Polyethylenimine Insulativity-Dominant Charge-Injection Balance for

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Polyethylenimine Insulativity Dominant Charge Injection Balance for Highly Efficient Inverted Quantum Dot Light Emitting Diodes Ke Ding, Hongting Chen, Lianwei Fan, Bo Wang, Zhi Huang, Shaoqing Zhuang, Bin Hu, and Lei Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 19 May 2017 Downloaded from http://pubs.acs.org on May 21, 2017

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ACS Applied Materials & Interfaces

Polyethylenimine Insulativity Dominant Charge Injection Balance for Highly Efficient Inverted Quantum Dot Light Emitting Diodes

Ke Ding, Hongting Chen, Lianwei Fan, Bo Wang, Zhi Huang, Shaoqing Zhuang, Bin Hu and Lei Wang*

Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China

* Correspondence author: [email protected]

ABSTRACT

Quantum dot light emitting diode (QLED) with an inverted architecture always suffers charge injection imbalance and severe quantum dot charging, which degrades device performance. Blocking excess injected electrons into quantum dots (QDs) is crucial for efficient inverted QLEDs. It is found that Polyethylenimine (PEI) has two opposite effect on electron injection, one is blocking electron injection by its intrinsic insulativity and the other one is promoting electron injection by the reduced work function of ZnO/PEI. This work dominantly

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utilizes the insulating nature of PEI to reduce electrons and the charge injection balance is realized when PEI becomes thicker and blocks more excess electrons. Furthermore, PEI contributes to quantum dot charging suppression and owns smoother surface morphology than ZnO nanoparticles, which is beneficial for leakage current reduction and QD deposition. As a result, the optimized QLED with 15 nm PEI shows a 2.5 times efficiency improvement compared to the QLED without PEI. And the QLED possesses the maximum external quantum efficiency (EQE) and current efficiency of 16.5% and 18.8 cd/A respectively, accompanied with a low efficiency roll-off of 15% at 1000 cd/m2 which is comparable to the reported inverted red QLED with the highest efficiency.

KEYWORDS: quantum dot light emitting diodes; intrinsic insulativity of PEI; blocking electron injection; charge injection balance; suppressing quantum dot charging

INTRODUCTION Since the first report appeared in 1994,1 light emitting diodes (LEDs) based on quantum dot (QD) have attracted much attention due to the highlighted properties of QDs, such as size-tunable emission wavelength,2 high photoluminescence (PL) quantum yield (QY),3 narrow emission bandwidth,4-6 good material photostability7-9 and solution-based synthesis process.10 And indeed, these outstanding properties endow QD based LEDs with a bright development prospect towards the proper candidate for the next generation high-color-quality display and solid-state lighting.11-13 During the past two decades, intensive researches on red quantum dot


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light emitting diodes (QLEDs) have greatly improved the device performance with the maximum external quantum efficiency (EQE) from less than 0.01%1 up to 20.5%,14 which is comparable to organic light emitting diodes (OLEDs).15 To improve the QLED performance, researchers always devote to balancing the injected charge in devices and suppressing QD charging by optimizing QD nanostructure and device architecture.14,

16-19

In these studies, Peng’ group

adopted an insulating polymer poly (methyl methacrylate) (PMMA) as electron blocking layer between the QD emitting layer and the electron transport layer (ETL) to optimize charge injection balance and gained the excellent device performance, making conventional QLEDs promising in applications in active-matrix (AM) OLED display technique. However, for high resolution large-size AM OLED display technique, the inverted QLEDs with a bottom cathode attract much more attention considering the manufacturing cost and the uniformity of the backplane, where the n-type amorphous silicon (a-Si) thin film transistor (TFT) backplane precedes commonly used low-temperature polycrystalline silicon (LTPS) backplane. In inverted QLED case, the bottom cathode can be directly connected with the drain line of the n-type TFT, resulting in the reduced driving voltage.20-22 Nevertheless, the inverted red QLEDs also face difficulties and challenges of the charge injection imbalance caused by the large potential energy barrier between QDs and HTLs as well as the carrier mobility difference between ETLs and HTLs. In particular, the electron injection from ETL to QDs is promoted due to a nearly barrier free transporting channel while the hole injection from HTLs to QDs is much difficult induced by a large energy barrier (0.6 eV or more). Moreover, the electron mobility of ETL (widely used



