UV Photoemission Study of Interfaces Related to Organic EL Devices
- PDF / 368,208 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 51 Downloads / 191 Views
.AI0
'N IN .z Y..0 ,01 OCH3 Alq3
T
A~q3"•TPD
S-s S~
H3C
CN NC•'NC) •CN
S'S
TCNQ
TTN
Fig. 1 Organic materials used in this study. by UPS. At first we measure the UPS spectrum of the clean metal substrate (Fig. 2(a)). Electrons below the Fermi level are excited by UV light, and emitted into vacuum. The kinetic energy distribution of photoelectrons (UPS spectrum) reflects the density of the valence states of the metal. The high energy cutoff corresponds to the emission from the Fermi level, and the low energy cutoff gives the energy of the vacuum level. From these data, we can determine the work function of the metal 4ým. Next we deposit the organic layer, and measure the UPS spectrum again (Fig. 2(b)). The fastest electron now comes from the highest occupied molecular orbital (HOMO) of the organic molecule. By comparing with the UPS spectrum of the metal, we can deduce the energy separation between the Fermi level of the metal and the HOMO (EVF). Also we can observe the change of the energy of the vacuum level (A), and also deduce the energy between the vacuum level and the Fermi level of the metal (Cvac F). For other interfaces (e.g. organic / organic), we can similarly deduce the energy alignment at the interface. EXPERIMENTAL The sample materials of TPD and Alq 3 were supplied from Toshiba R&D Center, and was purified by vacuum sublimation. TTN was supplied by Prof. T. Nogami of University of Electrocommunications, and was used without further purification. TCNQ was commercially supplied from Tokyo Chemical Industry, and was purified by sublimation.
hvi 1 Em F 7-
Metal Vacuum (a)
hv
~vac F ,,
'/ /HOMO
/*
Metal Organic Vacuum (b)
Fig. 2 Principle of the UPS study of an organic/metal interface. (a) Photoemission from the metal. (b) Photoemission from the organic layer deposited on the metal substrate.
720
The ITO substrate was supplied from Balzers Company (ITO thickness 45 nm, resistance 40 - 50 9 The ITO substrate was ultrasonically washed with acetone and ipropanol, and also by UV irradiation using a lamp (SEN Engineering SUV-40GS) for 5 to 10 minutes under air leading to the cleaning by ozone formation. The work function of ITO cleaned only by organic solvents was about 4.43 eV, while the value after UV cleaning was about 4.75 V. This change was accompanied by the removal of the carbon contamination as observed by XPS. The UPS spectra were measured on a retarding-field type electron spectrometer with a rare gas discharge lamp emitting Hel light of 21.2 eV. The energy analysis of photoelectrons was performed by ac modulation technique [ 16] with ac modulation of 0.2 eV peak to peak. The resolution was about 0.2 eV as judged from the spectrum of Fermi edge of Au. The base pressure of the vacuum chamber was I x 10-8 Torr. Since the UPS signal of the Fermi edge region of ITO was very weak, the location of the Fermi level was determined by measuring the Fermi edge of Au which was electrically connected with the ITO substrate. RESULTS AND DISCUSSION In Fig. 3 we show the UPS spectra of TPD with i
Data Loading...
