At present, p-type silicon is the main type of raw material for industrially produced solar cells. Although n-type silicon has better electrical properties, the most widely established, standard technology for cell production is for p-type Si. The combination of already widespread production processes with n-type material enables new, promising approaches for simply manufactured, highly efficient solar cells. In contrast to the complex cell structures on n-type Si which can already be found on the market, the concepts developed by Fraunhofer ISE now open the door to cost-effective production of n-type Si solar cells.
Most of the solar cells sold around the world are manufactured of crystalline p-type silicon (Si). However, the material itself is already a limiting factor today for good, industrially produced cells. At present, the highest efficiency values are achieved with cell structures on intrinsically higher-quality n-type Si, but after applying special or complex processes.
The easily implementable cell concepts developed at Fraunhofer ISE combine the well-proven production technology for p-type Si solar cells with n-type Si material. Aluminium-doped (Al) p+ layers as the emitter on the back surface of the cell are decisive in this concept. They are prepared from screen-printed, metallic pastes containing Al in a short, high-temperature firing step by alloying into the Si surface, and form the Al back surface field (BSF) in conventional p-type cells. The application of Al-alloyed emitters thus enables the production of highly efficient n-type cells on the basis of established production processes.
If the well-known PERC (Passivated Emitter and Rear Cell) technology from p-type Si solar cells is transferred to n-type Si material, a cell structure results which features many small local Al emitter regions, which are alloyed into the passivated Si back surface of the cell (Fig. 2). The structural and electrical properties of these point-formed or linear emitters were significantly improved by Al pastes that had been further developed at Fraunhofer ISE. As a result, our first large-area (144 cm2), n-type cells prepared in this way (Fig. 1) already achieve a peak efficiency value of 19.7%.