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Solar Energy Materials & Solar Cells 90 (2006) 3345–3355

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p-doped mc-Si:H window layers prepared by hot-wireCVD for amorphous solar cell application

P. Kumar, D. Bhusari, D. Grunsky, M. Kupich, B. Schroeder

Department of Physics, University of Kaiserslautern, PO Box 3049, D 67653 Kaiserslautern, Germany

Received 24 May 2005; accepted 23 September 2005

Available online 22 August 2006

Abstract

We report on boron-doped mc-Si:H films prepared by hot-wire chemical vapor deposition

(HWCVD) using silane as a source gas and trimethylboron (TMB) as a dopant gas and their

incorporation into all-HW amorphous silicon solar cells. The dark conductivity of these films was in

the range of 1–10 (O cm)1. The open circuit voltage Voc of the solar cells was found to decrease from

840mV at low hydrogen dilution H-dil ¼ 91% to 770mV at high H-dil ¼ 97% during p-layer

deposition which can be attributed to the increased crystallinity at higher H-dil and to subsequent

band edge discontinuity between mc-Si:H p- and amorphous i-layer. The short circuit current density

Jsc and the fill factor FF show an optimum at an intermediate H-dil and decrease for the highest H-

dil. To improve the conversion efficiency and the reproducibility of the solar cells, an amorphous-like

seed layer was incorporated between TCO and the bulk p-layer. The results obtained until now for

amorphous solar cells with and without the seed layer are presented. The I– V parameters for the best

p–i–n solar cell obtained so far are Jsc ¼ 13.95mA/cm2, Voc ¼ 834mV, FF ¼ 65% and Z ¼ 7.6%,

where the p-layers were prepared with 2% TMB. High open circuit voltages up to 847mV could be

achieved at higher TMB concentrations.

r 2006 Elsevier B.V. All rights reserved.

Keywords: Microcrystalline Si-1; Hotwire CVD-2; Seed layer-3

1. Introduction

Recently, the hot-wire chemical vapor deposition (HWCVD) method has attractedconsiderable attention for the preparation of amorphous and microcrystalline Si:H films

see front matter r 2006 Elsevier B.V. All rights reserved.

.solmat.2005.09.016

nding author. Current address: Department of Metallurgical and Materials Engineering, Colorado

ines, Golden, CO 80401, USA.

dress: pkumar@mines.edu (P. Kumar).

ARTICLE IN PRESSP. Kumar et al. / Solar Energy Materials & Solar Cells 90 (2006) 3345–33553346

because of the simple setup and the possibilities of high deposition rate compared to theplasma enhanced chemical vapor deposition (PECVD). Bauer et al. [1], Mahan et al. [2]and Iwaniczko et al. [3] reported on the growth of device quality a-Si:H and mc-Si:Hmaterial by HWCVD with low H-content, high stability and high ambipolar diffusionlength. Klein et al. have reported the successful inclusion of a HW prepared intrinsicabsorber layer in mc-Si:H solar cells where the mc-Si:H doped layers were prepared byPECVD [4].The p-layer plays a vital role in achieving high conversion efficiency in an a-Si:H solar

cell. Hence this layer needs to be highly conductive, highly transparent, should form agood substrate for the growth of the intrinsic collection layer and provide a low seriesresistance. Due to its high conductivity and high transparency p-mc-Si:H is considered tobe a better alternative to the wide band gap a-SiC:H, which often exhibits low quality whenprepared by HWCVD. Voz et al. [5] and Schubert et al. [6] investigated the mc-Si:H dopedlayers prepared by HWCVD but the solar cell efficiencies achieved with these layers werenot remarkable. Attempts were made to prepare doped mc-Si:H layers in our group byHWCVD for incorporation in p–i–n cells. However, p-layers prepared at high pressure andhigh filament temperature (in the following we call it ‘‘hard conditions’’) and the resultingp/i interface led to p–i–n solar cells with high initial values for Voc and FF, but were laterfound out to be unstable and showed irreversible degradation [7,8]. Therefore, the materialstudies and growth of mc-Si:H p-layers need to be readdressed. This paper describes thecharacterization of boron doped microcrystalline silicon films grown at low pressure andlow filament temperature (in the following we call it ‘‘soft conditions’’). Some of the p-mc-Si:H films were incorporated into single junction a-Si:H solar cells and the effects of thegrowth conditions are analyzed.

