Cu2AgInSe4 QDs sensitized electrospun porous TiO2 nanofibers as an efficient photoanode for quantum dot sensitized solar cells

Solar Energy 199 (2020) 317–325

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Cu2AgInSe4 QDs sensitized electrospun porous TiO2 nanofibers as an efficient photoanode for quantum dot sensitized solar cells

T

Roopakala Kottayia,b, Pratheep Panneerselvama, Vignesh Murugadossa, Ramadasse Sittaramaneb, ⁎ Subramania Angaiaha, a b

Electro-Materials Research Laboratory, Centre for Nanoscience and Technology, Pondicherry University, Puducherry 605014, India Department of Physics, Kanchi Mamunivar Govt. Institute for Post Graduate Studies and Research, Puducherry 605008, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Cu2AgInSe4 QDs Tauc plot Porous TiO2 Nanofibers Hot injection method QDSC

To obtain an efficient quantum dot sensitized solar cell (QDSC), a less toxic quaternary Cu2AgInSe4 QDs with 4.8 nm in size are synthesized by a simple hot injection method. The crystallite size and tetragonal structure are confirmed by XRD and HR-TEM analysis. Energy-dispersive X-ray spectroscopy analysis reveals that the atomic ratio of Cu: Ag: In: Se in the Cu2AgInSe4 QDs is 1.98:1.0:1.03:3.86. The oxidation state of the elements composed in Cu2AgInSe4 QDs is confirmed by XPS studies. Optical properties are studied from the UV–Vis–NIR absorption spectrum and photoluminescence emission spectrum. The porous TiO2 nanofibers (P-TiO2 NFs) are prepared from the conventional electrospun TiO2 NFs followed by the solvosonication process. The FE-SEM analysis is confirmed the porous texture of the TiO2 NFs. The bandgap of the Cu2AgInSe4 QDs and TiO2 NFs are determined from the Tauc plot and it was found to be 1.93 eV and 3.19 eV, respectively. QDSC is assembled using Cu2S counter electrode, polysulfide redox couple electrolyte and Cu2AgInSe4 QDs sensitized P-TiO2 NFs photoanode. The photoconversion efficiency (PCE) of the assembled QDSC is found to be 4.24%.

1. Introduction The urgent need for clean and renewable energy explores the reinvigorate approaches to develop a highly efficient and low-cost solar cell (Liu et al., 2012). Colloidal semiconductor quantum dots as solar harvesters constitute a promising approach toward the third-generation solar cells owing to their solution processability, high absorption coefficient, bandgap tunability and the possibility of multiple exciton generations (Duan et al., 2015; Hines and Kamat, 2014; Kim et al., 2003; Pan et al., 2014; Zhao et al., 2015). Despite these advantages, QDSC still exhibits only 13.85% PCE (Zhang et al., 2019). In the past decade, most of the QDSCs are mainly focused on cadmium-based and lead-based chalcogenide materials. But high toxicity of cadmium and lead restricts their commercial applications. Hence, the development of less-toxic binary and ternary QDs has been investigated (Branham et al., 2006; Coughlan et al., 2017). But binary and ternary QDs based QDSC have attained less PCE (Jing et al., 2016). Copper-based quaternary QDs are reported as promising sensitizers for QDSC due to its excellent optical and optoelectronic properties (Coughlan et al., 2017; Liu et al., 2012; Pan et al., 2018). Further to replacement of Indium (In) in the place of Tin (Sn) (Deng et al., 2013; Du et al., 2016; Pan et al., 2019;

⁎

Zhang et al., 2019, 2017), improved the light-harvesting capacity of quantum dots such as Cu-Zn-Sn-Se (Singh et al., 2018a) and Cu-Zn-Sn-S (Bai et al., 2015). But widening the light-harvesting range still remains as a challenging one. It has been demonstrated that the silver and copper-based QDs exhibit broad light-harvesting range from the UV–visible to the near-infrared region to their high conduction band edge and wide absorption range (Song et al., 2016; Zhang et al., 2018), which is very essential for capturing more photons and favourable for improving the PCE. In this aspect, herein we proposed to investigate the optical properties of Cu2AgInSe4 QDs for using sensitizer of QDSC. To the best of our knowledge, no data is available on Cu2AgInSe4 QDs as the sensitizer for QDSC. The photovoltaic performance of QDSC depends not only on the nature of the sensitizers but also on the properties of the counter electrolyte, redox electrolyte, and photoanode materials. In QDSC, photoanode should have a high surface area to occupy more amounts of sensitizers and enhance light-harvesting (Sudhagar et al., 2014). Subramania and co-workers demonstrated that electrospun porous TiO2 nanofibers possess a high surface area for maximum absorption of QDs (Singh et al., 2019, 2018b). In this work, the structural, morphological and optical properties of

