Recently, it was demonstrated that environmental heat can put a battery on the charged state by using the difference in the thermal coefficient alpha (= dV/dT) of the redox potential (V) between the cathode and anode materials. In this battery, alpha is the key parameter that determines the device performance. Here, we systematically investigated alpha in prototypical conjugated polymers, i.e. polythiophene and four fluorene-based polymers. We found that alpha steeply increases with the number ratio (r) of thiophene in the main chain from 0.19 mV K-1 at r= 0.0 to 1.08 mV K-1 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. (C) 2019 The Japan Society of Applied Physics