2. Experimental

The details of the ultra high vacuum (UHV) deposition system used in this study havebeen described elsewhere [9]. The i-chamber gas line was equipped with purifiers and W-shaped tantalum wire with 0.5mm in diameter was used as a filament. The system reachesa base pressure of 108mbar. All layers were prepared in a multi-chamber system with aload-lock and a central transfer chamber without air-break. mc-Si:H films were preparedusing SiH4 and H2 as a source gases and trimethylboron (TMB) and phosphine (PH3) as adopant gas for p- and n-layer deposition respectively. mc-Si:H p-layers were prepared atTf ¼ 1550 and 1650 1C, filament to substrate distance dfs ¼ 6.5 cm, low pressure,p ¼ 710 Pa and the substrate temperature Ts was kept constant at 180 1C. A systematicvariation in hydrogen dilution (H-dil ¼ F(H2)/F (SiH4+H2+TMB)) was carried out. Thei-layers were prepared at Tf ¼ 1550 1C keeping Ts and p at 170 1C and 1Pa, respectively.The samples were characterized by Raman and infrared (FTIR) spectroscopy. For

Raman measurement, 514.5 nm laser radiation was used as an excitation source and thepower density of the incident beam was kept below 15mW/mm2 to avoid thermallyinduced crystallization of the layer. The Raman spectra were deconvoluted into itsintegrated crystalline Gaussian peak Ic (520 cm1), amorphous Gaussian peak Ia(480 cm1) and intermediate Gaussian peak Im (500–510 cm1). The crystalline fractionwas calculated as Xc ¼ (Ic+Im)/(Ic+Im+Ia) [9]. The hydrogen bonding in the films wasinvestigated by FTIR and the thickness of the film was measured with a Dektak

ARTICLE IN PRESSP. Kumar et al. / Solar Energy Materials & Solar Cells 90 (2006) 3345–3355 3347

profilometer. The lateral dark conductivity was measured on glass with chromiumcontacts.

The superstrate type p–i–n single junction a-Si:H solar cells were prepared on ‘‘Asahi’’ UTCO-coated glass and the structures of cell were glass/SnO2:F/p-mc-Si:H/i-a-Si:H/n-mc-Si:H/Ag and glass/SnO2:F/seed layer/p-mc-Si:H/i-a-Si:H/n-mc-Si:H/Ag. Silver back con-tacts were made by thermal evaporation. The photo I– V measurements were carried outusing AM 1.5 illuminations.

3. Results and discussions

3.1. Microcrystalline p-layer

Concerning p-layer investigation, one needs to distinguish between investigations onbulk p-layers and the solar cells in which different p-layers are incorporated. We assumethat the results obtained from the measurement on thick p-layers (300 nm) are alsorepresentative for the thin film layer (15 nm) which is included in solar cells. However,the incorporation of preoptimized p-layers may not result in the optimum performance ofthe solar cell structure due to the complicated influence of the substrate, process chamberconditions and deposition parameters during solar cell preparation. Therefore, anadditional optimization of the p-layer in the p–i–n solar cell structure is needed.

The p-doped mc-Si:H films prepared at low dfso4 cm were found to be porous andunstable against atmospheric gases as Si-O peak was observed in the IR spectra afterexposure to air, whereas the films prepared at high dfs ¼ 6.5 cm were observed to be moredense and stable. The reason behind this observation could be the gas-phase reactions,which lead to different precursors particles for the film growth and the high substratesurface temperature due to excess heat transfer from the filament to substrate at such ashort dfs. This was reduced partially by increasing the dfs. Hence, all p-layers preparedfor layer characterizations and for solar cells (reported in this paper) were prepared at largedfs only.

