characterisation of light trapping in silicon films by spectral photoconductance measurements

*Corresponding author. Fax: #61-2-93855412.E-mail address: [email protected] (P. Campbell).

Solar Energy Materials & Solar Cells 66 (2001) 187}193

Characterisation of light trapping in silicon "lmsby spectral photoconductance measurements

Patrick Campbell*, Mark Keevers, Bernhard VoglPhotovoltaics Special Research Centre, University of New South Wales, Sydney 2052, Australia

Abstract

This paper presents progress made in developing a method of measuring light trappingintrinsically free of uncertainties associated with re#ector absorption and collection losses.These problems presently restrict analysis to uniformly absorbed wavelengths, which in thin"lms especially accounts for only a minor part of available light trapping bene"t. We aim toextend photoconductance measurements, which presently are also similarly restricted, tononuniformly absorbed wavelengths in order to fully characterise light trapping. Specimenpreparation and measurement guidelines are given. ( 2001 Elsevier Science B.V. All rightsreserved.

Keywords: Light trapping measurement; Photoconductance; Silicon "lms

1. Introduction

Although thin-"lm silicon solar cells depend on e!ective light trapping for highe$ciency, no relatively straightforward method is available for measuring the capabil-ity of a light trapping scheme to enhance short-circuit current. In the spectral region ofuniform absorption characterised by ad;1 (product of absorption coe$cient andthickness), re#ectance and spectral response measurements [1,2] are used to charac-terise near bandgap light trapping in terms of the pathlength enhancement byextraction of e!ective front and rear internal re#ectances. With spectral responsemeasurements, position-dependent collection losses have no spectral variationand can be factored out. However, light trapping contributes more signi"cantly to

0927-0248/01/$ - see front matter ( 2001 Elsevier Science B.V. All rights reserved.PII: S 0 9 2 7 - 0 2 4 8 ( 0 0 ) 0 0 1 7 2 - 0

Fig. 1. Absorbance (fraction of light absorbed) and absorbance enhancement curves for a c-Si layer 2 lmthick with and without randomising. An absorbance enhancement curve for a 200lm thick layerwith/without randomising is also shown. Light becomes uniformly absorbed above wavelengths 0.97 and1.14lm, respectively.

enhancing absorption where absorption is nonuniform, from around ad+0.02}1.The absence of a way to measure light trapping in this band has not been so importantfor wafer cells because surface texturing mainly improves performance by reducingfront re#ection, rather than trapping light. However this is not the case for thin "lms,as shown in Fig. 1 which compares absorption enhancement developed in a 2lm c-Si"lm with that in a 200lm substrate using a randomising texture, which raisesavailable AM1.5 J

4#by 56% and 4%, respectively. This paper presents the case for

fully evaluating light trapping by monitoring excess carrier generation from spectralphotoconductance measurements.

2. Photoconductance measurement

2.1. Background

Photoconductance measurements have been extensively applied to the ad;1region to study the absorption edge of a-Si:H [3,4]. Undoped material is used toobtain a maximum photoresponse. The technique relies on the direct relationshipbetween photoconductivity and free carrier generation

*p"Gknqn*n, (1)

where G is the generation rate, kn

the electron mobility and sn

the electron lifetime.Only electrons play a signi"cant part because their mobility in a-Si:H is about four

188 P. Campbell et al. / Solar Energy Materials & Solar Cells 66 (2001) 187}193

orders of magnitude higher than that for holes. Except at very low-energy levels andtemperatures, G is directly proportional to the incident photon #ux. Photoconduc-tance readings are scaled against values of a derived from transmittance readings,which can only be obtained down to ad+0.01, where absorption is of the order ofmeasurement error. Absorption takes place uniformly across the "lm in the ad;1region, so there is no need to account for spatial lifetime variation such as from surfacerecombination. Front re#ection corrections are not normally carried out becausephotoconductance-derived values of the absorption coe$cient span some ordersof magnitude over a wavelength range of low dispersion and only relative valuesare required. If necessary, interference fringes can be reduced by enlarging the slitwidth.

Photocurrent and illumination level for a-Si:H exhibit a nonlinear relationshipbecause lifetime is excitation-dependent. This relationship is often formulated by theequation

I1JFc, (2)

where I1

is the photocurrent, F the incident photon #ux and c an empirical exponent,0.5(c(1. The value of c also depends on excitation level. Measurements of theabove type are carried out using a relatively constant excitation level to establisha linear relationship between photon #ux and photocurrent. This is done either bymaintaining a constant photocurrent and varying the narrow band illumination level(CPM method) [3] or by simultaneously illuminating the "lm with uniformly ab-sorbed light at a much higher level than the narrow band source (red bias method) [4].In the bias method, narrow band light is chopped to remove both the dark and biassignals with a lock-in ampli"er. A stable lifetime can be obtained after long-wavelength light soaking [5].

2.2. Experimental setup

Although the bias method is inherently more noisy than the CPM method, wechose to use it as spectral response gear was available and because our purpose didnot require extracting very low signal levels. Fig. 2 shows the setup we have beenusing, which is aimed to determine secondary photoconductance [6]. The choppingrate is carefully set to around 7Hz because of the slow response time. A sensingresistor is chosen with a low enough value to not distort the intended linear relationbetween signal voltage and specimen photoconductance. The electric "eld appliedbetween specimen contacts is limited to less than 1000V/cm to maintain an ohmiccharacteristic [7]. Noise from the DC voltage source is minimised by using batteries.We use Mg ohmic contacts evaporated directly onto the a-Si:H [8] and test they areohmic by observing whether contact illumination has any e!ect on the current signal.The bias light is obtained from a white source using a high pass "lter with a cuto!around ad;1.

