Abstract
Organic materials have attracted recent interest as thermoelectric (TE) converters due to their low cost and ease of fabrication. We examine the effects of disorder on the TE properties of semiconducting polymers based on the Gaussian disorder model (GDM) for site energies while employing Pauli’s master equation approach to model hopping between localized sites. Our model is in good agreement with experimental results and a useful tool to study hopping transport. We show that stronger overlap between sites can improve the electrical conductivity without adversely affecting the Seebeck coefficient. We find that positional disorder aids the formation of new conduction paths with an increased probability of carriers in high energy sites, leading to an increase in electrical conductivity while leaving the Seebeck unchanged. On the other hand, energetic disorder leads to increased energy gaps between sites, hindering transport. This adversely affects conductivity while only slightly increasing Seebeck and results in lower TE power factors. Furthermore, positional correlation primarily affects conductivity, while correlation in site energies has no effect on TE properties of polymers. Our results also show that the Lorenz number increases with Seebeck coefficient, largely deviating from the Sommerfeld value, in agreement with experiments and in contrast to band conductors. We conclude that reducing energetic disorder and positional correlation, while increasing positional disorder can lead to higher TE power factors.
Highlights
Organic materials offer several advantages as they have an inherently low thermal conductivity on account of their disordered structure and do not require further processing such as nanostructuring to reduce it
We explore the effect of various manifestations of disorder, including positional disorder, energetic disorder, as well as correlation in both energy and wave-function overlap distributions, on the electrical conductivity, Seebeck coefficient and Lorenz number
We find that positional disorder aids the formation of new conduction paths with an increased probability of carriers in high energy sites leading to increase in electrical conductivity
Summary
Organic materials offer several advantages as they have an inherently low thermal conductivity on account of their disordered structure and do not require further processing such as nanostructuring to reduce it. Polymer systems do not possess the long-range order found in their inorganic counterparts; they are inherently disordered and charge transport can be described as a hopping process[26,27,28]. We explore the effect of various manifestations of disorder, including positional disorder, energetic disorder, as well as correlation in both energy and wave-function overlap distributions, on the electrical conductivity, Seebeck coefficient and Lorenz number. Weathers et al.[39] showed the electronic contribution to thermal conductivity was higher than previously reported, consistent with a large Lorenz number, while Lu et al.[40] reported a large deviation from the Wiedemann-Franz law under the effect of temperature, carrier concentration, energetic disorder, and electric field.
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