Nanomirror-Embedded Back Reflector Layer (BRL) for Advanced Light

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Cite This: Ind. Eng. Chem. Res. 2019, 58, 12678−12686

Nanomirror-Embedded Back Reflector Layer (BRL) for Advanced Light Management in Thin Silicon Solar Cells Sudarshana Banerjee,† Sourav Mandal,†,‡ Sukanta Dhar,†,§ Arijit Bardhan Roy,† and Nillohit Mukherjee*,†

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†

Centre of Excellence for Green Energy and Sensor Systems, Indian Institute of Engineering Science and Technology, Shibpur, Howrah 711103, West Bengal, India ‡ Centre for Energy Studies, Indian Institute of Technology Delhi, New Delhi 110016, India § Department of Electronics and Communication Engineering, National Institute of Technology Sikkim, Ravangla, South Sikkim 737139, India ABSTRACT: This work illustrates a technology for advanced light management by introducing a nonconventional back reflector layer (BRL) in amorphous silicon (a-Si:H) solar cells. To meet this, silver sulfide (Ag2S) nanoparticles with ∼50 nm diameter have been chosen as the nanomirror owing to its low parasitic absorption loss over a broad wavelength (300 to 1100 nm) region. The Ag2S NPs were sandwiched between two indium tin oxide (ITO) layers and placed as the back reflector layer of an a-Si:H solar cell to achieve better light trapping within the active layers. The embedded structure exhibited high reflectance (up to 93%) in the red and near-infrared region, the main working zone of a-Si:H cells. With the incorporation of such a state-of-the-art back reflector structure in a-Si:H solar cells, a photoconversion efficiency of 10.58% has been achieved, which is one of the best in this class.

1. INTRODUCTION The successful utilization of solar energy over other renewable energy sources significantly needs cost effectiveness, which can be achieved by exploring different methods in cell fabrication. Though crystalline silicon has been widely used as the absorber material for solar cells, thin film silicon, mainly the amorphous silicon (a-Si:H) solar cells, has the potential to be less expensive due to low material consumption and lower thermal budget. However, the commercially available stabilized efficiency and reliability of a-Si:H solar cells are greater than many of the third generation solar cells as reported so far. However, the main drawback of a-Si:H solar cells is lightinduced degradation (LID), which can be minimized by controlling the thickness of the intrinsic (i) layer.1−3 Normally, the LID decreases with decreasing the thickness of the i-layer, but the thickness of the i-layer cannot be lowered abruptly to minimize the LID as there are many other factors associated with this layer like absorption of light, etc. Among the three main layers of a-Si:H solar cell, the thickness of the i-layer controls the short-circuit current (Isc) and the thickness of the p- and n-layers along with their doping parameters determine the open circuit voltage (Voc) and short-circuit current (Isc). The total thickness of a-Si:H solar cell is less than 500 nm, which is notably thin, and a major portion of the incident light can pass through it. To facilitate light absorption, the cell thickness cannot be increased further, as the bulk defect parameters in a-Si:H are very high and increases with thickness. For this reason, some advanced tricks are required © 2019 American Chemical Society

to utilize the major portion of the incident light in the active layer of the cell.4 The issue can be resolved by utilizing light trapping techniques in order to enhance the optical path length (OPL) of photons inside the solar cell.5 Recently, various light trapping structures, viz., textured layers, plasmonics,6,7 diffractive structures,8 and layered media, have been utilized in a-Si:H solar cells.9−13 By suitably tuning the electrical and optical properties of several nanostructures, it is also possible to apply them as a light scattering layer for solar cell applications. Out of different ways, the optical absorption and carrier collection can be increased by improving the reflection characteristics of a back reflector layer (BRL, a layer between the metal back contact and the bottom n-layer in an aSi:H solar cell). This allows the incident light to have multiple numbers of bounces through the active layers, leading to an increase in the optical absorption of the device.13−15 Over the past few years, noble metal nanoparticles (NPs) like Ag and Au were normally used at the BRL.16−18 The major disadvantages in the use of such noble metal NPs is their high dissipative loss, and also, their optical properties cannot be adjusted easily as the carrier concentration in noble metals NPs cannot be tuned to a large extent.19 Though noble metal NPs can strongly scatter light,20 their application reduces the cell Received: Revised: Accepted: Published: 12678

