Research Article Effect of Hydrogen Content in Intrinsic a-Si:H ...downloads.hindawi.com/journals/ijp/2013/121875.pdfInternationalJournalofPhotoenergy 0 5 10 15 20 25 30 35 40 0 0.1

Hindawi Publishing CorporationInternational Journal of PhotoenergyVolume 2013 Article ID 121875 6 pageshttpdxdoiorg1011552013121875

Research ArticleEffect of Hydrogen Content in Intrinsic a-SiH onPerformances of Heterojunction Solar Cells

Yun-Shao Cho1 Chia-Hsun Hsu2 Shui-Yang Lien3 Dong-Sing Wuu1 and In-Cha Hsieh2

1 Department of Materials Science and Engineering National Chung Hsing University Taichung 402 Taiwan2Graduate Institute of Precision Engineering National Chung Hsing University Taichung 402 Taiwan3Department of Materials Science and Engineering DaYeh University ChungHua 51591 Taiwan

Correspondence should be addressed to Shui-Yang Lien sylmaildyuedutw

Received 14 June 2013 Accepted 9 August 2013

Academic Editor Gaetano Di Marco

Copyright copy 2013 Yun-Shao Cho et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Influences of hydrogen content in intrinsic hydrogenated amorphous silicon (i-a-SiH) on performances of heterojunction (HJ)solar cells are investigated The simulation result shows that in the range of 0ndash18 of the i-layer hydrogen content solar cells withhigher i-layer hydrogen content can have higher degree of dangling bond passivation on single crystalline silicon (c-Si) surfaceIn addition the experimental result shows that HJ solar cells with a low hydrogen content have a poor a-SiHc-Si interface Thedeteriorate interface is assumed to be attributed to (i) voids created by insufficiently passivated c-Si surface dangling bonds (ii) voidsformed by SiH

2clusters and (iii) Si particles caused by gas phase particle formation in silane plasma The proposed assumption is

well supported and explained from the plasma point of view using optical emission spectroscopy

1 Introduction

The hydrogenated amorphous siliconcrystalline silicon (a-SiHc-Si) heterojunction (HJ) solar cell is one of the mostinteresting technological solutions for the photovoltaic mar-ket owing to its excellent performance and also its low pro-cessing costs Some groups have already reported encourag-ing results for silicon HJ cells with a-SiH emitters depositedby plasma-enhanced chemical vapor deposition (PECVD)[1] Recently efficiencies of up to 23 using PECVD havebeen demonstrated [2 3] In HJ solar cells the control ofthe interface between different materials is an importantfactor for high-cell performances Particularly the reductionof carrier recombination at the pn interface is criticalExtensive studies have also demonstrated that the use of avery thin intrinsic (i-) a-SiH buffer layer inserted at theinterface can minimize the interface recombination and thusincrease the efficiency of HJ solar cells [4ndash6]

In i-a-SiH deposition process parameters should becarefully controlled since they determine the film propertiesIt is found that lowering the deposition temperature tendedto raise Si-H

2bond concentration and thus lowering the

dangling bond passivation quality [7] High temperature can

somewhat lead to partial epitaxial Si growth which con-tains more defects than amorphous films [8 9] Moreoverhydrogen dilution is usually adopted for lowering electrontemperature and suppressing poly silane radicals [10 11]A moderate dilution ratio is reported to result in a longereffective lifetime of Si wafer [12] However the effects of i-layer hydrogen content and plasma status on performancesof HJ solar cells are rarely reported

In this study the i-layer hydrogen content effect is inves-tigated by computational simulation Then we practicallyfabricate 156 times 156mm2 large-size HJ solar cells with high-and low-hydrogen content to validate the simulation resultFurthermore the interface quality between i-layer and c-Si inhigh- and low-hydrogen content cases is discussed from theplasma emission spectra point of view

2 Experimental

The two-dimensional commercially available software Sil-vaco technology computer aided design (TCAD) from Sil-vaco Inc was used to solve the Poisson equation coupledwiththe continuity equations of electrons and holes for virtualdevices by dividing the whole structure into finite elements

2 International Journal of Photoenergy

Table 1 TCAD input parameters for n-Sii-Sic-Si HJ solar cellswith various hydrogen content of i-layer

Device parameters ValueDevice area (cm2) 1Front surface TextureSolar cell structure n-Sii-Sic-SiBSFExterior rearreflectance () 90

Light source One sun (AM 15 100mWcm2)Layer parameter a-SiH(n) a-SiH(i) c-Si(p) Al-BSFThickness (120583m) 001 0005 250 5Band gap (eV) 192 17 112 112Dielectric constant 117 117 117 117Electron affinity (eV) 39 39 405 405Electron mobility(cm2Vminus1sminus1) 20 20 1417 200

