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Fourier-transform infrared spectroscopic studies of pristine polysilanes as precursor molecules for the solution deposition of amorphous silicon thin-films
Solar Energy Materials & Solar Cells 100 (2012) 61–64
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Fourier-transform infrared spectroscopic studies of pristine polysilanes as precursor molecules for the solution deposition of amorphous silicon thin-films Soo Kim, Chun-Young Lee, Michael H.-C. Jin n Department of Materials Science and Engineering, University of Texas, 500 West First Street, Arlington, TX 76019, USA
a r t i c l e i n f o
abstract
Article history: Received 1 September 2010 Received in revised form 15 March 2011 Accepted 15 April 2011 Available online 11 May 2011
Pristine polysilane containing only Si and H is considered as a potential precursor material that allows the solution-based deposition of hydrogenated amorphous Si (a-Si:H) thin-films reducing manufacturing cost of Si thin-film photovoltaic devices. This study explored three different synthetic routes ¨ including Wurtz-type reductive coupling reaction, hydrogenation of Si anionic compound, and the ¨ dehydrocoupling reaction in order to realize the soluble polysilane molecules. While Wurtz-type reaction of diiodosilane presented us a direct synthetic scheme for hydrogenated polysilane, the results indicated that an extremely controlled air-free system for the synthesis and sample handling would be necessary to prevent the formation of siloxane bonds from the spontaneous reaction between silanebased molecules and water in the air. While the hydrogenation of CaSi2 and the dehydrocoupling reaction of phenylsilane provided more stable forms of polysilane, dissolution of the polysilane from CaSi2 in dichlorobenzene was not successful possibly due to its layered structure succeeded from CaSi2 and the removal of the phenyl groups from the synthesized polyphenylsilane remains as a challenge to realize the polysilane precursor necessary for the solution-based thin-film process of a-Si:H. & 2011 Elsevier B.V. All rights reserved.
Keywords: Amorphous silicon Thin film Solution process Polysilane Solar cell Precursor
1. Introduction Hydrogenated amorphous silicon (a-Si:H) thin film is widely used in the thin-film transistors and photovoltaic devices, respectively, as a channel and an active layer on top of both rigid and flexible substrates because depositing an electronic-grade a-Si:H material is possible at low temperature ( o500 1C) [1]. The a-Si:H thin film is commonly deposited by plasma-enhanced chemical vapor deposition (PECVD) in which a monomeric silane gas flown into a vacuum chamber turns into reactive species in the plasma and they deposit on top of heated substrate forming Si thin films [2]. Since the conventional deposition method requires a complex vacuum and plasma tools and suffers from low deposi˚ tion rate in the order of A/s, the industry has been enduring high manufacturing cost and difficult process adaptation. In order to overcome those obstacles, solution-based deposition methods integrated with the printing and the roll-to-roll process is considered as very attractive alternatives because they can make the process adaptation easy, reduces cost, and allows a large area manufacturing process with high yield.
n
Corresponding author. Tel.: þ1 817 272 0759; fax: þ1 817 272 2538. E-mail address: [email protected] (M.H. Jin).
0927-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2011.04.023
For the solution-based deposition of a-Si:H thin film, one possible approach is to synthesize soluble pristine polysilane – polysilane only made of Si and H, to print it, and to reconstruct the structure into a-Si:H network by providing the necessary energy – the thermolysis of the polysilane, as an example, can be done for the reconstruction process. Successful formation of polycrystalline Si thin-films from the thermolysis of polymeric precursor [3] and their n-type doping [4] was recently demonstrated by Shimoda et al. In the study, the pristine polysilane precursor was synthesized by ring opening polymerization of cyclopentasilane under UV light and the fabrication of the polycrystalline Si thin-film transistor was successfully achieved. Yet, making stable electronic-grade a-Si:H thin films was not possible since, the films treated at high thermolysis temperature (540 1C for 2 h) lost too much hydrogen during the process and the films annealed at low temperatures (300 1C or less) were not stable in the air. Realizing a solution-based process making a-Si:H thin films can revolutionize highly cost-sensitive thin-film photovoltaic industry once one can identify most efficient and reliable synthetic route to the pristine polysilane and develop a controlled process that allows the reconstruction of the polysilane to a stable amorphous Si network with about 10 at% H, which is optimum for photovoltaic device [5]. Incorporating dopants during the reconstruction process is another important challenge, which will, once overcome, enable
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S. Kim et al. / Solar Energy Materials & Solar Cells 100 (2012) 61–64
the fabrication of a p–i–n junction generally used for a-Si:H thinfilm solar cells [5]. As the synthesis of pristine polysilane is seldom reported in the literature [3], this study has identified and explored three different synthetic routes of making pristine polysilane either for their simplicity and/or stability against oxidation. They include ¨ Wurtz-type reductive coupling reaction [6], hydrogenation of Si anionic compound [7], and the dehydrocoupling reaction [8]. ¨ Wurtz-type reductive coupling reaction and the dehydrocoupling reaction were chosen for their maturity in the field, and the motivation of using the hydrogenation of Si anionic compound will be explained later in the article.
2. Experimental details 2.1. W¨ urtz-type reductive coupling reaction and a-Si:H thin films thereafter The general procedure of performing this reaction is elucidated in the literature (Scheme 1) [6]. All the chemicals and reaction media were intentionally processed under Ar atmosphere to avoid any reaction with oxygen and water. The monomer, diiodosilanes (97%) and the catalyst, sodium (99.95%) were purchased from Sigma-Aldrich and used without further purification. Sodium cleaned with Hexane was added to 5 g of diiodosilanes in 10 ml dried toluene at room temperature and the reaction medium was stirred for 4 h. Drop-casting was employed for thin-film formation on a silicon wafer and Fourier-transform infrared (FTIR) spectroscopy was performed to monitor the change in the film during the subsequent thermal processes—drying at 150 1C for 10 min in a vacuum oven and thermal annealing at 300 1C for 1 h using a tube furnace under the flow of Ar.
2.2. Hydrogenation of CaSi2 The detailed procedure of the experiment is well described in the literature (Scheme 2) [9]. CaSi2, 32% HCl, and 48% HF were purchased, respectively, from Spectrum, Fluka, and Fisher scientific and used as received. Handling of all the chemicals and samples was conducted under air and only the hydrogenation reaction of CaSi2 was carried out under Ar. 100 mg of CaSi2 was added into 10 ml HCl at 0 1C under dark condition and the solution was stirred for 3 h. The yellow compound acquired as a form of powder was washed with de-ionized (DI) water to remove residual Ca and Cl ions and dried at 110 1C for 1 h in a vacuum oven. It was followed by HF treatment in order to remove oxygen at 0 1C. After rinsing with DI water, FTIR was conducted to confirm the hydrogenation. In order to probe its potential as a precursor for the solution-based thin-film process, the dissolution of the product in an organic solvent, dichlorobenzene was attempted at 140 1C under Ar atmosphere.
¨ Scheme 1. Reaction scheme of the Wurtz-type reductive coupling reaction to form polysilane.
Scheme 2. Reaction scheme of the hydrogenation of anionic Si compound.
Scheme 3. Reaction scheme of the dehydrocoupling reaction to form polysilane.
2.3. Dehydrocoupling reaction and a-Si:H thin films thereafter The general procedure of this reaction is explained in the literature (Scheme 3) [10]. Bis(cyclopentadienyl)dimethylzirconium (IV) as a catalyst and phenylsilane as a monomer were purchased from Sigma-Aldrich and used as received. The catalyst (40 mg) was added to the solution of phenylsilane (20 mmol) in 3 ml dried toluene under Ar atmosphere and stirred for 24 h. Florisil column chromatography was carried out to separate catalyst and polymer molecules followed by vacuum evaporation of the effluent (chloroform). The thin-films of the products were drop-casted on Si wafers and the attempt to remove phenyl groups in the films was made either thermally or by exposing them to light. The thermal dephenylation was performed at 300 1C for 1 h and the photo-dephenylation was carried out using a xenon arc lamp (450 W) for up to 2 h at room temperature. Both processes were conducted in a glass tube under Ar flow. FTIR was employed to monitor changes in bonding configuration through synthesis and the dephenylation processes.
3. Result and discussion 3.1. W¨ urtz-type reductive coupling reaction ¨ As-prepared polysilane thin-film from Wurtz-type reductive coupling reaction showed C–H bonds at the wavenumber of 2800–3000 cm 1, Si–H bonds at 2000–2100 cm 1, and siloxane bonds at 1000–1100 cm 1 [11] (Fig. 1a). The C–H bond was attributed to the residual toluene in the film as they disappeared after drying it at 150 1C for 10 min in a vacuum oven (Fig. 1b). The presence of Si–H bonds after the drying process also indicated that the film was made of polymer because no monomer was expected to be left in the film after drying. The presence of siloxane bonds was confirmed after removing the residual toluene because toluene also has the vibrational modes around 1000 cm 1 [12]. Due to the complication in transferring and handling thin-film sample between globe box and the vacuum oven for the toluene removal, there were a brief moments when the sample was exposed to air. In fact, the hydrolytic polycondensation reaction between water and diiodosilane (also polymer molecules from the synthesis) are expected to be extremely spontaneous [13], some degree of the siloxane bond formation may be even possible during the synthesis although the reactor was under Ar atmosphere. Preventing oxidation throughout the entire process is one of the challenging obstacles for developing any full line of device technology that utilizes the solution process of the pristine polysilane. Prevention of the siloxane formation seems to be possible only with an extremely controlled air-free system for the synthesis and the subsequent processes and characterizations. While Si–O bonds are considered as impurities in this study, the precursors containing oxygen could be also applicable to photovoltaic and transistor devices that require oxide thin films, respectively, for passivating surface defects to enhance carrier lifetime and as a gate dielectric material to eliminate junction capacitance [14,15].
S. Kim et al. / Solar Energy Materials & Solar Cells 100 (2012) 61–64
63
Transmittance (a.u.)
Transmittance (a.u.)
(a) C-H
(b) (c)
C-H Si-H Phenyl ring
Si-H Si-O-Si
4000 3500 3000 2500 2000 1500 1000
500
¨ Fig. 1. FTIR spectra of the polysilane thin films from Wurtz-type reductive coupling reaction of diiodosilanes: (a) before drying, (b) after drying at 150 1C for 10 min in a vacuum oven, and (c) after thermal annealing at 300 1C for 1 h under Ar.
Transmittance (a.u.)
4000 3500 3000 2500 2000 1500 1000
500
Wavenumber (cm-1)
Wavenumber (cm-1)
(b) (a) Si-H
Si-O-Si
4000 3500 3000 2500 2000 1500 1000
500
Wavenumber (cm-1) Fig. 2. FTIR spectra of the powder product of pristine polysilane from the hydrogenation of CaSi2: (a) before HF treatment and (b) after HF treatment.
Further annealing of the film at 300 1C reduced the concentration of Si–H bonds as Si–Si bonds were expected to start breaking before Si–H bonds at a temperature lower than 300 1C relieving several forms of silicon hydrides (Fig. 1c) [4]. At this stage, the vibrational mode of siloxane bonds at 1000–1100 cm 1 became much broader than before annealing and this can be attributed to the disordered Si–O–Si structure in the amorphous network produced by annealing. From the understanding of difficulty in preventing the hydrolytic polycondensation reaction, different approaches were sought in this study. The hydrogenation reaction of anionic Si compounds and the dehydrocoupling reaction are thought as possible synthetic routes because they can produce stable polysilanes in the beginning stage of the process allowing trouble-free handling of the material until the necessary modification is made to finalize the product in the form of soluble pristine polysilane precursor for the solution-based process. 3.2. Hydrogenation of CaSi2 The polysilane product as a form of solid powder obtained from the hydrogenation reaction of CaSi2 still showed the presence of the siloxane bonds together with Si–OH at the wavenumber higher than 3000 cm 1 (Fig. 2). Their presence can be ascribed to the reaction between water molecules and Si–H bonds in the hydrogenated product after the deintercalation of calcium
Fig. 3. FTIR spectra of the polysilane thin films from the dehydrocoupling reaction of phenylsilanes.
ions. However, it was possible to remove them by the HF treatment and only hydrogenated polysilane powder remained as a final product indicating that the siloxane and Si–OH bonds were mostly formed through the surface of the powder. In order to obtain a solution precursor for the thin-film deposition, the dissolution of the polysilane powder product in an organic solvent was attempted. Dichlorobenzene was selected for the experiment because it successfully dissolved the crosslinked polyethylene previously [16]. The attempt was not successful possibly due to the large molecular weight of the polysilane as it has a layered-network of Si inherited from the structure of CaSi2. According to the result, Si anionic compound with a linear Si structure as a starting material of the hydrogenation reaction would produce linear polysilane molecules with a higher solubility than the layered polysilane from CaSi2. Currently, commercial linear Si anionic compounds are not available according to Authors’ knowledge. 3.3. Dehydrocoupling reaction The selection of phenylsilane as a monomer in the dehydrocoupling reaction produces polysilane molecules stabilized by the phenyl groups in molecules (Scheme 3) and they can slow down the oxidation kinetics of the polysilane. Fig. 3 shows relatively lower concentration of siloxane bonds compared to, for example, ¨ the polysilane from Wurtz-type reductive coupling reaction. It should be noted that the brief exposure of the product to the air was unavoidable during the transport of the sample to the spectrometer. The gel-type polyphenylsilane was successfully synthesized in this study indicating the dehydrocoupling reaction can also provide better control over the molecular weight of the polymer compared to other synthetic methods tried. Controlling the molecular weight of the precursor polymer is important in the solution-based process because there is an optimum molecular weight necessary to prevent excessive evaporation of the precursor molecules during the reconstruction process of Si network through the thermolysis of the precursor polymer, for example. Dephenylation process was required in due course in order to obtain the pristine form of polysilane precursor and it would be desirable to achieve the removal of the phenyl groups during the later stage of process, reconstructing Si network by transforming the precursor thin film to a-Si:H thin film. In addition to thermal dephenylation at 300 1C, this study also tried photo-dephenylation at room temperature as a study reported previously showed that photon energy can detach phenyl rings from phenylsilanes [17]. The FTIR spectra of the polyphenlysilane molecules after both processes
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S. Kim et al. / Solar Energy Materials & Solar Cells 100 (2012) 61–64
Photo-dephenylation 2 hr Transmittance (a.u.)
Transmittance (a.u.)
Before thermal dephenylation
After thermal dephenylation
C-H
1 hr 30 min asdeposited
Si-H Phenyl ring
4000 3500 3000 2500 2000 1500 1000
C-H 500
Si-H
Phenyl ring
4000 3500 3000 2500 2000 1500 1000
-1
500
-1
Wavenumber (cm )
Wavenumber (cm )
Fig. 4. FTIR spectra of the polysilane thin films from the dehydrocoupling reaction of phenylsilanes: (a) after thermal dephenylation at 300 1C for 1 h under Ar and (b) after photo-dephenylation at room temperature for up to 2 h under Ar.
(Fig. 4) showed that the signature of phenyl groups at the wavenumbers of 2800–3000 cm 1 (C–H) and 1110–1430 cm 1 (Si–C in an aromatic ring) [11] was present after the processes. While there was an interesting change in the Si–H peak within the spectra during the dephenylation processes possibly indicating an overall change in the Si–H bonding configuration in the bulk of the films, the formation of Si–O bonds during the thermal dephenylation and their presence prior to the dephenylation process made the analysis on the structural change difficult. The future activities will include an effort to prevent the oxidation further and to find the optimum UV radiation that can selectively break the Si–C bonds.
4. Conclusions The a-Si:H thin-film deposition by a solution process from pristine polysilane precursor is an attractive way to prepare photovoltaic devices with a-Si:H films. The reconstruction of the precursor polymer into the amorphous network of Si should be possible by thermolysis of the precursor, for example. Preparation of the soluble and stable polysilane efficiently is the first challenge to overcome in order to achieve the aimed solution-based ¨ process. Studying from the Wurtz-type reductive coupling reaction of diiodosilanes, oxygen incorporation into the Si linkage due to spontaneous hydrolytic polycondensation reaction between diiodosilanes (and synthesized molecules) and water molecules turned out to be the major huddle for the approach. The topdown approach by the hydrogenation of CaSi2 allowed facile production of polysilanes with a built-in two-dimensional Si network inherited from CaSi2, which provided stability against the oxidation. The polyphenlysilane synthesized by hydrocoupling phenylsilanes also showed good tolerance against the oxidation. Future effort will be made toward (i) utilizing Si anionic compounds with a linear Si structure in order to improve the solubility of the polysilane product and (ii) developing a decomposition process of polyphenylsilane which is optimized for both efficient removal of the phenyl groups inside precursor polymer and the reconstruction of amorphous Si network.
Acknowledgements Authors acknowledge Characterization Center for Materials and Biology at the University of Texas at Arlington (UTA) and its
staff members for the assistance with FTIR and Prof. Jung-Il Jin from Korea University for invaluable discussion on the subject. The research is financially supported by UTA.
References [1] M.A. Green, Thin film solar cells: review of materials, technologies, and commercial status, J. Mater. Sci. 18 (2007) S15–S18. [2] D.L. Smith, Thin-film deposition: principles and practice, McGraw-Hill Professional, 1995. [3] T. Shimoda, Y. Matsuki, M. Furusawa, T. Aoki, I. Yudasaka, H. Tanaka, H. Iwasawa, D. Wang, M. Miyasaka, Y. Takeuchi, Solution-processed silicon films and transistor, Nature (London) 440 (2006) 783–786. [4] H. Tanaka, H. Iwasawa, D. Wang, N. Toyoda, T. Aoki, I. Yudasaka, Y. Matsuki, T. Shimoda, M. Furusawa, Spin-on n-type silicon films using phosphorousdoped polysilanes, Jpn. J. Appl. Phys. 46 (2007) L886–L888. [5] J. Ca´rabe, J.J. Gandia, Thin-film-silicon solar cells, Opto-Electron. Rev. 12 (2004) 1–6. [6] G.J. Richard, J.H. Simon, High-yield controlled syntheses of polysilanes by the ¨ Wurtz-type reductive coupling reaction, Polym. Int. 55 (2006) 711–718. [7] T.D. Tilley, The coordination polymerization of silanes to polysilanes by a ‘‘s-bond metathesis’’ mechanism. Implication for linear chain growth, Acc. Chem. Res. 26 (1993) 22–29. [8] J. He, J.S. Tse, D.D. Klug, K.F. Preston, Layered polysilane: thermolysis and photoluminescence, J. Mater. Chem. 8 (1998) 705–710. [9] U. Dettlaff-Weglikowska, W. Honle, A. Molassioti-Dohms, S. Finkbeiner, J. Weber, Structure and optical properties of the planar silicon compounds ¨ polysilane and Wohler siloxene, Phys. Rev. B 56 (1997) 13132–13140. [10] C. Aitken, J.F. Harrod, Polymerization of primary silanes to linear polysilanes catalyzed by titanocene derivatives, J. Organomet. Chem. 279 (1985) C11–C13. [11] C.J. Pouchert, Aldrich Library of FT-IR Spectra, Aldrich Chem. Co. Library (1989). [12] J. Turkevich, H.A. McKenzie, L. Friedmann, R. Spurr, Infrared spectra of ortho-, meta-, para- and omega-monodeuterotoluenes in the 2–16 micron region, J. Am. Chem. Soc. 71 (1949) 4045–4049. [13] R.G. Johnes, W. Ando, J. Chojnowski (Eds.), Silicon-Containing Polymers: The Science and Technology of Their Synthesis and Applications, Springer, 2001. [14] M. Hofmann, S. Glunz, R. Preu, G. Willeke, 21%-Efficient silicon solar cells using amorphous silicon rear side passivation, in: Proceedings of the 21st European Photovoltaic Solar Energy Conference, 4–8 September 2006, Barcelona, Spain, pp. 609–612. [15] C. Ye, Z. Ning, T. Wang, X. Yu, Y. Xin, Effect of Si–OH group on characteristics of SiCOH films prepared by decamethylcyclopentasiloxane electron cyclotron resonance plasma, Thin Solid Films 496 (2006) 221–226. [16] T. Macko, H. Pasch, Y.V. Kazakevich, A.Y. Fadeev, Elution behavior of polyethylene in polar mobile phases on a non-polar sorbent, J. Chroma. A 988 (2003) 69–76. [17] H. Ishikawa, O. Kajimoto, Photolysis of phenylsilane at 193 nm: energy distributions of the fragments, J. Phys. Chem. 98 (1994) 122–125.
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Fourier-transform infrared spectroscopic studies of pristine polysilanes as precursor molecules for the solution deposition of amorphous silicon thin-films Soo Kim, Chun-Young Lee, Michael H.-C. Jin n Department of Materials Science and Engineering, University of Texas, 500 West First Street, Arlington, TX 76019, USA
a r t i c l e i n f o
abstract
Article history: Received 1 September 2010 Received in revised form 15 March 2011 Accepted 15 April 2011 Available online 11 May 2011
Pristine polysilane containing only Si and H is considered as a potential precursor material that allows the solution-based deposition of hydrogenated amorphous Si (a-Si:H) thin-films reducing manufacturing cost of Si thin-film photovoltaic devices. This study explored three different synthetic routes ¨ including Wurtz-type reductive coupling reaction, hydrogenation of Si anionic compound, and the ¨ dehydrocoupling reaction in order to realize the soluble polysilane molecules. While Wurtz-type reaction of diiodosilane presented us a direct synthetic scheme for hydrogenated polysilane, the results indicated that an extremely controlled air-free system for the synthesis and sample handling would be necessary to prevent the formation of siloxane bonds from the spontaneous reaction between silanebased molecules and water in the air. While the hydrogenation of CaSi2 and the dehydrocoupling reaction of phenylsilane provided more stable forms of polysilane, dissolution of the polysilane from CaSi2 in dichlorobenzene was not successful possibly due to its layered structure succeeded from CaSi2 and the removal of the phenyl groups from the synthesized polyphenylsilane remains as a challenge to realize the polysilane precursor necessary for the solution-based thin-film process of a-Si:H. & 2011 Elsevier B.V. All rights reserved.
Keywords: Amorphous silicon Thin film Solution process Polysilane Solar cell Precursor
1. Introduction Hydrogenated amorphous silicon (a-Si:H) thin film is widely used in the thin-film transistors and photovoltaic devices, respectively, as a channel and an active layer on top of both rigid and flexible substrates because depositing an electronic-grade a-Si:H material is possible at low temperature ( o500 1C) [1]. The a-Si:H thin film is commonly deposited by plasma-enhanced chemical vapor deposition (PECVD) in which a monomeric silane gas flown into a vacuum chamber turns into reactive species in the plasma and they deposit on top of heated substrate forming Si thin films [2]. Since the conventional deposition method requires a complex vacuum and plasma tools and suffers from low deposi˚ tion rate in the order of A/s, the industry has been enduring high manufacturing cost and difficult process adaptation. In order to overcome those obstacles, solution-based deposition methods integrated with the printing and the roll-to-roll process is considered as very attractive alternatives because they can make the process adaptation easy, reduces cost, and allows a large area manufacturing process with high yield.
n
Corresponding author. Tel.: þ1 817 272 0759; fax: þ1 817 272 2538. E-mail address: [email protected] (M.H. Jin).
0927-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2011.04.023
For the solution-based deposition of a-Si:H thin film, one possible approach is to synthesize soluble pristine polysilane – polysilane only made of Si and H, to print it, and to reconstruct the structure into a-Si:H network by providing the necessary energy – the thermolysis of the polysilane, as an example, can be done for the reconstruction process. Successful formation of polycrystalline Si thin-films from the thermolysis of polymeric precursor [3] and their n-type doping [4] was recently demonstrated by Shimoda et al. In the study, the pristine polysilane precursor was synthesized by ring opening polymerization of cyclopentasilane under UV light and the fabrication of the polycrystalline Si thin-film transistor was successfully achieved. Yet, making stable electronic-grade a-Si:H thin films was not possible since, the films treated at high thermolysis temperature (540 1C for 2 h) lost too much hydrogen during the process and the films annealed at low temperatures (300 1C or less) were not stable in the air. Realizing a solution-based process making a-Si:H thin films can revolutionize highly cost-sensitive thin-film photovoltaic industry once one can identify most efficient and reliable synthetic route to the pristine polysilane and develop a controlled process that allows the reconstruction of the polysilane to a stable amorphous Si network with about 10 at% H, which is optimum for photovoltaic device [5]. Incorporating dopants during the reconstruction process is another important challenge, which will, once overcome, enable
62
S. Kim et al. / Solar Energy Materials & Solar Cells 100 (2012) 61–64
the fabrication of a p–i–n junction generally used for a-Si:H thinfilm solar cells [5]. As the synthesis of pristine polysilane is seldom reported in the literature [3], this study has identified and explored three different synthetic routes of making pristine polysilane either for their simplicity and/or stability against oxidation. They include ¨ Wurtz-type reductive coupling reaction [6], hydrogenation of Si anionic compound [7], and the dehydrocoupling reaction [8]. ¨ Wurtz-type reductive coupling reaction and the dehydrocoupling reaction were chosen for their maturity in the field, and the motivation of using the hydrogenation of Si anionic compound will be explained later in the article.
2. Experimental details 2.1. W¨ urtz-type reductive coupling reaction and a-Si:H thin films thereafter The general procedure of performing this reaction is elucidated in the literature (Scheme 1) [6]. All the chemicals and reaction media were intentionally processed under Ar atmosphere to avoid any reaction with oxygen and water. The monomer, diiodosilanes (97%) and the catalyst, sodium (99.95%) were purchased from Sigma-Aldrich and used without further purification. Sodium cleaned with Hexane was added to 5 g of diiodosilanes in 10 ml dried toluene at room temperature and the reaction medium was stirred for 4 h. Drop-casting was employed for thin-film formation on a silicon wafer and Fourier-transform infrared (FTIR) spectroscopy was performed to monitor the change in the film during the subsequent thermal processes—drying at 150 1C for 10 min in a vacuum oven and thermal annealing at 300 1C for 1 h using a tube furnace under the flow of Ar.
2.2. Hydrogenation of CaSi2 The detailed procedure of the experiment is well described in the literature (Scheme 2) [9]. CaSi2, 32% HCl, and 48% HF were purchased, respectively, from Spectrum, Fluka, and Fisher scientific and used as received. Handling of all the chemicals and samples was conducted under air and only the hydrogenation reaction of CaSi2 was carried out under Ar. 100 mg of CaSi2 was added into 10 ml HCl at 0 1C under dark condition and the solution was stirred for 3 h. The yellow compound acquired as a form of powder was washed with de-ionized (DI) water to remove residual Ca and Cl ions and dried at 110 1C for 1 h in a vacuum oven. It was followed by HF treatment in order to remove oxygen at 0 1C. After rinsing with DI water, FTIR was conducted to confirm the hydrogenation. In order to probe its potential as a precursor for the solution-based thin-film process, the dissolution of the product in an organic solvent, dichlorobenzene was attempted at 140 1C under Ar atmosphere.
¨ Scheme 1. Reaction scheme of the Wurtz-type reductive coupling reaction to form polysilane.
Scheme 2. Reaction scheme of the hydrogenation of anionic Si compound.
Scheme 3. Reaction scheme of the dehydrocoupling reaction to form polysilane.
2.3. Dehydrocoupling reaction and a-Si:H thin films thereafter The general procedure of this reaction is explained in the literature (Scheme 3) [10]. Bis(cyclopentadienyl)dimethylzirconium (IV) as a catalyst and phenylsilane as a monomer were purchased from Sigma-Aldrich and used as received. The catalyst (40 mg) was added to the solution of phenylsilane (20 mmol) in 3 ml dried toluene under Ar atmosphere and stirred for 24 h. Florisil column chromatography was carried out to separate catalyst and polymer molecules followed by vacuum evaporation of the effluent (chloroform). The thin-films of the products were drop-casted on Si wafers and the attempt to remove phenyl groups in the films was made either thermally or by exposing them to light. The thermal dephenylation was performed at 300 1C for 1 h and the photo-dephenylation was carried out using a xenon arc lamp (450 W) for up to 2 h at room temperature. Both processes were conducted in a glass tube under Ar flow. FTIR was employed to monitor changes in bonding configuration through synthesis and the dephenylation processes.
3. Result and discussion 3.1. W¨ urtz-type reductive coupling reaction ¨ As-prepared polysilane thin-film from Wurtz-type reductive coupling reaction showed C–H bonds at the wavenumber of 2800–3000 cm 1, Si–H bonds at 2000–2100 cm 1, and siloxane bonds at 1000–1100 cm 1 [11] (Fig. 1a). The C–H bond was attributed to the residual toluene in the film as they disappeared after drying it at 150 1C for 10 min in a vacuum oven (Fig. 1b). The presence of Si–H bonds after the drying process also indicated that the film was made of polymer because no monomer was expected to be left in the film after drying. The presence of siloxane bonds was confirmed after removing the residual toluene because toluene also has the vibrational modes around 1000 cm 1 [12]. Due to the complication in transferring and handling thin-film sample between globe box and the vacuum oven for the toluene removal, there were a brief moments when the sample was exposed to air. In fact, the hydrolytic polycondensation reaction between water and diiodosilane (also polymer molecules from the synthesis) are expected to be extremely spontaneous [13], some degree of the siloxane bond formation may be even possible during the synthesis although the reactor was under Ar atmosphere. Preventing oxidation throughout the entire process is one of the challenging obstacles for developing any full line of device technology that utilizes the solution process of the pristine polysilane. Prevention of the siloxane formation seems to be possible only with an extremely controlled air-free system for the synthesis and the subsequent processes and characterizations. While Si–O bonds are considered as impurities in this study, the precursors containing oxygen could be also applicable to photovoltaic and transistor devices that require oxide thin films, respectively, for passivating surface defects to enhance carrier lifetime and as a gate dielectric material to eliminate junction capacitance [14,15].
S. Kim et al. / Solar Energy Materials & Solar Cells 100 (2012) 61–64
63
Transmittance (a.u.)
Transmittance (a.u.)
(a) C-H
(b) (c)
C-H Si-H Phenyl ring
Si-H Si-O-Si
4000 3500 3000 2500 2000 1500 1000
500
¨ Fig. 1. FTIR spectra of the polysilane thin films from Wurtz-type reductive coupling reaction of diiodosilanes: (a) before drying, (b) after drying at 150 1C for 10 min in a vacuum oven, and (c) after thermal annealing at 300 1C for 1 h under Ar.
Transmittance (a.u.)
4000 3500 3000 2500 2000 1500 1000
500
Wavenumber (cm-1)
Wavenumber (cm-1)
(b) (a) Si-H
Si-O-Si
4000 3500 3000 2500 2000 1500 1000
500
Wavenumber (cm-1) Fig. 2. FTIR spectra of the powder product of pristine polysilane from the hydrogenation of CaSi2: (a) before HF treatment and (b) after HF treatment.
Further annealing of the film at 300 1C reduced the concentration of Si–H bonds as Si–Si bonds were expected to start breaking before Si–H bonds at a temperature lower than 300 1C relieving several forms of silicon hydrides (Fig. 1c) [4]. At this stage, the vibrational mode of siloxane bonds at 1000–1100 cm 1 became much broader than before annealing and this can be attributed to the disordered Si–O–Si structure in the amorphous network produced by annealing. From the understanding of difficulty in preventing the hydrolytic polycondensation reaction, different approaches were sought in this study. The hydrogenation reaction of anionic Si compounds and the dehydrocoupling reaction are thought as possible synthetic routes because they can produce stable polysilanes in the beginning stage of the process allowing trouble-free handling of the material until the necessary modification is made to finalize the product in the form of soluble pristine polysilane precursor for the solution-based process. 3.2. Hydrogenation of CaSi2 The polysilane product as a form of solid powder obtained from the hydrogenation reaction of CaSi2 still showed the presence of the siloxane bonds together with Si–OH at the wavenumber higher than 3000 cm 1 (Fig. 2). Their presence can be ascribed to the reaction between water molecules and Si–H bonds in the hydrogenated product after the deintercalation of calcium
Fig. 3. FTIR spectra of the polysilane thin films from the dehydrocoupling reaction of phenylsilanes.
ions. However, it was possible to remove them by the HF treatment and only hydrogenated polysilane powder remained as a final product indicating that the siloxane and Si–OH bonds were mostly formed through the surface of the powder. In order to obtain a solution precursor for the thin-film deposition, the dissolution of the polysilane powder product in an organic solvent was attempted. Dichlorobenzene was selected for the experiment because it successfully dissolved the crosslinked polyethylene previously [16]. The attempt was not successful possibly due to the large molecular weight of the polysilane as it has a layered-network of Si inherited from the structure of CaSi2. According to the result, Si anionic compound with a linear Si structure as a starting material of the hydrogenation reaction would produce linear polysilane molecules with a higher solubility than the layered polysilane from CaSi2. Currently, commercial linear Si anionic compounds are not available according to Authors’ knowledge. 3.3. Dehydrocoupling reaction The selection of phenylsilane as a monomer in the dehydrocoupling reaction produces polysilane molecules stabilized by the phenyl groups in molecules (Scheme 3) and they can slow down the oxidation kinetics of the polysilane. Fig. 3 shows relatively lower concentration of siloxane bonds compared to, for example, ¨ the polysilane from Wurtz-type reductive coupling reaction. It should be noted that the brief exposure of the product to the air was unavoidable during the transport of the sample to the spectrometer. The gel-type polyphenylsilane was successfully synthesized in this study indicating the dehydrocoupling reaction can also provide better control over the molecular weight of the polymer compared to other synthetic methods tried. Controlling the molecular weight of the precursor polymer is important in the solution-based process because there is an optimum molecular weight necessary to prevent excessive evaporation of the precursor molecules during the reconstruction process of Si network through the thermolysis of the precursor polymer, for example. Dephenylation process was required in due course in order to obtain the pristine form of polysilane precursor and it would be desirable to achieve the removal of the phenyl groups during the later stage of process, reconstructing Si network by transforming the precursor thin film to a-Si:H thin film. In addition to thermal dephenylation at 300 1C, this study also tried photo-dephenylation at room temperature as a study reported previously showed that photon energy can detach phenyl rings from phenylsilanes [17]. The FTIR spectra of the polyphenlysilane molecules after both processes
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S. Kim et al. / Solar Energy Materials & Solar Cells 100 (2012) 61–64
Photo-dephenylation 2 hr Transmittance (a.u.)
Transmittance (a.u.)
Before thermal dephenylation
After thermal dephenylation
C-H
1 hr 30 min asdeposited
Si-H Phenyl ring
4000 3500 3000 2500 2000 1500 1000
C-H 500
Si-H
Phenyl ring
4000 3500 3000 2500 2000 1500 1000
-1
500
-1
Wavenumber (cm )
Wavenumber (cm )
Fig. 4. FTIR spectra of the polysilane thin films from the dehydrocoupling reaction of phenylsilanes: (a) after thermal dephenylation at 300 1C for 1 h under Ar and (b) after photo-dephenylation at room temperature for up to 2 h under Ar.
(Fig. 4) showed that the signature of phenyl groups at the wavenumbers of 2800–3000 cm 1 (C–H) and 1110–1430 cm 1 (Si–C in an aromatic ring) [11] was present after the processes. While there was an interesting change in the Si–H peak within the spectra during the dephenylation processes possibly indicating an overall change in the Si–H bonding configuration in the bulk of the films, the formation of Si–O bonds during the thermal dephenylation and their presence prior to the dephenylation process made the analysis on the structural change difficult. The future activities will include an effort to prevent the oxidation further and to find the optimum UV radiation that can selectively break the Si–C bonds.
4. Conclusions The a-Si:H thin-film deposition by a solution process from pristine polysilane precursor is an attractive way to prepare photovoltaic devices with a-Si:H films. The reconstruction of the precursor polymer into the amorphous network of Si should be possible by thermolysis of the precursor, for example. Preparation of the soluble and stable polysilane efficiently is the first challenge to overcome in order to achieve the aimed solution-based ¨ process. Studying from the Wurtz-type reductive coupling reaction of diiodosilanes, oxygen incorporation into the Si linkage due to spontaneous hydrolytic polycondensation reaction between diiodosilanes (and synthesized molecules) and water molecules turned out to be the major huddle for the approach. The topdown approach by the hydrogenation of CaSi2 allowed facile production of polysilanes with a built-in two-dimensional Si network inherited from CaSi2, which provided stability against the oxidation. The polyphenlysilane synthesized by hydrocoupling phenylsilanes also showed good tolerance against the oxidation. Future effort will be made toward (i) utilizing Si anionic compounds with a linear Si structure in order to improve the solubility of the polysilane product and (ii) developing a decomposition process of polyphenylsilane which is optimized for both efficient removal of the phenyl groups inside precursor polymer and the reconstruction of amorphous Si network.
Acknowledgements Authors acknowledge Characterization Center for Materials and Biology at the University of Texas at Arlington (UTA) and its
staff members for the assistance with FTIR and Prof. Jung-Il Jin from Korea University for invaluable discussion on the subject. The research is financially supported by UTA.
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