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Efficiency improvement of a silicon-based thin-film solar cell using plasmonic silver nanoparticles and an antireflective layer
Optics Communications 454 (2020) 124437
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Efficiency improvement of a silicon-based thin-film solar cell using plasmonic silver nanoparticles and an antireflective layer Afsaneh Asgariyan Tabrizi a ,∗, Ali Pahlavan b a b
Department of Engineering, Sari Branch, Islamic Azad University, Sari, Iran Department of Physics, Sari Branch, Islamic Azad University, Sari, Iran
ARTICLE Keywords: Silicon Thin films Solar cells Silver nanoparticles Absorption Efficiency Fill-factor Plasmonics
INFO
ABSTRACT This paper presents a silicon thin-film solar cell (TFSC) integrated with the silver nanoparticles. It consists of anti-reflection, absorption and reflective layers in which the anti-reflective layer is made of pyramids of TiO2 . The purpose of this structure is to allow sunlight to enter the cell at any angle with the minimum reflection and absorption in the wavelength range of 300–1100 nm. The absorbing layer is composed of silicon and when sunlight enters this layer, the molecular bonds break down and release many electrons due to its high absorption coefficient. In this layer, silver spherical nanoparticles are placed to increase the absorption of solar energy by the localized surface plasmon resonances, which will increase the efficiency of the TFSC. The last layer of the structure is a reflective surface of aluminum, which aims to reflect the light into the upper layer to enhance its absorption. We will calculate the key performance metrics for a solar cell such as short-circuit current, open-circuit voltage, fill-factor, and photovoltaic efficiency considering the effects of recombination between silicon substrate and other materials. The numerical results based on the finite-difference time-domain method reveal that the proposed structure has much more absorption due to the anti-reflection layer and the presence of silver nanoparticles that leads to light scattering, light localization, and guided mode excitation compared to conventional TFSC. Our simulations based on the finite-element method show the presented TFSC integrated with silver nanoparticles has a fill-factor of 0.82 and an efficiency of 16.18%.
1. Introduction A solar cell is a photovoltaic component that converts solar energy into an electric current. This is done using semiconducting materials with photovoltaic properties [1]. The research groups have focused on the principles and concepts of new photovoltaics including thinfilm [2], organic [3], plasmonic [4–7], dye-sensitized [8–11], and photonic crystal [12,13] solar cells with the goal of reducing manufacturing costs and used materials (consumables), as well as increasing the efficiency of solar cells. When incorporating nanophotonic elements similar to metallic nanoparticles with plasmon resonances tuned to efficiently scatter the light into the absorbing layer, or nanotextured surface anti-reflection coatings to decrease undesirable back reflections that degrade cell performance, solar cell efficiency can be dramatically enhanced [7,14,15]. In recent years, thin-film silicon solar cells have attracted much attention due to their low manufacturing costs, but their efficiency is relatively low compared to other structures [16]. The main problem is very low absorption at the edge of the conduction band. Therefore, advanced schemes have been proposed to improve absorption based on light trapping in the active layer of a solar cell. In general, light-trapping methods are divided into (1):
reduce the reflection coefficient of the upper surface, and (2): increase the optical length inside the cell [17]. Various techniques have been presented by research groups to trap the sunlight including the use of anti-reflection coatings, rough surfaces [18,19], window grids [20], photonic crystals [21,22] and plasmon-based metallic nanoparticles [5, 23,24]. Among the above-mentioned methods, many studies are based on plasmonic nanoparticles. Indeed, plasmonic approach is one of the most desirable methods for improving localized light absorption [25, 26]. The study of metal nanoparticles in the upper layer of the solar cells shows a significant increase in the absorption of light [7]. It is caused by the scattering effect due to the excitation of surface plasmons. Increasing the absorption leads to lower recombination rate, higher open circuit voltage, higher conversion efficiency, and even new solar-cell designs. Since plasmonic metal nanostructures were used in the thin film solar cells, this led to strong light trapping due to the strong interaction of light in plasmonic nanostructures and surrounding media. Various structures have been proposed to increase the length of the optical pathway within the cell. Using structures with a periodic lattice is a simple way to increase the effective optical length because it can be contracted at both cell surfaces and increase the absorption
∗ Corresponding author. E-mail addresses: [email protected] (A.A. Tabrizi), [email protected] (A. Pahlavan).
https://doi.org/10.1016/j.optcom.2019.124437 Received 5 July 2019; Received in revised form 13 August 2019; Accepted 19 August 2019 Available online 22 August 2019 0030-4018/© 2019 Published by Elsevier B.V.
A.A. Tabrizi and A. Pahlavan
Optics Communications 454 (2020) 124437
Fig. 1. The perspective views of (a) TFSC-A, (b) TFSC-B, (c) TFSC-C, (d) (a) TFSC-D, (e) TFSC-E and (f) different layers of TFSC-E including protective layers (shown by light gray), anti-reflective layer (turquoise), absorption layer (red), and reflective layer (gray) . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
in a wide wavelength range. Li et al. designed and fabricated a flexible free-standing epsilon-near-zero metamaterial consisting of silver nanolayers [27]. They showed that the optical properties of the flexible metamaterial do not significantly change after repeated, macroscopic, and sustained mechanical deformations. The finite-difference time-domain (FDTD) is a high-performance optical solver which is used to simulate light interactions with a wide range of solar cell designs [28,29]. These designs can range from simple planar geometries to highly complex structures with a variety of materials including metals and organic. In this paper we present a thin-film silicon solar cell (TFSC) integrated with silver nanoparticles and demonstrate how to set up such a simulation and measure the
transmission and reflection versus the wavelength and investigate the possibility to enhance the absorption in solar cells by employing localized plasmon polaritons excited in silver nanoparticles. We will also gain an insight into where the electromagnetic energy is absorbed and where the photoelectrons are created. 2. Physical structure TFSCs are valuable devices to convert the sunlight energy into electrical energy [30]. Such devices include an absorbing layer composed of a semiconductor such as Silicon (Si) [31–33], germanium (Ge), gallium 2
A.A. Tabrizi and A. Pahlavan
Optics Communications 454 (2020) 124437
Fig. 2. (a) The real, and (b) Imaginary parts of the refractive indices of Si, TiO2 , SiO2 , Al, and Ag.
