Thin-Film Texture and Optical Properties of Donor/Acceptor Complexes. Diindenoperylene/F6TCNNQ vs Alpha-Sexithiophene/F6TCNNQ

Abstract

In this work, two novel donor/acceptor (D/A) complexes, namely, diindenoperylene (DIP)/1,3,4,5,7,8-hexafluoro-tetracyanonaphthoquinodimethane (F6TCNNQ) and alpha-sexithiophene (6T)/F6TCNNQ, are studied. The D/A complexes segregate in form of π–π stacked D/A cocrystals and can be observed by X-ray scattering. The different conformational degrees of freedom of the donor molecules, respectively, seem to affect the thin-film crystalline texture and composition of the D/A mixtures significantly. In equimolar mixtures, for DIP/F6TCNNQ, the crystallites are mostly uniaxially oriented and homogeneous, whereas for 6T/F6TCNNQ, a mostly 3D (isotropic) orientation of the crystallites and coexistence of domains of pristine compounds and D/A complex, respectively, are observed. Using optical absorption spectroscopy, we observe for each of the two mixed systems a set of new, strong transitions located in the near-IR range below the gap of the pristine compounds: such transitions are related to charge-transfer (CT) interactions between donor and acceptor. The optical anisotropy of domains of the D/A complexes with associated new electronic states is studied by ellipsometry. We infer that the CT-related transition dipole moment is perpendicular to the respective π-conjugated planes in the D/A complex.

Charge‐Transport Properties of F6TNAP‐Based Charge‐Transfer Cocrystals

“…Until very recently, reports on F 6 TNAP ( Figure ) were limited to demonstrating its use as a p ‐dopant ( i.e ., as a 1‐electron oxidant) for organic hole‐transport materials; however, its properties also suggest it to be an excellent candidate for formation of CT cocrystals. Indeed, although p‐doping ideally occurs through electron transfer from the hole‐transport material to the dopant, in the case of planar semiconductor and dopant molecules, CT complexes can be obtained instead with concomitantly less efficient generation of charge carriers; for example, CT complex formation has been observed when F 6 TNAP is used to p ‐dope diindeno[1,2,3‐ cd :1′,2′,3′‐ lm ]perylene or 2,2′:5′,2′′:5′′,2′′′:5′′′,2′′′′:5′′′′,2′′′′′‐sexithiophene, although the single‐crystal structures of the CT complexes were not determined . In 2018, Kloc and co‐workers reported crystal structures for mixed‐stack cocrystals of F 6 TNAP with four planar donor molecules—triphenylene (TP), pyrene (PY), phenanthrene, and naphtho[1,2‐ b :5,6‐ b ′]dithiophene—along with estimates of the extent of CT based on vibrational spectroscopy …”

Molecular Semiconductors for Logic Operations: Dead‐End or Bright Future?

“…Dopant density also determines doping efficiency because additional ionized dopants favor charge separation . It must be stressed that the open question of doping efficiency cannot be treated completely apart from those of molecular structures, miscibility, morphology, defects, impurities (notably water and oxygen), and device stability . Charge transport within doped organic semiconductors is thermally activated.…”

“…One can conclude from this short discussion that a plethora of electronic situations occur or even coexist depending on semiconductor‐dopant pairs, temperature, film thickness, and sample preparation conditions . Structural and morphological control upon doping polycrystalline or single crystal thin films of molecular semiconductors is particularly challenging, although not so often considered .…”

Electronic and Optical Properties of Oligothiophene-F4TCNQ Charge-Transfer Complexes: The Role of the Donor Conjugation Length

“…Doping in organic semiconductors is an ubiquitous phenomenon appearing when donor and acceptor molecules are combined together, and crucially determines the electronic, optical, and transport properties of the resulting materials. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] Two main mechanisms have been identified to be responsible for doping in organic semiconductors: Integer charge transfer (ICT) manifests itself when an electron is transferred from the donor to the acceptor, leading to the formation of ion pairs. Partial charge transfer occurs upon electronic hybridization of the frontier orbitals of the donor and the acceptor, giving rise to a charge-transfer complex (CTC).…”