# Study of Annealing Temperature Effect on the Photovoltaic Performance of BiOI-Based Materials
## 3 Results and Discussion
### Optical Study  
To obtain the optical property information, we used UV-Visible spectroscopy for its analysis. The absorbance spectra are displayed in Figure 4A. The absorbance value of each sample was determined from transmittance (T) and reflectance (R) values as the equation: \(A=1-(T+R)\)[44]. The spectra in Figure 4A show that all of annealed BiOI from 100 \({}^{\circ}\)C to 550 \({}^{\circ}\)C had absorption in the visible range. However, the annealing treatment provoked the blue-shift in its absorption along with the increase in temperature. From Figure 4A, we can see that the absorption edge of annealed BiOI at 100 \({}^{\circ}\)C and 200 \({}^{\circ}\)C overlapped at a wavelength of more than 600 nm. Due to this same absorption trend, we predict that the material composition of those treated samples may have been the same. When the annealing treatment was increased up to 300 \({}^{\circ}\)C and more, the absorption edge of those materials tended to be lower. The maximum absorption of each material was near 600 nm, and it was below 600 nm for annealed materials at 450 \({}^{\circ}\)C and 550 \({}^{\circ}\)C. Therefore, this supports the prediction that the material structure in the annealed BiOI at 100 \({}^{\circ}\)C, 200 \({}^{\circ}\)C, 300 \({}^{\circ}\)C, 450 \({}^{\circ}\)C, and 550 \({}^{\circ}\)C could be BiOI, BiOI, Bi\({}_{7}\)O\({}_{3}\)I, Bi\({}_{5}\)O\(\gamma\)I, and Bi\({}_{2}\)O\({}_{3}\). The oxidation reaction due to the treatment to BiOI at a high temperature allowed the BiOI to transform into the different materials, which have lower absorption abilities of visible light relative to the parent material [30].  
All those materials can be classified as semiconductor materials. The bandgap energy of bismuth-based materials is commonly determined by the Tauc plot, \(\alpha\)\(\lambda\4 was taken for this bandgap determination in our materials. The indirect bandgap calculation for annealed BiOI is shown in Figure 4B. The calculated bandgap energy of annealed BiOI at 100 \({}^{\circ}\)C, 200 \({}^{\circ}\)C, 300 \({}^{\circ}\)C, 450 \({}^{\circ}\)C, and 550 \({}^{\circ}\)C as BiOI, BiOI, Bi\({}_{\text{Y}}\)O\({}_{\text{J}}\)\({}_{\text{3}}\), Bi\({}_{\text{S}}\)O\(\gamma\)I, and Bi\({}_{\text{2}}\)O\({}_{\text{3}}\) were 1.76, 1.77, 1.84, 2.18, and 2.11, respectively. The bandgap energy order half a good agreement with the blue-shift phenomenon in the absorbance spectra of those annealed samples. Our bandgap energy of BiOI was also in line with the calculated BiOI from the previous report with order: \(BiOI<Bi_{\text{7}}O_{\text{9}}I_{\text{3}}<Bi_{\text{5}}O_{\text{7}}I\) which have the agreement with some references [26; 45]. The bandgap energy of Bi\({}_{\text{5}}\)O\(\gamma\)I was also reported to be slightly higher than Bi\({}_{\text{2}}\)O\({}_{\text{3}}\)[46], and for \(\beta\)-Bi\({}_{\text{2}}\)O\({}_{\text{3}}\), it was around 2.15 eV [47]. The structure changing in BiOI due to the annealing treatment influenced the bandgap energy as an effect of the different electronic structure in bismuth materials. The oxygen-deficient in the BiOI structure had a redshift response in the visible spectral range [48]. Thus, BiOI with oxygen-rich and iodine-deficient like in our materials had a blue shifting effect in their absorbance spectra.