Grazing Incidence Wide-angle X-ray Scattering Research Articles

Stepwise sulfurization of MoO3 to MoS2 thin films studied by real-time X-ray scattering

Chemical vapor deposition (CVD) is commonly used for the large-scale synthesis of the films of two-dimensional (2D) transition metal dichalcogenide (TMD) materials, including MoS2 thin films, which have potential applications in optoelectronics, catalysis, and nanoelectronics. As far as thin film of MoS2 is concerned, the influence of deposition parameters on the chemical reactions in the growth process via CVD synthesis has not yet been exhaustively studied. Here we present a comprehensive, time-resolved, in-situ study of the growth of MoS2 thin film from MoO3 by sulfurization using the grazing-incidence wide-angle X-ray scattering (GIWAXS) in laboratory conditions. We tracked the basic chemical reactions during the sulfurization process in real time by monitoring the phase transformations of MoO3 to MoO2 and MoO2 to MoS2 via a MoOS2 transient phase, although it was not possible to observe it directly. Preferential crystallographic orientation of MoO3, and MoO2 phases during the growth was confirmed. In addition, their activation energies were determined from the crystallization kinetics parameters. The present work highlights the importance of understanding the basic chemical reactions inside the CVD reactor as a prerequisite for optimizing the properties of the final 2D TMD films.

Confined crystallization and polymorphism in iPP thin films

The morphological and structural evolution of isotactic polypropylene (iPP) in thin and ultrathin films casted by hot-solution spin coating is investigated by means of atomic force microscopy (AFM) and grazing incidence wide angle X-ray scattering (GIWAXS). When reducing the film thickness from 300 nm to 15 nm, changes from a 3D to a quasi-2D morphology are observed. The quasi-2D confined morphology is found in the 220-40 nm thickness range and is characterized by a predominant edge-on orientation (EO) of the lamellae. A transition to the flat-on (FO) orientation paired to a drastic decrease of the sample roughness is observed for thicknesses below 30 nm. Surprisingly, a mixed morphology where orthorhombic γ-phase branches develop from FO α-phase lamellae is observed at the lowest prepared thickness of 15 nm. The unexpected appearance of the γ-phase for the iPP grade used here is due to the favourable flat-on lamellar orientation at the lowest film thickness. Thanks to a quantitative analysis of the GIWAXS patterns, the relative amount between edge-on and flat-on oriented lamellae (χ EO /χ FO ) is calculated, providing useful insights on the real morphological composition of the iPP thin films, a task that is very challenging to achieve with microscopy techniques. • Isotactic polypropylene thin film prepared by hot-solution spin coating present confined crystallization behaviour. • Morphology evolution is investigated via AFM, nanostructural composition and lamellar orientation is studied by means of GIWAXS. • Lamellae maintain a mixed morphological structure, preferential orientation changes from edge-on to flat-on on the substrate. • Formation of the orthorhombic γ-phase is induced by increasing the degree of confinement ( i.e. : decreasing film thickness).

Poly(vinylidene fluoride‐co‐trifluoroethylene) Thin Films after Dip‐ and Spin‐Coating

AbstractThe ferro‐, pyro‐ and piezoelectric properties of poly(vinylidene fluoride‐co‐trifluoroethylene) P(VDF‐TrFE) have created interest with regard to its application in aqueous and ambient surroundings for sensors, functional coatings, and in the field of life sciences. P(VDF‐TrFE) thin films are usually applied via spin‐coating, but dip‐coating will be advantageous especially for irregularly shaped substrates. The morphology of dip‐ and spin‐coated semi‐crystalline thin films is studied as a function of both the film thickness and the annealing temperature. The characterization of the films is carried out by grazing incidence wide‐angle X‐ray scattering (GIWAXS), X‐ray reflectometry (XRR), and infrared reflection absorption spectroscopy (IRRAS). Atomic force microscopy measurements (AFM) are used to examine the resulting topography. It is found that both spin‐ and dip‐coated thin films crystallize in the desired edge‐on orientation, but the overall crystallinity after dip‐coating is decreased compared to the spin‐coated films of comparable thickness and the resulting roughness is increased. The higher roughness is most probably caused by the slower evaporation of the solvents and a secondary crystallization process at the air‐polymer interface.

