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Stability, Scale-up and Performance of Quantum Dot Solar Cells with Carbonate Treated Titanium Oxide Films P. Naresh Kumar, Ankita Kolay, Melepurath Deepa, Sonnada Math Shivaprasad, and Avanish Kumar Srivastava ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b05726 • Publication Date (Web): 10 Jul 2017 Downloaded from http://pubs.acs.org on July 11, 2017

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ACS Applied Materials & Interfaces

Stability, Scale-up and Performance of Quantum Dot Solar Cells with Carbonate Treated Titanium Oxide Films P. Naresh Kumar,a Ankita Kolay,a Melepurath Deepa,a,* S. M. Shivaprasad,b A. K. Srivastavac a

Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi-502285, Sangareddy, Telangana (India) International Centre for Materials Science, Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore-560064 (India) c CSIR-National Physical Laboratory, Dr. K. S. Krishnan road, New Delhi-110012 (India) b

ABSTRACT: A novel yet simple approach of carbonate (CBN) treatment of TiO2 films is performed, and quantum dot solar cells (QDSCs) with high power conversion efficiencies (PCEs), reasonably good stabilities, and good fill factors (FFs) are fabricated with TiO2-CBN films. The ability of carbonate groups to passivate defects or oxygen vacancies of TiO2 is confirmed from a nominally enhanced band gap, a lowered defect induced fluorescence intensity, an additional Ti-OH signal obtained after carbonate decomposition, and a more capacitive low frequency electrochemical impedance behavior achieved for TiO2-CBN compared to untreated TiO2. A large area QDSC of 1 cm2 with a TiO2-CBN/CdS/Au@PAA (poly(acrylic acid)) photoanode delivers an enhanced PCE of 4.32% as opposed to 3.03% achieved for its’ analogous cell with untreated TiO2. Impedance analysis illustrates the role of carbonate treatment in increasing the recombination resistance at the photoanode/electrolyte interfaces, and in suppressing back electron transfer to the electrolyte, thus validating the superior PCE achieved for the cell with carbonate treated TiO2. QDSCs with the following configuration: TiO2-CBN/CdS/Au@PAA-polysulfide/SiO2 gel-Carbon-fabric/WO3-x and active areas of 0.2-0.3 cm2, yield efficiencies in the range of 5.16 to 6.3% and the average efficiency of the cells is 5.9%. The champion cell is characterized by the following photovoltaic parameters: JSC (short circuit current density): 11.04 mA cm-2, VOC (open circuit voltage): 0.9 V, FF: 0.63 and PCE: 6.3%. Stability tests performed on this cell show that dark storage has a less deleterious effect on cell performance compared to extended illumination. In dark, the PCE of the cell dropped from 5.69 to 5.52%, and under prolonged continuous irradiance of 5 h, it decreased from 5.91 to 4.83%. A scaled-up QDSC with the same architecture of 4 cm2 size showed a PCE of 1.06%, and the demonstration of the lighting of a LED accomplished using this cell, exemplifies that this cell can be used for powering electronic devices that require low power.

Keywords: Quantum dots; solar cells; plasmonic; passivation; stability

Introduction Quantum dot solar cells (QDSCs) are emerging as stable, low-cost, and highly efficient solar cells. Typically, for most of the high performance QDSCs, the power conversion efficiencies (PCEs) are reported for cells with active areas less than 0.5 cm2. A champion cell with Cu-In-Ga-Se QDs based photoanode and a mesoporous carbon/Ti counter electrode (CE) delivered a PCE of 11.49%,1 and in another study, ZnCu-In-Se QDs based photoanode coupled with a nitrogen doped mesoporous carbon CE gave a certified efficiency of 12.07%, with an active area of 0.236 cm2.2 Compared to small area cells, large area cells are more useful, for they can be used for practical applications like powering low power consuming electronic devices (e.g. bio-sensors), without any external power supply. PCE reduces as the device area increases, because of inefficient electron propagation towards the current collector. Efficient charge transfer to the external circuit, accompanied by a minimum loss via recombination, has been successfully realized in small area QDSCs, by the use of passivation layers of amorphous TiO2, SiO2, and ZnS.3,4,5 These coatings suppress recombination within the photoanode and also reduce back electron transfer to the oxidized electrolyte

by passivating the surface defects on QDs, and also by serving as a physical barrier at the photoanode, thin enough to allow hole transfer but thick enough to obstruct back electron movement. With a sequential inorganic ZnS/SiO2 double layertreatment applied to a CdSeTe QDs/TiO2 film, Zhao et al., obtained a PCE of 8.21%, compared to 2.53% without the passivation layers.5 This spectacular improvement in PCE indubitably brings out the role of passivation layers in improving charge collection. In another report, Yang et al., applied passivation layers of a-TiO2/ZnS/SiO2 to CdSeTe/CdS core/shell QDs/TiO2 films, and obtained a best PCE of 9.48%, much greater than 8.02%, achieved for an equivalent cell with just a ZnS layer.6 In another report, Cherie et al., introduced an extra buffer layer of CdSe QDs at the interface between ZnO NPs and PbS QD layers. The cell with the ZnO/CdSe/PbS photoanode showed a PCE of 7.5%, and without the CdSe buffering layer, it showed a PCE of 6.0%. Optimized band alignment across the junction, and passivation of interface traps by the buffer layer, improved the PCEs.7 Wei et al., studied the effect of fumed silica additive (in the polysulfide electrolyte) on cell performance. Fumed silica served not only as a

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gelatinizing agent but also imparted a 28.6% enhancement in PCE from 8.73 to 11.23%. The improvement was attributed to the passivation effect of SiO2 over TiO2/CdSexTe1-x QDs, which prevented back electron transfer to the electrolyte.8 Titanium oxide (TiO2) is the most widely used semiconducting scaffold in QDSCs and suffers from surface and subsurface defects, which increase the series resistance of the QDSC, and thus result in increased recombination and back electron transfer at TiO2/QDs and TiO2/electrolyte interfaces.9,10 During the processing of TiO2, an aqueous TiCl4 solution is applied over the mesoporous TiO2 (P25) layer, and the resulting film is annealed at 500 oC in air.11,12 Incomplete oxidation results in the formation of sub-stoichiometric TiO2-x.13 Intra-gap states in TiO2 are introduced by these oxygen vacancies. These defect states act as charge recombination centers and thus inhibit efficient charge transfer to the current collector (FTO or SnO2:F), which in turn reduces the short circuit current density (JSC) and therefore, the PCE. As the geometric size of the cell increases, the proportion of defect states also increases, and they impact PCEs adversely. Dette et al., studied TiO2 with a band gap in the visible region (Eg: 2 eV), and attributed this low gap to oxygen deficiency over the outer phase on the TiO2 crystal, and the presence of the Ti4+ interstitial particles by scanning tunneling microscopy (STM).14 Scheiber et al., studied the stability of oxygen vacancies at surface and bulk level; they found that, for the anatase polymorph, the (101) plane is energetically more favorable for holding the oxygen vacancies.15 DeJongh et al., studied photogenerated electron transport through nanometer-sized TiO2 (a suspension of TiO2 P25 in ultrapure water) and observed that defect states were located at 0.4 to 0.7 eV below the conduction band (CB) of TiO2 and while they enhanced the rate of charge collection from the photosensitizers in the QDSCs, they also reduced the charge transport to the FTO.9 ZnS has been employed as a passivation layer between the TiO2/QDs assembly and the electrolyte in previous reports.5,6 For the first time in 2002, Yang et al., studied the TiO2/PbS/CdS/ZnS photoanode system.16 Compared to other sulfides in QDSCs, the PbS photoanode experienced rapid photocurrent decay under irradiation. PbS was found dissociate to Pb2+ and S2- ions, which subsequently deposited on larger particles. As a consequence, the CB edge of PbS QDs shifted to more negative potentials (versus the normal hydrogen electrode (NHE)), relative to that of TiO2 and the charge carriers could not be transferred from the PbS QDs to TiO2 efficiently. Therefore, to prevent photoanode corrosion, 3 layers of ZnS were applied over the TiO2/PbS/CdS film and the stability of the photoanode and photocurrents improved.16 After this report, many research groups used ZnS to suppress the photo-corrosion of the anode. The power conversion efficiency improvement was also attributed to the ability of ZnS: (a) to serve as a barrier and prevent back electron transfer from the CB of TiO2 to the electrolyte and (b) to passivate the QD surface states, which reduced electron trapping.17-19 In a noteworthy study by Zhong’s group, first-principles density functional theory calculations were implemented to unravel the passivating effect of ZnS treatment on anatase TiO2. It was deduced that the ZnS layer deposition onto the TiO2 (101) plane, can achieve oxide surface passivation by forming wellordered Zn−O and S −Ti bonds that saturate all the dangling