arsenide (GaAs) [34], Chalcogenide [35–39], to create electron–hole pair. It contributes to a current in an external circuit. This is only possible if the electron–hole pair does not recombine along its path to the contacts. TFSCs have the potential to significantly decrease the fabrication cost of photovoltaics [40]. One of the important challenges of such devices is their small thickness of active layer [41]. Thus, using an approach to trap light in the solar cell is essential to increase light absorption that results in an increase in the conversion efficiency [42, 43]. In this section, we consider five TFSCs with various configurations as shown in Figs. 1(a)–1(e) whose active materials are all assumed to be made of Si. The first TFSC, TFSC-A, represented by Fig. 1(a), is assumed to have an active layer of Si, two short collecting junctions of Aluminum (Al) with the length of 2 μm and the thickness of 100 nm. The upper and lower layers of the structure is composed of silica as a protective layer [33,44–46]. Nevertheless, it also plays the role of anti-reflective layer at the top of the solar cell. The second TFSC, TFSC-B, represented by Fig. 1(b), is assumed to be the same as TFSC-A, except for the length of its P++ collecting junction that is 11.15 μm. The third TFSC, TFSC-C, depicted in Fig. 1(c), is similar to TFSC-A but it has an anti-reflective layer of TiO2 shown by turquoise. TFSC-D shown in Fig. 1(d) is as same as TFSC-C, except for the length of its P++ collecting junction that is 11.15 μm. Adding silver nanoparticles in a periodic lattice into the active layer of TFSC-D, TFSC-E is achieved. Fig. 1(f) shows the perspective views of the TFSC layers that all of them are used to form the TFSC-E. They include protective layers (shown by light gray), anti-reflective layer (turquoise), absorption layer (red), and reflective layer (gray). For more details, the TFSC-E contains anti-reflection, absorption, reflective and protective layers. The anti-reflective coating layer, shown by turquoise in Fig. 1, has been made of pyramids of TiO2. The purpose of this structure is to reduce reflection losses and to allow sunlight to enter the cell at any angle. It also has the least absorption and reflection in our desired wavelength range (visible and near-infrared, i.e. 300–1100 nm) and improves the conversion efficiency of solar cells. The proposed pyramid-based layer of TiO2 is a broad-band layer and even decreases the reflection for wavelengths outside the operating bandwidth. The absorbing layer shown by red in Fig. 1 is composed of Si with a thickness of 3 μm. When light enters this layer, due to its high absorption coefficient, the molecular bonds break down and
release many electrons. In this layer, silver spherical nanoparticles with radius of 50 nm (that is achieved by particle swarm algorithm) are periodically distributed to increase the absorption of solar energy by surface plasmon resonances, which will increase the efficiency of the solar cell. The third layer of the structure is the reflective layer composed of aluminum (Al) with a thickness of 100 nm, which aims to reflect the light into the upper layer to enhance its absorption. The physical structure of the solar cell is completely covered with silica, SiO2 , as a protective layer (two transparent squares in the upper and lower layers of the TFSCs with the thickness of 500 nm shown in Fig. 1). This layer should be used to prevent the surface contamination and oxidation. Fig. 2 demonstrates the real and imaginary parts of the refractive indices of Si, TiO2 , SiO2 , Al, and Ag. in terms of wavelength [47,48]. 3. Mathematical background The absorption of a solar cell is calculated as follows [49]: 𝑃𝑎𝑏𝑠 = −0.5𝜔 |𝐸|2 𝑖𝑚𝑎𝑔 (𝜀)
(1)
where 𝜔, |𝐸|2 , and 𝑖𝑚𝑎𝑔 (𝜀) are angular frequency, the intensity of electric field, and the imaginary part of the permittivity, respectively. Therefore, to measure the absorption in terms of angular frequency, we only need to know |𝐸|2 , and 𝑖𝑚𝑎𝑔 (𝜀). Using FDTD method, both quantities are easy to calculate. The number of absorbed photons in the active layer of solar cell composed of Si per unit volume can be computed by dividing 𝑃𝑎𝑏𝑠 by the energy per photon as [50]: 𝑃𝑎𝑏𝑠 −0.5 |𝐸|2 𝑖𝑚𝑎𝑔 (𝜀) = (2) ℎ𝜔 ℎ where ℎ is Plank’s constant. The generation rate is the integration of g over the spectrum under study (300–1100 nm). In the best case, if all absorbed photons generate electron–hole pairs, the current (A/m) is given by: 𝑔=
(3)
𝐼 = 𝑒𝑔
We can also calculate an accurate short-circuit current density using the CHARGE solver of Device software. The quantum efficiency is a key parameter of every solar cell which is defined by 𝑄𝐸 (𝜆) = 3
𝑃𝑎𝑏𝑠 (𝜆) 𝑃𝑖𝑛 (𝜆)
(4)
A.A. Tabrizi and A. Pahlavan
Optics Communications 454 (2020) 124437
evaluate the performance of a solar cell, the photovoltaic efficiency is calculated as the equation below [49]: 𝜂=
𝐹 𝐹 × 𝑉𝑂𝐶 × 𝐽𝑠𝑐 𝑆𝐴𝑀1.5𝐺
(5)
where 𝐹 𝐹 , 𝑉𝑂𝐶 , 𝐽𝑠𝑐 are the fill-factor, open-circuit voltage, the shortcircuit current density, respectively. 𝑆AM1.5G is the incident power of light from the AM1.5G solar irradiance spectrum model:100 mW/cm2 [51]. The fill-factor is given by [52,53] 𝐹𝐹 =
𝑃𝑚𝑎𝑥 𝑉𝑜𝑐 𝐽𝑠𝑐
𝑉𝑂𝐶 of a solar cell is calculated as ( ) 𝐼𝐿 𝑛𝑘𝑇 ln +1 𝑉𝑜𝑐 = 𝑞 𝐼0
(6)
(7)
where 𝑉𝑂𝐶 is found by setting the net current equal to zero and 𝑘, 𝑇 , 𝑞, 𝐼0 , and 𝐼𝐿 are Boltzmann’s constant, absolute temperature, absolute value of electron charge, dark saturation current, and light generated current. In a TFSC with a passive anti-reflective layer, 𝐽𝑠𝑐 can be approximated as: ( ) (8) 𝐽𝑠𝑐 = 𝑞𝐺 𝐿𝑛 + 𝐿𝑝 where 𝐺 is the generation rate, and 𝐿𝑛 and 𝐿𝑝 are the electron and hole diffusion lengths respectively. 