Solar Energy Materials & Solar Cells 100 (2012) 61–64

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Solar Energy Materials & Solar Cells

0927-02

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/solmat

Fourier-transform infrared spectroscopic studies of pristine polysilanesas precursor molecules for the solution deposition of amorphous siliconthin-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

Article history:

Received 1 September 2010

Received in revised form

15 March 2011

Accepted 15 April 2011Available online 11 May 2011

Keywords:

Amorphous silicon

Thin film

Solution process

Polysilane

Solar cell

Precursor

48/$ - see front matter & 2011 Elsevier B.V. A

016/j.solmat.2011.04.023

esponding author. Tel.: þ1 817 272 0759; fax

ail address: mjin@uta.edu (M.H. Jin).

a b s t r a c t

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 manufactur-

ing 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 silane-

based 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.

1. Introduction

Hydrogenated amorphous silicon (a-Si:H) thin film is widelyused in the thin-film transistors and photovoltaic devices, respec-tively, as a channel and an active layer on top of both rigid andflexible substrates because depositing an electronic-grade a-Si:Hmaterial is possible at low temperature (o500 1C) [1]. The a-Si:Hthin film is commonly deposited by plasma-enhanced chemicalvapor deposition (PECVD) in which a monomeric silane gas flowninto a vacuum chamber turns into reactive species in the plasmaand they deposit on top of heated substrate forming Si thinfilms [2]. Since the conventional deposition method requires acomplex vacuum and plasma tools and suffers from low deposi-tion rate in the order of A/s, the industry has been enduring highmanufacturing cost and difficult process adaptation. In order toovercome those obstacles, solution-based deposition methodsintegrated with the printing and the roll-to-roll process isconsidered as very attractive alternatives because they can makethe process adaptation easy, reduces cost, and allows a large areamanufacturing process with high yield.

ll rights reserved.

: þ1 817 272 2538.

For the solution-based deposition of a-Si:H thin film, onepossible approach is to synthesize soluble pristine polysilane –polysilane only made of Si and H, to print it, and to reconstructthe structure into a-Si:H network by providing the necessaryenergy – the thermolysis of the polysilane, as an example, can bedone for the reconstruction process. Successful formation ofpolycrystalline Si thin-films from the thermolysis of polymericprecursor [3] and their n-type doping [4] was recently demon-strated by Shimoda et al. In the study, the pristine polysilaneprecursor was synthesized by ring opening polymerization ofcyclopentasilane under UV light and the fabrication of the poly-crystalline Si thin-film transistor was successfully achieved. Yet,making stable electronic-grade a-Si:H thin films was not possiblesince, the films treated at high thermolysis temperature (540 1Cfor 2 h) lost too much hydrogen during the process and the filmsannealed at low temperatures (300 1C or less) were not stable inthe air.

Realizing a solution-based process making a-Si:H thin films canrevolutionize highly cost-sensitive thin-film photovoltaic industryonce one can identify most efficient and reliable synthetic route tothe pristine polysilane and develop a controlled process that allowsthe reconstruction of the polysilane to a stable amorphous Sinetwork with about 10 at% H, which is optimum for photovoltaicdevice [5]. Incorporating dopants during the reconstruction processis another important challenge, which will, once overcome, enable

Scheme 3. Reaction scheme of the dehydrocoupling reaction to form polysilane.

S. Kim et al. / Solar Energy Materials & Solar Cells 100 (2012) 61–6462

the fabrication of a p–i–n junction generally used for a-Si:H thin-film solar cells [5].

As the synthesis of pristine polysilane is seldom reported inthe literature [3], this study has identified and explored threedifferent synthetic routes of making pristine polysilane either fortheir simplicity and/or stability against oxidation. They includeWurtz-type reductive coupling reaction [6], hydrogenation of Sianionic compound [7], and the dehydrocoupling reaction [8].Wurtz-type reductive coupling reaction and the dehydrocouplingreaction were chosen for their maturity in the field, and themotivation of using the hydrogenation of Si anionic compoundwill be explained later in the article.

