Amorphous-Silicon-Based Thin-Film Solar Cells Exhibiting Low Light-Induced Degradation

Amorphous-Silicon-Based Thin-Film Solar Cells Exhibiting Low Light-Induced Degradation

Takuya Matsui 1?,Hitoshi Sai 1,Kimihiko Saito 2,and Michio Kondo 1

1

Research Center for Photovoltaic Technologies,National Institute of Advanced Industrial Science and Technology (AIST),Tsukuba,Ibaraki 305-8568,Japan 2

Thin Film Silicon Lab,Photovoltaic Power Generation Technology Research Association (PVTEC),Tsukuba,Ibaraki 305-8568,Japan Received December 20,2011;revised January 30,2012;accepted February 16,2012;published online October 22,2012

We have applied a triode electrode configuration in the plasma-enhanced chemical vapor deposition (PECVD)process to grow intrinsic hydrogenated amorphous silicon (a-Si:H)light absorbers for the fabrication of p–i–n junction solar cells.Although the deposition rate is lower (0.1–

0.3#A

/s)than that of the conventional diode PECVD process,the light-soaking stability of the solar cell is markedly improved and less sensitive to the cell thickness due to the reduced Si–H 2bond density in the a-Si:H i-layer.The a-Si:H single-junction solar cells exhibit low light-induced degradation of conversion efficiency (á = ini 10%)in comparison with that of high-efficiency solar cells reported to date.By applying the improved a-Si:H layers as top-cell absorbers in a-Si:H/hydrogenated microcrystalline silicon ( c-Si:H)tandem device,the light-induced degradation can be reduced even further (á = ini .5%).As a result,we obtain confirmed stabilized efficiencies of 9.6and 11.3%for a-Si:H single-junction and a-Si:H/ c-Si:H tandem solar cells,respectively.#2012The Japan Society of Applied Physics

1.Introduction

Thin-?lm silicon solar cells o?er several advantages over wafer-based crystalline silicon solar cells such as low silicon consumption,short energy-payback time,and better perfor-mance under high-temperature operation.Hydrogenated amorphous silicon (a-Si:H)and microcrystalline silicon ( c-Si:H)are extensively employed as light-absorbing materials in a tandem cell structure to attain higher conversion e?ciency than the corresponding single-junction devices.1,2)The state-of-the-art a-Si:H/ c-Si:H tandem solar cells are reported to provide stabilized e?ciencies of as high as 12%in laboratory small-area ($1cm 2)devices.3–5)Nevertheless,further progress in stabilized e?ciency toward 14–15%is essential to make thin-?lm silicon competitive with the other solar cell technologies.Among a variety of technological issues that need to be resolved,the light-induced degradation in a-Si:H is one of the biggest remaining issues that hinders the performance of a-Si:H based solar cells.

The light-induced degradation in a-Si:H,known as the Staebler–Wronski e?ect,6)has been a subject of research over the past three decades.Until now,no way of completely removing this adverse e?ect has been demonstrated in practical solar cell devices.It has been pointed out that the Si–H 2bond density (or density of Si–H n bonds in the internal void surface)in a-Si:H is correlated with the degree of light-induced degradation of a-Si:H ?lms 7)and solar cells.8)The Si–H 2bond density can be reduced by preventing the incorporation of higher silane radicals into the ?lm during the plasma-enhanced chemical vapor deposition (PECVD)process by using a triode-type electrode con?guration.9–11)In this study,we have applied the triode PECVD technique to grow intrinsic a-Si:H layers in the fabrication of p–i–n junction solar cells.It is demonstrated that high-e?ciency solar cells with low light-induced degradation can be attained in both a-Si:H single-junction and a-Si:H/ c-Si:H tandem solar cells.2.Experiment

Undoped a-Si:H layers were grown in a triode PECVD reactor in which a mesh electrode was placed between

powered and grounded electrodes.A dc voltage of à25V was applied to the mesh to con?ne the SiH 4–H 2glow discharge (60MHz)between the powered and mesh electrodes,which were separated by a distance of 20mm.The detail of the triode deposition technique has been described elsewhere.10,11)In this experiment,the deposition temperature was kept at 190 C while the distance between the mesh and substrate was varied from 35to 11mm,resulting in deposition rates in the range from $0:1to

$0:3#A

/s,respectively.As a reference material,a-Si:H ?lms were deposited at a higher rate (2.5#A

