Critical role of additive-induced molecular interaction on the operational stability of perovskite light-emitting diodes

•Interfacial reactions accelerate the degradation of the perovskites and the devices•Dicarboxylic-acid-induced amidation prohibits the interfacial reactions•Alkaline zinc oxide substrates catalyze the amidation process•Perovskite light-emitting diodes with notably improved stability are achieved Metal halide perovskites are emerging as promising emitters for high-performance light-emitting diodes because of their superior optical and electrical properties. Despite the rapidly improved efficiencies, state-of-the-art devices demonstrate poor stability, hindering their commercialization. Functional additives have been widely used to achieve high device efficiency; however, there is a lack of understanding of the additives’ role on the device operational stability. Here, we demonstrate different molecular interactions induced by additives of diamines and dicarboxylic acids and their effects on the device stability. Although the two types of molecules show similar effectiveness in reducing defects in perovskites, the dicarboxylic acids convert reactive organic ingredients in perovskites to stable amides, which prohibit detrimental interfacial reactions between the perovskites and the charge injection layers, leading to greatly improved stability of the resulting devices. Despite rapid improvements in efficiency and brightness of perovskite light-emitting diodes (PeLEDs), the poor operational stability remains a critical challenge hindering their practical applications. Here, we demonstrate greatly improved operational stability of high-efficiency PeLEDs, enabled by incorporating dicarboxylic acids into the precursor for perovskite depositions. We reveal that the dicarboxylic acids efficiently eliminate reactive organic ingredients in perovskite emissive layers through an in situ amidation process, which is catalyzed by the alkaline zinc oxide substrate. The formed stable amides prohibit detrimental reactions between the perovskites and the charge injection layer underneath, stabilizing the perovskites and the interfacial contacts and ensuring the excellent operational stability of the resulting PeLEDs. Through rationally optimizing the amidation reaction in the perovskite emissive layers, we achieve efficient PeLEDs with a peak external quantum efficiency of 18.6% and a long half-life time of 682 h at 20 mA cm−2, presenting an important breakthrough in PeLEDs. Despite rapid improvements in efficiency and brightness of perovskite light-emitting diodes (PeLEDs), the poor operational stability remains a critical challenge hindering their practical applications. Here, we demonstrate greatly improved operational stability of high-efficiency PeLEDs, enabled by incorporating dicarboxylic acids into the precursor for perovskite depositions. We reveal that the dicarboxylic acids efficiently eliminate reactive organic ingredients in perovskite emissive layers through an in situ amidation process, which is catalyzed by the alkaline zinc oxide substrate. The formed stable amides prohibit detrimental reactions between the perovskites and the charge injection layer underneath, stabilizing the perovskites and the interfacial contacts and ensuring the excellent operational stability of the resulting PeLEDs. Through rationally optimizing the amidation reaction in the perovskite emissive layers, we achieve efficient PeLEDs with a peak external quantum efficiency of 18.6% and a long half-life time of 682 h at 20 mA cm−2, presenting an important breakthrough in PeLEDs. 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Soc. 2018; 140: 562-565Crossref PubMed Scopus (459) Google Scholar,23Wu T. Li J. Zou Y. Xu H. Wen K. Wan S. Bai S. Song T. McLeod J.A. Duhm S. et al.High-performance perovskitelight-emitting diode with enhanced operational stability using lithium halide passivation.Angew. Chem. Int. Ed. Engl. 2020; 59: 4099-4105Crossref PubMed Scopus (55) Google Scholar,25Yuan Z. Miao Y. Hu Z. Xu W. Kuang C. Pan K. Liu P. Lai J. Sun B. Wang J. et al.Unveiling the synergistic effect of precursor stoichiometry and interfacial reactions for perovskite light-emitting diodes.Nat. Commun. 