Yong Qiu

Name:

Organization: Tsinghua University

Department: Key Laboratory of Organic Optoelectronics & Molecular Engineering of Ministry of Education, Department of Chemistry

Title:

Chemicals
References

Transient space-charge-perturbed currents in organic materials: A Monte Carlo study

Co-reporter:Haoyuan Li, Lian Duan, Chen Li, Liduo Wang, Yong Qiu

Organic Electronics 2014 Volume 15(Issue 2) pp:524-530

Publication Date(Web):February 2014

DOI:10.1016/j.orgel.2013.11.042

•We simulate multi-particle carrier movement in organic materials.•Space-charge perturbation facilitates charge transport by filling deep DOS.•Space-charge perturbation causes increased carrier packet spreading.•Mobilities extracted from transient SCP currents are lower for organic materials.The transient current holds rich information about carrier transport and is used to derive charge mobility in the time-of-flight (TOF) measurement. Because carriers have finite charge, all transient currents are space-charge-perturbed (SCP). Previous theories of transient SCP currents are derived by neglecting diffusion and assuming a constant mobility, which is unfit for organic materials because of the hopping behavior of carriers. Due to the lack of knowledge, we do not fully understand the results from TOF experiments, which hinders the understanding of the charge transport mechanisms. Here, we perform Monte Carlo simulations of multi-particle carrier movement to study the transient SCP currents in organic materials. Coulomb interactions are calculated, and it is assumed that multiple carriers cannot occupy the same site simultaneously. Our results show that space-charge perturbation has two opposite effects on charge transport. In most cases, the net result is slower carrier movement, which suggests that TOF measurements under SCP conditions underestimate the charge mobility of organic materials.Graphical abstract

Ambipolar Transporting 1,2-Benzanthracene Derivative with Efficient Green Excimer Emission for Single-Layer Organic Light-Emitting Diodes

Co-reporter:Lian Duan;Juan Qiao;Yongduo Sun;Deqiang Zhang;Guifang Dong;Liduo Wang

Advanced Optical Materials 2013 Volume 1( Issue 2) pp:167-172

Publication Date(Web):

DOI:10.1002/adom.201200024

Single-layer organic light-emitting diodes (OLEDs) are highly desired due to their potential advantages such as simplifying the fabrication process and lowering the cost. However, high-performance single-layer OLEDs are rare in the literature as they require bipolar transporting materials with balanced electron and hole mobilities, high fluorescent quantum yields, and good energy levels matching both electrodes. In this work, a bipolar transporting benzanthracene derivative, namely 7,12-bis-[4-(2,2-diphenylvinyl)-phenyl] benzanthracene (BDPBan), is designed and synthesized without any structural motifs for hole and electron transport. As electrons and holes can perform intermolecular hopping to similar spatial extents between the adjacent benzanthracene moieties, well-balanced hole and electron mobilities higher than 10⁻³ cm²/Vs are achieved. Though the strong intermolecular interactions facilitate the formation of excimers in concentrated solutions and thin films of BDPBan, efficient undoped single-layer OLEDs are fabricated with strong green emission from the excimers. A device with 90 nm BDPBan as the emitting layer shows a peak current efficiency of 11.4 cd/A: beyond the conventional upper limit calculated based on the photoluminescence quantum yield.

Bipolar charge transport property of N,N′-dicarbazolyl-1,4-dimethene-benzene: A study of the short range order model

Co-reporter:HaoYuan Li;Liang Chen;Juan Qiao;Lian Duan;GuiFang Dong

Science Bulletin 2013 Volume 58( Issue 1) pp:79-83

Publication Date(Web):2013 January

DOI:10.1007/s11434-012-5346-1

Short range order model is commonly used to explain the charge transport property of disordered organic semiconductors. However, its validity is not yet studied. In this paper, the hole and electron mobilities of a bipolar material, N,N′-dicarbazolyl-1,4-dimethene-benzene (DCB), were measured through time of flight method. The hole and electron mobilities of DCB based on the crystalline structure were calculated. In order to investigate the short range order model, the ratios of charge mobilities at zero electric field of holes to electrons were calculated. The results showed that this model cannot fully explain our case. The reason was discussed in detail, and a correction method was proposed. We showed that using the short range order model without preconditions to explain the charge transport property of amorphous materials may lead to deviations, which is often neglected in the past.

Study of the Hole and Electron Transport in Amorphous 9,10-Di-(2′-naphthyl)anthracene: The First-Principles Approach

Co-reporter:Haoyuan Li, Lian Duan, Yongduo Sun, Deqiang Zhang, Liduo Wang, and Yong Qiu

The Journal of Physical Chemistry C 2013 Volume 117(Issue 32) pp:16336-16342

Publication Date(Web):July 15, 2013

DOI:10.1021/jp4050868

An accurate description of charge transport in amorphous organic semiconductors is challenging. Many previously reported methods largely involve empirical parameters, which may hinder the understanding of the charge transport process in a specific material. In this paper, Born–Oppenheimer molecular dynamics (BOMD) is used to simulate the amorphous structure of a widely used small molecule 9,10-di-(2′-naphthyl)anthracene (ADN). Its hole and electron mobilities are calculated using an ab initio method. It is found that the inaccuracy in the calculation of the nonadiabatic couplings caused by the periodicity of the cell in the BOMD simulation can be greatly reduced by taking into account the mirror states in the surrounding cells. The calculated hole and electron mobilities both have the same order of magnitude with their corresponding experimental results, demonstrating the possibility to obtain reasonable charge transport mobilities for amorphous small-molecule semiconductors via the first-principles approach. Our work may shed light on the understanding of the charge transport process in amorphous organic semiconductors and the design of new charge transport materials.

