Quantum-Confined CdTe Films Deposited by SILAR and Their Photoelectrochemical Stability in the Presence of Se2- as a Hole Scavenger

Quantum-confined CdTe films were deposited by successive ionic layer adsorption and reaction (SILAR) on nc-TiO2 and on a conducting oxide electrode (PTO) from aqueous solutions of Cd2+ and Te2- prepared in situ under inert atmosphere. The films were characterized with UV-visible absorption, SEM, EDX, and XRD. CdTen films exhibited a zinc-blende structure and a red-shift in absorbance with increasing SILAR cycles (n) consistent with quantum size effects and featured either a mesoporous morphology on FTO or followed the contours of the titania nanoparticles on nc-TiO2 films. The films' photoelectrochemical behavior was studied in the presence of Se2- compared to S2- as hole scavengers. The incident-photon-to-current conversion efficiency reached ca. 16% at 460 nm and 9% at 500 nm at CdTe10/nc-TiO2 in alkaline Se2- electrolyte compared to 1% at 460 nm or 0.5% at 500 nm in S2-. CdTe10 films examined after acquiring a photoaction spectrum in Se2- still exhibited zinc-blende structure, EDX analysis showed Cd and Te peaks and no detectable Se, and the absorbance slightly increased with films remaining red-black. On the other hand, the absorbance edge and photocurrent onset shifted significantly to the blue and the films became yellow during the same measurement in S2-, indicating dissolution and formation of CdS, consistent with reports for CdTe single crystals and Q-CdTe. After hours of illumination at 500 nm at -0.55 V in Se2-, Se became incorporated in the films; however, the photocurrent decreased by only 5-8% after 2-3 h illumination, indicating significant photoelectrochemical stability. The results are attributed to effective quenching of the anodic dissolution of CdTe by Se2- scavenging the hole, and a slow growth of a protective overlayer possibly of CdTe1-xSex that does not block photocurrent generation, in contrast to the behavior of CdTe in sulfide electrolyte.