Quantum Dot DonorâPolymer Acceptor Architecture for a FRET

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Quantum Dot Donor-Polymer Acceptor Architecture for a FRET Enabled Solar Cell Ramesh K. Kokal, Sai Santosh Kumar Raavi, and Melepurath Deepa ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01792 • Publication Date (Web): 02 May 2019 Downloaded from http://pubs.acs.org on May 2, 2019

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ACS Applied Materials & Interfaces

Quantum Dot Donor-Polymer Acceptor Architecture for a FRET Enabled Solar Cell Ramesh K. Kokal,a,†, Sai Santosh Kumar Raavib,†, Melepurath Deepa,a,†,* Department of Chemistry, bDepartment of Physics, †Indian Institute of Technology Hyderabad, Kandi, Sangareddy, Telangana (India)-502285 a

*

Email: [email protected] Tel: +91-40-23016024, Fax: +91-40-23016003.

KEYWORDS Energy transfer, solar cell, quantum dot, donor, acceptor, efficiency ABSTRACT: Forster resonance energy transfer (FRET) based solution processed solar cell is fabricated with cadmium sulfide (CdS) as the energy donor and poly[N-9-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2,1,3-benzothiadiazole)] (PCDTBT) as the energy acceptor. Carbon dots (C-dots) deposited on carbon (C)-fabric is applied as a counter electrode (CE). While electron injection from CdS to PCDTBT is energetically disfavoured, evidences for energy transfer between the two components of the cell are obtained in terms of FRET parameters with the relative quantum yield of donor CdS QDs being ~0.3, a Forster radius of ~3.7 nm and an energy transfer efficiency of ~55%. Power conversion efficiency (PCE) of the TiO2/PCDTBT cell without the donor is 0.23% and when coupled with donor CdS QDs, the ensuing TiO 2/PCDTBT/CdS cell experiences a 23 times increment in PCE, reaching 5.3%. The complete FRET cell: TiO2/PCDTBT/CdS/ZnS-S2--C-dots/C-fabric produces a PCE of 7.42%, under 1 sun illumination. External quantum efficiency (EQE) studies reveal an enhanced spectral response spanning from 300 to 670 nm, with 300 and 175% increases attained for the FRET enabled TiO2/PCDTBT/CdS/ZnS photoanode compared to the TiO2/PCDTBT photoanode over the blue and green-red portions of the solar spectrum.

Introduction Quantum dot solar cells (QDSCs) are attractive for solar energy conversion into electricity due to some exciting advantages offered by quantum dots (QDs). QDs offer tuneable band gaps that allow maximum utilization of the solar spectrum,1–3 multiple exciton generation that produces greater than a single electron when a photon strikes the cell,4 and ease of processing by use of chemical bath deposition methods that can be implemented at room temperature.5,6 Most of the high performance QDSCs typically employ QDs based on cadmium (Cd) and lead (Pb) chalcogenides, due to their high absorption coefficients, short photoluminescence lifetimes thus affording rapid electron transfer into the electron transport film, and ease of fabrication.7–13 One interesting but underutilized strategy to maximize the power conversion efficiency (PCE) of a QDSC is Forster resonance energy transfer (FRET) which is more effective than the co-sensitization approach. Some systems where a donor-acceptor assembly yielded marked improvements in PCEs are: 5-carboxyl-2-[[3-[(1,3-dihydro-3,3-dimethyl-1-ethyl-2H-indol -2ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene] methyl]-3,3-trimethyl-1-octyl-3H-indolium; squaraine (SQ-1) dye (as acceptor)- 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM-pyran) (as donor),14 zinc phthalocyanine (as acceptor)- DCM-pyran (as donor)15 and CdSe (as acceptor)-CdSe/CdS/ZnS (as donor)16 where PCEs of 1.64, 3.68 and 0.05% were achieved respectively for the FRET enabled donor-acceptor cell, and ~0.5, 2.94 and 0.012% for the acceptor only cell. Another example is (2,2,7,7-tetrakis(3hexyl-5-(7-(4-hexylthiophen-2-yl)benzo[c][1,2,5]-thiadiazol4-yl)thiophen-2-yl)-9,9-spirobifluorene) (Spiro-TBT) (donor) poly[2,6-(4,4-bis-(2-ethylhexyl)-4Hcyclopenta[2,1-b;3,4b0]dithiophene)-alt-4,7-(2,1,3-benzothiadia-zole)] (PCPDTBT) (acceptor) sensitized ZnO based hybrid solar cell.17 Previously, a FRET based QDSC with CdS QDs as donors, and copper phthalocyanine (CuPc) as acceptor molecules, combined with carbon dots for efficient electron transfer,18

yielded a PCE of 0.34% compared to a PCE of 0.04% for the donor only cell. The donor ‘s quantum yield was found to be 0.28 and the Forster radius (Ro) was 4.25 nm. The FRET cell based on ZnS/CdS/ZnS/C-dot/CuPc showed a 5.75 times increase in the external quantum efficiency (EQE) when compared to that of the donor (ZnS/CdS/ZnS) only assembly.18 Gupta et al.,19 fabricated a ternary polymer (polythieno[3,4- b ] thiophene/benzodithiophene : poly[N-9-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2,1,3-benzothiadiazole)] : [6,6]- phenyl C 71 butyric acid methyl ester (PTB7:PCDTBT:PC71 BM )) in a 0.7:0.3:2 proportion based heterojunction cell, which delivered a PCE of 8.9% contrasting with lower PCEs of 6.2 and 6.8% for the donor (PCDTBT:PC 71 BM) only and acceptor (PTB7:PC71BM) only cells respectively. PCDTBT (donor) gave a quantum yield of 0.31 and the Forster radius was obtained as 0.7 nm.19 Hardin et al.,20 constructed a FRET based dye sensitized solar cell with a highly luminescent chromophore perylene-3,4,9,10-tetracarboxylic diimide (PTCDI) as a donor and a zinc phthalocyanine dye as an acceptor. They achieved a PCE of 3.21% in the presence of donor (13 mM PTCDI) and 2.55% in the absence of a donor (0 mM PTCDI). The average excitation transfer efficiency was observed to be 47%.20 Zhang et al.,21 demonstrated a FRET based ternary organic solar cell with poly[(2,6-(4,8-bis(5-(2ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2 ethylhexyl) benzo[1′,2′c:4′,5′-c′]dithiophene-4,8-dione))] (PBDB-T) as a donor and poly([2,6′-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b]dithiophene]{3-fluoro-2[(2-ethylhexyl)carbonyl] thieno[3,4-b]thiophenediyl}) (PTB7-Th:FOIC) as an acceptor and achieved a PCE of 12.02% for 50% of the donor content in the device. Warner and group 22 developed rhodamine B chloride, 1,1′-diethyl-2,2′-carbocyanine iodide, 3,3′-diethylthiacarbocyanine iodide, and meso-tetra(4-carboxyphenyl) porphine based group of materials based on organic salts (GUMBOS) for the application of energy relay dyes (ERD) in DSSCs. FRET occurred be-