2019; 10: 2818Crossref PubMed Scopus (61) Google Scholar Despite the effectiveness in enhancing the quantum efficiencies, it is largely unknown how these additives affect the operational stability of the devices. The fabricated PeLEDs with different additives in literature exhibit significantly different operational stability, even for devices with comparable device characteristics.6Cao Y. Wang N. Tian H. Guo J. Wei Y. Chen H. Miao Y. Zou W. Pan K. He Y. et al.Perovskite light-emitting diodes based on spontaneously formed submicrometre-scale structures.Nature. 2018; 562: 249-253Crossref PubMed Scopus (935) Google Scholar,7Xu W. Hu Q. Bai S. Bao C. Miao Y. Yuan Z. Borzda T. Barker A.J. Tyukalova E. Hu Z. et al.Rational molecular passivation for high-performance perovskite light-emitting diodes.Nat. Photonics. 2019; 13: 418-424Crossref Scopus (511) Google Scholar,17Wang H. Kosasih F.U. Yu H. Zheng G. Zhang J. Pozina G. Liu Y. Bao C. Hu Z. Liu X. et al.Perovskite-molecule composite thin films for efficient and stable light-emitting diodes.Nat. Commun. 2020; 11: 891Crossref PubMed Scopus (42) Google Scholar,25Yuan Z. Miao Y. Hu Z. Xu W. Kuang C. Pan K. Liu P. Lai J. Sun B. Wang J. et al.Unveiling the synergistic effect of precursor stoichiometry and interfacial reactions for perovskite light-emitting diodes.Nat. Commun. 2019; 10: 2818Crossref PubMed Scopus (61) Google Scholar Taking high-efficiency near-infrared (NIR) formamidinium (FA)-based PeLEDs as an example, with diamine-based additives, the measured device half-life (T50, the time taken for the radiance of a device to drop to 50% of its initial value) is less than 10 h under a mild driving current density (20 mA cm−2);25Yuan Z. Miao Y. Hu Z. Xu W. Kuang C. Pan K. Liu P. Lai J. Sun B. Wang J. et al.Unveiling the synergistic effect of precursor stoichiometry and interfacial reactions for perovskite light-emitting diodes.Nat. Commun. 2019; 10: 2818Crossref PubMed Scopus (61) Google Scholar in contrast, with amino acid-based additives, this value increases to around 20 h under a high current density (100 mA cm−2).6Cao Y. Wang N. Tian H. Guo J. Wei Y. Chen H. Miao Y. Zou W. Pan K. He Y. et al.Perovskite light-emitting diodes based on spontaneously formed submicrometre-scale structures.Nature. 2018; 562: 249-253Crossref PubMed Scopus (935) Google Scholar These results imply a general lack of understanding of the additives’ roles on the device stability; improved understanding of these effects is of critical importance to guide further improvement in the operational stability of state-of-the-art high-efficiency PeLEDs. In this work, we systematically investigated perovskite films and PeLEDs treated by additives consisting of representative amino and carboxyl groups and uncover the critical role of the additive-induced molecular interaction—a factor previously neglected—on the operational stability of PeLEDs. We focused our study on devices based on the benchmark formamidinium lead iodide (FAPbI3) perovskites with the incorporation of diamines and dicarboxylic acids. We observed comparable efficiencies of the PeLEDs because of the similar effectiveness on reducing the defects in the perovskites; however, the devices based on dicarboxylic-acid-treated perovskites demonstrate significantly longer operational stability than those of diamine-treated ones. We reveal that the carboxyl groups on the dicarboxylic acids convert reactive organic ingredients in perovskite emissive layers to amides during the film crystallization, catalyzed by the alkaline zinc oxide substrate underneath. The formed stable amides prohibit the detrimental reactions between the perovskite and the bottom interlayer, which greatly improves the stability of the perovskite emissive layers and the interfacial contacts, leading to notably enhanced operational stability of the resulting PeLEDs. Through judiciously engineering the amidation reaction in the perovskite emissive layers, we achieve highly efficient PeLEDs with a peak EQE of 18.6% and measure an outstandingly long T50 of 682 h at 20 mA cm−2, which represents a remarkable improvement in the operational stability of PeLEDs. We fabricated the PeLEDs based on a commonly used device structure of indium-doped tin oxide (ITO)/zinc oxide (ZnO)/polyethylenimine ethoxylated (PEIE)/FAPbI3 perovskite/poly (9,9-dioctyl-fluorene-co-N-(4-butylphenyl)diphenyl-amine) (TFB)/Au (Figure 1A).6Cao Y. Wang N. Tian H. Guo J. Wei Y. Chen H. Miao Y. Zou W. Pan K. He Y. et al.Perovskite light-emitting diodes based on spontaneously formed submicrometre-scale structures.Nature. 