plesiodon Verzi, Vucetich y Montalvo, 1994

36

3.2 a-Si:H/c-Si interface recombination

modeling results

This section presents the results of recombination modeling at a-Si:H/c-Si interface and compares them with the experimental results. Based on the molding results, the a-Si:H passivation mechanism is addressed. Finally, the effect of each passivation mechanism (addressed in section 2.2.1) on the injection level dependent is discussed.

3.2.1 Understanding a-Si:H passivation mechanism

It should be noted that the values for Ns and Qs obtained by our modeling are no absolute

values, however, they could be used for a relative comparison of various samples [85]. In our model, we have used the following values, taken from Refs [50,57,86]. The charge velocity at room temperature in Si is , the neutral electron to hole capture cross

section is , the charged to neutral capture cross section is and the hole capture cross section is .

Figure 3.5: The injection level dependent for samples deposited at various temperatures after the

post-deposition annealing. The data points represent the experimental results. The solid and dashed lines illustrate the simulation data. The Ns and Qs values, obtained from our molding are reported in

table 3.3.

Figure 3.5 shows the injection level dependent for the a-Si:H passivation layers, deposited at various temperatures. The other PECVD parameters are kept constant, as reported in table 3.2. The solid and dashed lines represent the modeling results whereas the data points show the experimental results. As figure 3.5 suggests, the experimental data points are match with the modeling results. The Ns and Qs values, obtained from our molding are

3.2 a-Si:H/c-Si interface recombination modeling results

37 Table 3.3: Interface defect density (Ns) and surface charge density (Qs) extracted from modeling for

the a-Si:H passivation layers, deposited at various temperatures.

Sample Deposition temperature (°C) Measurement mode Ns (1010 cm-2) Qs (109cm-2) X050-1-067 150 QSS 2.7 9 X050-1-067 150 Transient 3.4 7.9 X050-1-068 180 QSS 1.95 9.3 X050-1-068 180 Transient 2.8 8.8 X050-1-066 200 QSS 0.96 9.4 X050-1-066 200 Transient 1.5 9.2 X050-1-070 220 QSS 0.93 9.6 X050-1-070 220 Transient 1.4 9.6 X050-1-071 240 QSS 1.1 9.4 X050-1-071 240 Transient 1.7 9.1

Figure 3.6 demonstrates the Ns value (extracted from our modeling) with respect to the

deposition temperature for the QSS mode. While almost similar Qs values (see table 3.3) are

extracted from modeling for all the a-Si:H passivation layers, there is a direct relation between the improved passivation and the reduction in Ns (see figure 3.6). These results suggest that

the reduction of the interface defect density is the main mechanism of the surface passivation by using a-Si:H films. Thus, it can be concluded that the a-Si:H passivation mechanism is a chemical passivation.

As mentioned above, the closed-form recombination model allows us to distinguish between the two fundamental approaches for the surface recombination reduction; either reduction of the interface defect density (Ns parameter in the model) or reduction of the

surface density of one carrier type (Qs). In practice, the interface defect density reduction

could be achieved by reducing the recombination center density (dangling bonds at a-Si:H/c- Si interface). The reduction of one carrier type is technologically realized by implementing a dopant profile either underneath the c-Si surface or by creating an electric field by means of electrostatic charges with an overlaying doped layer. The injection level dependent are not equally affected by these passivation approaches.

3.2 a-Si:H/c-Si interface recombination modeling results

38 Figure 3.6: The variation of the measured (black square) and the interface defect density (Ns)

(blue triangle, extracted from modeling) with respect to the deposition temperature of the a-Si:H passivation layers. The improved with reduction of Ns indicates that passivation formed by using

intrinsic a-Si:H films, is a chemical passivation.

3.2.2 The effect of passivation mechanisms on the

injection level dependent lifetime

Figure 3.7(a) shows the modeled injection level dependent for the set Qs value of 9.7

×109 (cm-2) and various Ns values. As this graph suggests, the variation of interface defect

density (Ns) equally affects all the injection level dependent values for the injection levels lower than 1016. This can also be deduced from equation 2.18. As the recombination rate is in direct relation with Ns ( ), by reducing Ns, increases for all the

excess carrier densities with the same factor.

Figure 3.7: (a) The effect of variation of the interface defect density on the injection level dependent

.The effect of the interface defect density is observed on all injection levels. (b) The effect of the

field effect passivation on the injection level dependent compared to the effect of the interface defect density variation. The influence of the field effect passivation is more weighted at low injection levels [Courtesy of S. Glunz].

3.2 a-Si:H/c-Si interface recombination modeling results

39 The effect of the field effect passivation on the injection level dependent lifetime is different from the effect of the interface defect density variation. Figure 3.7(b) shows the effect of the field effect passivation on the injection level dependent lifetime compared to the effect of interface defect density variation. As this figure suggests, the effect of the field effect passivation is not the same on all the excess carrier densities and is more weighted at low injection levels [87]. The results of the current section have been used in chapter 4 to explain the passivation mechanism of the µc-Si:H emitter layers where both passivation mechanisms could simultaneously be obtained.

It should be noted that good passivation is the first essential step required for the fabrication of well-functioning SHJ solar cells. The results, presented in chapter 3, are well in-line with already published reports from other groups. The new results and achievements within the framework of this thesis are presented in the following chapters.