Decoupling Charge Carrier Collection and Metallization Geometry of Back-Contacted Back-Junction Silicon Solar Cells

In this thesis n-type back-contacted back-junction silicon solar cells have been investigated and optimized. Although these solar cells have no metal contacts on the front side and therefore no optical shading, the solar cells can suffer from electrical shading due to an increased recombination in the noncollecting base region on the rear side, which reduces the short-circuit current density. Therefore, special focus is put on the decoupling of the charge carrier collection, i. e. the diffused regions on the rear side of the solar cells, from the metallization geometry. This allows an optimization of the solar cell design without a trade-off between electrical shading losses and electrical series resistance losses in the metal fingers as well as in the base due to the lateral current transport. The decoupling is achieved by novel solar cell concepts where shunts due to an overlap of the positive and negative metal fingers and diffusions of the opposite polarity are avoided. Hence, electrical shading losses can be reduced by minimizing the lateral distance of charge carriers to the collecting emitter, i. e. by using silicon substrates with high diffusion length and by reducing and effectively passivating the noncollecting base region on the rear and front side. Two approaches to achieve the decoupling have been investigated in this thesis. On the one hand, a solar cell concept where the boron-doped emitter diffusion on the rear side is locally overcompensated by a phosphorus-doped BSF diffusion is analyzed. For this overcompensation approach, the emitter coverage on the rear side can be increased without inducing any shunts due to the overlap of the negative metal fingers and the emitter diffusion. On the other hand, an insulating thin film can be applied in order to avoid shunting between the negative metal finger and the emitter diffusion. For this insulating thin film approach, thin films are applied to a solar cell concept featuring a large or a small emitter coverage. For the first design nearly the whole rear side is covered by the emitter except for small point-like base regions whereas for the second design closely adjacent point-like emitter and point-like base regions exist. In the latter case almost the whole rear side consists of an undiffused base region and a small lateral distance is obtained by a small pitch distances.