Surface plasmon resonance effect of Cu nanoparticles in a dye sensitized solar cell

Accepted Manuscript Title: Surface plasmon resonance effect of Cu nanoparticles in a dye sensitized solar cell Authors: Mahesh Dhonde, Kirti Sahu, V.V.S. Murty, Siva Sankar Nemala, Parag Bhargava PII: DOI: Reference:

S0013-4686(17)31627-4 http://dx.doi.org/doi:10.1016/j.electacta.2017.07.187 EA 30004

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

13-5-2017 31-7-2017 31-7-2017

Please cite this article as: Mahesh Dhonde, Kirti Sahu, V.V.S.Murty, Siva Sankar Nemala, Parag Bhargava, Surface plasmon resonance effect of Cu nanoparticles in a dye sensitized solar cell, Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2017.07.187 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Surface plasmon resonance effect of Cu nanoparticles in a dye sensitized solar cell Mahesh Dhondea,*, Kirti Sahua, V.V.S. Murtya, Siva Sankar Nemalab, Parag Bhargavab aDepartment

of Physics, Govt. Holkar Science College, M.P. 452001, India

b

Department of Metallurgical Engineering & Materials Science, Indian Institute of Technology-Bombay, Mumbai,40076, India

*

Corresponding author. Email address: [email protected]

Graphical abstract

HIGHLIGHTS 

Pure and Cu-doped TiO2 Nanoparticles are synthesized and incorporated in DSSCs.



Addition of Cu provided high surface area and reduced charge recombination due to LSPR effect.



The highest photo conversion efficiency achieved is 8.65% with Jsc of 18.8 mA cm-2.



This efficiency is 26% higher than that of pure TiO2 DSSC.

Abstract

Pure and copper doped titanium dioxide nanoparticles (TiO2 NPs) for Dye Sensitized Solar Cell (DSSC) photo anodes with different doping amounts of copper (Cu) 0.1, 0.3 and 0.5 mole% are synthesized using modified sol-gel route. Addition of Cu in TiO2 matrix can enhance absorption towards visible spectrum and can reduce the charge carrier recombination due to Localized Surface Plasmon Resonance (LSPR). The samples are characterized by X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM), UV-Vis spectroscopy (UV-VIS), X-ray Photoelectron Spectroscopy (XPS), Electro Chemical Impedance Spectroscopy (EIS). The crystallite size is measured by XRD and surface morphology of the samples is analyzed using SEM. UV-Vis measurement shows that the influence of Cu in TiO2 lattice altered its optical properties and extended absorption in the visible region. The resistances between different junctions of the cell are measured by EIS. The J-V measurement of the cell prepared using pure and Cu-doped TiO2 NPs is carried out by solar simulator. The optimized Cu doped DSSC with 0.3 mole% Cu in TiO2 shows the best power conversion efficiency of 8.65% which is approximately 26% greater than the efficiency of undoped DSSC (6.41%).

Keywords: Cu-doped TiO2 NPs, dye sensitized solar cell, sol-gel, localized surface plasmon resonance.

1.

Introduction Titanium dioxides (TiO2) being nontoxic and environmentally benign in nature, have been the subject of

considerable attention in energy and environmental sciences, particularly due to their enhanced

use in present

generation DSSCs and photocatalysis for hydrogen production or water splitting, waste water treatment and air

