1. Introduction

Perovskite based thin film solar cell has attracted increasing attention as a novel emerged promising technology to harvest and convert sunlight to electricity efficiently [1, 2]. The efficiency of the perovskite solar cell has been improved from 3.8% to as high as 20.1% since the first published in 2009 [3–6]. However, there exists a severe challenge for the widespread application of perovskite solar cells. A major concern is the toxicity of lead in the perovskite materials. One way to solve this issue is to replace lead in the perovskite crystal with a less toxic metal such as Sn [7, 8]. However, the Sn based perovskite material only delivers the solar cell with an efficiency of around 6% due to the fast electron-hole recombination rate at the defect sites in the absorber [8]. An alternative approach is to maintain the high performance of the lead based perovskite solar cell with a much thinner absorber thereby reducing the amount of lead in the material to minimize the detrimental effect on environment. However previous studies have found that an optimum thickness for the perovskite layer is about 300 nm, and films thinner than this would result in lower efficiency due to insufficient light absorption [9, 10]. In this case, some novel light trapping strategies should be applied for designing perovskite solar cells with extremely thin absorber layers.

Recently, plasmonic nanoparticles and nanostructures have been proven to be a useful technology to enable solar cells to improve the efficiency with thin absorber film [11–16], with most investigation ascribing the improvement to the enhanced light absorbance via the plasmonic effects. The first mechanism is that plasmonic light scattering effect from the large size particle is capable of redirecting the incident light into solar cells with increased light path length [17, 18]. The second one is that when embedded with small metal particles, the light absorption of the absorber can be enhanced due to the near field light concentration of the nanoparticles through the localized surface plasmons [19, 20]. In this paper, to address the challenging trade-off between the high absorbance and thinner absorber required for the perovskite solar cells, plasmonic metal nanoparticles incorporated into the perovskite absorber are investigated. First, near field light concentration effects of small Au and Ag particles are explored with different particle radii, surface coverage (SC) densities and integrated positions as shown in Figs. 1(a) and 1(b). Based on the optimization results of small metal particles, a lumpy Ag nanoparticle model is proposed which consists of a large Ag particle as the core particle with many small Ag particles decorated evenly outside the core particle together, thereby providing both the plasmonic scattering and the plasmonic near field concentration mechanisms. To fully utilize the plasmonic effects of the lumpy nanoparticles, the nanoparticles are inserted in the perovskite absorber directly at the rear surface forming a nanoshell structure for the absorber layer as shown in Fig. 1(d).

 

Fig. 1 Device schematics of the plasmonic perovskite solar cells. (a) With small metal particles embedded in the perovskite absorber at the front surface. (b) With small metal particles at the rear surface. (c) With lumpy nanoparticles at the rear surface. (d) The cross-section of the solar cells embedded with the lumpy nanoparticle.

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2. Results

The optical performance of the designed plasmonic perovskite solar cell structure is conducted based on the finite difference time domain method (Lumerical FDTD solution). To fully optimize the lumpy nanoparticles for solar cells, the sizes of both core and surface nanoparticles need to be modified. Therefore, we first optimize the plasmonic light concentration effect of the small surface metal particles in the perovskite solar cells. Figures 1(a) and 1(b) illustrate the schemes of the model for the plasmonic perovskite solar cell with metal nanoparticles (Au or Ag) embedded in the perovskite absorber at front and rear surfaces, respectively. The solar cell is built up based on the common perovskite solar cell geometry (FTO: 700 nm, TiO₂: 50 nm, Sprio-OmeTAD: 250 nm and Au back reflector: 60 nm). The thickness of the perovskite absorber is varied from 10 nm to 400 nm for the investigation of the metal nanoparticle plasmonic effects on the solar cell performances. The refractive index and the extinction coefficient of the perovskite absorber is measured using ellipsometer as shown in Fig. 2(a), which corresponds well with the data in other publications [21]. The optical constants of the other materials in the solar cells are taken from the reference [22].

