Influence of Ag Nanostructure Location on the Absorption

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Influence of Ag Nanostructure Location on the Absorption Enhancement in Polymer Solar Cells Abhijith Thazhathe Nair, Shamjid Palappra, and Vari Sivaji Reddy ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13560 • Publication Date (Web): 31 Aug 2018 Downloaded from http://pubs.acs.org on August 31, 2018

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ACS Applied Materials & Interfaces

Influence of Ag Nanostructure Location on the Absorption Enhancement in Polymer Solar Cells Abhijith T, Shamjid P and V S Reddy* Organic and Nanoelectronics Laboratory, Department of Physics, National Institute of Technology Calicut (NITC), Calicut, Kerala, India-673 601 *Email: [email protected] ABSTRACT

The optical absorption enhancement in Ag nanocube (NC) and nanosphere (NS) embedded poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT):[6, 6]-phenyl C71-butyric acid methyl ester (PCBM) active layer was calculated using three dimensional finite-difference time-domain (3D-FDTD) simulations. The simulations were carried out by incorporating Ag nanostructures as a two dimensional (2D) array at various locations in the active layer matrix. A high absorption enhancement of 53% and 61% was achieved with NSs and NCs, respectively, when they were incorporated at the top portion of the active layer. The influence of various passivation layers on the absorption enhancement was also investigated. The simulation results revealed that the absorption enhancement is mainly due to the near field enhancement around the nanostructures and the backward reflection of incident light from the nanostructure array.

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KEYWORDS: Absorption enhancement, silver nanostructures, plasmonic effects, polymer solar cells, FDTD simulations I. INTRODUCTION

Polymer solar cells (PSCs) have attracted significant research attention due to their advantages like solution processability, mechanical flexibility, light weight and low cost manufacturing

1–5

.

However, the power conversion efficiency (PCE) of PSCs is not adequate to make them viable for commercial applications

6,7

. The poor performance of PSCs is mainly due to short exciton

diffusion length and low charge carrier mobility of donor and acceptor materials used for the device fabrication 1,7,8. The device performance can be improved by maximizing light absorption and charge carrier extraction at the same time 4. For efficient charge carrier extraction, the active layer thickness needs to be less than 100 nm due to low charge carrier mobility of organic semiconductors. Whereas for achieving high optical absorption, the active layer thickness should be very large 9,10. This thickness mismatch between light absorption and charge carrier extraction forced researchers to develop alternative methods such as diffraction gratings 11, V-shaped light traps

12

, photonic crystal nanostructures

13

and metal nanostructures

7,14–17

to improve light

absorption in PSCs without increasing the active layer thickness. Among these methods, utilizing plasmonic effects of metal nanostructures has gained wide acceptance as a simple and effective approach. Moreover, the metal nanostructures can improve the structural stability and the performance durability of the devices

18

. Metal nanostructures improve the active layer 2

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absorption via far field scattering or near field enhancement or both, depending on their size. Far field scattering is significant in case of larger size (> 50 nm) nanoparticles (NPs) and the near field effect dominates in smaller size (< 20 nm) NPs

19–21

. For utilizing both these plasmonic

effects simultaneously, metal nanostructures are usually included in the active layer. In recent years, substantial improvement in device performance has been achieved experimentally by incorporating various metal nanostructures into the active layer

22–26

. Choi et

al. increased the PCE of poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl})

(PTB7):PCBM

based

PSCs from 7.51% to 8.92% by embedding silica-coated Ag NPs on top of hole transport layer, the improvement in device performance is attributed to enhanced electric field distribution in the active layer 24. Park et al. achieved a PCE of 9.48% by embedding Au nanoclusters at the bottom of the PTB7:PCBM active layer. Significant improvement in the exciton generation and dissociation due to strong near field at the inter particle gaps of the nanocluster was found to be responsible for the high efficiency

26

. A record PCE of 10.42% was obtained by Liu et al. by

utilizing both the far field scattering and near field enhancement effects of Au-Ag nanocuboids embedded in the poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b;4,5-b']dithiophene2,6-diyl-alt-(4-(2-ethylhexyl)-3

fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)]

PBDTTT-EFT:PCBM active layer 23.



