## 3 Results and Discussions  
Bilayer Matrix of two TCOs (ITO and AZO) was introduced to form a heterojunction with n-Si. Each layer provides optical and electrical functionality to enhance photovoltaic performance. The ITO provides high visible transparency (75.3% averaged over 400 \(\sim\) 1100 nm of wavelength) and low resistivity. (\(\varrho\) = 1.53 \(\times\) 10\({}^{-4}\)\(\Omega\cdot\)cm and \(\mu\) = 42 cm\({}^{2}\)/V\(\cdot\)s) The Al modulated AZO layer was systematically investigated regarding optical and electrical properties. In Figure 1a, the measured photoluminescence (PL) spectra show two distinguishable peaks, broad green emission (2.6 \(\sim\) 2.7 eV), and near band-edge emission (3.16 \(\sim\) 3.4 eV). The defects or impurities related to green emission signals [25] were observed, and the signal strengths varied by different Al doping concentrations. The relative strength of the green emission was strongly dependent on the crystal surfaces [26]. The ratio of Igreen emission /Inear band edge was 0.42, 0.6, 1.82, and 0.56, for 0, 3.14, 4.07, and 5.22 wt% of Al doping, respectively. The intensity was enhanced with an increase of Al doping and abruptly diminished at 5.22 wt% of Al. It indicated that increased Al doping deteriorates the crystallinity of ZnO. However, when Al doping reached the proper amount--5.22 wt%--crystallinity was recovered with suppressed defects. This can be explained by released residual compressive stress by substituting the Zn sites with Al atom, and it resulted in improved crystallinity of ZnO14 [27]. The increase of Al doping shifted the peak of near band-edge emission from 3.16 eV (Al 0 wt%) to 3.4 eV (Al 5.22 wt%) due to Burstein-Moss shift for heavy Al doping regime [28; 29; 30; 31; 32]. The optical transmittance spectra of ZnO with different Al doping was shown in Figure 1b. The optical absorption coefficient (\(\mathbf{\alpha}\)) was calculated using Equation (1), where \(d\) is a ZnO thickness and \(T\) is an optical transmittance. \[\alpha=\frac{1}{d}\ln\left(\frac{1}{T}\right) \tag{1}\]  
The optical band gap of ZnO was extracted by linear interpolation of (\(\alpha\)_hv_)\({}^{2}\) versus _hv_, based on the \(\alpha\) given in Equation (1), since ZnO is direct bandgap material and it satisfies Equation (2) [33]. \[(\alpha\)_hv_)^2}=A\left(\textit{hv}-E_{g}\right) \tag{2}\]  
where _hv_ is photon energy, \(E_{g}\) is the bandgap of ZnO, and \(A\) is a constant that arises from [PERSON]'s golden rule for fundamental band-to-band electronic transitions. Over the visible range, optical transmittances of all AZO films were higher than 85%. The optical band gap (\(E_{g}\)) measured by absorption spectra was higher than \(E_{g}\) achieved from near band-edge by PL, with both presenting abrupt \(E_{g}\) widening beyond 4.65 wt% of Al. In Figure 1c, \(E_{g}\) achieved by emission spectra was gradually reduced when Al doping was higher than 5.78 wt%. This phenomenon was reported to have involved many-body effects [28; 29; 30]. The energy difference (\(\triangle E_{g}\)) of \(E_{g}\) measured by photoemission (by PL spectra) and photoabsorption (by transmittance spectra) clearly shows Stokes shift (0.06 eV at Al 0 wt% \(\sim\) 0.24 eV at Al 6.34 wt%). In Figure 1c, above 5.78 wt% of Al, stoke shift was drastically increased, which can be explained by the fact that excessive Zn interstitials induced inhomogeneous strain inside AZO [34; 35]. Electrical properties were characterized by Hall effect  
Figure 1: Optical and electrical properties of Al-doped ZnO (AZO). (**a**) Photoluminescence (PL) intensity of AZO with different Al doping (measured at room temperature, and excited by 4.4 eV). (**b**) Ultraviolet-Visible-Near Infrared (UV-Vis-NIR) spectra of AZOs with different Al doping. (**c**) Optical band gap widening, extracted from Figure 1a,b; Burstein–Moss effect was described in the inserted Figure (**d**) Electrical properties of AZO with different Al doping by using Hall effect measurement. measurement, as shown in Figure 1d. AZO films have n-type polarity.