Communication pubs.acs.org/cm
Low Temperature Solution-Phase Deposition of SnS Thin Films Priscilla D. Antunez,† Daniel A. Torelli,‡ Fan Yang,‡ Federico A. Rabuffetti,† Nathan S. Lewis,*,‡ and Richard L. Brutchey*,† †
Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States
‡
S Supporting Information *
T
(en) with alkanethiols.17,18 Consistently, bulk SnS powder readily dissolved in an 11:1 vol/vol solvent mixture of en and 1,2-ethanedithiol (edt) under flowing N2(g). A maximum concentration of 12 wt %, or 120 mg mL−1, dissolved SnS (at 25 °C, 1 atm) was observed after prolonged stirring and heating to 50 °C; however, the ink utilized herein was prepared by dissolving the bulk powder in en/edt at 50 °C for ca. 15 h, to produce a dissolved SnS concentration of 6 wt %, or 60 mg mL−1. The resulting SnS ink was stable and possessed a very faint yellow color, in contrast to the dark black color of bulk SnS powder. The solution was optically transparent and free of visible scattering. Thermogravimetric analysis (TGA) was used to determine the temperature needed to recover SnS from the en/edt solution. After drying the solution in situ at 125 °C for 15 min, the sample was further heated to 425 °C. A multistep mass loss of ca. 50 wt % was observed that is consistent with the loss of organic species originating from the solvent mixture, with an end point of decomposition measured at 350 °C (see Supporting Information, Figure S1). FT-IR spectroscopy corroborated the loss of organic species after mild heating of the dried SnS ink to 350 °C under flowing N2(g). Figure 1a
he solution-phase deposition of inorganic semiconductors is a promising, scalable method for the manufacture of thin film photovoltaics.1 Deposition of photovoltaic materials from molecular or colloidal inks offers the possibility of inexpensive, rapid, high-throughput thin film fabrication through processes such as spray coating. For example, CdTe, Cu(In,Ga)(S,Se)2 (CIGS), and CH3NH3Pb(Cl,I)3 perovskitebased thin film solar cells have been previously deposited using solution-based processes.1 Inks have also recently been developed for the solution deposition of Cu2ZnSn(S,Se)4 (CZTS)2 and FeS2 (iron pyrite)3 absorber layers for thin film solar applications, in order to provide sustainable alternatives to materials that contain environmentally harmful heavy metals (e.g., Cd, Pb) and/or scarce elements (e.g., Te, In).4 Tin monosulfide (SnS) is a potentially promising, yet relatively less studied, semiconductor comprised of earthabundant elements.5 Bulk SnS has both a direct (Eg,dir = 1.32 eV) and indirect (Eg,ind = 1.08 eV) band gap, with energies that make SnS an attractive candidate as an absorber layer in photovoltaic devices.6 SnS also exhibits appropriate carrier concentrations (1014−1017 cm−3), high (anisotropic) hole mobilities (90 cm2 V−1 s−1, ⊥ to c-axis), and a high absorption coefficient (>104 cm−1) at energies above the direct band edge for the bulk material.6,7 SnS possesses an orthorhombic (Pnma) layered crystal structure that is held together via van der Waals forces perpendicular to the c-axis, with the layers comprised of strongly bound double layers of Sn and S.6 A certified power conversion efficiency of 4.36% has been recently reported for a soda-lime glass/Mo/SnS (400 nm)/SnO2/Zn(O,S)/ZnO/ITO solar cell device architecture, in which the SnS layer was deposited by atomic layer deposition.8 Thin films of SnS have also been deposited using spray pyrolysis,9 electrochemical deposition,10 thermal and electron beam evaporation,11,12 chemical bath deposition,13 RF-sputtering,14 and chemical vapor deposition.15 Obtaining phase-pure SnS material is often challenging due to the wide variety of Sn− S phases (e.g., SnS, SnS2, Sn2S3, Sn3S4, Sn4S5) and the various oxidation states (0, 2+, 4+) available to tin.16 We report herein a solution-phase deposition route to phase-pure SnS thin films based on the facile dissolution of bulk SnS followed by deposition and crystallization via a mild annealing step. The potential viability of these solution-deposited thin films for use in solar energy-conversion devices has been established by the observation of significant photocurrent during illumination of a p-type SnS cathode in a photoelectrochemical cell. Gray Se, Te, and a series of metal chalcogenides can be readily dissolved in a binary solvent mixture of ethylenediamine © XXXX American Chemical Society
Figure 1. (a) FT-IR spectra of the SnS ink dried to 125 °C for 15 min and after annealing to 350 °C. (b) Powder XRD patterns of a commercial SnS standard and of a sample obtained after annealing the dried SnS ink to 350 °C under flowing N2(g).
shows the strong ν(C−H) and ν(N−H) stretching bands (at ca. 3350−2750 cm−1) originating from the en and edt that disappeared after annealing the dried SnS ink to 350 °C (Figure 1a). The absence of a ν(S−H) stretching band (ca. 2500−2600 cm−1) is in agreement with previous observations that indicate the nearly complete deprotonation of edt in the ink.17,18 Received: August 25, 2014 Revised: September 19, 2014
A
dx.doi.org/10.1021/cm503124u | Chem. Mater. XXXX, XXX, XXX−XXX
Chemistry of Materials
Communication
convert to black SnS and appear to be macroscopically free of defects (Figure 2b). SEM analysis indicated that the films were ca. 245 nm thick, were polycrystalline in nature, and showed no major microscopic cracks or pinholes (see Supporting Information, Figure S4). The transient photocurrent response of the SnS thin films was measured to assess the material’s potential for solar energy-conversion applications.21 The SnS thin films were evaluated in contact with 0.1 M Eu(NO3)3(aq), where Eu3+ served as a sacrificial oxidant, using an ELH-type tungsten−halogen lamp under 1 Sun chopped illumination, with a Pt-mesh counter electrode and a saturated calomel electrode (SCE) as a reference electrode. Cyclic voltammetry under chopped white-light illumination revealed a cathodic photocurrent that increased at potentials negative of −200 mV vs SCE (see Supporting Information, Figure S5), indicating that the SnS thin films exhibited p-type behavior. Figure 3 shows the
Powder X-ray diffraction (XRD) data confirmed that the material recovered after annealing the dried SnS ink at 350 °C was highly crystalline and phase-pure SnS. Crystalline oxide impurities were successfully avoided by drying and annealing the SnS ink under flowing N2(g). A mixture of crystalline SnS, SnS2, and SnO was produced when the dried ink was annealed in air at 350 °C. Figure 1b shows the matching diffraction patterns of both a commercial SnS standard and of recovered SnS after annealing at 350 °C. The diffraction patterns exhibited a 100% intensity peak (2θ = 32°), indexed to the (111) reflection of orthorhombic Pnma SnS (JCPDS no. 00039-0354), with all of the other peaks in Figure 1b also matching the SnS Herzenbergite phase. Rietveld analysis produced calculated lattice constants for the recovered SnS sample (a = 11.242(3) Å, b = 3.9943(11) Å, and c = 4.3102(12) Å) that closely matched the literature values of a = 11.20 Å, b = 3.99 Å, and c = 4.33 Å (see Supporting Information, Figure S2).19 An average grain size of 29 ± 4 nm was estimated from the Lorentzian isotropic broadening of the diffraction maxima. Analysis of 10 randomly selected areas of powdered samples by scanning-electron microscope energydispersive X-ray spectroscopy (SEM-EDX) gave an average elemental composition of 53 atom % Sn and 47 atom % S for the bulk commercial SnS standard and yielded 52 atom % Sn and 48 atom % S for the SnS recovered after annealing the dried ink to 350 °C. X-ray photoelectron spectropscopy (XPS) of the recovered material (see Supporting Information, Figure S3) yielded measured binding energies of 584.9 eV for the Sn 3d5/2 and 160.5 eV for the S 2p3/2 peaks, in close agreement with literature values,20 as well as a high-energy tail on the Sn 3d peaks that indicated a minor form of Sn(4+) at the surface. The optical properties of the recovered SnS were characterized by diffuse reflectance UV−vis−NIR spectroscopy using an integrating sphere. Figure 2a shows the absorption
Figure 3. Transient photocurrent response of SnS thin films under potential control at −700 mV vs SCE.
transient photocurrent response of a SnS thin film under 1 Sun chopped illumination. Chronoamperometric experiments yielded photocurrents >170 μA cm−2 at an applied bias of −700 mV vs SCE, while a bias of −500 mV vs SCE produced a photocurrent of ca. 36 μA cm−2. This current density compares favorably with the behavior of p-type SnS thin films prepared by electrodeposition, which exhibited a cathodic photocurrent density of 5−6 μA cm−2 at −600 mV vs Ag|AgCl under whitelight illumination.22 Here, the photoresponse was stable at ca. 150 μA cm−2 for >3 h during which >1.5 C of cathodic charge had been passed (see Supporting Information, Figure S6), suggesting that the photoresponse resulted from Eu3+ reduction rather than being dominated by a photocorrosion process. Similar results were obtained for the SnS photoelectrodes in contact with 0.05 M Eu(OTf)3 (OTf− = CF3SO3−) in CH3CN (see Supporting Information, Figure S7). To demonstrate that the SnS thin films were not simply photoconductive, the opencircuit potential (OCP) was monitored as a function of time under chopped illumination (see Supporting Information, Figure S8).23 Photovoltages of ca. 35 mV were obtained in a Cp2Co+/Cp2Co CH3CN solution (Cp2Co = cobaltocene). In summary, a solvent combination of en and edt facilitated the dissolution of bulk SnS powder, which was then utilized as an ink for solution processing. Phase-pure thin films of SnS were recovered from this ink at mild annealing temperatures, and the indirect and direct band gaps of the recovered material (Eg,dir = 1.3 eV and Eg,ind = 1.1 eV) were in accordance with literature values and were nearly optimal for solar energyconversion applications. Photoelectrochemical measurements showed that the solution-deposited SnS thin films exhibited ptype behavior with a stable current response under 1 Sun illumination, highlighting the potential of SnS processed from this solvent system for energy-conversion applications.
Figure 2. (a) Diffuse reflectance UV−vis−NIR spectrum of SnS recovered from annealing the dry ink to 350 °C. A Tauc plot of the corresponding indirect band gap transition ([F(R)hν]0.5 vs hν) is given as an inset. (b) Dissolved SnS in ethylenediamine and 1,2ethanedithiol (left) and a SnS thin film on a glass substrate (right).
spectrum of the recovered SnS material after annealing at 350 °C. Tauc plots of the Kubelka−Munk function were applied to estimate both the direct (Eg,dir = 1.3 eV) and indirect (Eg,ind = 1.1 eV) band gap transitions, which agree well with both literature values (vide supra) and the bulk commercial SnS standard. Thin films of SnS were prepared by spin-coating two coats of the 60 mg mL−1 ink directly onto fluorine-doped tin oxide (FTO) coated glass, with a 350 °C annealing step after each coat. The resulting SnS films were then annealed at 500 °C under flowing N2(g) to improve the mechanical robustness of the thin films on the FTO substrate. The thin films are colorless prior to thermal annealing, while after annealing the thin films B
dx.doi.org/10.1021/cm503124u | Chem. Mater. XXXX, XXX, XXX−XXX
Chemistry of Materials
■
Communication
(16) (a) Miles, R. W.; Ogah, O. E.; Zoppi, G.; Forbes, I. Thin Film Solids 2009, 517, 4702. (b) Mathews, N. R.; Anaya, H. B. M.; CortesJacome, M. A.; Angeles-Chavez, C. J. Electrochem. Soc. 2010, 157, H337. (c) Price, L. S.; Parkin, I. P.; Hardy, A. M. E.; Hibbert, T. G.; Molloy, K. C. Chem. Mater. 1999, 11, 1792. (d) Robles, V.; Trigo, J. F.; Guillen, C.; Herrero, J. J. Mater. Sci. 2013, 48, 3943. (17) Webber, D. H.; Buckley, J. J.; Antunez, P. D.; Brutchey, R. L. Chem. Sci. 2014, 5, 2498. (18) Webber, D. H.; Brutchey, R. L. J. Am. Chem. Soc. 2013, 135, 15722. (19) Wiedemeier, H.; von Schnering, H. G. Z. Kristallogr. 1978, 148, 295. (20) Price, L. S.; Parkin, I. P.; Hardy, A. M. E.; Clark, R. J. H. Chem. Mater. 1999, 11, 1792. (21) (a) Riha, S. C.; Fredrick, S. J.; Sambur, J. B.; Liu, Y.; Prieto, A. L.; Parkinson, B. A. ACS Appl. Mater. Interfaces 2011, 3, 58. (b) Ye, H.; Park, H. S.; Akhavan, V. A.; Goodfellow, B. W.; Panthani, M. G.; Korgel, B. A.; Bard, A. J. J. Phys. Chem. C 2011, 115, 234. (22) Steichen, M.; Djemour, R.; Gutay, L.; Guillot, J.; Siebentritt, S.; Dale, P. J. J. Phys. Chem. C 2013, 117, 4383. (23) Grimm, R. L.; Bierman, M. J.; O’Leary, L. E.; Strandwitz, N. C.; Brunschwig, B. S.; Lewis, N. S. J. Phys. Chem. C 2012, 116, 23569.
ASSOCIATED CONTENT
S Supporting Information *
Experimental details; TGA trace of the SnS ink; Rietveld refinement details; XPS spectra; SEM images; photoelectrochemical data. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (R.L.B.). *E-mail:
[email protected] (N.S.L.). Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported in part by the National Science Foundation under DMR-1205712. P.D.A. acknowledges the National Science Foundation for a Graduate Research Fellowship. N.S.L., D.A.T., and F.Y. acknowledge support work through the Office of Science of the U.S. Department of Energy (DOE) under Award No. DE-SC0004993 to the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub.
■
REFERENCES
(1) (a) Todorov, T. K.; Gunawan, O.; Gokmen, T.; Mitzi, D. B. Prog. Photovoltaics 2013, 21, 82. (b) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Science 2012, 338, 643. (c) Panthani, M. G.; Kurley, J. M.; Crisp, R. W.; Dietz, T. C.; Ezzyat, T.; Luther, J. M.; Talapin, D. V. Nano Lett. 2014, 14, 670. (2) (a) Guo, Q.; Ford, G. M.; Yang, W.-C.; Hages, C. J.; Hillhouse, H. W.; Agrawal, R. Sol. Energy Mater. Sol. Cells 2012, 105, 132. (b) Mitzi, D. B.; Gunawan, O.; Todorov, T. K.; Wang, K.; Guha, S. Sol. Energy Mater. Sol. Cells 2011, 95, 1421. (3) (a) Seefeld, S.; Limpinsel, M.; Liu, Y.; Farhi, N.; Weber, A.; Zhang, Y.; Berry, N.; Kwon, Y. J.; Perkins, C. J.; Hemminger, J. C.; Wu, R.; Law, M. J. Am. Chem. Soc. 2013, 135, 4412. (b) Puthussery, J.; Seefeld, S.; Berry, N.; Gibbs, M.; Law, M. J. Am. Chem. Soc. 2011, 133, 716. (4) Wadia, C.; Alivisatos, A. P.; Kammen, D. M. Environ. Sci. Technol. 2009, 43, 2072. (5) (a) Antunez, P. D.; Buckley, J. J.; Brutchey, R. L. Nanoscale 2011, 3, 2399. (b) Habas, S. E.; Platt, H. A. S.; van Hest, M. F. A. M.; Ginley, D. S. Chem. Rev. 2010, 110, 6571. (6) Albers, W.; Haas, C.; Vink, H. J.; Wasscher, J. D. J. Appl. Phys. 1961, 32, 2220. (7) (a) Noguchi, H.; Setiyadi, A.; Tanamura, H.; Nagatomo, T.; Omoto, O. Sol. Energy Mater. Sol. Cells 1994, 35, 325. (b) Vidal, J.; Lany, S.; d’Avezac, M.; Zunger, A.; Zakutayev, A.; Francis, J.; Tate, J. Appl. Phys. Lett. 2012, 100, 032104. (8) Sinsermsuksakul, P.; Sun, L.; Lee, S. W.; Park, H. H.; Kim, S. B; Yang, C. Y.; Gordon, R. G. Adv. Energy Mater. 2014, DOI: 10.1002/ aenm.201400496. (9) Reddy, K. T. R.; Reddy, N. K.; Miles, R. W. Sol. Energy Mater. Sol. Cells 2006, 90, 3041. (10) Ghazali, A.; Zainal, Z.; Hussein, M. Z.; Kassim, A. Sol. Energy Mater. Sol. Cells 1998, 55, 237. (11) Johnson, J. B.; Jones, H.; Latham, B. S.; Parker, J. D.; Engelken, R. D.; Barber, C. Semicond. Sci. Technol. 1999, 14, 501. (12) Tanusevski, A.; Poelman, D. Sol. Energy Mater. Sol. Cells 2003, 80, 297. (13) Tanusevski, A. Semicond. Sci. Technol. 2003, 18, 501. (14) Hartman, K.; Johnson, J. L.; Bertoni, M. I.; Recht, D.; Aziz, M. J.; Scarpulla, M. A.; Buonassisi, T. Thin Solid Films 2011, 519, 7421. (15) Sinsermsuksakul, P.; Hartman, K.; Kim, S. B.; Heo, J.; Sun, L.; Park, H. H.; Chakraborty, R.; Buonassisi, T.; Gordon, R. G. Appl. Phys. Lett. 2013, 102, 053901. C
dx.doi.org/10.1021/cm503124u | Chem. Mater. XXXX, XXX, XXX−XXX