Hole mobility (cm2Vminus1sminus1) 10 10 470 80Doping concentration(cmminus3) 1 times 10

191 times 10

191 times 10

161 times 10

19

Hydrogen content () mdash 0ndash18 mdash mdash

The physical models that we used were Shockley-Read-Hallrecombination model and concentration-dependent life-times The photogeneration model including a ray tracingalgorithm was used for calculating the transmission andabsorption of incident light in the semiconductor layers Thesolar cells considered here operate under the global standardsolar spectrum (AM15G) illumination with 100mWcm2total incident power density The model structure was (n)a-SiH(i) a-SiH(p) c-Sialuminum back surface field (Al-BSF) For the individual layer and the a-SiHc-Si interfacesthe distributions of defect states are specified Note that for a-SiH-related materials the correct material parameters couldbe varied subject to their preparation conditions Table 1 sum-marizes the main parameters used for calculation includingmeasured and assumed values which can be found elsewhere[13ndash16]The correlation between the i-layer hydrogen contentand performance indicators of solar cells is calculated

For HJ solar cell fabrication the cell structure wasAlindium tin oxide (ITO)(n) a-Si(i) a-Si(p) c-SiAl-BSFThe boron-doped c-Si wafers with thickness of 250 nmresistivity of 1ndash10 Ωcm and surface texturization were usedas a substrate The sample size was 156 times 156mm2 To formthe Al-BSF a 5 120583m Al film was sputtered on the waferbackside followed by annealing at 800∘C for 30 s The depo-sition of the n-type a-SiH was done in a PECVD systemwith the following process parameters plasma frequency of2712MHz power density of 375mWcm2 substrate temper-ature of 200∘C deposition pressure of 90 Pa SiH

4flow rate

of 40 sccm and total gases flow rate (PH3+ SiH

4+ H2) of

125 sccm The i-layers with low- and high-hydrogen contentwere prepared using hydrogen dilution ratios H

2SiH4 of

0 and 8 respectively Detailed parameters for the i-a-SiHlayers are summarised in Table 2 The thicknesses of n-layerand i-layer were 10 nm and 5 nm respectively The ITO with

Table 2 Deposition conditions of low-H and high-H

Parameter ValueLow-H High-H

Power density (mWcm2) 25 25Pressure (Pa) 90 90Electrode-to-substrate distance (mm) 30 30Substrate temperature (∘C) 180 180SiH4 flow rate (sccm) 40 40H2 flow rate (sccm) 0 320Hydrogen content () 6 18

thickness of 90 nm was formed using sputtering to enhancethe lateral conductivity and minimize reflection losses Fori-a-SiH film properties Fourier transform infrared (FTIR)spectroscopy was used to determine hydrogen content in thefilms The hydrogen content (119862H) was calculated from theintegrated intensity of the Si-H rockingwagging mode at640 cmminus1 in the FTIR spectra using the following formula[17]

119862H =119860120596

119873Siint

120596119903119908

120572 (120596)

120596

119889120596 (1)

where 120572(120596) is the absorption coefficient of the film at thewavenumber 120596 120596

119903119908stands for the rockingwagging bands

around640 cmminus1119860120596= 16 times 10

19 cmminus2 is the proportionalityconstant and 119873Si = 5 times 10

22 cmminus3 is the atomic density ofpure silicon The film thickness was evaluated by an alpha-step profiler (Tencor alpha-step 500) For HJ solar cellsJ-V characteristics were measured under AM15G (1 sun100mWcm2) solar simulator irradiation External quantumefficiencies (EQE) were measured using a lock-in amplifierwith a current preamplifier under short-circuit conditionsThe devices were illuminated by monochromatic light froma 150W xenon lamp passing through a monochromator Acalibrated single crystalline silicon photodiode with knownspectral response was used as a referenceThe cross-sectionalmicrostructure image was observed by high-resolution trans-mission electron microscopy (HRTEM) For plasma char-acterization optical emission spectroscopy (OES) [18 19]equipped with a spectra colorimeter was used to collectoptical emission spectra of the plasma glowedThemeasuringpoint was near the substrate surface

3 Results and Discussion

Figure 1 schematically shows themechanism of passivation ofdangling bonds at c-Si surface Without surface passivationthe dangling bonds act as recombination centers to trap elec-trons resulting in a large recombination rate at the a-SiHc-Si interface of final HJ solar cells These dangling bonds canbe passivated by H atoms to obtain a relatively high-qualityheterojunctionThis in turn implies that H content in the i-a-SiH layer plays a crucial role on the c-Si surface passivationfor HJ solar cells In this study the H content in the i-layer iscontrolled by adjusting the input hydrogen gas flow rate andtheir relationship is shown in Figure 1(b) It can be seen that

