Phosphorescence and delayed fluorescence
The OEM group has pioneered the use of gated luminescence measurements to study the dynamics of long lived excited states. Using a system consisting of an excitation source, subnanosecond pulsed YAG laser, using either 3rd or 2nd order harmonics (355 nm or 532 nm appropriately). The energy of each pulse can be tuned from nJ up to mJ, with the diameter of the beam falling on the sample normally in the range of from 0.1-1 cm (we can focus it depending on the sample size). Luminescence is collected onto a JY-190 spectrograph and detected with a single photon sensitive gated iCCD (intensified CCD) camera (Stanford Computer Optics 4 Picos) with 200 ps resolution. The delay time of camera can be changed from 0 to 80 s with minimum step of 0.1 ns, while gate time can be changed from 200 ps to 80 s with minimum step of 0.1 ns. Detection wavelength interval is roughly from 300nm to 900nm. We use calibration lamps to correct for the spectral response of camera and other optics, such as filters, lenses and spectrograph.
Decay measurements normally are performed by logarithmically increasing gate and delay times; consequentially one can record up to 11-12 orders of magnitude of intensity and around 10 order of magnitude of time, this cannot be achieved with any other decay measurement setup (for example using photodiode or photomultiplier tube as a detector instead of gated iCCD). Phosphorescence is identified by its long livetime, large spectral red shift and quenching by oxygen.
In many organic solids triiplet excitons are highly mobile leading to efficient triplet triplet annihilation. This process can be followed by monitoring the delayed fluorescence arising from triplet fusion. We have used this technique to elucidate in detail the dynalics of triplet excitons in organic small molecules and polymers.
OLED devices can be used. These can be excited optical as above or using a fast 10 ns HP pulse generator such that the electrically induced phosphorescence of delayed electroluminescence can be measured. Using a combination of both techniques enabled the OEM group to measure the singlet triplet ratio occuring at charge recombination in a working OLED. This gives us an extremely powerful way to characterise OLEDs.
Time resolved triplet induced absorption
Non emissive excitons can also be studied and their dynamics measured using nanosecond pump probe. Here, a combination og pump and probe lasers are used to monitor the induced absorption of the transient species. Ultrafast custom photodiodes are used for detection (Femto) allowing us to monitor the total trip;let population in a sample of device
ESKMA SL312 Nd:YAG laser (150 ps) home biult single pass dye laser
Crystallaser monolithic sub ns Nd:YAG 355 nm laser, 400 nm, 700 nm, 850 nm diode lasers (high stability)
Stanford computer optics 4Picos gated intensified CCD camera (200 ps) Jobin Yvon Triax 190 spectrograph
1 GHz Agilent Infinium oscilloscope and custom Femto detectors and amplifiers
HP 8114A pulse generator (bidiresctional up grade)
Cryophysics displex helium cryostat (10K)