Chapter 8. ORGANIC SOLAR CELLS - from and for SET students

Chapter 4
III-V Technologies
III-V Technologies are solar cells with high efficiencies due to high purities and multi junction technologies.
These films are very expensive though. The main applications are space and concentrated PV.
4.1 Theoretical limits of performances solar cells
Photons incident on the surface of a semiconductor will be either reflected from the top surface, will be absorbed
in the material or, failing either of the above two processes, will be transmitted through the material. For
photovoltaic devices, reflection and transmission are typically considered loss mechanisms as photons which are
not absorbed do not generate power. If the photon is absorbed it will raise an electron from the valence band
to the conduction band. A key factor in determining if a photon is absorbed or transmitted is the energy of the
photon. Photons falling onto a semiconductor material can be divided into three groups based on their energy
compared to that of the semiconductor band gap.
� Eph < EG: Photons with energy Eph less than the band gap energy EG interact only weakly with the
semiconductor, passing through it as if it were transparent.
� Eph = EG: Just enough energy to create an electron hole pair, efficiently absorbed.
� Eph > EG: Photons with energy much greater than the band gap are strongly absorbed, subsequently
thermal relaxation
The bandgap is the point from which a solar cell starts to absorb, energy below it is lost. Another part of
the photon energy is lost as heat (area above cell).
Figure 4.1: Spectral Mismatch
Photon utilization efficiency is the fraction of energy of photon which is used to create the electron
whole pair.
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