Enhanced absorption and carrier collection in Si wire arrays for photovoltaic applications

Journal name:
Nature Materials
Year published:
Published online
Corrected online

Si wire arrays are a promising architecture for solar-energy-harvesting applications, and may offer a mechanically flexible alternative to Si wafers for photovoltaics1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17. To achieve competitive conversion efficiencies, the wires must absorb sunlight over a broad range of wavelengths and incidence angles, despite occupying only a modest fraction of the array’s volume. Here, we show that arrays having less than 5% areal fraction of wires can achieve up to 96% peak absorption, and that they can absorb up to 85% of day-integrated, above-bandgap direct sunlight. In fact, these arrays show enhanced near-infrared absorption, which allows their overall sunlight absorption to exceed the ray-optics light-trapping absorption limit18 for an equivalent volume of randomly textured planar Si, over a broad range of incidence angles. We furthermore demonstrate that the light absorbed by Si wire arrays can be collected with a peak external quantum efficiency of 0.89, and that they show broadband, near-unity internal quantum efficiency for carrier collection through a radial semiconductor/liquid junction at the surface of each wire. The observed absorption enhancement and collection efficiency enable a cell geometry that not only uses 1/100th the material of traditional wafer-based devices, but also may offer increased photovoltaic efficiency owing to an effective optical concentration of up to 20 times.

At a glance


  1. Structure of Si wire arrays prepared for optical measurements.
    Figure 1: Structure of Si wire arrays prepared for optical measurements.

    a, SEM image of a peeled-off, polymer-embedded wire array, viewed upside-down (at 60° tilt) to illustrate the order and fidelity of the embedded wires. b, Schematic of the illumination conditions and definition of the incidence angles θx and θy.

  2. Representative composition and optical properties of each wire-array tiling pattern.
    Figure 2: Representative composition and optical properties of each wire-array tiling pattern.

    The scale bars in the left column apply to all images across each row. Top row: SEM images of as-grown wire arrays viewed from a top-down perspective. Second row: SEM images viewed at a 20° angle. Third row: Transmitted diffraction patterns of polymer-embedded wire arrays on a quartz slide, observed at λ=488nm. The axes indicate 4,000cm−1 in the direction of kx and ky. Bottom row: Integrated transmission of each wire array observed at λ=550nm as a function of the beam incidence angle (θx, θy).

  3. Light-trapping techniques and figure of merit (Aavg) calculation.
    Figure 3: Light-trapping techniques and figure of merit (Aavg) calculation.

    a,b, Schematic and measured absorption of a ηf = 4.2% square-tiled wire array on a quartz slide (a) and on a Ag back-reflector (b). c,d, Schematic and measured absorption of this array with an antireflective coating and embedded light-scatterers, measured on a quartz slide (c) and on a Ag back-reflector (d). e, A plot of Aavg calculations corresponding to each absorption measurement shown in ad, showing the incident sunlight and spectrally weighted absorption of each throughout the day, compared with the measured absorption of a commercial, antireflective-coated, polycrystalline Si solar cell.

  4. Measured Si wire-array absorption versus theoretical absorption of an equivalently thick, planar Si absorber.
    Figure 4: Measured Si wire-array absorption versus theoretical absorption of an equivalently thick, planar Si absorber.

    a, Measured absorption (AWA, red) of the Si wire array from Fig. 3d (which had an equivalent planar Si thickness of 2.8μm), at normal (solid) and 50° (dashed) incidence, versus the calculated normal-incidence absorption of a 2.8-μm-thick planar Si absorber, with an ideal back-reflector, assuming: bare, non-texturized surfaces (ASi, black) and ideally light-trapping, randomly textured surfaces (ALT, blue). b, Illustration of the normal-incidence, spectrally weighted absorption of the AM 1.5D reference spectrum, corresponding to each of the three absorption cases plotted above.

