Damaging Effect of Hot Metal Atoms on Organic Semiconducting Films during Top Contact Formation

The formation of a high quality interface between metallic and organic semiconducting thin films is critically important in achieving high-performance organic electronics devices. In this regard, the importance of understanding the multifaceted issue of structure damage incurred to organic films by the evaporated metal atoms cannot be overstated. In the present study, we have investigated the change of a structurally ordered, organic semiconducting (o.s.) thin film of 5,11-bis­(triethylsilylethynyl)­anthradithiophene (TESADT) effected by gold atoms by means of synchrotron-based soft X-ray spectroscopies including ultraviolet photoemission spectroscopy (UPS) and X-ray photoemission spectroscopies (XPS) with imaging capability, near edge X-ray absorption fine structure (NEXAFS) spectroscopy, and atomic force microscopy (AFM). This work shows that gold atoms readily diffuse into the organic films and nucleate into nanometer-size clusters, damage chemical structure, destroy structural ordering of the organic films, and shift relevant core level binding energy in accord with the expected interfacial band bending. Additionally, the patterned deposition performed via shadow mask is not reliable in confining Au deposit to the designated region due to the rapid diffusion of Au atoms. As a result, the real Au contacts should be treated as morphologically complicated gold films residing on top of structurally disordered organic film interspersed with Au clusters

Enhanced Stability of Organic Field-Effect Transistors with Blend Pentacene/Anthradithiophene Films

By taking advantage of the similarity between pentacene (PEN) and anthradithiophene (ADT) in molecular dimension and charge transport property, we have produced organic field-effect transistors (OFETs) with active layers consisting of well-blended PEN/ADT films. The blend-films were characterized by atomic force microscopy, X-ray diffraction, and soft X-ray spectroscopies. It is found that the blend-films containing no more than 10% of ADT exhibit a single-phase structure, large crystallinity, and improved oxidation resistance, as compared to PEN. The best performance achieved with 90% PEN-OFET gives a mobility of 0.37 cm² V^–1 s^–1 and an on/off current ratio of 10⁷. More importantly, this device provides a 3-fold improvement in operational stability as well as extended environmental stability. After the repetitive scanning between on and off states of OFET in ambient 940 times, the mobility decreases only to 0.33 cm² V^–1 s^–1. In comparison, the mobility of PEN-OFET decreases from 0.46 to 0.22 cm² V^–1 s^–1. After 3-month storage in ambient, the mobility of the optimal device decreases to 0.1 cm² V^–1 s^–1, whereas PEN-OFET almost loses its mobility

Core Dominated Surface Activity of Core–Shell Nanocatalysts on Methanol Electrooxidation

The activity of core–shell nanoparticles (NCs) in electrooxidation of methanol (MOR) was found to be dependent on the crystalline structure of the core and the lattice strain at the core–shell interface. Ru-core and Pt-shell NCs delivered 6.1-fold peak MOR current density at −135 mV than Pt NCs, while the Co-core and Pt-shell NCs showed a 1.4-fold peak MOR current density at 280 mV. The current density is improved by the compressive lattice strain of the surface that is caused by the lattice mismatch between the Pt shell and the Ru core. For Co-core NCs, the enhancement results from the ligand effect at surface Pt sites. In addition, the Ru-core NCs maintained a steady current density of 0.11 mA cm^–2 at 500 mV in a half-cell system for 2 h, which is 100-fold higher than that of Pt NCs and Co-core NCs. These results provide mechanistic information for the development of fuel cell catalysts along with reduced Pt utilization and programmable electrochemical performance

Interplay between Interfacial Structures and Device Performance in Organic Solar Cells: A Case Study with the Low Work Function Metal, Calcium

A better understanding of how interfacial structure affects charge carrier recombination would benefit the development of highly efficient organic photovoltaic (OPV) devices. In this paper, transient photovoltage (TPV) and charge extraction (CE) measurements are used in combination with synchrotron radiation photoemission spectroscopy (SRPES) to gain insight into the correlation between interfacial properties and device performance. OPV devices based on PCDTBT/PC₇₁BM with a Ca interlayer were studied as a reference system to investigate the interfacial effects on device performance. Devices with a Ca interlayer exhibit a lower recombination than devices with only an Al cathode at a given charge carrier density (<i>n</i>). In addition, the interfacial band structures indicate that the strong dipole moment produced by the Ca interlayer can facilitate the extraction of electrons and drive holes away from the cathode/polymer interface, resulting in beneficial reduction in interfacial recombination losses. These results help explain the higher efficiencies of devices made with Ca interlayers compared to that without the Ca interlayer

Crystalline Growth of Rubrene Film Enhanced by Vertical Ordering in Cadmium Arachidate Multilayer Substrate

