Pushing the Efficiency Envelope for Semiconductor Nanocrystal

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Perspective

Pushing the Efficiency Envelope for Semiconductor NanocrystalBased Electroluminescence Devices Using Anisotropic Nanocrystals Whi Dong Kim, Dahin Kim, Da-Eun Yoon, Hyeonjun Lee, Jaehoon Lim, Wan Ki Bae, and Doh C. Lee Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b05366 • Publication Date (Web): 22 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019

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Chemistry of Materials

Pushing the Efficiency Envelope for Semiconductor Nanocrystal-Based Electroluminescence Devices Using Anisotropic Nanocrystals Whi Dong Kim1, Dahin Kim1, Da-Eun Yoon1, Hyeonjun Lee1, Jaehoon Lim*,2, Wan Ki Bae*,3, and Doh C. Lee*,1 1Department

of Chemical and Biomolecular Engineering, KAIST Institute for the

Nanocentury, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Korea. 2Department

of Chemical Engineering and Department of Energy System Research, Ajou University, Suwon, Gyeonggi-do 16499, Korea.

3SKKU

Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon, Gyeonggi-do 16419, Korea.

E-mail: D.C.Lee ([email protected]), W.K.Bae ([email protected]), J.Lim ([email protected])

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Abstract Colloidal semiconductor nanocrystals hold great promise in display technologies, as the tunable energy levels and narrow emission bandwidth allow for wide gamut in color space. Impetus for energy-efficient, high-color-quality display has driven the surge of interest in electrically driven quantum dot based light-emitting diodes (QD-LEDs). While extensive efforts have led to synthesis of QDs with near-unity photoluminescence quantum yield and fabrication of QD-LEDs with external quantum efficiency reaching to the theoretical limit (~20%), low out-coupling factor poses a challenge in the way of improving the device performance when spherical QDs are used. Geometrically anisotropic nanocrystals (NCs), such as nanorods or nanoplatelets, represent a unique possible solution to enhancing light extraction efficiency.

In this Perspective, we highlight important design principles of

individual anisotropic NCs and their assembly in the context of LED applications.


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1. Introduction In the past few decades, the research field of colloidal quantum dots (QDs) has given us dazzling progress: from experimentally reproducible size-tunable growth to large-scale wetchemical synthesis of highly emissive QDs1-5 and commercial use in televisions.6 Tunable wavelength and narrow bandwidth of photoluminescence (PL) from QDs along with their nearunity PL quantum yield (QY) have rendered QDs powerful luminophores for high-color-purity displays.5, 7 Commercially available liquid crystal display (LCD) large-screen televisions using QD film as a down-conversion layer represents the pinnacle of the progress. Daunting as it may sound, the market is eyeing on QD-based light-emitting diodes (QD-LEDs), in which charge carriers are electrically injected into a QD layer and give off electroluminescence (EL). The EL devices can address fundamental limit of LCD, e.g., low energy efficiency, structural rigidness, poor viewing angle and contrast ratio.8 Recent publications report QD-LEDs with peak external quantum efficiencies (EQE, a ratio between photons generated from the device front and electrons injected to the device) reaching ~20%, a theoretical upper limit with an assumption of out-coupling efficiency (a fraction of photons escaping out of a LED) of 20%.9-14 In addition, maximum brightness reached ~319,000 cd m-2, ~460,000 cd m-2 and ~22,900 cd m-2 for red, green and blue emission, respectively,15-17 which are comparable to those of commercially available organic light emitting diodes (OLEDs).18 Following the remarkable progress in device efficiency and brightness, the field of QDLEDs faces a daunting challenge: Can QD-LEDs exhibit far better efficiency than the current state of the art? How can one operate the devices at a high current density without deteriorating the EL? These are legitimate device questions, yet the discussion has to come full circle and all eyes on QDs, the materials. Figures of merits in QD-LEDs relate to photophysics, colorimetry and their complicated combinations, but the metrics eventually boil down to how 3 ACS Paragon Plus Environment


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many photons are produced from electricity. While one should account for several physical processes taking place in devices, the emission of photon is a result of charge injection, exciton formation, and its radiative decay, all occurring inside QDs. Obviously, QDs must have high emission QY for QD-LEDs to reach high internal quantum efficiency (IQE). Yet, high emission QY is not necessarily sufficient to break through the current efficiency ceiling in QD-LEDs. While emission QY is typically measured by optically pumping QDs to generate electron–hole pairs, electrical pumping in QD-LEDs involves separate injection of electrons and holes by means of electric field, which could cause charged NCs as a result of uneven carrier injection rates. In addition to the field-induced exciton dissociation,19,

20

the generation of charged excitons and consequent nonradiative

Auger recombination (AR) have been identified as potential origins for premature onset of the efficiency roll-off, a reduction of device efficiency with increasing current density, and operational instability of devices.21-23 While some QD-LEDs show IQE (a ratio of generated photons to injected electrons) approaching 100%, the peak EQE is still largely restricted by out-coupling factor (ca. 0.2), a value mainly set by plasmonic loss through metallic electrodes and waveguiding along with charge transport layer/transparent electrode and glass substrates.24-26 Widespread use of QD-LEDs requiring high optical output (e.g., outdoor applications like signage, lighting, headlight, and so on) would require materials that can address these issues. For designing and synthesizing QD heterostructures, arrested precipitation of nanocrystals (NCs) in organic solution has been a powerful protocol with ever-expanding collection of understanding on precursor decomposition, crystal growth, and surface-ligand interaction.27 A branch of research effort is pointed toward the growth of anisotropic semiconductor NCs (referred to as aNCs hereafter), such as nanorods (NRs) or nanoplatelets (NPLs).28, 29 The use 4 ACS Paragon Plus Environment

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of aNCs as the emitters in LEDs could potentially address the aforementioned roadblocks (i.e., AR and light out-coupling factor) in QD-LEDs. First, anisotropic geometry of NRs and NPLs helps suppress the AR of charged excitons. Because charge carriers are delocalized along the long axis of NRs and the plane of NPLs, the AR, inelastic scattering of three charge carriers, becomes less probable in such nanostructures with an extended volume.30-35 Second, by way of dipole orientation in emitter layer, one can suppress dissipation of light by surface plasmon and total reflection, which can boost the out-coupling factor (ηout) from 0.2 to as high as 0.4.24, 25, 36, 37

The idea of the enhanced out-coupling factor using dipole alignment can be translated

into aNC-based emitters.

Anisotropic dielectric confinement and modified exciton fine

structure38-43 produce a transition dipole aligned to the longer axis of aNCs. In addition, ordered aNC assembly lying parallel to the substrate can allow for better light extraction. Guided self-assembly via controlling surface chemistry of aNCs, external fields, or physically or chemically patterned templates stands as a promising tool for a long-range ordering of NRs and NPLs.44-50 Several review articles have already covered growth, characterization and assembly of aNCs.51-67 Therefore, in this Perspective, we narrow our focus on the design of aNCs in the context of LED applications. Our discussion centers around CdE (E=S, Se, or Te) NRs and NPLs, where one can readily find breadth of publications on experimental and theoretical studies. We suggest important design principles on the growth of aNCs in light of controlling their transition dipoles and AR rate, and assembling them into films. Based on this discussion, we examine the prospect of aNCs as the emitters in LEDs compared to spherical QDs.

