Solar Energy Materials & Solar Cells 100 (2012) 57–60

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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

Sol–gel derived hydrogen annealed ZnO:Al films for silicon solarcell application

Firoz Khan a, Vandana a, S.N. Singh a, M. Husain b, P.K. Singh a,n

a National Physical Laboratory (CSIR), Dr. K.S. Krishnan Road, New Delhi 110012, Indiab Department of Physics, Jamia Millia Islamia, Jamia Nagar, New Delhi 110025, India

a r t i c l e i n f o

Available online 17 May 2011

Keywords:

Zinc oxide

Surface passivation

Antireflection coating

Minority carrier lifetime

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

016/j.solmat.2011.04.024

esponding author. Tel.: þ91 11 45608588; fa

ail address: pksingh@nplindia.org (P.K. Singh)

a b s t r a c t

A study of the effect of hydrogen annealing on antireflection and surface passivation properties of

sol–gel derived aluminum rich zinc oxide (AZO) films coated on silicon wafers has been done in

300–600 1C temperature range. The minority carrier lifetime in silicon wafers coated with AZO films

was measured using microwave photoconductive decay (m-PCD) technique. After annealing of AZO

coated silicon wafer in hydrogen ambient for 30 min between 400 and 600 1C the effective minority

carrier lifetime teff improved above the initial value of �16 ms and attained a maximal value of �71 ms

for annealing at 500 1C. It may be that above 400 1C the molecular hydrogen gets dissociated into

atomic hydrogen in presence of Al induced defects at the AZO coated silicon surface and passivates

them by formation of Si–H–Al complex. The annealing in air in the same temperature range did not

affect the lifetime. AZO films annealed at 500 1C in hydrogen or air show high transmittance in

400–1200 nm wavelength range and are suitable as AR coatings for silicon solar cells. We have applied

a hydrogen annealed AZO antireflection coating on nþ front surface and an equally thick hydrogen

annealed AZO coating on the p-back surface of an nþ-p multi crystalline silicon solar cell. The observed

improvement in Jsc, Voc values of the cell established that while AZO film on front acted as a good AR

coating the AZO film on the back surface passivated the back surface effectively.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

Zinc oxide (ZnO) is a wide and direct band gap (3.3 eV) II–IVgroup compound semiconductor with wurtzite as most stablestructure at ambient conditions with lattice spacing a¼3.25 A andc¼5.12 A [1]. Usually ZnO films are n-type conducting metal oxidesand are transparent to the visible radiations. Their useful electro-optical properties, stability and band gap arise from intrinsic defects,such as oxygen vacancies and zinc interstitials. ZnO has attractedintensive research efforts for its unique properties (such as largeexciton binding energy �60 meV [1]) and attractive applications inultraviolet (UV) light emitter, piezoelectric devices, chemical sensorsand spin electronics [2,3]. Pure ZnO is an unstable material due toadsorption of atmospheric oxygen, which decreases the conductivityand modifies its surface morphology [4]. To stabilize ZnO andenhance its conductivity the dopants of group III (boron [5],aluminum [4,6], indium [7], etc.) and group VII element (fluorine[8]) are added. Addition of a higher valence element such asaluminum, increases the conductivity by the replacement ofthe Zn2þ ions with Al3þ ions [9]. Aluminum rich zinc oxide (AZO)

ll rights reserved.

x: þ91 11 45609310.

.

films has been reported as antireflection coating for solar cellapplications [10].

The effective minority carrier lifetime (teff) of a semiconductoris a combination of bulk lifetime (tbulk) and surface lifetime(tsurface) and is given by [3]

1

teff¼

1

tbulkþ

1

tsurfaceð1Þ

Due to the presence of dangling bonds the minority carrier recom-bination is higher at the surfaces than in the bulk. Consequently,tsurfaceotbulk. Surface passivation enhances tsurface and then tsurface

becomes larger than tbulk and leads to teffEtbulk. The two surfaceshave high recombination due to presence of dangling bonds. Thereduction of the number of dangling bonds, and hence the lowsurface recombination velocity, is achieved by growing a layer ontop of the surface, which ties up some of these dangling bonds[11,12]. This reduction of dangling bonds is known as surfacepassivation. The surface passivation of c-Si solar cell improvesefficiency by suppressing the surface recombination velocity [13].It will be highly desirable if an antireflection (AR) coating, which isusually applied to silicon solar cells could also provide an effectivesurface passivation. In this paper, we report the effect of hydrogenannealing on antireflection and surface passivation properties of

F. Khan et al. / Solar Energy Materials & Solar Cells 100 (2012) 57–6058

sol–gel derived aluminum rich zinc oxide (AZO) thin films coated onsilicon wafers.

