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Two dimensional van der Waals heterostructures (2D are of significant interest due to their intriguing physical properties that are critically defined by the constituent monolayers and their interlayer coupling . However, typical inorganic 2 D vdWhs fall into the weakly coupled region, limiting efficient interfacial charge flow crucial for developing high performance quantum opto electronics. Here, we demonstrate strong interlayer coupling in Cu3 BHT (BHT = benzenehexathiol) graphene vdWhs an organic inorganic bilayer characterized by prominent interlayer charge transfer Monolayer Cu3 BHT with a Kagome lattice is synthesized on the water surface and then coupled with graphene to produce a cm2 scale 2D vdWh. Spectroscopic and electrical studies, along with theoretical calculation s show significant hole transfer from monolayer Cu3 BHT to graphene upon contact , being characteristic fingerprints for strong interlayer coupling This study unveils the great potential of integrating highly pi-conjugated 2D coordination polymers (2DCPs) into 2D vdWhs to explor e intriguing physical phenomena.

Quasi-two-dimensional conjugated polymers (q2DCPs) are polymers that consist of linear conjugated polymer chains assembled through non-covalent interactions to form a layered structure. In this work, w e report the synthesis of a novel crystalline q2D polypyrrole (q2DPPy) film at an air/H2 SO4 (95%) interface. The unique interfacial environment facilitated chain extension, prevented disorder, and resulted in a crystalline, layered assembly of protonated quinoidal chains with a fully extended conformation in its crystalline domains. This unique structure features highly delocalized π-electron systems within the extended chains, which is responsible for the low effective mass and narrow electronic bandgap. Thus w e investigated the temperature-dependent charge transport properties of q2DPPy using the van der Pauw (vdP) method and terahertz time-domain spectroscopy (THz-TDS). The vdP method revealed that the q2DPPy film exhibited a semiconducting behavior with a thermally activated hopping mechanism in long-range transport between the electrodes. Conversely, THz-TDS revealed a band-like transport, indicating intrinsic charge transport up to a record short-range high THz mobility of ∼107.1 cm2 V-1 s-1 . This article is protected by copyright. All rights reserved.

Heterogeneous reactions associated with porous solid films are ubiquitous and play an important role in both nature and industrial processes. However, due to the no-slip boundary condition in pressure-driven flows, the interfacial mass transfer between the porous solid surface and the environment is largely limited to slow molecular diffusion, which severely hinders the enhancement of heterogeneous reaction kinetics. Herein, we report a hierarchical-structure-accelerated interfacial dynamic strategy to improve interfacial gas transfer on hierarchical conductive metal-organic framework (c-MOF) films. Hierarchical c-MOF films are synthesized via the in-situ transformation of insulating MOF film precursors using π-conjugated ligands and comprise both a nanoporous shell and hollow inner voids. The introduction of hollow structures in the c-MOF films enables an increase of gas permeability, thus enhancing the motion velocity of gas molecules toward the c-MOF film surface, which is more than 8.0-fold higher than that of bulk-type film. The c-MOF film-based chemiresistive sensor exhibits a faster response towards ammonia than other reported chemiresistive ammonia sensors at room temperature and a response speed 10 times faster than that of the bulk-type film.

Surface-modification of phthalocyanine-based two-dimensional conjugated metal-organic framework (2D c-MOF) films by grafting aliphatic alkyl chains is developed for achieving high-performance polarity-selective chemiresistive sensing toward humidity and polar alcohols. 2D conjugated metal–organic frameworks (2D c-MOFs) are emerging as electroactive materials for chemiresistive sensors, but selective sensing with fast response/recovery is a challenge. Phthalocyanine-based Ni2[MPc(NH)8] 2D c-MOF films are presented as active layers for polarity-selective chemiresisitors toward water and volatile organic compounds (VOCs). Surface-hydrophobic modification by grafting aliphatic alkyl chains on 2D c-MOF films decreases diffused analytes into the MOF backbone, resulting in a considerably accelerated recovery progress (from ca. 50 to ca. 10 s) during humidity sensing. Toward VOCs, the sensors deliver a polarity-selective response among alcohols but no signal for low-polarity aprotic hydrocarbons. The octadecyltrimethoxysilane-modified Ni2[MPc(NH)8] based sensor displays high-performance methanol sensing with fast response (36 s)/recovery (13 s) and a detection limit as low as 10 ppm, surpassing reported room-temperature chemiresistors.

