Doping‐Induced Absorption Bands in P3HT: Polarons and Bipolarons

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.

Smaller Counter Cation for Higher Transconductance in Anionic Conjugated Polyelectrolytes

“…A similar behavior in spectroelectrochemical measurements is well known for poly(3‐hexylthiophene) (P3HT) . As the electronic and optical properties are mainly determined by the conjugated backbone, this is hardly surprising.…”

“…As the electronic and optical properties are mainly determined by the conjugated backbone, this is hardly surprising. The two simultaneously arising absorption bands, seen for all three polyelectrolyte films, represent the oxidized polythiophene backbone with polarons as charge carriers . This was demonstrated for P3HT experimentally as well as via quantum chemical calculations .…”

“…This was demonstrated for P3HT experimentally as well as via quantum chemical calculations . The conversion of the undoped sample into the oxidized species is denoted by the isosbestic point . As the polythiophene backbone is an electron donor, a positive bias has to be applied to obtain a change in the absorption spectrum.…”

“…By applying a varying bias (0–600 mV in steps of 100 mV) between the CPE film casted on indium tin oxide (ITO) substrate and a Pt‐counter electrode, while both are immersed in a 0.1 m NaCl aq electrolyte solution, oxidation of the polyelectrolyte is induced. These processes can be simultaneously monitored with UV–vis spectrometer as the charges formed on the polymer backbones lead to a structural and electronic distortion and consequently to a change in the spectra . Figure a shows the resulting plot for crosslinked PTHS − TMA + as a typical example.…”

The impact of [1,2,5]chalcogenazolo[3,4‐f]‐benzo[1,2,3]triazole structure on the optoelectronic properties of conjugated polymers

“…As seen from the figure, the UV–vis–NIR absorption spectra of the neutral films of both polymers exhibit three absorption peaks at wide absorption bands ranging from 300 to 1,200 nm for PSBT and 300 to 1,400 nm for PSeBT. During the stepwise oxidation of both polymers, these three absorption peaks were decreased while new transitions arose at higher wavelengths proving the formation of new charge carriers namely polarons and bipolarons on the polymer backbone . When the UV–vis–NIR spectra of the neutral state of both polymers investigated thoroughly, the two absorption peaks lie at 340 and 500 nm for PSBT and 360 and 515 nm for PSeBT could be attributed to π–π* transition within the polymer backbone.…”