TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The formulation additionally comprises at least one germanium compound, so that the layer formed comprises germanium in such a way that at least partially polymorphic and preferably a layer comprising silicon-germanium is present. The germanium-compound is a germanium-hydrogen compound of formula (GenH2n+2 or GenH2n), a germanium-halo, a germanium-organyl and/or an oligomeric germanium compound of formula (Gen1R2n1+2 or GenR2n1) or a mixture of germanium-silicon-hydrogen-organyl-compound, preferably the germanium-hydrogen compound with n of 4-8 and/or the oligomeric germanium compound with an average molecular weight of 500-10000 g/mole, preferably 800-5000 g/mole. The proportion of germanium in the formulation amounts to 0.5-15 mole-% based on the proportion of pure silicon and germanium. The proportion of germanium in the layer preferably comprising silicon-germanium amounts to 0.5-15.0 mole-% based on the proportion of pure silicon and germanium. The silicon-compound is a silicon-hydrogen compound of formula (Sin2H2n2+2 or Sin3H2n3), a silicon-halo, a silicon-organyl and/or an oligomeric silicon compound of formula (Sin1R2n1+2 or Sin1R2n1). The silicon- and germanium-containing formulation is a liquid formulation, which optionally includes a solvent, where the viscosity of the coating solution is 200-2000 mPas.n = 1-10;R = H, halo or organyl;n1 = 8-100;n2 = 3-10; andn3 = 4-8.Preferred Method: The solar cell is produced by providing a substrate, providing a formulation comprising at least one silicon compound, coating the substrate with the formulation, irradiating and/or thermally treating the coated substrate to form at least a partially polymorphic and preferably layer comprising silicon. The silicon-and germanium-containing formulation is prepared by oligomerization and/or polymerization of a mixture comprising at least one silane (Sin2H2n2+2 or Sin3H2n3), at least one germanium compound (GenH2n+2 with n of 3-10 or GenH2n with n of 4-8) optionally a solvent, dopant and/or other auxiliary material, preferably by irradiation and/or thermal treatment of the mixture. The silicon and germanium-containing formulation before, during and/or after the coating is optionally added with solvent, dopant and/or other auxiliary materials. The coating of the substrate is carried out by using spin-on deposition, casting, sputtering from the liquid phase, scraper application or roll coating. The coated substrate is irradiated or thermally treated at 300-1000 degrees C, preferably 500-800 degrees C. EXAMPLE - Cyclopentasilane (10.1 g) and phenyl-substituted silicon-germanium compound (PhH2GeSiH3) (0.76 g) in an open vessel were irradiated with UV lamp for 35 minutes. The high-molecular portion contained oligomers of the cyclopentasilane, into which partial germanium was embedded by co-oligomerization and the mixture contained the residue of monomer cyclopentasilane and unreacted PhH2GeSiH3. The mixture was diluted with 3 parts of toluene and was applied on a pre-purified quartz plate using a spin coater. The layer was heated to 500 degrees C using a hot plate. The layer was annealed for 30 minutes in a furnace under inert gas at 750 degrees C to obtain a dark silicon-germanium layer.