Influence of Acceptor Structure on Barriers to Charge

Influence of Acceptor Structure
on Barriers to Charge Separation
in Organic Photovoltaic Materials
Ryan D. Pensack†, Changhe Guo‡, Kiarash Vakhshouri‡,
Enrique D. Gomez‡, and John B. Asbury*†
†Department of Chemistry and ‡Department of Chemical
Engineering and the Materials Research Institute,
The Pennsylvania State University
• Charge Transfer States
– Bound electron/hole pair at the interface of the donor and
– CT state can be formed through electron transfer from donor to
acceptor or hole transfer from acceptor to donor
– The binding energy of this CT state is 0.0 -0.5 eV
– Additional studies have shown that excess energy is not require
to dissociate this CT state because of a minimal temperature
• Current work
– Identify the factors that may influence charge separation in OPV
materials using ultrafast vibrational spectroscopy
– P3HT as the Donor
– PC61BM and BTBP-PDI as acceptors
– Measure the rates of CT state dissociation by using an approach
based on solvatochromism assisted vibrational spectroscopy
– Films were prepared from
chlorobenzene with 300-500
nm thickness
– Vis-IR experiment designed to
only excite the donor
– Probe with IR
– 1 ps resolution
– 50 mW/cm2 power density
• X-Ray diffraction
– Examine the crystalline nature
of the films
Results: PDI
• P3HT:BTBP-PDI ultrafast C=O
dynamics 300K
– Broad feature contains positive
polarons in P3HT, negative polarons in
PDI and ground state bleach
– Fit each of the three components and
extract a center frequency for the
bleach which is traced by the dotted
– Shift to lower energy is indicative of CT
state dissociation
• Temperature Dependence on
center frequency of the bleach
– Cooler temperatures slow charge
transfer dynamics
Results: PCBM
• Broad feature contains polarons
and bleach 325 K
– Extract bleach from 3 component
– Bleach shift is must less sensitive
than in PDI
– Slower CT state dissociation (ns)
• Temperature Dependence
– CT state dissociation is not
sensitive to temperature
– Nanosecond timescale at all
Asymptotic Behavior of Shifts
• G(t) represents experimental frequency
shifts of the kinetic trace at a particular
• G(∞) represents the asymptotic shifts at
long times.
– PDI: 1709 to 1705 cm-1
– PCBM: 1739 to 1740 cm-1
– Consistent with previous experiments with
different donors
– Likely the result of a thermal contraction of the
– Origin of the greater sensitivity in PDI is
currently under investigation
Film Morphology
• Crystalline P3HT in all
• Amorphous PCBM and
• Crystalline C8:PDI
Reaction Barriers
• Plot reaction rate vs. 1/T
• PDI shows significant
dependence but with nonarrhenius behavior
– Extract a binding energy of
• PCBM shows no
dependence indicating a
barrierless pathway
BTBP:PDI Blend Barrier
• Conclude the charge separation is activated
• Time scale for CT state dissociation ranges from 1ps to
10ps depending on the temperature
• Excess energy model: Ground state, photosynthetic
reaction centers
– At lower temperature the dynamics are sufficiently slowed such
that vibrational energy redistribution dissipates the energy before
the CT state dissociates. Thus thermal energy is required.
• Competing Model
– Authors believe the system is activated at all temperatures
– Non-Arrhenius behavior arises from the dynamics being to fast
to be clearly resolved.
– The vibrational energy redistribution is much quicker in the
excited state vs the ground state
– Needs a faster experiment to make a confident assignment
PCBM Barrierless Separation
• Nanosecond timescale indicates excess energy
does not play a role since it would be dissipated
on that timescale
• Recent OPV device studies on the same blend
do not show temperature dependence of the VOC
• Literature Insights
– Electronic wave function of the PCBM is delocalized
and the reorganization energy is less because of this
– Molecular species show a larger coulombic attraction
because of smaller spatial confinement of the charges
– In PCBM, the wavefunctions are so delocalized that
Coulombic barriers and reorganization energies are
reduced below the energetic disorder of the system
Structure Influence on the Barrier
– 1 nm3 volume and 60 carbon atoms to distribute the charge
– Diffuse electron even if the charge is only on one fullerene
– Non-directional because of the spherical symmetry of PCBM
– 20 atoms and 1/3 the volume of PCBM
– Requires multiple PDI molecules to distribute the charge of the same
spatial volume as compared with PCBM
– The faster (1ps) charge separation PDI molecules; however, the lack
of a crystalline phase prevents delocalization
– The activation barrier combined with the lack of evidence of a
crystalline phase suggests that electron localization causes a greater
coulombic binding energy of the CT state
– Exploring C8:PDI blend because of its crystallinity
• Examined the dynamics of OPV materials by monitoring
the C=O vibration of photoreduced electron acceptors
using TRIR
• The PCBM blend shows a barrierless dissociation of the
CT state
• The BTBP:PDI requires energy to activate the CT state
dissociation as indicated by the temperature
• X-Ray measurements show the PCBM does not require
an ordered phase to achieve sufficient charge
• The PDI acceptors may require a greater degree of
crystallinity to achieve sufficient delocalization
• Proposed that C8:PDI blend would show less barriers to
charge separation
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