APL Mater. 3, 116105 (2015); https://doi.org/10.1063/1.4935125 3, 116105
© 2015 Author(s).
Environmental stability of high-mobility
indium-oxide based transparent electrodes
Cite as: APL Mater. 3, 116105 (2015); https://doi.org/10.1063/1.4935125Submitted: 02 September 2015 . Accepted: 22 October 2015 . Published Online: 09 November 2015 Thanaporn Tohsophon , Ali Dabirian, Stefaan De Wolf, Monica Morales-Masis , and Christophe Ballif
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Environmental stability of high-mobility indium-oxide based
transparent electrodes
Thanaporn Tohsophon, Ali Dabirian, Stefaan De Wolf, Monica Morales-Masis,aand Christophe Ballif
Ecole Polytechnique Fédérale de Lausanne (EPFL), Institute of Microengineering (IMT), Photovoltaics and Thin Film Electronics Laboratory, Rue de la Maladière 71b,
Neuchatel 2002, Switzerland
(Received 2 September 2015; accepted 22 October 2015; published online 9 November 2015)
Large-scale deployment of a wide range of optoelectronic devices, including solar cells, critically depends on the long-term stability of their front electrodes. Here, we investigate the performance of Sn-doped In2O3 (ITO), H-doped In2O3 (IO:H),
and Zn-doped In2O3(IZO) electrodes under damp heat (DH) conditions (85◦C, 85%
relative humidity). ITO, IO:H capped with ITO, and IZO show high stability with only 3%, 9%, and 13% sheet resistance (Rs) degradation after 1000 h of DH,
respec-tively. For uncapped IO:H, we find a 75% Rsdegradation, due to losses in electron
Hall mobility (µHall). We propose that this degradation results from chemisorbed
OH- or H2O-related species in the film, which is confirmed by thermal desorption
spectroscopy and x-ray photoelectron spectroscopy. While µHallstrongly degrades
during DH, the optical mobility (µoptical) remains unchanged, indicating that the
degradation mainly occurs at grain boundaries. C2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.[http://dx.doi.org/10.1063/1.4935125]
Transparent conductive oxides (TCOs) are used as electrodes in various optoelectronic devices, like light emitting diodes and photovoltaic devices, including high-efficiency silicon heterojunction (SHJ) solar cells.1 Sn doped In
2O3(ITO) is the most commonly used transparent front-contact in
SHJ technology; yet, the carrier mobility in ITO is low (typically 20–40 cm2/V s) and the films
can suffer from significant parasitic absorption losses in infrared (IR) wavelengths if a sufficiently high conductivity is required.2,3To lower such parasitic absorption losses, hydrogen doped In
2O3
(IO:H) is a suitable alternative: this material shows a significantly higher electron mobility (µe),
enabling high sheet conductivities with low free-carrier absorption and hence lower IR parasitic absorption losses. In the case of SHJ cells, applying IO:H with high µeas front contact showed an
improvement of short-circuit current density (Jsc) as compared to ITO, whereas it caused fill factor
(FF) losses because of an increase in contact resistance at the interface of IO:H and the silver front grid.4By adding an ultrathin ITO capping layer on the IO:H film before metallization—so called IO:H/ITO bilayers—a combination of high µeand low Rsis achieved, improving the performance
of such solar cells.4Recently, we have further confirmed the importance of high mobility TCOs in SHJ technology by demonstrating amorphous Zn-doped In2O3(IZO) as a front contact in SHJ5and
perovskite-SHJ tandem solar cells6since IZO equally combines high conductivity, high mobility, and high optical broadband transmittance, allowing the improvement in Jscwith respect to those
with ITO.5
These front TCO films “cap” the actual solar cell and can be subject to moisture ingress through the encapsulation polymer in the final photovoltaic module. Resilience of TCOs against such phenomenon and adverse environmental conditions in general is a critical factor for their large-scale deployment. The stability to heat and humidity of several TCOs, including ITO and
aAuthor to whom correspondence should be addressed. Electronic mail:monica.moralesmasis@epfl.ch
116105-2 Tohsophon et al. APL Mater. 3, 116105 (2015)
IZO, has already been studied, demonstrating a clear dependence on the deposition method,7,8 type of substrate,9,10film thickness,7,11free-carrier concentration,12microstructure and film crystal-linity.13–19To our knowledge however, the behavior of IO:H under damp heat (DH) conditions is yet to be discussed in the literature. Motivated by this, we compare directly the stability of ITO, IO:H, IO:H capped with 20 nm of ITO (IO:H/ITO bilayers), and IZO films and identify the main physical and chemical effects that impact their long-term stability.
