Electron transport mechanism in colloidal SnO₂ nanoparticle films and its implications for quantum-dot light-emitting diodes

Abstract

Charge transport behavior in SnO2 nanoparticle (NP) films is rather crucial to the optoelectronic devices. Temperature-dependent electrical results show that the electron transport in SnO2 NP films is dominated by the Mott variablerange hopping processes, i.e., the electrons are transported between different NPs through surface states rather than the conduction band of the nanocrystals, which is identical to the commonly used ZnO NP solids. Compared with ZnO, SnO2 films exhibit similar electron mobility but lower density of states. Therefore, we deduce that the low density of states in the SnO2 NP films should be the key factor limiting the device performance in compared with the ZnO as reported in most of the quantum-dot light-emitting diodes. Our work sheds light on optimizing SnO2 nanoparticle films for quantum-dot light-emitting diodes. Moreover, we believe that the SnO2 remains a desirable candidate as the electron transport material for the QLEDs due to its excellent physicochemical stability.

Efficient Quantum-Dot Light-Emitting Diodes Based on Solvent-Annealed SnO2 Electron-Transport Layers

“…Nevertheless, the current density (>2.0 V) of SnO 2 -based QLEDs remains lower than that of the ZnO device due to the lower conductivity of SnO 2 than that of ZnO. 32 Thus, the luminance of the SnO 2 -based QLEDs is lower than that of the ZnO-containing device. However, after SVA-SnO 2 ETL is employed, the peak luminance of SnO 2 devices is increased by 25.3%, from 24660 to 30900 cd/m 2 , comparable to that of ZnO QLEDs (39100 cd/m 2 ).…”

“…As shown in Figure 5b, the same delayed EL emission relative to the voltage rising (∼400 ns) is observed for these three devices, which is attributed to the comparable electron mobility of SnO 2 and ZnO as reported in our recent work. 32 However, the fast components are very different from each other. Compared with the ZnO QLED, the SnO 2 -based devices exhibit slower rising edges, which is attributed to the low conductivity of SnO 2 films due to their low density of states as demonstrated in our previous work.…”

“…Compared with the ZnO QLED, the SnO 2 -based devices exhibit slower rising edges, which is attributed to the low conductivity of SnO 2 films due to their low density of states as demonstrated in our previous work. 32 It has been reported that in the QLEDs the exciton is formed in a QD by injection of an electron and followed by a hole. This unique sequential charge injection requires the QDs are negatively charged to achieve highly efficient exciton formation.…”

“…ZnO and SnO 2 NPs were prepared according to our previous report. 32,39 The ZnO and SnO 2 NPs were about 3−5 nm in size, and the concentration was about 40 mg/mL in ethanol. All the materials were used as received without any further purification.…”

“…Tin oxide (SnO 2 ) NPs are considered as a promising ETL to replace ZnO due to their similar electron mobility. − Moreover, compared with ZnO, the higher conduction band of SnO 2 might facilitate the electron injection, hence reducing the turn-on voltage of QLEDs. To date, many reports have been conducted to improve device performance by enhancing the electronic properties of SnO 2 NP films, including energy level alignment, surface passivation, and ligand exchange. ,, Ethylenediamine­tetraacetic acid (EDTA) and tetramethyl­ammonium hydroxide were used to modify SnO 2 NPs, which suppress the agglomeration and enhance the electron mobility of the SnO 2 film. , The ZnO NP-modified SnO 2 film was reported to promote the performance of QLEDs .…”