Balancing the Electron and Hole Transfer for Efficient Quantum Dot

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Functional Inorganic Materials and Devices

Balancing the Electron and Hole Transfer for Efficient Quantum Dot Lightemitting Diodes by Employing a Versatile Organic Electron Blocking Layer Xiao Jin, Chun Chang, Weifeng Zhao, Shujuan Huang, Xiaobing Gu, Qin Zhang, Feng Li, Yubao Zhang, and Qinghua Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00729 • Publication Date (Web): 18 Apr 2018 Downloaded from http://pubs.acs.org on April 18, 2018

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ACS Applied Materials & Interfaces

Balancing the Electron and Hole Transfer for Efficient Quantum Dot Light-emitting Diodes by Employing a Versatile Organic Electron Blocking Layer Xiao Jin,† Chun Chang,‡ Weifeng Zhao,ǁ Shujuan Huang,§,* Xiaobing Gu,‡ Qin Zhang,‡ Feng Li,‡ Yubao Zhang,‡ Qinghua Li†,* †

School of Physics Science and Technology, Lingnan Normal University, Zhanjiang 524048,

P. R. China. ‡

Jiangxi Engineering Laboratory for Optoelectronics Testing Technology, Nanchang

Hangkong University, Nanchang 330063, P. R. China. ǁ

School of Materials and Chemical Engineering, Xi’an Technological University, Xi’an

710021, P. R. China §

School of Photovoltaic and Renewable Energy Engineering, University of New South Wales,

Sydney, NSW 2052, Australia

ABSTRACT: The electron blocking layer (EBL) is important to balance the charge carrier transfer and achieve highly efficient quantum dot light-emitting diodes (QLEDs). Here we report the utilization of a soluble tert-butyldimethylsilyl chloride-modified poly(p-phenylene benzobisoxazole) (TBS-PBO) as an EBL for simultaneous good charge carrier transfer balance while maintaining high current density. We show that the versatile TBS-PBO blocks excess electron injection into the QDs, thus leading to better charge carrier transfer balance. It also restricts the undesired QD to EBL electron transfer process, which preserves the superior emission capabilities of the emitter. As a consequence, the TBS-PBO device delivers an external quantum efficiency (EQE) maximum of 16.7% along with a remarkable current density as high as 139 mA/cm2 with a brightness of 5484 cd/m2. The current density of our device is higher than insulator EBL-based devices due to the higher conductivity of the TBS1

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PBO versus insulator EBL, thus helping achieve high luminance values ranging from 141420000 cd/cm2 with current densities ranging from 44 to 648 mA/cm2 and EQE>14%. We believe that these unconventional features of the present TBS-PBO-based QLEDs will expand the wide use of TBS-PBO as buffer layers in other advanced QLED applications. KEYWORDS: quantum dot light-emitting diodes, TBS-PBO, charge transfer balance, electron blocking layer, time-resolved PL

1. INTRODUCTION Quantum dot light-emitting diode (QLED) is one of the most promising candidates in nextgeneration displays and solid-state lightings owing to its high color purity, tunable emission wavelengths, flexible, good photo-stability, and ease of fabrication.1-6 Currently, most QLEDs use a planar sandwich-like structure where the QD emitter is sandwiched between the hole transport layer (HTL) and the electron transport layer (ETL). Under applied electric field, the electrons and holes are directed into the emitting materials where they are expected to be confined and achieve radiative recombination. As an emitter, the QD film is often very thin, typically within 50 nm and the mobility of the holes is much lower than electrons. This causes imbalanced injection of electrons and holes, which gives rise to QD layer charging.7-8 Under this circumstance, the Auger recombination occurs by which the electrons and holes recombine and release energy to excite electrons in the conduction band to higher energy level without emitting photons. These excited electrons then quickly thermalize back to the ground state losing the energy to heat.9-10 Heating up from non-radiative recombination not only further quenches the light emission because it can excite the charge carriers out of the QD into surface defect states but also decreases device lifetime.11

