High‐Performance Semitransparent Color Organic Photodiodes Enabled by Integrating Fabry–Perot and Solution‐Processed Distributed Bragg Reflectors

Organic photodiodes (OPDs) have recently garnered attention as a competitive alternative to their inorganic counterparts, given their inherent advantages in solution processability, mechanical flexibility, and cost‐effective manufacturing. In this work, a novel method for fabricating high‐performance semitransparent color OPDs by integrating Fabry–Perot (FP) interferometer‐based color‐filtering electrodes and solution‐processed distributed Bragg reflectors (sDBRs) is introduced. The FP electrode provides color control by modulating the metal oxide layer thickness, irrespective of the photoactive layer's color. To overcome limitations related to light absorption and device transparency, this work employs a sDBR as a selective window reflector, allowing the OPD to retain its color while preserving semitransparency. The experimental findings demonstrate the successful integration of these components, resulting in semitransparent red, green, and blue (RGB) OPDs exhibiting significantly improved detectivity. The fabricated RGB‐OPDs achieve detectivity values of 4.07, 3.49, and 4.22 × 1010 cm Hz1/2 W−1 for red, green, and blue, respectively. This research highlights the efficacy of FP and sDBR color filters in realizing high‐performance color sensors and offer novel opportunities for semitransparent OPD integration with other optoelectronic devices.

Usually, semi-T electrodes are utilized in these devices, and color is controlled by using different color photoactive materials.However, different color materials show different performance and require different processing conditions.Instead, a color filter electrode utilizing Fabry-Perot (FP) interferometer with Ag/metal oxide/Ag was used for semi-T color OPDs.The color of the semi-T color OPD was controlled by the thickness of the metal oxide layer without concerning the color of the photoactive layer. [24,25]28] In our previous work, we successfully demonstrated the fabrication of a semi-T color OPD utilizing a solution-processed TiO 2 nanoparticle (sTNP) layer to minimize damage to the underlying photoactive layer. [24]The color of the semi-T OPD was controlled by the thickness of the sTNP and independent of the color of the active layer materials.Despite these advancements, the semi-T color OPD faced limitations in effectively detecting light due to the presence of both semi-T electrodes.To address this issue and enhance device performance, it is crucial to reflect back the transmitted light to the photoactive layer while preserving the transparency of the semi-T color OPD.
In this study, we present an innovative approach aimed at significantly improving the detectivity of OPDs, all the while preserving their semi-T color.To achieve this, we incorporating a solution-processed distributed Bragg reflector (sDBR) in the device structure, positioning it beneath the device glass substrate.Distributed Bragg reflectors (DBRs) are optical structures renowned for their ability to selectively reflect specific wavelengths of light based on their periodic arrangement of materials with different refractive indices. [29][32][33] By utilizing the fundamental principle of DBRs, we coated alternating layers of materials with high and low refractive indices, varying their thicknesses and stacking numbers.This technique resulted in the creation of narrowband reflectors that exhibit high reflectance and captivating colors.Theoretically, increasing the number of layers would amplify the intensity of reflection.The coloration of the reflector is determined by the central wavelength of the peak reflection, which can be adjusted by altering the thickness of the reflector.The color purity is influenced by the full-width at half-maximum (FWHM) of the reflector.By reducing the ratio of the refractive indexes between the alternating materials, the FWHM of the DBR is narrower.Consequently, the reflection at a selective wavelength was intensified, leading to improved OPDs detectivity.Moreover, wavelengths falling outside the selective reflection window retained their semi-T nature, resulting in the existence of semi-T devices that exhibit vibrant colors.However, conventional DBRs are prepared by stacking layers of materials with different refractive indices alternately, which requires a vacuum-controlled, precise layer.This fabrication method not only increases the complexity of the process but also adds to the overall cost of fabricating DBRs.
Our sDBR is developed using sTNP and polymethyl methacrylate (PMMA) polymer, eliminating the need for high-temperature processing and enabling fabrication through a simple spincoating process.This cost-effective and facile approach significantly reduces the complexity of the fabrication process and expands the range of potential applications for semi-T color OPDs.The device structure consists of a color filter electrode, photoactive layer, indium tin oxide (ITO), glass substrate, and sDBR.The sDBR effectively reflects a specific color that is transmitted by the semi-T color OPD, significantly improving its detectivity while maintaining transparency and color.Our results demonstrate a promising strategy for enhancing the performance of semi-T color OPDs, paving the way for future advancements in organic photodetection technology.

Result and Discussion
Figure 1 demonstrates the idea of light transfer through the narrowband OPD device with the sDBR mirror.The OPD has a device structure of FP interferometer (Ag/TiO 2 /Ag) as an etalonelectrode, MoO 3 as a hole transport layer, PTB7-Th:PC 70 BM materials as a bulk-heterojunction photoactive layer, ZnO as an electron transport layer, and finally an ITO cathode on glass substrate.As the incident light was shone from the top, it was primarily filtered by the FP interferometer etalon-electrode.The FP electrode utilizes an FP resonance cavity structure where the widths and peak intensity of resonance peak can be tuned by varying the Ag metal meanwhile with different TiO 2 thicknesses, the reso-nance peak position can be adjusted since the FP resonant wavelengths are determined by the interference of the reflected halfwavelengths of the incident light between the two Ag layers. [24]ere, the device structure displays a descending TiO 2 layer thickness as shown by transmission electron microscope (TEM) image of focused ion beam (FIB) cross-section in Figure 1, which blue-shifted the transmitted light giving a R-color with 117 nm thickness, followed by G-color with 97 nm thickness and B-color with 75 nm thickness, respectively.The transmitted narrowband RGB-colored wavelength was absorbed by a photoactive layer and finally escaped through the translucent ITO electrode printed on glass substrate.This entire structure of RGB-colored FP interferometer on OPD device is later called as RGB-device.
To recapture the escaped light without losing the semi-T colorful effect, colored DBR mirrors have been placed under the glass substrate of each device according to the color of the respective device as shown in Figure 1.Alternated stacks were generated as (TiO 2 /PMMA/TiO 2 ) n with different thickness of total stacking layer will produce different color.According to the scanning electron microscope (SEM) image of the FIB cross-section result, 3729 nm thickness of multiple stacking layer produced an R-color followed by G-color (3260 nm) and B-color with total thickness of 2702 nm, respectively.RGB-device with DBR mirror will be referred to as RGB-mirrored device and detailed explanation about sDBR mirror will be described later in this manuscript.With the sDBR mirror, the transmitted light will reflect back and recapture by photoactive layer thus improving the detectivity.
The FP interferometer of Ag/TiO 2 /Ag was first calculated by using MATLAB programming software.The spectral position of the tip transmission wavelength depends on the thickness of TiO 2 optical cavity layer, meanwhile the peak intensity and resonance width can be adjusted by the two Ag reflective layer thicknesses.In order to change the color of the FP interferometer, the thickness of the Ag reflective layer was set to 25 nm and the thickness of dielectric TiO 2 layer was varied from 60 to 140 nm in increments of every 10 nm.As the thickness of the TiO 2 layer increased, the wavelength of the center of transmission was redshifted.The transmitted spectra showed a narrow bandwidth, with 40-59 nm FWHM and maximum transmission peaks between 73% and 84%.The simulation results were presented in the Commission Internationale de l'Éclairage (CIE) 1931 chromaticity color space indicating that the colors are located in a specific color region (Figure S1 and Table S1, Supporting Information).According to the calculation, the thickness of TiO 2 has been set to 120, 100, and 80 nm for R, G, and B colors at 616, 549, and 489 nm wavelengths, respectively.
