Dual-Band Plasmonic Perfect Absorber Based on the Hybrid Halide Perovskite in the Communication Regime

Abstract

Due to the weak absorption of (CH₃NH₃)PbI₃ in the communication regime, which restricts its optoelectronic applications, we design a adjustable dual-band perfect absorber based on the (CH₃NH₃)PbI₃ to significantly enhance its absorption capability. Since the localized plasmon (LP) mode and surface plasmon (SP) mode are excited in the structure, which can both greatly enhance light absorption of the (CH₃NH₃)PbI₃ layer, dual-band perfect absorption peaks are formed in the communication regime, and the absorption of (CH₃NH₃)PbI₃ layer is increased to 43.1% and 64.2% at the dual-band absorption peaks by using finite-difference time-domain (FDTD) methods, respectively. By varying some key structural parameters, the dual-band absorption peaks of (CH₃NH₃)PbI₃ can be separately shifted in a wide wavelength region. Moreover, the designed absorber can keep good performance under wide angles of incidence and manifested polarization correlation. Furthermore, not just for (CH₃NH₃)PbI₃, the physical mechanism in this absorber can also be utilized to strengthen the absorption of other halide perovskites.

1. Introduction

Nowadays, efficient light matter interaction is greatly conductive to develop optoelectronics and nanophotonics [1,2,3,4]. Many new materials, including transition-metal dichalcogenides [5,6,7], graphene [8,9,10], hexagonal boron nitride [11,12], black phosphorus [13,14] et al., have been widely reported for achieving the effective light matter interaction. Recently, the new halide perovskites materials described by the general formula ABZ₃ (A = organic ammonium cation, Cs⁺; B = Pb²⁺, Sn²⁺; Z = Cl, Br, I), have caused wide public concern for both photon sources [15,16,17] and photovoltaics devices [18,19], due to the unique photoelectric characteristics, including outstanding visible light response, strong photoluminescence, long carrier diffusion length, ultra-broadband spectral tunability, and so on [20,21,22]. In fact, the halide perovskites have been found for more than 30 years. Until 2009, A. Kojima et al. [23] proved that a solar cell based on (CH₃NH₃)PbI₃ can realize the power conversion efficiency (PCE) as 3.8%, which made the halide perovskites more and more popular in photovoltaic devices as a new generation of semiconductor material.

Recent years, halide perovskites have been made great progress in the solar field. By using (CH₃NH₃)PbI₃ nanoparticles as light harvesters in the solar cells, M. Gratzel et al. enhanced the PCE to 9.7% [24] in 2012. One year later, A. Hey et al. [25] increased the PCE to 12.3% by adjusting the processing temperature of (CH₃NH₃)PbI_3−xCl_x solar cells from 500 °C to below 150 °C. Further researches showed that changing the components of the halide perovskite and seriously selecting the other materials are very useful to increase PCE. For instance, without antireflective coating, PCE can be as high as 19.3% in a plane structure [26]. Until now, the PCE of halide perovskite solar cell has been reached 23%, and even with the potential exceeds 30% [27,28]. More importantly, the efficiency is already better than multi-crystalline silico-based solar cells (22.3%), which demonstrates great potential of halide perovskite solar cell in photovoltaic applications. Moreover, it is worth noting that the large optical absorption coefficient of halide perovskites in the visible regime takes an important part in these halide perovskite-based solar cells. However, the weak absorption coefficient limits the further applications of halide perovskites in the communication regime. Actually, halide perovskites are very instructive to research on photoelectric devices with excellent performance in the communication regime, although the related reports are still relatively rare. In this paper, we use the unique properties of metal metamaterials [29,30] to enhance the optical absorption of (CH₃NH₃)PbI₃ in the near-infrared regime. Then, we design and investigate a (CH₃NH₃)PbI₃-based optical absorber combined with metal metamaterial in the communication regime.