March 28, 2019 June 19, 2019 June 20, 2019 June 20, 2019 DOI: 10.1021/acs.iecr.9b01719 Ind. Eng. Chem. Res. 2019, 58, 12678−12686

Article

Industrial & Engineering Chemistry Research

ITO layer was deposited on the Al-coated glass substrate, on which 1 μL of the 99% diluted Ag2S solution was spin coated at 150 rpm. for 5 min and dried in a hot air oven at 50 °C for 10 min. Finally, another 30 nm ITO layer was deposited on the ITO/Ag2S using the same pulse DC magnetron sputtering technique. The following four structures were prepared to compare their light reflection properties, and the best has been chosen to apply as the BRL in an a-Si:H solar cell: Scheme A: Glass/Al/ITO Scheme B: Glass/Al/Ag2S/ITO Scheme C: Glass/Al/ITO/Ag2S/ITO Scheme D: Glass/Al/Ag2S In order to obtain structural, optical, and electrical properties of the synthesized Ag2S nanoparticles as well as ITO/Ag2S/ITO BRL and the overall a-Si:H cell, specific structural, optical, and electrical characterizations were carried out. To study the morphology of the dispersed Ag2S NPs on the glass substrates, field emission scanning electron microscopy (FESEM) was carried out using a Zeiss Sigma scanning electron microscope. The topography of the ITO/ Ag2S/ITO structure was investigated by an NT-MDT Solver NEXT atomic force microscope (AFM) with a scan area of 5 μm × 5 μm in semicontact mode. The depositions of the indium tin oxide (ITO) layers were carried out using a pulsed DC magnetron sputtering system (MM-196, MILMAN, India) on Asahi-U-type fluorine-doped tin oxide (SnO2:F)-coated glass substrates, i.e., FTO substrates. The a-Si:H solar cells were fabricated using Hind High Vac (HHV, India) made cluster tool plasma enhanced chemical vapor deposition (PECVD, CT-150) unit. Aluminum (Al) back contacts to the solar cells were deposited using a thermal evaporation system (Auto-500, HHV, India). To perform the reflectance measurements, a Shimadzu Solid Spec-3700 UV−vis−NIR spectrophotometer was used. The current density−voltage (JV) characteristics of devices were measured under AM 1.5G simulated illumination with an intensity of 100 mW/cm2 (Oriel Sol 3A Solar Simulator). External quantum efficiency (EQE) was measured using an incident photocurrent efficiency measurement system (Bentham PVE-300) with a perpendicular beam from the light probe. The film thickness, optical band gap, and refractive index were measured by spectroscopic ellipsometry (J.A. Woollam, ALPHA-SE series) using the Cody−Lorentz model.31 COMSOL 5.0 was used to validate the enhancement of the energy profile inside the a-Si:H cell caused by the application of Ag2S NP-embedded ITO as the BRL.

