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Apr 5, 2016 - Department of Chemistry, National Institute of Technology, Agartala, 799046 Tripura, India. ABSTRACT: We have reported the green synthes...
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Silver Nanoparticles: Synthesis and Its Nanocomposites for Heterojunction Polymer Solar Cells Prasanta Sutradhar, and Mitali Saha J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 05 Apr 2016 Downloaded from http://pubs.acs.org on April 5, 2016

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Silver Nanoparticles: Synthesis and Its Nanocomposites for Heterojunction Polymer Solar Cells Prasanta Sutradhar and Mitali Saha* Department of Chemistry, National Institute of Technology, Agartala -799046, Tripura, India

* Corresponding author: Dr. Mitali Saha, Assistant Professor, Department of Chemistry, NIT, Agartala; Email: [email protected] ; Tel: +918974006400

Abstract We have reported the green synthesis of silver nanoparticles (AgNPs) using the aqueous extract of Sapodilla (Manilkara zapota) fruit as non-toxic and eco-friendly reducing material. The synthesized AgNPs were characterised and used for preparing nanocomposites with GO and PEDOT:PSS. We have compared the performance of polymer cells based on Poly(3,4 ethylenedioxy- thiophene):poly(styrene sulfonate) (PEDOT:PSS) with AgNPs incorporated in graphene oxide (GO). The constructed structure of the solar devices was ITO-AgNPs-GOPEDOT:PSS-Al. We have investigated the influence of polymer as a hole extraction layer and GO as electron extraction layer on the performance of bulk heterojunction polymer solar cells. The maximum power conversion efficiency (PCE) was found to be 3.98% under illumination of 100 mW/cm2.

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Introduction Green synthesis of silver nanoparticles (AgNPs) using plant extracts has developed enormous interest1-12 as it has significant advantages in many aspects like yield, ease of preparation, rate of formation, health and environmental care, capital investment, etc. Although the exact mechanisms are still under dispute, it is suggested that these plant extracts act as both reducing and capping agents for the production of AgNPs. It is reported in a number of works that compounds containing carboxyl, amine, aldehyde/ketone groups, polyols and proteins, geranial or polysaccharide are somehow responsible for the formation and stabilization of NPs13,14. In the last decade, the processes and new materials for solar cells have been extensively studied in order to enhance the performance and stability of the cells15-17. Owing to its extraordinary mechanical, electrical, optical, and thermal properties, the twodimensional (2D) single atomic-thick sp2-hybridized carbon sheet of graphene has quickly emerged as an attractive candidate for energy applications23-26. However, graphene sheets without functionalization are insoluble and infusible with limited practical applications, whereas the functionalization of graphene, allows tunability of optoelectronic properties while retaining good solubility in water or polar organic solvents27, 28. Of particular interest, GO materials have been used in every part of photovoltaic solar cell (PSC) devices, including as electrodes, charge extraction layers, and in the active layer29-36. Recently, stable and highperformance PSCs have been successfully fabricated with stretchable GO and GO-based composites as electron- and/or hole-extraction layers37-43. Nanocomposites based on graphene oxide (GO) (or reduced GO), polymers and AgNPs have drawn wide attention due to their extensive applications in photovoltaic devices18-22. Yuan et al18, proposed a solution processable GO-AgNPs in photovoltaic solar cells (PSCs) at the interface of ITO-PEDOT:PSS with an enhanced SPR effect and they found that the incorporation of AgNPs-GO into PBDTTT-C-T:PC71BM-based PSCs dramatically improved the power conversion efficiency(PCE) up to 7.54%. Recently, combination of poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) has been found to be the most successful conducting polymer for practical applications. In addition, PEDOT:PSS is highly transparent in the visible range and generally has a work function of about 5.0 eV, so it is used for hole collection in organic solar cells44-47. It is well known that the photovoltaic (PV) device performance is mainly defined by the PCE which depends on several parameters such as light harvesting properties of the active