Solid Phase Epitaxy of Single Phase Two-Dimensional Layered InSe Grown by MBE.

Single-phase two-dimensional (2D) indium monoselenide (γ-InSe) film is successfully grown via solid phase epitaxy in the molecular beam epitaxy (MBE) system. Having high electron mobility and high photoresponsivity, ultrathin 2D γ-InSe semiconductors are attractive for future field-effect transistor and optoelectronic devices. However, growing single-phase γ-InSe film is a challenge due to the polymorphic nature of indium selenide (γ-InSe, α-In2Se3, β-In2Se3, γ-In2Se3, etc.). In this work, the 2D α-In2Se3 film was first grown on a sapphire substrate by MBE. Then, the high In/Se ratio sources were deposited on the α-In2Se3 surface, and an γ-InSe crystal emerged via solid-phase epitaxy. After 50 min of deposition, the initially 2D α-In2Se3 phase was also transformed into a 2D γ-InSe crystal. The phase transition from 2D α-In2Se3 to γ-InSe was confirmed by Raman, XRD, and TEM analysis. The structural ordering of 2D γ-InSe film was characterized by synchrotron-based grazing-incidence wide-angle X-ray scattering (GIWAXS).

Ruddlesden–Popper Perovskites with Narrow Phase Distribution for Air‐Stable Solar Cells

2D Ruddlesden–Popper perovskites have risen to prominence as stable and efficient photovoltaic materials because of their structural diversity, rich photophysics, and low moisture ingression. However, thin films processed from stoichiometric precursor solutions possess a broad phase distribution of different number of inorganic layers with random crystal orientation, crippling device performance. The effect of methylammonium chloride (MACl) and 3‐amino‐4‐phenolsulfonic acid (APSA) on the fabrication of perpendicularly oriented (PEA)2MA4Pb5I16 films with narrow phase distribution using antisolvent and hot‐casting processing techniques is investigated. MACl plays a critical role in suppressing parasitic n ≤ 2 and 3D‐like phases. APSA performs the dual function of trap passivation and further narrowing phase polydispersity through strong coordination with Pb2+. Ex situ grazing‐incident wide‐angle X‐Ray scattering (GIWAXS) and ultrafast spectroscopic characterization reveal uniformly mixed‐phase distribution with disordered orientation in antisolvent treated films, while additive‐assisted hot‐casting treatment results in oriented, reverse‐graded phase distribution, i.e., small‐n on the film surface and large‐n at the bottom. Arising thin films enable efficient p–i–n solar cells with an efficiency of 14.34%, and a Voc of 1.20 V, retaining 96% initial efficiency after 1440 h under ambient conditions (RH = 50–60%) without encapsulation.

18.55% Efficiency Polymer Solar Cells Based on a Small Molecule Acceptor with Alkylthienyl Outer Side Chains and a Low-Cost Polymer Donor PTQ10

18.55% Efficiency Polymer Solar Cells Based on a Small Molecule Acceptor with Alkylthienyl Outer Side Chains and a Low-Cost Polymer Donor PTQ10

Revealing the vertical structure of in-situ fabricated perovskite nanocrystals films toward efficient pure red light-emitting diodes

The development of efficient perovskite light-emitting diodes (PeLEDs) relies strongly on the fabrication of perovskite films with rationally designed structures (grain size, composition, surface, etc.). Therefore, an understanding of structure-performance relationships is of vital importance for developing high-performance perovskite devices, particularly for devices with in-situ fabricated perovskite nanocrystal films. In this study, we reveal the vertical structure of an in-situ fabricated quasi-two-dimensional perovskite film. By combining time-of-flight secondary ion mass spectrometry, energy dispersive spectroscopy, grazing incidence wide-angle X-ray scattering (GIWAXS), and low-temperature photoluminescence spectra, we illustrate that the resulting in-situ fabricated DPPA2Csn-1Pbn(Br0.3I0.7)3n+1 (DPPA+: 3,3-diphenylpropylammonium) film has a gradient structure with a very thin layer of ligands on the surface, predominantly small-n domains at the top, and predominantly large-n domains at the bottom owing to the solubility difference of the precursors. In addition, GIWAXS measurements show that the domain of n = 2 on the top layer has an ordered in-plane alignment. Based on the understanding of the film structure, we developed an in-situ fabrication process with ligand exchange to achieve efficient pure red PeLEDs at 638 nm with an average external quantum efficiency (EQE) of 7.4%. The optimized device had a maximum luminance of 623 cd/m2 with a peak EQE of 9.7%.