4. Modeling and simulations The modeling, analysis and simulation of a solar cell are necessary before the fabrication processes to predict its optical profiles and electrical parameters such as those discussed in Section 3. Finding an analytical method for a solar cell becomes more difficult by increasing the complexity of the structure, including the number of layers, the geometry, and the materials used in each layer. In real components, non-ideal processes such as bulk and surface recombination of electrical carriers (electrons and holes) reduce the efficiency of a TFSCs. The optical and electrical simulations are carried out on the structure for the design and characterization of solar cells considering the aforementioned destructive effects. The efficiency of a TFSC depends not only on high absorption spectrum, but also on effective charge transportation and the output power. Furthermore, the optical simulation analyzes the generation of electron–hole pairs in the semiconductor materials using the FDTD method and calculates the absorbed power in the active layers. This technique is a numerical method for solving Maxwell’s equations in complex geometries such as the TFSCs shown in Fig. 1. In other word, FDTD can also achieve the frequency solution using Fourier transform. Therefore, a full range of useful quantities such as the transmission or reflection of light can be computed. The finite-element method (FEM) in Device software is applied to every layer of the structure to determine how many pairs of electron–hole get collected at the collecting junctions and contribute to the output power. These two numerical methods are used to determine the efficiency of TFSCs under study. In every simulation, one of the TFSCs (shown in Fig. 1) was studied using FDTD solver. To do so, a TFSC, a plane wave source, a FDTD simulation region, two mesh override regions and several monitors such as electric field, generation rate, transmission and reflection monitors were used as our simulation objects. Herein, a TE-mode plane wave source in the wavelength range of 300–1100 nm which propagates in the 𝑧-backward axis was used as an incident light source. Threedimensional (3D) FDTD method was applied to one unit cell of the TFSC and perfectly matched layers (PMLs) with 32 layers were set as absorbing boundaries placed on top and bottom (𝑧-direction) of the physical structure while the boundary conditions are periodic in the 𝑥 and 𝑦 directions. In order to reduce the calculation time and speed up the simulation, the periodic boundary conditions in the 𝑥- and 𝑦-directions can be replaced by the asymmetric and symmetric boundary conditions
Fig. 3. (a) Absorption, (b) Reflection, and (c) Transmission spectra of all TFSCs presented in Fig. 1.
where 𝑃𝑎𝑏𝑠 (𝜆) and 𝑃𝑖𝑛 (𝜆) are the powers of the absorbed light and incident light within the Si layer of a solar cell, respectively, at 𝜆. To 4
A.A. Tabrizi and A. Pahlavan
Optics Communications 454 (2020) 124437
Fig. 4. Cross-sectional (xy) views of the electric-field (first row) and magnetic-field (second row) distributions at 𝑧 = −2 μm of the TE-polarized light in a unit cell of the TFSC-E at 300, 566, 832, and 1100 nm.
Table 1 Recombination properties between Si and other used materials including Al, Ag, TiO2 and SiO2 [54,55].
in the 𝑥- and 𝑦-directions, respectively. The PML boundary conditions were used along the injection direction to absorb the reflected and transmitted lights. In order to reduce meshing errors and increase the precision of the simulation, a non-uniform conformal meshing with the accuracy of 2 was also applied to all material interfaces. The maximum mesh step was set with the mesh sizes (point-to-point distance) of 𝑑𝑥 = 20 nm, 𝑑𝑦 = 20 nm, and 𝑑𝑧 = 20 nm. Although such small mesh sizes increase the simulation time, it increases the simulation accuracy. The calculation time was set as 3000 fs, and the transmission and reflection spectra were calculated using frequency-domain power monitors at 𝑧 = −6.5 μm and 𝑧 = 2.5 μm, respectively. The plane wave source was also placed at 𝑧 = 1.5 μm. Using these settings, the absorption, reflection and transmission in terms of wavelength are calculated. Taking the solar spectrum into account, the generation rate is calculated from the optical absorption monitor in FDTD solver and used as a source in the subsequent electrical simulation in CHARGE to calculate the quantum efficiency and characterize the photovoltaic performance.
Velocity (cm/s) Electron
Hole
Si-Al Si-Ag Si-TiO2 Si-SiO2
107 1000 1000 10
107 1000 1000 10
Table 2 Simulation region and properties of CTS. Simulation region
5. Results and discussion The reflection (𝑅) and transmission (𝑇 ) spectra of the TFSCs can be obtained by solving the Maxwell equations using FDTD method discussed in Section 4. In simulations, we need to know how much power is absorbed in the Si active layer. Thus, two frequency-domain power monitors were used to measures the total reflected power, and transmitted power. The total absorption is calculated as 𝐴 (𝜆) = 1 − 𝑅 (𝜆) − 𝑇 (𝜆)
Recombined materials
Properties
Simulation type
3D
Norm length (μm)
10000
Xmin (μm) Xmax (μm) Ymin (μm) Ymax (μm) Zmin (μm) Zmax (μm)
0 10.125 −5 5 −3.4 1.3
Solver model Temperature dependence Perturbation amplitude (V) Frequency (HZ) Solver type DC updated mode
Steady state Isothermal 0.001 1000 Gummel Self-consistent
localized surface plasmon polaritons. In fact, the presence of silver nanoparticles in the active layer of the structure leads to surface plasmons, which itself increases the absorption of the solar cell. By increasing the absorption of light in the active layer, more molecular bonds are broken, and more the electron–hole pairs are released, and they go to junctions and contribute to the flow of electric current. This leads to an increase in the efficiency of the solar cell. Although placing the last layer with aluminum sheet increases the solar cell fabrication cost, the light is reflected into the absorption (active) layer when it hits the aluminum surface. Fig. 3(c) shows the transmission spectra of TFSC-B, TFSC-D, and TFSC-E are zero due to the anti-reflective layer of Al with a length of 11.15 μm. This layer covers the substructure and it does not allow light to escape from the solar cell. We can also see from Fig. 3(a) that TFSC-C, TFSC-D, and
(9)
𝐴 (𝜆) of TFSC-E consists of an absorption in the active layer of Si that is common for all presented TFSCs (see Fig. 1), and the absorption due to silver nanoparticles [49]. Fig. 3 shows the absorption, reflection, and transmission curves of TFSCs under study. As can be observed in this figure, there are two reasons for relatively high absorption of TFSCE. The first one is the giant near-field enhancement and the second one is the enhanced scattering cross-section due to the excitation of 5
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Optics Communications 454 (2020) 124437
Table 3 Dopant concentrations of TFSCs in CTS of Device Software. Properties
Anode (Base) back diff (p++)*
Anode (Base) back diff (p++)**
Cathode (Emitter) nwell (n++)
Pepi
Dopant type Source face Junction width (μm) Distribution function Concentration (cm−3 ) Ref. concentration (cm−3 ) 𝑋min (μm) 𝑋max (μm) 𝑌min (μm) 𝑌max (μm) 𝑍min (μm) 𝑍max (μm)
p Lower Z 0.2 erfc 1019 1010
p Lower Z 0.2 erfc 2 × 1020 1010
n Upper Z 0.4 erfc 1019 1010
p All domains – – 2 × 1016 –
−2.2 12.2 −5 5 −3 2.8
−2.2 12.2 −5 −5 −3 2.8
−1.4 1.4 −5 5 −0.4 0
−3 13 −6 6 −5 1
*For TFSC-A, and TFSC-C. **For TFSC-B, TFSC-D, and TFSC-E.
