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Silicon_Nitride_Film_by_Inline_PECVD_for_Black_Silicon_Solar_Cells

Hindawi Publishing Corporation

International Journal of Photoenergy

Volume2012,Article ID971093,5pages

doi:10.1155/2012/971093

Research Article

Silicon Nitride Film by Inline PECVD for Black Silicon Solar Cells

Bangwu Liu,Sihua Zhong,Jinhu Liu,Yang Xia,and Chaobo Li

Key Laboratory of Microelectronics Devices&Integrated Technology,Institute of Microelectronics,Chinese Academy of Sciences,

Beijing100029,China

Correspondence should be addressed to Bangwu Liu,liubangwu@https://www.wendangku.net/doc/a512319326.html,

Received9May2012;Revised13July2012;Accepted15July2012

Academic Editor:F.Yakuphanoglu

Copyright?2012Bangwu Liu et al.This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use,distribution,and reproduction in any medium,provided the original work is properly cited.

The passivation process is of signi?cant importance to produce high-e?ciency black silicon solar cell due to its unique microstructure.The black silicon has been produced by plasma immersion ion implantation(PIII)process.And the Silicon nitride?lms were deposited by inline plasma-enhanced chemical vapor deposition(PECVD)to be used as the passivation layer for black silicon solar cell.The microstructure and physical properties of silicon nitride?lms were characterized by scanning electron microscope(SEM),Fourier transform infrared spectroscopy(FTIR),spectroscopic ellipsometry,and the microwave photoconductance decay(μ-PCD)method.With optimizing the PECVD parameters,the conversion e?ciency of black silicon solar cell can reach as high as16.25%.

1.Introduction

Black silicon is an e?ective method to reduce the surface re?ectivity for optoelectronic devices and solar cells appli-cation.Many black silicon methods have been developed, including reactive ion etching[1],metal-assisted chemical etching[2],and irradiating the silicon surface with femtosec-ond laser pulses[3].In our previous study[4,5],plasma immersion ion implantation(PIII)process has been put forward to produce black silicon with advantages of low cost and high throughput.During the PIII process,the reactive ions are injected into the silicon lattice and react with silicon, and then black silicon with various microstructures can be formed.Although the black silicon has very low surface re?ectivity,the conversion e?ciency can not be improved signi?cantly when the black silicon is used for solar cells application.In order to produce high-e?ciency black silicon solar cell,the matching process(e.g.,Phosphorus di?usion process,PECVD SiN x?lm process,and co?ring process) should be improved.Due to the unique microstructure of the black silicon,the passivation process is of signi?cant importance to produce high-e?ciency solar cell.PECVD SiN x?lm is widely used in photovoltaic industry,which cannot only be used as antire?ective coating(ARC)but also provide surface passivation e?ect and excellent bulk passivation for multicrystalline silicon solar cell due to a

large amount of hydrogen originating from plasma gas

dissociation and incorporated in the SiN x?lm[6].

In the present study,PECVD SiN x?lm is used to

passivate the black silicon for solar cells.A detailed study

on the physical properties of the as-grown SiN x?lms as

functions of the PECVD parameters will be carried out.The

passivation e?ects of the SiN x?lm on the black silicon solar

cell will also be discussed.

2.Experimental Details

The material used for experiments was commercially avail-

able boron-doped p-type multicrystalline silicon wafers

obtained from the ingot by wire sawing of thickness ～200μm,area156mm×156mm,and resistivity1～3Ωcm. Damage on the surface induced by wire cutting was removed

by etching in10%NaOH solution at80?C.The black silicon

was prepared by plasma immersion ion implantation process

on home-made equipment and subsequently subjected to

acid etching in2%HCl and then in10%HF to remove

the contamination and oxides.After that,the black silicon

wafers were phosphorus doped using phosphorous oxychlo-

ride(POCl3)as the dopant source and then subjected to

edge etching through reactive ion etching and removing

1 μm

(a)

100 nm

(b)

Figure1:The microstructure of the black silicon produced by PIII (a)top view and(b)cross-sectional view of the black silicon. phosphosilicate glass(PSG)layer with diluted HF(10%by volume).Silicon nitride?lm for passivation was grown by PECVD process with di?erent gas?ow ratios of NH3/SiH4, deposition time and deposition temperatures.Finally,the front and back metallization of all the black silicon wafers was carried out by screen-printing technique and followed by baking and co?ring in a conveyer belt furnace.

