Diblock Fullerene Derivatives for Organic Solar Cells.

[NNadir]

Just a few minutes ago, I posted a remark about indium tin oxide in this space, to excerpt it:

In cell phones, and in solar cells of the (CIGS) type, indium is in the form of ITO, indium tin oxide.

There are no pure ores of indium, and its solubility in ocean water is extremely low. It is a side product of the refining of zinc and a few other element ores.

In order to meet a requirement that we simply use indium that has already been isolated - the endless "we'll just recycle it" stuff that always ends up as a panacea statement whenever one makes a point that, for example, wind turbines and solar cells are not, in fact, "renewable" - it must be the case that 100% of the world's cell phones and 100% of its CIGS solar cells make it back to a recycling plant. Moreover indium, even if it is an essential component, is in low concentrations in these devices, and therefore one must invest considerable energy to recover it, not to mention using large amounts of materials, solvents, reagents, acids, bases, what have you.

A few minutes thereafter, just now, I came across a wonderful paper relevant to the general perception that solar cells will save the day even if they have not saved the day, are not saving the day - CO2 at Mauna Loa yesterday, January 29, 2081, was measured at 408.26 ppm - and, even if I'm swimming against the mainstream here, will not save the day.

But we have to try, people say, we have to try.

Here's the paper I just opened:

Amphiphilic Diblock Fullerene Derivatives as Cathode Interfacial Layers for Organic Solar Cells (Tu, Li, Li, Liu, Liu, ACS Appl. Mater. Interfaces, 2018, 10 (3), pp 2649–2657)

It's all about organic solar cells, which will of course, be green, green, green, green, because solar is sunlight and sunlight grows trees.

Some introductory text from the paper:

Because of the advantages of low cost, lightweight, flexibility, large-area fabrication, and semitransparency, organic solar cells (OSCs) have been a promising technology for clean and renewable energy conversion.(1-5) To improve the power conversion efficiency (PCE), much attention has been given on the interface control, material design, self-assembly of the donor and acceptor phases, and device fabrication.(6-8) In addition, the PCE of OSCs has reached over 13%.(9, 10) One of the strategies is to develop new donor or acceptor materials to enhance the short-circuit current density (JSC), open-circuit voltage (VOC), and fill factor (FF).(11, 12) On the other hand, approaches during the device fabrications, such as incorporation of additives, controlling the growth rate of films, and interface modifications, have also been well-studied and exhibited extremely important influences.(13, 14) The device geometry and interface properties are verified to be two main critical factors toward the preparation of high-performance OSCs...

Then there's some stuff about the work function of Zinc oxide, wonderful stuff too.

And then this:

In this work, we developed a new amphiphilic diblock fullerene derivative [6,6]-phenyl-C61-butyricacid-4-(9,9,9?,9?-tetrakis(3-bromopropyl)-9H,9?H-[2,2?-bifluoren]-7-yl)phenol-(N,N,N-trimethylpropan-1-aminium) bromide (C60-4TPB) with ammonium groups on the side chain of the fluorene block (Figure 1). This diblock molecule shows selected solubility and can support multilayer OSC structures by solution processing. Because of its rigid amphiphilic design, solvent annealing will be applied and expected to induce the self-assembly of the interface layer, which can help to investigate the influence of morphology in the interface layer on the performances of OSCs. The existence of large amount of fullerene in the cathode interface layer is expected to form multi-transmission channels for the electrons and to decrease the hole quenching at the cathode for the polymer: fullerene solar cells (Figure 1).

Fullerene, C60, was the subject of a huge amount of organic chemistry twenty or thirty years ago by all sorts of great organic chemists using very elegant syntheses involving cool cycloadditions and all kinds of other great stuff.

Eventually it was discovered - generating a Nobel Prize - not by synthesis but as common component of soot, specifically lampblack.

Nevertheless we can do all kinds of cool stuff with it now, apparently even make organic solar cells.

That's all great, but for me, I'll just go to the pictures from this paper.

Here's one:



The caption:

Figure 1. Chemical structures of the amphiphilic diblock fullerene derivative C60-4TPB and the structure of the inverted device.

That ITO at the bottom of the device, that's the indium tin oxide I talked about in my previous post and excerpted here.

There's silver on the top, and a layer of molybdenum trioxide as well. The PTB stuff is a C71 organic materials.

Here's the "green" chemistry for making the diblock fullerene:



Intermediate (e) is a bromotetraphenylene, in other words, a halotetraphenylene. Structurally it reminds me of two interesting sets of compounds that appear a great deal in the environmental literature.

One is the PCB's, which are halobiphenyls. These have made all of the fish in the Hudson River potentially carcinogenic to eat, since they are functionalized planar molecules that fit nicely into the minor groove of DNA and bond to it. They've done all sorts of things to remove PCB's, dumped by GE into the Hudson River as part of their manufacture of transformers and capacitors, dredging, pumping blah, blah, blah. Of course if you dredge, you need to need to dump the dredgings somewhere, and well, that's an expensive problem. (PCB's can be destroyed radiolytically, but nobody likes radioactive stuff.)

The second class of compounds of which this bromotetraphenylene reminds me is the brominated flame retardants, now banned in most countries but still found in all human (and most animal) flesh, in particularly the structurally close bromodiphenyl ethers, which are carcinogenic for the same reason, DNA grooves and planarity.

4-TPB has a 4-hydroxygroup, readily subject to oxidation to a quinone type system, conjugated with a planar tail, just a great system for forming DNA adducts.

Oxidization of aromatic rings is thought to be one reason that PAH's - another component of soot - is successful at forming DNA adducts that one can discover using high resolution mass in certain cancer cells.

For example, picked at random, here's an sample paper: Carcinogenic polycyclic aromatic hydrocarbon-DNA adducts and mechanism of action

The full synthetic process - an organic chemists wet dream with all kinds of wonderful solvents, separations, catalysts and chromatographic separations - is fully described in the supplemental information, which is, as always, open sourced.

If any of this sounds troubling, don't worry, be happy.

All solar chemistry is green, even when performed at a billion ton scale as we all hope it will be - well, as most of us hope with the exception of cranky old fat bald guys like me - because it's, um, "solar."

The conclusion of the paper is inspiring, even if some people can't get with the program and remain cynical:

In summary, an amphiphilic diblock fullerene derivative C60-4TPB has been developed and successfully employed with novel self-assembly properties by solvent annealing. Because of its interface modification properties, C60-4TPB can be applied as cathode interface layers between the ZnO and the active layers in the inverted OSCs to improve the interfacial compatibility between ZnO and the organic layer. Solvent annealing was carried out to increase the assembly of the fullerene block at the top surface of the C60-4TPB layer. The enriched C60 molecules were also expected to influence the distribution of PC71BM in the active layer and decrease the quenching of the hole at the cathode interface, resulting in an increased FF. For the device ITO/ZnO/C60-4TPB (0.5 mg/mL)/PTB7 : PC71BM/MoO3/Ag annealed by toluene solvent, an enhanced average PCE of 8.07% with a relatively long-term stable cathode interface was observed. The results demonstrated that the C60-4TPB layer is an ideal candidate to improve the photovoltaic performance of the inverted OSCs based on ZnO.

Again, don't worry, be happy. Everything will be wonderful in the grand solar renewable future. It must be true. I read about it on the internet.

Some people just don't get it.

We're saved.