Influence of Channel Surface with Ozone Annealing and UV Treatment on the Electrical Characteristics of Top-Gate InGaZnO Thin-Film Transistors.

The effect of the channel interface of top-gate InGaZnO (IGZO) thin film transistors (TFTs) on the electrical properties caused by exposure to various wet chemicals such as deionized water, photoresist (PR), and strippers during the photolithography process was studied. Contrary to the good electrical characteristics of TFTs including a protective layer (PL) to avoid interface damage by wet chemical processes, TFTs without PL showed a conductive behavior with a negative threshold voltage shift, in which the ratio of Ga and Zn on the IGZO top surface reduced due to exposure to a stripper. In addition, the wet process in photolithography increased oxygen vacancy and oxygen impurity on the IGZO surface. The photo-patterning process increased donor-like defects in IGZO due to organic contamination on the IGZO surface by PR, making the TFT characteristics more conductive. The introduction of ozone (O3) annealing after photo-patterning and stripping of IGZO reduced the increased defect states on the surface of IGZO due to the wet process and effectively eliminated organic contamination by PR. In particular, by controlling surface oxygens on top of the IGZO surface excessively generated with O3 annealing using UV irradiation of 185 and 254 nm, IGZO TFTs with excellent current-voltage characteristics and reliability could be realized comparable to IGZO TFTs containing PL.

Molecular Design of Triazole Based Thermally Activated Delayed Fluorescence Hosts for Blue Electrophosphorescence.

We designed novel thermally activated delayed fluorescence (TADF) host molecules for blue electrophosphorescence by combining electron donor acridine derivatives and the electron acceptor 3,4,5-triphenyl-1,2,4-triazole (TPT) unit into a single molecule based on density functional theory (DFT). We obtain the energies of the first singlet (S1) and first triplet (T1) excited states of TADF materials by performing DFT and time-dependent DFT (TD-DFT) calculations on the ground state using dependence on charge transfer amounts for the optimal Hartree-Fock percentage in the exchange-correlation of TD-DFT. The host molecules retained high triplet energy and showed great potential for use in blue phosphorescent organic light-emitting diodes. The results showed that these molecules are promising TADF host materials because they have a low barrier to hole and electron injection, a balanced charge transport for both holes and electrons, and a small energy difference (ΔEST) between the S1 and T1 states.

Thioxanthen-Based Blue Thermally Activated Delayed Fluorescence Emitters for Organic Light-Emitting Diodes.

Novel thermally activated delayed fluorescence (TADF) materials with 9,9-dimethyl-9H-thioxanthene 10,10-dioxide (SB) as an electron acceptor and andspiro[acridine-9,9'-fluorene] (D1) and spiro[acridine-9,9'-xanthene] (D2) as electron donors (2D1-SB and 2D2-SB) and their characteristics were compared with those of a reference material using 9,9-dimethylacridin (DMAC) as an electron donor (2DMAC-SB) for blue organic light-emitting diodes (OLEDs). We obtain the energies of the first singlet (S1) and first triplet (T1) excited states of TADF materials by performing density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations on the ground state using the dependence on charge transfer amounts for the optimal Hartree-Fock percentage in the exchange-correlation of TD-DFT. We show that 2D2-SB would be a suitable blue OLED emitter because it has sufficiently small value (0.064 eV) of energy difference (ΔEST) between the S1 and T1 states, which is favorable for a reverse inter system crossing process from the T1 to S1 states and an emission wavelength of 439.5 nm with sufficiently large oscillator strength (F) value.

Highly Charge Transport Thermally Activated Delayed Fluorescence Host Materials Based on Phenyl-Biscarbazolyl Derivatives.

We designed novel thermally activated delayed fluorescence (TADF) host molecules for blue electrophosphorescence by combining the electron donor 1,3-Bis(N-carbazolyl)benzene (mCP) unit and the electron acceptor diphenylphosphine oxide-4-(triphenylsilyl) phenyl (TSPO1) unit in a single molecule based on density functional theory. We obtained the energies of the first singlet (S1) and the first triplet (T1) excited states of the TADF materials by performing density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations to the ground state using dependence on charge transfer amounts for the optimal Hartree-Fock percentage in the exchange-correlation of TD-DFT. Using DFT and TD-DFT calculations, the large separation between the HOMO and LUMO caused a small difference in energy (ΔEST) between the S1 and T1 states. The host molecules retained high triplet energy and showed great potential for use in blue phosphorescent organic light-emitting diodes. The results showed that these molecules are promising TADF host materials because they have a low barrier to hole and electron injection, balanced charge transport for both holes and electrons, and small ΔEST.

Theoretical Study of Benzofuro-Pyridine Derivatives-Based Organic Light-Emitting Diodes Exhibiting Thermally Activated Delayed Fluorescence.

