Friction Force Mapping of Molecular Ordering and Mesoscopic Phase Transformations in Layered-Crystalline Organic Semiconductor Films

It is critical to understand molecular ordering processes in small-molecule organic semiconductor (OSC) films in optimizing electronic device applications, although it is difficult to observe and investigate the ordering characteristics at a mesoscopic or device scale. Here, we report that friction force microscopy (FFM) allows visualizing the ordering transformation process from a thermodynamically metastable phase to a stable phase at a mesoscopic scale. We utilized 2-octyl-benzothieno[3,2-b]naphtho[2,3-b]thiophene (2-C8-BTNT) as a typical highly layered-crystalline OSC. We found that the friction force between an AFM tip and spin-coated OSC films significantly depends on whether local film states are in metastable monolayer phase or stable bilayer-type herringbone (b-LHB) phase that exhibits high carrier mobility. The formation of the stable b-LHB phase leads to lower friction than the metastable monolayer phase, clearly visualizing the molecular order. Force map (Fmap) analysis indicates that the lower friction in the b-LHB phase should be associated with the reduction of interfacial adhesion force. Notably, the observed results demonstrate that the spin-coated thin film changes from continuous film with the monolayer phase to rugged microcrystal grains with the b-LHB phase when left at ambient conditions. By contrast, an appropriate post-thermal annealing process facilitates the phase transformation without inducing such morphological changes. The technique provides a unique and effective tool for revealing the relationship between processing conditions and device performance in polycrystalline OSC films.

Waterproof and Flexible Perovskite Photodetector Enabled By P-type Organic Molecular Rubrene with High Moisture and Mechanical Stability

Metal halide perovskite films have gained significant attention because of their remarkable optoelectronic performances. However, their poor stability upon the severe environment appears to be one of the main facets that impedes their further commercial applications. Herein, a method to improve the stability of flexible photodetectors under water and humidity environment without encapsulation is reported. The devices are fabricated using the physical vapor deposition method (Pulse Laser Deposition & Thermal Evaporation) under high-vacuum conditions. An amorphous organic Rubrene film with low molecular polarity and high elastic modulus serves as both a protective layer and hole transport layer. After immersed in water for 6000 min, the photoluminescence intensity attenuation of films only decreased by a maximum of 10%. The demonstrator device, based on Rubrene/CsPbBr3 /ZnO heterojunction confirms that the strategy not only enhances device moisture and mechanical stability but also achieves high sensitivity in optoelectronic detection. In self-powered mode, it has a fast response time of 79.4 µs /207.6 µs and a responsivity 124 mA W-1 . Additionally, the absence of encapsulation simplifies the fabrication of complex electrodes, making it suitable for various applications. This study highlights the potential use of amorphous organic films in improving the stability of perovskite-based flexible devices.

Impact of a rubrene buffer layer on the dynamic magnetic behavior of nickel layers on Si(100)

Interfaces of ferromagnetic/organic material hybrid structures refer to the spin interface that governs physical properties for achieving high spin polarization, low impedance mismatch, and long spin relaxation. Spintronics can add new functionalities to electronic devices by taking advantage of the spin degree of freedom of electrons, which makes understanding the dynamic magnetic properties of magnetic films important for spintronic device applications. Our knowledge regarding the magnetic dynamics and magnetic anisotropy of combining ferromagnetic layer and organic semiconductor by microwave-dependent magnetic measurements remains limited. Herein, we report the impact of an organic layer on the dynamic magnetic behavior of nickel/rubrene bilayers deposited on a Si(100) substrate. From magnetic dynamic measurements, opposite signs of effective magnetic fields between the in-plane (IP) and out-of-plane (OP) configurations suggest that the magnetization of Ni(x)/rubrene/Si prefers to coexist. A shift in OP resonance fields to higher values can mainly be attributed to the enhanced second-order anisotropy parameter K2 value. Based on IP measurements, a two-magnon scattering mechanism is dominant for thin Ni(x)/rubrene/Si bilayers. By adding a rubrene layer, the highly stable IP combined with the tunable OP ferromagnetic resonance spectra for Ni(x)/rubrene/Si bilayers make them promising materials for use in microwave magnetic devices and spintronics with controllable perpendicular magnetic anisotropy.

Structural characterization and electronic properties of Ni/rubrene bilayers with alternative stacking sequences

Recent progress in organic electronics has attracted interest due to their excellent characteristics that include photovoltaic, light emission, and semiconducting behaviours. Spin-induced properties play important roles in organic electronics, while introducing spin into an organic layer in which spin responses, such as a weak spin-orbital coupling and long spin-relaxation time, allows a variety of spintronic applications to be achieved. However, such spin responses are rapidly attenuated by misalignment in the electronic structure of hybrid structures. We report herein on the energy level diagrams of Ni/rubrene bilayers that can be tuned by an alternating stacking. The band edges of the highest occupied molecular orbital (HOMO) levels were determined to be 1.24 and 0.48 eV relative to the Fermi level for Ni/rubrene/Si and rubrene/Ni/Si bilayers, respectively. This raises a possibility of accumulating electric dipoles at the ferromagnetic/organic semiconductor (FM/OSC) interface, which would inhibit the transfer of spin in the OSC layer. This phenomenon is caused by the formation of a Schottky-like barrier in the rubrene/Ni heterostructures. According to the information about the band edges of the HOMO levels, schematic plots of the HOMO shift in the electronic structure of the bilayers are presented. Based on the lower value of the effective uniaxial anisotropy for Ni/rubrene/Si, the uniaxial anisotropy was suppressed compared to that of rubrene/Ni/Si. The characteristics of the formation of Schottky barriers at the FM/OSC interface have an impact on the temperature-dependent spin states in the bilayers.

