Impact of Glass Formation on the Thermal Stability of Non-Fullerene Solar Cells

The world is facing immense challenges such as climate change and the depletion of non-renewable resources, making renewable sources of energy essential for a sustainable future. Organic solar cells are emerging as a promising technology; however, their stability requires significant improvement. The nanostructure of the active layer evolves over time, especially during heating, leading to a degradation in device performance. The focus of this thesis is to improve the thermal stability of the active layer.Firstly, the thesis studies the impact of mixing on glass formation by introducing the concept of kinetic fragility to organic semiconductors. Model systems of up to eight perylene derivatives are investigated that demonstrate an unprecedented ability to form a stable molecular glass due to aggregate formation. Next, the thesis discusses the impact of isomers on glass formation, which is illustrated with an anthradithiophene-based compound. Binary mixtures of isomers were also found to form aggregates that stabilize the liquid state. In addition, the thesis describes fragility studies of doped systems and establishes that chemical doping can affect the glass formation of a semiconducting polymer. The doped polymer shows a strong tendency for glass formation which is assigned to restricted motion of oxidized polymer chains. Furthermore, the thesis analyzes mixtures of organic photovoltaic acceptors. Binary mixtures of two indacenodithienothiophene-based acceptors are found to co-crystallize, while mixtures of three to five fused-ring non-fullerene acceptors exhibited a reduced tendency to crystallize. Finally, the thesis discusses the use of acceptor mixtures for improving the thermal stability of organic photovoltaic devices. Ternary solar cell devices with two acceptors are discussed that show a stable nanostructure and improved thermal stability compared to binary devices. The thesis also explores hexanary devices that consist of five acceptor molecules, which exhibit excellent thermal stability. Therefore, the use of multicomponent acceptor mixtures is found to be a powerful tool for creating thermally stable organic solar cells

Room Temperature Dye Glasses: A Guideline Toward the Fabrication of Amorphous Dye Films with Monomeric Absorption and Emission

The morphology of films containing photoactive materials is crucial for the performance of solid-state dye applications. Organic dyes tend to crystallize due to their usually planar molecular structure and the resulting intermolecular interactions. This leads to inhomogeneous films with crystalline, aggregated, and amorphous regions, decreasing device efficiency and complicating spectral analysis. Improving the glass-forming ability of organic dyes therefore presents a major challenge for solid-state dye applications. Here, we present a guideline to create organic dye glasses using BODIPY as a model dye. The method is based on the strategic design of BODIPY derivatives, equipped with short alkyl chains, in combination with blending of two or more derivatives. Mixing increases the entropy of the liquid state and lowers the thermodynamic driving force for crystallization as well as the kinetic fragility of the system. This enables the fabrication of homogeneous thin films without any additives. In these films, the dye molecules are trapped in a glassy state, featuring monomeric absorption and emission. This strategy leads to a BODIPY material with an amorphous character in thin films, dropcast films, and bulk. Further, the strategy is based on thermodynamics and is therefore expected to be general, enabling the transformation of any dye molecule into a glass former

Roll-to-Roll Dyed Conducting Silk Yarns: A Versatile Material for E-Textile Devices

KGaA, Weinheim Textiles are a promising base material for flexible and wearable electronic applications such as sensors, actuators, and energy harvesters. An essential component in such electronic textiles (e-textiles) is electrically conducting yarns. Here, a continuous dyeing process is presented to convert an off-the-shelf silk sewing thread into a wash and wear resistant functional thread with a conductivity of about 70 S cm−1; a record high value for coated yarns. An aqueous ink based on the conducting polymer:polyelectrolyte complex poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is modified, to produce more than 100 m of dyed conducting threads, which are subsequently converted into e-textiles by both hand weaving and machine embroidery. The yarns are resistant to abrasion and wear, and can be machine washed at least 15 times with retained electronic properties. The woven fabric is used to design a capacitive touch sensor which functions as an e-textile keyboard

Vitrification of octonary perylene mixtures with ultralow fragility

Strong glass formers with a low fragility are highly sought-after because of the technological importance of vitrification. In the case of organic molecules and polymers, the lowest fragility values have been reported for single-component materials. Here, we establish that mixing of organic molecules can result in a marked reduction in fragility. Individual bay-substituted perylene derivatives display a high fragility of more than 70. Instead, slowly cooled perylene mixtures with more than three components undergo a liquid-liquid transition and turn into a strong glass former. Octonary perylene mixtures display a fragility of 13 \ub1 2, which not only is a record low value for organic molecules but also lies below values reported for the strongest known inorganic glass formers. Our work opens an avenue for the design of ultrastrong organic glass formers, which can be anticipated to find use in pharmaceutical science and organic electronics

Impact of oxidation-induced ordering on the electrical and mechanical properties of a polythiophene co-processed with bistriflimidic acid

The interplay between the nanostructure of a doped polythiophene with oligoether side chains and its electrical as well as mechanical properties is investigated. The degree of order of the polymer is found to strongly vary when co-processed with bistriflimidic acid (H-TFSI). The neat polythiophene as well as strongly oxidized material are largely disordered while intermediate concentrations of H-TFSI give rise to a high degree of π-stacking. The structural disorder of strongly oxidized material correlates with a decrease in the kinetic fragility with H-TFSI concentration, suggesting that positive interactions between TFSI anions and the polymer reduce the ability to crystallize. The electrical conductivity as well as the Young\u27s modulus first increase upon the addition of 4-10 mol% of H-TFSI, while the loss of π-stacking observed for strongly oxidized material more significantly affects the latter. As a result, material comprising 25 mol% H-TFSI displays an electrical conductivity of 58 S cm−1 but features a relatively low Young\u27s modulus of only 80 MPa. Decoupling of the electrical and mechanical properties of doped conjugated polymers may allow the design of soft conductors that are in high demand for wearable electronics and bioelectronics

