Catching Light: Making the Most of Solar Energy

Public lecture presented by Kristjan Kunnus

Developing cheaper and more efficient devices to harvest energy from the sun is a major scientific challenge. One way to increase the efficiency of solar cells – and even make solar cells out of otherwise inactive materials – is by covering them with a thin layer of dye that strongly absorbs sunlight and converts it into electrical current. This lecture describes our efforts to understand the basic chemical processes that take place within a dye molecule when it absorbs a photon, or particle of light.



material science at Potsdam in Germany and from there well slack is one of the

mecca's now of fast time photochemistry so he's come to take a postdoc at our

place and to use the accelerator synchrotron x-ray sources to see what he

can do with the problems that he's interested in so today he'll talk to you about catching light a very novel

approach to problems of solar energy so Kristian thank you very much

so thank you very much for the kind introduction and also thank you all the

organizers for really inviting me here I have to say this is my first time to

give such a public lecture and it's really it's a really a great honor to do

that here and I really I I had my PhD

defense about a little bit less than two years ago and I have a little bit of feeling like at the the PhD defense

right now so I haven't been so nervous for a long time but yeah so here's the

topic of my of my lecture catching light

so no this isn't haha sorry so catching

light making the most of solar energy so when I guess you have seen this poster

or this advertisement which the slack graphic designers dip here so which this

baseball guy catching the ball of light so I have to tell you this is not

actually what we do here so this is not actually how we catch the catch the light so I would like to tell you about

how we are planning to use San Francisco Giants to get more solar energy but this

is not what I'm going to tell you about so my talk is actually going to be about dyes so so here's a here is just a

picture which I which I got from the internet so it's a it's from from the Indian market so different dyes I guess

you all know a little bit what the dye is it's something which gives a color

it's a compound or chemical compound which gives a lot of color and maybe you also know that a color of a material or

a chemical compound depends on the how this material interacts with the light like how it

absorbs what what kind of light it's absorbs and what kind of light it reflects so I will tell you about how we use this

dice to catch light and when I talk about catching light and I mean about how this ties absorb light and what

happens next so here is a short outline

of my talk so I will give a short like a motivation why we should care about

solar energy and why do we need actually catch more light I will tell you about

how the dye is doing it and how we can use dyes to actually harvest these

photons and then thirdly I will tell you about okay what we are doing here it's

like and how we can actually use the facilities here and maybe know that

there are this the x-ray science to actually study these processes taking place in dyes so to start with here is a

figure which I guess social something you all know so this is a world total

energy production over a few tens of years and as you see it's constantly

growing so that so the unit's here are

the mega tons of oil equivalent so this kind of at year 2012 the world consumed

energy which was about 13,000 million of

tons of oil equivalent so and as you know also most of it comes from a fossil

fuels and this trend is expected to

continue so it's expected that the energy production it has to grow and it

has to double by the year 2050 so and this is not sustainable because we all

know that we don't have enough fossil fuels to actually do that they will run out at some point so but there is a good

news we have a lot of solar energy so every year

about 70 million mega tons of oil

equivalent energy is reaching the Earth's surface by this by some with

sunlight so this is about 5,000 times more energy than we are using annually

right now so there it's a lot of energy and of course the question comes ok can

we harness this energy and if you can do that then we have a we have a very

powerful energy source and it's nearly infinite because the Sun will hopefully stay here for at least few billion years

so the question is can we harness the solar energy like can we just turn this

Sun sunlight into electricity and for this the question is actually yes we can

I guess you all know about solar cells so this is here the typical silicon

based solar cells and this is these are getting more and more popular and you

see that in recent years there is a figure of the of the production of

electricity by the solar cells which has been exponentially growing over few years and as it is growing that the

price of the solar cells are also increasing exponentially so this is happening right now so more and more we

are producing actually electricity from solar from solar from sunlight directly

and by the year 2050 again it's expected that this will be one of the main

sources of electricity so this is really

a great news and these are really great devices however I will show you another graph so this looks very similar like

