The deposition of intrinsic hydrogenated amorphous silicon thin films incorporated with oxygen by plasma-enhanced vapor deposition

Solid State Sciences 20 (2013) 70e74

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The deposition of intrinsic hydrogenated amorphous silicon thin films incorporated with oxygen by plasma-enhanced vapor deposition Ji Eun Lee a, b, Joo Hyung Park a, Jinsu Yoo a, Kyung Hoon Yoon a, Donghwan Kim b, Jun-Sik Cho a, * a b

Solar Energy Department, Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon, Republic of Korea Department of Materials Science and Engineering, Korea University, Seoul, Republic of Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 January 2013 Received in revised form 8 March 2013 Accepted 13 March 2013 Available online 26 March 2013

Intrinsic hydrogenated amorphous silicon films incorporated with oxygen (i a-Si(H,O):H) were prepared using a plasma-enhanced chemical vapor deposition system with a carbon dioxide (CO2), silane (SiH4) and hydrogen (H2) gas mixture. The influence of oxygen incorporation on the chemical structure and on the optoelectronic properties of the deposited films was investigated. The performance of the solar cells that use these films as absorber layers was also evaluated. For the films incorporated with oxygen, local bonding configurations were identified in which H and O alloy atoms were bonded to the same Si site. With the incorporation of oxygen, the bandgap (Eopt) of the a-Si(H,O):H films increased significantly to 1.82 eV, while that of the pure hydrogenated amorphous (a-Si:H) films was 1.73 eV. The optoelectronic properties of the oxygen-incorporated films degraded due to the newly created dangling bonds that arose from an increased structural disorder. Increasing the hydrogen dilution in the plasma effectively reduced the defect density in the a-Si(H,O):H films, resulting in an improved photosensitivity. The solar cells that used wide-bandgap a-Si(H,O):H films as absorber layers exhibited a 26.3% higher open circuit voltage (Voc) than those that used pure a-Si:H films, mainly because of the increased Eopt of the films and the reduced defect density that was due to a high hydrogen dilution. Ó 2013 Elsevier Masson SAS. All rights reserved.

Keywords: Hydrogenated silicon film Defect density Solar cell Chemical structure Hydrogen passivation

1. Introduction Silicon-based thin-film photovoltaics are one of the most promising thin-film solar energy techniques because of its low cost and large-volume production capacity [1,2]. Single junction and multi-junction solar cells that use hydrogenated amorphous or microcrystalline silicon (a- or mc-Si:H) or hydrogenated amorphous or microcrystalline silicon germanium (a- or mc-SiGe:H) thin-films as active layers have been developed over the past several decades to achieve high cell performance [3e5]. Recently, a record initial efficiency of 16.4% was attained by United Solar Ovonic by using a triple junction structure with a-Si:H, a-SiGe:H and mc-Si:H absorbers in the top, middle and bottom cells [6]. However, the relatively low conversion efficiency of Si thin-film solar cells, compared with those of crystalline Si and Cu(InGa) Se2 thin-film solar cells, is still a drawback for industrial applications. According to a theoretical analysis [7], the use of a material with a bandgap larger than 1.7 eV, the bandgap of i a-Si:H, as an absorber in the top cell is required to achieve a high conversion

* Corresponding author. Tel.: þ82 42 860 3214; fax: þ82 42 860 3539. E-mail address: [email protected] (J.-S. Cho). 1293-2558/$ e see front matter Ó 2013 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.solidstatesciences.2013.03.015

efficiency of over 20% in triple junction solar cells. A variety of wide-bandgap silicon alloy materials, such as a-SiC:H, a-SiN:H, and a-SiO:H, have been considered for use in solar cells. Of these materials, a-SiO:H films have been attracting many attention because of their better optoelectronic properties [8e10]. Although a wide-bandgap silicon alloy incorporated with oxygen can be obtained by increasing the oxygen concentration, the degradation of film quality is inevitably accompanied by newly created defects in the deposited films. Oxygen incorporation into an a-Si:H network and the electronegativity of the oxygen atoms cause dangling bonds and a more disordered structure, resulting in the electrical properties of the films decaying [11e13]. Therefore, further studies are required to understand the influence of oxygen incorporation on the chemical structures and optoelectronic properties of a-Si(H,O):H films and to determine a method for improving the material quality of the films. In this paper, changes in the chemical structures of a-Si(H,O):H films as a function of the CO2/SiH4 ratio were investigated. The effect of oxygen incorporation on the electrical and optical properties of the films was systematically investigated. The use of hydrogen passivation to diminish the defect density and to improve the film quality was considered. The performances of solar cells with a-Si:H and a-Si(H,O):H absorbers were compared.

