Improved room-temperature thermoelectric characteristics in F4TCNQ-doped CNT yarn/P3HT composite by controlled doping

High room-temperature thermoelectric performance is important for low-grade waste heat power generation as there are plenty of heat thrown away uselessly in our daily life, most of which are below 100 °C. However, most of the thermoelectric materials are limited to high temperature application. In this work, room-temperature thermoelectric power factor of carbon nanotube (CNT) yarn is improved by controlled doping, which is achieved by making composite with poly 3-hexylthiophene −2, 5-diyl (P3HT) followed by doping with 2, 3, 5, 6-tetrafluo-7, 7, 8, 8-tetracyanoquinodimethane (F4TCNQ). The temperature-dependent Seebeck coefficient based on power–law model suggests that P3HT shifts the Fermi energy of CNT yarn towards the valence band edge, and reduces the ionic scattering and carrier relaxation time. As a result, the Seebeck coefficient is increased while the variation of Seebeck coefficient with temperature is reduced, and hence, the room-temperature thermoelectric power factor is improved. With controlled doping, the power factor of CNT yarn/P3HT composite reaches to 1640–2160 μW m⁻¹K⁻² at the temperature range of 25–100 °C, which is higher than that of CNT yarn alone.     

Tailoring Interfacial Charge Transfer for Optimizing Thermoelectric Performances of MnTe-Sb2Te3 Superlattice-Like Films

Interfacial charge transfer has a vital role in tailoring the thermoelectric performance of superlattices (SLs), which, however, is rarely clarified by experiments. Herein, based on epitaxially grown p-type (MnTe)_x(Sb₂Te₃)_y superlattice-like films, synergistically optimized thermoelectric parameters of carrier density, carrier mobility, and Seebeck coefficient are achieved by introducing interfacial charge transfer, in which effects of hole injection, modulation doping, and energy filtering are involved. Carrier transport measurements and angle-resolved photoemission spectroscopy (ARPES) characterizations reveal a strong hole injection from the MnTe layer to the Sb₂Te₃ layer in the SLs, originating from the work function difference between MnTe and Sb₂Te₃. By reducing the thickness of MnTe less than one monolayer, all electronic transport parameters are synergistically optimized in the quantum-dots (MnTe)_x(Sb₂Te₃)₁₂ superlattice-like films, leading to much improved thermoelectric power factors (PFs). The (MnTe)_0.1(Sb₂Te₃)₁₂ obtains the highest room-temperature PF of 2.50 mWm⁻¹K⁻², while the (MnTe)_0.25(Sb₂Te₃)₁₂ possesses the highest PF of 2.79 mWm⁻¹K⁻² at 381 K, remarkably superior to the values acquired in binary MnTe and Sb₂Te₃ films. This research provides valuable guidance on understanding and rationally tailoring the interfacial charge transfer of thermoelectric SLs to further enhance thermoelectric performances.     

The effect of thermal annealing on dopant site choice in conjugated polymers

Solution-processed organic electronic devices often consist of layers of polar and non-polar polymers. In addition, either of these layers could be doped with small molecular dopants. It is extremely important for device stability to understand the diffusion behavior of these molecular dopants under the thermal stress and whether the dopants have preference for the polar or the non-polar polymer layers. In this work, a widely used molecular dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) was chosen to investigate dopant site preference upon thermal annealing between the polar thiophene poly(thiophene-3-[2-(2-methoxy-ethoxy)ethoxy]-2,5-diyl) (S–P3MEET) and non-polar thiophene poly(3-hexylthiophene) (P3HT). F4TCNQ is able to p-type dope both P3HT and S–P3MEET. Further doping studies of S–P3MEET using near edge X-ray absorption fine structure spectroscopy, conductivity measurements and atomic force microscopy show that the F4TCNQ additive competes for doping sites with the covalently attached dopants on the S–P3MEET. Calorimetry measurements reveal that the F4TCNQ interacts strongly with the side-chains of the S–P3MEET, increasing the melting temperature of the side-chains by 30 °C with 5 wt% dopant loading. Next, the thermal stability of doping in the polar/non-polar (S–P3MEET/P3HT) bilayer architectures was investigated. Steady-state absorbance and fluorescence results show that F4TCNQ binds much more strongly in S–P3MEET than P3HT and very little F4TCNQ is found in the P3HT layer after annealing. In combination with reflectometry measurements, we show that F4TCNQ remains in the SP3MEET layer with annealing to 210 °C even though the sublimation temperature for neat F4TCNQ is about 80 °C. In contrast, F4TCNQ slowly diffuses out of P3HT at room temperature. We attribute this difference in binding the F4TCNQ anion to the ability of the ethyl-oxy side-chains of the S–P3MEET to orient around the charged dopant molecule and thereby to stabilize its position. This study suggests that polar side-chains could be engineered to increase the thermal stability of molecular dopant position.     

