Solar Energy Technologies Program Peer Review
Improved Fullerenes for OPV
PV
1 | Program Name or Ancillary Text
Michael D Diener
TDA Research
303 940 2314
May 26, 2010
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Overview
Timeline
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Project start date: 8/8/2007
Project end date: 8/7/2009
Percent complete: 100%
Budget
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Total project funding
– DOE share: $850,000
– Contractor share: $185,227
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Barriers
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OPV are not sufficiently
efficient; this project will
increase the efficiency of
organic photovoltaics.
Partners
TDA Research, lead
NREL, sub
DOE Funding received in
FY09: $379,490
DOE Funding for FY10: $0
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Challenges, Barriers or Problems
OPV Champion Device Efficiency by Year
• Though rapidly improving, the
efficiency of organic
photovoltaic (OPV) cells
remains low
• Due to their extremely versatile
and low-cost fabrication, a few
percent additional increase in
OPV efficiency will lead to their
wide-spread adoption in a
tremendous variety of powergeneration applications.
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Relevance
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Objective: To increase the efficiency of OPV by increasing the open circuit
voltage (Voc) through the synthesis of new electron-rich fullerenes, used as
acceptors in a variety of OPV architectures
– Voc in OPV  (ionization potential of the polymer) – (electron affinity of the
fullerene). Reduce the fullerene’s electron affinity, increase Voc.
– Current Voc is ~0.5 V
– 2020 target efficiency is 12% (2008-2012 MYPP)
• 2010 champion device efficiency is 7.4%
• Need new materials that maintain low-cost manufacturing
– 2009 objectives were the de novo synthesis and characterization of
electron-rich fullerene derivatives, followed by testing of the new fullerenes
in OPV devices
– New materials for OPV are created by synthetic organic chemistry.
• Good: incredibly large choice of materials; fine tailoring of properties
• Bad: de novo synthesis can be slow & costly
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Approach
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Summarized Project Tasks
– Optimize OPV performance from the materials developed in Phase I
– Perform quantum chemical modeling of new synthetic targets
– Synthesize the new electron-rich fullerene derivatives using the
methodology developed in Phase I
– Characterize the new fullerenes
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Electrochemistry
UV-vis absorbance
Solubility
Stability
– Test their performance in OPV
• Inverted bulk heterojunction (BHJ) cells with poly(3-hexylthiophene (P3HT)
• ITO/MgxZn1-xO/P3HT:fullerene/(PEDOT:PSS or oxide/)Ag device geometry
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Approach
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Electron-rich elements tend to react directly with electron-poor fullerenes,
without altering the electron affinity from that of PCBM
Must ensure that the extra electron density is present in the lowest
unoccupied molecular orbital (LUMO) of the resulting derivative
Quantum chemistry calculations allow for downselection of targets
– Example: C60C(CH2N(CH2)2)2
• Electron Affinity = 2.399 eV (vs. PCBM = 2.522 eV) MO52X/6-311++G(d,p) calculation
• The LUMO is on the fullerene:
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Collaborations
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NREL
– $100,000 subcontract using a CRADA
– Preparation and testing of the promising new fullerene derivatives in BJ
OPV using their state-of-the-art facilities
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Accomplishments / Progress /
Results
Initial Synthetic Strategy
Three step synthesis of PCPZEA. The resulting isomer mixture is converted to
pure (6,6)PCPZEA by stirring the purified isomer mixture under a sodium lamp
for four hours. 1: NEt3, CH2Cl2, 0 C, 1h; H2O, MgSO4, LC (silica); 32% yield. 2:
CH3OH, 6h reflux; -CH3OH, +CH2Cl2; H2O, MgSO4, LC (silica); 20% yield. 3:
NaOCH3, pyridine, oDCB, 70 C, 16h, dark; -pyridine, -oDCB, LC (silica) 2x; 50%
yield (consumed C60 basis).
