A universal soft upper limit to the Seebeck coefficient in organic thermoelectrics

The trade-off between conductivity ($\sigma$$\sigma$) and Seebeck coefficient ($S$$S$) is an ongoing challenge for organic thermoelectrics as it determines how far the power factor ($PF=\sigma S^2$$PF=\sigma S^2$) can ultimately be pushed. Comparing experimental data for different polymers at variable doping levels, we show that the $S$$S$ vs. $\sigma$$\sigma$ curve is universal up to the maximum $PF$$PF$, followed by a material-dependent roll-off, when $S$$S$ and $\sigma$$\sigma$ are normalized to their values at maximum $PF$$PF$ and find there is a soft upper limit for $S$$S$ (∼200 μV/K), where the optimal power factor is achieved. Combining tight-binding and kinetic Monte Carlo modeling, we quantitatively explain this behavior in terms of quasi-free charges moving in a renormalized density of states of Gaussian shape, where the renormalization accounts for the screened interaction with the ionized dopants. Our results imply that the trade-off exists only at the single-material level and leads to practical design rules.