Among all the different methods to enhance the optical absorption of photovoltaic devices. The plasmonic effect is one the most prominent and effective ways to capture more incident light and also provide good carrier dynamic management. Here, we systematically introduce spherical gold nanoparticles (Au NPs) with different radii in the absorber layer of perovskite solar cells (PSCs). The overall enhanced optical absorption around 14.20% and 20.02% is achieved for incorporated monolayer and bilayer Au NPs, respectively, in the active layer compared to the pure perovskite layer. Moreover, we employ the metal (Au)-dielectric (TiO2 and SiO2) nanoparticles in the absorber layer. The optical absorption increases as the core-shell size decreases. The optical absorption elevates in both [email protected]2 core-shell and [email protected]2 core-shell 17.5% and 3.5%, respectively. These results support superior separation and transfer of charge in the existence of plasmonic NPs. In addition, this study presents a very sophisticated approach in the optical enhancement of PSCs and thus helps to boost the overall photovoltaic device performance.
Research Article
The role of Plasmonic metal-oxides core-shell nanoparticles on the optical absorption of Perovskite solar cells
https://doi.org/10.21203/rs.3.rs-887610/v1
This work is licensed under a CC BY 4.0 License
Version 1
posted 10 Nov, 2021
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Gold nanoparticle
Perovskite solar cell
Surface plasmons
Optical absorption
Core-shell nanoparticle
The photovoltaic effect is the conversion of incident light into useful electricity. Among all solar cell materials, perovskite solar cells (PSCs) is considered one of the most prominent and promised candidate in solar cells industries owing to their excellent properties such as cheaper cost, low-temperature chemical processing and fabrication [1, 2], strong absorption of sunlight [3], higher mobility of carriers and low rate of non-radiative carrier recombination [4]. In addition, flexible PSCs of different colors can be fabricated effectively and advantageous to utilize large and wide wavelength ranges. The important parts of PSCs are transparent conductive oxides (FTO, ITO), electron transport layer (ETL), absorber layer (perovskite), hole transport layer (HTL), and the metal contact [5]. Miyasaka's group presented the first-ever report and replaced dye-sensitized solar cells with perovskite (Ch3NH3PbI3) used as liquid sensitizer and achieved power conversion efficiency (PCE) of 3.8%[6]. Kim et al. presented a successful report using perovskite as an active layer, and they also introduced Spiro-MeoTAD as a hole transport layer (HTL) and mesoporous TiO2 as an electron transport layer (ETL). The PCE was achieved 9.7%, almost three times higher than the one presented in the previous report[7]. Moreover, 22.1% certified PCE has been investigated for PSCs and is highly expected to jump over 30%[8]. The optimization of different parameters used in PSCs has become practicable by the rapid efficiency improvements. Each component has its role in improving optical absorption, PCEs, and stability. For instance, the perovskite layer thickness is a dominant factor that extensively affects the absorption of light and the charge separation in HTL and ETL, respectively [9]. The thin layer of perovskite has not the capability to utilize more light, but on the other hand, it provides a good pathway for charge separation. To resolve this hindrance an alternative solution is needed to enhance optical absorption and improve PSCs device performance without increasing the thickness of the perovskite absorber layer. In such circumstances, the surface plasmon resonance effect is one of the best solutions that utilize metal at nanoscale embedded in PSCs; when the incoming light from the sun strikes the metal nanoparticle inside the absorber layer; as a result, electrons start shining on the surface of metals, such phenomenon of collectively shining of electrons on the surface of metal is called localized surface plasmon resonance (LSPR) and thus enable the solar cells to harvest more light. These LSPR also provide strong electromagnetic field that leads to enhanced scattering cross sections and extinction cross section for larger atoms [10].
The effect of LSPR has extensively been studied in many photovoltaic devices besides PSCs, such as silicon-based solar cells [11], dye-sensitized solar cells (DSSC) [12], and organic solar cells (OSC) [13]. Zhang et al. first investigated the plasmonic effect in PSCs using metal nanoparticles (NPs) and enhanced PCE to 9.5 % as compared to controlled devices (8.4%) [14]. Furasova et al. studied the effect of silicon NPs in PSCs that optimize the PCE up to 18.8% compared to the controlled device without NPs at 17.7% [15]. Batmunkh et al. reported the influence of gold nanostars on mesoporous TiO2 photoanode in PSCs; they observed the PCE increased from 15.19–17.72%. Moreover, enhanced optical absorption and reduced charged recombination were also investigated [16]. Aeineh et al. introduced multifunctional [email protected]2 core-shell in PSCs and thus improved the device performance because of the plasmonic effect [17]. The [email protected]2 in PSCs is also capable of improved device stability. The stability is due to the SiO2 coating that acts as a shield and protects Au and the perovskite layer. However, a big gap still exists to wisely utilize the plasmonic effect of nanoparticles with various geometries in PSCs structure. These plasmonic nanoparticles can dramatically change the performance of PSCs in terms of absorption, scattering, efficiency, and stability. Moreover, LSPR caused by these nanoparticles depends on the shape, size, and dielectric function procured by the embedded nanoparticles [18, 19].