Corresponding author. E-mail address: [email protected] (S. Angaiah).

https://doi.org/10.1016/j.solener.2020.02.010 Received 26 November 2019; Received in revised form 14 January 2020; Accepted 3 February 2020 0038-092X/ © 2020 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.

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for 2 h. The above solution was transferred into an autoclave kept at 200 °C for 12 h. The obtained precipitate was washed several times with distilled water and ethanol and dried to obtain Cu2S NPs. Cu2S NPs' paste was prepared by mixing 95 wt% Cu2S NPs with 5 wt% of N-methyl −2-pyrrolidine. This paste was coated onto the FTO substrate to use as the counter electrode (CE).

Cu2AgInSe4 QDs and Cu2AgInSe4 QDs sensitized electrospun P-TiO2 NFs are studied. Further, QDSC is constructed with Cu2S counter electrode, Cu2AgInSe4 QDs/P-TiO2 NFs photoanode and polysulfide redox couple electrolyte and their photovoltaic performance is studied in detail. 2. Experimental

2.6. Fabrication of QDSC 2.1. Materials used Polyvinyl pyrrolidone (PVP), Titanium (IV) isopropoxide (TiP), Copper (III) chloride, Silver chloride, Indium (III) chloride, Selenium powder, Oleylamine (O Am), L- Cysteine, 1-Dodecanethiol (DDTh), Ethanol, Methanol, Chloroform, Glacial acetic acid, Terpinol, Dibutyl phthalate, Glycerol, and Ethylcellulose were purchased from SigmaAldrich. FTO glass plate is used as the substrate for the QDSC device.

QDSC was fabricated by using the following procedure; the prepared photoanode and the CE were sealed using a hot melt spacer. The polysulphide redox electrolyte containing 0.2 M KCl, 2 M S and 2 M Na2S in 7:3 vol mixtures of de-ionized water and methanol was injected through the predrilled holes in the CE and then the holes were closed by using a surlyn strip (Singh et al., 2018a). The active area of the cell was 0.5 × 0.5 cm2.

2.2. Preparation of P-TiO2 NFs

2.7. Physical characterization

The porous TiO2 NFs were obtained from electrospun TiO2 NFs by the solvosonication process as reported in our earlier work (Elayappan et al., 2015; Singh et al., 2019, 2018b, 20018a). Briefly, a solution of 0.005 M TiP, 0.02 M acetic acid and 5 wt% PVP in ethanol was loaded in a syringe with 27 G stainless steel needle and electrospinning was carried out at the potential of 15 kV at the flow rate of 1.5 mL/h to develop nanofibrous precursor mat, which was calcined at 450 °C for 5 h to get electrospun fibrous mat (TiO2 NFs). Finally, the resultant TiO2 NFs were ultrasonicated in glycerol medium for 90 min to obtain porous TiO2 NFs (P-TiO2 NFs).

The Rigaku Ultima (IV) X-ray diffraction analyzer (Cu-κα 1.54Å source) was used for the structural analysis of the synthesized porous TiO2 NFs, CAISe QDs, and CAISe QDs/P-TiO2 NFs. The Perkin Elmer (Model: L-650) spectrophotometer was used to obtain the UV–Vis-NIR spectra. The Horiba-Jobin-Yvon spectrometer (Xenon lamp as the source for excitation) was used to record photoluminescence (PL) emission spectra. Raman spectra were recorded by using Renishaw (Model: RM 2000) Raman spectrometer (532 nm laser with 0.05% optical power). Kratos Analytical Ltd (Ultra DLD) X-ray photoelectron spectrometer with X-mono A1- κα ray source at an operating power of 75 W was used to record the XPS spectrum. A field emission scanning electron microscope (Model: Zeiss Supra 55VP) was used to record the FE-SEM image for morphological studies of P-TiO2 NFs. A high-resolution transmission electron microscope (Model: JSM-7600F) was used to record the HR-TEM image of CAISe QDs. The energy dispersive X-ray spectrometer coupled with HR-TEM was used to record the EDX spectra of CAISe QDs.

