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Materials Chemistry and Physics 130 (2011) 1061– 1065

Contents lists available at SciVerse ScienceDirect

Materials Chemistry and Physics

j ourna l ho me pag e: www.elsev ier .com/ locate /matchemphys

ontrolled synthesis and characteristics of antireflection coatings of TiO2

roduced from a organometallic colloid

eha Batraa, Praveen Kumara, S.K. Srivastavaa, Vandanaa, Ravi Kumara, Ritu Srivastavaa,. Deepaa,b, B.R. Awasthya,1, P.K. Singha,∗

National Physical Laboratory (CSIR), Dr. K.S. Krishnan Road, New Delhi, 110012, IndiaDepartment of Chemistry, Indian Institute of Technology, Hyderabad, India

r t i c l e i n f o

rticle history:eceived 27 November 2010eceived in revised form 27 July 2011ccepted 21 August 2011

eywords:

a b s t r a c t

Antireflection titanium dioxide (TiO2) coatings have been developed on monocrystalline silicon by asol–gel spin-coating process using titanium di-isopropoxidebis(acetylacetonate) colloidal precursor solu-tion. The effect of titanium content in the precursor, spin rate, sintering duration and temperaturehave been studied and their effect on coating thickness and optical properties (i.e., refractive index andreflectivity) were investigated. The influence of post-deposition sintering temperature on the optical

hin filmsntireflection coatingsptical propertiesFMPSitanium dioxide

characteristics, composition and the microstructure of the coatings have been evaluated by UV–vis spec-troscopy, ellipsometry, X-ray photoelectron spectroscopy, atomic force microscopy and X-ray diffractiontechniques. Solar cells made on silicon wafers with TiO2 as antireflection layer showed enhancement ofmore than 20% in short circuit current density in comparison to a cell devoid of the TiO2 coating.

© 2011 Published by Elsevier B.V.

ilcon solar cells

. Introduction

Owing to their high refractive index, high transparency in theisible and near infrared wavelength regions, wide band gap, excel-ent chemical stability, titanium dioxide (TiO2) thin coatings can besed as antireflection coatings, layers, optical properties, AFM, inrystalline silicon solar cells [1–3]. A plethora of techniques arevailable for deposition of TiO2 thin coatings and these includehemical vapor deposition, vacuum evaporation, thermal oxida-ion, electro deposition, wet chemistry routes and so forth [4–7].mongst chemical routes, sol–gel methods are the most attractive,s they offer the advantage of producing metal oxides at fairly lowemperatures. Here, we employed wet chemical sol–gel process-ng to deposit TiO2 coatings, as the route is not only cost effectiveut also offers the benefit of better reproducibility in terms of sto-

chiometry, crystallinity, thickness and composition [8–12].The usual organometallic precursors, such as metal alkoxides

re highly hygroscopic and require a glove box for handling them;

owever, the precursor used in the present study is not as sen-itive to air/moisture as alkoxides thereby allowing its use undermbient conditions. We synthesized and used a relatively less used

∗ Corresponding author. Tel.: +91 1145608588; fax: +91 1145609310.E-mail address: pksingh@nplindia.org (P.K. Singh).

1 Retired from NPL.

254-0584/$ – see front matter © 2011 Published by Elsevier B.V.oi:10.1016/j.matchemphys.2011.08.035

organometallic precursor [13] for spin coating of TiO2 layers. Neatoxide layers were obtained after organic burnout using sinteringat elevated temperatures. The films were characterized in termsof optical (namely refractive index and reflectivity), and structuralproperties. The effect of these TiO2 coatings as an antireflectionlayer was clearly evidenced by their application on crystalline sili-con solar cells.

