Very High Brightness Quantum Dot Light-Emitting Devices via

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Very High Brightness Quantum Dot Light Emitting Devices via Enhanced Energy Transfer from a Phosphorescent Sensitizer Hossein Zamani Siboni, Bahareh Sadeghimakki, Siva Sivoththaman, and Hany Aziz ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08097 • Publication Date (Web): 02 Nov 2015 Downloaded from http://pubs.acs.org on November 8, 2015

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ACS Applied Materials & Interfaces

Very High Brightness Quantum Dot Light Emitting Devices via Enhanced Energy Transfer from a Phosphorescent Sensitizer Hossein Zamani Siboni* , Bahareh Sadeghimakki, Siva Sivoththaman and Hany Aziz

Department of Electrical and Computer Engineering, University of Waterloo, 200 University Avenue west, Waterloo, Ontario, N2L 3G1, Canada

KEYWORDS: quantum dot light emitting device, organic light emitting device, phosphorescent OLED, efficiency, forster energy transfer,

ABSTRACT

We demonstrate very efficient and bright quantum dot light emitting devices (QDLEDs) with the use of a phosphorescent sensitizer and a thermal annealing step. Utilizing CdSe/Cds core/shell quantum dots with 560nm emission peak, bis (4,6-difluorophenylpyridinatoN,C2) picolinatoiridium as a sensitizer and thermal annealing at 50oC for 30 minutes, green emitting QDLEDs with a maximum current efficiency of 23.9 cd/A, a power efficiency of 31 lm/W and a brightness of 65,000 cd/m2 are demonstrated. The high efficiency and brightness are attributed to annealing-induced enhancements in both the Forster resonance energy transfer (FRET) process from the phosphorescent energy donor to the *

[email protected] Phone: +1-519-888-4567-x32872 1 ACS Paragon Plus Environment


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QD acceptor and hole transport across the device. The FRET enhancement is attributed to annealinginduced diffusion of the phosphorescent material molecules from the sensitizer layer into the QD layer, which results in a shorter donor-acceptor distance. We also find, quite interestingly, that FRET to a QD acceptor is strongly influenced by the QD size, and is generally less efficient to QDs with larger sizes despite their narrower bandgaps.

INTRODUCTION

Quantum Dot light emitting devices (QDLEDs) have attracted significant attention recently for utilization in flat panel displays1-6. The narrow luminescence spectrum and size-controlled luminescence wavelength of nano-crystalline quantum dots make them capable of producing purer and easily tunable colors, hence a better image quality7-8. However, when compared to other electroluminescent devices (i.e. inorganic and organic LEDs (OLED)), QDLEDs exhibit very low efficiency

9-12

.

The low

efficiency is caused in part by the large misalignment between the energy levels of the Quantum Dot (QD) material and those of the adjacent charge transport layers, particularly the hole transport layer, which makes hole injection into them difficult and facilitates exciton quenching by un-recombined trapped electrons13-15.

Various approaches have been proposed for improving the charge balance and device efficiency in QDLEDs16-19. Among them, the use of an inverted device structure in which an inorganic electron transport layer (ETL) and an organic hole transport layer (HTL) are employed appears to be capable of providing significant electroluminescent (EL) efficiency enhancements. This is mainly due to the wider latitude it allows in choosing hole transport materials, hence the ability to utilize materials with deeper Highest Occupied Molecular Orbital (HOMO). Following this approach, Kwak et al. reported a green emitting QDLED with 19.2 cd/A maximum current efficiency

20

. Mashford et al. showed that energy 2

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alignment between the ETL and QDs can similarly impact efficiency and demonstrated a red-emitting QDLED with a 19 cd/A current efficiency through ETL and HTL optimization 21.

