We address the role of Sn-substitution and Pb-vacancy (Pb-$Box$) in regulating stability and carrier concentration of CH$_3$NH$_3$Pb$_{1-X-Y}$Sn$_X$$Box_Y$I$_3$ perovskite using density functional theory, where the performance of the exchange-correlation functional is carefully analyzed, and validated w.r.t. available experimental results. We find the most stable configuration does not prefer any Pb at 50% concentration of Sn. However, the Pb-$Box$s become unfavourable above 250K due to the reduced linearity of Sn-I bonds. For n-type host the Sn substitution is more preferable than Pb-$Box$ formation, while for p-type host the trend is exactly opposite. The charge states of both Sn and Pb-$Box$ are found to be dependent on the Sn concentration, which in turn alters the perovskite from n-type to p-type with increasing $X$ ($>$0.5).
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