All-conjugated diblock copolymer of poly(3-hexylthiophene)-block-poly(3-phenoxymethylthiophene) for field-effect transistor and photovoltaic applications

The electronic properties, morphology and optoelectronic device characteristics of conjugated diblock copolythiophene, poly(3-hexylthiophene)-block -poly(3-phenoxymethylthiophene) (P3HT-b-P3PT), are firstly reported. The polymer properties and structures were explored through different solvent mixtures of chloroform (CHCl₃), dichlorobenzene (DCB), and CHCl₃:DCB (1:1 ratio). The absorption maximum (λ_max) of P3HT-b-P3PT prepared from DCB was around 554 nm with a shoulder peak indicative for the highly crystalline structure around 604 nm while that from CHCl₃ was 516 nm without the clear shoulder peak. The field-effect hole mobility of P3HT-b-P3PT increased from ～6.0 × 10^?3, ～8.0 × 10^?3 to ～2.0 × 10^?2 cm² V^?1 s^?1 as the DCB content in the solvent mixture enhanced. The AFM images suggested that the highly volatile CHCl₃ processing solvent led to the amorphous structure, on the other hand, less volatile DCB resulted in the largely crystalline structure of the P3HT-b-P3PT. Such difference on the polymer structure and hole mobility led to the varied power conversion efficiency (PCE) of the photovoltaic cells fabricated from the blend of P3HT-b-P3PT/[6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) (1:1, w/w): 1.88 (CHCl₃), 2.13 (CHCl₃:DCB (1:1)), and 2.60% (DCB). The PCBM blend ratio also significantly affected the surface structure and the solar cell performance. The PCE of polymer/PCBM could be improved to 2.80% while the ratio of polymer to PCBM went to 1:0.7. The present study suggested that the surface structures and optoelectronic device characteristics of conjugated diblock copolymers could be easily manipulated by the processing solvent, the block segment characteristic, and blend composition.     

Self Encapsulated Poly(3‐hexylthiophene)‐poly(fluorinated alkyl methacrylate) Rod‐Coil Block Copolymers with High Field Effect Mobilities on Bare SiO2

Conjugated rod‐coil block copolymers provide an interesting route towards enhancing the properties of the conjugated block due to self‐assembly and the interplay of rod‐rod and rod‐coil interactions. Here, we demonstrate the ability of an attached semi‐fluorinated block to significantly improve upon the charge carrier properties of regioregular poly(3‐hexyl thiophene) (rr‐P3HT) materials on bare SiO₂. The thin film hole mobilities on bare SiO₂ dielectric surfaces of poly (3‐hexyl thiophene)‐block‐polyfluoromethacrylates (P3HT‐b‐PFMAs) can approach up to 0.12 cm² V^?1 s^?1 with only 33 wt% of the P3HT block incorporated in the copolymer, as compared to rr‐P3HT alone which typically has mobilities averaging 0.03 cm² V^?1 s^?1. To our knowledge, this is the highest mobility reported in literature for block copolymers containing a P3HT. More importantly, these high hole mobilities are achieved without multistep OTS treatments, argon protection, or post‐annealing conditions. Grazing incidence wide‐angle x‐ray scattering (GIWAX) data revealed that in the P3HT‐b‐PFMA copolymers, the P3HT rod block self‐assembles into highly ordered lamellar structures, similar to that of the rr‐P3HT homopolymer. Grazing incidence small‐angle x‐ray scattering (GISAXS) data revealed that lamellar structures are only observed in perpendicular direction with short PFMA blocks, while lamellae in both perpendicular and parallel directions are observed in polymers with longer PFMA blocks. AFM, GIWAXS, and contact angle measurements also indicate that PFMA block assembles at the polymer thin film surface and forms an encapsulation layer. The high charge carrier mobilities and the hydrophobic surface of the block copolymer films clearly demonstrates the influence of the coil block segment on device performance by balancing the crystallization and microphase separation in the bulk morphological structure.     

