| AVS 61st International Symposium & Exhibition | |
| Thin Film | Tuesday Sessions |
| Session TF+EN+PS-TuA |
| Session: | ALD for Energy |
| Presenter: | Bas van de Loo, Eindhoven University of Technology, Netherlands |
| Authors: | B.W.H. van de Loo, Eindhoven University of Technology, Netherlands H.C.M. Knoops, Eindhoven University of Technology, Netherlands G. Dingemans, ASM, Netherlands I.G. Romijn, ECN Solar Energy, Netherlands W.M.M. Kessels, Eindhoven University of Technology, Netherlands |
| Correspondent: | Click to Email |
Thin films of Al2O3 provide excellent passivation of heavily p-doped (p+) silicon surfaces and are therefore often applied in silicon solar cells to reach high efficiencies. The high level of passivation by Al2O3 can be attributed to its low interface defect density and high negative fixed charge density Qf. However, the negative fixed charge density of Al2O3 can be detrimental for the passivation of n+ surfaces [1]. Furthermore, in advanced cell architectures such as interdigitated back-contact (IBC) solar cells, both n+ and p+ surfaces are adjacent and are preferably passivated simultaneously. To this end, we systematically study the surface passivation by SiO2/Al2O3 stacks prepared by atomic layer deposition (ALD), which exhibit excellent chemical passivation while the effective fixed charge density can be tuned to zero by carefully tuning the SiO2 thickness.
Al2O3 and SiO2/Al2O3 film stacks with varying ALD SiO2 thickness (0-12 nm) were prepared by plasma-enhanced ALD at 200°C, with H2Si(N(C2H5)2)2 and Al(CH3)3 as metal-organic precursors and O2 plasma as oxidant. Moreover, a SiO2 ALD process using ozone was developed as this oxidant is more suitable for batch ALD. The relevant process parameters for surface passivation, such as ozone exposure time, were identified. The passivation of n+ and p+ doped surfaces was studied in detail, and results were compared with industrial passivation schemes, including PE-CVD SiNx and similar SiO2/Al2O3 stacks from a high-volume manufacturing ALD batch reactor.
A superior level of passivation of n+ surfaces (Rsheet = 100 Ω/sq) was obtained by SiO2/Al2O3 stacks as compared to single layer Al2O3, significantly reducing the recombination current density (J0) from (81±10) to (50±3) fA/cm2. On p+ surfaces (Rsheet= 60 Ohm/sq), J0increases with increasing SiO2 thickness. The results can be explained by an excellent level of chemical passivation, combined with a strongly reduced negative fixed charge density when increasing the SiO2 thickness. To fully exploit the virtues of ALD, the concept of using SiO2/Al2O3 stacks for the passivation of both the n+and p+doped surfaces in a single deposition run was demonstrated on (completed) n-type bifacial solar cells, reaching conversion efficiencies >19%. The results are promising for IBC solar cells, where n+ and p+surfaces are adjacent and care must be to achieve a low surface recombination, high shunt resistance and industrial feasibility.
[1] B. Hoex et al., Phys. status solidi - Rapid Res. Lett., vol. 6, no. 1, pp. 4–6, (2012).