Impact of Specifically Shaped Plasmonic Gold Nanoparticles and a

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Impact of Specific-Shaped Plasmonic Gold Nanoparticles and Double Cathode Interfacial Layer on the Performance of Conducting Polymer-Based Photovoltaics Ashish Singh, Anamika Dey, and Parameswar Krishnan Iyer ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01269 • Publication Date (Web): 07 Sep 2018 Downloaded from http://pubs.acs.org on September 9, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Nano Materials

Impact

of

Specific-Shaped

Plasmonic

Gold

Nanoparticles and Double Cathode Interfacial Layer on the Performance of Conducting Polymer-Based Photovoltaics Ashish Singh ,a Anamika Dey a and Parameswar Krishnan Iyer a,b* Centre for Nanotechnologya and Department of Chemistry,b Indian Institute of Technology Guwahati, Guwahati–781039, Assam, India. KEYWORDS. Polymer Solar Cells; Gold Nanoparticles; Anode and Cathode Interfaces; P3HT:PCBM; Active Conjugated Polymer Blend.

ABSTRACT. Specific shaped plasmonic gold nanoparticles (AuNPs) and their combined effect with double cathode interfacial layer for increasing the power conversion efficiency (PCE) of poly(3-hexylthiophene):(6,6)-phenylC61/71-butyric acid methyl ester (P3HT:PC61/71BM) based polymer solar cells (PSCs) are methodically established. Two diverse donor-acceptor blend polymers, P3HT:PC61BM and P3HT:PC71BM were used here along with BPhen and BCP as the buffer layers of the conventional LiF:Al cathode contact. Initially, four specific shaped AuNPs, viz. CTAB capped gold nanorod (AuNRs), gold nanosphere (AuNSs), gold nano-oval (AuNOs) 1

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and gold nano branch (AuNBs) were separately synthesized and blended alongwith poly(3,4ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) hole transporting layer to obtain four novel nanoparticle doped hole injecting layers and its effect on P3HT:PCBM based polymer photovoltaic device performance was methodically analyzed. The synthesized nanoparticles were characterized by various characterization technique viz., UV-Vis absorption study and TEM analysis to confirm their size and shape. The intention of varying the shape of the NPs was to improve the light scattering phenomenon within the polymer solar cell for the generation of higher photo current. From outputs of the fabricated solar cells, it has been found that both types of donor-acceptor polymer blends showed higher PCE value in presence of AuNRs modified PEDOT:PSS with Bathocuproine (BCP) as additional buffer layer and LiF-Al combination as an conventional cathode in comparison with Bathophenanthroline (BPhen) on the strong basis of better charge transport between BCP-LiF-Al and donor-acceptor blend layers with improved light scattering of the AuNRs at the solar absorption spectra region compared to AuNSs, AuNOs and

AuNBs.

Polymer

based

photovoltaic devices

with

the

following

architecture

ITO/PEDOT:PSS+AuNRs/P3HT:PC71BM/BCP-LiF-Al was observed to generate highest efficiency value of 5.83%, Voc=0.58 V, Jsc=15.80 mA/cm2 and FF=63%, whereas, for the structure ITO/PEDOT:PSS+AuNRs/P3HT:PC61BM/BCP-LiF-Al, highest solar cell outputs were 5.53 %, Voc=0.58 V, Jsc=16.28 mA/cm2 and FF=58% presenting the best values for unmodified PCBM with P3HT. These results confirm the significant influence of specific shapes of AuNPs to impact the PCE values of donor-acceptor P3HT:PCBM blend based polymer solar cells in presence of double cathode interfacial layer.

