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Working Principle of a Solar Cell
A solar panel/ module is a device that converts the sun’s energ into electricity by the so-called photovoltaic effect (“photo” means light and “voltaic” means electricit ). Solar modules are divided into cells units (solar cells) which are the basic component of a solar panel. Each cell produces its own electricity and is electrically connected to other solar cells of the panel, contributing this way to the global power of the module. There are basically two key processes involved in the conversion of solar energy into electricity. First, the photovoltaic cell is striked by the sun light, which results in the absorption of a certain fraction of the light by the material. As solar cells are typically made of semiconductors, this absorption results in the formation of positive and negative charges, commonly named an electron-hole pair. The second process is the separation and collection of these 2 charges at different electrodes. Separation of the charges is achieved by the action of a local electric field within the material. The flow of electrons and holes in opposite direction causes an electric current to flow across the device. This generated electric current is collected by contacts usually in the front and rear parts of the device. As mentioned above, solar cells are made of semiconductors, typically silicon wafers. A solar cell results from the close contact of two silicon layers with different properties: one layer has a high concentration of electrons (commonly named n-type wafer) and the other layer has a high concentration of holes (commonly named p-type wafers). This is the popular p-n junction. The difference of carrier concentrations between the n-type and p-type materials will result into the formation of a local electric field within the solar cells device, which will act as the driving force for the electron-hole pair separation. To obtain n-type and p-type materials, silicon wafers have to be doped by an appropriate element, phosphorus and boron respectively. Indeed, silicon belongs to group 4 in the periodic table, therefore its needs to form 4 covalent bonds to achieve chemical stability. When being doped by elements from group 3 (such as boron) or group 5 (such as phosphorus), the octet rule is no longer respected. As boron has only 3 electrons to share for covalent bonding, it lacks one electron: there is an empt space, named a “hole” here an extra electron should be. This missing electron – the hole – can wander freely around. Likewise, a silicon atom can be replaced by an element of the 5th group, such as phosphorus, which has one extra electron (5 valence electrons) compared to silicon. Silicon and phosphorus are sharing each 4 electrons to form 4 covalent bonds. However, phosphorus still has one more electron to share. At room temperature, the energy is enough for ionization. This extra electron can move freely in the conduction band and can therefore participate in conduction. When the p-type and n-type silicon materials are brought into contact, the difference in the concentrations between carriers of the two sides generates a diffusion current, aiming at equalizing the carriers’ concentration as illustrated in Figure 2-1 (a). Therefore, holes from the p-side will diffuse to the n-side, leaving behind them negatively ionized atoms (boron). Likewise, electrons from the n-side diffuse to the p-side, leaving behind positively ionized fixed atoms (phosphorous). Indeed dopant atoms are fixed and cannot diffuse. These permanent charges create an electrical field (due to the Coulomb force) in the region located between the n-type and p-type materials. We name this region the space charge region (SCR) or the depletion region as shown in Figure 2-1 (b) . This electrical field is essential to the functioning of a solar cell, since it is the driving force for electron-hole pair separation.
Silicon Nanowires: Methods of Fabrication and Properties
Many methods have been developed to fabricate silicon nanowires (SiNWs). Basically, they fall into two categories: bottom-up and top-down approaches. The former comprises mainly the vapor-liquid-solid (VLS) growth [41], [42], CVD [43] and Plasma Enhanced Chemical Vapor Deposition (PECVD) [44], [45] processes, while the latter includes dry reactive-ion-etching (RIE) [46]–[48], and solution based wet etching techniques [49].
Bottom-up
Chemical Vapor deposition via the Vapor-Liquid-Solid (VLS) process is a popular method used to fabricate dense, high aspect ratio, and vertical SiNWs [41], [42], [50]–[53]. In this approach, crystalline silicon wires are synthesized from gas particles through liquid metal-Si droplets as depicted in Figure 2-4 ([54]). A thin layer of catalyst agent (generally gold) is first deposited onto a silicon wafer. The temperature is then increased above the catalyst-Si eutectic point, generating alloy droplets on the surface of the wafer. The growth process begins with the decomposition of a precursor gas into a catalyst droplet to create an alloy. Silicon precursor molecules, i.e., monosilane (SiH4), predominantly crack at the droplet surface and silicon gets incorporated into the droplets by diffusion. Once supersaturation of the droplet is reached, a crystalline nanowire precipitates with a diameter similar to the droplet size at the liquid-solid interface (i.e. supersaturated droplet – substrate). The lateral growth is therefore limited by the droplet. There are two types of growth: the tip growth (the catalyst droplet is rooted at the top of the NWs) and the base growth (the catalyst droplet is rooted at the bottom of the NWs) which occur when the interaction between the metal and the substrate is weak and strong respectively. The bottom-up method can grow SiNWs whose diameter varies between 5 nm and 500 nm. LPICM has developed a strong expertise in the fabrication of SiNWs by PECVD from tin films [55], [56]. However this process induce unavoidable metallic contamination. In particular it is well-known that gold introduces deep-level electronic states in the Si band gap [57] which is detrimental to its use in electronic devices. Therefore a lot of effort has been put in the synthesis of SiNWs using other types of catalysts [44], [45], [55], [58] . Moreover, the control of the geometry (pitch, diameter and length) is challenging as it requires a perfect control of the size and position of the catalyst droplets during the growth.
