Effect of film thickness, type of buffer layer, and substrate temperature on the morphology of dicyanovinyl-substituted sexithiophene films

The influence of film thickness, type of buffer underlayer, and deposition substrate temperature on the crystal structure, microstructure, and morphology of the films of dicyanovinyl-substituted sexithiophene with four butyl-chains (DCV6T-Bu₄) is investigated by means of X-ray diffraction (XRD) and X-ray reflectivity methods. A neat Si wafer or a Si wafer covered by a 15 nm buffer underlayer of fullerene C₆₀ or 9,9-Bis[4-(N,N-bis-biphenyl-4-yl-amino)phenyl]-9H-fluorene (BPAPF) is used as a substrate. The crystalline nature and ordered molecular arrangement of the films are recorded down to 6 nm film thickness. By using substrates heated up to 90 °C during the film deposition, the size of the DCV6T-Bu₄ crystallites in direction perpendicular to the film surface increases up to value of the film thickness. With increasing deposition substrate temperature or film thickness, the DCV6T-Bu₄ film relaxes, resulting in reducing the interplane distances closer to the bulk values. For the films of the same thickness deposited at the same substrate temperature, the DCV6T-Bu₄ film relaxes for growth on Si to BPAPF to C₆₀. Thicker films grown at heated substrates are characterized by smaller density, higher roughness and crystallinity and better molecular ordering. A thin (up to about 6 nm-thick) intermediate layer with linear density-gradient is formed at the C₆₀/DCV6T-Bu₄ interface for the films with buffer C₆₀ layer. The XRD pattern of the DCV6T-Bu₄ powder is indexed using triclinic unit cell parameters.     

Comparison of p-channel transistors based on α,ω-hexyl-distyryl-bithiophene prepared using various film deposition methods

We present a series of organic thin film transistor (OTFT) devices realized by vacuum evaporation, spin-coating, drop casting and inkjet printing and a comparative analysis of their electrical response/behavior obtained under identical measurement conditions. A small molecule, α,ω-hexyl-distyryl-bithiophene DH-DS2T, was used as a hole transporting active layer. Structure and morphology of thin films have been studied by atomic force microscopy, X-ray diffraction and optical microscopy. Different parameters linked directly to the processes (solvent, concentration, deposition method, surface, post-treatment…) are identified as key factors controlling film quality/crystallinity and device performances. This systematic study reveals the factors that limit efficient charge transport at the macroscopic scale of the channel length in OTFT devices.     

Fabrication and characterization of tetracyanoquinodimethane/phthalocyanine solar cells

Fabrication and characterization of heterojunction solar cells of tetracyanoquinodimethane (TCNQ)/copper phthalocyanine (CuPc) and zinc phthalocyanine (ZnPc) were carried out. The light-induced charge separation with charge transfer was investigated by light-induced current density and optical absorption. In both cases of the TCNQ/CuPc and TCNQ/ZnPc solar cells, the TCNQ thin film worked for strong electron-accepting layer as n-type semiconductor. These behaviors would be originated in charge transfer of excited electron from CuPc and ZnPc to TCNQ. The photovoltaic mechanism was discussed on the basis of the experimental results.     

Effect of titanium oxide-polystyrene nanocomposite dielectrics on morphology and thin film transistor performance for organic and polymeric semiconductors

