Modeling and Simulation of Dual Gratings based
Ultrathin Amorphous Silicon Solar Cells

S S Saravanan, R S Dubey, S S Kalainathan

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S S Saravanan, R S Dubey, S S Kalainathan. Modeling and Simulation of Dual Gratings based Ultrathin Amorphous Silicon Solar Cells. Mechanics, Materials Science & Engineering Journal, 2017, 9, 10.2412/mmse.45.64.871. hal-01499538

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# Modeling and Simulation of Dual Gratings based Ultrathin Amorphous Silicon Solar Cells

S. Saravanan¹,ª, R.S. Dubey¹, S. Kalainathan²

1 – Advanced Research Laboratory for Nanomaterials and Devices, Department of Nanotechnology, Swarnandhra College of Engineering and Technology, Seetharampuram, Narsapur (A.P.), India

2 – School of Advanced Sciences, VIT University, Vellore, (T.N.), India

a – rag_pcw@yahoo.co.in

<figure><img src="image_2.png" /></figure>

DOI 10.2412/mmse.45.64.871 provided by [Seo4U.link](http://Seo4U.link)

**Keywords:** solar cell, RCWA method, dual gratings, plasmonic, photonic modes, Fabry-Perot resonance.

**ABSTRACT.** We present the modeling and simulation of a 50 nm ultrathin amorphous silicon solar cell using RCWA method. Optimized solar cell design showed enhanced cell efficiency up to 16.02 and 15.2% for the TE and TM polarization cases. An enhancement in optical performance is found that is associated with efficient light trapping design. The proposed design is observed to be supported with photonic and plasmonic modes. We have also explored the field distribution within the solar device with Fabry-Perot (FP) resonance and surface plasmon polariton (SPP) modes.

**Introduction.** Nowadays, there is a trend of making silicon solar cells by employing thin absorption layer in order to reduce the fabrication cost. But this absorber layer is inefficient for the absorption of high wavelength light [1]. According to the literature, the penetration depth of the photons in 180µm thick silicon solar cell was observed to be 3mm within the wavelength range 900-1100 nm. Therefore, the challenging issue is to design an efficient light trapping structure which can reuse the unabsorbed light coming after crossing the thin active region. Among various light trapping schemes, grating based design is found to be promising for the photons trapping. However, the metal and dielectric gratings at the bottom and top respectively are demanded for the better harvesting of light [2]. Ge et al. have proposed a solar cell design based on metallic gratings with one-dimensional (1D) photonic crystal and observed an enhancement in the optical path length. They have obtained a wide range of optical absorption for both TE and TM polarization modes using rigorous coupled wave analysis (RCWA) method. The designed hybrid solar cell showed enhancement in photon absorption over the entire spectral region irrespective to the angle of incidence [3]. Mutitu et al. have presented a design and fabrication of hybrid dielectric-metallic back reflectors for amorphous silicon solar cells and reported the enhanced reflectance with the use of more distributed Bragg layer (DBR) pairs. This proposed idea of solar cell design has explored the experimental realization of thin film-Si solar cells using hybrid dielectric-metallic back surface reflector [4]. Abass et al. have numerically studied the complex dual-interface grating systems (plasmonic Ag grating at the bottom and dielectric ITO gratings at the top) to enhance light absorption in silicon thin film solar cells. The proposed grating could felicitate the effect of both plasmonic and photonic modes [5]. Theuring et al. have presented the design and fabrication of plasmonic and photonic light trapping structure by using metallic (Ag) and non-metallic (SiO₂) nanoparticle respectively. The solar cell integrated with SiO₂ nanoparticles could give good result as comparison to the solar cell based on Ag nanoparticles [6].

In this paper, we propose a design of an ultrathin amorphous silicon solar cell which is integrated with a thin ITO (top) and Ag (bottom) gratings for the light trapping. In Section second designing
approach is described and simulated results are discussed in Section third. Finally, section fourth
concludes the paper.

**Designing Approach.** A schematic diagram of ultrathin a-Si solar cell structure is shown in Fig. 1. The designed solar cell is integrated with 70 nm ITO (ARC layer), 50 nm thin absorber (a-Si) and 200 nm Ag back reflector. Here, bottom Ag layer is considered as a perfect back reflector and top ITO layer acting as a suitable contact layer. Within absorber layer, bottom Ag (triangular) and top ITO (rectangular) gratings were embedded to reduce the reflection of light and to boost the path length of the photons [7]. Here, the structural parameters of both ITO and Ag gratings were the same. For the simulation, rigorous coupled wave analysis (RCWA) method is employed. This method also known as simple and fast method in which the periodic boundary conditions (PBC) were applied in x- and y-axis whereas the perfect match layer (PML) condition was in z-axis. The solar radiation (AM 1.5) was illuminated at normal incident angle from 300 to 1200nm spectral range. Below, we have investigated the different solar cell structures and compared for the both TE and TM polarizations conditions.

