Narrow bandgap lead sulfide (PbS) quantum dots (QDs) are solution-processed materials used for optoelectronic applications in the short-wavelength infrared (SWIR) range (1400 - 3000 nm). The PbS QDs based photodetector has achieved comparable detectivity with current commercial SWIR sensors. However, there are still obstacles towards commercialization in commonly used layer by layer (LbL) deposition, such as high material consumption and low reproducibility. Here, we developed a new ligand exchange strategy to prepare ligand exchanged QD inks for single-step PbS film deposition. Compared with LbL deposition, the EQE of PbS QD photodetector made by single-step deposition has improved from 31% to 53%. The EQE and responsivity can be further improved to 95% with IR transparent electrode.
In this work, we demonstrated that through the addition of large organic cation PEABr, the quality of cesium lead halide thin films has been enhanced by controlling the crystallization process and thus the domain size and morphology. As a result, by increasing the PEABr concentration to 40%, the perovskite thin films with a small domain size, highly oriented crystal structure and low defect density demonstrated a lowest amplified spontaneous emission threshold of 7 µJ/cm2. However, further increasing PEABr concentration will result in a slower energy transfer, which limits the population inversion process and thus showed a higher ASE threshold. Using this strategy, by tuning the halide ratio, the ASE emissions were observed over the broad visible range (from 490 nm to 680 nm), demonstrating its wide wavelength-tunability.
Quasi-2D Ruddlesden-Popper halide perovskites with a large exciton binding energy, self-assembled quantum wells and high quantum yield draw attention for optoelectronic device applications. Thin films of these quasi-2D perovskites consist of a mixture of domains having different dimensionality, allowing energy funneling from lower-dimensional nanosheets (narrower bandgap domains) to three-dimensional nanocrystals (wide bandgap domains). Here, the quality of quasi-2D perovskite (PEA)2(FA)3Pb4Br13 is controlled by solution engineering. Grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements were conducted to study the crystal orientation, and transient absorption spectroscopy (TAS) measurements were conducted to study the charge carrier dynamics. Our data show that the highly oriented 2D crystals have a faster energy transfer to the low bandgap regime (< 0.5 ps) compared to less oriented films. High-performance LEDs can be realized with these 2D films. Finally, amplified spontaneous emission is achieved with an ultralow threshold of 4.16 μJ/cm2 and distributed feedback lasers are realized.
Our results show that it is important to control the morphology and spatial domain distribution of the quasi-2D films to achieve efficient energy transfer, which is a critical requirement for optoelectronic devices.
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