412 The Photovoltaic Effect

When a photon of light is absorbed by a valence electron of an atom, the energy of the electron is increased by the amount of energy of the photon. If the energy of the photon is equal to or more than the band gap of the semiconductor, the electron with excess energy will jump into the conduction band where it can move freely. If, however, the photon energy is less than the band gap, the electron will not have sufficient energy to jump into the conduction band. In this case, the excess energy of the electrons is converted to excess kinetic energy of the electrons, which manifests at increased temperatures. If the absorbed photon has more energy than the band gap, the excess energy over the band gap simply increases the kinetic energy of the electron. Note that one photon can free up only one electron, even if the photon energy is greater than the band gap. This is a key limiting factor to PV cell efficiency.

Figure 4.6 shows a schematic diagram of a PV device and its load circuit. As free electrons are generated in the n layer by the action of photons, they can either pass through an external circuit, recombine with positive holes in the lateral direction, or move toward the p layer. The negative charges in the p layer at the p-n junction restrict their movement in that direction. If the n layer is made extremely thin, the movement of the electrons and, therefore, the probability of recombination within the n layer is greatly reduced unless the external circuit is open. If the external circuit is open, the electrons generated by the action of photons eventually recombine with the holes, resulting in an increase in the temperature of the device.

f photon

+ o+ o+\o +o 4

External S

P - type

Load <

Simple circuit showing PV cell and resistive load.

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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