Double layer devices fabricated by blade coating

Although the liquid buffer method can completely prevent the dissolution and achieve high-efficiency and stable PLEDs, material waste is however still a problem and it is non-trivial to scale up to very large areas. Blade coating is a common method to form large-area polymer films. After confirming the successful deposition of bilayer structure I turned to the more basic question of whether the film uniformity in blade coating was good enough for PLED. The uniformity was verified by comparison with the standard spin coated films. PFO thin film made by three process were compared, including spin coating, blade and spin coating, and blade coating on hot plate. In the blade and spin coating process the polymer wet film was first blade-coated then spun immediately to form the polymer dry thin film. This process is ideal for the first layer as it combines the advantages of fast drying for spin coating and high material usage of blade coating. On the other hand blade coating on hot plate is ideal for the second layer. The large scale uniformity in an area of 6 cm × 5 cm is 60±3 nm for spin coating, 60±2 nm for blade and

spin coating, and 60±10 nm for blade coating on hot plate at 70 . ℃ The microscopic uniformity is checked by SEM and AFM. The results are shown in Figure 3.7.

FIG. 3.7: Microscopic uniformity of PFO thin films checked by SEM of different processes (a) spin coating (b) blade and spin coating (c) blade coating on the hot plate (70 ). Microscopic uniformity of PFO thin films ch℃ ecked by AFM of different processes, (d) spin coating (e) blade and spin coating (f) blade coating on the hot plate (70 ).℃

There was no obvious difference among these three processes. The polymer film roughness in 0.5 μm × 1 μm area was 5.5 Å for spin coating, 3.6 Å for blade and spin coating, and 3.1 Å for blade coating on the hot plate. The single layer polymer thin film by blade coating is almost the same as that by spin coating in both macroscopic and microscopic scales. Blade coating therefore combine the advantages of multilayer deposition and efficient material usage without sacrificing the film quality. Then we turned to PLED performance. Fig 3.8 shows the results of single layer devices with structure of ITO/ PEDOT/ EML/ CsF/ Al. Three fabrication processes including spin coating, blade and spin coating, and blade coating on hot plate have been compared, which is described in page 16.

FIG. 3.8: Device performance of single layer S-Y and PFO PLED by spin coating (square), blade and spin coating (circle) and blade coating on hot plate (triangle). (a) The current efficiency. Inset are the electroluminescent spectrum of S-Y and PFO and the S-Y device by blade coating in operation. The active area of device is 4 cm × 7.5 cm.(b) The luminance. Inset are the current density and the PFO device by blade coating in operation.

The active area of device is 4 cm × 7.5 cm.

The maximum efficiencies of S-Y PLED are almost the same (about 9 cd/A at 3.5 V).

The maximum luminance are 69330 cd/m² for spin coating, 39830 cd/ m² for blade and spin coating, and 30190 cd/ m² for blade coating on the hot plate. The difference of luminance may be due to the variation of film thickness. The maximum efficiencies of PFO PLED is 1.1 cd/A for spin coating, 0.9 cd/A for blade and spin coating, and 1.7 cd/A for blade coating on hot plate. The maximum luminance is 3371 cd/ m² for spin coating, 2370 cd/ m² for blade and spin coating, and 4390 cd/ m² for blade on hot plate.

Surprisingly the performance of PFO PLED by blade coating on hot plate is the best.

Since the uniformity are almost the same for all PFO films, we speculate the chain entanglement of PFO in the nanometer scale by blade coating on hot plate is stronger than that of the other methods. Such entanglement enhancement is important for the low molecular weight PFO (Mw below 100,000) but not so for the high molecular weight S-Y (Mw about 1,000,000)[54]. Bilayer PFO devices with structures TFB/PFO and PFO/PBD are made by blade coating on hot plate for the second layer. The results are shown in Figure 3.9.

FIG. 3.9: Device performance of single layer PFO PLEDs and double layer TFB/PFO and PFO/PBD PLEDs. (a) The current efficiency. Inset is the electroluminescent spectra of the devices. The spectra of single layer PFO and double layer PFO/PBD are almost the same and normalized to 0.5 for clarity. (b) The luminance. Inset is the current density.

Single layer PFO devices by spin coating (solid square) and by blade coating on hot plate (empty square). Double layer TFB/PFO devices by liquid buffer method (solid circle) and by blade coating on hot plate (empty circle). Double layer PFO/PBD device by blade coating on hot plate (solid triangle).

The maximum efficiency is raised to 2.3 cd/A for TFB/PFO as compared to 1.05 cd/A for the single layer PFO device. The TFB/PFO made by liquid buffer has the efficiency of 1.7 cd/A which is lower than the same structure made by blade coating. This is probably because that TFB and PFO are more in contact with each other in blade coating on hot plate than in the liquid buffer process. Moreover the device efficiency is raised to 2.9 cd/A in PFO/PBD device. The maximum luminance is 8807 cd/ m² for TFB/PFO, about 2.5 times larger than the single layer PFO device (3371 cd/ m²). The maximum luminance is 4429 cd/ m² for PFO/PBD. The enhancement of TFB/PFO bilayer devices is due to that the electrons in PFO are blocked by TFB, which induce more holes to be injected and achieve higher efficiency and luminance. As for PFO/PBD device the holes are blocked by PBD. The efficiency are enhanced by separating the recombination zone from the cathode to reduce metal quenching. The performance of all the devices is listed in Table 3.2. S-Y and PFO are just two examples to demonstrate this new fabrication method.

Apparently blade coating can be applied to any kind of semiconducting polymers.

TABLE 3.2: Performance of PLEDs by blade coating.

Label Max. Current TFB/PFO(liquid buffer method) 1.7 (at 5.5V) 1.34 5575 (at 10.5V) TFB/PFO(blade on hot plate) 2.3 (at 4.5V) 2.2 8807 (at 10V) PFO/PBD(blade on hot plate) 2.9 (at 5.5V) 1.83 4429 (at 8.5V)

Hole-only devices and electron-only devices were also made to further study the basic transport properties. The device structures are ITO/PEDOT/EML/Al for hole-only device and Al/EML/CsF/Al for electron-only device. The results are shown in Fig. 5. In Fig.

3.10(a), it can be seen that hole currents of S-Y devices made by blade coating are slightly higher than those of the devices made by spin coating. Compared with hole currents, the electron currents in the blade coated S-Y devices are about the same as the spin coated S-Y devices. Similar phenomenon can also be seen in PFO devices, which is shown in Fig. 3.10(b).

FIG. 3.10: Hole-only and electron-only devices made by spin coating (square), blade and spin coating (circle) and blade coating on hot plate (triangle). (a) S-Y devices (b) PFO devices. Inset shows the photo-luminescent (PL) spectra.

We speculate that the stacking of polymer chains by blade coating could be more ordered than that by spin coating, which causes the increase of the hole mobility as well as the hole current. On the other hand, the electron current is mainly decided by the trap density in the bulk.[14] The photo-luminescent (PL) spectra are also shown in the inset of Fig.

3.10. The PL spectra of blade coated S-Y films are almost the same as the spin coated one. The PL spectra of blade coated PFO are slightly different from the spin coated one.

The peaks of 464 nm and 494 nm grow in blade coated thin film, which may also due to the ordered stacking polymer chains.