Functional optical design of thickness-optimized transparent conductive dielectric-metal-dielectric plasmonic structure

Dielectric/metal/dielectric plasmonic transparent structures play an important role in tailoring the high-optical performance of various optoelectronic devices. Though these structures are in significant demand in applications, including modification of the optical properties, average visible transmittance (AVT) and colour render index (CRI) and correlated colour temperature (CCT), obtaining optimal ones require precise thickness optimization. The overall objective of this study is the estimation of the optimal design concept of MoO3/Ag/WO3 (10/dAg/dWO3 nm) plasmonic structure. To explore the proper use in optoelectronic devices, we are motivated to conduct a rigorous optical evaluation on the thickness of layers. Having calculated optical characteristics and achieved the highest AVT of 97.3% for dAg = 4 nm and dWO3 = 6 nm by the transfer matrix method, it is quite possible to offer the potential of the structure acting as a transparent contact. Notably, the colour coordinates of the structure are x = 0.3110 and y = 0.3271, namely, it attributes very close to the Planckian locus. This superior colour performance displays that MoO3/Ag/WO3 shall undergo rapid development in neutral-colour windows and LED technologies. Structure with dAg = 6 nm and dWO3 = 16 nm exhibits the highest CRI of 98.58, thus identifying an optimal structure that can be integrated into LED lighting applications and imaging technologies. Besides the colour of structure with dAg = 4 nm and dWO3 = 8 nm is equal for D65 Standard Illuminant, the study reports that the range of CCTs are between 5000 and 6500 K. This optimization makes the structure employable as a near-daylight broadband illuminant. The study emphasizes that optimal MoO3/Ag/WO3 plasmonic structures can be used effectively to boost optoelectronic devices' performance.


Scientific Reports
| (2022) 12:8822 | https://doi.org/10.1038/s41598-022-13038-y www.nature.com/scientificreports/ this means, we have functionally realized the contact design with optimal values of optical parameters such as AVT, CRI, CCT and colour coordinates by thickness optimizing of MoO 3 /Ag/WO 3 in the mentioned application areas. We have employed the conventional Transfer Matrix Method (TMM) in calculations and made a detailed evaluation of the layer thickness dependent data. For the optimal MoO 3 /Ag/WO 3 (10/d Ag /d WO 3 nm) design, we have expressed the AVT, CRI and CCT from the findings of transfer matrix method. By its optical characteristic extracted, the current structure may be exhibited as a feasible transparent contact structure with a great colour rendering property. In this study, the methodology followed in the functional optical design of the transparent conductive plasmonic structure and the findings in its optical properties are given in Fig. 1.

Result and discussion
It is evident that DMD based transparent contact structures have low resistance value (< 10 sq −1 ), efficient charge injecting, and transmittance of more than 70%, particularly in the VR 16,41 . MoO 3 , which acts as an internal dielectric in the MoO 3 /Ag/WO 3 transparent contact design, has relatively high hole mobility and high transparency in the VR 10,42 . That is why the potential of using MoO 3 among TCOs with high work function in designing high-performance optoelectronic devices stands out. The use of MoO 3 , which provides a hole injection into active layers and acts as an anti-reflective layer for reflective metals such as Ag and Au, which have a high refractive index 43,44 , as HTL has become a well-established approach in inverted structures 10,[45][46][47] . For optimal electrical performance, the thickness of MoO 3 affected the electrical parameters of the devices is preferred around 10 nm 28 .
On the other hand, Ag is frequently preferred as a metal layer with a low absorption coefficient and high electrical conductivity than other common metals in DMD structures 48,49 . When an effective cap layer of WO 3 is put to use due to its high refractive index and prevention of Ag oxidation 50,51 , the radiation loss from the surface plasmon in the Ag layer can be suppressed, and thus the thickness of WO 3 ( d WO 3 ) and the permeability of the DMD structure can be modified 6,52 .
By arguments of the sort mentioned above, a detailed evaluation of optical parameters such as AVT, extended CRI (CRI ext ), CCT, and colour coordinates of the MoO 3 /Ag/WO 3 transparent contact designed within the scope of the study has been provided based on the thickness change of the metal and WO 3 layers. For MoO 3 /Ag/WO 3 (10/d Ag /d WO 3 nm), the theoretical reflectance, transmittance and absorption spectra were calculated by ranging of 0-20 nm, the effective range where the actual physical changes can be observed. Subsequently, optical parameters of the structures were estimated from the spectra obtained by TMM.