purification[1]. Due to various excellent characteristics of a DSSC such as semi-transparency, low-cost fabrication, high power conversion efficiency and environmental friendliness, it is considered as one of the most suitable source of electrical energy in third generation solar cells. A typical DSSC consists of a dye adhered nanocrystalline TiO2 photo anode on a Fluorine-doped Tin Oxide (FTO) substrate, Platinum (Pt) or carbon coated counter electrode and an iodidetriiodide (I-/I3-) based redox electrolyte. Out of these three components, photo anode plays a major role in photovoltaic performance of a DSSC. An ideal photo anode should possess proper morphology of nanoparticles, energy band gap suitable for visible spectrum absorption and average film porosity. Out of various oxide materials available for making different types of solar cells, TiO2 is the potential candidate with high photo catalytic activity and non toxicity [2]. However, wide band gap and charge carrier recombination between the TiO 2 coated photo anode and redox electrolyte are the downsides of TiO2. To overcome the problems associated with DSSCs based on pure TiO 2, doping of suitable transition metals such as Cu, Cr, Mn, V, Fe etc. and selection of suitable sensitizing dye and redox electrolyte are to be studied and explored. Cu doping can increase the light-harvesting efficiency of the sensitizing dye by enhancing far-field scattering, increasing near-field local surface plasmons or as an agent to induce charge separation [3-12]. Furthermore, LSPR of nanometals trap the light on the metal surface and induces a collective oscillations of metal free electrons, enhance the light absorption proficiency of dye molecules and boost the separation of carriers [13-18]. A schematic illustration of band gap reduction in pure TiO 2 and Cu-doped TiO2 is shown in fig.1. Therefore incorporation of Cu as dopant in TiO2 matrix along with ruthenium metal complex based sensitizing dye and iodide-triiodide (I-/I3-) based redox electrolyte can significantly improve the photo response of a DSSC [19-29,41]. Recent study shows that electronic structure of the TiO 2 could be altered by using Cu as dopant. Y. Recently Zhang and coworker’s achieved 9.44% power conversion efficiency by incorporating plasmonic Cu nanowires in TiO2 structure and J-Y Park and team accomplished 11.35% power conversion efficiency by Cu/N doping[30,31]. In the present work, we report the synthesis of plasmonic Cu nanoparticles and their application in TiO2 photoanode for enhancing the performance of DSSC. The Cu-doped TiO2 NPs are prepared via modified sol-gel method. Different physical, optical and electrical measurement techniques are used to investigate the effect of Cu doping on the photo voltaic performance of a DSSC. The photovoltaic properties such as Open Circuit Voltage (Voc), Short Circuit Current density (Jsc), Fill Factor (FF) and power conversion efficiency (η) of pure and Cu-doped photo electrodes with different doping concentrations are assessed.

2.

Experimental

2.1

Materials

Titanium Tetra Isopropoxide (TTIP) and Titanium tetra chloride (TiCl4) were purchased from Sigma Aldrich and Copper (II) nitrate tri hydrate (Cu(NO3)2.3H2O), deionized water, glacial acetic acid, ethanol, poly ethylene glycol (PEG) Triton X-100, isopropyl alcohol(IPA) and Hydrochloric acid (HCl) were supplied by Merck. All these reagents are of analytical grade and used without any further purification.

2.2

Synthesis of pure TiO2 nanoparticles

Pure TiO2 nanoparticles were prepared using TTIP as precursor. For the preparation of TiO2 NPs, 1 ml of HCl was slowly added to 40 ml ethanol and then magnetically stirred for some time. After stirring 5 ml TTIP was added to this solution drop wise under magnetic stirring to form a uniform solution. The resultant mixture was kept in hot air oven for 10 hours for drying. Obtained solids were ground well in alumina mortar for some time and then kept in muffle furnace for 30 minutes at 450oC for calcination.

2.3

Preparation of Cu-doped TiO2 nanoparticles

For the synthesis of Cu-doped TiO2 nanoparticles, Copper (II) nitrate with different mole % was added in TTIP acting as precursors for Cu and titania (Ti) respectively. Copper (II) nitrate with 0.1 to 0.5 mole % was dissolved in 100 ml of deionized water at room temperature, followed by addition of 0.5 ml glacial acetic acid to form a solution A. Another solution B was prepared by adding 80 ml of ethanol with 20 ml TTIP under constant stirring for one hour. Solution B was added drop wise into solution A, under constant magnetic stirring for two hours to form a uniform and homogeneous solution. The resultant mixture was kept for ageing at room temperature for a day and then dried in hot air oven at 100oC. Obtained solids were ground well and calcined at 450 oC at a ramp rate of 5oC min-1 for 1 hour.

2.4

Preparation of slurry for pure and Cu-doped TiO2 nanoparticles

A stoichiometric amount of ethanol, PEG and triton X-100 were mixed in the dried form of pure and Cudoped TiO2 nano particles to form a paste. To convert this paste into homogeneous slurry for doctor blade deposition, it was kept over pot miller for 5 days in polypropylene bottles with grinding media of Zirconia balls (3mm diameter).

2.5

Preparation of pure and Cu-doped photoanodes

To improve the performance of photo anode, a compact layer of TiCl4 of approximately 20 nm thickness was deposited on FTO coated glass (3 x 4cm) substrate by dip coating method to form uniform and flawless surface. After compact layer deposition, FTO substrates were heat treated at 450oC for 1 hour and cleansed by using ethanol and acetone in an ultrasonic bath for 10 minutes each. The mesoporous films of pure and Cu-doped TiO2 NPs pastes were formed on TiCl4 treated cleaned FTO by using doctor blade technique [32]. The films were dried at 60oC for 30 minutes and then calcined at 450oC for 1 hour in air at a ramp rate of 5oC min-1. For the sensitizer dye loading, the prepared photoanodes were immersed in N719 (0.3 mM) dye for 36 hours.

