Highly efficient blue light-emitting diodes with tunable wavelength using vertically graded bandgap quasi-2D perovskite films

Metal halide perovskite materials have emerged as a unique class of solution process compatible semiconductors and alluring candidates for high-performance optoelectronic applications1,2,3, especially light-emitting diodes (LEDs), owing to high quantum efficiency, facile color tunability, narrow emission line widths, as well as cost-effectiveness4,5,6. Despite of the great successes on green and red perovskite LEDs (PeLEDs), the advancement of external quantum efficiency (EQE) of blue PeLEDs still lags far behind those of green and red PeLEDs7,8. Here, we demonstrate color-tunable blue PeLEDs devices with high EQE of 16.1% and 10% for emission wavelengths of 472 nm and 461 nm, respectively. The efficient tunable wavelength electroluminescence (EL) and high (EQE) originate from the optimization of the recombination zone position to reach the charge injection balance in the vertically graded bandgap quasi-2D perovskite materials. Under the synergetic effect of lead chloride (PbCl2) doping and propane-1,3-diammonium (PDABr2) incorporation, the vertically graded bandgap perovskite materials can be prepared by the self-regulation of the reduced-dimensional perovskite during the annealing process. Our work here has significantly elevated the performance of the current blue PeLEDs. It opens up a novel avenue to fabricate high-performance blue PeLEDs that can match up the performance of the green and red PeLEDs for future lighting and display applications.

A Numerical Investigation on the Combined Effects of MoSe2 Interface Layer and Graded Bandgap Absorber in CIGS Thin Film Solar Cells

The influence of Molybdenum diselenide (MoSe2) as an interfacial layer between Cu(In,Ga)Se2 (CIGS) absorber layer and Molybdenum (Mo) back contact in a conventional CIGS thin-film solar cell was investigated numerically using SCAPS-1D (a Solar Cell Capacitance Simulator). Using graded bandgap profile of the absorber layer that consist of both back grading (BG) and front grading (FG), which is defined as double grading (DG), attribution to the variation in Ga content was studied. The key focus of this study is to explore the combinatorial effects of MoSe2 contact layer and Ga grading of the absorber to suppress carrier losses due to back contact recombination and resistance that usually occur in case of standard Mo thin films. Thickness, bandgap energy, electron affinity and carrier concentration of the MoSe2 layer were all varied to determine the best configuration for incorporating into the CIGS solar cell structure. A bandgap grading profile that offers optimum functionality in the proposed configuration with additional MoSe2 layer has also been investigated. From the overall results, CIGS solar cells with thin MoSe2 layer and high acceptor doping concentration have been found to outperform the devices without MoSe2 layer, with an increase in efficiency from 20.19% to 23.30%. The introduction of bandgap grading in the front and back interfaces of the absorber layer further improves both open-circuit voltage (VOC) and short-circuit current density (JSC), most likely due to the additional quasi-electric field beneficial for carrier collection and reduced back surface and bulk recombination. A maximum power conversion efficiency (PCE) of 28.06%, fill factor (FF) of 81.89%, JSC of 39.45 mA/cm2, and VOC of 0.868 V were achieved by optimizing the properties of MoSe2 layer and bandgap grading configuration of the absorber layer. This study provides an insight into the different possibilities for designing higher efficiency CIGS solar cell structure through the manipulation of naturally formed MoSe2 layer and absorber bandgap engineering that can be experimentally replicated.