Built-in Potential of a Pentacene Pin Homojunction Studied by Ultraviolet Photoemission Spectroscopy
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1270-II09-49
Built-in potential of a pentacene p-i-n homojunction studied by ultraviolet photoemission spectroscopy Selina Olthof, Hans Kleemann, Bjön Lüssem, and Karl Leo Institut für Angewandte Photophysik, Technische Universität Dresden, George Bähr Strasse 1, 01062 Dresden, Germany ABSTRACT In this paper we investigate the energetic alignment in an organic p-i-n homojunction using ultraviolet photoelectron spectroscopy. The device is made of pentacene and we emploay the small molecules NDN1 for n-doping and NDP2 for p-doping the layers. The full p-i-n structure is deposited stepwise on a silver substrate to learn about the interface dipoles and band bending effects present in the device. From the change in work function between the p- and ndoped layers we gain knowledge of the built-in potential of this junction. INTRODUCTION In the last years, organic semiconductors have been investigated intensively due to the interest in their basic physical principles and their large application potential in organic electronics and optoelectronics, allowing devices such as organic light emitting diodes [1] or organic solar cells [2]. Despite the already successful commercial application of such devices, the general state of the art of the field of organic semiconductors is still rather immature. For instance, many basic devices known from inorganic semiconductors have not been realized from organic semiconductors so far. Until recently, this applied also to the most fundamental semiconductor device of all, the pnhomojunction. A few years ago, Harada et al. [3] reported the first stable and reproducible organic homojunction based on p- and n-doped layers of the organic semiconductor zinc phthalocyanine. The challenge in realizing stable molecularly doped layers for a homojunction lies in the selection of dopant molecules which can dope the same host material both p- and ntype. This requires an n-dopant with very high lying LUMO (lowest unoccupied molecular orbital) and a p-dopant with very high-lying HOMO (highest occupied molecular orbital) [4]. Furthermore, the devices have to be realized in the form of p-i-n-junctions [3]. The intrinsic interlayer is needed to avoid tunnelling due to the rather narrow space-charge layers in doped organic semiconductors [5]. More recently, a p-i-n-homojunction could also be realized in pentacene [6], showing an exceptionally large built-in potential. The experimental results reported in Refs. [5,6] have been explained by the Shockley theory for pn-junctions where modifications due to the hopping transport in organic materials were taken into account. In this work, we investigate a p-i-n homojunction by ultraviolet photoelectron spectroscopy (UPS) to obtain further information on the basics alignment in the device, including interface dipoles, band bending effects and the built-in potential. As matrix material, we choose pentacene (PEN) since it has excellent hole and electron transporting properties [8] and is a popular material for organic field-effect transistors. As mentioned before, wh
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