Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session W25: Focus Session: Organic Electronics and Photonics - Thermoelectric Properties of Polymers |
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Sponsoring Units: DMP DPOLY Chair: Bryan Boudouris, Purdue University Room: 503 |
Thursday, March 6, 2014 2:30PM - 3:06PM |
W25.00001: The Prospects of Organic Semiconductors for Thermoelectrics Invited Speaker: Michael Chabinyc Organic semiconductors have moved from a laboratory curiosity to commercial use in displays with organic light emitting diodes. In comparison to inorganic semiconductors, a remaining challenge for organic materials is the rational control of their electrical conductivity by doping. Due to the low lattice thermal conductivity of organic materials and their high electrical conductivities, organic semiconductors represent a promising class of solution processable thermoelectrics. The state of organic thermoelectrics and work from our lab on electrical doping of both p- and n- type semiconducting polymers will be discussed. The phase behavior of blends of semiconducting polymers and molecular dopants plays a critical role in their ultimate performance. Recent work on charge transfer doping and self-doping of polymers and molecular materials will be presented. Data mining from the literature along with results on recently developed materials systems suggests directions for optimization of organic thermoelectrics. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W25.00002: Investigation of the Wiedemann-Franz law in the conducting polymer PEDOT Annie Weathers, Li Shi, Zia Ullah Khan, Olga Bubnova, Xavier Crispin The conducting polymer PEDOT:PSS (Poly-3,4-ethylenedioxythiophene poly-styrenesulfonate) has been shown to have promising thermoelectric properties for a polymer system, with a reported ZT on the order of 0.3 at room temperature. Previous measurements of the thermoelectric properties has suggested a violation of the Wiedemann-Franz law, with a reported total thermal conductivity less than the estimated electronic contribution. The validity of the Wiedemann-Franz law in these systems has remained an open question, as the charge transport mechanism can be different than in inorganic materials. However, no measurements have been done to measure directly all three thermoelectric properties in the same direction. We present the in-plane thermoelectric properties of suspended PEDOT samples of varying electrical conductivities and discuss the implications of the results on the validity of the Wiedemann-Franz law for conducting polymer systems.~ [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W25.00003: Optimization of thermoelectric performance in semiconducting polymers for understanding charge transport and flexible thermoelectric applications Anne Glaudell, Michael Chabinyc Organic electronic materials have been widely considered for a variety of energy conversion applications, from photovoltaics to LEDs. Only very recently have organic materials been considered for thermoelectric applications - converting between temperature gradients and electrical potential. The intrinsic disorder in semiconducting polymers leads to an inherently low thermal conductivity, a key parameter in thermoelectric performance. The ability to solution deposit on flexible substrates opens up niche applications including personal cooling and conformal devices. Here work is presented on the electrical conductivity and thermopower of thin film semiconducting polymers, including P3HT and PBTTT-C$_{14}$. Thermoelectric properties are explored over a wide range of conductivities, from nearly insulating to beyond 100 S/cm, enabled by employing different doping mechanisms, including molecular charge-transfer doping with F4TCNQ and vapor doping with a fluoroalkyl trichlorosilane (FTS). Temperature-dependent measurements suggest competing charge transport mechanisms, likely due to the mixed ordered/disordered character of these polymers. These results show promise for organic materials for thermoelectric applications, and recent results on thin film devices will also be presented. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W25.00004: Power factor enhancement in solution-processed organic n-type thermoelectric materials through side chain design Boris Russ, Maxwell J. Robb, Fulvio G. Brunetti, Levi Miller, Shrayesh Patel, Victor Ho, Jeffrey J. Urban, Michael L. Chabinyc, Craig J. Hawker, Rachel A. Segalman Building efficient organic thermoelectric architectures requires complementary p-type (hole transporting) and n-type (electron transporting) components. While several high performance hole-transporting polymers have been developed, the design of n-type organics has proven challenging, and thermoelectric studies of organic n-type systems are scarce. We investigate the properties of a series of charged perylene diimide (PDI) derivatives. Charged side chains in these materials enable both water solubility and self-doping. We show that changing the length of the alkyl spacer between the charged end groups and the PDI core dramatically improves thin film thermoelectric properties. The top derivatives in our study demonstrated the highest power factor reported for n-type solution-processed films. By complementing thermoelectric characterization of these variants with insight on the electronic and structural property changes from optical spectroscopy, EPR, and GIWAXS experiments, our findings shape a promising molecular design strategy for future enhancements in thermoelectric performance. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W25.00005: Thermoelectric Properties of Conjugated Polyelectrolytes Cynthia Chen, Cheng-Kang Mai, Michael Chabinyc, Jeffrey Urban, Guillermo Bazan, Rachel Segalman Conjugated polymers are emerging as promising thermoelectric materials due to their solution processability, low thermal conductivity, and tunability of electrical properties via chemical modification. For the first time, conjugated polyelectrolytes, which are conjugated polymers with charged side chains, are being explored for thermoelectric applications. Charged side chains may be able to dope directly conjugated polymers by stabilizing the radical cations on the $\pi $-conjugated backbone. In this work, we investigate the thermoelectric properties of a novel narrow band gap conjugated polyelectrolyte with anionic side chains, poly[2,6-(4,4-bis-potassiumbutanylsulfonate-4H-cyclopenta-[2,1-b;3,4-b']-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (CPE-K). We show that doping CPE-K with hydrochloric acid can raise electrical conductivity without significantly changing Seebeck coefficient, resulting in an overall increase in power factor and an indication of how molecular design can be used to increase thermoelectric efficiency. Our results also shed some light on the role of charged side chains and the mechanism of doping in conjugated polyelectrolytes, which is different from that of doping in inorganic materials. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W25.00006: Synthesis and Solid State Charge Transport in Radical Polymers Lizbeth Rostro, Aditya Baradwaj, Bryan Boudouris Conducting polymers have been studied extensively for their applicability in a wide range of electronic devices. Previously, $\pi $-conjugated polymers have dominated the research focus due to the high degree of electronic delocalization associated with their molecular structure; however many challenges continue to prevent their viability in consumer applications. Here, we report on an emerging class of transparent non-conjugated conducting polymers, radical polymers, which circumvent many of the challenges faced by $\pi $-conjugated polymers. Specifically, a model radical polymer, poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA), was synthesized in a controlled manner using the RAFT polymerization mechanism, which produced polymers with readily-tunable molecular weights and narrow molecular weight distributions. Additionally, the solid state charge transport ($i.e.,$ conductivity) was characterized in radical polymers. Furthermore, we demonstrate that the chemistries of the radical polymer functionalities can be tuned readily, and this tuning leads to critical changes in the charge transport ability of these types of macromolecules in the solid state; this tunability allows the materials to be used in high-performing photovoltaic and thermoelectric devices. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W25.00007: Anisotropic Thermal Conduction in Polymers and its Molecular Origins David Nieto Simavilla, David Venerus, Jay Schieber Anisotropy in thermal conductivity has a significant impact on both processing and final properties of materials. Simple molecular arguments suggest that Fourier?s law must be generalized to allow for anisotropic thermal conductivity. We present two complementary experimental methods to obtain quantitative measurements of the thermal diffusivity (conductivity) tensor. We report anisotropic thermal diffusivity and stress in molten, cross-linked and solid polymers under several types of flows. Our results support the validity of a linear relationship between stress and anisotropy in thermal conductivity. When the proportionality constant, the stress-thermal coefficient, is made dimensionless by the plateau modulus of the polymer melt, a universal value of approximately 0.03 is observed for all chemistries. Such a universality is surprising, since phonon transport mechanisms are sensitive to chemical structure. For instance, the analogous stress-optic coefficient depends strongly on chemistry, and can even change sign. Connecting these measurements with current theories for thermal transport in amorphous materials, such as Minimum Thermal Conductivity (MTC) model, is crucial to understand the molecular origins of anisotropic thermal conduction in polymers. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:54PM |