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bonds at the oxide surface, thus suppressing recombination, at defect states.20 It is obvious that back electron transfer and recombination are the two major pathways for current loss. To circumvent this issue, here we present the use of a yet unreported treatment for maximizing current collection in QDSCs. An aqueous carbonate (CBN) solution was applied to the TiO2 film, and the carbonate ions tend to fill the oxygen vacancies in TiO2, reduce the population of the defect states, which ensues in improved cell performance and stability as well. The role of carbonate, which has never been attempted before, was explained by emission quenching and decay studies, cyclic voltammetry, electrochemical impedance-, Raman- and X-ray photoelectron- spectroscopies. A polysulfide/fumed silica based gel electrolyte was used as the hole transport layer in QDSCs. In the TiO2/CdS photoanode, plasmonic nanoparticles of Au capped with poly(acrylic acid) or PAA were also incorporated to increase the light harvesting ability of CdS via near field enhancement and scattering effects.21 The Au@PAA NPs play a role similar to that of Au@TiO2 or Au@SiO2 NPs in photoanodes in dye sensitized solar cells (DSSCs).22,23 In a previous report, Au@SiO2 NPs increased the PCE of a DSSC due to near field plasmonic effect, by increasing the JSC of the cell, and Au@TiO2 NPs increased the PCE of a DSSC due to charging effect, by increasing the open circuit voltage (VOC) of the cell.22 While the core metal NP induces the plasmonic and charging effects, the shell (like PAA, in the present case) of the Au core is thin enough to induce plasmonic enhancement to vicinal CdS QDs, and thick enough to reduce the corrosion of the Au core by the sulfide electrolyte. Bare Au NPs, without shell are also known to trap electrons, which is a loss mechanism,23 and therefore capped NPs were used. In a previous elaborate study by our group on CEs, we found that Carbon (C)-fabric /WO3-x serves as an excellent CE, for the cell with this CE exhibited the highest PCE of 4.6% and this was attributed to the least polysulfide reduction potential, a reasonably good electrocatalytic activity, and a low sheet resistance of the C-fabric (9-15 Ω cm-2) and its’ mesh like morphology that maximized WO3-x loading.24 Here, QDSCs with the following configuration: TiO2CBN/CdS/Au@PAA-polysulfide/SiO2 gel-C-fabric/WO3-x, showed high PCEs, high fill factors (FFs) and reasonably good stabilities as well. The effect of carbonate treatment on stability and performance is explained, and how this treatment is effective in preserving high PCEs to some extent upon scaleup to large area cells with active areas of 1 cm2 and even 4 cm2 is also demonstrated. This report, we believe, will be of great use, especially for the deployment of large area QDSCs for real time applications. Experimental Photoanode preparation A TiO2 paste made of TiO2 NPs (0.3 g, P-25) dispersed uniformly in a clear solution of acetylacetone (1.5 mL), ultrapure water (8.5 mL) and Triton X-100 (20 mg) was applied over pre-cleaned FTO-coated conducting glass plates. The asfabricated TiO2 plates were heated at 60 oC for 30 min, and then annealed at 500 oC for another 30 min. Another layer of TiO2 was applied using steps detailed above, followed by heating and annealing. The TiO2-coated FTO substrate was then immersed in an aqueous TiCl4 (40 mM) solution for 30 min at

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70 oC, followed by washing in distilled water and annealing at 500 oC for 30 min. These films are labeled as TiO2. These films were immersed in an aqueous solution of 0.1 M potassium carbonate (K2CO3) solution for 5 min., and then rinsed in ultrapure water and heated to 150 oC, held at this temperature for 15 min., rinsed in water and the resulting film is labeled as TiO2-CBN. Over the TiO2 and TiO2-CBN films, six layers of CdS were deposited by successive ionic layer adsorption and reaction (SILAR) process using solutions of 0.1 M Cd(CH3COO)2 and 0.1 M Na2S salts in methanol placed in two separate beakers. TiO2-CBN or TiO2 films were dipped in the two beakers containing Cd2+ and S2- sequentially, with a rest time of two minutes in each beaker. After each dip, the coated substrates were rinsed in methanol to remove the excess ions, and dried for 2 minutes under hot air. The substrates turned slight yellow in color and this is noted as 1 SILAR cycle. The same procedure was followed five more times to deposit a total of six layers of CdS QDs over the TiO2 and TiO2CBN films. A solution of gold nanoparticles capped with poly(acrylic acid) (Au@PAA NPs) was prepared by using a previously reported method.25 Briefly, an aqueous clear solution of sodium citrate (2 mM) in ultrapure water (150 mL) was refluxed at 100 oC with continuous stirring for 15 min., and a solution of HAuCl4 (25 mM) in 1 mL was injected. The color of the solution changed from yellow to blue-green and consequently to pink. The colored solution was centrifuged and the clear supernatant liquid was extracted and diluted with an equal amount of ultrapure water. The residue was rejected. To this solution, an aqueous NaOH (0.5 M, 40 µL) solution was added and a solution of 10 µL of 25 wt.% PAA in ultrapure water was added and sonicated for 3 h at room temperature and the ensuing purple colloid of Au@PAA nanoparticles (NPs) was stored at 5 oC. A solution of Au@PAA NPs (5 mL) was added to 2 mL of toluene and sonicated for 2 minutes. The toluene layer from the solution was extracted, and it was drop-cast thrice (with intermittent drying) on the TiO2/CdS and TiO2CBN/CdS photoanodes. The resulting TiO2/CdS/Au@PAA and TiO2-CBN/CdS/Au@PAA photoanodes were stored under dark conditions at 45 oC for 20 days prior to solar cell characterizations. Counter electrode preparation Carbon-fabric/reduced tungsten oxide (C-fabric/WO3-x) films were employed as CEs in QDSCs.24 Tungsten metal powder (3 g) was dissolved in 30 mL of 30% H2O2; once the effervescence abated, the pale yellow solution was aged at room temperature under dark for 20 days. To this aged solution, 3 mL of poly(ethylene glycol) surfactant was added. A carbon fabric with the required dimensions, procured from Alibaba Pvt limited, was cleaned in acetone followed by drying under hot air for five min., and stuck onto a glass slide with glue. It was employed as a cathode in the polyperoxotungstate solution, and a blank FTO substrate was used as the anode for electrodeposition. An appropriate dc voltage was applied for 5 min., and a blue colored film of reduced tungsten oxide (WO3-x) was obtained over C-fabric. The film was rinsed in water, and dried in air. The film is labeled as C-fabric/WO3x and it was stored at room temperature overnight and used as a CE in QDSCs.

Electrolyte and fabrication of QDSCs A polysulfide/SiO2 gel was prepared by addition of sodium sulfide (1 M) and elemental sulfur (1 M) to ultrapure water, once it turned into a clear dark-yellow colored solution, 5 wt.% of fumed silica was added and it was dispersed by continuous stirring with a glass rod, till a bubble free homogeneous gel was obtained. A 2 mm thick para-film with a desired cavity, was affixed over the C-fabric/WO3-x film, and the cavity was filled with the polysulfide/SiO2 gel electrolyte. The photoanode (e.g., TiO2-CBN/CdS/Au@PAA) with the active layer facing the cavity, was affixed over this assembly, and the cells were used for photovoltaic measurements. Instrumentation The optical absorption spectra of the photoactive films were measured in absorbance mode, on a UV-Vis spectrophotometer (T90+UV/VIS Spectrometer). Fluorescence spectra of TiO2 and TiO2-CBN films were measured on a Horiba Fluoromax-4 fluorescence spectrometer; a filter was used during the measurement and background correction was applied. Timecorrelated single photon counting (TCSPC) method was used for determining emission lifetimes with a Horiba Jobin Yvon data station HUB functioning in the TCSPC mode. A nano LED emitting pulses at 370 nm with a 1 MHz repetition rate and a pulse duration of 1.3 ns was employed as an excitation source. Light-scattering Ludox solution (colloidal silica) was used to acquire the instrument response function (IRF/prompt). A long pass 450 nm filter was placed in front of the emission monochromator, for all measurements. Horiba Jobin Yvon DAS6 fluorescence decay analysis software was used to fit the model function (bi-exponential decays) to the experimental data, with appropriate correction for the instrument response. X-ray photoelectron spectroscopy (XPS) was done for films on an Omicron Nanotechnology GmbH system operating at a base pressure of ~6 × 10-11 Torr with a nonmonochromatized Mg Kα line at 1253.6 eV. The acquired resolution of core levels was 0.1 eV measured at 90% of peak height and the spectra were acquired at 25 eV pass energy. The survey spectra were acquired with 100 eV pass energy, and the resolution is 1 eV. The core level spectra were deconvoluted using a non-linear iterative least squares Gaussian fitting procedure. Corrections due to charging effects were taken care of by using C(1s) as an internal reference and the Fermi edge of a gold sample. A Jandel Peak FitTM (version 4.01) program was used for the analysis. A high resolution transmission electron microscope (HRTEM FEI Tecnai G2 F30 STWIN with a FEG source operating at 300 kV) was used for characterizing Au@PAA NPs. The NPs in toluene were drop-cast onto a carbon coated copper grid, and the solvent was evaporated prior to loading onto the microscope. A LOT-Oriel solar simulator with a 150 W Xe lamp, an AM 1.5 ﬁlter, capable of delivering an collimated output beam of 25 mm diameter, connected to a Metrohm Autolab PSTAT302N was used for measuring the current versus potential (I-V) data of QDSCs, under 1 sun (100 mW cm-2) illumination. The spatial uniformity of irradiance was confirmed by calibrating with a 2 cm × 2 cm Si reference cell and re-affirmed with an ILT1400 radiometer/photometer. Cyclic

