We focus upon the role of interfacial energetics and morphology in influencing the separation of CT states into dissociated charge carriers. In particular, we undertake transient optical studies of films comprising regioregular poly(3-hexylthiophene) (P3HT) blended with a series of perylene-3,4:9,10-tetracarboxydiimide (PDI) fullerene electron acceptors. For the PDI film series, we observe a close correlation between the PDI electron affinity and the efficiency of charge separation. This correlation is discussed in the context of studies of charge photogeneration for other organic donor/acceptor blend films, including other polymers, blend compositions, and the widely used electron phenyl-C61-butyric acid methyl ester(PCBM). Furthermore, we compare the charge recombination dynamics observed in films comprising P3HT blended with three fullerene derivatives: PCBM and two alternative pyrazolinofullerenes. Transient absorption data indicate that replacement of PCBM with either of the pyrazolinofullerene derivatives results in a transition from nongeminate to monomolecular (geminate) recombination dynamics. We show that this transition cannot be explained by a difference in interfacial energetics. However, this transition does correlate with nanomorphology data that indicate that both pyrazolinofullerenes yield a much finer phase segregation with correspondingly smaller domain sizes than observed with PCBM. Our results therefore provide clear evidence of the role of nanomorphology in determining the nature of recombination dynamics in such donor/acceptor blends.
set of novel regioregular poly(3-hexylthiophene)-based random copolymers containing varying ratios of
ester functionalized alkyl side chains were synthesized using the Rieke method. The percentage of functionalized side
chain varied between 10 and 50 mol% for each copolymer. Using post-polymerization reactions, the ester functions in
the alkyl side chain were hydrolyzed to yield an alcohol or acid group. These groups are available for further
functionalization reactions, so a wide variety of secondary functionalities may be covalently attached to the conjugated
polymer. The copolymers were applied in polymer: fullerene bulk heterojunction solar cells (BHJSCs) with [6,6]-phenyl
-C61-butyric acid methyl ester (PCBM) as electron acceptor. The influence of side-chain functionalities on absorption,
device performance and layer morphology depends on the ratio and nature of the functionalized side chains. For a 9/1
copolymer, containing 10% of functionalized side chains, behaviour and efficiency in BHJSCs comparable to
P3HT:PCBM solar cells were observed.