CdSe Cosensitized

ARTICLE pubs.acs.org/JPCC

Charge Transfer in the Heterointerfaces of CdS/CdSe Cosensitized TiO2 Photoelectrode Kung-Hsuan Lin,† Cho-Ying Chuang, Yu-Yang Lee, Fung-Chieh Li, and Yu-Ming Chang* Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan

I-Ping Liu, Shih-Chuan Chou, and Yuh-Lang Lee* Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan

bS Supporting Information ABSTRACT: One of the key issues affecting the performance of solar cells is the behavior of carrier transfer. In this work, the time-resolved photoluminescence (TRPL) technique was utilized to investigate the electron transfer at the CdS/CdSe, TiO2/CdS, and TiO2/CdSe heterointerfaces. By varying the excitation wavelengths, photoluminescence lifetimes of CdSe and CdS in TiO2/CdSe, TiO2/CdS, TiO2/CdS/CdSe, and TiO2/CdSe/CdS photoelectrodes were measured. The results show that, for the single sensitizer electrodes (TiO2 /CdS, TiO 2/CdSe), the average PL lifetime of CdS (0.69 ns) is shorter than CdSe (0.99 ns), suggesting that CdS has higher electron transfer rate toward TiO2 compared with CdSe. For the TiO2/CdSe/CdS electrode, the PL lifetime of CdSe exhibits an excitation-wavelength-dependent behavior. A shorter excitation wavelength leads to a longer PL lifetime of CdSe. This additional long lifetime is ascribed to the rapid carrier transfer from the photoexcited carriers in CdS layer into the CdSe layer. On the contrary, the PL lifetime of CdSe is independent of the excitation wavelength in the TiO2/CdS/CdSe electrode, indicating that the excited electrons in the CdS layer did not inject into the CdSe layer. This observation confirms that the charge transfer from the cosensitizers toward the TiO2 is much more efficient in the TiO2/CdS/CdSe electrode rather than in the TiO2/CdSe/CdS electrode.

’ INTRODUCTION Semiconductor-sensitized solar cell (SSC) is one of the promising third generation solar cells.1,2 In contrast to dyesensitized solar cell (DSSC), semiconductors such as quantum dot have the advantages of tunable bandgap and high extinction coefficients. Despite the many attractive properties for semiconductor sensitizers, the conversion efficiency of SSC is still low (45%)37 compared with DSSC (1112%)8,9 and Si-based solar cell (2425%).10 It still requires further fundamental investigation to realize and enhance the performance of SSC. In general, the charge separation and transfer mechanism is one of the key issues affecting the conversion efficiency of SSCs. The SSC photoelectrode is typically composed of sensitizers and wide-bandgap semiconducting oxides such as TiO2 and ZnO. A wide variety of semiconductor sensitizers such as CdS,1113 CdSe,6,1418 PbS,12,19 PbSe,20 InP,21 InAs,22 and Sb2S37 have been proposed and utilized to harvest photons. After electron hole pairs are generated in the sensitizers, the electrons subsequently inject to the semiconducting oxide electrode, and the holes in the sensitizers are subsequently extracted by the redox electrolyte. During this process, the loss of conversion efficiency results from the recombination of electronhole pairs in the sensitizers and the trap of electrons or holes in the defect states. r 2011 American Chemical Society

To evaluate the performance of semiconductor-sensitized photoelectrodes, the injection rate of electrons from the sensitizer to the electrode is one important factor. It can be experimentally studied by time-resolved photoluminescence (TRPL),14 transient absorption spectroscopy,18 and lens-free heterodyne detection transient grating techniques.23 Recently, SSCs with multiple semiconductor sensitizers, for example CdS/CdSe,4,5,24,25 CdSe/CdTe,2629 and semiconductor/dye sensitizers,30 have attracted a lot of attention. For a multiple semiconductor-sensitized photoelectrode, the charge transfer occurs at not only the sensitizer/TiO2 interface but also the cosensitizer interface. These additional heterointerfaces in fact complicate the charge separation process when compared with what happens in a single semiconductor-sensitized photoelectrode. In this paper, the excitation-wavelength-dependent TRPL technique was proposed and utilized to investigate the charge transfer at the CdS/CdSe, TiO2/CdS, and TiO2/CdSe heterointerfaces. By tuning the wavelength of excitation laser beam, the photoexcited carriers can be selectively populated Received: September 28, 2011 Revised: December 6, 2011 Published: December 15, 2011 1550

