The entangled triplet pair state in acene and heteroacene materials.

Entanglement of states is one of the most surprising and counter-intuitive consequences of quantum mechanics, with potent applications in cryptography and computing. In organic materials, one particularly significant manifestation is the spin-entangled triplet-pair state, which mediates the spin-conserving fission of one spin-0 singlet exciton into two spin-1 triplet excitons. Despite long theoretical and experimental exploration, the nature of the triplet-pair state and inter-triplet interactions have proved elusive. Here we use a range of organic semiconductors that undergo singlet exciton fission to reveal the photophysical properties of entangled triplet-pair states. We find that the triplet pair is bound with respect to free triplets with an energy that is largely material independent (∼30 meV). During its lifetime, the component triplets behave cooperatively as a singlet and emit light through a Herzberg-Teller-type mechanism, resulting in vibronically structured photoluminescence. In photovoltaic blends, charge transfer can occur from the bound triplet pairs with >100% photon-to-charge conversion efficiency.

Acido-basic control of the thermoelectric properties of poly(3,4-ethylenedioxythiophene)tosylate (PEDOT-Tos) thin films.

PEDOT-Tos is one of the conducting polymers that displays the most promising thermoelectric properties. Until now, it has been utterly difficult to control all the synthesis parameters and the morphology governing the thermoelectric properties. To improve our understanding of this material, we study the variation in the thermoelectric properties by a simple acido-basic treatment. The emphasis of this study is to elucidate the chemical changes induced by acid (HCl) or base (NaOH) treatment in PEDOT-Tos thin films using various spectroscopic and structural techniques. We could identify changes in the nanoscale morphology due to anion exchange between tosylate and Cl- or OH-. But, we identified that changing the pH leads to a tuning of the oxidation level of the polymer, which can explain the changes in thermoelectric properties. Hence, a simple acid-base treatment allows finding the optimum for the power factor in PEDOT-Tos thin films.

Semi-metallic polymers.

Polymers are lightweight, flexible, solution-processable materials that are promising for low-cost printed electronics as well as for mass-produced and large-area applications. Previous studies demonstrated that they can possess insulating, semiconducting or metallic properties; here we report that polymers can also be semi-metallic. Semi-metals, exemplified by bismuth, graphite and telluride alloys, have no energy bandgap and a very low density of states at the Fermi level. Furthermore, they typically have a higher Seebeck coefficient and lower thermal conductivities compared with metals, thus being suitable for thermoelectric applications. We measure the thermoelectric properties of various poly(3,4-ethylenedioxythiophene) samples, and observe a marked increase in the Seebeck coefficient when the electrical conductivity is enhanced through molecular organization. This initiates the transition from a Fermi glass to a semi-metal. The high Seebeck value, the metallic conductivity at room temperature and the absence of unpaired electron spins makes polymer semi-metals attractive for thermoelectrics and spintronics.

Tuning the thermoelectric properties of conducting polymers in an electrochemical transistor.

While organic field-effect transistors allow the investigation of interfacial charge transport at the semiconductor-dielectric interface, an electrochemical transistor truly modifies the oxidation level and conductivity throughout the bulk of an organic semiconductor. In this work, the thermoelectric properties of the bulk of the conducting polymer poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) were controlled electrically by varying the gate voltage. In light of the growing interest in conducting polymers as thermoelectric generators, this method provides an easy tool to study the physics behind the thermoelectric properties and to optimize polymer thermoelectrics.

Optimization of the thermoelectric figure of merit in the conducting polymer poly(3,4-ethylenedioxythiophene).

Thermoelectric generators (TEGs) transform a heat flow into electricity. Thermoelectric materials are being investigated for electricity production from waste heat (co-generation) and natural heat sources. For temperatures below 200 °C, the best commercially available inorganic semiconductors are bismuth telluride (Bi(2)Te(3))-based alloys, which possess a figure of merit ZT close to one. Most of the recently discovered thermoelectric materials with ZT>2 exhibit one common property, namely their low lattice thermal conductivities. Nevertheless, a high ZT value is not enough to create a viable technology platform for energy harvesting. To generate electricity from large volumes of warm fluids, heat exchangers must be functionalized with TEGs. This requires thermoelectric materials that are readily synthesized, air stable, environmentally friendly and solution processable to create patterns on large areas. Here we show that conducting polymers might be capable of meeting these demands. The accurate control of the oxidation level in poly(3,4-ethylenedioxythiophene) (PEDOT) combined with its low intrinsic thermal conductivity (λ=0.37 W m(-1) K(-1)) yields a ZT=0.25 at room temperature that approaches the values required for efficient devices.