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

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

Liang Xu 1 , Jian Zeng 1 , Xin Luo 2,* , Libin Xia 1,*, Zongle Ma 1, Bojun Peng 1, Zhengquan Li 1, Xiang Zhai 3 and Lingling Wang 3

1 Energy Materials Computing Center, School of Energy and Mechanical Engineering, Jiangxi University of Science and Technology, Nanchang 330013, China; [email protected] (L.X.); [email protected] (J.Z.); [email protected] (Z.M.); [email protected] (B.P.); [email protected] (Z.L.) 2 School of Science, East China Jiaotong University, Nanchang 330013, China 3 Key Laboratory for Micro-Nano Optoelectronic Devices of Ministry of Education, School of Physics and Electronics, Hunan University, Changsha 410082, China; [email protected] (X.Z.); [email protected] (L.W.) * Correspondence: [email protected] (X.L.); [email protected] (L.X.)

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 (CH NH )PbI can 3 3 3 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, Citation: Xu, L.; Zeng, J.; Luo, X.; Xia, L.; Ma, Z.; Peng, B.; Li, Z.; Zhai, X.; not just for (CH3NH3)PbI3, the physical mechanism in this absorber can also be utilized to strengthen Wang, L. Dual-Band Plasmonic Perfect the absorption of other halide perovskites. Absorber Based on the Hybrid Halide Perovskite in the Communication Keywords: plasmonics; absorption; metamaterial; halide perovskites Regime. Coatings 2021, 11, 67. https://doi.org/10.3390/coatings 11010067 1. Introduction Received: 13 November 2020 Nowadays, efficient light matter interaction is greatly conductive to develop opto- Accepted: 29 December 2020 electronics and nanophotonics [1–4]. Many new materials, including transition-metal Published: 8 January 2021 dichalcogenides [5–7], graphene [8–10], hexagonal boron nitride [11,12], black phosphorus [13,14] et al., have been widely reported for achieving the effective light matter inter- Publisher’s Note: MDPI stays neu- action. Recently, the new halide perovskites materials described by the general formula tral with regard to jurisdictional clai- ABZ (A = organic ammonium cation, Cs+; B = Pb2+, Sn2+; Z = Cl, Br, I), have caused wide ms in published maps and institutio- 3 nal affiliations. public concern for both photon sources [15–17] 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 [20–22]. In fact, the halide perovskites have been found for more than 30 years. Until Copyright: © 2021 by the authors. Li- 2009, A. Kojima et al. [23] proved that a solar cell based on (CH3NH3)PbI3 can realize the censee MDPI, Basel, Switzerland. power conversion efficiency (PCE) as 3.8%, which made the halide perovskites more and This article is an open access article more popular in photovoltaic devices as a new generation of semiconductor material. distributed under the terms and con- Recent years, halide perovskites have been made great progress in the solar field. By ditions of the Creative Commons At- using (CH3NH3)PbI3 nanoparticles as light harvesters in the solar cells, M. Gratzel et al. tribution (CC BY) license (https:// enhanced the PCE to 9.7% [24] in 2012. One year later, A. Hey et al. [25] increased the creativecommons.org/licenses/by/ PCE to 12.3% by adjusting the processing temperature of (CH3NH3)PbI3−xClx solar cells 4.0/).

Coatings 2021, 11, 67. https://doi.org/10.3390/coatings11010067 https://www.mdpi.com/journal/coatings Coatings 2021, 11, 67 2 of 9

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 Figure1, 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]: Z 4pc 2 A(l) = gRe(N)gIm(N)g |El| dV (1) l V

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 El 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. Coatings 2021, 11, x FOR PEER REVIEW 3 of 10 Coatings 2021, 11, x FOR PEER REVIEW 3 of 10

(CH3NH3)PbI3 can be separately shifted in a wide wavelength region. Moreover, the designed ab- Coatings 2021, 11, 67 sorber can keep(CH good3NH performance3)PbI3 can be under separately wide shifted angles inof a incidence wide wave andlength manifested region. Moreover,polarization the designed3 of ab- 9 correlation. sorber can keep good performance under wide angles of incidence and manifested polarization correlation.

