Abstract Co‐deposited molecular heterostructures with statistical intermixing of the constituents are attractive candidates to tune the optical and the transport properties, as well as the ability to promote photophysical processes like singlet fission. In order to comprehend and control the singlet fission mechanism in these systems, it is of utmost interest to study the underlying excited state dynamics. In this work, thin films of anthradithiophene blended with the efficient singlet fission material pentacene are investigated by means of time‐resolved and temperature‐dependent photoluminescence spectroscopy with a time resolution of a few picoseconds. The analysis of the photoluminescence dynamics points toward efficient funneling of excitons from anthradithiophene via isolated pentacene molecules to agglomerates of pentacene, where eventually singlet fission occurs. The efficient and largely temperature‐independent quenching of the luminescence in anthradithiophene is attributed to a favorable cascade‐like alignment of the energy levels, and it is hypothesized that Förster resonance energy transfer is the main driving mechanism for exciton transport to pentacene agglomerates. The system investigated here can serve as a blueprint for the design of other molecular heterostructures with spatially separated light harvesting and singlet fission regions.

Understanding the dynamics of excited states after optical excitation at donor-acceptor (D/A) interfaces is of paramount importance for improving the efficiency and performance of optoelectronic devices. Here, we studied the ultrafast excited state dynamics after optical excitation at interfaces between the electron donor (D) pentacene (PEN) and the electron acceptor (A) perfuoropentacene (PFP) as well as within the single compounds (PEN and PFP) using femtosecond (fs) time-resolved second harmonic generation (SHG). In the single compounds singlet fission is observed on a time scale of around 200 fs. In the bilayer systems a huge SHG intensity rise is observed due to the creation of charge transfer states at the interface and accordingly to formation of a local electric field within tens of picoseconds. The local electric field and therefore the SHG signal intensity from the interface of PEN/PFP bilayer is much more intense compared to the PFP/PEN system because the PFP and PEN intermixing at the PEN/PFP interface is higher. Accordingly a population of defect states on a time scale of 55±12 ps has been proposed for PEN/PFP. Our study provides important insights into D/A charge transfer properties, which is needed for the understanding of the interfacial photophysics of pentacene-based organic compounds.

Singlet fission is a promising mechanism to boost the performance of solar cells. In order to establish efficient singlet fission, it is essential to understand and control competing decay channels such as the occupation of trap states and the formation of excimers, for which the local ordering of the molecules plays an essential role. To systematically explore effects of molecular packing, a promising strategy is to blend the material of interest with another molecular compound. In the present study, we investigate binary thin films of anthradithiophene (ADT) and the high bandgap molecule hexaphenylbenzene with different compound ratios. By utilizing X-ray diffraction techniques and steady state optical spectroscopy, we identify a phase separating regime where the structural quality can be systematically tuned. Combining these subtle structural modifications with time-resolved and temperature-dependent photoluminescence spectroscopy allows us to independently manipulate the energetic landscape and thermal activation of kinetic processes. Eventually, our findings enable us to refine current models of the excited-state photophysics of ADT, suggesting that singlet fission critically depends on the number of neighboring ADT molecules, whereas excimer formation is not significantly affected by mesostructural changes. We propose that control of mesostructure and temperature can be a pathway toward controlling the balance between singlet fission and other competing channels.

Modifying the optical and electronic properties of crystalline organic thin films is of great interest for improving the performance of modern organic semiconductor devices. Therein, the statistical mixing of molecules to form a solid solution provides an opportunity to fine-tune optical and electronic properties. Unfortunately, the diversity of intermolecular interactions renders mixed organic crystals highly complex, and a holistic picture is still lacking. Here, we report a study of the optical absorption properties in solid solutions of pentacene and tetracene, two prototypical organic semiconductors. In the mixtures, the optical properties can be continuously modified by statistical mixing at the molecular level. Comparison with time-dependent density functional theory calculations on occupationally disordered clusters unravels the electronic origin of the low energy optical transitions. The disorder partially relaxes the selection rules, leading to additional optical transitions that manifest as optical broadening. Furthermore, the contribution of diabatic charge-transfer states is modified in the mixtures, reducing the observed splitting in the 0-0 vibronic transition. Additional comparisons with other blended systems generalize our results and indicate that changes in the polarizability of the molecular environment in organic thin-film blends induce shifts in the absorption spectrum.