International Journal of Photoenergy 3

Dangling bondCatch electron

No passivation Defect passivation

c-Si

Reduced interfacerecombination

Interfacerecombination

i-layer

Si H

(a)

Hydrogen gas flow rate (sccm)

Hyd

roge

n co

nten

t (

)

0

2

4

6

8

10

12

14

16

18

20

0 80 160 240 320 400 480

(b)

Figure 1 (a) Scheme of the mechanism of surface passivation onc-Si (b) The relationship between hydrogen flow rate and i-layerhydrogen content

the hydrogen content increases with increasing the hydrogengas flow rate due to more H atoms incorporated in the i-layerduring the deposition process However theH content finallysaturates to a value of about 18 at the hydrogen flow rate of320 sccm According to this result in TCAD simulation theH content is set to be in the range of 0ndash18 to investigate itseffect on the device performances

Figure 2 shows the TCAD simulation result of the deviceexternal parameters such as open-circuit voltage (119881oc) short-circuit current density (119869sc) fill factor (FF) and conversionefficiency (120578) as a function of the H content in the i-layerIt can be seen that 119881oc show a strong dependence on the i-layer H content The pure i-a-Si (H content = 0) leads to119881oc of 055V while the i-a-SiH with H content of 6ndash12results in 119881oc around 06V Increasing the H content up to18 further improves the 119881oc to 072V The better a-SiHc-Si interface passivation with an increase in i-layer H contentcould decrease the interface defect (dangling bond) densityand recombination for carriers that overcome the a-SiHc-Si barrier and thus could be the reason for the increase in119881oc with an increase in i-layer H content [20] 119869sc and FF also

05055

06065

07075

30

32

34

36

38

FF068

07

072

074

076

10

14

18

22120578

()

(V)

(mA

cm

2)

i-layer hydrogen content ()0 3 6 9 12 15 18

0 3 6 9 12 15 18

0 3 6 9 12 15 18

0 3 6 9 12 15 18

J sc

Voc

Figure 2 Simulation results of the performances of HJ solar cells asa function of the i-layer hydrogen content

show a similar tendency to that of119881oc Consequently 120578 can besignificantly improved from about 117 to 197 when the Hcontent increases from0 to 18 It is worth pointing out thathigh-H content i-layer leads to high-cell performances whichagrees well with the proposed mechanism of passivationmentioned above

To validate the simulation result experimental tests werecarried out The i-layers with low-H content of 6 and high-H content of 18 were selected for device fabrication Foreach type of i-layer 10 HJ solar cells were fabricated andtheir performances are quite close to each other with an errorof less than 3 Figure 3(a) illustrates the J-V characteristicsof the two types of HJ solar cells It is observed that thetendency of the experimental result is in good agreementwiththe simulation data The high-H cell has better conversionefficiency indicating an improved surface passivation Ascompared to the low-H cell 119881oc 119869sc and FF are improved by121 93 and 94 respectively This consequently leadsto an increase in 120578 by about 342 Note that FF values areslightly lower than the corresponding simulation values sincethe resistances of Al ITO and their interface are not takeninto account in the simulation work


0

5

10

15

20

25

30

35

40

0 01 02 03 04 05 06 07

Curr

ent d

ensit

y (m

Ac

m2)

Voltage (V)

Low-HHigh-H

Low-H High-HPerformance

0615 069333 364064 070131 1758

(V)(mAcm2)

FF120578 ()

Voc

Jsc

(a)

0

01

02

03

04

05

06

07

08

09

1

300 500 700 900 1100

EQE

()

Wavelength (nm)

Low-HHigh-H

(b)

Figure 3 Experimental results of (a) J-V characteristics and (b) EQE spectra response for HJ solar cells with different i-layer hydrogencontent

ITO

ITOn-a-SiH

interface

i-a-SiHc-Si interfacec-Si

5 nm

a-SiH

(a)

ITO

ITOn-a-SiH interface

i-a-SiHc-Si interface

c-Si

5 nm

a-SiH

(b)

Figure 4 HRTEM images of the i-a-SiHp c-Si interface for (a) low-H and (b) high-H HJ solar cells

The 119869sc of the fabricated low- and high-H solar cells canbe investigated byEQEmeasurement as shown inFigure 3(b)Clearly the EQE response can be divided into twowavelengthregions First in the wavelength range of 300 to 700 nm EQEvalues of the high-H cell are always higher than these of thelow-H cell Since blue light is absorbed close to the surfacethe higher blue portion of EQE reflects a lower surfacerecombination and thus reflect better collection efficiencyof minority carriers Note that although the absorption ofthe two a-SiH layers possibly may not be the same the

absorption loss due to very thin a-SiH is trivial and is not oneof the factors causing the observed EQE difference Secondin the wavelength range of 700 to 1100 nm the EQE valuesof the two cells become similar since the light absorptiondepth in this wavelength range corresponds to deeper regionin c-Si and thus irrelevant to the junction properties TheEQE at the wavelength of 1100 nm [21] close to the cut-off wavelength is still over 20 owing to contributions ofsurface texturing and back surface field [22] In summarythe EQE result indicates that the higher 119869sc of the high-H


