Improved Photoelectrochemical Cell Performance of Tin Oxide with

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Improved Photoelectrochemical Cell Performance of Tin Oxide with Functionalized-Multiwalled Carbon Nanotubes-Cadmium Selenide Sensitizer Sambhaji Shivajirao Bhande, Rohan B. Ambade, Dipak Vijaykumar Shinde, Swapnil B. Ambade, Supriya Patil, Mu. Naushad, Rajaram S. Mane, Zeid Abdullah Alothman, Soo-Hyoung Lee, and Sung-Hwan Han ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b05385 • Publication Date (Web): 03 Sep 2015 Downloaded from http://pubs.acs.org on September 6, 2015

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ACS Applied Materials & Interfaces

Improved Photoelectrochemical Cell Performance of Tin Oxide with Functionalized-Multiwalled Carbon Nanotubes-Cadmium Selenide Sensitizer Sambhaji S. Bhande, †, # Rohan B. Ambade, ‡, # Dipak V. Shinde, § Swapnil B. Ambade, ‡ Supriya A. Patil, § Mu. Naushad, ┴ Rajaram S. Mane, †, §,* Z. A. Alothman, ┴ Soo-Hyoung Lee ‡,* and Sung-Hwan Han §,*

†

‡

Center for Nanomaterials and Energy Devices, Swami Ramanand Teerth Marathwada University, Dnyanteerth, Vishnupuri, Nanded, 431606, India

School of Semiconductor and Chemical Engineering, Chonbuk National University, 664-14, 1ga Deokjin-dong, Deokjin-gu, Jeonju, Jeonbuk, 561-756, Republic of Korea

§

Department of Chemistry, Hanyang University, Seongdong-gu, Haengdang-dong 17, Seoul 133791, Republic of Korea ┴

#

Advanced Materials Research Chair, Department of Chemistry, College of Science, Bld-5, King Saud University, Riyadh, Saudi Arabia.

Contributed equally

*Corresponding author Emails: [email protected], Tel: +919850331971, Fax: +912462229574 (Rajaram S. Mane, Prof.), [email protected] Tel: +82 63 270 2435 Fax: +82 63 270 2306 (Soo-Hyoung Lee, Prof.) and [email protected] (Sung-Hwan Han, Prof.)

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ABSTRACT Here we report functionalized multiwalled carbon nanotubes (f-MWCNTs)-CdSe nanocrystals (NCs) as photosensitizer in photoelectrochemical cells, where f-MWCNTs was uniformly coated with CdSe NCs onto SnO2 upright standing nanosheets by using a simple electrodeposition method. The resultant blended photoanodes demonstrate extraordinary electrochemical properties including higher Stern-Volmer constant, higher absorbance, and positive quenching etc., caused by more accessibility of CdSe NCs compared with pristine SnO2-CdSe photoanode. Atomic and weight % changes of carbon with f-MWCNTs blending concentrations were confirmed from the energy dispersive X-ray (EDAX) analysis. The morphology images show a uniform coverage of CdSe NCs over f-MWCNTs forming a core-shell type structure as a blend. Compared to pristine CdSe, photoanode with f-MWCNTs demonstrated a 257% increase in overall power conversion efficiency (PCE). Obtained results were corroborated by the electrochemical impedance analysis (EIS). Higher scattering, more accessibility and hierarchical structure of SnO2-f-MWCNTs-blend-CdSe NCs photoanode is responsible for higher; a) electron mobility (6.89 x 10-4 to 10.89 x10-4 cm2 V-1 S-1), b) diffusion length (27x 10-6), c) average electron life time (32.2 ms) and transit time (1.15 ms).

KEYWORDS SnO2-CdSe, functionalized MWCNTs, SnO2-CdSe-MWCNTs, photoanodes, dye-sensitized solar cell

