Search Results (56 total)

We report on organic p-i-n homojunctions composed of differently doped vacuum-deposited pentacene layers. We observe a remarkably high built-in voltage of 1.65 V. An analysis of the current-voltage characteristics under dark and illuminated conditions reveals that the open-circuit voltage is directly related to the built-in voltage and that the recombination process is influenced by the distinct charge transport properties of electrons and holes in the pentacene film. By a comparison with p-i-p and n-i-n single-carrier homojunctions, deep trap states located around 0.63 eV below the electron transport level are shown to influence the properties.

A power conversion efficiency of 3.4% with an open-circuit voltage of 1  V was recently demonstrated in a thin film solar cell utilizing fullerene C₆₀ as acceptor and a new acceptor-substituted oligothiophene with an optical gap of 1.77  eV as donor [K. Schulze , Adv. Mater. (Weinheim, Ger.) 18, 2872 (2006)]. This prompted us to systematically study the energy- and electron transfer processes at the oligothiophene:fullerene heterojunction for a homologous series of these oligothiophenes. Cyclic voltammetry and ultraviolet photoelectron spectroscopy data show that the heterojunction is modified due to tuning of the highest occupied molecular orbital energy for different oligothiophene chain lengths, while the lowest unoccupied molecular orbital energy remains essentially fixed due to the presence of electron-withdrawing end groups (dicyanovinyl) attached to the oligothiophene. Use of photoinduced absorption (PA) allows the study of the electron transfer process at the heterojunction to C₆₀. Quantum-chemical calculations performed at the density functional theory and/or time-dependent density functional theory level and cation absorption spectra of diluted DCVnT provide an unambiguous identification of the transitions observed in the PA spectra. Upon increasing the effective energy gap of the donor-acceptor pair by increasing the ionization energy of the donor, photoinduced electron transfer is eventually replaced with energy transfer, which alters the photovoltaic operation conditions. The optimum open-circuit voltage of a solar cell is thus a trade-off between efficient charge separation at the interface and maximized effective gap. It appears that the open-circuit voltages of 1.0–1.1  V in our solar cell devices have reached an optimum since higher voltages result in a loss in charge separation efficiency.

We investigate triplet-triplet annihilation in molecular host-guest systems where triplets are localized on spatially separated guest molecules. Our results indicate that the dominant mechanism of annihilation is single-step long-range (Förster-type) energy transfer between two excited guests. This mechanism leads to a fundamental limit for the efficiency of phosphorescent organic light emitting diodes at high luminance. Our model is confirmed by photoluminescence decay experiments on 2,3,7,8,12,13,17,18-octaethylporphine platinum as guest in a host matrix of 4,4^′-N,N^′-dicarbazole-biphenyl.

We investigate quenching processes which contribute to the roll-off in quantum efficiency of phosphorescent organic light-emitting diodes (OLED’s) at high brightness: triplet-triplet annihilation, energy transfer to charged molecules (polarons), and dissociation of excitons into free charge carriers. The investigated OLED’s comprise a host-guest system as emission layer within a state-of-the-art OLED structure—i.e., a five-layer device including doped transport and thin charge carrier and exciton blocking layers. In a red phosphorescent device, N,N^′-di(naphthalen-2-yl)- N,N^′-diphenyl-benzidine is used as matrix and tris(1-phenylisoquinoline) iridium [Ir(piq)₃] as emitter molecule. This structure is compared to a green phosphorescent OLED with a host-guest system comprising the matrix 4,4^′,4^″-tris (N-carbazolyl)-triphenylamine and the well-known triplet emitter fac-tris(2-phenylpyridine) iridium [Ir(ppy)₃]. The triplet-triplet annihilation is characterized by the rate constant k_TT which is determined by time-resolved photoluminescence experiments. To investigate triplet-polaron quenching, unipolar devices were prepared. A certain exciton density, created by continuous-wave illumination, is analyzed as a function of current density flowing through the device. This delivers the corresponding rate constant k_P. Field-induced quenching is not observed under typical OLED operation conditions. The experimental data are implemented in an analytical model taking in account both triplet-triplet annihilation and triplet-polaron quenching. It shows that both processes strongly influence the OLED performance. Compared to the red Ir(piq)₃ OLED, the green Ir(ppy)₃ device shows a stronger efficiency roll-off which is mainly due to a longer phosphorescent lifetime τ and a thinner exciton formation zone w.

