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Models for organic solar cell and impedance spectroscopy results

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Presentation by J Bisquert about models of organic solar cells, presented at Boston MRS december 2011

Presentation by J Bisquert about models of organic solar cells, presented at Boston MRS december 2011

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  • 1. Characteristics of Organic Bulk Heterojunction Solar Cells Investigated by Impedance Spectroscopy. Juan Bisquert and Germà Garcia-Belmonte Photovoltaic and Optoelectronic Devices Group Universitat Jaume I 12071 Castelló Spain Boston 30-11-2011
  • 2. Cell mechanisms h + e - Poly(3-hexylthiophene), P3HT Fullerene derivative, PCBM Donor Acceptor
  • 3.
    • Exciton generation from absorved photon
    • Exciton disociation in the donor-acceptor interface
    • Electron transport (in the acceptor LUMO)
    • Electron extraction
    • Hole transport (in the donorHOMO)
    • Hole Extraction
    • Bimolecular recombination
    Cell mechanisms Acceptor HOMO Acceptor LUMO Donor HOMO Donor LUMO - - + E Fp (i) (ii) (iii) (iv) (v) Cathode (vi) - + (vi) E Fn V OC
  • 4. Charge separation by electrical field: a very popular idea P. Blom et al. PHYSICAL REVIEW B 72, 085205 (2005) Deibel and Dyakonov Rep. Prog. Phys. 73 (2010) 096401
  • 5. Two different pictures On Voltage, Photovoltage, and Photocurrent in Bulk Heterojunction Organic Solar Cells Juan Bisquert and Germa Garcia-Belmonte, J. Phys. Chem. Lett. 2011, 2, 1950–1964
  • 6. Electrical field model This model requires that (this is not usually remarked): The carrier density is fixed at the contacts Implies The voltage changes the bandbending But The voltage is not directly related to the drift field: On Voltage, Photovoltage, and Photocurrent in Bulk Heterojunction Organic Solar Cells Juan Bisquert and Germa Garcia-Belmonte, J. Phys. Chem. Lett. 2011, 2, 1950–1964
  • 7. The flatbands model On Voltage, Photovoltage, and Photocurrent in Bulk Heterojunction Organic Solar Cells Juan Bisquert and Germa Garcia-Belmonte, J. Phys. Chem. Lett. 2011, 2, 1950–1964 The carrier density is variable at the contacts The variation of potential can be absorbed at the interface The Fermi level goes up flat, the critical question is how the cathode works
  • 8. I ask myself the questions Am I totally sure that the BHJ blend is an insulator, that there is a built in potential in the dark that goes from one contact to the other? That Debye screening length is long (100 nm)? That the fill factor is caused by decreased photocurrent by less exciton dissociation So it’s all about the electrical field OR Maybe I have some doubts and I am inclined to believe that there could be some background doping density, that there are plenty of free carriers in the blend, that they shield and absorb electrical fields…
  • 9. Ask yourself the question The blend is an INSULATOR (no carriers at all, charges at the electrodes, field across layer) OR SEMICONDUCTOR (some carriers around, field at one contact)
  • 10. Band bending at the cathode
    • Now assume that the blend is a p-doped semiconductor
    • The equilibration occurs at the cathode
    • We proposed this in Organic Electronics 2008
    • The the built in voltage, is the difference between Fermi level of the blend and cathode workfunction, about 0.4 eV.
    Garcia-Belmonte, G.; Munar, A.; Barea, E. M.; Bisquert, J.; Ugarte, I.; Pacios, R. "Charge carrier mobility and lifetime of organic bulk heterojunctions analyzed by impedance spectroscopy". Organic Electronics 2008, 9, 847-851
  • 11. Band bending and Fermi levels: what happens when negative voltage is applied at the cathode Acceptor HOMO Acceptor LUMO Donor HOMO Donor LUMO E F Cathode - - - - - - - w V = 0
  • 12. w V < V fb Acceptor LUMO Donor LUMO - Band bending and Fermi levels Acceptor HOMO Donor HOMO E F - - - Cathode
  • 13. w V < V fb Acceptor LUMO Donor LUMO Cathode Cathode Band bending and Fermi levels Acceptor HOMO Donor HOMO E F - - -
  • 14. w V < V fb Acceptor LUMO Donor LUMO - Band bending and Fermi levels Acceptor HOMO Donor HOMO E F Cathode
  • 15. V = V fb Acceptor LUMO Donor LUMO Band bending and Fermi levels Acceptor HOMO Donor HOMO E F Cathode
  • 16. Using MS analysis we can determine the built in voltage (at the cathode) and the amount of doping. Depletion region – Mott Schottky plots Characterization of nanostructured hybrid and organic solar cells by impedance spectroscopyw Francisco Fabregat-Santiago, Germa Garcia-Belmonte, Ivan Mora-Sero´ and Juan Bisquert Phys. Chem. Chem. Phys., 2011, 13 , 9083–9118
  • 17. Depletion region: examples -P. P. Boix, G. Garcia-Belmonte , U. Muñecas, M.Neophytou, C. Waldauf, R. Pacios , Appl Phys. Lett 95, 1, (2009)
  • 18. Depletion region: examples
  • 19. The bands and the Fermi levels, again negative voltage (or photogeneration) The chemical capacitance Dielectric (depletion) capacitance
  • 20. Measured DOS of organic BHJ
    • PCBM-P3HT solar cell
    Measurement of the DOS The chemical capacitance Germà Garcia-Belmonte, Pablo P. Boix, Juan Bisquert, Michele Sessolo, and Henk J. Bolink Solar Energy Materials and Solar Cells , 94 , 366 (2010)
