Influence of potassium treatment on electronic properties of Cu(In<sub>1−</sub> <i> <sub>x</sub> </i>Ga<i> <sub>x</sub> </i>)(Se<sub>1−</sub> <i> <sub>y</sub> </i>S<i> <sub>y</sub> </i>)<sub>2</sub> solar cells studied by steady state photo-capacitance and admittance spectroscopy

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To clarify the mechanism of efficiency enhancement effect by potassium (K) treatment for Cu(In1− x Ga x )(Se1− y S y )2 (CIGSSe) solar cells, we investigated the physical properties of deep and shallow defect levels of CIGSSe solar cells without or with potassium fluoride treatment via using steady state photo-capacitance and admittance spectroscopy. The results show that the defect density of deep levels (such as 0.8 and 0.7 eV above valance band) are similar, suggesting that the K treatment does not drastically change the deep defect properties. For the shallow levels, the activation energy of interface-related shallow defect decreased from 183 to 120 meV upon K treatment, suggesting the modification of interface. The modified interface may be caused by the diffusion of potassium to grain boundaries and the formation of Cu-depleted layer in the near surface region of CIGSSe, resulting in the prolonged minority carrier lifetime. A new level with activation energy of 250 meV was found after K treatment, and it contributed to the increase of carrier density. These features well explained the enhanced device performance of CIGSSe solar cells by K treatment in terms of defects.

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