# Study of Annealing Temperature Effect on the Photovoltaic Performance of BiOI-Based Materials
## 3 Results and Discussion
### Photovoltaic Test  
We prepared the photovoltaic cells contained BiOI and its family members, as is illustrated in Figure 5A. The resulted current-voltage test of annealed BiOI in varied temperatures is displayed in Figure 6A and Table 1. Additionally, the dependence of short-circuit current (\(J_{\text{sc}}\)) and open-circuit voltage (\(V_{\text{oc}}\)) trend on the temperature is shown in Figure 6B. From Figure 6B, it can be seen that the increase in the temperature up to 300 \({}^{\circ}\)C resulted in the increasing of \(J_{\text{sc}}\) and \(V_{\text{oc}}\). Then, it was followed by a reduction in cell performance due to the treatment for more than 450 \({}^{\circ}\)C. In our work, we made two devices for photovoltaic cell measurement, and, here, we show better data in Figure 6A from two devices to represent the \(J_{\text{sc}}\) and \(V_{\text{oc}}\) trend, which is displayed in Figure 6B. The wider light absorption could have been responsible for the higher \(J_{\text{sc}}\) value, as this factor can affect the light-harvesting ability of semiconductor materials due to the redshift effect [49]. Other factors which may have an impact on the \(J_{\text{sc}}\) are grain size, internal resistance, film quality, charge transfer model, and electron mobility. Figure 5: Device structure (**A**) and band structure illustration of annealed BiOI in different temperatures (**B**). We observed that annealed material at 300 \({}^{\circ}\)C showed the best performance in comparison to the others. It was 0.438 mA/cm\({}^{2}\), which was almost four times higher than the resulted at 100 \({}^{\circ}\)C. In general, the photovoltaic properties of an electrochemical cell using porous material are affected by particle size, conductivity, lifetime, crystal orientation, etc. SEM images (Figure 3) and XRD patterns (Figure 1) show that the size of (001) nanosheets becomes larger and the FWHM of peaks becomes broader with increasing the annealing temperature from 100 \({}^{\circ}\)C to 300 \({}^{\circ}\)C, which suggests that the larger particle sizes with high conductivities were obtained at 300 \({}^{\circ}\)C annealing, which is suitable for electrochemical devices. The sample at 300 \({}^{\circ}\)C may also have had the bigger surface area for better contact with liquid electrolyte material. This phenomenon is likely to happen in the TiO\({}_{2}\) porous-based solar cell. As a result, Bi\({}_{7}\)O\({}_{9}\)I\({}_{3}\) could perform good interaction between the semiconductor material and iodine electrolyte in the photovoltaic device for exhibiting a higher \(J_{\rm sc}\) value. The contact angle check also revealed the increase of wettability due to the annealing treatment on BiOI from 100 \({}^{\circ}\)C to 300 \({}^{\circ}\)C (Figure 7). A declining contact angle occurred after the annealing treatment was set up to 300 \({}^{\circ}\)C. This indicated that Bi\({}_{7}\)O\({}_{9}\)I\({}_{3}\) had better contact with water than BiOI. As a consequence, the polar organic solvent, which is commonly used in the electrolyte liquid for dye-sensitized solar cells (DSSC), had more preference to make a good interaction with the more polar film surface. This trend was also matched to the previous report in BiOI solar cell [50]. Though overall \(J_{\rm sc}\) values in this report were lower than the reported BiOI solar cell [51] due to its bulky structure, it is interesting to note that Bi\({}_{7}\)O\({}_{9}\)I\({}_{3}\) had the better photovoltaic activity in comparison to its parent BiOI. This model is similar to the photocatalytic behavior of Bi\({}_{7}\)O\({}_{9}\)I\({}_{3}\)[1, 26, 29].