聚(3-己基)噻吩薄膜中电荷传导的研究

对两种具有相同化学结构的聚(3-己基)噻吩膜进行了电荷传导研究以检验膜的结构对载流子迁移率的影响. 一种膜是由3-己基噻吩单体经电化学合成直接制备的膜(原位生长膜); 另一种膜是将原位生长膜溶于三氯甲烷后重新滴涂而成的(滴涂膜). 研究表明, 虽然两种膜的制备方法不一样, 但在最低(0.02%)和较高(20%～30%)掺杂率下两膜中的载流子迁移率相一致; 然而在中等掺杂率区域, 两膜中的载流子迁移率明显不同. 对于原位生长膜, 载流子迁移率在低掺杂区域几乎保持不变, 当掺杂率大于1%后开始上升; 而在滴涂膜中, 随着掺杂率的增加, 迁移率先下降然后迅速升高. 上述两种迁移率变化特征分别与以前研究中观察到的电化学合成高分子膜和化学合成高分子旋涂或滴涂膜中迁移率的变化特征相一致, 表明了迁移率随掺杂率变化特征的改变是由膜的结构变化而引起的     

一系列单硅烷-寡聚噻吩共聚高分子膜中电荷传导研究

研究了一系列由单硅烷和寡聚噻吩组成的共聚高分子膜(PSnT, n表示寡聚噻吩单元中噻吩环的个数)在较宽掺杂率范围内载流子的迁移率变化规律. 结果表明, 掺杂率极低(<0.2%)时各膜中的载流子迁移率接近, 几乎不受n的影响; 随着膜的掺杂率的增加, 各PSnT膜中的迁移率相继增大, n增大, 迁移率在更低的掺杂率处开始增大, 其增幅随着n的增加而增大. PS14T迁移率的增幅超过4个数量级, 已与电化学合成的聚噻吩膜中观察到的迁移率增幅相当, 表明此共聚物中的π-共轭长度已足以再现聚噻吩传导性能.     

In-situ Apparent Mobility of Charge Carriers in Polyaniline Films Measured with a New Four-band Electrode

The in‐situ apparent mobility in polyaniline films was accurately measured in a wide doping region using a new four‐band electrode. It was found the apparent mobility in polyaniline films rises with increasing the doping level or carrier density. The influence of film thickness on the conductivity and apparent mobility of charge carriers was also investigated. The relative higher conductivity observed in a thinner film under low and intermediate doping potentials is assigned to the higher inter‐chain mobility related to the more ordered structure of the film. The mobility variations provide experimental evidence to confirm the inter‐chain path for hopping transport of polarons and the intra‐chain path for evolution of metallic conduction.     

Photoinduced charge carrier generation in a poly(3-hexylthiophene) and methanofullerene bulk heterojunction investigated by time-resolved terahertz spectroscopy

We report on the ultrafast photoinduced charge separation processes in varying compositions of poly(3-hexylthiophene) (P3HT) blended with the electron acceptor [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Through the use of time-resolved terahertz spectroscopy, the time- and frequency-dependent complex photoconductivity is measured for samples with PCBM weight fractions (WPCBM) of 0, 0.2, 0.5, and 0.8. By analysis of the frequency-dependent complex conductivity, both the charge carrier yield and the average charge carrier mobility have been determined analytically and indicate a short (<0.2 nm) carrier mean free path and a suppressed long-range transport that is characteristic of carrier localization. Studies on pure films of P3HT demonstrate that charge carrier generation is an intrinsic feature of the polymer that occurs on the time scale of the excitation light, and this is attributed to the dissociation of bound polaron pairs that reside on adjacent polymer chains due to interchain charge transfer. Both interchain and interfacial charge transfer contribute to the measured photoconductivity from the blended samples; interfacial charge transfer increases as a function of increasing PCBM. The addition of PCBM to the polymer films surprisingly does not dramatically increase the production of charge carriers within the first 2 ps. However, charge carriers in the 0.2 and 0.5 blended films survive to much longer times than those in the P3HT and 0.8 films.     

