# Ferroelectric Materials: A Novel Pathway for Efficient Solar Water Splitting
## 3 Solar Energy Conversion of Ferroelectric Materials
### Ferroelectric Photovoltaics--Phenomenology of Ferroelectric Solar Cells  
The photovoltaic effect in a semiconducting material occurs when an asymmetry in the electric potential occurs throughout the materials due to a flow of photogenerated electrons and holes. A photocurrent occurs as a result of the absorption of photons with high energy, which leads to the transfer of electrons from the valence band (VB) to the conduction band (CB). Conventional photovoltaic phenomenon could be observed in interfaced doped semiconductors such the p-n junction structure when the following three processes should be involved: high light absorption, generation of the electron-hole pairs, and separation of the free charges. Usually, hole (positive charge) and electron (negative charge) carriers are generated by high energy photons. Over time, the excited carriers return to the ground-state level, and the internally absorbed energies are released as light or heat. While most commercial solar cells are based on the p-n junction principle between two different metallic contacts, they are limited by the so-called Shockley-Queisser limit, which prevents any single p-n junction solar cell from converting more than 33.7% of the incident light [23]. Their heterojunction structure is limited with regard to the selection of potential materials due to dopant issues and the mismatched lattice of the structure. Ferroelectric materials have been recently proposed as excellent potential candidates to solve these problems. The phenomenon of a photovoltaic effect in a ferroelectric has been observed in BaTiO\({}_{3}\), LiNbO\({}_{3}\), and Pb(Zr,Ti)O\({}_{3}\) with weak pyroelectric currents (photovoltaic current above _Tc_). However, ferroelectrics have a significant disadvantage of large band gaps in the range of \(\sim\)3-4 eV, which is challenging for applications as photovoltaic materials. Their large band gaps values result in the degradation of efficient conversion because small current densities and large open circuit photovoltages are typical. In recent years, the higher efficiency of narrower bandgap ferroelectrics in ferroelectric solar cells such as those of BiFeO\({}_{3}\) (BFO, 2.2-2.7 eV) and multiferroic Bi\({}_{12}\)FeCrO\({}_{6}\) (BFCO, 1.9-2.1 eV), has been reported. Ferroelectric photovoltaic effects are divided into photoferroic phenomena including the bulk photovoltaic effect (BPE) and the anomalous photovoltaic effect (APE). Photoferroic phenomena could be demonstrated in hybrid organic-inorganic halide perovskites such as CH\({}_{3}\)NH\({}_{3}\)PbI\({}_{3}\) (MAPI) to fabricate high-efficiency photovoltaic devices. Ferroelectric domains as internal junctions can induce the generation of photoexcited electrons and hole pairs, and decrease recombination by dividing charged carriers. The effect could produce a higher open circuit voltage (V\({}_{\text{OC}}\)) and the current\(-\)voltage hysteresis observed in perovskite solar cells. The photoferroic system has an intricate interface between the photoresponse and the ferroelectric phase stability, single/multidomain, and distributions. The photoferroic system has recently achieved power efficacies in excess of 8% power for Bi\({}_{2}\)FeCrO\({}_{6}\)[26]. The larger bandgap (\(\sim\)3 eV) in the polar materials shows their potential for development as light absorbers [25; 26; 27]. The photovoltaic effect (BPE) is a phenomenon that is observed in an open circuit voltage under dark illumination. The BPE generates a photovoltaic current that results in an anomalous photovoltaic effect (APE), as a type of bulk photovoltaic effect that occurs between semiconductors and insulatorsas a result of an open-circuit voltage induced by light. Sometimes, the generated voltage approaches almost thousands of volts. The mechanisms of the anomalous photovoltaic effect (APE) are schematically represented in Figure 4. The generated photovoltage in the anomalous photovoltaic effect (APE) is limited by its bandgap.