1. Introduction

Recently, the organic-inorganic hybrid perovskite materials are gaining enormous attention across multi-disciplinary research fields due to their outstanding optical and electronic properties [1–5]. Since the first implementation of perovskite as the light absorber in liquid-junction solar cells in 2009, the power conversion efficiency of perovskite solar cells (PSCs) has been boosted from 3% to more than 20% [6–9]. The careful design of solar cell architecture and the optimization of perovskite film preparation methods have greatly facilitated the enhancement of device performance [10–13]. A vast amount of experimental and theoretical investigations indicate that the meteoric rise of hybrid perovskites is mainly attributed to their unique electrical properties, including the intrinsic ambipolar character, high charge carrier mobility and long electron/hole diffusion lengths [1,2,14]. Due to these remarkable features, the hybrid perovskite materials have also been introduced into other opto-electronic devices, such as photo-detectors, light-emitting diodes and field-effect transistors, with promising performances [4,5,15].

In contrast to the intensive efforts on optimizing the electrical properties of perovskite materials, less attention was paid to the constrains imposed by their intrinsic optical properties, especially considering their relatively low absorption efficiency at the long wavelengths of the solar spectrum (from 650 to 800 nm) [16–18]. Several initial attempts, including the introduction of photonic crystals or anti-reflection layers, demonstrate that enhancing optical absorption of PSCs in specific wavelength ranges can be realized, among which localized surface plasmon resonance (LSPR) effects characteristic of metallic nanostructures show significant promise and capability [19–21]. Metallic nanostructures display LSPRs in specific regions of the solar spectrum, producing near-field plasmonic effects that induce light absorption enhancement surrounding the nanostructures. Such effects have been successfully exploited in other applications, such as silicon and organic solar cells and metamaterials, with dramatically improved device performances [22–25]. Recently, metallic nanostructures have also been implemented in PSCs in order to alter their light absorption properties, including core-shell nanoparticles and alloyed nanostructures, where cell performance improvements were observed in both cases [26–29]. However, they are mainly attributed to either the lowering of exciton binding energy or the increasing of charge transfer kinetics, other than the enhancing of light absorption [26,27].

Herein, the light absorption property of perovskite films is optimized by incorporating plasmonic nanostructures of Ag nanowire (AgNW) arrays and nanowire crosses (nanocrosses). The alteration of the optical properties of perovskite films is investigated in three aspects through numerical simulations. Firstly, the solar absorption enhancement of perovskite films is evaluated when AgNW arrays of various sizes, periods and positions are introduced; an integrated solar absorption enhancement as high as 25.9% is obtained in the best case. Secondly, the polarization effect of light absorption for perovskite films with AgNWs is investigated; remarkably, when nanocrosses are presented, the polarization dependence of light absorption is greatly diminished. Finally, the light absorption property of perovskite films embedded with AgNW arrays and nanocrosses at various incident angles is evaluated, where the results show promising prospects towards omnidirectional light harvesting capability.

2. Results and discussion

The solar absorption property of perovskite films with periodic AgNW structures embedded is investigated by performing full-field electromagnetic simulations using finite-difference time-domain (FDTD) method. A simple stack model is constructed as depicted in Fig. 1(a), where a perovskite layer containing a medially positioned AgNW array is sandwiched between a spiro-OMeTAD layer and a glass substrate. The frequency-dependent dielectric constants of the widely adopted CH₃NH₃PbI₃ perovskite are obtained from spectroscopic ellipsometry measurements [30]. The periodic boundary conditions are implemented to model the AgNW arrays, and the perfect match layers are used at the top and bottom of the simulation volume. The cross-sectional views of the boundary conditions and the monitor positions used in the simulations are given in the Appendix (Fig. 8). The plane wave source is used in the simulations, and two light polarization directions, i. e. P_x and P_y, with the electric fields polarized along the x and y directions, are considered in calculating the perovskite solar absorption enhancement for randomly polarized sunlight.