J.E. Lee et al. / Solid State Sciences 20 (2013) 70e74

2. Experimental details

800

Intrinsic a-Si:H and a-Si(H,O):H films were prepared by using plasma-enhanced chemical vapor deposition (PECVD) with a 13.56 MHz radio frequency discharge. A gas mixture of SiH4, H2 and CO2 was used as the source gas. The deposition conditions for the films are summarized in Table 1. Crystalline silicon wafers were used for chemical structure studies, and non-alkali glass substrates were used for electrical and optical characterizations. The local atomic structures of the deposited films were investigated using Fourier transform infrared (FTIR) spectrophotometry measurements. The dark conductivity (sdark) and the photoconductivity (sphoto) were measured using an Al coplanar electrode configuration at room temperature. The film quality was evaluated using the ratio of sphoto to sdark, i.e., the photosensitivity. The optical bandgap (Eopt) was determined by spectroscopic ellipsometry (SE). The defect densities in the films were estimated from the sub-bandgap absorption spectra that were obtained using the constant photocurrent method (CPM) [14,15]. Peien single junction solar cells with i a-Si:H and a-Si:(H,O):H absorbers were fabricated on Asahi U-type glass using a multichamber PECVD system. The structure of the solar cells consisted of Asahi U-type glass/p-type amorphous silicon carbide (a-SiCx:H)/i a-Si:H or a-Si(H,O):H/n-type a-Si:H/Ag electrode. The p- and n-type films were prepared at 200  C with a 13.56 MHz radio frequency discharge and with SiH4, H2, B2H6 (1% in H2), CH4 (50% in H2) and PH3 (1% in H2) gases. The currentevoltage (JeV) characteristics of the solar cells were measured at 25  C using an AM 1.5G double beam solar simulator.

600

Changes in the chemical bonding properties of the films deposited with and without the introduction of CO2 gas into the plasma were investigated using FTIR spectroscopy. Fig. 1 shows the absorption bands over the range of 1900e2200 cm1 for the films deposited at different CO2/SiH4 (RO) ratios. Each spectrum has been deconvoluted into its probable satellite components, which can arise from the SieHn bonds and from the oxygen atoms being backbonded to SieHn [12,13,16]. For the films deposited at RO ¼ 0 (Fig. 1(a)), an absorption peak was observed at approximately 2000 cm1, which corresponds to the silicon monohydride (SieH) stretching vibration in pure a-Si:H films. With the introduction of CO2 gas into the plasma (Fig. 1(b) and (c)), the absorption spectrum extends to 2200 cm1 and a new absorption peak appears in the deconvoluted spectrum. As mentioned in the literature [12,17], this satellite spectrum with an absorption peak at 2075 cm1 can be assigned to the vibration stretching mode of SieH back-bonded to an oxygen atom in a HeSi(Si2O) configuration. Another origin of the absorption that is expected to occur at this wavenumber is the dihydride (SieH2) stretching vibration [12,18]. The polyhydrogenation of the films implies the addition of dangling bonds to the Si network due to the oxygen incorporation. At the growing surface of the film, oxygen atoms stick on the surface with almost no mobility or hopping, and the choice of a preferential site becomes almost impossible. The abstraction of hydrogen from the film surface by OH is dominant, creating more dangling bond sites [19]. Table 1 The deposition conditions for a-Si:H and a-Si(H,O):H films. Deposition parameter

Value

Substrate temperature ( C) Deposition pressure (mTorr) Power density (mW/cm2) CO2/SiH4 (RO) H2/SiH4 (RH)

300 300 60 0e0.14 1.43e8.57

(a)

RO = 0

(b)

RO = 0.09

(c)

RO = 0.14

400 200

Absorption coefficient (cm-1)

0 800 600 400 200 0 800 600 400 200 0 2200

2150

2100

2050

2000

1950

1900

Wave number (cm-1) Fig. 1. The absorption spectra over the range of 1900e2200 cm1 for a-Si(H,O):H films deposited at various RO ratios: (a) RO ¼ 0, (b) RO ¼ 0.09 and (c) RO ¼ 0.14.