the one I showed you a few slides before and this is final energy consumption

over a few tens of years and you see that about 18 percent of this is used as

electricity and most of it is still used as fuels so it's it's very likely that

this ratio will change in the near future in the

favor of electricity because more and more like electric cars and the electric motors will become more and more spread

but it is very unlikely that we can substitute all our energy consumption

what we get from fuels with electricity so it seems that we also need fuels and

the question is okay can we get views from Sun can we get create solar fields so-called

and this is now a really a kind of a

dream what many people have had for a fall for several years to have this kind

of so-called hydrogen society so to use sunlight and water and to create from

this hydrogen and oxygen and so that you can have a clean source of hydrogen and

you could so hydrogen it's a it's a

really good fuel so it's about three times more energetic when it's when it's

a pressurized that's around the bars so it contains more energy per weight then

then Casa lean and it's about 200 times more energy dense then leave to my own

battery right so this is a huge advantage so so batteries are great we

need more and more batteries but you see that actually their energy tends it is rather low compared to the fuels

secondly hydrogen can naturally stored so this is this is not the case I mean

we can put it electricity to the batteries but one needs a lot of lot of batteries to do that and of course it

will be a clean and carbon fee so the really the the question is so can we

have a such a device so which takes sunlight and water and

turn it into a hydrogen and oxygen so this is now a problem many

researchers and many scientists are working right now actually around the world and they have come up with with

new type of solar energy devices so which I have called a photo

electrochemical devices so this kind of devices you can use them as solar cells

so you can use them to create from sunlight electricity and they are they

are great devices they are actually now starting to maybe compete with silicon

based solar cells they have some advantages like there have a very low

production cost and investment costs and what is very important with this kind of

chemical electrochemical devices is that actually you could design them so that

they split water under sunlight so with

these devices you can design cells which split water into hydrogen and oxygen so

directly from sunlight and water so this is now very much in the research stage

so these are the sources you could in principle by but this is under a very

heavy research right now so these devices here these electrochemical devices they have a one thing in common

that they all used dyes so in all of this there is a dye is an important

component which is catching the light so and I will tell you now about how these

devices work and what the dyes are actually doing in there

so to do that I actually start with the with a normal silicon based solar cell I

don't know how much you know about this but it's it's a simple device so it's in

principle it's made out of a silicon so it's the same material like transistors are made out of computers so

in a silicon-based solar cell if a light hits silicon material what

happens history is he created the electron and a hole so charges are created in the

silicon material so in a normal state silicon has just containing a lot of a

lot of electrons they're bound to the do the atoms and they cannot conduct charge

but after the light absorption electron is promoted it gets more energy and it's

free to move in the in the material and at the same time where the electron was removed there is a vacancy of electron

which can also move around at these are the charges which can move around in the silicon so now the silicon is engineered

in such a way that these charges they separate in a material and electron

reaches one electrode and the whole so-called hole reaches to another electron and this means that in

principle you have a device which creates electricity so now you could like connect some kind of external

device to your electrodes and you could use it you could use the solar cell to -

yeah for the electricity right so that's

what in principle happens in the silicon based solar cell so now in in this

electrochemical devices so here I have a schematic picture of this one electrochemical device in principle same

processes take place but instead of having a silicon material there are

three different type of components in there so naturally there is a there is a

dye in there and die what it does it absorbs light right and

like in silicon in dye a hole and the electron is created but now different

from the silicon based solar cells these charges they cannot move around in a dye so in order to discharge

us to separate they have to move so the electron has to hope to this kind of a

nano porous titanium oxide which is a which is a semiconductor which can accept this electron and then from this

titanium oxide the electron can move to a electrode a hole on the other hand

this can be captured by the electrolyte which is a kind of a solution containing a chemical compound which can accept

