research article advantages of n-type hydrogenated … · 2020. 1. 13. · oxide films for...

Hindawi Publishing CorporationInternational Journal of PhotoenergyVolume 2013, Article ID 513284, 5 pageshttp://dx.doi.org/10.1155/2013/513284

Research ArticleAdvantages of N-Type Hydrogenated Microcrystalline SiliconOxide Films for Micromorph Silicon Solar Cells

Amornrat Limmanee, Songkiate Kittisontirak, Sorapong Inthisang,Taweewat Krajangsang, Jaran Sritharathikhun, and Kobsak Sriprapha

Solar Energy Technology Laboratory, National Electronics and Computer Technology Center, National Science and TechnologyDevelopment Agency, 112 Thailand Science Park, Phahonyothin Road, Klong 1, Klong Luang, PathumThani 12120, Thailand

Correspondence should be addressed to Amornrat Limmanee; [email protected]

Received 9 May 2013; Revised 30 May 2013; Accepted 30 May 2013

Academic Editor: Leonardo Palmisano

Copyright © 2013 Amornrat Limmanee et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

We report on the development and application of n-type hydrogenated microcrystalline silicon oxide films (n 𝜇c-SiO:H) inhydrogenated amorphous silicon oxide/hydrogenated microcrystalline silicon (a-SiO:H/𝜇c-Si:H) micromorph solar cells. The n𝜇c-SiO:H films with high optical bandgap and low refractive index could be obtained when a ratio of carbon dioxide (CO

2) to

silane (SiH4) flow rate was raised; however, a trade-off against electrical property was observed. We applied the n 𝜇c-SiO:H films

in the top a-SiO:H cell and investigated the changes in cell performance with respect to the electrical and optical properties of thefilms. It was found that all photovoltaic parameters of the micromorph silicon solar cells using the n top 𝜇c-SiO:H layer enhancedwith increasing theCO

2/SiH4ratio up to 0.23, where the highest initial cell efficiency of 10.7%was achieved.The enhancement of the

open circuit voltage (𝑉oc) was likely to be due to a reduction of reverse bias at subcell connection—n top/p bottom interface—and abetter tunnel recombination junction contributed to the improvement in the fill factor (FF). Furthermore, the quantum efficiency(QE) results also have demonstrated intermediate-reflector function of the n 𝜇c-SiO:H films.

1. Introduction

Wide-bandgap silicon oxide based materials have beenwidely studied for thin film silicon solar cell applicationsbecause of their attractive optical and electrical properties[1–4]. Characterizations of boron doped hydrogenated amor-phous and microcrystalline silicon oxide films (p a-SiO:Hand p 𝜇c-SiO:H) and their applications as window layer ofsolar cells have been reported bymany research groups [5, 6].N-type a-SiO:H and𝜇c-SiO:Hfilms also have been developedand applied in single junction and multijunction thin filmsilicon solar cells [7–11]. However, most works focused onan intermediate-reflector function of the n 𝜇c-SiO:H filmsin conventional a-Si:H/𝜇c-Si:H micromorph solar cells andpaid little attention to their effects on junction connectionand band diagram continuity [7–10]. Moreover, applicationof the n 𝜇c-SiO:H films to a-SiO:H based solar cells has notyet been reported; thus there is still much room for furtherresearch.

Our group has been investigating the wide bandgapSiO:H based materials and previously reported performanceof the a-SiO:H based solar cells with single junction andmultijunction structures—a-SiO:H, a-SiO:H/a-Si:H, and a-SiO:H/𝜇c-Si:H [12–15]. In this work, we focus on propertiesof the n 𝜇c-SiO:H films and their appropriateness to the useas the n top layer in the a-SiO:H/𝜇c-Si:Hmicromorph siliconsolar cells. Properties of the n 𝜇c-SiO:H films are presentedalong with the performance of the micromorph silicon solarcells.

