Carbon Dot Assisted Synthesis of Nanostructured Polyaniline for Dye

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Carbon Dot Assisted Synthesis of Nanostructured Polyaniline for Dye Sensitized Solar Cells Aniruddha Kundu, Arnab Shit, and Sudipta Nandi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00571 • Publication Date (Web): 16 Jun 2017 Downloaded from http://pubs.acs.org on June 20, 2017

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Carbon Dot Assisted Synthesis of Nanostructured Polyaniline for Dye Sensitized Solar Cells Aniruddha Kundua*, Arnab Shitb and Sudipta Nandib Abstract: Herein, we have reported a facile method to synthesize surface passivated carbon dots (CDs) with high quantum yield. The structure and optical properties of the CDs are successfully investigated using high-resolution transmission electron microscopy; dynamic light scattering; UV-vis, fluorescence and Fourier transform infrared spectroscopy (FTIR). The synthesized CDs with excellent water solubility have been utilized to prepare nanostructured polyaniline (PANI) where CDs act both as a dopant as well as nucleating agent. The synthesis of PANI is achieved through a simple and one pot chemical oxidative polymerization. The UV-vis and FTIR data clearly indicates the formation of highly doped PANI (emeraldine salt form); which will really be helpful for fabricating photovoltaic device. We have examined the current-voltage characteristics of CD-PANI samples both in dark and illuminated state, which evidently designates the enhancement of current in illuminated state. Hence, we have employed the CDPANI samples for photovoltaic study and the DSSCs are studied under illumination of 100 mW/cm2. Amongst, the CD-PANI samples CDPA10 shows a maximum power conversion efficiency of 3.65%. KEYWORDS: carbon dots, fluorescence, surface passivation, polyaniline, solar cell.

a

School of Mechanical Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, South Korea. b Polymer Science Unit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata-700032, India *Corresponding author email: [email protected] 1 ACS Paragon Plus Environment

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1. Introduction: The search for alternative energy sources, particularly renewable solar energy has massively motivated scientist due to the continuously growing global energy demands and the depletion of readily accessible fossil fuels. After some preliminary exploration,1,2 Gratzel and O’Regan3 for the first time put forwarded a novel solar cell: dye-sensitized titania nanocrystalline solar cell [named as, dye-sensitized solar cell (DSSC) or Gratzel cell] in 1991. Since this significant breakthrough, DSSCs have entered public view and garnered more and more research attention over the past two decades. DSSCs have received widespread attention in recent years and been regarded as one of the most prospective solar cells among the third-generation photovoltaic technologies due to low cost, easy preparation, good performance and environmental benignity compared with traditional photovoltaic devices.4,5 In DSSCs, TiO2 plays an important role since, the photoelectrons generated from the dye molecules are transported to the anode through it.6 But TiO2 based DSSCs suffers a severe problem due to the possibility of reverse transfer of photoinjected electrons from TiO2 to the iodide/triiodide (I-/I3-) redox couple or to the dye which may reduce the cell efficiency. Hence, recently different metal oxide/sulfide nanostructures are synthesized to replace traditional TiO2 for the achievement of good power conversion efficiency (PCE). Moreover, in DSSC systems, the platinum (Pt)-based counter electrode (CE) is one of the major obstacles for large-scale production of DSSCs owing to its high cost and limited resources; hence, alternative CE materials with low cost, high chemical stability and high electro-catalytic activity are extremely anticipated. Recently, several types of CEs based on carbon materials, transition metal complexes and conducting polymers have been utilized as promising candidates to substitute Pt-based CEs. In this regard, intrinsically conducting polymers (containing π2 ACS Paragon Plus Environment