this kind of this a hole and can lead it to the electrode so in principle the

same processes take place here and in the silicon solar cell only here that

the creation of the charges and their separation are kind of divided into a

different between different materials so and this makes this device a very

versatile because you can engineer this you could chemically engineer these different different dyes and different

semiconductors and electrolyte to make this work as you like now looking more

closely how this device actually looked like so here is this here is this this

nano material titanium oxide which contains dye so there are dye molecules

on the surface of this kind of titanium oxide nanoparticles so this is a very

small particle it's like usually now around 10 to 20 nanometers diameter and

it contains actually hundreds of time'll occurs on this surface if you look more

closely than i could see so this is a dye so this is a typical dye molecule how it looks like so to tell you more

about these dyes I show you that so here

is the absorption spectrum visible light absorption spectrum of a dye so this is

a red dye you see this is the visible spectrum and you can see that it

absorbs here it's absorbs only a little bit in the red range but it's absorbs more in the

green and blue range and therefore the dye it's a red colored this dye it's the

chemical formulas here so it's a it's a complicated looking structure but it's

actually not important what it exactly is important is that in the center here

you see there is a routining that's a metal that's a metal atom in the center

there ruthenium it's a noble metal and around this metal there are different

chemical groups which record which we call ligands so it's a dye which

consists of a metal central heavy metal and around this it's like carbon nitrogen different

light light atoms so in principle you can have a lot of different dyes I mean dyes can have a very different chemical

compositions but it has been shown that this kind of dies actually they work

very efficiently in these devices so that's why I'm showing it now how does

this dye actually work at the molecular level here if this dye absorbs a light

then electron from this central metal Rutina matin is transferred to a ligand

so there is a charge separation happening already within this molecule

so there is a vacancy in the metal and the electron is moved at the ligand now

in this kind of excited state if your dye is now attached to this nanoparticle

this electron at the ligand which is at the edges of this molecule it can hope

it can hope to the titanium oxide nano particle and here now it's when it's on

this particle it can travel along and hope from one nano particle to another and reach the

so that's how it happens so and it's similarly this hole here this hole can

hope to the to this electrolyte to the solution which is in there so the solution can cap it can capture

the hole and what you see is that you end up with a die in its initial State

before it actually absorbed the photon so so you end up again with die in a

radial state which means that it can absorb again a photon but you have in

your system you have two charges traveling so you have a which can create for example if you if you do this dies

in a in a solar in a in a solar cell device it can create charges so this

charge let's have a link they can travel to the electrodes and through your external device for example recombine

again creating a energy so in principle that's how at the

molecular level this tie works so it's kind of like a source of charges so it gives electrons and and holes and which

can then create electricity so yes but

what determines actually the efficiency of this this tie so I right now I just

told you that okay these processes are happening but is there something else what can happen

so if the dye works as it should then this electron hopes to the titanium

oxide so that's that's that's this is the process which is what we want

this will create electricity for us this will make the device efficient however

this process it's competing with the

other process which is a back transfer of this electron to the back to the

metal so if this happens if your electron goes

back to the metal then there is no electricity created so your your this

all just the photon which you came into your your die it just creates heat so no electricity so this in that means

inefficient so your device doesn't work in principle your your devices in a short circuit so what it means that

these two processes actually are always competing in your device and the the one

which is faster that one wins so although this process they're both

they're all possible there and they both can energetically take place there the

dynamics matters so what matters also is which one is faster so in typically in

case of this tie this process is about

100 femtoseconds to 10 picoseconds and

this packet transfer is about 100 nanoseconds so this means that this

process is much more faster and takes place so to give you the idea of the

time scales and what these numbers means so one second is about the time a light

travels from Earth to Sun sorry to moon from from Earth to moon nanosecond

which is 10 to minus 9 seconds is about the time light travels one foot like 30

centimeters so this is a much shorter time scale so the further so one

picosecond which is 10 to minus 12 seconds that

about the time a light travels about the three with hairs three widths of hair so

