Organic electronics has a problem with batch-to-batch variability in the quality of materials, particularly the active semiconducting layer. A fellow PhD student in my office described to me the trouble he often experiences. He made one batch of solar cells last week and measured an average efficiency of 5 %. This week, another batch had only 0.5 % efficiency, depsite having the same device structure and processing conditions. Further investigation revealed that the lab had recently switched to a new batch of polymer, even though they’re supposedly identical.
(This raises the question of how many positive literature reports describing organic electronic devices are “correct” and not anomalous? But here I want to focus on this problem in the context of large scale device production.)
Batch-to-batch variation is bad enough with the sub-gram quantities used for small area device fabrication. Printing optimised large area devices—measured in square metres/minute rather than square centimetres/day—would be next to impossible. You need to know that this week’s batch of polymer is the same as last week’s because different molecular weight size distributions, chemical defects and impurity levels have a big effect on the required processing conditions and final device performance.
It’s quite difficult to produce organic semiconductors on the large scale required for device printing because the polymerisation reactions scale up poorly from round bottom flasks to large batch reactors. You have to re-optimise at each scale, which costs time and money.
James Bannock, who is a PhD student in the same group as me at Imperial, has been working on the use of droplet flow reactors to make poly(3-hexylthiophene) and get around the problem of batch scale up. He’s recently published his work in Advanced Functional Materials (DOI: 10.1002/adfm.201203014) and it’s open access so you can read it yourself for free.
I encourage you to take advantage of the paper being open access and have a look. There’s lots of photographs and figures. Briefly, activated monomer solution is injected into a narrow plastic tube containing a flowing stream of immiscible carrier fluid dispersed with the catalyst. Microlitre-sized droplets form, like tiny individual reaction flasks, and travel through the tubing. Because the droplets are small the chemical environment is highly homogeneous and each droplet is essentially identical. The tubing is heated in an oil bath and the polymerisation reaction (a Grignard metathesis) happens as the droplets travel through the tubing.
You can produce more material by using longer tubing and a higher flow rate. Crucially, this can be done independently of droplet size. So unlike batch reactors, nothing happens to the chemical environment and you get exactly the same polymer, with a low polydispersity index and high regioregularity. The plan is to apply this method to other organic semiconducting polymers. It provides a way to produce device-grade polymer on large scale required for large area fabrication, with minimal batch-to-batch variation, and will hopefully drive the industrial production of organic electronic devices.
Reference: J. H. Bannock, S. H. Krishnadasan, A. M. Nightingale, C. P. Yau, K. Khaw, D. Burkitt, J. J. M. Halls, M. Heeney & J. C. de Mello, Continuous Synthesis of Device-Grade Semiconducting Polymers in Droplet-Based Microreactors, Advanced Functional Materials, 2012. DOI: 10.1002/adfm.201203014