Suppressing Competitive Coordination Reaction for Ohmic Cathode Contact Using Amino-Substituted Organic Ligands and Air-Stable Metals

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Dec 2021Suppressing Competitive Coordination Reaction for Ohmic Cathode Contact Using Amino-Substituted Organic Ligands and Air-Stable Metals Yang Lu, Ziyang Liu, Yuewei Zhang, Chen Zhang, Jinbei Wei, Zhengyang Bin, Xuewen Wang, Dongdong Zhang and Lian Duan Yang Lu Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084 , Ziyang Liu Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084 , Yuewei Zhang Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084 , Chen Zhang Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084 , Jinbei Wei Beijing National Laboratory for Molecular Sciences Institute of Chemistry, Chinese Academy of Sciences, Beijing 100049 , Zhengyang Bin College of Chemistry, Sichuan University, Chengdu, Sichuan 610064 , Xuewen Wang Foshan Xianhu Laboratory of the Advanced Energy Science and Technology Guangdong Laboratory, Xianhu Hydrogen Valley, Foshan 528200 , Dongdong Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084 and Lian Duan *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084 Center for Flexible Electronics Technology, Tsinghua University, Beijing 100084 https://doi.org/10.31635/ccschem.021.202000611 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Ohmic cathode contact can be formed readily via coordination-activated n-doping (CAN), by co-evaporating air-stable metals (e.g., silver) and organic ligands with coordination sites. It has been proposed that increasing the nucleophilicity of the main binding site of a ligand is essential for reducing the work function of the doped films. Here, engineering was conducted on phenanthroline-type ligands by introducing amino substitutes, which were stronger donors than MeO substitutes reported previously to increase the nucleophilicity of the ring nitrogen atoms binding-sites. Surprisingly, the dimethylamino group disabled the n-doping ability of silver via competitive coordination reaction with its sp3 nitrogen. Cyclic amino substitutes are thereof adopted to tune the p orbital of the exocyclic nitrogen coplanar with the phenanthroline ring to enhance the conjugative effect to significantly promote the coordination ability of the ring nitrogen. The optimal ligand renders ohmic contact with an even low concentration of silver and also lowered the work functions of copper and gold for a more-readily released free electrons than cesium. Electron injection layers doped in this manner could efficiently introduce electrons into organic semiconductors, typically, with low electron affinity of ∼2.2 eV, and the corresponding red organic light-emitting diodes (OLEDs) showed enhanced efficiency with a record-long lifetime. Download figure Download PowerPoint Introduction Ohmic contacts at organic/cathode interfaces in organic electronics are essential for efficient device performances, which are rather challenging due to the considerable energy gap between the high work function (φWF) of air-stable cathode metal and the low electron affinity (EA) of organic semiconductors.1,2 Controlled and stable n-doping has been regarded as an effective way to circumvent this issue, eliminating ohmic losses and increasing film conductivity of organic materials. The n-doping process involves the donation of free electrons released from low ionization energy (IE) n-dopants to the lowest unoccupied energy levels (LUMO) of electron-transporting materials (ETMs), resulting in significant Fermi level (EF) shifting toward LUMO of ETMs, thereby allowing efficient charge injection in the fabricated semiconductors.3,4 To make the electron-transfer process more efficient, the IE of n-dopant must lie above the EA of ETM. Though effective n-dopants for ETMs of large EAs (∼4.0 eV) used in organic photovoltaic cells (OPVs)5 and organic field-effect transistors (OFETs)6 have been developed rapidly, air-stable molecular n-dopants suitable for ETMs with low EA of <3.0 eV utilized in organic light-emitting diodes (OLEDs) are still underdeveloped, given that n-dopants with low IEs such as alkali metals and a few molecular compounds are easily oxidized in air.7,8 With conceptual and chemical novelties, obtaining air-stable precursors have been an on-going