1 research outputs found
Broadband Ultrafast Photoluminescence Spectroscopy Resolves Charge Photogeneration via Delocalized Hot Excitons in Polymer:Fullerene Photovoltaic Blends
Conventional
descriptions of excitons in semiconducting polymers
do not account for several important observations in polymer:fullerene
photovoltaic blends, including the ultrafast time scale of charge
photogeneration in phase separated blends and the intermediate role
of delocalized charge transfer states. We investigate the nature of
excitons in thin films of polymers and polymer:fullerene blends by
using broadband ultrafast photoluminescence spectroscopy. Our technique
enables us to resolve energetic relaxation, as well as the volume
of excitons and population dynamics on ultrafast time scales. We resolve
substantial high-energy emission from hot excitons prior to energetic
relaxation, which occurs predominantly on a subpicosecond time scale.
Consistent with quantum chemical calculations, ultrafast annihilation
measurements show that excitons initially extend along a substantial
chain length prior to localization induced by structural relaxation.
Moreover, we see that hot excitons are initially highly mobile and
the subsequent rapid decay in mobility is correlated with energetic
relaxation. The relevance of these measurements to charge photogeneration
is confirmed by our measurements in blends. We find that charge photogeneration
occurs predominately via these delocalized hot exciton states in competition
with relaxation and independently of temperature. As well as accounting
for the ultrafast time scale of charge generation across large polymer
phases, delocalized hot excitons may also account for the crucial
requirement that primary charge pairs are well separated in efficient
organic photovoltaic blends