efficiency due to absorption losses, back scattering of light, and nonradiative recombination of the photogenerated carriers at the metal−absorber interface.20−24 Compared to metal NPs, the advantages of using semiconductor nanoparticles lie in the low parasitic absorption loss over a broad wavelength region and better carrier collection. Throughout last couple of decades, several investigations have been conducted to quantify the parasitic absorption of light (parasitic absorption means the part of the incident light being absorbed by the device does not contribute to the obtainable photocurrent) in thin silicon films by incorporating optimized nanoparticle arrays, located at the rear surface, for improved light trapping via resonant plasmonics scattering.25,26 Plasmonics-based light trapping properties of the nanoparticle array is the trade-off between the beneficial effects of scattering and the deteriorating effects of parasitic absorption which can strictly hamper the overall photocurrent enhancement.27,28 To suppress the optical losses in the device, hence, properly designing the device is crucial. In this work, successful fabrication of single junction a-Si:H solar cells using a BRL consisting of spherical Ag2S nanoparticles in indium tin oxide (ITO) matrix has been demonstrated. To validate the optoelectronic performance of such BRL (ITO/Ag2S/ITO) incorporated a-Si:H solar cell, a two-dimensional finite-element model was employed. To sculpt the absorption of solar photons and subsequent electron−hole pair (EHP) generation, the frequency-domain Maxwell postulates were solved,29 which in turn supported the experimental outcome.

2. MATERIALS, METHODS, AND EXPERIMENTAL DETAILS Colloidal N-acetyl cysteine capped Ag2S NPs were synthesized using an in situ reduction-cum-capping technique. The chemicals used for this purpose were AgNO3 (99.9% SigmaAldrich), NaBH4 (99.99% Sigma-Aldrich), and N-acetyl cysteine (99.9% Sigma-Aldrich). All chemicals were of analytical reagent (AR) grade and used without further purification. Redistilled water (Millipore, 18 MΩ) was used to prepare the solutions. The comprehensive technique is reported in our previous work,30 where a detailed investigation on the structural and optical properties of the prepared colloidal Ag2S NPs was carried out, and it has been established that 99% diluted Ag2S mother solution is the most excellent candidate among all other probable concentrations to show the desired optical properties for its applications as a light trapping material. In this work, we have chosen 99% diluted mother solution to form the ITO/Ag2S/ITO BRL to cultivate their light harvesting properties in a-Si:H solar cells. The prepared Ag2S particles were embedded between two successive layers of 30 nm thick ITO matrix. These ITO layers were deposited through a pulse DC magnetron sputtering unit. An ITO sintered target with In2O3 and SnO2 in a weight proportion of 9:1 was used for this purpose. The base pressure of the sputtering system was 10−6 mbar. At the time of deposition, the chamber pressure was ∼10−2 mbar with a power density of 1 W/cm2. The argon and oxygen gas flow ratio was optimized at 100:1 to obtain ITO films with a resistivity of approximately 4.5 × 10−4 Ω cm and a transparency of about 85%. Thermally evaporated 500 nm thick metallic Al (99.999%)-coated glass (considered as the back contact for the amorphous Si cell) was taken as the substrate to fabricate the BRLs on it. Initially, a 30 nm thick

3. RESULTS AND DISCUSSION 3.1. Reflectance Study of the Ag2S-Embedded ITO Layer. Reflectance measurements are the primary way to understand whether a material/structure can be able to act as an efficient “mirror” if used in the BRL of an amorphous silicon solar cell. The conventional a-Si:H cells contain an Al layer, as mentioned earlier, as the back contact, which also helps in reflecting back a part of the incident light into the cell. However, this is not sufficient, as it fails to reflect back a major portion of light in the longer wavelength (675−800 nm) region, which could also be exploited by the active layers of the a-Si:H cells. This is also evident from the measured reflectance data (curve a, Figure 1) and many other previous reports. In this case, we can observe good reflection from the glass/Al/ ITO structure (i.e., Scheme A) within 400−675 nm, the conventional working zone for a-Si:H cells. However, the 12679

DOI: 10.1021/acs.iecr.9b01719 Ind. Eng. Chem. Res. 2019, 58, 12678−12686

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Table 2. Optimized Deposition Condition with Thickness of Each Layer for Fabricated Cellsa material p-aSiO:H i-a-SiO:H (Buffer) i-a-Si:H n-a-Si:H a

SiH4 (sccm)

1% PH3 in SiH4 H2 (sccm) (sccm)

1% B2H6 in H2 (sccm)

CO2 (sccm)

thickness (Å)

5

−

125

2.3

2.2

144

5

−

150

−

2.0

40

20 −

− 20

100 100

− −

− −

3500 250

Power density = 39 mW/cm2; pressure= 1 Torr.

