Size-Dependent Energy Transfer Pathways in CdSe Quantum Dot

Article pubs.acs.org/JPCC

Size-Dependent Energy Transfer Pathways in CdSe Quantum Dot−Squaraine Light-Harvesting Assemblies: Fö rster versus Dexter Jacob B. Hoffman, Hyunbong Choi, and Prashant V. Kamat* Radiation Laboratory, Department of Chemistry & Biochemistry University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: Energy transfer coupled with electron transfer is a convenient approach to mimic photosynthesis in light energy conversion. Better understanding of mechanistic details of energy transfer processes is important to enhance the performance of dye or quantum dot-sensitized solar cells. Energy transfer through both long-range dipole-based Förster resonance energy transfer (FRET) and shortrange Dexter energy transfer (DET) mechanisms have been identified to occur between CdSe quantum dots (QDs) linked to a red-infrared-absorbing squaraine dye through a short thiol functional group (SQSH). Solutions of SQSH linked to CdSe were investigated through steady-state and time-resolved spectroscopy experiments to explore both mechanisms. Photoluminescence studies revealed that smaller QDs had higher energy transfer efficiencies than predicted by FRET, and femtosecond transient absorption experiments revealed faster energy transfer rates in smaller donor QD sizes. These findings supported a DET process dominating at small donor sizes. The presence of both processes illustrates multiple strategies for utilizing energy transfer in light-harvesting assemblies and the required considerations in device design to maximize energy transfer gains through either mechanism.

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INTRODUCTION The demand for renewable clean energy has led to major scientific interest in researching new materials and processes to achieve efficient, cost-effective light energy conversion and storage.1−5 Semiconductor quantum dots (QDs) possess high customizability of band gap size and position through control of material size, shape, and composition.2,6−8 Coupled with other properties such as high extinction coefficients, low material cost, and the possibility of hot electron injection and multiple exciton generation make QDs ideal candidates as light harvesters in quantum dot solar cells (QDSCs).1,7,8,10−14 Despite their attractive properties, many QD species do not absorb significant portions of incident solar radiation in the infrared (IR). In an effort to extend the range of light harvesting in QDSCs, QDs are often coupled with an IR absorbing dye.15−17 Energy transfer is a process where an excited donor interacts with a relaxed acceptor, resulting in a relaxed donor and excited acceptor.18−20 Through the dipole−dipole coupling-based Förster resonance energy transfer (FRET) mechanism,18−22b energy transfer has been utilized extensively in biochemistry applications.23−29 In nature, energy transfer is a major process in photosynthesis.30 Recently, energy transfer via FRET has been used in many QD−dye paired,31−35 QD-QD,36−39 and dye−dye paired40−42 dual-sensitized devices by enhancing charge separation between sensitizers and directing charge carrier flow. Charge transfer and electron transfer processes have been shown to compete with FRET in QD-acceptor systems in previous studies.43−45 At donor and acceptor proximities under one nanometer, quantum mechanical effects such as orbital overlap and © XXXX American Chemical Society

tunneling can result in energy transfer through electron exchange interactions, known as Dexter energy transfer (DET).18,19,21,46 Although FRET has been well documented to occur in photoconversion devices, DET to our knowledge is yet to be considered as a possible energy transfer pathway in QD−dye lightharvesting assemblies. An example of FRET enhancing a QD−dye dual-sensitized solar cell was recently reported in earlier work by our group.33 A squaraine dye with a HS-R-COOH configuration (SQSH) was synthesized to perform a role similar to 3-mercaptopropionic acid (3-MPA), a linker molecule used to connect CdSe QDs to TiO2 in QDSCs.47 In our QD−dye system, SQSH performed the dual role of linking CdSe QDs to TiO2 and provided additional light harvesting in the near-infrared. In the assembly, CdSe transferred energy to SQSH via FRET, while SQSH was the sole injector of electrons into TiO2. In the present work, we investigate the presence of DET as a competitive energy transfer mechanism between CdSe QD donors and SQSH by tuning the CdSe band gap to produce conditions favorable for electron exchange. Using the efficiencies, rates, and well documented size-dependent optical and electronic properties inherent to quantum dots, differences in energy transfer mechanisms are investigated as a function of QD size (Scheme 1), and donor properties required to maximize gains from each mechanism are discussed. Received: July 7, 2014

