Although organic light-emitting diodes (OLEDs) are promising for use in applications such as in flexible displays, reports of long-lived flexible OLED-based devices are limited due to the poor environmental stability of OLEDs. Flexible substrates such as plastic allow ambient oxygen and moisture to permeate into devices, which degrades the alkali metals used for the electron-injection layer in conventional OLEDs (cOLEDs). Here, the fabrication of a long-lived flexible display is reported using efficient and stable inverted OLEDs (iOLEDs), in which electrons can be effectively injected without the use of alkali metals. The flexible display employing iOLEDs can emit light for over 1 year with simplified encapsulation, whereas a flexible display employing cOLEDs exhibits almost no luminescence after only 21 d with the same encapsulation. These results demonstrate the great potential of iOLEDs to replace cOLEDs employing alkali metals for use in a wide variety of flexible organic optoelectronic devices.
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/GaSx 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 GaSx 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 dots (QDs), which have high color purity, are expected to be applied as emitting materials to wide-color-gamut displays. To enable their use as an alternative to Cd-based QDs, it is necessary to improve the properties of QDs composed of low-toxicity materials. Although multielement QDs such as Ag–In–Ga–S are prone to spectrally broad emission from defect sites, a core/shell structure covered with a GaS x shell is expected to enable sharp emission from band-edge transitions. Here, QD light-emitting diodes (QD-LEDs) embedded with Ag–In–Ga–S/GaS x core/shell QDs (AIGS QDs) were fabricated, and their electroluminescence (EL) was observed. The EL spectra from the AIGS QD-LEDs were found to contain a large defect-related emission component not observed in the photoluminescence (PL) spectra of the AIGS QD films. This defect-related emission was caused by electrons injected into defect sites in the QDs. Therefore, the AIGS QDs and the electron injection layer (EIL) of ZnMgO were treated with Ga compounds such as gallium chloride (GaCl3) and gallium tris(N,N′-diethyldithiocarbamate) (Ga(DDTC)3) to improve the luminescence properties of the QD-LEDs. The added Ga compounds effectively compensated for defect sites on the surface of the QDs and suppressed direct electron injection from the EIL into defect sites. As a result, the defect-related emission components in the EL were successfully suppressed, and the EL exhibited a color purity comparable to the PL of the AIGS QD films. The QD-LEDs exhibited EL spectra with a full width at half-maximum of 33 nm, which is extremely sharp for a low-toxicity QD, and the chromaticity coordinates (0.260, 0.695) for green EL were achieved.
An 8-in. flexible active-matrix organic light-emitting diode (AMOLED) display driven by oxide thin-film transistors (TFTs) has been developed. In-Ga-Zn-O (IGZO)-TFTs used as driving devices were fabricated directly on a plastic film at a low temperature below 200°C. To form a SiO x layer for use as the gate insulator of the TFTs, direct current pulse sputtering was used for the deposition at a low temperature. The fabricated TFT shows a good transfer characteristic and enough carrier mobility to drive OLED displays with Video Graphic Array pixels. A solution-processable photo-sensitive polymer was also used as a passivation layer of the TFTs. Furthermore, a high-performance phosphorescent OLED was developed as a red-light-emitting device. Both lower power consumption and longer lifetime were achieved in the OLED, which used an efficient energy transfer from the host material to the guest material in the emission layer. By assembling these technologies, a flexible AMOLED display was fabricated on the plastic film. We obtained a clear and uniform moving color image on the display.
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