 

Fig. 1 Schematic diagrams of the glass-perovskite-spiro-OMeTAD model systems containing the AgNW array (a) and nanocross (b).

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The position-dependent absorption per unit volume,$P_{abs}$, normalized over the incoming radiation power,$P_{in}\left(\lambda \right)$, is calculated by the following equation,

The $\eta$ maps plotted as functions of the AgNW array parameters R and L for the 200 and 300-nm thick perovskite films under P_x polarized light are shown in Figs. 2(a) and 2(b), respectively. A broad peak exhibiting large absorption enhancement is observed in Fig. 2(a) for R between 40 and 80 nm and L less than 450 nm. The maximum enhancement of 24.7% compared to the reference is obtained when the periodic AgNW array embedded in a 200-nm thick perovskite film possesses a radius of 70 nm and a period of 400 nm. As shown in Fig. 2(b), a broadband absorption enhancement peak is observed for arrays with R ranging from 100 to 120 nm. In this case, the largest $\eta$ reaches 1.155 when the radius and period of the AgNW array are 110 and 400 nm, respectively. The $\eta$ maps of the 200 and 300-nm perovskite films under P_y polarized light are shown in Figs. 2(c) and 2(d), respectively. In the case H = 200 nm, the $\eta$ map shows the similar trend as that for P_x polarization except there are two peaks at (R, L) = (80, 300) and (70, 400) nm. For H = 300 nm, the $\eta$ map displays a different pattern when changing R as compared to that for P_x polarization. As can be seen from Fig. 2(d), there are two $\eta$ peaks for arrays with R in the range of 90-140 nm. The highest $\eta$ value of 1.122 is obtained at (R, L) = (120, 300) nm. Using the optimal radius and period parameters, the corresponding volume concentrations of AgNWs can be calculated.

 

Fig. 2 The integrated solar absorption enhancement $\eta$ of perovskite films plotted as functions of the AgNW radius, R, and the array period, L, for the film thickness, H = 200 nm, under P_x (a) and P_y (c), and H = 300 nm, under P_x (b) and P_y (d) polarized light.

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To understand the underlying physical mechanism of light absorption enhancement when AgNW arrays are introduced into perovskite films, the absorptance spectra of the 200-nm perovskite film with a AgNW array of R = 70 nm and L = 400 nm for P_x and P_y polarizations are calculated and shown in Fig. 3. For comparison, the absorptance spectrum of the perovskite film in the absence of AgNWs is also computed. The three-dimensional (3D) absorption ($P_{abs}$) profiles of a unit cell obtained at peaks “1” and “2” of the absorptance spectrum for P_x polarization and at peaks “3” and “4” for P_y polarization are shown in the insets of Fig. 3. For P_x polarization, the absorptance spectrum shows two peaks and a broadband enhancement at long wavelengths as compared to the reference. From the absorption profiles “1” and “2”, we find strong optical absorption localized around AgNWs due to the near-field plasmonic effect and at the front side of the array caused by back scattering (Mie scattering). For P_y polarization, the absorptance spectrum also exhibits large enhancement compared to that of the reference system. The absorption enhancement is mainly due to the strong back scattering from the AgNW array and the coupling to the waveguide modes in perovskite films as revealed by the 3D profiles “3” and “4” in Fig. 3. The resonant cavity mode of the planar structure is suppressed due to the large absorptance of perovskite films and the disruption from the AgNWs, while it is visible in the pure perovskite film (Appendix, Fig. 9).

 

Fig. 3 The absorptance spectra of the 200-nm perovskite film with AgNW arrays illuminated with P_x and Py polarized light propagating along z axis. The AgNW radius and the array period are 70 and 400 nm, respectively. The insets 1-4 are the 3D absorption ($P_{abs}$) profiles of a unit cell taken at the peak positions 1-4 with the corresponding wavelengths of 609, 675, 605 and 682 nm, respectively.