Based on simulation data, the device was fabricated on top of glass and as etalon-electrode on OPD devices.Figure 2a indicates the transmission spectra of the FP interferometer on glass and OPD devices.All peaks lie on exact wavelength as calculation with tip transmission efficiencies of 58%, 69%, and 70% and FWHM of 70, 79, and 86 nm for RGB-FP interferometer, respectively (Table S2, Supporting Information).Small differences in the peak characteristics can be attributed to inaccuracies in the material's refractive indices and mismatch between experimental and calculated film thicknesses.Meanwhile, with additional layers in a device structure, the change in dielectric function and environment will reduce the transmission peak intensity and introduce a spectral shift in tip transmission position.In order to get standard   intensity of RGB-device transmission, bulk heterojunction donor and acceptor PTB7-Th:PC 70 BM were used as photoactive layer since it featuring a uniform and broadband absorption across the visible range. [34]Therefore, considering the full device structure from top Ag to bottom glass, the transmission efficiency displays dispersion characteristics that closely resemble FP interferometer despite ≈10 nm shift of transmission position.The transmission intensity for RGB-device was recorded as a value of 33%, 34%, 29%, respectively, with all three color's FWHM was below 100 nm.Its transmission value above 25% is worth of being counted as semi-T device with FWHM below 100 nm resistant to reflect pure color (Table S2, Supporting Information). [35]Despite the difference in transmission intensity, plotting the measured chromaticity on the CIE color diagram, as shown in Figure 2b, shows an overlap for both values on the same color indicating no significant color change and this can be proven by photograph taken in Figure 2c which displayed the color is same for FP interferometer and RGB-devices.Interestingly, as shown in our previous work, color stability of FP interferometer was excellent after 4 months stored inside glove box under nitrogen condition. [25]or sDBR mirror, alternated stacks were generated as (TiO 2 /PMMA/TiO 2 ) n following same method as previously reported in our group. [36]Parameters such as material refractive indexes, thicknesses, and stacking numbers were inserted in MAT-LAB programming software to simulate the reflectance spectra of the RGB-sDBR mirror and the predicted colors were plotted on the CIE 1931 color space (Figure S2, Supporting Information).Since the high-refractive index material, TiO 2 thickness is fixed to 35 nm, the center of the reflected wavelength depends on the thickness of the PMMA layers.The simulated spectra showed that the sDBR mirror reflected a selective range of wavelengths when PMMA thickness is 170, 135, and 110 nm which gives a maximum reflection spectrum positioned at R, G, and B wave-length, respectively (Table S3, Supporting Information).Up until the stacking layer of n = 18, reflection tip reaches 94%, 98%, and 99% at 632, 530, and 457 nm wavelength corresponded to R, G, and B, respectively, which is enough to be use as the reflected mirror under the RGB-device.Thus, fabrication was proceeded to coat sDBR layer up until n = 18 corresponded to 37 layers.FWHM of sDBR mirror corresponds to the refractive index ratio of two materials.With high refractive index (n = 1.8) of TiO 2 [25] and low refractive index (n = 1.5) of PMMA, [37] the FWHM with 36 layers was 56, 55, and 56 nm for R-, G-, and B-sDBR mirror, respectively.Despite the narrow bandwidth of the reflection window, sDBR mirrors experience a ripple pattern outside the selective windows.The ripple pattern is significant at the short-wavelength bandpass due to the structure of sDBR mirror uses a high refractive index as an eight-wave layer. [38]Remarkably, the reflection intensity and color of sDBR mirror remain even after 6 months stored in ambient air.(FigureS3, Supporting Information).
The RGB-sDBR mirror was then fabricated on 2.5 × 2.5 cm glass substrates.Figure 3a shows the reflection spectra with n = 6, 12, and 18 and actual images of the fabricated RGB-sDBR mirror after 36 layers.As with the simulation, reflectance intensity increased for all colors as layers' stacking increased.After 36 layers, the central wavelength was positioned at 617, 547, and 487 nm, corresponding to R-, G-, and B-sDBR mirrors, respectively (Table S4, Supporting Information).The peak central wavelength of the actual RGB-sDBR mirror was shifted from the simulation due to the slightly different thickness.The actual thickness measured by atomic force microscopy (AFM) indicates that the TiO 2 thickness remained the same as 35 nm, but PMMA thickness was measured with 168, 138, and 115 nm for RGB-sDBR mirror, respectively.Importantly, the central wavelength of sDBR was located at the similar region with FP interferometer and RGB- device.The determination of this central wavelength peak is important to ensure the transmitted light falls on the correct window region and is reflected back to the photoactive layer.The reflection spectra with n = 6, 12, and 18 was converted to pixel coordinate (x,y) and was plotted in CIE 1931 color space (Figure 3b).The position of each color on each color spectrum indicates that the sDBR cells showed pure color with a narrow bandwidth.This was confirmed by the actual images of the sDBR mirror 36 layers coated on glass as shown in the inset in Figure 3a.
