Absorption enhancement analysis of crystalline Si thin film solar cells based on broadband antireflection nanocone grating

Abstract

1 μm thick Si solar cells based on nanocone grating (NCG) with height of 100-800 nm and period of 100, 500, and 800 nm are numerically investigated through reflectivities, absorption enhancement factors, absorption spectra, optical generation rates, ultimate efficiencies, and diffraction angles. Compared with the planar Si solar cell, absorption enhancement are observed in any solar cells with NCG surface. Their absorption enhancement mechanism varies with the incident wavelength range. When incident wavelength λ < 500 nm, antireflection of their front surface dominates the absorption enhancement behavior due to their stronger absorption coefficients. When 600 nm > λ > 500 nm, even though the absorption enhancement is still dominated by antireflection of the front surface, cavity-resonance effect and guided-mode excitation induced by high order diffraction start to make contribution. When λ > 600 nm, the contribution of guided-mode excitation induced by lower-order diffraction becomes larger and larger once the diffraction angle is larger than its critical angle. For the structure with P = 100 nm, high-order diffraction cut-off at the longer wavelength range is the main reason of its lower absorption enhancement and ultimate conversion efficiency. For P = 800 nm, the lower absorption enhancement and ultimate efficiency is also observed due to the high reflection loss and mode leakage induced by 1st order diffraction where its diffraction angle is lower than its critical angle. Higher absorption and ultimate conversion efficiencies are achieved in P = 500 nm due to the good balance between antireflection performance and guide-mode excitation induced by the high order diffraction is achieved. Moreover, such absorption enhancement is closely related with its height of NCG gratings. Reflection loss reduction, the interaction volume reduction between the incident light and Si material, and higher photon density in NCG structure coexists with H increasing, which results in absorption enhancement in P = 500 nm and P = 800 nm, but absorption reduction in P = 100 nm where high order diffraction cut-off. Based on these analysis, we do believe that high absorption and ultimate conversion efficiency should be achieved in NCG-based solar cells where both the lower reflection in short wavelength domain and guide-mode excitation induced by 1st and 2nd diffraction in longer wavelength domain can be achieved. According to this rule, the optimized structure is NCG with P = 559 nm and H = 500 nm, by which, the highest optical generation rate of 536.57 × 104 W/cm3 and ultimate efficiency of 28.132% are achieved. Such analysis should benefit the design of the thin film solar cells with nano-structured diffraction gratings.

Guaranteed global optimization of thin-film optical systems

“…Existing solutions for reducing reflection losses include surface texturing [16][17][18][19][20], used widely in commercial c-Si cells, in conjunction with a single-layer SiNx:H AR coatings, and gradient-index coatings [21,22], which approximate a smooth refractive index gradient from that of air to that of silicon. Surface texturing approaches employing nanocone gratings/arrays [23][24][25], and biomimetic nanostructures [26], have also been explored. Arguably, however, all of these approaches have some serious limitations.…”

Fabrication and characterization of Al2O3 /Si composite nanodome structures for high efficiency crystalline Si thin film solar cells

“…Recently, absorption improvement in c-Si thin film solar cells by means of photonic management (PM) engineering, such as photonic crystals, 4,5 nano-wires (nano-rods), 6,7 nanopores (nanoholes), 8,9 nanocones (nanodomes), [10][11][12][13][14][15] and inverted nano-pyramids 16 has been theoretically studied. However, all these light-trapping structures inevitably lead to significant increase in the device surface area and introduce surface defects, resulting in elevated carrier loss.…”

Enhanced Surface Properties of Light-Trapping Si Nanowires Using Synergetic Effects of Metal-Assisted and Anisotropic Chemical Etchings

“…The R avg values of all tapered nanowires fabricated by ACE, further decrease to less than 2% showing a minimum value of 1.1% for the nanowire of ρ = 0.95. Figure 3(b) shows the reflectance spectra of flat (solid lines) and tapered (dots lines) Si nanowires simulated with the FDTD method 3,6,27,28 . The lengths of the Si nanowires considered in the simulation are used according to the SEM images of the nanowires shown in Fig.…”