|Fig. 1: Nanoparticle based solar cell.|
Thin-film solar cells hold the promise of a cheap, renewable energy source that could contribute to reducing CO2 emission and more targeted use of fossil fuel energy. However, thus far, thin-film cells' reliance on rare elements, expensive vacuum deposition manufacturing, or low efficiencies has impeded their progress. A variety of solar cell architectures are being researched and many have been commercialized under government subsidies, however, a universal solution for efficient energy extraction from the Sun is yet to be put forth. Aside from the environmental impact of materials and scalability, a key figure of merit when talking about solar energy is cost per kilo watt of energy. The implication of this is that in addition to higher efficiencies, low-cost is essential in finding a universal solar cell solution.
Solution processed solar cells based on colloidal nanoparticles show great promise for low cost solar cells. A diagram of the different layers of such a solar cell is shown in Fig. 1. What distinguishes these solar cells from other thin-film cells is the low cost of the fabrication process and the nanoparticle nature of the absorbing layer of light.
There are quite a few advantages of using nanoparticles for solar cells. Nanoparticles facilitate solution processing through a quick and simple spin coat, or dip coat process followed by possibly baking to remove unwanted solution and materials. Additionally, nanoparticles of many materials have a well established synthetic rout with good control on composition, shape and size. This control can help produce high purity materials and well as strong absorption material for photons. Finally, due to quantum confinement of electrons in small dimensions (~ 5 nm), the bandgap of the material can be tuned to match the frequency of the light that is intended to be absorbed. This bandgap tuning in turn can be used to create tandem solar cells from a single material by simply varying the nanoparticle size. Tandem solar cells are designed by stacking multiple layers of materials each specializing at absorbing a specific region of the solar spectrum.
|Fig. 2: Energy band diagram of a nanocrystal based solar cell on ITO.|
Such solar cell devices have in fact been fabricated and have shown to have efficiencies of over 5%. An example of such a device is the all-inorganic metal/NC/metal sandwich photovoltaic (PV) cell, produced by Luther et al., that produces an exceptionally large short-circuit photocurrent (> 21 mA cm-2) by way of a Schottky junction at the negative electrode. The PV cell consists of a PbSe NC film, deposited via layer-by-layer (LbL) dip coating that yields an EQE of 55-65% in the visible and up to 25% in the infrared region of the solar spectrum, with a spectrally corrected AM1.5G power conversion efficiency of 2.1%. This NC device produces one of the largest short-circuit currents of any nanostructured solar cell, without the need for sintering, superlattice order or separate phases for electron and hole transport. 
Although solution processed colloidal nanoparticle solar cells are very low cost, significant work is being done to raise the power conversion efficiencies to over 10% in order to offer a solution for a universal solar cell which can provide the sustainable and scalable renewable energy we are all searching for.
© Asad Kalantarian. 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.
 J. M. Luther et al., "Schottky Solar Cells Based on Colloidal Nanocrystal Films," Nano Lett. 8, 3488 (2008).
 D. V. Talapin et al., "Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications," Chem. Rev. 110, 389 (2010).