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Controlling Triplet Excitons in Organic Semiconductors

Periodic Reporting for period 3 - CONTREX (Controlling Triplet Excitons in Organic Semiconductors)

Reporting period: 2019-04-01 to 2019-07-31

"The aim of the CONTREX project is to understand and control a particular type of photoexcited state in a class of material that have the potential to address the current worldwide problem surrounding clean energy generation.

Conjugated polymers are cheap, flexible, plastic semiconductors that can be used in a wide variety of applications, one of the most important of which is solar cell technology. Conjugated polymers offer the potential of thin, flexible devices (such as solar cells or LEDs) which can be processed using low cost methods such as roll to roll printing. The performance of these materials is steadily improving with efficiencies across many devices becoming comparable to traditional inorganic counterparts.

However, in all of these devices and applications when they are being operated (either as a solar cell or as an LED) they result in the formation of a type of excited state known as a ""triplet exciton"". Triplet excitons are lower in energy than the more common Singlet exciton, and importantly they cannot absorb or emit light. Therefore, they traditionally are a major loss mechanism in organic solar cells and light emitting diodes reducing the overall efficiency of the device.

This project directly address the need for new materials for both clean energy generation AND more efficient energy usage. Conjugated polymers show great promise as novel materials for solar cell technology and thus by understanding and controlling the role that triplet excited states play in their constructions will allow for improvements in efficiency and stability. The know-how generated throughout this project is also relevant to organic LED technology which would result in more efficient and therefore greener light emitting devices.
This project has two main aims.

i) To synthesise materials to aid the understanding of triplet excitons and ii) to make us of them

Little is known about the properties of triplet excited states in conjugated polymers. By developing materials which allow for their detailed understanding we can further understand their role in the function of optoelectronic devices, leading to an increase in their performance.

Furthermore, triplet excitons possess unique properties which may lead to novel types of devices which have ultimate efficiencies higher than the current theoretical maximum. After understanding the properties of the triplet excited state we will develop new materials which can harness the power of the triplet excited state to produce new materials with unprecedented properties such as photon multiplication or upconversion."
WP1

We have developed a series of materials designed to understand the mobility of triplet excited states in conjugated polymers. This has been a long standing debate in the literature and we hope to elucidate whether the triplet excitons are mobile or immobile. Through judicious design we have synthesised a series of electronically identical conjugated polymers which differ only in their level of crystallinity. These polymers have been modified such that we rapidly convert singlet excitations in triplets allowing them to be studied in isolation. This work will be an important milestone in the design of new materials where precise properties of a triplet can be controlled.

WP2

When two triplet states collide they produce one higher energy singlet which can then emit light. A fundamental problem is that these system which are always a complex mixtures function efficiently in solution. The main issues associated with solid state upconversion are twofold a) that the components undergo spontaneous phase separation preventing upconversion and b) in solid state most organic chromophores have significantly reduced emissive efficiency due to solid state interactions resulting in non-radiative decay processes. We have recently overcome both of these major issues through a new method to control the optical and structural properties of a conjugated polymer by encapsulating the polymer backbone with a macrocycle. These materials displayed superior optical properties and drastically reduced disorder meaning that they are highly luminescent in the solid state.

WP3

We shown for the first time that we can manipulate the energy difference between the singlet and triplet excited states through design of a conjugated polymer. It was previously thought to be impossible to manipulate this energy gap. Traditionally it was believed that the energetic difference between the first singlet and triplet excited states was fixed at approximately 0.7 eV. This represented an immense problem as it resulted in the triplet excited state acting as a loss mechanism in optoelectronic devices. Through a completely novel approach we introduced electron deficient groups orthogonal to the polymer backbone in contrast to traditional conjugated polymer design. The had the effect of reducing the S-T energy gap such that this material was the first conjugated polymer capable of undergoing reverse intersystem crossing.

WP4

We have developed important design rules for developed materials capable of singlet fission. The new design rules allows us to synthesise stable and tuneable materials for singlet fission. By carefully studying the relationship between aromaticity and antiaromaticity in organic chromophores we were able to understand why some molecules have larger than expected singlet-triplet energy gaps. This meant that we have now been able to design several families of novel singlet fission candidates. With our new design rule we in fact enhance the stability of the photoexcited state. This allows us to developed several new families of singlet fission candidates but also affects the photostability of ALL organic chromophores.
The project is complete and we have succeeded in shifting the paradigm in the design and understanding of triplet excitons in conjugated materials. We have shown that triplets in organic semiconductors can be used in a positive manner by reducing the energy gap between singlet and triplet excited states (to generate material for OLEDs), and simultaneously by increasing it (to generate materials for singlet fission)


Dissemination

The project has resulted in numerous publications covering several aspects of the work. In particular, there have been 4 key publications in the Journal of the American Chemical Society which have reported the success of the central objectives of the project. In addition to the other publications, the overall body of work resulting from this project has demonstrated that the properties of triplet excitons can be controlled to an extent beyond what was thought possible. This new tuneability and control has manifested in devices and films with new and improved properties.

In addition to the publications the PI and the research team have communicated the results at multiple invited and contributed conferences and networking events.


Exploitation

Beyond the scientific publications it is believed that there is strong potential commercial potential for some of the outputs from this publication. In particular, the possibility of accelerating this and achieving industrial engagement and success will be attempted through the submission of a ERC Proof of Concept Grant.