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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.

This dissertation will outline solution processable materials and fabrication techniques to manufacture flexible electronic devices from them. Conductive ink formulations and inkjet printing of gold and silver on plastic substrates were examined. Line patterning and mask printing methods were also investigated as a means of selective metal deposition on various flexible substrate materials. These solution-based manufacturing methods provided deposition of silver, gold and copper with a controlled spatial resolution and a very high electrical conductivity. All of these procedures not only reduce fabrication cost but also eliminate the time-consuming production steps to make basic electronic circuit components. Solution processable semiconductor materials and their composite films were also studied in this research. Electrically conductive, ductile, thermally and mechanically stable composite films of polyaniline and sulfonated poly (arylene ether sulfone) were introduced. A simple chemical route was followed to prepare composite films. The electrical conductivity of the films was controlled by changing the weight percent of conductive filler. Temperature dependent DC conductivity studies showed that the Mott three dimensional hopping mechanism can be used to explain the conduction mechanism in composite films. A molecular interaction between polyaniline and sulfonated poly (arylene ether sulfone) has been proven by Fourier Transform Infrared Spectroscopy and thermogravimetric analysis. Inkjet printing and line patterning methods also have been used to fabricate polymer resistors and field effect transistors on flexible substrates from poly-3-4-ethyleneoxythiophene/poly-4-sytrensulfonate. Ethylene glycol treatment enhanced the conductivity of line patterned and inkjet printed polymer thin films about 900 and 350 times, respectively. Polymer field effect transistors showed the characteristics of traditional p-type transistors. Inkjet printing technology provided the transfer of semiconductor polymer on to flexible substrates including paper, with high resolution in just seconds. / Ph. D.

Within the last decade, organic light-emitting diodes (OLEDs) have evolved to be one of the major players in the display panel market. For instance, in 2023, it is expected that about half of the produced smartphones incorporate an OLED display. This rapid development is based on the high image quality and fast response time of OLEDs compared to the previously dominating technology of liquid crystal displays. Additionally, OLEDs feature interesting properties like mechanical flexibility, areal light emission, and semi-transparency that allow for futuristic device designs in display or lighting applications. One of the major drawbacks of OLEDs is their limited efficiency compared to conventional light-emitting diodes (LEDs). Due to the high refractive index of their active, light-emitting layers, a large portion of the internally generated light is lost because of optical effects like total internal reflection. One promising approach, which increases the amount of outcoupled light, is to align the transition dipole moment (TDM) of the emitter molecules parallel to the interface planes of the device. In the best case, this method can yield an efficiency improvement of more than 50%. This thesis focuses on both the accurate measurement and the exploration of control strategies of the emitter orientation. Furthermore, a software tool is developed, which supports the device design and data evaluation. First, the state-of-the-art emitter orientation measurement technique is analyzed, which is angle-resolved photoluminescence spectroscopy (ARPS). A ray-optics model is developed in order to quantify the impact of experimental deviations from the ideal measurement configuration. In particular, a displacement of the light-emission spot from the rotation center of the measurement setup is investigated. The resulting alteration of the observable angle-resolved emission spectrum is calculated and the impact on the consequent orientation factor is estimated. Based on the optical model, a refined setup structure is proposed, which not only circumvents a part of the identified problems but also yields a ten-times increased signal-to-noise ratio (SNR). Subsequently, it is explored how the emitter orientation can be controlled by external physical parameters during and after processing. Selected phosphorescent organic model systems are exposed to elevated temperatures and electric fields. In the first measurement series, the impact of the substrate temperature during deposition (𝑇sub) is investigated. It is found that the emitter orientation can be tuned from a more horizontal configuration at room temperature (RT) to an isotropic distribution if 𝑇sub approaches the glass transition temperature (𝑇g) of the material. This observation fits well to previous results of glass physics obtained with similar materials. In a second, alternative test series, OLEDs are treated after processing. Here, an emitter system comprising the host material NPB and the emitter Ir(piq)3 is investigated. For a treatment temperature of 125℃ and a simultaneously applied reverse bias of -20V the external quantum efficiency (EQE) of the OLEDs is increased by more than 50%. The effect is observed for two different emitter concentrations of 1 wt% and 10 wt% and OLEDs in the optical minimum and maximum. Finally, the long-term stability of the emitter orientation is experimentally demonstrated over a time frame of 1.5 years and for storage temperatures up to 95% of the host material’s 𝑇g. Preceding the presentation of the experimental results, the software-tool simojio is introduced, which is developed in this thesis. It enables an efficient and convenient workflow throughout the emitter orientation investigations and supports various tasks such as the device and setup design, the processing and visualization of xperimental data, and the extraction of orientation factors from angle-resolved emission spectra. Simojio provides a graphical user interface (GUI), which enables a flexible configuration of input parameters and one-dimensional layer structures. The corresponding numerical and graphical results are neatly arranged in a separate, tab-structured window. The actual calculation and processing algorithms are implemented in custom-made python modules, which can be modified and extended by the users according to their specific needs. Simojio is applied to most of the emitter orientation related simulation and data-processing tasks, which are presented in this thesis. However, due to its flexible, modular architecture, it is not restricted to this use case but may be utilized for highly diverse numerical problems, which are based on the evaluation of generic parameters or one-dimensional structures.

