Modulating FRET in OrganicâInorganic Nanohybrids for Light

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Modulating FRET in Organic-Inorganic Nanohybrids for Light Harvesting Applications Arun Gopi, Sivasankaran Lingamoorthy, Suraj Soman, Karuvath Yoosaf, Reethu Haridas, and Suresh Das J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09867 • Publication Date (Web): 28 Oct 2016 Downloaded from http://pubs.acs.org on November 1, 2016

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The Journal of Physical Chemistry

Modulating

FRET

in

Organic-Inorganic

Nanohybrids for Light Harvesting Applications Arun Gopi,ab Sivasankaran Lingamoorthy,a Suraj Soman,*abc Karuvath Yoosaf,*abc Reethu Haridasab and Suresh Das*d a

Photosciences and Photonics Section, Chemical Sciences and Technology Division, CSIR-

National Institute for Interdisciplinary Science and Technology, Thiruvananthapuram 695 019, Kerala, India. E-mail: [email protected]; [email protected].

b

Academy of Scientific and

Innovative Research (AcSIR), New Delhi 110001, India. cCSIR-Network of Institutes for Solar Energy (CSIR-NISE), India. dKerala State Council for Science Technology and Environment, Sasthra

Bhavan,

Pattom,

Thiruvananthapuram

695004,

Kerala,

India.

E-mail:

[email protected].

Abstract: The energy transfer efficiencies of organic-inorganic nanohybrids comprising of two structurally similar squaraine dyes and CdSe nanoparticles were studied in detail and compared. Carbazole based unsymmetrical squaraine dyes (CTSQ-1 and CTSQ-2) having modified absorption characteristics were considered for modulating the effect of overlap integral on energy transfer rate with the designed QDs. CTSQ-2 with ~ 1.75 times higher molar extinction coefficient and 35 nm red-shift in absorption resulted ~ 2.4 times faster energy transfer rate with QD. The calculated energy transfer rates (kT =1.35 x 108 s-1 and 3.26 x 108 s-1 respectively for QD:CTSQ-1 and QD:CTSQ-2 nanohybrids) are at least one order of magnitude

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higher than both radiative (kr = 5.97 x 106 s-1) and nonradiative decay rate constants (knr =1.89 x 107 s-1) of QDs

yielding very high FRET efficiency. The Stern-Volmer analysis of the

quenching data indicated mainly static interaction of dyes with the QDs thus suggesting formation of organic-inorganic nanohybrids. When incorporated in dye-sensitized solar cells, the nanohybrids with 93 % FRET efficiency, exhibited an overall 43 % improvement in the photovoltaic performance. Among the two architectures employed for device fabrication the one with smallest donor-acceptor distance delivered the best performance. Due to increased contribution from QDs, the IPCE spectra clearly indicate panchromatic response from visible to NIR region. Thus photovoltaic performance of NIR absorbing dyes were successfully improved by constructing panchromatic organic-inorganic nanohybrid materials.

Introduction The quest for clean and renewable energy has resulted in immense growth and many breakthroughs in the research of photovoltaic technologies.1-5 Due to its simplicity of design, architecture and cost effectiveness, dye-sensitized solar cells (DSSC) has emerged as one of the leading 3rd generation photovoltaic technologies.6-9 At the heart, the working principle of DSSC is a photosensitizer that absorbs light and transfers electron to the conduction band of a metal oxide semiconductor; the dye gets regenerated by accepting an electron from the electrolyte which is then regenerated by the catalytic redox reaction at the counter electrode.10-12 Over the years several new design strategies have been practiced to improve the solar cell performance and efficiencies as high as 14 % have been achieved with careful molecular engineering.13-21 Squaraine dyes are one of the interesting classes of organic dyes which possess many desirable characteristics for solar cell application.22-24 These include simplicity in molecular structure, ease


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of synthesis, high molar extinction coefficient and strong NIR absorption. However, DSSCs fabricated out of these dyes suffer from low photovoltaic performance and is mainly attributed to their narrow absorption spectrum limiting the light harvesting to a specific solar spectral window. One of the possibilities to overcome this is by undertaking molecular engineering on designed squaraine dyes or by selecting an auxiliary absorber which can either directly contribute to the photocurrent or to improve the performance through sequential electronelectron or energy-electron transfer processes.25-31 Quantum dots (QDs) are tiny nanocrystals having strong absorption in the visible region, high molar extinction coefficient, large intrinsic dipole moment, quantum confined and tunable band gap.32-35 These when coupled with organic chromophores results in new types of materials often referred to as organic-inorganic nanohybrids. They possess advantages of both the components and are viable candidates for the new generation photovoltaics.36-39 Compared to dye alone DSSCs, they show enhanced light harvesting efficiency and usually contribute to the improved solar cell performance.40 However the underlying mechanism can vary widely depending upon the type and combination of materials employed. For example, Kamat et al. has identified the role of interfacial electron transfer as the contributing factor when near-infrared squaraine dye is used for supersensitization of CdS Quantum Dots.41 Whereas thiolated gold nanoclusters act both as a photosensitizer and voltage booster when employed in conjunction with the same dye.42 Differently, the direct covalent linking of phthalocyanine to CdS or CdSe QDs is found to be a good approach for reducing the surface trap states and thus reducing recombination centers leading to increased photocurrent.43 Among the various methods employed, the most popular one being Fluorescence Resonance Energy Transfer (FRET) followed by electron transfer process.44,45 However studies have shown that there are many determining factors in FRET