this is 300 microns and if you go to town to femtoseconds this is about a

light travels are like a length of a virus so it's it's smaller than a cell

so this is you see the very fast processes here now so I'm coming to the

part so this was in penal in a short

summary how this devices work and what kind of elementary processes take place

in there so what we did now we were thinking okay ruthenium nice ruthenium

dyes are very nice they are very efficient working nice but they are rather expensive actually so because of

this ruthenium metal you know it's it's a noble metal it's a very rare metal and

if you want to for example produce these devices maybe on a global scale

it will be you don't maybe probably have enough ruthenium on earth to do that so

that's of course a problem so what you were thinking okay but what about iron

so iron is a very abundant metal it's one of one of the most abundant metals

on earth if not the most abundant and it's so therefore it's very cheap and if

you look at the periodic table you see that iron is just above the ruthenium so that means that it's the the the

chemical properties and the properties of of these two dyes they are rather similar actually as you see these

molecules they're exactly the same except the metal so they can create same

chemical bonds that can create the same chemical structures this is one is also

it's a very strong light absorber so which is very important for efficient dye and it's of course it's cheap so we

were thinking okay maybe we can use iron instead why not and this we and when now when we tried

that okay in ruthenium this process takes 100 nanoseconds but in iron it's 100

femtoseconds so it's 1 million times faster so why is that it's it's it's the

same chemical structure but 1 million times faster this back transfer of

electron and this dye just doesn't work it just doesn't work it only creates

heat doesn't provide any electrons if you use it so our question okay what is

the mechanism actually what determines this timescale can we understand this

and how we can understand this

experimentally what is the challenge here so the challenge is that if you

look at this molecule molecules are right are very small so this bond distance here is like 2 times 10 to

minus 10 meters so this is about what is

a free throw distance compared to the earth to the Sun so that's how small

molecules are so so this is our way away from the human scale so when this is

what we have to distinguish in we have to understand okay is the electron here or here secondly of course these are

very fast so 100 femtosecond is this process so this is again if you compare

it to 1 minute it's like roughly like 1 minute is to 1/4 billion years so like a lifetime of

Earth so this is so fast process taking in so small scale how can you actually

see it like what's what's happening there at all right and well fortunately

we are at SLAC right so it's actually no problem

actually it's easy so maybe you maybe you know this a little bit so here is

that this is the slack site and there is this one suspiciously a straight line

here which is a linear accelerator of slack and and really a brilliant

scientist here it's like so I will zoom in so they use this linear accelerator

to create ultra short and ultra intense x-ray pulses so you have a accelerator

here so you have electron punches which are accelerated almost to the speed of light and then when they are at the

speed of light they are going through this kind of a magnetic periodic structure where this electron bunches

wiggling creating x-rays and amplifying them interacting with the x-rays as they

travel through the undulator creating a very short very intense x-ray pulse and then here there are the experimental

stations which can use these x-rays for four different experiments and that's

what we are doing so here I show

actually here is actually pulse here is the pulses that that's where I sit usually and we are here okay so how else

LS how this accelerator how it can actually do this so here is a

electromagnetic spectrum and if you look where the x-rays are the electromagnetic

waves else unless the leenock coherent light source here is creating the

wavelength of these x-rays are about the scale of item or the bond length so

that's where the sensitivity comes to the to the electron density in a

molecule because the x-rays have the same wavelength as the the bones as the items the distances of

bones or the radiuses of items leaks rays are sensitive actually to the to

these electronic densities in the in the molecule and secondly these x-rays are

coming with ultrashort pulses so they are shorter than 100 femtoseconds so

this means that if you think of your pulse as a shutter of a camera or as as

a stroboscopic light it's it's it's it's so short it's so fast so it can take a snapshot of a very fast process so

that's why LCLs can see this process and

so how we actually do this experiment is that we take we take a lot of these

molecules actually they are in in a solution like that so it's a it's a very nice colorful solution we use it and

