Cu 2 S as ohmic back contact for CdTe solar cells Johannes Türck, Sebastian Siol, Thomas Mayer, Andreas Klein, Wolfram Jaegermann ⁎ Technische Universität Darmstadt, Department of Materials and Earth Sciences, Surface Science Division, Jovanka-Bontschits-Str. 2, D-64287 Darmstadt, Germany abstract article info Available online xxxx Keywords: Cadmium telluride Solar cell Back contact Copper sulfide Interface X-ray photoelectron spectroscopy Valence band offset We prepared a back contact for CdTe solar cells with Cu 2 S as primary contact. Cu 2 S was evaporated on CdCl 2 treated CdTe solar cells in superstrate configuration. The CdTe and CdS layers were deposited by Closed Space Sublimation. Direct interface studies with X-ray photoelectron spectroscopy have revealed a strongly reactive interface between CdTe and Cu 2 S. A valence band offset of 0.4–0.6 eV has been determined. The performance of solar cells with Cu 2 S back contacts was studied in comparison to cells with an Au contact that deposited onto a CdCl 2 -treated CdTe surface that was chemically etched using a nitric-phosphoric etch. The solar cells were analyzed by current–voltage curves and external quantum efficiency measurements. After several post deposition annealing steps, 13% efficiency was reached with the Cu 2 S back contact, which was significantly higher than the ones obtained for the NP-etched back contacts. © 2014 Elsevier B.V. All rights reserved. 1. Introduction One of the big challenges in the fabrication of a CdTe solar cell is the back contact to p-CdTe. A direct CdTe/metal contact leads to large Schottky barrier heights in the valence band [1]. The formation of a tunnel junction is also impeded, due to the difficulties in doping CdTe p-type. The common back contacts applied to CdTe solar cells are a thin Te layer, formed by etching as a primary contact, which has a lower valence band offset [2], followed by a secondary metal contact. Cu containing back contacts are also often used. Cu diffuses into the CdTe and leads to p-doping, which improves the formation of the back contact [3–5]. To improve the back contact of a CdTe solar cell a contact interface layer with high ionization energy should lead to a small valence band offset. With an ionization energy of 5.4 eV Cu 2 S [6] is a promising candidate for such an application. 2. Experimental details The interface properties have been studied by stepwise deposi- tion of Cu 2 S onto a CdCl 2 treated and cleaned CdTe layer. After every deposition step the sample was studied by X-ray photoelec- tron spectroscopy (XPS) to investigate the band alignment. The sub- sequent analysis and deposition were carried out at the DArmstadt Integrated SYstem for MATerial research (DAISY-MAT), which com- bines a Physical Electronics PHI 5700 multi-technique surface analy- sis system with several deposition chambers. Monochromatic Al Kα radiation (1486.6 eV) has been used for the XPS measurements. Binding energies are given with respect to the Fermi level, calibrated by Ag and Cu references. The solar cells have been prepared on commercial Fluorine doped Tin Oxide (Pilkington TEC C15M) substrates. The CdS and CdTe layers were deposited by Closed Space Sublimation at a substrate tempera- ture of 520 °C and a film thickness of 100 nm and 5 μm, respectively. The cells were wet chemically activated with CdCl 2 soluted in meth- anol and annealed for 25 min at 400 °C in a tube furnace in an air at- mosphere. After activation the cells were cleaned in deionized water for 5 min in an ultrasonic bath to remove the surface oxidation on the CdTe layer [7]. A CdTe solar cell with a primary Cu 2 S back contact of 60 nm layer thickness has been prepared. The Cu 2 S layer was thermally evaporat- ed from Cu 2 S powder (abcr GmbH and Co. KG 99.5% purity). The sub- strate was not additionally heated during the deposition of the Cu 2 S layer. A more detailed description of the deposition parameters is given in [8]. As a secondary back contact 100 nm gold was deposited via sputtering. As a reference a CdTe solar cell with nitric-phosphoric (NP) etched back contact with a secondary contact of 100 nm Au has been manufactured. The composite of the NP etch was 70% H 3 PO 4 , 0.8% HNO 3 , and 29.2% H 2 O by weight and the etch process is contin- ued until bubbles cover the surface. The gold contacts were sputtered at room temperature on the solar cell covering the whole area. By mechanical scraping single solar cells with an area of 0.16 cm 2 have been prepared. The current–voltage (IV) behavior was measured with a Keithley 2400 under AM 1.5 spectra (solar sim- ulator LOT-Oriel Typ 81150). Also the external quantum efficiency (EQE) for the solar cells has been determined. The solar cells have been annealed several times on a hotplate at temperatures between 150 °C and 225 °C, followed by repeated light IV and EQE measurements. Thin Solid Films xxx (2014) xxx–xxx ⁎ Corresponding author. E-mail address: jaegermann@surface.tu-darmstadt.de (W. Jaegermann). TSF-33887; No of Pages 4 http://dx.doi.org/10.1016/j.tsf.2014.11.017 0040-6090/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf Please cite this article as: J. Türck, et al., Cu 2 S as ohmic back contact for CdTe solar cells, Thin Solid Films (2014), http://dx.doi.org/10.1016/ j.tsf.2014.11.017