2. Experimental details

2.1. Wurtz-type reductive coupling reaction and a-Si:H thin films

thereafter

The general procedure of performing this reaction is elucidatedin the literature (Scheme 1) [6]. All the chemicals and reactionmedia were intentionally processed under Ar atmosphere to avoidany reaction with oxygen and water. The monomer, diiodosilanes(97%) and the catalyst, sodium (99.95%) were purchased fromSigma-Aldrich and used without further purification. Sodiumcleaned with Hexane was added to 5 g of diiodosilanes in 10 mldried toluene at room temperature and the reaction medium wasstirred for 4 h. Drop-casting was employed for thin-film forma-tion on a silicon wafer and Fourier-transform infrared (FTIR)spectroscopy was performed to monitor the change in the filmduring the subsequent thermal processes—drying at 150 1C for10 min in a vacuum oven and thermal annealing at 300 1C for 1 husing a tube furnace under the flow of Ar.

2.2. Hydrogenation of CaSi2

The detailed procedure of the experiment is well described inthe literature (Scheme 2) [9]. CaSi2, 32% HCl, and 48% HF werepurchased, respectively, from Spectrum, Fluka, and Fisher scien-tific and used as received. Handling of all the chemicals andsamples was conducted under air and only the hydrogenationreaction of CaSi2 was carried out under Ar. 100 mg of CaSi2 wasadded into 10 ml HCl at 0 1C under dark condition and thesolution was stirred for 3 h. The yellow compound acquired as aform of powder was washed with de-ionized (DI) water toremove residual Ca and Cl ions and dried at 110 1C for 1 h in avacuum oven. It was followed by HF treatment in order to removeoxygen at 0 1C. After rinsing with DI water, FTIR was conducted toconfirm the hydrogenation. In order to probe its potential as aprecursor for the solution-based thin-film process, the dissolutionof the product in an organic solvent, dichlorobenzene wasattempted 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.

2.3. Dehydrocoupling reaction and a-Si:H thin films thereafter

The general procedure of this reaction is explained in theliterature (Scheme 3) [10]. Bis(cyclopentadienyl)dimethylzirco-nium (IV) as a catalyst and phenylsilane as a monomer werepurchased from Sigma-Aldrich and used as received. The catalyst(40 mg) was added to the solution of phenylsilane (20 mmol) in3 ml dried toluene under Ar atmosphere and stirred for 24 h.Florisil column chromatography was carried out to separatecatalyst and polymer molecules followed by vacuum evaporationof the effluent (chloroform). The thin-films of the products weredrop-casted on Si wafers and the attempt to remove phenylgroups in the films was made either thermally or by exposingthem to light. The thermal dephenylation was performed at300 1C for 1 h and the photo-dephenylation was carried out usinga xenon arc lamp (450 W) for up to 2 h at room temperature. Bothprocesses were conducted in a glass tube under Ar flow. FTIR wasemployed to monitor changes in bonding configuration throughsynthesis and the dephenylation processes.

3. Result and discussion

3.1. Wurtz-type reductive coupling reaction

As-prepared polysilane thin-film from Wurtz-type reductivecoupling reaction showed C–H bonds at the wavenumber of2800–3000 cm�1, Si–H bonds at 2000–2100 cm�1, and siloxanebonds at 1000–1100 cm�1 [11] (Fig. 1a). The C–H bond wasattributed to the residual toluene in the film as they disappearedafter drying it at 150 1C for 10 min in a vacuum oven (Fig. 1b). Thepresence of Si–H bonds after the drying process also indicatedthat the film was made of polymer because no monomer wasexpected to be left in the film after drying.

The presence of siloxane bonds was confirmed after removingthe residual toluene because toluene also has the vibrationalmodes around 1000 cm�1 [12]. Due to the complication intransferring and handling thin-film sample between globe boxand the vacuum oven for the toluene removal, there were a briefmoments when the sample was exposed to air. In fact, thehydrolytic polycondensation reaction between water and diiodo-silane (also polymer molecules from the synthesis) are expectedto be extremely spontaneous [13], some degree of the siloxanebond formation may be even possible during the synthesisalthough the reactor was under Ar atmosphere.