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We have shown that positioning AgNW arrays in the middle of the perovskite films results in the broadband and strong optical absorption enhancement. Next, we will investigate the effect of the AgNW array location (along the z-axis with its origin located at the center of the perovskite films) on the light absorption of perovskite films. The simulation parameters are set as following: for H = 200 nm, R = 70 nm and L = 400 nm; for H = 300 nm, R = 110 nm and L = 400 nm. As shown in Fig. 4(a), for P_x polarization, the similar $\eta$ variation curves are observed for H = 200 and 300 nm, both showing an inverted “v” shape as the AgNW array moving from one side to the other side of the perovskite film. No absorption enhancement is observed when the AgNW array is located at z = −30 nm for H = 200 nm and z = −40 nm for H = 300 nm. In the case H = 200 nm, the maximum enhancement factor of 25.9% is achieved for the AgNW array positioned at z = + 10 nm. Under these conditions, the corresponding total quantum efficiency is 78.9% as given in the Appendix. For H = 300 nm, a medially positioned AgNW array is more favorable for solar absorption enhancement. The effect of the AgNW array location is also considered for P_y polarization as shown in Fig. 4(a). The overall trends are similar to those for P_x polarization. The optimal positions are found to be z = 0 and −10 nm for H = 200 and 300 nm, respectively. By subtracting the AgNW radii, one notes that, regardless of perovskite film thicknesses, the highest $\eta$ values are obtained when the travel distance of light in perovskites reaches 30 nm before hitting the AgNW arrays. To visualize the alteration of light absorption and the excitation of different plasmonic modes at various locations, the absorptance spectra and the 3D absorption profiles of the 200-nm perovskite films containing the AgNW array of (R, L) = (70, 400) nm under P_x and P_y polarizations are shown in Figs. 4(b) and 4(c). As can be seen, the perovskite films with AgNW arrays show larger absorptance at the wavelengths of 600-770 nm, which is induced by activating the near-field plasmonic modes, the back scattering and the waveguide modes individually or in combination at different wavelengths.

 

Fig. 4 (a) The integrated solar absorption enhancement $\eta$ plotted as a function of the AgNW array position (along the z-axis and at the center of the films, z = 0) for perovskite films of thicknesses H = 200 and 300 nm under P_x and P_y polarized light. The absorptance spectra of the 200-nm perovskite films simulated under P_x (b) and P_y (c) polarizations at various z positions. The insets of (b) and (c) are the 3D absorption ($P_{abs}$) profiles of a unit cell taken at the peak positions 1-6.

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By far, the optimal AgNW array parameters for the highest absorption enhancement have been obtained for both P_x and P_y polarizations. Meanwhile, we find the polarization direction of the incident light has shown a vital influence on the optical absorption of perovskite films with AgNW arrays due to their built-in structural anisotropy. In order to eliminate the polarization dependence, nanocrosses consisting of two perpendicularly overlaid AgNW arrays are incorporated into perovskite films as schematically shown in Fig. 1(b). Such a net-like structure was previously shown to be capable of reducing polarization effect in optical devices [25]. As shown in Fig. 5(a), the $\eta$ values of the perovskite films with AgNW arrays and nanocrosses at different polarization angles are computed. The simulations are performed with the following parameters: for the AgNW array, R = 110 nm and L = 400 nm; for the nanocross, R = 60 nm and L = 400 nm; H = 300 nm for both cases. Clearly, the nanocross-embedded perovskite film is insensitive to the polarization direction with a stable $\eta$ value around 1.16. On the other hand, the $\eta$ of the perovskite film containing the AgNW array decreases from 1.155 to 1.074 as the polarization angle changes from 0 to 90°. As can be seen from Figs. 5(b) and 5(c), incorporating both AgNW arrays and crosses has significantly enhanced light absorption in the long wavelength range, while the absorptance spectra of the nanocross-embedded perovskite film show little variation as a function of the polarization angle. Therefore, integrating nanocrosses into perovskite films is not only able to enhance but also offer polarization-free light capture ability.

 

Fig. 5 (a) The $\eta$ values of perovskite films with AgNW arrays and nanocrosses are plotted as a function of the polarization angle. The absorptance spectra of the 300-nm perovskite films with AgNW crosses (b) and arrays (c) embedded for different polarization directions.

