Narrow‐Bandwidth I–III–VI Semiconductor Nanocrystals: Synthesis, Luminescence and Applications in Quantum‐Dot Light‐Emitting Diodes

Abstract

I‐III‐VI semiconductor nanocrystals (NCs) have emerged as promising candidates in quantum‐dot light‐emitting diodes (QLEDs) due to their environmental‐benign nature and capability for large‐scale tunable emission as well as straightforward synthesis. However, the photoluminescence (PL) emission of I–III–VI type NCs, as reported in numerous studies, exhibits a broader full width at half maximum (FWHM), adversely affecting their color purity. This review delineates the advancements in the development of narrow‐bandwidth I–III–VI NCs, focusing on their synthesis strategies, luminescence mechanisms, and applications in QLEDs. It concludes with a discussion on the challenges confronting narrow‐bandwidth I–III–VI‐based QLEDs and outlines potential strategies for improving device performance.

Introduction

Over the past few decades, semiconductor NCs have been extensively applied in biological applications,^[1–3] luminescent solar concentrators (LSCs)^[4–7] and light-emitting diodes (LEDs)^[8–12] owing to their exceptional optical properties, including color tunability, high brightness, and low processing cost. However, the prevalent II–VI type cadmium-based nanocrystals exhibit inherent high toxicity associated with cadmium, significantly limiting their advancement and practical deployment in display technologies.^[13–16] Moreover, II–VI type Zn-based NCs suffer from the longstanding problem of optical instability under aerobic conditions, which inevitably leads to an escalated manufacturing costs for display applications. Though III-V type InP-based NCs show remarkable performance in red- and green-emitting QLEDs, blue-emitting QLEDs are difficult to achieve due to their relatively small bulk bandgap of InP (1.35 eV).^[17,18] Accordingly, researchers have made great efforts to explore alternative nontoxic and low-cost NCs with large-scale tunable emission. In the recent years, I–III-VI NCs (I = Cu, Ag; III = Al, In, Ga; VI = S, Se, Te) have been generally regarded as one of the most promising and environmentally friendly materials for cadmium-free QLEDs for the next generation display technologies.^[19–33] However, the PL of typical I-III-VI NCs always displays a broader FWHM exceeding 80 nm, attributed to intrinsic defects, such as vacancy defects (e.g., V_{Cu, Ag}), substitution defects (e.g., In_{Cu, Ag}).^[34–37] These defects, serving as donor or acceptor sites within the bandgap, lead to a lower PLQY, broader FWHM, and larger Stokes shifts. The common strategy for achieving robust and stable emission from NCs involves the fabrication of a type I core/shell structure, wherein the bandgap of the shell covers that of the core. Zinc sulfide (ZnS) known for its chemical stability, exceptional crystallinity, and broad bandgap (3.7 eV),^[38] is frequently employed as a shell material for II-VI and III-V semiconductor NCs, forming type-I band alignment.^[39–48] Coating CuInS₂,^{[20,22,49,50]} CuGaS₂^{[27,51–53,12]} and AgInS₂^[54–56] ternary NCs with ZnS has also been attempted by reacting them with a mixed solution of zinc and sulfur sources. This treatment resulted in blue-shift of PL emission maximum, contrary to the red-shift generally observed when semiconductor NCs are coated with larger-bandgap semiconductors.^{[20–22,57–60]} The expected red-shift is typically attributed to the leakage of excitons from the core into the shell, where the bandgap is essentially smaller than the HOMO-LUMO gap of insulating organic ligands on its surface. In contrast, the blue-shift is often explained as the result of changes in surface defect levels by surface passivation with ZnS and/or the result of partial alloying by cation exchange from monovalent metal and trivalent metal into zinc. While ZnS passivation can significantly enhance the optical properties of I–III–VI NCs, the cation exchange of Zn²⁺ tends to introduce numerous transposition defects,^{[20–22,27,53]} resulting in a broader FWHM exceeding 80 nm in PL spectra. In 2018, Uematsu et al. innovatively coated an amorphous GaS_x shell on the surface of AgInS₂ NCs, achieving an FWHM of less than 35 nm.^[61] The introduction of GaS_x shell effectively avoids the problem of Zn ion diffusion and suppresses the radiation recombination of defect states. Subsequently, Motomura et al. achieved amber-emitting QLEDs with an FWHM of 44 nm and an external quantum efficiency (EQE) of 0.54%.^[62] And Uematsu et al. realized green-emitting QLEDs with an FWHM of 33 nm and an EQE of 1.1%.^[63,64] Afterward, Lee et al. further advanced the field by introducing Ag-In-Ga-S/AgGaS₂ core/shell NCs, showcasing a broad emission range from 468 to 610 nm alongside an improved photoluminescence quantum yield (PLQY) (Figure 1a,b).^[65] In 2022, Tang et al. developed an innovative one-pot method to synthesize alloyed narrow-bandwidth blue-emitting Ag-Ga-Zn-S NCs and successfully fabricated QLEDs with an FWHM of less than 50 nm.^[66]

