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

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.

💡 Research Summary

This work investigates the formation of donor‑acceptor complexes (DACs) between the strong electron donor dibenzotetrathiafulvalene (DBTTF) and the strong acceptor hexaazatriphenylene‑hexacarbonitrile (HATCN), and how these complexes influence the structural, morphological, optical, and electrical properties of thin films. Using a high‑throughput co‑evaporation setup, the authors prepared a 20 nm gradient film on silicon oxide in which the DBTTF:HATCN molar ratio varied continuously from pure DBTTF to pure HATCN across a 10 cm substrate. Spatially resolved measurements (GIWAXS, XRR, AFM, UV‑Vis‑NIR, UPS, and conductivity) were performed every 1 mm (≈4 mol % steps), allowing a direct correlation between composition and material characteristics.

GIWAXS revealed that the pristine components retain their known crystal structures (triclinic DBTTF, trigonal HATCN). In the intermediate region (≈40–55 mol % DBTTF) all Bragg peaks belonging to the pure phases disappear and new, broader peaks emerge, indicating the formation of a distinct co‑crystalline DAC phase with a smaller crystallite size and reduced orientation. The intensity analysis shows a linear increase of this new phase up to ~55 mol % DBTTF, after which HATCN‑related Debye‑Scherrer rings reappear, demonstrating a reversible phase transition as the composition is varied.

AFM imaging shows that DBTTF alone forms isolated islands (~100 nm high, 1 µm wide) rather than a continuous film, while HATCN yields a smooth (~1.5 nm RMS) continuous layer composed of ~330 nm grains. In the mixed region, large DBTTF‑rich islands coexist with smaller (~350 nm) elongated crystallites, which the authors assign to the DAC phase. Roughness exhibits a non‑monotonic trend: it rises to a maximum (~5 nm) around 0.6–0.7 mol % HATCN, then drops back to ~1 nm as elongated spherulitic structures dominate. This behavior reflects the competition between phase separation and co‑crystallization.

Optical spectroscopy uncovers two broad charge‑transfer (CT) absorption bands at 1.16 eV and 1.42 eV that appear only in the mixed region. Their oscillator strengths peak at the 1:1 composition, confirming that the DAC phase provides a new low‑energy electronic transition distinct from the parent materials. A weaker transition near 3 eV also follows the same compositional trend, suggesting additional CT sub‑structures.

Electrical measurements demonstrate a striking, non‑monotonic dependence of conductivity on composition. Pure DBTTF exhibits p‑type behavior (hole transport) whereas pure HATCN and all mixtures are n‑type (electron transport). Ultraviolet photoelectron spectroscopy shows that charge injection occurs via the lowest unoccupied molecular orbital (LUMO) of the DAC and of HATCN, explaining the universal n‑type response. At the 1:1 ratio the activation energy for carrier generation drops dramatically, carrier concentration and mobility both increase, and the conductivity exceeds that of the pristine films by a factor of 2–3. The authors attribute this enhancement to (i) the formation of a low‑band‑gap co‑crystalline DAC, (ii) reduced trap density at the numerous grain boundaries, and (iii) increased dielectric screening that weakens Coulomb binding of dopant‑induced ion pairs.

Overall, the study provides a comprehensive picture of how donor‑acceptor complex formation, beyond simple integer charge‑transfer doping, can be harnessed to engineer new crystalline phases with superior charge transport. The gradient‑film methodology offers a powerful, high‑throughput platform for mapping composition‑property relationships in organic semiconductor blends, paving the way for rational design of high‑performance organic electronic materials.