QDs exhibit tunable emission spectra, high photoluminescence (PL) QY, broad absorption spectra and increased environmental stability. These properties have generated significant attraction for the use of QD devices for light generation. To date, these QDs have been utilized in light-emitting diodes (LEDs) through two primary excitation schemes: color-conversion type  LEDs using QDs as photoluminescent materials and electrically driven  type LEDs using QDs via charge injection for electroluminescence. In these devices, QDs are integrated into different material systems to create a synergy via utilizing the advantages of the constituent materials, including other QDs, QWs, Qwires, carbon nanotubes (CNTs) and organic semiconductors. Here, we will review the excitonic processes in the various composites of the QDs.
4.1.1 QD–QD excitonic interactions
In colloidally synthesized semiconductor QD samples, there is always a finite size distribution, which inhomogeneously broadens the emission and absorption spectra of the QDs. Consequently, excitonic interactions arise in the distribution of the same QDs, which are referred to here as homoexcitonic interactions. The homoexcitonic interactions are important to understand the optical properties of the QD samples. Additionally, heteroexcitonic interactions, which can occur between different types, sizes and compositions of the QDs, are crucial towards engineering the excitonic operation in the QD composites.
Before discussing the homo- and heteroexcitonic interactions in the QD assemblies, it is worth discussing the effects of the different media (i.e. solution-phase or solid-state films) on the optical properties of QDs. In the solution phase, QDs are more isolated from each other, unless the solution is very dense or the QDs are chemically attracted to each other; therefore, the excitonic interactions between QDs are generally negligible in solution phase. However, when cast into the solid-state in the form of close-packed films, QDs come into intimate contact with each other, and they consequently exhibit complex excitonic properties. Distinct differences between solution-phase and solid-state films are that the photoluminescence emission is redshifted and the PL QY is reduced in solid-state films as compared with solution-phase films. The redshift in emission spectra involves both electromagnetic field effects on the transition dipoles in solid-state films due to the substrate and increased excitonic interactions (homoexciton interactions in the same QD batch) among the QDs in the form of exciton migration towards smaller energy gap (larger size) QDs. First, the substrate leads to a change in the dielectric medium around the QDs, which causes changes in the spontaneous decay rate and energy of the transition dipole, which is a well-known phenomenon and not limited to the QDs. Consequently, the radiative lifetime is shortened and the energy of the transition dipole is decreased, which leads to the redshift . Secondly, the existence of the size-distribution causes homo-NRET from smaller to larger QDs in the ensemble such that the exciton population in the QDs that are on the red tail of the spectrum. The reduction in PL-QY is attributed to the increased nonradiative recombination channels in solid-state phase. The surfaces of the QD are passivated with organic ligands. In the solution phase, these ligands can function properly such that surface traps are effectively passivated. However, in solid-state films, the stacking of the QDs may lead to improper passivation of the surfaces, which causes increased nonradiative decay channels and traps for the excitons.
The homoexcitonic effects have been shown to be important in the exciton migration in solid-state phase. For example, in the solid-state films of the highly confined silicon QDs long-range exciton transport was enabled through NRET . When smaller Si QDs were utilized, a longer transport was observed due to the higher NRET rates because small QDs can facilitate efficient NRET due to their sizes that are smaller than the Förster radius. The excitons hop between different QDs multiple times until they become trapped in a large size QD surrounded with smaller QDs (i.e. larger bandgap) . Accordingly, it was reported that in the QD ensembles, smaller QD lifetimes become shortened due to the exciton transfer to the larger QDs, whose lifetimes are increased due to the exciton feeding effect [63-66]. Recently, CdSe/CdS-based QDs were investigated in terms of their homoexcitonic interactions as a function of the CdS shell thickness. It was found that the homoexciton transfer in solid-state films is effectively suppressed because of the very thick CdS shells (16 monolayers, called giant QDs) . As shown in Fig. 1, the emission decay curves of the QD films exhibit large differences at the high-energy tail, peak and low-energy tail of the emission spectrum, which indicates the occurrence of a homoexciton transfer at the thin CdS shell but suppression of the homo-NRET in giant shells. As a result, the decay curves measured at different spectral positions of the giant-QDs become indistinguishable.