dx.doi.org/10.1021/jp209353j | J. Phys. Chem. C 2012, 116, 1550–1555

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either in the CdSe layer only or in both the CdSe and CdS layers. By measuring the photoluminescence (PL) lifetimes of CdSe or CdS at various excitation wavelengths, the charge transfer rates between the two sensitizers can be revealed. It is found that the cascade order of CdSe and CdS indeed affects the charge separation efficiency of CdS/CdSe cosensitized solar cells. The details will be discussed in the Results and Discussion section.

’ EXPERIMENTAL METHODS Sample Preparation. Indium-doped tin oxide (ITO, about 7 Ω/sq, RITEK) was used as transparent conducting oxide substrates for preparing TiO2 films. Mesoporous TiO2 films were prepared by spin-coating of TiO2 paste (Degussa P25) on ITO substrates, followed by sintering at 450 °C for 30 min. The thickness of a TiO2 film was about 810 μm, which was controlled by repeating the cycle of spin-coating procedure. Successive ionic layer adsorption and reaction (SILAR) was used to prepare CdS and CdSe semiconductor sensitizers. In the SILAR process, ethanol was employed to dissolve Cd(NO3)2 as the cation precursor solution. For CdS sensitizer, the TiO2 film was successively dipped into a 0.05 M cation solution and a 0.1 M Na2S methanol solution for 5 min each. Following each dipping procedure, rinsing and drying were undertaken by pure solvent and a dry air gun, respectively. All these procedures were considered one SILAR cycle. The incorporated amount of sensitizer could be increased by repeating the assembly cycle. For CdSe sensitizer, the anion source was sodium selenosulfate (Na2SeSO3) solution, which was prepared by refluxing 0.3 M Se and 0.6 M Na2SO3 in an ethanol/Milli-Q ultrapure water (3:7) solution at 70 °C for 2 h. The SILAR process of CdSe was similar to that of CdS, except that a longer time (20 min) and a higher temperature (50 °C) were required for dipping the sample in the anion solution. In this research, four types of sensitized photoelectrodes were fabricated, which were denoted TiO2/CdS, TiO2/CdSe, TiO2/ CdS/CdSe, and TiO2/CdSe/CdS. The SILAR cycles for preparing CdS and CdSe sensitizer were four and one, respectively. For the TiO2/CdS/CdSe photoelectrode, TiO2 thin film was sensitized by the CdS semiconductor, followed by introducing CdSe sensitizer. On the contrary, the CdS was adsorbed after the CdSe SILAR process for the TiO2/CdSe/CdS photoelectrode. After SILAR process, all samples were introduced into the temperature-controllable oven, where the heat treatment was carried out to remove the solvent inside sensitized thin film. The heat treatment was performed at 100 °C for 90 min and followed at 150 °C for 30 min. Absorption Spectra. UVvis absorption spectra were recorded with a UVvis spectrometer constructed by GBC Scientific Equipment, Australia (model GBC Cintra 10e). Steady-State Photoluminescence. A 405 nm continuouswave diode laser was used as the excitation light source. The laser power was set to 5 mW, and the beam was focused to the samples with a NA 0.5 objective lens. The PL signal was collected with the same objective lens and measured with the fiber-coupled spectrometer (Ocean Optics Inc.). Time-Resolved Photoluminescence. The excitation light source was a supercontinuum laser (Fianium Ltd.) with wavelengths between 450 and 2200 nm. The collimated laser beam was then sent to a homemade spectral filter for selecting the central wavelength and the spectral width for performing the TRPL experiment, where the wavelength tuning range was

Figure 1. Absorption spectra of TiO2/CdSe, TiO2/CdSe/CdS, TiO2/ CdS/CdSe, and TiO2/CdS structures.

Figure 2. PL of (a) TiO2/CdS (in black dashed line), TiO2/CdSe (in red solid line), (b) TiO2/CdSe/CdS (in black dashed line), and TiO2/ CdS/CdSe (in red solid line). The excitation wavelength is 405 nm.