Preventing oxidation throughout the entire process is one ofthe challenging obstacles for developing any full line of devicetechnology that utilizes the solution process of the pristinepolysilane. Prevention of the siloxane formation seems to bepossible only with an extremely controlled air-free system forthe synthesis and the subsequent processes and characterizations.While Si–O bonds are considered as impurities in this study, theprecursors containing oxygen could be also applicable to photo-voltaic and transistor devices that require oxide thin films,respectively, for passivating surface defects to enhance carrierlifetime and as a gate dielectric material to eliminate junctioncapacitance [14,15].

4000 3500 3000 2500 2000 1500 1000 500

(b)

(c)

(a)

Si-O-Si

Si-H

C-H

Tran

smitt

ance

(a.u

.)

Wavenumber (cm-1)

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.

(b)

(a)

4000 3500 3000 2500 2000 1500 1000 500

Tran

smitt

ance

(a.u

.)

Wavenumber (cm-1)

Si-H

Si-O-Si

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.

Si-HC-H

Phenyl ringTran

smitt

ance

(a.u

.)

4000

Wavenumber (cm-1)

3500 3000 2500 2000 1500 1000 500

Fig. 3. FTIR spectra of the polysilane thin films from the dehydrocoupling reaction

of phenylsilanes.

S. Kim et al. / Solar Energy Materials & Solar Cells 100 (2012) 61–64 63

Further annealing of the film at 300 1C reduced the concentra-tion of Si–H bonds as Si–Si bonds were expected to start breakingbefore Si–H bonds at a temperature lower than 300 1C relievingseveral forms of silicon hydrides (Fig. 1c) [4]. At this stage, thevibrational mode of siloxane bonds at 1000–1100 cm�1 becamemuch broader than before annealing and this can be attributed tothe disordered Si–O–Si structure in the amorphous networkproduced by annealing.

From the understanding of difficulty in preventing the hydro-lytic polycondensation reaction, different approaches were soughtin this study. The hydrogenation reaction of anionic Si compoundsand the dehydrocoupling reaction are thought as possible syn-thetic routes because they can produce stable polysilanes in thebeginning stage of the process allowing trouble-free handling ofthe material until the necessary modification is made to finalizethe product in the form of soluble pristine polysilane precursorfor the solution-based process.

3.2. Hydrogenation of CaSi2

The polysilane product as a form of solid powder obtainedfrom the hydrogenation reaction of CaSi2 still showed the pre-sence of the siloxane bonds together with Si–OH at the wave-number higher than 3000 cm�1 (Fig. 2). Their presence can beascribed to the reaction between water molecules and Si–H bondsin the hydrogenated product after the deintercalation of calcium

ions. However, it was possible to remove them by the HFtreatment and only hydrogenated polysilane powder remainedas a final product indicating that the siloxane and Si–OH bondswere mostly formed through the surface of the powder.

In order to obtain a solution precursor for the thin-filmdeposition, the dissolution of the polysilane powder product inan organic solvent was attempted. Dichlorobenzene was selectedfor the experiment because it successfully dissolved the cross-linked polyethylene previously [16]. The attempt was not suc-cessful possibly due to the large molecular weight of the poly-silane as it has a layered-network of Si inherited from thestructure of CaSi2. According to the result, Si anionic compoundwith a linear Si structure as a starting material of the hydrogena-tion reaction would produce linear polysilane molecules with ahigher solubility than the layered polysilane from CaSi2. Cur-rently, commercial linear Si anionic compounds are not availableaccording to Authors’ knowledge.