Figure 1. Time-resolved fluorescence decay measurements of the CdSe/CdS QDs depicted with respect to different CdS shell thicknesses (i.e. 4, 8, 13 and 16 monolayers). Decay measurements are performed for one QD distribution having only inhomogeneous broadening due to finite size distribution. Measurements were performed at three different spectral positions of the QD emission (i.e. higher- and lower-energy tails and at the peak) in thin film (green, black and red curves), and also at the peak position in the solution phase of the same QDs (gray curve). As the shell thickness is increased, NRET process is suggested to be suppressed in the solid-state films of the QDs because the decay curves at different positions of the QD emission spectrum become similar. Furthermore, as the shell thickness increases, the thin film and solution phase decay curves for the peak position become almost the same, which indicates the isolation of the emitting cores of the QDs due to the thick shells. [Reprinted (adapted) with permission from ref.  (Copyright 2012 American Chemical Society).]
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The heteroexcitonic interactions in the QD–QD structures were investigated for QDs in a wide variety of types, sizes and compositions. Rogach et al. reviewed some of the QD-based NRET structures . An exciton in a QD can be transferred to another QD if the donor QD emission spectrally overlaps with the acceptor QD absorption. The transferred exciton rapidly thermalizes to the band edge (on the order of ps) in the acceptor such that back-energy transfer is not possible, unless the transfer is coherent due to strong coupling, which is typically not the case for the QD systems. Therefore, excitons have the tendency to migrate towards smaller bandgap QDs in the heterostructures. The architecture of the heterostructure plays a crucial role in the emerging exciton dynamics. To date, different QD–QD-based structures have been studied in solid-state films using alternative deposition techniques, including layer-by-layer (LbL) [69, 70], Langmuir–Blodgett , spin coating , drop casting , and blending in the polymeric host matrix .
Utilization of layer-by-layer QD films with graded bandgap energy was exploited as a means of enhancing the light generation in QDs. This enhancement depends on the recycling of the trapped excitons [69, 70]. Figure 2 shows the designed cascaded energy transfer (CET) structure, which is composed of graded-bandgap LbL-deposited QDs, and a noncascaded reference structure (REF) that consists of only red-emitting QDs. In the CET structure, the steady-state PL emission was considerably increased as compared with the REF sample. This enhancement is attributed to the fact that the excitons, which were trapped in the subbandgap states of the QDs, can be transferred to smaller energy gap QDs. This recycling of the trapped excitons leads to a substantial increase in the PL emission of the acceptor QD. This scheme has been applied to color-conversion-based LEDs of QDs to enhance the conversion efficiency of the pump photons [72-75].
Figure 2. Two different QD structures are described: (a) Noncascaded reference (REF) structure and (b) cascaded energy transfer (CET) structure. (b) The CET structure consists of graded layer-by-layer assembled green/yellow/orange/red/orange/yellow/green QDs. (a) The RET structure consists of layered red QDs. On the left, electronic energy levels of the graded QD-employing CET structure and the only red-emitting QD-employing REF structure are shown. On the right, steady-state PL emission is depicted for the both structures. The CET structure exhibits substantial enhancement in the PL emission as compared with the REF structure due to the trapped exciton recycling effect. [Reprinted (adapted) with permission from ref.  (Copyright 2004 American Chemical Society).]
