## 3 Results and Discussions
Carrier concentration was increased from \(5\times 10^{16}\) cm\({}^{-3}\) (Al 0 wt%) to \(4.5\times 10^{20}\) cm\({}^{-3}\) (Al 6.34 wt%) by adding Al in AZO. Highest electron mobility, 15 cm\({}^{2}\)/V-s, was achieved at Al 5.78 wt%. The excessive Al doping with 6.89 wt% fully deteriorates crystal quality, as shown in quenched PL near band edge peak (Figure 1a), large Stoke shift (Figure 1c), and decreased electron mobility (Figure 1d). The proposed ITO/AZO/n-Si device structure and the cross-section scanning electron microscope (SEM) image were shown in Figure 2a,b, respectively. Bilayer TCO structure (ITO and AZO) was introduced to enhance both the optical and electrical performance of the photovoltaic devices [36]. In ITO/AZO/n-Si devices, the top ITO layer worked as a transparent conducting layer, providing high optical transparency and superior current collection efficiency. The device current transport mechanism was mainly determined by using the AZO/Si heterojunction, which provided the active junction built-in potential (\(qV_{bi}\)) for the photogenerated carrier separation under the illumination [37; 38]. The theoretical AZO/n-Si junction band diagram before and after equilibrium conditions are shown in Figure 2c [36; 37]. To better understand the ITO/AZO/Si structure properties, the chemical composition depth profiles of ITO/AZO/Si structures were characterized via a secondary ion mass spectrometer (SIMS). The AZO inter-layers with different Al doping concentrations were deposited via the dual-beam co-sputtering method (details in the Experimental Section). As shown in Figure 3a-c, they were prepared with the same ZnO sputtering power of 300 W, but different Al co-sputtering powers of 15, 25, and 35 W, respectively. One can see that the element distributions of In, O, and Zn were identical across samples, indicating a stable and uniform deposition condition. Due to the different Al co-sputtering powers, different signal intensities were observed, indicating different Al doping concentrations in AZO inter-layers. This demonstrated the flexible Al doping tunability in AZO thin films via the proposed dual-beam co-sputtering method. Based on the deposition rate, the Al doping in AZO inter-layers was calculated to be 4.07, 5.22, and 6.34 wt.%, as shown in Figure 3a-c, respectively. Figure 2: (**a**) Schematics of the In\({}_{2}\)O\({}_{3}\) (ITO)/ Al-doped ZnO (AZO)/Si heterostructure solar cell devices, (**b**) cross-sectional SEM image of the ITO/AZO/Si structure, and (**c**) theoretical energy band alignment of AZO/n-Si heterojunction before and after equilibrium condition. The level of Al doping in AZO thin films affects the AZO/n-Si heterojunction and interface properties. They are essentially important for AZO/Si heterojunction photovoltaic device performance. Proper junction and interface engineering can benefit the heterojunction built-in potential (\(qV_{bi}\)) and the device carrier transport mechanisms, providing enhanced V\({}_{\text{oc}}\) and FF performance. However, the actual energy band alignment between AZO and Si heterojunction can be complicated. The AZO doping concentration can affect not only the Fermi energy level (E\({}_{\text{F}}\)), but also other parameters such as electron affinity (\(\chi\)) [39]. The band alignment is also affected by the AZO thin film properties, such as the electrical conductivity, oxygen deficiency, and the deposition conditions [39,40]. In this work, the \(qV_{bi}\) values of AZO/n-Si heterojunctions were exacted via capacitance-voltage (C-V) measurement.