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voltammograms of TiO2 and TiO2-CBN films and electrochemical impedance spectroscopy (EIS) for films and QDSCs were recorded on the same electrochemical workstation. EIS studies were performed under forward bias ranging from 0.0 V to −0.6 V under dark-conditions and illumination (0.2 sun, using a LED). An ac amplitude of 20 mV was superimposed over different fixed values of dc voltages, over a frequency range of 1 MHz to 0.01 Hz. A Bruker Senterra dispersive Raman microscope spectrometer, with a 532 nm laser excitation was used to record the Raman spectra of TiO2. Incident photo to current conversion efficiency (IPCE) of devices were recorded using a QE Measurement System, Oriel IQE-200™ capable of measurements compliant to ASTM E1021-06. The light source was a 250 W quartz tungsten halogen lamp, the monochromator path length was 1/8M and the spot size was 1 mm × 2.5 mm, at focus. Results and discussion Effect of carbonate treatment on TiO2 The absorbance and diffuse reflectance spectra of TiO2 and TiO2-CBN films are shown in Figure 1a and b. Both films exhibit a very strong absorption peak with a λmax at 321 nm, which tapers off to much lower values in the visible region. However, the absorption band edge of TiO2-CBN is blueshifted in comparison to TiO2, and the absorbance decline in the visible region, between 400 -900 nm, is steeper for the TiO2-CBN film compared to the untreated TiO2 film. The band gaps calculated from the absorption band edges are 3.0 and 3.1 eV respectively for the TiO2 and TiO2-CBN films. The narrower band gap for the film without carbonate treatment and the broadened absorbance in the visible region are indicators of defects in untreated TiO2. The diffuse reflectance spectra in Figure 1b shows that TiO2 has a higher reflectance than TiO2CBN, and it varies from 42 to 27% (for TiO2) and from 33 to ~9% (for TiO2-CBN) ongoing from 400 to 800 nm. The magnitude of diffuse reflectance is a measure of the light scattered by a specimen, and here in TiO2, possibly due to the surface and bulk defects introduced by oxygen vacancies, light scattering is enhanced. TiO2 defect states were previously studied by Kumar et al., where, TiO2, upon excitation at 370 nm, showed a broad blue emission in the wavelength range of 430-470 nm. It was attributed to charge recombination at the shallow-trap surface states that originated from the oxygen vacancies.26 The fluorescence spectra of TiO2 and TiO2-CBN are shown in Figure 1c. These samples upon excitation with a 380 nm radiation, show a sharp intense emission peak with a λmax at ~430 nm, followed by three broad and low intensity peaks with λmax positioned at 470, 518 and 624 nm respectively. The high intensity, high energy peak is assigned to band edge recombination and the low energy, low intensity peaks are assigned to emissions corresponding to excited electron transitions from the CB of TiO2 to the mid-gap defect states formed by oxygen vacancies (particularly) in TiO2. While the emission intensities corresponding to the electron-hole recombination at the band gap are comparable for both films, the emission peaks due to defects in TiO2 are quenched in TiO2-CBN, thus indicating that carbonate treatment passivates the defects by an appreciable degree. The lifetimes of charge recombination processes

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are studied by measuring fluorescence decay for TiO2 and TiO2-CBN samples. The plots are displayed in Figure 1d, and the data are fit into a bi-exponential function, provided in equation 1. f(t ) = B1 e-t/τ1 + B2 e –t/τ2 =

∑Biτi2 /

∑Biτi

(1) (2)

In equation (1), B1 and B2 are the amplitudes, τ1 and τ2 are the electron lifetimes, and is the average lifetime. The fitting parameters are provided in Table 1. For TiO2, an average lifetime of 2.18 ns is obtained, which increased to 8.82 ns upon carbonate treatment of TiO2. The excited electron lifetime is shorter for untreated TiO2, due to fast electron injection into the mid-gap states, which are more in number due to a greater defect density. However, in TiO2-CBN, the average lifetime is largely a measure of electron-hole recombination at the band edge, and therefore excited electron lifetime is increased. Deconvoluted XPS spectra of TiO2 and TiO2-CBN films are shown in Figure 2. The Ti2p core level spectra of both films are almost similar; they show two spin-orbit split peaks at 463.5 and 458 eV (TiO2) and at 465 and 459 eV (TiO2-CBN), corresponding to 2p1/2 and 2p3/2 components of Ti4+, with energy separations of 5.5 and 6 eV. The slight shift of the Ti2p components to higher binding energies is attributed to the dominance of Ti4+ states (relative to pristine TiO2). The intensity ratio of I2p3/2/I2p1/2 is 1.75 and 1.92 for TiO2 and TiO2-CBN respectively. The O1s spectra are different; for TiO2, two components at 530 and 532 eV are identified due to Ti-O and adsorbed C-OH groups, and for TiO2-CBN, an additional component is obtained due to oxygen functionalities such as – OH groups that are possible residues of carbonate decomposition, and are bound to Ti4+.The integrated area for this component is 13.3%, which serves as an indirect evidence for passivation of oxygen vacancies by the CO32- groups, which thermally decompose to –OH groups after the 150 oC treatment, in TiO2-CBN films. A cartoon representation of the changes in TiO2 induced by carbonate treatment is presented in Scheme 1. The structure of defect free TiO2 (hypothetical case) is shown in Scheme 1a for comparison with untreated TiO2, which has oxygen vacancies (Scheme 1b). When the untreated TiO2 film is exposed to the aqueous K2CO3 solution, the CO32- ions, in all likelihood, occupy the vacant oxygen sites (Scheme 1c). Upon heat treatment at 150 oC, the K+ and CO32- decompose to KOH, H2O and CO2, leaving behind residual oxygens or –OH groups which then reside in the oxygen vacancies (Scheme 1d) and passivate the surface or subsurface defects. The subsequent water rinse removes any traces of alkali from the film. The deconvoluted C1s spectrum in TiO2 shows two peaks due to adsorbed C-C and C-OH groups at 284.6 and 286.5 eV. In TiO2-CBN, besides the C-C and C-OH peaks, an additional small component is identified at a higher binding energy of 289.3 eV, with an integrated area of 9.6%. This in all likelihood originates from the trapped C-OH residues after carbonate decomposition. The Raman spectra of TiO2 and TiO2-CBN samples are shown in Figure 3a. Both films show five active phonon modes at 143, 195, 396, 517 and 637 cm-1. The highest energy peak at 143 cm-1 is the most intense for both films. The peaks

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at 143, 195, and 637 cm-1 correspond to the Eg modes, and the one at 396 cm-1 is due to the B1g mode and the peak at 517 cm1 is regarded as a doublet of the A1g and B1g modes.27,28 The Eg, B1g and A1g modes are assigned to the symmetric stretching vibration and symmetric- and asymmetric- bending vibrations of O-Ti-O respectively in anatase TiO2.29 The only perceptible difference between the two spectra is lower full width at half maxima and a decreased broadness in the peaks ongoing from TiO2 to TiO2-CBN. The increased sharpness of peaks in TiO2CBN represents transitions between vibrational levels spread over a narrower energy range. Decreased defect density can cause this, and as a result, peak widths shrink. To compare the electroactivity of TiO2 and TiO2-CBN films, cyclic voltammetric (CV) plots were recorded (Figure 3b) for the two films (as working electrodes), a Pt rod as the CE and an Ag/AgCl/KCl as the reference, in an aqueous 0.1 M KOH solution. In CV plot, in the cathodic sweep, the current density profile represents the reduction current of TiO2 to form KxTiO2-x, and in the reverse (anodic) sweep, the oxidation peak at −1.28 V (versus Ag/Ag+), corresponds to the conversion of KxTiO2-x, back to TiO2. In the cathodic branch, for TiO2, a weak peak is observed at −1.03 V, which is not observed for TiO2-CBN. Pre-charging by K+ ions of the unpassivated TiO2 surface in all likelihood, produces this weak peak. Electrochemical impedance spectra for the two films are compared in Figure 3c. The plots were recorded in 0.1 M KOH, in a two electrode configuration with a Pt rod as the CE, over a frequency range of 1 MHz to 0.1 Hz at a Vac of 20 mV and under 0.0 V dc bias. The plots have an arc-like feature, which is indicative of very fast charge transfer at the TiO2 or TiO2-CBN/KOH interface. The impedance curve for the TiO2CBN film, especially over the intermediate to low frequency regime, makes an angle close to 90o with the real part of impedance (Z′), and thus confirming the capacitive nature of this film, and it is 40.2 µF. In comparison, the greater digression from 90o angle observed for the TiO2 film indicates it to be resistive than capacitive. The low frequency capacitance of untreated TiO2 is 31.2 µF. Fewer surface defects for the TiO2CBN film can manifest in this difference in their redox behavior, particularly at low frequencies. Photoanodes with TiO2-CBN and Au@PAA NPs Absorbance spectra of TiO2-CBN/CdS, and TiO2CBN/CdS/Au@PAA photoanodes, and of a solution of Au@PAA nanoparticles (NPs) are shown in Figure 4a. CdS is a photosensitizer, and when it is incorporated in the TiO2-CBN film, the absorption of the electrode is extended to the visible region, with an absorption onset at 546 nm, corresponding to a band gap of 2.27 eV. Au@PAA NPs show a distinct surface plasmon resonance peak in the wavelength range of 491 to 586 nm with a λmax at 533 nm, which matches with the reported peak for Au NPs.21,30 Upon inclusion of Au@PAA NPs in the TiO2-CBN/CdS photoanode, the resulting TiO2-CBN/CdS /Au@PAA photoanode shows an enhanced absorption in the visible region, extending even to longer wavelengths to ~700 nm. The increased absorption of CdS in the blue-green region is attributed to the near-field plasmonic enhancement. Here, CdS QDs, which are in the vicinity of Au@PAA nanoparticles, separated by distances no greater than a few na-

