## Photoexcitation

### Light Energy

Light is a form of energy. The frequency of light is directly proportional to energy and the wavelength of light is inversely proportional to energy. If we use kilojoules as our energy unit and measure the wavelength of light in nanometers, the proportionality constant is about 2 x 10-19.

Below is the spectrum of visible light along with the wavelength. The higher energy (blue) wavelengths are on the left and the lower energy (red) wavelengths are on the right.

 Earlier in the semester we discussed the energy in light from the sun. The yellow curve in the figure at left shows how the intensity of solar energy varies with wavelength of light. The maximum of solar radiation is centered around visible light.

### Silicon Light Absorption

 Silicon is a semiconductor with a band gap of 1.1 eV or 1100 nm. This means that light of 1100 nm is the lowest energy of light that can cause an electron from the valence band to be promoted to the conduction band. This wavelength is too low in energy for us to see. Silicon can absorb higher wavelengths of light as well. It can absorb light that corresponds to the energy difference between any molecular orbital in the valence band and the conduction band.

Now let's consider what happens when visible light is absorbed by a crystal of silicon with a few atoms of aluminum diffused into one side (p-doped) and a few atoms of phosphorus diffused into the other side (n-doped). An electrode on one side is connected to a wire that passes through a voltmeter, to measure the potential difference, to some load and then back to an electrode on the other side of the crystal. The load could be a light bulb, for example.

When visible light strikes the crystal, electrons from the valence bands of both the n-doped and p-doped silicon are transferred to the conduction band of the material.

The presence of the n-p junction causes electrons to flow in one direction. This is because the energy of the conduction band of one is a little lower than the other. Electrons flow so as to lower their overall energy and this small amount of energy is given up as heat. The electrons from both types of semiconductors travel through the wire in one direction.

The electrons travel through the wire through the voltmeter to the load. The electrons can lose their excess energy to the load (lighting the bulb) and then travel back to the crystal. At the crystal, holes travel to the electrode and the electrons combine with holes to make filled valence orbitals.

Professor Patricia Shapley, University of Illinois, 2012