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Light-Emitting Diodes Based on Colloidal Silicon Quantum Dots with Octyl and Phenylpropyl Ligands Xiangkai Liu, Shuangyi Zhao, Wei Gu, Yuting Zhang, Xvsheng Qiao, Zhenyi Ni, Xiaodong Pi, and Deren Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16980 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 19, 2018
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Light-Emitting Diodes Based on Colloidal Silicon Quantum Dots with Octyl and Phenylpropyl Ligands Xiangkai Liu, Shuangyi Zhao, Wei Gu, Yuting Zhang, Xvsheng Qiao, Zhenyi Ni, Xiaodong Pi* and Deren Yang State Key Laboratory of Silicon Material & School of Materials Science and Engineering Zhejiang University, Hangzhou 310027, China
ABSTRACT Colloidal silicon quantum dots (Si QDs) hold ever-growing promise for the development of novel optoelectronic devices such as light-emitting diodes (LEDs). Although it has been proposed that ligands at the surface of colloidal Si QDs may significantly impact the performance of LEDs based on colloidal Si QDs, little systematic work has been carried out to compare the performance of LEDs that are fabricated by using colloidal Si QDs with different ligands. Here colloidal Si QDs with rather short octyl ligands (Octyl-Si QDs) and phenylpropyl ligands (PhPr-Si QDs) are employed for the fabrication of LEDs. It is found that the optical power density of PhPr-Si QD LEDs is larger than that of Octyl-Si QD LEDs. This is due to the fact that the surface of PhPr-Si QDs is more oxidized and less defective than that of Octyl-Si QDs. Moreover, the benzene rings of phenylpropyl ligands significantly enhance the electron transport of QD LEDs. It is interesting that the external quantum efficiency (EQE) of PhPr-Si QD LEDs is lower than that of Octyl-Si QD LEDs because the benzene rings of phenylpropyl ligands suppress the hole transport of QD LEDs. The unbalance between the electron and hole injection in PhPr-Si QD LEDs is more serious than that in Octyl-Si QD LEDs. The currently obtained highest optical power density of ∼ 0.64 mW/cm2 from PhPr-Si QD LEDs and highest EQE of ∼ 6.2% from Octyl-Si QD LEDs should encourage efforts to further advance the development of high-performance optoelectronic devices based on colloidal Si QDs.
KEYWORDS: colloidal silicon quantum dots, ligands, surface defects, light-emitting diodes, charge transport
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INTRODUCTION: Colloidal semiconductor quantum dots (QDs) have been attracting intensive attention due to their great promise for the development of printed electronics, which features a variety of cost-effective and large-area semiconductor devices.1-8 Among all types of colloidal semiconductor QDs, colloidal silicon (Si) QDs deserve significant research efforts because they may enable the Si material that has already been the mainstay material for microelectronics and photovoltaics to also play an important role in printed electronics.9-12 Although research progress on colloidal Si QDs does not seem as impressive as that on archetypical colloidal semiconductor QDs such as Cd- and Pb-based QDs,13-19 a series of recent reports have demonstrated the intriguing potential of colloidal Si QDs for optoelectronic applications.20-22 For example, mid-infrared optical absorption induced by the localized surface plasmon resonance of colloidal Si QDs has been achieved by doping Si QDs with boron and phosphorus.23-26 Despite the quasi-direct bandgap of Si QDs, the quantum yield of photoluminescence (PL) from colloidal Si QDs may be further increased toward 100% as long as the QD surface is even better controlled to avoid defects.20, 22, 27, 28 Up to now, light-emitting diodes (LEDs) have been one type of the most important optoelectronic devices based on colloidal Si QDs.29-37 It has been realized that ligands at the surface of colloidal Si QDs are critical to the performance of Si QDs.38 On one hand, ligands should be long enough to render Si QDs excellent dispersibility,39-41 which facilitates the formation of high-quality Si-QD films in LED structures. On the other hand, ligands need to be short enough to ease charge transport because they are usually not conductive.32, 42, 43 Among hydrocarbon ligands that have been the most widely used at the surface of Si QDs,38, 39, 44-48 phenylpropyl and octyl ligands may be the shortest aryl and alkyl ligands to maintain the long-term stability of Si-QD colloids at room
