Efficient Singlet Fission Discovered in a Disordered Acene Film

Singlet exciton fission is a process that occurs in select
organic
semiconductors and entails the splitting of a singlet excited state
into two lower triplet excitons located on adjacent chromophores.
Research examining this phenomenon has recently seen a renaissance
due to the potential to exploit singlet fission within the context
of organic photovoltaics to prepare devices with the ability to circumvent
the Shockley–Queisser limit. To date, high singlet fission
yields have only been reported for crystalline or polycrystalline
materials, suggesting that molecular disorder inhibits singlet fission.
Here, we report the results of ultrafast transient absorption and
time-resolved emission experiments performed on 5,12-diphenyl tetracene
(DPT). Unlike tetracene, which tends to form polycrystalline films
when vapor deposited, DPT’s pendant phenyl groups frustrate
crystal growth, yielding amorphous films. Despite the high level of
disorder in these films, we find that DPT exhibits a surprisingly
high singlet fission yield, with 1.22 triplets being created per excited
singlet. This triplet production occurs over two principal time scales,
with ∼50% of the triplets appearing within 1 ps after photoexcitation
followed by a slower phase of triplet growth over a few hundred picoseconds.
To fit these kinetics, we have developed a model that assumes that
due to molecular disorder, only a subset of DPT dimer pairs adopt
configurations that promote fission. Singlet excitons directly excited
at these sites can undergo fission rapidly, while singlet excitons
created elsewhere in the film must diffuse to these sites to fission.