Xiao-Yuan Hou

In this study, a long-term misunderstanding of the role of extraction barriers in transport layer (TL) optimization, is clarified. It is generally believed that high extraction barriers (>0.026 eV) result in low power conversion efficiency (PCE). However, the present research revealed that efficient charge separation could be achieved even with barriers as high as 0.3 eV. A comprehensive computational analysis of over 10 000 simulated solar cells was then conducted, and a universal design principle was identified: minimizing the voltage drop across the TL is key for maintaining high PCE, regardless of the extraction barrier. Experimental validation of this principle was followed in perovskite solar cells, and exceptional efficiency was achieved for a 0.3 eV barrier, which was widely considered impossible before. To make these results practical, an evaluation factor ( θ ) was developed, which integrates various TL parameters, including extraction barrier, thickness, carrier mobility, and dielectric constant into a single, easy-to-calculate metric. This tool enables rapid and overall assessment of TL effectiveness, significantly accelerating the design and optimization process.

Side Lobe Level (SLL) suppression is a challenging but essential part of frequency selective surface (FSS) structural design. The SLL of the radiation pattern is a key parameter demonstrating the directional anti-interference ability and mainly depends on the geometry of the FSS. However, the correlation between SLL and FSS structural parameters is extremely complicated. A minor FSS structure change may result in a drastic diversion in the sidelobe’s direction or level, making the SLL the most difficult parameter to optimize. In this paper, an efficient optimization method specifically for SLL suppression is proposed. We discretized the edges of split square ring FSS structures by binary representation and generated new patterns based on the genetic algorithm. Optimization results showed that in the most optimal structure, the SLL was −23.41 dB, exhibiting a 5.17 dB reduction. Meanwhile, the center frequency variation was less than 2%. Moreover, the computation time cost was reduced by over 90% compared with that of the fully discretized pattern optimization method, showing that this novel method was truly effective in getting SLL suppressed FSS structures.

Multiple objectives optimization of frequency selective surface (FSS) structures is challenging in electromagnetic wave filter design. For example, one of the sub-objectives, the sidelobe level (SLL), is critical to directional anti-interference, which is complicated and becomes the bottleneck for radar design. Here, we established a dynamic algorithm for fitness function to automatically adjust the weights of multiple objectives in the optimization process of FSS structures. The dynamic algorithm could efficiently evaluate the achieving probability of sub-objectives according to the statistical analysis of the latest individual distribution so that the fitness function could automatically adjusted to focus on the sub-objective difficult to optimize, such as SLL. Computational results from the dynamic algorithm showed that the efficiency of multi-objective optimization was greatly improved by 213%, as compared to the fixed-weighted algorithm of the fitness function. Specifically for SLL, the efficiency rate increased even better, up to 315%. More interestingly, the FSS structures were most improved while picking median value or golden section value as the reference value. Taken together, the current study indicated that the dynamic algorithm with fitness function might be a better choice for FSS structural optimization with SLL suppression and potentially for the better design of lower SLL radar.

Multiple objectives optimization of frequency selective surface (FSS) structures is challenging in electromagnetic wave filter design. For example, one of the sub-objectives, the sidelobe level (SLL), is critical to directional anti-interference, which is complicated and becomes the bottleneck for radar design. Here, we established a dynamic algorithm for fitness function to automatically adjust the weights of multiple objectives in the optimization process of FSS structures. The dynamic algorithm could efficiently evaluate the difficulty of sub-objective optimization according to the statistical analysis of the latest individual distribution so that the optimization could be tightly focused on the sub-objective hard to optimize, such as SLL. Computational results from the dynamic algorithm showed that the efficiency of multi-objective optimization was greatly improved by 213%, as compared to the fixed-weighted algorithm of the fitness function. Specifically for SLL, the efficiency rate increased even better, up to 315%. More interestingly, the FSS structures were most improved while picking median value or golden section value as the reference value. Taken together, the current study indicated that the dynamic algorithm with fitness function might be a better choice for more efficient optimization of FSS structures and potentially for the better design of lower SLL radar.

To deal with the invalidation of commonly employed series model and parallel model in capacitance–voltage (C–V) characterization of organic thin films when current injection is significant, a three-element equivalent circuit model is proposed. On this basis, the expression of real capacitance in consideration of current injection is theoretically derived by small-signal analysis method. The validity of the proposed equivalent circuit and theoretical expression are verified by a simulating circuit consisting of a capacitor, a diode, and a resistor. Moreover, the accurate C–V characteristic of an organic thin film device is obtained via theoretical correction of the experimental measuring result, and the real capacitance is 35.7% higher than the directly measured capacitance at 5-V bias in the parallel mode. This work strongly demonstrates the necessity to consider current injection in C–V measurement and provides a strategy for accurate C–V characterization experimentally.

Organic–inorganic hybrid perovskite thin films with nanostructured polycrystalline grains have shown great potential in various nanoscale optoelectrical applications. Among them, the field of electrical memory has fallen behind due to insufficient knowledge of the related resistive switching characters and mechanisms. In the present work, switching behaviors of perovskite memory devices are systematically analyzed by comparing them with organic memory devices. We found that decreasing the conductivity of a polycrystalline perovskite thin layer would lead to unipolar switching behaviors, which is supplementary to the present perovskite memory family where bipolar switching is commonly reported. Moreover, our proposed symmetrical device with a nanoscale heterolayer structure enables us not only to achieve highly reproducible unipolar switching devices but also to settle the argument whether microconducting channels exist within perovskite memory devices through characterizing the microscopic morphological homogeneity. Surprisingly, the scanning electron microscopy results show that partial 10 μm large perovskite grains would be decomposed into various 100 nm small grains under high external bias, indicating the presence of microconducting channels. Furthermore, energy-dispersive X-ray spectroscopy results together with photoluminescence results of the perovskite thin film before and after applying bias are nearly identical, demonstrating that microconducting channels are formed without any difference in compositions or optical properties. Our discoveries provide a practical strategy to achieve electrical storage via organic–inorganic hybrid perovskite thin-film devices.

An ultra-thin molybdenum(VI) oxide (MoO₃) modification layer can significantly improve hole injection from an electrode even though the MoO₃ layer does not contact the electrode. We find that as the thickness of the organic layer between MoO₃ and the electrode increases, the hole injection first increases and it then decreases. The optimum thickness of 5 nm corresponds to the best current improvement 70%, higher than that in the device where MoO₃ directly contacts the Al electrode. According to the 4,4-bis[N-(1-naphthyl)-N-phenyl-amino] biphenyl (NPB)/MoO₃ interface charge transfer mechanism and the present experimental results, we propose a mechanism that mobile carriers generated at the interface and accumulated inside the device change the distribution of electric field inside the device, resulting in an increase of the probability of hole tunneling through the injection barrier from the electrode, which also explains the phenomenon of hole injection enhanced by MoO₃/NPB/Al composite anode. Based on this mechanism, different organic materials other than NPB were applied to form the composite electrode with MoO₃. Similar current enhancement effects are also observed.

In order to study the electric-field-assisted generation of mobile holes, a capacitance-voltage method is applied to asymmetric capacitancelike devices. The concentration of mobile holes generated from charge-transfer complexes at the CuPc/MoO₃ interface was studied both theoretically and experimentally. Furthermore, the efficiency of charge generation at various electric fields was also calculated. The capacitance of the device with only a 0.2 nm MoO₃ layer first increased and then decreased during bias sweeping from 0 V to −30 V. The capacitance variation is in good agreement with theoretical calculation, which in turn strongly confirms the existence of electric-field-assisted generation mechanism of the mobile holes.