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conjugated polymeric chain) in combination with metal nanoparticles or graphene are enormously studied.7,8 Conducting polymers have unique properties of light absorbance and also act as hole transporting materials (HTM) which make them a suitable candidate as the CE for DSSCs.8 Among the conducting polymer, polyaniline (PANI) is most extensively investigated in recent years, owing to its facile and low cost synthesis, good environmental stability, reversible electrochemistry, corrosion protection, and high instinct redox properties etc.9 Again, as a representative conducting polymer, PANI is suitable for CEs because of its superior catalytic activity for I3- reduction which may definitely contribute to the improvement of photovoltaic efficiency. Li et al. for the first time introduced, perchloride acid dopant microporous PANI nanoparticles as CE in DSSCs.10 Then various groups have synthesized nanostructured PANI (doped with different acids) for the employment as CEs in DSSCs.11-13 Carbon dots (CDs) are new class of carbonaceous material which has recently received growing interest as a potential candidate to conventional semiconductor quantum dots due to their ease of synthesis, chemical inertness and low toxicity.14 Although, fluorescent CDs have recently been utilized to make nanocomposite with PANI but these studies mainly focus on the sensing properties of the nanocomposite.15,16 Herein, we have reported a facile method to synthesize nanostructured PANI using surface passivated fluorescent CDs for high performance DSSCs. In this work, CDs display two important roles: acts as a dopant due to the presence of rich carboxylic groups on the surface of CDs and also nanometer-sized dots function as an effective nucleating agent for the primary growth of polymerization. Here, we have studied the photoreversibility of different CD doped PANI (CDPA) samples by measuring the relative photocurrent decay of CDPA over different cycles. Then, we have fabricated DSSCs, with different CDPA samples in the presence of N719 [cis-di(thiocyanate)bis-(2,20-bipyridyl-4,403 ACS Paragon Plus Environment

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dicarboxylate) ruthenium(II) (bistetrabutylammonium)] dye as a photosensitizer. Among the different CD doped PANI samples, one sample with optimum CD doping shows highest PCE upto 3.65%, which is also investigated using electrochemical impedance spectroscopy. 2. Experimental Section: 2.1 Materials Hyperbranched polyethyleneimine (PEI, Mw=25000) was purchased from Sigma Aldrich. Citric acid (CA), aniline and ammonium persulphate (APS) were purchased from Merck. Aniline was distilled before use and the middle fraction was used for the polymerization process. 2.2 Synthesis of Fluorescent CDs The fluorescent CDs passivated with hyperbranched PEI was synthesized by a facile green route of hydrothermal assisted pyrolysis method. For that purpose, at first CA (5.2 mmol) was dissolved in 10 mL of double distilled water (DD) and PEI (5.2 mmol) was dissolved in 10 mL of DD water in different glass vial. Next, these two solutions were mixed and sonicated for 5 min to get homogeneous mixture and refluxed in a preheated oil bath at 150 °C for 5 h. After the completion of reaction, the transparent viscous liquid turned into bright yellow color solution indicating the formation of carbon dots. The resulted solution was purified by filtering out largesized carbon nanoparticles using a syringe filter with pores of 0.1 µm. Finally, the product was subjected to dialysis (MWCO=1000 Da) in order to obtain the pure CDs and subsequently lyophilized to collect dry CDs for further use. 2.3 Synthesis of Polyaniline using Fluorescent CD as Dopant Here, polymerization of aniline was performed using CDs as dopant (without using any inorganic acid) via chemical oxidative polymerization technique. Briefly, at first required 4 ACS Paragon Plus Environment

