Surface plasmon enhanced Organic color image sensor with Ag nanoparticles coated with silicon oxynitride

As organic photodetectors with less than 1 μm pixel size are in demand, a new way of enhancing the sensitivity of the photodetectors is required to compensate for its degradation due to the reduction in pixel size. Here, we used Ag nanoparticles coated with SiOxNy as a light-absorbing layer to realize the scale-down of the pixel size without the loss of sensitivity. The surface plasmon resonance appeared at the interface between Ag nanoparticles and SiOxNy. The plasmon resonance endowed the organic photodetector with boosted photon absorption and external quantum efficiency. As the Ag nanoparticles with SiOxNy are easily deposited on ITO/SiO2, it can be adapted into various organic color image sensors. The plasmon-supported organic photodetector is a promising solution for realizing color image sensors with high resolution below 1 μm.

photons absorbed/used for charge carrier creation in the sensors becomes doubled. Furthermore, a higher spatial resolution is achieved with minimized color Moiré noise. However, organic photodetectors (OPDs) are now confronted with big challenges. First, new materials with narrowband and high absorption are in urgent need. Second, noise occurring from non-negligible absorption in unwanted regions should be minimized and the linear dynamic range should be improved. In this study, we are focused on controlling photon absorption and external quantum efficiency (EQE) using surface plasmon effect to realize high resolution with low noise.
Among the basic collective modes for improving photon absorption, surface plasmon-polariton has been investigated in various fields such as solar cell [16][17][18] , organic light-emitting diodes (OLEDs) [19][20][21][22] , and 2D materials [23][24][25][26] for improving photon absorption. Recently, it has been adapted in solar cells and OLEDs to enhance their efficiencies 17,18 . Moreover, light absorption, which is a vital factor for EQE, has been increased in OPD with metal layers by plasmon resonance effect [27][28][29] . Metal nanoparticles (NPs) such as silver and gold have been also used to increase the EQE and signal-to-noise ratio (SNR) via surface plasmon resonance (SPR) [30][31][32][33][34] . NPs in the active layer in OPDs absorb the incident photon and plasmon-assisted NPs absorb more photon when a polariton is created. The coupling between the plasmon and NPs depends on the size and shape of NPs. As the size of the NPs becomes smaller than the wavelength of light, a surface plasmon is confined in NPs, which is called a "localized surface plasmon resonance (LSPR)" 26,[35][36][37][38][39][40][41] . The LSPR has two main advantages: the electric field enhancement at the metal surface and the maximum optical absorption at the plasmon resonance frequency. The first accelerates the generation of excitons, while the second promotes the absorption of photons. Moreover, the coupling between the plasmon and incident photon is also influenced by the materials surrounding metal nanoparticles, i.e. the plasmon frequency may be tuned by the surrounding material. Therefore, the EQE can be improved along with the plasmon resonance if proper surrounding materials and NPs are chosen. Here we report that photon absorption in the OPD was surged by LSPR in Ag NPs coated with SiO x N y and the EQE was also improved. Figure 1 illustrates the structure of the OPD device. The SubPc-Cl:C60 is an organic semiconducting push-pull molecule, of which absorption band lies in the green-light region and energy level difference between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) is 1.9 eV. The photon absorption enhancement in this study was investigated with the structure shown in Fig. 1: ITO/Ag/SiO x N y /blend layer//ITO.  Fig. 2(c). Electron energy loss spectroscopy (EELS) mapping is superimposed on a TEM image in Fig. 2(e), where the mark denoted by a white line implies that a localized surface plasmon is formed on that mark and the energy is in the order of 2.4-2.6 eV. The non-retarded surface plasmon condition for the metal-dielectric interface is ε 1 + ε 2 = 0, where ε 1 and ε 2 are permittivities for each material. Since the permittivity of Ag at 543 nm for green light absorption is −10.2 42 , the amorphous silicon oxynitride film surrounding Ag NPs should have the permittivity of 9.8. To investigate the permittivities of Ag NPs and SiO x N y layer, EELS measurement with the help of Kramers-Kronig relation was carried out.