2018; 562: 249-253Crossref PubMed Scopus (935) Google Scholar,7Xu W. Hu Q. Bai S. Bao C. Miao Y. Yuan Z. Borzda T. Barker A.J. Tyukalova E. Hu Z. et al.Rational molecular passivation for high-performance perovskite light-emitting diodes.Nat. Photonics. 2019; 13: 418-424Crossref Scopus (511) Google Scholar We deposited the FAPbI3 perovskite emissive layers by spin coating the precursor solutions containing lead iodide (PbI2):formamidinium iodide (FAI):molecule additive with a molar ratio of 1:2:x (the optimized x value varies for different molecule additives). We chose two bidentate molecules of 2,2′-(ethylenedioxy) diethylamine (EDEA) and adipic acid (AAC), which possess similar molecular weights and carbon atoms but different terminal moieties, i.e., amino and carboxyl groups (inset of Figure 1B). The PeLEDs treated with EDEA molecules (denoted as EDEA-PeLEDs) are fabricated following the optimal procedures developed in our previous work.7Xu W. Hu Q. Bai S. Bao C. Miao Y. Yuan Z. Borzda T. Barker A.J. Tyukalova E. Hu Z. et al.Rational molecular passivation for high-performance perovskite light-emitting diodes.Nat. Photonics. 2019; 13: 418-424Crossref Scopus (511) Google Scholar For the PeLEDs treated with AAC molecules (denoted as AAC-PeLEDs), we systematically optimized the molar ratio of AAC and the film deposition conditions and achieved the optimal device performance with x = 0.5 (Figure S1). We present EQE statistics for the control devices (without any molecule additive), the EDEA- and AAC-PeLEDs with the optimized conditions in Figure 1B. The control devices exhibited an average peak EQE of around 7%, which is comparable with our previous results.7Xu W. Hu Q. Bai S. Bao C. Miao Y. Yuan Z. Borzda T. Barker A.J. Tyukalova E. Hu Z. et al.Rational molecular passivation for high-performance perovskite light-emitting diodes.Nat. Photonics. 2019; 13: 418-424Crossref Scopus (511) Google Scholar,17Wang H. Kosasih F.U. Yu H. Zheng G. Zhang J. Pozina G. Liu Y. Bao C. Hu Z. Liu X. et al.Perovskite-molecule composite thin films for efficient and stable light-emitting diodes.Nat. Commun. 2020; 11: 891Crossref PubMed Scopus (42) Google Scholar The incorporation of either EDEA (x = 0.3) or AAC (x = 0.5) greatly improves the average peak EQE to around 17%. All three types of devices demonstrate identical electroluminescence (EL) spectra with the emission peak locating at 802 nm (Figure 1C), consistent with the light emission from three-dimensional (3D) FAPbI3 perovskites. The 3D perovskite structure is also confirmed by the X-ray diffraction (XRD) results, which shows no detectable low-dimensional perovskites or impurity phases in the AAC-based perovskite films (Figure S2A). In addition, we observed notably improved average radiance from 119 W sr−1 m−2 for the control devices to 225 and 286 W sr−1 m−2 for the EDEA- and AAC-PeLEDs, respectively (Figure S3). We provide EQE versus current density (EQE-J) and current density-voltage-radiance (J-V-R) curves in Figures 1D and 1E, respectively, for the champion devices of EDEA- and AAC-PeLEDs. Both the devices demonstrate high peak EQEs of around 18%, low sub-band-gap turn-on voltages (defined as the voltage needed for reaching a radiance of 0.1 W sr−1 m−2) of below 1.5 V and high radiance values of around 200 W sr−1 m−2. Similar to the EDEA-PeLEDs demonstrated in our previous work,7Xu W. Hu Q. Bai S. Bao C. Miao Y. Yuan Z. Borzda T. Barker A.J. Tyukalova E. Hu Z. et al.Rational molecular passivation for high-performance perovskite light-emitting diodes.Nat. Photonics. 