Symposium S.EL14 : New Materials Design for Organic Semiconductors Through Multimodel Characterization and Computational Techniques​

Until today, the two-step processing method represents an attractive route for the thin film formation of halide perovskites. However, a fundamental understanding about the film formation dynamics in case of spin coating methylammonium iodide (MAI) on PbI₂ has not been established yet. Here we apply in-situ optical spectroscopy during the two-step film formation of the model halide perovskite MAPbI₃ via spin coating. We identify and analyze in detail the optical features that occur in photoluminescence and corresponding absorption spectra during processing. We find that the film formation takes place in five consecutive steps, including the formation of a MAPbI₃ capping layer via an interface crystallization and the occurrence of an intense dissolution-recrystallization process. Consideration of confinement and self-absorption effects in the PL spectra, together with consideration of the corresponding absorption spectra allows to quantify the growth rate of the initial interface crystallization to be 12 nm/s. We find the main dissolution recrystallization process to happen with a rate of 445 nm/s, emphasizing its importance to the overall processing.

The rational design of phase purity for two-phase conjugated polymer systems is challenging but crucial for organic/printed electronics. In this presentation we report a ‘mixed-flow design’ concept for printing two-phase conjugated polymer systems promoting phase purity for application in both bulk-heterojunction solar cells and thin-film transistors. The key aspect of this work lies in the mixed-flow design concept with the integration of both laminar and extensional flows using a unique designed microstructured shear blade. The fluid simulation is utilized as a tool for the flow pattern design to induce the shear, stretch and push-out effects to achieve optimal polymer chain conformation for phase purity. Experimentally, this mixed-flow strategy enhances semiconductor blend thin film crystallinity and increases phase purity with proper percolation. The improved morphology leads to higher short-circuit currents, enhanced fill factors, and significantly improved power conversion efficiency (PCE, enhanced by ~50% compared with conventional blade coating method) for printed all-polymer solar cells. In addition, this printing technique also enhances the performance of all-semiconductor polymer ambipolar transistors (mobility = + ~70%) as well as unipolar semiconductor polymer-insulating polymer transistors (mobility = + ~ 100%), suggesting the versatility of this methodology for various two-phase conjugated polymer systems.

The chemical structure of donors (Ds) and acceptors (As) has been a limitation to the achievable power conversion efficiencies (PCEs) of bulk heterojunction (BHJ) active layers of binary D-A mixtures in organic photovoltaics. While new syntheses can be used to generate Ds and As, a holistic strategy that simultaneously improves open circuit voltage, short circuit current, and fill factor is necessary and has been pursued using different approaches in organic photovoltaics (OPVs) to PCEs. This holistic approach, though, has been elusive, due to morphological constraints and the inherent electronic structures of the components, leading to performance trade-offs. Ideally, both the morphology and electronic structure would lead to a maximization of light absorption, enhancement of exciton splitting, and ease of carrier extraction. Adding a third component has been done, resulting enhancement of either the morphology or electronic structure leading to PCE improvement, but only incremental, at best. However, using quarternary D-A blends, double cascading energy level alignment in BHJ organic photovoltaic active layers are realized, enabling efficient carrier splitting and transport, without perturbing the desired BHJ morphology. This has led to record-breaking PCEs of 17.78% where, by electronic structure and morphology optimization, simultaneous improvements of the open-circuit voltage, short-circuit current and fill factor are realized. This strategy opens numerous avenues to optimize light absorption, carrier transport, and charge transfer state energy levels by the chemical constitution of the components. The chemical structures of the Ds and As offer control over electronic structure and charge transfer state energy levels, enabling manipulation of hole-transfer rates, carrier transport, and non-radiative recombination losses.

Resonant soft X-ray scattering (RSoXS) has proven to be a powerful technique to measure the structure of organic semiconductor systems. A particularly common application is found in the determination of relative domain purity in organic solar cells (OSC). In polymer-fullerene systems, RSoXS has shown that higher average domain purity is generally correlated to OSC device performance. A simple interpretation of RSoXS purity measurements is complicated by the consideration that real morphologies may be composed of three or more phases, with mixed amorphous regions in addition to relatively pure, aggregated donor and acceptor domains. Scattering originating from orientational contrast is convoluted with phase purity information in OSC systems, further complicating analysis. A complete model that includes both molecular orientation and composition fluctuations is required.

Phase information is lost in RSoXS pattern collection, as it is in most radiation scattering experiments, and algorithmic reconstruction of the reciprocal-space pattern into a real-space model is not straightforward. Model development and forward pattern simulation with iterative comparison to experimentally collected data is the most common interpretation method. In RSoXS analysis these approaches are greatly accelerated by multimodal measurements that directly provide a real-space model of composition fluctuations. By applying atomic force microscopy (AFM) and transmission electron microscopy (TEM), we can construct real-space models for RSoXS interpretation that allow for the extraction of information that the RSoXS technique uniquely provides: the nanoscale molecular orientation. A mathematical and computational framework for forward-simulating RSoXs patterns from these real-space models will be demonstrated. Results from these approaches explain trends in device performance for both OSC and organic transistor devices.

Reflecting the general data revolution, knowledge-based methods are now also entering theoretical catalysis and energy related research with full might. Automatized workflows and the training of machine learning approaches with first-principles data generate predictive-quality insight into elementary processes and process energetics at undreamed-of pace. Computational screening and data mining allows to explore these data bases for promising materials and extract correlations like structure-property relationships. In this talk I will illustrate this general development on the basis of an in-house database of >64,000 organic molecular crystals for which charge-transport descriptors have been calculated from first principles [1]. Suitable clustering of this data leads for instance to engineered molecular crystals, in which promising scaffolds are functionalized with favorable side groups [2]. At the same time, smart visualization techniques like chemical space networks identify scaffolds with most room for optimization in hitherto least explored parts of materials space [3].

[1] C. Schober, K. Reuter, and H. Oberhofer, J. Phys. Chem. Lett. 7, 3973 (2016).
[2] C. Kunkel, C. Schober, J.T. Margraf, K. Reuter, and H. Oberhofer, Chem. Mater. 31, 969 (2019).
[3] C. Kunkel, C. Schober, H. Oberhofer, and K. Reuter, J. Mol. Model. 25, 87 (2019).

The field of organic photovoltaics is currently enjoying a major resurgence thanks to the development of increasingly performant combinations of polymer donors and nonfullerene-based acceptors. Power conversation efficiencies now approach the 18% mark in single-junction devices.
In this contribution, we will provide a theoretical description of of the factors that have enabled such advances, including:
- hybridization of the strongly absorbing local-exciton electronic states with the charge-transfer electronic states appearing at the donor-acceptor interfaces [1-2];
- minimization of the voltage losses through reduction of the nonradiative recombination pathways [3]; and
- extent of order in the nonfullerene acceptor domains [4-5].

This work is supported by the Office of Naval Research.

