Atomic Layer Deposition and Its Application as a ... - ACS Publications

Apr 13, 2010 - UniVersity of Colorado at Boulder, Boulder, Colorado 80309. ReceiVed: September 8, 2009; ReVised Manuscript ReceiVed: March 18, 2010. I...
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J. Phys. Chem. C 2010, 114, 8032–8039

In2S3 Atomic Layer Deposition and Its Application as a Sensitizer on TiO2 Nanotube Arrays for Solar Energy Conversion Shaibal K. Sarkar,† Jin Young Kim,‡ David N. Goldstein,† Nathan R. Neale,‡ Kai Zhu,‡ C. Michael Elliott,§ Arthur J. Frank,‡ and Steven M. George*,†,| Department of Chemistry and Biochemistry, UniVersity of Colorado at Boulder, Boulder, Colorado 80309, National Renewable Energy Laboratory, Golden, Colorado 80401, Department of Chemistry, Colorado State UniVersity, Fort Collins, Colorado 80523, and Department of Chemical and Biological Engineering, UniVersity of Colorado at Boulder, Boulder, Colorado 80309 ReceiVed: September 8, 2009; ReVised Manuscript ReceiVed: March 18, 2010

In2S3 atomic layer deposition (ALD) with indium acetylacetonate (In(acac)3) and H2S was studied with quartz crystal microbalance (QCM), X-ray reflectivity (XRR), and Fourier transform infrared (FTIR) spectroscopy techniques. Subsequent In2S3 ALD on TiO2 nanotube arrays defined a model semiconductor sensitized solar cell. For In2S3 ALD on initial Al2O3 ALD surfaces, the In2S3 ALD displayed a nucleation period of ∼60-70 cycles followed by a linear growth region. These results were obtained under ALD conditions that were not completely self-limiting for the In(acac)3 exposure because of the low In(acac)3 vapor pressure. The growth per cycle decreased at higher temperature over the temperature range from 130 to 170 °C at these same reactant conditions. The growth per cycle was 0.30-0.35 Å per cycle at 150 °C as determined by QCM and XRR measurements at higher In(acac)3 exposures where the surface reactions were self-limiting chemistry versus In(acac)3 and H2S exposures. The FTIR examinations revealed that the nucleation period on Al2O3 ALD surfaces may be related to the formation of Al(acac)* species that act to poison the initial Al2O3 ALD surface. X-ray diffraction investigations revealed β-In2S3 ALD films and X-ray photoelectron measurements were consistent with In2S3 films. The In2S3 ALD was employed as a semiconductor sensitizer on TiO2 nanotube arrays for solar conversion. Scanning electron microscopy and energy dispersive X-ray analysis imaging revealed In2S3 over the full length of the TiO2 nanotube array after 175 cycles of In2S3 ALD at 150 °C at reactant exposure conditions that were self-limiting on flat substrates. The photoelectrochemical properties of these In2S3 ALD-sensitized TiO2 nanotube arrays with a Co2+/Co3+ electrolyte were then characterized by measuring the photocurrent density versus voltage and the external quantum efficiency versus photon energy. A small quantum efficiency of ∼10% was observed that can be attributed to charge recombination losses and charge injection/collection processes. I. Introduction Indium sulfide (In2S3) is an important semiconductor material that has received considerable attention as a buffer layer for Cu(In, Ga)Se2 (CIGS) photovoltaic cells.1 β-In2S3 is also a wellbehaved photoactive semiconductor with a direct bandgap of ∼2.0 eV displaying n-type conductivity with high carrier mobility.2 The capabilities of absorbing solar photons in the visible spectral region and good transport properties make In2S3 an attractive candidate to replace the molecular dye adsorbed to nanocrystalline TiO2 in dye-sensitized solar cells (DSSCs).3,4 However, covering the high surface area of nanoporous TiO2 films with a conformal semiconducting layer is more difficult than covering the surface area with discrete dye molecules. Simply soaking nanoporous TiO2 films in solutions containing the appropriate dye molecules can lead to high dye coverages on the TiO2 surface. In contrast, few options exist for the deposition of uniform semiconducting layers on nanoporous * To whom correspondence should be addressed. E-mail: steven.george@ colorado.edu. † Department of Chemistry and Biochemistry, University of Colorado at Boulder. ‡ National Renewable Energy Laboratory. § Department of Chemistry, Colorado State University. | Department of Chemical and Biological Engineering, University of Colorado at Boulder.