nometers, can benefit from the near field plasmonic effect of Au@PAA NPs. The mechanism of how the population of photo-generated electrons in CdS are enhanced when they are located in the proximity of Au@PAA NPs is explained with a schematic (inset of Figure 4a). The incident light is an electromagnetic (EM) radiation with electric and magnetic components oscillating perpendicular to each other. When the electric field component of this radiation interacts with an Au@PAA NP, the electron density (in the CB) redistributes within the NP as it increases on one side, and decreases on the other. This field acts in a direction opposite to the incident electric field (of light).31 The displaced electron density induces restoring oscillations within the metal NP which are in phase with the electric field component (of light). These coherent oscillations called plasmons dephase in roughly ten fs,31 and the optical energy is dissipated by the Au@PAA plasmons. This energy is picked up by the vicinal semiconductor (CdS, in the present case), and an electron is excited from the valence band (VB) of CdS to the CB of CdS. Therefore, in CdS, apart from the intrinsic excitation of electrons from the VB to the CB, by direct absorption of incident light, additional electrons are excited via the near field plasmonic energy of Au@PAA NPs. Only the CdS QDs which are located in the vicinity of the Au@PAA NPs benefit from this effect, for the plasmonic field decays exponentially with distance,31 and is therefore unavailable for the QDs located at longer distances. The plasmonic benefit is also reflected in the increased short circuit current density obtained for TiO2/CdS/Au@PAA cell compared to the TiO2/CdS cell. The cell efficiency is also improved due to Au@PAA NPs. X-ray diffraction patterns of CdS and TiO2 are shown in Figure 4b. For TiO2 film, peaks at 3.51, 2.45, 1.89, 1.69, 1.48, and 1.26 Å are observed, which match with the (101) (103) (200) (211) (204) and (215) planes of a body centered tetragonal crystal structure of TiO2, as per powder diffraction file (PDF) # 894921. For CdS, the peaks were broad, indicative of semi-polycrystalline nature. Peaks at 3.34, 2.06 and 1.76 Å are assigned to the lattice planes: (111), (220), and (311) of a face centered cubic (fcc) structure, in accordance with PDF # 652887. A Raman spectrum of CdS deposited on a glass slide by SILAR technique at room temperature (Figure 4c) shows peaks at 297 and 595 cm-1 assigned to the 1 LO (longitudinal optical) and 2 LO modes of lattice vibrations.32,33 TEM image of Au@PAA NPs is shown in Figure 4d, and the image shows the particles to have dimensions in the range of 20-40 nm. The NPs are discrete, and have definite geometric quasi-hexagonal shapes. Aggregation of the NPs is prevented significantly by the PAA caps. The corresponding lattice scale image extracted from one NP (Figure 4e), shows an inter-fringe separation of 0.24 nm, which matches with the (111) plane of the fcc lattice of Au (PDF # 652870). Role of WO3-x Prior to evaluating the solar cell performances, the role of WO3-x in the CE is studied. Previously, we had studied nine different CEs with the same photoanode and electrolyte, and found that the C-fabric/WO3-x electrode was the best among the lot,24 for it exhibits a high electrocatalytic activity for the reduction of polysulfide and a lower charge transfer resistance for electron transfer to oxidized polysulfide especially in com-

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parison to sole C-fabric. It was 45 Ω for C-fabric/WO3-x, and 7 Ω for C-fabric (obtained from the magnitude of the first arc in electrochemical impedance studies).24 To further illustrate the advantage of using WO3-x coated C-fabric as CE compared to C-fabric, the conductances (G) of the two electrodes were measured using linear sweep voltammetry (LSV) (Figure 5a). The conductances for C-fabric and C-fabric/WO3-x are calculated from the slopes of the straight-line fits of the I (current)V(voltage) plots, using the equation: G = 1/R = I/V, and are 0.01 and 2.21 S respectively. The conductance of C-fabric is improved by 221 times, by coating it with WO3-x. Reduced tungsten oxide is electrically conducting due to a large number of oxygen vacancies,34 and they provide the conductive paths for electron conduction. In literature, it is known to possess metal like conductivity (e.g., the conductivity of WO2 is ∼350 S cm−1)35,36 due to the strong metal-metal bonding in the crystal structures of tungsten sub-oxides. The oxygen vacancies also act as active sites and raise its electrocatalytic performance.37 These attributes help in efficient electron transfer to the oxidized polysulfide species during cell operation. The work functions were also compared for C-fabric and WO3-x, which were measured by cyclic voltammetry (CV) (Figure 5b and c). C-fabric is a p-type semiconductor, and therefore the work function is approximated as the VB, which is the oxidation potential. WO3-x is a n-type semiconductor, and its Fermi level (work function) is just below the CB, and this position is the reduction potential. The CV plots were recorded in an aqueous 0.2 M KOH solution, with C-fabric or WO3-x as the working electrode, a Pt rod as the counter-, and an Ag/AgCl/KCl as the reference- electrodes. The onset of reduction potential and oxidation potential of WO3-x and Cfabric are −0.55 and −0.62 V (versus Ag/Ag+), and they are −0.353 and −0.423 V versus normal hydrogen electrode (NHE), and from the corresponding energies or the work functions are at 4.077 and below 4.15 eV for C-fabric and WO3-x respectively, thus providing the energy level alignment as well required for electron cascade to the electrolyte (Figure 5d). JV characteristics of two cells (1 cm2, active area) were recorded (Figure 5e) with the same photoanode of TiO2/CdS, and polysulfide gel electrolyte, but having two different CEs: Cfabric and C-fabric/WO3-x. The PCEs were 1.78 and 2.22% respectively (Table S1, supporting information), and the fill factor was particularly higher when C-fabric/WO3-x was used. They were 0.44 and 0.55, with C-fabric and C-fabric/WO3-x, thus illustrating the significance of WO3-x in improving cell performance. Photovoltaic performance, stability and scale-up of QDSCs The performances of large area cells (1 cm2) with TiO2/CdS/Au@PAA-polysulfide/SiO2 gel-C-fabric/WO3-x and TiO2-CBN/CdS/Au@PAA-polysulfide/SiO2 gel-Cfabric/WO3-x configurations are compared to study the effect of the carbonate passivating layer. All measurements were performed under a solar irradiance of 100 mW cm-2, corresponding to an AM 1.5 spectrum. Large area photoanodes of TiO2/CdS/Au@PAA (~1 cm2) and TiO2-CBN/CdS/Au@PAA (~ 1 cm2 and ~ 4 cm2), and their cells were also fabricated, and their photovoltaic parameters were compared (Figure 6a and b and Table 2). We found that the photovoltaic performance

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improved by ~ 42.5%, by the application of the carbonate based passivation layer. For a 1 cm2 cell with a TiO2/CdS/Au@PAA photoanode, the PCE was 3.03%, and it increased to 4.32%, when a carbonate passivating layer was coated over TiO2. The dark current density for the same cell is approximately ~0.005 mA cm-2. This efficiency enhancement is driven by (a) the increased VOC, from 809 mV to 915 mV, after carbonate passivation of the TiO2 surface, and (b) the improved JSC, which increased from 6.37 to 7.97 mA cm-2 respectively. The current density enhancement with Au@PAA NPs incorporation in the TiO2/CdS film is 40.6%, which is higher compared to the JSC enhancement of 25.2% induced by the inclusion of Au@PAA NPs in the TiO2-CBN/CdS film, thus indicating that JSC is predominantly controlled by Au@PAA NPs rather than by carbonate treatment. Improvements in both VOC and JSC have been observed in the past due to passivation effects of ZnS layers in QDSC reports.6,38 Here, surprisingly, the FF did not change with passivation. From these solar cell parameters’ comparisons, it is clear that the upward shift of the Fermi energy level in the case of carbonate passivated solar cell, is induced by the increased electron accumulation over TiO2 by reduced charge trapping or recombination over the TiO2 surface or sub-surface. This leads to a higher VOC. In a previous report, Sargent’s group studied the surface passivation effect on the VOC and JSC of colloidal quantum dots based cells.39 They concluded that upon illumination, in a semiconductor with trap states in its’ bandgap, most of the photogenerated charge is consumed by filling mid-gap levels which results in a smaller VOC and JSC, compared to a cell based on QDs with a cleaner gap (where the trap states are passivated). Here, the defect states in TiO2 are most likely to be passivated by oxygens, as shown in Scheme 1. To confirm the shift of the CB of TiO2 upon carbonate treatment, to higher negative values (versus NHE) compared to TiO2, the VB (XPS) spectra were recorded for TiO2 and TiO2-CBN films (Figure 7). The point of intersection of two tangents, one drawn along the data points representing the density of states (DOS) approaching zero near zero eV, and the second drawn along the points where DOS rises steeply, is taken as the VB position. The VB positions for TiO2 and TiO2-CBN films are 7.23 and 7.1 eV. By subtracting the optical band gaps (obtained from the absorption spectra of the two films, Figure 1a in the MS) of 3.0 and 3.1 eV respectively from the VB positions of 7.23 and 7.1 eV, CB positions of 4.23 and 4.0 eV are obtained for the TiO2 and TiO2-CBN films. It is apparent from this experimental data that the CB edge is shifted upwards, as shown in a cartoon (inset of Figure 7a) for the TiO2-CBN film, which aligns with the increased VOC obtained for the cell based on this film. Faster charge injection into the TiO2 from the CdS QDs due to an improved interfacial contact (induced by carbonate) results in a larger JSC. The PCE of a 4 cm2 area cell with a TiO2CBN/CdS/Au@PAA-polysulfide/SiO2 gel-C-fabric/WO3-x configuration is 1.06%. This dramatic drop in PCE by approximately 75% on scaling up the cell dimensions from 1 cm2 to 4 cm2, is due to a less efficient packing or assembling of the photoactive layers (such as the CdS QDs, Au@PAA NPs, and TiO2) over the FTO substrate. Further, current collection by FTO is also affected adversely ongoing to large area cells; this is also reflected in the JSC values, for the JSC drops from ~8

