Rubrene Films Research Articles

Pretilt angle modulation of liquid crystals based on single crystal rubrene with incorporation of homeotropic polyimides

The alignment control of liquid crystals (LCs) is critical for various practical applications. The pretilt angle modulation of LCs typically requires a mechanical rubbing on substrates to orient the LCs. This study presents a contact-free approach to achieve pretilt angle modulation of LCs. Initially, a single crystal rubrene (SCR) film is deposited on the substrate. Subsequently, a mixture of nematic LCs and homeotropic polyimides (HPIs) is introduced between two SCR substrates via capillary action. During capillary filling, the synergy of capillary flow and the interaction between LCs and SCR ensures specific LC orientation. Subsequently, HPI dopants migrate toward and organize on the SCR through vertical phase separation, enhancing surface hydrophobicity and thereby increasing the pretilt angle of LCs. The pretilt angle of LCs can be continuously adjusted over a wide range from 2° to 90° by varying the HPI concentration. The contact-free process preserves against electrostatic charges, dust contamination, and surface damage typical of rubbed LC cells. This believed to be novel technique shows promise for developing no-bias-bend and bistable bend-splay LC displays.

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 6000min, 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 124mAW-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.

Kinetics of the Decay of Excited Singlet State into a Pair of T-Excitons in Rubrene Films: Mechanism and Manifestation of Exciton Migration

The kinetics of the decay (splitting) of the excited singlet -state of rubrene molecules into a pair oftriplet-excitons (T-excitons) in rubrene films, usually represented in terms of the kinetics of the decay of fluorescence(KDF) from the -state, is analyzed in detail. The KDF is known to be significantly controlled bythe process of diffusive migration and annihilation of the generated T-excitons. In the analysis, two migrationmodels are considered: the two-state model (TSM), treating the migration effect as a result of transitionsbetween the [TT] state of coupled T-excitons (at small TT-distances r) and the [T+T]-state of freely migratingЕ-excitons (at large distances r), as well as the free migration model (FMM), neglecting the effect of the [TT]state. Within the TSM and FMM, the expressions for are derived, which are applied to describe the KDF ,measured in amorphous rubrene films. Within the experimentally investigated range of times, , the TSM isshown to reproduce the behavior of the experimental KDF much more accurately than the FMM. At longertimes a substantial difference () between and the FMM-predicted KDF is found, which is far beyond theexperimental error (3%).

Kinetics of the Decay of Excited Singlet State into a Pair of T-Excitons in Rubrene Films: Mechanism and Manifestation of Exciton Migration

Kinetics of the Decay of Excited Singlet State into a Pair of T-Excitons in Rubrene Films: Mechanism and Manifestation of Exciton Migration

From Amorphous to Polycrystalline Rubrene: Charge Transport in Organic Semiconductors Paralleled with Silicon

AbstractWhile progress has been made in the design of organic semiconductors (OSCs) with improved transport properties, the understanding of the mechanisms involved is still limited, hindering further development. In this study, the interplay between structural order and transport considering one single OSC, analogous to past research on silicon is investigated. Rubrene (C42H28) is selected as it spans transport mechanisms from thermally activated hopping in its amorphous form to band‐like in highly ordered crystals in the orthorhombic polymorph. Transport characterizations including variable temperature conductivity, advanced Hall effect, and magnetoresistance measurements are performed on rubrene films with varying levels of order (polycrystalline vs amorphous), crystal phase (orthorhombic vs triclinic), and morphologies (platelet‐like vs spherulitic grains). A conductivity tuning range over four orders of magnitude between polycrystalline (platelet‐like) orthorhombic and amorphous films is reported. As observed in silicon, transport in polycrystalline orthorhombic rubrene is limited by energy barriers at grain boundaries. Additionally, a gradual transition from predominantly band‐like to predominantly hopping transport with increasing disorder, reminiscent of observations in silicon is shown. Nevertheless, OSCs differ from covalently bonded silicon by their weak intermolecular interaction. This study highlights that molecular packing must be optimized in OSCs to favor advantageous π‐orbital overlap and optimized transport properties.

