In the present study, Kelvin probe force microscopy (KPFM) has been used to study a prototype CdS-CZTS lengthwise single nanorod heterojunction. A shift in surface potential (SP) of the entire sample is observed as the device is taken from dark into light. It has been demonstrated that plots showing the number of pixels with different values of surface potential as a function of the SP itself, can be effectively used to topographically segregate regions composed of different semiconductors. On illumination, a new SP peak emerges and based on its topographical position and SP value, both of which lie in between that of CZTS and CdS, it has been assigned to the heterojunction interfacial layer. These observations have been discussed in terms of junction quality and charge generation and separation to gain a better understanding of the operation of nanorod based solar cell devices. (C) 2015 Elsevier B.V. All rights reserved.