TFSC-E have greater absorptions at wavelengths shorter than 600 nm due to anti-reflective layer of TiO2 at the top of the structure. It also demonstrates that TFSC-E has higher absorption at wavelengths greater than 790 nm, it is because of the simultaneous excitations of surface plasmon resonances due to the presence of silver nanoparticles in the active layer of Si. The maximum generation rates of TFSC-A, TFSCB, TFSC-C, TFSC-D and TFSC-E are 3.01 × 1027 , 1.99 × 1028 , 8.7 × 1027 , 7.44 × 1028 , and 6 × 1029 (1∕(m3 s)), respectively. Fig. 4 demonstrates the cross-sectional (𝑥𝑦) views of the electricfield (first row) and magnetic-field (second row) distributions at 𝑧 = −2 μm of the TE-polarized light in a unit cell of the TFSC-E at 300, 566, 832, and 1100 nm. This unit cell contains five silver nanoparticles. As can be seen, there is a relatively large absorption around nanoparticles at different wavelengths due to the surface plasmon resonances. In this study, the optical simulation using the FDTD method does not consider the effects of recombination between Si and other materials such as Al, Ag, TiO2 and SiO2 . Thus, this is not sufficient by itself to model the device characteristics. This optical simulation can obtain the required data for the electrical simulation based on the charge transport solver (CTS) to apply the recombination effects (shown in Table 1) to the numerical calculations. The CTS of the device software simultaneously solves the 3D equations for electrostatic potential, charge and heat transport. Herein, Gummel method is used to numerically solve equations of such systems in which by holding the charge with a fixed value, the electrostatic potential is calculated. Next, this achieved solution to the electrostatic potential is applied as a fixed input to the charge equations, which are updated. This process continues until the solution is self-consistent. Gummel technique is both stable and efficient for devices where the currents are small and the variations in the charge distribution are not too extreme such as solar cells and micro- and nanophotonic absorbers. Table 2 summarizes the parameters of the simulation region and more important properties of CTS. Aluminum is also used for both the cathode and anode contacts, which are specified as ohmic contacts using the boundary conditions. To define the space-charge regions in the TFSCs under study, first, the background doping concentration of Si is set to represent a ptype epitaxial layer with a concentration of 2 × 1016 cm−3 . This is accomplished by defining a region of constant doping that encompasses the entire geometry (all domains). Next, two diffusion doping regions are used to specify the n-type and p-type dopant concentrations under the cathode and anode respectively. Further details are given in Table 3. Fig. 5(a) shows current density versus voltage of all TFSCs. As can be seen, short-circuit current density of TFSC-E is 31.57 mA/cm2 that is
Table 4 The electrical simulation results based on the charge transport solver.
Structure
𝑉OC (V)
𝐽SC (mA/cm2 )
𝑃max (mW/cm2 )
FF
𝜂%
TFSC-A TFSC-B TFSC-C TFSC-D TFSC-E
0.56 0.593 0.578 0.601 0.626
18.38 17.26 26.9 26.71 31.57
8.47 8.5 12.88 13.36 16.17
0.82 0.83 0.827 0.83 0.82
8.4 8.5 12.88 13.36 16.18
71% more than TFSC-A as reference structure. The open-circuit voltage of TFSC-E is 0.626 which is 11.7% greater than TFSC-A. Fig. 5(b) demonstrates power as a function of voltage for all TFSCs under study. The maximum powers of 8.47, 8.5, 12.88, 13.36, and 16.17 mW/cm2 have been achieved for TFSC-A, TFSC-B, TFSC-C, TFSC-D, and TFSC-E, respectively. The photovoltaic energy conversion efficiencies of TFSC-A, TFSCB, TFSC-C, TFSC-D, and TFSC-E are 8.4, 8.5, 12.88, 13.36, and 16.18, respectively that have been presented in Table 4. Compared to TFSCD, the presence of nanoparticles in TFSC-E significantly increases the efficiency by 21%. TFSC-E gives rise to a 92% increase in the efficiency over the TFSC-A due to anti-reflective layer, silver nanoparticles, and a reflective surface of Al at the bottom of the structure. The electrical simulation results based on the CTS are summarized in Table 4. It can be shown (as further study) that by a careful adjustment of the silver nanoparticles using optimization algorithms such as particle swarm, an enhancement in the number of absorbed photons from the solar spectrum will be possible.
6. Conclusion In summary, we demonstrated the incorporation of plasmonic nanostructures in TFSC is a promising method to harvest light into the active layer and numerically demonstrated the presence of silver nanoparticles inside the Si active layer of TFSC enables the excitations of the localized surface plasmon resonances. Furthermore, we show that the coupling between photonic and plasmonic modes further improves the light absorption. It leads to have an efficiency of 16.18% with an improvement of 92.6% compared to the conventional TFSC (TFSC-A in this paper). 6
A.A. Tabrizi and A. Pahlavan
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Fig. 5. (a) Current density, and (b) Power versus voltage of all TFSCs using charge transport solver.
