Non-destructive determination of phase, size, and strain of individual grains in polycrystalline photovoltaic materials

We demonstrate a non-destructive approach to provide structural properties on the grain level for the absorber layer of kesterite solar cells. Kesterite solar cells are notoriously difficult to characterize structurally due to the co-existence of several phases with very similar lattice parameters. Specifically, we present a comprehensive study of 597 grains in the absorber layer of a 1.64% efficient Cu2ZnSnS4 (CZTS) thin-film solar cell, from which 15 grains correspond to the secondary phase ZnS. By means of three dimensional X-ray diffraction (3DXRD), we obtained statistics for the phase, size, orientation, and strain tensors of the grains, as well as their twin relations. We observe an average tensile stress in the plane of the film of ~ 70 MPa and a compressive stress along the normal to the film of ~ 145 MPa. At the grain level, we derive a 3D stress tensor that deviates from the biaxial model usually assumed for thin films. 41% of the grains are twins. We calculate the frequency of the six types of $\Sigma$3 boundaries, revealing that 180{\deg} rotations along axis <221> is the most frequent. This technique can be applied to polycrystalline thin film solar cells in general, where strain can influence the bandgap of the absorber layer material, and twin boundaries play a role in the charge transport mechanisms.