CuGaSe 2 -Based Solar Cells with High Open Circuit Voltage
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1012-Y12-38
CuGaSe2-Based Solar Cells with High Open Circuit Voltage Raquel Caballero1,2, Susanne Siebentritt2,3, Christian A. Kaufmann1, Carola Kelch2, Daniel Schweigert1, Thomas Unold1, Marin Rusu2, Hans-Werner Schock1, and Martha Ch Lux-Steiner2 1 Technology, Hahn-Meitner-Institut Berlin GmbH, Glienicker Strasse 100, Berlin, 14109, Germany 2 Heterogeneous Material Systems, Hahn-Meitner-Institut Berlin GmbH, Glienicker Strasse 100, Berlin, 14109, Germany 3 FacultÈ de Science, Technologie et Communication, UniversitÈ de Luxembourg, Campus Limpertsberg, Avenue de la FaÔencerie 162a, Luxembourg, L-1511, Luxembourg
ABSTRACT The objective of this work is to increase the open circuit voltage of CuGaSe2(CGS)-based solar cells without decreasing their efficiency. For that, the interface between the p-type CGS absorber and the n-type CdS/ZnO window layer is compared using three different recipes for the growth of the buffer layer. Results show the importance of the adaptation of the CdS buffer layer to the CuGaSe2 absorber film. A maximum open circuit voltage of 922 mV is achieved for the devices when using 60 ∞C as the chemical bath temperature and lower thiourea and ammonia concentrations. Drive-level capacitance profiling, external quantum efficiency and temperature dependent current-voltage measurements reveal a better quality of the CdS/CuGaSe2 interface for this buffer layer deposition condition. The lower doping level, reducing the tunnelling component, is pointed out as responsible of the higher Voc. INTRODUCTION CuGaSe2 (CGS) with a band gap of 1.7 eV is, in principal, a good candidate to be used as top cell in a mechanically stacked tandem structure [1]. The highest efficiency reported for a polycrystalline device is 9.5 % [2] and 9.7 % for a single crystal device [3]. However, the most critical disadvantage of CGS solar cells is the low open circuit voltage Voc compared to the band gap. This could be overcome if conduction type inversion at the surface of the absorber is obtained [4]. But this seems to be difficult to achieve with high gap chalcopyrites [5]. Therefore, a careful adaptation of the CdS growth onto CGS is necessary. In this work, different recipes for the CdS layer deposition on Ga-rich CuGaSe2 are used in order to enhance the ratio Eg/qVoc without reducing the efficiency of the solar cell. The absorber has been prepared by the three-stage co-evaporation process [6]. Drive-level capacitance profiling (DLCP), external quantum efficiency (EQE) and temperature dependent current-voltage (IV-T) measurements of different solar cells are carried out to study the influence of the buffer layer on the device parameters.
EXPERIMENTAL DETAILS CGS thin films are grown on Mo coated soda lime glass (SLG) using co-evaporation in a three-stage process, as described elsewhere [6]. Laser Light Scattering (LLS) together with a pyrometer are used as in-situ process controls [6, 7]. The nominal substrate temperature is measured by a thermocouple behind the substrate. Figure 1(a) illustrates the three-stage process t
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