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Next, we investigate the effect of the AgNW radius on the optical absorption property for nanocrosses located in the middle of perovskite films. Here we only consider P_x polarization, because $\eta$ of perovskite films with nanocrosses shows little polarization dependence as discussed in the previous section. The $\eta$ values as a function of R for nanocrosses of L = 400 nm inside the perovskite films of H = 200 and 300 nm are shown in Fig. 6(a). The $\eta$ curves of the two kinds of films with nanocrosses show the same trend that they first reach a peak value and then decline. For H = 200 nm, the optimum R is 40 nm, which gives a significant absorption enhancement of $\eta$ = 1.166. In the case H = 300 nm, the maximum $\eta$ = 1.164 is obtained at R = 60 nm. The absorptance spectra taken at the peaks “A” and “B” of Fig. 6(a) are plotted in Figs. 6(b) and 6(c), respectively. The results of the reference systems are also shown. Both the 200 and 300-nm perovskite films with nanocrosses exhibit higher intensities of light absorptance as compared to their references. The 3D absorption profiles of a unit cell acquired at peak positions “I” and “II” are shown in the insets of Figs. 6(b) and 6(c). As can be seen, for nanocrosses, the light absorption enhancement is achieved as a joint force of the near-field effects, the back scattering and the waveguide modes with varying strengths.

 

Fig. 6 (a) The integrated solar absorption enhancement $\eta$ of the 200 and 300-nm perovskite films plotted as a function of the AgNW radius R for nanocrosses with L = 400 nm. (b-c) The absorptance spectra taken at the conditions of the maximum enhancement points “A” and “B”. The insets are the corresponding 3D absorption ($P_{abs}$) profiles of a unit cell obtained at the peak positions “I” and “II”.

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Finally, we consider the influence of the incident angle of light on the optical absorption property of the AgNW-embedded perovskite films. Omnidirectional light harvesting capability of perovskite films is desirable for practical applications, because of the alteration of the incident angle of sunlight during a day. As shown in Fig. 7, the normalized integrated solar absorption of the perovskite films with and without AgNW arrays and nanocrosses are calculated as a function of the incident angle. The simulations are carried out at the following conditions: H = 300 nm, L = 400 nm, and R = 110 and 60 nm for the AgNW array and nanocross, respectively. The planes of incidence are defined by the normal direction of the perovskite films and the directions parallel and perpendicular to the AgNW axis for the P_x and P_y polarizations, respectively. As can be seen, for P_x polarization, the solar absorption enhancement is attained for the incidence angle between 0 and 60° when either the AgNW array or nanocross is introduced. This is due to the fact that the LSPR modes of the AgNW array and nanocross are excited for a broad range of incident angles. For P_y polarization, the AgNW array improves the optical absorption of perovskite films only when the incident angle is smaller than 30°; beyond this angle, it actually curtails the light absorption efficiency. These results demonstrate that integrating AgNWs, especially nanocrosses, into perovskite films has a significant potential in enhancing their solar absorption efficiency at oblique incidence conditions.

 

Fig. 7 The normalized integrated solar absorption as a function of the incident angle for perovskite films embedded with AgNW arrays and nanocrosses under P_x and P_y polarizations.

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3. Conclusions

We have demonstrated the feasibility of improving the solar absorption efficiency of perovskite films by incorporating periodic AgNW structures. The effects of the radius, period and position of AgNWs inside perovskite films are systematically investigated through numerical simulations and the optimal parameters are obtained. The AgNW crosses are introduced into perovskite films to achieve polarization-independent light harvesting capability. The effect of the incident angle on the optical absorption of perovskite films with AgNW arrays and nanocrosses embedded is also discussed. The full-field numerical simulations illustrate that the optical absorption enhancement is originated from the excitation of the LSPR modes in AgNWs, the back scattering from AgNW grids and the coupling to the waveguide modes in perovskite films. In view that metallic nanowire arrays and crosses of various dimensions can be facilely prepared [31,32], our findings point to a useful direction for optimizing the optical properties of perovskite films, making them amenable to not only photovoltaic cells but also other opto-electronic devices, such as photo-detectors and light emitting diodes.

