Session T71: Electronic Processes and Interfaces in Organic Semiconductors

Exciton Transport in Singlet Fission Materials: a New Hare and Tortoise Story

Singlet fission is promising for redistributing solar spectrum to overcome the Shockley-Queisser limit for single junction solar cells using molecular materials. Despite recent experimental and theoretical advances in understanding the underlying mechanisms, how exciton transport is coupled to singlet fission dynamics is much less explored. Here we examine exciton transport in singlet fission materials, highlighting the use of transient absorption microscopy (TAM) to track the population of different states in both spatial and temporal domains. In contrast to the conventional picture where singlet and triplet excitons migrate independently, TAM measurements of acene single crystals reveal cooperative transport between fast-moving singlet and slow-moving triplet excitons. Such cooperative transport is unique to singlet fission materials and allows hundreds of nanometers triplet migration on the nanosecond timescale, beneficial for solar cell applications. The transport of triplet pair intermediates and general criteria for achieving cooperative singlet-triplet transport are also discussed.   

Promoting singlet fission over exciton self-trapping by altering the packing morphology of anthradithiophene derivatives

When considering the criteria for singlet fission in solid-state thin-film organic photovoltaics, particular packing motifs play a significant role by modulating intermolecular electronic coupling. Using single-crystal ultrafast microscopy methods, we isolate the packing configuration that best promotes endoergic singlet fission.  Specifically, four fluorinated anthradithiophene (diF R-ADT) crystals are grown with differing functionalized derivatives [and packings]:  R = TES [brickwork], TSBS [sandwich herringbone], TDMS [brickwork] and TBDMS [twisted columnar]. In each ADT packing morphology, both the dynamic and spectral signatures of correlated triplet pair state (TT) formation are measured over a broad temperature range (1.5 to 295 K).  By varying the applied B-field (0 to 7 T), the photoluminescence emission and excitation spectra clearly isolate TT-state emission peaks for all ADT-crystals except sandwich herringbone.  Matching the singlet state depletion rate, a corresponding 70 ps rise in the TT population is prominent whenever the probe beam is polarized along the crystal CT-axis.  Conversely, ADT derivatives with a sandwich-herringbone packing morphology show no spectral or dynamic signatures of TT formation due to a preferential exciton self-trapping pathway.   

Direct Observations of Uniform Bulk-heterojunctions and the Energy Level Alignments in Non-Fullerene Organic Photovoltaic Active Layers

State-of-the-art organic photovoltaics cells (OPVCs) relies on the engineering of the energy levels of the organic molecules as well as the bulk-heterojunction nanomorphology to achieve high performance. However, both are difficult to measure inside the active layer where the electron donor and acceptor molecules are mingled. While the energy level alignments of the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) between the electron donors and acceptors may be altered in the mixed active layer compared to their pure forms, the nanomorphology of the donor and acceptor molecular domains are mostly studied in indirect means. Here, we present the direct observations of the nanomorphology of the molecular domains as well as the energy level alignments in the active layer of a non-fullerene based OPVC (donor: PBDB-T-2F and acceptor: IT-4Cl) using cross-sectional scanning tunneling microscopy and spectroscopy (XSTM/S). It is revealed that (1) the bulk-heterojunction (BHJ) structures are homogeneous and uniform throughout the ~1.2 μm thick active layer; (2) the energy alignments between the donor-rich and acceptor-rich domains are directly observed; (3) there exists the intermixing domains at the boundaries of the donor-rich and acceptor-rich domains with thickness in the nm scale; (4) the exciton binding energies in PBDB-T-2F and IT-4Cl are estimated to be 0.74 eV and 0.32 eV, respectively; and (5) there is a ~0.7 V loss in the open circuit voltage. The results provide nano-scale understanding of the OPVC active layers to guide further improvement of the OPVC performance.   