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layer, generation of excitons, exciton diffusion and separation, transportation and collection by the cell electrodes, etc48. Like many other polymeric thin film devices, the interfaces in photovoltaic solar cells (PSCs) play critical roles in regulating the charge separation and charge collection, and hence the overall device performance49,50. Moreover, a hole-extraction layer (HEL) between the anode and the active layer, as well as an electron-extraction layer (EEL) between the cathode and the active layer, are essential for achieving maximum PSC device efficiency and lifetime51-53. In continuation of our earlier studies on the green synthesis of metal nanoparticles5457

, we have now focused on the one pot synthesis of AgNPs using aqueous extract of

sapodilla fruits. Sapodilla (Manilkara Zapota) fruit is a good source of polyphenols and its leaf as well as seed extracts were already being used to synthesize AgNPs58,59. In this work, we have used GO and PEDOT:PSS with AgNPs, where PEDOT:PSS has acted as an excellent hole-extraction layer and GO has acted as an electron-extraction material in PSCs. The short circuit photocurrent (Jsc), open-circuit photovoltage (Voc), fill factor (FF) and PCE (η) were measured, in order to investigate the efficiency of the device. Experimental Materials and methods

The ripen sapodilla fruits were collected from local area. Silver nitrate and methanol were purchased from Sigma Aldrich. GO was prepared by modified hummers method60. Double distilled water was used throughout the experiments. The nano-morphologies of AgNPs and AgNPs-GO were characterized by atomic force microscopy (AFM, multimode V8), scanning electron microscopy (SEM, JSM-6360 JEOL), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM) (JEOL, JEM 2100F). The TEM and HRTEM samples were prepared by transferring AgNPs-GO onto a copper grid. The absorption spectra of AgNPs, GO and AgNPs-GO were collected from UV-Vis spectrophotometer (Shimadzu 1800). The steady-state photoluminescence spectra were measured using Florescence spectrophotometer (Perkin Elmer LS 55). The particle size distribution analysis was carried out by using Dynamic light scattering (DLS: Nanotrac wave W3222). The X-ray diffraction spectroscopy (XRD) was performed with Bruker D8 ADVANCE. The current density-voltage (J-V) curves were investigated under simulated AM 1.5 G (100 mW/cm2, PGSTAT 101 solar simulator) irradiation. The current–voltage

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characteristics of the cell were measured by applying external potential bias to the cell and measuring the generated photocurrent. The monochromator was incremented through the visible spectrum to generate the IPCE (incident photon to current conversion efficiency). Parameters such as short circuit photocurrent (Jsc), open-circuit photovoltage (Voc), fill factor (FF) and PCE were measured for the thin films. Preparation of aqueous fruit extract and synthesis of AgNPs

Sapodilla fruits were separately washed with double distilled water. The peels and seeds were removed from these fruits and the whole mass was squeezed to get the paste. 5 g of the paste, taken in 20 ml distilled water was stirred for 30 min at 600C using a magnetic stirrer and then filtered. The mixture of 1 g of AgNO3 and fruit extract was mixed and stirred at room temperature, resulting in the formation of AgNPs after 30 min. The solution was then centrifuged and filtered, washed with water and methanol to remove the organic groups from the surface of the AgNPs and dried in hot air oven for 4 hours. The possible mechanism of the formation of AgNPs from both the extracts was shown in reaction scheme-1.

Scheme-1: Reduction of Ag+ to Ag(0)

Preparation of AgNPs-GO nanocomposites and Device Fabrication To study the photovoltaic performances, nanocomposites were prepared using AgNPs, GO and PEDOT:PSS. 5 mg of GO was first sonicated in distilled water for 30 min, followed by addition of 10 mg of AgNPs with sonication to obtain a solution. Sonication was further continued for 1 h with simultaneous stirring. 1 mL of PEDOT:PSS and water was then

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added to the brown aqueous solutions of AgNPs-GO and again stirred for 1 h to prepare a thick slurry. The resultant thick solutions were spread on ITO coated glass substrate by doctor blade method and the thin films obtained were dried at 500-600C for 3 h. The preparation of nanocomposites of AgNPs-GO-PEDOT:PSS is presented in scheme-2. The device architecture of solar cell was presented as ITO-AgNPs-GO-PEDOT:PSS-Al. The band diagram of the device was also studied.