In this paper, we performed the optical simulation of embedded Au NPs with different radii (g), [email protected]2, and [email protected]2 with different shell thickness (Stk) in the absorber layer of PSC in the wavelength range, i.e., 300–800 nm. The plasmonic effect and the corresponding enhancement in the UV-Vis spectrum were systemically observed for each scheme by tailoring the radius (g) of Au NPs and shell thickness (Stk). Then we further calculated the bandgap energy for each geometry (Au NPs, [email protected]2, and [email protected]2 modified inside the perovskite layer with the help of Tauc’s curve. In addition, we finally compared all the results and investigated that the overall optical enhancement in PSC is dedicated to [email protected]2 as compared to simple Au NPs and [email protected]2, thus providing a useful track for enhancement and improved stability of PSCs.
We first systemically studied the influence of Au NPs, [email protected]2, and [email protected]2 on the optical properties of PSCs; we used the finite element method (FEM) in all numerical simulations. Our proposed PSC model is depicted in Fig. 1a. PSC structure normally consists of different functional layers like ITO/TiO2/Perovskite layer/Spiro-OMeTAD/Au. The thickness chosen for all the mentioned parameters were 150nm, 60nm, 250nm, 80, and 60nm, respectively. The refractive index assumed for glass substrate is 1.5. The dielectric function of gold used in the present work is according to the Johnson and Christy model presented in [20]. The complex refractive index of ITO, TiO2, Spiro-OMeTAD and perovskite are chosen from literature for simulation, respectively [21, 22]. In this simulating model, the entire simulation was performed in a unit cell. To obtain more precise and promised results, we employed tetrahedral meshing for the frequency-domain solver. The structure, as mentioned earlier, is composed of stacked layers obtained from distinct materials, which constitute a sandwich-type configuration. In this work, spherical-shaped plasmonic Au NPs with different sizes were incorporated in TiO2, and SiO2 of various shell thickness (Stk) of plasmonic nanoparticles is studied for the perovskite active layer shown in Fig. 1.
An array of equally distributed NPs inside the absorber layer of PSCs is studied. Therefore, symmetry boundary conditions were practiced in the x- and y- axes. The regular two-dimensional patterned structure is shown in Fig. 2 (a, b). To make our model more practical, the background environment is filled with air. The z-axis is kept “open (add space)” for an incoming electromagnetic wave to enable the perovskite layer to absorb the incident light. The simulated model shows that the incident electromagnetic wave propagates in the z-direction while x- and y- directions are dedicated to magnetic and electric fields, respectively. Radiofrequency (RF) module is used by enabling the scattered field formulation to model the optical properties of the full stack proposed device. The standard solar spectrum AM1.5 and irradiance 100 W/m2 is employed in the photovoltaic device. Moreover, the following mathematical equation is used to calculate optical absorption coefficient A(λ).
where lambda (λ), R, and T represent the wavelength, reflection, and transmission of light, respectively. The interaction of electromagnetic wave (EMW) with metal NPs that tend to excite electrons results in further generation of surface plasmon polariton. Maxwell’s equations are used to calculate it mathematically when EMW propagates between metal and dielectric.
Initially, we have studied the influence of Au NP, incorporated in the active layer of PSC, on optical absorption, as shown in Fig. 3a. The size of Au NP was first chosen as 30 nm and then increased to 40 nm and 50 nm, respectively. The results showed that as the Au NPs size increases, the absorption also increases compared to the neat PSC (without Au NPs), as shown in Fig. 3b. This optical absorption enhancement is mainly because of the strong near-field and far-field scattering caused by plasmonic NPs. The enhanced and broadband absorption, i.e., 550–800 nm investigate due to the dielectric properties of Au NP [23]. To further validate our results, we obtained the optical bandgap energy using the Tauc curve [24, 25]. The bandgap energy decreases from 1.69eV (without Au NPs) to 1.61eV, 1.53eV and 1.51eV as the nanoparticles size increased from 30 to 50 nm, respectively as shown in Fig .3c. As a result, the energy required to excite electrons is reduced, so electrons will easily jump from the valence band to the conduction band. Moreover, we further analyzed the optical absorption of the active layer with and without Au NPs in the simulations. Therefore, we calculated the percent amount of absorption enhancement of absorber layer by using Au NPs with various radii, taking the absorption value for the active layer without plasmonic effect as a reference for comparison. The corresponding enhancement in absorption values (6.2%, 10.92% and 18.32%) for the diameters 30nm, 40nm and 50nm, respectively as shown in Table.1.