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The rate of exciton transfer in the QD structures has been the subject of several studies [63, 71, 73]. Because of the size distribution of the QD samples, high NRET rates cannot be ensured in random assemblies of the QDs. However, NRET rates as high as 50 ps−1 with 80% efficiency were obtained using CdTe QDs with a narrow size distribution in LbL-assembled samples . In addition to intrinsic QD properties, organic ligands, which are in charge of passivating the QD surfaces, have also been shown to affect the exciton transfer. Ligands have been shown to change the nature of the transition dipole in the QD such that higher-order multipoles should be considered to account for the observed NRET in the QD–QD ensembles . Furthermore, the capability of ligands to passivate the defect and trap sites on the surfaces directly influence the competing exciton transfer rate because exciton decay pathways can be altered via extra nonradiative channels of the surface defects .
NRET between QDs has also been investigated from the theoretical point of view [77-80]. Förster resonance energy transfer is accounted primarily for the observed exciton transfer in the ensembles of the QDs due to the effects of the polydispersity and inhomogeneous broadening . However, NRET between single QDs could not be well described with classical FRET. In the case of molecular emitters such as dyes in the process of NRET, the resonance condition is satisfied by the existence of the spectral overlap between the donor emission and acceptor absorption. This resonance condition was also discussed in the NRET process for the QD–QD assemblies and was shown that totally resonant or slightly resonant electronic states could perform NRET through direct or phonon-assisted transfer of the excitons . Later, two studies questioned the validity of the dipole–dipole coupling approximation for QD structures, and it was shown that the dipole-approximation is valid for donor–acceptor separation distances that are considerably greater than the molecular dimensions [81, 82] therefore, the FRET approach generally provides results that are compatible with the experimental observations.
4.1.2 QD–QW excitonic interactions
Epitaxially grown QWs have importance for various optoelectronic devices, and they have already become the building blocks for various optoelectronic devices, such as LEDs, lasers, photodetectors, light modulators and photovoltaic devices . The current state-of-the-art inorganic LEDs are based on epitaxially grown QWs. These LEDs can be made very efficient, yet it is not easy to tune the emission color for the generation of white light. The common route to overcome this problem is the utilization of the color-conversion technique, which relies on a pump LED and color-converting phosphors. Multiple phosphors (green, yellow and red) are utilized on top of blue-emitting QW-LEDs to realize the color conversion. However, these phosphors are limited by their optical properties, such as their broad emission spectra that extend into the far red region in the case of red phosphors, which is spectrally out of the sensitivity of the human eye. By contrast, semiconductor QDs exhibit superior optical properties, including a very narrow full width at half-maximum (FWHM) and tunable emission spectrum in the visible range . Therefore, various QD–QW systems have been proposed as efficient color-conversion materials [84-90] and have recently been reviewed . These QD-integrated color-conversion LEDs only utilize the radiative energy transfer from the QWs to the QDs. Although high-quality white-light generation has been shown to be feasible, radiative-energy-transfer-based QW–QD color-conversion systems have some limitations. First, there is a loss mechanism of the pump photons due to the light outcoupling from the high refractive index pump LED into the QD-deposited color conversion layer (generally, QDs are encapsulated in a glass-like silicone resin that has a low refractive index). The other limitation is that the nonradiative recombination channels in the pump LED restrict the efficiency of the pumps’ photon usage. To overcome these problems, Achermann et al. experimentally demonstrated an alternative approach, which is pumping the QDs by QW excitons through NRET in the QW–QD architectures . This type of exciton pumping was first proposed by Basko et al. for QW-organic emitter system . The proposed exciton pumping of the QDs involve transfer of the excitation energy from a QW to a QD that is in close proximity. To achieve the NRET pumping of the QDs, an InGaN/GaN-based multi-QW system was used as a working pump LED platform. A GaN capping layer, which is used to passivate the QWs and provide electrical contacts, was thinned to a few nanometers to have an average donor (QW)–acceptor (QD) separation on the order of the Förster radius. With this excitonic pumping of the QDs, the light-outcoupling problem is surmounted because the pump photons are not needed to be emitted into the far field, but are transferred in the near-field via dipole–dipole coupling. Additionally, NRET creates a competing channel against the traps and the defects in the QWs such that some of the excitation energy, which was otherwise wasted, could be recycled by transferring them to the QD acceptors. Using this NRET pumping scheme, it was shown that the color-conversion efficiency can be boosted even utilizing a single monolayer of CdSe QDs on top of InGaN/GaN QWs capped with 3 nm of GaN. The color-conversion efficiency for this monolayer QD conversion layer was reported to be as high as 13% . Later, several groups demonstrated that NRET-facilitated pumping is not limited by only QD acceptors but organic emitters, such as conjugated polymers, can also be employed as efficient acceptors [94-99]. A similar scheme was even applied to light-harvesting systems by transferring the excitons from QDs to QWs [100-102]. Nevertheless, initial demonstrations of the exciton-pumped QD–QW-based color-conversion LED structures were limited in terms of the NRET rates and efficiencies because of the limited interaction volume between the QDs and the QWs. Although the GaN capping layer could be thinned to make the QWs and QDs closer, the resulting NRET was still restricted because only the top QW and the bottom QD layer could effectively interact. For the other QD and QW layers, NRET was not expected to be efficient due to separation distances greater than 10 nm.