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dx.doi.org/10.1021/jp506757a | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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sealed and bubbled under nitrogen for 15 min. An absorption spectrum was recorded after which the photoluminescence or transient absorption spectrum was obtained for CdSe alone. SQSH was added (8.0 μL for photoluminescence experiments, 20 μL for transient absorption experiments) from the dye stock solution to the CdSe solution in cuvette, resulting in equimolar concentrations of CdSe and SQSH. The mixtures were then resealed and rebubbled under nitrogen, and an absorption spectrum was recorded prior to the mixture photoluminescence or transient absorption spectrum being recorded. Optical Measurements. All steady-state spectroscopic experiments were carried out in 1 cm quartz cuvettes. A Cary 50 bio spectrophotometer was used to collect the UV−vis absorption spectra. Photoluminescence measurements were recorded with a Jobin Yvon Fluorolog-3 using 440 nm excitation with a 480 nm long pass filter before the detector. Emission lifetime measurements (in Supporting Information) were recorded using a Jobin Yvon single photon counting system with a 452 nm LED excitation source and conducted in a 1 cm quartz cuvette. Transient absorption experiments were performed using a Clark MXR-2010 laser system (775 nm fundamental, 1 mJ/pulse, fwhm = 130 fs, 1 kHz repetition rate) with Helios software from Ultrafast Systems. The fundamental was split 95/5 to generate a 387 nm pump through frequency doubling of the 95% portion. The probe was generated by focusing the 5% portion through a Ti:sapphire crystal to generate a white light continuum. Both beams were brought incident on the sample (in a 2 mm quartz cuvette) with the probe delayed via optical delay rail. Pump power density was maintained so that the QD carrier density was 0.03 excitions per QD.

Scheme 1. Various Sizes of CdSe QD Donors with SQSH Linked to the Surfacea

a

Varying the donor size induces conditions favorable for DET to occur.

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EXPERIMENTAL SECTION Materials. Materials used in CdSe synthesis: cadmium oxide (CdO, Alfa Aesar, 99.998%), selenium powder (Aldrich, 99.99%), trioctylphosphine (TOP, Aldrich, 90%), tetradecylphosphonic acid (TDPA, PCI Synthesis), toluene (Fisher Scientific, HPLC grade), and methanol (Fisher Scientific, Certified ACS grade) were used without further purification. Trioctylphosphine oxide (TOPO, Strem Chemicals, 99%) was purified via recrystallization in acetonitrile. The synthesis of the squaraine dye (SQSH), (E)-4-((5carboxy-3,3-dimethyl-1-octyl-3H-indolium-2-yl)methylene)-2((E)-(1-(3-mercaptopropyl)-3,3-dimethylindolin-2-ylidene)methyl)-3-oxocyclobut-1-enolate, was previously reported.33 The dye was synthesized and purified by this method without further modification before use. Synthesis of CdSe Quantum Dots. Synthesis of CdSe QDs was performed via a hot-injection method, similar to a previous published methodology.48 A mixture of CdO (0.1 g), TDPA (0.66 g), and TOPO (4.0 g) was purged under vacuum at 90 °C for 1 h. The reaction was exposed to a nitrogen atmosphere, and the temperature was increased to 320 °C until the mixture became clear. A solution of TOPSe (4.25 mL of TOP, 0.464 g of Se) was prepared in a glovebox and injected into the precursor mixture. Nucleation of CdSe QDs occurred, and the reaction progressed until it was quenched by rapid cooling when the desired QD size was reached. The QDs were then washed three times by inducing flocculation in toluene suspensions using methanol (1:4 ratio) and centrifugation. Six sizes of CdSe were synthesized (2.7, 2.9, 3.4, 3.9, 4.3, and 5.2 nm in diameter). Average size was determined through the absorbance of the first excitonic peak of each sample.14,49 Preparation of CdSe−SQSH Solutions. Stock solutions of CdSe (0.75 μM for photoluminescence experiments and 4.0 μM for transient absorption experiments) and SQSH (0.2 mM) in toluene were prepared prior to experiments using extinction coefficients determined from a previously published sizing curve14 (for CdSe) and absorption experiments (for SQSH, 68 000 M−1 cm−1). For each experiment, CdSe (2.5 mL photoluminescence experiments, 1.0 mL transient absorption experiments) stock solution was transferred to a cuvette and was then

