Quantum dots (QDs) are expected to be used as an emitting material in wide-color-gamut displays. However, the development of low-toxicity alternatives is necessary because QDs that exhibit high color purity and highly efficient emission contain toxic materials such as Cd. Here, QD light-emitting diodes (QD-LEDs) fabricated using AgInS₂/GaS_x core/shell QDs (AIS core/shell QDs) as low-toxicity QDs were investigated. The photoluminescence (PL) spectrum of an AIS core/shell QD dispersion showed a band edge emission with a peak wavelength of 560 nm and a full-width at half-maximum of 45 nm because the GaS_x shell suppressed the surface defects. Electroluminescence (EL) emission, which mainly comprises the band edge emission, was realized in the AIS-based QD-LED. However, the EL spectra included a large defect emission component, together with the band edge emission. The defect emission was attributed to electrons flowing in the emitting layer (EML) being easily trapped at defect levels in the QDs. The addition of tris(2,4,6-trimethyl-3-(pyridin-3-yl)phenyl)borane (3TPYMB) as an electron transporting material to the EML effectively suppressed the defect emission. The radiation-energy ratio of the band edge emission to the total emission was 50% in the QD-LED without 3TPYMB and was improved to 64% in the QD-LED with 3TPYMB; this ratio was comparable to that in the PL spectrum of the AIS core/shell QD film. The addition of 3TPYMB improved electron injection into the QDs and the carrier balance in the EML. As a result, the defect emission was suppressed and the EL characteristics were improved.

Quantum dot light-emitting diodes (QD-LEDs) are expected to be used in wide-color-gamut displays because the emission colors from QDs are highly saturated. InP-based QDs are one of the most promising candidates for low-toxicity QDs. Here, we report an efficient green QD-LED whose emitting layer was composed of InP-based QDs and an organic electron-transporting material (ETM). To investigate ETMs suitable for combining with the QDs, the device characteristics of QD-LEDs with various ETMs were compared. The external quantum efficiency (EQE) and the driving voltage were found to be markedly improved by the incorporation of suitable ETMs. We demonstrated green QD-LEDs with a high maximum EQE of 10.0% and a low turn-on voltage of 2.4 V by using 2,4,6-tris(3′-(pyridin-3-yl)biphenyl-3-yl)-1,3,5-triazine (TmPPPyTz) as an ETM. Furthermore, to clarify the origin of the difference in device characteristics, we investigated their hole- and electron-transporting properties. The results suggested that the hole-current leakage from the QD layer was substantially suppressed in the QD-LEDs with TmPPPyTz. Using an ETM with such hole-suppressing ability is demonstrated to be an effective approach to improving the EQEs of QD-LEDs.

This study focuses on the use of highly saturated and efficient red, green, and blue (RGB) cadmium (Cd)-free quantum dot light-emitting diodes (QD-LEDs). RGB QD-LEDs were fabricated using emitting layers (EMLs) comprising Cd-free QDs (red and green InP-based QDs and blue Zn–SeTe QDs) and organic electron-transporting materials (ETMs). The green QD-LED with high color purity was realized by adjusting the QD concentration in the EMLs and suppressing the defect-associated emission in the QD-LEDs. Narrow emissions of the full-width at half-maximum of 40, 34, and 23 nm were realized in the RGB QD-LEDs, respectively. Wide-area coverage of 80% of the available color reproduction area specified in Recommendation ITU-R BT.2020 was achieved using the developed RGB QD-LEDs without applying any color filter and cavity structure. Furthermore, the addition of organic ETMs into the EMLs improved the efficiency of all RGB QD-LEDs.

Highly efficient, operationally stable, and pure-color organic light-emitting diodes (OLEDs) are of considerable significance for developing practical wide-color-gamut displays. Further, we have demonstrated the feasibility of an efficient pure green phosphorescent OLED (PHOLED) by employing a narrow-band platinum complex and a top-emitting structure. The utilization of the thermally activated delayed fluorescence (TADF) material as the phosphorescent host is expected to serve as a promising solution for obtaining operationally stable PHOLEDs with high color purity. However, the emission spectrum of the platinum complex in the TADF host exhibits a considerably broad emission spectrum. This study investigates the cause of the spectral change by evaluating the photoluminescence spectra of the platinum complex in various hosts exhibiting different molecular structures. The triazine unit in the host material was observed to result in exciplex formation between the lowest unoccupied molecular orbital (LUMO) of the host and the highest occupied molecular orbital (HOMO) of the platinum complex. Therefore, the TADF material that sterically hinders the triazine unit is considered to be suitable to prevent both exciplex formation and spectral broadening.

This paper describes how to control strip gauge, looper angle, and strip tension for a hot strip finishing mill and the application result. This is based on the optimal servo theory and the model decoupling method. In the finishing mill process, there exists mutual interaction among strip gauge, looper angle, and strip tension. Conventionally, each of them is controlled independently. To improve the control performance, multivariable control considering this interaction is applied to all production.