actually we we run it in a jet so it's like as we create a little stream of

liquid which we then use in an experiment so with this little stream of

liquid first we take laser pulse come this it's ultra short it has to be short

it has to be also short in the time scale of the process we want to study and we come this laser pulse and we

excite all these molecules maybe not all of them but a lot of them so now after

that we come with the extra pass so we come and hit this stream of liquid

liquid jet we come with and hit with a filter extra pass which is also very short duration and it comes a little bit

after the after the laser pulse so it's actually well defined time delay between

the laser pulse and the x-ray probe and this this time delay here of course

gives us the time that's how we clock the time of how the process wolves so now we repeat this many times

actually we do this pump probe pump probe until we have a clear image of what takes place and then we shift the

time delay we do the pump pump probe again and that's how we create the movie so we like it change the time delay

between the pump which in each which initiates the process and then with the

x-rays become and provoke probe the system after a defined time delay that's

how we know for example within which state our system is after let's say 100

femtoseconds we excited it so then we know what what's taking place in there that's how we great we do this a very

many times we change this time delay between pump and probe and then that's

how we get the movie that's how we get the movie of how this electron is actually moving around in this molecular

yes and okay here I you don't have two pictures actually from the LCLs so this

is kind of how the experimental hutch looks like when we're doing experiments so you see it's a huge mess this is so

that's where the light is this is actually the experimental chamber and that's where we have we have a jet

running so why the light disturbed us this is just a lamp actually right now here there is no x-rays or or laser in

in there and x-rays they're coming kind of out from the out from this from this

wall and that's where we do the experiment and this when we have x-rays

and laser on here actually we we sit in a control room in a comfortable control

room and look at the screens and hopefully collecting a nice data and we

are collecting nice data then we are all happy and eating cookies and drinking

soda but if not then we are all running around and and shouting each other but

yeah usually it's we get something okay

so what do we actually learn from from this so from this experiment so we

learned that in an iron die so when we do this excitation and we separate these

charges we take the electron from the iron and put it at the ligand so what

actually happens is that it's not going to the to the its initial state very

fast there is an intermediate state in there between so the electron it's

moving back to the metal but it's not at the same it's not it's not filling the

hole at the metal it's it's going to the metal but it's still at its it has a different shape it has a different

energy and this state it kind of acts

like a like a ladder you know that's how the how it can step down so so this this

is the process which is very fast so this this is the the process which which

takes the electron from the ligand and put it back to the metal although it

doesn't put it at the same place that the metal doesn't fill the hole in there it still goes back to the metal and

makes this molecule inactive so cannot it cannot you don't have the electron at

the ligand and it cannot move to the titanium oxide so and that's just this

is not that's not what happening in ruthenium in ruthenium actually this

state is much higher we find out and so there is just just it's not

energetically it's not possible that it's it jumps there so that's the reason there is this kind of intermediate state

okay so of course we were thinking further

everything okay can we do actually something about this now if you look at

these two states they have a they have a rather different electron distribution right so so here as the electron at the

ligand that here it's at the metal so and now you can imagine because these electrons they are at different places

in the molecule they have a different shape of course they interact differently to the ligands in there

around there so they feel different forces from the ligands and if you actually maybe if you manipulate what

kind of ligands you have there in your molecule you can change the forces is

different to these electrons field and what is the relative energy so our

question is can we actually control this excited state dynamics and can we

somehow engineer this excited state chemistry so what's happening in there

and that's what we did so this is the

initial molecular set what we had which has a very fast decay to this metal

center state now when we substitute this this one ligand here with the to this

kind of carbon nitrogen carbon nitrogen ligands we see that we push up this

state this this metal center state in energy and if we substitute this second

ligand here with a carbon hydrogen carbonate in a group we even push it up

even further so this means how here okay

it's still very fast it still doesn't work but actually here so these states they are very close together so you can

