**1. Introduction**

302 Solar Cells – Thin-Film Technologies

News release by SANYO on 22nd May, 2009, SANYO Develops HIT Solar Cells with

World's Highest Energy Conversion Efficiency of 23.0%. < http://panasonic.net/sanyo/news/2009/05/22-1.html>.

6436-6443, ISSN 0021-8979.

interface roughness, *Journal Applied Physics* , Vol 88, No. 11 (December, 2000) pp.

A hygrogenated amorphous silicon (a-Si:H) thin film solar cell was first reported in 1976 [Carlson, & Wronski, 1976]. Since then, intensive works have been carried out for the improvement of its performances. Attempt to increase the conversion efficiencies of the thin film solar cells, a multi junction solar cell structure was proposed and has been investigated [Yang et al., 1997; Shah et al., 1999; Green, 2003; Shah et al., 2004]. It consists of the intrinsic layers having different optical bandgaps in order to absorb the sunlight efficiently in a wide spectrum range.

The density of photo-generated carriers is determined by the light absorption coefficient and the defect density of a material. The absorption coefficient of a-Si:H in a visible light region is one order magnitude higher than that of c-Si:H due to the direct transition phenomenon. Therefore, a thin a-Si:H layer absorbs sufficient photons. This is a huge advantage for the thin film based solar cell technology in which mass production should be definitely taken into account.

However, a-Si:H has another aspect known as a Staebler-Wronski effect, i.e., the number of unpaired Si dangling bonds increases with light soaking, which lowers photocarrier density by decreasing carrier lifetime [Staebler & Wronski, 1977]. Indeed, conversion efficiencies of a-Si:H based solar cells deteriorate generally by 15-20 % due to this phenomenon. On the other hand, it is possible to suppress this deterioration to some extent by reducing a film thickness of a-Si:H with efficient light-trapping structures [e.g., Müller et al., 2004]. Indeed, the fabrication of the highly stabilized a-Si:H single junction solar cell by the precise optimizations of the optical properties and the i-layer thickness has been reported [Borrello et al., 2011]. Besides those intensive efforts, establishing the technique for fabricating highly stable a-Si:H films is essentially very important to extract its maximum potential for the solar cell applications.

Fabrication of the Hydrogenated

the substrate (*d*ms) is adjustable.

**3.1.1 Hydrogen concentration** 

Amorphous Silicon Films Exhibiting High Stability Against Light Soaking 305

prepared with a conventional diode system where no mesh is installed. In this case, the

Fig. 1. Schematic of the a-Si:H growth chamber used in this study. A negatively dc-biased mesh is installed between the cathode and the substrate. The distance between the mesh and

The densities of Si–H and Si–H2 bonds in the resulting film deposited on a intrinsic Si substrate were calculated from the integrated intensities of the stretching modes in a Fourier transform infrared spectroscopy (FTIR) spectrum, where the proportional constants are 9.0×1019 cm2 for Si–H and 2.2×1020 cm2 for Si–H2, respectively [Langford et al., 1992]. The neutral spin density of the film deposited on a quartz substrate was measured by electron paramagnetic resonance (EPR). To study light-soaking stability of the film, a Schottky diode was fabricated on a phosphorous doped n+Si substrate (0.03 cm) with a half transparent Ni electrode on the top (n+Si/a-Si:H/Ni). The native surface oxide layer on the n+Si substrate

A p-i-n structured solar cell (5×5 mm2) was fabricated in a multi-chamber system. The doped layers were prepared in conventional diode system chambers, and the i-layer was fabricated in a triode system chamber at 180 oC. The distance between the mesh and the substrate is 1.5 cm. The other detailed conditions for the solar cell fabrication are described elsewhere [Sonobe et al., 2006]. The I–V characteristics of the solar cells were measured under an illumination of AM 1.5, 100 mW/cm2 white light. In every case, the light degradation

The hydrogen concentrations of the a-Si:H films prepared by the triode system were measured by FTIR. Figure 2 (a) shows the spectrum of the film prepared at 250 oC with the

was performed by illuminating intense 300 mW/cm2 white light for 6 h at 60 oC.

