Application Of Silicon Nanostructures To Solar Cells

Minimizing both optical and electrical losses are two major concerns to design the high efficiency solar cells. The enhancement of optical absorption usually focuses on anti-reflection and efficient light trapping of the incident light, while the electrical improvement concentrates on decreasing recombination rate throughout the device. Recently, the silicon nanostructure has attracted great attentions, because of its excellent anti-reflection and light trapping effect, which make this structure a promising candidate to lower both required quality and quantity of silicon material. Many groups have investigated the silicon nanostructured solar cells, providing valuable support for further improvement. However, the efficiency of silicon nanostructured solar cells still falls behind the conventional crystalline silicon cells, resulting from the limitation of the extreme high surface recombination in silicon nanostructure. Recent attempt has demonstrated that high Auger recombination at near surface plays a key role in the limitation of the photogenerated carrier collection and cell efficiency in the silicon nanostructured solar cells. The Auger recombination comes from the high doping related to in-diffusion through the large surface area of the silicon nanostructure. This near surface Auger recombination grows dominantly over the surface recombination especially for excessive doping condition in silicon nanostructured solar cells. Therefore, the electrical properties degrade for the accelerated recombination of photogenerated carriers which even counteracts the benefits of optical enhancement. The suppression of carrier recombination in silicon nanostructure turns out to be the primary focus on the performance improvement of silicon nanostructured solar cells.Surface passivations such as thermal oxidation, carbon thin film, and chlorine dielectric treatment have been widely studied to improve the electrical characteristics of silicon nanostructured solar cells. It should be noted that these passivations can only work on recombination at silicon nanostructure surface. In order to reduce recombination both at and near surface in silicon nanowires(Si NWs) while maintain good light trapping, we propose here an efficient way of dielectric passivation. The suppression of carrier recombination has been demonstrated through the combination of Si O2/Si Nx stack, which exhibits good passivation effect on heavily doped Si NWs via reducing both the Shockley-Read-Hall recombination and near-surface Auger recombination. We have examined in detail the effects of different passivations and Si NWs lengths on the effective minority carrier lifetime, reflectance and carrier recombination characteristics, as well as cell performance. We report the realization of high performance Si NWs based solar cells with a conversion efficiency of 17.11% in a large size of 125×125mm2, which is at that time(in 2013) the highest record for nanostructure based Si solar cell on large area of 125×125mm2(154.83cm2).The usage of a passivation layer to saturate the surface defects and dangling bonds, implementation of light and shallow doping to lower the Auger recombination as well as modification of nanostructures’ morphology have been proven to be effective methods to suppress the electrical loss. Among these three methods, surface morphology modification, including the control of the major structural parameters(filling ratio, periodicity, diameter, etc.), is a simple and adaptable way to adjust the light absorption and surface area enhancement in the present industrial manufacturing processes. We have presented a simple method to modify nanostructures’ surface morphology by Ag-catalyzed chemical etching and subsequent Na OH modification with diameter and depth changing from tens to hundreds of nanometers to realize of both excellent optical and electrical properties of nanostructured multicrystalline silicon solar cells. We have examined in detail the influence of different surface area enhancement ratios(AF/A) on reflectance, carrier recombination characteristics and cell performance. By conducting a quantitative analysis of these factors, we have successfully demonstrated a higher-than-traditional output performance of nanostructured multicrystalline silicon solar cells with a low average reflectance of 4.93%, a low effective surface recombination velocity of 6.59 m s-1, and a certified conversion efficiency of 17.75% on large size(156×156 mm2), which is ~0.3% higher than the acid textured counterparts.Besides, we also compare the optical and electrical properties of silicon nanowires and silicon nanopores based solar cells. It is found that under the same value of AF/A, silicon nanopores show better antireflection effect; while under the same value of reflectivity, silicon nanopores also show better electrical properties. Moreover, Si NWs structures partly destroyed during solar cell fabrication process. Finally, silicon nanopores based solar cells show much better cell performance.Since both the growth of silicon nanostructures and the fabrication of nanostructured silicon solar cells have been carried out in the present industrial manufacturing processes, the present work opens a potential prospect for the mass production of nanostructured solar cells with higher-than-traditional conversion efficiencies.