High Thermoelectric Performance in Earth-Abundant Cu3SbS4 by Promoting Doping Efficiency via Rational Vacancy Design

Sulfides are well investigated as thermoelectric materials but their performance is typically limited by low electrical conductivity. High electrical performance in Cu₃SbS₄ is reported by creating high valence vacancies, which efficiently provides multiple carriers. It is revealed from the perspective of a chemical bond by calculations that Al can serve as vacancy stabilizer as its entry into the lattice forms intensified bonds with neighboring atoms and lowers the vacancy formation energy. As a result, the average power factor of Cu₃SbS₄ with 9 wt% CuAlS₂ reaches 16.1 µW cm⁻¹ K⁻². Finally, by further addition of AgAlS₂, a peak zT of 1.3 and an average zT of 0.77 are obtained due to the reduced thermal conductivity. The attained average power factor and average zT are superior to other low-toxic thermoelectric sulfides. The findings shed light on the new strategy for creating favorable vacancies to realize high-efficiency doping in thermoelectric materials.     

A Large Bandgap Organic Salt Dopant for Sn-Based Perovskite Thin-Film Transistor

Metal halide perovskite optoelectronic devices have made significant progress over the past few years, but precise control of charge carrier density through doping is essential for optimizing these devices. In this study, the potential of using an organic salt, N,N-dimethylanilinium tetrakis(pentafluorophenyl)borate, as a dopant for Sn-based perovskite devices, is explored. Under optimized conditions, the thin film transistors based on the doped 2D/3D perovskite PEAFASnI₃ demonstrate remarkable improvement in hole mobility, reaching 7.45 cm²V⁻¹s⁻¹ with a low subthreshold swing and the smallest sweep hysteresis (ΔV_hysteresis = 2.27 V) and exceptional bias stability with the lowest contact resistance (2.2 kΩ cm). The bulky chemical structure of the dopant prevents it from penetrating the perovskite lattice and also surface passivation against Sn oxidation due to its hydrophobic nature surface. This improvement is attributed to the bifunctional effect of the dopant, which simultaneously passivates defects and improves crystal orientation. These findings provide new insights into potential molecular dopants that can be used in metal halide perovskite devices.     

Donor–Acceptor Poly(3‐hexylthiophene)‐block‐Pendent Poly(isoindigo) with Dual Roles of Charge Transporting and Storage Layer for High‐Performance Transistor‐Type Memory Applications

Field‐effect transistor memories usually require one additional charge storage layer between the gate contact and organic semiconductor channel. To avoid such complication, new donor–acceptor rod–coil diblock copolymers (P3HT₄₄‐b‐Piso_n) of poly(3‐hexylthiophene) (P3HT)‐block‐poly(pendent isoindigo) (Piso) are designed, which exhibit high performance transistor memory characteristics without additional charge storage layer. The P3HT and Piso blocks are acted as the charge transporting and storage elements, respectively. The prepared P3HT₄₄‐b‐Piso_n can be self‐assembled into fibrillar‐like nanostructures after the thermal annealing process, confirmed by atomic force microscopy and grazing‐incidence X‐ray diffraction. The lowest‐unoccupied molecular orbital levels of the studied polymers are significantly lowered as the block length of Piso increases, leading to a stronger electron affinity as well as charge storage capability. The field‐effect transistors (FETs) fabricated from P3HT₄₄‐b‐Piso_n possess p‐type mobilities up to 4.56 × 10^?2 cm² V^?1 s^?1, similar to that of the regioregular P3HT. More interestingly, the FET memory devices fabricated from P3HT₄₄‐b‐Piso_n exhibit a memory window ranging from 26 to 79 V by manipulating the block length of Piso, and showed stable long‐term data endurance. The results suggest that the FET characteristics and data storage capability can be effectively tuned simultaneously through donor/acceptor ratio and thin film morphology in the block copolymer system.     