3.1.1. p-layers grown with 2% TMB

Fig. 1 shows the dependence of dark conductivity and the corresponding crystallinevolume fraction of p-mc-Si:H films grown at different H-dil. The TMB concentration[TMB] used for this series of samples was 2% and the thickness of the films is nearly300 nm. The crystalline volume fraction, calculated from Raman scattering, can be seen toincrease with the H-dil and reaches a maximum of 76% at a H-dil of 97.5%. Thecorresponding dark conductivity also increases with the H-dil. At low H-dil of 91%, thefilms are amorphous with no detectable fraction of crystallites. Due to the fact that thisamorphous layer is of poor quality, the dark conductivity is quite low since the Fermi levelis not shifted significantly by doping. We also investigated very thin films of 20 nmthickness and found that the dark conductivity for high H-dil (495%) is about 102 S/cm.

Fig. 2 shows the IR absorbance spectra of p-layers prepared at different H-dil for thesample shown in Fig. 1. The Si-Hn stretching mode intensity at 2100 cm1 (due to Si-H1,2

on interfaces) increases at the cost of Si-H vibrational mode at 2000 cm1 (due to dissolvedSi-H bonds in the bulk) with increase in the H-dil. This 2100 cm1 peak is known to beassociated with hydrogen present at the grain boundaries of micro-crystallites and at the

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92 94 95 97

1

0

20

40

60

80D

ark

Con

duci

tivity

(Ω

cm

-1)

0.1

0.01

1E-3

1E-4

1E-5

1E-6

1E-7

1E-8

Hydrogen Dilution (%)90 91 98

Cry

stal

line

frac

tion

(%)

9693

Fig. 1. Dark conductivity and crystalline volume fraction of p-mc-Si:H as a function of hydrogen dilution where

Tf ¼ 1550 1C, Ts ¼ 180 1C, p ¼ 10Pa and [TMB] ¼ 2%.

Abs

orpt

ion

(arb

.uni

ts)

0.08

0.06

0.04

0.02

0.00500 1000

Wavenumber (cm-1)

1500 2000

91%

95.5%

97%

H-dil

97.5%

Fig. 2. IR absorbance spectra of p-mc-Si:H layers at different H-dil shown in the Fig. 1.

P. Kumar et al. / Solar Energy Materials & Solar Cells 90 (2006) 3345–33553348

surface. The 2100 cm1 peak is found to be more prominent than the 2000 cm1 at H-dil of97.5% indicating higher crystalline fraction.

3.1.2. p-layers grown with 8% TMB

Wang et al. [13] achieved very high Voc ¼ 0.94V for a-Si:H solar cell in n-i-p structureentirely prepared by HWCVD where the p-layer was prepared with 8% TMB. This gave usan impetus to prepare mc-Si:H p-layer at high TMB concentrations to obtain high Voc in

ARTICLE IN PRESSP. Kumar et al. / Solar Energy Materials & Solar Cells 90 (2006) 3345–3355 3349

superstrate type p–i–n solar cells. Fig. 3 shows the dark conductivity of the p-layerprepared at 8% TMB as a function of H-dil. The films at H-dil 95.5 and 96.5% were stillamorphous, as confirmed by Raman measurements as shown in Fig. 4, and consequentlyexhibit low conductivity. In comparison, the films prepared at the same H-dil for 2% TMBwere highly conductive and exhibited high crystalline volume fraction (as shown in Fig. 1).With increasing H-dil, the Raman spectra in Fig. 4 show crystalline peak at 520 cm1 witha weak shoulder at 480 cm1. The conductivity and crystalline volume fraction increased to

97.0

0.1

1

10

Dar

k C

ondu

ctiv

ity (

Ω c

m-1

)

0.01

1E-3

1E-4

1E-5

1E-6

1E-795.0 95.5 96.0 96.5

Hydrogen Dilution (%)

97.5

amorphous

Fig. 3. Dependence of dark conductivity of p-mc-Si:H prepared at 8% TMB as a function of H-dil where

Tf ¼ 1650 1C, Ts ¼ 180 1C, P ¼ 10 Pa.

