| AVS 62nd International Symposium & Exhibition | |
| Thin Film | Wednesday Sessions |
| Session TF+EN-WeM |
| 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 J. Melskens, Eindhoven University of Technology G.J.M. Janssen, ECN Solar Energy K.R.C. Mok, Delft University of Technology L.K. Nanver, Delft University of Technology A.H.G. Vlooswijk, Tempress Systems W.M.M. Kessels, Eindhoven University of Technology, Netherlands |
| Correspondent: | Click to Email |
Atomic layer deposition (ALD) of Al2O3 has been successfully implemented in silicon solar cell manufacturing, predominantly to passivate defects at the lowly-doped back surface of p-type solar cells. However, also for the passivation of highly-doped n+ or p+ type surfaces, present in e.g., high efficiency n-type solar cells, ALD films and stacks might become feasible. Yet, for such surfaces, the level of passivation strongly depends on the doping concentration and surface conditions. To allow for an even wider implementation of ALD-based passivation schemes in industrial solar cells, the work presented in this contribution aims to understand the passivation of highly-doped n+ and p+ type regions.
First of all, we observed that the passivation of boron-doped (p+) Si surfaces highly depends on the doping process and the fixed charge density Qf in the passivation layer. For instance, when a boron-rich layer (a BxSiy compound) was remained after the doping process, the surface passivation by ALD Al2O3 severely deteriorated. The formation of this undesirable layer could be avoided by using an oxidizing ambient during the drive-in of boron, although this results in a significant reduction in boron doping density at the surface. The latter impaired the level of passivation when Qf was virtually zero, such as for ALD SiO2/ Al2O3 stacks. Importantly, the level of surface passivation offered by ALD Al2O3 films remained high under these circumstances, due to the strong negative Qf.
Next, for phosphorous doped (n+) Si surfaces having surfaces densities of 1018-1020 cm-3, the passivation by dielectrics containing a negative Qf such as ALD Al2O3, can be severely compromised, as the negative Qf increases the minority carrier (i.e., hole-) concentration at and near the surface. Moreover, the negative Qf can invert the n+ Si surface, which triggers (undesirable) increased recombination at low injection levels and parasitic shunting. For those n+ Si surfaces, it is demonstrated that ALD stacks such as SiO2/ Al2O3 and HfO2/ Al2O3 are promising alternatives to Al2O3 single layers, due to the absence of a negative Qf. These stacks are particularly interesting from an industrial point of view, as they can make ALD viable for the passivation n+ Si surfaces.
Lastly, ALD-based passivation schemes also have the potential to fully replace the heavily-doped n+ and p+ Si regions in solar cells. In this new field of ‘passivating contacts’, the significant recombination losses in the highly-doped regions can be avoided due to accurate band-alignment. Preliminary but promising results on novel electron-selective, passivating stacks of HfO2 and TiO2 by ALD will also be presented.