Improving the efficiency of organic light emitting devices by using co-host electron transport layer

doi:10.1016/j.tsf.2005.09.135

Thin Solid Films

Volume 509, Issues 1–2, 19 June 2006, Pages 193-196

https://doi.org/10.1016/j.tsf.2005.09.135 Get rights and content

Abstract

By engineering a new cohosting system of tris(8-hydroxyquinoline) and 4,7-diphenyl-1,10-phenanthroline in the electron transport layer, the current efficiency of the organic light emitting diode is improved by 34% to 4.3 cd/A as compared to the device with a single host of Alq₃ as the electron transport layer. The maximum luminance is over 16,000 cd/m² at the bias of 22 V and the current of 475 mA/cm², which is ∼ 73% higher than the single host Alq₃ device without optimizing the layer thickness. The reasons for the improvement will be investigated. The results strongly indicate that the knowledge of bulk conductivity engineering of organic n-type transporters shows practical significance in OLED applications.

Introduction

Tris(8-hydroxyquinoline) aluminum (Alq₃) has been commonly used as an organic light-emitting host as well as electron transport layer (ETL) in organic light-emitting diodes (OLEDs). However, due to its lower electron mobility of 10^− 6 cm² V^− 1 s^− 1 [1], [2], [3], Alq₃ is one of the key sources responsible for the undesirable predominant voltage drop in Alq₃-based devices [4]. Furthermore, the operation lifetime of Alq₃ based devices is reduced due to the unbalanced charge accumulation originated from an excessive hole injection into the Alq₃ layer [5]. Extra amount of holes also generates non-emissive cationic species in the Alq₃ and result in the formation of dark spots [6]. Hence, enhancement of electron conduction in the ETL of OLEDs is a critical approach to both lowering the driving voltage and improving the operation lifetime. Recently, although some electron transporting materials such as phenanthroline and oxadiazole compounds were suggested for replacing the Alq₃ as the ETL due to their improved drift electron mobility [7], [8], [9], [10], the device reliability including film stability and thermal endurance have not yet fulfilled the requirement of commercial display application [10], [11]. The highly reactive metal (e.g. Li) doping reported by Kido et al. [12] is an alternative means to enhance the device electron conduction, although photoluminescence quenching occurs if the doped region exceeds 30 nm.

In our study, we establish an organic based co-host composite to replace the traditional single host ETL. The results conclude that our approach improves the electron conductivity on the ETL without affecting the film stability, resulting in an improved current efficiency of about 34% through employing a ET composite of phenanthroline: hydroxyquinoline. It is important to note that 4,7-diphenyl-1,10-phenanthroline (BPhen) is chosen for mixing with Alq₃ to form the co-host ETL since it possesses a considerable high electron mobility (∼5 × 10^− 4 cm² V^− 1 s^− 1) among the electron-dominant materials [10]. This report focuses mainly on the device aspect by applying the knowledge of bulk conduction presented previously. The composition of the BPhen:Alq₃ co-host electron transporter based devices will be investigated.

Section snippets

Experimental

The indium-tin-oxide (ITO) substrates with a size of 25 × 25 mm 2 and resistivity of 80 Ω/square were cleaned prior to loading into the evaporation chamber through scrubbing by detergent and soaking into de-ionized (DI) water for 10min in each step. The evaporation chamber is operated at ∼10^− 7 Torr. The substrates were then immersed into ultrasonic bath of DI water, ethanol and acetone for 20 min for each solvent. The solvent cleaned ITOs were further treated in UV ozone for 20 min.

The organic

Results and discussion

Fig. 2 shows the luminous efficiency of undoped devices with various compositions of ET hosts incorporating BPhen and Alq₃. In the typical device using Alq₃ as the ETL (device A), the efficiency is ∼ 3 cd/A at 20 mA/cm², similar to the reported values [13]. Replacing ETL by BPhen (device B), the efficiency is significantly enhanced by ∼ 34% to 4.3 cd/A. The time-of-flight results indicate that under an applied electric field of ∼ 1 MV/cm, the electron mobility of Alq₃ has a value of ∼ 10^− 6 cm² V^− 1 s

Conclusion

A new method of co-hosting hydroxyquinoline with phenanthroline derivative for improving the electron mobility of ETL has been introduced in this paper. Thanks to (a) the superior electron mobility of BPhen, (b) free electron hopping in ETL due to similar LUMO value of both Alq₃ and BPhen, and (c) high HUMO value of BPhen for reducing the hole injection into the ETL, the current efficiency of the cohost device is increased by 34% to 4.3 cd/A as compared to the conventional structure with single

Acknowledgement

The authors would like to acknowledge the support of the grant from the Research Grant Council of the Hong Kong Special Administrative Region, China, UDF grant and seed funding of The University of Hong Kong. P.C. Chui would also like to acknowledge the support of the university's Small Project Funding.

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Improving the efficiency of organic light emitting devices by using co-host electron transport layer

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusion

Acknowledgement

Appl. Phys. Lett.

Appl. Phys. Lett.

Jpn. J. Appl. Phys.

Appl. Phys. Lett.

Science

J. Appl. Phys.

Appl. Phys. Lett.