The cost of photovoltaic (PV) energy is reduced by increasing solar cell power conversion efficiency and decreasing manufacture cost. An effective way of lowering the cost of Si thin film solar cells (TFSC) is to grow panels on large-area substrates. In this paper we study the effect of air residual to Si TFSC grown on 5.7 m(2) glass in plasma-enhanced chemical vapor deposition (PECVD) chambers. Structural and chemical analysis show that oxygen incorporated into the Si films behaved as impurity dopant in the hydrogenated microcrystalline Si (mu c-Si) layers and reduced the efficiency of amorphous Si (a-Si)/mu c-Si tandem junction solar cells when the film had oxygen concentration > 2 x 10(19) atoms/cm(3). Higher oxygen content further suppressed the mu c-Si crystallization. We found that hydrogen plasma treatment of process chamber before Si film deposition effectively reduced the adverse effects of air exposure and improved both film quality and solar cell performance. The hydrogen-treated chamber produced contamination-free, solar cells with consistent, initial efficiency >10%. (C) 2010 Elsevier B.V. All rights reserved.