We present a comprehensive experimental and theoretical study of the optical properties of matrix-isolated molecules of the two perylene derivatives N,N^′-dimethylperylene-3,4,9,10-dicarboximide (MePTCDI) and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA). A solid solution of the dyes in an SiO₂ matrix exhibits monomer-like behavior. Transient absorption pump-probe spectroscopy in the range 1.2–2.6 eV has been performed on an ultrafast time scale. The differential transmittance reveals contributions from ground-state bleaching, stimulated emission, and excited-state absorption. Both systems exhibit broad excited-state absorption features below 2.0 eV with a clear peak around 1.8 eV. The spectra can be consistently explained by the results of quantum-chemical calculations. We have applied both the coupled cluster singles and doubles (CCSD) model and the multireference-determinant single and double configuration-interaction (MRD-CI) technique on the basis of the intermediate neglect of differential overlap (INDO) Hamiltonian. The results are insensitive to whether the geometry is optimized for the electronic ground state or first excited state. The experimental polarization anisotropies for the two major transitions are in agreement with the calculated polarizations.

We present a comprehensive study of ultrafast relaxation properties of optical excitations in thin films of quasi-1D stacked organic materials PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) and MePTCDI (N,N^′-dimethylperylene-3,4,9,10-dicarboximide) over five decades of time. Pump-probe experiments reveal excitonic intraband relaxation time constants of 65 fs for MePTCDI and 100 fs for PTCDA. The initial time-resolved luminescence anisotropy is consistent with the exciton model of Davydov-split states. The subsequent decay of the anisotropy can be explained with a thermally activated exciton hopping process. A full understanding of the pump-probe experiments calls for an explanation beyond the models presently available.

We report an investigation of the excitonic properties of thin crystalline films of the archetypal organic semiconductor PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) grown on poly- and single crystalline surfaces. A sensitive setup capable of measuring the optical properties of ultrathin organic molecular crystals via differential reflectance spectroscopy (DRS) is presented. This tool allows to carry out measurements in situ, i.e., during the actual film growth, and over a wide spectral range, even on single crystalline surfaces with high symmetry or metallic surfaces, where widely used techniques like reflection anisotropy spectroscopy (RAS) or fluorescence excitation spectroscopy fail. The spectra obtained by DRS resemble mainly the absorption of the films if transparent substrates are used, which simplifies the analysis. In the case of mono- to multilayer films of PTCDA on single crystalline muscovite mica(0001) and Au(111) substrates, the formation of the solid state absorption from monomer to dimer and further to crystal-like absorption spectra can be monitored.

We excite excitonic wave packets in biased semiconductor superlattices with spectrally shaped ultrashort optical pulses. We tailor the shape and phase of the pulse spectrum in order to control the coherent dynamics of the excitonic wave packets formed from a superposition of three excitonic states. Via careful shaping, we are able to excite either wave packets that exhibit standard Bloch oscillations (BO’s) or breathing-mode (BM) motion. These two types of motion are characterized by the presence (BO) or absence (BM) of an internal intraband polarization caused by the electron-hole separation within the excitonic wave packet. The wave packet evolution is monitored using spectrally resolved four-wave mixing. This ability to control the BO’s provides a way to control the emitted THz radiation.

We propose a new type of epitaxy, line-on-line coincidence (LOL), which explains the ordering in the organic-organic heterolayer system PTCDA on HBC on graphite. LOL epitaxy is similar to point-on-line coincidence (POL) in the sense that all overlayer molecules lie on parallel, equally spaced lines. The key difference to POL is that these lines are not restricted to primitive lattice lines of the substrate lattice. Potential energy calculations demonstrate that this new type of epitaxy is indeed characterized by a minimum in the overlayer-substrate interaction potential.

We realize p- and n-type doping of the organic semiconductor zinc-phthalocyanine using a novel strong organic donor. This allows us to demonstrate the first stable and reproducible organic p-n homojunctions. The diodes show very high built-in potentials, attractive, e.g., for organic solar cells. However, the diode characteristics cannot be described by the standard Shockley theory of the p-n junction since the ideality factor strongly increases with decreasing temperature. We show that this behavior can be explained by deviations from the Einstein relation for disordered materials.

