Doping-Induced Absorption Bands in P3HT: Polarons and Bipolarons.
In this work, we focus on the formation of different kinds of charge carriers such as polarons and bipolarons upon p-type doping (oxidation) of the organic semiconductor poly(3- hexylthiophene-2,5-diyl) (P3HT). We elucidate the cyclic voltammogram during oxidation of this polymer and present spectroscopic changes upon doping in the UV/Vis/near-IR range as well as in the mid-IR range. In the low-oxidation regime, two absorption bands related to sub-gap transitions appear, one in the UV/Vis range and another one in the mid-IR range. The UV/Vis absorption gradually decreases upon further doping while the mid-IR absorption shifts to lower energy. Additionally, electron paramagnetic resonance (EPR) measurements are performed, showing an increase of the EPR signal up to a certain doping level, which significantly decreases upon further doping. Furthermore, the absorption spectra in the UV/Vis range are analyzed in relation to the morphology (crystalline vs. amorphous) by using theoretical models. Finally, the calculated charge carriers from cyclic voltammogram are linked together with optical transitions as well as with the EPR signals upon p-type doping. We stress that our results indicate the formation of polarons at low doping levels and the existence of bipolarons at high doping levels. The presented spectroscopic data are an experimental evidence of the formation of bipolarons in P3HT.
Institute of Polymer Science, Johannes Kepler University Linz, Altenbergerstraße 69, 4040, Linz, Austria.|Linz Institute for Organic Solar Cells (LIOS), Physical Chemistry, Johannes Kepler University Linz, Altenbergerstraße 69, 4040, Linz, Austria.
Christina
C
Enengl
Institute of Polymer Science, Johannes Kepler University Linz, Altenbergerstraße 69, 4040, Linz, Austria.|Linz Institute for Organic Solar Cells (LIOS), Physical Chemistry, Johannes Kepler University Linz, Altenbergerstraße 69, 4040, Linz, Austria.
Sandra
S
Enengl
Department of Physical Chemistry and Technology of Polymers, Faculty of Chemistry, Silesian University of Technology, 9 Marcina Strzody street, 44-100, Gliwice, Poland.
Sandra
S
Pluczyk
Linz Institute for Organic Solar Cells (LIOS), Physical Chemistry, Johannes Kepler University Linz, Altenbergerstraße 69, 4040, Linz, Austria.|Czech Metrology Institute, Okruzni 31, 638 00, Brno, Czech Republic.
Marek
M
Havlicek
Department of Physical Chemistry and Technology of Polymers, Faculty of Chemistry, Silesian University of Technology, 9 Marcina Strzody street, 44-100, Gliwice, Poland.|Center of Polymer and Carbon Materials, Polish Academy of Sciences, 34M. Curie-Sklodowskiej street, 41-819, Zabrze, Poland.
Mieczyslaw
M
Lapkowski
Linz Institute for Organic Solar Cells (LIOS), Physical Chemistry, Johannes Kepler University Linz, Altenbergerstraße 69, 4040, Linz, Austria.
Helmut
H
Neugebauer
Department of Physics and Solid State Institute, Technion-Israel Institute of Technology, Technion City, 32000, Haifa, Israel.
Eitan
E
Ehrenfreund
bipolarons; doping; organic semiconductors; polarons; spectroelectrochemistry