[1] “Assessing the Nature of the Charge-Transfer Electronic States in Organic Solar Cells”, X.K. Chen, V. Coropceanu, and J.L. Brédas, Nature Communications, 9, DOI: 10.1038/s41467-018-07707-8 (2018).
[2] “Charge-Transfer Electronic States in Organic Solar Cells”, V. Coropceanu, X.K. Chen, T. Wang, Z. Zheng, and J.L. Brédas, Nature Reviews Materials, DOI: 10.1038/s41578-019-0137-9 (2019)
[3] “Design Rules for Minimizing Voltage Losses in High-Efficiency Organic Solar Cells”, D. Qian, Z. Zheng, H. Yao, W. Tress, T.R. Hopper, S. Chen, S. Li, J. Liu, S. Chen, J. Zhang, X.K. Liu, B. Gao, L. Ouyang, Y. Jin, G. Pozina, I. Buyanova, W. Chen, O. Inganäs, V. Coropceanu, J.L. Brédas, H. Yan, J. Hou, F. Zhang, A.A. Bakulin, and F. Gao, Nature Materials, 17, 703-709 (2018).
[4] “Low Energetic Disorder in Small-Molecule Non-Fullerene Electron Acceptors”, G. Kupgan, X.K. Chen, and J.L. Brédas, ACS Materials Letters, 1, 350-353 (2019).
[5] "An Extended Three-Dimensional Molecular Packing Structure Enables Highly Efficient Organic Solar Cells", G. Zhang, X.K.ai Chen, J. Xiao¹, P.C.Y. Chow, X. Jiao, G. Kupgan, C.C.S. Chan_, X. Du, M. Ren, R. Xia, Z. Chen, J. Yuan, Y. Zhang, S. Zhang, Y. Liu, Y. Zou, H. Yan, K.S. Wong, V. Coropceanu, N. Li, C.J. Brabec, J.L. Bredas, H.-L. Yip, and Y. Cao, submitted for publication (2019).

Organic solar cells (OSCs) are promising as an alternative solar energy technology and their efficiencies are continuously increasing, with record power conversion efficiency at ~ 17%. The processes involved in charge generation and recombination in the bulk heterojunction active layer govern device performance. However, these processes are all entangled and hard to measure quantitatively, which limits progress in optimizing the ultimate performance. We use time delay collection field (TDCF) to disentangle and fully quantify each process occurring in the current record OSC system PM6:Y6 as a function of processing co-solvent, chloronapthalene, concentration. TDCF is an advanced charge extraction technique that can separate charge generation and recombination processes. We use this to measure generation current, recombination current, and extraction current all at the operating condition of the OSC devices, fully quantifying each loss process. A field dependence of charge generation indicates geminate recombination and our measurements show that the additive eliminates this loss process in PM6:Y6. We additionally vary the excitation wavelength to selectively excite the accepter and the donor molecules. Our initial result suggests that the excited Y6 acceptor generates relatively less charge than the PM6 donor. This new understanding of charge generation and recombination processes will help in design of new materials and optimization of device fabrication for high-performance, economical, and massively scalable solar power.

Ion transport in organic materials is attracting tremendous attention due to the possibilities in applications such as electrochemical transistors, bioelectronics, sensors, soft robotics, and printed electronics. Poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS) shows promise as an ion transport material and has been integrated into biomedical applications while the nano-structure that enables this property is still unknown. Our previous study utilized Resonant Soft X-ray Scattering (RSoXS) which affords orders of magnitude higher contrast at an elemental absorption edge than electron or nonresonant X-ray techniques. There we showed the morphology consists of PEDOT-rich gel particles embedded within a PSS matrix and upon the addition of a co-solvent (Ethylene glycol) the PEDOT domains swell which was found to correlate with a reduction in ion flow. However, the measured nanostructures were dry and undoped, which do not directly demonstrate which domain facilitates ion flow. Here we will present our progress toward an in-situ RSoXS investigation of a PEDOT:PSS electrochemical transistor during ion transport. We will utilize an in-vacuum, microfluidic sample environment that is compatible with the newly developed RSoXS liquid-chamber at the Advanced Light Source. To achieve this goal, we modified the electrochemical device architecture and replaced the insulating ion barrier with no appreciable reduction in ion transport capabilities. Additionally, the fabrication of a miniaturized device on a micro-electromechanical system (MEMS) chip is under development. Understanding how the ions interact with the nano-structure to transduce an electric signal will enable these devices to potentially use in next generation medical devices.

Electronic devices based on organic semiconductors have great potential for future applications in devices for energy harvesting and electronics due to low costs, mechanical flexibility and tunable properties. Molecular doping of organic semiconductors is fundamental in order to optimize their electronic properties in high performance devices such as photovoltaic cells, thermoelectrics or light emitting diodes. However, the spatial distribution of these dopants is not completely controlled.[1] Previous studies have shown how density, distribution and diffusion of dopants can significantly affect electronic properties, disrupt the microstructure[2] and decrease long term stability of the host polymer.[3]

Electron microscopy has been used successfully in multiple studies characterizing organic semiconducting materials and extracting information with high spatial resolution. However, crucial information about the distribution of dopants and its dependence of the structure of the surrounding material still remains to be investigated with high spatial resolution. This work addresses the fine scale distribution of molecular dopants in a polar polythiophene with oligoethylene glycol side chains. The microstructure of the system and the distribution of dopants will be correlated to electronic properties at different doping concentrations. Concepts on how to control the distribution of the dopants will be presented.


[1] I. A. Jacobs, A. J. Moulé, Adv. Mater. 2017, 29, 1703063

[2] C.-Y. Chang et al., Nano Energy 55 (2019) 354–367

[3] L. Müller et al., Adv. Mater. 2017, 29, 1701466

SnO₂, as a crucial material in the field of energy storage, has always been the focus of scientific research. For organic-inorganic halide perovskite solar cells (PSCs) or dye-sensitized solar cells (DSSCs), SnO₂ affords high electron mobility and superior chemical stability amongst the various metal oxides. Moreover, as an alternative anode material for lithium-ion battery, it possesses a decent theoretical specific capacity of 1494 mAhg^-1. Compared with conventional bulk materials, mesoporous SnO₂ with high specific surface area affords more advantages when applied on either solar cells or lithium-ion batteries. By introducing the mesoporous structure, the recombination of injected electrons within the electron transport layer of the solar cell can be effectively inhibited. Furthermore, with this unique mesoporous structure, the structural collapse of lithium-ion battery anode provoked by significant volume expansion can also be effectively alleviated during the cycling process. In the present work, a novel amphiphilic block copolymer assisted sol-gel chemistry is used for the synthesis of porous SnO₂ nanostructure. Different kinds of solvent are used as a good solvent for both PS and PEO polymer chains, HCl is a selective poor solvent for PS chains and catalyst for the hydrolytic condensation reaction of SnO₂ precursor. With the progressive addition of HCl, the stretch of the hydrophobic PS chains is significantly restricted. As a result, different micelle structures of the block polymer are formed in the sol-gel solution, and the simultaneously hydrolysed SnO₂ nano-dot array is specifically confined in the PEO domains by hydrogen-bond interaction. With the evaporation of the solvent during spin-coating and annealing process, a distinct phase-separated SnO₂/PS-b-PEO composite thin film is obtained on the silicon substrate. For removing the organic polymer template, the composite thin film is calcined at 500 °C for two hours under ambient condition. The obtained nanostructure is visualized by scanning electron microscope (SEM). Crystallinity is characterized by X-ray diffraction (XRD). The buried structure within the thin film is investigated with grazing-incidence small-angle X-ray scattering (GISAXS) measurement.