/s)at a similar deposition temperature by our standard diode-type PECVD system (13.56MHz).The materials were characterized by transmission-re?ection spectroscopy and Fourier transform infrared spectroscopy (FTIR)for measurements of the optical band gap and Si–H n bond density,12)respectively.The intrinsic a-Si:H layers grown either by triode or diode PECVD were integrated into a p–i–n device in a superstrate con?guration.As the front transparent conductive oxide (TCO)substrate,commercially available SnO 2-coated glass (Asahi-VU)was used.To improve the light in-coupling at the TCO/Si interface,the SnO 2surface was covered with antire?ection layers consisting of TiO 2(30nm)–ZnO (10nm).13)For the doped layers,we deposited p-and n-layers consisting of a c-Si:H (p)/a-SiC:H (p)/a-SiC:H (undoped)triple layer and c-Si:H (n)single layer,respectively,in separate reactors.The current density–voltage (J –V )characteristics of the solar cells were measured under standard air mass 1.5(AM1.5)global illumination condition (irradiance:100mW/cm 2,cell tem-perature:T C ?25 C).To avoid overestimation of the photocurrent of the solar cells,we used a mask to de?ne an illumination area (1cm 2)that was slightly smaller than the device area (1.04cm 2).The solar cells were exposed to AM1.5illumination under 100mW/cm 2(T C ?50 C,1000h,open circuit)14)or 125mW/cm 2(T C ?48 C,310h,open circuit)15)intensities.In addition,some of the solar cells were exposed to 300mA/cm 2illumination at T c ?60 C for 6h as a quick degradation test.In our experiment,we found no notable di?erence in stabilized e?ciencies after light-soaking tests under these three degradation conditions.For some selected samples,J –V characteristics after light soaking were measured by the Calibration,Standards and Measurement Team of AIST.

?

E-mail address:t-matsui@aist.go.jp

Japanese Journal of Applied Physics 51(2012)10NB04

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https://www.360docs.net/doc/119853958.html,/10.1143/JJAP.51.10NB04

3.Results and Discussion

In the triode PECVD process,the growth of a-Si:H at the substrate surface is dominated by the di?usive transport of long-lifetime radicals such as SiH 3.Thus,the incorporation of short-lifetime radicals such as SiH 2and higher silane radicals (Si n H m ,n >2),which are believed to be the precursors of the Si–H 2bond structure in a-Si:H,16)can be reduced e?ectively by moving the surface of the growing ?lm away from the plasma zone.In Table I,the properties of the a-Si:H ?lms deposited by triode PECVD and our standard diode PECVD are summarized.Although the deposition rate is reduced by an order of magnitude for triode PECVD,the material parameters such as the optical band gap and hydrogen content are nearly identical to those of the reference a-Si:H ?lms.In the FTIR spectra,however,the a-Si:H ?lms grown by triode PECVD exhibit particularly low intensity of the silicon-hydride stretching mode ($2100cm à1),10)which is a signature of the material having a compact microstructure.7)The Si–H 2bond density,deduced from the high stretching mode in FTIR spectrum,is reduced from 2.1to 0.7at.%,while the Si–H bond density,deduced from the low stretching mode ($2000cm à1),remains almost constant at 10–11at.%.The slightly lower hydrogen content in the a-Si:H ?lm grown by triode PECVD re?ects the relatively narrower optical band gap.

Figure 1shows the degradation characteristics of the a-Si:H p–i–n solar cells with 250-nm-thick i-layers prepared by triode and diode PECVD.Although the initial e?ciencies are comparable between the two solar cells ( ini $10%),long-term light soaking results in markedly di?erent stabilized e?ciencies ( stb ).After 1000h of light exposure,

the solar cell prepared by triode PECVD shows a relative degradation of 11%in conversion e?ciency (de?ned as degradation ratio á = ini ?e ini à stb T= ini ),which is 8%smaller than the degradation ratio of our reference cell prepared by diode PECVD.Figure 2shows the illuminated J –V parameters of the a-Si:H p–i–n solar cells prepared by the two deposition techniques [(a)short-circuit current density J sc ,(b)open-circuit voltage V oc ,(c)?ll factor FF and (d)conversion e?ciency ]in the initial and stabilized states as a function of the i-layer thickness.In the initial state,the solar cells prepared by the two deposition techniques show almost exactly the same tendencies for all solar cell parameters upon variation of the cell thickness.With increasing thickness,J sc increases due to the increased light absorption,while V oc and FF stay nearly constant in the thickness range investigated (t i ?180{390nm).Conse-quently,the initial e?ciency can be increased gradually by increasing the absorber layer thickness.After light soaking,all solar cell parameters are degraded except for V oc for the solar cells prepared by triode PECVD.In particular,the reference solar cells show substantial degradation in FF,which is consistent with the results reported earlier.17)In comparison with these conventional solar cells,the solar cells prepared by triode PECVD show much lower degradation in FF.Furthermore,the gaps of the light-induced degradation in FF and e?ciency become larger between these two series solar cells as the i-layer thickness increases.It is worth mentioning that the solar cells prepared by triode PECVD maintain high stabilized e?ciencies ( stb >9%)even when the thickness of the i-layer is increased to $400nm.The high stability of a-Si:H at a greater cell thickness is of particular importance for its application to tandem solar cells because of the requirement of a high top-cell current while maintaining light-soaking stability.

