Indium Phosphide‐Based Quantum Dots with Shell‐Enhanced Absorption for Luminescent Down‐Conversion

It is shown that admixing small amounts of cadmium into the shell of InP/ZnSe core/shell quantum dots results in an increased absorption of blue light and a limited redshift of the band‐edge emission. These effects reflect the reduced bandgap of (Zn,Cd)Se alloys and their smaller conduction‐band offset with InP. Nevertheless, adjusting the InP core size enables InP/ZnSe and InP/(Zn,Cd)Se quantum dots with identical emission characteristics to be made. Processing both materials into remote phosphor disks, it is demonstrated that the shell‐enhanced absorbance of InP/(Zn,Cd)Se has the double benefit of suppressing self‐absorption and reducing the amount of quantum dots by weight needed to attain a given blue‐to‐red color conversion.


DOI: 10.1002/adma.201700686
(LEDs) used in display or lighting applications. This raises the amount of QDs InP-based down-convertors need and deteriorates their conversion efficiency; a double setback in performance and cost.
Here, we address these issues by introducing InP QDs shelled by (Zn,Cd) Se alloys since admixing of Cd is known to reduce the bandgap of ZnSe. [23,24] As QDs typically have higher absorption coefficients the more the photon energy exceeds the bandgap energy, [25] we expect that adding even small amounts of Cd can boost the absorbance at 450 nm and suppress self-absorption in remote phosphor films. In the case of ZnSe core QDs, bandgap tuning by addition of Cd has been achieved before via hot injection methods [26][27][28] or cation exchange reactions. [29,30] In addition, shells of Zn 0.5 Cd 0. 5 Se alloys have been grown around InP core QDs by Micic and co-workers, [31] where this particular composition was chosen to match the core and shell lattice parameter. In spite of this lattice-matched configuration, their approach resulted in rather polydisperse samples with a broad and moderately efficient photoluminescence. We therefore first propose a new method to form InP/(Zn,Cd)Se core/shell QDs, in which an intermediate ZnSe layer is grown between the InP core and the (Zn,Cd)Se shell. We show that such core/shell QDs feature an efficient photoluminescence in combination with a considerably enhanced absorption at 450 nm, similar to CdSe/CdS QDs. Comparing nanocomposites made of InP/ZnSe and InP/ (Zn,Cd)Se core/shell QDs with a 6% Cd content, we demonstrate that the latter overcomes the issues with conversion efficiency and material economy that arise when using QDs with shells that are transparent at the pump wavelength for optical down-conversion, as is the case for ZnS or ZnSe at 450 nm.
Core InP QDs were synthesized according to a recently established protocol where tris(diethylamino)phosphine reacts with indium chloride or indium bromide dissolved in oleylamine (OLA) in the presence of zinc chloride (see the Experimental Section and Section S1 in the Supporting Information for details). [32][33][34][35][36] Whereas this reaction mixture readily lends itself to ZnSe shelling, where the trioctylphosphine selenium (TOP-Se) added reacts with the ZnCl 2 already present, the formation of alloyed (Zn,Cd)Se shells requires more care. A mere partial replacement of ZnCl 2 by CdCl 2 results in a redshift and a broadening of the first exciton transition, especially during the shell growth stage through TOP-Se addition (see Section S2, Supporting Information). Whereas such changes are in line with reported effects of Cd carboxylate adsorption It is shown that admixing small amounts of cadmium into the shell of InP/ZnSe core/shell quantum dots results in an increased absorption of blue light and a limited redshift of the band-edge emission. These effects reflect the reduced bandgap of (Zn,Cd)Se alloys and their smaller conduction-band offset with InP. Nevertheless, adjusting the InP core size enables InP/ZnSe and InP/(Zn,Cd)Se quantum dots with identical emission characteristics to be made. Processing both materials into remote phosphor disks, it is demonstrated that the shell-enhanced absorbance of InP/(Zn,Cd)Se has the double benefit of suppressing self-absorption and reducing the amount of quantum dots by weight needed to attain a given blue-to-red color conversion.

