Side-group engineering: The influence of norbornadienyl substituents on the properties of ethynylated pentacene and tetraazapentacene

The chemical modifications of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-PEN) and 6,13-bis(triisopropylsilylethynyl)-5,7,12,14-tetraazapentacene (TIPS-TAP) by introduction of a large norbornadienyl substituent at the silyl atom of the side chain were examined and their effect on the charge transport properties in organic field-effect transistors (OFETs) was investigated. While the introduction of the norbornadienyl substituent resulted in only small changes in the crystallographic packing of these materials and did not affect the optical and electronic properties, the effect on the charge carrier mobility is significant. The hole mobility for the norbornadienyl substituted pentacene (Nor-PEN) is increased compared to similarly prepared TIPS-PEN devices, reaching up to 0.75 cm²/Vs. On the other hand, the electron mobility of the tetraaza derivative (Nor-TAP) is reduced by an order of magnitude compared to its parent TIPS-TAP. The strong effect of the norbornadienyl substitution on the charge carrier mobilities can be explained by the calculation of the transfer integrals and the microstructure of the resulting films of the Nor-derivatives.     

Electrical and optical analyses of trapping phenomenon with temperature dependence of organic device

Carrier mechanism in actual organic devices is not simple, owing to the dielectric nature of active organic semiconductor layers, the complexity of the organic device interface, carrier trapping effects by stress biasing, and others. By coupling the conventional electrical measurement, e.g., current–voltage, capacitance–voltage and capacitance-frequency measurements, with optical charge modulation spectroscopy (CMS) measurement, we studied the hysteresis behavior and the temperature dependence of indium tin oxide/polyimide/6,13-Bis(triisopropylsilylethynyl)pentacene(TIPS-pentacene)/Au diodes to understand the effect of carrier trapping caused by injected carriers. The coupled electrical and optical measurements were very helpful to clarify carrier injection that was followed by carrier trapping in the diode, in terms of hysteresis behavior. CMS measurement was used to observe energetic states of carriers in TIPS-pentacene double-layer diode. Finally, the carrier mechanism in organic diodes was discussed by analyzing the diodes as a Maxwell-Wagner effect element.     

Molecular packing correlated fluorescence in TIPS-pentacene films

The molecular packing and optical properties of exposed and buried layers (i.e. the layers at the top surface and near the substrate, respectively) were systematically studied in 6,13-Bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) films coated by spin-coated (SC) and droplet-pinned-crystallization (DPC) methods. Buried layers in both films exhibit intense photoluminescence (PL) resembling the behaviors of the molecules in dilute solution ascribing to weak π-π stacking, while the exposed layers show extremely weak PL due to strong crystallinity. Polarized excitation PL spectra demonstrate that molecular orientation of the buried layers is quasi-ordered in the film coated by DPC method and completely disordered in the film coated by SC method. Besides, the strong crystallinity of the exposed TIPS-pentacene is verified by grazing incident wide-angle X-ray scattering measurement. The distinct differences in optical and structural properties between the exposed and buried layers indicate that TIPS-pentacene films are inhomogeneous in vertical direction due to interfacial effect, which affects the performance of photodiode fabricated with both films. The understanding of the molecular packing correlated fluorescence in TIPS-pentacene films is vital for optimizing the film structure to achieve high performance organic electronic devices.     

Fabrication of organic semiconductor crystalline thin films and crystals from solution by confined crystallization

Highly crystalline thin films of organic semiconductors processed from solution for electronic devices are difficult to achieve due to a slow and preferential three-dimensional growth of the crystals. Here we describe the development of a processing technique to induce a preferential two-dimensional crystalline growth of organic semiconductors by means of minimizing one dimension and confining the solution in two dimensions into a thin layer. The versatility of the process is demonstrated by processing small molecules (TIPS-pentacene and C₆₀) and a polymer (P3HT), all from solvents with a relatively low boiling point, to obtain crystalline thin films. The thin films show an improved in-plane packing of the molecules compared to films processed under similar conditions by spin coating, which is beneficial for the use in organic field-effect transistors.     

