Localized surface plasmon resonance of Cu-doped ZnO nanostructures and the material's integration in dye sensitized solar cells (DSSCs) enabling high open-circuit potentials

Journal Pre-proof Localized surface plasmon resonance of Cu-doped ZnO nanostructures and the material's integration in dye sensitized solar cells (DSSCs) enabling high open-circuit potentials K. Rajan Aneesiya, Cindrella Louis PII:

S0925-8388(20)30860-4

DOI:

https://doi.org/10.1016/j.jallcom.2020.154497

Reference:

JALCOM 154497

To appear in:

Journal of Alloys and Compounds

Received Date: 28 September 2019 Revised Date:

14 February 2020

Accepted Date: 23 February 2020

Please cite this article as: K.R. Aneesiya, C. Louis, Localized surface plasmon resonance of Cu-doped ZnO nanostructures and the material's integration in dye sensitized solar cells (DSSCs) enabling high open-circuit potentials, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/ j.jallcom.2020.154497. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier B.V.

CRediT author statement

Aneesiya K Rajan: Conceptualization, Methodology, Investigation, Writing- Original draft preparation. Louis Cindrella: Supervision, Validation, Writing- Reviewing and Editing.

Graphical Abstract

•

Localized surface plasmon resonance enhanced visible and near IR absorption in ZCu3.

Localized surface plasmon resonance of Cu-doped ZnO nanostructures and the material’s integration in dye sensitized solar cells (DSSCs) enabling high open-circuit potentials. Aneesiya K Rajan and Louis Cindrella Fuel Cell, Energy Materials and Physical Chemistry Laboratory, Department of Chemistry, National Institute of Technology, Tiruchirappalli-15, India. Abstract: Herein, we report one step co-precipitation method of synthesis of ZnO and Cu-doped ZnO nanostructures with distinct amount (1, 3 and 5 mol %) of copper. All the synthesized samples are characterized by X-ray powder diffraction, Field emission scanning and Transmission emission microscopy. The optical properties of the samples were evaluated by UVvisible spectroscopy and photoluminescence studies. The presence of Cu atoms in the ZnO matrix enhances a broad spectral absorption upto near-IR region. Suppressed E2high phonon modes are observed in Cu-doped samples through Raman scattering measurements, which furnishes evidence for effective Cu-doping in ZnO crystal lattice. Un-doped and Cu-doped ZnO samples were integrated in Dye sensitized solar cells (DSSCs) as photoanode materials. The device performance and interface charge transfer properties were evaluated through photocurrentdensity-photovoltage measurements and electrochemical impedance spectroscopy respectively. It is noted that the incorporation of Cu2+ into the hexagonal wurtzite structure of ZnO considerably elevates the Fermi levels towards the conduction band edge which is the root cause for high open-circuit potentials of DSSCs integrated with Cu-doped ZnO photoanodes.

Keywords: Zinc oxide, Cu-doping, dye sensitized solar cell, localized surface plasmon resonance, open-circuit potential.

1. Introduction Seeking alternative energy resources to fossil fuel is a critical challenge for mankind for the past few decades. Biomimetics has given rise to new technologies to harness solar energy, of which dye sensitized solar cells based on solid/liquid junctions, co-invented by O’Regan and M.Grätzel in 199l, is the foremost one [1]. Recent progress in this field is very much fostering, but the photovoltaic conversion efficiency of DSSC is still behind that of Si-based solar cells. To overcome the limitations which bring down the efficiency, new approaches have been invented in the research of components of DSSCs. For example, implementation in the form of new semiconductor architectures for the fabrication of photoanodes (i), synthetic strategies for the preparation of highly efficient panchromatic sensitizers (ii), modifications in the nature of electrolyte (iii) and improved electro-catalytic property of the counter electrode materials (iv). Photoanode is one of the key components of DSSC, since photo-excited electrons are received in its conduction band and injected to the back electrode through the external circuit. An ideal photoanode must have distinct morphology of nanostructure with narrow band gap, suitable surface area and porosity for dye intake. Of the semiconductor materials integrated in photoanodes, ZnO is the incomparable material both ecologically and economically. ZnO offers wide band gap of ∼3.36 eV, large exciton binding energy of 60 meV and high electron mobility of 115-155 cm2 V-1s-1 [2]. The applications of ZnO nanostructures involve, Femto-second laser direct writing of microholes on roughened ZnO for output power enhancement of InGaN lightemitting diodes [3], NH4Cl-modified ZnO for high-performance CsPbIBr2 perovskite solar cells via low-temperature process [4], highly compact CsPbBr3 perovskite thin films decorated by ZnO nanoparticles for enhanced random lasing [5], the growth of ZnO nanorods on GaN-based LEDs increases light extraction efficiency up to 57% [6] etc. Transparent and light emitting

