High Performance Solar Cells Exceeding 17 ... - ISC Konstanz

Reverse current @-14.5V [A]
Figure 6: Concentrations of interstitial oxygen of wafers
from different ingot heights, measured using FTIR.
In consideration that bulk resistivity is lower (see table
IV) and respectively the doping level is higher for ESS
based solar cells, these results lead to the conclusion that
LID is rather relating to the net doping and the interstitial
oxygen concentration than to the absolute boron
concentration. This supports the assumption that
phosphorous in compensated silicon somehow prevents
the formation of BO 2i , what was stated by different
research groups before [2-5].
Table IV: Bulk resistivity of investigated wafers
Ingot
Resisitvity [Ω*cm]
Center brick Corner brick
ESS 1 1.0 – 1.2 1.0 – 1.4
ESS 2 1.4 – 2.3 1.4 – 2.0
ESS 3 1.5 – 1.7 1.5 – 1.7
poly 1.5 – 1.8 1.4 – 1.9
3.2 Results of reverse current-voltage characteristics
optimization
Table V shows the average solar cell parameters over
at least 12 solar cells of each group from positions
covering the complete ingot height, which were
processed using different texturisation methods. The soft
iso texturisation was processed with the texturisation
bath, but less silicon was removed compared to the
standard iso-texturisation. The ISC recipe is not disclosed
yet.
Table V: Average solar cell parameters of solar cells,
processed by different texturisations.
Ingot Texture V oc
[mV]
70%
ESS
100%
poly
J sc
[mA/cm²]
FF
[%]
Eta
[%]
Iso 618.7 33.1 79.2 16.2
Soft iso 615.7 33.2 79.4 16.2
ISC 616.8 33.3 78.8 16.2
Alkaline 617.8 33.4 79.1 16.3
NaOH 620.5 32.6 79.4 16.1
Iso 614.9 34.2 78.9 16.6
Soft iso 612.6 34.3 78.8 16.5
ISC 614.2 34.2 78.7 16.5
Alkaline 613.6 34.2 78.8 16.5
NaOH 616.0 33.5 78.8 16.3
As table V shows the solar cell efficiencies of almost all
groups are similar. Only the NaOH flat etched solar cells
show lower cell efficiencies, due to higher reflectivity
resulting in lower J sc values. Depending on the
texturisation the open-circuit of the solar cells is varying
as a consequence of the different surface areas and thus
different surface recombination velocities. The NaOH flat
etched cells show the highest V oc due to the lowest
surface area, whereas the wafers treated by the soft isotexturisation
show the lowest V oc , most likely because the
saw damage was not removed sufficiently. In general the
ESS based solar cells show lower performance than the
poly-Si based solar cells, due to significantly lower short
circuit currents.
The influence of the different texturisation methods
on the reverse current-voltage characteristics is shown in
figure 7. For the poly-Si solar cells there is only a slight
influence of the texturisation on the reverse current. Both
versions of the iso-texturisation lead to reverse current
values of 1.9 A and 1.5 A respectively, whereas the other
three texturisations lead to reverse currents below 1.2 A.
Even the higher values of the iso-texturisations are in a
non critical region regarding possible hot spots in solar
modules. For the ESS based solar cells the reverse
current generally is higher, due to higher doping levels
and assumable higher concentrations of impurities. The
SoG-Si solar cells treated by the two versions of the isotexturisation
show high reverse currents above 10 A,
measured at a reverse voltage of -14.5 V. The mixed
texturisation, the alkaline texturisation and the NaOH flat
etching leads to reduced reverse current values between
1.9 A and 3.6 A.
12
10
8
6
4
2
0
Figure 7: Average reverse currents, measured at -14.5 V,
depending on the used wafer material and the applied
texturisation.
The lowest reverse current was obtained on the NaOH
flat etched solar cells, due to the reduced influence of the
solar cells surface, but at the same time the solar cell
efficiency of those cell is the lowest. The alkaline
texturisation shows the best combination of solar cell
efficiency with 16.3% and a low reverse current of 3.2 A
at -14.5V reverse voltage and thus seems to be a
sufficient solution to prevent possible hot spots in ESS
based solar modules, while allowing for high efficiencies.
4 CONCLUSIONS
iso soft iso ISC alka NaOH
Texturisation
70% ESS
100% Poly
The high quality of ESS feedstock was confirmed
achieving solar cell efficiencies exceeding 17% with best
cells showing 17.1% cell efficiency. The obtained cell
results demonstrate that the ingots purified by IMCC
show high solar cell performance and low LID, indicating
that this technology can be an effective alternative for
final purification of SoG-Si. An industrially applicable,
regarding more efficient gettering optimized, POCl 3
diffusion was investigated leading to a gain of 0.1% in
absolute cell efficiency, due to higher V oc and J sc . The
reverse current could also be lowered by the optimized
diffusion, indicating a more efficient removal of
impurities. Generally V oc and FF is higher for ESS based
solar cell, because of the higher boron doping and thus a
Fermi level shift and low resistivity, whereas Jsc appears