## 2 Strategies and Device Designs for Efficient PEC Water Splitting
### Separation and Transport of Charge Carrier  
The photoexcited charge carriers in the photoelectrode need to move to active sites for participation in the water redox reaction. In order to efficiently split the water molecules and produce the hydrogen gas, these charge carriers avoid recombination before they reach the active sites. One promising solution is the creation internal built-in potential which forces the charge carriers to move to the surface. The internal potential can be created by forming heterojunctions with different oxides, doping of foreign elements, or introducing a metal onto the semiconductor [45]. The formation of heterojunction has been accepted as one of the most promising ways for the practical photocatalysts because of its effectiveness in separating electron-hole pairs. Therefore, much research has been reported on the enhancement of PEC performance in this manner.  
[PERSON] and co-workers reported on wavelength-dependent charge carrier separation in the WO\({}_{3}\)-BiVO\({}_{4}\) heterostructure film [11]. Although many research results have suggested the improved carrier separation of the WO\({}_{3}\)-BiVO\({}_{4}\) heterostructure, an understanding of the underlying dynamics is still elusive. Thus, they investigated the lifetime of photoexcited holes in the WO\({}_{3}\)-BiVO\({}_{4}\) heterojunction by femtosecond transient absorption (TA) measurement with tunable pump wavelength. The results showed that the charge carrier transfer between the WO\({}_{3}\) and the BiVO\({}_{4}\) depends on the wavelength of irradiation. At a wavelength that is longer than the absorption edge of the WO\({}_{3}\), the lifetime of photoexcited holes in the WO\({}_{3}\)-BiVO\({}_{4}\) was longer than that of the single BiVO\({}_{4}\) since the holes were effectively transferred into the WO\({}_{3}\). However, when the wavelength was tuned across the absorption edge of the WO\({}_{3}\), which were 460 nm and 387 nm in this report, the lifetime of holes in the WO\({}_{3}\)-BiVO\({}_{4}\) were decreased. The authors attributed the results to the increased electron-hole recombination at the interface between the BiVO\({}_{4}\) and the WO\({}_{3}\). That is, the electron excitation from valence band of the WO\({}_{3}\) at such conditions leads to the recombination of electrons from the WO\({}_{3}\), and holes in the BiVO\({}_{4}\). This tells us that another route for the recombination in the heterojunction of photocatalysts can negatively affect the whole PEC process, and thus, we need to carefully design the heterostructured PEC devices for efficient solar water splitting.  
The doping of the photocatalysts also has been explored as one of the promising solutions. Recently, [PERSON] et al. investigated the Ca-doped BiVO\({}_{4}\) system (Ca:BiVO\({}_{4}\)) [24]. The authors deposited the Ca-doped BiVO\({}_{4}\) thin films by spray pyrolysis. During the deposition process, interestingly, spontaneous out-diffusion of Ca took place, which was resulted in gradual band bending through the film. Then, internal bend banding in the Ca:BiVO\({}_{4}\) promoted the carrier separation in the bulk of film, since the out-diffusion of Ca occurred over the entire region. Moreover, the higher catalytic activity of the Ca-rich BiVO\({}_{4}\) surface for water splitting contributed the enhanced performance. In the past, they had reported on the gradient W-doped BiVO\({}_{4}\) (W:BiVO\({}_{4}\)) [6]. In that report, they deposited a 10-step gradient-doped W:BiVO\({}_{4}\). As a result, the photocurrent was enhanced by about 60%, in comparison with that of the homogeneously W-doped BiVO\({}_{4}\). Both results reported by the authors show that the poor carrier separation efficiency of the BiVO\({}_{4}\) can be overcome by forming a multi-step doping method, thus creating effective built-in band bending. It is notable that the proposed strategy may be generally applicable to various oxide photoelectrode materials.