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8
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Optics Communications journal homepage: www.elsevier.com/locate/optcom
Efficiency improvement of a silicon-based thin-film solar cell using plasmonic silver nanoparticles and an antireflective layer Afsaneh Asgariyan Tabrizi a ,∗, Ali Pahlavan b a b
Department of Engineering, Sari Branch, Islamic Azad University, Sari, Iran Department of Physics, Sari Branch, Islamic Azad University, Sari, Iran
ARTICLE Keywords: Silicon Thin films Solar cells Silver nanoparticles Absorption Efficiency Fill-factor Plasmonics
INFO
ABSTRACT This paper presents a silicon thin-film solar cell (TFSC) integrated with the silver nanoparticles. It consists of anti-reflection, absorption and reflective layers in which the anti-reflective layer is made of pyramids of TiO2 . The purpose of this structure is to allow sunlight to enter the cell at any angle with the minimum reflection and absorption in the wavelength range of 300–1100 nm. The absorbing layer is composed of silicon and when sunlight enters this layer, the molecular bonds break down and release many electrons due to its high absorption coefficient. In this layer, silver spherical nanoparticles are placed to increase the absorption of solar energy by the localized surface plasmon resonances, which will increase the efficiency of the TFSC. The last layer of the structure is a reflective surface of aluminum, which aims to reflect the light into the upper layer to enhance its absorption. We will calculate the key performance metrics for a solar cell such as short-circuit current, open-circuit voltage, fill-factor, and photovoltaic efficiency considering the effects of recombination between silicon substrate and other materials. The numerical results based on the finite-difference time-domain method reveal that the proposed structure has much more absorption due to the anti-reflection layer and the presence of silver nanoparticles that leads to light scattering, light localization, and guided mode excitation compared to conventional TFSC. Our simulations based on the finite-element method show the presented TFSC integrated with silver nanoparticles has a fill-factor of 0.82 and an efficiency of 16.18%.
1. Introduction A solar cell is a photovoltaic component that converts solar energy into an electric current. This is done using semiconducting materials with photovoltaic properties [1]. The research groups have focused on the principles and concepts of new photovoltaics including thinfilm [2], organic [3], plasmonic [4–7], dye-sensitized [8–11], and photonic crystal [12,13] solar cells with the goal of reducing manufacturing costs and used materials (consumables), as well as increasing the efficiency of solar cells. When incorporating nanophotonic elements similar to metallic nanoparticles with plasmon resonances tuned to efficiently scatter the light into the absorbing layer, or nanotextured surface anti-reflection coatings to decrease undesirable back reflections that degrade cell performance, solar cell efficiency can be dramatically enhanced [7,14,15]. In recent years, thin-film silicon solar cells have attracted much attention due to their low manufacturing costs, but their efficiency is relatively low compared to other structures [16]. The main problem is very low absorption at the edge of the conduction band. Therefore, advanced schemes have been proposed to improve absorption based on light trapping in the active layer of a solar cell. In general, light-trapping methods are divided into (1):
reduce the reflection coefficient of the upper surface, and (2): increase the optical length inside the cell [17]. Various techniques have been presented by research groups to trap the sunlight including the use of anti-reflection coatings, rough surfaces [18,19], window grids [20], photonic crystals [21,22] and plasmon-based metallic nanoparticles [5, 23,24]. Among the above-mentioned methods, many studies are based on plasmonic nanoparticles. Indeed, plasmonic approach is one of the most desirable methods for improving localized light absorption [25, 26]. The study of metal nanoparticles in the upper layer of the solar cells shows a significant increase in the absorption of light [7]. It is caused by the scattering effect due to the excitation of surface plasmons. Increasing the absorption leads to lower recombination rate, higher open circuit voltage, higher conversion efficiency, and even new solar-cell designs. Since plasmonic metal nanostructures were used in the thin film solar cells, this led to strong light trapping due to the strong interaction of light in plasmonic nanostructures and surrounding media. Various structures have been proposed to increase the length of the optical pathway within the cell. Using structures with a periodic lattice is a simple way to increase the effective optical length because it can be contracted at both cell surfaces and increase the absorption
∗ Corresponding author. E-mail addresses: [email protected] (A.A. Tabrizi), [email protected] (A. Pahlavan).
https://doi.org/10.1016/j.optcom.2019.124437 Received 5 July 2019; Received in revised form 13 August 2019; Accepted 19 August 2019 Available online 22 August 2019 0030-4018/© 2019 Published by Elsevier B.V.
A.A. Tabrizi and A. Pahlavan
Optics Communications 454 (2020) 124437
Fig. 1. The perspective views of (a) TFSC-A, (b) TFSC-B, (c) TFSC-C, (d) (a) TFSC-D, (e) TFSC-E and (f) different layers of TFSC-E including protective layers (shown by light gray), anti-reflective layer (turquoise), absorption layer (red), and reflective layer (gray) . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
in a wide wavelength range. Li et al. designed and fabricated a flexible free-standing epsilon-near-zero metamaterial consisting of silver nanolayers [27]. They showed that the optical properties of the flexible metamaterial do not significantly change after repeated, macroscopic, and sustained mechanical deformations. The finite-difference time-domain (FDTD) is a high-performance optical solver which is used to simulate light interactions with a wide range of solar cell designs [28,29]. These designs can range from simple planar geometries to highly complex structures with a variety of materials including metals and organic. In this paper we present a thin-film silicon solar cell (TFSC) integrated with silver nanoparticles and demonstrate how to set up such a simulation and measure the
transmission and reflection versus the wavelength and investigate the possibility to enhance the absorption in solar cells by employing localized plasmon polaritons excited in silver nanoparticles. We will also gain an insight into where the electromagnetic energy is absorbed and where the photoelectrons are created. 2. Physical structure TFSCs are valuable devices to convert the sunlight energy into electrical energy [30]. Such devices include an absorbing layer composed of a semiconductor such as Silicon (Si) [31–33], germanium (Ge), gallium 2
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Optics Communications 454 (2020) 124437
Fig. 2. (a) The real, and (b) Imaginary parts of the refractive indices of Si, TiO2 , SiO2 , Al, and Ag.