Evaluation on AVT values of MoO 3 /Ag/WO 3 structure
The first investigation for optimal MoO 3 /Ag/WO 3 (10/d Ag /d WO 3 nm) design was demonstrated depending on AVT. The d Ag and d WO 3 values dependence of the structure's associated AVT variation are exhibited in Fig. 2a. By varying d Ag and d WO 3 values in the range of 0-20 nm, the minimum AVT of the current structure was 32.48%, namely, it is widened to even higher levels than those of the lower limit (25%) for window applications. Based on our analysis, the MoO 3 /Ag/WO 3 structure is a convenient transparent contact.
Increasing d WO 3 at a constant value of d Ag has little or no overall effect on AVT levels and the highest AVT was achieved for d Ag =4 nm and d WO 3 =6 nm. On the contrary, the increase in d Ag considerably affects on AVT. Particularly for greater than 8 nm of d Ag , the AVT tends to drop below 80%. Therefore, it is identified that Ag in the transparent contact structure exerts a more dominant role over the AVT. The present case can be explained by the reflection characteristic of Ag known in the VR, the electric field provided by the high free electron density, and the SP effect. As seen in AVT distributions, a significant linearity is observed between d Ag and d WO 3 (Fig. 2a). When this relationship between d Ag and d WO 3 is maintained in the MoO 3 /Ag/WO 3 structure, the same AVTs can be obtained. In addition, for an electrical evaluation, in DMD structures in which Ag is a metal layer in the literature, with an increase of d Ag from 2 to 12 nm, resistivity and R sh decrease from 5.97 × 10 -5 cm to 0.97 × 10 -5 cm and from 18.66 sq −1 to 2.31 sq −1 , respectively 53 . Therefore, increasing d Ag significantly reduces AVT and increases conductivity.
In the AVT mapping of MoO 3 /Ag/WO 3 (10/d Ag /d WO 3 nm), the improvement in AVT with the increase of d WO 3 , especially for a specific d Ag larger than 6 nm, is due to the anti-reflection property of WO 3 with the waveguide effect. Because it acts as the anti-reflection layer of WO 3 for Ag with a high refraction index [1, 1:35, 1:36]. For a more effective evaluation of the anti-reflection feature, the change in the reflection spectra with the change of d WO 3 for 8 nm, 12 nm, 16 nm and 20 nm values of d Ag is given in Supplementary Fig. 4. In addition, a more effective anti-reflection feature with d WO 3 change by calculating the average reflection over AM1.5G for MoO 3 /Ag/WO 3 (10/d Ag /d WO 3 nm) is presented in Supplementary Fig. 5. AR AM1.5G is highly dependent on d Ag , but for a given Ag layer thicker than 6 nm, the reflection property of the structure is reduced by increasing d WO 3 .
CRI ext , CIE x and y, CCT, maximum transmittance (T max ) and wavelength ( T max ) at T max , AVTs according to the MoO 3 /Ag/WO 3 transparent contact structures change are presented in Table 1a. AVTs, estimated in this study, inside the areas with solid grey lines and grey dashed lines are greater than 97% and 95%, respectively (Fig. 2a). The lowest and highest AVTs of the DMD structure are 32.48% and 97.3% with d Ag =4 nm, d WO 3 =6 nm and with d Ag =20 nm, d WO 3 =2 nm respectively.
When both Table 1a and Fig. 2a are interpreted, an increment in AVTs occurs with increasing d WO 3 , particularly for d Ag thicker than 10 nm. For example, in the MoO 3 /Ag/WO 3 (10/20/20 nm), where the lowest AVT of 32.48% is observed, the AVT improves to 53.66% with the increase of d WO 3 to 20 nm. This is since the transparency reduced by the strong electric field and the plasmonic effect observed due to surface charges, especially in thicker metals, is improved with a thicker outer dielectric layer 6,52 . This effect is not dominant for the thin metal layer, and a reduction occurs with d WO 3 . In the high AVT region, while the AVT is 97% at d Ag =2 nm and d WO 3 =1 nm, the AVT decreases to 84.58% and contrarily d WO 3 increases to 20 nm.   Since the AVT has a maximum at 550 nm and is evaluated on AM1.5G, the deviation of T max from 550 nm reduces the AVTs even in the case of T max >99. Therefore, AVT is not at its maximum value in high-transparency structures, and T max values are considerably greater than 550 nm for these structures. With increasing d WO 3 from 2 to 20 nm, T max increases from 709 to 968 nm, and contrarily AVT decreases from 96.9% to 84.58%.