epoxy composites containing ZnO quantum dots as encapsulating materials have also been reported for solid state lighting [7]. Rectification behavior has been reported in diodes consisting of intrinsic ZnO nanoparticles and Al doped ZnO nanoparticles [8]. Device structures in which amplified spontaneous emission was observed from ZnO in n-ZnO/ ZnO nanodots- SiO2 composite/p-AlGaN heterojunction and these devices exhibited significantly different dependence of the output power on injection current compared to devices without ZnO:SiO2 nanocomposite layer [9]. Most of the researchers attained significantly high photo-current density in DSSCs by using new doped ZnO materials and device architectures. But enhancement in open-circuit potentials is less reported so far. As the maximum open-circuit potential (Voc) of the liquid junction DSSC is decided by the difference between the quasi-Fermi level (EF) of the metal oxide semiconductor under illumination and the redox potential of the electrolyte, Voc can be improved by moderating the electronic structure of the semiconductor by elemental doping. Improvement in Voc for TiO2 based DSSCs was reported by doping Zr [10], Al [11] and Ta [12]. The possible reasons behind this enhancement may be due to i) elevation of Fermi levels of the metal oxide by electrondonating elements, ii) highly suppressed electron recombination rate and iii) change in the band positions of the metal oxide semiconductor. The root of these effects is still under discussion. Due to the wide band gap energy, ZnO is benefited mostly in the UV-region. As per the reports [13-15], doping by precious metals like Au, Ag and Cu surprisingly enhance the visible light absorption by their unique localized surface plasmon resonance (LSPR). LSPRs are charge density oscillations confined to metallic nanostructure. Spherical nanoparticles exhibit single dipolar LSPR, while nanoparticles with anisotropic shapes possess multiple higher-order LSPR modes. Electromagnetic field enhancements are localized near the corners and edges of

nanostructures due to induced LSPR from the cragged charge distributions [16]. The present work focuses on extending the visible absorption edge of ZnO by Cu (transition metal) doping. Cu is highly conducting, less expensive and readily available transition metal on Earth’s crust. The advantages of Cu-doped nanostructures include its cost-effectiveness in comparison with other novel metals, it can inhibit the recombination of photo-generated charge carriers during the operations of devices under light [17], it is a prominent luminescence activator, which can modify the luminescence of ZnO crystals by creating localized impurity levels [18] and can increase the light-harvesting efficiency of the sensitizing dye by enhancing far-field scattering. Our study aims at the preparation of Cu-doped nanostructures with increased near field-LSPR through simple co-precipitation method in aqueous media at normal temperature (60 ˚C) and integration of the as formed structures into the photo-anodes of DSSCs to enhance the Voc beyond 1V. 2. Experimental section 2.1 Materials Zn(CH3COO)2.2H2O, Cu(CH3COO)2.2H2O and NaOH were procured from Merck. The chemicals used for constructing DSSCs are of analytical grade and are procured from Sigma Aldrich and TCI chemicals. Millipore water was used. 2.2 Synthesis of ZnO and Cu-doped ZnO nanostructures. Co-precipitation

method

was

adopted

for

the

synthesis.

For

this,

0.25

M

of

(Zn(CH3COO)2.2H2O) (precursor salt) and 0.625 M of NaOH (precipitating agent) were prepared separately in Millipore water and stirred for 30 min at normal temperature (30± ˚C) conditions. Then NaOH was added dropwise to the precursor solution to form the precipitate.

The resultant solution was stirred continuously for 30 min at ambient temperature conditions (30± ˚C) then at 60 ˚C for 5 h. The obtained white color precipitate was rinsed few times with ethanol followed by water and then dried at 80 ˚C for 6 h. The obtained product was grinded using mortar and pestle for further characterizations. For the preparation of Cu-doped ZnO nanostructures, the same method was adopted. Briefly, specific quantity of Cu-salt solution (0.001M, 0.003 M and 0.005 M) was prepared and stirred for 30 min at ambient temperature conditions (30± ˚C) then added to 0.0025M, 0.0075M and 0.0125 M of Zn-salt solution in drops respectively and again stirred for 30 min at normal temperature (30± ˚C). Then, 0.625 M of NaOH solution was added gradually to the corresponding solutions and stirred for 1 h at room temperature (30± ˚C) followed by stirring at 60 ˚C for 5 h. The formed precipitates were rinsed thoroughly with water and ethanol few times, and then dried in vacuum oven for 6 h at 80 ˚C. The samples are named as Z0, ZCu1, ZCu3 and ZCu5 for undoped, 1 mM, 3 mM and 5 mM Cudoped samples respectively. 3. Results and discussion 3.1 Structural studies X-ray powder diffraction analysis was done to identify the crystal structure and phase purity of the undoped and Cu-doped nanomaterials. XRD spectra are displayed in Fig.1a. Nine recognizable diffraction peaks are observed for Z0, ZCu1, ZCu3 and ZCu5 and are indexed to hexagonal wurtzite structure of ZnO with primitive lattice (JCPDS: 79-2205).