Sweet Spot of Intermolecular Coupling in Crystalline Rubrene: Intermolecular Separation to Minimize Singlet Fission and Retain Triplet–Triplet Annihilation

Singlet fission is detrimental to NIR-to-vis photon upconversion in the solid rubrene (Rub) films, as it diminishes photoluminescence efficiency. Previous studies have shown that thermally activated triplet energy transport drives singlet fission with nearly 100% efficiency in closely packed Rub crystals. Here, we examine triplet separation and recombination as a function of intermolecular distance in the crystalline films of Rub and the t-butyl substituted rubrene (tBRub) derivative. The increased intermolecular distance and altered molecular packing in tBRub films cause suppressed singlet dissociation into free triplets due to slower triplet energy transport. It was found that the formation of correlated triplet pairs 1(TT) and partial triplet separation 1(T···T) occurs in both Rub and tBRub films despite differences in intermolecular coupling. Under weak intermolecular coupling as in tBRub, geminate triplet annihilation of 1(T···T) outcompetes dissociation into free triplets, resulting in emission from the 1(TT) state. Essentially, increasing intermolecular distance up to a certain point (a sweet spot) is a good strategy for suppressing singlet fission and retaining triplet–triplet annihilation properties.

Raman spectroscopy approach to determine crystallographic and molecular orientation of rubrene molecular films

Raman spectroscopy approach to determine crystallographic and molecular orientation of rubrene molecular films

Molecular crystallization of rubrene thin films assisted by gold nanoparticles

We demonstrate that crystalline organic rubrene thin films can be obtained by a facile spin-coating method using gold (Au) nanoparticles (NPs). Dodecanethiol-functionalized Au NPs were dissolved with rubrene molecules in solvent and a thin film of Au/rubrene was prepared by a simple spin coating process. The results of confocal photoluminescence (PL) and absorption spectral mapping confirmed the local formation of orthorhombic crystalline structures of the Au/rubrene hybrid film, in contrast to the monoclinic structure of plain rubrene films. Further, the results of transmission electron microscopy (TEM) and X-ray diffraction analysis, as well as Raman spectroscopy measurements of the rubrene and Au/rubrene films suggested the formation of high crystalline Au/rubrene film. The molecular crystallization of the Au/rubrene hybrid film is attributed to the nucleation effect of the Au NPs.

Self-assembled and crystalline films of rubrene grown on Cd(0001) surface

We investigated the 2D self-assembled thin films of rubrene molecules on metallic Cd(0001) surface using low-temperature scanning tunneling microscopy. The isolated rubrene molecules adsorbed on Cd(0001) surface exhibit chiral characteristics. Three self-assembled structures, zigzag chain, linear chain and dimerized chain, have been identified in rubrene monolayer. The zigzag chain resembles the a-c plane of the orthorhombic crystal. Furthermore, we found the monoclinic phase of rubrene crystals in the second rubrene layer, theoretically predicted but not experimentally confirmed. These results provide new choices and ideas for preparing a new rubrene light-emitting device.

Growth Behavior of Rubrene Thin Films on Hexagonal Boron Nitride in the Early Stage

Comprehensive SummaryTwo‐dimensional materials, with an in‐plane ordered and dangling‐bonding‐free surface, are ideal substrates for fabricating high‐ quality crystalline thin films. Here, we show a systematic study on the growth of a benchmark organic semiconductor, rubrene, on hexagonal boron nitride (h‐BN) substrate via physical vapor deposition from the initial amorphous phase to the final crystalline phase; the role of temperature in such transition and the epitaxy relationship between rubrene and h‐BN are revealed. With the increase of substrate temperature, the critical thickness of amorphous‐crystalline‐transition decreases and the morphology of crystalline phase also evolves from porous to terrace‐like. When substrate temperature reaches >100°C, the critical thickness reduces to only 0.5 nm and a precise layer‐by‐layer growth from the very first layer is achieved, which is quite rare for rubrene growing on other substrates. The high ordering can be attributed to the fine epitaxy relationship between rubrene films and the h‐BN surface lattice, and this film demonstrates good charge transport ability with a p‐type field‐effect mobility of >1 cm2·V–1·s–1.