As seen in the AVT distribution of the MoO 3 /Ag/WO 3 (10/d Ag /d WO 3 nm), the intersection of the horizontal and vertical dashed lines corresponds to the values of d Ag =4 nm and d WO 3 =6 nm (Fig. 2a). The changes in AVT and T max along these horizontal and vertical lines for thicknesses varying in the range of 0-20 nm are introduced in Fig. 2b and c, respectively.
In a transparent contact structure, AVTs are more affected by the change of d Ag than T max . This optical characteristic can be understood when the absorption, reflectance and transmittance spectra of the MoO 3 /Ag/WO 3 ( Fig. 3a-c, respectively) are calculated by TMM according to the d Ag d WO 3 changes are examined. The AVTs are more sensitive to the change of d Ag than T max due to the significant increment in the reflection spectrum in the infrared region (IR) and VR. Because the absorption of the structure in this region is deficient and at wavelengths It should be noted here that Ag is a determining layer for both AVT and maximum transparency, and outer dielectric WO 3 acts as a waveguide. The encircling effect on the electromagnetic wave of WO 3 causes an increment in maximum transparency with the increase of d WO 3 . Even the d Ag and d WO 3 values of 20 nm, AVT is greater than 25% which is the lower limit for window applications. When the absorption spectra given in Fig. 3a are examined, it is seen that the MoO 3 /Ag/WO 3 transparent contact has very high absorption in the ultraviolet (UV) region. This is due to the known absorption characteristics of TMOs such as MoO 3 and WO 3 in the UV region. In the wavelength region more significant than 400 nm, MoO 3 and WO 3 have no absorption, and this characteristic indicates that the MoO 3 /Ag/WO 3 system is a perfect transparent contact in VR.
When the reflectance spectra given in Fig. 3b are examined, the wavelength value with the minimum reflection shifts to a lower wavelength with the increase of d Ag , while it shifts to higher wavelength values with the increase of d WO 3 . In addition, the increase in both d Ag and d WO 3 narrows the distribution of reflectance spectra. This behaviour observed in the reflection characteristic also manifests itself in the transmittance spectra, especially for the wavelength region with no absorption. The variation of wavelength values, at which minimum reflection and maximum transmittance are obtained, concerning d Ag and d WO 3 , are given in Fig. 4a and b, respectively.
The minimum reflectance and maximum transmittance values for d Ag = 4 nm and d WO 3 = 6 nm are around 550 nm. Considering that AM1.5G and human eye perception have a maximum value at 550 nm, MoO 3 /Ag/ WO 3 system designed with d Ag = 4 nm and d WO 3 =6 nm has the potential to both transparent contact with high AVT for semi-transparent optoelectronic devices and an anti-reflection system for photovoltaic-based optoelectronic devices. In addition, due to the absorption characteristics of TMO's in the UV region, there is a slight difference between the wavelengths belonging to the maximum transmittance and minimum reflectance for the same d Ag and d WO 3 .
The effect of the metal layer on the transmittance and reflection spectra of the transparent contact, compared to the outer dielectric, which acts as a waveguide and has a surrounding effect, shows itself in the variation of the colour coordinates depending on the thickness. The variation of the colour coordinates of the MoO 3 /Ag/WO 3 (10/d Ag /d WO 3 nm) transparent contact structure concerning d Ag and d WO 3 are given in the CIE 1931 chromaticity diagram in Fig. 5a and b, respectively. The colour coordinates of the transparent contact with the highest AVT designed at d Ag = 4 nm and d WO 3 = 6 nm are CIE x = 0.3110 and CIE y = 0.3271. They are quite close to the colour coordinates of achromatic point ( x ap =0.3333, y ap =0.3333), AM1.5G ( x AM1.5G =0.3202, y AM1.5G =0.3324) and D65 ( x D65 =0.3128, y D65 =0.3290). This superior performance, obtained in the colour characteristic of the MoO 3 /Ag/ WO 3 (10/4/6 nm) transparent contact structure with the highest AVT, increases the structure's potential to be used, especially in window applications that require neutral colour and in LED technologies.
With the increase of d Ag from 2 to 20 nm in the MoO 3 /Ag/WO 3 (10/d Ag /6 nm) transparent contact structure, The colour coordinates shifted from the achromatic point to the blue region along Planckian locus. CIE x decreased by 23.43% from 0.3154 to 0.2415, and CIE y decreased by 25.12% from 0.3299 to 0.2470. The blueshift is due to a serious decrease in the wavelengths responsible for the red colour and the IR region with the increase of d Ag , especially in the transmittance spectrum of the transparent contact structure. The effect of d WO 3 on colour coordinates is less than that of d Ag . This comparison can be better understood by examining the transmittance spectra given in Fig. 3b. With the increase of d Ag , the transmittance decreases, especially in the region where the colour matching functions are responsible for red, while the spectra of the transmittance spectra with d WO 3 change relatively at the same rate for all colours.
