On the role of the (de)localization length

Charge transport in structurally disordered or partially ordered organic semiconductors is typically understood in terms of thermally activated tunneling between localized states, or in short: hopping. The associated length scale, the localization length, is typically assumed to be between 0.1 and 1 nm. In the context of organic photovoltaics, the concept of charge delocalization appears on a regular basis to rationalize the high yield of charge generation. Here, the idea is that higher-lying states can have a much reduced localization length, or even be band-like in nature. In this talk, I will discuss our recent work on the role of the localization length in organic thermoelectrics. Specifically, I will show that the localization length is fundamentally energy dependent, with higher-energy states being more spread out. This allows to quantitatively explain an unexpected power-law dependence of the DC conductivity of doped organic semiconductors on charge carrier concentration. Moreover, I will argue that the counter-intuitive simultaneous increase in Seebeck coefficient and conductivity in uniaxially aligned polymer films can be explained in terms of an anisotropy in the localization length, but only for films that are not fully amorphous. Finally, I will discuss how the localization length can actually be measured and show first results of such measurements.