2.3. Synthesis of Cu2AgInSe4 QDs The Cu2AgInSe4 (CAISe) QDs were synthesized by a two-step simple hot injection method as follows. First step: 2 mL of OAm, 4 mmol of selenium metal powder and 2 mL DDTh was stirred for 2 h to obtain selenium precursor solution. Second step: 2 mL of DDTh and 10 mL of OAm were mixed with 2 mmol of CuCl2, 1 mmol of AgCl and 1 mmol of InCl3 and refluxed initially up to 100 °C under an inert atmosphere for 30 min. The temperature was gradually raised to 180 °C. At this temperature, the pre-prepared selenium solution was injected into it. This reaction temperature was maintained 10 min. It helps the nucleation and growth process. After 10 min, the reaction was stopped and then suddenly poured into the cold methanol. After washing several times with ethanol and methanol, the pure CAISe QDs were dispersed in chloroform.

2.8. Electrochemical and photovoltaic performance studies The electrochemical impedance studies were carried out with an AC voltage amplitude of 10 mV in the frequency range of 1 mHz to 100 kHz by using an electrochemical workstation (Biologic-VSP) to study about the electron recombination and charge transfer dynamics at the electrode(s)/electrolyte interface. The J-V characteristics (current density-voltage) of the QDSC was recorded by using AM 1.5G solar simulator (Newport, Model: 67005) with a light intensity of 100 mW cm−2 and its corresponding cell parameter values such as photocurrent density (Jsc) and open-circuit voltage (Voc) were measured (Murugadoss et al., 2019a). The photoconversion efficiency (η) and the fill factor (FF) were calculated by using the standard equations (Angaiah et al., 2018; Murugadoss et al., 2019a, 2019b, 2017).

2.4. Preparation of CAISe QDs/P-TiO2 NFs photoanode The CAISe QDs sensitized P-TiO2 NFs photoanode was fabricated by the following procedure. Firstly, a pre-cleaned FTO substrate coated with 5% TiP in ethanol was annealed at 450 °C for 30 min. Secondly, the prepared P-TiO2 NFs paste was coated on it at a thickness of 11–12 μm using the doctor blade technique. Thirdly, the above substrate was immersed in a solution containing 0.12 M TiCl4 for 30 min (Murugadoss et al., 2017). Finally, this was immersed in 10 mL of 1:9(v/v) ratio of 3-mercaptopropionic acid and acetonitrile for 48 h, followed by immersion in CAISe QDs colloidal solution for 48 h to obtain CAISe QDs sensitized P-TiO2 NFs (CAISe QDs/P-TiO2 NFs) as photoanode.

3. Results and discussion 3.1. Morphological studies of P-TiO2 NFs Morphological studies of electrospun TiO2 NFs and P- TiO2 NFs were examined by FE-SEM analysis. Fig. 1a shows the FE-SEM image of electrospun TiO2 NFs and Fig. 1b shows the FE-SEM image of electrospun P- TiO2 NFs. The electrospun P- TiO2 NFs show a relatively higher specific surface area and high pore volume than that of electrospun TiO2 NFs. It indicates that solvosonication plays a vital role in the pore formation that increases the surface area (Singh et al., 2018b). It helps to absorb more QDs and also react with more redox electrolytes onto PTiO2 NFs.

2.5. Preparation of Cu2S counter electrode Cu2S nanoparticles (NPs) were prepared by the hydrothermal method using CuCl2 and L-Cysteine as reported earlier (Salam et al., 2015; Saranya et al., 2015). Briefly, 2 mmol of CuCl2 and 1 mmol LCysteine were added to 50 mL deionized water and stirred continually 318

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Fig. 1. FE-SEM image of electrospun TiO2 NFs mat (a) before pore formation (b) after pore formation.

(105) (004)

(204)

(211)

(126)

2

(αhν)

Absorbance (a.u)

Intensity (a.u)

(101)

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 hν (eV)

(220)

10

20

30

40 50 2θ (deg)

60

70

80

200

Fig. 2. XRD Pattern of P- TiO2 NFs.

300

400 500 600 Wavelength (nm)

700

800

Fig. 3. UV–Vis NIR spectrum of P-TiO2 NFs. (Inset: Tauc plot).