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temperature, respectively. Please note that even shorter aryl ligands (e. g., phenylethyl ligands) and even shorter alkyl ligands (e. g., hexyl ligands) were once attached to the surface of Si QDs.41, 45 But the attachment processes could not be well controlled. The resulting Si-QD colloids did not possess long-term stability at room temperature. Mastronardi et al.32 have already fabricated LEDs by using colloidal Si QDs with phenylpropyl ligands (PhPr-Si QDs). Compared with those based on colloidal Si QDs with decyl ligands, LEDs based on PhPr-Si QDs showed better charge transport. Such improvement may be due to the reduced ligand length because the charge transport through a hydrocarbon ligand depends on its length.42 Benzene-ring-induced π-π stacking in phenylpropyl ligands should also contribute to the improved charge transport.32 If decyl ligands are replaced with octyl ligands, the alkyl ligand length is comparable with the length of the aromatic phenylpropyl ligands. The effect of the benzene ring in a phenylpropyl ligand on the charge transport of Si-QD LEDs may be then clearly elucidated by comparing LEDs based on PhPr-Si QDs and those based on colloidal Si QDs with octyl ligands (Octyl-Si QDs). In addition, it is apparent that phenylpropyl and octyl ligands may lead to different surface states for Si QDs. Since the electronic and optical properties of Si QDs are intimately related to their surface,28, 32, 38, 42, 48-50 LEDs based on PhPr-Si QDs and those based on Octyl-Si QDs may exhibit different performance. Therefore, it is interesting to carry out a comparative study on PhPr-Si QD and Octyl-Si QD LEDs. In this work, we have prepared Octyl-Si QDs and PhPr-Si QDs. It turns out that the surface of PhPr-Si QDs is more oxidized and less defective than that of Octyl-Si QDs. PhPr-Si QDs more efficiently emit PL than Octyl-Si QDs. LEDs based on PhPr-Si QDs exhibit larger current density than those based on Octyl-Si QDs mainly because the electron transport in the former is significantly better than that in the latter. It is interesting that the hole transport in PhPr-Si QD LEDs is actually
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not as good as that in Octyl-Si QD LEDs. This causes the unbalance between the electron and hole injection in PhPr-Si QD LEDs to be more serious than that in Octyl-Si QD LEDs. Therefore, the external quantum efficiency (EQE) of PhPr-Si QD LEDs is lower than that of Octyl-Si QD LEDs, although the optical power density of PhPr-Si QD LEDs is larger.
METHODS Preparation of Octyl-Si QDs and Phenylpropyl-Si QDs: Freestanding Si QDs were synthesized in a non-thermal plasma system10. The as-produced Si QDs were dispersed in methanol with the concentration of 5 mg/ml for surface hydrosilylation. The cloudy methanol solution containing Si QDs was etched by hydrofluoric acid (HF) to obtain hydrogen-passivated Si QDs (H-Si QDs). After centrifugation, H-Si QDs were added into the mixture of 1-octene and mesilytene (1:2 in volume) or the mixture of allylbenzene and mesilytene (1:5 in volume). The resulting solution was heated at 130 ℃ for 3 hours in the atmosphere of N2 to hydrosilylate H-Si QDs. During the hydrosilylation additional 1-octene was injected into the reaction system if the loss of 1-octene with the boiling point of 121 ℃ was large enough to impair the adequate hydrosilylation of H-Si QDs despite the refluxing. Hydrosilylated Si QDs were subsequently extracted using rotatory evaporation and finally dispersed in toluene for characterization and device fabrication. Light-emitting Diodes Fabrication: To fabricate Si-QD LEDs, pre-coated indium tin oxide (ITO) glass substrates were cleaned through ultrasonic by the order of detergent, deionized water, acetone and isopropanol for 20 min each. After filtration through a 0.45 µm N66 filter, the PEDOT:PSS solution was spin-coated on the ITO-coated substrate in air at the speed of 4,000 rpm for 60 s. The substrates were baked at 140 ℃ for 20 min and were then transferred into an Ar-filling glove box
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with O2
