This paper addresses the achievements and challenges in a-Si:H/μc-Si:H tandem solar cell technology including production aspects. As an example we show figures of the module production at Masdar PV, with module conversion efficiencies reaching 10%. The process integration was supported by PVcomB, namely, by developing improved PECVD processes and transferring them to Masdar PV. The important role of the TCO substrate for producing high-efficiency modules at lowest possible costs is discussed. We show the results of tandem cells on thermally annealed ZnO:Al substrates, resulting in an improvement of 0.4-0.5% (abs) as a result of the TCO annealing, reaching a stable efficiency of 12.1% for a 1 cm2 solar cell and 11.6% for a mini module. The challenges in developing thin film silicon solar modules with higher efficiency than presently reached are discussed. © 2013 Elsevier B.V. All rights reserved.