Alternative methods for pn-definition for fabrication of crystalline silicon solar cells

In this PhD thesis alternative pn-definition methods for the fabrication of crystalline silicon solar cells have been investigated. POCl3-emitter formation via the gas phase leads to short circuits in the contact region between the emitter and alloyed aluminium. POCl3-diffusion leads to a doping of the whole surface of the silicon wafer. By alloying the aluminium, deposited later on, at temperatures over 800°C, the emitter is locally overcompensated. During subsequent cooling, the highly doped p+-layer and a metallic back contact of eutectic composition are formed. The reason for the low shunt resistance is a direct connection along the wafer surface between the base contacting eutecticum and the neighbouring emitter.In principle there are two different methods to avoid short circuit formation: either by a local deposition of a diffusion prohibiting medium before emitter formation - or subsequently by removing the emitter at defined regions. These additional procedures become costly, particularly for solar cells with interdigitated p- and n-type doping regions as with Emitter Wrap Through (EWT) solar cells. The main focus of the work presented in this thesis is the Al/P-codiffusion process, leading to shunt resistances of more than 10kOhmcm2 without additional processing steps for junction isolation. The codiffusion process has proved to be successful with both mono- and multicrystalline silicon wafers, to be independent of the deposited Al layer thickness and the choice of the diffusion media.The independence of the shunt resistances from the length of the alloyed aluminium grid opens the possibility to use Al/P-codiffusion for pn-definition during the fabrication of solar cells with interdigitated p- and n-type doping regions.The main advantage of the codiffusion process is the significant simplification of the solar cell fabrication because three separate processing steps of the standard sequence - emitter formation, BSF formation and pn-junction isolation - can be realized in one single high temperature step.