It is also important to investigate the deposition rate dependence because the light-soaking stability of a-Si:H is known to depend strongly on its deposition rate.18)Since the growth of a-Si:H is governed by the di?usion of SiH 3radicals in triode PECVD,the deposition rate can be varied systematically by adjusting the mesh–substrate distance (L M {S )without changing the plasma conditions.9,10)Table II

Table I.Deposition rate R d ,optical band gap E g ,and SiH and SiH 2bond

densities measured by FTIR for the a-Si:H ?lms grown by di?erent deposition methods.

Diode

Triode R d (A /s) 2.50.34E g (eV) 1.77 1.74[Si–H](at.%)11.110.1[Si–H 2](at.%)

2.1

0.7

Exposure time (h)

24681012

E f f i c i e n c y (%)

Fig.1.(Color online)Stability of the a-Si:H solar cells under long-term

light soaking (AM1.5,100mW/cm 2,50 C,open circuit).The a-Si:H

absorbers were prepared by di?erent deposition techniques.The stability of the a-Si:H/ c-Si:H tandem cell whose top cell absorber was grown by the triode deposition system is also shown.

J s c (m A /c m 2

)

14

15

1617V o c (V )

0.800.820.840.860.880.900.92i layer thickness (nm)

200

300

400

200

300

400

F F

0.50

0.550.600.650.700.750.80η (%)

7

8910

11

Fig.2.(Color online)J –V parameters of the p–i–n junction a-Si:H solar cells in the initial (open symbols)and stabilized (?lled symbols)states as a function of thickness of the i-layer (light-soaking condition:AM1.5,

300mW/cm 2,T c ?60 C,6h,open circuit).The intrinsic light-absorbing layer was deposited either by diode (triangles)or triode (circles)PECVD.

shows the solar cell parameters of the initial and stabilized

states for di?erent deposition rates in the range from0.11

(L M{S?35mm)to0.34#A/s(L M{S?11mm).In this series, the thickness of the i-layer was kept constant at250nm.In

Table II,as expected,the lowest degradation can be found

for the sample with the longest mesh–substrate distance

(á = ini?9%for L M{S?35mm).Nevertheless,we found that the initial and stabilized e?ciencies in these three solar cells are almost independent of the deposition rate at such a low-deposition-rate regime.This indicates that the arrival rate of the higher silane radicals at the surface is low enough to sustain the high stability regardless of L M{S.Further investigation is necessary for solar cells prepared at higher deposition rates by changing L M{S and the plasma condi-tions.

By adopting antire?ective(AR)TiO2–SiO2multilayers on

the illumination surface of a glass substrate,so far we have

obtained a con?rmed stabilized e?ciency of9.6%(J sc?16:0mA/cm2,V oc?0:879V,FF?0:684)in a single-junction a-Si:H solar cell,as shown in Fig.3.For this solar cell,a250-nm-thick i-layer was deposited by triode PECVD

at a deposition rate of0.34#A/s.This solar cell showed an initial e?ciency of10.7%and a degradation ratio of10% under1sun illumination.Although the e?ciency of this solar cell is still lower than the highest reported stabilized e?ciency(10.09%),19)we emphasize that the degradation ratio of the solar cell e?ciency is5%smaller than that of high-e?ciency solar cells reported to date(e.g.,ref.20). The improved a-Si:H i-layer deposited by triode PECVD described above is applied in the top-cell absorber of an a-Si:H/ c-Si:H tandem solar cell.The solar cell consists of a250-nm-thick a-Si:H top cell and a2- m-thick c-Si:H bottom cell with neither AR layers at the glass substrate nor an intermediate re?ector between the top and bottom cells, which results in a current-matched cell.In Fig.1,the degradation behavior of this tandem solar cell is also shown. We measured11.8%(J sc?11:9mA/cm2,V oc?1:334V, FF?0:750)initial and11.3%(J sc?11:8mA/cm2,V oc?1:352V,FF?0:712)stabilized e?ciencies after1000h light exposure under1sun illumination.It is evident that the tandem cell exhibits even higher stability.The degradation ratio is found to be$4%in this solar cell,which is markedly lower than that of the single-junction solar cells($10%).

The better stability of the tandem solar cell can be explained

by the following reasons.Firstly,the power generated by the

a-Si:H top cell accounts for approximately2/3of the total

output of the tandem cell.Thus,the impact of the light-

induced degradation of the a-Si:H top cell on the total output

power is smaller than that for the a-Si:H single-junction

solar cells.Another e?ect is that the light absorption in the

a-Si:H top cell is less than that in single-junction solar cells

because the visible light is partly absorbed in the bottom cell

in the tandem device.