Theoretical simulations have been used to calculate the optical absorption enhancement induced by metal nanostructures in PSCs

27–31

. Shen et al. employed finite element method to

investigate the influence of Ag NPs on the optical absorption of poly(3-hexylthiophene-2,5-diyl) (P3HT):PCBM active layer

30

. An enhancement factor of 1.56 was obtained by embedding 24

nm sized NPs at the middle of the active layer. Duche et al. carried out FDTD simulations to confirm the experimentally obtained values of optical absorption by embedding 2D grating of Ag NSs in the active layer 28. In most of the simulation studies, metal nanostructures were embedded as a 2D array in one particular location in the active layer. Vedraine et al. calculated optical absorption by placing Ag NPs in front and back zones and found that the absorption enhancement is very sensitive to the location of NPs in the active layer 29. A detailed simulation study on the effect of nanostructure location is essential to clearly understand the light matter interaction and to achieve high absorption enhancement. In the present study, we performed 3DFDTD simulations to calculate the optical absorption in the Ag nanostructure embedded PCDTBT:PCBM active layer. The influence of shape and location of Ag nanostructures and the thickness of various passivation layers on the optical absorption enhancement has been systematically studied.


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II. SIMULATION METHODS

Numerical simulations were carried out using commercially available Lumerical FDTD Solutions software. Plasmonic PSCs with structure indium tin oxide (ITO)/ molybdenum trioxide (MoO3)(4 nm)/PCDTBT:PCBM (80 nm) +Ag nanostructures/LiF/Al were investigated in the present investigation. Schematic diagram of the device structure is shown in figure 1(a). Here, ITO and MoO3 were used as the anode and anode buffer layer (ABL), respectively. In PSCs, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is widely used as the ABL due to its promising properties like suitable work function, good optical transparency and solution based processability 32,33. However, the devices based on PEDOT:PSS degrade very fast due to its hygroscopic and acidic nature

34

. In recent years, an MoO3 layer with thickness less

than 10 nm is explored as an alternative to the PEDOT:PSS 35,36. We carried out simulations with various active layer thicknesses from 50 nm to 150 nm. These simulation results revealed that the absorption enhancement factor decreases continuously with increase in active layer thickness. Studies reported by other authors also indicate that the active layer thickness should be less than 100 nm for achieving high absorption enhancement due to plasmonic effects 30,37. Hence, a 4 nm thick MoO3 and an 80 nm thick PCDTBT:PCBM were used as the ABL and the photoactive layer, respectively in the present investigation. For enhancing the active layer absorption, Ag NSs or NCs were incorporated inside the active layer as a 2D array with periodicity in both x and y directions. Main aim of this study is to investigate the influence of Ag nanostructure location 5 ACS Paragon Plus Environment


on the optical absorption of PCDTBT:PCBM active layer. In order to include the nanostructure array at five different locations, without any overlapping, the diameter or edge length of the nanostructure was chosen as 15 nm. Moreover, the absorption enhancement factor obtained with 15 nm sized nanostructures is reasonably close to the value achieved with the optimum sized nanostructures as described later in the manuscript. Cross-sectional view of the unit cell in the xz plane is shown in figure 1 (b). Since the goal is to estimate active layer absorption enhancement due to plasmonic effects of Ag nanostructures, the LiF and the Al layers were not included in the unit cell which is used for performing simulations. The perfectly matched layers (PML) boundary conditions were used on the upper and lower boundaries of the unit cell along the z-direction to avoid reflection from boundaries. A plane wave light source with a wavelength range of 300 - 800 nm and polarization along the x-axis was used for the simulations. The simulation structure is illuminated from the ITO side in +z-direction. A mesh size of 0.2 nm was used in the region around the Ag nanostructures. The complex refractive indices of ITO, MoO3 and PCDTBT:PCBM were taken from the literature 38–40.