Fig. 2 shows the absorption spectra over the range of 750e 850 cm1 as a function of the RO ratio for the films prepared. In the case of the films deposited at RO ¼ 0 (Fig. 2(a)), no absorption peak was observed, indicating the absence of oxygen bonds in the films.

300

(a)

RO = 0

(b)

RO = 0.09

(c)

RO = 0.14

200 100

Absorption coefficient (cm-1)

3. Results and discussion

71

0 300 200 100 0 300 200 100 0 850

825

800

775 -1 Wave number (cm )

750

Fig. 2. The absorption spectra over the range of 750e850 cm1 for a-Si(H,O):H films deposited at various RO ratios: (a) RO ¼ 0, (b) RO ¼ 0.09 and (c) RO ¼ 0.14.

J.E. Lee et al. / Solid State Sciences 20 (2013) 70e74 6

-3

10

10

-4

10

-5

10

-6

10

5

10

σd

-7

10

σph

-8

10

σph/σd

Photosensitivity

With the introduction of a small amount of CO2 into the plasma, even for RO ¼ 0.09, a clear absorption peak appeared at 780 cm1. This vibration at 780 cm1 is due to the strongly coupled mode that involves the SieH and SieOeSi motions, which only occurs when the SieH bond is in the same plane as the SieOeSi bond and when the oxygen and hydrogen atoms are in a cis bonding geometry [18]. From Figs. 1 and 2, it can be verified that the addition of a small amount of CO2 gas into the plasma produces a major change in the chemical structure of the deposited film. The oxygen-related features can be explained by the local vibrations of a configuration in which the oxygen and hydrogen atoms are bonded to the same silicon atom. The changes in the optical bandgap (Eopt) and defect density (Nd) of the films prepared at various RO values are presented in Fig. 3. The Eopt and Nd of the films deposited at RO ¼ 0 were 1.73 eV and 1.65  1015/cm3, respectively. These values are usually obtained for i a-Si:H films that are prepared using a well-optimized deposition process. When RO was increased to 0.09, the Eopt of the films increased to 1.80 eV, accompanied by an abrupt increase in Nd to 2.96  1016/cm3. By further increasing RO to 0.14, a continuous increase in Nd to 7.18  1016/cm3 was obtained while Eg increased slightly to 1.82 eV. This increase in Eopt is closely related to the fact that the SieO bond is stronger than the SieSi bond and the SieH bond [20]. The bonding force constant of SieO is 6.0  105 dyn cm1, which is much higher than that of SieH, 2.5  105 dyn cm1 and that of SieSi, 1.4  105 dyn cm1. The Eopt value is a weighted average of the valence and conduction band states. When many SieO bonds are formed in the film, the respective bonding and antibonding states widen [21]. The inductive effect of the O atoms back-bonding to Si is attributed to the modification of the SieSi bonding force constant. It is thought that the significant increase in the Nd value of the a-Si(H,O):H films is due to the increased number of dangling bonds that was induced by an enhanced structural disorder. Fig. 4 shows the variations in the sd and sph of the films deposited at different RO ratios. The sph/sd ratio, i.e., the photosensitivity, is also shown in Fig. 4. The a-Si(H,O):H films prepared at RO ¼ 0.14 had a higher sd, 1.26  109 S/cm, than the other films deposited at RO ¼ 0 and 0.09. According to a previous report [19], the sd of a-Si(H,O):H films is enhanced by a quasi-doping effect.

Conductivity (S/cm)

72

-9

10

4

-10

10

10

-11

10

0.00

0.05

0.10

0.15

RO (CO2/SiH4) Fig. 4. The changes in the dark conductivity, photoconductivity and photosensitivity of a-Si(H,O):H films deposited at various RO ratios.