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ZnO) is higher than the hole mobility of HTLs.23-26 Hence, to eliminate the excess electron injection into the QD emissive layer is the key point to improve the device efficiency. The inverted device structure with ZnO as electron injection layer (EIL) or ETL plays an important role in combination of easy processability and good device performance due to the compatibility of solution-based processing, suitable energy level and good transport property. Besides, ZnO nanoparticles (NPs) can be spin-coated as a mechanically robust film after thermal annealing, which is quite beneficial for solution-based process. With these properties, modifying the interface between ZnO and QDs is thought to be a feasible method to acquire good device performance. Polyethylenimine

(PEI),

polyethylenimine

ethoxylated

(PEIE)

and

poly[(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-ioctylfluorene)

(PFN)

are widely reported for the effect of the intrinsic molecular dipole associated with the interface dipole on lowering the work function of the cathode contacts.27-29 As is known, polymer PEI that contains simple aliphatic amine groups is a large band-gap insulator although it can lower the work function of conductors and semiconductors. However, no literatures have been reported to utilize the insulating nature in inverted QLEDs to obtain charge balance, most of them make use of PEI or PEIE to reduce the work function of ZnO or electrodes and enhance electron injection.30, 31 The insulativity of PEI and the reduced work function of ETL by PEI in QLED are two opposite effects on electron injection, the former reduces electron injection while the latter enhances it. We speculated if both the insulativity and interface dipole can be utilized with the


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result of reducing electron injection (the former effect offsets the latter), the charge balance in PEI inserted QLEDs should be finally realized. Herein, we report a charge injection balanced inverted QLED by introducing a thick PEI layer (>10 nm) which is inserted between ETL and the QD layer. The optimized efficiency of the QLED is 16.5% (EQE) and 18.8 cd/A (current efficiency), which is 2.5 times higher than QLED without PEI. The great performance improvement is ascribed to the reduced electron injection and the suppressed QD charging with the insertion of PEI layer, where the insulativity is more prominent in contrast with the reduced work function of ZnO/PEI. Besides, the thick PEI layer deposited on ZnO contributes to the more uniform surface morphology by completely filling in the valleys at the surface of ZnO nanoparticle film. To the best of our knowledge, this work is the first one which utilizes PEI to reduce electron injection rather than enhance it. We believe that such a reasonable scheme in designing QLED device structure paves a way for enhancing the device performance to further realize the applications in next-generation QD-LED display and solid-state lighting. EXPERIMENTAL SECTION Synthesis of ZnO Nanoparticles. ZnO nanoparticles with the average diameter of about 4 nm were synthesized through a solution-precipitation process with some modifications. Zinc acetate dihydrate (0.672 g) was dissolved in dimethyl sulfoxide (30 mL) with tetramethylammonium hydroxide (1.02g) dissolved in ethanol (10 mL) in the meanwhile, then they were mixed together and stirred for 24h under ambient conditions. By adding ethyl acetate into the reaction mixture,



