Tuning the Dimensions of C60‐Based Needlelike Crystals in Blended Thin Films

A new ordered structure of the C₆₀ derivative PCBM ([6‐6]‐phenyl C₆₁‐butyric acid methyl ester) is obtained in thin films based on the blend PCBM:regioregular P3HT (poly(3‐hexylthiophene)). Rapid formation of needlelike crystalline PCBM structures of a few micrometers up to 100 μm in size is demonstrated by submitting the blended thin films to an appropriate thermal treatment. These structures can grow out to a 2D network of PCBM needles and, in specific cases, to spectacular PCBM fans. Key parameters to tune the dimensions and spatial distribution of the PCBM needles are blend ratio and annealing conditions. The as‐obtained blended films and crystals are probed using atomic force microscopy, transmission electron microscopy, selected area electron diffraction, optical microscopy, and confocal fluorescence microscopy. Based on the analytical results, the growth mechanism of the PCBM structures within the film is described in terms of diffusion of PCBM towards the PCBM crystals, leaving highly crystalline P3HT behind in the surrounding matrix.     

Cover Picture: Tuning the Dimensions of C60‐Based Needlelike Crystals in Blended Thin Films (Adv. Funct. Mater. 6/2006)

A new ordered structure of the C₆₀ derivative PCBM is obtained in thin films based on the blend PCBM:P3HT, as detailed by Swinnen, Manca, and co‐workers on p. 760. Needlelike crystalline PCBM structures, whose dimensions and spatial distribution ca be tuned by adjusting the blend ratio and annealing conditions, are formed. In typical solar‐cell applications of these blended films, these results indicate that during long‐term operation under normal conditions (50–70 °C) morphology changes and a decrease in cell performance could occur. A new ordered structure of the C₆₀ derivative PCBM ([6‐6]‐phenyl C₆₁‐butyric acid methyl ester) is obtained in thin films based on the blend PCBM:regioregular P3HT (poly(3‐hexylthiophene)). Rapid formation of needlelike crystalline PCBM structures of a few micrometers up to 100 μm in size is demonstrated by submitting the blended thin films to an appropriate thermal treatment. These structures can grow out to a 2D network of PCBM needles and, in specific cases, to spectacular PCBM fans. Key parameters to tune the dimensions and spatial distribution of the PCBM needles are blend ratio and annealing conditions. The as‐obtained blended films and crystals are probed using atomic force microscopy, transmission electron microscopy, selected area electron diffraction, optical microscopy, and confocal fluorescence microscopy. Based on the analytical results, the growth mechanism of the PCBM structures within the film is described in terms of diffusion of PCBM towards the PCBM crystals, leaving highly crystalline P3HT behind in the surrounding matrix.     

Millimeter-Size All-inorganic Perovskite Crystalline Thin Film Grown by Chemical Vapor Deposition

The chemical vapor deposition (CVD) method is a dry approach that can produce high quality crystals and thin films at large scale which can be easily adapted by industry. In this work, CVD technology is employed to grow high quality, large size all-inorganic cesium lead bromide perovskite crystalline film for the first time. The obtained films have millimeter size crystalline domains with high phase purity. The growth kinetics are examined in detail by optical microscopy and X-ray diffraction. The deposition rate and growth temperature are found to be the key parameters allowing to achieve large scale crystal growth. The large crystalline grains exhibit exceptional optical properties including negligible Stokes shift and uniform photoluminescence over a large scale. This suggests a high degree of crystallinity free from internal strain or defects. A lateral diode within one large crystalline grain is further fabricated and significant photo-generated voltage and short circuit current are observed, suggesting highly efficient carrier transport and collections without scattering within the grain. This demonstration suggests that the CVD grown all-inorganic perovskite thin films enable a promising fabrication route suitable for photovoltaic or photo-detector applications.     

