Energy conversion and transport in organic-inorganic heterojunctions

The interface between an organic semiconductor and inorganic electrodes crucially determines the performance of organic electronic, thermoelectric, and photovoltaic devices. In these systems discrete molecular orbitals mix with the continuum energy states in inorganic crystalline materials to create unique energy landscapes that can be sensitively tuned using chemistry. Energy transport and conversion in these hybrid materials and devices are defined at the organic-inorganic interface. One approach to study this interface, is to look at the smallest hybrid building block, i.e., the junction of a single organic molecule with inorganic contacts. Conductance of single molecule junctions has been extensively studied for molecular electronics, but several lingering questions motivated the present study of thermopower in molecular junctions. In particular, conductance measurements were unable to explicitly determine how hybridization and alignment between the molecular orbitals and the electrode states define transport in the junction.Thermopower was measured using a modified scanning tunneling microscope having high spatial resolution capable of isolating and trapping small molecules between its conductive tip and a conductive substrate that acted as a second electrode. In the presence of a temperature difference between the tip and the substrate, a thermoelectric voltage was measured across the junction. The magnitude and sign of this voltage were indicative of the junction thermopower. The thermopower can uniquely identify the molecule's dominant transport orbital, as well as its alignment and coupling to the electrode states. Phenylene, alkane, and fullerene molecules of varied size and chemistry were studied. Statistical variations in thermopower implied large variations in the offset of the molecular orbital relative to the chemical potential of the contacts. Thermopower proved to be a useful diagnostic tool for studying electron transport at the hybrid interface, but measurements of fullerene-metal junctions also suggest that a hybrid material built from junction ensembles could have competitive thermoelectric efficiency.