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ZnO particles were precipitated and then centrifuged before redispersed in ethanol. To accelerate the redispersing process and stabilize the nanoparticles, 2-ethanolamine was introduced into ethanol with the volume ratio as 160:1. The ZnO nanoparticles dispersed in ethanol at a concentration of ~ 50 mg/mL was filtered before use with a polytetrafluoroethylene (PTFE) filter. QLEDs Fabrication. Patterned indium tin oxide (ITO) glass substrates with the sheet resistance less than 15 Ω/sq were cleaned with alkaline leaner diluted with deionized water, deionized water, acetone, isopropanol in sequence and then oven-dried for more than 3 hours before use. ZnO NPs in ethanol (50 mg/mL) was spin-coated on ITO substrate treated with O2 plasma and baked at 150 oC for 30 minutes in a glove box filled with nitrogen. Then PEI in ethanol (0.1% wt) was deposited overlying the ZnO film and annealed at 100 oC for 5 minutes. Finally, CdSe/CdS/ZnS QDs (from Poly OptoElectronics Co., Ltd) in octane (5 mg/mL) was spin-coated on the PEI film and baked at 100 oC for 10 minutes. Finished with the solution process, the substrate was transferred to a vacuum chamber for the overlying organic film and Al electrode deposition with the base pressure of 4×10-4 Pa. Characterizations. The TEM and HRTEM images were obtained using Titan G2 60-300 Probe Cs Corrector HRSTEM operated at the accelerating voltage of 300 kV. The absorption and PL spectrum was measured by SHIMADZU UV-3600 UV-VIS-NIR spectrophotometer and Edinburgh FLS920 fluorescence spectrometer, respectively. Time-resolved photoluminance decay was measured using time-correlated single-photo counting (TCSPC) by Edinburgh


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FLS920 with a 375 nm laser. AFM images were obtained through Veeco Dimension 3100 NanoScope in a tapping mode measured by Bruker RTESPA-300 probes. The crystal structure of ZnO nanoparticles was measured by PANalytical PW3040-60 MRD X-ray diffractometer. For sample preparation, ZnO NP in ethanol was spin-coated on a washed and dried quartz substrate, then baked till the solvent was evaporated. The work function and the valence band maximum were obtained by UPS measured by Kratos AXIS-ULTRA DLD-600W using a helium discharge lamp (21.22 eV). The sample was spin-coated on a gold deposited silicon substrate and dried, then send to the vacuum chamber pumping overnight before measurement. Thicknesses of films were measured by Bruker DektakXT profilometer. To overcome the precision limit, for ultrathin film with the thickness less than 20 nm, we measured their thicknesses by depositing them on a thickness-known film (> 30 nm) and obtained thicknesses through a simple subtraction. With respect to QLED characteristics, the current density-voltage and luminance-voltage were measured by Keithley 2400 source measurement unite and a PhotoResearch spectroradiometer (PR655) with the efficiency (EQE, current efficiency) measured in the meanwhile. RESULTS AND DISCUSSION Generally speaking, ZnO NPs and QDs with excellent quality such as uniform size distribution and good crystallinity are definitely needed for good performance of QLEDs.32,

33

To

characterize ZnO NPs and CdSe/CdS/ZnS QDs used in the emitting device as ETL and the emissive layer respectively, we tested their transmission electron microscope (TEM) images. As is shown in Figure 1a and 1b, both ZnO NPs (about 4 nm) and CdSe/CdS/ZnS QDs (about 8nm)



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are uniformly distributed and well crystalized with clear lattice fringes. We also measured the X-ray diffraction (XRD) pattern of ZnO NPs (Figure S1, Supporting Information), which shows a wurtzite structure of ZnO NPs and proves its good crystallinity. As we all know, monodisperse QDs contributes to narrow bandwidth light emission, which can be seen from the PL spectra in Figure 1c, where the full width at half maximum (FWHM) is as narrow as 32 nm with the peak wavelength of QDs at 620 nm and 623 nm in toluene and solid state, respectively. The slight red-shift (3 nm) in solid state was generally caused by energy transfers from smaller dots to adjacent larger dots in a Förster resonate energy transfer process in the close-packed solid film.34 Figure 1c also shows the ultraviolet-visible absorption spectrum of QDs in toluene, which shows the absorption onset at 640 nm indicating a ~2.0 eV bandgap.

Figure 1. (a), (b) TEM images of ZnO NPs and CdSe/CdS/ZnS QDs, respectively. The insets are the related HRTEM images. (c) Ultraviolet-visible absorption and PL spectra of QDs, black solid line for absorption in toluene and red solid line for PL in solution (toluene) and blue dashed line for PL in solid state.


