π-Conjugated Nickel Complexes as n-Type Organometallic Semiconductors for Thermoelectric Generators

Flexible thermoelectric generators, which convert waste heat into electricity, have received increasing attention as convenient and low-cost energy-harvesting devices. p-Type and n-type organic and organometallic semiconducting materials are particularly suitable for thermoelectric materials on account of their very low thermal conductivity, which affords significant advantages not only in realizing high efficiency, but also in generating a temperature gradient. However, the development of n-type organic materials has been hampered partially on account of their instability in air upon doping with an n-dopant.<br/>One promising example of n-type organometallic semiconductors is π-conjugated nickel-ethenetetrathiolate (NiETT), which exhibits high air-stability and n-type thermoelectric behavior. We envisioned that the new design of π-conjugated ligands in nickel complexes should have a positive impact on the conductivity and the thermoelectric properties. In order to increase the accessible variety of this class of complexes, we have developed a versatile method for the synthesis of a variety of π-conjugated Ni complexes and examined their performance. Even though they are insoluble in virtually all common organic solvents, we found that the conductivity of the resulting films can be markedly improved by the choice of the π-conjugated core. For example, a nickel complex containing thieno[3,2-<i>b</i>]thiophene units was designed and synthesized. Composite films of the resulting nickel complex and polyvinylidene difluoride (PVDF), which can be fabricated via a simple solution process under atmospheric conditions, exhibit remarkably high n-type conductivity (&gt;200 S/cm). Moreover, the thermoelectric power factor of the n-type composite film was proven to be air stable. A grazing-incidence wide-angle X-ray diffraction analysis (GIWAXD) indicated a significant impact of introducing the thieno[3,2-<i>b</i>]thiophene core into the backbone of the nickel complex on the orientation within the composite films.<br/>These results on the design and synthesis of air-stable n-type materials should contribute to the generation of advanced materials with potential applications in flexible thermoelectric generators.