Si-based solar cell using metallic nanoparticles and interface texturization

Optik 153 (2018) 43–49

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Optik journal homepage: www.elsevier.de/ijleo

Original research article

Optimizing the optical performance of ZnO/Si-based solar cell using metallic nanoparticles and interface texturization H. Ferhati a , F. Djeffal a,b,∗ , K. Kacha a a b

LEA, Department of Electronics, University of Batna 2, Batna 05000, Algeria LEPCM, University of Batna 1, Batna 05000, Algeria

a r t i c l e

i n f o

Article history: Received 19 June 2017 Accepted 29 September 2017 Keywords: Nanoparticles Grooves Absorption Light trapping Optimization PSO

a b s t r a c t In this paper, we propose a new n-ZnO/p-Si hetero-junction solar cell design based on both interface engineering and metallic nanoparticles aspects. The merits of using both metallic nanoparticles and grooves morphology in the ZnO/p-Si interface to improve solar cell optical performance are investigated numerically using accurate solutions of Maxwell’s equations. It is found that the proposed structure suggests the possibility to achieve the dual role of improved light-scattering in the Si absorber layer as well as enhancing the absorption in the ZnO thin-film through the Surface Plasmon Resonance effect. Besides, the proposed design exhibits superior optical performance and offers improved total absorbance efficiency (TAE) as compared to the conventional counterpart. Moreover, particle swarm optimization (PSO)-based approach is exploited for the geometrical optimization of the proposed design to achieve higher light trapping capability. It is found that the optimized design yields 50% of relative improvement in the ZnO/p-Si-based solar cell TAE which confirms excellent capability of the proposed design approach for modulating the electric field behavior inside the solar cell structure. The obtained results indicate that the optimized n-ZnO/p-Si hetero-junction solar cell offer the potential for high conversion efficiency at low costs which make it valuable for photovoltaic application. © 2017 Elsevier GmbH. All rights reserved.

1. Introduction The sunlight has the prospective to power the Earth’s total energy requirements. However, photovoltaic panels still constitutes an extremely small portion of our power production owing to its high cost as compared to traditional energy sources [1]. Crystalline Silicon-based solar cells have enabled a huge growth in the solar cell conversion efficiency, which imposes a corresponding increase in solar cell fabrication cost [2–6]. In this context, the continuous requirement of high cost/efficiency ratio for achieving distinctive photovoltaic performance implies deficiencies mainly related to the discovery of low-cost materials that provide earth-abundant solar absorption. However, thin-film solar cells based on hydrogenated amorphous silicon present a viable alternative material due to their low-cost and acceptable conversion efficiency. Although, plasmaenhanced deposition is in fact needed for the perfect growing of the amorphous silicon that unfortunately can complicate the fabrication process [5–8]. In this light, holistic design of low cost solar cell structures with an in-depth understanding of the material optical proprieties brings the photovoltaic performance to unprecedented levels. For this purpose, the use of bilayer based on wide band gap materials such as ZnO and aluminum doped ZnO (AZO) with higher conductivity developed on the

∗ Corresponding author at: LEA, Department of Electronics, University of Batna 2, Batna, 05000, Algeria. E-mail addresses: [email protected], [email protected], [email protected] (F. Djeffal). https://doi.org/10.1016/j.ijleo.2017.09.127 0030-4026/© 2017 Elsevier GmbH. All rights reserved.