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In this review, we mainly focus on the synthesis strategies, luminescence mechanism of narrow-bandwidth I-III-VI NCs. Further, their potential applications in QLEDs are discussed. Finally, an overview is provided about the challenges encountered by narrow-bandwidth I–III–VI NCs, as well as potential future directions in display technologies.

Features of Narrow-Bandwidth I–III–VI NCs

Narrower Full Width at Half Maximum

Different from traditional I–III–VI NCs, narrow-bandwidth I–III–VI NCs are characterized by a narrower FWHM of less than 50 nm (Figure 2a), a shorter PL lifetime (40–200 ns) (Figure 2b) and a smaller Stokes shift. A narrower FWHM signifies enhanced color purity (Figure 1c) in the display technologies, presenting a key advantage. How to suppress the broader DAP emission and achieve narrow-bandwidth emission has remained an enormous challenge. Since Uematsu et al. first synthesized AIS/GaS_x core/shell NCs with an FWHM of 28 nm,^[61] there have been considerable advancements in developing narrow-bandwidth blue, green, and red I–III-VI NCs.^[61] The optical properties and synthetic methods reported in recent years have been summarized in Table 1.

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Table 1 Synthetic Methods and Optical properties of narrow-bandwidth emitting I-III-VI NCs.

NCs	Precursors, ligands, solvents	Methods	Temperature [°C]	Emission [nm]	FWHM [nm]	PLQY [%]	Reference
AIS/GaSx	Ag(OAc), In(OAc)3, Ga(acac)3, TU, DMTU, OAm, DDT, TOP	SM	200, 230–280	578	28	56	[61]
AIGS/GaSx	Ag(OAc), In(OAc)3, Ga(acac)3, TU, OAm, DDT	SM	250–300, 300	500–590	41	5–28	[67]
AIS/GaSx	Ag(OAc), In(OAc)3, Ga(acac)3, TU, DMTU, OAm, DDT, TOP	SM	120–140, 280	578–582	32	49	[68]
AIGSe/GaSx	Ag(OAc), In(OAc)3, In(acac)3, Ga(acac)3, Selenourea, TU, OAm, DDT, TOP	SM	250, 300	630–890	55–92	38	[69]
AIS/GaSx	Ag(OAc), In(OAc)3, Ga(acac)3, TU, DMTU, OAm, DDT	SM	135, 280	560	43	26	[62]
AIGS/GaSx	Ag(OAc), In(OAc)3, Ga(DDTC)3, Ga(acac)3, DMTU, OAm, DDT, TBP	SM	150, 280	498–602	32–42	68	[70]
AIGSSe/GaSx	Ag(OAc), In(OAc)3, Ga(acac)3, TU, Selenourea, OAm, DDT	SM	250, 300	580–790	45–125	15–50	[71]
AIGS/GaSx	Ag(OAc), In(OAc)3, Ga(DDTC)3, Ga(acac)3, DMTU, OAm, DDT, OA, TBP, TOP	SM	150, 280	514	36	73	[72]
AIS/GaSx	Ag(NO3), In(OAc)3, Ga(acac)3, S, DMTU, toluene, OAm	SM	130, 280	530–606	36–44	/	[73]
AIS/GaSx/ZnS	Ag(OAc), In(OAc)3, Ga(acac)3, Zn(DDTC)2, DMTU, ODE OAm, TOP	SM	200, 230, 140	575	45	60	[74]
Na-doped AIGS/Ga2O3	Ag(OAc), In(OAc)3, Ga(acac)3, Na(OAc), TU, OAm, DDT	SM	150, 300	513-579	41	58	[75]
AGSSe/ZnGa2S4	Ag(OAc), Ga(acac)3, Zn(OAc)2 TMTDS, OAm, ODE, DDT, OA TOP	SM	200–220, 240	491–651	31–55	19–69	[76]
AIGS/GaSx	Ag(OAc), In(DDTC)3, InCl3, Ga(DDTC)3, Ga(acac)3,GaCl3, DMTU, OAm, TBP	SM	200, 280	499–543	31–37	21–75	[63]
AIGS/GaSx	Ag(OAc), In(DDTC)3, InCl3, Ga(DDTC)3, Ga(acac)3,GaCl3, DMTU, OAm, TBP	SM	230, 280	532	32	53	[64]
AIGS/AgGaS2	AgI, In(acac)3, Ga(acac)3, S, ODE, OA, OAm, DDT	SM	210, 240	468–610	30–55	49–96	[65]
AGSe/ZnSe	AgI, ZnI2, Ga(acac)3, Se, ODE, OAm, DDT	SM	305, 120	632–675	42	/	[77]
AIGS	Ag(OAc), In(OAc)3, Ga(acac)3, DMTU, OA, OAm, DDT	SM	135, 280	586	38	/	[78]
AGZS	AgNO3, Ga(acac)3, Zn(st)2, S, OAm, DDT	OP	300	470	48	16	[66]
AIGS/AgGaS2	Ag(OAc), InCl3, Ga(acac)3, GaCl3,Ga(NO3)3·xH2O, OA, NaDDTC·xH2O, HF	SM	200, 240	532	33	45	[79]
AIGS/AGS	AgI, InI3, GaI3, S, ODE, OAm, DDT, OTT, TOP	SM	260–300, 240	528	33	95	[80]

