Electron Transfer Dynamics in a CdSe Quantum Dot

Nov 26, 2013 - and photoexcited PGR can inject an electron into CdSe QDs. ... Hole transfer time from the photoexcited CdSe QD to PGR is found to be 5...
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Ultrafast Hole/Electron Transfer Dynamics in a CdSe Quantum Dot Sensitized by Pyrogallol Red: A Super-Sensitization System Pallavi Singhal† and Hirendra N. Ghosh*,‡ †

Health Physics Division and ‡Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India S Supporting Information *

ABSTRACT: To find a suitable hole-transporting adsorbate for CdSe quantum dots (QDs), a pyrogallol red (PGR) molecule was chosen where PGR also can sensitize CdSe QDs. Energy level diagrams suggest that the photoexcited hole can be transferred to PGR and photoexcited PGR can inject an electron into CdSe QDs. Steady-state and timeresolved emission studies suggest that the photoexcited hole is transferred to PGR; however, the process is too fast to monitor with the subnanosecond time-resolution spectroscopic technique. Femtosecond transient absorption spectroscopy has been employed to monitor the charge-transfer behavior of the above system in an early time scale. Photoexcitation of pure PGR and CdSe QDs at 400 nm laser light gives the transient absorption due to the photoexcited singlet state of PGR and charge carriers (electron/ hole) in CdSe QDs, respectively, in the visible/near-IR region of the absorption spectra. However, on photoexcitation of the CdSe/PGR composite at 400 nm, the PGR cation radical and electron in the CdSe QD were detected in the transient absorption spectra. Hole transfer time from the photoexcited CdSe QD to PGR is found to be 500 fs. The transient signal due to the PGR cation and electron in the CdSe QD also contributed to photoexcitation of PGR on the CdSe QD, where electron injection is found to be 200 ps (85%) confirming a grand charge-separated state in the CdSe/PGR composite system.

1. INTRODUCTION Understanding charge-transfer dynamics between quantum dots (QDs) to molecule and molecule to quantum dots is essential for their application in many electronic devices like quantum dot based solar cells.1−10 The interest in charge transfer in the molecule−QD system has been intensified due to reports on multiple exciton generation (MEG) in quantum dots.11−21 It has been realized and partially verified that the efficiency of a solar module can be enhanced significantly by dissociating or separating the multiexciton before ultrafast exciton−exciton annihilation.22−24 Using a suitable adsorbate molecule (electron or hole acceptors) on photoexcited QD materials, ultrafast exciton dissociation can be a reality. Exciton dissociation in the fast and ultrafast time scale through electron transfer in QD/molecular adsorbate and QD/semiconductor nanoparticle (TiO2, ZnO etc.) systems has been widely investigated.4−10,25−30 Electron transfer time constants are reported to be 10 s of picosecond to 10 s of femtosecond, where most of the studies are carried out using fast and ultrafast transient absorption techniques and the electrons have been directly detected in the visible/near IR/IR region. However, not many reports are available on hole transfer dynamics from the photoexcited QD to a molecule. One of the main reasons is that holes are not characterized properly due to featureless weak absorption in the near IR region, and also the nature of the transition in transient absorption is not well understood.31−33 Hole transfer time constants are reported to be in nanosecond to 100 s of picoseconds time scale.10,34−36 Most of © XXXX American Chemical Society

the measurements were carried out by time-resolved emission quenching spectroscopy where time constants are determined after following the emission quenching of QDs in the presence of hole acceptor materials6,10,34−36 which is an indirect way to determine hole transfer dynamics. However, Kamat and coworkers10 reported a hole transfer time of 10 s of nanoseconds in CdSe QD−p-phenylenediamine after monitoring the adsorbate cation radical as measured by transient absorption spectroscopy. Similarly Lian and co-workers9 have reported a hole transfer time ∼300 ps in phenothiazine−CdSe QD and different core−shell composite materials as measured by transient absorption spectroscopy detecting the transients in the visible region, where the excited state and oxidized state of the molecular adsorbate are highly overlapped. Electron transfer takes place in the fast and ultrafast time scale in QD−dye composite materials, and at the same time it is quite strange that hole transfer takes place in such a longer and different range of time scales. The reason behind the occurrence of hole transfer time in different time scales needs to be addressed in the QD−dye composites. Recently, it was reported that organo metal halide perovskites, a new type of semiconductor material, can be used for designing highly efficient solar cells.37−39 Lee et al.37 demonstrated that in both Special Issue: Michael Grätzel Festschrift Received: August 31, 2013 Revised: November 15, 2013