We report theoretical and experimental results for the intraband dynamics of biased semiconductor superlattices excited by ultrashort optical pulses. The theoretical model used employs an excitonic basis that includes 1s and all higher-energy in-plane excitonic states. These excitonic states are used to calculate the intraband polarization and terahertz emission of the superlattice system in response to excitation via an ultrashort optical pulse. Our results show that the higher in-plane excitonic states often modify considerably the terahertz emission relative to the results obtained using a 1s exciton basis, but that under some excitation conditions a 1s exciton basis gives accurate results. Good agreement between experimental and theoretical results is obtained.

The Coulomb coupling between Wannier-Stark excitons and the continuum of lower-lying states in an electrically biased superlattice leads not only to pronounced Fano resonances, but has also a heavy impact on the nonlinear polarization dynamics. The strong variation of line broadening in linear spectra with the electric field, however, is not reflected in the dephasing rate. Moreover, increasing the excitation density leads to a linear growth of the time-resolved four-wave-mixing signal instead of a cubic dependence. We present a theory based upon the Born-Markov equations of a superlattice. The unusual polarization dynamics can be traced back to the interference resulting from the simultaneous occupation of different Wannier-Stark subbands and the relaxation of these nonequilibrium distributions.

For high electric fields, the lifetime of Wannier-Stark ladder states in a periodic potential is reduced by the fundamental process of Zener tunneling. We report on the analysis of the coherence lifetime of such states in semiconductor superlattices by interband spectroscopy. The reduction of lifetime by strong coupling between bands can only in the first approximation be described by the well-known Zener theory. A recently developed theoretical model is applied to calculate directly the tunneling probability of Wannier-Stark states as a function of the electric field. The theoretical results compare well with experiment, reproducing the complex interplay of both nonresonant and resonant Zener tunneling to higher bands. By comparing experiment and theory for a superlattice with a symmetric and one with a nonsymmetric potential, we can draw conclusions on a very general basis about the sensitive dependence of Zener tunneling on the specific dispersion relation of the carriers.

We present an electron energy-loss study of the nature and dispersion of excitons in quasi-one-dimensional 3,4:9,10-perylenetetracarboxylic dianhydride molecular crystals. The observed anisotropy of the excitation energies, the spectral shape and the dispersion indicates the presence of two types of excitons near the excitation onset. These are associated with mixed intramolecular Frenkel and intermolecular charge-transfer excitons. In particular, the excitons exhibit a negative dispersion along the molecular stacks which has important consequences for their recombination.

We present a study of the Zener effect in the optical absorption of strongly coupled superlattices with both a magnetic and an electric field in growth direction. The in-plane continuum of electron states is discretized due to Landau quantization, which allows to directly observe the transition from discrete to continuum states due to Zener tunneling in a true 1D system.

We investigate the dynamics of a Bloch-oscillating wave packet in the presence of strong coupling to delocalized above barrier states (Zener tunneling), using time-resolved intraband polarization-sensitive measurements. At a threshold electric field, the resonance of localized and delocalized states causes a quantum beating which is observed as a revival in the intraband polarization. Our numerical simulation visualizes the spatial wave packet decomposition and reformation. The wave packet moves on a ps time scale over a distance of more than 100 nm and sequentially undergoes Bloch oscillations in the below- and above-barrier bands.

We present a systematic study on p-type doping of zinc-phthalocyanine by tetrafluoro-tetracyano-quinodimethane as an example of controlled doping of thin organic films by cosublimation of matrix and dopant. The zinc-phthalocyanine layers are prepared both in polycrystalline and amorphous phase by variation of the sublimation conditions. The films are electrically characterized in situ by temperature dependent conductivity and Seebeck and field-effect measurements. In addition to previous work, we show that also amorphous phthalocyanine layers can be doped, i.e., their conductivity increases and their Seebeck coefficient decreases indicating a shift of the Fermi level towards the hole transport level. The field-effect mobility of the polycrystalline samples is in the range of 10^-4–10^-3 cm^-2/Vs and increases with increasing dopant concentration. Adapting a percolation model presented by Vissenberg and Matters [Phys. Rev. B, 57, 12 964 (1998)], which assumes hopping transport within a distribution of localized states, we can quantitatively describe the conductivity (in different organic layers) and the field-effect mobility.