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p-Si absorber layer could improve the efficiency/cost ratio [9,10]. Basically, the direct deposition at low temperatures of the ZnO layer on the c-Si absorber layer constitutes the major benefit of this alternative structure [9–12]. This advantage enables lower fabrication cost compared to both c-Si and thin film solar cell technologies. Besides, the ZnO/AZO bilayer provides the opportunity for double functionalities, where it behaves as a TCO, and solar cell emitter with perfect Ohmic contact. Despite the cost effectiveness of the n-ZnO/p-Si based solar cell, the low absorbance efficiency represents the most important issue that should be addressed for achieving superior photovoltaic performance. In this perspective, several recent works have concentrated on enhancing the n-ZnO/p-Si based solar cell conversion efficiency through originating innovative designs [11–14]. However, the incapability to highly improve the solar cell absorption behavior especially in both visible and nearinfrared regions still persist which leads to reduced its electrical efficiency. In this regard, it is of great importance to develop new design methodologies for recording the desired enhancement regarding the solar cell optical performance. To the best of our knowledge, no investigations have been carried out for improving the n-ZnO/p-Si optical performance by introducing both interface engineering and Ag metallic nanoparticles paradigms. In this framework, we propose in this paper a new approach based on n-ZnO/p-Si interface engineering and metallic nanoparticles to improve the light trapping capability. The merits of using both Ag nanoparticles and engineered ZnO/p-Si interface to enhance the photovoltaic performance are analyzed numerically using accurate solutions of Maxwell’s equations. It is found that the proposed structure suggests the possibility to achieve the dual role of improved light-scattering in the Si absorber layer as well as enhancing the absorption in the ZnO thin-film through the SPRE. Moreover, particle swarm optimization (PSO)-based approach is exploited for an eventual geometrical optimization of the proposed design to achieve highest possible optical performance. The obtained results indicate that the optimized n-ZnO/p-Si hetero-junction solar cell offers the potential for high conversion efficiency at low costs which make it valuable for photovoltaic application. 2. Numerical modeling Principally, the key idea behind the n-ZnO/p-Si based solar cell resides mainly on using bilayer based on low cost materials with wide band gap such as ZnO and aluminum doped ZnO (AZO) developed on the p-Si absorber layer. In the proposed design, versatile design amendments are adopted in order to improve the n-ZnO/p-Si based solar cell optical behavior. Firstly, the design alteration is made in ZnO/p-Si interface level by assuming textured c-Si with grooves morphology, where w and h are the grooves width and height, respectively. Secondly, we suggest introducing Ag metallic nanoparticles inside the ZnO layer with the aim of enhancing the solar cell optical performance. In this context, Fig. 1 illustrates schemas of both conventional n-ZnO/p-Si based solar cell and the proposed design including both interface engineering metallic nanoparticles aspects. For our numerical modeling tsi is the p-Si region thickness, tz denotes the ZnO material thickness R and P are the nanoparticles radius and position, respectively, and ta represents the AZO top layer thickness. The proposed design amendments of the n-ZnO/p-Si based solar cell impose many mathematical difficulties for an eventual analytical modeling, which are associated with the structural complexity that can obscure the analytical solution of the absorbance equations. Moreover, for the accurate modeling of the proposed design, we cannot ignore both diffraction and plasmonic effects which lead to some modeling bottlenecks mainly related to the accurate resolution of Maxwell’s equations. In this framework, numerical techniques can deal with the aforementioned critical problems and provide the possibility for modeling efficiently the proposed structure optical behavior. For this purpose, the discretization of Maxwell’s equations using the FDTD method provided by the ATLAS-2D device simulator using 2-D LUMINOUS module, [15], can be efficient for perfectly model the investigated solar cell optical behavior incorporating the impact of both interface texturization and Ag

Fig. 1. Cross-sectional view of both n-ZnO/p-Si based hetero-junction solar cell and the proposed design including interface texturization and Ag nano particles.

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nano particles. It is worth mentioning that the solar cell materials, (AZO, c-Si and ZnO), optical constants (n, K) used in our numerical modeling are normally wavelength dependent and are extracted from the SOPRA database. For the modeling procedure, we assume a plane wave conveyed at normal incidence to the AZO surface. Further, to express the periodicity, we set the periodic boundary conditions. Moreover, the details about the calculation methodology concerning the estimation of the device optical parameters namely, the total reflectance and the integral absorbance are provided in [16,17]. Thus, the final formulations of both the integral absorbance and reflectance can be given in the following equations A () =

 1 2  |E y r | ωε0 ε () dV    V1 2    

S 2



 ∗ r Re E y r × H

port1

R () =

(Ec − E1 ) E1 ∗ dA1

(E1 E1 ∗ ) dA1 port1

dS

=

reflectedpower Incident power

(1)

(2)

where ␧ represents the imaginary part of the complex material dielectric constant and ␧0 refers to the permittivity of the  ∗ is the complex Magnetic field conjugate, Ec represents the calculated electric field on vacuum. It worth mentioning that H the port which relies on the excitation added to the reflected field and E1 denotes the electric pattern on the first port. In order to get a profound insight about the solar cell conversion efficiency, it is important to estimate the total absorption efficiency (TAE). The latter can be expressed as follows



max  A () I () d hc

TAE =

min

(3)