4000

1

2

3

4

5

Ram

an in

tens

ity (

arb.

uni

ts)

97.3%

H-dil

96.5

440Wavenumber (cm-1)

480 520

95%

Fig. 4. Raman spectra of mc-Si:H p-layer for the samples shown in Fig. 3.

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1500

Abs

orpt

ion

(arb

. uni

ts)

95%

0.08

0.06

0.04

0.02

0.00500 1000

Wavenumber (cm-1)

2000

96.5%

H-dil

97.3%

Fig. 5. IR absorbance spectra of the sample shown in Fig. 3.

P. Kumar et al. / Solar Energy Materials & Solar Cells 90 (2006) 3345–33553350

0.5 S/cm and 68%, respectively, at H-dil ¼ 97%. One can, thus, conclude that a highmagnitude of boron in the film suppresses the growth of microcrystalline film and thusshifts the amorphous–microcrystalline phase transitions to higher values of H-dil. Thisresult is in accordance with the result obtained by e.g., Voz et al. [5]. The IR spectrum (ofthe sample shown in Fig. 3) grown at H-dil of 95%, as shown in Fig. 5, exhibits mainly2000 cm1 peak but as the H-dil is increased to 96.5%, the 2100 cm1 peak and 2000 cm1

peak become almost equal in intensity. This indicates the presence of microvoids in the filmwhich is correlated with a bad microstructure. With further increase in H-dil to 97%, theIR spectra show only a weak shoulder at 2000 cm1 together with a prominent peak at2100 cm1, the typical IR spectrum of mc-Si:H.

3.2. Solar cell results

3.2.1. Films grown with 2% TMB

In Fig. 6, representative data for the optimization of the thickness of the mc-Si:H p-layerfor amorphous solar cells are shown. With decreasing deposition time of the p-layer, the Jscincreases due to lower absorption in the p-layer, whereas Voc decreases because in this casethe p-layer cannot build up a sufficient built-in potential in the solar cells. The maximumefficiency is obtained for 5min deposition time of the p-layer resulting in an optimumthickness.Fig. 7 shows variations in the I–V parameters of amorphous pin solar cells as a function

of H-dil during the p-layer deposition. The p-layer thickness in these cells is estimated to beabout 15 nm. The Voc of the cells decrease from 840mV at H-dil ¼ 91% to 770mV at H-dil ¼ 97%. The Voc of the pin solar cell is known to depend strongly on the resulting p/iinterface. The decrease in Voc observed here can be attributed to the band edgediscontinuity between the mc p-layer and the amorphous i-layer [10] that increases withincreasing crystalline fraction at higher H-dil. In Fig. 7 it is seen that the parameters Jsc,

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352

56

60

64

68

Jsc

(mA

/cm

2 )

Effi

cien

cy (

%)

Voc

(V

)

0.80

0.78

0.76

0.74

0.72

0.70

6.6

6.4

6.2

6.0

Deposition time (min)

4 5 6 7

12.8

13.0

13.2

13.4

13.6

FF

(%

)Fig. 6. Variations in the parameters of amorphous pin solar cells as a function of p-layer deposition time keeping

Tf and Ts constant at 1550 and 180 1C, respectively.

P. Kumar et al. / Solar Energy Materials & Solar Cells 90 (2006) 3345–3355 3351

FF and (Z) efficiency exhibit a maxima in the region of amorphous–microcrystallinetransition. A similar behavior of these parameters was also observed for films grown atTf ¼ 1650 1C, although the position of the maxima was somewhat shifted to lower H-dil.With increasing H-dil for p-layer preparation, the crystalline volume fraction and darkconductivity increase as shown in Fig. 1. A higher crystalline volume fraction is expected toenhance the transparency of the p-layer, which should in-turn result in an increase in Jsc.But the Jsc decreases for the p-layer prepared at a very high hydrogen dilution. This can beattributed to (1) the darkening of the TCO (chemical reduction of tin) due to reaction withatomic hydrogen generated at the filament [11,12] and, (2) a high band gap offset due tohigher crystalline fraction. It is important to note that the p-layer serves as a substrate fori-layer deposition. So the p-layer prepared at high pressure with a growth rate of 1 A/smay probably result in different substrates for i-layer deposition compared to the p-layerprepared at low pressure (see Section 3.2.3).

