Short Communication for Accelerated Publication

TiO2 DLAR Coatings for Planar Silicon Solar

Cells

B.S. Richards1, S.F. Rowlands2, C.B. Honsberg1, and J.E. Cotter1

1Centre for Photovoltaic Engineering, University of New South Wales, Sydney, NSW 2052,

Australia

2Department of Electrical and Electronic Engineering, University of Western Australia,

Nedlands, Perth, WA 6907, Australia

In this paper we demonstrate that a double-layer antireection (DLAR)

coating can be fabricated using only titanium dioxide (TiO2). Two TiO2

thin lms were deposited onto planar silicon wafers using a simple at-

mospheric pressure chemical vapour deposition (APCVD) system un-

der dierent deposition conditions. Weighted average reectances of

6.5% (measured) and 7.0% (calculated) were achieved for TiO2 DLAR

coatings in air and under glass, respectively. An increase in the short-

circuit current density of Jsc = 2.5mA/cm2 can be expected for an

optimised TiO2 DLAR coating when compared to a commercial TiO2

single-layer antireection coating.

Correspondence to: B.S. Richards, Centre for Photovoltaic Engineering, The University of

New South Wales, Sydney, NSW 2052, Australia, tel: +61-2-9385-5914, fax: +61-2-9662-4240,

email: b.richards@unsw.edu.au

Contract/grant sponsor: ARC; contract/grant number: A00104665.

1

BryceText BoxProgress in Photovoltaics 2003; 11(1): 2732

INTRODUCTION

Controlling the reectance of commercial solar cells fabricated on multicrystalline

silicon (mc-Si) wafers is dicult, primarily due to the inability to adequately tex-

ture mc-Si wafers with the standard alkaline etch solution. This work investigates

increasing the optical performance of planar Si solar cells by using a double-layer an-

tireection (DLAR) coating fabricated completely from APCVD-deposited titanium

dioxide (TiO2). Two advantages of DLAR coatings over single-layer antireection

(SLAR) coatings are, rstly, that two minima can be achieved in the reectance

spectrum, leading to a lower spectral-weighted reectance. Secondly, that their op-

tical performance is less sensitive to layer thickness variations.1 One disadvantage of

DLAR coatings is that their performance decreases rapidly with increasing thickness

of an intermediary, low-refractive index surface passivation layer.2 However, the use

of such a passivation layer at the AR coating:Si interface, such as silicon dioxide

(SiO2), is essential to achieve high conversion eciencies.

In the past, DLAR coatings have been primarily applied to laboratory-scale pho-

tovoltaic (PV) devices using two dierent evaporated materials. Common mate-

rials that have been used for DLAR coatings of non-encapsulated cells include

SiO2/TiO22 and magnesium uoride/zinc sulphide.3 For encapsulated solar cells,

aluminium oxide/TiO2 DLAR coatings have been implemented.4 Recently, plasma-

enhanced chemical vapour deposited (PECVD) silicon nitride (SiN) has become

popular as an AR coating material due to additional electronic benets aorded by

the presence of hydrogen in these lms.5 Research has also been performed on SiN

2

DLAR coatings, however results show that at high refractive indices (n > 2.4 at

= 600 nm) these lms becoming highly absorbing with extinction coecients of

0.4 at = 400 nm.57

The DLAR coatings presented here rely on TiO2 for both layers. TiO2 has several

advantages: i) it is familiar to the PV industry; ii) depositions can be performed

at atmospheric pressure and low temperatures using a non-toxic liquid precursor;

iii) it has excellent chemical resistance;8,9 iv) it has a high refractive index and low

absorption coecient; v) materials and processes are low cost; and, vi) methods

for achieving excellent surface passivation with TiO2 coated Si wafers have been

demonstrated.10

DLAR COATING DESIGN

For two quarter-wavelength coatings to achieve zero reectance at two wavelengths,

the necessary refractive indices for the top (nAR1) and bottom (nAR2) layers of the

DLAR coating are determined by11

n3AR1 = n20 nSi and n

3AR2 = n0 n

2Si. (1)

while the thickness of each of the layers is given by

dAR1 =0

4nAR1and dAR2 =

04nAR2

. (2)

In Equation 1, which does not account for any absorption in the lm, the refractive

indices of air and silicon are assumed to be n0 = 1 and nSi = 3.939 at = 600

nm, respectively. For the case of a DLAR coating in air, the required refractive

3

indices are nAR1 = 1.579 and nAR2 = 2.494. From Equation 2, the optimum layer

thicknesses are dAR1 = 95.0 nm and nAR2 = 60.1 nm at the design wavelength of

0 = 600 nm. For a glass-encapsulated solar cell the desired parameters change

to nAR1 = 2.07 (72.5 nm) and nAR2 = 2.86 (52.4 nm). In comparison, the highest

achievable n values ( = 600 nm) for the anatase and rutile phases of TiO2 are

2.53212 and 2.70,13 respectively.

