A process for producing multi-junction solar cells involves forming first and second pin solar cells with an intermediate tunnel junction, which connects the two cells and which consists of the n-layer of one cell, the p-layer of the other cell and an interposed tunnel junction layer. The tunnel junction layer is formed by carrying out (a) plasma etching of the surface of one of the doped layers in the tunnel junction in the absence of the previous surface deposition starting material; and then (b) nucleation and growth on the etched layer to form a microcrystalline tunnel junction layer by plasma-assisted CVD with a dopant and a diluted starting material. Also claimed is a process for producing multi-junction tandem solar cells by a plasma-assisted CVD process involving (a') depositing a conductive tin or zinc oxide layer on a transparent or translucent glass substrate; (b') forming a first pin solar cell by successively depositing p-, i- and n-layers; (c') subjecting the n-lay er to hydrogen plasma etching in the absence of silane for 150-750 secs., without affecting the optical and electrical properties of the layer; (d') nucleating a 50-120 thick, n-doped microcrystalline tunnel junction layer with a n-doped hydrogen plasma and a silicon-containing starting material; (e') forming a second pin solar cell by depositing p-, i- and n-layers; and (f') positioning a conductive back face plate e.g. of aluminium or silver on the n-layer of the second cell. Additionally claimed is a solar cell production process involving (a") producing solar cell p-, i- and n-conductive semiconductor layers by plasma-assisted CVD from a diluted starting material; (b") plasma etching a surface of the doped layers with the diluent in the absence of the starting material; and (c") carrying out nucleation and growth of a microcrystalline semiconductor layer on the plasma etched layer. Further claimed are (i) multi-junction photovoltaic solar cell components which may be in the form of a tandem solar cell; and (ii) a photovoltaic component with one or more solar cells.    ADVANTAGE -   The processes allow production of improved multi-junction or single junction, microcrystalline silica solar cells exhibiting improved conversion efficiency, filling factor and open circuit voltage.