The method to reach low-cost thin-film solar cells studied here involves the deposition of a thin layer of crystalline Si directly on cheap foreign substrates with CVD at high temperatures. Our investigation deals with controlling physical phenomena such as nucleation. layer growth and dopant diffusion to achieve specific properties enabling better performance. effectively designing both the polysilicon material and the device structure. The required grain size is obtained by depositing small isolated crystallites with the right surface density on the substrate and subsequently growing a closed layer from these nuclei. To reduce recombination velocity at grain boundaries the effect of the growth rate and post-deposition treatments are investigated. The high second diode current that usually plagues such devices can be reduced by improving the material quality and optimising the base doping level. We introduce the TREBLE concept in which carrier collection relies on the presence of deep peaks of preferential doping at grain boundaries.