Origin of charge generation efficiency of metal oxide p-dopants in organic semiconductors Jae-Hyun Lee, Hyun-Mi Kim, Ki-Bum Kim ⇑ , Jang-Joo Kim ⇑ Department of Materials Science and Engineering, Seoul National University, Seoul 151-742, Republic of Korea article info Article history: Received 25 January 2011 Received in revised form 20 February 2011 Accepted 5 March 2011 Available online 21 March 2011 Keywords: Organic semiconductor Electrical doping Metal oxide Nanocluster Transmission electron microscopy abstract We report that the low charge generation efficiency of metal oxide dopants in organic semi- conductor hosts is originated from the formation of dopant nanoclusters. Dopant nanoclus- ters of rhenium oxide (ReO 3 ) and molybdenum oxide (MoO 3 ) exhibit log-normal size distributions with an average radius of 0.7–1.4 nm and standard deviation of 1.5–2.6 nm, depending on the dopant and their concentration in 1,4-bis[N-(1-naphthyl)-N 0 -phenylami- no]-4,4 0 diamine (NPB). The number ratio of the nanoclusters to dopant molecules (disper- sion efficiency) was measured to be 0.62–1.3% for the ReO 3 -doped NPB. The low dispersion efficiency is close to the charge generation efficiency of 0.7–1.1%, which is defined as the ratio of generated carriers to dopant molecules, indicating that charge generation efficiency in the dopants is predominantly controlled by the dispersion of dopants in organic semiconductors. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction Electrical doping is one of the most important and fun- damental technologies in organic semiconductor devices such as organic light emitting diodes, organic photovolta- ics and organic field effect transistors for efficient charge injection and charge transport. Various metal oxides such as molybdenum oxide (MoO 3 ), tungsten oxide, vanadium oxide and rhenium oxide (ReO 3 ) along with organic dopants including tetrafluorotetracyanoquinodimethane (F 4 -TCNQ) and molybdenum tris-[1,2-bis(trifluoromethyl) ethane-1,2-dithiolene] have been widely investigated as p-type dopants [1–4], while alkali metals and alkali metal carbonates are generally employed as n-type dopants [3,5]. In contrast to inorganic semiconductors, however, the doping concentrations in organic semiconductors for organic devices are in the range of approximately a few to tens of percent because of their low charge generation efficiency, which is defined as the ratio of carrier density to the number of dopant atoms or molecules [5–8]. We have shown that a diverse range of charge generation efficiencies are obtained for different combinations of do- pants and organic semiconductor hosts [7,9]. However, the charge generation efficiencies of metal oxide dopants re- main very low, in the range of 0.3–4% [7]. The organic dopant F 4 -TCNQ also exhibits a low charge generation efficiency [10]. The low charge generation efficiency not only seriously limits the application of doping technology in organic semi- conductors because the diffusion of dopants can destabilize the device [11,12], but also hinders further progress toward more advanced devices such as doping controlled diodes, transistors and integrated circuits. It is, therefore, important to investigate the origin of the low charge generation effi- ciency of metal oxide dopants in organic hosts to aid in improving the charge generation efficiency. The origin of low charge generation efficiency has not yet been suitably clarified to our best knowledge [13,14]. Here we report the formation of dopant nanoclusters as an origin of low charge generation efficiency in organic semiconductors using MoO 3 and ReO 3 as the dopants. Trans- mission electron microscopy studies revealed a very good consistency between the charge generation efficiencies and the dispersion efficiencies defined as the number ratio of the nanoclusters to dopant molecules, demonstrating 1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2011.03.008 ⇑ Corresponding authors. Tel.: +82 2 880 7893. E-mail addresses: kibum@snu.ac.kr (K.-B. Kim), jjkim@snu.ac.kr (J.-J. Kim). Organic Electronics 12 (2011) 950–954 Contents lists available at ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel