Strong coupling between plasmon resonance and optical cavity resonance provides more energy in ultrathin solar-cell materials.

In plasmonic solar cells, plasmon resonances are used to capture light and utilize its energy to create and separate energy rich charge carriers, so that they can be extracted in a circuit. To achieve this, one approach is to use metallic nanoparticles, whose localized surface plasmons give rise to a very strong electromagnetic field in their near vicinity. Within reach of this strong near-field, the plasmon energy can be transferred to electron–hole pairs generated over the bandgap of an ultrathin layer of semiconductor material, so that charge separation can eventually be accomplished across a solar cell junction.

In connection to this research, the complementary use of optical cavity resonances has been investigated, where the ultrathin plasmonic absorber layer (around 10 nm) is deposited on top of a transparent layer that separates it from a highly reflective, mirror-like layer on the back. By varying the thickness of the cavity, the stack can be brought to fulfill the condition for optical impedance matching, so that reflections from the front side are eliminated and light is completely absorbed. Impedance matching is typically fulfilled near a so-called Fabry–Pérot resonance (FP resonance) for the semi-open nanocavity created between the absorber layer and the reflector.

By combining plasmon-active structures with nanocavities, extremely high optical absorption with a large spectral bandwidth can be achieved through contributions from the two different types of resonances and their different modes. However, one wants to limit the complexity of the system and there is a challenge in simultaneously impedance-matching several different resonances at different wavelengths with one and the same geometry.

In this study it was demonstrated that strong coupling between fundamental resonance modes gives rise to new, hybridized resonance modes that constitute a possibility to build systems with impedance matching and efficient absorption at two or more wavelengths simultaneously.

In the experiments, nanoparticles of gold were used, distributed in a regular pattern on a surface. These gold particles efficiently capture light at their plasmon resonances. The gold particles were coated with a layer of the semiconductor tin sulfide (SnS). Together with the gold particles, the SnS layer functioned as an ultrathin, nanostructured absorber layer. This absorber layer was placed on top of a transparent layer of silicon dioxide (SiO₂), which in turn lay on a relatively thick and reflective layer of aluminum. In this way, a semi-open nanocavity with conditions for FP resonances is formed.

Samples of SnS were produced by atomic layer deposition (ALD) on silicon and glass respectively and were first investigated on their own. The optical properties were found to vary due to crystallization being sensitive to substrate and other parameters that were not fully controlled in this study, but in general the SnS layer showed a high absorption capability. On top of the gold structure, the SnS coating showed an increasing absorption up to a thickness of 8.6 nm. Compared with other forms of tin sulfide (SnSx, with x≈2) studied previously, SnS in this study gave a significantly broader absorption peak, which is an advantage since more light is then absorbed over the solar spectrum. Interestingly, this absorption was distributed over two peaks for certain SnS thicknesses, which could not be explained by different modes of the plasmon resonance in the nanostructure since spectroscopic ellipsometry showed that the absorption capability in this layer was concentrated to a single peak.

It turned out that the origin of the two peaks derives from strong coupling between a plasmon resonance in the absorber layer and an FP resonance in the nanocavity formed by the structure with SnS–Au/spacer layer/reflector. The coupling was found to be unusually strong due to a high internal reflection in the SnS layer, which produced a large overlap between the fields of the FP resonance and the plasmon resonance. Strong coupling between resonances leads to new, mixed (hybridized) modes arising. The hybridized modes inherit properties from the original, pure resonance modes, and thus become a mixture of these.

By varying the thickness of the cavity, the degree of mixing between the modes can be controlled, and the different peaks come to be dominated by properties from one or the other of the original modes. At a certain thickness, the two hybridized modes become equally influenced by the original modes so that they obtain very similar properties. By adapting one of the hybrid modes to, for example, optical impedance matching, the other mode will automatically fulfill a similar condition. Thereby, the structure obtains two peaks where close to 100% absorption is achieved. The hybridization could furthermore be generalized to even more resonance modes so that properties of the system could be matched to desirable conditions at more than two wavelengths.

With hybridization of plasmon and FP resonances, high absorption can therefore be achieved around two or more wavelengths. With broad absorption peaks associated with materials like SnS, a large portion of the wavelengths of sunlight can be covered and contribute to very efficient light absorption and therefore also efficient solar cells. In this study, SnS was especially identified as a good material for ultrathin solar cells based on this concept, thanks to its high absorption coefficient and high refractive index. More than 60% of the photons of sunlight with energies higher than the bandgap of SnS were shown to be able to contribute to the generation of electricity, which gives a theoretical efficiency of up to 19% for the approximately 10 nm thick semiconductor. These results can therefore be important steps toward inexpensive and efficient solar cells based on nanotechnology.

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