Formation of a Secondary Phase in Thermally Evaporated MAPbI3 and Its Effects on Solar Cell Performance.

Thermal evaporation is a promising deposition technique to scale up perovskite solar cells (PSCs) to large areas, but the lack of understanding of the mechanisms that lead to high-quality evaporated methylammonium lead triiodide (MAPbI3) films gives rise to devices with efficiencies lower than those obtained by spin coating. This work investigates the crystalline properties of MAPbI3 deposited by the thermal coevaporation of PbI2 and MAI, where the MAI evaporation rate is controlled by setting different temperatures for the MAI source and the PbI2 deposition rate is controlled with a quartz crystal microbalance (QCM). Using grazing incident wide-angle X-ray scattering (GIWAXS) and X-ray diffraction (XRD), we identify the formation of a secondary orthorhombic phase (with a Pnma space group) that appears at MAI source temperatures below 155 °C. With synchrotron-based X-ray fluorescence (XRF) microscopy, we show that the changes in crystalline phases are not necessarily due to changes in stoichiometry. The films show a stochiometric composition when the MAI source is heated between 140 to 155 °C, and the samples become slightly MAI rich at 165 °C. Increasing the MAI temperature beyond 165 °C introduces an excess of MAI in the film, which promotes the formation of films with low crystallinity that contain low-dimensional perovskites. When they are incorporated in solar cells, the films deposited at 165 °C result in the champion power conversion efficiency, although the presence of a small amount of low-dimensional perovskite may lead to a lower open-circuit voltage. We hypothesize that the formation of secondary phases in evaporated films limits the performance of PSCs and that their formation can be suppressed by controlling the MAI source temperature, bringing the film toward a phase-pure tetragonal structure. Control of the phases during perovskite evaporation is therefore crucial to obtain high-performance solar cells.

Fine Tuning Alkyl Substituents on Dithienoquinoxaline-Based Wide-Bandgap Polymer Donors for Organic Photovoltaics.

The molecular design of wide-bandgap conjugated polymer donors (WB-CPDs) is a promising strategy for tuning the bulk heterojunction blend film morphologies to achieve high-performance organic photovoltaic (OPV) devices. Herein, we synthesize two WB-CPDs, namely, PBQ-H and PBQ-M, with and without methyl groups on the fused-dithieno[3,2-f:2',3'-h]quinoxaline (DTQx) moiety. We systematically investigate their structure-property relationship and OPV performances. The AFM and 2D grazing-incidence wide-angle X-ray scattering (GIWAXS) studies reveal that the PBQ-H:BO-4Cl BHJ blend shows strengthened aggregation behavior and stronger π-π stacking on face-on orientation compared with the PBQ-M:BO-4Cl BHJ blend, enhancing the phase separation, charge transport, and fill factor (FF). Blend film absorption spectra, however, show that the PBQ-H:BO-4Cl BHJ blend exhibits a lower absorption coefficient than that of the PBQ-M:BO-4Cl BHJ blend, which decreases the short-circuit current density (JSC). As a consequence, the optimized PBQ-H:BO-4Cl BHJ blend delivers a higher power conversion efficiency (PCE) of 12.88% with a JSC of 23.97 mA/cm2, an open-circuit voltage (VOC) of 0.86 V, and an FF of 62.46%, compared with the PBQ-M:BO-4Cl BHJ blend (PCE of 11.81% with a JSC of 24.78 mA/cm2, a VOC of 0.85 V, and an FF of 56.11%). Overall, this work demonstrates that alkyl group substitution on the DTQx moiety on the basis of WB-CPDs is critical for controlling the film morphology and thus obtaining high OPV performances.

