Initial JV parameters for filtered mc-Si:H bottom cells (diamonds, primary axis) and a-Si:H/mc-Si:H tandem cells (squares, secondary axis) as a function of NCZnO filling layer thickness on a 1.8 mm thick base layer (B2H6/DEZ ¼ 4.2 Á 10 À 4). (a) open circuit voltage (Voc), (b) fill factor (FF), (c) short circuit current density (Jsc) and (d) conversion efficiency (η). All data points show average values obtained from 8 submodules per experiment.