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Strategies to balance electron and hole transport include energy level engineering of the transport layer via ion doping or surface modification to enhance the charge injection efficiency,12-15 introducing an electron blocking layer to slow down the electron injection rate,16-17 and utilizing a more robust HTL to increase the hole mobility.18 Among these, incorporating an electron-blocking layer (EBL) is the most promising way to improve the charge transport balance because the electron injection rate can be greatly reduced by introducing a high physical barrier from the blocking material. It also functions well in maintaining the charge neutrality of the QD emitters improving the confinement of the electron and hole in the QD layers, thereby preserving the superior emission efficiencies of the QDs. In addition, charge carrier blocking layers can also act as buffer layers to prevent the reverse transfer of electrons (holes) from QDs to ETL (HTL). To date, many efforts have been devoted to exploring new device architectures with EBLs. For instance, a deoxyribonucleic acid and cetyltrimetylammonium (DNA-CTMA) complex layer has been used as an EBL to prevent the electrons from being injected into the HTL; this offered a luminance of 6580 cd/m2 and an external quantum efficiency (EQE) of 4%.17 Incorporation of a poly(N,N-bis(4-butylphenyl)-N,N-bis(phenyl)benzidine) (poly-TPD) facilitates electron and holes injection into the CdSe/ZnS QD layer leading to maximum luminance values of 4200 cd/m2, 68,000 cd/m2, and 31000 cd/m2 for the blue, green, and red QLEDs, respectively.19 By inserting an insulating polymer buffer layer such as poly(methyl methacrylate) (PMMA) between the QD emitter and the ETL, the EQE of red QLED achieved a milestone of 20.5%.1 However, the current densities of these insulated blocking layer-based devices are often limited due to their relatively low conductivities. Hence, identifying a proper blocking material with electron blocking behaviors while maintaining current density at a high level is very important and urgent for designing robust QLEDs.

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Heterocyclic aromatic polymer poly(p-phenylene benzobisoxazole) (PBO) has a ladder-like rigid chemical structure that offers many outstanding properties such as high mechanical strength, high heat resistance relative to other organics, and high thermal conductivity.20-22 Its lowest unoccupied molecular orbital (LUMO) is -3.02 eV, and the highest occupied molecular orbital (HOMO) level is -5.74 eV. These results suggest that this material can block electrons injecting into CdSe-based QDs due to its high LUMO level while maintaining efficient hole transfer because its HOMO level is very close to that of the QDs. However, the use of PBO as a buffer layer in QLEDs is rare—partially because it lacks a glass transition and has a fairly low solubility in organic solvents for obtaining desired shapes or films.22-24 Here, we demonstrate improved solubility of PBO using tert-butyldimethylsilyl (TBS) chloride modified precursor and successfully fabrication of uniform TBS-PBO thin film. We for the first time apply this thin layer as an EBL between the QDs and ETL in a QLED device. The QLED demonstrates a very high current density over 139 mA/cm2 at a bias voltage of 3.2 V. This is much higher than that of a QLED using insulating electron blocking layer attributed to the high conductivity of the TBS-PBO. The optimized QLED delivers a CE over 4.67 Cd/A, a luminance of 5484 cd/m2, and an impressive EQE of 16.7%.

2. RESULTS AND DISCUSSION 2.1. Device architecture. The as-prepared TBS-PBO exhibit good solubility with an intrinsic viscosity of 0.36 dL g-1 in DMF due to the 4,6-diaminoresorcinol dihydrochloride (DAR) function group, which guarantees its excellent filming properties. In this work, we deposit a thin film of TBS-PBO on the QD layer to form a device structure of ITO/PEDOT:PSS-polyTPD(50 nm)/QDs(50 nm)/PBO(60 nm)/ZnO:Mg(20 nm)/Al, as shown by the transmission electron microscopy (TEM) image in Figure 1a. The QD layer was fabricated by spin coating of CdSe/CdS/ZnS core/shell QDs solution. The QDs are capped with long chain oleic acid 4


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(OA) ligand to ensure their dispersion in solvents and prevent the harmful energy transfer between the QDs. The inter-dot interactions are greatly reduced by this long ligand. The asprepared QDs show non-obvious interfaces at the core-shell contact indicating that a continuous composition structure is formed, as shown in Figure 2. This composition gradient structure not only benefits from relaxed lattice strain25-27 but also leads to a low Auger nonradiative recombination rate,28 which is essential for high photoluminescence (PL) quantum yields (QYs). The average diameter of the QDs is 12.4±1.3 nm based on at least 10 TEM images. With a narrow size distribution, the QDs exhibit bright emission at 626 nm with a QY of about 70% and a full width at half maximum (FWHM) of 32 nm. The detailed methods for device fabrication and QD synthesis are presented in Experimental Section.

