Nanostructured surface 

Optical and Electrical Properties of Nanowires Based Solar Cell with Radial p-n Junction 
Evtukh A.A.1,2, Parfenyuk P.V.1, Pylypova O.V.2, Ivanov I.I.2, 
Korobchuk I.M.2, Havryliuk O.O.3, Semchuk O.Yu.3
1 V. Lashkaryov Institute of Semiconductor Physics NAS of Ukraine. 41 pr. Nauki,  Kyiv-03028, Ukraine. E-mail: anatoliy.evtukh@gmail.com
2 Taras Shevchenko National University of Kyiv. 60 Volodymyrska Str, Kyiv-01033, Ukraine. 
3 Chuiko Institute of Surface Chemistry NAS of Ukraine. 17 Ganeral Naumova Str., Kyiv-03164, Ukraine. 

Silicon nanowires (SiNWs) array shows unique electrical and optical properties compared to bulk silicon, which makes it to be a potential candidate for photovoltaic applications. The radial (coaxial) p–n junction can enable efficient carrier collection, and the open array structure can significantly enhance optical absorption. Absorption enhancement can be attributed to three major mechanisms. First, the NWs array structure has remarkably small reflectivity owing to the large open area on the frontal surface. Second, each individual NW is a nanoscale cylindrical resonator, which can trap light by multiple total internal reflections. Third, strong interwire light scattering occurs to cause further light trapping because the NWs diameter and period are normally of the same order as the optical wavelength. 
In our studies the absorption, transmittance and reflectance spectra for periodic nanostructures with different parameters were calculated by the FDTD (Finite-Difference Time-Domain) method. It is shown that the proportion of reflected light in periodic structures is smaller than in case of thin films. The experimental results showed the light reflectance in the spectral range 400-900 nm lower than 1% and it was significantly lower in comparison with surface texturing by pyramids or porous silicon. 
Silicon nanowires on p-type Si substrate were formed by the Metal-Assisted Chemical Etching method (MecEtch). At solar cells with radial p-n junction formation the thermal diffusion of phosphorus has been used at 790 ºC. Such low temperature ensures the formation of the ultra-shallow p-n junction. Investigation of the photoelectrical properties of solar cells was carried out under light illumination with intensity of 100 mW/cm2. The obtained parameters of NWs solar cell were Isc= 29 mA/cm2, Uoc= 0.5 V, FF = 0.34 for an overall efficiency η = 4.97%. The relatively low efficiency of obtained SiNWs solar cells is attributed to the excessive surface recombination at high surface areas of SiNWs and high series resistance.