Non-Stoichiometry Effect

Non-stoichiometry is a prominent feature of I-III-VI NCs, arising from the substitution between monovalent cations (Cu⁺, Ag⁺) and trivalent cations (In³⁺, Ga³⁺). Previous studies have demonstrated that the non-stoichiometric compositions can considerably improve the optical properties of I-III-VI NCs, including an increase in bandgap and an improvement in PLQY.^[81–83] In the case of AGS NCs, the conduction band minimum primarily consists of Ga 4s4p and S3p orbitals, whereas the valence band maximum is composed of Ag 4d and S 3p orbitals.^[82–84] Thus, the deficiency of Ag⁺ can decrease the valence band maximum, thereby expanding the bandgap. Moreover, the scarcity of monovalent cations significantly contributes to the narrowing of FWHM. As shown in Figure 2c,d, Tang et al. revealed that the feeding ratio of Ag/Ga is conductive to the narrow-bandwidth emission.^[66] The deficiency of Ag⁺ can produce a large amount of Ag vacancies, thus enhancing free-to-bound recombination and suppressing DAP recombination with broader FWHM. Tang et al. also observed a greater substitution of Ag⁺ with Ga³⁺ during the narrowing process. They successfully obtained alloyed AIGS NCs with an FWHM of 38 nm, attributing the narrow-bandwidth emission to the radiative recombination related with Ag vacancies.^[78]

The Passivation Effect of Ga Ion

All studies reporting the synthesis of narrow-bandwidth I-III-VI NCs have noted the involvement of the Ga element. The AIS, Ag-In-Zn-S (AIZS), and AGS NCs without GaS_x shell coating commonly exhibit a broader FWHM. In Uematsu's work, the AIS/InS_x core/shell NCs exhibited an obvious DAP emission compared to AIS/GaS_x.^[61] Zhang et al. reported red-emitting AIGZS/InS_x with an FWHM of 78 nm, suggesting that the passivation effect of Ga element cannot be substituted by In element for narrow-bandwidth emission.^[85] Tang et al. revealed that narrow-bandwidth emission is related with diffusion of Ga³⁺.^[78] During the growth process of AIGS NCs, Ga³⁺ gradually diffuses into the interior of AIS NCs by cation exchange, forming alloyed AIGS NCs rather than AIS/GaS_x NCs (Figure 3a). They further investigated excited-state relaxation dynamics of AIS and AIGS NCs through Transient Absorption (TA) measurement (Figure 3b,c). They discovered that the diffusion of Ga ion can effectively passivate both S-states and shallow donor states near the conduction band. The passivation of intrinsic defects served as donor level would diminish the contribution of DAP radiation recombination (Figure 3d). Subsequently, the hot electrons concentrate in the recombination process between the CB bottom and the V_Ag level, leading to narrow-band luminescence.

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Synthesis Strategies of Narrow-Bandwidth I–III–VI NCs

Up to now, core/shell and alloyed NCs are the most common structures for narrow-bandwidth I-III-VI NCs. The Seed-Mediated method (SM) is utilized for synthesizing core–shell structures (Figure 4a), and the One-Pot method (OP) is used to synthesize nanocrystals with alloy structures.