Appendix Photovoltaic performance of perovskite films for the proposed solar cell model

The total quantum efficiency, TQE , which takes the solar spectral irradiance into account, determines the overall photovoltaic performances of perovskite films with proposed models. TQE is the fraction of the incident photons that are absorbed by perovskite films

$TQE=\frac{{\displaystyle {\int }_{330}^{800}\frac{\lambda }{hc}{\alpha }_{{}_P}(\lambda )AM1.5d\lambda }}{{\displaystyle {\int }_{330}^{800}\frac{\lambda }{hc}AM1.5d\lambda }},$where h is Planck’s constant, c is the speed of light in the free space, α_P is the absorption spectrum of perovskite film excluding the absorption contribution from the AgNWs, and the AM1.5 is the normalized solar spectrum irradiated on the Earth’s surface. We calculate the total quantum efficiency for the proposed model with the optimal parameters given as following: H = 200 nm, R = 70 nm, L = 400 nm and z = +10 nm for the AgNW array. The maximum TQE of 78.9% is obtained for the proposed model.

 

Fig. 8 Cross-sectional views of the boundary conditions and the monitor positions used in the simulations for the perovskite films containing the AgNW array (a) and nanocross (b). For AgNW arrays, in order to reduce computation time, the 2D power absorption monitor was used. For nanocrosses, the 3D power absorption monitor was applied in the simulations [only the cross-sectional view is given in (b)]

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Fig. 9 2D absorption profile (in the x-z plane) for a unit cell of the 200-nm thick pure perovskite ^{film (planar structure) taken at the wavelength of 609 nm. The light is P}x ^{polarization and propagates along} the z axis.

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Funding

National Natural Science Foundation of China (NSFC) (11375256, U1632265); the Science and Technology Commission of Shanghai Municipality (14JC1493300).

References and links

1. D. Shi, V. Adinolfi, R. Comin, M. Yuan, E. Alarousu, A. Buin, Y. Chen, S. Hoogland, A. Rothenberger, K. Katsiev, Y. Losovyj, X. Zhang, P. A. Dowben, O. F. Mohammed, E. H. Sargent, and O. M. Bakr, “Low trap-state density and long carrier diffusion in organolead trihalide perovskite single crystals,” Science 347(6221), 519–522 (2015). [CrossRef]   [PubMed]  

2. S. D. Stranks, G. E. Eperon, G. Grancini, C. Menelaou, M. J. Alcocer, T. Leijtens, L. M. Herz, A. Petrozza, and H. J. Snaith, “Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber,” Science 342(6156), 341–344 (2013). [CrossRef]   [PubMed]  

3. M. Grätzel, “The light and shade of perovskite solar cells,” Nat. Mater. 13(9), 838–842 (2014). [CrossRef]   [PubMed]  

4. J. Luo, J.-H. Im, M. T. Mayer, M. Schreier, M. K. Nazeeruddin, N.-G. Park, S. D. Tilley, H. J. Fan, and M. Grätzel, “Water photolysis at 12.3% efficiency via perovskite photovoltaics and Earth-abundant catalysts,” Science 345(6204), 1593–1596 (2014). [CrossRef]   [PubMed]  

5. Z.-K. Tan, R. S. Moghaddam, M. L. Lai, P. Docampo, R. Higler, F. Deschler, M. Price, A. Sadhanala, L. M. Pazos, D. Credgington, F. Hanusch, T. Bein, H. J. Snaith, and R. H. Friend, “Bright light-emitting diodes based on organometal halide perovskite,” Nat. Nanotechnol. 9(9), 687–692 (2014). [CrossRef]   [PubMed]  

6. A. Kojima, K. Teshima, Y. Shirai, and T. Miyasaka, “Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells,” J. Am. Chem. Soc. 131(17), 6050–6051 (2009). [CrossRef]   [PubMed]  