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RESULTS AND DISCUSSION Energy Transfer from 4.3 nm CdSe QDs to SQSH. Energy transfer was examined using 4.3 nm QDs that exhibited optical properties similar to that of the QD donors used in the original assembly for solar cell application.33 In solution, SQSH was covalently linked to CdSe QD surfaces due to the affinity that thiol functional groups have for the CdSe surface (Scheme 2).50 An increase in SQSH absorbance at 648 nm (Figure 1A) is interpreted as SQSH bound to the surface of CdSe, similar to absorbance changes in squaraine dyes when complexed with β-cyclodextrin.51 Energy transfer was probed in 4.3 nm CdSe QDs covalently linked to SQSH using steady-state photoluminescence experiments with 440 nm excitation. This setting allowed selective excitation of QDs due to minimal SQSH absorbance at 440 nm (Figure 1A). Figure 1B shows steady-state emission quenching of CdSe QDs (577 nm) in a solution of equimolar CdSe and SQSH concentrations. The quenching of CdSe emission was accompanied by a large promotion of SQSH emission at 654 nm, indicative of energy transfer from CdSe to SQSH.

Scheme 2. SQSH Binding to the Surface of CdSea

a

The alkane chain separates the dye from the surface of CdSe by approximately 0.7 nm, a distance where energy transfer can occur through either a FRET or DET mechanism. B



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Energy transfer was completely attributed to SQSH linked to the CdSe surface, as contributions to energy transfer from unbound dye were shown to be insignificant from Stern−Volmer analysis (supplementary Figure S1, Supporting Information), due to the limitation of diffusional processes at the concentrations used. Size Dependence of Energy Transfer Mechanisms. Varying CdSe size drastically impacts the optical and electrical properties of the nanoparticle, affecting the bandgap size and the absolute energetic positions of the conduction and valence band. As these properties change, energy transfer is impacted because the FRET dipole−dipole interaction and DET electron exchange mechanisms are fundamentally different, making use of different donor properties. kFRET =

2 ⎞ 9(ln 10) ⎛ ϕf κ ⎜ ⎜ 4 6 J(εA )⎟⎟ 5 128π Na ⎝ n τDr ⎠

kDET = KJe−2r / r °

(1) (2)

Rate equations for both FRET (presented as the case of point dipoles) (eq 1) and DET (eq 2) both depend on the spectral overlap (J) and donor−acceptor separation (r). However, in FRET J is convoluted with a dependence on acceptor extinction (εA), and each mechanism has different donor/acceptor separation dependencies, (1/r6) for FRET and (e−r) for DET.18,21,22,46 Due to the dipole−dipole based nature of FRET, kFRET includes dependencies on solvent refractive index (n4), dipole orientations (κ2), donor lifetime (τD), acceptor extinction coefficient (εA), and donor quantum yield of emission (ϕf). Likewise, the electron-exchange-based nature of DET requires dependence on orbital interactions (K), and the distance between donor and acceptor when in van der Waals contact (r°).18,19,22,46 When bound to the QD surface, SQSH is separated from CdSe by a short alkane chain approximately 0.7 nm long. Due to this close proximity of bound acceptor, CdSe QDs could potentially serve as an energy transfer donor through either mechanism.18,21 Spectral overlap is related to the energetic matching of donor excited-state relaxation pathways with acceptor excitation pathways and is necessary for both energy transfer mechanisms. This is commonly represented as the normalized emission of the donor compared to normalized acceptor absorption.18,19 Scheme 3A illustrates that spectral overlap between QD emission and SQSH absorbance decreases with decreasing QD size. Because both mechanisms depend on spectral overlap, it is expected that energy transfer will be maximal for either mechanism at larger QD sizes when considering spectral overlap alone. Orbital interaction is related to how energetically favorable electron transfer is from the donor to acceptor excited states and from the acceptor to donor ground states.18,19 The relative orbital interaction is shown in Scheme 3B by comparing previously published values of the QDs bands52 and SQSH molecular orbitals.33 As QD size approaches 2.8 nm, electron transfer becomes energetically favorable from the CdSe conduction band to SQSH LUMO, thus allowing electron exchange (DET) to occur. On the basis of these arguments, we can expect that for large donor sizes (including the previous example of 4.3 nm CdSe) FRET will be the dominate energy transfer pathway, while at small donor sizes DET will be a viable mechanism (Scheme 3C). Energy Transfer Efficiencies Using Different Sized CdSe QD Donors. Steady-state photoluminescence experiments were used to compare energy transfer in 2.7, 2.9, 3.4, 3.9, 4.3, and 5.2 nm diameter donor QDs. To quantify energy transfer from the photoluminescence data, energy transfer efficiency (ϕET),