Another approach for enhancing efficiency involves incorporating a sensitizer that transfers the excitation energy to the QDs via Forster Resonance Energy Transfer (FRET)22. Phosphorescent organic emitters are considered to be ideal candidates for that purpose, as they have nearly 100% internal quantum efficiency23-24. Anikeeva et al. observed an enhancement in the photoluminescence (PL) intensity of QDs made of CdSe/ZnS core/shell on using fac tris(2-phenylpyridine) iridium (Ir(ppy)3) as a sensitizer22. They found that the lifetime of Ir(ppy)3 became shorter whereas that of the QDs became longer, a sign of energy transfer from the phosphorescent sensitizer (which acts as an energy donor, D) to the QD (which acts as an energy acceptor, A). Zhang et al. later showed that the use of bis (4,6difluorophenylpyridinatoN,C2) picolinatoiridium (FIrpic) phosphorescent sensitizer in a non-inverted CdSe/ZnS QDLED could increase electroluminescence efficiency25. The devices showed a 0.55% external quantum efficiency compared to only about 0.22 % in case of a control device without the sensitizer layer. Later, Mutlugun et al. demonstrated a QDLED with enhanced color purity by incorporating an intermediate layer between the QD and the Ir(ppy)3 sensitizer26. They concluded that the use of an intermediate layer between the QD layer and the phosphorescent emitter was important for reducing the non-radiative dissociation of the triplet excitons at the QD/sensitizer interface.

The

observation of significant luminescence from Ir(ppy)3 when the intermediate layer was not used in their devices however suggested that energy transfer from the sensitizer to the QD

was limited and

incomplete.

In this work we demonstrate very efficient and bright green-emitting QDLEDs utilizing CdSe/CdS core/shell quantum dots as light emitting material. The quantum dots have an emission peak at 560nm. The efficiency enhancement is achieved through the use of a phosphorescent sensitizer and thermal 3 ACS Paragon Plus Environment


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annealing. In these devices, Firpic is used as the sensitizer and thermal annealing is done by heating the completed devices, post fabrication, to 50oC for 30 minutes. The devices demonstrate a maximum current efficiency of 23.9 cd/A, a power efficiency of 31 lm/W and a brightness of 65,000 cd/m2. The high efficiency and brightness are attributed to enhancements in hole transport as well as in the FRET process from the phosphorescent sensitizer to the QD by annealing. We also find, quite interestingly that FRET to a QD acceptor is strongly influenced by the QD size, and is generally less efficient to QDs with larger sizes despite their narrower bandgap.

RESULTS AND DISCUSSIONS

In this work two CdSe/CdS core/shell nanorod QD materials (from CAN GmbH) with maximum luminescence wavelengths of 560nm and 620nm are used, and are referred to here as 560QD and 620QD, respectively (see the supporting information for a schematic illustration of the nanorod structure).

The materials have diameters of 4.5 nm and 5.5nm, respectively, but the same nanorod

length (~20nm), and are surface functionalized with tri-n-octylphosphine oxide (TOPO). The diameters of the cores are approximately 2.5 nm and 3.5 nm, respectively. The nanorods are grown by a seeded growth mechanism in which the CdSe cores are first produced as seeds and isolated, before the CdS shells are grown on them. The mechanism produces relatively abrupt core/shell interfaces with almost no Se in the shell or S in the core (from Vendor’s information). The QD materials are used in an inverted QDLEDs of structure as shown in the schematic diagram in figure 1. In these devices IndiumTin-Oxide (ITO) is used as a cathode and a 40nm thick ZnO is employed as an ETL. A ~15nm thick QD layer (2-3 monolayers) is spin coated on the ZnO film to form the emission layer. A 5nm thick layer of 4,4r-bis(carbazol-9-yl)biphenyl (CBP):FIrpic host:guest mixture with different FIrpic concentrations is deposited on top of the QD layer by thermal co-evaporation, and functions as a

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sensitizer layer. A 35nm thick neat CBP layer is used as a HTL. MoO3 layer with a thickness of 5 nm is employed as a hole injection layer and Al is used as an anode.

Al Al MoO3 (5nm) CBP (35nm) CBP:Firpic (x%) (5nm) QD (15nm) ZnO(40nm) GLASS

ITO

Figure 1. Schematic structure of the QDLED used in this study.