The photophysics of organic semiconductor (OSC) thin films or crystals has garnered significant attention in recent years since a comprehensive theoretical understanding of the various processes occurring upon photoexcitation is crucial for assessing the efficiency of OSC materials. To date, research in this area has relied on methods using Frenkel-Holstein Hamiltonians, calculations of the GW-Bethe-Salpeter equation with periodic boundaries, or cluster-based approaches using quantum chemical methods, with each of the three approaches having distinct advantages and disadvantages. In this work, we introduce an optimally tuned, range-separated time-dependent density functional theory approach to accurately reproduce the total and polarization-resolved absorption spectra of pentacene, tetracene, and perylene thin films, all representative OSC materials. Our approach achieves excellent agreement with experimental data (mostly ≤0.1 eV) when combined with the utilization of clusters comprising multiple monomers and a standard polarizable continuum model to simulate the thin-film environment. Our protocol therefore addresses a major drawback of cluster-based approaches and makes them attractive tools for OSC investigations. Its key advantages include its independence from external, system-specific fitting parameters and its straightforward application with well-known quantum chemical program codes. It demonstrates how chemical intuition can help to reduce computational cost and still arrive at chemically meaningful and almost quantitative results.

Evidence shows that charge carriers in organic semiconductors self-localize because of dynamic disorder. Nevertheless, some organic semiconductors feature reduced mobility at increasing temperature, a hallmark for delocalized band transport. Here we present the temperature-dependent mobility in two record-mobility organic semiconductors: dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]-thiophene (DNTT) and its alkylated derivative, C8-DNTT-C8. By combining terahertz photoconductivity measurements with atomistic non-adiabatic molecular dynamics simulations, we show that while both crystals display a power-law decrease of the mobility (μ) with temperature (T) following μ ∝ T -n, the exponent n differs substantially. Modelling reveals that the differences between the two chemically similar semiconductors can be traced to the delocalization of the different states that are thermally accessible by charge carriers, which in turn depends on their specific electronic band structure. The emerging picture is that of holes surfing on a dynamic manifold of vibrationally dressed extended states with a temperature-dependent mobility that provides a sensitive fingerprint for the underlying density of states.

Utilizing strong light-matter coupling is an elegant and powerful way to modify the energy landscapes of excited states of organic semiconductors. Consequently, the chemical and photophysical properties of these organic semiconductors can be influenced without the need of chemical modification but simply by implementing them in optical microcavities. This has so far mostly been shown in Fabry-Pérot cavities and with organic single crystals or diluted molecules in a host matrix. Here, we demonstrate strong, simultaneous coupling of the two Davydov transitions in polycrystalline pentacene thin films to surface lattice resonances supported by open cavities made of silver nanoparticle arrays. Such thin films are more easily fabricated and, together with the open architecture, more suitable for device applications.

Perylene-based organic semiconductors are widely used in organic electronic devices. Here, we studied the ultrafast excited state dynamics after optical excitation at interfaces between the electron donor (D) diindenoperylene (DIP) and the electron acceptor (A) dicyano-perylene-bis(dicarboximide) (PDIR-CN2 ) using femtosecond time-resolved second harmonic generation (SHG) in combination with large scale quantum chemical calculations. Thereby, we varied in bilayer structures of DIP and PDIR-CN2 the interfacial molecular geometry. For an interfacial configuration which contains a edge-on geometry but also additional face-on domains an optically induced charge transfer (CT) is observed, which leads to a pronounced increase of the SHG signal intensity due to electric field induced second harmonic generation. The interfacial CT state decays within 7.5±0.7 ps, while the creation of hot CT states leads to a faster decay (5.3±0.2 ps). For the bilayer structures with mainly edge-on geometries interfacial CT formation is suppressed since π-π overlap perpendicular to the interface is missing. Our combined experimental and theoretical study provides important insights into D/A charge transfer properties, which is needed for the understanding of the interfacial photophysics of these molecules.

Conjugated polymer field-effect transistors are emerging as an enabling technology for flexible electronics due to their excellent mechanical properties combined with sufficiently high charge-carrier mobilities and compatibility with large-area, low-temperature processing. However, their electrical stability remains a concern. ON-state (accumulation mode) bias-stress instabilities in organic semiconductors have been widely studied, and multiple mitigation strategies have been suggested. In contrast, OFF-state (depletion mode) bias-stress instabilities remain poorly understood despite being crucial for many applications in which the transistors are held in their OFF-state for most of the time. Here, a simple method of using an antisolvent treatment is presented to achieve significant improvements in OFF-state bias-stress and environmental stability as well as general device performance for one of the best performing polymers, solution-processable indacenodithiophene- co -benzothiadiazole (IDT-BT). IDT-BT is weakly crystalline, and the notable improvements to an antisolvent-induced, increased degree of crystallinity, resulting in a lower probability of electron trapping and the removal of charge traps is attributed. The work highlights the importance of the microstructure in weakly crystalline polymer films and offers a simple processing strategy for achieving the reliability required for applications in flexible electronics.

