Effects of Exciton Polarity in Charge-Transfer Polymer/PCBM Bulk Heterojunction Films.
Charge-transfer copolymers with local electron density gradients, systematically modified by quantity and position of fluorination, result in widely variable (2-8%) power conversion efficiencies (PCEs). Ultrafast, near-infrared, transient absorption spectroscopy on the corresponding films reveals the influence of exciton polarity on ultrafast populations and decay dynamics for the charge-separated and charge-transfer states as well as their strong correlation to device PCEs. By using an excitation energy-dependent, dynamic red shift in the transient absorption signal for the polymer cation, the exciton polarity induced by push-pull interactions within each polymer fragment is shown to enhance charge dissociation on time scales of tens to hundreds of picoseconds after excitation. These results suggest the important role played by the local electronic structure not only for exciton dissociation but also for device performance.
§Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States.
Brian S
BS
Rolczynski
§Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States.
Jodi M
JM
Szarko
∥Department of Chemistry and The James Franck Institute, The University of Chicago, 929 East 57th Street, Chicago, Illinois 60637, United States.
Hae Jung
HJ
Son
∥Department of Chemistry and The James Franck Institute, The University of Chicago, 929 East 57th Street, Chicago, Illinois 60637, United States.
Luping
L
Yu
§Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States.
Lin X
LX
Chen
carrier diffusion; charge-transfer polymer; exciton dissociation dynamics; organic photovoltaics; polarized exciton; ultrafast transient absorption