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Seed-Mediated Method

For the synthesis of core/shell structure NCs, the predominant method involves initially synthesizing Ag-In-S (AIS) or Ag-In-Ga-S (AIGS) core, subsequently coated with GaS_x,^[61–63,67] ZnGa₂S₃,^[76] or AgGaS₂^[65] shell. In 2018, Uematsu et al. innovatively synthesized AgInS₂ NCs with Ag(OAc)₂ and In(OAc)₃ as precursors, then coated an amorphous GaS_x shell on the surface of AgInS₂ NCs.^[61] This innovation significantly reduced the PL FWHM to less than 35 nm. Building on this, Kameyama et al. further developed a novel strategy for incorporating Ga³⁺ into AIS nanoparticles.^[67] They tuned the PL peak wavelength of the AIGS NCs by controlling the In/Ga ratio. The absorption spectra exhibit an extended redshift with increasing the content of In precursor (Figure 4b). The corresponding PL of AIGS NCs coated with GaS_x shell exhibited a PL emission ranging from 500 to 610 nm with a FWHM less than 50 nm (Figure 4c,d). A higher temperature for the synthesis of AIGS NCs is necessary due to the lower reactivity of Ga³⁺. Consequently, quaternary AIGS NCs were synthesized at 300 °C and then coated with GaS_x similar to AIS@GaS_x NCs.

However, realizing perfect passivation with shell materials for AIGS/GaS_x core/shell NCs is challenging due to a significant lattice mismatch (≈8.5%) between the tetragonal AgIn_xGa_1-xS₂ core and the hexagonal Ga₂S₃ shell. This mismatch results in poor optical properties, such as quantum yield (QY) below 50% and FWHM wider than 40 nm. Afterward, Lee et al. devised a novel synthesis method for achieving I–III–VI-based core/shell NCs with a very narrow FWHM (<30 nm), which was accomplished through passivation using a shell material (ZnGa₂S₄) with a matching tetragonal crystalline structure to the AgGaS_2-xSe_x core NCs.^[76] This approach minimized the lattice mismatch (≈6.5%) between core and shell materials. They initially synthesized AgGaS_2-xSe_x core NCs using Ag(OAc), Ga(acac)₃, tetramethylthiuram disulfide(TMTDS), and Se-OAm as precursors. Then AgGaS_2-xSe_x core NCs were coated with ZnGa₂S₄ shell at 240 °C. The synthesized AgGaS_2-xSe_x/ZnGa₂S₄ NCs exhibited a narrower FWHM (28 nm) and a higher PLQY (69%). Subsequently, Lee et al. devised a chemical route for high-quality AIGS/AGS core/shell NCs. They first synthesized a molecular precursor containing Ag, Ga, and S (Ag-S-Ga(OA)₂) for both AIGS core synthesis and AGS shell growth.^[65] The growth of AIGS/AGS includes 3 steps: i) The preparation of precursors. They first synthesized In(OA)₃, Ga(OA)₃, Zn(OA)₂, and S-OAm stock solutions with In(acac)₃, Ga(acac)₃, Zn(acac)₂, S power, OA and OAm. Then the Ag-S-Ga(OA)₂ stock solution was prepared by injecting Ga(OA)₃ and S-OAm into Ag₂S solution at 210 °C. ii) The synthesis of AIGS core NCs. The In precursor (In(OA)₃) was injected into Ag-S-Ga(OA)₂ stock solution at an elevated temperature (T = 210 °C) to burst AIGS NCs nucleation and subsequent growth. iii) The heteroepitaxial growth of AGS shell. The Ag-S-Ga(OA)₂ stock solution was injected into the AIGS core solution to grow the AGS shell on the surface of the AIGS core. Despite the amorphous GaS_x shell suppress the broadband emission of AIGS cores, but only marginal PLQY enhancement (≈70%) is achieved due to structural imperfection. In contrast, heteroepitaxy with AGS shell guarantees both bright and narrowband PL emission for AIGS/AGS NCs. The synthesized AIGS/AGS NCs exhibited superior optical properties with an FWHM of less than 40 nm and a PLQY over 70%, akin to the state-of-the-art InP NCs. Kim et al. reported the AIGS NCs coated inner quasi-AGS and outer GaS_x shell via In³⁺ to Ga³⁺ cation exchange.^[80] They first synthesized AIGS core NCs, which were then precipitated and redispersed in ODE. Mixing this with a toluene solution containing GaCl₃ and heating to 240 °C enabled the formation of AIGS/AGS core/shell NCs. Coated with heteroepitaxial growth of AGS shell, AIGS/AGS NCs achieved a narrower FWHM of 33 nm and a high PLQY of 95%, effectively suppressing broader DAP emission. Li et al. reported green emitting AIGS/AGS core/shell NCs with a FWHM of 33 nm via HF-assisted synthesis strategy.^[79] They initially synthesized AIGS core NCs followed by HF etching treatment. Then the treated AIGS NCs were coated with a compatible AGS shell. The HF treatment played an important role in alleviating core/shell mismatch in AIGS/AGS core/shell NCs, facilitating uniform growth of shell layer and surface defects passivation. However, the complexity of the precursor preparation and synthesis process, as well as the requirement for glove box conditions, poses challenges to large-scale production.

