Chan-mo Kang

Improved Efficiency of Inverted Organic Light-Emitting Diodes Using Tin Dioxide Nanoparticles as an Electron Injection Layer

We demonstrated highly efficient inverted bottom-emission organic light-emitting diodes (IBOLEDs) using tin dioxide (SnO2) nanoparticles (NPs) as an electron injection layer at the interface between the indium tin oxide (ITO) cathode and the organic electron transport layer. The SnO2 NP layer can facilitate the electron injection since the conduction band energy level of SnO2 NPs (-3.6 eV) is located between the work function of ITO (4.8 eV) and the lowest unoccupied molecular orbital (LUMO) energy level of typical electron transporting molecules (-2.5 to -3.5 eV). As a result, the IBOLEDs with the SnO2 NPs exhibited a decrease of the driving voltage by 7 V at 1000 cd/m(2) compared to the device without SnO2 NPs. They also showed a significantly enhanced luminous current efficiency of 51.1 cd/A (corresponds to the external quantum efficiency of 15.6%) at the same brightness, which is about two times higher values than that of the device without SnO2 NPs. We also measured the angular dependence of irradiance and electroluminescence (EL) spectra in the devices with SnO2 NPs and found that they had a nearly Lambertian emission profile and few shift in EL spectrum through the entire viewing angles, which are considered as remarkable and essential results for the application of OLEDs to display devices.

High performance inverted organic solar cells with solution processed Ga-doped ZnO as an interfacial electron transport layer

We demonstrate solution-processed Ga-doped ZnO incorporated as an interfacial electron transport layer into inverted organic solar cells with active layers comprising either PCDTBT or PTB7 mixed with PC71BM. The 5.03 at% Ga-doped ZnO showed the best efficiencies of 5.56% and 7.34% for PCDTBT and PTB7 polymers respectively.

Cross-Stacked Single-Crystal Organic Nanowire p−n Nanojunction Arrays by Nanotransfer Printing

We fabricated cross-stacked organic p-n nanojunction arrays made of single-crystal 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-PEN) and fullerene (C60) nanowires as p-type and n-type semiconductors, respectively, by using a nanotransfer printing technique. Single-crystal C60 nanowires were synthesized inside nanoscale channels of a mold and directly transferred onto a desired position of a flexible substrate by a lubricant liquid layer. In the consecutive printing process, single-crystal TIPS-PEN nanowires were grown in the same way and then perpendicularly aligned and placed onto the C60 nanowire arrays, resulting in a cross-stacked single-crystal organic p-n nanojunction array. The cross-stacked single-crystal TIPS-PEN/C60 nanowire p-n nanojunction devices show rectifying behavior with on/off ratio of ∼ 13 as well as photodiode characteristic with photogain of ∼ 2 under a light intensity of 12.2 mW/cm(2). Our study provides a facile, solution-processed approach to fabricate a large-area array of organic crystal nanojunction devices in a desired arrangement for future nanoscale electronics.

Overcoming tradeoff between mobility and bias stability in organic field-effect transistors according to the self-assembled monolayer chain lengths

This study examined the relationship between the mobility and bias stability of pentacene-based organic field-effect transistors (OFETs) regarding a self-assembled monolayer (SAM) treatment. For this systematic study, four types of silazane-based SAMs with different alkyl chain lengths in the range of 1–8 were used. Silazane-based SAMs have an advantage of processability due to the mild reaction conditions. The mobility was increased from 0.29 without SAM to 0.46, 0.61, 0.65, and 0.84 cm2/V s after the SAM-treatment with an alkyl chain length of 1, 3, 4, and 8, respectively. On the other hand, inverse proportional relationship was observed between the bias stability and SAM alkyl chain length. Under high gate bias stress (equivalent to electric field of 3 MV/cm) for 2 h, the threshold voltage shift of the OFET was decreased from 12.19 V without SAM to 5.69 V with a short SAM-treatment (alkyl chain length of 1) and 7.14 V with a long SAM-treatment (alkyl chain length of 8). This is the significant finding ...

Injection-modulated polarity conversion by charge carrier density control via a self-assembled monolayer for all-solution-processed organic field-effect transistors

We demonstrated modulation of charge carrier densities in all-solution-processed organic field-effect transistors (OFETs) by modifying the injection properties with self-assembled monolayers (SAMs). The all-solution-processed OFETs based on an n-type polymer with inkjet-printed Ag electrodes were fabricated as a test platform, and the injection properties were modified by the SAMs. Two types of SAMs with different dipole direction, thiophenol (TP) and pentafluorobenzene thiol (PFBT) were employed, modifying the work function of the inkjet-printed Ag (4.9 eV) to 4.66 eV and 5.24 eV with TP and PFBT treatments, respectively. The charge carrier densities were controlled by the SAM treatment in both dominant and non-dominant carrier-channel regimes. This work demonstrates that control of the charge carrier densities can be efficiently achieved by modifying the injection property with SAM treatment; thus, this approach can achieve polarity conversion of the OFETs.