Top down
Top down approach prepares SiNWs via the « sculpting » of the bulk Si material by lithography and etching or just catalyzed etching. The reactive ion etching (RIE) and the metal-assisted chemical etching (MACE) techniques are generally used to produce SiNWs. RIE is an etching process used in microfabrication which uses synergistic chemical (reactive species) and physical etching (ion bombardment) at the wafer surface to achieve etching with good anisotropy and selectivity. The plasma is generated at low pressure, usually 10-100 mTorr to achieve a more directional etching. By combining the RIE method with lithography, the size and density of SiNWs array can be well controlled [46]–[48]. Another advantage of this technique is that the SiNWs have the same doping as the original wafer. However the process is time consuming and expensive. Moreover, the roughness of the sidewall is also a problem as the surface is physically damaged by the ion bombardment [59].
The MACE method [49], [60] has been newly developed and is really promising for future industrial applications. This method, being simple and low cost, is able to produce SiNWs with a high aspect ratio over a large area. All there to do is to immerse Si wafers into a HF-AgNO3 solution at room temperature. The mechanism proposed is a galvanic displacement reaction [61]–[63]. Silver plays the role of etching catalyst in the oxidizing HF solution. The final reaction is composed of the following cathodic and anodic reactions: The synthesis of SiNWs is based on a redox reaction between Ag+ and Si0. As Ag/Ag+ has a more positive redox level than the energy of the valence band edge of Si, there is a preferential charge transfer from Si to Ag+ as illustrated in step (a) of Figure 2-5. The reduction of Ag+ into Ag nanoparticles occurs at the Si surface. The surface Si atoms are oxidized (anodic reaction) and supply the electrons for the Ag+ reduction (cathodic reaction). The silicon surface is therefore locally oxidized (only where Ag is directly contacting the Si). The SiO2 formed is next etched by HF, as shown in step (c) of Figure 2-5. We then observe the formation of silver nanoclusters on the surface due to the initial cathodic reaction of Ag+. These nanoclusters are restricting further oxidation of the Si surface. Indeed further reduction of Ag+ in the etching solution occurs on the Ag nanocluster itself as shown in step (b) of Figure 2-5. This latter is relatively more electronegative than silicon (easy injection path for holes) and plays the role of a cathode by transferring electrons from the wafer below. The continuous reduction of the silver ions results in the formation of the so-called silver dentrites that are observed at the top of the nanowires. The silver nanoparticles captured in pores drill progressively, leading to the formation of a SiNW array.
Solar Cells based on Silicon Nanowires: a review
Table 2-1 summarizes the application of SiNWs fabricated by top-down (MACE and RIE) and bottom up approaches (PECVD) in different solar cell technologies. Generally it has been used as an antireflective layer by taking advantage of the low optical reflection (axial and substrate junction) of SiNWs or as a radial / core-shell p-n junction to make the best use of the short collection distance offered by the radial geometry.