450700 nm, the bandwidth was set to 20 nm, and the selected laser output power was in the range 0.52 mW. The pulse duration of the selected laser beam was about 30 ps, and its repetition rate was 20 MHz. The beam was focused onto the studied samples with a NA 0.5 objective lens, and the PL was collected with the same objective in backscattering configuration. Here a 527 nm bandpass filter with 20 nm bandwidth was used to measure the CdS PL signal. To measure the CdSe PL signal, a 650 nm bandpass filter with 13 nm bandwidth was used instead. The collected PL signals were then measured with time-correlated single photon counting (TCSPC) technique (PicoHarp 300, PicoQuant GmbH) to determine the PL lifetimes. The instrument response function (IRF) of our TRPL system was determined to be ∼70 ps.

’ RESULTS AND DISCUSSION Absorption and Steady-State PL Spectra. Figure 1 shows the absorption spectra of the samples. For TiO2/CdS electrode, the absorption curve has a cutoff at ∼540 nm, which reflects the band edge of CdS (2.29 eV). For the other three structures with CdSe, the absorption extends from 540 nm to longer wavelength and show a cutoff at ∼700 nm, which correspond to the band 1551

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Table 1. Average PL Lifetimes of the Samples Studied in This Work excitation

detection

average PL

wavelength (nm)

wavelength (nm)

lifetimes (ns)

TiO2/CdSe

600 450600

650 (CdSe) 650 (CdSe)

0.99 1.01.2

TiO2/CdS

450

527 (CdS)

0.69

TiO2/CdS/CdSe

600

650 (CdSe)

1.86

450

527 (CdS)

1.37

450600

650 (CdSe)

1.72.0

600

650 (CdSe)

0.83

450

527 (CdS)

0.85

450600

650 (CdSe)

3.920.83

sample

TiO2/CdSe/CdS

Figure 3. Band diagrams of (a) TiO2/CdSe, (b) TiO2/CdS/CdSe, (c) TiO2/CdSe/CdS, and (d) TiO2/CdS. (e) The schematic diagram of charge transfer model. The definition of the notations is mentioned in the main text.

TiO2/CdSe/CdS electrodes. The schematic diagrams of band alignment, based on our previous measurements,31 are shown in Figure 3ad for reference. Figure 4 shows the TRPL results. It clearly reveals that TiO2/CdS/CdSe has the longest CdSe PL lifetime and TiO2/CdSe/CdS has the shortest one. To quantitatively analyze the PL lifetime based on the TRPL measurement, one can use exponential functions to fit the TRPL curves14 IðtÞ ¼

n

∑ Ai eτ=τ i¼1

i

ð1Þ

For TiO2/CdS/CdSe, the TRPL curve needs to be fitted with three-exponential decay function to optimize the fitting accuracy. The retrieved three time constants τ1, τ2, and τ3 are 0.18, 1.06, and 3.86 ns, respectively. Here we further defined the intensityweighted average PL lifetime τAV as14 n

τAV ¼

Figure 4. TRPL of CdSe from TiO2/CdSe, TiO2/CdSe/CdS, and TiO2/CdS/CdSe structures. The excitation wavelength is 600 nm, and the PL wavelength is 650 nm.

edge of CdSe (1.77 eV). Figure 2 shows the PL spectra of the electrodes, where the excitation wavelength is 405 nm. For TiO2/CdS and TiO2/CdSe, the PL spectra show peaks at ∼500 and ∼620 nm, respectively. It is found that the CdS and CdSe band edges are close to the values of bulk materials (i.e., 2.25 and 1.7 eV, respectively), but the measured PL peaks were blue-shifted compared with the cutoff energy in absorption spectra. This infers that the semiconductor particles, prepared by the SILAR method, have a broad distribution of particle size. Smaller particles with quantum confinement effect contribute to the blue shift of the emission spectra. In Figure 2b, the PL spectra of TiO2/CdS/CdSe and TiO2/ CdSe/CdS reveal the coexistence of CdS and CdSe PL contributions. It is worth to note that the PL intensity of CdSe is much lower than that of CdS in the single sensitizer electrodes. However, the PL intensity of CdSe increases significantly in the TiO2/CdS/CdSe electrode, which implies the photoexcited electrons in the CdSe is more difficult to inject into CdS than to the TiO2. Since the PL peak positions of CdS and CdSe are quite separated, it becomes possible to selectively monitor the TRPL of CdSe or CdS in cosensitized electrodes. PL Lifetime of CdSe. First of all, 600 nm laser pulses were used to photoexcite free carriers only in the CdSe layer and monitored the TRPL of CdSe in TiO2/CdSe, TiO2/CdS/CdSe, and

