*2.2.2 Evolution of photoluminescence spectra as a function of the anodization current density*

**Figure 4** shows the PL spectra of p-type samples that were obtained from different etching current densities, 5, 10, 15 and 20 mA/cm<sup>2</sup> , and etching time of 180 s. The presented serial in **Figure 1** indicates near similar variations of the PL

*Evolution of photoluminescence spectra of P-type porous layers prepared at different etching current density.*

**113**

**Table 3.**

*different current densities [31].*

*Optical Study of Porous Silicon Layers Produced Electrochemically for Photovoltaic Application*

intensity. However, a remarkable increase of the PL intensity ranges from 2.59 to

The two spectra show an improvement in the PL intensity as a function of etching current density and etching time. On the other hand, we noticed that the

tm = 240 s. The slight blue-shift energy of PL band allow us to attribute a confinement of more and more wells and wire that leads to diminution of nanocrystallite

The porous layer obtained from p-type silicon wafer is presented as cylindrical and spherical crystallites [29, 30]. Therefore, PS is defined as a mixture of quantum wells (QWs) and quantum wire following different concentrations and sizes. In the case of the increased etching time, the thickness of porous silicon layer increases

**Figure 5** shows the variation of the PL intensity and the width at half height

The width at half height (FWHM) decreases according to the current density; this is due to the increase in porosity, therefore the increase in the intensity of PL. **Figure 6** depicts that the thickness and the integral intensity of the PL increase as function of etching time, meanwhile the porosity and the integral intensity of PL increase as a function of the current density **Figure 7**. On the other hand, the PL intensity of the porous layer increases as a function of etching time and current

**Samples Current density (mA/Cm2) λPeak (nm) FWHM (meV) IPLMax (u,a) Eg (eV)** S5 5 682 249 2.59 1.81 S6 10 665 210 4.12 1.87 S7 15 652 122 6.81 1.91 S8 20 655 111 11.72 1.90

*Optimized fitting parameters corresponding to the theoretical curves of porous silicon samples prepared at* 

S1 60 15 0.1063 36.02 / / S2 120 15 0.7061 58.84 / / S3 180 15 0.9058 70.20 / / S4 240 15 1.0272 76.89 / / S5 180 5 0.0985 34.53 1.77 0.0035 S6 180 10 0.7221 61.45 1.43 0.0022 S7 180 15 0.9058 70.20 1.33 0.0018 S8 180 20 1.0311 78.23 1.22 0.0014

*Different parameters evaluated from the optical model of samples obtained at different etching times and* 

**Thickness (μm) (d)**

**Porosity (%) (P)**

**n k**

**Current density (mA/Cm2 )**

, with

PL intensity reached the maximum value of JCM, that equals 20 mA/cm<sup>2</sup>

(FWHM) as a function of the anodization current density.

*DOI: http://dx.doi.org/10.5772/intechopen.93720*

11.72 was noticed (**Table 2**).

from 106.3 to 1027.2 nm (**Table 3**).

*different etching current densities and etching time of 180 s.*

**time (s)**

**Samples Etching** 

size [27, 28].

density.

**Table 2.**

*Optical Study of Porous Silicon Layers Produced Electrochemically for Photovoltaic Application DOI: http://dx.doi.org/10.5772/intechopen.93720*

intensity. However, a remarkable increase of the PL intensity ranges from 2.59 to 11.72 was noticed (**Table 2**).

The two spectra show an improvement in the PL intensity as a function of etching current density and etching time. On the other hand, we noticed that the PL intensity reached the maximum value of JCM, that equals 20 mA/cm<sup>2</sup> , with tm = 240 s. The slight blue-shift energy of PL band allow us to attribute a confinement of more and more wells and wire that leads to diminution of nanocrystallite size [27, 28].

