1、 Development process of solar cells
(1) The first generation solar cells, represented by crystalline silicon materials, include monocrystalline silicon and polycrystalline silicon solar cells. This kind of solar cell has the advantages of mature manufacturing technology and has always occupied the main market of solar cells. At present, in the laboratory, the maximum efficiency of monocrystalline silicon solar cells is 25% (AM1.5), which has been maintained for a long time and is very close to the theoretical efficiency. The efficiency of large-scale production line is generally 17% – 20%.
(2) The second generation solar cells are mainly characterized by thin-film materials, including amorphous silicon (a-Si), cadmium telluride (CdTe), copper indium gallium selenium (culngase), copper indium sulfur (culns2) and other solar cells. The battery production technology is a new technology developed in recent 30 years. Thin film solar cells can not only reduce the amount of semiconductor materials, but also deposit semiconductor materials on cheap substrates, thereby reducing the cost of solar cells.
(3) The third generation solar cell the third generation solar cell uses more sophisticated technology to improve efficiency, but the cost only increases slightly. Professor Martin Green believes that the third generation solar cells must have the characteristics of thin film, high conversion efficiency, rich raw materials and non-toxic. The third generation solar cells mainly include tandem solar cells, multiple band gap solar cells, hot carrier solar cells, dye-sensitized solar cells, impact ionization solar cells (also known as quantum dot solar cells), etc. Among them, the multi junction solar cell has achieved high efficiency.
The idea of the third generation solar cell constructed by the American renewable laboratory is shown in Figure 2-1. It is hoped that the third generation solar cell will not only significantly improve its efficiency, but also have a relatively low cost. From the perspective of industrialization, most of the third-generation solar cells are still in the stage of laboratory research and development or concept, and industrialization still needs a long way to go, while the first and second-generation solar cells, which have been studied more mature, have a more promising industrialization development. Of course, the United States has set the spotlight solar cell as a major direction of future industrial development, which has effectively promoted the innovation and application of spotlight solar cell technology.
2、 Characteristics of silicon nanowire solar cells
Silicon based solar cell is one of the solar cells with the most research and the most mature technology. Due to the rich sources of silicon materials and mature production technology, although new solar cells continue to be produced, the vitality of silicon-based solar cells has not been weakened, and the research on silicon-based solar cells has not stopped. For example, nanowire silicon-based solar cells are the latest new research direction, which broadens the research field of silicon-based solar cells.
Nanometer is a unit of length, and its magnitude is near the atomic size. Because the physical, chemical and biological properties of substances will be different from those of the original substances at the nano scale, nanotechnology has attracted much attention and developed rapidly recently. Similarly, semiconductor silicon nanowires also have unique optical (nonlinear optical effect) and electrical (Coulomb blocking effect) characteristics different from silicon, which have attracted extensive attention of scientific researchers. In 2002, c.ji et al. Proposed the idea of metal induced growth of silicon nanowires for solar cells to increase the current density of solar cells. In 2005, b.m.kayes of California Institute of technology and others published an article on the simulation of radial p-n junction new solar cells based on nano Si and as in the Journal of Applied Physics. The simulation of radial p-n junction new solar cells based on nano Si and as is shown in Figure 2-2. In the same year, k.peng successfully developed silicon crystal solar cells on silicon wafers and Multichips using electroplating and etching technology. In 2007, Professor Lieber of Harvard University successfully developed coaxial single silicon nanowire pin solar cells by using vapor liquid solid (VLS) deposition growth mode, as shown in Figure 2-3. In 2008, the team developed a single silicon nanowire laminated solar cell, which promoted the rapid development of silicon nano solar cell research. Because silicon nanowires have the characteristics of simple fabrication and mature technology, scientific and technological workers all over the world have invested in the research and development and production of such solar cells. At present, silicon nano array solar cells and single silicon nano solar cells are more studied. In 2010, Erik Garnett of California Institute of Technology prepared silicon nanowire solar cells using the first silicon nanowire solar cell deposition reactive ion etching method of SF6 gas. French Grenoble laboratory used chemical deposition technology to deposit n-type silicon nanowires on p-type substrate, and manufactured silicon nanowire solar cells with power generation efficiency of 1.9%. The Jena Institute of photonics in Germany has manufactured axial nanowire array solar cells with a conversion efficiency of 4.4% by using the electrodeless chemical etching method.
Different laboratories have different methods and techniques to study nano solar cells, which is a manifestation of the immaturity of silicon nanowire solar cells. But it also proves the vitality of nanowire solar cells.
3、 Silicon nanowire array solar cell
Silicon based meter wire solar cell is a solar cell with a longitudinal structure. As shown in Figure 2-4, a metal bottom electrode and an n+ layer isolated by SiO2 as a contact electrode can be formed on the glass substrate first, so as to continuously deposit a radial NC Si / a-si:h/ito longitudinal junction centered on columnar n-type nanowires (single crystal characteristics). Silicon nanowire solar cells can be directly prepared on traditional crystalline silicon cells. As a light trapping part, nanowires can increase the absorption of light by solar cells, thereby increasing the short-circuit current of solar cells.
