The major influence of the conduction-band-offset on Zn(O, S)/CuIn0.7Ga0.3Se2 solar cells

Abstract

Conversion efficiencies greater than 19% for thin-film solar cells based on (CIGS) cells with CdS buffer layers prepared by different methods are reported by several groups in recent years (Ramanathan et al., 2003, Contreras et al., 2005). However, the toxicity of cadmium leads the research community to replace the CdS buffer layer with another alternative while preserving the performance already achieved in different laboratories. Our work is focused on this type of CIGS based solar cells with Zn(O, S) as a buffer layer in place of CdS to avoid the toxic effect of cadmium. In this study, we simulated this kind of CIGS thin-film solar cell with the variation of Zn(O, S) affinity until an optimal interval which corresponds to the best efficiency of 20.63% where the affinity of CIGS remains constant. Its optimal value is around 4.58 eV for a (gallium/indium) ratio of 0.3 eV corresponds to a band gap energy of 1.17 eV as it is already demonstrated by several groups (Repins et al., 2008, Powalla et al., 2009) and the resulting Conduction Band Offsets (CBO) between CIGS and Zn(O, S) can range from −0.2 to +1.2 eV if the full range of the (oxygen/sulfur) ratio is considered from the pure ZnO to the pure ZnS. The simulation tool used for this study is Atlas of Silvaco-package based on the digital resolution 2D transport equations governing the conduction mechanisms in semiconductor devices as described in Section “Atlas numerical model description”. The J-V characteristics are simulated under standard AM1.5G Illumination.