Type I: QLEDs with polymer charge transport layers

The earliest generation of QLEDs (Type I) were a natural progression from polymer LEDs, comprising active layers of either bilayers or blends of CdSe QDs and polymers.^{Reference Colvin, Schlamp and Alivisatos23,Reference Dabbousi, Bawendi, Onitsuka and Rubner24} Their EQEs were low (typically <0.1%) mainly because QDs were forced to play dual roles of charge conduction and light emission; while thick wide-bandgap shells and long organic ligands generally serve to passivate a QD and enhance its η_PL, they also markedly detract from the mobility of the QD film.

Type II: QLEDs with organic small molecule charge transport layers

Type II QLEDs were first introduced by Coe et al. in 2002 and comprise a monolayer of QDs at the interface of a bilayer OLED.^{Reference Coe, Woo, Bawendi and Bulović25} These devices demonstrated record EQEs of 0.5%, an efficiency that has since been augmented by an additional factor-of-ten through optimizations. The enhanced performance was attributed to the use of a single monolayer of QDs (enabled by the development of spin-coating^{Reference Coe, Woo, Bawendi and Bulović25} and microcontact printing^{Reference Kim, Anikeeva, Coe-Sullivan, Steckel, Bawendi and Bulović26–Reference Rizzo, Mazzeo, Palumbo, Lerario, D’Amone, Cingolani and Gigli28} techniques), which decouples the luminescence process in the QDs from charge transport through the organic layers.^{Reference Coe, Woo, Bawendi and Bulović25,Reference Steckel, Snee, Coe-Sullivan, Zimmer, Halpert, Anikeeva, Kim, Bulović and Bawendi27,Reference Mueller, Petruska, Achermann, Werder, Akhadov, Koleske, Hoffbauer and Klimov29–Reference Anikeeva, Madigan, Halpert, Bawendi and Bulović32}

Type III: QLEDs with inorganic charge transport layers

Type II devices boast all of the advantages of OLEDs, with the added benefits of enhanced spectral purity and tunability, as illustrated by the tunable EL shown in Figure 2b–c. However, Type III QLEDs, which replace the organic charge transport layers (CTLs) of Type II QLEDs with inorganic CTLs, can potentially lead to greater device stability in air^{Reference Friend, Gymer, Holmes, Burroughes, Marks, Taliani, Bradley, Dos Santos, Bredas, Lögdlund and Salaneck33,Reference Burrows, Bulović, Forrest, Sapochak and McCarty34} and should enable the passage of higher current densities and therefore brighter emission. Initial demonstrations of Type III QLEDs based on sputtered metal oxides have indeed displayed high current densities (up to 4 A cm^–2), but EQEs to date remain less than 0.1%.^{Reference Caruge, Halpert, Wood, Bulović and Bawendi35} Over the past few years, similarly structured all-inorganic (except for organic ligands) QLEDs that operate by the altogether different excitation mechanism of QD ionization have also emerged^{Reference Shirasaki, Supran, Bawendi and Bulović1,Reference Wood, Panzer, Halpert, Caruge, Bawendi and Bulović36,Reference Cho, Sung, Hwang, Kim, Choi, Jo, Lee and Park37} (Figure 4d).

Figure 4.

Quantum dot (QD) excitation mechanisms. There are four routes to generating excitons in QDs that have been used in quantum dot light-emitting devices (QLEDs). (a) Optical excitation: An exciton is formed in a QD by absorbing a high-energy photon. (b) Charge injection: Neighboring charge transport layers inject charge carriers directly into a QD. (c) Energy transfer: An exciton is transferred from a nearby donor molecule to a QD via Förster resonant energy transfer. (d) Ionization: A large electric field ionizes an electron from one QD to another, thereby generating a hole. When these ionization events occur throughout a QD film, generated electrons and holes can meet on the same QD to form excitons. (e) Exciton formation is followed by exciton recombination, generating light. (f) Energy band diagram of a typical QLED outlines the two suspected routes to electrical exciton formation in QDs: (green arrows) charge injection; and (blue arrows) energy transfer.

