Hong Goo Jeon

Solution processable single layer organic light-emitting devices with a single small molecular ionic iridium compound

We herein report on the occurrence of bright and efficient electrophosphorescence from a simple organic light-emitting diode (OLED) with a single organic layer comprised of a small molecular ionic iridium compound, formed using a solution process. The studied small molecular ionic iridium compound is [Ir(dfppy)2(bpy)]+PF6−, which exhibits excellent film-forming properties, bright green photoluminescence, and efficient bipolar carrier transport with balanced electron and hole mobilities of about 10−5 cm2/(V s). A high performance of the device was achieved by using a phosphorescent OLED (PHOLED) that was fabricated using the [Ir(dfppy)2(bpy)]+PF6− compound, with a peak brightness of about 18 000 cd/m2 and a peak current efficiency of 12 cd/A. A peak power efficiency of 2.5 lm/W was measured at 2800 cd/m2. These results suggest that the small molecular ionic iridium compound is a promising material for bright and efficient PHOLEDs manufactured using a simple solution process.

Improved homogeneity and surface coverage of graphene oxide layers fabricated by horizontal-dip-coating for solution-processable organic semiconducting devices

We herein report an investigation of graphene oxide (GO) thin layers fabricated by simple horizontal-dip (H-dip) coating on an indium-tin-oxide (ITO) anode as used in solution-processable organic semiconducting devices. Homogeneous and smooth GO thin films were successfully deposited via an aqueous dispersion of GO on an ITO electrode, with a high surface coverage and low surface roughness, and a thickness controlled by H-dip coating. The use of an H-dip-coated GO film as a hole-injecting interfacial layer (IFL) in organic light-emitting diodes (OLEDs) resulted in a remarkable improvement in device performance (17 cd A−1), better than that (12 cd A−1) of a reference OLED with a spin-coated GO IFL. Stacked bi-IFLs of GO/poly(ethylenedioxythiophene):poly(styrene sulfonate) (GO/PEDOT:PSS) fabricated by H-dip coating were also investigated as hole-injecting IFLs in OLEDs, and these showed an even better device performance (23 cd A−1). Furthermore, it was also shown that polymer solar cells with H-dip-coated GO/PEDOT:PSS hole-collecting bi-IFLs exhibited a remarkable improvement in power conversion efficiency (6.9%), which was also higher than that (4.8%) obtained with spin-coated bi-IFLs. These results clearly indicate that the H-dip-coating of GO(/PEDOT:PSS) can effectively modify the ITO interface to yield efficient hole-selective IFLs, representing considerable promise for use as an alternative to spin-coated IFLs in the mass production of solution-processable organic semiconducting devices.

High-performance organic thin-film transistors with polymer-blended small-molecular semiconductor films, fabricated using a pre-metered coating process

We herein present results obtained from our study of flat and uniform polymer-blended small-molecular semiconductor thin films. These films were produced for organic thin film transistors (OTFTs), using a simple pre-metered horizontal dipping process. The organic semiconductor thin films were composed of a small molecular 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-PEN) composite blended with a polymer binder of poly(α-methylstyrene) (PaMS). We show that the pre-metered solution-coating process allowed the critical control of the thickness of the TIPS-PEN:PaMS film; extremely thin films could be produced using the downstream meniscus of the solution at high speeds (of the order of a few metres per minute). The fabricated TIPS-PEN:PaMS OTFTs exhibited a maximum field-effect mobility of 0.22 cm2 V−1 s−1, and on/off ratios of over 105, that were consistently superior to those of conventional spin-cast devices. These results demonstrate that horizontal dip-coated TIPS-PEN:PaMS films show considerable promise for the production of reliable, reproducible, high-performance OTFTs.

