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Energetics of States in Conjugated Polymer P3HT

Energetics of States in Conjugated Polymer P3HT. Ziran Gu 01/18/10. Process: Photon Absorbed Exciton created and separated. Charges and holes collected at the electrodes. . A Typical Organic Solar Cell BHJ Structure. Bilayer Structure. LUMO: Lowest Unoccupied Molecular Orbital

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Energetics of States in Conjugated Polymer P3HT

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  1. Energetics of States in Conjugated Polymer P3HT ZiranGu 01/18/10

  2. Process: Photon Absorbed Exciton created and separated. Charges and holes collected at the electrodes. A Typical Organic Solar Cell BHJ Structure

  3. Bilayer Structure LUMO: Lowest Unoccupied Molecular Orbital HOMO: Highest Occupied Molecular Orbital PCBM (Acceptor) P3HT (Donor) PEDOT:PSS (Hole Transporter)

  4. Measurement of HOMO and LUMO levels:Photoelectron Spectroscopy(PES or UPS) and IPES Occupied electronic states measured by Photoelectron process: Density of the states indicated by the intensity of spectrum IPES measures unoccupied states, which make a perfect complementary to PES.

  5. Taking Fermi level as ground,

  6. Polaronic State. Ionic crystal Electron cloud modified. Atomic positions modified. In polymer: Atomic positions change more

  7. Time Scales of the photon absorption: Photon absorption: Electronic relaxation: Geometric relaxation: UPS measures LUMO- and HOMO+ states (just the electronic cloud relaxation) S.Krause et. al. New Journal of Physics(2008)

  8. PES: Electrons are Kicked outPhoton absorbed does NOT mean electron is Freed. Absorption gap vs. Transport gap Absorption begins from 1.85eV, instead of 2.6eV External Quantum Efficiency: No. of charges collected in device over No. of photons (no matter absorbed or not) as a function of the energy EQE also begins from 1.85eV, but very limited, due to impurities or electrodes Binding energy = 2.6eV – 1.85eV = 0.75 eV

  9. Polaron levels: electron transported and ready to be collected at electrodes Optical gap: electron is excited, but still bonded. Polaron pair: electron transported, two polarons formed and bonded due to coulomb attraction.

  10. Measurement of the exciton binding energy PL quenching by doping Field depend photoluminescence quenching: to prevent recombination of exciton by giving binding energy through the E field

  11. Onsager-Braun Model Dissociation Probability Dissociation rate F: electric field Eb= Binding energy b = Bessel Function Parameter denpendant on E-Field Parameters:

  12. Based on different binding energies, quenching yield could be achieved.The fitting of the experiment data gives us0.42ev as the exciton binding energy.What is the binding energy?difference between optical gap and polaron pairs?Or between optical gap and transport gap?

  13. Low field discrepancy: binding energy over finite range.Difference between modeled and experimental magnitudes: Transition from exciton to single polaron states requires more energy than to polaron pair (0.75eV vs. 0.42eV)

  14. The actually binding energy could be 0.55 ev!This could be the difference from exciton to polaron pair. Which is still less than 2.6ev-1.85ev = 0.75evAlso: this empirical model is developed from ion pair dissociation, so the binding energy form is really simple, which is not necessarily applicable here:

  15. CONCLUSION

  16. In other Measurements.Vacuum level shifted due charge transfer between Polymer/Sub

  17. (a) Wf > Eict+ Fermi level pinned to Eict+(b) Eict+ <Wf < Eict- Vacuum level aligned(c) Wf < Eict- Fermi level pinned to Eict-

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