2. Prospect of anisotropic nanocrystals in nanocrystal-based light emitting diodes 5 ACS Paragon Plus Environment


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LED performance is usually evaluated in terms of EQE, a ratio of the number of photons emitted out of a device to the number of injected carriers. EQE of a QD-LED is described by the following expression: EQE = η𝑜𝑢𝑡 × IQE = η𝑜𝑢𝑡 × (𝑟 × 𝛾 × 𝑞)

(1)

where ηout denotes an out-coupling efficiency, and r, γ and q are a fraction of excitons enabling optically-allowed transition, charge balance, and radiative recombination efficiency of emitter, respectively. When photons are emitted from a thin luminescent layer, a large portion of the photons are bounced back or lost via substrate-, waveguide- and surface plasmon-modes.25 In the substrate and waveguide modes, light emitted with an incident angle exceeding the critical angle is trapped inside the glass substrate and interlayers (i.e., organic and inorganic charge transport layers, QD film and ITO), respectively. Evanescent wave at the electrode surface is responsible for another light loss, referred to as surface plasmon-mode. Figure 1a illustrates the various modes in which the generated photons from the emitting layer are dissipated. Approximation based on QD-LED architecture has yielded the out-coupling factor of 0.2.68-70 Remarkable progress in QD-LEDs that has elevated their EQE to ~20%9-14 paradoxically casts a question that the QD-LEDs have already hit the limit with their out-coupling factor fixed at 0.2.


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Figure 1. (a) Schematic illustration of light loss processes in typical LED structure. (b) Vertical (left) and horizontal (right) dipole orientation of the emitters on the metal electrode and corresponding transverse mode. Red arrows indicate the dipole orientation of the emitter and the red curves represent the propagation of light. Green contours show radiation patterns of the corresponding orientation of dipole. TM and TE emission of the vertical and horizontal dipole propagates along the interface of the metal electrode and perpendicular to metal electrode, respectively. (c) Schematic showing propagation of light from the emitters with the vertical (left) and horizontal (right) dipole orientation to the transparent substrate. Red arrows are the dipole direction of the emitter, and θc is critical angle. Green contours show radiation patterns of the corresponding orientation of dipole. (d) Distribution of electrons and holes wavefunctions of NRs and NPLs according to orientation and applied electric fields. Blue and red solid line is the wavefunction of electrons and holes, respectively. Black arrows indicate the direction of the external electric field. To overcome the out-coupling efficiency limit, several light-extraction approaches have been utilized in LEDs.68, 71-75 For instance, patterning of light-emitting layer composed of organic light-emitting polymers and QDs results in the Bragg scattering of waveguided light, 7 ACS Paragon Plus Environment


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leading to constructive interference of emission in the out-of-plane direction, while the in-plane momentum is conserved.76 By using components of different refractive indices, hemispheretype macroextractors or nano-pillar arrays, one can suppress total internal reflection resulting from substrate- and waveguide-modes.68, 71-74 The surface plasmon-mode may tune out with the emitting layer and the metallic electrode further apart, while the effect usually comes at the expense of increasing ratio of waveguide mode.75 Experimental and theoretical studies indicate that orientation of emitters have significant bearing on surface plasmon-mode and ultimately out-coupling factor.24, 77, 78 Light loss due to surface plasmon coupling is mainly caused by transverse magnetic (TM) mode waves traveling at the interface between a metal and a dielectric. The TM mode wave, which represents polarized light with its electric field along the plane of incidence, is generated from vertical dipole orientation of emitter.26 On the other hand, plasmon coupling is expected to diminish in the case of horizontal dipole orientation of emitter, which leads to half-TM mode wave and half-transverse electric (TE) mode wave.25 Figure 1b shows the light propagation wave mode at dielectric/metal interface depending on orientation of emitter dipole. In addition, the dipole orientation also comes into play for the substrate- and waveguide-modes. Since the photon can be extracted when the angle of emission is smaller than the critical angle, dipole orientation largely influences light extraction efficiency. Emitting dipoles oriented parallel to the substrate leads to high light extraction efficiency, because the emission intensity of a linear dipole relates to sin2𝜃 distribution, where 𝜃 is the angle between the dipolar axis and the direction of light propagation. On the other hand, large emission angle from vertical dipole leads to increase in total internal reflection of the photons. The fraction of escaped photons upward at angles less than the critical angle for the horizontal and vertical dipole orientations, is given by 𝑏𝑣(𝜃𝑐) 𝑏0

1

= 4(2 + 𝑐𝑜𝑠3𝜃𝑐 ―3cos𝜃𝑐) (2) 8

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𝑏h(𝜃𝑐) 𝑏0

1

= 8(4 ― 𝑐𝑜𝑠3𝜃𝑐 ―3cos𝜃𝑐) (3)

where 𝑏𝑣/𝑏0 and 𝑏h/𝑏0 are the fraction of escaped photon for vertical and horizontal dipole orientations, respectively, and 𝜃𝑐is the critical angle.79 Figure 1c depicts strong correlation between the fractions of escaped light and dipole orientation.

Simply put, horizontal

orientation of emitting dipoles effectively increases out-coupling efficiency. This orientation dependence suggests that QD-LEDs based on spherical QDs are subjected to the nearly fixed value of out-coupling efficiency at 0.2, because of the random dipole orientation in QDs. The anisotropic geometry in aNCs, NRs and NPLs alike, can come into effect for the enhancement of out-coupling factor. As the horizontal dipole orientation has proven to double the out-coupling factor in the research of OLED,26,

80-86

the horizontal orientation of the

transition dipole in aNCs would boost out-coupling efficiency as well. Unlike spherical NCs, NRs exhibit linearly polarized emission resulting from transition dipole. The exciton fine structure splitting and the anisotropic dielectric confinement are indeed responsible for the polarized emission in NRs.38-42 For wurtzite CdSe QDs, the electron ground state has ssymmetry and presents a double degeneracy, while the first hole level with a p-symmetry is four-fold degenerate. Thus, the first excited state (1S(e)-1S3/2(h)) has an eight-fold degeneracy. Because of uniaxial crystal lattice (wurtzite), shape anisotropy and electron-hole exchange, initially eight-fold degenerate band-edge is split into five sublevels with total angular momenta, |Nm| = 0U, 1U, 0L, 1L, and 2.87, 88 Only three (0U, 1U and 1L) of them are optically active, whereas 0L and 2 are forbidden. For perfectly spherical CdSe NCs, 2 is the lowest, optically forbidden exciton state and the next optically allowed state is 1L.87 Therefore, circularly polarized emission has been predicted and reported for 1L exciton in the case of spherical NCs. Asymmetry in NC shape would lead to significant change in the above model. Efros et al. 9 ACS Paragon Plus Environment


