Silicon Material Considerations for Heterojunction Solar Cells: Potential and Limitations
Creators: name: Bätzner, D.L.
name: Andreetta, L.
name: Lachenal, D.
name: Frammelsberger, W.
name: Legradic, B.
name: Meixenberger, J.
name: Papet, P.
name: Strahm, B.
name: Wahli, G.
Publisher: name: WIP
Year: 2014
Subjects: WAFER-BASED SILICON SOLAR CELLS AND MATERIALS TECHNOLOGY
Silicon Feedstock, Crystallisation and Wafering

description: 29th European Photovoltaic Solar Energy Conference and Exhibition; 789-792
description: High efficiency silicon heterojunction solar cells such as Meyer Burger’s HeteroJunction Technology
(HJT) are typically processed on Cz grown mono crystalline n-type wafers. Batch Cz n-type ingots have naturally
distributions in a variety of their electrical key specifications such as –but not exclusively - resistivity, interstitial
oxygen concentration, carbon concentration and metal impurities concentration. Variations in bulk properties cause a
distribution of the bulk life time mainly along the growth-axis of the ingot which is expected to impact on device
performance. Often a strong variation of life time perpendicular to the growth axis is observed, which can be avoided
or minimised in proper growth conditions. Since all high efficiency solar cell technologies and especially HJT rely on
a high quality surface passivation the limitation on cell performance caused by the quality of the bulk material
becomes dominant. Therefore, it is of great interest for the production of such cells to know how the device
performance is impacted by the material specifications and variations in the material due to the silicon crystal growth.
HJT cells on 6-inch n-type wafers have exceeded 23% and 60 cell heterojunction modules made with Meyer Burger’s
Smart Wire Contacting Technology (SWCT) exceed 325W. Commercial batch n-type ingots were evaluated in a
wide range of available wafer resistivities. We fund that Meyer Burger’s latest HJT accepts a large variation of
resistivity without losses in performance. Resistivities quoted are measured on as cut wafers, i.e. before any thermal
treatment such as thermal donor killing (TDK). The range investigated was from 1cm - 6cm., which corresponds
at least twice the resistivity range of typical commercial n-type ingots. HJT cells were processed on wafers spanning
the entire resistivity range with effective life times measured on those cell precursors after passivation showing a
large distribution. Cells across the entire resistivity and effective life time range showed the same high igh efficiency
of around 23%. For all HJT cells with such a high performance a correlation between resistivity and life time was
maintained that is governed by the dopand inflicted Shockley-Read-Hall recombination in the bulk and the surface
recombination after passivation that is typically characterised by a J0e of around 5fA/cm2 for Meyer Burger’s HJT.
Therefore, neither resistivity nor effective life time separately are suited parameters to characterise material quality. It
rather requires the knowledge of both parameters to predict performance potential of a given silicon material quality.
The HJT cell performance across a complete ingot was also demonstrated to stay well within a variation of ±2%
relative efficiency. A common observation on various tested ingots is that the lower top and middle of the ingot
perform best with a slight performance loss towards the upper top and tail of the ingot. This is expected since owing
to the Cz growth the upper top will have crystal imperfections and a high concentration of thermal donors while most
recombination-active contaminants and dopants aggregate in the liquid silicon melt that is accordingly affecting the
tail.
These findings indicate, that commercial n-type ingots can be used entirely for the production of HJT cells with little
impact on performance distribution. This is also valid for a wide parameter window of wafer resistivites.