There is an interesting post on The Scholarly Kitchen on Talking About Science vs. Doing Science, a critical view on Web 2.0 for Scientists.
Every second spent blogging, chatting on FriendFeed, or leaving comments on a PLoS paper is a second taken away from other activities. Those other activities have direct rewards towards advancement.
Actually, this is one of the reasons for the low activity in recent months: I just do not have time for the blog right now – I believe I can write again end of March or so. Nevertheless, I am active, having written proposals (1 won, 1 open) as well as two review-like papers. One of them is on Organic Bulk Heterojunction Solar Cells, the other on the Role of the Charge Transfer State for Organic Photovoltaics. Writing a third one right now… Once they are published (if they ever are), I will link to them. You are likely to find some figures etc familiar from the blog… and finally something to properly reference :-)
[Update 25th March 2010] 2nd proposal won as well;-) Also, the CT review was accepted by Adv Mater: if you are interested in the preprint, drop me a line.
26 thoughts on “Science talking vs doing – and status”
I hope you are not going to talk about ground state charge transfer states. There is ample evidence that all those observations could be accounted for in other ways.
Only one or two references on ground state charge transfer states;-) It is rather about CT complexes as precursor of free charges, and their role in determining photocurrent as well as open circuit voltage. In any case, there is evidence that the CT state can be occupied nicely by a direct (sub S0-S1 gap) optical transition. I would be interested to hear what evidence you are referring to!
I don’t quite understand what you mean , probably because we speak slightly different languages. For me, in the absence of any ground state charge transfer interaction there would be negligible to none, subbandgap optical transitions leading to promotion of an electron from the HOMO of one molecule to the LUMO of the next. I thought this was by definition, a ground state charge transfer complex has some CT absorption, without it , there is no complex. Of course there is varying degrees of charge transfer but still its either a ground state charge transfer complex or not.
Moving back to the point of why I think there is no CT interaction between , say, PCBM and P3HT, is because, if there was some CT character, then recombination of charges across the PCBM-P3HT heterojunction should give rise to CT emission. We can detect single photons of emission however to my knowledge no one has reported seeing emission from P3HT-PCBM at the expected CT wavelength of around 0.6 eV. If there is no emission (even accounting for non-radiative decay losses) there likewise can be no absorption, as absorption and emission are linked by the Einstein relation. Hence although I cannot rule out any CT absorption (anything is possible with defects etc) I doubt that it plays any significant role in the device operation.
I have seen lots of papers to date on CT states between a polymer and PCBM, however most of these reports have weaknesses. One common weakness is that it is not well known that PCBM shows absorption down as low as around 1 eV because it has midbandgap states. As an example, if you look at the IPCE spectra of a P3HT:PCBM device you will see photogenerated current (albeit small) for wavelengths down to around 900 nm. I see it at least a few times a week. That is from absorption in the PCBM. These midbandgap states are very important to the properties of fullerene however I havent had time to write up any of my ideas.
Anyway sorry for talking your ear off – congratulations on your grants!
Thanks for your thoughts! Hmm, you are right, we come from different directions;-) So let’s meet at the middle. If I consider the data of Vandewal et al. (Adv Funct Mater 2008, DOI:10.1002/adfm.200800056) — say Fig. 1, it seems to me that MDMO-PPV and PCBM alone do not show any absorption (or photon conversion efficiency, measured by Fourier transform photocurrent spectroscopy) at 1.3eV, whereas the blend shows a distinct additional feature… looks like an additional feature just due to blending, and might very well (i.e., with good reason, I think) be interpreted as ground state CT! Have you seen something similar?
I just googled the paper you mentioned. I will be critical and say that they show poor evidence for any CT states, even the abstract itself hints at what is most important cause of the observed changes , the PCBM itself. I have particular gripes with Fig.1 in that they cut the EQE spectra for PCBM at 1.6 eV. Why is this given that PCBM has absorption must lower than this? When they have resolution to spare in their measurement down to EQE 10-4 they didnt bother to check see if PCBM continued to give some current?? I am sure if they looked they might have found that PCBM did show current below 1.6 eV possibly complicating their end analysis.
Figure 4 and Figure 5, the tail in EQE down to low wavelengths just seems to correlate to how efficient the device is, with a more efficient device showing higher EQE at long wavelengths, which is not surprising. Unfortunately they cannot deconvolute absorption out of the EQE directly and so it is very feasible that all blends have the same absorption but that they only see heightened EQE at low wavelengths for the best devices. An equally likely explanation to the one they give is that PCBM is absorbing down to 1 eV in all devices and that how high the EQE gets depends on how good the device is.
