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Solar Energy Materials & Solar Cells 94 (2010) 1329–1332

Contents lists available at ScienceDirect

Solar Energy Materials & Solar Cells

0927-02

doi:10.1

� Corr

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

Performance enhancement of mc-Si solar cells due to synergetic effect ofplasma texturization and SiNx:H AR coating

B. Prasad �, S. Bhattacharya, A.K. Saxena, S.R. Reddy, R.K. Bhogra

BHEL-ASSCP, C/o BHEL House, Siri Fort, New Delhi 110049, India

a r t i c l e i n f o

Article history:

Received 2 July 2008

Received in revised form

25 March 2009

Accepted 30 June 2009Available online 30 July 2009

Keywords:

Multicrystalline silicon

Solar cells

Surface texturization

Plasma texturization

48/$ - see front matter & 2009 Elsevier B.V. A

016/j.solmat.2009.06.026

esponding author. Tel.: +91124 2579222; fax

ail address: bprasad@bhel.in (B. Prasad).

a b s t r a c t

The present paper discusses the plausible physical processes dominant during plasma texturization of

multicrystalline silicon (mc-Si) wafers, deposition of silicon nitride (SiNx) antireflection (AR) coating

and firing of contacts through it. During plasma texturization, it is observed that by using low RF power

density and loading wafers on the ground electrode, the texturization process is dominated by chemical

etching. The resulting surface of the wafer shows low-reflectivity (o10% in wavelength range

350–800 nm) and low-defect density leading to improved minority carrier lifetime. It is postulated that

plasma-etched nanoscale structures accelerate the migration of hydrogen released during firing of

contacts. As a result of these physical processes, an improvement up to �2.4% in absolute efficiency of

large area (�149 cm2) multicrystalline silicon solar cells has been achieved.

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

The performance of a solar cell is critically dependent on thereflectivity of the wafer surface, absorption of incident photonsand their conversion into electrical current. Significant R&Defforts have been made towards reducing the reflectivity througha combination of geometrical texturization and antireflection (AR)coating. The latter, however, has a resonant structure that limitstheir effectiveness to a narrow range of wavelengths [1,2]. Plasmatexturization, on the other hand, provides a convenient way ofobtaining isotropically texturized multicrystalline silicon (mc-Si)surface that is not possible with the methods adopted for c-Siwafers. In a recent publication on the subject [3], development ofa plasma texturization process has been reported that shows asubstantial improvement in efficiency (�20%) of multicrystallinesilicon solar cells compared to the ones with alkali-polishedsurface. The improvement in efficiency of mc-Si solar cells hasbeen attributed to: (a) complete suppression of reflectivity(o10%) in a broad spectral range (350–800 nm) leading to blacksilicon surface, (b) implementation of the texturization process inthe chemical-etching-dominated mode that results in reducedsurface damage and (c) passivation of the surface and bulk defectsin silicon by silicon nitride AR coating.

In this paper, an attempt has been made to understand the roleof possible physical processes dominant during the plasmatexturization and effect of subsequent synergetic processes giving

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rise to a substantial improvement in the performance of mc-Sisolar cells.

2. Experimental

The plasma-based, maskless texturization process was devel-oped in an industrial vacuum chamber using reactive plasma(13.56 MHz) with sulphur hexafluoride (SF6) and oxygen (O2) asthe reactant gases. The RF-powered electrode formed the lowerplate in a parallel plate configuration and the wafers fortexturization were kept either on the top-grounded electrode(reactive species-dominated plasma etching mode as shown inFig. 1) or on the powered electrode (sputtering-dominatedreactive ion etching or RIE mode). A roots blower–dry pumpcombination was used for evacuation of residual and reactionbyproducts and the chamber pressure was controlled with thehelp of a throttle valve. The electrode area and RF power weresuitably chosen to produce a defect-free texturized wafer surface.

The wafers were alkali-polished prior to texturization andsubjected to a damage removal etch (DRE) after plasma texturiza-tion. The texturized wafers were subjected to standard industrialprocessing for crystalline silicon solar cells with alkali-polishedmc-Si wafers acting as the control wafers. The process entailedjunction formation by diffusion, deposition of plasma-depositedsilicon nitride AR coating and contact formation by screenprinting and firing of Ag fingers through AR coating on the frontand of Al contact at the back. The plasma-texturized wafers wereviewed under optical as well as scanning electron microscope andthe reflection spectra of the wafers at various stages of processing

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Fig. 1. Schematic of a plasma texturization system.

B. Prasad et al. / Solar Energy Materials & Solar Cells 94 (2010) 1329–13321330

were measured. The finished solar cells were electrically tested forlight and dark I–V characteristics as well as for minority carrier lifetimes using standard Suns-Voc measurement setup under QuasiSteady-State (QSS) condition. Details of these measurementsetups are given below.

2.1. Details of the I–V measurement setup

The I–V measurement set up [4] has provision to measure I–V

characteristics of a solar cell in dark and under illumination. Thesolar cell to be tested is firmly held on a nickel-plated,temperature-controlled vacuum chuck and the measurement ofcurrent and voltage are made with a 4-wire arrangement (2 wiresfor current and 2 separate wires for voltage).

For all measurements under illumination, first the lightintensity is checked and adjusted to 100 mW/cm2. Also, thetemperature of the chuck is set at 2571 1C and monitoredcontinuously using a PT100 probe inserted horizontally insidethe chuck. Calibration of intensity is performed with the help ofreference cell calibrated at NREL, USA in December 2007 (Make:PV Measurements Inc. USA, Model no. PVM 230, 4 cm2 area, Isc:107 mA at 2570.2 1C, mounted on an Al block with BK7 glassprotective window, with 4 wire contacts). The reference cell iscertified for use to quantify or set the irradiance level of a lightsource used for testing solar cells and modules. When the short-circuit current output of the reference cell is equal to its calibratedvalue of short-circuit current, it indicates that the irradiancereaching the reference cell is equivalent to the irradiance (usuallyone sun) that was present during its calibration. The I–V

measurement system comprises a controller module, whichinterfaces with a power supply/electronic load on one hand anda PC on the other, from where the parameters of measurementsare set. The system operates with the help of embedded dataacquisition and analysis software to sweep the entire I–V curvefrom Voc to Isc value and also displays all the performanceparameters of the solar cell.

2.2. Details of the Suns-Voc measurement setup

In Suns-Voc measurement setup [5,6] open-circuit voltage ismeasured as a function of light intensity using the relationVoc ¼ kT/q ln{np/ni2} where n and p are the total electron and holeconcentrations. For p-type silicon wafer, p ¼ NA+Dn and n ¼ Dn.Excess minority carrier density Dn is dependent on light intensity.In this condition the carrier concentration is essentially in steady

state, with generation and recombination in balance, and lifetimeis calculated by t ¼ Dn/G, where G is the carrier generation rate.This minority carrier data is used directly to indicate the materialand passivation quality. A sample having a p–n junction andcontactable on both sides of the junction is used to perform theSuns-Voc measurements. A fully metallized finished solar cell istherefore easily measured. The quality of passivation of the solarcell is, therefore, reflected in terms of minority carrier lifetime ofthe solar cells.

3. Results and discussions

3.1. Effect of loading of wafers on the ground electrode

It was observed that uniformly texturized surface withminimum reflectivity was obtained only in the plasma mode,i.e., when the wafers were kept on the ground electrode. Non-uniform darkening of the silicon surface, confined mostly at thecenter of the wafer, resulted on placing the wafers on the poweredelectrode. While the exact reason for this was not clearlyunderstood, the phenomenon augured well for the ultimateperformance of solar cells as plasma mode configuration togetherwith the low RF power density helped keep the surface damage toa minimum. This can be explained considering the fact that in theRIE mode, the ‘‘negative self-bias’’ acquired by powered electroderesulted in bombardment of the wafers placed on the poweredelectrode with heavy ions. The result was physical sputtering inaddition to chemical etching and consequently more damage tothe evolving surface. On the other hand, in the plasma mode,texturization/blackening of the wafer was more due to ‘‘chemicaletching’’ with self-masking due to the reaction products and lessdue to ‘‘physical sputtering’’. Technical literature on the subjectalso corroborates our explanation of physical and chemical modesof plasma etching. The texturization process was thus controlledand made to originate from a combination of least physicalsputtering and maximum chemical etching due to reactive speciesas described in Ref. [3]. The surface produced, therefore, had lessdamage and was worthy of device fabrication.

3.2. Reduction in reflectivity of plasma-textured surface of mc-Si

wafer

Fig. 2 presents pictures of a typical mc-Si wafer before andafter plasma texturization. The uniform dark appearance of thetexturized wafer indicated enhanced absorption of the incidentlight. Fig. 3 shows the reflectance spectra in the wavelength range350–800 nm for mc-Si samples at four different stages of solar cellprocessing, viz., alkali-polished wafer, plasma-texturized wafer,plasma-texturized wafer with DRE and finally texturized waferwith DRE+SiNx AR coating. For alkali-polished/saw-damage-removed wafer surface, effective reflectance data have beentaken from the literature [7]. The results show that plasmatexturization is able to reduce the reflectance up to 5% and plasmatexturization+SiNx AR coating reduces the reflectance up to 2% at600 nm. The reflectance curve for the plasma-texturized sample isflat at �5% value throughout the wavelength range used formeasurement. The observed trend is inline with the data reportedin the literature [1]. The intermediate step of DRE aftertexturization enhanced reflection due to ‘‘rounding off’’ of thesharp peaks produced during plasma texturization. DRE is achemical solution with a composition of HF:HNO3:H2O in 2:48:50ratio maintained at around 8–10 1C, and is used for damageremoval. The duration of etch is 1–3 min.

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0

10

20

30

40

50

60

350 400 450 500 550 600 650 700 750 800Wavelength (nm)

Ref

lect

ance

(%)

Virgin alkali polished mc-Si wafer

plasma textured mc-Si wafer

Plasma text +DRE mc-Si wafer

Plasma Textured + DRE+SiNx ARC

Fig. 3. Diffused reflectance of mc-Si wafer/cell at different stages of cell

processing.

Fig. 4. SEM micrograph of plasma-textured mc-Si wafer surface (a) before and

(b) after DRE (Magnification: 15k� ).

0

1

2

3

4

5

6

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Voltage (V)

Cur

rent

(A) with Plasma Texturization

w/o Plasma Texturization

Voc (V) η (%) W/0 Plasma Text 0.58 4.52 0.71 12.7 With Plasma Text 0.60 5.07 0.73 15.1

Isc (A) FF

Fig. 5. Comparison of light I–V characteristics of mc-Si solar cell with and without

plasma texturization.

Fig. 2. Typical appearance of mc-Si wafers (a) before and (b) after plasma

texturization.

B. Prasad et al. / Solar Energy Materials & Solar Cells 94 (2010) 1329–1332 1331

3.3. Effect of AR coating and contact firing on defect passivation in

mc-Si wafer

The physical reason behind the drastic reduction of reflectionfrom the texturized surface is evident from the SEM micrographsgiven as Fig. 4a—before DRE and Fig. 4b—after DRE treatment,which clearly shows the nanostructure produced as a result ofplasma texturization. It is clear from the figure that sponge-like[8] microstructure resulted in an increased absorption of theincident light due to multiple reflections and also increased theeffective surface area. Both these factors coupled with the SiNx ARcoating should lead to an increase in short-circuit current and Iscof the solar cell. A comparison of light I–V data in Fig. 5 shows thatthere was a gain of �12% in Isc for plasma-texturized waferscompared to alkali-polished wafers. With similar AR coatingpresent on both the cells, this gain was clearly on account ofplasma texturization. However, the increase in Voc by 20 mVcould not have been due to increase in Isc alone and must havehad the beneficial effect of reduced dark reverse saturationcurrent density J0. Going by the hypothesis of enhanced surfacearea after texturization, dark current should also have increasedsimultaneously with Isc, and have negative effect on Voc.However, the dark I–V measurements on cells with and withouttexturization, as depicted in Fig. 6 revealed that value of reversesaturation current density J0, indeed decreased on solar cells withtexturization. It can be seen from these plots and the comparative

data derived from these (shown in the inset) that there has beenan improvement in the dark characteristic of the device aftertexturization despite an increase in the surface area. A decrease inthe value of reverse saturation current density J01 (from 8.37 to1.28 nA/cm2) and diode ideality factor n (from 1.6 to 1.4)contributed towards improvement in the cell Voc and FF, whichis corroborated by the data from the light I–V measurements.

This is indicative of a defect-free RIE-texturized surface withlow surface recombination velocity in which the surface damagedue to ion bombardment during RIE-assisted texturization iseffectively reduced by hydrogen atoms liberated during plasmadeposition of SiNx AR coating and subsequent firing of the screen-printed contacts through it [9]. Also, the sponge-like surfacestructures of the texturized wafers provided an increased cell-areathat enhanced the possibility of migration of hydrogen atoms forpassivation of surface and bulk defects in silicon. This enhancedthe minority carrier lifetimes for mc-Si solar cells with RIE-assisted texturization as revealed by measurements on minoritycarrier lifetime employing the QSS Sun-Voc technique. Fig. 7depicts variation of minority carrier life times in mc-Si solar cells

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1.00E-06

6.00E-06

1.10E-05

1.60E-05

2.10E-05

2.60E-05

3.10E-05

3.60E-05

4.10E-05

4.60E-05

Log (minority carrier density)

Life

tim

e (s

ec)

w/o Plasmatexturing

with Plasmatexturing

12 13 14 15 16

Fig. 7. Effective lifetime v/s carrier density for mc-Si solar cell with and without

plasma texturization.