2019; 13: 418-424Crossref Scopus (511) Google Scholar we ascribe the enhanced EQEs of AAC-PeLEDs mainly to notably reduced defects in the perovskite emissive layers, evidenced by the increased PL lifetime (Figure S4A) and the obviously enhanced PL QYs under different excitation densities (Figure S4B) of the AAC-based perovskites compared with the control films. The enhanced PL QYs are also consistent with the notably enhanced PL emission (and at the same time a negligible change to the absorption spectrum) for the AAC-based perovskite films in comparison with the control film (Figure S2B). Although the island features in AAC-based perovskite films (Figure S2C) might also help to enhance the device efficiency due to the increased light outcoupling,6Cao Y. Wang N. Tian H. Guo J. Wei Y. Chen H. Miao Y. Zou W. Pan K. He Y. et al.Perovskite light-emitting diodes based on spontaneously formed submicrometre-scale structures.Nature. 2018; 562: 249-253Crossref PubMed Scopus (935) Google Scholar the outcoupling effect itself would not lead to such significant improvement of EQEs from 7% to over 17% in our devices. Despite the comparable device performance parameters, we observed strikingly different operational stability for the EDEA- and AAC-PeLEDs. As shown in Figure 1F, a typical EDEA-PeLEDs demonstrates a measured T50 of 26.5 h under a constant current density of 20 mA cm−2 corresponding to an initial radiance of around 17 W sr−1 m−2, a comparable operational lifetime with our previous report.7Xu W. Hu Q. Bai S. Bao C. Miao Y. Yuan Z. Borzda T. Barker A.J. Tyukalova E. Hu Z. et al.Rational molecular passivation for high-performance perovskite light-emitting diodes.Nat. Photonics. 2019; 13: 418-424Crossref Scopus (511) Google Scholar In order to achieve a fair comparison between the stability performance for different devices, we measured the operational stability of all PeLEDs in this work by fixing the light output of the devices at the same level (around 17 W sr−1 m−2). Impressively, the AAC-based device exhibits a greatly improved operational stability at the identical radiance, delivering a measured T50 of around 178.9 h, which is around 6 times longer than that of the EDEA-PeLEDs. We started with assessing the thermal stability of the perovskite emissive layers to understand the reasons behind the dramatically different operational stability of the EDEA- and AAC-PeLEDs, as the Joule heating induced elevated temperature during the device operation was regarded as a critical factor affecting the device stability.4Liu X.K. Xu W. Bai S. Jin Y. Wang J. Friend R.H. Gao F. Metal halide perovskites for light-emitting diodes.Nat. Mater. 2021; 20: 10-21Crossref PubMed Scopus (136) Google Scholar We carried out thermal stress treatments for the perovskite films under 100°C in the glovebox. As shown in Figure 2A, both the control and the EDEA-based perovskite films showed obvious color fading during the thermal stress, implying poor thermal stability of the perovskite emissive layers. The XRD results in Figures 2B and 2C confirm the decomposition and phase transition of the control and the EDEA-based perovskites, showing detectable and gradually increasing diffraction peaks of PbI2 and yellow non-perovskite phase after around 30 min during the thermal stress. On the contrary, the AAC-based perovskite films demonstrate superior thermal stability, exhibiting negligible color fading (Figure 2A) and almost no change to the main XRD diffraction peaks (Figure 2D) during the whole thermal stress period for over 300 min. Given that the thermal degradation of the perovskite emissive layer is arguably one of the critical factors deteriorating the device performance of PeLEDs,26Zhao L. Roh K. Kacmoli S. Al Kurdi K. Jhulki S. Barlow S. Marder S.R. Gmachl C. Rand B.P. Thermal management enables bright and stable perovskite light-emitting diodes.Adv. Mater. 