Organic photovoltaics based on non-fullerene acceptors (NFAs) show record efficiency of 16 to 17% and increased photovoltage owing to the low driving force for interfacial charge-transfer. However, the low driving force potentially slows down charge generation, leading to a tradeoff between voltage and current. Here, we disentangle the intrinsic charge-transfer rates from morphology-dependent exciton diffusion for a series of polymer:NFA systems. Moreover, we establish the influence of the interfacial energetics on the electron and hole transfer rates separately. We demonstrate that charge-transfer timescales remain at a few hundred femtoseconds even at near-zero driving force, which is consistent with the rates predicted by Marcus theory in the normal region, at moderate electronic coupling and at low re-organization energy. Thus, in the design of highly efficient devices, the energy offset at the donor:acceptor interface can be minimized without jeopardizing the charge-transfer rate and without concerns about a current-voltage tradeoff., It has been commonly believed that the driving force at the donor-acceptor heterojunction is vital to efficient charge separation in organic solar cells. Here Zhong et al. show that the driving force can be as small as 0.05 eV without compromising the charge transfer rate and efficiency.

Two-dimensional polymers (2DPs) are a class of atomically/molecularly thin crystalline organic 2D materials. They are intriguing candidates for the development of unprecedented organic���inorganic 2D van der Waals heterostructures (vdWHs) with exotic physicochemical properties. In this work, we demonstrate the on-water surface synthesis of large-area (cm2), monolayer 2D polyimide (2DPI) with 3.1-nm lattice. Such 2DPI comprises metal-free porphyrin and perylene units linked by imide bonds. We further achieve a scalable synthesis of 2DPI-graphene (2DPI-G) vdWHs via a face-to-face co-assembly of graphene and 2DPI on the water surface. Remarkably, femtosecond transient absorption spectroscopy reveals an ultra-fast interlayer charge transfer (ca. 60 fs) in the resultant 2DPI-G vdWH upon protonation by acid, which is equivalent to that of the fastest reports among inorganic 2D vdWHs. Such large interlayer electronic coupling is ascribed to the interlayer cation����� interaction between 2DP and graphene., publishedVersion

Diketopyrrolopyrrole (DPP)-based donor acceptor copolymers have gained a significant amount of research interest in the organic electronics community because of their high charge carrier mobilities in organic field-effect transistors (OFETs) and their ability to harvest near-infrared (NIR) photons in solar cells. In this study, we have synthesized four DPP based donor-acceptor copolymers with variations in the donor unit and the branching point of the solubilizing alkyl chains (at the second or sixth carbon position). Grazing incidence wide-angle X-ray scattering (GIWAXS) results suggest that moving the branching point further away from the polymer backbone increases the tendency for aggregation and yields polymer phases with a higher degree of crystallinity (DoC). The polymers were blended with PC70BM and used as active layers in solar cells. A careful analysis of the energetics of the neat polymer and blend films reveals that the charge-transfer state energy (E-CT) of the blend films lies exceptionally close to the singlet energy of the donor (E-D*), indicating near zero electron transfer losses. The difference between the optical gap and open-circuit voltage (V-OC) is therefore determined to be due to rather high nonradiative 418 +/- 13 mV) and unavoidable radiative voltage losses (approximate to 255 +/- 8 mV). Even though the four materials have similar optical gaps, the short-circuit current density (J(SC)) covers a vast span from 7 to 18 mA cm(-2) for the best performing system. Using photoluminescence (PL) quenching and transient charge extraction techniques, we quantify geminate and nongeminate losses and find that fewer excitons reach the donor-acceptor interface in polymers with further away branching points due to larger aggregate sizes. In these material systems, the photogeneration is therefore mainly limited by exciton harvesting efficiency.

We report on the graphene-assisted growth, crystallization, and phase transition of zinc phthalocyanine (ZnPc) vertically oriented single crystal nanopillars. Postcrystallization thermal annealing of the nanostructures results in a molecular packing change while maintaining the vertical orientation of the single crystals orthogonal to the underlying substrate. Grazing incidence X-ray diffraction and highresolution TEM studies characterized this phase transition from a metastable crystal phase to the more stable beta-phase commonly observed in bulk crystals. These vertical arrays of crystalline nanopillars exhibit a high-surface-to-volume ratio, which is advantageous for applications such as gas sensors. We fabricated chemiresistor sensors with ZnPc nanopillars grown on graphene and demonstrated its selectivity for ammonia vapors, and improvement in sensitivity in the beta-phase crystal packing pillars due to their molecular orientation increasing the exposure of the Zn2+ ion to the ammonia analyte. This work highlights the first morphology-retentive phase transition in organic single crystal nanopillars through simple postprocessing thermal annealing. This study opens up the possibility of molecular packing control without large variations in morphology, a necessity for high-performance devices and establishing structure-property relations.