2.6

Preparation of Pt counter electrode

On a cleaned compact layer deposited FTO glass, Pt film of 40 nm thickness was deposited using Orion sputter at 3m torr pressure and deposition rate of 6 nm min-1.

2.7

Preparation of DSSC assembly

DSSC was fabricated by joining the dye loaded photo anode and Pt counter electrode. A thin Surlyn spacer (DuPont) of approximately 60μm was used to maintain a space between the dye loaded photoanode and counter electrode. An iodide-triiodide (I-/I3-) based redox electrolyte containing 0.5 M of LiI, 0.05 M of I2 and 0.5 M of 4-tertbutylpyridine and 0.5 M of 1-butyl-3-methylimidazolium iodide in acetonitrile was then introduced into the vacant space between photoanode and counter electrode[43]. Finally the device was sealed with a Surlyn sheet (DuPont) by heating. The schematic of complete assembly of a plasmonic DSSC is shown in fig.2.

2.8

Photovoltaic performance evaluation

The power conversion parameters of the pure and Cu-doped TiO2 DSSCs were evaluated by using solar simulator with a light intensity of 100 mW cm-2, calibrated at AM 1.5 with a digital source meter (Newport, oriel instruments Model: 67005 and Keithley, Model: 2420). Various cell parameters such as fill factor (FF), open circuit

voltage(Voc), short circuit current density (Jsc) and incident optical power (Pin) were used to evaluate the power conversion efficiency (η) of the pure and Cu-doped TiO2 based DSSCs . The incident photon-to-current conversion efficiencies (IPCE) of the DSSCs were measured by Bentham PVE300 measurement system. The active area of cell was 0.16 cm2.

3.

Results and discussion

3.1

Surface Characterization

3.1.1

X-ray diffraction studies

The X-ray diffraction spectra of the pure and Cu-doped samples were measured by X-ray diffractometer (Rigaku-smart Lab) in the range of 20-80o using Cu-kα radiation. Fig. 3 shows the XRD patterns for pure and Cudoped TiO2 NPs with varying Cu concentrations. All the XRD patterns are in the agreement with JCPDS card no 211272. Presence of anatase phase confirms that only crystalline phases were formed during the synthesis. The crystallite size of all the samples was measured using Scherrer’s equation shown in equation (1).

D=

0.9λ

(1)

βcosϕ

Where λ is the X-ray wavelength, β is the full width at half maximum of the peak and 𝜙 is the diffraction angle.

The crystallite size of all the prepared samples was calculated in the range of 10-13 nm. The crystallite sizes of pure and Cu-doped TiO2 with Cu doping concentrations of 0.1, 0.3 and 0.5mole% were 10.31 nm, 11.23 nm, 10.35 nm and 12.9 nm respectively. No deviation in the peak position was found in all XRD patterns suggesting effective susbstituional doping of Cu in the TiO2 lattice [33]. Photo catalytic activity of doped TiO2 depends on Cu content. Agglomeration occurs due to high concentration of Cu doping resulting decrease in surface area available for effective absorption. Nevertheless low Cu content leads to decrease in plasmonic effect due to reduced scattering.

Table 1: Particle size of pure and Cu-doped TiO2

Sample Pure TiO2 Cu doped TiO2 Cu doped TiO2

Mole% Cu 0 0.1 0.3

Crystallite size(nm) 10.31 11.23 10.35

Cu doped TiO2

3.1.2

0.5

12.9

Scanning Electron Microscopy

Fig. 4.1 and fig. 4.2 show the surface morphology and cross sectional SEM micrographs of the pure and optimized Cu-doped samples respectively. The surface view revealed that the films were porous and uniform. The thickness of the films measured by surface profilometer was similar to that observed by cross sectional SEM micrographs and it was found to be approximately 14-16 μm. This morphology was detected for all the samples regardless to the dopant concentrations. Furthermore a change in the film color was observed with increase in Cu content. The film with highest Cu content was found to be the darkest. This variation in film color is mainly due to the presence of Cu in TiO2.

3.1.3

X-ray Photoelectron Spectroscopy

In order to understand the chemical composition of the pure and Cu-doped TiO2 NPs XPS measurement were performed. Fig. 5(a) shows the survey spectrum of the optimized 0.3 mole% Cu. The core level spectra of Cu 2p1, 2p3, Ti 2p1 and 2p3, were shown in fig. 5(b) and 5(c) respectively. The peaks of Ti 2p1 and Ti 2p3 were observed at 463.2 and 457.9 eV respectively. For Cu 2p1 and 2p3 peaks were observed at 933.6 and 951.3 eV, which is comparable to the binding energy of Cu indicating presence of Cu elements within the structure of Cu-doped TiO2 NPs. Another peak at 529 eV shows the O 1s (fig. 5d). A small deviation in binding energy of Ti atom clearly suggests the substitutions of few sites of Ti4+ ions by Cu2+ ions.

