Ambiplar vs bipolar charge transport - whats the difference?

I am writing this open letter to raise the awareness of all scientists who do not differentiate between ambipolar and bipolar charge transport. My deepest respect to James G. Champlain who understands that well and explained it in his publication, but here is my triple C explanation.

Please, do not use terminology "ambipolar charge transport", "ambipolar mobility" in disordered systems, such as organic semiconductors. In general, these Langevin-type materials typically are undoped, there are no minority carriers nor their transport in these films, therefore "ambipolar transport" terminology does not apply here. You can call charge transport as bipolar, or name charge carrier mobility as bipolar, and use definitions of bipolar electron and hole movement.

Be careful, because this goes as far as if you state (god forbid in the title of your paper) "ambipolar charge transport" in organic field effect transistors, you show that there is some space left for you to learn the majority/minority carrier concepts in solid state and semiconductor device physics.

Hehe, now when I scared you enough, the fact is that it is still safe to call organic Field Effect Transistor (FET) as "ambipolar FET". When talking about Langevin-type organic systems, "ambipolar FET" is ok, but "ambipolar charge transport" is NOT ok. Charge transport and FET are two different things.

Record high electron and hole mobility for organic solar cells, transistors and other optoelectronic applications

I think everyone agrees - charge transport is a key parameter of any electronic device, because it defines the material's ability to conduct the electrical current. In other words, charge carrier mobility, in particular electron and hole mobility, must be sufficiently high in order for electronic devices to reach specific performance requirements, say high power conversion efficiency for solar cells or high on/off ratio and frequency for field effect transistors, or high brightness of organic light emitting diodes etc.

But, if the charge transport is so important, let me ask you then, how come that I so rarely see the scientific papers published with such keywords as "charge transport", "carrier mobility" in academic journals? Nature or Science journals nearly never publishes articles about charge transport (maybe once a year or so). I believe this needs to change, because no improvements in charge transport - no improvements in performance of electronic devices.

Personally, my main academic focus is on charge transport and mechanisms of electrical conduction. My last ten years I have been working on trying to understand the electron and hole movement in organic semiconductors better. And let me tell you what I think about this research topic: not so many people actually even know how to measure charge carrier mobility in disordered organic films reliably! There are so many reasons why classical techniques (current-voltage or I-V is one of them) can not be applied in these materials. And naturally, the lack of techniques and the complexity of the charge transport phenomena scares scientists away from it.

But this can change, the significance of charge transport can be brought back from the cellar into the daylight. I believe that the accelerated progress is only possible when issues are being addressed through understanding and not just by poking in the dark on random and hoping to synthesize materials with good mobility for instance.

Lately, I have been developing novel techniques, such as Flash Photovoltage (FPV) and Metal-Insulator-Semiconductor Charge Extraction by Linearly Increasing Voltage (MIS-CELIV). These techniques allow to study the charge transport in disordered (organic and inorganic) semiconductors and electronic devices much more reliably than any technique previously known (including photo-CELIV). For instance, FPV is perfect for actual and operational organic solar cells or light emitting diodes. MIS-CELIV is perfect for quantifying the electron and hole mobility in novel and unknown organic materials.

So, folks stay tuned, I have submitted the papers and hopefully that rotten anonymous academic peer-review process will not delay them for too long.


And, if you have arrived to this page as a results of Google search and hoping to find here those "record high mobility" values, you just have to wait for those "records" - wont be too long anymore :)

Photocurrent mapping of large area organic solar cells

I love this idea of my colleague, Dr. Hui Jin (Hellen), to map the large area organic solar cells and determine the series resistance of electrodes.


The photocurrent mapping I believe is very helpful approach when fabricating large area devices academically or industrially, where the contact resistance is a typical problem :)

How to hire motivated PhD students?

This is my simple but tough-enough procedure used when I hire undergraduate or postgraduate and students, including PhD applicants.

1. Read this article about scientific way of life. Be prepared to reflect on it, give your opinion, highlight the points in this article that you agree or disagree.
2. Answer a list of questions, relevant to PhD studies.
3. Prepare for a video-assisted discussion using either Skype.
4. In general, the scientific knowledge or the previous work experience of applicants will be the least judging factor, because I strongly believe that pertinacity and perseverance combined with humble and reverential personal character can go a long way ...

Doping and performance of organic solar cells

We have pioneered the work quantifying the doping levels in polymer-based organic photovoltaic solar cells. The conclusions is: "The results demonstrate thatthe suppressed (non-Langevin) bimolecular recombination rate and charge collection are not strongly affected by native doping levels in this materials combination."