ITO, IO:H, IZO, and IO:H/ITO bilayer films were deposited by dc or rf magnetron sputtering on 0.5 mm thick alkali free glass (AF45) substrates under argon and oxygen atmosphere (see details on TableI). For IO:H films, water vapor, which plays the role of H source, is introduced into the reactor during the sputtering process.4,20 More details on the TCO film depositions can be found elsewhere.4The films were then annealed in air (190◦C for 20 min) to simulate the curing process needed for the screen-printed silver grid fabricated on top of the TCO front contact.1Specifically for silicon heterojunction solar cell front contact formation, the sputtering parameters of each TCO film were optimized to achieve 100 nm-thick films with carrier concentrations (Ne) of 1–2.5 × 1020cm−3
after the annealing step. These specific properties are tuned to minimize the optical reflectance of the used silicon substrates in the visible range and to maximize the optical transmittance in the IR range. Stability to heat and humidity of the annealed films was tested through DH treatment at 85◦C and 85% relative humidity (RH). The evolution of the electrical and optical properties of the annealed films during the 1000 h of DH treatment was studied by Hall effect measurements and UV-Vis-NIR spectroscopy (using an integrating sphere) respectively. The optical mobility (µoptical)
was calculated by graph fitting of reflectance spectra in the range of 400–8000 cm−1, recorded by Fourier transform IR (FTIR) spectrometer.21 The structural properties of the annealed films were characterized by grazing-incidence x-ray diffraction (GI-XRD). The chemical state of the annealed films before and after DH was carried out by x-ray photoelectron spectroscopy (XPS). The film surfaces were Ar etched for 1 min (7 nm in depth), before analyzing the survey spectra. The high-resolution core-level O1s spectra were then recorded and Gauss-Lorentz fitted, which is the standard procedure for In2O3 compounds.22 The H2O and H2 desorption of the films was
investigated by thermal desorption spectroscopy (TDS). For this an ESCO EMD-WA1000S system operated at ultrahigh vacuum (<1.0 × 10−9Torr) was used in which the samples are lamp heated
up to 1000 ◦C, with a linear temperature ramp of 20 K min−1. During the annealing, a Balzers AG
QMG 421 quadrupole mass spectrometer was used to determine the H2O and H2effusion rates from
the respective films.
TableIsummarizes the electrical properties and crystal structure before and after annealing of ITO, IO:H, IZO, and IO:H/ITO bilayers. Before annealing, ITO is polycrystalline while IO:H and IZO are amorphous. After annealing, IO:H crystallizes, while the IZO films remain amorphous. The polycrystalline ITO films do not present changes with the annealing step.5Details of the XRD spectra are presented in Fig. S1 of the supplementary material.23The results in TableIshow that
TABLE I. Comparison of the deposition parameters, crystal structure, and electrical properties before and after annealing of the studied ITO, IO:H, IZO, and IO:H/ITO bilayers films.