2. Experimental

2.1. Synthesis of TiO2 precursor and deposition on Si wafers

All chemicals and reagents were procured from Merck, Germany and used asreceived. We have used titanium di-isopropoxidebis(acetylacetonate) precursorwhich is not exhaustively used as its conventional counterpart, namely, titaniumtetraiso-propoxide (Ti(OPri)4). Chemical modification of a metal alkoxide (Ti(OPri)4)with a chelating ligand such as acetylacetone (acac) leads to the formation of theaforesaid precursor: Ti(OC3H7)n(CH3–CO–CH C(CH3)O)m (where n + m = 4, and typ-ically n = 2 and m = 2). The reaction between Ti(OPri)4 and excess of acetyl acetone(molar ratio Ti(OPri)4:acac = 1:4), under the conditions mentioned in the text, causesthe preferential hydrolysis of OPri ligands and specifically leads to the formation oftitanium di-isopropoxide bis(acetylacetonate) [9,13]. The salient advantage of usingthis precursor, in contrast to a traditional titanium alkoxide is that the chelatedalkoxide is less reactive and therefore it controls the condensation rate and the evo-lution of the final polymer, thus leading to improved properties of the final product

(TiO2). To synthesize titanium di-isopropoxidebis(acetylacetonate), the procedureused in the present study is as follows. 50 ml (0.17 mol) of tetra isopropyl titanate,70 ml (0.68 mol) of acetyl acetonate and 100 ml of isopropyl alcohol (IPA) weremixed under an inert atmosphere of N2. 2.5 g polyethylene glycol in 50 ml IPA wasadded to this solution, which served as a binder. The mixture was refluxed in the

1062 N. Batra et al. / Materials Chemistry and Physics 130 (2011) 1061– 1065

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For the samples (diced from the same wafer), the variation of coat-ing thickness as a function of sintering temperature is shown in theinset (a) of Fig. 3. As anticipated, the coating thickness decreaseswith sintering temperatures. This could be ascribed to the thermal

ig. 1. Variation of coating thickness and refractive index as a function of Ti con-entration in the precursor solution.

nert atmosphere for 5 h at 45 ◦C. The solution was filtered and few drops of nitriccid were added for hydrolysis and condensation to ensure the formation of col-oidal solution. The amount of nitric acid was just enough to avoid precipitation.equisite amount of IPA was added to the precursor in order to get solutions ofifferent titanium concentrations. The decomposition temperature and weight per-entage of TiO2 was determined by gravimetric measurements. Chemical estimatef the solutions provided 5% difference with theoretical yield.

In the present study polished silicon wafers p-type (boron doped) of 〈1 0 0〉 ori-ntation, 50 mm diameter and a thickness of 300 �m were used. A standard cleaningrocedure [14] developed at the Radio Corporation of America (RCA) was followedo clean wafers. After soaking in deionized (DI) water, the wafers were immersedn NH4OH:H2O2:H2O (1:1:5 vol%) solution (for 10 min at 80 ◦C) in order to removerganic impurities. This treatment results in the formation of a thin silicon dioxideayer on the silicon surface which was removed by treating them in a 1:50 (vol%)olution of HF and H2O at 25 ◦C. After several rinses in DI water the wafers werereated in HCl:H2O2:H2O (1:1:5 vol%) at 80 ◦C for 10 min. This treatment effectivelyemoves the remaining traces of metallic (ionic) contaminants. Again wafers wereubjected to HF treatment to remove native oxide. Finally the wafers were washedhoroughly in DI water and dried with nitrogen gas.

The wafers were coated with the titanium containing precursors using spin-oating method at different speeds (2000–10,000 rpm) for different durations15–60 s). Titanium di-isopropoxidebis (acetylacetonate) decomposed thermallyithout going through the powder stage and formed a thin coating of TiO2 on the

ubstrate. All the samples were dried in an oven at 200 ◦C for 15 min before finalintering in a tube furnace in oxygen ambient for coating densification and crystal-ization. The resulting coatings were transparent and showed good adherence withhe substrate.