2. Experimental Details

2.1. Preparation of n 𝜇c-SiO:H Films. N 𝜇c-SiO:H films havebeen prepared by very high frequency plasma enhancedchemical vapor deposition (60MHz VHF-PECVD) tech-nique.The gas sources were silane (SiH

4), hydrogen (H

2), and

carbon dioxide (CO2), and phosphine (PH

3) was employed

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2 International Journal of Photoenergy

as a doping source. For film characterizations, the n 𝜇c-SiO:H films were deposited on Corning glass substrates atthe deposition temperature of 180∘C, a plasma power of70mW/cm2, deposition pressure of 0.5 Torr, H

2/SiH4ratio

of 35, PH3/SiH4ratio of 0.38, and CO

2/SiH4ratio in the

range of 0∼0.28. The thickness of the films was kept atabout 350 nm, which was measured by step profilometer.The crystalline volume fraction (𝑋

𝑐) of the n 𝜇c-SiO:H

films was estimated by Raman scattering experiment. TheRaman scattering spectra of the n 𝜇c-SiO:H films in the 400–600 cm−1 region can be deconvoluted into three spectra. Apeak distribution around 470–475 cm−1 is assigned to thetransverse optical (TO) mode of amorphous silicon, whosecorresponding integrated area is identified as 𝐼(𝑎). A sharppeak arising at around 519–522 cm−1 corresponds to thetransverse optical vibrational mode of crystalline silicon, andthe associated integrated area is identified as 𝐼(𝑐). And theintermediate component corresponding to a peak at around506–510 cm−1 is identified as 𝐼(𝑏). The crystalline volumefraction is calculated by using the simplified empirical rela-tion as follows [16]:

𝑋𝑐=[𝐼 (𝑐) + 𝐼 (𝑏)]

[𝐼 (𝑎) + 𝐼 (𝑏) + 𝐼 (𝑐)]. (1)

We have measured the absorption data (𝛼) of the filmsat visible range by UV/Visible spectrophotometer. Due tothe varying structure of the films from microcrystalline toamorphous phase, we avoided Tauc’s plots, and to give anumerical presentation of the shift in the absorption spectrawe determined 𝐸

04, that is, the energy corresponding to 𝛼 =

104 cm−1, as an indicator of relative optical bandgap (𝐸op).Refractive index (𝑛) spectra of the films were estimated bySpectroscopic Ellipsometry (SE) using Tauc-Lorentz model[17]. The dark conductivity (𝜎

𝑑) of the films was measured

in a coplanar configuration with Al electrode at roomtemperature.

2.2. Fabrication of a-SiO:H/𝜇c-Si:H Micromorph Silicon SolarCells. We have applied the n 𝜇c-SiO:H films as the ntop layer of the micromorph silicon solar cells with thestructure of TCO glass/ZnO/p-𝜇c-SiO:H/i-a-SiO:H/n-𝜇c-SiO:H/p-𝜇c-SiO:H/i-𝜇c-Si:H/n-𝜇c-Si:H/ZnO/Ag (cell activearea was 0.75 cm2). Note that absorber layer of the topcell was wide-bandgap a-SiO:H film, and p 𝜇c-SiO:H filmswere used as p layer in both top and the bottom cells.There was no intermediate layer at the junction connectionbetween the top and bottom cells. Thicknesses of the i topa-SiO:H and i bottom 𝜇c-Si:H layers were 400 and 1500 nm,respectively.TheCO

2/SiH4ratio for the n top layer deposition

was varied from 0 to 0.28, while other conditions in cellfabrication were kept as the same. The thickness of the ntop layerwas approximately 30 nm.The current-voltage (𝐼-𝑉)characteristics of the solar cells have been investigated underthe standard testing conditions—AM1.5, 100mW/cm2, and25∘C—in aWacom solar simulator. Quantum efficiency (QE)of the solar cells also has been evaluated by spectral responsemeasurements.

Inte

nsity

(a.u

.)In

tens

ity (a

.u.)

(a)

(b)

(c)

Inte

nsity

(a.u

.)In

tens

ity (a

.u.)

