Organic Semiconductors

In The Hedley Lab we primarily look at organic semiconductors. These are carbon based materials that are electrical semiconductors which also absorb and emit visible light. Dissolved in a solvent, they can be painted or printed to allow incredibly thin functioning light emitting diodes, solar cells and transistors to be created. These are not only very thin, but can be made on flexible backing materials to allow bendable electronics.

Commercialisation of organic semiconductors has been impressive over the last 15 years, with the technology going from simple lab devices to a commercial market worth multi-tens of billions of dollars per year. Organic light emitting diode (OLED) displays are now commonplace in smartphones and high-end televisions.

Time Resolved Photophysics

The light absorbed and emitted by the organic semiconducting materials can be used to understand how they behave to investigate their photophysics. When the material absorbs light it is moved into an excited state that will then reorganise before it emits light.

We are particularly interested in time-resolving this light emission to tell us about the reorganisation processes in the excited state. This can involve energy transfer between molecules, structural changes or the opening up of loss pathways. Each of these processes can affect the efficiency that can be realised in a material in an OLED or solar cell, so understanding and minimising such losses by using time-resolved photophysics is key to future advances in organic semiconductor materials.

Single Molecule Spectroscopy

At The Hedley Lab we specialise in single molecule spectroscopy of organic semiconductors. Here the idea is that instead of measuring many billions of molecules at once, we reduce the material concentration until we are exciting and observing photoluminescence from only one molecule at a time when using a confocal microscope.

We can thus investigate how the photophysical properties of a single molecule compares to the ensemble, measuring the emission spectrum (colour), lifetime (how long it takes the excited state to decay) and photon statistics (see below). These measurements allow us to understand the important excited state properties that govern how an organic semiconductor works, and connect the single molecule behaviour to the ensemble.

Photon Correlations

When addressing one or a few emitting molecules, the fact that there is only one or a few emitters present can be used to record correlations in the actual photons emitted by those molecules. This can be done by, for example, splitting the photon stream in a Hanbury-Brown & Twiss arrangement such that correlations between two channels are possible. We are interested in developing advanced measurements of these photon correlations, and using them to tell us unique and valuable information about the excited states in organic semiconductors.

By combining such advanced measurements with the development of powerful software analysis methods (see below) we are aiming to open up a new field of single molecule spectroscopy.

Advanced Software Analysis

The power of single molecule spectroscopy is that individual photon events are recorded. At a single molecule level we can record tens to hundreds of thousands of photons being emitted from each molecule per second - and knowing the arrival time of each photon with a 10-100 picosecond accuracy, we can construct a very detailed timeline of emission events - known as the photon stream - this can generate many gigabytes of raw data.

We have developed advanced MATLAB code to work with and analyse these large photon stream datasets. In combination with the advanced photon correlation measurements discussed above, we are able to extract unique information out of the photon stream that goes well beyond what would typically be deduced.