Controlling the Recombination Zone of White Organic Light-Emitting Diodes with Extremely Long Lifetimes

Co-reporter:Lian Duan;Deqiang Zhang;Kongwu Wu;Xiuqi Huang;Liduo Wang

Advanced Functional Materials 2011 Volume 21( Issue 18) pp:3540-3545

Publication Date(Web):

DOI:10.1002/adfm.201100943

Abstract

The lifetime of the organic devices remains a major challenge that must be overcome before the wide application of white organic light-emitting diodes (WOLEDs) technology. In this work, we present a new strategy to achieve WOLEDs with an extremely long lifetime by wisely control of the recombination zone. A blue emitting layer of 6,6′-(1,2-ethenediyl)bis(N-2-naphthalenyl-N-phenyl-2-naphthalenamine doped 9-(1-naphthyl)-10-(2-naphthyl)-anthracene was deposited on top of the mixed host blue emitting layer to prevent hole penetration into the electron transporting layer and to attain better confinement of carrier recombination. In this way, we obtained a WOLED with a record high lifetime of over 150 000 hours at an initial brightness of 1000 cd m⁻², 40 times longer than the conventional bilayer WOLED. The electroluminescent spectra of the long-lived WOLED showed almost no color-shifting after accelerated aging. It is anticipated that these results might be a starting point for further research towards ultrastable OLED displays and lightings.

Transparent organic light-emitting diodes based on Cs2CO3:Ag/Ag composite cathode

Co-reporter:Qian Liu;Lian Duan;DeQiang Zhang;Juan Qiao;LiDuo Wang

Science Bulletin 2010 Volume 55( Issue 15) pp:1479-1482

Publication Date(Web):2010 May

DOI:10.1007/s11434-010-3150-3

The transparent organic light-emitting diodes (TOLEDs) based on Cs2CO3:Ag/Ag composite cathode are reported in this paper. They show higher efficiency and better stability than the control devices with the Cs2CO3/Ag cathode. The devices are highly transparent, and show total efficiencies from both sides of the devices comparable with conventional bottom-emitting devices. The electroluminescence (EL) spectrum from the cathode side of the device is closer to the photoluminescence (PL) spectrum of the light-emitting material than that from the anode side. The effect of the cathode Ag thickness on the device performance has been investigated. It is observed that a critical Ag thickness of 15 nm is required to form a good Ag film with high optical transparency and electrical conductivity.

Thermally Decomposable Lithium Nitride as an Electron Injection Material for Highly Efficient and Stable OLEDs

Co-reporter:Lian Duan, Qian Liu, Yang Li, Yudi Gao, Guohui Zhang, Deqiang Zhang, Liduo Wang and Yong Qiu

The Journal of Physical Chemistry C 2009 Volume 113(Issue 30) pp:13386-13390

Publication Date(Web):July 2, 2009

DOI:10.1021/jp901510j

Improvement of electron injection is essential for highly efficient low-voltage organic light-emitting diodes (OLEDs). In this work, we report our study on thermally decomposable Li3N as the electron injection layer (EIL) for OLEDs. We use a quartz crystal microbalance method to investigate the decomposition process of Li3N during vacuum thermal evaporation in situ. Thermodynamics study also proves that Li3N decomposes in vacuum to form metallic lithium and nitrogen. OLEDs with Li3N as the EIL outperform those with conventional LiF in every respect of view. An optimized green OLED with a Li3N EIL shows 35% higher efficiency and more than doubled lifetime compared with the control device with a LiF EIL.

Nanocomposite Thin Film Based on Ytterbium Fluoride and N,N′-Bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine and Its Application in Organic Light Emitting Diodes as Hole Transport Layer

Co-reporter:Lian Duan ; Jing Xie ; DeQiang Zhang ; LiDuo Wang ; GuiFang Dong ; Juan Qiao

The Journal of Physical Chemistry C 2008 Volume 112(Issue 31) pp:11985-11990

Publication Date(Web):July 15, 2008

DOI:10.1021/jp8040555

We present a nanocomposite thin film based on ytterbium fluoride (YbF3) and N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB). The transmission electron microscopy study testified the presence of YbF3 nanoparticles with a size of 3−5nm, and these nanoparticles were observed to be uniformly dispersed in NPB. The conductance of YbF3 (50%)-doped NPB was measured to be 1.76 × 10−5 S, much higher than the value of 5.81 × 10−8 S for pure NPB film. However, unlike other p-doped systems, the absorption spectrum study shows that there is no obvious interaction between the host (NPB) and the dopant (YbF3). The hole mobility of YbF3 (50%)-doped NPB decreases to 7.31 × 10−5 cm2/(V s) at an electric field of 4 × 105 V/cm, while that of pure NPB film is 5.87 × 10−4 cm2/(V s). We believe that the improved film conductivity and decreased hole mobility can enhance the built-in field and thus favor the electron injection. Due to the improved balance of holes and electrons, a double-layer device with YbF3-doped NPB as the HTL and tris(8-hydroxyquinolinolato)aluminum (Alq3) as the electron transport and light-emitting layer shows a luminous efficiency of 4.50 cd/A at 300 mA/cm2, 20% higher than that of control device with pure NPB as the HTL.