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enabled systems and there are instances in which efficiency is reduced. The investigations on the effect of QD size on FRET based hybrid DSSCs with D719 dye reported a maximum 14 % improvement in power conversion efficiency (PCE) with largest spectral match combination whereas decrease in PCE is observed when energy transfer efficiency is less.46 Similarly, M. G. Bawendi et al. investigated the effect of two differently sized quantum dots (CdSe495 and CdSe545; number refers to the emission wavelength maximum) on the performance of DSSCs fabricated using dyes N719 or TBTCA. The solar cell efficiency was boosted only in the combination of smaller QDs with TBTCA dye whereas it got adversely affected for the rest of the three.47 These studies show a mere occurrence of FRET alone cannot cause PCE enhancement and there need to be more efforts to understand this complex phenomenon for achieving a rational design of solar cells with improved performance. In one of the earlier attempts Suraj et al. had presented a detailed insight into the need for band gap tuning through molecular engineering leading to improved electron injection and better performance for carbazole based unsymmetrical squaraine sensitizers.25 However, this had to compromise with the lower absorption coefficient. In the present study, we constructed organicinorganic nanohybrids of these interesting NIR absorbing dyes with CdSe quantum dots and thoroughly investigated the various parameters of Förster Resonance Energy Transfer (FRET) occurring in these systems. Further an organic-inorganic nanohybrid solar cell was fabricated such that CdSe quantum dots absorb light and transfers energy to the sensitizing dye. This FRET process results in enhanced dye excited state population thereby prompting more electron injection into the conduction band of TiO2.48 In order to have efficient energy transfer it is necessary to tune spectral match between donor emission and acceptor absorption and the distance between them. Here in we show that how the spectral overlap and molar extinction


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coefficient of the dyes largely influence the FRET processes and hence the performance of organic-inorganic nanohybrid solar cells. The judicious selection of squaraine dyes and QDs helped us in achieving more than 90 % FRET efficiency and thus panchromatic spectral response. This led to 43 % improvement in photovoltaic performance of the nanohybrid device in comparison to the dye alone solar cell.

Experimental Section Materials and methods: Chemicals such as cadmium oxide, oleylamine, selenium powder, oleic acid, trioctylphosphine (TOP) and octadecene were purchased from M/s Sigma Aldrich. HPLC grade solvent such as dichloromethane (DCM), chloroform (CHCl3), ethanol, toluene, methanol (MeOH), acetone and isopropanol were procured from M/s Merck Pvt. Ltd. All chemicals were used as received without further purification and experiments were performed using standard glass apparatus, unless otherwise stated. Stock solutions were prepared in dichloromethane and titration experiments were performed by adding aliquot amounts to 3 mL (1 cm) quartz cuvette. UV-visible absorption spectra were recorded on either Shimadzu UV-2401PC or UV-2600 spectrophotometers. Fluorescence measurements were carried out using Fluorolog-3 FL3-221 spectrofluorimeter equipped with 450 W Xenon arc lamp (Horiba Jobin Yvon). Excitation spectra were taken by collecting the emission at 725 nm and the excitation wavelength selected for emission spectra was 440 nm, unless otherwise specified. Fluorescence quantum yields were calculated by relative method49,50 (using cresyl violet in ethanol; f = 0.54 as standard).51 IBH (Fluorocube) time-correlated single photon counting (TCSPC) system was used for determining fluorescence lifetimes. All optical measurements were performed at 23 °C under ambient conditions.