-6

-5

-4

-3

-2

-1

0

Bias Voltage (V)

Log

J

Rev with Plasma TextRev w/o Plasma TextFWD with Plasma TextFWD w/o Plasma Text

n J01 (nA/ cm2) J02 (uA/ cm2) Rsh(Ω-cm2)

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0

W/o Plasma Text 2062 27.68.371.6With Plasma Text 250715.61.281.4

Fig. 6. Comparison of dark I–V characteristics of mc-Si solar cell with and without

plasma texturization.

B. Prasad et al. / Solar Energy Materials & Solar Cells 94 (2010) 1329–13321332

with and without plasma texturization. It is seen that for reasonsmentioned above there has been improvement of the effectiveminority carrier lifetime in these solar cells by almost 100%, i.e.,from 20 to 40ms when compared at a typical carrier density of5�1014 cm�3.

A significant improvement in efficiency of mc-Si solar cells (upto 2.4% absolute) has been obtained due to cumulative beneficialeffects of plasma texturization and plasma-enhanced chemicalvapor deposited (PECVD) SiNx AR coating with firing of screen-printed metallization.

4. Conclusion

An attempt has been made to explain the various physicalprocesses responsible for substantial improvement, observed inthe performance of large area (149 cm2) mc-Si solar cells due toplasma texturization and plasma-deposited SiNx:H AR coatingwith screen-printed metallization. The enhancement has primar-ily been due to (a) loading of the mc-Si wafers on the groundelectrode resulting in plasma texturization in the chemical-etching-dominated mode producing reduced reflectance of in-cident photons over a broad spectral range, (b) use of low RFpower density for reduced surface damage and also treating thewafers with a suitable damage removal etch before junctionformation and (c) employing a plasma-deposited SiNx:H ARcoating followed by fired-through, screen-printed Ag contacts.The last step resulted in the release of hydrogen leading toeffective passivation of surface and bulk defects in siliconrendering high minority carrier lifetime. The synergy of all theseprocesses has been responsible for an improvement of �2.4% inthe absolute efficiency of large area mc-Si solar cells.

Acknowledgement

The authors are thankful to the BHEL management for theirconstant encouragement and permission to publish this work.

References

[1] B.O. Seraphin, A.B. Meinel, in: Optical Properties of Solids, North-Holland,Amsterdam, 1974.

[2] P. Campbell, M.A. Green, Light trapping properties of pyramidally texturedsurfaces, J. Appl. Phys. 62 (1987) 243–249.

[3] B. Prasad, S. Bhattacharya, A.K. Saxena, S.R. Reddy, B.L. Bedi, R.K. Bhogra, Effectof self-masking, low-energy RIE texturization process on the performance oflarge-area multi-crystalline silicon solar cells. in: Proceedings of the 22ndEuropean Photovoltaic Solar Energy Conference and Exhibition, 3–7 September2007, WIP Renewable Energies Press, Milan, Italy, 2007, pp. 1477–1479.

[4] S. Bhattacharya, B. Prasad, R. K. Bhogra, Automated test set up for industrialsolar cells, BHEL-ASSCP Doc# T&C/01/ 2008 (unpublished Internal Report).

[5] M.J. Kerr, A. Cuevas, Generalized analysis of the illumination intensity vs. open-circuit voltage of solar cells, Solar Energy 76 (1–4) (2004) 263–267 (January-March).

[6] WCT-120 Photoconductance Lifetime Tester and optional Suns-Voc Stage-UserManual, M/s Sinton Consulting, Inc. CO, USA, Web: /www.sintonconsulting.comS.

[7] Jessica D. Hylton, Light coupling and light trapping in alkaline etchedmulticrystalline silicon wafers for solar cells, Ph. D. Thesis, Utrecht University,Netherlands, 2006, p. 74.

[8] H.F.W. Dekkers, F. Duerinckx, L. Carnel, G. Agostinelli, G. Beaucarne, Plasmatexturing processes for the next generations of crystalline Si Solar Cells, in:Proceedings of the 21st European Photovoltaic Solar Energy Conference andExhibition, 4-8 September 2006, WIP Renewable Energies Press, Dresden,Germany, 2006, pp. 754-757.

[9] F. Duerinckx, J. Szlufcik, Defect passivation of industrial multicrystalline solarcells based on PECVD silicon nitride, Solar Energy Mater. Solar Cells 72 (2002)231–246.