3.1. ZAZ electrode modeling with one silver nanowire
In this section, we investigate plasmon resonance around a single silver nanowire in order to observe the behavior of the electric field as a function of the wavelength. The diameter of the silver nanowires is 35 nm, which corresponds approximately to the diameter of the AgNWs used experimentally [
13]. The lower layer of ZnO nanoparticles (20 nm) is located below the AgNW and the upper layer is modeled by the ring around the AgNW with a diameter greater than 45 nm (
Figure 3a).
Figure 3a shows the absorption of the ZAZ electrode, with an absorption peak at around 347 nm (typical resonance of silver nanosphere); the resulting electric field is shown in
Figure 3b. The localized field inside the AgNW is intense due to the absorption of the nanowire at the chosen wavelength. A high electric field intensity is noted at the top of AgNW/ZnO interface, given the difference in permittivity between the two materials, but a low intensity on the edges. Its radiation extends over a length of around 30 nm from the ZnO interface and spreads out into the air. The ZnO layer seems to favor plasmon propagation in air, as the electric field is stronger there than in ZnO. This effect is particularly interesting and will enable us to take advantage of this surface plasmon in the active layer that will fill this space.
The LSPR is the privileged plasmonic resonance mode in this configuration. A multi-wavelengths investigation (
Figure 4) represent the electric field distributed around the cross-section of the silver nanowire. At 302 nm, the electric field takes the form of two lobes positioned on either side of the silver nanowire directed towards the air, under the influence of the substrate. These lobes show a strong presence of scattering compared with plasmon resonance. This radiation is very similar to that observed with a metal sphere, as demonstrated in several research studies [
41]. Although this electric field is localized around the AgNW, its amplitude is very low at this wavelength. At 316 nm, the electric field reaches greater distances vertically, but its intensity is still very low. Part of the electric field starts to be absorbed by the AgNW.
At 347 nm, corresponding to the AgNW absorption peak, a significant portion of the electric field remains confined within the AgNW, with a high concentration at the AgNW/ZnO interface, as mentioned in the paragraph above. Absorption inside the nanowires is limited after 387 nm which make possible for the most part of the light to be absorbed. Radiation is almost uniform around the AgNW, with intensities low but higher than those observed before the absorption wavelength. There is a strong localization of the field, which means a high proportion of plasmonic resonance.
Beyond 347 nm, we see a significant increase in the electric field up to 608 nm from which lobes due the scattering tends to be majority. At 416 nm, we find the maximum electric field intensity (311 V2.m-2). The electric field distribution changes shape as a function of wavelength. This is due to the interaction of light with silver being strongly related to wavelength. For wavelengths between 347 and 387 nm, the field distribution is almost uniform around the AgNWs. However, for wavelengths above 387 nm, we note an increasingly localized distribution towards the lower section of the AgNW. High-intensity hotspots at the AgNW/ZnO interface reflect very intense local confinement of the electric field, at certain wavelengths.
All these electric field exaltation phenomena contribute to increasing the absorption of the active layer of an organic solar cell over a wide wavelength spectrum, as long as the field is present beyond the ZnO.
We have just seen the behavior of the electric field on the cross-section of an AgNW and the associated plasmonic resonance mode. We will now investigate the distribution of the electric field over the longitudinal section of the AgNW.
Figure 5 shows the longitudinal distribution of the electric field. Below the absorption wavelength, the electric field intensity is very low, as we noticed with the LSPR mode. At the absorption peak (347 nm), the electric field is confined solely to the AgNW and its surroundings.
In the following section, we develop a hypothesis to explain the interpretation of the electric field distribution along the silver nanowire.
At 450 nm, we observe a wave propagating along the AgNW with high intensity. We assume that this is the waveguide mode. This mode appears at the metal-dielectric interface (
Figure 1d) and is noted in our structures with cylindrical geometry. This also explains why we observe a strong confinement of the electric field in the lower section of the AgNWs when we observe its cross-section (
Figure 7f). At 520 nm, this mode continues to propagate with a different frequency and a weaker but uniformly distributed intensity. We obtain the same behavior for the wavelength at 550 nm.