Charge transport in monolayer and few-layer organic semiconductor field-effect transistors

Like many other field effect transistor material systems, the contact resistance between the electrode and the active layer plays a dominating role in the performance of organic field effect transistors. In order to miniaturize the device and increase their density, a contact resistance down to 0.01 W-cm (100  W-mm) is highly desired. However, today’s devices are still orders of magnitudes higher than this desired value. In this talk, I will report the methodology of using ultrathin monolayer organic semiconductors with the van der Waals electrodes to address the contact resistance in OFETs. The transferred gold electrodes can eliminate the defects or trap states induced in the conventional vacuum deposition methods. By using the C10-DNTT as the channel material and solution shearing deposition method, we have achieved an intrinsic carrier mobility of 12 cm² V-s and a contact resistance of 40 W-cm. Furthermore, I will also demonstrate the individual contribution of the access resistance (R_A) and the interface resistance (R_INTF) in the total contact resistance of the OFETs. The electrical performance and the fabrication process of the monolayer OFETs will also be discuss in detail.   

Seed-induced Nanostructuring of Heteroepitaxial Molecular Semiconductor Thin Films for Enhanced Optoelectronic Response

Controlled growth of molecular semiconductor thin films with specific ordering and nanostructuring is crucial for devising next-generation organic optoelectronics with enhanced performance. A highly ordered crystalline organic thin films can be an ideal active layer for electronic devices owing to its improved charge transport ability. Despite tremendous efforts to achieve high quality crystalline active layers, most superior performances were realized in single crystal states rather than films, even though the film is more desirable for the device applications. This is critical for most molecular semiconductors with complicated molecular structure, such as twisted conformation or with bulky side chains, which show greater electronic properties in its crystalline phase, but it is particularly difficult to form crystalline films directly on inert substrates. Here, we demonstrate an unprecedented approach by the incorporation of nucleation seed layer that creates crystalline nanostructures of a molecular semiconductor that is observed to have an intrinsic proclivity to form amorphous films; our technique is also readily adaptable to scale-up ultrahigh vacuum deposition processes and can be immediately integrated into existing diode manufacturing routines to create uniform crystalline nanostructure over large areas. Crystalline films exhibited enhanced photophysical and electronic characteristics, compared to the amorphous counterparts. Finally, we show that large-area flexible optoelectronic devices including organic solar cells and light emitting diodes can be realized.   

Time resolved electric field-induced second harmonic generation imaging from organic thin-film devices

Transient electric field-induced second harmonic generation (EFISHG) method, based on the third-order susceptibility, allows direct and selective probing of dynamic carrier motion in the active channel region of a field-effect transistor (FET). This technique provides a powerful tool for visualizing the carrier transport and predicting the carrier mobilities, free from contact resistance issues and device geometrical factors. We have developed a nonlinear microscopic imaging system using a tunable femtosecond laser and a pulse compensation arrangement, which compensates for any group velocity dispersion of the pulses in the imaging system. We apply the EFISHG technique to pentacene and other polymer-based FETs. By varying the time delay between the laser and the voltage pulse, the carrier motion across the FET channel is observed from which the carrier mobilities are deduced.   

Isoporous membranes as a platform to understand nanoscale transport behavior in aqueous environments

A thorough understanding of transport behavior of water and solutes through membranes is central to scientific and technological processes such as water treatment, electrochemical energy conversion, and biosensors. As the dimensions of pores shrink to the nanoscale, the larger surface-to-volume ratio of the material enhances the role of interfaces in interactions between the membrane and solutes as well as the water itself, resulting in different physicochemical properties from the bulk. Electrostatics, van der Waals forces, hydrogen bonding, and other forces between particles and walls each play an important role in transport behavior. Traditional membrane systems, however, have substantial structural and chemical heterogeneity that hinders deciphering the interplay of these contributing mechanisms. Using a nanofabrication process centered on block copolymer lithography, we fabricate nearly ideal isoporous membranes as an experimental platform to drive insights into fundamental transport processes. Through tuning pore size and surface chemistry of isoporous membranes, we explore how parameters such as pore size affect transport behavior independently from contributions from other factors.   