  5. Photoelectrochemical characterization of Si wire arrays.
    Figure 5: Photoelectrochemical characterization of Si wire arrays.

    a, Schematic of the photoelectrochemical cell and definition of illumination angles. b, Effect of light-scattering particles on wire-array electrode EQE. Top: SEM image; centre: two-dimensional angle-resolved EQE at λ=550nm; bottom: wavelength-angle-resolved EQE at θy=0°, of a Si wire-array electrode without (left) and with (right) Al2O3 light-scattering particles. c, Normal-incidence absorption measurement of a polymer-embedded wire array (black) and normal-incidence EQE of a wire-array electrode (blue). A 50 nm running average was applied to the absorption, to reduce interference fringes in the experimental data. The resulting IQE is plotted in red. Inset: SEM images of the wire-array electrode (left) and the polymer-embedded wire array (right).

Change history

Corrected online 19 February 2010
In the version of this Letter originally published, the first sentence in the Acknowledgements should have been: “This work was supported by BP and in part by the Department of Energy EFRC program under grant DE-SC0001293, and made use of facilities supported by the Center for Science and Engineering of Materials, an NSF Materials Research Science and Engineering Center at Caltech.”
This has been corrected in the HTML and PDF versions of this Letter.


  1. Kayes, B. M., Atwater, H. A. & Lewis, N. S. Comparison of the device physics principles of planar and radial p–n junction nanorod solar cells. J. Appl. Phys. 97, 114302114311 (2005).
  2. Garnett, E. C. & Yang, P. Silicon nanowire radial p–n junction solar cells. J. Am. Chem. Soc. 130, 92249225 (2008).
  3. Kelzenberg, M. D., Putnam, M. C., Turner-Evans, D. B., Lewis, N. S. & Atwater, H. A. Proc. 34th IEEE Photovoltaic Specialists Conference 16 (IEEE, 2009).
  4. Tsakalakos, L. et al. Silicon nanowire solar cells. Appl. Phys. Lett. 91, 233117 (2007).
  5. Stelzner, T. et al. Silicon nanowire-based solar cells. Nanotechnology 19, 295203 (2008).
  6. Gunawan, O. & Guha, S. Characteristics of vapor–liquid–solid grown silicon nanowire solar cells. Sol. Energy Mater. Sol. Cells 93, 13881393 (2009).
  7. Peng, K. et al. Aligned single-crystalline Si nanowire arrays for photovoltaic applications. Small 1, 10621067 (2005).
  8. Sivakov, V. et al. Silicon nanowire-based solar cells on glass: Synthesis, optical properties, and cell parameters. Nano Lett. 9, 15491554 (2009).
  9. Altermatt, P. P., Yang, Y., Langer, T., Schenk, A. & Brendel, R. Proc. 34th IEEE Photovoltaic Specialists Conference 16 (IEEE, 2009).
  10. Hu, L. & Chen, G. Analysis of optical absorption in silicon nanowire arrays for photovoltaic applications. Nano Lett. 7, 32493252 (2007).
  11. Muskens, O. L., Rivas, J. G. m., Algra, R. E., Bakkers, E. P. A. M. & Lagendijk, A. Design of light scattering in nanowire materials for photovoltaic applications. Nano Lett. 8, 26382642 (2008).
  12. Zhu, J. et al. Optical absorption enhancement in amorphous silicon nanowire and nanocone arrays. Nano Lett. 9, 279282 (2009).
  13. Tian, B. et al. Coaxial silicon nanowires as solar cells and nanoelectronic power sources. Nature 449, 885889 (2007).
  14. Goodey, A. P., Eichfeld, S. M., Lew, K.-K., Redwing, J. M. & Mallouk, T. E. Silicon nanowire array photoelectrochemical cells. J. Am. Chem. Soc. 129, 1234412345 (2007).
  15. Maiolo, J. R. I. et al. High aspect ratio silicon wire array photoelectrochemical cells. J. Am. Chem. Soc. 129, 1234612347 (2007).
  16. Plass, K. E. et al. Flexible polymer-embedded Si wire arrays. Adv. Mater. 21, 325328 (2009).
  17. Spurgeon, J. M. et al. Repeated epitaxial growth and transfer of arrays of patterned, vertically aligned, crystalline Si wires from a single Si(111) substrate. Appl. Phys. Lett. 93, 032112032113 (2008).
  18. Tiedje, T., Yablonovitch, E., Cody, G. D. & Brooks, B. G. Limiting efficiency of silicon solar-cells. IEEE Trans. Electron Devices 31, 711716 (1984).
  19. Wagner, R. S. & Ellis, W. C. Vapor–liquid–solid mechanism of single crystal growth. Appl. Phys. Lett. 4, 8990 (1964).
  20. Putnam, M. C. et al. 10μm minority-carrier diffusion lengths in Si wires synthesized by Cu-catalyzed vapor–liquid–solid growth. Appl. Phys. Lett. 95, 163116 (2009).
  21. Kelzenberg, M. D. et al. Photovoltaic measurements in single-nanowire silicon solar cells. Nano Lett. 8, 710714 (2008).
  22. Kelzenberg, M. D. et al. Proc. 33rd IEEE Photovoltaic Specialists Conference 16 (IEEE, 2008).
  23. Kayes, B. M. et al. Growth of vertically aligned Si wire arrays over large areas (>1cm2) with Au and Cu catalysts. Appl. Phys. Lett. 91, 103110103113 (2007).
  24. Marion, B. et al. Validation of a photovoltaic module energy ratings procedure at NREL. Report No. NREL/TP-520-26909 (1999).
  25. Aspnes, D. E. in Properties of Crystalline Silicon (ed. Robert, H.) 683690 (INSPEC, IEE, 1999).
  26. Yablonovitch, E. Statistical ray optics. J. Opt. Soc. Am. 72, 899907 (1982).
  27. Boettcher, S. W. et al. Energy-conversion properties of vapor–liquid–solid-grown silicon wire-array photocathodes. Science 327, 185187 (2010).
  28. Tsakalakos, L. et al. Strong broadband optical absorption in silicon nanowire films. J. Nanophoton. 1, 013552 (2007).
  29. Campbell, P. & Green, M. A. The limiting efficiency of silicon solar cells under concentrated sunlight. IEEE Trans. Electron Devices 33, 234239 (1986).
  30. Kupec, J. & Witzigmann, B. Dispersion, wave propagation and efficiency analysis of nanowire solar cells. Opt. Express 17, 1039910410 (2009).
  31. Yoon, J. et al. Ultrathin silicon solar microcells for semitransparent, mechanically flexible and microconcentrator module designs. Nature Mater. 7, 907915 (2008).
  32. Fan, Z. et al. Three-dimensional nanopillar-array photovoltaics on low-cost and flexible substrates. Nature Mater. 8, 648653 (2009).
  33. Putnam, M. C. et al. Secondary ion mass spectrometry of vapor–liquid–solid grown, Au-catalyzed, Si wires. Nano Lett. 8, 31093113 (2008).

