Lead sulfide nanoparticles (PbS NPs) are used in the short-wavelength infrared photodetectors because of their excellent photosensitivity, band gap tunability, and solution processability. It has ...been a challenge to synthesize high-quality PbS NPs with an absorption peak beyond 2000 nm. In this work, using PbS seed crystals with an absorption peak at 1960 nm, we report a successful synthesis of very large monodispersed PbS NPs having a diameter up to 16 nm by multiple injections. The resulting NPs have an absorption peak over 2500 nm with a small full width at half-maximum of 24 meV. To demonstrate the applications of such large quantum dots (QDs), broadband heterojunction photodetectors are fabricated with the large PbS QDs of an absorption peak at 2100 nm. The resulting devices have an external quantum efficiency (EQE) of 25% (over 50% internal quantum efficiency) at 2100 nm corresponding to a responsivity of 0.385 A/W and an EQE of ∼60% in the visible range.
Lead sulfide (PbS) quantum dots are promising solution-processed materials for short-wave infrared (SWIR) photodetectors due to their tunable band gap and solution processability. Phase-transfer ...ligand exchange is a common method to prepare quantum dot (QD) inks used in device fabrication. For large-sized PbS QDs used for SWIR detection, the conventional phase-transfer ligand exchange has been problematic due to the densely packed organic ligands and charge-neutral (100) facets. Here, we report a new strategy to carry out the efficient phase-transfer ligand exchange in large-sized QDs. Specifically, using lead acetate trihydrate (PbAc2·3H2O) as a precursor and methylammonium acetate (MAAc) as an additive in the ligand solution, we can facilitate the efficient phase-transfer ligand exchange and epitaxial growth of perovskite intermediate (MAPbI3–x Ac x ) on the (100) facets, resulting in a significant improvement in film quality suitable for device fabrication. The resulting photodiodes show a 2.5× enhancement in external quantum efficiency (EQE) compared to devices using QD inks obtained using the conventional method. Considering the low transmittance of the ITO electrode in the SWIR regime, our devices exhibit an internal quantum efficiency of over 90%.
The light outcoupling efficiency of a top-emitting organic light-emitting diode (OLED) is only about 20%, and the majority of the light is trapped in the waveguide modes and surface plasmon polariton ...(SPP) modes. Extracting the trapped modes can reduce the device power consumption and improve the operating lifetime. In this study, we demonstrate a top-emitting OLED structure with a dielectric spacer to suppress the SPP mode and with a patterned back mirror to extract the waveguide modes. We examine and compare several curved mirror arrays and conclude that a micromirror array (μMA) can efficiently extract the waveguide modes while minimizing the absorption loss. The optimized μMA device with a semi-transparent top electrode shows a 36% external quantum efficiency, 2 times higher than the referenced device. This optical design can be easily incorporated into a top-emitting device and has a great potential for displays and lighting applications.
Organic light emitting diodes (OLEDs) with controlled light emission have many photonic applications. In this paper, using OLEDs for demonstrations and different photonic structures, we demonstrated ...highly polarized light emission from an OLED. The highly selective diffraction of only the TE waveguide mode is possible by selectively diffracting the TE waveguide mode while suppressing other optical modes. Similar devices are made with perovskites to achieve a current efficiency of 56 cd/A.
Light‐emitting diodes (LEDs) with directional and polarized light emission have many photonic applications, and beam shaping of these devices is fundamentally challenging because they are Lambertian ...light sources. In this work, using organic and perovskite LEDs (PeLEDs) for demonstrations, by selectively diffracting the transverse electric (TE) waveguide mode while suppressing other optical modes in a nanostructured LED, the authors first demonstrate highly directional light emission from a full‐area organic LED with a small divergence angle less than 3° and a TE to transverse magnetic (TM) polarization extinction ratio of 13. The highly selective diffraction of only the TE waveguide mode is possible due to the planarization of the device stack by thermal evaporation and solution processing. Using this strategy, directional and polarized emission from a perovskite LED having a current efficiency 2.6 times compared to the reference planar device is further demonstrated. This large enhancement in efficiency in the PeLED is attributed to a larger contribution from the TE waveguide mode resulting from the high refractive index in perovskite materials.
Highly directional and polarized light emission is demonstrated in thin‐film light‐emitting diodes (LEDs) by suppressing the background emission and extracting only the transverse electric (TE) waveguide mode. It is found that the TE waveguide mode increases with the refractive index of the emitting layer and a 2.6 times higher current efficiency is achieved in a perovskite LED.
Thin-film light-emitting diodes (LEDs) with linearly polarized emission enable advanced photonic applications. Here, we demonstrate a linearly polarized light emission organic LED by utilizing the ...intrinsic polarization nature of the transverse electric (TE) waveguide mode present in the device and extracting it using a linear grating. With a device having 67.6% of the light trapped in the TE waveguide mode, we demonstrated a linearly polarized organic LED with a high current efficiency (CE) of 136 cd/A and a large polarization ratio above 30. Our analysis indicates that the CE can further be increased at least by 2-fold to 280 cd/A. Our device architecture can easily be adopted for other thin-film LED platforms for photonic applications.
Perovskite light‐emitting diodes (PeLEDs) have received great attention for their potential as next‐generation display technology. While remarkable progress has been achieved in green, red, and ...near‐infrared PeLEDs with external quantum efficiencies (EQEs) exceeding 20%, obtaining high performance blue PeLEDs remains a challenge. Poor charge balance due to large charge injection barriers in blue PeLEDs has been identified as one of the major roadblocks to achieve high efficiency. Here band edge control of perovskite emitting layers for blue PeLEDs with enhanced charge balance and device performance is reported. By using organic spacer cations with different dipole moments, that is, phenethyl ammonium (PEA), methoxy phenethyl ammonium (MePEA), and 4‐fluoro phenethyl ammonium (4FPEA), the band edges of quasi‐2D perovskites are tuned without affecting their band gaps. Detailed characterization and computational studies have confirmed the effect of dipole moment modification to be mostly electrostatic, resulting in changes in the ionization energies of ≈0.45 eV for MePEA and ≈ −0.65 eV for 4FPEA based thin films relative to PEA‐based thin films. With improved charge balance, blue PeLEDs based on MePEA quasi‐2D perovskites show twofold increase of the EQE as compared to the control PEA based devices.
Rational band edge energy level control of quasi‐2D metal halide perovskites by modifying the dipole moments of organic spacer cations is shown to improve the external quantum efficiency of blue perovskite light‐emitting diodes by more than twofold.
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