Ultra-low EQE roll-off and marvelous efficiency perovskite quantum-dots light-emitting-diodes achieved by ligand passivation

Highlights

• Perovskite QDs are synthesized by using a room-temperature ligand-assisted reprecipitation (LARP) methods with sodium dodecyl sulfate as ligand.

• PQDs films exhibit suppressed non-radiative recombination process, decreased trap density, smooth surface and augmented carrier mobility.

• HHigh efficiency QLEDs based on SDS capped PQDs with low roll-off of 1.5% at 200 mA/cm² are fabricated by a modified device structure.

Abstract

Recently, perovskite quantum dots (PQDs) were employed as promising emitters in light-emitting diode devices (LEDs) mainly owing to their marvelous photoelectronics properties. However, these perovskite QLEDs suffered from the inefficient performance, especially their serious efficiency roll-off probably due to the exciton recombination dynamic in the interface of PQDs films and charge imbalance resulting from low electrical transportation. Herein, sodium dodecyl sulfate (SDS) was effectively introduced to synthesize PQDs with many excellent properties and eventually suppressed the efficiency roll-off of QLEDs. SDS-based PQDs films exhibited obviously enhanced radiative recombination, reduced trap density, and increased carrier mobility, especially the transportation of electrons. Therefore, the charge carrier was balanced at high current density together with the remarkably suppressed efficiency roll-off. QLEDs fabricated by SDS-capped PQDs exhibited significantly improved maximum brightness of 193,810 cd/m², and achieve the external quantum efficiency of 10.13%. More importantly, the EQE roll-off of QLED optimized in the thickness of emitting layer was only 1.5% at the current density of 200 mA/cm², representing a prominent achievement of our strategy.

Introduction

Perovskite quantum-dots (PQDs) were studied and employed as promising materials for it exhibited excellent photoelectronics properties in the application of optoelectronic devices recently [1], [2], [3], [4], [5]. Because of their unique characteristics in tunable color, excellent luminescence, and high color purity, PQDs were widely used in various kinds of light-emitting diodes (LEDs) [6], [7], [8], [9]. Although the external quantum efficiency (EQE) of PQDs-LEDs (QLEDs) was enhanced to over 20% [10], [11], there still remained many fatal weaknesses for those reported PQDs-LEDs devices, especially in very poor stability for their photoelectronics performance since their high efficiency only occurred at low current density while some serious efficiency roll-off could be observed at high current density. Similar to the perovskite film-based LEDs, the efficiency roll-off usually resulted from the non-radiative Auger recombination, charge imbalance, and joule heating [12], [13], [14], [15]. Some methods were introduced to minimize the EQE roll-off of perovskite film-based LEDs. Huang et al. believed that it was the luminescence quenching resulting from non-radiative recombination that affected the efficiency roll-off [16]. To suppress the Auger recombination in perovskite film-based LEDs, various methods were applied such as passivation strategy of the defect and ion migration [17], [18], surface treatment by using the phenylalkylammonium molecules [19], [20], [21]. These studies indicate that both the defect and the charge imbalance altogether exerted a great impact on the roll-off performance in perovskite LEDs.

Although it could significantly promote the efficiency roll-off in perovskite film-based LEDs, there were still many insurmountable difficulties to introduce these methods into QLEDs mainly owing to the complex surface trap state of PQDs caused by surface ligands [22]. Thus, the ligand engineering strategy which could effectively passivate the surface trap state of PQDs become the key factor to improve the efficiency roll-off of QLEDs. Recently, multiple methods were employed to eliminate the surface trap state of PQDs, such as introducing ligands that strongly binding to PQDs surface [23], [24] especially the ligands with –SO₃- molecules [25], [26], [27], [28], demonstrating a passive surface [29], [30], [31], [32], applying post-treatment strategy [33] and inserting a passivation layer[34]. Although these methods efficiently passivated the surface of PQDs or significantly suppressed the surface trap states, which resulted in excellent luminescent properties, there still exhibited many obvious stability issues for the PQDs based QLEDs probably due to the non-radiative recombination generated in PQDs films and imbalanced charge carriers in QLEDs.

In this work, sodium dodecyl sulfate with a molecule of –OSO₃-, which could not only effectively promote the stability of PQDs proved by Zhang and co-workers [35] but also inhibited the non-radiative recombination evidenced by Wang et al. [36], was employed as a surface ligand to synthesis PQDs. The experimental results indicated that the greatly promoted performance of PQDs not only in optical, electrical aspects but also in stability just by introducing SDS was achieved. The non-radiative recombination process was effectively restrained while the trap state density was also significantly decreased mainly after the SDS passivation, which brought about the greatly enhanced luminescent properties. Meanwhile, the PQDs films with much smoother surface and higher carrier mobility were effectively achieved after the SDS passivation, which extremely balanced the charge carrier and greatly boosted the performance of QLEDs. As a result, the lifetime (T₅₀, time to half of the initial brightness) under 100 cd/m² of the SDS3 device was enhanced to 13.51 h, which is about 4.5-fold improvement compared to that of SDS0 device with a T₅₀ of 2.96 h. Besides, the performance of the QLEDs was further promoted by optimizing the thickness of PQDs films. The peak brightness of 193,810 cd/m² was achieved when increasing the spin-coating speed from 2000 rpm to 3000 rpm while the EQE significantly enhanced to 10.13% at the speed of 4000 rpm. More importantly, the efficiency roll-off value of the 3000 rpm sample only decreased a lot to ~ 1.5% at the current density of 200 mA/cm², which was much superior to the most reported perovskite-based LEDs (Table S1).