TECHNOLOGY I MANUFACTURING
less sensitive than p-type to a wide range of metallic impurities and degrades much less under the influence of light. It is also expected that n-type wafers will have a similar manufacturing cost as p-type silicon once it is produced in larger quantities. ITRPV further predicts that n-type solar cells will become more important in the future, particularly in the segment of a high efficiency solar cells.
A final important characteristic for the imec nPERT cells is that the emitter is made at the rear side of the solar cell instead of the more commonly used front side. Firstly, this was done to be able to build further upon the research results that imec achieved in earlier research projects on p-type PERL cells. In both cases (p-type PERL with front emitter and n-type PERT with rear emitter), Ni/Cu contacts are applied to the phosphor-doped front side of the cell.
Apart from this, there are other potential advantages for a rear-side emitter. One of them is the larger distance between the junction and the Cu contacts which can have beneficial influence on the reliability of the solar cells. Also, a rear side p+ emitter can be less sensitive to degradation by light than a front side emitter. However, with a rear-side emitter, measures have to be taken to minimize recombination at the front side (because the charge carriers have to travel a longer distance to the rear p-n junction ). Therefore, a Front Surface Field (FSF) and a high quality passivation stack of SiO2/ SiNx is used.
An n-PERT cell with 21.5% conversion efficiency At imec, three research routes are followed in the study of nPERT cells. The difference between them lies in the way the emitter is realized.
A first route that is followed to make the rear side emitter is diffusion of boron (from BBr3
emitter either SiO2 or Al2
). For the passivation of the O3
is used. The
former is thermally grown while the thin Al2O3 layer is applied via Atomic Layer Deposition (ALD). In the latter case, the dielectric layer is thickened with an additional SiOx layer, deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD).
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www.solar-international.net I Issue IV 2014
Figure 3: SEM picture of the textured rear side of an n-PERT solar cell before (left) and after (right) epitaxial growth of the emitter.
This resulted in n-PERT cells with respectively 21.3% (SiO2
(Al2O3 ) and 21.5%
) efficiency. This conversion efficiency of 21.5% was independently confirmed by ISE CalLab and is, to the best of our knowledge, a record efficiency for this type of solar cells.
The process to make the n-PERT cells with a diffused emitter is currently still too complex to use at an industrial scale. Therefore, further research concentrates on simplifying the processing sequence. Next to this, alternative routes to make the rear side emitter are explored. One of these alternatives is to use epitaxy
with in-situ doping instead of diffusion. In this case thin silicon epitaxial layers are grown in the presence of boron doping precursors. The advantage of epitaxy is that it can be done only on one side (in this case the rear side of the solar cell) whereas gas phase diffusion is a 2-sided process. In the latter case, the layers at the front side have to be removed by etching , or avoided by masking, which are extra process steps.
Another advantage of epitaxy is the flexibility in doping profile. A passivation layer stack of ALD-Al2
O3 +PECVD-SiOx also gave the best results for the PERT
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