Figure 1. Structural figure of the dual-band (CH3NH3)PbI3-based perfect absorber. 3 3 3 FigureFigure 1.1. StructuralStructural figurefigure ofof thethe dual-banddual-band (CH(CH3NHNH3)PbI)PbI3-based perfect absorber. And we used the first-principles to calculate the dielectric function (𝜀=𝜀 −𝑖𝜀) of (CH3NH3)PbI3 in softwareAndAnd wewe usedVASPused thethe [34,35]. first-principlesfirst-principles The electron toto calculatecalculate exchange-correlation thethe dielectricdielectric functionfunction ((εis𝜀=𝜀= ε1−−𝑖𝜀iε2)) ofof adopted by the(CH(CH generalized33NHNH33)PbI3 iningradient software approximation VASPVASP [[34,35].34,35 ].(GGA) TheThe electronelectron in the Perdew exchange-correlationexchange-correlation Burke-Ern- functionfunction isis zerhof (PBE) methodadoptedadopted [36]. byby the Inthe generalizedthe generalized calculation, gradient gradient the first approximation Brillouinapproximation zone (GGA) is (GGA) performed in the in Perdew the with Perdew the Burke-Ernzerhof Burke-Ern- Monkhorst-Pack(PBE)zerhof k-points method (PBE) grid method [36 of]. 15 In[36]. × the 15 In calculation,× the 15, calculation, and the the basis the first setfirst Brillouin energy Brillouin cutoff zone zone isof is performedplane- performed with with the the wave is set asMonkhorst-Pack Monkhorst-Pack500 eV. The convergence k-points k-points grid critgrid oferion of 15 15× of 15× energy 15× 15,× 15, andand and the force the basis arebasis set assumed energyset energy cutoff as 1 cutoff× of plane-wave of plane- −8 10−8 eV and 1 iswave× set10− as8 iseV/Å, 500set eV.as respectively. 500 The eV. convergence The Meanwhile, convergence criterion the crit of usual energyerion Kramers-Kronigof and energy force and are assumedforce transfor- are asassumed1 × 10 as eV1 × −8 and10−8 1eV× and10 1eV/Å × 10−,8 respectively.eV/Å,ε respectively. Meanwhile, Meanwhile, the usual the Kramers-Kronig usual Kramers-Kronig transformation transfor- is mation is used to obtain the real part 1 of the dielectric function, while the imaginary used to obtain the real part ε1 of the dielectricε function, while the imaginary part ε2 is solved part ε is solvedmation from is usedthe summation to obtain theover real empt party states.1 of the The dielectric calculation function, results while of the the imaginary 2 from the summation over empty states. The calculation results of the dielectric function are part ε is solved from the summation over empty states. The calculation results of the dielectric functionshown are in2 shown Figure in2, Figure which are2, which almost are consistent almost consistent with the with previously the previously reported results [ 37]. reported resultsClearly,dielectric [37]. theClearly, function value the of are imaginaryvalue shown of imaginary in part Figure (ε ) in2,part thewhich ( visibleε ) are in thealmost band visible is consistent high, band which is withhigh, demonstrates the previously the 2 2 ε which demonstrates(CHreported3NH the3 )PbIresults (CH3 is3NH [37]. a good3)PbI Clearly,3 material is a good the invalue material photovoltaic of imaginary in ph field.otovoltaic part However, ( field.2 ) in its However,the further visible application band is high, in thewhich optical demonstrates communication the (CH is3 limitedNH3)PbI by3 is the a good low valuematerial of εin inph theotovoltaic communicationε field. However, band. its further application in the optical communication is limited by the low2 value of 2 in Therefore, it is of great significance to study and improve the absorption of (CH NH )PbIε the communicationits further band. application Therefore, in it theis of optical great communicationsignificance to study is limited and improveby the low the value 3 of 3 2 in3 in the communication regime. absorption of (CHthe communication3NH3)PbI3 in the band. communication Therefore, regime.it is of great significance to study and improve the absorption of (CH3NH3)PbI3 in the communication regime.