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amount of CDs were dispersed in water to get homogenous dispersion. Then, 100 µL of aniline was slowly added to the CDs in each case and stirred continuously for one hour at room temperature. Subsequently, the mixture was cooled to 10 0C and the polymerization of aniline was initiated by using equimolar solution of APS as an oxidant. The polymerization was continued for 24h at ~5 0C temperature. After completion of reaction, dark green colored precipitate was obtained. The precipitate was washed several times with water and methanol to remove the oligoanilines and excess APS. Finally, the product was dried at room temperature in vacuum for 24h to obtain CD doped PANI samples with different yield. The samples were designated as CDPA5, CDPA10 and CDPA20 depending on the weight percentage of CD with respect to aniline. 2.4 Quantum Yield Measurement The relative fluorescence quantum yield (QY) of CD was measured using quinine sulfate in 0.1 M H2SO4 (quantum yield 54% at 360 nm) as a standard. The value of QY was calculated according to the following equation: QYsample = QYstd. [(I/A)sample × (A/I)std.] (η2sample/ η2std.) where A is absorbance at the excitation wavelength, η is the refractive index of solvent and I is the integrated emission intensity calculated from the area under the emission peak on the same wavelength scale. The absorbance values of both sample and standard solutions were kept below 0.1 to minimize the re-absorption effects at the excitation wavelength (360 nm). 2.5 dc-conductivity Measurement

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The dc-conductivities of the samples were measured by a two-probe method by making a pellet of the samples with 0.25 cm diameter using an electrometer (Keithley, model 617). The conductivity values were calculated directly from the measured resistance and sample dimensions using the equation: Ω = 1/R × L/a where ‘L’ is the thickness, ‘a’ is the area and ‘R’ is the resistance of the sample. The I-V characteristics curves of the samples were studied at 25 °C using the same samples by applying voltage from -5 to +5 V, and the current was measured at each applied voltage. 2.6 Electrochemical Impedance Spectroscopy Electrochemical impedance spectroscopy (EIS) measurements for the DSSCs were performed under open circuit voltage bias and 10 mV voltage perturbation conditions in the frequency range from 1 MHz to 1 mHz using Solartron SI1260 impedance analyzer. All the impedance spectra were obtained by simulation of the experimental data using Z-view software and the respective equivalent circuits were drawn. 2.7 Fabrication of DSSCs The electrodes, with an active cell area of 0.28 cm2, were fabricated on low resistance FTO glass (2×2 cm2, 8-12 Ω/sq) by drop-casting CD doped PANI dispersions in chlorobenzene. Here, Scotch tape was used as a frame and spacer to control the film thickness as well as area and subsequently they were dried in air. The films were sensitized by keeping them immersed in a 0.5 mM solution of N719 dye in ethanol. The solar cells were fabricated by assembling these electrodes with a graphite coated FTO counter electrode and were filled with the I-/I3- redox electrolyte solution, which consists of 0.5 M KI, 0.05 M I2 in γ-butyrolactone. The cells were


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sealed with parafilm strip and were pressed together with clips to form a sandwich-type configuration. A small portion of bare FTO glass and graphite-coated glass was uncovered for wire connection to the anode. Current density-voltage (J-V) characteristics of solar cell devices were measured using Keithley 2401 source meter. The cells were illuminated from the semiconductor side using a 150 W Xe-lamp equipped with an AM 1.5G filter (Newport, 67005) at a calibrated intensity of 100 mW/cm2, as determined by a standard silicon reference cell (91150V Oriel Instruments). 3. Characterization Techniques UV-Vis absorption features of aqueous solutions of the samples were recorded with a UV-Vis spectrophotometer (Hewlett-Packard, model 8453). The fluorescence characteristics of the samples were studied in a fluorescence spectrophotometer (Horiba Jobin Yvon Fluoromax 3) using a quartz cuvette of 1 cm path length. Fourier transform infrared (FTIR) spectroscopy was studied on a Perkin Elmer Spectrum 100 FTIR spectrometer with solid KBr pellet. The structures of the CDs were observed with a high-resolution transmission electron microscope (UHRFEGTEM) instrument at an accelerating voltage of 200 kV. Dynamic light scattering (DLS) was measured using a NanoZS (Malvern) instrument. 4. Results and Discussion: 4.1 Synthesis, Characterization and Optical Properties of CDs The synthesis of water soluble, blue light emitting fluorescent CDs were obtained by a simple, one-step hydrothermal route (shown in Scheme 1), where CA was used as carbon source and hyperbranched PEI acted as surface passivation agent. Here, we propose that there may exist three steps in the whole pyrolysis process, (i) dehydration of CA and nanoparticle formation, (ii)