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mA cm-2 to 2.22 mA cm-2, when the active electrode area is ramped from 1 to 4 cm2 (Figure 6b and Table 2). The VOC decreases by 152 mV on upscaling, which is not a very significant decrease, indicating that hole scavenging by the electrolyte, and charge separation (Fermi level position) are not severely affected by the increase in electrode area. Another point of note is the small increase in the FF on upscaling. It is therefore evident, that the photoanode architecture predominantly controls cell performance, which implies that (i) the loading of CdS QDs has to be high for high photocurrents, (ii) pinholes or cracks or any discontinuity in the TiO2 film or in the TiO2/CdS film decrease photocurrents, and therefore must be avoided and (iii) the uniformity of the coating of Au@PAA NPs over the photoanode has to be high to ensure a uniform plasmonic enhancement across the entire cross-sectional area of the cell. These parameters are better controlled when the cell dimensions are small, and become increasingly difficult to manage when the cell size is increased. The photographs of a large area (2 cm × 2 cm) TiO2-CBN/CdS/Au@PAA film, a polysulfide/SiO2 gel, a C-fabric/WO3-x film, and the resulting cell are shown in Figure 6c. A real time application, i.e., demonstration of a lighting a red LED using 3 such cells connected in series, under illumination is also shown therein, thus illustrating the promise QDSCs have for powering practical low power consuming electronic devices, such as a bio-sensor, or a blue-tooth enabled keyboard. QDSCs were fabricated in different configurations. A schematic (Figure 8a) shows the architecture of the TiO2CBN/CdS/Au@PAA-polysulfide/SiO2 gel-C-fabric/WO3-x cell and the remaining cells assembled in this study are derivatives of this cell. The advantages of (a) including Au@PAA NPs in the photoanode, and (b) carbonate treatment of TiO2, were further analyzed by comparing the IPCE spectra of cell. The IPCE plots for cells with TiO2/CdS, TiO2/CdS/Au@PAA, TiO2-CBN/CdS and TiO2-CBN/CdS/Au@PAA are shown in Figure 8b. The electrolyte and CE are the polysulfide/SiO2 gel, and C-fabric/WO3-x respectively. IPCE values are the highest for the TiO2-CBN/CdS/Au@PAA based cell, and the values are in the range of 67-68% in the 450-510 nm wavelength regime. Maximum IPCE of the TiO2/CdS cell is about 40% at 470 nm, and after carbonate treatment, as in the TiO2CBN/CdS cell, IPCE increased to ~48% at the same wavelength, indicating that the carbonate treatment helps in improving charge collection. By incorporation of Au@PAA NPs in the photoanode, the IPCE of TiO2/CdS and TiO2-CBN/CdS cells is extended to ~540 nm, due to near field plasmonic effects and scattering effects. Both these effects increase the light absorption capability of the photoanode. It is evident from the plots, that the carbonate treatment and inclusion of Au@PAA NPs improve the IPCE of CdS based QDSCs. When the active electrode areas are decreased, the photovoltaic response is excellent for the cells with carbonate treated TiO2. The QDSC with the following configuration: TiO2CBN/CdS/Au-polysulfide/SiO2 gel-C-fabric/WO3-x, yielded efficiencies in the range of 5.61 to 6.3% (area: 0.2-0.3 cm2) and the average efficiency of the cells is 5.9% (Figure 8c and Table 3). The champion cell is characterized by the following photovoltaic parameters: JSC: 11.04 mA cm-2, VOC: 0.9 V, FF: 0.63 and PCE: 6.3%. The PCEs obtained in this study are lower compared to the record PCE of 12.45% reported by Zhong’s

group.2 In their work, the active area of an equivalent cell was 0.236 cm2 which delivered a certified PCE of 12.07%.2 The key difference is in the photoanode. Authors2 used Zn-Cu-InSe QDs, which harvest the solar spectrum upto 1000 nm, spanning from visible to near infrared region. The lower PCE obtained here is due to the limited solar spectral absorption by the CdS QDs, extended from 500 to ~550 nm by Au@PAA NPs in the TiO2-CBN/CdS/Au@PAA-polysulfide/SiO2 gel-Cfabric/WO3-x cell. The absorption onset of CdS QDs coupled with Au@PAA is 580 nm, and therefore the maximum JSC is 11 mA cm-2. In contrast, for the reported 12.07% cell,2 the absorption onset is close to 1000 nm, and therefore the corresponding JSC is 25 mA cm-2. The FF was 0.63, and the VOC was 765 mV.2 For the champion cell here, the FF was 0.63 and the VOC was in fact higher (899 mV), indicating that the CE of C-fabric/WO3-x used here is an efficient CE. It must be noted that we used the SILAR method for the deposition of CdS QDs, which is easy to scale-up for it is performed at room temperature in air and just involves the immersion of the TiO2 coated substrates in cadmium and sulfide salt solutions. This process does not require any inert conditions, or expensive equipment or vacuum or any complicated processing step. However the Zn-Cu-In-Se QDs used in the 12.45% cell,2 are prepared by the hot injection method, which requires an inert or evacuated atmosphere, and also high temperatures, so scale-up of the process for the synthesis of QDs, and subsequent deposition over large area substrates are expected to be challenging, in terms of cost, and uniformity of deposit (the latter controls cell performance). Cells with CdSe QDs were also attempted and it was observed that a CdS based cell outperformed a CdSe based cell (Figure S1 and Table S2, supporting information), thus validating the use of the former in this study. Stability tests are performed for smaller area cells with the active areas ranging from 0.2 to 0.3 cm2 by using the polysulfide/SiO2 gel electrolyte and C-Fabric/WO3-x as CE. A cell with the configuration: TiO2-CBN/CdS/Au@PAApolysulfide/SiO2 gel-C-fabric/WO3-x, and having a PCE of 5.69%, was stored in dark, under ambient conditions in air for one month, and the efficiency dropped to 5.52% (by 3% of the original value) (Figure 8d and Table 4). The effect of extended illumination on cell performance was also studied. A cell with this configuration was kept under direct sunlight for 5 h (irradiance measured by a power meter: 1-2 Sun). The variation in solar cell parameters as a function of illumination time, are shown in Figure 8e-h and the parameters are listed in Table 5. Initially, prior to extended exposure, the cell efficiency was 5.91%, and after 5 h, it declined by 18%, to 4.835%. The JSC of the cell after exposure to the one sun for 5 h is found to be largely invariant, but the VOC reduced by 100 mV and FF decreased from 0.64 to 0.58. The drop in the latter two resulted in the overall PCE decline. The decreased efficiency is perhaps due to the lowered performance of the CE: C-fabric/WO3x; detachment of the WO3-x particles from C-fabric and slow photo-corrosion of WO3-x by the alkaline polysulfide electrolyte, upon prolonged exposure unfavorably impacts electron injection (into electrolyte) kinetics and therefore VOC and FF. In the TiO2-CBN/CdS based cell, during the prolonged exposure to sunlight, the TiO2-CBN/CdS film is exposed to a strongly alkaline electrolyte of polysulfide under intense (1-2

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sun) irradiance. Under these severe conditions, the CdS QDs undergo photo-corrosion, and dissociate into Cd2+ and S2-. These ions either co-deposit onto the CdS QDs which are still tethered to the TiO2 scaffold or leach into the electrolyte. The enlargement of CdS QDs by this re-deposition affects the QDpolysulfide electrolyte contact, which adversely affects hole transfer from the VB of the QD to the electrolyte and thus lowers VOC. FF depends largely on the shunt resistance (RSH) of the cell. Upon continuous irradiance, since the hole transfer is impacted, it follows that the degree of polysulfide reduction at the CE is lowered, and as a consequence, RSH reduces, and FF decreases. The shunt resistances are calculated from their J-V curves and are: 42 and 13 Ω initially and after 5 h of exposure respectively, which is in line with this reasoning. We find that dark storage has a less deleterious effect on the QDSC performance compared to light exposure. A possible way to further boost stability will be to use more number of thin passivating layers not only at the photoanode, but also at the CE, so that photo-induced degradation processes are suppressed. A comparison of stability performances of QDSCs is presented in Table 6. From literature survey on stability tests, it is obvious that dark storage does not decrease the cell performance drastically,40 which aligns with our observations. In a noteworthy study on a TiO2/CdS/CdSe based cell with a CuS CE, the initial PCE is 4.22%, and after 2 h of continuous illumination, it drops by a nominal degree, to 4.0%.41 In another work, a cell with a TiO2/CdSeS-ZnS photoanode, a Pt CE, and an electrolyte gel, the PCE decreased from 4.23 to 2.32% after 8 h of irradiation,42 In two other reports, where Cu2S was employed as the CE, the PCEs increased by a small extent, when the cells were stored at room temperature.43,44 Considering the very low volume of work done on stability evaluation of QDSCs, the work done in this study fills up an important gap. Charge transfer kinetics by impedance EIS studies on QDSCs are shown in Figure 9. The plots were fitted into a circuit similar to the one used by Santiago et al.,45 and comprise of R(RC)(RC) elements and the parameters: charge recombination resistance (Rrec), chemical capacitance (Cµ) and electron lifetimes (τn) are plotted (Figure 9a-e). The additional (RC) elements are attributed to the charge transfer resistance (RCE) and the double layer capacitance (CCE) at the CE (C-fabric/WO3-x)/polysulfide gel electrolyte interface. In dark, a marginally increased chemical capacitance is observed for the carbonate buffered photoanode, which indicates a slightly increased electron density in the Fermi level of TiO2-CBN, compared to TiO2.The decreased surface defects on TiO2-CBN, restrict electron loss via accumulation in the trap states. In dark, under a bias of −0.6 V, the chemical capacitance increased from 1.66 mF to 1.77 mF with carbonate treatment. The increased Cµ supports the upshift of the CB to lower negative values (versus vacuum), which is reflected in the increased Voc for the QDSC with carbonate treated TiO2. Since the Rrec is inversely proportional to the recombination rate, the TiO2-CBN based cell shows a reduced charge recombination rate, compared to the TiO2 based cell. Recombination is suppressed in the cells with carbonate treated TiO2, due to a reduced number of intra-gap states for excited electrons to

