Rubrene Films Research Articles

Crystalline rubrene via a novel process and realization of a pyro-phototronic device with a rubrene-based film

A single-step solvent-free process for crystalline rubrene film where surface layer polarization results in the pyro-phototronic effect.

Effect of silver nanoplate on singlet exciton fission in rubrene polymer-composite films

The effect of silver nanoplate (AgPL) on the singlet exciton fission (SF) of rubrene (Rub) was verified by examining the magnetic field effects (MFEs) on the SF of Rub through measuring magnetic field dependence of fluorescence intensity of Rub in two thick Rub polymer-composite films in the absence and the presence of AgPL. The fluorescence intensity of Rub in the Rub-AgPL polymer-composite film was larger than that in the Rub polymer-composite film. The enhancement (1.4 times) must be caused by strong electric fields due to localized surface plasmon resonance (LSPR) of the embedded AgPL near the Rub in polymer matrix. The magnitude of the MFEs in the Rub-AgPL polymer-composite film was almost same as those in the Rub polymer-composite film. These results strongly indicate that the efficiency of SF was enhanced by the LSPR due to the embedded AgPL into the polymer matrix in the Rub-AgPL polymer-composite film.

Routes to singlet exciton fission in rubrene crystals and amorphous films

By using wavelength-tunable, low fluence picosecond pulses at a 1 kHz repetition rate in a transient grating pump and probe configuration, we show that photoexcitation to higher vibrational levels leads to slower singlet exciton fission rates in rubrene. After the 1 ps photoexcitation, the initial growth of the triplet exciton population is exponential, with its time-constant systematically varying from 2.5 ± 0.7 ps to 40 ± 15 ps as the excitation photon energy is increased from the lowest to the third main vibrational band in the vibrational progression of rubrene’s absorption spectrum. We also determine that short-pulse-induced fluorescence in amorphous rubrene films deposited by molecular beam epitaxy in high vacuum decays as a single exponential with a lifetime of 15.2 ns, close to rubrene’s radiative lifetime for molecules in solution. This shows that singlet fission is non-existent in fully amorphous rubrene films, despite the close molecular packing, which indicates that in these films there is no short range molecular order matching the molecular arrangement in orthorhombic rubrene crystals.

Quantitative study of spin relaxation in rubrene thin films by inverse spin Hall effect

Spin relaxation properties of π-conjugated organic semiconductors are key indicators of the performance of organic spintronic devices. However, reliable determination of spin relaxation parameters in organic materials is hindered by complex interfacial phenomena at organic/ferromagnetic metal interfaces that couple spin injection with charge injection. Here, we study the spin pumping induced pure spin transport in Permalloy/rubrene/Pt trilayers and determine the spin diffusion length λs = 132 ± 9 nm and the spin relaxation time τs = 3.8 ± 0.5 ms in rubrene films at room temperature by using the inverse spin Hall effect. The determined spin diffusion length λs is found to be almost two times larger than that of ∼46.3 nm at 100 K extracted from rubrene spin valve devices in which charge carrier injection/detection occurs at organic/ferromagnetic metal interfaces. Our results demonstrate experimentally that the efficiency and the rate of spin polarized charge transport through the organic/ferromagnetic metal interface play a dominant role in determining the spin relaxation process of spin valve devices in which charge and spin currents are coupled.

Hall Effect in Polycrystalline Organic Semiconductors: The Effect of Grain Boundaries

AbstractHighly crystalline thin films in organic semiconductors are important for applications in high‐performance organic optoelectronics. Here, the effect of grain boundaries on the Hall effect and charge transport properties of organic transistors based on two exemplary benchmark systems is elucidated: (1) solution‐processed blends of 2,7‐dioctyl[1]benzothieno[3,2‐b][1]benzothiophene (C8‐BTBT) small molecule and indacenodithiophene‐benzothiadiazole (C16IDT‐BT) conjugated polymer, and (2) large‐area vacuum evaporated polycrystalline thin films of rubrene (C42H28). It is discovered that, despite the high field‐effect mobilities of up to 6 cm2 V−1 s−1 and the evidence of a delocalized band‐like charge transport, the Hall effect in polycrystalline organic transistors is systematically and significantly underdeveloped, with the carrier coherence factor α < 1 (i.e., yields an underestimated Hall mobility and an overestimated carrier density). A model based on capacitively charged grain boundaries explaining this unusual behavior is described. This work significantly advances the understanding of magneto‐transport properties of organic semiconductor thin films.

