Natural random nano-texturing of the Au interface for light

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Natural random nano-texturing of the Au interface for light backscattering enhanced performance in perovskite solar cells Hui Zhang, Mariia Kramarenko, Johann Osmond, Johann Toudert, and Jordi Martorell ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00099 • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on March 27, 2018

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Natural random nano-texturing of the Au interface for light backscattering enhanced performance in perovskite solar cells Hui Zhang,*,† Mariia Kramarenko,† Johann Osmond,† Johann Toudert,*,† and Jordi Martorell*,†,‡ †

ICFO – Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels, Barcelona, Spain. ‡

Departament de Física, Universitat Politècnica de Catalunya, 08222 Terrassa, Spain.

KEYWORDS: photovoltaics, light trapping, light management, perovskite, light harvesting

ABSTRACT: As the efficiency of a solar cell approaches its limits, photonic considerations to further enhance its performance overtake electronic ones. It has been theoretically shown for GaAs solar cells that with the combined effects of a surface random texturing and a perfectly reflecting rear mirror, efficiencies close to the Shockley-Queisser limit can be reached, even when the absorber layer is very thin. In here, we demonstrate a method for taking advantage of surface random texturing to enhance the efficiency of planar perovskite solar cells. By naturally transferring the perovskite random nano-texturing to the back semiconductor/metal interface, where the contrast in the imaginary part of the refractive index is very large, backscattering reduces light escape from the solar cell structure. This leads to a close to optimal light absorption that allows bringing the cell efficiency from 19.3% to 19.8%. Such path we opened towards an ergodic behaviour for maximum light absorption in perovskite cells may lead to the most efficient perovskite cells ever.

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INTRODUCTION Hybrid metal halide perovskites have attracted increasing attention in recent years due to a simple solution-processed fabrication procedure, unique photo-physical properties and suitability for a large range of optoelectronic applications. Since the first attempt to use a perovskite layer as the active solar cell material in 2009,1 perovskites have brought a gold rush period in the photovoltaic research field. Until very recently most of the effort focused on the improvement of the mesoporous configuration provided power conversion efficiencies (PCEs) for such type of devices were considerably larger than the PCEs measured from planar ones.2-6 However, because of a simpler low temperature fabrication process, adaptable with fast manufacturing technology which may significantly reduce the eventual commercial cell fabrication cost, part of that original interest on the mesoporous structure has now shifted towards the planar one.7-10 Despite recent progress made in the performance of planar cells, certified PCEs for such cells are,8-10 on average, close to 2 percentage points below the largest certified PCE for a mesoporous cell.11 When comparing the external quantum efficiency (EQE) spectra of planar cells8-10,

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those from mesoporous cells, one observes an oscillatory behaviour of the EQE in the red part of the spectrum for the former class of cells. These oscillations result from the interference between forward and backward propagating waves in the cell,12 and thus depend on the layer thicknesses, refractive indices and extinction coefficients. They are typically more pronounced for cells with a perovskite layer thinner than 500 nm where the backward propagating wave has a significant amplitude. In ref. 12, such oscillations translate into a diminished absorption in the near-bandgap spectral region in addition to a dip at around 620 nm. Such two features indicate a far from optimal light absorption for the cell, which can be ascribed to an insufficient path length for the light. A recent computational study by our group estimated that using a two-resonance tapping


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cavity may partially fix this problem by better trapping the light in the perovskite layer of a planar cell, leading to short circuit current enhancements larger than 10%.13 However, a maximum enhancement of the internal field intensity should be expected in an ergodic optical medium incorporating a randomization of the light propagation. Indeed, in a landmark paper published in 1982, Yablonovitch established that a maximum light absorption in an absorber layer can be achieved by a random texturing of this layer’s interfaces.14 When a random surface texturing is combined with a perfectly reflecting rear mirror in solar cells, it was shown for GaAs cells that PCEs close to the Shockley-Queisser limit can be reached, even when the absorber layer thickness is as small as 100 nm15 In perovskite cells, the intrinsic random nanoscale texturing of the perovskite at its interface with the hole transporting layer (HTL) may be potentially used to achieve a broad angular randomization. However, taking advantage of such intrinsic randomness has been largely unexplored for boosting the efficiency of perovskite cells essentially because the refractive index contrast is very limited at such perovskite/HTL interface.16 To obtain a broad angular randomization, a textured interface with a high refractive index contrast would be desirable. In principle, it is possible to naturally transfer the perovskite nano-texture to the interface separating the HTL from the back metal electrode of the cell; a semiconductor/metal interface where the contrast in the imaginary part of the refractive index can be dramatically large. This interface will be called hereafter “back interface”. Unlike vacuum-processed polycrystalline materials, perovskite crystals are grown on a substrate through solution processing and several growth control parameters can be used to tailor the average crystal size and orientation during the stochastic crystallization events.17-20 The surface of the so-obtained perovskite layers can therefore be designed to have a certain random