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For transmission electron microscopy (TEM) studies, samples were drop casted on a carbon coated Cu grid and imaged with FEI 300 kV high resolution transmission electron microscope (FEI-Tecnai G2-30 with EDAX) equipped with Gatan camera. Data were analysed using Gatan’s digital micrograph software. For XRD, samples were coated onto a precleaned glass plate and data were acquired using a Panalytical PW 3040/60 X'Pert Pro powder diffractometer with Cu K1 radiation. Zetasizer nanoseries (Zeta Nano-ZS, Ms. Malvern Instruments) was used for Dynamic light scattering (DLS) experiments. To ensure statistical significance at least five measurements were taken for each sample. Cyclic voltammetry analysis of CdSe QDs was carried out by BAS CV-50W instrument using 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) in DCM as the supporting electrolyte, glassy carbon working electrode, Ag/AgCl reference electrode, and platinum wire counter electrode at room temperature and argon atmosphere with a scan rate of 30 mV/s. Synthesis of CdSe Quantum dots: Oleylamine capped CdSe QDs with average size in the range of 5 - 6 nm and corresponding first excitonic absorption peak at 615 nm and emission maximum at 635 nm were prepared according to literature method.52,53 In a typical procedure, selenium powder (0.166 g, 2 mmol) in oleylamine (OAm) (20 mL, 61 mmol), and trioctylphosphine (1 mL, 2 mmol) were heated at 80 oC under nitrogen atmosphere with stirring. Temperature of this solution was then raised up to 300 oC in vacuum condition and kept for 5 minutes. Cadmium precursor was prepared by refluxing cadmium oxide (0.256 g, 2 mmol), oleic acid (2.5 mL, 8 mmol) and octadecene (2.5 mL, 8 mmol) at 280 oC in nitrogen atmosphere. When this solution became colourless, it was quickly injected into the selenium precursor maintained at 300 oC. Afterwards the reaction temperature was maintained at 280 oC for subsequent growth and annealing of nanocrystals. The growth of the nanocrystals was monitored visually as well as


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spectroscopically. When the QDs attained desired optical properties the growth was arrested by cooling to room temperature. They were purified by repeated precipitation and centrifugation in toluene-methanol mixture. The purified oleylamine capped CdSe QDs were dissolved in DCM and used for further studies. Fabrication and characterization of DSSC devices: FTO glass plates (TEC15, thickness, 2.2 mm; 15 Ω/cm2) obtained from Dyesol were initially cleaned by a stepwise procedure. This involves sonication with detergent solution, successive washing with distilled water, acetone and isopropanol followed by UV-O3 treatment. The plates were then immersed in 40 mM aqueous TiCl4 solution at 70 oC for 30 min, washed with distilled water and ethanol and annealed at 500 o

C for 30 min. These plates were doctor bladed with 20 nm sized TiO2 paste (Dyesol) and

annealed at 125 oC for 10 min to obtain a transparent nanocrystalline film (Thickness ~10 µm). These electrodes were then again subjected to another TiCl4 treatment followed by programmed heating at 325 oC for 15 min, 450 oC for 15 min, and 500 oC for 30 min. The final thickness of the film was measured (using Bruker’s Dektaxt profilometer) to be 12 μm. For device architecture C, the sintered plates were slowly cooled to 70 oC and CdSe QDs are first spin coated which are then soaked in squaraine dye solution (CHCl3/MeOH (1:1) along with 10 mM chenodeoxycholic acid (CDCA). In case of architecture D, the TiO2 electrodes are immersed in dye solution (Squaraine + CDCA), kept at room temperature for 12 h. The CdSe QD solution was then spin coated on top of the dye adsorbed TiO2 film. Pre-drilled and cleaned counter electrodes were coated with a drop of H2PtCl6 solution (2 mg of Pt in 1 mL of ethanol). The dye adsorbed TiO2 electrodes and Pt counter electrodes were assembled with a hot press using surlyn spacer. I-/I3- liquid electrolyte (0.6 M 1-butyl-3-methylimidazolium iodide, 0.03 M I2 and 0.1 M


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LiI in acetonitrile) were filled in space between the electrodes. The drilled holes were sealed with a microscopic cover slide and surlyn to avoid electrolyte leakage. The photocurrent-voltage (J–V) measurements of the fabricated devices were measured using an AM 1.5 solar simulator (Newport Instruments, USA) which makes use of a 450 W Xenon lamp equipped with a Keithley 2400 digital source meter. To reduce the mismatch between the simulated light and AM 1.5 simulated sunlight a reference Si solar cell (supplied by Newport instruments) was used for calibration. The incident photon-to-current conversion efficiency (IPCE) measurement of the devices was performed under DC mode using a 250W Xenon lamp coupled with Newport monochromator. The J-V properties of cells were measured by using square shade mask with active area 0.25 cm2 (without mask active area is 0.36 cm2).