Figure 1. Reflectance from different systems with ITO films and Ag2S nanoparticles.

Table 1. Average Reflectance for Different Schemes sample

average reflection

glass/Al/ITO glass/Al/Ag2S/ITO glass/Al/ITO/Ag2S/ITO glass/Al/Ag2S

78% 62% 84% 63%

Figure 4. Schematic of cells with Structure A and Structure B.

Figure 2. (a) Surface morphology of the glass/Al/ITO substrate spin coated with Ag2S nanoparticles. (b) Cross-sectional view of the glass/ Al/ITO/Ag2S/ITO sandwich pattern. The segment within the rectangle shows the Ag2S layer sandwiched between two ITO layers.

Figure 5. Reflectance property of the cells with Structure A and Structure B.

Al/Ag2S structures, respectively, failed to show any promising reflectance property in any of the wavelength regions (curves b and d of Figure 1) and, hence, have not been included in further considerations. Now, when the case for Scheme C, i.e., glass/Al/ITO/Ag2S/ITO structure, comes, we can see a sharp maximum in the reflectance curve (curve c) at around 500 nm, which is above curve a at that position. This reflectance curve was associated with a fall after 500 nm followed by two crossovers with curve a at ∼525 and 675 nm. After 675 nm, curve c again goes over the curve a with a rising trend. This creates three major zones highlighted with green and red colors that give us a quantitative idea about back reflections from Scheme A and Scheme C within 450 to 800 nm, the

Figure 3. XRD pattern of the BRL under Scheme C, i.e., glass/Al/ ITO/Ag2S/ITO structure.

region above 675 nm is not well covered by reflection from this glass/Al/ITO structure, as a steady fall in reflectance starts after this particular point. On covering up this region, we can expect some additional photon collection by the active layers of a-Si:H cell, leading to a probable increase in the photoconversion efficiency. The other two schemes, viz., Scheme B and Scheme D with glass/Al/Ag2S/ITO and glass/ 12680


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Figure 6. J-V curves with error bars for cells with Structure A and Structure B under illumination.

Table 3. Detailed Photovoltaic Parameters of the Prototype Cells structure

Jsc (mA/cm2)

cell with Structure A cell with Structure B percentage increase

14.62 17.26 18.05

Voc (V)

fill factor (%)

efficiency (%)

0.880 0.870 −1.13

73.31 70.52 −2.79

9.43 10.58 12.19

Figure 7. EQE curves for the cells with Structure A and Structure B.

photon flux density of the AM 1.5G spectrum (both as a function of wavelength λ). The presence of encapsulated Ag2S NPs in the ITO matrix plays an important role in back reflecting the light, which might be attributed to the near-field coupling effect.33−35 The organic encapsulation of the Ag2S NPs in this regard plays an important role by reducing the surface states of the semiconductor and lowering the built-in barrier height.36,37 Again, the resistive encapsulation of the semiconductor NPs restricts the migration of photogenerated excitons (electron− hole pairs), thereby concentrating them in a particular crystallite.36,37 These factors all together cause an enhancement in near-field coupling, which in turn scatter back the longer wavelengths of light, as evident from curve c of Figure 1. Normally, metal NPs are known to exhibit near-field coupling more prominently; however, semiconductor NPs with a specific shape, size, and environment (like organic encapsulation and distribution) can also exert the same if the excitons within them are cultivated properly,38−41 as is the case here. Encapsulation and embedding between two ITO layers also give the Ag2S NPs chemical stability. It is worth mentioning here that the Ag2S layer has been deposited from a colloid on the first ITO layer. So, when metallic Al was thermally evaporated directly on this Ag2S layer, there is a good chance that the organic encapsulation of the Ag2S NPs gets ruptured

