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Investigation of (Leaky) ALD TiO Protection Layers for Water Splitting Photoelectrodes Thomas Moehl, Jihye Suh, Laurent Sévery, René Wick-Joliat, and S. David Tilley ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12564 • Publication Date (Web): 30 Nov 2017 Downloaded from http://pubs.acs.org on December 2, 2017

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ACS Applied Materials & Interfaces

Investigation of (Leaky) ALD TiO2 Protection Layers for Water Splitting Photoelectrodes Thomas Moehl, Jihye Suh, Laurent Sévery, René Wick-Joliat, S. David Tilley,* Department of Chemistry, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich *E-mail: [email protected].

ABSTRACT

Protective overlayers for light absorbers in photoelectrochemical (PEC) water splitting devices have gained considerable attention in recent years. They stabilize light absorbers which would normally be prone to chemical side reaction leading to degradation of the absorber. Atomic layer deposition (ALD) enables conformal and reproducible ultrathin protective layer growth even on highly structured substrates. One of the most widely investigated protective layers is amorphous TiO2, deposited by ALD at relatively low temperature (120-150°C). We have deposited protective layers from tetrakis(dimethylamido)titanium(IV) at two different temperatures and investigated their chemical composition as well as optical and electrochemical properties. Our main findings reveal a change of the flat band potential with thickness, reaching a stable value of about -50 to -100 mV vs RHE for films >30 nm, with doping densities of ~ 1020 cm3. Practical thicknesses to achieve pinhole-free films are evaluated and discussed.

KEYWORDS: water splitting, protection layer, ALD, TiO2, flat band potential, leakage current

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INTRODUCTION Photoelectrochemical (PEC) water splitting, next to photovoltaics, is one of the most promising techniques to harvest the energy from the sun, the largest source of renewable and sustainable energy available on Earth1-2. PEC water splitting devices convert the solar energy directly into chemical energy in an integrated fashion, which can in some circumstances be more cost competitive than solar cells coupled with water electrolyzers.3 In PEC water splitting the absorbers are immersed directly in an aqueous electrolyte solutions of usually very high or very low pH, in order to minimize ionic resistance losses in the system.4 This design imposes high stability constraints to the materials used. Several light absorber materials such as Cu2O, CuO, silicon and III-V semiconductors have regained attention for PEC application due to the concept of applying a thin protecting overlayer to separate the semiconductor from the harsh (electro-) chemical conditions of water oxidation or reduction reactions.5-8 The atomic layer deposition (ALD) process enables a precise and reproducible protective metal oxide layer growth with conformal coverage, even on mesoporous samples.9 And, as can be observed in recent years, ALD deposited metal oxide layers are increasingly in the focus of research in several renewable energy related fields such as photovoltaics10-11, solar fuels12, fuel cells13 and batteries14. One of the most commonly used protection layer materials is TiO2 as it has a favorable energetic position of the bands and is stable in a wide range of pH and potential. Tetrakis(dimethylamido)titanium (TDMAT) has evolved as one of the standard precursors for TiO2 in renewable energy research as it is readily available, shows good process control, and the byproducts released during the process do not damage the growing film. Literature reports for ALD TiO2 protective layers in PEC applications show that relatively thick layers are usually applied for reaching long term stability (in the range of hours, see Table