Quantum Dots
Colloidal quantum dots (QDs) have become an enabling material for applications relying on luminescent down-conversion. Displays, LEDs, or solar concentrators all make use of the efficient and spectrally narrow QD photoluminescence (PL) to convert short wavelength incident light to nearly monochromatic emitted light at a longer wavelength of choice, as determined by the dimensions and shape of the QDs. [1][2][3][4][5][6][7] In this respect, CdSe/CdS core/shell QDs stand out. Having a bandgap of 2.42 eV (512 nm), a CdS shell can enhance both the photoluminescence quantum yield (PLQY), by passivating the CdSe outer surface, and the absorption cross section of the QDs at blue, violet, and UV wavelengths. [8][9][10][11][12] This combination makes for high performing and economical QD down-convertors, where suppressed self-absorption translates the high PLQY of the QDs into a high internal and external PLQY of the down-convertor, while using a minimal amount of QDs. [13][14][15] A major drawback of this approach, however, is its reliance on compounds rich in Cd, which is a toxic heavy metal that is restricted in several countries. Hence, the rise of InP-based QDs as an alternative. [16] Like Cd-based QDs, InP QDs feature a photoluminescence tunable across the visible spectrum with a high PLQY following shelling by ZnS or ZnSe. [17][18][19][20][21][22] Unfortunately, neither of these materials contributes much to the absorbance of ≈450 nm blue light, the typical pump wavelength of light emitting diodes www.advmat.de www.advancedsciencenews.com to InP QDs [37] -indicating that CdSe shell growth is feasible with the same procedure-this is better avoided here. Indeed, also during the formation of (Zn,Cd)Se QDs, [38,39] it is typically seen that the Cd precursor reacts more easily with Se than the Zn precursor and the ensuing InP/CdSe interface will exhibit a staggered, type 2 band alignment that comes with an undesired redshift and broadening of the emission. [40] We therefore developed an alternative approach where first an initial ZnSe layer is grown around the InP core QDs at relatively low temperature. Next, a mixture of Zn and Cd carboxylates is injected, the temperature is set to increase, and TOP-Se is added dropwise to form an alloyed (Zn,Cd)Se shell. Eventually, the reaction mixture is kept for 3 h at 320 °C.
In the case of a reaction that uses a molar fraction of Cd to Cd+Zn precursors of 3.2%, this protocol results in a 50 nm redshift of the band-edge absorption, which hardly broadens, and a strong enhancement of the absorbance at shorter wavelengths (see Figure 1a). Transmission electron microscopy (TEM) images indicate that the thus formed QDs have an average projected diameter of 13.5 nm as compared to the 10.2 nm found for core/shell QDs grown using a Zn-only procedure (see Figure 1b,c and Section S3 in the Supporting Information). Similar results are obtained upon increasing the Cd precursor fraction to 6.4% and 13.8% (see Figure 1d,e and Section S3 in the Supporting Information). Moreover, according to TEM-based energy dispersive X-ray spectroscopy, the Cd to Cd+Zn ratio (x Cd ) of these core/shell QDs largely corresponds to what is used in the synthesis (see Section S4, Supporting Information). X-ray diffraction patterns of InP/(Zn,Cd)Se with different Cd composition are shown in Section S5 (Supporting Information). The patterns indicate a progressive transition from zinc blende to wurtzite upon increasing the Cd content, yet the gradual shift of the zinc blende (220)/wurtzite (110) diffraction peak toward a larger lattice spacing is in line with the supposed formation of an alloyed (Zn,Cd)Se shell with increasing Cd content. We thus conclude that the implemented synthesis protocol results in InP/(Zn,Cd)Se core/shell QDs, where the composition of the shell can be readily tuned by varying the Cd precursor fraction in the synthesis. Figure 2a shows absorbance spectra, normalized to the maximum absorbance at the band-edge transition and PL spectra of InP/(Zn,Cd)Se core/shell QDs with x Cd increasing from 0 to 0.025, 0.05, and 0.13. It follows that the increase of x Cd concurs with a marked enhancement of the absorbance in the blue part of the visible spectrum. In addition, it leads to a progressive redshift of the band-edge PL, which only shows some notable broadening at the highest molar fraction of Cd (x Cd = 0.13). Both trends are in line with what can be expected for InP core QDs shelled with (Zn,Cd)Se alloys. Indeed, a simple linear interpolation of semiconductor properties [41] already suggests that admixing Cd into a ZnSe shell will reduce the bandgap of the shell and lower the core/shell conduction band offset, see Figure 2b. Moreover, both effects will be enhanced by the positive bandgap bowing of (Zn,Cd) Se alloys. [42] With the (Zn,Cd)Se bandgap shifting from 2.72 (456 nm) to 1.74 eV (710 nm), the bandgap reduction accounts for the enhanced absorption at short wavelengths. Moreover, the reduced conduction-band offset will promote the spreading of the electron wavefunction in the shell. This leads to an enhanced red shift with increasing shell thickness-an effect also seen in CdSe/CdS QDs [9] -and increasing Cd content.
