with a time decay of 15 ms. This indicates that benzil can efficiently sensitize the phosphorescence of the polymer. In addition, a broad and featureless emission is observed in the delayed luminescence spectra of benzil-doped MeLPPP, which is attributed to an exciplex formed between the polymer host and the dopant. We further observe that the delayed fluorescence is enhanced by the addition of benzil. It is concluded that the delayed fluorescence of benzil-doped MeLPPP is mainly due to the annihilation of triplet excitons on the polymer. Finally, efficient triplet-triplet energy transfer from the benzil-doped polymer to the red-emitting phosphorescent dye Pt(II)octaethylporphyrin is established. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Stationary photoconduction in either a phenyl-substituted poly-phenylenevinylene copolymer (PhPPV) or an alkylated hexa-peri-hexabenzocoronene (HBC) doped by perylenediimide (PdI) at a broad range of dopant concentrations has been measured and compared to previous results on PhPPV doped by trinitrofluorenone (TNF). At moderate concentration, the yield of the photogeneration is virtually invariant regardless of the system and blend ratio because practically every primary excitation dissociates at a donor-acceptor site and the subsequent escape from the coulombic potential is virtually system-independent. At dopant concentrations >10% the yield of that escape process increases by two to three orders of magnitude. [ABSTRACT FROM AUTHOR]

The properties of light-emitting diodes based on the electrophorescent platinum-porphyrin dye PtOEP blended into a ladder-type poly(para-phenylene) (LPPP) polymer host are presented. Due to the small difference between the highest occupied molecular orbital (HOMO) levels of the guest and the host, the operating voltage of single layer devices is almost independent of the dopant concentration in clear contrast to what has been observed in the case of wide-band gap host polymers. However, the efficiency and the color purity of these single-layer devices is quite poor, which can be attributed to the weak trapping of carriers on the phosphorescent dye. Incorporating an electron-transporting/hole-blocking layer greatly increases the efficiency of the devices but at the same time emission from the host becomes more significant. Adding a hole-transporting/ electron-blocking layer further increases the efficiency of the devices. Pure red emission is, however, only obtained if the HOMO of the hole-transporting layer is well above that of the LPPP host. This effect is interpreted in terms of a direct injection of holes from the hole-transporting layer into the HOMO level of the dopant. Further, prompt fluorescence and phosphorescence studies indicate that energy transfer from LPPP singlet excitons contributes to the emission from the dopant, but that the efficiency of singlet exciton transfer is too small to explain the almost complete absence of host emission in the electroluminescence spectra of single- and three-layer devices. [ABSTRACT FROM AUTHOR]

We present a comprehensive experimental and computational study on fs-relaxational dynamics of optical excitations in the conjugated polymer poly(p-phenylenevinylene) (PPV) under selective excitation tuning conditions into the long-wavelength, low-vibrational S[sup v=0, sub 1]-density-of-states (DOS). The dependence of single-wavelength luminescence kinetics and time-windowed spectral transients on distinct, initial excitation boundaries at 1.4 K and at room temperature was measured applying the luminescence up-conversion technique. The typical energy-dispersive intra-DOS energy transfer was simulated by a combination of static Monte Carlo method with a dynamical algorithm for solving the energy-space transport Master-Equation in population-space. For various, selective excitations that give rise to specific S[sub l]-population distributions in distinct spatial and energetic subspaces inside the DOS, simulations confirm the experimental results and show that the subsequent, energy-dissipative, multilevel relaxation is hierarchically constrained, and reveals a pronounced site-energy memory effect with a migration-threshold, characteristic of the (dressed) excitation dynamics in the disordered PPV many-body system. [ABSTRACT FROM AUTHOR]

The question of the origin of delayed fluorescence from a phenyl-substituted poly(p-phenylenevinylene)-type copolymer (Ph-PPV) is addressed. Two origins have to be considered, i.e., triplet-triplet annihilation and recombination of geminate electron-hole pairs. Ph-PPV is investigated in the form of two different samples, namely diluted in 2-methyltetrahydrofuran (MTHF) glass at 80 K and as a thin film of optical density of 0.6. It will be shown that in the diluted solution, delayed fluorescence is due to triplet-triplet annihilation and in the thin film, it is due to geminate pair recombination. The assignment of delayed fluorescence to geminate pair recombination in the thin film could be made due to delayed fluorescence experiments under applying an electric field of 2 × 10[SUP6] V/cm. The delayed fluorescence is strongly influenced by the electric field which is a sign that charged species, i.e., geminate electron-hole pairs, are involved in the process of delayed fluorescence. [ABSTRACT FROM AUTHOR]

The effect of permanent dipoles on the density-of-states (DOS) function of an array of hopping sites is examined by summing the contributions of individual dipoles to the total potential at a given site. For relative dipolar site concentrations >=0.1, the DOS function is of Gaussian shape. At lower concentrations, it approaches a Lorentzian. Good agreement with literature data on the dipolar contribution to the DOS function for hole transport in molecularly doped polymers and molecular glasses differing with regard to dipole moment and concentration of dipolar moieties is found. [ABSTRACT FROM AUTHOR]