Figure 6 they anneal the films causing PCBM crystallisation heightening its absorption in the region around 1 eV causing the observed change (note the absorption comes from the PCBM aggregate and not the molecule, molecule band gap is fixed at 1.7 eV).
Figure 7 increasing PCBM content causes more PCBM aggregates but also again, the more efficient device would show greater EQE at the long wavelength region. The explanation for this effect in the paper is particularly weak. How exactly is adding more PCBM meant to shift a CT state energy? A CT state is an interaction between one molecule and the next. Adding a higher concentration of one component should not shift the CT absorption band. If adding more PCBM shifts the PCBM LUMO, then suddenly we have a CT state between one polymer and a PCBM aggregate?
I have seen similar evidence like this elsewhere also for reports on absorption but the underlying factor in all of this is the PCBM; no-one yet is to show CT state evidence that goes lower than 1 eV beyond the tail of absorption in PCBM, and all this despite the fact that below 1eV is where we all expect the CT absorption to be. The energy of the CT state in PPV:PCBM should be 0.8eV and P3HT:PCBM at 0.6eV the same as the device voltage (HOMO donor-LUMO acceptor gap) but in this paper they dont even look there which is weird.
Furthermore they overlook the fact that the CT states could be coming from anything else but PCBM and MDMOPPV. The CT absorption is only weak, the absorption could be coming from an PPV defect complexing to a PCBM defect, nothing to do with PCBM and polymer at all.
I’ll take the other stance;-)
Even from the data shown, if I make a linear superposition of P3HT and PCBM single phase data shown in Fig. 1, and compare it to the blend data, I see that the low energy part in the blend is a new feature. Actually, the results from electroluminescence are similar (Tvingstedt, JACS 2008 and Vandewal, Nature Materials 2009). As a CT state is a hybrid, the donor-acceptor properties have an influence. For instance, the constant dielectric of the blend is increased when raising the PCBM fraction, as PCBM has epsilon of about 4 and P3HT of only 3.5 or so. Thus, changed screening, changed CT binding energy, changed CT energy.
Nevertheless, I hear your criticism loud and clear… Maybe you should finally write a paper about it;-) Most people (indeed, including myself) are pretty convinced of the CT story, at the latest since the above mentioned Nat Mater in which CT EQE, CT emission and injection current to drive the emission are used to predict the open circuit voltage! The latter is based on the Shockley–Queisser detailed balance limit, adapted 2007 by Rau (Phys Rev B). I reckon your controversial view will raise the interest in your paper-to-be;-)
I will naturally play the part of the sceptic ;-)
I have just read the papers you mentioned. It seems the story to CTC states has changed a little since I remember, or at least for these papers. Now there is no longer need to find CT absorption as now we are more interested in CT emission (Tvingstedt JACS 2008) as CT emission is all that is important and radiative recombination decay is the principal recombination mechanism that controls Voc.
Or is it actually? The Nat.Mater paper closes by saying ‘Therefore to improve Voc, the exact origin of these non-radiative recombination pathways should be investigated.’ So in other words , the radiative recombination doesn’t mean much at all as there are other recombination pathways. So in fact it is likely that
non-radiative recombination controls Voc which in turn controls the CT emission. Not the other way around.
Even this last part, that Voc controls CT emission is an open question. Why is the CT emission band higher in energy than the Voc of the device? Assuming some exciton binding for hole and electron, the emission should be at lower energy than that required to push the initial charges into the device (Voc). I believe its true for OLED’s, can’t get blue emission out of a device which has only a 2 eV bias across it!? Could it be that this CT emission is actually coming out of defects. (e.g photochemically bound polymer and fullerene), hence the higher emission energy than expected and the need for a larger driving force. Defects could be discounted if PL could be seen matching the EL, as then the electrons and hole recombination would be occuring at the same site as photo-induced charge carrier generation. With just EL we could be looking at voltage driving current to unusual places that are not a D-A junction. Defect emission and Voc could be related because one half of the defect is the polymer and has a changing HOMO.
I may not be making sense because I just had a cup of strong coffee and I have the jitters but basically I don’t see the significance of these findings. CT absorption is probably from PCBM and weak emission could be from anything. There is a correlation true but it doesn’t tell you what controls what.
