Thermochromism in poly(3-hexylthiophene) in the solid state: A spectroscopic study of temperature-dependent conformational defects

Abstract

Reversible thermochromism in thin solid films of poly(3-hexylthiophene), or P3HT, has been studied using ultraviolet and x-ray photoelectron spectroscopies (UPS and XPS, respectively). The UPS and XPS spectra, as well as previously published optical absorption spectra, are analyzed using the results of valence effective Hamiltonian (VEH) quantum chemical calculations of the electronic structure of isolated polymer chains. The analysis of the spectra indicates that at elevated temperatures thermally induced electronic localization occurs as a consequence of thermally induced conformational disorder.

Photohole Screening Effects in Polythiophenes with Pendant Groups

“…We assign this feature of -1.7 eV to a defect state, which could be a consequence of impurities and/or "chemical" defects, although this feature has been ascribed to band structure effects accompanying the ordering of the aromatic chain backbone. 1 The nature of the "defects" that lead to this state cannot be uniquely identified from our data, although there are several possibilities. As seen in Figure 3, limited loss of regioregularity (Figure 3a), an oxygen bridging two pendant groups (Figure 3b), selected dehydrogenation resulting in a pendant group linking two adjacent thiophenes (Figure 3c), and loss of π-π stacking either through tilting of adjacent polymer chains (Figure 3d) or as a result of a lateral displacement of adjacent polymer chains (Figure 3e) could well result in the appearance of defects states within the HOMO-LUMO gap of regioregular poly(3-hexylthiophene-2,5 diyl) (P3HT).…”

“…On the basis of the photoemission results, Salaneck and co-workers 1,2 proposed that the thiophene rings are coplanar at low temperatures and the torsion angle or disorder along the aromatic backbone increases with increasing temperature, and these structure changes occur near or above room temperature. [1][2][3] From the light polarizationdependent photoemission, we infer that changes in the regioregular P3HT polymer backbone geometry occur well below room temperature for P3HT, and the structure changes are gradual. Other conducting polymers exhibit changes in the orientation of the aromatic rings in the polymer backbone with temperature similar to those reported here for P3HT, including polypyrrole, 4 so P3HT is not unusual in this regard.…”

“…Changes in band gap with temperature are often observed with semiconducting polymers [1][2][3][4][5][6] and can be useful not only as optical temperature indicators but also for thermal recording. 7 Moreover, thermal control of the band gap allows for a continuous variation of electroluminence energies (wavelengths) for polymer light-emitting diodes.…”

“…1,2,6 Thermochromism has been inferred from photoemission studies of undoped P3HT, as a function of temperature, 1,2 combined with the optical absorption spectra, 2 and explained on the basis of valence effective Hamiltonian (VEH) quantum chemical calculations, 1,11 without invoking chemical defects.…”

Side chain engineering of poly-thiophene and its impact on crystalline silicon based hybrid solar cells

“…Thermochroism studies on poly(3-alkylthiophenes) identified the cause for the strong hypsochromic shift with increasing temperature to be an increasing twist between adjacent thiophene units. 17 A similar effect comes into play for P3TOT. For this type of molecule, a preferential ordering of the side chains within the plane of the main polymer backbone is expected.…”