Deposition processes
Electrical properties before and after annealing
Sample
Magnetron
sputtering T (◦C) Ceramic target
Rs(Ω/) before/after µHall(cm2/V s) before/after Ne(1020cm−3) before/after Crystal structure before/after annealing ITO dc Room temperature In2O3with 10 wt.% SnO 125/110 40/25 1.1/2.5 Crystalline/ crystalline IO:H rf with water
vapor Room temperature In2O3 28/35 50/115 3.6/1.5 Amorphous/ crystalline IZO rf 60 In2O3with 10 wt.% ZnO 80/50 50/60 1.5/1.9 Amorphous/ amorphous IO:H/ITO bilayers rf with water vapor/dc Room temperature In2O3/In2O3with 10 wt.% SnO 55/90 55/85 2.5/1.0 Amorphous/ crystalline
for these respective materials high electron Hall mobility (µHall) (IO:H: 115 cm2/V s and IO:H/ITO
bilayers: 85 cm2/V s, integrated value), low µ
Hall(ITO: 25 cm2/V s), and intermediate µHall(IZO:
60 cm2/V s) are achieved after annealing. Conversely, the N
edecreases in IO:H and IO:H/ITO
bila-yers but increases in ITO after annealing. The decrease in Ne in IO:H is described by OH radicals
and H2O desorption from the amorphous film by annealing,24as well as a possible oxidation of the
films.25The crystallization, decrease in Ne, and possible H passivation at grain boundaries result in
a strong increase in µHall.26,27For the amorphous IZO layer, the µHalland Neslightly increase after
annealing.
Figure1 displays the relative change (%) in sheet resistance (Rs), carrier concentration (Ne),
and electron Hall mobility (µHall) of annealed ITO, IO:H, IZO, and IO:H/ITO bilayers as a function
of DH time. For ITO, IO:H/ITO bilayers, and IZO, Rs changes by 3%, 9%, and 13% from the
initial value, respectively, while for IO:H, the relative change is more than 70% after 1000 h of DH (Fig.1(a)). For Ne, all films show less than 10% relative change (Fig.1(b)). In terms of µHall
FIG. 1. Relative change in electrical properties of annealed ITO, IO:H, IZO, and IO:H/ITO bilayers as a function of damp heat (DH) time: (a) sheet resistance, Rs, (b) carrier concentration, Ne, and (c) Hall mobility, µHall.
116105-4 Tohsophon et al. APL Mater. 3, 116105 (2015)
(Fig.1(c)), we observe only 1% decrease after 1000 h of DH for ITO, while for IZO and IO:H/ITO bilayers, the µHalldecreases by 5% and 10%, respectively. Figure S2 of the supplementary material
shows that increasing the carrier density of IZO results in a further stability improvement, with no change in hall mobility even after 2000 h of DH.23Remarkably, for uncapped IO:H, we find a drastic reduction of µHall(40% in relative change) after DH treatment, playing the main role in the
Rsdegradation. This result underscores the importance of capping the IO:H layers with ITO.4
Figure 2 presents the optical transmittance and absorptance of the layers before and after 1000 h of DH. The films show over 80% transmittance and less than 5% absorptance in the wave-length range of 380–1100 nm. No significant change in the transmittance and absorptance after the DH treatment is observed for ITO and IZO, and only a slight decrease in absorptance at wavelengths above 1500 nm is observed for the IO:H and IO:H/ITO bilayers after DH. This slight decrease in absorptance is linked to a shift in the reflectance of the layers, probably due to a small change of the refractive index of the films.
Comparing all annealed films, IO:H presents the highest sensitivity to heat and humidity, evidenced by the strongest degradation in µHall, Rs, and optical properties, while ITO, IZO, and
IO:H/ITO show much higher stability under DH treatment.
To probe the chemisorption of H2O-groups in the TCOs, we conducted XPS on the annealed
samples before and after DH test (Fig. 3). The high-resolution core-level O1s spectra were fitted
FIG. 2. Optical (a) transmittance, (b) reflectance and (c) absorptance of ITO, IO:H, IZO, and IO:H/ITO bilayers before (full lines) and after 1000 h of DH (dashed lines).
