Modulated amplitude reflectance spectroscopy to map in-channel charge carrier concentration and drift velocity in organic field effect transistors

Organic semiconductors have a range of applications, including flexible electronics and spectrum-optimized photovoltaics. However, applications of these materials have been constrained by low device efficiencies and an incomplete understanding of their intricate charge transport mechanisms. Traditional current–voltage transfer curve analysis is a bulk method and can lead to unreliable parameter measurements when faced with non-ideal circumstances such as contact resistance. In this work, we use modulated amplitude reflectance spectroscopy (MARS) amplitude measurements to obtain high-resolution, two-dimensional maps of the carrier distribution in the channel of an organic field effect transistor under varying bias conditions. These results are consistent with measured IV and transfer curves and are used to validate a theoretical model of the spatially varying electric potential and field in the transistor channel under operating conditions. MARS phase images provide spatially resolved maps of charge carrier transport dynamics in the transistor channel and enable quantitative measurement of the drift velocity under operating conditions. By enabling spatially resolved characterization of both charge carrier distribution and transport dynamics in an actively biased transistor, MARS imaging is a valuable experimental technique for advancing the study of organic semiconductors.