Several groups proposed and demonstrated nanostructured pump LED architectures to promote the NRET between QWs and QDs as compared with the NRET in the geometrically limited planar architectures [103-105]. These nanostructured pump LED architectures generally employ top-down fabricated nanopillars or nanoholes of the InGaN/GaN multi-QWs. Nizamoglu and coworkers reported a nanopillar architecture of InGaN/GaN QWs, which is intimately integrated with CdSe/ZnS QDs, resulting in NRET efficiencies up to 83% for red, 80% for orange and 79% for yellow-emitting QD acceptors [104, 106]. Figure 3 presents a schematic of the nanostructured (i.e. nanopillar) QW architecture with integrated QDs. A scanning electron microscopy image of the top-down fabricated InGaN/GaN nanopillars, which enable a large interaction volume between the donor and acceptor, is also shown in this figure. Furthermore, all the multi-QWs in the pump LED can now contribute to the pumping of the QDs because the QDs completely surround the nanopillars. In Figure 3 (bottom), time-resolved and steady-state PL measurements of the QW–QD structure are presented. The exciton decay of the QWs becomes faster upon incorporation with the QDs, which indicates that an efficient NRET channel has been created. From the steady-state PL spectrum of the hybrid QD–QW structure, almost totally quenched emission of the QWs can be observed upon introduction of a thin QD layer (several monolayers) on the nanopillar structure.
Figure 3. (Top) Schematic illustration of the InGaN/GaN multi-QW architecture and the QD-integrated hybrid. The scanning electron micrograph of the fabricated nanopillar structure is also shown. (Bottom) Time-resolved and steady-state PL spectra of the hybrid structure. In the time-resolved PL, exciton decay in the QW is measured before and after the incorporation of the QD. The steady-state PL measurement indicates that the QW emission is almost quenched due to the efficient NRET. [Reprinted (adapted) with permission from ref.  (Copyright 2012 Optical Society of America).]
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Recently, exciton pumping in the LbL-deposited graded energy gap CdTe QDs on planar InGaN/GaN QWs were investigated and compared with a nongraded QD acceptor layer. The graded bilayer of the CdTe QDs that consisted of green- and red-emitting QDs (QW–green QD–red QD) exhibited enhanced exciton pumping into the top red QDs (NRET efficiency of 83.3%) as compared with the reference sample of a bilayer of red-emitting QDs exhibiting much lower NRET efficiency of 50.7% . The underlying reason was explained via theoretical modeling of the exciton population evolution in the near-field. The gradient structure enabled faster and unidirectional transfer of the excitons from the QWs into the red-emitting QDs via channeling through the green QDs. In the case of the control sample, the back-and-forth NRET was theoretically shown to slow the exciton flow from the QW into QDs.
