Dandelion-Inspired, Wind-Dispersed Polymer-Assembly Controlled by Light

Abstract The rise of stimuli-responsive polymers has brought about a wealth of materials for small-scale, wirelessly controlled soft-bodied robots. Thinking beyond conventional robotic mobilities already demonstrated in synthetic systems, such as walking, swimming and jumping, flying in air by dispersal, gliding, or even hovering is a frontier yet to be explored by responsive materials. The demanding requirements for actuator's performance, lightweight, and effective aerodynamic design underlie the grand challenges. Here, a soft matter-based porous structure capable of wind-assisted dispersal and lift-off/landing action under the control of a light beam is reported. The design is inspired by the seed of dandelion, resembling several biomimetic features, i.e., high porosity, lightweight, and separated vortex ring generation under a steady wind flow. Superior to its natural counterparts, this artificial seed is equipped with a soft actuator made of light-responsive liquid crystalline elastomer, which induces reversible opening/closing actions of the bristles upon visible light excitation. This shape-morphing enables manual tuning of terminal velocity, drag coefficient, and wind threshold for dispersal. Optically controlled wind-assisted lift-off and landing actions, and a light-induced local accumulation in descending structures are demonstrated. The results offer novel approaches for wirelessly controlled, miniatured devices that can passively navigate over a large aerial space.publishedVersionPeer reviewe

Hydrogel Lasers Via Supramolecular Host-Guest Complexation

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Enhancing the Microstructure of Perovskite-Inspired Cu-Ag-Bi-I Absorber for Efficient Indoor Photovoltaics

Lead-free perovskite-inspired materials (PIMs) are gaining attention in optoelectronics due to their low toxicity and inherent air stability. Their wide bandgaps (≈2 eV) make them ideal for indoor light harvesting. However, the investigation of PIMs for indoor photovoltaics (IPVs) is still in its infancy. Herein, the IPV potential of a quaternary PIM, Cu2AgBiI6 (CABI), is demonstrated upon controlling the film crystallization dynamics via additive engineering. The addition of 1.5 vol% hydroiodic acid (HI) leads to films with improved surface coverage and large crystalline domains. The morphologically-enhanced CABI+HI absorber leads to photovoltaic cells with a power conversion efficiency of 1.3% under 1 sun illumination-the highest efficiency ever reported for CABI cells and of 4.7% under indoor white light-emitting diode lighting-that is, within the same range of commercial IPVs. This work highlights the great potential of CABI for IPVs and paves the way for future performance improvements through effective passivation strategies.</p

Flexible organic photovoltaics with star‐shaped non‐fullerene acceptors end‐capped with indene malononitrile and barbiturate derivatives

We report the design and synthesis of three star-shaped non-fullerene (NFA) acceptors, TPA-2T-INCN, TPA-2T-BAB, and TPA-T-INCN, based on triphenylamine (TPA) core and linked through π-conjugated thiophene (T) spacers to different terminal units (3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile, INCN, and 1,3-dimethylbarbituric acid, BAB). These materials were blended with the widely used poly(3-hexylthiophene-2,5-diyl) (P3HT) donor polymer and tested in flexible organic photovoltaics (OPVs). The NFAs capped with the strong electron withdrawing INCN unit performed best in OPVs. Both P3HT:TPA-T-INCN, and P3HT:TPA-2T-INCN blends also showed the highest photoluminescence quenching efficiency (95.8% and 92.6%, respectively). Surprisingly, when reducing the number of T spacers from 2 to 1, the solubility of the NFAs in o-dichlorobenzene increased, leading to easier processing during the OPV fabrication and better surface morphology. This explains the best performance of TPA-T-INCN-based blends in OPVs, with a champion power conversion efficiency of 1.13%.publishedVersionPeer reviewe

Wavelength-tunable polymer distributed feedback lasers controlled by dielectric elastomer actuators