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tween different GUMBOS based-ERDs (donors), and the photosensitizing dye, N719, [di-tetrabutylammonium cisbis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II)] (acceptor). Authors observed that, in the presence of GUMBOS-based ERDs, DSSCs have enhanced efficiencies when compared to that of the cells without the ERDs.22 Gao at el. 23 fabricated FRET based DSSCs with 4-(dicyanomethylene)-2-t-butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) as an ERD (donor) and cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II) (N3) dye as an acceptor. The efficiency enhanced from 4.2% (only acceptor) to 5.64% in the presence of the donor.23 In view of the above described improvements realized in dye or polymer based solar cells which employed energy relay photo-sensitizers, here we demonstrate the design and implementation of a FRET enabled solar cell with CdS QDs as the energy relay donor and poly[N-9-heptadecanyl-2,7-carbazolealt-5,5-(4,7-di-2-thienyl-2,1,3-benzothiadiazole)] (PCDTBT) as the energy acceptor. Since the counter electrode (CE) also governs the device performance, carbon (C)-dots tethered to a carbon (C)-fabric electrode was utilized as counter electrode. C-dots offer high electrical conductivity, high electrocatalytic activity towards polysulfide reduction, are cheap and do not corrode in a strong alkaline medium (like sulfide), thus rendering them to be apt as CEs for the FRET enabled cell. Light absorption, fluorescence emission, and decay analysis, the quantum yield of the donor, Forster radius, charge transfer, energy transfer mechanisms and electrochemical impedance studies were performed to understand how FRET assists in improving the PCE of the TiO2/PCDTBT/CdS/ZnS based cell. Experimental section Fabrication of electrodes and cells Carbon dots (C-dots) were synthesized by using a procedure proposed by Zhu et al.24 In this method, 2 g of D-(+)-Glucose (Sigma Aldrich) and 10 mL of PEG-400 were mixed with 3 mL of ultrapure water till a clear and colorless solution was obtained. The solution was kept in a microwave oven (operating at a power of 500 W) for 10 min, and a dark brown colored product was obtained. The resultant product was washed three to four times with ultrapure water, and dried at 50 oC for 10 h, and was labelled as C-dots. TiO2 films from P25 (Evonik, free of cost) were deposited over FTO substrates using a previously reported procedure.25,26 Multiple clear purple colored solutions of PCDTBT (Luminescence Technology Corp) were prepared in chlorobenzene (with varying PCDTBT content), and they were was spin-coated on a Spektrospin machine in the air over the TiO2 films at 5000 rpm for a 60 s. The electrodes were heated at 80 oC for 10 min in air. Cadmium sulfide was deposited over TiO2 or TiO2/PCDTBT films by using a wet chemical method which is described in detail elsewhere.26 The photoanodes were coated with 4 ZnS layers. 9 mg of C-dots were added to 1 mL of ultrapure water by and subjected to ultra-sonication for 20 min. The ensuing colloid was drop-cast over a C-fabric, and dried in air, prior to use as a CE in QDSCs. QDSCs were fabricated in the following sandwich configuration: photoanode/0.1 M Na2S + 0.1 M KCl/CE (C-fabric or C-dots/C-fabric). The cells were illuminated from the back-side during the measurements. Scheme 1

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shows the fabrication protocols of the photoanode, CE and QDSC. Characterization techniques Details of UV-Vis-NIR spectrophotometer, X-ray diffractometer, fluorescence spectrometer with lifetime capability, scanning electron microscope (SEM) and transmission electron microscope (TEM), and the modes of measurement are provided in our earlier works. Complete details of solar cell parameters measurement (current density versus applied bias), and EQE measurements are also provided in a previous study.10 Electrochemical measurements (Cyclic voltammetry (CV) plots, Mott-Schottky (M-S) profile, impedance spectra) were recorded on an Autolab PSTAT302N, with a frequency response analyser (FRA) machine. Results and discussion Structural studies of photoanode The FE-SEM images of TiO2 and TiO2/PCDTBT/CdS films are displayed in Figure 1. TiO2 (Figure 1a, b) is made up of interconnected particles with conjoined pores of 20 to 100 nm dimensions. The mean pore magnitude is ~30 nm. TiO2 allows easy percolation and adsorption of the CdS precursor solutions or the PCDTBT solution. After the sequential deposition of PCDTBT and CdS on TiO2 electrode, the resulting TiO2/PCDTBT/CdS electrode (Figure 1c, d), again conveys a particulate morphology, with pores of sizes in the range of 50 to 500 nm. Such a morphology is suitable for electrolyte (0.1 M Na2S) penetration, during cell operation. Cross-sectional images of TiO2/PCDTBT/CdS deposited over FTO substrate are shown in the Figure 1e, f. The 3 components: TiO2, PCDTBT, and CdS co-exist as a composite with no inter-layer boundaries. The thickness of the deposit is ~4.5 µm. TEM images of C-dots is displayed in Figure 2a, b. The images show carbon nanoparticles of 4-6 nm dimensions. The lattice scale image is shown in Figure 2c and the inter-fringe separation is 0.34 nm. This aligns with d = 0.34 nm for graphite (JCPDS # 75-1621). The TEM images of TiO2/PCDTBT/CdS are shown in Figure 2d-h. The images show discrete crystallites of TiO2, PCDTBT and CdS QDs in some regions, and I in other regions, they are overlapping. Crystallites from TiO2, PCDTBT, and CdS QDs have been marked with red, blue and yellow colored dotted circles or ellipses respectively. Figure 2e, clearly shows the lattice fringes from all the 3 components. Inter-fringe separations of 0.35 nm due to the (101) plane of TiO2 and 0.34 nm is ascribed to the (111) plane of CdS. The inter-fringe spacing of 0.3 nm corresponds to PCDTBT with 2θ = 26.47 o. The distances between the donor CdS and the acceptor PCDTBT were calculated from the TEM image shown in Figure 2h. The average distance between the donor and the acceptor was found to be 5.9 nm. The selected area electron diffraction (SAED) pattern for TiO2/PCDTBT/CdS shows spotty rings (Figure 2i,j). TiO2 gives rise to the spot indexed at the (101) plane (PDF # 89-4921). The (220) plane stems from CdS (PDF # 65-2887). PCDTBT yields a spot assigned to a d of 0.34 nm. In Figure 2j, spots are indexed to the (101) plane from TiO2, (200) plane from CdS, and a spot corresponding to d = 0.15 nm arises from PCDTBT. The XRD patterns of TiO2, PCDTBT and CdS are presented and discussed in the supporting information (Figure S1).

