Now that we’ve studied carrier transport, and the way in which current flows through doped semiconductors, we can now discuss how semiconductors absorb photons from sunlight and generate free charge carriers. This involves a study of the behavior of charged particles within the crystal lattice upon photon interaction, and an analysis of the relationship between the energy levels in various semiconductor materials. By discussing the dynamics of photon capture and not merely thermal excitations, we can then begin to understand how sunlight in particular generates a usable current. In this section, we will cover absorption and generation; although absorption and recombination are intimately related, the latter will be covered in the next section.
Absorption in semiconductors occurs, of course, when an electron absorbs a photon. Often the electron is excited into the conduction band, leaving a hole in the valence band1. Absorption therefore often leads to generation, or the production of free charge carriers (we call the excited electron and resulting hole an electron-hole pair). Absorption in the case of solar cells usually leads to generation, but it doesn't always (such processes are discussed in Other Absorption Processes). The process of absorption does have some peculiarities, however we can generally treat the electrons as we do in empty space with the modification of an effective mass. Photons, which can have a substantial energy but very small momentum (where photon momentum is h/λ and photon energy is hc/λ, where λ is the wavelength of the light and c is the speed of light), impart a substantial energy to an electron by exciting it, whereas it gives negligible momentum to the crystal. This crystal momentum will be discussed in the following pages.
Of course, not all photons are absorbed. Photons, depending on their energy/corresponding wavelength, as well as on the thickness and type of semiconductor material, have various options: photons can be transmitted and passed right through the semiconductor (in which case the semiconductor is considered transparent), they can be reflected from the top surface, or they can be absorbed. Transmitted or reflected light is considered a loss, and therefore optical science is extremely important for minimizing the percentage of reflected light. For wavelengths of interest in solar work, over 30% of light is reflected from silicon, a number which greatly hinders our efficiency2.
Below are the basic possibilities of a photon’s fate based on its energy, where EP is the energy of the incoming photon and EG is the energy required to cross the band gap3:
- EP < EG : The photon’s low energy/long wavelength makes the semiconductor transparent, and it passes right through.
- EP = EG : The photon has just enough energy to excite an electron to the conduction band, so there is little wasted energy.
- EP > EG : The photon has more than enough energy to excite an electron; therefore, any electron that is excited quickly gives off that energy as heat and returns to the conduction band edge.
Above: An incoming photon is absorbed, exciting an electron to the conduction band (making it a free charge carrier) and thus generating an electron-hole pair. Because this semiconductor device has a P-N junction, where there is a P-type material next to an N-type material, there is a potential difference. This electric field then causes electrons to move to the N-type and holes to the P-type.
1. Goetzberger, Adolf et.al. Crystalline Silicon Solar Cells. Chichester: John Wiley & Sons Ltd., 1998.
2. Green, Martin A. Solar Cells: Operating Principles, Technology, and System Applications. Englewood Cliffs: Prentice-Hall, Inc., 1982.Full book ordering information at www.pv.unsw.edu.au.
3. Honsberg, Christiana, and Stuart Bowden. "Absorption of Light.” PVEducation.org. Web. 28 July 2012. <www.pveducation.org/pvcdrom/p...ption-of-light>