High Efficiency Solar Cells

High efficiency cells can cost considerably more to produce than standard silicon cells and are typically used in solar cars or space applications.

Honda dream, the winning car in the 1996 World Solar Challenge. The custom made cells for the car were greater than 20% efficient, which was quite high for that time. (Photograph PVSRC)

Some of the techniques and design features used in the laboratory fabrication of early silicon solar cells to produce the highest possible efficiencies included:

1. lightly phosphorus diffused emitters, to minimise recombination losses and avoid the existence of a "dead layer" at the cell surface;
2. closely spaced metal lines, to minimise emitter lateral resistive power losses;
3. very fine metal lines, typically less than 20 µm wide, to minimise shading losses;
4. polished or lapped surfaces to allow top metal grid patterning via photolithography;
5. small area devices and good metal conductivities, to minimise resistive losses in the metal grid;
6. low metal contact areas and heavy doping at the surface of the silicon beneath the metal contact to minimise recombination;
7. use of elaborate metallization schemes, such as titanium/palladium/silver, that give very low contact resistances;
8. good rear surface passivation, to reduce recombination;
9. use of anti-reflection coatings, which can reduce surface reflection from 30% to well below 10%.

Early cell designs incorporated advanced laboratory features. Two approaches that were used by niche markets such as solar cars were 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 used 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.

Schematic of high efficiency laboratory cell.

Electron microscope image of the top surface of a PERL Cell showing a broken electroplated finger. The total finger width is less than 20 µm and the contact width is 3 µ>m.

A cell for a solar car in the 1990s had the following characteristics:

Area: 22 cm²
Efficiency: 23.5%
V_oc: 703 mV
I_sc: 914 mA
J_sc: 41.3 mA
V_mp: 600 mV
FF: 0.81
I_mp: 868 mA

IV curve for a solar car cell.

Today, PERC cells are the most common commercial cells, but a number of advanced cell designs are being explored for efficiencies > 25%.