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nano-texture,21,22 which, as indicated above, can be transferred to the back interface of a solar cell. This natural nano-texturing of the back interface is clearly less invasive to the electrical device performance than other artificial procedures that use nanoparticles,23-25 nano-lithography26 or nano-imprinting27 to incorporate a tailored nano-structuration within the thin-film stack of the cell. Furthermore this natural nano-structuration is very suitable for an effective light absorption enhancement in solar cells, since a random nano-structuration has been shown to provide enhancements comparable with those of a periodic one.28-31 Note also that, from the point of view of fabrication, such nano-structuration of the back interface may appear more direct and practical than other light trapping strategies such as those requiring additional steps to structure the substrate surface.32, 33 In here, we used the back interface random nano-texturing to effectively backscatter light in a broad angular range. We departed from a reference cell with no back interface texturing but with an optimal performance exhibiting PCEs slightly higher than 19%. When the reference cell fabrication procedure was modified to naturally transfer the perovskite texture to the back interface we demonstrated that light trapping can compete with light escape in the entire wavelength range of the perovskite absorption. For the TM polarization such light trapping is enhanced because of a broader scattering pattern thus promoting a most effective light absorption in the perovskite layer, which may be further enhanced by coupling to the surface plasmon at the semiconductor/metal interface. The random texturing we implemented has no detrimental effect on the cell electrical parameters while providing an optimal light absorption in planar cells allowing for PCEs close to 20%. The new path we opened is promising for taking perovskite cells towards the Yablonovitch limit of light absorption.


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RANDOM NANO-TEXTURING OF THE BACK INTERFACE OF PEROVSKITE SOLAR CELLS AND SURFACE SCATTERING CHARACTERISTICS The concept behind the current approach is depicted in Figure 1a. Normally incident light penetrates into the perovskite absorber layer where it gets fully or partially absorbed, depending on the layer thickness and wavelength. Light with a wavelength larger than 650 nm is only partially absorbed by the perovskite and thus reaches the back interface.

Figure 1. Optical path in perovskite solar cells: flat versus randomly nano-textured back interface. (a) Scheme of light propagation in cells with flat (left) and nano-textured (right) back interface, respectively. The inset shows a scheme of the angular patterns of the TM-polarized and TE-polarized light backscattered at the nano-textured back interface. (b) Cross-section scanning electron microscope images of cells with (left) a flat and (right) a nano-textured back interface. (c) Experimental angular patterns of the TE-polarized and TM-polarized light backscattered at the surface of a randomly nano- textured back Au electrode: evidence of the broader scattering pattern for TM-polarized light. Note that the specular reflection contribution has been eliminated from the pattern shown.


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In a cell with a flat back interface (Figure 1a, left), such light is reflected specularly making a second pass in the absorber layer during which it is further absorbed, before escaping the cell through the front glass. In this configuration, light absorption is inefficient at the spectral vicinity of the perovskite bandgap, unless the layer thickness is increased up to a few microns, i.e. above the photo-carrier diffusion length. In a cell with a randomly nano-textured back interface (Figure 1a, right), the light not absorbed after one pass through the active layer is backscattered into it with an angular distribution that depends on the characteristic structural parameters of the back interface and the polarization of the backscattered light (Supporting Information S1). The backscattered light propagates along non-normal directions (path 1) and thus shows an increased optical path in the absorber layer when compared to specularly reflected light. Furthermore, the light backscattered at angles outside the escape cone of the cell (path 2) is totally internal reflected at the front glass substrate/air interface and thus can be trapped inside the cell structure in between such interface and the back interface. For the TM polarization, trapping is further enhanced by a broader scattering pattern and thus a broader backscattering cone. This is exemplified in the inset of Figure 1a that depicts the angular patterns for the TE-polarized and TM-polarized backscattered light (see further details in Supporting Information S1), together with the corresponding escape cone (aperture: 36º). Such broader pattern for the TM polarization can already originate from the first-order mechanism of scattering at the semiconductor/metal interface, i.e. light radiation by polarization currents, which yields a strong and predominant TMpolarized radiation at large scattering angles.34-37 TM-polarized scattering may also be further enhanced through the excitation and radiative desexcitation of the surface plasmon at this


