Oxidation of silicon nanowire can transport much more light into silicon substrate.

Silicon nanowire (SiNW) has been widely used for light-trapping in photovoltaics, optical sensors, and other optoelectronic devices. However, we found that 58.4% of the light trapped by a SiNW with a diameter of 60 nm and a length of 1 µm will be wasted: 64.5% of the trapped light will be absorbed within itself, and 90.5% of carriers excited by this part of light will recombine before being transported to the silicon substrate. In this work, it is shown that oxidation of SiNW can transport much more light into the silicon substrate. At first, our simulation results demonstrate that oxidation can dramatically reduce the percentage of absorbed light. In an oxidized SiNW (O-SiNW) with a total and silicon core diameter of 60 nm and 30 nm, respectively, the percentage is about 44.5%. Next, a low carrier recombination ratio, about 27.3%, can be obtained in O-SiNW due to the passivation effect of the oxide layer. As a result, oxidation of SiNW can reduce the proportion of wasted light from 58.4% to 12.1%. More importantly, oxidation almost doesn't sacrifice the light-trapping ability: experimental measurements demonstrate that the average reflectance of an O-SiNW array is only slightly higher than that of a SiNW array, 3.9% vs. 3.0%. Such O-SiNW is promising to be used for low-loss light-trapping in specially designed photovoltaic devices.

Specific distribution of the light captured by silver nanowire.

The silver nanowire (AgNW) has excellent light capture ability, showing great prospects in many fields. Based on discrete dipole approximation simulations, it is found that the captured light can be subdivided into three parts: the near-field light occupies ~27.3%, mainly confined around the nanowire with a distance <20nm; the far-field part occupies ~59.6%, showing a dramatic conical distribution; and ~13.1% is ohmically absorbed. These insights are helpful to estimate the limited performance of AgNW-based device utilizing each subdivision, and locate the functional zone. Besides, we found that the light capture efficiency of AgNW can be easily controlled as it increases linearly with nanowire length.

Light harvesting of silicon nanostructure for solar cells application.

Silicon nanostructures have light-harvesting effects for enhancing the performance of solar cells. Based on theoretical investigations on the optical properties of silicon nanowire (Si NW), the influencing laws of the size of Si NW on its light-harvesting effect are proposed. For the first time, we reveal that the resonant wavelength of Si NW predicted by the leaky mode theory does not correspond to the actual resonant wavelength calculated by the discrete dipole approximation method, but exactly coincides with the leftmost wavelength of the resonance peak. Then, the size dependency of the resonant intensity and width of Si NW is different from that of spherical nanoparticles, which can be deduced from the Mie theory. The size dependencies of resonant intensity and width are also applicative for silver/silicon composite nanowires. In addition, it is found that the harvested light by the Si and Ag/Si NW both show significant radial locality feature. The insight in this work is fundamental for the design and fabrication of efficient light -harvesting nanostructures for photovoltaic devices.

Exact comprehensive equations for the photon management properties of silicon nanowire.

Unique photon management (PM) properties of silicon nanowire (SiNW) make it an attractive building block for a host of nanowire photonic devices including photodetectors, chemical and gas sensors, waveguides, optical switches, solar cells, and lasers. However, the lack of efficient equations for the quantitative estimation of the SiNW's PM properties limits the rational design of such devices. Herein, we establish comprehensive equations to evaluate several important performance features for the PM properties of SiNW, based on theoretical simulations. Firstly, the relationships between the resonant wavelengths (RW), where SiNW can harvest light most effectively, and the size of SiNW are formulized. Then, equations for the light-harvesting efficiency at RW, which determines the single-frequency performance limit of SiNW-based photonic devices, are established. Finally, equations for the light-harvesting efficiency of SiNW in full-spectrum, which are of great significance in photovoltaics, are established. Furthermore, using these equations, we have derived four extra formulas to estimate the optimal size of SiNW in light-harvesting. These equations can reproduce majority of the reported experimental and theoretical results with only ~5% error deviations. Our study fills up a gap in quantitatively predicting the SiNW's PM properties, which will contribute significantly to its practical applications.