Scheme 2:- Preparation of nanocomposites AgNPs-GO-PEDOT:PSS.

Results and discussion Characterization of AgNPs

The reaction mechanism in the scheme-1 showed that, AgNPs can be obtained through the reduction of Ag+ using the fruit extract, containing ascorbic acid, which acted as both reducing as well as capping agent during the synthesis. As illustrated in the scheme-1, ascorbic acid served as a stable (electron + proton) donor during interactions and was first converted into the radical ion called “semihydro-ascorbic acid” and then dehydro-ascorbic acid through oxidation. It was suggested that dehydro-ascorbic acid and ascorbic acid together constituted the redox system which was enough to reduce Ag+ to Ag (0). Also, the

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lone pair electrons at the polar groups of ascorbic acid may have occupied two sp orbitals of the Ag ion to form a complex compound. Thus, ascorbic acid was capped with silver ions and then gave Ag(0) nanoparticles through reduction of Ag+ inside the nanoscopic templates. In the presence of nanoscopic templates, small AgNPs were easily formed. The UV-visible spectrum (Figure 1a) of neat aqueous extracts of sapodilla fruits showed maximum absorption peaks at 257 nm, which confirmed the presence of active compound, ascorbic acid in the fruit extracts. The synthesis was initially monitored by the colour change (to dark brown) occurring during the reaction period (Figure 1b) and this colour change was obviously due to excitation of surface plasmon resonance in the metal nanoparticles, indicating the formation of AgNPs. The UV-Vis spectra of AgNPs showed the absorption peak at 426 nm (Figure 1c), which confirmed the formation of AgNPs. Figure 1d showed the room temperature photoluminescence spectra of AgNPs with excitation peaks at 667 nm for AgNPs.

Figure 1:- (a) UV-Vis spectra of Sapodila fruit extract; (b) Colour change photo of 1aqueous solution of silver nitrate, 2- aqueous solution of Sapodila fruit, 3- synthesised AgNPs; (c) UV-Vis spectra of synthesised AgNPs; (d) Fluorescence spectra of synthesised AgNPs.

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The particle size distribution of the AgNPs was carried out at 250C, which confirmed the size of AgNPs within 40-70 nm, where the average particle size of the AgNPs was around 60 nm (Figure 2a). Figure 2(b & c) showed the surface morphology of synthesized AgNPs. The SEM image clearly indicated the formation of spherical AgNPs having size of 50-60 nm. AFM showed (Figure 3(a- d) the topographic image of well-dispersed AgNPs, which further confirmed the size of the AgNPs (Figure 4a) of around 50-70 nm.

Figure 2:- (a) DLS; (b) & (c) SEM images of synthesised AgNPs.

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Figure 3:- (a)-(d) AFM images of synthesised AgNPs.

Figure 4(a- c) showed the TEM and HRTEM images of AgNPs synthesized at 400C. The TEM micrographs of the AgNPs confirmed that the particles were spherical in shape having the size of 45-60 nm. However, a maximum number of particles were found to be of 50 nm. From Figure 4c it was found that the lattice fringe spacing in the HTREM image is 0.42 nm. The selected area diffraction patterns (Figure 4d) of AgNPs revealed that the particles are crystalline in nature. XRD pattern (Figure 5) showed the crystalline nature of AgNPs as face centered cubic. The intense diffraction peaks at 38°, 43°, 65° and 73° corresponded to the planes (111), (200), (220) and (311), respectively.

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Figure 4:- (a) & (b) TEM; (c) HR-TEM; (d) SAED pattern images of synthesised AgNPs.

Fig. 5:- XRD pattern of AgNPs

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Figure 6(a- c) showed the TEM images of AgNPs/GO nanocomposite. From the data, it can be seen that the AgNPs were distributed nicely on the surface of the GO and the diameter of the AgNPs were matched with the previous figure, where the size of the particles were around 50-60 nm. Figure 6(d & e) showed the HRTEM image of the nanocomposite. The selected area diffraction patterns (Figure 6f) of AgNPs/GO nanocomposite also revealed that the silver nanoparticles are crystalline in nature.

