  Donor-acceptor (D-A) type copolymers show great potential for the application
in the active layer of organic solar cells. Nevertheless the nature of the
excited states, the coupling mechanism and the relaxation pathways following
photoexcitation are yet to be clarified. We carried out comparative
measurements of the steady state absorption and photoluminescence (PL) on the
copolymer poly[N-(1-octylnonyl)-2,7-carbazole] -alt-5,5-[4',7'
-di(thien-2-yl)-2',1',3' -benzothiadiazole] (PCDTBT), its building blocks as
well as on the newly synthesized
N-(1-octylnonyl)-2,7-bis-[(5-phenyl)thien-2-yl)carbazole (BPT-carbazole) (see
Figure 1). The high-energy absorption band (HEB) of PCDTBT was identified with
absorption of carbazoles with adjacent thiophene rings while the low-energy
band (LEB) originates instead from the charge transfer (CT) state delocalized
over the aforementioned unit with adjacent benzothiadiazole group.
Photoexcitation of the HEB is followed by internal relaxation prior the
radiative decay to the ground state. Adding PC70BM results in the efficient PL
quenching within the first 50 ps after excitation. From the PL excitation
experiments no evidence for a direct electron transfer from the HEB of PCDTBT
towards the fullerene acceptor was found, therefore the internal relaxation
mechanisms within PCDTBT can be assumed to precede. Our findings indicate that
effective coupling between copolymer building blocks governs the photovoltaic
performance of the blends.