To evaluate the optical absorption enhancement factor, optical absorption of the active layer with and without metal nanostructures was calculated between the wavelengths 350 nm and 700 nm. The optical absorption enhancement factor (η) is given by 41,

η =

A λ . AM1.5G λ . dλ

A λ . AM1.5G λ . dλ

− − − − − 1


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where A λ and A λ denote the optical absorption of the active layer with and without metal nanostructures, respectively. The term AM1.5G λ denotes the solar spectral irradiance at the earth’s surface. The optical absorption, A λ and A λ were calculated independently using the following equation 41, Optical Absorption, A = ωε + |E x, y, z, ω |1 n ω k ω dV − − − − − 2 where ω, ε , E x, y, z, ω , n ω and k ω represent the angular frequency, permittivity of the free space, electric field in the active layer, real and imaginary parts of the complex refractive index of the PCDTBT:PCBM, respectively.

Figure 1. Schematic diagrams of (a) model device, (b) cross-sectional view of the unit cell and (c) PCDTBT:PCBM active layer with nanostructures embedded at different locations.



III. RESULTS AND DISCUSSIONS

To investigate the influence of nanostructure location on the optical absorption enhancement, simulations were carried out by incorporating a 2D array of metal nanostructures at different locations (z = -30, -15, 0, 15 and 30 nm) in the active layer as shown in the figure 1 (c). Here, the location z = 0 indicates the middle of the active layer. At each location, the period of Ag nanostructures was optimized to obtain the maximum absorption enhancement. Figure 2 (a) and (b) show the variation of absorption enhancement factor with period at different locations for Ag NSs and Ag NCs, respectively. At all the locations, enhancement factor increases till the optimum period and it decreases with further rise in period value. When the period is less than the optimum value, the nanostructure array acts like a mirror and reflects large portion of incident light and limits the amount of light entering into the active layer 27,29. When the period is increased above the optimum value, the enhancement factor decreases and approaches a value of 1 at larger periods due to significant reduction in concentration of nanostructures in the active layer. It was noticed that the optimum period (P = Px = Py) is very sensitive to the location and the shape of nanostructures. The optimum period is found to be large when nanostructures are embedded in the bottom portion of the active layer. Since light enters into the device from the bottom side, larger periods are essential at this location to decrease the backward reflection and to increase the amount of light entering into the active layer. Significant reduction in optimum period is noticed when nanostructure array is moved in the +z direction. The optical absorption 8 ACS Paragon Plus Environment

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spectra of the PCDTBT:PCBM active layer with Ag NSs and NCs at the optimum periods are shown in figures 2 (c) and (d). The absorption spectrum of active layer without nanostructures is also included for comparison purpose. When the nanostructures are embedded in the bottom part of the active layer at locations z = -30 nm and -15 nm, no improvement in absorption is observed at smaller wavelengths and slight increase in absorption is noticed above 550 nm. Significant improvement in absorption in a broad wavelength range 350-700 nm is obtained for locations z = 15 nm and 30 nm. The volume fraction and concentration of Ag nanostructures and the absorption enhancement factor at the optimum period for Ag NSs and NCs at various locations are summarized in Table 1. The enhancement factor was found to be very small for the location z = -30 nm and it increases steadily when the nanostructure array is moved in the +z direction. The Ag NCs provided better absorption enhancement compared to NSs at all the locations. A highest absorption enhancement of 53 % and 61 % was achieved at location z = 30 nm for NSs and NCs, respectively. The Ag nanostructures themselves absorb considerable fraction of incident light when they are incorporated in the active layer. Therefore, definition of integral domain is very crucial while calculating the optical absorption enhancement. Figure S1 (a) and (b) show the absorption spectra of plasmonic devices simulated by considering only PCDTBT:PCBM and PCDTBT:PCBM + Ag nanostructures (Ag NSs or Ag NCs) as the integral domain for the location z = 30 nm. When nanostructures were included in the integral domain, a high enhancement factor of 1.84 and 1.99 was obtained for Ag NSs and Ag NCs, respectively. 9 ACS Paragon Plus Environment


However, light power absorbed by the nanostructures does not contribute to the performance enhancement of PSCs. Hence, absorption enhancement must be calculated by considering only photovoltaic materials as the integral domain. The enhancement factor reduces to 1.53 and 1.61 when the absorption due to Ag NSs and Ag NCs is excluded, respectively.