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calculated the exciton fine structure in elongated wurtzite CdSe NCs.87 From the estimation, it was proposed that the exciton state with zero angular momentum projection (0L) along the axis of rod growth becomes the ground state in elongated NCs and induces a strong linearly polarized emission.87 Therefore, the shape asymmetry results in transposition of the lowest exciton state in NRs, thereby causing linearly polarized emission. Besides exciton fine structures, dielectric confinement can complement the increase in polarization of absorption and emission. Typically, the dielectric constant of semiconductor NCs is considerably larger than that of surrounding organic ligands. In the case of NRs, this difference leads to the formation of 1D exciton along the long axis, further enhancing the polarized emission of NRs.42 The anisotropic geometry of NPLs is responsible for direction-dependent electronic structures and polarized emission. In CdSe NPLs, the top of the valence band is mainly contributed by heavy-hole states as a result of large energy splitting between heavy-hole and light-hole transitions (e.g., 167 meV at 298 K for NPLs with 5 Cd and 4 Se layers).89, 90 Since the heavy-hole exciton forms in-plane electronic dipole, NPLs have predominant orientation of the transition dipole moment in their in-plane direction.43, 91 The polarized emission of NPLs can be further enhanced by the dielectric contrast between the NPL and the surrounding medium (organic ligands). In addition to light out-coupling efficiency, IQE is an obviously important parameter for EQE. Typically, high PL QY of emitters equates to high IQE, so strategies to increase IQE revolve around making high-quality NC samples. However, notwithstanding high PL QY, some samples exhibit progressive deterioration of EQE at high operation voltage and current density, which is referred to as efficiency roll-off or efficiency droop.15, 20, 22, 23, 92 The operation of QD-LEDs is hamstrung by the roll-off, which escalates power consumption and reduces the longevity of the devices. The roll-off is attributed to the imbalance of charge carriers injected into NCs.93 The discrepancy between electron and hole injection rates yields the presence of 10 ACS Paragon Plus Environment

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excess charge carriers along with electron and hole pair and this leads to non-radiative AR process, in which the exciton recombination energy is transferred to the extra carriers rather than being released as a photon.4, 92, 94 In addition, dissociation of electron and hole due to induced electric field could also contribute to efficiency roll-off.19, 20 The core/shell NCs with type-I band offset would confine electrons and holes in the core and suppresses the exciton dissociation, but strong Coulomb interaction would promote AR process.95 On the flip side, in quasi-type II band offset, exciton dissociation at high operating voltage poses a predicament. Again, aligned aNCs can be a potential solution to the efficiency roll-off issue. The carriers in aNCs experience strong confinement along diameter and thickness of NRs and NPLs, respectively, whereas they delocalize along the long axis of the NR and lateral axis of NPLs. Therefore, engineering of lateral dimension of NPLs or length of NRs can enable the mitigation of AR while retaining band gap tunability. By contrast, from the viewpoint of spatial separation of carriers, this efficient carrier delocalization in aNCs gives rise to a larger PL reduction under an applied electric field than in spherical QDs.96, 97 Alignment of aNCs holds promise in search of a balance between spatial separation of carrier and AR process. When NRs and NPLs are placed on the substrate, the applied electric field no longer has a significant effect on the carrier separation, if the delocalization direction of the carriers is perpendicular to the direction of the electric field as shown in Figure 1d.97 Therefore, a strategy based on both synthesis of geometry-controlled aNCs and their assembly can lead to suppression of both AR processes and charge separation, mitigating efficiency roll-off. Elongated geometry in aNCs and their alignment would make aNCs attractive candidate materials as emitting layer in LEDs. Then, a more important question becomes how one could realize the promise by incorporating aNCs into LEDs. In the following sections, we overview the status quo in the research progress on aNC-LEDs and discuss the performance of aNC11 ACS Paragon Plus Environment


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LEDs in the context of photon extraction efficiency and nonradiative AR processes.

3. Efficiency envelope of anisotropic nanocrystal-based light emitting diodes: current status and roadblocks

Figure 2. (a) State of the art of external quantum efficiency (EQE) of EL devices based on spherical (black),9-11, 13, 14 rod-shape (blue)98-101 and plate-shape (red)102-105 NCs as a function of PL QY of NC emitters. Purple and green dashed line represents the theoretical EQE limit depending on PL QY of emitters, assuming that the out-coupling efficiency of device is 20% and 40%, respectively. (b) Current EQE values of OLEDs based on homoleptic iridium complex and thermally activated delayed fluorescence (TADF) materials are plotted as a function of PL QY and dipole orientation with a color index of EQE. Adapted with permission from Ref 25. Copyright 2017 Wiley‐VCH.

Several unique characteristics of aNCs highlight the prospect of their use as emissive materials in LEDs: high PL QY, reduced AR rate, and transition dipole orientation.104 Despite the promise, the progress has amounted to LEDs based on aNCs with considerably lower efficiency than LEDs based on spherical QDs. Figure 2a and Table 1 summarize some of reported EQE values for NC-based state-of-the-art LEDs as a function of PL QY of emitters.9-11, 12 ACS Paragon Plus Environment

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13, 14, 98-104

While QD-LEDs using spherical NCs exhibit their peak EQEs approaching the

projection line with ηout = 0.2, most aNC-LEDs have fallen way short of their potential efficiency limit with ηout = ~0.4 and even less than that of QDs, regardless of their PL QY. Recently, Shim and co-workers reported the use of double heterojunction NRs (DHNRs),106 in which CdSe QD emitters are located at the tips of CdSe NRs and the emitters are covered with ZnSe shell. In spite of moderate PL QY of DHNRs, balanced charge injection through two distinctive phases (i.e., electron injection through CdS and hole injection through ZnSe) and enhanced out-coupling efficiency by randomized in-plane arrangement of NRs allow to increasing a peak EQE of 12%. Assuming IQE = 40% (PL QY of DHNRs), this peak EQE at least corresponds to ηout > 30%, considering the devices were not fully optimized. This particular work provokes several questions: Why do NRs with better PL QY still exhibit far lower EQE than DHNRs? Will the device results using DHNRs apply to different types of aNCs? Can aNCs really help push the efficiency envelop of QD-LEDs? In fact, questions similar to these helped increasing EQE of OLEDs (Figure 2b).25 Multilateral efforts have led to high QY of emitters and overcoming spin statistics of triplet exciton as well as to align the transition dipoles of emitters in the in-plane direction. In addition, alignment of the emitting molecules and their dipole moments resulted in high out-coupling efficiency, boosting EQE of OLEDs up to 39%, nearing the upper boundary of ηout = 40% (see Figure 2b and Table 1). On the other hand, relatively few studies examine relationship between transition dipole and light out-coupling in aNC-LEDs.

Now that the operational principles and device

architectures are nearly identical between OLEDs and QD-LEDs, these lessons learned from OLED technology could save the researchers tiresome optimization. Table 1. External quantum efficiency and out-coupling efficiency of EL devices collected from previous studies depending on type of emitter Type

Shape

Chemical

Device

PL

EQE

Estimated

Ref. 13

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composition structure QY (%) (structure) (%) CdSe/CdS Conventional ~90 18.5 (core/shell) CdSe/CdS/ZnS Inverted 85 16.8 (core/shell/shell) CdSe/ZnS Spherical Inverted ~90 15.6 (core/shell) ZnCdS/ZnS Conventional 91 15.4 (core/shell) CdSe@ZnS/ZnS Conventional 83 12.6 (alloyed core/shell) CdS/CdSe/ZnSe (double Conventional ~40 12.1 heterojunction nanorod) CdS/CdSe/ZnSe NC(double LEDs Conventional ~40 10.7 Rod heterojunction nanorod) CdSe/CdS Conventional 38 5.4 (dot-in-rod) CdSe/CdS Conventional 54 6.1 (dot-in-rod) CdSe/CdZnS Conventional 40 8.39 (core/shell) CdSe/CdS Conventional 60 5.0 (core/shell) Plate CdSe/CdSeTe Inverted 85 3.75 (core/shell) CdSe/CdS Conventional 30 0.63 (core/shell) Pt(fppz)2 Conventional 96 38.8 Ir(dmppy-ph)2tmd Conventional 96 38.1 SpiroAC-TRZ Conventional 100 36.7 OLEDs Ir(3′,5′,4Conventional 97 34.1 mppy)2tmd mCP:B3PYMPM:(5 Conventional 97 29.6 wt% 4CzIPN) [a]Estimated out-coupling efficiency under the assumption (IQE = PL QY of emitter) [b]Calculated out-coupling efficiency value using measured IQE value