The lattice

constants a and c were calculated with respect to the (100) and (002) planes of Z0 and are 3.253 Å and 5.206 Å respectively. Since the Cu-dopant is well incorporated in the host matrix lattice sites, no other phases of Cu or its oxides or binary compounds are observed. To describe peak intensity and peak shift, the primary (100), (002) and (101) diffraction planes were separately

analyzed (Fig. 1b shows (100) plane). The peak intensity of the ZnO samples increases from from undoped samples to ZCu3. This is due to the increase in crystallinity after Cu-doping. Due to comparatively low ionic radii, Cu2+ ions can be substituted in place of Zn2+ easily. The increased covalent nature of Cu-O bonds further decrease oxygen defects in the crystal lattice and promote crystallinity and c-axis growth [19]. But the intensity of the peaks is suppressed by further doping of Cu and fall below that of undoped sample in ZCu5. The excess doping of Cu2+ in ZnO crystal lattice creates lattice deformations in small areas around Cu2+ and also induces movement of Cu2+ ions to the interstitial sites. Excessive Cu-doping obstruct the preferential orientation along the c-axis [20]. The primary diffractions at (100), (002) and (101) planes are shifted to higher angle in ZCu3 and lower angles in ZCu1 and ZCu5. This is due to the lattice tensile or compressive stress induced in the Zn-sites of the crystal lattice by substitution of Cu atoms [21]. The average crystallite size of the nanostructures were calculated by Scherrer equation, =

.



(1)

Where ‘λ’ is 0.154 nm (wavelength of X-ray), ‘θ’ is the Bragg’s angle and ‘β’ is the broadening of the diffraction line signified by full width at half maximum (FWHM). The crystallite size for Z0, ZCu1, ZCu3 and ZCu5 are 32.071 nm, 33.668 nm, 32.830 nm and 23.020 nm respectively. The increased crystallite size refers to superior crystal quality which further enhance the nucleation and subsequent crystal growth [22]. The crystallite size for ZCu5 is lower than that of Z0, which would deteriorate further crystal growth.

Fig. 1 (a) Powder X-ray diffractograms of pure ZnO and Cu-doped ZnO nanostructures and (b) Enlarged view of (100) plane displaying diffraction peak shift. 3.2 Morphological studies FESEM analysis was performed to analyze the size and morphology of the pure ZnO and Cudoped ZnO nanostructures. Fig. 2 displays the FESEM images of Z0, ZCu1, ZCu3 and ZCu5 of different magnifications. Morphology of the nanostructures is entirely different from each other. In Z0, the nanoparticles are aggregated in the form coral reef like structures and the particles are observed to have the size of about 15-20 nm. In ZCu1, the product morphology was a mixture of nanoparticle aggregates in the form of nanospheres and nanorods. The length and diameter of the nanorods varied between 246 and 253 nm and 18 and 25 nm respectively. In ZCu3, a drastic change in morphology was observed. The nanoparticles are enlarged in the form of nanoflowers with spindle shaped petals originating from a central zone. The diameter of the central zone is about 161-163 nm and the length of the petals is around 160-190 nm. The morphology of ZCu5 is almost similar to that of ZCu1, but the particles are aggregated in the form of large spheres. To interpret the morphology in detail, TEM analysis was executed (Fig. 3 a-d). The as obtained images confirm the coral reef like Z0, mixture of spherical aggregates and scattered aggregates

of nanorods in ZCu1, flower like architecture with spindle shaped petals in ZCu3 and large spherical aggregates in ZCu5. The maximum frequency of the particle size of Z0, ZCu1, ZCu3 and ZCu5 are in the range of 15-20 nm, 15-20 nm, 10-15 nm and 25-30 nm respectively and are shown graphically in Fig. S1. The lowest partile size is observed for flower shaped nanostructure. Fig 3(e-h) represent the HRTEM images of synthesized nanorods. The distance between ZnO fringes that are perpendicular to the rod axis of Z0, ZCu1, ZCu3 and ZCu5 are separated by 0.18 nm, 0.19 nm, 0.25 nm and 0.17 nm respectively. Fig. 3(i-l) represent the selected area diffraction patterns of the synthesized nanostructures. From the results, it could be seen that all the doped samples and the un-doped one are single crystalline in nature. Elemental compositions of Z0, ZCu1, ZCu3 and ZCu5 were evaluated through EDAX analysis (Fig. S2). In Z0, the atomic ratio of Zn:O is 1:0.95. In ZCu1, the atomic ratio of Zn:Cu:O is 1:0.03:1.1 while in ZCu3, the ratio of Zn:Cu:O is 1: 0.05: 1.25 and in ZCu5, the atomic ratio of Zn:Cu:O is 1:0.07:1.27.

