Diketopyrrolopyrrole (DPP)
-
based polymers have been consistently used for the
fabrication of solar cell devi
ces and transistors, due to
the existence of intermolecular
short
contacts,
resulting in high electron and hole mobilities. However, they also often show limit
ed
external quantum efficiencies (EQEs). In this contribution we analyze the limitations on EQE
by a combined study of exciton dissociation efficiency, charge separation and recombination
kinetics in thin films
and solar devices
of a DPP
-
based donor polyme
r, DPPTT
-
T
(
thieno[3,2
-
b]thiophene
-
diketopyrrolopyrrole
copolymer
)
blended with varying weight fractions of the
fullerene acceptor PC
70
BM. From the correlations between photoluminescence quenching
(PLQ), transient absorption studies an
d
EQE
measurements, w
e conclude
that the main limitation of photon
-
to
-
charge conversion in DPPTT
-
T/PC
70
BM devices is poor exciton
dissociation. This exciton quenching limit is related to the low affinity/miscibility of the
materials, as confirmed by
WAXD
diffraction and t
ransmission electron microscopy data, but
also to the relatively short DPPTT
-
T singlet exciton lifetime
,
possibly
associated with
high
non
-
radiative
losses
. A further strategy to improve EQE in this class of polymers without
sacrificing the good extraction
properties in optimized blends is therefore to limit
those
non
-
radiative decay processes.