<figure><img src="image_3.png" /><figcaption>Fig. 1. Schematic diagram of ultra-thin a-Si solar cell integrated with metal and dielectric gratings.</figcaption></figure>

**Results and Discussion.** Fig. 2 (a) shows the absorption spectra of proposed solar cell design in which high absorption can be observed from 550 to 610 nm. Fig. 2 (b)-(f) shows the transverse electric field intensity profile at various wavelengths. In Fig. 2 (b) and 2 (c), at wavelength 550 and 610 nm the surface reflection is suppressed and hence, light absorption is enhanced.

The use of front grating could give strong absorption and also extended towards the bottom of the
solar cell. We have also observed that when the wavelength is increased, a strong peak is appeared in
the vertical axis refer to Fig. 2 (d) and 2 (f). Subsequently, in longer wavelength 1090 and 1140 nm
strong absorption peaks are observed because of the induced Fabry-Perot resonance as shown in Fig.
2 (e) and 2 (f).

Fig. 3 shows the absorption curve of TM polarization and field distribution at various wavelengths. The absorption curve shown in Fig. 3 (a) reveals the decreased light absorption as comparison to TE mode. At 530 nm, light interaction is high with surface guided resonance with field excitation at the tip of the grating as shown in Fig. 3 (b). Fig. 3 (c) depicts strong field intensity between the metal and dielectric interface at wavelength 670 nm. However, at wavelength 700 nm stronger field is observed which is due to the plasmonic effect as can be seen in Fig. 3 (d). For the longer wavelength i.e. 780 and 1030 nm, localized surface plasmon resonance (LSPR) and surface plasmon polariton are observed on the metal gratings as shown in Fig. 3 (e) and 3 (f). Furthermore, this excitation of plasmonic effect is appropriate for the scattering by the metal triangular gratings [8].
<figure><img src="image_4.png" /><figcaption>Fig. 2. Absorption spectra for TE polarization (a) and electric field distribution at λ= 550nm (b), λ=610nm (c), λ= 750nm (d), λ=1090nm (e) and λ=1140nm (f) respectively.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 3. Absorption spectra for TM polarization (a) and magnetic field distribution at (b) λ= 530nm, (c) λ=670nm, (d) λ= 700nm (e) λ=780nm (f) λ=1030nm respectively.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Fig. 4. Absorption spectra of various solar cell structures for TE (a) and TM (b) respectively.</figcaption></figure>

Fig. 4 shows the absorption spectra of various solar cells for TE and TM polarization conditions. The use of bottom Ag (triangular) grating yields sharp absorption peaks (green and blue lines) in the near IR spectral region for the TE mode as depicted in Fig. 4 (a). For the TM mode, the collection of the photons are increased in visible region while sharp and wider absorption peaks are obtained in IR region as shown in Fig. 4 (b) which is associated with the effect of metal grating. The dual grating based solar cell design shows enhanced absorption for the TM polarization because of the induced plasmonic modes. However, as comparison to TE polarization, the light absorption is less in TM case. This performance has been attributed to the strong surface guided and Fabry-Perot resonance modes. In simple words, here TE mode is more dominant as a result of combined effect of metal and dielectric gratings.

**Summary.** We have investigated the performance of an ultrathin-Si solar cell for both TE and TM polarization conditions using RCWA method. The photonic and plasmonic modes showed optimal performance within 50 nm absorber region due to the use of dual gratings. Further, the dual gratings based cell design could yield efficiency 16.02% (TE), 15.2% (TM) with short-circuit current density 24.35 and 23.13 mA/cm², respectively. An excellent relative absorption enhancement ~172% is achieved as compared to the reference solar cell. The combination of metal and dielectric gratings could give enhanced performance of the solar cells due to the assisted plasmonic and photonic modes respectively.

**Acknowledgement**

The authors gratefully acknowledge the financial support from the Defence Research Development Organization (DRDO), New Delhi (India) for the financial support.

**References**

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Cite the paper

S. Saravanan, R.S. Dubey, S. Kalainathan (2017). *Modeling and Simulation of Dual Gratings based Ultrathin Amorphous Silicon Solar Cells*. *Mechanics, Materials Science & Engineering*, Vol 9. doi:10.2412/mmse.45.64.871