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ηout[a] 20.6

9

19.8

11

17.3

10

16.7

13

15.2

14

31.3

100

26.7

98

14.2

101

11.3

99

20.9

104

8.3

102

4.4

105

2.1

103

40.4 38.9 38.3[b]

83 84 82

35.1

85

30.3

86

To come up with aNC-LEDs with EQE of 40%, we need to identify the efficiency-limiting factors and their origin. Basically, NC-based LEDs share their fundamental working principles regardless of NC morphology. In eqn. 1, we classify the efficiency-determining factors and express the EQE as a function of variables, ηout, r, γ and q. This simple expression describes how the separate processes in a LED amount to its efficiency, yet the processes are intertwined 14 ACS Paragon Plus Environment

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with one another and complicated phenomena can arise when each variable is not correctly quantified. For example, shell composition is one of determining factors for high PL QY (e.g., introduction of wide band gap materials for type-I configuration) but also affects charge injection efficiency and the AR rate. In-plane orientation of NRs not only improves the outcoupling factor but also lowers PL QY by promoting energy transfer (ET) processes among NRs when they are closely packed. While progress in wet-chemical approaches has enabled the synthesis of NCs with PL QY of over 80%, their emissive property is vulnerable to numerous deterioration pathways and lowers the device efficiency as experienced in early-stage QD-LED studies.107, 108 To make matters worse, NRs and NPLs have larger surface area per a NC compared to QDs due to their elongated morphology, which is likely to amplify nonradiative decay of exciton via surface defects potentially generated during surface modification, purification, and device fabrication steps. Moreover, additional PL QY reduction takes place when the assembly of aNCs in the emissive layer aligns the transition dipole of aNCs in parallel direction (e.g., parallel packing of NRs or face-to-face alignment of NPLs).109 These fundamental limitations have posed a roadblock in the performance of aNC-based LEDs, notwithstanding the promises shown in the studies on photophysical properties.110 Electron oversupply and consequent poor radiative exciton production efficiency have long been an issue in QD-LEDs owing to the mismatch of electron and hole injection barrier. This mismatch of carrier injection rate can become even worse in the presence of passivation shell if it provides additional asymmetric carrier injection barrier.21, 104, 111, 112 A representative example demonstrated in QD-LEDs is CdSe/CdS core/shell QDs in which CdS provides additional large hole injection barrier (ca. 0.6 eV) but negligible electron injection barrier (ca. 0.1 eV or less).110 Unfortunately, most aNCs are subject to this problem: Majority of aNCs 15 ACS Paragon Plus Environment


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introduces CdSe and CdS as main components for emissive core and passivation shell (e.g., CdSe/CdS DIR, CdSe/CdS core/crown NPLs, etc.). Despite the rapid progress in the study of the effects of intrinsic characteristics of aNCs on aNC-LEDs, relatively smaller attention has been given to investigation of the correlation between assembly structure of aNCs and device performance, particularly in the frame of ηout. An emitting aNC layer is mostly formed by spin-casting process. In the case of NRs, it is reported that majority of the NRs lie on the substrate after the spin-coating process.106 Orientation of NRs lying mostly in-plane could then give rise to transition dipole being oriented mainly parallel to the substrate, which leads to enhanced out-coupling efficiency. On the other hand, in the case of NPLs, stacked NPLs are vertically aligned vertically to the substrate after the spin coating process,113, 114 because of strong van der Waals interaction between large faces of NPLs. This assembly structure of NPLs not only hinders the increase of the out-coupling efficiency because of vertical transition dipole (ηout) but also causes a serious reduction of the radiative exciton production efficiency owing to the accelerated ET process. Although it is evident that the ensemble properties of the emitting layer dramatically vary depending on the assembly structure,115, 116 the effect of assembly structure on the device performance is largely underexplored. Therefore, we anticipate that research on control of assembly structure in terms of dipole orientation and ET will drive further improvement of device performance.

4. Toward high-efficiency anisotropic nanocrystal-based light emitting diodes The performance of aNC-LEDs is not up to par yet. Comparatively speaking, aNC-LEDs have shown device performance inferior to QD-LEDs based on spherical NCs. The lower 16 ACS Paragon Plus Environment

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device efficiency appears to run counter to the promise that we have highlighted in Section 2. The relatively slow progress in the development of aNC-LEDs leaves us wondering whether the anisotropy in aNCs would lead us to wiggle into a marginal improvement or there is a sweet spot for significant leap in LED performance. 4.1 Structural design of anisotropic nanocrystals It would be a grave understatement to say that the research community has seen progress in the synthesis of colloidal NRs and NPLs. There is a rich body of published reports on the formation of heterostructure NCs with a variety of compositions and morphology, and the number is growing as we speak. While it seems impossible to cover the comprehensive list of reports on synthesis, we elect to highlight important milestones in the progress of synthetic control of size and shape of colloidal aNCs. From the discussion, we call attention to the photophysical properties of aNCs, such as PL QY, AR rate, orientation of transition dipole, and other key characteristics relevant to the use of NCs in LED applications.

4.1.1 Nanorods The morphology of inorganic NCs is subject to surface energy of NC facets. This principle of NC growth becomes complicated upon the introduction of organic ligands. If surface ligands selectively adsorb on specific facets of NCs and thus reduce the surface energy of certain facets, the NCs can grow with high portion of these facets exposed to surface.117, 118 In addition, the relatively higher ligand density of a certain facet inhibits the supply of precursors to that particular surface plane.119 These thermodynamic and kinetic factors permit spontaneous anisotropic growth into elongated geometry. As for colloidal NRs of wurtzite CdE (E = S, Se, -

-

-

or Te), three pair of sidewalls usually consist of facets of (1100), (0110) and (1010), while tips -

of NRs are terminated by the (0001) and (0001) planes.56, 120, 121 Phosphonic acids selectively 17 ACS Paragon Plus Environment


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passivate the side facets, stabilizes the surface reactivity of side facets than that of end facets, and serve as a blocking layer for precursor penetration, which result in the anisotropic growth of CdE NRs. Since the first report of colloidal synthesis of NRs,28 surfactant-controlled growth approaches have allowed for the precise control of the length and width of NRs.122-125 PL QY is the most obvious figure of merit for one to consider when designing emitters for LEDs. The PL QY of bare CdE NRs is typically lower than that of spherical NCs (QDs) having the same composition. Similarly to spherical core/shell NCs, seeded growth of rod-shape CdS shell on CdSe NCs to form CdSe/CdS dot-in-rod (DIR) heterostructures leads to improved PL QY (40~75%).126-128 Recently, Coropceanu et al. reported near-unity PL QY of CdSe/CdS DIR by growing additional outer CdS shell on top of DIR for a prolonged time at high temperature, at which crystalline defects are healed.129 Interestingly, the PL QY of this specific CdSe/CdS DIR remains high even when CdS shell is photo-excited, implying that the excitons generated in the shell are effectively transferred to the core without carrier trapping on surface. Efficient shell-to-core carrier transfer makes DIR promising for LED applications.

