Fabrication of Pixelated Organic Light‐Emitting Transistor (OLET) with a Pure Red‐Emitting Organic Semiconductor

Abstract

semiconductors with carrier mobilities insufficient to be useful for OLET applications. Therefore, developing high-purity red-emitting materials showing a high transistor mobility at the same time is a primary challenge to embodying highperformance OLETs in display devices.We have been exploring a broad library of dicyanodistyrylbenzene (DCS)-type materials that show excellent photoluminescence (PL) quantum efficiency and/or charge carrier mobility in the solid state by virtue of their unique stacking modes and propensity for strong self-assembly. [12,[18][19][20][21][22] Recently, we have specifically demonstrated that a charge-transfer (CT) cocrystal of DCS derivatives can be a light-emitting ambipolar semiconductor, which is ideally suitable for OLET applications. [23] Aiming at finding a potentially pure red-emitting organic semiconductor, we reviewed the whole DCS library, particularly for the π-extended ones. Fortunately, we came across Hex-4-TFPTA ((2E,2′E)-3,3′-(2,5-bis(hexyloxy)-1,4-phenylene) bis(2-(5-(4-(trifluoromethyl)phenyl) thiophen-2-yl)acrylonitrile) (Figure 1a), which has reportedly shown an excellent n-type mobility in a vacuum-evaporated film (µ e ≈ 2 cm 2 V −1 s −1 ) but with an unexplored photoluminescence property. [21] In this work, we decided to first investigate the photoluminescence properties of Hex-4-TFPTA and its application in the red-emitting OLET devices. We thoroughly investigated the photophysical properties of the crystal and solid thin film, in terms of intermolecular stacking mode (J-/H-aggregation), size of the crystal, and excimeric contributions. In addition, the electrical properties of Hex-4-TFPTA were properly reevaluated by introducing different metal electrodes to ensure a small contact resistance and to prevent the probable mobility overestimation [24,25] of organic field-effect transistors (OFETs) by their nonlinear transfer curve.Subsequently, Hex-4-TFPTA single-layered OLET devices were fabricated and characterized in terms of their charge transport and electroluminescence (EL). Finally, micropatterned pixels of the Hex-4-TFPTA OLET were fabricated by using a soft-lithographic technique called "patterned taping" [26] and employing a semitransparent electrode. [27,28] This report is actually the first example of pixelated emission from an OLET which was demonstrated by patterning the organic layer with a lithographic technique.The Hex-4-TFPTA crystalline solid states (Figure 1a-c) showed a deep red (DR) emission of which the peak wavelengths were 683 and 631 nm (Figure 1d) for the single crystal A pure red-emitting organic light-emitting transistor (OLET) is successfully fabricated using a π-extended dicyanodistyrylbenzene-type organic semiconductor material (Hex-4-TFPTA), which shows outstanding charge transport and solid-state luminescence at the same time. Based on the structural (X-ray) and photophysical analyses, it is found that the appropriate molecular stacking and highly allowed S 1 →S 0 transition of Hex-4-TFPTA created the ambidextrous balance between el...

Organic Laser Molecule with High Mobility, High Photoluminescence Quantum Yield, and Deep-Blue Lasing Characteristics

“…Organic laser semiconducting molecules are the basis of organic light-emitting transistors (OLETs) and electrically pumped organic lasers (EPOLs), which demonstrate great promise for smart display technology, organic lasers, biosensing, and other related optoelectronic circuits. − However, it remains a big challenge for the development of molecules for OLETs and EPOLs over the past decades. − One of the key restrictive factors is the design motif integrating high mobility, strong emission for efficient electro-optic conversion, and ideal laser characteristics to achieve a sufficiently high number of excited states under high current density to initiate lasing. ,,, Designing highly π-extended fused conjugated systems is one effective approach to increasing charge transport for high mobility; − however, such kinds of molecules always show very weak fluorescence in the solid state due to the significant quenching of excited states induced by condensed molecular packing and remarkable singlet fissions. , Reducing the π-conjugation and intermolecular interactions enough may enhance the fluorescence efficiency of organic materials but generally at the cost of efficient charge transport property. − More recently, achievement of high-mobility emissive organic semiconductors, − high-efficiency OLETs, ,,, and the exciting indication of current-injection lasing from an organic semiconductor have been demonstrated, which brings the hope and passion of scientists from different fields to this field of research. Generally speaking, to date, such materials are still very limited, and only very few of them could meanwhile possess the characteristic of an amplified spontaneous emission (ASE) phenomenon at reasonably low pump intensities, not to mention simultaneously possessing good optical gain for lasing character. ,,− …”

“…17,18 Reducing the π-conjugation and intermolecular interactions enough may enhance the fluorescence efficiency of organic materials but generally at the cost of efficient charge transport property. 19−21 More recently, achievement of high-mobility emissive organic semiconductors, 22−26 high-efficiency OLETs, 6,7,27,28 and the exciting indication of current-injection lasing from an organic semiconductor 29 have been demonstrated, which brings the hope and passion of scientists from different fields to this field of research. Generally speaking, to date, such materials are still very limited, and only very few of them could meanwhile possess the characteristic of an amplified spontaneous emission (ASE) phenomenon at reasonably low pump intensities, not to mention simultaneously possessing good optical gain for lasing character.…”

Efficient Organic Light-Emitting Transistors Based on High-Quality Ambipolar Single Crystals

“…Ambipolar OLETs are capable of balanced injection and migration for two opposite carriers as well as adjustable emission position within the channel, which avoid exciton quenching by carriers or metal electrodes and result in higher quantum efficiency . Although there have been some unipolar materials for OLETs, their light emission is merely closed to the metal electrodes, which leads to the serious loss of carriers, exciton quenching, and low EQE. , It is still a bottleneck for single compound with high yet balanced mobility and strong fluorescence simultaneously because the two properties are generally regarded as contradictory aspects. The materials with high mobility are usually featured with strong intra- and intermolecular interactions as well as compact molecular packing, giving rise to an increasing non-radiative decay rate and decreasing their luminescence efficiency.…”

Organic Light-Emitting Transistors: From Understanding to Molecular Design and Architecture

“…Pixelated deep red emission from a solid thin film layer of 62 is reported by Oh et al for its exceptional n-channel mobility and uncharted luminescent property (Figure ). Line emission from a single layer of 62 was observed without any of the additional supporting layers having an equivalent maximum current but enhanced mobility.…”

“…This lower performance of the CYTOP/C-PVA dielectric may be because of the lower dielectric constant and nonpolarization of the materials. Thus, the design strategy and 103 for its exceptional n-channel mobility and uncharted luminescent property (Figure 6). Line emission from a single layer of 62 was observed without any of the additional supporting layers having an equivalent maximum current but enhanced mobility.…”