Silicon nanowires arrays (SiNWs) are nanowire arrays formed by one-dimensional silicon nanowires arranged in a certain order. Silicon solar cells made of nanowires or arrays are called silicon nanowire solar cells. The light receiving surface of this kind of solar cells is distributed with nanowire arrays perpendicular to the surface, similar to the distribution of toothbrush bristles. This distribution can effectively increase the absorption of light. The nanowire arrays with wide wave band and incident angle have good light trapping characteristics. For 300~800nm incident light, the nanowire array can make the absorption rate reach more than 80%. The reason why it has good light trapping characteristics is that silicon nanowire array solar cells use this radial PN junction nanowire array to absorb sunlight and transport carriers. Joan Redwing of the University of Pennsylvania in the United States wrapped a thin layer of amorphous silicon on silicon nanorods with a vertical distribution and high aspect ratio with a new structure to form an amorphous silicon film / crystalline silicon nanorod heterojunction.
4、 Preparation of silicon nanowire arrays
Silicon nanowire array solar cells are usually prepared in two ways: top-down and bottom-up. Top down fabrication refers to selective etching from bulk silicon to prepare nanowire arrays; Bottom up preparation refers to the growth of nanowire arrays from molecules or atoms.
(1) There are many methods to prepare nanowire arrays from top to bottom, including solution method, electrochemical method and laser ablation method, but the order of the prepared nanowire arrays is poor. The typical method is metal catalytic chemical etching, which has the characteristics of simple preparation, not affected by the crystal form and orientation of the chip, and can be prepared in a large area. The specific operation methods are as follows
① Cleaning of silicon wafer and H termination of silicon wafer surface. Immerse the silicon wafer in HF solution to form Si-H bond on the surface of the silicon wafer. AG metal particles are deposited on the surface of silicon wafer. H-terminated silicon wafers are immersed in a mixed solution containing HF and AgNO3, and discontinuous Ag particle films are deposited on the surface of silicon wafers
② Chemically etch silicon wafer. Put the silicon wafer deposited with Ag metal particles into the etching solution for etching, and the position covered by Ag metal particles on the surface will be gradually etched, and the position without Ag metal particles will remain the same. Dilute nitric acid dissolves and removes Ag particles,
(2) Bottom up growth of nanowire arrays bottom up growth of nanowire arrays generally adopts chemical vapor deposition (CVD) technology and catalyst driven molecular beam deposition, which can directly grow nanowire arrays on substrates. However, the order of the nanowire arrays grown in this way is poor. In view of the existing problems, scientific and technological workers have also explored the combination of CVD and template technology, CVD and Langmuir Blodgett technology.
1) CVD and template combination technology. The main function of template is to limit the diameter, growth direction and growth position of nanowires. In 2001, Zhang Lide of the Institute of solid state physics of the Chinese Academy of Sciences successfully grew monocrystalline silicon nanowire arrays using closely aligned hexagonal nanopore alumina.
2) CVD is combined with Langmuir Blodgett technology. Professor Lieber of Harvard University first used CVD method to grow nanowires, and then made silicon nanowires into suspension in non-polar solvent. In Langmuir Blodgett tank, the nanowires were arranged in parallel, and then transferred to a smooth substrate. Then, another parallel nanowire is placed on the first layer of nanowire array in a cross manner, which can form a cross nanowire array.
3) Gas liquid solid (VLS) deposition growth technology. The process of making nanowire solar cells by VSL is shown in Figure 2-5, and the operation process is as follows.
① Gold plated film, use standard steps to clean the silicon wafer, and then plating 5nm thick gold film on the substrate as a catalyst.
② Put the silicon wafer in a quartz furnace to make silicon nanowires. Working conditions in the quartz furnace: the temperature rises by 30 ℃ every 1min, the air pressure is 350mtorr, the ammonia flow rate is 100sccm, and the silicon burning flow rate is 100sccm.
③ The sod layer diffuses, and the liquid solution of phosphorus doped SiO2 is coated on the nanowires. Bake at 90 ℃ for 30min, and then place in 950 ℃ horizontal annealing furnace for annealing for 4H. Then use 10% HF solution to clean SiO2, and then use deionized water to clean the silicon wafer.
④ Sputter aluminum electrode, directly sputter 600nm aluminum layer, and then anneal in hydrogen for 1H. At the same time, make positive electrode.
5、 Application of silicon nanowire array solar cells
Due to the unique antireflection of silicon nanowires and arrays, silicon nanowires and arrays also have potential value in the fields of optoelectronics, photochemistry and photocatalysis. The application of silicon nanowires or arrays in solar cells not only promotes the technological innovation of solar cells, but also promotes the improvement of solar cell structure.
Under standard conditions, the single crystal conversion efficiency of silicon nanowire array solar cells is 9.31%, and the polycrystalline conversion efficiency is 4.73%. The main reason for the low conversion efficiency is that the grid electrode film deposited on the upper surface is not dense enough, resulting in high resistance, which reduces the current collection efficiency. At the same time, the large surface area ratio has more hanging bonds and defects. These hanging bonds are often the composite center of carriers, which greatly accelerates the recombination of carriers.
Coaxial core-shell heterojunction silicon nanowire solar cell core-shell heterojunction is used to absorb photons, short axis separation and collect photogenerated carriers. In addition, compared with graphene and silicon heterojunction solar cells, the heterojunction solar cells made by wrapping graphene on the surface of silicon nanowire arrays have the characteristics of enhanced light capture and carrier transfer.
Michael Naughton of solasta company in the United States deposited amorphous silicon films on the surface of carbon nanotube arrays to prepare nanostructured amorphous silicon films / carbon nanotube heterojunction solar cells, which completely separated the transmission path of photons from the transmission path of photogenerated carriers, and is expected to improve the conversion efficiency.
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