## 1 Introduction  
ZnO, a transparent conducting oxide (TCO) material with a wide direct bandgap and a high exciton binding energy has been widely studied for various optoelectronic applications such as light emitting diodes (LEDs), laser diodes, photodetectors, and photovoltaics [1; 2; 3; 4; 5; 6; 7; 8; 9]. Typically, group III (B, Al, Ga, In) elements in the periodic table are employed as dopants in ZnO. Undoped ZnO has an electron affinity of 4.35 eV and direct bandgap energy of 3.28 eV with \(\sim 10^{17}\) cm\({}^{-3}\) of an electron concentration. However, transparent heavily Al-doped and Ga-doped ZnO have been demonstrated to have low resistivity with \(10^{20}\sim 10^{21}\) cm\({}^{-3}\) of electron concentration. Very low resistivities of Al-doped ZnO (AZO) and Ga doped ZnO (GZO) were reported to be \(\sim 8.5\times 10^{-5}\)\(\Omega\)-cm and \(\sim 8.1\times 10^{-5}\)\(\Omega\)-cm, respectively, which are comparable with \(\sim 7.7\times 10^{-5}\)\(\Omega\)-cm of resistivity of In\({}_{2}\)O\({}_{3}\) (ITO) [10]. Moreover, higher transparency, better thermal stability, and low growth temperature make AZO and GZO much more promising [11; 12]. ZnO-based TCOs have a relatively large refractive index in the range of 2.1 \(\sim\) 1.9 at the visible light spectra. This optical property can improve internal light absorption in conventional semiconductor materials. In recent researches, Al-doped ZnO (AZO)/Si heterostructures demonstrated reliable rectifying junction behaviors with improved optical absorption and current collection for enhanced photovoltaic performance [13; 14]. The TCO/Si device structure could provide opportunities to overcome some challenges of conventional solar cell structures, such as the lateral component of photogenerated current reduction due to emitter layer Ohmic loss. Although such a loss could be compensated for by using a heavily doped emitter layer, it could still cause higher carrier recombination with decreased light absorption. These limitations could be addressed by using well-designed TCO/Si heterojunction device structures, attributed to the superior optical transparency and electrical conductivity of TCO materials. In addition, the device fabrication process could also be simplified compared to conventional solar cells [15]. However, a main challenge of TCO/Si devices is the relatively low open circuit voltage (V\({}_{\rm oc}\)) and fill factor (FF) performance due to the intrinsic defects and lattice mismatch at the heterojunction interfaces [11]. Thus, improving junction interfacial qualities with higher junction built-in potential values (\(qV_{bi}\)) is essentially important [16; 17; 18]. Several demonstrations of ZnO/Si heterojunction solar cells have been reported. [PERSON] et al. utilized ZnO by MOCVD method and obtained conversion efficiency of 2.82% with V\({}_{\rm oc}\): 294.4 mV, J\({}_{\rm sc}\): 26.108 mA/cm\({}^{2}\), and FF: 36.66%, respectively [19]. [PERSON] et al. reported a 1.14% of conversion efficiency of AZO/Si solar cell using Atomic Layer Deposition [20]. [PERSON] et al. used thermal oxidation of Zn dots and recorded conversion efficiency of 6.79% with V\({}_{\rm oc}\): 375 mV, J\({}_{\rm sc}\): 25 mA/cm\({}^{2}\), and FF: 72%, respectively [21]. [PERSON] et al. reported a conversion efficiency of 2.51% from Si doped ZnO with a sputtering method on Si [22]. The record high efficiency has been reported by [PERSON] with the efficiency of 14% (V\({}_{\rm oc}\): 520 mV, J\({}_{\rm sc}\): 38 mA/cm\({}^{2}\), and FF: 71%) and 10% (V\({}_{\rm oc}\): 470 mV, J\({}_{\rm sc}\): 32 mA/cm\({}^{2}\), and FF: 69%) for textured and planar structures, respectively, which jump up to theoretical efficiency of 21%. In [PERSON]'s report, the hydrothermal method and the ALD method were used to grow AZO/Zn1-xMgxO films on a Si [23]. In our previous work, a bi-layer TCO structure (ITO on AZO) was reported with an improved figure of merit for both electrical and optical properties.