Results and Discussion
The direct investigation of the surface plasmon in the OPDs was carried out by TEM-EELS and the results are shown in Fig. 3. Figure 3(a) shows the image of SiO x N y on ITO. Figure 3  www.nature.com/scientificreports www.nature.com/scientificreports/ absolute value of the permittivity of Ag NP is 10.25, while SiO x N y has a range of permittivities from 9.39 (S1) to 9.75 (S2). Therefore, the surface plasmon appears in the region where the sum of the permittivities of Ag NP and SiO x N y is zero.
The transmittances of the OPDs are compared in Fig. 4(a). In the green wavelength range of 460 nm to 590 nm, the reference OPD does not show uniform transmittance and the transmittance goes very high near 600 nm, while the transmittance is the lowest at 520 nm. Meanwhile, the Ag/SiO x N y OPD shows uniform transmittance in the green wavelength range. If the transmittance and absorbance curves are like a rectangular function, the color selectivity and color uniformity will be enhanced. However, in real life, the transmittance and absorbance curves are different from a rectangular function. On the contrary, in several cases, they show a peaky or triangular shape, implying that the OPD becomes sensitive only at about 520 nm centered in the green wavelength and less sensitive at about 480 nm and 590 nm. Figure 4(b) shows the absorbance spectra of the OPD devices. The blue line is from the conventional OPD, illustrating narrow bandwidth of 80 nm centered at 526 nm, while the red line comes from the OPD with Ag/ SiO x N y which exhibits a slightly broad bandwidth of ~120 nm centered at 524 nm. Compared with the absorption of the conventional OPD, the Ag/ SiO x N y OPD displays 8% higher absorption (85->93%). The absorption difference is caused by the plasmon effect in Ag/SiO x N y . The Ag/SiO x N y OPDs also display much more improved EQE characteristic with a maximum of 65% at 580 nm, while the conventional OPDs have 55% of the EQE.
To understand the role of Ag NPs in the OCIS for absorption enhancement, the excitation of LSPR as a function of wavelength of the incident photon with varying diameters of Ag-NP coated with SiO x N y was simulated with the finite difference time domain (FDTD) method and the results are shown in Fig. 5. When UV is incident on Ag NP, the excitation of LSPR is negligible and slightly appears at the interface between Ag NP and the substrate (ITO). With the incidence of 500 nm photons, the LSPR modes still remain near the interface between the Ag NP and the substrate. However, the strong excitation of the LSPR spreads along the surface of the Ag NP when 600 nm photons are incident. When the wavelength of the incident photon is 700 nm, the LSPR modes are excited on the upper surface.
Generally, the light intensity of a localized surface plasmon at metal nanoparticles/insulator interface is proportional to that inside the metal nanoparticles, i.e. the stronger the LSPR is formed at the interface, the more the light is absorbed and confined inside the metal nanoparticles. In Fig. 5, the excitation of LSPR creates strong   www.nature.com/scientificreports www.nature.com/scientificreports/ Band structures of the reference OPD and the OPD with Ag NP coated with SiO x N y layer are constructed and shown in Fig. 6. Both carriers of electrons and holes flow from the ITO anode and cathode to the acceptor and donor with negative reverse bias for the reference. The origin of the leakage current under reverse bias is due to electrons rather than holes because the barrier for electrons between the work-function of ITO (4.7 eV) and the LUMO of C60 (4.5 eV) is 0.4 eV, which is considerably smaller than the hole barrier which is 0.6 eV between the work function of ITO (4.7 eV) and the HOMO level of SubPc-Cl (5.3 eV). However, the SiO x N y film in the OPD acts as electron blocking layer (EBL) since the LUMO level of SiO x N y is higher than that of SubPc-Cl:C60(1:1) layer. Once Ag NPs coated with SiO x N y are inserted between ITO and SubPc-Cl, the barrier for electrons becomes higher and prevents the electrons from transporting from ITO to the acceptor layer, reducing the leakage  www.nature.com/scientificreports www.nature.com/scientificreports/ current. However, when SPR appears at the interface between Ag and SiO x N y , it generates electrons and holes. SPR-assisted electrons occupy the plasmon energy states, while holes stay near the Fermi level of Ag. Therefore, the SPR facilitates the flow of electrons to the cathode, indicating the increase in the leakage current. In addition, SPR-generated holes staying near the Fermi level of Ag also promote the hole-current toward the anode. Nonetheless, the photon absorption by the LSPR outnumbers the leakage current, causing the leap in the EQE. Figure 7 illustrates the incident photon absorption enhancement by Ag NP coated with SiO x N y . The energy of the SPR in Fig. 3 measured by TEM-EELS is around 2.2 eV. The excited electrons by SPR have higher energy by 2.2 eV than the Fermi level of Ag. Since the energy difference between the plasmon state and the conduction band of SiO x N y is 2.3 eV, the excited electrons by SPR can transfer to the exciton states in SiO x N y layer and they can easily arrive at the cathode electrode (ITO) under illumination.

conclusion
In summary, the OPD with Ag NPs coated with SiO x N y was fabricated. The SPR formed at Ag NPs coated with SiO x N y accelerated the photon absorption and improved the EQE. The SPR was investigated and visualized by TEM-EELS. The formation of the SPR at the interface of Ag NP and SiO x N y was experimentally confirmed. Although the effective area for receiving the incident photon is expected to decrease with the scaling-down of the pixels, the introduction of the SPR in OCIS counters the problem without losing the spatial resolution. With further systematic research conducted on the pattern and size of Ag NPs, the SPR is likely to be the sole solution for realizing OCISs with high resolution below 1 μm.

Methods
Device fabrication. The deposition of the SiO x N y buffer layer on ITO glass was sequentially carried out at 180 °C by Plasma-Enhanced Chemical Vapor Deposition (PECVD) using various SiH 4 :NH 3 :NO 2 gas mixtures with carrier N 2 gas. The thickness of the SiO x N y layer was 10 nm. The ratios of x (O/Si) and y (N/Si) were 0.16 and 0.66, respectively. The OPD layer, the organic blend layer of SubPc-Cl and C60 was deposited. Lastly, the capping layer of ITO was deposited, as shown in Fig. 1. characterization. Compositional analysis was performed by reflection electron energy loss spectroscopy (REELS) using auger electron spectroscopy (AES, PHI-4700, Concentric hemispherical analyzer) and X-ray photoelectron spectroscopy (XPS, PHI Quantera II Scanning XPS Microprobe), respectively. REELS spectra were collected using the primary electron energy of 1.5 eV for excitation and constant analyzer pass energy of 10 eV. The full width at half maximum (FWHM) of the elastic peak was 0.8 eV. TEM-EELS characterization of Ag NPs was conducted using a high-resolution transmission electron microscopy (TEM, Titan 20-200ST, FEI).