Type IV: QLEDs with hybrid organic-inorganic charge transport layers

The hybrid structure of Type IV QLEDs offers a compromise between Type II and Type III QLEDs, often comprising an inorganic metal oxide electron transport layer (notably solution processed ZnO^{Reference Qian, Zheng, Xue and Holloway5,Reference Kwak, Bae, Lee, Park, Lim, Park, Cho, Woo, Yoon, Char, Lee and Lee7} ) and an organic small molecule hole transport layer.^{Reference Qian, Zheng, Xue and Holloway5,Reference Kwak, Bae, Lee, Park, Lim, Park, Cho, Woo, Yoon, Char, Lee and Lee7,Reference Stouwdam and Janssen38–Reference Mashford, Stevenson, Popovic, Hamilton, Zhou, Breen, Steckel, Bulović, Bawendi, Coe-Sullivan and Kazlas40} A typical device structure and its corresponding EL are shown in Figure 2a–b. Significant efficiency gains have resulted, with recent visible-emitting QLEDs reaching EQEs as high as 18%^{Reference Mashford, Stevenson, Popovic, Hamilton, Zhou, Breen, Steckel, Bulović, Bawendi, Coe-Sullivan and Kazlas40} (Figure 3a and 5a), approaching those of commercially mature phosphorescent OLEDs^{Reference Meerheim, Scholz, Olthof, Schwartz, Reineke, Walzer and Leo41} as well as the outcoupling-limited EQE ceiling of ∼20–25% (without targeted device engineering, the fraction of emitted photons that are coupled out of a planar QD-LED is limited by wave guided and surface plasmon modes to ∼5–50%^{Reference Shirasaki, Supran, Bawendi and Bulović1}).^{Reference Meerheim, Furno, Hofmann, Lüssem and Leo42} The brightness of Type IV QLEDs has also reached record levels of 218,800 cd m^–2.^{Reference Kwak, Bae, Lee, Park, Lim, Park, Cho, Woo, Yoon, Char, Lee and Lee7} High-resolution microcontact printing of QD films (>1000 pixels per inch)^{Reference Kim, Anikeeva, Coe-Sullivan, Steckel, Bawendi and Bulović26} (Figure 5b) has already enabled the fabrication of full-color 4-inch QLED displays^{Reference Kim, Cho, Lee, Lee, Chae, Kim, Kim, Kwon, Amaratunga, Lee, Choi, Kuk, Kim and Kim43} and, by mixing different compositions^{Reference Anikeeva, Halpert, Bawendi and Bulović44} or sizes^{Reference Li, Rizzo, Cingolani and Gigli45} of QDs, white-emitting QLEDs that are highly amenable to SSL have been demonstrated with excellent color quality (color rendering index, CRI ∼90 [out of 100]—see Figure 1b). White-emitting QLEDs on flexible substrates (Figure 5c) have also been realized at QD Vision.^{Reference Shirasaki, Supran, Bawendi and Bulović1,Reference Coe-Sullivan46}

Figure 5.

State-of-the-art quantum dot light-emitting devices (QLEDs). (a) External quantum efficiency (EQE) versus applied voltage for record high performance (peak EQE of 18%) red-emitting (photograph in inset) QLEDs. (b) The first demonstration of red-green-blue electroluminescence from (Type II) QLED pixels, patterned using microcontact printing (scale bar, 100 μm). (c) Flexible white-emitting Type II QLED. (d) Large-area infrared illumination by an infrared-emitting QLED, as seen through an infrared camera. (a) Adapted with permission from Reference 40. (b) Adapted with permission from Reference 26. (c) Adapted with permission from Reference 46.