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INTRODUCTION. Over the past two decades, major interest has been devoted for enhancing the performance of third generation- organic/polymer bulk hetero junction (BHJ) solar cells because of its capability to obtain a more expeditious, light weight and economical device in contrast to the first and second generation-silicon wafer as well as CdTe and CIGS photovoltaic technologies. However, their power conversion efficiency (PCE) is still not commensurable to their silicon counterparts. The PCE of organic BHJ solar cell essentially depends on two factors- (1) the photo-absorption capacity of the active blended thin film layers, and (2) the photo-generated charge collecting capacity of the device contact material. Already several approaches exist in literature to improve the photovoltaic properties by reducing these problems. For improving the point (1), the most commonly used techniques are the inclusion of diffraction gratings, periodic nanostructures, plasmonic excitation using metallic nanoparticles (NPs) and a combination of metallic NPs structure and gratings.1-4 It has been found recently that by trapping light on inducing the plasmonic metal nanoparticles in various thin films of BHJ solar cell is a useful approach for better photo absorption in BHJ solar cell devices.5-7 The incorporation of metal NPs can improve the light absorption by following two different mechanisms: first the enhancement in the forward scattering cross section and second the near-field improvement. When the NPs are smaller and analogous in dimensions than the incident light wavelength, a much stronger interaction among the free conduction electrons within the metal as well as the electromagnetic radiation results. This resonance results when the frequencies of the light and oscillating electrons match and are called localized surface plasmon resonance (LSPR) in nanometer dimensions. When excited, the surface plasmon can decay by radiative or non-radiative method, followed by scattering 3



or absorption of light, due to which there is an increase in the internal absorption path length. It is known that the electromagnetic field can be enhanced by the LSPR from nanostructures and noble metal NPs thereby facilitating light absorption and hence generating excess excitons within the solar cell device. Further, the nanostructures and the metal NPs are able to scatter the incident photons and generate longer propagation pathways.8,9 Additionally, another main advantage with metallic nanoparticles is that it does not compromise the structure of the BHJ solar cell since they can be easily mixed in any thin film layers of the device. Intriguingly, it has been found that specific shapes and sizes of the metallic nanoparticles significantly exerts its influence on the PCE of BHJ solar cell as the plasmonic property of any metallic NPs directly depends on its dimensionality. It also depends on the size, concentration as well as their location in the polymer solar cell film to influence and improve the performance of the device. If the size of AuNPs decrease, the aspect ratio will be reduced. Generally, the plasmonicity of AuNRs is directly proportional to its aspect ratio. Therefore, by reducing the size of gold nanorods further it is difficult to improve the performance of polymer solar cell. It is already reported that gold nanorods (AuNR) are more plasmonic than the other different shaped gold nanoparticles. Again, due to the geometrical configuration it can scatter more incident photon compared to others.10,11 Further, in order to improvise point (2), the interfacial layers of the contact materials are of very important significance for increasing the overall efficiency of organic solar cell devices by modifying metal work function of the contact and enhancing the selectivity of specific charge carrier.12-19 The judicious combination of a specific additional interfacial layer and a cathode electrode improves the short circuit current density (Jsc) and the open circuit voltage (Voc) on varying the built-in4


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potential produced by the difference in contact work function which drives the electrons and holes towards the direction of their respective contacts because of which it helps overall to increase the PCE. Further BCP or BPhen layer can improve the hole blocking capacity at the interface of active layer and LiF/Al contact. Additionally, BCP or BPhen also prevent direct penetration of Al in the active blend polymer layer. This study demonstrates the combined impact of using specific shaped plasmonic gold nanoparticles (AuNPs) alongwith double cathode interfacial layer for enhancing the PCE of P3HT:PCBM based bulk hetero junction (BHJ) solar cell devices. Two different donor-acceptor blend polymers, P3HT:PC61BM and P3HT:PC71BM along with BPhen and BCP as the buffer layers of the conventional LiF-Al cathode contact were used in order to demonstrate the effects. Initially, four specific types of AuNPs, viz. CTAB capped gold nanorod (AuNRs), gold nanosphere (AuNSs), gold nano-oval (AuNOs) and gold nano branch (AuNBs) were synthesized separately and then blend along with PEDOT:PSS hole transport layer to form four newly doped hole injecting layers and their effect on improving the PCE of donor-acceptor based polymer solar cell was systematically analyzed. From outputs of the fabricated solar cells, it has been found that both types of donor-acceptor polymer blends showed higher PCE value in presence of AuNRs modified PEDOT:PSS with BCP as an additional buffer layer and LiF-Al combination as conventional cathode electrode compared with BPhen on the strength of better charge transport between BCP-LiF-Al and donor-acceptor blend layers with improved light scattering of the AuNRs at the solar absorption spectra region compared to AuNSs, AuNOs and AuNBs. BHJ devices with ITO/AuNRs+PEDOT:PSS/P3HT:PC71BM/BCP-LiF-Al architecture demonstrated highest efficiency values of 5.83%, Voc=0.58 V, Jsc=15.80 mA/cm2 and FF=63%, whereas, for the architecture ITO/AuNRs+PEDOT:PSS/P3HT:PC61BM/BCP-LiF-Al, solar cell outputs were 5