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interface.36-38 In addition, surface plasmons may also contribute to the enhancement of absorption in the perovskite absorber layer through their evanescent field.39-41 Note that, as indicated above, the backscattering at the back interface is several times stronger than at the perovskite/HTL interface with similar characteristic structural parameters (Supporting Information S1). Indeed, the large imaginary part for the refractive index of the back electrode (generally made of Au) results into a strong backscattering with a broad angular distribution. To naturally achieve a marked nano-texturing of the back interface, we engineered the surface morphology of the solution-processed perovskite layer and transferred the obtained nano-texture to the interfaces of the subsequently deposited layers, i.e. HTL and back Au electrode. Our perovskite solar cells were fabricated using spiro-OMeTAD as HTL because of its suitability for achieving high performance devices. Nano-texture transfer is a critical step when using this HTL material due to its excellent wetting properties that make it prone to surface smoothening.42 As shown in Figure 1b (left), a “thick” HTL of approximately 200 nm thickness smoothens the back interface of the cell. In contrast, an effective nano-texture transfer from the perovskite/HTL to the back interface can be achieved upon depositing a “thin” HTL, with a thickness of a few tens of nm, as seen in Figure 1b (right). It is worth noting that this nano-texture is also effectively transferred to the Au/air interface. To roughly determine the full angular range backscattering pattern we may shine light directly onto that Au/air interface from the air side. The angular scattering patterns from that latter interface for the TE-polarized and TM-polarized near-infrared light (wavelength of 830 nm), shown in Figure 1c, roughly reproduce the ones from the back interface, provided for the thin Au layer we used, the texture of an underlying perovskite layer is replicated on both sides of the Au


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layer. Both polarizations show a broad angular distribution, but more pronounced for the TMpolarized light than for the TE-polarized one.

EFFECTIVENESS OF THE NANO-TEXTURE TRANSFER FROM THE PEROVSKITE TO THE BACK INTERFACE In order to quantify the effectiveness of the nano-texture transfer from the perovskite surface to the back interface of a solar cell, we first studied the surface nano-texture from structures consisting of: an uncovered perovskite layer, a perovskite layer covered with a spiro-OMeTAD layer, and with a spiro-OMeTAD layer capped with an Au layer. The thickness of the spiroOMeTAD layer was approximately 80 nm. Figures 2a & b show the corresponding threedimensional atomic force microscopy (3D AFM) and scanning electron microscope (SEM) images that evidence a marked surface nano-structuration for the uncovered perovskite layer, with peak-valley heights reaching 250 nm. A marked nano-texturing is also seen for the “covered” structures, with peak-valley heights reaching 110 nm in both cases.


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Figure 2. Transfer of the perovskite surface nano-texture to the HTL and back metal electrode surface: (a) Threedimensional atomic force microscopy images of a perovskite layer: uncovered (blue), covered with an aprox. 80 nm spiro-OMeTAD layer (perovskite/80 nm spiro-OMeTAD (green)), covered with an aprox. 80 nm spiro-OMeTAD layer plus an Au layer (perovskite/80 nm spiro-OMeTAD/Au (orange)). (b) Top-view scanning electron microscope image of an uncovered perovskite layer. (c) Experimental angular patterns of the light backscattered by these structures with the incident light impinging through the front glass. (d) Integrated backscattered intensity measured on perovskite/spiro-OMeTAD (green squares) and perovskite/spiro-OMeTAD/Au (orange circles) structures with different thicknesses of the spiro-OMeTAD layer.

