In donor:acceptor bulk heterojunction organic solar cells, the chemical miscibility between different components and phase evolution dynamics within thin films often induce phase segregation and molecular aggregation/ orientation, both of which are film-depth-dependent. This leads to strong variations of molecular energy levels, photon absorption, exciton generation, charge transfer, and transport along film-depth direction. However, currently there is a lack of comprehensive investigation of filmdepth-dependent optical and electronic variations on the photovoltaic performance. In this work, using the recently developed film-depthdependent light absorption spectroscopy which simultaneously reveals vertical optical and electronic variations, the performance of organic solar cells is correlated with film-depth-dependent profiles of photon absorption and charge transport energy levels, which is subsequently compared with experimentally observed open-circuit voltage, short-circuit current, and efficiency. Because both light interference and vertical material variations contribute to film-depth-dependent exciton generation profiles, the local gradient of transport energy levels which provides extra built-in electric force could accelerate the dissociation of excitons and transport of free charges to avoid recombination, leading to high photovoltaic performance. A new method is therefore proposed to improve the photovoltaic performance by simultaneously tuning the film-depth-dependent optical and electronic distributions.
The recent discovery of higher-order topology has largely enriched the classification of topological materials. Theoretical and experimental studies have unveiled various higher-order topological insulators that exhibit topologically protected corner or hinge states. More recently, higher-order topology has been introduced to topological semimetals. Thus far, realistic models and experimental verifications on higher-order topological semimetals are still very limited. Here we design and demonstrate a three-dimensional photonic crystal that realizes a higher-order Dirac semimetal phase. Numerical results on the band structure show that the designed three-dimensional photonic crystal is able to host two fourfold Dirac points, which are connected in the momentum-space projections via higher-order hinge states localized at the hinge. The higher-order topology can be characterized by the topological invariant χ (6) at different values of k z . An experiment at microwave frequencies is also presented to measure the hinge state dispersion. Our work demonstrates the physical realization of a higher-order Dirac semimetal phase and paves the way to explore higher-order topological semimetal phases in three-dimensional photonic systems.
Thin film organic field-effect transistors (OFETs) are usually featured with charge traps, limiting charge transport and leading to low mobility especially at low gate voltages. In this work, doping process of a model organic semiconductor 2,7-didodecyl[1]benzothieno [3,2-b][1]benzothiophene (C12-BTBT) has been manipulated, using low-cost reactive oxygen namely oxygen plasma or ozone. It is found that low-pressure (20 Pa) oxygen plasma can induce a low-moderate doping concentration, which is, however, sufficient to warrant field-effect mobilities over 5 cm 2 V À1 s À1 in a large gate voltage range and sharp subthreshold swings, without degradation of on/off ratio. Lowpressure oxygen plasma can also positively shift the threshold voltage of the device, thus largely reducing the working voltages. Oxygen plasma with higher pressure than 100 Pa induces higher doping concentration, more significantly shifting the threshold voltage and deteriorating on/off ratio due to substantial bulk electrical conductivity, which is similar to the treatment by UV-ozone at atmospheric pressure. As compared with the well-studied organic dopant F4-TCNQ, the doping process of reactive oxygen can be more easily in situ manipulated to reach an appropriate range of doping concentration, warranting higher performance and larger tunability for OFETs.
Spin–orbit coupling, a fundamental mechanism underlying topological insulators, has been introduced to construct the latter’s photonic analogs, or photonic topological insulators (PTIs). However, the intrinsic lack of electronic spin in photonic systems leads to various imperfections in emulating the behaviors of topological insulators. For example, in the recently demonstrated three-dimensional (3D) PTI, the topological surface states emerge, not on the surface of a single crystal as in a 3D topological insulator, but along an internal domain wall between two PTIs. Here, by fully abolishing spin–orbit coupling, we design and demonstrate a 3D PTI whose topological surface states are self-guided on its surface, without extra confinement by another PTI or any other cladding. The topological phase follows the original Fu’s model for the topological crystalline insulator without spin–orbit coupling. Unlike conventional linear Dirac cones, a unique quadratic dispersion of topological surface states is directly observed with microwave measurement. Our work opens routes to the topological manipulation of photons at the outer surface of photonic bandgap materials.
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