| Structure | Values of normalized absorption of neat PSC (%) | Values of normalized absorption of PSC with Au NP (%) | Enhancement in normalized absorption (%) |
|---|---|---|---|
| Pure Perovskite | 76.84 | - | - |
| Au NP (g = 30nm) | - | 82.36 | 7.18 |
| Au NP (g = 40nm) | - | 84.87 | 10.92 |
| Au NP (g = 50nm) | - | 87.60 | 14.20 |
| Bilayer NPs (50nm) | - | 91.46 | 20.02 |
In addition, we further explored the influence of bilayer Au NPs placed inside the absorber layer in the z-direction. The size of 50nm (AuNPs) was chosen for the bilayer; this is the extreme size of NPs because there was not enough space to put more NPs in the unit space designed. The enhancement in UV-Vis spectrum for bilayer Au NPs incorporated in the perovskite layer is depicted in Fig. 4a.
The optical absorption spectrum of the bilayer Au NPs embedded perovskite layer is compared with single Au NPs incorporated and without Au NP in the absorber layer. The results showed further enhancement in optical absorption up to 91.46%, as reported in Table 1. The percent increment is also investigated in the case of bilayer Au NPs up to 6% and 14% compared to single NPs embedded in the active layer and pure perovskite, respectively. The Tauc curve is depicted in Fig. 4b. The Tauc curve showed that the decrease in bandgap energy strongly supports our previous results because of the optimization of Au NPs and LSPR effects.
4.1 Effect of [email protected]2 shell thickness:
In this section, we have studied the influence of [email protected]O2 shell thickness (Stk) on the optical properties of PSCs. The schematic of [email protected]2 embedded in the perovskite layer is shown in Fig. 5a. The optical absorption spectrum, as shown in Fig. 5b suggests that the Stk has a positive effect on the optical performance of PSCs. The broad-spectrum is achieved between 630 and 800 nm as the Stk decreases; at the same time, the shell layer improves the stability of NPs and minimizes the recombination effect by providing safety to avoid unnecessary interaction between NP and absorber layer [26]. Moreover, the converted hot electrons originated from the absorbed photons are linked to the Landau damping, which happens when the phase velocity of electromagnetic approaches that of the plasma particles and possesses superior coupling with each other [27]. When these hot electrons arrive at the perovskite layer, they provide a very sophisticated path to utilize more useful energy around the electromagnetic waves and thus improve the carrier dynamics in the device. In addition, the Tauc curve, as shown in Fig. 5c, also provided information about bandgap and the comparison of different shell Stk used in the simulation, the bandgap energy of PSC without [email protected]2 is about 1.89eV, with embeded only TiO2 inside the absorber layer is 1.85eV and with [email protected]2.
The bandgap is reduced to 1.6eV, 1.55eV, and 1.52eV for shell thickness Stk=40nm, 30nm, and 20nm, respectively. Moreover, the percentage increase in the UV-Vis spectrum is listed in the Table. 2. The normalized absorption of neat PSC is the same as the embedded only spherical TiO2 NPs while using [email protected]2 inside the absorber layer; the overall absorbance increased to 9.52%, 15.64%, and 17.23% for different Stk 40nm, 30nm, and 20nm respectively. The enhancement of the UV-Vis spectrum in PSC is because of the transfer of electrons from Au NPs to TiO2 and is also dedicated to the light scattering phenomenon in metallic NP[28, 29].
| Structure | Values of normalized absorption of neat PSC (%) | Values of normalized absorption of PSC with core-shell (%) | Enhancement in normalized absorption (%) |
|---|---|---|---|
| Pure Perovskite | 76.84 | ||
| Pure TiO2 | 76.84 | - | 0 |
| [email protected]2 (40nm) | - | 84.26 | 9.52 |
| [email protected]2 (30nm) | - | 88.84 | 15.64 |
| [email protected]2 (20nm) | - | 90.02 | 17.23 |
4.2 Effect of [email protected]2 shell thickness
Here we performed the detailed simulations of [email protected]2 incorporated in the perovskite layer as depicted in Fig. 6a, and their impact on the optical absorption spectrum of PSC. The absorption spectrum for this scheme is shown in Fig. 6a. A very slight increase is observed in the optical spectrum of PSCs with [email protected]2, which means that the silica shell has no prominent impact on the optical enhancement, or the shell layer has a very small influence on the optical properties due to the dielectric properties of surrounding NP after coating. However, dielectric silica offered both thermal and structural stability. The enhancement, in this case, is mainly due to the increased Au NPs size [14]. The Tauc’s curve in Fig. 6b further validated the previous result. There is no such difference obtained in the bandgap energy by using the [email protected]2 metal-dielectric core-shell.
