Nickel Oxide Photocathode Boosts the

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New Antimony Selenide / Nickel Oxide Photocathode Boosts the Efficiency of Graphene Quantum Dots Co-sensitized Solar Cell Ankita Kolay, Ramesh K Kokal, Ankarao Kalluri, Isaac Macwan, Prabir K Patra, Partha Ghosal, and Melepurath Deepa ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09754 • Publication Date (Web): 18 Sep 2017 Downloaded from http://pubs.acs.org on September 19, 2017

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ACS Applied Materials & Interfaces

New Antimony Selenide / Nickel Oxide Photocathode Boosts the Efficiency of Graphene Quantum Dots Co-sensitized Solar Cell Ankita Kolay,a Ramesh K. Kokal,a Ankarao Kalluri,b Isaac Macwan,b Prabir K. Patra,b,c Partha Ghosald and Melepurath Deepa a,* a

Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi-502285, Sangareddy, Telangana (India).

b

Department of Biomedical Engineering, c Department of Mechanical Engineering, b,c University of Bridgeport, Bridgeport, CT 06604. d

Defence Metallurgical Research Laboratory, Defence Research & Development Organisation (DRDO), Hyderabad500058, Telangana (India).

KEYWORDS: Solar cells, photocathode, antimony selenide, graphene quantum dots, efficiency

ABSTRACT: A novel assembly of a photocathode and photoanode is investigated to explore their complementary effects in enhancing the photovoltaic performance of a quantum dot solar cell (QDSC). While p-type nickel oxide (NiO) has been used previously, antimony selenide (Sb2Se3) has not been used in a QDSC, especially as a component of a counter electrode (CE) architecture that doubles up as the photocathode. Here, near infrared (NIR) light absorbing Sb2Se3 nanoparticles (NPs) coated over electrodeposited NiO nanofibers on a carbon (C)-fabric substrate was employed as the highly efficient photocathode. Quasi-spherical Sb2Se3 NPs, with a band gap of 1.13 eV, upon illumination release photoexcited electrons in addition to other charge carriers at the CE to further enhance the reduction of the oxidized polysulfide. The p-type conducting behavior of Sb2Se3, coupled with a work function at 4.63 eV, also facilitate electron injection to polysulfide. The effect of graphene quantum dots (GQDs) as co-sensitizers as well as electron conduits is also investigated where a TiO 2/CdS/GQDs photoanode structure in combination with a C-fabric CE delivered a power conversion efficiency (PCE) of 5.28%, which is a vast improvement over the 4.23 % that is obtained by using a TiO 2/CdS photoanode (without GQDs) with the same CE. GQDs due to a superior conductance, impact efficiency more than Sb2Se3 NPs do. The best PCE of a TiO2/CdS/GQDs-nS2/Sn2--Sb2Se3/NiO/C-fabric cell is 5.96% (0.11 cm2 area), which when replicated on a smaller area of 0.06 cm2, is seen to increase dramatically to 7.19%. The cell is also tested for 6 h of continuous irradiance. The rationalization for the channelized photogenerated electron movement which augments the cell performance is furnished in detail in these studies.

Introduction A quantum dot solar cell (QDSC) is one of the most promising examples of third-generation solar cells, wherein quantum dots (QDs) are employed as the light-absorbing photovoltaic material. QDSCs hold great potential due to certain attractive features that have placed these photovoltaic devices in the limelight in recent years. Various novel concepts have been implemented in order to achieve higher power conversion efficiencies (PCEs) in QDSCs. One of the prime approaches has been that of cosensitization 1, which involves combining two QDs with different bandgaps to expand light absorption over both visible and near infrared (NIR) regions to harvest maximum portions of the solar spectrum. This may be performed by various means such as core shell 2, doping3, alloying to form binary or ternary QDs4. Another convenient approach is the incorporation of highly conducting

carbon nanostructures5 (carbon nanotubes or reduced graphene oxide or fullerene), which are chemically compatible with the QDs, and can boost the current collection capability of the electrode. Studies have also been undertaken to explore the potential of counter electrodes (CEs) by employing metal chalcogenides 6, conducting polymers7 and carbon-based materials8, among others. The thirst for increasingly higher efficiencies of QDSCs has given rise to yet another approach wherein innovations are incorporated at both the photoanode and the CE of the same cell in order to take maximum advantage of their synergy. An example of this is seen in a series of works by Zhong’s group. In one study, CEs with mesoporous carbon supported on Ti mesh (MC/Ti) were developed, and CdSeTe QD sensitized QDSCs employing this CE were found to yield a PCE of 11.16%9. In another work, green Zn-Cu-In-Se alloyed QDs (capped with oleylamine) on TiO2 were used with the aforementioned CE, giving an

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increased efficiency of 11.66% 10. Subsequently, in a slight modification, nitrogen-based mesoporous carbons (N-MCs) supported on TiO2 mesh substrates were used as the CEs in Zn-Cu-In-Se QDSCs. The cell with the N-MC having a nitrogen content of 8.58 wt% gave a PCE of 12.23%, setting a new record for QDSCs 11. Graphene-based nanoassemblies have appreciable potential to boost the efficiency of various catalytic and storage reactions in energy conversion applications due to high carrier mobility 12. Moreover, their bandgap tunability and large optical absorptivity are valuable properties to enhance the efficiency of light harvesting13. Newly emerging low cost and environment-friendly graphene quantum dots (GQDs) are thus unique semiconducting materials for optoelectronic applications. Striking properties of GQDs such as photoluminescence and band gap engineering have already been explored widely in the recent past14,15. Ultrahigh efficiency silicon solar cells were developed by employing GQDs. Reported in recent literature, n-type Si heterojunction solar cells achieved efficiency as high as 16.55% by the addition of GQDs16. Likewise, the inclusion of GQDs in dye sensitized solar cells (DSSCs) was found to yield a maximum efficiency of 6.10% 17, compared to 5.10% for a DSSC devoid of GQDs. GQDs act as a bridge between the two concepts of co-sensitization and better electron conductivity. They not only enable broader solar spectrum utilization, owing to a band gap in the visible region but also increase current collection, and these cumulatively manifest in a high performance QDSC. It is apparent that a photoanode architecture which employs CdS QDs as the blue-green light harvesters and GQDs (as electron conduits as well as red light absorber) with TiO2 as the wide band gap semiconductor, may be expected to deliver a high PCE. Such a photoanode design has not been attempted before, and the above-discussed developments provided the impetus to use the same in a QDSC. Nickel oxide (NiO) has long been considered an attractive electrode material for solar cells18. It is an intrinsic p-type semiconductor having a wide band gap of ∼3.6 eV and shows appreciable thermal and chemical stability. Its valence-band (VB) potential makes it a suitable electron donor for many photosensitizers. The possibility of using p-type semiconductor photocathodes in DSSCs has been a subject of investigation for the last couple of decades. Dye-sensitized NiO has been employed in combination with a photoanode which allows light harvesting from a broader part of the electromagentic spectrum due to the use of two sensitizers, one at each electrode19. However, the overall solar energy conversion efficiencies of these devices were found to be low due to poor sensitizer loading, recombination of holes in the VB of NiO with reduced dye molecules.

