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Page 2 of 9 Wang. Soft Sci 2024;4:25 https://dx.doi.org/10.20517/ss.2024.14

[2-4]
small-scale applications . Among these emerging technologies, thermoelectric cooling based on the Peltier
effect has the following advantages: lightweight, compact, and simple device structure, making it suitable for
portable refrigeration equipment. The Peltier effect was first discovered in 1,834 where the application of a
current through a conductor/semiconductor transfers the heat from one side to the other, hence creating a
hot end and a cold end. The advantages of the Peltier cooling elements have triggered substantial research
[5]
since the demonstration of thermoelectric cooling based on bismuth telluride, Bi Te , in 1954 . However,
3
2
one of the key challenges of Peltier cooling technology is its relatively low efficiency, limiting its applications
to niche areas in portable devices, such as containers in automobiles with thermal regulation functions,
where lower efficiency can be accepted .
[6]
The rise of flexible and wearable electronics in recent years has opened up many niche application areas that
conventional mechanical refrigeration could not be used such as temperature regulation of the human body
and flexible cooling devices for analgesia . Generally, two methods can introduce mechanical flexibility to
[7]
thermoelectric devices: reducing the thickness of inorganic thermoelectric materials and working with
materials systems that possess intrinsic mechanical flexibility such as organic semiconductors, 2D materials,
etc. While the development of thin inorganic thermoelectric materials on soft substrates has already led to
numerous flexible cooling devices including high performance wearable thermoelectric devices with
temperature differences over 10 °C and coefficients of performance > 1.5 [8-10] flexible organic cooling devices
are scarce due to various challenges such as high conductivity anisotropy . The inorganic thermoelectric
[11]
approach could attain higher cooling performance as inorganic thermoelectric materials are more
established and possess higher ZT (thermoelectric figure of merit) factor over 1. Since the existing reviews
mostly covered Peltier cooling based on inorganic thermoelectric materials , we focus on organic materials
[1,9]
for Peltier cooling applications. We aim to provide a comprehensive overview of the state-of-the-art organic
Peltier cooling work and insights into materials and device design aspects required for advancing organic
thermoelectric coolers.

ORGANIC PELTIER COOLERS
Organic semiconductors have enabled a paradigm shift in the small to medium display market due to their
excellent optoelectronic properties, ease of processing and low cost. They continue to find applications in
areas of emerging technologies such as photodetectors, bioelectronics and thermoelectrics [12-14] . Organic
thermoelectrics research is fueled by the advances in carrier mobility and efficient doping strategies over the
past two decades through synthetic design and morphology control, hence boosting the thermoelectric
power ,
[12]

where S and σ are the Seebeck coefficient and electrical conductivity, respectively. For thermoelectric
materials, their thermoelectric performance is determined by a dimensionless figure of merit ,
[15]

where k is thermal conductivity. Given the typical low thermal conductivity in organic semiconductors due
to the weak Van der Waals interactions that limit the heat transfer mediated by phonons, the improvement
in thermoelectric power factor leads to a high ZT value approaching 1 for both p- and n-type organic
semiconductors [16-19] . The intrinsic mechanical flexibility of organic semiconductors makes them particularly
attractive for flexible thermoelectric applications.