Tailoring the Work Function of Chalcopyrite Thin Films with Self-Assembled Monolayers of Thiols
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1012-Y13-08
Tailoring the Work Function of Chalcopyrite Thin Films with Self-Assembled Monolayers of Thiols Sebastian Lehmann1, David Fuertes MarrÛn1, Marcus B‰r2, Iver Lauermann1, Harry Mˆnig1, and Martha Ch. Lux-Steiner1,3 1 Solar Energy Research, Hahn-Meitner Institut Berlin, Glienicker Strasse 100, Berlin, 14109, Germany 2 Department of Chemistry, University of Nevada, 4505 Maryland Parkway, Box 454003, Las Vegas, NV, 89154-4003 3 Freie Universit‰t Berlin, Kaiserswerther Str. 16/18, Berlin, 14195, Germany ABSTRACT Self-assembled monolayers of fluorinated thiols have been used as a means of surface conditioning to modify the work function of polycrystalline, wide-gap, chalcopyrite thin films. The molecular dipole, characteristic of such polar molecules, could be transferred to the surface of the semiconductor. Self-arrangement and orientation of the molecules upon adsorption ensured a net dipole contribution that was observed by means of ultra-violet photoemission spectroscopy. Such an approach offers a simple way for interface engineering, with a potential impact on the design of compound-specific band alignments of chalcopyrite-based devices. Molecular mechanics calculations of the expected molecular geometries complemented this work.
INTRODUCTION The control of the electrical properties at interfaces building up the heterojunctions is most prominent among the critical issues for further progress in device performance of chalcopyrite-based devices. High-efficiency devices based on low-gap absorbers comply with the required design rules of heterojunctions [1]. However, the design of the device is no longer optimal for wide-gap counterparts, for which a reduction of the energy barrier for carrier recombination appears at the absorber/buffer interface, which in turn limits the output voltage and fill factor of related solar cells. Electronic losses associated with non-ideal band alignments have been found experimentally in devices based on wide gap absorbers [1,2], that make them interface- rather than bulk-limited. The difficulty encountered so far in finding an optimal junction partner for wide-gap absorbers is unfortunate. The obvious consequence of lacking an optimal junction partner is that devices based on wide-gap absorbers do not fully profit from their potential high open-circuit voltages. In this scenario, approaches oriented toward surface modification and interface engineering are required, aiming at the modification of the electronics of the absorber at the near-surface region [3]. The use of molecules to shape the electronic properties of surfaces and interfaces of solid-state-based devices appears advantageous [4,5]. Firstly, deposition methods of molecular layers are simple and can be accomplished at room temperature and pressure by, e.g., immersion of the samples into solutions and subsequent rinsing [6]. Secondly, the manifold nature of molecular properties to be transferred to the solid-
state surface ensures in practice an unlimited and flexible way of manipulating and engineering surface a
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