(Invited) Integrated Qled-Thin Film Electronic Driving Device Fabricated Fully in Solution
Authors: Mohammed-Brahim Tayeb , Hussain Sajid , Wei Lei 
Published: December 2024

Usual Light Emitting Transistor (LET) integrates Organic Light Emitting Diode (OLED) with a Vertical Filed Effect Transistor (VFET) made in organic materials mainly. The needed driving current is high, that can be got at high voltage only. When using Quantum dots LED (QLED) the needed current is higher. The main purpose of the present work is to jump this issue and to find a device that is able to produce a high current as low voltage with the particular difficulty of using low temperature solution process.

We fabricate a QLED based on inorganic light emitting layer that allows a luminance of more than 4000 Cd/m2. This QLED is driven by an integrated device made by additive process.

The inverted QLED is based on inorganic light emitting CdSe-ZnS. Its structure is ITO/ ZnO/ CdSe-ZnS/ PVK/ Poly-TPD/ Au. Except Au film that is evaporated, all other layers are spin-coated and then annealed. However the Au evaporation can be replaced by a spin-coating of Au nanowires solution.

Figure 1 shows the current and luminance of this QLED as a function of the applied voltage

To drive this QLED, a device consisting on a stack of bottom contact, low conductive layer, middle porous contact, semiconducting layer and top contact, is proposed (Figure 2)

Thanks to the porous middle contact (M), the current can flow from the bottom contact (B) to the top contact (T) if a voltage VB is applied to B and a different voltage VT is applied to T. Both voltages are referred to the contact M that is always fixed at 0V. By this mean, the current can be modulated independently by VB or VT.

For example, we can take PCBM as low conductive layer and ZnO as semiconductor. The porous layer is a spin-coated ITO nanoparticles layer that provides some holes inside as observed by SEM (Figure 3).

The current ITM between the top contact and the middle one as a function of the voltage between the bottom contact and the middle one VBM is shown in figure 4 when the voltage between T and M is fixed to +1V and -1V.

The current ITM is high, more than 200 mAmp/cm2, and is null when VTM= VBM. The characteristic shifts when VTM changes. This means the current that varies due to the variation of the bottom voltage is modulated by the voltage of the third contact. Then, for example, the current, null at VBM=-1V and VTM=-1V, jump to 250 mAmp/cm2 if VTM moves suddenly to +1V for the same value of VBM. This gives an idea to drive the QLED, particularly as the current is very high.

An integrated vertical light emitting device, constituted by a stack electron device – QLED, was fabricated. No light is emitted by this device in the range -6V< VBM <-7.5V for VTM =-2V. But the device lights on in the same range for VTM =+2V. Fixing VBM at -6.5V, we checked that the device lights-on progressively when VTM is swept from -3V to +2V as shown in the Figure 5. This figure reports the photocurrent of a photodetector when VTM is swept.

Very simple and low cost fully in solution processed integrated light emitting device was presented. The device is very simple. It leads to a control of the luminance of a QLED thanks to the modulation of the current by a second applied voltage.

Figure 1