200

400

600

800

1000

1200

200 300 400 500 600 700 800Wavelength (nm)

Inte

nsity

(a u

)

Low-H

High-H

SiHlowast(414)

Silowast(252)

Hlowast(656)

Hlowast

(722

)

Hlowast

(772

)

Figure 5 Optical emission spectra for the plasmas during low- andhigh-H i-layer deposition

cells as observed in Figure 3(a) is a consequence of themuchhigher spectral response in the short wavelengths due tobetter surface passivation

The cross-sectional HRTEM images of a-SiHc-Si inter-face in low- and high-H cells are shown in Figures 4(a) and4(b) respectively The a-SiH layer (including i- and n-layer)thickness of about 15 nm can also be evidenced by the TEMimages Interestingly the low-H interface is deterioratedwhereas the interface region in the high-H case is quitesmooth The poor interface region can be assumed to mainlyresult from two factors First it is likely that the interfaceregion contains voids or vacancies related to unpassivateddangling bonds on c-Si surface illustrated in Figure 1(a)or from SiH

2bonds clustered in the i-layer [23] Second

it seems that particles occur around the interface and thisphenomenon can be linked to plasma gas particle formationto c-Si substrate interaction The assumptions are furtherdiscussed and explained below using the OES spectra fromthe plasma point of view

Figure 5 compares the OES spectra between plasmasduring i-layer deposition of low- and high-H solar cells Thepeaks shown in the spectra result from several collision ofenergetic electrons with gas molecules involving the forma-tion of neutral (SiH

3 SiH2 SiH Si) emissive (Silowast SiHlowast) and

ionic (SiH +119909) species as given by [24 25]

SiH4+ e

997888rarr SiH3+H + e (40 eV 120591 gt 20ms)

997888rarr SiH2+H2+ e (22 eV 120591 lt 3ms)

997888rarr SiH +H2+H + e (57 eV 120591 lt 3ms)

997888rarr Si + 2H2+ e (42 eV 120591 lt 3ms)

(2)

SiH2997888rarr Silowast +H

2 (3)

Silowast 997888rarr Si + ℎ] (49ndash51 eV) (4)

SiH3997888rarr SiHlowast +H

2 (5)

SiHlowast 997888rarr SiH + ℎ] (3 eV) (6)

where e is the electron h] is the released photon energyand 120591 is the life time of the Si-based radicals Among thevarious radicals SiH

2-related reactions described in (3) and

(4) finally release a photon energy from Silowast radiative decaycorresponding to a wavelength of 252 nm Interestingly thisreleased photon is detected by OES only in the plasma for thelow-H cell while this peak vanishes for the high-H cellThusthe plasma in low-H i-layer deposition contains amuch largeramount of SiH

2 finally leading to void-rich and low-quality

i-layer [26 27] In addition another effect accompanied withhigh SiH

2amount is the increased generation rate of poly

silane (gas particle formation) Si2H6 according to [28]

SiH2+ SiH

4997888rarr Si

2H6 (7)

The resultant Si2H6species will interfere with film deposition

and thus lead to a poor interface Therefore the OES spectraconfirm the assumptions as mentioned in Figure 4 On thecontrary the Si-related peak found in the spectra of the high-H cell is only SiHlowast at 414 nm (see (5) and (6)) and thus theincreased proportion of SiH

3is beneficial to the film growth

with high qualities [29] leading to a smooth interface shownin Figure 4(b)

4 Conclusions

In this study the effect of a-SiH i-layer hydrogen contentof 0ndash18 on performances of HJ solar cells is investigatedBoth of the experimental and simulation results indicatethat the i-layer with high-hydrogen content can lead to ahigher degree of dangling bond passivation on c-Si surfaceCompared to low-H i-layer HJ solar cells with a high-H i-layer is found to have the i-layerc-Si interface with reducedsurface recombination and increased collection efficiency forblue light generated carriers In addition to surface danglingbond passivation the low-H solar cell exhibits a deterioratei-layerc-Si interface The OES spectra clearly show thepresence of Silowast peak only found in the plasma of low-H i-layerdeposition suggesting that the large SiH

2portion may finally

create voids and Si particles and thus decrease the i-layerc-Siinterface quality

Acknowledgment

This work was supported by the National Science Coun-cil (102-2622-E-451-002-CC3 102-2622-E-451-001-CC2 and100-2628-E-451-002-MY2)

