Vol 449 18 October 2007 doi: 10.1038/ nature06181 nature LETTERS Coaxial silicon nanowires as solar cells and nanoelectronic power sources Bozhi Tian *, Xiaolin Zheng *, Thomas J Kempa, Ying Fang!, Nanfang Yu, Guihua Yu, Jinlin Huang Charles m. Lieber 2 cells are attractive candidates for clean and renewable minimal amounts of metal catalyst were incorporated into the silicon s with miniaturization, they might also serve as integrated nanowire structure. Scanning electron microscopy ( SEm)images of a sources for nanoelectronic systems. The use of nano- typical p-i-n coaxial silicon nanowire recorded in the back-scattered structures or nanostructured materials represents a general electron mode(Fig. Ib)highlight several key features. First, the approach to reduce both cost and size and to improve efficiency uniform contrast of the nanowire core is consistent with a single in photovoltaics-. Nanoparticles, nanorods and nanowires have crystalline structure expected for silicon nanowires obtained by the been used to improve charge collection efficiency in polymer- VLS method.ls. Second, contrast variation observed in the shells multiplication, and to enable low-temperature processing of pho- 30-80 nm. Third, the core/shell silicon nanowires have uniform tovoltaic devices-. Moreover, recent theoretical studies have indi- cated that coaxial nanowire structures could improve carrier collection and overall efficiency with respect to single-crystal bulk a semiconductors of the same materials. However solar cell based on hybrid nanoarchitectures suffer from relatively low effi ciencies and poor stabilities. In addition, previous studies have not yet addressed their use as photovoltaic power elements in nano- Liquid.(si electronics. Here we report the realization of p-type/intrinsic/ n-typep-i-n)coaxial silicon nanowire solar cells Under one solar equivalent(l-sun)illumination, the p-i-n silicon nanowire ele- ments yield a maximum power output of up to 200 pw per nano- wire device and an apparent energy conversion efficiency of up to 3.4 per cent, with stable and improved efficiencies achievable at b Percentage of si atoms high-flux illuminations. Furthermore, we show that individual and interconnected silicon nanowire photovoltaic elements can serve as robust power sources to drive functional nanoelectronic sensors andlogic gates. These coaxial silicon nanowire photovoltaic elements provide a new nanoscale test bed for studies of photo- induced energy/charge transport and artificial photosynthesis and might find general usage as elements for powering ultralow- power electronics and diverse nanosystems We have focused on p-i-n coaxial silicon nanowire structures(Fig la)consisting of a p-type silicon nanowire core capped with i-and n-type silicon shells. An advantage of this core/shell architecture is hat carrier separation takes place in the radial versus the longer axial direction, with a carrier collection distance smaller or comparable to Figure 1 Schematics and electron microscopy images of the p-i-n coaxial the minority carrier diffusion length. Hence, photogenerated car- its cross-sectional diagram shows that the photogenerated electrons ( e riers can reach the p-i-n junction with high efficiency without sub- and holes(h+)are swept into the n-shell and p-core, respectively, by the stantial bulk recombination. An additional consequence of this built-in electric field. The phase diagram of gold(Au)-silicon (Si)alloy or geometry is that material quality can be lower than in a traditional the right panel illustrates that the core is grown by means of the VLS p-n junction device without causing large bulk recombination mechanism, whereas the shells are deposited at higher temperature n nanowire res were synthesized by means of a lower to inhibit further nanowire axial elongation. b, SEM nocluster-catalysed vapour-liquid-solid(VLS) method (back-scattered electron mode)of the p-i-n coaxial silicon nanowir con shells were then deposited at a higher temperature and lower different magnifications Scale bar, lum(top), 200 nm(bottom). The p-i-n pressure than for p-core growth(Fig. la, right panel)to inhibit axial and n-shell growth times of 60 min and 30 min, respectively. The feeding elongation of the silicon nanowire core during the shell deposition, ratios of silicon: boron and silicon phosphorus are 500: 1 and 200:1 where phosphine was used as the n-type dopant in the outer shell. respectively. c, High-resolution TEM image(spherical-aberration The growth temperatures were sufficiently low to ensure that corrected)of the p-i-n coaxial silicon nanowire Scale bar, 5nm. School of Engineering and Applied Sciences, Harvard University. Cambridge, Massachusetts 02138, USA. ese authors contributed equally to this 8s5 E2007 Nature Publishing GroupLETTERS Coaxial silicon nanowires as solar cells and nanoelectronic power sources Bozhi Tian1 *, Xiaolin Zheng1 *, Thomas J. Kempa1 , Ying Fang1 , Nanfang Yu2 , Guihua Yu1 , Jinlin Huang1 & Charles M. Lieber1,2 Solar cells are attractive candidates for clean and renewable