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Optimization of concentration of PCDTBT on TiO2 electrodes To identify the concentration of PCDTBT capable of delivering the highest PCE, QDSCs were fabricated by employing the following photoanodes: TiO2/PCDTBT (x mg mL-1), where x = 2, 4, 6, 8, 10. C-fabric was employed as the counter electrode, and an aqueous solution of 0.1 M Na2S and 0.1 M KCl was used as the electrolyte in all the cells. Current density-voltage (J-V) characteristics recorded under 1 sun, are presented in Figure 3a. The simulated device parameters are provided in Table 1. At x = 2 mg mL-1 of PCDTBT, the open circuit voltage (VOC), shortcircuit current density (JSC), fill factor (FF) and power conversion efficiency (PCE), are 316 mV, 1 mA cm-2, 0.42 and 0.13% respectively. As x or PCDTBT concentration is progressively increased from 4 to 8 mg mL-1, the PCE increases further from 0.14 and 0.23% respectively. These enhancements are attributable to an increase in the number of photoactive species undergoing charge separation. However, when x is further raised to 10 mg mL-1, the PCE dropped to 0.18%, possibly due to the aggregation of PCDTBT which obstructs efficient electron transfer to TiO2, causing electrons to recombine instead of reaching the external circuit, thereby inducing the efficiency decline. Therefore, the concentration of PCDTBT is fixed at x = 8 mg mL-1, for all characterizations. To affirm the electron conducting capability of the PCDTBT polymer, Mott-Schottky plots were recorded at two different frequencies in dark and are discussed in the supporting information (Figure S2). PCDTBT is an n-type polymer which is evidenced from the positive slopes obtained for the polymer in the Mott-Schottky plots. Optical studies UV-visible absorption measurements of the photosensitizers: TiO2, PCDTBT, CdS, PCDTBT/CdS and the TiO2/PCDTBT, TiO2/CdS, TiO2/PCDTBT/CdS photoanodes are shown in Figure 3b and c. From the absorption spectrum of TiO2 and by employing the expression, Eg (eV) = 1240/ (nm), a band gap of 3.18 eV is calculated for TiO2. PCDTBT is characterized by two broad absorption peaks spread over 350 to 440 nm and 490 to 670 nm with λmax positioned at 400 and 580 nm respectively. The optical band gaps are estimated to be 1.85 and 2.26 for PCDTBT and CdS. The PCDTBT/CdS film shows a combined visible light absorption from the two components. Similarly, the spectra of the TiO2/PCDTBT and TiO2/CdS photoanodes reflect a combination of the absorption features of their components. Figure 3d illustrate the energy band diagram of the photoanode TiO2/PCDTBT/CdS/ZnS. Cyclic voltammetry (Figure S3, supporting information) and absorption studies are used to determine the VB and CB positons of the different components. The CBs of CdS, PCDTBT, and TiO2 are positioned at 3.9, 3.6 and 4.12 eV (versus vacuum) respectively. Upon irradiation, the energy level offsets permit photoexcited electrons to be injected from the CB of PCDTBT to the CB of TiO2. Since the CB of PCDTBT lies above the CB of CdS, electron transfer from CdS to PCDTBT is thermodynamically disfavoured, however, the reverse injection (of electrons) is allowed from the CB of PCDTBT polymer to the CB of CdS QD. Since CdS and PCDTBT are in direct contact with each other, energy transfer from the donor CdS (having a higher band gap) to the acceptor PCDTBT (with a narrower bandgap) is feasible. PCDTBT is, therefore, expected to undergo enhanced charge separation in the presence of the vicinal CdS QDs.

