High efficiency rear a-Si:H/SiNx:H passivated local-contacted P-type CZ silicon solar cells

Abstract

To reach the goal of grid parity, technology improvements to enhance the conversion efficiency of solar cells at similar or reduced manufacturing cost must be found. The research presented in this thesis describes industrially applicable high efficient Passivated Emitter and Rear Locally Diffused and Contacted (PERL) cell technology with a hydrogenated amorphous silicon and silicon nitride (a-Si:H/SiNx:H) layer stack for rear surface passivation.

This thesis starts with studying the dependence of the film physical characteristics, chemical bond configurations, surface passivation quality and the thermal stability of the intrinsic a-Si:H on the deposition conditions. The deposition conditions include deposition time, deposition temperature and microwave power. This was done for the purpose of growing uniform films with good film quality and ideal surface passivation. The surface passivation properties of the intrinsic a-Si:H on a textured surface with relatively small pyramid size was also studied. As a result, an effective surface recombination velocity (Seff) below 8 cm/s was achieved on 1 Ω.cm boron doped p-type FZ wafers.

The second part of this work focuses on developing a thermally stable a-Si:H/SiNx:H stack with good surface passivating property. The dependence of the surface passivating property and the thermal stability of the a-Si:H/SiNx:H stack on the deposition conditions of both layers was investigated for the purpose of optimizing these two aspects. With the optimized deposition conditions, excellent τeff values of 1.7 ms was achieved on 1 Ω.cm p-type FZ wafers with an upper bound for the corresponding Seff of 5.8 cm/s. The surface passivating quality of the optimized a-Si:H/SiNx:H stack remains stable for over 90 min at 400°C which is sufficiently stable for the formation of metal contacting schemes that require relatively low metal sintering temperatures and the implementation of hydrogenation passivation processes. Optimized iVoc of 717 mV was achieved on cell precursors prior to laser-doping processes. As high as 54% reflectance at 1200 nm indicates that the a-Si:/SiNx:H stack acts as a good rear reflector when combined with an overlying thin layer of Al.

The third part of this work focuses on developing a rear localised metal contact in conjunction with the a-Si:H/SiNx:H passivated rear surface, based on laser-doping combined with a low temperature annealing step for an overlying Al layer. A 355 nm Q-Switched laser was selected as a suitable tool to form high quality local back surface field (LBSF) through the a-Si:H/SiNx:H stack. The quality of the LBSF, including the sheet resistance (R□), doping profile and the recombination properties of the laser-doped LBSF were investigated in a wide range of laser parameters to optimize the process. As a result, an LBSF with R□ of about 5 Ω/□, a surface doping concentration above 1020cm-3 and the lowest effective SRV in the LBSF region of 6400 cm/s was obtained with the lowest laser scanning speed and a relatively low laser power. The laser-induced defects in the LBSF region can be hydrogenated by the H atoms released from the passivating layers during a thermal annealing process, resulting in a remarkably reduced effective SRV to about 4600 cm/s. To avoid potential damage induced by Al during the thermal annealing process for ohmic contact formation, the influence of the capping SiNx:H layer thickness and the characteristics of the underlying a-Si:H layer on the performance of cell test structures was studied. Contact resistance (Rc) values as low as 2 mΩ.cm2 were achieved after annealing for 20 min at temperatures ranging from 350°C to 450°C.

The final stage of this work presents the development of the rear a-Si:H/SiNx:H passivated PERL cells with the developed a-Si:H/SiNx:H rear surface passivating stack and the rear local contacting scheme. Best cell efficiency of 20.24% with a Jsc of 40.2 mA/cm2 and Voc of 671 mV was achieved on 1.8-2.4 Ω.cm p-type commercial CZ wafers. To optimize the cell performance, the influence of the rear contact pattern on the electrical performance, including point contact patterns and line contact patterns, was studied. It was found that, by using closer spaced point contact pitch, the Rs can be effectively reduced, which therefore improves the FF. However this leads to increased dark saturation current and hence reduced Voc and Jsc. As a compromise, the highest efficiency of 20.3% was achieved on the cell with an optimal point contact pitch of 0.4 mm. The industrial feasibility of the developed technique is discussed.