Fig. 1. Variation of refractive index and thickness of air (–K–) and hydrogen

(__’__) annealed AZO films with the annealing temperature. For annealing at

500 1C in hydrogen ambient, the refractive index of AZO film is 1.85.

Fig. 2. Variation of teff of AZO coated Si wafers with annealing in (a) air (–K–) and

(b) hydrogen (__’__) ambients.

2. Experimental

We have used FZ grown polished monocrystalline siliconsubstrates of 3 cm�3 cm area, laser scribed from middle portionof a big wafer of size 150 mm diameter and 625 mm thickness.The wafers were p–type (B–doped), /1 0 0S oriented and of4.4 O cm resistivity. Samples were first cleaned in Piranha(H2SO4:H2O2) solution and then used for coating of AZO films.AZO solutions were prepared in ethanol using zinc acetatedihydrate and aluminum nitrate nonahydrate in different (0–20%)molar ratios. A small amount of diethanolamine (DEA) was addedas stabilizer to clear the solution. These AZO films were spin coatedonto polished silicon wafers at 1500 rpm, which were found to beuniform for different concentrations range. The 5:1 molar ratio ofzinc acetate dihydrate and aluminum nitrate nonahydrate gave Alrich zinc oxide (AZO) thin films with 20% atomic Al/Zn ratio. Thesefilms were initially dried in air in an oven at 200 1C and, then, thesamples were heat treated separately at a constant temperaturebetween 300 and 600 1C in air or hydrogen ambient for 30 min tostudy the effect of ambient on the structural, optical and electronicproperties of the films. Henceforth, we shall refer the 20% atomicAl/Zn ratio spin coated films simply as AZO film in the text.

The refractive index and thickness of the AZO films weremeasured at 632.8 nm wavelength (l) with an ellipsometer (Gaert-ner Model L117). The lifetime of the minority carriers was measuredusing microwave photoconductivity decay (m-PCD) technique(Semilab system, Model WT-2000). The optical transmittance ofAZO films coated on fused silica in the 200–1200 nm wavelengthrange and the diffused reflectivity of AZO films coated on siliconwafers in 400–1100 nm wavelength range were measured with aspectrophotometer (Shimadzu, Model UV-3101-PC). An integratingsphere was used for the reflectivity measurement. X-ray diffractionpatterns of the AZO films coated on silicon were recorded in 2yrange of 201–801 by an X-ray diffractometer (Bruker, Model AXS D8).Fourier transform infrared (FTIR) spectra of the films were recordedin transmission mode with a spectrophotometer (Perkin-Elmer,Model BX2) in the wave number range 400–4000 cm�1. All themeasurements were carried out at room temperature.

3. Results and discussion

Fig. 1 shows the variations in the refractive index and thick-ness of AZO films as function of annealing temperature (T) in300–600 1C range in air and hydrogen ambients for 30 min. It canbe seen that refractive index (m) and thickness (t) of the films donot change for annealing up to T¼300 1C in air or hydrogen;thereafter the thickness of both the films decreases from theinitial value of 220 to 140 nm (at 500 1C). Further increase intemperature beyond 500 1C the thickness of the hydrogenannealed film increases e.g., the thickness is 180 nm atT¼600 1C. The refractive index (m) also decreases from 2.05 to1.9 when temperature is increased from 300 to 525 1C andbecome constant in 525–600 1C range for air annealed samples.However, the value of m decreases sharply from 2.05 to 1.55 for300oTo525 1C of the samples annealed in hydrogen ambient.Finally, m¼1.8 is obtained at T¼600 1C. The above values of t andm of air and hydrogen annealed AZO layer indicate that these filmsmay be suitable for the use as antireflection coating on solar cells.

Fig. 2 shows the variation in teff with annealing temperaturebetween 300 and 600 1C in AZO coated silicon wafers in the twoambients for 30 min. teff had a value of �16 ms in both the virgin