  • 21. Recombination in organic BHJ
  • 22. How does organic BHJ work
    • With good selective contacts
    • And charge separation and mutual shielding
    • We have flat Fermi levels
    • And flat bands
    • No electrical fields! Do not worry so much about drift-diffusion models, the critical question:
    • The current-potential curve, and consequently photovoltage, is entirely determined by
    • recombination
  • 23. How is Voc limited (1)?
    • Voc is given by the difference of Fermi levels.
    • So the Schottky barrier measured by MS does not pose a limitation to Voc (in fact the flatband of the barrier is about 0.4 V, and photovoltage can be much higher!)
    • This is because the cathode is not a pn junction, it is a heterojunction, it allows the Fermi level to rise past the flatband value. See again pages 11-15.
  • 24. How is Voc limited (2)?
    • Voc is given by the difference of Fermi levels.
    • Photovoltage, is set by recombination, that establishes the number of carriers in each separate material.
    • Important for the final Voc value are two factors
    • The relative energetics of the materials. If the LUMO of the fullerene is high, Voc will tend to be higher
    • The distribution of states at the Fermi levels. If there are few trap states, the Fermi level will rise higher
    Current-Voltage Characteristics of Bulk Heterojunction Organic Solar Cells: Connection Between Light and Dark Curves Pablo P. Boix, Antonio Guerrero, Luís F. Marchesi, Germà Garcia-Belmonte and Juan Bisquert Advabced Energy Materials 2011
  • 25.
    • DPM6 has a lower density of states in the region where the Fermi level moves, so higher Voc
    Comparison of different fullerenes with similar reduction potential but different DOS PCBM DPM6 G. Gar Germa Garcia-Belmonte Pablo P. Boix, Juan Bisquert, Martijn Lenes, Henk J. Bolink, Andrea La Rosa, Salvatore Filippone,and Nazario Martín§, J. Phys. Chem. Lett. 1 (2010) 2566-2571
  • 26. The diode equation for a solar cell At V = 0 equilibrium of generation and recombination Equilibrium of generation and recombination Rise of the Fermi level enhances recombination Dark Sunlight Can be measured directly by recombination resistance Also can be expressed in terms of carrier density (Durrant)
  • 27. Recombination in organic BHJ
    • Here Durrant group reconstructs Voc from carrier density using the simple diode model. Works very well.
    C. G. Shuttle, R. Hamilton, B. C. O’Regan, J. Nelson, and J. R. Durrant 16448–16452 ∣ PNAS, 2010 ∣ vol. 107
  • 28. 3 selected topics for investigation
    • assuming
    • flat Fermi levels
    • flat bands, homogeneous carrier distribution!
    • The current-potential curve determined by recombination
    • What is the role of disorder, especially on Voc?
    • Does voltage fix carrier density (Rau reciprocity)
    • How does recombination correlate with fullerene energetics
  • 29. 1. Why is Voc less than HOMO-LUMO difference? This is explained by disorder The Fermi levels lie below LUMO and above HOMO Energy loss related to disorder G. Garcia-Belmonte, J. Bisquert, Appl. Phys. Lett. 96 (2010) 113301
  • 30. 2. Reconstruction of current voltage curves from recombination resistance data Current-Voltage Characteristics of Bulk Heterojunction Organic Solar Cells: Connection Between Light and Dark Curves Pablo P. Boix, Antonio Guerrero, Luís F. Marchesi, Germà Garcia-Belmonte and Juan Bisquert Advabced Energy Materials 2011 T. Kirchartz, U. Rau Detailed balance and reciprocity in solar cells physica status solidi (a). 2008, 205, 2737-2751. Here you have low mobilities, or poor morphology, you may worry about transport Transport is fast, each phase is well interconnected, Fermi levels flat: recombination is the key issue
  • 31. 2. Reconstruction of current voltage curves from recombination resistance data Thuc-Quyen Nguyen et al., Advanced Energy Materials 2011 a) DPP(TBFu) 2 :PC 60 BM, and,b) P3HT:PC 70 BM solar cells under different illumination intensities. Same Voc at all light intensities: the simple diode model donm’t work
  • 32. 2. Reconstruction of current voltage curves from recombination resistance data Current-Voltage Characteristics of Bulk Heterojunction Organic Solar Cells: Connection Between Light and Dark Curves Pablo P. Boix, Antonio Guerrero, Luís F. Marchesi, Germà Garcia-Belmonte and Juan Bisquert Advabced Energy Materials 2011 Here we show, that recombination resistance is independent of illumination, satisfying reciprocity. See that series resistance is variable! Further, from recombination, we can construct the phtocurrent at open circuit, and we see it is the same as that at short circuit.
  • 33. 3. Energetics of recombination Recombination in Organic Bulk Heterojunction Solar Cells: Small Dependence of Interfacial Charge Transfer Kinetics on Fullerene Affinity   Antonio Guerrero, Luis F. Marchesi, Pablo P. Boix, Juan Bisquert and Germa Garcia-Belmonte, submitted
  • 34. Acknowledgments
      • Antonio Guerrero Pablo P. Boix
      • Homepage: www.elp.uji.es/jb.htm
      • E-mail: [email_address]
    www.hopv.org
  • 35. You are invited to participate in the 4th international Conference on Hybrid and Organic Photovoltaics, from 16 to 19 May 2012, Uppsala, Sweden. www.nanoge.org