Enhancement of electron injection in organic light-emitting devices using an Ag/LiF cathode

X. J. Wang, J. M. Zhao, Y. C. Zhou, X. Z. Wang, S. T. Zhang, Y. Q. Zhan, Z. Xu, H. J. Ding, G. Y. Zhong, H. Z. Shi, Z. H. Xiong, Y. Liu (+5 others)
2004 Journal of Applied Physics  
A LiF-buffered silver cathode has been used in organic light-emitting devices ͑OLEDs͒ with structure indium-tin-oxide/N,NЈ-bis-͑1-naphthl͒-diphenyl-1,1Ј-biphenyl-4,4Ј-diamine (50 nm)/Alq 3 ͑100 nm͒/cathode. The efficiency of electron injection from the cathode is strongly dependent on the thickness of the LiF buffer layer. While a LiF layer thinner than 1.0 nm leads to higher turn-on voltage and decreased electroluminescent ͑EL͒ efficiency, a LiF layer of 3.0 nm significantly enhances the
more » ... enhances the electron injection and results in lower turn-on voltage and increased EL efficiency. A brightness of 16 000 cd/m 2 and EL efficiency of 4.8 cd/A can be achieved with an Ag/LiF cathode. This dependence of electron injection on the LiF thickness is quite different from that reported for OLEDs with a Al/LiF cathode, but can be well understood using the tunneling model. To achieve excellent performance in organic lightemitting devices ͑OLEDs͒, it is desirable to use a low work function metal ͑Mg, Li, Ca, etc͒ or metal alloy ͑e.g., Mg:Ag͒ as the cathode. This gives a small effective injection barrier height at the organic/metal electrode interface. However, these metals or alloys are not well fit for the operation environment because of their relatively high chemical reactivity in air. If less active metals such as aluminum ͑Al͒, and silver ͑Ag͒ are used as the cathodes, however, the devices have given poor light output and low electroluminescent ͑EL͒ efficiency. In 1997, by inserting a thin buffer layer of LiF between Alq 3 and Al, a great enhancement in EL performance of OLEDs with these stable metals was first reported. 1 Thereafter, many insulating materials such as MgO, Al 2 O 3 , MgF 2 , and NaSt were also found to be either suitable or effective buffers in small molecule-based OLEDs, 1-4 where the optimal thickness of the insulating layer was usually less than 1.0 nm. In the case of LiF as a buffer layer, Stobel et al. 5 reported that among Mg, Ag, Ca, Li, and Al, only aluminum can be used as the effective cathode to improve the performance of OLEDs. A very poor electron injection of a device having 0.3 nm LiF between Alq and Ag cathode was also demonstrated by Hung et al. 6 Recently, H. Heil et al. 7 fabricated a device with a layer sequence of Ag/LiF (0.2 nm)/Alq 3 ͑200 nm͒/LiF ͑0.2 nm͒/Ag, which did not show any improved electron injection. Based on these ex-perimental results in which the adopted thickness of LiF in the Ag/LiF cathode is mostly less than 1.0 nm, silver, a noble metal, has been so far judged as a poor cathode material to combine with a LiF buffer layer. We nevertheless report the excellent EL performance of the devices containing a Ag/LiF cathode structure. The thickness of the LiF layer we adopted is varied in a range of 0-5.0 nm. The presence of an ultrathin LiF layer of 0.6 nm between Alq and Ag will lead to a poorer EL performance than that of the device without LiF. With the thickness of the buffer layer being further increased, however, the current injection is gradually improved. The device with a 3.0-nmthick LiF layer exhibits excellent EL characteristic. A significantly reduced turn-on voltage, a brightness of 16 000 cd/m 2 , and an EL efficiency of 4.8 cd/A, comparable to the results of the devices with an Al/LiF cathode, 8 are achieved. The dependence of device performance on the LiF thickness is tentatively ascribed to the effect of electron tunneling through LiF layers. Indium-tin-oxide ͑ITO͒-coated glass with a sheet resistance of 20 ⍀ per square was used for the device fabrication. The routine cleaning procedure included sonication in detergent, rinsing in de-ionized water, and a final UV ozone treatment to remove remaining organic materials. During the fabrication process, a quartz-oscillator thickness monitor was used to detect the deposition rate. A multilayer structure of N,NЈ-bis-͑1-naphthl͒-diphenyl-1,1Ј-biphenyl-4,4Ј-diamine ͑NPB͒ (50 nm)/Alq 3 ͑100 nm͒/LiF ͑0-5 nm͒/Ag ͑110 nm͒ was sequentially deposited on the cleaned ITO substrate. The use of relatively thick Alq 3 layer in the device was for sure a͒
doi:10.1063/1.1655676 fatcat:kxvwpsfpfnb3zkxh2dgpgufuii