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Therefore, employing QDs as alternatives to the organic dye appears to be a promising strategy to improve the performance of NiO photocathodes 20. Inorganic semiconductors as sensitizers offer several important advantages such as high extinction coefficient for improved light absorption, tunability of band gap energies by controlling the size of QDs, and smaller hole transport resistance for faster transport 18. The concept of photocathode is being recently scrutinized in the quest for a higher photocurrent efficiency in QDSCs. NiO sensitized by visible-light responsive chalcogenides such as CdS18, CdSe20, Cu2S21 is believed to be one of the most useful photocathode materials. Besides, these, lead sulfide (PbS) electrode can serve as both, the CE as well as a photocathode to promote electron injection to the electrolyte with a view to increasing the short circuit current density (JSC) as well as open circuit voltage (V OC) for optimum performance22. Here we present Sb2Se3 QDs as photosensitizers for the NiO photocathode. Sb 2Se3 can be regarded as a new non-toxic and earth abundant NIR region light absorber. Thin film of Sb2Se3 with an optimal solar bandgap of ∼1.1 eV displayed a compelling photoelectrochemical performance23. These Sb2Se3 thin film solar cells displayed a certified device efficiency of 5.6%23. Recent studies on the photovoltaic activity of Sb2Se3-based heterojunction solar cells24 and thin film solar cells25,26 are also noteworthy. Furthermore, Sb2Se3 is also a promising candidate as a light harvesting material in the design of low-cost, robust, and high efficiency spiroOMeTAD/Sb2Se3/metal oxide solid-state semiconductor sensitized solar cells27. Yet it continues to be unexplored in the field of QDSCs. In this work, Sb2Se3 QDs deposited by successive ionic layer adsorption and reaction (SILAR) over electrodeposited NiO nanofibers on carbon (C)-fabric substrate is used as the photocathode. C-fabric is a cost effective highly conducting (with low sheet resistance) current collector that requires no elaborate treatment prior to use. The unhindered charge propagation in the TiO 2/CdS/GQDs photoanode assembly is augmented by an energetically favorable electron cascade at the photocathode. The synergy that prevails between the photoanode and photocathode in this cell architecture leads to an impressive performance improvement.

Experimental section Materials Fluorine doped tin oxide (FTO) glass with a sheet resistance of ~25  cm-2 was purchased from Pilkington and cleaned consecutively in soap solution, 10% HCl solution, 10% NaOH solution, distilled water

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and acetone/ethanol (v/v: 1:1). TiO2 P25 and fumed silica were free gifts from Evonik and Cabosil respectively. Titanium chloride (TiCl 4), Triton X-100, sodium hydroxide (NaOH, 99%), hydrochloric acid (HCl), methanol, ethanol, acetone, nickel sulfate heptahydrate (NiSO4.7H2O), polyethylene glycol (PEG-1000) and sulfur powder, were purchased from Merck; acetyl acetone, cadmium acetate [Cd(CH3COO)2], sodium sulfide (Na2S), diethanolamine (DEA), antimony chloride (SbCl3), selenium dioxide (SeO2) and sodium borohydride (NaBH4), and dialysis bags (2000 Da MWCO, product number D2272) were acquired from Sigma Aldrich. Pitch carbon fiber (Fibre Glast Development Corporation), and H2SO4 (95-98%), HNO3 (70%), sodium carbonate (Na 2CO3, 70%) from FisherScientific were used directly. C-fabric (sheet resistance ~10  cm−2) was procured from Alibaba Pvt. Ltd. Ultrapure water with a resistivity of ~18.2 M cm was obtained through a Millipore Direct-Q3 UV system. Photoanode preparation TiO2 paste was made with TiO2 P25 powder (0.3 g), dispersed uniformly in a clear solution of acetylacetone (1.5 mL), ultrapure water (8.5 mL) and Triton X-100 (20 mg), and applied on cleaned FTOcoated conducting glass plates by screen-printing method. The as-fabricated TiO2 plates were heated at 60 °C for 30 min, and then annealed at 500 °C for another 30 min. A second layer of TiO 2 was applied in the same manner, followed by heating and annealing. The TiO2 coated FTO substrate was immersed in an aqueous TiCl4 (40 mM) solution for 30 min at 70 °C. It was then washed in distilled water and subsequently annealed for 30 min at 500 °C. For the deposition of CdS QDs over the TiO 2 film, 0.1 M cadmium acetate and 0.1 M sodium sulfide were taken in two separate beakers and dissolved in methanol, to be employed as the cadmium and sulfide precursors, respectively. A given TiO2 film was initially immersed in the cadmium precursor for 2 min, rinsed in methanol to remove excess ions, and then dried in a hot air oven at 60 °C. This was followed by immersing the same film in the sulfide precursor solution for 2 min and then rinsing in methanol. This constitutes one SILAR cycle. A total of six SILAR cycles were performed, and the film obtained on completion was labelled as a TiO2/CdS film. Graphene quantum dots (GQDs) were synthesized by the chemical oxidation and an acidic treatment of carbon fibers (CFs, 300 mg) with 60 mL of H 2SO4 and 20 mL of HNO3. The solution was sonicated for 2 h and the reaction mixture was stirred for 24 h at 100 oC. After that, the mixture was cooled down to room temperature and diluted with 800 mL of deionized water. The pH was adjusted to 8 with Na 2CO3. The final product solution was dialyzed in a dialysis bag

with a molecular cut off of 2000 Da for 3 days to get a purified GQDs solution.The GQDs colloid was further diluted in deionized water, drop-cast on the TiO2/CdS film and dried at 70 oC for 5 min on a hotplate. Photocathode or CE preparation A 0.2 M solution of nickel sulfate heptahydrate was prepared in deionized water (20 mL). To this green colored solution, 5 mL of diethanolamine was added and stirred for 5 min to form a bright blue solution. Few drops of the surfactant PEG 1000 were mixed thoroughly in the resultant solution. The nickel oxide film was electrochemically deposited on C-fabric (glued to a glass substrate for mechanical support) at +0.9 V vsersus an Ag/AgCl/KCl reference electrode against Pt CE for a fixed duration. This process is analogous to the one reported in literature 28. The NiO coated C-fabric electrode was dipped in a solution of 10 mM Sb(CH3CO2)3 in acetone for 30 s, rinsed in acetone and dried for 2 min. A 30 mM selenide solution was prepared by the reduction of selenium dioxide with NaBH 4 in ethanol under an inert atmosphere27. The deep red solution turned transparent which indicated the formation of Se 2- from Se4- in the reaction: SeO2 + 2NaBH4 + 6C2H5OH → Se2- + 2Na+ + 2B(OC2H5)3 + 5H2 + 2H2O (1) After the deposition of the Sb 3+ cationic layer over the NiO/C-fabric, the electrode was dipped in the selenide solution for 30 s, rinsed in ethanol, dried for 2 min and a Sb2Se3 layer was obtained. The dipping of the electrode in two different precursors, Sb 3+and Se2solutions constitutes one SILAR cycle. The electrode is labelled as Sb2Se3/NiO/C-fabric. The NiO deposition time (1, 3, 5, 8 and 10 min.) and the number of Sb 2Se3 SILAR layers were optimized depending on photovoltaic performances. Cell fabrication A polysulfide gel electrolyte was prepared by vigorously mixing 1 M Na2S and 1 M sulfur powder in deionized water (10 mL) till they completely dissolved. To the above solution, 4 wt% of fumed silica was added and stirred for 20 min. and a gel was obtained. A 2 mm thick and 2 mm wide para-film spacer was placed between the photoanode and CE, which were then clamped together with binder clips after filling the gaps between active sites with the aqueous polysulfide gel electrolyte. The stepwise construction of the solar cell is presented as a cartoon illustration in Scheme 1. Instrumental methods Raman spectrum of GQDs was recorded on a Bruker Senterra dispersive Raman microscope spectrometer, with a 532 nm laser excitation. A Horiba Fluoromax-4 spectrometer was used for recording fluorescence spectra. Time-correlated single photon counting (TCSPC) method with a Horiba Jobin Yvon data station HUB operating in the TCSPC mode, was

