Effect of Al doping on the conductivity type inversion and electro-optical properties of SnO2 thin films synthesized by sol-gel technique

Al doped SnO₂ thin films have been synthesized by a sol-gel dip coating technique with different percentages of Al on glass and silicon substrates. X-ray diffraction studies confirmed the proper phase formation in the films and atomic percentage of aluminium doping in the films was obtained by energy dispersive X-ray studies. SEM studies showed the particle sizes lying in the range 100–150 nm for the undoped films and it decreased with increase of Al doping. Optical transmittance spectra of the films showed high transparency (∼80%) in the visible region and the transparency increases with the increase of Al doping in the films. The direct allowed bandgap of the films have been measured for different Al concentration and they lie within the range of 3.87–4.21 eV. FTIR studies depicted the presence of Sn–O, Al–O, bonding within the films. The room temperature electrical conductivities of the films are obtained in the range of 0.21 S cm⁻¹ to 1.36 S cm⁻¹ for variation of Al doping in the films 2.31–18.56%. Room temperature Seebeck coefficients, S_RT of the films were found in the range +56.0 μVK⁻¹ to −23.3 μVK⁻¹ for variation of Al doping in the films 18.56–8.16%. It is observed that the Seebeck coefficient changes its sign at 12.05% of Al in the films indicating that below 12.05% of Al doping, SnO₂:Al behaves as an n-type material and above this percentage it is a p-type material.     

Electrochemical Treatment for Effectively Tuning Thermoelectric Properties of Free‐Standing Poly(3‐methylthiophene) Films

The degree of oxidation of conducting polymers has great influence on their thermoelectric properties. Free‐standing poly(3‐methylthiophene) (P3MeT) films were prepared by electrochemical polymerization in boron trifluoride diethyl etherate, and the fresh films were treated electrochemically with a solution of propylene carbonate/lithium perchlorate as mediator. The conductivity of the resultant P3MeT films depends on the doping level, which is controlled by a constant potential from ?0.5 to 1.4 V. The optimum electrical conductivity (78.9 S cm^?1 at 0.5 V) and a significant increase in the Seebeck coefficient (64.3 μV K^?1 at ?0.5 V) are important for achieving an optimum power factor at an optimal potential. The power factor of electrochemically treated P3MeT films reached its maximum value of 4.03 μW m^?1 K^?2 at 0.5 V. Moreover, after two months, it still exhibited a value of 3.75 μW m^?1 K^?2, and thus was more stable than pristine P3MeT due to exchange of doping ions in films under ambient conditions. This electrochemical treatment is a significant alternative method for optimizing the thermoelectric power factor of conducting polymer films.     

Bismuth Interfacial Doping of Organic Small Molecules for High Performance n‐type Thermoelectric Materials

Development of chemically doped high performance n‐type organic thermoelectric (TE) materials is of vital importance for flexible power generating applications. For the first time, bismuth (Bi) n‐type chemical doping of organic semiconductors is described, enabling high performance TE materials. The Bi interfacial doping of thiophene‐diketopyrrolopyrrole‐based quinoidal (TDPPQ) molecules endows the film with a balanced electrical conductivity of 3.3 S cm^?1 and a Seebeck coefficient of 585 μV K^?1. The newly developed TE material possesses a maximum power factor of 113 μW m^?1 K^?2, which is at the forefront for organic small molecule‐based n‐type TE materials. These studies reveal that fine‐tuning of the heavy metal doping of organic semiconductors opens up a new strategy for exploring high performance organic TE materials.     

Highly Converged Valence Bands and Ultralow Lattice Thermal Conductivity for High‐Performance SnTe Thermoelectrics

A two‐step optimization strategy is used to improve the thermoelectric performance of SnTe via modulating the electronic structure and phonon transport. The electrical transport of self‐compensated SnTe (that is, Sn_1.03Te) was first optimized by Ag doping, which resulted in an optimized carrier concentration. Subsequently, Mn doping in Sn_1.03?xAg_xTe resulted in highly converged valence bands, which improved the Seebeck coefficient. The energy gap between the light and heavy hole bands, i.e. ΔE_v decreases to 0.10 eV in Sn_0.83Ag_0.03Mn_0.17Te compared to the value of 0.35 eV in pristine SnTe. As a result, a high power factor of ca. 24.8 μW cm^?1 K^?2 at 816 K in Sn_0.83Ag_0.03Mn_0.17Te was attained. The lattice thermal conductivity of Sn_0.83Ag_0.03Mn_0.17Te reached to an ultralow value (ca. 0.3 W m^?1 K^?1) at 865 K, owing to the formation of Ag₇Te₄ nanoprecipitates in SnTe matrix. A high thermoelectric figure of merit (z T≈1.45 at 865 K) was obtained in Sn_0.83Ag_0.03Mn_0.17Te.     