Redox Poly-Counterion Doped Conducting Polymers for Pseudocapacitive Energy Storage

Conducting polymers (CPs) have been widely studied for electrochemical energy storage. However, the dopants in CPs are often electrochemically inactive, introducing “dead-weight” to the materials. Moreover, commercial-level electrode materials with high mass loadings (e.g., >10 mg cm⁻²) often encounter the problems of inferior electrical and ionic conductivity. Here, a redox-active poly-counterion doping concept is proposed to improve the electrochemical performance of CPs with ultra-high mass loadings. As a study prototype, heptamolybdate anion (Mo₇O₂₄⁶⁻) doped polypyrrole (PPy) is synthesized by electro-polymerization. A 2 mm thick PPy electrode with mass loading of ≈192 mg cm⁻² reaches a record-high areal capacitance of ≈47 F cm⁻², competitive gravimetric capacitance of 235 F g⁻¹, and volumetric capacitance of 235 F cm⁻³. With poly-counterion doping, the dopants also undergo redox reactions during charge/discharge processes, providing additional capacitance to the electrode. The interaction between polymer chains and the poly-counterions enhances the electrical conductivity of CPs. Besides, the poly-counterions with large steric hindrance could act as structural pillars and endow CPs with open structures for facile ion transport. The concept proposed in this work enriches the electrochemistry of CPs and promotes their practical applications.     

Strain-Mediated Lattice Rotation Design for Enhancing Thermoelectric Performance in Bi2S2Se

A unique strain-mediated lattice rotation strategy is introduced via nanocompositing to upsurge the optimized limits in the composition-to-structural pathway on rationally engineering the efficient thermoelectric material. In this study, a special lattice rotation via strain engineering is realized to optimize the desired electronic and chemical environment for enhancing thermoelectric properties in n-type Bi₂S₂Se. This approach results in a unique transport phenomenon to assist high-energy electrons in transferring through the optimized transport channels, and appropriate structure disparity to significantly localize phonons. As a result, Sb over Cl doping in Bi₂S₂Se gently reduces E_g and introduces defect states in bandgap with shifting down the Fermi level, thus causing increase in carrier concentration, which contributes to a higher power factor of ≈7.18 µW cm⁻¹ K⁻² (at T = 773 K). Besides, a lower thermal conductivity of ≈0.49 W m⁻¹ K⁻¹ is driven through lattice strain and defect engineering. Consequently, an ultra-high ZT_max = 1.13 (at T = 773 K) and a high ZT_ave = 0.54 (323 K-773 K) are realized. This study not only leads to an extraordinary thermoelectric performance but also reveals a unique paradigm for electron transportation and phonon localization via lattice strain engineering.     

High mobility and low operation voltage organic field effect transistors by using polymer-gel dielectric and molecular doping

In this work, we present a method to increase the performance in solution processed organic field effect transistors (OFET) by using gel as dielectric and molecular doping to the active organic semiconductor. In order to compare the performance improvement, Poly (methylmethacrylate) (PMMA) and Poly (3-hexylthiophene-2,5-diyl) P3HT material system were used as a reference. Propylene carbonate (PC) is introduced into PMMA to form the gel for using as gate dielectric. The mobility increases from 5.72×10⁻³ to 0.26 cm² V s^–1 and operation voltage decreases from −60 to −0.8 with gel dielectric. Then, the molecular dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) is introduced into P3HT via co-solution. The mobility increases up to 1.1 cm² V s^–1 and the threshold voltage downs to −0.09 V with doping. The increase in performance is discussed in terms of better charge inducing by high dielectric properties of gel and trap filling due to the increased carrier density in active semiconductor by molecular doping.     