 

Fig. 2 (a) The real and imaginary parts of the complex refractive index of the perovskite material measured by ellipsometer. (b)-(e) Absorption enhancement with Au and Ag nanoparticles at different positions in 10 nm thick perovskite layer as a function of the radius and the SC density.

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To find out the optimum condition of the metal nanoparticles for a highest absorption enhancement in the ultrathin perovskite absorber, the radius and the SC density of the nanoparticles are systematically investigated. The thickness of the perovskite is selected to be 10 nm to demonstrate an ultrathin case. The radius of the metal nanoparticle is varied from 5 nm to 15 nm while the SC density is changed from 10% to 50%. Periodic boundary conditions (PBCs) are used at the lateral boundaries of the simulation model to mimic the infinite periodic arrays of nanoparticles. To quantify the influence of the nanoparticles on the performance of perovskite solar cells, the absorbance of the perovskite absorber is first calculated. Then by integrating the absorbance with the AM 1.5 solar spectrum, the area independent J_SC can be obtained, assuming that all generated electron-hole pairs contribute to the photocurrent. Through mapping the perovskite absorption enhancement with nanoparticles as a function of the particle radius and the SC density, the optimized condition for the highest absorption enhancement in the perovskite absorber is achieved as shown in Figs. 2(b)-2(e).

Through the comparison of the perovskite absorption enhancement with Au nanoparticles in Figs. 2(b)-2(c) to that with Ag nanoparticles in Figs. 2(d)-2(e), the highest absorption enhancement of 323% is obtained with the optimized Ag nanoparticles at the rear surface of the solar cell with the particle radius of 10 nm and the SC density of 30%. The best position of the integrated nanoparticles is found to be at the rear surface in the absorber thereby avoiding the nanoparticle absorption loss when incorporated at the front surface, which can also be confirmed from the absorbance curves in Fig. 3(a). The Ag nanoparticles integrated at the rear surface can further boost the light absorbance in the short wavelength below 500 nm compared with the case with Ag nanoparticles on top. From Figs. 2(b)-2(e), it can also be observed that the Ag nanoparticles can enhance the absorbance more than the Au nanoparticles. That is mainly due to a stronger plasmonic near field light concentration effect from the Ag nanoparticle, which is illustrated in Figs. 3(b) and 3(c). From the comparison, it is clear that light intensity of Ag nanoparticles is much stronger than that of Au nanoparticles with the same size at the wavelength of 630 nm. The light energy is confined effectively around the Ag nanoparticle near the plasmon resonance frequency thereby further improving the light absorption of the perovskite material surrounding the nanoparticle.

 

Fig. 3 (a) Absorbance in perovskite layer with/without Ag nanoparticles at the rear surface or on the top surface. (b)-(c) Normalized light intensity around Au and Ag nanoparticle in solar cells at the wavelength of 630 nm.

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Based on the optimum condition of Ag nanoparticles in perovskite solar cells with thin absorber for the enhanced light concentration effect, a lumpy Ag nanoparticle model is designed with small Ag particles at a radius of 10 nm decorated on the surface of a large Ag core particle. To optimize the lumpy nanoparticle model, we define the radius ratio α as the ratio of the radius of the Ag core particle to that of the Ag surface particles and incorporate the lumpy nanoparticles into the thin perovskite absorber at the rear surface as being shown in Figs. 1(c) and 1(d). The radius ratios, α, are selected to be 75/10, 100/10 and 125/10 to find out the best nanoparticle parameters for the highest absorption enhancement in solar cells. Through the calculation of the absorption enhancement as a function of the radius ratio and the SC density, we find that the highest absorption enhancement of 431% is obtained under conditions with a radius ratio α = 100/10 and the SC density of 40%, as shown in Fig. 4(a).

 

Fig. 4 (a) Absorption enhancement mapping results in the perovskite layer with lumpy Ag nanoparticles. (b) Light absorbance of the perovskite absorber with/without metal nanoparticles at the optimized conditions.

