Theory of non-equilibrium diffusive transport in disordered materials

Influence of Energetic Disorder on Exciton Lifetime and Photoluminescence Efficiency in Conjugated Polymers

Using time-resolved photoluminescence (TRPL) spectroscopy the exciton lifetime in a range of conjugated polymers is investigated. For poly(p-phenylenevinylene) (PPV)-based derivatives and a polyspirobifluorene copolymer (PSBF) we find that the exciton lifetime is correlated with the energetic disorder. Better ordered polymers exhibit a single exponential PL decay with exciton lifetimes of a few hundred picoseconds, whereas polymers with a larger degree of disorder show multiexponential PL decays with exciton lifetimes in the nanosecond regime. These observations are consistent with diffusion-limited exciton quenching at nonradiative recombination centers. The measured PL decay time reflects the time that excitons need to diffuse toward these quenching sites. Conjugated polymers with large energetic disorder and thus longer exciton lifetime also exhibit a higher photoluminescence quantum yield due to the slower exciton diffusion toward nonradiative quenching sites.

Charge Carrier Transport in Disordered Solids: Dispersive Versus Gaussian

We present a critical review of numerous time-of-flight and radiation-induced conductivity measurements in disordered solids as well as of the theoretical models used to explain them. Recent experiments employing both techniques simultaneously resulted in conflicting conclusions about charge carrier transport in molecularly doped polymers. Whereas the latter method gives a consistent representation of it as a highly non-equilibrium phenomenon of yet unspecified duration (dispersive process), the former is notorious for a number of ambiguous results, some of which are still present, and it is also known to generate the Gaussian disorder model predicting the prevalent steady-state mobility (Gaussian transport). A way to reconcile these seemingly contradictory data has been suggested. This consists of taking due account of surface effects arising from the presence of surface regions (layers) in the dielectric sample with somewhat inferior electronic properties compared with the bulk. Preliminary calculations lend credit to this idea. It is argued that the time-of-flight method should be supplemented by some bulk excitation technique, which allows us to minimize or even remove surface effects. The methodological basis of the former method should be revisited. On the weight of the available information, preference should be given to the dispersive rather than the Gaussian transport but a major problem still exists.

25th anniversary article: microstructure dependent bias stability of organic transistors

Recent studies of the bias-stress-driven electrical instability of organic field-effect transistors (OFETs) are reviewed. OFETs are operated under continuous gate and source/drain biases and these bias stresses degrade device performance. The principles underlying this bias instability are discussed, particularly the mechanisms of charge trapping. There are three main charge-trapping sites: the semiconductor, the dielectric, and the semiconductor-dielectric interface. The charge-trapping phenomena in these three regions are analyzed with special attention to the microstructural dependence of bias instability. Finally, possibilities for future research in this field are presented. This critical review aims to enhance our insight into bias-stress-induced charge trapping in OFETs with the aim of minimizing operational instability.

Dynamics of triplet migration in films of N, N'-diphenyl-N, N'-bis(1-naphthyl)-1, 1'-biphenyl-4, 4''-diamine

We study triplet migration properties in NPB (N, N'-diphenyl-N, N'-bis(1-naphthyl)-1, 1'-biphenyl-4, 4''-diamine) films using time resolved gated spectroscopy and dispersive migration theory as our main tools of analysis. We show that in NPB, a well-known hole transporter in organic light emitting diodes, at high excitation densities triplet migration follows two regimes--a dispersive non-equilibrium regime (distinguished by exciton energetical relaxation within the distribution of hopping sites and as a consequence the hopping frequency being time dependent) that evolves into a second, non-dispersive equilibrium regime. Further, we observe a third region, which we term acceleration. From the turning over time between dispersive and non-dispersive dynamics, we deduce the width of the triplet density of states (DOS). We observe how the DOS variance changes when one decreases the thickness of the NPB film and note how surface effects are becoming important. Furthermore, the DOS variance of NPB changes when another organic layer is evaporated on top, namely Ir(piq)3 (tris(1-phenylisoquinoline)iridium(III)). We believe that these changes are due to the different polarizable media in contact with the NPB film, either vacuum or Ir(piq)3. We also show in this paper that the triplet level when time approaches zero is much higher in energy than the relaxed triplet levels, as quoted in most published papers; these values are thus incorrect for NPB. Lastly, it is possible that even at room temperature, the dispersive regime might be important for triplet migration at high initial triplet concentrations and might affect the diffusion length of triplets to a certain extent. However, more experimentation needs to be performed in order to address this question. Overall, we have characterized the triplet migration dynamics of NPB fully and shown that it agrees with previously published observations for other organic semiconductors and theoretical considerations.

Nonexponential relaxation and hierarchically constrained dynamics in a protein

A scaling analysis within a model of hierarchically constrained dynamics is shown to reproduce the main features of nonexponential relaxation observed in kinetic studies of carbonmonoxymyoglobin.