Recently, it was demonstrated that environmental heat can put a battery on the charged state by using the difference in the thermal coefficient α (= dV /dT) of the redox potential (V) between the cathode and anode materials. In this battery, α is the key parameter that determines the device performance. Here, we systematically investigated α in prototypical conjugated polymers, i.e., polythiophene and four fluorene-based polymers. We found that α steeply increases with the number ratio (r) of thiophene in the main chain from 0.19 mV/K at r = 0.0 to 1.08 mV/K at r = 1.0. This trend is well understood in terms of variation of the vibrational entropy S vib between the neutral and oxidized states of the constituent molecule.

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