| Perovskite absorber layer | Values of normalized absorption of neat PSC (%) | Values of normalized absorption of PSC with core-shell (%) | Enhancement in normalized absorption (%) |
|---|---|---|---|
| Pure Perovskite | 76.84 | ||
| Pure SiO2 | 76.42 | - | - |
| [email protected]2 (40nm) | - | 77.64 | 1.04 |
| [email protected]2 (30nm) | - | 78.90 | 2.6 |
| [email protected]2 (20nm) | - | 79.53 | 3.5 |
Moreover, the enhancement in the absorption with and without assuming [email protected]2 is shown in Table.3. The maximum enhancement of 3.5% was achieved in the overall optical absorption for [email protected]2 for Stk of 20nm. While in the other two cases for Stk of 30nm and 40nm, a very small enhancement was observed 1.04% and 2.6%, respectively.
Finally, we compared all the simulated schemes embedded in the perovskite layer i.e., Au NPs, [email protected]2, and [email protected]2 core shells. In the first scenario, we introduced monolayer and bilayer Au NPs embedded in the perovskite layer and investigated about 14.02% and 20.20% optical enhancement due to the LSPR effect, near field enhancement, and far-field scattering. In the second case, we introduced plasmonic Au NPs in pure TiO2 to form core-shell nanostructure; the results are shown in Fig. 7a. a redshift, as well as enhancement in optical absorption, was observed by wisely incorporated Au NPs in TiO2 nanoparticle to enhance the index of refraction of the insulting medium around the Au NPs. Because pure TiO2 has the capability to absorbed less than 5% of the solar spectrum [30]. We found the superior optical enhancement around 17.23% for embedding [email protected]2 core-shell in the perovskite layer is shown in Fig. 7a. Finally, in the case of incorporating [email protected]2 core-shell in the perovskite layer, a very slight redshift and very small 3.5% optical enhancement were observed in the absorption spectrum of PSC for the same Stk of 20 nm in both cases. The small increment in optical absorption is due to the low dielectric constant of silica as compared to the titania dielectric medium. Moreover, the reduced bandgap calculation from Tauc’s curve also confirmed our previous outcomes. Similarly, the same concept was followed for the bandgap calculation of [email protected]2 core-shell and [email protected]2 core-shell. The decrease in bandgap achieved as the shell thickness decreases, for minimum shell thickness (Stk =20nm) of both [email protected]2 and [email protected]2 core-shell, the optimized reduced bandgap 1.52eV investigated as compared to pure perovskite.
In conclusion, we theoretically designed plasmonic and metal-dielectric core-shell nanostructure put in the active layer of PSCs and very wisely examined the plasmonics effect. Our optimized approach showed that the plasmonic effect strongly relies on the NPs size, shape, geometry, and the metal NPs and dielectric interaction. Firstly we have reported the enhancement in optical absorption of PSCs by inserting the Au NPs with different radii in the absorber layer due to the plasmonic effect. The Au NPs with increasing radii showed about 14% (g = 50nm) improved optical enhancement and achieved broad spectrum in PSCs as compared to neat PSCs without plasmonic effect. Secondly, we have also investigated the effect of Stk of [email protected]2 and [email protected]2 incorporated in the active perovskite layers and their influence on the optical absorption of PSCs. Our findings depicted that the Stk has a positive impact on the UV- Vis absorption of PSCs. The UV-Vis absorption increased as the size of the shell decreased. Finally, we compared the UV- Vis absorption of [email protected]2 and [email protected]2 for the same radii. The best optimum optical absorption is achieved in the case of [email protected]2 rather than in [email protected]2. The smart tailoring optical properties of Au NPs and their coupling with core-shell nanostructure open huge opportunities for future aspects to improve the technology in terms of chemical and thermal stability.
Authors contribution
All authors systematically designed the conceptual study, developed the model and investigated the results. I.Ullah simulated the completed model from scratched and got the results. H. Saghaei and I.Ullah developed the methodology. I.Ullah has written the manuscript, H. Saghaei and S.K.Shah revised, edit and review the manuscript.
Corresponding author
Ihsan Ullah
Institute of Functional Nano and Soft Materials, Soochow University, Suzhou 215123, China
Email: [email protected]
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