References

[1] Y Tsunomura Y Yoshimine M Taguchi et al ldquoTwenty-twopercent efficiency HIT solar cellrdquo Solar Energy Materials andSolar Cells vol 93 no 6-7 pp 670ndash673 2009


[2] T Mishima M Taguchi H Sakata and E Maruyama ldquoDevel-opment status of high-efficiency HIT solar cellsrdquo Solar EnergyMaterials and Solar Cells vol 95 no 1 pp 18ndash21 2011

[3] T Kinoshita D Fujishima A Yano et al ldquoThe approachesfor high efficiency HITTM solar cell with very thin (lt100120583m)silicon wafer over 23rdquo in Proceedings of the 26th EuropeanPhotovoltaic Solar Energy Conference and Exhibition pp 871ndash874 Hamburg Germany 2011

[4] K V Maydell H Windgassen W A Nositschka et al ldquoBasicelectronic properties and technology of TCOa-SiH(n)c-Si(p)heterostructure solar cells a german network projectrdquo inProceedings of the 20th European Photovoltaic Solar EnergyConference and Exhibition p 822 Barcelona Spain 2005

[5] H Fujiwara and M Kondo ldquoEffects of a-SiH layer thicknesseson the performance of a-SiHc-Si heterojunction solar cellsrdquoJournal of Applied Physics vol 101 Article ID 054516 2007

[6] T HWang E Iwaniczko M R Page et al ldquoEffective interfacesin silicon heterojunction solar cellsrdquo in Proceedings of the 31stIEEE Photovoltaic Specialists Conference and Exhibition pp955ndash958 Orlando Fla USA January 2005

[7] V A Dao N V Duy J Heo et al ldquoHydrogenated amorphoussilicon layer formation by inductively coupled plasma chemicalvapor deposition and its application for surface passivation ofp-type crystalline siliconrdquo Japanese Journal of Applied Physicsvol 48 no 6 Article ID 066509 2009

[8] N Dwivedi S Kumar A Bisht K Patel and S SudhakarldquoSimulation approach for optimization of device structure andthickness of HIT solar cells to achieve 27 efficiencyrdquo SolarEnergy vol 88 pp 31ndash41 2013

[9] N Hernandez-Como A Morales-Acevedo and Y MatsumotoldquoI-V characteristics of a-Si-c-Si hetero-junction diodes made byhot wire CVDrdquo Solar Energy Materials and Solar Cells vol 95no 8 pp 1996ndash2000 2011

[10] A Matsuda M Takai T Nishimoto and M Kondo ldquoControlof plasma chemistry for preparing highly stabilized amorphoussilicon at high growth raterdquo Solar Energy Materials and SolarCells vol 78 no 1ndash4 pp 3ndash26 2003

[11] W M M Kessels R J Severens A H M Smets et al ldquoHydro-genated amorphous silicon deposited at very high growthrates by an expanding ArndashH

2ndashSiH4plasmardquo Journal of Applied

Physics vol 89 no 4 pp 2404ndash2413 2001[12] J Gea Z P Linga J Wonga T Muellera and A G Aberlea

ldquoOptimisation of intrinsic a-SiH passivation layers in crystal-line-amorphous silicon heterojunction solar cellsrdquo Energy Pro-cedia vol 15 pp 107ndash117 2012

[13] L Zhao H L Li C L Zhou H W Diao and W J WangldquoOptimized resistivity of p-type Si substrate for HIT solarcell with Al back surface field by computer simulationrdquo SolarEnergy vol 83 no 6 pp 812ndash816 2009

[14] ADattaM RahmouniMNath R Boubekri P R Cabarrocasand P Chatterjee ldquoInsights gained from computer modeling ofheterojunction with instrinsic thin layer ldquohITrdquo solar cellsrdquo SolarEnergy Materials and Solar Cells vol 94 no 9 pp 1457ndash14622010

[15] L Zhao C L Zhou H L Li H W Diao and W J WangldquoDesign optimization of bifacial HIT solar cells on p-typesilicon substrates by simulationrdquo Solar Energy Materials andSolar Cells vol 92 no 6 pp 673ndash681 2008

[16] V A Dao J Heo H Choi et al ldquoSimulation and study of theinfluence of the buffer intrinsic layer back-surface field densi-ties of interface defects resistivity of p-type silicon substrate and

transparent conductive oxide on heterojunction with intrinsicthin-layer (HIT) solar cellrdquo Solar Energy vol 84 no 5 pp 777ndash783 2010

[17] J Gopea S Kumar S Sudhakar K Lodhi C M S Rauthanaand P C Srivastava ldquoInfluence of argon dilution on the growthof amorphous to ultra nanocrystalline silicon films using VHFPECVD processrdquo Journal of Alloys and Compounds vol 577 pp710ndash716 2013