and AZO coated silicon wafers before heat-treatment. It can beseen that teff values remain practically constant at �16 ms overthe entire range (300–600 1C) of heat treatment in air. Similarlyno significant change is observed in teff values up to 400 1C inhydrogen ambient. However, teff starts increasing in the samplesannealed above 400 1C. The maximum teff value is �71 ms at500 1C. The lifetime starts decreasing on further increase intemperature and acquires a value as low as 25 ms at 600 1C. Theincreased teff after heat treatment above 400 1C in hydrogen maybe attributed to surface passivation by AZO layer. According toEstreicher and Hastings [14] this occurs because molecularhydrogen gets dissociated into atomic hydrogen in presence ofAl induced vacancies or interstitials at temperature above�230 1C. According to Chia et al. [15] atomic hydrogen leads toformation of Si–H–Al complex that passivates the defects. Thedecrease in lifetime for annealing temperature above 500 1C maybe due to desorption of hydrogen. A similar observation was alsomade by Hirayama and Tatsumi [16] in silicon films made by MBEand annealed in hydrogen above 460 1C. The increase in teff withhydrogen annealing of AZO film at 500 1C indicates the effectivepassivation of Si surface. It is to be mentioned that Yelundur et al.[17] also found lifetime enhancement in string ribbon siliconwhen a back side screen-printed aluminum layer and a frontcoated thin film of PECVD SiNx, which contained atomic hydrogen,were annealed at 850 1C in air.

The transmittance of AZO films annealed in air and hydrogen isshown in Fig. 3 in 200–1200 nm wavelength range. It is found

Fig. 3. Transmittance spectra of AZO films on fused silica (a) as-deposited (yy)

one and those annealed at 500 1C for 30 min in (b) air (———) and (c) hydrogen

(_____) ambients.

Fig. 4. X-ray diffraction pattern of (a) as-deposited and (b) air and (c) hydrogen

annealed AZO films coated on silicon wafers.

Fig. 5. FTIR transmission spectra of (a) as-deposited and (b) air and (c) hydrogen

annealed AZO films coated on polished silicon wafers.

F. Khan et al. / Solar Energy Materials & Solar Cells 100 (2012) 57–60 59

that between 435 and 1200 nm the average transmittance is morethan 90% for both the films, although the hydrogen annealed filmis �2% less transparent in comparison to the air annealed film.The loss in transmittance is �9%, which is primarily due toreflection rather than absorption by the films. This indicates thatthe transmittance of the AZO films for l4435 nm is nearly equalto that of the non-absorbing fused silica. The air and hydrogenannealed films show significantly reduced transmittance below400 and 380 nm wavelengths, respectively, that is an indication ofsignificant absorption in the 250olo400 nm spectral range.As-deposited AZO film shows high transmittance for l4270 nmnearly equal to that of the fused silica. The difference in thetransmittance of annealed and unannealed (as-deposited) AZOfilms in 200olo400 nm wavelength range is due to absorptionof the light, which is owing to the crystallization of ZnO duringheat-treatment. The crystalline nature of the annealed AZO filmsis confirmed by XRD spectra depicted in Fig. 4, the details ofwhich are discussed below.

Fig. 4 shows the XRD patterns for as-deposited, air annealedand hydrogen annealed AZO films on silicon substrate. XRD peaksare observed at 2y value of 62.141 and 66.31, which correspond toshifted (1 0 3) plane and (2 0 0) plane, respectively, of hexagonalwurtzite structure of zinc oxide (JCPDS 36-1451). The slight shiftin these peaks may be due to strain in the films. However, highintensity peaks corresponding to (1 0 1), (1 0 0), (0 0 2) and (1 1 0)planes are absent in XRD data. Annealing in air or hydrogen doesnot show any change in the XRD pattern and is indicative of thesame in the crystal structure of the two films. Hexagonal wurtziteunit cell parameters calculated using peaks corresponding to

(1 0 3) and (2 0 0) planes are a¼3.25 A and c¼5.28 A, respec-tively. The calculated ‘a’ value matches with the standard JCPDS36-1451 a-value (3.2498 A), but the calculated ‘c’ value is largerthan the JCPDS 36-1451 c-value (5.026 A) in Ref. [1]. As-depositedsample with AZO film shows no peak corresponding to zinc oxide.This observation corroborates the higher transmittance of as-deposited film on fused silica substrates in 200olo400 nmwavelength range compared with the annealed AZO films (shownin Fig. 3).

Fig. 5 shows the FTIR transmittance spectra of the AZO films,which are as-deposited, air annealed and hydrogen annealed at500 1C. For as-deposited film, FTIR data shows main absorptionbands at �3400, �2920, �1570 and �1400 cm�1, which corre-sponds to the O–H mode, C–H mode, C¼C mode and C¼Ostretching modes of zinc acetate, respectively. Absorption bandsat �1050 and �670 cm�1 belong to other organic groups [18,19].These bands are attenuated after annealing the films at 500 1Ceither in air or hydrogen. It can be seen from the figure that theabsorption peak at �870 cm�1 (corresponding to Si–H3 sym-metric mode [18,19]) in hydrogen annealed AZO film is strongerthan for as-deposited and air annealed AZO films. The increasedSi–H3 bonding in H2 annealed AZO film may be responsible forincrease in the minority carrier lifetime of AZO coated siliconwafers. The inset in the Fig. 5 shows an absorption band at�487 cm�1, which corresponds to the ZnO stretching mode. TheXRD and FTIR results of Figs. 4 and 5 confirm formation of zincoxide films on silicon surface after 500 1C annealing in both theair and the hydrogen ambients.

