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J. Phys. Chem. C 2008, 112, 7742–7749
Efficient Photoinduced Charge Injection from Chemical Bath Deposited CdS into Mesoporous TiO2 Probed with Time-Resolved Microwave Conductivity Jorge Piris,† Andrew J. Ferguson, Jeff L. Blackburn, Andrew G. Norman, Garry Rumbles, Don C. Selmarten, and Nikos Kopidakis* National Renewable Energy Laboratory, 1617 Cole BouleVard, Golden, Colorado 80401 ReceiVed: January 18, 2008; ReVised Manuscript ReceiVed: March 5, 2008
We use the contactless time-resolved microwave conductivity (TRMC) technique to investigate the photoconductance of CdS-coated mesoporous TiO2 and ZrO2. The CdS domains were grown directly on the surface of the oxide by a chemical bath deposition method. Mobile charge carriers are generated with a low yield following photoexcitation of CdS grown on ZrO2; there is no injection into the oxide and the shortlived photoconductance signal is attributed to the hole in CdS. In contrast, illumination of CdS domains grown on TiO2 results in very efficient electron injection into the TiO2 nanoparticles, producing long-lived charge carriers. We show that the quantum yield for electron injection into TiO2 per photon absorbed by the CdS sensitizer is close to unity. The photoconductance action spectrum follows the absorption of the CdS domains, which indicates that the charge injection efficiency does not depend on wavelength in the range where the CdS absorbs (425–550 nm). 1. Introduction Mesoporous TiO2 offers a robust template for the assembly and testing of sensitizers for solar energy conversion applications. Quantum-confined inorganic semiconductor sensitizers are attractive because of their size-dependent properties that allow tuning of the absorption spectrum and energetics of the sensitizer-oxide system. In particular, CdS and CdSe can relatively easily be deposited onto TiO2 with chemical bath techniques1–7 or linked as nanoparticles to the oxide with bifunctional linker molecules.8,9 Electron injection from the photoexcited sensitizer into TiO2 is the primary step in the photocurrent generation process and is typically studied by optical probe techniques such as steady-state and transient photoluminescence7,10 and transient absorption.7–9 These techniques provide compelling qualitative evidence for charge injection from the photoexcited CdS sensitizer to TiO2 and a quantitative measure of the related characteristic times or chargetransfer rate constants. The photoconductivity of complete photovoltaic devices based on CdS-sensitized TiO2 has been reported.1–6,9 In these reports, sensitization is demonstrated by the shift of the photoconductivity action spectrum toward visible wavelengths compared to the bare (unsensitized) oxide. Measurements of the incident photon to current conversion efficiency (IPCE) give the yield of electrons extracted from the device per photon incident on the sample, which provides a lower limit for the quantum yield for photoinduced electron injection from the sensitizer to the oxide.11 In a previous report we presented photoluminescence spectroscopy and transient absorption measurements on CdS domains grown onto mesoporous TiO2 and ZrO2 by successive ionic layer adsorption and reaction (SILAR).7 Steady-state photolumines* To whom correspondence should be addressed. E-mail: nikos_kopidakis@ nrel.gov. † Current address: Opto-electronic Materials, DelftChemTech, Faculty of Applied Sciences, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands.
Figure 1. The different photoexcitation schemes presented in this paper. Direct band gap photoexcitation of TiO2 (with and without CdS sensitizer) is shown by process I (λ1 ) 300 nm). Photoexcitation of the CdS sensitizer on TiO2 in the visible is shown by process II (λ2 ) 425–550 nm). The vertical dotted arrows in processes I and II represent the trapping and detrapping of electrons in TiO2. Process III represents photoexcitation of the CdS sensitizer on ZrO2, where electron injection to the oxide does not occur.
