Article pubs.acs.org/JPCC
Performance of Inverted Quantum Dot Light-Emitting Diodes Enhanced by Using Phosphorescent Molecules as Exciton Harvesters Guohong Liu,†,‡ Xiang Zhou,*,† Xiaowei Sun,‡ and Shuming Chen*,‡ †
State Key Lab of Optoelectronic Materials and Technologies, School of Materials Science and Engineering, Sun Yat-Sen University, Guangzhou, 510275, People’s Republic of China ‡ Department of Electrical and Electronics Engineering, Southern University of Science and Technology, Shenzhen, 518055, People’s Republic of China S Supporting Information *
ABSTRACT: Exciton harvesters based on blue phosphorescent molecules bis(4,6-difluorophenylpyridinato-N,C2)picolinatoiridium (FIrpic) doped in 4,4′,4″-tris(carbazol-9yl)triphenylamine (TCTA) are used to enhance the performance of inverted quantum dot light-emitting diodes (QDLEDs). In the proposed device structures, electrons that leak to the TCTA layer can be effectively captured by FIrpic and subsequently can recombine in the TCTA:FIrpic layer. The harvested energy is then nonradiatively transferred to the adjacent QDs via Förster dipole−dipole coupling mechanism. Because of effective harvest of leaked electrons and complete energy transfer from FIrpic to the adjacent QDs, the demonstrated QD-LEDs exhibit pure QD emission, higher efficiency (1.62-fold improvement), and longer lifetime.
1. INTRODUCTION Colloidal quantum dot light-emitting diodes (QD-LEDs) have motivated increasingly active research owing to their attractive properties such as saturated color emission, tunable colors, high luminescence quantum yield, good material stability, and simple fabrication process.1−5 All these attractive characteristics make QD-LEDs excellent candidates for the development of nextgeneration display technologies. In typical QD-LEDs, ZnO nanocrystal is commonly used as electron injection and transporting material because of its high electron mobility and matched energy levels with that of QDs,6,7 which greatly favors the injection of electrons to QDs. However, hole injection to QDs is quite difficult because of the high injection barrier caused by the unmatched energy levels between QDs and typical organic hole transporting materials.8 The imbalance of carrier injection leads to a low device performance because the excess electrons, either charging the QDs and consequently enhancing the nonradiative Auger recombination8−10 or leaking to the counter electrode without recombination, greatly reduce the efficiency and stability of the devices. Designing devices that rely on precise charge balance and direct exciton formation on QDs could be difficult. Though by employing a 4,4′,4″-tris(carbazol-9-yl)triphenylamine (TCTA) electron-blocking layer, the device performance has been improved greatly because of reduction of electron overflow and improvement of hole-injection as demonstrated by Peng et al.,11 charge imbalance still exists and the excess electrons have not been fully exploited. © XXXX American Chemical Society
To fully exploit the leakage electrons, we herein propose using a blue phosphorescent molecule bis(4,6-difluorophenylpyridinato-N,C2)picolinatoiridium (FIrpic) doped in TCTA to effectively capture the leakage electrons and subsequently to harvest the excitons. The harvested excitons can transfer their energy to the adjacent QDs via near-field dipole−dipole coupling mechanism known as Förster resonant energy transfer (FRET).12 FRET was demonstrated to be a favorable mechanism that can contribute significantly to device performance of QD-LEDs employing appropriate charge transport layers (CTLs).8 For FRET mechanism, an exciton is initially formed in the adjacent CTLs. The exciton’s energy is then nonradiatively transferred to the QDs via near-field dipole−dipole coupling which relies on the spectral overlap between the absorption of QDs and the photoluminescence (PL) of the CTLs. Generally, the organic CTLs are comprised of fluorescent small organic molecules or conjugated polymers with which only one-quarter of the singlet states can be generated according to spin statistics.13 The other three-fourths of the triplet excitons formed in these fluorescent CTLs will not be transferred to the QDs via FRET because they have zero oscillatory strength and thus will be dissipated as heat.14 On the other hand, phosphorescent materials can achieve nearly 100% internal quantum efficiency owing to the Received: December 29, 2015 Revised: February 8, 2016
A
DOI: 10.1021/acs.jpcc.5b12692 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
with a peak photoluminescence (PL) wavelength of 630 nm were dissolved in octane (15 mg/mL) and then were spincoated at 2000 rpm, followed by annealing at 100 °C for 5 min. The thicknesses of ZnO and QD layers were determined to be around 60 and 24 nm, respectively, by Dektak Step Profiler. The subsequent layers were thermally evaporated in sequence in a vacuum chamber with a base pressure of 5 × 10−4 Pa. The energy level alignment of the devices is shown in Figure 1.
strong spin−orbit coupling and efficient intersystem crossing.15,16 Hence, utilization of phosphorescent small organic molecules in the adjacent CTL is expected to boost greatly the energy transfer to QDs in QD-LEDs and, accordingly, the device efficiency. The excitonic processes of semiconductor quantum dots have been discussed in detail in a recent review.17 Efficient exciton transfer has been demonstrated in the nanocomposites of QDs uniformly dispersed into conjugated polymers which showed great potential of the doped system.18,19 To make full use of the triplet excitons for enhanced light generation, either FRET in phosphorescent organic-material based hybrids or Dexter energy transfer in fluorescent organic-material based hybrids can be used.19 The potential for phosphorescent molecules to harvest excitons and to transfer energy to QDs was explored first by both steady-state and time-resolved photoluminescence study20 and was utilized later in hybrid LEDs for white light generation.21,22 Only Zhang and Cao23 and Mutlugun et al.24 reported great enhancement of the external quantum efficiency in a noninverted QD-LED device architecture by employing phosphorescent molecules as exciton harvesters. However, the color (spectral) purity of the QD-LEDs could not be fully preserved even for a very thin (