7. N. J. Jeon, J. H. Noh, W. S. Yang, Y. C. Kim, S. Ryu, J. Seo, and S. I. Seok, “Compositional engineering of perovskite materials for high-performance solar cells,” Nature 517(7535), 476–480 (2015). [CrossRef]   [PubMed]  

8. M. A. Green, K. Emery, Y. Hishikawa, W. Warta, and E. D. Dunlop, “Solar cell efficiency tables (Version 45),” Prog. Photovolt. Res. Appl. 23(1), 1–9 (2015). [CrossRef]  

9. K. Meng, S. Gao, L. Wu, G. Wang, X. Liu, G. Chen, Z. Liu, and G. Chen, “Two-dimensional organic-inorganic hybrid perovskite photonic films,” Nano Lett. 16(7), 4166–4173 (2016). [CrossRef]   [PubMed]  

10. M. Liu, M. B. Johnston, and H. J. Snaith, “Efficient planar heterojunction perovskite solar cells by vapour deposition,” Nature 501(7467), 395–398 (2013). [CrossRef]   [PubMed]  

11. D. Liu and T. L. Kelly, “Perovskite solar cells with a planar heterojunction structure prepared using room-temperature solution processing techniques,” Nat. Photonics 8(2), 133–138 (2013). [CrossRef]  

12. M. Xiao, F. Huang, W. Huang, Y. Dkhissi, Y. Zhu, J. Etheridge, A. Gray-Weale, U. Bach, Y. B. Cheng, and L. Spiccia, “A fast deposition-crystallization procedure for highly efficient lead iodide perovskite thin-film solar cells,” Angew. Chem. Int. Ed. Engl. 53(37), 9898–9903 (2014). [CrossRef]   [PubMed]  

13. J. Burschka, N. Pellet, S.-J. Moon, R. Humphry-Baker, P. Gao, M. K. Nazeeruddin, and M. Grätzel, “Sequential deposition as a route to high-performance perovskite-sensitized solar cells,” Nature 499(7458), 316–319 (2013). [CrossRef]   [PubMed]  

14. T. C. Sum and N. Mathews, “Advancements in perovskite solar cells: photophysics behind the photovoltaics,” Energy Environ. Sci. 7(8), 2518–2534 (2014). [CrossRef]  

15. Q. Lin, A. Armin, P. L. Burn, and P. Meredith, “Filterless narrowband visible photodetectors,” Nat. Photonics 9(10), 687–694 (2015). [CrossRef]  

16. J. M. Ball, S. D. Stranks, M. T. Hörantner, S. Hüttner, W. Zhang, E. J. Crossland, I. Ramirez, M. Riede, M. B. Johnston, R. H. Friend, and H. J. Snaith, “Optical properties and limiting photocurrent of thin-film perovskite solar cells,” Energy Environ. Sci. 8(2), 602–609 (2015). [CrossRef]  

17. M. Anaya, G. Lozano, M. E. Calvo, W. Zhang, M. B. Johnston, H. J. Snaith, and H. Míguez, “Optical description of mesostructured organic-inorganic halide perovskite solar cells,” J. Phys. Chem. Lett. 6(1), 48–53 (2015). [CrossRef]   [PubMed]  

18. Q. Lin, A. Armin, R. C. R. Nagiri, P. L. Burn, and P. Meredith, “Electro-optics of perovskite solar cells,” Nat. Photonics 9(2), 106–112 (2014). [CrossRef]  

19. W. Zhang, M. Anaya, G. Lozano, M. E. Calvo, M. B. Johnston, H. Míguez, and H. J. Snaith, “Highly efficient perovskite solar cells with tunable structural color,” Nano Lett. 15(3), 1698–1702 (2015). [CrossRef]   [PubMed]  

20. M. M. Tavakoli, K.-H. Tsui, Q. Zhang, J. He, Y. Yao, D. Li, and Z. Fan, “Highly efficient flexible perovskite solar cells with antireflection and self-cleaning nanostructures,” ACS Nano 9(10), 10287–10295 (2015). [CrossRef]   [PubMed]  

