# Study of Annealing Temperature Effect on the Photovoltaic Performance of BiOI-Based Materials
## 1 Introduction  
Bismuth oxyiodide (BiOI) is a p-type semiconductor which is commonly applied as a photocatalyst for photocatalytic reaction during the last decade [1; 2; 3; 4; 5; 6; 7; 8; 9]. Since it has the bandgap energy \(\sim\)1.8 eV, this material has good absorption in the visible spectral range. It is interesting to note that this oxyhalide semiconductor also has an environmentally benign character [10]. The wider application of BiOI can extend for a photoanode used in photoelectrochemical devices [11; 12; 13; 14; 15; 16; 17]. Several ways are common to synthesize BiOI, either including the solvent usage or solvent-less process [5; 11; 16; 18; 19; 20]. Generally, hydrothermal and solvothermal methods are used to produce BiOI powder for photocatalytic application [9; 18; 19]. However, successive ionic layer adsorption and reaction, as well as chemical-based deposition have been employed to deposit BiOI films for photovoltaic study [13; 14; 16]. Nevertheless, as an alternative, the BiOI film's preparation for solar cell application also can be carried out via the doctor blade method which will be discussed in this report. Some key aspects of semiconductor materials' properties are noted to be a crucial matter for the device performance as different parameters in synthesis can result in different physical properties of materials. The morphology, size, crystallinity, and optical properties are needed to be considered, since those characteristics can be affected by the procedure of materials preparation, the concentration of precursors [21; 22; 23], surfactant selection, and solvent selection [24; 25]. The post-treatment after thematerial synthesis process, like calcination, also can change the material properties. It seems to be similar to the BiOI structure, since it consists of [Bi2O2]2+ slabs that allow for stoichiometric changing due to the calcination for producing the derivate of BiOI [26]. Moreover, the pH adjustment during the synthesis process is an important factor which affects BiOI structure [1]. BiOI and its derivation have been reported mostly for photocatalytic application due to their effectivity in the separation of electron-hole pairs [27]. Bi4O5I2, with its smaller size and wider valence band, had a better photocatalytic performance than BiOI because it had a lower electron-hole pairs recombination rate [28]. Bi7O9I3, which is often difficult to differ with BiOI, was also reported to have excellent photocatalytic activity under visible light irradiation [26; 29]. Another BiOI family member is Bi5O7I, which exhibited a twice higher photocatalytic activity than BiOI because of its pores and different band structure [30]. Moreover, polymorphic Bi2O3 could show its ability as photocatalyst with performance order: \(\beta-Bi2O_{3}>\alpha-Bi2O_{3}>\delta-Bi2O_{3}\)[31], but it has not been compared yet with BiOI activity. Many reports on BiOI and its derivatives for photocatalytic application have been reported. However, to the best our knowledge, there is no study in the screening of BiOI and its derivations for photovoltaic device application, even though BiOI can be considered as the promising material for the future photovoltaic devices owing to its longer lifetime which up to 2.7 ns [11]. In this study, we investigated the effect of annealing treatment in air at different temperatures on the structural, morphological, optical, and photovoltaic properties of BiOI films. By changing the temperature from 100 \({}^{\circ}\)C to 550 \({}^{\circ}\)C, we could obtain different characteristics of bismuth-based material, both chemically and physically. The heating process in the air let the reaction occur by turning into the oxygen-rich materials as a consequence of losing some iodine in the parent BiOI structure.