But please don't get confused. This conclusion only applies to the specific (low) doping range that we have measured. If the films are doped strongly, I suspect that it will negatively impact the solar cell performance - a statements to be proven in the future.

Why do I think this work is significant? Because the impact of classical doping has not yet been widely explored yet in organic semiconductors, while in classical electronics it forms a base of every electronic device.

Do organic solar cells need a balanced carrier mobility?

No they don't!

What they need is a "good enough" or "high enough" mobility of slower carriers, typically hole in polymer-PCBM based organic photovoltaic devices.

Our latest work utilizes MIS-CELIV to measure the mobilities of both, electrons and holes as a function of blend ratio in OPV to proves this point.

What equation is correct to calculate the carrier mobility in photo-CELIV

Our not so latest :) work describes how to use CORRECTION FACTOR to calculate the carrier mobility correctly in photo-CELIV in case there is NO TRAPPING. I repeat, when there is no trapping!!! If there is trapping, all this approach fails, and remember, there is always trapping in organic semiconductors (more or less of it). Such is a nature or disordered semiconducting films.

So, what to do? Use either Flash Photovoltage or MIS-CELIV - our recently developed new techniques to solve problems of photo-CELIV. FPV and MIS-CELIV are superior to photo-CELIV. But don't get me wrong about the weakness of CELIV, because dark-CELIV is still a killer technique I use every day to check the level of doping, film thickness or dielectric constant.

Searching for content inside pdf files in Windows

What a misery Windows operating system is?

By default, the pdf file contents are NOT searched, you need to turn this feature on! Here is how to do it in Windows 7:
1.   Open Indexing Options by clicking the Start button, and then clicking Control Panel. In the search box, typeindexing options, and then clickIndexing Options.
2.   Click Advanced. If you're prompted for an administrator password or confirmation, type the password or provide confirmation.
3.   In the Advanced Options dialog box, click the File Types tab.
4. Check "Index Properties and File Contents".

Hot excitons - are they real in efficient organic solar cells?

Our latest Nature Materials paper shows that hot exciton effects are non-existent in PCPDTBT and PC60BM blends, at least within the given visible range of photon energies.

The same is true in several other efficient polymer-based organic solar cells blends, such as PCDTBT blends.

Having said that, in the past I reported the energy dependent exciton dissociation myself, but it was in pristine polymers.

Therefore, it is possible that in inefficient organic solar cells there might be photon energy dependence of photocarrier generation.

Further research following our work of IQE measurements aiming is required to conclude on the importance of hot-excitons. Personally, I believe the community should dedicate more attention to charge transport and electrical conduction because regardless of the presence of hot or excitons :), if these systems are like wood (non-conducting), they are useless!

The return of mainframes and centralized computing - will the Moore's law ever end or saturate?

Still remember VM's and terminals used to connect to a mainframe computers? Well guess what, I predict they will return by 2023, that is in 10 years from now. Here is my reasoning.

Moore's law predicts, that the number of transistors on integrated circuits roughly doubles every two years. If we use a processor's clock rate (frequency at which a CPU is running) as an ultimate parameter describing the processing speed, the lets look what happens in a very near future.

Standard CPU frequency today is 3 Ghz. After two years it is predicted to be 6 Ghz, after four years  - 16 Ghz, after six years - 256 GHz, after eight years - 65536 Ghz, after 10 years - 4.3 e21 Hz... According to Moore's law, after a few decades we should have computers that process information at the scale of individual atoms [doi:10.1038/35023282] - a scenario that probably will not happen :) Even more interesting and funny statement is concluded by Lawrence M. Krauss et al.: "Our estimate for the total information processing capability of any system in our Universe implies an ultimate limit on the processing capability of any system in the future, independent of its physical manifestation and implies that Moore’s Law cannot continue unabated for more than 600 years for any technological civilization" [arXiv:astro-ph/0404510].

To catch up with the predictions of Moore's law, a qualitative step in technology is necessary, and very soon. But, there is no novel technology today, which could substitute old good field-effect transistors. Whether thats Silicon or Graphene, I believe it will still be FETs that rule the world, for some time at least :)

So, what will happen in the next decades? I believe parallel computing will happen. The computational power will increase due to increased number of processors working in parallel. 8-core processors, 16-core, 32-core, 64-core, 128-core, 256-core, 65536-core CPU, ... where is an end? The interesting thing though is that from this point, processors would start to grow in size - they will get larger, bulkier, heavier and whats for worst, more power hungry.

