Preparation of plasmonic monolayer with Ag and Au nanoparticles for dye-sensitized solar cells

Accepted Manuscript Research paper Preparation of plasmonic monolayer with Ag and Au nanoparticles for dyesensitized solar cells Da Hyun Song, Hyun-Young Kim, Ho-Sub Kim, Jung Sang Suh, Bong-Hyun Jun, Won-Yeop Rho PII: DOI: Reference:

S0009-2614(17)30812-6 http://dx.doi.org/10.1016/j.cplett.2017.08.051 CPLETT 35061

To appear in:

Chemical Physics Letters

Received Date: Revised Date: Accepted Date:

30 May 2017 21 August 2017 23 August 2017

Please cite this article as: D. Hyun Song, H-Y. Kim, H-S. Kim, J. Sang Suh, B-H. Jun, W-Y. Rho, Preparation of plasmonic monolayer with Ag and Au nanoparticles for dye-sensitized solar cells, Chemical Physics Letters (2017), doi: http://dx.doi.org/10.1016/j.cplett.2017.08.051

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Preparation of plasmonic monolayer with Ag and Au nanoparticles for dye-sensitized solar cells Da Hyun Song1, Hyun-Young Kim,1 Ho-Sub Kim1, Jung Sang Suh1, Bong-Hyun Jun2*and Won-Yeop Rho1,2* 1

Department of Chemistry, Seoul National University, Seoul 151-747, Republic of Korea; [email protected]

2

Department of Bioscience and Biotechnology, Konkuk University, Seoul 143-701, Republic of Korea; [email protected]

* Correspondence: Bong-Hyun Jun: [email protected], Won-Yeop Rho: [email protected]; Tel: +82-2-450-0521, Fax: +82-2-3437-1977

Abstract: We fabricated a plasmonic layer by immobilizing both Au and Ag nanoparticles via P4VP on a photoactive layer for dye-sensitized solar cells (DSSCs) to prevent aggregation of metal nanoparticles, to increase absorbance of N719 dye, and to enhance light harvesting and power conversion efficiency (PCE). The optimal conditions for immobilizing these nanoparticles were also examined. With plasmonic Au and Ag nanoparticles, the PCE increased by 8.05% and 5.78%, respectively (plasmonic Au nanoparticles: from 8.82% to 9.53%; plasmonic Ag nanoparticles: from 8.82% to 9.33%). When both Au and Ag nanoparticles were employed in the plasmonic layer, the PCE showed further improvement of 10.17%, corresponding to a 15.31% enhancement. This significant improvement of the PCE could be explained by a broader range of light absorption resulting from the presence of the plasmonic layer.

Keywords: Plasmonic layer, silver nanoparticles, gold nanoparticles, power conversion efficiency, dye-sensitized solar cells (DSSCs)

1. Introduction Dye-sensitized solar cells (DSSCs) have received widespread attention due to their low cost, light weight, low toxicity, ease of fabrication, customizable design with flexibility, and good performance under diverse illumination conditions [1-5]. The efficiency of the device is however far from optimal, and hence numerous recent studies have focused on developing high-efficiency DSSCs. The power conversion efficiency (PCE) of DSSCs can be improved by several parameters for example, enhancement in the range of solar spectral absorption by employing dyes with strong absorption [6-8], efficient generation of electron-hole pairs [9,10], transportation of generated charge carriers [11-14], and reduction of the loss of charge carriers at the interface or the defects [15-17]. One very promising approach is plasmon coupling [18] which uses plasmonic nanoparticles to trap light in DSSCs. The efficiency of such device depends strongly on the type of plasmonic materials and DSSCs structures [19-23]. Recently, DSSCs with a panchromatic plasmonic monolayer have been demonstrated to endow plasmonic DSSCs with highest efficiency when a mixture of three different types of Ag nanoparticles, including Ag nanoplates, is used [24]. However, individual fabrication of different nanoparticles and their mixing involve a fairly complex and time-consuming process which have limited practical applications. In this study, we fabricated a plasmonic layer in DSSCs by immobilizing both Au and Ag nanoparticles, which are most commonly used nanoparticles in plasmonic DSSCs [25, 26], on a photoactive layer. We explored optimal immobilization conditions of nanoparticles on the plasmonic layer, and subsequently, the effect of the plasmonic layer on the PCE of the device was studied in detail.