300 400 500 600 700

CO2/SiH4 = 0

CO2/SiH4 = 0.18

CO2/SiH4 = 0.23

CO2/SiH4 = 0.28

Xc = 69%

Xc = 48%

Xc = 35%

Xc = 12%

Raman shift (cm−1)

Figure 1: Raman spectra of n 𝜇c-SiO:H films deposited with differ-ent CO

2/SiH2ratios.

3. Results and Discussion

3.1. Properties of n 𝜇c-SiO:H Films. Figure 1 shows Ramanspectra of the n 𝜇c-SiO:H films deposited with differentCO2/SiH4ratios. It is obviously shown that the peak corre-

sponding to crystalline phase, peak (𝑐), gradually decreasedwith increasing the CO

2/SiH4ratio, and the amorphous

silicon (𝑎) became a dominant phase at the ratio above 0.23.With no CO

2addition the𝑋

𝑐of the filmwas 69%, decreasing

to 12% at the CO2/SiH4ratio of 0.28.

The optical bandgap of the films tended to increase whilethe refractive index measured at the wavelength of 550 nmshowed an opposite change when the CO

2/SiH4ratio became

higher, as shown in Figure 2. Incorporation of oxygen intothe Si:H network has a direct consequence for optical gapwidening. A component of the increase in optical bandgapis associated with the Si–O bonds because of the strongerbond energy of Si–O compared to those of Si–Si and Si–H[18]. Addition of oxygen atoms to Si:H films canwiden opticalbandgap; however, the more the participation of the oxygenatoms, the lower the conductivity of the films, as indicated inFigure 3.

3.2. Characteristics of a-SiO:H/𝜇c-Si:H Micromorph SiliconSolar Cells. As shown in Figure 4, open circuit voltage(𝑉oc), short circuit current density (𝐽sc), and fill factor (FF)

International Journal of Photoenergy 3

1.8

1.9

2

2.1

2.2

2.3

2.4

0 0.1 0.2 0.33.1

3.2

3.3

3.4

3.5

3.6

3.7

CO2/SiH4

E04

n

Refr

activ

e ind

exn

E04

(eV

)

Figure 2: Optical bandgap (𝐸04) and refractive index (𝑛) of n 𝜇c-

SiO:H films as a function of CO2/SiH2ratio.

0 0.1 0.2 0.3

Con

duct

ivity

(S/c

m)

102

101

100

10−1

10−2

CO2/SiH4

Figure 3: Dark conductivity of n 𝜇c-SiO:H films as a function ofCO2/SiH2ratio.

of the solar cells obviously improved when the n 𝜇c-SiO:Hfilm was applied as the n top layer instead of the n 𝜇c-Si:Hfilm (CO

2/SiH4= 0). The best cell with initial conversion

efficiency of 10.7% with 𝑉oc = 1.47V, 𝐽sc = 10.6mA/m2, andFF = 0.67 has been achieved at the CO

2/SiH4ratio of 0.23,

where the𝑋𝑐of the filmwas approximately 35%.At the higher

ratio, the 𝐽sc of the cell began to drop, resulting in a decreasein the cell efficiency. Since the n 𝜇c-SiO:H films possess wideoptical bandgap of about 2.3 eV and higher defect densitycompared to the n 𝜇c-Si:H film, these are supposed to allowa better continuity of band diagram and also a better tunnelrecombination junction at the connection between the topand the bottom cells. As mentioned previously, the i top andp bottom layers in these solar cells were wide bandgap SiO:Hbased materials. The 𝐸

04of the p bottom 𝜇c-SiO:H layer was

estimated to be about 2.25 eV; thus the n 𝜇c-SiO:H film withthe 𝐸04of 2.3 eV was probably better suited to the n top layer

application for this cell structure. The enhancement in the𝑉oc was supposed to be due to a reduction of reverse bias

1.4

1.45

1.5

1.55

9.5

10

10.5

11

0.65

0.6

0.7

0 0.1 0.2 0.3

9.5

10

10.5

11Effi

cien

cy (%

)J s

c(m

A/c

m2)

Voc

(V)