Multi-color organic light emitting diodes (OLEDs) comprising just one active electro-optical material define the next vital step in the fast-developing field of photonic devices for optical data communication. Taking into account the spectral range of relevant transmission windows as well as the feasibility of coupling light to plasmonic nanostructures, an OLED device emitting at different wavelengths in the near infra-red (NIR) would be highly desirable. In this contribution we cope with these challenges by demonstrating a stable, dual luminescent OLED based on just one electro-optical active zinc phthalocyanine (ZnPc) layer [1].
ZnPc turns out to be a promising candidate for such single layer devices due to its different polymorphs [2] combined with specific emission characteristics, its photo-stability as well as luminescence in the long wavelength range between 800 and 900 nm which allows for efficient coupling to plasmonic excitations in metallic nanostructures [3].
Performing photoluminescence (PL) studies on ZnPc thin films over the full course of the phase transition between α- and β-polymorph, we are able to optically monitor the thermally induced structural changes and their impact on the respective emission over time. The Johnson-Mehl-Kolmogorov-Avrami model [4] can consistently describe the observed behavior of the transition kinetics derived by the distinct emission features of the involved α- and β-polymorphs. In combination with complementary structural studies and temperature dependent PL measurements from 4 K - 360 K this enables us to provide a comprehensive picture of the excited state emission and its correlation to the crystalline packing on molecular length scales. Vice versa, we exploit these morphology dependent emission characteristics to demonstrate a prototypical dual luminescent OLED comprising just one active layer of ZnPc. As will be shown, the emission properties can be spectrally tuned on demand between the excimer emission of the α-phase at 930 nm and the Frenkel emission of the β-phase at 780 nm by a suited thermal treatment. Considering the long-term stability of the adjusted spectral intensity distribution of the devices our work presents an approach towards simple organic photonic devices being of interest for future optical information technology.

[1] S. Hammer et al, Appl. Phys. Lett. 116, 1 (2020)
[2] J. M. Assour, The Journal of Physical Chemistry 69, 2295 (1965)
[3] V. Kolb and J. Pflaum, Optical Express 25, 6678 (2017)
[4] M. Avrami, The Journal of Chemical Physics 8, 212 (1940)

Electrochemical control of electronic charge (electrolyte gating) has opened a broad array of applications for organic electronic materials including printed electronics, biosensors, neuromophics, electrochromics, supercapacitors, and actuators to name a few. Structure-property relationships are critical for the advancement of these materials across application. Because electrolyte gating necessarily requires dynamic ion and solvent intercalation and expulsion during device operation, static structural measurements, including steady-state in situ and operando measurements, are critically limited. Understanding electrolyte gated organic materials requires understanding their dynamic structure, which in turn requires time-resolved in situ or operando techniques. Grazing incident wide angle X-ray scattering is a powerful tool to for accessing the microstructure of organic electronic materials. In particular, PEDOT:PSS is ubiquitous in organic electrolyte gated devices. Here the dynamic time-resolved operando GIWAXS of acid crystallized PEDOT:PSS films during aqeous electrolyte gating is reported. The majority of the structural transformation occurs over a narrow range of potential and the doping-dedoping structure dynamics are not symmetric, displaying very different time dependences. Coupling the time-resolved operando GIWAXS with time-resolved UV-vis-NIR reveals that the PEDOT:PSS structure depends not on the absolute charge density, but on the relative population dynamics of neutral, polaronic, and bipolaronic species. Even after the relative populations of charge species equilibrates, time-resolved GIWAXS shows continued long-lived structural relaxations. This complicated phenomena, which is inaccessible with steady-state ex situ, in situ, or operando methods, gives a concrete physical origin of the commonly observed asymmetry of electrolyte gated transistor turn on and turn off. Further it identifies the structural origin of the rate limiting physical processes to be targeted for rational materials design. Finally, this work represents the first report time-resolved X-ray scattering of an organic electronic material during electrochemical gating.

The use of aqueous / alcohol-based nanoparticulate dispersions in printable optoelectronics offers a promising approach to control the donor: acceptor morphology on the nanoscale with the benefit of environmentally-friendly, solution-based fabrication. Appropriate nanoscale morphology of the donor: acceptor composite nanoparticles (NPs), such as Janus structure, is the prerequisite for a well-suited mesoscale morphology formation to ensure an efficient charge transport. The final nanostructure of a composite NP is determined by the competition between thermodynamics and kinetics during the particle formation. However, fine tuning and control of these variables require prior observations and in-situ measurements. In this presentation, we will discuss our achieved results on the in-situ analysis of the size growth and morphology evolution of the colloidal organic composite NPs by employing a stopped-flow apparatus equipped with various optical spectroscopic methods.

The extrinsic properties of organic semiconductors (OSC) are deeply interconnected to their thin-film morphologies. To implement high performance and reproducible device fabrication guidelines and for synthetic regulation of the thin film morphologies, a rich understanding of the structure–processing–function relationships needs to be established. However, there currently exists limited insight to elucidate the physicochemical mechanisms that determine aggregation and self-assembly of OSC. Through molecular dynamics (MD) simulations, we aim to systematically understand how molecular design and processing chemistry impact the behavior of donor–acceptor (D–A) type oligomers in solution. In particular, we isolate the roles of the oligomer chemistry, oligomer concentration, and host solvent. Variations in these factors lead to a variety of rotational isomer preferences that, in turn, impacts the capacity of the oligomers to aggregate in solution. In total, the results provide molecular-scale foundations to allow for kinetic and thermodynamic control of OSC morphology development through solvent optimization.

The potential to modulate material (opto)electronic properties through well-established synthetic chemistry methods has made organic semiconductors (OSC) a scientific playground. Limited knowledge among the relationships that connect chemical composition and molecular architecture, materials processing, and the solid-state packing arrangements that define the underlying physicochemical processes that determine OSC performance, however, renders OSC design to be highly Edisonian. We seek to address these connections to facilitate OSC design and deployment through the development and application of multiscale, theoretical materials chemistry approaches that build upon principles from organic and physical chemistry, condensed matter physics, and materials and polymer science. In this presentation, we will demonstrate how such approaches can reveal the striking influence that seemingly modest changes in chemical structure can have on the dynamics and solid-state packing of OSC active layers and resulting materials characteristics.

Organic solar cells (OSCs) are considered one of the most promising cost-effective options for utilizing solar energy in high energy/weight or semi-transparent applications. Recently, the OSC field has been revolutionized by the development of novel non-fullerene small molecular acceptors with efficiencies now reaching >16%. The device stability and mechanical durability of non-fullerene OPVs have received less attention and developing devices with both high performance and long-term stability remains challenging, particularly if the material choice is restricted by roll-to-roll and benign solvent processing requirements and desirable ductility requirements. Yet, morphological and mechanical stability is a prerequisite for OPV commercialization. Here, we report our current understanding of the phase behavior of OPV mixtures and the relation of phase behavior to performance, processing needs (e.g., kinetic quenches), and morphological stability via meta-stability or vitrification. Characterization methods range from SIMS and DSC measurements to delineate phase diagrams and miscibility to soft x-ray scattering and WAXS to determine critical morphology parameters and molecule packing. A large range of miscibility (from hyper-miscibility to strong hypo-miscibility) is observed, including complex temperature dependence that can be a mixture of upper- and lower critical solution temperature behavior for both the binodal and the liquidus. The measurements presented should help to create molecular structure-function relationships that would allow some predictive guidance on how desired phase behavior and vitrification properties can be targeted by specific chemical design. They also allow to predict how unstable binary systems can be stabilized with the correct ternary compound.

Conjugated polymers have generated great interest due to their potential in the fabrication of deformable logic circuits that can be integrated into portable/wearable electronics. Here, we developed a beamline setup that can be capable of probing the bulk- and surface-sensitive data of polymer thin films during tensile test using soft x-ray. The combination of experimental and simulated x-ray spectroscopies assists to uncover the fingerprint of molecular behaviors under strain-stress measurement. Furthermore, to establish the relationship of the molecular evolution and device performances, the charge transport properties of the static strained thin film are also evaluated. We expect this work can contribute to understand the molecular origins of mechanical behaviors associated with the resulting device performances.