3.2

Optical Characterization

3.2.1

UV-visible spectroscopy

The UV-visible spectra of pure and Cu-doped photoanodes were measured in the range of 300-800 nm using UV-Vis spectrophotometer (Perkin Elmer, Lambda25). Fig.6 (a) and fig.6 (b) show the UV-visible absorption spectra and Tauc plot respectively of all the samples. The peak absorption edge is found to be shifted towards longer wavelength side with increase in Cu content in TiO2. This enhanced light absorption is occurring due to the LSPR

effect[34], which sets up a strong electromagnetic field around Cu NPs and boost their interaction with dye molecules resulting in effective resonant energy transfer between excited state of dye and surface plasmons[35,36]. Thus it can be concluded from the results that doping of transition metal as Cu in TiO2 lattice is effective for visible light response [37]. The Band gap energies and UV-Vis absorption spectra of the samples were calculated using the Tauc plots and Kubelka-Munk function respectively. Here the maximum absorption peak was observed for 0.5mole% Cu-doped TiO2 photoanode. The band gap energies of pure and Cu-doped TiO2 with 0.1mole%, 0.3mole% and 0.5mole% were found to be 3.2eV, 2.8eV, 2.7eV and 2.6eV respectively. Furthermore UV-Vis spectroscopy was used to estimate the amount of dye adsorbed in various samples. Using the desorbed dye from the pure and Cu-doped samples with 1mM NaoH adsorption spectra were observed. The dye adsorption as a function of Cu content is shown in fig.7. The specific surface areas of pure and Cu-doped TiO2 were measured by BET. The BET surface area of pure TiO2 NPs was 47 g m-2. An increase in the surface area was found as the Cu content increased up to 0.3 mole%. However surface area were dramatically reduced as the Cu content exceeded beyond optimum value. The surface areas of 0.1, 0.3, 0.5 mole% Cu-doped TiO2 were 62, 86 and 28 g m-2 respectively. It is found that as the Cu content increased, the amount of dye adsorption decreased because of aggregation of nano particles, decrease in nanoparticles size and decreased specific surface area. The maximum adsorption of dye was found in optimized 0.3 mole% Cudoped TiO2 sample due to appropriate particle size, proper morphology and maximum specific surface area. The obtained results were summarized in table 3.

3.3

Electrical Characterization

3.3.1

Electrochemical Impedance Spectroscopy

To understand the influence of Cu on dye loaded TiO2 photoanodes with different interfacial properties, EIS was performed. Fig.8 shows the EIS spectra of pure and Cu-doped DSSCs carried out in the dark environment at bias potential of -0.7 V. From left to right the spectra show three semicircles Rt, Rr and Rd where Rt, Rr and Rd are transport, recombination and diffusion resistances respectively which represent resistance at the Pt counter electrode, charge transfer resistance of TiO2/dye/electrolyte and Nernst diffusion in I-/I3- electrolyte[38]. Thus the total resistance of the DSSC is the sum of the resistance at the Pt electrode, resistance due to diffusion of tri-iodide in the electrolyte

and characteristics of the resistance of the TiO2 electrode. The obtained data from different DSSCs are summarized in table 2. From the EIS spectra it is inferred that the doping of Cu mainly affects the charge transfer resistance (R t) and charge recombination resistance (Rr) due to LSPR effect [39-42]. The first semicircle (Rt) lying towards the higher frequency range suggests charge transfer resistance which should be as low as possible. The increase in the diameter of the middle semicircle (Rr) indicates higher value of charge recombination resistance, increased charge transfer mechanism and hence increased power conversion efficiency. For the optimized sample with 0.3 mole% of Cu doping in TiO2, the values of Rt and Rr are found to be 1.10 Ω and 35.43 Ω respectively. These results are in agreement with the J-V results of the cell.