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5.53 %, Voc=0.58 V, Jsc=16.28 mA/cm2 and FF=58%. These result are amongst the maximum reported values with P3HT:PCBM polymer based solar cells and these enhanced values confirm the effect of shape variation of AuNPs to impact the PCE of donor-acceptor based polymer solar cells in presence of double cathode interfacial layer. EXPERIMENTAL SECTION. Synthesis of Specific Shaped AuNPs. Four specific types of AuNPs namely, AuNBs, AuNSs, AuNOs and AuNRs were synthesized in this study by seed-mediated method.20,21 The schematic representation of the theme of this work is demonstrated in Figure 1.

Figure 1. Step-by-step schematic representation of the theme of the present work. The detail synthesis protocols of all the NPs are given in the supporting information file.

6


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Figure 2. Transmission Electron Microscopic (TEM) images of the synthesized (a) AuNBs (b) AuNSs (c) AuNOs and (d) AuNRs respectively. The recorded UV-Vis spectra and the TEM images of each of the NPs is presented in Figure 2 and Figure S1 (Supporting Information) respectively. The plasmonic peaks for AuNBs, AuNSs, AuNOs and AuNRs were observed at 636 nm, 541 nm, 543 nm and (532 nm, 762 nm) respectively [Figure S1] and their average sizes were nearly 20 nm-AuNBs, 10 nm-AuNSs, 15 nm-AuNOs and 18 nm-AuNRs (Figure 2). Thin Film Morphology Study. The plasmonic BHJ devices were fabricated over commercial ITO-coated on glass substrates (RSheet ≈15Ω/sq., Sigma Aldrich, India) and the device architectures with the structure of P3HT and PCBM are schematically represented in Figure 3.

7



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Figure 3. Schematic of the fabricated device structures with modified HIL by (a) AuNBs, (b) AuNSs (c) AuNOs and (d) AuNRs respectively. The structure of (e) P3HT donor, (f) PC61BM acceptor and (g) PC71BM acceptor are also represented, combination of which are used for making the active layer blend solutions. The detailed explanation of the device fabrication method is mentioned in the SI file. Before fabricating the devices, the influence of these four specific shaped AuNPs on the active layer thin film morphology were studied via GISAXS (Grazing Incidence Small Angle X-ray Scattering) and the 2D q-plots are shown in Figure 4.22-27 The synthesized four specific types of AuNPs were first blended with PEDOT:PSS, hole injecting layer at an doping concentration of 20% (v/v), to obtain four specific NPs doped HIL. Following this, all the HILs were carefully spin casted over the ITO films at 3000 rpm followed by heating up to 120°C for 30 minutes. After cooling down the substrates, P3HT:PC71BM layer (~110 nm) was spin coated over it.

8


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Figure 4. GISAXS images P3HT:PC71BM blend polymer on top of specific shaped nanoparticle modified HIL (a) without NPs, (b) AuNBs, (c) AuNSs, (d) AuNOs and (e) AuNRs respectively. From the Figure 4b to Figure 4e it was confirmed that all the four different shaped AuNPs doped films of PEDOT:PSS were identical with each other whereas the sample with bare PEDOT:PSS showed a distinct scattering pattern (Figure 4a). A correlation peak observed for all the four films are evidence for a somewhat distorted morphology and was confirmed by plotting the azimuthal integrated correlation peaks [Figure S2, SI]. Thus, the sample with bare PEDOT:PSS displayed a narrower peak whereas the PEDOT:PSS film having AuNPs were broader which indicated that the AuNPs led to a minor disorder in the active layer morphology. By incorporating AuNPs in the PEDOT:PSS layer here, two physical phenomenon are taking places simultaneously- (a) 9



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absorption of the incident photon and (b) scattering of incident light. By studying the GISAXS measurement it was proved that since the size of AuNPs were very small compared to PEDOT:PSS it does not affect the morphology of the HIL or active layer, whereas it increases the absorption path length through the scattering phenomena. There were total sixteen specific device architectures fabricated in order to analyze this dual effect as listed in Table 1. For reference purpose we have also recorded the performance of the BHJ devices without AuNPs along with double cathode interfacial layer, which are listed in the supporting information file (Table S1). The J-V characteristics and the EQE of the photovoltaic cells were characterized using Oriel Sol 3A solar simulator and Oriel IQE-200 instrument respectively. The effective area of all the cells are 3 mm × 2 mm which is defined by the cross-sectional area of the cathode and anode electrodes deposited using shadow mask. Table 1. Fabricated plasmonic BHJ solar cell configurations with specific shaped AuNPs and double cathode interfacial layer. Polymer Cathode blended active interfacial layer HIL= PEDOT:PSS+ layer (PBAL) (CIL)

