# The Effect of Local Doping of the Polymer-Polymer Interface Using Cu\({}_{2}\)O Particles
## 2 Materials and Methods
For this purpose, the Cu\({}_{2}\)O films on a quartz substrate were made in parallel with the formation of island films on polymer. The thickness of the film and the manufacturing method were the same. The spectra of the UV-visible region were recorded in the wavelength range of 250-800 nm in air at room temperature (Figure 3a). It is revealed that Cu\({}_{2}\)O films are characterized by high transparency in most of the visible range. The optical width of the band gap is estimated according to the absorption coefficient. The band gap energy is the energy difference between the bound state (valence band) and the free state (conduction band). The minimum energy required to excite an electron from a bound state to a free state and to participate in the conduction process must be at least equal to the width of the band gap. The band gap width of the copper oxide film was determined using [PERSON]'s method from the optical absorption spectrum [29] using the equation:  
\[\alpha\;h\
u=B\left(h\
u-E_{g}\right)\;^{n}, \tag{2}\]  
where \(B\) is the Tauc's constant, \(h\) is the photon energy, \(E_{g}\) is the optical band gap of the material, \(\alpha\) is the absorption coefficient and the value of \(n=\frac{1}{2}\) or \(2\) for direct and indirect allowed transitions, respectively. Figure 3: (**a**) Absorption coefficient \(\alpha\) with photon energy and (**b**) [PERSON]’s plot of thin copper oxide film. The absorption coefficient \(\alpha\) is calculated from the transmission spectrum data according to the Beer-Lambert law (3):  
\[\alpha(\lambda)=-\frac{1}{d}\ln\biggl{(}\frac{T}{100}\biggr{)} \tag{3}\]  
where \(T\) is the transmittance in \(\%\) at the wavelength \(\lambda\) and \(d\) is the film thickness. The absorption coefficient \(\alpha\) lies in the range from \(10^{4}\) to \(10^{5}\) cm\({}^{-1}\) (Figure 3a). The absorption region is at a wavelength less than 500 nm. It is known that a value showing \(\alpha>10^{4}\) cm\({}^{-1}\) indicates the presence of direct electronic transitions [30; 31] occurring from dipole-resolved blue and indigo transitions. A direct band gap is typical for thin films, whereas transitions with an indirect band gap are usually detected for relatively thick films [32]. In this case, the energy of the forbidden zone (\(E_{g}\)) was estimated under the assumption of a permitted forward transition (\(n=\frac{1}{2}\)). The value of the band gap width is located at the intersection of the linear approximation of the edge of the main absorption (Figure 3b) and the spectrum below the main absorption [29]. Thus, the \(E_{g}\) value of the Cu\({}_{2}\)O film is 2.65 eV. It may seem that this value is somewhat overstated. However, it is known that with a decrease in the thickness of the amorphous Cu\({}_{2}\)O film, the band gap increases [21]. For example, in [33] it was found that a decrease in the size of Cu\({}_{2}\)O nanoparticles from 3.2 to 2.1 nm leads to an increase in the band gap of 2.15 to 2.71 eV due to the effect of dimensional quantization. Therefore, an increase in \(E_{g}\) in our case may be evidence that the Cu\({}_{2}\)O film is an island, which is in close agreement with other experimental results [21; 33]. Electrophysical measurements were carried out using the Keysight B2902A precision parametric analyzer (Keysight Technologies, Santa Rosa, CA, USA) in the atmosphere under normal conditions.