Polymer-Nanocrystal Hybrid Materials for Light Conversion Applications

In this mini-review we report on current developments of hybrid materials based on semiconductor nanocrystals integrated into polymer matrices for direct light conversion, their present limitations, as well as their high potential for future applications.

Tailoring the properties of these materials can be achieved by simply tuning the particle size without introducing any changes in the chemical composition, or by changing the material composition.By changing the material composition as well as the size of the individual NCs the range for PL emission can be tuned covering the entire optical spectrum starting from UV to the NIR (Figure 2).The high quantum yield (QY), extraordinary photostability, pure color and the PL emission tunabilty of semiconductor NCs make them an obvious target of investigation for applications of white light generation.Examples for light emitting NCs are ZnO, CdS, CdSe, CdTe and CdSe core, core-shell or core-multishell structures which absorb UV or blue light and convert it into light of longer wavelengths.

Semiconductor NCs/Polymer Nanocomposites
Colloidal semiconductor NCs can form various hybrid materials by e.g., being integrated into different host materials including polymers.The incorporation of NCs into solid matrices from their growth solution is of interest for technologically useful applications as well as for fundamental studies of NC-matrix interactions.Polymers offer opportunities for flexible, lightweight, and mechanically stable NC nanocomposites [19].Semiconductor NC-polymer composites combine the advantages of both components and have been realized for light emitting displays [7], light conversion layer [20] such as LED covers or in solar concentrators [21], optical bar coding [22], photocatalyst [23] and photovoltaics [4].The main challenge in the preparation of NC-polymer composites is preventing a macroscopic phase separation and the aggregation of NCs in the hybrid material which would lead to film inhomogeneities and fluorescent quenching effects limiting the respective optical device performance.
Colvin et al. reported on the first hybrid NC-polymer light emitting diodes (LEDs) in 1994 [7].A thin layer of CdSe NCs was deposited on a conductive support, and combined with a 100 nm thick soluble poly(p-phenylenevinylene) PPV derivative layer.Since then, a lot of progress has been achieved for optimizing all parameters of NC-polymer hybrid LEDs.The introduction of CdSe@CdS core-shell NCs made a significant improvement for the NCs-polymer hybrid LED [24].The efficiency was increased twenty times by increasing the efficiency of the radiative recombination and device internal quantum efficiency, while the lifetime was increased as well by a factor of hundred.Other potential application for NC-polymer based LEDs are offering large area lighting systems and backlighting for flat panel displays.Such applications require LEDs emitting multi-color light or white light.Recently, Wood et al. fabricated a full color AC-driven display based on inject-printed NCs/polymer composites [25].Semiconductor colloidal NCs integrated in solvent based polymers have a potential to compete with other technologies such as OLEDs and full-color quantum dot displays [26].The lifetime of hybrid organic-NC based LED devices is still limited to some extend by the instability of the metal contacts and degradation of organic components under high current operation conditions [11].In order to avoid such limitations, some non-conductive polymers are combined with semiconductor NCs forming a photoluminescent conversion layer for commercially available e.g., blue LEDs [20,27].In the following we concentrate on the description of NC-polymer hybrid materials based on non photoactive polymers for light conversion applications.
The first down-conversion LED was achieved by coating a transparent CdSe@ZnS core-shell polylaurylmethacrylate (PLMA) hybrid composite on the surface of a GaN light-emitting diode.Saturated-color light with different wavelengths has been generated by tuning the size of NCs (Figure 3).The chemical structure of various polymers utilized as polymeric hosts for luminescent semiconductor NCs are shown in Figure 5.

Synthesis Approaches for NC Polymer Based Hybrid Materials
There are in principle three distinguishable approaches for obtaining polymer NC hybrid materials.