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Introduction Solar energy is envisioned to be an important energy source for the next generation due to its renewable and clean energy characteristics. Light to electric power conversion efficiency (PCE) as high as 12% has already been achieved in dye-sensitized solar cells (DSSCs) with organometallic complexes as sensitizers.1 Nevertheless, the key problem to achieve higher PCE in nanostructured electrodes remains unaddressed.2 Two of the major limiting factors for PCE in DSSCs is the transport of photogenerated electrons through the randomized network of semiconductor nanoparticles electrode that eventually recombine and the uncontrolled oxidation of organometallic sensitizer. One of the effective ways to smoothen the transport of photogenerated electrons is to replace semiconductor nanoparticles by 1D nanostructures as they provide direct electrical pathways for the photogenerated carriers. Several efforts have been made in the development of 1D nanomaterials for application in solar cells3-5 Moreover, such 1D nanostructures provide high junction areas and low reflectance owing to their ability to scatter and trap light.6 Secondly, to address the issue of organometallic sensitizers that are susceptible to oxidation, quantum dots (QDs) are quite promising alternatives, as they prominently possess intrinsic properties such as tunable band gap, high extinction coefficient and large intrinsic dipole moment etc.7-9 Thus, quantum dot-sensitized solar cells (QDSSCs) are of particular interest in the development of environmental friendly renewable energy source due to their facile fabrication processes, low-cost and theoretical estimation of high efficiency.10-19 Moreover, the theoretical demonstration of multiple exciton generations by impact ionization could push the thermodynamic efficiency limit of these devices up to 44%20 instead of the presently estimated 31%21 imposed on DSSCs by the Schockley-Queisser limit. Various QDs, such as CdS, CdSe, In2S3, ZnS, PbS, PbSe, InP, etc., 22-26 have been demonstrated for QDSSCs. Multiwalled carbon nanotubes (MWCNTs) as a new class of 1D nanomaterials have been exploited for various

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electrode materials due to their unique electronic, chemical, and mechanical properties. Thus, MWCNT-modified electrodes exhibit superior ability compared with pristine one 27, 28 and have been extensively used to study conventional electrochemistry.29–31 The use of MWCNTs-blended electrode with large band gap semiconductors in the DSSCs for enhancing photoelectron transportation has been proposed in recent reports.32-34 Contrarily; there are only a few reports on the use of MWCNTs into the photoactive materials due to challenge in synthesis of semiconductor photosensitizer and MWCNT blend with uniform coating. Kamat et al. suggested that the electrons take random paths before entering into large band gap semiconductor35 as well as semiconductor sensitizer (TiO2, ZnO, SnO2, etc.) and thus, have maximum probability to recombine due to the intrinsic defects within the semiconductor. It is thus possible to achieve high photocurrent and faster-photoexcited electron transfer through such 1D network of carbon materials viz, MWCNTs and graphene for enhancing the overall PCE. However, the challenge still remains to use this 1D network as support to anchor light harvesting semiconductor nanoparticles and provide direct pathways to these photogenerated electrons in semiconductor photosensitizer towards the charge collecting electrode. As within the photoanode of a QDSSC, photoinduced electrons must overcome grain boundaries to be collected by a working electrode. The low transfer efficiency of photo-induced electrons across the base semiconductor matrix represents a major limitation of such nanostructured photoanodes.36 The inefficient charge transfer path results in recombination of photoinduced electrons with the oxidizing species of the electrolyte, resulting in a decrease of photocurrent and thereby PCE. Controlling charge recombination can eventually improve the photo-induced transfer of electrons that can otherwise be achieved by using 1D nanostructure network; however there are only a few reports on directing the quantum dots photogenerated electrons.37,38 Hence, it would be interesting to use

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these 1D based electrode of large band gap semiconductor such as nanotubes, nanorods and upright standing nanosheets (NSs) and then using MWCNTs and CdSe NCs-blend for sensitizing Gratzel cell, so that there will be less probability of the recombination of photogenerated electrons and a directed pathway, which will help to get large current across the junction with reduced charge transport resistance. Various methods proposed in the literature are either tedious or expensive to develop mass-scale electrodes of composite blend.39 Although, TiO2 by far has remained the workhorse for DSSC photoanodes, its weaker electron mobility (~0.1–1 cm2 V-1 s-1) is a serious drawback. In this context, SnO2 is a promising alternative as a photoanode in DSSCs owing to its higher electron mobility (~100–200 cm2 V-1 s1 40, 41

).

