Systematic Studies were carried out to optimize microcrystalline (pc-Si:H) pin and tandem ("micromorph") pinpin solar cells entirely prepared by HWCVD at low substrate temperature (T-s < 180 degrees C). A hydrogen treatment after deposition of the p-layer was found to improve the quality of TCO-p-i interface of mu c-Si:H pin cells. The best p-i-n mu c-Si:H cell prepared on textured ZnO substrates (with simple Ag back contact and an i-layer thickness of 0.9 mu m) showed an initial conversion efficiency of eta = 6.7%. First attempts to prepare tandem pinpin cells show low V-OC values with a s-shape of the I-V characteristic indicating that the n/p recombination tunnel junction does not work properly. Different p-layers and hydrogen treatment were used to improve the quality of the junction. However until now, only when an ITO layer is deposited between n- and p-layer we obtained a reliable n/p recombination junction. The best "micromorph" cell obtained so far had an initial conversion efficiency of eta = 6.4%. However the "micromorph" cell still degraded due to the structural instability. (c) 2005 Elsevier B.V. All rights reserved.