3.3. Dehydrocoupling reaction

The selection of phenylsilane as a monomer in the dehydro-coupling reaction produces polysilane molecules stabilized by thephenyl groups in molecules (Scheme 3) and they can slow downthe oxidation kinetics of the polysilane. Fig. 3 shows relativelylower concentration of siloxane bonds compared to, for example,the polysilane from Wurtz-type reductive coupling reaction. Itshould be noted that the brief exposure of the product to the airwas unavoidable during the transport of the sample to thespectrometer.

The gel-type polyphenylsilane was successfully synthesized inthis study indicating the dehydrocoupling reaction can alsoprovide better control over the molecular weight of the polymercompared to other synthetic methods tried. Controlling themolecular weight of the precursor polymer is important in thesolution-based process because there is an optimum molecularweight necessary to prevent excessive evaporation of the pre-cursor molecules during the reconstruction process of Si networkthrough the thermolysis of the precursor polymer, for example.

Dephenylation process was required in due course in order toobtain the pristine form of polysilane precursor and it would bedesirable to achieve the removal of the phenyl groups during thelater stage of process, reconstructing Si network by transforming theprecursor thin film to a-Si:H thin film. In addition to thermaldephenylation at 300 1C, this study also tried photo-dephenylationat room temperature as a study reported previously showed thatphoton energy can detach phenyl rings from phenylsilanes [17]. TheFTIR spectra of the polyphenlysilane molecules after both processes

BeforePhoto-dephenylation

thermal dephenylation 2 hr

After

1 hr

thermal dephenylation 30 min

as-

Si-HC-H deposited

4000 3500 3000 2500 2000 1500 1000 500

ringPhenyl

4000 3500 3000 2500 2000 1500 1000 500

Phenyl ringSi-HC-H

Tran

smitt

ance

(a.u

.)

Wavenumber (cm-1)

Tran

smitt

ance

(a.u

.)

Wavenumber (cm-1)

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.

S. Kim et al. / Solar Energy Materials & Solar Cells 100 (2012) 61–6464

(Fig. 4) showed that the signature of phenyl groups at the wave-numbers of 2800–3000 cm�1 (C–H) and 1110–1430 cm�1 (Si–C inan aromatic ring) [11] was present after the processes. While therewas an interesting change in the Si–H peak within the spectraduring the dephenylation processes possibly indicating an overallchange in the Si–H bonding configuration in the bulk of the films,the formation of Si–O bonds during the thermal dephenylation andtheir presence prior to the dephenylation process made the analysison the structural change difficult. The future activities will includean effort to prevent the oxidation further and to find the optimumUV radiation that can selectively break the Si–C bonds.

4. Conclusions

The a-Si:H thin-film deposition by a solution process frompristine polysilane precursor is an attractive way to preparephotovoltaic devices with a-Si:H films. The reconstruction of theprecursor polymer into the amorphous network of Si should bepossible by thermolysis of the precursor, for example. Preparationof the soluble and stable polysilane efficiently is the first chal-lenge to overcome in order to achieve the aimed solution-basedprocess. Studying from the Wurtz-type reductive coupling reac-tion of diiodosilanes, oxygen incorporation into the Si linkage dueto spontaneous hydrolytic polycondensation reaction betweendiiodosilanes (and synthesized molecules) and water moleculesturned out to be the major huddle for the approach. The top-down approach by the hydrogenation of CaSi2 allowed facileproduction of polysilanes with a built-in two-dimensional Sinetwork inherited from CaSi2, which provided stability againstthe oxidation. The polyphenlysilane synthesized by hydrocou-pling phenylsilanes also showed good tolerance against theoxidation. Future effort will be made toward (i) utilizing Sianionic compounds with a linear Si structure in order to improvethe solubility of the polysilane product and (ii) developing adecomposition process of polyphenylsilane which is optimized forboth efficient removal of the phenyl groups inside precursorpolymer and the reconstruction of amorphous Si network.

Acknowledgements

Authors acknowledge Characterization Center for Materialsand Biology at the University of Texas at Arlington (UTA) and its

staff members for the assistance with FTIR and Prof. Jung-Il Jinfrom Korea University for invaluable discussion on the subject.The research is financially supported by UTA.

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