Fig. 2. FESEM images of un-doped ZnO (a and e) and Cu-doped samples ZCu1 (b and f), ZCu3 (c and g), ZCu5 (d and h). The scale bar for first column images is 1 µm and second column images is 200 nm.

Fig. 3. TEM images of un-doped ZnO (a) and Cu-doped samples ZCu1 (b), ZCu3 (c), ZCu5 (d). HRTEM images are shown as lattice fringes of (e) Z0, (f) ZCu1, (g) ZCu3 and (h) ZCu5. The SAED patterns of (i) Z0, (j) ZCu1, (k) ZCu3 and (l) ZCu5.

3.3 Raman spectral studies Raman spectral analysis is an excellent probe to analyze the phase and purity along with phonon interactions with the free carriers. Optical phonons at the C point of Brillouin zone can be represented by the irreducible representation Γopt = A1(z) + 2B1 + E1(x, y) + 2E2 in (x,y,z) polarization directions as per Group theoretical selection rules [23]. The polar modes A1 and E1 are split into transverse optical (TO) and longitudinal optical (LO) components. A1, E1 and E2 modes are Raman active and B1 modes are Raman inactive. The non-polar E2low and E2high modes

are representing the phonon vibrations of heavy zinc sub-lattice and oxygen atoms of wurtzite structure respectively [24]. Fig. 4 represents the Raman spectra of Z0, ZCu1, ZCu3 and ZCu5. All the spectra are split upto 3 portions for Lorentzian fitting. The combined Raman spectra are given in Fig. S3. No additional peaks of secondary phase for CuO or Cu2O are observed in the results which are consistent with XRD data. A very narrow and highly intense peak detected at ∼97.17 cm-1 corresponds to E2low modes of phonon vibrations associated with zinc sub-lattice [25]. Acoustic phonon overtones 2TA or 2 E2low modes are observed at ∼202.91 cm-1 in Z0 and are blue shifted in all the Cu-doped samples [26]. The intense peak at ∼328.83 cm-1 of Z0 due to second order multi-phonon Raman scattering of E2high-E2low zone boundary phonons of wurtzite structure is intense and blue shifted in Cu-doped samples [27]. The vibrational modes

at

∼378.96 cm-1 and ∼406.90 cm-1 of Z0 are assigned to A1(TO) and E1(TO) respectively [28]. These modes are not observed in Cu-doped samples. This may have been merged with E2high. The E2high peak, distinctive of non-polar optical phonones of wurtzite structure observed at ∼435.84 cm-1 in Z0 is suppressed, broadened and blue shifted as we go on to ZCu5 from Z0. This is due to the lattice deformations of ZnO by Cu-doping which would result in the formation of complex defects [CuZn- Zni]x in Cu-ZnO and gradually increase in defect densities due to oxygen vacancies [29]. The highest blue shift at ∼421.91 cm-1 is observed for ZCu3. This is preferably due to its anharmonic interaction with transverse acoustic (TA) and longitudinal acoustic (LA) phonon modes rather than lattice disorder [30]. Persistence of E2high mode even in ZCu5 is an indication of the stable wurtzite structure. A1(LO) modes and E1(LO) modes originated from second order Raman scattering are identified at ∼538.21 cm-1 and ∼583.75 cm-1 respectively for Z0. These modes are blue shifted in Cu-doped samples. A combination of transverse acoustic (TA) and longitudinal optical (LO) modes is observed at ∼651.14 cm-1 [31].

The broad peaks observed at ∼982.03 cm-1 and ∼1112.74 cm-1 are due to the overlapping of 2TO and 2LO modes respectively in Z0. These modes are also blue shifted in Cu-doped samples. The broad peak at ∼1156.48 cm-1 is the reflection of 2A1(LO)/E1(LO)/2(LO) modes [32]. The Cu2+ units in ZnO lattice is predicted to affect Raman features significantly in lower frequencies of Cu-doped samples. In the Cu-doped samples, the intensity of all the phonon modes except E2high is extreme especially in middle and high frequency phonon modes in ZCu3. This is because of the greatest amount of defect population at intermediate Zn/Cu ratios (3M % doping in ZCu3) due to significant amount of mixing of the two cations [33]. This would further affect the local polarizability and scattering cross section through the charge redistribution and hence the vibrational modes are also affected drastically. In ZCu3 and in ZCu5, comparatively intense peak is observed at ∼487.64 cm-1 and ∼497.07 cm-1 due to surface optical phonon modes [34]. The absence of certain vibrational modes corresponds to some secondary phases and the blue shift of almost all the phonon modes in Cu-doped samples further confirm the effective doping of Cu2+ ions in the crystal lattice of ZnO [35]. The wavenumbers of Raman active Γ-point phonon vibrational frequencies for wurtzite structure of Z0, ZCu1, ZCu3 and ZCu5 are listed in Table 1.