2. Materials and Methods

2.1 Synthesis of Ag nanoparticles

Ag nanoparticles were synthesized via the seed-growth method [25]. To prepare seed solution, 0.30 mL of 10 mM silver nitrate (AgNO3) solution was injected into 20 mL of 1 mM trisodium citrate solution and mixed with vigorous stirring for 5 min. Then, 1.8 mL of 10 mM sodium borohydride (NaBH4) was added to the mixed solution and stirred vigorously for another 5 min. The Ag seed solution was aged for 3 h at room temperature. Next, 9 mL of Ag seed solution, 11 mL of distilled water, and 1.2 mL of 20 mM sodium ascorbate solution were mixed together, and then 1.2 mL of 10 mM AgNO3 solution was rapidly injected. The resulting mixture was stirred vigorously.

2.2 Synthesis of Au nanoparticles

Au nanoparticles were synthesized via the Turkevich method [26] - 50 mL of 0.5 mM HAuCl4 was heated to the boiling point for 1 h, and 0.3 mL of 1% sodium citrate solution was added into the heated solution. The color of the colloid solution turned purple after injection and slowly changed to red. After the secondary color change, the solution was stirred at 100 °C for 30 min. Finally, the Au colloid solution was cooled down slowly with stirring.

2.3 Fabrication of plasmonic enhanced dye-sensitized solar cells

A fluorine-doped tin oxide (FTO) glass was washed sequentially by sonication method in a detergent solution, in distilled water, and in acetone, each for 10 min. A TiO2 blocking layer was deposited

on

the

cleaned

FTO

glass

by

spin-coating

with

5

wt%

of

titanium

di-isopropoxidebis(acetylacetonate) in butanol and annealed in air at 450 ºC for 30 min. A TiO2 paste (T/SP, Solaronix) was doctor-bladed on a blocking layer and annealed in air at 450 °C for 30 min to form a TiO2 nanoparticle photoactive layer. Poly (4-vinylpyridine) (P4VP) was coated on the photoactive layer by dipping in P4VP solution (0.15 g of P4VP in 50 mL ethanol) for 10 s. Then, the sample was washed with ethanol and dried in air. To immobilize Au and Ag nanoparticles on the

photoactive layer, the P4VP-coated TiO2 nanoparticle film was dipped successively in Au and Ag colloid solutions, each for a given duration, and then washed by H2O and ethanol. After drying, a scattering layer (DSL 18NR-AO, Dyesol) was doctor-bladed and annealed at 450 °C for 30 min. The resulting film was treated by titanium isopropoxide (TIP) solution (0.1 M in isopropyl alcohol) at 90 °C for 30 min and then annealed at 450 °C for 30 min. Films that included metal nanoparticles were annealed additionally in hydrogen atmosphere at 450 °C for 10 min.

The prepared photoanode was immersed into 0.5 mM of N719 (Solaronix) dye solution in ethanol at 50 °C overnight. A Pt counter electrode was prepared by dropping H2PtCl6 solution (0.5 M in ethanol) on another piece of FTO glass and annealing it at 450 °C for 30 min. Each of the photoanode and the counter electrode was sandwiched by surlyn. Then, electrolyte was injected between the photoanode and the Pt counter electrode. The electrolyte was composed of 0.7 M of 1-butyl-3-methyl imidazolium iodide (BMII), 0.03 M of I2, 0.1 M of guanidium thiocyanate (GSCN), and 0.5 M of 4-tertbutyl pyridine (TBP) in a mixture of acetonitrile and valeronitrile (85:15 v/v).

2.4 Characterization of dye-sensitized solar cells

The morphology of the surface-immobilized metal nanoparticles was examined by a field emission scanning electron microscope (FE-SEM, JSM-6330F, JEOL Inc.). The UV-visible absorption spectra of Au and Ag colloid solutions and films with immobilized Au and Ag nanoparticles were analyzed with a Neosys-2000 system (Scinco). The current density-voltage (I-V) characteristics of the DSSCs were measured using an electrometer (KEITHLEY 2400) under AM 1.5 illumination (100 mW/cm2) delivered by a solar simulator (1 KW xenon with AM 1.5 filter, PEC-L01, Peccell Technologies).

