Coaxial multishell nanowires with high-quality electronic interfaces and tunable optical cavities for ultrathin photovoltaics

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 109; no. 5; pp. 1407 - 1412
• Main Authors: Kempa, Thomas J, Cahoon, James F, Kim, Sun-Kyung, Day, Robert W, Bell, David C, Park, Hong-Gyu, Lieber, Charles M
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 31.01.2012; National Acad Sciences
• Subjects: Crystals; Current density; Electric current; Electronics; horizontal integration; lighting; Materials; Microscopy, Electron, Transmission; Nanostructured materials; Nanowires; Narrative devices; Photochemistry; Photovoltaic cells; Photovoltaics; Physical Sciences; Quantum efficiency; Silicon; Solar energy; Wavelengths
• ISSN: 0027-8424; 1091-6490; 1091-6490
• DOI: 10.1073/pnas.1120415109

Silicon 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 Formula (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.