In 2020, Luo et al. were studying in the semiconductor/catalyst heterojunctions for solar water splitting and solar rechargeable device. They found that a photoinduced reversible coupled electron-ion transfer behavior in Ni(OH)₂ on semiconductors, and the redox potential window of Ni(OH)₂ depends on the semiconductor energy band position (iScience 2020, 23, 100949). It clearly differs from conventional semiconductor physical junctions, in which the charge carries are electrons or holes. Therefore, Luo et al. proposed a new concept of a Faradaic junction (Fig. 1). Since then, they found that the Faradaic junction charge transfer not only happens at the interface of semiconductor/catalyst, but also at the interfaces of semiconductor/electrolyte, semiconductor/semiconductor, semiconductor/gas, semiconductor/solid electrolyte and metal/hydroxide (Nat. Commun. 2021, 12, 6363; Chem. Sci. 2020, 11, 6297; CCS Chem. 2021, 3, 1670).

The Faradaic junction theory can explain some conflicting results in photovoltaic and photoelectrochemical fields. Luo et al. found an isoenergetic charge transfer mechanism in a Faradaic junction, which successfully elucidated abnormal high V_OC in quantum-dot sensitized solar cells, perovskite solar cells and high performance in photo(electro)catalysis (Nat. Commun. 2021, 12, 6363). Moreover, by controlling the interface charge transfer direction, Faradaic junctions can realize reversible charge storage. Therefore, they developed two-electrode solar rechargeable devices based on Faradaic junctions for direct solar energy conversion and storage (Angew. Chem. Int. Ed. 2021, 60, 1390; J. Mater. Chem. A, 2022, 10, 1802). The previous studies suggest that the photovoltage in a Faradaic junction device is equal to the difference between the Fermi level of a semiconductor and the onset potential of a Faradaic material. However, it is still unclear about the influence factors of the dark output voltage in the devices, and the low volumetric energy densities also limits their practical applications. Therefore, it is desirable to clarify the working mechanism on the dark output voltage and construct a Faradaic junction solar rechargeable device with high volumetric energy density.

In this study, Luo et al. used n-Si/CoO_x as the photoelectrode and MnO_x as the counter electrode to assemble a portable two-electrode Faradaic junction solar rechargeable device (Fig. 2a). They found a characteristic of photovoltage memory effect in the Si/CoO_x/KBi_(aq)/MnO_x device that the dark output voltage can precisely record the value of the photovoltage (Fig. 2b). In contrast, a commercial Si photovoltaic cell only generated photovoltage when illuminated, but negligible dark output voltage. To study the mechanism of the photovoltage memory effect, they developed a method to monitor the potentials of the photoelectrode and counter electrode in real time. The results suggest that the electron and hole quasi-Fermi levels in the Si photoelectrode under illumination are recorded by the Faradaic reactions of MnO_x and CoO_x, respectively (Fig. 2c). Since both MnO_x and CoO_x have high pseudocapacitance, the photo-induced potentials can be maintained even after the light is off. This photovoltage memory effect can minimize interfacial energy loss and lead to higher performance of solar rechargeable devices. In addition, by replacing the opaque counter electrode with a transparent one, a portable solar rechargeable device with high volumetric energy density was achieved.

The novel photovoltage memory effect of the Faradaic junction device in this study offers guidance to improve the performance of a solar rechargeable device, and provides a possibility to design other new photoelectric devices, such as photodetectors, smart windows, optoelectronic synaptic devices and so on.

You can read more about the work in the article in Nature Communications following the link: https://doi.org/10.1038/s41467-022-30346-z.