The thin film growth behaviors of ADT and β-MT-ADT are studied by X-ray diffraction techniques and atomic force microscopy, and the templating effect on the thin film growth process of β-MT-ADT is investigated with DIP as the templating layer.

For electron donor–acceptor complexes a link will be established between optical, structural and vibrational properties of EDA complexes as well as the electrical doping by them.

The interactions between organic donor and acceptor molecules and the related charge transfer (CT) effects are of great interest in organic optoelectronics. Here, we present a comprehensive investigation of cocrystal formation and charge transfer effects in weakly interacting organic semiconductor mixtures. As a model system, we choose dinaphthothienothiophene (DNTT) as a donor molecule and two different perylene diimide derivatives (PTCDI-C-8-CN2 and PDIF-CN2) as acceptors, which differ in the fluorination of the side chains in the imide position. Experimentally, both systems show a small ground-state CT governed by hybridized HOMO-1 and LUMO+1 levels. In contrast, the respective HOMO and LUMO levels of the complex are localized on the acceptor and donor molecule. This leads to the observation of a nearly pure charge transfer excitation from the acceptor to the donor in the absorption spectra. We discuss the general impact of localized HOMO and LUMO levels on the optoelectronic properties in CT complexes dependent on comparison with first-principles calculations based on density functional theory and many-body perturbation theory.

Pentacene (PEN) is a prototypical small-molecule organic semiconductor and a singlet fission (SF) material, but the details of its charge-transfer (CT) interactions with electron acceptors are still under debate. Here, we revisit the CT interactions in thin film blends of PEN and 2,3,5,6-tetrafluoro-7,7,8,8,-tetracyanoquinodimethane (F4-TCNQ) and find experimental evidence for the coexistence of integer CT (ICT) and charge-transfer complex (CTC) formation, with the relative amount depending on the mixing ratio. In contrast, in blends of PEN and 2,2′-(perfluoronaphthalene-2,6-diylidene)dimalononitrile (F6-TCNNQ), only ICT is found, illustrating the special behavior of PEN/F4-TCNQ at the border between CTC formation and ICT. We further study the photophysics of these donor/acceptor blends with respect to the importance of PEN as a SF material and find that SF is robust against moderate amounts of acceptors in the blends. Triplet pair formation and separation, as well as CT from PEN triplets to CTCs and acceptor molecules, is observed.

Organic semiconductors offer a flexibility in band gap tuning through different molecular lengths and have shown to be promising candidates for high‐performance electronic devices. While electronic properties have been extensively studied in many publications, the topic of thin film structure and molecular packing is still quite neglected. In this work, the thin film crystal structure of [6]phenacene (Fulminene) and [7]phenacene deposited on silicon substrates using the OMBD method as well as further studies with potassium deposition on [6]phenacene are investigated. Ex‐situ X‐ray and optical methods are employed to obtain an insight into the crystal structure and optical properties. The orientation of the molecules and the unit cell structure is calculated from the measured reciprocal space maps. Additionally, the influence of a possible interfacial doping mechanism by deposition of potassium on top of [6]phenacene is investigated. Thin films of [6]phenacene show a high crystallinity with a standing‐up configuration and a similar molecular structure and symmetry compared to [4]phenacene. [7]phenacene features two different apparently thickness‐dependent polymorphs (H and L) with a structure similar to [5]phenacene. These results reveal that odd/even parity, that is, even or odd number of benzene rings, directly influences the phenacene thin film structure.