However, due to the lower isoelectric point and occurrence of faster interfacial electron

recombination owing to a 0.4 eV negative shift of the conduction-band edge of SnO2 with respect to that of nanocrystalline TiO2, SnO2-based DSSCs have not been developed as expected. To solve these issues, Xie et al., explained the significance of multi-heterojunction structure in CdS sensitized photoelectrochemical system for the 1D TiO2 branched SnO2 structure to serve as faster electron transport network.42 Such multi-heterojunction structures are beneficial to improve the usually limited charge separation and transport dynamics of single component electrodes like SnO2 only. In their another work, Xie et al proposed another multi-junction system comprising of porous SnO2 nanotube–TiO2 (SnO2 NT–TiO2) core–shell structured photoanodes for DSSCs to solve the issue of faster interfacial electron recombination and lower trapping density.43 Another favorable approach is that of SnO2-CdSe sensitized semiconductor solar cells. In the first of its kind representative study on SnO2-CdSe sensitized solar cells, Wang et al., synthesized mesoporous SnO2 spheres by anodization of tin foil and CdSe was coated using the successive ionic layer adsorption and reaction (SILAR) method.44 The regenerative

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photoelectrochemical cell exhibited a PCE of ~1.9 % owing to efficient charge separation and inhibition of charge recombination as the semiconductor light absorber (CdSe) acts as a passivation layer. Hossain et al., demonstrated another strategy of co-sensitizing CdS/CdSe with SnO2 to improve photocurrent in the semiconductor solar cells.45 Despite such interesting approaches, it remains a challenge to achieve higher PCE in SnO2-CdSe sensitized solar cells. In this work, we synthesized f-MWCNT blended SnO2-CdSe photoanodes with various concentrations of f-MWCNT for application in DSSCs. A facile, low-temperature and costeffective electrodeposition method is used for the synthesis of hierarchical CdSe network sensitizer onto SnO2 NSs where the functionalized group is used for branching the MWCNTs through CdSe. The unidirectional flow of electrons by using a small amount of graphene is previously reported for electron transportation and collection for inorganic photosensitizer.29 This study exhibits the apparent enhancement in photovoltaic characteristics using f-MWCNT blended SnO2 photoanodes in comparison to pristine SnO2 in CdSe-sensitized solar cells.This increment is mainly attributed to the maximum utilization of photogenerated electrons through recombination with electrolyte and trap sights.

Experimental details: Synthesis of tin oxide: SnO2 (tin oxide) NSs were prepared by using low-temperature chemical bath deposition method. In a typical procedure; 0.3M SnCl4 was initially dissolved in an ethanol solvent and then 0.6M thioacetamide was added into the same solution at room temperature. The as-prepared transparent solution was transferred into air-sealed Teflon tubes with pieces of FTO (fluorine-tinoxide) in a vertical direction. Initially, these pieces of FTO were cleaned with acetone and

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ethanol for 30 min by ultrasonication and further dried under argon flow. This as-prepared solution was further maintained at 70 °C for 3 h. The FTO with SnO2 films deposited was then removed and annealed at 500 °C for 1 h followed by plasma treatment for 10 min.

Synthesis of SnO2-CdSe f-MWCNTs blend electrode: Pristine MWCNT (provided by Iljin Nanotech Korea, purity of 90%) were functionalized by refluxing in H2SO4:HNO3 (3:1, v/v) at 80 °C for 6 h to obtain the carboxylic acid functionalized carbon nanotubes (CNT-COOH), then washed with distilled water until neutral, and finally dried under vacuum at 50 °C for 24 h [see Fig S1-S2, Supporting Information (SI)].46, 47 Filtration of fMWCNT was carried out through either a 100-nm pore polycarbonate membrane (Millipore) or a 200-nm-pore Teflon membrane (Millipore). Electrodeposition method was operated at room temperature for loading CdSe sensitizer. Initially, an aqueous solution of 0.05M of cadmium sulfate that acts as a source of metallic ions and 0.01 M selenium oxide releasing cationic ions was prepared as a solution for deposition. The electrodeposition was carried in three electrodes system for 30 min, to control the monolayer coverage of CdSe NCs over SnO2 NSs supported by FTO substrate. A constant current density of 2 mA/cm2 was applied in the presence of platinum plate as a counter electrode and Ag/AgCl as a reference electrode for 30 min. For the uniform coverage of the film, the distance between working and the counter electrode was kept small (0.5cm) with a parallel arrangement to each other. The obtained film electrodes were dark brownish and shiny. For the synthesis of f-MWCNTs-CdSe blend sensitizer electrodes, same procedure was followed for electrodeposition with 0, 20, 40, 60, 80 and 100 μl f-MWCNTs in electrolyte solution and named as R0, R1, R2, R3, R4 and R5, respectively. However, the solutions after f-MWCNTs dispersion prepared were kept for 30 min of stirring in order to provide the maximum probability for Cd+2 ions to attach to the functionalized group of

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MWCNTs there by importing anodic charge on f-MWCNTs and can be easily deposited by applying current.