Device architecture on ITO coated glass substrate

(a) AuNBs

(1) HIL (a) / PBAL (i) / CIL (A)

(b) AuNSs

(2) HIL (b) / PBAL (i) / CIL (A)

(c) AuNOs

(3) HIL (c) / PBAL (i) / CIL (A)

(i) rrP3HT:

(d) AuNRs

(4) HIL (d) / PBAL (i) / CIL (A)

PC61BM

(a) AuNBs

(5) HIL (a) / PBAL (i) / CIL (B)

(b) AuNSs

(6) HIL (b) / PBAL (i) / CIL (B)

(c) AuNOs

(7) HIL (c) / PBAL (i) / CIL (B)

(d) AuNRs

(8) HIL (d) / PBAL (i) / CIL (B)

(A)BPhen-LiFAl

(B)BCP-LiF-Al

10


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(a) AuNBs

(9) HIL (a) / PBAL (ii) / CIL (A)

(b) AuNSs

(10) HIL (b) / PBAL (ii) / CIL (A)

(c) AuNOs

(11) HIL (c) / PBAL (ii) / CIL (A)

(ii) rrP3HT:

(d) AuNRs

(12) HIL (d) / PBAL (ii) / CIL (A)

PC71BM

(a) AuNBs

(13) HIL (a) / PBAL (ii) / CIL (B)

(b) AuNSs

(14) HIL (b) / PBAL (ii) / CIL (B)

(c) AuNOs

(15) HIL (c) / PBAL (ii) / CIL (B)

(d) AuNRs

(16) HIL (d) / PBAL (ii) / CIL (B)

(A)BPhen-LiFAl

(B)BCP-LiF-Al

RESULTS AND DISCUSSION Photovoltaic Characterizations. The mechanism involved in the P3HT:PC71BM plasmonic photovoltaic devices, with four specific shaped AuNPs doped hole injecting layers with double cathode interfacial layer are schematically depicted in Figure 5 and Figure 6. In this study, two different physical phenomenon, i.e., (1) the plasmonic effect of specific shaped AuNPs and (2) the influence of double cathode interfacial layer, are simultaneously acting together and helping to improve the photovoltaic parameters of the BHJ cells. It has been found that in both the polymer active blend materials the PCE value rises with the combination of PEDOT:PSS+AuNRs based anode buffer layer and BCPLiF double cathode interfacial layer. Figure 7 and Figure 8 represent the J-V characteristics and EQE graph of plasmonic BHJ solar cell devices. For devices having configuration (8) the maximum efficiency ɳ = 5.53%, Jsc = 16.28 mA/cm2, Voc= 0.58 V, and FF= 58.6% for P3HT:PC61BM was observed, while, highest ɳ seen in device architecture (16) with P3HT:PC71BM, is 5.83%, Voc=0.58 V, Jsc=15.77 mA/cm2and FF= 11



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63.5%. An analogues behavior was seen when BCP-LiF-Al is used instead of BPhen-LiFAl.

Figure 5. Schematic representation of the mechanism involved for P3HT:PC71BM BHJ solar cell with specific shaped AuNPs viz., (a) AuNBs, (b) AuNSs, (c) AuNOs and (d) AuNRs and the double cathode interfacial layer (BPhen-LiF).