QLED operating mechanisms

First, most routes to higher efficiency QLEDs—especially those addressing the quenching and lifetime of QDs under operational conditions—will be predicated on a better understanding of their operating mechanisms. In all four types of QLEDs, QD EL has been speculated to be driven by direct charge injection^{Reference Mueller, Petruska, Achermann, Werder, Akhadov, Koleske, Hoffbauer and Klimov29} (Figure 4b), Förster resonant energy transfer (FRET) (Figure 4c), or both, with the relative contribution of these mechanisms remaining unclear.^{Reference Qian, Zheng, Xue and Holloway5,Reference Solomeshch, Kigel, Saschiuk, Medvedev, Aharoni, Razin, Eichen, Banin, Lifshitz and Tessler57,Reference Zhang and Cao61,Reference Bae, Kwak, Lim, Lee, Nam, Char, Lee and Lee62} In the case of direct charge injection, an electron and a hole are injected from CTLs into a QD, forming an exciton that subsequently recombines via emission of a photon (Figure 4f, green arrows). In the FRET scheme, an exciton is first formed on a luminescent CTL. Thereafter, the exciton energy is non-radiatively transferred to a QD via dipole-dipole coupling (Figure 4f, blue arrows).

Studies in our group have indicated that, at least in certain Type II QLED geometries, FRET is the dominant QD excitation mechanism.^{Reference Anikeeva, Madigan, Halpert, Bawendi and Bulović32} Yet, for example, the achievement of EQEs >2% in QD monolayer-based devices comprising organic donor materials with very low η_PL^{Reference Jing, Zheng, Zeng, Zhang, Liu, Liu, Kong and Zhao63} challenges the universality of the FRET model. Moreover, since Type III and Type IV QLEDs, in contrast with Type II QLEDs, employ QD films thicker than one monolayer (up to ∼50 nm), the working mechanism of Type IV QLEDs is more compatible with a charge injection model.

QD PL quenching

From a device efficiency perspective, a second central consideration is QD PL quenching, since EQE is directly proportional to η_PL. While our previous review focused on the challenges and opportunities in improving the intrinsic (zero bias) η_PL of QDs in QLEDs,^{Reference Shirasaki, Supran, Bawendi and Bulović1} there are also ongoing efforts to understand the impact of bias-dependence of QD η_PL on EQE.^{Reference Shirasaki, Supran, Tisdale and Bulović64,Reference Bozyigit, Yarema and Wood65} This is of interest because application of voltage to QLEDs can lead to further reductions in η_PL due to injected-charge-induced Auger recombination^{Reference Wood and Bulović8,Reference Wood, Panzer, Halpert, Caruge, Bawendi and Bulović36,Reference Zhao, Bardecker, Munro, Liu, Niu, Ding, Luo, Chen, Jen and Ginger66,Reference Stouwdam and Janssen67} and electric field-induced exciton quenching.^{Reference Jing, Zheng, Zeng, Zhang, Liu, Liu, Kong and Zhao63,Reference Huang, Dorn, Nair, Bulović and Bawendi68–Reference Woo, Shimizu, Jarosz, Neuhauser, Leatherdale, Rubner and Bawendi71} A major consequence of these bias-induced instabilities may be a reduction in the operational lifetimes of QLEDs.

By simultaneously measuring the PL and EL of operating QLEDs, our recent work indicates that the roll-off in efficiency typically observed at high applied biases (e.g., see Figure 5a) in a Type IV QLED can be wholly explained by a simultaneous roll-off in QD η_PL.^{Reference Shirasaki, Supran, Tisdale and Bulović64} From the electric field dependence of QD PL we are able to quantitatively predict the EQE roll-off (Figure 6) and therefore to deduce that it is largely governed by electric field-induced QD luminescence quenching, and not carrier leakage or QD charging. Bozyigit et al. have likewise observed a substantial roll-off in QD η_PL when the QDs are exposed to an electric field.^{Reference Bozyigit, Yarema and Wood65} Transient PL measurements of QDs under varying electric fields^{Reference Shirasaki, Supran, Tisdale and Bulović64,Reference Bozyigit, Yarema and Wood65} suggest that the cause of quenching is either a decrease in radiative exciton recombination rate (for example, due to a decrease in the overlap of electron and hole wave functions)^{Reference Bozyigit, Yarema and Wood65} or a decrease in the efficiency of thermalized-exciton formation (e.g., trapping of hot charge carriers by QD surface traps).^{Reference Galland, Ghosh, Steinbrück, Sykora, Hollingsworth, Klimov and Htoon72,Reference Pandey and Guyot-Sionnest73}

Figure 6.