3.2. Structural analysis of P- TiO2 NFs

CAISe QDs

(112)

Intensity (a.u)

The XRD pattern of P- TiO2 NFs is shown in Fig. 2. The diffraction peaks at 25.2°, 38.0°, 48.0°, 54.1°, 55.0°, 62.9o, and 70.2°, correspond to the (1 0 1), (0 0 4), (1 0 5), (2 1 1), (2 0 4), (1 2 6) and (2 2 0) planes of the pure anatase structure of TiO2 (JCPDS 89-4921) (Karthick et al., 2012, 2011). The absence of peaks at 27° and 31° indicate that the prepared TiO2 NFs have no rutile phase. 3.3. Optical studies of P-TiO2 NFs The UV–Vis NIR spectrum of P-TiO2 NFs (Fig. 3) shows an absorption onset of less than 400 nm with a broadband absorption in the nearultraviolet and visible. This is due to the transition of electrons from the antibonding of O2− to the lowest unoccupied orbital of Ti4+ (Karthick et al., 2012). From the Tauc plot (Inset of Fig. 2) of the P-TiO2, its bandgap energy is found to be 3.19 eV.

10

(220)

(312)

20

30

40 50 2θ (deg)

60

70

80

Fig. 4. XRD pattern of CAISe QDs.

3.4. Structural studies of CAISe QDs

3.5. Spectral studies of CAISe QDs

The XRD pattern of CAISe QDs is shown in Fig. 4. The three major peaks at 2θ = 27.2°, 45.2° and 53.3° are consistent with (1 1 2), (2 2 0/ 2 0 4) and (3 1 2) planes, respectively, of tetragonal kesterite phase (Engberg et al., 2015; Liu et al., 2012; Singh et al., 2016). All three peaks are broad and strong because of the small size and crystalline nature of QDs. There are no impurity peaks observed. By using DebyeScherer’s equation the average diameter of these QDs is calculated to be 4.8 nm (Das et al., 2018; Gantassi et al., 2015; Lox et al., 2018).

Raman spectrum of CAISe QDs is shown in Fig. 5. The peaks at 266 cm−1, 233 cm−1, and 183 cm−1 correspond to the crystalline nature of CAISe QDs. No other secondary phase peaks correspond to Cu2Se (345 cm−1), Ag2Se (176 cm−1) (Dalmases et al., 2016; Tubtimtae et al., 2011; Zhu et al., 2013) and CuInSe (154 cm−1) (Li et al., 2015) are observed. This indicates the high purity of synthesized 319

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CAISe QDs

Intensity (a.u)

233 cm-1 266 cm-1 183cm-1

100

150

200

250

300

Raman shift (cm-1)

350

Fig. 5. Raman spectrum of CAISe QDs.

Fig. 7. UV–Vis-NIR spectrum of CAISe QDs (Inset: Tauc plot).

QDs. XPS is used to determine the valence band-edge and oxidation states of CAISe QDs. Fig. 6a–f shows the XPS spectra of survey scan, Cu, Ag, In, Se and band edge, respectively. In Fig. 6a, the peaks of Se 3d, Ag 3d, In 3d and Cu 2p are originating from CAISe QDs. The O 1s peak is due to the adsorption of oxygen from the laboratory environment (Moulder et al., 2000, Murugadoss et al., 2017). In Fig. 6b, two strong peaks at the binding energies of 951.23 eV and 931.10 eV correspond to Cu2p1/2 and Cu2p3/2, which arises due to the spin-orbit doublet of the 2p core shells. Here the binding energy difference is greater than 19.8 eV, which attributes the presence of Cu2+ ions (Ghodselahi et al., 2008; Moulder et al., 2000; Shaikh et al., 2011; Velásquez et al., 2000). In Fig. 6c, the two peaks at 373.83 eV and 366.94 eV correspond to the

binding energies of Ag 3d3/2 and Ag 3d5/2, respectively indicates the presence of Ag+ ions (Branham et al., 2006; Moulder et al., 2000). In Fig. 6d, the In3d spectrum exhibits peaks at 451.38 eV and 443.78 eV, which correspond to In 3d3/2 and In 3d5/2. It indicates the presence of In3+ ions (Kim et al., 2015; Moulder et al., 2000). In Fig. 6e, the peak corresponds to Se 3d5/2 at 53.4 eV indicates Se2− oxidation state of selenium (Moulder et al., 2000; Subila et al., 2013) in CAISe QDs. The valence band edge of CAISe QDs is calculated from the XPS survey spectrum (Fig. 6f) is −1.86 eV (Carey et al., 2017; Kraut et al., 1983; Yu et al., 1990). Hence, the valence band maximum is found to be −5.57 eV (Johnson et al., 2009; Kraut et al., 1983; Liang et al., 2017). The UV–Vis-NIR spectrum of the CAISe QDs is shown in Fig. 7. This spectrum shows the strong absorbance from the visible region to the