2. Structure Design

As seen from Figure 1, the designed absorber is composed of periodic silver (Ag) strips on the SiO₂ layer, which is coated with (CH₃NH₃)PbI₃ layer supported by the Ag substrate, and the (CH₃NH₃)PbI₃ layer is sandwiched by two anisotropic lossless hexagonal boron nitride (h-BN) films. Compared with only utilizing a SiO₂ layer, the h-BN films and (CH₃NH₃)PbI₃ layer together constitute the heterojunction to maintain higher carrier mobility in structure. The thickness of SiO₂ layer is D₁ = 230 nm, the thickness of Ag substrate is D₂ = 800 nm, and the thickness (z direction) of each h-BN film is 10 nm. Since the thickness of the Ag substrate is too thick to let the incident light penetrate through it, the transmission in the whole structure is near zero, that is T = 0. In this condition, the absorption of this structure can be expressed by A = 1 − R (R is the reflection from the whole structure). Simultaneously, the widths (in the x direction) and thicknesses of the Ag strips are expressed by w and h, respectively. Meanwhile P is the period of the Ag strips in the x direction. Then, the filling factor of the Ag strips is described by $F=w/p$. In calculation, w = 550 nm, P = 800 nm, h = 15 nm, and the (CH₃NH₃)PbI₃ layer is employed with thickness of 100 nm in this structure. The permittivity of Ag is calculated by the Drude formula [31], and the optical parameters of h-BN films in our simulation are obtained from the paper [32]. Moreover, the absorption of (CH₃NH₃)PbI₃ is described by the equation [33]:

$$A\left(l\right)=\frac{4pc}{l}gRe(N)gIm(N)g{{\displaystyle \int }}_V{\left|E_l\right|}^{2}dV$$

(1)

where N is the refractive index of (CH₃NH₃)PbI₃, λ is the wavelength of incident light, c is velocity of light in vacuum, V is the volume of (CH₃NH₃)PbI₃, and $E_l$ is the local electric field. Then, dual-band perfect absorption peaks are formed in the communication regime through the use of finite-difference time-domain (FDTD) methods, and the absorption of (CH₃NH₃)PbI₃ is increased to 43.1% and 64.2% at the dual-band absorption peaks, respectively. Meanwhile, by varying some key structural parameters, the dual-band absorption peaks of (CH₃NH₃)PbI₃ can be separately shifted in a wide wavelength region. Moreover, the designed absorber can keep good performance under wide angles of incidence and manifested polarization correlation.

And we used the first-principles to calculate the dielectric function ($\epsilon ={\epsilon }_1-i{\epsilon }_2$) of (CH₃NH₃)PbI₃ in software VASP [34,35]. The electron exchange-correlation function is adopted by the generalized gradient approximation (GGA) in the Perdew Burke-Ernzerhof (PBE) method [36]. In the calculation, the first Brillouin zone is performed with the Monkhorst-Pack k-points grid of 15 × 15 × 15, and the basis set energy cutoff of plane-wave is set as 500 eV. The convergence criterion of energy and force are assumed as 1 × 10⁻⁸ eV and 1 × 10⁻⁸ eV/Å, respectively. Meanwhile, the usual Kramers-Kronig transformation is used to obtain the real part ${\epsilon }_1$ of the dielectric function, while the imaginary part ${\epsilon }_2$ is solved from the summation over empty states. The calculation results of the dielectric function are shown in Figure 2, which are almost consistent with the previously reported results [37]. Clearly, the value of imaginary part (${\epsilon }_2$) in the visible band is high, which demonstrates the (CH₃NH₃)PbI₃ is a good material in photovoltaic field. However, its further application in the optical communication is limited by the low value of ${\epsilon }_2$ in the communication band. Therefore, it is of great significance to study and improve the absorption of (CH₃NH₃)PbI₃ in the communication regime.