Table 2. EIS parameters of pure TiO2 and different amounts of Cu-doped TiO2 DSSCs

3.3.2

Sample

Rt (Ω)

Rr (Ω)

Rd (Ω)

Jsc (mA cm-2)

Pure TiO2

1.67

23.43

10.53

13.9

0.1 mole% Cu

1.54

29.08

8.62

14.1

0.3 mole% Cu

1.10

35.43

6.97

18.8

0.5 mole% Cu

1.36

32.73

7.74

11.8

J-V Characteristic

The photo voltaic performance and efficiency of the pure and Cu-doped DSSCs were measured under standard temperature condition (1 sun; input power 100mW cm-2; AM 1.5) using J-V characteristics as shown in fig.9. The short circuit current density (Jsc), open circuit voltage (Voc), maximum power point values, fill factor (FF) and power conversion efficiency (η) of different cells were calculated using solar simulator when sunlight is simulated on an active area of 0.16 cm2.The results were tabulated in table 3. In the present work 0.3 mole% Cu-doped TiO2 photo electrode displays a power conversion efficiency of 8.65% which is 26% higher than that of pure TiO2. The 0.3 mole% Cu-doped TiO2 NPs showed suitable particle size and a reduced band gap value, promising increased dye adsorption, enhanced visible light absorption and improved short circuit current. However high concentration of Cu in TiO2 photo electrodes leads to decrease in specific surface area and hence minimizing dye adsorption resulting in low efficiency and high Voc as observed in case of 0.5 mole% Cu-doped TiO2 photo electrode.

Table 3: Performance of DSSCs based on pure and Cu-doped TiO2 photoanodes Adsorbed dye Sample

Jsc (mA cm-2)

Voc (V)

FF (%)

(η ± 0.07) % (mole cm-2)

Pure TiO2

13.9

0.71

64.0

6.41

4.23x10-8

0.1 mole% Cu

14.1

0.70

66.3

6.58

4.87x10-8

0.3 mole% Cu

18.8

0.71

64.2

8.65

22.37x10-8

0.5 mole% Cu

11.8

0.76

60.5

5.48

3.84x10-8

3.3.3

IPCE measurement

The incident photon-to-electron conversion efficiency (IPCE) spectra of the pure and Cu-doped DSSCs are shown in the fig 10. A significant improvement in IPCE in the visible region is observed in Cu-doped TiO2 DSSCs except 0.5 mole% Cu-doped TiO2 due to aggregation of nanoparticles, increased recombination and lower dye adsorption. This enhanced IPCE is due to enhanced electron injection, charge-transfer efficiency and increased dye loading [44]. The improvement in the electron injection and charge transfer efficiency is mainly due to the doping of Cu metal in TiO2 semiconductor promoting LSPR effect and hence contributing enhanced IPCE. For the efficient electron injection, the energy gap between the Lowest Unoccupied Molecular Orbital (LUMO) of the sensitizing dye and Conduction Band (CB) edge of the TiO2 semiconductor should be sufficiently high (i.e., >0.2 eV). A decrease in energy gap (i.e., <0.2 eV) between LUMO of the dye and CB of TiO 2 will lead to decrease in injection efficiency and hence IPCE [45]. In the present work IPCE of pure TiO2 based DSSC was found to be approximately 68% but it was improved significantly with Cu doping and has a maximum value of approximately 75% in 0.3 mole % Cu-doped TiO2 DSSC. Similar IPCEs was observed for 0.1 mole% and 0.5 mole% Cu-doped TiO2 DSSCs but the later showed lower IPCE due to less dye loading as discussed earlier.

4.

Conclusions

In the present work pure and Cu-doped TiO2 NPs were synthesized by modified sol-gel route and successfully incorporated in DSSCs. The addition of suitable amount of Cu content in TiO2 lattice effectively altered the optical properties of TiO2 photo anodes by extending the optical response towards visible range of solar spectrum. Cu-doped TiO2 NPs provided higher surface area for dye adsorption, active charge transfer sites and reduced charge carrier recombinations due to LSPR effect except 0.5 mole% Cu sample. The DSSC incorporating 0.3mole% Cu-doped TiO2 photo anode showed the highest power conversion efficiency of 8.65% which is 26% higher than undoped TiO2 DSSC (6.41%). The addition of suitable amount of Cu content played a vital role for improving the performance of the DSSC. This addition of Cu-doped TiO2 NPs in DSSC could be a step towards an efficient and cost effective DSSC.

Acknowledgements The Authors are thankful to INUP for availing lab facilities for nano fabrication and characterizations at the CEN, IITB under INUP which is sponsored by DIT, MCIT, Government of India” and department of Metallurgical Engineering and Materials science IITB. Authors are also thankful to V. Ganeshan and Mukul Gupta Scientists IUCDAE CSR, Indore for providing guidance and lab support.

