Impacts of plasmonic nanoparticles incorporation and interface energy alignment for highly efficient carbon-based perovskite solar cells

This work utilizes a realistic electro-optical coupled simulation to study the (i) impact of mesoporous TiO2 removal; (ii) the embedding of Ag@SiO2 and SiO2@Ag@SiO2 plasmonic nanoparticles; (iii) utilization of solution-processed inorganic p-type copper(I) thiocyanate (CuSCN) layer at the perovskite/carbon interface; and (iv) the increase of the work function of carbon electrodes (via incorporation of suitable additives/binders to the carbon ink) on the performance of carbon-based PSCs. Removal of mesoporous TiO2 increased the power conversion efficiency (PCE) of the device from 14.83 to 16.50% due to the increase in exciton generation rate and charge carriers’ mobility in the vicinity of the perovskite-compact TiO2 interface. Subsequently, variable mass ratios of Ag@SiO2 and SiO2@Ag@SiO2 plasmonic nanoparticles are embedded in the vicinity of the perovskite-compact TiO2 interface. In the optimum cases, the PCE of the devices increased to 19.72% and 18.92%, respectively, due to light trapping, scattering, and strong plasmonic fields produced by the plasmonic nanoparticles. Furthermore, adding the CuSCN layer remarkably increased the PCE of the device with a 0.93% mass ratio of Ag@SiO2 nanoparticles from 19.72 to 26.58% by a significant improvement of Voc and FF, due to the proper interfacial energy band alignment and the reduction of the recombination current density. Similar results were obtained by increasing the carbon work function, and the cell PCE was enhanced up to 26% in the optimal scenario. Our results pave the way to achieve high efficiencies in remarkably stable printable carbon-based PSCs.

www.nature.com/scientificreports/ TiO 2 17 . However, the presence of isolated TiO 2 nanoparticles can lead to charge accumulation and increased recombination due to the trapping of electron carriers and the low mobility of carriers in the perovskite-filled mesoporous structure. This, in turn, affects hysteresis behavior in measuring the efficiency and instability of the device due to the ion migration 18 . Therefore, providing a carbon-based planar structure by removing mesoporous TiO 2 can not only reduce the device's hysteresis behavior 19 but also take another step toward commercializing perovskite-based solar cells by reducing the cost of mass production.
One of the most effective approaches to increase the efficiency of carbon-based perovskite solar cells is to use plasmonic nanoparticles within the device structure [20][21][22][23][24][25][26] . Under light illumination, metal nanoparticles create strongly localized fields around them during the coherent oscillations of their conducting electrons 27,28 , which can not only increase the photon flux inside the perovskite as a secondary light source but also increase the generation rate of free carriers by reducing the exciton binding energy 29 . In addition, the light scattering by resonated nanoparticles can increase the absorption via enlarging of the light pathway, even at thin perovskite thicknesses 30 . On the other hand, plasmonic nanoparticles generate hot electrons during plasmon resonance which can be directly injected into their surrounding perovskite environment and increase photo-generated electrons mobility by filling the trap states 31 .
To take advantage of local surface plasmon resonance (LSPR) effects, metal nanoparticles must be added to the photoactive layer. First, however, a capping layer over the metallic nanoparticles must be used due to the corrosive properties of CH 3 NH 3 PbI 3 , which lead to the decomposition of noble metals 32 . Then, heating the nanoparticles metallic surface during plasmon resonance will lead to the recombination of excitons at their surface 33 . Here, the usage of a thin layer of SiO 2 as a capping layer not only stabilizes the noble metal nanoparticles against thermal and chemical interactions but also creates the slightest manipulation in the LSPR properties of the nanoparticles 21 . The presence of a dielectric coating layer can attenuate the plasmonic properties of the nanoparticles. In some studies 21,34,35 , to improve the plasmonic effects of nanoparticles, triple core-shell nanostructures with a metal nanoshell sandwiched between the dielectric outer shell and core (e.g. SiO 2 @Ag@SiO 2 and TiO 2 @Au@TiO 2 ) have been proposed, which can produce better LSPR properties thanks to their plasmon hybridization mechanism.