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used for studying emission decay. The excitation source was a nano LED, pulsed at 370 nm with a 1 MHz repetition rate, and the pulse duration was 1.3 ns. A Ludox solution was used to acquire the instrument response function (prompt). Horiba Jobin Yvon DAS6 fluorescence decay analysis software was used to fit the model function to the experimental data. Absorption spectra were recorded on a UV−VIS-NIR spectrophotometer (T90+ of PG Instruments). Current versus potential (I-V) data of QDSCs were measured using a LOT-Oriel solar simulator coupled with a Metrohm Autolab PSTAT302N. The light source was a 150 W Xenon arc lamp, which delivered a collimated output beam of 25 mm diameter through Air Mass (AM) 1.5 ﬁlter, providing a light intensity of 100 mW cm-2 (1 sun). The spatial uniformity of irradiance was confirmed by calibrating with a 2 cm  2 cm Si reference cell and re-affirmed with a Newport power meter. Electrochemical impedance spectra (EIS) for photoanodes were recorded on an Autolab PGSTAT 302N equipped with a frequency response analyser (FRA) and a NOVA 1.11 software, under an ac amplitude of 20 mV over the frequency range of 1 MHz to 0.1 Hz, and a white LED (irradiance: 22 mW cm -2) was used as the light source. Mott-Schottky plots were derived from dark EIS measurements using 0.1 M KOH electrolyte, Pt CE and Ag/AgCl/KCl reference. Linear sweep voltammetry (LSV) was performed using a three-electrode system where the bare C-fabric and Cfabric with NiO and Sb2Se3 served as the working electrode, Pt as the CE and Ag/AgCl/KCl was used as a reference electrode at a scan rate of 20 mV s -1. An aqueous polysulfide electrolyte containing 0.2 M KCl as supporting electrolyte was used. LSV plots were also recorded for the CEs in a neutral pH electrolyte under irradiance (1 sun). EIS measurements for CEs were performed by assembling symmetrical cells with sandwich structure, i.e., two identical CEs face-to-face and the polysulfide electrolyte in between. TEM (transmission electron microscopy) images of GQDs, NiO, Sb2Se3 were obtained a JEOL 2100 microscope operating at 200 kV using samples deposited on carbon coated copper grids. A Bruker Multimode 8 microscope in ScanAsyst mode (Nanoscope 8.10 software) was used for Kelvin probe force microscopy (KPFM) measurements. In KPFM, a line scan was produced first in tapping mode, and then the same line was re-scanned in the lift mode. Sb doped Si cantilevers with tips coated with Co/Cr were used. The sample was affixed onto a stainless steel disk with a conducting carbon tape and pin-hole free silver paste applied for making contacts. In KPFM, an alternating current (ac) voltage with an adjustable direct current (dc) offset potential was applied between a conducting tip and the sample surface and the resulting electrostatic force was detected by a lock-in amplifier.

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The feedback loop controls the dc tip potential until the contact potential difference (VCPD) is annulled. Topography and surface potential maps were obtained as a function of the lateral coordinate. Conductances (G) were measured by LSV using two stainless steel (SS) electrodes with a 2 mm wide para-film spacer in between.

Results and Discussion 1. Photoanode characterization 1.1 Structural features of GQDs The TEM images of GQDs (Figure 1a,b) show them to be in the form of agglomerated particles. The individual GQDs, each of which measures 10-15 nm in size, coalesce to form the larger particle. The corresponding selected area electron diffraction (SAED) pattern (inset of Figure 1a) of diffuse rings with bright spots is observed and they are indexed to the (002) and (101) lattice planes of carbon matching with the interplanar spacings of 3.39 Å and 2.04 Å (JCPDS card # 75-1621). For conductance, an I (current)-V (voltage) plot of GQDs was recorded over a potential range of 0.5 V to +0.6 V (Figure 1c). The dependence is linear over a range of 0.2 to 0.6 V, and using Ohm’s law (slope = I/V = 1/R = G), the conductance of GQDs is calculated to be 10.2 S. The impact of conductance on cell perfromance is discussed later. The Raman spectrum of GQDs is exhibited in Figure 1d. The D and G bands are typically observed due to defects and the first-order scattering of the E2g mode of the sp2 hybridized carbon atoms. An intense but broad disorder-induced D peak appears around 1365 cm -1 and the graphitic G peak appears around 1585 cm -1. The ID/IG ratio is ~2.17 which is suggestive of the presence of more structural defects in the graphene sample (perhaps caused by the mineral acid treatment, during exfoliation of the carbon fibers, used during their synthesis). The integrated area ratio of D and G bands indicates the extent of disorder in graphene. 1.2 Effect of GQDs on absorption and emission The absorption spectra of the wide band gap semiconductor TiO2 and photosensitizers: CdS and GQDs and the photoanodes are shown in Figure 2a and b. The spectrum of sole TiO2 showed a pronounced absorption band below 400 nm and from the absorption edge, the band gap (Eg) of TiO2 is calculated to be 3.18 eV. Pristine CdS QDs shows a broad absorption extending from 310 to 510 nm, and then it tapered off. The band gap is deduced to be 2.33 eV. The tunable bandgap of GQDs depends on the size of the quantum dots tailored directly by synthesis temperature. At low synthesis temperatures, the absorption tends to be broad across the visible region. The absorption profile of GQDs synthesised at 100 ˚C exhibits an expansive absorption band in the wavelength range of 330-545 nm. The band gap is