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cascade into. The τn was calculated following the equation τn = Rrec × Cµ; the lifetimes were calculated and plotted. This lifetime is the measure of the time taken by the electrons in the Fermi level or CB of the photoanode to transfer back to the oxidized electrolyte species in the electrolyte. This value is more than twice for the cell with carbonate treated TiO2 relative to the cell with untreated TiO2, thus indicating that back electron transfer is also inhibited to some extent. These studies clearly bring out the ability of a simple approach such as carbonate treatment in improving both stability and performance of QDSCs. EIS spectra under illumination (0.2 sun), at −0.6 V are shown for the cells with the TiO2/CdS/Au@PAA and TiO2CBN/CdS/Au@PAA films in Figure 9f. The magnitudes of Rrec as a function of applied bias are lower under irradiance compared to that under dark (Figure 9g). Under forward bias and illumination, charges are injected into the cell, in addition to the photo-generated ones, thus reducing the resistance to recombination at the photoanode/electrolyte interface, in comparison to Rrec under dark. Similar differences in light and dark responses were observed by Santiago et al.45 Chemical capacitance, shows an insignificant change with applied bias (Figure 9h), under irradiance for the cell with the carbonate treated TiO2 film, whereas it increases monotonically with bias for the cell based on sole TiO2, thus indicating that under irradiance, the electron density accumulated in the Fermi level of TiO2 is largely controlled by irradiance (which is fixed), and not by the magnitude of injected charge via bias for the TiO2-CBN film. In contrast, the Fermi level of sole TiO2 increasingly populates with bias, a behavior almost similar to that observed under dark. While the effect of carbonate is reflected in the impedance data, the effect of Au@PAA NPs, are primarily revealed in the JSC values of the cells, for these NPs via plasmonic effects increase the optical absorption of CdS QDs, which increases the photocurrent density. Conclusions By employing a yet unused approach of a facile carbonate treatment of TiO2 films, quasi solid-state QDSCs with high efficiencies, and large FFs (> 0.5), and reasonable stabilities were prepared successfully. The effect of carbonate treatment on TiO2, as a passivating agent, was reflected in terms of a slightly wider band gap, decreased fluorescence (due to oxygen vacancies) intensity, and a Ti-OH signal originating from decomposed CO32- groups, obtained for the TiO2-CBN films compared to untreated TiO2 films besides a capacitive impedance response at low frequencies registered for TiO2-CBN as opposed to a resistive one obtained for TiO2. Large area QDSCs of 1 cm2 dimensions with TiO2/CdS/Au@PAA and TiO2-CBN/CdS/Au@PAA photoanodes yielded PCEs of 3.03 and 4.32%, which confirmed the worth of carbonate treatment in improving cell performance. This cell with carbonate treated TiO2 also showed higher recombination resistance, and a longer back electron transfer time relative to the untreated TiO2 based cell, which are responsible for the its’ superior PCE. Quasi solid-state QDSCs with the following configuration: TiO2-CBN/CdS/Au@PAA-polysulfide/SiO2 gel-Cfabric/WO3-x, yielded efficiencies in the range of 5.16 to 6.3% (area: 0.2-0.3 cm2) and the average efficiency of the cells is 5.9%. Stability analysis showed that dark storage had a less

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detrimental impact on cell performance compared to extended illumination. Under dark, the PCE of the cell dropped from 5.69 to 5.52%, and under prolonged irradiance, it decreased from 5.91 to 4.83%. A PCE of 1.06% is achieved for a 4 cm2 QDSC with carbonate treated TiO2, thus indicating that the films are scalable. However, controlled optimization of the layers is required to enable the retention of high efficiencies with scale-up. These cells can be applied to niche applications, such as powering low power consuming devices like a biosensor or a blue tooth enable keyboard. Associated content Supporting Information. Solar cell parameters of QDSCs with C-fabric and C-fabric/WO3-x as the CEs, absorbance spectrum of CdSe QDs, J-V characteristics and solar cell parameters of QDSCs with TiO2/CdS and TiO2/CdSe photoanodes. This material is available free of charge via the Internet at http://pubs.acs.org.

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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: +91-40-23016003. Acknowledgements Financial support from the Solar Energy Research InitiativeDepartment of Science & Technology (DST/TM/SERI/2K1211(G)) is gratefully acknowledged. PNK and AK are thankful to UGC for research fellowships.

References (1)

(2)

(3)

(4)

(5)

(6)

(7)

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(15)

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(17)

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Peng, W.; Du, J.; Pan, Z.; Nakazawa, N.; Sun, J.; Du, Z.; Shen, G.; Yu, J.; Hu, J. S.; Shen, Q.; Zhong, X. Alloying Strategy in Cu–In–Ga–Se Quantum Dots for High Efficiency Quantum Dot Sensitized Solar Cells. ACS Appl. Mater. Interfaces 2017, 9, 5328–5336. Jiao, S.; Du, J.; Du, Z.; Long, D.; Jiang, W.; Pan, Z.; Li, Y.; Zhong, X. Nitrogen-Doped Mesoporous Carbons as Counter Electrodes in Quantum Dot Sensitized Solar Cells with a Conversion Efficiency Exceeding 12%. J. Phys. Chem. Lett. 2017, 8, 559–564. Solis, M.; Fuente, D.; Sa, R. S.; Gonza, V.; Boix, P. P.; Mhaisalkar, S. G.; Rinco, M. E.; Bisquert, J.; Mora-sero, I. Effect of Organic and Inorganic Passivation in Quantum-DotSensitized Solar Cells. J. Phys. Chem. Lett. 2013, 4, 1519−1525. Ren, Z.; Wang, J.; Pan, Z.; Zhao, K.; Zhang, H.; Li, Y.; Zhao, Y.; Mora-Sero, I.; Bisquert, J.; Zhong, X. Amorphous TiO2 Buffer Layer Boosts Efficiency of Quantum Dot Sensitized Solar Cells to over 9%. Chem. Mater. 2015, 27 (24), 8398– 8405. Zhao, K.; Pan, Z.; Mora-Seró, I.; Cánovas, E.; Wang, H.; Song, Y.; Gong, X.; Wang, J.; Bonn, M.; Bisquert, J.; Zhong, X. Boosting Power Conversion Efficiencies of Quantum-DotSensitized Solar Cells beyond 8% by Recombination Control. J. Am. Chem. Soc. 2015, 137 (16), 5602–5609. Yang, J.; Wang, J.; Zhao, K.; Izuishi, T.; Li, Y.; Shen, Q.; Zhong, X. CdSeTe/CdS Type-I Core/Shell Quantum Dot Sensitized Solar Cells with Efficiency over 9%. J. Phys. Chem. C 2015, 119 (52), 28800–28808. Zhao, T.; Goodwin, E. D.; Guo, J.; Wang, H.; Diroll, B. T.; Murray, C. B.; Kagan, C. R. Advanced Architecture for

(19)

(20)

(21)

(22)

(23)

(24)

(25)