∑ Ai τi 2 i¼1 n

∑

i¼1

Ai τi

ð2Þ

to describe the overall TRPL character, and the average PL lifetime τAV is determined to be 1.86 ns. For the TRPL curve of TiO2/CdSe, the fitting result shows two time constants 0.14 and 1.23 ns, and the average PL lifetime τAV is 0.99 ns. For the TiO2/ CdSe/CdS case, two time constants 0.11 and 0.97 ns are obtained, and the average PL lifetime τAV is 0.83 ns. The average PL lifetimes of the studied samples are summarized in Table 1. In general, the obtained average PL lifetime (τAV) can be related to radiative lifetime (τR) and nonradiative lifetime (τNR) and expressed as 1/τAV = 1/τR + 1/τNR. Here, the nonradiative decay rate (= 1/τNR) is primarily due to the carrier injection into the adjacent materials.14 Since the intrinsic radiative lifetimes of CdSe and CdS are relatively long, the obtained τAV is dominated by the electron transfer rate.14 In TiO2/CdS/CdSe electrode, the average PL lifetime of CdSe can therefore reflect the carrier transfer rate from the CdSe layer to the CdS layer. Compared with the case of TiO2/CdSe electrode, the longer PL lifetime (τAV = 1.86 ns) of CdSe in TiO2/CdS/CdSe indicates that electron transfer rate from the CdSe layer to the CdS layer is slower than that from the CdSe layer directly to the TiO2 (τAV = 0.99 ns). This argument also consists with the steady-state PL spectra shown in Figure 2, where the CdSe PL intensity in TiO2/ CdS/CdSe is higher than that in TiO2/CdSe. This PL emission strength is associated with the slower carrier transfer rate into the 1552



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TiO2/CdS/CdSe electrode and prolong the PL lifetime of CdS. Carrier Transport Model. A simple carrier transfer model as shown in Figure 3e is constructed to understand the involved electron population kinetics in these electrode structures. The electrode layer structure is ordered as T/N/M, where T, N, and M denote the TiO2 and two sensitizer (either CdS or CdSe) layers. The time-dependent electron populations in N and M layers are denoted as N(t) and M(t). The corresponding rate equations to describe the electron population kinetics are then expressed as Figure 5. TRPL of CdS from TiO2/CdS, TiO2/CdSe/CdS, and TiO2/ CdS/CdSe structures. The excitation wavelength is 450 nm, and the PL wavelength is 527 nm.