3. Results and Discussion

Firstly, under normal incidence condition, we consider the absorption of this absorber and (CH₃NH₃)PbI₃ layer by shining from p-polarized and s-polarized light separately, as exhibited in Figure 3. When the structure is shined from s-polarized light, both the absorption of the absorber and (CH₃NH₃)PbI₃ are rather weak in this condition. It is because plasmonic resonance is hardly excited when the electric field of s-polarized light is parallel to the Ag strips. On the other hand, when the structure is shined from p-polarized light, both the surface plasmonic resonance and localized plasmonic resonance can be excited in the proposed absorber. Meanwhile, the absorption of (CH₃NH₃)PbI₃ has been obviously increased to 43.1% at λ₁ = 1291 nm and 64.2% at λ₂ = 1548 nm, respectively. Thus, the performance of our proposed absorber is remarkably polarization-sensitive, s-polarized light is not conducive to increase absorption in (CH₃NH₃)PbI_3. For this reason, the impacts of p-polarized light are our main concern on the next content.

Next, we will go through how to realize the function of the dual-band perfect absorption for this structure. As seen from Figure 1, the whole structure can be approximately regarded as a periodic metal-insulator-metal (MIM) structure, which can suppress the reflection at the resonant wavelength. As mentioned before, the absorption of this structure can be expressed by A = 1 − R, that is, the absorption peak corresponds to the reflection dip when the reverse light fields are superimposed on each other in the absorber. Thus, we can explain the absorption enhancement for the (CH₃NH₃)PbI₃ layer by analyzing the light field distributions. As depicted from Figure 4, at the peak wavelength of λ₁ = 1291 nm, the skin depth of Ag is $S_1\approx \frac{\lambda }{2\pi Im(Ag)}\approx 23.16$ nm, as the complex refractive index of Ag is $0.3534+8.876i$. Simultaneously, at the absorption peak of λ₂ = 1548 nm, the skin depth of Ag is $S_2\approx \frac{\lambda }{2\pi Im(Ag)}\approx 22.80$ nm, as the complex refractive index of sliver is $0.5378+10.81i$. Consequently, the thicknesses of Ag strips are much less than both $S_1$ and $S_2$ leading to the interaction of optical fields at the interface of Ag strip/air separatrix and Ag strip/hBN interface. Thus, the electric fields surrounding the Ag strips are accumulated and strengthened, as described in Figure 4a,c. And the magnetic field is assembled between each Ag strip and substrate for the resonance mode at λ₁, although both of them are also gathered around the Ag strips, as described in Figure 4a,b. Actually, these characteristics refer to the excitation of localized plasmon (LP) mode. On the other hand, parallel electromagnetic field stripes extending the z-axis are taken shape for the resonance mode at λ₂, as seen in Figure 4d. This means that the resonance mode at λ₂ as a surface plasmon (SP) mode accumulates the light energy surrounding the (CH₃NH₃)PbI₃ layer and enhances the near field. Thus, both LP mode and SP mode can greatly enhance light absorption of the (CH₃NH₃)PbI₃ layer. However, as seen in Figure 4e,f, at the wavelength of λ₃ = 1400 nm, which is away from resonance wavelength, light energies and optical fields are not concentrated and enhance the absorption of (CH₃NH₃)PbI₃ layer.

Subsequently, we study the effect of different widths of Ag strips on the absorption peaks of (CH₃NH₃)PbI₃ layer, as shown in Figure 5a. Since every Ag strip exhibits as a Fabry-Perot (F-P) cavity for the LP mode, the wavelengths of absorption peaks are significantly correlated with the width of Ag strip. Therefore, when the width of Ag strip is increased, the absorption peak at λ₁ will be remarkably red-shifted owe to the gradually increasing effective wavelength of LP mode, which further leads to the enhancement and concentration of optical fields inside (CH₃NH₃)PbI₃ layer and between adjacent Ag strips. Thus, the absorption will firstly increase with w. Unfortunately, if the width of Ag strip continues to be increased, too much Ag strips will cover the (CH₃NH₃)PbI₃ layer. The absorption efficiency will stop increasing and turn into reduce. Meanwhile, since the wavelength of SP mode is not sensitive to w, when we change the width of Ag strip, the absorption peak at λ₂ has only a slight shift, which depends heavily on the period P of Ag strips. As seen from Figure 5c, both the absorption peaks at λ₁ and λ₂ will be obviously red-shifted with the increase of P, due to the resonance wavelengths of LP and SP modes become longer and longer. By the same token, the absorption will firstly increase and then decline in whole process of increasing P.