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S1). A few reports, however, show hours of stability for very thin TiO2 layers (< 10 nm), probably supported by the native interlayer of SiO2 or by a thicker catalyst overlayer (Ni of 7-9 nm).15-16 From the literature overview in Table S1 it is apparent that for over 50 h of stability all authors used thicknesses of the protection layers of at least 40 nm and usually >100 nm. The question arises which thickness is actually necessary for a pin-hole free layer. Furthermore, there are widespread values reported for the flat band potential of the TiO2 protection layers ranging from +0.3 to -0.8 V vs RHE (see Table S2 and Figure S1), which has implications for the longterm stability in water splitting experiments (e.g. whether the TiO2 is in depletion or accumulation while evolving H2 at the surface). The knowledge of the properties (flat band potential (EFB), doping density and also impurities) of such protective layers is crucial for the understanding of their behavior and performance and, ultimately, for their optimization and implementation into practical PEC devices. As there is thus far no conclusive in-depth investigation of thicker ALD TiO2 with the focus on the use in PEC applications, we analyzed these layers deposited at two different temperatures (120 and 150 °C) and investigated their chemical composition as well as optical, electrochemical and blocking properties. Fluorine-doped SnO2 (FTO) was used as a model substrate for our investigation as several of the newly investigated absorber materials are oxides (CuO,6 Cu2O,5 ZnO,17, BiVO416) or can have a thin oxide layer on the actual absorber like in the case of silicon.18 This transparent conductive oxide is widely used in photovoltaic and PEC research and, as can be seen in Table S2, most of the values in the literature for the flat band potential of ALD TiO2 were determined on FTO. Furthermore, the FTO is slightly rough (roughness factor 1.5)19, as most oxide semiconductor surfaces used for PEC.


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EXPERIMENTAL SECTION CHEMICALS. All electrolyte solutions were prepared using Milli-Q water (>18 Mohm). Tetrakis(dimethylamido)titanium (TDMAT) (99.999% trace metal basis), K3[Fe(CN)6] (>99%) and K4[Fe(CN)6] (>99%) were purchased from Sigma Aldrich. KCl (>99%) was obtained from Acros Organics. 1 M sulfuric acid was prepared from 95-97% H2SO4 from Merck (ENSURE). 1 M KOH was prepared from extra pure KOH pellets from Honeywell. 1 M HCl was prepared from 32% hydrochloric acid from Merck (ENSURE). PREPARATION OF THE ALD TiO2 LAYERS. Microscopy glass slides and FTO (TEC 15, Pilkington) were first cleaned mechanically with soap (deconex 0.5 vol% in Milli-Q water) and afterwards sequentially in acetone, deconex (0.5%), water, and ethanol (ultrasonic bath, 10 min each step). ALD was performed with a Picosun R-200 tool using TDMAT and water as reactants at 120 and 150 °C. The TDMAT recipe was used under variation of the reaction chamber temperature but maintaining all other parameters. The TDMAT precursor was held at 85 °C in the cylinder. A “boost” sequence was used for the Ti precursor cylinder, whereby nitrogen gas was introduced into the cylinder following each pulse by increasing the line pressure while the ALD valve remained open. A 0.5 s dosing pulse (to the reactor) was used for the TDMAT followed by the “boost” increased line pressure for 1.2 s (with an overall opening time of the ALD valve of 1.6 s). Water was held at room temperature and a 0.1 s pulse was used. A 6 s purge was used following both precursor pulses. The non-uniformity was tested with varying the dosing pulse length (0.4, 0.5, 0.6 and 1 s for the dosing pulse while maintaining the software boost function at 1.2 s and adjusting the overall pulse time so that it was 0.1 s shorter than the sum of the dosing pulse and cylinder back filling). The non-uniformity was between 0.3 and 1.7 % on a 4” wafer (see Table S3).


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For the Al2O3 the reaction chamber temperature was at 300 °C and the TMA precursor was kept at room temperature. The pulse of the TMA precursor was 0.1 s with a purge time of 3 s. The water was pulsed with 0.1 s with a purge time of 4 s. ELLIPSOMETRY, XRD and XPS MEASUREMENTS. The thickness of the layers deposited by ALD was determined with an alpha-SE Ellipsometer from J.A. Woollam Co. During depositions on FTO and glass, a piece of silicon witness wafer was placed in the reaction chamber for thickness confirmation. The ellipsometry data were fitted with the completeEASE software from J.A. Woollam Co. using a model for transparent films. The error of the thickness is ± 0.1 nm. To confirm the uninhibited growth of TiO2 also on a transparent conductive oxide, ITO was used as a substrate (30 ohm cm) as it is less rough than FTO and therefore suitable as a substrate for measurements by ellipsometry. The fitting procedure used for the ITO/ALD-TiO2 films first included the fitting of the bare ITO substrate (model used in the CompleteEase software: ITO (thin) on glass). To prevent backside reflection, scotch tape (Magic Tape) was placed on the bottom of the substrate. The ITO parameters in the model were then fixed and a Cauchy layer was implemented to fit the ALD TiO2 layer on top. Two different thicknesses were measured on ITO to confirm the growth rate (300 and 1860 cycles). On silicon, 300 cycles gave 16.3 nm and 1860 cycles 99.95 nm. The thickness of the ALD TiO2 layer on ITO was, with 16.28 nm and 100.25 nm, in close agreement to the observed thicknesses on silicon, confirming a similar growth rate. Morphologies of the films were examined by scanning electron microscopy (SEM) using a Zeiss Supra 50 VP microscope. Crystal structures of the fabricated films were evaluated by Xray diffraction (XRD) using a Bruker AXS D8Advance diffractometer. XPS measurements and XPS depth profiling were performed with a Thermo Fisher Scientific ESCALAB 250. The