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It is known that PEIE exerts more and more influence on lowering the work function of metal oxides with increased thickness.27 We measured the work function of ZnO coated by PEI with increased PEI thickness (from 0 to 20 nm), and found that the work function became much lower when PEI got thicker, consistent with PEIE (Figure S2, Supporting Information). By virtue of the reduced work function of ZnO/PEI, QLEDs should exhibit increased current density.35 To verify this effect of ZnO/PEI on QLED current density, we fabricated QLEDs with the structure of ITO/ZnO NPs (70 nm) /PEI (x) /QDs (10 nm) / N,N′-dicarbazolyl-3,5-benzene (mCP) (10 nm) /4,4′-N,N′-dicarbazole-biphenyl (CBP) (50 nm) /MoO3 (10 nm) /Al (100 nm), which is shown in Figure 2a. PEI thickness was set as 0, 1, 5, 8, 15 and 20 nm respectively and the optimization of ZnO thickness was presented in Figure S3, Supporting Information. It is clearly showed in Figure 2b that QLED with (5-20 nm) PEI has lower current density than QLED without PEI, even 5 nm PEI insertion reduces the device current density. While the QLED with 1 nm PEI shows higher current density than the QLED without PEI, coincident with the previous report.28 To clarify whether there exists electron injection barrier at ZnO/QD and ZnO/PEI/QD interfaces, UPS spectra of QD deposited on ZnO and ZnO/PEI are shown in Figure S4, Supporting Information. It is clearly showed that the energy levels of QDs are almost unaffected with the conduction band maximum (CBM) of 4.81+0.01 eV and the valence band maximum (VBM) of 6.81+0.01 eV. ZnO and ZnO/PEI (5 nm) as well as the energy diagram of the corresponding QLED is given in Figure S5, Supporting Information, where the energy level of HTLs and MoO3 were referenced from the previous reports.

26, 36-38


It shows the down-shifted


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vacuum level and reduced CBM of ZnO/PEI which is beneficial for electron injection.35 The nearly unchanged QD energy level is also shown in Figure S5b, Supporting Information, indicating that electrons can be injected from both ZnO and ZnO/PEI into QD without energy barrier. For detailed discussion, we further compared the current density of QLEDs and found the current density became lower with the PEI thickness increasing. The reduced current density prompted us to take the intrinsic insulativity of PEI into consideration. As is known, PEI as an insulator shows larger tunneling barrier for electron transport with the increased thickness, and the QLED current density must be reduced if the insulativity dominantly determines electron injection into the QD layer. We ascribed the current density variation to the combined effect on electron injection of PEI. In the 1 nm PEI insertion situation, the tunneling barrier for electrons can be ignored so that electron injection into QDs is promoted by the lowered work function with higher current density, and for QLEDs with 5-20 nm PEI, the tunneling barrier increases and the enhanced electron injection by ZnO/PEI becomes less prominent compared with the impeded electron injection by the insulativity of PEI, thus resulting in the reduced current density. To confirm this mechanism, we also fabricated QLEDs with PMMA which is insulating and doesn’t form interface dipole (Figure S6, Supporting Information). As is shown, QLEDs with PMMA show obvious decrease in current density compared with QLED without PMMA, regardless of its thickness. And Von is much higher than the QLED without PMMA. Even compared to QLEDs with the same thickness of PEI, the contrastive consequence is very similar to the


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comparison situation between QLEDs with PMMA and the pristine QLED mentioned above. This is a forceful evident for the work mechanism of PEI, as the PMMA just act as a pure insulator between ZnO and QDs, which induced over-reduction in electron injection in QLEDs. The over-reduced electron injection sacrifices charge balance with severe efficiency roll-off, shown in Figure S6b, 6c and 6f. Figure 2c shows typical EL spectra of QLEDs with different PEI thicknesses, which has no difference, indicating a complete emission of QDs without disturbance by the insertion of PEI. The insert typically exhibits the uniform and deep red emission of the operating QLED with a Commission International de l’Eclairage (CIE) colour coordinates of (0.68, 0.31) (Figure S7, Supporting Information). Figure 2d-f clearly shows that QLEDs with increased PEI thickness from 0 to 15 nm had enhanced performance and gained the highest efficiency when PEI thickness reached 15 nm. And further increasing PEI thickness didn’t guarantee any efficiency improvement, that means neither excess electron injection nor too less electron current could improve the device efficiency due to the broken charge injection balance. We also note that the maximum EQE and current efficiency of QLED with 1 nm PEI decreased in contrast with QLED without PEI, which also demonstrates the over-injected electrons in the 1 nm PEI inserted QLED aggravate charge injection imbalance. The charge transfer process in energy levels of QLEDs with different PEI thicknesses is illustrated in Figure S8, Supporting Information.



