Dual-functional quantum-dots light emitting diodes based on solution processable vanadium oxide hole injection layer

Abstract

Dual-functional quantum-dots light emitting diodes (QLEDs) have been fabricated using solution processable vanadium oxide (V₂O₅) hole injection layer to control the carrier transport behavior. The device shows selectable functionalities of photo-detecting and light-emitting behaviors according to the different operating voltage conditions. The device emitted a bright green light at the wavelength of 536 nm, and with the maximum luminance of 31,668 cd/m² in a forward bias of 8.6 V. Meanwhile, the device could operate as a photodetector in a reverse bias condition. The device was perfectly turned off in a reverse bias, while an increase of photocurrent was observed during the illumination of 520 nm wavelength light on the device. The interfacial electronic structure of the device prepared with different concentration V₂O₅ solution was measured in detail using x-ray and ultraviolet photoelectron spectroscopy. Both the highest occupied molecular orbital and the gap state levels were moved closer to the Fermi level, according to increase the concentration of V₂O₅ solution. The change of gap state position enables to fabricate a dual-functional QLEDs. Therefore, the device could operate both as a photodetector and as a light-emitting diode with different applied bias. The result suggests that QLEDs can be used as a photosensor and as a light-emitting diode for the future display industry.

Figure 1

(a) Current density and luminance versus applied voltage curves of QLEDs. (J–L–V) Inset shows a schematic illustration of the QLEDs under forward bias for the light emission. (b) Electroluminescence spectra of QLEDs. Inset shows the light emitting QLEDs. (c) Energy level diagram of the QLEDs measured from UPS. The concentration of V₂O₅ was 1 wt%.

Figure 2

(a) UPS spectra of ITO and various concentration V₂O₅ films on ITO at the secondary cutoff and E_F regions. To clarify the gap state, inset shows near the E_F. (b) Energy level diagram of the hole injection region of the QLEDs. Inside the dotted square, gap states and HOMO levels of different concentration V₂O₅ films on ITO are shown.

Figure 3

Measured V 2p_3/2 and O 1s core level spectra of (a), (d) 1 wt% V₂O₅ film on ITO, (b), (e) 3 wt% V₂O₅ film on ITO, and (c), (f) 5 wt% V₂O₅ film on ITO. In the V 2p core-level spectra, the ratio of V⁴⁺ and V⁵⁺ are shown, while the ratio of oxygen binding ratio with V⁴⁺ and V⁵⁺ is shown in O 1s core-level spectra.

Figure 4

(a) Transfer curves of the device in a dark state and with the illumination of various wavelengths light. Inset shows a schematic illustration of the QLEDs with 5 wt% V₂O₅ for the light detecting process. (b) Photo response characteristics of the device. Rising and falling time was defined as the interval between 10 and 90% of the signal. (c) Normalized photo responses according to the different speed of turn on and off the 520 nm wavelength light signal.

Figure 5

(a) Time-resolved photoluminescence decay results of QDs on quartz and full devices with zero bias, light-emitting mode (+ 3.8 V) and light-detecting mode (− 3.8 V), respectively.

Figure 6

(a) Circuit diagram that used two dual-functional QLEDs. Input laser signal enables to produce enough electrical signal from the first device (PD) to turn-on the second device (QLEDs) for the light emission. (b) Periodic photoresponses of the PD according to the 520 and 635 nm wavelength light signal measured at (ii) V_out of the circuit diagram. (c) Luminescence characteristics of the QLEDs according to the periodic input laser signal (λ = 520 nm) into the PD.

Figure 7

(a) Circuit diagram to generate an electrical signal using PD from the light emission of QLEDs. (b) Periodic output electrical signal of PD measured at (ii) V_out from the input light generated from the (i) dual-functional QLEDs. Inset shows the image of turn-on state QLEDs to generate electrical signal from the PD and turn-off state QLEDs to turn-off the PD.