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PUBLISHED ONLINE: 5 MAY 2013 | DOI: http://www.nature.com/doifinder/10.1038/nmat3635
Web End =10.1038/NMAT3635
Engineered doping of organic semiconductors for enhanced thermoelectric efciency
G-H. Kim1, L. Shao1, K. Zhang1 and K. P. Pipe1,2*
Signicant improvements to the thermoelectric gure of merit ZT have emerged in recent years, primarily due to the engineering of material composition and nanostructure in inorganic semiconductors1 (ISCs). However, many present high-ZT materials are based on low-abundance elements that pose challenges for scale-up, as they entail high material costs in addition to brittleness and difculty in large-area deposition. Here we demonstrate a strategy to improve ZT in conductive polymers and other organic semiconductors (OSCs) for which the base elements are earth-abundant. By minimizing total dopant volume, we show that all three parameters constituting ZT vary in a manner so that ZT increases; this stands in sharp contrast to ISCs, for which these parameters have trade-offs. Reducing dopant volume is found to be as important as optimizing carrier concentration when maximizing ZT in OSCs. Implementing this strategy with the dopant poly(styrenesulphonate) in poly(3,4-ethylenedioxythiophene), we achieve ZT = 0.42
at room temperature.
Thermoelectric devices directly convert heat to electricity and vice versa without moving parts or working fluids, making them reliable and compact compared with conventional heat engines. OSCs offer numerous advantages over ISCs for thermoelectric applications, such as low cost, large-area deposition, high toughness and elasticity, material abundance and low weight. Furthermore, OSCs have low thermal conductivity ( ), which increases their energy conversion efficiency as defined by the thermoelectric figure-of-merit ZT = S2T/ , where S is the Seebeck coefficient,
is the electrical conductivity and T is the absolute temperature. As the material parameters constituting ZT do not suffer from the same trade-offs in OSCs as they do in ISCs (for example, OSCs do not typically obey the WiedemannFranz law, as the correlation between and is weak), they offer new routes to optimization of thermoelectric efficiency that remain for the most part unexplored.
Doping is key to maximizing the thermoelectric power factor (S2), because it determines free-carrier concentration (and hence S; ref. 2) and affects carrier mobility (). For OSCs, which traditionally suffer from low thermoelectric power factor, the effect of dopants on mobility is especially large, because dopants in van der Waals bonded...