With the rapid development of non-fullerene acceptors, polymer donor design is critical for the realization of significantly higher power conversion efficiencies in organic solar cells (OSCs). The donor–acceptor (D–A) alternating structure is an effective strategy for the design of polymers due to its advantages in optical absorption, energy level modulation, charge mobilities, etc. Nevertheless, the current development of polymers is restricted in terms of modification of the acceptor unit. In this work, a 2D conjugated acceptor unit of benzobis(thiazole) was designed for constructing wide bandgap polymers. Fluorine and chlorine atoms were introduced into the polymers to modulate the energy levels and film morphology. The photovoltaic performances of the polymers were examined by fabricating devices employing IT-4F as the acceptor. All three polymers exhibit high short-circuit densities (J_SCs) of over 21 mA cm⁻² in the devices which could be attributed to high extinction coefficient and beneficial face-on orientation, as featured by the polymers based on the 2D conjugated BBT unit. Incorporation of F- or Cl-atoms effectively improved all three photovoltaic parameters open-circuit voltage (V_OC), J_SC and fill factor. As a result, a high power conversion efficiency of 14.8% was achieved for the chlorinated polymer (PBB-Cl). A high V_OC of 0.94 V and J_SC of 21.8 mA cm⁻² were realized in the devices, indicating well-balanced V_OC and J_SC values. Dominant face-on orientation was observed for all three polymer films, which is beneficial for charge transport in the vertical direction of the substrate. Given the excellent photovoltaic performance achieved in the devices, the polymer based on the 2D conjugated BBT unit would be a very promising candidate for high performance organic solar cells.

ConspectusHigh-precision neuromodulation plays a pivotal role in elucidating fundamental principles of neuroscience and treating specific neurological disorders. Optical neuromodulation, enabled by spatial resolution defined by the diffraction limit at the submicrometer scale, is a general strategy to achieve such precision. Optogenetics offers single-neuron spatial resolution with cellular specificity, whereas the requirement of genetic transfection hinders its clinical application. Direct photothermal modulation, an alternative nongenetic optical approach, often associates a large temperature increase with the risk of thermal damage to surrounding tissues.Photoacoustic (also called optoacoustic) neural stimulation is an emerging technology for neural stimulation with the following key features demonstrated. First, the photoacoustic approach demonstrated high efficacy without the need for genetic modification. The generated pulsed ultrasound upon ns laser pulses with energy ranging from a few μJ to tens of μJ is sufficient to activate wild-type neurons. Second, the photoacoustic approach provides sub-100-μm spatial precision. It overcomes the fundamental wave diffraction limit of ultrasound by harnessing the localized ultrasound field generated through light absorption. A spatial precision of 400 μm has been achieved in rodent brains using a fiber-based photoacoustic emitter. Single-cell stimulation in neuronal cultures in vitro and in brain slices ex vivo is achieved using tapered fiber-based photoacoustic emitters. This precision is 10 to 100 times better than for piezo-based low-frequency ultrasound and is essential to pinpoint a specific region or cell population in a living brain. Third, compared to direct photothermal stimulation via temperature increase, photoacoustic stimulation requires a 40 times less laser energy dose to evoke neuron activities and is associated with a minimal temperature increase of less than 1 °C, preventing potential thermal damage to neurons. Fourth, photoacoustics is a versatile approach and can be designed in various platforms aiming at specific applications. Our team has shown the design of fiber-based photoacoustic emitters, photoacoustic nanotransducers, soft biocompatible photoacoustic films, and soft photoacoustic lenses. Since they interact with neurons through ultrasound without the need for direct contact, photoacoustic enables noninvasive transcranial and dura-penetrating brain stimulation without compromising high precision.In this Account, we will first review the basic principles of photoacoustic and discuss the key design elements of PA transducers for neural modulation guided by the principle. We will also highlight how these design goals were achieved from a materials chemistry perspective. The design of different PA interfaces, their unique capability, and their applications in neural systems will be reviewed. In the end, we will discuss the remaining challenges and future perspectives for this technology.

A bidirectional brain interface with both “write” and “read” functions can be an important tool for fundamental studies and potential clinical treatments for neurological diseases. Herein, a miniaturized multifunctional fiber‐based optoacoustic emitter (mFOE) is reported thatintegrates simultaneous optoacoustic stimulation for “write” and electrophysiology recording of neural circuits for “read”. Because of the intrinsic ability of neurons to respond to acoustic wave, there is no requirement of the viral transfection. The orthogonality between optoacoustic waves and electrical field provides a solution to avoid the interference between electrical stimulation and recording. The stimulation function of the mFOE is first validated in cultured ratcortical neurons using calcium imaging. In vivo application of mFOE for successful simultaneous optoacoustic stimulation and electrical recording of brain activities is confirmed in mouse hippocampus in both acute and chronical applications up to 1 month. Minor brain tissue damage is confirmed after these applications. The capability of simultaneous neural stimulation and recording enabled by mFOE opens up new possibilities for the investigation of neural circuits and brings new insights into the study of ultrasound neurostimulation.

Neuromodulation at high spatial precision poses great significance in understanding the flow of information in the nervous system, thus providing enticing opportunities to decipher and manipulate the intricate organization of the mammalian brain. Clinically, neural stimulation has been successfully applied for treating neurological and psychiatric disorders. For example, electrode-based deep brain stimulation (DBS) has been applied to humans with epilepsy,¹ Alzheimer’s disease,² Parkinson’s disease,³ and treatment-resistant depression.⁴ Transcranial magnetic stimulation has been used for major depression⁵ and transcranial direct current stimulation works for the treatment of anxiety,⁶ pain,⁷ and chronic motor stroke.⁷ Besides, vision restoration has been achieved by stimulation of retinal ganglion cells (RGC) evoked by the optogenetic actuator ChrimsonR (or ChrimsonR fused to tdTomato, ChR-tdT), which served as an appealing modality in non-human primates^8,9 and blind patients¹⁰ with ongoing clinical trials (ClinicalTrials.gov identifier NCT02556736). Toward surgical applications, electrical stimulation of the individual dorsal root nerves has been applied to identify the abnormal one in selective dorsal rhizotomy (SDR) on children.^11,12