adjacent layer and the higher probability of electronhole pair recombination in the CdSe layer. For the TiO2/CdSe/CdS electrode, the CdSe layer has the shortest PL lifetime (τAV = 0.83 ns) when compared with the CdSe PL lifetimes in the other two electrodes. This result should be primarily attributed to the multiple carrier injection channels when CdSe is sandwiched between TiO2 and CdS as shown in Figure 3c. The major scenario is suggested as the following: when the free electrons are photoexcited in the CdSe layer, they can inject into either the CdS layer or TiO2 and lead to the highest electron transfer rate of CdSe layer in these three electrode structures shown in Figures 3a,b,c. Of course, the other process, such as the photoexcited electrons trapped in the interfacial defect states, also influences the charge transfer and make the PL lifetime shorter. PL Lifetime of CdS. Second, in order to investigate the carrier transfer occurring in the CdS layer, 450 nm laser pulses were used to photoexcite free carriers in three designed electrodes: TiO2/ CdS, TiO2/CdSe/CdS, and TiO2/CdS/CdSe. Figure 5 shows the corresponding TRPL curves of the CdS layer and clearly reveals that TiO2/CdS/CdSe electrode has the longest CdS PL lifetime among these three structures. Here the two-exponential decay function was used to curve fit the TRPL curve of TiO2/ CdS/CdSe. The time constants were determined to be 0.36 and 1.88 ns, respectively, and the average PL lifetime is 1.37 ns. For the other two electrodes, the average PL lifetimes for TiO2/CdS and TiO2/CdSe/CdS are retrieved to be 0.69 and 0.85 ns, respectively. Compared among these three electrodes, the TiO2/CdS electrode has the shortest CdS PL lifetime (τAV = 0.69 ns). This result clearly indicates the carrier transfer from the CdS toward the TiO2 is the most efficient pathway, which is consistent with the result in our previous work.4 For TiO2/CdS/CdSe, the sandwiched CdS has the longest PL lifetime (τAV = 1.37 ns), which is contrary to the aforementioned PL lifetime of CdSe in the TiO2/CdSe/CdS electrode. It should be noted that the bandgap difference between the CdSe and CdS must be taken into consideration to realize this discrepancy. Since the 450 nm laser pulses can photoexcite free carriers not only in the CdS layer but also in the CdSe layer, the carrier transfer dynamics occurring in the CdS/CdSe heterointerface becomes very different from that in the previous CdSe case, where the photoexcited carriers can only populate in the CdSe layer. The origin of this longest CdS PL lifetime for the sandwiched CdS layer is ascribed to the carrier injection from the CdSe layer toward the CdS layer (see Figure 3b). This observation also supports that the photoexcited carriers in the CdSe layer can efficiently inject into the CdS layer in the

dNðtÞ ¼ ðkNR þ kNM þ kNT þ kNO ÞNðtÞ þ kMN MðtÞ dt ð3aÞ dMðtÞ ¼ ðkMR þ kMN þ kMO ÞMðtÞ þ kNM NðtÞ dt

ð3bÞ

In eq 3a, kNR is the electronhole recombination rate, kNM is the electron transfer rate from N to M, kNT is the electron transfer rate from N to TiO2, kNO is the summation of other possible nonradiative relaxation rates such as the trapping rate in N and its heterointerfaces, and kMN is the electron transfer rate from M to N. Similarly in eq 3b, kMR, kMN, and kMO are the electronhole recombination rate in M, electron transfer rate from M to N, and the summation of other possible nonradiative relaxation rates in M and its heterointerfaces, respectively. In this proposed model, both electron transfer directions between the M and N layer are taken into consideration (but kMN 6¼ kNM). When electrons are only generated in single layer, it is clear that the electron injection direction can only be from the photoexcited layer to the other layers. According to our experimental results, when the photoexcited carriers populate in both the N and M layers, the carrier transfer direction is still from M to N because the majority of the photoexcited electrons are drained from N toward TiO2 in the T/N/M electrode. Our experimental evidence indicate that kNT > kMN and kNM can be neglected in eq 3. On the basis of these arguments, one can solve eq 3b and obtain M(t) = M0ekmt, where M0 is constant and kM denotes the sum of all relaxation rates, kM kMR + kMN + kMO. Note that the time-dependent PL emission strength of the M layer is proportional to the electron population M(t), and the PL lifetime is therefore governed by this rate constant kM. Similarly, one can solve eq 3a and obtain the expression for N(t) as NðtÞ ¼ N0 ekN t þ M0

kMN kM t e kN kM

ð4Þ

where N0 is constant and kN kNR + kNT + kNO. According to our TRPL experimental conditions and results, it is expected that kN kM > 0 and the electron population in the N layer, N(t), is therefore composed of two exponential decay functions. On the right-hand side of eq 4, the first term ekNt describes the decay of electron population originally photoexcited in the N layer, and the second term results from the electron transfer from the M layer into the N layer. Since the PL lifetime of the N layer can correlate to the time evolution of N(t), the TRPL measurement can reveal the time evolution of carrier population in the N layer. By curve-fitting the TRPL curve of the N layer, the decay rate kN of carrier population in the N layer, and the carrier transfer rate, kM, from the M layer to the N layer, can be realized experimentally. 1553



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Figure 6. TRPL of CdSe from TiO2/CdSe/CdS. The excitation wavelengths are 450, 500, 510, and 600 nm. The PL wavelength is 650 nm.