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radiation source used a monochromatic Aluminum Kα X-ray (hν = 1486.6eV) with a power of 200 W. AFM MEASUREMENTS. An Asylum AFM (MFP-3D) was used to measure the surface morphology of the ALD layers. Open source Gwyddion software package was used to further analyze the AFM pictures. UV/Vis MEASUREMENTS. UV/Vis measurements were performed with a UV-3600 Plus from Shimadzu equipped with an integrating sphere. The ALD layers used for UV/Vis measurements were deposited on microscopy slide glass. ELECTROCHEMICAL MEASUREMENTS. For all measurements in electrolyte solution the samples were placed in a PEC cell made out of Teflon. The sample was fixed with an O-ring to define the area. The electrochemical measurements were performed with a Bio-Logic potentiostat (SP200) and an Ag/AgCl reference electrode with saturated KCl as electrolyte solution (Koslow Scientific). Electrolyte solutions used were 1 M H2SO4, 1 M KOH, and for the measurements with redox system 0.5 M KCl with 5 mM K4[Fe(CN)6] and 5 mM K3[Fe(CN)6] (pH 2.5, adjusted with 1 M HCl). The scan velocity of the cyclic voltammetry measurements was 100 mV/s. The geometric area of the electrodes was measured to be 0.29 cm2. For measurement of the ALD Al2O3 films, the electrolyte solution containing redox system was adjusted to pH 6 with 1 M KOH. All measurements were performed in a three electrode setup against Ag/AgCl reference electrode. To convert these potential values at a certain pH to RHE, equation (1) was used: E RHE = E Ag / AgCl + 0.059 pH + E °Ag / AgCl

(1)

where ERHE is the converted potential vs. RHE, E°Ag/AgCl = 0.197 V at 25 °C, and EAg/AgCl is the experimentally measured potential against Ag/AgCl reference electrode. All measurements were


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performed in a (photo)electrochemical cell as described in Scheme S1 using a Viton o-ring, 7 mm diameter, to fix the sample. Electrochemical impedance measurements were performed with at least 20 s equilibrium time at each bias potential step. Bias potential steps were between 25 and 40 mV in the desired potential window. Frequencies applied ranged from 1 MHz down to 0.1 mHz. The Nyquist plots (Figure S2) of the ALD TiO2 usually showed only one semicircle and were fitted with a Randels circuit (without the Warburg element, see inset Figure S2). Rct represents the charge transfer resistance and Csc the space charge capacity. The series resistance from cables, contacts and the electrolyte solution is represented by a resistance in series to the RC element, Rseries. The data from the EIS measurements were fitted with the ZView software from Scribner. In all fitting procedures, the capacity is modeled by a CPE to account for e.g. the roughness of the electrode. After the fitting of the Nyquist plots, the CPE values were converted into the space charge capacitance: 1 ( )

( R × Q) n C SC = ct (2) Rct with Rct as the resistance in parallel to the desired capacitance, Q as the charge from the CPE and n as the exponent of the CPE. The ideality factor of the CPE, n, was in all fitting procedures about 0.9 and higher, so close to ideality. EIS measurements in conjunction with the hexacyanoferrate solution used a stabilization time of 59 s at each bias potential. To extract the DC current values from the EIS measurements we used the current value from the lowest frequency (0.1 mHz), which represents a stabilization for the DC current of roughly 220 s (stabilization time plus measurement time). As a protective layer for photocathodes, protection and conductivity of the ALD TiO2 for photoelectrons was demonstrated for CuO-based materials in Reference 6.6 To demonstrate that


