PHOTOVOLTAICS & ENERGY HARVESTING
common-mode noise along the DC link and
consequently all over the solar panel array.
Solutions to this problem include adding
a fourth inverter leg, which adds hardware
without improving efficiency, adding a linefrequency
isolation transformer, which adds
unacceptable weight and cost, using an
isolated boost converter, or going back with
a three-level inverter.
The lowest cost solution comes back to
the common topology of figure 1, with the
IGBTs and anti-parallel diodes replaced by
cascodes, with the TNPC inverter commutated
in a way that cancels the commonmode
voltage swings.
Fig. 2: SiC cascode solution.
Trade-off
The three-level inverter topology cuts switching loss in half (because
half the voltage is switched), which benefits SiC devices
less than lossy IGBTs, but is still a benefit. The main advantage
with SiC devices is the ability to squash common-mode noise.
The lower losses from SiC purchase greatly reduced size and
weight, with equal or reduced system-level cost. This “expenditure”
is very slight because at IGBT switching frequencies,
the switching loss of SiC devices is a very small portion of total
semiconductor losses. The switching frequency can often be
doubled without significantly affecting overall efficiency. The
impact on passive components and heat sinking is substantial
however, greatly shrinking overall size, weight, and installation
cost.
Boost for perovskite solar cell efficiency
A By Nick Flaherty team of German researchers have identified the losses in
perovskite solar cells, boosting the efficiency to over 20
percent for a basic cell.
Organometallic perovskite absorber layers are regarded as
a particularly exciting new material class for solar cells, and
Prof Dieter Neher at the University of Potsdam and Dr. Thomas
Unold at the Helmholtz Centre in Berlin (HZB) looked in detail at
the various defects in solar cells and determine which ones lead
to losses and how. This was used to boost the efficiency of a
1cm2 perovskite solar cell to well over 20 percent.
At certain defects in the crystal lattice of the perovskite layer,
the charge carriers that have just been released by sunlight can
recombine again and thus be lost. But whether these defects
were preferentially located within the perovskite layer, or instead
at the interface between the perovskite layer and the transport
layer was unclear until now.
The team used photoluminescence techniques with high
precision, spatial and temporal resolution to examine the materials.
Using laser light, they excited the square-centimetre-sized
perovskite layer and detected where and when the material
emitted light in response to the excitation. “This measurement
method at our lab is so precise, we can determine the exact
number of photons that have been emitted”, said Unold. The
energy of the emitted photons was precisely recorded and analyzed
as well using a hyperspectral CCD camera.
“In this way, we were able to calculate the losses at every
point of the cell and thereby determine that the most harmful
defects are located at the interfaces between the perovskite
absorber layer and the charge transport layers,” he said. This
is important information for further improving perovskite solar
cells, for instance by means of intermediate layers that have a
positive effect or through modified fabrication methods.
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