O 1s

Cu 2p

Ag 3d

366.94 eV

Ag 3d3/2 373.83 eV Intensity (a.u)

In 3d

Ag 3d5/2

(c)

Ag 3d

2p1/2 951.23 eV

Intensity (a.u)

Cu 2p

Intensity (a.u)

2p3/2 931.10 eV

(b)

CAISe QDs Survey

(a)

Se 3d

1000

800 600 400 Binding energy (eV)

200

0

960

In 3d5/2

(d)

950

945 940 935 Binding energy (eV)

930

925 380 378 376 374 372 370 368 366 364 362 360 Binding energy (eV)

3d5/2 53.40 eV

(e)

443.78 eV

In 3d

955

(f)

Se 3d

In 3d3/2

Intensity (a.u)

Intensity (a.u)

451.38 eV

Intensity (a.u)

1200

-1.86 eV

460

455

450 445 Binding energy (eV)

440 60

58

56 54 Binding Energy (eV)

52

50

-4

-2

0 2 4 Binding energy (eV)

6

8

10

Fig. 6. (a) The survey XPS spectrum, (b) Cu 2p XPS spectrum, (c) Ag 3d XPS spectrum, (d) In 3d XPS spectrum, (e) Se 3d XPS spectrum, (f) Band edge XPS spectrum. 320

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CAISe QDs/P-TiO2 P-TiO2 NFs CAISe QDs

(101)

(112) (004)

Intensity (a.u)

Intensity (a.u)

845 nm

(200)

(101)

780

810

840

870

900

Wavelength (nm)

930

960

990

10

(200)

(004)

(112)

750

(105)

(220)

20

30

(220)

40

50

2θ (deg)

(105) (312)

60

70

80

Fig. 8. PL emission spectrum of CAISe QDs.

Fig. 10. XRD patterns of CAISe QDs/P-TiO2 NFs, P-TiO2 NFs, and CAISe QDs.

infrared region. This indicates the high photon capturing capacity of the CAISe QDs. From the Tauc plot (Inset of Fig. 7), the bandgap of CAISe QDs is estimated to be 1.93 eV. Fig. 8 shows the PL emission spectrum of CAISe QDs. It exhibits emission at the excitation wavelength of 295 nm. It shows the symmetrical single peaks in the range between 800 nm and 950 nm regions with a maximum PL emission intensity at 845 nm. The narrow band with a high intense PL peak revealed that the synthesized QDs are free from defects.

that the synthesized QDs are well-formed and the average crystallite size is about 4.8 nm. From lattice fringes, the interplanar distance is observed to be 0.31 nm, which corresponds to the (1 1 2) plane of tetragonal kesterite phase crystals (Ajjammouri et al., 2016; Baláž et al., 2018; Bree et al., 2018; Li et al., 2012; Narayana et al., 2013; Prabhakar et al., 2016; Singh et al., 2016; Wang et al., 2015). Well-defined concentric rings in the SAED pattern (Fig. 9b) confirm the crystalline nature of CAISe QDs. This SAED pattern firmly supports the XRD result, which shows the rings at (1 1 2), (2 2 0), (3 1 2) planes. Fig. 9c depicts the EDX analysis of the synthesized CAISe QDs. It shows the atomic ratio of Cu/Ag/In/Se is 1.98/1.0/1.03/3.86 that is nearer to the 2/1/1/ 4 ideal ratio.

3.6. TEM studies of CAISe QDs The HR-TEM image of CAISe QDs is shown in Fig. 9a. It indicates

(a) Cu AgInSe QDs 2 4

(b)

0.31nm (112)

(c)

Element Cu Ag In Se

Atomic % 26.03 13.10 13.61 47.26

Fig. 9. (a) HR-TEM image, (b) SAED pattern and (c) EDX spectrum of CAISe QDs. 321

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Table 1 Electrochemical impedance parameters of CAISe QDs/P-TiO2 NFs photoanode based QDSCs.