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the ALD TiO2 layers fabricated in our system are also suitable for photoanodes (“leaky” TiO2), a protected silicon photoanode was prepared. Fifty nm ALD-TiO2 was deposited onto a np+-Si wafer (0.1-0.2 ohm-cm, oriented, single side polished) at 120 °C. Prior to the ALD of TiO2, the Si surface was cleaned by the following procedure: 10 min RCA I (5:1:1 of H2O:H2O2:NH4OH) at 50 °C; 10 min RCA II (5:1:1 of H2O:H2O2:HCl) at 50 °C; and 30 s in 2% hydrofluoric acid etch, with deionized water rinses after each step. After drying under a stream of N2, the Si substrates were immediately loaded into the ALD chamber. Then ALD TiO2 with a thickness of 50 nm was deposited by the ALD procedure described above. Afterwards, 2 nm of Ni was sputtered (Model CCU-010 HV from Safematic) on top of the TiO2 film. For the photoelectrode preparation, the back side of the Si sample was scratched and In-Ga eutectic was applied to form an ohmic contact to Cu foil which was fixed to a microscope slide. The sample was then sealed to the microscope slide using grey epoxy (Loctite EA9466 and EA9461 mixed). An O-ring was used to define the exposed electrode surface area, which was 0.283 cm2. The fabricated photoelectrode was placed in a Teflon cell for electrochemical measurements in 1.0 M KOH. An Ag/AgCl electrode was used as a reference electrode and Pt wire as a counter electrode. The working electrode was held at 0.6 V vs Ag/AgCl during the long term stability test (1.62 V vs RHE). The light source for the stability measurements with a silicon photoanode was a Prizmatix ultra high power white light LED (UHP-F-HCRI, color temperature: 5700 K). The photocurrent for the stability measurements was adjusted to the same value as that obtained under AM 1.5 G (100 mW/cm2) from a solar simulator with xenon lamp. The spectrum of the white light LED does not contain UV light and can be found on the webpage of the producer.


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RESULTS AND DISCUSSION. Before going into detail about the analysis of the deposited ALD layers, we have tested them as protection for both photocathodes and photoanodes. For photocathodes, the reader is referred to the protection of CuO/CdS heterojunctions as presented by Septina et al.,6 in which we have used our ALD TiO2 layers in a system for water reduction. For the case of photoanodes, we have deposited the ALD layer onto np+-Si absorbers together with a nickel water oxidation catalyst. Stability measurements of water oxidation with this system are presented in Figure S3 of the supporting information. Clearly the system is able to oxidize water with high current densities, indicating the “leaky” character of our ALD deposited TiO2 layers, in line with reports by Hu et al.20 and Mei et al.21 The term “leaky” is used to indicate that the n-type ALD TiO2 is able to conduct not only photo-generated electrons but also photo-generated holes, though the mechanism of the transport of these photo-generated holes under such conditions is still under debate.

(a)

(b)

Figure 1. (a) Transmission and total reflection of the samples with 1280 cycles deposited at both 120 and 150 °C on microscopy slide glass. (b) Absorption of the TiO2 layers with 1280 cycles. Inset shows the change of transmission with thickness of the layers.