TiO2 films. The development of a general synthetic method for depositing conformal coatings of well-defined semiconductor layers on TiO2 nanostructures may be important for the fabrication of semiconductor sensitized solar cells (SSSCs). With this goal in mind, In2S3 atomic layer deposition (ALD) has been investigated as a sensitizer on the surface of nanoporous TiO2 nanostructured films to improve upon the current DSSCs based on organic dyes and liquid electrolytes.5,6 The TiO2 photoelectrodes in DSSCs or quantum dotsensitized solar cells (QDSSCs) are typically comprised of a sintered film of randomly packed TiO2 nanoparticles or, more recently, oriented arrays of TiO2 nanotubes.4,7-9 Photoelectrodes prepared from TiO2 nanotube arrays are particularly attractive because of their improved charge collection efficiency compared with their nanoparticle counterparts.9,10 Furthermore, their welldefined and ordered pore structure makes the pores more amenable to coating by semiconductor sensitizers than the disordered, random pore structure in nanoporous TiO2 nanoparticle films.11-14 Uniform coating of nanotube arrays is difficult because standard techniques, such as physical vapor deposition (PVD) or chemical vapor deposition (CVD), do not produce conformal deposition. The high aspect ratio of these structures and nonline-of-sight conditions leads to very nonuniform deposition.

10.1021/jp9086943  2010 American Chemical Society Published on Web 04/13/2010

In2S3 Atomic Layer Deposition

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In contrast, atomic layer deposition (ALD) is well suited to depositing conformal thin films on very porous or high aspect ratio structures.15 Recognizing this possible application of ALD, earlier work has explored the ALD of CuInS2 on nanostructured TiO2 to define an inorganic three-dimensional solar cell.16 In2S3 ALD was first accomplished with use of InCl3 and H2S precursors.17 A growth rate of 1.4 Å per cycle was observed at 300 °C.17 In2S3 ALD was also performed with indium acetylacetonate (In(acac)3) and H2S.18 A rather unusual temperature dependence was observed and a maximum growth rate of 0.6 Å per cycle was obtained at 180 °C.18 In2S3 ALD with In(acac)3 and H2S has been used as a buffer layer in CIGS thin film solar cells.19-21 This In2S3 ALD layer can replace the environmentally unfriendly CdS buffer layer.20 High device efficiencies have been obtained with the In2S3 ALD buffer layer.19-21 In this paper, In2S3 ALD is examined by using In(acac)3 and H2S as the reactants. Quartz crystal microbalance (QCM) and X-ray reflectivity (XRR) investigations are employed to measure the growth per cycle and the temperature dependence of the growth per cycle. The QCM and XRR studies also verify that In2S3 ALD can occur with self-limiting surface chemistry. In situ Fourier transform (FTIR) vibration spectroscopy is also employed to monitor the surface species during the sequential surface reactions during In2S3 ALD. The In2S3 ALD is then used to deposit a thin layer of a semiconductor onto a metal oxide photoelectrode consisting of an ordered array of TiO2 nanotubes. The photoconversion performance is explored for photovoltaic devices consisting of In2S3 semiconductor-sensitized TiO2 nanotube arrays with a Co2+/Co3+ redox electrolyte. II. Experimental Section The binary reaction for In2S3 growth with In(acac)3 and H2S can be written as

2In(acac)3 + 3H2S f In2S3 + 6(acacH)

(1)

For In2S3 ALD, this binary reaction is split into two reactions and the proposed surface reactions are:18

(A)

InSH* + In(acac)3 f InS-In(acac)*2 + acacH

(2) (B)

In(acac)* + H2S f InSH* + acacH

(3)

The asterisks (*) designate the surface species. The repetition of this ABAB... sequence should lead to the controlled growth of In2S3 films. Earlier studies reported ALD growth per cycle between 0.6 and 0.2 Å per cycle depending on temperature.18 In this study, thin films of In2S3 were deposited by sequential exposure of In(acac)3 and H2S in a hot-wall viscous flow reactor.22 This reactor was equipped with a quartz crystal microbalance (QCM) for monitoring the mass deposited during each reactant exposure.22 The In(acac)3 was obtained from Strem Chemicals. The H2S was received from Sigma Aldrich. The chemicals were introduced to the reactor with a N2 carrier gas. The In(acac)3 precursor has a low vapor pressure and was heated to 80 ( 2 °C. Higher In(acac)3 source temperatures were not employed to avoid thermal decomposition. The N2 carrier gas was flowed through the head space of the In(acac)3 source vessel during the In(acac)3 exposure to increase the efficiency of In(acac)3 delivery to the reactor.

