## 1 Introduction  
Since we are heavily dependent on fossil fuels, harvesting clean energy in an efficient way is a pressing problem for mankind. Photoelectrochemical water splitting offers the capabilities to harness sunlight and convert it in the form of chemical storage through chemical bonds in the form of hydrogen [1; 2; 3; 4].  
Among various metal oxides such as Fe2O3, TiO2, WO3, and BiVO4, BiVO4 have attracted immense attention due to their promising photoelectrochemical performance, photostability, and non-toxicity [5; 6; 7; 8; 9]. BiVO4 has been recognized as a promising photocatalyst, as it possesses a relatively small band gap. However, BiVO4 suffers from low efficiency due to poor electron transport and slow water oxidation kinetics [10; 11; 12]. Therefore, various strategies have been adopted to overcome the limitations of BiVO4. Doping with Mo and W, and surface modifications with co-catalysts are the most common strategies adopted to improve the water oxidation efficiencies [13; 14; 15; 16]. The main disadvantage of BiVO4 is the short electron diffusion length, ranging from 10-70 nm, which is lesser than the light absorption depth (~250 nm) [11; 17; 18]. The hole diffusion length of BiVO4 is around 100 nm; therefore, it is of less concern. Due to short electron diffusion length, BiVO4 exhibits high photocurrent density under backside illumination. Photogenerated electrons at the surface need to travel across a BiVO4 film during front side illumination to reach the current collector, which usually results in bulk recombination. The tandem cells have been fabricated using BiVO4 with a superior photocurrent under back illumination [19]. Performance under backside illumination is limited due to the absorption of UV photons and blue light from the transparent conducting surface. Most of the reports on BiVO\({}_{4}\) focused on improving performance under back side illumination [20; 21]. Therefore, it is necessary to fabricate BiVO\({}_{4}\) photoanode which delivers superior pec performance during front side illumination. This would help in designing the future energy materials for the industrial scale applications.  
Recently, there have been few reports of BiVO\({}_{4}\) with heterojunction showing enhanced water oxidation performance under front illumination [22; 23]. A thin film film of BiVO\({}_{4}\) was found to exhibit superior performance during front illumination [24]. BiVO\({}_{4}\)/WO\({}_{3}\) type II heterojunctions have been intensively constructed in order to improve the light absorption capacity [25]. SnO\({}_{2}\) thin films were found to be a hole blocking layer for BiVO\({}_{4}\)[26]. Nanords of SnO\({}_{2}\) have also been used to improve the performance of BiVO\({}_{4}\), especially under front illumination [27; 28; 29]. Double heterojunction of BiVO\({}_{4}\) with SnO\({}_{2}\) and WO\({}_{3}\) has been fabricated to achieve an unassisted water splitting reaction [30]. However, studies on heterojunction photoanodes tuning the front illumination are rarely found in the literature.  
In this work, we focus on the different thickness of double heterojunction photoanode and their influence on the performance under front illumination. We have studied SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\) triple planar double heterojunction with different thicknesses and observed the change in the behavior of front illumination performance. This study would be beneficial for the construction of efficient tandem devices.