Firstly, the PCDTBT concentration is optimized by recording fluorescence spectra of TiO2/PCDTBT with 6, 8 and 10 mg mL1 polymer concentrations, which are furnished in the supporting information (Figure S4). As the concentration increases from 6 to 8 mg mL-1, the fluorescence intensity increases by 25%, when compared with that obtained with 6 mg mL-1. In general, the fluorescence intensity shows a direct dependence on the concentration of the fluorophore up to a certain concentration. Upon further increasing concentration of PCDTBT on the TiO2 electrode, the emission intensity decreased by 11% when compared to that at 8 mg mL-1. It is due to the aggregation of molecules, self-quenching between the fluorophores, and hence there is a decline in the fluorescence intensity. The average excited lifetimes for TiO2/PCDTBT at 6, 8 and 10 mg mL-1 concentration are 2.0, 2.5 and 2.2 ns respectively (Table S1). Since maximum quenching was achieved at 8 mg mL-1, this was taken as the optimum value, and further studies are reported for this concentration. Fluorescence spectra of PCDTBT, CdS, PCDTBT/CdS deposited on glass and TiO2/CdS, TiO2/PCDTBT and TiO2/PCDTBT/CdS deposited on FTO are provided in Figure 4a. CdS is excited at 370 nm. It shows broad emission peak over a wavelength span of 450 to 600 nm, due to band gap transitions. Another weak peak with a max at ~660 nm, is due to the transitions from the VB to the trap states that are created by the surface defects in QDs.27 PCDTBT also exhibits a strong emission response in the red region over 600 to 800 nm, peaking at ~674 nm, due to the radiative band-edge recombination in the polymer.28 TiO2/CdS and TiO2/PCDTBT films incur 31 and 24.5% losses in their respective intensities, owing to photo-excited electron injection from the CB of CdS and PCDTBT to the CB of TiO2. PCDTBT/CdS shows a couple of emission peaks, with max at ~500 and ~670 nm, and these arise from CdS and PCDTBT respectively.25 Their intensities are shrunk by 11.4 and 22% respectively with respect to the fluorescence peak intensities of CdS and PCDTBT. The intensity of CdS emission is diminished by the band-edge excitation energy transfer via FRET mode, from CdS to the vicinal PCDTBT species. The emission intensity of PCDTBT is reduced possibly owing to photoexcited electron cascade from the CB of PCDTBT to the CB of CdS. The curve profile of the TiO2/PCDTBT/CdS film resembles that of the PCDTBT/CdS film, but the fluorescence peak intensities are reduced in intensity by 73 and 25% respectively at CdS and PCDTBT emission positions. The dramatic drop in the emission intensity for CdS, is due to enhanced energy transfer to PCDTBT in the presence of TiO2. Figure 4b and c shows the emission decay profiles for the above stated samples (ex: 370 nm and em: 520 and 660 nm for the samples with CdS and PCDTBT).The following equations were used for determining lifetimes. I =  Bi exp(-t/i) (1) In (1), I is the intensity of fluorescence at time t, Bi is amplitude of the electron lifetime (i). Equation 2 was used to determine the average electron lifetime. =  Bii2 /  Bii (2) Table S2 shows the fitted data. CdS and PCDTBT exhibit electron lifetimes of 11.2 and 6.2 ns. The CdS lifetime is reduced in the presence of TiO2 to 5 ns due to excited electron injection into the oxide. The average excited electron lifetime



for the TiO2/PCDTBT (concentration of PCDTBT is 8 mg mL) is 2.52 ns. The electron injection from the CB of PCDTBT to the CB of TiO2 produces the fast decay contribution of 0.26 ns, while radiative electron-hole recombination results in the slow decay component of 2.9 ns. The average lifetime for the excited charge carrier in CdS/PCDTBT is 1.5 ns (which is shorter than that of CdS) at em = 520 nm (em of CdS). The reduced lifetime implies energy transfer from CdS to PCDTBT. The same assembly, at em = 660 nm (em of PCDTBT), yielded an average lifetime of 0.61 ns (which is again, shorter than that of PCDTBT). The TiO2/PCDTBT/CdS at em = 520 nm, shows a lifetime of 0.75 ns. This is ascribed to energy transfer from CdS to PCDTBT. When explored at em = 660 nm, photoexcited electron lifetime is 0.47 ns. The rapid and slow decay contributions of 0.017 ps and 0.48 ns are assigned to electron injection from the CB of PCDTBT to the CB of CdS, and band edge recombination respectively. The driving force for injection of an electron into TiO2, in terms of E is defined as ECB(PCDTBT)ECB(TiO2)), and it is 0.55 eV. It is greater than that for electron relay into CdS (E = ECB(PCDTBT)- ECB(CdS) = 0.3 eV). FRET parameters The quantum yield of the donor CdS QDs was estimated by measuring the absorption and fluorescence of CdS at different concentrations at a 370 nm excitation wavelength, and the same data was collected for a standard/reference Rhodamine 6G (Figure 4d). Rhodamine 6G was dissolved in ethanol and has a known quantum yield of 95%. CdS was scraped off from the electrode and was dispersed in ethanol. The quantum yield (Φ) of the donor was calculated using the following formula. ΦX = ΦR(GradX/GradR)(ηX2/ηR2) (3) In equation (3), X and R represent the donor (CdS) and the reference fluorophore (Rhodamine 6G). The slope of the straight line obtained from the integrated fluorescence intensity versus absorption is “Grad” and  is the refractive index of the solvent. Since the solvent (ethanol) is the same for both fluorophores, ηX2/ηR2 = 1. By substituting the experimental values in the equation (3), equation (4) was obtained. ΦX = 0.95(55.45/185)(1) (4) Using equation (4), the quantum yield of the donor CdS is calculated to be ~0.3 (Table S3, supporting information). For FRET to occur, the absorption spectrum of the acceptor (PCDTBT) and the fluorescence spectrum of the donor (CdS) must overlap, as shown in Figure 4e. Forster distance in Å is given by the following equation (5) Ro6 = 9000lnk2QDJ/128π5n4NA (5) In equation (5), k2 represents the dipole orientation factor of the donor and the acceptor, and generally the magnitude of k2 is assumed to be 2/3,18,19 QD is the quantum yield of the donor when the acceptor is not present. J gives the spectral overlap integral between the emission of the donor and the absorbance of the acceptor (Figure 4e). NA is the Avogadro’s number and n is the refractive index of the solvent. 1

∞

∞

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J (λ) = ∫0 FD (λ) εA (λ) λ4 dλ = = ∫0 FD (λ) εA (λ) λ4 dλ/∫0 FD (λ) (6) In (6), FD is the donor’s (CdS) emission intensity and εA is the molar extinction coefficient of the acceptor (PCDTBT). FD is a