FIG. 3. Graph fitting of high resolution O1s XPS spectra of annealed IO:H, ITO, and IZO ((a), (c), and (e)) before and ((b), (d), and (f)) after DH test. C1s peak of IO:H (g) before and (h) after DH.
with four Gauss-Lorentz peaks. Peak A observed at a binding energy (BE) of 529.5 eV is an oxide peak, related to the oxygen (O) attached to the metallic elements (here In, Sn, or Zn). The slightly shifted peak about 530.5 eV, namely, peak B, is also a metal oxide peak but ascribed to O atoms in the vicinity of an oxygen vacancy (VO).28 Therefore, the area of this peak is related
to the concentration of free-carriers originating from VO. Peak C at BE of 532 eV is attributed to
chemisorbed OH groups on the surface, often shown as indium oxyhydroxide (InOOH).22,27,29This peak is indicative for the amount of chemisorbed OH in the layers, surface, and grain boundaries. The fourth peak at 533 eV, peak D, we assign to O in metal carbonates (C—O—M)29or molecularly chemisorbed H2O.30 The C1s peak of these compounds is usually fitted with three components,
in which peak E corresponds to graphitic carbon and peaks F and G correspond to C—O—C and O—C==C, respectively.
Comparing the XPS spectra of the layers before and after DH, we observe that the fraction of O near VOdecreases in IO:H and IZO, indicating lower Nein these DH treated films, which is
116105-6 Tohsophon et al. APL Mater. 3, 116105 (2015)
FIG. 4. XRD spectra of IO:H films before and after 1000 h of DH treatment.
in good agreement with our Hall effect data (Fig.1). Additionally, the fraction of adsorbed OH, present as InOOH, increases significantly in all films after DH, which is in agreement with previous publications,29 further confirming the OH-groups incorporation in the layers after DH test. While chemisorbed H2O or OH groups strongly affect the electrical properties of IO:H, notably, in ITO
and IZO, the electrical properties do not degrade (Fig. 1), probably due to accumulation of OH groups on the surface but not diffusion into the bulk.
To deepen our understanding of the degradation mechanism in IO:H during DH, we inves-tigated the film properties by XRD, atomic force microscopy (AFM), and FTIR spectroscopy. Analyzing our XRD data (Fig.4), no structural change is observed in the DH treated film, indi-cating that the adsorbed H2O-related species do not deteriorate the crystallinity of the film. This is
supported by AFM measurements (Fig. S2 in supplementary material23). AFM topography scans indicate that the root-mean-square (RMS) surface roughness of the films changes only very slightly from 0.5 nm RMS before DH to 0.7 nm RMS after DH. Consequently, this suggests that degra-dation in Rsis not caused by a modification in the microstructure of the films (amorphization for
example27) but points to H
2O or OH-species adsorption at the grain boundaries.14The creation or
annihilation of point defects in IO:H caused by DH is equally discarded based on the unchanged Ne
before and after DH (Nechanges by <10%).
The contribution of grain boundary scattering to the degradation of the electrical properties of IO:H before and after DH was analyzed by comparing the optical (µoptical) and Hall (µHall)
mobilities as shown in Fig. 5. The optical mobility was obtained by fitting the FTIR reflectance spectra (Fig.5(a)) by a commercial software using the Drude model with effective electron mass (m∗
) = 0.3 me, with me the electron mass.24,31 The details of graph fitting can be found
else-where.21 Reference data on ITO are also presented. It is well known that in macroscopic Hall effect measurements, the mobility of bulk materials is influenced by all possibly present scattering mechanisms—including grain boundary scattering.21 For the optical measurement, the mean free path of the electron in the IR region is much smaller than the average grain size, accordingly only intragrain scattering will affect the µoptical.21 As shown in Fig.5, before DH treatment, the µoptical
and the µHall values of IO:H and ITO are almost identical. For ITO, the µoptical, and µHallafter
DH feature similar values, indicating that no degradation of grain boundaries occurs in this film. Contrastingly, for IO:H, after DH treatment, µHallshows a remarkable decrease, while µopticalonly
show a slight decrease. This indicates a clear degradation at grain boundaries. We propose that this degradation originates from the removal of H atoms at grain boundaries by adsorbed OH-radicals from the DH chamber, forming H2O molecules.32 The formation of H2O molecules, and their
possible desorption from the films, results in the loss of passivation at grain boundaries provided by the H-doping, i.e., higher trap density or potential barriers for electron transport, decreasing
FIG. 5. (a) Infrared reflectance spectra of IO:H and ITO before and after DH treatment. The full line represents the curve obtained by fitting with the Drude model as described by Ref.21. (b) Comparison of the optical (µoptical) and Hall mobility (µHall) of IO:H and ITO before and after 1000 h of DH treatment. The optical mobility was obtained from the curve fitting presented in (a).