Although the Eopt of the films is widened by the oxygen incorporation, the position of the Fermi level in relation to the conduction band edge remains almost unchanged. However, a gradual decrease in sph was observed with increasing RO. This result can be explained by the increased transparency and decreased optical absorption of the films due to the wider Eopt. Additionally, it is expected that the enhanced recombination of the photocarriers, which is due to the increased number of defects, contributes to the reduction in sph. The photosensitivity of the films significantly decreased from 4.61  105 to 8.07  103 when RO was increased from 0 to 0.14. This result indicates that a-Si(H,O):H films have poor photo-gain and are therefore not suitable for use as absorber layers in solar cells. As demonstrated above, when oxygen atoms are introduced into the Si

7

10 17

1.9

10

6

-1 3

Nd

Defect density, Nd( /cm )

Optical bandgap, Eopt (eV)

Eopt

Absorption coefficient (cm )

10

1.8 16

10 1.7

5

10

4

10

3

10

2

10

RH = 1.43

1

10

RH = 3.57 RH = 8.57

0

10 1.6 0.00

15

0.05

0.10

10 0.15

RO (CO2/SiH4) Fig. 3. The changes in the optical bandgap (Eopt) and in the defect density (Nd) of aSi(H,O):H films as a function of the RO ratio.

-1

10

1.0

1.5

2.0

2.5

3.0

h (eV) Fig. 5. The sub-bandgap optical absorption spectra of a-Si(H,O):H films deposited at various RH ratios.

J.E. Lee et al. / Solid State Sciences 20 (2013) 70e74

73

Table 2 The chemical, structural and optoelectronic properties and defect densities of a-Si:H and a-Si(H,O):H films deposited at various RH ratios.

Pure a-Si:H (RO ¼ 0) a-Si(H,O):H (RO ¼ 0.14)

RH (H2/SiH4)

CH (%)

1.43 1.43 3.57 8.57

9.02 9.80 10.92 12.02

I2075/I2000    

0.10 0.10 0.10 0.10

0 3.16  0.10 2.80  0.10 1.66  0.10

1.73 1.82 1.80 1.81

network, dangling bonds are essentially created due to structural imperfections. Therefore, to improve the optoelectronic properties of a-Si(H,O):H absorber layers, defects in the films should be removed as much as possible. Under moderately hydrogen-dilute plasma conditions, in which polyhydrogenation does not occur, the rehydrogenation of dangling bonds is expected to help reduce the defect density in the films. Fig. 5 shows the sub-bandgap optical absorption spectra that were estimated using the CPM for the aSi(H,O):H films prepared at RO ¼ 0.14 as a function of the H2/SiH4 (RH) ratio. The calculated Nd values of the films are listed in Table 2. By increasing RH from 1.43 to 8.57, significantly less absorption was observed within the sub-bandgaps of the films, as shown in Fig. 5, indicating that the defect density in the films decreased. For the film deposited at RH ¼ 1.43, Nd is 7.18  1016/cm3, as shown in Table 2. When RH was increased to 3.57, Nd decreased significantly to 2.35  1016/cm3. By further increasing RH to 8.57, a gradual decrease in Nd to 1.33  1016/cm3 was obtained. This result revealed that the defect density in a-Si(H,O):H films can be effectively reduced by using hydrogen passivation, confirmed by the increased hydrogen concentration (CH) in a-Si(H,O):H from 9.80% for the film prepared at RH ¼ 1.43 to 12.02% for that at RH ¼ 8.57. The reduction in Nd with increasing RH can also be identified by using the absorption spectra that have the range of 1900e2200 cm1, which are shown in Fig. 6. When RH was increased from 1.43 to 8.57, the peak intensity at 2075 cm1 decreased gradually. For the films deposited at a high hydrogen dilution (RH ¼ 3.57 and 8.57), an increased peak intensity at 2000 cm1, compared with that of the film deposited at a low hydrogen dilution (RH ¼ 1.43), was observed. The peak intensity ratio of 2075 cm1 to 2000 cm1 (I2075/I2000) decreased from 3.16 to 1.66 by increasing RH from 1.43 to 8.57. This result indicates that the number of the dangling bonds that induced dihydride stretching was reduced by the high hydrogen dilution, in good agreement with the Nd reduction and CH increase of the films. Table 2 also lists the optoelectronic properties of the films deposited at different RH. As shown in Table 2, the dependence of the Eopt