wavelength zone of our interest. It is evident from Figure 1 that the overall reflection from Scheme C is more than that of other three schemes, specifically, Scheme A, within 450 to 800 nm. The capability of reflecting back good amount of photons in the longer wavelengths to the active layer of a-Si:H solar cells indicates that Scheme C would be worth considering as the back reflector-cum-contact layer for fabricating a-Si:H solar cells. The average reflection for all the schemes over the region of 300 to 800 nm was also calculated with the help of eq 1 as proposed by Zhang et al.32 and has been summarized in Table 1. 1100

R̅(%) =

∫300 R(λ)ϕ(λ) dλ 1100

× 100%

∫300 ϕ(λ) dλ

(1)

It has been observed that the average reflectance of 84% was achieved for Scheme C, which is highest among all the structures. Here, R(λ) is the reflectance and Φ(λ) is the

Table 4. Comparison of Parameters with Other Thin Silicon Solar Cells Fabricated with Various Back Reflector Layers cell structure a-Si:H a-Si:H a-Si:H a-Si:H a-Si:H a-Si:H a-Si:H a-Si:H a-Si:H a-Si:H a-Si:H a-Si:H a-Si:H

(p-i-n) (p-i-n) (p-i-n) (p-i-n) (p-i-n) (p-i-n) (p-i-n) (n-i-p) (p-i-n) (p-i-n) (p-i-n) (p-i-n)

flat textured flat flat

BRL material

efficiency (%)

Voc (mV)

Jsc (mA)

fill factor (%)

ref

n-μc-SiO:H/Al ZnO:Al n-μc-SiOx ITO NPs Ag NPs ZnO/nucleated Ag NPs Ag NPs 1-D photonic crystals Ag NPs flat Ti Ti nanodent Ti nanodent pillar ITO/Ag2S NPs/ITO