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determined to be 1.97 eV, which is close to an earlier reported value of 2.2 eV15. The absorption features of TiO2/CdS and TiO2/CdS/GQDs films reflect the cumulative effects of both TiO2 and the QDs. Both curves display a wide absorption in the visible region over the wavelength range of 330-550 nm with λmax at aproximately 380 nm. Both films show an additional absorption hump in the visible region, spanning 440520 nm, due the band-edge excitation of CdS. The absorption spectrum of TiO2/CdS/GQDs photoanode has a minor peak with a λmax at 548 nm closely resembling GQDs. Although the band gaps of GQDs and CdS are not far apart, yet, a dramatic increase in photovoltaic properties is observed by the inclusion of GQDs in the CdS based photoanode. The fluorescence spectra of CdS/glass, GQDs/glass and electrodes of TiO 2/CdS, TiO2/GQDs, and TiO2/CdS/GQDs deposited on FTO/glass, obtained at an excitation wavelength of 370 nm are shown in Figure 2c and d. Pristine CdS QDs, upon excitation at 370 nm, show a strong emission above 470 nm, with a λmax at ~530 nm and this is due to the band edge emission in CdS QDs. Similarly, GQDs upon excitation at 370 nm, show a strong broad emission above 370 nm, with a λmax at ~475 nm. The emission peak is more intense and broader for GQDs compared to CdS, indicating a high proportion of intra-gap states in GQDs, which is correlated to a lower recombination resistance for electrons at the interfaces wihin the photoanode, and at the anode/electrolyte interface in EIS studies. In the TiO2/CdS and the TiO2/GQDs films, the emission intensity of CdS is quenched by ~41% when the CdS QDs are anchored to titania in the TiO2/CdS electrode whereas GQDs retain about 61% of their original intensity in the presence of titania. The CBs of TiO 2, CdS and GQDs are poised at 4.15, 3.83 and 3.18 eV, and these are determined by cyclic voltammetry experiments (Figure S1, and Table S1, supporting information). The quenching is attributed to the photoexcited electron transfer from the CB of CdS or the CB of GQDs to the CB of TiO2, in TiO2/CdS and TiO2/GQDs films. Upon further introduction of GQDs into the TiO2/CdS assembly, the emission peak is blueshifted to λmax ~470 nm and the fluorescence peak intensity is quenched by ~ 69% compared to the TiO2/CdS assembly devoid of GQDs. Since the emission of the TiO2/CdS/GQDs system was the lowest among all the electrodes, it is obvious that maximum charge transfer occurs herein and therefore this photoanode delivers high photocurrent upon illumination. The energy level alignment in this ternary photoanode is conducive for efficient charge transfer to the current collector, as can be judged from Figure 2e. 1.3 Effect of GQDs on excited electron lifetime

The electron deactivation pathways in the different photoactive material based films were assessed by time resolved emission decay measurements recorded at an excitation wavelength of 370 nm and emission wavelengths of 530 nm. Figure 2f depicts the emission decay plots of pristine CdS and GQDs as well as of the different photoanode assemblies, TiO2/CdS, TiO2/GQDs and TiO2/CdS/GQDs. The emission decay profiles were fitted to biexponential curves on the basis of χ2 values, and the fitted parameters are summarized in Table 1. The average lifetime can be determined by using the equations: I = B1 exp(-t/τ1) + B2 exp(-t/τ2) (2) ⟨τ⟩ = Σi Biτi2 /Σi Biτi (3) In the above equation, I is the normalized fluorescence intensity, τi and Bi are decay time constants and amplitudes, respectively, of the individual decay components, and ⟨τ⟩ is average electron lifetime. The excited electron lifetime for pristine CdS QDs is 16.3 ns, and for pristine GQDs it is 7.9 ns. Here, the slow and fast decay components are attributed to the band edge electron-hole recombination and excited electron transfer. The average electron lifetimes for TiO2/CdS and TiO2/GQDs are 0.63 and 0.77 ns respectively, which are relatively lower as compared to those of the pristine photosensitizers. This is due to fast electron injection from the CBs of CdS or GQDs into the CB of TiO2. The short-lived components here arise due to direct contact of the CdS QDs or GQDs with TiO2 and the long-lived component is ascribed to QDs that are not in direct contact with the oxide particles. The average electron lifetime for the TiO2/CdS/GQDs is 0.17 ns. Here, the short lived component of 6.01 ps is assigned to electron transfer from GQDs to CdS, and the long lived component of 0.17 ns is assigned to electron transfer from CdS to TiO2. This is indicative of a more efficient charge transfer, aided by GQDs that, by virtue of their high electrical conductivity, provide channels for unhindered electron transport, and further promoted by CdS QDs that provide an additional energy level for accepting electrons from GQDs as opposed to recombination. The electrons received from the GQDs are then shuttled to the current collector, thereby enhancing the rate of electron injection. This is in good correspondence with the fluorescence emission trend. 1.4 Effect of GQDs on charge transport Electrochemical impedance spectra provides several parameters such as chemical capacitance, C μ, which is a measure of the electron density in the conduction band (CB) of TiO 2; recombination resistance Rrec, which relates to the recombination of electrons in TiO2 with acceptor species in the electrolyte and/or sensitizer at the

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TiO2/sensitizer/electrolyte interfaces; electron transport resistance, Rt and dark current for photoanode cells. These can be determined from Nyquist curves using a transmission line circuit, as described in detail in earlier reports 29. EIS were recorded at different forward biases ranging from 0.3 to 0.7 V under both dark and 0.1 sun light conditions to compare and construe the mechanism of the enhanced photovoltaic performance for the TiO2/CdS and TiO2/CdS/GQDs photoanode based cells. All Nyquist plots show two skewed semi-circles; the one at high frequencies has a smaller diameter, and is followed by an incomplete large arc in the mid- to lowfrequency range. EIS fitted parameters are summarized in Table 2. Distinction of Cμ, Rrec, and dark current as a function of applied bias and Nyquist plots (under 0.4 and 0.6 V), are shown in Figure 3. We find that C μ increases with applied negative bias for a given TiO2/CdS or a TiO2/CdS/GQDs film both under dark and illumination (Figure 3a). The Cμ value for TiO2/CdS is higher than that for TiO2/CdS/GQDs in dark conditions, while the reverse is true under 0.1 sun light. Rrec being inversely proportional to the recombination rate, decreases with higher negative potential as shown in Figure 3b. The R rec values are in good correlation with the C μ behavior for the same photoanode cells. In the dark, greater R rec value for TiO2/CdS compared to TiO2/CdS/GQDs implies reduced charge recombination rates in the former for electron (from CB of TiO 2)-hole (VB of CdS) recombination at the TiO 2/CdS QDs interface, and electron (from CB of TiO2)-oxidized electrolyte species recombination at the (TiO2/CdS)/S2- interface. In contrast, for TiO2/CdS/GQDs, the chances of recombination are greater owing to (i) the presence of several trap states in the GQDs, which serve as electron sinks, (ii) the high electrical conductance of GQDs, and (iii) a higher number of interfaces. Interfaces have more defects than bulk, and as a consequence, these states, which have energies lower than the CB edge of TiO2 can trap the photoexcited electrons. Rrec, which is sensitive to surface processes, exhibits a strong dependence on illumination but a lower dependence on voltage, indicating that charge transfer is affected by the photogenerated holes 29. On illumination, the recombination rate is enhanced as the presence of photogenerated holes accelerates the recapture of CB electrons by the redox electrolyte, reducing the Rrec30. The poorer dark current of TiO2/CdS/GQDs cells as against the bare TiO 2/CdS at the same potential concurs with the PCEs obtained for the cells (Figure 3c). The length of a 45° line observed at higher frequencies corresponds to one third of the transport resistance- Rt/3, signifying electron diffusion through

