Uspostavljanje strujnih staza i pogoršanje svojstava elektronskih sklopova zasnovanih na amorfnim organskim slojevima

The recent advent of new flat-panel organic displays follows a long struggle for the extended device lifetime. Many modifications proposed along this way have been based on trial and error experimentation. In this paper, we show that some recent major improvements may have been implicitly related to fighting against the strong current filamentation, intrinsic for charge injection and conduction in organic amorphous thin films. We first recapitulate some major causes of current filamentation in thin amorphous organic layers. Then we consider the charge transport in devices with a high-mobility injection layer, with a smoothened organic heterojunction surface, that operate at lower electric fields, and the devices with doped transport layers. We show that these conditions, known to decrease the device degradation rates, may separately lead to the current homogenization in organic films.Nedavni uzlet u proizvodnji organskih zaslona uslijedio je nakon duge borbe za produljenje vijeka trajanja tih uređaja. Mnoga su poboljšanja pri tom nastala kao rezultat eksperimentalnih pokušaja i promašaja. U ovom članku pokazujemo da su najvažnije nedavne preinake vrlo vjerojatno mikroskopski povezane s prostornim ujednačavanjem električne struje. Pri tom smo najprije ponovili glavne poznate razloge za pojavu prostornog uspostavljanja strujnih staza, pojave svojstvene za injekciju i vođenje u tankim organskim amorfnim slojevima. Potom smo numeričkim simulacijama redom razmotrili utjecaje: tankog injekcijskog sloja velike elektronske pokretljivosti; zaglađene naspram hrapave granice organskih slojeva; slabog nasuprot jakog operativnog električnog polja, te utjecaj punjenja na transport u organskim transportnim slojevima. Pokazujemo da sve te preinake u strukturi i načinu rada, znane da dovode do produljenja vijeka trajanja uređaja, također uzrokuju prostorno ujednačavanje struje u tankim slojevima

Current filamentation and degradation in electronic devices based on amorphous organic layers

The recent advent of new flat-panel organic displays follows a long struggle for the extended device lifetime. Many modifications proposed along this way have been based on trial and error experimentation. In this paper, we show that some recent major improvements may have been implicitly related to fighting against the strong current filamentation, intrinsic for charge injection and conduction in organic amorphous thin films. We first recapitulate some major causes of current filamentation in thin amorphous organic layers. Then we consider the charge transport in devices with a high-mobility injection layer, with a smoothened organic heterojunction surface, that operate at lower electric fields, and the devices with doped transport layers. We show that these conditions, known to decrease the device degradation rates, may separately lead to the current homogenization in organic films

Injection and strong current channeling in organic disordered media

We consider charge injection from a metal into amorphous organic molecular media with correlated disorder. It is shown that correlations, known to be essential for understanding the field dependence of the carrier mobility, also strongly influence the injection current distribution. In particular, we find that the injection hot spots are intrinsic for metal/organic interfaces, even for perfectly flat surfaces. The current density variations reach several orders of magnitude for realistic material parameters. The injection hot spots further induce current channels in the bulk of the material that extend a hundred nanometers beyond the injection surface. For electronic devices based on thin amorphous organic films, as are the organic light-emitting diodes, this current channeling is expected to have a serious impact on device characteristic and performance

Particle-Energy Distribution and Effective Temperature for the Hopping Transport in One-dimensional Disordered System

Recent numerical simulations of the electronic hopping conduction in disordered organics have indicated that the effective temperature may be suitable to describe the ensemble of electrons in the electric field. However, the reasons for the emergence of the effective temperature have not been clarified, and only phenomenological expressions for the effective temperature have been proposed based on numerical simulations. We address these questions here for an analytically tractable one-dimensional model with uncorrelated Gaussian disorder. The exact relation for the distributions of particles in energy is obtained, the analytical expression for the effective temperature is derived, and the limits of the applicability of the concept of the effective temperature are drawn

Recombination at heterojunctions in disordered organic media: Modeling and numerical simulations

Multilayer organic electroluminescent devices derive their advantages over their single-layer counterparts from the processes occurring at heterojunctions in organic media. These processes significantly differ from those in the bulk of the material. This paper presents three-dimensional modeling, numerical simulations, and a discussion of transport and recombination in a system with a heterojunction. We consider partial cross sections for the creation of excitons and exciplexes, and probabilities for recombination in the respective channels. We examine the influence of the energy barrier, electric field, site-energy disorder, and structural disorder at an organic-organic interface on transport and recombination. In particular, we investigate optimal parameter domains for recombination in the exciton channel. The interface roughness, unlike the site-energy disorder, is found to strongly affect the partial cross sections

Beyond the effective temperature: The electron ensemble at high electric fields in disordered organics

Hopping between localized polaronic states, with a Gaussian distribution in energy, is regarded as the main mechanism of electric conduction in disordered organics. Several authors have recently suggested that the hopping electrons, subjected to an electric field, can be described as a homogeneous ‘overheated’ gas, with its “effective temperature” sufficient for a parametrization of the ensemble and the current. It is not clear how such a picture could be reconciled with the observed strongly oriented filamentization of the flow. We show, through extensive numeric simulations, that the picture is misleading as it can overestimate the electron mobility by orders of magnitude. The reason lies in deviations of the average site occupancies from the effective Boltzmann distribution. The ensemble can be described by a distribution function with two parameters—the effective temperature and the variance of the occupancy deviations. The two are connected by a simple universal relation. The spatial structure of the occupancy deviations is found to be connected to the current filaments and its neglect is recognized as the cause for the failure of mobility calculations based on the overheated gas concept. Thus we identify those aspects, lying beyond the overheated gas picture, that are of a fundamental importance to a proper understanding of the transport in disordered organics

Transport and Spectral Properties of Taylor-phase T-Al73Mn27 Complex Intermetallic

We report the experimental results for the electrical conductivity and the thermoelectric power of the complex intermetallic polygrain compound T-Al73Mn27. The electrical conductivity shows the non-metallic behavior, but with finite value in the T = 0 limit, and with unusual √T term appearing at the low temperature. This indicates the existence of an electronic pseudo-gap in the system, whereas more detailed theoretical analysis reveals a non-analytic behavior of the spectral conductivity function in the vicinity of the Fermi energy.</p