Single-chamber processes for the deposition of high efficiency thin-film silicon tandem cells of an a-Si:H p-i-n (top cell)/mu c-Si:H p-i-n (bottom cell) structure involving short fabrication time are reported. An industry relevant reactor and an excitation frequency of 13.56 MHz were used. The conversion efficiency is found to be highly sensitive to dopant cross contamination into the mu c-Si:H i-layer of the bottom cell and within the n/p-interface of the tunnel recombination junction (TRJ). Different reactor treatments at the p/i-interfaces of the top and bottom cells and at the n/p-interface of the TRJ were applied, aiming to prevent dopant cross contamination. The phosphorus and the boron concentrations were evaluated by secondary ion mass spectrometry measurements. Phosphorus cross contamination after TRJ n-layer deposition is found to result in significant n-type doping of the mu c-Si:H i-layer of the bottom cell if no reactor treatment is applied. In situ reactor treatment via an Ar flush and pumping step of 15 min applied at the n/p-interface of TRJ results in reduction of the mu c-Si:H i-layer phosphorus concentration to values below 10(17) cm(-3). A conversion efficiency of 11.8% for such tandem cells is demonstrated. Shorter interface treatment time with phosphorus concentrations in the mu c-Si:H i-layer of about 5 x 10(17) cm(-3) results in lower conversion efficiencies of 10.6%, mainly due to the decrease of open-circuit voltage and fill factor. (C) 2013 Elsevier B.V. All rights reserved.