Previous studies have shown that organic thin film transistors with pentacene deposited on gate dielectrics composed of a blend of high K titanium oxide-polystyrene core-shell nanocomposite (TiO₂-PS) with polystyrene (PS) perform with an order of magnitude increase in saturation mobility for TiO₂-PS (K = 8) as compared to PS devices (K = 2.5). The current study finds that this performance enhancement can be translated to alternative small single crystal organics such as α-sexithiophene (α-6T) (enhancement factor for field effect mobility ranging from 30-100× higher on TiO₂-PS/PS blended dielectrics as compared to homogenous PS dielectrics). Interestingly however, in the case of semicrystalline polymers such as (poly-3-hexylthiophene) P3HT, this dramatic enhancement is not observed, possibly due to the difference in processing conditions used to fabricate these devices (film transfer as opposed to thermal evaporation). The morphology for α-sexithiophene (α-6T) grown by thermal evaporation on TiO₂-PS/PS blended dielectrics parallels that observed in pentacene devices. Smaller grain size is observed for films grown on dielectrics with higher TiO₂-PS content. In the case of poly(3-hexylthiophene) (P3HT) devices, constructed via film transfer, morphological differences exist for the P3HT on different substrates, as discerned by atomic force microscopy studies. However, these devices only exhibit a modest (2×) increase in mobility with increasing TiO₂-PS content in the films. After annealing of the transferred P3HT thin film transistor (TFT) devices, no appreciable enhancement in mobility is observed across the different blended dielectrics. Overall the results support the hypothesis that nucleation rate is responsible for changes in film morphology and device performance in thermally evaporated small molecule crystalline organic semiconductor TFTs. The increased nucleation rate produces organic polycrystalline films with small grain size which are better connected and exhibit lower barriers for charge transport and as such higher field effect mobilities are measured in these devices.     

Influence of target to substrate distance on the sputtered CuCl film properties

CuCl is a potential candidate for UV optoelectronic devices due to its superior optical properties and lattice matching with Si. Stoichiometric CuCl thin films of polycrystalline nature were grown by RF magnetron sputtering technique. The effect of varying the target to substrate distance on the compositional, structural and optoelectronic properties of the sputtered films was analysed. A critical target to substrate distance (d_ts) was observed and the film properties were clearly different above and below this distance. Based on the film properties, the optimum spacing of d_ts = 6 cm was found to yield stoichiometric and high optical quality films. The existence of more than one chemical bonding state was identified in nonstoichiometric, chlorine rich, films by analysing the Cu 2p_3/2 core level XPS spectra. Chlorine rich samples were found to show a noticeable emission from deep levels at ∼ 515 nm in cathodoluminescence (CL) spectroscopy. An exciton mediated sharp UV luminescence (385 nm) emission was realized at room temperature in the stoichiometric CuCl thin films.     

Electrodeposition and morphology analysis of Bi-Te thermoelectric alloy nanoparticles on copper substrate

Nanoscale Bi-Te particles with thermoelectric properties on copper substrate were investigated. The substrate was prepared by electroplating copper layer on a copper zinc alloy plate in a copper sulfate solution. Electrodeposition of the Bi-Te alloy particles was then performed in a nitrate bath. The electrolyte is composed of 0.05 M bismuth nitrate and 0.01 M tellurium dioxide dissolved in 2.0 M HNO₃. Cyclic voltammetry and quartz microbalance tests associated with the electrodeposition process were conducted to show the mechanism and kinetics of the deposition. The morphology and compositional analysis of Bi-Te was obtained using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) respectively. The morphology analysis suggested that nanoscale Bi-Te particles were obtained and the EDS results indicated that the surface of the copper substrate contained Cu₂O. The atomic ratio 1:1 for Bi:Te in the alloy, which is equivalent to the weight percentage of Bi:Te = 62%:38% was confirmed from the data obtained.     

Layer-by-layer deposition films of copper phthalocyanine derivative; their photoelectrochemical properties and application to solution-processed thin-film organic solar cells

Ultrathin films serving as a light-harvesting and hole-transporting material were fabricated by layer-by-layer deposition of a water-soluble phthalocyanine derivative, copper(II) phthalocyanine-3,4′,4″,4″′-tetrasulfonic acid tetrasodium salt (CuPcTS), and poly(diallyldimethylammonium chloride). The blue-shift of absorption peak and the absorption dichroism of the Q band indicated that CuPcTS molecules in the layer-by-layer films form cofacial dimers or oligomers and that their molecular planes take a three- or two-dimensional orientation in a direction parallel to the substrate depending on a drying process of the film during the deposition. The diffusion constant of hole carriers among CuPcTS molecules in the film was evaluated to be 6.5 × 10^− 11 cm² s^− 1 in an acetonitrile solution from potential step chronoamperometry measurement. Solution-processed thin-film organic solar cells with a triple-layered structure were developed by combining a hole-transporting layer made of poly(3,4-ethylenedioxythiophene) oxidized with poly(4-styrenesulfonate), a light-harvesting layer of CuPcTS, and an electron-transporting layer of fullerene, in this sequence. Photovoltaic properties of the cells strongly depended on the thickness of CuPcTS films and can be maximized by controlling the thickness at ca. 10 nm.     

