research industry
rhombus4
reduce the voltage as well as the
The challenge of this structure is to completely relax the
current of the device. For this reason it
was believed for a long time that high
mismatched crystal layers in a region of the device
efficiency solar cells need to be built
from semiconductors with the same where no photocurrent is generated. In the ideal case
lattice-constant to avoid the formation
of defects due to lattice relaxation.
misfit dislocations are only formed inside a spatially
The global efficiency maximum for a
defined buffer region and threading dislocations are
triple-junction solar cell is found for a
sufficiently suppressed
bandgap combination of 1.74, 1.17
and 0.70 eV. Germanium with a
bandgap of 0.67 eV turns out to be a the success of the metamorphic to further reduce defect densities
good candidate for the bottom approach relies on the material in the active solar cell layers
junction but unfortunately for the quality that can be achieved in the ring6 high bandgap tunnel diodes
middle cell there is no lattice- photoactive parts of the solar cell between the subcells to reduce
matched semiconductor with a structure. Therefore, the graded losses due to absorption and
bandgap between 0.67 and 1.41 eV buffer layer is a key for achieving reflection
(see Fig. 3 left). Therefore, until excellent device performance. ring6 back surface passivation of the
recently the most successful material Ge bottom cell to improve the
combination was a lattice-matched Our best metamorphic triple-junction photocurrent of this subcell
Ga0.5In0.5P/Ga0.99In0.01As/Ge (1.88, solar cells are reaching efficiencies of ring6 a broad-band anti-reflection
1.41, 0.68 eV) solar cell which has 41.1 % (see Fig. 5) under 454 suns coating which covers the
reached efficiencies up to 40.1%.
[3]
concentration
[4]
, and they full spectral range between
demonstrate the high theoretical 300 – 1850 nm
At Fraunhofer ISE we have followed potential as well as excellent material ring6 contact fingers with higher
an alternative approach which quality which can be realized with aspect ratio to minimize
enables the use of lattice- metamorphic growth. shadowing losses
mismatched semiconductor materials. Still there is sufficient room for
In this case the lattice constant is further improvements of this The potential of the metamorphic
intentionally graded between Ge and structure. As an example some of the Ga0.35In0.65P/Ga0.83In0.17As/Ge
- in our case - Ga0.83In0.17As with a most important R&D topics are the triple-junction solar cell has been
bandgap energy of 1.17 eV. development of: shown to be high and excellent
ring6 dislocation blocking layers device characteristics can already be
The challenge of this structure is to surrounding the buffer structure achieved. Figure 4
completely relax the mismatched
crystal layers in a region of the
device where no photocurrent is
generated. In the ideal case misfit
dislocations are only formed inside a
spatially defined buffer region (see
Fig. 4) and threading dislocations are
sufficiently suppressed. This
approach is called metamorphic as
starting from Ge the crystal is
transformed into a virtually new lattice
with a 1.2 % larger lattice constant.
This new lattice acts as the template
for the growth of a Ga0.35In0.65P top
and Ga0.83In0.17As middle cell structure
which are both lattice-matched to
each other. The resulting solar cell is
composed of semiconductors with
bandgap energies of 1.65, 1.17, 0.68
eV which is close to the theoretical
optimum for a device with three pn-
junctions.
It has to be emphasized again that
October 2009
www.compoundsemiconductor.net 23
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