Intrachain versus Interchain Electron Transport in Poly(fluorene‐alt‐benzothiadiazole): A Quantum‐Chemical Insight

Poly(9,9‐di‐n‐octylfluorene‐alt‐benzothiadiazole) [F8BT], displays very different charge‐transport properties for holes versus electrons when comparing annealed and pristine thin films and transport parallel (intrachain) and perpendicular (interchain) to the polymer axes. The present theoretical contribution focuses on the electron‐transport properties of F8BT chains and compares the efficiency of intrachain versus interchain transport in the hopping regime. The theoretical results rationalize significantly lowered electron mobility in annealed F8BT thin films and the smaller mobility anisotropy (μ_∥/μ_⊥) measured for electrons in aligned films (i.e. 5–7 compared to 10–15 for holes).     

Charge transport properties of MDMO PPV thin films cast in different solvents

Charge transport properties in thin films of Poly(2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylenevinylene) (MDMO PPV) cast using either chloroform (CF), toluene (TOL), or chlorobenzene (CB) as solvent were investigated. Hole mobility (μ) in these thin films measured using time‐of‐flight transient photoconductivity showed an increasing trend with respect to the solvent used in the same order, that is, μ_CF (2.4 × 10^?7 cm²/Vs) < μ_TOL (6.9 × 10^?7 cm²/Vs) < μ_CB (2.3 × 10^?6 cm²/Vs). Observed variations in mobilities were attributed to different morphologies of MDMO PPV chains in thin films cast using the aforesaid solvents. Nature of the interchain interactions and aggregate formation were obtained using photoluminescence (PL), Raman spectroscopy, and AFM studies. Ratio of PL peak intensities of 0–0 and 0–1 transitions, which is a direct measure of interchain interaction, was the highest in CB and lowest in CF. Variation in the relative intensities of out‐of‐plane wagging of vinylene group (～963 cm^?1 mode) in Raman spectra suggested different extent of coiling of polymer chains in these thin films. From these observations, it was elicited that aggregate size and interchain interactions are highest in CB and least in CF. AFM‐based topographic images of thin films further supported these variations in the size of aggregates. Variation in the aggregate sizes and interchain interactions explained the corresponding variation in the mobility. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1431–1439     

Influence of extended π-conjugation units on carrier mobilities in conducting polymers

Carrier mobilities in thin films of copolymers with repeat units consisting of oligothiophenes bridged by Si atoms are measured over a range of doping levels, where the numbers of thienylenes in the repeat unit are 7, 8, 10, 12, and 14. The mobilities for these polymer films increased with the increase in doping level and the mobility enhancement followed an increasing order of the π-conjugation length. The magnitude of the mobility increase for the Si polymer comprising 14 thiophene units reached ca. 10⁴, implying that this π-conjugation length is almost sufficient to reproduce transport properties of polythiophenes.     

Temperature dependence of molecular dynamics and supramolecular aggregation in MEH-PPV films: A solid-state NMR, X-ray and fluorescence spectroscopy study

This article presents an investigation of the temperature induced modification in the microstructure and dynamics of poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) cast films using Wide-Angle X-ray Scattering (WAXS), solid-state Nuclear Magnetic Resonance (NMR), and Fluorescence Spectroscopy (PL). MEH-PPV chain motions were characterized as a function of temperature by NMR. The results indicated that the solvent used to cast the films influences the activation energy of the side-chain motions. This was concluded from the comparison of the activation energy of the toluene cast film, E_a = (54 ± 8) kJ/mol, and chloroform cast film, E_a = (69 ± 5) kJ/mol, and could be attributed to the higher side-chain packing provided by chloroform, that preferentially solvates the side chain in contrast to toluene that solvates mainly the backbone. Concerning the backbone mobility, it was observed that the torsional motions in the MEH-PPV have average amplitude of ∼10° at 300 K, which was found to be independent of the solvent used to cast the films. In order to correlate the molecular dynamics processes with the changes in the microstructure of the polymer, in situ WAXS experiments as a function of temperature were performed and revealed that the interchain spacing in the MEH-PPV molecular aggregates increases as a function of temperature, particularly at temperatures where molecular relaxations occur. It was also observed that the WAXS peak associated with the bilayer spacing becomes narrower and its intensity increases whereas the peak associated with the interbackbone planes reduces its intensity for higher temperatures. This last result could be interpreted as a decrease in the number of aggregates and the reduction of the interchain species during the MEH-PPV relaxation processes. These WAXS results were correlated with PL spectra modifications observed upon temperature treatments.     