Single‐Crystal Germanium Growth on Amorphous Silicon

Growing single‐crystal semiconductors directly on an amorphous substrate without epitaxy or wafer bonding has long been a significant fundamental challenge in materials science. Such technology is especially important for semiconductor devices that require cost‐effective, high‐throughput fabrication, including thin‐film solar cells and transistors on glass substrates as well as large‐scale active photonic circuits on Si using back‐end‐of‐line CMOS technology. This work demonstrates a CMOS‐compatible method of fabricating high‐quality germanium single crystals on amorphous silicon at low temperatures of <450 °C. Grain orientation selection by geometric confinement of polycrystalline germanium films selectively grown on amorphous silicon by chemical vapor deposition is presented, where the confinement selects the fast‐growing grains for extended growth and eventually leads to single crystalline material. Germanium crystals grown using this method exhibit (110) texture and twin‐mediated growth. A model of confined growth is developed to predict the optimal confining channel dimensions for consistent, single‐crystal growth. Germanium films grown from one‐dimensional confinement exhibit a 200% grain size increase at 1 μm film thickness compared to unconfined films, while 2D confinement growth achieved single crystal Ge. The area of single crystalline Ge on amorphous layers is only limited by the growth time. Significant enhancement in room temperature photoluminescence and reduction in residual carrier density have been achieved using confined growth, demonstrating excellent optoelectronic properties. This growth method is readily extensible to any materials system capable of selective non‐epitaxial deposition, thus allowing for the fabrication of devices from high‐quality single crystal material when only an amorphous substrate is available.     

Atomically Smooth Graphene-Based Hybrid Template for the Epitaxial Growth of Organic Semiconductor Crystals

Atomically thin 2D materials are good templates to grow organic semiconductor thin films with desirable features. However, the 2D materials typically exhibit surface roughness and spatial charge inhomogeneity due to nonuniform doping, which can affect the uniform assembly of organic thin films on the 2D materials. A hybrid template is presented for preparation of highly crystalline small-molecule organic semiconductor thin film that is fabricated by transferring graphene onto a highly ordered self-assembled monolayer. This hybrid graphene template has low surface roughness and spatially uniform doping, and it yields highly crystalline fullerene thin films with grain sizes >300 nm, which is the largest reported grain size for C₆₀ thin films on 2D materials. A graphene/fullerene/pentacene phototransistor fabricated directly on the hybrid template has five times higher photoresponsivity than a phototransistor fabricated on a conventional graphene template supported by a SiO₂ wafer.     

Fabrication of organic semiconductor crystalline thin films and crystals from solution by confined crystallization

Highly crystalline thin films of organic semiconductors processed from solution for electronic devices are difficult to achieve due to a slow and preferential three-dimensional growth of the crystals. Here we describe the development of a processing technique to induce a preferential two-dimensional crystalline growth of organic semiconductors by means of minimizing one dimension and confining the solution in two dimensions into a thin layer. The versatility of the process is demonstrated by processing small molecules (TIPS-pentacene and C₆₀) and a polymer (P3HT), all from solvents with a relatively low boiling point, to obtain crystalline thin films. The thin films show an improved in-plane packing of the molecules compared to films processed under similar conditions by spin coating, which is beneficial for the use in organic field-effect transistors.     

Optical properties of thin GaSe/<Emphasis Type="Italic">n</Emphasis>-Si(111) films

Morphological and optical studies (ellipsometry and reflectance spectroscopy in the ranges 400–750 nm and 1.4–25 μm) of thin GaSe films fabricated by thermal evaporation on the n-Si (111) single-crystal substrates are reported. The film thickness was 15–60 nm. It is established that, in the initial stage of growth, the growth of GaSe on the n-Si (111) substrates occurs via formation of islands (three-dimensional growth). It is shown that, as the thickness increases, the physical parameters of the film change and the films approach single crystals in crystalline and energy band structure. For films with a thickness of 60 nm, the reflectance band peak is attributed to indirect optical transitions enhanced by reflection from the film-substrate interface. From the results of optical studies, quantum effects in the surface region of the thin films are conjectured.     

Determination of crystal orientation in organic thin films using optical microscopy

The electrical behavior of devices based on highly crystalline thin films of organic semiconductors is inherently anisotropic. Thin film optimization requires simple and accessible means to characterize the orientation of the constituent crystals. The standard polarized light microscopy (PLM) provides a contrast between different crystallites but fails to distinguish crystals with relative orientation of 90°. In this paper, we discuss two methods that enable the unambiguous identification of crystal orientation in thin films of optically anisotropic materials: PLM with a full-wave retardation plate and differential interference contrast (DIC). The latter is standard on most microscopes and delivers images with high contrast and good color balance.As an illustration, we use DIC to extract the optical properties of highly crystalline thin films of three high-performance organic semiconductors: rubrene, 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) and 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C₈-BTBT). Building on the relation between optical properties and crystal orientation, we demonstrate how DIC characterizes the in-plane crystal orientation of these thin films. This leads to the identification of the fast growth direction of the crystal front.     