Figure 3. Schematic illustration of emission polarization depending on geometric anisotropy of emissive NRs and core position in DIR: (a) aspect ratio of NR; (b) shape of core dots in DIR structure; and (c) position of core in DIR. 18 ACS Paragon Plus Environment

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Structural characteristics of NRs allow for the control of the dipole moment. Previous studies have established several important designing principles for high degree of fluorescence polarization. Hu et al. experimentally demonstrated the effect of aspect ratio on polarized emission of NRs.39 The single-NC emission spectroscopy measurements on CdSe NRs confirm a sharp transition from random polarization to linearly polarized emission at an aspect ratio of 2 (See Figure 3a).39 Polarization, 𝑃 = (𝐼 ∥ ― 𝐼 ⊥ )/ (𝐼 ∥ + 𝐼 ⊥ ), is used for measurement of the extent of polarized emission, where 𝐼 ∥ and 𝐼 ⊥ are the emission intensity of parallel and perpendicular to the excitation polarization, respectively.

Empirical pseudopotential

calculation suggests that the change of the lowest ground state is mainly responsible for this dramatic change of emission property. In the case of DIRs, the degree of polarization from CdSe/CdS DIRs measured in several different methods yield very similar value of emission polarization of ~0.75.41, 130, 131 Banin and co-workers investigated the effect of the shape of core NCs on the degree of polarization (p).41 DIR and rod-in-rod (RIR) heterostructures prepared by CdS NR shell growth on spherical and rod-shaped CdSe NCs, respectively, contrast with respect to the emission polarization depending on the degree of elongation of the emissive core: DIR (p = 0.75) versus RIR (p = 0.83) (see Figure 3b). In the case of RIR, the core CdSe NRs are already projected to exhibit linearly polarized emission along the long axis without CdS shell, and the growth of the rod-shape shell reinforces the polarization. The position of the core turns out to be critical in the degree of polarization in the DIR heterostructures.132

In our recent study, we demonstrate CdSe/CdS DIRs with precisely

controlled CdSe core position by regulating the growth rates of the NR shell toward [0001] and -

[0001] directions.132 Interestingly, DIR with core located near the tip shows a low polarization response. A weak polarized field in the short NR part and change of strain in the shell CdS 19 ACS Paragon Plus Environment


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region near the core are responsible for the low polarization. Fluorescence anisotropy, defined as (𝐼 ∥ ― 𝐼 ⊥ )/ (𝐼 ∥ +2𝐼 ⊥ ) where 𝐼 ∥ and 𝐼 ⊥ are the emission intensity of parallel and perpendicular to the excitation polarization, respectively, increases as the core locates near the center of the NR shell, and eventually reaches saturation (See Figure 3c). Fluorescence anisotropy (r) can be interchanged with polarization (𝑟 = 2𝑃/(3 ― 𝑃)). Further experimental and theoretical study is required to account for the change in the polarization depending on the core position. Strong quantum confinement in NCs enhances the Coulomb interaction between charge carriers, relieves translational momentum conservation, and, in turn, promotes non-radiative AR processes.30,

133

Considering that AR processes take place in tens to hundreds of

picoseconds in comparison to radiative decay lifetime of few nanoseconds, the AR processes, involving parasitic carrier losses, are deemed to be responsible for the efficiency roll-off of LEDs operated at high carrier density regimes.134 How can one suppress AR? A design strategy has been proposed from the study of spherical NCs: thick CdS shell grown on CdSe core NC results in the suppressed AR processes.4 This “giant” QD with quasi-type-II band alignment leads to weaker Coulomb interaction among charge carriers and hence slower AR via spatial separation between electrons and holes (i.e., reduced wavefunction overlap between strongly localized hole in the core and delocalized electron over entire NC). Rabouw et al. observed that the introduction of rod-shape large-volume CdS shell on CdSe seed similarly impedes AR rates as resulting of formation of 1D exciton along the long axis (See Figure 4a).33 Studies of the multicarrier dynamics in the DIR or RIR structures encourage designing aNC for the purpose of AR suppression. Indeed, the energy band structure is the keys to altering AR in our favor. For CdSe/CdS DIRs, AR processes pronounce for larges core ones in which electron and hole 20 ACS Paragon Plus Environment

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wavefunction overlap becomes greater, whereas AR is suppressed for smaller core ones owing to the delocalization of electron wavefunction toward the rod growth direction.135 (See Figure 4b) Following the new strategy arising from spherical NCs to reduce AR processes, recently, Hadar et al. proposed a novel DIR heterostructure, in which rod-shape shell with a radially graded composition is grown on seeded core.136 Considering that the graded composition of the shell provides a smooth confinement potential that effectively reduces AR processes in spherical NCs, introduction of rod shell with graded composition could result in more efficient reduction of AR rates than the previously studied DIR with an abrupt interface (e.g., CdSe/CdS DIR).

Figure 4. (a) Sketch of exciton extension in spherical (left) and rod-shaped (right) nanocrystals and relative implications in Auger processes. (b) Change of wavefunction overlap of electron and hole according to the size of core in dot-in-rod (DIR) structures. Adapted with permission from Ref 135. Copyright 2009 American Chemical Society.

4.1.2 Nanoplatelets Since the discovery of quantum confinement effect in a heterostructure quantum well and the ensuing invention of quantum well laser in the 1970s, quantum wells have inspired a 21 ACS Paragon Plus Environment


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generation of study on quantum confined nanostructures.137 Epitaxial growth has allowed for controlling the thickness of the two-dimensional structures at nanoscale, thus modulating the quantum confinement of charge carriers. The stunning advance in wet chemistry of colloidal NCs has recently expanded into the growth of two-dimensional NPLs.138 Colloidal CdSe NPLs instantaneously captivated the interest of research society with their unique optical properties, such as ultra-narrow emission bandwidth, giant oscillator transition, and linearly polarized emission.44, 50, 139 Extensive efforts on the colloidal NPL synthesis have led to precise control of morphology of both core-only NPLs and heterostructured NPLs (e.g., core/shell and core/crown architectures).140-142 CdSe NPLs exhibit decent PL QY (30~50 %) even without a shell, which is attributed to the cadmium-rich (100) planes passivated by a dense layer of carboxylate ligands.143 PL QY of NPLs can be further improved by passivation with an inorganic shell. Considering that NPLs are susceptible to crystalline strain due to their thin and flat geometry, structural stress caused by the lattice mismatch between core and shell and its detrimental effect on the photophysical properties become more pronounced in NPLs. For example, ZnS, a widely used shell material for spherical CdSe NCs, results in only mediocre enhancement of PL QY (~60%) when used as a shell for CdSe NPLs due to the large lattice mismatch between CdSe and ZnS (12%).144 Instead, the use of a CdZnS alloy shell with smaller lattice mismatch (~8%) has proven to increase PL QY up to 80%.141 Another alternative approach is engineering band gap of NPLs, which can provide a defect tolerance characteristic if carriers can be confined within the NPL rather than transfer to trap site. For example, growth of CdSeTe crown on the CdSe NPLs results in strong localization of holes in the crown region rather than trapping at surface defects because of low valence band potential of CdSeTe crown compared with CdSe core. By controlling the composition of crown, CdSe/CdSeTe core/crown NPLs showed considerably higher PL QY (~95%).145 22 ACS Paragon Plus Environment

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Figure 5. (a) Schematic drawing of NCs with different shapes showing varying effect of dielectric confinement. (b) (left) TEM image of microneedles made of nanoplatelets. Scale bar is 100 nm. (right) Fluorescence intensity of microneedles as a function of the polarization direction. Fluorescence intensity in false color for a single microneedle depending on the polarization direction. Scale bar is 10 μm. Adapted with permission from Ref 50. Copyright 2014 American Chemical Society. (c) Fluorescence polarization of CdSe NPLs with respect to the lateral aspect ratio. Adapted with permission from Ref 147. Copyright 2017 American Chemical Society.