Triplet Sensitization by Lead Halide Perovskite Thin Films for Efficient Solid-State Photon Upconversion at Subsolar Fluxes

Summary We investigate the rubrene triplet sensitization by perovskite thin films based on methylammonium formamidinium lead triiodide (MAFA) of varying thicknesses. The power-law dependence of both the MAFA photoluminescence (PL) intensity and upconverted emission is tracked as a function of the incident power density. Bimolecular triplet-triplet annihilation (TTA) exhibits a unique power-law dependence with a slope change from quadratic-to-linear at the threshold Ith. The underlying MAFA PL power-law dependence dictates the power law of the upconverted PL: (1) below Ith, the slope of the upconverted PL is twice the value of the MAFA PL; (2) above Ith, it follows the same power law as the underlying recombination of mobile electrons and holes in the MAFA films. We find that the Ith shifts to subsolar incident laser powers when increasing the MAFA thickness above 30 nm. For the thickest MAFA film of 380 nm we find an upconversion threshold of Ith = 7.1 mW/cm2.

Using magneto-electroluminescence to analyze the influence of high temperature environment on exciton evolution processes in organic light-emitting diodes

An organic light-emitting diode (OLED) based on rubrene was fabricated, and the magneto-electroluminescence (MEL) was used to analyze the exciton evolutions in the device under high temperature environment. The MEL characteristics of the device in the high and low applied magnetic fields indicate that the exciton evolution at room temperature is dominated by singlet exciton splitting (STT) processes, 420 K ambient temperature the STT process weakens, but the exciton evolution process of intersystem crossing (ISC) occurs. Combined with the surface morphology, luminescence-current characteristics, current efficiency and spectrum of the device, we believe that the high temperature environment causes a large number of structure defect in the rubrene film. The defect limits the diffusion of the polaron pairs and excitons inside the device, and improves the conversion efficiency between the singlet polaron pairs and triplet polaron pairs, and results in an ISC process that is not conducive to internal quantum efficiency in high temperature. The capture of excitons inhibits the conversion of singlet excitons to triplet excitons, resulting in a weakening of the STT process in high temperature environments, thereby increasing the quantum efficiency of the device. It not only deepens the understanding of the influence of high temperature environment on the exciton evolution process in rubrene type OLED devices, but also provides a technical solution to monitor the structural changes of devices by using organic luminescence magnetic effect.

Solution-processed organic phototransistors based on controllable crystal growth of rubrene thin films with polymer-assisted layer

Crystalline rubrene films were fabricated by polymer-assisted layer in a sequential solution deposition process. Controllable crystal growth was achieved through varying the solution concentration of the polystyrene layers of the bilayer thin films. And the transitions from dendrites to spherulites were observed. The formation mechanism of rubrene crystals was proposed. Phototransistors based on rubrene thin film modified by PS-assisted layer showed a mobility of 1.67 × 10−3 cm2/Vs, a photoresponsivity of 4.7 μA/W, and a photosensitivity of 6900 under 200 mW/cm2 light power conditions. This process provides a valuable platform for studying the organic crystal growth and its photoelectric properties.

Variations of the elastic modulus perpendicular to the surface of rubrene bilayer films.

Investigations exploring the inherent mechanical properties of electronic materials have grown rapidly in recent years largely because they are important in developing flexible electronics, organic displays and sensors. However, our understanding of the mechanical properties of organic semiconductors with a thin-film form remains limited. We report herein on an investigation of the structures and related elastic moduli perpendicular to the surface of a rubrene thin film. A rubrene/Si(100) film typically has a cluster-type morphology mainly comprising crystalline nanodomains within the film. We propose a structural bilayer model that can be used to explain the layered nature or characteristics of the rubrene films. As the film thickness is increased, the enhancement in elastic modulus can be attributed to the presence of a soft surface layer on a hard underlayer. Based on four-point probe measurements, the bilayered nature of such materials can be used to characterize their electrical resistive behavior while interfacial roughness is sensitive to the transport paths of conduction electrons. This information is valuable for future applications of organic semiconductors in flexible devices.

