Fluctuations Near Thin Films Of Polymers, Organic Photovoltaics, And Organic Semiconductors Probed By Electric Force Microscopy

Abstract

Scanned probe microscopy has the ability to image a surface by probing dynamic fluctuations. In this work, we measure surface-induced fluctuations as noise in the cantilever resonance frequency. We provide a theoretical basis of surface-induced cantilever frequency noise, which we then use to study thin polymer films, organic photovoltaics, and organic semiconductors. Over polymer films we demonstrate that the observed frequency noise is due to fluctuations in the sample's electric polarization. We have developed a theory that links these fluctuations to the dielectric function of the polymer. Our theory correctly predicts the magnitude and spectral shape of the observed frequency noise, as well as its dependence on distance and tip-voltage. Over polymer-blend heterojunction solar cells we find that in the presence of light, cantilever frequency noise increases by almost two orders of magnitude and, remarkably, shows a wavelength dependence that follows the absorption spectrum of one of the polymer components. We attribute the light-induced noise to charge trapping and detrapping. In molecular organic semiconductors, we investigate charge-induced frequency noise. Charge motion in these materials has to date been described using microscopic chargehopping models, which essentially neglect long-range inter-carrier interactions. Here we demonstrate that inter-carrier interactions cannot be ignored in a frequency noise experi- ment because these interactions suppress fluctuations in the electrostatic potential by several orders of magnitude. keywords: transistor, poly(3-hexylthiophene) (P3HT), N,N'-Diphenyl-N,N'-di(3-tolyl)-4benzidine (TPD),