3. Results and Discussion The fabrication process of the DSSCs including a plasmonic layer with Au and Ag nanoparticles is shown in Scheme 1. The photoanode is composed of a blocking layer, a mesoporous TiO2 photoactive layer, and a scattering layer. In the plasmon-enhanced DSSCs, the mesoporous TiO2 layer was first coated with P4VP [27] as shown in Scheme 1(a). Au nanoparticles were then immobilized on the surface by dipping the TiO2 layer in Au colloid solution as shown in Scheme 1(b). Next, Ag nanoparticles were immobilized on the TiO2 layer, which was previously coated with P4VP and Au nanoparticles, by dipping the TiO2 layer in Ag colloid solution (Scheme 1[c]). After immobilizing the Au and Ag nanoparticles as the plasmonic layer, P4VP was removed by a sintering step. The DSSCs were subsequently fabricated by assembling the photoanode and the Pt counter electrode as shown in Scheme 1(d). The SEM image in Figure 1 (a) shows the immobilized Au nanoparticles on the film for 70 min without aggregation. Then, the Ag nanoparticles were immobilized on the same film for 10 min (Figure 1[b]). Figure 1(c) shows the comparison of UV-Vis spectra of N719 dye, immobilized Au nanoparticles for 70 min, and Au and Ag nanoparticles for 70 min and 10 min. The maximum absorbance wavelength (λmax) of the film with Au nanoparticles was ca 530 nm. After further immobilization of Ag nanoparticles, which exhibited λmax ≈ 400 nm, the absorbance of the film with both nanoparticles showed a red-shifted peak for Au nanoparticles at 554 nm and another peak for Ag nanoparticles at 436 nm. These shifts might indicate that the red-shifted SPR bands of Au and Ag nanoparticles depend highly on the refractive index of the matrix [28-31]. To visualize the changes in their absorption spectra, we also compared the absorption spectra of colloidal Au, colloidal Ag, TiO2 film incorporated with Au nanoparticles, TiO2 film incorporated with Ag nanoparticles, and TiO2 film incorporated with Au and Ag nanoparticles (see Figure S1). The ranges of absorption peaks of both Au and Ag nanoparticles broadened when they were simultaneously immobilized on the film. The resulting broad ranges of absorbance were able to covere the absorbance of the N719 dye sufficiently enough.

The PCE and short-circuit current density (Jsc) of DSSCs depend on factors related to plasmonic layer, such as the type of metal nanoparticles [19, 23] and their immobilization time [24]. We first studied an optimal immobilization condition for the Au and Ag nanoparticles. As shown in Figure 2, the photocurrent density-voltage (I-V) curves of DSSCs using plasmonic layer with Au nanoparticles alone immobilized for different durations (i.e., the immobilization time) were measured in AM 1.5 sunlight. The changes in the PCE and Jsc with the immobilization time of the Au nanoparticles are represented in Figure 3, and the values of short-circuit current density (Jsc), open-circuit voltage (Voc), fill factor (ff), and PCE are summarized in Table 1. The PCE gradually increased from 8.82% up to 9.53% as the immobilization time was increased to 70 min, and then it decreased to 9.19% when the immobilization time was further extended to 120 min. A similar trend was observed in the Jsc value, which increased from 16.58 mA/cm2 to 17.66 mA/cm2 and then dropped to 17.03 mA/cm2, respectively at 70 min and 120 min. A longer immobilizing time affected the performance of DSSCs negatively due to reduction of light absorption at the TiO2 layer and increase in the recombination rate of the charge carriers [32]. Figure S2 shows the SEM images of the immobilized Au nanoparticles across different durations. All DSSCs with Au nanoparticles, immobilized for 30–120 min, showed higher Jsc and PCE values than those without Au nanoparticles. Under the optimal condition, which was 70 min of immobilization time, Jsc and the PCE were enhanced by 6.51% and 8.05%, respectively. Figure 4 shows the I-V curves of DSSCs with the plasmonic layer containing only Ag nanoparticles across various immobilization times. The photovoltaic parameters are listed in Table 2. The effects of the immobilization time on Jsc and PCE are represented in Figure 5. From 0 to 10 min, the Jsc and PCE values increased from 16.58 mA/cm2 to 17.09 mA/cm 2 and 8.82% to 9.33%, representing a 7.28% and a 5.78% enhancement, respectively. Both values decreased when the Ag nanoparticle immobilization time was extended to 15 min. The plasmonic effect became negligible with the immobilization time beyond 15 min.

