Solar energy offer the promise of a reliable, renewable source of energy. However, solar energy is currently infeasible as a primary energy source because of its relatively high cost compared to nonrenewable energy sources. [1] One factor contributing to this cost is the inefficiency of solar cells. Typical cells operate at around 15-20% efficiency. [2] Traditionally, it was believed that the efficiency of solar cells was limited, due to the process by which photons of sunlight are converted to electrical current. However, recent research has found that these limitations may not in fact exist, potentially leading the way to significantly more efficient, and therefore inexpensive, harvesting of solar energy.
When a photon of sunlight hits a solar cell, the energy of the photon is transferred to an electron in the semiconductive material of the cell. This energy causes the electron to be released from the atom of the material. The released electron and the hole in the atom's structure left behind are called an exciton.
In theory, the energy of a single photon from the sun has the ability to cause more than one exciton to form. In a process called impact ionization, it is possible for the first electron to hit another electron as it moves, transferring some of the energy to that electron and causing it to be released from the atom as well. [1] This process can continue as long as there is enough energy left to create another exciton.
In traditional solar cells, scientists have been unable to apply this idea. Each photon from the sun is only able to create one exciton; the excess energy is dissipated in the form of heat as the electron collides with other parts of the atom. However, recent research has shown that multiple exciton generation (MEG) is in fact possible. [1] Rather than use regular semiconductor materials, researchers use quantum dots - small pieces of semiconductive material on the order of 8 nanometers wide. Because quantum dots are so small, they confine the electron's motion, allowing the designer of the solar cell to make the electron collide with another, creating another exciton.
Researchers at the National Renewable Energy Laboratory have successfully created seven excitons from a single photon similar to those produced by the sun. From this, they estimate that by using quantum dots in solar cells, we can achieve efficiencies of around 42%. [1] In spite of these promising results, questions remain as to the exact process behind MEG and whether it can actually be applied to create usable solar cells. [3]
In 2012, researchers demonstrated a solar cell that achieved an external quantum efficiency (EQE) of 114%. [4] The EQE of a solar cell measures the ratio of electrons released to photons that hit the cell. Thus, an EQE exceeding 100% shows that some photons must be responsible for generating more than one exciton. As these results were demonstrated on a functional solar cell, MEG may in fact allow us to create highly-efficient solar cells, decreasing the cost of solar energy enough to compete with nonrenewable energy sources.
© Michael Chang. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] A. Aguilar, "Solar Power Conversion," Physics 240, Stanford University, Fall 2011.
[2] P. Weiss, "Quantum-Dot Leap," ScienceNews, 3 Jun 06.
[3] R. F. Service, "Shortfalls in Electron Production Dim Hopes for MEG Solar Cells," Science 322, 1784 (2008).
[4] J. Miller, "Multiple Exciton Generation Enhances a Working Solar Cell," Physics Today 65, No. 2, 17 (February 2012).