Integration of as Synthesized Semiconductor NCs into Polymers by Physical Mixing
Physical mixing of NC solutions and polymer solutions is a first obvious approach for obtaining NC-polymer hybrid materials, especially for thin films.Even when serious chemical attacks to the NC surface are avoided during the solution mixing, PL QY is often reduced by the agglomeration of NCs due to phase segregation processes.An example is shown in Figure 6.Physical mixing is often used with organic soluble polymers, such as PMMA [29,31], cellulose [23,32] and so on.In order to maintain or even increase the PL QY, additional protective shells e.g., out of CdS [32], ZnS [20] and/or silica [33] are utilized for covering the core NCs.Bomm et al. [32] prepared CdSe@CdS NCs and cellulose triacetate (CTA) nanocomposites with a PL OY of 52% by using this physical mixing  Additionally the protecting trioctylphosphine (TOP) ligand was also used to prevent the agglomeration and chemical attack on CdSe/ZnS QDs.Woelfle et al. introduced a poly(methyl methacrylate) (PMMA) compatible ionic liquid to protect the CdSe/ZnS QDs [38].However, the chemical attack during the thermal polymerization process could not be completely avoided.In both cases the resulting nanocomposites exhibited photoluminescence QYs of less than 40%.Recently, Bomm et al. reported a new method to incorporate CdSe@CdS core @shell nanorods into P(LMA-co-EGDM) by UV-polymerization lauryl methacrylate (LMA) monomers with the cross-linking agent ethylene glycol dimethacrylate (EGDM) monomers in the presence of the CdS@CdS NRs [32].They found that a high concentration of a liquid UV-initiator led to a significant decrease of PL QY. 70% PL QY was observed for this hybrid material by using only 0.1 wt % of a UV-initiator.Figure 8 shows the effect of the concentration of the UV-initiator on the PL behavior of the nanocomposites.A decrease in PL QY is observed for high UV-initiator concentration.After incorporation of NCs into polymer, to maintain the optical properties of NCs in the hybrid materials is of outermost importance for light conversion application.As we mentioned before, the NCs phase separation and agglomeration, which could significantly reduce the transparency and PL QY of the NCs/polymer hybrid, should be avoided.NCs should be homogenously distributed in the polymer matrix without forming agglomerates.TEM investigation were carried out on this CdSe@CdS NR/P(LMA-co-EGDM) nanocomposites confirming the absence of agglomerates within the hybrid material (Figure 9).Without an additional protecting shell for the CdSe core material, the QY of CdSe-PMMA or CdSe-BP-PFCB hybrid materials is rather limited.Very recently, we developed a simple reproducible and up-scalable one pot approach for the incorporation of crude CdSe core QDs into nylon without the need of further purification steps.60% PL QY was reached by this CdSe core NCs nylon composite [18].No significant loss in PL intensity was observed for the hybrid material compared to NC solutions and no additional protective shell was needed for the NCs which makes the process easy and up-scalable and.In Figure 10(a) the principle of the in-situ polymerization process of nylon in the presence of CdSe QDs is shown.The resulting hybrid material is moldable in any shape, transparent and highly luminescent (Figure 10(b)).As it is depicted in Figure 10(a), the polymerization of 6-aminocaproic acid monomers was performed at 220-250 °C in the presence of as-prepared unpurified TOPO and HDA capped core CdSe QDs using a straight forward process under nitrogen atmosphere [18].The excess ligand molecules are separated during solidification due to phase separation between the QD-nylon phase and the ligand phase and can be easily removed.Therefore even unpurified crude QDs can be used directly after synthesis without applying any extra purification step.Differently sized QDs can be incorporated into the nylon polymer resulting in different color emitting hybrid materials and laser scanning microscopical (LSM) investigation of the hybrid films revealed a homogenious PL emission at the microscopic level [18].The transparency of the resulting product can be increased by fast cooling of the liquid phase.The QD hybrid materials can be processed while kept in liquid phase above 150 °C and different forms and shapes are available.After incorporation of the QDs into the polymer matrix, no significant loss of PL QY was observed.
Another successful approach for obtaining functional NC-polymer hybrid films is the attachment of polymerizable capping ligands directly onto the NC surface, which can lead to a strong binding of NCs to the resulting polymer matrices.Zhang et al. capped octadecyl-p-vinyl-benzyl dimethylammonium chloride (OVDAC) on CdTe NCs in aqueous solution via electrostatic interactions [39].CdTe NC-polymer bulk composites were obtained after the radical polymerization reaction was induced by azobis-isobutyronitrile (AIBN) as initiator.The phase separation and agglomeration of the NCs from the polymer host was avoided by the strong attachment of CdTe NCs to the polymer matrix during  by NC-/hybrid materials.Current research focuses on two directions, one is to increase the quality of the emission color (CRI and CCT), and another is to obtain higher luminescence efficiency.
For obtaining a good emission color, the most common approach is mixing different emitting NCs in the polymer.Chung et al. reported a white emission by using mixtures of differently sized CdSe core NCs incorporated in PMMA as a phosphor [27].The white LED was fabricated by covering a 460 nm emitting blue LED with a CdSe NC-PMMA nanocomposite layer.Two white light emitting realizations were presented, the first one was based on a single phosphor film containing CdSe NCs emitting at 580 nm and the second one was based on a dual phosphor film containing CdSe NCs emitting at 555 and 625 nm respectively.In the single phosphor realization the CRI is with 15.7 very low.The CRI value was increased to 61.1 after choosing the dual phosphor realization.In order to avoid the reabsorption effect by mixing differently emitting NCs together in one polymer layer, a double approach was introduced where the longer emission wavelength NC-polymer layer was coated on top of the shorter emission wavelength NC-polymer layer.However, limited by the low PL QY of CdSe NCs, the luminous efficiency of white LEDs were lower than 6 lm/W at an operating current for the blue LED of 20 mA.The design of NCs with a broad emission signal is an alternative approach for achieving white light spectra.Schreuder et al, examined thirteen dissimilar polymers as potential encapsulates for special designed broad emitting (white-light emitting) NCs [31].They found that encapsulates based on cyclosiloxane or bisphenol-A type epoxy structures caused extensive aggregation of the NCs even at low loading levels (less than 0.5% w/w) due to the solubility difference between the polymers and the NCs.Biphenylperfluorocyclobutyl (BP-PFCM) exhibited the most robust, color stable, and homogenous encapsulation properties enabling a high loading of NCs as well.White-light emitting CdSe NCs encapsulated in the BP-PFCB polymer were coated on various UV-LEDs creating a white light source with chromaticity coordinates of (0.324, 0.322) and a high color-rendering index of 93 with a luminescence efficiency below 1 lm/W since the original PL QY of such white-light emitting CdSe NCs is already very low.The luminescence efficiency of such CdSe NCs-BP-BFCB nanocomposite covered LEDs increased up to 5.3 lm/W by optimizing the thickness of the hybrid films and the utilization of different types of UV LEDs [43].A lot of researchers also choose the CCT which is a criterion for the quality for warm white light as overall quality parameter for optimizations.Chandramohan et al. demonstrated a warm white light (CCT of 3,436 to 4,500 K) emitting LED with a CRI of 87.4 using green-light-emitting CdSe NC-PMMA hybrid materials covering a InGaN/GaN based blue emitting LED [44].Very recently, Chung et al. fabricated a warm (CCT of 3,237.4K) white light emitting LED with a CRI of 83.8 and a luminescence efficiency of 4.14 lm/W by combining blue LED emitting at 430 nm with a nanocomposite film based on (Poly[(9,9-dioctyl-2,7-divinylenefluorenylene)-alto-co-(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene)] (PFPV) and CuInS 2 -ZnS nanoparticles [45].
In order to improve the overall luminescence efficiency, a lot of research groups try to increase the PL QY of the light converting hybrid material.Weaver et al. for example studied the fluorescence quenching behavior during the incorporation of CdSe@ZnS NCs into cyanoacrylate, epoxy and silicone [46].They found that excess of amines in the NCs solution passivated the NCs surface and preserved the PL QY of the NCs-polymer composites.Yu et al. reported on a white light LED with a luminescence efficiency of 44.2 lm/W by mixing a relatively high amount of CdSe NCs (20% by weight) into PFPV [47].Since the luminescence efficiencies achieved for light conversion layers so far are still low, efforts have to be dedicated for the development of NCs/polymer composites with high QY.Low cost and high PL QY hybrid materials would be the key parameters for NC based white light conversion materials towards their utilization and for commercializing.First commercially available NCs/polymer based white light converter plates are already on the market (Figure 11).