max  I () d hc min

3. Results and discussions To get a profound insight into the impact of both interface grooves morphology and Ag nanoparticles in improving the ZnO/p-Si based hetero-junction solar cell photovoltaic performance, it seems to be crucial to show the absorbance spectra of the proposed design. In this perspective, Fig. 2 compares the absorption spectra of the conventional ZnO/p-Si based heterojunction solar cell with that of the proposed structures based on different design amendments. It is clearly seen from the spectra that an improved absorbance behavior can be reached by including both interface engineering and Ag nanoparticles in comparison to that offered by the conventional n-ZnO/p-Si based solar cell. Moreover, the proposed design with interface texturization exhibits superior TAE as compared to that provided by the ZnO/p-Si based solar cell with Ag nanoparticles. Furthermore, it can be noticed also from this figure that some peak absorption appeared in the visible and near-infrared ranges of the spectrum. These peaks may be ascribed to the thin-film interference effect occurred from coherent wave propagation imposed by the textured ZnO/c-Si interface. To this extent, the modified interface with grooves morphology enables achieving superior light trapping capability leading to facilitate light-scattering inside the absorber layer. Likewise, the introduced Ag nanoparticles in the n-ZnO material arise the localized surface plasma resonance effect which can, in

Fig. 2. Absorbance versus the free space wavelength for the conventional design compared to the designs including interface texturization and Ag nano particles, with h = 100 nm, w = 100 nm, R = 20 nm and P = 100 nm.

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Fig. 3. Electric field profile of the conventional design compared to that of the proposed designs including interface texturization and Ag nano particles for a specific wavelength ( = 500 nm), with tz = 150 nm, ta = 20 nm and tSi = 500 nm.

turn, improve the solar cell optical performance. Therefore, the proposed design alterations have a profound implication in modulating the optical behavior of the ZnO/p-Si based solar cell. Aiming at exploring the main reason behind the improved absorbance behavior of the proposed structure, Fig. 3 illustrates the electric field distribution of the conventional design and the proposed designs including the impact of both interface texturization and metallic nano particles paradigms for the free space wavelength value of 500 nm. It is obvious from this figure that significant changes in the electric field profile can be achieved by introducing the proposed features. Moreover, the adopted design amendments arise strong electric fields intensities mainly confined in the Si absorber region leading to enhance the solar cell TAE. To this extent, the proposed design provides wider possibilities to achieve superior light trapping capability and to modulate the electric field behavior in the Si absorber region through the optical confinement effect. This phenomenon enables reaching prospective enhancements of the solar cell optical behavior and hence improved conversion efficiency. The obtained results indicate that the proposed design geometrical parameters play an important role in enhancing the ZnO/p-Si based solar cell optical performance. In this perspective, the usefulness of the proposed design alterations depends on the ability to design the grooves and the metallic nanoparticles to boost the light trapping capability. In this framework, the carful adjustment of the proposed design geometrical parameters such as: the grooves width and height and the Ag nanoparticles position and radius can be in fact valuable for achieving superior improvements regarding the total absorbance efficiency. For this purpose, new insight based on a global optimization approach can be intuitively effective for improving the solar cell optical characteristic which constitutes the main objective of the next subsection. 3.1. Optimization of ZnO/p-Si based hetero-junction solar cell optical performance The geometrical optimization of the proposed ZnO/p-Si based solar cell including both interface engineering and metallic nanoparticles arises as a paramount solution to boost the solar cell total absorbance efficiency. In this regard, the particle swarm optimization (PSO) approach appears to be of valuable help for recording the desired optical behavior of the investigated structure. In this perspective, PSO approach is a heuristic algorithm originated in the last few years by Eberhart and Kennedy [18]. Basically, this algorithm reproduces the metaphor of natural social behavior of bird flocking. Notably, PSO-based approach exhibits high ability for providing the global solution to very hard and complex mathematical issues in

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Table 1 Overall optical performance comparison between the conventional n-ZnO/p-Si based solar cell and the proposed design including both interface texturization and Ag nano particles aspects with and without optimization. Conventional n-ZnO/p-Si based solar cell

Proposed design with interface engineering and Ag nano particles aspects

Optimized n-ZnO/p-Si based solar cell

Design variables: Wavelength  (nm) p-c-Si thickness tc-Si (nm) ZnO thickness tZnO (nm) AZO layer thickness tAZO (nm) Ag nano-particles radius R (nm) Ag nano-particles position P(nm) Ag nano-particles period T(nm) Grooves width w(nm) Grooves height h(m)

400 500 150 20 / / / / /

400 500 150 20 20 100 100 100 150

400 415 112 23 30 26 145 140 253

Performance parameters: Absorbance (%) Reflection (%) Total absorbance efficiency TAE (%)