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Figure 6. Schematic representation of the mechanism involved for P3HT:PC71BM BHJ solar cell with specific shaped AuNPs viz., (a) AuNBs, (b) AuNSs, (c) AuNOs and (d) AuNRs and the double cathode interfacial layer (BCP-LiF). It was observed that for device (4) the maximum ɳ=5.23%, Voc=0.57 V, Jsc=16.17 mA/cm2 and FF= 56.7% for P3HT:PC61BM system. While, the maximum ɳ for device (12) with P3HT:PC71BM, was 5.55%, Voc=0.57 V, Jsc=15.75 mA/cm2, and FF= 61.9%. Incorporation of metallic nanoparticle in PEDOT:PSS layer have direct effect in increasing the crystallinity of the active layer morphology. As the crystallinity increases, 13



donor-acceptor interface also increases as a result of which enhancement in short-circuit current was observed in case of the fabricated devices. But at the same time, we can have a negligible or even negative impact on the device Voc which is due to the increased recombination losses at the donor-acceptor interfaces. Due to these reasons even with a higher Jsc value we observed relatively lower Voc. Similarly, incremental patterns were observed in the EQE measurements also. From EQE studies it has been noticed that when BCP was used as the second cathode layer highest quantum efficiency was observed as compared to BPhen in between the ultraviolet to near infrared wavelength range (300-700 nm), with EQE of ≥ 60% for P3HT:PC61BM and ≥ 65% for P3HT:PC71BM with all specific shaped metal NPs HIL. EQE studies for all the devices were done under ambient conditions (outside the glove box) without encapsulation at higher humidity levels (~80%). The natural humidity level of the geographical location our laboratory is situated is very high and it is very difficult to control (Figure S3) the humidity levels. Further our EQE instrument is presently situated outside the glove box. Hence, due to the fast degradation of BCP, BPhen and the blend active layers, very high increments were not observed in the EQE spectra. This statement is also supported by the integrated Jsc value obtained from EQE spectra. For analyzing the effect of double cathode interfacial layer, it was observed that, BCP containing double layer cathode compared to BPhen showed better device performance which is because of the improved charge transport between BCP-LiF-Al electrode and the donor-acceptor polymer blend layers compared to the cathode contacts BPhen-LiF-Al.28 Furthermore, the selectivity towards electrons in case of BCP is higher because of the >3eV energy gap (Eg) and a deeper 7eV HOMO energy level due to which it can easily allow to pass electrons and block holes by diminishing the 14


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charge carrier recombination between donor-acceptor polymer blend active layers and the cathode contacts [Figure 5, 6 and Figure S3, S4]. Again for analyzing the effect of specific plasmonic AuNPs, it was noticed that devices with AuNBs+PEDOT:PSS showed inferior performance on equating with AuNSs+PEDOT:PSS and AuNOs+PEDOT:PSS, whereas with AuNRs+PEDOT:PSS the devices showed best performances compared to all. This can be because of higher surface plasmon influence of AuNRs compared to others thereby enhancing the scattering, trapping and absorption length of the incident photon during its path via the new HIL [Figure 5, 6 and Figure S3, S4]. This statement was also supported by UV-Vis study [Figure S1] from which it was clearly visible that compared to the other AuNPs, AuNRs have been observed to cover almost the entire UVVis spectrum (400nm-900nm) by its two characteristic surface plasmon resonance peaks. Hence, AuNRs were able to increase the photo absorption by enhancing the scattering in forward cross section and the near-field improvement which further helped to enhance their PCE values. The device parameters obtained from all the devices with different configurations, mentioned in Table 1, are summarized in Table 2. These results are amongst the maximum reported values with P3HT:PCBM based BHJ devices and the results successively explain the influence of shape of AuNPs to magnify the PCE value of donor-acceptor based polymer solar cells in presence of double cathode interfacial layer and the morphology very well characterized by GISAXS measurements. Different approaches are mentioned in literature for improving the polymer solar cell performances by designing new molecules and via device structure modulations. Table S2 provides a summary of recent data on P3HT:PCBM based bulk heterojunction solar cells having different device architecture modification in cathode buffer layers and incorporation of 15



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metallic NPs in various layer of BHJ solar cell for getting efficient device performance. Among them modulation in cathode contact by inserting small molecule or polymer buffer layer29-33 and in the hole injecting layer with plasmonic NPs are among the most able approaches.34-38 Coalescing metallic NPs in the HIL is relatively the most facile approach. Since the polymer PEDOT:PSS are usually dispersed in water medium and because the different shaped AuNPs were easily synthesized from solution method, these NPs can be combined easily with hole injecting material PEDOT:PSS and not requiring any further functionalization.

Figure 7. Current density-Voltage characteristics of (i) P3HT:PC61BM and (ii) P3HT:PC71BM plasmonic BHJ solar cells containing specific shaped AuNPs along with cathode interfacial layers (a) BPhen-LiF-Al, as well as (b) BCP-LiF-Al.