Efficiency roll-off in quantum dot light-emitting devices (QLEDs). Measured (black) and predicted (red) external quantum efficiency (EQE) as a function of voltage for a red-emitting QLED. The roll-off in EQE at high biases commonly observed in QLEDs is shown to be consistent with that predicted by accounting for reductions in QD photoluminescence quantum yield under the associated electric fields in an operating device. Adapted with permission from Reference 64.

These findings pose a tall order for the redesign of QLEDs with reduced EQE roll-off; efficient QD excitation by FRET^{Reference Anikeeva, Madigan, Halpert, Bawendi and Bulović32,Reference Chang, Musikhin, Bakueva, Levina, Hines, Cyr and Sargent74–Reference Shik, Konstantatos, Sargent and Ruda76} (rather than direct charge injection) may help to decouple the efficiency of exciton formation from η_PL so that bias-induced PL quenching can be minimized.

QLED lifetime

Operational lifetime improvements are perhaps the greatest technological hurdle to the commercialization of electrically driven QLEDs. Demonstrations of intrinsic QD PL lifetimes of >14,000 hours and PL thermal stabilities (12% fall-off at 140°C) comparable with red inorganic phosphors^{Reference Coe-Sullivan77} already render QDs commercially viable in lower flux/temperature optical down-conversion applications.^{Reference Coe-Sullivan, Liu, Allen and Steckel10} Yet the lifetimes of today’s electrically driven QLEDs (mainly Type IV) operated at initial video brightness (of 100 cd m^–2) are usually on the order of only 100 to 1000 hours^{Reference Qian, Zheng, Xue and Holloway5–Reference Kwak, Bae, Lee, Park, Lim, Park, Cho, Woo, Yoon, Char, Lee and Lee7,Reference Kim, Cho, Lee, Lee, Chae, Kim, Kim, Kwon, Amaratunga, Lee, Choi, Kuk, Kim and Kim43,Reference Cho, Lee, Joo, Jang, Kim, Lee, Kwon, Han, Kim, Choi and Kim78} (>10,000 hours is required for displays) compared with 10³ to 10⁶ hours^{Reference Meerheim, Scholz, Olthof, Schwartz, Reineke, Walzer and Leo79} for state-of-the-art OLEDs. Encouragingly, QD Vision recently reported a Type IV visible-emitting QLED with a half-life (the time after which brightness has fallen to half its initial [time zero] value) of >10,000 hours when operated at 100 cd m^–2 initial brightness,^{Reference Coe-Sullivan77} and have achieved record lifetimes for NIR QLEDs of up to ∼1700 hours (at an initial radiance of ∼3W sr^–1 m^–2). We have previously outlined some possible strategies for addressing the short lifetimes of most QLEDs,^{Reference Shirasaki, Supran, Bawendi and Bulović1} but the absence of in-depth studies focused on QLED stability is conspicuous.

QLED cost

From a manufacturing standpoint, QLEDs may be approximated as QD-enhanced OLEDs. The manufacturing cost of QLEDs can be broadly divided into the cost of raw materials and the fabrication costs of processing these materials. The similarity of the constituent materials of QLEDs and OLEDs means that they are fabricated using a similar toolbox of thin-film processing techniques, so that QLED commercialization would benefit from the manufacturing infrastructure and expertise developed for OLED production. Aside from the QDs themselves, the materials typically employed in QLEDs (metals, metal oxides, and organic small molecules) are also very similar to those found in OLEDs. Their materials costs should therefore be commensurate with those that are enabling the growth of OLED markets and would benefit from their economies of scale.

