Study of charge separation and interface formation in a single nanorod CdS-CuxS heterojunction solar cell using Kelvin probe force microscopy

In the present investigation, Kelvin probe force microscopy (KPFM) is used to study the charge separation, shift in Fermi level position and interfacial depletion region formation in a single cadmium sulfide (CdS)-copper sulfide (CuxS) nanorod heterojunction fabricated using hydrothermal synthesis and a topotaxial conversion reaction. A detailed analysis of KPFM images in the dark shows work function (or Fermi energy) values of CdS and CuxS regions consistent with the energy band diagram of the CdS-CuxS junction. Under illumination, Fermi energy levels of both the CuxS and CdS shift away from the vacuum level by 0.2 and 0.4 eV, respectively, which is very different from the behaviour expected in the case of a bulk p-n junction. The existence of interfacial regions topographically placed between ITO-CdS and CdS-CuxS with intermediate work function values as well as the observed narrowing of the work function spread under illumination are important for understanding the fundamental process of charge separation and junction formation in semiconductor nanorod solar cells.