9.12 8.97 9.30 8.27 7.90 8.10 6.76 8.56 7.51 5.68 7.58 8.05 10.58

880 870 501 870 810 ∼810 854 836 816 850 850 860 870

14.9 14.7 27.4 13.5 15.1 ∼14.2 11.4 15.1 13.7 10.2 13.6 14.6 17.2

70 70 68 70 65 data not available 69 69 67 65 65 64 70

43 43 44 45 46 47 48 48 48 49 49 49 this work

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2). The morphological view (Figure 2a) of the glass/Al/ITO substrate spin coated with Ag2S indicates uniform distribution with random orientation of the sphere-like nanoparticles. The major number of particles was found to have an average diameter of 50 nm, though some slightly overgrown particles were also found. When another 30 nm thick ITO layer was coated on this to produce the glass/Al/ITO/Ag2S/ITO sandwich structure, the cross-section appears as Figure 2b. It can clearly be noticed that the Ag2S NPs are sandwiched between two 30 nm thick ITO layers. 3.2.2. X-ray Diffraction. Apart from morphological aspects, it is also important to investigate the composition of various layers of the BRL. As the BRL under Scheme C (i.e., glass/Al/ ITO/Ag2S/ITO structures) contains all the chosen materials, it has been subjected to X-ray diffraction (XRD) analysis, and the resulting diffraction pattern is shown in Figure 3, which identified the presence of four different materials in the taken BRL. The signature of the ITO layers was detected by the presence of both In2O3 and SnO2. The diffractions from In2O3 were obtained at 2θ values of 21.4, 30.6, 35.5, and 51.05° for the planes (211), (222), (400), and (440), respectively (JCPDS no. 71-2194), whereas SnO2 showed only one diffraction at 2θ = 51.5° which closely matches with a (211) diffraction (JCPDS no. 77-0452). A diffraction peak centered at 2θ = 38.65° can be assigned to the (111) plane of metallic Al used as the back contact in this scheme (JCPDS no. 021109). On the other hand, four major diffractions from (110), (−113), (−104), and (−214) planes along with one small diffraction from the (−116) plane were obtained from the XRD pattern, which could easily be identified as the monoclinic phase of Ag2S (JCPDS no. 24-0715) where the diffractions occurred at 2θ = 29.08, 31.7, 37.8, 47.8, and 61.6°, respectively. A similar monoclinic phase for Ag2S was also reported earlier.30 In this case, the phase purity of chemically synthesized Ag2S is confirmed due to the absence of any other peaks from silver-related compounds. It is also confirmed from the XRD analysis that DC magnetron sputtering of ITO on the spin coated and chemically synthesized Ag2S layer did not carry out any undesired compositional change in Ag2S. 3.2.3. Fabrication of the Solar Cell. Two sets of p-i-n single junction amorphous silicon solar cell prototypes have been fabricated for comparative study; one using the conventional glass/FTO/p-a-SiO:H/i-a-SiO:H(buffer)/i-a-Si:H/n-a-Si:H structure (i.e., Structure A) and the other using the optimized BRL structure as glass/FTO/p-a-SiO:H/i-a-SiO:H(buffer)/i-aSi:H/n-a-Si:H/ITO/Ag2S/ITO/Al (i.e., Structure B). A capacitively coupled multi chamber PECVD reactor employing 13.56 MHz RF excitation source was used for this purpose. Different intrinsic and doped a-Si:H layers were deposited by using a SiH4, H2, PH3, and CO2 gas mixture at optimum power and pressure. Before the layers were deposited, all of the process chambers were evacuated up to 10−7 Torr to minimize any probable contamination. The optimized process conditions are summarized in Table 2, and the properties of the optimized layers are described elsewhere.42 After fabrication of the conventional a-Si:H solar cells following Structure A, a set of cells was taken to fabricate the cells with Structure B as follows. First, a 30 nm ITO layer was deposited on it by DC magnetron sputtering and then Ag2S particles were spin-coated, followed by the deposition of another 30 nm ITO layer. Finally, the back Al electrode was deposited by a thermal evaporation system. The schematic of the two cells are given in Figure 4.

Figure 8. The 3D visualization of the cell (Structure B) taken up for performing the optical simulations.

Figure 9. Refractive index and extinction coefficient (n-k) vs wavelength plot of synthesized Ag2S nanoparticles.

by reacting with the thermally excited Al atoms. There is also a chance of chemical reaction between thermally excited Al atoms and Ag2S NPs. On the other hand, the second ITO layer on Ag2S NPs acts as a protective layer and restricts any unwanted interaction between the thermally excited Al atoms and Ag2S NPs. So, the polymer encapsulation on Ag2S NPs remains intact to exert light scattering by near-field coupling. For these reasons, Scheme C has a better reflectance property than Scheme B. 3.2. Structural Properties. 3.2.1. Morphology and Layer. To understand about the distribution, shape, and size of the Ag2S NPs deposited on the ITO layer, field emission scanning electron microscopic (FESEM) images were captured (Figure 12682


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Figure 10. Electric field profiles of the prototype cell with Structure B at wavelengths of (a) 500, (b) 600, (c) 700, and (d) 800 nm.