Table 1. The wavenumbers of Raman active wurtzite ZnO and Cu-doped ZnO Γ-point vibrational frequencies for Z0, ZCu1, ZCu3 and ZCu5. Wavenumber (cm-1) Process

Z0

ZCu1

ZCu3

ZCu5

E2low

∼97.17

98.41

95.85

95.87

2TA,2E2low

∼202.91

197.52

198.14

192.13

E2high – E2low

∼328.83

320.76

312.27

312.69

A1(TO)

∼378.96

…………

…………

…………

E1(TO)

∼406.90

…………

…………

…………

E2high

∼435.84

428.53

421.91

422.42

SOP

…………

…………

487.64

497.09

A1(LO)

∼538.21

528.58

…………

…………

E1(LO)

∼583.75

569.76

556.11

553.07

TA+LO

∼651.14

…………

…………

…………

2TO

∼982.03

928.43

941.15

935.76

2LO

∼1112.74

1115.45

1098.27

1099.83

2A1(LO),

∼1156.48

…………

…………

…………

E1(LO), 2(LO)

Fig. 4. Raman spectra of un-doped ZnO (Z0) (a,e and i) and Cu-doped ZnO samples ZCu1 (b, f and j), ZCu3 (c, g and k), ZCu5 (d, h and i) (All the spectra are Lorentzian fitted).

3.4 Optical properties The optical properties of the un-doped and Cu-doped samples were studied by UV-visible absorption spectroscopy (Fig. 5a) and diffuse reflectance (DRS) spectroscopy (Fig. S4). All the

nanostructures exhibit strong absorption in the UV-range. This is assigned to the electronic transitions from the valence band (VB) to conduction band (CB). The absorption wavelengths from 400-1000 nm is mainly due to sub band transitions and correlate surface oxygen vacancies [36]. A pronounced increase of visible range absorption is observed in Cu-doped samples. More than 35 % increment in visible light absorption is observed in ZCu3 and in ZCu5 with maximum visible light absorption in between ∼ 500-600 nm. This is due to LSPR absorption induced by Cu-ions in the ZnO lattice. Relative to pure ZnO, the optical absorption edges of Cu-doped samples are red shifted. The surface plasmon band of ZCu3 is comparatively red shifted by ∼24 nm from Z0 and ∼12 nm from ZCu5. The UV-range absorption is also highest in ZCu3. The band gap energies of the nanostructures were calculated from Kubelka-Munk plot (Fig. 5b), where [F(R)hυ]2 is plotted against hυ. Here F(R) = (1-R)2/2R, R is the reflectance of the samples obtained from DRS measurements. The band gap values of Z0, ZCu1, ZCu3 and ZCu5 are 3.32 eV, 3.30 eV, 3.28 eV and 3.29 eV respectively. A shift in absorption maxima of doped samples along with band gap shrinkage is due to Cu2+ induced two lone pair states above the VB. In this case Cu2+ ions are either located on surface sites or in bulk ZnO lattice which cause lattice deformations and creation of excess oxygen vacancies especially in ZCu3 by charge compensation effect [37]. The work function of Cu (4.7 eV) is lower than ZnO (5.3 eV). So the Cu-doped samples construct a constant Fermi level during the formation of metal-semiconductor junction by electron transfer from Cu to ZnO [38]. The Fermi level of ZCu3 doped electrode is more negatively shifted (upward shift) and the band gap energy decreases when the dopant concentration is more than the Mott critical density under working conditions in comparison with other electrodes fabricated with Z0, ZCu1 and ZCu5 [39]. Here strong spin exchange interactions of Cu-3d electrons and O-2p band electrons takes place [40].

Fig. 5. UV-visible absorption spectra (a) and Kubelka-Munk plot (b) for un-doped ZnO and Cudoped ZnO nanostructures. 3.5 Photoluminescence properties Photoluminescence spectra (PL) of pure ZnO and Cu-doped ZnO nanostructures were recorded at normal temperature (30 ˚C) with 350 nm as excitation wavelength and are depicted in Fig. 6. The individual PL spectrum is fitted by Gaussian function.

Fig. 6. Photoluminescence (PL) spectra of pure ZnO and Cu-doped ZnO nanostructures.