One-Pot Method

In 2022, Tang et al. developed a one-pot method to synthesize narrow-bandwidth blue-emitting Ag-Ga-Zn-S NCs with an FWHM of 48 nm.^[66] Compared with the traditional synthesis of AGS NCs, they added a small amount of Zn precursors in the reaction mixture. The introduction of a small amount of Zn precursors could produce lots of Ag vacancies and avoid the diffusion of Zn²⁺, thereby narrowing the PL linewidth. They first put AgNO₃, Ga(acac)₃ and Zn(St)₂ into the four-necked flask, which was then preheated at 90 °C and vacuumed for 1 h. After fast injection of S-OAm precursor, the reaction was elevated to 300 °C, and maintained for a few minutes. Compared with seed-mediated method, the one-pot method has the advantages of simplicity, low cost, and low requirements for the synthesis environment, which are beneficial for future large-scale production. However, the surface defects of alloy NCs could not be passivated effectively, thus leading to relatively lower PLQY compared with core/shell NCs.

Strategies for Improving Luminescence Properties

Surface Ligands Exchange

Surface ligands, pivotal in the synthesis and stabilization of NCs, are categorized into L-, X-, and Z-types based on the electron donation characteristics in their neutral states.^[86–88] L-type ligands, exemplified by oleylamine, possess paired electrons and thus attaching to electron-accepting cations on the NC surface. Conversely, Z-type ligands, such as metal halides, contain paired holes and bind with electron-donating anions like sulfur and selenium. X-type ligands, represented by oleic acid, donate one electron to form a NC-ligand bond. In the synthetic process of NCs, oleylamine (OLA) and oleic acid (OA) are frequently employed as long-chain ligands to prevent the aggregation of NCs and passivate surface defects. OLA provides a weak reducing effect, which assists the formation of metal and metal chalcogenide nanoparticles, whereas OA (when deprotonated) provides a stronger coordinating force to metal sites, resulting in better passivation of nanoparticles. Both ligand types play a critical role in passivating surface trap states, thereby amplifying the PL intensity of quantum dots, and they have occasionally been utilized as a ligand (solvent) mixture with the aim of combining their properties. Furthermore, Z-type ligands have been recognized for their significant contribution to the surface modification of indium phosphide (InP) and cadmium selenide (CdSe) NCs, particularly in post-treatment processes.^[86–88] The above discussion underscores the indispensable role of ligand engineering as a fundamental approach toward enhancing PL efficiency, demonstrating its critical importance in the development of high-performance NCs.

