Charge Transport in Disordered Organic Photoconductors a Monte Carlo Simulation Study

Semiconducting polymers and small molecules form an extremely flexible class of amorphous materials that can be used in a wide range of applications, some of which are display, radio-frequency tags, and solar cells. The rapid progress towards functional devices is occurring despite the lack of sufficient understanding of the physical processes and very little experience in device engineering. This tutorial review aims to provide sufficient intuitive background to draw more researchers to look into the fundamental aspects of device physics and engineering.

Charge Transport in Disordered Organic Materials and Its Relevance to Thin‐Film Devices: A Tutorial Review

Since the discovery of electroluminescence in conjugated polymers it has been recognized that charge transport is a key ingredient for the efficiency of the polymer light-emitting diodes (PLEDs). This review focuses on the charge transport properties of these materials. From temperature dependent current density–voltage characteristics it has been obtained that the hole transport in poly(dialkoxy-p-phenylene vinylene) (PPV) is governed by a combination of space-charge effects and a field- and temperature-dependent mobility. The origin of the hole mobility, which seems to be generic for a large class of disordered materials, arises from hopping in a system with both energetic and structural disorder. The response time of PPV-based PLEDs is governed by the dispersive transport of holes towards the cathode. Based on the results of the electron- and hole-transport a device model for PLEDs is proposed in which the light generation is due to bimolecular recombination between the injected electrons and holes. The unbalanced electron and hole transport gives rise to a bias dependent efficiency. By comparison with experiment it is found that the bimolecular recombination process is of the Langevin-type, in which the rate-limiting step is the diffusion of electrons and holes towards each other. The occurrence of Langevin recombination explains why the conversion efficiency of current into light of a PLED is temperature independent. The understanding of the device operation of PLEDs indicates directions for further improvement of the performance.

Charge transport in poly(p-phenylene vinylene) light-emitting diodes

Twenty years ago Heinz Bassler published in this journal the seminal review article on charge transport in disordered organic semiconductors [Phys. Status Solidi B 175, 15 (1993)], which has become one of the most popular references in this research field. Thanks to this paper, our understanding of charge transport in disordered organic materials has been essentially improved in the past two decades. New theoretical methods have been developed and new results on various phenomena related to charge transport in disordered organic materials have been obtained. The aim of the current review is to present these new theoretical methods and to highlight the most essential results obtained in their framework. While theoretical consideration in the article by Bassler was based on computer simulations, particular attention in the current review is given to the development of analytical theories. Dependences of charge carrier mobility and diffusivity on temperature, electric field, carrier concentration and on material and sample parameters are discussed in detail. Schematic behaviour of charge carriers within the Gaussian density of states (DOS)

https://onlinelibrary.wiley.com/doi/pdf/10.1002/pssb.201350339

Theoretical description of charge transport in disordered organic semiconductors

Detailed experimental and theoretical analysis of the pulsed excitation of polymer light emitting diodes is presented. We find a set of universal transient features for a variety of device configurations (different polymers/cathodes) which can be reproduced using our phenomenological numerical model. We find that the temporal evolution of the electroluminescence in response to a step voltage pulse is characterized by: (i) a delay followed by; (ii) a fast initial rise at turn-on followed by; (iii) a slow rise (slower by at least one order of magnitude). The large mobility mismatch between holes and electrons in conjugated polymers allows us to separately time resolve the motion of holes and electrons. We suggest a method for extracting mobility values that takes into account the possible field-induced broadening of carrier fronts, and which is found to be compatible with mobilities determined from constant wave measurements. By using appropriate device configurations it is possible to determine the mobilities of both holes and electrons from a single device. Mobilities for holes and electrons are extracted for a poly(p-phenylenevinylene) copolymer and poly(di-octyl fluorene).

Transient electroluminescence of polymer light emitting diodes using electrical pulses

One of the most puzzling aspects of the hole mobility in molecularly doped polymers, its electric field dependence, is studied. New data over the widest electric field range yet reported have been obtained and are compared with various functions proposed in the literature. Comprehensive linear and nonlinear statistical analysis reveals that only the function exp(E)1/2 is consistent with the data. The theoretical implications of this result are discussed.

The electric field dependence of the mobility in molecularly doped polymers

The hole mobilities \ensuremath{\mu} of solution cast 1-phenyl-3-p-diethylaminostyryl-5-p-diethylphenyl-pyrazoline:polycarbonate (DEASP:PC) thin films have been characterized by the time-of-flight technique. The mobilities were measured for DEASP doping concentrations in the range 10\char21{}90 % over a wide range of temperatures and electric fields, including measurements above the glass transition temperature ${\mathit{T}}_{\mathit{g}}$, and analyzed with use of recently suggested deconvolution procedures. The independence of the activation energy on the calculated distance between DEASP molecules \ensuremath{\rho} and the exponential dependence of \ensuremath{\mu} on \ensuremath{\rho} suggest nonadiabatic small-polaron hopping is occurring, as was concluded for the molecularly doped polymer hydrazone:PC. The decrease in activation energy above the glass transition temperature is clear evidence of the role of the polymer matrix in determining the activation energy. The electric-field dependence of \ensuremath{\mu}, while similar to previous measurements, i.e., ln\ensuremath{\mu}\ensuremath{\propto} \ensuremath{\surd}E , provides significant new information. We report the first characterization of the field dependence of \ensuremath{\mu} above the glass transition temperature: It appears to be unaffected by ${\mathit{T}}_{\mathit{g}}$, in contrast to the behavior of the activation energy. We also report the first observation of a mobility that decreases as the electric field increases at 10% DEASP concentration above 370 K. This result is contrary to the predictions of several investigators, but consistent with empirical equations suggested earlier.

D. Glatz

Papers

The electric field dependence of the mobility in molecularly doped polymers

Hole mobilities in films of a pyrazoline:polycarbonate molecularly doped polymer.