Results and discussion The organic dyes selected for the present study includes unsymmetrical squaraine dyes differing only by the substitution of t-butyl group, CTSQ-1 and CTSQ-2 (Figure 1A). Both of them displayed absorption in the range of 500 nm to 700 nm (Figure 1B) with maximum around 610 nm and 645 nm respectively. Their corresponding fluorescence maxima in DCM are 670 nm and 690 nm. i.e. squaraine dye with tertiary butyl group present on the carbazole moiety termed as CTSQ-2 exhibited 35 nm and 20 nm red shift in the absorption and emission bands respectively as compared to the unsubstituted derivative, CTSQ-1. The molar extinction coefficient (ε) at their absorption maxima is estimated to be 3.14 × 104 M−1 cm−1 and 5.64 × 104 M−1 cm−1 respectively. However, it has to be noted that both these dyes have considerably low absorption characteristics below 500 nm. Thus the main rationale for the selection of system includes (i)


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their structural similarity, (ii) shift in absorption maximum, (iii) ~ 1.8 times difference in molar extinction coefficient and (iv) negligible absorbance in 350−500 nm spectral region.

Figure 1. (A) Molecular structures (B) corresponding absorption and emission spectra of squaraine dyes CTSQ-1 and CTSQ-2, (C,D) TEM images (E) SAED, (F) XRD and (G) absorption (blue) and emission (red) spectrum of oleylamine capped CdSe QDs; Emission spectra were collected after exciting at 440 nm. In order to tap the full potential of energy transfer, we carefully chose the right sized CdSe quantum dots whose emission profile matches with the absorption spectra of the squaraine dyes. Both TEM and DLS analysis showed that synthesized CdSe nanoparticles have an average


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size of ~ 5.5 nm with narrow size distribution (Figures 1C, 1D and S1 - S3, ESI). The d spacing values were extracted to be 0.18 ± 0.01 nm, 0.34 ± 0.01 nm and 0.21 ± 0.01 nm from the HRTEM and XRD measurements (Figures 1E and 1F) which confirms the wurtzite crystal structure of the QDs. Table 1. Summary of the photophysical and electrochemical properties of squaraine dyes and QDs in dichloromethane. λabs

max

λem

(nm)

(M-1cm-1)

(nm)

CdSe

615

5.2 x 105

635

CTSQ-1

610

3.2 x 104

CTSQ-2

645

5.6 x 104

em (ns)

EVB

ECB

Eg

(NHE)

(NHE)

(NHE)

40.2

+1.35

-0.62

1.97

670

0.6

+1.19

-0.74

1.93

690

1.4

+1.21

-0.65

1.86

The conduction band (CB) and valence band (VB) energies of these QDs were estimated to be -0.62 V and +1.35 V vs. NHE respectively. A comparison of the photophysical and electrochemical properties of the QDs with that of dyes is summarized in Table 1. The QDs exhibit broad excitonic spectra in the UV-Vis region with maximum centered around 615 nm (Figures 1G and S4, ESI). They possess very high molar extinction coefficient (~ 5 x 105 M-1cm1

) which is at least one order of magnitude higher than that of dyes. Thus, they can perform as a

better solar light harvester. Their fluorescence profile is very narrow and lies in the range 600 680 nm with FWHM value of around 30 nm and has a quantum yield of 23%. Thus, the emission spectrum of QDs matches well with the absorption of selected squaraine dyes and constitute a suitable FRET pair.


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The rate of energy transfer between a donor and acceptor can be expressed as54

( )

(



(

)

) ( )

( )

Where QD and D are respectively the fluorescence quantum yield and lifetime of donor (CdSe QDs) in the absence of acceptor (squaraine dye). K2 is the factor describing relative orientation in space of transition dipoles of donor and acceptor whose value can be approximated to 2/3 for dynamic random orientation. n is the refractive index of the medium, N is the Avagodro’s number and r is the distance between donor and acceptor. J(λ) is the degree of spectral overlap between the donor emission and the acceptor absorption. Thus for specific donor-acceptor pair the major factors affecting the efficiency of FRET are (i) the extent of overlap between the donor emission and acceptor absorption, (ii) the distance between donors and acceptors and (iii) the orientation of transition dipole moment of donor and acceptor system. In the present case, both the squaraine dyes have similar structure and their binding with QDs is expected to occur via carboxylic acid resulting in similar r value. Therefore the only factor which affects the rate and hence the efficiency of energy transfer is spectral overlap which can be expressed as

( )

( ) ( )

∫ ∫

( )

( ) ( )

( )

Where FD(λ) and A(λ) are the fluorescence intensity of the donor and molar extinction coefficient of the acceptor respectively at the wavelength, λ. Figure 2 illustrates the degree of overlap between the donor (QDs) emission with the acceptor (squaraine dyes) absorbance. It could be noticed that, due to its narrowness and position of the wavelength maximum, the


