Photovoltaic cells implementing high-index planar materials suffer from reflection losses. Micro-sized scattering particles as well as antireflection coatings have been applied to lessen this reflection. Nanoimprinted dielectric materials have been used to enhance the in-coupling of light in planar materials. The enhanced coupling can be discussed in terms of Mie resonances that scatter the light in the forward direction.
Nanowire solar panels are an important field owing to intrinsic benefits over the planar solar devices, leading to higher photocurrent and photovoltage. However, the size and shape of the nanowire are necessary for attaining high absorption efficiency and the highest photocurrent of the III-V nanowire solar cell has been below that of the optimum planar cells fabricated with the same materials.
Dutch scientists led by professor Jos Haverkort at the Eindhoven University of Technology developed a InP nanowire solar cell where the indium-tin-oxide reflection loss was reduced significantly and the nanowire absorption enhanced with respect to a planar contact by applying Mie scattering of the nanostructured transparent contact layer. Their work is published in peer-reviewed journal, ACS Nano.
In a bid to produce the cells, n-and p-doped InP layers were fabricated on an InP substrate. An etching mask was made of a silicon nitride layer was realized by nanoimprint lithography. The nanowires were then etched in a plasma etching chamber. The side walls were then etched by surface oxidation. Chemical etching followed to scrap the oxide. The nanowires were covered with silicon oxide and the spaces between them filled with benzocyclobutene. Finally, an indium-tin-oxide layer was deposited as a conductive layer. The full cell was exposed to light. Photocurrent density was then calculated.
The authors fabricated solar cells based on InP nanowires, which were then covered with silicon oxide to enhance adhesion with the material surrounding the layer. Benzocyclobutene layer was applied for insulation and planarization. It was then etched back to expose the ends of the nanowires. Benzocyclobutene was selected owing to its electrical insulation properties and transparency in the visible infrared. The indium-tin-oxide formed spherical particles because the deposited material stuck better on the InP than on the benzocyclobutene surface. The current-voltage of the nanowire solar cell was measured independently.
This paper shows that appropriately designed hemispherical nano-particles on top of the transparent top contact of the nanowire solar cell can enhance the absorption efficiency for the full wavelength and incident angle gap. Calculations as well as simulations indicated that forward Mie scattering was the approach behind this improvement. The absorption improvement that is wavelength averaged at normal incidence makes the self-aligned indium-tin-oxide hemispheres elementary design building blocks for solar cells based on nanowires. In this approach, the authors demonstrated a nanowire solar cell that was 17.8% efficient with a short-circuit current of 29.3 mA/cm2 measured at 1 sun incident light. This record efficiency is among the highest of III-V solar cells.
“Nanowire solar cells show enhanced light incoupling due to forward Mie scattering of the hemispherical ITO nanoparticles, but they also fundamentally outperform planar solar cells due to an enhanced light outcoupling in comparison with a planar solar cell. At V=Voc, a solar cell is not generating any current, implying that in a lossless cell, all absorbed light should be emitted externally”. Said Dr. Jos Haverkort.
Dick van Dam† , Niels J. J. van Hoof†‡, Yingchao Cui†, Peter J. van Veldhoven†, Erik P. A. M. Bakkers†§ , Jaime Gómez Rivas†‡, and Jos E. M. Haverkort*†. High-efficiency nanowire solar cells with omnidirectionally enhanced absorption due to self-aligned indium-tin-oxide mie scatters. ACS Nano, volume 10 (2016), pages 11414-11419.Show Affiliations
† Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
‡ Dutch Institute for Fundamental Energy Research DIFFER, P.O. Box 6336, 5600 HH Eindhoven, The Netherlands
§ Kavli Institute of Nanoscience, Delft University of Technology, 2600 GA Delft, The Netherlands
Y. Cui, D. van Dam, S.A. Mann, N.J.J. van Hoof, P.J. van Veldhoven, E.C. Garnett, E.P.A.M. Bakkers, J.E.M. Haverkort, Boosting solar cell photovoltage via nanophotonic engineering, Nano Lett 16, 6467-6471 (2016).