Thermally Activated Delayed Fluorescence (TADF) is one of the most promising mechanisms to realize high efficiency Organic Light Emitting Diodes (OLEDs) without the use of heavy transition metals and offers flexibility to fine-tune the electronic and optical properties of purely organic molecules. In TADF, nonemissive triplet excitons are harvested through a designed molecular emitter that undergoes efficient thermally activated reverse intersystem crossing (RISC) to a singlet manifold, which is followed by a radiative relaxation to the ground state. The rate of the RISC depends on the singlet-triplet energy gap, deltaE(ST), which needs to be minimized for an efficient TADF process. To minimize deltaE(ST), organic molecules with electron donor (D) and acceptor (A) groups are utilized, such that, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are localized on the donor and acceptor, respectively, giving spatially separated HOMOs and LUMOs, giving in a small deltaE(ST). While D-A molecules were the first examples of effective TADF materials, how this efficiency is achieved in devices is not understood. We use multiscale quantum-chemical methods to characterize the (opto)electronic properties of carbazole-based TADF emitters with known D-A groups. We benchmark our calculations with these systems. We will develop: (1) a comprehensive description, at the molecular level, of the fundamental photophysical processes of TADF emitters; (2) a modelling protocol that can accurately describe the electronic structure of such emitters; (3) structure-property relationships; and provide theoretical guidelines for the design of new materials and/or selection of existing materials with well-defined properties leading to devices with improved performances.

The electronic structure and exciton dynamics of the molecules and polymers that form the active layer in organic electronic devices can change dramatically during solution deposition. As solvent vaporizes, molecules aggregate and become electronically coupled, sometimes dramatically changing the exciton dynamics and thus the suitability of the material for electronic devices. The exciton dynamics of molecules in solution and in films of aggregates can be measured using transient absorption spectroscopy. However, the progression of exciton dynamics during film formation is unknown since measurements typically cannot be performed quickly enough to collect accurate transient absorption spectra of these species. The exciton dynamics of evolving material systems can be measured by increasing the speed of data collection. A novel implementation of transient absorption spectroscopy is introduced that can measure transient spectra with up to a 60 ps pump-probe time delay in one shot. The exciton dynamics of intermediate aggregation states are revealed during the formation of an organic film. The information gained using this technique can be used to modify environmental parameters during the film formation process to kinetically trap aggregates with exciton dynamics tailored for particular types of electronic devices.

Organic solar cells (OSCs) shows continued progress as an alternative photovoltaic technology, now with power conversion efficiency record ~ 17% and rising. For successful industrialization of OSCs, a few challenges need to be overcome such as high performance with thick active layers. Intense research has shown that active-layer nanostructure governs performance in bulk-heterojunction (BHJ) photovoltaics and uncovered some guidelines to optimum nanostructure. Unfortunately, many of those guidelines only apply to specific organic systems, are not quantitative, and do not target donor-acceptor (DA) interfaces.
Here, we present quantitative studies of active-layer domains and DA interfaces in multiple classes of organic photovoltaic systems, thus establishing universal structure-performance relationships. We utilize soft x-ray techniques (spectroscopy, microscopy, and scattering) to quantify domain size and composition, crystallinity, and mixing of donor-acceptor interfaces. We find that these parameters, specifically sharpness of DA interfaces have strong impacts on charge generation and recombination in both small-molecule and polymer based OSCs. In particular, our study shed light on a recent novel system that has high efficiency (>10%) with suitable BHJ thickness (~ 1 μm) for large-scale production. Our established structure-performance relationships will help in guiding molecular design and device engineering for industrial production of solar power.

For the last few decades, bulk heterojunction (BHJ) organic photovoltaics (OPVs) have been considered as strong candidates for next generation power source for portable or wearable electronic devices with their versatile properties such as low-cost, large-area solution based fabrication, light weight, and flexibility. For general binary blend based OPVs, rational design of polymer donor for optimization of its physical and chemical properties is probably the most decisive step to improve the efficiency. In this sense, introduction of halogen atoms like fluorine and chlorine to either backbone or side chain of conjugated polymer is one of the most favored strategies. Halogen atoms have strong electronegativity values so that fluorinated or chlorinated polymers have downshifted HOMO and LUMO energy levels. This is particularly beneficial for donor polymers since downshifted HOMO can contribute to increase in open circuit voltage (V_OC). Furthermore, small size fluorine atoms can induce molecular planarity to enhance the long-range ordering in the blend morphology, while large size chlorine atoms can reduce the unnecessary molecular aggregation. In other words, we can precisely control the blend morphology with fluorine and chlorine atoms. Herein, we demonstrated the positive effects of halogenation strategy to the OPV performance with three polymer donor series PBDT-X (X=H, F, and Cl). We could successfully achieve simultaneous control in the HOMO energy level and aggregation behavior of BDT-based polymer via introducing fluorine and chlorine atoms to side thienyl groups. Interestingly, PBDT-Cl showed a much enhanced processability and we could prepare super high M_n of PBDT-Cl for device fabrication. The PCEs dramatically increased from 9.03% for PBDT-H:Y6 to 12.68% for PBDT-F:Y6 and 15.11% PBDT-Cl:Y6 based OPV devices. To our best knowledge, the PCE over 15% is comparably high with state-of-the-art highly efficient binary OPVs.

Solution processing of electrically active layers is a promising route to sustainable manufacturing of functional components on diverse substrates such as flexible foils and textiles. Typically, solution processing does not result in the thermodynamic equilibrium form; instead metastable, kinetically trapped structures dominate. This allows great flexibility in the ability to tailor film structure (and performance) by processing details. In general, ink formulation (solvent, additives), processing conditions (casting temperature), and post processing (thermal and solvent annealing) are empirically optimized. The transition to rational process design requires in-situ measurements to determine the complex trajectory of the system through phase separation and domain refinement. Photon-based techniques are ideally suited to this problem, yet no single measurement provides the required knowledge of composition, nano- and meso- scale structure. Lab-scale techniques, such as UV-vis or IR spectroscopy, ellipsometry, and photoluminescence are easily implemented and rapid, yet rely on phenomenological relationships between observable and structure. Synchrotron based grazing incidence X-ray scattering techniques (both wide angle diffraction and small angle scattering) are more rigorous in interpretation but can be limited in contrast. By performing multiple measurements on a single system, either simultaneously, or separately, detailed models for the kinetic evolution of film structure can be developed. Results will be presented from a number of studies of solution processing of organic semiconducting layers for both photovoltaic and transistor applications. Studies of both ink deposition, via blade coating and post processing (thermal and solvent vapor annealing) will highlight insights into phase separation, liquid-liquid vs liquid-solid, and kinetics, nucleation and vitrification, afforded by multimodal analysis for each highlighted system. The role of fundamental material properties: semi-crystallinity, liquid crystallinity, glass transition temperature, etc. as guides to ink processing will be developed.

Organic mixed ionic/electronic conductors have gained considerable interest in bioelectronics, power electronics, circuits and neuromorphic computing. These organic, often polymer-based, semiconductors rely on a combination of ionic transport, electronic transport, and high volumetric charge storage capacity. Despite recent progress and a rapidly expanding library of new materials, the understanding of stability and transport/coupling of ionic and electronic carriers remain largely unexplored. We highlight recent synthetic and processing approaches used to tailor electrochemical device properties and stability, as well as new opportunities enabled by such advances. Our understanding of critical processes in electrochemical devices further requires us to study these materials in device-relevant conditions, fully considering the effects of ions and solvent on microstructure and transport. To this end, we report on recent efforts towards structure-property relations in high performance organic mixed conductors using ex-situ, in-situ, and operando scattering and spectroscopic techniques.