References [1] L.S. Yoong, F.K. Chong, Binay K. Dutta, Development of copper-doped TiO2 photocatalyst for hydrogen production under visible light, Energy 34 (2009) 1652–1661. [2] Nam-Gyu Park, Light management in dye sensitized solar cell, Korean J. Chem. Eng., 27 (2) (2010) 375-384. [3] Y. Xie, N. Huang, S. You, Y. Liu, B. Sebo, L. Liang, X. Fang, W. Liu, S. Guo, X.Z. Zhao, Improved performance of dyesensitized solar cells by trace amount Cr-doped TiO2 photoelectrodes, J. Power Sources 224 (2013) 168. [4] M.D. Brown, T. Suteewong, R.S.S. Kumar, V. D'Innocenzo, A. Petrozza, M.M. Lee, U. Wiesner, H.J. Snaith, Identity profiling of cell surface markers by multiplex gold nanorod probes, Nano Lett. 11 (2011) 438-445. [5] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Highly efficient multiple exciton generation in colloidal PbSe and PbS quantum dots, Chem. Rev. 110 (2010) 6595-6663. [6] F. Meng, Y. Luo, Y. Zhou, J. Zhang, Y. Zheng, G. Cao, X. Tao, Integrated Plasmonic and Upconversion starlike Y2O3:Er/Au@TiO2 Composites for Enhanced-Photon Harvesting in Dye-sensitized Solar Cells, J. Power Sources 316 (2016) 207-214. [7] W. Zhang, M. Saliba, S.D. Stranks, Y. Sun, X. Shi, U. Wiesner, H.J. Snaith, Enhancement of Perovskite-Based Solar Cells Employing Core-Shell Metal Nanoparticles, Nano Lett. 13 (2013) 4505-4510. [8] H. Choi, W.T. Chen, P.V. Kamat, Know Thy Nano Neighbor. Plasmonic versus Electron Charging Effects of Metal Nanoparticles in Dye-Sensitized Solar Cells, ACS Nano 6 (2012) 4418-4427. [9] Li Zhou, Ligou Wei, Yulin Yang, Xue Xia, Ping Wang, Jia yu, Tianzhu Luan, Improved performance of dye-sensitized solar cells using Cu-doped photoanode materials: Band edge movement study by spectroelectrochemistry, Chem. Phy. 475 (2016) 1-8. [10] K. Guo, M. Li, X. Fang, X. Liu, B. Sebo, Y. Zhu, Z. Hu, X. Zhao, Enhanced photovoltaic performance of silver@titania plasmonic photoanode in dye-sensitized solar cells, J. Power Sources 230 (2013) 155-160. [11] W.R. Erwin, H.F. Zarick, E.M. Talbert, R. Bardhan, Light trapping in mesoporous solar cells with plasmonic