arsenide (GaAs) [34], Chalcogenide [35–39], to create electron–hole pair. It contributes to a current in an external circuit. This is only possible if the electron–hole pair does not recombine along its path to the contacts. TFSCs have the potential to significantly decrease the fabrication cost of photovoltaics [40]. One of the important challenges of such devices is their small thickness of active layer [41]. Thus, using an approach to trap light in the solar cell is essential to increase light absorption that results in an increase in the conversion efficiency [42, 43]. In this section, we consider five TFSCs with various configurations as shown in Figs. 1(a)–1(e) whose active materials are all assumed to be made of Si. The first TFSC, TFSC-A, represented by Fig. 1(a), is assumed to have an active layer of Si, two short collecting junctions of Aluminum (Al) with the length of 2 μm and the thickness of 100 nm. The upper and lower layers of the structure is composed of silica as a protective layer [33,44–46]. Nevertheless, it also plays the role of anti-reflective layer at the top of the solar cell. The second TFSC, TFSC-B, represented by Fig. 1(b), is assumed to be the same as TFSC-A, except for the length of its P++ collecting junction that is 11.15 μm. The third TFSC, TFSC-C, depicted in Fig. 1(c), is similar to TFSC-A but it has an anti-reflective layer of TiO2 shown by turquoise. TFSC-D shown in Fig. 1(d) is as same as TFSC-C, except for the length of its P++ collecting junction that is 11.15 μm. Adding silver nanoparticles in a periodic lattice into the active layer of TFSC-D, TFSC-E is achieved. Fig. 1(f) shows the perspective views of the TFSC layers that all of them are used to form the TFSC-E. They include protective layers (shown by light gray), anti-reflective layer (turquoise), absorption layer (red), and reflective layer (gray). For more details, the TFSC-E contains anti-reflection, absorption, reflective and protective layers. The anti-reflective coating layer, shown by turquoise in Fig. 1, has been made of pyramids of TiO2. The purpose of this structure is to reduce reflection losses and to allow sunlight to enter the cell at any angle. It also has the least absorption and reflection in our desired wavelength range (visible and near-infrared, i.e. 300–1100 nm) and improves the conversion efficiency of solar cells. The proposed pyramid-based layer of TiO2 is a broad-band layer and even decreases the reflection for wavelengths outside the operating bandwidth. The absorbing layer shown by red in Fig. 1 is composed of Si with a thickness of 3 μm. When light enters this layer, due to its high absorption coefficient, the molecular bonds break down and
release many electrons. In this layer, silver spherical nanoparticles with radius of 50 nm (that is achieved by particle swarm algorithm) are periodically distributed to increase the absorption of solar energy by surface plasmon resonances, which will increase the efficiency of the solar cell. The third layer of the structure is the reflective layer composed of aluminum (Al) with a thickness of 100 nm, which aims to reflect the light into the upper layer to enhance its absorption. The physical structure of the solar cell is completely covered with silica, SiO2 , as a protective layer (two transparent squares in the upper and lower layers of the TFSCs with the thickness of 500 nm shown in Fig. 1). This layer should be used to prevent the surface contamination and oxidation. Fig. 2 demonstrates the real and imaginary parts of the refractive indices of Si, TiO2 , SiO2 , Al, and Ag. in terms of wavelength [47,48]. 3. Mathematical background The absorption of a solar cell is calculated as follows [49]: 𝑃𝑎𝑏𝑠 = −0.5𝜔 |𝐸|2 𝑖𝑚𝑎𝑔 (𝜀)
(1)
where 𝜔, |𝐸|2 , and 𝑖𝑚𝑎𝑔 (𝜀) are angular frequency, the intensity of electric field, and the imaginary part of the permittivity, respectively. Therefore, to measure the absorption in terms of angular frequency, we only need to know |𝐸|2 , and 𝑖𝑚𝑎𝑔 (𝜀). Using FDTD method, both quantities are easy to calculate. The number of absorbed photons in the active layer of solar cell composed of Si per unit volume can be computed by dividing 𝑃𝑎𝑏𝑠 by the energy per photon as [50]: 𝑃𝑎𝑏𝑠 −0.5 |𝐸|2 𝑖𝑚𝑎𝑔 (𝜀) = (2) ℎ𝜔 ℎ where ℎ is Plank’s constant. The generation rate is the integration of g over the spectrum under study (300–1100 nm). In the best case, if all absorbed photons generate electron–hole pairs, the current (A/m) is given by: 𝑔=
(3)
𝐼 = 𝑒𝑔
We can also calculate an accurate short-circuit current density using the CHARGE solver of Device software. The quantum efficiency is a key parameter of every solar cell which is defined by 𝑄𝐸 (𝜆) = 3
𝑃𝑎𝑏𝑠 (𝜆) 𝑃𝑖𝑛 (𝜆)
(4)
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Optics Communications 454 (2020) 124437
evaluate the performance of a solar cell, the photovoltaic efficiency is calculated as the equation below [49]: 𝜂=
𝐹 𝐹 × 𝑉𝑂𝐶 × 𝐽𝑠𝑐 𝑆𝐴𝑀1.5𝐺
(5)
where 𝐹 𝐹 , 𝑉𝑂𝐶 , 𝐽𝑠𝑐 are the fill-factor, open-circuit voltage, the shortcircuit current density, respectively. 𝑆AM1.5G is the incident power of light from the AM1.5G solar irradiance spectrum model:100 mW/cm2 [51]. The fill-factor is given by [52,53] 𝐹𝐹 =
𝑃𝑚𝑎𝑥 𝑉𝑜𝑐 𝐽𝑠𝑐
𝑉𝑂𝐶 of a solar cell is calculated as ( ) 𝐼𝐿 𝑛𝑘𝑇 ln +1 𝑉𝑜𝑐 = 𝑞 𝐼0
(6)
(7)
where 𝑉𝑂𝐶 is found by setting the net current equal to zero and 𝑘, 𝑇 , 𝑞, 𝐼0 , and 𝐼𝐿 are Boltzmann’s constant, absolute temperature, absolute value of electron charge, dark saturation current, and light generated current. In a TFSC with a passive anti-reflective layer, 𝐽𝑠𝑐 can be approximated as: ( ) (8) 𝐽𝑠𝑐 = 𝑞𝐺 𝐿𝑛 + 𝐿𝑝 where 𝐺 is the generation rate, and 𝐿𝑛 and 𝐿𝑝 are the electron and hole diffusion lengths respectively. 