Figure 1. (A) Absorbance, (B) photoluminescence (excitation 440 nm), and (C) excitation acquisition monitoring 654 nm emission spectra of (a) 0.75 μM 4.3 nm CdSe QDs, (b) equimolar mixtures of 4.3 nm QDs and SQSH (0.75 μM each), and (c) 0.75 μM SQSH in toluene (normalized to SQSH absorbance).

Energy transfer was confirmed through excitation acquisition experiments (Figure 1C) in which SQSH emission was monitored while varying the excitation wavelength. The spectra recorded for CdSe−SQSH (a) and SQSH alone (c) show clear difference in excitation spectra. The SQSH emission response to excitation below 600 nm in the presence of CdSe demonstrated that CdSe absorbance contributed to the increased 654 nm emission. C



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Scheme 3. (A) Relative Spectral Overlap for 4.2, 3.6, and 2.8 nm CdSe QDs Represented by the Normalized Emission of CdSe Compared to the Absorbance of SQSH. (B) Comparison of Relative Orbital Interaction of CdSe and SQSH Interacting States. (C) As Donor Size Approaches 2.8 nm, DET Becomes a Viable Mechanism Compared to FRET in Larger Sizes

ϕET = 1 −

kET kD + kET

(4)

By definition, ϕET can be expressed as the ratio of the energy transfer rate (kET) over the rates of all other processes present in the system (kD + kET) (eq 3).18,19 This quantity can be calculated using steady-state photoluminescence data (eq 4) where FDA and FD represent the donor emission peak intensities in the presence of acceptor and in the absence of acceptor, respectively.19 Figure 2A shows ϕET calculated from steady-state photoluminescence data as a function of QD sizes. The smallest donor, 2.7 nm, had the highest value of ϕET despite having the lowest spectral overlap of all donor sizes investigated. This combined with the required orbital interactions was suggestive of a DET process contributing to the energy transfer. As donor size increased, changing QD energetics made DET drastically less probable, while FRET became more probable with increased spectral overlap. Variations in ϕET in large donor sizes were attributed to differences in donor quantum yield of emission (ϕf) (Table 1), a quantity that impacts FRET only. To show differences in donor ϕf did not also account for the high value of ϕET observed for 2.7 nm donors, values of ϕET were normalized by ϕf and J(εA) using values in Table 1. The values of ϕET normalized by FRETdependent variables (ϕf and (εA)) are plotted in Figure 2B. We would expect similar values of energy transfer efficiency if FRET was the only energy transfer process present, assuming equal SQSH loading between donors. The normalized values of ϕET remain higher for 2.7 nm QDs (Figure 2B) and then decreased with increasing QD size before becoming similar for dots 3.4 nm in diameter and greater. This phenomenon is consistent with FRET as the only energy transfer process occurring in larger donor sizes. Conversely, larger normalized ϕET values for smaller QD sizes suggested that there is an additional effect not explained by FRET alone. The higher normalized values of ϕET observed for 2.7 and 2.9 nm donors is close to the predicted onset of sufficient CdSe conduction band and SQSH LUMO overlap. This shows that DET is a contributing process to the overall energy transfer in QDs 2.9 nm in diameter and smaller. Time-Resolved Analysis of Energy Transfer Using Ultrafast Transient Absorption Spectroscopy. Femtosecond transient absorption spectroscopy allows deeper insight into processes occurring on the ultrafast time scale. This technique was used to further investigate the presence of DET and FRET processes between QDs donors and SQSH acceptors by directly probing energy transfer rates. The difference absorption spectra of an equimolar solution of CdSe with SQSH following 387 nm laser pulse excitation are shown in Figure 3 of 4.3 nm (3A) and 2.7 nm (3C) donors. Figure 3 also shows the absorption recovery for the corresponding CdSe exciton bleaches of 4.3 nm (3B) (at 568 nm) and 2.7 nm (3D) (at 488 nm) donors for the cases of (a) CdSe and (b) CdSe with equimolar SQSH. In the case of larger (4.3 nm) CdSe donors, there was little difference observed in CdSe recovery kinetics between donor alone and CdSe SQSH mixture during the time frame probed (1.6 ns) (Figure 3B). The presence of a small SQSH bleach at 648 nm (Figure 3A) increasing over the course of the scan suggests that energy transfer is present in the system. This was confirmed when no significant bleach was observed in the dye absorbance when dye was excited alone (supplementary Figure S2, Supporting Information), ruling out direct dye excitation being responsible for the observed signal. The growth kinetics of this SQSH bleach