Anyway I will be staying well clear of any discussion of CT states in my papers. I only pray that reviewers leave me in peace to do my work. My idea with PCBM is interesting but its main focus is not CT states , its why you can’t make a good organic solar cell using a polymer with a HOMO higher than 5.3 eV. This is experimentally validated but nobody knows yet why this is and I think I know. It is purely a thought experiment (there is enough literature evidence) and would be happy to publish it on your blog as an exclusive! Ha ha.
Hi! Actually, the difference between CT state energy and Voc is due to both, radiative and nonradiative recombination! The way to go for improving Voc is probably indeed reducing the nonradiative part. However, the maximum (theoretically possible) Voc is given by the CT state energy. Clearly, CT determines Voc, and recombination leads to Voc being lower than CT;-) Nevertheless, I’ll have also a strong coffee tomorrow in the morning on your health and our discussions:-) And if you find that ou might become interested in CT again, and can think of complementary experiments between you and me, I’d welcome that! Nice evening (~ GMT ;-)
For the lumberjack wanna-be, I respect your skepticism of the evidence for CT states. At the same time, I’d love to see the outline of your arguments that give you the numbers that you’ve quoted. I must say I am surprised how confidently you dismiss the potential importance of CT states given that they are rather ubiquitous in pi-conjugated organic-organic interfaces. I am also curious why you consider PCBM in-gap defect states as more likely precursor states to the onset of photocurrent on the red side of the IPCE spectrum rather than CT excitons. What do you think is the molecular nature of these defect states?
To deibel, I’ve finally signed up for an account, but I’ve been reading your blog that I’d accidentally discovered for some time now. Just wanted to let you know that I’ve really enjoyed your posts.
Hi glidera, thanks! Are you from LA? ;-) I think I recently read a JPCC by you… Concerning signing up, does that imply you are also going to blog? ;-)
I am from LA, and I did recently publish a paper in JPCC. I am working on a follow-up or two to a lot of the open questions that were left. Would love to hear your thoughts in the meantime. I am definitely not going to blog for similar due to lack of time and desire. Signing up was fully just to be able to post to your blog to be honest. =)
Yes I think the red-edge to IPCE spectra comes from PCBM. I cant really explain it well here more than I have above, did you have some specific question in mind? If you look in the literature you will see parallels with C60, and there is a lot more data there on c60 than PCBM. The nature of the midbandgap states in fullerene , I have no idea (C60 and PCBM both show the same evidence for midbandgap states). It could be intercalated compounds or gases in the C60 crystal (that add their electrons), it could be intercalated compounds in the bucky ball, it could be C60 polymerised defects, or least likely it could be natural for C60 to have midbandgap states. I want to say that it is somekind of intercalated compounds, because otherwise the energy models for C60 will all be wrong, but I have a horrible sinking feeling that it is natural.
CT states are fairly common, I agree but we should be careful about the degree of charge transfer. I did not know that CT states were common at organic organic interfaces, how can you eaily measure something which is only one layer thick?
Anyway the evidence I want to see is easy to get. I would like to see the EL data shown in the papers mentioned above repeated with PL measurements. There should be no difficulty in doing so, under the current given explanation, right?
First off, let me say that I appreciate this exchange and the questions that you’ve raised. It’s nice to have one’s conventional understanding challenged on occasion.
Here is another recent paper on CT excitons that after a quick scan I think deibel might have missed (forgive me if I’m wrong): Adv. Funct. Mater. 2009, 19, 1–7. It is all about the emission band that’s red of the emission of the individual components, and it arises only in the blend. Here, I believe you’d be hard-pressed to argue that the effective interfacial area over which one expects to observe CT exciton states is small.
Your ideas about defect states being intrinsic to fullerenes are certainly interesting. PCBM has been shown to assume a weakly H-bonded motif through the ester groups when assembled as a monolayer on a (cold!) Cu surface. While this is far from the spin-coated BHJ conditions, it at least suggests that there could be aggregates of PCBM with varying degrees of these sidechain interactions, which could lead to differing degrees of local polarization and thus site energies for the various nanocrystallites. I’ve yet to see anyone take these thoughts any further.
I just realized that I had the paper saved before final page numbers had been assigned, so here are the correct numbers: 3662-3668. It came out at the end of November.
The paper you mentioned is not convincing. I thought I would have had to dig up some old references to show you, but luckily you can already see it in Figure 2(inset). PCBM shows emission down to the NIR (thus proving it also has absorption, yeah!), and the blends show emission down to the NIR, in fact the shape of PCBM emission and the shape of the blend emission are similar. Why do we need to believe in complicated CT states when we could just say we have selective quenching of the PCBM emission. The authors of this paper fall back on an APL they published to ‘prove’ that the emission is from CT states based on electric field effects. The cause of this effect would also seem obvious, belowbandgap emission is a highly forbidden process hence applying a disturbing electric field will effect this emission more than natural HOMO-LUMO transitions.
