Can a solar cell be transparent?

The answer is yes, but you wouldn’t particularly want to unless you have some application like solar windows. The sun is a black body radiator at 5777 K. For a solar to be transparent it has to allow the visible spectrum (380 to 750 nm) through, but absorb the rest.

If we look at where the energy from the sun lies, you get

• 10% in the short wavelength
• 44% in the visible
• 46% in the long wavelength

So there is about just as much energy in the infrared and longer as there is in the visible, but the peak energy is in the visible. The way inorganic solar cells work is that they start absorbing at a certain wavelength and lower wavelengths with less efficiency.

MIT is working on solar cells that can be integrated into windows. Here’s a treehugger article.

One thing that decreases long wavelength energy production is that molecules in the atmosphere absorb certain frequencies as vibrational modes.