Phase behavior and electrical transport in DBTTF-HATCN donor-acceptor mixtures

TL;DR

This work investigates phase behavior and electrical transport in mixtures of the donor DBTTF and the acceptor HATCN, focusing on donor-acceptor complex (DAC) formation. Using gradient co-deposition, GIWAXS, AFM, UV–Vis, UPS, FTIR/Raman, XPS, and MIS/OFET devices, the authors identify a distinct DAC-containing crystalline phase that forms near equimolar composition and exhibits sub-gap optical absorption at ~1.16 and ~1.42 eV, with a LUMO pinned ~0.45 eV above the Fermi level, promoting n-type conduction. Electrical transport is nonmonotonic with composition: carrier density peaks around 60–70% HATCN, mobility shows dual maxima, and a pronounced conductivity minimum occurs at equimolar composition due to morphological fragmentation of the DAC-rich phase; overall, DACs enhance conductivity but morphology governs macroscopic transport. These findings illuminate how DAC formation and film morphology jointly influence charge transport and offer design principles for infrared-responsive organic electronics. The results also show a relatively small Arrhenius prefactor compared with ICT doping, suggesting lower doping efficiency in complex doping scenarios.

Abstract

The formation of donor-acceptor complexes (DACs) between the electron donor Dibenzotetrathiafulvalene (DBTTF) and the acceptor Hexaaza\-triphenylene\-hexacarbo\-nitrile (HATCN) results in a new phase with a distinctly different crystal structure as well as new optical absorption bands below the energy gaps of the two pristine materials. X-ray scattering and atomic force microscopy provide detailed insights into the film structure and morphology by systematic variation of the mixing ratio from pristine DBTTF to pristine HATCN. The measured electrical conductivity of thin films depends in a highly non-monotonic manner on the composition of the mixture and shows significantly improved charge transport compared to the pristine films. The temperature-dependent conductivity, charge carrier concentration, and mobility were investigated across these compositions. Surprisingly, all compositions exhibited n-type behavior, except for pristine DBTTF. This behavior is explained by the electronic structure of the mixtures, as revealed by ultraviolet photoelectron spectroscopy, which indicates that charge injection and transport occur via the lowest unoccupied molecular orbital of the DAC and HATCN. Additionally, the observed electrical conductivity is strongly influenced by morphology and structural ordering of the films. These findings offer valuable insights for the design of advanced materials with enhanced electrical performance.

Figures (9)

• Figure 1: a) Chemical structures of DBTTF (electron donor) and HATCN (electron acceptor). b) Possible formation of a donor-acceptor complex (DAC) by mutually overlapping $\pi$-orbitals in a face-to-face arrangement.
• Figure 2: Crystalline structure of the DBTTF:HATCN film with gradient in relative concentrations. (a)-(c) Reciprocal space maps measured at the DBTTF edge (a), HATCN edge (c), and in the mixed part of the sample (b). Arcs and circles indicate the simulated peak positions for the previously reported structures of the pristine DBTTF and HATCN phases and the fitted peak positions for the mixed phase. (d) Square root of the average fitted intensity of selected Bragg peaks corresponding to the crystalline phases shown in panels (a)-(c). The intensity corresponding to the pristine DBTTF phase is shown in orange, the pristine HATCN in blue, and the mixed phase in purple.
• Figure 3: Morphology of the DBTTF:HATCN gradient film. (a) Roughness values extracted from AFM scans at different spatial positions along the gradient. The upper curve (circles) is extracted from large (10$\times$10 $\mathrm{\mu}$m$^2$) scans, the bottom (squares) one from smaller (3$\times$3 $\mathrm{\mu}$m$^2$) scans between large particles. The letters next to the curves denote the positions where the AFM maps in (b)--(h) were measured. (b), (c) AFM maps of pristine DBTTF (b) and HATCN (c) measured at the edges of the gradient sample. (d)--(h) AFM maps measured at different positions in the mixed part of the film. Inset (e) shows a smaller scan measured between the large particles as shown in (d) with a green square. The zero value corresponds to the mean height value for each map.
• Figure 4: (a) UV-Vis absorption spectra of the DBTTF:HATCN gradient film. The inset highlights the low-energy region of the spectra, where the CT absorption peak is located. The curves are color-coded according to the position along the film, as indicated by the color bar. (b) Extracted oscillator strength for the two CT absorption peaks at 1.16 eV (circles) and 1.42 eV (squares).
• Figure 5: (a) Secondary electron-cutoff (SECO) and (b) valence spectra measured for the ITO substrate (only SECO), the pristine films and 3 mixed films. (c) Energy level diagram of frontier orbitals deduced from SECO and valence spectra (for details see text). The mixing fractions are given in mol%.