Bad news for mobile devices, no? Not necessarily, if high speed data communication protocols will improve. Why bother carrying extra mass and volume of batteries if we can do computation on the mainframe and transfer the data to the mobile devices at high speed? It seems that near future will bring lots of focus and attention to better networks - cellular, long and short range wireless, cable and all types of networks.

But can the wireless networks sustain the large amount of data transfer without local interference? What is the theoretical data transfer speed limit on wireless bandwidth? What is the required data transfer rate for the future devices? Lets say we will have fancy pancy phones in the near future with screen resolutions of 2560x1400 pixels which require 3.7e6 bits to be transferred at 50 Hertz refresh rate to make that next level shooter game of yours so real :) This implies that only 200 Mbps (3.7e6*50) data transfer speed is required for a single device.

An interesting article about the fundamental limits of computations, which are not that far away...
http://www.nature.com/nature/journal/v406/n6799/full/4061047a0.html

Hackers and scientists - do they have someting in common?

I have just realized: true scientists are like "hackers". Or at least thats what follows fromPaul Graham visionary video about hackers and start-ups. Check this video for your erudition, its awesome.

I could not agree more with this quote: "Most start-ups fail because they did not make something that people wanted, they made something that they THOUGH people WOULD WANT."

The only question I have is how to know a priory WHAT PEOPLE WANT? :)
As Paul states in this article about how to generate successful ideas, "The way to get startup ideas is not to try to think of startup ideas. It's to look for problems, preferably problems you have yourself. The very best startup ideas tend to have three things in common: they're something the founders themselves want, that they themselves can build, and that few others realize are worth doing."

Here is a must read from Paul Graham about why start-ups fail. I personally love this quote: "Startups are more like science, where you need to follow the trail wherever it leads."

Also check out some focused advise for start-ups.

In search for compact yet powerful binoculars

I am on the market to purchase binoculars. My requirements are:
- Waterproof
- Soft and smooth zoom and focus function, "one-finger operable" so to speak :)

So the following list might be helpful if you are looking for binoculars too. I was choosing from these models:

- Barska 12-60x70 Escape, advertized as better than “Barska 12-60x70 Gladiator”
http://www.shopbarska.com/AB11052.html

- Celestron 20-100x70 SkyMaster
http://www.celestron.com/sports_outdoors/celestron-skymaster-20-100x70-zoom.html
Celestron 25-125x80 SkyMaster
http://www.celestron.com/sports_outdoors/celestron-skymaster-25-125x80-zoom.html

- Vivitar 20-100x70 HD Zoom
http://www.vivitar.com/products/18/binoculars/1091/mv-20100

- Sunagor 20-100x50 Mega Zoom, Sunagor 30-160x70 Mega Zoom
http://www.sunagor.com/acatalog/high-quality-binoculars.html

- Sunagor 15-70x27 Maxima, Sunagor 25-110x30 Super Zoom
http://www.sunagor.com/acatalog/pocket-zoom-binoculars.html



And, the winner iiiiis: Sunagor 25-110x30 Super Zoom. Compact size and super-zoom won the prize. Probably its shitty quality on the far end of zoom, but I wont make award winning pictures through it anyway

Predicting the future citations of scientific publications

A new paper in Science magazine has just published a very interesting article (DOI: 10.1126/science.1237825) about quantifying and predicting long-term scientific impact of a publication (and a commentary about it for non-experts DOI: 10.1126/science.1245218). Authors developed a model allowing to predict the future citations for a scientific paper.

I am amazed - its a simple formula, a few years of history (actually they got less uncertainty and errors using 10 years of history) and voila, here you have it - the nature of the scientific publishing is predictable! Or at least it is predictable now, basing the model on past history.

Anyway, authors write: "In contrast with the IF (Impact Factor) and short-term citations that lack predictive power, we find that c∞ (their specific parameter, which means total number of citations a paper acquires during its lifetime) offers a journal-independent assessment of a paper’s long term impact, with a meaningful interpretation: It captures the total number of citations a paper will ever acquire or the discovery’s ultimate impact.

However, I believe, it is important to remember that times are changing, publishing habits are changing, the nature of how scientists cite papers is changing all the time. So, this specific model might not be a panacea for the long term future predictions. But, for shot term I believe it will hold well, and that where the whole fun and new horizons come from!

As a final note, authors write: "... an ultimate understanding of long-term impact will benefit from a mechanistic understanding of the factors that govern the research community’s response to a discovery." Well well, I believe those factors are pretty clean, no? In general, the research community has a very simple response to a discovery - either they find it useful for their work or not. I always say: "Science is like business, the more people happy you make, the greater the success!" :)