Rubrene Single-crystal Transistors Research Articles

(Keynote) Physics, Materials and Applications of High-Mobility Organic Transistor Circuits

Development of functional materials and understanding of the microscopic mechanisms mutually benefit through their close interaction. To accelerate development of organic semiconductor devices for industrial application to flexible and printed electronics, it has been essential to understand the mechanism of charge transport in conjunction with molecular-scale charge transfer. We examine the idea that high-mobility charge transport in newly developed solution-crystalized organic transistors is caused by band-like charge transport, using several different methods to probe coherence in the electronic charge. These materials, being essentially different from conventional organic semiconductors with basically incoherent hopping-like transport, often show high values of mobility exceeding 10 cm2/Vs so that high-speed organic transistor circuits are realized. We first employ Hall-effect measurement which differentiates the coherent band transport from site-to-site hopping. Rubrene single crystal transistors were the first to show the Hall voltage obviously indicating the band transport with the mobility exceeding 10 cm2/Vs, whereas the result of pentacene single crystal transistors with the mobility around 1 cm2/Vs suggested somewhat intermediate between coherent and incoherent charge transport. An interesting crossover from hopping-like to band-like transport is also found by gradually changing temperature and external pressure, clearly indicating involvement of intermolecular phonons. Stimulated by the experiments in the early stage, such materials as decyldinaphthobenzo-dithiophene derivatives (Cn-DNBDT) are newly synthesized to stabilize the molecular position in the crystal form by introducing steric hindrance in their p-conjugated cores. With the development of solution-crystallization method, arrays of the single crystal transistors are formed to the size of wafer scale on plastic substrates. The mobility is as high as 20 cm2/Vs for p-type compound and 4 cm2/Vs for an n-type compound. We measured spin relaxation time of the electric-field induced carriers down to 4.2 K using the large-area ultra-thin single-crystal transistors, so that electron-spin relaxiation is precisely investigated. We found that the spin-relaxation follows the Elliott-Yafet mechanism so that the spin life time is consistently elongated at low temperatures due to reduced phonon scattering via spin-orbit coupling. The result suggest that charge carrier mobility can as high as 650 cm2/Vs with minimized phonon scattering at the low temperature. This presentation also focuses on recent development of key technologies for printed LSIs which can provide future low-cost platforms for RFID tags, AD converters, data processors, and sensing circuitries. Such prospect bears increasing reality because of recent research innovations in the field of material chemistry, charge transport physics, and solution processes of printable organic semiconductors. With excellent chemical and thermal stability in recently developed new materials, we are developing simple integrated devices based on CMOS using p-type and n-type printed organic FETs. Particularly important are new processing technologies for continuous growth of the organic single-crystalline semiconductor “wafers” from solution and for lithographical patterning of semiconductors and metal electrodes. Successful rectification and identification are demonstrated at 13.56 MHz with printed organic CMOS circuits.

Direct Observation of the Dipole-Induced Energetic Disorder in Rubrene Single-Crystal Transistors by Scanning Kelvin Probe Microscopy.

It is commonly accepted that gate dielectric dipoles can induce energetic disorder in organic field-effect transistors. However, convincing experimental evidence that directly demonstrate this effect are still in lack. In this work, we present a combined experimental and theoretical study to reveal this effect. We have investigated the temperature-dependent mobility of two rubrene single-crystal transistors with different polymer dielectrics. Model fittings of the data indicate there is higher energetic disorder in the device on dielectric with larger permittivity. Scanning Kelvin probe microscopy was then employed to directly characterize the density of tail states, which is correlated with energetic disorder, in the devices. The results further confirm that dielectric dipoles can increase energetic disorder in organic semiconductors.

Rubrene Single-Crystal Transistors with Perfluoropolyether Liquid Dielectric: Exploiting Free Dipoles to Induce Charge Carriers at Organic Surfaces

We report the use of perfluoropolyether (PFPE) as a liquid dielectric for use in rubrene single-crystal transistors. In particular, we explore the effect of the free, permanent dipoles in PFPE on the charge carrier accumulation and transport at the rubrene/PFPE interface. This provides a complementary approach to ionic liquid dielectrics where charge accumulation is achieved via mobile ions. Large hysteresis (i.e., a memory effect) in transistor transfer curves and peaks in the gate displacement current curves are observed and interpreted in terms of the dipolar response of PFPE to the gate electric field. The orientation of free dipoles in PFPE is found to have a significant influence on the formation and annihilation of the rubrene conducting channel. Hole densities on the order of 1011 cm–2 are achieved at the surface of rubrene, and a transition from band-like to hopping transport near the freezing point of PFPE is evidenced by temperature-dependent Hall effect and resistance measurements. Overall, th...