Results and discussion Figure 1(a) represents the X-ray diffraction spectra of pristine and f-MWCNTs-blended CdSe sensitized electrodes. A peak of (002) for f-MWCNTs is shifted to a higher two theta (2 by 0.58° as compared to its standard value of 26.02 (2). Moreover, (110) peak of SnO2 has also been shifted and merged into f-MWCNTs (002) peak making it more intense. Along with (110) and (101) of SnO2 and (102) of CdSe are shifted with 0.56 and 0.53 2 values, respectively. Other (101) (211) and (310) reflection peaks confirm the presence of SnO2 as the base electrode for supporting and anchoring CdSe NCs; whereas peaks resembling to (100), (002), (102), (110), (202) and (210) are assigned to CdSe with the help of JCPDS data card no. 77-2307. Change in XRD patterns signifies an involvement of SnO2, CdSe, and f-MWCNTs. It is difficult to provide evidence for the presence of the f- MWCNTs quantitatively into the CdSe matrix with the help of XRD spectra only, therefore EDAX analysis (Fig. 1c) was carried out and estimated atomic and molecular weight percentages are summarized in Table 1. A maximum 5.16 at. % blending of fMWCNTs in SnO2-CdSe is confirmed. Furthermore, the elemental mapping (Figure 2) was used to confirm the presence of f-MWCNTs in CdSe-SnO2. The uniform distribution of elemental carbon () in the f-MWCNT blended SnO2-CdSe samples (Figure 2b-f) along with other constituent elements tin (Sn), oxygen (O), cadmium (Cd) and selenium (Se) confirms the presence of MWCNT in all of the blended samples. The morphological progression of blend structures has been illustrated using Scheme S1, SI. FE-SEM images [Figure 3 (a-r)] were obtained to understand the influence of f-MWCNT

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blending on the morphology of nanostructures. Figure 3a-b show FE-SEM images of upright standing SnO2 NSs. FE-SEM image in Figure 3c shows electrodeposited CdSe NCs on upright standing SnO2 NSs. Figure 3d-f corresponding to the f-MWCNT blending concentration of 20 µl shows the presence of f-MWCNTs on SnO2-CdSe. The embedding of f-MWCNTs on SnO2CdSe even at such low concentration (20 µl) suggests that the electrodeposition conditions used in our work were extremely crucial in directing the migration of f-MWCNTs over the surface of SnO2 NSs in conjunction with CdSe. With the increase in f-MWCNT concentration to 40 µL (Figure 3g-i), it is observed that CdSe NCs tend to deposit on the walls of SnO2 NSs as well as on f-MWCNTs. Despite the increase in the concentration of f-MWCNTs, the initial structure of SnO2 is still maintained suggesting that SnO2 structure is rigid enough to withstand even higher concentrations of f-MWCNTs. As a result, the concentration of f-MWCNTs was further increased to 60 µl, which leads to the growth of f-MWCNT covered CdSe NCs over the SnO2 NSs (Figure 3j-l). This interconnected form of photosensitized NCs could be beneficial for absorbing the maximum available sunlight along with the directed flow of photoexcited electrons making them less susceptible for the recombinations to occur. Furthermore, a densely interconnected nanochains network is obtained for the f-MWCNT concentration of 80 μl (Figure 3m-o). Wide surface, cross-sectional views and effect of f-MWCNT on surface morphology of SnO2-CdSe are presented in Figure S4, SI. It is quite evident that in the blended electrodes (Figure S3b-f) the growth of CdSe nanochains along with f-MWCNTs forms a much denser and homogeneous morphology compared to pristine electrode (Figure S3a, SI). Such dense and homogeneous morphology would ultimately lead to directed and unperturbed electron transportation, desired for many electronic devices. Further increase in the concentration of fMWCNTs to 100 µl (Figure 3p-r) leads to aggregation of f-MWCNTs over SnO2-CdSe. Under

