Solar Energy Technologies Program Peer Review

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Solar Energy Technologies Program Peer Review

• Michael D Diener

• Improved Fullerenes for OPV

• TDA Research
• 303 940 2314
• May 26, 2010

• PV

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Overview
Timeline
Barriers

• Project start date 8/8/2007
• Project end date 8/7/2009
• Percent complete 100

• OPV are not sufficiently efficient this project
  will increase the efficiency of organic
  photovoltaics.

Budget
Partners

• Total project funding
• DOE share 850,000
• Contractor share 185,227
• DOE Funding received in FY09 379,490
• DOE Funding for FY10 0

• TDA Research, lead
• NREL, sub

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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
  power-generation applications.

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Relevance

• 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 fullerenes 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

• 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
• Electrochemistry
• UV-vis absorbance
• Solubility
• Stability
• Test their performance in OPV
• Inverted bulk heterojunction (BHJ) cells with
  poly(3-hexylthiophene (P3HT)
• ITO/MgxZn1-xO/P3HTfullerene/(PEDOTPSS or
  oxide/)Ag device geometry

Iterate
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Approach

• 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/6-311G(d,p) calculation
• The LUMO is on the fullerene

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Collaborations

• 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).
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/PEDOTPSS/(fullerene)P3HT/Ba/Al
ITO/PEDOTPSS/PCPZEAP3HT/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/P3HTPCPZEA/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/6-311G(d,p)
• Rather little difference in electron affinity
  between
• PCBM and PCSME (either conformer) or TCSMe
• C60C(CH2N(CH2)2)2 still looks good

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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, its 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
3,6-diamine substituted cyclohexyl (A Diels-Alder
adduct?) Electron Affinity 2.354 eV
Imidazoline Adduct Electron Affinity 2.436 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

• 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
• 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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