Fig. 3 shows an example of measurements we have made where values of afor a 0.7lm a-Si:H "lm are determined from transmittance measurementsdown to ad"0.007 [9] using a planar "lm, then extended using photoconductance

P. Campbell et al. / Solar Energy Materials & Solar Cells 66 (2001) 187}193 189

Fig. 2. The experimental setup for photoconductance measurement using the red bias method.

Fig. 3. Values of a derived from transmittance measurement (squares) and photoconductance readings(diamonds).

measurements to one order of magnitude lower. We used a sandblasted specimen forthe photoconductance measurements to get a stronger signal and to eliminate inter-ference e!ects, with the knowledge that the e!ective thickness enhancement due tolight trapping is not wavelength dependent in the ad;1 region. Note that thephotoconductance curve diverges from the transmittance curve at shorterwavelengths because absorption is nonuniform. We carried out these measurementsboth to characterise our material and to enable phtoconductance enhancementmeasurements to be expressed as a function of a, in order to evaluate light trappingperformance of a c-Si "lm with the same geometry. We only needed to measurea down to around 10/cm as this range accounts for collection of over 99% of AM1.5J4#

for a c-Si "lm with random scatter, up to10lm thickness.


Fig. 4. Comparison of re#ection corrected photconductance response for a sandblasted and planar a-Si:H"lm. The term Rf refers to front re#ectance.

2.3. Light trapping characterisation

Photoconductance measurement was demonstrated some time ago for characteris-ing light trapping in an undoped a-Si:H "lm [10]. The authors scaled photoconduc-tance readings to unity at 2 eV where absorption was complete over the 1.4lmthickness as a way to "nd absorbance (fraction of light absorbed) at light trappingwavelengths. One shortcoming of this scaling method is that dispersion is high around2 eV and leads to spectral variation of front re#ection: this means that scaling shouldbe carried out after correcting for front re#ection. In the region where absorption iscomplete (ad<1), the photocurrent falls below a certain wavelength because anincreasing fraction of absorption takes place near the front surface, where carrierdensities are lowered by surface recombination [5]. The penetration depth at thewavelength where this begins gives a good idea of how much of the "lm is a!ected bythe front surface. Rear surface recombination can be tested similarly using rearillumination. Unless both surface a!ected regions are con"ned to a relatively smallportion of the "lm, scaled photoconductance readings for the nonuniformly absorbedwavelengths of interest here, nominally 0.02(ad(1, give an unreliable representa-tion of absorbance.

Our own measurements show that surface recombination can signi"cantly a!ectreadings where no surface passivation is used. Fig. 4 shows results obtained fora 0.7lm a-Si:H "lm on planar/sandblasted glass. Photoconductance readings were"rst corrected for front re#ection by extrapolating the linear region of re#ectancemeasurements taken at fully absorbed wavelengths, then the peak value at around630nm was scaled to unity. Texturing can be seen to provide an enhanced response


Fig. 5. Comparison of re#ectance corrected photoconductance readings for a 0.7lm thick a-Si:H "lmwithout passivation and with a SiN

xpassivation "lm on both surfaces.

above 630nm where ad:1 which we attribute to light trapping. Shorter wavelengthsdevelop a declining response, which we attribute to surface recombination. This is ata penetration depth of around 300 nm, so about half the "lm thickness is a!ected.Because surface recombination is likely to be worse at the rear, we expect most of the"lm was a!ected by surfaces.

2.4. Improvements in specimen preparation

Our a-Si:H "lms were deposited by PECVD in conditions known for producinggood quality material [11]. The fact that the front surface is `seena at a penetrationdepth of 300 nm suggests a bulk di!usion length of at least this value. Having observedan unacceptable level of surface recombination, we chose to passivate both surfaceswith in situ PECVD deposited SiN

x[12]. Deposition conditions were chosen

which have been optimised in our laboratory for passivating c-Si wafers, using a70nm thick SiN

x"lm and a substrate temperature of 4003C [13]. Contacting

was made using a lifto! process after lithographically de"ning Mg evaporationwindows. A thick Hoechst AZ4620 photoresist was used to accommodate the unevensurface of textured areas. The SiN

xwas removed from the evaporation windows

after postbake using a HF dip, then the Mg evaporation was carried out. Fig. 5shows front re#ection corrected measurements for the passivated and unpassivatedplanar "lms. The peak response shifts down to 420nm, at a penetration deptharound 30nm, which at "rst suggested much less front surface recombination.This may be so, however it appears that deposition of the "nal SiN

xlayer has

broadened the silicon "lm bandgap, perhaps through formation of a-SiNx:H [14].

A repeated deposition run gave the same result. We intend to characterise the "lm andwill either modify the SiN

xdeposition conditions or will try passivating with an

oxide.


3. Conclusion

We have selected photoconductance measurement for light trapping characterisa-tion because free carrier densities are not subject to the optical or collection losses thatrestrict existing methods to uniformly absorbed wavelengths. An equivalent problemexists with this electrical form of measurement, where surface recombination presentsa nonuniform lifetime pro"le across the "lm. Use of surface passivating "lms shouldhowever permit light trapping measurement where most of the "lm thickness hasa constant lifetime.

Acknowledgements

We thank Steven Hegedus and Mike Gal for their help in decoding the secrets ofa-Si:H and Tom Puzzer for reviving our ailing evaporator. Patrick Campbell andMark Keevers are supported by Australian Research Council Fellowships.

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characterisation of light trapping in silicon films by spectral photoconductance measurements

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