.2. Characterization techniques

Refractive index (ns) and thickness (d) of TiO2 coatings were measured byn ellipsometer (Gaertner; Model L117) at an incidence angle of 70◦ using aelium–neon laser light source of wavelength 6328 A. Reflectivity (R�) mea-urements were performed on a double beam spectrophotometer (Beckman;odel 5240) with diffuse reflectance integrating sphere in the wavelength range

00–1300 nm. A “halon” sample plate (R� = 100%) was used as a standard reference.ecomposition temperature and quantitative estimation of TiO2 was determinedy a thermal and differential analysis (TGA and DTA) system (Stanton Redcroft;odel 1000). X-ray diffraction (XRD) patterns were recorded on a X-Ray diffrac-

ometer (Rigaku; Model Mini flex-II,), at a grazing angle of 2◦ . Surface morphologyf the TiO2 coatings was investigated by using atomic force microscopy (AFM, Solvercanning Probe Microscope from NT-MDT Co.) and a scanning electron microscopySEM system from Carl Zeiss Model EVO MA10) working at 10 kV in secondary emis-ion mode. The X-ray photoelectron spectroscopy (XPS) studies were carried out bysing a system (PerkinElmer Corp., Physical Electronics with an ultra high vacuumystem, (Model PHI-1257) operating at a base pressure of 5 × 10−10 Torr and spectraere recorded using Mg K� as an X-ray source.

. Results and discussion

.1. Optical characterization

Fig. 1 shows the variation of TiO2 coating thickness and refrac-ive index as a function of titanium concentration in the precursor.oating thickness increases linearly from 480 to 1250 A as Ti con-entration was raised from 12 to 30 vol% under a constant spinning

Fig. 2. Variation of coating thickness and refractive index as a function of spin speed.

speed (3000 rpm) and time (30 s). On the other hand, the refrac-tive index does not change for different Ti concentrations, thusimplying that refractive index is pre-dominantly independent ofcoating thickness and Ti content in the precursor. Fig. 2 shows thevariation in coating thickness and refractive index as a function ofspin speed which were made using solution with a fixed Ti con-centration (20 vol%). An increase in the spin speed brought aboutan exponential decrease in TiO2 coating thickness but the refrac-tive index remained almost the same. The exponential drop in thecoating thickness from 1160 to 700 A and slight decrease in refrac-tive index from 2.3 to 2.2 for spin speed variation from 2000 to10,000 rpm, again reiterates that refractive index is independent ofboth coating thickness and Ti content in the coating.

Fig. 3 shows the effect of sintering temperatures (Ts) on therefractive index of coatings made from a solution containing pre-cursor having fixed Ti content (20 vol%). The samples were sinteredfor a fixed duration of time (ts = 15 min). The refractive indexincreases from 1.96 (at 400 ◦C) to acquire a maximum of 2.7 (at865 ◦C) and then subsequently declined to 2.6 (at 1117 ◦C). Increasein refractive index could be largely due to loss of porosity, as thecoating tends to acquire a dense, compact structure with increasein sintering temperature [15]. It is to be remarked that change inthe degree of crystallinity was also observed (XRD and AFM results)which might also affect ns values with Ts though by a lesser extent.

Fig. 3. Change in refractive index as a function of sintering temperature. Insets show(a) the variation of coating thickness as a function of sintering temperature and (b)variation of coating thickness and refractive index as a function of sintering time.

N. Batra et al. / Materials Chemistry and Physics 130 (2011) 1061– 1065 1063

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ig. 4. Variation of reflectivity as a function of wavelength for samples made usingolutions containing different vol% of Ti, but spun at a fixed speed and time.

ecomposition of organics from the coating microstructure. Theecrease in thickness occurs by a larger magnitude from 995 A (at80 ◦C) to 847 A (at 760 ◦C) and thereafter, the extent of decrease inhickness is less, indicating that most of organic burnout occurs inhe low temperature region. The other inset (b) of the figure showshe variation of film thickness and refractive index in the samples

ade from a solution with fixed Ti concentration (20 vol%) and sin-ered at a constant temperature of 622 ◦C but for different timeurations. The coating thickness changes from 1080 to 1180 A whenintering time increased from 15 to 60 min, which is rather sur-rising. The increase in coating thickness may be attributed to theossible oxidation of silicon underneath the oxide layer when theiO2 layer were subjected to prolonged durations at elevated tem-eratures. In this case, the measured thickness may be of TiO2–SiO2ulti layer composite. It may also be the cause of marginal decrease