To study the semi-T colorful effect of RGB-device with RGB-sDBR mirror, transmission measurements were measured for both devices.As shown in Figure 4a, the RGB-device shows high transmission intensity for each color similar to as described in Figure 2 and Table S2, Supporting Information.With the sDBR mirror placed under the glass, the transmission intensity drops significantly indicating that the transmitted light within the measured area is reflected back.For devices with sDBR mirror, the minimum transmission peaks were 2.49%, 1.56%, and 1.46% at 621, 550, and 480 nm for R-, G-, and B-mirrored devices, respectively.
Despite the difference in transmission intensity, plotting the measured chromaticity on the CIE color diagram, as shown in Figure 4b, shows both values placed on almost similar color regions.This is due to the ripple pattern of the RGB-mirrored devices showing high transmission on both sides of the outside reflected region.As for R-mirrored devices, peak transmission at 588 and 652 nm gives orange-red color.For G-mirrored device, peak transmission at 516 and 576 nm gives paler shades of green color and for B-mirrored device, peak transmission of ripple pattern at 455 and 518 nm gives cyan color of the final device.
In Figure 4c, all of the devices were brought outside in ambient light and put under the camera lens to capture the building images.Both photographs left and right display the clear shape of the building in bright and vibrant colors.All three RGBdevices display a pure and strong R, G, and B hue, reinforcing the fact that light has been successfully emitted by the glass substrate.With the sDBR mirror, the semi-T effect was not demolished where the building still shows the color differences according to the RGB-mirrored device used.This color photograph of the building shows the same colors as plotted in CIE 1931 chromaticity.Even though there is a slight change in color between the device and the mirrored device, both devices maintain the concept of semi-T color, allowing for new functions when this device is used in conjunction with other devices.Exciting future possibilities and prospective applications in a variety of industries, including display technologies, can be provided by the integration of colorful semi-T OPD with other optoelectronic devices.Transparent displays enabling the creation of vibrant and visually appealing color displays with high transparency.This can be used in areas such as wearable devices, smart glasses, headsup displays, and transparent signage, where color information needs to be displayed without obstructing the view.On the other hand, it can be used in art, design, and wearable fashion technology.The integration of colorful semi-T OPDs with artistic installation or architectural designs can create visually stunning effects by producing dynamic and interactive color displays or lightning systems that enhance aesthetics and provide unique visual experiences in various settings such as museums, galleries, or public spaces.Additionally, the integration with wearable devices and fashion accessories adds a visual appeal and functionality.They can be used to create interactive displays or lighting elements in clothing, jewelry, or accessories, allowing for personalized expression, notifications, or health monitoring in a stylish manner.Besides, it is useful in the entertainment and gaming field.The integration of colorful semi-T OPDs into gaming consoles, virtual reality headsets, or entertainment systems can enhance the immersive experience by providing vibrant and dynamic visual effects.These OPDs can be used to create colorful lighting patterns, responsive displays, or interac-tive elements that enhance the overall gaming or entertainment experience.
