1 Introduction

Tang and Slyke [1] demonstrated the first organic light emitting diode (OLED) by using Alq3 in 1987. From there on, a lot of research has been reported on OLEDs [2, 3]. The conjugated polymer devices have advantages like easy fabrication, low cost, lightweight, human friendly, sustainable raw materials, very thin, large view angle, and fast response [4, 5]. The solution-based polymers make the fabrication process easier as compared to the inorganic conventional LEDs [6]. The LED lamps today achieve the most efficient lighting but OLEDs are even outperforming LEDs in performance [7]. Therefore, the future of lighting is flexible, thin, light, dimmable, low cost, solution processable and large area OLEDs. OLED lighting is highly efficient and has closest spectral distribution without any heat, UV emission, or glare [5, 8, 9].

Indium tin oxide (ITO) is generally used as the transparent bottom electrode of the device. ITO as anode provides holes to the device. Emissive layer is deposited between hole transport layers and electron transport layers. Top electrode is usually of reflective metal with low work function to provide electrons to the device [10–12].

Quantum dots are the nanocrystal with a quantum confinement [13, 14]. This quantum confinement enables them to narrow down the emission wavelengths that are tune able throughout the visible range [15]. The QDs have tunable band gap and are solution processable. These qualities makes them excellent candidates to be used in OLEDs [13, 16–18], solar cells [19–21], electroactive polymer artificial muscles [22], biological imaging [23–26] and composite materials. The quantum dots formed from heavy metals are used for imaging, diagnostics, and sensing purposes in medical science and are believed to be toxic. Ye et al. [27] show in a pilot study that rhesus macaques injected with phospholipid micelle encapsulated CdSe/CdS/ZnS quantum dots do not exhibit evidence of toxicity.

OLED thin films can be deposited by different printed electronics approaches like spin coating [4, 7, 11, 28, 29], transfer printing [30, 31], screen printing [32] and spray [33]. Here the focus is on large area OLED fabrication. Spin coating offers easy fabrication process but it is not suitable for large area applications. Spin coating has huge ink wastage, the process is not compatible with roll-to-roll system, masking is required and has low throughput [34]. In transfer printing, a thin films by spin coating or any other process is required before the material is transferred by using a stamp [35].

Electrohydrodynamics spray technique was used to fabricate the thin films. This technique is simple, easy, completed in a single step, and is performed in atmospheric temperature and pressure conditions. Various organic, inorganic inks have been sprayed by electrohydrodynamics spray in our previous work like poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), zinc oxide (ZnO), zirconium dioxide (ZrO2), carbon nanotubes, graphene flakes [28, 36–42].

In this work large area OLED is fabricated by using organic layers as hole and electron transport layers while quantum dots are used as emissive layer. Electrohydrodynamics spray process is used for the deposition of organic layers and quantum dots thin films with a device active area of 3 × 3 cm2. The poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) ink has been optimized by adding ZnO nanoparticles for better spray. The thin layers and device are characterized for their surface, optical and electrical properties.

2 Experimental setup

2.1 Materials

The flexible polyethylene terephthalate (PET) substrate coated with indium tin oxide (ITO) was used with of surface resistivity 12 Ω/cm2, MEH-PPV powder with average molecular weight around 40,000–70,000, cadmium oxide (CdO, 99.99 %), zinc acetate (Zn(CH3COO)2 99.99 %, powder), oleic acid (OA, 90 %), trioctylphosphine (TOP, 90 %), selenium (Se, 99.99 %, powder), sulfur (S, 99.9 %, powder), 1-octadecene (90 %), dodecanethiol (99 %) and dimethyl sulfoxide (DMSO, 99.9 %) were purchased from Sigma Aldrich (South Korea). PEDOT:PSS was obtained from Agfa Materials Japan. Isopropanol and acetone were purchased from Deajon Chemicals and Metal Co, Ltd, South Korea. All chemical were used as received.

