Veni Quid Veniat

     Faster than a speeding bullet, able to soar higher than any plane... no, not a superhero: light. Light’s unique and intriguing properties have long tantalized scientists. Indeed, if the glimpses of potential it presents were ever fully utilized, the possibilities would be nearly endless. Significant advances have been made in harnessing light - the development of fiber optic cables and photovoltaic cells come to mind - but these are merely ripples on the surface of what could be. The development which could change all that, though, is an increase in the understanding and utilization of photonic crystals. In a general sense, the operation of photonic crystals is extremely simple: they act as optical equivalents of semiconductors. In semiconductors, variations in the arrangement of ions control the movement of charge carriers. In photonic crystals, the periodic arrangement of areas of high and low indices of refraction (a measure of how much light is bent when entering or exiting the material) control the movement of photons. In both cases energy bands of allowed energy levels - in the case of light, wavelengths - are created, which control the motion of either electrons or photons, depending on the substance in question.
   
In practice, although the theory behind photonic crystals is reasonably complex, the actual application of photonic crystals is even simpler. Photonic crystals are used to exert control over the movement of photons, a task which they can accomplish much more effectively than other methods of controlling the path of light because of their unique abilities. Much more could be said on the behavior and characteristics of photonic crystals, but that is all we need to know at the moment.

     That ability to change the path of light presented by three-dimensional photonic crystals has incredible potential. Recent research (Guldin et al. 2010) suggests that photonic crystals could be utilized in building more efficient solar cells. Currently, silicon is used in most photovoltaic cells to absorb sunlight and convert it into electricity. The silicon absorbs highly energetic photons and uses the energy from those photons to create free charge carriers, which can then be extracted to an external circuit. This process generally works well, however, it can be improved. The silicon does not absorb all the photons that impact it - some pass through. Although it is backed with aluminum to reflect some of the photons which pass through, these are reflected at a high angle and have a reasonable chance of passing through the silicon again. On the other hand, a three-dimensional photonic crystal, with its ability to control the paths of photons, could reflect more light than aluminum and diffract the light as it did so, causing it to re-enter the silicon at a much lower angle than it would otherwise and increasing the chance of reabsorption. By attaching photonic crystals, rather than aluminum, to the back of the silicon, the efficiency of the cell could be improved significantly.



Figure 1: A photovoltaic cell with a backing of three-dimensional photonic crystals. From Guildin et al. 2010.

     The increased efficiency inherent in the system is not its only advantage. Notably, unlike aluminum, photonic crystals do not have to be opaque. Technically, nothing prevents the entire solar cell from being completely transparent if photonic crystals are used instead of aluminum. This could provide the potential for using solar cells in windows, to produce windows that generate electricity. Given that solar sources cannot provide baseload power, it is as a supplementary power source that solar power is most attractive. Incorporating solar power generation into something as common as windows would provide a heretofore unheard of capability for solar energy to be used as a supplementary power source, and would remove many of the difficulties (for example, transportation and storage) which plague solar energy.

     Appealing as the concept of applying photonic crystals to increase the efficiency and utility of photovoltaic cells may be, it is not without its challenges. As previously mentioned, three-dimensional photonic crystals are difficult to fabricate, whatever purpose they are intended for. A further difficulty arises from the fact that the particular form of photovoltaic cell used - a diblock-copolymer based dye-sensitized solar cell - is limited in its effectiveness by its tendency to crack and delaminate, making it of dubious usefulness at present. Efforts to remedy both of these deficiencies are, however, ongoing. Efforts to improve diblock-copolymer based dye-sensitized solar cells show promise, and it seems more than likely that three-dimensional photonic cells will be fabricated much more readily in the not-too-distant future, making the possibility that solar cells could be far more efficient and widespread through the use of photonic crystals more likely than not.

      If there is to be an insurmountable challenge to the use of photonic crystals in solar cells, it will likely come from the invention of other methods of generating energy which make solar power unnecessary. Here photonic crystals are seen again. Researchers at MIT (Yi Xiang Yeng et al. 2011) have been quite successful in fabricating photonic crystals using tungsten and titanium which can withstand temperatures of up to 1200 degrees Celsius. In theory, these can absorb infrared radiation which will subsequently be converted to electricity. The actual process is essentially the same as that used in solar cells, the only difference is that the potential applications are much broader. Indeed, the possible applications for such technology are nearly endless: any heat source can be used, even heat sources already used in the process of generating energy whose heat is not completely consumed. This would include nuclear, geothermal - here photonic crystals could play a particularly significant role, since the chief challenge to geothermal energy at the moment is collection - coal, oil, and other hydrocarbons, and even as means of regaining heat lost through processes not intended to generate energy. Even the heat of a summer day could be converted to electricity, given the right conditions. Perhaps the most appealing option, however, is as a sort of “nuclear battery.” The concept originates from existing ideas. Many of NASA’s deep space missions make use of the concept - the Curiosity rover, for example, uses radioisotope thermal generators to generate electricity. Heat is generated by the decay of a radioactive element such as plutonium, but in present-day radioisotope thermal generators, this heat is converted to electricity using a thermocouple, which achieves less than 10% efficiency. That is, less than 10% of the thermal energy produced by radioactive decay in the apparatus is converted to electrical energy by the thermocouple. The process is still useful, but it could be far more efficient with the use of photonic crystals. It is unclear exactly how much photonic crystals could improve efficiency, but the improvement would be significant.

      Nor are those squeamish about using nuclear energy shut out of the market for batteries that utilize photonic crystals. Burning butane to produce heat has also been proposed, and batteries combining the combustion of butane to produce heat with photonic cells to collect heat could potentially last ten times as long as conventional batteries while providing a cleaner energy source (butane is produced as a byproduct of refining natural gas and is a far cleaner energy source than, say, coal or oil). An improvement in battery technology of this magnitude would have a tremendous impact on every field that uses batteries, but most notably on electric cars. It is not unthinkable that in the next several decades cars with the improved efficiency and emissions of electric cars but the range of petroleum-fueled cars could come on the market. Further, it takes little imagination to envision an incredible range of applications for longer-lasting batteries - the implications are positively staggering.

      Perhaps most thrilling, there are no clear obstacles to the development of batteries using photonic crystals. The team of researchers at MIT established a simple way of generating durable three-dimensional photonic crystals for the high-temperature environments required. That achievement is the bulk of the theoretical work in designing batteries using photonic crystals. What remains, although important and difficult, is comparatively simple. In fact, the MIT team estimated that such devices would be viable within the next two years. That estimate is perhaps optimistic - the MIT study was the last major development in the field, and that came in 2011 - but the fact remains that the most daunting hurdle has already been cleared.

     In the closely related fields of photovoltaic and thermovoltaic cells there are few developments more exciting than photonic crystals. Use of these materials in collecting energy could, if all went as expected, provide a significantly cheaper and cleaner energy source, particularly for small scale, localized energy production. While putting the theory into practice will require effort and may take a few years, the concept itself represents a tantalizing glimpse into what might be.




Yi Xiang Yeng, Michael Ghebrebrhan, Peter Bermel, Walker R. Chan, John D. Joannopoulos, Marin 
             Soljačić, and Ivan Celanovic. 2011. “Enabling high-temperature nanophotonics for energy 
             applications.” PNAS 2011 109 (7), 2280-2285