Fundamentals of Molecular Electrical Doping of Organic Semiconductors (FoMEDOS)

• Applicants: Professorin Dr. Caterina Cocchi, since 10/2017; Professor Dr. Norbert Koch, since 10/2017; Professor Dr. Dieter Neher
• Subject Area: Experimental and Theoretical Physics of Polymers ; Experimental Condensed Matter Physics ; Theoretical Chemistry: Molecules, Materials, Surfaces
• Term: from 2015 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 286798544

• Final Report Year: 2021

Final Report Abstract

Our joint experimental-theoretical work revealed important inside into the fundamental mechanisms of molecular doping of organic semiconductors. One part of the work concerned the nature of doping-induced species when two forms of poly(3- hexylthiophen-2,5-diyl) (P3HT) – regioregular (rreP3HT) and regiorandom (rraP3HT), whose chains differ in their ability to planarize and aggregate were combined with and different dopants (F4TCNQ, Mo(tfd- CO2Me)3 and BCF). Our studies showed that irrespective of their shape/size, all three dopants lead to delocalized polarons on the rreP3HT backbone, with identical extents of charge delocalization. This was despite marked differences in the dopant-polymer interactions as shown by EPR spectroscopy. However, when the ability of the polymer chains to aggregate was inhibited, either intrinsically as in rraP3HT, or extrinsically by elevating the solution temperature, the nature of the dopant-polymer interactions changed. This suggests that rather than the dopant size or type, the polymer backbone arrangement plays a more crucial role in determining the nature of the doping-induced species (doping mechanism) and the ionization efficiency. Importantly, charge transfer complex formation was only observed when combining rra-P3HT with the planar dopant F4TCNQ. Although limited by the sample size, this is an important insight for the design and selection of semiconducting polymers for doping in the future. By employing BCF as dopant, which does not form charge transfer complexes with P3HT, we could unambiguously differentiate correlate the optical signatures of polarons to the degree of polymer aggregation. By performing optical studies on filtered solutions, we found that the characteristic optical spectra of doped P3HT films are related with doped aggregates, while the spectra of individual (nonaggregated) P3HT chains are distinctly different. The results of this study help understanding the impact of solution pre-aggregation on thin film properties of molecularly doped P3HT, and highlight the importance of considering such aggregation for other doped conjugated polymers in general. Regarding the molecular doping of solid rre-P3HT with F4TCNQ, our studies showed that the crystalline portion of p-doped P3HT comprise regions where dopant anions pack with the polymer chains in a metastable, co-crystalline structure while further ionized dopants ae dispersed in the alkyl chain region of P3HT. Simultaneously, regions exist where the pristine polymer backbones closely pack. Importantly, De-doping via dopant desorption through thermal annealing revealed the dopants crystallized within the mixed phase to be the thermally least stable. Notably, their initial desorption did not alter the thin film conductivity, which indicates this phase to be not crucial for charge transport. Regarding doping of molecular semiconductors, we studied p-type doping of the planar organic semiconductor dibenzo-tetrathiafulvalene (DBTTF) in combination with the electron acceptors tetracyano-naphthoquinodimethane (TCNNQ) and hexafluoro-tetracyano-naphthoquinodimethane (F6TCNNQ) as planar dopants. No signature of charge transfer interaction is found for DBTTF and TCNNQ in solution, whereas IPA formation only is observed for DBTTF and F6TCNNQ. Different fundamental semiconductor-dopant interactions thus prevail in solution as compared to the solid-state. Through first-principles simulations we unraveled the nature of the optical excitations in different kinds of donor/acceptor complexes, formed by oligothiophenes doped by either F4TCNQ or BCF. These findings complemented the experimental studies carried out in this project and, at the same time, provided standalone advances that contributed to clarify the atomistic electronic structure of these complex materials.

Publications

• Orientation-Dependent Work-Function Modification Using Substituted Pyrene-Based Acceptors. J. Phys. Chem. C 2017, 121, 24657-24668
  Hofmann, O.T.; Glowatzki, H.; Bürker, C.; Rangger, G. M.; Bröker, B.; Niederhausen, J.; Hosokai, T.; Salzmann, I.; Blum, R.-P.; Rieger, R.; Vollmer, A.; Rajput, P.; Gerlach, A.; Müllen, K.; Schreiber, F.; Zojer, E.; Koch, N.; Duhm, S.
  
