Nanostructured surfaces

Investigation of the influence of geometric sizes and periodicity of silicon nanowires on the performance of photovoltaic cells on their basis
Nichkalo S.I., Druzhinin A.A., Yerokhov V.Yu., Syven’ka L.I.
Department of Semiconductor Electronics, Lviv Polytechnic National University, 
S. Bandera Str., 12, Lviv-79013, Ukraine
E-mail: nichkalo@gmail.com

It is known that optical absorption is one of the main factors that determine the efficiency of a photovoltaic cell based on silicon nanowires (Si NWs). Therefore, the characteristics of optical absorption must be investigated to optimize the geometric parameters of nanowires, such as periodicity, length, and diameter. However, the parasitic absorption losses on light trapping by substrates and surface defects during sample preparation may make ambiguous an accurate analysis of the optical characteristics of nanowires. 
In this paper, the characteristics of optical absorption of photovoltaic cells based on Si NWs were analyzed. Using the transfer-matrix method, we investigated how the period of nanowires' location depending on the wavelength of incident light affects the capture of light. The influence of geometry, period, diameter, and height of Si NWs on their optical properties is also analyzed. Silicon nanowires were prepared by conventional metal-assisted chemical etching method on Si substrate. Ag-nanoparticles deposited from an AgNO3 salt on the surface of Si substrate were used as a catalyst in the etching process.
As a result, it was found that periodicity affects light absorption. The analysis of the dependence of the light conversion efficiency on the periodicity of Si NWs showed that in the energy region of 2.5 eV the best period of nanowire arrangement was 550 nm. The conversion efficiency was 26.3%. These results are close to those calculated by the finite element method in [1, 2]. In addition, to ensure optimal conversion of solar energy, the ratio of the nanowires’ diameter to their periodicity should be greater than 0.5 (more precisely, 0.8). In our case, such an indicator is observed only for the nanowires with a periodicity of 300 nm.

1.	Schinke C., Peest P.C., Schmidt J., Brendel R., et al. Uncertainty analysis for the coefficient of band-to-band absorption of crystalline silicon // AIP Advances.– 2015.– 5.– P. 067168-1–067168-22.
2.	Hu L., Chen G. Analysis of optical absorption in silicon nanowire arrays for photovoltaic applications // Nano Lett.– 2007.– 7.– P. 3249–3252.