The photocarrier generation and recombination mechanism in double layer cells of dye sensitized liquid crystalline hexahexylthiotriphenylene (HHTT) have been studied by steady-state photoconduction measurements. From the dependence of the photocurrent on light intensity we could determine the charge carrier generation mechanism in an azo pigment 4,4’-[(2,5-diphenyl1,3,4-oxadiazol)-bis(azo)]-bis[3-hydroxy-(2H-benzimidazo[2,1-a]benz[d,e]isoquinolin-7-one)] as surface enhanced whereas in titanylphthalocyanine charge generation takes place in the bulk of the dye. We investigated the actionspectra of a single HHTT cell and an azo sensitized cell in the different phases of HHTT to show the influence of azo on the spectral response of the photocurrent. The quantum efficiency in a single HHTT cell as well as in a double layer cell with azo as charge generation layer was determined. By solving the diffusion equation for excitons we carried out an estimation of the exciton diffusion length in azo.

Photogeneration of charge carriers in a methyl-substituted ladder-poly(para-phenylene) (LPPP) has been investigated by xerographic experiments. The quantum efficiency of charge carrier generation was calculated by evaluating the photoinduced discharge curves (PIDCs) and found to be independent of both excitation wavelength and temperature. The results can be described by a modified Braun theory with a simple exponential distribution of radii of thermalized electrons. LPPP shows a very high primary quantum efficiency of 80%, a low residual potential, and a small dark decay. Thus, LPPP represents a promising material to build single component single-layer photoreceptors.

The xerographic technique was used to investigate the photogeneration of charge carriers in a methyl-substituted ladder-poly(para-phenylene) (LPPP). The quantum efficiency of charge carrier generation was calculated from photoinduced-discharge curves (PIDCs) and found to be independent of the excitation wavelength and temperature. This is in contrast to the 3D Onsager theory which predicts a wavelength and temperature dependent photogeneration of free charge carriers. LPPP shows a high overall quantum efficiency up to 25%, a very low residual potential and a small dark decay. Thus, LPPP provides a promising way to build a single component one-layer photoreceptor sensitive to blue light.

We report on a novel binary glass consisting of an electron donor and an electron acceptor molecule. It contains up to 25% of charge-transfer complexes and displays excellent hole as well as electron transport properties. Charge carrier mobilities have been measured by time-of-flight for various donor–acceptor ratios and are discussed within the framework of the disorder model of Bässler and co-workers. The results demonstrate that the presence of the donor molecules has no influence on the electron transport properties. In contrast to this, the hole transport is apparently affected by dipolar interactions between donor and acceptor molecules. Surprisingly the highest nondiagonal disorder for hole transport of about Σ=5 is found in samples with similar amounts of donor and acceptor indicating that the formation of charge-transfer complexes causes additional positional disorder.

We report on photoelectric and holographic investigations of an organic photorefractive material based on a low molar mass glass with both photoconductive and nonlinear optical properties. By implementing a suitable plasticizer we obtained a composite system which shows extremely fast initial response times down to 450 mu s at writing beam intensities of I-write = 10.8 W/cm(2) and 2.5 ms at the canonical intensity of I-write = 1 W/cm(2). Furthermore, high refractive index modulations up to Delta n = 6.1 x 10(-3), long lifetimes and high optical quality of the samples are observed. In comparison to a second similar composite system, which was plasticized by a more polar dopand, we demonstrate the crucial role of this functional constituent on the photoelectric properties. Time-of-flight measurements show a major impact of the plasticizer on the dispersivity of charge-carrier transport. Subsequently the buildup and decay dynamics of the photorefractive grating are Substantially affected. Holographic time-of-flight measurements confirm these observations.

Photorefractive organic systems presently show much longer response times than their inorganic counterparts. The origin of this limitation is not yet fully understood. We present a detailed investigation of the photoelectric processes involved in the photorefractive effect, namely charge generation and charge transport. A comparison between conventional (TOF) and holographic time-of-flight (HTOF) experiments, which were used to determine the charge carrier mobility, is presented. The mobility determined by TOF is shown to depend on the sample thickness. The results show that charge carrier generation and transport are not the limiting factors for the cw-response time of holographic experiments on this system.

One challenge for photorefractive organic materials is to overcome the problem of their relatively low speed. The latter may be caused by either the photoelectric properties of the material or by the reorientation of nonlinear optical chromophores in a space-charge field. This contribution focuses on the first aspect. Using holographic and pulsed experiments, we determine the charge carrier mobility in a bifunctional glass. A comparison of both techniques shows the critical influence of several parameters, such as the concentration of the photoconductor and the sample thickness, on the photoelectric properties of the material.