The root mean square height deviation to average (σ) and height-to-height correlation length (l) values of the surface height-height correlation function (see more details in Supporting Information S2-1), given in Figure 2a, clearly demonstrate that the surface nano-texture of the perovskite layer has been to a large extent transferred to the surface of the spiro-OMeTAD layer. Furthermore, the surface nano-texture of the Au layer perfectly replicates that of the underlying spiro-OMeTAD layer. Therefore, it can be concluded that the 80 nm-thick spiro-OMeTAD layer


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is suitable for an efficient natural transfer of the perovskite surface nano-texture to the back interface of a solar cell. As indicated above, the efficiency of nano-texture transfer from the perovskite surface to the back interface is dictated by the thickness of the spiro-OMeTAD layer. Decreasing this thickness, the transfer efficiency increases, and becomes optimal when Au is deposited directly on top of the perovskite (Supporting Information S2-1). On the contrary, for a thick spiroOMeTAD layer, the Au layer is almost flat, indicating an inefficient transfer. Therefore, the spiro-OMeTAD thickness has to be controlled accurately for an efficient nano-texture transfer. In order to quantify the surface nano-texture transfer effectiveness to achieve a broad angular range light backscattering from the back interface of perovskite solar cells, we have measured the angular patterns of the light backscattered by the three structures shown in Figure 2a, and by perovskite layers covered with spiro-OMeTAD of different thicknesses, with or without Au capping layer. All the measurements were done with the incident beam impinging onto the front glass substrate. Near infrared light at a wavelength of 830 nm was used to ensure no absorption in the perovskite layer (Supporting Information S2-2). Figure 2c depicts the angular patterns for the three structures shown in Figure 2a. The strongest intensity is observed for the perovskite layer covered with spiro-OMeTAD and Au. This shows the excellent backscattering efficiency of the back interface achieved using a thin spiro-OMeTAD layer. In agreement with the AFM data, the thickness of this layer affects markedly the backscattering efficiency, as seen in Figure 2d. Depositing a thinner spiro-OMeTAD layer enhances the nano-texture transfer from the perovskite surface to the back interface, thus enhancing the backscattering efficiency at this interface. This relation is further supported by optical specular reflectance measurements (Supporting Information S2-3) that show a value around 40% for an 80 nm thickness (suggesting


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that 60% of the incident light is scattered or absorbed) and around 20% when the spiroOMeTAD thickness approaches zero (near 80% scattered or absorbed). For achieving an optimal backscattering efficiency at the back interface in a fully stacked solar cell and thus a maximized light trapping and optical absorption, it is necessary to decrease as much as possible the spiro-OMeTAD thickness. However, there is a limit to such a decrease from an electrical point of view that must be respected in order to design devices that will profit from light backscattering effects.

BACKSCATTERING-INDUCED ENHANCED EFFICIENCY IN HIGH PERFORMANCE PEROVSKITE SOLAR CELLS In a standard device, a “thick” spiro-OMeTAD layer is used to facilitate the charge separation and avoid the thermal damage from the electrode deposition. Such a thick spiro-OMeTAD coated from solution prevents the nano-texture transfer from the perovskite surface to the back interface, which is almost completely flat as shown in Figure 1b (left). For such a cell (called hereafter “reference” cell), no efficient backscattering takes place. The optimal thickness of the spiro-OMeTAD layer is determined to be around 200 nm, considering the balance between optical interference effects and electrical conductivity. In the optimal reference device configuration, the fabricated best device displayed short-circuit current density (JSC), open circuit voltage (VOC), fill factor (FF) and PCE values of 22.7 mA/cm2, 1.08 V, 78.6% and 19.3% respectively as shown in Figure 3a. It should be noted that optimal perovskite solar cells with similar structures and fabrication conditions reported by others have a similar performance.43,44


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Figure 3. Device performance analysis. (a) Current density–voltage (J-V) curves, photovoltaic parameters, (b) steady-state measurements and (c) statistical deviation of the photovoltaic parameters of reference (black) and scattering (red) devices (40 cells of each type).