Despite significant advances in transmission electron microscopes, the resolution limit of polymers remains limited by the electron dose the sample can handle. We thus propose that revealing the mechanisms for radiation damage can yield new methods for imaging at length scales not previously achievable. We have characterized the effect of beam damage on various conjugated polymers, including poly(3-hexylthiophene-2,5-diyl) (P3HT), poly(3-dodecylthiophene-2,5-diyl) (P3DDT), poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT), and poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl]and poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3’’’-di(2-octyldodecyl)-2,2’;5’,2’’;5’’,2’’’-quaterthiophene-5,5’’’-diyl)] (PffBT4T-2OD), via electron diffraction and low-loss electron energy-loss spectroscopy (EELS). We find that the critical dose for damage depends on dose rate, temperature and beam size. Altogether, our results suggest that a significant mechanism for beam damage in conjugated polymers is diffusion of secondary reactive species, such as free radicals. These new concepts in beam damage reveal strategies to push the resolution in the TEM, allowing us to image 3.6 Å π-π stacking in PffBT4T-2OD with high-resolution TEM (HRTEM) at cryogenic conditions, 4D STEM again at cryo temperatures, and HRTEM at room temperature using antioxidants to limit damage. We use high resolution information to reveal pathways for charge conduction, including the type of grain boundaries present, as well as the effect of liquid crystallinity on promoting orientational correlations between domains. Together with our work on predicting liquid crystalline order in conjugated polymers, we use microscopy to develop strategies for designing new molecules with long-range order and strong orientational correlations that lead to enhancement of macroscopic charge transport.

Multiple exciton generation (MEG) through singlet fission (SF) is a spin allowed process whereby a singlet excited state is split into two triplet excitons. Inclusion of SF chromophores into solar cells raises the maximum theoretical efficiency of a solar cell from the Shockley-Queisser limit of 33% to around 45% by effectively harvesting the energy from high energy photons.
We have been examining self-assembly of intra-molecular singlet fission materials as a pathway to control local energy landscapes and assist the dephasing of the correlated triplet pair [1,2]. Decoupling “isolated” triplet generation from the initially generated correlated triplet pair remains a significant issue as long lived “dephased” triplets allow for a higher chance of exciton harvesting. We have demonstrated high solid-state SF yield (170%) in solution processed intra-molecular SF materials, with isolated triplet lifetimes in the microseconds [2], using pi-pi-stacking to generate hexagonal columnar structures with the diketopyrrolopyrrole (TDPP) triplet host chromophores on the outside of the stacks. New transient EPR measurements indicate support for a S₁→¹(TT)→⁵(T-T)→ T₃+T₃ mechanism, that is a pathway to the quintet coupled triplet pair, then to the isolated triplets from the initially generated correlated triplet pair.
In this presentation we will discuss the recent results from our structure property studies in the impact of self-assembly on SF yield and triplet lifetimes.

[1] Masoomi-Godarzi, S.; Liu, M. N.; Tachibana, Y.; Goerigk, L.; Ghiggino, K. P.; Smith, T. A.; Jones, D. J., "Solution−Processable, Solid−State Donor−Acceptor Materials for Singlet Fission" Adv. Energy Mater. 2018, 8 (30), 1801720
[2] Masoomi−Godarzi, S.; Liu, M.; Tachibana, Y.; Mitchell, V. D.; Goerigk, L.; Ghiggino, K. P.; Smith, T. A.; Jones, D. J., "Liquid−Crystallinity as a Self−Assembly Motif for High−Efficiency, Solution−Processed, Solid−State Singlet Fission Materials" Adv. Energy Mater. 2019, 9 (31), 1901069.

Structure formation is a complex interplay of physical interactions between the molecules of a system. In thin films structure formation can be triggered e.g. by the evaporation of the solvent or by heat treatments and is further influenced by a multitude of external parameters. Such external parameters can influence the nanostructure significantly and therefore alter the function of the involved materials. For some external parameters already small changes are sufficient to alter the final nanostructure within the thin film. If these are unidentified this results in poor reproducibility of the final thin film structure. For other external parameters the processing is rather stable. Often it is unclear what the physical reason for the sensitivity or stability towards the external parameters is. We want to show that time-resolved measurements, ideally on multi-length scales and with complementary methods will help to resolve fundamental structure formation processes in thin films. Comparing different material systems from printed organic photovoltaics to hybrid perovskite solar cells, I will point out the opportunities we have in interfering with the structure formation process to tune the material properties by external processing parameters.

Perovskite solar cells (PSCs) have gained tremendous attention as potential materials for photovoltaics due to their high efficiencies approaching the best silicon solar cells and their compatibility with low-cost low-temperature fabrication methods (such as solution processing). Many solution-processing approaches have been adopted to manipulate perovskite formation including anti-solvent processing, inert-gas jet treatment, and additive-assisted processing. Some of these approaches allow room-temperature processing of perovskite thin films, while other approaches require post-deposition annealing. Perovskite film formation is complex, involving the formation of intermediates and/or metastable phases that strongly affect the final perovskite film microstructure. Therefore, understanding the mechanism of perovskite formation and the crystallization pathways is key for more facile control of perovskite formation. An effective way to understand the mechanisms of perovskite formation is using real-time X-ray scattering. Here, we use time-resolved x-ray scattering to investigate the perovskite formation of MAPbI₃-based perovskites and mixed cation (Cs, FA)PbI₃ perovskites in-situ during spin coating and the subsequent post-deposition treatments with different processing approaches such as additive-assisted processing, anti-solvent processing and N₂-gas jet treatment.

Time-resolved monitoring of the perovskite thin film processing reveals the formation of intermediate phases on the route of perovskite formation, whereas perovskite formation is dominated by a sol-gel process. For MAPbI₃-based perovskites processed with the MASCN additive, we show that the nature of the intermediate precursor phases and their formation/dissociation dynamics have an impact on the extent of nucleation and growth of perovskite phase affecting the microstructure of the perovskite film. Our findings reveal that the combination of two precursors (MASCN-precursor and DMSO-precursor) with fast and slow transformation rates contributes to the formation of micron-sized perovskite crystals, through seeding nuclei combined with the slow growth of the perovskite phase. For Cs-FA-containing perovskites, we show how the competition between the non-perovskite and perovskite phases formation are affected by the processing treatments. We find that dropping anti-solvent induces immediate crystallization from the bulk wet film, whereas applying N₂-gas works by depleting volatiles from the top-surface leading to interface-induced crystallization that occurs after reaching supersaturation, while both treatments result in dominant crystallization of non-perovskite hexagonal phase with the formation of seeds of perovskite phase. When neither treatment is applied, the as-cast film is mostly amorphous with little non-perovskite phase formation. We further show how the initial structure of the as-cast precursor film impacts the perovskite formation during the subsequent annealing treatment.

Our work highlights the importance of real-time investigation of perovskite film formation which can aid in establishing processing-microstructure-functionality relationships and help to provide a fundamental understanding of the mechanisms of perovskite formation.

