|Fig. 1: Diagram of solar cell. Light strikes the solar cell and produced an exciton. The electron and hole are pushed to negative and positive electrode by the electric field created by the p-n junctions. The electrons then travel through the load to recombine with holes. (Source: Wikimedia Commons)|
As the world's energy needs keep growing, the need for alternative, renewable energy sources such as solar, wind, geothermal, etc has become increasingly apparent. Solar energy is by far the most viable option but it currently meets less than 1% the world's energy needs. Although recent technological advancements and subsidies have made it commercially available, the technology is still expensive and too inefficient. In most cases, it would takes homes more than 10 years for the investment to pay off. In order to meet real demands, new initiatives are necessary to increase the efficiency of solar cells. One such approach being researched is the introduction of nanostructures in semi-conductors to fabricate quantum dot solar cells.
Conventional solar cells are photovoltaic devices which convert solar energy into electrical energy. They are composed of a semiconductor material such as silicon or gallium arsenide (GaAs), with a p-n junction. When light strikes a solar cell, some percentage of energy is absorbed by the material to produce an electron-hole (exciton) pair by knocking an electron from the valence to the conductance band. The electron and hole are then separated by the electric field created by the p-n junction, pushing the electrons to the top of the cell towards the n-type and the holes to the bottom towards the p-type. Typically, the electron and hole would recombine but in a solar cell, the electrons are passed through a load as a current and then recombined with holes at the bottom of the cell. These processes are visible in Fig. 1.
The efficacy of solar cells is limited by a number of factors. First, for a photovoltaic cell to work properly, the energy of the incoming photon must equal the energy of the bandgap in order to push electrons from the valence to conduction band. If the energy is too low, then nothing will happen, and if the energy is too high, the excess energy is lost through collisions, which heats the cell and represents thermal loss. This significantly limits the amount of incoming energy that can used by the device.  Other sources of loss include blackbody radiation (7%) and recombination (10%). In recombination, electrons produced by incoming photons sometimes recombine with holes left behind by previous incidents, and thus represent thermal loss.  Additionally, there is the problem of placing contacts on the solar device. We cannot place contacts on top of the cell since it would limit the amount of photons that pass through to the device, and it is disadvantageous to place contacts on the side because that would require electrons traveling a long distance to reach the contacts. This is a problem since most semiconductors have high internal resistances so the longer the electrons must travel, the higher the energy losses. Thus, contacts are generally placed on top in a grid arrangement that limits the amount of covered surface area and minimizes electron travel distance. These losses limit the theoretical efficiency of solar cells to about 31%. 
So far, the main approach to make solar cells more efficient has been experimenting with cells with multiple band gaps, each tuned to a different wavelength. This configuration allows the cell to capture energy from a larger percentage of photons. One way to implement multiple band gaps is to stack p-n junctions on top of one another with the shortest wavelength on top. Each junction is tuned to a different wavelength of light so more photon energy can be utilized for conversion. Theoretically, an infinite-junction cell is calculated to be 87% efficient when exposed to concentrated sunlight. Experimentally, the highest efficiency multi-junction solar device was fabricated by Solar Junction with 43.5% efficiency using three-layer InGaAs/GaAs/InGaP cells.  The disadvantage with such cells is the complex structure and high cost associated with fabrication materials. Thus, such cells are rarely used outside of space applications, where the power-to-weight ratio is worth the cost.
An alternate solution to the solar cell efficiency problem is quantum dot solar cells, proposed in 1990 by Barnham and Duggan.  A quantum dot (QD) is a nanocrystal made of semiconductor material that is characterized by 3D potential well for excitons. In such a crystal, the diameter of the nanocrystal is smaller than the exciton Bohr radius so the exciton is quantum confined. This allows us to model the energy levels using the particle in the box model, which means the energy levels and band gap energy are tunable according to the box length instead of relying upon the bulk material, as is the case with conventional solar cells.  This ability of quantum dot cells allows for greater photon absorption and makes them highly desirable for use in solar energy applications. Additionally, it was suggested by Nozik et al. in 1997 that quantum dot solar cells were capable of producing multiple low-energy excitons (electron-hole pairs) from a single photon incidence versus one high-energy using conventional solar cells.  This theory was corroborated through experimental efforts by Klimov at the Los Alamos National Laboratory using PbSe, PbTe and PbS quantum dot cells that generated up to seven exciton per incident photon.  Unfortunately, this effect has yet to be implemented in a solar device. It was also discovered by Sargent in 2005 that PbS quantum dots have bandgap energies that can tuned into the infrared region of the electromagnetic spectrum. This is not possible with conventional solar cells, but is an advantageous feature of quantum dot solar cells since nearly 50% of the solar energy received by the Earth is in the infrared and near-infrared region.  In terms of fabrication, quantum dot solar cells are inexpensive to grow and offer the same or better efficiencies as conventional solar cells, making them a highly desirable alternative.
In recent years, there has been great speculation as to whether investing in solar power is wise. There have been a number of solar companies that have gone bankrupt in recent years, the largest of which was the government-loan supported Solyndra. It was later determined however that the company provided faulty information in its loan-guarantee application and its review by the government committee was rushed.  Despite these problems, the industry for solar PVs has started to recover since 2013. It has seen tremendous growth in the past year, especially in China, Japan and the US.  At this point, the question is not whether solar power is a viable alternative to coal, oil and gas (it already is), but how do we keep sustaining its growth and development. The fuel source for solar power is free, its operating costs are lower, and with further research, production costs can keep decreasing and efficiency of devices can keep increasing. By carefully continuing to invest in new research such as quantum dot solar cells, we continue to invest in a clean, green future for ourselves.
© Prastuti Singh. 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.
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