Organic photovoltaics based on non-fullerene acceptors (NFAs) show record efficiency of 16 to 17% and increased photovoltage owing to the low driving force for interfacial charge- transfer. However, the low driving force potentially slows down charge generation, leading to a tradeoff between voltage and current. Here, we disentangle the intrinsic charge-transfer rates from morphology-dependent exciton diffusion for a series of polymer:NFA systems. Moreover, we establish the influence of the interfacial energetics on the electron and hole transfer rates separately. We demonstrate that charge-transfer timescales remain at a few hundred femtoseconds even at near-zero driving force, which is consistent with the rates predicted by Marcus theory in the normal region, at moderate electronic coupling and at low re-organization energy. Thus, in the design of highly efficient devices, the energy offset at the donor:acceptor interface can be minimized without jeopardizing the charge-transfer rate and without concerns about a current-voltage tradeoff.

Organic and molecular electronics is a promising field for many applications, particularly flexible electronics, since it allows for the processing of electronic circuits at low temperatures on a variety of different substrates, including flexible sheets. Despite continuous improvements in the charge carrier mobility of both n- and p-type thin-film transistors, the performance of these devices is not yet attractive for commercial applications. A substantial hurdle to the realization of effective organic-based digital circuits is the lack of scalable, reproducible high-charge carrier-mobility organic semiconductors with a low contact resistance and controllable threshold voltage in transistors. The threshold voltage control is required to optimize the performance of digital circuits. Previous approaches used doping or self-assembled monolayers to provide threshold voltage control in organic field-effect transistors (OFETs). However, neither of these methods offers a proper fine-tuning of the threshold voltage or a substantial on/off ratio. Both of these problems have been successfully solved in inorganic electronics by using the concept of remote doping. This doping results in a so-called modulation-doped field-effect transistor (MODFET). Thus, in the first part of this work, we present the concept of remote doping for controlling and fine-tuning the threshold voltage without compromising charge carrier mobility by using a hole-type conduction architecture at the junction between two organic semiconductors. The second part of this work is dedicated to developing, fabricating, and characterizing high-mobility single and polycrystalline rubrene OFETs by using scalable growth methods. Finally, in the last part, we apply the knowledge gained from the first two parts and design digital circuits of rubrene OFETs aiming for high-frequency applications.

Organic solar cells usually utilise a heterojunction between electron-donating (D) and electron-accepting (A) materials to split excitons into charges. However, the use of D-A blends intrinsically limits the photovoltage and introduces morphological instability. Here, we demonstrate that polycrystalline films of chemically identical molecules offer a promising alternative and show that photoexcitation of α-sexithiophene (α-6T) films results in efficient charge generation. This leads to α-6T based homojunction organic solar cells with an external quantum efficiency reaching up to 44% and an open-circuit voltage of 1.61 V. Morphological, photoemission, and modelling studies show that boundaries between α-6T crystalline domains with different orientations generate an electrostatic landscape with an interfacial energy offset of 0.4 eV, which promotes the formation of hybridised exciton/charge-transfer states at the interface, dissociating efficiently into free charges. Our findings open new avenues for organic solar cell design where material energetics are tuned through molecular electrostatic engineering and mesoscale structural control., Though single-material organic solar cells are attractive for next-generation photovoltaic technologies, designing new materials with ideal properties remains a challenge. Here, the authors report the use of orientation-dependent molecular electrostatics to realise efficient homojunction devices.