The evolution of surface roughness in binary mixtures of the two molecular organic semiconductors (OSCs) diindenoperylene (DIP) as electron-donor and 1, 3, 4, 5, 7, 8-hexafluoro-tetracyano naphthoquinodimethane (F6TCNNQ) as electron-acceptor is studied. We co-deposit DIP and F6TCNNQ in vacuum with varying relative molar content while keeping a molar excess of DIP in order to produce phase-heterogeneous mixtures. The excess DIP phase segregates in pristine crystallites, whereas the remaining mixed phase is constituted by DIP:F6TCNNQ co-crystallites. We calculate the surface roughness as function of film thickness by modelling x-ray reflectivity data acquired in situ and in real-time during film growth. To model the experimental data, two distinct approaches, namely the kinematic approximation and the Parratt formalism, are applied. A comparative study of surface roughness evolution as function of DIP:F6TCNNQ mixing ratio is carried out implementing the Trofimov growth model within the kinematic approximation. Depending on the thickness regime, mixing ratio-specific trends are identified and discussed. To explain them, a growth mechanism for binary heterogeneous mixtures of strongly interacting OSCs is proposed.

Blended organic thin films have been studied during the last decades due to their applicability in organic solar cells. Although their optical and electronic features have been examined intensively, there is still lack of detailed knowledge about their growth processes and resulting morphologies, which play a key role for the efficiency of optoelectronic devices such as organic solar cells. In this study, pure and blended thin films of copper phthalocyanine (CuPc) and the Buckminster fullerene (C60) were grown by vacuum deposition onto a native silicon oxide substrate at two different substrate temperatures, 310 K and 400 K. The evolution of roughness was followed by in-situ real-time X-ray reflectivity. Crystal orientation, island densities and morphology were examined after the growth by X-ray diffraction experiments and microscopy techniques. The formation of a smooth wetting layer followed by rapid roughening was found in pure CuPc thin films, whereas C60 shows a fast formation of distinct islands at a very early stage of growth. The growth of needle-like CuPc crystals loosing their alignment with the substrate was identified in co-deposited thin films. Furthermore, the data demonstrates that structural features become larger and more pronounced and that the island density decreases by a factor of four when going from 310 K to 400 K. Finally, the key parameters roughness and island density were well reproduced on a smaller scale by kinetic Monte-Carlo simulations of a generic, binary lattice model with simple nearest-neighbor interaction energies.

Many polymorphic crystal structures of copper phthalocyanine (CuPc) have been reported over the past few decades, but despite its manifold applicability, the structure of the frequently mentioned α polymorph remained unclear. The base-centered unit cell (space group C2/c) suggested in 1966 was ruled out in 2003 and was replaced by a primitive triclinic unit cell (space group P 1). This study proves unequivocally that both α structures coexist in vacuum-deposited CuPc thin films on native silicon oxide by reciprocal space mapping using synchrotron radiation in grazing incidence. The unit-cell parameters and the space group were determined by kinematic scattering theory and provide possible molecular arrangements within the unit cell of the C2/c structure by excluded-volume considerations. In situ X-ray diffraction experiments and ex situ atomic force microscopy complement the experimental data further and provide insight into the formation of a smooth thin film by a temperature-driven downward diffusion of CuPc molecules during growth.

Electronic states within the HOMO-LUMO gap of organic semiconductors play a key role in the energy level alignment of substrate-organic and organic-organic interfaces and therefore are a defining parameter for device functionality and efficiency. They are thought to result from structural defects influencing the specific environment of a molecule. Varying the substrate temperature for samples grown by molecular beam deposition, we are able to control their density. Using atomic force microscopy and X-ray scattering techniques, we can differentiate defects depending on their length scale and effective direction. Comparison of the respective defect density with the density of gap states, measured directly via ultra-low-background ultraviolet photoelectron spectroscopy, enables to correlate structural and electronic properties for different prototypical organic semiconductors. We investigate the impact of gap states on the energy level alignment and find a direct link between structural defects and the interface dipole.