Figure 1. (a) The cross-sectional TEM image of the champion device showing the device structure of ITO/PEDOT:PSS-poly-TPD(50 nm)/QDs(50 nm)/TBS-PBO(60 nm)/ZnO:Mg(20 nm)/Al. (b) Schematic of the energy levels of the device.

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Figure 2. (a) TEM and (b) HR-TEM images of the QDs. (c) Size distribution of the QDs. (d) Absorption and PL spectra of the QD solution. The schematic energy diagram of our device is shown in Figure 1b where the TBS-PBO layer is inserted between the QD emitter and the ZnO:Mg ETL. It was found that Mg dopant is capable turning the electronic properties of ZnO. Recent works reveal that solution processed Mg dopant is capable of enlarging the band gap of ZnO and improving the balance the electron and hole transport in the QLED devices.14,15,29 Therefore, we employed ZnO:Mg fabricated by a sol-gel method as an electron transport layer to fabricate the QLEDs. The energy level values of ITO, PEDOT:PSS, and poly-TPD are cited from Ref. 1, and the energy level values of ZnO:Mg are taken from Ref. 14. The LUMO and HOMO level values of the 6


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TBS-PBO film are obtained by electrochemical cyclic voltammetry (CV) measurements as presented in Figure 3a and 3b. The redox waves are obvious implying that TBS-PBO can transport both electrons and holes. In detail, the onset oxidation and reduction potential Eonset are 1.03 and -1.69 V, respectively. The corresponding energy levels with respect to the vacuum level are calculated using the formula: CB (or VB) (eV) =-4.8-(E-E1/2) (eV), where E is the peak point of the redox potential30 and E1/2 is the potential difference between the formal potential of the Fc/Fc+ system and against reference Ag/Ag+ system (E1/2 =0.09 V).28, 31-32

The HOMO and LUMO values of the TBS-PBO are -5.74 and -3.02 eV, respectively.

The bandgap obtained from CV results is 2.72 eV, which agrees well with UV−vis optical spectrum as shown in Figure 3c. As depicted in the energy diagram of our QLED, the electrons are easily injected from Al to QDs via the conduction band of the ZnO:Mg layer because of the positive driving force of the built-in electric field. The injection barrier is negligible between the QDs and ZnO:Mg. After incorporating a TBS-PBO layer, the electron injection barrier increases to 0.46 eV, which is expected to suppress the electron injection. In this architecture, a poly-TPD hole transport layer is employed to cooperate with PEDOT:PSS and enhances hole transport due to its deep HOMO level as well as its high hole mobility. Colloidal ZnO:Mg nanocrystals act as an electron transport layer because Mg-doped ZnO improves the charge injection efficiency.14

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Figure 3. (a-b) CV behaviours of bare TBS-PBO film in a 0.1 M TBAPF6 acetonitrile solution (sweep rate, 50 mV/s). (c) Absorption spectrum of the TBS-PBO film. 2.2. Device performance. The inset in Figure 4a displays bright and uniform red emission from the QLED at a bias voltage of 4.0 V. The normalized electroluminescence (EL) spectrum from the QLED and PL spectrum from QD solution are shown in Figure 4a, where a 6-nm red shift to PL of the QD solution is observed due to the inter-dot interaction enhanced by the closely packing or dielectric dispersion of solvent.33 However, the EL peak wavelengths and the FWHM line widths are unaffected from 3.0-8.0 V as shown in Figure 4b. This phenomenon is likely from the screening effect of the wide bandgap shell ZnS to the applied electric field. This prevents the electron hole pairs from being polarized. The current

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density and luminance versus voltages of the champion device (60-nm thick TBS-PBO EBL) and device without an EBL are shown in Figure 4c. All devices show a steep increase in current density after 2.0 V, while their luminance increases steadily. Overall, the luminance of the device with a TBS-PBO EBL is much higher than those without an EBL throughout the applied bias voltage. The current density is lower than its counterpart implying that while the charge carrier transport is slowed down by introducing the EBL, the energy conversion into photons is markedly enhanced. Figure 4d shows that the EQE maximum is 16.7% at 3.2 V with a current density of 139 mA/cm2, a current efficiency (CE) of 4.59 cd/A and a brightness of 5484 cd/m2. Although the EQE maximum is lower than the highest reported red QLED (20.5%),1 the brightness reported here is almost three-fold higher than its value. The EBL used here has a higher conductivity than insulating EBL leading to a greater current density (139 mA/cm2 compared to 7 mA/cm2) and resulting in brighter emission. The conversion efficiency of our device can be held at a high level at EQE>14% despite current densities varying from 44 to 648 mA/cm2 and luminance values of 1414-20000 cd/cm2. This indicates that after introducing the TBS-PBO EBL, the efficiency roll-off effect is suppressed, suggesting the devices with TBS-PBO EBL are promising for practical commercialization.