We designed novel thermally activated delayed fluorescence (TADF) materials with benzofuro[2,3-b]pyridine (BFP) as an electron acceptor and spiro[acridine-9,9'-fluorene] (D1) and spiro[acridine-9,9'-xanthene] (D2) as electron donors (BFP-D1 and BFP-D2) and compared their characteristics with those of a reference material using 9-phenylcarbazole (pCz) as an electron donor (BFP-pCz) for blue organic light-emitting diodes (OLEDs). Using density functional theory (DFT) and time-dependent DFT calculations, we obtained the electron distributions of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), and the energies of the lowest singlet (S1) and lowest triplet (T1) excited states. The calculated difference in energy (ΔEST) between the S1 and T1 states of BFP-D1 (0.06 eV) and BFP-D2 (0.07 eV) was smaller than that of BFP-pCz (0.89 eV). The results showed that BFP-D2 is a suitable blue OLED emitter because it has sufficiently small ΔEST values, which is favorable in a reverse-intersystem process crossing from the T1 state to S1 states, as well as an emission wavelength of 465.9 nm.

Pyridine-, Pyrimidine-, and Triazine-Based Thermally Activated Delayed Fluorescence Emitters.

Novel thermally activated delayed fluorescence (TADF) materials with pyridine (Pd), pyrimidine (Pm), and triazine (Trz) as electron acceptors and carbazole (Cz) as an electron donor (TmCzPd, TmCzPm, and TmCzTrz) were designed, and the effect of the number of nitrogen (N) atoms with acceptor units was investigated by comparing their electronic and optical properties for blue organic light-emitting diodes (OLEDs). Using density functional theory (DFT) and time-dependent DFT calculations, we obtained the electron distributions of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), and the energies of the lowest singlet (S1)and lowest triplet (T1) excited states. The calculated energy difference (ΔEST) between the S1 and T1 states of TmCzPm (0.200 eV) and TmCzTrz (0.186 eV) were smaller than that of TmCzPd (0.395 eV). We show that TmCzPm would be a suitable blue OLED emitter because it has sufficiently small ΔEST values, which is favorable for a reverse intersystem crossing process from the T1 to S1 states, and an emission wavelength of 473.1 nm with sufficiently large oscillator strength for fluorescence.

The Effect of Heteroleptic Dual Acceptors on Dye-Sensitized Solar Cells.

A novel carbazole-based organic dye with heteroleptic dual electron acceptors (cyanoacrylic acid and thieno[3,4-b]pyrazine acid) on each side of a quinoxaline was designed, and its electronic and optical properties were investigated theoretically for dye-sensitized solar cells (DSSCs). To gain insight into the factors responsible for photovoltaic efficiency, density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations of these dyes were performed. The absorption spectrum of the dyes showed different forms because of the different energy levels of the molecular orbital (MO) of each dye and the intramolecular energy transfer (EnT). Considering the overall properties, the asymmetric organic dye containing the heteroleptic dual acceptors showed a competitive conversion efficiency, greater red-shift, broader absorption spectra, and higher molar extinction coefficient compared to a single acceptor in the conversion efficiency of the DSSCs. These results indicate that heteroleptic dual acceptors produces good photovoltaic properties for DSSC.

Study of Phenoxaborin Derivatives for Thermally Activated Delayed Fluorescence Emitters.

Novel thermally activated delayed fluorescence (TADF) materials including 10H-phenoxaborin (PXB) as an electron acceptor and carbazole derivatives [carbazole (Cz), 1-methyl-carbazole (1-m-Cz), and 3,6-bis(3,6-diphenylcarbazolyl)carbazole (BDPCC)] as electron donors (Cz-PXB, 1-m-Cz-PXB, and BDPCC-PXB) were designed and investigated theoretically for deep blue organic light-emitting diode (OLED). Using density functional theory (DFT) and time-dependent DFT calculations, we obtained the electron distribution of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) and the energy of the lowest singlet (S1) and the lowest triplet (T1) excited states. Values for the calculated energy difference between the S1 state and the T1 state (ΔEST) of 1-m-Cz-PXB (0.158 eV) and BDPCC-PXB (0.058 eV) were smaller than for Cz-PXB (0.265 eV). Both materials had sufficiently small ΔEST values, which is favorable for a reverse intersystem crossing process (RISC) from the T1 to the S1 states. 1-m-Cz-PXB (0.1067) and BDPCC-PXB (0.0857) also have larger oscillator strengths (F) than reference material DMAC-PXB (0.0001). Our results showed that 1-m-Cz-PXB and BDPCC-PXB would have highly efficient TADF properties in terms of a small-enough energy difference (ΔEST) between the S1 state and the T1 state and a large F for the OLED.

Benzofuropyridine-Based Highly Efficient Thermally Activated Delayed Fluorescence Emitters.

