Breaking the 100% Quantum Efficiency in Solar Cells

Researchers have found the way to convert light into electricity much more efficiently than current photovoltaic devices can achieve.


Being available in a never-ending supply, the sun is considered as one of the most powerful renewable energy sources.
When the first solar cell was introduced into the photovoltaic field, converting the energy of sunlight into electricity seemed to be a great idea of scientists. Soon after, however, it was realized that the materials used in the active layer of photovoltaic cells limit the conversion of photons in the sunlight to current, which proves the low efficiency of solar cells.
In principle, solar spectrum involves a range of photons with different energies. However, low energy photons cannot be used in energy conversion and high energy photons produce heat with the excessive energy that they carry.
A solar cell can convert only a narrow band of photons to electrons although it is exposed to all at the same time by the sunlight. The reason for that is the bandgap (or energy gap) which is equivalent to the energy required to free an outer shell electron from its orbit to become a mobile charge carrier. Device only uses photons which have energy close to bandgap, the rest is either not used or converted to heat.  
Quantum efficiency for a solar cell indicates ratio of the number of charge carriers collected by the solar cell to the number of photons hitting the device's photoactive surface. In order to calculate the current that will be produced under exposure of sunlight for a photovoltaic cell, quantum efficiency is integrated over entire solar spectrum. It is generally assumed that the device would yield the maximum energy production value, if quantum efficiency was 100% over the entire spectrum. Thus, actually quantum efficiency of the device brings a theoretical barrier to the maximum efficiency to be obtained.      
Prof. Marc Baldo and his coworkers, at Massachusetts Institute of Technology’s Center forExcitonics have recently published the results of a study conducted in Science. They have brought a solution to the challenge for obtaining high efficiency solar cell, with an organic dye pentacene, utilizing an old phenomenon called “singlet-exciton-fission”.
Pentacene can generate two excitons from a single exciton, which means increasing the energy production value for the device containing pentacene, enormously. In the process of singlet exciton fission, a high-energy excited state splits into two low-energy states, generating one extra exciton per absorbed photon, which would have been otherwise converted to thermal energy as above mentioned.
Paper claims that “Analysis of the magnetic field effect on photocurrent suggests that the triplet yield approaches 200% for pentacene films thicker than 5 nanometers”, which increases the chance for solar cells to exceed the theoretical limits assumed to be existed up to now.
As same group reviewed in Accounts of Chemical Research, it is a huge achievement in the field of organic solar cells. Nevertheless, it can also lead to boosting the efficiency of the silicon cells since singlet fission materials may possibly be integrated with them.

 written by  Derya Baran find her on Facebook
 edited by Abidin Balan find him on Facebook


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