A study of size dependent structure, morphology and luminescence behavior of CdS films on Si substrate

Size tunable cadmium sulfide (CdS) films deposited by a dip coating technique on silicon (100) and indium tin oxide/glass substrates have been characterized using X-ray diffraction, X-ray reflectivity, transmission electron microscopy, atomic force microscopy and photoluminescence spectroscopy. The structural characterization indicated growth of an oriented phase of cadmium sulfide. Transmission electron microscopy used to calculate the particle size indicated narrow size dispersion. The tendency of nanocrystalline CdS films to form ordered clusters of CdS quantum dots on silicon (100) substrate has been revealed by morphological studies using atomic force microscopy. The photoluminescence emission spectroscopy of the cadmium sulfide films has also been investigated. It is shown that the nanocrystalline CdS exhibit intense photoluminescence as compared to the large grained polycrystalline CdS films. The effect of quantum confinement also manifested as a blue shift of photoluminescence emission. It is shown that the observed photoluminescence behavior of CdS is substantially enhanced when the nanocrystallites are assembled on silicon (100) substrate.     

Influence of substrate temperature and film thickness on the structure of reactively evaporated In2O3 films

Indium oxide films 25–550 Å thick were reactively evaporated at an oxygen pressure of about 0.27 Pa and at a substrate temperature between room temperature and 400°C. The dependence of the structure of the films on the substrate temperature and on the film thickness was studied using transmission electron microscopy and electron diffraction. It was found that thick films (about 550 Å) were amorphous at room temperature, partially crystallized at 50–125°C and crystalline at 150–400°C. The crystallinity of the films deposited at 150–250°C also depended markedly on the film thickness. Very thin films about 25 Å thick were quasi-amorphous, but with increasing film thickness the amorphous phase transformed into a crystalline phase.The thermal transformation of the amorphous films after deposition was also studied. Amorphous films about 550 Å thick deposited at room temperature and 100°C crystallized at 230°C and 210°C respectively.     

Influence of substrate temperature on the physical properties of thermally evaporated nanocrystalline bismuth sulfide thin films

Nanocrystalline bismuth sulfide thin films were deposited on glass substrate by thermal evaporation technique using the solvothermally synthesized nanometer-sized bismuth sulfide powder as the source material. X-ray diffraction (XRD) analysis revealed that the films are polycrystalline in nature with orthorhombic structure. The crystallinity of the thin films improved with substrate temperature, and the estimated crystallite size are in the nanometer regime. Scanning electron microscope (SEM) analysis showed homogenous distribution of grains with well defined grain boundaries. The optical transmittance of the nanocrystalline bismuth sulfide thin films increases with the increase in substrate temperature, and the optical transition was found to be direct and allowed. The estimated optical band gap energy was found to decrease with the increase in substrate temperature. The electrical resistivity of the bismuth sulfide thin films is of the order of 10⁻⁴ Ω-cm and exhibits semiconductor nature. Experimental results demonstrate that the structural, optical and electrical properties of bismuth sulfide thin films have strong dependence on the substrate temperature.     

Core-cyanated distyryl-bithiophene: Synthesis and impact on charge transport in field-effect transistors

We report on the synthesis of two new distyryl-oligothiophenes with cyano groups at different positions of the ethylene linkage: E,E-5,5′-bis((2-cyano-2-phenyl)-ethenyl)-2,2′-bithiophene 1 and E,E-5,5′-bis((1-cyano-2-phenyl)-ethenyl)-2,2′-bithiophene 2. The impacts of cyanated substitutions on molecular orbitals, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), of these new oligomers investigated by the experimental data are confronted in a theoretical study. Despite lower LUMO level, introduction of electron-withdrawing groups in conjugated cores creates a local hindrance in the π-electron delocalization. Detrimental effects are observed on thin-film morphology studied by scanning electron microscopy and the crystalline structure characterization by X-ray diffraction. Organic thin-film transistors made from 1 showed only a low p-channel transistor activity in air whereas devices made from 2 showed neither a n- or a p-channel activity. Analysis of these materials reveals a direct relationship between the molecular structure and solid state properties.     