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TiO2 and Al2O3 (wide bad gap semiconductor) film higher conversion efficiency is obtained due to fast release of hole to the hole-transport materials in the solar devices. So it is essential to understand hole transfer dynamics in early time scales in all the above composite materials. So far, the highest conversion efficiency in quantum dot solar cells (QDSCs) has been reported by Kamat and co-workers40 to be ∼5.2% which is much lower than conventional dyesensitized solar cells (DSSCs).41 The main factors for the overall lower efficiency are mainly due to limited absorption of solar radiation by the QD materials and slow hole transfer rate. Again the size of the QD is larger (sometimes more than 2−3 nm larger) as compared to the dye molecule (size is less than 1 nm), and as a result QD loading on the TiO2 electrode is much less as compared to dye loading in DSSCs. Due to lower loading, injected electrons in QDSCs are in direct contact with the electrolyte which is undesirable. All these problems can be tackled by introducing the concept of super sensitization in solar cells.42−44 In supersensitization, the quantum dot and molecular adsorbate can exchange charge carriers where molecular adsorbate in addition to photosensitizing the quantum dot material can also act as a hole-transporting material, where holes are generated out of photoexcitation of the QD. As a result grand charge separation can take place in the QD−molecular composite material, and it can be used as a supersensitizer. Until now not many reports are available in the literature on charge-transfer dynamics in the ultrafast time scale in supersensitizer materials, except the work reported by Kamat and co-workers42 in CdS QD sensitized squaraine dye molecules. However, the dynamics of the hole and electron transfer process in supersensitizers (QD−dye composite) are not reported precisely in the literature. In the present investigation, we have chosen CdSe as the quantum dot and pyrogallol red (PGR) as the molecular adsorbate where both CdSe QD1−3 and PGR45 individually can sensitize TiO2 nanoparticles. Therefore, the CdSe/PGR composite material can be used as a supersensitizer on TiO2 electrodes. The redox energy level of the CdSe QD and PGR molecule suggest that the conduction band of CdSe lies below the LUMO level (both S2 and S1 states) of PGR, and the HOMO level of PGR lies above the valence band of the CdSe QD (Scheme 1). In this situation the photoexcited hole in the CdSe QD can be captured by PGR molecules, and photoexcited PGR can inject an electron in the conduction band of the CdSe QD. We have carried out steady state and timeresolved emission measurements to confirm the hole transfer process in the above system. Femtosecond transient absorption spectroscopy has been employed to monitor the charge (both electron and hole) transfer dynamics in the CdSe/PGR composite system in an ultrafast time scale, and grand charge separation has been demonstrated.

Scheme 1. Charge-Transfer Process in CdSe/PGR Composite Materialsa

a

Photoexcitation of the CdSe QD below 550 nm radiation generates electron (e−) and hole (h+) pairs where the hole (h+) is captured by the PGR molecule; on the other hand, photoexcited PGR by below 650 nm radiation injects an electron in the conduction band of CdSe. The molecular structure of PGR is shown in the scheme.

nanopure water. Mercaptopropionic acid (7.05 mmol) was added to this solution under stirring conditions. pH of the reaction mixture was increased slowly to 9−10 by using 1 M NaOH. Selenium precursor (NaHSe) was prepared separately by reducing selenium powder using sodium borohydride at 3−4 °C. This selenium precursor (NaHSe) was added to the reaction mixture, and reaction temperature was maintained at 90 °C for 4 h. The growth of the CdSe QD was monitored by measuring absorbance at different time intervals. After the reaction was over, the aliquot was concentrated to one-thrid of its original volume by using a rotary evaporator. Isopropyl alcohol was added to this concentrated solution as nonsolvent. After addition of isopropyl alcohol, QD gets precipitated. The supernatant solution at this stage was thrown out, and the precipitated QD was redissolved in nanopure water. This step was repeated two more times. The cleaned QD obtained was used for further experiments. The exciton peak for the CdSe QD was found to be at ∼463 nm. B). Techniques. a). Steady State Absorption and Emission Spectrometer. Steady state absorption spectra were recorded on a Thermo-Electron model Biomate spectrophotometer. Fluorescence spectra, which were corrected for the wavelength dependence of the instrument sensitivity, were recorded using a Hitachi model 4010 spectrofluorimeter. b). Time-Correlated Single-Photon Counting (TCSPC). Time-resolved fluorescence measurements were carried out using a diode laser-based spectrofluorometer from IBH (U.K.). The instrument works on the principle of time-correlated single-photon counting (TCSPC). In the present work, a 406 nm laser light (fwhm < 100 ps) was used as the excitation light sources, and a TBX4 detection module (IBH) coupled with a special Hamamatsu PMT was used for fluorescence detection. c). Femtosecond Transient Absorption Studies. The femtosecond tunable visible spectrometer has been developed