• Unraveling the Microstructure of Molecularly Doped Poly(3-hexylthiophene) by Thermally Induced Dedoping. J. Phys. Chem. C 2018, 122, 45, 25893–25899
  Hase, H.; O’Neill, K.; Frisch, J.; Opitz, A.; Koch, N.; Salzmann, I.
  
• Electronic and optical properties of oligothiophene-F4TCNQ chargetransfer complexes: The role of donor conjugation length. J. Phys. Chem. C 2019, 129, 9617— 9623
  Valencia, A.M.; Cocchi, C.
  
• Nat. Mater. 2019, 18, 1327-1334
  Yurash, B.; Cao, D. X.; Brus, V. V.; Leifert, D.; Wang, M.; Dixon, A.; Seifrid, M.; Mansour, A. E.; Lungwitz, D.; Liu, T.; Santiago, P. J.; Graham, K. R.; Koch, N.; Bazan, G. C.; Nguyen, T.-Q.
  
• State-of-Matter-Dependent Charge-Transfer Interactions between Planar Molecules for Doping Applications. Chem. Mater. 2019, 31, 1237-1249
  Beyer, P; Pham, D; Peter, C.; Koch, N.; Meister, E.; Brütting, W.; Grubert, L.; Hecht, S.; Nabok, D.; Cocchi, C.; Draxl, C.; Opitz, A.
  
• Ab initio modelling of local interfaces in doped organic semiconductors. Phys. Chem. Chem. Phys. 2020, 22, 3527-3538
  Valencia, A.M.; Guerrini, M.; Cocchi, C.
  
• An Organic Borate Salt with Superior p-Doping Capability for Organic Semiconductors. Adv. Sci. 2020, 7, 2001322
  Wegner, B.; Lungwitz, D.; Mansour, A. E.; Tait, C. E.; Tanaka, N.; Zhai, T.; Duhm, S.; Forster, M.; Behrends, J.; Shoji, Y.; Opitz, A.; Scherf, U.; List-Kratochvil, E. J. W.; Fukushima, T.; Koch, N.
  
• Microscopic Insight into the Electronic Structure of BCF- Doped Oligothiophenes from Ab initio Many-Body Theory. J. Phys. Chem. C 2020, 124, 14363– 14370
  Schier, R., Valencia, A.M.; Cocchi, C.
  
• Quantitative Analysis of Doping- Induced Polarons and Charge-Transfer Complexes of Poly(3-Hexylthiophene) in Solution. J. Phys. Chem. B 2020, 124, 7694–7708
  Arvind, M.; Tait, C. E.; Guerrini, M.; Krumland, J.; Valencia, A. M.; Cocchi, C.; Mansour, A. E.; Koch, N.; Barlow, S.; Marder, S. R.; Behrends, J.; Neher, D.
  
• Single-Step Formation of a Low Work Function Cathode Interlayer and n-type Bulk Doping from Semiconducting Polymer/Polyethylenimine Blend Solution. ACS Appl. Mater. Interfaces 2020, 12, 25, 28801–28807
  Seidel, K. F.; Lungwitz, D.; Opitz, A.; Krüger, T.; Behrends, J.; Marder, S. R.; Koch, N.
  
• The Optical Signatures of Molecular-Doping Induced Polarons in Poly(3- Hexylthiophene-2,5-Diyl): Individual Polymer Chains versus Aggregates. J. Mater. Chem. C 2020, 1, 3777
  Mansour, A.; Lungwitz, D.; Schultz, T.; Arvind, M.; Valencia, A.; Cocchi, C.; Opitz, A.; Neher, D.; Koch, N.
  
• Exploring organic semiconductors in solution: The effects of solvation, alkylization, and doping. Phys. Chem. Chem. Phys. 2021
  Krumland, J., Valencia, A.M.; Cocchi, C.