2.2 Inks preparation

The viscosity of PEDOT:PSS was reduced to 20 mPa s for better spray performance using isopropanol [43, 44]. The DMSO was added in ink for conductivity enhancement. The polymeric nature of PEDOT:PSS caused agglomerations that were reduced by using Triton as a surfactant. The synthesis of CdSe/ZnS QDs is already reported in our previous work [33]. 5 wt% of quantum dots were dispersed in toluene while DMSO was used as co-solvent. The viscosity of quantum dots ink was 0.56 mPa s.

Different recipes for MEH-PPV electrohydrodynamics spray were used. In optimized recipe 0.2 % concentrated MEH-PPV solution was prepared form MEH-PPV powder by bath sonication with ice packs for 5 min and then magnetic stirring for 2 h. DMF was added to this solution in the ratio of 3:1. Solution was stirred for 30 min on magnetic stirrer. 0.05 % ZnO nanoparticles of average size of 40 nm were dispersed in the solution. The final viscosity of the solution was 6.2 mPa s. The ZnO nanoparticles were added to the ink solution to eliminate the electrospinning phenomena during electrohydrodynamics spray. The recipes for PEDOT:PSS, QDs and MEH-PPV inks are shown in Table 1.

Table 1 Recipe for PEDOT:PSS, quantum dots and MEH-PPV inks for EHDA spray
Full size table

2.3 Device fabrication

The device structure of fabricated OLED was ITO/PEDOT:PSS/(CdSe/ZnS QDs)/MEH-PPV/Al as shown in Fig. 1. The device was fabricated by electrohydrodynamics spray process. Initially ITO coated PET was cleaned by bath sonication using acetone and then isopropanol for 15 min each. After cleaning the ITO coated PET was rinsed with deionized water and dried by hot air. The organic contaminations present on substrate were removed by UV (ultraviolet) treatment for 120 s. The fabrication process is shown in Fig. 1. PEDOT:PSS was the first layer to be deposited on the ITO coated PET on an area of 4 × 4 cm2. The thin film was deposited by electrohydrodynamics spray process. The layer was dried at 100 °C for 30 min. CdSe/ZnS QDs were sprayed onto the dried PEDOT:PSS thin film by electrohydrodynamics spray system. The QDs thin film, used as emissive layer, was dried at 100 °C for an hour. MEH-PPV acted as electron transport layer. MEH-PPV was also sprayed by electrohydrodynamics spray. Thin film was dried after deposition at 100 °C for 30 min. The top electrode was deposited by thermal evaporation of aluminum. The deposition pressure was 10−6 torr with deposition rate of 3 Å/s. The thickness of top electrode was 100 nm with active area of 3 × 3 cm2. Finally the device was encapsulated by a flexible epoxy binder.

Fig. 1
figure 1

OLED step by step fabrication process and the structure of device

Full size image

2.4 Electrohydrodynamics spray setup

ESD setup is shown in Fig. 2. ESD was performed at room temperature and atmospheric pressure (25 °C, 50 % humidity). The in-house developed ESD system was fitted on a roll-to-roll system. The ESD system consists of a metallic capillary, nozzles with different internal diameters such as 110, 210 and 310 µm were selected on the bases ink parameters like viscosity, molecular weights of organic inks, particle size of inorganic materials and standoff distances. A nozzle adaptor (NanoNC) was connected to a syringe (Hamilton, Model 1001 GASTIGHT syringe). This syringe was connected to an infusion pump (Harward Apparatus, PHD 2000 Infusion) by Teflon tubing. The infusion pump controlled the flow rate of inks. The nozzle adopter was attached to a head holder that can move in x, y and z axis using linear servomotors. LabVIEW (version 8.6) was used to control the motion of head holder using a motion controller card (NI PXI 7340) installed in PXI (NI-PXI-8110). To interface the linear servomotor with motion controller card, an interface board (CW7764YS3) was used. A program was written in LabVIEW to move the head holder in prescribed path by coordinates mentioned in a text file. The metallic nozzle was connected to high voltage supply (TREK Model 610E) while the base was connected to ground on which substrate was placed. The voltage signal was generated by a function generator (Hewlett Packard 33120A). The electrohydrodynamics phenomena was observed by a high speed camera (Motion Pro X) laminated by a light source (Moritex MLET-A080W1LRD).