For the QCM and XRR studies on flat substrates, many of the exposure sequences of In(acac)3 dose, N2 purge, H2S dose, N2 purge were defined by (1, 30, 1, 30) where the numbers indicate the times in seconds. The long purge times of 30 s were required to obtain stable linear growth without evidence of In2S3 CVD. The In2S3 CVD results from incomplete reactant removal and adds to the In2S3 ALD growth at shorter purge times. Because of the low vapor pressure of In(acac)3, multiple In(acac)3 doses were used to obtain higher In(acac)3 exposures. For these experiments, the exposure sequence was (n × (1, 30), 1, 30) where n is the number of In(acac)3 doses per cycle. Separate In(acac)3 exposures were used instead of one longer In(acac)3 exposure because the In(acac)3 source could not sustain its vapor pressure over dosing periods longer than 1 s. For the In2S3 ALD on the TiO2 nanotube arrays, longer exposures and purges may be necessary to ensure conformal deposition inside the array of TiO2 nanotubes.15,23 For the TiO2 nanotube arrays, the deposition sequence was modified compared with the deposition sequence for the flat substrates. A sequential exposure of 3 doses of In(acac)3 per cycle was employed to lengthen the In(acac)3 exposure. Likewise, the H2S exposure was increased to 3 s. Purge steps of 30 s were utilized between the two reactants to ensure evacuation of the unreacted precursors and reaction products. This exposure sequence for In2S3 ALD on the TiO2 nanotube arrays was (3 × (1, 30), 3, 30). The FTIR investigations were performed in an ALD reactor designed for in situ FTIR spectroscopy studies.24 This reactor is a “warm-wall” reactor with the walls heated to 373 K. In contrast, the sample could be independently heated to >900 K. High surface area particles facilitate transmission FTIR spectroscopy studies.24-26 The In2S3 ALD films were deposited on zirconia nanopowders coated with Al2O3 ALD that were supported in a 2 × 3 cm tungsten grid. An infrared beam from a Nicolet Magna 560 Fourier transform infrared (FTIR) spectrometer was aligned to pass through the tungsten grid sample in the reactor. The FTIR spectra were recorded at 4 cm-1 resolution over 200 averaged scans with a mirror speed of 3.6 cm s-1. The reactant exposures during the FTIR investigations were determined to be self-limiting based on the absorbance changes in the vibrational spectra. The XRR scans were performed with a Bede D1 high resolution X-ray diffractometer from Bede Scientific Inc. This X-ray diffractometer was equipped with a Cu X-ray tube working at λ ) 1.54 Å. The filament current was 40 mA and the voltage was 40 kV. Raw data were fit with use of the REFS fitting software from Bede Scientific Inc. The XRR fits revealed the film thicknesses, film densities, and surface roughness. X-ray photoelectron spectra (XPS) were acquired with a Perkin-Elmer 5600 photoelectron spectrometer with a monochromatic KR source (12.5 mA, 12 kV), using a 1.1 µm diameter spot size and operating at a base pressure of 2 × 10-10 Torr. Survey spectra were acquired by using a pass energy of 187 eV. High-resolution spectra of the C 1s, O 1s, and Al 2p regions were collected with a pass energy of 23 eV. The crystallinity of the samples was characterized with a Bruker Scintag X-ray diffraction (XRD) system (Cu KR radiation). The microstructure and composition of the nanotube arrays were investigated with a field-emission scanning electron microscopy (FE-SEM, JEOL JSM-7000F with EDAX Genesis Energy Dispersive X-ray Spectrometer). For preparing the TiO2 nanotube arrays, Ti foils (Aldrich, 0.25 mm thick, 99.7%) were anodized in a two-electrode cell at 20 V for 4 h. This anodization was performed at room

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Figure 1. Mass gain versus time for QCM measurements during 100 cycles of In2S3 ALD on an Al2O3 ALD surface at 150 °C, using a pulse sequence of (1, 30, 1, 30). Linear growth is observed following a nucleation period of about 50 ALD cycles.