Perspectives of crystalline Si thin film solar cells: a new era of thin monocrystalline Si films?

A large number of competing approaches are currently being investigated around the world to develop crystalline silicon thin film solar cells on foreign substrates. These approaches can be broadly classified according to the crystalline state of the Si films employed: (i) thin film solar cells based on nano‐ or microcrystalline Si‐films; (ii) cells fabricated from large‐grained polycrystalline Si and (iii) recent approaches utilizing the transfer of monocrystalline Si films. The paper discusses prospects and limitations of these approaches and describes device results based on the transfer of quasi‐monocrystalline Si films. Using Si absorber films epitaxially grown on quasi‐monocrystalline Si, we achieve a conversion efficiency of 13˙6% for a 4 cm² sized thin film solar cell on glass. In contrast to the limited performance of polycrystalline Si thin film solar cells imposed by the presence of grain boundaries, transfer approaches are expected to result in thin film solar cell efficiencies in the range of 15 – 18% depending on process maturity and complexity. The transfer of monocrystalline Si films therefore opens a new avenue to an efficient and competitive Si‐based thin film technology. Copyright © 2000 John Wiley & Sons, Ltd     

Microstructures and electrical properties of SrRuO3 thin films on LaAIO3 substrates

Conductive SrRuO₃ thin films have been deposited using pulsed laser deposition on LaA10₃ substrates at different substrate temperatures. Structural and microstructural properties of the SrRuO₃/LaAlO₃ system have been studied using x-ray diffraction, scanning electron microscopy, and scanning tunneling microscopy. Electrical properties of SrRuO₃ thin films have been measured. It was found that the film deposited at 250°C is amorphous, showing semiconductor-like temperature dependence of electrical conductivity. The film deposited at 425°C is crystalline with very fine grain size (100∼200?), showing both metallic and semiconductor-like temperature dependence of electrical conductivity in different temperature regions. The film deposited at 775°C shows a resistivity of 280 μΩ.cm at room temperature and a residual resistivity ratio of 8.4. Optimized deposition conditions to grow SrRuO₃ thin films on LaA10₃ substrates have been found. Possible engineering applications of SrRuO₃ thin films deposited at different temperatures are discussed. Bulk and surface electronic structures of SrRuO₃ are calculated using a semi-empirical valence electron linear combination of atomic orbitals approach. The theoretical calculation results are employed to understand the electrical properties of SrRuO₃ thin films.     

Effect of annealing on physical properties of CBD synthesized nanocrystalline FeSe thin films

Nanocrystalline FeSe thin films were successfully prepared by solution growth method using ferric chloride and sodium selenosulphate as cationic and anionic precursors along with complexing agent oxalic acid. The thickness dependent physical properties of FeSe thin films prepared by varying deposition time are discussed. The FeSe films of thickness 161 nm were further annealed to investigate its impact on physical properties. The X-ray diffraction studies showed that, as deposited FeSe films are nano crystalline in nature and their crystallinity increases with thickness as well as with annealing temperature. The morphological studies showed that FeSe exhibits granular surface with channel like features at higher thickness. The electrical resistivity and thermo-emf measurements confirmed that, FeSe films are semiconducting in nature with P-type conductivity. The activation and band gap energies of FeSe films are found dependent on film thickness as well as on annealing temperature.     

Vertically Segregated Structure and Properties of Small Molecule–Polymer Blend Semiconductors for Organic Thin‐Film Transistors

A comprehensive structure and performance study of thin blend films of the small‐molecule semiconductor, 2,8‐difluoro‐5,11‐bis(triethylsilylethynyl)anthradithiophene (diF‐TESADT), with various insulating binder polymers in organic thin‐film transistors is reported. The vertically segregated composition profile and nanostructure in the blend films are characterized by a combination of complementary experimental methods including grazing incidence X‐ray diffraction, neutron reflectivity, variable angle spectroscopic ellipsometry, and near edge X‐ray absorption fine structure spectroscopy. Three polymer binders are considered: atactic poly(α‐methylstyrene), atactic poly(methylmethacrylate), and syndiotactic polystyrene. The choice of polymer can strongly affect the vertical composition profile and the extent of crystalline order in blend films due to the competing effects of confinement entropy, interaction energy with substrate surfaces, and solidification kinetics. The variations in the vertically segregated composition profile and crystalline order in thin blend films explain the significant impacts of binder polymer choice on the charge carrier mobility of these films in the solution‐processed bottom‐gate/bottom‐contact thin‐film transistors.     