In ZnO, PL spectra are characterized by two distinct emissions. One sharp and highly intense near band emission (UV-region) and a broad visible emission constituting green yellow emissions and deep level emissions (DLE) [41]. The UV peak arises as the result of exciton recombination at the band gap and green emissions are mainly due to oxygen or zinc vacancies or due to surface defects [42]. The UV emission peak of Z0, ZCu1, ZCu3 and ZCu5 are located at ∼391nm, ∼399 nm, ∼401 nm and ∼394 nm respectively. All the UV-peaks are red shifted in Cu-doped samples in comparison with Z0. The maximum red shift of about ∼10 nm is observed in the case of ZCu3, showing obvious shrinkage in band gap. Also the intensity ratio of UV to

visible peak (IUV/IVis) is much higher in Cu-doped nanostructures indicating an enhanced luminescent property. The peak corresponding to violet emission is centered at ∼457 nm, ∼428 nm, ∼424 nm and ∼429 nm respectively for Z0, ZCu1, ZCu3 and ZCu5. These emissions arise due to electronic transition from shallow donor level of the zinc interstitials (Zin) to the top of the valence band [43]. The blue emission band arises due to singly ionized Zn vacancies centered at ∼464 nm, ∼468 nm, ∼467 nm and ∼465 nm respectively in Z0, ZCu1, ZCu3 and ZCu5 [44]. The green emission bands centered at ∼564 nm, ∼519 nm, ∼524 nm and ∼518 nm respectively in Z0, ZCu1, ZCu3 and ZCu5 arose due to green emissions. Quenching of green emissions in Cu-doped samples is attributed to the blocked defect sites by Cu deposition on the nanoparticle surface as well as in ZnO crystal lattice [34]. The CIE 1931 color space chromaticity diagram in the (x, y) co-ordinates system for Z0, ZCu1, ZCu3 and ZCu5 are shown in Fig. S6. The color purity for the nanostructures; Z0, ZCu1, ZCu3 and ZCu5 can be visualized as a combination of red, green and blue vertex regions with the chromaticity co-ordinates (0.32, 0.32), (0.19, 0.10), (0.20, 0.14) and (0.19, 0.11) respectively. The co-ordinates for Z0 are located at blue-red region and co-ordinates for ZCu1, ZCu3 and ZCu5 are located at blue region due to the decrement in emission peaks. The samples shine in accordance with the color co-ordinates under PL emission. 3.6 N2 adsorption-desorption analysis High specific surface area is one of the favorable factors for enhanced photocurrent density. To evaluate the adsorption nature of pure and Cu-doped nanostructures, BET analysis was done. The N2 adsorption-desorption isotherm is depicted in Fig. 7. All the isotherms exhibit reversible Type-IV adsorption-desorption characteristics. It represents the unrestricted monolayermultilayer adsorption. The monolayer adsorption takes place upto relative pressure of 0.1 and the quantity adsorbed is ∼4.00 cm3g-1 at STP (shown as inset in Fig.7) and after that the multilayer

adsorption begins. All the isotherms exhibit type H3 hysteresis loop at P/P0 ranging from 0.7 to 1.0 in Z0 and 0.85 to 0.1 in Cu-doped samples (shown as inset in Fig. 7) due to the capillary condensation occurreing in the mesopores. This hysteresis loop is an indication of aggregates of particles forming slit-like pore connectivity networks [45]. The surface area of Z0 is 10.4052 cm2g-1. All the Cu-doped samples exhibit larger surface area than that of un-doped ZnO. The highest BET surface area of 20.0433 cm2g-1 is observed with ZCu1. For ZCu1, the surface area is 19.2136 cm2g-1. ZCu3 exhibits an intermediate surface area of 15.0644 cm2g-1. Besides BET surface area measurements, BJH pore size distribution analysis was also performed for un-doped ZnO and Cu-doped ZnO nanostructures to evaluate the diameter and volume of mesopores. The pore size distribution curves of synthesized nanostructures are displayed in Fig. S5. The BET surface area, average pore area, average pore diameter and average pore volume of Z0, ZCu1, Zcu3 and Zcu5 are given in Table 2.

Fig.7. N2 adsorption-desorption isotherms of pure ZnO and Cu-doped ZnO nanostructures (insets: (a) monolayer adsorption and (b) hysteresis loop).

The evaluated pore volumes of Cu-doped samples are more than double the volume of pores present in undoped ZnO (Z0). The maximum pore volume of 0.100172 cm3g-1 was observed in ZCu1. Formation of two peaks is observed in the pore size distribution curve of Cu-doped samples. The first peak centered at ∼180 Å represents the existence of supermicropores and the broad peak centered at ∼1036 Å is an indication of wide range of mesoporosity in the Cu-doped samples [46].

