H. Bässler

Charge transport in disordered molecular solids

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.

Charge transfer transitions in solid tetracene and pentacene studied by electroabsorption

Abstract Electric field modulated absorption spectra of vapor deposited layers of tetracene and pentacene indicate existence of charge transfer transitions. In tetracene the transitions from a molecule located at (0, 0, 0) to molecules located at ( 1 2 , 1 2 , 0), (0, 1, 0), (1, 0, 0), and (1, 1, 0) or 1 2 , 3 2 , 0) occur at 2.71 eV, 2.779 eV, 2.895 eV and 3.063 eV, respectively. The dependence of the electron—hole binding energy on the pair separation is coulombic and yields a bandgap Eg= 3.4 = 0.05 eV. The oscillator strengths of the transition between nearest neighbors is about 0.03. In pentacene the transition between molecules located at (0, 0, 0) and ( 1 2 , 1 2 , 0), (0, 1, 0), (1, 0, 0) could be resolved. They appear at 2.120 eV, 2.270 eV and 2.345 eV, respectively, and are the dominant features in the absorption spectrum. The electron—hole pair binding energy decreases faster than rCT−1 with increasing separation. This effect explains the high carrier generation efficiency in pentacene. The results are in accord with recent work of Bounds and Siebrand suggesting that CT excitons are generated by direct transitions rather than by autoionization of a localized excited molecular state followed by thermalization. The influence of the external field on the singlet exciton transition is explained in terms of a quadratic Stark effect.

CHARGE INJECTION INTO LIGHT-EMITTING DIODES : THEORY AND EXPERIMENT

An analytic theory is presented for dark injection from a metallic electrode into a random hopping system, e.g., a conjugated polymer or a molecularly doped polymer. It encompasses injection of a charge carrier from the Fermi level of the electrode into tail states of the distribution of hopping states of the dielectric followed by either return of the charge carrier to the electrode or diffusive escape from the attractive image potential. The latter process resembles Onsager-type geminate pair dissociation in one dimension. The theory yields the injection current as a function of electric field, temperature and energetic width of the distribution of hopping states. At high electric fields it resembles that the current calculated from Fowler-Nordheim tunneling theory although tunneling transitions are not included in the theory. Good agreement with experimental data obtained for diode structures with conjugated polymers as a dielectric is found.

Exciton Fission and Charge Generation via Triplet Excitons in Pentacene/C60 Bilayers

Organic photovoltaic devices are currently studied due to their potential suitability for flexible and large-area applications, though efficiencies are presently low. Here we study pentacene/C(60) bilayers using transient optical absorption spectroscopy; such structures exhibit anomalously high quantum efficiencies. We show that charge generation primarily occurs 2-10 ns after photoexcitation. This supports a model where charge is generated following the slow diffusion of triplet excitons to the heterojunction. These triplets are shown to be present from early times (<200 fs) and result from the fission of a spin-singlet exciton to form two spin-triplet excitons. These results elucidate exciton and charge generation dynamics in the pentacene/C(60) system and demonstrate that the tuning of the energetic levels of organic molecules to take advantages of singlet fission could lead to greatly enhanced photocurrent in future OPVs.

Why is exciton dissociation so efficient at the interface between a conjugated polymer and an electron acceptor

Although doping of a conjugated polymer by electron acceptors strongly facilitates exciton dissociation into geminate pairs of carriers, the yield of free carrier photogeneration can be high only at high doping levels, that is, in polymer/acceptor blends. We suggest a model that explains how excitons can efficiently dissociate into free carriers at an intrinsic polymer/acceptor interface despite the Coulomb interaction between the charges within precursor geminate pairs.

