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interdigitated back contact or IBC
architectures are the best performing
technology in silicon solar cells
although perovskite solar cell
efficiencies have now surpassed
mainstream silicon efficiencies most of
the research has mainly focused on
planner heads Road junction structures
the number of studies involving IBC
devices is still limited and the
proposed architectures are unfeasible
for large-scale manufacturing in this
work we discuss the importance of IBC
solar cells as a powerful tool for
investigating the fundamental working
mechanisms of perovskite materials and
show a fabrication protocol for IBC
devices that does not involve
photolithography and metal evaporation
the most widespread flexible and
low-cost deposition technique for
parofsky materials is solution
processing which is fast and messy and
does not lead to the formation of single
crystals but rather polycrystalline
films with very high trap densities if
you look at consolidated device physics
and electronics theories which are
taught at undergraduate university
courses you would think that devices
based on solution process per off skite
could not possibly work and in fact
solar cells based on gallium arsenide
with analogous levels of doping would
not work at all however perovskites
display exceptional opto electronic
properties which are very similar to
those of gallium arsenide including
strong light absorption non excitonic
behaviour low recombination rates
long carry a lifetime's and diffusion
lengths and photon recycling in the
following section professor richard
friend and dr. luis pesos discussed
defect tolerance in per offs guide
materials we kind of agree that one of
the key challenges for any new
semiconductor technologies to actually
achieve a very good passivation strategy
so you can get rid of traps do you think
there is anything in pair of skates that
actually allowed them to have such a
high performance this is probably the
absolute core of why the prof skies turn
out to be so surprising that in spite of
the fact that they have many many
structural and chemical defects that
must have because of the way they're
made and the love of impurities that are
present the luminescence which is the
most sensitive test remains extremely
efficient and that's unprecedented where
if you compare other non crystalline
thin-film semiconductors they generally
show really low luminescence heals so we
start with a surprise that no matter how
you make a LED halide profs kite film
it's actually very ribbon essence and
then of course what's been interesting
is that there all sorts of approaches
have been taken and now in the
literature that show you can improve on
it and by almost certainly a passivating
that is removing chem dates that are
entered in restaurants around the
crystallites maybe in the bowel but
almost certain it's the surfaces that
are being cleaned up with these
approaches it's just very surprising
really is different
well there are other key aspects right
because to achieve a very high
luminescence you certainly benefit from
having very low trap densities and low
shockley-read-hall recombination rates
but also having a very strong radiative
rate it's beneficial do you have an
explanation of why the relative rate is
so strong well
the very strong relative recombination
again is something which we find
surprising the recombination where
electron hole come together again we
know that the kinetics of by molecular
so the the rate depends on the electron
concentration times the hole
concentration usually that does not give
very efficient luminescent because those
collisions don't happen all that quickly
but when they do they certainly produce
photons I think the key is to ratio the
Reg ative recombination processes
against everything else going on which
can quench the luminescence and it is
again surprising that the non radiative
channels are relatively unimportant also
because you know this paper that came on
I think it was nature photonics a few
years ago about laser cooling so they
actually showed something in the order
of 99.8% external luminescence
efficiency and that's really
extraordinary and they've shown that on
a single crystal if they have achieved
that in a single crystal doesn't that
make you think that perhaps the the
field should move from solution process
materials there are poly crystalline and
therefore will most likely have higher
defects in to actually making devices on
single crystals wouldn't that be a
better approach and your question was
should we work with single crystals I
mean the reality is we're not going to
be able to if I want to take solar cells
because we need to have those absorbing
material that are quite thin at the
order of a fraction of a micron and
unless someone finds a remarkably easy
way of growing thin crystals on
institude that's not going to happen and
back to the surprise and which is that
the performance of poly crystalline
materials turns out to be particularly