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New products compared
to PCBM – similar solubility,
similar morphology expected
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Accomplishments / Progress /
Results
The new fullerenes do not work in normal devices with low work function metals
ITO/PEDOT:PSS/(fullerene):P3HT/Ba/Al
ITO/PEDOT:PSS/PCPZEA:P3HT/LiF/Al
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Accomplishments / Progress /
Results
PCPZEA does work in inverted devices
Tuning the work function of the TCO electrode greatly enhances efficiency
ITO/Zn1-xMgxO/P3HT:PCPZEA/Ag devices cast from
ODCB
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Accomplishments / Progress /
Results
Synthesis of PCSME
Thermally unstable
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Accomplishments / Progress /
Results
Quantum Chemical Calculations for PCSME: Structure Determination
• Molecular mechanics conformational analysis:
vary the dihedral angles, minimize the energy
• 10,000 optimizations:
each of the 200 lowest energy structures was found about 50 times
• The six lowest energy conformers (of 200) all had the N pointing away from C60
N in blue, O in red
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Accomplishments / Progress /
Results
Quantum Chemical Calculations: Electron Affinity
• Four lowest energy conformers + #7 (amine down) geometry optimized
with M052X/6-31G
• #7 is 5 kcal/mol higher in energy than #1
• Thermal energy at ambient temperature = 0.59 kcal/mol
• Not much #7 likely to be present
• Unless the crystal lattice energy imposes a higher energy
conformation…
• Single point energy calculated at M052X/6-311++G(d,p)
• Rather little difference in electron affinity between
PCBM and PCSME (either conformer) or TCSMe
C60
• C60C(CH2N(CH2)2)2 still looks good
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EA (eV)
2.607
PCBM
2.522
C60CH2
2.523
PCSME conformer 1
2.486
PCSME conformer 7
2.488
TCSME conformer 1
2.522
C60C(CH2N(CH3)2)2
2.399
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Accomplishments / Progress /
Results
Where Are the Electrons Going?
Idea #1: The ester is stealing them
• Replace the ester with an alkyl chain
• Conformational analysis & structure optimization
• Electron Affinity is now 2.521 eV
• Same as PCBM
• Not the ester
Idea #2: The phenyl ring is stealing them
• Replace the phenyl ring with a t-butyl group
• Conformational analysis & structure optimization
• Electron affinity is now 2.459 eV
• Halfway between PCBM and C60C(CH2N(CH2)2)2
• Yes, it’s the phenyl ring, combined with having an
amine on both sides of the vertex carbon
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Accomplishments / Progress /
Results
Other Synthetic Targets with Amines
Imidazoline Adduct
Electron Affinity = 2.436 eV
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3,6-diamine substituted cyclohexyl
(A Diels-Alder adduct?)
Electron Affinity = 2.354 eV
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Accomplishments / Progress /
Results
Silyl Adducts
C60C(CH3)2
Electron Affinity = 2.497
(CH3)2SiC60
Electron Affinity = 2.393 eV
((CH3)2Si)2CC60
Electron Affinity = 2.501 eV
(Not useful)
Recent work from Japan shows SIMEF has ~0.1 eV lower
electron affinity than PCBM (JACS 131, 16048), and OPV
with phthalocyanine has PCE = 5.2%
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Accomplishments / Progress /
Results
Other syntheses, other calculations, other devices not yet IP-protected
Stability of electron-rich fullerene derivatives is clearly an issue:
rearrangements and oxidations are frequent (and frustrating)
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Budget Status and Potential for
Expansion
• DOE $750,000 Phase II + $100,000 Phase I
• TDA $185,227: Equipment $60,283 + Labor
– Project and budget are complete
– Additional funding would allow us to pursue new derivatives
• Enhance the stability of the new derivatives through the introduction
of bulky substituents and/or other chemical motifs
• Increased purity of the new derivatives
– Only ~98% achieved routinely, impairing performance
– Commercial electronic grade PCBM is 99.5%
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Future Plans (FY 2011 and beyond)
• Pursue patent protection on the composition of matter of
the new fullerenes, as well as the synthetic methodology
• Market the materials to OPV manufacturers
• Attempt to further enhance purity of the stable new
fullerene derivatives
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Summary
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OPV is swiftly advancing – efficiency has doubled in ~6 years and there is no
sign of advancements slowing down
– Expect to meet the 12% goal by ~2015 at this pace, ahead of the MYPP target of 2020
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While cell construction can enhance efficiency, the big steps are taken with new
materials
Excellent progress has been made with low bandgap polymers to enhance
currents, but little published work has appeared with new fullerenes to enhance
Voc
QC calculations prove that significant enhancements in performance are
possible, but new derivatives must also have proper solubility and stability
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