The coherent distribution of an electronic excitation over multiple organic molecules in the solid state, namely excited-state delocalization, plays an important role in photophysical processes such as singlet fission. However, experimental studies of the influence of excited-state delocalization on singlet fission have been challenging mainly for two reasons. First, there is no easy way of measuring the excited-state delocalization, and second, tracking the resulting changes for singlet fission is demanding due to the triplet-pair state, which is a crucial intermediate in singlet fission, being an optically dark state and hence hard to access experimentally. Binary systems offer a way to adapt the growth conditions of a singlet fission material, which enables tuning of the excited-state delocalization, possibly due to the impact of structural disorder on exciton localization. By varying the growth conditions, we demonstrate that emission from the triplet-pair state via Herzberg-Teller coupling is detectable in films with low growth rates of the singlet fission material, while the triplet-pair state shows no luminescence in the other cases due to triplet dissociation outcompeting the luminescent decay. With this we find that triplet-pair state luminescence correlates with higher excited-state delocalization.

Hybrid organic-inorganic nanomaterials composed of organic semiconductors and inorganic quantum dots (QDs) are promising candidates for opto-electronic devices in a sustainable internet of things. Especially their ability to combine the advantages of both compounds in one material with new functionality, the energy-efficient production possibility and the applicability in thin films with little resource consumption are key benefits of these materials. However, a major challenge one is facing for these hybrid materials is the lack of a detailed understanding of the organic-inorganic interface which hampers the widespread application in devices. We advance the understanding of this interface by studying the short-range organization and binding motif of aryleneethynylenes coupled to CdSe QDs as an example system with various experimental methods. Clear evidence for an incorporation of the organic ligands in between the inorganic QDs is found, and polarization-modulation infrared reflection-absorption spectroscopy is shown to be a powerful technique to directly detect the binding in such hybrid thin-film systems. A monodentate binding and a connection of neighboring QDs by the aryleneethynylene molecules is identified. Using steady-state and time resolved spectroscopy, we further investigated the photophysics of these hybrid systems. Different passivation capabilities resulting in different decay dynamics of the QDs turned out to be the main influence of the ligands on the photophysics.

The application potential of singlet fission (SF), describing the spontaneous conversion of an excited singlet into two triplets, underlines the necessity to independently control SF rates, energetics and the optical band gap. Heterofission, whereby the singlet splits into triplets on chemically distinct chromophores, is a promising approach to control the above-mentioned parameters, but its details are not yet fully understood. Here, we investigate the photophysics of blends of two prototypical SF chromophores, tetracene (TET) and rubrene (RUB) using time-resolved photoluminescence spectroscopy and time-correlated single photon counting to explore the potential for heterofission in combinations of endothermic SF chromophores.

Planar organic heterostructures are widely explored and employed in photovoltaic cells, light‐emitting diodes, and bilayer field‐effect transistors. An important role for device performance plays the energy level alignment at the inorganic–organic and organic–organic interfaces. In this work, incremental ultraviolet photoelectron spectroscopy measurements and real‐time X‐ray scattering experiments are used to thoroughly investigate the thickness‐dependent electronic and structural properties of a perfluoropentacene (PFP)‐on‐[6]phenacene heterostructure. For both materials an incremental increase of the material work function (positive interface dipole) is found. For [6]phenacene, this can be assigned to a thickness‐dependent change of molecular arrangement evident from a change of the unit cell volume and a consequential alteration of the ionization energy. In the case of PFP the interface dipole stems from charge transfer from the substrate into unoccupied molecular orbitals resulting in an electrostatic potential on the surface. The magnitude of this potential can be correlated with an increased gap state density resulting from templated structural defects mediated by the bottom layer.

Theoretical studies using clusters as model systems have been extremely successful in explaining various photophysical phenomena in organic semiconductor (OSC) thin films. But they have not been able to satisfactorily simulate total and polarization-resolved absorption spectra of OSCs so far. In this work, we demonstrate that accurate spectra are predicted by time-dependent density functional theory (TD-DFT) when the employed cluster reflects the symmetry of the crystal structure and all monomers feel the same environment. Additionally, long-range corrected optimal tuned functionals are mandatory. For pentacene thin films, the computed electronic spectra for thin films then reach an impressive accuracy compared with experimental data with a deviation of less than 0.1 eV. This allows for accurate peak assignments and mechanistic studies, which paves the way for a comprehensive understanding of OSCs using an affordable and easy-to-use cluster approach.