21. S. Carretero-Palacios, M. E. Calvo, and H. Míguez, “Absorption enhancement in organic-inorganic halide perovskite films with embedded plasmonic gold nanoparticles,” J Phys Chem C Nanomater Interfaces 119(32), 18635–18640 (2015). [CrossRef]   [PubMed]  

22. H. A. Atwater and A. Polman, “Plasmonics for improved photovoltaic devices,” Nat. Mater. 9(3), 205–213 (2010). [CrossRef]   [PubMed]  

23. S. Pillai, K. Catchpole, T. Trupke, and M. Green, “Surface plasmon enhanced silicon solar cells,” J. Appl. Phys. 101(9), 093105 (2007). [CrossRef]  

24. M. G. Kang, T. Xu, H. J. Park, X. Luo, and L. J. Guo, “Efficiency enhancement of organic solar cells using transparent plasmonic Ag nanowire electrodes,” Adv. Mater. 22(39), 4378–4383 (2010). [CrossRef]   [PubMed]  

25. K. Aydin, V. E. Ferry, R. M. Briggs, and H. A. Atwater, “Broadband polarization-independent resonant light absorption using ultrathin plasmonic super absorbers,” Nat. Commun. 2, 517 (2011). [CrossRef]   [PubMed]  

26. W. Zhang, M. Saliba, S. D. Stranks, Y. Sun, X. Shi, U. Wiesner, and H. J. Snaith, “Enhancement of perovskite-based solar cells employing core-shell metal nanoparticles,” Nano Lett. 13(9), 4505–4510 (2013). [CrossRef]   [PubMed]  

27. Z. Yuan, Z. Wu, S. Bai, Z. Xia, W. Xu, T. Song, H. Wu, L. Xu, J. Si, Y. Jin, and B. Sun, “Hot-Electron Injection in a Sandwiched TiOx-Au-TiOx Structure for High-Performance Planar Perovskite Solar Cells,” Adv. Energy Mater. 5(10), 1500038 (2015). [CrossRef]  

28. Z. Lu, X. Pan, Y. Ma, Y. Li, L. Zheng, D. Zhang, Q. Xu, Z. Chen, S. Wang, B. Qu, F. Liu, Y. Huang, L. Xiao, and Q. Gong, “Plasmonic-enhanced perovskite solar cells using alloy popcorn nanoparticles,” RSC Advances 5(15), 11175–11179 (2015). [CrossRef]  

29. H.-L. Hsu, T.-Y. Juang, C.-P. Chen, C.-M. Hsieh, C.-C. Yang, C.-L. Huang, and R.-J. Jeng, “Enhanced efficiency of organic and perovskite photovoltaics from shape-dependent broadband plasmonic effects of silver nanoplates,” Sol. Energy Mater. Sol. Cells 140, 224–231 (2015). [CrossRef]  

30. P. Löper, M. Stuckelberger, B. Niesen, J. Werner, M. Filipič, S.-J. Moon, J.-H. M. Yum, M. Topič, S. De Wolf, and C. Ballif, “Complex refractive index spectra of CH3NH3PbI3 perovskite thin films determined by spectroscopic ellipsometry and spectrophotometry,” J. Phys. Chem. Lett. 6(1), 66–71 (2015). [CrossRef]   [PubMed]  

31. F. Chen, J. Li, F. Yu, D. Zhao, F. Wang, Y. Chen, R.-W. Peng, and M. Wang, “Construction of 3D Metallic Nanostructures on an Arbitrarily Shaped Substrate,” Adv. Mater. 28(33), 7193–7199 (2016). [CrossRef]   [PubMed]  

32. J.-W. Liu, H.-W. Liang, and S.-H. Yu, “Macroscopic-Scale Assembled Nanowire Thin Films and Their Functionalities,” Chem. Rev. 112(8), 4770–4799 (2012). [CrossRef]   [PubMed]