(a) (b) (a) (b) Figure 2. (a)Figure Real part 2. (εa) Realand (partb) imaginaryε and (b) partimaginaryε of dielectric part ε of function dielectric of fu (CHnctionNH of)PbI (CH3NHbased3)PbI on3 based the ﬁrst-principles on 1 1 ε2 2 ε 3 3 3 calculation. the first-principlesFigure calculation. 2. (a) Real part 1 and (b) imaginary part 2 of dielectric function of (CH3NH3)PbI3 based on the first-principles calculation. 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 Figure3. When the structure is shined from s-polarized light, both the absorption of the absorber and (CH NH )PbI are rather weak in this condition. It is 3 3 3 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 Coatings 2021, 11, x FOR PEER REVIEW 4 of 10

3. Results and Discussions 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 Coatings 2021, 11, 67 4 of 9 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 beexcited excited in in the the proposed proposed absorber. absorber. Meanwhile, Meanwhile, the the absorption absorption of of (CH (CH3NH3NH33)PbI)PbI33 has been obviouslyobviously increased to 43.1% atat λλ11 == 1291 1291 nm nm and and 64.2% 64.2% at λλ22 == 1548 1548 nm, nm, respectively. respectively. Thus, Thus, thethe performance ofof ourour proposed proposed absorber absorber is is remarkably remarkably polarization-sensitive, polarization-sensitive, s-polarized s-polar- izedlight light is not is conducive not conducive to increase to increase absorption absorption in (CH 3inNH (CH3)PbI3NH3. For3)PbI this3. For reason, this thereason, impacts the impactsof p-polarized of p-polarized light are light our mainare our concern main concern on the next on the content. next content.

Figure 3. Absorption of the proposed structure and (CH NH )PbI layer under illumination of Figure 3. Absorption of the proposed structure and (CH3NH3 3)PbI3 3 layer3 under illumination of p- polarizedp-polarized and and s-polarized s-polarized normal normal incident incident light, light, respectively, respectively, where where P = 800P = nm, 800 w nm, = 550w nm,= 550 h = nm, 15 nm.h = 15 nm.

Next,Next, we we will will go go through through how how to to realize realize the the function function of of the the dual-band dual-band perfect perfect absorp- absorp- tiontion for for this this structure. As As seen seen from from Figure Figure 11,, thethe wholewhole structurestructure cancan bebe approximatelyapproximately regardedregarded as as a a periodic periodic metal-insulator-metal metal-insulator-metal (MIM) (MIM) structure, structure, which which can can suppress suppress the there- flectionreflection at atthe the resonant resonant wavelength. wavelength. As As mentio mentionedned before, before, the the absorption absorption of of this this structure structure can be expressed by A = 1 − R, that is, the absorption peak corresponds to the reflection can be expressed by A = 1−R, that is, the absorption peak corresponds to the reflection dip dip when the reverse light fields are superimposed on each other in the absorber. Thus, we 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 can explain the absorption enhancement for the (CH3NH3)PbI3 layer by analyzing the light field distributions. As depicted from Figure4, at the peak wavelength of λ1 = 1291 nm, field distributions. As depicted from Figure 4, at the peak wavelength of λ1 = 1291 nm, the ≈ λ ≈ the skin depth of Ag is S1 2πIm(Ag) 23.16 nm, as the complex refractive index of skin depth of Ag is 𝑆 ≈ ≈ 23.16nm, as the complex refractive index of Ag is Ag is 0.3534 + 8.876i. Simultaneously, (Ag) at the absorption peak of λ2 = 1548 nm, the skin 0.3534+ 8.876i λ 2 depth of Ag is. Simultaneously,S2 ≈ ≈ at22.80 the absorptionnm, as the peak complex of λ = refractive 1548 nm, indexthe skin of depth sliver of is 2πIm(Ag) Ag is 𝑆 ≈ ≈ 22.80nm, as the complex refractive index of sliver is 0.5378 + 0.5378 + 10.81 (Ag)i. Consequently, the thicknesses of Ag strips are much less than both S1 and S leading to the interaction of optical fields at the interface of Ag strip/air separa- 10.81𝑖2. Consequently, the thicknesses of Ag strips are much less than both S1 and S2 lead- ingtrix to and the Ag interaction strip/hBN of interface. optical fields Thus, at the the electric interface fields of Ag surrounding strip/air separatrix the Ag strips and areAg strip/hBNaccumulated interface. and strengthened, Thus, the electric as described fields surrounding in Figure4a,c. the And Agthe strips magnetic are accumulated field is as- andsembled strengthened, between each as described Ag strip andin Figure substrate 4a,c. for And the resonancethe magnetic mode field at λis1 ,assembled although both be- of them are also gathered around the Ag strips, as described in Figure4a,b. Actually, these tween each Ag strip and substrate for the resonance mode at λ1, although both of them characteristics refer to the excitation of localized plasmon (LP) mode. On the other hand, are also gathered around the Ag strips, as described in Figure 4a,b. Actually, these char- parallel electromagnetic field stripes extending the z-axis are taken shape for the resonance mode at λ2, as seen in Figure4d . 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 Figure4e,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. Coatings 2021, 11, x FOR PEER REVIEW 5 of 10

