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
Background
• Charge Transfer States
– Bound electron/hole pair at the interface of the donor and
acceptor
– 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
dependence.
• 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
(SAVS)
Experimental
• SAVS (TRIR)
– 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
line
– 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
fit
– 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
temperatures
Asymptotic Behavior of Shifts
• G(t) represents experimental frequency
shifts of the kinetic trace at a particular
temperature
• 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
donor
– Origin of the greater sensitivity in PDI is
currently under investigation
Film Morphology
• Crystalline P3HT in all
films
• Amorphous PCBM and
BTBP:PDI
• Crystalline C8:PDI
Reaction Barriers
• Plot reaction rate vs. 1/T
• PDI shows significant
dependence but with nonarrhenius behavior
– Extract a binding energy of
0.1eV
• 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
delocalization
– 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
• PCBM
– 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
• BTBP:PDI
– 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
Conclusion
• 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
dependence
• X-Ray measurements show the PCBM does not require
an ordered phase to achieve sufficient charge
delocalization
• 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