The field and temperature dependencies of the hole mobility of 1,1-bis(di-4-tolylaminophenyl)cyclohexane (TAPC) doped polystyrene have been measured and compared to results obtained for the TAPC doped polycarbonate and pure TAPC. The results are described by the disorder formalism, due to Bässler and co-workers. The mobility of TAPC doped polystyrene is approximately 100-fold greater than that observed for TAPC doped polycarbonate. This effect is interpreted in terms of (1) the elimination of random dipolar fields due to static dipole moments of the polycarbonate that affect the energetic disorder, and (2) improved electronic intermolecular coupling with a concomitant reduction of positional disorder. [ABSTRACT FROM AUTHOR]

Develops a model for charge recombination in double-layer organic light-emitting diodes in which charge transport across the interface between the anodic and cathodic cell compartments is impeded by energy barriers. Basis of the functioning of organic light-emitting diodes; Description of the model; Effect of increased electron injection barrier relative to the hole injection barrier on the light output; Information on the characteristic time of luminescence kinetics.

Provides information on a study that investigated the tail broadening of nondispursive photocurrent transients over a range of 1,1-bis(di-4-tolylaminophenyl)cyclohexane-doped polystyrene over a range of concentrations, fields and temperatures. Details on the experiment; Results and discussion; Conclusions.

Time dependent transport of triplet excitations in amorphous benzophenone, anthraquinone, and phenanthrene has been investigated in the temperature range 4.2 K≤T≤100 K by monitoring diffusion-controlled decay of the inhomogeneously broadened 0–0 phosphorescence band of bulk states. The results are in excellent agreement with both the analytic theory of Grünewald et al. and computer simulation of nonequilibrium transport. They indicate the applicability of a stochastic transport model based on asymmetric hopping rates with unrestricted energy relaxation. The time dependence of the ensemble averaged hopping frequency has been determined within three decades of time. It can be approximated by a ν(t)∼(t/t0)α-1 law with time dependent dispersion parameter α. [ABSTRACT FROM AUTHOR]

The benzophenone molecule has been treated in MNDO, MNDOC, and MINDO/3 approximation as a function of the twist angle α of the phenyl rings. Calculated were (i) the energies of S0 and T1 state, (ii) delocalization of the n and π* orbitals involved in the 3nπ* state, (iii) net atomic charges,(iv) the force constants of the C=O vibration in the S0 and T1 state, respectively, (v) vertical transition energies for T1←S0 and S1←S0, and (vi) the position of upper triplet states. The results are discussed in relation to the spectroscopic properties of the T1 state of a vitreous benzophenone phase. It is concluded that diagonal disorder is due to a significant fluctuation of α causing both a spread of the site energies of the glass-forming molecules and a variation of spin-orbit coupling. [ABSTRACT FROM AUTHOR]

By time-of-flight photocurrent techniques, hole transport has been investigated in vapor deposited films of the title compound. The measurements were made over a wide range of fields and temperatures which encompassed the glass transition temperature. The results are described within the framework of the disorder formalism due to Bässler and co-workers. The formalism is based on the assumption that transport occurs by hopping in a manifold of states subject to both energetic (diagonal) and positional (off-diagonal) disorder. In this paper, we describe (1) the field and temperature dependencies of the photocurrent transients, (2) the role of polymer dynamics on charge transport at temperatures above the glass transition temperature, and (3) the spectroscopy of interfacial hole states derived from an analysis of photocurrent transients in terms of computer simulations conducted under the premise of energy selective carrier injection into a Gaussian distribution of states. [ABSTRACT FROM AUTHOR]

Hole mobilities have been measured in vapor deposited films of 1,1-bis(di-4-tolylaminophenyl)cyclohexane over an extended range of fields and temperatures. At 295 K, the mobility is approximately 10-2 cm2/V s, the highest value for a disordered molecular solid reported thus far. Monte Carlo simulations of random walk in a geometrically and energetically disordered medium demonstrate that the time dependence of the photocurrent and the field and temperature dependencies of the mobility can be described quantitatively in terms of the inherent disorder and its effect on charge transport. No polaronic or trapping phenomena need to be invoked to reproduce even subtle features of the experimental results. [ABSTRACT FROM AUTHOR]

Hole mobilities have been measured in vapor deposited films of 1,1-bis(di-4-tolylaminophenyl)cyclohexane (TAPC) and TAPC-doped bisphenol-A-polycarbonate (BPPC). Over an extended range of temperatures, the mobilities decrease with increasing field at low fields. At high fields, a log μ∝E1/2 relationship is observed with a slope that approaches zero at high temperatures. The results are described within the framework of the disorder transport formalism. By comparison of the experimental results with Monte Carlo simulations, we show that the observed behavior is a signature of the simultaneous presence of diagonal and off-diagonal disorder. Agreement between simulation results and experiment is excellent. Generalizing these results provides a framework for determining the magnitude of the relevant diagonal and off-diagonal disorder parameters from an analysis of mobility measurements. [ABSTRACT FROM AUTHOR]

We report on the influence of the lateral electric field on the charge mobility in organic field-effect transistors (OFET) based on C60films with multigrain morphology. The experimental data were quantitatively described using a recent analytical model by accounting for the strong local electric fields in a multigrain transistor channel and for the energy correlation effects. To rationalize the presence of a correlated disorder in a non-polar C60material, we show that randomly oriented permanent dipoles in organic gate dielectric layers can generate a significant dipolar disorder in an adjacent nonpolar semiconductor layer. [ABSTRACT FROM PUBLISHER]

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]