Doping Effect on Chloroindium Phthalocyanine (ClInPc)/C60 Solar Cells
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Doping Effect on Chloroindium Phthalocyanine (ClInPc)/C60 Solar Cells Weining Wang1 and Neal Armstrong2 1 Physics Department, Seton Hall University, 400 South Orange Ave. South Orange, NJ 07079, U. S. A. 2 Department of Chemistry and Biochemistry, University of Arizona, 1306 East University Blvd, Tucson, AZ 85721, U.S.A. ABSTRACT For inorganic semiconductor solar cells, controlled doping is important because it can cause Fermi level shift of the inorganic semiconductor and achieve ohmic contact at the metalsemiconductor interface. In this paper we show that doping can also be used to shift Fermi level in organic semiconductors and cause changes in solar cell performance. We have made chloroindium phthalocyanine (ClInPc)/C60 heterojunction solar cells, where tetrafluoroteracyano-quinodimethane (F4-TCNQ) is used to dope ClInPc layer. Ultraviolet photoemission spectroscopy (UPS) is used to investigate the ITO/ClInPc interfaces. The result shows that doping causes a Fermi level shift at the ITO/ClInPc interface as it does for inorganic semiconductors. As the doping increases, dark saturation current J0 of the solar cell increases, while open-circuit voltage Voc, short-circuit current Jsc and fill factor decreases. As a result, the efficiency of the solar cell decreases as doping increases. More UPS studies on ClInPc (doped with F4TCNQ)/C60 junction are needed to correlate the energy band diagram of the whole solar cell structure with the J-V characteristics. INTRODUCTION To improve the efficiency of organic photovoltaics (OPVs), it is critical to understand and control the Transparent Conducting Oxide (TCO)/ organic semiconductor interface. However, unlike metal-semiconductor interface, interface between contact and organic semiconductor is not well understood. Doping is a well-known method to shift Fermi –level and achieve ohmic contact at the metal-semiconductor interface, improving the solar cell performance [1-2]. While several studies have shown that doping can cause Fermi-level shift at the interface between contact and organic semiconductor as well [3-5], few of them have shown how the performance of those organic solar cell depend on different doping levels. In this paper, we report our investigation via ultraviolet photoemission spectroscopy (UPS) of chloroindium phthalocyanine (ClInPc) doped with the molecular acceptor tetrafluorotetracyanoquinodimethane (F4-TCNQ) and the performance of corresponding ClInPc (doped with F4-TCNQ)/C60 solar cells at different doping levels (0.3% and 3%） EXPERIMENTAL METHODS Commercial ITO was obtained from Colorado Concept Coating, LLC, with a sheet resistance of ca. 15 Ω/ □. ITO substrates were cleaned with a micro-fiber cloth using 10% Triton-X100, followed by successive sonications in 10% Triton-X100 for 15 mins, nanopure water for 5
minutes, and pure ethanol for 15 minutes. Prior to the deposition of organic thin films, substrates were dried under a stream of nitrogen and then etched via oxygen plasma cleaning (Harrick PDC-32G) for 15 minutes. Chloroindium phthalocyanine (C
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