No abstract

The development of organic semiconductors for use in thermoelectrics requires the optimization of both their thermopower and electrical conductivity. Here two fundamentally different doping mechanisms are used to investigate the thermoelectric properties of known high hole mobility polymers: poly 3‐hexylthiophene (P3HT), poly(2,5‐bis(3‐tetradecylthiophen‐2‐yl)thieno[3,2‐b]thiophene) (PBTTT‐C14), and poly(2,5‐bis(thiphen‐2‐yl)‐(3,7‐diheptadecantyltetrathienoacene)) (P2TDC17‐FT4). The small molecule tetrafluorotetracyanoquinodimethane (F4TCNQ) is known to effectively dope these polymers, and the thermoelectric properties are studied as a function of the ratio of dopant to polymer repeat unit. Higher electrical conductivity and values of the thermoelectric power factor are achieved by doping with vapor‐deposited fluoroalkyl trichlorosilanes. The combination of these data reveals a striking relationship between thermopower and conductivity in thiophene‐based polymers over a large range of electrical conductivity that is independent of the means of electrical doping. This relationship is not predicted by commonly used transport models for semiconducting polymers and is demonstrated to hold for other semiconducting polymers as well.

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.

The orientational correlation length of domains in a semiconducting polymer controls its thermoelectric performance.

Identifying how small molecular acceptors pack with polymer donors in thin and thick (bulk) films is critical to understanding the nature of electrical doping by charge transfer. In this study, the packing structure of the molecular acceptor tetrafluorotetracyanoquinodimethane (F4TCNQ) with the semiconducting polymer poly­(2,5-bis­(3-tetradecylthiophen-2-yl)­thieno­[3,2-b]­thiophene) (PBTTT-C14) is examined. A combination of solid-state NMR, synchrotron X-ray scattering, and optical spectroscopy was used to determine the packing motif for blends of PBTTT-C14 and F4TCNQ in thin and bulk films. These results indicate that F4TCNQ and PBTTT-C14 order in a cofacial arrangement where charge transfer is near 100% efficient in the solid state. These results provide crucial insights into the structures and compositions of ordered domains in doped semiconducting polymers and suggest a model for the microstructure where the location of the molecular acceptors are correlated rather than randomly dispersed.

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.

ABSTRACT: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)PbI 3 ) 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)PbI 3 at temporal and spatial scales that significantly impact functional behavior. The relative PCE of (MA)PbI 3 photovoltaic cells is observed to reduce drastically with decreasing temperature, suggesting that such polarization effects could be a prerequisite for high-performance device operation. The pace at which new materials and designs for solar cells emerge is very slow, [1][2][3][4][5][6] and is arguably comparable to the discovery of high T c superconductors. [7][8] The finding that hybrid organic metal halide solar cells based on CH 3 NH 3 PbI 3 can lead to high power conversion efficiency (PCE) using simple coating methods, in a material comprising earth-abundant elements has therefore garnered significant interest. 6, 9-10 Not only has the peak reported PCE exceeded 20% in a short time, [11][12][13] but the processing techniques widely employed suggest that commercial products could be fabricated using low-cost, large-area techniques, potentially compatible with flexible substrates. 14The ABX 3 perovskite crystal structure is characterized by three-dimensionally corner connected network of BX 6/2 octahedra that is filled by the A ions. Perovskite compounds where the A is an organic cation, B is usually a main group element, and X is a group 7 anion (halide) We found that the electrical characteristics of (MA)PbI 3 had a complex temperature dependence. At room temperature these devices exhibit low source-drain currents and no fieldinduced current modulation (see Figure 1(a)). However when the temperature is reduced below 220K, a field-effect is observed and the drain current continues to increase as the temperature is reduced. Where the drain current is observed to be substantially in excess of gate current the gradual-channel approximation 44 has been applied to extract a value of field-effect mobility. The gradual channel approximation for extraction of carrier mobility in field-effect devices assumes a single electronic charge carrier, uniform carrier accumulation in the channel, and timeindependent behavior and consequently does not provide a simple estimation of the carrier motion for measurements with non-ideal effects. This is consistent with our observation of absolute values of electron mobility substantially lower than those previously reported using other techniques. 16 Nonetheless it can be applied as a proxy for relative transistor performance, assuming a high transcondu...

The thermoelectric properties of semiconducting polymers are influenced by both the carrier concentration and the morphology that sets the pathways for charge transport. A combination of optical, morphological, and electrical characterization was used to assess the effect of the role of disorder on the thermoelectric properties in thin films of poly(3-hexylthiophene) (P3HT) doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4 TCNQ). Controlled morphologies were formed by casting blends of regioregular (RR-P3HT) and regiorandom (RRa-P3HT) and then subsequently doping with F 4 TCNQ from the vapor phase. Optical spectroscopy and X-ray scattering show that vapor phase doping induces order in the disordered regions of thin films and increases the long-range connectivity of the film. The thermoelectric properties were assessed and show that while the Seebeck coefficient is affected by structural ordering, the electrical conductivity and power factor are more dominantly correlated with the long-range connectivity of domains.