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the mesoporous film. The length of this line corresponds to Rt = L/σ where σ is the electron conductivity and L the film thickness29. The Rt values for TiO2/CdS/GQDs and TiO2/CdS at 0.6 V and 0.4 V (Figure 3d and e), both under dark and 0.1 sun, affirm that lower the transport resistance, more facile is the electron transfer. In agreement with earlier reports31, at more negative potentials, both R t and Rrec values become smaller as listed in Table 2 owing to the increasing electron density in TiO2. Under illumination and in the presence of electrically conductive GQDs, Rt is reduced validating the presence of excess electrons in the bulk of the material. The lowering of Rt and Rrec upon illumination of 0.1 sun complements the PCEs of the TiO 2/CdS photoanodes with and without GQDs. 2. Counter electrode characterization 2.1 NiO and Sb2Se3: Structures and morphology The TEM images of NiO and Sb2Se3 are shown in Figure 4. The image of NiO reveals a speckled fibrous morphology (Figure 4a and b). The high magnification image presented as an inset of Figure 4b clearly shows the specks. Its appears as if small particles align vertically to form discrete fibers. These nanofibers are of different dimensions, are oriented haphazardly and also tend to cluster up. The high magnification TEM image of the NiO nanofibers show the length of the fibers to be in the range of 140-160 nm and width between 25-30 nm. The SAED pattern affirming the crystal structure of NiO is shown in Figure 4c. A pattern of concentric diffuse rings with bright spots superimposed over the rings is observed which were indexed to the lattice planes. The (200), (111) and (220) planes originate from the NiO nanofibers with a face centered cubic (fcc) lattice corresponding to d values of 2.09, 2.41, and 1.48 Å as per JCPDS card # 47-1049. The bright field image of randomly scattered Sb 2Se3 NPs grown by SILAR is displayed in Figure 4d. Under higher resolution (Figure 4e), the Sb2Se3 particles are observed as slightly distorted spheres with dimensions in the range of 35-45 nm. Bright spots in the corresponding SAED pattern (Figure 4f) are assigned to (230), (211), (221), (301) and (240) planes of the primitive orthorhombic lattice that match with the inter-layer spacings of 3.25, 3.16, 2.89, 2.78, 2.63 Å respectively as per JCPDS card # 15-0861. Since the Sb2Se3 NPs have dimensions less than 50 nm, when they are drop-cast on the NiO/C-fabric electrode, they can either be accomodated in the gaps between the NiO nanofibers, or get deposited over the regions of Cfabric which are uncoated with NiO, or can even flank to NiO fibers via van der Waals’ interactions. 2.2 NiO: Optimization, p-type behavior and reflectance The current versus time transients recorded during the potentiostatic electrodeposition of NiO over C-

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fabric for different time spans are illustrated in the Figure 5a. At t = 0 s, upon application of a fixed dc bias of +0.9 V, a current peak is observed, which corresponds to the capacitive current whereas the latter half of the decay represent the Faradaic current. The capacitive current exponentially decays with time whereas the Faradaic current decays with the square root of time (inset of Figure 5a). Among the time spans of 60, 180, 300, 480 and 600 s of electrochemical deposition of nickel oxide from the Ni(II)─diethanolamine complex solution over Cfabric, NiO films for 60, 180 and 300 seconds deposition were found to have marginally higher peak currents of 4.6-4.9 mA, making them the most promising as well as in good agreement with the J-V parameters. Optimization of the electrodeposition time of NiO was accomplished from J-V characterization for QDSCs containing NiO/C-fabric as the CE (where NiO was deposited for different durations), a TiO2/CdS photoanode, and a polysulfide gel electrolyte. The plots are shown in Figure 5b. Photovoltaic parameters acquired under 1 sun illumination (100 mW cm-2) for the meticulous comparison of the time for deposition are listed in Table S2. It was seen that the cell containing C-fabric with NiO deposited for 5 min gave marginally higher VOC and JSC but lower fill factor (FF) of 31.73% as compared to that with a 3 min deposition. Therefore, the NiO/C-fabric obtained by 3 min of electrodeposition was selected for further studies as this shows a much higher FF of 47.52% (a 49.76% increase in FF over the 5 min NiO deposit), since compromising on the FF was not considered desirable. The reflectance of NiO film obtained after 3 min or 180 s of electrodeposition time, shows a monotonous rise in %R from 6.8% to 34.5%, when wavelength increases from 300 to 800 nm as shown in Figure 5c. For reflectance measurement, the NiO film was electrodeposited over a FTO/glass substrate under exactly similar conditions that were employed when it was deposited over C-fabric. Since NiO shows a reasonably good reflectance, this is advantageous, for when NiO is employed as the CE in a solar cell, upon rear illumination, it is possible that some part of the light unabsorbed by the photoanode will still be available when it gets reflected back to the photoanode by NiO. This is also confirmed indirectly from the higher photocurrent density obtained for the cell with NiO relative to the cell without NiO. A minor shoulder is also observed in the wavelength range 420550 nm. The absorption spectrum of NiO (inset of Figure 5c) exhibits a sharp peak with a λ max at 315 nm. The band gap of NiO was calculated to be 3.57 eV which coincides with earlier reports21. To ascertain the semiconducting (type) behavior of NiO, a Mott-Schottky plot (Figure 5d) was recorded

and it reveals that the as-deposited NiO on FTO glass behaves as a p-type semiconductor. The flat band potential of NiO, estimated from the point where the line meets the abscissa (by extrapolation) was found to be 2.4 V (versus the normal hydrogen electrode or NHE) in a 0.1 M KOH electrolyte. The negative slope of the curve at a frequency of 10 5 Hz affirms that the semiconductor oxide is p-type, and it will have a workfunction position conducive for accepting electrons from the external circuit, via the current collector (or C-fabric, in this case), when it is used as a CE in a QDSC, upon illumination. 2.3 Sb2Se3 NPs: Band gap, p-type nature and optimization and their role The band gap of Sb2Se3 NPs was determined by cyclic voltammetry (Figure 6a and b), for the absorption spectrum yielded an inconlusive flat band (Figure 6c). Sb2Se3 NPs were drop-cast on a FTO substrate, and used as the working electrode; a Pt sheet and an Ag/AgCl/KCl electrode served as counter and reference respectively.The oxidation and reduction peaks in the cyclic volammograms recorded at 10 mV s-1 in a 0.1 M KCl electrolyte (Figure 6a and b), correspond to the valence and conduction band positions. From the CV plots, by using the equations provided below, the VB and CB positions were obtained, and this VB-CB interval (in eV) is the band gap (Eg). ECB = Ered (eV) = 4.5 eV (≡ 0 V versus NHE) – (Ered(peak) (V) vs. Ag/AgCl/KCl + 0.197 V) (4) EVB = Eox (eV) = 4.5 eV (≡ 0 V versus NHE) – (Eox((peak) (V) vs. Ag/AgCl/KCl + 0.197 V) (5) The CB and VB positions of Sb2Se3 are 4 and 5.14 eV respectively, thus implying a band gap of 1.14 eV, that corresponds to a NIR wavelength of 1088 nm, by using a relation: Eg (eV) = 1240/ (nm). The absorption of Sb2Se3 commences at this wavelength, indicating that it can harvest red photons. To confirm its ability to act as a photocathode, a Mott-Schottky plot for a Sb2Se3 film was recorded. The negative slope of the 1/C 2 versus E plot of Sb2Se3 at 105 Hz in Figure 6d confirms its p-type nature, having holes as the majority carriers, thus proving it be a photocathode material. To optimize the thickness of Sb2Se3 deposit over NiO/Cfabric, J-V characteristics were recorded for QDSCs with Sb2Se3 (x SILAR cycles)/NiO (3 min)/C-fabric electrodes as the CEs (where x = 0,1, 2 and 3), a TiO2/CdS photoanode and a polysulfide gel electrolyte, under 1 sun illumination. The photovoltaic parameters are listed in Table S3 and the plots are shown in Figure 6e. Blank C-fabric/NiO (without Sb2Se3) as CE showed an efficiency of 3.40% with TiO2/CdS photoanode. Upon incorporation of Sb 2Se3 NPs into this electrode by 1 SILAR, efficiency improved to 4.53%. This is due to the conversion of a CE from a