In the photoluminescence spectra of thinfilms made of singletfission (SF)materials emission features that are red-shifted from the free exciton emission are of particularinterest. They can befingerprints of the correlated triplet-pair state and as such offer insights intothe mechanisms of the multistep SF process. However, excimer formation or trap-state populationcan also cause such features and a clear disentanglement of the various contributions can bechallenging. Here, we use blends of anthradithiophene (ADT) and weakly interacting organicsemiconductors to control the polarizability of the molecular environment and, thus, todistinguish between excimer emission and emission from the correlated triplet-pair state. Usingtime-resolved photoluminescence spectroscopy measurements, we clarify the relation betweenexcimer formation and SF in ADT andfind that excimer formation constitutes a parallel relaxationchannel for the exciton and neither mediates nor hinders SF.

Vibronic coupling has been proposed to play a decisive role in promoting ultrafast singlet fission (SF), the conversion of a singlet exciton into two triplet excitons. Its inherent complexity is challenging to explore, both from a theoretical and an experimental point of view, due to the variety of potentially relevant vibrational modes. Here, we report a study on blends of the prototypical SF chromophore pentacene in which we engineer the polarizability of the molecular environment to scan the energy of the excited singlet state (S1) continuously over a narrow energy range, covering vibrational sublevels of the triplet-pair state (1(TT)). Using femtosecond transient absorption spectroscopy, we probe the dependence of the SF rate on energetic resonance between vibronic states and, by comparison with simulation, identify vibrational modes near 1150 cm-1 as key in facilitating ultrafast SF in pentacene.

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.

While the charge transport properties of organic semiconductors have been extensively studied over the recent years, the field of organics‐based thermoelectrics is still limited by a lack of experimental data on thermal transport and of understanding of the associated structure–property relationships. To fill this gap, a comprehensive experimental and theoretical investigation of the lattice thermal conductivity in polycrystalline thin films of dinaphtho[2,3‐b:2′,3′‐f]thieno[3,2‐b]thiophene (Cn‐DNTT‐Cn with n = 0, 8) semiconductors is reported. Strikingly, thermal conductivity appears to be much more isotropic than charge transport, which is confined to the 2D molecular layers. A direct comparison between experimental measurements (3ω–Völklein method) and theoretical estimations (approach‐to‐equilibrium molecular dynamics (AEMD) method) indicates that the in‐plane thermal conductivity is strongly reduced in the presence of the long terminal alkyl chains. This evolution can be rationalized by the strong localization of the intermolecular vibrational modes in C8‐DNTT‐C8 in comparison to unsubstituted DNTT cores, as evidenced by a vibrational mode analysis. Combined with the enhanced charge transport properties of alkylated DNTT systems, this opens the possibility to decouple electron and phonon transport in these materials, which provides great potential for enhancing the thermoelectric figure of merit ZT.

Organic charge transfer complexes (CTCs) with near-infrared absorption received growing interest in the past years, but the details of their photophysics, especially in thin films, remain largely unknown. We combined experimental and computational methods to thoroughly investigate and compare CTCs formed by tetracene with 2,2'-(perfluoronaphthalene-2,6-diylidene)di-malononitrile and 2,3,5,6-tetrafluoro-7,7,8,8,-tetracyanoquinodimethane, respectively. Using ultrafast transient absorption spectroscopy, the photophysics of these small bandgap CTCs was revealed, which is dominated by a sub-picosecond relaxation of the excitons back to the ground state. In equimolar blends, tetracene singlet fission is suppressed while in blends with excess of tetracene reduced lifetimes of tetracene, singlet and triplet excitons were found.

The fission of singlet excitons into triplet pairs in organic materials holds great technological promise, but the rational application of this phenomenon is hampered by a lack of understanding of its complex photophysics. Here, we use the controlled introduction of vacancies by means of spacer molecules in tetracene and pentacene thin films as a tuning parameter complementing experimental observables to identify the operating principles of different singlet fission pathways. Time-resolved spectroscopic measurements in combination with microscopic modelling enables us to demonstrate distinct scenarios, resulting from different singlet-to-triplet pair energy alignments. For pentacene, where fission is exothermic, coherent mixing between the photoexcited singlet and triplet-pair states is promoted by vibronic resonances, which drives the fission process with little sensitivity to the vacancy concentration. Such vibronic resonances do not occur for endothermic materials such as tetracene, for which we find fission to be fully incoherent; a process that is shown to slow down with increasing vacancy concentration.