Figure 5a shows the J-V curve of RGB-device and RGBmirrored device under dark and 1 sun illumination.Power conversion efficiency (PCE) of RGB-device was 0.48%, 0.46%, and 0.52% and PCE was increased with sDBR mirror to 0.63%, 0.57%, and 0.65% for RGB-mirrored device, respectively (Table 1).The increment in PCE is due to the improvement in short current density (J sc ) with sDBR mirror meanwhile no significant changes were detected in open circuit voltage and fill factor.No current was detected for devices under dark condition meanwhile significant increase of J sc was noticed for RGBmirrored devices under 1 sun intensity measurement.1.26, 1.21, and 1.35 mA cm −2 J sc detected for RGB-device and current value was increased to 1.62, 1.53, and 1.63 mA cm −2 for RGB-mirrored Table 1.Device parameter of colorful RGB device with and without sDBR mirror.devices.This significant improvement was predicted as the light penetrating through the transparent ITO substrate was reflected back toward the photoactive layer by sDBR mirror.This increment in J sc was supported by external quantum efficiency (EQE) results for all devices under negative 1 bias in Figure 5b.Based on J-V characteristic under dark and illumination at −1 V, the on/off ratio of RGB-mirrored devices was increased by 25% compared to RGB-device itself.Improvement in current density together with low dark current later enhances the responsivity and detectivity as shown in Figure 5c,d Based on Equation (1), the responsivity was calculated by dividing a Planck's constant (h) and speed of light (c) with elemental charge (q) and peak wavelength () of each color.Responsivity was improved from 0.07, 0.07, and 0.06 A W −1 for RGB-device to 0.11, 0.09, and 0.11 A W −1 for RGB-mirrored devices Meanwhile detectivity was calculated from Equation ( 2) where responsivity was divided with square root of 2q and dark current (J d ).Detectivity was 2.70, 2.65, and 2.42 × 10 10 Jones for RGBdevice and it was improved to 4.07, 3.49, and 4.22 × 10 10 Jones for RGB-mirrored device, respectively.

Conclusion
In summary, a semi-T colorful R-, G-, and B-device was successfully fabricated by using an FP interferometer on top of an organic photoactive layer.The RGB-mirrored devices exhibited excellent spectral responsivity and detectivity yet still maintained the semi-T colorful properties.
These results indicate that the RGB-mirrored device is a promising candidate for use in high-resolution PD and has potential to be integrated with other optoelectronic devices to make new functionalities such as in display technologies, art, design, wearable fashion, entertainment, and gaming field.Due to the noninvasive process, we also expect that OPDs can be fabricated on large-area flexible plastic using our developed sDBR mirror.The quest for excellence encounters challenges when it comes to scaling up and integrating devices with flexible substrates, primarily within the realm of manufacturing techniques.The intricacies lie in navigating the lower thermal conductivity of flexible substrates, which poses a formidable hurdle for heat dissipation.However, our vibrant semi-T device boasts organic materials as the photoactive layer, basking in the advantages of lowtemperature annealing.Moreover, the utilization of a solutionbased process with gentle annealing temperature for sDBR showcases its prowess in coating plastic substrates with unwavering resilience, effortlessly embracing thermal management.Preparation of ZnO Sol-Gel and TiO 2 Nanoparticle: Preparation of ZnO sol-gel and TiO 2 nanoparticle was followed by previous study. [25]reparation of Spin-Coated FP Interferometer: FP interferometer consisted of Ag/TiO 2 /Ag layers.Both the Ag layers were coated by thermal evaporation with a rate of 0.4 Ǻ s −1 resulting in 25 nm thickness.The TiO 2 solution (40 mg mL −1 in n-butanol) was spin-coated and dried on a hot plate at 70 °C for 30 min.

Experimental Section
Fabrication of FP-OPD: The device had an inverted structure with a bulk-heterojunction active layer.The electron transport layer, ZnO (30 nm) was spin-coated on a pattern ITO glass and dried on a hot plate at 200 °C for 1 h.Then, an active layer PTB7-Th:PC 70 BM (1:1.5, total concentration 20 mg mL −1 ) mixed solution was spin-coated and dried for 1 h at room temperature in a vacuum chamber.Hole transport layer, MoO 3 (10 nm) was thermally evaporated with a rate of 0.2 Ǻ s −1 .The FP layer was fabricated on top of the MoO3 layer by following above methodology.
