## 17 August 2005

### Electron-volts

Just a quick comment. I am trying to discuss some issues in solar power but I find my arguments quickly degenerating into science babble.

Most people who are reading this blog probably know that the visual spectrum varies from about 700 nm (red) to 400 nm (violet). Beyond those limits are the near-infrared and ultraviolet spectrums, respectively. Unsuprisingly the majority of the energy emitted by the sun is in these wavelengths -- our eyes have evolved to see it. The Sun has a surface temperature of about 5750 K (it varies somewhat). This corresponds to a peak wavelength of about 504 nm (green).

There is a fairly simple way to relate the energy of a photon to that of an electron. Energy relates to wavelength by the formula:

E_n [eV] = 1240 nm / lambda

So a photon with a wavelength of 1240 nm in the infrared has an energy of 1 eV. An electron volt (abbreviated eV) is the energy gained by an electron accelerated through an electrostatic potential of one Volt. We can convert 1 eV = 1.6 x 10^-19 Joules. As you can see an eV is a small quantity of energy.

You'll often see band gaps in photovoltaics expressed in terms of eV. Silicon is characterized as having a band gap of 1.1 eV. In comparison the sun's peak wavelength has an energy of about 2.5 eV.

I think now is a good time to shutup.