  We report on infrared (IR) spectro-microscopy of the electronic excitations
in nanometer-thick accumulation layers in FET devices based on
poly(3-hexylthiophene). IR data allows us to explore the charge injection
landscape and uncovers the critical role of the gate insulator in defining
relevant length scales. This work demonstrates the unique potential of IR
spectroscopy for the investigation of physical phenomena at the nanoscale
occurring at the semiconductor-insulator interface in FET devices.
