Results of a study on the interdiffusion of Al and alpha-Si:H, occurring in cells grown on a KAPTON/Al substrate, are presented. Cell parameters of a series of cells grown at temperatures ranging from 100 degrees C to 200 degrees C are presented for KAPTON/Al and KAPTON/Al/ZnO substrates, where a thin ZnO buffer layer was inserted for the latter series of substrates. It is demonstrated that the implementation of this buffer layer results in a significant increase in cell efficiency. With SIMS and SXPS depth profiles, it is demonstrated that cells grown on substrates without the ZnO layer are destroyed as a result of the interdiffusion of alpha-Si:H and Al, and that the presence of the ZnO buffer layer impedes this process. It is argued that, apart from the well-known enhanced reflectance and resulting improved cell characteristics caused by the insertion of this buffer layer, the main effect of the buffer layer (in cells grown on aluminized flexible substrates) is to impede the interdiffusion of Al and alpha-Si:H at the semi-conductor/metal interface.