Grain-Boundary-Enhanced Carrier Collection in CdTe Solar Cells

Abstract

When CdTe solar cells are doped with Cl, the grain boundaries no longer act as recombination centers but actively contribute to carrier collection efficiency. The physical origin of this remarkable effect has been determined through a combination of aberration-corrected scanning transmission electron microscopy, electron energy loss spectroscopy, and first-principles theory. Cl substitutes for a large proportion of the Te atoms within a few unit cells of the grain boundaries. Density functional calculations reveal the mechanism, and further indicate the grain boundaries are inverted to n type, establishing local p-n junctions which assist electron-hole pair separation. The mechanism is electrostatic, and hence independent of the geometry of the boundary, thereby explaining the universally high collection efficiency of Cl-doped CdTe solar cells.

The effect on CdS/CdTe solar cell conversion efficiency of the presence of fluorine in the usual CdCl2 treatment of CdTe

“…This phenomenon together with the overall improvement in the performance of fully fabricated solar cells, have been attributed to Cl diffusion through these grain boundaries during the post-deposition CdCl 2 treatment [8,7]. This effect should be more pronounced when F is involved in the post-deposition heat treatment of CdTe as has been observed in the present work.…”

“…Recent studies of current collection mechanism in CdTe using electron beam induced current (EBIC) technique have revealed high current collection efficiency at the CdTe grain boundaries [8]. This phenomenon together with the overall improvement in the performance of fully fabricated solar cells, have been attributed to Cl diffusion through these grain boundaries during the post-deposition CdCl 2 treatment [8,7].…”

“…Apart from generally improving the efficiency of CdTe-based solar cells, this treatment has been said to result in the conversion of as-grown n-type CdTe thin films to p-type CdTe [4,5] and in the improvement of n-type conductivity of CdTe layers [6]. There are also recent reports of conversion of p-type CdTe to n-type [3,7,5] or at least the conversion of thin outer layer of ptype CdTe grains to n-type [8] and thin outer layer of n-CdTe to p-type [7]. These cases however, depend on the nature of the defect distribution in the as-grown CdTe material [7].…”

Thin-film Sb2Se3 photovoltaics with oriented one-dimensional ribbons and benign grain boundaries

“…Bright areas in an EBIC image indicate regions of high collection efficiency for minority carriers. Whereas in CdTe and CIGS, high contrast indicates non-uniform collection efficiency 26 , the case of Sb2Se3 solar cell exhibits no appreciable variation in EBIC signal across the absorber layer close to the CdS layer (Fig. 4e) and more importantly also across a 2 μm long line scan (Fig.…”

Electron beam induced current in the high injection regime

“…EBIC is naturally suited to measure the minority carrier diffusion length and the surface recombination velocity [5][6][7]. Recent work also studies grain boundary properties using EBIC [8], identifying CdCl 2 treatment as the key to changing grain boundaries from regions of increased recombination to regions of increased charge collection [9,10].…”