Morphology Associated Positive Correlation between Carrier Mobility and Carrier Density in Highly Doped Donor–Acceptor Conjugated Polymers

Molecular doping in conjugated polymers (CPs) has recently received intensive attention for its potential to achieve high electrical conductivity in organic thermoelectric materials. In particular, it affects not only the carrier density n but also the carrier mobility µ because high degree of molecular doping changes the morphological properties. Herein, the effect of molecular doping in CP thin films on the pathways and mechanisms of charge transport is investigated, which govern the µ-n relationship. Two representative donor–acceptor type CPs with similar µ but different molecular assembly in an undoped state, that is poly[2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrrole-alt-5,5-(2,5-di(thien-2-yl)thieno[3,2-b]thiophene)] (DPPDTT) and indacenodithiophene-co-benzothiadiazole (IDTBT), are prepared. Heavy doping with iron chloride (FeCl₃) induced DPPDTT with highly crystalline edge-on orientation to increase its µ up to 19.6 cm² V⁻¹ s⁻¹, whereas IDTBT with irregular intermolecular stacking showed little change in µ. It is revealed that this different µ-n relationship is highly attributed to the initial molecular ordering of CP films. The charge transport mechanism also becomes significantly different: both coherent and incoherent transports are observed in the doped DPPDTT, whereas incoherent transport is only found in the doped IDTBT. This study suggests guidelines for enhancing charge transport of CPs under doping in terms of structural disorder.     

Light-induced electron spin resonance study of galvinoxyl-doped P3HT/PCBM bulk heterojunctions

We report the effects of doping of P3HT/PCBM layers with spin 1/2 radicals of galvinoxyl (Gx) based on light-induced electron spin resonance (LESR), photoluminescence-detected magnetic resonance (PLDMR), and post-annealing experiments.LESR showed both a P3HT⁺ and PCBM⁻ signal for undoped P3HT/PCBM; however, as Gx doping increased (above ∼1 wt%), only the P3HT⁺ signal was evident in the LESR spectra, with no PCBM⁻ signal.The PLDMR exhibited a strong narrow signal at g = 2.002 that originates from nongeminate polaron pairs; no triplet PLDMR signal has been observed throughout the whole range of Gx concentrations (x = 0, 0.1, 1, 2, 4, 12 wt%). Adding Gx to ∼3 wt% led to a decrease of the PL-enhancement.There was big difference between the slow-dried P3HT/PCBM samples and the post-annealed samples. For the slow-dried samples, efficiency monotonously decreased with Gx additives. When post-annealed, however, an enhancement in η was observed at ∼2 wt% for P3HT/PCBM(1:2) samples.The LESR spectra for post-annealed samples revealed disappearance of Gx spin signals, and thus no spin interactions with PCBM⁻ spins. It is unlikely that the increase of efficiency after Gx doping of P3HT/PCBM solar cell is due to an increase of triplet states.     

2,2-Dicyanovinyl-end-capped oligothiophenes as electron acceptor in solution processed bulk-heterojunction organic solar cells

Three 2,2-dicyanovinyl (DCV) end-capped A-π-D-π-A type oligothiophenes (DCV-OTs) containing dithieno[3,2-b:2′,3′-d]silole (DTSi), cyclopenta[1,2-b:3,4-b′]dithiophene (DTCP) or dithieno[3,2-b:2′,3′-d]pyrrole (DTPy) unit as the central donor part, mono-thiophene as the π-conjugation bridge were synthesized. The absorption spectroscopies, cyclic voltammetry of these compounds were characterized. Results showed that all these compounds have intensive absorption band over 500–680 nm with a LUMO energy level around −3.80 eV, which is slightly higher than that of [6,6]phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM, E_LUMO = −4.01 eV), but lower than that of poly(3-hexylthiophene) (P3HT, E_LUMO = −2.91 eV). Solution processed bulk heterojunction “all-thiophene” solar cells using P3HT as electron donor and the above mentioned oligothiophenes as electron acceptor were fabricated and tested. The highest power conversion efficiency (PCE) of 1.31% was achieved for DTSi-cored compound DTSi(THDCV)₂, whereas PTB7:DTSi(THDCV)₂ based device showed slightly higher PCE of 1.56%. Electron mobilities of these three compounds were measured to be around 10⁻⁵ cm² V⁻¹ s⁻¹ by space charge limited current method, which is much lower than that of PC₆₁BM, and was considered as one of the reason for the low photovoltaic performance.     