3.2.4. Effect of Ag2S Nanoparticles Embedded in the ITO Matrix of a-Si:H Cells. From the reflectance data of the finished cell (Figure 5) it is clear that the reflection spectrum from the cell, with our proposed back reflector (Structure B), is more than that of Structure A, in the longer wavelength range, specifically between 600 to 800 nm. Within this range, the increase in reflection of structure B is ∼ 22%; this means ∼22% more light is being reflected from the back surface of the device and again passing through the active layer of the cell. These facts suggest significant back scattering of light by ITO embedded with Ag2S nanoparticles in the longer wavelength region, which results in enhanced absorption and carrier collection within this range. This is also in good agreement with the reflectance data as shown in Figure 1. The current−voltage characteristics of the cells with Structure A and Structure B were measured under AM 1.5 G illumination. A set of 10 prototype cells for both Structure A and Structure B have been prepared and were subjected to J-V measurements to investigate the repeatability and reproducibility of the fabricated devices. In Figure 6, the illuminated J-V curves with the error bar are presented for both types of cells. The error in the overall efficiency for Structure A was 9.43 ± 0.4%, and for Structure B it was 10.58 ± 0.3%. This indicates significant repeatability in the performance of the fabricated cells. From the figure, we can see that the short circuit current density was improved significantly for the case of Structure B, which can be attributed to the broadband reflection (back scattering) of light from the modified BRL to the active layers of the cell, leading to better carrier generation. The short circuit current density (Jsc) was found to be improved by 18.05%, which is due to greater amount of carrier generation. However, the open circuit voltage (Voc) was found to be almost same. The fill factor (FF) of cell Structure B was decreased by 2.79% as compared to that of Structure A, which might be attributed to the presence of semiconducting Ag2S particles within the ITO layers that may affect the carrier mobility in this layer. The overall efficiency of Structure B was found to be 10.58%, which is 12.19% higher than that of Structure A (efficiency = 9.43%). The detailed photovoltaic parameters are given in Table 3. A comparison between the parameters obtained from different thin film solar cells with various BRL structures has been elaborated in Table 4. To validate the changes in cell parameters, external quantum efficiency (EQE) values were measured for the cells with

Structure A and Structure B within the wavelength region of 350 to 800 nm, and the result is shown in Figure 7. In the longer wavelengths, specifically between 600 to 800 nm, the quantum response of the cell with ITO/Ag2S/ITO back reflector layer (Structure B) is better than that of the cell without it (Structure A). The increase in photocurrent depicted by EQE measurements is also found to be in good agreement with the increase in current as seen from the J-V characteristics (Figure 6). This might be attributed to the better scattering of light from the back reflector layer of Structure B due to the presence of ITO-embedded Ag2S NPs. The increased amount of reflected light put its signature in the J-V characteristics of the cells. Since the basic cell structure is the same, it is evident that the EQE enhancements in the longer wavelength region are due to the corresponding enhancements in the internal quantum efficiency and, in turn, to both the enhanced absorption and carrier collection in these wavelength regions. It is worth mentioning here that the blue response of an amorphous silicon solar cell depends on the properties of the p-layer and the p/i interface. The bandgap energy and the thickness of the p-layer play an important role in this regard. For the case reported here, the bandgap energy of the deposited p-layer was ∼1.9 eV with a thickness of 14.4 nm. Efforts have been made to further reduce the p-layer thickness and increase the bandgap energy; however, the cells fabricated with such variations showed poor fill factors either with thicknesses less than 14.4 nm or with a p-layer bandgap energy greater than 1.9 eV, or with both. So, these two conditions came out as the optimized conditions for this case. Thus, it may be inferred that the bandgap energy of the p-layer might be responsible for such less significant blue response of the device. Thus, from our preliminary studies, it appears that the Ag2S nanoparticle-embedded ITO matrix plays a crucial role in light reflection by back scattering to the active layer of the cell, leading to greater carrier generation. However, better results can be obtained by tuning the particle size and their area of coverage. 3.2.5. Rationale of Advanced Light Management. To reconfirm the proposed light scattering schemes by investigating the electromagnetic fields inside the active layers of the solar cell, numerical calculations were performed using a COMSOL Multiphysics wave optics module by solving the three-dimensional vector Maxwell’s equations. The advantage 12683


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layer of the cell, thus enhancing the performance. With this aim, a set of a-Si:H solar cells has been prepared incorporating such a BRL, which came out with 10.58 ± 0.3% photoconversion efficiency and good reproducibility. This might be attributed to the low parasitic absorption loss of encapsulated Ag2S nanoparticles over a broad wavelength (300 to 1100 nm) region and near-field coupling effect of the compact nanoparticle array. The experimental outcomes were also validated using simulations, where encapsulated Ag2S nanoparticles were found to exert good scattering of light within 700−800 nm without any notable absorption by the nanoparticles themselves.