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non-photoactive one to a photocathode with light sensitive Sb2Se3. 1 SILAR cycle of Sb2Se3 over NiO/Cfabric gave not only the best PCE of 4.53% but also higher values of VOC, JSC and FF of 0.60 V, 17.50 mA cm 2 and 43.40%, respectively as compared to those with cells with thicker Sb2Se3 deposits grown by 2 and 3 SILAR cycles, thus making it (~1 SILAR cycle) the obvious choice for further analysis. The I-V plot of Sb2Se3 NPs sandwiched between two SS plates is shown in Figure 6f the conductance is calculated to be 0.44 S respectively. Relatively minor photovoltaic performance for QDSCs with Sb 2Se3 can be attributed to the lower conductance compared to GQDs (10.2 S). The electrical conductance of GQDs is 23-times greater than that of Sb 2Se3; this factor comes to fore during photo-excitation, followed by electron transfer and transport. The latter are facilitated by the higher electrical conductance of GQDs. As a consequence, by incorporation of GQDs in the photoanode, a 28% increase in the average PCE is achieved. In contrast, with the inclusion of Sb2Se3 in the photocathode, the increment is only 7.43%. This is tabulated in Table S4 (supporting information). 2.4 Electrocatalytic activities of NiO-CE and Sb2Se3/NiO-photocathode The electrocatalytic activities of C-fabric and of Cfabric with NiO and Sb2Se3 deposited electrodes for reduction of Sn2− in the electrolyte were assessed as depicted in Figure 7. The linear sweep voltammograms of the electrodes with a polysulfide electrolyte are shown in Figure 7a. The standard reduction potential of the polysulfide redox couple is 0.5 V versus NHE. This is analogous to the apparent reduction peaks obtained for C-fabric and C-fabric/NiO CEs at around 0.52 V and NiO/Sb2Se3/C-fabric electrode at 0.505 V. The reduction peak is credited to the reduction of Sn2into S2- according to the following equation: Sn2- + 2e- → Sn-12- + S2(6) The reduction peak position of Sn2- at the NiO/Sb2Se3/C-fabric electrode shifts to a rather positive potential side compared to the other two electrodes, which reveals that the reduction occurred at a lower overpotential of only 0.005 V as compared to the other two having overpotentials of ~0.02 V. A higher peak current density of 4.7 mA cm -2 of the NiO/Sb2Se3/C-fabric electrode also validates its superior electrocatalytic activity. This is in contrast to the other two electrodes whose lower peak current densities and higher overpotentials testify to their inferior activities32. To further investigate the electrochemical activities precisely, Nyquist plots were generated from symmetric cells of each CE/electrolyte/CE combination and are shown in Figure 7b. In characteristic EIS analysis R s is the ohmic series resistance in the high frequency range which comprises of the sheet resistance of two identical

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electrodes, and the electrolytic resistance. Rce is the charge-transfer processes occurring at the CE/electrolyte, as determined from the first semicircles in the frequency region of 1–100 KHz and inter-related with the electrocatalytic activity for the nS2-/Sn2- redox reaction. Zw is the Warburg impedance obtained from the low frequency arc indicating the resistance to electrolyte diffusion33. The much lower Rs value for the NiO/Sb2Se3/Cfabric electrode in comparison to NiO/C-fabric and Cfabric alone denotes higher conductivity of the former. Similarly the smallest Rce, 2.24  of the NiO/Sb2Se3/Cfabric CE connotes superior electron transfer kinetics for the reduction of Sn2- species in the polysulfide electrolyte. The higher Rce ~7  of the other two CEs adversely affects the charge transfer as well as the overall efficacy. The negligible Z w for the NiO/Sb2Se3/C-fabric photocathode cum counter electrode out of the three infers minimum hindrance to the diffusion of polysulfide ions or sulfide ions in the electrolyte and also across the cross-section of the CE which would otherwise deteriorate the electrocatalytic activity whereas the electrolyte diffusion for C-fabric and NiO/C-fabric is not as facile. All these parameters are crucial for the improvement in the solar cell performances. LSV scans of C-fabric, C-fabric/NiO and Cfabric/NiO/Sb2Se3 photocathodes in a 0.1 M NaNO3 (neutral pH of 7) electrolyte with a Pt rod as the CE and Ag/AgCl/KCl as reference electrode under 1 sun (100 mW cm-2) light intensity are presented in Figure 7c, for comparing the photoelectrochemical performance of the photocathodes. From the I (current density) -V (voltage) curves, C-fabric/NiO/Sb2Se3 photocathode as well as the C-fabric/NiO are seen to have slightly more positive photocurrent onset potential than bare Cfabric. Addition of Sb2Se3 NPs further improved the photocurrent onset potential of C-fabric/NiO. Cfabric/NiO/Sb2Se3 is a much better photocathode for the hydrogen evolution reaction since it gives higher limiting photocurrent of 11.5 mA cm -2 compared to that of 6.05 mA cm -2 of C-fabric in potentials more negative than 0.0 V versus NHE. This affirms Cfabric/NiO/Sb2Se3 to be an effective photocathode. 2.5 Charge transport in the photoanodephotocathode cell Cyclic voltammograms of pristine TiO 2, CdS, GQDs, and NiO films are shown in the Figure S1, and the positions of the CB and VB were calculated from the oxidation and reduction potentials. These data along with the band gaps obtained from absorption spectra are listed in the Table S1, and were used to determine the energy level offsets for the excited electrons relay in the photoanode and photocathode assemblies. To study the electron flow direction in the

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photocathode, the work functions of NiO and Sb 2Se3 were determined by KPFM measurements. The topography and surface potential maps of electrodeposited NiO and Sb2Se3 (grown in-situ by SILAR) were acquired and are presented in Figure 8 (ad). The corresponding section profiles are shown in Figure 8 (aʹ-dʹ). For NiO, the topography image shows the presence of discrete particles, and the corresponding surface potential image shows large bright spot in the centre, embedded in a dark contrast. The bright and dark regions correspond to high and low surface potentials (on a relative scale). The surface potential is color-scaled at the right side of the potential maps. Similarly, for Sb2Se3, aggregates of nanoparticles are perceived. The topography and potential maps are almost similar. The protuding bright particles have higher surface potentials, and the dark particles in the background, have lower surface potentials. The VCPD or surface potential is plotted as a function of the lateral position coordinate. The conducting Co/Cr tip was first calibrated with a reference sample of highly ordered pyrolytic graphite (HOPG) of a known work function (ϕ) of 4.6 eV. The VCPD for HOPG was 90 mV, and using the following equation, the work functions of unknown samples are calculated. ϕ (sample) = 4.6 + VCPD (HOPG) - VCPD (sample) (7) The maximum values of surface potentials for NiO and Sb2Se3 are 180 and 60 mV. By using these potentials, the work function values are calculated to be 4.51 eV and 4.63 eV respectively which is indicative of both NiO and Sb2Se3 being p-type semiconductors. These values suggest better uptake and simultaneous donation of electrons making them suitable as CEs or photocathode. The energy band diagram in Figure 7c depicts the same. 3. Solar cells with NiO-CE and Sb2Se3/NiOphotocathode QDSCs were constructed using two different photoanodes, TiO2/CdS and TiO2/CdS/GQDs paired with different CEs such as blank C-fabric, NiO on Cfabric and NiO as well as Sb 2Se3 on C-fabric. An aqueous polysulfide gel electrolyte was used in the fabrication along with a para-film spacer. The cells were illuminated from the rear-side. Current density (J) versus voltage (V) plots, obtained under 1 sun irradiance (100 mW cm-2) are shown in Figure 9 (a-c). Average solar cell parameters for 3 different cells are summarized in Table 3, and the individual values for 3-cells with different combinations of photoanode and CE are provided in Table S5 (supporting information). Average cell parameters are reliable in providing a more appropriate insight into this study. TiO 2/CdS alone with bare C-fabric gave an average efficiency of 4.06%, while incorporation of GQDs on the