Herein, we improve the performance of HTL-free carbon-based planar perovskite solar cells using plasmonic NPs. Firstly, the photovoltaic measurements of the fabricated HTL-free carbon-based perovskite solar cell have been utilized for simulation and extract the electrical parameters via fitting its corresponding J-V characteristics. Then, a detailed analysis of the device performance in the presence of mesoporous TiO 2 and planar structure has been presented using electro-optical coupled simulation regimes. Next, the effect of realistic embedding of Ag@ SiO 2 and SiO 2 @Ag@SiO 2 nanoparticles with different concentrations on the optical and electrical performance of the carbon-based planar perovskite solar cell has been investigated. Finally, since the presence of a carbon electrode (work function of ~ 5 eV) as the non-ideal anode limits the device efficiency, for increasing the transfer rate of hole carriers and blocking the electron carriers, we investigate the effects of adding a copper(I) thiocyanate (CuSCN) interlayer at the perovskite-carbon interface and increasing the working function of carbon electrode on the performance of carbon-based perovskite solar cells.

Results and discussion
The structure of the fabricated and modeled device is Glass/FTO/Compact TiO 2 /Mesoporous TiO 2 /CH 3 NH 3 PbI 3 / Carbon (see Fig. 1a). Here, the mesoporous TiO 2 is considered inside the perovskite photoactive layer. In other words, the pore sites of the mesoporous TiO 2 are filled with perovskite material. In the simulated structures, mesoporous TiO 2 is made of deformed TiO 2 nanospheres with a size distribution of 7-10 nm that can overlap www.nature.com/scientificreports/ with each other and randomly distribute within an equivalent thickness of ~ 250 nm with a porosity of ~ 39% (Fig. 1a). This modeling is based on the results of FESEM and ellipsometry analyzes of the fabricated solar cell 40 . Accurate modeling of the structure of the mesoporous TiO 2 is crucial because it plays a vital role in the distribution of the field profile within the perovskite layer. This directly affects the exciton distribution, generation, and recombination rates of charge carriers along with the photoactive layer. Essential details considered in this modeling have not been reported in any of the previously published works. Figure 1b shows the measured current-voltage curves from the experimental device compared to the simulation results, wherein the simulation results are in complete agreement with the experimental data. The power conversion efficiency (PCE) of 14.93% and 14.83% for the HTL-free carbon-based perovskite solar cell has been measured and obtained from the simulation, respectively. Furthermore, all the electrical characteristics obtained from the simulation (V oc , J sc , FF, and PCE) are in good agreement with the experimental data, which shows that the modeling performed in this work can explain the experimental optical-electrical characteristics. Therefore, it provides a good description of the device performance.
The first perovskite solar cell manufacture was based on dye-sensitized solar cells in which the TiO 2 mesoporous was used as the electron transfer layer. The main reason for using the mesoporous structure was to assume the short diffusion length of the carriers in the perovskite. Researchers later found that the perovskite diffusion length is at least more than 100 nm. Accordingly, TiO 2 mesoporous is removed from the device structure, and its effect on the performance of the solar cell is investigated in Fig. 2 using the developed modeling. Embedding the mesoporous TiO 2 in the perovskite-compact TiO 2 interface can reduce the mobility of charge carriers in this region. This is the reason for obtaining the low charge carriers mobility during the fitting of the simulation results with those of the experimental data (see Fig. 1b and Table S1). Removal of TiO 2 mesoporous has increased the power conversion efficiency (PCE) from 14.83 to 16.50% due to J sc and FF improvement (Fig. 2a). Investigation of the exciton generation rate profile along the perovskite layer shows that the improved device performance is due to the increased exciton generation rate and charge carrier mobility near compact TiO 2 (Fig. 2b).
To benefit from the plasmonic properties of metallic nanoparticles in improving the performance of carbonbased planer perovskite solar cells, Ag@SiO 2 core-shell nanoparticles, and SiO 2 @Ag@SiO 2 triple core-shell nanoparticles have been embedded at the interface of CH 3 NH 3 PbI 3 -compact TiO 2 (Fig. 3). These plasmonic nanostructures have been designed and optimized in our previous studies and introduced as effective plasmonic nanoparticles 21 . Here, the deformed nanospheres capable of overlapping with each other are randomly distributed in a volumetric space with varying mass ratios (the volume of distributed plasmonic nanoparticles/perovskite layer volume × 100) using a realistic simulation. The overall dimensions of the nanoparticles, the thicknesses of Ag inner shell and SiO 2 outer shell are 32 nm, 2.5 nm, and 1 nm, respectively.
