Polytypic ZnCdSe shell layer on a ZnO nanowire array for enhanced solar cell efficiency

Wurtzite–zinc blende (WZ–ZB) polytypic ZnxCd1−xSe layers were deposited on a ZnO nanowire array with full composition tuning by the chemical vapor transport method. As the composition tuned, three distinctive WZ–ZB polytypic structures appear. (1) At x = 0.2–0.3, the WZ and ZB domains exist separately in the inner and outer regions, respectively, forming a unique double shell structure. Their [0001]WZ and [111]ZB (or [100]ZB) directions are aligned along the WZ [0001] growth direction of the ZnO nanowire. (2) At x = 0.5, these WZ and ZB domains line up along the axial direction, producing a superlattice structure. (3) When x = 0.7–0.8, unique twinned superlattice structures, which are composed of ZB twinned segments having alternating orientations along the axial [111] direction, were identified. The outer region of the shell contains the higher Cd content than the inner region. In the fabricated photoelectrochemical cells, the double shell structure (x = 0.2–0.3) produced a highest photoconversion efficiency, indicating the most effective band alignments of the Cd-rich ZB and Zn-rich WZ domains. However, the twinned superlattice structure exhibits the lowest photoconversion efficiency, probably due to the larger number of interfacial defects between the twinned segments.