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

摘要

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 (F(6)TCNNQ) using Polarization-Modulation Infrared Reflection-Absorption Spectroscopy (PMIRRAS). For DIP:F(6)TCNNQ 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 (CN) stretching of F(6)TCNNQ in the different mixtures with DIP, we quantify the average ground-state charge-transfer (GS-CT) to be rho(avg) = (0.84 +/- 0.04) e. The PMIRRAS data for P3HT:F(6)TCNNQ blended films reveal nearly the same shift of the CT-affected CN 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 CN to the CC stretching modes of F(6)TCNNQ in the mixtures with DIP and P3HT, respectively, and support it with DFT calculations for the isolated F(6)TCNNQ. Such comparison allows to identify the vibrational signatures of the acceptor mono-anion in P3HT:F(6)TCNNQ, 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 CN and CC stretching vibrations in F(6)TCNNQ, or similar quinoid systems, for a reliable picture of the nature of GS-CT interactions.