2. EXPERIMENTAL SECTION A). Synthesis. a). Chemicals. Cadmium chloride (≥99%), selenium powder (≥98), 1-mercaptopropionic acid (≥99%), and sodium borohydrate were purchased from Aldrich and used as received without further purification. Isopropyl alcohol was used as received from s.d. fine chemicals. Pyrogallol red (PGR) dye was obtained from Aldrich and used without further purification. b). Synthesis of CdSe Quantum Dots. The water-soluble CdSe QD was prepared by a previously reported method.46 In brief, 2.35 mmol (1.58g) of cadmium chloride was dissolved in B

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based on a multipass amplified femtosecond Ti:sapphire laser system supplied by Thales, France. The pulses of 20 fs duration and 4 nJ energy per pulse at 800 nm obtained from a self-modelocked Ti-sapphire laser oscillator (Synergy 20, Femtolaser, Austria) were amplified in a regenerative and two-pass amplifier pumped by a 20 W DPSS laser (Jade) to generate 40 fs laser pulses of about 1.2 mJ energy at a repetition rate of 1 kHz. The 800 nm output pulse from the multipass amplifier is split into two parts to generate pump and probe pulses. In the present investigation we have used frequency doubled 400 nm as excitation sources. To generate pump pulses at 400 nm one part of 800 nm with 200 μJ/pulse is frequency doubled in BBO crystals. To generate visible probe pulses, about 3 μJ of the 800 nm beam is focused onto a 1.5 mm thick sapphire window. The intensity of the 800 nm beam is adjusted by iris size and ND filters to obtain a stable white light continuum in the 400 nm to over 1000 nm region. The probe pulses are split into the signal and reference beams and are detected by two matched photodiodes with variable gain. We have kept the spot sizes of the pump beam and probe beam at the crossing point around 500 and 300 μm, respectively. The noise level of the white light is about ∼0.5% with occasional spikes due to oscillator fluctuation. We have noticed that most laser noise is low-frequency noise and can be eliminated by comparing the adjacent probe laser pulses (pump blocked vs unblocked using a mechanical chopper). The typical noise in the measured absorbance change is about 30 ns (7%) with τavg = 2.14 ns. Finally in the presence of 100 μM PGR concentration, it is clearly seen that emission decay is extremely fast with pulse-width limited decay (τavg ∼ 0.12 ns). We have also carried out time-resolved studies for the CdSe/PGR composite system at lower PGR concentration and observed relatively faster decay as compared to pure CdSe QDs. It indicates that hole transfer still occurs at lower concentration of PGR, but surface modification dominates over hole transfer; therefore, we have seen an increase in luminescence intensity of the CdSe QD (Figure 2). As the emission quenching is very rapid, the fluorescence decay process can be a good approximation to the charge-transfer (CT) process. However, with nanosecond time resolution it will not be judicious to determine hole transfer time in the CdSe/PGR system. So to determine hole transfer dynamics correctly it is very important to monitor these processes in an ultrafast time scale. It is clear from Scheme 1 that on photoexcitation of CdSe the photoexcited hole can be transferred to PGR; however, at the same time, laser light also can excite the PGR molecule, and therefore photoexcited PGR can also inject an electron into the conduction band of CdSe. However, in the CdSe/PGR system we have only confirmed hole transfer dynamics after monitoring the luminescence quenching of CdSe QDs. It is also important to monitor electron transfer dynamics in the above system. As PGR is a nonluminescent molecule in water, the electron transfer process could not be monitored by using luminescence spectroscopy. Now to determine both electron and hole transfer dynamics in the early time scale, femtosecond transient absorption spectroscopic measurements have been carried out and described in detail in the next section. c). Femtosecond Transient Absorption Spectroscopy. To understand interfacial charge (both electron and hole) transfer dynamics in the CdSe/PGR system in the ultrafast time scale we have carried out femtosecond transient absorption spectroscopy on CdSe QD, PGR, and CdSe/PGR composite systems. Figure 4A shows the transient absorption spectra of photoexcited CdSe QD materials in different time delays, which comprise a broad positive absorption band in the 500−900 nm region. The first exciton position for the CdSe core samples (Figure 1a) appears at ∼463 nm; however, in the transient spectrum we could show only from 490 nm due to the low intensity of the probe light below 490 nm in our experimental