remarkably good and there appear to be
methods to improve on the original
recipes of course the other
the factor that comes into play when we
make a device of beer - the light
emitting diode or a solar cell is that
the luminescence can easily be quenched
at the interface the semiconductor the
probe Skype makes with the two
electrodes for electron and hole
management and the current probe Scout
literature shows that there is still
some quenching it's easy to suggest why
that might be that there are some
chemical defects
whatever-whatever chemistry there is at
that interface with tio2 for electron
extraction it is possible to apparently
produce energy states that lie in the
gap and there is some quenching
irrespective whether it's a single
crystal or a thin film which is poly
crystalline and i think a lot of the
future efforts are going to be to
understand that inter facial chemistry
to be able to remove quenching there
we've tended at the moment to
concentrate more on the pure material
and as the field becomes more developed
we will pay more attention to what
happens at interfaces so what do you
think it's actually helping to passivate
the defects is that there is lab on it
or actually the same effect is observed
for different materials well the the
passivation appears to be something that
can be done with with with almost any of
the compositions if one is looking at
the sort of fundamental issues the
electronic structure the band structure
is really determined by see the LED and
the bromide and iodide and the the third
cation probably doesn't do a lot more
than you control the lattice parameter
so I wouldn't expect to see a
fundamental difference as we tune the
composition of the cation I know it is
advantageous for other reasons
so I don't think that will change but
your work rather elegantly brings into
play that we have to look at at ha-has
we had two operating devices at higher
brightnesses and say under concentration
the non-radiative decay of electrons and
holes through O'Shay processes becomes
quite important and I think you've shown
that that can be important
generally most semiconductors the one of
the key requirements is to actually be
able to dope the material effectively
but in porosity it seems to be kind of
difficult to actually control doping so
I wonder if you had tried any
experiments on that and if you have some
conclusions about it the interesting
thing about the profs guys is that they
break just about every rule and they
work very well the much cleaner than
they should be they're very very hard to
dope and most semiconductor technologies
have depended on the ability to
extrinsic ly p-type or n-type dope
that's often critically important for
getting a really good quality ohmic
electrode for organic semiconductors as
doping has been engineered after a few
decades of hard work and is alive and
well in the O LEDs and LED displays for
the profs guides it may well be that the
ability of the materials of the Prophet
material to adjust itself locally to
neutralize the effect of a defect which
we suspect is why they are so
luminescent and so clean will also mean
that you can't dope them as if you try
to put a wrong valence cation in the
local chemistry will adjust itself and
you won't create the doping and the
literature really suggests that may well
be the case and we've done a lot of work
putting in mono valent cations that in
principle might substitute for the LED
if anything those materials rather than
being extrinsically doped or even less
doped
and the materials by themselves the
literature that also tests that is
looking for an electric field induced
charging in the material in a field
effect device a field effect transistor
and that literature is relatively small
when it is possible to see a field
effect what we're not sure about is
whether that field effect eventually is
reduced because the induced charges can
be compensated by structural
rearrangement around the charge again
it's been very surprising that the profs
guides seemed to show relatively good
electrode behavior against rather
straightforwardly selected off-the-shelf
whole or electron extracting materials
ultimately they might not be as good as
we need but the reality is that for most
anything works to a degree yeah it's
really interesting that the same process
that prevents doping also allows you to
not be near building I think that's the
case yes I know I haven't seen a lot of
explicit discussion of that in the
literature but I think that's the case
okay well thank you very much Richard
hey this is great to see you back in
Cambridge and good to chat
in the following section dr. Mitch doll
sorry
and dr. Mojtaba Abdi jalebi talk about
interdigitated back contact solar cells
so the common Piroska solar cell
architecture is normally represented by
either amethyst Copic or a planar
heterojunction we have a prescribed film
as the intrinsic semiconductor absorber
layer which is a sandwich between two
transport materials one is for electrons