The geometric anisotropy of NPLs is responsible for anisotropic dielectric environment (See Figure 5a), which results in polarized emission. Abécassis et al. reported polarized emission from needle-like superstructures of NPLs in their lateral direction (Figure 5b).50 The beauty of the superstructures is that the coherent orientation of the NPLs permits accurate 23 ACS Paragon Plus Environment


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analysis of the anisotropic optical properties of NPLs. Individual NPLs are stacked against one another within superparticles deposited on a substrate, and the superstructures exhibit polarized emission. Tisdale and co-workers reported that the orientation of the transition dipole moment in CdSe NPLs is isotropic within the plane with no preference for the long- or short axis of NPLs.146 Recently, our group observed that the degree of fluorescence polarization in NPLs depends on the lateral aspect ratio of the NPLs: CdSe NPLs with higher lateral aspect ratios exhibit higher fluorescence polarization (Figure 5c).147 This shape-dependent polarization behavior is mainly attributed to the local field effect originating from dielectric contrast between NPLs and the surrounding media. While the dominant direction of dipole orientation in NPLs is ambiguous, it is commonly agreed that the transition dipole is mainly distributed within the plane of a NPL. Therefore, as long as NPLs are placed on the substrate with the large faces against it, the dipoles would align parallel to the substrate, leading to enhanced out-coupling factor of LEDs.


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Figure 6. Schematic of biexciton annihilation process in colloidal CdSe NPLs, showing both the collision and Auger recombination processes. Size and thickness dependence of biexciton collision in NPLs. (bottom left) Biexciton lifetime (symbols) as a function of ANPL and their linear fits for 3, 4, and 5 ML NPLs. (bottom right) Biexciton lifetime of different NPL samples as a function of the product of ANPL and the reciprocal of Ek(e) to the 7/2 order. Adapted with permission from Ref 32. Copyright 2017 American Chemical Society.

Because of the anisotropic geometry of NPLs, electrons and holes have well-defined momenta along the plane axes and the requirement of momentum conservation leads to very different AR pathways. Recently, Li et al. reported that biexciton lifetime of NPLs does not increase linearly with their volume, deviating from the “universal volume scaling law” which is observed from AR rate in spherical NCs or NRs.32 Instead, the biexciton lifetime scales linearly with ANPL·(1/Ek(e))7/2 (ANPL is lateral area of NPLs and Ek(e) is the electron confinement energy) (see Figure 6).32 The linear relationship between AR rate and NPL lateral areas manifests the inverse proportionality between the lateral area and the binary collision frequency 25 ACS Paragon Plus Environment


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for 2D excitons. This fundamental study suggests that the biexciton lifetime of NPL can be easily controlled with the lateral area without losing of band gap tunability, a major advantage of NCs in display applications. In addition, multiple independent studies show that AR rate of NPLs is an order of magnitude smaller than that in spherical NCs of an equal volume, promising the use of NPLs as emissive materials in high-power LEDs or lasers whose efficiency is mainly dictated by AR processes.34, 35

4.2 Expanding chemical composition Most of the heterostructure aNCs reported to date have consisted of CdSe and CdS. This specific pair has the edge over other combinations in the study of aNCs because of wellestablished chemical synthesis, small lattice mismatch, and abundant database on their physical properties. However, small conduction band offset between CdSe and CdS is likely to cause delocalization of electron wavefunction depending on CdSe core size. Although the small band offset is key to core size-dependent suppression of AR in NRs, carrier delocalization is responsible for electron trapping to surface defects. From the viewpoint of carrier injection in LEDs, such an electronic structure catalyzes the formation of negative trions and subsequent nonradiative AR. As demonstrated in research of QDs and QD-LEDs, diversifying chemical composition of aNCs can offer a solution to this issue. For example, type-I heterostructures using wider bandgap shell materials, such as ZnSe, ZnS, or alloyed shells (e.g., CdSeS, CdZnSe, and ZnSeS), can help regulate carrier injection in aNC-LEDs, let alone improve PL QY and photochemical stability via stronger carrier confinement.15, 106 Notably, employing CdZnS shell on CdSe/CdS NRs leads to improved PL QY, suppressed blinking,21 and even enhanced optical polarization.136 In addition, the hybrid composition approach opens the opportunity for designing graded confinement potential, which can alleviate the issue of AR along the axis of strong confinement (e.g., x- and y-direction in NRs).15, 148 26 ACS Paragon Plus Environment

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The anisotropic geometry enables one to expand the engineering parameters from materials choice to morphological variations in aNCs (e.g., core/crown NPLs,140, 149 core/shell NPLs,150, 151 core/shell/crown NPLs113, 152 and platelet-in-box153, 154). Combination of the shell and crown compositions can create an interesting set of energy band alignments involving both type-I and quasi type-II in core/shell/crown NPLs. For example, in the case of CdSe/ZnS/CdS core/shell/crown NPLs, type-I band offset in core/shell offers tunability in electron injection barrier and at the same time quasi type-II band alignment between crown and core suppresses the AR process.113 In addition, synthetic strategies enable the control of the dielectric screening effect, ET, applied strain, exposed surface area and PL QY from heterostructure NPLs.

4.3 Directed, long-range assembly of anisotropic nanocrystals 4.3.1. Photophysical properties of anisotropic nanocrystals assembly Transition dipoles in aNCs can ultimately lead to a higher out-coupling factor in LEDs, as long as the dipoles are aligned to in-plane direction. Generally, emissive layers composed of colloidal NRs or NPLs in LEDs are prepared via spin coating process, resulting in random spatial orientation.113, 114 While approaches to aligned aNCs in LEDs are called for, induced self-assembly has been regarded as the most promising means to such orientation in a long range. Since NCs are soluble in organic solvents and NC films are prepared from the solution via spin-coating, dip-coating,155-157 drop-casting,158-160 or Langmuir-Blodgett deposition,45, 161 interparticle interactions during solvent evaporation are of paramount importance. To achieve reliable control of large-area spatial and orientational ordering of aNCs, one can tune the interparticle forces by modifying NC surface and the surrounding media, applying external fields on drying solution, or using physically or chemically patterned templates.44, 48, 49, 157, 162 Despite the tremendous progress in assembly of aNCs, the precision and reliability of the 27 ACS Paragon Plus Environment


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proposed approaches have not reached the quality and predictability gained in commercially available thin film LEDs, let alone in microelectronic circuits and devices. In a typical LED fabrication process, 1-3 layers of colloidal NCs are assembled onto various types of charge transport layers (e.g., metal oxides or conducting polymers).163, 164 Interaction between aNCs and substrate has primary bearing on the resulting assembly structure;49, 116 therefore, different charge transport layers would require a new set of engineering parameters for reliable assembly. Alternatively, transfer of assembled film onto a device has been proposed to minimize the effect of charge transport materials or device configuration.165

Figure 7. (a) Schematic illustration of self-assembly process of aNCs at liquid/liquid interface. 28 ACS Paragon Plus Environment

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TEM images of (b) vertically and (c) horizontally aligned superlattice of CdSe/CdS NRs on liquid subphase. Adapted with permission from Ref 49. Copyright 2015 American Chemical Society. TEM images of NPLs assembled into (d) lamellar and (e) columnar superlattices through a liquid interfacial assembly technique. Adapted with permission from Ref 173. Copyright 2011 American Chemical Society. Slow evaporation of a NC solution on a polar-liquid subphase produces thin film of assembled NCs.49,