pursuit in those days involving one species that liberate strong alkalis during their insertion into the host, including Li3N,9 LiH,10 Cs2CO3,11 and CsN3,12 and a few organic precursors, Liq13,14, or those that react in a fashion, whereby, bond cleavage and/or formation accompanies electron transfer, such as dimers formed by certain 19-electron organometallic sandwich compounds15 or organic radicals.16–18 Several approaches have been utilized to provide ohmic contacts to organic semiconductors with very low electron affinity. These include the efficient photoactivation of n-doping with a cleavable air-stable dimeric dopant reported by Kahn an coworkers.19 However, most of these processes are incompatible with mass OLEDs production vacuum evaporation, owing to either the generation of undesired side products and massive out-gassing or harsh treatments, such as UV irradiation, required to overcome the thermodynamic limit and activate the arene rhodocene dimer ([Rh(C5H5)2]2) dimers.19,20 To the best of our knowledge, the OLEDs industry with multibillion outputs still relies on highly reactive metals to ensure desirable device performances, which, unfortunately, could not prevent the migration issue of the small size metal atoms under an electrical field that tend to deteriorate a device efficiency and long-term stability severely. In commercialized OLEDs, the most commonly adopted cathode is air-stable silver (Ag), particularly for the transparent and flexible ones. Bearing a high IE of 7.6 eV, Ag as a cathode usually features a large energy barrier for electron injection into ETMs in OLEDs.20 Interestingly, electron injection from Ag cathode to electron transport layers, such as bathocuproine (BCP) or Bathophenanthroline (BPhen), has been observed without the assistance of electron injection layers.21–23 Our previous work revealed that featuring rigid heterocyclic planar structure with preorganized coordinating nitrogen donors, Bphen, could form strong in situ coordination with Ag, consequently pushing the equilibrium between metals and metal ions to the forward direction to release free electrons readily.24 In this scenario, the IE of Ag can be essentially lowered to function as a strong n-dopant. Further, studies have also revealed that the IE shift of Ag is closely related to the nucleophilicity of the main binding sites. Though the phenanthroline ring has been proven to be a promising ligand, it is a weak donor, limiting its ability to donate electrons to metal ions. A natural choice is to introduce electron-donating moieties and 4,7-dimethoxy-1,10-phenanthroline (p-Meo-phen) with electron-donating methoxy groups on 4,7-positions of the phenanthroline ring, which has thereof been developed, consequently leading to a much lower IE of Ag than that of the Bphen ligand. Also, compared with easy-to-migrate alkalis, the coordinating interaction with organic ligands could limit the migration of Ag, preventing the exciton-quenching issues by metal atoms. We considered an in situ coordination-activated n-doping (CAN) strategy, which generated byproduct-free, air-stable OLEDs that were also compatible for mass production vacuum evaporation. With all these viable potentials, the CAN strategy still faces formidable challenges. On the one hand, those developed ligands still rely on high doping concentration of Ag (20 wt %) to realize ohmic contact, which contributes to both plasmonic absorption and hindrance of the transparency of doped films, thereby limiting their applications in top-emitting devices.25 On the other hand, although other air-stable metals, such as copper (Cu) and gold (Au), have also proven their potential as n-dopants in organic ligands, they show inferior performance than the commonly used reactive cesium (Cs), thereby limiting the applications of CAN strategy with different cathodes.24 The reasons for all these issues could be attributed to the still unsatisfied coordinating ability of phenanthroline ligands, which require a relatively high number of Ag atoms to achieve efficient n-doping. Thus, it has been a demanding task to engineer phenanthroline-type ligands for enhanced nucleophilicity of the ring sp2 nitrogens (sp2-Ns). Here, the phenanthroline-type ligand engineering was conducted by introducing electron-donating amino substitutes to increase the nucleophilicity