Based on these results, we prepared DSSCs incorporating a plasmonic layer with both Au and Ag nanoparticles immobilized for the optimal durations (70 min and 10 min, respectively). The I-V curves are shown in Figure 6. The photovoltaic parameters are summarized in Table 3. Between the devices using nil nanoparticles and using both types of nanoparticles, the values of Jsc, Voc, ff, and PCE increased from 16.75 mA/cm2 to 17.88 mA/cm2, 0.78 V to 0.81 V, 0.68 to 0.70, and 8.82% to 10.17%, respectively. In the presence of the plasmonic layer, greater amount of electrons was generated from the dye by the plasmonic effect [33, 34], increasing the electron density of the mesoporous TiO2 layer with Au and Ag nanoparticles. This change increased the value of ff and induced the Fermi level to be shifted to more negative in potential, as reflected by the increased Voc [33, 34]. As a result of the stronger plasmonic effect, the DSSCs based on plasmonic layer with Au and Ag nanoparticles exhibited the highest PCE value of 10.17%, which is a 15.31% enhancement compared to those with the plasmonic layer without metal nanoparticles. To confirm the plasmonic effect of the plasmonic layer with Au and Ag nanoparticles, we investigated the change of optical density between DSSCs with and without metal nanoparticles. The results are shown in Figure S3. Figure 7 shows the incident photon to electron conversion efficiency (IPCE) spectra of the DSSCs. The intensity of IPCE in DSSCs with Ag and Au nanoparticles is higher than those without Ag and Au nanoparticles. This is due to the fact that the light absorption of Ag and Au nanoparticles is overlapped with that of the N719 dye from 350 nm to 700 nm. The result indicates that increase in generated electron yields improved not only the Jsc but also the electron density that is related to Voc and ff values. As a result, the DSSCs with Ag and Au nanoparticles exhibited superior PCE.

4. Conclusions To improve the PCE of DSSC devices, a plasmonic layer with Au and Ag nanoparticles was prepared with ease by dipping a P4VP-covered mesoporous TiO2 layer in colloid solutions.

Individual experiments with Au and Ag nanoparticles showed the optimal immobilization times of 70 min for Au and 10 min for Ag, which resulted in a 8.05% (from 8.82% to 9.53%) and a 5.78% (from 8.82% to 9.33%) enhancement of the PCE, respectively. When both types of nanoparticles were used in the plasmonic layer, increase in the values of Jsc, Voc, and ff brought about a further increase in the PCE to 10.17%, which corresponds to a 15.31% enhancement. The increase in Jsc from 16.75 mA/cm2 to 17.88 mA/cm2 was due to the broader light absorption range, covering the N719 dye absorption bands, which is caused by the plasmonic layer on the mesoporous TiO2 layer. This simple approach can be extended to other organic, hybrid, and perovskite solar cells.

Acknowledgments: This work was supported by the Bio & Medical Technology Development Program of the National Research Foundation (NRF) funded by the Korean government (MSIP & MOHW) (2016-A423-0045) and by Basic Study program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (2015R1D1A1A09058247).

Scheme 1. Fabrication of dye-sensitized solar cells (DSSCs) using plasmonic layer with Au and Ag nanoparticles: (a) coating of P4VP, (b) immobilization of Au nanoparticles, (c) immobilization of Ag nanoparticles, and (d) assembly of DSSCs.