67, 166-169

Depending on the initial concentration of NCs solutions, the

floating NC film on subphase liquid exhibits controllably varying areal coverage and thicknesses. The beauty of this approach is that the assembled NC layers stay nearly equal regardless of device substrate, suggesting high predictability. In the assembly process on a subphase, aNCs undergo two-dimensional alignment in a way of minimizing interfacial free energy between aNCs and subphase liquid that competes with interaction between among aNCs.67 Figure 7a illustrates assembly process in which aNCs are trapped at liquid/liquid interface and progresses in two dimension because of increased concentration of trapped NCs by solvent evaporation. The principle leaves us a notion that we can expand the opportunity space for the assembly of aNCs simply by finding subphase liquid with adequate polarity and surface tension.49, 162, 168, 170-173 For instance, Diroll et al. prepared film of assembled NRs with alignment varying from horizontal to vertical depending on the subphase solvent.49 The surface tension of the subphase influences the orientational ordering of the NR superlattices as shown in Figure 7b and 7c. NRs lie in-plane at the liquid−liquid interface to reduce surface tension of subphase liquid with large surface tension, while dominant NR-NR interaction due to the lower surface tension of subphase liquid leads to vertical alignment of NRs. Rizzo et al. demonstrated the transfer of a floating film of ordered CdSe/CdS core/shell NRs from water subphase surface onto a device using a poly(dimethylsiloxane) (PDMS) stamp.165 The ordered NR film in LEDs exhibit polarized emission from the devices starkly contrasted by similar devices made of randomly oriented NR film. This control principle for NRs also works for the 29 ACS Paragon Plus Environment


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alignment of NPLs as shown in Figure 7d and 7e.168, 172, 173 It is important to validate that the NRs and NPLs lying on the substrate through assembly processes indeed induces the in-plane orientation of the ensemble dipole and thus contributes to the increase in out-coupling efficiency. A signature of in-plane dipole orientation of NR or NPLs assembled parallel to the substrate has been experimentally observed by angle-dependent PL measurement, variable angle spectroscopic ellipsometry and back focal plane imaging (see Figure 8).43, 88, 100, 135 This in-plane dipole orientation is predicted to contribute to the increase of out-coupling efficiency in aNC-LEDs by the same principle manifested in the studies of OLEDs. Yet, empirical observation about correlation between the dipole orientation and outcoupling efficiency of aNC-LEDs relies on EQE and PL QY values based on equation 1. On the other hand, correlation between in-plane orientation and out-coupling efficiency has been clearly revealed by angle-resolved PL or EL measurements in OLED.82, 83 Therefore, more research efforts are demanded to experimentally demonstrate this correlation in aNC-LEDs throughout the control of dipole orientation and diversification of measurement method. One caveat of the translational order in the assembly is the accelerated fluorescence resonance ET (FRET). In dense NC films, the probability that exciton undergoes non-radiative recombination processes is amplified, since excitons likely funnel into surface trap sites of NCs via ET process.23, 134 A relatively small Stokes shift in NCs leads to large spectral overlap of absorption and emission between NCs and thus high ET probability. Now that both ET (a few hundred ps for spherical CdSe QDs) and carrier trapping (typically a few ten ps for hole trapping spherical CdSe QDs) beat the radiative decay (~20 ns for spherical CdSe QDs) by a few orders of magnitude.21 As a result, ET leads to lower PL QY. Under the assumption that ET continues to occur in dense NC film until exciton recombines, the overall PL QY can be expressed: 30 ACS Paragon Plus Environment

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𝑄𝑠𝑜𝑙𝑖𝑑 = 𝑄0𝑘𝑟/[𝑘𝑟 + 𝑘𝐸𝑇(1 ― 𝑄0)]

(4)

where 𝑄𝑠𝑜𝑙𝑖𝑑 is the PL QY of dense NC film, 𝑄0 is the initial PL QY of dispersed NC, 𝑘𝑟 is the radiative recombination rate and 𝑘𝐸𝑇 is the ET rate.134 Considering that the ET rate is proportional to the inverse six-power of distance between NCs, it is obvious that the distance between the NCs sensitively determines the PL QY of the film, and thus greatly affects the EL performance. Recently, we reported a systematical study that relates ET and performance of QD-LEDs using a series of shell thickness controlled QDs.23 PL QY and device IQE increase as a result of suppressed ET, all enabled by design of shell dimension. The ramification of ET in QD-LEDs is obvious, and, in the case of spherical QDs, a thick shell has proven to slow down the ET. ET poses even more significant impediment in the case of aNC-based LEDs. In general, ET rate is given by

KT(r) =

QD k2 9000(ln 10) τDr6

(

128π5Nn4

)∫ F (λ)ε (λ)λ dλ ∞

0 D

A

4

(5)

where QD is the quantum yield of the donor in the absence of an acceptor, n is the refractive index of the medium, N is Avogadro’s number, r is the distance between a donor and an acceptor, τD is the lifetime of the donor in the absence of an acceptor, FD(λ) is the florescence intensity at the wavelength of λ with total area under the florescence curve normalized to unity, ɛA(λ) is the extinction coefficient of the acceptor at the wavelength of λ, and the term of k2 is a factor describing the relative orientation of the transition dipoles of the donor and acceptor.174 For spherical NCs, orientation factor of transition dipoles is generally assumed to be 2/3, reflecting random orientation of transition dipoles.174 On the other hand, the alignment of aNCs induces the increased orientation factor (k2) with strong coupling of the emitting dipole in an 31 ACS Paragon Plus Environment


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excited NC with the absorption dipole of an adjacent NC, resulting in a more efficient ET.109, 175

Furthermore, giant oscillator strength due to the anisotropic structure leads to a large

extinction coefficient of NRs and NPLs, which makes ET faster than in the case of spherical NCs.109, 114, 175, 176 Guzelturk et al. observed efficient FRET in stacked NPLs, in which excitons migrate over the length of 100 nm with 4 ps of an ET characteristic time.109 FRET efficiency of stacked NPLs reaches as high as 99.9% at room temperature within the close-packed collinear orientation of the NPLs along with their large extinction coefficient. As a result of the efficient FRET, the PL QY of NPLs decreases from 30% in solution to >5% in the stacks. This dramatic decrease in PL QY brings to an alarming notion that assembly of aNCs can cost device performance in QD-LEDs. Therefore, the suppression of ET should weigh as much as the increasing out-coupling efficiency in consideration of plotting a new assembly protocol.

Figure 8. (a,b) Angle-dependent emission of (a) NPLs and (b) spherical QDs with theory curve. Insets: distribution of dipole moments showing strong anisotropic distribution from NPLs and random orientation from spherical QDs. Adapted with permission from Ref 43. Copyright 2017 Nature Publishing Group. (c-e) Interfacial assembly of CdSe NPLs and their optical property. 32 ACS Paragon Plus Environment

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(c) TEM image of face-down assembly (left) and their back focal plane imaging (BFP) image (right). (d) TEM image of edge-up assembly (left) and their BFP image (right). (e)Transient PL spectra for the face-down assembly, edge-up assembly, and drop-casted film. The inset shows decay kinetics at early times. Adapted with permission from Ref 146. Copyright 2017 American Chemical Society. (f) Hydrophobic attraction-assisted assembly of CdS NRs. (top line) TEM images of (left) vortex and (right) vertical structure of assembled CdS NRs. (bottom line) PL lifetime microscopy images of vortex and vertical NR formations with PL lifetime represented by gradient color bar. (g) Ensemble PL decay curves of NRs assembled in vertical and vortex structures. Adapted with permission from Ref 116. Copyright 2015 American Chemical Society.