600

Absorption coefficient (cm

-1

)

RH = 1.43

500 400

RH = 3.57 RH = 8.57

300 200 100 0 2200

sdark

Eopt (eV)

(  1010 S/cm)    

0.02 0.02 0.02 0.05

3.18 12.65 2.75 1.99

   

0.20 0.20 0.20 0.20

sphoto

(  105 S/cm) 14.67 1.01 5.83 5.40

   

0.20 0.20 0.20 0.20

sphoto/sdark (  104)

Nd (  1016/cm3)

46.13 7.98 21.20 27.14

0.16 7.18 2.35 1.33

   

0.01 0.10 0.10 0.10

of the films on RH is almost negligible under the experimental conditions used in our study. For the films deposited at a high hydrogen dilution (RH ¼ 3.57 and 8.57), a decrease in sdark and an increase in sphoto, compared with the values of the films deposited at a low hydrogen dilution (RH ¼ 1.43), were observed. This phenomenon leads to an enhanced photo-gain (sphoto/sdark), from 7.98  104 for the film prepared at RH ¼ 1.43 to 2.71  105 for that at RH ¼ 8.57, indicating that the film quality is significantly improved by the high hydrogen dilution condition. To estimate the quality of the a-Si(H,O):H films as absorber layers, peien single junction solar cells were fabricated. For comparison, a solar cell with pure a-Si:H layers was also prepared under identical experimental conditions. Compared with the solar cell with the a-Si:H absorber, the cells with the a-Si(H,O):H films exhibited a 36.7% lower Jsc and a 7.7% lower FF. This result is mainly due to the decreased photo-gain and decreased conductivity of the a-Si(H,O):H films due to the increased Eopt of the films and to the inferior film quality. However, in terms of the open circuit voltage (Voc), the cells with the aSi(H,O):H films, even the cell with the film deposited at a low hydrogen dilution (RH ¼ 1.43), exhibited an increased Voc value of at least 0.86 V; the cell with the pure a-Si:H layer exhibited a Voc value of 0.77 V. This enhanced Voc for the cells with the a-Si(H,O):H films is induced by the increased Eopt of the absorber layers from 1.73 eV to 1.82 eV. For the cell with the film deposited at a high H dilution (RH ¼ 8.57), an additional increase in Voc to 0.91 V was obtained. As shown in Table 2, the number of defects induced by the oxygen incorporation was effectively reduced by the hydrogen passivation method. The decrease in the defect density that was due to a high hydrogen dilution contributed to the increase in the Voc of the solar cells. 4. Conclusions The influences of oxygen incorporation and hydrogen dilution on i a-Si(H,O):H films that were deposited using PECVD with a gas mixture of SiH4, CO2 and H2 were investigated. By introducing CO2 into the plasma, the optoelectronic properties and the chemical structures of the deposited films were strongly affected. Si atoms that were back-bonded to O atoms were newly created by the oxygen incorporation. These chemical bonds caused the Eopt of the deposited films to increase from 1.73 eV to 1.82 eV. A degradation in the photosensitivity of the films was observed due to newly formed dangling bonds. By increasing RH from 1.43 to 8.57, the defect density in the a-Si(H,O):H films significantly decreased from 7.18  1016/cm3 to 1.33  1016/cm3, resulting in an improved film quality. In the solar cells with the wide-bandgap i a-Si(H,O):H films that were deposited at a high RH, an enhanced Voc of 0.91 V was achieved, while a Voc of 0.77 V was obtained for the cells with pure i a-Si:H films. Acknowledgment

2150

2100

2050

2000

1950

1900

-1

Wave number (cm

)

Fig. 6. The absorption spectra over the range of 1900e2200 cm1 for a-Si(H,O):H films deposited at various RH ratios.

This work was supported by the Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Education, Science and Technology, Korea (2011-0031578).

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J.E. Lee et al. / Solid State Sciences 20 (2013) 70e74

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