Figure 1. SEM images of the surface immobilized (a) Au nanoparticles for 70 min, (b) Au and Ag nanoparticles for 70 min and 10 min, respectively, (c) UV-Vis spectra of N719 dye, immobilized Au nanoparticles for 70 min, and Au and Ag nanoparticles for 70 min and 10 min.

Figure 2. I-V curves of DSSCs with Au nanoparticles by immobilization times. (a) 0 min (no dipping), (b) 30 min, (c) 50 min, (d) 70 min, (e) 100 min, and (f) 120 min.

Figure 3. Dependence of Jsc (blue) and PCE (red) on immobilization time, for DSSCs using a plasmonic layer of Au nanoparticles.

Figure 4. I-V curves of DSSCs with Ag nanoparticles by immobilization times. (a) 0 min (no dipping), (b) 5 min, (c) 10 min, (d) 15 min, (e) 20 min, and (f) 25 min.

Figure 5. Dependence of short-circuit current density (Jsc)(blue) and power conversion efficiency (PCE) (red) on immobilization time, for DSSCs using a plasmonic layer of Ag nanoparticles.

Figure 6. I-V curves of DSSCs (a) without plasmonic layer and (b) with Au and Ag nanoparticles immobilized for 70 min and 10 min, respectively.

Figure 7. Incident photon to electron conversion efficiency (IPCE) spectra of the DSSCs (a) without plasmonic layer and (b) with Au and Ag nanoparticles immobilized for 70 min and 10 min, respectively.

Table 1. Photovoltaic properties (short-circuit current density [Jsc], open-circuit voltage [Voc], fill factor [ff], and power conversion efficiency []) of DSSCs with Au nanoparticles across immobilization time.



DSSCs with Au nanoparticles across immobilization time

Jsc

Voc

(mA/cm2)

(V)

0 min

16.58

0.76

0.70

8.82

30 min

16.92

0.76

0.70

9.00

50 min

17.44

0.76

0.70

9.28

70 min

17.66

0.76

0.71

9.53

100 min

17.16

0.76

0.71

9.26

120 min

17.03

0.76

0.71

9.19

ff

(%)

Table 2. Photovoltaic properties (short-circuit current density [Jsc], open-circuit voltage [Voc], fill factor [ff], and power conversion efficiency []) of DSSCs with Ag nanoparticles across immobilization time.



DSSCs with Ag nanoparticles across immobilization time

Jsc

Voc

(mA/cm2)

(V)

0 min

15.93

0.78

0.71

8.82

5 min

16.87

0.78

0.70

9.21

10 min

17.09

0.78

0.70

9.33

15 min

16.08

0.79

0.71

9.02

20 min

15.85

0.78

0.71

8.78

25 min

15.23

0.80

0.72

8.77

ff

(%)

Table 3. Photovoltaic properties (short-circuit current density [Jsc], open-circuit voltage [Voc], fill factor [ff], and power conversion efficiency []) of DSSCs with Ag and Au nanoparticles across immobilization time. DSSCs with Au nanoparticles immobilization time

and Ag across



Jsc

Voc

(mA/cm2)

(V)

0 min

16.75

0.78

0.68

8.82

70 min and 10 min

17.88

0.81

0.70

10.17

ff

(%)

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Highlight - Plasmonic-layer-based DSSCs were prepared with Au and Ag nanoparticles - Optimal conditions for immobilization of Au and Ag nanoparticles for the DSSC plasmonic layer were studied. - The DSSC with a plasmonic layer with Au and Ag nanoparticles exhibited better PCE than that without a plasmonic layer

Graphical Abstract Preparation of plasmonic monolayer with Ag and Au nanoparticles for dye-sensitized solar cells Da Hyun Song1, Hyun-Young Kim,1 Ho-Sub Kim1, Jung Sang Suh1, Bong-Hyun Jun2*and Won-Yeop Rho1,2* 1

2

Department of Chemistry, Seoul National University, Seoul 151-747, Republic of Korea; [email protected] Department of Bioscience and Biotechnology, Konkuk University, Seoul 143-701, Republic of Korea;

[email protected] * Correspondence: Bong-Hyun Jun: [email protected], Won-Yeop Rho: [email protected]; Tel.: +82-2-450-0521