Structural transitions of nanocrystalline domains in regioregular poly(3‐hexyl thiophene) thin films

The effects of solution processing and thermal annealing on thin film morphology and crystalline structures of regioregular poly(3‐hexyl thiophene) (RR P3HT) are studied in terms of molecular weight (M_w). Using grazing‐incidence X‐ray diffraction, π‐conjugated planes in drop‐cast films from chloroform solutions are found to be preferentially oriented parallel to the substrates regardless of M_w. However, the mesoscale nanocrystalline morphology of the drop‐cast films is significantly affected by M_w, exhibiting a distinctive morphological transition from short nanorods to long nanofibrils above a critical number‐averaged M_w (～ 3.6 kDa). This is probably due to the change in a conformation change from an extended‐chain to a folded‐chain, as M_w of RR P3HT increases. In contrast, spin‐casting of high M_w RR P3HT produces less ordered films with a lower crystallinity and mixed parallel/perpendicular orientations of π‐conjugated planes. The crystallinity and parallel π‐conjugated orientation of RR P3HT in spin‐cast films could be improved by thermal treatments at high‐temperatures either (1) above the glass transition temperature or (2) above the melting temperature of RR 3PHT followed by recrystallization upon cooling under vacuum. However, the charge mobility of the spin‐cast films for a field‐effect transistor application is still lower than that of the drop‐cast films. This would be attributed to the chain oxidation and the development of distinct grain boundaries between RR P3HT nanofibrils during the thermal treatments. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1303–1312, 2007     

Comparative study of guest charge-charge interactions within silica sol-gel

We investigated the effect of charge-charge interactions on the mobilities of rhodamine 6G (R6G), Nile Red, sulforhodamine B, and Oregon Green 514 (ORG) guest molecules within a silica sol-gel host as the guest charge progressed from positive to neutral to negative. Through classification of the mobility as fixed, tumbling, or intermediate behavior, we were able to distinguish differences in surface attraction as the guest charge was varied. On the basis of our results, an attractive charge (as tested by cationic R6G) does not contribute significantly to mobility within dry films. However, an increase in the cationic influence is observed in water-equilibrated environments. A comparison of ORG in dry and water- and phosphate-buffer-equilibrated films indicates that charge repulsion does significantly increase dye rotational mobility (to a maximum of 24 +/- 3% tumbling molecules). However, in view of the percentage of tumbling molecules found, charge-charge interactions do not appear to be the dominant force controlling guest mobility.     

Correlation of charge transport with structural order in highly ordered melt‐crystallized poly(3‐hexylthiophene) thin films

A series of well‐defined poly(3‐hexylthiophene)s (P3HT) of different molecular weight (MW) and high regioregularity was investigated for charge transport properties in as‐cast and melt‐crystallized films. The semicrystalline structure of the P3HT was characterized by X‐ray scattering and Atomic force microscopy. Crystallization by cooling from the melt led to a substantial increase in crystallinity and a stronger alignment of the crystals in comparison to as‐cast films. The increase in crystallinity went along with an increase in hole mobility of up to an order of magnitude as measured by the space charge limited current method. Additionally, the hole mobility depended on the long period of P3HT lamellae and consequently on the MW. In compliance with the long period, the charge carrier mobility first increased with the MW before decreasing again at the onset of chain folding. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 943–951     

Redox potential of bacteriorhodopsin in oriented purple membrane films

The redox potentials of bacteriorhodopsin (BR) in oriented purple membrane (PM) films formed by the electrophoretic sedimentation (EPS) technique and the Langmuir–Blodgett (LB) technique were measured. The PM LB film and PM EPS film formed at 0.4 V versus SCE electrode potential exhibited two redox potentials at −0.48 and −0.76 V for the cis- and trans-isomers of the retinal Schiff base in the BR, respectively. The reduction current of BR in the PM LB film was greater than that in the PM EPS film and the reduction current of BR in the PM cast film was ca. 70% smaller values of the PM LB film. The results indicate that the electron transfer was affected by a difference in the orientation of PM patches in the films.     

