As the world has started to adopt solar technologies increasingly in their businesses and their lives, the research and development on improving its features must be carried out. Solar panels can produce electricity by absorbing light energy and each layer of these panels play an integral role in the efficient functioning of the device. Improving its features will make solar technology more accessible, affordable and widespread through all parts of the world. Some of the key areas where solar technologies are constantly being improved are light absorption capacity, decreased manufacturing costs, adaptability of solar technology to versatile conditions and increasing the output power produced by solar modules. This is where nanotechnology comes into action. Therefore, this article will be covering the role of nanotechnology in solar PV systems and the recent advancements in solar technology using nanoparticles such as self-cleaning solar panels, Dye-Sensitized solar cells, Quantum Dot sensitized solar cells, Perovskite-sensitized solar cells and the effect of silicon nanoparticles on the efficiency of solar cells.
Role of Nanotechnology in Solar Technologies
Nanomaterials are those materials which have a physical size between 1nm-100nm. These materials can exist in the shape of nanorods, nanotubes, nanoparticles, nanospheres, nanofibers and nanoflowers. The small size of these particles allows for a large surface-to-volume ratio. The surface of these materials contains dangling bonds which form bonds with molecules on the neighbouring side. These bonds help in stabilizing the material by reducing the total energy. This further leads to low melting points, high solubility and high chemical stability when we compare them to bulk materials. An effect called the quantum tunnelling effect causes these materials to be quantized or discrete which leads to the ability to optimize or change the optical properties and intrinsic properties of the material such as Bohr’s radius and reduced distance of migration. Nanomaterials can reduce the manufacturing costs of the material because of the ability to modify cheaper materials to suit the purpose of the application. The maximum efficiency obtained by a conventional silicon solar cell is 33.7%. Nanotechnology has played a vital role by enabling the modification of solar cells by improving the light absorption, energy conversion and efficiency of the cell. The ability to modify different components of a solar cell from the transparent coatings, electrodes and the wafers, each layer can contribute to the increased efficiency of nano-enhanced solar cells. However, nanomaterials are prone to agglomeration and increased side-reactions due to availability of larger surface area. They are also prone to homogeneity control issues but recent developments in manufacturing processes such as roll-to-roll manufacturing and inkjet printing have helped to limit the drawbacks significantly [1-5].
Recent Advancements in Nano-Enhanced Solar Technologies
The development in the field of nanotechnology has led to multi-disciplinary applications which include solar technology. Listed below are some of the recent progressions of Nano-enhanced solar applications.
1] Nano-Coatings for Solar Panels: In this technology, solar panels are coated with a protective coating which helps to keep the solar modules free from dust and dirt. Regions which are industrial and rural have a lot of pollution and dust in the air which makes it more likely for panels to get dirt. If panels get dirty, the layer of dust on the panel will block the sunlight penetrating the panel which will reduce light absorption and consequently reduce charge generation and output efficiency of the solar panel. Also, another challenge with solar panels is that it is prone to superficial damages such as scratches and damages caused by moss, etc. These scratches can also inhibit the efficient performance of the panel. Due to issues like these, scientists and researchers have developed nano-coatings which can be applied on the surface of the panels. These coatings prevent dust and dirt from sticking onto the panel and can easily roll down to prevent blockage of sunlight. The surface also inhibits moss or lichens from growing on the panel thereby ‘self-cleaning’ the panel. These coatings also prevent the surface from being covered by ice or snow during winter conditions and protect the panels from external damages such as scratches or other minor superficial disfigurations. Also, nano-coatings have an additional advantage of being anti-reflective which helps to increase the transmittance through the panel. Solar modules coated with nano-coatings have shown to yield higher efficiencies by 7-12%. The anti-reflective property of the coating also enables low sunlight regions or weak-radiation areas to be implemented with solar technology and obtain a decent power output from such installations [6, 7].