Uematsu et al. reported the PLQY of AIS/GaS_x core/shell NCs can considerably increase from 28% to 56% via post-treatment with TOP.^[61] They proposed that the poor optical performance of NCs can be attributed to exciton leakage due to an inadequate potential barrier between the core and its thin amorphous GaS_x shell. TOP as L-type ligand can coordinate with zero-valent sulfur sites existing on the surface of GaS_x shell, thereby passivating the electron traps and increasing the PLQY. Hirase et al. observed a significant increase in band-edge emission from the AIS/GaS_x core/shell NCs, which was attributed to the passivation of the surface S sites acting as electron traps facilitated by the formation of a stable TOP sulfide.^[89] This process obstructed the excited carrier pathways to the surface defect sites, leading to an appearance of the band-edge emission peak. Hoisang’ research highlighted substituting pure OLA with a mixed solvent of the OA and OLA led to an increase in the PLQY of the AIGS QDs from 11.5% to 35.2%.^[72] This enhancement was considered to be attributed to the stronger coordination capability of X-type OA with multicomponent nanocrystals compared to the L-type OLA. Further investigation into the effect of Z-type ligands (metal halides and carboxylate) on the PL revealed that post-treatment with zinc chloride (ZnCl₂), a potent Lewis acid, significantly boosts the PLQY. As shown in Figure 5a, ZnCl₂ uniquely targets the electron-donating sites of the NCs. Aside from binding to the dangling sulfur sites, it introduces X-type ligands (chloride ions), which facilitate the removal of original ligands via ligand exchange processes. As shown in Figure 5b,c, the absorption spectra remain unchanged while the PL intensity of AIGS/GaS_x NCs is increased significantly. The PL decay lifetimes become longer with increasing ZnCl₂ amount during the post-treatment, indicating the noncombination related to surface defects with shorter lifetime is suppressed effectively (Figure 5d). Additionally, Uematsu's group achieved substantial improvements in the PLQY of green-emitting AIGS/GaS_x NCs through post-treatment with gallium chloride (GaCl₃).^[63] This was achieved by rapidly injecting a mixture of olamine and GaCl₃ during the synthesis phase, simultaneously enhancing the band-edge PL ratio and the PLQY of the AIGS/GaS_x NCs. Here, chloride ions also served as X-type ligands, further passivating surface defects. Kim et al. injected TOP solution during the nucleation of AIGS core NCs and subsequent AGS shell coating process.^[80] They revealed that AIGS core NCs without TOP surface passivation become more vulnerable to degradation and oxidation when exposing to the air during the purification process. After TOP treatment, the PLQY of AIGS/AGS NCs can be considerably improved from 78% to 95%.

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Epitaxial Growth of ZnS Shell

The epitaxial growth of a ZnS shell with a larger bandgap on core NCs has been recognized as a potent strategy to enhance their optical properties significantly. The type I core/shell NCs can efficiently confine the wave functions of electrons and holes, which can help to improve the performance of NCs. The diffusion of active Zn²⁺ ions into the interior of NCs during high-temperature synthesis process presents a significant challenge for the narrow-bandwidth I–III–VI NCs. Cation exchange involving Zn²⁺ can lead to the generation of numerous substitution defects within the I–III–VI semiconductor compounds, such as Zn substituting for Cu (Zn_Cu) or In (Zn_In), etc. These defects introduce unwanted electronic states within the crystal structure of the NCs, thereby disrupting the homogeneity and optical properties of NCs.^[90] The GaS_x, AgGaS₂ and ZnGa₂S₄ seem to be ideal shells for the improvement of optical properties of narrow-bandwidth I-III-VI NCs. Moreover, Loan et al. devised a new structure: AgInS₂/GaS_x/ZnS NCs, which demonstrated a higher luminescence quantum yield of ≈60% and photochemical stability over 12 months.^[74] They initially synthesized AIS/GaS_x core/shell NCs with two layers of GaS_x shell, which were subsequently coated with ZnS shell. This innovative core/double-shell configuration resulted in AIS/GaS_x/ZnS NCs emitting solely bright excitonic luminescence at 575 nm, with no broad luminescence in the lower energy region. Moreover, the PLQY was significantly increased from 35% for AIS/GaS_x NCs to 60% for AIS/GaS_x/ZnS NCs. Obviously, efficient double-shelling AIS NCs with GaS_x and ZnS contributes to complete passivation of the surface defects and enhancing the localization of the photo-generated electrons and holes in the AIS NCs, thus emitting more excitonic luminescence.

Luminescent Mechanism for Narrow-Bandwidth I-III-VI NCs

The photoluminescence mechanism of I–III–VI type NCs distinguishes from that of II–VI (e.g., CdSe) or III–V (e.g., InP) types, primarily arising from the recombination of charge carriers at intragap defect states due to the nonstoichiometric composition. To date, researchers has elucidated two primary models to describe the luminescent mechanisms of I–III–VI NCs: free-to-bound (FTB) and donor–acceptor pair (DAP) radiative recombination processes.^{[34–37,91,92]} As shown in Figure 6a, free-to-bound recombination is characterized by the recombination of a delocalized electron from the conduction band with a hole localized at a point defect, which is mainly associated with vacancies of copper (V_Cu) or silver (V_Ag) serving as the acceptor sites. Conversely, DAP recombination represents the transition from a donor level (e.g., V_S, In_Ag) to an acceptor level (Figure 6b). Notably, a competitive relationship exists between free-to-bound and DAP recombination. Our previous work has confirmed that the diffusion of Ga³⁺ can passivate the intrinsic defects served as donor level, suppressing the broadening DAP emission.^[66,78] Simultaneously, non-stoichiometric ratios will generate a large number of monovalent metal vacancies (such as V_Cu or V_Ag), thus enhancing the free-to-bound recombination. Dominance of free-to-bound recombination typically leads to narrowband emission, while obvious DAP recombination can broaden the PL spectrum, considerably damaging the high color purity of NCs. Adjusting the stoichiometry and defect content of I–III–VI NCs could be a powerful strategy to control and optimize their luminescent properties.