power1,2; with miniaturization, they might also serve as integrated power sources for nanoelectronic systems. The use of nano￾structures or nanostructured materials represents a general approach to reduce both cost and size and to improve efficiency in photovoltaics1–9. Nanoparticles, nanorods and nanowires have been used to improve charge collection efficiency in polymer￾blend4 and dye-sensitized solar cells5,6, to demonstrate carrier multiplication7 , and to enable low-temperature processing of pho￾tovoltaic devices3–6. Moreover, recent theoretical studies have indi￾cated that coaxial nanowire structures could improve carrier collection and overall efficiency with respect to single-crystal bulk semiconductors of the same materials8,9. However, solar cells based on hybrid nanoarchitectures suffer from relatively low effi￾ciencies and poor stabilities1 . In addition, previous studies have not yet addressed their use as photovoltaic power elements in nano￾electronics. Here we report the realization of p-type/intrinsic/ n-type (p-i-n) coaxial silicon nanowire solar cells. Under one solar equivalent (1-sun) illumination, the p-i-n silicon nanowire ele￾ments yield a maximum power output of up to 200 pW per nano￾wire device and an apparent energy conversion efficiency of up to 3.4 per cent, with stable and improved efficiencies achievable at high-flux illuminations. Furthermore, we show that individual and interconnected silicon nanowire photovoltaic elements can serve as robust power sources to drive functional nanoelectronic sensors andlogic gates. These coaxial silicon nanowire photovoltaic elements provide a new nanoscale test bed for studies of photo￾induced energy/charge transport and artificial photosynthesis10, and might find general usage as elements for powering ultralow￾power electronics11 and diverse nanosystems12,13. We have focused on p-i-n coaxial silicon nanowire structures (Fig. 1a) consisting of a p-type silicon nanowire core capped with i- and n-type silicon shells. An advantage of this core/shell architecture is that carrier separation takes place in the radial versus the longer axial direction, with a carrier collection distance smaller or comparable to the minority carrier diffusion length8 . Hence, photogenerated car￾riers can reach the p-i-n junction with high efficiency without sub￾stantial bulk recombination. An additional consequence of this geometry is that material quality can be lower than in a traditional p-n junction device without causing large bulk recombination1 . Silicon nanowire p-cores were synthesized by means of a nanocluster-catalysed vapour–liquid–solid (VLS) method14,15. Sili￾con shells were then deposited at a higher temperature and lower pressure than for p-core growth (Fig. 1a, right panel) to inhibit axial elongation of the silicon nanowire core during the shell deposition, where phosphine was used as the n-type dopant in the outer shell15. The growth temperatures were sufficiently low to ensure that minimal amounts of metal catalyst were incorporated into the silicon nanowire structure. Scanning electron microscopy (SEM) images of a typical p-i-n coaxial silicon nanowire recorded in the back-scattered electron mode (Fig. 1b) highlight several key features. First, the uniform contrast of the nanowire core is consistent with a single￾crystalline structure expected for silicon nanowires obtained by the VLS method14,15. Second, contrast variation observed in the shells is indicative of a polycrystalline structure grain of the order of 30–80 nm. Third, the core/shell silicon nanowires have uniform *These authors contributed equally to this work. 1 Department of Chemistry and Chemical Biology, 2 School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA. Shell growth Core growth 1,500 300 Au Si 0 100 Percentage of Si atoms n Temperature (°C) i p a b c Liquid + (Si) Liquid (Au) + Liquid h+ e– Figure 1 | Schematics and electron microscopy images of the p-i-n coaxial silicon nanowire. a, Illustrations of the core/shell silicon nanowire structure; its cross-sectional diagram shows that the photogenerated electrons (e2) and holes (h1) are swept into the n-shell and p-core, respectively, by the built-in electric field. The phase diagram of gold (Au)–silicon (Si) alloy on the right panel illustrates that the core is grown by means of the VLS mechanism, whereas the shells are deposited at higher temperature and lower pressure to inhibit further nanowire axial elongation. b, SEM images (back-scattered electron mode) of the p-i-n coaxial silicon nanowire at two different magnifications. Scale bar, 1 mm (top), 200 nm (bottom). The p-i-n silicon nanowire was grown with 100-nm-diameter gold catalyst, and with i￾and n-shell growth times of 60 min and 30 min, respectively. The feeding ratios of silicon:boron and silicon:phosphorus are 500:1 and 200:1, respectively. c, High-resolution TEM image (spherical-aberration￾corrected) of the p-i-n coaxial silicon nanowire. Scale bar, 5 nm. Vol 449| 18 October 2007| doi:10.1038/nature06181 885 ©2007 NaturePublishingGroup