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Figure 2. (a) Schematic of the QLED configuration. (b) Current density and luminance versus bias voltage characteristics of different PEI thicknesses based QLEDs, respectively (c) EL spectra of QLEDs, the inset shows the photographic of the operating device of 3 mm × 3 mm at 4.0 V. (d), (e), EQE and Current efficiency versus luminance characteristics, respectively. (f) The maximum EQE as a function of different PEI thicknesses.

As a result, with the insertion of 15 nm PEI, the most efficient QLED possessed a maximum EQE of 16.5% (corresponding to 18.8 cd/A) at the luminance of 153 cd/m2 with the Von of 2.1 V close to the optical band gap. The efficiency roll-off was low (15%) even when the luminance reached 1000 cd/m2 (EQE, 14% and current efficiency, 16.1cd/A), which is comparable to the most efficient inverted QLED reported so far.39 We noticed the relatively low luminance of QLEDs, it may be induced by high moisture (about 50%) in test environment, as QLEDs can’t be operated to a higher voltage due to the moisture damage on devices especially on organic HTLs


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(mCP/CBP). To confirm the explanation, QLEDs test under different moisture conditions was conducted, shown in Figure S9, Supporting Information. To test whether this inserted insulating PEI layer could offer common efficiency improvement, we summarized the maximum EQEs of 23 QLEDs all tested without encapsulation at ambient conditions in histogram (Figure 3). The average peak EQE was 14.5% with the relative standard deviation of only 6.5%, which is promising and encouraging.

Figure 3. Histogram of the maximum EQE of 23 unencapsulated QLEDs.

To further confirm the effect of PEI on reducing injected electrons for charge injection balance, we also fabricated the electron-only and hole-only devices for comparison (electron-only:

Al/ZnO/QDs/Al

and

Al/ZnO/PEI/QDs/Al,

hole-only:

ITO/MoO3/QDs/mCP/CBP/MoO3/Al). As is showed in Figure 4a, the current density of the 15 nm PEI modulated electron-only device became lower than that of the electron-only device without PEI and got much closer to that of the hole-only device, proving the electron injection reducing effect of PEI. We also fabricated electron-only devices with different PEI thicknesses,



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corresponding to the emitting devices. Figure 4b shows the comparison of these devices in current density. As can be seen, the current density of the electron-only device with 1 nm PEI is higher than PEI-free device, it becomes lower with the PEI thickness reaching 5 nm and keeps decreasing when PEI thickness further increases. It is in good correspondence with the situation in the related QLEDs and further validates the working mechanism of PEI in QLEDs

Figure 4. (a) Comparison of current density as a function of bias voltage of electron and hole-only devices, electron-only: Al (100 nm)/ZnO (70 nm)/QDs (30 nm)/Al (100 nm) (black), Al (100 nm)/ZnO (70 nm)/PEI (15 nm)/QDs (30 nm)/Al (100 nm) (red); hole-only: ITO/MoO3 (12 nm)/QDs (30 nm)/mCP (10 nm)/CBP (50 nm)/MoO3 (10 nm)/Al (100 nm) (blue). (b) Current density of electron-only devices with different PEI thicknesses. As is detailedly discussed above, PEI reduces electron injection into QDs, and is supposed to contributing to the suppression of QD charging. It is known that ZnO can degrade QLED performance through Auger non-radiative decay by making QDs charged39,