acteristics 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 Coatings 2021, 11, 67 absorption of the (CH3NH3)PbI3 layer. However, as seen in Figure 4e,f, at the wavelength 5 of 9 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.

Figure 4. ContourFigure profiles 4. Contour of normalized profiles fieldof normalized of the structure field of under the structure p-polarized under incident p-polarized light. (a )incident Electric light. and ( b(a)) magnetic Electric and (b) magnetic fields at λ1 = 1291 nm. (c) Electric and (d) magnetic fields at λ2 = 1548 nm. fields at λ1 = 1291 nm. (c) Electric and (d) magnetic fields at λ2 = 1548 nm. (e) Electric and (f) magnetic fields at λ3 = 1400 nm. (e) Electric and (f) magnetic fields at λ3 = 1400 nm. Subsequently, we study the effect of different widths of Ag strips on the absorption Subsequently, we study the effect of different widths of Ag strips on the absorption peaks of (CH3NH3)PbI3 layer, as shown in Figure5a. Since every Ag strip exhibits as peaks of (CHa3 Fabry-PerotNH3)PbI3 layer, (F-P) as cavityshown for in theFigure LP 5a. mode, Since the every wavelengths Ag strip exhibits of absorption as a peaks are Fabry-Perot significantly(F-P) cavity for correlated the LP mode, with the the width wavelengths of Ag strip. of absorption Therefore, peaks when are the signif- width of Ag strip icantly correlatedis increased, with the the width absorption of Ag peak strip. at Therefore,λ1 will be remarkablywhen the width red-shifted of Ag owe strip to is the gradually increased, theincreasing absorption effective peak at wavelength λ1 will be remarkably of LP mode, red-shifted which further owe leads to the to gradually the enhancement and increasing effectiveconcentration wavelength of optical of LP fields mode, inside which (CH further3NH3)PbI leads3 layer to the and enhancement between adjacent and Ag strips. concentrationThus, of optical the absorption fields inside will (CH firstly3NH increase3)PbI3 layer with andw. between Unfortunately, adjacent if Ag the strips. width of Ag strip Thus, the absorptioncontinues will to befirstly increased, increase too with much w. AgUnfortunately, strips willcover if the thewidth (CH of3NH Ag 3strip)PbI3 layer. The continues toabsorption be increased, efficiency too much will Ag stop strips increasing will cover and the turn(CH3 intoNH3)PbI reduce.3 layer. Meanwhile, The ab- 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 Figure5c, 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. Coatings 2021, 11, x FOR PEER REVIEW 6 of 10

sorption 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 be- Coatings 2021, 11, 67 6 of 9 come longer and longer. By the same token, the absorption will firstly increase and then decline in whole process of increasing P.

Figure 5. The absorption of (CH3NH3)PbI3 layer under normal incident p-polarized light with different (a) widths w, Figure 5. The absorption of (CH3NH3)PbI3 layer under normal incident p-polarized light with dif- (b) thicknessesferenth, and (a) (widthsc) periods w, (Pb)of thicknesses silver strips, h, respectively.and (c) periods (d) P Absorption of silver strips, spectra respectively. of different (d halide) Absorption perovskites. The other parametersspectra are of the different same as halide Figure perovskites.3. The other parameters are the same as Figure 3.