Simultaneous measurement of X-ray scattering and photoluminescence during molecular deposition

We present a simultaneous in situ measurement of X-ray scattering and photoluminescence (PL) spectroscopy during organic thin-film growth in a vacuum. We have designed a modified deposition chamber that broadens its utilization and enables complementary information on the structure and optoelectronic properties. In a pilot experiment, we investigated the growth of diindenoperylene (DIP) thin films in real time. We found a direct correlation between PL and grazing-incidence wide-angle X-ray scattering (GIWAXS), showing the formation of a new DIP polymorph starting at a layer thickness of ∼14 nm. Furthermore, we show that the PL signal is more sensitive to the arrangement of the molecules than GIWAXS in the limit of small layer thicknesses. The presented experimental setup paves the way for performing simultaneous PL and X-ray scattering measurements during the growth of various materials suitable for optoelectronic applications.

Near-Infrared All-Fused-Ring Nonfullerene Acceptors Achieving an Optimal Efficiency-Cost-Stability Balance in Organic Solar Cells

Near-Infrared All-Fused-Ring Nonfullerene Acceptors Achieving an Optimal Efficiency-Cost-Stability Balance in Organic Solar Cells

Structure-Transport Properties Governing the Interplay in Humidity-Dependent Mixed Ionic and Electronic Conduction of Conjugated Polyelectrolytes.

Polymeric mixed ionic-electronic conductors (MIECs) are of broad interest in the field of energy storage and conversion, optoelectronics, and bioelectronics. A class of polymeric MIECs are conjugated polyelectrolytes (CPEs), which possess a π-conjugated backbone imparting electronic transport characteristics along with side chains composed of a pendant ionic group to allow for ionic transport. Here, our study focuses on the humidity-dependent structure-transport properties of poly[3-(potassium-n-alkanoate) thiophene-2,5-diyl] (P3KnT) CPEs with varied side-chain lengths of n = 4-7. UV-vis spectroscopy along with electronic paramagnetic resonance (EPR) spectroscopy reveals that the infiltration of water leads to a hydrated, self-doped state that allows for electronic transport. The resulting humidity-dependent ionic conductivity (σi) of the thin films shows a monotonic increase with relative humidity (RH) while electronic conductivity (σe) follows a non-monotonic profile. The values of σe continue to rise with increasing RH reaching a local maximum after which σe begins to decrease. P3KnTs with higher n values demonstrate greater resiliency to increasing RH before suffering a decrease in σe. This drop in σe is attributed to two factors. First, disruption of the locally ordered π-stacked domains observed through in situ humidity-dependent grazing incidence wide-angle X-ray scattering (GIWAXS) experiments can account for some of the decrease in σe. A second and more dominant factor is attributed to the swelling of the amorphous domains where electronic transport pathways connecting ordered domains are impeded. P3K7T is most resilient to swelling (based on ellipsometry and water uptake measurements) where sufficient hydration allows for high σi (1.0 × 10-1 S/cm at 95% RH) while not substantially disrupting σe (1.7 × 10-2 S/cm at 85% RH and 8.0 × 10-3 S/cm at 95% RH). Overall, our study highlights the complexity of balancing electronic and ionic transport in hydrated CPEs.

Increased Efficiency of Organic Solar Cells by Seeded Control of the Molecular Morphology in the Active Layer

The performance of non‐fullerene, polymer bulk heterojunction (BHJ) organic photovoltaic devices has a significant correlation with the molecular morphology of the donor and acceptor. The authors show that small organic molecules coordinated to a metal oxide, an electron transport seed layer (ETSL), can profoundly modify the donor:acceptor molecular morphology of inverted organic photovoltaic (OPV) devices. Using grazing incidence wide angle X‐ray scattering (GIWAXS), the authors show that a PTB7‐Th:IEICO‐4F BHJ active layer has a higher degree of face‐on molecular alignment on ETSL‐1 (biphenyl‐4,4′‐dicarboxylic acid, coordinated to ZnO), whilst for naphthalene‐2,6‐dicarboxylic acid coordinated to ZnO (ETSL‐2), it is reduced. Devices of PTB7‐Th:IEICO‐4F BHJ prepared on ETSL‐1 had a 19.91% increase in the average power conversion efficiency (PCE), a 1.56% increase in the fill factor (FF), and a 16.66 ± 0.2% enhancement in the short circuit current density. The observed improvements are believed to be due to significant modifications to the oxide‐BHJ interfacial region of ETSL‐1, namely the elimination of nano‐ridges and defect centers, along with an enhanced wettability. These factors can be correlated with the enhanced device performances, leading to the conclusion that the modulation of the molecular morphology of donor:acceptor blends by ETSL‐1 has a broad impact on improving OPV cell efficiencies.

