Effect of Three-Stage Growth Modification on a CIGS Microstructure

Cu(In,Ga)Se ₂ (CIGS) thin-film solar cells from a Global Solar Energy (GSE) production line with 17.7% cell efficiency have been analyzed by a variety of imaging techniques, which include high-resolution electroluminescence and 3-D optical profilometry. The microstructure of supergrown CIGS nodules has been observed by an optical microscope and a scanning electron microscope (SEM). Statistical distribution of the nodule microstructure matched well with the bright spots in electroluminescence. A variation of several orders of magnitude in electroluminescence intensity indicates a wide margin of the quasi-Fermi level splitting potential and a possibility of efficiency improvement. Substrate temperatures of (In,Ga) ₂ Se ₃ precursor in the three-stage growth process have been modified to eliminate the nodule growth, which resulted in V _OC increase from the baseline value of 653 to 677 mV for a CIGS optical bandgap of 1.15 eV. Additional measurements, such as conductive-tip atomic force microscopy, energy-dispersive X-ray spectroscopy, and glow-discharge optical emission spectroscopy, are presented, and the results are discussed.