In the following, what influences on the absorption of (CH NH )PbI layer caused by In the following, what influences on the absorption of (CH3NH3)PbI3 layer3 caused3 3 by changing the changingthickness theof Ag thickness strips ofare Ag considered, strips are considered, as shown in as Figure shown 5b. in Figure When5 b.h = When 15 h = 15 nm, nm, which iswhich much isthinner much thinnerthan the than depth the of depth penetration, of penetration, the optical the optical fields fieldsinside inside the the air/Ag strip interface and h-BN/Ag strip interface are efficiently interacted with each other to air/Ag strip interface and h-BN/Ag strip interface are efficiently interacted with each other improve the absorption of (CH3NH3)PbI3 layer, as mentioned above. When h is increased to improve the absorption of (CH3NH3)PbI3 layer, as mentioned above. When h is in- from 15 nm to 20 nm, the optical loss in the Ag strips is increased, and the coupling intensity creased from 15 nm to 20 nm, the optical loss in the Ag strips is increased, and the coupling between the top and bottom of Ag strips is weakened. As a result, both the absorption intensity between the top and bottom of Ag strips is weakened. As a result, both the ab- peaks of (CH3NH3)PbI3 layer at λ1 and λ2 are attenuated and blue-shifted slightly due sorption peaks of (CH3NH3)PbI3 layer at λ1 and λ2 are attenuated and blue-shifted slightly to the reduction of resonance wavelength. However, much too thin Ag strips are not 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 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 optical fields around the (CH3NH3)PbI3 layer, and the strong light diffraction around the the Ag strips. Consequently, when h is reduced from 15 nm to 10 nm, the absorption of 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 (CH3NH3)PbI3 layer is obviously reduced. Furthermore, not just for (CH3NH3)PbI3, the physi- physical mechanism in this absorber can also be utilized to strengthen the absorption of cal mechanism in this absorber can also be utilized to strengthen the absorption of other halide other halide perovskites, such as (CH3NH3)PbBr3, and (CH3NH3)PbCl3. As shown in perovskites, such as (CH3NH3)PbBr3, and (CH3NH3)PbCl3. As shown in Figure 5d, when Figure5d, when the (CH 3NH3)PbI3 layer in this absorber is changed by (CH3NH3)PbBr3 the (CH3NH3)PbI3 layer in this absorber is changed by (CH3NH3)PbBr3 and and (CH3NH3)PbCl3 respectively, both the absorption peaks of (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 (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 peaks of (CH3theyNH3)PbBr can be3 and optimized (CH3NH by3)PbCl manipulating3 are not high the enough other structure in this case, parameters. we believe 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 Figure6. 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 Coatings 2021, 11, x FOR PEER REVIEW 7 of 10

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

Coatings 2021, 11, 67 nm to 10 nm, the intensities and wavelengths of LP and SP modes are also increased,7 of 9 which lead to the increase and red-shift of absorption peaks of (CH3NH3)PbI3 layer. How- ever, 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 op- ticalaround fields the around (CH3NH the3 )PbI(CH33NHlayer.3)PbI Thus,3 layer. when Thus, the when thicknesses the thic ofknesses h-BN layers of h-BN are layers increased are increasedfrom 10 nm from to 1410 nm,nm to the 14 absorption nm, the absorption peaks of peaks (CH3NH of (CH3)PbI3NH3 layer3)PbI are3 layer decreased. are decreased.

