Low Resistance with the High Transparency of Al-Doped ZnO TCO Thin-Films from ALD Procedure for c-Si-Based Passivated Contact Solar Cell Applications

8th World Conference on Photovoltaic Energy Conversion; 133-137
The study aims to develop a suitable TCO via different modes of the ALD process for c-Si based passivating contact solar cells (PCS) and we have proposed a new type of deposition scheme. This TCO deposition, along with other layers (passivation and electron/hole transport layers) can be deposited in a single ALD run, which helps to fabricate a complete PCS device without exposing the device outside of the vacuum chamber. ZnO:Al (AZO) films on the glass/c-Si substrates from thermal (TH) ALD and plasma enhanced (PE) ALD processes were deposited at different temperatures (100-250 °C), and the growth per cycle (GPC) of these layers were calculated from spectroscopic ellipsometry and X-ray reflectometry. The GPC of films deposited by PE ALD at 200 C is measured to be 1.45±0.1 Å for Al2O3 and 1.78±0.2 Å for ZnO films; similarly for the TH ALD deposited films, a GPC of 1.00±0.1 Å and 1.20±0.1 Å, respectively. The (1: n) (Al2O3: ZnO) pulse ratio was used for the film deposition and optimized GPC of Al2O3 and ZnO was used to deposit about 200 nm AZO films for different Al concentrations (0.5 to 5 at %). The (1:21) (Al - 2%) ratio provides an optimum film property for the PE ALD process, and the same ratio was tried with TH ALD, where the film properties were significantly enhanced. Further, a new mode of deposition named “DTH” (DEZ-TMA-H2O) was explored, and an additional decrease in the film resistivity was observed. We have altered the sequence step of precursor dose and co-reactant in this mode. Compared to the conventional TH and PE ALD technique, the DTH mode of deposition proves to be the best fit for the TCO application here. A 200nm AZO film deposited by DTH ALD has a minimum resistivity in the range of 1.91×10-3 Ω-cm and high transparency of more than 85% in the visible region. DTH approach in the ALD proves to be an innovative deposition technique for developing films with enhanced quality, indicating the potential use in PCS device applications.