2020; 32: 2000752Crossref Scopus (45) Google Scholar we suggest that the superior thermal stability of the AAC-based perovskite films would be the main reason for the notably enhanced operational stability of the AAC-PeLEDs. The strikingly different thermal stability of the perovskite emissive layers also indicates that molecular interactions between the perovskite components and the additives (EDEA and AAC) could be fundamentally different. We proceeded to perform attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy characterizations on the perovskite films to obtain an in-depth understanding of the molecular interactions induced by the two molecules. As shown in Figure 2E, the control film shows clear molecular vibration features of C–N asymmetric stretching signals νas (C–N) at 1,715 cm−1 and –NH2 scissoring signals δs (NH2) at 1,671 cm−1.27Taylor V.C.A. Tiwari D. Duchi M. Donaldson P.M. Clark I.P. Fermin D.J. Oliver T.A.A. Investigating the role of the organic cation in formamidinium lead iodide perovskite using ultrafast spectroscopy.J. Phys. Chem. Lett. 2018; 9: 895-901Crossref PubMed Scopus (47) Google Scholar As revealed in our previous work,25Yuan Z. Miao Y. Hu Z. Xu W. Kuang C. Pan K. Liu P. Lai J. Sun B. Wang J. et al.Unveiling the synergistic effect of precursor stoichiometry and interfacial reactions for perovskite light-emitting diodes.Nat. Commun. 2019; 10: 2818Crossref PubMed Scopus (61) Google Scholar the obvious δs (NH2) signals in the FTIR spectrum of the perovskite films mainly originate from the excessively incorporated FA+ cations, which are excluded outside the perovskite crystals during the film crystallization and mainly located at the grain boundaries of the as-crystalline perovskites. With the addition of EDEA in perovskites, we observed slightly broadened and shifted δs (NH2) signals to a lower wavenumber of 1,665 cm−1 due to the formation of hydrogen bonds between the FA+ cations and the EDEA molecules.7Xu W. Hu Q. Bai S. Bao C. Miao Y. Yuan Z. Borzda T. Barker A.J. Tyukalova E. Hu Z. et al.Rational molecular passivation for high-performance perovskite light-emitting diodes.Nat. Photonics. 2019; 13: 418-424Crossref Scopus (511) Google Scholar Surprisingly, the AAC-based perovskite film exhibits a strikingly different FTIR spectrum than the control and EDEA-based ones (Figure 2E), showing almost negligible δs (NH2) signals but two new signals at 1,221 and 1,548 cm−1. The signals at 1,221 cm−1 can be indexed to the fingerprint of amide III, which results from the coupling between C–N–H in-plane deformation and the in-phase C–N stretching. The strong absorption at 1,548 cm−1 can be ascribed to amide II signal originating from C–N–H in-plane bending coupled with out-of-phase C–N stretching vibration.28Kuodis Z. Matulaitienė I. Špandyreva M. Labanauskas L. Stončius S. Eicher-Lorka O. Sadzevičienė R. Niaura G. Reflection absorption infrared spectroscopy characterization of SAM formation from 8-mercapto- N -(phenethyl)octanamide thiols with phe ring and amide groups.Molecules. 2020; 25: 5633Crossref Scopus (4) Google Scholar,29Colthup N. Introduction to Infrared and Raman Spectroscopy. Elsevier, 2012Google Scholar In addition, the signal at around 1,700 cm−1 is associated with the C=O stretching vibrations (amide I), which overlaps with the νas (C–N) signal of the FA+ cations in the perovskites.30Lin-Vien D. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules. Academic Press, 1991Google Scholar The results provide clear evidence of the formation of amides in the AAC-based perovskite films, a process that can be rationalized by a chemical reaction between the carboxyl groups on the incorporated AAC molecules and the amino groups on the excess FA+ cations outside the perovskite grains. We reveal that the in situ amidation is the key to the superior thermal stability of the AAC-based perovskites by monitoring the evolution of the FTIR spectra under thermal stress. As shown in Figures 2F and 2G, both the control and the EDEA-based perovskite films exhibit obviously decreased characteristics of the δs (NH2) in the FTIR spectra during the thermal stress at 100°C, indicating a continuous loss of the excess FA+ cations. On the contrary, the FTIR spectrum of the AAC-based perovskite films, in which almost