Current transport in ZnO/ZnS/Cu(In,Ga)(S,Se) 2 solar cell M. Rusu a , W. Eisele a , R. Wu ¨rz a , A. Ennaoui a, * , M.Ch. Lux-Steiner a , T.P. Niesen b , F. Karg b a Hahn-Meitner Institut, Bereich Solarenergieforschung SE2, Glienicker Strasse 100, Berlin D-14109, Germany b Shell Solar GmbH, D-81739 Mu ¨nchen, Germany Abstract Temperature-dependent current-voltage measurements are used to determine the dominant recombination mechanism in thin-film heterojunction solar cells based on Cu(In,Ga)(S,Se) 2 absorbers with chemical bath deposited ZnS buffer layer. The measurements are carried out in the dark and under illumination in the temperature range 200 – 320 K. The activation energy of the recombination under illumination follows the absorber band gap energy E g ¼ 1:07 eV of bulk Cu(In,Ga)(S,Se) 2 . The thermal dependence of the diode ideality factor is described by classical Shockley – Read – Hall (SRH) recombination via an exponential distribution of trap states in the bulk of the absorber. In the dark, the current flow is dominated by tunnelling enhanced bulk recombination with a tunnelling energy E 00 ¼ 18 meV. Two activation energies higher than E g ; namely 1.21 and 1.40 eV, have been found. These results may be explained by dominant recombination in a region close to the surface of the Cu(In,Ga)(S,Se) 2 absorber with an enlarged band gap. Thus, the electronic loss in the ZnO/Zn(S,OH)/Cu(In,Ga)(S,Se) 2 solar cell takes place mainly in the absorber and is determined by tunnelling enhanced bulk recombination with a tunnelling energy E 00 influenced by illumination. q 2003 Elsevier Ltd. All rights reserved. 1. Introduction Thin-film ternary Cu-chalcopyrite semiconductors CuInSe 2 , CuGaSe 2 , CuInS 2 and their alloys are among the leading absorber materials for low-cost and most efficient terrestrial solar cells. Photovoltaic conversion efficiencies up to 18.9% on a laboratory scale and 12% for large area monolithically integrated submodules [1] have been achieved by using Cu(In,Ga)Se 2 absorbers and CdS buffer layers at the absorber – window interface. The usage of the Cu(In,Ga)(S,Se) 2 (CIGSSe) alloys as absorber with a wider band gap is in progress to rise the open circuit voltage of the solar cells and, finally, their efficiency. Strong research effort is forwarded towards replacement of the CdS by a less toxic alternative buffer material. Solar cells based on CIGSSe have already achieved an efficiency of 15% using such buffer materials as Zn(X,OH) or In(X,OH) with X ¼ S, Se prepared by chemical bath deposition (CBD) [2–7]. However, the knowledge about current transport and recombination mechanisms in Cd-free CIGSSe-based devices is still limited. The aim of this contribution is to evaluate the dominating recombination mechanism which limits the device perform- ance. In order to determine the transport mechanism in ZnO/ZnS/Cu(In,Ga)(S,Se) 2 structures, the temperature dependence of dark JV curves and short-circuit current ðJ sc Þ versus open circuit voltage ðV oc Þ plots were analysed. 2. Experimental Solar cells based on CIGSSe absorbers were processed by the following procedure. A chemical bath was used for the deposition of Zn(S,OH) buffer layer on the CIGSSe/- Mo/glass substrates provided by Shell Solar GmbH (Mu ¨nchen). Details concerning Zn(S,OH) preparation can be found elsewhere [7]. A double ZnO (100 nm intrinsic and 400 nm Ga-doped) window layer was subsequently sputtered. Ni – Al grids were deposited by e-beam evapor- ation through a shadow mask as front contacts. Solar cell 0022-3697/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0022-3697(03)00175-6 Journal of Physics and Chemistry of Solids 64 (2003) 2037–2040 www.elsevier.com/locate/jpcs * Corresponding author. Tel.: þ49-30-8062-3038; fax: þ 49-30- 8062-3199. E-mail address: ennaoui@hmi.de (A. Ennaoui).