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Wang. Soft Sci 2024;4:25 https://dx.doi.org/10.20517/ss.2024.14 Page 3 of 9

While research on organic semiconductors-based thermoelectric energy harvesting (Seebeck effect) has
[20]
steadily increased over the past decade , experimental work on the Peltier effect in organic semiconductors
remains relatively unexplored. While the Seebeck effect is directly linked to the Peltier effect by
thermodynamics, different to Seebeck effect measurement that simply involves the measurement of thermo-
voltage under a temperature gradient, Peltier effect measurement is more complex as passing an electrical
current through a thermoelectric material would involve not only the heat transfer from one end to the
other end due to the Peltier effect but also Joule heating and heat conduction . A major challenge with
[1]
organic semiconductors is their relatively high electrical resistivity and charge transport anisotropy such
that their out-of-plane resistivity is often the highest. This limitation makes the amount of Joule heating in
vertical organic semiconductor junction to be very significant and could easily overshadow the Peltier effect
as the cooling achieved by the Peltier effect linearly depends on electrical current whereas heating generated
by the Joule effect has a quadratic dependence on the applied electrical current. For thermoelectric cooling,
the device performance is often determined by several parameters such as the maximum cooling
temperature achievable, response time, coefficient of performance, and maximum cooling capacity . The
[9]
derivation of these parameters and their dependence on thermoelectric materials parameters are well
covered by other review papers [1,4,6] . Nevertheless, in order to achieve efficient thermoelectric cooling, it is
important to work with high ZT materials considering whether the ZT value is anisotropic and the electrical
resistivity of the thermoelectric material along the direction of the current.

State-of-the-art organic Peltier coolers
The first work experimented with Peltier cooling in organic semiconductors was performed by Hu et al. in
[21]
2005 . They observed the first signs of Peltier cooling in polypyrrole (PPy) powder despite the weak and
unstable effect due to material degradation under electrical current. Similarly, the experimental setup used
to probe the cooling effect is also non-optimal as the polymer particles are directly placed between two
metal plates, and the thermal insulation chamber is not well-designed, making measurement of tiny
temperature changes using thermocouple attached/thermal camera challenging [Figure 1A]. It is worth
mentioning that the Seebeck coefficient for PPy is only 10 µV/K, which is small compared with current high
performance organic thermoelectric materials. Since then, organic semiconductor research has accelerated,
motivated by commercializing organic light-emitting diodes with several high ZT materials developed by
[12]
optimizing the charge transport and doping in the organic materials . These high ZT organic
thermoelectric materials show promising performance for energy harvesting applications. However,
research in Peltier cooling with organic semiconductors did not progress significantly despite the discovery
of these high ZT organic thermoelectric materials due to large Joule heating coming from the relatively high
electrical resistivity and heat conduction with the material [1,11] .

Jin et al. have made major progress in fundamental understanding of the Peltier effect in high ZT organic
semiconductors [poly(Ni-ett)] in 2018, where they used a suspended device design under vacuum to
minimize heat dissipation together with an alternative current measurement scheme to separate the
temperature change induced by Joule heating and Peltier effect . In particular, they showed the possibility
[11]
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to achieve large Peltier cooling around 40 K under high applied current density of 5 A·mm highlighting the
potential of organic Peltier cooling devices [Figure 1B]. Wang et al. tried to overcome the typical electrical
conductivity anisotropy in organic semiconductors by working with n-type doped C thin films with decent
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ZT factor of around 0.1 where the buckyball structure of C allows the charge transport to be nearly
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isotropic within the film . The results show net cooling under electrical current excitation implying that
[22]
vertical electrical conductivity is an important roadblock in organic Peltier cooling devices [Figure 1C].
Moreover, they showed that the vertical organic Peltier devices have a fast response time of around 25 µs,
making them one of the fastest micro-thermoelectric coolers . Both studies use thermal reflectance
[9]
microscopy techniques to measure the temperature change induced by Peltier cooling. The thermal