Highly enhanced electron injection in organic light-emitting diodes with an n-type semiconducting MnO 2 layer Hyunbok Lee a , Jeihyun Lee a , Pyungeun Jeon a , Kwangho Jeong a , Yeonjin Yi a,⇑ , Tae Gun Kim b,c , Jeong Won Kim b,c , Jin Woo Lee d a Institute of Physics and Applied Physics, Yonsei University, 50 Yonsei-ro, Seodaemoon-Gu, Seoul 120-749, Republic of Korea b Korea Research Institute of Standards and Science, 267 Gajeong-ro, Daejeon 305-340, Republic of Korea c University of Science and Technology, 217 Gajeong-ro, Daejeon 305-350, Republic of Korea d LG Innotek, 379, Gasoo-Dong, Osan-City, Gyeonggi-Do 447-705, Republic of Korea article info Article history: Received 15 October 2011 Received in revised form 14 January 2012 Accepted 15 January 2012 Available online 11 February 2012 Keywords: OLED Electron injection layer MnO 2 UPS XPS Electronic structure abstract Highly enhanced electron injection is demonstrated with a thin manganese dioxide (MnO 2 ) electron injection layer (EIL) in Alq 3 -based organic light-emitting diodes. Insertion of the MnO 2 EIL between the Al cathode and Alq 3 results in highly improved device characteris- tics. In situ photoelectron spectroscopy shows remarkable reduction of the electron injec- tion barrier without significant chemical reactions between Alq 3 and MnO 2 , which could induce Alq 3 destruction. The reduction of the electron injection barrier is due to the n-type doping effect, and the lack of strong interfacial reaction is advantageous with regards to more efficient electron injection than a conventional LiF EIL. These properties render the MnO 2 , a potential EIL. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Organic light-emitting diodes (OLEDs) have been remarkably attractive for the last two decades due to their unique advantages, such as simple fabrication processes, wide viewing angles, light weight, and mechanical flexibil- ity [1]. However, high charge-injection barrier from the electrode to organic semiconducting materials is one of the inveterate obstacles to design highly efficient OLEDs. Lowering the electron injection barrier in OLEDs especially is a prerequisite to balancing electrons with holes within an emission layer to maximize light emission because most organic semiconducting materials have lower elec- tron mobility than that of hole. It has been reported that the insertion of an appropriate electron injection layer (EIL) reduces the electron injection barrier efficiently. Alkali metals and alkali metal halides have been used as a conventional EIL through the n-type doping effect on an electron transport layer (ETL), such as Alq 3 [2,3]. How- ever, alkali metals are not tractable as an evaporation source and are destructively reactive to the organic mole- cules. On the other hand, alkali metal halides often show variable performance depending upon the cathode choice [4]. Recently, several alternative EILs, such as alkali metal carbonates, nitride and quinolates [5–11] have been stud- ied and show good electron injection performances. Metal oxides such as ZnO, TiO 2 and ZrO 2 are also candidates for alternative EILs [12–14]. Recently, Luo et al. reported highly efficient OLEDs with insulating manganese monoxide (MnO) EIL [15]. However, its detailed working mechanism is not yet understood. Furthermore, the semi- conducting phase of metal oxide would be more suitable for device performance than the insulating phase. In this paper, we propose that semiconducting manga- nese dioxide (MnO 2 ) could be a potential candidate as a 1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2012.01.012 ⇑ Corresponding author. E-mail address: yeonjin@yonsei.ac.kr (Y. Yi). Organic Electronics 13 (2012) 820–825 Contents lists available at SciVerse ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel