Multishell nanowires for next-generation photovoltaics

Silicon nanowires (Si NWs) can enable low-cost and efficient photovoltaics, although nonideal electrical characteristics and insufficient control over absorption properties have precluded the promise of Si NWs for next-generation solar cells. We have overcome these limitations through controlled synthesis of core/multishell Si NWs (i.e., p-type/intrinsic/n-type NWs) with highly crystalline, well-defined and faceted surfaces [1]. Photovoltaic devices fabricated from the core/shell NWs with a diameter of ∼ 300nm exhibit good electrical characteristics, including ultralow leakage currents of ∼ 1 fA, and open-circuit voltages and fill factors up to 0.5V and 73%, respectively, under one-sun illumination. Photocurrent measurements of the single NWs reveal several unique absorption properties, characterized by size-dependent optical resonances and external quantum efficiencies exceeding unity resulting from optical antenna effects. These unconventional optical properties lead to current densities of a NW twice larger than that of silicon film with an equivalent thickness. To verify the experimental results and further understand light-matter interaction in NWs, we performed finite-difference time-domain (FDTD) simulations for the measured NW structures. The photocurrent measurements agree well with the numerical simulations, including peak amplitudes and positions. In addition, the measured peaks can be assigned to particular resonant modes (i.e., Fabry-Perot, whispering-gallery and higher order complex modes) by comparing the experimental spectrum to simulated one. Further optimized NW devices have achieved current densities up to 17mA/cm
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and power conversion efficiencies of ∼ 6%. We also presented preliminary results toward larger-scale NW photovoltaic arrays. For example, assembly of vertically-stacked NWs yielded current densities of 25mA/cm
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and further simulations predicted that the power conversion efficiency would approach ∼ 15% for 1 µm thick assemblies. Lastly, we investigated how morphological changes (i.e., different sizes and cross-sectional shapes) in NWs influence the absorption characteristics. To modulate these morphological parameters, we have developed new synthesis protocol for NW growth. Experimental, polarization-resolved external quantum efficiency spectra showed noticeable absorption properties depending on the size of NWs, including red-shifted resonant modes, increased spectral density and weakened optical antenna effects by increasing the size of NWs [2]. Additionally, we demonstrated that NWs with approximately rectangular cross-sections exhibit sharp and high amplitude absorption peaks at red to near-infrared wavelengths, as compared to absorption peaks at the similar wavelengths from hexagonal NWs. The ability to modulate absorption through subtle and controllable changes in NWs represents a promising route for developing new photovoltaic and optoelectronic devices.