Figure 2. The variation of enhancement factor with period of nanostructures at different locations for (a) Ag NSs and (b) Ag NCs. The optical absorption spectra of the PCDTBT:PCBM active layer with (c) Ag NSs and (d) Ag NCs at the optimum periods for different locations.


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Table 1. Summary of results obtained from FDTD simulations for Ag NSs and NCs embedded at different locations.

Nano structure

Nanosphere

Nanocube

Location, z (nm)

Optimum Period, P (nm)

Enhancement Factor, η

Volume Fraction (%)

Concentration (particles/cm3) ×1016

30

18

1.53

6.81

3.86

15

18

1.40

6.81

3.86

0

18

1.20

6.81

3.86

-15

26

1.06

3.27

1.85

-30

32

1.03

2.16

1.22

30

22

1.61

8.71

2.58

15

22

1.46

8.71

2.58

0

25

1.27

6.75

2.00

-15

30

1.11

4.69

1.39

-30

36

1.09

3.26

0.96

To understand the effect of nanostructure size on the enhancement factor, simulations were repeated with different diameters or edge lengths. Figure S2 (a) shows the variation of absorption enhancement factor with nanostructure size (diameter or edge length) at the top location. For this study, the nanostructure array is included inside the active layer at 2.5 nm below the top surface as shown in figure S2 (b). The optimum size was found to be 20 nm for both the nanostructures. The highest enhancement factor achieved with the optimum size nanospheres and nanocubes is 1.57 and 1.66, respectively.



Two plasmonic effects, far field scattering of light and near field enrichment of electromagnetic fields in the vicinity of metal nanostructures, usually contribute to the absorption enhancement of active layer. The near field effect enhances optical absorption only in the close surroundings of the nanostructure. On the other hand, the far field scattering can enhance the absorption in both the close surroundings and also in regions away from the nanostructure. In order to understand the absorption enhancement mechanism, the active layer is divided into three sublayers (bottom, middle and top) of equal thickness around 26.67 nm. The 2D array of nanostructures is included in one of the sublayers and the absorption enhancement has been calculated in all the three sublayers. Figure 3 (a) shows the optical absorption spectra of the sublayers without and with 2D array of NSs included in the middle sublayer. The dashed and solid lines represent the absorption spectra with (w) and without (w/o) nanostructures, respectively. The highest enhancement factor (1.89) is obtained for the middle sublayer in which NS array is included. The enhancement is mainly at wavelengths above 500 nm. In case of top sublayer, the optical absorption decreased in a broad wavelength range after incorporating NSs leading to a small enhancement factor of 0.41. When a NS array with a period 18 nm is included in the middle sublayer, the array acts like a mirror to the incident light and limits the amount of light entering into the top sublayer 27,29. Due to this backward reflection of incident light, optical absorption decreases for the top sublayer and increases for the bottom sublayer in a broad wavelength range from 350 nm to 650 nm. Similar behavior has been observed for NC array as 12 ACS Paragon Plus Environment

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shown in Figure 3(b). Optical absorption spectra of the sublayers when nanostructure array is embedded in top and bottom sublayers are shown in Figure S3 and Figure S4, respectively. In every case, the highest enhancement factor is obtained in a sublayer in which the nanostructure array is included confirming that the main enhancement mechanism is near-field concentration around the nanostructures. When the nanostructure array is included in the bottom sublayer, no improvement in absorption is noticed in middle and top sublayers indicating that the contribution of far field scattering to absorption enhancement is negligible. Significant improvement in absorption is achieved in middle and bottom sublayers in a broad wavelength range when the array is included in the top sublayer. These results clearly indicate that in addition to near-field enhancement, the backward reflection of light from the nanostructure array also contributes significantly to the active layer absorption enhancement when the nanostructure array is incorporated in the top sublayer. Though the optimum period of NC system is large compared to NS system at the top location, the NC system induced significantly higher enhancement factor. This result shows that the NC array exhibits higher backward reflection compared to the NS array due to difference in shape of the nanostructures.