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surface passivation of nanoparticle and (iii) growth of carbon dots. Since, these steps have no well-defined boundary; employing this one pot strategy, the formation of CDs and the surface passivation with PEI are achieved simultaneously. The morphology and structures of the synthesized CDs are confirmed by TEM image and demonstrated in Figure 1. The statistical particle size distributions of the CDs clearly exhibit uniform dispersion and a relatively narrow size distribution ranging from 2-5 nm with an average diameter of 2.47 nm (Figure 1b). Again, the HRTEM image (Figure 1c) does not exhibit noticeable fringe pattern which discloses the amorphous nature of the CDs. Dynamic light scattering (DLS) study was performed to determine the hydrodynamic size of the synthesized CD and the average hydrodynamic diameter of the CD is calculated to be 12.9 nm (Figure 1d). The differences in average diameter are attributed to the different surface states of the CDs under two measurement conditions (TEM and DLS). FTIR spectra were employed to gain structural insight i.e. to identify the surface chemistry of the passivated CDs. As depicted in Figure 2, after the hydrothermal treatment, the O-H stretching of CA (Figure S1) becomes weak while a strong peak at 1395 cm-1 appears due to the N-H in-plane bending of the amine of PEI. The N-H in-plane bending of the secondary amine at 1575 cm-1 and the weak C-N stretching of the primary amine around 1130 cm-1 provide significant information about the in situ grafting of PEI molecules onto the surface of CDs. Furthermore, the C-O-C stretching at 1232 cm-1 verifies the incomplete dehydration reaction of CA. The absorption around 2850 and 2965 cm-1 represent C-H antisymmetric and symmetric stretching which comes from the backbone of PEI as well as the CDs. Again, the doublet observed for CA (Figure S1) change into one peak at 1710 cm-1 for the CDs, which may be attributed to the formation of


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amide groups or associated carboxyl groups, which appear on the surface of CDs. Thus, the grafted PEI molecules provide surface passivation, rendering some advantages to CDs like, water solubility, water stability and strong fluorescence, which are discussed below. The optical properties of the CDs were characterized by UV-vis and fluorescence spectroscopic studies. The synthesized aqueous solution of CD exhibits two absorption bands at 242 and 349 nm respectively (Figure 3), which are attributed to π-π* and n-π* transition.17 The aqueous solution of CD displays bright yellow color (Figure 3 inset) in day light while bright blue emission under UV light (365 nm, Figure 3 inset). The synthesized CD shows blue fluorescence with λmax at 448 nm (at excitation of 360 nm) and the full width at half maximum (FWHM) is about 82 nm (Figure 3), indicating a narrow size distribution of CD. CDs show fascinating fluorescence properties with λex dependent emission intensity, similar to semiconductor QDs but the origin is still a debatable subject.18-20 Though, recent efforts provide mounting evidence that emission arises from quantum effect, emissive traps and radiative recombination of excitons located at surface energy traps, more research is needed in this regard to get clear insight about the origin of fluorescence from CDs.21-23 Now, the fluorescence spectra of CD solution were recorded by changing the excitation wavelengths to obtain clear picture about the wavelength dependence feature of synthesized CDs. The excitation wavelength dependent fluorescence spectra of CD solution (1 mg/mL) are shown in Figure 4, which clearly depicts that when the excitation wavelengths changes from 330 nm to 440 nm, the emission peak position is red-shifted from 445 to 490 nm and the fluorescence intensity decreased remarkably. The red shift of emission spectra is more obvious from the normalized fluorescence spectra (Figure S2), indicating a strong dependence on the excitation