TiO2 THIN FILM DEPOSITION

The TiO2 thin lms in this work were deposited using a simple APCVD system. The

TiO2 liquid precursor, tetraisopropyl titanate (97% purity Tyzor TPT, DuPont), was

maintained at 50C in a stainless steel bubbler, resulting in a vapour pressure of

1mbar. The TPT vapour is transported via heated lines to a stainless steel nozzle

situated above the Si substrate. The Si wafers are maintained at a temperature of

150 450C. Deposition temperatures (Tdep) less than 300C result in amorphous

TiO2 thin lms, while Tdep = 300 450C favours the formation of polycrystalline

TiO2 lms of the anatase phase.

The TPT can either react via hydrolysis (in the presence of water vapour) or pyrol-

ysis (in the absence of oxygen) to form TiO2. The bottom TiO2 layer is formed by

pyrolysis at 450C onto a polished silicon wafer. As deposited, the sample exhibits

a refractive index of about 2.1 (at 600 nm), however after sintering (Tsin = 900C

for 6 hr) the n increased to 2.489. Scanning electron microscopy (SEM) images of

the lower lm, before and after sintering are shown in Figures 1(a) and (b), respec-

tively, illustrating that signicant lm densication has occurred during sintering.

4

The top TiO2 layer was prepared in the presence of a high concentration of water

(H2O) vapour at a deposition temperature of 250C. This resulted in a soft and

porous TiO2 lm as shown in Figure 1(c). The lm hardness is greatly improved by

performing a sinter at Tsin = 700C for 2 hr, although even extensive sinting of the

porous lm does not increase the lm density, as illustrated in Figure 1(d). This is

attributed to the high void fraction retarding the sintering mechanism.

TiO2 DLAR COATINGS

The DLAR coatings were characterised using reectance spectrophotometry and

variable-angle spectroscopic ellipsometry (VASE, J.A. Woollam Co., Inc). VASE

measurements were performed over the wavelength range 350 1150 nm and at

angles of 65 80. A double Lorentz oscillator model was used for tting the ellip-

sometric parameters and and extracting the dispersive refractive indices n()

and extinction coecients k(). A 50% void/50% TiO2 Bruggeman eective medium

approximation (EMA) layer was used to model the surface roughness layers.14 The

optical modelling was performed using the software package WVASE32.15 Figure 2

illustrates the TiO2 DLAR coating sample structure. The optical constants of the

porous top layer and the denser bottom layer are each modelled by a EMA and solid

TiO2 layer. By varying the deposition and sintering condtions, a wide range of TiO2

refractive indices could be achieved, nT iO2 = 1.73 2.63. The n=600nm, k=400nm

and thickness d of the four modelled layers for a typical TiO2 DLAR coating are

plotted in Figure 3.

Experimental and modelled reectance data for the TiO2 DLAR coating exhibiting

5

the lowest weighted-average reectance (Rw) to date is shown in Figure 4. The

characteristic double minimum of a DLAR coating is clearly seen and the modelled

data clearly provides an excellent t. The noisy data in the near infrared (800

1000 nm) is due to poor sensitivity in the spectrophotometers infrared detector.

The lowest weighted-average reectance (3501150 nm, including back reectance)

measured for a TiO2 DLAR coating in air in this work is Rw = 6.5%, compared to

Rw = 8.6% for a typical commercial TiO2 SLAR coating.

It should be emphasized, rstly, that the performance of the TiO2 DLAR coating is

less than optimal due to the high ntopT iO2 value. Secondly, it is important to realise

that while high-temperature processing was used to achieve dense (high n) layers,

this is primarily due to the limitation of the deposition technology available to the

authors. While such DLAR coatings are compatible with evaporated metal contacts,

alternate deposition methods would need to be used in a production environment to

avoid the incompatibility of screen-printed metallic contacts with high-temperature

processing. For example, high n and low k TiO2 thin lms have been achieved using

alternate deposition methods, such as spray deposition8 and ltered arc deposition.16

The most interesting application for a TiO2 DLAR coating is for planar mc-Si solar

cells encapsulated under glass. Modelling was performed, using the software package

TFCalc,17 with data from experimental coatings to determine the optimum perfor-

mance that could be expected for this case. The optical models for the 2mm-thick

B270 Crown glass and 1mm-thick ethyl-vinyl-acetate (EVA) layers were taken from

Nagel et al.5 As previously discussed, the ideal nAR2 value is nAR2 = 2.86, but as

this is not achievable in practice, the highest value observed for our TiO2 lms was

6

used, nAR2 = 2.63. This subsequently reduces the optimal nAR1 value from 2.07 to

1.95. This model predicts a weighted average reectance of Rw = 7.0% (including

back reectance), transmittance Tw = 91.0% and absorptance Aw = 2.0%. The

reectance spectrum, plotted in Figure 4, is extremely at and lies between 4.7%

and 7.7% for all wavelengths in the range 4101040 nm. This is excellent, consider-

ing that about 4.3% (absolute) is unavoidably lost at the air:glass interface. When

a 10 nm-thick SiO2 surface passivation layer is included in the model, the average

weighted reectance and absorptance increase to Rw = 7.57% and Aw = 2.10%,

respectively, while Tw = 90.33%.