Download references

Author information


  1. California Institute of Technology, 1200 E California Blvd, MC 129-95, Pasadena, California 91125, USA

    • Michael D. Kelzenberg,
    • Shannon W. Boettcher,
    • Jan A. Petykiewicz,
    • Daniel B. Turner-Evans,
    • Morgan C. Putnam,
    • Emily L. Warren,
    • Joshua M. Spurgeon,
    • Ryan M. Briggs,
    • Nathan S. Lewis &
    • Harry A. Atwater


M.D.K. participated in the design and execution of the experiments, analysed the results and prepared the manuscript under the advisement of H.A.A. and the guidance of N.S.L. and S.W.B. J.A.P. contributed to the design and fabrication of the array template photomasks, the integrating-sphere apparatus and the image processing software. S.W.B., J.M.S., J.A.P., M.C.P. and D.B.T-E. assisted in the fabrication of the wire arrays and R.M.B. carried out the deposition and characterization of the SiNx antireflective coating. S.W.B., E.L.W. and J.M.S. assisted with the photoelectrochemical measurements and fabricated the electrodes. All authors discussed the results and commented on the manuscript.

Competing financial interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to:

Author details

Supplementary information

PDF files

  1. Supplementary Information (1.78 MB)

    Supplementary Information

Additional data