Metal halide perovskite nanocrystals (NCs) have the advantages of high fluorescence quantum yield (PLQY), continuously adjustable band gap, high color purity, and solution processability, and are considered to be the next generation materials for light-emitting diodes (LEDs). CsPbI₃ perovskite NCs have excellent thermal stability and ideal band gap (~1.7 eV) and are widely used in red light-emitting diodes and solar cells (SCs). Size-controlled small-sized CsPbI₃ quantum dots (QDs) enable them to meet the red standard for wide-gamut high-definition Rec. 2020 displays through quantum confinement effects. However, the high specific surface area of small-sized CsPbI₃ QDs brings high density of non-radiative recombination defects. Traditional long-chain organic ligands, such as oleylamine (OA) and oleylamine (OAm), are difficult to fully passivate defects on the surface of small-sized QDs due to large steric hindrance. In addition, long fatty chains also hinder charge transfer between QDs. Therefore, replacing long-chain ligands with short ligands is the key to improving the performance of LED devices based on small-sized CsPbI₃QDs.
For traditional OA and OAm-modified QDs, there is an equilibrium between the ionized and molecular forms of these ligands (OA⁻+OAmH⁺⇌OAH+OAm or OAmH⁺+I⁻⇌OAm+HI). This dynamic binding reaction leads to the desorption of ligands during the separation and purification process of QDs, thereby reducing the stability and optical properties of quantum dots. Figure 1a illustrates the HI-driven in situ ligand exchange strategy of 5AVA ligand with OA/OAm ligand. In the proton-promoting strategy, the protons provided by excess HI drive the OA⁻ and OAmH⁺ ligands bound to the surface of QDs to move toward a dynamic equilibrium form of the molecules. The 5AVAI ligand with bifunctional groups then binds to the surface sites left by the OA and OAm ligands. In addition, I- from HI can fill the I-vacancy defects on the surface of QDs. As the amount of 5AVAI increases, the first exciton absorption peak and PL peak shift to shorter wavelengths, as shown in Figure 1b. The blue shift of the absorption peak and PL peak is attributed to the decrease in the size of QDs caused by the increase in iodide ions in the reaction system. Figure 1c shows photos of CsPbI₃ QDs synthesized with different amounts of 5AVAI under UV light (365 nm). As the amount of 5AVAI increases, the PLQY of QDs increases, reaching a maximum value of 95% at 0.2 M (Figure 1d). These results show that the introduction of 5AVAI during the synthesis process can reduce the size of CsPbI₃ QDs, effectively passivate the surface defects of CsPbI₃ QDs, and improve the optical properties of QDs.
Figure 1. (a) Strategy diagram for HI-driven in situ exchange of 5AVA ligand and OA/OAm ligand. (b) UV-visible absorption and fluorescence spectra of primary purified CsPbI₃ QDs with different 5AVAI amounts. (c) Photographs of CsPbI₃ QDs treated with different amounts of 5AVAI under UV light (365 nm). (d) PLQY of CsPbI₃ QDs treated with different amounts of 5AVAI.
The optical properties and morphology of CsPbI₃ QDs synthesized with 0.2M 5AVAI and without 5AVAI were compared. As shown in Figure 2a, the half-height width of QDs treated with 5AVAI becomes narrower (38 and 42 nm, respectively), indicating that the size of QDs treated with 5AVAI is smaller and the size distribution is more uniform. The PLQY of QDs treated with 5AVAI is 87%, while the PLQY of QDs without 5AVAI treatment is 69%, indicating that 5AVAI effectively passivates the surface defects of QDs. In addition, the stability of QDs under environmental conditions was also studied (Fig. 2b). The PLQY of QDs treated with 5AVAI remained above 70% after 20 d, while QDs without 5AVAI treatment decomposed after 20 d. This shows that 5AVAI ligands exchange dynamically bound OA and OAm ligands on the surface of QDs, effectively passivating surface defects of QDs and improving the stability of QDs. The effect of 5AVAI on the morphology of QDs was characterized by TEM. Figure 2c,d shows the TEM images of CsPbI₃ QDs with and without 5AVAI. The QDs treated with 5AVAI maintain the cubic shape and good monodispersity.
Figure 2. (a) UV-visible absorption and fluorescence spectra of twice-purified CsPbI₃ QDs with and without 5AVAI. The inset shows the photo of QDs (left: without 5AVAI; right: with 5AVAI) under UV (365 nm) and the corresponding PLQY values. (b) Stability of CsPbI₃ QDs with and without 5AVAI under ambient conditions (temperature 25±5°C, humidity 50±10%). The inset shows photos of the two QDs after different storage days. TEM images of (c) CsPbI₃ QDs with and (d) without 5AVAI.