In determining the optimal conditions for the MoO 3 /Ag/WO 3 transparent contact, the adjustment of d Ag should be made by evaluating the colour coordinates and AVTs together since the metal layer has a limiting effect on the AVT. However, even for d Ag =20 nm, the AVT of the transparent contact is 36.24%. This value is greater than 25%, considered the maximum limit for window applications. Therefore, this feature indicates that colour modification can be achieved with a metal layer in the MoO 3 /Ag/WO 3 (10/d Ag /6 nm) transparent contact structure. Especially for an LED that emits light in a different colour, integrating the MoO 3 /Ag/WO 3 transparent contact into the structure with convenient design parameters can change the colour of the light emitted by the LED. In particular, the shift to blue offers significant potential for blue LED technology. In addition, the fact that the outer dielectric does not seriously affect the colour coordinates requires d WO 3 to be considered an effective parameter only in the evaluation of AVT. Optical spectra of the transparent contact structure. Variations of (a) absorption, (b) reflectance and (c) transmittance spectra dependent on d Ag and d WO 3 of MoO 3 /Ag/WO 3 (10/d Ag /d WO 3 nm) transparent contact structure. In a, wavelength dependent the refractive index ( n ) changes and extinction coefficient ( k ex ) of MoO 3 , WO 3 , and Ag are presented. It should be noted that the difference between the n values of MoO 3 and WO 3 drastically changes the n distribution in the structure and allows for ultra-thin designs that allow easier modification of thickness control and optical characteristics. This enables the design of lighter and more flexible structures with the desired optical properties by using less material. The k ex values in the VR for MoO 3 and WO 3 are very close to zero, allowing the evaluation of the direct trade-off between transmittance and reflectance without absorption. In b, the visible spectrum and the wavelength range from which the AVTs are calculated are given. In c, the photonic response of the human eye and the VR are presented as enveloped by AM1.5G. When the maximum value of the transparency coincides with the AM1.5G maximum, the highest AVT should be expected in the structure. In addition, the photonic response of human eye determines a wavelength range in the AVT calculations, and an evaluation should be made in this range.    The CRI ext of the MoO 3 /Ag/WO 3 transparent contact structure, which is designed based on the highest AVT and neutral colour coordinates in the d Ag =4 nm and d WO 3 =6 nm parameters, is 95.75% and is not at its maximum value. Therefore, the optimal structure ( d Ag =4 nm, d WO 3 =6 nm) determined based on AVT and neutral colour values may not show high performance when evaluated in terms of CRI ext . Therefore, evaluating the AVT given in Fig. 2 and the CRI ext distributions given in Fig. 6 together provides a more optimal structure. When Fig. 3 and Table 1b are examined, it is evident that a thick outer dielectric layer is required to achieve high CRI ext values. In this case, when the AVT distribution given in Fig. 2 is examined, d Ag should be in the range of 2-7 nm for high d WO 3 values. With a more detailed analysis, it is understood that d Ag = 6 nm and d WO 3 = 16 nm for both AVT and CRI ext and neutral colour. For d Ag = 6 nm and d WO 3 = 16 nm, the AVT is 95.38%, the CRI ext is maximum 98.58, and the CIE x and y colour coordinates are 0.3168 and 0.3350, respectively. In addition, in these www.nature.com/scientificreports/ structure parameters, T max is very close to 550 nm and has a value of 582.9 nm. Considering that commercial LEDs have reached 98 CRI today, with the achieved CRI of 98.58 and TCS09 of 94.80, MoO 3 /Ag/WO 3 transparent contact for d Ag = 6 nm and d WO 3 = 16 has the potential to be highly applicable both as a transparent contact structure and conductive sheath for existing commercial LEDs and imaging technologies provided with them.