Thin films of small or ganic semi con duct ing mol e cules ex hibit at trac tive op ti cal and elec tronic prop er ties de pend ing on their mo lec u lar struc ture. Pentacene (PEN) mol e cules are known to form a ly ing-down phase on graphene that im proves ver ti cal elec tron trans port and in creases op ti cal ab sorp tion and light har vest ing. Here, we stud ied the mech a nism of epitaxial PEN growth on graphene by in-situ GISAXS and GIWAXS tech niques com ple mented by ex-situ AFM and po lar ized con fo cal Raman mi cros copy. Two prin ci pal stages of the growth were ob served. First, nu cle ation and growth of PEN is lands sat u rat ing in width and height at 1 monolayer thick ness was ob served. Later on, the is lands con tinue to grow only along [100] di rec tion of the PEN triclinic lat tice adopt ing nee dle-like shape and copy ing hex ag o nal sym me try of the un der ly ing graphene. The c* axis of PEN re cip ro cal lat tice was found to be par al lel to the [210] (arm chair) di rec tion of graphene and tilted by 18° with re spect to the graphene sur face. This sug gests a »11° de vi a tion of the PEN mo lec u lar chains from the graphene sur face driven by the en ergy minimization in later stages of the is lands growth.

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 ρavg = (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.

Many organic semiconductors (OSCs) feature strong optical anisotropy that can be exploited to increase the efficiency of optoelectronic devices. We demonstrate that for the technologically relevant, rod-like OSCs diindenoperylene (DIP), pentacene (PEN), and α-sexithiophene (6T) deposited on silicon oxide surfaces it is possible to prepare polycrystalline thin films in which the long molecular axis is oriented parallel to the substrate plane in a template-free fashion. In films grown by organic molecular beam deposition at room temperature or higher, the molecules are oriented upright standing (σ-orientation). Instead, the here-presented growth at low temperatures followed by slow annealing up to a temperature near molecular desorption has the effect of "freezing" the molecules with their long axis oriented parallel to the substrate plane (λ-orientation) while conferring them crystalline long-range order. We discuss the huge impact on the optical anisotropy of the films observed as a consequence of the orientation transition. Finally, we propose a mechanism for explaining the achieved λ-orientation, which is stable under environmental conditions.

The interaction between a graphene layer and pentacene (PEN) molecules leads to the formation of a lying-down phase, which can improve charge transport for organic vertical field effect transistors and enhance the optical absorption for increased light harvesting in organic solar cells. Here, we present a comprehensive study of PEN growth on epitaxial graphene on silicon carbide (SiC). Simultaneous grazing-incidence small- and wide-angle X-ray scattering (GISAXS/GIWAXS) were used in situ for real-time monitoring of the PEN crystal growth with millisecond time resolution to identify two distinct anisotropic growth stages after the nucleation of the first monolayer (ML). In the first stage up to 1.5 nm, we observe rapid growth of pentacene domains along the (010) and (001) facets. This growth behavior is saturating after 1.5 nm. In a second stage, this is followed by continuous lateral crystal growth in only one in-plane direction (100) forming needle-shaped domains. In the second stage, an uninterrupted linear growth of the lying-down PEN phase is found based on the (001) diffraction up to 15 nm. Ex situ atomic force microscopy and polarized confocal Raman microscopy were used to further support the real-time observations of aligned PEN films on graphene.

We present a combined experimental and theoretical study to solve the unit-cell and molecular arrangement of the tetracene thin film (TF) phase. TF phases, also known as substrate induced phases (SIPs), are polymorphs that exist at interfaces and decisively impact the functionality of organic thin films, e.g., in a transistor channel, but also change the optical spectra due to the different molecular packing. As SIPs only exist in textured ultrathin films, their structure determination remains challenging compared to bulk materials. Here, we use grazing incidence X-ray diffraction and atomistic simulations to extract the TF unit-cell parameters of tetracene together with the atomic positions within the unit-cell.

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) inter...