The device efficiency has increased to 19.72% and 18.20% after adding a 0.93% mass ratio of Ag@SiO 2 and SiO 2 @Ag@SiO 2 NPs, respectively, due to the J sc increment shown in Fig. 3. Further increment of the mass ratio of embedded Ag@SiO 2 nanoparticles decreases the efficiency stemming from the J sc drop (Table 1). This could be due to the enhancement of the reflectivity behavior of nanoparticles, which prevents light from penetrating deep into the perovskite layer. This, in turn, can weaken the performance of the device by reducing the optical absorption. However, the increase in the mass ratio of SiO 2 @Ag@SiO 2 nanoparticles has increased the device efficiency, and the maximum power conversion efficiency of 18.92% is obtained for the devices incorporated with the mass ratios of 2.79% ( Fig. 3b and Table 1). Removal of mesoporous TiO 2 and adding 0.93% mass ratio of Ag@SiO 2 NPs between perovskite and compact-TiO 2 have increased the device efficiency by 33%. The presence of mesoporous TiO 2 and the addition of NPs within the mesoporous structure could only increase the device efficiency by 17% ( Fig. 3c and Table 2). This suggests that the presence of TiO 2 mesoporous as the surrounding environment of doped plasmonic nanoparticles can weaken their plasmonic enhancement.
In Figs. 4 and 5, the LSPR properties of SiO 2 @Ag@SiO 2 and Ag@SiO 2 nanoparticles embedded in the perovskite environment are investigated, respectively, as single-particle and mass during plasmon resonance. Compared  www.nature.com/scientificreports/ to the single nanoparticle, mass nanoparticles show a larger absorption cross-section due to the production of new plasmonic peaks and their increase in spectral width due to plasmonic interactions of neighboring nanoparticles ( Fig. 4a and c). This is a reason for further improvement in the efficiency of the device incorporated with SiO 2 @Ag@SiO 2 nanoparticles by increasing the mass ratio of embedded nanoparticles (Fig. 3b). Plasmon resonance of SiO 2 @Ag@SiO 2 nanoparticles occurs due to the plasmon hybridization mechanism at higher wavelengths than Ag@SiO 2 nanoparticles (Figs. 4 and 5), where the photon flux produced by the nanoparticle is unable to transfer electrons from the valance band to the conduction band of perovskite. This results in less plasmonic improvement of the device in the presence of SiO 2 @Ag@SiO 2 nanoparticles compared to Ag@SiO 2 nanoparticles. In addition, Ag@SiO 2 nanoparticles generate stronger plasmonic fields in the perovskite environment compared to SiO 2 @Ag@SiO 2 nanoparticles. Figure S2 shows the absorption spectrum of perovskite in the presence of a 0.93% mass ratio of Ag@SiO 2 and SiO 2 @Ag@SiO 2 nanoparticles compared to the reference device (without nanoparticles). As expected, perovskite absorption in the presence of Ag@SiO 2 nanoparticles shows further improvement. The presence of a carbon electrode (work function of ~ 5 eV) as the non-ideal anode and the absence of an electron blocking layer at the perovskite-carbon interface limits the device efficiency by enhancing the probability of electron recombination at the interface. Adding an appropriate interlayer at the perovskite-carbon interface and/or adequate increment of the working function of carbon by using suitable additives/binders in carbon ink to increase the transfer rate of hole carriers while blocking electron carriers are other approaches to improve the performance of carbon-based perovskite solar cells. In metal electrode-based perovskite solar cells, spiro-OMeTAD has been widely used as a representative of hole transport material 41,42 . However, there are significant limitations to the use of this material (and generally organic and conductive polymer-based molecules) in terms of cost and stability 43,44 . In addition, previous studies have shown that spiro-OMeTAD-equipped carbon electrode-based perovskite solar cells offer lower efficiencies than a metal electrode-based device 45 . This is due to the incompatibility of some organic solvents in carbon paste with spiro-OMeTAD, which leads to its degradation 46 . Therefore, the effect of the inorganic CuSCN layer with excellent semiconducting properties such as high hole mobility, wide bandgap, optical transparency, thermal and chemical stability, and solution processability 44,47 on the performance of the device has been investigated. It should be noted that CuSCN can be deposited at low temperatures using various techniques including spin and spray coatings, doctor Blade, and electro-deposition 44,[48][49][50] . Figure 6a shows the J-V characteristic calculated under AM 1.5G irradiation for the device comprised of FTO/TiO 2 /Ag@SiO 2 NPs: CH 3    www.nature.com/scientificreports/ On the other hand, the used solution-processed inorganic p-type CuSCN layer with a thickness of 30 nm at the perovskite-carbon interface has increased the device PCE by ~ 35% to 26.58% thanks to the increased V oc and FF to 1.156 V and 81.64%, respectively ( Table 3). The remarkable improvement of the V oc is attributed to better interfacial energy alignment between the perovskite and CuSCN (see Figure S3). Figure 6b shows the recombination current density for the device equipped with a CuSCN layer compared to the reference device. Adding the CuSCN layer not only has reduced the SRH recombination by producing a back surface field and blocking electrons toward the TiO 2 ETL but also has reduced the current density of the back surface recombination down to ~ 10 −22 mA/cm 2 , even at high voltages, by appropriately aligning the interfacial energy. Furthermore, Figs. 6c and d show the performance of the optimized plasmonic device as a function of the back contact work function. As the working function of the carbon electrode increases, which is possible by using suitable additives/binders in the carbon ink, V oc and FF have increased thanks to the excellent alignment of the interfacial energy and the reduction of the recombination current density. As a result, a PCE above 26% is obtained for the work function of 5.4 eV (Table 3).