P = 18

(a)

P = 25

Top

η = 0.41

Middle

η = 1.89

Bottom

η = 1.25

(b)

Light

0.35

0.20 0.15 0.10 0.05

η = 0.50

Middle

η = 2.09

Bottom

η = 1.21

Top (w/o) Top (w) Middle (w/o) Middle (w) Bottom (w/o) Bottom (w)

0.30 Optical Absorption

0.25

Top

Light

0.35

Top (w/o) Top (w) Middle (w/o) Middle (w) Bottom (w/o) Bottom (w)

0.30 Optical Absorption

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.25 0.20 0.15 0.10 0.05 0.00

0.00 400

500 600 700 Wavelength (nm)

400

800

500 600 700 Wavelength (nm)

800

Figure 3. The optical absorption spectra of the sublayers without and with (a) Ag NSs and (b) Ag NCs embedded in the middle sublayer.

The optimum period for the NS and NC array at the location z = -30 nm was found to be 32 nm and 36 nm, respectively. At this location, the nanostructures are completely isolated in the PCDTBT:PCBM matrix and hence the near-field enhancement around the isolated nanostructures solely contributes to the absorption enhancement. Figure 4(a) and (b) show the near field intensity distribution around the NS and NC on x-y, y-z and x-z cross-sections. In the case of NS, the electric field is present on x-y and x-z cross sections and no significant field is found on y-z cross-section. This indicates that the field is concentrated mainly in the direction of incident electric field oscillation. On the other hand, the field is primarily concentrated at the corners and to a lesser extent along the sharp edges in the case of NC as shown in figure 4(b). 14 ACS Paragon Plus Environment

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The electric field intensity around the NC is found to be high compared to that around the NS due to the presence of sharp corners and edges in the NC resulting in a slightly higher absorption enhancement factor. These results reveal that the absorption enhancement obtained at the locations z = -30 nm and -15 nm is mainly due to the interaction of near-field with the active layer in the vicinity of the isolated nanostructures.

Figure 4. The near field intensity distribution on different cross-sections for (a) Ag NS and (b) Ag NC at a wavelength of 600 nm. The optimum period at the location z = 30 nm is found to be 18 nm for NSs and 22 nm for NCs. Therefore, the gap between the adjacent nanostructures is 3 nm and 7 nm for NSs and NCs, respectively. At this location, coupling of the plasmon oscillations of the adjacent NSs resulted in a hot spot formation at the narrow gap between the NSs. Figure 5 (a) and (c) show the near field intensity distribution in the x-y and the x-z cross-sections for an array of NSs at 600 15 ACS Paragon Plus Environment


nm. Similar near field images for a system of NCs are shown in Figure 5(b) and (d). Significant field enhancement at the corners of nanocube is observed at wavelengths above 450 nm. The NS system exhibited higher field intensity compared to the NC system due to smaller inter-particle separation.

Figure 5. The near field intensity distribution in the x-y cross-section for a system of (a) Ag NSs and (b) Ag NCs. Intensity distribution in the x-z cross-section for a system of (c) Ag NSs

and (d) Ag NCs. The gap between nanostructures is filled with the photoactive layer. Interaction of hot spot field with the active layer increases optical absorption. The extent of absorption enhancement depends on the intensity and distribution of near field at the gap. Figure 6 (a) and (b) show the intensity distribution at different wavelengths on a y-z plane kept at the center of the gap between adjacent NSs and NCs. For NSs, the intensity is maximum in a circular region at the center of this plane and decreases outside this region. When the wavelength is increased from 500 nm to 700 nm, intensity of the field increased continuously. The NC system exhibits lower 16 ACS Paragon Plus Environment

