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Coaxial silicon nanowires as solar cells and nanoelectronic power sources
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Letter
Nature 449, 885-889 (18 October 2007) | doidoidoidoidoidoidoidoi:10.1038/nature06181; Received 15 May 2007; Accepted 7 August 2007
Coaxial silicon nanowires as solar cells and nanoelectronic power sources
Bozhi Tian1,3, Xiaolin Zheng1,3, Thomas J. Kempa1, Ying Fang1, Nanfang Yu2, Guihua Yu1, Jinlin Huang1 & Charles M. Lieber1,2
- Department of Chemistry and Chemical Biology,
- School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA
- These authors contributed equally to this work.
Correspondence to: Charles M. Lieber1,2 Correspondence and requests for materials should be addressed to C.M.L. (Email: cml@cmliris.harvard.edu).
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 nanostructures or nanostructured materials represents a general approach to reduce both cost and size and to improve efficiency in photovoltaics1, 2, 3, 4, 5, 6, 7, 8, 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 photovoltaic devices3, 4, 5, 6. Moreover, recent theoretical studies have indicated 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 efficiencies and poor stabilities1. In addition, previous studies have not yet addressed their use as photovoltaic power elements in nanoelectronics. 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 elements yield a maximum power output of up to 200 pW per nanowire 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 and logic gates. These coaxial silicon nanowire photovoltaic elements provide a new nanoscale test bed for studies of photoinduced energy/charge transport and artificial photosynthesis10, and might find general usage as elements for powering ultralow-power electronics11 and diverse nanosystems12, 13.
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), 200nm (bottom). The p-i-n
silicon nanowire was grown with 100-nm-diameter gold catalyst, and with iand
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-aberrationcorrected)
of the p-i-n coaxial silicon nanowire. Scale bar, 5 nm.
Figure 2 | Device fabrication and diode characterization. a, Schematics of
device fabrication. Left, pink, yellow, cyan and green layers correspond to the
p-core, i-shell, n-shell and PECVD-coated SiO2, respectively. Middle,
selective etching to expose the p-core. Right, metal contacts deposited on the
p-core and n-shell. b, SEM images corresponding to schematics in a. Scale
bars are 100nm (left), 200nm (middle) and 1.5 mm (right).
认真读完这篇文章.觉得他们的这个观点的提出很不错.以前就有理论预言, coaxial纳米线有利于载流子的高效快速分离和收集，他们在这文章里就给出了一个p-i-n结构的好例子．文章的新意估计不外乎两点：1,首次实现单根的 solar cell.2, 提出了将这种优异的结构作为纳米级器件的power source. 说到这里，我想到了zhonglin wang在 science上的ZnO纳米发电机.在那里，王利用压电效应能够使机械能转换电能；在这里，lieber他们是将光能转换为电能，目的一样：都是为纳米级的器件提供电源。看来，大牛们早就有这个共同的思想了，呵呵。事实上，lieber的这文章恰好引用了王的两篇 science. 另外，lieber还是文章撰写的主要人物。
总结了下他们在文章提到的几个关键因素：
1，p-i-n的形成很重要， intrinsic硅的存在对二极管的特性影响很大．有了这一层，击穿电压变大。
2，在太阳能电池的效率提高上，外层的多晶n tpye硅的形成至关重要。We note that this nanocrystalline
shell structure could enhance light absorption in the nanowires。其次，外层的结构表现出很好的导电性， 有利于载流子的传输，分离收集。 The highly conductive n-shell will reduce or eliminate
potential drop along the shell, thereby enabling uniform radial
carrier separation and collection when illuminated。（纳米材料的合成设计还是相当重要，呵呵）
3. 太阳光照射，并且电池很稳定，七个月功率还保持不变。
从纳米线的诞生，到单根纳米线激光器，纳米LED激光器，纳米压电发电机，再到现在的单根太阳能电池，似乎更坚信纳米的广阔前景了。
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