Novel thermally activated delayed fluorescence (TADF) materials with benzofuro[2,3-b]pyridine (BFPd) as an electron acceptor and carbazole derivatives [carbazole (Cz), 1,3,6,8-tetramethyl-9H-carbazole (tmCz), and 9'H-9,3':6',9''-tercarbazole (terCz)] as electron donors are designed, and their electronic and optical properties are investigated theoretically for a deep blue organic light-emitting diode (OLED). We obtain the energies of the first singlet (S1) and first triplet (T1) excited states of TADF materials by performing density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations on the ground state by using the dependence on charge transfer amounts for the optimal Hartree-Fock percentage in the exchange-correlation of TD-DFT. We show that terCz-BFPd would be a suitable blue OLED emitter because it has sufficiently small ΔEST value (0.048 eV), which is favorable for a reverse intersystem crossing process from the T1 to S1 states and an emission wavelength of 432.8 nm with sufficiently large oscillator strength (F) value.

Theoretical Study of Effect of Introducing π-Conjugation on Efficiency of Dye-Sensitized Solar Cell.

In this study, phenoxazine (PXZ)-based dye sensitizers with triphenylamine (TPA) as a dual-electron donor and thiophen and benzothiadiazole (BTD) or 4,7-diethynylbenzo[c][1,2,5]thiadiazole (DEBT) as an electron acceptor (dye1, dye2, and dye3) were designed and investigated. dye3 can significantly stabilize the lowest unoccupied molecular orbital (LUMO) energy level of an organic dye. We used density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations to better understand the factors responsible for the photovoltaic performance. The absorption spectrum of the dyes showed different forms because of the different energy levels of the molecular orbital (MO) of each dye and the intramolecular energy transfer (EnT). Among the three dyes, dye3 showed greater red-shift, broader absorption spectra, and higher molar extinction coefficient. These results indicate that adding a withdrawing unit and π-conjugation to a dye can result in good photovoltaic properties for dye-sensitized solar cells (DSSCs).

Novel Quinoxaline-Based Organic Dye with Heteroleptic Dual Electron Donor for Dye-Sensitized Solar Cells.

A novel quinoxaline-based organic dye with heteroleptic dual electron donors (phenoxazine and carbazole) on each side of a quinoxaline was designed, and its electronic and optical properties were investigated theoretically for applications to dye-sensitized solar cells (DSSCs). To understand the factors influencing the photovoltaic efficiency, density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations of these dyes were performed. The absorption spectrum of the dyes showed different forms because of the different energy levels of the molecular orbital (MO) of each dye and the intramolecular charge-transfer (ICT) characteristics. The asymmetric organic dye with the heteroleptic dual donor showed competitive conversion efficiency, greater red-shift, broader absorption spectra, and higher molar extinction coefficient compared to dyes with homoleptic dual donors. These results indicate that adding a withdrawing unit and heteroleptic donor produces good photovoltaic properties for DSSCs.

Theoretical Study on Benzazole Derivatives for Use in Blue Thermally Activated Delayed Fluorescence Emitters.

Four novel thermally activated delayed fluorescence (TADF) materials with 9,10-dihydro-9,9-dimethylacridine (DMAC) and phenylindolo(2,3-a)carbazole (PIC) as electron donors and benzazole derivatives (BO, and BT) as electron acceptors (DMAC-BO, DMAC-BT, PIC-BO, and PIC-BT) were designed and theoretically investigated for use as a blue organic light emitting diode (OLED) emitter. Using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations, we calculated the electron distribution of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), and the energy of the lowest singlet (S1) and the lowest triplet (T1) excited states. All the dyes had a small spatial overlap between the HOMO and LUMO because of the relatively large dihedral angle between the phenyl ring and the acceptor moiety. In terms of the energy difference (ΔEST) between the S1 state and the T1 state, DMAC-BO and DMAC-BT showed the small ΔEST (0.18 eV and 0.21 eV, respectively). However, PIC-BO and PIC-BT showed the large ΔEST (0.62 eV and 0.61 eV, respectively). Among the TADF materials, we showed that DMAC-BO would have the best TADF properties in terms of small ΔEST and blue OLED emitters.

Study of an Oxadiazole Derivative for a Blue Thermally Activated Delayed Fluorescence Emitter.

Novel thermally activated delayed fluorescence (TADF) materials (ACR-OXD, 2ACR-OXD) with 9,10-dihydro-9,9-dimethylacridine (ACR) as an electron donor and oxadiazole derivative (OXD) as an electron acceptor were designed and theoretically investigated for blue OLED emitter. Using DFT and TDDFT calculations, we gained the electron distribution of HOMO and LUMO and the energy of the lowest singlet (S1) and the lowest triplet (T1) excited states. In comparison with the previously reported a xanthen derivative (ACR-XTN), ACR-OXD exhibits a promising blue TADF emitter because of destabilizing the LUMO of ACR-OXD by the change of the electron accepting group and maintaining the steric hindrance between donor and acceptor moieties which lead to efficient TADF due to the small energy gap between the lowest excited singlet (S1) state and the lowest excited triplet (T1) state.