still have a transition from from this

to here but because the states they're closed and there is like a barrier

between this going from one to another this process is actually much faster so

it's actually 100 times faster here so this is quite good so this all almost

works not quite it's it's still the life time here it's still way shorter than in

a ruthenium compound but this just shows you how actually one can engineer these

dynamics in this molecules and ties to maybe make them more efficient right and

actually here I come already to the summary of my talk so so there is a

major scientific challenge here we are facing right if you want to make these

devices work and efficient we need to know how to engineer this charge

separation dynamics in this flies in these photosensitizers and if you know

that we can use them to more efficiently catch the light right and even further I

mean I didn't tell you about I didn't tell you about how this device is exactly work but to make these devices

work you have to also engineer for example how how well you can like let's

say transfer a hole to the chemical compound which is doing the water

splitting for example so you need to have you need to do a lot of engineering to get this process match so that the

device works efficiently so this is a really a huge problem with lots of

chemists and material scientists are working it to understand how to do this and and the kind of molecular scale

knowledge is actually I think it really are necessary to understand how to do

this better so and secondly of course there is this huge experimental

challenge we are facing right how do you actually look at these processes now that we if you want to study them how

these electrons actually move in these molecules and how structures rearranged in in a

femtosecond time scales right so and for that we need this kind of a huge I mean

huge devices huge accelerators to do that there is no other ways to do that

actually and another thing I want to do

like a take-home message is so what makes a good what isn't necessary for a good

photosensitizer for a good diet to work in the systems so first of course it has

to absorb light very well but this is not enough in addition it has two discs

I it has to be such that this excitation leads to the charge separation so it's

not enough if you just like get hit by the ball you have to catch it right so

if you get hit by the ball okay you take the energy but it's just nothing useful

happens right so it's not enough if the dye absorbs light and creates heat it

has to absorb it and it had to be able to do with something with this two greatest choices and here's the last

slide of my my talk so I'm working in a solution faced chemistry group at pulse

so and I have a we have two bosses it's Kelly and Amy and and this work which I

actually showed it it's a lot it's a work of mostly Kelly and Robert and

Casper has have been working on this for for our many years so I I came to this

group only less than two years ago and here is a group picture we actually did

today so this is made specially for this talk so you can you can recognize these

people by these dirty clothes probably they're sitting somewhere here in this auditorium and and I also of course want to thank

all the collaborators because all these experiments they have been a big project collab collaborative efforts to make

this really work so thank you very much for your attention

okay so I think we have some time for questions I warned you that don't ask

the question until you have the microphone in your hand because this will be videotaped and we'd like to have

your question on the recording so why don't we start over here oh by the way

could you put this way the previous slide back sure so you'll notice that

the people in this picture are also the people running the microphones so you can thank them okay please sir

no thank you for the nice time I was wondering if you happen to have a copy of the video movie that you created

using the x-ray yeah unfortunately I don't have yeah I guess maybe we should

do that we haven't we haven't done really a movie so that would be of course a great thing to have yeah it

takes a certain amount of effort to reduce the data to a movie we've done that for some reactions but not for this

one yeah we have graphs I mean I didn't

show any data here that's right yeah I didn't so I maybe I should have done it but hi I have a question I'm confused

about the whole and electron transfer an electron is excited out of the ruthenium

goes to the titanium dioxide the whole goes the other way and you say the dye

molecules return to its normal state it sounds like a dye molecule is returned

to a state of charge zero yeah but we're still missing a Truong am i are they depleted am i

completely missing something well well what what do you do I mean you

kind of like yeah I mean you take an electron from a metal you put it on a ligand so the whole what I what we call

hole it's it's a vacancy of electron so then there is like electron missing so

when now from the ligand so the electron goes to the titanium oxide right there

is still a electron missing from the metal so what now happens I mean I mean

okay you can use the term hole but what you can also think of is that from this electrolyte an electron comes and fills

this so that's and that's how you are in the initial state so yeah I mean use

different languages sometimes we talk about holes and which is actually just the absence of electron but yeah so