To estimate the materials costs of QDs typical in light-emitting applications when produced in large quantities, we perform a quantitative analysis based on published small-scale synthetic procedures, which we have previously used to asses the commercial viability of organic materials for photovoltaics.^{Reference Osedach, Andrew and Bulović80} We consider a few of the most common and promising QLED materials and synthetic preparations: red-emitting “legacy” CdSe QDs,^{Reference Murray, Norris and Bawendi81} “modern” CdSe QDs,^{Reference Chen, Zhao, Chauhan, Cui, Wong, Harris, Wei, Han, Fukumura, Jain and Bawendi16} “legacy” CdSe-ZnS core–shell QDs,^{Reference Hines and Guyot-Sionnest82} and “modern” CdSe-CdS core–shell QDs^{Reference Chen, Zhao, Chauhan, Cui, Wong, Harris, Wei, Han, Fukumura, Jain and Bawendi16} (all with trioctylphosphine oxide (TOPO) ligands); NIR-emitting PbS-CdS core–shell QDs (with oleic acid ligands); and PbS QDs (with oleic acid ligands), which are not only commonly used in NIR QLEDs but have also garnered tremendous interest as an active material in QD-based solar cells. The “legacy” and “modern” labels refer to the synthetic recipe evaluated, as discussed later. As detailed in Reference 80, our cost analysis takes into account all of the material inputs to these procedures in order to estimate the total material costs for each type of QD (based on our assembled database of quotations from major chemical suppliers for each of the input materials). As an example, our model for the synthesis of PbS-CdS is represented graphically as a flowchart in Figure 7. The first box in the flowchart represents the starting material, lead (II) oxide. Red arrows indicate reagents, green arrows indicate solvents, and blue arrows indicate additional materials required for workup and purification (“crash out”). The indicated quantities of input materials and waste are calculated to produce one kilogram of product.

Figure 7.

Synthesis of core–shell PbS-CdS quantum dots (QDs). The flowchart describes the synthesis of 1 kg of PbS-CdS core–shell QDs. The requisite quantities of (red arrow) reagent; (green arrow) solvents; and (blue arrow) workup (“crash out”) materials are indicated for this single-step process. Note that a quantitative yield is assumed, as discussed in the caption of Table I.

The materials costs results from our models for each synthetic procedure are summarized in Table I. We note that the material costs that we consider represent only one component of the overall cost to produce these materials. In the case of pharmaceutical drugs, for example, materials only account for 20–45% of the cost of drug synthesis. The balance includes contributions for labor, capital, utilities, maintenance, waste treatment, taxes, insurance, and various overhead charges.^{Reference Osedach, Andrew and Bulović80}

Table I.

Calculated large-scale chemical synthesis costs for red- and near-infrared-emitting quantum dots (QDs) and for two archetypal organic dyes.

Cost-per-gram for “legacy” CdSe QDs (with trioctylphosphine oxide (TOPO) ligands); “modern” CdSe QDs (TOPO ligands; wurtzite-CdSe synthesis); “legacy” CdSe-ZnS core–shell QDs (TOPO ligands); “modern” CdSe-CdS core–shell QDs (TOPO ligands; wurtzite-CdSe synthesis); PbS (via two methods) and PbS-CdS core–shell QDs (oleic acid ligands); fluorescent dye, tris(8-hydroxyquinolinato) aluminium (Alq₃); and phosphorescent dye, acetylacetonatobis(2-phenylpyridine) iridium (Ir(ppy)₂(acac)). The cost analysis accounts for all material inputs (reagents, solvents, and workup [“crash out”]), yielding total costs-per-gram both without and with workup. For the QDs, these have been converted into costs-per-area assuming a QD film of 25 nm thickness, as detailed in the text.

* In the absence of literature yields, we assume quantitative yields; although this is clearly a slight over-estimate, it is a reasonable approximation given the synthetic refinements and waste recycling that will surely accompany scale-ups in QD synthesis.