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dimensionless quantity and εA is expressed in M-1c m-1, λ is the wavelength in nm and J has units of M-1 cm-1 nm4. PhotochemCAD software was used for FRET calculations and the obtained values are given in the supporting information. The spectral integral (J) is calculated to be 2.37 × 10-13 M-1 cm-1 nm4 and the Forster radius (Ro) is 3.67 nm. The acceptable separation for the effective dipole-dipole interaction can lie anywhere between 1 to 10 nm, and the obtained Ro of ~3.7 nm is within this range, thus implying that FRET is possible from CdS to PCDTBT. FRET efﬁciency (E) was calculated using a relation: E = 1/[1+(r/Ro)6]. Here ‘r’ is the actual distance between the donor and acceptor and ‘Ro’ is the Forster radius. By substituting, r = 5.9 nm (from TEM studies) and Ro = 3.67 nm, E is found to be 54.7%. Our value is higher than FRET efficiencies of 47, 40 and 27% reported previously for donor-acceptor systems of PTCDIzinc pthalocyanine20, ZnS/CdS/ZnS-CuPc18 and graphene quantum dots -N719.29 As seen from the energy alignment diagram in Figure 3(d), since the CB of PCDTBT (3.6 eV) is higher than the CB of TiO2 (4.15 eV) and CB of CdS (3.9 eV), there is finite probability of electron being injected from the photo-excited PCDTBT into both TiO2 and CdS. However the |ΔECB| is 0.55 eV at PCDTBT/TiO2 interface and 0.3 eV at PCDTBT/CdS interface. The driving force for electron transfer is much higher at the PCDTBT/TiO2 interface compared to PCDTBT/CdS interface. However, the electron injection at the PCDTBT/CdS cannot be ruled out completely and is one of the loss-mechanism of the device. Solar cell performance QDSCs were fabricated using the following photoanodes: TiO2/PCDTBT, TiO2/CdS, TiO2/PCDTBT/CdS and TiO2/CdS/PCDTBT. An aqueous solution of 0.1 M Na2S and 0.1 M KCl served as the electrolyte and C-fabric or C-dots/Cfabric was the CE. To minimize back electron transfer to the electrolyte, and photocorrosion of the QDs, four ZnS layers were applied as passivation layers on the photoanodes in all QDSCs. PCE is calculated using the equation: JSC×VOC×FF/Pinput. In this expression, VOC is the open-circuit voltage, JSC is the short circuit current density, and FF is the fill factor. Pinput is the input light power (100 mW cm-2). The J-V profiles are shown in Figure 5a; the parameters are discussed in the Table 2, and the average values are given in Table S4, and the PCE with error bar plots are shown in Figure S5. The acceptor only cell (TiO2/PCDTBT-S2--C-dots/C-fabric) delivers a PCE of 0.24%, with a VOC = 407 mV, JSC = 1.45 mA cm-2 and FF = 40.7%. The analogous donor only cell (TiO2/CdS-S2--Cdots/C-fabric) delivers a PCE of ~6%. When the CE is substituted with plain C-fabric, the PCE is only 5.3%; a 13.4% increase in PCE is therefore realized by coating the C-fabric with C-dots. The donor-acceptor cell (TiO2/PCDTBT/CdS-S2--Cdots/C-fabric) produces a PCE of 7.42%, and when the CE was sole C-fabric, the PCE is ~6.4%, again illustrating that C-dots improve the charge collection efficiency of the cell. The same cell was run in the reverse scan direction and the resultant J-V profile (Figure S6) is almost similar to the forward scan profile. J-V profile is independent of the scan direction. The same donor-acceptor cell was fabricated by following an altered deposition sequence of the first TiO2, followed by CdS, and the final layer being PCDTBT. The TiO2/CdS/PCDTBT-S2-C-fabric gave a PCE of 4.95%, which was lower by ~22% in contrast to the TiO2/PCDTBT/CdS-S2--C-fabric cell. In


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TiO2/CdS/PCDTBT, upon illumination, two modes of excited electron depopulation are activated. (1) Photo-excited electrons from the CB of PCDTBT injected into the CB of CdS, which further cascade into the CB of TiO2 via favorably aligned energy levels. (2) CdS upon excitation conveys its’ excitation energy to PCDTBT, which undergoes photo-excitation. These FRET generated electrons in PCDTBT follow the process (1). To maximize charge collection via FRET, the acceptor (PCDTBT) should be anchored to the electron transport layer or TiO2. Since it is the donor (CdS) which is attached to TiO2, in this case, PCE decreases. FRET was also confirmed by studying the external quantum efficiency (EQE) variation with wavelength for the cells TiO2/PCDTBT, TiO2/CdS and TiO2/PCDTBT/CdS photoanodes, and C-dots/C-fabric as the counter electrode (Figure 5b). The acceptor only cell with TiO2/PCDTBT photoanode shows a maximum EQE of ~21% at 390 nm and 17% at 580 nm. EQE varies between 5-21% and 5-17% over the wavelength ranges of 350 to 450 nm and 460 to 660 nm respectively. EQE lies in the span of 37-56% over the wavelength range of 320 to 520 nm for the donor only TiO2/CdS cell, and it is highest (56%) at λ = 430 nm. The donor-acceptor cell (TiO2/PCDTBT/CdS-S2--C-dots/C-fabric) shows enhanced EQE values compared to the donor (CdS) and acceptor (PCDTBT) only cells. EQE shows a maximum of 85% at 400 nm. EQE lies between 40 to 85% in the 320 to 550 nm wavelength range. EQE increases slightly to 50% at 610 nm, and gradually decreases to 9% at 670 nm and becomes negligible thereafter. The EQE spectrum of complete cell resembles the absorption spectrum of the TiO2/PCDTBT/CdS photoanode. Significant increments in EQE are manifested in the blue-green and red regions of the visible spectrum. The increase in the blue-green region is due to FRET from CdS to PCDTBT. As a consequence, PCDTBT undergoes charge separation by indirectly absorbing photons to which it is otherwise unresponsive (in the absence of CdS). In the red region, in the presence of CdS, it undergoes enhanced inherent charge separation by directly absorbing the red photons. Electrochemical impedance spectra of solar cells with different photoanodes and Ces and 0.1 M Na2S and 0.1 M KCl as electrolyte were recorded over a frequency range of 1 MHz to 0.1 Hz, at an AC amplitude of 20 mV, and under dark conditions. The Nyquist plots (Figure 5c and d) are recorded under short circuit. The equivalent circuit is provided as an inset of Figure 5d. A distorted semicircle in the high frequency region is followed by a sloping straight line in the low frequency region. The high-frequency intercept (Rb) has contributions from the uncompensated bulk resistance of the electrolyte and the resistance at the contacts. The diameter of the semi-circle corresponds to the electron transport resistance, Rt. Rt for the TiO2/PCDTBT-C-fabric cell is 95 Ω cm2 (Table 3) which is more than that achieved for the same cell with the C-dots/Cfabric CE (90 Ω cm2). Rt for TiO2/CdS-C-fabric cell is 96 Ω cm2 which is higher than that of TiO2/PCDTBT/CdS-C-fabric cell (65 Ω cm2), which implies that faster electron injection into TiO2 is achieved in the donor-acceptor cell, compared to the donor only or acceptor only cells. The advantage of FRET is realized here. Cµ corresponds to the chemical capacitance. Cµ for the TiO2/PCDTBT-C-fabric cell is 31 µF cm-2 and this value is lower than that for the TiO2/CdS-C-fabric (65 µF cm-2) and TiO2/PCDTBT/CdS-C-fabric (86 µF cm-2) cells. For each of the

