1 | Program Name or Ancillary Texteere.energy.gov Solar Energy Technologies Program Peer Review...

1 | Program Name or Ancillary Text eere.energy.gov

Solar Energy Technologies Program Peer Review

Improved Fullerenes for OPV Michael D Diener

TDA Research303 940 2314May 26, 2010

PV

2 | Solar Energy Technologies Program eere.energy.gov

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• 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.

• Total project funding– DOE share: $850,000

– Contractor share: $185,227

• DOE Funding received in FY09: $379,490

• DOE Funding for FY10: $0

Timeline

Budget

Barriers

• TDA Research, lead• NREL, sub

Partners

Overview


Challenges, Barriers or Problems

• 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.

OPV Champion Device Efficiency by Year


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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

Relevance


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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• 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

ApproachIt

erat

e


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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:

Approach


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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

Collaborations


Accomplishments / Progress / Results

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).

Initial Synthetic Strategy

New products comparedto PCBM – similar solubility,similar morphology expected



ITO/PEDOT:PSS/PCPZEA:P3HT/LiF/Al

The new fullerenes do not work in normal devices with low work function metals

ITO/PEDOT:PSS/(fullerene):P3HT/Ba/Al



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



Synthesis of PCSME

Thermally unstable



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



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• C60C(CH2N(CH2)2)2 still looks good EA (eV)

C60 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



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



Other Synthetic Targets with Amines

Imidazoline AdductElectron Affinity = 2.436 eV

3,6-diamine substituted cyclohexyl(A Diels-Alder adduct?)

Electron Affinity = 2.354 eV



Silyl Adducts

(CH3)2SiC60

Electron Affinity = 2.393 eV ((CH3)2Si)2CC60

Electron Affinity = 2.501 eV(Not useful)

C60C(CH3)2

Electron Affinity = 2.497

Recent work from Japan shows SIMEF has ~0.1 eV lowerelectron affinity than PCBM (JACS 131, 16048), and OPVwith phthalocyanine has PCE = 5.2%



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)


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%


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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• 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

Summary

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