n refractive index as its value decreases from 2.27 to 2.25 with Ts.Reflection from non-absorptive multi-layer thin film coatings

s defined by standard optical theory [16]. In the case of a sin-le layer coating over a substrate, the reflected light from secondnterface (thin film-substrate) strikes back the first interface (thinlm-ambient) with a phase change of 180◦ and will interfere withhe reflected light from first interface in opposite phase. In thisondition R is described as;

=[

(n2ARC − nambnsub)

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here namb, nARC and nsub are the refractive indices of the ambi-nt, film material and substrate respectively. For zero reflectionhe above equation reduces to

ARC = √nambnsub (2)

Hence, Eq. (2) must be satisfied for an optimized single layerntireflection coating (ARC). In the present case namb = nair andsub = nSi which are equal to 1 and 3.85 respectively. For normal

ncidence of light, the quarter wavelength thickness gives the min-mum reflection [17] and is defined as;

ARC = �i

(4nARC)(3)

here �i is the wavelength where minimum reflectivity is desired.n the above equation dARC = d and nARC = ns.

Fig. 4 shows the variation of reflectivity as a function ofavelength for TiO2 coating made using precursors containingifferent Ti concentrations. All coatings were spun at a fixedpeed (3000 rpm) and time (30 s); and the samples were sin-

ered at 622 ◦C. As can be judged from the figure, the coatingsf different Ti content showed minimum reflectivity at differentavelengths; and the minima shifts towards higher wavelengthsith the increase of Ti concentration (i.e., 470, 590, 780, and

Fig. 5. XRD pattern of TiO2 coating sintered at different temperatures (a) 680 ◦C (b)860 ◦C (c) 1117 ◦C.

960 nm for 12, 15, 18.7 and 25 vol% of titanium content respec-tively), however, their minimum value remained practically thesame (3 ± 0.2%). It is to be noted here that the position of the min-imum could be cross-verified using Eq. (3) with ns and d shown inFig. 1. The two values were found within 5% of each other. Thus, thecoatings obtained from solutions encompassing 12–20 vol% of Ticontent could be used as an effective antireflection layers. Further,in this range the average R� values were low in 450–1100 nm wave-length regime compared to the coating made from higher Ti contentsolutions. The measurements of spectral variation of reflectivity onthe samples made at different spin speeds (2000–10,000 rpm) froma solution with 20 vol% of Ti were also made. The minimum reflec-tivity for the samples made at high spin speeds (i.e., >6000 rpm) wasfound in 635–695 nm wavelength range with comparable reflec-tivity values (3 ± 0.2%). This indeed is a manifestation of the datashown in Fig. 2 where ns is practically the same and d decreasesmarginally for coatings made at higher spin speeds. On the otherhand, the minima occur at higher wavelengths for lower speeds(<4000 rpm) and the values can be derived from corresponding ns

and d in Fig. 2

3.2. Structural characterization

For structural (XRD and AFM) and compositional analysis, TiO2coatings prepared with 20 vol% Ti precursor and spin speed of3000 rpm for 30 s were used. These samples were sintered at dif-ferent temperatures (T = 680 ◦C, 860 ◦C and 1117 ◦C) for 15 min. andtheir XRD patterns are shown in Fig. 5. The coating sintered at680 ◦C has two peaks at 2� = 25.5◦ and 58.6◦ which correspond toA(1 0 1) and A(2 1 1) planes of anatase crystalline phase of titaniumdioxide. The strongest peak at 2� = 28.30◦ is due to the silicon sub-strate. The coating sintered at 860 ◦C did not induce any significantchange in the crystal structure. However, for the coating sintered at1117 ◦C, new peaks at 2� = 27.3◦ and 38.1◦ corresponding to R(1 1 0)and R(2 0 0) planes of the rutile phase of TiO2 emerge along withsmall intensity (1 0 1) plane of anatase phase. This indicates that atlower sintering temperature dominant phase is anatase, however at

higher sintering temperature it transforms to predominantly rutilephase.

