Advanced Electron Transport Materials for Application in Organic Photovoltaics (OPV)

Transcription

1 Eight19 Limited Advanced Electron Transport Materials for Application in rganic Photovoltaics (PV) Alan Sellinger GCEP Research Symposium 2011 ctober 5,

2 Why Solar? 1 % of land with solar cells could meet our electricity needs 6 Boxes at 3.3 TW Each Amount of solar energy in hours, received each day on an optimally tilted surface during the worst month of the year.

3 Total PV installation: Germany 50% USA 9%! 3

4 Types of Solar Technologies Lowest cost $/W 4

5 Why rganic Semiconductors? Attractive properties: Abundant: ~100,000 tons/year Mature industry/markets Low materials cost: ~$1/g 15 /m 2 Low-cost processing: printing Excellent stability on-toxic CuPc Copper Phthalocyanine 5

6 rganic LEDs for Displays

7 rganic LEDs for Lighting

8 ew Applications for rganic Solar Cells

9 Chemistry obel Prize Day! rganic Electronics: Combination of 3 obel Prizes! 1996 obel Prize in Chemistry - Robert F. Curl Jr., Sir Harold Kroto, Richard E. Smalley for their discovery of fullerenes (C60) C60 C obel Prize in Chemistry - Alan Heeger, Alan G. MacDiarmid, Hideki Shirakawa for their discovery and development of conductive polymers S n PEDT 2010 obel Prize in Chemistry - Richard F. Heck, Ei-ichi egishi, Akira Suzuki for palladium-catalyzed cross couplings in organic synthesis C-C bond formation S S C 6 H 13 S C 6 H 13 S C 6 H 13 S n/4 C 6 H 13 9

10 Motivation for rganic-based PV Lower production cost Roll-to-roll coating Inexpensive active materials Inexpensive processing Lower capital cost Lower environmental impact - $/W Inhabitat.com Energy Payback Time (yrs) C 2 Emission in Production (g) Crystalline silicon Thin film (CIGS, CdTe, etc.) Polymer PV (Glass substrate) Polymer PV (Plastic substrate) Roes et al., Prog. Photovoltaics (17)

11 Efficiency Cost Analysis for Solar Cell Technologies Thin Film Crystalline Si rganic/hybrid near term/today today future $/W basis competitive Paint Material and Manufacturing Cost Ingot

12 Anode What is an rganic Photovoltaic? Simplified Working Principle Recombination Photon + - Exciton + - Hole Charge Separation + + Electron Cathode Photon + - Generation Diffusion Drift/Diffusion Donor Material nm Acceptor Material Kietzke, T., Adv. ptoelect., 2007, Article ID

13 Progress in rganic Solar Cells 10%? Mitsubishi 2011 Solution processes (Polymer, PCBM) Vacuum processed 9% Solarmer 2011? 8.3% Heliatek Similar development for organic solar cells as for amorphous silicon 20 years ago Both approaches are reaching nearly similar efficiencies Improvements are largely materials discovery based

14 Estimated lifetimes of PVs Burn-in loss Burn-in time Lifetime in linear regime* P3HT:PCBM 16% 55 days 3.5 years PCDTBT:PC70BM 27% 38 days 6.7 years *Lifetime assumes 5.5 hrs/day of one-sun intensity 1 Heliatek (BASF and Bosch) has achieved 30 year lifetimes with tandem PVs 2 1. C. Peters, M. McGehee, Adv. Energy Mater. in press,

15 Bulk heterojunction (BHJ) Polymer Solar Cell Advantages: Solution processing (roll-to-roll), increased contact area between active phases, Disadvantages: Current world record: 8-10% Extrapolated lifetimes >6 years Difficult to achieve optimum morphology, polymers generally less pure than molecules, solvent dependence

16 High-efficiency BHJ solar cells PBT7:PC 70 BM 7.4% PCE PCDTBT:PC 70 BM 6.1% PCE PC70BM PC 70 BM Liang et al., Adv. Mat (22) E135-E138 Park et al., at. Phot (3) P3HT:ICBA* 6.5% PCE * ICBA: Indene-C60 Bisadduct PC 60 BM Zhao et al., Adv. Mat (22) Record PV cells ALL use fullerene derivatives Recent development based on donor materials discovery 16

17 Absorption Coefficient (M -1 cm -1 ) Drawbacks for fullerenes 2.5 x ew Acceptor PC 60 BM PC 70 BM -3.7 ev -4.3 ev -3.1 ev P3HT -3.4 ev -3.9 ev -3.1 ev P3HT Wavelength (nm) -5.6 ev -6.1 ev PCBM -5.1 ev -5.9 ev -6.1 ev ew Acceptor -5.1 ev Reduced solar spectrum absorption Lower V C Difficult synthesis and purification = Higher cost PC 60 BM: $50/g CuPc: $1-5/g Fullerene acceptor up to 10% of total PV system cost Roes et al., Prog. Photovoltaics (17)

18 Effect of Production Steps on Cost PC60BM PC70BM Significant increases (2X) for higher purification PC 71 BM approximately 3.4X more expensive than PC 61 BM 18 Anctil et al., Environ. Sci. Technol. 45, (2011)

19 on-fullerene acceptors P3HT:99 BF ~2% PCE C PPT:C-PPV 2% PCE C Holcombe et al., J. Am. Chem. Soc (40) F F F F F B Cl Cl F F F F F F Verreet et al., Adv. Ener. Mat (1) n SubPc:FSubPc dimer 2.5% PCE (vacuum) F B F F F F C C Brunetti et al., Ang. Chem. Int. Ed (3) S Woo et al., Chem. Mater (5) C C PPT:EV-BT 1.4% PCE Lower efficiencies due to lower J SC and lower FF 19

