Plasma Polymer Films Prepared in a Triode Glow Discharge

Plasma-polymerized p-xylene (PPPX) and hexamethyldisiloxane (PPHMDS) films were prepared in a triode glow disharge. Formation characteristic, Fourier transform infrared spectroscopy (FTIR), photoluminescence (PL), relative dielectric constant (2r) of PPPX films, and application to the encapsulation of organic light-emitting diodes (OLEDs) were investigated. The thickness and 2r of PPPX films increased and the FTIR and PL spectra decreased with increasing discharge current. The luminance of encapsulated OLEDs with PPPX and PPHMDS films was higher than that of non-encapsulated OLED after the preparation of PPPX and PPHMDS films. [DOI: 10.1380/ejssnt.2009.760]


I. INTRODUCTION
Since plasma polymer films prepared in a electric discharge are pinhole-free and have superior thermal stability, wide applications of them have been found such as dielectrics in the electronic devices and protective coating for metal and reactive surfaces [1].
In this paper plasma-polymerized p-xylene (PPPX) and PPHMDS films were prepared in a triode glow disharge. The formation characteristic, Fourier transform infrared spectroscopy (FTIR), photoluminescence (PL), relative dielectric constant ( r ) of PPPX films, and application to the encapsulation of OLEDs were investigated.

II. EXPERIMENTAL
A. Sample preparation PPPX and PPHMDS films were prepared on three kinds of substrate (Al deposited glass, ITO coated glass and quartz), and on one type of device (OLEDs), for measurements of thickness and capacitance, FTIR and PL spectra, and luminous characteristics of encapsulated OLEDs.
Prior to polymerization, p-xylene or hexamethyldisiloxane monomer was degassed by repeated freezing and pumping. Plasma po1ymerization was carried out by applying an rf voltage (100 kHz) between the grounded anode and the substrate electrode through a 24 µF capacitor [3,5].

B. Measurements
The thickness (d) of plasma-polymerized films was measured by means of multiple beam interferometry. Measurements of the capacitance (C) of the PPPX condenser which has two parallel electrodes were carried out at 1 kHz using an Ando TR-IB bridge. Relative dielectric constant ( r ) was calculated from C = r 0 S/d, where 0 and S are dielectric constant of vacuum and electrode area, respectively.
FTIR spectra of PPPX films were acquired using a Shimadzu FTIR-8600PC spectrophotometer with an RAS-8000 reflectance attachment. 100 scans were accumulated for each spectrum. The luminance and current of OLEDs, and PL spectra of PPPX films were measured using a homemade system [6]. All measurements were performed at room temperature in air.

III. RESULTS AND DISCUSSION
A. Formation characteristic Figure 2 shows the relationship between the thickness of PPPX films and polymerization time. The thickness changed almost linearly with time, and increased with increasing discharge current. The increase of the thickness of PPPX films is considered to be due to the increases of the decomposition rates of the p-xylene molecules. cm −1 ) are observed for monomer [7]. Large band in the region 2800-3000 cm −1 and very small band in the region 3000-3010 cm −1 appeared in PPPX film prepared at 8 mA. However, very small band in the region 2800-3000 cm −1 appeared in PPPX film prepared at 23 mA and band in the region 3000-3010 cm −1 disappeared. It has been reported that the benzene ring was destroyed by glow discharge polymerization and functional groups, such as -CH 2 -CH 2 -CH 2 -and -C=C=C-appeared [8]. The increase of dischrge current is accompanied by disappearance of the absorption peaks, suggesting that the PPPX film shows highly cross-linked structure. Figure 4 shows the changes in PL spectra of PPPX films with dischrge current. The peak at 470 nm decreased with increasing dischrge current, which might be due to the disappearance of benzene rings as suggested by the FTIR spectra. increased to 2.2 from 1.9 with increasing dischrge current, since the density of PPPX film increased with increasing cross-linked structure. r of PPPX films is smaller than that of conventional poly(p-xylen) films 2.6 [9].

B. FTIR and PL spectra and relative dielectric constant
C. Characteristics of OLEDs Figure 6 shows the relationship between luminance of the uncoated OLED and current density. The luminance decreased with elapsed time. The luminance and current density of OLEDs at constant voltage were measured and decreased with elapsed time. Therefore, the reduction of luminance could be considered to be due to the reduction of current density of OLED, which was affected by oxygen and moisture in air. Figure 7 shows the relationship between luminance of the uncoated and coated OLEDs with PPPX or PPHMDS film and current density after 20 h. The luminance of coated OLED with PPPX or PPHMDS film was higher than that of uncoated OLED.

IV. CONCLUSION
PPPX and PPHMDS films were prepared in a triode glow disharge. Formation characteristic, FTIR and PL spectra, r , and application to the encapsulation of OLEDs were investigated.
The thickness and r of PPPX films increased and the FTIR and PL spectra of PPPX films decreased with increasing discharge current. The luminance of encapsulated OLEDs with PPPX and PPHMDS films was higher than non-encapsulated OLED after the preparation of PPPX and PPHMDS films.