From 630 nm onwards, a second oscillation superposed to the first oscillation of the electric field appears around the AgNW, with smaller wavelengths superposed on the waveguide mode. This is an SPP mode (
Figure 5), as the waveform (visible at 637 nm) corresponds to the propagation of the SPP mode as illustrated in
Figure 1c Furthermore, the AgNWs are coated and in contact with the ZnO, which can be likened to the Otto configuration, as discussed by Liqun Sun group [
42], enabling the attenuated total reflection (ATR) condition to optically excite the SPP mode in very restricted positions. Since the propagation and localized modes (waveguide, LSPR and SPP) are superposed, it is complex to distinguish the single contribution of each mode. However, we noted a positive impact of these modes, as areas of low electric field intensity, when the waveguide mode is propagating alone, present a high intensity when the SPP mode is present.
The waveguide mode continues to propagate up to 800 nm, with increasing wavelengths as it moves away from the absorption peak.
Figure 6 shows the behavior of the electric field as a function of the distance between the AgNW/ZnO interface and the monitor. By varying the position of the monitor up to 40 nm, we notice a certain increase in the electric field, which reaches its maximum value at around 20 nm above the interface. This seems to be in line with our hypothesis concerning the propagation of SPP and waveguide modes, since, in accordance with the Otto configuration, the distance between the dielectric and the metal is a few tens of nanometers; in our case, this distance is 20 nm.
To sum up, we have demonstrated that for a silver nanowire, several plasmonic resonance modes can take place. Each of these modes can have its own peak in electric field intensity, with the LSPR mode peaking at 416 nm and the waveguide mode at 450 nm.
3.2. Modeling a ZAZ electrode with multiple silver nanowires
The aim of this numerical study is to propose a numerical model of the ZAZ electrode that approximates that of the experimental one, in order to demonstrate plasmon resonance. To this end, we have used a script to generate a semi-random array of ZnO-coated silver nanowires.
The numerical model structure closest to the filling factor of an experimental ZAZ electrode is that with five AgNWs. The electric field is maximum at 454 nm, close to the peak of the waveguide mode.
Figure 7.
Normalized sqared electric field distribution for a wavelength of 454 nm and a ZAZ electrode composed of: (a) one AgNW; (b) two AgNWs; (c) three AgNWs; (d) four AgNWs; (e) 5 AgNWs.; (f) Normalized squared electric field distribution on the cross-section of an AgNW. The wavelength at which the field is observed is 454 nm, with the monitor at 10 nm above the AgNWs.
Figure 7.
Normalized sqared electric field distribution for a wavelength of 454 nm and a ZAZ electrode composed of: (a) one AgNW; (b) two AgNWs; (c) three AgNWs; (d) four AgNWs; (e) 5 AgNWs.; (f) Normalized squared electric field distribution on the cross-section of an AgNW. The wavelength at which the field is observed is 454 nm, with the monitor at 10 nm above the AgNWs.
The two modes of plasmon propagation (SPP and waveguide) are also present for several AgNWs through the ondulation of the electric field along the wires. For example, in
Figure 7b we show both propagation modes, with the slightly vertical nanowire showing waveguide mode propagation and the slightly horizontal nanowire showing the superposition of both propagation modes. As we saw before, the waveguide mode is independent of polarization and angle of incidence, which is one of the reasons why we can see it on AgNWs regardless of wavelength.
In
Figure 7c,d. and e., the electric field propagation around the silver nanowires differs at each AgNW/AgNW intersection. At some intersections, we see areas of low electric intensity, while at others, we see areas of very high intensity. This is due to the interference of electrical waves from plasmon resonance. At intersections with low electric field intensity, interference with waves from different AgNWs is destructive, whereas at intersections with high electric field intensity, interference is constructive.
Figure 7f illustrates the electric field distribution associated with the LSPR mode. A high electric field intensity is located on either side of the AgNW, as discussed above. This mode is clearly perceptible at the end of the AgNWs, where a strong confinement of the electric field is observed (
Figure 7d,e).
The simultaneous existence of these different modes of plasmon propagation is very interesting for organic solar cells, as it would allow us to increase their absorption without increasing the thickness of the active layer.