Conducting polymers as electron glasses: surface charge domains and slow relaxation.

Conducting and semiconducting polymers have been proposed as the fourth generation of polymeric materials1. Due to their easy processability, high tunability and low-cost production, they have a great potential for a variety of applications. Plastic electronics devices, such as organic field-effect transistors, organic light-emitting diodes, and organic solar cells, are already fabricated and commercialized2. Apart from their technological applications, conducting polymers offer a wealth of interesting and challenging basic phenomena from the fundamental point of view. Typical conducting polymers are semi-crystalline in the sense that locally the chains align in an ordered structure forming nano-crystallites which are surrounded by amorphous regions. This nanostructure determines the intra and inter-chain interactions that govern the electronic properties of the material3,4,5. It is well accepted that, due to the high degree of disorder, in polimeric materials the conduction occurs by phonon assisted hopping between localized states, involving the formation of polarons or bipolarons6,7. Although it has been profoundly studied for the last decades, the complexity of conducting polymers prevent a full consensus about the underlying conduction mechanisms and in particular the way they relax after excitation far from equilibrium. Interactions between carriers are likely to be significant enough to affect their conduction mechanism, as evidenced by the T−1/2 dependence of the logarithm of the conductivity in the variable-range hopping regime8, and makes the conduccting polymers excellent candidates to be electron glasses. Electron glasses are systems with states localized by the disorder and with long-range Coulomb interactions between carriers. Disorder produces localization of the wavefunctions, which in turn results in a lack of screening and an increase in the importance of Coulomb interactions. Very slow relaxation rates are commonly observed in these systems due to the exponential dependence of the transition rates on hopping length and energy and to the many-valley structure of the phase space produced by the interactions9. Typical glassy phenomena observed in electron glasses include a slow logarithmic decrease of the conductivity10,11, a memory dip11,12,13 and aging12,14,15. These phenomena have been observed in a great variety of materials, such as indium oxides10, granular metals11, thin metal films13,16, and recently GeSbTe films, where glassy phenomena coexist with persistent photoconductivity17. Electron glasses have been excited with electromagnetic radiation, and for high enough frequency, slow relaxation is observed10,17,18. These results are consistent with the following picture. At equilibrium, charges are arranged in low energy configurations that minimize the Coulomb energy and present complex correlations between carriers and a Coulomb gap in the single-particle density of states19,20. If the radiation frequency exceeds the Coulomb gap energy, charges are randomized and reestablishing the equilibrium configuration is usually a very slow process. Slow logarithmic relaxation of the electrochemical doping potential was observed over nine orders of magnitude in polymeric materials21. A dependence of the logarithmic shift with scan rate and aging phenomena were also observed. The universal features of slow relaxation were attributed to a hierarchical series of processes, following ideas from kinetic studies of the decay of persistent photoconductivity in semiconductor structures22,23. Slow relaxation in the photoconductivity in poly(phenylenevinylene) (PPV) films was studied by Lee et al.24. A roughly logarithmic behavior was observed, as well as an ω0.66 dependence of the photoconductivity with chopping frequency, which was explained again in terms of hierarchical processes working in series. Logarithmic relaxation over many orders of magnitude was observed in photoinduced conductivity in organic field effect transistor25. Under illumination, the photoinduced excess current also increases logarithmically. All experiments mentioned describe conductivity at the macroscopic scale. Some mesoscopic properties have also been analyzed13,26,27, but the nanoscale has never been reached in electron glass studies. The scanning force microscopy (SFM) technique offers the double opportunity of being able to observe nanoscopic properties of interacting systems and to follow the relaxation behavior for a quantity other than conductivity. In this letter, we use the scanning Kelvin probe microscopy (SKPM) to measure the surface potential (SP) in order to explore a possible, very natural explanation for slow relaxation in conducting polymers based on electron glass ideas. Firstly, we study the SP distribution in equilibrium and see a well defined domain structure with spatial and temporal correlations fully compatible with the electron glass model. Secondly, the sample is excited by irradiation with green light for a certain time and we monitor how the sample SP relaxes to equilibrium once the excitation is switched off. We have observed a logarithmic behavior characteristic of glassy interacting systems.