The porous layer obtained from p-type silicon wafer is presented as cylindrical and spherical crystallites [29, 30]. Therefore, PS is defined as a mixture of quantum wells (QWs) and quantum wire following different concentrations and sizes. In the case of the increased etching time, the thickness of porous silicon layer increases from 106.3 to 1027.2 nm (**Table 3**).

**Figure 5** shows the variation of the PL intensity and the width at half height (FWHM) as a function of the anodization current density.

The width at half height (FWHM) decreases according to the current density; this is due to the increase in porosity, therefore the increase in the intensity of PL.

**Figure 6** depicts that the thickness and the integral intensity of the PL increase as function of etching time, meanwhile the porosity and the integral intensity of PL increase as a function of the current density **Figure 7**. On the other hand, the PL intensity of the porous layer increases as a function of etching time and current density.


#### **Table 2.**

*Solar Cells - Theory, Materials and Recent Advances*

(FWHM) as a function of the anodization time.

**Figure 3** shows the variation of the PL intensity and the width at half height

From **Figure 3**, we notice that the width at half height of the PL spectra decreases as a function of the anodization time. As known the thickness of the porous layer increases as a function of the anodization time, then deduces that the width at half height decreases, and the intensity of PL increases as a function of the thickness of the porous layer. This is due to the decrease in the sizes of nanocrystallites. However, note that the intensity of PL increases as a function of the

*The variation of the intensity, PL, and the width at half height (FWHM) as a function of the anodization* 

*2.2.2 Evolution of photoluminescence spectra as a function of the anodization* 

different etching current densities, 5, 10, 15 and 20 mA/cm<sup>2</sup>

**Figure 4** shows the PL spectra of p-type samples that were obtained from

180 s. The presented serial in **Figure 1** indicates near similar variations of the PL

*Evolution of photoluminescence spectra of P-type porous layers prepared at different etching current* 

, and etching time of

**112**

**Figure 4.**

*density.*

anodization time.

**Figure 3.**

*time.*

*current density*

*Optimized fitting parameters corresponding to the theoretical curves of porous silicon samples prepared at different etching current densities and etching time of 180 s.*


#### **Table 3.**

*Different parameters evaluated from the optical model of samples obtained at different etching times and different current densities [31].*

**Figure 5.**

*The variation of the PL intensity and the width at half-height (FWHM) as a function of the anodization current density.*

#### **Figure 6.**

*Variation of thickness and integrated PL intensity according to etching time [31].*

**Figure 7.** *Variation of porosity and integrated PL intensity according to etching current [31].*

#### **2.3 Study of low temperature photoluminescence**

This part is devoted to the study of the evolution of the PL band as a function of temperature in order to identify the nature of the energy levels which are at the origin of this PL and to understand the mechanisms of radiative recombinations which participate in this PL. We have studied the variation of intensity and the integrated intensity all as a function of temperature in the range [10–300 K].

**115**

**Figure 8.**

*substrate (100).*

processes:

temperatures.

*Optical Study of Porous Silicon Layers Produced Electrochemically for Photovoltaic Application*

The sample studied is of type N (100), produced under these conditions:

.

i.At low temperatures (T < 50 K), the decrease in IPL as a function of the increase in temperature is attributed to the transfer of excitons to states of lower energies in a non-radiative manner by thermal activation (**Figure 8**).

(50 K ≤ T ≤ 80 K) indicates that the excitons are trapped in the lower localized states are thermally activated toward the higher states and then recom-

iii.At higher temperatures (T ≥ 80 K), thermal activation becomes more dominant and localized excitons become free and can diffuse in a non-radiative manner in the structure leading to a decrease in the intensity of PL [32].

iv.In the same figure, we notice the appearance of an intensity peak of PL at a

ii.The second is transfer by thermal dissociation [34] which is negligible at low

But we have shown that the variation in the intensity of PL with temperature is

Ten et al. have shown that the increase in temperature could increase the tunneling process [35]. However, Hua et al. confirm that thermal dissociation of excitons increases with temperature, which favors the leakage of excitons from QDs to QWs at lower energy levels [36]. In **Figure 9**, we represent the integrated

*Evolution of PL intensities as a function of the temperature of a SiP layer produced on an N-type* 

According to Zhao et al. and Weng et al., there are two types of transfer

ii.While increasing the intensity of PL in the temperature range of

bine radiatively and generate an increase in IPL intensity [32].

