Overcoming theoretical limits on solar cell capacity
Solar photo voltaics have inherent limitations which are the subject of research.
In photovoltaics – i.e. the study of conversion of light energy into electrical energy — the ShockleyQueisser limit is a theoretical concept that defines the maximum capacity of a solar cell to produce electricity.
The Sun is the primary source of light and energy on the earth. Photovoltaic devices like solar cells allow us to harness this energy. Photovoltaic cells are made of semiconducting materials like (doped) silicon. When sunlight interacts with a semiconductor, it excites electrons from the lowerenergy valence band to the higherenergy conduction band.
This transition leaves behind a vacancy in the valence band called a hole. (To be more precise, a hole is a vacant site where an electron is supposed to be. Since it denotes the absence of an electron, a hole is also a place with positive charge.)
The process of an electron moving to the conduction band and leaving a hole behind in the valence band creates an electronhole pair. These pairs are the fundamental charge carriers in
Researchers have been trying to surpass the ShockleyQueisser limit and use more solar energy, and thus improve the cells’ efficiency, but this has been easier said than done
semiconductors and play a crucial role in the operation of electronic devices.
In short, electronhole pairs create the photocurrent — an electric current created as a result of radiation — in the semiconductor.
We know that the efficiency with which a solar cell can produce an electric current when sunlight is incident on it can’t be 100% because some light particles (photons) pass through the material without interacting with it (i.e. transparency loss, around 25%) and some energy simply heats up the material without exciting the electrons (thermalisation, around 30%). As a result, the maximum efficiency of a conventional solar cell is confined, and this range is called the ShockleyQueisser limit. It is named after the physicists William Shockley (of the U.S.) and HansJoachim Queisser (Germany).
These days, a solar cell can convert only a third of the incident solar energy into electric energy. The semiconductor can’t make use of photons with less energy than that required by electrons to jump across the band gap. Similarly, photons carrying significantly more energy than the size of the band gap only heat the device.
Researchers have been trying to find ways to surpass the ShockleyQueisser limit and use more solar energy, and thus improve the cells’ efficiency, but this has been easier said than done.
South Korean physicist Young Hee Lee has said carrier multiplication and hot carrier extraction are two promising pathways. In the former, a cell allows a photon to create multiple electronhole pairs. The latter aims to quickly capture photons with ‘too much’ energy before they dissipate as heat.