Colloidal PbS Quantum Dot Solar Cells Exploiting a CdSe Quantum Dot Buffer Layer. ACS Nano 2016, 10, 9267–9273. Wei, H.; Wang, G.; Shi, J.; Wu, H.; Luo, Y.; Li, D.; Meng, Q. Fumed SiO2 Modified Electrolytes for Quantum Dot Sensitized Solar Cells with Efficiency Exceeding 11% and Better Stability. J. Mater. Chem. A 2016, 4, 14194–14203. de Jongh, P. E.; Vanmaekelbergh, D. Trap-Limited Electronic Transport in Assemblies of Nanometer-Size TiO2 Particles. Phys. Rev. Lett. 1996, 77 (16), 3427–3430. Gregg, B. a.; Pichot, F.; Ferrere, S.; Fields, C. L. Interfacial Recombination Processes in Dye-Sensitized Solar Cells and Methods to Passivate the Interfaces. J. Phys. Chem. B 2001, 105 (7), 1422–1429. Kumar, P. N.; Deepa, M.; Srivastava, A. K. Ag Plasmonic Nanostructures and a Novel Gel Electrolyte in a High Efficiency TiO2/CdS Solar Cell. Phys. Chem. Chem. Phys. 2015, 17 (15), 10040–10052. Kumar, P. N.; Das, A.; Deepa, M.; Ghosal, P.; Srivastava, A. K. Bimetallic Au-Ag Alloy Nanoparticles Improve Energy Harvesting of a TiO2/CdS Film. ChemistrySelect 2016, 1 (16), 5320–5330. West, R. H.; Celnik, M. S.; Inderwildi, O. R.; Kraft, M.; Beran, G. J. O.; Green, W. H. Toward a Comprehensive Model of the Synthesis of TiO2 Particles from TiCl4. Ind. Eng. Chem. Res. 2007, 46, 6147–6156. Dette, C.; Pérez-Osorio, M. a.; Kley, C. S.; Punke, P.; Patrick, C. E.; Jacobson, P.; Giustino, F.; Jung, S. J.; Kern, K. TiO2 Anatase with a Bandgap in the Visible Region. Nano Lett. 2014, 14 (11), 6533–6538. Scheiber, P.; Fidler, M.; Dulub, O.; Schmid, M.; Diebold, U.; Hou, W.; Aschauer, U.; Selloni, A. (Sub)Surface Mobility of Oxygen Vacancies at the TiO2 Anatase (101) Surface. Phys. Rev. Lett. 2012, 109 (13), 136103(1–5). Yang, S. M.; Huang, C. H.; Zhai, J.; Wang, Z. S.; Jiang, L. High Photostability and Quantum Yield of Nanoporous TiO2 Thin Film Electrodes Co-Sensitized with Capped Sulfides. J. Mater. Chem. 2002, 12, 1459–1464. Mora-Sero, I.; Gimenez, S.; Fabregat-Santiago, F.; Gomez, R.; Shen, Q.; Toyoda, T.; Bisquert, J. Recombination in Quantum Dot Sensitized Solar Cells. Acc. Chem. Res. 2009, 42 (11), 1848–1857. Shen, Q.; Kobayashi, J.; Diguna, L. J.; Toyoda, T. Effect of ZnS Coating on the Photovoltaic Properties of CdSe Quantum DotSensitized Solar Cells. J. Appl. Phys. 2008, 103 (8), 084304(1– 5). Guijarro, N.; Campiña, J. M.; Shen, Q.; Toyoda, T.; LanaVillarreal, T.; Gómez, R. Uncovering the Role of the ZnS Treatment in the Performance of Quantum Dot Sensitized Solar Cells. Phys. Chem. Chem. Phys. 2011, 13 (25), 12024–12032. Gong, X.; 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. Kumar, P. N.; Narayanan, R.; Deepa, M.; Srivastava, A. K. Au@poly(acrylic Acid) Plasmons and C60 Improve the Light Harvesting Capability of a TiO2/CdS/CdSeS Photoanode. J. Mater. Chem. A 2014, 2 (25), 9771–9783. Choi, H.; Chen, W. T.; Kamat, P. V. Know Thy Nano Neighbor. Plasmonic versus Electron Charging Effects of Metal Nanoparticles in Dye-Sensitized Solar Cells. ACS Nano 2012, 6 (5), 4418–4427. Sheehan, S. W.; Noh, H.; Brudvig, G. W.; Cao, H.; Schmuttenmaer, C. A. Plasmonic Enhancement of DyeSensitized Solar Cells Using Core− Shell−Shell Nanostructures. J. Phys. Chem. C 2013, 117, 927−934. Kumar, P. N.; Kolay, A.; Kumar, S. K.; Patra, P.; Aphale, A.; Srivastava, A. K.; Deepa, M. Counter Electrode Impact on Quantum Dot Solar Cell Efficiencies. ACS Appl. Mater. Interfaces 2016, 8 (41), 27688–27700. Jans, H.; Jans, K.; Lagae, L.; Borghs, G.; Maes, G.; Huo, Q. Poly(acrylic Acid)-Stabilized Colloidal Gold Nanoparticles: Synthesis and Properties. Nanotechnology 2010, 21, 455702(1– 7).

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ACS Applied Materials & Interfaces (26)

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(27)

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(29) (30)

(31) (32)

(33)

(34)

(35)

(36)

(37)

(38)

Kumar, S.; Verma, N. K.; Singla, M. L. Size Dependent Reflective Properties of TiO2 Nanoparticles and Reflectors Made Thereof. Dig. J. Nanomater. Biostructures 2012, 7 (2), 607–619. Šćepanović, M.; Grujić-Brojčin, M.; Dohčević-Mitrović, Z.; Popović, Z. V. Effects of Confinement, Strain and Nonstoichiometry on Raman Spectra of Anatase TiO2 Nanopowders. Mater. Sci. Forum 2006, 518, 101–106. Grujić-Brojčin, M.; Armaković, S.; Tomić, N.; Abramović, B.; Golubović, A.; Stojadinović, B.; Kremenović, A.; Babić, B.; Dohčević-Mitrović, Z.; Šćepanović, M. Surface Modification of Sol-Gel Synthesized TiO2 Nanoparticles Induced by La-Doping. Mater. Charact. 2014, 88, 30–41. Ohsaka, T.; Izumi, F.; Fujiki, Y. Raman Spectrum of Anatase, TiO2. J. Raman Spectrosc. 1978, 7 (6), 321–324. Bastus, N. G.; Comenge, J.; Puntes, V. Kinetically Controlled Seeded Growth Synthesis of Citrate-Stabilized Gold Nanoparticles of up to 200 nm : Size Focusing versus Ostwald Ripening. Langmuir 2011, 27 (July 2015), 11098–11105. Warren S. C.; Thimsen, E. Plasmonic Solar Water Splitting. Energy Environ. Sci. 2012, 5, 5133–5146. Trajić, J.; Gilić, M.; Romčević, N.; Romčević, M.; Stanišić, G.; Hadžić, B.; Petrović, M.; Yahia, Y. S. Raman Spectroscopy of Optical Properties in CdS Thin Films. Sci. Sintering. 2015, 47 (2), 145–152. Hu, C.; Zeng, X.; Cui, J.; Chen, H.; Lu, J. Size Effects of Raman and Photoluminescence Spectra of CdS Nanobelts. J. Phys. Chem. C 2013, 117 (40), 20998–21005. Wu, W-T.; Wu, J-J.; Chen, J-S. Resistive Switching Behavior and Multiple Transmittance States in Solution-Processed Tungsten Oxide. ACS Appl. Mater. Interfaces 2011, 3, 2616– 2621. Xi, G.; Ouyang, S.; Li, P.; Ye, J.; Ma, Q.; Su, N.; Bai, H.; Wang, C. Ultrathin W18O49 Nanowires with Diameters below 1 Nm: Synthesis, near-Infrared Absorption, Photoluminescence, and Photochemical Reduction of Carbon Dioxide. Angew. Chem., Int. Ed. 2012, 51 (10), 2395–2399. Zhou, Y.; Hu, X.-C.; Liu, X.-H.; Wen, H.-R. Core–shell Hierarchical WO2 / WO3 Microspheres as an Electrocatalyst Support for Methanol Electrooxidation. Chem. Commun. 2015, 51 (83), 15297–15299. Chen, J.; Yu, D.; Liao, W.; Zheng, M.; Xiao, L.; Zhu, H.; Zhang, M.; Du, M.; Yao, J. WO3-X Nanoplates Grown on Carbon Nanofibers for an Efficient Electrocatalytic Hydrogen Evolution Reaction. ACS Appl. Mater. Interfaces 2016, 8 (28), 18132–18139. Hossain, M. A.; Jennings, J. R.; Shen, C.; Pan, J. H.; Koh, Z. Y.;

(39)

(40)

(41)

(42)

(43)

(44)

(45)

(46)

(47)

Page 10 of 24

Mathews, N.; Wang, Q. CdSe-Sensitized Mesoscopic TiO2 Solar Cells Exhibiting >5% Efficiency: Redundancy of CdS Buffer Layer. J. Mater. Chem. 2012, 22 (32), 16235–16242. Ip, A. H.; Thon, S. M.; Hoogland, S.; Voznyy, O.; Zhitomirsky, D.; Debnath, R.; Levina, L.; Rollny, L. R.; Carey, G. H.; Fischer, A.; Kemp, K. W.; Kramer, I. J.; Ning, Z.; Labelle, A. J.; Chou, K. W.; Amassian, A.; Sargent, E. H. Hybrid Passivated Colloidal Quantum Dot Solids. Nat. Nanotechnol. 2012, 7 (9), 577–582. Zhang, X.; Huang, X.; Yang, Y.; Wang, S.; Gong, Y.; Luo, Y.; Li, D.; Meng, Q. Investigation on New CuInS2 / Carbon Composite Counter Electrodes for CdS / CdSe Cosensitized Solar Cells. ACS Appl. Mater. Interfaces 2013, 5, 5954–5960. Wang, F.; Dong, H.; Pan, J.; Li, J.; Li, Q.; Xu, D. One-Step Electrochemical Deposition of Hierarchical CuS Nanostructures on Conductive Substrates as Robust , High- Performance Counter Electrodes for Quantum-Dot-Sensitized Solar Cells. J. Phys. Chem. C 2014, 118, 19589–19598. Yan, K.; Chen, W.; Yang, S. Significantly Enhanced Open Circuit Voltage and Fill Factor of Quantum Dot Sensitized Solar Cells by Linker Seeding Chemical Bath Deposition. J. Phys. Chem. C 2013, 117, 92–99. Zhao, K.; Yu, H.; Zhang, H.; Zhong, X. Electroplating Cuprous Sulfide Counter Electrode for High-efficifiency Long-Term Stability Quantum Dot Sensitized Solar Cells. J. Phys. Chem. C 2014, 118, 5683–5690. Pan, Z.; Zhao, K.; Wang, J.; Zhang, H.; Feng, Y.; Zhong, X. Near Infrared Absorption of CdSe X Te1-X Alloyed Quantum Dot Sensitized Solar Cells with More than. ACS Nano 2013, 7 (6), 5215–5222. Fabregat-Santiago, F.; Bisquert, J.; Garcia-Belmonte, G.; Boschloo, G.; Hagfeldt, A. Influence of Electrolyte in Transport and Recombination in Dye-Sensitized Solar Cells Studied by Impedance Spectroscopy. Sol. Energy Mater. Sol. Cells 2005, 87 (1-4), 117–131. Yang, Y.; Zhu, L.; Sun, H.; Huang, X.; Luo, Y.; Li, D.; Meng, Q. Composite Counter Electrode Based on Nanoparticulate PbS and Carbon Black : Towards Quantum Dot-Sensitized Solar Cells with Both High Efficiency and Stability. ACS Appl. Mater. Interfaces 2012, 4, 6162–6168. Shen, H.; Lin, H.; Liu, Y.; Li, J.; Oron, D. Study of Quantum Dot / Inorganic Layer / Dye Molecule Sandwich Structure for Electrochemical Solar Cells. J. Phys. Chem. 2012, 116, 15185– 15191.