photoanode side enhanced the average efficiency to 5.21% along with an increased open-circuit voltage of 740 mV, a 12.34% upsurge in FF, as well as a slight increase in the average JSC values. Only NiO on Cfabric as the CE and TiO2/CdS as photoanode gave a PCE of 4.44%, but replacing the CE with a photocathode by simply depositing the NIR light harvesting Sb2Se3 NPs over NiO/C-fabric, improved the PCE to 4.77%. For the cell with a Sb2Se3/NiO photocathode, the VOC increased from 670 mV to 690 mV with 2.32% increase in FF and slight boost in the current. The effect of photocathode is more prominent in presence of GQDs in the TiO2/CdS photoanode assembly. In the photoanode array of TiO2/CdS/GQDs, maximum electron injection occurs from the CB of CdS to CB of TiO2 that finally reaches the current collector (FTO) in conjunction with electron transfer from CB of GQDs to CdS. GQDs not only accelerate the charge mobility but also, to a certain extent, act as light absorbers themselves, which is confirmed from the PCE values of only TiO2/GQDs. Average efficacy of TiO 2/GQDs acquired was 0.35% (parameters listed in Table S6). On the CE side, electrons from the external circuit reach the C-fabric which get carried to the 4.51 eV level of NiO and 4.63 eV level of Sb2Se3 that are deposited within the pores of the C-fabric. Excess charge carriers are also generated due to intrinsic photoexcitation of Sb2Se3, which further facilitate faster electron transfer and transport to the holes in the VB of CdS as well as to GQDs in the photoanode via the nS2-/Sn2- electrolyte species. Therefore the average PCE of the TiO2/CdS/GQDs-nS2-/Sn2--NiO/C-fabric cell escalated from 5.34% to 5.62% with the substitution of NiO/Cfabric CE with a Sb2Se3/NiO/C-fabric photocathode. The observed increase in V OC, JSC as well as FF clearly represents the photovoltaic behavior of the photocathode. The complete cell performance with a smaller photoanode area of 0.06 cm 2 gives an excellent efficiency of 7.19% which accentuates the synergistic impact from both the photocathode and photoanode. In a significant study by Jovanovski et al., 34 a sulfide/polysulfide-based ionic liquid electrolyte was used as the hole transport material in QDSCs. Authors observed that ionic liquid based cells showed less degradation compared to cells with aqueous electrolytes. Ionic liquids are non-volatile with high boiling point, high ionic conductivity and are electrochemically extremely robust. In comparison, aqueous electrolytes pose limitations such as evaporation of solvent (water) with time, water reduction at 0.83 V (in an alkaline medium) and the possibility of phase separation. Notwithstanding these drawbacks of aqueous electrolyte, here, solar cell durability assessment of TiO2/CdS/GQDs photoanode with Sb2Se3/NiO/C-fabric photocathode in the 1 M

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aqueous S2-/Sn2- gel electrolyte was performed and the cell was perceived to be stable over a few hours on prolonged illumination of approximately 1 sun. The J (current density)-V (voltage) data listed in Table S7 (supporting information) and shown in Figure 9d confirm the stability of the aforementioned cell as the drop in the PCE is minimal over 6 hours of constant illumination. This stability is perhaps due to the presence of SiO2 nanoparticles in the electrolyte; they are known to induce a passivating effect, and also have the capability to restrict the photo-corrosion of QDs35. Detailed studies on stability will be undertaken in future.

Conclusions Engineering each component in the solar cell contributed in its own way to enhancing the efficiency of the complete cell. The role of conductive GQDs as light harvesters in the CdS quantum dots sensitized titania photoanode has been studied extensively in this work. The solar cell responses for champion cell of TiO2/CdS/GQDs photoanode structure with a C-fabric CE delivered a PCE of 5.28%, as opposed to a 4.23% value that is obtained by using a bare TiO 2/CdS photoanode with the same CE. This 1.25 times rise in the cell efficacy due to the addition of GQDs is reinforced by the photoanode characterizations performed, wherein the electron lifetime is found to reduce along with charge transfer resistance being minimised, notwithstanding the slight decrease in recombination resistance induced by the highly conductive GQDs. GQDs alone anchored to TiO 2 gave 0.36% as best efficiency. On implementing a new photocathode structure with Sb2Se3 nanoparticles as the NIR-photosensitive material on electrodeposited NiO nanofibres over C-fabric substrate and pairing it with the TiO2/CdS/GQDs photoanode, a remarkable cell performance with a PCE of 5.96% was observed. The complementary effect of the photocathode and photoanode lead to good increment in V OC, JSC as well as PCE as compared to bare NiO/C-fabric CE. However, GQDs enhance the PCE more than Sb 2Se3 NPs do. The best PCE achieved for the TiO2/CdS/GQDs-nS2-/Sn2-- Sb2Se3/NiO/C-fabric cell with a smaller area was 7.19%. The excess electrons at the CE side due to the electronic excitation of Sb 2Se3 lead to more rapid overall charge transfer and transport phenomena alongside GQDs in the photoanode matrix, giving a boost to the photovoltaic performance of the solar cell.

ASSOCIATED CONTENT Supporting Information

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Tables of solar cell parameters and cyclic voltammograms. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Melepurath Deepa Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi-502285, Sangareddy, Telangana (India). Email: [email protected], Tel: +91-40-23016024, Fax: +91-4023016003.

ACKNOWLEDGEMENT Financial support from the Solar Energy Research Initiative-Department of Science & Technology (DST/TM/SERI/2K12-11(G)) is gratefully acknowledged. Author AK is thankful to University Grants Commission (UGC) for the grant of junior research fellowship.

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2012, 22, 9745-9750. Jovanovski, V.; González-Pedro, V.; Giménez, S.; Azaceta, E.; Cabañero, G.; Grande, H.; Tena-Zaera, R.; Mora-Seró, I.; Bisquert, J. A sulfide/polysulfide-based ionic liquid electrolyte for quantum dot-sensitized solar cells. J. Am. Chem. Soc. 2011, 133, 20156-20159. Zhao, K.; Pan, Z. X.; Mora-Sero, I.; Ca ́novas, E.; Wang, H.; Song, Y.; Gong, X. Q.; Wang, J.; Bonn, M.; Bisquert, J.; Zhong, X. Boosting Power Conversion Efficiencies of Quantum-Dot-Sensitized Solar Cells Beyond 8% by Recombination Control. J. Am. Chem. Soc. 2015, 137, 5602−5609.