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The ALD process showed reproducible layer growth with growth rates of 0.55 and 0.47 Å/cycle at 120 °C and 150 °C, respectively (see Figure S4 and Table 1). The growth rate at 150 °C is diminished by the higher temperature in the ALD chamber, likely due to increased desorption of the precursor from the surface.22-23 SEM as well as AFM showed that the roughness is mainly determined by the FTO substrate (Figure S5). With increasing thickness the TiO2 deposition softens out the edges of the underlying substrate and the roughness is reduced from 11.72 nm for the FTO to ∼ 10.04 nm for a layer of 1280 cycles deposited at 150°C (expressed as root mean square roughness). TiO2 as wide band gap semiconductor absorbs in the near UV. To determine the absorption of the ALD TiO2 layers, the transmission and the total reflection were measured (A=1-T-Rtotal, see Figure 1 and Figure S6). As the incoming wavelength reaches an integer multiple of the thickness of the TiO2 layers, constructive thin film interference effects are observed, leading to a coloring of the thicker samples. Leveraging this phenomenon further one can imagine that such protective overlayers could also serve as a light “managing” layer or Bragg mirror for tandem PEC cells.24-25 The broad absorption below bandgap (>∼390 nm) observed in Figure 1b can either be due to free charge carrier absorption or due to intra-band gap states and impurities. Our TiO2 layers are not degenerately doped as they do not show metallic character like e.g. FTO and free carrier absorption would show a power law dependence of the form λn (with n≈1.7)26. The exponent n from a power law fit of the absorption between 500 and 900 nm yields 0.3 and 0.8, so clearly below the expected 1.7. Trapped holes and electrons are normally assigned to a broad absorption at about 500 nm and 700 nm respectively.27 As our TiO2 films contain nitrogen and carbon as impurities in the low percentage range (as will be shown below), the broad absorption in the


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visible range is trap and impurity induced. Such an extrinsic doping is sometimes used to “sensitize” TiO2 in the visible range (e.g. nitrogen, carbon or metal doping of TiO2).28-29 For PEC applications, however, light absorption by protective overlayers is parasitic, leading to a lower final efficiency of the devices. Kim et al. investigated the relationship between transmission and thickness (at 542 nm) and found a similar reduction of the light intensity, though they seem not to have observed the thin film interference effect.30 The absorption at a wavelength of 542 nm of ALD deposited TiO2 (with 1280 cycles) at 120°C and 150°C showed 3.2% and 7.8%, respectively. Generally the deposition at 120°C showed lower absorption in the visible range which would make the layers produced at this deposition temperature more suitable for PEC applications, from an optical point of view. The deposited layers are amorphous as no peaks other than FTO could be detected in XRD. When the samples were heated to 550 °C, the amorphous TiO2 could be converted into crystalline anatase TiO2 (Figure S7). XPS in conjunction with argon ion etching depth profiling yielded a ratio of oxygen to titanium of roughly 2:1. This ratio can be observed in the bulk of the film (Figure S8). On the surface absorbed water or hydroxyl-groups lead to a surplus of oxygen. As impurities we observe carbon and nitrogen impurities in the low % range, in agreement with previous reports.22, 31-35 A further discussion of the XPS results can be found in the supporting information (Figure S9). As the TiO2 protection layer directly interfaces with the electrolyte solution, its electrochemical properties play a crucial role for the PEC device performance. The space charge capacitance was extracted from the fitting of the Nyquist plots. In contrast to a “normal” MottSchottky (MS) analysis which is usually performed only at one frequency, in EIS analysis no frequency dispersion appears as one determines directly the space charge capacitance at each


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potential. The space charge capacitance is used to make a MS analysis, relating 1/Csc2 plotted vs the applied potential to the flat band potential (x axis intercept) and doping (∼slope, see also Figure 2a and equation (3)):

1 2 kT ( E − EFB − ) = (3) 2 q CSC N D εε 0 q where Csc is the space-charge capacitance, q is the charge of an electron, ε is the relative dielectric constant of the materials (here assumed to be 38),36 ε0 is the permittivity of free space, ND donor density (for an n-type semiconductor), E is the potential applied, EFB is the flat band potential, k is the Boltzmann constant and T is the temperature. The value of the dielectric constant shows a wide variation in literature depending on deposition process and thermal treatment. Generally the amorphous TiO2 films have a lower dielectric constant than crystallized films. A value of 38 was used for the dielectric constant in this study as this value is reported for ALD films prepared in a similar manner.36 First, the dependence of the flat band on pH was investigated. Normally, under Nernstian

(b)

(a)

Figure 2. (a) Mott-Schottky plots for different temperatures and thicknesses, measured in 1M H2SO4. (b) Evolution of the EFB with thickness of the TiO2 layer (inset shows the data with the x-axis plotted in log scale).


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behavior, oxide surfaces show a flat band potential shift of 59 mV per pH unit, and this was indeed observed (Figure S10).

