Tuning the Seebeck coefficient of naphthalenediimide by electrochemical gating and doping

We investigate the sign and magnitude of the single-molecule Seebeck coefficient of naphthalenediimide (NDI) under the influence of electrochemical gating and doping. The molecule consists of a NDI core with two alkyl chains in the bay-area position, connected to gold electrodes via benzothiophene (DBT) anchor groups. By switching between the neutral, radical and di-anion charge states, we are able to tune the molecular energy levels relative to the Fermi energy of the electrodes. The resulting single-molecule room-temperature Seebeck coefficents of the three charge states are -294.5 μV K^-1, 122 μV K^-1 and 144 μV K^-1 respectively and the room-temperature power factors are 4.4 × 10^-5 W m^-1 K^-2, 3 × 10^-5 W m^-1 K^-2 and 8.2 × 10^-4 W m^-1 K^-2. As a further strategy for optimising thermoelectric properties, we also investigate the effect on both phonon and electron transport of doping the NDI with either an electron donor (TTF) or an electron acceptor (TCNE). We find that doping by TTF increases the room-temperature Seebeck coefficient and power factor from -73.7 μV K^-1 and 2.6 × 10^-7 W m^-1 K^-2 for bare NDI to -105 μV K^-1 and 3.6 × 10^-4 W m^-1 K^-2 in presence of TTF. The low thermal conductance of NDI-TTF, combined with the higher Seebeck coefficient and higher electrical conductance lead to a maximum thermoelectric figure of merit of ZT = 1.2, which is higher than that of bare NDI in several orders of magnitude. This demonstrates that both the sign and magnitude of NDI Seebeck coefficient can be tuned reversibly by electrochemical gating and doping, suggesting that such redox active molecules are attractive materials for ultra-thin-film thermoelectric devices.