above cells, Rt decreases further ongoing from C-fabric to Cdots/C-fabric as the CE. TiO2/PCDTBT/CdS-C-fabric cell (65 Ω cm2) shows higher charge injection resistance than theTiO2/PCDTBT/CdS-C-dots/C-fabric cell (51 Ω cm2). Counter electrode studies Morphologies of carbon fabric and C-dots coated C-fabric are shown in Figure 6a and b. The SEM image of C-fabric (Figure 6a), shows a network of criss-cross carbon fibers with smooth surfaces. The width of each fiber is approximately 5 µm, and the lengths of the fibers go up to a few mm. The C-dots/C-fabric CE (Figure 6b), shows that the C-dots are coated over the Cfabric and they tend to aggregate on the surface of C-fibers. The absorption spectrum of C-dots (Figure 6c) shows a broad absorption band in the 200 to 400 nm wavelength range. The corresponding fluorescence spectra at different excitation wavelengths are shown in the same figure. When the wavelength of excitation is gradually enhanced from 350 to 420 nm, the max corresponding to the emission peak redshifts from 450 to 502 nm, and the emission intensity also reduces progressively. This behaviour aligns with the decrease in absorption intensity as a function of wavelength. The Raman spectrum of C-dots is shown in Figure 6d. It shows the two peaks at 1330 and 1586 cm-1, arising from the D and the G bands. The D band is due to the structural defects caused by the covalently bound oxygens to the carbon framework. The G band is due to the stretching vibrations of the sp2 hybridized carbon atoms in plane. The ID/IG ratio is calculated to be 1.15. The efficacy of C-dots as a counter electrode for solar cell is evaluated. I-V plots of C-fabric and C-dots/C-fabric were recorded by contacting two stainless steel point probes over the sample surface. The contacts were separated by 1 cm. The scan rate was 10 mV s-1, and the plots were recorded over a voltage range of 1 to +1 V (Figure 6e). An Ohmic behaviour is registered over an applied potential span of 0.25 to +0.25 V. The slopes acquired from the straight-line fits in the Ohmic domains were employed for calculating the conductance of the CE materials. The relation: I/V = 1/R = G (-1) was used. The conductance’s of C-fabric and C-dots/C-fabric are ~0.02 and ~0.05 -1. C-dots enhance the conduction capability of the CE. This conduction helps with injecting electrons into the electrolyte during cell operation. Nyquist plots and table of symmetric cells of both the CEs are presented in Figure 6f, Table S5. Both plots show two semicircles succeeded by a straight line at low frequencies. Rb is the bulk resistance of the electrolyte, Rct corresponds to the charge transfer resistance at the working electrode/electrolyte interface and Rgb is the grain boundary resistance. Cdl and Cgb represent the double layer capacitances. The charge transfer resistance at the C-dots/C-fabric/electrolyte interface is 1.2  cm2, which is lower than that at the C-fabric/electrolyte interface (2.3  cm2), which explains that C-dots deposited on C-fabric is more effective at bringing about the electrocatalytic reduction of the oxidized sulfide species, in a QDSC. The grain boundary resistance, Rgb at the CE/electrolyte interface for C-dots/Cfabric is lower than that for C-fabric alone. Higher the capacitance, higher is the electrocatalytic activity of CE. The corresponding capacitances for C-dots/C-fabric are higher than that of C-fabric. The slanting line at low frequencies is characteristic of a Z = (1-j)    -1/2 dependence, where,  is the angular



frequency given by 2f, and  is 1/(21/2  Yo). The parameter, Yo, gives the ion-diffusional conductance. Since Yo is larger for the C-dots/C-fabric cell in comparison to the C-fabric cell, iontransport is relatively more facile for the former. Conclusions FRET enabled solar cell with a TiO2/PCDTBT/CdS photoanode, where the narrow band gap polymer, PCDTBT serves as an energy acceptor and the inorganic chalcogenide, CdS acts as the energy donor was coupled with a C-dots/Cfabric based CE. FRET parameters such as the quantum yield of the donor (CdS) was calculated to be 0.3, the Forster radius (or CdS-PCDTBT separation) was ~3.7 nm and the energy transfer efficiency was ~55%. EQE studies and fluorescence quenching and decay analysis confirm energy relay from CdS to PCDTBT, in the form of (1) an enhanced EQE over a wide spectral range spanning from 350 to 550 nm, compared to the donor only cell, (2) a reduced average excited electron lifetime registered for the donor (CdS)-acceptor (PCDTBT) assembly compared to the donor only and acceptor only systems. PCE of the champion cell with the following configuration: TiO2PCDTBT-CdS/S2-/C-dots-C-fabric CE is 7.42% (under 1 sun irradiance). It is superior by ~23 and ~2991% respectively than the analogous donor only (TiO2/CdS) and acceptor only (TiO2/PCDTBT) cells. The CE: C-dots/C-fabric also assists in imparting greater VOC and JSC compared to C-fabric alone because of higher conductivity and rapid electron injection to the electrolyte. This study shows how FRET from a QD to a polymer results in a solar cell performance superior to that of the QD or polymer based cell. Acknowledgments