In the following, what influences on the absorption of (CH₃NH₃)PbI₃ layer caused by changing the thickness of Ag strips are considered, as shown in Figure 5b. When h = 15 nm, which is much thinner than the depth of penetration, the optical fields inside the air/Ag strip interface and h-BN/Ag strip interface are efficiently interacted with each other to improve the absorption of (CH₃NH₃)PbI₃ layer, as mentioned above. When h is increased from 15 nm to 20 nm, the optical loss in the Ag strips is increased, and the coupling intensity between the top and bottom of Ag strips is weakened. As a result, both the absorption peaks of (CH₃NH₃)PbI₃ layer at λ₁ and λ₂ are attenuated and blue-shifted slightly due to the reduction of resonance wavelength. However, much too thin Ag strips are not conducive to improve the absorption of (CH₃NH₃)PbI₃ layer, owing to the reduction of optical fields around the (CH₃NH₃)PbI₃ layer, and the strong light diffraction around the Ag strips. Consequently, when h is reduced from 15 nm to 10 nm, the absorption of (CH₃NH₃)PbI₃ layer is obviously reduced. Furthermore, not just for (CH₃NH₃)PbI₃, the physical mechanism in this absorber can also be utilized to strengthen the absorption of other halide perovskites, such as (CH₃NH₃)PbBr₃, and (CH₃NH₃)PbCl₃. As shown in Figure 5d, when the (CH₃NH₃)PbI₃ layer in this absorber is changed by (CH₃NH₃)PbBr₃ and (CH₃NH₃)PbCl₃ respectively, both the absorption peaks of (CH₃NH₃)PbBr₃ and (CH₃NH₃)PbCl₃ layer also appear in the communication regime. Although the absorption peaks of (CH₃NH₃)PbBr₃ and (CH₃NH₃)PbCl₃ are not high enough in this case, we believe they can be optimized by manipulating the other structure parameters. Therefore, this method can be considered as a general approach to enhance absorption of most halide perovskites. Meanwhile, we also consider the effects of changing the thicknesses of h-BN layers, as shown in Figure 6. When the thicknesses of h-BN layers are increased from 6 nm to 10 nm, the intensities and wavelengths of LP and SP modes are also increased, which lead to the increase and red-shift of absorption peaks of (CH₃NH₃)PbI₃ layer. However, much too thick h-BN layers will weaken the coupling of plasmonic near fields between each Ag strip and the continuous Ag substrate, which lead to the reduction of optical fields around the (CH₃NH₃)PbI₃ layer. Thus, when the thicknesses of h-BN layers are increased from 10 nm to 14 nm, the absorption peaks of (CH₃NH₃)PbI₃ layer are decreased.

In the next, it should be emphasized that all of results are obtained under the condition of normal incidence, however, the absorber in the application need keep good performance under wide angles of incidence. Then, we investigate the optical absorption of (CH₃NH₃)PbI₃ as a function of angle and the wavelength of incident light. As shown in Figure 7, the absorption of (CH₃NH₃)PbI₃ at λ₁ and λ₂ are still more than 22% and 30%, even if the angle of incident light is increased to 50°. Therefore, our designed absorber can keep good performance under wide angles of incidence in practical applications.

4. Conclusions

In the summary, this paper designs a adjustable dual-band perfect absorber based on (CH₃NH₃)PbI₃ in the communication regime, and the corresponding features are investigated through the use of FDTD methods. It is shown that the absorption of (CH₃NH₃)PbI₃ layer is enhanced up to 43.1% and 64.2% at the dual-band absorption peaks in the communication regime, respectively. By varying some key structural parameters, the dual-band absorption peaks of (CH₃NH₃)PbI₃ can be separately shifted in a wide wavelength region. Moreover, the designed absorber can keep good performance under wide angles of incidence and manifested polarization correlation. Furthermore, not just for (CH₃NH₃)PbI₃, the physical mechanism in this absorber can also be utilized to strengthen the absorption of other halide perovskites. Thus, the proposed absorber has great application potential for dual-frequency and frequency selective photovoltaic devices in the communication regime.