The MoSe₂ Electrochemical Solar Cell: Anodic Coupling of Electron Transfer to d → d Photo‐Transitions in Layer Crystals

Abstract

Abstract n‐type MoSe 2 layer crystals were successfully used as photoelectrodes in electrochemical solar cells. Their mechanism is based on d → d phototransitions in the electrode in combination with anodic electron transfer from suitable redox couples in the adjoining electrolyte (Fe 2+/3+ , I − /I 2 ). Redox agents with potentials more negative than that of water can be oxidized by holes from the low lying Mo 4d energy band clearly before the anodic onset of the photoreaction with water molecules which would lead to the formation of selenic acid and to a gradual photodissolution of the electrode. In this way it is possible to operate regenerative electrochemical solar cells by‐passing corrosive anodic reactions. With MoSe 2 : I − /I 2 solar cells photocurrent densities of 22 mA/cm 2 , photovoltages of 0.55 V and energy conversion efficiencies between 4 and 5% were obtained in the red and near infrared spectral region. Their stability behaviour was tested at photocurrent densities between 11 and 12 mA/cm 2 over a period of more than 1200 hours (50 days). No deterioration of the photoelement was observed. Capacity measurements indicate that reducing agents such as iodide ions are chemisorbed to surface states formed by trapped holes on d‐orbitals. The resulting surface charge redistribution causes a relative shift of energy levels which suppresses the reaction with water and induces a space charge polarization that facilitates photoactivated electron transport from iodide.