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EQE measurements were done on a machine bought from the grant: DST/TM/SERI/2K12-11(G). We acknowledge IIT Hyderabad for the TEM characterization. RSSK acknowledges financial support from DST project no. YSS/2015/000008. (12)

SUPPORTING INFORMATION XRD patterns of photoanode materials, Mott-Schottky plots of PCDTBT, cyclic voltammograms profiles and calculation of the valance band and the conduction band positions, fitted fluorescence decay parameters of films/solution, quantum yield calculations of the donor CdS, FRET parameters, J-V and EIS fitted parameters of cell, and PCE with error bars plots. 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: +9140-23016003. REFERENCES (1)

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Yu, W. W.; Peng, X. Formation of High-Quality CdS and Other II–VI Semiconductor Nanocrystals in Noncoordinating Solvents: Tunable Reactivity of Monomers. Angewandte Chemie International Edition 2002, 41 (13), 2368–2371. Beard, M. C. Multiple Exciton Generation in Semiconductor Quantum Dots. J. Phys. Chem. Lett. 2011, 2 (11), 1282–1288. Chang, C.-H.; Lee, Y.-L. Chemical Bath Deposition of CdS Quantum Dots onto Mesoscopic TiO2 Films for Application in Quantum-Dot-Sensitized Solar Cells. Appl. Phys. Lett. 2007, 91 (5), 053503. Lee, W.; Min, S. K.; Dhas, V.; Ogale, S. B.; Han, S.-H. Chemical Bath Deposition of CdS Quantum Dots on Vertically Aligned ZnO Nanorods for Quantum Dots-Sensitized Solar Cells. Electrochemistry Communications 2009, 11 (1), 103– 106. Pan, Z.; Zhao, K.; Wang, J.; Zhang, H.; Feng, Y.; Zhong, X. Near Infrared Absorption of CdSexTe1–x Alloyed Quantum Dot Sensitized Solar Cells with More than 6% Efficiency and High Stability. ACS Nano 2013, 7 (6), 5215–5222. 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. 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 (37), 14194–14203. Kokal, R. K.; Deepa, M.; Kalluri, A.; Singh, S.; Macwan, I.; Patra, P. K.; Gilarde, J. Solar Cells with PbS Quantum Dot Sensitized TiO2–Multiwalled Carbon Nanotube Composites, Sulfide-Titania Gel and Tin Sulfide Coated C-Fabric. Phys. Chem. Chem. Phys. 2017, 19 (38), 26330–26345. Kim, T.; Firdaus, Y.; Kirmani, A. R.; Liang, R.-Z.; Hu, H.; Liu, M.; El Labban, A.; Hoogland, S.; Beaujuge, P. M.; Sargent, E. H.; Amassian, A. Hybrid Tandem Quantum Dot/Organic Solar Cells with Enhanced Photocurrent and Efficiency via Ink and Interlayer Engineering. ACS Energy Lett. 2018, 3 (6), 1307–1314. Gao, Y.; Patterson, R.; Hu, L.; Yuan, L.; Zhang, Z.; Hu, Y.; Chen, Z.; Teh, Z. L.; Gavin Conibeer; Huang, S. MgCl2 Passivated ZnO Electron Transporting Layer to Improve PbS Quantum Dot Solar Cells. Nanotechnology 2019, 30 (8), 085403. Pei, Q.; Chen, Z.; Wang, S.; Zhang, D.; Ma, P.; Li, S.; Zhou, X.; Lin, Y. PbS Decorated Multi-Walled Carbon Nanotube/Ti Mesh Films as Efficient Counter Electrodes for Quantum Dots Sensitized Solar Cells. Solar Energy 2019, 178, 108– 113. Mor, G. K.; Basham, J.; Paulose, M.; Kim, S.; Varghese, O. K.; Vaish, A.; Yoriya, S.; Grimes, C. A. High-Efficiency Förster Resonance Energy Transfer in Solid-State Dye Sensitized Solar Cells. Nano Lett. 2010, 10 (7), 2387–2394. Yum, J.-H.; Hardin, B. E.; Hoke, E. T.; Baranoff, E.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Torres, T.; McGehee, M. D.; Grätzel, M. Incorporating Multiple Energy Relay Dyes in Liquid Dye-Sensitized Solar Cells. ChemPhysChem 2011, 12 (3), 657–661. Choi, S.; Jin, H.; Bang, J.; Kim, S. Layer-by-Layer Quantum Dot Assemblies for the Enhanced Energy Transfers and Their Applications toward Efficient Solar Cells. J. Phys. Chem. Lett. 2012, 3 (23), 3442–3447. Grancini, G.; Sai Santosh Kumar, R.; Maiuri, M.; Fang, J.; Huck, W. T. S.; Alcocer, M. J. P.; Lanzani, G.; Cerullo, G.; Petrozza, A.; Snaith, H. J. Panchromatic “Dye-Doped” Polymer Solar Cells: From Femtosecond Energy Relays to Enhanced Photo-Response. J. Phys. Chem. Lett. 2013, 4 (3), 442–447.


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Scheme 1 Cartoon illustrating the preparation of photoanode and cell fabrication.


