MATERIALS
limit cell efficiency (and yield levels with thin- rear passivation layers are not constrained to an Figure 2: Examining
wafers). Yield-related concerns include direct AR-coating role, so are optimized for passivation the effects of sub-
‘contact’ to surfaces, and high temperatures with and reflectivity. surface laser-induced
potential wafer ‘bowing’. Efficiency constraints are material modification.
imposed by resolution limits on line thickness Opening the passivation layers 2a uses data from
(~100µm), and difficulty in optimizing aspect ratio, It sounds so simple! Selectively (and locally) Engelhart
9
showing the
series resistance, passivation material, or local remove lines or holes from the surfaces. But here change in carrier
(selective) dopant diffusion levels. begins the challenge since reliable solutions have lifetimes as function of
hitherto been elusive. First, consider the degree of depth in the bulk c-Si,
Next-generation concepts allow for optimizing all ‘selective’ removal, and the layer thickness in when using
aspects pertinent to high-efficiency. As noted by question. Knorz
3
sets the scene admirably: “the nanosecond lasers in
Fork
1
: “the tool set for the next generation solar cell laser ablation process must be accomplished the infra-red (1064),
processing line is expected to look very different without inducing significant damage in the space green (532), and UV
21
from screen printing.” Key to high-efficiency charge region.” From a laser micromachining (355). 2b and 2c show
concepts are ‘passivation layers’ and means to perspective, the process finds resonance within a SIMS data presented
www
locally ‘open’ these to create lines or holes, prior to generic category ‘thin-film’ removal; differentiating by Neckermann
4
and
.solar
new direct-write, multi-layer, individually-optimized it from scribing or ‘mini-via’ hole-drilling. Selective Rana
14
metallization techniques
2
(e.g. ink-jetting or nickel thin-film removal has two requirements aside from
-pv-management.com
plating with a subsequent silver or copper merely ‘blasting’ away a layer of material: (i)
electroplating step
3
)*. eliminate surface damage by way of debris or
affected zones around the lines or holes; (ii)
Blanket passivation (or diffusion barrier) layers prevent sub-surface damage such as
4
“material
already feature on the front surface of standard modifications into silicon wafers including lifetime
cells. Generally, a thin layer of SiNx (~75-90nm) limiting defects and internal stress”, particularly
performs a dual-role: as an anti-reflection (AR) difficult when other (thin) regions are located
Issue II 2009
coating (increased light transmission) and as a directly below. Fork
1
provides a deadly warning: “if
‘passivation’ layer. Passivation prevents excessive an ablated contact opening in the passivation were
recombination losses at the surfaces which would to extend through the emitter, the metallization
otherwise reduce efficiency detrimentally. At the could form a shunt to the p-type material below the
rear surface, passivation layers on standard cell emitter, ruining the device.” Sentiments echoed by
types are rare today - but again - they form part of Glunz
5
: “although laser ablation is a very elegant
next-generation cells. Currently, a full back- and stress-free process, it is important to control
surface-field (BSF) is employed in which aluminum the process parameters very carefully so that no
is deposited directly onto the rear and fired. laser damage is introduced into the sensitive
Inherent limitations abound here too: first, space charge region.”
passivation is not great (merely adequate) and
insufficient for thin wafers where surface-to-volume So, why lasers for this process! Lasers offer ultra-
ratio’s are higher; second, the internal reflectivity is high resolution – repeatable and controllable – to
not particularly high, with low reflectivity at long remove thin lines down to 10µm or less. By
wavelengths. Therefore, next-generation cells use controlling the energy, wavelength, pulsewidth,
rear passivation layers, typically comprised of and beam-profile from the laser, highly-precise
SiNx/SiOx stacks. In contrast to front-surface SiNx, material selectivity can be achieved reliably
* New metallization schemes generally includes two parts: a contact (silicide producing) material deposited into the openings,
followed by a highly conductive metal deposited on the contact material.
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