We have directly time resolved coherent phonon oscillations in quasi-one-dimensional organic crystals of MePTCDI ( N-N^′-dimethylperylene-3,4,9,10-dicarboximide), using femtosecond pump-probe experiments. We observe both higher-energy oscillations caused by intramolecular vibrations (internal phonons) and, for the first time in a quasi-one-dimensional organic system, lower-energy modulations which are related to coherent lattice phonons (external phonons). For internal A_g vibrations, the coherence decay time of about 2 ps is almost independent of the mode. In contrast, the damping time of the external phonons increases strongly with decreasing energy.

Hexa-peri-hexabenzocoronene (C₄₂H₁₈, HBC) films adsorbed on the Au(111) surface were investigated by means of ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling spectroscopy (STS). The results show that both methods give comparable results for the electronic structure of the occupied states, if the STS is performed at appropriate parameters. Additionally, STS gives an immediate insight into the unoccupied states. The highest occupied state of multilayer films is found 1.3–1.4 eV below the Fermi levels and the lowest unoccupied state 1.8 eV above the Fermi level. The resulting transport gap is compared to optical absorption measurements. Work-function changes of -0.8 eV indicate an interface dipole, i.e., vacuum level alignment does not occur at the HBC-Au interface. Compared to UPS, the voltage range of the STS measurements is limited to prevent sample damage.

We investigate the energy spectrum and the electron dynamics of a band in a semiconductor superlattice as a function of the electric field. Linear optical spectroscopy shows that, for high fields, the well-known localization of the Bloch states is followed by a field-induced delocalization, associated with Zener breakdown. Using time-resolved measurements, we observe Bloch oscillations in a regime where they are damped by Zener breakdown.

The Zener effect in the optical absorption of shallow superlattices is studied theoretically and experimentally. The comparison between the Kane approximation, which is the widely accepted picture so far, and a numerical calculation including all subbands, provides clear evidence for the Zener effect. The experimental spectra are in good agreement with the theory. The measured line broadening as a function of the electric field behaves as predicted by the Zener theory, and is of the same order of magnitude as calculated from the sample parameters. We also suggest absorption measurements in a magnetic field in growth direction, which would provide an even clearer demonstration for the transition between discrete transitions and the continuum due to Zener tunneling.

We observe that the oscillatory motion of photoinjected electron-hole pairs in a biased semiconductor superlattice (Bloch oscillation) is accompanied by a coherent quasi-dc current that is generated by the interaction of the carriers with the self-induced oscillating field. It is shown that this novel macroscopic quantum effect, which is a coherent analog of the Shapiro effect observed in Josephson junctions, can be controlled by changing the spectral position of the exciting laser pulse, which in turn determines the amplitude and phase of the wave packet oscillations. It is thereby possible to coherently drive the electrons either downwards or upwards in the potential of the static field.

We investigate the influence of scattering and coherent plasmon coupling on the temporal and spatial dynamics of Bloch-oscillating electrons in a semiconductor superlattice. We demonstrate that the dynamics are strongly influenced due to the scattering processes. For higher carrier densities, coupling to coherent plasmons leads to anharmonic Bloch oscillations since the static bias field is considerably changed by the oscillating carriers.

We report the first observation of Fano resonances in biased semiconductor superlattices: The excitonic Wannier-Stark ladder transitions show asymmetric absorption due to the coupling to the continua of lower transitions. In contrast to other known examples of Fano resonances, the Fano coupling can be continuously tuned in this system by changing the static field across the superlattice. The line shapes and their coupling dependence are in excellent agreement with theory. We also investigate the dephasing dynamics of the resonances and observe an increase in dephasing with increasing Fano coupling.

We use a technique to measure the spatial dynamics of Bloch wave packets in semiconductor superlattices and investigate the dependence of the dynamics on the optical excitation conditions. For excitations well above or below the center of the Wannier-Stark ladder (WSL), the wave packets perform harmonic oscillations following the prediction of Zener; for excitations near the center of the WSL, the wave packets undergo a symmetric oscillation with virtually zero center-of-mass amplitude.