Fabrication of solar cells on cheap plastics, due to its demand for a low temperature processing (~100?C) needs adaptation of the cell structure of the state-of-the-art high efficiency thin film silicon solar cells made at high temperature (200?C). Use of a double n-layer (n-?c-Si/n-a-Si) instead of a single n-?c-Si layer improves Voc from a low 0.66 V to 0.82 V in the as deposited state of an n-i-p type a-Si cell made at 100?C. In a high temperature deposited cell, where a p-type ?c-Si (band gap Eg =1.1 eV) is used in combination with a-Si i-layer (Eg =1.8 eV), a thin buffer a-Si layer made at 100?C acts as a barrier for electron back diffusion from the i-layer. For the low temperature deposited i-layers this type of buffer layer loses its advantage of band offset and fails to stop the electron back diffusion. To verify this hypothesis, a-Si n-i-p cells were made with a heavily doped amorphous silicon p-layer at high hydrogen dilution (leaving out the ineffective buffer layer). We obtained indeed a high Voc of 0.89 V in as deposited state and after annealing at 100?C the Voc reached 0.92 V. An efficiency of 6.6% was obtained for a cell made on a flat Ag/ZnO:Al back reflector (BR) on a glass substrate. A low current density of 12.5 mA/cm2 is attributed to the fact that it was a smooth BR. The high Voc of 0.92 V was reproduced in a similar cell on stainless steel foil substrate. Cells with similar structure and deposition condition were fabricated on plastic substrates, namely polyethylene naphthalene (.PEN) and polyethylene terephthalate (PET). Deposition conditions of ZnO:Al by magnetron sputtering were adapted to reduce the deposition related stress on the plastic foil while the Ag layer was thermally evaporated. A comparable Voc of 0.89 V is obtained in as deposited state and 0.92 V after annealing. The cells on PEN and PET showed initial efficiencies of 6.3% and 5.9% respectively.