Polymer Light-Emitting Diode Interlayers’ Formation Studied by Current-Sensing Atomic Force Microscopy and Scaling Laws

This work describes the use of current-sensing atomic force microscopy and dynamic scaling laws to characterize the surface morphologies of polymer light-emitting diode interlayers formed by poly(9,9-dioctylfluorene), PFO, on top of poly(3,4-ethylene dioxythiophene) doped with poly(styrene sulfonic acid), PEDOT:PSS. Two types of PFO differing in molecular weight are compared. Surface current maps and calculated energy gaps of PEDOT:PSS evidence surface segregation of the two components of the blend, being PEDOT preferentially located in the surface valleys. Upon formation of PFO interlayers, an overall current decrease occurs, with this decrease being more pronounced for the interlayer based on PFO with higher molecular weight. It is observed that, under the preparation conditions used, neither of the two PFO samples leads to full coverage of the surface. The submonolayer nature of these interlayers has allowed us to establish that PFO chains are preferentially deposited in the valleys of the PEDOT:PSS surface; that is, they are anchored at doped PEDOT domains. The mechanism involved in the PFO deposition was studied using power spectral density analysis with scaling laws. This study provided quantitative information on the surface growth for each interlayer.