Yaxing Zhu

We elucidate a photocarrier collection mechanism in intermediate band solar cells (IBSCs) with InAs-quantum dots (QDs)-in-an-Al _0.3 Ga _0.7 As/GaAs-quantum well structures. When the Al _0.3 Ga _0.7 As barrier is excited, the device electrical output can be varied by additional infrared light for the electron intraband optical transition in QDs. The photocurrent in IBSC with a single QDs-in-a-well structure shows a monotonic increase with the intraband-excitation density. Conversely, IBSC with a multilayered QDs-in-a-well structure exhibits a photocurrent reduction when electrons in QDs are optically pumped out. The simultaneously measured photoluminescence spectra proved that the polarity of QD states changes depending on the intraband-excitation density. We discuss the drift and diffusion current components and point out that the hole diffusion current is significantly influenced by carriers inside the confinement structure. Under strong intraband excitations, we consider an increased hole diffusion current occurs by blocking hole-capture in the quantum structures. This causes unexpected photocurrent reduction in the multilayered device.

The generation of two-photon photocurrent plays an essential role in realizing intermediate-band solar cells (IBSCs) with high conversion efficiencies. This current generation process strongly depends on the photocarrier dynamics in the intermediate levels, which can sometimes give rise to a deficient output current unexpectedly. In this work, we investigated the two-photon photocurrent generation process in InAs quantum dot (QD)-in-well intermediate-band solar cells. The two-photon photocurrent is generated by an interband transition in the structure (for example, in the well or the QD) and a subsequent intraband transition in the QD, and we used two different light sources to separately control these transitions. We found that, in the case of QD interband excitation in a sample with multiple wells, the carrier collection efficiency does not simply increase with the intraband excitation intensity; in the range from about 0.08 to 0.5 W/cm², the collection efficiency decreases with increasing intraband excitation density. A comparison between samples with different numbers of wells revealed that the repetition of carrier trapping and detrapping during the transport in a multi-well structure can effectively modulate the recombination rate. This modulation induces a reduction of the current yield under certain illumination conditions. We propose a model to explain this phenomenon and verify it by investigating the bias dependence of the two-photon photocurrent from the QD.

A two-step photon upconversion (TPU) solar cell is a single-junction solar cell with a suitably designed heterointerface in the intrinsic layer. At this interface, upconversion can take place by absorbing infrared light, which excites carriers to energies above the heterojunction barrier, and then an electric field extracts these upconverted carriers to the barrier material. In this work, we study this photoabsorption process in a TPU solar cell with a modulation-doped structure. The modulation doping technique enables us to control the electric field at the heterointerface, and we demonstrate that the efficiency of the TPU process strongly depends on the electric field at the heterointerface. While we show that the TPU process induced by infrared light leads to a significant increase in the external quantum efficiency, we also note that this TPU process is an adiabatic process. It is shown that the adiabatic intraband photoexcitation at the heterointerface is able to increase the open-circuit voltage even above the open-circuit voltage that we predict from the diode equation and the actual current increase. We demonstrate obvious voltage boost effects and elucidate the relationship between the current and voltage.

Owing to the additional usage of sub-bandgap photons, the intermediate-band solar cell has been regarded as a promising device design to exceed the conversion limits of conventional photovoltaic devices. An output-voltage preservation is theoretically possible in this kind of device in the case of independent quasi-Fermi levels. This phenomenon manifests experimentally in a voltage recovery induced by supplementary two-step photon absorption processes. Here, we study the excitation-power and temperature dependences of the voltage performance in an intermediate-band solar cell containing InAs quantum dots in Al_0.3Ga_0.7As/GaAs quantum wells. The two-color photoexcitation method is used to separately control the interband and quantum dot-conduction band transitions. The output voltage is sensitive to the balance between the two excitation densities and the cell temperature. It is found that a strongly asymmetric irradiation can even lead to a voltage decrease. The temperature-dependent data suggest a faster electron–hole annihilation at lower temperatures. We introduce a new characteristic index to qualitatively evaluate the carrier loss in the intermediate band.

CoS₂/MoS₂ heteronanosheet arrays with vertically aligned flower-like architecture are fabricated through in situ topotactic sulfurization of CoMoO₄ nanosheet arrays.

Exploring earth-abundant and noble metal-free catalysts for water electrolysis is pivotal in renewable hydrogen production. Herein, a highly active electrocatalyst of nitrogen-doped porous carbon nanosheets coupled with Mo₂C nanoparticles (Mo₂C/NPC) was synthesized by a novel method with high BET surface area of 1380 m² g⁻¹ using KOH to activate carbon composite materials. The KOH plays a key role in etching out MoS₂ to produce Mo precursor; simultaneously, it corrodes carbon to form porous structure and produce reducing gas such as H₂ and CO. The resulting Mo₂C/NPC hybrid demonstrated superior HER activity in acid solution, with the overpotential of 166 mV at current density of 10 mA cm⁻², onset overpotential of 93 mV, Tafel slope of 68 mV dec⁻¹, and remarkable long-term cycling stability. The present strategy may provide a promising strategy to fabricate other metal carbide/carbon hybrids for energy conversion and storage.

Micro/nano scale architectures found in nature have unique functionalities, and their discovery has led to significant advancements in various fields of natural and engineering sciences, such as electronics, machinery, materials and energy. The piezoelectric micro/nano semiconductors, comprised of unique hierarchical structures and strain induced charges, are representative examples which inspired the development of various mechanics, electrical and environmental coupling systems, owing to their high sensitivity and fast responsivity to mechanical and humidity. In this paper, we use a hydrothermal method to synthesize Tellurium (Te) microwires, and fabricate a high sensitive humidity sensor based on single Te microwire through piezoelectric modulation. When applying stress to the Te microwire, the output current of the humidity sensor dramatically increased, experimental results and theoretical simulations indicated that the piezotronic effect could effectively tune the Schottky barrier height (SBH), thus enhancing the sensitivity of humidity sensor. The optimized sensitivity reaches to 1.6 × 10 ⁵ % through applied external strain at low humidity. This result demonstrates that with piezoelectric modulation, Te microwire can be serve as an effective semiconductor gas sensor.