cence (PL) spectroscopy showed a strong PL signal when CdS was deposited on ZrO2, and that the PL was quenched when CdS was deposited on TiO2. This observation indicates that, when deposited on zirconia, CdS forms isolated domains that do not communicate electronically with each other or with the underlying oxide. When CdS is grown onto TiO2, electronic communication between CdS and the oxide becomes possible, due to the higher electron affinity of TiO2. The underlying assumption that CdS, grown by SILAR, has the same structure on zirconia and titania is substantiated by the similar surface chemistry of the two oxides. The scheme shown in Figure 1 summarizes the results from that work and sets the stage for the studies presented here. Transient absorption (TA) measure-
10.1021/jp800527r CCC: $40.75 2008 American Chemical Society Published on Web 04/24/2008
Charge Injection from CdS into Mesoporous TiO2 ments probing the time evolution of the population of the photoexcited state of CdS showed that charge injection from CdS to TiO2 (through process II of Figure 1) occurs in time scales ca. 10–50 ps.7 In contrast, photoinduced charge injection to ZrO2 does not occur (process III of Figure 1), in which case the excited state of CdS has a lifetime exceeding 80 ps.7 In this work we present direct photoconductance measurements of the TiO2/CdS composite and focus on electron injection from the CdS sensitizer to the oxide, which is the primary step in photocurrent generation in a complete photovoltaic device. To isolate the sensitizer/oxide system and study it with no additional components (e.g., hole-transporting media in the pores of the oxide), we use the contactless time-resolved microwave conductivity (TRMC) technique,12 which is based on exciting the sample with a laser pulse and probing the photoconductance with a microwave beam. The technique allows us to investigate carrier generation efficiencies, carrier mobilities, trapping processes, and the lifetime of free carriers. We use ultraviolet excitation to primarily excite the oxide (direct band gap excitation, process I in Figure 1) and produce an electron and a hole in TiO2. Tuning the excitation wavelength toward the visible, where the oxide is transparent, we can excite the CdS nanocrystals on the surface (processes II and III in Figure 1) and investigate electron injection to the oxide. Our measurements confirm that charge injection from CdS to TiO2 occurs (process II in Figure 1), but does not take place from CdS to ZrO2 (process III in Figure 1). We also investigate the trapping of electrons generated or injected in TiO2, shown by the vertical dotted arrows in processes I and II, and show that the CdS nanocrystals do not change surface trapping of electrons in TiO2. The high-frequency ac probe allows us to measure the hole mobility in the confined CdS domains, which is found to be higher than the electron mobility in the TiO2 nanoparticles. The latter observation distinguishes the present study from previous work on sensitizing TiO2 with organic sensitizers and probing free carrier generation with microwaves.13–21 In those studies the carrier mobility in the organic sensitizer was much lower than the electron mobility in TiO2 and microwave absorption was dominated by electrons in TiO2, making it possible to directly quantify the efficiency of charge injection from the sensitizer. In the present work the microwave absorption signal is more complicated and a deconvolution procedure is needed to quantify the efficiency of charge injection to TiO2 per photon absorbed in CdS. As we will show, this efficiency is very close to 100% for the TiO2/CdS system presented here, and the efficiency does not change with wavelength in the range where the sensitizer absorbs. 2. Materials and Methods 2.1. Sample Preparation. The TiO2 and ZrO2 colloids with average particle size of ca. 20 nm were synthesized as described elsewhere.7 The resulting colloidal suspension was mixed with poly(ethylene glycol) to form a slurry, which was doctor bladed onto a 25 × 11 × 1 mm quartz substrate and sintered at 450 °C for 1 h. After performing microwave photoconductance measurements of the bare layers, the nanoparticle films were then exposed to the successive ionic layer adsorption and reaction (SILAR) process to grow the CdS sensitizer, as reported previously.2,3,7 In brief, the oxide film is dipped for 2 min into a saturated cadmium acetate solution, rinsed with deionized water, then dipped for 2 min into a 0.1 M Na2S solution, and rinsed again. For the samples presented here this reaction cycle was repeated eight times. With this process, nanocrystalline CdS domains are grown as disklike domains directly on the surface
