**6. Conclusions**

316 Solar Cells – Thin-Film Technologies

radicals and ions in the plasma are basically the same in the both cases. When the VHF power is reduced to 2 W with a single mesh, the observed growth rate is 0.2 Å/s which is the similar value observed at the VHF power of 10 W with the double mesh (0.1 Å/s). On the other hand, the observed Si-H and Si-H2 bond densities are different each other where lower values are observed in the low power case in which less higher-ordered silane radicals are produced due to a low electron temperature effect [Matsuda, 2004]. The results indicate that the gas phase condition is very important for determining a hydrogen concentration in the resulting film. Note that, when a plasma is unstable even in the triode case due to the lack of electrical matching, the observed Si-H and Si-H2 densities are higher than the expected values shown in

The properties and the stabilities of the a-Si:H films prepared by the triode deposition system have been demonstrated in this study. The quality of the film is very good, and it exhibits very high stability against light soaking. Although the triode method reduces a growth rate of a film due to its configuration, which is a disadvantage for mass productions, we used this system to study the fundamental features. Several results indicate that the control of the gas phase condition is one of the essential factors to obtain a stable a-Si:H film. Based on this knowledge, one could establish the alternative fabrication methods which can

In our result, the degree of degradation correlates well with Si-H2 bond density in the film, which also corresponds to the former works. Beside the possibility of micro-void structure formation, one can propose the existence of chain-like Si-Si structures when the film contains large Si-H2 bond density. Since it is a flexible structure, it can cause instability against light soaking. In figure 12 the *FF* (= *FF*ini – *FF*deg) of the Schottky diode is plotted against the Si-H2 bond density. It is a re-plot of figure 8 in a semi-log scale. The extrapolated line shows that the *FF* value is zero at the Si-H2 bond density of c.a. 1.3×1019 cm-3 (≈ 0.03 at.%). Although it is a hypothesis, the correlation indicates the guideline for the fabrication of stable a-Si:H films.

Fig. 12. Light-induced change in the fill-factor (*FF = FF*ini*-FF*deg) of the Schottky diode as a

function of the Si-H2 bond density (re-plot of fig. 8 in a semi-log scale).

produce the preferable gas phase condition for a stable a-Si:H fabrication.

figure 3 (the higher value date are not presented).

**5. Prospects for the future applications** 

Stable a-Si:H films against light soaking are prepared with adopting a triode deposition method where a mesh is placed between a cathode and a substrate. The resulting films contain very low Si-H and Si-H2 bond densities compared with those observed in the films prepared by a conventional diode electrode method at the same substrate temperature. The hydrogen reduction effect is higher when the distance between the mesh and the substrate is increased. The films exhibit low initial defect densities and high photosensitivities. After the light soaking, high stabilities are observed in the films prepared by the triode system. The high stabilities of the films are also confirmed with the device configurations. It is most likely that the density of the precursors that reach to the growing surface is different each other in the triode and the diode systems. Control of gas phase condition is one of the key issues to fabricate stable a-Si:H films and related solar cells.