We present a detailed study of the photoelectric as well as the holographic properties of a novel organic photorefractive glass based on triphenylamine. We studied the quantum efficiency Φ of the photogeneration of charges by means of photoinduced discharge measurements. The photoconductivity σ and the charge carrier mobility μ were obtained via dc photoconduction and pulsed time-of-flight experiments, respectively. The holographic characterization was performed by two-wave and degenerate four-wave mixing experiments allowing for the determination of properties such as diffraction efficiency η, modulation of the refractive index Δn, gain coefficient Γ, and phase-shift φp of the investigated system. The experimental data for Φ could be successfully described by the Onsager formalism with a thermalization radius of r0=24 Å and a primary quantum yield of Φ0=40%. We evaluated the E field and temperature-dependent measurements of μ using the Bässler formalism yielding a width of the density of states of σ=0.13 eV and a disorder parameter Σ=3.6. On this basis the lifetime and the average drift length of the charge carriers could be estimated from the dc photoconduction experiments. From the photoelectric measurements we also calculated the holographic response time that matched very well to the measured response time and described the E-field dependence satisfactorily. The presented photorefractive system shows outstanding optical properties and stability with respect to degradation. We measured a gain coefficient of Γ=90 cm-1, and a diffraction efficiency of η=27% at a response time of 30 ms for only 40-μm-thick samples. Orientational enhancement was observed and evaluated quantitatively. To our knowledge, this work presents the first determination of each of the above quantities all in one single organic photorefractive material.

Charge transport is a basic process for the photorefractive (PR) effect and has a strong influence on the grating formation speed. We investigate the transient hole transport in three organic low-molecular PR glasses by the well known time-of-flight technique. We determine the energetic parameters in terms of the empirical Gill formalism. The characteristic depth of the trapping sites correlates with the response time of the PR effect in a holographic experiment. Thus, the introduction of deeper traps, i.e., a broadening of the density of transport states, leads to faster PR response of our systems by spreading the release-time distribution and causing dispersive transport. Consequently not only the absolute value of the mobility but also the transport mechanism - providing adequate immobilization of the charge carriers - influences the dynamics of the grating formation. A detailed analysis of the transport mechanism confirms the predictions of the stochastic model of Scher and Montroll: The trace of the current transients indicates dispersive transport and the dependence of the transit time on the applied electric field and the sample thickness obeys the same nonlinear scaling law. [S0163-1829(99)02147-5].

Charge carrier generation and charge carrier transport in binary mixtures of the electron acceptor 4H-1,1-dioxo-4- dicyanomehtylidene-2-p-tolyl-6-phenylthiopyran (PTS) and the electron donor N,N'-bis(3-methylphenyl)-(1,1'-bi-phenyl)- 4,4'-diamine (TPD) have been studied. It is found that charge generation in TPD:PTS is sensitized by the formation of exciplexes and charge-transfer complexes. Moreover, TPD:PTS displays hole transport as well as electron transport. Charge carrier mobilities have been measured for different TPD:PTS ratios and are discussed within the framework of the disorder model of Baessler and coworkers. TPD:PTS displays trap-free electron transport over a wide range of electric fields, temperatures, and mixture ratios. The results demonstrate that the presence of the donor molecules has no influence on the electron transport properties. In contrast to this, the hole transport is apparently affected by dipolar interactions between TPD and PTS molecules. THus the width of DOS increases from 0.08 eV in TPD to 0.12 eV upon adding PTS. Agreement with experiment requires that the van der Waals component is dependent on the concentration of the transport molecules, increasing with increasing dilution. Surprisingly the highest non- diagonal disorder for hole transport of about (Sigma) equals 5 is found in mixtures of similar TPD and PTS ratio indicating that the formation of charge-transfer complexes causes additional disorder.

One of the reasons for the missing technological implementation of photorefractive organic materials is their relatively low speed as compared to inorganic systems. The origin of the speed limit can be related to either the photoelectric properties of the materials or to reorientation of nonlinear optical chromophores in a space-charge field. This contribution focuses on the first aspect. Using holographic and pulsed experiments, we determine the charge carrier mobility in two bifunctional glasses. The charge generation efficiency is measured by a xerographic technique. Both properties are related to a holographic characterization of the sample by cw two-beam coupling and degenerate four-wave mixing experiments. We discuss the limiting factors for the holographic response and ways how to overcome them.

The performance of organic light-emitting devices (OLEDs) is strongly influenced by the electronic properties of the employed materials. In order to determine the effect of these materials' parameters, several different hole-transporting 1,3,5-tris(4-diphenylaminophenyl)benzenes (TDAPBs) were synthesised. These TDAPBs contained different substituents, different numbers of substituents and different positions of theses substituents. For the evaluation of the electronic properties, cyclic voltammetry was employed in order to determine the HOMO values, and time-of-flight (TOF) measurements to obtain the hole mobilities. OLEDs were prepared consisting of the TDAPBs blended in a polymer matrix, and of Alq3 as electron-conducting and light-emitting layer. These devices were investigated regarding their current density/voltage characteristics, efficiencies, onset voltages for electroluminescence, and lifetimes. For hole-transporting blend systems an exponential relationship between the current density and the HOMO levels of the TDAPBs was found. However, even though the HOMO values cover a range from −5.09 to −5.35 eV, no effects on the performance of the OLEDs were detected for electroluminescent two-layer systems. In this case the initial voltage seems to be a determining parameter for the behaviour of the devices during operation. Copyright © 1999 John Wiley & Sons, Ltd.