Reasons for “disappearance” phenomenon of both intersystem crossing of polaron-pair states and reverse intersystem crossing of high-lying triplet excitons in pure Rubrene-based OLEDs

With unique advantages of high sensitivity, no-contact, and non-destructiveness, magneto-electroluminescence (MEL) is usually employed as an effective detection tool to visualize the microscopic mechanisms of excited states existing in organic light-emitting diodes (OLEDs) because their evolution channels of many spin-pair states in OLEDs have the fingerprint MEL line-shapes even with opposite signs. The recently-published MEL results (Tang X T, Pan R H, Zhao X, Jia W Y, Wang Y, Ma C H 2020 <i>Adv. Funct. Mater.</i> <b>5</b> 765) have demonstrated the existence of high-level reverse intersystem crossing process (HL-RISC, S<sub>1,Rub</sub> ← T<sub>2,Rub</sub>) of high-lying triplet excitons (T<sub>2,Rub</sub>) in Rubrene when Rubrene with a typical value of several percent in content is doped into a host with high triplet exciton energy and there are no energy loss channels of triplet excitons from charge-carrier transporting layers either. Furthermore, this HL-RISC process can considerably increase the efficiency and brightness of OLEDs operated at room temperature, for example, high external quantum efficiency up to 16.1% and ten thousands of brightness have been achieved in Rubrene-doped OLEDs with a co-host of exciplex. Herein, surprisingly, in the pure Rubrene-based OLEDs (i.e. the pure Rubrene film is used as an emissive layer) with no energy loss channels of triplet excitons from charge-carrier transporting layers, only strong singlet fission (S<sub>1,Rub</sub>+S<sub>0,Rub</sub> → T<sub>1,Rub</sub>+T<sub>1,Rub</sub>) processes are detected at room temperature, but this HL-RISC process is not observed. Moreover, even the most usual evolution process of intersystem crossing of polaron-pair (ISC, PP<sub>1</sub> → PP<sub>3</sub>) cannot be observed in this pure Rubrene-based OLEDs, where the polaron-pair is generated through the recombination of the injected electrons and holes in the pure Rubrene emissive layer. To determine the cause of the underlying physical mechanism behind this abnormal and fascinating experimental phenomena, two kinds of devices with pure Rubrene and 5% Rubrene-dopant as emissive layers are fabricated and their current- and temperature- dependent MEL responses are systematically investigated. By comparing and analyzing these tremendously different MEL curves of these two types of devices, we find that the positive Lorentzian MEL curves induced from <i>B</i>-mediated ISC of polaron-pair just completely cancel out the negative Lorentzian MEL curves induced from <i>B</i>-mediated HL-RISC process of T<sub>2,Rub</sub> excitons. Note that such an abnormal and coincidental experimental phenomenon is the physical reason why the ISC process and HL-RISC process cannot be observed simultaneously in the pure Rubrene-based OLEDs, and this phenomenon has not been found in the literature. Clearly, this work further deepeneds our understanding of some unique microscopic processes and physical phenomena in organic semiconductor “star” material of Rubrene (such as the energy resonance between 2T<sub>1</sub> and S<sub>1</sub> and the energy approach between T<sub>2</sub> and S<sub>1</sub>).

Triplet and singlet exciton diffusion in disordered rubrene films: implications for photon upconversion.

Triplet and singlet exciton diffusion plays a decisive role in triplet-triplet annihilation (TTA) and singlet fission (SF) processes of rubrene (Rub) films at low excitation power, and therefore has an important implication for TTA-mediated photon upconversion (UC). Although triplet diffusion in crystalline Rub was studied before, there is no quantitative data on diffusion in disordered Rub films most widely employed for NIR-to-Vis UC. The lack of these data hinders the progress of TTA-UC applications relying on a Rub annihilator (emitter). Herein, a time-resolved PL bulk-quenching technique was employed to estimate the exciton diffusion coefficient (D) and diffusion length (LD) in the neat Rub films as well as Rub-doped PS films at 80 wt% doping concentration, previously reported to be optimal in terms of UC efficiency. The impact of commonly utilized singlet energy collector (sink) DBP on exciton diffusion was also assessed, highlighting its importance exclusively on the dynamics of singlets in Rub films. Our study revealed that triplet diffusion lengths (LTD) of 25-30 nm estimated for the disordered Rub films are sufficient for encountering triplets from the neighboring sensitizer molecules at a low sensitizer PdPc concentration (0.1 wt%), thereby enabling the desired TTA domination regime to be reached. Essentially, the performance of Rub-based UC systems was found to be limited by the modest maximal LTD (up to ∼55 nm) in disordered films resulting from a short maximum triplet lifetime τT (∼100 μs) inherent to this emitter. Thus, to enhance the NIR-to-Vis TTA-UC performance, new emitters with a longer triplet lifetime in the solid state are required.

Efficient NIR-to-vis photon upconversion in binary rubrene films deposited by simplified thermal evaporation

Simplified thermal evaporation of rubrene in an inert atmosphere is demonstrated to alter the morphology of photon upconverting (UC) films, and subsequently ramp their NIR-to-vis UC quantum yield up to a record value of 1.2% (out of maximum 50%).

