Nanosecond laser-induced selective removal of the active layer of CuInGaSe${}_2$ solar cells by stress-assisted ablation

We demonstrate that laser pulses of nanosecond duration ($\lambda =1064$ nm, $\tau =25$ ns, PRR $=5$ kHz) are capable of the clean removal of the CuInGaSe${}_2$ (CIGS) and ZnO:Al layers in the layer structure of chalcogenide-based solar cells, leaving the underlying Mo layer undamaged and producing excellent crater morphology. Our results prove that the material removal process is governed by the thermomechanical stress developing in the CIGS layer due to rapid laser heating. In the mechanical ablation of the active layer, three phenomena play a crucial role, namely, delamination, buckling, and fracture. Morphological and compositional analysis of the laser-processed areas is used to identify the experimental parameters where clean mechanical ablation can be achieved. Numerical calculations, performed in the comsol software environment, are also presented to complement the experimental tendencies and verify the proposed model. Our calculation proves the development of a stress distribution that drives the delamination of the CIGS and Mo layers. As the delamination front proceeds radially outward, the separation of the layers ceases in the colder outer regions according to the Griffith's criterion and defines the size of the craters produced afterwards. The free-standing chalcogenide layer continues to deform, and buckling results in a growing tensile stress at the perimeter of the delaminated area, where ultimately fracture will finalize the removal process and facilitate the clean ablation of the laser-irradiated area.

Figure 1

Process geometry.

Figure 2

Measured (solid symbols) and calculated (continuous curve) beam radii at different axial positions (corresponding to different working distances).

Figure 3

Calculated temperature distributions (at $F=0.75$ J cm${}^{-2}$, $w=39$ $\mu$m) (a) along the axis of symmetry and (b) radially, at the ZnO/CIGS interface.

Figure 4

Calculated distribution (a) of the radial and azimuthal components of the stress along the axis of symmetry, (b) radial component of the stress in radial direction at 0.4 $\mu$m below the ZnO/CIGS interface at 43 ns. Normalized distribution of the temperature and radial components of the stress field in the (c) axial and (d) radial directions at 0.4 $\mu$m below the ZnO/CIGS interface. In graphs (c) and (d), continuous curves refer to temperature, while dotted ones indicate the stress. All data refer to $F=0.75$ J cm${}^{-2}$ and $w=39$ $\mu$m.

Figure 5

Secondary electron micrograph of an ablation crater produced by the (a) evaporation ($F=4.86$ J cm${}^{-2}$ and $w=11.5$ $\mu$m), (b) thermally driven mechanical stress ($F=0.842$ J cm${}^{-2}$ and $w=44$ $\mu$m), and (c) competing evaporative and mechanical stress ($F=2.41$ J cm${}^{-2}$ and $w=26$ $\mu$m). Corresponding profilometric traces are also shown below each SEM image.

Figure 6

Area ratio AR of the molten residue covered inner area to the total area of the crater as a function of laser fluence used for creating them. Different colors depict craters made with different beam sizes. Closed and semiclosed symbols identify $S$- and $M$-type craters, respectively.

Figure 7

Spread of crater morphology type on the semilogarithmic beam radius vs fluence plane. Ablation was made in a glass/Mo/CIGS/ZnO:Al layer structure by a Nd:YAG laser ($\lambda =1064$ nm, $\tau =25$ ns). The typical error of the laser fluence and beam radii is indicated at several selected points.

Figure 8

EDS line scans of six elements in two craters having (a) $S$-type ($F=0.7$ J cm${}^{-2}$) and (b) $M$-type morphology ($F=1.50$ J cm${}^{-2}$). All data were acquired at an accelerating voltage of 20 kV.

Figure 9

(a) Calculated radii fitted to the measured data (closed symbols: crater radius; open symbols: radii of buckled regions) and (b) the fluence threshold where fracture and delamination are observed at different beam radii.

Figure 10

Secondary electron micrograph of a sample surface, which became bulged and partially fractured due to laser irradiation ($F=0.66$ J cm${}^{-2}$, $w=44$ $\mu$m).