Figure 2a and 2b show luminance and current efficiency versus current density characteristics (the traces with the solid symbols), respectively, of devices with the 560QDs and various FIrpic concentrations. The EL characteristics of the devices are summarized in Table 1. As can be seen, increasing FIrpic concentration leads to an increase in device efficiency. For example, at 20mA/cm2, luminance from the device with 20% FIrpic is 2,450 cd/m2 versus only 1,740 cd/m2 in case of a control device without FIrpic. It is important to note that the device with 20% FIrpic does not show any noticeable differences in color in comparison to the control device, indicating that the increase in efficiency is not due to any luminescence from FIrpic. The increase in efficiency is therefore due to an increase in QD luminescence, suggesting it may be the result of FRET from FIrpic to the QDs, which increases with concentration. Increasing the FIrpic concentration beyond 20% brings about a gradual decrease in efficiency however (data not shown here) which may be due to increased concentration quenching effects in FIrpic.



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Figure 2. a) Luminance and b) Current Efficiency versus Current Density, and c) Current-Voltage characteristics of the 560QD devices at various FIrpic concentrations before and after annealing.

Table 1. EL characteristics of the 560QD device at various FIrpic concentrations at 20 mA/cm2 current density.

Before annealing FIrpic Luminance Voltage concentration (cd/m2) (V) (vol. %)

After Annealing

CIE coordinates x,y

Luminance (cd/m2)

Voltage (V)

CIE coordinates x,y

6%

2050

4.3

0.381, 0.617

3210

3.8

0.375, 0.621

12%

2210

4.5

0.381, 0.615

3760

3.7

0.375, 0.622

20%

2450

4.57

0.380, 0.617

4053

3.8

0.377, 0.620

0% (control)

1740

4.5

0.382, 0.616

2000

3.7

0.377, 0.622

In order to see if thermal annealing can bring about further enhancements in QDLED efficiency, for example by means of increasing the contact between the QDs and the FIrpic sensitizer (due to inter-layer diffusion) or improving charge injection into the QD layer, all devices are then annealed at a 6 ACS Paragon Plus Environment

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temperature of 50oC for a period of 30 mins, and then retested. The traces with the open symbols on the same figure represent the characteristics of the same devices after annealing. As can be seen clearly, annealing results in a significant increase in the efficiency of all devices.

The extent of this

enhancement increases as the FIrpic concentration increases. For example, the luminance from the 20% FIrpic device increases by almost 65% after annealing, becoming 4,053 cd/m2 at 20mA/cm2 versus an increase of only about ~15% in case of the 0% FIrpic device (control device). In this regard, the device with the 20% FIrpic displays a maximum current efficiency and brightness of 23.9 cd/A and 65,000 cd/m2, respectively. Quite notably, changes in color coordinates (Table 1) relative to the control device remain insignificant, indicating that luminescence still arises almost exclusively from the QDs.

Figure 2c shows the current density versus voltage (J-V) characteristics of the devices before and after annealing. Clearly, annealing increases the slope of the J-V characteristics without altering the turn-on voltage. The steeper characteristics suggest annealing improves charge transport across the devices27. In QDLEDs, we can expect the QD emission layers to have relatively high electron-to-hole ratios. This is due to the relatively deep conduction band level of QDs which makes electron injection into them from the adjacent ZnO ETL relatively easy21. In contrast, the large hole injection barrier at the QD/HTL interface makes hole injection from the HTL into the QDs much more difficult. We do not expect changes in the ZnO layer or at the ITO/ZnO and ZnO/QD layer interfaces to be behind this annealing-induced improvement in charge conduction. This is because the annealing temperature is only 50oC, much lower than the 180oC temperature that the ZnO film was cured at prior to coating the QD layer. The annealing temperature also does not exceed the temperature used in drying the QD layer. It follows that the increase in charge conduction after annealing must be mainly due to increased conduction in the hole conduction path. Such increased conduction subsequently results in improved charge balance manifested by the increase in the efficiency of all devices, including the control device.