Figure 2. Mass gain versus time for QCM measurements of In2S3 ALD during four cycles in the linear growth region of Figure 1 showing the individual mass gains and losses from In(acac)3 and H2S exposures.

temperature with a Pt plate counter electrode, using 0.15 M NH4F (Aldrich, 99.9%) electrolyte in formamide (Aldrich, 99%) with 3.5% of H2O.27 The as-prepared samples were rinsed with water and ethanol, and subsequently annealed at 400 °C for 1 h in a tube furnace under air flow. The In2S3-coated film was assembled as a sandwich-type solar cell with a Pt-loaded TCO counter electrode. A cobalt electrolyte consisting of 0.2 M Co2+ complex ((Co(phen)3)(ClO4)2, tris(1,10phenanthroline)cobalt(II) perchlorate), 0.02 M Co3+ complex ((Co(dtb-bpy)3)(ClO4)3, tris(4,4′-ditert-butyl-2,2′-bipyridine)cobalt(III) perchlorate), and 0.2 M LiClO4 in a solvent mixture composed of acetonitrile and ethylene carbonate (4:6 in volume ratio) was selected instead of the conventional iodide electrolyte.28 This choice was made because the In2S3 was found to degrade very quickly in the iodide electrolyte in preliminary experiments. Photocurrent-voltage measurements were performed with a Keithley Model 236 source measure unit under a simulated AM1.5 solar irradiance as described elsewhere.29 III. Results and Discussion A. Quartz Crystal Microbalance and X-ray Reflectivity. Figure 1 shows the mass gain during the sequential exposures of In(acac)3 and H2S obtained by QCM measurements during the first 170 cycles of In2S3 ALD at 150 °C. The In2S3 ALD films were grown on an initial Al2O3 ALD surface prepared by

Sarkar et al.

Figure 3. XRR scan of In2S3 ALD film on a silicon wafer after 500 cycles at 150 °C, using a pulse sequence of (1, 30, 1, 30). The XRR scan yields a film thickness of 82 Å.

using sequential exposures of trimethylaluminum (TMA) and H2O at 150 °C. The exposure sequence for these initial experiments was (1, 30, 1, 30). The In2S3 ALD growth displays a nucleation period during the first ∼60-70 cycles. After this nucleation period, the In2S3 ALD shows very linear growth. Figure 2 shows the mass gains more clearly for four individual In(acac)3 and H2S exposures during the (1, 30, 1, 30) exposure sequence in the linear growth region at 150 °C. The In(acac)3 exposure leads to a pronounced mass gain as expected based on the proposed surface reaction given in eq 2. After the In(acac)3 exposure, the slow mass decrease indicates the slow desorption of either the reaction products or the parent In(acac)3 molecule. This slow mass decrease required the purge times to be 30 s following the In(acac)3 exposures. The mass gain settles to a steady-state value of ∆m1 ) 15.1 ng/cm2 prior to the H2S exposure. The H2S exposure produces a mass decrease as expected based on the proposed surface reaction in eq 3. This mass decrease results from the removal of In(acac)* surface species and their replacement with InSH* surface species. The mass decrease following the H2S exposure is ∆m2 ) -6.6 ng/cm2. The mass gain with the In(acac)3 exposure and mass loss with the H2S exposure produce an overall mass gain per cycle at 150 °C of ∆m ) ∆m1 + ∆m2 ) 8.5 ng/cm2. On the basis of a density of 4.31 g/cm3 for the In2S3 ALD films determined from XRR analysis, the mass gain of 8.5 ng/cm2 is consistent with a thickness increase of 0.20 Å per cycle at 150 °C. The mass gain during In(acac)3 exposure and mass loss during H2S exposure can be used to determine the stoichiometry of the reactions during In2S3 ALD. On the basis of the proposed surface reactions given by eqs 2 and 3, the ratio of mass gain to mass loss should be 1.9. In comparison, the ratio of mass gain for In(acac)3 exposures and mass loss for H2S exposures was experimentally observed to be 2.3. There could be slightly more mass gain during In(acac)3 exposures resulting from molecular adsorption of In(acac)3 on the InS-In(acac)2* surface. Alternatively, there could be slightly more mass loss during H2S exposures resulting from the loss of product InSH* species from cross-linking to form In-S-In* + H2S. An XRR scan of an In2S3 ALD film deposited on a bare silicon wafer with native oxide surface at 150 °C is shown in Figure 3. The In2S3 ALD film was deposited by using 500 cycles with a pulse sequence of (1, 30, 1, 30). Figure 3 also shows the fit of the XRR scan, using the REFS fitting software from Bede Scientific Inc. The fitting is consistent with a film thickness of

In2S3 Atomic Layer Deposition

Figure 4. Growth per cycle for In2S3 ALD versus deposition temperature, using a pulse sequence of (1, 30, 1, 30). The growth per cycle is measured by both XRR and QCM measurements.