Figure 6. The absorption of (CH3NH3)PbI3 layer under normal incident p-polarized light with Figure 6. The absorption of (CH3NH3)PbI3 layer under normal incident p-polarized light with dif- different thicknesses of h-BN layers. ferent thicknesses of h-BN layers. In the next, it should be emphasized that all of results are obtained under the con- ditionIn ofthe normal next, it incidence,should be emphasized however, the that absorber all of results in the are application obtained under need keepthe condi- good tionperformance of normal under incidence, wide angleshowever, of incidence.the absorber Then, in the we investigateapplication theneed optical keep absorptiongood performance under wide angles of incidence. Then, we investigate the optical absorption of of (CH3NH3)PbI3 as a function of angle and the wavelength of incident light. As shown in (CHFigure3NH7,3 the)PbI absorption3 as a function of (CH of angleNH and)PbI theat waveleλ andngthλ are of still incident more light. than 22%As shown and 30%, in Coatings 2021, 11, x FOR PEER REVIEW 3 3 3 1 2 8 of 10 Figureeven if 7, the the angle absorption of incident of (CH light3NH is increased3)PbI3 at λ to1 50and◦. Therefore,λ2 are still ourmore designed than 22% absorber and 30%, can

evenkeep if good the angle performance of incident under light wide is increased angles of incidenceto 50°. Therefore, in practical our applications.designed absorber can keep good performance under wide angles of incidence in practical applications.

FigureFigure 7. 7.The The absorption absorption of of (CH (CH3NH3NH3)PbI3)PbI3 3as as a a function function of of angle angle and and the the wavelength wavelength of ofp -polarizedp-polar- incidentized incident light. light.

4.4. Conclusions Conclusions InIn the the summary, summary, this this paper paper designs designs a a adjustable adjustable dual-band dual-band perfect perfect absorber absorber based based on on (CH(CH3NH3NH3)PbI3)PbI33in in thethe communication communication regime, regime, and and the the corresponding corresponding features features are are investi- investi- gatedgated through through the the use use of of FDTD FDTD methods. methods. It It is is shown shown that that the the absorption absorption of of (CH (CH3NH3NH3)PbI3)PbI3 3 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.

Author Contributions: Conceptualization, L.X. (Liang Xu) and X.L.; methodology, L.X. (Liang Xu); software, X.L.; validation, J.Z., Z.M. and B.P.; formal analysis, L.X. (Libin Xia); investigation, L.X.(Liang Xu); resources, X.L.; data curation, X.Z.; writing—original draft preparation, L.X.(Libin Xia); writing—review and editing, X.L.; visualization, L.W.; supervision, Z.L.; project administration, Z.L.; funding acquisition, L.W. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Natural Science Foundation of China (Grant No. 11764018 and 61505052); Natural Science Foundation of Jiangxi Province (Grant No. 20192BAB212003, 20202ACBL211004); Science and Technology Project of the Education Department of Jiangxi Province (Grant No. GJJ180426 and GJJ200655). Data Availability Statement: Data available in a publicly accessible repository. Acknowledgments: This work was supported by the National Natural Science Foundation of China (Grant No. 11764018 and 61505052); Natural Science Foundation of Jiangxi Province (Grant No. 20192BAB212003, 20202ACBL211004); Science and Technology Project of the Education Department of Jiangxi Province (GJJ180426). Conflicts of Interest: The authors declare no conflict of interest.

References 1. Geim, K.; Grigorieva, I.V. Van der Waals heterostructures. Nature 2013, 499, 419.

Coatings 2021, 11, 67 8 of 9

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.

Author Contributions: Conceptualization, L.X. (Liang Xu) and X.L.; methodology, L.X. (Liang Xu); software, X.L.; validation, J.Z., Z.M. and B.P.; formal analysis, L.X. (Libin Xia); investigation, L.X. (Liang Xu); resources, X.L.; data curation, X.Z.; writing—original draft preparation, L.X. (Libin Xia); writing—review and editing, X.L.; visualization, L.W.; supervision, Z.L.; project administration, Z.L.; funding acquisition, L.W. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Natural Science Foundation of China (Grant No. 11764018 and 61505052); Natural Science Foundation of Jiangxi Province (Grant No. 20192BAB212003, 20202ACBL211004); Science and Technology Project of the Education Department of Jiangxi Province (Grant No. GJJ180426 and GJJ200655). Data Availability Statement: Data available in a publicly accessible repository. Acknowledgments: This work was supported by the National Natural Science Foundation of China (Grant No. 11764018 and 61505052); Natural Science Foundation of Jiangxi Province (Grant No. 20192BAB212003, 20202ACBL211004); Science and Technology Project of the Education Department of Jiangxi Province (GJJ180426). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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