Morphology controls the thermoelectric power factor of a doped semiconducting polymer.

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 F n TCNQ (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

Morphology controls the thermoelectric power factor of a doped semiconducting polymer

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 (s) 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 (m). The Seebeck coefficient (a) is largely independent of OCL. This reveals that, unlike s, leveraging strategies to improve m have a smaller impact on a. Our best-performing sample with the largest OCL, vapor-doped PBTTT: F4TCNQ thin film, has a s of 670 S/cm and an a of 42 μV/K, which translates to a large PF of 120 mW 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

Increasing the Thermoelectric Power Factor of a Semiconducting Polymer by Doping from the Vapor Phase

We demonstrate how processing methods affect the thermoelectric properties of thin films of a high mobility semiconducting polymer, PBTTT. Two doping methods were compared: vapor deposition of (tridecafluoro-1,1,2,2-tetrahydrooctyl)­trichlorosilane (FTS) or immersion in a solvent containing 4-ethylbenzenesulfonic acid (EBSA). Thermally annealed, thin films doped by FTS deposited from vapor yield a high Seebeck coefficient (α) at high electronic conductivity (σ) and, in turn, a large power factor (PF = α²σ) of ∼100 μW m^–1 K^–2. The FTS-doped films yield α values that are a factor of 2 higher than the EBSA-doped films at comparable high value of σ. A detailed analysis of X-ray scattering experiments indicates that perturbations in the local structure from either dopant are not significant enough to account for the difference in α. Therefore, we postulate that an increase in α arises from the entropic vibrational component of α or changes in scattering of carriers in disordered regions in the film

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

High conductivity in a nonplanar n-doped ambipolar semiconducting polymer

n-Doping of P(BTP-DPP) with the organometallic dimer (RuCp*mes)(2), processed through sequential casting, is reported. Maximum conductivities of 0.45 S cm(-1) were achieved that are relatively high for n-type semiconducting polymers. Electron paramagnetic resonance spectroscopy, ultraviolet visible spectroscopy, and ultraviolet photoemission spectroscopy are consistent with the introduction of high carrier concentrations by sequential processing, leading to bipolaronic, or otherwise spin-paired carriers. P(BTP-DPP) has glassy ordering in thin films, observed using wide angle X-ray scattering, that allows efficient incorporation of the dopant as a function of processing condition. The changes in electrical conductivity as a function of the dopant concentration are proposed to occur by charge percolation through domains with a mixture of polaronic and bipolaronic carriers

High Conductivity in a Nonplanar <i>n</i>‑Doped Ambipolar Semiconducting Polymer

<i>n</i>-Doping of P­(BTP-DPP) with the organometallic dimer (RuCp*mes)₂, processed through sequential casting, is reported. Maximum conductivities of 0.45 S cm^–1 were achieved that are relatively high for <i>n</i>-type semiconducting polymers. Electron paramagnetic resonance spectroscopy, ultraviolet visible spectroscopy, and ultraviolet photoemission spectroscopy are consistent with the introduction of high carrier concentrations by sequential processing, leading to bipolaronic, or otherwise spin-paired carriers. P­(BTP-DPP) has glassy ordering in thin films, observed using wide angle X-ray scattering, that allows efficient incorporation of the dopant as a function of processing condition. The changes in electrical conductivity as a function of the dopant concentration are proposed to occur by charge percolation through domains with a mixture of polaronic and bipolaronic carriers