Study of stress biasing effect observed in C–V characteristics of ITO/PI/TIPS-pentacene/Au diodes by electric field induced optical second harmonic generation

To clarify the capacitance–voltage (C–V  ) characteristics of organic double-layer diodes with a structure of ITO/PI/TIPS-pentacene/Au, time-resolved electric field induced optical second harmonic generation (TR-EFISHG) measurement was employed. The TR-EFISHG measurement probed the change of electric fields in the TIPS-pentacene organic layer, which originates from accumulated charges at the PI/TIPS-pentacene double-layer interface. Consequently, the shift of threshold voltage ΔV_th$\mathrm{\Delta }{V}_{\mathit{th}}$ of the C–V curves caused by stress negative-biasing was well explained, and a potential well model was proposed on the basis of the TR-EFISHG measurement. TR-EFISHG is a very useful way to study the carrier behaviors, in terms of the C–V characteristics.     

TIPS-pentacene crystalline thin film growth

The crystalline film growth of TIPS-pentacene thin films by confined solution deposition is investigated. The crystalline thin films grow dendritic in the initial stage and continue to grow to elongated plate-like crystals when the solution is deposited in a confined space in-plane. The majority of the thin film, containing smaller thin crystals, is formed within the first 10 s after depositing the solution and continues to grow in minutes to millimeter sized single crystals. By atomic force microscopy we show that impurities are expelled by the growing crystals and clusters accumulate at step edges on the surface of the larger crystals. The influence of crystal thickness and orientation on the electronic transport in field-effect transistors is studied, and shows an optimum performance for devices with thin elongated crystals that are aligned parallel to the electric field between the source–drain electrodes.     

Performance enhancement in mechanically stable flexible organic-field effect transistors with TIPS-pentacene:polymer blend

Flexible organic field-effect transistors (OFETs) with TIPS-pentacene: polystyrene (PS) blend are demonstrated to exhibit enhanced mobility and significantly improved electrical stability compared to neat TIPS-pentacene on poly(4-vinylphenol) (PVP) dielectric (bi-layer OFETs), along with high mechanical stability. Due to merit of high quality dielectric-semiconductor interface, pristine TIPS-pentacene: PS blend OFETs exhibited maximum mobility of 0.93 cm² V⁻¹ s⁻¹ with average of 0.44(±0.25) cm² V⁻¹ s⁻¹ compared to 0.14(±0.10) cm² V⁻¹ s⁻¹ for bi-layer OFETs with high current on-off ratios on the order 10⁵ for both. Both types of devices exhibited high electrical stability upon bending with increasing magnitude of strain or its duration up to 5 days. However, significant differences in electrical stability of devices were observed upon applying constant bias-stress for 40 min to 1 h. Pristine blend devices exhibited outstanding electrical stability with very low drain current decay of <5% compared to ∼30% for bi-layer devices. Even upon bias-stress after 5 days of bending, the drain current decay levels were only changed to <10% and ∼50% for blend and bi-layer devices respectively.     

Self-alignment of 6,13-bis(triisopropylsilylethynyl)pentacene molecules through magnetic flux-affected nanoparticle motion in solution-processed transistors

We propose a viable method of inducing the self-alignment of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) molecules using magnetic nanoparticles under a controlled magnetic field. An essential feature of the self-alignment of TIPS-pentacene containing magnetic nanoparticles is systematically studied through meticulous understanding of gradient solvent evaporation in a magnetic nanoparticle-dispersed droplet guided by a static magnetic field. It is found that such TIPS-pentacene molecules are spontaneously aligned by magnetic flux-affected nanoparticle motion with the generation of gradual anisotropic solvent evaporation. In particular, by controlling the effective magnetic force applied to a magnetic nanoparticle-containing TIPS-pentacene solution, TIPS-pentacene droplet flow can be induced, causing greater alignment. We fabricate TIPS-pentacene-based transistors to confirm the effect of this aligned state on device performance, using both optical and electrical methods.     

一种基于聚合物绝缘层的TIPS-pentacene晶体管性质研究

采用顶接触结构研究制备了以TIPS-pentacene为有源层、聚甲基丙烯酸甲酯(PMMA)为绝缘层的有机场效应晶体管(OFET),其中绝缘层采用溶液旋涂法制备,电极采用Au电极。通过原子力显微镜(AFM)和X射线衍射(XRD)技术对TIPS-pentacene在PMMA上的生长特性进行了详细分析,结果表明,器件获得了良好的电学特性,其场效应迁移率、阈值电压以及开关电流比分别为0.137 cm2/Vs、-19 V和9.74×104。对器件的稳定性也做了详细研究。