The Best of Both Worlds

Crystalline materials such as silicon, cadmium telluride and copper indium gallium diselenide (CIGS) currently dominate the solar-cell market, with organic and dye-sensitized devices being regarded as the emerging technologies. However, large areas of crystalline solar cells are inherently diffi cult and expensive to manufacture, and organic technologies have so far been unable to compete in terms of power-generation effi ciency. Fortunately, there is another technology on the horizon that promises to deliver the best of both worlds — the ease-ofmanufacturing of organic solar cells, combined with effi ciencies approaching those of crystalline technologies. Aft er many years of research and the development of a cost-eff ective production technique, quantum dot solar cells based on semiconductor nanocrystals embedded in an appropriate medium are now becoming a commercial reality. Until now, the most limiting factor in the development of commercial quantum dot solar cells has been their cost. Th e historically high prices for the quantum dot feedstock have meant that a cell could not be fabricated at a cost low enough to compete with conventional silicon solar cells, let alone with fossil fuel energy sources. However, the capacity to now produce industrial amounts of quantum dots is fi nally making it possible to fabricate high volumes of quantum dot solar cells at competitive prices. Advances in chemistry and nanotechnology have also made it possible to manufacture quantum dots from diff erent types of semiconductor nanocrystals easily and uniformly, avoiding the need for a clean room, a high-temperature process and ultrahigh-vacuum equipment. To appreciate the attraction and potential of quantum dot solar cells, it is fi rst necessary to understand the limitations of existing photovoltaic technology. Conventional silicon solar cells do not absorb the entire spectrum of the sun’s energy. Electron–hole pairs are generated when photons with energies more than the bandgap of silicon (1.1 eV ~ 1.1 μm) are absorbed, with electrons being excited to the conduction band and holes being created in the valence band. However, a signifi cant part of solar radiation is composed of visibleand ultraviolet-wavelength photons, which have energies far exceeding the bandgap of silicon. Such energetic, shorter wavelength photons excite electrons into higher levels of the conduction band. Th ese ‘hot’ electrons then relax to the bottom of the conduction band (the associated holes relax to the top of the valence band) by giving up phonons, thus heating up the silicon crystal but not bringing any useful benefi t for electricity generation. Such heating can also degrade the performance of the cell. Th ese problems can all be solved using quantum dot technology. Th e bandgap of a quantum dot can be precisely controlled by its size, meaning that diff erent sizes of quantum dots have diff erent absorption band edges. It is therefore possible to synthesize quantum dots of various sizes that absorb most, if not all, of the sun’s spectrum — something than cannot be achieved using the conventional approaches of crystalline silicon solar-cell fabrication. One can then envision a multistack solar cell in which the top layer absorbs the highest energy (shortest wavelength) photons and the bottom layer absorbs the lowest energy photons. Th is approach maximizes the absorption of sunlight by utilizing the photons that cannot be collected by single-layer crystalline solar cells. Although the multistack scheme can also be achieved using several diff erent traditional semiconductor materials (each with a diff erent bandgap), the big advantage of quantum dots is that a single material is used for all of the layers comprising the solar cell (except for the electrodes). Th e electrons and holes generated in a solar cell must travel to their respective electrodes for the electrical potential to be useful and drive a load. Th e process of charge transport within quantum dot solar cells can be enhanced in several ways, including through the use of materials that provide quantum dots with a large Bohr radius mixed with (or in the proximity of) an electronaccepting and electron-transporting material The size tunability of quantum dots enables photovoltaic devices to harvest a broad range of wavelengths over the solar spectrum. Here, various phials containing quantum dots of diff erent sizes (in solution) can be seen. The diff erent colours indicate diff erent absorption bands of light. RI C E U N IV ER SI TY