SCANNING CURRENT SPECTROSCOPY: A CONDUCTING PROBE ATOMIC FORCE MICROSCOPY TECHNIQUE FOR EXPLORING THE PHYSICAL AND ELECTRONIC PROPERTIES OF METAL OXIDE/ORGANIC INTERFACES

Abstract

Organic photovoltaics (OPVs) offer the prospect of inexpensive processing compared with conventional crystalline semiconductor cells. These cells are still lower in efficiency than their inorganic counterparts, in part because a detailed understanding of the role that interfaces play in these devices is lacking. The electronic properties of the surface of the common transparent electrode Indium:Tin Oxide (ITO) have been studied both on a macroscopic and nanoscopic scale, and the interface between ITO and organic materials has been studied on a macroscopic scale as well. Little work has been done on the nanoscopic properties of the ITO/organic interface. This dissertation introduces a new conducting-probe atomic force microscope (CP-AFM) technique, Scanning Current Spectroscopy (SCS), for probing the nanoscopic lateral variation in the electronic properties of this interface, and demonstrates its utility by examining the ITO/copperphthalocyanine (CuPc) interface. SCS demonstrates large lateral variations in the hole collection efficiency at that interface on a nanometer length scale, and that the distribution of these variations is affected by ITO pretreatment. Measurements on OPVs demonstrate that the performance of these devices is dependant on the nanoscopic lateral variation in surface properties that SCS measures, and that in the case of the ITO/CuPcinterface SCS explains the observed device behavior better than techniques that yield macroscopic average electronic properties, such as photoelectron spectroscopy. Additionally, this dissertation discusses advances made in the design of an integrated optical refractive index sensor. The sensor uses organic light-emitting diodes (OLEDs) and OPVs as integrated light-sources and detectors on top of a slab waveguide substrate. The platform offers potentially high sensitivities to refractive index changes (and the selective binding of chemical and biological analytes), and is amenable to largescale integration for on-chip multiplexed detection. The refractive index response has been demonstrated previously, but the performance was limited by electrical noise and OLED drift. The use of different absorbing species in the OPV, integration of multiplesensors on a single substrate, addition of a reference channel to monitor OLED drift andthe use of lock-in amplification for signal processing allow the sensor to detect changesof 10-4 refractive index units.