called electron transport layer and the
other is for hole which is like the hole
transport material on the other hand the
market leading technology crystalline
silicon the main architecture for this
technology is interdigitated back
contact solar cell which can you explain
bits about this IVC architecture in the
IVC architecture we have the electron
and hole selective electrodes Co
positioned on the backside of the cell
in an interdigitated fashion and the IBC
solar cell is completed by depositing
the active layer on the inter digitated
electrodes with this architecture
optical transmission losses caused by
the top contact can be avoided by bottom
illumination by illuminating the solar
cell from the absorber layer side and
with this architecture because
electrodes transparency is not required
a wide variety of materials can be
employed without compromising
conductivity in addition these
architectures offer the possibility to
maintain the interconnection circuitry
on the rear side of the cells on one
common surface facilitating solar cell
module assembly processes the IBC
concept was originally developed for
concentrating photovoltaics by Schwarz
and Lambert in 1975 and has been
successfully utilized in silicon based
CAD
teller ID dye sensitized organic and
most recently profs get solar cells so
what has been done so far with IBC
perovskite solar cells so there have
been a few reports on IBC solar cells
employing perovskites some reports
focused on device optimization and
others focused on the device physics of
profs care materials however the
fabrication of these devices involve
rather complicated steps which is a
major issue for large-scale production
can you talk about how the IBC
architecture was used in our recent
publication so in our two recent studies
we use the IBC perovskite structure to
investigate the charge dynamics and as
well as to investigate the correlations
between the structural and upto
electrical properties of these solar
cells during film formation and in these
two studies we use a simple IBC
structure for the second study we used a
structure where we have commercially
available inter digitated IPO substrates
we also use electrodeposited titanium
dioxide electro polymerize P dot and
spin coated perovskite one of the
advantages of using this architecture is
the fact that the peroxide deposition
represents the last fabrication step and
because the absorber layer is not locked
by any other layer it can be directly
probed during solar cell operation and
that can be coupled with many
characterization techniques such as
x-ray diffraction photo current and
photo luminescence mapping and others
can you talk is more detail about the
work that has been done at this RF so in
this work we used these IBC structures
to investigate the structural and
photovoltaic properties of paurav skate
solar cells
during the Anil so after fabricating
these perovskite back contact devices
here in Cambridge I went to Grenoble to
the Europeans neutron radiation facility
the es RF and we used these structures
to investigate the relationship between
the structural properties and the device
performance of prof. skate solar cells
during the during film formation during
the perab skate formation so we found
that the precursor phase displays clean
semiconducting behavior and we also saw
that the VLC Peaks before the precursor
fully converts into perovskite and that
is probably due to a favorable
combination between the band gaps of the
precursor and the peroxide phase we we
also found analogous trends between the
prof skate peak intensity which was
extracted from G wax measurements with
the power conversion efficiency we were
able to confirm these trends by
observing similar profiles for planner
devices with these IBC devices we were
able to simultaneously measure the
structural properties and the
photovoltaic properties of perovskite
solar cells during the formation of the
Perot skate and that wouldn't have been
possible without the IBC structure in
the following section dr. Felix Tesla
and dr. Lewis Pazzo's discussed photon
recycling and porous guide materials so
Felix nice to see you again licensee
universe tell me something about like
photo recycling these things can you
explain some of the basic concepts
behind it right in the process
specifically here can you describe how
the phenomenon occurs when we study the
photo luminescence in these metal halide
perovskites
initially we found that they are very
bright materials and
also the luminescence overlaps quite a
lot with the absorption edge materials
and so with this combination of factors
you could try and see this photon
recycling effects where you generate
charge pairs by absorbing a photon in
the material and then the recombination
of this charge pair generates another
photo in the material that doesn't out
couple directly doesn't leave the
material directly but gets reabsorbed
and then generates another electron hole
pair and by this sequence of processes
you get this photon recycling effect and
