**3.2.3 Solar cell**

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 [Sonobe et al., 2006].

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

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

Fabrication of the Hydrogenated

**4.2 Hydrogen elimination process during film growth** 

d. hydrogen elimination process through a cross-linking reaction

following and are schematically shown in figure 11. a. hydrogen abstraction reaction by an atomic hydrogen b. spontaneous thermal desorption of surface hydrogen c. hydrogen abstraction reaction by a SiH3 radical

Amorphous Silicon Films Exhibiting High Stability Against Light Soaking 313

The possible hydrogen elimination processes during the a-Si:H film growth are the

Fig. 11. Schematic of the hydrogen elimination processes during the growth of a-Si:H.

generated at the rate constant of ~ 3×10-12 cm3/s [Kushner, 1988; Perrin et al., 1996]:

elimination process is not dominated by atomic hydrogen in the triode system.

Atomic hydrogen exists in an silane plasma [e.g., Matsuda, 2004]. It reacts with a bonded hydrogen of a film and forms H2 molecule, resulting in a hydrogen elimination. The probability of this reaction should be proportional to the flux of atomic hydrogen. In a silane plasma, generated radicals and ions collide with SiH4 molecule of which density is high in the gas phase. When the atomic hydrogen reacts with SiH4, SiH3 radical and H2 molecule are

The stable H2 molecule does not contribute to the abstraction of the bonded hydrogen. In the triode system, basically no atomic hydrogen is generated but only the collisions take place in the region between the mesh and the substrate, indicating that the density of atomic hydrogen near the substrate is low. Therefore, it is natural to say that the hydrogen

The hydrogen desorption process from Si-H bond has been studied elsewhere [Toyoshima et al., 1991]. The activation energy of this reaction is estimated as 2 - 3 eV, and the reaction takes place only in the temperature range higher than 400 oC [Beyer & Wagner, 1983]. Therefore, it is unlikely that the spontaneous hydrogen desorption takes place under the

H + SiH4 SiH3 + H2. (1)

a. Hydrogen abstraction reaction by an atomic hydrogen

b. Spontaneous thermal desorption of surface hydrogen

substrate temperature of 250 oC as in our case.

method with the same i-layer thickness. While further optimization is necessary to achieve higher stabilized efficiency, the result demonstrates the low degradation ratio of the a-Si:H solar cell with improving the stability of the i-layer itself, which is one of the essential solutions to obtain a stable a-Si:H solar cell.

### **4. Hydrogen elimination process**

#### **4.1 Hydrogen elimination process – post annealing**

It is observed that the films grown by the triode system contain very low hydrogen concentrations, namely Si-H2 bond densities. Those values change with the distance between the mesh and the substrate where the lowest hydrogen concentration is observed at the largest distance between the mesh and the substrate. In this section, we will discuss the possible mechanism for the reduction of Si-H and Si-H2 bond densities in the triode deposition system.

Fig. 10. Thermal effusion of hydrogen from the a-Si:H films deposited at 110 o C. The *C*H is the sum of the Si-H and the Si-H2 bond densities [Shimizu et al., 2007].

Hydrogen elimination takes place both in a film growth state and in a post annealing state when a substrate temperature is high. To distinguish it in our case, at first, the thermal annealing tests were performed on the a-Si:H films prepared at the low substrate temperature of 110 oC using the diode system. The as-deposited films contain the large initial hydrogen concentrations (*C*H) of c.a. 27 at.%. After the growth, the individual film was kept in the deposition chamber and was annealed for 30 minutes at the certain temperature. The result is shown in figure 10 [Shimizu et al., 2007]. One can see that the hydrogen concentration is reduced at the high annealing temperatures. On the other hand, at the temperature of 250 oC, which is the substrate temperature used in our triode deposition system, no *C*H reduction takes place at least from the bulk. The result shows that under the substrate temperature of 250 oC, the hydrogen elimination process takes place during the film growth, i.e., most likely with gas reactions.