Fig. 2
figure 2

Electrohydrodynamics spray deposition (ESD) system setup

Full size image

2.5 Characterization

Thin films surface morphology was analyzed by field emission scanning electron microscopy (FE-SEM Model JEOL JSM-7600F, Japan). The FE-SEM was operated at accelerating voltage of 5 kV. The size and lattice spacing of quantum dots was observed by high resolution transmission electron microscope (HRTEM Model JEOL JEM 2100F). Thickness of thin films was measured by reflectometer (K-MAC ST4000-DLX). The optical analysis of thin films on ITO coated PET were done by using a ultra-violet/visible spectrometer (Shimadzu UV-3150) in range of 300–800 nm UV–visible (UV–Vis) data. Photoluminance of thin films was performed by photoluminance spectroscope (Parkin Elmer LS-55) with 365 nm excitation wavelength. The electrical analysis of devices was performed by Agilent B1500A Semiconductor Device Analyzer coupled with MST8000C Probe Station having a current resolution of 1 fA. The HOMO and LUMO values of the QDs were determined from the cyclic voltammetry (CV) for which a three electrode electrochemical cell was used with a potentiostat (VSP, Bio-Logic). We used a glassy carbon disk as a working electrode, a Pt wire as a counter electrode, and a Ag/0.01 M AgNO3 + 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) acetonitrile (Ag/Ag+) as a reference electrode. The CV measurements were done from −2.0 to 1.7 V at a scan rate of 20 mV/s. The electroluminance spectra of the films were measured by an Avantes AVA-SPEC 2048 calibrated spectrometer. Minolta LS-100 luminance meter was used to measure luminous intensity and to collect the Commission International d’Eclairage (CIE) coordinators (x,y) for the identification of color.

3 Results and discussion

3.1 Electrohydrodynamics spray

Electrohydrodynamics atomization process is used to spray the functional inks onto the substrate under the effect of electric field. Without any electric field, functional ink flowing through the nozzle results in drops larger than the external diameter of nozzle. The phenomenon is known as dripping mode of electrohydrodynamics. As the electric field is applied and increased gradually, the drop size starts to decrease. This mode of atomization is known as the microdripping mode. With further increase in voltage, a cone is observed at the meniscus in an unstable form. Increase in voltage takes the unable cone jet to a stable jet. Further increase in voltage results in multijet.

MEH-PPV was used as electron transport layer. The conductivity of this polymer was increased by DMF for better spray performance. According to the classical electrohydrodynamic atomization of the stable cone jet mode, the electrical relaxation time, Te, must be very much smaller than the hydrodynamic time, Th. It is established by the inequality [45].

$$\frac{{\beta \varepsilon_{o} }}{K} \ll \frac{LD}{Q}$$
(1)

where β is the relative permittivity, K is the electrical conductivity, L is the axial length of the jet, D is the jet diameter, Q is the given flow rate and ε0 is the permittivity of vacuum (8.85 × 10−12 F/m). Here DMSO increases the conductivity of ink, which results in reduction of the electrical relaxation time. The lower relaxation time results in satisfying the inequality and achieve better spray results. ZnO nanoparticles were added in small quantity to remove electrospinning phenomena. Electrospinning for even a very small duration throughout spray process would result in porous thin film. The different recipes used for the electrospray are shown in Table 2. The spray of the different functional inks was performed in stable cone jet mode. The parameters for the spray of PEDOT:PSS, CdSe/ZnS QDs and MEH-PPV are presented in Table 3.