setup. The transient positive absorption beyond the 500 nm region can be attributed to absorption of light by photogenerated charge carriers (both electrons and holes), free or trapped in the surface states. Again, Figure 4B shows the transient absorption spectra of photoexcited PGR in water in different time delays, which comprise a negative absorption band in the 500−600 nm region and a broad positive absorption band in the 620−900 nm region. The negative absorption (bleach) appears due to photoexcitation of the ground state molecules, where the molecules have optical absorption in the same spectral region (Figure 1f). The transient absorption signal of PGR can be attributed to the excited singlet (S1) state absorption. Now to understand charge-transfer dynamics in the ultrafast time scale in the CdSe/PGR system we have excited the composite system by 400 nm laser light. Figure 4C shows the transient absorption spectra of photoexcited CdSe/PGR composite materials in different time delays, which comprise a small bleach at 510 nm and two broad absorption bands at 600−850 nm and 850− 1000 nm, respectively. The broad spectral absorption in the 850−1000 nm regions can be attributed to the electrons in the conduction band of the CdSe QD. We have already reported that electrons in the conduction band of QD materials can be detected by visible and near IR absorption bands.59,60 The transient absorption peak at 600−800 nm can be attributed to the PGR cation radical.45 The band having a maximum at 690 nm is assigned to the PGR cation radical (PGR•+). Assignment of this band has been made on the basis of the results obtained in separate pulse radiolysis experiments (Supporting Information), where PGR•+ was generated selectively by the reaction of the N3• radical with the PGR molecule in N2O saturated aqueous solution. In our earlier investigation on the PGR/TiO2 system,45 we have observed that on excitation of the PGR molecule it injects an electron into the conduction band of TiO2 and it is converted to a PGR•+ cation radical which has a transient absorption band at 690 nm. To understand the charge carrier dynamics in photoexcited CdSe and excited state dynamics of PGR we have monitored the transients in both the systems, as shown in Figure 5. The kinetic traces for CdSe QD at 690 nm can be fitted with time constants τ1 = 0.5 ps (33%), τ2 = 2.5 ps (36%), and τ3 > 200 ps (31%) (Figure 5a) and at 900 nm with time constants τ1 = 0.3 ps (42%), τ2 = 2 ps (44%), and τ3 > 200 ps (14%) (Figure 5b). The shorter components at both the wavelengths can be

Figure 4. Transient absorption spectra of (A) CdSe (top panel), (B) PGR (middle panel), and (C) CdSe/PGR composite materials (bottom panel) in water at different time delays after excitation at 400 nm laser light.

Figure 5. Kinetic decay traces at panel A: (a) 690 nm and (b) 900 nm for CdSe; at panel B: (c) 690 nm and (d) 900 nm for PGR; and at panel C: (e) 690 nm and (f) 900 nm for the CdSe/PGR system in water after exciting the samples at 400 nm laser light. E

