Impurities in the absorber material of chalcopyrite (Cu(InxGa1-x)Se2) PV devices can have substantial influence on the device characteristics. Therefore a better understanding of the influence of impurities on the performance of CIGS solar cell is needed. Usually impurities are not from one single element, but rather a combination of different elements. There are elements like Fe, who are well known to severely deteriorate the efficiency of photovoltaic devices when contained in the absorber material in relevant concentrations. On the other hand, elements like Na and Sb are known to improve the device properties by improving the crystalline structure of the absorber. The sources of impurities in the absorber material can be either the substrate material like for example the Na diffusing from a soda-lime glass into the absorber or Fe coming from a steel substrate if no sufficient diffusion barrier is present. Another important source of impurities which is in the main focus of the presented work are impurities contained in the precursor materials forming the absorber themselves. Depending on the deposition method used, a concentration or dilution of the impurity concentrations in the source material is possible. The transfer of different impurities contained in the precursor material for the absorber formations by using sputtering as well as thermal evaporation was studied. Measurements of the impurities in the precursor material as well as in the prepared absorber material have been carried out by LA ICP-MS (Laser Ablation Inductively Coupled Plasma - Mass Spectrometry). As a result of this work information about pathways and sources of the different impurities in the absorber material can be given. This information will be useful for selecting suitable precursor materials depending on the deposition method used. Furthermore lowering the purity of the precursor material can have a significant contribution on reducing the overall manufacturing costs of solar module production.
Hinrichs, Volker; Handke, Volker; Chikhaoui, Liliana; Lux-Steiner, Martha C.