We study the growth of two n-type small-molecule organic semiconductors from the perylene diimide family: N,N'-bis-(2-ethylhexyl)dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDIR-CN2) and N,N'-1H,1H-perfluorobutyl-dicjyanoperylene-3,4: 9,10-bis (dicarboximide) (PDIF-CN2) whose chemical structures differ only in the imide substituents, branched alkyl chains -C8H16 and linear fluoroalkyl chains -C4F7H2, respectively. Both types of substituents introduce some degree of steric hindrance for intermolecular interactions, affecting solid-state packing during thin film formation, and thus induce specific structure-dependent optoelectronic properties in thin films. The transition from an amorphous structure to crystalline domains with strong intermolecular coupling was followed in situ and in real time during growth. We investigated the structural and morphological properties by X-ray diffraction and atomic force microscopy as a function of the substrate temperature and chemical structure. We examined the relationship between the structural properties and thin film optical signatures probed via differential reflectance spectroscopy, ellipsometry, and temperature-dependent photoluminescence. A new crystalline PDIR-CN2 polymorph at high temperatures emerges. In addition, we observed in PDIF-CN2 that the fluorinated chains contribute to crystallization inhibition because of the higher overall steric hindrance compared to the alkyl chains.

Binary mixed thin films of picene (C22H14, PIC) and pentacene (C22H14, PEN) consist of crystallites with a statistical occupation of the lattice sites by either PEN or PIC and unit cell parameters continuously changing with the mixing ratio. For high PIC ratios a PIC phase forms which corresponds to a limited intermixing of the two compounds. The growth behavior of these mixtures is investigated in situ and in real‐time using grazing incidence X‐ray diffraction. We observe a delayed phase separation in PIC‐rich blends, i.e. complete intermixing in the monolayer range and the nucleation of a pure PIC‐phase in addition to the intermixed phase starting from the second monolayer. Growth scenario of picene‐rich pentacene‐picene blends.

We report on the growth and characterization of molecular mixed thin films of a-sexithiophene (6T), a well-known organic p-type semiconductor with high hole mobility, together with its perfluorinated counterpart, the so far rarely studied tetradecafluoro-a-sexithiophene (PF6T). Pure and blended thin films of these two molecules with different mixing ratios were grown on silicon oxide in ultrahigh vacuum by coevaporation. The effect of perfluorination and mixing on crystal structure, morphology, electronic, and optical properties was examined. The evolution of the PF6T crystal structure was followed in situ in real time by X-ray scattering. We found a new thin film structure different from the reported bulk phase with molecules either standing-up or lying-down depending on the growth temperature. The different morphologies of pure films and blends were investigated with atomic force microscopy. The impact of mixing on the core-levels and on the highest occupied molecular orbitals of 6T and PF6T is discussed.

Anthradithiophene (ADT) and its functionalized derivatives have proven to be attractive for high-performance electronic devices based on small-molecule organic semiconductors. In this manuscript we investigate the structural and optical properties of thin films of difluoro-anthradithiophene (diF-ADT), an ADT derivative, grown by organic molecular beam deposition (OMBD). By grazing incidence X-ray diffraction and reciprocal space maps, we show that diF-ADT crystallizes in a thin film structure similar to the single crystal unit cell. In addition, we investigate the growth characteristics with atomic force microscopy (AFM) and show an increase of surface mound sizes with elevated substrate temperature. Optical absorption measurements reveal a clear vibronic progression in both solution and thin film spectra along with a distinct optical anisotropy related to the molecular orientation in thin films.

We report on the growth and characterization of molecular mixed thin films of α-sexithiophene (6T), a well-known organic p-type semiconductor with high hole mobility, together with its perfluorinated counterpart, the so far rarely studied tetradecafluoro-α-sexithiophene (PF6T). Pure and blended thin films of these two molecules with different mixing ratios were grown on silicon oxide in ultrahigh vacuum by coevaporation. The effect of perfluorination and mixing on crystal structure, morphology, electronic, and optical properties was examined. The evolution of the PF6T crystal structure was followed in situ in real time by X-ray scattering. We found a new thin film structure different from the reported bulk phase with molecules either standing-up or lying-down depending on the growth temperature. The different morphologies of pure films and blends were investigated with atomic force microscopy. The impact of mixing on the core-levels and on the highest occupied molecular orbitals of 6T and PF6T is discussed.