it more applicable for light trapping in photoelectrochemical devices. Therefore, TiO2 is an ideal choice material for coupling of light to waveguides in nanocones. Ni metal is widely used as an earth abundant catalyst for oxygen evolution reaction (OER) which is an important part of both water splitting [209, 232, 233], and CO2 reduction devices [210], therefore in this study we utilize Ni metal as the front contact deposited between hexagonally-packed TiO2 nanocones on Si.
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Figure 6.2: Simulated transmission (T), absorption (A), and reflection (R) spectra of the three configurations in Figure 6.1. (a), (b), and (c) plots the spectra in air for TiO2 cones on Si, TiO2
cones on Si with Ni, and Ni hole array on Si respectively. (d), (e), and (f) plot the same in water.
Figure 6.1: (a), (b), and (c) show the schematics of the three configurations that are simulated
Figure 6.2 plots the transmission, absorption, and reflection spectra for the three cases in Figure 6.1 in air and water. The simulations were performed with a background index of n = 1 and n= 1.33 to understand the behavior in air and water respectively. Figure 6.2 (a) and (d) show that the TiO2
nanocones act as an excellent broadband antireflection structures that can enable 97.5%, and 96.9%
of the total number of photons in 400 nm – 1100 nm range in air and water respectively. When 50 nm of Ni was incorporated into the space between the nanocones over the Si substrate the transmitted photon flux was reduced to 86.2%, and 84.7% in air and water respectively. Compared to the structure in Figure 6.1 (c) with no nanocones and same amount of Ni loading as Figure 6.1 (b) the transmitted photon flux was 23.3% and 24.8% in air and water respectively. Therefore, the TiO2
nanocones enable transmission of more than 3 times the amount of light to pass through in this specific configuration. In case of Ni hole array, the reflection and parasitic absorption account for above 75% of optical losses, whereas when coupled with TiO2 nanocones the total loss was < 15 % indicating that the TiO2 nanocones are enabling the light to be transmitted into the Si substrate below Ni with minimal interaction with Ni. Figure 6.3 plots the transmitted photon flux data along with AM 1.5 spectrum using which the percentage of transmitted photon flux was calculated.
Figure 6.3: Area plot of simulated transmitted spectral photon flux in air and water for the three configurations in Figure 6.1. Blue represents AM 1.5G spectral photon flux. Orange, yellow, and purple plot the transmitted spectral photon flux into Si for nanocones on Si, nanocones with Ni on Si, and Ni hole array on Si respectively.
To understand the interactions of light with TiO2 nanocones, frequency domain field and power monitor was utilized to record electric field data in FDTD simulations. Figure 6.4 plots the electric field profiles along a nanocone cross section as shown in Figure 6.4 (a). In Figure 6.4 plots the field profiles at wavelengths 484 nm, 552 nm, 628 nm, and 770 nm. These wavelengths correspond to the maxima in transmission spectra into Si with TiO2 nanocones and Ni as shown in Figure 6.1 (b). It can be observed that most of the electric field is confined in the nanocone. Strong coupling of incident light occurs at different radii for different wavelengths as expected from a nanoconical Figure 6.4: (c), (d), (e), and (f) show the electric field profiles along the cross section shown in (a) at maxima in transmission spectrum into Si with TiO2 nanocones and Ni in air. The selected wavelengths are highlighted using the yellow stars in (b).
structure [31, 183, 184]. The light propagates in the nanocone and is transmitted into the Si substrate below, where decay in field intensity can be observed due to absorption.
Similar plots are shown in Figure 6.5 and this time the wavelengths 442 nm, 584 nm, 738 nm, and 940 nm are chosen from the minima of the transmission spectrum. Compared to field profiles in Figure 6.4 the profiles in Figure 6.5 show a higher intensity of electric field in the middle of the nanocones. The minima in transmission spectrum primarily correspond to absorption peaks in Ni and these peaks shift depending on the background index as shown in Figure 6.2 (b) and (e).
Figure 6.5: (c), (d), (e), and (f) show the electric field profiles along the cross section shown in (a) at minima in transmission spectrum into Si with TiO2 nanocones and Ni in air. The selected wavelengths are highlighted using the yellow stars in (b).
The field profiles in Figure 6.5 show that even at the minima, the transmission is 60 % or above, which is still higher compared to the transmission spectra in case of Ni hole array on Si without TiO2
nanocones as shown in Figure 6.2. This indicates that the incident light couples efficiently to the nanocones which guide the light into the underlying Si substrate. Depending on the slope of the nanocone, electric field associated with some wavelengths of the light are highly confined inside the TiO2 nanocone while some wavelengths are partially confined. Absorption in Ni is enhanced when the light is not completely confined inside the nanocones causing a potential photocurrent loss. Using the transmitted spectral photon flux, it can be estimated that in air the maximum photocurrent that is achievable in Si with TiO2 nanocones, in Si with TiO2 nanocones and Ni, and in Si with Ni hole arrays is 42.9 mA cm-2, 37.9 mA cm-2, and 10.9 mA cm-2 respectively. In water the photocurrent respectively corresponds to 41.8 mA cm-2, 36.5 mA cm-2, and 10.7 mA cm-2. Therefore, incorporation of Ni in between TiO2 nanocones results in ~ 5 mA cm-2 photocurrent loss but compared to a Ni hole array with no TiO2 nanocones the photocurrent enhancement was > 300%.
This behavior of TiO2 nanocones cannot be explained using effective medium theory and wave-optic simulations show that the nanocones act as an antenna for the incoming radiation, coupling the light to the waveguide modes and providing a route for the light to reach underlying Si substrate despite 54 % of the surface being covered by Ni.