TR Photo- Luminescence
TR-PL is a powerful technique for probing the dynamics of excitonic transitions within
semiconductor materials such as those used in thin-film based photovoltaics.
Dr Enrico De Como and co-workers at the Ludwig-Maximilians-
University Munich, Germany, are using the technique to investigate
the efficiency of organic solar cells. Organic solar cells may offer key
benefits such as high efficiencies, low costs and ease of production.
Such cells typically consist of thin films of conjugated polymer with
a fullerene layer. They rely upon photo-induced charge transfer
occurring between the light harvesting conjugated polymer and the
strong acceptor fullerene layer. The charge transfer excitons (CTEs)
formed in this process have a limited lifetime. Photoluminescence from
de-excitation of excitons is one of the most important recombination
mechanisms that occurs; recombination impacts directly on the
efficiency of such structures.
The decay dynamics of the CTE of the cells were investigated by
analysing the photoluminescence with an iStar ICCD camera coupled
to a Shamrock 500i spectrometer. A pulsed laser (Ti-Sapphire at 540
nm, rep rate 90 kHz) was used to excite the sample and trigger the
iStar. Figure 2 shows a kinetic series captured by the DH340T-18U-03
camera of the photoluminescence spectra in the visible region. Each
photoluminescence spectrum was aquired with a 5 ns gatewidth and
a linear step delay of 5 ns. Signal for each delay was accumulated at
the frequency of the laser i.e. 90 kHz. By considering a time slice of
integrated intensity through the profiles, the decay characteristics can
be plotted and lifetimes determined from power-law models. Decay
time between 20 ns and 40 ns were measured during this experiment.