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 televisions.

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 Statistics

When addressing one or a few emitting molecules, the fact that there is only one or a few emitters present can be used to identify the discreet emission of individual photons. Consequently, second-order intensity correlations as a function of a lag time can be calculated that tell us about the self-similarity of photon emission as a function of that lag time. This can be done experimentally by, for example, splitting the photon stream in a Hanbury Brown & Twiss arrangement such that correlations between two detection channels are made.

It is then possible to identify molecules or particles that are single photon emitters or measure the diffusion times of molecules as they move through the laser focus when in solution.

Time-Resolved Photon Antibunching

We are pioneering the application of time-resolving photon statistics to understand loss mechanisms in organic semiconductors that can affect their performance. These techniques are a novel extension of conventional photon antibunching. Now we time-resolve this process, as devised in our new paper published in Nature Communications, and in doing so we can monitor the evolution in the number of emitters on a picosecond timescale.

These new techniques can then be used to observe how excitons move and interact with each other through exciton-exciton annihilation in organic semiconductors, a loss mechanism, and consequently we can deduce materials that have reduced losses and thus better device performance.

Quantum Behaviour

We have began to explore the quantum mechanical behaviour of organic semiconductors. We do this by exploiting the discreet emission of photons from single molecules, as discussed above. By measuring photon emission from single molecules very precisely we aim to explore the nature of the earliest physics in excited states, where the exciton is a coherent quantum mechanical object. In time this dephases into a classical quasiparticle and we want to determine how long this takes and whether protection against it can be engineered.

These coherent states are potentially important in determining energy transfer efficiencies and may also be exploited in the generation of entangled states, useful for quantum cryptography or computing.

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.

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 python code to make customised advanced measurements, using computer control of lasers, spectrographs and optical components. We have also developed a novel analysis suite in python and C to work with and analyse these large photon stream datasets, for example to extract the time-resolved photon antibunching and quantum coherence results mentioned above.