Preparation of Al2O3 Thin Films by Atomic Layer Deposition Applied to Passivated Emitter and Rear Cell

• Main Authors: Chih-Hsiang Yang, 楊智翔
• Other Authors: Chung-Yuan Kung
• Format: Others
• Language: en_US
• Published: 2017
• Online Access: http://ndltd.ncl.edu.tw/handle/38550395546487981695

博士 === 國立中興大學 === 電機工程學系所 === 105 === Compared to traditional monocrystalline silicon solar cells, passivated emitter and rear cells (PERC) feature as its rear-side passivation stacks of aluminum oxide and silicon nitride (Al2O3/SiNx), which can reduce the recombination velocity and enhance the absorption of long-wavelength incident light. Currently plasma-enhanced chemical vapor deposition (PECVD) technique becomes a good choice instead of plasma-assisted atomic layer deposition system (ALD) for depositing Al2O3 films due to capacity requirement. In this thesis, high quality Al2O3 films are prepared by using self-developed non-vacuum spatial ALD with deposition rate of 0.16 nm/cycle. The deposition and annealing conditions are investigated to estimate properties of Al2O3 films. We also have investigated other structural topics such as rear-side surface morphologies, novel rear-side opening technique and the antireflective coating (ARC) material. Finally all the concepts are merged to fabricate a PERC. The passivation effect of Al2O3 films could be divided into chemical passivation and field effect passivation, which are mainly related to interfacial trap density (Dit) and negative charge (Qf) of Al2O3 films, respectively. The interfacial SiO2 films and hydrogen atoms can effectively passivate dangling bonds to prevent carriers being trapped. The root cause for the charges of Al2O3 is determined by fourfold-coordinated AlO4 tetrahedral configuration. Another stable sixfold-coordinated AlO6 octahedra also exist within the Al2O3 bulk. The more the ratio of AlO4 sites to AlO6 sites, the higher the negative charges. Experimental result shows that as H2O carrier flow reaches 500 sccm, the Qf and Dit are -2.7×1012 cm-2 and 7.15 ×1012 eV-1cm-2, respectively, leading to the surface recombination velocity (Smax) 40.5 cm/s; When deposition temperature is 170°C, the Qf and Dit are -4.59×1012 cm-2 and 6.98 ×1012 eV-1 cm-2, with a corresponded Smax of 41.55 cm/s; The Qf and Dit are -1.25×1012 cm-2 and 6.71 ×1012 eV-1cm-2, the corresponded Smax is 37.5 cm/s, after the post-annealing treatment was performed to Al2O3 films. The blisters which form at the Si/Al2O3 interface occur under an external load in the presence of a tensile residual stress due to the effusion of H2 and H2O. Two approaches are proposed to solve it. First a stoichiometric silicon is deposited on silicon surface by inductively coupled plasma chemical vapor deposition to block blisters. The other method is to reduce the thickness of Al2O3 as well as increase the post-annealing temperature to out-gassing the interior gases. The optimized PERC with the improved triple-layer SiO2 /Al2O3 /SiNx:H stacked passivation film has an obvious gain in open-circuit voltage (Voc) and short-circuit current (Jsc). The electrical performance of the optimized PERC with the Voc of 0.647 V, Jsc of 38.2 mA/cm2, fill factor of 0.776, and the efficiency of 19.18 % can be achieved. Various rear-side surface morphologies were obtained through different etching treatments. We compare the PERCs with standard etching treatment and further polishing processes on rear-side surfaces. Experimental results show that compared with the unpolished cell, the polished cell attains superior electrical performance, particularly in Voc and Jsc, because of the more effective rear-side surface passivation and reabsorption of long-wavelength light. Both improvements raise the conversion efficiency to 19.27 %, with the Voc of 0.662 V, Jsc of 36.69 mA/cm2, and FF of 0.793. Instead of using the traditional laser ablation process, this thesis demonstrates spin-coated polystyrene spheres (PS) to create local openings on the rear side of PERCs. Effects of PS concentration and post-annealing temperature on PERC performance are investigated. The experimental results show that the PS are randomly distributed on wafers and no PS are joined together at a spin rate of 2000 rpm. The PS can be removed at a temperature of 350°C, leaving holes on the passivation layers without damaging the wafer surfaces. As compared to the laser opening technique with the same contact fraction, the PS opening technique can yield a higher minority effective lifetime, a higher Voc, and a slightly higher Jsc. Although the fill factor of the PS opening technique is lower owing to non-optimized distribution of the openings, the conversion efficiency of the devices is comparable to that of devices prepared via the laser opening process. Composite silicon dioxide-titanium dioxide (SiO2-TiO2) films are deposited on a large area of 15.6 × 15.6 cm2 textured multicrystalline silicon solar cells to increase the incident light trapped within the device. For further improvement of the antireflective coatings (ARCs) quality, dimethylformamide (DMF) solution is added to the original SiO2-TiO2 solutions. DMF solution solves the cracking problem, thus effectively decreasing reflectance as well as surface recombination. The ARCs prepared by sol-gel process and PECVD on multicrystalline silicon substrate are compared. The average efficiency of the devices with improved sol-gel ARCs is 16.3 %, only 0.5 % lower than 16.8 % of devices with PECVD ARCs. Eventually a PERC based on all concepts mentioned above is realized on a 15.6 × 15.6 cm2 p-type solar grade silicon wafer. The conversion efficiency is 20.5 %, slightly lower than 21.2 % of the PERC from the industrial. The main factor in around 0.7 % difference can be attributed to the amounts of front side bus bars, which collect minority carrier lifetime. The results represents that the spatial ALD utilized in this thesis has high potential to be used in industrial production line.