Table 2. Fitting Parameters of TRPL of CdSe from TiO2/ CdSe/CdS According to eqs 1 and 2 wavelength (nm)

A1

τ1 (ns)

A2

τ2 (ns)

A3

τ3 (ns) τAV (ns)

450 475

308.5 285.3

0.35 0.28

257.3 233.8

1.60 1.41

29.12 26.38

500

205.2

0.45

92.10

2.06

510

117.7

0.28

75.92

1.40

1.14

525

60.5

0.21

55.29

1.14

0.99

550

84.7

0.08

53.25

0.91

0.80

600

66.7

0.11

39.12

0.97

0.83

9.01 6.95

3.92 2.97 1.53

Excitation-Wavelength-Dependent TRPL of CdSe. To furthermore investigate the carrier injection effect in the electrode heterointerfaces, an excitation wavelength dependent TRPL experiment was performed. Since the bandgap of CdS is around 540 nm, the photoexcited carrier population in the CdS layer can thus be controlled by tuning the excitation laser wavelength across the CdS bandgap. First of all, the excitationwavelength-dependent experiment was performed on the TiO2/CdSe electrode. The TRPL curves of CdSe with four different excitation wavelengths at 450, 500, 550, and 600 nm are shown in Figure S1 of the Supporting Information. The curves do not show significant difference, and the intensityweighted average PL lifetime is about 1.01.2 ns. Since the photoexcited carriers typically relax to the conduction band valley on the time scale of 0.110 ps via carrier-phonon scattering,32 our TRPL technique (IRF ∼ 70 ps) is not capable of resolving this kind of ultrafast carrier dynamics. On the other hand, it is suitable to monitor the charge transfer or electronhole recombination process happening in the CdSe layer on the subnanosecond and nanosecond time scale. This result reveals that the carrier transfer rate from the CdSe toward the TiO2 layer is about the same no matter the excitation laser wavelength or photon energy. Secondly, the excitation-wavelength-dependent TRPL experiment was performed on the TiO2/CdSe/CdS electrode. The TRPL curves shown in Figure 6 indicate that the PL decay rate of the sandwiched CdSe layer becomes slower when the excitation wavelength gets shorter. These curves were curve-fitted based on eq 1, and the obtained fitting parameters are summarized in Table 2. Note that the TRPL curves can be fitted with a dualexponential decay function as the excitation laser wavelength is larger than 500 nm. But it becomes necessary to adopt a triple-exponential decay function to achieve reasonable fitting result for the TRPL curves measured with 450 and

Figure 7. PL average lifetime of CdSe in TiO2/CdSe/CdS as a function of excitation wavelength. The PL wavelength is 650 nm.

475 nm excitation laser wavelength. The additional long lifetime component (79 ns) is assigned to the second term on the right-hand side of eq 4. kM , the reciprocal of the lifetime, can thus be obtained as 0.110.14 ns1 and is attributed to the carrier transfer rate from the CdS layer toward the CdSe layer. The wavelength dependence becomes very clear when the average PL lifetime is plotted as a function of the excitation wavelength as shown in Figure 7. The average PL lifetime rapidly increases when the excitation wavelength is less than 520 nm but remains about the same value (0.81.0 ns) when the excitation wavelength is longer than 520 nm. This crossover wavelength ∼520 nm can be associated with the bandgap of CdS. When the excitation photon energy is higher than the bandgap of CdS, photoexcited carriers can populate the CdS layer and then transfer into the CdSe layer via the kMN term in eq 3a. This additional charge transport channel therefore influences the carrier population dynamics of the CdSe layer and results in a longer average PL lifetime. Furthermore, we did excitation-wavelength-dependent TRPL of CdSe in the TiO2/CdS/CdSe electrode. These curves, as shown in Figure S2 of the Supporting Information, have no dependence on the excitation wavelength. These curves were fitted with a triple-exponential decay function to determine the PL lifetimes. The fitting result reveals the average PL lifetime is 1.72.0 ns. This experimental result clearly indicates that the photoexcited carriers in the CdS layer make no significant contribution to the PL lifetime of CdSe in the TiO2/CdS/CdSe electrode. This evidence confirms that the dominant electron transfer direction is from the CdSe layer toward the TiO2 layer due to the highest electron drain capability of TiO2 and the kNM term in eq 3b indeed can be neglected in the TiO2/CdS/CdSe electrode. It has been reported that the energy conversion efficiency of TiO2/CdS/CdSe electrode is higher than that of TiO2/CdSe/ CdS electrode.4 Our TRPL can fully explain this observation. First of all, in TiO2/CdS and TiO2/CdSe electrodes, the average PL lifetimes of CdS and CdSe (please see Table 1) are 0.69 and 0.99 ns, respectively. It suggests that CdS has higher electron transfer rate toward TiO2 than CdSe does. The physical origin of this higher electron transfer rate is attributed to the band alignment31 shown in Figure 3a,d. Meanwhile, the average PL lifetime of CdS (1.37 ns) in TiO2/CdS/CdSe electrode is much shorter than that of CdSe (3.92 ns) in the TiO2/CdSe/CdS structure when the photoexcited carriers populate in both sensitizer layers. According to eq 4, the PL lifetime of the sandwiched layer is governed by two decay rates, kM and kN, in 1554