The structure and physical properties of silicon nitride were characterized by scanning electron microscope(SEM), Fourier transform infrared spectroscopy(FTIR),spectro-scopic ellipsometry,and the microwave photoconductance decay(μ-PCD)method.The surface re?ectance was exam-ined by a UV-VIS-NIR spectrophotometer(Varian Cary 5000)equipped with an integrating sphere detector in the wavelength from300to1100nm.I-V characterization was used to evaluate the black silicon solar cell under one sun global solar spectrum of AM1.5at25?C.

3.Results and Discussion

3.1.The Microstructure of Black Silicon.The microstructure of the black silicon produced by PIII is shown in Figure1. The surface of the black silicon appears to be porous sponge like microstructure.The evaluated dimension of randomly distributed structures is less than1μm(varied from100nm to300nm).Due to such morphology,a reduced e?ective

20040060080010001200

Wavelength (nm)

(

)

PIII texture wafer

Planar wafer

Figure2:The re?ectance of the PIII textured wafer and planar wafer varying with wavelength from300nm to1100

nm.

2.3

2.25

2.2

2.15

2.1

2.05

0.75

0.7

0.65

0.6

0.55

Ratio of SiH4: NH3

Figure3:The refractive index and stoichiometry of SiN x varying with the ratio of SiH4:NH3.

re?ectance value about7%in the wavelength from300nm～1100nm is obtained,presented in Figure2.

3.2.Gas Flow Ratio.The NH3/SiH4gas?ow ratio is the most important factor a?ecting the properties of the silicon nitride(SiN x)?lm.Keeping other parameters(deposition temperature,deposition time,plasma power,etc.)constant, the NH3/SiH4gas?ow ratio varied from5to9.The refractive index of the SiN x?lm as a function of the NH3/SiH4gas?ow ratio is shown in Figure3.It can be seen that the refractive index of SiN x?lm increases with decreasing the gas?ow ratio of NH3/SiH4,indicating that increasing Si content will increase the refractive index.There is an empirical equation to link SiN x stoichiometry with refractive index(n):[7]

n=1.22+0.61?

,(1)

1μm

(a)

1μm

(b)

Figure 4:The microstructure of the as-deposited SiN x ?lm (a)NH 3/SiH 4ratio =9and (b)NH 3/SiH 4ratio =

1615.915.815.715.615.5

0.614

0.612

0.61

0.608

The NH 3/SiH 4gas ?ow ratio

O p e n -c i r c u i t v o l t a g e (m V )

C o n v e r s i o n e ?i c i e n c y (%)

Figure 5:The conversion e ?ciency and open-circuit voltage of the black silicon solar cell varying with the NH 3/SiH 4gas ?ow ratio.

where n is the refractive index,and x is [N]/[Si]ratio.From Figure 3,the stoichiometries ranging from 0.59to 0.73for the as-deposited SiN x ?lm vary with the NH 3/SiH 4gas ?ow ratio varying from 5to 9.Figure 4shows the microstructure of the deposited SiN x ?lm.One can obviously see that the ?lm becomes denser when the NH 3/SiH 4gas ?ow ratio increases.As the NH 3/SiH 4gas ?ow ratio increase,the Si content decreases,consequently,the growth rate of the ?lm becomes slow,resulting in a much denser ?lm.

16015014013012011010090807060

2000

1800

1600

140012001000800600

400

425?C

450?C 475?C T r a n s m i s s i o n (a .u .)

Wavenumbers (cm ?1)

Figure 6:FTIR transmission spectra of SiN x ?lms deposited at 425?C,450?C,and 475?C,

respectively.

1211

1098

7616

15.9

15.815.7

15.6Deposition temperature (?C)

C o n v e r s i o n e ?i c i e n c y (%)

M i n o r i t y c a r r i e r l i f e t i m e (μs )

Figure 7:The minority carrier lifetime and conversion e ?ciency versus deposition temperature.

