In order to reduce the costs of thin-film silicon solar cell production, a manufacturing concept in which amorphous silicon (a-Si) solar cells are produced in a roll-to-roll manner on a temporary metal substrate (temporary-superstrate process) has been introduced in the Helianthos Project. Later in the process the device is transferred to a permanent plastic substrate on which cells are monolithically series connected, resulting in PV modules. In the Helianthos pilot line, modules with efficiencies above 7% are obtained on 1-foot-wide foil. On 30 × 30 cm2 modules, initial efficiencies of 5.7% have been reached, while 60 cm2 modules cut from the 35 cm wide foil have reached stabilized efficiencies of 5.8%. Lab modules with the deposition of a-Si in a batch process on roll-to-roll deposited SnO2, have reached stabilized efficiencies of 6.7%. In order to increase efficiency, amorphous/microcrystalline (a-Si/μc-Si) tandem solar cells have been implemented in the temporary-superstrate process to fabricate flexible, lightweight tandem modules with monolithic series integration. A first module with an aperture area efficiency of 8.6% has been presented earlier. At present, an initial aperture area efficiency of 9.4% has been reached on a 60 cm 2 module. © 2006 IEEE.