16


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Our observations indicate that aqueous PEDOT:PSS containing AuNPs can significantly enhance the P3HT:PCBM polymer solar cell PCE nearly to 3.19% with conventional cathode contact.37 Further, it was noticed that the PCE of plasmonic solar cell is also influenced by the metal NPs size. AuNPs of 50 nm sized contained within the hole injecting layer are likely to enhance the overall efficiency of the blended BHJ solar cell by up to 4.24%.35 Another group also reported that the amalgamation of gold and silver NPs based alloy in the polymer layer amended the efficiency up to 4.73% with low work function metal (Ca) and aluminum as cathode contact.33

Figure 8. External quantum efficiency spectra of (i) P3HT:PC61BM and (ii) P3HT:PC71BM plasmonic BHJ solar cells containing specific shaped AuNPs along with double cathode interfacial layers (a) BPhen-LiF-Al, as well as (b) BCP-LiF-Al. 17



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Table 2. Summary of BHJ photovoltaic device. Polymer Blended active layer (PBAL)

Cathode HIL= Interfaci al layer PEDOT:PS S+ (CIL)

PCE,ηmax (#ηavg)

Device Config uration

Jsc,max (#Jsc,avg)

Jsc,EQE

(mA.cm-2)

(mA.cm-2) (V)

(%)

(a) AuNBs

(1)

14.51(13.98)

8.65

0.56

53.6

4.36(4.19)

(b) AuNSs

(2)

12.14(11.63)

9.53

0.58

63.3

4.46(4.27)

(c) AuNOs

(3)

15.29(14.35)

10.03

0.58

57.7

5.12(4.79)

(d) AuNRs

(4)

16.17(15.31)

11.27

0.57

56.7

5.23(5.03)

(a) AuNBs

(5)

12.69(12.04)

8.71

0.57

61.2

4.43(4.27)

(b) AuNSs

(6)

13.92(13.56)

9.82

0.58

61.3

4.94(4.82)

(c) AuNOs

(7)

16.04(15.58)

10.26

0.58

57.6

5.36(5.07)

(d) AuNRs

(8)

16.28(15.87)

11.39

0.58

58.6

5.53(5.26)

(a) AuNBs

(9)

14.98(13.83)

9.44

0.57

56.7

4.78(4.47)

(b) AuNSs

(10)

14.78(14.01)

10.93

0.58

62.0

5.31(5.04)

(c) AuNOs

(11)

15.35(15.10)

11.54

0.58

59.8

5.32(5.24)

(ii) P3HT:

(d) AuNRs

(12)

15.75(15.08)

12.01

0.57

61.9

5.55(5.35)

PC71BM

(a) AuNBs

(13)

16.15(15.32)

9.32

0.57

54.2

4.97(4.74)

(b) AuNSs

(14)

16.08(15.46)

11.12

0.58

60.1

5.61(5.39)

(c) AuNOs

(15)

15.87(15.23)

11.64

0.58

61.7

5.68(5.45)

(d) AuNRs

(16)

15.77(15.41)

12.41

0.58

63.5

5.83(5.67)

(A) BPhenLiF-Al (i) P3HT: PC61BM (B)BCP -LiF-Al

(A)BPh en-LiFAl

(B)BCP -LiF-Al

Voc,max

FFmax

(%)

# Average value of ten different devices. These results confirm that the PCE values for both the active layer of polymer blended devices along with the combined effects of specific shaped plasmonic gold nanoparticles (AuNPs) and the double cathode interfacial layer has been successfully achieved. Initially, four specific types AuNPs, viz. CTAB capped gold nanorod (AuNRs), gold 18