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smaller fraction (25%) of light at 400 nm; so, it is expected that the majority of the cation radical will be formed through electron injection from photoexcited PGR to CdSe QD which is clearly reflected in our time-resolved absorption data. In the present studies the major charge separation process in the CdSe/PGR composite system takes place by electron injection from photoexcited PGR to the CdSe QD which is clearly seen from the growth kinetics of the PGR cation radical. Now it is very interesting to see that in addition to difference in growth dynamics of the transients at 690 and 900 nm we have also observed a difference in decay dynamics at both the wavelengths. We have observed the decay kinetics at 690 nm fitted biexponentially with τ1 = 4 ps (15%) and τ2 = >200 ps (85%) which can be attributed to the recombination reaction between the PGR cation and electron in the CdSe QD. The 900 nm transient which has been attributed to the electron in the CdSe QD decays with time constants τ1 = 600 fs (38%), τ2 = 2 ps (26%), and τ1 = >200 ps (36%). The faster components like 600 fs and 2 ps at 900 nm can be attributed to electron trapping dynamics in the CdSe QD. However, the long time component (>200 ps) can be attributed to recombination dynamics between the PGR cation radical and the electron in the CdSe QD. As we have already mentioned in the present investigation, CdSe QDs have defect states which has been confirmed from CdSe QD surface state emission. So it is quite obvious that once the electrons are excited from the valence band or injected into the conduction band it will go for trapping process. It is interesting to see that early time dynamics at 900 nm is quite similar in pure CdSe QDs (Figure 5b) and the CdSe/PGR composite (Figure 5f). However, in longer time scales it is clear that the kinetic trace for the pure CdSe QD decays much faster as compared to that of the CdSe/ PGR composite. The most interesting observation in the present investigation is the slow recombination dynamics between the electron in the CdSe QD and PGR cation radical. This observation confirms that the CdSe/PGR composite system can be used as a supersensitizer in quantum dot solar cells as a conduction band of the CdSe QD and LUMO level (both S1 and S2 levels) of PGR lie above the conduction band of the TiO2 nanoparticle.

attributed to trapping of charge carriers, and the longer component (>200 ps) can be attributed to recombination dynamics of photoexcited carriers. On the other hand, the kinetics for PGR at 690 nm can be fitted with 0.5 ps (60%) and 2.5 ps (40%) growth components and with decay of >200 ps (Figure 5c), and the kinetic trace at 900 nm can be fitted with 3.6 ps growth and >200 ps decay components (Figure 5d). The growth time constants 2−3 ps can be attributed to vibrational relaxation. Now to understand the charge-transfer dynamics in the CdSe/PGR composite system we have monitored the kinetics at 690 and 900 nm, as shown in Figure 5C. The kinetic trace at 690 nm can be fitted with biexponential growth with time constants of 150 fs (85%) and 500 fs (15%) and eventually decays with time constants τ1 = 4 ps (15%) and τ2 = >200 ps (85%) (Figure 5e). However, the kinetic trace at 900 nm can be fitted with ∼150 fs growth time and multiexponential decay with time constants of τ1 = 600 fs (38%), τ2 = 2 ps (26%), and τ1 = >200 ps (36%) (Figure 5f). It is very interesting to see that growth and decay dynamics are completely different at 690 and 900 nm in the CdSe/PGR composite system. This observation clearly suggests that generation of electron in the conduction band and formation of the PGR cation radical might be taking place through more than one process. In the next section we have demonstrated charge (both electron and hole) transfer dynamics in the CdSe/ PGR composite system where photoexcited PGR injects an electron in the CdSe QD and holes generated from photoexcited CdSe QDs can be transferred to PGR with the formation of a PGR cation radical. d). Charge-Transfer Dynamics in the CdSe/PGR Supersensitizer. It has been realized that one of the main problems in quantum dot solar cells for lower efficiency is slow transport of a photogenerated hole from the QD. In the present investigation we are using PGR as a hole-quenching material where PGR can serve as a hole transporter material and at the same time can also sensitize the QD material. So it is very important to understand the charge-transfer dynamics in the CdSe/PGR composite system. Scheme 1 shows the energy level diagram of the CdSe/PGR quantum dot−molecular adsorbate material. It is clearly seen that the valence band and conduction band of CdSe lie below the HOMO and LUMO (both S1 and S 2 ) level of PGR, respectively. So on photoexcitation of the CdSe QD by radiation below 550 nm (Figure 1a) light electrons and holes are generated where holes can be transferred to PGR molecule as it is thermodynamically viable. On the other hand, photoexcitation of the PGR molecule below 650 nm light (Figure 1f) (both S1 and S2 states) can inject an electron into the conduction band of the CdSe QD. As a result, photoexcitation of either the PGR molecule or CdSe QD leads to charge separation in the CdSe/ PGR composite system, where in both cases electrons are localized in the CdSe QD and holes are localized in PGR with the formation of a PGR cation radical. In femtosecond timeresolved absorption studies we can clearly see the appearance of an electron at 900 nm (near IR region) in the ∼150 fs time scale; however, the PGR cation radical appears at 690 nm biexponentially with time constants of 150 fs (85%) and 500 fs (15%). A longer growth time (500 fs) of the PGR cation radical is attributed to hole transfer time from the CdSe QD to PGR molecule, and shorter growth time (150 fs) is attributed to the electron injection from excited PGR to CdSe. We have observed in Table 1 that PGR absorbs a larger fraction of light in the composite system (∼75%), and the CdSe QD absorbs a

