## 3 Challenging PEC Device Designs toward Practical Solar Water Splitting
### Development of Ferroelectric Coupled PEC Device
On the other hand, the high built-in-potential by the ferroelectric polarization causes upward (downward) band bending toward the electrolyte/photoanode (photoanode/bottom electrode) interface. In this case that the upward ferroelectric switching of the BiFeO\({}_{3}\) is induced, each band bending operates as the barrier for photoexcited carriers, and this deteriorates PEC performances (Figure 7c). In addition, interestingly, BiFeO\({}_{3}\) has been widely reported to have different polarization values with different crystallographic orientations, as shown in Figure 7d [74; 75; 76; 77; 78]. The different ferroelectric polarization values of the BiFeO\({}_{3}\), which depend on different crystallographic orientations, induce the differences in the energy band bending height. Figure 7e indicates the difference in the energy band bending height induced by the different spontaneous polarization values in the downward polarization state. Especially, (111)\({}_{\text{pc}}\)-oriented BiFeO\({}_{3}\) has the highest energy band bending height because of its highest spontaneous polarization value among all crystallographic orientations. In conclusion, the most efficient charge separation and collection of the photogenerated carriers can be obtained in the (111)\({}_{\text{pc}}\) BiFeO\({}_{3}\) photoanode. There are many limitations inherent in the ferroelectric materials in terms of the light absorption and the photoactivity when using ferroelectric materials alone as PEC water splitting photoelectrodes. Thus, in recent years, the ferroelectric materials have not been used as a single material for the PEC water splitting, but as a part of the heterojunction structures to harness the ferroelectric energy band bending. For instance, [PERSON] et al. synthesized the ferroelectric BaTiO\({}_{3}\)/TiO\({}_{2}\) heterojunction structure, which exhibited the improved PEC performances as a result of the enhanced charge separation efficiency induced by the ferroelectric polarization of BaTiO\({}_{3}\)[79]. In addition, [PERSON] et al. reported the PEC performance of the ferroelectric BiFeO\({}_{3}\)/BiVO\({}_{4}\) composite photoanode fabricated by the chemical solution deposition [34]. The result showed that the photoanodic property of the ferroelectric BiFeO\({}_{3}\)/BiVO\({}_{4}\) heterostructure film was significantly improved by the effect of the surface passivation and the reduced charge recombination by the BiFeO\({}_{3}\). To date, it is known that the built-in band bending  
Figure 7: Schematic representations of the built-in polarization field in (**a**) no polarization state, (**b**) downward polarization state, and (**c**) upward polarization state of ferroelectric BiFeO\({}_{3}\) thin films. (**d**) Different polarization values of BiFeO\({}_{3}\) with different crystallographic orientations. (**e**) Polarization direction of the BiFeO\({}_{3}\) in different crystallographic orientations. Reprinted with permission from [35]. Copyright 2018, Tsinghua University Press and Springer-Verlag GmbH Germany. induced by the spontaneous polarization of the ferroelectric materials enables the improved charge separation. Because the application of ferroelectric materials into PEC water splitting has plentiful of undiscovered strategies that are yet to be explored, it is expected to be intensively investigated.