that's how it's ends up in the initial state okay thank you and standard P V

cells you get a voltage of about 0.6 volts you know across the the junction

what do you get with in your in the in the ruthenium and in this iron and first

and second what kind of efficiencies are you far enough along to measure efficiencies I think we are talking

about here like efficiencies are around ten percent maybe so they are not as

efficient actually as as the silicon based solar so silicon resource you can make efficient but I mean the the big

advantage of this type of devices is release that they are so versatile so you can chemically engineer them to do

for example this water splitting or something and potentially in the future I mean I mean this thing depends on the

economic situation and so forth they have potential to become much more cheaper because they don't require any

this kind of you know like I mean

creating this Oh - pure silicon you know you need you need the huge factories and so on

I mean to make these devices it's actually very easy it's kind of you can kind of spray your paint them on one

substrate and and take it for it I mean what is the exact the voltage I can I

don't remember right now by heart ie

actually I haven't I haven't we haven't made it made it with iron we haven't made it into like a cellular device so

so what we did we only investigated this dye so far on page 18 what is the co t

ba okay okay I mean it's it's just the

the one chemical group of in this in this in this molecule so it's I didn't

talk about because it's just for example here yeah you're talking about for example this so I mean this this dye

it's actually it's actually a salt I think so it's a if you this TVA it's if

this is one cation I mean it's a positive ion actually which is but in a solution actually this this TVA part

it's in in the solution I believe I mean it's just CLO it's a carbon oxygen

oxygen it's like carboxyl yeah yeah

could ask another question sorry so I

was curious when you were showing the periodic table and there were some other elements besides iron that were

surrounding ruthenium have you ever gonna look at any of those other elements so far

okay here I mean when we are talking

about molecular photosensitizers so we have only looked at iron ruthenium I mean actually you

can I mean you can use different dyes or different photosensitizers for example

you can use nanoparticles photosensitizer also which we have which

we have tried to study and if you look at the other other metals here these are

metals for example which we are also we are studying the complexes of for example cobalt to look at the catalytic

or the chemical activity of these complexes but we haven't looked at like other molecular photosensitizers other

than that use manganese which is right

next door mm-hmm but generally the chemistry is the same when you go down the periodic table and it changes when you go across

so I don't know how that works with yeah I think it's about it's about right yeah I mean it's it's very I hesitate to say

something very generally right here but but it is true that this this group is kind of its kind of there it's kind of

photosensitizers and for example cobalt and manganese and they are kind of this kind of which initiate or catalyze

reactions chemical reactions

so so when you change that iron-based molecule to make the energy Delta's

better or whatever it was he did how did he decide how to change the molecule oh that's a good question I mean you're

kind of trying out a little bit I was only who's more trial and error or if there's well with with this with this

what I showed here I mean the kind of

their ID was and I think it was actually a correct ID is that so if you if you

have this let's say you have this two metal centered electron state at the metal like one is where you create a

hole and the other one is this you know when the electron goes back but it doesn't fill the hole but it's still at

the metal so these are this two different different state and having

different ligands using using this CN ligand which is actually it which has a

which has a effect to these metal orbitals that it tends to make them make

them energetically further apart so this is something we knew we know that if you

if you substitute these ligands with the C and you make these two two metal

centers electron orbitals electron States further apart and that's why we

used it and I think actually it was a correct ID

okay um so when using the I think you mentioned for the x-ray spectroscopy

techniques is it really wouldn't it be really hard to distinguish scattering atoms with little difference in atomic

number so I think that's a correct

comment but I mean what I didn't tell you here about is that we didn't look at the scattering your fix rays actually we

did a spectroscope yeah and and we used this kind of I didn't go into this

detail I really did try to avoid any this kind of technical x-ray spectroscopy with specific details but