To determine up to what extent the efficiency enhancement can be linked to the random nanotexturing of the back interface of the cell, a series of devices with different spiro-OMeTAD thicknesses were fabricated, and their photovoltaic performance were summarized in Supporting information S3-1. Decreasing this thickness below 200 nm leads to a drop in device performance. This is because the spiro-OMeTAD layer is not thick enough to homogeneously cover the surface nano-texture of the underlying perovskite layer and to avoid the penetration of Au atoms into the perovskite. The direct contact between the perovskite and Au leads to a reduction in shunt resistance and therefore lower VOC and FF (Supporting Information S3-1). In order to reduce this direct contact and avoid the damage from the thermally evaporated Au, an


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additional 5 nm-thick MoO3 HTL was deposited onto the Spiro-OMeTAD before Au deposition. Following this approach, decreasing the spiro-OMeTAD thickness down to around 80 nm allowed to clearly enhance JSC and bring it up to 23.6 mA/cm2 (4% relative increase). This was achieved together with a small decrease in the VOC (2% relative) and no significant variation in the FF (0.5%), resulting in a higher efficiency of 19.8% (2.6% relative increase) as shown in Figure 3a. This highest performance cell, which presents a markedly nano-textured back interface with efficient backscattering, will be called hereafter “scattering” device. Further thinning down the spiro-OMeTAD layer would lead to lower VOC and FF due to the decrease of shunt resistance, while the current enhancement would stay constant (Supporting Information S3-1). Steady state measurements were then performed at the maximum output point, and the efficiency stabilized to 18.9% and 18.2% for the scattering device and the reference device, respectively, as seen in Figure 3b, which shows the evolution of PCE and current density during a 60 seconds period under illumination. For both devices, the PCE first increases to reach a maximum value after a few seconds, and then decreases to reach a stabilized value at the end of the 60 seconds period. The smaller relative PCE decrease of the scattering device (PCE decrease from 19.5 to 18.9% - 3% relative) compared with that of the reference one (PCE decrease from 19.3 to 18.2% - 6% relative) indicates a better stability for the scattering device. This might be due to the presence of the compact MoO3 layer, which can protect the cell from humidity thus enhancing its stability. However, similar to many other planar perovskite solar cells, both devices in our study showed a small hysteresis (Supporting Information S3-2).45 To check the result reproducibility and verify the JSC enhancement, 40 cells from five different batches were fabricated and their performance data are summarized in Figure 3c. It is clearly shown that the


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enhancement in performance is produced by the increased JSC, which increases of ~1 mA/cm2 without sacrificing the other photovoltaic parameters. We relate this enhancement with an improved optical absorption in the perovskite, allowed by the backscattering of light at the random nano-textured back interface of the cell. The EQE measurements carried out confirmed the JSC increase for the scattering device. Note that the measured JSC from the J-V curve agrees well with the integrated JSC from the EQE spectra seen in Figure 4a. Compared with the reference device, an overall enhancement in EQE at all wavelengths was obtained in the scattering device, especially in the near infrared region where the perovskite has less absorption.

Figure 4. External quantum efficiency analysis. (a) Experimental measurement and (b) optical simulation of external quantum efficiency and integrated JSC of the best performing reference (black) and scattering device (red).

When the spiro-OMeTAD layer thickness was further reduced to 20 nm, even though a decay in VOC and FF occurred, the enhancement in EQE became more pronounced and formed a plateau at wavelengths between 710 and 780 nm (Supporting Information S3-3). This is consistent with an increased transfer efficiency of the perovskite surface nano-texture to the back interface, and an increased backscattering efficiency at this interface.


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Although a significant EQE enhancement, seen in Figure 4a, was attained by reducing the spiro-OMeTAD thickness and transferring the perovskite texture randomness to the back interface, one must confirm that this enhancement comes from the backscattering of light at the back interface and subsequent increased light trapping. At such aim, we developed an optical model based on a modified transfer-matrix method to include light scattering effects when numerically computing the EQE. To properly account for multiple light scattering at the back interface we computed the intensity that escapes from the cell structure through the front glass/air interface. By energy conservation, the rest of the scattered light intensity remains trapped inside the cell structure defined by all the layers contained in between the back and the front glass/air interfaces. The fraction of light intensity that is extracted from the normally incident beam is defined by the unit-less parameter s which we determine by adjusting the numerical value of the computed JSC to the experimentally measured one. The EQE can be easily computed assuming a homogenous distribution of the scattered light that remains trapped in the cell structure and determining which part of that light is actually absorbed in the perovskite layer, as indicated in Supporting Information S4-2. The architecture we consider for the numerical computation of the EQE for the reference device is the following: soda lime glass/ITO (100 nm)/TiO2 (50 nm)/perovskite (660 nm)/spiroOMeTAD (190 nm)/Au (100 nm). The experimental EQE for this configuration can be accurately reproduced by the numerical prediction assuming s = 0. If we decrease the spiroOMeTAD thickness to 80 nm and still consider s = 0, the simulated EQE is then moved far away from the experimental spectrum of the scattering device (Supporting Information S4-1). As shown in Figure 4b, when the s parameter is increased up to 0.3, the agreement between the numerically computed and the experimentally measured EQEs is remarkable. Indeed, it