4. Modeling and simulations The modeling, analysis and simulation of a solar cell are necessary before the fabrication processes to predict its optical profiles and electrical parameters such as those discussed in Section 3. Finding an analytical method for a solar cell becomes more difficult by increasing the complexity of the structure, including the number of layers, the geometry, and the materials used in each layer. In real components, non-ideal processes such as bulk and surface recombination of electrical carriers (electrons and holes) reduce the efficiency of a TFSCs. The optical and electrical simulations are carried out on the structure for the design and characterization of solar cells considering the aforementioned destructive effects. The efficiency of a TFSC depends not only on high absorption spectrum, but also on effective charge transportation and the output power. Furthermore, the optical simulation analyzes the generation of electron–hole pairs in the semiconductor materials using the FDTD method and calculates the absorbed power in the active layers. This technique is a numerical method for solving Maxwell’s equations in complex geometries such as the TFSCs shown in Fig. 1. In other word, FDTD can also achieve the frequency solution using Fourier transform. Therefore, a full range of useful quantities such as the transmission or reflection of light can be computed. The finite-element method (FEM) in Device software is applied to every layer of the structure to determine how many pairs of electron–hole get collected at the collecting junctions and contribute to the output power. These two numerical methods are used to determine the efficiency of TFSCs under study. In every simulation, one of the TFSCs (shown in Fig. 1) was studied using FDTD solver. To do so, a TFSC, a plane wave source, a FDTD simulation region, two mesh override regions and several monitors such as electric field, generation rate, transmission and reflection monitors were used as our simulation objects. Herein, a TE-mode plane wave source in the wavelength range of 300–1100 nm which propagates in the 𝑧-backward axis was used as an incident light source. Threedimensional (3D) FDTD method was applied to one unit cell of the TFSC and perfectly matched layers (PMLs) with 32 layers were set as absorbing boundaries placed on top and bottom (𝑧-direction) of the physical structure while the boundary conditions are periodic in the 𝑥 and 𝑦 directions. In order to reduce the calculation time and speed up the simulation, the periodic boundary conditions in the 𝑥- and 𝑦-directions can be replaced by the asymmetric and symmetric boundary conditions
Fig. 3. (a) Absorption, (b) Reflection, and (c) Transmission spectra of all TFSCs presented in Fig. 1.
where 𝑃𝑎𝑏𝑠 (𝜆) and 𝑃𝑖𝑛 (𝜆) are the powers of the absorbed light and incident light within the Si layer of a solar cell, respectively, at 𝜆. To 4
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Fig. 4. Cross-sectional (xy) views of the electric-field (first row) and magnetic-field (second row) distributions at 𝑧 = −2 μm of the TE-polarized light in a unit cell of the TFSC-E at 300, 566, 832, and 1100 nm.
Table 1 Recombination properties between Si and other used materials including Al, Ag, TiO2 and SiO2 [54,55].
in the 𝑥- and 𝑦-directions, respectively. The PML boundary conditions were used along the injection direction to absorb the reflected and transmitted lights. In order to reduce meshing errors and increase the precision of the simulation, a non-uniform conformal meshing with the accuracy of 2 was also applied to all material interfaces. The maximum mesh step was set with the mesh sizes (point-to-point distance) of 𝑑𝑥 = 20 nm, 𝑑𝑦 = 20 nm, and 𝑑𝑧 = 20 nm. Although such small mesh sizes increase the simulation time, it increases the simulation accuracy. The calculation time was set as 3000 fs, and the transmission and reflection spectra were calculated using frequency-domain power monitors at 𝑧 = −6.5 μm and 𝑧 = 2.5 μm, respectively. The plane wave source was also placed at 𝑧 = 1.5 μm. Using these settings, the absorption, reflection and transmission in terms of wavelength are calculated. Taking the solar spectrum into account, the generation rate is calculated from the optical absorption monitor in FDTD solver and used as a source in the subsequent electrical simulation in CHARGE to calculate the quantum efficiency and characterize the photovoltaic performance.
Velocity (cm/s) Electron
Hole
Si-Al Si-Ag Si-TiO2 Si-SiO2
107 1000 1000 10
107 1000 1000 10
Table 2 Simulation region and properties of CTS. Simulation region
5. Results and discussion The reflection (𝑅) and transmission (𝑇 ) spectra of the TFSCs can be obtained by solving the Maxwell equations using FDTD method discussed in Section 4. In simulations, we need to know how much power is absorbed in the Si active layer. Thus, two frequency-domain power monitors were used to measures the total reflected power, and transmitted power. The total absorption is calculated as 𝐴 (𝜆) = 1 − 𝑅 (𝜆) − 𝑇 (𝜆)
Recombined materials
Properties
Simulation type
3D
Norm length (μm)
10000
Xmin (μm) Xmax (μm) Ymin (μm) Ymax (μm) Zmin (μm) Zmax (μm)
0 10.125 −5 5 −3.4 1.3
Solver model Temperature dependence Perturbation amplitude (V) Frequency (HZ) Solver type DC updated mode
Steady state Isothermal 0.001 1000 Gummel Self-consistent
localized surface plasmon polaritons. In fact, the presence of silver nanoparticles in the active layer of the structure leads to surface plasmons, which itself increases the absorption of the solar cell. By increasing the absorption of light in the active layer, more molecular bonds are broken, and more the electron–hole pairs are released, and they go to junctions and contribute to the flow of electric current. This leads to an increase in the efficiency of the solar cell. Although placing the last layer with aluminum sheet increases the solar cell fabrication cost, the light is reflected into the absorption (active) layer when it hits the aluminum surface. Fig. 3(c) shows the transmission spectra of TFSC-B, TFSC-D, and TFSC-E are zero due to the anti-reflective layer of Al with a length of 11.15 μm. This layer covers the substructure and it does not allow light to escape from the solar cell. We can also see from Fig. 3(a) that TFSC-C, TFSC-D, and
(9)
𝐴 (𝜆) of TFSC-E consists of an absorption in the active layer of Si that is common for all presented TFSCs (see Fig. 1), and the absorption due to silver nanoparticles [49]. Fig. 3 shows the absorption, reflection, and transmission curves of TFSCs under study. As can be observed in this figure, there are two reasons for relatively high absorption of TFSCE. The first one is the giant near-field enhancement and the second one is the enhanced scattering cross-section due to the excitation of 5
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Optics Communications 454 (2020) 124437
Table 3 Dopant concentrations of TFSCs in CTS of Device Software. Properties
Anode (Base) back diff (p++)*
Anode (Base) back diff (p++)**
Cathode (Emitter) nwell (n++)
Pepi
Dopant type Source face Junction width (μm) Distribution function Concentration (cm−3 ) Ref. concentration (cm−3 ) 𝑋min (μm) 𝑋max (μm) 𝑌min (μm) 𝑌max (μm) 𝑍min (μm) 𝑍max (μm)
p Lower Z 0.2 erfc 1019 1010
p Lower Z 0.2 erfc 2 × 1020 1010
n Upper Z 0.4 erfc 1019 1010
p All domains – – 2 × 1016 –
−2.2 12.2 −5 5 −3 2.8
−2.2 12.2 −5 −5 −3 2.8
−1.4 1.4 −5 5 −0.4 0
−3 13 −6 6 −5 1
*For TFSC-A, and TFSC-C. **For TFSC-B, TFSC-D, and TFSC-E.