we use the x-ray spectroscopy technique which is just sensitive to the to the

actually kind of how many electrons you have at the metal so it was we kind of

selected a technique

do your theoretical physicist friends have any kind of theory or model that

you could pursue to choose a different kind of ligand either a different number

of ligands or a different our compound yeah I mean that would be really great if you could just calculate everything

and we would know everything this is in principle yes we would like to do that

we would like to be able to calculate everything and we are actually working in close collaboration with with I mean

it's a quantum chemistry of quantum chemistry groups who are doing all this very complicated calculations but I mean

one problem with this these systems is actually that it's it's and not only

these systems but actually in general it's it's very difficult to actually get accurate description of of all these

excited state dynamics I mean this this I would I could say that with lots of

molecules for example they take this kind of ground state calculations work very well but if you if you make

expectations and you want to understand the dynamics this is very hard so this

is hard to hard to really calculate so we can calculate lots of things and we it's helpful but we cannot do the whole

we cannot do the whole calculation what

is the voltage it takes to split water into hydrogen and oxygen and how well does this match the voltage you can get

from a photo electrochemical cell I mean

when we are talking about these these water splitting devices what I was so so

it's important to to actually point out that this this is not electrolysis so this is not just like because you could

just you know put like two electrodes in a water just metal electrodes and just

if you put enough voltage you could just split your water you could do that I don't know what's the voltage exactly is

yeah yeah but yeah yeah but but with this with these systems the the aim is

actually they were due to use these to this designed you know catalytic

molecules which which can do it with like that's very low very low voltages I

believe so that so that you can drive this process with this with the source and in principle also kind of like based

on how like you look at the atoms and like can like the electrons um would

also like the distance determinations be limited because of like the presence of the K edge of one of the metals that use

sorry I didn't get this like um when so when you're using like the x-ray for

measuring like the atoms and like the amount of electrons when also like the

distance determinations like be limited due to like like a K edge of one of the metals that you're using so I mean III

sorry I didn't I didn't I just don't hear very well your question but I mean we are using like a different technique

so I don't know are you asking still about scattering or or for example or distances so yeah kind of well like yeah

yeah something yeah I mean actually I mean we don't use only spectroscopy we

have been using also like scattering which is kind of sensitive also to the chest to the like distances in your

molecule and and Vav RB we have been able to for example look directly at the structural dynamics how the how these

molecules like wobble around and what they do and also like what what actually

solvent around this is doing so there is really different things to look at so not only not only this yeah it's all

that the answers to in that case when you're using with the scattering back to like kind of like the first question how

do you like work around like it being really similar to other molecules with

little like number I mean this is a really important questions right yes I mean it is well I

can just tell you so much that I mean

it's a question of sensitivity right so we just have used this spectroscopy

technique which we which we know we have like measured like at lab before that

there is the sensitivity to this to this you know electron count at the metal and

we know that there is a there is we are sensitive to that of course when we do this experiment

so if our sensitivity is very low then we just have to pump and pump and probe

a lot of times and but yeah I mean if

you do it right if you select your system right you can you can see it okay Thank You Vicki so that's not Christian

SLAC National Accelerator Laboratory

All content is © SLAC National Accelerator Laboratory. Downloading, displaying, using or copying of any visuals in this archive indicates your agreement to be bound by SLAC's media use guidelines

For questions, please contact SLAC’s media relations manager: 
Manuel Gnida 
(650) 926-2632 

SLAC is a vibrant multiprogram laboratory that explores how the universe works at the biggest, smallest and fastest scales and invents powerful tools used by scientists around the globe. With research spanning particle physics, astrophysics and cosmology, materials, chemistry, bio- and energy sciences and scientific computing, we help solve real-world problems and advance the interests of the nation.

SLAC is operated by Stanford University for the U.S. Department of Energy’s Office of Science. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time.

Featured in

Related event

Developing cheaper and more efficient devices to harvest energy from the sun is a major scientific challenge. One way to increase the efficiency of solar cells ­– and even make solar cells out of otherwise inactive materials – is by...
stillframe catching light