Highly efficient red fluorescent organic light-emitting diodes by sorbitol-doped PEDOT:PSS

This work shows a promising approach to improve device performance by optimizing the electron transport and hole injection layers for tetraphenyldibenzoperiflanthene (DBP):rubrene-based red fluorescent organic light-emitting diodes (OLEDs). We compared the effect of two electron transport layers (ETLs), and found that the rubrene/bathophenanthroline (Bphen) ETL-based OLED showed a much higher external quantum efficiency (EQE) (4.67%) than the Alq3 ETL-based OLED (EQE of 3.08%). The doping ratio of DBP in rubrene was tuned from 1.0 wt% to 4.5 wt%, and the 1.5 wt%-DBP:rubrene-based OLED demonstrated the highest EQE of 5.24% and lowest turn-on voltage of 2.2 V. Atomic force microscopy images indicated that 1.5 wt% DBP-doped rubrene film exhibited a regular strip shape, and this regular surface was favorable to the hole and electron recombination in the emitting layer. Finally, the sorbitol-doped poly(3, 4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was used to further improve the EQE; doping with 6 wt% sorbitol achieved the highest current efficiency of 7.03 cd A−1 and an EQE of 7.50%. The significantly enhanced performance implies that the hole injection is a limiting factor for DBP:rubrene-based red fluorescent OLEDs.

33‐1: Invited Paper: Exploring the Formation and Growth of Organic Semiconductors with mm‐Scale Grains

While record mobilities have been reported for organic semiconductors in their single crystal form, the bulk nature of such crystals prohibit their practical application in devices. Here, we discuss our efforts to realize pinhole free films of organic semiconductors with grains of up to 1 mm in extent. One such material is rubrene, but we will also show our efforts on other materials applicable to organic light emitting diodes, such as the electron transport layer 2,2′,2″‐(1,3,5‐benzinetriyl)‐tris(1‐phenyl‐1‐H‐benzimidazole) (TPBi). For rubrene, we will show our efforts to understand crystal formation, epitaxy, and transistors. Homoepitaxial studies uncover evidence of point and line defect formation in these films, indicating that homoepitaxy is not at equilibrium or strain‐free. Point defects that are resolved as screw dislocations can be eliminated under closer‐to‐equilibrium conditions, whereas we are not able to eliminate the formation of line defects. We are, however, able to eliminate these line defects by growing on a bulk single crystal of rubrene, indicating that the line defects are a result strain built into the thin film template, indicating that, in general, organic crystalline thin films may not adopt the exact lattice of a bulk crystal. Transistors made out of these large‐grained films of rubrene display charge carrier mobility of up to 3.5 cm2 V−1s−1, very close to single crystal values, highlighting their potential for practical application.

A new phase of rubrene monoclinic crystals on Cd(0001)

The self-assembly 2D thin films of rubrene molecules on a metallic Cd(0001) surface have been investigated using a low-temperature scanning tunneling microscopy. Compared with other precious metals, the divalent hexagonal close packed metal Cd are less reminiscent of a free electron metal because of the strong repulsive interaction between the 4d state and the conduction bands. It manifests some anisotropic behaviors in thermal conductivity, electron mean free path, and superconducting gaps. So in this paper, Cd was used as a substrate to study rubrene. First, isolated rubrene molecules reveal a tilted orientation on the substrate surface and exhibit three distinct protrusions with chiral feature. When the rubrene coverage increased to 0.4 ML, rubrene molecules begin to assemble into monolayer structure on the Cd(0001) substrate. Rubrene molecules adopt a side-lying orientation, and each rubrene molecule takes the form of zigzag with three bright spots. Upon further deposition of rubrene to 1.5 ML, the orientation transition from titled standing-up monolayer to standing-up monoclinic crystals was observed. There are two kinds of structures in the second layer. The first one is chainlike structure, where rubrene molecules are supported by two phenyls and the planes of tetracene are parallel to each other. The plane of tetracene backbone is perpendicular to the substrate plane. Another novel structure in the second layer of rubrene film is a new monoclinic phase. Furthermore, the rubrene monoclinic phase has a slight compressive strain in the a -axis direction, whereas the compressive strain in the c -axis direction becomes more pronounced. In addition, we noticed that one c -axis of a - c plane of monoclinic phase is parallel to the crystal orientation of the substrate. In particular, this monoclinic phase of rubrene was predicated by DFT calculation but has never been confirmed in experiment.