We find that the materials costs of visible-emitting “legacy” CdSe QDs ($569–660 g^–1; lower value is without workup, upper value is with workup) exceed those of “modern” CdSe QDs ($58–59 g^–1) by an order-of-magnitude as a consequence of similarly sizeable differences in the costs of reagents, solvents, and workup materials. This reflects the 20-year evolution in synthesis procedures that has led to the use of significantly smaller quantities of more economical input materials, notably a significantly cheaper and air-stable source of Cd (cadmium oxide replaces dimethyl cadmium). These advances carry forward to the “modern” CdSe-CdS core–shell QDs ($61–65 g^–1), which cost only fractionally more than their core-only equivalents, again owing to the use of an economical source of Cd for the shell. In contrast, the “legacy” CdSe-ZnS core–shell QDs ($1884–1996 g^–1) inherit the 10-fold higher costs of their starting CdSe QDs and require the use of an expensive source of zinc for their shell (replacement of dimethyl zinc is key to lower costs). It is possible that reports of QD costs of up to $10,000 g^–1 (Reference 9) may result from evaluation of antiquated “legacy” syntheses rather than more economical state-of-the-art approaches. The materials costs of NIR-emitting QDs (PbS, Method 1: $18–29 g^–1; PbS, Method 2: $45–68 g^–1; and PbS-CdS: $68–97 g^–1) are roughly commensurate with those of the “modern” visible-emitting QDs.

Current target prices for QDs synthesized via scaled-up continuous processes are ∼$10 g^–1 (Reference 21). As a guide, the materials cost of Alq₃—an archetypal organic dye used in OLEDs since the 1980s, and therefore subject to considerable economies of scale—is ∼$4 g^–1 (Reference 80). However, most heavy-metal-based phosphors found in high-performance OLEDs are considerably more expensive. A representative example is Ir(ppy)₂(acac) (Reference 83), for which we calculate a materials cost of $658–1297 g^–1. Nevertheless, direct comparison of the materials costs of QDs with those of organic dyes is complicated by the specifics of a given application, which determine how much material is consumed. Considerations include whether it is used as a neat film or as a dopant dispersed in a host matrix, the thickness of such a film, and the wastefulness of the deposition technique employed. As is to be expected, the first QD-based products address optical down-conversion by using a compact edge-mounted geometry^{Reference Coe-Sullivan, Liu, Allen and Steckel10} (Figure 1b, inset) that requires relatively small amounts of QDs (often dispersed to maximize η_PL).

One way to try to assess our results is to translate them into approximate materials costs-per-unit-area (Table I). The per-area costs are based on the assumption that a typical QLED might comprise a 25 nm film of hexagonally close-packed QDs separated by ∼0.5 nm due to their surrounding organic ligands. As expected, we obtain similar values for “modern” CdSe ($3 m^–2) and CdSe-CdS ($4 m^–2) QDs as for PbS (Method 1: $1–2 m^–2; Method 2: $3–5 m^–2) and PbS-CdS ($4–6 m^–2) QDs. “Legacy” CdSe and CdSe-ZnS QDs are significantly more expensive ($35–41 m^–2 and $90–95 m^–2, respectively). Especially given the likelihood that QD syntheses will be subject to some economies-of-scale,^{Reference Bourzac9} this is competitive, for example, with the DOE’s 2015 SSL target cost of organic materials of $40 m^–2 for OLEDs.⁸⁴ Moreover, this is just one possible metric, which does not necessarily reflect the higher material costs that luxury items such as displays might be able to shoulder.

One significant assumption that has so far been made, however, is that we can deposit QDs with 100% efficiency. In reality, the spin-casting technique (and therefore microcontact printing, which in published studies involves a spin-casting step) that has so far dominated laboratory demonstrations of QLEDs wastes ∼95% of the starting solution.^{Reference Haverinen, Myllylä and Jabbour85} Unless the QD waste is recyclable, the associated 20-fold increase in QD materials costs could render some applications economically unviable. This points to the importance of developing low-waste QD-deposition techniques. For example, ink-jet printing of multi-colored pixel arrays of QDs for both down-conversion^{Reference Panzer, Wood, Geyer, Bawendi and Bulović86} and RGB QLED^{Reference Haverinen, Myllylä and Jabbour85} technologies has been demonstrated, but further refinements in film quality and device performance are required.