The efficiency of planar perovskite solar cells (PSCs) with SnO2 as electron transport layer is already more than 19% achieved under controlled atmosphere. PSCs with solution processed SnO2 show high hysteresis and low fill factor. One way to improve the planar PSCs is using buffer layer between electron transport layer and perovskite to enhance the photo-electron extraction process. In this study, SnO2 and SnO2/CdS layers were fabricated by solution process using a suspension including CdS nanoparticles synthesized via a simple solution route. Then planar PSCs with the structure of Glass/FTO/ETL/Perovskite/Sprio-OMeTAD/Au were fabricated in ambient air condition using SnO2 and SnO2/CdS as ETL. It is shown that a thin interface layer of CdS nanoparticles on top of SnO2 layer consistently improves the electron transporting properties of SnO2 layer. Mott-Schottky analysis shows a gradual change of electron affinity takes place by deposition of CdS nano particles. CdS interface layer can act as an intermediate step to facilitate electron transfer from perovskite layer to SnO2. The hysteresis index reduces from 0.17 to 0.05 and the efficiency improves from 15.0% to 17.18%. Impedance spectroscopy indicates that interface resistance is reduced by incorporating CdS nanoparticles.