**3. Properties and stabilities of the triode-deposited a-Si:H 3.1 Properties of the a-Si:H films prepared by the triode system** 

was etched with diluted HF solution before the growth of a-Si:H.

distance between the cathode and the substrate is fixed at 2 cm.

Phenomenologically, a good correlation is observed between degradation ratio of a-Si:H and its hydrogen concentration, namely Si-H2 bond density where a low Si-H2 bond density film exhibits high stability [Takai et el., 2000]. Although the detailed microscopic model for explaining this correlation has not been revealed yet, the tendency is observed in the films prepared under the wide range of fabrication conditions [Nishimoto et al., 2002]. One of the methods to reduce a hydrogen concentration is to increase a substrate temperature. However, a high processing temperature results in increasing initial defect density. Additionally, it is preferable to use the processing temperature of around or less than 200 oC from the viewpoint of low cost fabrications. Reducing Si-H2 bond density without increasing a substrate temperature is one of the key issues for the fabrication of stable a-Si:H films.

In a chemical vapor deposition process, there are mainly two steps to be considered, i.e., 1) gas phase reactions and 2) surface reactions. In the first step, depending on the electron temperature in a silane plasma, several types of precursors are generated, and they play an important role on the properties of resulting films [Matsuda, 2004]. For example, the a-Si:H films prepared under a powder rich gas condition have very high initial defect densities, namely at the low substrate temperatures [e.g., Roca i Cabarrocas, 2000]. Those powders or so-called higher-ordered silane radicals are created by the insertion reactions of SiH2 radicals produced generally under a high electron temperature condition in a silane plasma. This insertion reaction is a rapid process. The SiH2 radicals are created even under a relatively low electron temperature condition because it is statistically difficult to eliminate only high energy electrons from the system. A higher-ordered silane radical causes a steric hindrance and inhibits short range-ordered sp3 bond formations on the film growing surface. For example, it is observed that the Si-H2 bond density in the film, which has correlation with light-induced degradation of a-Si:H, increases when the density of the higher-ordered silane radicals in a gas phase is high [Takai et al., 2000].

In this work, to study the effect of precursors in a gas phase on the properties of the resulting film, a triode deposition system is applied for the growth of a-Si:H films where a mesh is installed between a cathode and a substrate. With such a configuration, a long lifetime radical such as SiH3 mainly contributes to the film growth [Matsuda & Tanaka, 1986]. The properties and the stabilities of the resulting films are evaluated.

#### **2. Fabrication and evaluation methods**

The preparations of a-Si:H films were performed using a triode deposition system. Figure 1 shows the schematic of the system. A mesh is placed between the cathode and the substrate scepter in which a heater is mounted. VHF (100 MHz) voltage is applied on the cathode with the 20 sccm of SiH4 gas flow, and a silane plasma is generated between the cathode and the negatively dc-biased mesh. All the films were prepared at 100 mTorr (13.3 Pa). The deposition precursors pass through the mesh and reach to the substrate. The substrate scepter is movable, and the distance between the mesh and the substrate (*d*ms) is one of the important deposition parameters. The distance between the cathode and the mesh is fixed at 2 cm. In some cases, an additional mesh is installed behind the pre-existing mesh with the distance of 1.5 mm at which no plasma is generated between the two mesh under our conditions. The volume of the chamber is c.a. 1.1×104 cm3, and its base pressure is c.a. 3×10-8 Torr. The diameters of the electrodes are 10 cm. As a comparison, a-Si:H films were also

Phenomenologically, a good correlation is observed between degradation ratio of a-Si:H and its hydrogen concentration, namely Si-H2 bond density where a low Si-H2 bond density film exhibits high stability [Takai et el., 2000]. Although the detailed microscopic model for explaining this correlation has not been revealed yet, the tendency is observed in the films prepared under the wide range of fabrication conditions [Nishimoto et al., 2002]. One of the methods to reduce a hydrogen concentration is to increase a substrate temperature. However, a high processing temperature results in increasing initial defect density. Additionally, it is preferable to use the processing temperature of around or less than 200 oC from the viewpoint of low cost fabrications. Reducing Si-H2 bond density without increasing a substrate temperature is one of the key issues for the fabrication of