Thin- film organic distributed feedback (DFB) dye lasers are a topic of continuous research and development due to their quality to enable a narrow-band, single-mode emission applicable e.g. as compact biosensors or integrated lab-on-a-chip systems. If such DFB dye lasers are processed with elastomeric polymers they offer possibilities to continuously tune their spectral emission properties due the flexibility and stretchability of the elastomers. The combination with a dielectric elastomer actuator (DEA) enables the control of the emission wavelength by means of electro-mechanical actuation. This thesis presents effective and reproducible technological methods for fabrication, including new replication processes, of DFB laser and DEA module components, followed by techniques for their as sembling and integration to a single device. Within the scope of this work, a selection of compatible organic laser dyes, polymer matrices and solvents were investigated to achieve lasing in a broad spectral range of 575 700 nm. The modification of the polymer matrix through addition of plasticizers, as well as the development of a new ultra-thin DFB laser design leads to lower impact of the mechanical properties during actuation. Different measurement setups are used to examine the laser characteristics regarding the lifetime, optical gain, threshold and the efficiency. The e ort results in several wavelength tunable laser devices, which show emission within a spectral range of 615 - 692 nm, a total of 77 nm by means of mechanical tuning and within a spectral range of 608 - 656 nm, a total of 48 nm by means of electromechanical actuation. Further research was focused on the combination of distinct active layer systems with different periods and dyes onto a single DEA module in order to form a laser array with extended laser emission tuning range. The experiments reveal a tuning range of 573 - 659 nm, a total of 86 nm, the highest yet reported emission tuning on a single module

Digital holographic microscopy for real-time observation of surface-relief grating formation on azobenzene-containing films

Light-induced surface structuring of azobenzene-containing films allows for creation of complex surface relief patterns with varying heights, patterns which would be difficult to create using conventional lithography tools. In order to realize the full potential of these patternable surfaces, understanding their formation dynamics and response to different types of light fields is crucial. In the present work we introduce digital holographic microscopy (DHM) for real time, in-situ observation of surface-relief grating (SRG) formation on azobenzene-containing films. This instrument allows us to measure the surface topography of films while illuminating them with two individually controlled laser beams for creating periodically varying patterns. By utilizing the information of the grating formation dynamics, we combine multiple grating patterns to create pixels with wide gamut structural colors as well as blazed grating structures on the film surface. As long as the material behaviour is linear, any Fourier optical surface can be created utilizing this multiple patterning approach. The DHM instrument presented here has the potential for creating complex 3D surface reliefs with nanometric precision.publishedVersionPeer reviewe

Analysis of light diffraction by azobenzene-based photoalignment layers

Photoalignment materials, such as the azobenzene-based PAAD series studied here, are becoming increasingly important in liquid crystal-based optical devices and displays. Yet their properties and, in particular, their response to light, are still not fully understood. We investigate, experimentally and theoretically, the photoinduced birefringence, the order parameter and the formation of surface relief gratings, as well as the diffraction caused by them. We show that some of the azobenzene PAAD materials are suitable for the formation of surface relief gratings with high modulation depth, while others exhibit strong photoinduced birefringence. The two effects are inversely correlated: the stronger the surface relief grating is, the weaker is photoinduced birefringence. Analytical formulas based on the Raman-Nath approximation and numerical simulations of Maxwell’s equations are used to quantify the diffraction caused by the induced diffraction gratings, showing excellent agreement between theory and experiment

Enhancing the Microstructure of Perovskite-Inspired Cu-Ag-Bi-I Absorber for Efficient Indoor Photovoltaics

Lead-free perovskite-inspired materials (PIMs) are gaining attention in optoelectronics due to their low toxicity and inherent air stability. Their wide bandgaps (≈2 eV) make them ideal for indoor light harvesting. However, the investigation of PIMs for indoor photovoltaics (IPVs) is still in its infancy. Herein, the IPV potential of a quaternary PIM, Cu2AgBiI6 (CABI), is demonstrated upon controlling the film crystallization dynamics via additive engineering. The addition of 1.5 vol% hydroiodic acid (HI) leads to films with improved surface coverage and large crystalline domains. The morphologically-enhanced CABI+HI absorber leads to photovoltaic cells with a power conversion efficiency of 1.3% under 1 sun illumination—the highest efficiency ever reported for CABI cells and of 4.7% under indoor white light-emitting diode lighting—that is, within the same range of commercial IPVs. This work highlights the great potential of CABI for IPVs and paves the way for future performance improvements through effective passivation strategies.publishedVersionPeer reviewe