Evaluation of Effective Field-Effect Mobility in Thin-Film and Single-Crystal Transistors for Revisiting Various Phenacene-Type Molecules

The magnitude of the field-effect mobility μ of organic thin-film and single-crystal field-effect transistors (FETs) has been overestimated in certain recent studies. These reports set alarm bells ringing in the research field of organic electronics. Herein, we report a precise evaluation of the μ values using the effective field-effect mobility, μ_eff, a new indicator that is recently designed to prevent the FET performance of thin-film and single-crystal FETs based on various phenacene molecules from being overestimated. The transfer curves of a range of FETs based on phenacene are carefully categorized on the basis of a previous report. The exact evaluation of the value of μ_eff depends on the exact classification of each transfer curve. The transfer curves of all our phenacene FETs could be successfully classified based on the method indicated in the aforementioned report, which made it possible to evaluate the exact value of μ_eff for each FET. The FET performance based on the values of μ_eff obtained in this study is discussed in detail. In particular, the μ_eff values of single-crystal FETs are almost consistent with the μ values that were reported previously, but the μ_eff values of thin-film FETs were much lower than those previously reported for μ, owing to a high absolute threshold voltage, |V _th|. The increase in the field-effect mobility as a function of the number of benzene rings, which was previously demonstrated based on the μ values of single-crystal FETs with phenacene molecules, is well reproduced from the μ_eff values. The FET performance is discussed based on the newly evaluated μ_eff values, and the future prospects of using phenacene molecules in FET devices are demonstrated.

Photochemical synthesis and device application of acene-phenacene hybrid molecules, dibenzo[n]phenacenes (n = 5-7)

Dibenzo[n]phenecenes (DBnPs, n = 5-7) were conveniently synthesised through Mallory photocyclization as the key step. Effective mobilities of single-crystal field-effect transistors of DBnPs were evaluated to demonstrate that C2h-symmetrical DB6P shows higher performance than C2v-symmetrical DB5P and DB7P.

Fabrication of ring oscillators using organic molecules of phenacene and perylenedicarboximide

Organic field-effect transistors (FETs) can be applied to radio-frequency identification tags (RFIDs) and active-matrix flat-panel displays. For RFID application, a cardinal functional block is a ring oscillator using an odd number of inverters to convert DC voltage to AC. Herein, we report the properties of two ring oscillators, one formed with [6]phenacene for a p-channel FET and N,N′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) for an n-channel FET, and one formed with 3,10-ditetradecylpicene ((C₁₄H₂₉)₂-picene) for a p-channel FET and PTCDI-C8 for an n-channel FET. The former ring oscillator provided a maximum oscillation frequency, f_osc of 26 Hz, and the latter a maximum f_osc of 21 Hz. The drain–drain voltage, V_DD, applied to these ring oscillators was 100 V. This may be the first step towards a future practical ring oscillator using phenacene molecules. The values of field-effect mobility, μ in the p-channel [6]phenacene FET and n-channel PTCDI-C8 FET, which form the building blocks in the ring oscillator with an f_osc value of 26 Hz, are 1.19 and 1.50 × 10⁻¹ cm² V⁻¹ s⁻¹, respectively, while the values in the p-channel (C₁₄H₂₉)₂-picene FET and n-channel PTCDI-C8 FET, which form the ring oscillator with an f_osc of 21 Hz, are 1.85 and 1.54 × 10⁻¹ cm² V⁻¹ s⁻¹, respectively. The μ values in the p-channel FETs are higher by one order of magnitude than those of the n-channel FET, which must be addressed to increase the value of f_osc. Finally, we fabricated a ring oscillator with ZrO₂ instead of parylene for the gate dielectric, which provided the low-voltage operation of the ring oscillator, in which [6]phenacene and PTCDI-C8 thin-film FETs were employed. The value of f_osc obtained in the ring oscillator was 24 Hz. In this ring oscillator, the V_DD value applied was limited to 20 V. The durability of the ring oscillators was also investigated, and the bias stress effect on the f_osc and the amplitude of the output voltage, V_out are discussed. This successful operation of ring oscillators represents an important step towards the realization of future practical integrated logic gate circuits using phenacene molecules.

A ring oscillator consisting of p-channel and n-channel organic FETs.

