  In organic semiconductor based bulk heterojunction solar cells, the presence
of acceptor increases the formation of charge transfer (CT) excitons, thereby
leading to higher exciton dissociation probabilities. In this work we used
steady state EA measurements to probe the change in the nature of excitons as
the blend composition of the solar cell active layer material is varied. We
investigated blends of
poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c]-[1,2,5]thiadiazole)]
(PPDT2FBT) and (6,6)-Phenyl C71 butyric acid methyl ester (PCBM). Analysis of
the EA spectra showed that in presence of fullerene based acceptor, like PCBM,
CT characteristics of the excitons were modified, though, no new CT signature
was observed in the blend. Enhancement in the CT characteristic in the blend
was reflected in the photoluminescence (PL) measurements of the blends, where,
PL quenching of $\sim$ 63\% was observed for 1\% PCBM. The quenching reaches
saturation at about 20\% PCBM. However, the maximum efficiency of the devices
was obtained for the blend having 50\% PCBM. Comparing experimental results
with simulations, the variation of the device efficiency with PCBM percentage
was shown to be arising from multiple factors like increase in polarizability
and dipole moment of excitons, and the efficiency of the carrier collection
from the bulk of the active layer.
