Sliding Defects in Planar Aromatic Films

Organic semiconducting thin films are likely to play an important role in new optoelectronic technologies including photovoltaics and lighting [1-3]. Such devices involve the very complex physical process of charge transport in molecular solids that are often structurally disordered [4].

Quantitative understanding of charge transport in organic semiconducting films requires consideration of strong disorder in the electronic density of states and in the spatial location of molecules [4]. Bassler et al. carried out seminal Monte Carlo simulations illustrating the impact of disorder on charge transport by considering a Gaussian distribution of charge transport states and of intermolecular distances [5]. Later work expanded these ideas to include correlations in disorder due to instrinsic molecular dipole moments [6] and also to include disorder at interfaces in injection-limited devices [7,8]. This has led to an increasingly accurate ability to phenomenologically model organic device behavior in terms of the detailed shape and temperature dependence of current-voltage characteristics [9]. A key basic question is: What are the typical molecular-scale features that give rise to phenomenological disorder parameters needed for organic device modeling?

Our group uses STM to seek out examples of strongly disordered molecular systems [10] that can provide insights into the microscopic underpininngs of device models. A particularly interesting example arises in the case of transport in single layer sexithiophene films in a field effect transistor geometry. These very thin devices show a novel power-law temperature dependence of charge carrier mobility that is different than thicker counterparts and that indicates 2D transport in a disordered layer [11].

We constructed a model system to reveal microscopic details of this unique 2D transport. By depositing sexithiophene onto a Au(111) crystal coated with a perfect single monolayer of C60, we could establish an upright molecular orientation very similar to the structure of films in the recent 2D FET devices [11]. This upright geometry is indicated by the very large apparent height observe at sexithiphene island boundaries.

Figure 1. a) STM image at 130 K of 6T on C60/Au(111); Each round protrusion is a single molecule; b) Schematic depiction of molecular sliding defects that give rise to apparent height variations.

Within these islands, evidence of structural disorder is readily visible through spatially-random apparent height differences between sexithiophene molecules on top of C60 as shown in Figure 1a. We attribute these differences to "sliding defects" where adjacent molecules are slipped with respect to one anothe along their long axes (Figure 1b). Such defects were first discovered for pentacene films grown on a thiolate self-assembled monolayer on Cu(111) [12] and also later observed for pentacene grown on a monolayer of C60 on Ag(111) [13]. The fact that similar defects can be found in a different planar aromatic molecular system suggests that they may be a common feature in many organic thin films.

To understand the impact on density of electronic states, we carried out local tunneling spectrscopy in z(V) mode [14] on the sexithiophene films. Figure 2 shows a typical z(V) trace, with a very strong step feature that is assigned as the HOMO orbital by reference to related upright sexithiophene films [13]. This feature exhibits negligible variations for sexithiophene molecules with different long-axis slip distances, indicating that these structural defects have an electronic effect that is on the scale of only 10-50 meV. However, electronic disorder in 2D SAMFET's has been inferred to require a distribution of HOMO states with a width of more like 0.2 eV []. In our experiments, variations in HOMO energy of this size have only been observed due to random tip changes that apparently shift the measured energy of the HOMO peak due to a "Stark" effect. This implies that in 2D molecular layers such as those observed here, the primary source of electronic disorder is extrinsic to the organic film.

Figure 2. Constant current distance-voltage spectrum of a-6T on C60/Au(111) shwoing a sharp feature assigned as the HOMO-derived state.

More details can be found in Reference [15] and Jiuyang Wang's Ph.D thesis.

This work is funded by a CAREER award from the National Science Foundation Division of Materials Research (DMR-1056861).

References

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[15] Wang and Dougherty, J. Vac. Sci. Tech. B 32, 030601 (2014).