**3.3 The effect of surface molecular modification on space charge characteristics of multilayer PI films**

The samples used in the section are multilayer PI films without nanoparticles and fluorinated for 15, 30, 45, and 60 min, respectively, which are named as PI15, PI30, PI45, and PI60. In addition, the samples without fluorination named PI10 are

### **Figure 11.**

*The space charge distribution of multilayered PI films by stacking (a) five samples without fluorination, (b) five samples fluorinated for 30 min, (c) five samples fluorinated for 60 min.*

used as the contrast group. The testing system is the PEA method in **Figure 1(b)**. During the measurement, five samples with the same fluorination time were stacked into single multilayer film, and the silicone oil was applied between the interfaces to remove air. The applied voltage time was 3600 s, and the amplitude was 5 kV.

The space charge distribution of multilayered PI films is shown in **Figure 11**. In the figure, the thickness of each layer is 25 μm. Each layer of the sample is marked with PI0, PI30, and PI60. It can be seen from **Figure 11(a)** that when the voltage time is 10 s, a certain number of homosexual charges appear near the anode and cathode. As the voltage time increases, the space charge distribution changes. When the voltage time is 1800 and 3600 s, a lot of positive charge occur near cathode, followed by the negative charge accumulating near the next interface, and then the

### **Figure 12.**

*The electrical distribution of multilayered PI films by stacking (a) five sample without fluorination, (b) five sample with fluorinated for 30 min, and (c) five sample with fluorinated for 60 min.*

*Effect of Molecular Structure Modification and Nano-Doping on Charge Transportation… DOI: http://dx.doi.org/10.5772/intechopen.92024*

**Figure 13.** *Relationship between the charge of multilayer samples and the fluorination time.*

positive and negative charges alternately appear, until the small amount of positive charge exists near the anode. The largest space charge density amplitude near the cathode is 0.3 C/m<sup>3</sup> .

According to **Figure 11(b)**, the charge distribution is very different from the space charge distribution of the untreated samples. The space charge distribution of the five layers of PI30 has the same trend from 10 to 3600 s. A certain amount of positive charge accumulates near the anode, and a large amount of negative charge accumulates at 210–220 μm near the interface near the anode. The maximum space charge density amplitude is 0.21 C/m<sup>3</sup> . In **Figure 11(c)**, the space charge distribution trend of the five-layer PI60 sample is similar with the fivelayer PI30 film. In addition, the space charge distribution of the five-layer PI15 and PI45 samples has the similar trend with five-layer PI30 and PI60 samples. To avoid repetition, the space charge distribution of the two is not given in this chapter.

**Figure 12** shows the distribution of the electric field of the sample after different fluorination times. In **Figure 12(a)**, the sample accumulates a large number of homosexual charges near the cathode and anode during the initial stage of applying voltage, resulting in the high electric field strength with 7.0 <sup>10</sup><sup>7</sup> V/m. As the voltage time increases, the charge is transferred and accumulated inside the sample, leading to the change of electrical distribution. **Figure 11(b)** and **(c)** shows the internal electric field strength of the five-layer PI30 and PI60 samples. It can be found that the electric field strength of PI30 is much lower than PI10, which means surface molecular modification can optimize the electric field. The highest internal electric field strength of PI30 is 1.6 <sup>10</sup><sup>7</sup> V/m, and PI60 is 1.8 <sup>10</sup><sup>7</sup> V/m.

The studies in **Figures 11** and **12** show that the surface structure and internal trap energy level of the sample after surface molecular modification is adjusted can make the sample effectively inhibit the injection of charge into the sample, thereby suppressing the space charge accumulation in the sample.

In order to analyze the amount of space charge injected into the internal of sample, the total amount is calculated according to formula (4):

$$\underline{Q} = \int\_{d\_2}^{d\_1} \left| \rho(\mathbf{x}) \right| \text{Sdx} \tag{4}$$

In the formula, *S* is the effective area, and *d1* and *d2* are the thickness start and end positions of the sample. *ρ(x)* is the charge density at x.

The charge amount of the sample with different fluorination times is shown in **Figure 13**. It can be seen that the sample without fluorination has a charge of 135 nC, and the amount of space charge of all sample with fluorination is below 80 nC. With the increase of the fluorination time, the amount of charge decreases slightly and then increases again. The charge amount of sample fluorinated for 45 min is the lowest (75 nC) among all the fluorinated samples.
