We report on recent developments of a-Si:H/a-Si:H based thin film solar cells on Gen8.5 module area. The initial efficiency of 5.7 m2 modules and 1.4 m2 sub-modules has been increased from 5% to 8% using standard floatline, low-cost glass substrates coated with SnO2:F front TCO. The improvements based on optimizations of the n1/p2 tunnel recombination junction and of the p/i interfaces. The device simulation software AFORS-HET has been extended with an improved model for the n/p junction to describe measured module properties. Model input parameters are taken from measured properties of single thin films. The presented progress indicates the high potential of thin-film silicon based solar cells produced on large area. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.