Table 2. BET surface area characteristics and BJH pore-size distribution analysis of pure ZnO and Cu-doped ZnO nanostructures. Average pore area (m2.g-1)

ZCu5

BET surface area (m2.g-1) 19.2136

ZCu3

20.5116

Average pore diameter (Å) 149.527

Average pore volume (cm3.g-1) 0.076676

15.0644

16.2136

151.276

0.061318

ZCu1

20.0433

20.2125

198.238

0.100172

Z0

10.4052

8.6934

182.934

0.039758

Nanostructure

High surface area and pore volume respectively are remarkable criteria to host the dye molecules and enable dye infiltration process within the mesoporous thin films during sensitization [47]. Increased pore size enhances the electrolyte diffusion and also prevents the formation of Zn2+ /dye complex in Cu-doped nanostructures. 3.7 Photovoltaic studies The aforementioned characteristic properties of the un-doped ZnO (Z0) and the Cu-doped nanostructures (ZCu1, ZCu3 and Zcu5) provide confirmations for essential characteristic features of photo-anode materials in DSSCs. The fabricated DSSCs (The fabrication methods are given in supporting information) are evaluated from the photocurrent-photovoltage curves constructed under test conditions, and are represented in Fig. 8. The photovoltaic performance parameters such as photocurrent density (Jsc), photovoltage (Voc), fill factor (ff) and photovoltaic conversion efficiency (η) are tabulated in Table 3. The DSSCs constructed with Cu-doped ZnO based photoanodes show enhanced efficiency in comparison with un-doped ZnO based one.

Fig. 8. J-V curves of DSSCs fabricated with pure ZnO (Z0) and Cu-doped ZnO nanostructures (ZCu1, ZCu3 and ZCu5) constituting photoanodes.

Table 3. Photovoltaic performance parameters of DSSCs fabricated with pure ZnO (Z0) and Cudoped ZnO nanostructures (ZCu1, ZCu3 and ZCu5) constituting photoanodes. DSSC

Jsc (mA.cm-2)

Voc (mV)

ff

η (%)

ZCu5

1.600

991

0.688

1.092

ZCu3

1.800

1086

0.686

1.342

ZCu1

1.674

964

0.714

1.151

ZNP

1.573

875

0.690

0.950

Upon Cu-doping, the absorption edge of ZnO is shifted to visible region and an intense absorption in the entire visible range and near infra-red is observed. A four-fold improvement in the visible absorption is noticed for ZCu5 and a three-fold improvement in ZCu3. The maximum

short-circuit current density of 1.800 mA cm-2 and open-circuit voltage of 1.086 V were recorded for the DSSC fabricated with ZCu3. The 3 molar % Cu-doping is found to be the optimum doping level due to optimal carrier concentration in the semiconductor, significant morphology and relevant particle size distribution. Cu-doping induces LSPRs when the nanostructures allowed the incident photons to combine with CB electrons. These LSPRs augment the far field light scattering and electromagnetic near fields around Cu-doped nanostructures [48]. Unlike the spherical aggregates of Z0, ZCu1 and ZCu5; ZCu3 displays an isotropic and anisotropic plasmonic structure which generate intense fields near the sharp edges and corners of the spindle shaped nanoflowers with a central zone due to the quasi-static lighting rod effect [49]. And these localized fields resemble “nano-sized light concentrators” which harvest and scatter sunlight and functions as antennae for enhanced light absorption by re-absorbing light from the dye monolayer and also cause separation of charge carriers. The as formed hot electrons are rapidly consumed in the CB which amplify the number of charge carriers available for photo-current generation [50]. The mechanism of plasmonic enhancement of Cu-doped ZnO based DSSCs performance can be interpreted through the energy diagram represented in Fig. 9. Upon illumination, the Cu atoms present in the wurtzite ZnO preferentially absorb incident light and generate confined electromagnetic fields on the semiconductor surfaces especially on the sharp edges and corners of ZCu3. The conduction band of ZnO is activated due the presence of LSPR and there is an accumulation of large number of electrons in Cu-doped ones. As a result, the Fermi level of Cu-doped ZnO gets tuned towards an equilibrium position (negative shift). Also, the nearby sensitizer molecules use the intense plasmonic near fields as secondary light source and cause enhanced light absorption, which further generate more number of electron-hole pairs

in the dye. Therefore large numbers of electrons from LUMO of the dye to the conduction band of Cu-ZnO are transferred.

Fig. 9. Energy diagram describing plasmonic enhancement absorption by sensitizer in Cu-doped ZnO based DSSCs. Due to the average particle size of 15-20 nm in ZCu3, the ratio of reflectance to absorption is very low in ZCu3. The aforesaid properties contribute well to the photo-current improvement in ZCu3 based DSSC even though it possesses less surface area for dye intake in comparison with ZCu1. In ZCu1 based DSSC, the high surface area for dye adsorption is the major factor for increased photovoltaic conversion efficiency in contrast to ZCu5 based one. When the Cudoping level surpassed 5 molar %, the electron-hole recombination increases.