82 Å, a film density of 4.31 g/cm3, and a surface roughness of 5 Å. The In2S3 ALD film density of 4.31 g/cm3 is 88% of the reported density of 4.90 g/cm3 for bulk In2S3. On the basis of the film thickness after 500 cycles, the growth per cycle is determined to be 0.16 Å per cycle. The slight difference between the In2S3 ALD growth rate of 0.20 Å/cycle measured by the QCM and the In2S3 ALD growth rate of 0.16 Å/cycle measured by XRR may result from the different starting surfaces. Differences between growth rates measured during separate experiments may also be attributed to the low In(acac)3 vapor pressure if the In(acac)3 exposure is not completely self-limiting. Slight differences in In(acac)3 source temperature could lead to different In(acac)3 vapor pressure, changing In(acac)3 exposures, and slightly different In2S3 ALD growth rates. To determine the temperature dependence of the In2S3 ALD growth per cycle, a variety of temperatures were examined between 130 and 170 °C, using both QCM and XRR measurements with a pulse sequence of (1, 30, 1, 30). The XRR measurements were performed by depositing 300 cycles of In2S3 ALD on an initial Al2O3 ALD surface on the QCM or on silicon wafer samples and then determining the total film thickness. The growth per cycle was obtained by dividing the total film thickness by 300. Figure 4 shows that the In2S3 ALD growth per cycle for the pulse sequence of (1, 30, 1, 30) was observed to decrease steadily with increasing temperature from ∼0.28 Å per cycle at 130 °C to ∼0.15 Å per cycle at 170 °C. The In2S3 ALD growth rates shown in Figure 4 are lower than the previous results.18 The earlier reported In2S3 ALD growth rates increased from 0.5 to 0.6 Å per cycle between 140 and 180 °C.18 These results were obtained with short purge times of only 1 s. These short purge times may have led to the larger growth rates resulting from some accompanying CVD due to an incomplete purge. In contrast, the growth rates in Figure 4 were obtained with much longer purge times of 30 s. The growth rates in Figure 4 also do not increase with temperature between 130 and 170 °C. The higher In2S3 ALD growth rates observed in the earlier studies were also dependent on the In(acac)3 source temperature.18 The In2S3 ALD growth rate was observed to increase with source temperature and reached a maximum at 135 °C and then decreased rapidly above 145 °C.18 These source temperatures for the maximum In2S3 ALD growth rate are much higher than the source temperature of 80 °C used in this study. Higher source temperatures were not utilized in the current study to

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Figure 5. Growth per cycle for In2S3 ALD at 150 °C versus number of In(acac)3 doses per cycle. The growth per cycle is measured by both XRR and QCM measurements.

Figure 6. FTIR difference spectra showing the infrared absorbance after the (a) 11th In(acac)3 exposure, (b) 11th H2S exposure, (c) 61st In(acac)3 exposure, and (d) 61st H2S exposure. (e) FTIR difference spectrum after acetylacetone adsorption on an Al2O3 ALD surface.

avoid In(acac)3 decomposition. Earlier studies have attributed inconsistent In2O3 ALD growth rates when using indium β-diketonates to precursor thermal decomposition.30 The growth per cycle shown in Figure 4 may also be lower if the reactant exposures are not self-limiting. Figure 5 shows the growth per cycle at 150 °C versus the In(acac)3 exposure defined by n, the number of In(acac)3 doses of 1 s during each cycle. For these results, the H2S exposure was sufficient for the H2S reaction to reach completion. The QCM measurements indicate that the growth per cycle levels off at ∼0.35 Å per cycle after 4-5 In(acac)3 doses per cycle. In approximate agreement with the QCM results, the XRR measurements reveal that the growth per cycle levels off at ∼0.30 Å per cycle at the higher In(acac)3 exposures. Both the In(acac)3 and H2S reactions are observed to be self-limiting for sufficient reactant exposures. The results in Figure 5 indicate that the results in Figures 1-4 may not have been obtained under completely self-limiting conditions. The pulse sequence of (1, 30, 1, 30) uses only one 1 s dose of In(acac)3 per cycle. Figure 5 reveals that one 1 s dose of In(acac)3 reaches a growth per cycle that is ∼2/3 of the growth per cycle for a self-limiting In(acac)3 reaction at 150 °C. Long times of 8.4-34.4 h were required to complete