The materials discovery cycle contains many different components, including synthesis, characterization and data analysis and interpretation. In the past few decades, automatic synthesis pipelines have been established for many chemistry and materials systems. For characterization, many advanced techniques, such as X-ray scattering and NMR crystallography, have enabled the structure identification of various chemical, biological and materials systems, including polymers, inorganic materials, and proteins. These techniques have been developed and improved substantially over the past few decades, which brings high-throughput experimental discovery into reach. Meanwhile, these breakthroughs produce enormous data amounts. However, the process of understanding the structural features from data is still very labor-intensive. It requires many man-hours of work by highly specialized and trained scientific staff to interpret the data and identify the structure correctly. Recently, machine learning, a branch of artificial intelligence, has demonstrated the potential to tackle and accelerate the analysis of common techniques such as tomography, scattering, and spectroscopy.

Bulk heterojunction (BHJ) polymer solar cells have gained significant improvements via both novel organic synthesis and easy fabrication methods. Especially the fabrication through solution allows for large scale deposition processes such as roll to roll printing. Furthermore, the application of conjugated polymers as functional layers makes organic solar cells an attractive approach for a cost-effective solution to the current energy-shortage issue. Recently, the efficiencies of non-fullerene organic solar cells (OSCs) with small molecule acceptors rapidly increased to over 16%, which makes OSCs competitive to commercial available solid-state solar cells [1]. However, to make OPVs commercial competitive in the long term, overcoming degradation and achieving long device lifetime is of significant interest. The peeling-off of the electrode from the contact layer surface caused by aging stresses lead to an intrinsic degradation, which is one of the main reasons for the performance loss of OSCs [2]. Although thermal evaporation is a common method of fabricating the electrode due to its low cost, this technique brings several problems for the device such as soft contact between electrode and function layer interface. Another disadvantage is the inevitable high temperature during the fabrication process. Magnetron sputtering technique is a promising technique to solve these issues.
For understanding the mechanism of the metal cluster growth on the thin films with various morphologies, we introduce in-situ grazing incidence small angle X-ray scattering (GISAXS) is a technique to observe the morphology change during sputter process. In-situ GISAXS measurements during sputter deposition of Al nanolayers are conducted at the beamline P03 of DESY in Hamburg via using a DC magnetron sputter chamber. Active layers contain a PffDT4T-2OD donor with a small molecule accepter EH-IDTBR blender dissolved in 1,2,4-trimethyl benzene and chlorobenzene respectively to obtain different morphology. Then 10 nm MoO₃ blocking layers were deposited on their top surface, which usually acts as the electron blocking layer for the invert solar cell device. A 20 nm Al layer deposited on these four different layers through the sputtering technique. The morphology evolution during Al nanolayers growth observed in the out-of-plane cuts. The plane referred to is defined by the sample’s surface normal and the wave vector of the incident X-ray beam. All peaks of different thin films appear at lower effective thickness value. These peaks can be assigned to Al particles and clusters appearing during the sputtering process. It should be noted that the formation of Al layers with one layer MoO₃ deposited on the active layer is slower than the active layer without deposition of MoO₃.In Addition, SEM and AFM images indicate that the morphology impact on Al growth significantly.

[1]. Fan B, Zhang D, Li M, Zhong W, Zeng Z, Ying L, et al. Achieving over 16% efficiency for single-junction organic solar cells. Science China Chemistry. 2019.
[2]. Mateker WR, McGehee MD. Progress in Understanding Degradation Mechanisms and Improving Stability in Organic Photovoltaics. Advanced Materials. 2017;29(10):1603940.

In this presentation, I shall discuss what we can learn from comparatively simple spectroscopic measurements regarding the formation of aggregates in polymers and oligomers when spin-coating. I will demonstrate that the aggregation process is preceded by a planarization of the polymer backbone, and that it can be described like a coil-globule transitions [1]. In particular, the nature of the resulting aggregates can be controlled through the substrate temperature [2,3] which changes the nucleation process. This is illustrated for a range of widely used polymers such as P3HT, PFO, PCPDTBT, PCE11 and others.



[1] F. Panzer, H. Bässler, A. Köhler, JPC Lett 8 (2017) 114
[2] M. Reichenberger et al, JPolySci B 56 (2018) 532
[3] M. Reichenberger et al., Macromolecules 49 (2016) 6420-6430

Getting accurate performance predictions of organic semiconductors is crucial for the development of organic devices. Existing multi-scale analysis often relies on computationally expensive quantum-chemical calculations [1]. Machine learning approaches have been proposed to efficiently predict quantum-chemical quantities [2]. In this project, we present a multi-scale simulation for charge transport in an amorphous organic thin film of pentacene. The molecular structure of the pentacene film is obtained by Molecular Dynamics simulations. The transfer integrals between molecules are calculated with Density Functional Theory (DFT) methods and passed to a kinetic Monte Carlo simulation to calculate charge carrier mobility. Since DFT simulations for every possible molecule orientation are not feasible we use machine learning using kernel ridge regression to predict the transfer integrals [3]. Critical steps to obtain a well trained, highly predictive model is the feature design, the prior information embedding and the structure of the training dataset. Several techniques will be discussed involving these three aspects of Machine Learning applied to organic semiconductor properties.

[1] J. Kirkpatrick, et. al., PRL, 2007, 98, 227402
[2] K. Hansen, et. al., J. Chem. Theory Comput. 2013, 9, 3404
[3] J. Lederer, et. al., Adv. Theory Simul. 2018, 102, 1800136

A detailed understanding of the molecular and mesoscopic structure of a solution-processed organic, semiconducting thin film is still lacking, mostly due to the complicated film morphology. Recent progress, however, demonstrated the possibility of tuning the electronic properties of either short or long-range local molecular aggregates in films where high-performing semiconducting polymers are dispersed in a majority insulator and utilized in organic field effect transistors. Such a diluted blend matrix offers opportunities not only to reduce the cost associated with the semiconducting polymer significantly but also to systematically explore the fundamental molecular structure-function relationships. Herein, basic thermodynamic material interaction principles, in conjunction with detailed morphological studies, are invoked to understand the mesoscale structures leading to more efficient charge transport in the ultrathin films applicable for transparent and flexible electronic applications. Exploring two high-performing polymers, PDPP3T and N2200, we report a drastic variance in the change in charge transport properties of these two polymers when dispersed in an insulator matrix, due to the large difference in thermodynamics, resulting differences in molecular interaction and dimension of their fibrillar morphology. This basic finding provides design guidelines for semiconductor: insulator blends to achieve long-range ordered polymers by selecting compatible blends, and hence, high mobility in organic transparent ultrathin films.

Organic solar cells are an interesting alternative to conventional silicon based solar cells as the feature new possibilities introduced by using a different class of materials namely polymers. Instead of expensive ultra-high vacuum technologies, fabrication can be done at room temperature, using wet chemical processing, and thereby enabling usage of methods such as roll-to-roll printing. As a consequence, the production of organic solar cells has the potential to become very cheap and easy. With in-situ grazing incidence small and wide angle X-ray scattering (GISAXS and GIWAXS) studies, we gain information on the kinetics of morphology formation of electrodes, blocking layers and active layers of the solar cells during processing.
In terms of large scale usability, one of the major challenges for organic solar cells is to overcome their relatively short lifetime, as compared to their inorganic counterparts. To gain a deeper understanding of organic solar cell degradation with respect to changes in the active layer nano-morphology, we present an in-situ study on model polymer-fullerene solar cells during the first hours of operation. Thein- operando study reveals information on both, its evolving current-voltage characteristics and active layer nano-morphology. For that purpose, GISAXS / GIWAXS measurements and current-voltage (IV) tracking of an operating solar cell are performed simultaneously to gain fundamental understanding. Starting from an optimized morphology of the active layers in terms of highest efficiencies for organic solar cells, depending on the system, a mixing or demixing process are identified to cause changes of the morphology. The altered morphology is less optimal for charge transport through the active layer due to poor percolation in a too fine morphology or poor splitting of excitons in a too coarse morphology.