of optical simulation is that the light trapping mechanisms/ effects can be studied as a natural extension of the performed experiments. The unstructured grids of COMSOL enable studies of a relatively large domain (>103/μm) in visible wavelengths. A perfectly matched layer and a self-adapted conformal mesh region (very fine mesh size was chosen) were taken to define the structure. The following a-Si:H cell structure was chosen to validate the light management as observed from the previous sections. In order to understand the effect of scattering properties of Ag2S nanoparticles embedded in ITO matrix vide simulation, we have considered spheres of Ag2S nanoparticles with a ∼50 nm diameter and placed in an ITO 2D matrix with a maximum center-to-center spacing of 60 nm. The thickness of the successive n-i-buffer-p layers were taken as 25, 350, 4.0, and 14.4 nm, respectively. The 3D visualization of the model taken up to perform the simulation is shown in Figure 8. In order to obtain the electric field characteristics of the proposed structure, perfectly matched layer boundary conditions with the wave vector provided by the periodic ports were used.50 Perfect electric contacts (PEC) were assumed in the simulation to take the effect of back metal contact of the sample. In the simulation model, the refractive indices (R.I.) of FTO, a-Si:H, and ITO were taken from the ellipsometric measurements which have good agreement with those of previous reports.36,37,51 The refractive index-extinction coefficient (nk) of Ag2S NPs has been determined experimentally (ellipsometry) from a coating on clinical glass substrate and is shown in Figure 9. Figure 10 shows the electric field profiles of the prototype aSi:H solar cell with ITO/Ag2S/ITO as BRL (Structure B) at four different wavelengths (500, 600, 700, and 800 nm) to cover the whole region of interest for a-Si:H. As per our experimental observation, it has been quite established that due to the back scattering from the ITO-embedded Ag2S NPs, the unabsorbed photons were trapped into the cell, mainly within 700 and 800 nm, which is denoted by the increased number of “red” zones in the electric field profile. To confirm the back scattering phenomena of the synthesized nanoparticles in the ITO matrix, we first inserted the refractive index (n) and extinction coefficient (k) data obtained from ellipsometry into the simulation software. The red regions were found to be generated in the a-Si:H cell; however, the Ag2S NPs were basically free from such red electric field hotspots in the longer wavelengths (Figure 10c, d). This indicates that the Ag2S NPs are significantly scattering light in this wavelength region (700−800 nm) rather than absorbing light. This observation is well in agreement with the reflectance and EQE results and confirms the role of ITO/Ag2S/ITO as BRL for a-Si:H cells. So, we can say that a low k value of semiconductor NPs (in this case Ag2S) offers less joule heating and provides better back scattering, which may beneficial for the thin silicon solar cell light-trapping schemes.52 This is the main reason behind the enhancement of short circuit current in J-V measurements.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +91-33-2668-2915. ORCID

Sourav Mandal: 0000-0001-7859-9648 Nillohit Mukherjee: 0000-0001-8662-7565 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Gufran Ahmad, IIT Kharagpur, for carrying out the ellipsometry. The authors thank DST (GoI) for providing instrumental support through the DST-IIEST Solar Hub.

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REFERENCES

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4. CONCLUSION Ag2S nanoparticles were used as nanomirrors by embedding them in an ITO matrix to form an ITO/Ag2S/ITO back reflector layer (BRL) which showed good reflectance within the longer wavelength region (700−800 nm). As the longer wavelengths are not normally well exploited in thin siliconbased solar cells, mainly in a-Si:H solar cells, such BRL can help immensely in scattering more light back in to the active 12684


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Nanomirror-Embedded Back Reflector Layer (BRL) for Advanced Light

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