Two-Dimensional Electronic Excitations in Self-Assembled Conjugated Polymer Nanocrystals

Abstract

Several spectroscopic methods were applied to study the characteristic properties of the electronic excitations in thin films of regioregular and regiorandom polythiophene polymers. In the regioregular polymers, which form two-dimensional lamellar structures, increased interchain coupling strongly influences the traditional one-dimensional electronic properties of the polymer chains. The photogenerated charge excitations (polarons) show two-dimensional delocalization that results in a relatively small polaronic energy, multiple absorption bands in the gap where the lowest energy band becomes dominant, and associated infrared active vibrations with reverse absorption bands caused by electron-vibration interferences. The relatively weak absorption bands of the delocalized polaron in the visible and near-infrared spectral ranges may help to achieve laser action in nanocrystalline polymer devices using current injection.

Structural correlations in the generation of polaron pairs in low-bandgap polymers for photovoltaics

“…5e), monitored at 0.41 eV (ref. 30), exhibits the lowest yield among the materials presented here. It is gratifying to note that the yield we report, η = 8 ± 2 %, is comparable to values obtained by means of complementary techniques 38,39 .…”

“…A more exact statement on the nature of the primary photoexcitations, that is, whether excitons decay into polaron pairs or the latter are the primary photoexcitations, would require a time resolution well below 100 fs from the visible to mid-infrared. The distribution of the initial photoexcitations between these two species depends on the electronic structure of the polymer and eventually the chain conformation in the solid state 30 . Effects of chain conformation and mesoscopic morphology typically have a strong influence on the delayed formation of interchain polaron pairs 30 .…”

“…The distribution of the initial photoexcitations between these two species depends on the electronic structure of the polymer and eventually the chain conformation in the solid state 30 . Effects of chain conformation and mesoscopic morphology typically have a strong influence on the delayed formation of interchain polaron pairs 30 . On light absorption, a chromophore in the polymer chain is excited resulting in a photoexcitation with Coulomb-bound electron-hole pair, delocalized over several repeat units 41 .…”

“…Optical signatures of polarons are evident in the infrared and originate from the bandgap renormalization on removal of charge neutrality on the π-conjugated system 30,31 . We have determined the energetic position of the polaron absorption bands in the infrared by chemical doping with the oxidizing dopant SbCl 5 and quantum-chemical calculations 32,33 .…”

Optical measurement of doping efficiency in poly(3-hexylthiophene) solutions and thin films

“…Previously, photoinduced absorption (PIA) [31][32][33] and charge modulation spectroscopy (CMS) 30,34,35 have been used to characterize optical absorption in charged P3HT. In the IR region, absorption peaks with energy lower than 0.2 eV were observed in both cases in regio-regular P3HT.…”