On stabilizing spin crossover complex [Fe(tBu2qsal)2] on suitable supports: insights from ab initio studies

Selecting suitable supports that can preserve intrinsic properties of spin cross over (SCO) complexes is an essential step for enabling their applications. Here, for the SCO complex [Fe(tBu₂qsal)₂], on the basis of density functional theory calculations, we conclude that while the Au(111) surface may not be a suitable support for the molecule, it may be so for their  monolayer (ML). Our rationale is based on charge transfer characteristics from the highest occupied molecular orbital of the molecule to the substrate which is noticeable for a single molecule but minimal for a monolayer on Au(111), leaving the electronic structure of the molecules unaltered. We also find a small magnetic exchange coupling (~0.01 meV) between the molecules in the ML indicating a weak magnetic coupling. These results suggest Au(111) as a plausible support for a monolayer of [Fe(tBu₂qsal)­₂], suitable for electronic and spin transport applications. As for [Fe(tBu₂qsal)­₂] SCO molecules themselves, we find h-BN to be a viable support for them, as there is hardly any charge transfer between the molecule and h-BN, unlike the case of graphene which displays stronger interaction with the molecule resulting in larger charge transfer from the molecule to graphene.   

Substrates influencing thin-film geometry through charge-transfer: a computational study with DFT and Machine Learning

Organic thin-films are known for their versitile applications due to their tuneable properties. Most properties are strongly dependent on the polymorph geometry the film assumes, which is mainly determined by processing conditions and by the employed substrate. Traditional ab-initio studies of thin films geometries remain computationally prohibitive, due to the immense number of possible configurations. However, recent developments in machine-learning assisted structure search have made structure-to-property investigations accessible. In this contribution we demonstrate the impact that different substrates can have on the geometry of the first two layers of a thin film using the prototypical systems of benzoquinone on Ag(111) and on graphene. We identify the energetically most favourable geometries for both systems and compare them. While the polymorphs formed in the first layer of benzoquinone are very similar, we find differences in the second layer. Further analysis reveals the origin of the deviations to arise from the substrate-induced charge transfer into the first layer: Silver, which transfers more charge to the organic molecules, systamtically favors bilayers with larger electronic interactions than graphene, for which the charge-transfer is negligible.   

The role of adatoms for F4TCNQ adsorption on Au(111)

Organic adlayers on inorganic surfaces often contain adatoms, which can be incorporated within the adlayer and form well‑ordered frameworks. This causes notable modification of interface properties such as the adsorption geometries or the electronic structure of the adsorbed molecules.

Here we use DFT to study prototypical monolayers of F4TCNQ on Au(111), a system that is well‑known for embedding adatoms. In detail we investigate adatom‑caused modifications in the adsorption geometry, adsorption energy, charge transfer, and general electronic structure.

To study the adsorption geometry within the experimentally confirmed unit cell, geometry optimizations were performed for the adlayer with and without adatoms, yielding different adsorption geometries due to alteration of the electronic structure.

An analysis of the electronic states reveals an adatom‑driven modification of the occupation of adlayer states. This causes a substantial change in the net charge transfer at the interface, and, therefore, the work function at the interface. Furthermore, adatoms introduce interactions between neighboring adsorbate molecules via covalent coupling to the molecules. This covalent bonding character within the adlayer causes significant dispersion for adlayer bands at the Fermi level.   

Reversibly switching the work function of tetrachloropyrazine on Pt(111)

The functionality of organic electronic devices is to a large extent governed by the properties of its organic-inorganic interfaces. These properties are mainly determined by the structure and chemical composition of the interface. To date, research has mainly focused on controlling the structure and chemistry of the adlayer to tailor the interface properties. Here, we go one step further and investigate a system with switchable interface properties.

We realize such a switchable interface with tetrachloropyrazine (TCP) on Pt(111). TCP can either chemisorb or physisorb on the Pt(111) surface, forming a double-well potential. The two minima exhibit different adsorption geometries. These allow forming diverse interface structures with notably different work function changes and coherent fractions (obtained by X-ray standing wave measurements) which can be reversibly switched.

To model this switchable interface, we apply an extended version of the SAMPLE approach which facilitates structure search for commensurate as well as higher-order commensurate adlayers. This yields three different classes of interface structures with varying work function changes and coherent fractions. We demonstrate that external stimuli, such as temperature and pressure, enable reversible switching between these different classes in order to create a dynamic interface for applications in organic electronics.