11


fluorescence profile of the QDs is completely engulfed within the absorption profile of the highly absorbing dye. Assuming equal extinction coefficient for both dyes, the 35 nm redshift in the absorption maximum will account for 1.4 times difference in spectral overlap. This along with 1.75 times better extinction coefficient, CTSQ-2 will have more degree of spectral overlap (~2.4 times) and hence have a larger possibility of energy transfer from the CdSe QDs. In fact, the value for J(λ) estimated using the equation (2) yielded values 3.44 x 1016 M-1 cm-1 nm4 and 8.31 x 1016 M-1 cm-1 nm4 for CTSQ-1 and CTSQ-2 dyes respectively. Due to their similar molecular structure it could be assumed that both the dyes will have same separation and orientation of dipoles with the QDs. Thus, as per equation 1, one could conclude that the excited state energy is transferred almost 2.4 times faster to CTSQ-2 than CTSQ-1. The expected Forster distance for the QD:CTSQ-1 and QD:CTSQ-2 nanohybrids were estimated to be 47.7 Å and 55.3 Å respectively using the equation (3)

15

-1

-1

(B)

4

1.0

0.5

0.5

600 Wavelength (nm)

700

Norm. Absorbance

1.0

500

( )

J() = 3.44 x 10 M cm nm

Norm. Fl. Int. (a.u.)

(A)

( ))

15

-1

-1

4

J() = 8.31 x 10 M cm nm

1.0

1.0

0.5

0.5

0.0 500

Norm. Fl. Int. (a.u.)

(

Norm. Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.0 550

600

650

700

Wavelength (nm)

Figure 2. Spectral overlap between the emission spectrum of QDs (red) with the absorption spectra (black) of squaraine dyes (A) CTSQ-1 and (B) CTSQ-2.


12

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This implies that for QD:CTSQ-2 combination 50 % energy transfer can occur at 7.6 Å more longer distance than that compared to QD:CTSQ-1 hybrid. Or in other words, for a fixed donoracceptor distance, which is the case in rigid bound systems, the energy transfer efficiency will be the highest for QD:CTSQ-2 combination. Table 2. Calculated FRET parameters for the two organic-inorganic nanohybrid combinations Hybrid Systems

J(λ) (M-1 cm-1 nm4)

R0 (‎Å)

r (‎Å)

kT (s-1)

E (%)

QD:CTSQ-1

3.44 x 1016

47.7

36.0

1.35 x 108

84

QD:CTSQ-2

8.31 x 1016

55.3

36.0

3.26x 108

93

In order to get an actual scenario on these current systems, titration experiments were performed with QDs in presence of squaraine dyes. The addition of either CTSQ-1 or CTSQ-2 into quantum dot solution caused raise in absorbance in the 500 – 700 nm range. On the contrary there were negligible changes below 500 nm owing to their very low absorbance (Figures S5 and S6, ESI). This allowed us to choose a selective excitation wavelength wherein the donor, QDs, alone absorbs. At the same time, presence of both the dyes caused a decrease in the emission intensity of QDs along with the concomitant emergence of squaraine centered emission (Figures 3A and S5, ESI) suggesting the occurrence of FRET process. This was further supported by the reverse titration experiments, wherein addition of CdSe nanoparticles into squaraine solution resulted in enhanced quantum dot centered absorption and dye centered emission (Figures S7 and S8, ESI). A clear proof for the occurrence of FRET was obtained through both excitation spectra and lifetime analysis. The excitation spectra collected by monitoring emission at 750 nm exhibited considerable contribution from QD absorption (Figures 3B and S9, ESI). Similarly, the lifetime analysis showed a continuous decrease in lifetime for QD fluorescence and an increase


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(A) 3 8.3 M

2

[CTSQ-2] 0 M

1

600

4 2 0

650 700 750 Wavelength (nm)

(C)

CTSQ-2 CTSQ-2 QD Hybrid QD

6

Fl. Intensity (a.u.)

Fl. Intensity (a.u.)

(B)

0

400

500 600 Wavelength (nm)

700

(D)

4000

1.0

2000

0.5 0

100

50

100

150

200

250

Time (ps)

W 0

100

200 Time (ns)

300

0.0

600

av el en

(a.u.)

1000

10000 8000 6000

Fl. Int.