Enhanced mobility and increased gas sorption capacity in thin film and nanoconduit confined polymers

Poly(tert butyl acrylate) (PTBA) is found to exhibit enhanced mobility when spun cast into thin films or impregnated into cylindrical anodic aluminum oxide (AAO) nanoscale pores. In a thin film configuration, the glass transition temperature of 20 nm thick PTBA is found to decrease almost 20 °C compared to the bulk. Consistent with this mobility increase, an increased volume fraction of interphase polymer leads to at least a 2.4 times viscosity reduction when PTBA is impregnated in 100 nm pores versus 200 nm pores. Such increases in mobility result in a 15‐fold increase in CO₂ permeability for an AAO confined geometry compared to a bulk film. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 434–441, 2010     

Structural and optoelectrical properties of nanocrystalline Gd-doped CdO films prepared by sol gel method

Gd-doped CdO thin films with various Gd concentrations have been prepared on glass and Si wafer substrates using sol gel technique. The films were characterised by X-ray fluorescence (XRF), X-ray diffraction (XRD), optical absorption spectroscopy, and dc-electrical measurements. XRF method was used to determine the %Gd content in the films while XRD was used to study the influence of Gd doping on the detailed crystalline structure. Experimental data indicate that Gd³⁺ doping with level of less than 2.4% slightly enlarge the CdO crystalline unit cell. The bandgap (E _g) of Gd-doped CdO suffers narrowing by about 13% due to a small (0.2%) doping level but with %Gd doping level larger than 2.4%, E _g becomes wider than that of undoped CdO. The electrical behaviours of the Gd-doped CdO films show that they are degenerate semiconductors. The 2% Gd-doped CdO film shows increase in its mobility by about 92%, conductivity by 320%, and carrier concentration by 127%, relative to undoped CdO film. From transparent-conducting-oxide point of view, the Gd doping of CdO by sol gel method is not effective. Finally, the absorption in the NIR spectral region was investigated to be due to the free electrons.     

Effect of chain aggregation on the conductivity and ESR spectra of polyaniline

Solutions of polyaniline in m-cresol with and without camphorsulfonic acid (CSA), as well as films cast from these solutions were studied by ESR spectroscopy at 133–423 K and by optical spectroscopy in the range λ = 350–1100 nm. An analysis of the optical and ESR spectra shows that in the solutions and films without CSA polyaniline is fully doped but the conductivity of these films is low (∼10⁻⁸ S cm⁻¹; cf. 100 S cm⁻¹ for the films with CSA). Compared with the CSA-containing samples, the samples without CSA are characterized by broader ESR lines and higher contribution of the Curie spins to the magnetic susceptibility. These facts indicate a weak aggregation of polyaniline chains without CSA, which leads to low conductivity. A formula was proposed, which describes the temperature dependence of the polyaniline ESR linewidth and allows the interchain distance and the mobility of electrons moving along polymer chains to be determined. The conductivity of polyaniline films is affected by moderate heating (363–388 K) of the films and solutions from which the films were cast. It was found that the interchain distances correlate with the conductivity of the films and with the broadening of their ESR lines caused by the effect of O₂. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2701–2711, December, 2005.     

Polythiophenes via thiophene, bithiophene and terthiophene in propylene carbonate: an electrochemical and in-situ FTIR study

Polythiophene films obtained by the galvanostatic polymerization of thiophene (Th), bithiophene (BTh) and terthiophene (TTh) in 0.5 M LiClO₄ + propylene carbonate have been investigated by cyclic voltammetry and in-situ Fourier transform IR (FTIR) spectroscopy. The polymerization potential in the propylene carbonate electrolyte decreases in the order Th BTh TTh. Both the charge capacity and the doping level of the resulting polymers increase in the order polythiophene (PTh) < polybithiophene (PBTH) polyterthiophene (PTTh). For PTTh, a doping level of 37% is obtained during cycle 10 and 31% during cycle 1000.In-situ FTIR spectra of the neutral PBTh and PTTh films show a single band at 3063 cm⁻¹ (corresponding to aromatic β-(C---H)-groups) which suggests a regular α,α′-linking of the monomeric units. For thin PTh films the intensity of this band is very weak, indicating a disordered cross-linked polymer structure. During the electrochemical oxidation of the three polythiophenes investigated from 3.3 V up to 4.4 V vs. Li/Li⁺ the intensity of the electronic absorption band (above 2000 cm⁻¹) increases with increasing potential, but it strongly decreases at potentials above about 4.4 V, indicating a strong irreversible oxidation of the polymer films. Furthermore, strong CO₂ evolution is observed at potentials above 4.2 V.     

