Electrodeposited Poly(thiophene) Thin Film Contacts for Organic Photovoltaics and Organic Light Emitting Diodes

摘要

Selective ohmic contacts for the harvesting and/or injection of both holes and electrons at metal oxide or metal electrode contacts, to ensure good diode electrical device properties, is essential to the successful implementation of emerging thin film organic photovoltaics (OPV) and organic light emitting diodes (OLED). These contacts must satisfy the following criteria: i) compatibility (wettability) between the non-polar active layers and the more polar contact materials (metal oxides and metals), ii) control of effective work function, and high rates of charge transfer at the organic/contact interface, producing "ohmic" electrical behavior for both holes and electrons." In both OPVs and OLEDs, the implementation of transparent conducting oxide (TCO) contacts (e.g. ITO) has lead to problems due to the heterogeneity of the surface and electrical properties of the electrode on the nanometer scale. Typically, conducting polymer thin films are added at the oxide/organic active layer interface in an effort to homogenize the interface between the electrode and the active organic layer; the most prominent of these conductive polymer layers is PEDOT:PSS, a dispersion of high molecular weight poly(stryenesulfonate) and partially doped oligomers of poly(dioxyethylthiophene). By electrochemically doping the PEDOT:PSS polymer layer, it is possible to tune its effective work function, and thus control the injection barrier between the oxide and the active organic layer. Recent reports, however, have indicated that PEDOT:PSS is not a long term solution to the correction of electrode heterogeneity: recent conductive tip atomic force microscopy (AFM) measurements that the PEDOT:PSS layer is mimicking the underlying electrical properties of indium tin oxide. Electrical heterogeneity combined with less than optimized rates for charge injection and the inclusion of acidic PSS groups which may degrade the oxide have yielded devices that suffer in efficiency and lifetime stability