116105-8 Tohsophon et al. APL Mater. 3, 116105 (2015) µHall.24–27,33 The H2O desorption from the IO:H films at low temperatures is confirmed by TDS
measurements (Fig.6).
Finally, capping the IO:H film with a thin layer of ITO significantly improves the stability to heat and humidity. ITO functions as a protective layer, preventing water from reaching the IO:H layer and suppressing desorption of hydrogen species from the IO:H film.4,27Therefore, the improved stability of the IO:H/ITO bilayers under DH treatment can be directly related to the pres-ence of the ITO layer (Fig.1). In fact, capping of DH-sensitive TCOs to improve their stability to heat and humidity is a known effective approach, as demonstrated earlier for ZnO electrodes.16,34,35
In summary, we have studied the sensitivity to heat and humidity of sputtered ITO, IO:H, IZO, and IO:H/ITO bilayers front contacts used in solar cells by DH test. We found that polycrystalline ITO and amorphous IZO films feature high stability to heat and humidity, thus promising long term stability, needed for large-scale deployment of solar cells, whereas polycrystalline IO:H films degrade under DH exposure. The degradation of IO:H films stems from H removal at the grain boundaries by bonding with chemisorbed OH groups to create H2O, increasing the potential barrier
for electron transport and thus decreasing the µHall. Capping the IO:H with a thin layer of ITO is an
effective strategy to obtain also for this material long-term stable and highly transparent films. This work has been supported by the Swiss National Science Foundation (SNSF) through Marie Heim-Vögtlin grants (MHV), under Project Nos. PMPDP2_145468 and CCEM CONNECT PV. We gratefully acknowledge T. Matsui, Research Center for Photovoltaics, National Institute of Advanced Industrial Science and Technology, Japan for TDS measurement and Professor H.-A. Klok, Molecular and Hybrid Materials Characterization Center (MHMC), Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland for XPS measurement. T. Tohsophon wishes to thank S. Martin de Nicolas, B. Paviet-Salomon, and Z. C. Holman for useful discussions and J. Geissbühler and B. Delaup for technical assistance.
1S. De Wolf, A. Descoeudres, Z. C. Holman, and C. Ballif,Green2, 7 (2012).
2Z. C. Holman, M. Filipic, A. Descoeudres, S. De Wolf, F. Smole, M. Topic, and C. Ballif,J. Appl. Phys.113, 013107 (2013). 3T. Koida, H. Fujiwara, and M. Kondo,Sol. Energy Mater. Sol. Cells93, 851 (2009).
4L. Barraud, Z. C. Holman, N. Badel, P. Reiss, A. Descoeudres, C. Battaglia, S. De Wolf, and C. Ballif,Sol. Energy Mater. Sol. Cells115, 151 (2013).
5M. Morales-Masis, S. Martin de Nicolas, J. Holovsky, S. De Wolf, and C. Ballif,IEEE J. Photovoltaics5, 1340 (2015). 6J. Werner, G. Dubuis, A. Walter, P. Löper, S.-J. Moon, S. Nicolay, M. Morales-Masis, S. De Wolf, B. Niesen, and C. Ballif,
Sol. Energy Mater. Sol. Cells141, 407 (2015).
7T. Tohsophon, J. Hüpkes, S. Calnan, W. Reetz, B. Rech, W. Beyer, and N. Sirikulrat,Thin Solid Films511–512, 673 (2006). 8F. Matsumoto, M. Tani, and T. Enomoto, U.S. patent No. 5,105,291 (14 April 1992).
9D. E. Carlson, R. Romero, F. Willing, D. Meakin, L. Gonzalez, R. Murphy, H. R. Moutinho, and M. Al-Jassim,Prog. Photovoltaics11, 377 (2003).