Absorbance (a.u)

CAISeQDs/P-TiO2 NFs P-TiO2 NFs

QDSC

Rs (Ω)

R1 (Ω)

C1 (F)

R2 (Ω)

C2 (F)

CAISe QDs/P-TiO2 NFs

6.33

6.18

0.70 × 10−6

20.99

4.65 × 10−6

300

400

500 600 700 Wavelength (nm)

800

Photocurrent density (mA/cm2)

16

900

Fig. 11. UV–Vis spectra of CAISe QDs/P-TiO2 NFs and P-TiO2 NFs.

P-TiO2 NFs CAISe QDs/P-TiO2 NFs

CAISe QDs

14 12 10 8 6 4 2

Intensity (a.u)

0 0.0

0.1

0.2

0.3 0.4 Voltage (V)

0.5

0.6

Fig. 14. J-V curve of fabricated QDSC.

352

368

384 400 416 Wavelength (nm)

432

448

Fig. 12. PL spectra of P-TiO2 NFs and CAISe QDs/P-TiO2 NFs.

Fig. 15. Schematic representation of the fabricated QDSC. Table 2 Photovoltaic parameters of CZTSe QDs/P-TiO2 NFs and CAISe QDs/P-TiO2 NFs based QDSC. QDSC

Jsc (mA cm−2)

Voc (V)

FF

Ƞ (%)

Ref.

CZTSe QDs/P-TiO2 NFs CAISe QDs/P-TiO2 NFs

13.65 12.86

0.47 0.52

0.55 0.63

3.61 4.24

Singh et al This work

3.7. Structural and optical studies of CAISe QDs/P-TiO2 NFs The XRD patterns of CAISe QDs/P-TiO2 NFs, P-TiO2 NFs, and CAISe QDs are shown in Fig. 10. The peaks at 27.45° and 45.5° in CAISe QDs/ P-TiO2 NFs ensure the presence of CAISe QDs loaded onto P-TiO2 NFs. Fig. 11 shows the UV–Vis absorption spectra of CAISe QDs sensitized PTiO2 NFs compared with P-TiO2 NFs. The absorption spectrum of CAISe

Fig. 13. Nyquist plot of CAISe sensitized P-TiO2 NFs based QDSC.

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Table 3 Comparison of the photoconversion efficiency of QDSC fabricated in this study with the previously reported QDSCs. S. No.

QDs

Photoanode

Counter Electrode

η (%)

Ref.

1. 2. 3. 4. 5. 6. 7. 8. 9.

CuInS2/CdS Ag2Se/ZnS AgInS2/ZnS Zn diffused AgInSe2 CuInSe2 AgInSe2/CdS AgInSe2 AgSe2 Cu2AgInSe4

TiO2 TiO2 TiO2 TiO2 ZnO TiO2 TiO2 P-TiO2 NFs P-TiO2 NFs

Cu2S/FTO Pt/FTO Cu2S/Brass CuS/FTO Au/FTO Cu2S/FTO Cu2S/FTO Cu2S/FTO Cu2S/FTO

2.61 3.12 2.91 3.07 0.30 4.17 1.02 2.50 4.24

Pralay et al. (2013) Tubtimtae et al. (2011) Cai et al. (2017) Halder and Bhattacharyya (2017) Zhang et al. (2012) Abate and Chang (2018) Abate and Chang (2018) Singh et al. (2019) This work

TiO2 NFs (CZTSe QDs/P-TiO2 NFs) photoanode based QDSC is 3.61% and PCE of Cu2ZnSnSe4 QDs sensitized conventional TiO2 NFs (CZTSe QDs/P-TiO2 NFs) based QDSC is 2.84% (Singh et al., 2018a). They interpreted that the porous nature of TiO2 NFs affords adequate area to absorb more QDs and also helps to interact with the electrolyte. It reduces the electrode resistance and improves the PCE. The photovoltaic parameters of the synthesized QDSC and the reported CZTSe QDs/PTiO2 NFs based QDSC are given in Table 2. QDSC based on Cu2AgInSe4 QDs reported in this study exhibited superior photoconversion efficiency than that based on the reported based on binary QDs and ternary QDs as listed in Table 3. The increased photoconversion efficiency is attributed to the wide absorption range of prepared Cu2AgInSe4 QDs and the excellent charge transport properties of P- TiO2 NFs.