Table 1. Thickness, number of cycles, doping density and EFB of the investigated layers deposited at 120 °C and 150 °C in 1 M H2SO4 Temperature

Thickness ALD Cycles

Doping

EFB vs RHE

/1020 cm-3

/V

/°C

/nm

120

136.04

2500

3.2 (± 0.8)

-56 (± 06)

70.92

1280

3.4 (± 2.9)

-72 (± 20)

35.58

640

2.6 (± 0.7)

-64 (± 22)

17.25

320

2.6 (± 0.7)

-69 (± 23)

8.61

160

2.8 (± 1.5)

-51 (± 21)

4.74

80

1.9 (± 0.3)

20 (± 10)

2.46

40

3.3 (± 1.4)a

25 (± 36) a

1.41

20

1.9 (± 0.3)a

88 (± 18) a

121.80

2500

4.8 (± 1.1)

-65 (± 13)

59.92

1280

3.9 (± 1.1)

-91 (± 25)

30.54

640

4.4 (± 1.2)

-81 (± 18)

15.02

320

3.4 (± 2.0)

-54 (± 36)

7.48

160

2.9 (± 1.1)

-22 (± 10)

3.61

80

1.9 (± 0.9)

6 (± 20)

1.77

40

2.4 (± 1.1)a

18 (± 11) a

1.01

20

1.9 (± 0.2)a

125 (± 21 ) a

FTO in H2SO4

14.1

130

FTO in KOH

14.0

95

150

a

Only two samples were measured


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Interestingly, there is little information on the EFB of FTO, which is probably due to the fact that depending on the producer of the FTO, different production processes and impurities can lead to variations of the EFB. Kavan et al. reported a value of 50 mV to 150 mV vs RHE.37,11 We have determined the EFB to be between 95 and 130 mV vs RHE (Table 1). The MS analysis results of the different ALD TiO2 films can be found in Table 1. An average of four measurements for each thickness is presented (except for the 20 and 40 cycle depositions in which case there were only two measurements). The EFB for the thin layers is close to the one of FTO (see Figure 2b), and shifts to more negative potentials with increasing thickness of the TiO2 (∼ -60 mV and ∼ -70 mV vs RHE for the deposition at 120 °C and 150 °C, respectively). The EFB evolution with increasing layer thickness stabilizes between 9 to 17 nm and 15 to 31 nm for 120 and 150 °C, respectively, indicating that the dielectric properties of the TiO2 are now dominating the interface properties. The doping densities are in the low 1020 cm-3 range, which is a relatively high doping density (most probably due to the amorphous character and the impurities from e.g. nitrogen) but has been observed also by other authors.5, 37 Also, it must be considered that at such high doping densities the Helmholtz capacitance can influence the flat band potential (see Table S4 and accompanying text). Comparing the EFB and the doping density with the literature values one can observe only a small variation of the EFB with temperature in our measurements. We observed that for thinner ALD layers the flat band potential is in large part determined by the FTO substrate, which can explain the low value determined by Kavan et al.37, as the TiO2 layers used had thicknesses ≤ 6 nm. Kavan et al. further showed in a different publication that the EFB for thin amorphous ALD SnO2 layers (< 15 nm) is near the one of FTO though for crystalline SnO2 it is several 100 mV more positive.11


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The actual conduction band position (as the MS analysis yields the position of the Fermi level at which there is no band bending) can be calculated by equations 4 and 5:

kT  N C  ln  (4)  q  ND  where NC is the number of available states in the conduction band. In most cases, authors ECB − EFB =