4.1.3 QD–Qwire excitonic interactions
QDs integrated into Qwires were demonstrated and investigated for optoelectronics with more emphasis on light-harvesting applications due to the synergistic combination of the strong light-absorption properties of the QDs and the superior electrical transport properties of the Qwires. QDs have limited electrical transport properties due to their organic ligands acting as barriers for the carrier transport. Thus, highly conductive and confined Qwires have great interest as potential hybrid systems when combined with QDs for photovoltaics and photodetectors. Kotov and coworkers investigated semiconductor CdTe Qwires as exciton acceptors, where the colloidal CdTe QDs function as strong light absorber and exciton donor in the specifically functionalized hybrid structure, as shown in the inset of Figure 4 . As the QDs are integrated into the Qwires, their PL emission spectrum exhibited changes, which are explained by the exciton transfer from the QDs into the Qwires. To further enhance the sensitization of the CdTe Qwires, a cascaded energy system, which consists of green- and orange-emitting CdTe QDs, was utilized. The excitons were efficiently funnelled to the Qwires via a two-step NRET process. Later, Madhukar and coworkers demonstrated a QD–Qwire light-harvesting system and verified that the sensitization of the Qwires principally occurs via NRET, which was understood through time-resolved photocurrent spectroscopy [109, 110]. Dorn et al. proposed and investigated CdSe/CdS QDs integrated into CdSe Qwires as an efficient exciton-harvesting platform . Furthermore, Hernandez-Martinez and Govorov investigated the NRET dynamics between QD donors and Qwire acceptors with a theoretical model and revealed that quantum confinement of the acceptor Qwire alters the distance dependence to be R−5 .
Figure 4. Exciton energy transfer sensitization of the CdTe Qwires by CdTe QDs of two different sizes (orange- and green-emitting) that are specifically attached to the Qwires with an energy gradient structure (Qwire–orange QD–green QD). Steady-state PL spectra are shown for different cases, which indicate that the emission of the QDs is quenched but the emission of the Qwire is enhanced due to the exciton funneling. These systems are promising for excitonically enabled light-harvesting systems. [Reprinted (adapted) with permission from ref.  (Copyright 2005 American Chemical Society).]
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The use of QD–Qwire hybrids towards light generation was also investigated. The transfer of the Qwire excitons into QDs has been realized, especially for ZnO-based Qwires, which can pump the QDs excitonically through NRET [113-115]. ZnO is one of the most suitable materials for this type of excitonic operation due to its very large exciton binding energy. However, in addition to the proof-of-concept demonstration of the exciton transfer from ZnO Qwires to semiconductor QDs, the full potential of the exciton pumping via 2D confined structures should be investigated and compared to QW–QD-based schemes.
Another important class of 2D confined Qwire structures are carbon nanotubes (CNTs). The excitonic nature of CNTs will be discussed in the section on Qwire excitonic interactions. Here, we will describe the QD–CNT-based nanostructures and the underlying excitonic operation. The composite structures of the QDs and CNTs have been characterized by several groups, and two reviews highlight the possible schemes of creating hybrid composites of the QDs and the CNTs [116, 117]. In these composite structures excitonic transfer from the QDs to CNTs could be facilitated, and this was generally studied through steady-state photoluminescence quenching of the QDs when the QDs are in close proximity to the CNTs [118, 119]. Systematic studies on the separation distance vs. PL quenching of QDs revealed that efficient exciton energy transfer from QDs to CNTs is possible . This exciton transfer increases the photoconductance of the CNTs, which could be beneficial for light-harvesting or light-detection systems . Recently, the NRET process was accomplished from QDs into several carbon-based nanostructures, including graphene oxide , graphite , carbon nanofiber  and even amorphous carbon thin films .