Double Doping of Semiconducting Polymers Using Ion-Exchange with a Dianion

The interactions between counterions and electronic carriers in electrically doped semiconducting polymers are important for delocalization of charge carriers, electronic conductivity, and thermal stability. The introduction of a dianions in semiconducting polymers leads to double doping where there is one counterion for two charge carriers. Double doping minimizes structural distortions, but changes the electrostatic interactions between the carriers and counterions. Polymeric ionic liquids (PIL) with croconate dianions are helpful to investigate the role of the counterion in p-type semiconducting polymers. PILs prevent diffusion of the cation into the semiconducting polymers during ion exchange. The redox-active croconate dianions undergo ion exchange with doped semiconducting polymers depending on their ionization energy. Croconate dianions are found to reduce doped films of poly(3-hexyl thiophene), but undergo ion exchange with a polythiophene with tetraethylene glycol side chains, P(g₄2T-T), that has a lower ionization energy. The croconate dianion maintains crystalline order in P(g₄2T-T) and leads to a lower activation energy for the electrical conductivity than PF₆⁻ counterions. The control of the doping level with croconate allows optimization of the thermoelectric performance of the semiconducting polymer. The thermal stability of the doped films of P(g₄2T-T) is found to depend strongly on the nature of the counterion.     

Enhancement of the field-effect mobility of poly(3-hexylthiophene)/functionalized carbon nanotube hybrid transistors

With the aim of enhancing the field-effect mobility of poly(3-hexylthiophene) (P3HT) field-effect transistors (FETs), we added functionalized multiwalled carbon nanotubes (CNTs) to the P3HT solution prior to film formation. The nanotubes were found to be homogeneously dispersed in the P3HT films because of their functional groups. We found that at the appropriate CNT concentration (up to 10 wt% CNT), the P3HT FETs have a high field-effect mobility of 0.04 cm² V⁻¹ s⁻¹, which is an improvement by a factor of more than 10. This remarkable increase in the field-effect mobility over that of the pristine P3HT film is due to the high conductivity of the CNTs which act as conducting bridges between the crystalline regions of the P3HT film, and the reduction in the hole-injection barrier due to the low work function of CNTs, which results in more efficient carrier injection.     

Synthesis,Morphology, and Properties of Poly(3‐hexylthiophene)‐block‐Poly(vinylphenyl oxadiazole) Donor–Acceptor Rod–Coil Block Copolymers and Their Memory Device Applications

Novel donor–acceptor rod–coil diblock copolymers of regioregular poly(3‐hexylthiophene) ( P3HT )‐block‐poly(2‐phenyl‐5‐(4‐vinylphenyl)‐1,3,4‐oxadiaz‐ole) ( POXD ) are successfully synthesized by the combination of a modified Grignard metathesis reaction ( GRIM ) and atom transfer radical polymerization ( ATRP ). The effects of the block ratios of the P3HT donor and POXD pendant acceptor blocks on the morphology, field effect transistor mobility, and memory device characteristics are explored. The TEM, SAXS, WAXS, and AFM results suggest that the coil block fraction significantly affects the chain packing of the P3HT block and depresses its crystallinity. The optical absorption spectra indicate that the intramolecular charge transfer between the main chain P3HT donor and the side chain POXD acceptor is relatively weak and the level of order of P3HT chains is reduced by the incorporation of the POXD acceptor. The field effect transistor (FET) hole mobility of the system exhibits a similar trend on the optical properties, which are also decreased with the reduced ordered P3HT crystallinity. The low‐lying highest occupied molecular orbital (HOMO) energy level (–6.08 eV) of POXD is employed as charge trap for the electrical switching memory devices. P3HT‐ b ‐POXD exhibits a non‐volatile bistable memory or insulator behavior depending on the P3HT / POXD block ratio and the resulting morphology. The ITO/ P3HT₄₄ ‐b‐ POXD₁₈ /Al memory device shows a non‐volatile switching characteristic with negative differential resistance (NDR) effect due to the charge trapped POXD block. These experimental results provide the new strategies for the design of donor‐acceptor rod‐coil block copolymers for controlling morphology and physical properties as well as advanced memory device applications.     