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intensity compared to the NS system in the absorption range of the active layer. However, the NC system induces higher absorption enhancement compared to the NS system due to broader intensity distribution on the y-z plane (figure 6(b)) and higher backward reflection of light. In the case of NS system, the optimum period remains 18 nm at all the three locations z = 30 nm, 15 nm and 0 nm. Therefore, the amount of light reflected from the NS array is assumed to be nearly same at all these locations. Some portion of the backward reflected light gets absorbed by the active layer and the remaining light escapes from the active layer through ITO. The active layer absorption enhancement depends on the portion of the active layer thickness through which the reflected light travels. Therefore, reflection loss is more for the location z = 0 nm as the optical path length of the reflected light inside the active layer is less. Though the increase in period decreases reflection losses, it also adversely affects the hot spot field between adjacent nanospheres. Therefore, the optimum period remains 18 nm at all the three locations. The enhancement factor decreases continuously from 1.53 to 1.20 when the NS array is moved from z = 30 nm to z = 0 nm due to rise in reflection losses. The NC array exhibits higher reflection compared to the NS array due to its favorable shape. When the NC array is moved in the downward direction from the location z = 30 nm to z = 0 nm, the optimum period increases from 22 nm to 25 nm in order to reduce reflection losses. This rise in optimum period decreases the intensity of electric field between adjacent NCs leading to a reduction in enhancement factor from 1.61 to 1.27. 17 ACS Paragon Plus Environment


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Figure 6. The near field intensity distribution on a y-z plane kept at the center of the gap between adjacent (a) Ag NSs and (b) Ag NCs at different wavelengths.

Though the bare metal nanostructures significantly increase optical absorption, direct contact between metal surface and active layer induces electrical losses such as exciton quenching and charge recombination that are detrimental to the device performance

42–44

. To

avoid such electrical losses, metal nanostructures are generally coated with passivation layers like SiO2 and TiO2 20,24. In the present investigation, the influence of SiO2 shell thickness on the optical absorption enhancement has been studied. After incorporating SiO2 shell on the nanostructures, the period has been separately optimized at different locations for each shell thickness. Figure 7 (a) and (b) show the variation of enhancement factor with the location of Ag NSs and NCs in the active layer for different SiO2 shell thicknesses. When the Ag NSs are coated with a 1 nm SiO2 shell, the enhancement factor drastically decreases to 1.24 at the location z = 18 ACS Paragon Plus Environment

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30 nm and it approaches 1 for z = 0, -15 and -30 nm. The enhancement factor remains nearly 1 at all the locations when the SiO2 shell thickness is increased above 1 nm indicating that there is no improvement in optical absorption. The Ag NC system with 1 nm SiO2 shell was able to exhibit an enhancement factor of 1.43 and 1.23 at locations z = 30 nm and 15 nm, respectively as shown in figure 7(b).

Figure 7. Variation of absorption enhancement factor with the location of (a) Ag NS and (b) Ag NC array in PCDTBT:PCBM active layer for different SiO2 shell thicknesses.

Results presented in figure 7 indicate that the nanostructures inserted at locations below z = 0 nm (bottom part of the active layer) do not enhance the optical absorption significantly when they are coated with SiO2 shell. At all these locations, the optimum period is very large and the particles are completely isolated in the active layer medium. Figures 8(a) and (b) show the variation of near field intensity as a function of distance from the surface of isolated NS and NC without and with 1 nm thick SiO2 shell. The Ag NC exhibited higher field intensity compared to 19 ACS Paragon Plus Environment


Ag NS. Sharp reduction in intensity near the surface is observed for both the nanostructures after depositing 1 nm thick SiO2 shell, which decreases the enhancement factor to 1.

Figure 8. Variation of near field intensity with distance from the surface of (a) Ag NS and (b) Ag NC with and without SiO2 shell. The green arrows indicate the direction in which the field intensity is measured. Figure 9 (a) and (b) show the absorption spectra of active layer with SiO2 coated Ag NSs and NCs embedded at the location z = 30 nm, respectively. The spectrum of active layer without Ag nanostructures is also given in the figures as reference. After coating 1 nm thick SiO2 layer on NSs, the absorption enhancement significantly reduced at wavelengths above 500 nm. Sharp reduction in absorption enhancement was noticed with further increase in SiO2 thickness and the enhancement factor approaches 1 for SiO2 thicknesses larger than 3 nm. On the other hand, the NC system could retain significant absorption enhancement in the wavelength range 400 to 650 nm even after coating 2 nm thick SiO2 layer resulting in an enhancement factor of 1.25. The NC system was able to retain higher enhancement factor compared to the NS system for all SiO2 20 ACS Paragon Plus Environment