Table 2 MEH-PPV ink recipes for electrohydrodynamics spray
Full size table
Table 3 Parameters of electrohydrodynamics spray of PEDOT:PSS, CdSe/ZnS QD and MEH-PPV
Full size table

3.2 Surface morphology

The surface morphology of organic thin films was analyzed by FE-SEM. Figure 3 shows the FE-SEM images of PEDOT:PSS and MEH-PPV. The FE-SEM images showed no signs of pores or defects on the thin films. The CdSe/ZnS QDs were analyzed by HR-TEM as shown in Fig. 4. The quantum dots showed average particle size of 8 nm. The lattice spacing was found out to be 0.4 nm. The thickness of thin films of PEDOT:PSS, QDs and MEH-PPV were 100, 105 and 120 nm respectively.

Fig. 3
figure 3

SEM images of a PEDOT:PSS layer and b MEH:PPV layer

Full size image
Fig. 4
figure 4

TEM images of the CdSe quantum dots showing their size

Full size image

3.3 Optical analysis

The photoluminescence of QDs showed a peak at wavelength of 634 nm as shown in Fig. 5. The electroluminance of the OLED device also showed a wavelength around 634 nm. The electroluminescence produced by OLED showed that QD acted as the emissive layer and MEH-PPV acted as electron transport layer. This electroluminescence at 634 nm wavelength lies in red color region. The electroluminance is shown in Fig. 6.

Fig. 5
figure 5

Photo luminescence curve of the quantum dots

Full size image
Fig. 6
figure 6

Electro luminescence curve of the fabricated OLED

Full size image

3.4 Electrical characterization

The electrical analysis of OLED was performed as shown in the Fig. 7. I–V shows the turn on voltage of 5 V and maximum current density of 143 mA/cm2. Here it can be seen that device (ITO/PEDOT:PSS/QDs/MEH-PPV/Al) shows non-ideal, schottky diode behavior with hopping charge transport mechanism [15, 46]. The energy level of QDs was measured and HOMO/LUMO values of QDs were −5.785 and −3.55 eV. The energy level diagram of the device is shown in Fig. 8. The luminance of the OLED was measured by lux meter and maximum 0.5 lx was obtained as shown in Fig. 9. The luminance of OLED was uniform and without any defects. The lifetime of fabricated devices was measured to be more than an hour. The Commission International d’Eclairage (CIE) coordinates were obtained by the lux meter which were (0.6, 0.39) which is in red color range as shown in Fig. 10. This large area, flexible, solution based, roll-to-roll processed OLED offers the answer to the high demands of lighting applications in near future.

Fig. 7
figure 7

I–V characteristic curve of the organic light emitting diode

Full size image
Fig. 8
figure 8

Layer by layer energy band diagram of the device

Full size image
Fig. 9
figure 9

Luminescence versus voltage curve of the OLED

Full size image
Fig. 10
figure 10

Commission International d’Eclairage (CIE) coordinates for the OLED

Full size image

4 Conclusions

The large area OLED fabricated by ESD process with an active area of OLED is 3 × 3 cm2 has been fabricated and characterized. CdSe/ZnS QDs are used as the emissive layer with emission wavelength of 634 nm lying in red color region. The conjugate polymers of PEDOT:PSS and MEH-PPV have been used as hole and electron transport layers to facilitate the movement of charge carriers from electrodes to emissive layer. The average size of QDs was 8 nm with lattice spacing of 0.4 nm. The spray of MEH-PPV has been optimized by adding nanoparticles of ZnO. The OLED device has shown the current density of 143 mA/cm2 at 20 V. Maximum luminance of 0.5 lx has been achieved with CIE color coordinates (0.6, 0.39). The encapsulated device remained active for more than an hour. Thus, the simple structure device with optimized charge transport and emissive layers fabricated by spray system installed on roll-to-roll system offer a good potential for future lighting applications.