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CONCLUSION In conclusion, in search of suitable hole-transporting adsorbate and at the same time sensitizing the quantum dot material we have carried out steady state and time-resolved spectroscopic studies of a CdSe quantum dot sensitized PGR molecular system. Energy level diagrams of the CdSe QD and PGR suggest that photoexcited hole can be transferred from the CdSe QD to the PGR molecule, and photoexcited PGR can inject an electron into the conduction band of the CdSe QD. At a lower concentration range (≤10 μM) the PGR molecule was found to modify the CdSe QD surface resulting in an increase in photoluminescence of the QD. However, at higher PGR concentration photoluminescence of the CdSe QD gets completely quenched indicating hole transfer from the photoexcited CdSe QD to the PGR molecule. Femtosecond transient absorption studies reveal that on exciting the PGR molecule the transient spectra show a bleach below the 600 nm region and weak transient absorption due to the excited singlet state in the 600−900 nm region. The excited state absorption shows 2−3 ps growth which can be attributed to vibrational relaxation and decays with time constant >200 ps. On photoexcitation of the CdSe QD transient absorption due to F

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(7) Boulesbaa, A.; Huang, Z.; Wu, D.; Lian, T. Competition Between Energy and Electron Transfer from CdSe QDs to Adsorbed Rhodamine B. J. Phys. Chem. C 2010, 114, 962−969. (8) Huang, J.; Huang, Z.; Yang, Y.; Zhu, H.; Lian, T. Multiple Exciton Dissociation in CdSe Quantum Dots by Ultrafast Electron Transfer to Adsorbed Methylene Blue. J. Am. Chem. Soc. 2010, 132, 4858−4864. (9) Huang, J.; Huang, Z.; Jin, S.; Lian, T. Exciton Dissociation in CdSe Quantum Dots by Hole Transfer to Phenothiazine. J. Phys. Chem. C 2008, 112, 19734−19738. (10) 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. (11) Schaller, R. D.; Klimov, V. I. High Efficiency Carrier Multiplication in PbSe Nanocrystals: Implications for Solar Energy Conversion. Phys. Rev. Lett. 2004, 92, 186601. (12) Ellingson, R. J.; Beard, M. C.; Johnson, J. C.; Yu, P.; Micic, O. I.; Nozik, A. J.; Shabaev, A.; Efros, A. L. Highly Efficient Multiple Exciton Generation in Colloidal PbSe and PbS Quantum Dots. Nano Lett. 2005, 5, 865−871. (13) Luther, J. M.; Beard, M. C.; Song, Q.; Law, M.; Ellingson, R. J.; Nozik, A. J. Multiple Exciton Generation in Films of Electronically Coupled PbSe Quantum Dots. Nano Lett. 2007, 7, 1779−1784. (14) Schaller, R. D.; Sykora, M.; Pietryga, J. M.; Klimov, V. I. Seven Excitons at a Cost of One: Redefining the Limits for Conversion Efficiency of Photons into Charge Carriers. Nano Lett. 2006, 6, 424− 429. (15) Trinh, M. T.; Polak, L.; Schins, J. M.; Houtepen, A. J.; Vaxenburg, R.; Maikov, G. I.; Grinbom, G.; Midgett, A. G.; Luther, J. M.; Beard, M. C.; et al. Anomalous Independence of Multiple Exciton Generation on Different Group IV−VI Quantum Dot Architectures. Nano Lett. 2011, 11, 1623−1629. (16) Murphy, J. E.; Beard, M. C.; Norman, A. G.; Ahrenkiel, S. P.; Johnson, J. C.; Yu, P.; Micic, O. I.; Ellingson, R. J.; Nozik, A. J. PbTe Colloidal Nanocrystals: Synthesis, Characterization, and Multiple Exciton Generation. J. Am. Chem. Soc. 2006, 128, 3241−3247. (17) Stewart, J. T.; Padilha, L. A.; Bae, W. K.; Koh, W.