Table 1:The in?uence of deposition time on the properties of black silicon solar cells.V oc (V)I sc (A)R s (m Ω)E ?(%)3000.6058.01 3.1915.235000.6118.25 3.2115.91700

0.613

8.22

4.62

15.66

Figure 5shows the conversion e ?ciency and open-circuit voltage of the black silicon solar cell varying with the NH 3/SiH 4gas ?ow ratio.The conversion e ?ciency and open-circuit voltage reach their highest value when the NH 3/SiH 4gas ?ow ratio is 6,indicating that the SiN x ?lm has a good passivation e ?ect on black silicon when the NH 3/SiH 4gas ?ow ratio is 6.

3.3.Deposition Temperature.Deposition temperature is also a crucial parameter to determine the passivation e ?ect for black silicon in the process of PECVD SiN x ?lm.Keeping

100 nm (a)

100 nm

(b)

100 nm

(c)

100 nm (d)

100 nm

(e)

100 nm

(f)

Figure8:The microstructure of SiN?lm varying with deposition time,Top view:(a)300s,(b)500s,and(c)700s,Side view:(d)300s,(e) 500s,and(f)700s.

the deposition time at500s,deposition power at3000W and the NH3/SiH4gas?ow ratio at6,the deposition temperature is varied from425?C to475?C.Figure6shows the FTIR transmission spectra of the SiN x?lms grown at425?C, 450?C,and475?C,respectively.All the spectra show a strong absorption band centered at approximately840–890cm?1 that can be identi?ed with the asymmetric stretching mode of vibration of the Si–N bond.This band shifts very slightly varying with the deposition temperature,indicating that the deposition temperature a?ects the chemical composition very slightly.Figure7shows the minority carrier lifetime and conversion e?ciency of the black silicon after PECVD the SiN x?lms at425?C,450?C,and475?C,respectively. Higher deposition temperature will result in higher minority carrier lifetime and conversion e?ciency.It is believed that relatively high deposition temperature facilitates hydrogen atoms di?use into silicon substrate and consequently reduces the recombination center,contributing to an increase of conversion e?ciency.Considering equipment factor,450?C is preferable deposition temperature.

3.4.Deposition Time.Figure8shows the microstructure of the SiN x?lms varying with deposition time.It can be seen that the SiN x?lm becomes denser,and the thickness increases with the deposition time.Also it can be obviously seen that the SiN x?lm has a good coverage on black silicon surface,and the thickness is more uniform when the deposition time is500s.Table1shows the in?uence of deposition time on the properties of black silicon solar cells.The open circuit voltage gradually increases varying with deposition time from300s to700s.The increase of V oc can be attributed to the improvement of passivation. As increasing the deposition time,more hydrogen atoms are di?used into silicon bulk to passivate defects and dislocation, leading to a decrease of recombination center.It can also be seen that the series resistance increases with increasing the deposition time.This is due to that,with increasing the thickness of SiN x?lm,Ag paste is hard to?re through thick SiN x?lm.

With optimizing the PECVD parameters(the NH3/SiH4 gas?ow ratio:6,deposition temperature:450?C,and depo-sition time:500s),the conversion e?ciency of the black silicon solar cell can reach as high as16.25%.The current-voltage(I-V)characteristics of black silicon solar cells compared with acid-textured Si solar cell are shown in Figure9.

4.Conclusion

The black silicon has been produced by plasma immersion ion implantation(PIII),and its surface appears to be porous sponge like microstructure.To passivate the black silicon, SiN x?lm was deposited by inline PECVD using ammonia and silane as the reactant gas.The black silicon solar cell is high when the NH3/SiH4gas?ow ratio is6,the deposition temperature is450?C,and the deposition time is500s.With optimizing these parameters,the conversion e?ciency of black silicon solar cell reaches as high as16.25%.This work constitutes the most important step to optimize the SiN x ?lm,which allows us to design a desirable?lm for black

876543210Voltage (V)

C u r r e n t (A )

Black silicon solar cell Acid texturing solar cell V oc =613mV I sc =8.3A E ?=16.25%

V oc =615mV I sc =8.18A E ?=16.14%FF =78.1FF =77.6Figure 9:The current-voltage (I -V )characteristics of black silicon solar cell compared with acid-textured Si solar cell.

silicon solar cell.Matched by adequate metallization step,the conversion e ?ciency of the black silicon solar cell can be further improved.

Acknowledgment

This work was supported by the National Natural Science Foundation of China (no.61106060)and the Instrument Developing Project of the Chinese Academy Sciences (no.YZ200755).

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