For higher Tf ¼ 1650 1C and keeping TMB concentration constant at 2%, the Voc doesnot exceed 800mV. At this Tf, the maximum of the other parameters are shifted to lowerH-dil. At high Tf and high H-dil, the p-layers always show a higher crystallinity, thusstrongly affecting the p/i interface, and consequently Voc is reduced.

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91 93

11

12

13

14

60

64

68

Voc

(V

)η

(%)

J sc

(mA

/cm

2 )

-- -- 7 Pa

.... .. .. 10 Pa

amorphousmicrocrystalline

0.84

0.80

0.76

9796959492

6.4

6.8

7.2

Hydrogen Dilution (%)

FF

(%

)

Fig. 7. Variations in the parameters of amorphous pin solar cells as a function of hydrogen dilution during p-

layer deposition keeping Tf and Ts constant at 1550 and 180 1C, respectively. TMB concentration is 2%. The

dotted lines are the guidelines for the eyes.

P. Kumar et al. / Solar Energy Materials & Solar Cells 90 (2006) 3345–33553352

3.2.2. Films grown with 8% TMB

p-layers prepared at 8% TMB were found to exhibit higher amorphous fractions at thesame H-dil compared to those at 2% TMB p-layer. The Voc is generally above 800mV anda maximum Voc850mV is observed near H-dil ¼ 95.5%. The FF at H-dil ¼ 95 and 96%is low compared to H-dil ¼ 98%. This low FF was caused by high series resistance, whichis not related to the deposition conditions of the p-layer, especially the H-dil. Therefore, weadded in Table 1, a solar cell with a large i-layer thickness, although we know thisthickness leads to high degradation. We include this cell to show that high FF can beobserved at this H-dil, although it is expected to get a lower FF for larger i-layer thickness.The results are summarized in Table 1. The solar cells were annealed at 150 1C to increasethe FF, but the FF decreased for some samples. Fig. 8 shows the secondary ion massspectroscopy (SIMS) results of pin cell before and after annealing at 150 1C. The p-layergrown at 2% TMB can be seen to be more stable compared to that at 8% TMB afterannealing. Moreover, the films prepared at 8% TMB show a significant boron in-diffusionin the i-layer which can be attributed to a more void rich structure of the p-layer. Thus it

ARTICLE IN PRESS

Table 1

Solar cell parameters where p-layer prepared with TMB ¼ 8% at different H-dil, Tf ¼ 16501C

H-dil (%) i-layer (nm) Jsc (mA/cm2) Voc (mV) FF (%) Z (%)

95 400 13.60 834 61 7.2

95.5 800 14.85 847 65 8.2

96 400 12.74 847 (61) (6.6)

98 400 13.17 825 68 7.4

200 400 600 800

TMB2% (after annealing)

TMB8% (after annealing)

TMB8% (after annealing)

TMB8% (no annealing)

Bor

on c

once

ntra

tion

(arb

. uni

ts)

Sputter time (s)

p-layer

Fig. 8. Boron concentration in different pin cells measured by SIMS before and after annealing the solar cell at

150 1C for 1 h.

P. Kumar et al. / Solar Energy Materials & Solar Cells 90 (2006) 3345–3355 3353

may be concluded that the p-layer prepared at 8% TMB at these deposition parametersdoes not bring significant benefit for the solar cell efficiency compared to that p-layerprepared at 2% TMB.

