Open Access Publications from the University of California

## Understanding Charge Transport in Polymers for Thermoelectric Applications

Abstract

There is a great need to recycle energy wasted as heat into usable energy using clean, low-cost, and innovative technologies. Thermoelectric materials convert between heat and energy with no moving parts, and can be used to passively capture waste heat from industrial processes and for local fast temperature control. Traditional inorganic thermoelectric materials are efficient at high temperature and large temperature gradients, but are brittle and inflexible, often using environmentally harmful materials. Organic semiconductors have proved successful in commercial applications such as light-emitting diodes (OLEDs), solar cells (OPVs), and thin film transistors (OTFTs or OFETs), and there has recent been a significant research effort on organic materials for thermoelectric applications. Organic thermoelectric materials have the potential to fill voids that inorganic thermoelectric devices cannot. These materials can be solution deposited near room temperature onto flexible substrates, and generally perform well at room temperature. Flexible devices open up new applications for near room temperature thermoelectrics, and enable creative solutions for current applications. The challenge for efficient organic thermoelectrics is developing highly doped, stable p- and n-type materials, as intrinsic disorder in organic semiconductors makes predictive design non-trivial.

The effort is two fold: to assess the viability of high-mobility semiconducting polymers for thermoelectric applications, and to gain a deeper understanding of the underlying transport mechanism that governs electronic, thermoelectric, and optical properties. First, we evaluate the thermoelectric properties of a consistent set of polythiophenes varying the dopant molecule and method. By comparing the electronic and thermoelectric properties studied here and the many existing OTE compounds in the literature, we demonstrate that a general relationship between electrical conductivity $\sigma$ and thermopower $\alpha$, $\alpha\propto\sigma^{-1/4}$, emerges over a large range of conductivity, polymers, and doping schemes. This empirical relationship can function as an approximate metric for assessing new materials as well as to guide improvements in thermoelectric performance in organics. Next, the temperature-dependent electronic and thermoelectric transport properties of the well-studied high-mobility polymer poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno-

[3,2-b]thiophene) (PBTTT-C14) were studied for a range of processing conditions, dopants, and doping methods. In conjunction with microstructural characterization, we demonstrate how rational processing improves performance above what is expected from the empirical relationship, a key result in interpreting the transport measurements and understanding charge transport in PBTTT and other semiconducting polymers.