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To further illustrate the influence of the lumpy nanoparticle on the absorbance in the perovskite layer, light absorbance curves of 10 nm thick absorbers integrated with small Au particles, small Ag particles and lumpy Ag nanoparticles are plotted respectively and compared with the flat reference case in Fig. 4(b). It can be found that with the small Au particles in the perovskite layer, the absorbance in the longer wavelength region is improved significantly due to the plasmonic light concentration effect while the light absorbance in the same wavelength region can be further increased with the small Ag particles. In contrast, the lumpy Ag nanoparticles can further boost the absorbance in the perovskite layer over the entire wavelength region significantly. The improvement of perovskite film’s light absorbance with lumpy nanoparticles can be ascribed to two possible reasons. One is that plasmonic light scattering effect from the large core particle and light concentration effect from the small surface particles being provided by lumpy Ag nanoparticles boost the longer wavelength light absorption. The other one is that the perovskite nanoshell structure induced by the lumpy nanoparticle shown in Fig. 1(d) can enhance the short wavelength absorbance due to the modes coupling into the shell structure and the enlarged effective perovskite material absorption area [23].

The achieved enormous absorption enhancement allows us to further investigate short circuit currents of solar cells with different absorber thicknesses. Based on the previous nanoparticle optimization results, J_SC values in nanoparticle incorporated solar cells with different thick perovskite layers (from 10 nm to 400 nm) are calculated and compared to the flat reference solar cells as shown in Fig. 5.

 

Fig. 5 J_SC of the perovskite solar cells with surface engineered metal nanoparticles as a function of the perovskite absorber thickness.

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It can be observed that the J_SC enhancement is much higher when absorber is thinner. When the thickness is below 100 nm, J_SC can be dramatically improved with rear surface engineered metal nanoparticles. CH₃NH₃PbI₃ is proved to be a good light absorber with visible light absorption coefficient of around 2.5 x 10⁵ cm⁻¹ [24], therefore, most visible light is fully absorbed already when the film is thicker than 100 nm. This situation is more significant in terms of the case with lumpy Ag nanoparticles. When the thickness of the absorber is increased from 10 nm to 50 nm, the J_SC values are improved from 3.44 mA/cm² and 10.52 mA/cm² without nanoparticles to 18.29 mA/cm² and 22.5 mA/cm² respectively with lumpy nanoparticles. And after the rapid increase in J_SC with respect to the absorber thickness in this range, J_SC tends to saturate at around 24.5 mA/cm² with further increased thicknesses. Rather than a dramatical improvement in J_SC, further increasing the perovskite layer thickness does not significantly improve the solar cell performance. In addition, the solar cell performance with lumpy Ag nanoparticles outweighs that with small Ag and Au particles, especially at the absorber thickness of less than 50 nm. In contrast, in terms of the solar cells with small Ag particles at front, J_SC decreases with increasing the thickness to over 200 nm compared to the flat reference case. This is mainly due to that the absorption loss from nanoparticles overweighs the nanoparticle induced absorption gain in the absorber. These findings indicate a preferable absorber thickness choice with designed lumpy nanoparticles for high performance ultrathin solar cells. At the point of the 50 nm thick perovskite layer, J_SC is increased from 10.52 mA/cm² to 22.5 mA/cm², which is even higher than the maximum achievable Jsc value in a flat 300 nm thick perovskite solar cell (21.6 mA/cm²).

3. Conclusion

In conclusion, it has been theoretically found that the radii of the core and the surface particles and the surface coverage density of the lumpy Ag nanoparticles play an imperative role in the light absorption enhancement of ultra-thin perovskite solar cells. A maximum short circuit current density (J_SC) gain of 431% relative to the reference flat perovskite solar cell with the absorber layer thickness of 10 nm can be achieved through optimizing the lumpy Ag nanoparticles. Under the optimized condition, we have found that the solar energy absorbed by a 50 nm thick perovskite layer is even higher than that for a 300 nm thick perovskite absorber without the modified nanoparticles.