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thicknesses. Figure S5 (a) and (b) shows absorption spectra of the sublayers when SiO2 coated nanostructures are embedded in the top sublayer. The enhancement factor decreases drastically in the top sublayer as a result of sharp reduction in the near field intensity after including 1 nm thick SiO2 shell on the nanostructures. Whereas in the middle and bottom sublayers, significant absorption enhancement still remains due to backward reflection of incident light.

Figure 9. The optical absorption spectra of the active layer with SiO2 coated (a) Ag NSs and

(b) Ag NCs embedded at the location z = 30 nm.

To understand the influence of the refractive index of passivation shell material on absorption enhancement, simulations were also performed with other shell materials such as Al2O3, ZnO and TiO2. For this study, nanostructures coated with various shell materials of thickness 1 nm were included in the active layer at the location z= +30 nm. The refractive indices of SiO2, Al2O3, ZnO and TiO2 were taken as 1.46, 1.77, 2 and 2.5, respectively 27,30. All these materials were assumed to be perfectly transparent in the simulation wavelength range and 21 ACS Paragon Plus Environment


hence the imaginary part of the refractive index was taken as zero. Figure S6 shows the absorption spectra of the active layer with NSs and NCs coated with various dielectric materials. The optical properties of NS system were found to be more sensitive to the refractive index of shell material compared to those of NC system. The enhancement factor of NS system increased from 1.24 to 1.41 when the refractive index of the shell material was increased from 1.46 to 2.5. The SiO2 and TiO2 shells provided the lowest and the highest enhancement factors for NS system. The NC system coated with SiO2 and TiO2 shells produced enhancement factors around 1.43 and 1.36, respectively. These results indicate that for achieving better absorption enhancement, the SiO2 shell is suitable for NC system and the TiO2 shell for NS system. For including nanostructures in the active layer, required quantity of nanostructures are usually mixed with the active layer blend and this mixture is deposited on the substrate by spin coating technique. This method leads to random distribution of nanostructures in the active layer. Therefore, some new experimental techniques need to be developed for incorporating nanostructures as a 2D periodic array inside the active layer at a specific location.

CONCLUSION

In summary, the influence of Ag nanostructure location on the optical absorption of PCDTBT:PCBM active layer was numerically investigated. The absorption enhancement was found to be sensitive to the shape and location of the nanostructures. The highest enhancement


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factor of 1.53 and 1.61 was obtained for Ag NSs and NCs, respectively, at the location z = 30 nm. The larger absorption enhancement obtained at the top location is attributed to the formation of hot-spot due to the plasmon coupling of adjacent nanostructures. The Ag NC system provided higher enhancement factor than the NS system due to broader field intensity distribution at the gap between adjacent nanostructures and higher backward reflection of light. Significant reduction in absorption enhancement was noticed when nanostructures were coated with various passivation layers. The TiO2 shell provided better absorption enhancement for NS system and SiO2 shell for NC system. Present investigation provides valuable information which may be useful to design highly efficient PSCs based on plasmonic effects of Ag nanostructures within the active layer matrix.

Supporting Information. Absorption spectra simulated by considering only PCDTBT:PCBM and PCDTBT:PCBM + Ag nanostructures as the integral domain, variation of enhancement factor with size of Ag nanostructures, sub layer analysis for Ag and Ag@SiO2 nanostructures and optical absorption spectra of active layer with Ag nanostructures coated with various dielectric materials.



ACKNOWLEDGEMENT

The authors acknowledge the Kerala State Council for Science, Technology and Environment (KSCSTE), Government of Kerala for partially supporting this work under SRS program [019/SRSPS/2013/CSTE].

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ABSTRACT GRAPHIC


Influence of Ag Nanostructure Location on the Absorption

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