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One prevailing theory suggests that the narrow-bandwidth emission observed in I–III–VI NCs mainly originate from band-edge emission (Figure 6c). Uematsu et al. revealed that AIS/GaS_x core/shell NCs had a FWHM of 28 nm, which is much narrower than that of bare AIS core NCs (220 nm).^[61] Further, the PL lifetimes of AIS/GaS_x core/shell NCs are comparable with that of the band-edge emission from CdSe NCs, indicating that the narrow-bandwidth emission is likely derived from the band-edge emission. Additionally, the Stokes shift between the bandgap and PL peak energy is about 0.06 eV, suggesting a band-edge emission. Lee et al. revealed that the PL emission of individual AIGS/AGS NCs is corresponding to the band-edge emission along with a FWHM of 17 nm by single particle PL spectra.^[65] The defect emission is typically associated with the existence of localized donor defect or acceptor defect states, which arise from vacancies, interstitials or anti-sites within the crystal lattice or from structural imperfections on the surface. The GaS_x or AgGaS₂ shell passivates the surface defects, thus leading to the generation of the band-edge emission.

At present, there is still no consensus on the mechanism of narrow-bandwidth luminescence. Klimov et al. discovered that numerous of copper vacancies will induce the conversion of monovalent copper to divalent copper during the reaction process.^[93] Thus, the synthesis of narrow-bandwidth emitting I-III-VI NCs may involves changes in valence states if free-to-bound recombination corresponds to the luminance mechanism of narrow-bandwidth emission. Whether the narrow-bandwidth emission source originates from the band-edge or free-to-bound recombination, it can be confirmed that the appearance of narrow-bandwidth emission can be attributed to the passivation effect of Ga³⁺. The passivation of interior and surface defects can effectively suppress DAP radiation recombination, which is an important way to achieve narrow-bandwidth emitting I–III–VI NCs.

Applications of Narrow-Bandwidth I–III–VI NCs in QLEDs

In the recent years, I-III-VI NCs (e.g., CuInS₂, CuGaS₂, and AgInS₂) have emerged as promising and environmentally friendly materials for the next generation cadmium-free QLEDs.^{[10–12,19–37]} Up to now, the efficiencies of QLEDs based on I–III–VI NCs have reached 7.1%, 7.3% and 7.8% for blue, yellow and red emissions, respectively.^[51,49,94] The efficiencies and stabilities of red and green QLEDs are close to the standards requirement of commercial applications. However, I–III–VI NCs generally exhibit broad PL characteristics due to their intrinsic defects, thereby limiting the color purity of QLEDs.

In 2020, Motomura et al. first fabricated narrow-bandwidth emitting QLEDs based on AIS/GaS_x core/shell NCs, comprising ITO/ZnO/AIS NCs/TCTA/MoO₃/Al, as depicted in Figure 7a.^[62] The device exhibited a narrower FWHM of 45 nm and a peak EQE of 0.6% (Figure 7c). They attributed defect emission in EL spectra to electrons flowing in the emitting layer being easily trapped at defect levels in the NCs. They added tris(2,4,6-trimethyl-3-(pyridine-3-yl)phenyl)borane (3TPYMB) into AIS/GaS_x NCs solutions as the mixing emitting layer (EML) in QLEDs. This resulted in an EQE of 0.54% and a narrower FWHM in EL spectra, whereas the device without 3TPYMB exhibited an EQE of 0.13%. The observed improvement was attributed to a better balance between electrons and holes in the EML due to the hole-blocking property of the 3TPYMB. Based on this work, Motomura et al. continuously fabricated QLEDs based on green emitting AIGS/GaS_x core/shell NCs.^[64] They discovered that the contact between AIGS QDs and ZnMgO electron transport layer was a critical factor of the weak luminescence. Lots of defect sites existed at the interface between QDs and the ZnMgO electron transport layer, leading to the generation of defect-related emission in EL spectra. Then they treated the ZnMgO layer with Ga compound, such as GaCl₃ and Ga(DDTC)₃, which successfully suppressed the defect-related emission and enhanced the device efficiency, achieving an EQE of 1.1% and a narrow FWHM of 33 nm (Figure 7d). Subsequently, Tozawa et al. fabricated QLEDs based on Na-doped AIGS/Ga₂S₃ NCs with an EQE of 0.6% (Figure 7e).^[75] The addition of Na⁺ during the reaction process contributed to the passivation of surface defects effectively. However, the introduction of Ga₂S₃ with larger bandgap caused a larger turn-on voltage and the accumulation of charge carriers within the EML. To further explore the narrow-bandwidth potential, Tang et al. synthesized narrow-bandwidth blue-emitting Ag-Ga-Zn-S NCs using facile one-pot method and fabricated blue QLEDs for the first time.^[66] The AGZS QLEDs demonstrated a FWHM less than 50 nm and a peak EQE of 0.40%, comprising ITO/PSS:PEDOT/TFB/AGZS/ZnMgO/Al (Figure 7b,f). The performance of QLEDs based on I-III-VI NCs has been summarized in Table 2. In conclusion, to put the narrow bandwidth I–III–VI nanocrystals to practical applications in the future, surface defects of quantum dots and the unbalanced charge carriers between interfaces are the key aspects that need to be broken through.