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wavelengths. The origin of excitation wavelength dependent fluorescence in CDs are not very clear; we assume that this phenomenon may come from the different distribution of emissive energy traps on the surface of CD, since it has been proposed that both surface state and size of CDs may affect the emission spectra of polymer passivated CDs.24 The relative fluorescence QY (excited at 360 nm) of the CDs in aqueous solution was calculated to be 41.64% (Table S1), which is much higher than that obtained by bottom-up methods.25-27 The high QY of the synthesized CDs signifies that the incorporation of PEI renders better surface passivation to the CDs, since without passivation we could have obtained very weak emission.24 Thus it can be concluded that in this hybrid CD, the grafted PEI displays two key roles: (i) acts as a surface passivation agent (nitrogen rich compound) to deliver CDs with strong blue fluorescence, since upon surface passivation or functionalization, the surface defects may become more stable to facilitate more effective radiative recombination of surface-confined electrons and holes, (ii) forms a protecting layer which endows CDs excellent water solubility. 4.2 Synthesis and Characterization of CD Doped PANI The synthesis of nanostructured PANI using CD as dopant as well as nucleating agent is illustrated in Scheme S1. CD with rich carboxylic acid groups on the surface interacts with aniline to form CD-anilinium ion complex and as APS solution was added to produce radicals, the polymerization of the aniline monomers proceeded to produce nanostructured PANI. The synthesized CD doped PANI sample shows good dispersibility in water (Scheme S1). The morphology of CD doped PANI samples were characterized with TEM as shown in Figure 5. All the CDPA samples exhibit nanotubular morphology which confirms that here, CD acts both as a dopant and nucleation agent. Initially, when the amount of CD is low (Figure 5a) it shows 10 ACS Paragon Plus Environment

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very few nanostructured PANI due to the lower extent of doping. But, as the amount of CD increases the nanotubular morphology becomes more prominent due to the better doping phenomenon (Figure 5b,c and Figure S3). FTIR and UV-vis spectroscopy were employed to get structural insights about the CD doped PANI samples. FTIR spectra are depicted in Figure 6, which show two characteristic peaks at 1572, 1492 cm-1 due to the C=N, C=C stretching of the quinoid (Q) structure and aromatic ring stretching of the benzenoid (B) structure of synthesized PANI, respectively.28-32 The peaks at 1300, 1238 cm-1 are ascribed to the C-N secondary aromatic amine and C=N stretching modes. The peak at 1110 cm-1 appears for the polaron band vibration (Q=N+H–B or B–N+H–B) due to doping with CD whereas the peak at 800 cm-1 is the distinctive feature of C-H out-of-plane bending mode of the 1,4-disubstituted benzene ring.29 The many low-intensity peaks ranging from 500 to 690 cm-1 can be assigned to the vibrations of the C-H bonds in the benzene rings. In addition, no peaks appear around 1710, 1395 cm-1 (the characteristic peaks of CDs) in the spectra of the CD doped PANI samples, which suggests involvement of CDs in the polymerization process and confirms that the surface of the CDs was successfully modified with PANI. The UVvis spectra of CD doped PANI samples are interpreted in Figure 7. From the Figure 7 it is evident that all the CDPA samples exhibit four characteristic peaks and the first two peaks originate from the CDs. The absorption peak of the CDs for π-π* transition (shown in Figure 3) moved to 230 nm in the CDPA samples, which may be due to the interaction between the CDs and growing PANI chains. Furthermore, the characteristic peak of the CDs at around 349 nm (Figure 3) corresponding to the n-π* transition shifted to lower wavelength (297 nm) value for all CDPA samples. This may be ascribed to the hydrogen bonding interaction between the hydroxyl groups of the CDs and the secondary amine of PANI, resulting in the lowering of 11 ACS Paragon Plus Environment