TFSC-E have greater absorptions at wavelengths shorter than 600 nm due to anti-reflective layer of TiO2 at the top of the structure. It also demonstrates that TFSC-E has higher absorption at wavelengths greater than 790 nm, it is because of the simultaneous excitations of surface plasmon resonances due to the presence of silver nanoparticles in the active layer of Si. The maximum generation rates of TFSC-A, TFSCB, TFSC-C, TFSC-D and TFSC-E are 3.01 × 1027 , 1.99 × 1028 , 8.7 × 1027 , 7.44 × 1028 , and 6 × 1029 (1∕(m3 s)), respectively. Fig. 4 demonstrates the cross-sectional (𝑥𝑦) views of the electricfield (first row) and magnetic-field (second row) distributions at 𝑧 = −2 μm of the TE-polarized light in a unit cell of the TFSC-E at 300, 566, 832, and 1100 nm. This unit cell contains five silver nanoparticles. As can be seen, there is a relatively large absorption around nanoparticles at different wavelengths due to the surface plasmon resonances. In this study, the optical simulation using the FDTD method does not consider the effects of recombination between Si and other materials such as Al, Ag, TiO2 and SiO2 . Thus, this is not sufficient by itself to model the device characteristics. This optical simulation can obtain the required data for the electrical simulation based on the charge transport solver (CTS) to apply the recombination effects (shown in Table 1) to the numerical calculations. The CTS of the device software simultaneously solves the 3D equations for electrostatic potential, charge and heat transport. Herein, Gummel method is used to numerically solve equations of such systems in which by holding the charge with a fixed value, the electrostatic potential is calculated. Next, this achieved solution to the electrostatic potential is applied as a fixed input to the charge equations, which are updated. This process continues until the solution is self-consistent. Gummel technique is both stable and efficient for devices where the currents are small and the variations in the charge distribution are not too extreme such as solar cells and micro- and nanophotonic absorbers. Table 2 summarizes the parameters of the simulation region and more important properties of CTS. Aluminum is also used for both the cathode and anode contacts, which are specified as ohmic contacts using the boundary conditions. To define the space-charge regions in the TFSCs under study, first, the background doping concentration of Si is set to represent a ptype epitaxial layer with a concentration of 2 × 1016 cm−3 . This is accomplished by defining a region of constant doping that encompasses the entire geometry (all domains). Next, two diffusion doping regions are used to specify the n-type and p-type dopant concentrations under the cathode and anode respectively. Further details are given in Table 3. Fig. 5(a) shows current density versus voltage of all TFSCs. As can be seen, short-circuit current density of TFSC-E is 31.57 mA/cm2 that is
Table 4 The electrical simulation results based on the charge transport solver.
Structure
𝑉OC (V)
𝐽SC (mA/cm2 )
𝑃max (mW/cm2 )
FF
𝜂%
TFSC-A TFSC-B TFSC-C TFSC-D TFSC-E
0.56 0.593 0.578 0.601 0.626
18.38 17.26 26.9 26.71 31.57
8.47 8.5 12.88 13.36 16.17
0.82 0.83 0.827 0.83 0.82
8.4 8.5 12.88 13.36 16.18
71% more than TFSC-A as reference structure. The open-circuit voltage of TFSC-E is 0.626 which is 11.7% greater than TFSC-A. Fig. 5(b) demonstrates power as a function of voltage for all TFSCs under study. The maximum powers of 8.47, 8.5, 12.88, 13.36, and 16.17 mW/cm2 have been achieved for TFSC-A, TFSC-B, TFSC-C, TFSC-D, and TFSC-E, respectively. The photovoltaic energy conversion efficiencies of TFSC-A, TFSCB, TFSC-C, TFSC-D, and TFSC-E are 8.4, 8.5, 12.88, 13.36, and 16.18, respectively that have been presented in Table 4. Compared to TFSCD, the presence of nanoparticles in TFSC-E significantly increases the efficiency by 21%. TFSC-E gives rise to a 92% increase in the efficiency over the TFSC-A due to anti-reflective layer, silver nanoparticles, and a reflective surface of Al at the bottom of the structure. The electrical simulation results based on the CTS are summarized in Table 4. It can be shown (as further study) that by a careful adjustment of the silver nanoparticles using optimization algorithms such as particle swarm, an enhancement in the number of absorbed photons from the solar spectrum will be possible.
6. Conclusion In summary, we demonstrated the incorporation of plasmonic nanostructures in TFSC is a promising method to harvest light into the active layer and numerically demonstrated the presence of silver nanoparticles inside the Si active layer of TFSC enables the excitations of the localized surface plasmon resonances. Furthermore, we show that the coupling between photonic and plasmonic modes further improves the light absorption. It leads to have an efficiency of 16.18% with an improvement of 92.6% compared to the conventional TFSC (TFSC-A in this paper). 6
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Fig. 5. (a) Current density, and (b) Power versus voltage of all TFSCs using charge transport solver.
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