This thesis presents novel materials for photovoltaic conversion. The materials described are solution-processable organic semiconductors and have been used in the fabrication of organic photovoltaic cells (OPVs). The widely used PEDOT:PSS layer was investigated in P3HT and PTB7 photovoltaics. By doping, the efficiencies recorded were amongst the highest reported in the field using a conventional architecture. Two low band-gap BODIPY-based polymers were introduced and shown to have properties favourable for optoelectronics. Photovoltaics consisting solely of the polymers as the active component surpassed the performance expected without the use of an acceptor, indicating ambipolar behaviour, which was verified by charge carrier mobility measurements. When blended with an acceptor, the devices demonstrated a short-circuit current density similar to that of P3HT, a well-studied and successful OPV material. They also revealed a broad spectral response and were shown to operate as photodiodes. Two small molecules containing diketopyrrolopyrrole (DPP) and BODIPY were introduced and characterised. The addition of thiophenes red shifted the absorption but did not result in a sufficient bathochromic shift. Instead, a propensity to aggregate limited the performance. PLQY measurements showed the aggregation to quench luminescence. The study demonstrated the importance of controlling aggregation for efficient devices. Two solution-processable small molecules with a germanium-bridged spiro centre were investigated, and the molecular, electrochemical and optical properties discussed. The small molecule with shorter conjugation length exhibited an interesting packing motif shown to be favourable for charge transport. The mobility measurements were an order of magnitude higher than those reported for sexithiophene, a small molecule analogue, and the same order of magnitude as P3HT. The two-dimensional charge transporting nature of the material was verified with two independent techniques: time of flight (TOF) and organic field-effect transistor (OFET) measurements. The mobility of the material was found to vary with annealing, a result of morphological changes. These were studied with optical, electron and scanning probe microscopies. By controlling the morphology with the implementation of a well-defined annealing method, it was possible to improve the performance of OFETs and planar-heterojunction OPVs. Solution-processed bulk-heterojunction OPVs were fabricated, characterised and optimised with Ge spiro molecules. A PCE similar to that of P3HT, 2.66 %, was achieved for the one, whilst a PCE of 1.60 % was obtained for the other. The results are encouraging, and there is scope for improvement by increasing the overlap between the absorption and solar spectrum, for example.

This thesis describes a number of studies on organic semiconductors as laser gain media with the aim of simplifying the excitation scheme and exploring potential applications. A hybrid device taking the advantage of high power inorganic light emitting diodes (LEDs) and low threshold organic distributed feedback lasers is demonstrated to realize a LED pumped organic laser. When the drive current is higher than 152 A, a sharp peak is clearly observed in the laser output spectrum, implying the LED successfully pumps the polymer laser above threshold. This is the first time an incoherent LED has been used as the excitation source for an organic semiconductor laser. A strategy for further improving the performance of the hybrid device is explored with the use of a luminescent concentrator made of a dye doped SU8 film, to intensify the power density from the inorganic LED. The luminescent concentrator is capable of increasing the incident power density by a factor of 9 and reducing the lasing threshold density by 4.5 times. As a preliminary investigation towards mode-locked polymer lasers, the impact of a solid state saturable absorber on a solution based organic semiconductor laser is explored. The dye doped polystyrene thin film saturable absorber exhibits a saturation intensity of a few MW/cm². When it is placed into the laser cavity, a train of short pulses is generated and the underlying mechanism is discussed. Finally, the potential of using organic semiconductor lasers in the detection of nitro-aromatic explosive vapours is studied in distributed feedback polyfluorene lasers. A high sensing efficiency and fast response from the laser prove polyfluorene lasers can be used as disposal and low cost devices in explosive chemosensing.