*DOI: http://dx.doi.org/10.5772/intechopen.93720*

[HF] = 16%, t = 3 min and j = 20 mA/cm2

*2.3.1 Evolution of PL intensities as a function of temperature*

characteristic temperature TM = 80 K.

i.The first is a tunnel transfer [33, 34].

*2.3.2 The intensity of PL as a function of the inverse of temperature*

due to the presence of these two exciton injection processes.

*Optical Study of Porous Silicon Layers Produced Electrochemically for Photovoltaic Application DOI: http://dx.doi.org/10.5772/intechopen.93720*

The sample studied is of type N (100), produced under these conditions: [HF] = 16%, t = 3 min and j = 20 mA/cm2 .

*2.3.1 Evolution of PL intensities as a function of temperature*

*Solar Cells - Theory, Materials and Recent Advances*

**114**

**Figure 7.**

**Figure 5.**

**Figure 6.**

*current density.*

**2.3 Study of low temperature photoluminescence**

*Variation of porosity and integrated PL intensity according to etching current [31].*

*Variation of thickness and integrated PL intensity according to etching time [31].*

This part is devoted to the study of the evolution of the PL band as a function of temperature in order to identify the nature of the energy levels which are at the origin of this PL and to understand the mechanisms of radiative recombinations which participate in this PL. We have studied the variation of intensity and the integrated intensity all as a function of temperature in the range [10–300 K].

*The variation of the PL intensity and the width at half-height (FWHM) as a function of the anodization* 


According to Zhao et al. and Weng et al., there are two types of transfer processes:


But we have shown that the variation in the intensity of PL with temperature is due to the presence of these two exciton injection processes.

Ten et al. have shown that the increase in temperature could increase the tunneling process [35]. However, Hua et al. confirm that thermal dissociation of excitons increases with temperature, which favors the leakage of excitons from QDs to QWs at lower energy levels [36]. In **Figure 9**, we represent the integrated

#### **Figure 8.**

*Evolution of PL intensities as a function of the temperature of a SiP layer produced on an N-type substrate (100).*

**Figure 9.** *Variation of the integrated intensity of PL as a function of the inverse of the temperature.*


#### **Table 4.**

*The values obtained from the parameters of the equation.*

intensity of PL as a function of the inverse of the temperature. Taking into account these two transfer processes, we fitted the experimental curve using the twoenergy model, this empirical model is presented in Eq. (1) below [37]:

$$\mathbf{I}\_{\rm PL} \left( \mathbf{T} \right) = \frac{\mathbf{I}\_{\rm PL} \left( \mathbf{O} \right)}{\left[ \mathbf{1} + \mathbf{a}\_1 \exp \left( -\frac{\mathbf{e}\_1}{\mathbf{kT}} \right) \right]^2} \times \left( \mathbf{1} + \frac{\mathbf{A}}{\left[ \mathbf{1} + \frac{\mathbf{1}}{\mathbf{a}\_2} \times \exp \left( \frac{\mathbf{e}\_2}{\mathbf{kT}} \right) \right]} \right) \tag{1}$$

where e1: first thermal activation energy; e2: second thermal activation energy; A, a1 and a2: the fixing parameters; and IPL: the intensity PL.

The different values found are presented in **Table 4**.

From the results, it can be seen that the thermal activation energy of one energy level is different from that of another level. This phenomenon can be explained by the increase in the Tunnel process with the increase in temperature [35]. We can also know the thermal activation energy of phonons in porous silicon.