[18] A A Howling B Strahm P Colsters L Sansonnens and CHollenstein ldquoFast equilibration of silanehydrogen plasmas inlarge area RF capacitive reactors monitored by optical emissionspectroscopyrdquo Plasma Sources Science and Technology vol 16no 4 pp 679ndash696 2007

[19] A A Howling B Strahm and C Hollenstein ldquoNon-intrusiveplasma diagnostics for the deposition of large area thin filmsiliconrdquoThin Solid Films vol 517 no 23 pp 6218ndash6224 2009

[20] N Hernandez-Como and A Morales-Acevedo ldquoSimulation ofhetero-junction silicon solar cells with AMPS-1Drdquo Solar EnergyMaterials and Solar Cells vol 94 no 1 pp 62ndash67 2010

[21] M Schaper J Schmidt H Plagwitz and R Brendel ldquo201-efficient crystalline silicon solar cell with amorphous siliconrear-surface passivationrdquoProgress in Photovoltaics Research andApplications vol 13 no 5 pp 381ndash386 2005


[23] D-S Wuu S-Y Lien H-Y Mao et al ldquoGrowth and characteri-zation of polycrystalline Si films prepared by hot-wire chemicalvapor depositionrdquo Thin Solid Films vol 498 no 1-2 pp 9ndash132006

[24] J C Knights ldquoCharacterization of plasma-deposited amor-phous Si H thin filmsrdquo Japanese Journal of Applied Physics vol18 pp 101ndash108 1979

[25] F J Kampas ldquoReactions of atomic hydrogen in the deposition ofhydrogenated amorphous silicon by glowdischarge and reactivesputteringrdquo Journal of Applied Physics vol 53 no 9 pp 6408ndash6412 1982

[26] H-Y Kim K-Y Lee and J-Y Lee ldquoThe influence of hydrogendilution ratio on the crystallization of hydrogenated amorphoussilicon films prepared by plasma-enhanced chemical vapordepositionrdquoThin Solid Films vol 302 no 1-2 pp 17ndash24 1997

[27] M Jeon S Yoshiba and K Kamisako ldquoHydrogenated amor-phous silicon film as intrinsic passivation layer deposited atvarious temperatures using RF remote-PECVD techniquerdquoCurrent Applied Physics vol 10 no 2 pp S237ndashS240 2010

[28] A Matsuda ldquoControl of plasma and surface conditions for lowdefect density a-SiH at high growth ratesrdquo in Proceedings ofthe 25th IEEE Photovoltaic Specialists Conference pp 1029ndash1034May 1996

[29] W M M Kessels J P M Hoefnagels P J van den Oever YBarrell andM CM van de Sanden ldquoTemperature dependenceof the surface reactivity of SiH

3radicals and the surface

silicon hydride composition during amorphous silicon growthrdquoSurface Science vol 547 no 3 pp L865ndashL870 2003

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Inorganic ChemistryInternational Journal of

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal ofPhotoenergy


Carbohydrate Chemistry

International Journal of


Journal of

Chemistry


Advances in

Physical Chemistry

Hindawi Publishing Corporationhttpwwwhindawicom

Analytical Methods in Chemistry

Journal of

Volume 2014

Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

SpectroscopyInternational Journal of


The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Medicinal ChemistryInternational Journal of


Chromatography Research International


Applied ChemistryJournal of



Theoretical ChemistryJournal of


Journal of

Spectroscopy

Analytical ChemistryInternational Journal of


Journal of


Quantum Chemistry


Organic Chemistry International

ElectrochemistryInternational Journal of



CatalystsJournal of


Table 1 TCAD input parameters for n-Sii-Sic-Si HJ solar cellswith various hydrogen content of i-layer

Device parameters ValueDevice area (cm2) 1Front surface TextureSolar cell structure n-Sii-Sic-SiBSFExterior rearreflectance () 90

Light source One sun (AM 15 100mWcm2)Layer parameter a-SiH(n) a-SiH(i) c-Si(p) Al-BSFThickness (120583m) 001 0005 250 5Band gap (eV) 192 17 112 112Dielectric constant 117 117 117 117Electron affinity (eV) 39 39 405 405Electron mobility(cm2Vminus1sminus1) 20 20 1417 200

Hole mobility (cm2Vminus1sminus1) 10 10 470 80Doping concentration(cmminus3) 1 times 10

