Improved Design of Slope-Shaped Hole-Blocking Layer and Electron-Blocking Layer in AlGaN-Based Near-Ultraviolet Laser Diodes.

The injection and leakage of charge carriers have a significant impact on the optoelectronic performance of GaN-based lasers. In order to improve the limitation of the laser on charge carriers, a slope-shape hole-barrier layer (HBL) and electron-barrier layer (EBL) structure are proposed for near-UV (NUV) GaN-based lasers. We used Crosslight LASTIP for the simulation and theoretical analysis of the energy bands of HBL and EBL. Our simulations suggest that the energy bands of slope-shape HBL and EBL structures are modulated, which could effectively suppress carrier leakage, improve carrier injection efficiency, increase stimulated radiation recombination rate in quantum wells, reduce the threshold current, improve optical field distribution, and, ultimately, improve laser output power. Therefore, using slope-shape HBL and EBL structures can achieve the superior electrical and optical performance of lasers.

Morphologies and optical and electrical properties of InGaN/GaN micro-square array light-emitting diode chips.

InGaN/GaN micro-square array light-emitting diode (LED) chips (micro-chips) have been prepared via the focused ion beam (FIB) etching technique, which can not only reduce ohmic contact degradation but also control the aspect ratio precisely in three-dimensional (3D) structure LED (3D-LED) device fabrication. The effects of FIB beam current and micro-square array depth on morphologies and optical and electrical properties of the micro-chips have been studied. Our results show that sidewall surface morphology and optical and electrical properties of the micro-chips degrade with increased beam current. After potassium hydroxide etching with different times, an optimal current-voltage and luminescence performance can be obtained. Combining the results of cathodoluminescence mappings and light output-current characteristics, the light extraction efficiency of the micro-chips is reduced as FIB etch depth increases. The mechanisms of micro-square depth on light extraction have been revealed by 3D finite difference time domain.

Enhanced device performance and stability of perovskite solar cells with low-temperature ZnO/TiO2 bilayered electron transport layers.

The instability of perovskite films is a major issue for perovskite solar cells based on ZnO electron transport layers (ETLs). Here, ZnO nanoparticle (NP)- and ZnO sol-gel layers capped with low-temperature processed TiO2, namely ZnO/TiO2 bilayered films, have been successfully employed as ETLs in highly efficient MAPbI3-based perovskite solar cells. It is demonstrated that these ZnO/TiO2 bilayered ETLs are not only capable of enhancing photovoltaic performance, but also capable of improving device stability. The best device based on the ZnO/TiO2 bilayered ETL exhibits an efficiency of ∼15% under standard test conditions and can retain nearly 100% of its initial efficiency after 30 days of atmosphere storage, showing much higher device performance and stability compared to those devices based on ZnO single-layer ETLs. Moreover, it is found that perovskite films and devices prepared on the single ZnO sol-gel ETLs are much superior to those deposited on the single ZnO NP-ETLs in both stability and performance, which can be ascribed to fewer surface hydroxyl groups and much smoother surface morphology of the ZnO sol-gel films. The results pave the way for ZnO to be used as an effective ETL of low-temperature processed, efficient and stable PSCs compatible with flexible substrates.

Towards understanding the initial performance improvement of PbS quantum dot solar cells upon short-term air exposure.

An initial improvement in performance of PbS quantum dot solar cells composed of one thick layer of PbS quantum dots (QDs) treated with tetrabutylammonium iodide (PbS-TBAI) and one thin layer of PbS QDs capped with 1,2-ethanedithiol (PbS-EDT) over short-term air exposure is widely observed. However, the underlying mechanisms still remain elusive. In the work, we sought to understand the mechanisms as well as their physicochemical origins using a combination of X-ray photoelectron spectroscopy (XPS) and various electronic measurements. It is found that the PbS-TBAI film plays a dominant role in the initial device performance improvement compared with the PbS-EDT film. The PbS-TBAI film is compensation doped upon short-term air exposure (one to three days) owing to the increase of Pb-O and/or Pb-OH species, enabling its energy band to align better with the electron transport layer for more efficient charge extraction. Moreover, it is demonstrated that the short-term air exposure is capable of reducing defects in the devices and improving the diode quality, resulting in an initial increase in device performance. This work contributes to the fundamental understanding of the surface chemistry changes of PbS quantum dots treated by different ligands over air-exposure and the role of surface chemistry of quantum dots in optimizing their photovoltaic performance.

Quantum dot PbS(0.9)Se(0.1)/TiO2 heterojunction solar cells.

We report on photovoltaic cells based on ternary PbS(0.9)Se(0.1) quantum dots utilizing a heterojunction type device configuration. The best device shows an AM 1.5 power conversion efficiency of 4.25%. Furthermore, this ternary PbS(x)Se(1-x) quantum dot heterojunction device has a peak external quantum efficiency above 100% at 2.76 eV, approximately 2.7× the bandgap energy. The ternary quantum dots combine the higher short circuit currents of the binary PbSe system with the higher open circuit voltages of the binary PbS system.