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accurately reproduces the infrared shift at the band edge and the increase in the red and infrared part of the spectrum. In other words, 30% of the normally incident light intensity is scattered by the back interface, partially trapped in the cell structure and, a significant portion of it, eventually absorbed by the perovskite layer. An EQE very close to the internal quantum efficiency (IQE), i.e. an absorbance tending toward 100%, would be achieved when the s parameter is increased up to 0.9, as can be seen in the Figure S4.6. In that event, the JSC would be 26.5 mA/cm2, very close to the short circuit current upper limit for a photovoltaic material with a band gap close to 1.5 eV.

CONCLUSION In conclusion, a random nano-texturing of the HTL/Au electrode interface at the back of planar perovskite solar cells was achieved by naturally transferring the textured surface morphology of the perovskite absorber layer to such back interface. The large contrast in the refractive index imaginary part at that back interface leads to a strong backscattering and more effective absorption of the incident light, which may be enhanced by the excitation of a surface plasmon. When a planar perovskite solar cell incorporating this nano-textured back interface was fabricated, an optimal light harvesting and absorption could be obtained resulting in ~1 mA/cm2 gain in JSC relative to high performance reference cells. This was achieved without altering any other photovoltaic parameters relevant for the performance of such reference cells. Highly efficient perovskite solar cells with a champion efficiency of 19.8% were obtained by this method, which did not require using any lithography or nano-imprinting technique. Instead, it was based on the control of the perovskite morphology by self-assembly methods that are highly attractive for their ample applicability. The purely photonic path that we proposed and


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implemented towards an ergodic optical medium to enhance light absorption is a demonstration that when a given solar cell gets closer to its efficiency limit photonic considerations overtake electronic ones.15

METHODS Device fabrication. Commercial ITO (15 Ω/sq, Stuttgart) covered glass substrates were purchased from the Institute for Large Area Microelectronics (IGM). The ITO substrate was sequentially washed with detergent, distilled water, acetone, and ethanol. The ETL of Titanium diisopropoxide bis(acetylacetonate) (Sigma-Aldrich) stabilized TiO2 nanoparticle solution (4-8 nm diameter, 20 wt% in water, PlasmaChem GmbH) was coated on ITO substrate at 6000 r.p.m., and annealed at 150 °C for 30 min in air. The thickness of the ETL is around 50 nm. The perovskite layer was then deposited by a sequential two-step spin coating method; first, 1.0 M of PbI2 (99%, Sigma-Aldrich) in anhydrous N, N- Dimethylformamide (99.8%, Sigma Aldrich) was spin coated onto the ETL at 1500 r.p.m. for 45 s, then annealed at 100 °C for 10 min to remove the remaining solvent; second, after the PbI2 coated substrates cooling to room temperature (25 °C), 0.3 M of CH(NH2)2I (98%, Sigma Aldrich) in 1 ml isopropanol with 10 mol% of CH3NH3Cl (Sigma Aldrich) was spin coated onto the PbI2 at 1500 r.p.m. for 45 s, and then the film was annealed at 150 °C for 20 min. The HTL solution was prepared by dissolving 72.3 mg (2,2’,7,7’tetrakis-(N,N-dimethoxyphenylamine)-9,9’-spirobifluorene) (Spiro-OMeTAD, Merck), 28.9 mL 4-tert-butylpyridine (99.9%, Sigma-Aldrich) and 17.5 mL of a stock solution of 520 mg/ml lithium bis(trifluoromethylsulphonyl)imide in acetonitrile (99.9%, Sigma-Aldrich) in 1 mL chlorobenzene (99.9%, Sigma-Aldrich). The ~200 nm thick HTL was deposited by spin coating the solution at 3000 r.p.m. for 45 s, and the solution concentration and spin coating speed were


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varied to achieve thinner thickness. Finally, the MoO3 (5 nm, 0.5 Å·s-1) and Au (80 nm, 1 Å·s-1) was sequentially deposited by thermal evaporation under a pressure of

Natural random nano-texturing of the Au interface for light

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