With a bulk bandgap of about 2.72 eV (456 nm), the InP/ZnSe shells will not benefit from an enhanced shell  All spectra are recorded on reaction aliquots taken at the end of the reaction as described in Section S1 (Supporting Information). All spectra have been normalized relative to the absorbance maximum A 1S-1S of the band-edge feature. The inset shows a zoom on the band-edge transition. b) Simplified energy gap and band alignment diagram of (Zn,Cd)Se alloys constructed as a linear interpolation between the energy levels of ZnSe and CdSe as calculated in ref. [41]. The bold horizontal lines represent the energy levels of InP, showing a transition from type I to type II alignment when changing from InP/ZnSe to InP/CdSe. www.advmat.de www.advancedsciencenews.com absorbance at around 450 nm, the typical wavelength range of blue LEDs used in lighting and display applications. This has several drawbacks. With only the InP cores contri buting to the absorption of the blue pump light, a larger mass or volume of QDs is needed to reach a preset absorptance at 450 nm. Moreover, as the absorption cross section at 450 nm (pump LED) and at the wavelength of the band-edge photoluminescence will be similar, InP/ZnSe-based nanocomposites are prone to self-absorption and the concomitant efficiency loss. Similar to CdSe/CdS QDs-where the CdS shell enhances the QD absorption cross section below 500 nm- Figure 2a shows that alloyed (Zn,Cd)Se shells can overcome this issue in the case of InP core/shell QDs.
To demonstrate this point, we synthesized InP/ZnSe and x Cd = 0.06 InP/(Zn,Cd)Se core/shell QDs, where different InP core diameters were selected to attain a similar bandedge absorption and photoluminescence. Absorption and emission spectra of both QDs, dispersed in toluene, are represented in Figure 3a. The spectra confirm that both samples exhibit an almost identical band-edge absorbance and a highly similar PL spectrum with a peak intensity λ max at around 631 nm. Admixing Cd, however, results in a somewhat lower PLQY of 45% as compared to the 60% measured for InP/ZnSe with rhodamine 6G as reference. Possibly, this reflects the enhanced delocalization of the electron wavefunction in the shell, which makes charge carrier trapping at the QD outer surface more likely, and higher PLQYs may result from further ZnSe or ZnS shell growth. Moreover, the selective boosting of the absorbance of blue light as compared to red and green makes such core/ shell QDs ideal as light convertors for devices pumped by blue LEDs.
Both types of QDs were processed in remote phosphor coatings by dispersing a predetermined amount of QDs in Kraton FG1901X, a triblock copolymer based on styrene and ethylene/butylene, with a polystyrene content of 30%, see Figure 3b. Figure 4a shows the emission spectrum obtained when pumping such layers containing InP/ZnSe QDs in a remote phosphor configuration with a blue LED and recorded in an integrating sphere. Here, the bands at around 450 and 640 nm correspond to the transmitted blue pump light and the QD photoluminescence, respectively. As expected, less blue light is transmitted and more red light emitted upon increasing the QD loading in the film. However, the emission spectra already indicate that the increase of the integrated emission intensity from the QDs does not match the reduction in the intensity of the blue pump light, while at the same time λ max exhibits a progressive redshift. Both trends can be seen more clearly in Figure 4b,c, showing that at an absorptance of 94,5%, the internal PLQY of the InP/ZnSe emission (see the Experimental Section for a definition of internal and external PLQY) has dropped from the originally 60% in suspension to a mere 38%, whereas λ max has shifted to 650 nm, 21 nm to the red as compared to a suspension of the same InP/ZnSe QDs. Both effects are a signature of self-absorption, where QDs absorb the luminescent light emitted by other QDs. Indeed, after each absorption and reemission cycle, only a fraction of the light intensity, corresponding to the PLQY measured in solution, is retained. Moreover, the reduced QD absorbance at longer wavelengths makes that self-absorption mostly affects the blue side of the QD photoluminescence. Hence, the efficiency drops and the remaining luminescence shifts to longer wavelengths.
