## 1 Introduction  
Energy harvesting technology with a clean and renewable source of energy has drawn significant attention because of its possibility for solving the global energy challenge. Among the renewable energy sources including solar, wind, geothermal, and hydropower energy, solar seems the most desirable since it can support future societies in a sustainable way with the use of hydrogen gas as an energy carrier [1]. Along with that consideration, the solar-derived hydrogen production from water such as photoelectrochemical (PEC) water splitting has made significant progress [2; 3]. In general, the PEC water splitting system consists of photoelectrodes (photoanode or photocathode), electrolyte, and a light source. Mostly, light-absorbing semiconductors have been used as the photoelectrode, and the electrode materials have been being of great interest in the field of PEC water splitting. Among various materials, this review focuses on bismuth vanadate (BiVO4) and bismuth ferrite (BiFeO3) which are small band gap oxide semiconductors. There are some advantages of using BiVO4 and BiFeO3 as the photoanode. Briefly, they are composed of inexpensive elements and they can utilize visible light. The latter becomes more important when they utilize the solar radiation on earth, since about half of the radiation on earth corresponds to the visible light. However, they suffer from excessive charge carrier recombination, poor charge transport, and slow water oxidation kinetics. In order to solve the problems, various efforts have been investigated and the detailed strategies will be discussed further in Section 2. The electrolyte in the PEC water splitting mainly consists of water, and the use of seawater as the unlimited feedstockfor the production of hydrogen might be allowed through the investigation, using an aqueous solution of salts as the electrolyte. With a simulated one sun radiation from the light source, the PEC water splitting processes take place with several steps as follows (also see Figure 1):  
1. Absorption of solar radiation by the photoelectrode, followed by charge carrier generation as a result of photoexcitation of electrons in the valence band of photoelectrode;
2. Separation and transport of charge carriers to the electrode surface by the internal or external bias applied through the circuit;
3. Oxidation of water at the anode by the photoexcited holes. Simultaneously, transport of H\({}^{+}\) ions from the anode to the cathode, and transport of the photoexcited electrons to the cathode through an external circuit, followed by the reduction of H\({}^{+}\) ions into hydrogen gas at the cathode by the photoexcited electrons. As a result of the series of steps in the PEC water splitting, the solar energy can be converted into the hydrogen gas as the chemical fuel. The bottleneck for the realization of efficient and practical water splitting systems lies on the way to (1) effective charge carrier generation and separation, and (2) rapid water redox kinetics. The overall process is represented in Figure 1 with the PEC water splitting system consists of the photoanode and the metal counterpart as the cathode. More detailed issues on each step process and the strategies for efficient PEC devices will be discussed in Sections 2 and 3. This review seeks to provide the reader with the PEC device designs to improve the water splitting activity, and thereby aids the design of a highly efficient PEC device for practical solar water splitting. On one hand, the inherent problems including electron-hole recombination in bulk and on a small surface area per unit volume, which limits the activity of thin film photocatalysts in comparison with the porous and nanostructured morphologies (e.g., nanowires). On the other hand, as a model structure, the thin film configuration gives advantages that the understanding of complex relations between the light absorption, the charge separation, and the water redox kinetics becomes simple. In this regard, the review focuses on these investigations using thin film photocatalysts.