Two-dimensional versus three-dimensional post-deposition grain growth in epitaxial oxide thin films: Influence of the substrate surface roughness

Epitaxial thin films made of nanosized yttria-stabilized zirconia islands deposited on (0001) sapphire substrates are synthesized by sol-gel dip-coating followed by a high-temperature post-deposition thermal annealing procedure. At high temperatures, a competitive growth process takes place that allows to obtain thin films made of atomically flat islands with an in-plane diameter typically ten times higher than the thickness or on the contrary inducing the formation of dome-shaped islands. Apart from having a different shape, these islands are also characterized by a different crystallographic orientation with respect to the substrates respectively (001) and (111). In this paper, we investigate the influence of the substrate surface roughness on this competitive grain growth process. The deposition on epi-polished substrates results in a two-dimensional (2D) island growth, whereas the deposition on rough substrates results in a three-dimensional (3D) growth of dome-shaped nanosized islands. The films have been characterized by atomic force microscopy and high-resolution X-ray diffraction using the reciprocal space mapping technique.     

柔性ITO衬底上铜酞菁薄膜有序生长的X射线衍射研究

采用石英晶体微天平实时监测薄膜生长速率,通过控制衬底温度与薄膜生长速率,在柔性ITO导电衬底上真空蒸发沉积了铜酞菁薄膜.X射线衍射分析表明,适当提高衬底温度与薄膜生长速率,可促进薄膜的有序生长.当衬底温度为90℃,生长速率为10nm/min时,薄膜的有序度最高,薄膜晶型呈(相和(200)晶面.     

Shelf-life time test of p- and n-channel organic thin film transistors using copper phthalocyanines

P- and n-type channel thin film transistors (OTFTs) were fabricated by using hexadecahydrogen copper phthalocyanine (H₁₆CuPc) and hexadecafluoro copper phthalocyanine (F₁₆CuPc) molecules, respectively. Top-contact and bottom-contact source-drain configurations were used for both semiconductors. Furthermore, the temperature and film thickness dependences on the mobility values were measured in the saturation regime of source-drain current. Unipolar mobilities in such single-layer OTFTs were correlated to thin film morphology by X-ray diffraction analysis and atomic force microscopy measurements. Shelf-life time tests of p-type and n-type OTFTs are detailed as OTFT configuration and substrate temperature dependence over a time period of 100 days.     

Structure, surface morphology, composition and optical properties of dip coated CdS films

Cadmium sulphide is a promising semiconductor material. In the photovoltaic solar cells it can be used as a window material. In this paper the preparation of CdS film by chemical bath deposition and its solid state properties by taking XRD, SEM, XPS and optical properties has been reported. The XRD and SEM analyses of as deposited and annealed at 400°C clearly indicate the polycrystalline hexagonal phase of the film with (002) orientation. The surfaces are uniform. The XPS measurements indicate the ratio of Cd/S in both the films to be equal to 1·1 at the surface and bulk. It is observed that the thickness of the film depends upon the deposition duration and 5 min deposited films are good for device applications.     

Influence of substrate doping on the surface chemistry and morphology of Copper Phthalocyanine ultra thin films on Si (111) substrates

