We have developed zinc oxide (ZnO) film and CO2 plasma treatment for the use as an intermediate layer between top and bottom cell in order to improve performance of micromorph silicon solar cells. The CO2 plasma treatment was performed by very high frequency plasma-enhanced chemical vapor deposition (VHF PECVD) technique, and the ZnO interface layer was deposited by DC-magnetron sputtering method. Effects of both techniques on the cell performance were comparatively investigated. We found that the ZnO interface layer and CO2 plasma treatment were effective in enhancing V-oc, J(sc) as well as FF of the cells as the same. The micromorph solar cells using an optimized ZnO interface layer and the CO2 plasma treatment indicated initial conversion efficiency of 11.4% and 11.2%, respectively. Experimental results indicated that the CO2 plasma treatment technique is more suitable for using in cell fabrication process than the ZnO interface layer since it is simpler and has no negative impact of possible shunts. (c) 2010 Elsevier B.V. All rights reserved.