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In order to explore whether the use of a phosphorescent sensitizer and thermal annealing can bring about similar efficiency enhancements in case of red-emitting QDLEDs, we fabricated and tested another group of devices in which 560QD is replaced by 620QD. Figure 3 shows the current efficiency vs current density of these devices with 20% and 0% FIrpic, the latter to act as a control. As can be seen from the figure, the use of FIrpic as a sensitizer again brings about an increase in device efficiency. However the efficiency enhancement is much less than in case of 560QD devices (the corresponding data from the 560QD devices is shown again in Figure 3 to facilitate the comparison) suggesting that FRET from FIrpic to 620QD may not be as efficient as to 560QD. Here again, thermal annealing brings about a further increase in efficiency, but the enhancement is smaller relative to the case of 560QD device. The EL spectra of the devices before and after annealing are presented in Figure 4. As can be seen from the figure, the EL spectrum of the 620QD device shows some luminescence in the 460-520 nm range, which corresponds to FIrpic luminescence. The presence of FIrpic luminescence indicates that energy transfer from FIrpic to the QD620 is incomplete. In contrast, the spectra from the 560QD devices show negligible luminescence in this range indicating that energy transfer is complete in this case. This observation is somewhat surprising as it is contrary to the general presumption that energy transfer will generally become more efficient as the bandgap of the acceptor decreases (i.e. should be more efficient in case of 620QD, being with a narrower bandgap relative to 560QD).


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Figure 3. Current efficiency versus current density of the 620QD and 560QD devices at 20% FIrpic concentration.

Figure 4. Normalized EL spectra of the 560QD and 620QD device with 20%. FIrpic before and after annealing. The inset shows the enlarged EL spectra

In order to gain insights about the role of the sensitizer in the efficiency enhancement and its different effects in case of the 560QD vs the 620QD materials, photophysical measurements on thin films of the neat QD and FIrpic materials as well as of QD:FIrpic mixtures are carried out. Figure 5 shows normalized optical absorption and PL spectra of the QDs and FIrpic. Both QDs have broad absorption 9 ACS Paragon Plus Environment


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spectra and overlap with the PL luminescence spectrum of FIrpic indicating that efficient energy transfer from FIrpic to both 560QD and 620QD should be possible from a spectral overlap standpoint. Figures 6a and 6b present results from time-resolved PL measurements on the FIrpic:QD mixture films showing the decay rate of PL at 470, 560 and 620 nm, respectively, which correspond to luminescence from FIrpic, 560QD and 620QD, respectively. As can be seen from figure 6a, mixing QDs with FIrpic accelerates the FIrpic PL decay rate, indicating that energy transfer from FIrpic to the QDs indeed occurs, hence the creation of an additional pathway that leads to the faster decline in the FIrpic exciton population. More notably however, the change in FIrpic PL decay rate is more significant when mixed with 560QD than when mixed with 620QD, consistent with the earlier conclusion that energy transfer from FIrpic to 560QD must be more efficient. In the same time, we can see from figure 6b that the PL decay rate of the QDs becomes slower when mixed with FIrpic. The slower decay rate is evident in the slower decaying tails which can be attributed to their slower excitation via the longer-lived FIrpic excitons through FRET (The initial fast decay is likely from QD excitons that get created via direct excitation by the incident laser beam, rather than via energy transfer from FIrpic, hence exhibit the typical unperturbed lifetime of QDs (~20 ns)). Again, from the figure, it is clear that the QD PL lifetime is much longer in case of 560QD:FIrpic than in case of the FIrpic:620QD mixture, mirroring the more significant FIrpic to QD FRET in the first system. We tested a third FIrpic:QD system using QDs with an even longer peak emission wavelength (i.e. an even narrower bandgap). These QDs are composed of CdSe/ZnS core/shell (7.5nm diameter) and have an emission peak of 660nm. As can be seen from figures 6a and 6b, the PL decay rates of both FIrpic and the QD in the mixture films versus the neat films remain essentially unchanged, indicating that energy transfer from FIrpic to the QDs is indeed insignificant in this case. The results, therefore, convincingly show that energy transfer from FIrpic to the QDs is quite efficient in case of 560QD but becomes increasingly less efficient for the QDs with the longer emission wavelengths despite the increase in the spectral overlap. This behaviour can perhaps be attributed to QD size effects. Aside from the need for a large spectral overlap between the donor PL 10 ACS Paragon Plus Environment

