Injection level dependent effective lifetime
of a p-type c-Si wafer symmetrically passivated
by a 30 nm Al2O3 film synthesized by plasma
assisted ALD and thermal ALD on the FlexAL reactor.
By Knut Beekmann, Product Manager, OIPT
In the battle to improve efficiency in crystalline silicon (c-Si) solar cells, one of the critical factors is avoiding recombination losses of the charge carriers created in the base material.
One of the problems is that sooner or later, these carriers meet a surface or interface and any defects at the surface or interface can increase recombination rates. A good surface passivation is required to achieve very efficient c-Si solar cells. A cost driven reduction in the cell thickness leads to an increase in the relative surface area to volume. This in turn makes a high quality surface passivation a critical factor to improve the efficiency for conventional solar cells.
Typically, the c-Si surface is passivated using a thin film of either silicon dioxide or silicon nitride. Silicon oxide would normally be thermally grown and undergoes additional chemical annealing to provide a very high quality interface. The high temperature required to thermally grow the silicon dioxide however; generally limits its applicability to high quality c-Si material. Silicon nitride, such as that deposited by plasma enhanced chemical vapour deposition as an anti reflection coating, has also gained ground as a commonly used passivation material however; provides inferior passivation to thermally grown silicon dioxide.
Both silicon oxide and silicon nitride have positive fixed charge densities. These contribute to improving the passivation quality through establishing an electric field at and below the silicon surface. This is particularly useful in the case of n-type c-Si, where recombination is reduced by the minority charge carrying holes being shielded from the surface. For p-type c-Si, such positive charge passivation films will have the opposite effect. The electrons will be attracted to the surface, increasing minority carrier concentration and increasing recombination rates.
The critical importance of achieving a high quality surface passivation has opened the field of development to potential new materials. One such material that has been investigated is aluminium oxide and specifically, atomic layer deposition (ALD) of aluminium oxide. Recently, the relative passivation properties of thermally grown ALD films have also been compared with those of plasma assisted ALD layers deposited on a FlexAL reactor1.
Films were analysed post annealing and showed identical stoichiometry and structural properties however; C-V analyses of both types of film have yielded significant differences in the flatband voltage shifts. The plasma assisted film had a much greater negative fixed charge density which in turn can explain significantly higher measured effective lifetimes of the charge carriers of p-type c-Si and a better surface passivation. Similar plasma assisted ALD films have been used in the passivation of highly doped p-type emitters on n-type solar cells yielding a cell efficiency of 23.2%2.
References
1 Silicon Surface Passivation by Atomic Layer Deposited Al2O3 – B. Hoex, J. Schmidt, P. Pohl, M. C. M. van de Sanden and W. M. M. Kessels. Journal of Applied Physics 104 044903 (2008)
2 High Efficiency n-type Si Cells Based on Al2O3 Passivated Boron Emitters – J. Benick, B. Hoex, M. C. M. van de Sanden, W. M. M. Kessels, O. Schulz, and S. W. Glunz. Applied Physics Letters 92 253504 (2008)
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