Organic Photovoltaics 02/28/2014 Philip Schulz Department of Electrical Engineering Princeton University, NJ-08544 Talking Points Introduction Organic Semiconductors | Plastic Electronics Organic Solar Cells Challenges Interface Energetics | Charge Selective Interlayers The Next Big Thing Metal-Organic Halide Perovskites 2 Organic Photovoltaics | Philip Schulz Talking Points Introduction Organic Semicondutors Plastic Electronics Organic Solar Cells 3 Organic Photovoltaics | Philip Schulz Talking Points from organic semiconductors… Organic = (Hydrocarbon) molecular-models.com Bio X (Life) materialviews.com …to plastic electronics Philips pubs.acs.org Sony Samsung polyIC Organic Photovoltaics | Philip Schulz LG Grossiord 2013 Solarmer 4 Organic Semiconductors Brief History – towards OPV: 1906 – Photoconductivity in organic solid 1953 – Dark conductivity in organic single crystals 1977 – Conductivity in polymers 1986 – First heterojunction OPV 1987 – First organic light emitting diode (OLED) 1993 – First OPV from solution processing 2001 – First certified organic solar cell with 2.5% PCE nobelprize.org wikipedia.com:OLED Unique properties of organic materials: Soft. Flexible. Light-weight. Inexpensive. Earth abundant. Organic Photovoltaics | Philip Schulz Pochettino 1906, Mette 1953, Chiang 1977, Tang 1986, Tang 1987, Sariciftci 1993, Shaheen 2001 5 Organic Semiconductors Molecular properties sigmaaldrich.com INTRAmolecular structure tailors optoelectronic properties INTERmolecular bonds convey „soft matter“ properties Organic Photovoltaics | Philip Schulz Gavezotti 1988 6 Organic Semiconductors Frontier molecular orbitals Energy E / eV EVAC F ELUMO EF EHOMO LUMO: lowest unoccupied molecular orbital (corresponds to conduction band) HOMO: highest occupied molecular orbital (corresponds to valence band) F: work function surface (defines difference between Fermi level EF and Vacuum level Evac at surface) 7 Organic Photovoltaics | Philip Schulz Plastic Electronics Organic Electronics Organic Field Effect Transistors .OFET, OTFT .RF-ID tag, sensor e-paper Hybrid Solar Cells .solid state DyeSensitized Solar Cells (ss-DSSC) .perovskite solar cells Organic Light Emitting Diodes .OLED, AMOLED .flexible display, large area lighting Organic Solar Cells .OSC, OPV Polymer Solar Cells Small Molecule Solar Cells .PSC .long chained molecules .good solubility .large area processing .„classic“ OSC .well defined molecule .vacuum evaporated .high purity and control 8 Organic Photovoltaics | Philip Schulz Organic Solar Cells Organic Photovoltaics | Philip Schulz NREL chart 2013 9 Organic Solar Cells Working Principle „excitonic“ cell i) Light absorption – exciton generation ii) Exciton diffusion to D/A interface iii) Exciton dissociation – charge carrier transport iv) Charge carrier extraction at electrode interface 10 Organic Photovoltaics | Philip Schulz Organic Solar Cells The Bulk Heterojunction (BHJ) e- h+ h+ e- .One layer containing donor and acceptor .Maximize D/A interface area .Blend materials forming continous domains .Co-evaporation, precursor mixing .Control over morphology and carrier extraction required Organic Photovoltaics | Philip Schulz Li 2012 11 Talking Points Challenges Structure and morphology on the nanoscale Encapsulation and integration Interface energetics 12 Organic Photovoltaics | Philip Schulz Interface Energetics Energy Diagram Energy E / eV EVAC high work function - - ELUMOA low work function D/A interface: .Optimize HOMO-LUMO gap EHOMOD + + TCO donor metal anode acceptor cathode high F low F Organic/electrode interface: .Detrimental electron and hole reverse current .Charge selectivity required z / nm 13 Organic Photovoltaics | Philip Schulz Interface Energetics HOMO-LUMO interface ≈ 2.0 Donor; Hole Transport Layer LUMOD Energy (eV) 3.0 LUMOA - EFn ELUMOA – EHOMOD correlates to Voc 4.0 5.0 ELUMOD – ELUMOA impacts Jsc EFp + HOMOD 6.0 HOMOA Acceptor Electron Transport Layer 14 Organic Photovoltaics | Philip Schulz Interface Energetics Electron and hole collection ≈ Donor; Hole Transport Layer 2.0 LUMOD LUMOA - Energy (eV) 3.0 EFn ELUMOD – ELUMOA impacts Jsc ELUMOA – EHOMOD correlates to Voc 4.0 5.0 EFp VB 6.0 CB + HOMOD HOMOA Acceptor Electron Transport Layer 15 Organic Photovoltaics | Philip Schulz Interface Energetics Charge selective contacts ≈ Donor; Hole Transport Layer 2.0 Electron selective interlayer CB Transparent contact CB LUMOA - EF Top contact EFn EFp EF VB 6.0 ELUMOA – EHOMOD correlates to Voc + HOMOD HOMOA Hole selective interlayer ELUMOD – ELUMOA impacts Jsc X Energy (eV) 4.0 5.0 X 3.0 LUMOD Acceptor Electron Transport Layer VB 16 Organic Photovoltaics | Philip Schulz Interface Energetics Charge selective contacts – Material choices 0 ≈ Evac C60PCBM ICBA ClInPc CuPc PEIE + ……. 2.1 -2 Ag, Al < 3.6 Energy (eV) -3 -4 4.1 4.4 TiO2 ZnO 4.2 5.3 -5 -6 4.2 5.5 NiO 6.6 -7 -8 6.8 ITO P3HT -9 PEIE PCDTBT MoO3 DONORS ACCEPTORS Organic Photovoltaics | Philip Schulz Ratcliff 2011 17 Interface Energetics Band offset measurements .Direct determination of energy diagram by electron spectroscopy .Ultraviolet photoemission spectroscopy (UPS) yields work function (WF), valence band information, position of the HOMO and ionization energy (IE) .Inverse photoemission spectroscopy (IPES) yields conduction band information, position of the LUMO and electron affinity (EA) 18 Organic Photovoltaics | Philip Schulz Talking Points The Next Big Thing Metal-Organic Halide Peroskite Solar Cells 19 Organic Photovoltaics | Philip Schulz Metal-Organic Halide Perovskites A new player in the field Organic Photovoltaics | Philip Schulz Hodes 2013 20 Metal-Organic Halide Perovskites Composition and structure .Stacked between organic hole transport layer and oxide electron transport layer .Most promising candidate: Methylammonium (MA) Lead (Pb) Halide (I3-xClx) Organic Photovoltaics | Philip Schulz Rhee 2013, Lee 2012 21 Metal-Organic Halide Perovskites Working principle & advantages .Good optical absorption .Ambipolar charge transport .Large charge carrier diffusion length .Low recombination rates .Small „loss-in-potential“ .High efficiencies (>16%) .All earth-abundant materials .Low temperature processing (max 150°C) .Low production costs .Long device lifetime …but still needs enhancement Organic Photovoltaics | Philip Schulz Snaith 2013 22 Metal-Organic Halide Perovskites Track interface energetics .next step: Identification of optimal electron & hole transporters Organic Photovoltaics | Philip Schulz Schulz 2014 23 Talking Points Summary .Organic photovoltaics… …exhibit different underlying physics („excitonic“ cell) …require careful interface engineering …have potential for inexpensive production and specialized applications Outlook .OPV efficiencies are constantly rising .Perovskite solar cells already on the verge to utility-scale application Thank you for your attention! Organic Photovoltaics | Philip Schulz 24 W References References Grossiord 2013 – N. Grossiord et al., Org. Electronics 13, 