defined as the fraction of the excited state that decays via energy transfer (i.e., a quantum yield of energy transfer), was determined. ϕET =

FDA FD

(3) D



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Figure 2. (A) ϕET vs QD donor size for 0.75 μM equimolar solutions of CdSe and SQSH, calculated from steady-state photoluminescence experiments. The highest value was observed for 2.7 nm donors, which when combined with the possibility for electron exchange was suggestive of DET contributing to the overall energy transfer. In larger sized donors values began to vary due to differences in ϕf. (B) Values for ϕET normalized by J(εA) and FRETdependent donor property ϕf vs donor size. The normalized values were higher at small donor sizes >2.9 nm, supporting an additional process contributing to the energy transfer. In larger donor sizes values are similar, indicating that FRET is responsible for all energy transfer.

Table 1. Band Edge Emission Spectral Overlap, ϕf, and ϕET of Each Donor QD Sizea size (nm)

J(εA) (M−1 cm3)

ϕf

ϕET

2.7 2.9 3.4 3.9 4.3 5.2

3.63 × 10−15 5.57 × 10−15 1.03 × 10−14 3.21 × 10−14 7.27 × 10−14 1.87 × 10−13

0.055 0.023 0.012 0.008 0.003 0.001

0.37 ± 0.005 0.16 ± 0.017 0.12 ± 0.011 0.21 ± 0.017 0.22 ± 0.043 0.09 ± 0.028

seen in 4.3 (Figure 3C) and 5.2 nm donors (supplementary Figure S3, Supporting Information). The case of 2.9 nm donors was interesting because a large fast quenching (∼200 ps) was observed initially, and then a second slower component (∼1 ns) was observed across the remaining recovery (Figure 4). These multiple components are suggestive of DET and FRET both contributing to the energy transfer during the time scale probed (1.6 ns), representative of the QD size limit where DET becomes a competitive process. Size Dependence of Energy Transfer Rate Constants. To further quantify the rate of the observed energy transfer, the formation of excited state was monitored through SQSH bleach growth. Figure 5A shows the kinetic traces of the bleach growth at 648 nm recorded following the laser pulse excitation of different QD donor sizes. With increasing QD size, the SQSH bleach grew at a slower rate. The bleach growth was multiexponential and can be fitted to biexponential growth kinetics. Such a complex kinetic behavior mainly arises from the donor (excited QD) which has been shown to deactivate via multiexponential decay.52 Table 2 lists the parameters obtained using a biexponential kinetic fit along with average lifetimes, and energy transfer rates for the QD donor sizes. Energy transfer rate constants were directly estimated as the inverse of the time constants obtained from the average lifetime. The rate constant for 5.2 nm QDs could not be calculated, as the small signal and very slow growth of the SQSH bleach prohibited accurate fitting. Energy transfer rate constants drastically decreased as the QD donor size increased (Figure 5B), differing by almost 1 order of magnitude between 2.7 and 4.3 nm QD donors. Although the rate constants in Table 2 showed that energy transfer occurs faster in small QD systems, we cannot explain this phenomenon based on the FRET mechanism. Because the overlap between donor emission and acceptor absorption remains minimal with small QDs, we can rule out FRET as the dominating factor in energy transfer. Hence, we consider DET as the major contributing factor in energy transfer for small donors in the CdSe−SQSH assembly. The ability to harvest high energy photons and transfer them to a donor molecule at a rate faster than that of donors dependent on FRET suggests the usefulness of DET processes in designing light-harvesting assemblies.