SOLAR CELLS WITH TiO2 DLAR COATINGS

To predict the performance of solar cells with these DLAR coatings, the weighted

average reectance curves were used as an input to a PC1D18 simulation. The

intensity of the AM1.5 global spectrum was reduced to 98.35mW/cm2 to account

for absorption in the glass, EVA and TiO2 layers. The main source of absorptance

is the 1mm thick EVA layer. The electrical parameters for a standard buried-

contact solar cell were taken from Honsberg et al.19 On 1 cm p-type Si, this

modelled device resulted in a Jsc of 37.5mA/cm2 without a surface passivation layer

present, reducing slightly to Jsc = 37.2mA/cm2 with the inclusion of 10 nm SiO2

at the TiO2:Si interface. The emitter dark saturation current density (J0e) was

kept constant for TiO2 DLAR coatings both with and without an SiO2 passivation

layer present. While it is not realistic to expect a J0e of 1 1013 A/cm2 with no

SiO2 layer, this value was used to indicate the maximum optical performance of the

7

TiO2 DLAR coating. For the device with a surface passivation layer, an open-circuit

voltage (Voc) of 640.6mV and eciency of = 18.6% were calculated. The ll-factor

of this solar cell was 78.1%, which the authors believe can be achieved for a BC solar

cell fabricated on mc-Si substrates. The bulk minority carrier lifetime was assumed

to be b = 100s. The DLAR coating results represent a 7% improvement in Jsc over

a typical commercial APCVD TiO2 SLAR coating (Tdep = 320C and Tann = 850C

for 1min to simulate ring of the front screen-printed contacts). Using the same

PC1D model, the latter SLAR coating (n600nm = 2.275 and k400nm = 0) results in

a Jsc of 34.7mA/cm2 (Voc = 621.3mV and = 16.8%).

SUMMARY

In this work we have demonstrated that a simple, low-cost deposition system can

be used to realise a DLAR coating, fabricated from a single material, TiO2. Initial

experimental results from TiO2 DLAR coatings, measured in air, indicate that a

non-optimized weighted average reectance of 6.5% can be achieved. This compares

favourably with a typical commercial SLAR coating (Rw = 8.6%). Of primary

interest is the performance of such a DLAR coating under glass, and our results

have shown that an extremely low weighted average reectance of 7.0% (including

the silicon back reectance) can be achieved using TiO2 layers. When implemented

into a buried-contact solar cell model, the TiO2 DLAR coating results in increase

in Jsc of 2.5mA/cm2 (7%) compared to a commercial TiO2 SLAR coating. SLight

losses due to the inclusion of 10 nm-thick SiO2 passivation layer at the TiO2:Si

interface were taken into consideration.

8

Acknowledgments

B.S. Richards would like to thank Prof. Trevor Redgrave (Department of Physiology,

UWA) for generous access to the VASE instrument and the Faculty of Engineering

(UNSW) and the Centre for Photovoltaic Engineering for nancial support. This

work was supported by an Australian Research Council 2001 Large Grant number

A00104665.

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11

Figure 1: SEM images of the lower TiO2 film (Tdep = 450C) (a) before and (b)

after sintering; and the porous, upper TiO2 film (Tdep = 250C in H2O vapour) (c)

before and (d) after sintering (Tsin = 1000C for 6 hr).

12

silicon nSi , kSi

EMA nbotEMA , kbotEMA

TiO2 nbotTiO2 , kbotTiO2

EMA ntopEMA , ktopEMA

TiO2 ntopTiO2 , ktopTiO2

air or glass n0

Figure 2: TiO2 DLAR coating sample structure, showing the link between the n and

k nomenclature and the layers in the optical model.

13

Figure 3: Optical constants for the two pairs of dense and EMA layers in a TiO2

DLAR coating.

14

300 400 500 600 700 800 900 1000 1100 12000

10

20

30

40

50

Reflectance, R (%

)

W avelength, (nm )

TiO2 SLAR in air (R

w=8.6% )

TiO2 DLAR in air (R

w=6.5% )

Encapsulated TiO2 DLAR (R

w=7.0% )

Figure 4: Experimental (points) and modelled reflectance data (solid curve) for a

TiO2 DLAR coating, compared to a typical commercial TiO2 SLAR coating (dashed

curve). The modelled reflectance spectrum (dotted curve) of an encapsulated TiO2

DLAR coating is also plotted.

15