QDs/P-TiO2 NFs is broader than that of P-TiO2 NFs. It indicates the high photon gathering capacity of CAISe QDs/P-TiO2 NFs that attributes due to the high loading capacity of P-TiO2 NFs. The PL emission studies reveal that the emission intensity of CAISe QDs/P-TiO2 NFs is quenched and maintains almost the same peak position with respect to P-TiO2 NFs (Fig. 12). Quenching of PL intensity implies the high effective electron transfer capacity of excited CAISe QDs/P-TiO2 NFs and efficient photo-generated electron-hole separation. An intense peak observed at 397 nm corresponds to the band to band transitions in the TiO2 NFs (Abazović et al., 2006; Pallotti et al., 2017). 3.8. Electrochemical impedance studies

4. Conclusion

Fig. 13 shows the Nyquist plot of CAISe/P-TiO2 NFs based QDSC. The inset of Fig. 13 shows the equivalent circuit used to fit the plot obtained by Biologic software, where, Rs is the resistance of the substrate, R1 is the charge transfer resistances at CE/electrolyte, R2 is the charge transfer resistance at photoanode/electrolyte interface, C1 is the double layer capacitance at CE/electrolyte and C2 is the double layer capacitance at photoanode/electrolyte interface. A small arc in the high-frequency range represents the impedance related to the charge transfer at the CE/polysulfide redox electrolyte interface. The larger arc in the lower frequency range represents the impedance related to the charge transfer at photoanode (CAISe QDs/P-TiO2 NFs)/polysulfide electrolyte interface. The corresponding impedance parameters are given in Table 1. In this table, the value of R2 is 20.9 Ω. This charge transport resistance at the photoanode/electrolyte indicates the suppressed electron recombination between CAISe QDs and electrolytes. This phenomenon may be due to the high QDs absorption capacity of porous TiO2 NFs that helps to transport electrons smoothly without any hindrance in the path (Singh et al., 2018b).

Cu2AgInSe4 QDs were synthesized by a simple hot injection method and P-TiO2 NFs were prepared by the electrospinning method. Structural, optical and morphological analyses were carried out. The bandgap of the synthesized QDs and P-TiO2 NFs were found to be 1.93 eV and 3.19 eV, respectively. Due to the high optical properties of the synthesized QDs, it is used as a sensitizer for QDSCs. Cu2AgInSe4 QDs sensitized P-TiO2 NFs showed a high light-harvesting capacity. Hence the fabricated QDSC showed an enhanced photocurrent density of 12.87 mA/cm2. The PCE of the fabricated QDSC was found to be 4.24%. Declaration of Competing Interest 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. Acknowledgments

3.9. Photovoltaic performance Dr.AS gratefully acknowledge the Council of Scientific and Industrial Research (CSIR) New Delhi (Ref.No.01 (2810)14/EMR-II, dt. 26/04/2017) and UGC-BSR-Mid Career Award (No. F. 19-214/ 2018(BSR)) for their financial supports. The authors also gratefully appreciate the Central Instrumentation Facility of Pondicherry University for providing the instrumentation facility.

Fig. 14 shows the current density-voltage (J-V) characteristics of the QDSC. The CAISe QDs/P-TiO2 NFs based QDSC exhibits a PCE of 4.24%. The optimized performance of CAISe QDs/P-TiO2 NFs based QDSC is attributed due to the following factors: (i) the high loading capacity of QDs onto P-TiO2 NFs; (ii) the significant photon harvesting capability of CAISe QDs; (iii) the appropriate band alignment at the interface of TiO2 NFs and CAISe QDs. It should be noted that the absorption onset of CAISe QDs towards the NIR region resulted in the high quantum of the light-harvesting capacity of CAISe QDs. This will increase the Fermi-energy (Ef) level and also helps to an upward shift in the conduction band (CB) of CAISe QDs. The upshift in CB of CAISe QDs, in turn, acts as a driving force for electron transfer from the conduction band of CAISe QDs to the conduction band of TiO2 NFs. It leads to an increase in the current density as well as open-circuit voltage. Fig. 15 shows the schematic representation of the fabricated QDSC. Singh et al reported that the PCE of Cu2ZnSnSe4 QDs sensitized P-

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