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such aggregated condition, it is likely for photo-excited electrons to travel a random path either to reach the photoanode or towards incorporated f-MWCNTs, which could be detrimental to obtain the desired photovoltaic effect.48, 49 Much closer morphological evaluation for our best performing sample (80 µl f-MWCNT) was carried out using transmission electron microscopy (TEM) analysis. TEM analysis presented in Figure 4a for the sample not containing f-MWCNT reveals the presence of two distinct nanoforms, one nanocrystal and second, larger sized walls corresponding respectively to CdSe and SnO2. CdSe NCs appear to form long chains surrounding the walls of SnO2. On introducing f-MWCNT into SnO2 –CdSe, f-MWCNTs appear to have tethered SnO2 –CdSe (Figure 4b). The selected area electron diffraction (SAED) pattern in samples without (Figure 4c) and with fMWCNT (Figure 4d) reveals the occurrence of entirely different structure for f-MWCNT blended SnO2-CdSe. The corresponding ring-like SAED pattern from the sample containing fMWCNT (Figure 4d) can be indexed (inside to the outside) to the (100), (002), (110), (101) and (102) planes of SnO2-CdSe blended with f-MWCNT, which is consistent with the findings of XRD. The planes corresponding to CdSe are marked in white while those of SnO 2 and fMWCNT are marked in pink and green, respectively. Blending is further confirmed using the X-ray photoelectron spectroscopy (XPS) carried out for the R4 photoanode (80 µl f-MWCNT) that showed the best PCE. The XPS survey spectrum of the 80 µl f-MWCNT blend photoanode presented in Figure 5 shows prominent peaks for elements C, Sn, O, Cd and Se signifying the blend of SnO2-CdSe –MWCNT. Figure 5b-f shows the narrow scan spectra of C1s, Sn3d, O1s, Cd3d and Se3d regions respectively. The XPS spectra exhibited by C1s core is presented in Figure 5b, which shows the location of C1s at the binding energy of 284.6 eV. The C1s emission is attributed to the carbon from MWCNTs, which

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is one of the components for blending. The deconvoluted three C 1s peaks at 284.6, 286.29, and 288.09 eV typically correspond to the positions of sp 2-hybridized carbon (C-C) in the MWCNTs, C-O and C=O oxygen containing carbonaceous bands appeared as a result of oxidizing acid treatment of MWCNTs.38-39 The analysis of C peaks clearly indicates the presence of carboxylic groups on the MWCNTs, which is in accordance with the appearance of O signals at their known positions (Figure 5d). The two peaks arisen for the Sn region at 486.3 and 494.73 eV correspond to Sn3d5/2 and Sn3d3/2 binding energies (Figure 5c). The binding energies of Cd (Figure 5e) corresponding to Cd3d 5/2 and Cd3d3/2 were found to appear at 404.97 and 411.75 eV, respectively. The Se3d transition peaks at ~54 & 58 eV (Figure 5f) are attributed to the binding energies of selenium. The energies for C, Sn, O, Cd and Se are in good agreement with the reported values in literature.50-53 Figure 6a-b represents the J-V analysis taken at 1 sun intensity and the external quantum efficiency (EQE), respectively. The results indicate that the current density (Jsc) and fill factor (FF) is increased appreciably while the open circuit voltage (Voc) increased gradually. This result proved that the addition of f-MWCNTs into photo-sensitizer reduces the charge transport resistance. The electrochemical parameters of all photoanode cells are summarized in Table 1. The current density is increased from 11.06 mA/cm2 for pristine electrode to 16.75 mA/cm2 for the 80 µl f-MWCNTs blended electrode, whereas the Voc is also increased from 0.43 to 0.49 V. The slight change in Voc is suggestive of negative shift (towards vacuum) of conduction band for the f-MWCNT blended electrode. The conduction band edge (CBE) of SnO2 is much shallow than TiO2 or ZnO and hence Voc in DSSCs of SnO2 is comparatively lower.54 In a blended electrode, the resulting charge equilibration due to the difference in CBE of MWCNT and SnO2 would cause the obvious shift of Fermi level and the hybrid conduction band arising from the

