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The introduction of annealing treatment in CdCl2 atmosphere has been demonstrated to be an effective method in improving the efficiency of CdTe solar cells. Kelvin Probe Force Microscopy (KPFM) was used to study the effect of the CdCl2 treatment process by spatially resolved imaging of the surface contact potential difference (CPD). The results indicated that the grains were spatial depleted by grain boundaries (GBs). The quantitatively CPD distribution at GBs was analyzed by Gaussian fitting. During annealing process the carrier density was decreased and the electronic properties of the GBs were modified. The electronic structure changes resulted from the CdCl2 treatment at GBs were put forward to explain the efficiency enhancement in device performance. The results suggested that GB depletion regions were narrowed down and barrier height at the GBs was also significantly decreased. Therefore the carrier transport can be enhanced. A GB model including the electron traps was introduced. It was found that the barrier height at GBs can reach the maximum value at a specific hole density. [ABSTRACT FROM AUTHOR]