Fig. 6 shows reflectivity of the as-deposited and the air andhydrogen annealed AZO films coated on silicon substrates.Reflectivity of polished silicon surface is also shown in Fig. 6 forthe sake of comparison. All AZO coated silicon wafers have lowerreflectivity than polished silicon wafers. Amongst the AZO coatedsilicon wafers the reflectivity of hydrogen annealed film is lowestin the 460–1100 nm range. All AZO films had the same initialthickness (�220 nm) but the thickness obtained after annealingin hydrogen ambient was close to the value of optical thickness(¼product of m and t) required for the single layer antireflectioncoating whereas, the thickness of the air annealed film was more.In the latter, similar reflectivity values could be realized bytailoring the initial AZO film thickness (as-deposited) to get afinal thickness equal to l/4 after annealing. The maximum Jsc

values in a silicon solar cell can be calculated by using thefollowing equation:

Jsc

Zlg

l0

qlhc

1�Rlð ÞEldl ð2Þ

Fig. 6. Reflectance spectra of (a) bare polished silicon (-.-.-) and of (b) as-deposited

(yy), (c) air annealed (———) and (d) hydrogen annealed (_____) AZO films coated

on polished silicon wafers.

F. Khan et al. / Solar Energy Materials & Solar Cells 100 (2012) 57–6060

where l0¼400 nm and lg¼1100 nm, El is solar irradiance for agiven l (AM1.5G), Rl is reflectivity of front surface and othersymbols have their usual meaning. The maximum Jsc values thatcould be realized corresponding to the reflectivity values (asshown in Fig. 6) under AM1.5G condition of polished (Jscb), AZOcoated air annealed (Jsca) and hydrogen annealed (Jsch) nþ-psilicon solar could be 29, 36 and 40 mA/cm2, respectively. Thusthe maximum possible improvement in Jsc of the solar cell withhydrogen annealed AZO ARC (applied on the front surface) can be�37% relative to device without ARC. This improvement in the Jsc

value can cause an enhancement of 8.14 mV in the Voc

(DVoc¼nkT/qnln(Jsch/Jscb), (where n is the ideality factor of thecell) for n¼1 and 12.2 mV for n¼1.5 [20]. However, the passiva-tion effect of an additional hydrogen annealed AZO film appliedon back p-Si surface of the silicon solar cell may increase Voc by astill higher value.

We applied the AZO films on the front and back surface of afew nþ-p multicrystalline silicon solar cells having griddedmetallic contacts on both sides to harness their antireflectionand passivation properties. The Jsc values in the cells before andafter AZO film (annealed in hydrogen ambient at 500 1C) were18.61 and 23.31 mA/cm2, respectively, (measured under AM 1.5 Gsolar spectrum of 100 mW/cm2 intensity) that is an effectiveimprovement of �25%. The n value was �1.5 in both the cells.Further an improvement of 12.4 mV in Voc was realized, which issignificantly higher than 8.7 mV, which could result from �25%enhancement in Jsc. The observed increase in Voc above 8.7 mVcould be assigned to passivation of the back surface. The pre-liminarily results indicate the dual role of hydrogen annealed AZOfilms as anti-reflection and passivation layer. A greater enhance-ment in Jsc and Voc may be expected after optimization of processparameters and grid design.

4. Conclusions

We have found that sol–gel derived aluminum rich zinc oxidefilms annealed in the temperature range 400–600 1C in hydrogenambient passivate the p-silicon surface and this effect is moreprominent for annealing at 500 1C. From the above we find thatthe optical properties of AZO films are suitable for use of thesefilms as anti-reflection coatings on silicon. Moreover, hydrogen

annealed films also provide effective passivation of p-Si surface.It has been shown that when applied on both sides of nþ-p backilluminated silicon solar cell, AZO films act as AR coating on thefront and the passivating layer at the rear side. Therefore, suchfilms could be highly suitable for applications in silicon solar cells.

Acknowledgments

Authors are thankful to Dr. N. Vijayan for XRD, Dr. S.N. Sharmafor FTIR and Dr. M. Kar for reflectivity and transmittance mea-surements. Authors FK and SNS also thank CSIR for the financialsupport. The facilities used to conduct this work were createdunder CSIR project SIP-17.

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