Fabrication of Spin-Coated sDBR: sDBR was fabricated by orthogonal spin coating of two solutions; TiO 2 and PMMA.TiO 2 nanoparticles were dissolved in n-butanol (20 mg mL −1 ) and filtered twice with a 0.45 μm polytetrafluoroethylene.The PMMA solution was diluted with anisole (1:0.6 vol%) and filtered with a 0.45 μm polyvinylidene fluoride.First, TiO 2 layer was coated on bare glass, producing a 35 nm layer.TiO 2 layer was dried on a hot plate at 70 °C for 2, 4, and 5 min for attaining B, G, and R color, respectively.Next, the PMMA layer was generated on TiO 2 layer through an identical drying process, resulting in 110, 135, and 170 nm layers corresponding to blue, green, and red color.Alternate coating ended when a pure color was achieved.
Device Characterization: The single layer thickness was measured by AFM (AFM5100N, Hitachi).The cross-sectional image of FP interferometer was measured after sampling with FIB (Helios NanoLab) and image was captured using a TEM (JEM-2100F HR, Jeol) and cross-sectional image of sDBR mirror was measured using an FIB equipped with SEM image processing technology (ZEISS).The reflectance of the sDBR and FP interferometers films was obtained using UV-vis spectrophotometers (UV/VIS Lambda 365, PerkinElmer and UV-2450, Shimadzu).The electrical properties of the current voltage (J-V) curves of the device were measured under 1 sun (AM 1.5G irradiation of 100 mW cm −2 ) sources by a 150 W Xenon lamp using a Solar simulator (K201 LAB50, McScience) and Sourcemeter (Model 2400, Keithley instrument Inc.).The EQE measurements were conducted using a K3100IQX (McScience) at the National Research Facilities and Equipment Center (NanoBio Energy Materials Center), Ewha Womans University, South Korea.
Simulation: The transmission of FP interferometer and reflection of sDBR were simulated in MATLAB program by using a transfer-matrix-method model through the calculation of diffraction by stack of lamellar 1D gratings in the z-direction which have all identical periods in the x-direction and are invariant in the y-direction.The (x,y) plane referred to the transverse plane and the z-direction as longitudinal directions.For transmission of FP interferometer, metal/insulator/metal (MIM) multilayer with n of Ag as metal and TiO 2 as dielectric layer was used.For reflectance of sDBR spectra, alternate stacking layer of (high refractive index material/low refractive index material/high refractive index material) n of TiO 2 as high index material and PMMA polymer as low index material with refractive index values of 1.8 and 1.5, respectively, was used.
Preliminary input parameter in Matlab script is as follows: wavelength= visible region of 400 to 800 nm; period = in the x-direction; nn = Fourier harmonic run from [-40,40], for large n values, a high accuracy for the calculated data is achieved; parm = res0(1) for TE polarization; parm = res0(-1) for TM polarization;

Figure 1 .
Figure 1.Light pathway of RGB-device with RGB-sDBR mirror with TEM image of FIB cross-sectional of RGB-FP interferometer and SEM image of FIB cross-sectional of RGB-sDBR mirror layers.

Figure 2 .
Figure 2. a) Transmission of RGB FP-interferometer and RGB-device, b) plotting of the colors from the transmission peak onto a CIE 1931 chromaticity diagram (small letter refers to FP-interferometer and capital letter refers to RGB-device), and c) digital images of fabricated device taken through a lightning white sheet of paper (top is FP-interferometer and bottom is RGB-device).

Figure 3 .
Figure 3. a) Reflection spectra of different stacking number of RGB-sDBR mirror on top of glass substrate.Inset is a photograph of sDBR mirror up to 36 layers taken on top of black surface.b) Plotting of the colors from the reflection peak onto a CIE 1931 chromaticity diagram.

Figure 4 .
Figure 4. a) Transmission of RGB-device and RGB-mirrored device, b) transmission peak transfer to color coordinate and plot on CIE 1931 chromaticity, and c) colorful building images captured through the RGB-device and RGB-mirrored device taken at Building D, Chemistry and Nanoscience Department, Ewha Womans University, South Korea.

Figure 5 .
Figure 5. a) J-V curve of RGB-devices and mirrored devices under dark and 1 sun illumination.b) EQE spectra of RGB-devices and mirrored devices at −1 V. Calculated c) responsivity and d) detectivity of RGB-devices and mirrored devices.