The chemical structure of donors (Ds) and acceptors (As) has been a limitation to the achievable power conversion efficiencies (PCEs) of bulk heterojunction (BHJ) active layers of binary D-A mixtures in organic photovoltaics. While new syntheses can be used to generate Ds and As, a holistic strategy that simultaneously improves open circuit voltage, short circuit current, and fill factor is necessary and has been pursued using different approaches in organic photovoltaics (OPVs) to PCEs. This holistic approach, though, has been elusive, due to morphological constraints and the inherent electronic structures of the components, leading to performance trade-offs. Ideally, both the morphology and electronic structure would lead to a maximization of light absorption, enhancement of exciton splitting, and ease of carrier extraction. Adding a third component has been done, resulting enhancement of either the morphology or electronic structure leading to PCE improvement, but only incremental, at best. However, using quarternary D-A blends, double cascading energy level alignment in BHJ organic photovoltaic active layers are realized, enabling efficient carrier splitting and transport, without perturbing the desired BHJ morphology. This has led to record-breaking PCEs of 17.78% where, by electronic structure and morphology optimization, simultaneous improvements of the open-circuit voltage, short-circuit current and fill factor are realized. This strategy opens numerous avenues to optimize light absorption, carrier transport, and charge transfer state energy levels by the chemical constitution of the components. The chemical structures of the Ds and As offer control over electronic structure and charge transfer state energy levels, enabling manipulation of hole-transfer rates, carrier transport, and non-radiative recombination losses.

Organic solar cells (OSCs) are considered one of the most promising cost-effective options for utilizing solar energy in high energy/weight or semi-transparent applications. Recently, the OSC field has been revolutionized by the development of novel non-fullerene small molecular acceptors with efficiencies now reaching >16%. The device stability and mechanical durability of non-fullerene OPVs have received less attention and developing devices with both high performance and long-term stability remains challenging, particularly if the material choice is restricted by roll-to-roll and benign solvent processing requirements and desirable ductility requirements. Yet, morphological and mechanical stability is a prerequisite for OPV commercialization. Here, we report our current understanding of the phase behavior of OPV mixtures and the relation of phase behavior to performance, processing needs (e.g., kinetic quenches), and morphological stability via meta-stability or vitrification. Characterization methods range from SIMS and DSC measurements to delineate phase diagrams and miscibility to soft x-ray scattering and WAXS to determine critical morphology parameters and molecule packing. A large range of miscibility (from hyper-miscibility to strong hypo-miscibility) is observed, including complex temperature dependence that can be a mixture of upper- and lower critical solution temperature behavior for both the binodal and the liquidus. The measurements presented should help to create molecular structure-function relationships that would allow some predictive guidance on how desired phase behavior and vitrification properties can be targeted by specific chemical design. They also allow to predict how unstable binary systems can be stabilized with the correct ternary compound.

Solution processing of electrically active layers is a promising route to sustainable manufacturing of functional components on diverse substrates such as flexible foils and textiles. Typically, solution processing does not result in the thermodynamic equilibrium form; instead metastable, kinetically trapped structures dominate. This allows great flexibility in the ability to tailor film structure (and performance) by processing details. In general, ink formulation (solvent, additives), processing conditions (casting temperature), and post processing (thermal and solvent annealing) are empirically optimized. The transition to rational process design requires in-situ measurements to determine the complex trajectory of the system through phase separation and domain refinement. Photon-based techniques are ideally suited to this problem, yet no single measurement provides the required knowledge of composition, nano- and meso- scale structure. Lab-scale techniques, such as UV-vis or IR spectroscopy, ellipsometry, and photoluminescence are easily implemented and rapid, yet rely on phenomenological relationships between observable and structure. Synchrotron based grazing incidence X-ray scattering techniques (both wide angle diffraction and small angle scattering) are more rigorous in interpretation but can be limited in contrast. By performing multiple measurements on a single system, either simultaneously, or separately, detailed models for the kinetic evolution of film structure can be developed. Results will be presented from a number of studies of solution processing of organic semiconducting layers for both photovoltaic and transistor applications. Studies of both ink deposition, via blade coating and post processing (thermal and solvent vapor annealing) will highlight insights into phase separation, liquid-liquid vs liquid-solid, and kinetics, nucleation and vitrification, afforded by multimodal analysis for each highlighted system. The role of fundamental material properties: semi-crystallinity, liquid crystallinity, glass transition temperature, etc. as guides to ink processing will be developed.

Organic solar cells are an interesting alternative to conventional silicon based solar cells as the feature new possibilities introduced by using a different class of materials namely polymers. Instead of expensive ultra-high vacuum technologies, fabrication can be done at room temperature, using wet chemical processing, and thereby enabling usage of methods such as roll-to-roll printing. As a consequence, the production of organic solar cells has the potential to become very cheap and easy. With in-situ grazing incidence small and wide angle X-ray scattering (GISAXS and GIWAXS) studies, we gain information on the kinetics of morphology formation of electrodes, blocking layers and active layers of the solar cells during processing.
In terms of large scale usability, one of the major challenges for organic solar cells is to overcome their relatively short lifetime, as compared to their inorganic counterparts. To gain a deeper understanding of organic solar cell degradation with respect to changes in the active layer nano-morphology, we present an in-situ study on model polymer-fullerene solar cells during the first hours of operation. Thein- operando study reveals information on both, its evolving current-voltage characteristics and active layer nano-morphology. For that purpose, GISAXS / GIWAXS measurements and current-voltage (IV) tracking of an operating solar cell are performed simultaneously to gain fundamental understanding. Starting from an optimized morphology of the active layers in terms of highest efficiencies for organic solar cells, depending on the system, a mixing or demixing process are identified to cause changes of the morphology. The altered morphology is less optimal for charge transport through the active layer due to poor percolation in a too fine morphology or poor splitting of excitons in a too coarse morphology.

Perovskite solar cells (PSCs) have gained tremendous attention as potential materials for photovoltaics due to their high efficiencies approaching the best silicon solar cells and their compatibility with low-cost low-temperature fabrication methods (such as solution processing). Many solution-processing approaches have been adopted to manipulate perovskite formation including anti-solvent processing, inert-gas jet treatment, and additive-assisted processing. Some of these approaches allow room-temperature processing of perovskite thin films, while other approaches require post-deposition annealing. Perovskite film formation is complex, involving the formation of intermediates and/or metastable phases that strongly affect the final perovskite film microstructure. Therefore, understanding the mechanism of perovskite formation and the crystallization pathways is key for more facile control of perovskite formation. An effective way to understand the mechanisms of perovskite formation is using real-time X-ray scattering. Here, we use time-resolved x-ray scattering to investigate the perovskite formation of MAPbI₃-based perovskites and mixed cation (Cs, FA)PbI₃ perovskites in-situ during spin coating and the subsequent post-deposition treatments with different processing approaches such as additive-assisted processing, anti-solvent processing and N₂-gas jet treatment.