Kinetics of Prompt Fluorescence Decay in Rubrene Films. Manifestations of the Migration of T Excitons

The kinetics of fast fluorescence decay in organic semiconductors due to the splitting of the excited singlet state S1 into a pair of triplet (T) excitons is significantly influenced by the process of reverse TT annihilation. It is shown that a correct interpretation of this effect requires taking into account the stochastic migration of T excitons. A two-state model (TSM) is proposed for describing the effect of migration on the kinetics of TT annihilation and, thus, on the kinetics of the fission of the S1 state. In the TSM, the migration effect is interpreted in terms of transitions between the [TT] state of the interacting excitons (at small T−T distances) and the [T + T] state of freely diffusing excitons (at large T−T distances). Within the framework of the TSM, an analytical expression for the fluorescence decay kinetics (FDK) $$I_{S_1 } (t)$$ from the S1 state is derived. This expression is used to describe the FDK measured in amorphous rubrene films in the absence of the magnetic field (B = 0) and in the magnetic field B = 8.1 kG. Adjusting the parameters of the model makes it possible to reproduce the experimentally measured FDK with good accuracy. An analysis of the theoretical FDK obtained revealed a significant contribution of T migration to $$I_{S_1 } (t)$$ , which manifests itself, in particular, in the characteristic dependence $$I_{S_1 } (t) \sim t^{ - 3/2}$$ at long times.

Effect of silver nanoparticle on singlet exciton fission in rubrene films

ABSTRACTThe fluorescence intensity from rubrene (Rub) in Rub-silver nanoparticle (AgNP) composite film was larger than that in Rub film. The enhancement is most likely attributable to large electric fields due to localized surface plasmon resonance of AgNP. The magnitude of the magnetic field effects in the Rub-AgNP composite film was almost same as those in the Rub film. These results indicate that the triplet excited states of Rub due to the singlet exciton fission in the presence of AgNP are generated much more than those in the absence of AgNP.

Use of an Underlayer for Large Area Crystallization of Rubrene Thin Films

In this work, we discovered a very efficient method of crystallization of thermally evaporated rubrene, resulting in ultrathin, large-area, fully connected, and highly crystalline thin films of this organic semiconductor with a grain size of up to 500 μm and charge carrier mobility of up to 3.5 cm2 V–1 s–1. We found that it is critical to use a 5 nm-thick organic underlayer on which a thin film of amorphous rubrene is evaporated and then annealed to dramatically influence the ability of rubrene to crystallize. The underlayer property that controls this influence is the glass transition temperature. By experimenting with different underlayers with glass transition temperatures varying over 120 °C, we identified the molecules leading to the best crystallinity of rubrene films and explained why values both above and below the optimum result in poor crystallinity. We discuss the formation of different crystalline morphologies of rubrene produced by this method and show that field-effect transistors made with ...

Rubrene on differently treated SiO2/Si substrates: A comparative study by atomic force microscopy, X-ray absorption and photoemission spectroscopies techniques

The performances of organic thin film transistor devices are crucially connected with electronic and structural properties at organic semiconductor/dielectric interfaces. In our studies, Silicon substrates with native SiO2 layer were made hydrophilic according to Radio Corporation of America (RCA) cleaning procedure (RCA treated SiO2, RCA/SiO2) and hydrophobic by following octadecyltrichlorosilane (OTS) modification method (OTS treated SiO2, OTS/SiO2). The surface morphology, molecular orientation and nature of interaction have been studied at rubrene/RCA treated SiO2 and rubrene/OTS treated SiO2 interfaces with increasing thickness of rubrene film from sub-monolayer to multilayer. The angle dependent near edge x-ray absorption fine structure (NEXAFS) spectroscopy and atomic force microscopy (AFM) were used to probe the evolution of molecular configuration and surface morphology of rubrene thin films respectively. The nature of interaction was investigated by X-ray photoemission spectroscopy (XPS) techniques. XPS and NEXAFS studies indicate that the rubrene molecules were physically adsorbed onto both substrates. Angle dependent NEXAFS data suggest that OTS molecules were in standing up configuration on SiO2 surface while rubrene molecules were randomly oriented on both RCA and OTS treated SiO2 substrates. On RCA/SiO2 substrate the percentage of rubrene molecules with twisted backbone was higher compared to that on OTS/SiO2 for 20nm thick films. The rubrene films were found to grow with different features onto two studied substrates due to their difference in surface energy. At lower coverage, the grain size was larger and nucleation density was smaller onto hydrophilic RCA/SiO2 surface, compared to hydrophobic OTS/SiO2. Hydrophobation of the dielectric substrate surface tends to hinder the diffusion of rubrene molecules resulting in smaller grains for hydrophobic substrate compared to hydrophilic one.