40

and the PL

intensity of QD (proportional to PLQY) decreases due to the Auger recombination.41, 42 In our


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work, we found that exciton quenching existed even at the interface of QD/HTLs (mCP/CBP). As can be seen in Figure 5a, b and the decay fitting curves in Figure S10, Supporting Information, QDs adjacent to HTLs shows a faster PL decay and a lower PL intensity in contrast with the pure QD film and the decay curve shows a three-exponent function fitting, which is attributed to the decay of neutral exciton (X0), singly negatively charged exciton (X-) and doubly negatively charged exciton (X2-) states.41 We ascribe this faster PL decay and lower PL intensity to the ionized QDs by hole transportation to the HOMO of HTLs. QD emission decay in ZnO/QD/HTLs also fit with a three-exponent function and interestingly, a large ratio of X2- state and an extremely long-lived X0 state was observed (Table 1). The increased ratio of X2- state lifetime could be induced by spontaneous electron injection from ZnO to QDs. As X2- state emerges when a single QD is under large negative bias voltage,41 the large ratio of X2- state of QD emission in ZnO/QD/HTLs demonstrate QDs between ZnO and HTLs are seriously negatively charged. We deduced that the excessively injected electron into QDs could enhance the spatial extension of the electron wave function resulting in a reduction in electron-hole wave function overlap and lengthening of lifetime,43,

44

which explains the long-lived X0 state.

However, extremely long-lived excitons can lower the radiative recombination rate and cause exciton annihilation45, 46 which degrades efficiency and further explains why ZnO/QD/HTLs film shows the lowest QD PL intensity and QLED without PEI exhibits lower efficiency. With respect to QD emission in ZnO/PEI/QD/HTLs film, X2- decay state is disappeared with the PL decay curves preferably fitted by a two-exponent function (Figure S10) and the PL intensity is



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enhanced (Figure 5b). The disappearance of X2- decay state means the suppression of spontaneous electron injection from ZnO to QDs by the insertion of PEI, which blocks excess electrons by the insulating nature and by separating QD from ZnO to break electron charging channel as well as ligand modification on the surface of QDs of amine-functionalized ligands of PEI.47 What’s more, the X0 state emission ratio reaches 35.2% just close to the X0 state ratio in pure QD film. As a consequence, the disappearance of X2- decay state, increasement of X0 state ratio and the enhancement of PL intensity of QD emission in ZnO/PEI/QD/HTLs film further proves the efficient QD charging suppression by PEI, indicating the insulativity dominant electron injection reducing effect. To further illuminate the effect of PEI on suppressing QD charging, we also measured the lifetime and PL spectra of QD emission in ZnO/QD and ZnO/PEI/QD films, which clearly shows the lengthened lifetime (18.6 ns for ZnO/PEI/QD film and 16.4 ns for ZnO/QD film) and enhanced PL intensity of QD in ZnO/PEI/QD film (Figure S11, Supporting Information).


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Figure 5. (a) Time-resolved photoluminance decay for QD, QD/HTLs, ZnO/QD/HTLs and ZnO/PEI/QD/HTLs films respectively, the inset shows the enlarged photoluminance decay curves. (b) Stable-state PL spectrum of QD, QD/HTLs, ZnO/QD/HTLs and ZnO/PEI/QD/HTLs films, respectively. Table 1. Lifetime

