Revealing and Identifying Laser-Induced Damages in CIGSe Solar Cells by Photoluminescence Spectroscopy

Laser-based patterning for serial interconnection of chalcopyrite (i.e., Cu(In ${}_{\rm{x}},\text{Ga}_{\rm{1-x}}$)Se2 or CIGSe) solar cells was obtained by 1) laser ablation using picosecond (ps) pulses, 2) local phase transformation using nanosecond (ns) laser pulses, and 3) conventional needle-based patterning. All three patterning approaches cause a modification of the material properties in the vicinity of the actual P2 scribing lines, which affects and limits the electrical functionality of the interconnection, and thus has to be considered for positioning the P3 scribe. Thus, the extension and the properties of the affected zone aside the P2 scribe was investigated through spectral and spatial photoluminescence (PL). From the depletion of the PL intensity when approaching the scribing line and a peak shift analysis it is concluded that the laser-affected zone is distinctively larger than visual inspections suggest. Even putatively ultrashort, nonthermal ps pulses cause material modifications which might be facilitating recombination losses and thus limiting solar cell efficiencies. For ps laser patterning the affected area is even larger than for the ns laser patterning, due to a modification of the band structure and to thermal decomposition. Evolving subpeaks at the low energy tails are found to originate from Cu-related flat defect levels, i.e., Cu vacancies ($V_{{\rm{Cu}}}$) and antisites (Cu In), created upon laser impact. These findings provide insights into laser-based material modification and provide beneficial information for minimizing the dead area resulting from laser-based monolithic interconnection.