In contrast to the layers containing InP/ZnSe QDs, the InP/(Zn,Cd)Se QD layers show no dropping internal PLQY and feature a strongly reduced redshift of λ max with increased QD loading (see Figure 4a-c). For a 91% absorptance, a PLQY of 43% is measured, while λ max has shifted to the red by only 8 nm as compared to the emission of dilute QD suspensions. Both effects point toward an effective suppression of selfabsorption, in line with the enhanced absorption cross section due to Cd admixing at 450 nm. The consequence of this is best appreciated by focusing on the example of ≈77% absorptance layers. Despite the original difference in PLQY-60% versus 45%-both remote phosphor layers effectuate a similar color conversion (see Figure 4d). Moreover, thanks to the absorptionenhancing alloyed shells, InP/(Zn,Cd)Se layers with x Cd = 0.06 achieve this with a solid loading, i.e., the sheer weight of the QDs incorporated, that is only 26% of what InP/ZnSe layers need. Clearly, this weight saving directly translates into a reduced use of InP in InP/(Zn,Cd)Se-based remote phosphors. Moreover, a solid loading of 0.9 g m -2 as needed here for InP/(Zn,Cd)Se QDs is comparable to the 0.5-1.5 g m -2 solid loading needed in CdSe/CdS-based color convertors. [15] Hence, while not entirely Cd-free, a 20-fold reduction of the Cd content in remote phosphor disks can be attained by the approach presented here, while preserving the exquisite self-absorption suppressing characteristics of CdSe/CdS-based QDs.
Summing up, we have presented an approach to enhance the absorption cross section of InP-based QDs in the blue region of the visible spectrum by shelling InP core QDs with a (Zn,Cd) Se alloy. The optical properties of these core/shell QDs point toward a reduced shell bandgap and an enhanced electron delocalization, similar to the seminal CdSe/CdS core/shell  QDs. We have demonstrated the impact of shell-enhanced absorption on the economy of QD-based color conversion by studying QD-loaded polymer films as remote phosphors. Comparing InP/ZnSe and InP/(Zn,Cd)Se loaded films, we find that the latter significantly suppresses the detrimental effects of selfabsorption and can achieve the same color conversion using 74% less QDs by mass. Often acclaimed as an alternative for Cd-based QDs, InP QDs face issues for widespread use as color convertors in view of the cost and scarcity of indium. Hence, a strategy such as shell-enhanced absorption that lowers the amount of InP QD-based color convertors need makes a difference. Moreover, the approach leaves ample room for further progress. Either increasing the shell volume or raising the Cd fraction in the shell will further enhance the shell absorption. In addition, admixing other elements can have similar effects, where especially a partial replacement of selenium by tellurium is a possible alternative.
Full Chemical Yield Synthesis of InP QDs with First Exciton at 560 nm (Estimated Diameter: 3.2 nm): 50 mg (0.225 mmol) of indium(III) chloride and 150 mg (1.1 mmol) of zinc(II) chloride were mixed in 2.5 mL (7.5 mmol) of technical OLA. The reaction mixture was stirred and degassed at 120 °C for an hour and then heated to 180 °C under inert atmosphere. Upon reaching 180 °C, a volume of 0.23 mL (0.8 mmol) of tris(diethylamino)phosphine was quickly injected in the above mixture and InP nanocrystals synthesis proceeded. The reaction occurred during 30 min. At the end of the reaction, the temperature was lowered. InP nanocrystals were then precipitated in ethanol and suspended in toluene. This synthesis provided InP nanocrystals with a diameter of 3.2 nm (band-edge absorption (first exciton) at 560 nm).   QD-Doped Remote Phosphor Layers-Formation: Remote phosphor layers containing either InP/ZnSe or InP/Cd 0.06 Zn 0.94 Se QDs were prepared by mixing an appropriate amount of these materials with 80 mg of Kraton FG1901X in 0.5 mL of toluene, stirring, and drop casting on a circular glass substrate with a diameter of 1.8 cm and a corresponding area of 10.18 cm 2 . After evaporation of the solvents, transparent QD phosphor layers were obtained.
QD-Doped Remote Phosphor Layers-Characterization: Layer efficiency measurements were performed inside an integrating sphere (152 mm, Spectralon coated). Excitation of the samples was done with a blue LED (λ max of 446.5 nm, full width at half maximum of 19.2 nm and a luminous efficacy of 37 lm W -1 ) and detection of outgoing light by a CCD camera (Princeton Instruments ProEM 16002), attached to a spectrograph (Princeton Instruments Acton SP2358). A baffle was mounted between the sample and the detection port of the integrating sphere. Internal and external quantum efficiency were determined by the two measurement approach. [43,44] The external quantum yield is defined as the ratio between the numbers of photons emitted by and incident on the phosphor layer, whereas the internal quantum yield is the ratio between the number of photons emitted and photons absorbed by the remote phosphor layer. In this work, always the internal PLQY is reported. QDs layers were analyzed by introducing the circular layer in a cylindrical, white teflon mixing chamber with a height of 20 mm, which contains the blue LED in the bottom center for excitation. The measurements were operated at a constant current of 20 mA, which translates in a luminous efficacy of the blue LED of 7.26 lm W -1 .

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.