In this sense, it is critical to control the distance and orientation between individual aNCs in a film and investigate the ET process depending on the assembly structure. During assembly process, van der Waals attraction tends to hold NRs aligned side-to-side and NPLs stacked face-to-face. Most-frequently reported, thermodynamically stable superstructures of NRs and NPLs include a sematic or hexagonal close packed structure of NRs or a face-to-face arrangement of NPLs, respectively. To systemically investigate the ET depending on assembly structure, it is necessary to provide the diversity of the assembly structure by controlling the complex interaction of NCs. Gao et al. reported controlled shift of edge-down and face-down assembly configuration by tuning interaction strength between NPL−NPL and NPL−subphase by varying the concentration of excess oleic acid (See Figure 8c and d).146 Relatively large center-to-center spacing between NPLs considerably reduces the ET rate in the face-down assembly, resulting in a longer exciton lifetime as shown in Figure 8e. As for NRs, we stumbled upon vortex structure in NR assembly, which provides a platform for quantitative analysis on the ET rate in relation to the dipole orientation and distance between neighboring NRs (Figure 8g).116, 163 Fluorescence lifetime imaging microscopy helps visualize that ET rate is contingent upon NR-NR separation even within a cluster of NRs (Figure 8f), reflecting the strong dependence of ET rate on inter-NR distance. While such vortex structure would hardly be an ideal superstructure in the context of the out-coupling efficiency, the results of assembly33 ACS Paragon Plus Environment


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dependent ET shed light on the design of the assembly structure. 4.3.2. Toward long-range anisotropic nanocrystal assemblies While the field of synthesis has had a steep learning curve, long-range assembly of aNCs has not come close to the caliber of top-down approaches like in microelectronics. Now that an ideal alignment of aNCs would give rise to significantly higher out-coupling efficiency, it is of paramount importance to find ways to align aNCs as desired orientation on a substrate. As we discussed in Section 4.3.1, self-assembly of aNCs at the liquid/liquid or liquid/air interface holds promise in formation of two-dimensional thin films with the in-plane dipole orientation of NRs and NPLs. Yet, the understanding on interfacial energy and orientation of aNCs is far from complete, rather extending invitation to further study. Several parameters including geometry and surface ligands of the aNCs, and the dielectric constant, surface tension, and viscosity of subphase liquid come into play for interfacial energy and thus the orientation of aNCs.168

Figure 9. Simplified design parameters for aNCs and their assembly in the context of highperformance aNC-LEDs. Assembly of aNCs into close-packed film would cause significant ET. Considering the dependence of ET on distance, a strategy to creating separation between neighboring aNCs while maintaining in-plane orientation would be desirable. Selective ligand peeling or partial 34 ACS Paragon Plus Environment

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ligand exchange near the tip of aNC have proven to induce strong tip-to-tip attraction rather than side-by-side interaction, as illustrated Figure 9. We observed that even slight tilting between neighboring NRs results in drastic decrease in ET rate.116 Obviously, this approach would require such a delicate control as to avoid losing the density of the emitters to void space. An ultimate solution involves the design of heterostructure aNC in a way to separate emitting cores further apart. Similar to the thick-shell spherical NCs,23 thick shell on aNCs would slow down ET process between neighboring aNCs. For example, the core/crown heterostructure of NPLs can increase the spacing between the neighboring cores in closed packed NPLs lying face-down on a substrate. Likewise, increasing the diameter of the rod-shaped shell is expected to suppress the ET in the case of NRs. In addition, control of core position in NR shell can be a simple method to increase the average separation distance between core emitters in NR ensemble films.132 Exploiting both in-plane orientation of dipoles and separation of neighboring emitters could result in LEDs based on aNCs with surpassing out-coupling efficiency, and hence superb device performance.

Acknowledgements This work was supported by the National Research Foundation (NRF) grants funded by the Korean government (NRF-2016M3A7B4910618, NRF-2017R1A2B2011066, and NRF2019R1C1C1006481). The authors acknowledge the support by the Ministry of Trade, Industry & Energy (MOTIE, Korea) under Industrial Strategic Technology Development Program. No. 10077471, 'Development of core technology for highly efficient and stable, non-cadmium QLED materials', and the grant (No. 20173010013200) funded by Korea Institute of Energy Technology Evaluation and Planning (KETEP) and MOTIE.

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Table of Contents Figure

Biographies Whi Dong Kim received his B.S. (2009) in Nano Science and Technology from Pusan National University and his Ph.D (2016) in Chemical and Biomolecular Engineering from Korea Advanced Institute of Science and Technology (KAIST) in Korea. He is a Postdoctoral Fellow in the Chemistry Division at Los Alamos National Laboratory. His research interests include synthesis and characterization of semiconductor nanocrystals and assembly of nanocrystals in the context of the exciton dynamics in nanocrystal solid film. Dahin Kim received her B.S. (2012) in chemical engineering at Hanyang University and Ph. D. (2018) in chemical and biomolecular engineering at KAIST in South Korea. She conducted postdoctoral research work at KAIST (2018), and currently she is a Postdoctoral Fellow in the 44 ACS Paragon Plus Environment

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Department of Chemical Engineering at Massachusetts Institute of Technology. Her research interest focuses on the synthesis, assembly, and characterization of semiconductor nanocrystals. Da-Eun Yoon received her B.S. (2015) in Chemical Engineering from Ulsan National Institute of Science and Technology and M.S. (2017) in Chemical and Biomolecular Engineering from KAIST. Currently a Ph.D student at KAIST, she is interested in research on the synthesis and optoelectronic properties of anisotropic nanocrystals, especially nanoplatelets. Hyeonjun Lee received his B.S. (2017) in Chemical Engineering from Sungkyunkwan University and M.S. (2019) in Chemical and Biomolecular Engineering from KAIST under the supervision of Prof. Doh C. Lee. His research interests are the synthesis and characterization of colloidal semiconductor nanocrystals and application in LEDs. Jaehoon Lim received his B.S. (2007) and Ph.D (2013) in Chemical and Biological Engineering from Seoul National University in Korea. He conducted postdoctoral research work within the Department of Electrical Engineering and Computer Science at Seoul National University (2013 – 2014) and within the Chemistry Division of Los Alamos National Laboratory (2013 – 2017). In 2018, he began his independent career in the Department of Chemical Engineering and the Department of Energy System Research at Ajou University, South Korea. Wan Ki Bae is an assistant professor at SAINT, Sungkyunkwan University (SKKU). He received B.S. (2003), M.S. (2005) and Ph.D. (2009) in Chemical and Biological Engineering at Seoul National University in Korea. He conducted postdoctoral study at Los Alamos National Laboratory (2010-2013)). He was a senior researcher at Korea Institute of Science and Technology (2013–2018), before he joined the faculty at SKKU in 2018. Doh C. Lee is an associate professor at KAIST. He received B.S. and M.S. in chemical engineering at Seoul National University and his Ph.D in chemical engineering at the 45 ACS Paragon Plus Environment


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University of Texas at Austin. He was a Director’s Postdoctoral Fellow at Los Alamos National Laboratory between 2007 and 2010, before he joined the faculty at KAIST in 2010.


Pushing the Efficiency Envelope for Semiconductor Nanocrystal

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