Disordered photonics: A New Strategy for Light Trapping in Thin Films

Left to right: Kevin Vynck, Matteo Burresi, Francesco Riboli, and Diederik S. Wiersma

Authors: 
Kevin Vynck, Matteo Burresi, Francesco Riboli, and Diederik S. Wiersma

Affiliation: 
European Laboratory for Non-linear Spectroscopy (LENS) &
National Institute of Optics (CNR-INO), Florence, Italy

Thin-film solar cells nowadays represent a promising alternative to more conventional, thick, silicon panels. Using less material for solar cells allows for a significant saving of natural resources and a lowering of the production costs. A counter-effect of using thinner films is that the amount of light that is absorbed and eventually converted into electricity is significantly reduced. For this reason, improving the absorption of light by thin dielectric films constitutes a challenge of paramount importance in the development of high-efficiency, cost-effective, photovoltaic technologies [1].

Significant efforts have been made in recent years to design structures on the scale of the wavelength that are able to efficiently “trap” light in thin films. Among the various techniques proposed [2], great attention has been given to so-called photonic crystals, made, for instance, by creating a periodic array of holes in the film. At well-defined frequencies and angles of incidence, light impinging on the film can couple to the optical modes created by the nanostructuring and be trapped in the absorbing medium for a long time, thereby significantly increasing the light absorption [3]. Alternatively, randomly textured surfaces have been designed to efficiently spread light in the film on broad spectral and angular ranges, leading as well to an overall increase of the light absorption [4].

In a recent Letter published in Nature Materials [5], our team has presented a new strategy for light trapping thin films that takes advantage of both the efficient light trapping of photonic structures and the broadband/wide-angle properties of random media. The solution that we proposed relies on the use of two-dimensional disordered photonic structures, such as that shown in Figure 1 (higher panel), which exhibit complex electromagnetic modes to which coupling from free space is possible.

Figure 1: (Upper panel) Schematic view of a thin film containing a random pattern of holes. (Lower panel) Top and side views of the electromagnetic energy density in a randomly-nanopatterned film at two different frequencies (where t is the film thickness and λ, the wavelength of light). Light is efficiently trapped in the film, due to the light coupling to disordered optical modes. (Figures adapted from Ref. [5])

To understand the physical process involved, it is instructive to consider how light behaves in such a film. The dielectric film naturally acts as a waveguide for light, confining it in the plane of the film and preventing any out-of-plane loss. Placing holes at random positions in the film makes such that light is multiply-scattered in the plane, in a similar way as a two-dimensional random walk. Multiple scattering and wave interference lead to the formation of optical modes, the characteristics of which (e.g., their spatial extent) are intimately related to the structural properties of the disordered system. A key feature of these modes is that they are leaky, due to the finite thickness of the film, meaning that they are accessible from the third dimension, and thus, can be used for light trapping purposes.

For illustration, the electromagnetic energy density produced by a plane wave at normal incidence on a thin film containing a random pattern of holes (air filling fraction of 30%) is shown in Figure 1 (lower panel) at two different frequencies. The very high energy density in the film is a clear indication of an efficient light trapping effect. The speckle patterns observed arise from the interference between the multiply-scattered waves in the plane of the film, as described above.

The main results of our work are given in Figure 2, showing the absorption spectra of a bare (unpatterned) film with a moderate absorption efficiency (< 5%) and of the same film containing the random pattern of holes considered above. A strong enhancement of the absorption efficiency is observed over a broad range of frequencies, as well as for wide incidence angles and both polarizations of light (see the inset). These are very important properties for solar panels since they should ideally be efficient in all circumstances.

Figure 2: Absorption spectra of the bare (unpatterned) film (black curve) and the films containing random and amorphous patterns of holes (blue and gray curves, respectively). The inset shows the angular dependence of the absorption of the randomly-nanopatterned film at t/λ=0.15 for both polarizations of light. The random pattern of holes leads to a large absorption of the incident light over broad spectral and angular ranges. Disorder correlations in the amorphous pattern allow for a fine-tuning of the absorption spectrum. (Figure adapted from Ref. [5])

Since, as stated above, the coupling process is mediated by the optical modes, which intrinsically depend on the type of disorder considered, we further investigated the possibility to tune the light absorption by engineering the disorder. More particularly, we considered the case of an “amorphous” structure, characterized by a short-range correlation in the position of the holes, as periodic patterns, yet lacking any long-range order. The results on the absorption efficiency, shown in Figure 2, are remarkable: while the absorption is diminished at lower frequencies, becoming quite close to that of the bare slab, it is significantly increased at higher frequencies. The absorption enhancement occurs when the wavelength in the material approximately equals the typical distance between holes, proving that disorder correlations provide us with an important degree of control over the light absorption spectrum.

A final test to conclude our work has been to simulate the absorption of a film of amorphous silicon in the red part of the solar spectrum, where efficient light trapping is generally needed. We observed that the absorption efficiency of the films containing the disordered hole patterns (random and amorphous) was at least as high as that of the film containing the periodic hole pattern. This is an important result as it shows that periodic nanostructuring does not necessarily guarantee the best possible outcome. The lack of periodicity in photonic structures and the robustness of the properties of the films to structural imperfections could lead to the development of low-cost solar panels with a higher efficiency.

References:
[1] Albert Polman and Harry A. Atwater, "Photonic design principles for ultrahigh-efficiency photovoltaics", Nature Materials, 11, 174-177 (2012). Abstract.
[2] Shrestha Basu Mallick, Nicholas P. Sergeant, Mukul Agrawal, Jung-Yong Lee and Peter Peumans, "Coherent light trapping in thin-film photovoltaics", MRS Bulletin 36, 453-460 (2011). Abstract.
[3] Xianqin Meng, Guillaume Gomard, Ounsi El Daif, Emmanuel Drouard, Regis Orobtchouk, Anne Kaminski, Alain Fave, Mustapha Lemiti, Alexei Abramov, Pere Roca i Cabarrocas, Christian Seassal, "Absorbing photonic crystals for silicon thin-film solar cells: design, fabrication and experimental investigation", Solar Energy Materials and Solar Cells, 95, S32-S38 (2011). Abstract.
[4] C. Rockstuhl, S. Fahr, K. Bittkau, T. Beckers, R. Carius, F.-J. Haug, T. Söderström, C. Ballif, F. Lederer, "Comparison and optimization of randomly textured surfaces in thin-film solar cells", Opics Express, 18, A335-A341 (2010). Abstract.
[5] Kevin Vynck, Matteo Burresi, Francesco Riboli, Diederik S. Wiersma, "Photon management in two-dimensional disordered media", Nature Materials, 11, 1017-1022 (2012). Abstract.