None of the solar cells produced to date shows external photocurrent quantum efficiencies greater than 100 percent from any portion of the solar spectral irradiance band.
In fact, for math dummies like me, the possibility seems inconceivable, yet researchers from the National Renewable Energy Laboratory, or NREL, located in Boulder, Colorado, have reported external quantum efficiencies above 100 percent in solar cells “excited” with photons from the high-energy region of the solar spectrum. That is, from the near ultraviolet (350-400 nanometers) through the visible light spectrum (400-700 nanometers).
In simple terms, the solar cell in question is able to produce more energy than can be accounted for by the light hitting the cell. Researchers at NREL achieved this with a layered quantum dot cell – a surface of anti-reflective glass, a thin layer of semiconductor material like silicon, a zinc oxide layer “textured” at the nano level, a quantum dot (QD) layer of lead selenide (PbSE) “doped” with ethanedithol (a bonding agent)and hydrazine (a deposition stabilizer), and a thin layer of gold for the uppermost electrode.
The resultant efficiency, 114 percent, is the “spectrally resolved ratio of collected charge carriers (electrons or electron holes) to incident photons” as a result of multiple exciton generation, or MEG, which creates many electron-hole pairs from a single photon – an effect that some quantum physics experts have ascribed to superposition.
The electron-hole pairs, typically one per photon, result when a photon enters the solar cell’s structure and bounces into a valence electron which is part of the semiconductor layer’s material structure. This freed electron leaves behind an electron-hole, which is pulled toward the N-side, while the electron moves toward the P-side, forming the electric current.
The abstract to the paper, called “Peak External Photocurrent Quantum Efficiency Exceeding 100% via MEG in a Quantum Dot Solar Cell,” is available here. The paper itself is only available for a fee. The authors are NREL scientists Joseph M. Luther, Sukgeun Choi, Hsiang-Yu Chen, Jianbo Gao, Matthew C. Beard, Octavi E. Semonin and Arthur J. Nozik; Semonin and Nozik are also affiliated with the University of Colorado at Boulder.
The secret behind the phoenomenal efficiency is attributed to the quantum dot structure of the solar cells themselves. Quantum dots (1-20 nanometers) have a number of interesting properties. For one thing, their minute size allows them to capture the excess energy that would, in other types of solar cells, be lost as heat. They also:
Form wider bandgaps as size decreases
Create excitons at normal temperatures
Enhance particle coupling
Improve MEG processes
Perhaps the most important part of this demonstration is that it opens formerly unexamined approaches to improving solar cell efficiencies. And improved solar cells can’t help but achieve the grid parity so sought after by solar researchers. In fact, according to a new report from researchers at Queen’s University (Ontario), residential solar PV has already achieved parity in terms of the cost of electricity.