when we maybe try to see this together
in our paper and we use this photo
luminescence microscopy to look at the
transport of charges and photons in the
material and we found that the transport
occurs of a length case that are much
beyond the expected length case from
just absorption and we were really
excited to see that and concluded that
photo recycling has to happen in this
case it was super exciting to see this
effect because it's a relevant for
devices right but the key to actually
get efficient photon recycling to for
this effect to be relevant you need to
have a very internal very high internal
luminescence efficiency right that will
mean that you know you need to have a
very accurate measurement of the
recombination rates you need to know how
much is going into radiation how much is
going into chakra hole and well you've
done quite a lot of work on that so can
you tell us a little bit of how you've
done those measurements where the
conclusions are so in order to to really
record the recombination process of
excited charge carriers electrons and
holes in the brass gate we use
ultra-fast laser spectroscopy techniques
to study what happens when we use
femtosecond pulses to put a certain
number of carriers into the material and
how do they then relax to equilibrium
conditions in the dark
these femtosecond pulses that we use are
generating large laser systems so-called
amplifiers for which this year the Nobel
Prize was awarded
it's very nice to hear that it's a
useful technique now for us as well and
what we do is we excite the material
with short pulses we use a technique
called pump probe spectroscopy where we
measure the absorption of light through
the material if we have excited States
in the materials we get a little bit
more light
through the peroxides if there are a lot
of electrons in the material you get
more light through it and then you can
see how the the amount of light going
through it changes over time exactly
what you do and that's what we can use
to quantify how many charge carriers
they are and at the same time you can
measure the luminescence that we get out
from these carrots and this tells us all
the information we really need to know
how the recombination works and that was
especially relevant for this wooden
recycling and photoluminescence
yields because when we did our
measurements it didn't agree with the
quantum yields we measured externally
exactly and so we realized some other
process has to happen and that allowed
us then to quantify really the amount of
auto recycling that on first in the
material in this section we illustrate
the protocol for the fabrication of ibc
perovskite solar cells discussed in our
publications in our work on photon
recycling we patterned the eye teos up
straits in our lab due to the
unavailability of commercial for micron
gap inter digitated i teo substrates
this was time-consuming and involved
photolithography
in our recent paper we demonstrate that
100 micron gap substrates work as well
as the 4 micron gap substrates when
employed in the fabrication of solar
cells electron and hole selective layers
of titanium dioxide and P dot were
respectively electrodeposited and
electro polymerized on the inter
digitated i teo electrodes we used inter
digitated i teo substrates with variable
gaps ranging from 50 to 200 microns
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the substrates need to be cleaned before
use place the substrates on a substrate
holder and immerse it in a beaker filled
with acetone place the beaker in a
sonicator filled with water and Sonic
ate for 10 minutes transfer the
substrate holder to another beaker
containing isopropanol and Sonic 8 again
for 10 minutes finally dry the
substrates with nitrogen stream we
cathodically electro deposit titanium
dioxide from an aqueous peroxo titanium
solution place a 25 milliliter glass
beaker under a retort stand connect a
five centimeter silver wire to a small
crocodile clip connect the clip to a
long aluminium wire and mount this on
the stand this is the reference
electrode repeat these steps for a
platinum foil which is the counter
electrode connect one side of the
substrate to electrical connection legs
for IT o glass substrates
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then bend the leg so that it fits in the
beaker and wrap it with parafilm mount
the ITA substrate on the retort stand
this is the working electrode connect
the three electrodes to the Galvan estat
turn on the auto lab galvano stat and
open the GPS software go to method
chrono methods and / AMA tree in the
manual control pop-up select a current
range of 1 milli ampair type a potential
of minus one point zero five volts a
duration of 800 seconds and a sampling
time of 0.05 seconds pour 20 milliliters
of deionized water into a 50 milliliter
centrifuge tube weigh sixty four
milligrams of titanium oxide sulfur salt
with precision scales and transfer it to
the centrifuge tube add three hundred
and sixty-six microliters of hydrogen
