Fundamental studies of charge transport through structures of nanoparticles stabilized with organic molecules, especially at the interface between organic molecules and metals (or semiconductors), are necessary today for innovations in nanoelectronics and photovoltaics. We focus on mainly two topics: charge transport through nanoparticle chains and organic crystals.
Due to their optical and electronic properties, arrays of metallic nanoparticles (NPs) have attracted much attention in materials science. Charge transport through nanoparticle assemblies represents a fundamental process that controls their physical properties which are determined by coupling and arrangement of individual NPs, and depend on their size, shape, and composition. The coupling can be influenced by inter-NP spacing or by stabilizer molecules capping the NPs. The possibility of switching of molecular conformations represents one peculiarity of these molecules which can strongly influence charge transport through nanoparticle assemblies.
As examples, two representative samples investigated in our group are shown in Fig. 1, where the gold electrodes are bridged by a chain of gold particles stabilized with citrate molecules.
Fig. 1. (a) Schematic illustration of the NP chain located between two gold electrodes. (b) and (c) Scanning electron microscopy images of the samples investigated.
Conjugated polymers are excellent candidates for use in low-cost electronics and photovoltaics, because they can show a high absorption, typically fall in the visible spectral range, and the power conversion efficiency reaches 5 % today. Charge carrier mobility of such polymers is usually in the range of 10-7 – 10-3 cm2/Vs. If the latter will reach the value 10 cm2/Vs, then the polymers can be competitive with amorphous silicon. While charge transport along an individual conjugated chain is predicted to be extremely rapid, over longer distances charge must also pass between the chains. Crystallization of polymers leads to the ordering of the chains in regular, more intimate contact, and as result, the charge carrier mobility between the chains increases.
To answer the basic question how a ordering of polymer molecules in a crystal can be used to increase the charge mobility, a simplified device configuration “electrode/molecule crystal/electrode” as model system is chosen, where the crystal (40 nm thick, 1 µm wide, 40 µm long) of poly(3-hexylthiophene) (P3HT) bridges four gold electrodes. Figure 2 illustrates the samples investigated, where the P3HT crystals are grown in the group of Prof. G. Reiter, Institute of Physics, University of Freiburg.
Fig. 2. (a) Schematic illustration of the four point geometry used for measurement of the P3HT crystal. (b) Atomic force microscopy image of the P3HT crystal investigated. (c) and (d) Schematic illustration of the molecule ordering in the P3HT crystal. [Figs. 2(b)-2(d) reprinted with permission from G. Reiter, Institute of Physics, University of Freiburg].