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luminescence and acceptor absorption, the distance between the donor and the acceptor must also be sufficiently small for efficient FRET to occur. Since the typical radius of a core/shell type QD with surface functionalization (such as TOPO) is in the range of a few nanometers, i.e. of the same order of the Forster critical radius7, it is possible that the FRET rate from an energy donor located near the QD surface to the QD core decreases as the QD radius (hence size) increases. In this regard QD size becomes a limiting factor for the FRET process 28-29. It should be pointed out that although the amount of TOPO and surface passivation will generally increase when QD size becomes smaller (due to an increase n surface to volume ratio), the TOPO length remains essentially unchanged and therefore the influence of TOPO on the energy transfer process should not be expected to change appreciably with the QD size. The increase in FRET rate as the QDs become smaller can thus be attributed primarily to the decrease in the D-A distance due to the smaller QD radius.

Figure 5. UV-Vis absorption and PL spectra of the QDs and phosphorescent sensitizers.



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Figure 6. Time resolved PL lifetime decay rates of a)FIrpic and b) QDs in the films..

Figure 7. High resolution SEM image of 560QD film on the ZnO layer. The inset shows HRTEM image of 560QD.


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Figure 7 shows a high-resolution scanning electron microscope (HRSEM) image of a 560QD film deposited on a ZnO layer. As can be seen from the figure, most of the nanorod QDs acquire a horizontal alignment, lying with their long axis parallel to the film plane.

In order to understand the role of the annealing in the efficiency enhancement, time-resolved PL measurements on 560QD (~15nm)/CBP:FIrpic (5nm) bilayer stacks with 20% and 30% FIrpic before and after annealing at 50oC for 30 minutes are carried out. Figures 8 shows the PL decay lifetimes of FIrpic (figs 8 a and c) and 560QDs (figs 8 b and d) in the bilayer films with the 20% and 30% FIrpic, respectively. In these figures the control data corresponds to PL decay lifetime from 560QD single films without the CBP:FIrpic sensitizer layer. As can be seen from figures 8a and 8b, annealing results in a decrease in the FIrpic PL decay rate and an increase in the QD PL decay rate indicating an enhancement in the energy transfer from FIrpic to the QDs due to annealing. The QD decay lifetimes in the control films on the other hand remain essentially unchanged after annealing. The fact that the changes in PL lifetime are more pronounced in FIrpic than in the QDs likely arises from the higher optical density of the QDs which allows for a larger fraction of the QDs to be excited directly by the laser (i.e. not via energy transfer from FIrpic) and thus do not exhibit a change in lifetime by annealing, consistent with earlier reports30. The results, therefore, suggest that annealing enhances the FRET process from the sensitizer to the QD, possibly as a result of reduced donor-acceptor distance due to annealing-induced diffusion of the phosphorescent sensitizer into the QD layer. As most excitons in case of EL are created within a close proximity of the CBP:FIrpic/QD interface, where this annealing-induced inter-diffusion can be expected to be more significant, changes in EL due to annealing are generally more significant in comparison to changes in PL.



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Figure 8. Pl decay lifetime of a) FIrpic and b)560QDs in the 560QD (~15nm)/CBP:FIrpic (20%) (5nm)

bilayer and c) FIrpic and d)560QDs in the 560QD (~15nm)/CBP:FIrpic (30%) (5nm) bilayer film before and after annealing. The insets show enlarged QD PL decay lifetimes.

Figures 9a and 9b show HRTEM cross-sectional images of an un-annealed (i.e. as fabricated) 560QD device and of an identical but annealed device, respectively. A comparison between the images shows that the overall thickness of the organic stack is reduced in the annealed device. A close examination of the figures reveals that the difference is mainly in the CBP:FIrpic/CBP layers which become thinner after annealing. The decrease in thickness suggests CBP:FIrpic layer indeed diffuses into the QD layer, consistent with the conclusion that FRET enhancement after annealing is due to a reduction of RD-A. The decrease in thickness can also – at least in part – be the result of a more compacted morphology in the CBP and CBP:FIrpic layers due to annealing. Figures 9c and 9d show higher magnification TEM images of the CBP/MoO3 interface in the two devices.

