Investigation Of High-efficiency Nanotextured Silicon Solar Cells

As a kind of clean, safe and sustainable energy technology, solar cells have become more and more popular with the aggravated contradiction between the increased demand of energy and the limit of fossil energy together with people’s heightened awareness of environmental conservation. Solar cell is a kind of photoelectric devices converting light into electricity. The key point of the mass applications of solar cells lies in their high photoelectric conversion efficiency with low cost. There are many kinds of solar cells, among which silicon solar cells dominate the photovoltaic market due to its extensive materials source, mature technology, high efficiency and low cost. Now, people are trying to develop silicon solar cells with efficiency higher than 22%.It is anticipated that nanomaterials will play an important role in the next-generation photovoltaics due to their extraordinary optical and electrical properties. For example, silicon nanostructures have attracted considerable interests owing to their superior optical performance. Through a rational design, silicon nanostructure arrays can realize a reflectance of nearly 0 over a broad wavelength and incident angle, so it is an ideal antireflection technique. However, when it is utilized as the surface textures for solar cells, its excellent antireflection effect doesn’t lead to the improvement of the cell performance, even lower as compared to the traditional ones. These mainly attribute to the greatly increased surface from nanostructures, and hence results in a severe surface and Auger recombination. Therefore, in the thesis, we will focus on the suppression of carrier recombination while maintaining the superior optical properties, together with adopting a novel device structure, to realize high-efficiency nanotextured silicon solar cells.Firstly, we have investigated buried Mie resonator arrays, which is composed of silicon nanostructures atop a silicon substrate and buried with a dielectric film, by the combination of theoretical and experimental method and thus found that the antireflection from Mie resonance and the antireflection from destructive interference are two competing processes as increasing the thickness of dielectric layer. The novel antireflection mechanism provides us a design guideline to realize a superior broadband antireflection in buried Mie resonator arrays. We have also found that the buried Mie resonator arrays mainly play a role as a transparent antireflection layer. The light absorbed in the silicon nanostructures can be neglected. Moreover, their antireflection effect is insensitive to the nanostructure height when it is higher than 150 nm, which are of prominent significance for photovoltaic applications in the reduction of photoexcited carrier recombination.Secondly, we have adopted various techniques to optimize the nanotextured silicon solar cells in order to minimize their carrier recombination loss. We have adopted multi-scale textures and focused on optimizing the distribution of silicon nanostructures to reduce the surface area, and thus realized both low reflection and recombination. By controlling the nanostructure height(only 100 nm), the photoexcited carrier recombination at the surface and in the emitter bulk is effectively decreased. The sheet resistance is optimized(80Ω/□) to reduce Auger recombination in the emitter. The silicon nanostructures are further conformally coated by SiN_x layer to achieve excellent surface passivation effect. Meanwhile, this structure becomes to be the buried Mie resonator arrays, and hence we can obtain an ultra low solar spectrum averaged reflectance(2.43% over the wavelength from 400 to 1100 nm) by adjusting the thickness of SiN_x layer. Owing to the overall optimization to realize both the outstanding optical and electrical properties, we have successfully fabricated an 18.5%-efficient nanotextured silicon solar cells on a large-size wafer.In the next step, we have proposed a self-masking method to fabricate nanopyramids as the surface texture for further decreasing the surface increment ratio and thus the carrier recombination. Through the investigation of the forming mechanism of the nanopyramids, we have found that it is mainly based on the self-masking effect from Ag nanoparticle and the anisotropic etching from alkaline solution. Therefore, this method is much easier, cheaper and possesses larger process window, no need of masking layer, photolithograph, electron beam etching or ion beam etching, which are very important for mass production. We applied nanopyramid texture to both diffused-junction solar cells and heterojunction solar cells, and have demonstrated its low surface increment and superior electric performance. As a result, an efficiency as high as 19.2% is obtained, which is the record efficiency of the nanotextured solar cells. For thin film silicon solar cells, nanopyramid texture can reduce the surface reflectance and realize excellent light trapping effect, thus increasing light absorption and short circuit current density.Finally, we have proposed nanotextured homo-hetero junctions solar cells. In order to better understand the physical aspect of the homo-hetero junctons structure, we have simulated an ideal planar solar cell and found that the homojunction provides an excellent field-effect passivation. Hence, the homo-hetero junctions solar cells have better tolerance for interfacial defects. Moreover, it has lower series resistance and higher fill factor due to the absence of intrinsic a-Si layer. To realize high-efficiency nanotextured silicon solar cells, the front homojunction should be heavily doped(such as 5×1018cm-3) with small thickness(such as 10 nm), while the back homojunction should be heavily doped(such as 5×1018cm-3) with large thickness(such as 10 nm). Through the overall optimization, the nanotextured silicon solar cells can achieve an efficiency higher than 22%, even reaching 25%, with the benefits of superior field-effect passivation and low series resistance from the homo-hetero junctions structure.