peroxide to the centrifuge tube mix the
solution with the centrifuge weigh 202
milligrams of potassium nitrate and
transfer it to the centrifuge tube mix
the solution again finally pour the
solution into the beaker in the galvano
stat control software press start during
the deposition monitor current versus
time this gives an indication of how
good the deposition is if the curve is
flat there won't be any deposition
taking place once the deposition is
completed rinse the substrate with water
and ethanol then dry it with nitrogen
stream inspect the substrate and make
sure the deposited titanium dioxide is
uniform across the substrate anneal the
substrate at 300 degrees for one hour
under ambient conditions we deposited P
dot through electro polymerization after
cleaning the cell
and platinum electrodes placed them back
to the previous configuration in a clean
beaker connect the other side of the ITA
substrate to a new electrical connection
leg go to methods chrono methods and /
AMA tree and type a potential of 1.25
volts a duration of 1 second and a
sampling time of 10 milliseconds pour 20
milliliters of propylene carbonate into
a 50 milliliter glass vial way to point
12 grams of lithium perchlorate and
transfer it into the vial Sonic 8 the
vial with the sonicator for 30 minutes
add 20 1.4 microliters of e dot close
the vial and shake it pour the solution
into the beaker in the galvano stat
control software press start during the
deposition monitor current versus time
after completion rinse the substrate
with acetonitrile and dry it with a
nitrogen stream the precursor solution
was spin coated at 6,000 RPM for 30
seconds under ambient conditions turn on
the solar simulator and let it warm up
set the irradiance to 1 son connect the
IBC solar cell electrodes corresponding
to the 100 micron gap to the multimeter
using electrical connection legs
set the following sweeper parameters
sweep range from minus 0.2 volts to 1
volt voltage step 10 millivolts 50
milliseconds delay and an active area of
0.03 96 square centimeter the solar cell
measured here was fabricated at the xmas
bean line and had been stored under dark
ambient conditions for almost two years
interestingly the device still works and
shows a relatively high open circuit
voltage a common aspect of all JV curves
reported in our works is that the fill
factor is very low and that the JV
curves are straight lines this is likely
to be caused by a high series resistance
more selectively in two layers and
optimized electro deposition could
improve the efficiency of these IBC
solar cells in the last parts of this
work dr. Felix Dessler and dr. Luiz
Passos discussed the future developments
of IBC perovskite solar cells at the
University of Cambridge you actually
have a project now on the back contact
solar cells right yeah because there is
an interesting aspect of it that you
mentioned earlier that in these kind of
solar cells you actually have all of the
electrodes in the back therefore you can
actually tune the top surface in
whichever way you find yes it's a very
interesting structure where we basically
take our vertical device and we flip it
over 90 degrees and try to make all the
electrons inclined that helps us to make
the solar cell more transparent because
we can minimize the area of the
reflective electrodes potentially they
could be quite good for applications in
Windows or wherever you want to transmit
some light through your solar cell
additionally it gives us access to the
surface of the perovskite film which is
usually the most effective area and you
mentioned on radiative decays which are
the main loss for luminescence so now we
have a
a surface that we can address and we can
use chemical passivation other
fabrication techniques to reduce the
number of defects that we have on this
dominant surface area of the Perot skate
films that you can use passivation
passivation agents that are not even
conducted right exact there can be
insulators you don't care exact we don't
need to care about extracting charges
from this interface which usually in the
vertical devices is the largest
interface but you need to get charges
through and we try it already a few
approaches and it seems as if we can
efficiently passivate these films very
well and get very high luminescent seeds
in them and now we are trying to combine
this all and make one big full structure
and then try with photonic structures on
top to to modulate the photon density
internally so that we can boost the
photo recycling effects particularly
something that is really nice about the
back context is that the surface
structure doesn't have to be functional
so you don't have to put a structured
electrode on top we can just put some
non conductive dielectric stack on top
that has a photonic bandgap in the right
energy range and then hopefully we will
improve the efficiency of the devices
okay well thank you very much for all
these explanations Thank You Luis
see you soon Photoshop see you soon
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