8

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Table 1. Solar cell parameters of cells with a 0.1 M Na 2S and 0.1 M KCl as electrolyte, exposed cell area: 0.13 to 0.15 cm2, under 1 sun illumination (100 mW cm-2) with a TiO2/PCDTBT photoanode with different concentrations of PCDTBT and Cfabric as the CE. x (PCDTBT) mg mL-1

VOC (mV)

JSC (mA cm-2)

FF

best (%)

avg (%)

2

316

1

42

0.13

0.12 ± 0.01

4

324

1.1

39

0.14

0.13 ± 0.01

6

336

1.3

39

0.17

0.16 ± 0.01

8

383

1.43

42

0.23

0.22 ± 0.01

10

353

1.33

38

0.18

0.16 ± 0.02

Table 2. Solar cell parameters of cells with a 0.1 M Na2S and 0.1 M KCl as an electrolyte, exposed cell area: 0.13 to 0.15 cm2, under 1 sun illumination. Photoanode configuration

Counter Electrode

VOC (mV)

JSC (mA cm-2)

FF

best (%)

avg (%)

TiO2/PCDTBT

C-fabric

383

1.43

42

0.23

0.220.01

C-dots/C-fabric

407

1.45

40.7

0.24

0.230.01

C-fabric

791

17.4

38.5

5.3

5.20.1

C-dots/C-fabric

820

19.23

38.1

6

5.80.2

C-fabric

857

20.32

36.7

6.4

6.30.1

C-dots/C-fabric

877

25.3

33.44

7.42

7.320.1

C-fabric

776

17.25

37

4.95

4.90.05

C-dots/C-fabric

807

17.8

38.3

5.5

5.40.1

TiO2/CdS

TiO2/PCDTBT/CdS

TiO2/CdS/PCDTBT

Table 3. EIS parameters of solar cells with a 0.1 M Na2S and 0.1 M KCl electrolyte under dark. Photoanode

TiO2/PCDTBT

TiO2/CdS

TiO2/PCDTBT/CdS

Counter Electrode

Rb

Rt

( cm2)

( cm2)

C (µF cm-2)

Yo (S s1/2)

C-fabric

13

95

31

9.4×10-3

C-dots/C-fabric

12.5

90

85

7.6×10-3

C-fabric

13.1

96

64.8

5×10-3

C-dots/C-fabric

11.7

54

63.2

5×10-3

C-fabric

11.1

65

86

6×10-3

C-dots/C-fabric

10.5

51

41.6

5×10-3


9


Page 10 of 16

Figures

Figure 1 SEM images of (a, b) TiO2, and (c,d) TiO2/PCDTBT/CdS electrodes. Cross-sectional images of (e, f) TiO2/PCDTBT/CdS films at distinct magnifications. The white colored (a,b) and the red colored dotted (c,d) circles or ellipses confines the pores in the films.


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Figure 2 TEM images of (a-c) C-dots at low and high magnifications. TEM images of (d-h) TiO2/PCDTBT/CdS, (i-j) SAED patterns of TiO2/PCDTBT/CdS assembly. The doted eclipses/circles enclose crystallites of TiO2 (red), PCDTBT (light blue) and CdS (yellow).


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Figure 3 (a) Current density-voltage (J-V) characteristics of QDSCs with TiO2/PCDTBT photoanodes under a uniform solar irradiance (AM 1.5G, 100 mW cm-2) were presented with varying concentration (2, 4, 6, 8, 10 mg mL-1) of PCDTBT loadings; a C-fabric was used as the counter electrode. Absorption spectra of (b) TiO2, PCDTBT, CdS, PCDTBT/CdS films and (c) TiO2/PCDTBT, TiO2/CdS, TiO2/PCDTBT/CdS films on glass substrates. (d) Energy band diagram of the photoanode.


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Figure 4 (a) Fluorescence spectra of CdS, TiO2/PCDTBT, TiO2/CdS, PCDTBT/CdS and TiO2/PCDTBT/CdS recorded at λex = 370 nm. Time resolved fluorescence decay traces of (b) PCDTBT, CdS, TiO2/PCDTBT, TiO2/CdS and (c) PCDTBT/CdS and TiO2/PCDTBT/CdS recorded at λex = 370 nm, and at the specified emission wavelengths. (d) Integrated fluorescence intensity versus absorbance of donor (CdS, λex = 370 nm) and reference (Rhodamine 6G, λex = 367 nm). (e) Absorption spectrum of acceptor (PCDTBT) and emission spectrum of the donor (CdS, λex = 370 nm).


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Figure 5 (a) J-V characteristics of QDSCs under 1 sun (100 mW cm-2) illumination (AM 1.5G) with: TiO2/PCDTBT (P), TiO2/CdS (C), TiO2/PCDTBT/CdS (P/C), TiO2/CdS/PCDTBT (C/P) and different CEs: C-dots/C-fabric (C/C) and C-fabric (C). (b) EQE measurements of devices with individual photoanodes: TiO2/PCDTBT, TiO2/CdS and TiO2/PCDTBT/CdS and C-dots/C-fabric as the CE. EIS spectra of QDSCs with different photoanodes: TiO2/PCDTBT, TiO2/CdS and TiO2/PCDTBT/CdS, and with: (c) C-fabric and (d) C-dots/C-fabric as the CEs, recorded under dark; the corresponding equivalent circuit is displayed in (d). All measurements were performed with a 0.1 M Na2S + 0.1 M KCl solution as the electrolyte.


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Figure 6 SEM morphologies of (a) C-fabric and (b) C-dots/C-fabric CEs. (c) Absorption and fluorescence spectra of C-dots (λex = 350, 360, 370, 380, 390, 400, 410 and 420 nm). (d) Raman spectrum of C-dots. (e) I-V profiles of C-fabric and C-dots/C-fabric using a two-probe DC measurement. (f) Nyquist plots for C-fabric-S2--C-fabric and C-dots/C-fabric-S2--C-dots/C-fabric in symmetric measurements; inset of (f) shows an equivalent Randles circuit.


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Graphical Abstract


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Quantum Dot DonorâPolymer Acceptor Architecture for a FRET

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