The Journal of Physical Chemistry C which kMN and kNT are the dominated terms, respectively. The shorter average PL lifetime in the sandwiched layer means higher carrier transfer rates between the heterointerfaces of sensitizer/ sensitizer and TiO2/sensitizer. It is generally believed that the charge transfer rate is governed by the band alignment at the heterointerfaces, which is determined by the energy level alignment4,31 and the interfacial electronic states. Our work therefore clearly explains why the TiO2/CdS/CdSe electrode has a higher energy conversion efficiency than that of TiO 2 /CdSe/CdS. 31

’ CONCLUSION We have demonstrated that the carrier transfer dynamics in the heterointerfaces of CdS/CdSe cosensitized TiO2 photoelectrode can be realized experimentally. The excitation-wavelengthdependent TRPL experiment clearly reveals the carrier transfer efficiency under various sensitizer layer structures and ordering. It was found that the carrier transfer between CdS and CdSe can significantly affect the PL lifetime of the sandwiched layer and influence the performance of CdS/CdSe cosensitized electrodes. This research work explains why the TiO2/CdS/CdSe electrode has better energy conversion efficiency than the TiO2/CdSe/ CdSe electrode in cosensitizer SSC. This experimental approach also opens a new window to investigate the role of heterointerface in the carrier transfer mechanism and to identify the possible transfer channel when one tries to design and fabricate semiconductor-sensitized solar cells in the future. ’ ASSOCIATED CONTENT

bS

Supporting Information. Figures S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (Y.-M.C.); yllee@mail. ncku.edu.tw (Y.-L.L.). Present Address †

Institute of Physics, Academia Sinica, Taipei 11529, Taiwan.

’ ACKNOWLEDGMENT The authors acknowledge the sharing of supercontinuum laser system by Prof. Chun-Wei Chen and the financial support of National Science Council of Taiwan under Grants NSC99-2112M-002-008-MY3 and NSC100-3113-E-007-008 and Bureau of Energy, Ministry of Economic Affairs, Taiwan, under Grant 100-D0204-2. ’ REFERENCES (1) Kamat, P. V. J. Phys. Chem. C 2008, 112, 18737–18753. (2) Klimov, V. I. J. Phys. Chem. B 2006, 110, 16827–16845. (3) Gonzalez-Pedro, V.; Xu, X. Q.; Mora-Sero, I.; Bisquert, J. ACS Nano 2010, 4, 5783–5790. (4) Lee, Y. L.; Lo, Y. S. Adv. Funct. Mater. 2009, 19, 604–609. (5) Zhang, Q. X.; Guo, X. Z.; Huang, X. M.; Huang, S. Q.; Li, D. M.; Luo, Y. H.; Shen, Q.; Toyoda, T.; Meng, Q. B. Phys. Chem. Chem. Phys. 2011, 13, 4659–4667. (6) Fan, S. Q.; Fang, B.; Kim, J. H.; Kim, J. J.; Yu, J. S.; Ko, J. Appl. Phys. Lett. 2010, 96, 063501. (7) Chang, J. A.; Rhee, J. H.; Im, S. H.; Lee, Y. H.; Kim, H. J.; Seok, S. I.; Nazeeruddin, M. K.; Gratzel, M. Nano Lett. 2010, 10, 2609–2612.

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