W25.00008: Thermal Conductivities of Crystalline Organic Semiconductors Invited Speaker: Joseph Brill As applications for organic semiconductors grow, it is becoming increasingly important to know their thermal conductivities, k. For example, for sub-micron electronic devices, values of k\textgreater k$_{0}$ $\sim$ 5 mW/cm/K are needed, while values k\textless k$_{0}$ are required for desired thermoelectric applications. Whereas it is not surprising that semiconducting polymers typically have room temperature thermal conductivities below k$_{0}$, many molecular organic crystals also have values of k below this value. We have started measurements of both the in-plane and interplane thermal diffusivities of layered crystalline organic semiconductors using frequency\footnote{H. Zhang and J.W. Brill, J. Appl. Phys. \textbf{114}, 043508 (2013).} and position dependent\footnote{I. Hatta \textit{et al}, Jpn. Jnl Appl. Phys. \textbf{25}, L493 (1986).} ac-calorimetry; the thermal conductivities are then determined from the specific heats measured with differential scanning calorimetry. For rubrene, which has k\textless k$_{0}$, the interplane thermal conductivity is several times smaller than the in-plane value, although its temperature dependence indicates that the phonon mean-free path is at least a few layers.\footnote{H. Zhang and J.W. Brill} On the other hand, the in-plane thermal conductivity of TIPS-pentacene,\footnote{J.E. Anthony, Chem. Rev. \textbf{106}, 5028 (2006).} is several times greater than k$_{0}$, similar to that of the quasi-one dimensional organic metal TTF-TCNQ.\footnote{M.B. Salamon \textit{et al};, Phys. Rev. B \textbf{11}, 619 (1975).} Remarkably, its interlayer thermal conductivity is several times larger than its in-plane value,\footnote{H. Zhang and J.W. Brill} perhaps due to interactions between the large (triisopropylsilylethynyl) side groups on the pentacene backbone. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W25.00009: Measurements of In-Plane Thermal Diffusivities of Layered Organic Semiconductors by ac-Calorimetry Hao Zhang, Yulong Yao, Joseph Brill We are using the position-dependent ac-calorimetric technique of Hatta \textit{et al}\footnote{I. Hatta \textit{et al}, Jpn. Jnl Appl. Phys. \textbf{25}, L493 (1986).} to measure the in-plane thermal conductivity of layered organic semiconductors, such as 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pn).\footnote{J.E. Anthony, Chem. Rev. \textbf{106}, 5028 (2006).} Chopped light is used to heat the sample, part of which is screened from the light, with the thermometer placed on the back of the sample in the screened region. In the ``infinite crystal length'' limit, the logarithm of the oscillating temperature as well as its phase shift decrease linearly with distance of the thermometer from the edge of the screen, with a slope inversely related to the thermal diffusivity. Materials like TIPS-pn have surprisingly large values of thermal diffusivity, D \textgreater 1 mm$^{2}$/s, making finite size effects important, since crystal lengths are typically \textless 1 cm. We will discuss our technique and results in detail, including the effects of finite crystal size on the measurements of phase and magnitude of the oscillating temperature. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W25.00010: Polymer Thermoelectric Generators: Device Considerations Shannon Yee Recent control of the transport properties in polymers has encouraged the development of polymer thermoelectric (TE) devices. Polymer TEs are thought to be less expensive and more scalable than their inorganic counterparts. The cost of the raw material is less and polymer TEs can leverage the large areal manufacturing technique established by the plastics industry. Additionally, while the overall ZT of polymer TEs appears attractive, individual polymer properties have a very different scale than their inorganic counterparts (i.e., the thermal conductivity and electrical conductivity are approximately one and two orders of magnitude smaller, respectively). Furthermore, the majority of TE measurements on polymers have been limited to thin-films where traditional TE materials are measured in bulk. So why should it be expected that polymer TE devices resemble traditional TE devices? Given the uniqueness of polymers, different device architectures are proposed that can leverage the unique strengths of polymer films. It will be shown that by logically considering device requirements, new polymer TE devices have non-linear features that are more attractive than linear inorganic TE devices. This leads to very different device optimizations that favor polymer TEs. [Preview Abstract] |
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