Magnetic-Field-Switchable Laser via Optical Pumping of Rubrene.

Volumetric optical imaging of magnetic fields is challenging with existing magneto-optical materials, motivating the search for dyes with strong magnetic field interactions, distinct emission spectra, and an ability to withstand high photon flux and incorporation within samples. Here, the magnetic field effect on singlet-exciton fission is exploited to demonstrate spatial imaging of magnetic fields in a thin film of rubrene. Doping rubrene with the high-quantum yield dye dibenzotetraphenylperiflanthene (DBP) is shown to enable optically pumped, slab waveguide lasing. This laser is magnetic-field-switchable: when operated just below the lasing threshold, application of a 0.4 T magnetic field switches the device between nonlasing and lasing modes, accompanied by an intensity modulation of +360%. This is thought to be the first demonstration of a magnetically switchable laser, as well as the largest magnetically induced change in emission brightness in a singlet-fission material to date. These results demonstrate that singlet-fission materials are promising materials for magnetic sensing applications and could inspire a new class of magneto-optical modulators.

Rubrene Thin Films with Viably Enhanced Charge Transport Fabricated by Cryo-Matrix-Assisted Laser Evaporation.

Among organic semiconductors, rubrene (RB; C42H28) is of rapidly growing interest for the development of organic and hybrid electronics due to exceptionally long spin diffusion length and carrier mobility up to 20 cm2V−1s−1 in single crystals. However, the fabrication of RB thin films resembling properties of the bulk remains challenging, mainly because of the RB molecule’s twisted conformation. This hinders the formation of orthorhombic crystals with strong π–π interactions that support the band transport. In this work, RB films with a high crystalline content were fabricated by matrix-assisted laser evaporation and the associated structure, composition, and transport properties are investigated. Enhanced charge transport is ascribed to the crystalline content of the film. Spherulitic structures are observed on top of an amorphous RB layer formed in the initial deposition stage. In spherulites, orthorhombic crystals dominate, as confirmed by X-ray diffraction and the absorption and Raman spectra. Surprisingly, nanowires several microns in length are also detected. The desorption/ionization mass and X-ray photoelectron spectra consistently show minimal material decomposition and absence of RB peroxides. The observed carrier mobility up to 0.13 cm2V−1s−1, is close to the technologically accepted level, making these rubrene films attractive for spintronic and optoelectronic applications.

Tunable self-trapped excitons in 2D layered rubrene

As emerging efficient emitters, two-dimensional (2D) organic semiconductors offer an intriguing potential to the low-cost and high-performance light emitting devices. However, organic semiconductors usually suffer from serious luminescence quenching owing to the self-trapped exciton formation which prevalently occur in materials with soft lattice and strong exciton-phonon coupling. Therefore, revealing the underlying mechanism that leads to self-trapped excitons is a prerequisite for increasing the photoluminescence (PL) efficiency in organic materials. Here, we grew high-quality layered rubrene films on hexagonal boron nitride through a physical vapor transport method. Combined with time-resolved photoluminescence (TRPL) spectra and laser power dependent TRPL spectra, we confirmed the free exciton (FE) and self-trapped exciton (STE) emission. We found that STEs evolved from FEs from the temperature dependent PL characterization. In addition, we observed tunable STEs in a 2D layered rubrene: reducing the layer number could strongly suppress the exciton transferring efficiency and the rate from a free exciton state to a self-trapped exciton state which result from the decreased self-trapping depth and increased barrier height. Consequently, the average PL intensity is strongly enhanced about seven times, whereas STE emission is quenched. The results provide a method for suppressing the STE formation process and contribute to improving the photoluminescence efficiency in optoelectronic applications.

High-performance self-powered photodetector based on organic/inorganic hybrid van der Waals heterojunction of rubrene/silicon* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11604228, 11774208, and 11974222), the Science and Technology Planning Foundation of Shandong Province, China (Grant No. J18KA219).

