Two-dimensional Grazing Incidence X-ray Diffraction Research Articles

Molecular Orientation and Phase Separation by Controlling Chain Segment and Molecule Movement in P3HT/N2200 Blends

The molecular orientation and phase separation of poly(3-hexylthiophene) (P3HT) and poly[[N,N-bis(2-octyldodecyl)-napthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5, 5′-(2,2′-bithiophene)] (N2200) blends are investigated by in situ temperature-resolved two-dimensional grazing incidence X-ray diffraction (2D GIXD) in step-by-step heating and cooling processes. When pristine amorphous films are thermally annealed below the melting temperature of P3HT, only the chain segment of P3HT moves, while P3HT molecule is crystallized, induced by N2200 with face-on orientation (thermodynamics unsteady state). The interpenetrating network forms during the crystallization process of the polymers. When thermally annealed above melting temperature of P3HT, the whole molecular chain of P3HT moves, the space restriction of N2200 could be broken. The P3HT crystallizes with edge-on orientation (thermodynamics steady state) while N2200 crystallizes with face-on orientation. The hierarchical morphology forms by nucleation an...

A new instrumentation for in situ characterization of the charge transport and crystallographic properties in co-evaporated organic thin film transistor

ABSTRACTA new instrumentation was developed to study the in situ electrical and crystallographic properties of organic thin film transistor during vacuum deposition. We characterized pentacene (PEN) and perfluoro-pentacene (PFP) co-deposited organic thin film transistor with various mixing ratio using this equipment. Lattice parameters and crystal orientations of PEN and PFP alloyed phases (named σ-phase and λ-phase) were determined using an in situ two-dimensional grazing incidence X-ray diffraction (2D-GIXD) measurement. The observed 2D-GIXD patterns clarified that the σ-phase is in triclinic unit cell with the following lattice parameters: a = 0.67 nm, b = 0.75 nm, c = 1.58 nm, α = 95.7° β = 94.2° and γ = 84.0°. The c plane of σ-phase crystal is parallel to the substrate surface. The λ-phase is also in triclinic with the following lattice parameters: a = 0.66 nm, b = 0.69 nm, c = 1.56 nm, α = 109.5° β = 113.0° and γ = 81.5°. The a plane of λ-phase crystal was parallel to the substrate surface. It was also found the best symmetric hole and electron mobility (μh = 5.5 × 10−4 cm2V−1s−1 and μe = 5.1 × 10−4 cm2V−1s−1) were obtained at PEN: PFP = 1:1.

Evolution of Intermediate and Highly Ordered Crystalline States under Spatial Confinement in Poly(3-hydroxybutyrate) Ultrathin Films

The crystallization behavior and crystalline structures of poly(3-hydroxybutyrate) (PHB) ultrathin films (∼52 nm) under spatial confinement were investigated using infrared reflection–absorption spectroscopy (IR-RAS) and two-dimensional grazing incidence X-ray diffraction (2D-GIXD). Intermediate and highly ordered crystalline states were observed during heating and melt-cooling of these films. In the ultrathin films, the intermediate state was noticeably stable at lower temperatures, whereas the highly ordered state was more stable at higher temperatures. A transformation from the intermediate state into the highly ordered state occurred as the temperature increased, as the crystals in the intermediate state acquire sufficient thermal energy to overcome the energy barrier. 2D-GIXD results show that the intermediate state was dominant in edge-on lamellae configuration where the crystallographic b-axis is normal to the film surface. Meanwhile, the highly ordered state was predominant in flat-on lamellae con...

Face-On and Edge-On Orientation Transition and Self-Epitaxial Crystallization of All-Conjugated Diblock Copolymer

The orientation transition and self-epitaxial crystallization of all-conjugated diblock copolymers poly(p-phenylene)-block-(3-hexylthiophene) (PPP-b-P3HT) were systematically investigated by in situ temperature-resolved two-dimensional grazing incidence X-ray diffraction (2D GIXD) in step-by-step heating and cooling process. B39T18 was selected; the results of 2D GIXD showed that the PPP block crystal adopted a face-on orientation while the crystallization of P3HT block was hindered in as-casted films. Three different molecular orientations transition were obtained in self-epitaxial crystallization circles. First, P3HT crystallize with edge-on during the heating process and induced the PPP blocks crystallized with edge-on during the cooling process. Then, the as-casted film was heated in the melting temperature region of PPP blocks and isothermally crystallized. The partial melting of PPP blocks promoted the P3HT blocks crystallize in a face-on due to the steric limitation effect; PPP blocks crystallized ...

