Vibrational entropy as an indicator of temperature coefficient of redox potential in conjugated polymers

Abstract

Recently, it was demonstrated that environmental heat can put a battery on the charged state by using the difference in the thermal coefficient α (= d V/d T) 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-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.