a

and the corresponding ratio of QD emission in QD, QD/HTLs,

ZnO/QD/HTLs and ZnO/PEI/QD/HTLs films, respectively. sample

߬1 (ns) ratio

߬2 (ns) ratio

߬3 (ns) ratio

QD

13.9

63.6%

36.2

36.4%

QD/HTLs

4.1

28.8%

12.6

59.4%

42

11.8%

ZnO/QD/HTLs

4.3

51.8%

14.4

31.2%

94.9

17%

ZnO/PEI/QD/HTLs 9.6 64.8% 26.7 35.2% The PL decay curves of the QD emission in QD and ZnO/PEI/QD/HTLs films show a two-exponent function fitting, which has two different decay channels corresponding to lifetime ߬1 and ߬2. While the fitting function turns to be a three-exponent for QD emission decay in QD/HTLs and ZnO/QD/HTLs, a new decay channel occurs corresponding to the X2- state. In the table, for QD and ZnO/PEI/QD/HTLs, ߬ 1 and ߬ 2 represent for lifetime of X- and X0 state respectively, and for QD/HTLs and ZnO/QD/HTLs, ߬1, ߬2, ߬3 represent for lifetime of X2-, Xand X0 state, respectively. The appearance of X2- decay state represents for over-charged QDs. PEI inserted device shows a similar X0 state emission ratio with the pure QD film and a total suppression of X2- decay state revealing a positive effect of PEI on maintaining charge neutrality of QDs. a

Besides, the inserted PEI layer also offers lower surface roughness than ZnO layer (Figure 6 and Figure S12, Supporting Information). As is shown in Figure 6a, ZnO film owns a root mean square (RMS) roughness of 2.31 nm, while the peak to valley height of ZnO film reaches 11.4 nm, indicating that a uniform overlying film can be formed only if the deep valleys in ZnO film



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are all filled. Figure 6b shows the AFM image of ZnO/PEI (15 nm) film and the corresponding line-scan profile. The RMS roughness is further reduced to 1.46 nm and the peak to valley roughness of ZnO/PEI is greatly lowered due to the completely filling in the valleys of ZnO film by PEI. The smoother surface morphology of ZnO/PEI benefits QD film deposition, as is showed in Figure 2c, d and Figure S12, where no QD stacking is found on ZnO/PEI film while the stacking situation is severe on ZnO layer. The uniform QD distribution on ZnO/PEI ensured low leakage current for QLED as is shown in Figure S13, Supporting Information, which is crucial for QLED performance improvement, as the reduction of leakage current means the reduction of current flowing into device without emission. 48, 49

Figure 6. (a)-(d) AFM images of ZnO, ZnO/PEI (15 nm), ZnO/QD, ZnO/PEI (15 nm)/QD and the corresponding line-scan profiles, the red arrows point to the QD stacking areas.


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CONCLUSIONS In summary, we have demonstrated the highly efficient, low efficiency roll-off red QLED with an inverted device architecture, by inserting a 15 nm PEI layer. This work emphasizes the combined effect of the intrinsic insulativity and the lowering work function property of PEI, which are two opposite effects on electron injection. The insulativity of PEI impedes electron tunneling from ETL to QDs, which is more effective than the promoted electron injection by the reduced work function. Thus, the charge injection balance is realized. What’ more, the PEI layer also offers lower surface roughness, guarantees uniform distribution of QD layer for leakage current reduction and suppresses QD charging. As a result, the efficiency of the QLED with PEI is 2.5 times improved compared with the QLED without PEI and a peak EQE of 16.5% and a low efficiency roll-off of 15% at 1000 cd/m2 are achieved, showing a great promise for the applications in future display and lighting.



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ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website.

XRD pattern of ZnO NPs; UPS spectra of PEI coated ZnO with increased thickness; characterizations of QLEDs with different ZnO thicknesses and QLEDs with PMMA; energy levels of ZnO, ZnO/PEI and the corresponding QLED; CIE coordinate of QLED; schematics of electron transport in PEI inserted device; exponent fitting curves of QD PL decay; PL decay curves and PL spectra of QD in QD, ZnO/QD and ZnO/PEI/QD films; AFM images of ZnO/PEI and ZnO/PEI/QD with different PEI thicknesses; leakage current in QLEDs. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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The work was supported by the National Basic Research Program of China (973 Program 2013CB922104). We thank Dr. Qingwu Huang for the UPS test in the Analytical and Testing Center of Huazhong University of Science and Technology (HUST) and engineer Huan Lian for the support in XRD test in the Center of Micro-Fabrication and Characterization (CMFC) of Wuhan National laboratory for Optoelectronics (WNLO) of HUST.



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TOC


Polyethylenimine Insulativity-Dominant Charge-Injection Balance for

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