FF

CO2/SiH4

Figure 4: Photovoltaic parameters of a-SiO:H/𝜇c-Si:Hmicromorphsilicon solar cells using n top 𝜇c-Si(O):H layer deposited withvarious CO

2/SiH2ratios.

at subcell connection—n top/p bottom interface. The seriesresistance (𝑅

𝑠) slightly increased while the shunt resistance

(𝑅sh) significantly enhanced from 1500 to 3200Ω whenthe CO

2/SiH4ratio increased from 0 to 0.28, as shown in

Figure 5. The increase of the 𝑅sh was supposed to be causedby the better tunnel recombination junction, contributing tothe improvement in the FF.

According to the QE results shown in Figure 6, thespectrum response corresponding to the top a-SiO:H cellslightly enhanced while those of the bottom 𝜇c-Si:H celldecreased with increasing the CO

2/SiH2ratio.This suggested

that, besides allowing ohmic and low resistive electricalconnection between the two adjacent cells in the a-SiO:H/𝜇c-Si:H micromorph silicon solar cell, the n top 𝜇c-SiO:Hfilm also worked as an intermediate reflector to enhancelight scattering, as verified by the increase of the spectrumresponse corresponding to the top cell. The drop of the 𝐽sc atthe CO

2/SiH2ratio of 0.28 was thought to be due to current

mismatch between the top and the bottom cells.Experimental results have verified the excellent multi-

function of the n 𝜇c-SiO:H films when they are appliedin the a-SiO:H/𝜇c-Si:H micromorph solar cells, in additionto the conventional a-Si:H/𝜇c-Si:H structure. Interestingly,

4 International Journal of Photoenergy

0 0.1 0.2 0.3

0 0.1 0.2 0.3

1000

2000

3000

5

10

15

20

25

CO2/SiH4

CO2/SiH4

Rsh

(Ω)

Rs

(Ω)

Figure 5: 𝑅𝑠and 𝑅sh of a-SiO:H/𝜇c-Si:H micromorph silicon solar

cells using n top 𝜇c-Si(O):H layer deposited with various CO2/SiH2

ratios.

400 600 800 1000Wavelength (nm)

00.18

0.230.28

0

0.2

0.4

0.6

0.8

Qua

ntum

effici

ency

(QE)

CO2/SiH4

Figure 6: QE of a-SiO:H/𝜇c-Si:H micromorph silicon solar cellsusing n top 𝜇c-Si(O):H layer deposited with various CO

2/SiH2

ratios.

the 𝑉oc of our a-SiO:H/𝜇c-Si:H micromorph solar cells wasfound to be high, compared to the conventional micromorphsolar cells [7–10], and was further improved when the ntop 𝜇c-SiO:H layer was used. The 𝑉oc of the conventionalcells was about 1.38–1.42V, while the a-SiO:H/𝜇c-Si solarcells showed the 𝑉oc as high as 1.47–1.49V.The multijunctionthin film silicon solar cells with high 𝑉oc are considered to

have advantage of low temperature coefficients (TC) [15].Although, at present, the efficiency of the a-SiO:H/𝜇c-Simicromorph solar cells is lower than that of the micromorphsolar cells using conventional structure, their advantagesare expected to become more obvious when the cells areoperating in high-temperature environment.

4. Conclusion

We have developed the n-type 𝜇c-SiO:H films and appliedthem as the n top layer of the a-SiO:H/𝜇c-Si:H micromorphsilicon solar cells. The solar cells using the n top 𝜇c-SiO:Hlayer showed higher 𝑉oc, 𝐽sc, and FF than the cell with then top 𝜇c-Si:H layer. Enhancements in the cell parameterswere supposed to be due to the better tunnel recombinationjunction, the better continuity of band diagram at the subcellconnection, and, equally importantly, the more efficientintermediate reflector, all of which were mainly owing to then 𝜇c-SiO:H films in the top cell.

Acknowledgment

This work was supported by Cluster and Program Manage-ment Office (CPM) of NSTDA,Thailand (P-00-10470).

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