Ground-state charge-transfer interactions in donor:acceptor pairs of organic semiconductors - a spectroscopic study of two representative systems

Giuliano Duva; Paul Beyer; Reinhard Scholz; Valentina Belova; Andreas Opitz; Alexander Hinderhofer; Alexander Gerlach; Frank Schreiber

doi:10.1039/c9cp02939g

Ground-state charge-transfer interactions in donor:acceptor pairs of organic semiconductors - a spectroscopic study of two representative systems

Phys Chem Chem Phys. 2019 Aug 21;21(31):17190-17199. doi: 10.1039/c9cp02939g. Epub 2019 Jul 31.

Authors

Giuliano Duva¹, Paul Beyer², Reinhard Scholz³, Valentina Belova¹, Andreas Opitz², Alexander Hinderhofer¹, Alexander Gerlach¹, Frank Schreiber⁴

Affiliations

¹ University of Tübingen, Institute for Applied Physics, Auf der Morgenstelle 10, 72076 Tübingen, Germany. giuliano.duva@uni-tuebingen.de.
² Humboldt-Universität zu Berlin, Department of Physics, Newtonstraße 15, 12489 Berlin, Germany.
³ Dresden Integrated Center for Applied Physics and Photonic Materials, Nöthnitzer Str. 61, 01187 Dresden, Germany.
⁴ University of Tübingen, Institute for Applied Physics, Auf der Morgenstelle 10, 72076 Tübingen, Germany. giuliano.duva@uni-tuebingen.de and Center for Light-Matter Interactions, Sensors & Analytics (LISA+), Auf der Morgenstelle 15, 72076 Tübingen, Germany.

PMID: 31364636
DOI: 10.1039/c9cp02939g

Abstract

We investigate blended donor:acceptor (D:A) thin films of the two donors diindenoperylene (DIP) and poly(3-hexylthiophene) (P3HT) mixed with the strong acceptor 1,3,4,5,7,8-hexafluorotetracyanonaphthoquinodimethane (F₆TCNNQ) using Polarization-Modulation Infrared Reflection-Absorption Spectroscopy (PMIRRAS). For DIP:F₆TCNNQ thin films we first carry out a comprehensive study of the structure as a function of the D : A mixing ratio, which guides the analysis of the PMIRRAS spectra. In particular, from the red-shift of the nitrile (C[triple bond, length as m-dash]N) stretching of F₆TCNNQ in the different mixtures with DIP, we quantify the average ground-state charge-transfer (GS-CT) to be ρ_avg = (0.84 ± 0.04) e. The PMIRRAS data for P3HT:F₆TCNNQ blended films reveal nearly the same shift of the CT-affected C[triple bond, length as m-dash]N stretching peak for this system. This points towards a very similar CT strength for the two systems. We extend the analysis to the relative intensity of the C[triple bond, length as m-dash]N to the C[double bond, length as m-dash]C stretching modes of F₆TCNNQ in the mixtures with DIP and P3HT, respectively, and support it with DFT calculations for the isolated F₆TCNNQ. Such comparison allows to identify the vibrational signatures of the acceptor mono-anion in P3HT:F₆TCNNQ, thus indicating a much stronger, integer CT-type interactions for this system, in agreement with available optical spectroscopy data. Our findings stress the importance of a simultaneous analysis of C[triple bond, length as m-dash]N and C[double bond, length as m-dash]C stretching vibrations in F₆TCNNQ, or similar quinoid systems, for a reliable picture of the nature of GS-CT interactions.