Anne M. Glaudell

Solubility-limited extrinsic n-type doping of a high electron mobility polymer for thermoelectric applications.

The thermoelectric properties of a highperformance electron-conducting polymer, (P(NDIOD-T2), extrinsically doped with dihydro-1H-benzoimidazol-2-yl (NDBI) derivatives, are reported. The highest thermoelectric power factor that has been reported for a solution-processed n-type polymer is achieved; and it is concluded that engineering polymerdopant miscibility is essential for the development of organic thermoelectrics.

Temperature-Dependent Polarization in Field-Effect Transport and Photovoltaic Measurements of Methylammonium Lead Iodide

While recent improvements in the reported peak power conversion efficiency (PCE) of hybrid organic-inorganic perovskite solar cells have been truly astonishing, there are many fundamental questions about the electronic behavior of these materials. Here we have studied a set of electronic devices employing methylammonium lead iodide ((MA)PbI3) as the active material and conducted a series of temperature-dependent measurements. Field-effect transistor, capacitor, and photovoltaic cell measurements all reveal behavior consistent with substantial and strongly temperature-dependent polarization susceptibility in (MA)PbI3 at temporal and spatial scales that significantly impact functional behavior. The relative PCE of (MA)PbI3 photovoltaic cells is observed to reduce drastically with decreasing temperature, suggesting that such polarization effects could be a prerequisite for high-performance device operation.

Morphology controls the thermoelectric power factor of a doped semiconducting polymer

The orientational correlation length of domains in a semiconducting polymer controls its thermoelectric performance. The electrical performance of doped semiconducting polymers is strongly governed by processing methods and underlying thin-film microstructure. We report on the influence of different doping methods (solution versus vapor) on the thermoelectric power factor (PF) of PBTTT molecularly p-doped with FnTCNQ (n = 2 or 4). The vapor-doped films have more than two orders of magnitude higher electronic conductivity (σ) relative to solution-doped films. On the basis of resonant soft x-ray scattering, vapor-doped samples are shown to have a large orientational correlation length (OCL) (that is, length scale of aligned backbones) that correlates to a high apparent charge carrier mobility (μ). The Seebeck coefficient (α) is largely independent of OCL. This reveals that, unlike σ, leveraging strategies to improve μ have a smaller impact on α. Our best-performing sample with the largest OCL, vapor-doped PBTTT:F4TCNQ thin film, has a σ of 670 S/cm and an α of 42 μV/K, which translates to a large PF of 120 μW m−1 K−2. In addition, despite the unfavorable offset for charge transfer, doping by F2TCNQ also leads to a large PF of 70 μW m−1 K−2, which reveals the potential utility of weak molecular dopants. Overall, our work introduces important general processing guidelines for the continued development of doped semiconducting polymers for thermoelectrics.

The electrochemical impact on electrostatic modulation of the metal-insulator transition in nickelates

For physical studies of correlated electron systems and for realizing novel device concepts, electrostatic modulation of metal-insulator transitions (MITs) is desired. The inherently high charge densities needed to modulate MITs make this difficult to achieve. The high capacitance of ionic liquids are attractive but, voltages are needed that can be in excess of the electrochemical stability of the system. Here, we show temperature/resistivity data that suggest electrostatic modulation of the MIT temperature of NdNiO3 in a wide regime. However, additional voltammetric and x-ray photoelectron spectroscopy measurements demonstrate the electrochemical impact of the electrostatic doping approach with ionic liquids.

Electrically conductive polymers convert heat to electricity

Organic semiconductors are electrically conducting carbonbased materials with structures similar to dyes and plastics. They can be dissolved in solvents, and so they can be printed at room temperature like inks using jet-printing or roll-to-roll coating (like newspapers) onto a flexible substrate.1 As a result, they are attractive as a basis for low-cost displays, sensors, and solar cells. Organic light-emitting diodes are already commercial products in displays for mobile phones, and much effort has been directed at expanding their use. Very recently, organic semiconductors have been considered as thermoelectric materials,2 which convert heat to electrical energy (and the reverse) with no moving parts. Thermoelectric modules can be used for waste heat scavenging as well as local cooling applications, such as temperature control of optical switches. The power-conversion efficiency of a thermoelectric material is related to the figure of merit ZT D . ̨2 /T= , where ̨ is the thermopower, is electrical conductivity, is thermal conductivity, and T is temperature. ZT of more than 0.5 is considered useful for applications, and a ZT as high as 0.4 has recently been demonstrated in a common organic semiconductor named PEDOT.3 A critical issue in the development of organic thermoelectrics is a means to control the number of charge carriers.4 The three parameters ̨, , and are interrelated as a function of carrier concentration such that the electrical and thermal conductivities increase with the number of carriers while the thermopower decreases, so that maximum ZT results from intermediate carrier concentration. Therefore, we need organic semiconductors that can be stably doped by other chemical species to obtain a suitable charge carrier concentration to maximize ZT. Semiconducting organic polymers are poorly ordered in comparison with inorganic crystals, but recent advances in controlling this ordering have led to highly efficient charge Figure 1. (top) Structural model of the interaction between the semiconducting polymer, PBTTT, and the molecular dopant, F4TCNQ. (bottom) Grazing incidence x-ray scattering shows that blends of PBTTT and F4TCNQ are highly ordered.