# Defect Processes in Halogen Doped SnO2
## 3 Results
### Electrical Properties  
We examine the effect of halogen doping on the electronic properties and band gap of SnO\({}_{2}\) by calculating the Density of States (DOS) of perfect and doped SnO\({}_{2}\). The bandgap of the undoped SnO\({}_{2}\) is calculated using the PBE0 functional, at 3.35 eV (Figure 2a) close (but still underestimated) to the experimental value of 3.7 eV [32], in agreement with other theoretical studies [41; 42; 43]. As it is observed from Figure 2b, the F interstitial in SnO\({}_{2}\) gives rise to energy states inside the band gap, near the band edges with a small reduction of 0.2 eV of the bandgap width, calculated at 3.12 eV (Figure 2b). When F is situated in an O-substitutional position, the bandgap is notably reduced to 2.90 eV without the presence of gap states (Figure 2c). \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **a (Å)** & **b (Å)** & **c (Å)** & **Vol (Å3)** \\ \hline SnO\({}_{2}\) & 4.717 & 4.717 & 3.189 & 70.956 \\ \hline F\({}_{\text{i}}\): SnO\({}_{2}\) & 4.739 & 4.715 & 3.195 & 71.390 \\ \hline Fo: SnO\({}_{2}\) & 4.735 & 4.735 & 3.205 & 71.857 \\ \hline Cl\({}_{\text{i}}\): SnO\({}_{2}\) & 4.789 & 4.790 & 3.201 & 73.429 \\ \hline Cl\({}_{\text{o}}\)SnO\({}_{2}\) & 4.778 & 4.778 & 3.213 & 73.350 \\ \hline Br\({}_{\text{i}}\): SnO\({}_{2}\) & 4.795 & 4.845 & 3.194 & 74.202 \\ \hline Br\({}_{\text{o}}\): SnO\({}_{2}\) & 4.795 & 4.795 & 3.218 & 73.988 \\ \hline I\({}_{\text{i}}\): SnO\({}_{2}\) & 5.055 & 4.830 & 3.165 & 77.276 \\ \hline I\({}_{\text{o}}\): SnO\({}_{2}\) & 4.840 & 4.840 & 3.215 & 75.313 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The computed lattice parameters and cell volumes for every doping case. Figure 2: _Cont._Regarding the Cl doping, in the case of interstitial doping, Cl\({}_{1}\); SnO\({}_{2}\), the bandgap is further reduced to 2.85 eV with gap states present near the valence band. When Cl is at an O-substitutional position, Cl\({}_{o}\), a more significant bandgap reduction occurs, to 2.70 eV (Figure 2d,e). In a previous DFT research on F and Cl doped TiO\({}_{2}\)[44], we also observed a significant bandgap reduction when a Cl atom was inserted in an interstitial position. Regarding the effect of a Br interstitial intercalation (Figure 2f), the calculated bandgap is reduced to 2.90 eV with gap states near the valence band edge (minimum) are created. In comparison, in the case of substitutional Br\({}_{o}\), the bandgap reaches a value of 2.71 eV(Figure 2g). It is evident that regarding the electronic structure, Br doping has the same effect as Cl doping in terms of bandgap reduction. Lastly, I interstitial and substitutional doping is examined. We calculated that the bandgap is reduced to a value of 2.62 eV for the interstitial and 2.61 eV for the substitutional case, respectively, while states near the valence band are again presented. In this case, the substitutional doping has a smaller bandgap value than the interstitial case. This can be attributed to the states that are created to the valence band edge, which is due to the hybridization of the 2p orbitals of the nearest oxygen atom with the iodine 5p orbitals. As iodine is larger than the other halogens, even in the substitutional position, its orbitals are mixing with the nearest oxygen atom, giving rise to the states near the valence band. These results indicate that halogen interstitial dopants serve as single acceptors when they are intercalated within the SnO\({}_{2}\) host lattice. Such gap states near the oxide's valence band maximum and those near the conduction band position may act as shallow and deep acceptors, respectively, hence contributing to performance enhancements of devices based on doped SnO\({}_{2}\). However, the gap states may also act as traps for the photogenerated carriers in specific photovoltaic devices such as organic and halide perovskite photovoltaics, thus decreasing the device photocurrent and overall performance. However, the incorporation of substitutional halogen atoms in oxygen position does not introduce any gap states.