Charge motion and trapping in molecularly doped hole transporters
- PDF / 493,990 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 37 Downloads / 191 Views
Charge motion and trapping in molecularly doped hole transporters H.H. Fong, K.C. Lun, and S.K. So Department of Physics, Hong Kong Baptist University, Kowloon Tong, Hong Kong, China. ABSTRACT The charge transporting properties of N, N’-dipheny l–N, N’-bis(3-methylphenyl)(1,1’-biphenyl)-4,4’-diamine (TPD), TPD doped with 5,6,11,12- tetraphenylnaphthacene (rubrene), and TPD doped with 4-(dicyanomethylene)-2-methyl-6-(pdimethylaminostyryle) 4H-pyran (DCM1) were examined by time-of-flight (TOF) technique between 180-300K. The dependence of the mobility on electric field and temperature for undoped and doped TPD was investigated. Reductions in hole mobility shows that both dopants act as hole traps in TPD. Computational results of TPD also account for the effective hole conduction in pristine TPD film. INTRODUCTION Organic light-emitting diode (OLED) has become highly promising in display industry [1]. It was fabricated from multi-layers of organic thin films sandwiched between two electrodes. Both hole- and electron- conducting organic materials are required. The electron mobilities of many electron-transporting materials (ETMs) are low. Tris-(8-hydroxyquinoline) aluminum (Alq) is one of the classical ETMs and its electron mobility is in the order of 10-5 to 10-6 cm2V-1s-1 under an applied electric field of 0.5 MV cm-1 at room temperature [2]. On the order hand, many important holetransporting materials (HTMs) such as TPD and N, N’-diphenyl–N, N’-(2-naphthyl)(1,1’-phenyl)-4,4’-diamine (NPB) have higher hole mobility. Typical values are in the range of 10-3 cm-2V-1s-1 [3,4]. Generally, hole transporters are better conductors than electron transporters. One consequence of the large difference in hole and electron mobility is imbalance carrier populations in an OLED [5]. Selective doping of HTM was reported to help significantly improve the device lifetime. This report aims at clarifying the effect of doping by rubrene (RB) and DCM1 on the hole mobility of a classic HTM -- TPD (Fig. 1). Mechanisms and microscopic descriptions of carrier transport will be discussed. EXPERIMENTAL DETAILS The TOF samples had a structure of ITO/undoped or doped TPD/Al. ITO on glass (2.5×2.5cm2, 72Ω/sq) was used as the substrate. The substrate was cleaned by standard procedures [6]. Organic thin films were deposited inside a high vacuum evaporator. The coating rate for TPD was 19Å/s while the coating rate for the dopants [RB or DCM1] was about 1Å/s. The doping concentration was fixed at 5% for all RB- or DCM1-doped TPD films. A semi-transparent aluminum contact of thickness of 15nm was coated on the top of the organic film. The sample area was 0.36 cm2. After coating, the sample was P8.1.1 Downloaded from https://www.cambridge.org/core. Iowa State University Library, on 29 Jan 2019 at 23:40:21, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/PROC-725-P8.1
NC
N
N
CH3
H3C
CN
O N
CH3
TPD
Rubrene (RB)
CH3
CH3
DCM1
Figure 1. Molecular structures of TPD, rubrene (R
Data Loading...
