Selective p+ emitter formation for n-type silicon solar cells

Presently, the majority of commercially available PV modules  are made using front junction silicon solar cells based on p-type Czochralski (Cz) substrate with full aluminum BSF and more recently with local rear contacts (PERC – Passivated Emitter and Rear Contact - cells). This transition is propelled by the need to reduce metallization related recombination and optical losses on the rear side of the cell thereby improving the efficiency. In order to reduce recombination losses from Ag front metallization, various methods to form phosphorus based selective emitter have been implemented in p-type solar cells. These selective emitter technologies feature heavy phosphorus doping under screen printed contacts and shallow doping elsewhere. These  offer the advantages of having both a low specific contact resistivity and low recombinations for passivated surface and metal contacts without compromise.

The next technology transition towards cells based on  n-type substrates is starting. Solar cells based on n-type substrates offer advantages such as absence of light induced degradation, higher tolerance to many transition metal impurities (including Fe, Ti, V, Cr) and much lower sensitivity  to  the presence of the laser induced dislocations. The share of n-type silicon based solar cells is expected to increase in the coming decade according to the International Technology Roadmap for photovoltaics (ITRPV). For front junction n-PERT solar cells, a cost effective and simple selective emitter technology for boron doping needs to be developed. A new idea (currently under study for a potential patent application) will be investigated to obtain both a highly doped p++ region in the contact area and a more lowly doped p+ region in the passivated area in a simple, industrially applicable manner. Various parameters of these 2 different regions will be characterized. These include:

1. Sheet resistance of highly and lowly doped boron regions
2. Recombinations of passivated regions characterized by dark saturation current density,
3. Contact resistivity to highly doped boron regions
4.  Recombinations of metallized regions characterized by dark saturation current density