Investigation of nature of excitons in PPDT2FBT and effect of optical interference

Abstract

Electroabsorption (EA) measurements can be used to identify the type of excitons contributing to the absorption spectra of semiconductors. However, the shape of the EA spectrum may vary depending on the mode of measurement due to the optical interference effects. Analysis without considering these effects may lead to erroneous conclusions. In this work, we present EA measurements and analysis for reflection mode measurements considering optical interference effects. We compared the inferences with transmission mode measurements and discuss the limitations. We identified the nature of excitons associated with each transition in the absorption spectrum of poly[(2,5-bis(2-hexyldecyloxy)-phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c]-[1,2,5]-thiadiazole)] thin film from EA measurements. The bands at 1.89, 2.05, and 2.27 eV had a mixed nature consisting of charge transfer and Frenkel characteristics. Of these, the band at 2.05 eV showed the strongest charge transfer characteristic. From thickness dependent measurements, we showed that the interference effects increase with the thickness of the semiconductor layer. The nature of excitons, however, could still be deduced qualitatively from reflection mode EA measurements.

The nature of excitons in PPDT2FBT:PCBM solar cells: Role played by PCBM

“…Therefore, the presence of acceptor in the blend helps in formation of interfacial charge transfer excitons [3]. In our earlier studies [14], we showed that, for pristine PPDT2FBT, the generated excitons had mixed nature i.e. both Frenkel and charge transfer (CT) characteristics.…”

“…In the case of Frenkel excitons, the change in polarizability (∆P ) between the ground state and excited state energy level is significant. Whereas the change in the dipole moment (∆µ) indicates the formation of CT excitons in a transition [8,14]. Both these contributions have distinct lineshape, revealing the nature of the exciton related to any particular transition.…”

“…Being a modulated measurement, the line shape of the EA spectrum resembles the derivatives of the zero-field absorption spectrum. From the spectrum analysis, the type of excitons generated in the system can be identified [14].…”