All-Polymer Solar Cells Incorporating Readily Accessible Naphthalene Diimide and Isoindigo Acceptor Polymers for Improved Light Harvesting

The development of readily accessible polymer acceptors is imperative to preserve the guiding principles of all-polymer solar cells as a low-cost and sustainable technology for alternative energy production. In this study, we report a computationally guided design and facile synthesis of new acceptor polymers comprising the alternating copolymer, PNIT, and the corresponding random copolymer, r-PNIT, which incorporate structurally simple naphthalene diimide (NDI) and isoindigo (IID) units. PNIT was prepared via direct arylation polymerization (DArP), which proceeds via C–H activation and avoids the use of toxic reagents and extended synthetic pathways for the monomer synthesis. PNIT has broad optical absorption in the 300–850 nm range and an enhanced absorption coefficient (47 × 103 cm–1) compared to r-PNIT (300–850 nm; 40 × 103 cm–1). When incorporated in all-polymer solar cells (APSCs) using PBDB-T as the donor polymer, PBDB-T:PNIT provides greater than two times the average (maximum) power conversion efficiency (PCE) of 5.18 ± 0.08 (5.32)% compared to that of PBDB-T:r-PNIT, 2.38 ± 0.16 (2.57)%, due to increased Jsc (10.36 vs 5.45 mA cm–2) and fill factor (FF) (0.58 vs 0.50) metrics. The PBDB-T:PNIT PCE is among the highest reported for an IID-based APSC and demonstrates the viability of DArP for the synthesis of new APSC acceptor polymers. Detailed morphological and microstructural investigations using atomic force microscopy (AFM) and grazing-incidence wide-angle X-ray scattering (GIWAXS), respectively, reveal enhanced texturing for PNIT, which enhances charge transport properties as supported by space-charge-limited current (SCLC) mobility measurements.

Enhancing the Intermolecular Interactions of Ladder-Type Heteroheptacene-Based Nonfullerene Acceptors for Efficient Polymer Solar Cells by Incorporating Asymmetric Side Chains

Enhancing the Intermolecular Interactions of Ladder-Type Heteroheptacene-Based Nonfullerene Acceptors for Efficient Polymer Solar Cells by Incorporating Asymmetric Side Chains

Phenylene-Bridged Perylene Monoimides as Acceptors for Organic Solar Cells: A Study on the Structure-Property Relationship.

A series of non‐fullerene acceptors based on perylene monoimides coupled in the peri position through phenylene linkers were synthesized via Suzuki‐coupling reactions. Various substitution patterns were investigated using density functional theory (DFT) calculations in combination with experimental data to elucidate the geometry and their optical and electrochemical properties. Further investigations of the bulk properties with grazing incidence wide angle X‐ray scattering (GIWAXS) gave insight into the stacking behavior of the acceptor thin films. Electrochemical and morphological properties correlate with the photovoltaic performance of devices with the polymeric donor PBDB‐T and a maximum efficiency of 3.17 % was reached. The study gives detailed information about structure–property relationships of perylene‐linker‐perylene compounds.

Cl-Assisted Perovskite Crystallization Pathway in the Confined Space of Mesoporous Metal Oxides Unveiled by In Situ Grazing Incidence Wide-Angle X-ray Scattering

Cl-Assisted Perovskite Crystallization Pathway in the Confined Space of Mesoporous Metal Oxides Unveiled by In Situ Grazing Incidence Wide-Angle X-ray Scattering

Bithienopyrroledione-based polymeric donors for efficient fullerene- and non-fullerene-based organic photovoltaic cells

