Theoretical boost for pi-conjugated polymers
The scientists from the Georgia Institute of Technology in the USA, the University of Mons-Hainaut in Belgium and the Chinese Academy of Sciences hope that the study will encourage experimental researchers to pursue techniques that could improve efficiency of polymer devices.
"These results are important in the sense that they lead to an understanding of why polymer LEDs can have an efficiency that goes beyond the 25% limit predicted on the basis of simple spin statistics," says Jean-Luc Bredas, a professor of chemistry and biochemistry at Georgia Tech and also part of a research team at Mons-Hainaut. "It’s important to show that there are ways past this theoretical limit."
Polymer LEDs consist of a 0.1micron thin film of a polymer such as polyparaphenylene vinylene sandwiched between two electrodes. They are usually built on transparent substrates of glass or flexible plastic. When a voltage is applied to the electrodes, the top electrode (cathode) injects electrons into the polymer film, while the bottom electrode (anode) injects positive charges, also known as holes. These charges migrate along the polymer chains until they meet.
When the charges meet, a two-step charge-recombination process takes place in which the opposite charges neutralise one another, producing an excited state (an exciton) in the polymer. The decay of that excited state can then produce light.
During the charge-recombination process, the spin directions of the electrons involved can orient themselves into four possible combinations, each with an equal statistical likelihood. These orientations form excitons in two different patterns - one "singlet" state and three "triplet" states. Only the singlet state, which according to spin statistics should be created in just 25% of the recombinations, can produce light in pi-conjugated polymers - 75% of the charge recombinations are then wasted in terms of light production.
"We really need these polymers to go beyond 25% for the devices to be more efficient," says Bredas. "Our theoretical work is oriented at how we might have deviations in that statistical limit – how we can bias the spin statistics."
By taking advantage of complex restrictions on the amount of energy that can be released by the materials during the recombination process, Bredas and his colleagues show theoretically that systems built from long polymer chains should be able to boost the percentage of light-emitting singlets to as high as 50%. That should be possible because in long chains, triplets are believed to take much longer to convert into neutral excitons after they initially meet to form a loosely-bound "charge transfer state". If another process, such as intersystem crossing or dissociation, intervenes before the internal conversion takes place, the loosely-bound positive and negative charges in a triplet charge transfer state may transform into a singlet that can then emit light. By contrast, the singlet charge transfer states decay quickly, allowing no time for other processes to intervene.
"The calculations we have done based on electron transfer theory show that in short chains, the rates (to go from the charge transfer state to the neutral exciton state) are very fast for both singlets and triplets," explains Bredas. "But when you get to longer chains, the rate of formation of singlets remains large, but the rate of formation of triplets slows considerably. This means that in longer chains, you can bias the spin statistics to produce more singlet neutral excitons than would be predicted."
The ACS meeting paper also suggests avenues that could boost these efficiency improvements such as re-ordering the polymers or making substitutions in their backbones. Bredas is working with experimental researchers to pursue that goal, and to study other applications for the materials in photovoltaic power applications.
Organic light-emitting diodes (OLEDs) based on pi-conjugated polymers offer significant advantages over other display materials. They are lightweight, flexible, easily tailored, operate on low voltages and can be deposited on large areas using simple techniques such as ink-jet printing or spin-coating. Until recently, however, many researchers believed that these light-emitting polymers were limited in efficiency, able to convert no more than 25% of their energy into light.
Alternative OLEDs are based on molecular materials and produced through a more complex vapour deposition process. However, these molecular materials can incorporate heavy-metal atoms that allow triplet excitons to electroluminesce, theoretically allowing 100% efficiency.
