The absorption and emission of light by materials is a very powerful tool for discovering the properties of materials. Photophysics is the study of those absorption and emission processes, looking at them on different timescales and under different environmental conditions, all in an attempt to understand how the materials behave. In the organic materials we study (including conjugated polymers, small molecules and dendrimers) photophysics plays an important role in understanding both their fundamental properties and their performance and suitability for use in OLEDs, Lasers and Solar Cells.

Upon suitable absorption of light in a conjugated organic material an electron moves from the highest occupied molecular orbital (HOMO) to the lowest unnocupied molecular orbital (LUMO), forming an excited state. From there the excited state population can undergo a number of different relaxation processes. Some of these processes result in the emission of light and some do not, with the properties of the material defining how much, how fast and how frequently such events occur.

Steady state spectra of the phosphorescent OLED material Ir(ppy)3 in solution, the absorption spectrum is shown in blue and the photoluminescence in green.

We can observe the various excited state processes using a number of techniques:

Time resolved:
            Time correlated single photon counting
            Gated CCD
            Streak camera
            Femtosecond optical gating
            Ultrafast transient absorption
     Charge Transport:
Steady state:
      Absorption and photoluminescence spectra
      Photoluminescence quantum yield

Ultrafast relaxation

Luminescence decay of the phosphorescent molecule Ir(ppy)3. The excited state shows two relaxation processes, a fast 230 femtosecond decay and a slower 3 picosecond decay.

Observing how an optically excited system relaxes on very fast timescales can tell us interesting things about how a material is behaving. The kinetics shown to the left illustrate an ultrafast relaxation recorded in solution for the phosphorescent metal complex Ir(ppy)3, which is commonly used as an efficient OLED material. Two distinct processes can be seen, a fast decay of 230 femtoseconds and a slower decay of 3 picoseconds. The fast decay is representative of relaxation to two closely spaced lower electronic states, and the slower decay represents vibrational relaxation of the excited state, with the molecule dissipating excess energy to the solvent molecules that surround it. This work contributes to the understanding of how phosphorescent metal complexes behave.

Polymer dynamics

Time resolved data of the polymer F8BT in solution (upper trace, green and red) and in thin film (lower trace, grey and red). The repeat unit of F8BT is shown lower left.

Time resolved photophysical studies can tell us a great deal about not only the evolution of the excited states in a system, but also how the material is arranged at the molecular level. Shown on the right are the kinetic traces from the streak camera for the polymer F8BT - the top dataset (red) is for the material in solution, and the lower (blue) is for it in thin film. Fitting to the data with exponential functions tells us that this material decays with a time constant of 2.3 nanoseconds in solution and 1.4 ns in film. The large difference between these two lifetimes can be attributed to how the polymer chains themselves are arranged in each case. In solution the chain is surrounded by solvent molecules - this isolates the chain and it decays slower. In thin film, however, the long chains are now pinned to a substrate as well as criss-crossing over each other, this allows a faster decay due to crossing of the excitation between the polymer chains.