## 3 Challenging PEC Device Designs toward Practical Solar Water Splitting
### Development of Ferroelectric Coupled PEC Device  
In order to efficiently convert solar energy into electrical or chemical energy, the charge carrier separation and collection has generally been considered as one of the most decisive factors. Also, it is well known that the internal electric field developed in conventional p-n junctions enables efficient charge separation and collection. Hence, attempts to utilize the unusual photovoltaic effect of the ferroelectric oxides have been widely reported. The studies have shown that the spontaneous polarization of the ferroelectric oxides, which creates an internal electric field, provides significant improvements in the charge carrier separation and collection. Herein, the correlation between the spontaneous polarization and the internal electric field can be explained as follows. The high built-in-potential at the interface is determined by quantifying the polarization effect, which can be indicated as the potential height of ferroelectric band bending (\(P\delta/\varepsilon\), Equation (4)). Additionally, the electric field at the interface can be determined by Equation (5) [64]. \[{V_{\text{bi}}}^{\prime}={V_{\text{bi}}}\pm\frac{\text{P}}{\varepsilon}\delta \tag{4}\]  
\[\text{E}=\sqrt{\frac{2\text{qN}_{\text{eff}}\left(\text{V}+{V_{\text{bi}}}^{ \prime}\right)}{\varepsilon}}\pm\frac{\text{P}}{\varepsilon} \tag{5}\]  
where \({V_{\text{bi}}}^{\prime}\) is the built-in potential at the interface, \(\text{V}_{\text{bi}}\) is the built-in potential, P is the polarization of ferroelectrics, \(\varepsilon\) is the dielectirc constant, \(\delta\) is the distance between the polarization charge and the interface with the electrode, E is the electric field at the interface, q is the electric charge, and \(N_{\textit{eff}}\) is the effective charge density. Note that the spontaneous polarization and the internal electric field are linearly aligned. Recently, many studies have been widely conducted to apply this unique property of the ferroelectric materials to PEC water splitting. However, in spite of the advantageous photovoltaic effect, most ferroelectric materials, including BaTiO\({}_{3}\), PbTiO\({}_{3}\), and Pb(Zr, Ti)O\({}_{3}\) typically have the wide bandgap that absorbs only a short range of the visible spectrum [65]. Fortunately, among the ferroelectric materials, the BiFeO\({}_{3}\) have a relatively narrow bandgap in the range 2.2-2.7 eV [66; 67]. For that reason, various studies on the ferroelectric BiFeO\({}_{3}\) photoelectrodes have been reported for PEC water splitting [35; 68; 69; 70; 71; 72; 73]. Recently, our group reported that the effect of applying ferroelectric BiFeO\({}_{3}\) thin films to PEC water splitting, which could be systematically summarized as shown in Figure 7[35]. As described in Figure 7a-c, the schematic shows the energy band diagram of the BiFeO\({}_{3}\) thin film photoanodes in the PEC water splitting system to help with understanding on the effect of ferroelectric switching with different polarization states. In the no-polarization state, each photoexcited carriers are separated inefficiently on both sides of the photoanode without the energy band bending (Figure 7a). However, the downward ferroelectric switching of BiFeO\({}_{3}\) induces negative (positive) bound charges toward the electrolyte/photoanode (photoanode/bottom electrode)  
Figure 6: (**a**) PEC performances of 100 nm-thick epitaxial BiVO\({}_{4}\) films with different crystallographic orientations. The electrolyte of 0.5 M Na\({}_{2}\)SO\({}_{4}\) aqueous solutions in the presence or absence of 0.5 M Na\({}_{2}\)SO\({}_{3}\) was used for the measurement. (**b**) EIS spectra and (**c**) IPCE of the BiVO\({}_{4}\) with different crystallographic orientations. Inset of (**b**) shows the equivalent circuit for the data. Reprinted with permission from [19]. Copyright 2018, American Chemical Society. interface (Figure 7b). As a result, each band bending significantly enhances PEC performances.