AFM images of TiO2 coatings sintered at different temperaturesare shown in Fig. 6 (a, b and c for T = 680 ◦C, 860 ◦C and 1117 ◦C

1064 N. Batra et al. / Materials Chemistry and Physics 130 (2011) 1061– 1065

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ig. 6. AFM images of TiO2 coatings sintered at different temperatures (a) 680 ◦C (b)

espectively). The as deposited coatings were found to be smoothith no visible grains. After sintering at 680 ◦C regular ring shapedicrostructures (Fig. 6a) appeared to evolve all over the surface.

he average and rms roughness values were found to be 2.8 nmnd 3.9 nm respectively. The coating sintered at 860 ◦C (Fig. 6b)howed regular hollow ring like shapes consisting of nano-sizedrains of size approximately 80–150 nm. The average and rmsoughness were 7 nm and 8.78 nm respectively. A dramatic changen the coating topography was observed in the coating sintered at117 ◦C (Fig. 6c) as well-defined, almost perfectly spherical rings,ompactly packed with clearly demarked grain boundaries wereeen. This clearly reveals the increase of crystallinity in the coatingshich corroborates with the change in crystal structure observed

y the XRD (Fig. 5). The average and rms roughness of the coatingere 13 nm and 16 nm respectively. Relativily large grains of about

00–200 nm in size were seen on both inside and outside the ringss well as on the peripheral regions. SEM micrograph of the TiO2oating sintered at 860 ◦C is as shown in Fig. 6d. The microghrapheveals that the regular ring formation occurs as a result of heatreatment in the entire area of the coating uniformly and is, indeed,ot a localized feature.

The XPS spectra were recorded to find the correlation betweenhe chemical and structural properties, by using Mg K� as an X-rayource. Survey scan has been used for compositional analysis, while(1s) and Ti(2p3/2) core level spectra have been deconvoluted into

heir components to find out the contribution from different oxi-ation states. Fig. 7 shows the deconvolution of O(1s) and Ti(2p)ore level spectra, where curves (a–c) correspond to the samplesintered at 680 ◦C, 860 ◦C and 1117 ◦C respectively. The deconvolu-ion can be interpreted as following: for the anatase phase we haveonsidered the existence of Ti in +2 and +3 oxidation states, whileor rutile it is +3 and +4 [18]. Therefore, the peak at 459.7 eV corre-ponds to the anatase phase, while the peak at 457.6 eV is attributedo the rutile phase in Ti(2p) core level spectra. The deconvolution

anifests the existence of anatase phase, for the samples sintered

t 680 ◦C and 860 ◦C while we have observed the appearance ofutile phase in the samples sintered at temperature of 1117 ◦C,ndicating the transformation of the anatase phase into rutile atigher temperature. The deconvolution of O(1s) core level spec-

C (c) 1117 ◦C (d) representative SEM micrograph of TiO2 coatings sintered at 860 ◦C.

tra also led to the same conclusion, wherein a peak at 530.9 eVand 532.6 eV correspond to the anatase structure, while the one533.6 eV is after the rutile phase. Thus it can be concluded that uponincrease in temperature, the degree of crystallinity increases andthe crystalline phase also undergoes a transformation from a pre-dominantly anatase (above 500 ◦C) to the formation of a new phase,i.e., rutile (above 1000 ◦C). Hence, the XPS results also support tothe observations made by XRD.