In this thesis, methods for improving charge carrier mobility and deposition conditions for the solution shearing of organic semiconductors for organic field-effect transistors (OFETs) are investigated. Electrical performance for OFETs is currently still limited by the charge carrier mobility, especially when high fabrication speeds are required. In this work, adaptations are made to the solution shearing method to enhance charge carrier mobility values and to increase the deposition speed and film uniformity of semiconductor films. The solution shearing method can be easily adapted to large-scale roll-to-roll fabrication, a low-cost and high throughput fabrication process. In this work, the fabrication of OFETs with both crystalline small-molecule and donor-acceptor polymer semiconductors as the active layer is performed, and significant improvements in charge carrier mobility and film formation are achieved. Specifically, the crystalline small-molecule semiconductor TIPS-pentacene is blended with the inert dielectric polystyrene, and solution shearing parameters are optimized to obtain highly-aligned crystalline films. The thin film with optimized morphology is deposited on a very thin polymer dielectric film, demonstrating the feasibility of high-performance OFETs (effective mobility of ~1.2 cm2 V-1s-1) and an ultra-low operating voltage (~1 V) – at the time a record value. To improve crystal growth, the solution shearing method is modified to add vibrations to the liquid during the coating process. The new coating method, named “piezoshearing”, allows the application of vibrations to the liquid during deposition through the attachment of a piezo actuator to the shearing blade. The piezoshearing is implemented to enhance crystal growth during the solution shearing of crystalline materials, and tests of piezoshearing for the material 2,7-Dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) demonstrate that substrate coverage can be increased due to induced stick-and-slip caused by the piezoshearing. Due to the unfavorable wetting conditions of semiconducting donor-acceptor polymer solutions on the commonly used low surface energy OFET substrates, conventional solution shearing is problematic. With piezoshearing, film deposition can be significantly improved. In particular, through piezoshearing the so-called stick-and-slip instabilities are mitigated, allowing the doubling of the shearing speed, and the deposition of smooth and ultrathin films (~7 nm). In addition to enabling higher coating speeds, piezoshearing also lowers the polymer material consumption by up to ~ 70% in comparison to the conventional solution shearing method. For some materials, piezoshearing is also found to increase the charge carrier mobility in OFET devices by up to two orders of magnitude. The piezoshearing is utilized for viscous polymer solutions, which are challenging to coat, and usually, result in non-uniform films. Three donor-acceptor polymer systems were tested, and morphology changes are observed for all materials when piezoshearing is applied. For one of the polymeric solutions, an increase in crystallinity is achieved, possibly accompanied by a change in the degree of alignment of the polymer chains. For two other polymer solutions with higher molecular weight chains, very smooth films were obtained with the piezoshearing – saving 30% of material. Without the application of vibrations, such materials yield very non-uniform films, with significant thickness variations, which is unsuitable for OFET devices. In summary, this work leads to significant improvements in the solution shearing of organic semiconductor materials by adding vibrations in the kHz range to the deposition process. The effects and benefits of utilizing the piezoshearing are demonstrated, and suggestions for further improvement and studies are made.:Contents 7 1.Introduction 11 Motivation 11 Outline 12 2.Theoretical Principles of Organic Electronic Materials and Devices 13 Organic Electronics 13 Organic Semiconductors 14 Charge Transport Mechanisms in Organic Semiconductors 16 Organic Field-effect Transistors 19 Operation 19 The Metal-Semiconductor Interface 22 The Dielectric 25 Film Morphology and Charge Transport in OFETs 27 Small Molecules 27 Semicrystalline Polymers 29 3.Solution Shearing and Control of Film Morphology 33 The Solution Shearing Method 34 Capillary Flow and the Pinned Contact Line. 36 Marangoni Flow 36 Shear Flow 37 Film Formation in Solution Shearing 38 Small Molecules 38 Polymers 43 Stick-and-slip Instabilities 50 Contact Angle Hysteresis and Stick-and-slip 52 Vibration-assisted Thin-film Solution Fabrication Methods 53 Effects on a Liquid stemming from Vibration 53 Relevant Characteristics 57 Vibrations and Thin-film Formation 58 Combining the Solution Shearing and Vibrations 61 4.Experimental Methods 63 Device Fabrication 63 Substrate Preparation 63 Electrode Evaporation . 65 Piezoshearing Setup 65 Thin-film Characterization 68 Cross-Polarized Optical Microscopy 68 Grazing Incidence Wide-Angle X-ray Scattering 71 Electrical characterization 77 Characterization 77 Mobility estimation and overestimation discussion 77 5.Alignment Improvement from Blending the Small molecule TIPS- pentacene with an inert Polymer 81 Introduction 81 Optimization of film morphology for TIPS-pentacene . 82 Device Fabrication 82 Electrical Characterization .. 83 Film morphology characterization 86 Fabrication of Ultra-low-voltage Operation Devices 96 Figure of Merit of this Study 97 6.Piezoshearing of Crystalline Materials 101 Introduction 101 Piezoshearing of Pristine TIPS-pentacene 102 Film Fabrication 102 Thin-film Characterization 102 TIPS-pentacene blended with PS in Toluene: Better Performing Devices 104 Piezoshearing of C8-BTBT 105 7.Addressing Stick-and-Slip Instabilities in solution-sheared films for Introduction 109 Device Fabrication 110 The Effect of Piezoshearing on Stick-and-Slip Instabilities 111 Increasing Shearing Speed 111 Thin-film Characterization 114 Electrical Characterization 116 Energy Barriers and Overcoming them with Vibration 119 Acceleration Threshold for Mitigating Stick-and-slip 122 8.Piezoshearing of Viscous Polymer Solutions 127 Introduction 127 Device Fabrication 128 DPP4DE-TT and Film Morphology 129 DPP6DO-TT, DPP6DO-T, and Faraday Instabilities 137 Thin-film Characterization 141 Piezoshearing as a Parametric Oscillator System 145 Solid Friction 146 Viscosity 146 Transition Between Regimes 147 9.Conclusion and Outlook 149 Conclusion 149 Outlook 150