3.2.3. Incorporation of p-doped seed layer

Attempts were made to prepare p-mc-Si:H films with a more amorphous like seed layeron TCO. This seed layer between TCO and bulk p-layer was prepared by reducing thepressure to 1 Pa while keeping the TMB concentration at 2%. The pressure was reducedfor the growth of seed layer for the following reasons:

(i)

Low pressure yielded films with more amorphous like character as confirmed byellipsometry. Ellipsometry was particularly employed to understand the phase of thisvery thin film since this technique is known to be highly sensitive to the surface. In Fig.9, the ellipsometric spectra of about 80 nm thin layers at different H-dil are shownwhich indicate that at low pressure the growth of both p-layers is amorphous.

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(ii)

Fig.

energ

Tabl

Para

Seed

No

Yes

No

Yes

Yes

Low growth rate may lead to formation of a smooth surface structure that serves as abetter substrate for the bulk p-layer growth.

(iii)

The ‘‘soft deposition’’ seed layer protects the TCO for the more hard depositioncondition (more atomic hydrogen) of the bulk p-layer.

The seed p-layer thickness, prepared near the TCO at low pressure, was around 2 nm.The results obtained so far for the best amorphous pin solar cells with seed layer aresummarized in Table 2. Improvement in the Jsc with incorporation of this moreamorphous-like (a-like) seed layer is observed, while Voc and FF decrease slightly. The FFdecrease could be due to higher serial resistivity of the resulting p-layer.

3.2.4. Yield of solar cells at soft conditions

It seems from many experiences we got that the p–i–n structure with ‘‘soft’’ p-layer aremore reproducible than using ‘‘hard’’ conditions (low distance between filament tosubstrate distance and high pressure) for p-layer. All cells with soft p-layer are working

0

4

8

12

16 H-dil =91% H-dil =95%

Pse

udo-

diel

ectr

ic c

onst

ant ε

i

Photon energy(eV)1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

9. Ellipsometric pseudo-dielectric constant of thin films prepared on corning glass as a function of photon

y for thin p-layer prepared at p ¼ 1Pa.

e 2

meters of solar cells where the p-mc-Si:H layer was prepared with and without ‘‘soft deposition’’ seed layer

layer H-dil (Seed layer/

bulk p-layer)

Jsc (mA/cm2) Voc (mV) FF (%) Z (%)

91 11.63 840 68 6.7

91/91 13.95 834 65 7.6

95 12.89 820 69 7.3

95/95 14.02 808 64 7.3

91/95 14.22 810 66 7.6

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compared to about 30% shunted cells with hard p-layer. Also the deviation of fill factor forcells deposited in same run is higher for the hard conditions (75% instead of 72% for‘‘soft’’ conditions). After light soaking procedure situation is even more serious. Only fewcells with p-layer prepared at extreme conditions are working and most of the cells areshunted or show irreversible change. These changes, which may also occur after some timewithout light soaking, have to be attributed to structural changes or boron diffusion.

4. Conclusions

We have investigated the properties of mc-Si: H p-layers as a function of H-dil duringpreparation, TMB doping gas concentration and some other preparation parameters.After preoptimization of the p-layers in direct film measurements, we did the finaloptimization in the solar cell structure. Voc is found to decrease with increasing H-dilduring the p-layer deposition which is attributed to larger band offset between mc-Si:H p-layer and amorphous absorber layer which is in turn related to the crystalline fractionpresent at the p/i interface. For the FF and Jsc, a maximum has been found in the a-Si/mc-Si transition range. At a TMB concentration of 8%, higher Voc is iobtained compared to2% TMB. To improve the conversion efficiency and the reproducibility of the solar cell, amore amorphous-like seed layer was deposited between TCO and p-layer. With this seedlayer, an improvement in the Jsc was observed. Finally, the solar cells with the p-layerprepared at large dfs were found out to be more reproducible compared to the cellsprepared at smaller dfs.

Acknowledgement

The authors are grateful to Jorg Kreutz for his help in carrying out Ramanmeasurements, Dr. Bock, IFOS Kaiserslautern, for carrying out SIMS measurement andBernd Hofferberth for technical assistance. We gratefully acknowledge the support for thiswork from the German Ministry of Economics and Technology (BMWI), and RWESchott Solutions GmbH, Product Center Phototronics, Putzbrunn.

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