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blended electrode will generate a more negative shift (towards vacuum) compared to that of pristine f-MWCNT.55 The fact that f-MWCNT blended SnO2-CdSe photoanodes exhibit increased photovoltage compared to pristine SnO2, suggests closed binding of f-MWCNTs to SnO2-CdSe leading to more efficient charge transfer process and higher Voc. All blended photoanodes show trend of increasing fill factor from 34% to 46% to that of pristine; thereby increasing the overall PCE from 1.59% to 4.17%, for the best photoanode. Incorporation of fMWCNTs into the CdSe sensitizer has demonstrated an increase in the performance about 257% to that of pristine photoanode. Further increase in f-MWCNT blending concentration (100 µl) has dropped Jsc and FF with lower PCE (3.13%) which is well-justified as higher concentration of f-MWCNTs would incorporate an additional resistance to electron transport thereby leading to a loss of electrons. This is attributed to competition of light harvesting between dye molecules and MWCNT, as a result of which the dye loading capacity, charge collection and thus the efficiency are both reduced for DSSCs of photoanodes with higher concentrations of MWCNT. This is well in accordance to the observations of Vomiero et al.56 EQE plots are shown in Figure 6b. It is seen that f-MWCNT blended SnO2-CdSe photoanodes exhibited EQE as high as 73% in contrast to a lower EQE of 40% for pristine photoanodes. The increase in the EQE is attributed to three major factors; (a) minimization of random paths for photogenerated electrons in CdSe in presence of fMWCNTs as core structure for guiding electron flow, (b) band position difference between the SnO2 and the CdSe that might prohibit injected photo-electrons flow in reverse direction, and (c) direct pathway in SnO2 NSs, is free from the random percolation of photo-injected electrons. This results in the least charge transfer resistance followed by prolonged electron lifetime.

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The enhancement in photocurrent can be explained by the photoluminescence (PL) quenching studies carried out using the pristine as well as f-MWCNTs modified photoanodes. PL quenching of SnO2-CdSe and f-MWCNTs blended SnO2-CdSe provides valuable information on electron transfer between excited states of SnO2-CdSe towards f-MWCNTs. PL quenching of SnO2-CdSe and f-MWCNTs blended SnO2-CdSe provides valuable information on electron transfer between excited states of SnO2-CdSe towards f-MWCNTs. PL quenching is a process that decreases the intensity of the PL emission, which may occur by several mechanisms:57 quenching by energy transfer, charge-transfer reactions, dynamic quenching, static quenching. The PL spectra of the pristine and f-MWCNTs blended CdSe photosensitized SnO2 photoanodes are shown in Figure 7. Positive quenching was obtained as the concentration of f-MWCNTs increased into the blend until the addition of 80 µl of f-MWCNTs. This positive quenching was attributed to the fact that the photogenerated electron-hole pairs in the CdSe NCs prefer separate transferring to fMWCNTs. The positive quenching effect is mainly due to the well-directed growth of NCs that are stable and connected to upright standing NSs of SnO2. The bulk nature of CdSe photosensitizer covers the entire visible region with a bandgap of 1.7 eV. Whereas, f-MWCNTs that act as transporting medium for photogenerated electrons could guide them through proper channels with minimization of charge transport resistance. Hence, it is presumed that photoexcited electrons have efficient pathways that could eventually enhance the photoresponse to great extent.58-60 Thus; it is fair to believe that quenching in the presence of MWCNTs is likely caused by dynamic quenching and energy transfer mechanisms. Further increase in the blending concentration of f-MWCNT has promoted an increase in the PL intensity; indicating that the electron transportation is limited. It is justified from the FE-SEM images that further increase in f-MWCNT concentration is responsible for thick coverage of CdSe NCs, restricting photo-

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excited electrons to travel towards f-MWCNTs due to closed pack structure of CdSe NCs over fMWCNTs. This might not allow the electrolyte to perform redox reaction with the central CdSe NCs where the only outer surface could be exposed to the electrolyte solution. PL quenching of CdSe and CdSe blended f-MWCNTs provided valuable information on electron transfer between excited states of CdSe towards f-MWCNT. PL of CdSe NCs is quenched as the concentration of f-MWCNTs is increased from 0 to 80 µl and start to rise with a further increase in the concentration to 100 µl. The rate of PL quenching is continued linearly until all of the QDs are essentially quenched whereas further addition of MWCNTs has confirmed increased PL intensity, suggesting negative quenching effect. Thus, f-MWCNTs have provided a wide accessibility to the CdSe NCs, which might help in higher photo-absorbance and higher photocurrent generation. Figure 7b displays the Stern-Volmer plot of SnO2-CdSe quenched by MWCNT. The Stern-Volmer constant (Ksv) obtained using the equation, F0/F = 1+ Ksv [CNT] [38] describes the accessibility of CdSe NCs towards f-MWCNTs. The linear fit of the SternVolmer data is suggestive of a simple dynamic quenching mechanism.57 In the present case, fMWCNTs provided a Stern-Volmer constant (Ksv) of 4.60 Lmg-1, which was 30 times higher than those previously reported.61 Thus, it is confirmed that f-MWCNTs offered more accessibility to the CdSe NCs, which might help in higher photo-absorbance and higher photocurrent generation and subsequent PL quenching. Moreover, to evaluate the better charge transfer dynamics, electrochemical impedance spectroscopy (EIS) was carried out on photoanodes. Figure 8a shows the Nyquist spectra for pristine and f-MWCNTs blended CdSe photosensitized photoanode. EIS plots were fitted using an equivalent circuit as illustrated in Figure S5, SI. The high and low-frequency arcs in EIS correspond to the charge transport behavior at platinum/electrolyte interface and second