Electron mobility in tris(8-hydroxy-quinoline)aluminum thin films determined via transient electroluminescence from single- and multilayer organic light-emitting diodes

Transient electroluminescence (EL) from single- and multilayer organic light-emitting diodes (OLEDs) was investigated by driving the devices with short, rectangular voltage pulses. The single-layer devices consist of indium-tin oxide (ITO)/tris(8-hydroxy-quinoline)aluminum (Alq3)/magnesium (Mg):silver (Ag), whereas the structure of the multilayer OLEDs are ITO/copper phthalocyanine (CuPc)/N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB)/Alq3/Mg:Ag. Apparent model-dependent values of the electron mobility (μe) in Alq3 have been calculated from the onset of EL for both device structures upon invoking different internal electric field distributions. For the single-layer OLEDs, transient experiments with different dc bias voltages indicated that the EL delay time is determined by the accumulation of charge carriers inside the device rather than by transport of the latter. This interpretation is supported by the observation of delayed EL after the voltage pulse is turned off. In the multilayer OLED the ...

Conformational effects in poly(p-phenylene vinylene)s revealed by low-temperature site-selective fluorescence

Low-temperature site-selective fluorescence (SSF) spectroscopy is employed to study morphological effects on the conformation of poly(p-phenylene vinylene) (PPV) and its phenyl-substituted, soluble derivative poly(phenylphenylenevinylene) (PPPV). Samples of PPV prepared as spin-coated thin films and stretch-aligned free-standing films, and samples of PPPV prepared as cast films and as blends with poly(methylmethacrylate) and polycarbonate have been studied. The results that the authors present are considered with the notion that each polymer sample consists of an array of ordered chain segments whose average length reflects the perfection of the local structure. The statistical distribution of the segment lengths is responsible for inhomogeneous broadening of the optical spectra (absorption and emission). The dominant electronic excitation created by photoexcitation across the pi - pi * energy gap is a singlet exciton that can execute a random walk among the chain segments. SSF spectroscopy allows the authors to distinguish the contributions to the apparent fluorescence Stokes shift that arise from energy relaxation through excitation migration (spectral diffusion) and from structural relaxation of the polymer chain (self-localization). The structural contribution to the Stokes shift approaches zero in well aligned PPV and reaches values of up to 500 cm-1 in highly disordered PPPV films. The SSF method also provides a means of assessing the extent of phase separation that occurs in PPPV blends.

Charge-transfer transitions in crystalline anthracene and their role in photoconductivity

Abstract Electric-field modulated absorption spectra of polycrystalline anthracene layers delineate existence of a series of five charge-transfer bands that can be assigned to transitions within the ab crystal plane. The energy versus distance relationship is coulombic (e = 3.2) yielding an optical band gap Egopt = 4.4±0.05 eV. Absorption coefficients are about one order of magnitude lower than calculated by Bounds et al. and indicate a coupling constant A ∼ 0.15 for interaction between CT and Frenkel exciton states. Previous data for intrinsic free-carrier production, in particular the energy dependence of the “thermalization” distance, can be consistently interpreted in terms of dissociation of CT pairs if the assumption is made that the vibrational CT energy (0.3 eV) can fully or in part be used for additional separation of the electron-hole pair. The adiabatic (electrical) band gap is Ege1 = 4.1±1 eV.

Hole transport in 1,1-bis(di-4-tolylaminophenyl)cyclohexane

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.

Charge transport in organic semiconductors.

Modern optoelectronic devices, such as light-emitting diodes, field-effect transistors and organic solar cells require well controlled motion of charges for their efficient operation. The understanding of the processes that determine charge transport is therefore of paramount importance for designing materials with improved structure-property relationships. Before discussing different regimes of charge transport in organic semiconductors, we present a brief introduction into the conceptual framework in which we interpret the relevant photophysical processes. That is, we compare a molecular picture of electronic excitations against the Su-Schrieffer-Heeger semiconductor band model. After a brief description of experimental techniques needed to measure charge mobilities, we then elaborate on the parameters controlling charge transport in technologically relevant materials. Thus, we consider the influences of electronic coupling between molecular units, disorder, polaronic effects and space charge. A particular focus is given to the recent progress made in understanding charge transport on short time scales and short length scales. The mechanism for charge injection is briefly addressed towards the end of this chapter.