a Spectral overlap was calculated according to Förster theory.19,22 The normalized emission of the donor,FD, the extinction of the acceptor, εA(λ), and wavelength to the fourth power, λ4 were integrated, with respect to wavelength. The ϕf of CdSe QDs was found experimentally by comparing CdSe emission to a reference dye of known ϕf at concentrations with matched excitation absorbance.

0

J(εA ) =

∫∞ FDεA(λ) λ 4 dλ

are shown in the inset of Figure 3B to further illustrate the dye bleach after CdSe excitation. In the case of 2.7 nm donors, faster QD bleach recovery was observed for CdSe when in the presence of SQSH. An additional relaxation process within the first several hundred picoseconds for CdSe in the presence of SQSH could be clearly seen in Figure 3D. This fast recovery component is the result of energy transfer to the acceptor, independently confirmed by the SQSH absorbance bleach at 648 nm that grew in over the course of the experiment (Figure 3C). The possibility of one-way charge transfer processes were ruled out due to lack of CdSe or SQSH radical signals in the difference absorption spectra. The kinetics of the increasing SQSH ground-state bleach (inset of Figure 3B) show that the change in SQSH absorbance plateaus during the time frame probed, again suggestive of a single and complete energy transfer. We conclude on the basis of transient absorption and the previous photoluminescence experiments that this single fast energy transfer in the ultrafast time scale is from DET. In general, energy transfer observable in ultrafast time scale decreased as donor size increased, with no significant quenching E



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Figure 3. Difference absorption spectra of equimolar QD−SQSH solutions (4 μM each) for (A) 4.3 nm and (C) 2.7 nm normalized to minimum CdSe bleach. Kinetic traces normalized to minimum of (B) 4.3 QDs (568 nm) and (D) 2.7 nm (488 nm) for (a) CdSe alone and (b) equimolar CdSe SQSH solutions. Insets: Bleach growths for SQSH (monitored at 648 nm) in the presence of (B) 4.3 nm and (D) 2.7 nm donors.

the donors used in this study could be a reason for such differences in rate constants between those reliant on DET and those reliant on FRET mechanisms because ϕf directly impacts FRET rate. In cases where FRET is the targeted energy transfer mechanism, consideration of donor emissive properties is important for creating an efficient and fast process. In contrast, when designing a system where DET is the targeted mechanism, sufficient energetic conditions for electron exchange and short donor−acceptor distances are necessary.

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CONCLUSIONS Steady-state photoluminescence experiments revealed multiple size-dependent energy transfer mechanisms in the CdSe−SQSH system after accounting for differences in donor ϕf. DET was the dominant system consisting of 2.7 and 2.9 nm diameter QDs due to orbital interaction of the CdSe and SQSH interacting states. As donor size increased, DET became insignificant because interaction between the conduction band of CdSe and the LUMO of SQSH was lost. FRET remained the major energy transfer pathway in larger QD−SQSH systems where there was no orbital interaction and maximal spectral overlap. Femtosecond transient absorption experiments showed that energy transfer was occurring much faster in CdSe−SQSH systems where DET was dominant, suggesting that DET could lead to higher gains in light harvesting due to energy transfer.

Figure 4. Recovery of 2.9 nm donors for (a) equimolar CdSe−SQSH mixture and (b) CdSe donors. An initial fast quenching (∼200 ps) is attributed to DET, while a slow slower quenching (∼1 ns) is attributed to FRET. Both mechanisms competing is representative of the donor size where DET begins to be competitive process.