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energy level of the n-orbital. Again, there are two new absorption bands at 426-436 and 893-919 nm with a free tail extended to the IR region, which are attributed to the polaron band to π* band transition and π band to polaron band transition of PANI, respectively. These peaks essentially characterize the formation of the emeraldine salt (ES) form of PANI, which is typical for conducting state PANI. Therefore, PANI in the nanocomposite is highly doped which will be beneficial for opto-electronic devices. Again, it is interesting to note that both polaron band to π* band and π band to polaron band shows gradual red shift with increasing the amount of CD. This is because of the lowering of both π* and π energy level, which might be as a result of enhanced conjugation length of the PANI polymer backbone due to the nucleation on CDs. 4.3 Photo-electronic Properties The electrical conductivity of the samples are measured and depicted in Table S2, which obviously demonstrates that CDPA10 possess higher conductivity than the other two samples. From this it can be concluded that in CDPA10, CD induced better doping to the growing polymer. The photo-response properties of the CDPA samples are determined from the I-V plot under dark and illuminated conditions of 100 mW/cm2 (AM 1.5) irradiation using FTO electrodes. Figure 8a shows the I-V characteristics of CDPA10 whereas Figure S4 (a,c) depict the I-V characteristics of CDPA5 and CDPA20, respectively. All the samples exhibit non-Ohmic relation and this type of non-linear increase in current with applied potential may be due to the contribution from the charge carried by polarons and bipolarons in the doped PANI. The I-V plot clearly designates that the photocurrent is higher than the dark current in all the cases. Again, the separation between the dark and photocurrent reaches maximum in the case of CDPA10 (Figure 8a). To gain better insight about the reversibility of the photocurrent developed by illumination


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of white light irradiation of 1 sun, the photocurrent growth for 100 s and decay for the same time are recorded at a bias of 2 V with the time gap of 1000 ms for several cycles (Figure 8b and Figure S4 b,d). The reversibility is found to be meager for CDPA5 and CDPA20 (Figure S4 b,d) but CDPA10 (Figure 8b) shows good reversible behavior with the white light illumination on and off, respectively. To further confirm this photo-reversibility behavior we have calculated relative photocurrent decay from first to sixth cycle and presented in Table S3. It is clear from the Table S3 that after sixth cycle both CDPA5 and CDPA20 exhibits a substantial decay in photocurrent but interestingly photocurrent decay is negative for CDPA10. Hence, it is obvious that CDPA10 is ideal for fabricating DSSC device, since it shows better photo-stability than the other two samples. 4.4 Application in Dye-sensitized Solar Cell After obtaining information about the I-V characteristics as well as photocurrent response of the samples we have fabricated dye-sensitized solar cells (DSSCs) with N719 dye as sensitizer. The fabrication of devices for the DSSCs has been demonstrated in the experimental section. The performance of the DSSCs is estimated in terms of four key factors namely, open circuit voltage (VOC), short circuit current (JSC), fill factor (FF) and PCE (ɳ) and these important parameters are summarized in Table S4. The CDPA samples exhibit a short-circuit current (Jsc) in a range of 8.26-9.37 mA/cm2 with the highest value of 9.37 mA/cm2 for CDPA10 (Figure 9, Table S4). The fill factor and PCE values are also evaluated which follows the same tradition as that of JSC with a maximum fill factor of 0.59 and highest ɳ value of 3.65% for CDPA10 (Table S4). This observation clearly signifies that more currents are collected at the CDPA10 which suggests that CD nucleation may have an effect on the photo-electronic activity of PANI. The open-circuit voltages (Voc) of the samples are calculated to be 0.66 V. The lower values of JSC for CDPA5 13 ACS Paragon Plus Environment