The growth of highly crystalline rubrene thin films for organic field effect transistor (OFET) application remains a challenge. Here, we report on the vapor-deposited growth of rubrene films on the substrates made of cadmium arachidate (CdA) multilayers deposited onto SiO₂/Si­(100) via the Langmuir–Blodgett technique. The CdA films, containing 2<i>n</i>+1 layers, with integer <i>n</i> ranging from 0 to 4, are surface-terminated identically by the methyl group but exhibit the thickness-dependent morphology. The morphology and structure of both CdA and rubrene films are characterized by X-ray reflectivity (XRR), X-ray diffraction (XRD), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and atomic force microscopy (AFM). Crystalline rubrene films, evidenced by XRD and marked by platelet features in AFM images, become observable when grown onto the CdA layer thicker than 5L. XRD data show that vertical ordering, that is, ordering along surface normal, of CdA multilayer substrates exerts a strong influence in promoting the crystalline growth of rubrene films. This promoted growth is not due to the surface energy of CdA layer but derived from the additional interaction localized between rubrene and CdA island sidewall and presumably strengthened by a close dimensional match between the <i>a</i>-axis of rubrene lattice and the layer spacing of CdA multilayer. The best OFET mobility is recorded for 9L CdA substrate and reaches 6.7 × 10^–2 cm² V^–1 s^–1, presumably limited by the roughness of the interface between CdA and rubrene films

Thermal Reaction of 2,4-Dibromopyridine on Cu(100)

Nitrogen-containing aromatics have potential applications in surface functionalization, corrosion inhibition, and carbon-nitride materials. Reflection–absorption infrared spectroscopy (RAIRS), X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), and temperature-programmed reaction/desorption (TPR/D) have been employed to study the system of 2,4-C₅NH₃Br₂/Cu­(100). Our experimental results indicate that 2,4-C₅NH₃Br₂ is adsorbed predominantly in molecular form on Cu(100) at 100 K; however, a tiny fraction of the adsorbed molecules is subjected to debromination. The 2,4-C₅NH₃Br₂ undergoes partial C–Br dissociation below 400 K, forming C₅NH₃Br intermediate. Although after breaking both the C–Br bonds (>400 K), 2,4-pyridyne (C₅NH₃) can be formed, the possibility of Ullmann coupling reaction cannot be excluded. The NEXFAS study shows a ∼ 35° average inclination of the aromatic plane, with respect to the surface, in a packed 2,4-pyridyne adsorption layer. Thermal decomposition of the C₅NH₃ or its coupling reaction products on the Br/Cu(100) surface mainly occurs at a temperature higher than 550 K, generating H₂, HCN, HBr, and (CN)₂

Role of Tin Chloride in Tin-Rich Mixed-Halide Perovskites Applied as Mesoscopic Solar Cells with a Carbon Counter Electrode

We report the synthesis and characterization of alloyed Sn–Pb methylammonium mixed-halide perovskites (CH₃NH₃Sn_<i>y</i>Pb_1–<i>y</i>I_3–<i>x</i>Cl_<i>x</i>) to extend light harvesting toward the near-infrared region for carbon-based mesoscopic solar cells free of organic hole-transport layers. The proportions of Sn in perovskites are well-controlled by mixing tin chloride (SnCl₂) and lead iodide (PbI₂) in varied stoichiometric ratios (<i>y</i> = 0–1). SnCl₂ plays a key role in modifying the lattice structure of the perovskite, showing anomalous optical and optoelectronic properties; upon increasing the concentration of SnCl₂, the variation of the band gap and band energy differed from those of the SnI₂ precursor. The CH₃NH₃Sn_<i>y</i>Pb_1–<i>y</i>I_3–<i>x</i>Cl_<i>x</i> devices showed enhanced photovoltaic performance upon increasing the proportion of SnCl₂ until <i>y</i> = 0.75, consistent with the corresponding potential energy levels. The photovoltaic performance was further improved upon adding 30 mol % tin fluoride (SnF₂) with device configuration FTO/TiO₂/Al₂O₃/NiO/C, producing the best power conversion efficiency, 5.13%, with great reproducibility and intrinsic stability

Selective Hydrogen Etching Leads to 2D Bi(111) Bilayers on Bi₂Se₃: Large Rashba Splitting in Topological Insulator Heterostructure

Ultrathin bilayers (BLs) of bismuth have been predicated to be a two-dimensional (2D) topological insulator. Here we report on a new route to manufacture the high-quality Bi bilayers from a 3D topological insulator, a top-down approach to prepare large-area and well-ordered Bi(111) BL with deliberate hydrogen etching on epitaxial Bi₂Se₃ films. With scanning tunneling microscopy (STM) and X-ray photoelectron spectra (XPS) <i>in situ</i>, we confirm that the removal of Se from the top of a quintuple layer (QL) is the key factor, leading to a uniform formation of Bi(111) BL in the van der Waals gap between the first and second QL of Bi₂Se₃. The angle resolved photoemission spectroscopy (ARPES) <i>in situ</i> and complementary density functional theory (DFT) calculations show a giant Rashba splitting with a coupling constant of 4.5 eV Å in the Bi(111) BL on Bi₂Se₃. Moreover, the thickness of Bi BLs can be tuned by the amount of hydrogen exposure. Our ARPES and DFT study indicated that the Bi hole-like bands increase with increasing the Bi BL thickness. The selective hydrogen etching is a promising route to produce a uniform ultrathin 2D topological insulator (TI) that is useful for fundamental investigations and applications in spintronics and valleytronics