Counts

10000

Counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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120 90

650

gt h

(n

m )

60 700

30 0

e m Ti

s) (p

Figure 3. (A) Emission spectral changes of 0.34 µM CdSe QDs in presence of 0-8.3 µM CTSQ-2 (B) Excitation spectra of CTSQ-2, QDs, and QD:CTSQ-2 hybrid collected by monitoring emission at 750 nm (C) Fluorescence lifetime changes of 0.24 µM QDs in presence of 0-3 µM CTSQ-2. Data were collected by exciting at 440 nm and monitoring emission at 610 nm; Inset shows the raise component for the squaraine centered emission (em = 670 nm) from the hybrid. (D) Time resolved emission spectral changes of QD:CTSQ-2 hybrids.

for squaraine centered fluorescence (Figures 3C, S10 and S11, ESI). The lifetime curve of the hybrid recorded at 670 nm shows an initial fast raise component (inset of Figure 3C and S12, ESI) clearly suggesting the formation of dye excited state via FRET process. A more conclusive evidence for the FRET was obtained from time resolved emission spectral measurements. Following the excitation with a pulsed laser at 440 nm a rapid decrease in the intensity of QD centered fluorescence was observed with concomitant raise of squaraine centered emission (Figure 3D). The raise time was found to be around 120 ps for QD:CTSQ-2 system. Even though


14

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we tried similar experiments with CTSQ-1 closeness of the donor and acceptor emission bands restricted to draw any conclusive inference. Further to understand the nature of quenching and to check the possibility of organic-inorganic nanohybrid formation the fluorescence quenching data were further analysed. In both cases the Stern-Volmer plots extracted from emission intensity changes showed an upward curvature (Figures 4A and S13, ESI) suggesting the occurrence of both static and dynamic processes in the current FRET.

Figure 4. (A) Stern-Volmer and (B) modified Stern-Volmer plots showing the variation of fluorescence intensity as a function of acceptor concentration; (C) Stern-Volmer plots derived from fluorescence lifetime changes of QDs in presence of CTSQ-2 and (D) schematic representation of organic-inorganic nanohybrids.


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However, the collisional part of the process can be extracted from the plot of 0/τ vs. squaraine dye concentration (Figures 4C and S14, ESI) which gave a dynamic quenching constant of KD = 0.102 x 106 M-1. From this, bimolecular quenching constant (kq) is estimated to be 2.54 x 1012 M1 -1

s using the equation (KD = kq x τ0). Such a high value suggests that the organic dye must be

bound to the quantum dot surface. Finally using the modified Stern-Volmer plots depicted in Figures 4B and S13, ESI the static component is estimated to be in the range of ~1-2 x 106 M-1. Thus, these studies gave a clear indication that squaraine dyes can bind to the surface of QDs and form an organic-inorganic nanohybrid material as is represented in Figure 4D. By taking to account of (i) optimized molecular structure, (ii) average of nanoparticles as 5.5 nm and (iii) assuming that the molecule bind orthogonal to the QD surface the donor acceptor distance in the organic inorganic hybrids is estimated as 36 Å. Therefore, in the present case, the rate of Forster Resonance Energy Transfer (kT) is calculated to be 1.35 x 108 s-1 and 3.26 x 108 s-1 respectively for QD:CTSQ-1 and QD:CTSQ-2 nanohybrids. These values are at least one order of magnitude higher than the both radiative and nonradiative decay rate constants of QDs (kr = 5.97 x 106 s-1 and knr =1.89 x 107 s-1). Thus it is clear that energy transfer can effectively compete with these processes thus yielding a very high FRET efficiency of ~ 84 % and 93 % respectively for QD:CTSQ-1 and QD:CTSQ-2. In the present case each quantum dots can interact with multiple number of squaraine acceptor molecules. In such cases, the FRET efficiency can be expressed as

Accordingly it can be estimated that for having the same FRET efficiency as that of CTSQ-2 approximately two CTSQ-1 dyes have to interact with QDs. In fact it is noted that under similar


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concentrations, CTSQ-1 is only half efficient as that of CTSQ-2 to have the same fluorescence quenching.

Scheme 1. (A, B) Energy level diagram illustrating concerted energy and electron transfer process in (A) TiO2/CdSe/CTSQ-1 and (B) TiO2/CdSe/CTSQ-2 hybrid systems and (C, D) schematic representation of two different solar architectures employed in the current work (C) squaraine dyes are adsorbed onto TiO2/CdSe surface and (D) CdSe QDs are spin casted onto TiO2/CTSQ electrodes. Further to utilize these FRET enabled organic-inorganic nanohybrids in a real energy harvesting application and hence to study FRET as a means to improve the performance, dyesensitized solar cell devices were constructed, and the results are presented in Figure 5 and Table 3. Fabrication and characterization of solar cell devices have been described in detail in the