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TABLE 1: Assignment of Vibrational Modes Monitored during In2S3 ALD Showing Experimentally Observed Frequencies and Previous Literature Frequency Assignments Based on In(acac)3 and Al(acac)3 vibrational modes

In(acac)3 (cm-1)30,31

Al(acac)3 (cm-1)29,31

exptl (cm-1)

ν(CdO) + ν(CdC) ν(CdC) + δ(CH) δas(CH3) ν(CdO), ν(CdC) δs(CH3) ν(CdC) + ν(C-CH3) δ(CH)

1575 1524 1452 1395 1370 1262 1195

1599 1535 1454 1416 1363 1288 1192

1618 1529 1456 (br) 1400 1363 1292 1199

500-2000 cycles, respectively, of In2S3 ALD using the pulse sequence of (1, 30, 1, 30) for the XRR and XRD investigations. These long times prohibited using completely self-limiting ALD conditions for all the experiments. B. FTIR Spectroscopy. FTIR spectroscopy was used to monitor the vibrations of the surface species during In2S3 ALD at 150 °C under self-limiting surface reaction conditions. The In2S3 ALD was performed on an initial Al2O3 ALD layer on the ZrO2 nanoparticles. Figure 6 shows FTIR difference spectra during In2S3 growth at 150 °C obtained by subtracting the spectrum after each previous reaction or starting condition. Difference spectra observe the addition of surface species as positive features and the removal of surface species as negative features. The spectra have been displaced for clarity in presentation. The difference spectra were obtained after saturating exposures of In(acac)3 and H2S during the 11th and 61st AB cycles of In2S3 ALD on an initial Al2O3 ALD surface. The FTIR spectra in Figure 6 are shown in the CdO and CdC stretching regions from 1200 to 1800 cm-1 that characterize the gain and loss of In(acac)* and Al(acac)* surface species. The 11th In(acac)3 exposure shown in Figure 6a shows the appearance of vibrational frequencies from new surface species. The observed frequencies are listed in Table 1 together with assignments based on experimental observations and calculations for In(acac)3 and Al(acac)3.31-33 The spectrum in Figure 6a is very similar to the spectrum of acetylacetone adsorbed on Al2O3 ALD surfaces at 150 °C.34,35 Figure 6e shows the measured vibrational spectrum for acetylacetone (acacH) absorbed on an Al2O3 ALD surface at 150 °C. Acetylacetone is expected to yield a similar spectrum on the In2S3 surface. After the 11th In(acac)3 exposure, the In2S3 ALD film is still in the nucleation region as suggested by Figure 1. The vibrational features in Figure 6a are consistent with In(acac)3 dissociatively adsorbing to release acetylacetonate (acac) that forms Al(acac)* species on the initial Al2O3 ALD surface. Alternatively, the vibrational features may also be consistent with intact In(acac)* surface species. The peaks at 1618 and 1529 cm-1 are broadened and shifted compared with the reference spectrum for acetylacetone on Al2O3 ALD surfaces. These two vibrations are combination bands containing CdO and CdC stretching vibrations. These vibrations are sensitive to their local environments and shift when the metal center changes from In to Al.36 The other observed vibrations in Figure 6a are consistent with intact adsorption of acetylacetonate on either Al or In surface sites. After the 11th H2S exposure shown in Figure 6b, the H2S does not completely remove all of the vibrational absorbance added during the previous In(acac)3 exposures. This implies that the H2S was not able to react with all the Al(acac)* or In(acac)* surface species added during the previous In(acac)3 exposure. Similar behavior was observed recently for Pd ALD on Al2O3 ALD surfaces with Pd(acac)2 and formalin.34,35 This lack of removal of all Al(acac)* or In(acac)* surface species may lead

