Techniques Used for producing high efficiency solar cells
To produce the high efficiency solar cells, some of the design features and techniques used in fabrication of silicon solar cells in the laboratory include:
- Use of lightly phosphorus diffused emitters, so as to minimize recombination losses and avoid the creation of a dead layer at the cell surface
- narrowly spaced metal lines, so as to minimize the lateral resistive power losses of the emitter
- really fine metal lines, usually with a width less than 20 µm, so as to minimize the shading losses
- polished or lapped surfaces that can allow patterning of top metal grid via photolithography
- small area devices and good metal conductivities, which minimize the resistive losses in the metal grid
- heavy doping at the surface of the silicon beneath the metal contact and low metal contact areas which minimize recombination;
- use of titanium/palladium/silver or similar elaborate metallization schemes, that provide very low contact resistances
- good passivation of the rear surface, which reduces recombination
- using anti reflection coatings, that can reduce surface reflection from 30% , all the way to well below 10%.
High efficiency solar cells technologies
There are various existing cell designs, which incorporate the advanced laboratory features. Two approaches which have been used in niche markets like solar cars are the University of New South Wales produced PERL cells, and Stanford University and SunPower produced rear contact cells.
PERL Solar Cells
PERL cells are the passivated emitter with rear locally diffused cells. They use micro electronic techniques that produce high efficiency solar cells approaching 25% under the standard AM1.5 spectrum. The high quality oxide at the front surface which significantly lowers the number of carriers recombining at the surface is the passivated emitter. The rear emitter is locally diffused only at the metal contacts so as to minimize the recombination at the rear while maintaining good electrical contact.
Another way for increasing cell efficiency is by decreasing the surface recombination at the rear of the cell. The passivated emitter and rear contact (PERC) cell reduces rear recombination by placing a patterned dielectric layer between silicon and aluminum. So, aluminum only comes in contact with a small fraction of the cell area. The extra SiO2 or Al2O3 dielectric layer reduces electron surface recombination considerably, which results in increased cell efficiency.
Rear Contact cells
The highest power conversion efficiencies of silicon wafer based solar cell have been achieved with the IBC (interdigitated back contact) architecture. IBC solar cells require striped (interdigitated) doping on the rear surface and have contacts only on the rear. This doping can be realized by masked diffusion, laser doping or masked ion implantation. Then the solar cells are metallized by formation of metal fingers along each diffused region.
Other than the TOPCon (Tunnel Oxide Passivated Contact) and PERL solar cell architectures, this architecture of silicon wafer based solar cell structure is the only one to achieve or improve on efficiency of 25 %.