Diindenoperylene derivatives: A model to investigate the path from molecular structure via morphology to solar cell performance

Efficient organic electronic devices require a detailed understanding of the relation between molecular structure, thin film growth, and device performance, which is only partially understood at present. Here, we show that small changes in molecular structure of a donor absorber material lead to significant changes in the intermolecular arrangement within organic solar cells. For this purpose, phenyl rings and propyl side chains are fused to the diindenoperylene (DIP) molecule. Grazing incidence X-ray diffraction and variable angle spectroscopic ellipsometry turned out to be a powerful combination to gain detailed information about the thin film growth. Planar and bulk heterojunction solar cells with C60 as acceptor and the DIP derivatives as donor are fabricated to investigate the influence of film morphology on the device performance. Due to its planar structure, DIP is found to be highly crystalline in pristine and DIP:C60 blend films while its derivatives grow liquid-like crystalline. This indicates that the molecular arrangement is strongly disturbed by the steric hindrance induced by the phenyl rings. The high fill factor (FF) of more than 75% in planar heterojunction solar cells of the DIP derivatives indicates excellent charge transport in the pristine liquid-like crystalline absorber layers. However, bulk heterojunctions of these materials surprisingly result in a low FF of only 54% caused by a weak phase separation and thus poor charge carrier percolation paths due to the lower ordered thin film growth. In contrast, crystalline DIP:C60 heterojunctions lead to high FF of up to 65% as the crystalline growth induces better percolation for the charge carriers. However, the major drawback of this crystalline growth mode is the nearly upright standing orientation of the DIP molecules in both pristine and blend films. This arrangement results in low absorption and thus a photocurrent which is significantly lower than in the DIP derivative devices, where the liquid-like crystalline growth leads to a more horizontal molecular alignment. Our results underline the complexity of the molecular structure-device performance relation in organic semiconductor devices.     

Influence of substrate on the growth of microcrystalline silicon thin films deposited by plasma enhanced chemical vapor deposition

Microcrystalline silicon (μc-Si) thin films are widely used for silicon thin film solar cells, especially in the high performance tandem solar cells which comprise an amorphous silicon junction at the top and a μc-Si junction at the bottom. One of the major factors affecting the photovoltaic properties of μc-Si thin film solar cells of thin films is the quality of the μc-Si thin films. In this work, we investigated the effect of substrates on the crystallization characteristics and growth behaviors of μc-Si thin films grown by the plasma enhanced chemical vapor deposition method (PECVD), and found that substrates have a strong effect on the crystallization characteristics of μc-Si thin films. In addition, the growth rate of μc-Si thin films was also highly influenced by the substrates. Three types of substrates, quartz glass, single crystalline silicon and thermally oxidized single crystalline silicon, were used for growing μc-Si thin films from SiH₄/H₂ with a flow rate ratio 2:98 at different temperatures. Crystallization characteristics of these μc-Si thin films were studied by Raman scattering and X-ray diffraction techniques.     

Stretchable Mesh-Patterned Organic Semiconducting Thin Films on Creased Elastomeric Substrates

Recently, many researchers have tried to develop stretchable semiconducting thin films that can maintain their electrical performance under stretching. However, the fabrication processes have not been sufficiently practical and feasible to be used for soft electronics. Here, a stretchable high-performance organic semiconducting thin film is fabricated by exploiting simultaneous patterning and pinning of a polymer semiconductor solution on an elastomeric substrate in which creasing-instability has occurred. As a result, a mesh-like polymer semiconducting thin film having vacant regions in the crease centers and surrounding crystalline regions near them can be fabricated. Due to the mesh-like morphology and the percolated crystalline regions, the polymer semiconducting thin film shows superior stretchability and charge-transport performance compared to the reference flat polymer thin film. When incorporated into organic thin-film transistors, the DPP-DTT polymer semiconducting thin film maintains its high field-effect carrier mobility (0.53 ± 0.03 cm² (V s)⁻¹) under a strain ε of 80% and is highly stable under repeated stretching cycles at an ε of 50%.     