Scientific
In addition to the CRI ext evaluation, it is essential to know how the colour renderings of the optimally determined structures are for each test colour sample (TCS). TCS analysis determines which transparent contact structure to be used in various applications can give specific standard colour with which rendering and how faithfully it is. Within the scope of the study, colour renders analysis was performed by making calculations over 15 TCS for the MoO 3 /Ag/WO 3 transparent contact structure. The TCS values for the MoO 3 /Ag/WO 3 transparent contact structure with the highest AVT ( d Ag = 4 nm, d WO 3 = 6 nm), the highest CRI ext (d Ag = 6 nm, d WO 3 = 16 nm), and the highest transparency ( d Ag = 2 nm, d WO 3 = 2 nm) are given in Fig. 7.
As expected for the structure with optimal values of d Ag = 6 nm and d WO 3 = 16 nm based on CRI ext , there is a colour rendering index of over 90 for all TCSs. For structures designed based on AVT and T max , the values of all TCSs except TCS09 are pretty high and more significant than the perfection limit of 90. Therefore, the contact structure designed in these parameters is very suitable for optoelectronic applications such as lighting and display technologies that include specific colour applications. An electrical evaluation of MoO 3 /Ag/WO 3 (10/6/16 nm), which is optimally presented for CRI, is also noteworthy in the literature. Especially, for d Ag > 4 nm, the R sh decreases and becomes around 4 sq −1 in d Ag = 6 nm 53 . Therefore, the MoO 3 /Ag/WO 3 (10/6/16 nm) also has a very convenient contact properties for electrical performance.
TC09 measures how well a light source or transparent structure can reproduce red. The red colour is critical in applications such as photography, textiles and the production of human colour tones. Many objects around us appear as a mixture of colours, including red. For example, skin tone is very sensitive to the red colour of blood flowing under the skin. With all these features and its proximity to daylight or incandescent bulbs, the TCS09 stands out as a special consideration among other TCSs. A system with a low TCS09 score will display red as far from its colour, or even green. Therefore, the TCS09 is significant for LEDs, which form the backbone of daily or professional lighting technology today.
In analysis, it is not easy to obtain TCS09 at a high value compared to other TCSs, especially for mathematical evaluation over spectra. In addition, the TCS09 value is highly dependent on the spectral characteristics of the device. Therefore, the TCS09 rating is classified as suitable for values of 50 and above and excellent for values of 90 and above, unlike the CRI or CRI ext scale. TCS09 values for MoO 3 /Ag/WO 3 transparent contact designed based on AVT and T max are quite high and are 88.67 and 85.74, respectively. For the structure designed based on CRI ext , the TCS09 metric is rated excellent and has a value of 94.80. Therefore, MoO 3 /Ag/WO 3 (10/d Ag /d WO 3 nm) transparent contact structure for d Ag = 6 nm and d WO 3 = 16 nm parameters is a high-performance contact system that can be integrated into optoelectronic devices based on LED lighting applications and imaging technologies.
When the d Ag are examined for the optimal structures presented in Fig. 7, it is remarkable that ultra-thin metal layer such as 2 nm, 4 nm and 6 nm is optimally presented. In practice, properties such as roughness and thickness of ultra-thin Ag films can significantly degrade the resistivity and sheet resistance. This is due to the www.nature.com/scientificreports/ formation process of the Ag layer, especially in physical vapor deposition techniques. Because it is usual for Ag particles to accumulate in islands during the deposition process. This requires the investigation of different transport mechanisms for the movement of electrons in the metal layer. Therefore, using various techniques such as sputtering and electron beam evaporation in the literature and improving the deposition parameters in these techniques, ultra-thin, ultra-smooth, continuous, low-loss and low-R sh Ag thin films can be produced 53,54 . Especially in the sputter system, by increasing the sputtering time for Ag and narrowing the gap between Ag islands, R sh for 4 nm Ag thin-film could be reduced considerably 53 . In addition, with the electron beam evaporation system, the root mean square roughness values could be improved from 0.73 nm to 0.22 nm even with the thinning of the thickness from 6 to 1 nm for ultra-thin Ag 54 . With all these developments and improvements in different deposition techniques for the production of ultra-fine Ags, the optimal structures presented in the study have the potential to be produced and offer good electrical properties.