Amphiphilic semiconducting copolymer as compatibility layer for printing polyelectrolyte-gated OFETs

We report a method for inkjet-printing an organic semiconductor layer on top of the electrolyte insulator layer in polyelectrolyte-gated OFETs by using a surface modification treatment to overcome the underlying wettability problem at this interface. The method includes depositing an amphiphilic diblock copolymer (P3HT-b-PDMAEMA). This material is designed to have one set of blocks that mimics the hydrophobic properties of the semiconductor (poly(3-hexylthiophene) or P3HT), while the other set of blocks include polar components that improve adhesion to the polyelectrolyte insulator. Contact angle measurements, atomic force microscopy, and X-ray photoelectron spectroscopy confirm formation of the desired surface modification film. Successful inkjet printing of a smooth semiconductor layer allows us to manufacture complete transistor structures that exhibit low-voltage operation in the range of 1 V.     

Processing and doping of thick polymer active layers for flexible organic thermoelectric modules

While the majority of research on organic thermoelectric generators has focused on individual devices with organic films having thicknesses of several hundred nanometers (nano-films), films with micrometer-scale thicknesses (micro-films) provide a longer thermal conduction path that results in a larger temperature gradient and higher thermoelectric voltages in modules. In this study, the properties of solution-processed nano- and micro-films of the p-type semiconductor P3HT doped with two different dopants, F₄-TCNQ and Fe³⁺-tos₃·6H₂O, were investigated. While doping with F₄-TCNQ resulted in high electrical conductivity only in nano-films, doping with Fe³⁺-tos₃·6H₂O from a 25 mM solution yielded power factors of up to ∼30 μWm⁻¹ K⁻² with a conductivity of 55.4 Scm⁻¹ in micro-films. Changes in the molecular packing were compared based on X-ray diffraction, and the best operational stability in air was found for the Fe³⁺-tos₃·6H₂O-doped micro-films. Using Fe³⁺-tos₃·6H₂O as dopant, flexible thermoelectric modules with solution-processed micro-films patterned by a photo-etching technique that does not require alignment and assembly of individual devices were demonstrated, exhibiting a maximum power output of 1.94 nWK⁻² for a uni-leg module with 48 elements. Analysis of the flexible module performance showed that the performance is limited by the contact resistance, which must be taken into consideration when optimizing module structure.     

The Effects of Crystallinity on Charge Transport and the Structure of Sequentially Processed F4TCNQ‐Doped Conjugated Polymer Films

The properties of molecularly doped films of conjugated polymers are explored as the crystallinity of the polymer is systematically varied. Solution sequential processing (SqP) was used to introduce 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F₄TCNQ) into poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) while preserving the pristine polymer's degree of crystallinity. X‐ray data suggest that F₄TCNQ anions reside primarily in the amorphous regions of the film as well as in the P3HT lamellae between the side chains, but do not π‐stack within the polymer crystallites. Optical spectroscopy shows that the polaron absorption redshifts with increasing polymer crystallinity and increases in cross section. Theoretical modeling suggests that the polaron spectrum is inhomogeneously broadened by the presence of the anions, which reside on average 6–8 Å from the polymer backbone. Electrical measurements show that the conductivity of P3HT films doped by F₄TCNQ via SqP can be improved by increasing the polymer crystallinity. AC magnetic field Hall measurements show that the increased conductivity results from improved mobility of the carriers with increasing crystallinity, reaching over 0.1 cm² V^?1 s^?1 in the most crystalline P3HT samples. Temperature‐dependent conductivity measurements show that polaron mobility in SqP‐doped P3HT is still dominated by hopping transport, but that more crystalline samples are on the edge of a transition to diffusive transport at room temperature.