The light absorbing semiconductor layers of solar cells are traditionally made to absorb nearly all photons with energies exceeding the bandgap, more or less through choice of thickness according to the inverse of the absorption coefficient. A drastically different approach is to design a nanocomposite material with effective optical constants such that they approach the ideal conditions for light absorption for a given, very small thickness. By combining semiconducting layers suitable for solar energy conversion with noble metal nanoparticles supporting localized surface plasmon resonances in the visible to near infrared range, it has been theoretically and experimentally demonstrated that such conditions can readily be accomplished with mass equivalent thicknesses on the nanoscale. Effective absorption coefficients exceeding 1 nm-1, or an order of magnitude higher than for bulk metals, were observed.1 In order to exploit the light absorption in such nanocomposites for efficient solar energy conversion, several important challenges must be successfully addressed. Firstly, the absorption must cover a broad spectral range, typically between 2 and 3 eV in width depending on bandgap. Further, losses in the form of Joule heating of the metal component must be minimized. Finally, efficient charge carrier separation must be accomplished. One approach that may prove useful in these regards is to exploit plasmon near-field induced absorption in a strongly coupled, highly damped semiconductor material.2 A high damping of the plasmon resonance both broadens the absorption peak and causes a high fraction of the absorption to take place in the semiconductor layer, thus avoiding Joule heating and producing more long lived electron hole pairs that can feasibly be separated with higher yield. Recent experimental results in this area, based on ultrathin semiconductor layers grown on metal nanoparticle arrays by atomic layer deposition, will be presented.
1 Hägglund, C., Zeltzer, G., Ruiz, R., Thomann, I., Lee, H.-B.-R., Brongersma, M. L. & Bent, S. F. Self-assembly based plasmonic arrays tuned by atomic layer deposition for extreme visible light absorption. Nano Lett. 13, 3352-3357 (2013). DOI: 10.1021/nl401641v
2 Hägglund, C. & Apell, S. P. Plasmonic near-field absorbers for ultrathin solar cells. The Journal of Physical Chemistry Letters 3, 1275-1285 (2012). DOI: 10.1021/jz300290d