Real-time observation of graphene layer growth: Coupling of the interlayer spacing with thickness

In-situ two-dimensional grazing incidence X-ray diffraction was applied in isothermal growth of graphene on nickel through chemical vapor deposition. The analyses showed that the graphene interlayer spacing decreases with the increasing number of graphene layers (growth time increase). Moreover, the as-deposited graphene can be gradually removed through in-situ Ar annealing treatment and inverse interlayer spacing change was also observed, i.e., the interlayer spacing of graphene increases with the number of graphene layers decrease. These results prove that there is a strong coupling effect between the interlayer spacing and the number of graphene layers. The weak interlayer interactions between graphene layers and the interface effects may contribute to this phenomenon.

Naphthodithiophene Diimide (NDTI)-Based Semiconducting Copolymers: From Ambipolar to Unipolar n-Type Polymers

A series of naphthodithiophene diimid (NDTI) based semiconducting polymers with various comonomer units, i.e., thienylenevinylene (TV), naphtho[1,2-b:5,6-b′]dithiophene (NDT), vinylene (V), benzo[c][1,2,5]thiadiazole (BTz), and naphtho[1,2-c:5,6-c′]bis[1,2,5]thiadiazole (NTz), were synthesized by Stille coupling or copper iodide-assisted Stille coupling using 2,7-dibromo-NDTI or 2,7-bis(trimethylstannyl)-NDTI, respectively. Their HOMO and LUMO energy levels were estimated by cyclic volttammetry and photoelectron spectroscopy. The HOMO energy levels of the NDTI-based polymers were strongly affected by the electronic nature of the comonomer units. In contrast, their LUMO energy levels were almost identical, likely owing to their localized LUMOs on the NDTI moiety. All the polymers showed air-stable electron transport in the field-effect transistors (FETs), thanks to their low-lying LUMO (∼−4.0 eV), as ambipolar (PNDTI-TV, -NDT, ∼0.082 cm2 V–1 s–1, μe = ∼0.029 cm2 V–1 s–1) or unipolar n-channel materials (PNDTI-V, -BTz, PNDTI-NTz, μe = ∼0.21 cm2 V–1 s–1), depending on their HOMO energy levels. These results indicate that the type of active carrier in the NDTI-based polymers can be controlled by the nature of comonomer units. Moreover, the two-dimensional grazing incidence X-ray diffraction (2D GIXD) indicated that the polymers with linear backbone structures and large space between the branched alkyl chains tend to afford well-organized crystalline thin film with the edge-on orientation, consistent with good transport characteristics in their FET devices.

Fluorination of Benzothiadiazole–Benzobisthiazole Copolymer Leads to Additive-Free Processing with Meliorated Solar Cell Performance

Processing solvents and conditions have unique importance in the performance of bulk heterojunction organic solar cells. In the present work, we have investigated the role of a primary solvent and solvent additive in the device performance of two benzobisthiazole (BBTz)-based push–pull type polymers. In an inverted cell structure, the BBTz-co-fluorinated benzothiadiazole polymer (PBBTzFT) with a PC71BM acceptor showed additive-free enhanced performance with a power conversion efficiency (PCE) of 6.4% from a 1,2-dichlorobenzene solvent, while the BBTz-co-pyridylthiadiazole polymer (PBBTzPT) showed maximum performance from a chlorobenzene (CB) solution with a 1,8-diiodooctane (DIO) additive (PCE = 2.3%). The detailed investigation by atomic force microscopy and two-dimensional grazing incidence X-ray diffraction corroborates that the fluorination of benzothiadiazole brought about optimal morphology without a solvent additive, the PCE of which is comparable with the previous nonfluorinated analogue (PCE = 6.5%) processed from CB with DIO.

