Thin film Encapsulations of Flexible Organic Light Emitting Diodes

Various encapsulated films for flexible organic light emitting diodes (OLEDs) were studied in this work, where gas barrier layers including inorganic Al2O3 thin films prepared by atomic layer deposition, organic Parylene C thin films prepared by chemical vapor deposition, and their combination were considered. The transmittance and water vapor transmission rate of the various organic and inorgabic encapsulated films were tested. The effects of the encapsulated films on the luminance and current density of the OLEDs were discussed, and the life time experiments of the OLEDs with these encapsulated films were also conducted. The results showed that the transmittance are acceptable even the PET substrate were coated two Al2O3 and Parylene C layers. The results also indicated the WVTR of the PET substrate improved by coating the barrier layers. In the encapsulation performance, it indicates the OLED with Al2O3 /PET, 1 pair/PET, and 2 pairs/PET presents similarly higher luminance than the other two cases. Although the 1 pair/PET encapsulation behaves a litter better luminance than the 2 pairs/PET encapsulation, the 2 pairs/PET encapsulation has much better life time. The OLED with 2 pairs/PET encapsulation behaves near double life time to the 1 pair encapsulation, and four times to none encapsulation.


Introduction
Due to the simplicity, organic light emitting diodes (OLEDs) can be easily built on not only glass substrates but also flexible substrates.However, the short lifetime and low efficiency of the flexible OLEDs are still the major problem to be solved.It is distributed to the penetration of oxygen and water vapor to the organic layers through the substrate and the encapsulated film [1][2][3].
The basic encapsulation method is used glass and epoxy adhesive in a N 2 -filled glove box.However, the glass is not suitable for flexible encapsulation because of its rigid nature.The polymer substrates are the most promising candidate for the flexible encapsulation.The water vapor transmission rate (WVTR) of the polymer substrate is substantially below the requirement of OLED encapsulation.Therefore, polymer substrates coated barrier thin films to improve the WVTR are essential for flexible OLED encapsulation [4,5].
Inorganic thin films have shown the potential to be the barrier layer for flexible OLED encapsulation [6].Although these inorganic thin films have low WVTR, the defects of pinholes cannot be avoided because of the limitation of the vacuum deposition process.An organicinorganic multi-barrier become an alternative to prevent the defects.[7][8][9][10][11] In this work, to prevent the penetration of water vapor into the OLEDs, various encapsulated films for flexible OLEDs were studied, where gas barrier layers including inorganic Al 2 O 3 thin films prepared by atomic layer deposition, organic Parylene C thin films prepared by chemical vapor deposition, and their combination were considered.The transmittance and water vapor transmission rate of the encapsulated films were tested.The effects of various encapsulated films on the luminance and current density of the OLEDs were compared.The life time experiments of the OLEDs with these encapsulated films were also conducted in the final.

Specimen Preparation
The structure of the OLED is shown in Fig. 1, where a flexible PET substrate, an ITO anode, a PEDOT:PSS (Clevios AI4083) hole injecting layer (HIL), a NPB hole transporting layer (HTL), an Alq 3 emitting layer (EL), an aluminum cathode, and a PET cover were stacked in sequence.The ITO-coated PET substrate with size 300u200u0.175mm was treated by standard cleaning procedure including ultrasonic bath and oxygen plasma before deposition.To fabricate the OLED, a PEDOT:PSS film was deposited on the ITO/PET substrate by spin coating at the first.The specimen was followed by drying process at 70 qC for 10 min.And then, the NPB and Alq 3 were thermally evaporated with 1 Å/s deposition rate.And, an aluminum cathode was successively evaporated.Finally, the device was covered with the encapsulated film and packaged by UV glue, and cured by UV light for 1 hour.
To prepare the encapsulated films, Al 2 O 3 thin films were fabricated on the PET substrates by ALD.The

Result and Discussion
Optical transmittance of the encapsulated films were measured by using an UV/VIS/NIR spectrophotometer (BWTEK BTC112) with normal incidence in the visible wavelength ranged from 400 to 800 nm, taking the air as reference.The results are shown in Fig. 2, where the average transmittance of the PET substrate is 88.162%, PET coated Al 2 O 3 is 87.05%, 1 pair/PET is 86.56%, and 2 pairs/PET is 84.65%.Obviously, the transmittance decreases as the barrier layer increases.However, there is only 1~5% transmittance loss by adding the barrier layers.