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representing the contribution of electron transfer through the photoanode consisting of the nanocrystalline semiconductor.45 As second semicircle represents the charge transport resistance, it is of prime importance to discuss the mean electron life time (n), electron transit time (d), electron mobility () and other various related parameters.

While the electron life time (n) is obtained from the charge transport resistance that is estimated from the frequency at the highest value of recombination region arc (fmax) in the second semicircle in a Nyquist plot using the correlation, 62 n = (2 fmax)-1

(1)

the mean electron transit time (τd) is obtained from the relation between the electron transport resistance (Rt) and interfacial charge transport resistance (Rct) using the relation, (τd,EIS / τ n, EIS) = (Rt / Rct)

(2)

The electron diffusion length (Ln) was calculated by using, Ln =√(Dn x τn) = L√(R ct/Rt)

(3)

Where, Dn is diffusion coefficient, and L is the thickness of the photoanode. Einstein’s relation presented in Equation 4 was used to calculate the electronic mobility (μ) μ = (Dn.e/KBT)

(4)

where, e is the elementary charge of the electron, KB is Boltzmann constant and T is absolute temperature in Kelvin.

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Electronic parameters calculated by using above equations are summarized in Table 2. The charge transport resistance (Rct) is decreased to half (from 82.71 for CdSe sensitized to 41.86 for f-MWCNTs blended CdSe photosensitized photoanode) which directly supported for incorporation of f-MWCNTs into the semiconductor photosensitizer; the charge transport resistance is decreased drastically. This is mainly due to the effective charge separation of electron and hole pairs at the junction of f-MWCNT and CdSe. Moreover, f-MWCNTs might allow the directed flow of the photogenerated electrons by providing a minimal probability of recombination with the defect sites of CdSe, thereby high charge collection efficiency and mean electron lifetime. Figure 8b represents the plot of electron mobility (µ) and chemical capacitance (Cµ) with respect to blending concentration (electrodes). The values of µ and Cµ of all the photosensitizer studied highlight that the electron mobility for f-MWCNTs blend CdSe photoanodes is much larger than that of the pristine CdSe, indicating that the f-MWCNTs-doped CdSe electrodes experience relatively higher intrinsic electron mobilities. This must favor the electron transport through a longer distance with less diffusive hindrance. The value of Cµ gives the total density of free electrons into photoanode conduction band and localized electrons in the trap states.61 The capacitance values are decreased with respect to increase in f-MWCNTs blending concentration into CdSe NCs matrix (from 5.159 x 10 -4 F for pristine photoanode to 1.125 x 10 -4 F for f-MWCNTs doped photoanode); due to a reduction of trapping sites for photogenerated electrons as observed in PL study. The electron mobility is increased from 6.89 x 10 -4 to 10.89 x 10-4 cm2V-1S-1, supporting a reduction in tapping states. Hence, both results i.e. chemical capacitance and electron mobility of the photoanodes present reduction in trapping states where f-MWCNTs worked as an electron extractor into the CdSe photosensitizer. Figure 78c represents a comparison between the electron mobility and the diffusion length depending on

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the blending concentration of f-MWCNTs. Interestingly, the diffusion length of the photoanode is decreased from 33.8 to 28 μm that is still higher as compared to the thickness of photoanode, and accordingly the electron mobility is increased. Figure 8d represents the comparison between the efficiency of the photoanodes and the electron lifetime. The efficiency is linearly increased from 1.6% to 4.17% whereas the electron lifetime is decreased from 6.46 x 10 -2 to 3.22 x 10-2 s. The decrease in electron lifetime could be due to a well-directed photocarrier flow i.e. without hindrance to the collecting electrode.

