Department of Physics, Lancaster University, Lancaster, LA1 4YB, UK.
Department of Electronic & Electrical Engineering - Photonics Group, University College London, UK.
Nanoscale. 2017 Apr 6;9(14):4819-4825. doi: 10.1039/c7nr00571g.
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, 122 μV K and 144 μV K respectively and the room-temperature power factors are 4.4 × 10 W m K, 3 × 10 W m K and 8.2 × 10 W m K. 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 and 2.6 × 10 W m K for bare NDI to -105 μV K and 3.6 × 10 W m K 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.
我们研究了电化学门控和掺杂对萘二酰亚胺（NDI）的单分子塞贝克系数的符号和大小的影响。该分子由 NDI 核心和两个在海湾区域的烷基链组成，通过苯并噻吩（DBT）锚定基团与金电极相连。通过在中性、自由基和二阴离子电荷态之间切换，我们能够调整分子能级相对于电极的费米能。三种电荷态的单分子室温塞贝克系数分别为-294.5 μV K、122 μV K 和 144 μV K，室温功率因子分别为 4.4 × 10-3 W m K、3 × 10-3 W m K 和 8.2 × 10-3 W m K。作为优化热电器件性能的进一步策略，我们还研究了用电子给体（TTF）或电子受体（TCNE）掺杂 NDI 对声子和电子输运的影响。我们发现，用 TTF 掺杂会增加室温塞贝克系数和功率因子，从裸 NDI 的-73.7 μV K 和 2.6 × 10-3 W m K 增加到存在 TTF 时的-105 μV K 和 3.6 × 10-3 W m K。NDI-TTF 的低热导与更高的塞贝克系数和更高的电导率相结合，导致最大热电优值 ZT = 1.2，比裸 NDI 高出几个数量级。这表明，电化学门控和掺杂可以可逆地调节 NDI 塞贝克系数的符号和大小，这表明这种氧化还原活性分子是超薄薄膜热电器件的有吸引力的材料。
