The effect of molybdenum trioxide inter-layer between indium tin oxide (ITO) and organic semiconductor on the energy lev
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1212-S08-05
The effect of molybdenum trioxide inter-layer between indium tin oxide (ITO) and organic semiconductor on the energy level alignment Irfan1, Huanjun Ding1, Yongli Gao1, Do Young Kim2, Jegadesan Subbiah2 and Franky So2 1
Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627.
2
Department of Material Science and Engineering, University of Florida, Gainesville, FL 326116400 ABSTRACT We investigated 0 to 300 Å thick stepped molybdenum trioxide (MoO3) inter-layer between insitu oxygen plasma treated conducting indium tin oxide (ITO) and layer by layer evaporated chloro-aluminum pthalocyanine (AlPc-Cl) up to 228 Å, with ultra-violet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES). The MoO3 inter-layers were observed to increase the surface workfunction (WF). The WF shift was observed to saturate at 20 Å of MoO3 coverage. The increased surface WF causes hole accumulation and band bending in the subsequently deposited AlPc-Cl. A possible explanation of reduction in series resistance by the insertion of the MoO3 insulating layer is discussed based on these observations and energy level alignment. INTRODUCTION The potential application of organic semiconductors have been established including organic photovoltaic (OPV)[1] and organic light emitting diode (OLED)[2]. Lower production costs light-weight and flexible solar panels are added advantages of organic photovoltaic cells over crystalline inorganic photovoltaic cells. A lot of efforts have been made in order to improve the charge transport inside organic semiconductors and collection at the electrodes. One attempt was made by Tokito et al.[3] to introduce transition metal oxides with high work function (WF) as an inter-layer between the indium-tin-oxide (ITO) and the hole injection layer. More recently, it was reported that improved hole injection and stable device performance can be achieved by the insertion of MoO3 inter-layer in OLED[4]. In OPV researches, metal oxide as an inter-layer was successfully demonstrated by Shrotriya et al.[5]. Our very recent results show 20% improvement in the fill factor and 35% reduction in the series resistance for MoO3 inter-layer between ITO anode and chloro-aluminum pthalocyanine (AlPc-Cl)[6]. The observations raised a question on how an insulating layer may reduce the resistivity and what is the mechanism of the improvement. Most recently, M. Kroger et al. reported a possible mechanism for hole injection improvement originating form electron extraction from the highest occupied molecular orbital (HOMO) of organic semiconductor to the anode through conduction band of MoO3[7]. In this paper we report thickness dependence of MoO3 interlayer between AlPc-Cl and ITO anode, on the electronic energy level alignment using ultraviolet photoemission
spectroscopy (UPS) and inverse photoemission spectroscopy (IPES). We monitored the changes in the valence electronic structure, WF, and ionization potential of the surface as AlPc-Cl was deposited on MoO3/ITO. The interfa
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