3.3. Application on silicon solar cell as ARC

Finally application of TiO2 coatings on crystalline silicon solarcells was explored. Solar cells used in this study were based onn+–p–p+ structure. The starting material was (1 0 0) oriented p-type 300 �m thick Cz silicon of 0.5–1.0 � cm resistivity. The wafers(78 cm2) were chemically mechanically polished on one side anddamage-free etched on the other. Prior to the junction formationwafers were cleaned using RCA procedure and were subjected tophosphorous diffusion (polished side) using POCl3 liquid source.The contacts were made by screen printing of silver paste on the(p-n junction) front and silver/aluminum paste on the back side ofthe cells. The samples were co-fired at 700 ◦C in air followed byannealing in forming gas at 420 ◦C for 45 min and edge isolationwas done to complete the cell fabrication. I–V characteristics of thecells were measured under simulated AM1.5 (Air Mass 1.5) spec-trum (100 mW/cm2) and the cell performance parameters namely,open-circuit voltage (Voc), short-circuit current density (Jsc) and fillfactor (FF) were determined.

Subsequently, desired thickness of TiO2 film was applied on thefront side by spin coating of the solution to investigate effect ofthe dielectric layer on the solar cell parameters. It is known thatfor the case of single layer ARC on silicon solar cells, the mini-mum reflectivity should be in 550–600 nm range. In this spectralrange the number of photon are maximum. Considering this, thecells were coated with the solution having 15% Ti concentration

(�min = 580 nm from Fig. 4) at 3000 rpm and followed by sinteringat 680 ◦C. I–V characteristics of the cells were again measured undersimilar conditions. An improvement in the range of 20–22% in shortcircuit current (e.g., from 21.1 to 25.8 mA cm−2) and a marginal

N. Batra et al. / Materials Chemistry and Physics 130 (2011) 1061– 1065 1065

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ig. 7. (i) XPS spectra of TiO2 coatings sintered at different temperatures (a) 680 ◦

oatings sintered at different temperatures (a) 680 ◦C (b) 860 ◦C (c) 1117 ◦C showin

ncrease in open circuit voltage (by 4–5 mV from the initial val-es ∼565 ± 2 mV) was observed after TiO2 coating on solar cells asompared to uncoated cells. The marked improvement in Jsc is dueo the reduced reflectivity as TiO2 layer which acts as an effectiventi-reflection coating while the marginal enhancement in Voc maye attributed to surface passivation [19,20]. The other parametersuch as fill factor, (which was rather low ∼0.60), series (Rs) andhunt (Rsh) resistances were remained practically the same afterhe TiO2 coating. It is important to mention here that the solar cells,eported here, are fabricated using moderate processing facilities.etter fabrication environment and proper optimization of processarameters (of the cell) may yield desired results in-terms of solarell performance. This will be a subject matter of our future publica-ion. The real advantage of this compound, however, lies in the facthat it is simple to synthesize and is well suited for mass productionf photovoltaic cells.

. Conclusions

Preparation of TiO2 coatings by sol–gel spin coating oningle-crystalline silicon substrates by using a less used organo-etallic compound, titanium di-isopropoxidebis(acetylacetonate),

as been demonstrated. Systematic investigations on the effectf material and process parameters (such as titanium contentn the precursor, spin speed, sintering time and temperature,tc.) on structural and optical (particularly reflectivity, refractivendex, thickness) properties of the coatings have been made. Thetructural analysis reveals that at lower sintering temperaturesominant phase is anatase whereas at higher sintering temperature

t transforms to rutile phase. It is found that coating thickness hastrong dependence on titanium content in the precursor whereasefractive index has strong dependence on sintering temperature.

he refractive indices variation from 2.0 to 2.7 is due to phase trans-ormation from anatase to rutile. A correlation between coating

icrostructure and sintering temperature has been successfullyelineated. The application of TiO2 coatings as an antireflection

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60 ◦C (c) 1117 ◦C showing the Ti(2p3/2) core level spectra. (ii) XPS spectra of TiO2

(1s) core level spectra.

layer on single-crystalline silicon solar cell was demonstrated. Thecoated cells showed more than 20% increase in short-circuit cur-rent density in comparison to a cell devoid of the coating withoutaffecting other cell parameters. It demonstrates the tremendouspotential of these coatings as ARC.

Acknowledgements

The work was carried out under SIP-17 grant from Council ofScientific and Industrial Research. The authors are also thankful toMr. K.N. Sood for SEM measurements.

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