Coaxial multishell nanowires with high-quality electronic interfaces and tunable optical cavities for ultrathin photovoltaics
Kempa, T. J.
Cahoon, J. F.
Day, R. W.
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CitationKempa, T. J., J. F. Cahoon, S.-K. Kim, R. W. Day, D. C. Bell, H.-G. Park, and C. M. Lieber. 2012. “Coaxial Multishell Nanowires with High-Quality Electronic Interfaces and Tunable Optical Cavities for Ultrathin Photovoltaics.” Proceedings of the National Academy of Sciences 109 (5) (January 19): 1407–1412. doi:10.1073/pnas.1120415109.
AbstractSilicon nanowires (NWs) could enable low-cost and efficient photovoltaics, though their performance has been limited by nonideal electrical characteristics and an inability to tune absorption properties. We overcome these limitations through controlled synthesis of a series of polymorphic core/multishell NWs with highly crystalline, hexagonally-faceted shells, and well-defined coaxial Graphic (p/n) and p/intrinsic/n (p/i/n) diode junctions. Designed 200–300 nm diameter p/i/n NW diodes exhibit ultralow leakage currents of approximately 1 fA, and open-circuit voltages and fill-factors up to 0.5 V and 73%, respectively, under one-sun illumination. Single-NW wavelength-dependent photocurrent measurements reveal size-tunable optical resonances, external quantum efficiencies greater than unity, and current densities double those for silicon films of comparable thickness. In addition, finite-difference-time-domain simulations for the measured NW structures agree quantitatively with the photocurrent measurements, and demonstrate that the optical resonances are due to Fabry-Perot and whispering-gallery cavity modes supported in the high-quality faceted nanostructures. Synthetically optimized NW devices achieve current densities of 17 mA/cm2 and power-conversion efficiencies of 6%. Horizontal integration of multiple NWs demonstrates linear scaling of the absolute photocurrent with number of NWs, as well as retention of the high open-circuit voltages and short-circuit current densities measured for single NW devices. Notably, assembly of 2 NW elements into vertical stacks yields short-circuit current densities of 25 mA/cm2 with a backside reflector, and simulations further show that such stacking represents an attractive approach for further enhancing performance with projected efficiencies of > 15% for 1.2 μm thick 5 NW stacks.
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