Unipolar Injection and Bipolar Transport in Electroluminescent Ru-Centered Molecular Electronic Junctions

Abstract

Bias-induced light emission and light-induced photocurrents were used as independent probes of charge transport in carbon-based molecular junctions containing Ru­(bpy)3. The thickness, bias, and temperature dependence of both the total device current and photoemission were compared, as well as their response to bias pulses lasting from a few milliseconds to several seconds. The device current was exponentially dependent on the square root of the applied electric field, with weak dependence on thickness when compared at a constant field. In contrast, light emission was strongly dependent on thickness at a given electric field, with a thickness-independent onset for light emission and a large intensity increase when the bias exceeded the 2.7 V HOMO–LUMO gap of Ru­(bpy)3. The apparent activation energies for light emission and current were similar but much smaller than those expected for thermionic emission or redox exchange. Light emission lagged current by several milliseconds but reached maximum emission in 5–10 ms and then decreased slowly for 1 s, in contrast to previously reported solid-state Ru­(bpy)3 light-emitting devices that relied on electrochemical charge injection. We conclude that at least two transport mechanisms are present, that is, “unipolar injection” initiated by electron transfer from a Ru­(bpy)3 HOMO to the positive electrode and “bipolar injection” involving hole and electron injection followed by migration, recombination, and light emission. The unipolar mechanism is field-driven and the majority of the device is current, while the bipolar mechanism is bias-driven and involves electrode screening by PF6 ions or mobile charges. In addition, significant changes in thickness and temperature dependence for thicknesses exceeding 15 nm imply a change from injection-limited transport to bulk-limited transport. The current results establish unequivocally that electrons and holes reside in the molecular layer during transport once the transport distance exceeds the ∼5 nm limit for coherent tunneling and that redox events involving nuclear reorganization accompany transport. In addition, they demonstrate luminescence in a single organometallic layer without hole or electron transport layers, thicknesses below 30 nm, and symmetric electrodes with similar work functions