Light Conversion Layers in Concentrator Solar Cells
One main driving force of photovoltaic (PV) research and development is attaining higher power conversion efficiencies at lower costs.Compared with expensive prices of high efficient PV modules, it is preferable to convert incident light from the solar spectrum collected at large areas to monochromatic light and concentrate the converted light onto a small area of a high efficient solar cells with optimized power conversion efficiencies for the chosen monochromatic light .Based on this concept, luminescent solar concentrators (LSC) were developed in the late 1970s as an alternative approach to lower the costs of PV.The achievable efficiency of LSCs is dependent on how well it is spectrally matched with the attached absorbing fluorescent material.Recently, Goldschmidt et al. reported two independent methods to increase the collection efficiency of LSCs.One is to combine two different dyes to enlarge the utilizable spectral range.The other is to increase the collection efficiency by using a photonic structure acting as a band stop reflection filter in the emission range of the dye. Figure 12 demonstrates the principle of a multi-fluorescent concentrator cell based on Goldschmidt's concept [48].
However, over 60% of the total solar photon flux occurs at wavelengths above 600 nm, which is beyond to the absorption range of most organic dyes.Novel luminescent photo stable materials with absorptions reaching to the infrared are needed for increasing the efficiency of LSCs.Semiconductor NC based hybrid materials have good potential to match this need.NCs can provide excellent PL QY with sufficient long term stabilities.For example PLQE of 85% has been reported for core-shell NCs [49], and PL QY of 70% for the transparent NCs/polymer composites so far [32].Since cadmium and lead based materials are considered toxic and therefore not environmental friendly, the development of cadmium and lead-free NC materials are of high importance for future applications.Luminescent NCs based on Y 2 O 3 @Eu 2 O 3 , InP@ZnS, MnSe@ZnSe might be promising materials to be incorporated into polymers for various light conversion applications [28,33,51].
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Figure 11 .
Figure 11.Photograph of Quantum LightTM optics from the company QD Vision, Lexington, MA, USA for converting cold white light emitting LEDs into warm white light sources (Image source: www.qdvision.com,2010, copyright QD Vision).
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