Interfacial Structure Modifying Interlayers Equalize Substrate Performance: The Case of PEDOT:PSS

Variability of indium tin oxide properties can be a significant concern for wide-scale manufacturing of organic devices. Many treatments and interlayers have been proposed to optimize the electrode surface for high device performance. One of the proposed mechanisms is smoothing of the anode to achieve better interfacial contact with the active layers. To examine the impact of substrate roughness on the structure, morphology, stability and device performance of organic diodes, ITO substrates of varying roughness, but similar chemical and electronic properties, were tested, with and without a poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) interlayer. Atomic force microscopy (AFM), X-ray diffraction (XRD) and electrical measurements, indicate that ITO substrate roughness variations caused dramatic differences in film ordering, morphology, stability and device performance. Counter-intuitively, rougher surfaces, with reduced film ordering and smaller crystalline domains, lead to better device performance. For highly anisotropic molecules, which can crystallize in both edge-on (long axis perpendicular to the substrate) or face-on (long axis parallel to the substrate) configurations such as poly(3-hexyl-thiophene) (P3HT) or diindenoperylene (DIP), high substrate roughness encourages better contact formation. Although the introduction of a PEDOT:PSS interlayer does not uniformly lead to smoother surfaces, it does equalize the behaviour of different ITO substrates. A slight decrease in the device performance with PEDOT:PSS for the rough ITO substrates suggests that different strategies that preserve good contact formation can be used to replace this conventional interlayer.