191 times 10

191 times 10

161 times 10

19

Hydrogen content () mdash 0ndash18 mdash mdash

The physical models that we used were Shockley-Read-Hallrecombination model and concentration-dependent life-times The photogeneration model including a ray tracingalgorithm was used for calculating the transmission andabsorption of incident light in the semiconductor layers Thesolar cells considered here operate under the global standardsolar spectrum (AM15G) illumination with 100mWcm2total incident power density The model structure was (n)a-SiH(i) a-SiH(p) c-Sialuminum back surface field (Al-BSF) For the individual layer and the a-SiHc-Si interfacesthe distributions of defect states are specified Note that for a-SiH-related materials the correct material parameters couldbe varied subject to their preparation conditions Table 1 sum-marizes the main parameters used for calculation includingmeasured and assumed values which can be found elsewhere[13ndash16]The correlation between the i-layer hydrogen contentand performance indicators of solar cells is calculated

For HJ solar cell fabrication the cell structure wasAlindium tin oxide (ITO)(n) a-Si(i) a-Si(p) c-SiAl-BSFThe boron-doped c-Si wafers with thickness of 250 nmresistivity of 1ndash10 Ωcm and surface texturization were usedas a substrate The sample size was 156 times 156mm2 To formthe Al-BSF a 5 120583m Al film was sputtered on the waferbackside followed by annealing at 800∘C for 30 s The depo-sition of the n-type a-SiH was done in a PECVD systemwith the following process parameters plasma frequency of2712MHz power density of 375mWcm2 substrate temper-ature of 200∘C deposition pressure of 90 Pa SiH

4flow rate

of 40 sccm and total gases flow rate (PH3+ SiH

4+ H2) of

125 sccm The i-layers with low- and high-hydrogen contentwere prepared using hydrogen dilution ratios H

2SiH4 of

0 and 8 respectively Detailed parameters for the i-a-SiHlayers are summarised in Table 2 The thicknesses of n-layerand i-layer were 10 nm and 5 nm respectively The ITO with

Table 2 Deposition conditions of low-H and high-H

Parameter ValueLow-H High-H

Power density (mWcm2) 25 25Pressure (Pa) 90 90Electrode-to-substrate distance (mm) 30 30Substrate temperature (∘C) 180 180SiH4 flow rate (sccm) 40 40H2 flow rate (sccm) 0 320Hydrogen content () 6 18

thickness of 90 nm was formed using sputtering to enhancethe lateral conductivity and minimize reflection losses Fori-a-SiH film properties Fourier transform infrared (FTIR)spectroscopy was used to determine hydrogen content in thefilms The hydrogen content (119862H) was calculated from theintegrated intensity of the Si-H rockingwagging mode at640 cmminus1 in the FTIR spectra using the following formula[17]

119862H =119860120596

119873Siint

120596119903119908

120572 (120596)

120596

119889120596 (1)

where 120572(120596) is the absorption coefficient of the film at thewavenumber 120596 120596

119903119908stands for the rockingwagging bands

around640 cmminus1119860120596= 16 times 10

19 cmminus2 is the proportionalityconstant and 119873Si = 5 times 10

22 cmminus3 is the atomic density ofpure silicon The film thickness was evaluated by an alpha-step profiler (Tencor alpha-step 500) For HJ solar cellsJ-V characteristics were measured under AM15G (1 sun100mWcm2) solar simulator irradiation External quantumefficiencies (EQE) were measured using a lock-in amplifierwith a current preamplifier under short-circuit conditionsThe devices were illuminated by monochromatic light froma 150W xenon lamp passing through a monochromator Acalibrated single crystalline silicon photodiode with knownspectral response was used as a referenceThe cross-sectionalmicrostructure image was observed by high-resolution trans-mission electron microscopy (HRTEM) For plasma char-acterization optical emission spectroscopy (OES) [18 19]equipped with a spectra colorimeter was used to collectoptical emission spectra of the plasma glowedThemeasuringpoint was near the substrate surface

3 Results and Discussion

Figure 1 schematically shows themechanism of passivation ofdangling bonds at c-Si surface Without surface passivationthe dangling bonds act as recombination centers to trap elec-trons resulting in a large recombination rate at the a-SiHc-Si interface of final HJ solar cells These dangling bonds canbe passivated by H atoms to obtain a relatively high-qualityheterojunctionThis in turn implies that H content in the i-a-SiH layer plays a crucial role on the c-Si surface passivationfor HJ solar cells In this study the H content in the i-layer iscontrolled by adjusting the input hydrogen gas flow rate andtheir relationship is shown in Figure 1(b) It can be seen that




c-Si



i-layer

Si H

(a)


Hyd

roge

n co

nten

t (

)

0

2

4

6

8

10

12

14

16

18

20

0 80 160 240 320 400 480

(b)




05055

06065

07075

30

32

34

36

38

FF068

07

072

074

076

10

14

18

22120578

()

(V)

(mA

cm

2)