A Record Chromophore Density in High-Entropy Liquids of Two Low-Melting Perylenes: A New Strategy for Liquid Chromophores

Weinheim Liquid chromophores constitute a rare but intriguing class of molecules that are in high demand for the design of luminescent inks, liquid semiconductors, and solar energy storage materials. The most common way to achieve liquid chromophores involves the introduction of long alkyl chains, which, however, significantly reduces the chromophore density. Here, strategy is presented that allows for the preparation of liquid chromophores with a minimal increase in molecular weight, using the important class of perylenes as an example. Two synergistic effects are harnessed: (1) the judicious positioning of short alkyl substituents, and (2) equimolar mixing, which in unison results in a liquid material. A series of 1-alkyl perylene derivatives is synthesized and it is found that short ethyl or butyl chains reduce the melting temperature from 278 \ub0C to as little as 70 \ub0C. Then, two low-melting derivatives are mixed, which results in materials that do not crystallize due to the increased configurational entropy of the system. As a result, liquid chromophores with the lowest reported molecular weight increase compared to the neat chromophore are obtained. The mixing strategy is readily applicable to other π-conjugated systems and, hence, promises to yield a wide range of low molecular weight liquid chromophores

Tuning of the elastic modulus of a soft polythiophene through molecular doping

Molecular doping of a polythiophene with oligoethylene glycol side chains is found to strongly modulate not only the electrical but also the mechanical properties of the polymer. An oxidation level of up to 18% results in an electrical conductivity of more than 52 S cm(-1) and at the same time significantly enhances the elastic modulus from 8 to more than 200 MPa and toughness from 0.5 to 5.1 MJ m(-3). These changes arise because molecular doping strongly influences the glass transition temperature T-g and the degree of pi-stacking of the polymer, as indicated by both X-ray diffraction and molecular dynamics simulations. Surprisingly, a comparison of doped materials containing mono- or dianions reveals that - for a comparable oxidation level - the presence of multivalent counterions has little effect on the stiffness. Evidently, molecular doping is a powerful tool that can be used for the design of mechanically robust conducting materials, which may find use within the field of flexible and stretchable electronics

Origins of the open-circuit voltage in ternary organic solar cells and design rules for minimized voltage losses

The power conversion efficiency of ternary organic solar cells (TOSCs), consisting of one host binary blend and one guest component, remains limited by large voltage losses. The fundamental understanding of the open-circuit voltage (V OC) in TOSCs is controversial, limiting rational design of the guest component. In this study, we systematically investigate how the guest component affects the radiative and non-radiative related parts of V OC of a series of TOSCs using the detailed balanced principle. We highlight that the thermal population of charge-transfer and local exciton states provided by the guest binary blend (that is, the guest-component-based binary blend) has a significant influence on the non-radiative voltage losses. Ultimately, we provide two design rules for enhancing the V OC in TOSCs: high emission yield for the guest binary blend and similar charge-transfer-state energies for host/guest binary blends; high miscibility of the guest component with the low gap component in the host binary blend

Hexanary blends: a strategy towards thermally stable organic photovoltaics

Non-fullerene based organic solar cells display a high initial power conversion efficiency but continue to suffer from poor thermal stability, especially in case of devices with thick active layers. Mixing of five structurally similar acceptors with similar electron affinities, and blending with a donor polymer is explored, yielding devices with a power conversion efficiency of up to 17.6%. The hexanary device performance is unaffected by thermal annealing of the bulk-heterojunction active layer for at least 23 days at 130 \ub0C in the dark and an inert atmosphere. Moreover, hexanary blends offer a high degree of thermal stability for an active layer thickness of up to 390 nm, which is advantageous for high-throughput processing of organic solar cells. Here, a generic strategy based on multi-component acceptor mixtures is presented that permits to considerably improve the thermal stability of non-fullerene based devices and thus paves the way for large-area organic solar cells

Impact of Glass Formation on the Thermal Stability of Non-Fullerene Solar Cells

As the world faces major challenges with the climate change and the depletion of non-renewable resources, development of renewable sources is crucial towards a more sustainable future. Organic solar cell is an excellent alternative as a renewable resource with many advantages. In contrast to conventional inorganic solar cells organic solar cells can be produced with printing techniques. Making up-scaling of the production possible, which reduces the price. In addition, organic solar cells are lightweight, flexible and semi-transparent. One drawback is the stability of organic solar cells which needs to be improved to achieve longer lifetime and higher durability.\ua0 The aim of this thesis is to investigate the use of multicomponent mixtures to improve the thermal stability of organic solar cells. The first part of the thesis explores the relation between entropy of mixing and glass formation. Perylene molecules with varying sidechains were mixed and it was found that mixing increases the glass formation, especially at low cooling rates where dimers where formed in the mixture. In the last part of the thesis, this mixing concept was used for organic solar cells. Binary devices of PTB7-Th: ITIC-4F and PTB7-Th: ITIC-4Cl were compared with the corresponding ternary solar cell PTB7-Th: ITIC-4F: ITIC-4Cl.\ua0 It was found that the crystals were suppressed in the ternary resulting in stable photovoltaic performance up to 170 \ubaC and in addition stable performance at 130 \ubaC for more than 200 h, while binary devices suffer crystallization in the active layer