Facile synthesis of picenes incorporating imide moieties at both edges of the molecule and their application to n-channel field-effect transistors

Picene derivatives incorporating imide moieties along the long-axis direction of the picene core (C_n-PicDIs) were conveniently synthesized through a four-step synthesis. Photochemical cyclization of dinaphthylethenes was used as the key step for constructing the picene skeleton. Field-effect transistor (FET) devices of C_n-PicDIs were fabricated by using ZrO₂ as a gate substrate and their FET characteristics were investigated. The FET devices showed normally-off n-channel operation; the averaged electron mobility (μ) was evaluated to be 2(1) × 10⁻⁴, 1.0(6) × 10⁻¹ and 1.4(3) × 10⁻² cm² V⁻¹ s⁻¹ for C₄-PicDI, C₈-PicDI and C₁₂-PicDI, respectively. The maximum μ value as high as 2.0 × 10⁻¹ cm² V⁻¹ s⁻¹ was observed for C₈-PicDI. The electronic spectra of C_n-PicDIs in solution showed the same profiles irrespective of the alkyl chain lengths. In contrast, in thin films, the UV absorption and photoelectron yield spectroscopy (PYS) indicated that the lowest unoccupied molecular orbital (LUMO) level of C_n-PicDIs gradually lowered upon the elongation of the alkyl chains, suggesting that the alkyl chains modify intermolecular interactions between the C_n-PicDI molecules in thin films. The present results provide a new strategy for constructing a high performance n-channel organic semiconductor material by utilizing the electronic features of phenacenes.

Picenediimide derivatives serve as the active layer of n-channel thin-film field-effect transistors displaying a maximum charge carrier mobility as high as 2.0 × 10⁻¹ cm² V⁻¹ s⁻¹.

Synthesis of the extended phenacene molecules, [10]phenacene and [11]phenacene, and their performance in a field-effect transistor

The [10]phenacene and [11]phenacene molecules have been synthesized using a simple repetition of Wittig reactions followed by photocyclization. Sufficient amounts of [10]phenacene and [11]phenacene were obtained, and thin-film FETs using these molecules have been fabricated with SiO₂ and ionic liquid gate dielectrics. These FETs operated in p-channel. The averaged measurements of field-effect mobility, <μ>, were 3.1(7) × 10⁻² and 1.11(4) × 10⁻¹ cm² V⁻¹ s⁻¹, respectively, for [10]phenacene and [11]phenacene thin-film FETs with SiO₂ gate dielectrics. Furthermore, [10]phenacene and [11]phenacene thin-film electric-double-layer (EDL) FETs with ionic liquid showed low-voltage p-channel FET properties, with <μ> values of 3(1) and 1(1) cm² V⁻¹ s⁻¹, respectively. This study also discusses the future utility of the extremely extended π-network molecules [10]phenacene and [11]phenacene as the active layer of FET devices, based on the experimental results obtained.

Recent progress on carbon-based superconductors

This article reviews new superconducting phases of carbon-based materials. During the past decade, new carbon-based superconductors have been extensively developed through the use of intercalation chemistry, electrostatic carrier doping, and surface-proving techniques. The superconducting transition temperature T c of these materials has been rapidly elevated, and the variety of superconductors has been increased. This review fully introduces graphite, graphene, and hydrocarbon superconductors and future perspectives of high-T c superconductors based on these materials, including present problems. Carbon-based superconductors show various types of interesting behavior, such as a positive pressure dependence of T c. At present, experimental information on superconductors is still insufficient, and theoretical treatment is also incomplete. In particular, experimental results are still lacking for graphene and hydrocarbon superconductors. Therefore, it is very important to review experimental results in detail and introduce theoretical approaches, for the sake of advances in condensed matter physics. Furthermore, the recent experimental results on hydrocarbon superconductors obtained by our group are also included in this article. Consequently, this review article may provide a hint to designing new carbon-based superconductors exhibiting higher T c and interesting physical features.

Transistor application of alkyl-substituted picene

Field-effect transistors (FETs) were fabricated with a thin film of 3,10-ditetradecylpicene, picene-(C₁₄H₂₉)₂, formed using either a thermal deposition or a deposition from solution (solution process). All FETs showed p-channel normally-off characteristics. The field-effect mobility, μ, in a picene-(C₁₄H₂₉)₂ thin-film FET with PbZr_0.52Ti_0.48O₃ (PZT) gate dielectric reached ~21 cm² V⁻¹ s⁻¹, which is the highest μ value recorded for organic thin-film FETs; the average μ value (<μ>) evaluated from twelve FET devices was 14(4) cm² V⁻¹ s⁻¹. The <μ> values for picene-(C₁₄H₂₉)₂ thin-film FETs with other gate dielectrics such as SiO₂, Ta₂O₅, ZrO₂ and HfO₂ were greater than 5 cm² V⁻¹ s⁻¹, and the lowest absolute threshold voltage, |V_th|, (5.2 V) was recorded with a PZT gate dielectric; the average |V_th| for PZT gate dielectric is 7(1) V. The solution-processed picene-(C₁₄H₂₉)₂ FET was also fabricated with an SiO₂ gate dielectric, yielding μ = 3.4 × 10⁻² cm² V⁻¹ s⁻¹. These results verify the effectiveness of picene-(C₁₄H₂₉)₂ for electronics applications.