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FIGURES

Figure 1 (a) TEM image of GQDs, (b) the corresponding high magnification image and inset of (a) is SAED pattern of GQDs. (c) I-V characteristics of GQDs recorded in the configuration shown as an inset; the dotted line is the linear fit. (d) Raman spectrum of GQDs; inset is the photograph of the GQDs solution.



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Figure 2 Absorbance spectra of (a) TiO2, CdS, GQDs and (b) TiO2/CdS (TC) and TiO2/CdS/GQDs (TCG) films, (c) fluorescence spectra of CdS, and GQDs and (d) TiO2/GQDs (TG), TiO2/CdS (TC) and TiO2/CdS/GQDs (TCG) films recorded at λex = 370 nm. (e) Energy levels of the photoanode components and (f) fluorescence decay plots of the films and reference recorded at λex = 370 nm and λem = 530 nm.


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Figure 3 Variation of (a) Cμ, (b) Rrec, (c) dark current- as a function of applied forward bias and Nyquist plots under (d) 0.6 and (e) 0.4 V for TiO2 /CdS/GQDs (TCG) and TiO2/CdS (TC) ─ nS2-/Sn2- gel electrolyte ─ Sb2Se3/NiO/C-fabric cells in dark (D) and 0.1 sun light (L).

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Figure 4 (a) TEM image of pristine NiO nanofibers, (b) the corresponding high magnification image, and inset shows the enlarged view of the speckled fibres, (c) the SAED pattern of NiO, (d) TEM image of pristine Sb2Se3, (e) the corresponding high magnification image and (f) the SAED pattern of Sb2Se3.

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Figure 5 (a) Electrodeposition curves of NiO. Inset shows current decay with the square root of time for 3 min NiO (b) J-V characteristics of NiO (deposited for different times on carbon fabric) counter electrodes based cells (under 1 sun illumination, AM 1.5) with a TiO2/CdS photoanode and a polysulfide gel electrolyte. (c) Reflectance spectrum of NiO and inset shows the absorbance spectrum of NiO. (d) Mott-Schottky plot of NiO, deposited for 3 min (as the working electrode), Pt and Ag/AgCl/KCl as counter and reference electrodes, and 0.1 M KCl as the electrolyte (in dark).

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Figure 6 (a,b) Cyclic voltammograms of Sb2Se3 serving as the working electrode with an area 1 cm2, recorded in a 0.1 M KCl solution as electrolyte, with a Pt sheet as the counter electrode and an Ag/AgCl/KCl as the reference electrode, at a scan rate of 10 mV s-1. (c) Absorption spectrum of Sb2Se3. (d) Mott-Schottky plot of Sb2Se3 (measurement set-up shown as an inset) and (e) J-V characteristics of solar cells without Sb2Se3 (0 SILAR) and with Sb2Se3 (x SILAR cycles, x: 1, 2, and 3) deposited on NiO/C-fabric as photocathodes with a TiO2/CdS photoanode and a polysulfide gel electrolyte (under 1 sun illumination, AM 1.5). (f) I-V characteristics of Sb2Se3 recorded in the configuration shown as an inset; the dotted line is the linear fit.

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Figure 7 (a) Linear sweep voltammograms of different CEs: C-fabric (C), NiO/C-fabric (CN) and Sb2Se3/NiO/C-fabric (CNS) (as working electrodes) in a 0.02 M polysulfide solution in ultrapure water containing 0.2 M KCl as supporting electrolyte in dark. (b) Nyquist plots for symmetric cells of C-fabric (C), NiO/C-fabric (CN) and Sb2Se3/NiO/C-fabric (CNS) and inset shows the enlarged views of the plots. (c) LSV scans of C-fabric (C), NiO/C-fabric (CN) and Sb2Se3/NiO/C-fabric (CNS) in 0.1M NaNO3 electrolyte under illumination of 1 sun. (d) Energy levels of the photocathode components.

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Figure 8 Topography and surface potential maps (a,b) and the corresponding section profiles (aʹ,bʹ) of NiO, topography and surface potential maps (c,d) and the corresponding section profiles (cʹ,dʹ) of Sb2Se3.

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Figure 9 (a,b) J-V characteristics for TiO2/CdS/GQDs (TCG) and TiO2/CdS (TC) photoanode based cells each with three different counter electrodes: C-fabric (C), NiO/C-fabric (CN) and Sb2Se3/NiO/C-fabric (CNS) (polysulfide gel electrolyte, under 1 sun illumination, AM 1.5). (c) J-V characteristics for a small area cell with the following architecture: TiO2/CdS/GQDs (TCG) ─ S2-/Sn2- gel electrolyte ─ Sb2Se3/NiO/C-fabric (CNS). (d) Variation of cell parameters: VOC, JSC, FF and PCE on continuous illumination for 6 hours.

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SCHEME

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Scheme 1 Cartoon illustrating the preparation of photoanode, photocathode and complete cell.

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TABLES Table 1 Kinetic parameters of emission decay analysis of photosensitizer films deduced from double exponential fits. B1

1 (ns)

B2

CdS

26.79

0.00747

73.21

16.30

16.31

1.15

GQDs

0.53

0.00436

99.47

7.89

7.88

1.08

TiO2/CdS

6.45

0.00299

93.55

0.63

0.63

0.99

TiO2/GQDs

65.65

0.04090

34.35

0.84

0.77

1.12

TiO2/CdS/GQDs

2.75

0.00601

97.25

0.17

0.17

1.02

Sample

2 (ns)

χ2

(ns)

Table 2 EIS parameters extracted from Nyquist plots of different photoanodes and counter electrodes under dark and 0.1 sun illumination.

Cells

Rt ( cm2)

Rt ( cm2)

Rrec ( cm2)

Rrec ( cm2)

0.6 V

0.4 V

0.6 V

0.4 V

TiO2/CdS/GQDs (Light)

1.81

1.93

4.8

16.2

TiO2/CdS (Light)

1.94

2.01

2.9

13.4

TiO2/CdS/GQDs (Dark)

2.05

2.19

5.2

29.7

TiO2/CdS (Dark)

2.20

2.32

26.9

112.1

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Table 3 Average solar cell parameters of 3-cells with 1 M polysulfide gel electrolyte, under 1 sun illumination (AM 1.5, 100 mW cm-2) with the listed combinations of photoanodes and counter electrodes, exposed cell area: 0.100.12 cm2. Cells

VOC (V)

JSC (mA cm-2)

FF (%)

Efficiency ( %)

TiO2/CdS – C-fabric

0.66

11.10

55.57

4.06

TiO2/CdS – C-fabric/NiO

0.67

12.48

53.51

4.44

TiO2/CdS – C-fabric/NiO/Sb2Se3

0.69

12.74

54.75

4.77

TiO2/CdS/GQDs – C-fabric

0.74

11.53

62.43

5.21

TiO2/CdS/GQDs – C-fabric/NiO

0.66

16.42

49.78

5.34

TiO2/CdS/GQDs – C-fabric/NiO/Sb2Se3

0.68

16.53

50.31

5.62

TiO2/CdS/GQDs – C-fabric/NiO/Sb2Se3

0.63a

25.49a

45.00a

7.19a

a: Cell parameters for the champion cell with smaller exposed area of 0.06cm 2

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Graphical Abstract

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Nickel Oxide Photocathode Boosts the

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