Time-resolved monitoring of the perovskite thin film processing reveals the formation of intermediate phases on the route of perovskite formation, whereas perovskite formation is dominated by a sol-gel process. For MAPbI₃-based perovskites processed with the MASCN additive, we show that the nature of the intermediate precursor phases and their formation/dissociation dynamics have an impact on the extent of nucleation and growth of perovskite phase affecting the microstructure of the perovskite film. Our findings reveal that the combination of two precursors (MASCN-precursor and DMSO-precursor) with fast and slow transformation rates contributes to the formation of micron-sized perovskite crystals, through seeding nuclei combined with the slow growth of the perovskite phase. For Cs-FA-containing perovskites, we show how the competition between the non-perovskite and perovskite phases formation are affected by the processing treatments. We find that dropping anti-solvent induces immediate crystallization from the bulk wet film, whereas applying N₂-gas works by depleting volatiles from the top-surface leading to interface-induced crystallization that occurs after reaching supersaturation, while both treatments result in dominant crystallization of non-perovskite hexagonal phase with the formation of seeds of perovskite phase. When neither treatment is applied, the as-cast film is mostly amorphous with little non-perovskite phase formation. We further show how the initial structure of the as-cast precursor film impacts the perovskite formation during the subsequent annealing treatment.

Our work highlights the importance of real-time investigation of perovskite film formation which can aid in establishing processing-microstructure-functionality relationships and help to provide a fundamental understanding of the mechanisms of perovskite formation.

The field of organic photovoltaics is currently enjoying a major resurgence thanks to the development of increasingly performant combinations of polymer donors and nonfullerene-based acceptors. Power conversation efficiencies now approach the 18% mark in single-junction devices.
In this contribution, we will provide a theoretical description of of the factors that have enabled such advances, including:
- hybridization of the strongly absorbing local-exciton electronic states with the charge-transfer electronic states appearing at the donor-acceptor interfaces [1-2];
- minimization of the voltage losses through reduction of the nonradiative recombination pathways [3]; and
- extent of order in the nonfullerene acceptor domains [4-5].

This work is supported by the Office of Naval Research.

[1] “Assessing the Nature of the Charge-Transfer Electronic States in Organic Solar Cells”, X.K. Chen, V. Coropceanu, and J.L. Brédas, Nature Communications, 9, DOI: 10.1038/s41467-018-07707-8 (2018).
[2] “Charge-Transfer Electronic States in Organic Solar Cells”, V. Coropceanu, X.K. Chen, T. Wang, Z. Zheng, and J.L. Brédas, Nature Reviews Materials, DOI: 10.1038/s41578-019-0137-9 (2019)
[3] “Design Rules for Minimizing Voltage Losses in High-Efficiency Organic Solar Cells”, D. Qian, Z. Zheng, H. Yao, W. Tress, T.R. Hopper, S. Chen, S. Li, J. Liu, S. Chen, J. Zhang, X.K. Liu, B. Gao, L. Ouyang, Y. Jin, G. Pozina, I. Buyanova, W. Chen, O. Inganäs, V. Coropceanu, J.L. Brédas, H. Yan, J. Hou, F. Zhang, A.A. Bakulin, and F. Gao, Nature Materials, 17, 703-709 (2018).
[4] “Low Energetic Disorder in Small-Molecule Non-Fullerene Electron Acceptors”, G. Kupgan, X.K. Chen, and J.L. Brédas, ACS Materials Letters, 1, 350-353 (2019).
[5] "An Extended Three-Dimensional Molecular Packing Structure Enables Highly Efficient Organic Solar Cells", G. Zhang, X.K.ai Chen, J. Xiao¹, P.C.Y. Chow, X. Jiao, G. Kupgan, C.C.S. Chan_, X. Du, M. Ren, R. Xia, Z. Chen, J. Yuan, Y. Zhang, S. Zhang, Y. Liu, Y. Zou, H. Yan, K.S. Wong, V. Coropceanu, N. Li, C.J. Brabec, J.L. Bredas, H.-L. Yip, and Y. Cao, submitted for publication (2019).

Despite significant advances in transmission electron microscopes, the resolution limit of polymers remains limited by the electron dose the sample can handle. We thus propose that revealing the mechanisms for radiation damage can yield new methods for imaging at length scales not previously achievable. We have characterized the effect of beam damage on various conjugated polymers, including poly(3-hexylthiophene-2,5-diyl) (P3HT), poly(3-dodecylthiophene-2,5-diyl) (P3DDT), poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT), and poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl]and poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3’’’-di(2-octyldodecyl)-2,2’;5’,2’’;5’’,2’’’-quaterthiophene-5,5’’’-diyl)] (PffBT4T-2OD), via electron diffraction and low-loss electron energy-loss spectroscopy (EELS). We find that the critical dose for damage depends on dose rate, temperature and beam size. Altogether, our results suggest that a significant mechanism for beam damage in conjugated polymers is diffusion of secondary reactive species, such as free radicals. These new concepts in beam damage reveal strategies to push the resolution in the TEM, allowing us to image 3.6 Å π-π stacking in PffBT4T-2OD with high-resolution TEM (HRTEM) at cryogenic conditions, 4D STEM again at cryo temperatures, and HRTEM at room temperature using antioxidants to limit damage. We use high resolution information to reveal pathways for charge conduction, including the type of grain boundaries present, as well as the effect of liquid crystallinity on promoting orientational correlations between domains. Together with our work on predicting liquid crystalline order in conjugated polymers, we use microscopy to develop strategies for designing new molecules with long-range order and strong orientational correlations that lead to enhancement of macroscopic charge transport.

The electronic structure and exciton dynamics of the molecules and polymers that form the active layer in organic electronic devices can change dramatically during solution deposition. As solvent vaporizes, molecules aggregate and become electronically coupled, sometimes dramatically changing the exciton dynamics and thus the suitability of the material for electronic devices. The exciton dynamics of molecules in solution and in films of aggregates can be measured using transient absorption spectroscopy. However, the progression of exciton dynamics during film formation is unknown since measurements typically cannot be performed quickly enough to collect accurate transient absorption spectra of these species. The exciton dynamics of evolving material systems can be measured by increasing the speed of data collection. A novel implementation of transient absorption spectroscopy is introduced that can measure transient spectra with up to a 60 ps pump-probe time delay in one shot. The exciton dynamics of intermediate aggregation states are revealed during the formation of an organic film. The information gained using this technique can be used to modify environmental parameters during the film formation process to kinetically trap aggregates with exciton dynamics tailored for particular types of electronic devices.

The potential to modulate material (opto)electronic properties through well-established synthetic chemistry methods has made organic semiconductors (OSC) a scientific playground. Limited knowledge among the relationships that connect chemical composition and molecular architecture, materials processing, and the solid-state packing arrangements that define the underlying physicochemical processes that determine OSC performance, however, renders OSC design to be highly Edisonian. We seek to address these connections to facilitate OSC design and deployment through the development and application of multiscale, theoretical materials chemistry approaches that build upon principles from organic and physical chemistry, condensed matter physics, and materials and polymer science. In this presentation, we will demonstrate how such approaches can reveal the striking influence that seemingly modest changes in chemical structure can have on the dynamics and solid-state packing of OSC active layers and resulting materials characteristics.

The materials discovery cycle contains many different components, including synthesis, characterization and data analysis and interpretation. In the past few decades, automatic synthesis pipelines have been established for many chemistry and materials systems. For characterization, many advanced techniques, such as X-ray scattering and NMR crystallography, have enabled the structure identification of various chemical, biological and materials systems, including polymers, inorganic materials, and proteins. These techniques have been developed and improved substantially over the past few decades, which brings high-throughput experimental discovery into reach. Meanwhile, these breakthroughs produce enormous data amounts. However, the process of understanding the structural features from data is still very labor-intensive. It requires many man-hours of work by highly specialized and trained scientific staff to interpret the data and identify the structure correctly. Recently, machine learning, a branch of artificial intelligence, has demonstrated the potential to tackle and accelerate the analysis of common techniques such as tomography, scattering, and spectroscopy.