to the lower growth per cycle observed during the nucleation period observed by the QCM measurements in Figure 1. Spectra c and d in Figure 6 are the difference spectra recorded after the half-reactions during the 61st cycle. The In2S3 ALD is now in the linear growth region. In contrast to the 11th cycle results shown in Figure 6a,b, the H2S exposure now is able to remove nearly all of the vibrational absorbance added by the previous In(acac)3 exposure. The difference spectra c and d in Figure 6 are nearly mirror images of each other. This behavior is typically observed for self-limiting ALD reactions24,37,38 and argues that the H2S can remove all of the In(acac)* surface species. There are also some minor shifts in the main vibrational peaks at 1618 and 1529 cm-1. These shifts may be consistent with the dominance of In(acac)* surface species in the linear growth region. C. Film Composition and Deposition on TiO2 Nanotube Arrays. XRD studies performed in a Θ-2Θ configuration were used to study the In2S3 ALD films deposited on a microscope slide after 2000 cycles with pulse sequences of (1, 30, 1, 30). These XRD investigations revealed that the In2S3 ALD films had no preferred crystallographic orientation. The XRD scan and the assignment of the various diffraction peaks corresponding to the β-In2S3 phase (JCPDS 32-0456) are shown in Figure 7. The broad background is from the amorphous glass substrate. XPS studies were also performed to confirm the stoichiometry of the In2S3 ALD films. These XPS investigations revealed that the S/In ratio in the films was 1.42. This ratio is close to the S/In ratio of 1.50 expected for In2S3. The carbon composition in the films was 5.1 atom % after argon sputtering into the bulk of the In2S3 ALD film.

Figure 7. XRD scan of an In2S3 ALD film deposited at 150 °C by 2000 cycles, using a pulse sequence of (1, 30, 1, 30) on a microscope glass slide.

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Figure 10. External quantum efficiency versus photon energy. The spectrum yields an optical band gap of 2.07 eV.

Figure 8. SEM images of the TiO2 nanowire arrays (a) before In2S3 ALD coating and (b) after In2S3 ALD coating.

Figure 9. Photocurrent density versus voltage for AM 1.5 solar illumination.

SEM and EDAX were employed to characterize the initial and ALD-coated TiO2 nanotube arrays. The TiO2 nanotube arrays had a length of 4 µm. Figure 8 shows the SEM image of the TiO2 nanotube arrays before and after the In2S3 ALD with a pulse sequence of (3 × (1, 30), 3, 30). Although this pulse sequence is sufficient for self-limiting In(acac)3 exposures on flat substrates, these In(acac)3 exposures may not be adequate for self-limiting conditions over the full length of the TiO2 nanotube array. The deposited In2S3 ALD film thickness was ∼5.2 nm after 175 cycles at 150 °C. This thickness was measured by XRR analysis of In2S3 ALD films on silicon wafers

that were coated at the same time as the TiO2 nanotube arrays. The SEM images indicate that the In2S3 ALD film does not significantly change the TiO2 nanotube array. The thin In2S3 ALD film only produces a slightly fuzzier SEM image. The cross-section EDAX analysis revealed that the In2S3 films were deposited over the full length of the nanotube array. However, the In2S3 ALD film thickness may be less in the interior of the TiO2 nanotube array. D. Photoelectrochemical Properties of In2S3-Sensitized TiO2 Nanotube Arrays. Figure 9 shows the photocurrent density versus voltage for an In2S3-sensitized TiO2 nanotube solar cell with an active area of 0.3 cm2 measured under simulated AM 1.5 solar irradiance. The In2S3 ALD film was deposited at 150 °C, using 175 cycles with a pulse sequence of (3 × (1, 30), 3, 30). These reaction conditions and resulting film thicknesses may not be optimum for cell performance. The short circuit current density (Jsc), open circuit voltage (Voc), fill factor, and the conversion efficiency of the cell were 4.91 mA/ cm2, 0.22 V, 0.33, and 0.36%, respectively. The small Voc, compared with other sensitized solar cells, is ascribed, in part, to the relatively negative redox potential of the cobalt electrolyte used in this study. This electrolyte is more negative by approximately 0.21 V compared with the conventional iodide electrolyte and results in a lower Voc by ∼0.2 V relative to devices made with the traditional triiodide/ iodide redox couple.39 Figure 10 displays the external quantum efficiency versus photon energy for the In2S3-sensitized TiO2 nanotube solar cell. The external quantum efficiency is ∼10% for photon energies >2.5 eV. This spectrum also reveals that the optical band gap of the ALD-In2S3 layer is 2.07 eV. This optical band gap is very similar to the reported values of 2.0 eV for bulk β-In2S3 single crystals2 and 2.01 eV for β-In2S3 thin films.40 In contrast, the previous investigations of In2S3 ALD with use of In(acac)3 and H2S observed a much higher band gap of 2.7-2.8 eV.18 These In2S3 ALD films may have contained oxygen impurities that are known to increase the band gap.41 The external quantum efficiency of ∼10% for photon energies >2.5 eV in Figure 10 can be compared with the predicted light absorption by the In2S3 ALD film. The amount of In2S3 in the light beam can be determined from the estimated thickness of the In2S3 layer of ∼5.2 nm, the roughness factor of the nanotube film9 of 31.9 per micrometer of nanotube length, and the nanotube length of 4 µm. These conditions yield an effective