[IMAGE OMITTED. SEE PDF]

Table 2 Performance Summary of QLEDs Based on narrow-bandwidth emitting I-III-VI NCs.

NCs	EL [nm]	FWHM [nm]	Von [V]	Lmax [cd/m2]	EQE [%]	Reference
AIS/GaSx	572	44	2.8	60	0.54	[62]
AIGS/GaSx	531	33	2.4	175	1.1	[64]
Na-doped AIGS/Ga2O3	563	45	3.2	/	0.6	[75]
AIGS/AGS	535	36	2.2	2747	0.75	[79]
AGZS	482	48	3.2	123	0.40	[66]

Conclusions and Prospects

In summary, as the promising candidate for display technologies, narrow-bandwidth I-III-VI NCs have attracted considerable attention due to their high purity, high PLQY, non-toxic, and low-cost properties. This review focused on the advancements in the synthetic methods, the luminescence mechanisms, as well as the applications of narrow-bandwidth I–III–VI NCs. Despite the narrow-bandwidth I–III–VI NCs have made significant progress in recent years, there are still some remaining issues to be addressed by further research. As follows: 1) There is no consensus on the mechanism of narrow-bandwidth luminescence. Whether the narrow-bandwidth emission source originates from the band edge or free to bound recombination, it can be confirmed that suppressing DAP emission is an efficient strategy to achieve narrowband luminescence. 2) Narrow-bandwidth Cu-based I–III–VI NCs have rarely been reported. Compared with Ag-based I-III-VI NCs, the Cu⁺ with higher mobility at high temperature is prone to ion exchange, thus generating numerous substitution defects and enhancing broader DAP emission. Moreover, the random position of the Cu-related emitting center within the Cu-based I-III-VI NCs leads to large variations in the contribution from the electron-hole Coulomb coupling to the PL energy.^[34] 3) The instability of narrow-bandwidth I-III-VI NCs in air is also a noteworthy issue. Loan et al. observed a decrease in the PLQY from 77% to 64% when AIGS/AGS NCs were transferred from solution to air.^[74] Uematsu et al. also reported a similar problem with AIS/GaS_x and AIGS/GaS_x NCs, wherein the intensity of the band-edge emission significantly diminished under conditions such as light exposure, moisture, oxygen or merely after 15 days due to the sensitive GaS_x shell.^[63,70] 4) The performance of QLEDs based on narrow-bandwidth I–III–VI NCs lags far behind that of traditional I–III–VI NCs. In recent years, the applications of narrow-bandwidth I–III–VI NCs in display technologies have generally aroused the interest of researchers, the efficiencies of QLEDs still remain unsatisfactory. Furthermore, shorter device lifetimes and unbalanced carrier transport are still urgent issues to be addressed.

Overall, despite a few issues awaiting thorough exploration in the near future, narrow-bandwidth I–III–VI NCs have made significant breakthroughs in synthesis and device applications. It is foreseen that their development will offer innovative opportunities for the next generation of display technologies.

Acknowledgements

This work was partly supported by the National Natural Science Foundation of China (12274021, 62375012) and Beijing Natural Science Foundation (Z220007).

Conflict of Interest

The authors declare no conflict of interest.

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