nanostructures, Energy Environ. Sci. 9 (2016) 1577-1601. [12] N. Zhou, V. Lopez-Puente, Q. Wang, L. Polavarapu, I. Pastoriza-Santos, Q.-H. Xu, Plasmon-enhanced light harvesting: applications in enhanced photocatalysis, photodynamic therapy and photovoltaics, RSC Adv. 5 (2015) 29076-29097. [13] M. Ihara, K. Tanaka, K. Sakaki, I. Honma, K. Yamada, Enhancement of the Absorption Coefficient of cis(NCS)2 Bis(2,2‘-bipyridyl-4,4‘-dicarboxylate)ruthenium(II) Dye in Dye-Sensitized Solar Cells by a Silver Island Film, J. Phys. Chem. B 101 (1997) 5153-5157. [14] S.D. Standridge, G.C. Schatz, J.T. Hupp, Toward plasmonic solar cells: Protection of silver nanoparticles via atomic layer deposition of TiO2, Langmuir 25 (2009) 2596-2600. [15] S.D. Standridge, G.C. Schatz, J.T. Hupp, Distance Dependence of Plasmon-Enhanced Photocurrent in Dye-Sensitized Solar Cells, J. Am. Chem. Soc. 131 (2009) 8407-8409 [16] H.-W. Chen, C.-Y. Hong, C.-W. Kung, C.-Y. Mou, K.C.W. Wu, K.-C. Ho, A gold surface plasmon enhanced mesoporous Titanium dioxide photoelectrode for the plastic-based flexible dye-sensitized solar cells, J. Power Sources 288 (2015) 221-228. [17] X. Dang, J. Qi, M.T. Klug, P.-Y. Chen, D.S. Yun, N.X. Fang, P.T. Hammond, A.M. Belcher, Tunable localized surface \ plasmon-enabled broadband light-harvesting enhancement for high-efficiency panchromatic dye-sensitized solar cells, Nano Lett. 13 (2013) 637-642. [18] D. Kuang, C. Klein, S. Ito, J.E. Moser, R. Humphry-Baker, N. Evans, F. Duriaux, C. Greatzel, S.M. Zakeeruddin, M. Greatzel, High-Efficiency and Stable Mesoscopic Dye-Sensitized Solar Cells Based on a High Molar Extinction Coefficient Ruthenium Sensitizer and Nonvolatile Electrolyte, Adv. Mater. 19 (2007) 1133-1137. [19] Jun-Yong Park, Chan-Soo Kim, Kikuo Okuyama, Hye-Moon Lee, Hee-Dong Jang, Sung-Eun Lee, Tae-Oh Kim, Copper and nitrogen doping on TiO2 photoelectrodes and their functions in dye-sensitized solar cells, Journal of Power Sources 306 (2016) 764-771. [20] G. Campet, J. Veriolle, J.P. Doumerc, Photoelectronic processes in transition-element doped n-type TiO2 electrode, Mater. Res. Bull., 15 (1980) 1135. [21] J.Y. Park, K.H. Lee, B.S. Kim, C.S. Kim, S.E. Lee, K. Okuyama, H.D. Jang, T.O. Kim, Enhancement of dye-sensitized solar cells using Zr/N-doped TiO2 composites as photoelectrodes, RSC Adv. 4 (2014) 9946. [22] Ye Zhang, Ning Zhou, Keqin Zhang, Feng Yan, Plasmonic copper nanowire@TiO2 nanostructures for improving the performance of dye-sensitized solar cells, Journal of Power Sources 342 (2017) 292-300 [23] C.S. Kim, J.W. Shin, S.H. An, H.D. Jang, T.O. Kim, Photodegradation of volatile organic compounds using zirconiumdoped TiO2/SiO2 visible light photocatalysts, Chem. Eng. J. 204e206 (2012) 40. [24] J.M. Herrmann, J. Disdier, P. Pichat, Effect of chromium doping on the electrical and catalytic properties of powder titania under UV and visible illumination, Chem. Phys. Lett. 108 (1984) 618-622. [25] Y. Li, L. Jia, C. Wu, S. Han, Y. Gong, B. Chi, J. Pu, L. Jian, Mesoporous (N, S)-codoped TiO2 nanoparticles as effective photoanode for dye-sensitized solar cells, J. Alloys Comp. 512 (2012) 23.. [26] K.T. Ranjit, B. Viswanathan, Synthesis, characterization and photocatalytic properties of iron-doped TiO2 catalysts, J. Photochem. Photobiol., A: Chem. 107 (1997) 215. [27] K. Wilke, H.D. Breuer, The influence of transition metal doping on the physical and photocatalytic properties of titania, J. Photchem. Photobiol., A: Chem. (1999) 49-53. [28] C.S. Kim, J.W. Shin, Y.H. Cho, H.D. Jang, H.S. Byund, T.O. Kim, Synthesis and characterization of Cu/N-doped mesoporous TiO2 visible light photocatalysts, Appl. Catal. A General 455 (2013) 211. [29] Y. Wang, H. Cheng, Y. Hao, J. Ma, W. Li, S. Cai, Photoelectrochemical properties of metal-ion-doped TiO2 nanocrystalline electrodes, Thin Solid Films 349 (1999) 120-125. [30] Ye Zhang, Ning Zhou, Keqin Zhang, Feng Yan, Plasmonic copper nanowires @TiO2 nanostructures for improving performance of dye-sensitized solar cells, J. Power Sources 342 (2017) 292-300. [31] Copper and nitrogen doping on TiO2 photoelectrodes and their functions in dye sensitized solar cells, J. Power Sources 306 (2016) 764-771. [32] J.Y. Park, K.H. Lee, B.S. Kim, C.S. Kim, S.E. Lee, K. Okuyama, H.D. Jang, T.O. Kim, Enhancement of dye-sensitized solar cells using Zr/N-doped TiO2 composites as photoelectrodes, RSC Adv. 4 (2014) 9946. [33] K.V. Baiju, S. Shukla, K.S. Sandhya, J. James, K.G.K. Warrier, Photocatalytic Activity of Sol−Gel-Derived Nanocrystalline Titania, J. Phys. Chem. C 111 (2007) 7612–7622. [34] R. Dahal, B. Pantha, J. Li, J. Lin, H. Jiang, InGaN/GaN multiple quantum well solar cells with long operating wavelengths, Appl. Phys. Lett. 94 (2009) 063505. [35] H.F. Zarick, O. Hurd, J.A. Webb, C. Hungerford, W.R. Erwin, R. Bardhan, Enhanced efficiency in dye-sensitized solar cells with shape-controlled plasmonic nanostructures, ACS Photon. 1 (2014) 806-811. [36] S. Son, S.H. Hwang, C. Kim, J.Y. Yun, J. Jang, Designed Synthesis of SiO 2 /TiO 2Core/Shell Structure As Light

