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

OutlineLiterature 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.

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.

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.

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

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 Voc = 0.260V;

V is essentially independent of length.

Isc = 0.503 nA;

Ffill = 55.0 %

Maximum Power = 72 pW, stable for seven months

Power Conversion Efficiency = ~ 3%

Radial and axial p-i-n structural

JscRadial =24 mA cm-2

JscAxial =3.5 mA cm-2

VocRadial = 0.26VVocAxial =0.29 V Efficiencies ;ηRadial =3.4%ηAxial =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)

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.

Supplementary Nanowire pH sensor

Capacitance Equations of Core shell

Capacitance Equations of Core shell