## 4 Conclusions  
SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\) planar heterojunction has been fabricated through a combination of the e-beam deposition method and the MOD technique. Photocurrent behavior under back and front illumination was studied for different layers of SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\) planar heterojunction. When a WO\({}_{3}\) layer was introduced between SnO\({}_{2}\)/BiVO\({}_{4}\), photocurrent density under front illumination increased compared to that under back illumination. This indicates that poor charge carrier transport property of BiVO\({}_{4}\) can be successively suppressed by triple planar double heterojunction. By inserting WO\({}_{3}\) in SnO\({}_{2}\)/BiVO\({}_{4}\), the charge transport property was improved considerably. Remarkably high IPCE of ~80% during back side and ~90% upon front side illumination was witnessed for SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\) at 1.23 V vs. RHE. This study gives insights to tuning photocurrent density under directional illumination by constructing triple planar double heterojunction.  
The following are available online at [[http://www.mdpi.com/2076-3417/8/10/1765/s1](http://www.mdpi.com/2076-3417/8/10/1765/s1)]([http://www.mdpi.com/2076-3417/8/10/1765/s1](http://www.mdpi.com/2076-3417/8/10/1765/s1)), Figure S1: (a) LSV of SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\) measured using a three-electrode configuration in aqueous phosphate buffer (pH 7.0) with 0.5 M Na\({}_{2}\)SO\({}_{4}\) in the absence of a hole scavenger. (b) LSV of SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\) in an aqueous phosphate buffer (pH 7.0) with 0.5 M Na\({}_{2}\)SO\({}_{3}\) under chopped illumination. (c) Stability measurements of SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\) under chopped illumination in an aqueous phosphate buffer (pH 7.0) with 0.5 M Na\({}_{2}\)SO\({}_{3}\)/BiVO\({}_{4}\) under chopped illumination in an aqueous phosphate buffer (pH 7.0) with 0.5 M Na\({}_{2}\)SO\({}_{3}\)/BiVO\({}_{4}\) without chopped illumination. Figure S2: Equivalent circuit model of Nyquist plot for SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\) photoanode, Figure S3: SEM images of SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\) (a) before and (b) after photostability measurement, Table S1: Nyquist plot fitted results for SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\) photoanode.  
Conceptualization, H.W.J. and S.S.M.B.; Methodology, S.S.M.B.; Software, S.S.M.B.; Validation, S.S.M.B. and H.W.J.; Formal Analysis, S.S.M.B.; Investigation, S.S.M.B.; Resources, S.A.L., J.M.S. and S.-P.H.; Data Curation, S.S.M.B., S.AL.; Writing-Original Draft Preparation, S.S.M.B.; Writing-Review & Editing, S.S.M.B.; Visualization, S.S.M.B.; Supervision, H.W.J.; Project Administration, H.W.J.; Funding Acquisition, H.W.J.  
This work was financially supported by the Basic Science Research Program (2017R1A2B3009135), the Future Material Discovery Program (2016M3D1A1027666), the Nano Material Technology Development Program (2016M3A7B4910) through the National Research Foundation of Korea, and the International Energy Joint R&D Program of the Korea Institute of Energy Technology Evaluation and Planning (20168510011350).  
The authors declare no conflict of interest.