16 nm thick Copper Phthalocyanine (CuPc) films were deposited at room temperature in Ultra High Vacuum onto “n” and “p” type doped Si(111) substrates covered with a native SiO₂ overlayer. Atomic Force Microscopy indicates that the two substrates are both atomically flat (0.15 nm root mean square roughness). Angle dependent X-ray photoemission spectroscopy shows that the thickness of the native SiO₂ over-layer is 0.8 nm (both for the “n” and “p” type Si substrate). Despite the identity of the substrate roughness, of the SiO₂ thickness, and of the CuPc film growth conditions, the organic films (made out of crystallites in the α-phase, as checked with X-ray Diffraction) grown on the “p” and “n” type substrate show clearly different morphological features (determined with Atomic Force Microscopy and Scanning Electron Microscopy measurements). While the CuPc film on “p” Si(111) shows a compact network of densely packed crystallites with sizes (along the substrate plane) ranging from 50 to 100 nm, the CuPc film on “n” Si(111) shows a slightly more open network of larger crystallites (with 75-150 nm size range). Accordingly, the CuPc film roughness is 0.67 nm and 1.15 nm for the “p” and “n” type substrate respectively. Due to the increased surface to volume effects (lower crystallite size) affecting the CuPc film on “p” Si(111), this film exhibits stronger interaction with oxygen and water vapor of the ambient air, as determined by photoemission spectroscopy experiments performed on samples as grown “in situ” and after prolonged (1 year) exposure to air.     

Influence of substrate temperature on phase formation in Al-Cr thin films

Thin films (of the order of nm) of Al-Cr alloys were prepared by successive depositions by an electron gun in a vacuum chamber. Three Al and two Cr layers, of thicknesses as to yield the final composition, were deposited on both hot (350 °C and 440 °C) and cold (70 °C and 108 °C) substrates and the phases formed were characterized in each case by X-ray diffraction and TEM observations, both in bright and in dark field conditions. The results show that on the hot substrates, Bragg peaks that do not correspond to any reported crystalline or quasi-crystalline phase appear. Both the samples on cold substrates and those heated afterwards showed an amorphous structure by X-ray diffractomery but TEM demonstrated the presence of tiny faulted crystallites, with the same Bragg reflections than those deposited on hot substrates. The results show that, for the composition range studied, a new phase not reported before appears and the substrate temperature only produces crystallite growth, not the formation of new phases. Received: 26 May 2000 / Reviewed and accepted: 31 May 2000     

Morphology and molecular orientation in sexithiophene and N,N′-bis (n-octyl)-dicyanoperylenediimide heterostructures

We report on the growth mode of N,N′-bis (n-octyl)-dicyanoperylenediimide (PDI-8CN₂) on sexithiophene (T6) thin films, studied with different structural, morphological and optical techniques. We aim to individuate the most favorable conditions for the realization of heterostructure devices. The crystalline quality was established by X-ray patterns and Atomic Force Microscopy (AFM) images, and found to be generally high. The anisotropic optical constants extracted from ellipsometry measurements shed light on the mean molecular orientation in the PDI-8CN₂ film. AFM images evidence two different growth modes: at T6 thickness less than 2 monolayers (ML), the growth of PDI-8CN₂ on T6 is favored with respect to SiO₂, while, at higher thickness (2-6 ML), the situation is reversed. An optimum T6 underlayer thickness of approximately 1 ML provides the best quality of PDI-8CN₂ layer corresponding to the highest island dimension, the highest molecular order parameter, and the lowest roughness. Spectrum broadening was observed for extinction coefficient of PDI-8CN₂ in the heterostructures, as compared with a sole material film, and explained by two effects: increase in molecular disorder and formation of charge transfer complexes.     

Thin film encapsulation of DSSCs on plastic substrate

Aluminum oxynitride (AlO_xN_y) films were deposited on polyethylene naphthalate (PEN) substrates using a reactive radio frequency (RF) magnetron sputtering system by varying the nitrogen flow rate. Experimental results show that the AlO_xN_y films deposited on PEN substrate exhibit a pebble-like surface morphology. The deposition rate decreases slightly upon increasing the nitrogen flow rate. The surface roughness of the deposited AlO_xN_y films also decreases upon increasing the nitrogen flow rate. The AlO_xN_y film deposited at a nitrogen flow rate of 15 sccm exhibited the lowest water vapor transmission rate of 0.02 g/m²·day. Meanwhile, the passivation of AlO_xN_y films can effectively improve the long-term stability of plastic DSSC. Their power conversion efficiency can sustain 50% of the initial values even after 300 h.