The exciton diffusion length, which is the distance an exciton can diffuse in its lifetime, is an important parameter that has a critical impact on the operation of many organic optoelectronic devices, including organic solar cells, light emitting diodes and lasers. Knowledge of the exciton diffusion length can be a powerful aid for the design and optimisation of these devices. This thesis details the development of techniques based on time-resolved fluorescence for measuring the exciton diffusion in organic semiconductors. Two main methods were used to investigate exciton diffusion in the conjugated polymers P3HT, MEH-PPV and F8BT: the surface quenching technique and exciton-exciton annihilation. In particular, the surface quenching technique was adapted to avoid some of the potential pitfalls that have plagued earlier measurements. Using a titania quencher, measurements were performed using the surface quenching technique and fitted with an exciton diffusion model, allowing the calculation of the exciton diffusion length. Results from measurements of the exciton-exciton annihilation rate, which is a diffusion controlled process, where in good agreement with those from surface quenching, confirming the robustness of this twofold approach. A novel method for the control of the β-phase conformation in PFO films was used to produce films containing varying concentrations of β-phase. Exciton-exciton annihilation was used to investigate exciton diffusion in these films, revealing a gradual rise with increasing β-phase fraction due to improved interconnectivity. This work demonstrates how simple processing techniques can be used to control both film morphology and the exciton diffusion. The thickness dependence of the photoluminescence lifetime in conjugated polymers is a phenomenon that has so far received little attention and, thus, remained unexplained. This study demonstrates that it is not due to exciton quenching by external factors, but can be explained by a change in the morphology with decreasing film thickness.

FLUORINATED ARENE, IMIDE AND LACTAM-FUNCTIONALIZED DONOR ACCEPTOR CONJUGATED POLYMERS: SYNTHESIS, STRUCTURE-PROPERTY AND DEVICE STUDIES After the discovery of doped polyacetylene, organic semiconductor materials are widely studied as high impending active components in consumer electronics. They have received substantial consideration due to their potential for structural tailoring, low cost, large area and mechanically flexible alternatives to common inorganic semiconductors. To acquire maximum use of these materials, it is essential to get a strong idea about their chemical and physical nature. Material chemist has an enormous role to play in this novel area, including development of efficient synthetic methodologies and control the molecular self-assembly and (opto)-electronic properties. The body of this thesis mainly focuses on the substituent effects: how different substituent’s affect the (opto)-electronic properties of the donor-acceptor (D-A) conjugated polymers. The main priority goes to understand, how different alkyl substituent effect to the polymer solubility, crystallinity, thermal properties (eg: glass transition temperature) and morphological order. Three classes of D-A systems were extensively studied in this work. The second chapter mainly focuses on the synthesis and structure-property study of fluorinated arene (TFB) base polymers. Here we used commercially available 1,4-dibromo-2,3,5,6-tetrafluorobenzene (TFB) as the acceptor material and prepare several polymers using 3,3’-dialkyl(3,3’-R2T2) or 3,3’-dialkoxy bithiophene (3,3’-RO2T2) units as electron donors. A detail study was done using 3,3’-bithiophene donor units incorporating branched alkoxy-functionalities by systematic variation of branching position and chain length. The study allowed disentangling the branching effects on (i) aggregation tendency, intermolecular arrangement, (iii) solid state optical energy gaps, and (iv) electronic properties in an overall consistent picture, which might guide future polymer synthesis towards optimized materials for opto-electronic applications. The third chapter mainly focused on the structure-property study of imide functionalized D-A polymers. Here we used thiophene-imide (TPD) as the acceptor moiety and prepare several D-A polymers by varying the donor units. When selecting the donor units, more priority goes to the fused ring systems. One main reason to use imide functionality is due to the, open position of the imide nitrogen, which provides an attaching position to alkyl substituent. Through this we can easily manipulate solubility and solid state packing arrangement. Also these imide acceptors have low-lying LUMOs due to their electron deficient nature and this will allow tuning the optical energy gap by careful choice of donor materials with different electron donating ability. The fourth chapter mainly contribute to the synthesis and structure property study of a completely novel electron acceptor moiety consist of a unsaturated pyrrolidinone unit known as Pechmann dye (PD) core. Pechmann dyes are closely related to the Indigo family. This can refer as 3-butenolide dimer connected via an alkene bridge, containing a benzene ring at the 5 and 5’ positions of the lactone rings. We have prepared several D-A polymers using this PD system with benzodithiophene (BDT) as the donor unit. Different to common D-A polymers the HOMO and LUMO of the PD acceptor moiety are energetically located within the gap of the BDT, so that the electronic and optical properties (HOMO-LUMO transition) are dictated by the PD properties. The promising electronic properties, band gaps, high absorption coefficients and broad absorption suggest this new D-A polymers as an interesting donor material for organic solar cell (OSC) applications.

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