J. Phys. Chem. C, Vol. 112, No. 20, 2008 7743 of the oxide throughout the entire thickness of the porous film. The thickness of the films used in this work was between 2 and 4 µm. 2.2. Transmission Electron Microscopy (TEM) Imaging. TEM characterization was carried out on a Philips CM30 TEM operated at 300 kV. Lattice images were acquired with an objective aperture that admitted contributions from low-index reflections. The samples were prepared in the following manner. Eight cycles of CdS deposition were carried out on a TiO2 film on a glass substrate. After deposition, the film was scraped off the glass substrate into a vial. Approximately 0.5 mL of toluene was added to the vial, and this suspension was sonicated for several minutes in a bath sonicator. Approximately 4 µL of the sonicated suspension was micropipetted onto a carbon-coated Cu grid for TEM analysis. 2.3. Optical Absorption and Microwave Photoconductance Measurements. The fraction of incident light transmitted, FT, and reflected, FR, by each sample was measured with a Cary 500 spectrophotometer fitted with an integrating sphere. The fraction, FA, of incident photons actually absorbed within the sample, usually called the attenuation spectrum, was calculated as
FA ) 1 - (FT + FR)
(1)
The photoconductance of the films was measured using the flash-photolysis time-resolved microwave conductivity (FPTRMC) technique as described previously by Warman and coworkers.12 The experimental setup is similar to that described elsewhere.18 For photoexcitation in the visible, the third harmonic at 355 nm of a Q-switched Nd:YAG laser (Surelite, Continuum) was used to pump an optical parametric oscillator (OPO) (Panther, Continuum), yielding 5 ns pulses tunable in the range 420–700 nm. To obtain pulses in the UV, the output of the OPO was frequency-doubled by a second harmonic generator. The output beam was directed and expanded using fused silica prisms and lenses. A set of neutral density filters was used to attenuate the beam and vary its intensity over 4 orders of magnitude. The pulse energy was continuously monitored by directing a small portion of the beam (ca. 10%) onto a pyroelectric sensor. The samples were mounted inside a resonant cavity at the end of an X-band microwave waveguide. One end of the cavity was closed with a grating end plate, through which the samples were photoexcited. The grating is an almost perfect mirror for the TE10 mode microwaves that propagate in the waveguides. The other end of the cavity was closed with an iris plate that partially reflects the microwaves. The frequency of the microwaves (typically around 9 GHz) is tuned so that the sample is at the position of the maximum electric field of the standing wave pattern. Because of the creation of oxygen vacancies in TiO2 at low oxygen partial pressure,22 measurements were performed in air. An electron scavenging mixture of 10% of SF6 in N2 at 1 atm of pressure was used to test for the presence of photoemitted electrons from the surface of the film, which was found to be negligible for these samples.23 The microwaves were generated by a Gunn diode (Midcentury MC16/34B) mounted in a resonant cavity. The resonant frequency could be adjusted between 8.4 and 11 GHz by changing the inner dimensions of the cavity with a plunger. The output power was directed into the cell by a circulator. The remaining power not absorbed by the sample was reflected and directed by the circulator to the detector (IN23 Schottky barrier diode). The output voltage of the diode is amplified with a 1.1 GHz amplifier (Femto HSA-X-1-40) and recorded with a
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digital oscilloscope (TDS1234, Tektronix). To minimize electromagnetic pickup, the microwave source, power supply, detector, and amplifier were enclosed in a Faraday cage. 2.4. Photoconductance Data Analysis. Transient changes in the conductance of the sample upon illumination with short laser pulses were monitored with nanosecond time resolution as variations in the microwave power, ∆P. The relative change in microwave power can be directly related to the photoinduced conductance, ∆G, of the films by
∆P ) -K∆G P
(2)
where K is an experimentally determined calibration factor derived from the resonance characteristics of the cavity and the dielectric properties of the samples used.24 Since charge carriers are generated during the pulse and eventually decay, ∆G is time dependent and is referred to as the “photoconductance transient”, ∆G(t). The peak, or end-of-pulse (EOP), value of the conductance can be related to the sum of the mobilities of charge carriers (electrons and holes), ∑µ, by18