Figure 6:- (a)-(c) TEM; (d) & (e) HRTEM; (f) SAED pattern images of AgNPs/GO nanocomposites.

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Figure 7a showed the AFM image of AgNPs/GO nanocomposite, where the nanoparticles were nicely distributed on the surface of the GO layer and the diameter (40-60 nm) were also matched with the synthesised AgNPs. XRD pattern of AgNPs/GO nanocomposite (Figure 7b) showed the characteristics peaks of crystal planes of AgNPs at 37.2°, 45°, 63°, and 74.47° corresponding to the planes (111), (200), (220) and (311), respectively, whereas the peaks at 10.83° was due to (001) plane of GO. The slight difference in the diffraction angles between synthesised AgNPs and AgNPs in nanocomposite, may be attributed due to the preferred orientation of the crystalline silver nanoparticles.

Figure 7:- (a) AFM & (b) XRD pattern of AgNPs/GO nanocomposites

Photovoltaic performances The photovoltaic measurements were carried out with the thin films of different nanocomposites using two electrode systems. Xenon lamp was used as a light source and the incident light intensity was maintained at 100 mWcm−2. It is well known that the photovoltaic device performance is mainly defined by the PCE which depends on several parameters such as light harvesting properties of the active layer, generation of excitons, exciton diffusion and separation, transportation and collection by the cell electrodes, etc. Moreover, a holeextraction layer (HEL) between the anode and the active layer, as well as an electronextraction layer (EEL) between the cathode and the active layer, are essential for achieving maximum PSC device efficiency and lifetime. Like

many

other

polymeric

thin

film

devices,

here

also

poly(3,4-

ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) worked as hole transporting

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layer, whereas GO worked as electron transporting layer. The band diagram studies and device structure were shown in Figure 8(a & b). Electrons generated from AgNPs on illumination of light were excited to LUMO energy level and then goes to the ITO cathode through the GO layer. The holes generated on the HUMO energy level of the AgNPs were transported onto the Al (anode) through the PEDOT:PSS layer. Figure 9 showed the current density vs. voltage (J-V) curves of different thin film. Detailed device parameters were presented in the table 1. The power conversion efficiency (PCE) value has dramatically enhanced upon the addition of PEDOT:PSS in the AgNPs-GO nanocomposites.

Figure 8:- (a) band diagram studies; (b) device structure of the fabricated AgNPs-GOPEDOT:PSS.

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Figure 9:- J-V curve of thin film nanocomposites. The device based on GO is the best reference device (type-a) having a Jsc value of 5.66 mAcm-2, Voc value of 0.603 V and FF of 54% resulting in a PCE value of 1.84%. For device type-b (GO/PEDOT:PSS), the increased value of PCE (2.93%) was observed, resulting from the enhancement of Jsc value (7.48 mAcm-2) and FF (56%). AgNPs/GO device (type-c) showed the Jsc value of 8.28 mAcm-2, Voc of 0.722 V and FF of 59% yielding a PCE value of 3.52%. Interestingly, the device with PEDOT:PSS exhibited a better photovoltaic performance, showing Jsc value of

9.20 mAcm-2, Voc value of 0.722 V and FF value of

60% , resulting in remarkable increase of PCE value upto 3.98%. The results showed that the device AgNPs/GO/PEDOT:PSS (type-d) showed a higher PCE value of 3.98% than the devices with only AgNPs/GO. These results indicated that the higher mobility of charges towards both the electrodes were aroused due to the strong anchoring effect of AgNPs on the GO and PEDOT:PSS surface and synergistic effect of nanocomposite thin film which are resoponsible for the enhancement of PCE value. The strong coupling between the SPR effect of AgNPs/GO/PEDOT:PSS and incident light offers the possibility of improved light absorption capacity and corresponding exciton generation rate with enhanced charge collection, resulting in significant promotion in Jsc and thus PCE.