report the EFB directly without further implementation of the potential difference between the ECB and EF, the reason probably being that especially in amorphous and multicrystalline semiconductors the amount of available states is higher than the classically defined NC, since numerous states exist in the forbidden zone between the bands. For crystalline semiconductors, NC can be calculated for an n-type semiconductor by 2π m* kT 3/ 2 N C = 2( ) (5) h2 where m* is the effective mass of the electrons and h is the Planck constant. For TiO2, 7.8×1020 cm-3 is calculated for an effective mass of 10m0.5, 38 In amorphous semiconductors, the states inside the band gap are numerous and the real amount is unknown. The estimated distance of the conduction band to the EF based on Equation 4, yields offsets of 20 to 50 mV, placing the ECB for the thicker films (≥30 nm) at about -100 mV vs RHE (see Figure S11). We could not detect any significant difference between the EFB positions for the depositions at 120 or 150 °C, independent of layer thickness. The changes in doping with variation of the thickness are small, especially for the 120 °C deposition. In the case for 150 °C it seems though that there is an increase in doping with increasing thickness of the ALD layers. The wide range of values especially for the flat band position in the literature likely results from insufficient sample statistics and/or a high sensitivity of the properties on preparation conditions of the films.


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In practical work and for future PEC applications, the protection of the underlying semiconductor is crucial for the long-term stability of the devices. Therefore information on the minimal layer thickness required to reproducibly deposit pin hole free layers by ALD is essential. Furthermore, the measurement technique has to be sufficiently sensitive to detect even small leakage currents. We have investigated the ALD TiO2 layers towards their blocking ability against the ferrocyanide redox couple by cyclic voltammetry, by the current flow driving the impedance measurement, as well as through the charge transfer resistance from EIS.

(a)

(b)

(c)

(d)

Figure 3. (a) CVs of the different layer thicknesses with ferrocyanide redox couple for the depositions at 120 °C in 5 mM hexacyanoferrate solution (0.5 M KCL at pH=2.5, scan velocity 100 mV/s). (b) CVs with the current plotted in log scale. (c) DC currents from the EIS measurements. For comparison also FTO and the ALD Al2O3 layers are presented. (d) Charge transfer resistance determined from the EIS measurements.


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By CV (Figure 3a) we observed, as expected, that with increasing thickness of the ALD layer the current flowing over the electrode interface is reduced and a higher overpotential must be applied to drive the oxidation reaction. The oxidation reaction is strongly suppressed with 4.7 nm of deposited TiO2. This observation seems to indicate that the charge transfer is already completely blocked and that the ALD films are pin hole free. However, plotting the current of the CV measurements in logarithmic scale (Figure 3b) reveals that even the layer with 17 nm thickness shows a small but clear current increase due to the oxidation reaction. This implies that a layer thickness of 17 nm is not sufficient to act as a complete barrier for the redox reaction. Only the layers of 30 nm and thicker showed no further decrease in the oxidation current by CV in the potential range investigated. It must be considered that the measured current in CV contains a capacitive contribution due to the sweeping of the potential that can mask a small contribution from a Faradaic current. Since during the EIS measurement 59 s of stabilization time were applied at each bias potential step, and as the potential perturbation is small during the actual impedance measurement, the current at the lowest frequency (0.1 mHz) can be considered similar to a stepped DC current measurement (Figure S12). The DC current and the charge transfer resistance determined by the EIS measurement provide a more precise measure of the leakiness of these layers. Thicknesses above 17 nm, which appeared to be completely blocking by CV, are now seen to have a small but definite charge transfer (Figure 3c,d). Making an analogous comparison with the measurements at 150 °C yields similar observations (Figure S13a and b) although the higher temperature deposition shows generally higher leakage currents. As TiO2 naturally contains oxygen vacancies, charge compensation is achieved by Ti3+ states, which are shallow states below the conduction band.39-40 Henkel et al. showed that the


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amount of Ti3+ is proportional to the leakage current of ALD TiO2 layers, which points to a shallow trap assisted charge transport level.41 Whether or not this transport channel also enables hole conduction is also still under discussion.12, 18, 31, 42-45 The relation between the amount of Ti3+ states and leakage current can explain the higher leakage currents observed for the 150 °C deposition of the ALD TiO2 layers as we have detected higher optical absorption in the visible (indication of intra-band states) and a higher doping density in the TiO2 made at 150 °C. A comparison of the DC current flowing at 0.6 V vs Ag/AgCl, where the ferrocyanide is oxidized, is shown in Figure 4a for both deposition temperatures. The current flow with increasing thickness can be divided in three regimes: (1) tunneling with pinholes; (2) pinholes and (3) finally leakiness. The ALD layers

Leaky - ACS Publications - American Chemical Society

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