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experimental section. Mainly, two different architectures have been employed for device fabrication as shown in Scheme 1 (C, D). In the first case (C) dyes were adsorbed directly onto CdSe QDs spin casted TiO2 substrate. The second architecture (D) involves spin casting of CdSe QDs on the dye adsorbed TiO2 layers. The former architecture will have the lowest possible donor-acceptor distance (r ≈ 3.6 nm) since squaraine dyes efficiently bind to the QD surface via carboxylic acid group and hence lead to highest energy transfer efficiency. Whereas in the latter case (D) the carboxylic group of dyes are already engaged in anchoring with TiO 2. Thus, direct chemical linkage with QD is not possible, and the presence of bulky capping ligand sterically imposes an increase in donor-acceptor distance by ~ 2 nm. This in effect can reduce the FRET efficiency by a factor of ~ 50%. Since the CdSe QD layer is sandwiched in between the TiO2 and the dye in first architecture, it also acts as a barrier to recombination of electrons from the conduction band (CB) of TiO2 back to electrolyte resulting in improved voltage (Voc increases by 1.89 % and 5.77 % for CTSQ-1 and CTSQ-2 respectively, vide infra). This configuration also has the added advantage of minimizing direct interaction between CdSe QDs and I-/I3- electrolyte thus improving their stability. On the contrary, the second cell structure exposes the QDs directly to the electrolyte environment thereby creating new charge recombination channels. This results in decomposition of QDs thereby leading to defective device performance (Table 3, Figures S15 and S16, ESI). As reported previously, the solar cell efficiencies of 2.94 % and 2.23 % were obtained for devices fabricated using CTSQ-1 and CTSQ-2 squaraine dyes alone.25 Both the dyes when adsorbed in presence of QDs showed an enhancement in its photovoltaic performance (Figure 5 and Table 3). In the case of unsubstituted squaraine dye, CTSQ-1, there was only 1.9 %, 1.89 %


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CTSQ-1 QD QD:CTSQ-1

10

5

0 0.0

(B)

0.2

0.4

CTSQ-1 QD:CTSQ-1

40

IPCE (%)

2

Current density (mA/cm )

(A)

20

0

0.6

400

Voltage (V)

(C)

CTSQ-2 QD QD:CTSQ-2

10

5

0 0.0

0.2 0.4 Voltage (V)

500

600

700

Wavelength (nm)

(D)

0.6

CTSQ-2 QD:CTSQ-2

40

IPCE (%)

2

Current density (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20

0

400

500 600 Wavelength (nm)

700

Figure 5. Photocurrent density-voltage characteristics and incident photon-to-current conversion efficiency (IPCE) spectra of DSSCs comprising of (A, B) CTSQ-1 and (C, D) CTSQ-2 dyes in the presence and absence of CdSe QDs; For comparison corresponding J-V curve of QDs alone (orange) is also provided and 3.08 % increase in Jsc, Voc and FF respectively contributing to slight enhancement (5.44 %) in power conversion efficiency. On the contrary, CTSQ-2, which has more than two times larger spectral overlap performed much better in the presence of QDs. The power conversion efficiency got augmented from 2.23 % to 3.19 % amounting to an overall gain of 43 %. The analysis of the data reveals that a major contribution to this boost arises from enhanced (31.8 %) current


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density. This is in agreement with the IPCE spectra of QD:CTSQ-2 hybrid solar cell (Figure 5D) that showed an

Table 3. Summary of photovoltaic performance data for CTSQ-1 and CTSQ-2, hybrid cells and QD cell under AM 1.5 G illumination. The term delta () represents percentage of enhancement in the photovoltaic performance of hybrid. Device

Jsc

Voc -2

FF

Efficiency

Architecture

(mA cm )

(V)

(%)

CTSQ-1

8.42

0.53

0.65

2.94

CTSQ2

6.76

0.52

0.64

2.23

CdSe QD

0.29

0.56

0.64

0.104

Hybrid Architecture I QD:CTSQ-1

8.58

0.54

0.67

3.1

 (%)

1.9

1.89

3.08

5.44

QD:CTSQ2

8.91

0.55

0.65

3.19

 (%)

31.8

5.77

1.56

43.05

Hybrid Architecture II CTSQ-1+QD

8.29

0.49

0.46

1.89

 (%)

-1.54

-7.55

-29.23

-35.71

CTSQ-2+QD

6.73

0.50

0.51

1.69

 (%)

-0.44

-3.85

-20.31

-24.22

intensified response from 380 nm to 700 nm region. Most probably, this can be attributed to the efficient FRET resulting in cascade electron transfer from the LUMO of the dye to the CB of TiO2. Alternatively, this can also be due to co-sensitization process wherein QDs independently contribute to the photocurrent. In such cases, one would expect to have similar performance in