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Table-1:- Photovoltaic Properties of the Devices Type-(a-d) under AM 1.5G Illumination (100 mW cm−2)

Device

Voc

Jsc -2

FF

η

(mAcm )

(V)

Type-a (GO)

5.66

0.603

0.54

1.84

Type-b (GO/PEDOT:PSS)

7.48

0.700

0.56

2.93

Type-c (AgNPs/GO)

8.28

0.722

0.59

3.52

Type-d (AgNPs/GO/PEDOT:PSS)

9.20

0.722

0.60

3.98

(nanocomoposite)

(%)

Figure 10:- (a) Jsc Vs IPCE & (b) FF Vs IPCE of Type-(a-d) thin film. From the figure 10(a & b) we have seen that on increasing the value of Jsc, and FF, incident power conversion efficiency (IPCE) value also increased. Final increments of IPCE (3.98%) are seen when we incorporated PEDOT:PSS in the device (type-d). Where the Jsc increases upto 5.66 to 9.20 mA/cm2, while the FF improves from 0.54 to 0.60. Thus, the charges (electrons and the holes) transfer is more if we use PEDOT:PSS as hole transporting layer.

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However, all parameters (Jsc, Voc, and FF) not very much progressive may be due to the absence of photoactive layer.

Conclusions

In summary, we have developed a facile route for one pot synthesis of well-defined dimensions of AgNPs in bulk amount, using aqueous extract of sapodilla fruit, acting as reducing as well as capping agents. The excellent reproducibility of these nanoparticles, without the use of any additional capping agent or stabilizer will have a great advantage in comparison to microbial synthesis, avoiding all the tedious and hygienic complications. A solution processable AgNPs/GO/PEDOT:PSS nanocomposite thin film was successfully prepared in situ and employed in PSCs results in the improvement of PCE due to the enhanced of SPR effect. The incorporation of Ag NPs-GO into PEDOT:PSS based PSCs dramatically improved the PCE to 3.98% from 3.52%. These results demonstrated that the SPR silver nanocomposite exhibited great potential as an effective alternative approach for high-efficiency PSCs. In addition, the synthetically simple, aqueous-solution processable, cost-effective, and environmentally friendly silver nano-composites make it compatible with the roll-to-roll commercial manufacturing of printable PSCs.

Acknowledgements

We acknowledge Central research facility (CRF), NIT Agartala and NEHU Shillong for characterizations of nanoparticles. Financial assistance from Central Power Research Institute (CPRI), Bangalore is greatly acknowledged. References (1) Balaji, D. S.; Basavaraja, S.; Deshpande, R.; Bedre, M. D.; Prabhakar, B. K.; Venkataraman,

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Figure caption: Figure 1:- (a) UV-Vis spectra of Sapodila fruit extract; (b) Colour change photo of 1aqueous solution of silver nitrate, 2- aqueous solution of Sapodila fruit, 3- synthesised AgNPs; (c) UV-Vis spectra of synthesised AgNPs; (d) Fluorescence spectra of synthesised AgNPs. Figure 2:- (a) DLS; (b) & (c) SEM images of synthesised AgNPs. Figure 3:- (a)-(d) AFM images of synthesised AgNPs. Figure 4:- (a) & (b) TEM; (c) HR-TEM; (d) SAED pattern images of synthesised AgNPs. Figure 5:- XRD pattern of synthesised AgNPs. Figure 6:- (a)-(c) TEM; (d) & (e) HRTEM; (f) SAED pattern images of AgNPs/GO nanocomposites. Figure 7:- (a) AFM & (b) XRD pattern of AgNPs/GO nanocomposites Figure

8:- (a) band diagram

studies; (b) device

structure

of

the fabricated

AgNPs/GO/PEDOT:PSS. Figure 9:- J-V curve of thin film nanocomposites. Figure 10:- (a) Jsc Vs IPCE & (b) FF Vs IPCE of Type-(a-d) thin film. Scheme 1:- Plausible reaction scheme for the Reduction of Ag+ to Ag(0). Scheme 2:- Preparation of nanocomposites AgNPs/GO/PEDOT:PSS. Table-1:- Photovoltaic Properties of the Devices Type-(a-d) under AM 1.5G Illumination (100 mW cm−2)

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