Measuring carrier mobility in a thin film organic semiconductor

Semiconducting organic polymers are promising candidates for fabrication of inexpensive, mechanically flexible optoelectronic devices such as photovoltaic cells and transistors. Widespread adoption of these materials has been limited by poor charge carrier mobility. Charge transport in semiconducting polymers is not well understood, and conventional experimental techniques often lead to conflicting measurements, in which it can be difficult to distinguish between the effects of electric field and charge carrier density. In this work, we use poly(3-hexylthiophene) organic thin film transistors as a model system and demonstrate the use of modulated amplitude reflectance spectroscopy (MARS) to spatially quantify charge carrier distribution and drift velocity in the channel of the transistors under operating conditions. Combining these measurements with a model of the electric field distribution enables us to extract the carrier mobility in the channel. The values of carrier mobility measured using MARS are consistent with those extracted more conventionally from measured transfer curves of the transistors. However, our spatially resolved MARS technique enables us to examine the effects of charge carrier density and electric field on the mobility values.

This work was supported by NSF Award Number 1919282.