Potential of Electrochemical Charge Injection for Quantum Dot Light-Emitting Devices

Abstract

The efficiency of quantum dot (QD) light-emitting diodes is limited by inefficient hole injection into the valence levels of the QDs. Electrochemical doping, where mobile ions form electrical double layers (EDLs) at electrodes, offers a route to removing injection barriers. While QD light-emitting electrochemical cells (QLECs) have shown promise, prior studies relied on additional charge injection layers, complicating the study of charge injection into QDs. In this work, devices with a simple ITO/QD active layer/Al structure were fabricated using highly photoluminescent ligand-exchanged CdSe/CdS/ZnS QDs, poly-(ethylene oxide), and lithium trifluoromethanesulfonate as electrolyte. We show that the dense QD films in these QLECs can be electrochemically doped, transport charges, and exhibit electroluminescence. Symmetrical cyclic voltammograms and operando photoluminescence measurements prove that these devices function as electrochemically doped LECs. Spectroelectrochemical experiments on separately n- and p-doped QD films indicate that hole injection remains the primary limitation in QLEC performance. These findings demonstrate that using EDLs to facilitate charge injection in QD light-emitting devices is promising, but significant challenges remain to be solved before electron and hole injections are balanced.

1. Working Mechanism of LECs, Illustrating the Formation of EDLs, Electrochemical Doping, and EL

1

(a) Absorption (solid lines) and PL (dash lines) spectra, (b) ¹H NMR spectra, (c) X-ray diffraction patterns, and (d) TEM images of CdSe/CdS/ZnS QDs before and after ligand exchange. Inset in (a) photographs of QDs transferred from hexane (top) to DMF (bottom) after a two-phase ligand exchange. The standard PDF cards of CdSe, CdS, and ZnS in (c) are 19–0191, 75–1546, and 01–0792, respectively. Scale bars in (d) 20 nm.

2

(a) Device structure of the QLECS: ITO/QD active layer/Al. (b) Experimental and simulated current density–voltage-luminance curves. EDL, ECD and EL denote electrical double layer, electrochemical doping and electroluminescence, respectively. Inset: photograph of a red-emitting QLEC in operation. (c) EQE and (d) normalized PL spectra of QDs dispersion and normalized EL spectra of QLECs at various voltages. ITO was biased positively in these measurements and the scan rate was 50 mV/s.

3

(a) Cyclic voltammogram of a QLEC. The scan rate was 50 mV/s. (b) Experimental and simulated electrical response of the device under a constant bias of −5 V. The ITO electrode was biased positively. (c) Simulated n-i-p junction evolution in an LEC under a constant bias of −5 V. Top panel: distribution of carriers, ions, and recombination concentrations from cathode to anode. Bottom panel: distribution of electrostatic potential from the cathode to anode.

4

(a) Scheme of experimental setup for operando PL measurement. Operando PL (black solid lines) and EL (red solid lines) measurements were made when (b, c) ITO electrode was the positive anode and (d, e) ITO electrode was the negative cathode. During the measurement, the devices were electrically scanned from 0 to 5 V and back to 0 V for three cycles and then connected at 0 V for 10–30 min.

5

Cyclic voltammograms, 2D differential absorbance, 2D normalized PL and 1D plot of differential absorbance at the CdSe 1S transition and PL maxima of QD films/ITO scanned (a–d) negatively and (e–h) positively in the 0.1 M LiCF₃SO₃ PEO (Mn ≈ 600 g/mol) solution. The scan rate was 5 mV/s.