by Katherine Derbyshire, Contributing Editor, Solid State Technology
Efficiency is one of the biggest challenges for solar cells: most of the sun's incoming light is dissipated as heat, not converted to electricity. Yet researchers think the strange behavior of tiny particles known as quantum dots may help break the efficiency barrier. In quantum dots, energy that might otherwise be lost can excite additional free carriers.
Though the sun bathes our planet in about a thousand watts of energy per square meter, the best silicon solar cells achieve efficiencies of only about 24.7% 1. (All efficiencies quoted in this article are for individual cells. Fully integrated modules suffer additional resistive losses.) The maximum possible efficiency, the Shockley-Queisser limit, is only about 31% 2. After decades of effort, conventional silicon cells are approaching their theoretical limits.
One of the main reasons for the Shockley-Queisser limit is the mismatch between the energy of incoming photons and the band structure of the cell. If a photon has more energy than needed to generate a free carrier, the excess energy is lost: it simply dissipates as heat. If a photon has less energy than the band gap, it fails to excite a carrier, and its energy is also lost as heat. Since only a small fraction of the sun's output lies precisely at the silicon band gap, substantial energy is lost through mismatch alone.
One partial solution, the so-called tandem cell, stacks several junctions, each with a different band gap. Though the Shockley-Queisser limit applies to each junction individually, combining junctions allows the cell to capture a larger fraction of the sun's output. At this writing, the world's record for cell efficiency is 40.7%, held by a multi-junction cell based on gallium arsenide 1. Still, tandem cells don't address the basic problem of supra band gap photon energies. They still dissipate the excess energy of such photons as heat.
A phenomenon known as impact ionization may be able to help. In impact ionization, hot carriers generated by a high-energy photon transfer some of their energy to another carrier, exciting it to the conduction band and creating an electron-hole pair. In bulk materials, impact ionization is rare. There simply aren't enough such events to offset electron relaxation back to the conduction band.
Carrier confinement in quantum dots produces a number of useful effects, however. First, as Eun-Chel Cho and coworkers at the University of New South Wales explained, restricting at least one dimension to less than the Bohr radius of silicon increases the band gap. In a closely spaced array, where all three dimensions are constrained and the wave functions of adjacent dots overlap, the band gap depends on the spacing of the resulting super-lattice. Thus, a material with arrays of embedded quantum dots of various dimensions functions as a tandem cell, capturing several slices of the total solar spectrum. 3
Second, and even more interesting, as Antonio Luque of the Polytechnic University of Madrid explained, the carrier confinement increases interaction between electrons and holes, greatly increasing the impact ionization rate. A single photon can generate two or even three or more photocarriers. Though not all will ultimately contribute to the photocurrent, calculations from the Shockley-Quiesser model put the maximum theoretical efficiency for quantum dot solar cells around 45% 4. As in tandem cells, this limit would apply to each sub-array separately, so the total efficiency could be higher.
Multi-exciton generation is not a new phenomenon, having been demonstrated in PbSe, PbS, and PbTe over the last several years. (See references to [4]) More recently, though, studies of silicon quantum dots have shown that this most ubiquitous of semiconductors can achieve multiple exciton generation as well. Using silicon allows manufacturers to deploy the full array of silicon deposition and patterning technologies, while still working with a much more environmentally neutral material than lead.
According to Cho, alternating layers of silicon-rich and silicon-poor dielectrics can be annealed to create a super-lattice. Annealing precipitates quantum dots out of the silicon-rich phase, while the dielectric phase isolates successive layers from each other.
Though this work is interesting, it doesn't mean that we should expect 40% efficient silicon cells anytime soon. Actually extracting carriers from quantum dots into the circuit is difficult. Quantum dots, by definition, are completely surrounded by insulating materials?a direct conducting path would break the confinement responsible for their special properties. To escape, each carrier must make a series of tunneling steps, jumping from one quantum dot to the next until it reaches the cell's electrodes. The odds against such a trip increase with distance. Rather than placing a grid of electrodes above and below the super-lattice of quantum dots, a more effective design might intermingle quantum dots with a low conductivity semiconductor, such as a conjugated polymer semiconductor. The polymer's low conductivity preserves the dot's isolation, while providing a direct connection to the cell electrodes.
Luque cautioned that such designs remain hypothetical at this point. Though photovoltaic effects have been seen in quantum dot-polymer hybrid cells, and spectroscopy has confirmed the existence of the MEG effect, no quantum cell design has yet demonstrated that "extra" MEG carriers can be extracted to the circuit. Quantum dot cells offer exciting potential, but also enormous uncertainty. -- K.D.
IMAGE CAPTION:
In multiple exciton generation, an incoming photon generates an electron-hole pair within a quantum dot. The electron loses its excess energy through impact ionization, generating a second electron-hole pair. Multiple ionization events are possible as long as any of the free electrons have excess energy. Figure derived from reference [4].
References:
1 M. A. Green, et. al., "Solar cell efficiency tables (version 30)," Prog. in Photovoltaics: Research and Applications, vol.15(5), pp 425-430 (2007)
2 W. Shockley and H.J. Queisser, J. Appl. Phys., vol.32 (3) (1961) p. 510.
3 Eun-Chel Cho, et. al., "Silicon quantum dots in a dielectric matrix for all-silicon tandem solar cells," Adv.OptoElec., vol. 2007, article 69578.
4 Antonio Luque, Antonio Martí, and Arthur J. Nozik, "Solar cells based on quantum dots: multiple exciton generation and intermediate bands," MRS Bulletin, vol.32, pp 236-241 (March, 2007).
