Use of Nanostructures in Harvesting Solar Energygreenready
Use of Nanostructures in Harvesting Solar Energy
There is an increasing demand for clean renewable energy to counter pollution caused by carbonaceous fossil fuels such as coal and petroleum. Solar energy offers a promising alternative fuel because it’s cheap, abundant and renewable. The current commercially produced solar modules, made of doped silicon cells, have an average maximum conversion efficiency of 20%. Nanostructured semiconductors are deemed as possible low cost solution to low efficiency in solar modules. Nanostructures energy harvesters work under three principles, photovoltaic effect, photo electrochemical effect, and solar hydrogen production. This paper explores the application of these three principles in solar energy harvesting. These are nanostructures technology application in the design of nanostructure semiconductor solar photovoltaic cells, photosynthesis based donor-acceptor molecular assemblies, and semiconductor based photocatylists for fuel production such as hydrogen.
Currently, economies depend heavily on fossil fuels for industrial energy requirements. Apart from being a major source of carbon dioxide emissions, fossil fuels are getting depleted at an alarming rate. Hence, there is an urgent need for clean and sustainable fuel. Solar energy on the other hand is a clean form of energy which cannot be depleted. However, solar energy has several shortcomings which limit its application in an industrial scale. First, the energy density in solar is very low compared to that of fossil fuels. Also, solar energy is intermittent; it is only available during the day, affected by the vagaries of the weather, and also the availability of sunlight in a certain location. Finally, solar energy storage poses a major challenge to its technology compared to fossil fuels, which store energy in chemical bonds. Consequently, solar energy application has failed to take off and accounts for only around 0.015% of the total energy used (Fan, Johnny and Baoling 150). Nanostructures are intended to mitigate these pitfalls in solar technology through increased efficiency which will make it competitive against fossil fuels.
Properties of nanostructure materials
Conventional solar energy harvesting involves generation of electrons and holes in a semiconductor through photovoltaic effect. The electrons are knocked off from their molecules due to photo excitation and move through an energy band gap which creates the electric potential. The electrons are then absorbed into the holes on the other side of the doped semiconductor. The use of nanostructures ensures high efficiency in solar cells because they have energy gap geometrical configurations which effectively capture photons more effectively during the electrons generation stage (Pan 55). Also, their dimensions are similar to the carrier diffusion length, which ensures efficient collection of free carries in the electron separation step. These two factors raise the overall efficiency of the cell above that of a conventional solar cell.
Efficiency in current solar cells is limited by two of their aspects pertaining to photon absorption; photons with energy lower than the energy gap do not get absorbed and those that contain higher energy lead in excitation of hot electrons (Ren et al. 150). Also photons get deflected back by the surface of the solar structure. Nanostrcutures are engineered to overcome this pitfall by having a tunable bandgap designed to overlap the solar spectra and utilize most of the received irradiation.
The nanostructures in a solar cell can be arranged into several shapes such as tubes, pillars, nanofibers, rods, films, wires or spheres and networks (Rathnayake, Ramana, and Xu 25).
The dual diameter nanostructure pillars shown in figure 1 are more efficient because the smaller diameter of 60nm have better absorbance to high frequency irradiance of 300 to 600nm while the higher filling factor of 130nm diameter perform better in low frequency irradiation with wavelengths ranging from 600 to 900 nm. The resulting array is more efficient than a single diameter nanostructure array as shown by the graph on figure 2. Also, one dimensional nanostructure offer better movement of electrons within the array than zero dimensional structures as shown in figure 3 and 4 (Ren et al. 150).
Recent developments in nanostructure solar harvesting
The most recent development in nanostructure technology as a solar material is its application in water splitting to hydrogen and oxygen where the latter are used as forms of energy storage. Plasmonic nanostructures are a new type of gold nanorods, which donate electrons to hydrogen and oxygen co-catalysts. The donated electrons leads to separation of hydrogen and oxygen, hence water splitting occurs. Plasmon resonances can also be tuned to harvest a wide range of solar spectrum (Pan 5.
Also, the photoelctrochemical properties of ZnO have been investigated for use in nanostructure solar harvesting in the UV and visible spectrums. 15nm thin film electrodes made from undoped crystallites were observed to achieve a maximum conversion efficiency of 58%. The overall efficiency obtained from thin film was 2% (Pan, Hui 6). It was discovered that nanostructured ZnO exhibits high efficiency in the UV spectrum than in the visible spectrum.
Challenges and future prospects
Mass production of nanostructure solar modules is hindered by the lack of an inexpensive process to synthesize and purify nano-materials. Deposition and fabrication of thin film solar structures remains a complex process which cannot be applied for large scale production. Nanotechnology solar harvesting technology is also still in its nascent stages of development and the fabrication of a solar structure with high performance and acceptable tolerance and performance is still a mirage (Pan 5).
Incorporation of nanotechnology into thin film solar technology is a promising step to improving solar cell conversion efficiency and reducing manufacturing cost. Future work on nanostructure solar structures will be focused on improving irradiation absorption efficiency, which is currently under research. Also, research is underway to improve conversion of absorbed energy into electricity. More materials are being investigated for their suitability in designing solar nanostructures and the technology is expected to pick pace and compete with silicon crystalline solar cells in the near future.
Nanotechnology application in solar harvesting will improve efficiency of solar technology as a source of energy. The increased absorption cross sections will lower the required thickness of the structures below the diffusion length of the charge carrier while retaining harvesting efficiency as before. Cheaper, thinner, and more efficient nanostructure solar modules will have a wide range of applications which will conserve the environment. Low solar energy cost for commercial and domestic applications will improve living standards especially in rural areas.
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Pan, Hui. “Design of Nanostructures for Solar Energy Conversion: a Case Study on Silicon Nitride Nanowire.” Nanoscale Research Letters. 9.1 (2014): 1-9. Print.
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