High efficiency cells cost considerably more to produce than standard silicon cells and are typically used in solar cars or space applications.
Some of the techniques and design features used in the laboratory fabrication of silicon solar cells, to produce the highest possible efficiencies include:
- lightly phosphorus diffused emitters, to minimise recombination losses and avoid the existence of a "dead layer" at the cell surface;
- closely spaced metal lines, to minimise emitter lateral resistive power losses;
- very fine metal lines, typically less than 20 µm wide, to minimise shading losses;
- polished or lapped surfaces to allow top metal grid patterning via photolithography;
- small area devices and good metal conductivities, to minimise resistive losses in the metal grid;
- low metal contact areas and heavy doping at the surface of the silicon beneath the metal contact to minimise recombination;
- use of elaborate metallization schemes, such as titanium/palladium/silver, that give very low contact resistances;
- good rear surface passivation, to reduce recombination;
- use of anti-reflection coatings, which can reduce surface reflection from 30% to well below 10%.
A number of cell designs exist which incorporate advanced laboratory features. Two approaches that have been used by niche markets such as solar cars are the PERL cells produced at University of New South Wales, and the rear-contact cells developed at Stanford University and SunPower.
PERL Solar Cells
The passivated emitter with rear locally diffused (PERL) cell uses micro-electronic techniques to produce cells with efficiencies approaching 25% under the standard AM1.5 spectrum. The passivated emitter refers to the high quality oxide at the front surface that significantly lowers the number of carriers recombining at the surface. The rear is locally diffused only at the metal contacts to minimise recombination at the rear while maintaining good electrical contact.
While expensive to fabricate, it is possible to produce cells with very high efficiencies. A cell for a solar car had the following characteristics:
Area: 22 cm2
Voc: 703 mV
Isc: 914 mA
Jsc: 41.3 mA
Vmp: 600 mV
Imp: 868 mA