The light-induced degradation of the tandem cell is also

a?ected by the current-matching condition.We applied an

intermediate re?ector based on aluminum-doped ZnO

(AZO)between the top and bottom cells and varied its

thickness(t AZO?0;25;50nm)in order to obtain current-matched and-mismatched cells.AR multilayers were also

applied to the illumination surface of the glass substrates.In

Table III,the illuminated J–V characteristics of the a-Si:H

(0.25 m)/ c-Si:H(3 m)solar cells are summarized for

initial and stabilized states and various intermediate re?ector

Table II.Illuminated J–V characteristics of the p–i–n junction a-Si:H solar cells in the initial(ini)and light-soaked(LS)states for di?erent deposition rates of the i-layer(light-soaking condition:AM1.5,

100mW/cm2,50 C,1000h,open circuit).The deposition rate was changed by varying the distance between the mesh and substrate L M{S in the triode deposition system.

L M{S (mm)

R d

(#A/s)

J sc

(mA/cm2)

V oc

(V)

FF

(%) ini15.40.8910.75410.3

110.34LS15.00.8820.6879.12á(%)à2:60à1:01à8:89à11:4

ini15.60.8800.75610.4 170.21LS15.10.8820.6799.08á(%)à3:210.23à10:1à12:6

ini15.20.8710.75710.0 350.12LS14.90.8740.7019.13á(%)à1:90.34à7:40à8:

70Fig.3.(Color online)Illuminated J–V characteristics of the a-Si:H single-junction solar cells after light soaking(AM1.5,100mW/cm2,50 C, 1000h,open circuit)measured by the Calibration,Standards and Measurement Team of AIST.

Table III.Illuminated J–V characteristics of the p–i–n junction a-Si:H/ c-Si:H tandem solar cells in the initial(ini)and light-soaked(LS)states for di?erent intermediate re?ector thicknesses(t AZO)between the top and bottom cells(light-soaking condition:AM1.5,125mW/cm2,48 C,open circuit,310h).The photocurrent densities of the top(J top)and bottom (J bottom)cells in the initial state were measured by quantum e?ciency measurement.

t AZO

(nm)

J top=J bottom

(mA/cm2)

J sc

(mA/cm2)

V oc

(V)

FF

(%)

ini12.0 1.3140.74211.7 012.1/14.7LS11.9 1.3430.71511.2

á(%)à0:83 2.21à3:64à4:27

ini12.8 1.3000.72712.0 2512.9/13.4LS12.6 1.3300.68711.4

á(%)à1:56 2.31à5:50à5:00

ini11.3 1.3240.78011.7 5013.7/11.7LS11.3 1.3300.75511.3

á(%)0.000.45à3:21à3:42

thicknesses.According to the quantum e?ciency measure-

ments of each component cell,the cell without an

intermediate re?ector behaves as a top-current-limited cell,

i.e.,J sc is limited by the smaller photocurrent of the a-Si:H

top cell.We obtain current-matched and bottom-current-

limited cells when t AZO is25and50nm,respectively.

Among the three cells,the most stable cell is found to be the

bottom-current-limited cell that exhibits FF as high as75%

even after light soaking.On the other hand,the highest

stabilized e?ciency is obtained for the current-matched cell,

while showing the relatively high degradation ratio of5%.

For this solar cell,a stabilized e?ciency of11.3%(J sc?12:1mA/cm2,V oc?1:340V,FF?0:696)has been con-?rmed.Further experiments are in progress to improve the

stabilized e?ciency of tandem cells using a thicker top cell

by taking advantage of the better stability of the a-Si:H

absorber.

4.Conclusions

Intrinsic a-Si:H?lms containing a lower Si–H2bond density

are prepared by using the triode PECVD technique and

applied as light absorbers in p–i–n solar cells.In comparison

with our standard cell and high-e?ciency solar cells

reported to date,the solar cells exhibit better stability

(á = ini$10%for single-junction cells and$5%for tandem cells)under prolonged illumination.As a result, we have obtained con?rmed stabilized e?ciencies of9.6and 11.3%for a-Si:H single-junction and a-Si:H/ c-Si:H tandem solar cells,respectively,using a commercially available TCO substrate.

Acknowledgements

The authors are grateful to Ms.Hozuki for the help in

sample preparation.They also thank Dr.Hishikawa,Ms.

Tobita,and Ms.Ishitsuka in the Calibration,Standards and

Measurement Team of AIST for J–V measurements.This

work was supported by the New Energy and Industrial

Technology Development Organization(NEDO).

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