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and CDPA20 samples may be accounted for the lower conductivity and a higher recombination rate during electron transport, which in turn provides poorer PCE than CDPA10. The electronic and ionic transport process within the DSSCs can be well understood by carrying out electrochemical impedance spectroscopy (EIS) of the DSSC devices. Figure 10 shows the Nyquist plots and equivalent circuit of the DSSCs under open circuit voltage bias condition. All the Nyquist plots are simulated using Z-view software and the results are depicted in Table S5. The high frequency arc corresponds to the graphite counter electrode charge transfer resistance and Helmholtz capacitance (Rce and Cce) in parallel combination. In the middle frequency region the second arc is the result of the recombination resistance (Rrc) at the PANI-CD-dye/electrolyte interface and the chemical capacitance (Cµ). Finally, the third arc, appearing at the lower frequency region, is due to the parallel combination of I3- ion diffusion resistance (Rd) and corresponding diffusion capacitance (Cd). The initial shift of the arcs from the origin is corresponding to the contribution from Rs.33,34 The lifetime (τ) of photo-injected electrons has been calculated using equation: τ = Rrc×Cµ The recombination resistance is highest for CDPA10 sample which means there is less recombination of electrons at the PANI–CD–dye/electrolyte interface. The chemical capacitance is also found to be higher for the CDPA10 sample and the combination of these two yields the longest lifetime (τ) of photo-injected electrons showing the highest efficiency.35


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5. Conclusions In summary, we have demonstrated the synthesis of highly fluorescent CD via a one pot surface passivation approach. We have tactfully utilized the synthesized CD for the preparation of nanostructured PANI in which, CD induced homogeneous nucleation as well as growth for the polymerization process. The success of polymerization using CD as dopant as well as nucleation agent is clearly understood from the UV-vis and FTIR study. Among the different CD doped PANI samples, one possess good electrical conductivity as well as excellent photo-reversibility, which clearly suggests that CD plays important role for the polymerization process and also render better electrical conductivity to PANI due to nucleation. Hence, it has been successfully utilized to fabricate DSSC device and it demonstrate higher power conversion efficiency (3.65%) than others, which has been validated from EIS spectroscopy. We strongly believe that this efficient and facile means will pave a new way for producing high-performance DSSCs or other applications, in near future. Supporting Information: FTIR spectrum of CA; Normalized Fluorescence spectra of CD; Table for quantum yield, dc-conductivity, photocurrent response and photovoltaic performances; schematic for the synthesis of PANI; I-V characteristics and photocurrent response for CDPA5, CDPA20; table for the electrochemical parameters calculated from EIS. Author Information Corresponding Author E-mail: [email protected] ORCID 15 ACS Paragon Plus Environment

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Aniruddha Kundu: 0000-0002-6508-6728 Acknowledgements A. S. acknowledges DST-INSPIRE Program. S. Nandi acknowledges IACS for the fellowship during this period of research. A. Kundu likes to acknowledge Prof. A. K. Nandi for providing solar cell facility and director of IACS for the central facility.


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O

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OH

HO OH

HO

0

O

150 C, 5h

O

+

Surface Passivated CDs

Scheme 1. Synthetic scheme for the preparation of surface passivated CDs.


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(c)

Figure 1. (a) TEM image, (b) Size histogram calculated from TEM, (c) HRETM image and (d) DLS hydrodynamic diameter size distributions of synthesized CD.


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Figure 2. FTIR spectrum of the synthesized CD.

Figure 3. UV-Vis absorption (black line) and emission (blue line) spectrum of CDs in dilute aqueous solution. [Inset: photographs of the fluorescent CDs in aqueous solution under (a) visible light and (b) 365 nm UV light].


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Figure 4. Fluorescence emission spectra of CD under different excitation wavelengths (from 330 to 440 nm excitation with 10 nm increment). (b)

(a)

Figure 5. TEM images of (a) CDPA5, (b) CDPA10 and (c) CDPA20.


(c)

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(c)

(b)

(a)

Figure 6. FTIR spectra of (a) CDPA5, (b) CDPA10 and (c) CDPA20.


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Figure 7. UV-vis spectra of different CD doped PANI samples.


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(a)

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(b)

Figure 8. (a) I-V characteristics under dark, illuminated conditions and (b) photocurrent response of CDPA10.

Figure 9. J-V characteristics of different CD doped PANI samples.


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Figure 10. Nyquist plots from EIS measurements of different CD doped PANI samples (Inset: equivalent circuit model).


Carbon Dot Assisted Synthesis of Nanostructured Polyaniline for Dye

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