Organic/inorganic hybrid van der Waals heterostructure with an atomically abrupt interface has attracted great research interests within the field of multifunctional electronic and optoelectronic devices. The integration of organic rubrene films with inorganic Si semiconductors can avoid the atomic mutual-diffusion at the interface, and provide the possibility of forming two-dimensional van der Waals heterojunction accompanied with the type-II energy band alignment, due to the transfer behaviors of majority carriers at the interface. In this study, the high-quality rubrene/Si van der Waals heterostructure with an electronically abrupt junction was prepared, and a self-powered photodetector was then constructed based on this hybrid heterojunction. The photodetector demonstrated an excellent switching response to the 1064 nm monochromatic light with large on/off current ratio of 7.0 × 103, the maximum photocurrent of 14.62 mA, the maximum responsivity of 2.07 A/W, the maximum detectivity of 2.9 × 1011 Jones, and a fast response time of 13.0 μs. This study offers important guidance for preparing high-quality rubrene/Si hybrid van der Waals heterostructure with desirable band alignment, and the designed heterojunction photodetector has an important application prospect in the field of multifunctional optoelectronics.

Singlet fission from upper excited singlet states and polaron formation in rubrene film.

Femtosecond fluorescence up-conversion and transient absorption pump-probe setups are applied to study the relaxation dynamics of the lower and upper excited singlet electronic states in easy-to-make rubrene films. Upon 250 nm (4.96 eV) excitation, singlet fission was observed directly from S2 state bypassing S1 state within 30 fs i.e. breaking the classical Kasha rule. From the transient absorption measurements, polaron formation was also detected on the same time scale. Both singlet fission and polaron formation are accelerated from upper excited states compared with S1 state. Our work shows that rubrene films with low degree of crystallinity could display efficient singlet fission, notably in the case of excitation to upper lying electronic states. This can strongly expand the applications of rubrene in organic electronics. Moreover, our results will provide a new direction for synthesizing novel materials with optimized excited state properties for organic photovoltaic applications.

Morphology and transport characterization of solution-processed rubrene thin films on polymer-modified substrates

In this report, the morpho-structural peculiarities and the crystallization mechanisms in solution-processed, solvent vapor annealed (SVA) thin films of rubrene (5,6,11,12-tetraphenylnaphthacene) on different substrates were investigated. The high-quality rubrene crystal films with a triclinic crystal structure were successfully prepared on the FTO substrates (glass slide coated with fluorine-tin-oxide) modified by PLA (polylactic acid) for the first time. The area coverage of rubrene crystal and the sizes of rubrene dendritic crystals increased with increasing thickness of PLA film and concentration of rubrene solution. For rubrene molecules, FTO wafers with rough surface provided the possibility of heterogeneous nucleation. During the SVA process, there were two kinds of forces acting on the diffusion of rubrene molecules: one force was provided by the residual chloroform solvent, which was perpendicular to the substrate, and the other force was provided by gaseous dichloromethane, which was parallel to the substrate. The synergy of these two forces was proposed to explain the nucleation and the crystallization processes of rubrene films. The higher nucleus of PLA/rubrene dendrites and the layer-by-layer stacking of needle-shaped nanocrystalline PLA/rubrene were important for exploring their kinetic formation process.

Impact of t-butyl substitution in a rubrene emitter for solid state NIR-to-visible photon upconversion.

Solid state NIR-to-visible photon upconversion (UC) mediated by triplet-triplet annihilation (TTA) is necessitated by numerous practical applications. Yet, efficient TTA-UC remains a highly challenging task. In this work palladium phthalocyanine-sensitized NIR-to-vis solid UC films based on a popular rubrene emitter are thoroughly studied with the primary focus on revealing the impact of t-butyl substitution in rubrene on the TTA-UC performance. The solution-processed UC films were additionally doped with a small amount of emissive singlet sink tetraphenyldibenzoperiflanthene (DBP) for collecting upconverted singlets from rubrene and in this way diminishing detrimental singlet fission. Irrespective of the excitation conditions used, t-butyl-substituted rubrene (TBR) was found to exhibit enhanced TTA-UC performance as compared to that of rubrene at an optimal emitter doping of 80 wt% in polystyrene films. Explicitly, in the TTA dominated regime attained at high excitation densities, 4-fold higher UC quantum yield (ΦUC) achieved in TBR-based films was caused by the reduced fluorescence concentration quenching mainly due to suppressed singlet fission. Under low light conditions, i.e. in the regime governed by spontaneous triplet decay, even though triplet exciton diffusion was obstructed in TBR films by t-butyl moieties, the subsequently reduced TTA rate was counterbalanced by both suppressed singlet fission and non-radiative triplet quenching, still ensuring higher ΦUC of these films as compared to those of unsubstituted rubrene films.