Conclusions Facile, low-temperature and low-cost wet chemical and electrodeposition methods are employed for synthesizing SnO2 and CdSe-f-MWCNTs electrodes, respectively. Uniform coatings of CdSe NCs over SnO2 nanosheets and MWCNTs are obtained. The f-MWCNT concentration dependent morphological change and development of hierarchical structure are confirmed from the FESEM images of SnO2-CdSe electrodes. Due to change in morphology, in the presence of f-MWCNTs, XRD peak positions of both SnO2 and CdSe are shifted to a higher 2θ side. The blend-sensitizer photoanode corroborated extraordinary properties such as higher electron mobility, reduced chemical capacitance, and charge transport resistance. On account of above favorable properties, a 257% increase in overall PCE in the presence of f-MWCNTs was obtained for photoanodes of SnO2-CdSe.

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Acknowledgements The authors would like to extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for funding this work through the Research Group NO. RG1436-034. This research was supported by the Pioneer Research Center Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (NRF-2013M3C1A3065528).

Supplementary Information Structural characterizations, supplementary figures, and tables. This information is available free of charge via the Internet at http://pubs.acs.org/.

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Figure 1 X-ray spectra of; (a) SnO2-CdSe and SnO2-CdSe/f-MWCNT electrodes, and (b) magnified spectrum indicating a shift in 2 values (details of R0-R5 electrodes are given in Table 1). c) EDAX analysis for Sn, O, Cd, Se and C elements on the surfaces of R0-R5.

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Figure 2. EADX mapping of all the photoanodes (a-a5) SnO2-CdSe, (b-b5) R1, (c-c5) R2, (dd5) R3, (e-e5) R4, and (f-f5) R5.

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Figure 3. FE-SEM top-view digital photoimages of (a-b) SnO2 NSs, (c) SnO2-CdSe, (d-f) R1, (g-i) R2, (j-l) R3, (m-o) R4, and (p-r) R5 photoanodes at different magnifications.

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Figure 4. TEM and SAED pattern images of; (a and c) R0 and (b and d) R4 photoanodes.

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Figure 5. XPS spectra of SnO2-CdSe-MWCNT for R4 photoanode; (a) wide scan survey spectra, (b) C1s, (c) Sn3d, (d) O1s, (e) Cd3d, and (f) Se3d.

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Figure 6. (a) J-V characteristics (under simulated AM 1.5 irradiation), and (b) corresponding EQE measurements of R0-R5 photoanodes.

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Figure 7. (a) PL spectra, and (b) Stern-volmer plot of R0-R5 photoanodes.

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Figure 8. (a) EIS spectra, (b) chemical capacitance and electron mobility, (c) diffusion length and mobility, and (d) PCE efficiency and mean electron lifetime obtained for R0-R5 photoanodes.

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Table 1. Carbon concentration (as obtained from EDAX analysis) presents in R0-R5 photoanodes as a function f-MWCNTs concentration blending level and their photovoltaic parameters.

Sample R0 (0 μl f-MWCNTs)

‘C’ Concentration through EDX At. (%) Wt.% 00 00

J sc (mA/cm2) 11.0

Voc (V) 0.43

FF

η (%)

0.34

1.59

R1 (20 μl f-MWCNTs)

0.18

1.44

12.2

0.47

0.39

2.24


0.36

2.83

12.8

0.49

0.46

2.88


0.52

4.03

14.6

0.49

0.48

3.44


0.67

5.16

15.8

0.49

0.54

4.17

R5 (100 μl f-WCNTs)

0.86

6.56

13.9

0.49

0.46

3.13

36


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Table 2. Various electrochemical parameters of R0-R5 electrodes derived from impedance spectroscopy analysis.

Sample

Rs

Rt

Rct

Cµ * -4

τn *

τd *

µ*

-2

-3

-4

10

10

10

10

Ln * 10

DL * -6 10

-6

Ro

28.36

2.02

82.71

5.159

6.46

1.58

6.89

33.79

33.8

R1

15.75

1.37

61.22

2.612

5.18

1.16

8.79

35.29

35.2

R2

20.62

1.72

56.51

2.327

4.05

1.23

8.83

30.26

30.2

R3

25.01

1.52

50.10

1.668

4.05

1.23

8.95

30.31

30.3

R4

21.17

1.52

41.86

1.125

2.56

0.93

9.34

27.71

24.3

R5

19.39

1.68

47.18

1.132

3.22

1.15

10.89

27.98

28.0

37



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254x190mm (150 x 150 DPI)


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Improved Photoelectrochemical Cell Performance of Tin Oxide with

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