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Figure 4. (a) ELspectrum of the TBS-PBO-based QLED and PL spectrum of the QD solution. The inset highlights the emission photograph of the QLED operating at 4.0 V. (b) EL spectra of the TBS-PBO based device at different voltages. (c) Luminance–current–voltage (L–I–V) characteristics of the QLEDs with and without TBS-PBO EBL. (d) EQE-V and CE-V characteristics of the devices. We have also investigated the effect of TBS-PBO thickness on device performance. Figure 5a shows the current densities of the devices with different TBS-PBO thickness. The current densities of the QLEDs decrease gradually with the increase of the thickness of TBS-PBO. This suggests that we can optimize the electron/hole injection and reduce the redundant electron currents by adjusting the thickness of the TBS-PBO EBL. As shown in Figure 5b, the luminance of the QLED with 60-nm TBS-PBO is much higher than that of the QLEDs 10


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with 50-nm TBS-PBO at all voltages, despite the lower current density of the 60-nm TBSPBO device. Therefore, blocking the redundant electron injection is useful to improve the efficiency of the device. However, further increase in the thickness leads to a notable decrease in luminance. A similar trend is also observed in the EQE-V and CE-V curves in Figure 5c. The EQEs and CEs show high sensitivity to the thickness of TBS-PBO EBL. The EQE increases from 6.9% to 16.7%, and the CE increases from 1.98 cd/A to 4.59 cd/A when the TBS-PBO thickness increases from 50 nm to 60 nm. However, further increases in thickness lead to a decline in EQE and CE. The turn on voltage of the champion device (60 nm EBL) is the lowest at 1.8 V compared to 2.0 V (50 nm EBL) and 2.2 V (70 nm EBL). Furthermore, TBS-PBO also improves the stability of the QLEDs as shown in Figure 5d. The luminance of the QLED with and without TBS-PBO layer as a function of time (hours) were measured at a fixed current density (100 mA cm-2). The half lifetime T is the time when the luminance L decreases to the half of the initial value, i.e., L0/2. As shown in Figure 5d, the half lifetime T of the device without TBS-PBO is only 25 h. However, after incorporating a TBS-PBO layer, the half lifetime T is greatly extended to 75 h. According to the relationship

L0nT=const (n=1.5), we converted the T values of the two devices at the same initial brightness of 100 cd m-2.34 After equivalent calculations, the T value of the device with TBSPBO is over 20000 hours, more than 20 times greater than its counterpart without TBS-PBO (875 hours). This implies that TBS-PBO EBL plays an important role in improvement of the stability of the QLEDs. Because TBS-PBO layer blocks excess electrons undergoing nonradiative recombination such as Auger recombination and releasing heat, thus improving the longevity of the device.

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Figure 5. (a) Current density, (b) luminance (c) current efficiency (CE) and EQE versus driving voltage of the QLEDs with different thicknesses of TBS-PBO. (d) Stability data for QLED devices with and without TBS-PBO (L, luminance). The devices were test at ambient conditions. 2.3. Effects of TBS-PBO EBL on carrier transport. Efficient QLEDs require the electrons and holes injected from the electrodes to be confined in the QD emitter followed by emission of photons by radiative recombination. Nevertheless, the reverse QD to EBL electron transfer may occur, which will cause a decrease of charge carriers in the active layer and finally quench the emission.35 Introducing a TBS-PBO EBL layer between the QDs and ETL strengthens the physical barrier at the interface. Therefore, the reverse charge transfer is 12


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likely diminished. To explain this superior performance of TBS-PBO based QLEDs, timeresolved PL (tr-PL) curves were measured to identify the electron-blocking properties of this material. The role of EBL in modifying the electron transfer can be identified by comparing the dynamic kinetics of pure QD film and the QD/ZnO:Mg with and without a TBS-PBO EBL.32, 36-37 Figure 6a shows the decay curves of the QD solution, QD film, QD/ZnO:Mg, and QD/TBSPBO/ZnO:Mg blend films on glass substrates. Although the PL decay profile exhibits a series of heterogeneous kinetics, we found that a bi-exponential function was satisfied in fitting the emission decay:38 +∞

