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The mechanism and the nature of the species formed by molecular doping of the model polymer poly(3-hexylthiophene) (P3HT) in its regioregular (rre-) and regiorandom (rra-) forms in solution are investigated for three different dopants: the prototypical π-electron acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), the strong Lewis acid tris(pentafluorophenyl)borane (BCF), and the strongly oxidizing complex molybdenum tris[1-(methoxycarbonyl)-2-(trifluoromethyl)-ethane-1,2-dithiolene] (Mo(tfd-CO2Me)3). In a combined optical and electron paramagnetic resonance study, we show that the doping of rreP3HT in solution occurs by integer charge transfer, resulting in the formation of P3HT radical cations (polarons) for all the dopants considered here. Remarkably, despite the different chemical nature of the dopants and dopant-polymer interaction, the formed polarons exhibit essentially identical optical absorption spectra. The situation is very different for the doping of rraP3HT, where we observe the formation of a charge-transfer complex with F4TCNQ and formation of a “localized” P3HT polaron on non-aggregated chains upon doping with BCF, while there is no indication of dopant-induced species in case of Mo(tfd-CO2Me)3. We estimate the ionization efficiency of the respective dopants for the two polymers in solution and report the molar extinction coefficient spectra of the three different species. Finally, we observe increased spin delocalization in regioregular compared to regiorandom P3HT by electron nuclear double resonance, suggesting that the ability of the charge to delocalize on aggregates of planarized polymer backbones plays a significant role in determining the doping mechanism.
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Probing transport energies and defect states in organic semiconductors using energy resolved electrochemical impedance spectroscopy
Abstract Determining the relative energies of transport states in organic semiconductors is critical to understanding the properties of electronic devices and in designing device stacks. Futhermore, defect states are also highly important and can greatly impact material properties and device performance. Recently, energy‐resolved electrochemical impedance spectroscopy (ER‐EIS) is developed to probe both the ionization energy (IE) and electron affinity (EA) as well as sub‐bandgap defect states in organic semiconductors. Herein, ER‐EIS is compared to cyclic voltammetry (CV) and photoemission spectroscopies for extracting IE and EA values, and to photothermal deflection spectroscopy (PDS) for probing defect states in both polymer and molecular organic semiconductors. The results show that ER‐EIS determined IE and EA are in better agreement with photoemission spectroscopy measurements as compared to CV for both polymer and molecular materials. Furthermore, the defect states detected by ER‐EIS agree with sub‐bandgap features detected by PDS. Surprisingly, ER‐EIS measurements of regiorandom and regioregular poly(3‐hexylthiophene) (P3HT) show clear defect bands that occur at significantly different energies. In regioregular P3HT the defect band is near the edge of the occupied states while it is near the edge of the unoccupied states in regiorandom P3HT.
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- Award ID(s):
- 1905734
- PAR ID:
- 10419258
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Advanced Materials Interfaces
- Volume:
- 10
- Issue:
- 19
- ISSN:
- 2196-7350
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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