Image
The life time of the OLEDs with various encapsulated films is shown in Fig. 6.The life time of the OLED is indicated by the normalized luminance (L/L 0 ) that presents by the ratio of the luminance after usage hours (L) to the luminance at the finished moment (L 0 ).For the OLED without encapsulation, the normalized luminance is 0.26 for 12 hrs, 0.10 for 24 hrs, and 0.03 for 36 hrs.For the OLED with PET encapsulation, the normalized luminance are 0.37, 0.16, 0.08, and 0.02 for 12, 24, 36, and 48 hrs, respectively.The OLEDs with Al 2 O 3 /PET and 1 pair/PET encapsulation behave the similar life time.The OLED with 2 pairs/PET presents the best life time, where the normalized luminance are 0.80, 0.66, 0.53, 0.43, 0.33, and 0.03 for 12, 24, 36, 48, 60, and 120 hrs, respectively.Note that, although the 1 pair/PET encapsulation behaves a litter better luminance than the 2 pairs/PET encapsulation, the 2 pairs/PET encapsulation has much better life time.The OLED with 2 pairs/PET encapsulation behave near double life time to the 1 pair encapsulation.

DOI: 10
gas of the ALD reactor are controlled by switching valves and flow past the substrate surface and into a mechanical pump.The substrate temperature at 60 qC, and the precursor pulse time with 10 msec were considered, where the chamber pressure is controlled at 9×10 -3 Torr.The precursor H 2 O, purging N 2 gas, precursor TMA, and purging N 2 gas are controlled to flow into the chamber in sequence.The Al 2 O 3 film sequentially grows with 10 msec precursor of H 2 O, 6000 msec of N 2 purging, 10 msec precursor of TMA, and 6000 msec of N 2 purging.The film thickness is controlled as 50 nm, consequently near 500 cycles were needed.Parylene C thin films with 100 nm thickness were deposited by a chemical vapor deposition system.Parylene C powders were evaporated at 150 qC and pyrolyzed at 650 qC.The deposition temperature was room temperature and the deposition time was 1 hour.In this work, five encapsulated types were considered, including none, PET, Al 2 O 3 /PET, Parylene C /Al 2 O 3 /PET, and Parylene C /Al 2 O 3 /Parylene C /Al 2 O 3 /PET, respectively.In the following discussion, the fourth and fifth types are simply denoted as 1 pair/PET and 2 pairs/PET.

Figure 1 .
Figure 1.Sketch of the flexible OLED.

Figure 2 .
Figure 2. Transmittance of the encapsulated films.The water vapor transmission rate (WVTR) of the encapsulated films is shown in Fig.3.It shows the average WVTR of PET is 11.08 g/m 2 day, where Al 2 O 3 /PET is 3.44 g/m 2 day, 1 pair/PET is 3.36 g/m 2 day, and 2 pairs/PET is 2.77 g/m 2 day.The results indicate 2 pairs/PET has the best water and vapor barrier performance, which is 4 times better than the bare PET.It also reveals that the PET substrate coated with a barrier layer will efficiently improve the water-vapor resistance.

Figure 3 .
Figure 3. Water vapor transmission rate (WVTR) of the encapsulated films.Luminance of the OLEDs with various encapsulated films is shown in Fig. 4. The results show that the OLEDs work as the applied voltage higher than 3V, and the luminance increases quickly as the voltage increases.It shows the luminance of the OLED without the encapsulated film is 111.82 cd/m 2 at 8V, and the luminance of OLED with PET, Al 2 O 3 /PET, 1 pair/PET, and 2 pairs/PET are 162.18,540.58, 557.23, and 544.93 cd/m 2 at 8V, respectively.It indicates the OLED with Al 2 O 3 /PET, 1 pair/PET, and 2 pairs/PET presents similarly higher luminance than the other two cases.This phenomenon is consistent with the result of WVTR

Figure 4 .
Figure 4. Luminance of the OLEDs with various encapsulated films.The Current density vs. voltage diagram of the OLEDs with various encapsulated films is shown in Fig.5, which is similar to the luminance diagram shown in Fig.4.The Luminance and corresponding images of OLEDs with various encapsulated types at 8V are list in Table1.Each luminant area of the OLED is 2u2 mm.The images

Figure 5 .
Figure 5. Current density vs. voltage diagram of the OLEDs with various encapsulated films.

Figure 6 .
Figure 6.Life time of the OLEDs with various encapsulated films

Table 1 .
Luminance and corresponding images of OLEDs with various encapsulated films at 8 V.