0 3 6 9 12 15 18

0 3 6 9 12 15 18

0 3 6 9 12 15 18

J sc

Voc





0

5

10

15

20

25

30

35

40

0 01 02 03 04 05 06 07

Curr

ent d

ensit

y (m

Ac

m2)

Voltage (V)

Low-HHigh-H


0615 069333 364064 070131 1758

(V)(mAcm2)

FF120578 ()

Voc

Jsc

(a)

0

01

02

03

04

05

06

07

08

09

1

300 500 700 900 1100

EQE

()

Wavelength (nm)

Low-HHigh-H

(b)


ITO

ITOn-a-SiH

interface


5 nm

a-SiH

(a)

ITO



c-Si

5 nm

a-SiH

(b)





200

400

600

800

1000

1200

200 300 400 500 600 700 800Wavelength (nm)

Inte

nsity

(a u

)

Low-H

High-H

SiHlowast(414)

Silowast(252)

Hlowast(656)

Hlowast

(722

)

Hlowast

(772

)









SiH4+ e




997888rarr Si + 2H2+ e (42 eV 120591 lt 3ms)

(2)


2 (3)



2 (5)









SiH2+ SiH

4997888rarr Si

2H6 (7)





4 Conclusions




Acknowledgment


References













































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of




c-Si



i-layer

Si H

(a)


Hyd

roge

n co

nten

t (

)

0

2

4

6

8

10

12

14

16

18

20

0 80 160 240 320 400 480

(b)




05055

06065

07075

30

32

34

36

38

FF068

07

072

074

076

10

14

18

22120578

()

(V)

(mA

cm

2)


0 3 6 9 12 15 18

0 3 6 9 12 15 18

0 3 6 9 12 15 18

J sc

Voc





0

5

10

15

20

25

30

35

40

0 01 02 03 04 05 06 07

Curr

ent d

ensit

y (m

Ac

m2)

Voltage (V)

Low-HHigh-H


0615 069333 364064 070131 1758

(V)(mAcm2)

FF120578 ()

Voc

Jsc

(a)

0

01

02

03

04

05

06

07

08

09

1

300 500 700 900 1100

EQE

()

Wavelength (nm)

Low-HHigh-H

(b)


ITO

ITOn-a-SiH

interface


5 nm

a-SiH

(a)

ITO



c-Si

5 nm

a-SiH

(b)





200

400

600

800

1000

1200

200 300 400 500 600 700 800Wavelength (nm)

Inte

nsity

(a u

)

Low-H

High-H

SiHlowast(414)

Silowast(252)

Hlowast(656)

Hlowast

(722

)

Hlowast

(772

)









SiH4+ e




997888rarr Si + 2H2+ e (42 eV 120591 lt 3ms)

(2)


2 (3)



2 (5)









SiH2+ SiH

4997888rarr Si

2H6 (7)





4 Conclusions




Acknowledgment


References













































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


0

5

10

15

20

25

30

35

40

0 01 02 03 04 05 06 07

Curr

ent d

ensit

y (m

Ac

m2)

Voltage (V)

Low-HHigh-H


0615 069333 364064 070131 1758

(V)(mAcm2)

FF120578 ()

Voc

Jsc

(a)

0

01

02

03

04

05

06

07

08

09

1

300 500 700 900 1100

EQE

()

Wavelength (nm)

Low-HHigh-H

(b)


ITO

ITOn-a-SiH

interface


5 nm

a-SiH

(a)

ITO



c-Si

5 nm

a-SiH

(b)





200

400

600

800

1000

1200

200 300 400 500 600 700 800Wavelength (nm)

Inte

nsity

(a u

)

Low-H

High-H

SiHlowast(414)

Silowast(252)

Hlowast(656)

Hlowast

(722

)

Hlowast

(772

)









SiH4+ e




997888rarr Si + 2H2+ e (42 eV 120591 lt 3ms)

(2)


2 (3)



2 (5)









SiH2+ SiH

4997888rarr Si

2H6 (7)





4 Conclusions




Acknowledgment


References













































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


200

400

600

800

1000

1200

200 300 400 500 600 700 800Wavelength (nm)

Inte

nsity

(a u

)

Low-H

High-H

SiHlowast(414)

Silowast(252)

Hlowast(656)

Hlowast

(722

)

Hlowast

(772

)









SiH4+ e




997888rarr Si + 2H2+ e (42 eV 120591 lt 3ms)

(2)


2 (3)



2 (5)









SiH2+ SiH

4997888rarr Si

2H6 (7)





4 Conclusions




Acknowledgment


References













































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of












































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of










Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of

Documents

Research Article Effect of Hydrogen Content in Intrinsic a-Si:H ...downloads.hindawi.com/journals/ijp/2013/121875.pdfInternationalJournalofPhotoenergy 0 5 10 15 20 25 30 35 40 0 0.1