application of nanostructure materials in solar cells

TÜBAV Bilim 1(1) 2010 1-6

H.Movla, N.Gorji, F.Sohrabi, A.Hosseinpour, H.Babaei

APPLICATION OF NANOSTRUCTURE MATERIALS IN SOLAR CELLS Hossein Movla 1, Nima Es’haghi Gorji2, Foozieh Sohrabi3, Ahmad Hosseinpour4, Hassan Babaei3 1

Department of Solid State Physics, Faculty of Physics, University of Tabriz, Tabriz, 51566, Iran Depatment of Optics and Laser, Technical and Engineering Faculty of Bonab, Bonab, 55517, Iran 3 Department of Theoretical Physics, Faculty of Physics, University of Tabriz, Tabriz, 51566, Iran 4 Department of Mining, Faculty of Engineering, Urmia University, Urmia, 57154, Iran

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Abstract In recent years, nanostructure materials have opened a promising route to future renewable sources, especially the solar cells. This article considers the advantages of nanostructure materials in improving solar cell structures. These structures have been employed for various performance/energy conversion enhancement strategies. Here, we have investigated four types of nanostructures applied in solar cells, where all of them are named as quantum solar cells. We have also discussed recent development of quantum solar cells enabling quantum solar cells to be competitive with the conventional solar cells. Furthermore, the advantages, disadvantages and industrializing challenges of quantum solar cells have been investigated.

Keywords: Nanostructure Solar Cells, Solar Energy Conversion, Photovoltaic devices. 1. Introduction Solar Cells (SCs) or Photovoltaics (PVs) are the devices which harvest sunlight energy and convert it directly into electrical power. Fossil fuels such as petroleum, tar and etc. have been used for many years as the main resource of energy; however fossil fuels have limited resources, annually increasing costs of supplying owing to environmental degradation. Technologies and infrastructures have been developed or been improved to the alternative and renewable energy sources, such as, solar energy, wind power, geothermal energy, biomass, hydropower, nuclear power, etc [1]. PV technology is only one of the alternative renewable energies such as wind, biomass, nuclear and hydropower. This technology has many advantages being relative to other renewable energy resources such as, generating directly electricity from sunlight, supplying electrical power in the form of portable modules, having small-scale up to the large-scale mega-watt power plants and not being restricted to the special regions. All of these technologies are expected to contribute significantly to the world’s energy supply in the next decades. Improving the energy conversion efficiency of PVs by developing the technology and concepts must be increasingly extended as one of the key components in our future global energy supplement, but, the main problem of PVs is their rather high production and energy cost. 2. History and Development of PVs The first commercial PV in the world is based on the crystalline Si and it is made by researchers at Bell Labs in 1954 [2]. First Si cell had poor efficiency which is about 4% and then, many researchers made an attempt to demonstrate the efficiency of about 25% [3] very close to the theoretical limit for a single junction under one sun illumination of 31% [4,5]. Nowadays, most of today’s commercial crystalline Si-based solar cells which are packaged in modules have 14–20% sunlight conversion efficiency.

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E-mail: [email protected]

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Today, the main reason as employing Si as the most used materials in the PV industry is the fact that it is the second most abundant element (after Oxygen) in the earth’s crust which make it relatively inexpensive semiconductor. Production of cost crystalline Si is relatively low and it is 0.2 $ per kilogram. However, optical and electronical properties of Si are not very good for semiconductor technology and PVs for examples, indirect bandgap of Si makes the inefficient optical absorption and poor optical transmition [6] and also in the fabrication process, Si requires a minimum purity level. These are the most important reason why Si is not the ideal semiconductor material for solar energy conversion. The main reasons that the efficiency of single junction solar cells (like Si) are limited to 31% are given below as: * Not absorbing the significant fraction (20%) of the photons in the solar spectrum that are below the bandgap, * High energy photons loss due to thermalization of high-energy charge carriers in the conduction band due to phonon scattering, * Reflection from the surface of the cell, * Non-radiative recombinations in the transitions between in the band gaps. Bulk Si solar cells are considered to be Generation I technologies, whereas Generation II cells are based on thin film technologies that allow for the use of thinner materials as absorber deposited on substrates with lower cost and hence reduced cell cost. Generation III cells are based on Nanostructures and energy conversion concepts that have the potential to achieve limited efficiencies greater than the single junction limit. It is expected that such structures based on Nanoparticles will be able to achieve the cost levels similar to or better than Generation II cell technologies. Generation II cells, like Multi-Junction (MJ) PVs, allow the absorption of a wider range of wavelengths in the solar spectrum by combining different bandgap materials in a tandem stack and can attain a theoretical efficiency of 71% under maximum concentration based on 2–4 and beyond number of different bandgaps [7,8]. Nowadays, these cells are expensive (>$7W-1) and they are used in space applications, as well as terrestrial concentrator systems in solar power stations in which the small area is necessary [9-11]. 3. Nonastructure Solar Cells All this problems drift us to a new technology and concept. This is the way that the effect of tandem cells can be combined in one material for high-efficiency and low-cost semiconductors. It must be noticed that the Nanotechnology power is introduced in the field of Solar Energy Conversion. Four following types of Nanostructures applied in solar cells have been investigated; * Nanocomposites [3D], * Quantum wells [2D], * Nanowire and nanotubes [(quasi) 1D], * Nanoparticles and Quantum Dots (QDs) [(quasi) 0D]. These structures have been employed for various performance/energy conversion enhancement strategies. Each type of mentioned phenomena will be explained shortly; Nano-sized Phosphor particles absorb part of the spectrum and convert it to the more suitable energy for the solar cell as Nanocomposite systems called as up [12] or down [13] energy convertors. One of the first examples of 3D nanostructured applications in PV devices is the Dye-Sensitized Solar Cell (DSC) which is originally developed by Gratzel and O’Regan [14]. Such structures, in combination with new absorbing molecules, have been contributed to improve the performance of DSCs [15, 16] from 2% to 11% which leads some companies to make an attempt scale-up to the module level. Furthermore, the presence of a liquid electrolyte in DSCs has inspired labors to develop polymer-based solar cells [17, 18]. Nanu et al. [19] spray coated a composite of CuInS2 (CIS) and TiO2 Nanoparticles onto a graphite/Nanocrystalline Titania electrode structure and completed the structure with a dense Titania film followed by a Transparent Conducting Oxide (TCO) with energy conversion efficiency of about 5%. The authors have showed that the act of creating a Nanocomposite [20]. Another form of bulk-nanostructured solar cells is the use of nanocrystalline silicon to replace the amorphous Si or to combine them together in tandem structures [21]. This kind of structures have a high surface area of grain boundaries that significantly increases the density of recombination centers as well as the probability of recombination because charge carriers should traverse so many boundaries.

