Air Mass (solar Energy) - Solar Cell Efficiency

Solar Cell Efficiency

Silicon solar cells are not very sensitive to the portions of the spectrum lost in the atmosphere. The resulting spectrum at the Earth's surface more closely matches the bandgap of silicon so silicon solar cells are more efficient at AM1 than AM0. This apparently counter-intuitive result arises simply because silicon cells can't make much use of the high energy radiation which the atmosphere filters out. As illustrated below, even though the efficiency is lower at AM0 the total output power (Pout) for a typical solar cell is still highest at AM0. Conversely, the shape of the spectrum does not significantly change with further increases in atmospheric thickness, and hence cell efficiency does not greatly change for AM numbers above 1.

Output power vs airmass coefficient
AM Solar intensity Output power Efficiency
Pin W/m2 Pout W/m2 Pout / Pin
0 1350 160 12%
1 1000 150 15%
2 800 120 15%

This illustrates the more general point that given that solar energy is "free", and where available space is not a limitation, other factors such as total Pout and Pout/$ are often more important considerations than efficiency (Pout/Pin).

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