Also the

possibility of dissipation of heat (due to light energy to heat conversion) by large nanostructure aggregates (25-30 nm diameter) will further enhance phonon density and reduce the number of charge carriers responsible for current generation [51]. A noticeable increase in Voc has been manifested by the DSSCs made up of Cu-doped nanostructures. A very high open-circuit potential of 1.086 V is recorded for device fabricated with ZCu3 constituting photoanode. This

is due to shrinkage in band gap energy of ZCu3. As a result, the negative shift of Fermi levels as compared to Z0, ZCu1 and ZCu5 might be possible in ZCu3 due to optimum Cu-doping [36]. 3.8 Electrochemical interface charge transfer studies of ZnO DSSCs The influence of Cu-doping on the charge transfer properties of electrical double layers of the constructed DSSCs were assessed in terms of electrochemical impedance spectroscopic studies (EIS). The representative Nyquist plots of EIS measurements are shown in Fig. 9. A typical Nyquist plot of DSSC is composed of three semicircles. The high frequency interception on the real axis is ascribed to counter electrode/electrolyte resistance (R1). The larger semicircle at the middle frequency region is related to the charge transfer resistance (R2) at ZnO/dye/electrolyte interfaces [52]. The small semicircle appearing at lower frequency region corresponds to Warburg diffusion of I-/I3- redox couple within the electrolyte. In the procured spectra, the first semicircle is merged with middle semi-circle. The R2 values of the DSSCs are in the order, Z0 > ZCu5 > ZCu1 > ZCu3. From the results, it could be inferred that the DSSC fabricated with ZCu3 based photoanode experiences the least charge transfer resistance across the ZCu3/N3dye/electrolye interfaces, which will further reduce the charge recombination and enhance the fast electron transfer. In plasmonic nanostructures based DSSCs, hot electrons can be generated via exothermic chemical process. Followed by light absorption and LSPR excitation in these structures, electromagnetic decay takes place either through re-emitted photons or through nonradiative transfer of energy to hot-electrons. After the surface plasmon decay, the electrons from occupied energy levels are excited above the Fermi energy. The LSPR can able to transfer approximately 1-4 eV of energy to the hot electrons. This energy depends on the size and shape of the nanostructures. An efficient mechanism for capturing such hot electrons is to form a Schottky barrier with an appropriate semiconductor. In the present study it is possible with ZnO.

These barriers can induce separation of photo-generated charge carriers and build-up a better charge density in the in the CB of ZnO and obstruct the electron back flow. In the case of ZCu3, due to the specific shape of the structure with sharp edges and corners, the energy transfer capacity of the LSPRs might be maximum among the other structures since it depends on shape to a large extent [53]. This might be the reason for least charge transfer resistance across the ZCu3/N3- dye/electrolyte interfaces in ZCu3 based DSSC. The device fabricated with Z0 possessed the high charge transfer resistance, which will further affect the electron recombination rate and overall efficiency of the device.

Fig. 10. Nyquist plots for electrochemical impedance spectroscopic analysis of DSSCs fabricated with pure ZnO (Z0) and Cu-doped ZnO nanostructures (ZCu1, ZCu3 and ZCu5) constituting photoanodes.

4. Conclusions In summary, we demonstrated a simple and economical way of synthesis of Cu-doped ZnO nanostructures which display localized surface plasmon resonances (LSPRs).

The optical

absorption properties of ZnO were considerably extended upto near IR wavelengths. The LSPRs set up confined electromagnetic fields upon illumination on the nanostructure surfaces and the sensitizer molecules utilizing it as secondary light source. Enormous number of electron-hole pairs are generated which boost electron transfer from excited dye molecules to CB of the metal oxide. The ZCu3 with optimum concentration of Cu-dopant substantially enhance the photovoltaic conversion efficiency (1.342 %) with very high open-circuit potential (Voc) of 1.086 V. The band gap narrowing and the negative shift of Fermi level upon 3 molar % Cu-doping is the root cause for the high Voc. The aforementioned study addresses a new concept of introducing LSPR effect and band gap tuning in Cu-doped ZnO for the construction of plasmon enhanced DSSCs with high open circuit voltages. Acknowledgements The authors are grateful to Ministry of Human Resource Development, Government of India for financial assistance and NIT Tiruchirapalli for research facilities. Supporting Information Details of chemicals used for DSSC fabrication, method of fabrication of DSSCs, analytical instruments used for characterization of nanostructures, average particle size distribution graph of pure ZnO and Cu-doped ZnO, EDAX spectra, combined Raman spectra, diffuse reflectance spectra (DRS), CIE plot and the pore size distribution curves.

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Highlights •

Effect of copper doping of ZnO.

•

Enhancement in broad spectral absorption upto near-IR region of Cu doped ZnO.

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Suppressed E2high phonon modes as evidence of effective Cu-doping in ZnO crystal lattice.

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High open-circuit potentials of DSSCs integrated with Cu-doped ZnO photoanodes.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: