1. Onur Ergen “ “ Coaxial Silicon Nanowires as Solar Cells and Nanoelectronic Power Sources” EE235 Student Presentation 2 march 2009 Electirical Engineering and computer science University of California, Berkeley onur@eecs.berkeley.edu

2. Outline Literature review: “Coaxial Silicon Nanowires as Solar Cells and Nanoelectronic Power Sources”  Advantage of this core/shell architecture.  Diode characteristic of p-i-n nanowire core shell strucuture.  Photovoltaic properties of the p-i-n coaxial silicon nanowire diodes.

4. Silicon p-i-n Nanowires Advantage of this core/shellstructure, ◦ Charge separation along radial vs.longer axial direction ◦ Photo generated carriers can reach the p-i-n junction without bulk recombination. ◦ Material quality can be lower Grown by VLS followed by CVD Single crystalline core, polycrystalline shell Tian, B.; Zheng, X.; Kempa, T.; Fang, Y.; Yu, N.; Yu, G.; Huang, J.; Lieber, C. M. Nature, 2007, 449, 885- 890.

5. Contact Formation by using Lithography Tian, B.; Zheng, X.; Kempa, T.; Fang, Y.; Yu, N.; Yu, G.; Huang, J.; Lieber, C. M. Nature, 2007, 449, 885-890.

6. Diode characteristic Highly conductive n-shell eliminate potential drop along the shell, ◦ enabling carrier separation and collection when illuminated. “ ‘AND’ and ‘OR’ diode logic gates. Ideality factor N, ◦ Np-i-n=1.96 for Np-n=4.52 p-i-n diode breaks down much larger. Low temperature high breakdown voltage. Tunnelling or leakage currents are more significiant for p-n

7. IV Curves Tian, B.; Zheng, X.; Kempa, T.; Fang, Y.; Yu, N.; Yu, G.; Huang, J.; Lieber, C. M. Nature, 2007, 449, 885-890. Illuminated under AM1.5 conditions V oc = 0.260V; V is essentially independent of length. I sc = 0.503 nA; F fill = 55.0 % Maximum Power = 72 pW, stable for seven months Power Conversion Efficiency = ~ 3%

8. Radial and axial p-i-n structural Jsc Radial =24 mA cm- 2 Jsc Axial =3.5 mA cm- 2 Voc Radial = 0.26V Voc Axial =0.29 V E ffi ciencies ; η Radial =3.4% η Axial =0.5%, Radial and Axial Single Nanowire Photovoltaics, B. Tian, T.J. Kempa and C.M. Lieber, "Single Nanowire Photovoltaics," Chem. Soc. Rev. 38, 16-24 (2009)

9. Conclusions Reducing recombination can increase device performance ◦ Nanowire is 6.6 % efficient at 80 K Possible to used as power sources in nanoelectronics ◦ Nanowire PV was used to drive a nanowire pH sensor without external power. Controlling the thickness of p-i-n layers have crucial importance for Voc. Tian, B.; Zheng, X.; Kempa, T.; Fang, Y.; Yu, N.; Yu, G.; Huang, J.; Lieber, C. M. Nature, 2007, 449, 885-890.

10. Supplementary Nanowire pH sensor

12. Capacitance Equations of Core shell