Synthesis and characterization of oligo(2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene)s: effect of the chain length and end-groups on their optical and charge transport properties

Oligo(2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene)s Tn and Tn-2CN (n = 4, 8, 12, 20, n represents the number of thiophene rings), which have H and CN end-groups, respectively, were synthesized, and the effect of chain-length and end-groups on their properties was studied. For both Tn and Tn-2CN, a red-shift of absorption spectra was observed as the chain length increased. Introduction of electron-withdrawing CN end-groups resulted in a remarkable red-shift of absorption spectra in both solution and film states for shorter oligomers (n = 4 and 8). However, for oligomers with n = 12 and 20, obvious red-shift of absorption spectra was only observed in the film state. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels were both lowered by introducing CN end-groups, and the level of reduction decreased with an increase of the chain length. Both the chain length and end-groups influenced the thin film microstructures of the oligomers as revealed by two-dimensional grazing incidence X-ray diffraction (2D-GIXD) and atomic force microscopy (AFM) studies. The oligomers with n = 4 and 8 stood on the substrates with a tilt angle, and the films of T4-2CN and T8-2CN were characterized with 3D crystal structures. In contrast, the oligomers with n = 12 and 20 adopted an edge-on arrangement with the long axes of the molecules parallel to the substrates, like polymeric counterparts. Compared to the films of T12 and T20, those of T12-2CN and T20-2CN comprised larger and more oriented crystal-domains to the substrates along with shorter lamellar distances. Charge-transport properties of the oligomers were influenced by both the chain length and end-groups due to their remarkable effect on the film morphology and packing structures. The film of T8-2CN combined three features: large domain sizes, low surface-roughness and diminished grain boundaries, and therefore exhibited the highest field-effect mobility.

Fluorinated Benzoselenadiazole-Based Low-Band-Gap Polymers for High Efficiency Inverted Single and Tandem Organic Photovoltaic Cells

We designed and synthesized two low-band-gap conjugated copolymers with alternating difluorinated benzoselenadiazole (DFDTBSe) and ethylhexyloxy (EH)- or octyldodecyloxy (OD)-substituted benzo[1,2-b:4,5-b′]dithiophene (BDT) building blocks. PEHBDT-DFDTBSe and PODBDT-DFDTBSe have optical band gap energies of 1.66 and 1.69 eV, respectively, and HOMO energy levels of −5.44 and −5.43 eV, respectively. The different alkyloxy side chains in the polymers affect the molecular packing and ordering in active-layer films blended with [6,6]-phenyl-C71 butyric acid methyl ester (PC71BM). The PEHBDT-DFDTBSe:PC71BM film comprises predominantly “face-on” crystal structures with short π–π stacking distances (3.69 A) while PODBDT-DFDTBSe:PC71BM has mostly “edge-on” structures according to two-dimensional grazing-incidence X-ray diffraction analysis. Bulk heterojunction solar cells were fabricated with an inverted structure of ITO/ethoxylated polyethylenimine/polymer:PC71BM/MoO3/Ag. The device fabricated using the PEHBDT-DF...

Crystal structure of oligothiophene thin films characterized by two-dimensional grazing incidence X-ray diffraction

Crystal structure in an highly oriented organic thin film was determined using two dimensional grazing incidence X-ray diffraction (2D-GIXD). α,ω-Hexyl-distyryl-bithiophene (DH-DS2T) was chosen as the material for the structural analysis, because it is a typical organic semiconductor showing a high crystallinity and an ordered layer-growth behavior. A 2D-GIXD pattern over large range of scattering angles was obtained by using high-brightness synchrotron radiation in SPring-8 and high-sensitive 2D X-ray detector (PILATUS 300 K). The analysis of the observed 2D-GIXD pattern was made to clarify that the crystal structure of highly oriented DH-DS2T thin films belongs to a monoclinic unit-cell with a = 0.58 nm, b = 0.78 nm, c = 3.43 nm, and β = 94.3°, and the space group is determined to be P21/a by considering the extinction rule. Furthermore, by fitting the simulated Bragg peak intensities to the experimental data, the molecular structure is determined. In this structure, two molecules are included in the unit cell and they tilt about 25° against substrate normal.

Essential Differences of Organic Films at the Molecular Level via Vacuum Deposition and Solution Processes for Organic Light-Emitting Diodes

This paper presents an in-depth understanding of the essential differences of organic small-molecule thin films at the molecular level via vacuum deposition and solution processes for organic light-emitting diodes (OLEDs). Synchrotron-based two-dimensional grazing incidence X-ray diffraction has been used to investigate the essential difference. The result reveals that tris(4-carbazoyl-9-ylphenyl)amine (TCTA) molecules show highly oriented arrangements, that is, face-to-face π–π stacking, in vacuum-deposited films, unlike the randomly arranged molecules in spin-coated films. The face-to-face π–π stacking behavior of the molecules in a vacuum-deposited TCTA film leads to higher hole-transport mobility, which is the essential reason for the higher efficiency of a vacuum-deposited OLED compared with that of a solution-processed counterpart, consistent with the calculation results.