60 0.31 19

90 0.05 42

99 0.00012 75

Symbol

a large research space. Moreover, this heuristic algorithm has confirmed its capability to be implemented for solving diverse optimization problems associated with engineering fields [16–19]. Principally, the PSO-based approach suggests the use of random techniques to minimize a well defined fitness function that enables achieving the global solution of the optimization problem. Besides, candidate particles are affected by two factors in the research space namely the best position of each particle in the swarm (Plik ) and the best position of the particles k ). In this regard, to adjust and update the particles position and velocity, we use the relationship inspired from group (Pgi

the behavioral models of bird flocking. In this light, the ith particle which includes the ZnO/p-Si based solar cell geometrical design parameters vector Xi = (tAZO , tc−Si , Rcore , Rshell , P, ta-Si:H ), the position and velocity of each particle can be expected using the following equations: Vik+1 = wVik + c1 r1k (pkli − Xik ) + c2 r2k (pkgi − Xik )

(4a)

Xik+1 = Xik + Vik+1

(4b)

with i = 1...n

Where n refers to the swarm size, Xik and Vik denote the particle position and velocity, respectively, c1 and c2 are the cognitive and social acceleration factors; r1 and r2 denote the random numbers distributed in the range of [0,1] andXik is the actual position of the particle in swarm. Therefore, to achieve the highest possible conversion efficiency, the geometrical parameters of the proposed solar cell including textured interface and Ag nano particles will be optimized regarding the following aspects

• Maximizing the total absorbance efficiency • Minimizing the total reflection In this perspective, the well known “weighted sum approach method” can be exploited to formulate the fitness function by including weighting coefficients. Hence, the objective function for the geometrical optimization can be given by





F(Xi ) = w1 1/R () + w2 TAE

(5)

where wj (j = 1, 2) is the weighting coefficient equals to 1/2. For the geometrical optimization development, we consider the swarm with 30 particles, the stall generation for the global optimization process is equal to 500 for which the error is considered infinitesimal and a quick stabilization of the fitness function can be reached. As expected, after performing the global geometrical optimization of the ZnO/p-Si based solar cell including the adopted design amendments, improved absorbance behavior can be achieved as it is revealed in Fig. 4(a). In addition, the optimized solar cell exhibits superior TAE in comparison with that offered by the conventional counterpart. The obtained optical behavior confirms the effectiveness of the suggested global optimization in selecting the appropriate geometrical parameters for which an advantageous light-scattering improvement over the conventional structure can be reached. More importantly, the electric field profile of the optimized design can give us a profound insight about the obtained optical behavior. In this context, Fig. 4(b) shows the electric field distribution of the optimized ZnO/p-Si based solar cell. It is obvious from this figure that the optimized design exhibits an improved electric field behavior, where the extremely dense electric fields intensities inside the Si absorber layer can explain the improved absorbance characteristics. In order to get a global idea about the optimized solar cell structure optical performance, Table 1 compares the performance metrics obtained from our optimized design with that of the conventional ZnO/p-Si based hetero-junction solar cell.

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Fig. 4. (a) Absorbance as function of the free space wavelength for the proposed n-ZnO/p-Si based solar cell including interface texturization and Ag nanoparticles with and without optimization and the conventional designs. (b) Electric field distribution of the optimized design.

From this table, it is clearly shown that the optimized solar cell yields total absorbance efficiency with 43% of relative improvement as compared to the conventional counterpart. Therefore, the overall solar cell optical and electrical performances for photovoltaic applications may be consolidated by introducing the optimized design. Substantially, it is evidently confirmed that the optimized ZnO/p-Si based hetero-junction solar cell including the proposed features can dramatically improve the solar cell optical characteristics, not only enables achieving effective absorption of the sunlight, but also leading to distinctive light-scattering improvement over the conventional design.

4. Conclusion In this work, the role of both interface engineering and metallic nanoparticles in improving the n-ZnO/p-Si based hetero-junction solar cell optical performance has been analyzed numerically by exploiting FDTD-based computation. Our investigation has been focused on studying the solar cell optical behavior incorporating the effect of the proposed features. It was revealed from the obtained results that the adopted design amendments have a profound implication in achieving superior light trapping capability, which enables the possibility for improving the total absorbance efficiency. In order to boost the solar cell optical performance, a new metaheuristic technique based on PSO approach has been successfully developed in the context of our study. Promising results have been obtained where the optimized solar cell design outperforms greatly the conventional n-ZnO/p-Si based solar cell. It was demonstrated that the TAE has been enhanced by about 40% by including the adopted optimization approach. It was deduced also that the improved optical behavior of the optimized design brings the opportunity for overcoming the trade-off between high efficiency solar cells and the low fabrication cost. The results make the optimized n-ZnO/p-Si based hetero-junction solar cell valuable for developing high optical performance and low cost solar cells.

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