Torque-dense photomechanical actuation.

Contactless actuation powered using light is shown to generate torque densities approaching 10 N m kg-1 at angular velocities ∼102 rad s-1: metrics that compare favorably against tethered electromechanical systems. This is possible even though the extinction of actinic light limits the characteristic thickness of photoresponse in polymers to tens of µm. Confinement of molecularly patterned developable shells fabricated from azobenzene-functionalized liquid crystalline polymers encodes torque-dense photoactuation. Photostrain gradients from unstructured irradiation segment this geometry into two oppositely curved regions connected by a curved crease. A monolithic curved shell spontaneously bifurcates into a jointed, arm-like mechanism that generates flexure over sweep angles exceeding a radian. Strain focusing at the crease is hierarchical: an integral crease nucleates at smaller magnitudes of the prebiased curvature, while a crease decorated with point-like defects emerges at larger curvatures. The phase-space of morphogenesis is traceable to the competition between stretch and bending energies and is parameterizable as a function of the geometry. The framework for generating repetitive torque-dense actuation from slender light-powered actuators holds broader implications for the design of soft, remotely operated machines. Here, it is harnessed in illustrative mechanisms including levers, lifters and grabbers that are powered and regulated exclusively using light.

Thermomechanically active electrodes power work-dense soft actuators.

The effect of chain extender structure and composition on the thermomechanical properties of liquid crystal elastomers (LCE) synthesized using thiol-acrylate Michael addition is presented. The intrinsic molecular stiffness of the thiol chain extender and its relative molar ratio to acrylate-based host mesogens determine the magnitudes of the thermomechanical strains, temperatures at which they are realized and the mechanical work-content. A non-linear structure-property relationship emerges, wherein higher concentrations of flexible extenders first magnify the thermomechanical sensitivity, but a continued increase leads to weaker actuation. Understanding this interplay leads to a composite material platform, enabling a peak specific work production of ∼2 J kg-1 using ∼115 mW of electrical power supplied at 2 V. Composites of LCE with eGaIn liquid metal (LM) are prepared, which act as heaters, while being capable of actuation themselves. The thermomechanically active electrodes convert the electrical power into Joule heat, which they efficiently couple with the neat LCE to which they are bound. This system harnesses the nascent responsiveness of the LCE using electrodes that work with them, instead of fighting against them (or passively standing in the way). Specific work generated increases when subjected to increasing levels of load, reaching a peak at loads ∼260× the actuator weight. These ideas are extended to tri-layered actuators, where LCE films with orthogonal molecular orientations sandwich LCE-LM composite heaters. Torsional actuation modes are harnessed to twist under load.

Investigating ultrafast carrier dynamics in perovskite solar cells with an extended π-conjugated polymeric diketopyrrolopyrrole layer for hole transportation.

Here, we show a new diketopyrrole based polymeric hole-transport material (PBDTP-DTDPP, (poly[[2,5-bis(2-hexyldecyl)-2,3,5,6-tetrahydro-3,6-dioxopyrrolo[3,4-c]pyrrole-1,4-diyl]-alt-[[2,2'-(4,8-bis(4-ethylhexyl-1-phenyl)-benzo[1,2-b:4,5-b']dithiophene)bis-thieno[3,2-b]thiophen]-5,5'-diyl]])) for application in perovskite solar cells. The material performance was tested in a solar cell with an optimized configuration, FTO/SnO2/perovskite/PBDTP-DTDPP/Au, and the device showed a power conversion efficiency of 14.78%. The device charge carrier dynamics were investigated using transient absorption spectroscopy. The charge separation and recombination kinetics were determined in a device with PBDTP-DTDPP and the obtained results were compared to a reference device. We find that PBDTP-DTDPP enables similar charge separation time (<∼4.8 ps) to the spiro-OMeTAD but the amount of nongeminate recombination is different. Specifically, we find that the polymeric PBDTP-DTDPP hole-transport layer (HTL) slows-down the second-order recombination much less than spiro-OMeTAD. This effect is of particular importance in studying the charge transportation in optimized solar cell devices with diketopyrrole based HTL materials.