Further criticism, although I don’t think its relevant to the data. They measured the PL spectra in air. Why didn’t they realise that any defect emission could obscure there desired signal, why test in air?? Also if they are using the lamp, why didn’t they provide us with an excitation spectra for the emission? (preferably for different emission wavelengths as well).
You are right in that the midbandgap states could be coming from the sidechain but actually I see plenty of evidence that C60 also has midbandgap states. Take for instance that C60 will form an ohmic contact with metal such as gold. Workfunction of gold is 5.0eV , while the HOMO of C60 is 6.1 eV. Thats a big difference over which to form an ohmic contact unless of course there is a midgap state with which to push electrons into/out of. Plus researchers now in OLED are starting to use C60 as a HOLE injecting material because its so good.
my mistake they did measure PL under vacuum.
Come now, you humbly asked for PL and you were given PL. =) Surely you could add some detail to your disagreements on top of mentioning that the PCBM emission band has a red shoulder.
First, with your permission let me ask you a couple of questions about some of the points you brought up in your earlier discussion with deibel. You mentioned that one would expect a CT band to not shift with PCBM concentration. I don’t see this as that obvious, because one could easily imagine that the polarization that a charge experiences at a polymer-fullerene interface may be different if you have one or two fullerene molecules participating in the complex, especially if they have polar sidechains. Also, what is so peculiar about having a CTE state between polymer and a nanoscale aggregate of PCBM, which could have a modified band structure and thus a shifted CT band?
Now on to your criticisms of the paper I suggested. You say that what lets you dismiss the authors is that Fig. 2 shows PCBM emission in the red. While this is
certainly true, keep in mind that fullerene emission has a very small quantum yield, so to see PCBM emission takes a sizable amount of fullerene. I personally think it’s very hard to reconcile the intensity of what they assign to CTE emission (relative to the albeit significantly quenched MDMO-PPV PL) with the very low quantum yield of PCBM at the relatively low 20% PCBM weight ratio.
Next comes the question of regiorandom vs. regioregular P3HT CTE emission. If you claim the red emission is due to the in-gap PCBM defect state, then why would the intensity of this emission be so sensisitve to the polymer self-organization? To me, to explain that you have to invoke some interaction between this defect state that you say is intrinsic to PCBM and the polymer. In that case, this rogue state isn’t as native to the fullerene as you suggest and might itself give rise to new polymer-fullerene CTE-type states. These authors also observe a shift in the “CTE” emission that at least goes in the right direction upon going from a PPV to a polythiophene. Again, why would the energy of the defect emission be so sensitive to the polymer if it’s intrinsic to the fullerene? I will grant you that the authors could have done better in terms of control samples. I would have loved to have seen PL of PCBM in an inert PMMA or polystyrene-type matrix as a function of fullerene loading to isolate the effects of PCBM emission.
Finally, your statement that C60 can be used as a hole-transporting material in conjuction with gold does not necessitate invoking the existance of a mid-gap state. The energy levels that you quote were measured in ultrahigh vacuum conditions, and the penetration depth in those experiments isn’t that large so the 6.1 eV number measured for the top several layers in contact with vacuum may not at all be representative of the energetics at the buried fullerene-metal interface, with its own charge-transfer processes. Similarly, the workfunction of gold is likely no longer 5 eV when in contact with one or more fullerene layers.
You have some good comments, I relish the challenge. Yes I asked for PL, but I asked for PL to match the EL, preferably all from the same group in the same paper. In any case the PL from the Adv. Funct. Mater. paper for MDMOPPV:PCBM doesnt seem to match the PL from MDMOPPV:PCBM in the JACS paper. In the Adv.Funct. Mater. paper the supposedly CT peak is at 1.35-1.40 eV (885 -915 nm) while the CT peak in the JACS paper is 960-970 nm. I don’t think that the shape of the emission is similar neither. You may think that 50 nm is only a small shift but really given the precision of the equipment it makes you doubt. (Given the similar loadings of PCBM – it cannot be ascribed to any red-shift)
There is a similar story in the RR-P3HT:PCBM CT state emission, in JACS the peak is at 1200 nm while in Adv.Funct.Mater the peak is at 1000 nm??? ( data is unclear here – anyway its not at 1200 nm). Another question is why JACS dont mention a problem in seeing P3HT:PCBM CT while Adv.Funct.Mat. say it is difficult to see.