Evaluation on colour Coordinates of MoO 3 /Ag/WO 3 structure. The colour perception and CRI ext characteristics are also imperative to examine the colour coordinates of the MoO 3 /Ag/WO 3 according to the d Ag and d WO 3 changes. The distribution of colour coordinates obtained from optical characteristics by TMM for MoO 3 /Ag/WO 3 (10/d Ag /d WO 3 nm) according to d Ag and d WO 3 was studied on. For d Ag and d WO 3 in the range of 0-20 nm is represented in Fig. 8. As mentioned in the AVT characteristic, examining the Fig. 8, it is seen that the metal layer is more effective than the outer dielectric on the colour coordinates. Also, the colour coordinates dependent on d Ag decrease after 7 nm for CIE x and 9 nm for CIE y. The obtainment colour coordinates very close to D65 and the achromatic point is possible for a wide range of d Ag and d WO 3 . On the other hand, the CIE x coordinate for the MoO 3 /Ag/WO 3 (10/d Ag /d WO 3 nm) contact cannot be at the Planckian locus value, but this is possible for the CIE y. A structure with colour characteristics of D65 coordinate can also be stated for MoO 3 /Ag/WO 3 (10/d Ag /d WO 3 nm), as seen from the D65 contour line in Fig. 8a and b. The optical parameters of the structures corresponding to the exact values of d Ag and d WO 3 are noted in Tables 2a and b. In addition, the change of colour coordinates and their relations with each other and together with d Ag and d WO 3 are given in Fig. 9a.
As d Ag and d WO 3 decrease together in structures where the CIE x is equal to D65's x value, the CIE y coordinate also approaches its value in the D65 line. The same behaviour is obtained for structures where the coordinate is equal to D65. With this examination, as shown in Fig. 9a, the colour coordinate of the MoO 3 /Ag/WO 3 transparent contact structure designed for d Ag nm and d WO 3 = 8 nm is equal to the D65 colour coordinates. In addition, the AVT and CRI ext values for this structure are pretty high, with values of 96.97% and 95.76%, respectively.
The colour coordinates equal to D65 are the characteristic properties that make the current structure suitable for use in various optoelectronic applications, especially acting as a light source. Moreover, CCTs are noteworthy in optoelectronic devices designed for lighting technology, various light sources and illuminations closest to daylight. Here, CCTs were calculated from the optical characteristics of the MoO 3 /Ag/WO 3 transparent contact structure obtained by TMM. The distribution of the values corresponding to d Ag and d WO 3 in the 0-20 nm range is given in Fig. 9b.

Method
Calculation of optic spectrum. The design of DMDs, which are transparent top contacts with MoO 3 /Ag/ WO 3 structure, was carried out by calculating the absorption, transmittance, reflection spectra and evaluating the thickness parameters. The transfer matrix method (TMM), a very effective one used in the simulations of optoelectronic devices, was applied to perform the calculations. It is the most potent and widely used method to analyze how the electromagnetic wave propagates in the structure and to determine the optical characteristics of the structure theoretically, especially in structures such as DMD, where different dielectric layers are grown on top of each other 10 . The method followed, and the equations used in calculating made with TMM are given in detail in our previous study 10   www.nature.com/scientificreports/ into account the photonic response of the human eye. The transmittance spectra of the MoO 3 /Ag/WO 3 system calculated by TMM were employed when making AVT calculations. The method followed, and the equations used in calculating AVTs are given in detail in our previous study 10 and Supplementary Information.

Calculation of CIE 1931 colour coordinates. Another significant characteristic of transparent contact
structures is the colour coordinates in the CIE 1931 chromaticity diagram (CIE x and y) as vital as AVT. Often used to determine the colour properties of transparent optoelectronic devices, the design of this diagram is based on the photonic response of the human eye. The method followed, and the equations used in calculating the CIE 1931 colour coordinates of the MoO 3 /Ag/WO 3 transparent contact system are given in detail in our previous study 10 and Supplementary Information.
Calculation of colour render ındex. Colour render index (CRI) measures how precise the colour of an object is when it is illuminated with an ideal or natural light source 22 . CRIs of 90 and above are considered excellent, while CRIs below 80 is generally considered poor 40 . Therefore, CRI creates a measurement system that gives whether the colours of illuminated objects are true to their originality and takes values in the range of 0-100. A CRI of 100 can be seen in standardised daylight sources or incandescent lamps. The method followed, and the equations used in calculating the CRIs of the MoO 3 /Ag/WO 3 transparent contact system are given in detail in our previous study 10 and Supplementary Information.

Calculation of correlated colour temperature.
One key criterion of defining predominantly white light sources is that the associated colour temperature (CCT), a complete, one-dimensional measurement system, identifies a particular point along the blackbody curve on the CIE 1931 chromaticity diagram. CCT also emerges as a system that shows the similarity of optoelectronic devices with transparent or semi-transparent characteristics to light emitters in various technological applications. The method followed, and the equations used in calculating the CCTs of the MoO 3 /Ag/WO 3 transparent contact system are given in detail in the Supplementary Information.

Data availability
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