Investigation of Singlet Fission-Halide Perovskite Interfaces

A method for improving the efficiency of solar cells is combining a low-band-gap semiconductor with a singlet fission material (which converts one high-energy singlet into two low-energy triplets following photoexcitation). Here, we present a study of the interface between singlet fission molecules and low-band-gap halide pervoskites. We briefly present 150 experiments screening for triplet transfer into a halide perovskite. However, in all cases, triplet transfer was not observed. This motivated us to understand the halide perovskite-singlet fission interface better by carrying out first-principles calculations using tetracene and cesium lead iodide. We found that tetracene molecules/thin films preferentially orient themselves parallel to/perpendicular to the halide perovskite's surface. This result is in agreement with simulations of tetracene (and other rodlike molecules) on a wide range of inorganic semiconductors. We present formation energies of all interfaces, which are significantly less favorable than for bulk tetracene, indicative of weak interaction at the interface. It was not possible to calculate excitonic states at the full interface due to computational limitations, so we instead present highly speculative toy interfaces between tetracene and a halide-perovskite-like structure. In these models, we focus on replicating tetracene's electronic states correctly. We find that tetracene's singlet and triplet energies are comparable to that of bulk tetracene, and the triplet is strongly localized on a single tetracene molecule, even at an interface. Our work provides new understanding of the interface between tetracene and halide perovskites, explores the potential for modeling excitons at interfaces, and begins to explain the difficulties in extracting triplets directly into inorganic semiconductors.

Rubbing-free liquid crystal electro-optic device based on organic single-crystal rubrene

Liquid crystals (LCs) have been a vital component of modern communication and photonic technologies. However, traditional LC alignment on polyimide (PI) requires mechanically rubbing treatment to control LC orientation, suffering from dust particles, surface damage, and electrostatic charges. In this paper, LC alignment on organic single-crystal rubrene (SCR) has been studied and used to fabricate rubbing-free LC devices. A rubrene/toluene solution is spin-coated on the indium-tin-oxide (ITO) substrate and transformed thereafter to the orthorhombic SCR after annealing. Experimental result reveals that SCR-based LC cell has a homogeneous alignment geometry, the pretilt angle of LCs is low and the orientation of LCs is determined with capillary filling action of LCs. LC alignment on SCR performs a wider thermal tolerance than that on PI by virtue of the strong anchoring nature of LCs on SCR due to van der Waals and π-π electron stacking interactions between the rubrene and LCs. SCR-based LC cell performs a lower operation voltage, faster response time, and higher voltage holding ratio than the traditional PI-based LC cell. Organic SCR enables to play a role as weakly conductive alignment layer without rubbing treatment and offers versatile function to develop novel LC devices.

Comparisons of magnetic defects and coercive forces for Co/Si(100) and Co/rubrene/Si(100)

Spintronics can add new functionalities to electronic devices by utilizing the spin degree of freedom of electrons. Investigating magnetic defects is crucial for the performance of spintronics devices. However, the effects of magnetic defects that are introduced by the presence of organic materials on their magnetic properties remain unclear. Herein, we report on a novel method using rubrene combined with Kerr microscopy that enables quantitative and direct measurements of magnetic defect density. For Co/Si(100) at a magnetic field near the coercivity value, Kerr microscopy images show a dark image with some magnetic defects. Because of domain wall motion, small patches gradually change the contrast. These magnetic defects are immovable at different magnetic fields and serve as pinning sites for domain wall motion. Experimental evidence shows that coercive force can be reduced by controlling the magnetic defect density by introducing small amounts of rubrene into the films. Furthermore, direct quantitative measurements of magnetic defects show both a one-dimensional bowing of domain walls and strong defect-domain wall interactions in the films. Based on these findings, we propose a viable strategy for reducing the coercive force of Co/Si(100) by controlling the magnetic defect density and this new information promises to be valuable for future applications of spintronics devices.

Correlation between Nanoscale Elasticity, Semiconductivity, and Structural Order in Functionalized Polyaniline Thin Films

The correlation between structural order, elasticity, and semiconductivity for butylthio-functionalized polyaniline (PANI-SBu) thin films was investigated using atomic force microscopy (AFM)-based techniques with X-ray diffraction (XRD) and scanning electron microscopy (SEM). After different stirring times, the thin films were cast from the solution of PANI-SBu in N-methyl-2-pyrrolidone that was continuously stirred at a constant rate of 150 rpm in an airtight round-bottom flask. According to the XRD and SEM results, the cross-sectional film structure evolved from being generally holey to highly lamellar with an increase in the stirring time. However, some new types of disordered structures began emerging beyond the optimal stirring time, possibly caused by the formation of disordered packing structures as contributed from the overoxidized polyaniline backbones during the additional stirring time. Moreover, according to the investigation results obtained using AFM-based techniques, the out-of-plane elastic moduli and charge mobilities of the PANI-SBu films were consistently smaller for disordered thin films and larger for structurally more ordered ones. The shear force resulting from the mechanical stirring of the PANI-SBu solution may gradually disentangle the polymer chains and thus help transform the individual polyaniline molecule from a coil-like chain conformation to a better extended rodlike chain conformation. Therefore, when cast into a film, the stretched polymer chains facilitate self-organization among the PANI-SBu backbones during the film formation process. Thus, an improved structural order in the film is attained. Our results demonstrate an unambiguous correlation between the structure order, elasticity, and conductivity in PANI-SBu thin films, which may have useful applications in conducting polymer-based flexible electronics.