I I The role of 5AVAI and CsPbI₃ QDs
In order to determine the interaction between QDs and 5AVAI ligands, the ligands on the surface of QDs were characterized. As shown in Figure 3a, the signal peak at 1639cm⁻¹ represents the N–H bending vibration of the amine functional group, and the peak at 1571cm⁻¹ is caused by the asymmetric stretching vibration peak of carboxylate (COO⁻), which shows that There are OA, OAm or 5AVAI ligands on the surface of QDs. After treatment with 5AVAI ligand, the peak value of the CN stretching vibration signal at 907cm⁻¹ is enhanced, indicating that the binding of 5AVAI ligand to QDs increases the amino-containing ligands on the surface of QDs. Thermogravimetric analysis showed that at 500°C, the weight of CsPbI₃ QDs containing 5AVAI was reduced by 32%, while the weight of CsPbI₃ QDs without 5AVAI was reduced by 34%, indicating a reduction in long-chain ligands (Figure 3b). The XRD pattern (Figure 3c) confirms that the crystal structure of QDs does not change after 5AVAI ligand treatment, and they all have the same cubic crystal structure. The time-resolved PL decay spectrum (Figure 3d) shows that the average fluorescence lifetimes of CsPbI₃ QDs with and without 5AVAI are 7.1 and 5.5 ns, respectively, which proves that 5VAI ligand treatment can effectively reduce the defects of CsPbI₃ QDs, thereby enhancing radiative recombination.
Figure 3. (a) FTIR spectra of CsPbI₃ QDs with and without 5AVAI. (b) TGA of CsPbI₃ QDs with and without 5AVAI. (c) XRD spectra of CsPbI₃ QDs films with and without 5AVAI. (d) Time-resolved photoluminescence (TRPL) decay of CsPbI₃ QD films with and without 5AVAI.
A QLED device was prepared using the CsPbI₃ QDs film before and after 5AVAI treatment as the light-emitting layer. Figure 4a illustrates the QLED device structure composed of ITO/PEDOT:PSS/PTAA/CsPbI₃ QDs/TmPyPB/PO-T2T/LiF/Al. Figure 4b shows the normalized electroluminescence spectra of CsPbI₃ QDs films with and without 5AVAI ligand treatment. The electroluminescence peaks are located at 645 and 651 nm respectively. The CIE1931 color coordinates of CsPbI₃ QDs treated with 5AVAI ligand are (0.710, 0.289) (Figure 4c). Figure 4d shows the current density-voltage-brightness (JVL) characteristics of the champion QLED based on these two CsPbI₃ QDs. The high-quality QDs film treated with 5AVAI enables the device to have lower leakage current density. QLED based on 5AVAI processing shows higher current density and brightness, with the maximum brightness increasing from 1090cd m⁻² to 7494 cd m⁻². This is attributed to the exchange between short-chain 5AVAI ligands and long-chain OA and OAm, which increases the charge transfer between CsPbI₃ QDs and improves the performance of the device. The peak EQE of QLEDs based on CsPbI₃ QDs with and without 5AVAI is 24.45% and 18.63%, respectively (Fig. 4e), and the 5AVAI-treated devices exhibit lower efficiency roll-off at high current density. The improvement in device performance is attributed to 5AVAI passivating surface defects of QDs and suppressing non-radiative recombination. Finally, the operational stability of the QLED was evaluated (Figure 4f). At an initial brightness of 209cd m⁻², the T₅₀ (the time required to decay to 50% of its initial value) of the CsPbI₃ QDs device treated with 5AVAI is approximately 10.79h, while the T₅₀ of the QDs device without 5AVAI treatment is approximately 0.15 h (initial brightness is 204cd m⁻²). After treatment with 5AVAI ligand, the T₅₀ of the device was increased by about 70 times, and the operating stability of the device was significantly improved. The performance of our LED devices represents the best performance of bright red (EL<650 nm) PeLEDs.
Figure 4. (a) QLED device structure diagram. (b) Normalized electroluminescence spectra of CsPbI₃ QDs devices before and after 5AVAI treatment. (c) CIE coordinates correspond to the electroluminescence spectrum of CsPbI₃ QDs devices based on 5AVAI treatment. QLED operating characteristics based on CsPbI₃ QDs with and without 5AVAI: (d) current density and brightness-voltage curve; (e) EQE-current density curve. (f) Stability of QLEDs based on CsPbI₃ QDs with and without 5AVAI treatment.
Yanming Li, Ming Deng, Xuanyu Zhang, Lei Qian, Chaoyu Xiang* Proton-Prompted Ligand Exchange to Achieve High-Efficiency CsPbI₃ Quantum Dot Light-Emitting Diodes, Nano-Micro Letters (2024)16: 105