Substrate induced molecular conformations in rubrene thin films: A thickness dependent study

A systematic study about substrate induced molecular conformation in rubrene thin films by varying film thickness from sub-monolayer to multilayer, which currently attracts substantial attention with regard to its application in organic electronics, is performed. The clean polycrystalline Au and Ag were used as noble-metals, whereas, H passivated and SiO2 terminated Si (100) were used as dielectric substrates. Angle dependent near edge x-ray absorption fine structure spectroscopy (NEXAFS) was employed to understand the molecular conformation and orientation whereas atomic force microscopy (AFM) was used to investigate the surface morphologies of the film. X-ray absorption spectrum (XAS) of rubrene molecules with twisted conformation was calculated using density functional theory (DFT). Percentage of twisted rubrene molecules are gradually reducing with increasing film thickness. But the change of degree of molecular conformationa is found to depend on substrate surface properties. Deposited molecules retain the twisted geometry of the free rubrene molecule at sub-monolayer coverage on four studied substrates. At 200Å thick rubrene film, molecules are in lying configuration on the Ag substrate and randomly oriented on other three substrates. The Ag surface comprised of comparatively wider, flatter terrains separated by large height differences help to achieve rubrene film with larger closely packed grains. The results have important implications for the understanding of the substrate induced molecular conformations in rubrene thin films with its thickness and are beneficial for the improvement of the device performance.

Interference-enhanced infrared-to-visible upconversion in solid-state thin films sensitized by colloidal nanocrystals

Infrared-to-visible photon upconversion has potential applications in photovoltaics, sensing, and bioimaging. We demonstrate a solid-state thin-film device that utilizes sensitized triplet-triplet exciton annihilation, converting infrared photons absorbed by colloidal lead sulfide nanocrystals (NCs) into visible photons emitted from a luminescent dopant in rubrene at low incident light intensities. A typical bilayer device consisting of a monolayer of NCs and a doped film of rubrene is limited by low infrared absorption in the thin NC film. Here, we augment the bilayer with an optical spacer layer and a silver-film back reflector, resulting in interference effects that enhance the optical field and thus the absorption in the NC film. The interference-enhanced device shows an order-of-magnitude increase in the upconverted emission at the wavelength of λ = 610 nm when excited at λ = 980 nm. At incident light intensities above 1.1 W/cm2, the device attains maximum efficiency, converting (1.6 ± 0.2)% of absorbed infrared photons into higher-energy singlet excitons in rubrene.

Homoepitaxy of Crystalline Rubrene Thin Films.

The smooth surface of crystalline rubrene films formed through an abrupt heating process provides a valuable platform to study organic homoepitaxy. By varying growth rate and substrate temperature, we are able to manipulate the onset of a transition from layer-by-layer to island growth modes, while the crystalline thin films maintain a remarkably smooth surface (less than 2.3 nm root-mean-square roughness) even with thick (80 nm) adlayers. We also uncover evidence of point and line defect formation in these films, indicating that homoepitaxy under our conditions is not at equilibrium or strain-free. Point defects that are resolved as screw dislocations can be eliminated under closer-to-equilibrium conditions, whereas we are not able to eliminate the formation of line defects within our experimental constraints at adlayer thicknesses above ∼25 nm. We are, however, able to eliminate these line defects by growing on a bulk single crystal of rubrene, indicating that the line defects are a result of strain built into the thin film template. We utilize electron backscatter diffraction, which is a first for organics, to investigate the origin of these line defects and find that they preferentially occur parallel to the (002) plane, which is in agreement with expectations based on calculated surface energies of various rubrene crystal facets. By combining the benefits of crystallinity, low surface roughness, and thickness-tunability, this system provides an important study of attributes valuable to high-performance organic electronic devices.

Role of molecular conformations in rubrene polycrystalline films growth from vacuum deposition at various substrate temperatures

We report on the optical and structural characterization of rubrene polycrystalline films fabricated from vacuum deposition with various substrate temperatures (Tsub). Depending on Tsub, the role of twisted and planar rubrene conformational isomers on the properties of rubrene films is focused. The temperature (T)-dependent inverse optical transmission (IOT) and photoluminescence (PL) spectra were performed on these rubrene films. The origins of these IOT and PL peaks are explained in terms of the features from twisted and planar rubrene molecules and of the band characteristics from rubrene molecular solid films. Here, two rarely reported weak-peaks at 2.431 and 2.605 eV were observed from IOT spectra, which are associated with planar rubrene. Besides, the T-dependence of optical bandgap deduced from IOT spectra is discussed with respect to Tsub. Together with IOT and PL spectra, for Tsub > 170 °C, the changes in surface morphology and unit cell volume were observed for the first time, and are attributed to the isomeric transformation from twisted to planar rubrenes during the deposition processes. Furthermore, a unified schematic diagram in terms of Frenkel exciton recombination is suggested to explain the origins of the dominant PL peaks performed on these rubrene films at 15 K.