Modification on the Unoccupied Electronic Structure of Organic Semiconductor by Alkali Metal
- PDF / 695,630 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 53 Downloads / 190 Views
1029-F03-34
Modification on the Unoccupied Electronic Structure of Organic Semiconductor by Alkali Metal Huanjun Ding, Kiwan Park, and Yongli Gao Department of Physics and Astronomy, University of Rochester, Rochester, NY, 14627 ABSTRACT We have investigated the evolution of both the occupied and unoccupied states for alkali metal (Cs and Na) doped Copper-Phthalocyanine (CuPc) with photoemission and inverse photoemission spectroscopy. As the doping ratio increases, the lowest unoccupied molecular orbital (LUMO) of CuPc shifts downward, reaching the Fermi level. After the saturation, the LUMO intensity decreases monotonically, while a gap state grows in the valence spectra, which gives direct evidence for the origin of the doping-induced gap state in CuPc molecules. INTRODUCTION To successfully employ the organic molecules as the semiconducting thin films in the devices, such as organic light emitting diodes (OLEDs) [1], organic thin film transistors (OTFOs) [2], and organic photovoltaic cells (OPVs) [3], it is essential to be able to modify the electronic structures of these molecules in a controlled way. Doping has proven to be one of the most efficient techniques to adjust the electronic properties of the organic molecules [4-6]. Alkali metals, which can be easily used as electron donors, are good candidates for prototypical studies due to their simple electronic structures and low electron affinity. Alkali metal doping in commonly used organic semiconductors, such as copper phthalocyanine (CuPc) [7,8], tris (8hydroxyquinoline) aluminum (Alq) [9,10], and perylene-3,4,9,10-tetracarboxylic-dianhydride (PTCDA) [11,12], has been investigated by several groups. The experiments suggest that doping of alkali metals in organic semiconductors induces energy level shift due to electron donation from the outmost s orbital. The shift can be seen in two stages. At the first stage, the Fermi level shifts linearly within the band gap as a function of the logarithm of the doping ratio, whose slope is likely dominated by the interaction between the alkali metal and organics. At the second stage, the shift of the energy levels becomes saturated, followed by the appearance of the gap state. However, most of the investigations so far are focused on the evolution of the occupied states by means of photoemission. It is therefore important to investigate directly the evolution of the unoccupied states in doped organic semiconductor. In this paper, we report our investigations on the evolution of both the unoccupied and occupied states for Cs and Na doped CuPc with inverse photoemission spectroscopy (IPES), and ultraviolet and X-ray photoemission spectroscopy (UPS and XPS). As the doping ratio increases, the lowest unoccupied molecular orbital (LUMO) of CuPc shifts toward the Fermi level, which is quite consistent with all other occupied energy levels. Its intensity remains relatively the same until the shift becomes saturated. A gap state appears in the UPS spectra at the same time when a reduction can be observed for the CuPc LUMO, wh
Data Loading...