Organic semiconducting layers fabricated by self-metered slot-die coating for solution-processable organic light-emitting devices

We present the results of a study of flat, uniform, and stripe-patternable organic semiconducting layers produced by a slot-die coating method using a self-metered coating mode with blended solutions for the fabrication of bright, efficient, and large-area organic light emitting devices (OLEDs). It is shown that the self-metered slot-die coating process can produce high quality, homogeneous, and stripe-patterned thin films using the downstream meniscus of a blended solution, which can be controlled by adjusting the coating parameters of the capillary number of the coating solution by varying the gap height and coating speed. It is shown that very bright and efficient OLEDs (peak brightness ∼50 000 cd m−2 with maximum efficiencies of ∼29 cd A−1 and ∼14 lm W−1) were successfully demonstrated by manipulating the slot-die coated hole-injecting and electroluminescent layers that contained the phosphorescent Ir complex. In view of these results, we believe that the fabrication of organic semiconducting layers using the self-metered slot-die coating process is a promising new technique for high-throughput manufacturing such as via the roll-to-roll process.

Spontaneous buckling in flexible organic light-emitting devices for enhanced light extraction

We herein present the results of a study of the direct fabrication of buckled patterns in flexible organic light-emitting devices (FOLEDs) that had a conducting polymer anode on a polyethersulfone substrate. These patterns were produced spontaneously by the thermal deposition of an aluminum cathode on an electroluminescent (EL) composite layer. The polymer used for the anode was modified poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) and the EL layer was composed of a solution-processable small molecular composite including phosphorescent Iridium complex mixed with a poly(vinylcarbazole) host. It is shown that FOLEDs produced with buckled patterns can exhibit a luminance as high as ca. 14,900 cd/m(2) with a peak efficiency of 50.5 cd/A. The patterned structure formed by the buckling of the EL layer allows FOLEDs to be produced with a high peak external quantum efficiency of 15% with an increase in light extraction by a factor of ca. 3.1. These results show that spontaneous buckling yields patterned structures that offer considerable promise for the production of high performance, reproducible and reliable FOLEDs.

Polarized electroluminescence from organic light-emitting devices using photon recycling

We present results that show highly polarized electroluminescence (EL) from an organic light-emitting device (OLED) by using a quarter-wave (λ/4) retardation plate (QWP) film and a giant birefringent optical (GBO) photonic reflective polarizer. Polarized EL light of 13,400 cd/m(2) with high peak efficiencies (greater than 10 cd/A and 3.5 lm/W) was obtained from an OLED in this way. These values are almost double those of a polarized OLED that only uses a polarizer. The direction of polarization of the emitted EL light from the polarized OLED corresponded to the passing axis of the GBO reflective polarizer. Furthermore, the degree of linear polarization obtained, i.e. the ratio between the brightness of two linearly polarized EL emissions parallel and perpendicular to the passing axis, is greater than 40 over the whole range of emitted luminance.

Multiple horizontal-dip-coating of small molecular emission layers for solution-processable organic light-emitting devices

We report an investigation of small molecular organic light-emitting diodes (SM-OLEDs) which consist of solution-processable light-emitting layers (EMLs) fabricated using a horizontal-dip- (H-dip-)coating method. The EMLs used were composed of a co-mixed small molecular host matrix of hole-transporting 4,4′,4′′-tris(N-carbazolyl)-triphenylamine and electron-transporting 2,7-bis (diphenylphosphoryl)-9,9′-spirobifluorene, doped with blue-, green-, and red-emitting iridium phosphors. To investigate the film-forming ability, the film quality levels of H-dip-coated small molecular EMLs with multiple coatings were determined when increasing the number of coating cycles. It was found that the thickness of the EML increases as the number of cycles of H-dip-coating increases. Moreover, the formation of film defects in the form of nano-pinholes in the EMLs was found to decrease dramatically with an increase in the number of H-dip-coating cycles. By applying three H-dip-coatings of EML solutions, highly homogeneous small molecular EMLs were successfully deposited, demonstrating that the use of the triple-H-dip-coated EMLs in SM-OLEDs results in good device performance, with maximum luminance levels of 25 000 cd m−2, 79 000 cd m−2, and 15 000 cd m−2, with corresponding peak current efficiencies of 15.8 cd A−1, 23.2 cd A−1, and 5.7 cd A−1, for blue, green, and red SM-OLEDs, respectively. These results clearly indicate that H-dip-coated EMLs with multiple coatings will yield bright and efficient all-solution-processable SM-OLEDs.

