Double layer devices

In my work, huge amounts of devices were made to verify the feasibility of the multilayer fabrication process, including EML/HBL and EBL/EML device structures. Figure 2.4 shows the schematic energy profiles of the structure EML/HBL and EBL/EML. First, the double layer devices were made by liquid buffer layer method and then I studied the difference between liquid buffer layer method and blade coating method.

FIG. 2.4： Schematic electronic energy profile for the double-layer device structure of (a) EML/HBL, and (b) EBL/EML.

Since many emissive conjugated polymers are p-type, and the hole mobility is two or three orders lager than electron mobility. Therefore the device structure ITO/PEDOT:PSS/EML/HBL/cathode to block the hole current and improve the device efficiency was adopted. Four typical p-type polymers were used as EML, including MEH-PPV, PFO, TFB and DPOC10-DOMe-PPV. For HBL, we chose two materials with good electron transport characteristic and large ionization potential (IP) to block holes.

One was S-Y by Covion Organic Semiconductors[54], and the other was the small molecule TPBi. With the unusually large IP, TPBI is commonly used as the HBL in OLEDs by evaporation[55]. The structure of the device without HBL was glass/ITO/PEDOT/EML/LiF/Ca/Al, and the baking condition of EML was 180 ℃ for TFB and 120 ℃ for the other EML for 40 minutes in vacuum. The structure of the device with HBL was glass/ITO/PEDOT/EML/HBL/LiF/Ca/Al, and the baking condition of EML was the same as the devices without HBL. After the HBL was spin-coated on the EML using BL method, the devices were baked in vacuum for 60 minutes at 200 ℃ for DPOC10-DOMe-PPV and 120 ℃ for the other EML. Then the devices were coated with LiF/Ca/Al cathodes and packaged in the glove box.

(a) (b)

In addition to the EML/HBL structure, EBL/EML structure was also made. In this experiment, TFB was chosen as the material of EBL and LUMATION BP105 was chosen as the blue polymer. BP105 is currently very good blue polymer for PLED. Even so the electron-hole balance is far from perfect. In single layer LED the current is dominated by electron current because of the high electron mobility and large hole injection barrier.

Such imbalance was evidenced by the much smaller hole-only current than the LED current to be determined by the comparison of hole-only and electron-only devices. Four kinds of devices were made to present the advantage of the buffer method. The first two were ITO/PEDOT: PSS/TFB/EML/Cathode made by the BL method and the spin-rinse method.[52] The second two were ITO/PEDOT: PSS/EML/Cathode made by pure BP105 or BP105: TFB blend (10:1). The ITO substrates were cleaned and the surfaces were treated by oxygen plasma followed by spin-coating 50 nm of PEDOT:PSS. PEDOT:PSS film was baked at 200 ℃ for 5 minutes in vacuum environment (10⁻³ torr). TFB film (30 nm) was spin-coated on PEDOT: PSS film (1% in xylene) and annealed at 180 ℃ in vacuum for 40 minutes. For the BL method, a 67 nm BP105 film was spin-coated (2% in xylene) over the liquid buffer covering TFB. The double-layer device was baked at 120

℃ for 1 hour in vacuum to remove the residue 1,2-propylene glycol. Since the size of 1,2-propylene glycol molecules is about the same as common organic solvents like xylene, there should be no problem for them to diffuse across the layer 2 polymer during annealing. In fact any significant residue of 1,2-propylene glycol at the interface would be detrimental to the device performance. The excellent performance to be discussed below is another evidence for the complete removal of the buffer liquid. There was no loss in the double-layer film thickness compared with the sum of the two individual films within the error of thickness measurement. In other words, the inter-diffusion of the two polymers was less than the experimental error of the individual thickness, which was about 5 nm.

For the spin-rinsing method, a very thin TFB layer (about 10 nm) was achieved by

spin-rinsing the TFB layer with pure xylene to remove the remaining soluble part. A 67 nm BP105 film was then spin-coated on the spin-rinsed TFB layer and baked at 120 ℃ for 40 minutes in vacuum. For the other devices the EML films were baked in vacuum for 40 min at 120 ℃. Finally all the devices were coated with LiF/Ca/Al cathode and packaged in the glove box.

In order to verify the feasibility of multilayer PLED by blade coating, bilayer PFO devices were made with the structures ITO/PEDOT:PSS/TFB/PFO/CsF/Al and ITO/PEDOT/PFO/PBD/CsF/Al. Both TFB and PBD were dissolved in toluene. TFB film (30 nm) was spin-coated on PEDOT:PSS film and annealed at 180 ℃ in vacuum for 40 minutes. PFO was blade coated on top of TFB layer on hot plate at 70 ℃. PBD was blade coated on PFO layer on hot plate 100 ℃. The thickness of PFO was about 80 nm. All the devices are coated with CsF(2 nm)/Al(100 nm) cathode and packaged in a glove box.

2.4