20 ew Acceptors based on Vinazene riginal applications as high nitrogen containing materials for reduced flammability. Without vinyl groups has been used in the agriculture sector (fertilizers) Their highly electron deficient properties make them candidates as acceptor materials in organic electronic applications R R H H H C C C Vinazene 2-vinyl-4,5-dicyanoimidazole C C C 4,5-dicyanoimidazole Commercially available

21 Alkylation of 2-Vinyl-4,5-dicyanoimidazole Acetone, K 2 C 3 Reflux I C C 1-Butylvinazene (90%) H Acetone, K 2 C 3 I C Vinazene C Reflux C C 1-Hexylvinazene (90%) Br DMF, K 2 C 3, 70 o C C C 1-Ethylhexylvinazene (56%)

22 Tunable Synthesis: Heck Chemistry Shin R.Y.C., Sonar, P., Siew, P.S., Chen, Z.C., Sellinger, A., J. rg. Chem., 2009, 74 (9),

23 ormalised Intensity (a.u.) ormalised Intensity (a.u.) ptical properties of selected vinazene derivatives S S S (1) (4) S (5) (10) (13) Wavelength (nm) UV spectra of the molecules in toluene Wavelength (nm) PL spectra of the molecules in toluene Significant absorption in visible spectrum

24 Vinazenes used for PVs Solubilizing groups for solution processing Conjugated chemical links S HV-BT C C C C Electron accepting sites EV-BT Better solubility Can be thermally sublimed as well

25 Preparation of PV Devices PPT donor polymer ew Electron Acceptor Solution processed active layer 25

26 Kietzke, T., Shin R.Y.C., Egbe, D.A.M., Chen, Z.K., and Sellinger, A., Macromolecules, 2007, 40, Shin R.Y.C., Kietzke, T., Sudhakar, S., Dodabalapur A., Chen, Z.K., and Sellinger, A., Chem. Mater., 2007, 19(8), Woo, C.W., Holcombe, T.W., Tam, T.L., Sellinger, A., Frechet, J.M.J., Chem. Mater. 2010, 22 (5), PPT with HV-BT PPT Voc = 0.62 V, FF = 0.40, PCE = 1.41%

27 Can we make better acceptors? Solubilizing groups for solution processing Conjugated chemical links S C C C C Electron accepting sites Corresponding electron donor has charge mobility (cm 2 /V sec) in 10-4 range. Is charge transport mis-match a problem?

28 Improving rganic Acceptor Materials

29 Improved Acceptor Materials? S

30 Acceptor Materials: Computational Studies Ground-state geometries LUM HM

31 Synthetic Scheme for PI-BT/I-BT PI-BT PI-BT I-BT I-BT 31

32 ormalized Absorption (arb. units.) ew Acceptor Properties -3.1 ev -3.1 ev Thin Film PI-BT I-BT ev P3HT ev P3HT 0.6 PI-BT -5.1 ev I-BT -5.1 ev ev ev Wavelength (nm) E LUM ~ 0.6 ev E HM ~ 0.9 ev E LUM ~ 0.7 ev E HM ~ 0.9 ev Peak acceptor absorption is in visible spectrum Larger E LUM,Acc E HM,Don than for P3HT:PC 60 BM Tunable LUM expected higher V oc than fullerenes 32

33 Current Density (ma cm -2 ) J-V Curve ptimal Devices PI-BT I-BT Voltage (V) P3HT:PI-BT P3HT:I-BT Solvent Chlorobenzene Chloroform Thickness (nm) ~ 90 ~ 120 Cathode LiF(1nm)/Al Ca(7nm)/Al Anneal Pre/Post Post Pre Ann. Temp/Time 110 C/3min 110 C/10min Jsc (ma/cm 2 ) Voc (V) FF PCE (%) Believed highest efficiency BHJ with non-fullerene acceptor and commercially available P3HT donor High voltage as expected with higher-lying LUM Why is the efficiency for I-BT 10X lower? 33

34 EQE (%) or Absorption (arb. units) EQE Spectra of P3HT:PI-BT Device P3HT:PI-BT EQE P3HT Absorption PI-BT Absorption Wavelength (nm) Significant photocurrent generation from acceptor absorption 34

35 Molecular Simulation Ground State Geometry PI-BT I-BT Twisting of I-BT molecule due to steric interactions may prevent crystallization in films 35

36 GIXS Acceptor Crystallization P3HT:PI-BT (CB) P3HT:I-BT (CF) P3HT nly - CB P3HT nly - CF More acceptor peaks in PI-BT sample than I-BT 36

37 PV Conclusions Very promising new acceptor materials as potential replacement for fullerene derivatives Synthesized in minimal step/moderate yield reactions Tunable energy levels Anticipated to be much less expensive than fullerenes Initial device performance very promising From initial PCE of 0.1% >2.54% PCE max, 5 years of R&D, (5-7 total researchers) Close interaction between chemists and device engineers/physicists

38 Acknowledgements Co-PI - Prof. Michael McGehee Synthesis Dr. Xu Han (now at DuPont R&D Shanghai), Dr. Andrew Higgs Device Fabrication/Characterization Jason Bloking, Jack Kastrop Quantum Calculations Dr. Chad Risko, Dr. Joe orton, Laxman Pandey, Dr. Jean-Luc Bredas (Georgia Tech) Funding Sources: 38