The operation of all organic optoelectronic devices – such as organic light-emitting diodes, solar cells and lasers depends on excitons. The EXCITON project is an Advanced Grant funded by the European Research Council that aims to greatly advance our understanding of excitons in organic semiconductors by developing methods for measuring and controlling exciton diffusion.

 

There is great interest in organic materials with semiconducting electronic properties. This arises from both a scientific point of view (how can a plastic be a semiconductor?) and a technological point of view as these materials can be used to make light-emitting diodes, lasers and solar cells. The performance of all these devices is strongly affected by exciton diffusion, a process that is little studied or understood (particularly compared with charge transport) largely because of the lack of reliable measurement techniques.

 

The purpose of this project is to make a breakthrough in the measurement, understanding and control of exciton diffusion in organic semiconductors, and so create a new generation of materials and devices with enhanced performance due to control of exciton diffusion. The key elements of the study are first to develop and validate advanced measurements of exciton diffusion. This will open up the whole topic of exciton “transport” and provide the tools for us (and others) to explore the physics of exciton diffusion and how it is affected by a range of factors relating to the structure of the materials and how they are processed.

The following phase of work will use information about the main factors affecting exciton diffusion to develop strategies for controlling it. A particular challenge is to increase exciton diffusion which will then lead to improved efficiency of organic solar cells. We aim to address this both by applying the structure-property relations we develop and by developing directional exciton transfer, including quantum coherent energy transfer.

This is an unconventional approach to improving organic solar cells, which could not only improve their efficiency, but also greatly simplify their structure, leading to a breakthrough in their manufacturability. The proposed research will also lead to strategies for reducing exciton-exciton annihilation in organic light-emitting diodes and lasers, leading to (for example) increased power output from organic semiconductor lasers

Research Programme

The programme consists of four main tasks:

1. Developing, testing and validating measurements of exciton diffusion
2. Applying the measurements to understand exciton diffusion and develop structure-property relations
3. Developing strategies to control exciton diffusion
4. Demonstrate enhanced device performance via control of exciton diffusion.

Examples of Current Publications

So far the project has led to more than 20 refereed journal publications, including in prestigious journals such as Nature Communications, Science Advances, Advanced Materials and Chemical Reviews. Selected outputs are described below.

Light Harvesting for Organic Photovoltaics
Gordon J. Hedley, Arvydas Ruseckas, and Ifor D. W. Samuel
Chemical Reviews, DOI: 10.1021/acs.chemrev.6b00215

This invited contribution to Chemical Reviews describes the processes from light absorption to charge generation in organic photovoltaics, including a comparison of the various ways of measuring excition diffusion.

Molecular Weight Dependence of Exciton Diffusion in Poly(3-hexylthiophene)
Zarifi Masri, Arvydas Ruseckas, Evguenia V. Emelianova, Linjun Wang, Ashu K. Bansal, Andrew Matheson, Henrik T. Lemke, Martin M. Nielsen, Ha Nguyen, Olivier Coulembier, Philippe Dubois, David Beljonne, Ifor D. W. Samuel
Advanced Energy Materials
DOI: 10.1002/aenm.201300210

An important aspect of the EXCITON project is to understand how the structure of materials affects exciton diffusion. Here the influence of molecular weight on exciton diffusion in spin-coated films of the polymer poly (3-hexylthiophene) (P3HT) films is reported Exciton diffusion was found to be higher in medium molecular weight samples, and correlated with the degree of aggregation of chromophores, and exciton delocalisation along the polymer chain, suggesting that exciton diffusion length can be enhanced by tailored synthesis and processing conditions.

The Impact of Driving Force on Electron Transfer Rates in Photovoltaic Donor–Acceptor Blends Alexander J. Ward, Arvydas Ruseckas, Mohanad Mousa Kareem, Bernd Ebenhoch, Luis A. Serrano, Manal Al-Eid, Brian Fitzpatrick, Vincent M. Rotello, Graeme Cooke, Ifor D. W. Samuel
Advanced Materials, DOI: 10.1002/adma.201405623

In organic solar cells, charge generation occurs by charge transfer at a donor acceptor interface. In this paper we studied the important problem of how large the energy level offset between donor and acceptor should be to optimise electron transfer. We measured electron transfer rates as a function of energy level offset and found an optimum driving force. The results can be described by Marcus theory and a rare Marcus inverted regime was observed. They suggest that the best organic solar cell materials will have small reorganisation energies.

Controlling Exciton Diffusion and Fullerene Distribution in Photovoltaic Blends by Side Chain Modification
Muhammad T. Sajjad, Alexander J. Ward, Christian Kästner, Arvydas Ruseckas, Harald Hoppe, and Ifor D. W. Samuel
Journal of Physical Chemistry Letters, DOI: 10.1021/acs.jpclett.5b01059

We established the structure-property relationship and showed how crystallinity affects exciton diffusion and fullerene distribution in organic solar cell materials by blending amorphous and semi-crystalline polymers. We found 2 times higher exciton diffusion length in semi-crystalline polymer compared to amorphous and found fullerene mixes preferentially into disordered region of polymer film.

Determining the optimum morphology in high-performance polymer-fullerene organic photovoltaic cells
Gordon J. Hedley, Alexander J. Ward, Alexander Alekseev, Calvyn T. Howells, Emiliano R. Martins, Luis A. Serrano, Graeme Cooke, Arvydas Ruseckas & Ifor D. W. Samuel
Nature Communications
DOI: 10.1038/ncomms3867

Here we showed how exciton diffusion measurements can be used to understand the operation of the high performance blend PTB7:PC71BM. We performed exciton diffusion measurements on PC71BM and then used them to probe the morphology of the blend on a lengthscale that is otherwise difficult to access. By combining nanoscale photocurrent mapping, ultrafast fluorescence and exciton diffusion we showed that optimum blends have elongated fullerene-rich and polymer-rich fibre-like domains (10–50 nm wide and 200–400 nm long) suggesting how efficient charge generation can occur without excessive recombination.

Direct observation of intersystem crossing in a thermally activated delayed fluorescence copper complex in the solid state
Larissa Bergmann, Gordon J. Hedley, Thomas Baumann, Stefan Bräse and Ifor D. W. Samuel
Science Advances, DOI: 10.1126/sciadv.1500889

Here we studied exciton dynamics in an organic LED material. We made the first direct observation of intersystem crossing in a thermally activated delayed fluorescence (TADF) material – a very important class of OLED material. We observed fast intersystem crossing in a copper complex TADF material with a time constant of 27 ps.