Revisiting Metal Sulfide Semiconductors: A Solution‐Based General Protocol for Thin Film Formation,Hall Effect Measurement,and Application Prospects

Nanostructured thin films of metal sulfides (MS) are highly desirable materials for various optoelectronic device applications. However, a general low‐temperature protocol that describes deposition of varieties of MS structures, especially in their film form is still not available in literatures. Here, a simple and highly effective general solution‐based deposition protocol for highly crystalline and well‐defined nanostructured MS thin films from ethanol on variety of conducting and non‐conducting substrates is presented. The films display remarkable electronic properties such as high carrier mobility and high conductivity. When NiS thin film deposited on a flexible polyethylene terephthalate (PET) substrate is used as a fluorine doped tin oxide (FTO)‐free counter electrode in dye‐sensitized solar cells, it exhibits a solar‐to‐electric power conversion efficiency of 9.27 ± 0.26% with the highest conversion efficiency as high as 9.50% (vs 8.97 ± 0.07% exhibited by Pt‐electrode). In addition, the NiS film deposited on a Ti‐foil has demonstrated an outstanding catalytic activity for the hydrogen and oxygen evolution reactions from water.     

HfO2薄膜折射率非均质性生长特性研究

在加热的BK7基板上,采用电子束蒸发(EB)工艺制备了一系列不同厚度的HfO2单层膜,对HfO2薄膜生长过程中的折射率非均质性进行了研究。光谱分析表明薄膜非均质性与其厚度息息相关。X射线衍射(XRD)测试表明不同非均质性薄膜对应不同的微观结构;薄膜的微观结构主要由薄膜的生长机制决定。当膜厚较薄时,薄膜不易结晶,呈无定形态,此时薄膜呈正非均质性。如果沉积温度足够高,则薄膜达到一定厚度后开始结晶,此后薄膜折射率就会逐渐下降。随着薄膜继续生长,薄膜晶态结构保持恒定不再变化,非均质性也会因此保持不变达到极值。     

Impact of vacuum and nitrogen annealing on HVE SnS photoabsorber films

Researchers worldwide focus on new earth abundant and cheap absorber materials for use in thin film solar cells that allow wider use of photovoltaics in energy production. SnS is one of such promising absorber materials that comprises earth abundant elements (Sn, S). We describe here the effect of annealing of high vacuum evaporated (HVE) SnS thin films in vacuum and nitrogen atmosphere with relatively high pressures of nitrogen. SnS thin films with a thickness of 500 nm were deposited onto the surface of glass by HVE at a substrate temperature of 300 °C. The as-deposited SnS thin films were annealed at 500 °C and 550 °C for 1 h in vacuum as well as in nitrogen with respect to ambient (N₂) pressure that varied in the range of 500–2000 mbar. We analyze crystalline quality, crystal structure, elemental and phase compositions, and electrical properties of SnS films before and after the annealing process and their changes. Our results show that the use of pressurized inert ambient, such as nitrogen, improves the crystalline quality as well as the electrical properties of SnS thin films. The enhanced growth of crystals and modification of microstructural properties of SnS thin films as a function of annealing conditions (type of ambient, annealing temperature and ambient pressure) are discussed in detail.     

Sn掺杂量对Sn-Al共掺ZnO薄膜光学性能的影响

利用溶胶-凝胶(sol-gel)法在玻璃和硅衬底上制备了不同Sn掺杂量的Sn-Al共掺的ZnO薄膜。采用X射线衍射仪(XRD)、扫描电子显微镜(SEM)、紫外可见分光光度计(UV-Vis)、光致发光谱(PL)等测试手段,对薄膜的结构、形貌和光学性能进行了表征。结果表明:所制备的样品晶粒均沿(002)方向择优生长,且随着Sn元素掺杂量的增加,择优取向性先增强后减弱,同时薄膜的半高宽先减小后增大,半高宽最小时,薄膜的结晶质量最好。与只掺Al元素的ZnO薄膜相比,共掺后的薄膜近紫外发光峰的强度明显降低,出现了轻微的蓝移,且在600 nm处的缺陷发光强度明显增强;随着Sn掺杂量的增加薄膜的透过率先增加后减小。与AZO薄膜相比,当Sn的掺杂量为0.020时,薄膜的结晶质量更好,缺陷发光更强,光透过率更高。     

Micro-Raman characterization of Germanium thin films evaporated on various substrates

We perform an extensive micro-Raman analysis of Germanium thin films physically evaporated on several substrates including silicon, silicon oxide and glass. We investigate the dependence of crystal quality on thin film deposition parameters such as substrate temperature and growth rate. We also study the continuous transitional change of the material structure from amorphous to crystalline phases. Ge films obtained by this simple and low cost technique are a viable solution towards the realization of virtual substrates and devices.