**3.1.4 Conductivity**

308 Solar Cells – Thin-Film Technologies

Fig. 4. Si-H and Si-H2 bond densities in the a-Si:H films fabricated under the various conditions. Open and closed circle: VHF = 2 W, with a single mesh (2 W, SM), open and closed square: VHF = 10 W, with a single mesh (10 W, SM), open and closed triangle: VHF = 10 W, with double mesh (10 W, DM). As a comparison, those of the conventionally prepared

films without the mesh are also shown (diode, *d*ms = 0 cm). [Shimizu et al., 2007]

Fig. 5. FWHM of the Si-H and Si-H2 stretching mode peaks in the FTIR spectra platted against the density of Si-H and Si-H2, respectively. The films were prepared at the VHF input power of 2 or 10 W using the each electrode configuration (triode or diode) as

indicated.

The conductivities of the a-Si:H films fabricated using the triode system are measured. Figure 6 shows the dark and photoconductivities of the films. The photoconductivity was measured under the illumination of 100 mW/cm2 white light. The observed darkconductivities are of the order of 10-11 S/cm. The deposition rate of the triode system is typically less than 1 Å/s, which may cause unfavorable impurity incorporations during the film growth, causing the reduction of photosensitivity due to the increase of darkconductivity. The dark-conductivity of the triode-deposited a-Si:H is, however, in the range equivalent to that observed in the diode-deposited film grown at 7.3 Å/s, and the photoconductivities of those films are of the order of 10-5 S/cm. The result indicates that the triode-deposited a-Si:H films do not contain substantial number of impurities which deteriorates photosensitivity.

Fig. 6. The dark and photoconductivities of the a-Si:H films prepared either by a triode or a diode deposition system (*d*ms = 0 cm).

### **3.2 Stabilities of the triode-deposited a-Si:H films 3.2.1 Spin density**

Degradation of the film prepare by the triode system is checked by measuring the change of neutral spin density by light soaking. Figure 7 shows the result [Shimizu et al., 2008]. All the films were prepared at 250 oC, and as a comparison, the results of the diode-deposited films are also shown. The spin density is plotted against Si-H2 bond density. The initial defect densities are almost the same throughout the samples (≈ 2×1015 cm-3). On the other hand,

Fabrication of the Hydrogenated

**3.2.2 Schottky diode**

**3.2.3 Solar cell** 

[Sonobe et al., 2006].

substrate is 1.5 cm. [Sonobe et al., 2006]

Amorphous Silicon Films Exhibiting High Stability Against Light Soaking 311

Furthermore, the stabilities of the triode-deposited a-Si:H films were studied with fabricating the Schottky diodes where their fill-factor (*FF*) changes were evaluated as a measure of degradation. The intrinsic layer of the Schottky diode was fabricated either by a triode or a diode system under the various conditions. The fill-factors in the initial state (*FF*ini) are almost the same throughout the samples: 52 - 54 %. On the other hand, the fill-factors in the degraded state (*FF*deg) are different each other. In figure 8, the change in the fill-factor (*FF* = *FF*ini – *FF*deg) is plotted against Si-H2 bond density [Shimizu et al., 2005]. For comparison, those of the films prepared with the diode system under the various conditions are also shown [Nishimoto et el., 2002]. One can see that the triode-deposited a-Si:H films contain low Si-H2 bond densities, and correspondingly, the observed *FF*s are low. Note that, the scattered correlation is observed

The stability of the triode-deposited a-Si:H is checked with fabricating a p-i-n solar cell where the i-layer is deposited with a triode system. Since a multi chamber was used to prepare the solar cell, the i-layer fabrication conditions including the chamber geometry are different from those used in the previous sections. Especially, the distance between the mesh and the substrate is short as 1.5 cm which lowers the effect of Si-H2 bond elimination than that achieved at larger distances as shown in figure 3. Additionally, the i-layer growth temperature of 180 oC was chosen. Therefore, the Si-H2 bond density in the i-layer is slightly high as indicated in figure 3. On the other hand, we chose this temperature from the viewpoint of the device applications in which low temperature operations are preferable. The i-layer thickness is 250 nm. The I-V characteristic of the solar cell is shown in figure 9

Fig. 9. The I-V characteristic of the p-i-n solar cell. The i-layer was prepared with the triode system at the substrate temperature of 180 oC. The distance between the mesh and the

The initial conversion efficiency is 10.0 %, and after the light soaking, the stabilized efficiency of 9.2 % is achieved. The degradation ratio is 7.8 % which is the lower value compared with that generally observed in the a-Si:H solar cell prepared by a conventional

when *FF*s are plotted against the Si-H densities of the films [Shimizu et al., 2005].

more stable behaviors are observed in the triode-deposited a-Si:H films in the degraded states. The trend is best seen in the film prepared at the *d*ms of 4 cm where the lowest Si-H2 bond density is observed as shown in figure 3.

Fig. 7. Change in the neutral spin density (*N*s) due to light soaking as a function of Si-H2 bond density in the film. [Shimizu et al., 2008]

Fig. 8. Light-induced change in the fill-factor (*FF = FFini - FFdeg*) of the Schottky diode having the intrinsic layer produced at the each condition. Closed circle: triode-deposited film (triode), open circle: conventionally prepared film (diode). [Shimizu et al., 2005]
