Photovoltaic properties of axial in-situ doped SiGe heteronanowires

We report on vapor-liquid-solid (VLS) growth and photovoltaic properties of axial in-situ doped Ge-Si heteronanowire (hetero-NW) solar cells. Modern VLS growth has recently demonstrated the growth of Si-Ge axial NWs with abrupt heterojunctions [1], and simultaneous control of both material composition and doping profiles. This advance allows for the integration of Ge with Si in a hetero-NW structure for broad-spectrum and high absorption efficiency solar cells. Our preliminary optical measurements on $p$-Ge/$i$-Si/$n-$Si wires provide important ingredients for a hetero-NW photovoltaic device. We achieved good rectification with $\sim $ 10$^{3}$ ratio between forward and reverse bias currents at moderate voltage. Under laser illumination (532 nm), we measured a large open circuit voltage $V_{OC} \quad \sim $ 0.54 V and a high short-circuit current density $J_{SC} \quad \sim $ 4x10$^{3}$A/cm$^{2}$, comparable to state-of-the-art reported single NW results [2]. Additional optimization of separate Ge and Si \textit{pin} NW structures and their integration in a combined Ge/Si tandem hetero-NW solar cell will be reported. \\[4pt] [1] D. E. Perea \textit{et al.}, \textit{Nano Lett.} \textbf{11}, 3117 (2011). \\[0pt] [2] B. Tian \textit{et al.}, \textit{Chem. Soc. Rev.} \textbf{38}, 16 (2009).