Abstract
Femtosecond transient absorption spectroscopy is performed to assess bridge effects on energy transfer and charge separation in molecular junctions. A short, conjugated bridge can facilitate charge separation from both donor and acceptor, whereas in longer bridges charge separation only occurs from the excited donor.
Highlights
Controlling charge separation on an intramolecular scale in molecular p-n junctions will be important for increasing the efficiency of solar cell devices [1,2]
To assess the effect of conjugation, length, and rigidity of the bridge between donor and acceptor on charge separation and recombination, we conducted transient absorption (TA) measurements on molecular PN junctions consisting of a hexathiophene (6T) electron donor and a naphthalene bisimide (NDI)perylene bisimide (PDI) dimer electron acceptor
Following excitation of the hexathiophene (6T) at 400 nm, all three molecules exhibited an excited state absorption (ESA) in the range between 720 and 960 nm, which we ascribe to the ESA of 6T [8]. This feature decayed at a comparable rate to the appearance of ground state bleach (GSB)/stimulated emission (SE) signals at 540 nm (PDI GSB and SE) and 580 nm, indicating energy transfer from 6T to PDI (Fig. 1c)
Summary
Controlling charge separation on an intramolecular scale in molecular p-n junctions will be important for increasing the efficiency of solar cell devices [1,2]. The effect of the connection between electron donor and electron acceptor materials on the efficiency and yield of charge separation and charge recombination is still a topic of ongoing investigation [3]. To assess the effect of conjugation, length, and rigidity of the bridge between donor and acceptor on charge separation and recombination, we conducted transient absorption (TA) measurements on molecular PN junctions consisting of a hexathiophene (6T) electron donor and a naphthalene bisimide (NDI)perylene bisimide (PDI) dimer electron acceptor
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