Improved output characteristics of organic thin film transistors by using an insulator/protein overlayer and their applications

We herein present our study of the effect of an insulator/protein overlayer deposited onto semiconducting active layers in organic thin film transistors (OTFTs) with regard to their electrical performance. The active layers used were composed of 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-PEN) blended with poly(α-methylstyrene) (PaMS), and the overlayer consisted of a bottom insulating (protecting) layer of an amorphous perfluoropolymer (Cytop) and a top protein layer of negatively charged bovine serum albumin (BSA). The functional layers were deposited using a simple solution-coating method which involved a horizontal dip coating process. We show that the Cytop/BSA overlayer on the TIPS-PEN:PaMS active layer improved the electrical performance of the OTFTs; the devices with the Cytop/BSA overlayer exhibited an average effective mobility of 0.25 cm2 V−1 s−1, which was higher than that of bare devices without any overlayer (0.20 cm2 V−1 s−1). This improved performance of OTFTs with the overlayer was successfully simulated and was found to stem from the formation of a second current channel in the TIPS-PEN:PaMS active layer via the electric field effect of the negatively charged BSA overlayer. These results demonstrate that OTFTs employing charged protein overlayers show considerable promise for the production of high-performance OTFT devices.

Organic thin-film transistors fabricated using a slot-die-coating process and related sensing applications

We herein present the results of a study involving the fabrication of semiconductor thin films for organic thin-film transistors (OTFTs) composed of a small molecular 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-PEN) composite blended with a polymer binder of poly(α-methylstyrene) (PaMS), i.e., TIPS-PEN:PaMS. Thin TIPS-PEN:PaMS semiconducting films were effectively produced using a simple slot-die-coating process operating in a dip-coating mode. It is shown that the slot-die-coating process used here allows critical control of the thickness of the TIPS-PEN:PaMS film; nanoscale thin films could be produced using the downstream meniscus of the blended solution at speeds of the order of a few meters per minute. It is also shown that the slot-die-coated TIPS-PEN:PaMS OTFTs exhibited maximum field-effect mobility of 0.33 cm2 V−1 s−1 and on/off ratios which exceeded 105, both of which are superior to those of conventional spin-cast devices. We also provide an example of an application of the slot-die-coated OTFTs to demonstrate that the OTFTs investigated here can be used to operate a protein sensor device. Considering these results, we believe that the slot-die-coating method with the blended composite of TIPS-PEN:PaMS shows considerable promise for the high-throughput production of reliable, reproducible, and high-performance OTFTs.

Organic light-emitting devices based on solution-processable small molecular emissive layers doped with interface-engineering additives

In this study, we investigate small molecular organic light-emitting diodes (SM-OLEDs) consisting of emission layers (EMLs) fabricated using a solution-coating process of self-metered horizontal dip- (H-dip-) coating. The EML used was composed of a co-mixed small molecular host matrix of hole-transporting 4,4′,4′′-tris(9-carbazolyl)-triphenylamine (TcTa) and electron-transporting 2,7-bis (diphenyl phosphoryl)-9,9′-spirobifluorene (SPPO13) doped with blue-, green-, and/or red-emitting phosphorescent iridium complexes. To improve the electron-injecting and hole-blocking properties at the cathode interface and to enhance the film-forming capabilities, an interface-engineering additive of poly(oxyethylene tridecyl ether) (PTE) was mixed with the small molecular EMLs. Using PTE additives was shown to reduce dramatically the formation of film defects such as nano-pinholes in the EMLs, resulting in thin and homogeneous PTE-mixed EMLs with smooth surface morphologies, even when using a single H-dip-coating process. The use of simple H-dip-coated EMLs mixed with PTEs in SM-OLEDs resulted in good device performance, with maximum luminance levels of 29 200 cd m−2, 115 000 cd m−2, and 16 400 cd m−2, with corresponding peak current efficiencies of 18.8 cd A−1, 31.2 cd A−1, and 10.0 cd A−1, for blue, green, and red SM-OLEDs, respectively. Furthermore, we demonstrated the feasibility of fabricating large-area and high-performance solution-processable SM-OLEDs using H-dip-coated EMLs doped with PTEs. These results clearly indicate that H-dip-coated small molecular EMLs mixed with PTE can be used to yield simple, bright, and efficient solution-processable SM-OLEDs.