Blue-Emitting CdSe Nanoplatelets Enabled by Sulfur-Alloyed Heterostructures for Light-Emitting Diodes with Low Turn-on Voltage

Abstract

Colloidal nanoplatelets (NPLs) have emerged as the last class of semiconductor nanocrystals for their potential optoelectronic applications. The heterostructures of these nanocrystals can achieve high photoluminescence quantum yield and enhanced photostability, along with color purity. Such advantages make them a promising candidate for solution-processable lightemitting diodes (LEDs). However, to date, blue-emitting CdSe nanoplatelets (NPLs) exhibit poor photoluminescence quantum yield and also typically suffer from a rolled-up morphology. To mitigate these problems in this work, we propose and demonstrate efficient alloyed 4 ML CdSe 1−x S x nanoplatelets having a CdS crown with enhanced photoluminescence quantum yields (up to 60%) in the blue region (462−487 nm). We successfully used these NPLs as an electrically driven active emitter in the blue-emitting NPL-LEDs with a low turn-on voltage of ∼4 V. The Commission Internationale de L'Eclairage (CIE) coordinates of (0.23, 0.14) were obtained for these blue-emitting NPL-LEDs. These emitters could potentially open up the opportunity for full-color displays using these NPL-based blue LEDs in conjunction with the red and green ones.

Pushing the Emission Envelope for Full-Color Realization of Colloidal Semiconductor Core/Shell Nanoplatelets

“…Both EL spectra of green- and cyan-emitting CdZnSe/ZnS NPLs were slightly red-shifted compared to the PL spectra (by 6 nm) and had a narrow fwhm of 30 nm (Figure d). These results are better than or comparable to the previous NPL-LED studies with the best performance for the similar emission colors using CdSe 1– x S x /Cd y Zn 1– y S core/shell NPLs (PL at 554 nm, turn-on voltage = 2.4 V, and peak luminance = 21,280 cd/m 2 ) and CdSe 1– x /CdS core/crown NPLs (PL at 468 nm, turn-on voltage = 4 V, and peak luminance = 12 cd/m 2 ) . Our results ensure the potential of CdZnSe/ZnS NPLs in EL application.…”

“…Therefore, to extend the emission wavelength of NPLs, in particular toward bluish color, composition control is required using materials with a large band gap, such as ZnSe or CdS. So far, CdSeS alloy-based core/shell NPLs can only cover the yellowish green-to-red region (554–615 nm), and CdSeS alloy-based core/crown NPLs can emit blue light at 462–487 nm but have poor device performance due to the absence of shell passivation . In the case of ZnSe, 2D nanosheets (NSs) can be synthesized through synthesis with a lamellar mesophase, but lateral growth control is difficult .…”

“…So far, CdSeS alloy-based core/shell NPLs can only cover the yellowish green-to-red region (554−615 nm), 23 and CdSeS alloy-based core/crown NPLs can emit blue light at 462−487 nm but have poor device performance due to the absence of shell passivation. 24 In the case of ZnSe, 2D nanosheets (NSs) can be synthesized through synthesis with a lamellar mesophase, but lateral growth control is difficult. 25 Therefore, ZnSe NSs have poor colloidal stability due to their large lateral size of several tens of nanometers.…”

Observation of optical gain from aqueous quantum well heterostructures in water

“…Recently, colloidal quantum wells (CQWs) have emerged as a favorable optical gain medium since they possess large absorption cross-sections, [16][17][18] with continuously tunable emission, 19,20 suppressed Auger recombination rates, 17,21 low optical gain thresholds, 17,22,23 long gain lifetimes, 17 and large gain cross-sections. 14,15 Especially, their large gain crosssection makes them excellent candidates for in-solution lasing by supporting the gain condition and feasible levels of gain coefficients in solution.…”

Elastic, Electrostatic and Magnetic Properties in Colloidal Quantum Wells

“…Consequently, it has so far possessed one of the highest quantum yields among heterostructures (Φ ∼ 1). [89,90,94,95,96] The origin of such an elevated quantum yield is twofold: i) the high overlap between electrons and holes which results in fast radiative recombinations; ii) the effective passivation of the CdSe core which suppresses superficial traps acting as non-radiative recombination channels.…”