Role of mesoscopic molecular organization in organic-based thin film transistors

Organic-based thin film transistors have been realized from various organic conjugated materials, which can be gathered into two categories, according to the mechanism describing charge transport. In conjugated polymers and amorphous materials, occurrence of a variable range hopping mechanism leads to a direct relationship between doping level, conductivity and carrier mobility, which explains the difficulty for achieving materials possessing, at the same time, a high mobility and a low conductivity. On the other hand, the trap-limited mechanism of charge transport in conjugated oligomers allows a distinct control of carrier mobility and conductivity. Carrier mobility in thin films of conjugated oligomers can be increased by lowering the concentration of grain boundaries, which can be readily achieved by imposing long range structural ordering of oligomer molecules. Thin films of oligomer with a liquid crystal-like structure have thus been realized, using a self-assembly approach, which presents a mobility close to that of a single crystal of this oligomer. On the other hand, conductivity of these oligomers can be decreased by controlling the purity of these materials. High mobility and low conductivity values can thus be achieved with conjugated oligomers, allowing the realization of organic thin-film transistors presenting characteristics close to those of amorphous-silicon based ones.     

Enhanced electrochemical response of solution‐deposited n‐doping polymer via cocasting with ionic liquid

A novel cocasting approach is presented for improving electroactivity of solution‐cast films of conducting polymers. Solutions of the n‐doping polymer poly(benzimidazobenzophenanthroline) (BBL) were co‐deposited with the ionic liquid electrolyte 1‐ethyl‐3‐methyl‐imidazolium bis(trifluoromethylsulfonyl)imide (EMIBTI). The resultant co‐continuous mixture yielded highly porous polymer films (CC‐BBL) upon removal of solvent and EMIBTI. Electrochemical quartz crystal microgravimetry revealed that the n‐doping process in neat ionic liquid is anion‐dominant, which is contrary to what is observed in dilute electrolyte solutions. The CC‐BBL films exhibit a thirty‐fold increase in initial current response and capacity relative to non‐cocast BBL films. While current response and capacity of the non‐cocast BBL improve with cycling, they level out after 800 cycles at 35% of those of the CC‐BBL. CC‐BBL shows high n‐doping stability; no decrease in electroactivity is seen after 1000 cycles. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Molecular organization in MAPLE‐deposited conjugated polymer thin films and the implications for carrier transport characteristics

The morphological structure of poly(3‐hexylthiophene) (P3HT) thin films deposited by both Matrix Assisted Pulsed Laser Evaporation (MAPLE) and solution spin‐casting methods are investigated. The MAPLE samples possessed a higher degree of disorder, with random orientations of polymer crystallites along the side‐chain stacking, π–π stacking, and conjugated backbone directions. Moreover, the average molecular orientations and relative degrees of crystallinity of MAPLE‐deposited polymer films are insensitive to the chemistries of the substrates onto which they were deposited; this is in stark contrast to the films prepared by the conventional spin‐casting technique. Despite the seemingly unfavorable molecular orientations and the highly disordered morphologies, the in‐plane charge carrier transport characteristics of the MAPLE samples are comparable to those of spin‐cast samples, exhibiting similar transport activation energies (56 vs. 54 meV) to those reported in the literature for high mobility polymers. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 39–48     

Solution derived Al-doped zinc oxide films: doping effect,microstructure and electrical property

This study investigates the influences of the microstructures and doping effect on electrical and optical properties of ZnO:Al films deposited by sol–gel method. Experimental results showed that aluminum concentration affected the crystallite size obviously and enhanced the relative intensity i ₍₀₀₂₎ faintly. Based on photoluminescence results, too much doping atoms generally can cause film crystallinity to deteriorate. Hall measurements indicated the carrier concentration rose only to a certain level after several coating processes. According to ellipsometric data, higher carrier mobility was mainly caused by the escalating density resulted by the increasing film thickness. However, the formation mechanism of charge carrier by doping technique in the sol–gel process is different from that of sputtering technique. The best sample having a sheet resistance of 182 Ω/sq and a transmittance of over 80% in visible region was obtained in aluminum concentration of 1.0 at.%.