The formation of CuInSe2-based thin-film solar cell absorbers from alternative low-cost precursors

The compound semiconductor CuInSe2 and its multinary alloys are successfully applied as absorber material in thin-film solar cells. In one technologically realised production technique, the thin-film semiconductor CuInSe2 is crystallised during a fast annealing step of precursors, which consist of the elements copper, indium and selenium. This work deals with real-time investigations concerning the crystallisation process of CuInSe2-based thin-film solar cell absorbers while annealing differently produced and composed “low-cost” precursors. Various types of precursors have been investigated concerning their crystallisation behaviour. Three groups of experiments have been performed: (i) Investigations concerning the crystallisation process of the quaternary chalcopyrite Cu(In,Al)Se2 and Cu(In,Al)S2, (ii) investigations concerning the formation process of the compound semiconductor CuInSe2 from electroplated precursors, and (iii) investigations concerning the crystallisation of Cu(In,Ga)Se2 using precursors with thermally evaporated indium. All these “alternative” precursors have in common, that a distinct decrease of production costs has been expected from their successful application for the production of thin-film solar cells. A specific sample surrounding has been constructed, which enables to perform time-resolved angle-dispersive X-ray powder diffraction experiments during the annealing process of precursor samples. A thorough analysis of subsequently recorded diffraction patterns using the Rietveld method provides a detailed knowledge about the semiconductor crystallisation process while annealing. Based on these fundamental investigations, conclusions have been drawn concerning an adaptation of the precursor deposition process in order to optimise the final solar cell results. Especially the investigations concerning electroplated precursors could impressively show the importance of a fundamental understanding of the chalcopyrite crystallisation process as obtained by the conducted experiments. The investigations have shown, that one class of electroplated precursors shows a crystallisation behaviour identical to the one known for vacuum-deposited precursors. Further experiments could clarify, that a distinctly reduced amount of electrochemically deposited selenium is the decisive parameter for this formation behaviour, which results in an improved absorber morphology. The anticipated benefits were confirmed by distinctly improved solar cell results. The investigations concerning the crystallisation process of the quaternary chalcopyrite Cu(In,Al)Se2 revealed, that the chalcopyrite forms from the ternary selenide (Al,In)2Se3 and Cu2Se at elevated process temperatures. This result is used to explain the separation of the absorber layer into an aluminum-rich and an indium-rich chalcopyrite phase, which has been observed at processed Cu(In,Al)Se2 absorbers from several research groups. In addition, differences concerning the selenisation and sulfurisation of metallic precursor films are pointed out. The third group of experiments has shown, that the selenisation behaviour of copper-indium-selenium precursors with thermally evaporated indium is similar to that using precursors with sputtered indium. The investigation of the selenisation process of a copper-indium-gallium-selenium precursor revealed distinct differences concerning the selenisation kinetics of gallium containing and gallium free intermetallic precursor phases. These results explain the observed phase segregation into a gallium-rich and an indium-rich chalcopyrite phase. The results of the three groups of experiments are summarised and evaluated in the last chapter of this work.