High performance of rubrene thin film transistor by weak epitaxy growth method

Rubrene single-crystal transistors have achieved one of the highest carrier mobilities in organic semiconductors. However its thin film transistor usually shows inferior performance due to the poor film quality. Therefore how to obtain large-area and high quality rubrene thin film has become a prominent challenge. This work utilized weak epitaxy growth method with new inducing layer 1,3-di(terphenyl) benzene (m-7P), and lager-area highly ordered terrace rubrene film was obtained. Based on this high quality film, the hole mobility of rubrene polycrystalline thin film transistor has been enhanced to 11.6cm2V−1s−1 with VOPc as buffer layer between semiconductor layer and electrodes. This high device performance was attributed to the flat inducing layer and the single orientation of rubrene domains on m-7P layer, which may reduce grain boundaries and improve the film quality. This easy process to prepare large-area high performance rubrene device supplies a good opportunity for large-area electronic device manufacture.

Charge transport and Hall effect in rubrene single-crystal transistors under high pressure

Four-terminal conductivity and Hall coefficient are measured in high-mobility rubrene single-crystal field-effect transistors as functions of gate electric field and external hydrostatic pressure up to 900 MPa. Estimating charge density directly from the measured Hall coefficient regardless of contraction in the gate dielectric layer, genuine pressure dependence of the mobility is extracted. It turned out that enhanced intermolecular charge transfer causes an increase in the mobility of the bandlike charge carriers by 20% GPa up to 600 MPa, which is typically one order of magnitude larger than that in inorganic semiconductors. We also found negative pressure coefficient for the mobility above 600 MPa, suggesting that the external hydrostatic force causes additional molecular displacement due to a specific molecular shape.

Extraction of the contact resistance from the saturation region of rubrene single-crystal transistors

A modified transmission-line method (TLM) is proposed to extract the contact resistance from the transistor saturation region. The conventional TLM requires a linear current–voltage characteristic, and this requirement strongly limits its application. In this study, we focused on the pinch-off point of the output characteristics and analyzed the contact resistance using nonlinear output curves. We applied the modified TLM to both p- and n-type rubrene single-crystal transistors and compared the mobility differences in terms of both the intrinsic bandwidth and the extrinsic carrier trap density.

Microscopic mechanisms behind the high mobility in rubrene single-crystal transistors as revealed by field-induced electron spin resonance

Microscopic mechanisms behind the high mobility in rubrene single-crystal transistors as revealed by field-induced electron spin resonance

High-performance organic field-effect transistors with binary ionic liquids

High-performance rubrene single-crystal field-effect transistors are developed with binary ionic liquid electrolytes used for gating. Inclusion of small amount of inorganic salts in the ionic liquids enhances the degree of dissociation for the organic ions and accelerates formation of the electric-double-layers in response to the gate voltage. High carrier mobility of 2.9 cm 2/Vs is achieved in the rubrene single-crystal transistors with the mixture ionic liquid. In addition to the advantage of the low-voltage operation due to concentrated field in ultra-thin electric-double-layers, drastically increased capacitance at above 100 Hz makes the technique of the ionic liquid gating more attractive for fast-switching devices.

Steady-state and transient photocurrents in rubrene single crystal free-space dielectric transistors

We report results of photocurrent studies performed on rubrene single crystal transistors in the air-gap configuration under 405nm irradiation. The phototransistors show a strong photoresponse indicative of photoconduction. Under illumination, the phototransistors show an average threshold voltage shift of 22V and a maximum photosensistivity of 2.65×103. A small persistent photoconductivity effect is observed in the transistors tested under continuous illumination which is explained by delayed recombination aided by spatial separation of the photocarriers. Photocurrent transients measured by applying short pulses on the other hand show a complete recovery in the microsecond regime implying immediate recombination.

Scaling effect on the operation stability of short-channel organic single-crystal transistors

Organic single-crystal transistors allowed the authors to investigate the essential features of short-channel devices. Rubrene single-crystal transistors with channel lengths of 500 and 100nm exhibited good field-effect characteristics under extremely low operation voltages, although space charge limited current degrades the subthreshold properties of 100nm devices. Furthermore, bias-stress measurements revealed the remarkable stability of organic single-crystal transistors regardless of device size. The bias-stress effect was explained by the trapping of gate-induced charges into localized density of states in the single-crystal channel.

Effect of metal electrodes on rubrene single-crystal transistors

The authors herein have investigated the effect of the metal work function on the performance of rubrene single-crystal transistors using gold and calcium metal electrodes. The current-voltage characteristic is controlled by the metal work function, which offers the possibility of controlling the Schottky barrier height by the choice of the metal. In the process of the study of metal-rubrene contacts, the authors have realized an ambipolar transistor and a Schottky diode in an identical single-crystal device with asymmetric electrodes. These data provide direct evidence of the weak Fermi level pinning and formation of depletion layer on metal-rubrene contacts.