Improved photovoltaic performance of inverted polymer solar cells through a sol-gel processed Al-doped ZnO electron extraction layer

We demonstrate that nanocrystalline Al-doped zinc oxide (n-AZO) thin film used as an electron-extraction layer can significantly enhance the performance of inverted polymer solar cells based on the bulk heterojunction of poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT) and [6,6]-phenyl C(71)-butyric acid methyl ester (PC(70)BM). A synergistic study with both simulation and experiment on n-AZO was carried out to offer a rational guidance for the efficiency improvement. As a result, An n-AZO film with an average grain size of 13 to 22 nm was prepared by a sol-gel spin-coating method, and a minimum resistivity of 2.1 × 10(-3) Ω·cm was obtained for an Al-doping concentration of 5.83 at.%. When an n-AZO film with a 5.83 at.% Al concentration was inserted between the ITO electrode and the active layer (PCDTBT:PC(70)BM), the power conversion efficiency increased from 3.7 to 5.6%.

Trap-level-engineered common red layer for fabricating red, green, and blue subpixels of full-color organic light-emitting diode displays.

We report a novel strategy to reduce one fine metal mask (FMM) step in a full-color organic light-emitting diode (OLED) display by introducing a common red layer (CRL) which replaces a hole transporting layer (HTL) with the same thickness of a red phosphorescent dye-doped layer. Because the dopant in the HTL acts as a hole trap, careful trap-level engineering is required for achieving efficient green and blue emission from the emitting layer while minimizing the red emission from the CRL. We investigated the characteristics of OLEDs depending on hole trap levels of the CRL with five different organic HTLs, and demonstrated efficient red, green and blue (RGB) emitting devices using the CRL. The electroluminescence spectrum of the devices with the CRL is nearly identical with those of the devices without the CRL. These results open up the possibility of simplified fabrication of practical full-color OLED displays with the reduced FMM steps, resulting in lower manufacturing cost.

Application of linear annealing method to Si||SiO2/Si wafer direct bonding

A linear annealing method for silicon wafer direct bonding was developed and applied for silicon-on-insulator (SOI) substrate fabrication. The new annealing method was similar to a conventional zone-melting system, except with no additional lower heating modules. The bonded interface, observed using an infrared camera and a transmission electron microscope, showed no gaseous defects. The bonding strength was high enough to separate the thermal oxide layer from the Si wafer surface, which was observed through the tensile test. The bond strength increased with increasing annealing temperature. The new annealing method was thought to be more effective compared with the conventional furnace annealing, as it offered low temperature and fast processing time without the decrease of bond strength.

Organic complementary inverter and ring oscillator on a flexible substrate

A complementary inverter was fabricated using pentacene and N-N′-dioctyl-3,4,9,10-perylene tetracarboxylic diimide-C8 (PTCDI-C8) for p- and n-type transistors on a poly(ether sulfone) substrate, respectively. The mobilities of the p- and n-type transistors were 0.056 and 0.013 cm2/V s, respectively. The inverter, which was composed of p- and n-type transistors, showed a gain of 48.6 when V DD=−40 V and at the maximum noise margin of V DD/2. A ring oscillator was also fabricated by cascading five inverters. The five-stage ring oscillator showed the maximum output frequency of 10 kHz when V DD=−170 V.

Frequency Performance Optimization of Flexible Pentacene Rectifier by Varying the Thickness of Active Layer

We studied the thickness dependence on the high-frequency performance of pentacene-based diodes on plastic substrates. The current–voltage characteristics, forward- and reverse-bias breakdown voltages, and the frequency performance of the pentacene rectifiers were measured for diodes with the pentacene layer of various thicknesses (14, 28, 56, 84, and 104 nm) and the MoO3 buffer layer. Although the rectifier with the 56nm-thick pentacene diode shows a highest output voltage at the low frequency, its output voltage rapidly decreases compared with others. At high frequency region up to 25 MHz, the 84-nm-thick pentacene diode shows best performance. The behavior is analyzed with the current scaling with the pentacene thickness and the frequency response of the impedance of the diode capacitance. # 2010 The Japan Society of Applied Physics