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In2S3 film thickness of 664 nm. The absorption coefficients have been measured recently for β-In2S3 thin films grown by coevaporation.40 The absorption coefficient increases with photon energy and is R ≈ 3.8 × 104 cm-1 at 2.6 eV and R ≈ 1.6 × 105 cm-1 at 3.0 eV.40 The light absorption can be determined from I/I0 ) exp(-Rx), where I is the transmitted light intensity, I0 is the incident light intensity, and x is the In2S3 film thickness. The predicted transmitted light fraction is I/I0 ≈ 8.0 × 10-2 (i.e., absorption of 92%) with R ≈ 3.8 × 104 cm-1 at 2.6 eV. In comparison, the predicted transmitted light fraction is I/I0 ≈ 2.4 × 10-5 (i.e., essentially 100% absorption) with R ≈ 1.6 × 105 cm-1 at 3.0 eV. These predictions indicate that the In2S3 ALD film should be absorbing light very effectively at photon energies >2.5 eV. Assuming 25% optical loss from TCO and 100% charge injection/collection efficiencies, the maximum external quantum efficiencies should be 69% at 2.6 eV and 75% at 3.0 eV. These maximum external quantum efficiencies are significantly higher than the measured quantum efficiency of ∼10%. This comparison indicates that charge recombination and charge injection/ collection processes must also limit the device performance. For example, the charge recombination at the In2S3/TiO2 or In2S3/electrolyte interfaces, together with recombination inside the In2S3 layer, may reduce the quantum efficiency and Jsc of the solar cell. IV. Conclusions In2S3 atomic layer deposition (ALD) was performed with In(acac)3 and H2S as the reactants. The In2S3 ALD was studied with use of quartz crystal microbalance (QCM), X-ray reflectivity (XRR), and Fourier transform infrared (FTIR) spectroscopy techniques. Under conditions that were not completely self-limiting for the In(acac)3 exposure, the In2S3 ALD displayed a nucleation period of ∼60-70 cycles on initial Al2O3 ALD surfaces. The nucleation period was followed by linear growth that decreased at higher temperatures between 130 and 170 °C. The growth per cycle was 0.32-0.35 Å per cycle at 150 °C under self-limiting exposure conditions as determined by QCM and XRR studies at higher In(acac)3 and H2S exposures. The FTIR investigations observed the acac ligand on the surface following In(acac)3 exposures. The formation of Al(acac)* species following In(acac)3 adsorption may act to poison the initial Al2O3 ALD surface and lead to the nucleation period. The In2S3 ALD films were crystalline and X-ray diffraction studies observed β-In2S3. In agreement with the S/In ratio of 1.5 expected for In2S3, an S/In atomic ratio of 1.42 was observed for the In2S3 ALD films by X-ray photoelectron measurements. The In2S3 ALD was used as a sensitizer on TiO2 nanotube arrays to fabricate a photoelectrochemical semiconductor sensitized solar cell. Under conditions that were self-limiting for In(acac)3 exposures on flat substrates, In2S3 ALD was observed to cover the full length of the TiO2 nanotube array after 175 cycles of In2S3 ALD at 150 °C. The photoelectrochemical properties were then characterized for these In2S3 ALD-sensitized TiO2 nanotube arrays. The photocurrent density versus voltage and the quantum efficiency versus photon energy were measured for cells employing a Co2+/ Co3+ electrolyte. A low quantum efficiency of ∼10% was derived that may be attributed to recombination losses and charge injection/collection processes. Acknowledgment. This research was funded by a seed grant from the University of Colorado-National Renewable

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