4.1.4 QD–organics excitonic interactions
Colloidal QDs are solution-processable materials, which make them compatible with the majority of the organic materials, such as conjugated polymers, dyes and proteins. These QD–organic hybrid nanocomposites find applications in bioimaging and sensing, light-emitting devices (LEDs and lasers) and photovoltaics [53, 54, 124, 125]. In addition, such inorganic–organic composites offer rapid and inexpensive processing techniques (roll-to-roll processing), even on flexible substrates. In this part of the review, we will focus on the excitonically tailored QD–organic composite material systems. Organic materials have active excitonic properties due to the strongly bound nature of the excitons, which are called Frenkel excitons. There is a recent review paper on the excitonic interactions among organic systems .
Integrating QDs into conjugated polymers is a common technique for preparing solid-state films of the QDs. The excitonic interactions make these nanocomposites interesting for light generation due to the possibilities of combining the better mechanical and electrical properties of the conjugated polymers with the better optical properties of the QDs. First, Colvin et al. demonstrated a conjugated polymer-QD-based LED , which utilized a conjugated polymer as a host charge-transporting matrix. Later, exciton transfer from the conjugated polymers to QDs was identified as a possible scheme for the excitation of the QDs for light-emitting devices . Spectroscopic evidence of this type of exciton transfer has been reported by several groups. Anni et al. demonstrated that the blue-emitting polyfluorene-type conjugated polymer can transfer the optically created excitons into the visible emitting CdSe/ZnS core/shell QDs via FRET . Similarly, exciton transfer was reported for infrared-emitting PbS QDs composed of different conjugated polymers [129-131]. Following these initial reports, several studies have focused on developing a deeper understanding of the excitonic processes between conjugated polymers and QDs [132-134]. Stöferle et al. demonstrated that diffusion of the exciton in the conjugated polymer is a vital process for NRET to occur from conjugated polymers to QDs, especially at low QD loading levels in the polymeric films . Lutich et al. revealed the excitonic interactions in an electrostatically bound QD-conjugated polymer hybrid in solution phase; although there is a type II band alignment in the QD-conjugated polymer composite, the dominant excitonic process is found to be NRET rather than charge transfer or Dexter energy-transfer processes . Figure 5 presents the time-resolved fluorescence decay of the donor polyelectrolyte poly[9,9-bis(3′-((N,N-dimethyl)-N-ethyla-mmonium)-propyl)-2,7-fluorene-alt-1,4-phenylene] dibromide (PDFD) polymer and acceptor CdTe QDs that have negatively charged ligands before and after the integration in solution phase. The PDFD conjugated polymer has a single exponential lifetime in the absence of the acceptors, but a double-exponential fit could only account for the measured decay curve in the presence of the acceptors. The newly appeared decay path has the same lifetime scale as the exciton feeding process in QDs (see Fig. 5 top), which confirms that the excitons are transferred from the PDFD to the QDs. The efficiency of the NRET process was measured to be 70%. Ultimately, the interaction zone of the long-range NRET and short-range Dexter energy transfer can be seen in Fig. 5 (bottom).
Figure 5. Time-resolved fluorescence decays for the donor PDFD and acceptor CdTe QDs in the PDFD–CdTe QD hybrid nanocomposite (solution phase) are shown before and after the incorporation. The decay of the PDFD becomes significantly faster upon QD integration due to the efficient NRET. The decay of the QD shows the exciton feeding on the same time scale of the NRET via slowing in the decay curve. Although there is a type-II band alignment in the nanocomposite, the dominant excitonic interaction is NRET with 70% efficiency. Other excitonic interactions, such as Dexter energy transfer and charge separation, are limited due to their short-range operation, as shown in the bottom schematic of the hybrid. [Reprinted (adapted) with permission from ref.  (Copyright 2009 American Chemical Society).]