Several works have been reported on the use of SiNWs for solar cell applications. Our group has investigated thin film hydrogenated amorphous silicon (a-Si:H) radial junction solar cells by VLS-grown SiNWs with a highest reported efficiency of 9.2% on glass substrate [55], [56]. This architecture uses a 100 nm thick a-Si:H as the active material and the c-SiNWs as the conducting channel. c-SiNWs have also been used as the active material. In this type of devices, the p-n junction is typically formed by the high temperature phosphorus diffusion process. As a consequence, the NWs might be fully converted to n-type (or p-type) materials, making an axial or substrate p-n junction rather than a radial p-n junction. In this case the SiNWs are used for their good anti-reflection and enhanced light absorption, but the benefit of the radial junction is lost [27], [46], [62], [69], [98]–[101]. For example, highly efficient substrate junction solar cells based on SiNWs fabricated by MACE and diffusion process were fabricated with a reported efficiency of 17.11% for an area of 154.83 cm2. Moreover an efficiency of 18.2% [102], and more recently an efficiency of 22.1% [103], was also achieved for black silicon solar cells nanostructured by a wet etching process on a c-Si wafer. Nevertheless, it is very challenging to fabricate a radial junction by the diffusion process since the latter tends to produce axial or substrate junction. However, SiNWs with larger diameter fabricated by deep reactive ion etching (DRIE) have been successfully used in solar cells with a core shell structure fabricated using the phosphorus diffusion process with a reported efficiency of 10.8 % for a few micrometer thick c-Si films [104]. SiNWs have also been used in heterojunction devices resulting in radial type junction. Heterojunction solar cells are devices where the p-side and n-side of the junction are made of different materials. For example heterojunction with intrinsic thin layer (HIT) device is a solar cell where the n-side (respectively p-side) of the junction is made of c-Si while the p-side of the junction (respectively n-side) is made of hydrogenated amorphous silicon [105]. Hybrid devices are also another popular type of heterojunction solar cells where one of the semiconductor of the junction is made of inorganic material like silicon while the other consists of an organic layer. Basically heterojunction solar cells are made by depositing an amorphous silicon or an organic layer on a c-Si substrate. The use of SiNWs in heterojunction solar cells brings an additional challenge of passivation compared to axial or radial homojunctions solar cells mentioned above. Indeed, while diffusion-fabricated p-n junctions are formed inside SiNW bulk, for HIT or hybrid devices the p-n junction is formed at the surface of the SiNW which may present many surface defects. Today, only few works have been published on HIT solar cells based on SiNWs [106]–[109]. So far, the best efficiency reported is 10.04% for SiNWs upon addition of a thin insulating layer of Al2O3 to passivate the SiNWs surface [107]. An atomic layer deposition (ALD) technique was used for the Al2O3 and contact deposition. Another interesting work claimed an efficiency of 12.2 % for HIT solar cells based on silicon microwires (SiMWs) [110] with radii ranging from 1.5m to 50m fabricated by DRIE. Nevertheless, SiMWs do not boost the light trapping inside the cells as much as SiNWs can. So far, all the studies published on SiNW based HIT solar cells are based on thick crystalline silicon wafers, while the final objective is to apply this type of architecture to Si thin films.
Table of contents :
Acknowledgements
Abstract
Substancial Summary in French / Résumé substanciel en français
Table of content
List of Acronyms:
List of Figures
List of Tables
Chapter 1 : Introduction
1.1 Solar energy: the Reality and the Future of Photovoltaics
1.2 PV Technologies
1.3 Three Categories of Solar Cells
1.4 Motivation, Objectives and Scope of this Thesis
1.5 Main Contributions of the Thesis
1.6 Organization of this Thesis
Chapter 2 : Literature review
2.1 Basics of Solar Cells
2.2 Silicon Nanowires: Methods of Fabrication and Properties
2.3 Silicon Nanowire based Solar Cells
2.4 Heterojunction with Intrinsic Thin Layer Solar Cells
2.5 Hybrid Solar Cells
Chapter 3 : Fabrication and Characterization Techniques
3.1 Silicon Nanowires Fabrication by Wet Etching
3.2 Fabrication of Solar Cells based on Silicon Nanowires
3.3 Characterization Techniques
Chapter 4 : Solving the Agglomeration Effect of Silicon Nanowires fabricated by MACE
4.1 Theoretical Basis
4.2 Investigation of the Solutions proposed in the Literature to avoid the Bundling of Silicon Nanowires
4.3 Effect of the wettability on the agglomeration of SiNWs
Chapter 5 : Optical Properties of SiNW arrays
5.1 Comparison between SiNWs and commercial pyramids
5.2 Ordered vs Disordered SiNWs: effect of the geometry
5.3 Effect of ITO coating
5.4 Angle-resolved MM polarimeter
5.5 RCWA analysis
5.6 Dual diameter
Chapter 6 : HIT Solar Cells based on SiNWs
6.1 Understanding the Challenge and Complexity of HIT Solar Cells based on SiNWs
6.2 Fabrication, Optimization and Characterization of HIT Solar Cells based on random SiNWs
6.3 Comparison between Ordered and Disordered SiNWs
Chapter 7 : Hybrid Solar Cells based on SiNWs and Advanced Concepts
7.1 Hybrid Solar cells based on Disordered and Ordered SiNWs
7.2 Hybrid Solar Cells based on amorphous thin films
7.3 LiF/Al as a back contact for thin film solar cells based on a-Si:H
Chapter 8 : Conclusion and Future Work
8.1 Achievements:
8.2 Future Work
References:
