Balancing Electron Transfer and Surface Passivation in Gradient

Letter pubs.acs.org/JPCL

Balancing Electron Transfer and Surface Passivation in Gradient CdSe/ZnS Core−Shell Quantum Dots Attached to ZnO Mohamed Abdellah,†,§ Karel Ž ídek,† Kaibo Zheng,† Pavel Chábera,† Maria E. Messing,‡ and Tõnu Pullerits*,† †

Department of Chemical Physics, Lund University, Box 124, 22100 Lund, Sweden Solid State Physics, Lund University, Box 118, 22100 Lund, Sweden § Department of Chemistry, Qena Faculty of Science, South Valley University, Qena 83523, Egypt ‡

S Supporting Information *

ABSTRACT: Core−shell (CS) quantum dots (QDs) are promising light absorbers for solar cell applications mainly because of their enhanced photostability compared with bare QDs. Moreover, the superb photostability can be combined with a low number of defects by using CSQDs with a gradient composition change from the core to the shell. Here, we study electron injection from the gradient CSQDs to ZnO nanoparticles. We observe the typical exponential injection rate dependence on the shell thickness (β = 0.51 Å−1) and discuss it in light of previously published results on step-like CSQDs. Despite the rapid drop in injection rates with shell thickness, we find that there exists an optimum thickness of the shell layer at ∼1 nm, which combines high injection efficiency (>90%) with a superior passivation of QDs. SECTION: Physical Processes in Nanomaterials and Nanostructures

S

yield, and as a side effect, it also reduces the Auger recombination rate compared with step-like CSQDs.14,15 A key process for the QD-based solar cells is the initial charge separation. It is maintained by electron injection from QDs to metal oxide (MO) nanoparticles (NPs). On one hand, the shell layer of the CSQDs acts as a potential barrier and slows down the electron injection. On the other hand, the shell protects the QD and reduces the unwanted back-recombination. Intuitively, we expect that there exists an optimal shell thickness providing sufficient protection, whereas the electron injection is still efficient enough. Finding the conditions where these two processes are well balanced is the main aim of the current work. We studied the electron injection from the gradient CdSe/ ZnS CSQDs to ZnO NPs using femtosecond transient absorption spectroscopy. The injection rate exhibited strong exponential shell thickness dependence. By analyzing the injection rate and emission quantum yield, we found an optimum shell thickness that combines a high-quality QD passivation with efficient charge separation of almost 100%. Gradient CSQDs were prepared by following a method described by Bae et al.12,16 and reproduced in our recent work.4 Briefly, we used a single-step hot injection method. Both Cd2+

olar energy with its abundance and longevity is considered to be a likely solution for the global energy crisis. Among various kinds of light-harvesting materials, semiconductor nanocrystals, the so-called quantum dots (QDs), have attracted increasing attention due to their superb features, such as high extinction coefficient, a tunable absorption edge, and the possibility of multiple exciton generation and collection, which can lead to photon-to-current conversion quantum efficiencies over 100%.1−6 A crucial factor for application of QDs in photovoltaics is their long-term stability, which is mostly limited by a slow, yet important, degradation of the QDs’ surface.7 One solution to this problem is using core−shell QDs (CSQDs), wherein a protective shell with a wider band gap semiconductor is employed to shield the core from surface oxidation.8 Consequently, CSQDs feature superior thermal, chemical, and photochemical stability compared with organically capped QDs.9−11 Generally, the core and the shell of CSQDs are synthesized in sequence (step-like CSQDs). Such a core−shell interface will suffer from surface defects induced during the shell growth, mainly due to the lattice mismatch between core and shell layers.12 Recent reports have revealed that the number of interfacial defects in CSQDs can be minimized by using a gradual transition between the core and shell materials (gradient CSQDs). 13 The gradual change in chemical composition results in an enhanced fluorescence quantum © XXXX American Chemical Society

Received: March 24, 2013 Accepted: April 30, 2013

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dx.doi.org/10.1021/jz4006459 | J. Phys. Chem. Lett. 2013, 4, 1760−1765

The Journal of Physical Chemistry Letters

Letter

and Zn2+ oleate in 1-octadecene solution were heated to 325 °C. Then, Se2− and S2− in 3 mL of trioctylphosphine (TOP) solution were swiftly injected into the cation precursor solution. To obtain gradient CSQDs with different shell thicknesses, the growth process was terminated by cooling the solution down using an ice bath after 5 s and 1, 3, 5, 10, and 15 min. For sensitization, the surface capping of purified QDs was exchanged from oleic acid to a bifunctional linker molecule, 2-mercaptopropionic acid (2-MPA).17 ZnO nanoparticle mesoporous films were prepared via doctor blading. As-obtained ZnO NP films were immersed into the QD solution in EtOH for 2 h in the dark. The resulting films were washed with EtOH and dried using N2 air to avoid aggregation of residual QDs in the solution. Absorption and emission spectra were measured using an Agilent 845x spectrophotometer and Spex 1681 spectrophotometer, respectively. The photoluminescence (PL) quantum yield (QY) of the QDs was measured and estimated by comparing the QDs’ PL intensities with that of standard rhodamine 6G dye at the same optical density (0.07).18 The high-resolution analytical transmission electron microscopy images (HR-TEM) of the QDs were obtained using a JEOL3000F microscope equipped with an Oxford SDD X-ray analyzer to obtain the average size of the QDs. Transient absorption measurements were carried out on a femtosecond laser setup based on the MaiTai-pumped Spitfire Pro XP (Spectra Physics) with the central output wavelength of 795 nm and a 1 kHz repetition rate delivering ∼80 fs pulses. The beam was split into two parts, one for pumping a collinear optical parametric amplifier (TOPAS-C, Light Conversion) to generate the pump beam (470 nm) and the second one either focused onto a thin sapphire plate to generate a white light continuum (for transient spectra measurements) or used for pumping a second TOPAS-C to generate narrow-band probe pulses at 540 nm (for kinetics measurements). The delay between the pump and probe pulses was introduced by a delay line. The mutual polarization between pump and probe beams was set to the magic angle (54.7°) by placing a Berek compensator in the pump beam. We used thin-film samples of QDs on ZnO NPs (OD ≈ 0.05) and colloidal samples in the 1 mm quartz cell (OD between 0.2 and 0.4). During measurements, the samples were kept in a N2 atmosphere to avoid possible photodegradation.19 Steady-state optical spectroscopy was used to characterize the gradient CSQDs and confirm their attachment to ZnO NPs. Figure 1 shows the absorption and the emission spectra of the purified QDs in toluene at different shell growth times (i.e., for different shell thicknesses). Due to the significant difference in the precursors’ reactivity, the CdSe core is formed during the very early stage of the reaction (5 s growth time; see ref 12 for details). We use this sample as the reference for the core QD. The mean size of the core QDs was measured in the HR-TEM image to be 3 nm, which is in perfect agreement with the estimated diameter from the position of lowest absorption band (3 nm);3 see Figure 1C. By careful analysis of HR-TEM images, we determined the mean diameters of gradient CSQDs for 1, 3, 5, 10, and 15 min growth times to be 4.2, 5.6, 6.1, 6.8, and 7.6 nm, respectively; see Figure. 1C. Subtracting the core size (3 nm) provides the corresponding shell thicknesses as 0.6, 1.3, 1.6, 1.9, and 2.3 nm; see Figure 1D. The red shift in spectral position of the lowest absorption band (∼30 nm) indicates that the effective size of the QD is increasing with shell layer thickness. This effect

Figure 1. (A) Normalized absorption and PL spectra of the core (i) and gradient CSQDs of different shell thicknesses of (ii) 0.6, (iii) 1.3, and (iv) 2.3 nm. (B) Steady-state absorption spectra of a core QD (in toluene) and a core QD attached on ZnO NPs and bare ZnO NPs. (C) HR-TEM of the core QDs and gradient CSQDs of different shell thicknesses. (D) Scheme of the shell layer (i.e., potential barrier) formation around the 5 s core sample.

originates from the fact that electron and hole wave functions can extend to the shell layer. The shift is analogous (including its magnitude) to the same effect observed for the step-like CSQDs.20 This implies that even for our case of gradient CSQDs, we can define the core and the shell size because the charges at the lowest excited state stay well localized within the core. For further details and discussion about the core size, see the Supporting Information (Figure SI-1). For all samples, the PL spectrum consists of a single band (FWHM < 33 nm) originating from emission from core states of QDs. A gradually increasing red shift in the emission exciton band can be observed with growing the thicker shell around the core due to the increase of the effective size of the QDs. The prepared QDs were used to sensitize a layer of ZnO NPs. After sensitization, the measured absorbance spectrum becomes a sum of losses due to ZnO NPs scattering (background signal) and absorption of the QDs. The deposited QDs retain the band-like absorption structure, which is even more pronounced in the transient absorption spectra described in the next paragraph. ZnO NPs do not absorb in the studied spectral region (see Figure 1B). Ultrafast photoinduced dynamics of the colloidal QDs and QDs attached to ZnO NPs were investigated using TA spectroscopy under low excitation (∼1014 photons/cm2/ pulse, ∼0.1 e−h/QD/pulse).4 The TA spectra (see Figure 2A,B) show an instantaneous bleach of the lowest excitonic states caused by the electrons’ state filling. Under low 1761



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same character as previously reported decays of single exciton in CdSe QDs affected by fluctuating rates of nonradiative processes (QDs charging, change in surroundings, etc.).22,23 The long-lived recombination shows only minor changes with shell thickness. In contrast, the shell layer clearly affects the TA kinetics of CSQDs attached to ZnO (Figure 2C). To obtain the electron injection time, we first attempted to fit the TA kinetics using the bi- and triexponential decay function;21,24 however, the simple fit clearly could not reproduce the experimental data. Triexponential function fit could be applied only for the core (5 s growth time), where the effect of the shell is not prominent. Instead, we employed fitting by a stretched-exponential decay20 ΔA(t) = ΔA(0) × exp[−(t/τSE)B], where τSE denotes the decay time and B is the dispersion factor. Fitting parameters are summarized in Table 1. The stretched-exponential function has been previously used to describe the dynamics in disordered nanomaterials as well as electron injection in QDMO systems.25,26 In the case of the studied gradient CSQDs, the disorder is induced by the shell layer; its thickness, as well as composition, might vary from QD to QD. The dispersion factor 0 < B < 1 reflects the degree of disorder, and it decreases with increasing shell thickness as the longer shell growth time introduces more disorder into the system. It should be noted that we use B instead of commonly used β to avoid confusion with the later used β factor. By using the fitting data, we calculated the mean lifetime of the stretched-exponential function ⟨τ⟩CSQDs−ZnO = (τSE/B) ∗Γ(1/B), where Γ is the Gamma function. This allowed us to compare lifetimes of the TA kinetics measured for pure CSQDs and CSQDs attached to ZnO. The calculated ⟨τ⟩CSQDs−ZnO shows a clear dependence on the shell thickness, ranging from ∼6 ps for the core CdSe QDs to ∼15 ns for the thickest shell (see Figure 3A). We attribute the observed change in TA kinetics to electron injection into ZnO, analogously to the previous reports.20,24 Surface trapping of electrons can be ruled out by the fact that no fast decay is observed in the unattached QDs. From the mean TA decay times of the unattached CSQDs, ⟨τ⟩CSQDs, and CSQDs attached to ZnO, ⟨τ⟩CSQDs−NPs, we can determine the injection rate through the shell as4 kET(d) = (1/⟨τ⟩CSQDs−ZnO) − (1/⟨τ⟩CSQDs). For the thickest shell layers, the TA mean decay time of the CSQDs becomes seemingly even slower after attachment to ZnO (i.e., the injection rate would have to be negative). This is due to the uncertainty in the stretched-exponential fit (see Figure 3A and Table 1). Therefore, we consider values of the injection rate for the thickest shell layers as ∼0 (see the Supporting Information for detailed discussion).

Figure 2. Transient absorption spectra of the CdSe core in toluene (A) and CdSe−ZnO NPs (B) at different delay times. (C) Transient absorption kinetics of gradient CSQDs in toluene (red open circles and solid red line for fitted data) and gradient CSQDs attached to ZnO NPs (black open circles and solid black line for fitted data) with different shell thicknesses (i), (ii), (iii), and (iv) for the core: without any shell, 1.3, 1.6, and 1.9 nm shell thicknesses, respectively.

excitation, the bleach directly reflects the number of excited electrons in the QDs.19 As the shape of the TA spectra is not changing with the pump−probe delay (see Figure 2A,B), we will focus only on the TA kinetics at the maximum of the TA spectrum (Figure 2C). Comparing the TA decay of unattached CSQDs with the dots attached to ZnO enables us to distinguish the intrinsic electron−hole recombination in the QD from electron transfer to ZnO. The analysis is analogous to previous reports in QDacceptor systems.4,21 Normalized TA kinetics for both unattached and attached QDs with different shell thicknesses are shown in Figure 2C. First, we will discuss the TA kinetics for unattached CSQDs. We used a biexponential function to fit the TA decay, resulting in a minor subnanosecond and a dominant nanosecond component (A1 = 20−45%, τ1 = 0.1−0.3 ns; A2 = 55−80%, τ2 = 12−18 ns). The observed multiexponential decay has the

Table 1. Stretched-Exponential Function Parameters τCSQDs−ZnO and B, ⟨τ⟩CSQDs−ZnO Mean Injection Time, ⟨τ⟩CSQDs Recombination Lifetime, kET Electron Injection Rate Constant, ηinj Electron Injection Efficiency, and QY Luminescence Quantum Yield shell thickness (nm)

τSE (ps)

B

⟨τ⟩CSQDs−ZnO (ps)

⟨τ⟩CSQDs (ps)

0 0.6 1.3 1.6 1.9 2.3

6 9 700 3000 1000 30

1 0.31 0.39 0.37 0.25 0.17

6 67 2600 13000 23000 15000

12000 18000 17000 14500 13700 12400 1762

kET (ps)−1 0.167 0.015 (3.2 (8.1 (−3 (−1

± ± ± ± ± ±

0.003 0.008 1.1) × 10−4 0.3) × 10−6 4) × 10−5 10) × 10−5

ηinj (%)

QY (%)

99 99 84 11 0 0

29 42 63 67 45 52



Letter

Figure 3. (A) Mean decay rate dependence on the shell thickness for both pure QDs in toluene (red), QDs attached to ZnO NPs (black), and reported values for step-like CSQDs (ref 24). (B) Energetic scheme for gradient CSQDs−ZnO NPs and step-like CSQD systems (C). The dependence of injection efficiency ηinj from gradient CSQDs to ZnO NPs and the QY on the shell thickness.

the electron injection rate kET and exciton recombination rate kR, we can calculate the injection efficiency as ηinj = kET/(kET + kR). Even though the electron injection rate decreases extremely rapidly with the shell thickness, the injection efficiency stays almost unaffected for shell thicknesses up to 1.2 nm. This is due to the long recombination lifetimes in QDs; see Figure 3C. As a consequence, it is possible to use QD protection by a shell without losing the electron injection efficiency. Moreover, the back-recombination rate of the injected electrons is also decreased in the same manner. Therefore, a shell can improve solar cell parameters, even if the injection efficiency slightly decreases.2 At the same time, an important question is, whether the ∼1 nm thick shell can already passivate the QD sufficiently. In order to answer the question, we studied the dependence of the luminescence QY on the gradient shell thickness. The QY directly reflects carrier trapping on QD surface defects affected by the number of surface traps and electron wave function density close to surface of the QD. Although a high QY does not imply better photovoltaic parameters,27 the high value of the QY is a good measure of the degree of QD passivation, which is important for photostability of the QDs and secures that charges will not be trapped on the QD surface instead of being injected into ZnO. As is illustrated in Figure 3C, the QY for a 1.2 nm thick shell is about two times higher (∼60%) compared to that of the core only (30%). For the thicker shell layers (>1.6 nm), the QY does not increase any more due to lattice imperfections within the shell.20,28 Using a ∼1 nm thick shell is therefore also optimal from the point of view of QD passivation, which however does not come at the expense of electron injection efficiency, as we showed above.

Due to the fact that the electron has to tunnel through the shell barrier in order to be injected into ZnO, the chargetransfer rate follows the expected exponential dependence kET(d) ∝ (e−βd); see Figure 3A, black line.20,22 β denotes a parameter characterizing the shell properties.20 In the step-like CSQDs-acceptor system, the β value is related to the shell materials, and the typical values are reported to be 0.33−0.35 Å−1, where only the energy barrier width changes with the shell growth.20,22 However, in our system (gradient CSQDs−ZnO NPs), the gradual change of the band gap adds an additional variation, which occurs within the shell material in the radial direction (see Figure 3B). Consequently, the injection rate dependence on the shell thickness is enhanced, leading to β = 0.51 ± 0.05 Å−1 (see the Supporting Information, Figure SI-2 for the error estimate) as a result of both the barriers’ width and height change with the shell growth (Figure 3B). The red shift of the spectrum indicates that the effective core size is changing with the shell layer thickness. Because the band gap changes, even the driving force of the electron transfer changes, resulting in a slow-down of the transfer. We estimated the magnitude of this effect based on the previously published measurements of Robel et al.25 According to the published results, the change in the driving force should lead to less than four times the change in the electron injection rate. In contrast, we observe in our samples a change of the injection rate over more than 3 orders of magnitude. This implies that the change in the core size (driving force) represents only a minor correction, and it is neglected. The more detailed discussion of the effect can be found in the Supporting Information. Nevertheless, the important question for solar cell application is not the injection rate itself but rather the efficiency of electron injection (ηinj) into ZnO. By comparing 1763



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Dynamics of CdSe−ZnS Quantum Dots. J. Phys. Chem. C 2009, 114, 627−632. (8) Baranov, A. V.; Rakovich, Y. P.; Donegan, J. F.; Perova, T. S.; Moore, R. A.; Talapin, D. V.; Rogach, A. L.; Masumoto, Y.; Nabiev, I. Effect of ZnS Shell Thickness on the Phonon Spectra in CdSe Quantum Dots. Phys. Rev. B 2003, 68, 165306−165313. (9) Li, J. J.; Wang, Y. A.; Guo, W.; Keay, J. C.; Mishima, T. D.; Johnson, M. B.; Peng, X. Large-Scale Synthesis of Nearly Monodisperse CdSe/CdS Core/Shell Nanocrystals Using Air-Stable Reagents via Successive Ion Layer Adsorption and Reaction. J. Am. Chem. Soc. 2003, 125, 12567−12575. (10) Guo, W.; Li, J. J.; Wang, Y. A.; Peng, X. Luminescent CdSe/CdS Core/Shell Nanocrystals in Dendron Boxes: Superior Chemical, Photochemical and Thermal Stability. J. Am. Chem. Soc. 2003, 125, 3901−3909. (11) Peng, X.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. Epitaxial Growth of Highly Luminescent CdSe/CdS Core/Shell Nanocrystals with Photostability and Electronic Accessibility. J. Am. Chem. Soc. 1997, 119, 7019−7029. (12) Bae, W. K.; Char, K.; Hur, H.; Lee, S. Single-Step Synthesis of Quantum Dots with Chemical Composition Gradients. Chem. Mater. 2008, 20, 531−539. (13) Ratchford, D.; Dziatkowski, K.; Hartsfield, T.; Li, X.; Gao, Y.; Tang, Z. Photoluminescence Dynamics of Ensemble and Individual CdSe/ZnS Quantum Dots with an Alloyed Core/shell Interface. J. Appl. Phys. 2011, 109, 103509−103515. (14) Smith, A. M.; Nie, S. Semiconductor Nanocrystals: Structure, Properties, and Band Gap Engineering. Acc. Chem. Res. 2009, 43, 190− 200. (15) Cragg, G. E.; Efros, A. L. Suppression of Auger Processes in Confined Structures. Nano Lett. 2009, 10, 313−317. (16) Bae, W. K.; Kwak, J.; Park, J. W.; Char, K.; Lee, C.; Lee, S. Highly Efficient Green-Light-Emitting Diodes Based on CdSe@ZnS Quantum Dots with a Chemical-Composition Gradient. Adv. Mater. 2009, 21, 1690−1694. (17) Zheng, K.; Ž ídek, K.; Abdellah, M.; Torbjörnsson, M.; Chábera, P.; Shao, S.; Zhang, F.; Pullerits, T. Fast Monolayer Adsorption and Slow Energy Transfer in CdSe Quantum Dot Sensitized ZnO Nanowires. J. Phys. Chem. A 2012, DOI: 10.1021/jp3098632. (18) Fischer, M.; Georges, J. Fluorescence Quantum Yield of Rhodamine 6G in Ethanol as a Function of Concentration Using Thermal Lens Spectrometry. Chem. Phys. Lett. 1996, 260, 115−118. (19) Ž ídek, K.; Zheng, K.; Chabera, P.; Abdellah, M.; Pullerits, T. Quantum Dot Photodegradation Due to CdSe−ZnO Charge Transfer: Transient absorption study. Appl. Phys. Lett. 2012, 100, 243111− 243115. (20) Zhu, H.; Song, N.; Lian, T. Controlling Charge Separation and Recombination Rates in CdSe/ZnS Type I Core−Shell Quantum Dots by Shell Thicknesses. J. Am. Chem. Soc. 2010, 132, 15038−15045. (21) Klimov, V. I. Spectral and Dynamical Properties of Multiexcitons in Semiconductor Nanocrystals. Annu. Rev. Phys. Chem. 2007, 58, 635−673. (22) Dworak, L.; Matylitsky, V. V.; Breus, V. V.; Braun, M.; Basché, T.; Wachtveitl, J. Ultrafast Charge Separation at the CdSe/CdS Core/ Shell Quantum Dot/Methylviologen Interface: Implications for Nanocrystal Solar Cells. J. Phys. Chem. C 2011, 115, 3949−3955. (23) Sharma, S. N.; Pillai, Z. S.; Kamat, P. V. Photoinduced Charge Transfer between CdSe Quantum Dots and p-Phenylenediamine. J. Phys. Chem. B 2003, 107, 10088−10093. (24) Ž ídek, K.; Zheng, K.; Ponseca, C. S.; Messing, M. E.; Wallenberg, L. R.; Chábera, P.; Abdellah, M.; Sundström, V.; Pullerits, T. Electron Transfer in Quantum-Dot-Sensitized ZnO Nanowires: Ultrafast Time-Resolved Absorption and Terahertz Study. J. Am. Chem. Soc. 2012, 134, 12110−12117. (25) Robel, I.; Kuno, M.; Kamat, P. V. Size-Dependent Electron Injection from Excited CdSe Quantum Dots into TiO2 Nanoparticles. J. Am. Chem. Soc. 2007, 129, 4136−4137. (26) Phillips, J. C. Stretched Exponential Relaxation in Molecular and Electronic Glasses. Rep. Prog. Phys. 1996, 59, 1133−1207.

In conclusion, we studied the electron injection from unconventional gradient CSQDs with an alloyed interface between the core and shell to ZnO NPs. The main advantage of using this type of CSQDs is to reduce the interface defects that can be induced from the sudden change in the materials with a step-like composition change. We observed a significant improvement in QD passivation for shell layer of about 1 nm. Noteworthy, the electron injection rate constant shows a strong exponential dependence on the shell thickness with a factor of β = 0.51 ± 0.05 Å−1. However, even for the 1 nm thick shell layer, the electron injection efficiency stays above 90% due to the long exciton recombination time in gradient CSQDs. As the shell layer on the top of that also slows down backrecombination of the injected electron, the resulting performance of the solar cell can even improve. Hence, the photostable gradual CSQDs can be indeed applied in QD-based solar cells, but careful control over the used shell thickness with respect to the CSQDs’ photostability and the electron injection efficiency is indispensable.

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ASSOCIATED CONTENT

S Supporting Information *

Discussion of the core size dependence on the core−shell quantum dot growth time and estimation of the error in the β value. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support of the Knut and Alice Wallenberg Foundation, Crafoord Foundation, Swedish Energy Agency, the Swedish Foundation for Strategic Research, and Erasmus Mundus program. We thank Prof. Reine Wallenberg for fruitful discussion about HR-TEM images and Dr. Jens Uhlig for critical reading of the manuscript. Collaboration within nmC@LU is acknowledged.

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REFERENCES

(1) Kamat, P. V. Quantum Dot Solar Cells. Semiconductor Nanocrystals as Light Harvesters. J. Phys. Chem. C 2008, 112, 18737−18753. (2) Guijarro, N.; Campina, J. M.; Shen, Q.; Toyoda, T.; LanaVillarreal, T.; Gomez, R. Uncovering the Role of the ZnS Treatment in the Performance of Quantum Dot Sensitized Solar Cells. Phys. Chem. Chem. Phys. 2011, 13, 12024−12032. (3) Yu, W. W.; Qu, L.; Guo, W.; Peng, X. Experimental Determination of the Extinction Coefficient of CdTe, CdSe, and CdS Nanocrystals. Chem. Mater. 2003, 15, 2854−2860. (4) Ž ídek, K.; Zheng, K.; Abdellah, M.; Lenngren, N.; Chábera, P.; Pullerits, T. Ultrafast Dynamics of Multiple Exciton Harvesting in the CdSe−ZnO System: Electron Injection versus Auger Recombination. Nano Lett. 2012, 12, 6393−6399. (5) Sambur, J. B.; Novet, T.; Parkinson, B. A. Multiple Exciton Collection in a Sensitized Photovoltaic System. Science 2010, 330, 63− 69. (6) Kamat, P. V. Quantum Dot Solar Cells. The Next Big Thing in Photovoltaics. J. Phys. Chem. Lett. 2013, 4, 908−918. (7) Makhal, A.; Yan, H.; Lemmens, P.; Pal, S. K. Light Harvesting Semiconductor Core−Shell Nanocrystals: Ultrafast Charge Transport 1764



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(27) Kim, S.; Fisher, B.; Eisler, H.; Bawendi, M. Type-II Quantum Dots: CdTe/CdSe(Core/Shell) and CdSe/ZnTe(Core/Shell) Heterostructures. J. Am. Chem. Soc. 2003, 125, 11466−11467. (28) Xie, R.; Kolb, U.; Li, J.; Basché, T.; Mews, A. Synthesis and Characterization of Highly Luminescent CdSe−Core CdS/ Zn0.5Cd0.5S/ZnS Multishell Nanocrystals. J. Am. Chem. Soc. 2005, 127, 7480−7488.

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Balancing Electron Transfer and Surface Passivation in Gradient

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