Silicon nanowire arrays with passivated axial p-i-n junctions for photovoltaic applications

Metal catalyst-assisted vapor-liquid-solid mechanism can be used to grow large areas of nanowires (NWs) with compositional and doping control in either axial or core-shell geometries. Here, we report on vertical arrays of Si axial $p$-$i$-$n$ oxide-passivated NWs that were 12 microns long with a 4 micron intrinsic section. The NW arrays were planarized using SU-8 photoresist, followed by reactive ion etching to expose the NW tips. Top $n$-contact was realized by sputter deposition of a 200 nm IZO layer. The $p$-contact was made by backside metallization of the $p$-Si substrate. Under AM 1.5 illumination, unpassivated NW arrays exhibited an open-circuit voltage, $V_{\mathrm{OC}}$ of 170 mV, a short-circuit current density $J_{\mathrm{SC}}$ \textgreater 3.7 mA/cm$^{2}$ (with uncertainty due to the unknown fraction of properly contacted NWs), and a fill factor of 28.9{\%}. After the passivation, $V_{\mathrm{OC}}$, $J_{\mathrm{SC}}$ and FF increased to 250 mV, \textgreater 9.2 mA/cm$^{2}$ and 35.7{\%}, respectively. The measured normal reflectance was around 6{\%} over the 400--1000 nm spectral range, whereas the diffuse reflectance was around 20{\%} over the same range, indicating strong light scattering and absorption by the NWs. The photovoltaic performance of passivated single NWs and NW arrays were compared using a 532 nm laser with a power density of about 10 W/cm$^{\mathrm{2}}$. Higher values of $V_{\mathrm{OC}}$ and FF obtained for the latter are explained by light trapping in the NW arrays.