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nanosphere (AuNSs), gold nano-oval (AuNOs) and gold nano branch (AuNBs) were synthesized separately. These four coalesced NPs solutions are then commixed together with the hole injecting PEDOT:PSS layer to compose four new NPs doped hole injecting layers for BHJ solar cell. The plasmonic metal NPs were introduced within the PEDOT:PSS thin film layer to increase the photocurrent of the fabricated BHJ solar cell by increasing optical absorption and scattering in both the UV and visible wavelength range inside the devices. For dual cathode interfacial layer, two organic small molecules, BPhen and BCP were used with LiF and aluminum cathode as contact to improve the charge collection. For both the active layers systems the efficiency enhances significantly with gold nano-rods contained hole injecting layer and double cathode buffer layer contact as BCP-LiF-Al. Moreover, the results summarized in this article describe profoundly a simplistic and very easy technique for high performance P3HT:PC71BM and P3HT:PC61BM based polymer solar cell with PCE close to 6 %, which is among highest for P3HT based photovoltaics and unmodified PCBM. This technique can also be useful for improving the PCE value of other low band gap polymer and non-fullerene based organic solar cells. CONCLUSIONS Combined effects of incorporating specific shaped plasmonic gold nanoparticles (AuNPs) and the double cathode interfacial layer in an P3HT:PC61BM and P3HT:PC71BM derived bulk heterojunction solar cell devices was methodically studied. Initially, four specific types AuNPs, viz. CTAB capped gold nanorod (AuNRs), gold nanosphere (AuNSs), gold nano-oval (AuNOs) and gold nano branch (AuNBs) were synthesized separately and then 19



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blend with PEDOT:PSS hole transport layer to form four newly doped hole injecting layers and their effect on improving the overall performance of the P3HT:PCBM BHJ solar cell devices were methodically analyzed. The significance and impact of NPs mixed HIL on the overall device performances were carefully studied and compared with state of the art devices. It was noticed that the PCE was enhanced remarkably in case of both the active blend systems, in presence of AuNRs modified PEDOT:PSS with Bathocuproine (BCP) as additional buffer layer and LiF-Al as an conventional cathode electrode compared to Bathophenanthroline (BPhen) on the strength of better charge transport between BCP-LiF-Al and donor-acceptor blended layers with improved light scattering of the AuNRs at the solar absorption spectra region compared to AuNSs, AuNOs

and

AuNBs.

Devices

with

architecture

ITO/AuNRs+PEDOT:PSS/

P3HT:PC71BM/BCP-LiF-Al showed highest efficiency value 5.83% with Jsc=15.80 mA/cm2, Voc=0.58 V, and FF=63%, whereas, for the structure ITO/AuNRs+PEDOT:PSS/ P3HT:PC61BM/BCP-LiF-Al, the solar cell outputs showed highest efficiency of 5.53 %, Jsc=16.28 mA/cm2, Voc=0.58 V, and FF =58%. These outputs conclusively describe a very simple method involving the combination of specific shaped plasmonic gold nanoparticles (AuNPs) plus the double cathode interfacial layer to improve the PCE as well as the overall device performance of this multilayer polymer bulk heterojunction solar cells with P3HT-PCBM and that can be extended to other systems as well. The presented method can also be applicable for other non-fullerene acceptor systems. However, the cathode interfacial layer may have to change according to the band energy alignment of the non-fullerene acceptor. ASSOCIATED CONTENT 20


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Supporting Information. The Supporting information file consists of the synthesis protocol, device fabrication method, GISAXS images and UV-Vis spectra of metal NPs. Alongside, the energy band diagram of P3HT:PC61BM blend polymers with BCP and BPhen contact are also given. “This material is available free of charge via the Internet at http://pubs.acs.org.” AUTHOR INFORMATION Corresponding Author *E-mail id: [email protected] ORCID Ashish Singh- (0000-0002-5705-4267), Anamika Dey- (0000-0002-1959-9223) and Prof. Parameswar K. Iyer (0000-0003-4126-3774) Funding Sources 1. Department of Electronics & Information Technology, DeitY Project No. 5(9)/2012-NANO (Vol. II), 2. Department of Science and Technology (DST) (No. DST/SERB/EMR/2014/000034), and 3. Max-Planck-Gesellschaft, Germany (No. IGSTC/MPG/PG(PKI)/2011A/48) for financial support. 4. Ministry of Human Resource Development (FAST) Notes Authors declare no competing financial interests.

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ACKNOWLEDGMENT This work has been financially supported by Department of Electronics & Information Technology [5(9)/2012-NANO (Vol. II)], Ministry of Human Resource Development, Department of Science and Technology (DST) sponsored projects [DST/TSG/PT/2009/23, DST/SERB/EMR/2014/000034] and CIF, IIT Guwahati for providing instrument facilities. The authors remain thankful to Dr. H. M. A. Ehmann, Dr. Prasad Gosavi and Mr. Rajkumar Somendrajit Singh of Anton Paar for GISAXS measurements. REFERENCES

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Table of Contents Entry

28


Impact of Specifically Shaped Plasmonic Gold Nanoparticles and a

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