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The exciton transfer dynamics were also modified due to the architecture of the inorganic–organic nanostructure, where a LBL-deposited hybrid assembly of CdTe-QDs and polyelectrolyte conjugated polymer, showed suppression of nonradiative channels in the polymer . Furthermore, in the conjugated polymer–QD mixtures, one important effect should be considered; the phase segregation of the constituent materials. This segregation is observed in the mechanically blended QD–conjugated polymer systems such that the QDs tend to form aggregates in the solid-state films. The phase segregation restricts the NRET in the QD–polymer films via suppressing the interaction volume. Therefore, it is crucial to control the nanoscale interactions in these hybrids to achieve the desired excitonic operation [138-140].
Small organic molecules are frequently employed in the OLED and OPV architectures as electron/hole transport or emissive layers. Furthermore, these molecules are employed in the QD-based LEDs; therefore, it is important to understand the excitonic interactions between these small organic molecules and the QDs to engineer QD-based LEDs [53, 141, 142]. The charge injection from the adjacent organic layers into the QDs is not efficient due to unbalanced injection, leading to Auger recombination in the QDs . By contrast, excitonic injection could resolve this charging and subsequent Auger recombination problem. Therefore, maximizing the excitonic injection from the adjacent small organic molecule layers into QDs is vital. For example, TPBi (1,3,5-tris(N-phenylbenzimidizol-2-yl)benzene), which is one of the most frequently used electron-transport and hole-blocking layer, was shown to possess an exciton transfer efficiency up to 50% into core/multishell CdSe/CdS/ZnS QDs . The engineering of the shell composition and thickness to match with the TPBi emission was shown to lead to strengthened excitonic interactions. Later, TPD (N′-diphenyl-N, N′-bis(3-methylphenyl) 1, 1′-biphenyl-4, 4′ diamine) and TcTa (4,4′,4′′-Tri(9-carbazoyl)triphenylamine), which are widely used for hole-transport purposes, were also shown to have a large exciton transfer capability when they are adjacent to QDs . Furthermore, phosphorescent molecules, where heavy-metal atoms create a strong spin-orbit coupling and intersystem crossing, have highly emissive triplet states. These phosphorescent molecules are promising candidates for exciton injection to QDs. It was demonstrated that an iridium complex phosphorescent molecule called Ir(ppy)3 (fac-tris(2-phenylpyridine)iridium) can enhance the steady-state PL emission of the CdSe/ZnS core/shell QDs in a bilayer film structure of QDs and Ir(ppy)3 in CBP (4,4′-N,N′-dicarbazolyl-1,1′-biphenyl) . However, the underlying physics of the exciton transfer between the QDs and organic molecules is still unknown, whether the main transfer route is through NRET or Dexter transfer. However, this scheme was applied to hybrid QD-LEDs and descent enhancements were observed in the external quantum efficiencies (EQEs) of the devices [147-149]. Although there are concomitant enhancements in the device performances, the efficiencies are still well below the EQEs of those of only phosphorescent devices (>20% EQE). More suitable architectures, rather than simple bilayers of the QDs and phosphorescent molecules, are desired for efficient excitonic operation.
Additionally, bioconjugates of the QDs with proteins have been investigated for imaging, labeling and sensing applications in biology . These bioconjugates of the QDs are excitonically active such that the QDs can function as both the exciton donor and acceptor [150-155]. These hybrids could be promising for future lighting and light-harvesting systems. For example, chemical and biological systems can produce light upon molecular-level interactions so-called chemiluminescence and bioluminescence. These systems can be used as novel light-generation structures with the incorporation of QDs due to their superior color control and tuning abilities. Through bio- or chemiluminescence resonance energy transfer (BRET or CRET), the excitation energy created in the bio- or chemiluminescent system can be transferred to the QDs [156-158]. These initial demonstrations were targeted for applications as external light sources for bioimaging and sensing applications. Furthermore, electrically activated chemiluminescence and the transfer of the excitons to QDs were shown to be favorable for sensing applications [159, 160]. In addition to bioconjugates, QD–dye hybrids show promise for biological sensing and labeling applications. NRET between luminescent dyes and QDs have been studied in detail to elucidate the effects of concentration, shape and structure of the hybrids [161-165].