Thermoelectricity at the Organic-Inorganic Interface

Electronic transport in molecular junctions has been studied through measurements of junction thermopower to evaluate the feasibility of thermoelectric (TE) energy generation using organic-inorganic hybrid materials. Energy transport and conversion in these junctions are heavily influenced by transport interactions at the metal-molecule interface. At this interface the discrete molecular orbitals overlap with continuum electronic states in the inorganic electrodes to create unique energy landscapes that cannot be realized in the organic or inorganic components alone. Over the past decade, scanning probe microscopes have been used to study the electronic conductance of single-molecule junctions[1–5]. Recently, we conducted measurements of junction thermopower using a modified scanning tunneling microscope (STM)[6]. Through our investigations, we have determined: (i) how the addition of molecular substituent groups can be used to predictably tune the TE properties of phenylenedithiol (PDT) junctions[7], (ii) how the length, molecular backbone, and end groups affect junction thermopower[8], and (iii) where electronic transport variations originate[9]. Furthermore, we have recently found that large (10 fold) TE enhancement can be achieved by effectively altering a (noble) metal junction using fullerenes (i.e., C₆₀, PCBM, and C₇₀). We associate the enhancement with the alignment of the frontier orbitals of the fullerene to the chemical potential of the inorganic electrodes. We further found that the thermopower can be predictably tuned by varying the work function of the contacts. This yields considerable promise for altering the surface states at interfaces for enhanced electronic and thermal transport. This paper highlights our work using thermopower as a probe for electronic transport, and reports preliminary results of TE conversion in fullerene-metal junctions.