2] Dye-Sensitized Solar Cells (DSSCs): These solar cells were developed by Brian O’Regan and Michael Grätzel at UC Berkeley. They are made with transparent conducting oxide layer such as Indium Tin Oxide (ITO) or Fluorine doped Tin Oxide (FTO), a photo-anode, sensitizer dye, electrolytes such as Iodide-Triiodide and counter electrodes coated with catalysts such as Platinum. In this technology, the photo-anode which is commonly made from nano-enhanced TiO2 is coated with Ruthenium dye nanoparticles. The combination of these nano-enhanced materials helps to effectively absorb the light and convert it into charge carriers. The working principle initially deals with the absorption of photon energy from sunlight. The photon energy excites an electron from the Highest Occupied Molecular Orbital (HOMO) level to the Lowest Unoccupied Molecular Level (LUMO) level which is analogous to the valence band and conduction band of a semiconductor respectively. The excitation of the electron leads to the oxidation of the dye. The electrolyte donates its electron to the dye to remove the dye from its oxidized state so that it can participate in conduction again and the electrolyte which donated its electron is consequently replenished with the help of the counter electrode coated with a Platinum catalyst. Meanwhile, the electrons which were excited from the dye gets transferred to the photo-anode (TiO2). These electrons under optimum conditions get collected at the terminals for charge collection at the photo-anode. In this manner, the electrons flowing through the circuit is recycled. Dye-sensitized solar cells (DSSCs) are known to resemble photosynthetic processes observed in nature and these cells offer low cost and eco-friendliness as compared to conventional silicon solar cells. These cells are also flexible which makes them adaptable to various surfaces but their efficiencies are quite low averaging at around 12%. Nonetheless, the constant development in optimizing nanoparticles in DSSC’s have found to show potential in increasing the efficiency of these cells and make them commercially competitive in the coming years [8, 9].
3] Quantum-Dot Sensitized Solar Cells (QDSSCs): These solar cells are a variation of the DSSCs which was discussed above. The significant difference between QDSSCs and DSSCs is the nanocrystal sensitizer that’s used which differs in surface conditions. This difference considerably affects the properties of charge transfer that takes place at the interface of the photo-anode/sensitizer dye/electrolyte. Another significant difference is that the lifetime of recombination is faster in QDSSCs which make it prone to higher levels of recombination which is a substantial shortcoming. The photo-anodes used in QDDSCs are ZnO and TiO2 which is similar to that of DSSCs. The sensitizers for QDSSCs are quantum dots made of materials such as CDs, ZnSe and CdTe. Their efficiencies are at an average of 4.4%. They are economically viable and their bandgaps can be subject to modification to accept a wide range of spectrums. The QD sensitizers have a higher extinguishing coefficient as compared to DSSCs and the working principle of QDSSCs is the same as that of DSSCs [10-12].
4] Perovskite-Sensitized Solar Cells (PSCs): These cells have been the focus of research in the recent past because of their ability to have exceptional light absorption properties, efficient charge carrier mobility, high lifetimes of charges, low cost and technology which is industrially scalable. PSCs have an ABX3 structure and are derived from the Calcium titanate (CaTiO3) compound. These cells have a very small thickness which reduces heat losses. These cells consist of a transparent conducting oxide made from materials such as Fluorine tin oxide FTO, a photo-anode, a metal electrode, a hole transport layer (HTM), an electron transport layer (ETM) and a perovskite layer. The perovskite layer first absorbs photon energy and then generates excitons. These excitons can either recombine or be used to produce current. The ETM and the HTM effectively absorb the electrons and holes from the excitons and the electrons are transported to the TiO2 anode layer which is further collected by the FTO whereas the holes are transported and collected by the metal electrode. PSCs have found to have low binding energy, high optical coefficients and large dielectric constant which help in the transmission of electrons and holes consequently increasing the open-circuit voltage (Voc) and the short-circuit current density (Jsc) which increases efficiency up to an average value of 33%. PSCs have also been manufactured on flexible substrates which increase its adaptability to different surfaces [13-16].
5] Nano-Silicon Solar Cells: These solar cells involve fabricating and synthesizing silicon monoxide (SiO) in solid form by thermally treating it which leads to its disproportionation. This leads to the separation of nano-silicon particles. The fine silicon particles were bound to the matrix of SiO until they were intentionally removed through chemical etching techniques. This ensured that the particle dimensions were within the range of 2nm-10nm which improved its anti-reflective and passivating properties. This manufacturing technique also maintained the purity of the cell. These cells show superior properties such as low bulk density, active surface state, photoluminescence and biocompatibility [17-22].
To conclude, the research and development of Nano-enhanced solar technology is still a work in progress. The various technologies that were covered in this article such as QDSSCS, DSSC’S, Nano-silicon solar cells and Nano-coatings facilitated the understanding of how nanotechnology can play a role in the field of solar technology. Many developments and optimizations are taking place for these applications and in the future, the symbiosis of nanotechnology with solar technology may be the solution for cutting-edge, low cost and efficient power generation in the coming decades!
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