On the Photopotential Output of Electrochemical Solar Cells Based on Layer‐Type d‐Band Semiconductors

A comparative study of molybdenum‐ and tungsten‐dichalcogenides in contact with various redox electrolytes revealed that in absolute values as well as in relation to the energy gap, n‐type WSe 2 is producing the largest photopotential output. It exceeds 0.55 Volt in presence of several redox couples (Fe 2+/3+ , H 2 Q/Q pH = 10, Fe(CN) 3‐/4‐ , 6 , Ru 3‐/4+ , Br − /Br 2 , Ce 3+/4+ ) and amounts to more than 0.7 Volt in presence of I − /I 2 . Addition of small quantities of iodide can increase the photopotential output in presence of redox systems with lower redox potential by up to 0.1 Volt. This gives further evidence for the specific photochemical surface activity of iodide on layer‐type electrodes and is an example for the influence of surface states on the efficiency of energy conversion. ‐ The higher photopotential output of WSe 2 as compared to that of MoSe 2 (maximum value I − /I 2 :0.56 Volt) is explained in terms of a negative shift of the conduction band edge. With respect to the efficiency of electrochemical solar energy conversion WSe 2 is considered to be the most promising compound among layer‐type d‐band semiconductors.