Thin films of CZTS and CZTO for solar cells produced by pulsed laser deposition

Silicon solar cells are presently dominant for harvesting solar energy because of the well-known production technology, but silicon has a low absorption such that a cell requires a layer of up to 200 µm silicon for sufficient light absorption. During the latest decades new thin-film semiconductor cells with a four-component absorber, e.g. CZTS (Cu2ZnSnS4), have emerged as promising candidates for solar energy. This material consists of abundant and non-toxic elements. The material has a direct absorption and the absorber works perfectly with a thickness of 1-2 µm. Films of this four-component material are difficult to make by pulsed laser deposition (PLD), because of the different physical properties of the elements in the target both for one-phase targets of CZTS and for composite targets of sulfides. One further complication is that the stoichiometry of the most efficient absorbers are different from the nominal composition mentioned above, i.e. the film has to be Cu-poor and Zn rich. After the production at room temperature the (amorphous) film has to be annealed in a furnace at a temperature up to 600 C with a sulfur atmosphere in order to form CZTS. In addition, Sn has to be added during the annealing as well. At low fluence it was possible to obtain a Cu-poor composition for CZTS such that a cell of more than 5 % efficiency could be produced [1]. Also the usual problem for PLD, large droplets, could be reduced at low fluence.
In order to avoid evaporation of the volatile SnS from the composite target during deposition we have replaced the sulfide target with a target of copper zinc tin oxide (CZTO). The SnO binary compound in CZTOis much less volatile than SnS, such that the final content of Sn in the deposited film can be controlled much better. During the annealing in the sulfur atmosphere the oxide in the film is completely converted to sulfide. A general trend is that the Cu/Sn ratio of the film decreases strongly with decreasing fluence for the oxide film similar to the behavior of the sulfide film previously reported [2]. The underlying physics of the behavior of the film composition as a function of laser fluence for a number of chalcogenides will be discussed in terms of the physical properties of the materials, in particular the cohesive energy. With the oxide target we has obtained the world record, 5.4 % , in efficiency for solar cell absorbers of CZTS produced by PLD