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Quantum Wells also have potential for high absorption due to the higher density of states at the band edges which lead to high short circuit current. Multi quantum wells inserted in the active region of the Solar Cell devices based on III–V materials mainly GaAs and the related alloys such as AlGaAs and InGaAs, had been concerned several times [22]. The next 2D confined-nanostructures, i.e. nanowire and nanotubes provide quasi or true 1D structure. The nanowires act as a direct path for charge transport without the presence of grain boundaries, and thus leading to an enhanced performance compared to solar cells employing nanostructures with aspect ratios approaching 1:1 [23]. They provide one more direct path for charge transport to the contacts whereas dendritic nanowire structures are allowed to enhance light harvesting. For instance, Yang and coworkers have shown two increases in the short circuit current density in comparison with a nanocrystalline Titania-based DSC with maximum power conversion efficiency under AM1.5 light of 1.5% [24]. In addition, nanowires also offer the potential for enhanced optical absorption characteristics. Tsakalakos et al. [25] have shown this effect directly in Si nanowires which are fabricated directly on fused silica substrates. Alivisatos and co-workers [26] demonstrated an all-inorganic nanorod solar cell consisting of CdSe nanorods deposited on a layer of CdTe nanorods. These cells produced power conversion efficiency of 1% and upon further sintering of the nanowire composite film to allow better adhesion/bonding of the nanowire interfaces yielded cells with an efficiency of about 3%. In summary, nanowires and nanorods show great promise for future solar cell devices. Remaining technical challenges include proper surface passivation, shunting, and high quality contacts. Carbon Nanotubes (CNT) have also been shown to yield a photovoltaic effect. Lee et al. [27] showed that an individual CNT diode, electrostatically doped in a split-gate field effect transistor configuration ,is an ideal p–n junction with an ideality factor of one. They have concluded that there is no surface state on a nanotube since the carbon bonds are well satisfied in the graphene structure of the CNT. Such device was shown to possess the small photovoltaic effect of about 5% as estimated power conversion efficiency [28]. CNTs are also being explored as electrodes for solar cells [29]. Among all mentioned Nanostructures, QDs have become so renowned due to their potential to implementation in various PV applications and enhancement schemes. This property of intering QDs in the solar cell structures based on tow substantial concepts can be modified by the special properties of following practical Nanostructures; * Multi Exiton Generation (MEG) or Impact Ionization (II): In this concept, the higher energy photons with energies greater than tow bandgap should generate two or even more electron-hole pairs through impact ionization effect [30, 31]. The quantized level of Nanoparticle Quantum Dots is an excellent idea to enhance the probability of this effect [33]. Nozik proposed this theoretically concept first [34]. Recent experiments on PbSe and PbS Nanocrystals have shown that it is indeed possible to multiply the number of excitons to 7 for a single absorbed photon [35]. The practical aspects, including charge separation [36], etc, require additional basic research. * Intermediate Band Concept: The intermediate band originates from integrated Quantum Dots (QDs) quantized levels. This concept can solve a big problem that has a long history in solar cell material design. The photon with energy less than the gap could be utilized if an intermediate energy band around midband were present through the host material bandgap. Such band structure would have a limited efficiency more than 60% and thus it has great promise [37,38]. This promising approach, based on the structures in which the QDs are embedded in the active region of a p-i-n structure, introduce a middle band in the gap of the host semiconductor that is similar to the approach of Luque and co-workers. 4. Conclusion There is a realization in the PV technical and customer/user communities that increases the cell efficiency while decreasing cost will be critical if PV technology is widely utilized for primary or secondary energy needs. Future development of Nanostructure solar cells will now be concerned to predict the future development of solar cell efficiency. The motivation for using nanostructure materials emerges from the specific physical and chemical properties of nanostructures. Manifold research activities have been focusing on the application of nanostructure materials in the field of solar energy conversion. Special physical effects , related to the nanometer-sized scale , increase interesting macroscopic properties. As discussed above, new phenomena directly related to nanostructure materials are currently

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investigated. Since nanotechnology is a field of considerable research activity, the findings may become gain the major interest of solar energy technology.

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