Analysis the Actual Nanostructure of α Phase Polyoctylfluorene Thin Film via Synchrotron Grazing-Incidence X-Ray Diffraction

The nanostructure of α phase polyoctylfluorene thin film was characterized using normal X-ray diffraction, one-dimensional out-of-plane grazing incidence X-ray diffraction and two-dimensional grazing incidence X-ray diffraction with lab diffractometer and synchrotron diffractometer. The results show that using grazing incidence X-ray diffraction the weak diffraction signal of thin film can be observed after the elimination of background signals. Incorrect (h10) diffraction signals can be collected by lab diffractometer due to its low collimation and resolution, which can be overcome by using synchrotron diffractometer with high collimation and resolution that reveal the actual microstructure of polyoctylfluorene thin film.

Uniaxially aligned nucleation of vinylidene fluoride oligomer single-crystals on highly ordered ultrathin films of poly(vinylidene fluoride-trifluoroethylene) copolymer

Uniaxially aligned single-crystals of ferroelectric vinylidene fluoride (VDF) oligomer were obtained on a highly-oriented ultra-thin film of vinylidene fluoride–trifluoroethylene copolymer P(VDF/TrFE). Trapezoidal shaped plate-like VDF oligomer crystals with 20nm thickness were selectively grown perpendicularly on P(VDF/TrFE) lamella crystals as a structural template. Fourier transform infrared reflection absorption spectroscopy (FTIR-RAS) and synchrotron two-dimensional grazing incidence X-ray diffraction (2D-GIXD) revealed that the molecular chain axis (c-axis) of P(VDF/TrFE) oriented to the friction transfer direction with the a-axis perpendicular to the film surface, and the friction-transferred P(VDF/TrFE) film induced the uniaxially aligned nucleation of the VDF oligomer with an epitaxial relationship (010)[001]VDF-oligomer//(100)[001]P(VDF/TrFE) in initial crystal growth of VDF oligomer.

A molecular design principle of lyotropic liquid-crystalline conjugated polymers with directed alignment capability for plastic electronics

Conjugated polymers with a one-dimensional p-orbital overlap exhibit optoelectronic anisotropy. Their unique anisotropic properties can be fully realized in device applications only when the conjugated chains are aligned. Here, we report a molecular design principle of conjugated polymers to achieve concentration-regulated chain planarization, self-assembly, liquid-crystal-like good mobility and non-interdigitated side chains. As a consequence of these intra- and intermolecular attributes, chain alignment along an applied flow field occurs. This liquid-crystalline conjugated polymer was realized by incorporating intramolecular sulphur-fluorine interactions and bulky side chains linked to a tetrahedral carbon having a large form factor. By optimizing the polymer concentration and the flow field, we could achieve a high dichroic ratio of 16.67 in emission from conducting conjugated polymer films. Two-dimensional grazing-incidence X-ray diffraction was performed to analyse a well-defined conjugated polymer alignment. Thin-film transistors built on highly aligned conjugated polymer films showed more than three orders of magnitude faster carrier mobility along the conjugated polymer alignment direction than the perpendicular direction.

Phase and Texture of Solution-Processed Copper Phthalocyanine Thin Films Investigated by Two-Dimensional Grazing Incidence X-Ray Diffraction

The phase and texture of a newly developed solution-processed copper phthalocyanine (CuPc) thin film have been investigated by two-dimensional grazing incidence X-ray diffraction. The results show that it has β phase crystalline structure, with crystallinity greater than 80%. The average size of the crystallites is found to be about 24 nm. There are two different arrangements of crystallites, with one dominating the diffraction pattern. Both of them have preferred orientation along the thin film normal. Based on the similarities to the vacuum deposited CuPc thin films, the new solution processing method is verified to offer a good alternative to vacuum process, for the fabrication of low cost small molecule based organic photovoltaics.

In-Situ Synchrotron X-Ray Scattering Study of Thin Film Growth by Atomic Layer Deposition

We report an atomic layer deposition chamber for in-situ synchrotron X-ray scattering study of thin film growth. The chamber was designed for combined synchrotron X-ray reflectivity and two-dimensional grazing-incidence X-ray diffraction measurement to do a in-situ monitoring of ALD growth. We demonstrate ruthenium thermal ALD growth for the performance of the chamber. 10, 20, 30, 50, 70, 100, 150 and 250-cycled states are measured by X-ray scattering methods during ALD growth process. Growth rate is calculated from thickness values and the surface roughness of each state is estimated by X-ray reflectivity analysis. The crystal structure of initial growth state is observed by Grazing-incidence X-ray diffraction. These results indicate that in-situ X-ray scattering method is a promising analysis technique to investigate the initial physical morphology of ALD films.