I (t) = ∫ G (t − τ )[ a1 exp(−τ / τ 1 ) + a2 exp( −τ / τ 2 )]d τ −∞

(1)

where G is background-free laser pulse autocorrelation function related with half width at half-maximum (HWHM) of the pulse duration, a (a1 and a2) is the amplitude, τ1 and τ2 are the PL lifetime constants. The kinetic parameters were derived from bi-exponential fitting of the tr-PL trace, as listed in Table 1. Then the average lifetimes τave of QD solution, QD film and the QD/ZnO:Mg blend film with and without the TBS-PBO layer were obtained by the relationship:

τ ave =

a1τ 12 + a2τ 22 a1τ 1 + a2τ 2

(2)

The average PL lifetime of the QD film is 21.1 ns, which is shorter than the QD solution of 31.7 ns. Besides, the PL QY of the QD film was tested to be 34%, which was much lower than its solution counterpart (70%). The lower QY together with faster decay rate observed in QD solid are probably due to the enhanced diffusion to nonluminescent states of the excitons, because the QDs are closely packed in a QD solid and the inter dot interactions are greatly 13



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enhanced.3,39 It is worth noting that after interfacing with ZnO:Mg, the τave value of the QD film further decreased from 21.1 to 14.6 ns. Such a notable decrease in PL lifetime suggests a fast and severe transport of the electrons from QDs to ETL. However, the insertion of a TBSPBO layer modifies the QD/ZnO:Mg interracial interaction, increasing the lifetime to 16.8 ns. For bare QD films, the radiative recombination contributes solely to the PL decays, whereas there are two processes contribute to tr-PL curves of the blend films: one is the radiative recombination of the electron-hole pairs and the other one is the charge transfer process from QDs to ZnO:Mg layer. Then, one can use the formulas = 1 − / and =

1/ − 1/ to estimate the electron transfer efficiency and the electron transfer rate , respectively.28 After inserting a TBS-PBO, the electron transfer rate declines by more than 40%, that is, from 2.11×107 s-1 to 1.21×107 s-1, and the electron transfer efficiency decreases from 30.8% to 20.4%. In fact, the QD to ZnO:Mg electron transfer process is an undesired process that is inverse from the charge injection from electrodes to QDs. These results suggest that the TBS-PBO layer is helpful in blocking the reverse electron transfer confining the electrons in the active layer, hence and preserving the efficient light emission of the QDs. Table 1. The amplitudes and lifetimes obtained from bi-exponential fitting. Kinetic parameters

a1

a2

τ1 (ns)

τ2 (ns)

τave (ns)

QD solution

0.98

0.02

25± 1

108± 6

31.7

QD film

0.97

0.03

14±1

68±5

21.1

QDs/ZnO:Mg film

0.98

0.02

12±1

47± 4

14.6

QDs/TBS-PBO/ZnO:Mg film

0.97

0.03

16±1

31± 3

16.8

To reveal the influence of the TBS-PBO layer on the balance of the electron and hole transfer, electron-transport-only or hole-transport-only devices were fabricated and characterized.40-42 The electron-transport-only structure of the ITO/ZnO:Mg (20 nm)/TBS14


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PBO (60 nm)/QDs (50 nm)/ZnO:Mg (20 nm)/Al with TBS-PBO layer between the QDs and ZnO:Mg was fabricated to compare with its counterpart without a TBS-PBO layer where the hole injection is blocked by the two ZnO:Mg layers. The hole-transport-only devices have a structure of ITO/PEDOT:PSS poly-TPD (50 nm)/QDs (50 nm)/TBS-PBO (60 nm)/MoO3/Al and a counterpart without a TBS-PBO layer where the electrons are blocked. The log-log scale plot of the J-V curves of single carrier devices are shown in Figure 6b. The J−V slope of the electron-only device without TBS-PBO shows a unity gradient at all voltages including an ohmic region (J∝V). The J−V curve of the electron-only device with TBS-PBO clearly displays different regions, i.e., an ohmic-like region (J ∝V0.85) at low voltage (V< 1.0 V), a space-charge limited current (SCLC) region19 (J ∝ V2) at high voltage (V> 2 V), and a transition region with J∝Vn, n>2. At low voltages (V

Balancing the Electron and Hole Transfer for Efficient Quantum Dot

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