Charge transport modulation of silicon nanowire by O2 plasma

The modification of the electrical characteristics of field-effect transistors (FETs) with channels composed of n- or p-type silicon nanowires (SiNWs) by oxygen plasma treatment is investigated in this study. The SiNWs obtained from silicon bulk wafers are <111> surface-oriented and their doping concentrations are 10²¹ and 10¹⁷ cm⁻³ for the n- and p-type SiNWs, respectively. After the back-gate SiNWFETs were subjected to oxygen plasma treatment, the magnitude of the drain current of the n-type SiNWs was decreased, whereas that of the p-type SiNWs was increased, while the gate-dependent characteristics of both of types of SiNWs were improved. The changes in the electrical characteristics are due to the adsorption of oxygen ions on the surface of the SiNWs. To verify the effect of the oxygen ions, the SiNWFETs were kept in a vacuum for 24 h whereupon their electrical characteristics tended to revert to their inherent state.     

Electronic structure and properties of some oligomers based on fluorinated 1H-phospholes: n- versus p-type materials

A series of oligothiophenes and novel oligophospholes, consisting of fluorinated and perfluoroarene-substituted structures, were investigated using density functional theory (DFT) methods. The study focused on the geometrical structures and electronic properties. The degree of π-conjugation in the neutral oligomers was probed by different approaches including analysis of predicted Raman spectra. The character of the charge carrier of the new substituted oligomers, either electron (n-type doping) or hole (p-type doping) transport, was predicted by comparing their properties, including the frontier orbital HOMO and LUMO, excitation, and reorganization energies, with those of their non-substituted parent oligomers. The quantum chemical DFT results are consistent with available experimental data on the oligothiophenes for both geometries and conductivity properties. The results strongly suggest that an effective way of designing new materials with n-type conductivity is to introduce electron-withdrawing groups into the oligomer backbone. Calculated results were subsequently obtained for oligomers based on 1H-phospholes, which are predicted to have potentially useful properties as novel semiconductor materials.     

Synthesis and electrical conductivity of polymers containing trans-1,2-bis(9-carbazolyl)cyclobutane units

Novel poly-Schiff bases (PSB's) that contain trans-1,2-bis(9-carbazolyl)cyclobutane(DCZB) units were synthesized by the direct polycondensation of trans-1,2-bis(3-formyl-9-carbazolyl)cyclobutane with aromatic diamines in n-amyl alcohol at 160°C. Complexation of these PSB's and of poly(vinyl DCZB) (PVDCZB) with iodine produced cation-radical salts which resulted form the transfer of an electron from DCZB moieties to iodine. All the undoped polymers were insulators having electrical conductivity of the order of 10^?10–10^?12 S cm^?1 depending on the structure of polymers. By doping with iodine, the electrical conductivity increased by several orders of magnitude and reached a value of 10^?3 S cm^?1 in the case of PVDCZB and 10^?5–10^?6 S cm^?1 in the case of PSB's. The electrical conductivity of doped PSB's increased with decreasing diamine length. PVDCZB having the same iodine content per carbazole unit as poly(9-vinyl-carbazole) (PVK) has a greater electrical conductivity than PVK.     

Highly enhanced thermoelectric performance of WS2 nanosheets upon embedding PEDOT:PSS

Two‐dimensional (2D) WS₂ nanosheets (NSs) as a promising thermoelectric (TE) material have gained great concern recently. The low electrical conductivity significantly limits its further development. Herein, we reported an effective method to enhance the TE performance of WS₂ NSs by combining poly(3,4‐ethylenedioxythiophene):poly(4‐styrenesulfonate) (PEDOT:PSS). The restacked WS₂ NSs thin film with 1T phase structure obtained by a common chemical lithium intercalation show a high Seebeck coefficient of 98 μV K^?1 and a poor electrical conductivity of 12.5 S cm^?1. The introduction of PEDOT:PSS with different contents obviously improve the electrical conductivity of WS₂ NSs thin films. Although a declining Seebeck coefficient was observed, an optimized TE power factor of 45.2 μW m^?1 k^?1 was achieved for WS₂/PEDOT:PSS composite thin film. Moreover, the as‐prepared WS₂/PEDOT:PSS thin film can be easily peeled off and transferred to other substrate leading to a more promising application. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55 , 997–1004     