-K.; Pietryga, J. M.; Klimov, V. I. Carrier Multiplication in Quantum Dots within the Framework of Two Competing Energy Relaxation Mechanisms. J. Phys. Chem. Lett. 2013, 4, 2061−2068. (18) Schaller, R. D.; Sykora, M.; Jeong, S.; Klimov, V. I. HighEfficiency Carrier Multiplication and Ultrafast Charge Separation in Semiconductor Nanocrystals Studied via Time-Resolved Photoluminescence. J. Phys. Chem. B 2006, 110, 25332−25338. (19) Schaller, R. D.; Pietryga, J. M.; Klimov, V. I. Carrier Multiplication in InAs Nanocrystal Quantum Dots with an Onset Defined by the Energy Conservation Limit. Nano Lett. 2007, 7, 3469− 3476. (20) Pijpers, J. J. H.; Hendry, E.; Milder, M. T. W.; Fanciulli, R.; Savolainen, J.; Herek, J. L.; Vanmaekelbergh, D.; Ruhman, S.; Mocatta, D.; Oron, D.; et al. Carrier Multiplication and Its Reduction by Photodoping in Colloidal InAs Quantum Dots. J. Phys. Chem. C 2007, 111, 4146−4152. (21) Knowles, K. E.; Peterson, M. D.; McPhail, M. R.; Weiss, E. A. Exciton Dissociation within Quantum Dot−Organic Complexes: Mechanisms, Use as a Probe of Interfacial Structure, and Applications. J. Phys. Chem. C 2013, 117, 10229−10243. (22) Klimov, V. I. Spectral and Dynamical Properties of Multiexcitons in Semiconductor Nanocrystals. Annu. Rev. Phys. Chem. 2007, 58, 635−673. (23) Nozik, A. J. Spectroscopy and Hot Hole Relaxation Dynamics in Semiconductor Quantum Wells and Quantum Dots. Annu. Rev. Phys. Chem. 2001, 52, 193−231. (24) Beard, M. C.; Luther, J. M.; Semonin, O. E.; Nozik, A. J. Third Generation Photovoltaics Based on Multiple Exciton Generation in Quantum Confined Semiconductors. Acc. Chem. Res. 2013, 46, 1252− 1260. (25) Spanhel, l.; Weller, H.; Henglein, A. Photochemistry of Semiconductor Colloids. 22. Electron Injection from Illuminated

charge carrier (both electron and hole) is observed in the 500− 900 nm region which decays multiexponentially. Trapping dynamics of the charge carriers of photoexcited CdSe QDs was found to be 500 fs−2 ps. Photoexcitation of the CdSe/PGR composite shows a transient absorption band at 600−850 nm peaking at 690 nm due to the PGR cation radical and in the 850−1000 nm region due to the electron in the CdSe QD. The formation of the PGR cation radical is found to be a biexponential process with time constants of 150 and 500 fs, where the 500 fs component is attributed to the transfer of the hole from photoexcited CdSe to PGR and the 150 fs component is attributed to electron injection from photoexcited PGR to the CdSe QD. Charge recombination dynamics were found to be extremely slow (≫200 ps) confirming grand charge separation in the CdSe/PGR composite system and also proving its utility as a supersensitizer in quantum dot solar cells.

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ASSOCIATED CONTENT

* Supporting Information S

Transient absorption spectrum of the cation radical of pyrogallol red (PGR) obtained from one electron pulse radiolysis. 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]. Tel.: (91)-22-2559-0300. Fax: (91)- 22-2550-5151. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.S. thanks Dr. Sandeep Verma, Tushar Debnath. and Partha Maity of Radiation & Photochemistry Division for their help with experimental measurements. P.S. also thanks Dr. P. M. Ravi, Dr. R. M. Tripathi, and Dr. D. N. Sharma of Health Safety and Enviornment Group, Bhabha Atomic Research Centre, for their kind support and encouragement. Both the authors thank Dr. D. K. Palit and Dr. B. N. Jagatap of Bhabha Atomic Research Centre of Mumbai for their encouragement.

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