In a chemical vapor deposition process, there are mainly two steps to be considered, i.e., 1) gas phase reactions and 2) surface reactions. In the first step, depending on the electron temperature in a silane plasma, several types of precursors are generated, and they play an important role on the properties of resulting films [Matsuda, 2004]. For example, the a-Si:H films prepared under a powder rich gas condition have very high initial defect densities, namely at the low substrate temperatures [e.g., Roca i Cabarrocas, 2000]. Those powders or so-called higher-ordered silane radicals are created by the insertion reactions of SiH2 radicals produced generally under a high electron temperature condition in a silane plasma. This insertion reaction is a rapid process. The SiH2 radicals are created even under a relatively low electron temperature condition because it is statistically difficult to eliminate only high energy electrons from the system. A higher-ordered silane radical causes a steric hindrance and inhibits short range-ordered sp3 bond formations on the film growing surface. For example, it is observed that the Si-H2 bond density in the film, which has correlation with light-induced degradation of a-Si:H, increases when the density of the

In this work, to study the effect of precursors in a gas phase on the properties of the resulting film, a triode deposition system is applied for the growth of a-Si:H films where a mesh is installed between a cathode and a substrate. With such a configuration, a long lifetime radical such as SiH3 mainly contributes to the film growth [Matsuda & Tanaka,

The preparations of a-Si:H films were performed using a triode deposition system. Figure 1 shows the schematic of the system. A mesh is placed between the cathode and the substrate scepter in which a heater is mounted. VHF (100 MHz) voltage is applied on the cathode with the 20 sccm of SiH4 gas flow, and a silane plasma is generated between the cathode and the negatively dc-biased mesh. All the films were prepared at 100 mTorr (13.3 Pa). The deposition precursors pass through the mesh and reach to the substrate. The substrate scepter is movable, and the distance between the mesh and the substrate (*d*ms) is one of the important deposition parameters. The distance between the cathode and the mesh is fixed at 2 cm. In some cases, an additional mesh is installed behind the pre-existing mesh with the distance of 1.5 mm at which no plasma is generated between the two mesh under our conditions. The volume of the chamber is c.a. 1.1×104 cm3, and its base pressure is c.a. 3×10-8 Torr. The diameters of the electrodes are 10 cm. As a comparison, a-Si:H films were also

higher-ordered silane radicals in a gas phase is high [Takai et al., 2000].

1986]. The properties and the stabilities of the resulting films are evaluated.

**2. Fabrication and evaluation methods**

stable a-Si:H films.

prepared with a conventional diode system where no mesh is installed. In this case, the distance between the cathode and the substrate is fixed at 2 cm.

Fig. 1. Schematic of the a-Si:H growth chamber used in this study. A negatively dc-biased mesh is installed between the cathode and the substrate. The distance between the mesh and the substrate (*d*ms) is adjustable.

The densities of Si–H and Si–H2 bonds in the resulting film deposited on a intrinsic Si substrate were calculated from the integrated intensities of the stretching modes in a Fourier transform infrared spectroscopy (FTIR) spectrum, where the proportional constants are 9.0×1019 cm2 for Si–H and 2.2×1020 cm2 for Si–H2, respectively [Langford et al., 1992]. The neutral spin density of the film deposited on a quartz substrate was measured by electron paramagnetic resonance (EPR). To study light-soaking stability of the film, a Schottky diode was fabricated on a phosphorous doped n+Si substrate (0.03 cm) with a half transparent Ni electrode on the top (n+Si/a-Si:H/Ni). The native surface oxide layer on the n+Si substrate was etched with diluted HF solution before the growth of a-Si:H.

A p-i-n structured solar cell (5×5 mm2) was fabricated in a multi-chamber system. The doped layers were prepared in conventional diode system chambers, and the i-layer was fabricated in a triode system chamber at 180 oC. The distance between the mesh and the substrate is 1.5 cm. The other detailed conditions for the solar cell fabrication are described elsewhere [Sonobe et al., 2006]. The I–V characteristics of the solar cells were measured under an illumination of AM 1.5, 100 mW/cm2 white light. In every case, the light degradation was performed by illuminating intense 300 mW/cm2 white light for 6 h at 60 oC.