20

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both the hybrid systems. Therefore, this argument could not substantiate the observed difference in the performance of the solar cells. Moreover, the quantum dot solar cells constructed alone with similar procedure in the absence of dye showed very poor performance having Jsc only 0.29 mAcm-2 and efficiency of 0.104 % which is only 4.3 % of that of CTSQ-2 solar cells. The other probabilities are either a photo-induced sequential electron-electron or energy-electron transfer (FRET) process. The electrochemical analysis (Table 1) shows that LUMO of the dye is placed at higher energy compared to the CB of quantum dots suggesting that electron transfer is less thermodynamically feasible (Schemes 1A and 1B). Thus the greater chance is for FRET process which can create surplus dye excited state which in turn drive more electrons to TiO2 and contribute to the enhanced photocurrent. With a better overlap integral QD:CTSQ-2 hybrid exhibited better FRET thereby improving the photovoltaic performance in comparison to that of

CdSe@OAm QDs QD:CTSQ-1 QD:CTSQ-2 CTSQ-1 CTSQ-2

4 3

x 1/6

(B)

CTSQ-1 CTSQ-2 QD:CTSQ-1 QD:CTSQ-2

1000 100

2

Counts

(A) Fl. Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 0 550

10 1

600

650

700

750

0

5

Wavelength (nm)

10 Time (ns)

15

20

Figure 6. (A) Steady state and (B) time resolved photoluminescence spectra of CTSQ-1 (green, 0.2 mM) and CTSQ-2 (black, 0. 38 mM) adsorbed onto bare TiO2 and those spin casted with QDs, 0.012 mM (red; QD:CTSQ-1 hybrid and blue; QD:CTSQ-2 hybrid). For fluorescence lifetime measurements samples were excited with 440 nm diode laser and the signals were collected at 685 nm. For comparison corresponding spectrum of QDs alone is also provided.


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QD:CTSQ-1 hybrid which is in tune with the photophysical data. The estimated values indicate more than double faster energy transfer rate for the QD:CTSQ-2 hybrid combination. Further to support our argument, that FRET is the main cause for improved solar cell efficiency, detailed photophysical investigations has been carried out on the nanohybrid sensitized TiO2 electrodes (Figure 6). Both dyes showed quenched emission when anchored onto TiO2 surface mainly due to the excited state electron transfer from its LUMO to the CB of the metal oxide semiconductor. On the contrary, considerable fluorescence was observed from CdSe QDs when they are spin casted onto TiO2 surface suggesting less efficient electron injection. However enhancement in fluorescence intensity was observed when squaraine dyes were adsorbed onto FTO/TiO2/QD architecture (Figure 6A). The fluorescence intensity got amplified ten times for CTSQ-1 and seventeen times for CTSQ-2. This is well in agreement with the calculated overlap integral. Moreover, the fluorescence lifetime values for both dyes got increased in the presence of CdSe QDs confirming the energy transfer (Figure 6B). These studies clearly show that the energy transfer is more efficient in QD:CTSQ-2 nanohybrid system than that for QD:CTSQ-1 which contributed to a better photovoltaic performance.

Conclusions In summary, we have investigated and compared the energy transfer efficiencies in organicinorganic nanohybrids comprising of QDs and squaraine dyes having the same π-backbone. The excited state energy of QDs is transferred with more than double efficiency to the dye bearing tbutyl substitution (CTSQ-2). This is due to its 1.75 times higher extinction coefficient and unique match of donor emission and acceptor absorption spectral profiles. The direct linkage of dyes with QDs via carboxylic group ensured the smallest donor-acceptor distance thereby offering highest FRET efficiency. The more than one order of higher energy transfer rate


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compared to radiative and nonradiative decay rates resulted in FRET efficiencies as high as 93 %. These distinctive properties resulted in an overall 43 % increase in DSSC performance (PCE=2.1 % for CTSQ-2 and 3.19 % for QD:CTSQ-2 nanohybrid). The analysis of the data revealed that a major contribution to this arises from enhanced current density (31 %) and can be attributed to improved light harvesting capability. The complementarities of the absorption of QDs with that of dyes resulted in panchromatic response in the range 350 to 700 nm. Thus, the studies presented here reveal that how the molecular design, spectral positioning and molar extinction coefficients are important in creating an efficient organic-inorganic nanohybrid FRET pair which can be directly applied in various solar light harvesting application niches.

ASSOCIATED CONTENT Supporting Information. QDs Size distribution analysis, excitation spectra of CTSQ1, QD, QD:CTSQ-1 hybrid, absorption, emission and lifetime titration plots, Stern-Volmer plots, Photovoltaic characteristics of solar cell fabricated device architecture D. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors: [email protected], [email protected], [email protected]

Acknowledgements We thank Mr. Robert Philip and Mr. Kiran Mohan for their help in recording TEM images. SS and SL gratefully acknowledge financial support from DST-INSPIRE Faculty Award (IFA 13-


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CH-115). We also thank DST for the Indo-European collaborative project, OISC/LARGECELLS and CSIR-TAPSUN programs NWP0054 and CLP-133739.

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Modulating FRET in OrganicâInorganic Nanohybrids for Light

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