An effective approach to enhanced thermoelectric properties of PEDOT:PSS films by a DES post‐treatment

As conventional organic solvents present inherent toxicity, deep eutectic solvents (DES) have been considered as excellent candidates due to their green characteristics. In this work, thermoelectric properties enhancement of PEDOT:PSS films is achieved by introducing DES as an additive and post‐treatment reagent. Direct addition and post‐treatment approaches lead to a maximum Seebeck coefficient of 29.1 μV K^?1 and electrical conductivity of 620.6 S cm^?1, respectively. In addition, an optimal power factor is obtained by DES post‐treatment, reaching up to 24.08 μW m^?1 K^?2, which is approximately four orders of magnitude higher than the pure PEDOT:PSS. Assuming a thermal conductivity of 0.17 W m^?1 K^?1, the maximum ZT value is estimated to be 0.042 at 300 K. Further, atomic force microscopy and X‐ray photoelectron spectroscopy are performed and suggest that the remarkably enhanced electrical conductivity originates from the removal of the excess insulating PSS and the phase separation between the PEDOT and PSS chains. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 885–892     

Doping Effects in CMOS-compatible CoSi Thin Films for Thermoelectric and Sensor Applications

Abstract . We report on semi-metallic cobalt monosilicide (CoSi) as a CMOS-compatible thermoelectric (TE) material and discuss the effect of n- and p-type dopants on its transport properties. Thin films of CoSi are developed using chemical vapor deposition tools and subsequent rapid thermal processing. Film properties such as microstructure, crystallinity and elemental distribution are studied via electron microscopy, X-ray diffraction and time-of-flight secondary ion mass spectroscopy. Doping silicon with boron prior to silicidation impedes the Co-Si diffusion process, while phosphorus atoms distribute uniformly in silicides with no voids or agglomerations. CoSi makes a suitable n-type TE candidate and provides an alternative to Si or SiGe materials. Transport properties of undoped CoSi exhibit a linear dependence within the investigated temperature window, whereas dopants in CoSi increase the number of electron carriers that contribute to charge transport and thereby influence the Seebeck coefficient. Thus, TE characteristics of thin CoSi films can be tuned via (i) the type of dopants used and/or (ii) varying the residual silicon thickness post silicidation.     

Air‐Stable n‐Type Thermoelectric Materials Enabled by Organic Diradicaloids

Air‐stable n‐type thermoelectric materials are recognized as an important and challenging topic in organic thermoelectrics (OTEs) because conventional n‐type OTE materials prepared by chemical doping are highly volatile upon exposure to air. Besides, doping efficiency and microstructure are hard to control with the incorporation of external dopants. We report herein the design and synthesis of unconventional n‐type OTE materials based on the diradicaloids 2DQQT‐S and 2DQQT‐Se, which are proved to be neutral single‐component organic conductors that exhibit an unprecedented air stability. Without external n‐doping, a pristine film of 2DQQT‐Se shows an electrical conductivity as high as 0.29 S cm^?1 delivering a power factor of 1.4 μW m^?1 K^?2. Under ambient conditions, no decay in electrical conductivity is observed for over 260 hours. This work demonstrates that diradicaloids are promising candidates for air‐stable and high‐performance OTE materials.     

Novel conjugated polymer containing anthracene backbone: Addition polymer of 9, 10-diethynylanthracene with 9, 10-anthracenedithiol and its properties

9,10-Diethynylanthracene was prepared by the alkaline hydrolysis of 9,10-bis (trimethylsilylethynyl) anthracene. Another new monomer of 9, 10-anthracenedithiol was prepared by the reduction of anthracene polydisulfide. A crystalline conjugated polymer of 9,10-diethynylanthracene with 9,10-anthracenedithiol was synthesized in a THF solution at 50°C by UV irradiation or by using radical initiators. The molecular weight (M?_n) of the insoluble polymer in THF is about 20000–30000 and the soluble is about 4000. From the sulfur content and IR spectrum of the insoluble polymer, it is realized that the obtained polymer has the alternating structure consisting of 9,10-diethynylanthracene and 9,10-anthracenedithiol units. X-ray pattern indicated that the polymer has a layer structure. The conductivity of the undoped polymer was about 10^?11S/cm, but enhanced up to 10^?6 S/cm by doping with iodine. The enhancement of the conductivity seems to be the existence of the CT complex among the polymer backbone and iodine or iodine anion.     