Development of efficient and clean energy sources to meet the ever-increasing de- mand of humankind is one of the greatest challenges of the 21st century. There is a dire need to decarbonise the power sector, and the focus needs to shift to re- newable resources such as wind and solar energy. In this regard, organic solar cells are a promising and novel technology owing to its low carbon footprint, innovative applications, and possible integration into the current infrastructure. Due to its unique advantages, a considerable research effort has been put into its development in the last decades. As a result, the power conversion efficiency (PCE) of the organic photovoltaics has steadily risen from as low as 0.5% to around 17 % at the current stage. This improvement primarily originates from the better understanding of the underlying physical processes and as a result of extensive material development. In the most general case, organic solar cells consist of a binary blend of an electron donating and an electron accepting organic semiconductor forming the so-called ‘bulk-heterojunction’ (BHJ) morphology. Thermodynamics places an upper limit on the power conversion efficiency (PCE) of binary blend BHJ devices and for further enhancement in efficiency novel device concepts like the use of ternary blends and tandem device architectures is being investigated. In relation to these approaches, the development of low optical-gap (Eopt ≤ 1.5 eV) organic semiconductors has gained importance as these materials provide for the complementary absorption with respect to the other components and better harvesting of the solar spectrum. This work mainly deals with the investigation of low optical gap donor and acceptor materials for organic solar cells. We investigate the effect of the molecular structure on the device performance and the photophysical processes in the binary and ternary blend configuration. In the first part of the thesis, we study a family of low optical- gap diketopyrrolopyrrole (DPP) based polymers while varying the conjugated core and the branching position and length of the solubilizing alkyl side chains. The branching position of the side chains is found to have a significant influence on the polymers ability to crystallize, which in turn influences the mobility of free charge carriers. The branching position also affects the solubility of the polymer, which in turn influences the morphology of the bulk-heterojunction (BHJ) and ultimately the yield of photogenerated charge carriers. To investigate the electron transfer and charge separation dynamics in the blends consisting of DPP polymers and fullerene, we employed ultrafast pump-probe spec- troscopic techniques. In the spectroscopy data, we observe signatures suggesting an ultrafast electron transfer process and an efficient charge separation process due to the high mobility of the free charge carriers shortly after separation (∼10-100 ps). Lastly, we investigated indacenodithiophene (IDT) based non-fullerene acceptor (NFA) molecules. In particular, we studied the effect of fluorination on the device performance when these acceptors are blended with PTB7-Th and P3HT donor polymers. The kinetics of the photophysical processes in the binary and ternary blends are characterized using ultrafast spectroscopy and related to the morphology of the blend and the molecular structure of the acceptors. Overall, we investigated the structural variations in the DPP polymers and flu- orinated non-fullerene acceptor (NFA) molecules and suggest design rules for the synthesis of optimal DPP polymers and non-fullerene acceptors to achieve supe- rior device performance. Additionally, we also shed light on the phenomenological processes happening on an ultrafast time scale (0.2-1000 ps) in the binary and the ternary blends with the aim of developing a better understanding of the photophys- ical processes in these promising material systems.