Thienopyrroledione (TPD) is a promising electron-accepting core building block of donor-acceptor-type semiconducting polymers for use in high-performance organic photovoltaic (OPV) cells. In this study, bithienopyrroledione (biTPD) derivatives with different lengths of side alkyl chains were designed to utilize specific ability including stronger electron-withdrawing strength and extended π-conjugation compared with the mono TPD unit. The synthesized copolymers, poly(3-(5-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophen-2-yl)-6-octylthieno[3,2-b]thiophen-2-yl)-5,5′-bis(2-butyloctyl)-3'-(6-octylthieno[3,2-b]thiophen-2-yl)-4H,4′H-[1,1′-bithieno[3,4-c]pyrrole]-4,4′,6,6′(5H,5′H)-tetraone) (PBDT-biTPD(BO)) and poly(3-(5-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophen-2-yl)-6-octylthieno[3,2-b]thiophen-2-yl)-5,5′-bis(2-hexyldecyl)-3'-(6-octylthieno[3,2-b]thiophen-2-yl)-4H,4′H-[1,1′-bithieno[3,4-c]pyrrole]-4,4′,6,6′(5H,5′H)-tetraone) (PBDT-biTPD(HD)), composed of biTPD and typical benzo[1,2-b:4,5-b’]dithiophene (BDT) units had broad absorption with deep frontier molecular orbital energy levels. In addition, PBDT-biTPD(BO) and PBDT-biTPD(HD) showed predominant face-on orientation confirmed by the grazing-incidence wide-angle X-ray scattering (GIWAXS). The OPV device exhibited the maximum power conversion efficiency (PCE) of 8.48% and 8.99% for PBDT-biTPD(BO):PC71BM and PBDT-biTPD(HD):PC71BM, respectively, with a high open-circuit voltage (VOC). In particular, PBDT-biTPD(HD):IT-4F showed the best PCE of 9.49% with improved short-circuit current, despite the slightly decreased VOC compared to that of PBDT-biTPD(HD):PC71BM.

Additive Effect on the Structure of PEDOT:PSS Dispersions and Its Correlation with the Structure and Morphology of Thin Films.

We reported on the interaction between poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and high-boiling-point additives in PEDOT:PSS aqueous dispersions and in the final polymer films with the aim of stablishing correlations between the structure of both inks and solid thin films. By Small-Angle X-ray Scattering (SAXS) using synchrotron radiation, it was found that the structural changes of dispersions of PEDOT:PSS with high-boiling-point additives can be explained as a two-step mechanism depending on the additive concentration. A compaction of PEDOT:PSS grains was observed at low concentrations while a swelling of the grains together with a phase segregation between PEDOT and PSS segments was evidenced at larger concentrations. Thin films’ morphology and structure were investigated by atomic force microscopy (AFM) and synchrotron Grazing Incidence Wide-Angle X-ray Scattering (GIWAXS) respectively. Our two-step model provides an explanation for the small and sharp domains of PEDOT:PSS thin films observed for low-additive concentrations (first step) and larger domains and roughness found for higher-additive concentrations (second step). A reduction of the ratio of PSS in PEDOT:PSS thin films upon the presence of additives was also observed. This can be related to a thinning of the PSS shells of PEDOT:PSS grains in the dispersion. The results discussed in this work provide the basis for a controlled tuning of PEDOT:PSS thin films structure and the subsequent electrical properties.

Solid-solvent hybrid additive for the simultaneous control of the macro- and micro-morphology in non-fullerene-based organic solar cells

Most organic solar cells based on a bulk-heterojunction active layer are fabricated with the assistance of high boiling point solvent additives to optimize the phase separation of the donor and acceptor. The macroscopic phase separation can be controlled by this solvent additive. However, the control of the microscopic morphology (e.g., π-π stacking, orientation) of the inside phase is still dependent on the interaction and self-assembly characteristics of each donor and acceptor. In this work, we introduce a solid-solvent hybrid additive on PM6:Y6 solar cells to optimize both the macroscopic phase separation and the microscopic morphology at the same time. For the solvent additive, the well-known 1-CN solvent additive was used. For the solid additive, newly synthesized 3D star-shaped solid additives (Star-A and Star-F), which were delicately designed to achieve adequate electrical properties, electrostatic potential, and geometrical structure, were used to further optimize the microscopic morphology. Grazing-incidence small-angle X-ray scattering (GISAXS) and grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements showed that the star additives not only induce the dense and enhanced microscopic intermolecular π-π stacking within the phase, but also further optimize the phase separation. By adding only 1% star-series solid additives, a significantly enhanced efficiency was achieved from PM6:Y6 solar cells.