Effect of Ligands on Formation of Photosensitive Oxide Layers on Copper Electrode*

Photosensitive oxide layers are found to develop on copper electrode exposed to solutions containing Cu(II), different ligands, and K₂SO₄ as a supporting electrolyte. Two mechanisms of Cu₂O formation are discussed: corrosion of copper in naturally aerated Cu(II)-free solutions, and interaction between Cu and Cu²⁺ yielding intermediate Cu⁺ ions. Oxide layers formed in the supporting electrolyte at pH 5 and 7 exhibit n-type conduction; the n–p transition is observed at pH 10. An addition of ligands suppresses the oxide formation. The correlation between the photoelectrochemical effects and the stability of Cu(II) complexes is revealed: the higher the complexation degree, the lower the level of photoresponse. A model of nonuniform Cu₂O-containing layer with predominant n- and p-type properties at copper/oxide and oxide/solution interfaces, respectively, is discussed.     

Chemical Bonding Tuned Lattice Anharmonicity Leads to a High Thermoelectric Performance in Cubic AgSnSbTe3

Comprehension of chemical bonding and its intertwined relation with charge carriers and heat propagation through a crystal lattice is imperative to design compounds for thermoelectric energy conversion. Here, we report the synthesis of large single crystal of new p-type cubic AgSnSbTe₃ which shows an innately ultra-low lattice thermal conductivity (κ_lat) of 0.47–0.27 Wm⁻¹ K⁻¹ and a high electrical conductivity (1238 – 800 S cm⁻¹) in the temperature range 294–723 K. We investigated the origin of the low κ_lat by analysing the nature of the chemical bonding and its crystal structure. The interaction between Sn(5 s)/Ag(4d) and Te(5p) orbitals was found to generate antibonding states just below the Fermi level in the electronic band structure, resulting in a softening of the lattice in AgSnSbTe₃. Furthermore, the compound exhibits metavalent bonding which provides highly polarizable bonds with a strong lattice anharmonicity while maintaining the superior electrical conductivity. The electronic band structure exhibits nearly degenerate valence-band maxima that help to achieve a high Seebeck coefficient throughout the measured temperature range and, as a result, the maximum thermoelectric figure of merit reaches to ≈1.2 at 661 K in pristine single crystal of AgSnSbTe₃.     

Concerted Rattling in CsAg5Te3 Leading to Ultralow Thermal Conductivity and High Thermoelectric Performance

Thermoelectric (TE) materials convert heat energy directly into electricity, and introducing new materials with high conversion efficiency is a great challenge because of the rare combination of interdependent electrical and thermal transport properties required to be present in a single material. The TE efficiency is defined by the figure of merit ZT=(S²σ) T/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the total thermal conductivity, and T is the absolute temperature. A new p‐type thermoelectric material, CsAg₅Te₃, is presented that exhibits ultralow lattice thermal conductivity (ca. 0.18 Wm^?1 K^?1) and a high figure of merit of about 1.5 at 727 K. The lattice thermal conductivity is the lowest among state‐of‐the‐art thermoelectrics; it is attributed to a previously unrecognized phonon scattering mechanism that involves the concerted rattling of a group of Ag ions that strongly raises the Grüneisen parameters of the material.     

Electric conductivity of TCNQ salts of ionene polymers containing triethylenediammonium or 4,4′-bipyridilium ring

The ionene polymers were prepared by the Menshutkin reaction of α,ω-dibromoalkane (n) with triethylenediamine (TDA) or 4,4′-bipyridil (BP). Resistivities (p) and activation energies of conduction (E_a) were measured for the polymeric 7,7,8,8-tetracyanoquinodimethan (TCNQ) salts with these ionenes. The correlation between the chemical structure of the ionenes and the conductivity was discussed. In the TDA,n-TCNQ complex salts and the BP,n-TCNQ simple salts the salts of the ionenes containing even numbers of CH₂ groups showed higher conductivities than those of the ionenes containing odd numbers of CH₂ groups. The conductivities determined by the narrower interval between the N⁺ cations of the main chains were measured in the simple salts. In the complex salts the conductivities determined by the larger interval were measured. The conformational change of the matrix ionenes affected the arrangement of the TCNQ molecules. The values of p were 79.7 and 12.5 Ω cm, and the values of E_a were 0.122 and 0.063 eV for TDA,4-TCNQ complex salt and BP,5-TCNQ complex salt, respectively.     