3 (2012) Pochettino 1906 – A. Pochettino, Acad. Lincei Redoconti 15, 1 (1906) Mette 1953 – H. Mette, H. Pick, Z. f. Physik 134, 5 (1953) Chiang 1977 – C. K. Chiang et al., Phys. Rev. Lett. 39, 17 (1977) Tang 1986 – C. W. Tang, Appl. Phys. Lett. 86, 183 (1986) Tang 1987 – C. W. Tang, S. A. VanSlyke, Appl. Phys. Lett. 51, 913 (1987) Sariciftci 1993 – Sariciftci et al., Science 258 (5087) Shaheen 2001 – S. Shaheen et al., Appl. Phys. Lett. 78, 841 (2001) Gavezotti 1988, A. Gavezotti, R. Desirajou, Acta Cryst., 427 – 434 (1988) Li 2012 – G. Li et al., Nature Photonics 6, 153 (2012) Ratcliff 2011 – E. Ratcliff et al., J. Phys. Chem. Lett. Perspective 2, 1337 (2011) Schulz 2013 – P. Schulz et al., Adv. Funct. Mater. 24, 5 (2014) Schulz & Kelly 2014 – P. Schulz, L. Kelly et al., in preparation (2014) Hodes 2013 – G. Hodes, Science 342, 317 (2013) Rhee 2013 – L. Rhee et al., NPG Asia Materials 5, (2013) Lee 2012 – M. Lee et al., Science 338, 643 (2012) Snaith 2013 – H. Snaith, J. Phys. Chem. Lett. 4, 3623−3630 (2013) Schulz 2014 – P. Schulz et al., Energy Env. Sci. (2014) 25 Organic Photovoltaics | Philip Schulz Organic Semiconductors Charge carrier transport Band transport in highly crystalline and pristine compounds Hopping transport in disordered systems (OLED, OPV) Organic Photovoltaics | Philip Schulz 26 http://www.engineersgarage.com/articles/oled-working-application-future?page=4 Organic Solar Cells Inorganic Organic .direct generation of free charge carriers .„excitonic“ solar cells with complex charge carrier dynamics .high efficiencies .approaching competitive efficiencies .long life time .sensitive to ambient conditions .heavy weight, non-flexible, non-transparent .light-weight, flexible, semi-transparent .high energy consumption in fabrication, small wafer size .facile and fast large area printing techniques .rare materials .potentially inexpensive materials 27 Organic Photovoltaics | Philip Schulz Interface Energetics HOMO-LUMO interface – Material choices Organic Photovoltaics | Philip Schulz refs 28 Photoemission vs. Inverse Interface Energetics Photoemission Band offset measurementsInverse – spectrometer Photoemission setup e- e- DOS Channeltron EVAC h h KCl film 800A (IE 8.6 eV) Shield IPES SrF2 window (cut off 9.7 eV) EF Energy Gap Discharging mesh photons electron gun esample PES 5 - 20 eV Transmission Electron emission SrF2 KCl SrF 2 = low pass filter with cutoff at 9.7 eV KCl = high pass filter with cutoff at 8.6 eV 7 8 9 10 Energy (eV) 11 29 Organic Photovoltaics | Philip Schulz Charge Selective Interlayers Hole collection – sNiOx/MoO3 bi-layers .Hole extraction due to MoO3 .Electron blocking due to sNiOx .Increase in PCE and FF .Reduced dark currents Organic Photovoltaics | Philip Schulz Schulz 2013 30 Charge Selective Interlayers Electron collection – ZnO from peALD .plasma enhanced Atomic Layer Deposition .Zinc oxide with shallow donors .Tuned ZnO surface with varied defect composition .Identification of Contact barrier Organic Photovoltaics | Philip Schulz Schulz & Kelly 2014 31 Interface Energetics Charge selective contacts Evac ≈ Donor; Hole Transport Layer 2.0 Electron selective interlayer EA Evac 5.0 CB LUMOA - EF EFn Top contact 4.0 IE LUMOD EFp EF VB HOMOA Organic Photovoltaics | Philip Schulz ELUMOA – EHOMOD Voc + HOMOD 6.0 Hole selective interlayer ELUMOD – ELUMOA controls Jsc X Energy (eV) 3.0 X Transparent contact CB Acceptor Electron Transport Layer refs VB 32
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