The images clearly demonstrate that the 14

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MoO3/CBP interface is significantly less abrupt in case of the annealed device, suggesting that some inter-diffusion between the MoO3 and CBP layers may be occurring due to annealing. The less abrupt MoO3/CBP interface as well as the more compact CBP layer brought about by the annealing are perhaps behind the improved hole transport in the annealed devices pointed to earlier (Fig 2c). The results therefore suggest that the efficiency enhancement by annealing is the result of both (i) a reduction in the energy transfer distance from the sensitizer to the QDs and (ii) morphological improvements in the hole transport path that bring about improved charge balance.

Figure 9. HRTEM images of the 560QD device before (a and c) and after (b and d) annealing.

CONCLUSION

In conclusion, we demonstrate very efficient and bright QDLEDs with the use of a phosphorescent sensitizer and a thermal annealing step. Green-emitting QDLEDs with a maximum current efficiency of 23.9 cd/A, a power efficiency of 31 lm/W and a brightness of 65,000 cd/m2 are demonstrated using this approach. The high efficiency and brightness are attributed to annealing-induced enhancements in both the FRET process from the sensitizer to the QD and the transport of holes across the device. The efficiency of the FRET process from the sensitizer to the QDs is found to be strongly dependent on the 15 ACS Paragon Plus Environment


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QD size, and is generally less efficient to QDs with larger sizes despite their narrower bandgaps. The results suggest that this behavior could be due to the generally large FRET donor-acceptor distances in case of QD systems, resulting from the relatively large size of QDs, which make FRET rate decrease as QDs become larger. Annealing the devices results in the diffusion of the sensitizer into the QD layer leading to a shorter donor-acceptor distance, and hence can enhance the FRET rate. Finally, this work demonstrates that the combined use of a phosphorescent sensitizer and thermal annealing is an effective approach for achieving very high brightness and efficiency QDLEDs, and thus opens up opportunities for utilizing them in a wide range of technological applications including QD display panels.

EXPERIMENTAL PROCEDURE

In this work QDLEDs with the structure of ITO/ZnO/QD/CBP:FIrpic/CBP/MoO3/Al are fabricated and tested. Prior to the device fabrication ITO coated glass substrates are cleaned with acetone, and isopropanol in an ultrasonic bath. Zinc Oxide (ZnO) precursor has been prepared by mixing 0.194 g zinc acetate dihydrate and 54 µa ethanolamine in 6 mL ethanol and stirred at 45 oC for 2 hours. 40 nm thick ZnO ETL has been formed by spin coating of ZnO solution at a rate of 800 RPM for 1 min and subsequent annealing at 180 oC for 1 hour. Nanorod CdSe/CdS quantum Dots (QDs) with wurtzite crystal structure that are dissolved in HPLC grade hexane (i.e. ≥98.5% purity) have been purchased from CAN GmbH and used as received without further purification. Quantum yield of the studied highly crystalline QDs is 85% (from the supplier’s data sheet and our verification measurement). QD emission layer has been deposited by spin coating of 0.6 mg/mL QD concentration in hexane at a rate of 2000 rpm for 1 min. The deposited film is then annealed in the glovebox under nitrogen ambient at 50 oC for 30 mins in order to facilitate the solvent evaporation. The substrate is then transferred to the thermal evaporation system for the deposition of organic layers at a base pressure of 5 e 10-6 Torr. Organic layers have been deposited at a rate of 1 Ao/s. In this work all organic materials have been purchased 16 ACS Paragon Plus Environment

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from Lumtec Co. and used as received. Current density- measurements were carried out using Agilent 4156C semiconductor analyzer and Minolta Chroma Meter CS-100. The devices were stored and characterized under nitrogen ambient without encapsulation.

Time resolved photoluminescence (PL) lifetime measurements are carried out using Edinburgh instruments FL920 spectrometer equipped with picosecond pulsed laser diode with the peak luminescence at 375 nm. Samples are prepared by spin coating of a glass substrate with a 2 mg/ml FIrpic and 0.05 mg/ml QDs mixture in 1 ml toluene at a rate of 700 rpm for 30 second.

Corresponding Author [email protected] ACKNOWLEDGMENT

Financial support by the Natural Science and Engineering Research Council of Canada is acknowledged. SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website at DOI: Current efficiency versus current density of the device after annealing at 70oC, schematic illustrating the microstructure of nanorod.



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