Conformal zinc sulfide coating of vertically aligned ZnO nanorods by two-step hydrothermal synthesis on wide bandgap seed layers for lead-free perovskite solar cells

Vertically aligned ZnO nanorods (NRs) were grown hydrothermally on the wide bandgap (~ 3.86 - 4.04 eV) seed layers (SLs) of grain size ~162±35 nm, prepared using ball-milled derived ZnO powder. The synthesized ZnO NRs were further decorated with ZnS nanocrystals to achieve a ZnO NR-ZnS core-shell (CS)-like nano-scaffolds by a subsequent hydrothermal synthesis at 70 ºC for 1 h. UV-Vis-NIR spectroscopy, X-ray diffractometry (XRD), Raman spectroscopy, and Field emission scanning electron microscopy (FESEM) coupled with Energy dispersive X-ray spectroscopy (EDX) analyses confirmed the formation of ZnS atop the vertically aligned ZnO NR arrays of ~1.79±0.17 µm length and ~165±27 nm diameter. Transmission electron microscopy (TEM)/EDX analyses revealed that vertically aligned ZnO NRs (core dia. ~181±12 nm) arrays are conformally coated by an ultrathin ZnS (~25±7 nm) shell layer with a preferential ZnS{111}/ZnO{10-10}-like partial epitaxy. The ZnO NRs exhibited a sharp band edge near ~384 nm having optical bandgap energy (Eg) of ~3.23 eV. However, the ZnO NR-ZnS CS exhibited double absorption bands at Eg~3.20 eV (ZnO-core) and Eg~3.78 eV (ZnS-shell). The ZnS{111}/ZnO{10-10}-nano-scaffolds could be utilized to facilitate the enhanced absorption of UV photons as well as the radial junction formation between the pb-free perovskite absorber and ZnS/ZnO NRs layers.&#xD.