10D. Greiner, N. Papathanasiou, A. Pflug, F. Ruske, and R. Klenk,Thin Solid Films520(4), 1285 (2011). 11J. S. Hong, B. R. Rhee, H. M. Kim, K. Ch. Je., Y. J. Kang, and J. S. Ahn,Thin Solid Films467, 158 (2004). 12Y. Kishimoto, O. Nakagawara, H. Seto, Y. Koshido, and Y. Yoshino,Vacuum83, 544 (2009).
13C. Guillén and J. Herrero,Surf. Coat. Technol.201, 309 (2006).
14A. Kaijou, M. Ohyama, M. Shibata, and K. Inoue, U.S. patent No. 5,972,527 (26 October 1999).
15J. I. Kim, W. Lee, T. Hwang, J. Kim, S.-Y. Lee, S. Kang, H. Choi, S. Hong, H. H. Park, T. Moon, and B. Park,Sol. Energy Mater. Sol. Cells122, 282 (2014).
16J. Hüpkes, J. I. Owen, M. Wimmer, F. Ruske, D. Greiner, R. Klenk, U. Zastrow, and J. Hotovy,Thin Solid Films555, 48 (2014).
17R. Sundaramoorthy, F. J. Pern, C. DeHart, T. Gennett, F. Y. Meng, M. Contreras, and T. Gessert, “Stability of TCO window layers for thin-film CIGS solar cells upon damp heat exposure: Part II,”Proc. SPIE7412, 74120J (2009).
18M. Theelen, T. Boumans, F. Stegeman, F. Colberts, A. Illiberi, J. van Berkum, N. Barreau, Z. Vroon, and M. Zeman,Thin Solid Films550, 530 (2014).
19S. Tabassum, E. Yamasue, H. Okumura, and K. N. Ishihara,J. Mater. Sci.: Mater. Electron.25, 3203 (2014). 20T. Koida, H. Fujiwara, and M. Kondo,Jpn. J. Appl. Phys., Part 246, L685 (2007).
21J. Steinhauser, S. Faÿ, N. Oliveira, E. Vallat-Sauvain, and C. Ballif,Appl. Phys. Lett.90, 142107 (2007).
22C. Donley, D. Dunphy, D. Paine, C. Carter, K. Nebesny, P. Lee, D. Alloway, and N. R. Armstrong,Langmuir18, 450 (2002). 23See supplementary material athttp://dx.doi.org/10.1063/1.4935125for XRD spectra of the annealed IZO, ITO and IOH
films and AFM surface scans of IO:H and ITO before and after damp heat treatment.
24T. Koida, H. Shibata, M. Kondo, K. Tsutsumi, A. Sakaguchi, M. Suzuki, and H. Fujiwara,J. Appl. Phys.111, 063721 (2012). 25T. Koida, M. Kondo, K. Tsutsumi, A. Sakaguchi, M. Suzuki, and H. Fujiwara,J. Appl. Phys.107, 033514 (2010). 26B. Macco, H. C. M. Knoops, and W. M. M. Kessels,ACS Appl. Mater. Interfaces7(30), 16723 (2015).
28K. L. Purvis, G. Lu, J. Schwartz, and S. L. Bernasek,J. Am. Chem. Soc.122, 1808 (2000). 29A. Forget, B. Limoges, and V. Balland,Langmuir31, 1931 (2015).
30J. S. Kim, P. K. H. Ho, D. S. Thomas, R. H. Friend, F. Cacialli, G.-W. Bao, and S. F. Y. Li,Chem. Phys. Lett.315, 307 (1999).
31Z. M. Jarzebski,Phys. Status Solidi A71, 131982 (1982).
32M. Morales-Masis, L. Ding, F. Dauzou, Q. Jeangros, A. Hessler-Wyser, S. Nicolay, and C. Ballif,APL Mater.2, 096113 (2014).
33J. Steinhauser, S. Meyer, M. Schwab, S. Faÿ, C. Ballif, U. Kroll, and D. Borrello,Thin Solid Films520, 558 (2011). 34T. L. Chen, R. Betancur, D. S. Ghosh, J. Martorell, and V. Pruneri,Appl. Phys. Lett.100, 013310 (2012).