Enhanced thermoelectric performance of hydrothermally synthesized polycrystalline Te-doped SnSe

In this study,large-scale Te-doped polycrystalline SnSe nanopowders were synthesized by a facile hydrothermal approach and the effect of Te doping on the thermoelectric properties of SnSe was fully investigated.It is found that the carrier concentration increases due to the reduction of band gap by alloying with Te,which contributes to significant enhancement of electrical conductivity especially at room temperature.Combined with the moderated Seebeck coefficient,a high power factor of 4.59μW cm ~1 K ~2 is obtained at 773 K.Furthermore,the lattice the rmal conductivity is greatly reduced upon Te substitution owing to the atomic point defect scattering.Benefiting from the synergistically optimized both electrical-and thermal-transport properties by Te-doping,thermoelectric performance of polycrystalline SnSe is enhanced in the whole temperature range with a maximum ZT of-0.79 at a relatively low temperature(773 K) for SnSe_(0.85)Te_(0.15).This study provides a low-cost and simple lowtemperature method to mass production of SnSe with high thermoelectric performance for practical applications     

Etude structurale de la bipyrimidine par RMN en phase nématique

The 240 MHz NMR spectra of bipyrimidine in the nematic phase of p-methoxybenzylidene-p-n-butylaniline is analysed. The para H,H inter-ring distance is determined. The barrier to internal rotation of the inter-ring C? C bond is studied. Excellent agreement between experimental and calculated spectra is obtained using potential function V(?) = ½ Σ_NV_n (1 – cosn ?) with V₁ = V₂ = V₃ = 0 cal mol^?1 and V₄ = 500 cal mol^?1.     

Oxygen effects on the interfacial electronic structure of titanyl phthalocyanine film: p-Type doping,band bending and Fermi level alignment

The effect of oxygen doping on titanyl phthalocyanine (TiOPc) film was investigated by ultraviolet photoelectron spectroscopy (UPS). The electronic structure of the interface formed between TiOPc films deposited on highly oriented pyrolytic graphite (HOPG) was clearly different between the films prepared in ultrahigh vacuum (UHV) and under O₂ atmosphere (1.3 × 10⁻² Pa). The film deposited in UHV showed downward band bending characteristic of n-type semiconductor, possibly due to residual impurities working as unintentional n-type dopants. On the other hand, the film deposited under O₂ atmosphere showed upward band bending characteristic of p-type semiconductor. Such trends, including the conversion from n- to p-type, are in excellent correspondence with reported field effect transistor characteristics of TiOPc, and clearly demonstrates that bulk TiOPc film was p-doped with oxygen. In order to examine the Fermi level alignment between TiOPc film and the substrate, the energy of the highest occupied molecular orbital (HOMO) of TiOPc relative to the Fermi level of the conductive substrate was determined for various substrates. The alignment between the Fermi level of conductive substrate and Fermi level of TiOPc film at fixed energy in the bandgap was not observed for the TiOPc film prepared in UHV, possibly because of insufficient charge density in the TiOPc film. This situation was drastically changed when the TiOPc film exposed to O₂, and clear alignment of the Fermi level fixed at 0.6 eV above the HOMO with the Fermi level of the conducting substrate was observed, probably by p-type doping effect of oxygen. These are the first direct and quantitative information about bulk oxygen doping from the viewpoint of the electronic structure. These results suggest that similar band bending with Fermi level alignment may be also achieved for other organic semiconductors under practical device conditions, and also call for caution at the comparison of experimental results obtained under UHV and ambient atmosphere.     

Résonance Magnétique Nucléaire en phase nématique. Exploration de la barrière d'empêchement à la libre rotation de la monofluoroacétone

The NMR spectra of monochloro-, monobromo- and monofluoroacetone (CH₃? CO? CH₂X with X = Cl, Br, F) oriented in a nematic phase have been measured and the direct dipolar coupling constants determined. The barrier to internal rotation for the CH₂F group has been studied for fluorine compound using various hypotheses. The best agreement with IR data has been obtained using the potential equation V(θ) = Σ_n V_n × (1 – cos nθ)/2 and a Boltzmann distribution of the CH₂F group (V₁ = 250 ± 50 cal.mol^?1, V₂ = 1650 ± 100 cal.mol^?1, V₃ = ?1000 ± 100 cal.mol^?1).