Doping‐Induced Absorption Bands in P3HT: Polarons and Bipolarons1. Issue 23 (26th October 2016)

• Record Type: Journal Article
• Authors: Enengl, Christina; Enengl, Sandra; Pluczyk, Sandra; Havlicek, Marek; Lapkowski, Mieczyslaw; Neugebauer, Helmut; Ehrenfreund, Eitan
• Is Part Of: Chemphyschem. Volume 17:Issue 23(2016)
• Page Start: 3836
• Page End: 3844
• Publication Date: 2016-10-26
• Subjects: bipolarons -- doping -- organic semiconductors -- polarons -- spectroelectrochemistry; Chemistry, Physical and theoretical -- Periodicals; 541.05
• DOI: 10.1002/cphc.201600961
• Languages: English
• ISSNs: 1439-4235

Abstract

Abstract: 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. Abstract : Spinless charge carriers (bipolarons) are spectroscopically determined in poly(3‐hexylthiophene‐2, 5‐diyl) (P3HT) by means of in situ spectroelectrochemical measurements. In the low‐doping regime, polarons are formed, whereas in the high‐doping case, bipolarons are the main charge carriers.