Conclusions
In summary, we have investigated the effects of the removal of mesoporous TiO 2 , plasmonic nanoparticle embedding, and utilization of solution-processed inorganic p-type CuSCN as an HTL on the performance of carbon-based PSCs in an electro-optical study. We have shown that removing mesoporous TiO 2 can increase the device PCE by increasing the exciton generation rate and charge carriers' mobility in the vicinity of the CH 3 NH 3 PbI 3 -compact TiO 2 interface. Furthermore, we have shown that the embedding of Ag@SiO 2 and SiO 2 @ Ag@SiO 2 plasmonic nanoparticles can increase the device PCE from 16.50% to 19.72% and 18.92% (in the optimum mass ratio), respectively, thanks to benefiting from the light scattering, trapping, and strong plasmonic fields produced by the nanoparticles. On the other hand, non-ideal carbon electrode has limited the device efficiency by increasing the interface recombination. Adding a solution-processed inorganic p-type CuSCN layer at the perovskite-carbon interface increased the device PCE to 26.58% by significantly improving the V oc and FF thanks to the proper alignment of the interfacial energy and the reduction of the recombination current density. The same results were obtained by increasing the carbon working function (through incorporating suitable additives/ binders to the carbon ink), and the cell PCE was enhanced up to 26% in the optimal case. These results pave the ways to achieve high efficiencies in carbon-based PSCs.    www.nature.com/scientificreports/ the carbon back electrode. The carbon paste has been painted on the perovskite film followed by spraying an ethanol solution of 1H, 1H,2H,2H-perfluouorodecyltriethoxysilane (Sigma-Aldrich, 5% v/v) on the paste and heating at 100 °C for 1 h 7 .
Numerical calculations. Optical-electrically coupled simulation regimes have been used to investigate the plasmonic nanoparticles embedding effects on the performance of carbon-based perovskite solar cells. The rate of exciton generation dependent on the absorption profile of the cell was calculated using FDTD software based on solving Maxwell equations 38 . The complex refractive indices obtained from the ellipsometric analyzes of the fabricated device were utilized to simulate and calculate the exciton generation rate profile optically. Perfectly matched layer (PML) boundary conditions are set at the top and bottom of the unit cell along the Z direction. Periodic boundary conditions are located along x and y directions. A plane wave light source propagated along the Z direction is used; the photon flux density is calculated based on standard solar spectrum AM1.5. Experimental data of the materials' complex refractive indices used for optical simulation are shown in Figure S1. To model the electrical response of the cell, the exciton generation rate profile is directly imported into the Solar Cell Capacitance Simulator (SCAPS-1D) software. Photovoltaic measurements of the fabricated devices were utilized to extract electrical parameters by fitting their corresponding J-V characteristics. The software uses the finite difference approach to solve electron and hole drift-diffusion equations to describe the motion of charge carriers inside the photoactive layer. Here Shockley-Read-Hall (SRH) model has been used to describe recombination current density 39 . The electrical parameters used for the simulation, which are extracted by fitting the simulation results with experimental data, are shown in Table S1. www.nature.com/scientificreports/ Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.