Scattering Material for Highly Efficient Dye-Sensitized Solar Cells, ACS Appl. Mater. Inter. 5 (2013) 4815-20. [37] Akihiko K, Ryo N, Akihide I, Hideki K., Effects of doping of metal cations on morphology, activity, and visible light response of photocatalysts, Chem Phys 339 (2007) 104–110. [38] K.K. Wong, A. Ng, X.Y. Chen, Y.H. Ng, Y.H. Leung, K.H. Ho, A.B. Djuri_si_c, A.M.C. Ng, W.K. Chan, L.Yu, D.L. Phillips, Effect of ZnO nanoparticles properties on dye sensitized solar cell performance, ACS Appl. Mater. Interfaces 4 (2012) 1254-1261. [39] Y.B. Tang, C.S. Lee, J. Xu, Z.T. Liu, Z.H. Chen, Z. He, Y.L. Cao, G. Yuan, H. Song, L. Chen, L. Luo, H.M. Cheng, W.J. Zhang, I. Bello, S.T. Lee, Incorporation of graphenes in nanostructured TiO2 films via molecular grafting for dyesensitized solar cell application, ACS Nano 4 (2010) 3482. [40] Y. Lou, S. Yuan, Y. Zhao, P. Hu, Z. Wang, M. Zhang, L. Shi, D. Li, A simple route for decorating TiO2 nanoparticle over ZnO aggregates dye-sensitized solar cell, Chem. Eng. J. 229 (2013) 190-196. [41] Q. Zhang, T. Gao, J.M. Andino, Y. Li, Copper and iodine co-modified TiO2 nanoparticles for improved activity of CO2 photoreduction with water vapor, Appl. Catal. B Environ. 123-124 (2012) 257-264. [42] Simon Mathew, Aswani Yell, Peng Gao, Robin Humphry-Baker, Basile F.E.Curchod, Negar Ashari-Astanil, Ivano Tavernelli, Ursula Rothlisberger, Md.Khaja Nazeeruddin and M.Gratzel, Dye sensitized solar cells with 13% efficiency achieved through the molecular engineering of porphyrin sensitizers, Nature chemistry 6 (2014) 242-247. [43] K. Saranya, N. Sivasankar and A. Subramania, Microwave-assisted exfoliation method to develop platinum-decorated graphene nanosheets as a low cost counter electrode for dye-sensitized solar cells, RSC Adv. 4 (2014) 36226–36233. [44] J. Qi, X. Dang, P.T. Hammond, A.M. Belcher, Highly efficient plasmon-enhanced dye-sensitized solar cells through metal@oxide core-shell nanostructure. ACS Nano 5 (2011) 7108-7116 [45] Hara, K.; Sato, T.; Katoh, R.; Furube, A.; Ohga, Y.; Shinpo, A.; Suga, S.; Sayama, K.; Sugihara, H.; Arakawa, H., J. Phys. Chem. B Molecular Design of Coumarin Dyes for Efficient Dye-Sensitized Solar Cells,107 (2003) 597-606.

Figures

Fig.1. Schematic illustration of a) undoped DSSC; b) Cu-doped TiO2 DSSC. A shift in Fermi level in (b) Cu-doped DSSC can be seen due to LSPR effect

Fig. 2. Schematic of a plasmonic DSSC

Fig. 3. XRD patterns of pure and Cu-doped TiO2

Fig.4.1 Surface view of pure (a) and optimized 0.3 mole% Cu-doped TiO2 (b)

Fig.4.2 Cross sectional view of pure and optimized 0.3 mole% Cu-doped TiO2

Fig. 7. XPS spectra of Cu-doped TiO2 (a) Survey; (b) Cu 2p peaks; (c) Ti 2p peaks; (d) O 1s peak.

Fig. 6 (a) Absorbance spectra of pure and Cu-doped TiO2. (b) Tauc plot of pure and Cu-doped TiO2

Fig.7. N719 dye (0.3mM) adsorption onto the surface of pure and Cu-doped TiO2

Fig.8. EIS spectra of pure and Cu-doped DSSCs in the dark at bias potential of -0.7 V.

Fig.9. J-V characteristics of DSSCs based on pure and various Cu-doped TiO2 photoanodes.

Fig.10. IPCE spectra of pure and Cu-doped TiO2 DSSCs