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SCHEME

Scheme 1. Cartoon showing changes in TiO2 induced by carbonate treatment: (a) Defect free TiO2 (hypothetical), (b) TiO2 with oxygen vacancies, (c) oxygen vacancies in TiO2 occupied by CO32- molecules, and (d) oxygen vacancies in TiO2 film passivated after heat treatment.


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Figures Figure 1. (a) Absorption and (b) diffuse reflectance spectra of TiO2 and TiO2-CBN films. (c) Fluorescence spectra and (d) fluorescence decay spectra of TiO2 and TiO2-CBN films, recorded at λex = 380 nm and decay probed at λem = 430 nm.


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Figure 2. Deconvoluted core level spectra of (a-c) TiO2 and (d-f) TiO2-CBN films: (a,d) Ti2p, (b,e) O1s and (c,f) C1s.


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Figure 3. (a) Raman spectra of TiO2 and TiO2-CBN samples. (b) Cyclic voltammograms of TiO2 and TiO2-CBN films in a 0.1 M KOH solution with Ag/AgCl/KCl and Pt as the reference and counter electrodes. (c) Nyquist plots of TiO2 and TiO2-CBN films in a 0.1 M KOH solution with Pt as counter electrode, over a frequency range of 1MHz-0.1Hz; with the high frequency response as an inset.


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Figure 4. (a) Absorbance spectra of a colloid of Au@PAA NPs, and TiO2-CBN/CdS and TiO2-CBN/CdS/Au@PAA films. (b) XRD patterns of TiO2 and CdS. (c) Raman spectrum of CdS. (d) TEM and (e) lattice scale image of Au@PAA NPs. Inset of (a) is a schematic illustrating the generation of electron-hole pairs in CdS QDs located in the proximity of Au@PAA NPs via SPR effect.


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Figure 5. (a) LSV plots of C-fabric and C-fabric/WO3-x electrodes. (b, c) CV plots measured for C-fabric and WO3-x in a 0.2 M KOH electrolyte. (d) Energy level diagram showing the work functions of C-fabric and WO3-x. (e) J-V characteristics of 1 cm2 cells with TiO2/CdS photoanodes, under 100 mW cm-2 irradiance (AM 1.5), employing C-fabric and C-fabric/WO3-x CEs.


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Figure 6. J-V characteristics of (a) 1 cm2 cells with TiO2/CdS, TiO2/CdS/Au@PAA and TiO2-CBN/CdS/Au@PAA photoanodes, under 100 mW cm-2 irradiance (AM 1.5); dark current of latter is also shown and (b) a 4 cm2 cell with TiO2-CBN/CdS/Au@PAA photoanode. A polysulfide/SiO2 gel and C-fabric/WO3-x were used as the electrolyte and the CE. (c) Photographs of a large area (2 cm × 2 cm) TiO2CBN/CdS/Au@PAA film, a polysulfide/SiO2 gel, a C-Fabric/WO3-x film, and the resulting cell. Lighting of a LED with 3 such cells connected in series under illumination.


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Figure 7. Valence band (XPS) spectra of (a) TiO2-CBN and (b) TiO2 films. Inset of (a): cartoon showing the difference in the CB edges.


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Figure 8. (a) Schematic of a TiO2-CBN/CdS/Au@PAA photoanode based cell. (b) IPCE versus wavelength spectra of cells with different photoanodes. (c) J-V characteristics of 5 cells with active areas of 0.2 - 0.3 cm2, and recorded under 1 sun, (d) effect of dark storage, and variation of solar cell parameters with illumination time: (e) PCE, (f) VOC, (g) FF and (h) JSC. In (c-h) the cell configuration is: TiO2CBN/CdS/Au@PAA-polysulfide/SiO2 gel-C-fabric/WO3-x.


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Figure 9. (a) Nyquist plots measured under dark conditions for cells with TiO2/CdS/Au@PAA and TiO2-CBN/CdS/Au@PAA photoanodes at −0.6 V; enlarged views of high frequency regions are shown in (a′) and (a″). (b) Equivalent circuit used for fitting the Nyquist curves. Variation of (c) Rrec, (d) Cµ and (e) τn as a function of applied (dc) forward bias, under dark. (f) Nyquist plots under 0.2 sun irradiance, for cells with TiO2/CdS/Au@PAA and TiO2-CBN/CdS/Au@PAA photoanodes at −0.6 V; enlarged views of high frequency regions are shown as insets. Variation of (g) Rrec and (h) Cµ as a function of applied (dc) forward bias under irradiance.


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Tables Table 1. Emission decay (fitted) parameters for TiO2 and TiO2-CBN films. Decay monitored at 430 nm. Sample

B1

τ1 (ns)

B2

τ2 (ns)

(ns)

χ2

TiO2

73.9

0.21

26.06

2.64

2.18

1.25

TiO2-CBN

66.39

0.35

33.61

9.45

8.82

1.33

Table 2. Effect of carbonate passivating layer on cell performance, and parameters for large area cells (under 1 sun illumination). A polysulfide/SiO2 silica gel is the electrolyte and the CE is C-fabric/WO3-x. Jsc (mA cm-2)

Voc (mV)

FF

PCE (%)

4.529

885

0.55

2.225

TiO2/CdS/Au@PAA(1 cm )

6.370

809

0.59

3.03

TiO2-CBN/CdS/Au@PAA (1 cm2)

7.977

915

0.59

4.32

TiO2-CBN/CdS/Au@PAA (4 cm2)

2.218

763

0.62

1.06

Sample TiO2/CdS (1 cm2) 2

Table 3. Photovoltaic parameters of five QDSCs with the same TiO2-CBN/CdS/Au@PAA-polysulfide/SiO2 gel-C-fabric/WO3-x configuration, under an irradiance of 100 mW cm-2 (AM 1.5 spectrum). Cell

Jsc (mA cm-2)

Voc (mV)

FF

PCE (%)

Cell-1

10.525

868

0.61

5.616

Cell-2

11.863

903

0.54

5.829

Cell-3

11.445

925

0.55

5.855

Cell-4

11.045

899

0.63

6.277

Cell-5

11.226

869

0.61

5.990

Table 4. Effect of dark storage on the solar cell performances of a cell with a TiO2-CBN/CdS/Au-polysulfide/SiO2 gel-Cfabric/WO3-x configuration. Days

Jsc (mA cm-2)

Voc (mV)

FF

PCE (%)

1st

10.28

902

0.61

5.69

19th

10.16

882

0.63

5.646

30th

10.26

886

0.61

5.52


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Table 5. Effect of prolonged continuous illumination on a cell with a TiO2-CBN/CdS/Au-polysulfide/SiO2 gel-C-fabric/WO3-x configuration (Irradiance: 1-2 Sun). Illumination

Jsc (mA cm-2)

Voc (mV)

FF

PCE (%)

0h

10.385

891

0.64

5.909

1h

10.385

891

0.62

5.772

2h

9.865

886

0.64

5.607

3h

10.747

843

0.60

5.463

4h

10.621

827

0.57

5.010

5h

10.534

791

0.58

4.835


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Table 6. Comparison of stability performances of QDSCs from literature. Photoanode Configuration

Electrolyte

Counter Electrode

Storage Conditions

Reference

Initial

Final

3.0

2.7

46

4.32

Almost same

40 47

TiO2/CdS/CdSe

Polysulfide

PbS/Carbon black

TiO2/CdS/CdSe

Polysulfide

CuInS2/Carbon

TiO2/CdS/ZnS-N719

I−/I3 −

Pt/FTO

Solar irradiation (400 s)

4.27

TiO2/CdS/CdSe

Polysulfide

CuS

Solar irradiation (2 h)

4.22

Almost same 4.00

TiO2/CdSeS-ZnS

Pt/FTO

Solar irradiation (8 h)

4.23

2.32

42

TiO2/CdSe

1 M Na2S + 0.25 M S gel with PEG Polysulfide

Cu2S/FTO

RT conditions (10 h)

5.21

5.73

43

TiO2/CdSeTe TiO2CBN/CdS/Au@PAA TiO2CBN/CdS/Au@PAA

Polysulfide Polysulfide/fumed SiO2 gel Polysulfide/fumed SiO2 gel

Cu2S/FTO C-fabric/WO3-x

RT conditions (500 h) Dark storage at RT (720 h) 1-2 Sun (5 h, outside)

4.5 5.69

5.4 5.52

44 This work

5.9

4.83

This work

C-fabric/WO3-x

RT conditions (1000 h) Dark storage at RT (1000 h)

PCE (%)

41

RT: Room temperature, PEG: Poly(ethylene glycol)


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SYNOPSIS TOC


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Stability, Scale-up, and Performance of Quantum ... - ACS Publications

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