Second-harmonic generation in conjugated polymer films: A sensitive probe of how bulk polymer crystallinity changes with spin speed

In this paper, we examine the second-harmonic generation (SHG) from spin-cast films of the conjugated polymer poly(2-methoxy-5-(2(')-ethylhexyloxy)para-phenylenevinylene) (MEH-PPV). We find that the SHG intensity depends strongly on the speed used to spin cast the films. Two-dimensional grazing incidence x-ray diffraction (XRD) experiments show that the bulk crystallinity of the MEH-PPV films varies in the same way with spin speed as the SHG intensity. This strongly suggests that instead of being interface specific, the second-harmonic signal from conjugated polymer films is dominated by the crystalline domains in the bulk. The nonmonotonic dependence of both the SHG intensity and the degree of MEH-PPV crystallinity results from a competition between the shear forces and the solvent evaporation rate during spin coating, which produces a maximum degree of crystallinity for MEH-PPV films spin cast at around 1400 rpm. We also use XRD to show that thermal annealing produces MEH-PPV films with a single degree of bulk crystallinity, independent of how they were originally cast. This allows us to model the angle- and thickness-dependent SHG from annealed MEH-PPV films with a single polarizability tensor. We find that the SHG from MEH-PPV films fits best to a bulk-allowed electric quadrupole mechanism, consistent with the bulk SHG seen in other pi-stacked aromatic molecules. Thus, rather than providing information about conjugated polymer interfaces, SHG can be used as a sensitive probe of the local degree of crystallinity in the bulk of conjugated polymer films.

Solvent vapor annealing of an insoluble molecular semiconductor

Solvent vapor annealing has been proposed as a low-cost, highly versatile, and room-temperature alternative to thermal annealing of organic semiconductors and devices. In this article, we investigate the solvent vapor annealing process of a model insoluble molecular semiconductor thin film—pentacene on SiO2 exposed to acetone vapor—using a combination of optical reflectance and two-dimensional grazing incidence X-ray diffraction measurements performed in situ, during processing. These measurements provide valuable and new insight into the solvent vapor annealing process; they demonstrate that solvent molecules interact mainly with the surface of the film to induce a solid–solid transition without noticeable swelling, dissolving or melting of the molecular material.

Selectively patterned highly conductive poly(3,4-ethylenedioxythiophene)-tosylate electrodes for high performance organic field-effect transistors

We have improved the performance of pentacene field-effect transistors by using highly conductive poly(3,4-ethylenedioxythiophene)-tosylate (PEDOT-Tos) source-drain electrodes (∼103 S/cm) formed by a simple solution-based process. A high field-effect mobility of 0.25 cm2/Vs and an ON/OFF current ratio of 107 were obtained in pentacene-based bottom contact organic field-effect transistors (OFETs), which constitutes an improvement over OFETs based on Au and PEDOT:PSS electrodes. Two-dimensional grazing incidence x-ray diffraction and ultraviolet photoemission spectroscopy results confirmed that the crystalline properties of the pentacene film and the hole injection from the PEDOT-Tos electrode to the pentacene layer are more efficient than those from Au and PEDOT:PSS electrodes.

In situ structural analysis of BPDA-PPD polyimide thin film using two-dimensional grazing incidence X-ray diffraction

Structural development of BPDA-PPD polyimide thin film has been investigated by in situ grazing incidence X-ray diffraction at the BL24XU beamline of the SPring-8. Optimizing the sample shape, two-dimensional images were measured successfully without sacrificing angle resolution. It has been clearly shown that the crystallization first begins in the in-plane direction, at the curing temperature of 180 °C, in which the periodic structure of the molecular chain axis (c axis) is developed. The crystallization in the surface normal (out-of-plane) direction is observed later, at the curing temperature above 300 °C. A slight increase of the d spacing of the c axis during heating process has been observed, suggesting the stretching of the contracted molecular chain in accordance with the curing process. In the cooling process, the decrease of the d spacings for a and b axes was considerable, which indicates thermal expansion of the crystals at high temperatures. The increases in the peak intensities during the cooling process have been observed, which indicate the d spacing of each axis becomes close to the equilibrium value to produce higher periodicity.