The holographic properties of the photorefractive polymer composite PSX:DMNPAA:TNF are determined as a function of the grating spacing by use of two-beam coupling and degenerate four-wave mixing measurements. The phase shift was quantified with the grating translation technique. In particular, an almost constant mean drift length of the charge carriers of about 0.5 μm was obtained. The resulting discrepancies between the standard theory of crystals, which is generally used as an approximation for amorphous organic materials, and the experimental data are discussed and attributed to the different mechanisms of charge transport in amorphous organic materials. To verify our conclusions we performed time-of-flight and degenerate four-wave mixing measurements on the two photorefractive organic glasses DRDCTA:DOP:C60 and DRDCTA:EHMPA:C60, which differ in the degree of dispersivity of charge carrier transport and, hence, the involved trap energy distributions. Comparing the behavior of the two materials under investigation, we obtained a qualitative relationship between dispersivity and a theoretically unexpected drop in the diffraction efficiency: The larger the dispersivity the earlier the drop in the efficiency.

Photogeneration of charge carriers in a methyl-substituted ladder-poly(para-phenylene) (LPPP), which possesses a high intrachain order, has been studied by xerographic experiments. The Time-of-Flight technique was used to study charge carrier transport in LPPP. The quantum efficiency of charge carrier generation was calculated by evaluating the photoinduced-discharge curves (PIDCs) and found to be independent of both excitation wavelength and temperature. The results can be described by a modified Braun theory with a simple exponential distribution of radii of thermalized electrons. LPPP shows a high charge carrier mobility up to 10(-3) cm(2)/V.s, a high primary quantum efficiency of 80%, a low residual potential and a small dark decay. Thus, LPPP represents a promising material to build single component single-layer photoreceptors. For a comparison, another polymer ladder-type poly(para-phenylene thienylene) LPPPT, which possesses low intrachain order, is also studied. The quantum efficiency of free charge generation in LPPPT is about one order of magnitude smaller as compared to that in LPPP.

We present an organic photorefractive material based on a low molar mass glass with both photoconductive and nonlinear optical properties. By implementing a novel plasticizer and doping with the well known sensitizer C-60 we obtained a composite material, which shows extremely fast initial response times of 2.5 ms at writing beam intensities of I-write = 1 W/cm(2) and 450 mu s at I-write = 10.8 W/cm(2). Combined with high refractive index modulations of up to Delta n = 6 . 10(-3) and sample lifetimes of over 6 months this material exhibits an excellent overall performance.In comparison to another similar composite system which was plasticized by a more polar dopand, we demonstrate the crucial role of this functionality in the dispersivity of charge transport and the photorefractive grating dynamics. The complicated temporal behavior of grating build-up and decay which was investigated by degenerate four-wave mixing experiments is discussed in detail. We propose a new way to describe these processes in a more general manner as it is done up to date, using an inverse Laplace transform (ILT) analysis.

Holographic studies of a new guest-host system based on a highly photoconducting fluorene-triarylamine copolymer are presented. The photorefractive grating dynamics are investigated in detail. We propose a new approach to characterize the temporal behavior of these processes. By performing an inverse Laplace transform analysis using the algorithm CONTIN, we were able to identify several processes and to evaluate the according time constants. We compare this method with a conventional procedure to prove its applicability. Both approaches yield almost identical results for the fast time constant which is down to 1 ms for this material at a writing beam intensity of I-wb = 1.44 W/cm(2). (C) 1999 Elsevier Science B.V, All rights reserved.

Photorefractive guest-host systems based on photoconducting polymers such as poly(N-vinylcabazol) or polysiloxane generally exhibit large diffraction efficiencies and photorefractive gain coefficients. Their response times however, are limited by the orientation of the nonlinear optical chromophores and by the photoconducting properties of the polymer. Rise times down to 50 ms have been observed until now.

Two-layer organic photoreceptors consisting of a dye sensitizer acting as the charge generation layer (CGL) and a hole transport layer (HTL) have been investigated by steady-state photoconduction measurements and time-of-flight experiments. With these techniques effects of exciton diffusion, charge-carrier generation, charge injection and charge transport can be studied. The measurements were carried out with (i) single generation layers consisting of Ate-pigment dispersion and Titanylphthalocyanine pigment dispersion and (ii) with two-layered photoreceptors using Azo and TiOPc as charge generation layer and TDAPB and hexa(hexylthio)triphenylene (HHTT) as charge transport materials. We found that in the Phthalocyanine/HTL-system the efficiency for charge injection into the HTL is transport-limited due to the hole mobility of the Titanylphthalocyanine dispersion, whereas the efficiency in the Azo/HTL receptor is limited by the diffusion of excitons and (or) holes towards the CGL/HTL-interface. We draw to the conclusion that illumination of the Phthalocyanine sensitizer leads to the formation of charge-transfer excitons which subsequently dissociate into free charges. In contrast to this, the Ate-pigment as a sensitizer seems to form strongly bound excitons. Hence no dissociation in free charge-carriers occurs except on the CGL/HTL-interface where the hole transport molecules act as electron donors. From steady-state photocurrent measurements we calculate diffusion lengths belonging to the diffusion of excitons and minority charge-carriers respectively.