Inconsistency in reports of CT is one reason why I find it hard to believe.
You asked about why CT emission should not shift with PCBM concentration. These are soft materials and prefer to localise charges, relaxing their electornic structure and shape. For a neighbouring molecule to begin to distort a CT state it would have to be able to offer greater stabilisation for the polaron first and then secondly it would have to offer enough stabilisation to offset the decrease in coulombic attraction that would happen when you move the charges become further apart on average. Given the large binding energy in these soft materials (say 0.5 eV) and the weak interaction between fullerene molecule in the crystal I doubt that neighbouring fullerenes have an influence (although I wouldn’t bet my house on that). I would say that the physical spread in distances between hole and electron is more likely to control the emission energy. Which bring us nicely to the next point, as CT emission is typically so broad and featureless, why does CT emission in Adv.Funct.Mater. seem to have fine structure. If you look at Fig.2 the emission clearly has spikes? shoulders which is not like CT emission at all, and is more like emission seem from a rigid molecule, like for instance fullerene.
You asked about the intensity of the CT emission versus intensity of PCBM emission, the answer is the authors know because they measured them both. They dont tell us so as a sceptic I assume they are similar and so didn’t provide any additional evidence either way. Anyway I doubt if PCBM emission is negligible, I can measure it here on an old RT fluorimeter (although I must admit it doesnt look as clear as that.)
Regiorandom versus regioregular , I would say that the regioregular data is so unclear that it is almost worthless. If you stare at that data for long enough you will see anything you want. Furthermore the JACS paper doesnt seem to be having a problem seeing P3HT:PCBM CT emission, although of course they see it at a different wavelength.
You asked about peak shifts as a function of polymer, there is little evidence for any peak shift in the Adv.Funct.Mater paper (at least negligible to what the see in the JACS paper).
You asked about organic-metal interfaces. I agree who knows what goes on at those kind of interfaces, and most knowledge at this point is only speculation. Personally I think the first layer of organic on a metal undergoes a CT reaction and basically is changed. Whether that first layer can then distort the second layer however is more difficult. The point is that while the interface between metal-organic is probably doped, the bulk property of C60 at 6.1eV remains unchanged. How does a metal-organic electrode shift holes into that at space charge limited current? Or more potently, if 1 eV barrier is not a problem for getting ohmic contacts with organics then why do we see evidence for this in the literature. I have only a casual interest in metal organic interfaces however so I don’t know much.
I guess we will agree to disagree for the moment. I think this is still a rather nascent offshoot of the polymer-fullerene PV literature, and much still has to be done, but I believe we should at least keep an open mind and not reject the possibilities so quickly.
I believe the spikes in the PL spectrum from the Adv. Funct. Materials paper are only observed at the high fullerene loading and are in fact due to PCBM emission. I apologize, but I will have to embark on a completely thorough CTE literature exploration at some later time, so I can’t directly comment at the moment on the JACS paper you referred to.
In terms of the fullerene energy levels, yes it’s very possible that the bulk HOMO level for PCBM is around 6.1 eV. But presumably the transition from interfacial energetics to the bulk is continuous, so I guess I don’t see it as that troubling that once the barrier is lowered and the applied bias high enough, one could achieve SCL currents. At the same time, I would not call an interface with any significant barrier Ohmic by definition. Again, that doesn’t mean injection from gold to C60 requires overcoming a barrier because once the two interfaces are brought into contact, I believe that the nominal barriers calculated from UPS-type measurements are simply not applicable to the states that emerge due to the interaction between the two layers. Check out this one out of a gazillion papers on the subject: http://pubs.acs.org/doi/abs/10.1021/la981114f
Hi guys, I appreciate your open way of discussing… and it is all the more interesting if you take different stances (as did I;-) ! As I do not have much time for discussions now, I wonder if any of you will be at the MRS Spring meeting in SF or at the SPIE in San Diego in August?
Yes! My talk is two talks after yours in the same session if you can believe it. I would love to get together and get acquainted a little more. My friend and colleague will also be there presenting on the same day, and we have been following/discussing your blog for a while now so maybe we can all grab some food or beer or both after the talks.
Thanks for giving us a forum to have these discussions.
Excellent, looking forward to it! :-)
I am not sure if I will be attending or not, hopefully I will go though. I will look out for you if I do go!
That’d be splendid! :)
Unfortunately couldnt make it :-( maybe next year.