Collaborators: Pengyu Fan, Alok Vasudev, Justin S. White, Zongfu Yu, Wenshan Cai, Jon A Schuller, Shanhui Fan.

Semiconductor Nanowire Antenna Solar Cells Linyou Cao, Mark L. Brongersma

Geballe Lab for Advanced Materials, Stanford University, CA 94305

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Reference Linyou. Cao, Pengyu Fan, Alok Vasudev, Justin S. White, Zongfu Yu, Wenshan Cai, Jon A Schuller, Shanhui Fan. Mark L. Brongersma. Nano. Lett. Submitted

Leaky Mode Resonances(LMRs)

Acknowledgement

1. Nanowires may support leaky optical modes.

2. When incident wavelengths match any of the leaky modes (leaky mode resonances), the nanowires acs as cylindrical cavity antennas that confines incident radiation inside.

A. Enhanced short circuit current density

Resonancecondi+on:

k0, free space wavevector J( ) Bessel function H( ) Hankel function of first kind The prime indicates differentiation n, integer number m,refractive index of nanowire λ, wavelength in free space

n(λ/m)≈2πr

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Jn (mk0r)Hn' (k0r) −mJn

' (mk0r)Hn (k0r) = 0

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mHn' (k0r)Jn (mk0r) − Jn

' (mk0r)Hn (k0r) = 0

TMLMR

TELMR

Approximately,

LMRs in Si Nanowires(SiNWs)

1. The observed absorption spectra of SiNWs dominantly feature with resonant absorption peaks.

2. The nice consistence between experimental observations and theoretical calculations using Lorentz-Mie formalism indicates that the absorption peaks are due to (LMRs in the NWs.

B. Minimized angle-dependence

Generality in Other Nanowires

Generality in Other Geometries

Design for Practical Solar Cells

The short circuit current density of SiNWs is signficantly larger than that of Si thin film with comparale thickness.

100 200 300 400 500

1

3

5

7

Diameter (nm)

Phot

ocur

rent

enha

ncem

ent (

%)

Nanowire SiBulk Si

4

8

x 100%

Photocurrent JSC (mA /cm

2)

The angle-dependence of the photocurrent in NWs can be dramatically minimized if the diameter is properly designed, e.g. ~80 nm for SiNWs. That is due to the unique angle-dependence of LMRs (detail see Ref.).

A. The enhancement of JSC in NW is generally expected for any semiconductor solar cell materials

B.Tthe minimized angle-dependence applies to any solar cell materials as well. The specific diameters for the given results are, CuInGaSe, 180 nm, Ge, 140 nm, α-Si, 120 nm, CdTe, 140 nm, and GaAs, 120 nm.

Similar enhancement of JSC can be generally seen in other one-dimensional semiconductor nanostructures.

By patterning the semiconductor layer in a thin film PV cell into an array of nanowires (NWs), one can boost the short-circuit current by 25% while utilizing less than half of the semiconductor material (by 150% per unit volume material).

Conclusion

glass

α-Si

θ Improved angle-dependence can be also seen in the nanowire array. The given numbers refer to the spacing between the NWs.

We demonstrate optical antenna effects (leaky mode resonance) generally in semiconductor nanowires, and show that the optical resonance can dramatically boost solar absorption and decrease the angle-depence of the absorption.