[ ∑ µ]
∆GEOP ) I0FAβqe φ
EOP
(3)
with a dimensionless parameter φ discussed in more detail below. In eq 3, I0 is the incident photon flux, FA is the fraction of incident light absorbed by the film, β is a dimensionless geometric parameter of the waveguide (on the order of 2 for the X-band waveguide used here), and qe is the elementary charge. Assuming no recombination during the pulse, the parameter φ represents the quantum yield for free carrier generation per photon absorbed.18,20 Conversely, if significant decay occurs within the duration of the pulse, the values of φ calculated from eq 3 will be a lower limit to the yield. In general, φ signifies the yield for free carrier generation that we measure at the peak of the photoconductivity with the TRMC technique. In the absence of an independent measurement of the mobility of free carriers at microwave frequencies, it is not possible to separate the yield and the mobility in eq 3. As will be discussed below, with appropriate comparisons between samples, one can still draw conclusions regarding the values of the yield or the mobility separately. The data in this paper are presented in one of three forms: (1) Time-dependent data are presented as the absorbed-intensitynormalized photoconductance, given by ∆G(t)/(I0FAβqe), so that the peak value corresponds to the EOP value of φ∑µ, (2) the φ∑µ product at EOP is used to show the light-intensitydependence data, and (3) the photoconductance action spectrum is presented as the incident-intensity-normalized photoconductance, given by ∆GEOP/(I0βqe). 3. Results 3.1. TEM Imaging. High-resolution TEM images of CdS domains on TiO2 nanoparticles, grown with eight reaction cycles, are shown in Figure 2. The CdS domains were identified by measuring the lattice plane spacing, which was in very close agreement with that expected for wurtzite CdS. The domains grow as disks onto the TiO2 nanoparticles and for eight reaction cycles their average dimensions are ca. 15 nm in diameter and 5 nm in thickness. 3.2. Absorption Spectra. The optical attenuation spectra of the samples studied are shown in Figure 3. The contribution of the CdS nanocrystals was estimated by subtracting the spectra measured for bare ZrO2 from the same sample after the SILAR
Figure 2. High-resolution TEM images of CdS domains grown by SILAR (eight reaction cycles) onto a TiO2 nanoparticle film.
Figure 3. Attenuation spectra, FA, of the samples studied. Squares indicate samples based on TiO2 and circles indicate samples based on ZrO2; empty symbols denote the bare oxide and filled symbols denote the oxide coated with CdS domains. Also shown with a green line is the difference between the spectra measured for the ZrO2 sample before and after CdS deposition. The CdS domains were grown with eight reaction cycles.
process, and is also shown. The CdS domains extend the absorbance of the TiO2 and ZrO2 samples from their onsets in the UV well into the visible. The broad absorption reflects the wide distribution of the CdS domain sizes. The onset of absorption from CdS in Figure is ca. 510 nm, corresponding to an energy of 2.43 eV, which is close to the 2.5 eV band gap of bulk CdS. Indeed at eight reaction cycles the CdS domains approach their bulk absorption behavior.7 3.3. Photoconductance Measurements. Photoconductance transients measured under similar photon fluxes for TiO2 and TiO2/CdS are shown in Figure 4. A transient from the ZrO2/ CdS sample excited at 425 nm is also shown for comparison.
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Figure 5. Absorbed-intensity-normalized photoconductance transients obtained upon irradiation of CdS-coated ZrO2 (green line) and bare ZrO2 (black line) with 425 nm laser pulses. The incident photon flux, I0, was 5 × 1015 cm-2.
how the two contributions to φ∑µ (holes in CdS at early times and electrons in TiO2 at longer time) can be extracted from the signal to elucidate the yield for electron injection to TiO2 after photoexcitation of CdS. The photoconductance action spectrum for the CdS-sensitized TiO2 sample between 425 and 550 nm is shown in Figure 7. The attenuation spectrum of the sample is also shown for comparison. Figure 4. Absorbed-intensity-normalized photoconductance transients: (a) 300 nm excitation of bare TiO2 (black) and CdS-sensitized TiO2 (red), and (b) 425 nm excitation of bare TiO2 (black), CdS-sensitized TiO2 (red), and CdS-coated ZrO2 (green). All transients were measured at similar incident photon fluences of I0 ≈ 5 × 1014 cm-2. Each trace shown is composed of two measurements: one for short times (