The kinetic and emission measurements point to the inherent differences between the two mechanisms and what parameters need to be considered when designing light-harvesting assemblies that utilize energy transfer. For example, the low ϕf of F



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Figure 5. (A) Kinetic traces of the SQSH bleach growth (at 648 nm) for various sizes of QD donors normalized to minimum. The resulting biexponential fits (Table 2) were used to estimate energy transfer rates for when using each donor size. (B) Energy transfer rates vs donor QD size. Rates differed by almost 1 order of magnitude between 2.7 and 4.3 nm donors, showing that energy transfer was much faster in donors with DET.

Table 2. Biexponential Fitting Parameters of SQSH Kinetic Traces with Average Lifetimes and Energy Transfer Rates for Various QD Donor Sizesa

a

size (nm)

A1

τ1 (ps)

A2

τ2 (ps)

Tavg (ps)

kET (×10−10)

R2

2.7 2.9 3.4 3.9 4.3 5.2

0.36 0.33 0.24 0.23 0.26 −

9.47 ± 0.65 12.1 ± 1.6 13.2 ± 1.2 39.1 ± 11 25.3 ± 3.6 −

0.64 0.67 0.76 0.77 0.74 −

149 ± 5.2 309 ± 22 311 ± 10 525 ± 75 730 ± 80 −

97.9 ± 3.6 211 ± 15 239 ± 8.2 413 ± 60 547 ± 60 −

1.03 ± 0.036 0.474 ± 0.034 0.418 ± 0.14 0.242 ± 0.035 0.183 ± 0.020 −

0.997 0.986 0.984 0.997 0.989 −

SQSH growths, normalized to the maximum, were fit using the following biexponential equation.

y = A1e X / τ1 + A 2 e X / τ2 τavg = A1τ1 + A 2 τ2 where A1 and A2 are weighted coefficients and τ1 and τ2 are lifetimes of components 1 and 2, respectively. Average lifetime was calculated as a weighted average of the two components. Energy transfer rates were found by taking the inverse of the average lifetimes.

Both energy transfer mechanisms can be utilized in energy conversion or sensing devices. However, it is important to consider donor and acceptor properties so that energy transfer can be maximized. Notably, high donor ϕf is important when using FRET in a system, due to a direct dependence on donor ϕf in kFRET. Conversely, for DET to occur, donor and acceptor must be in close proximity and the interacting states must overlap to allow for electron exchange. Although there are numerous examples of FRET being utilized in light-harvesting assemblies, the prospect of utilizing DET processes has not been explored fully. As discussed in this study, DET represents a new strategy for enhanced light harvesting in multisensitized devices and should be further investigated for implementation in devices where components are in close proximity and possess the necessary energy levels for electron exchange.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research described herein was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through award DE-FC02-04ER15533. This is contribution number NDRL no. 5026 from the Notre Dame Radiation Laboratory. The authors thank Dr. Kevin Stamplecoskie for helpful discussions.

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(1) Kamat, P. V. Quantum Dot Solar Cells. Semiconductor Nanocrystals as Light Harvesters. J. Phys. Chem. C 2008, 112, 18737− 18753. (2) Hägglund, C.; Apell, S. P. Plasmonic Near-Field Absorbers for Ultrathin Solar Cells. J. Phys. Chem. Lett. 2012, 3, 1275−1285. (3) Snaith, H. J. Perovskites: The Emergence of a New Era for LowCost, High-Efficiency Solar Cells. J. Phys. Chem. Lett. 2013, 4, 3623− 3630. (4) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. DyeSensitized Solar Cells. Chem. Rev. 2010, 110, 6595−6663. (5) Graetzel, M.; Janssen, R. a J.; Mitzi, D. B.; Sargent, E. H. Materials Interface Engineering for Solution-Processed Photovoltaics. Nature 2012, 488, 304−312. (6) Alivisatos, A. P. Perspectives on the Physical Chemistry of Semiconductor Nanocrystals. J. Phys. Chem. 1996, 3654, 13226−13239.

ASSOCIATED CONTENT

S Supporting Information *

Stern−Volmer analysis of CdSe−SQSH, transient spectra of SQSH alone, and kinetic recoveries for 3.4, 3.9, and 5.2 nm CdSe−SQSH systems. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. G



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