of ring nitrogen binding sites. Surprisingly, the dimethylamino group disabled the n-doping ability of Ag by a competitive chelating reaction with its sp3 nitrogen (sp3-N). Cyclic amino substitutes were thereof adopted to tune the p orbital of the exocyclic nitrogen coplanar with phenanthroline ring to enhance the conjugative effect, significantly promoting the chelating ability of ring nitrogen. The optimal ligand renders Ohmic contact with an even low concentration of Ag and lowered the work functions of Cu and Au for more readily released free electrons than Cs. Electron injection layers doped in this manner, in turn, efficiently injected electrons into organic semiconductors with low EA of ∼2.2 eV and promoted both efficiency and long-lifetimes of OLEDs. Experimental Methods Theoretical calculations The geometrical, electronic properties and ionization energies were performed using the Gaussian 09 program package ( https://www.cmu.edu/) using Becke three parameters hybrid functional with Lee–Yang–Perdew correlation functions (B3LYP) and the 6-31G(d) atomic basis set. The electrostatic potential surfaces were visualized using the GaussView program ( https://gaussian.com/gaussview6/). Device fabrication and measurement The indium tin oxide (ITO) glass substrates were precleaned carefully with oxygen plasma before devices' fabrication. The organic materials used were purified by a vacuum sublimation approach and thermally evaporated at a rate of 1.0 Å s−1 in a vacuum at a pressure of 5 × 10−5 Torr. The electron injection layers of electron-only devices (EODs) were prepared by co-evaporation with the rate of 0.01–0.02 Å s−1 for metals and 0.15–0.35 Å s−1 for organic ligands. The electrical characteristics of the encapsulated devices were recorded using a Keithley 2400 SourceMeter instrument (Global Sources, Guangdong, China). The electroluminescence spectra were attained on a PR650 spectrometer (Inc. of Chatsworth, Calif). All the device fabrication and characterization steps were performed at room temperature under ambient laboratory conditions. Measurement of optical properties UV–vis absorption spectra and experimental transmittance spectra were measured using an Agilent 8453 spectrophotometer (Palo Alto, California, USA). X-ray photoelectron spectroscopy (XPS) measurements were performed using the ESCALAB 250Xi equipment (ThermoFisher Scientific, Shanghai, China). Ultraviolet photoelectron spectroscopy (UPS; ThermoFisher Scientific) characterizations were performed using monochromatized HeI radiation at 21.2 eV. Results and Discussion Compared with the oxygen atom, the nitrogen atom possesses a much lower electronegativity and lone pair of electrons, facilitating the electron-donating effect.26 Besides, the steric hindrance of substituents for coordination reaction should be simultaneously taken into consideration.27 With all these in mind, amino moieties were introduced as substituents in the initial development of N4,N4,N7,N7-tetramethyl-1,10-phenanthroline-4,7-diamine (p-Amn-phen). To evaluate its performance as a ligand, EODs with Bphen as an electron-transporting layer (ETL) was created, while Ag-doped p-Amn-phen as n-doping electron injection layer was constructed with structure, ITO/Bphen (100 nm)/p-Amn-phen:20 wt % Ag (5 nm)/Al (150 nm). For comparison, EODs with Bphen or p-Meo-phen alone as ligands were also fabricated. EOD is widely adopted and represents a direct approach to investigate the electron-injection capability of an electrode into an organic semiconductor. When applying a voltage across the device, the measured current is carried exclusively by electrons.28–30 Since the current is dependent exponentially on the injection barrier, the measured current is very sensitive to changes at the barrier height. Thus, it is a measure of the electron-injection capabilities of n-doping layers of a cathode. Figure 1a displays the current density through those EODs. Consistent with previous results, an enhanced current density was observed for EOD with p-Meo-phen as ligand than that based on Bphen, owing to the improved coordinating ability of p-Meo-phen. Curiously, despite the strong electron-donating ability of the Amn-group, almost no current was recorded for EOD with p-Amn-phen as ligand, suggesting that there was no apparent n-doping effect, explained by measuring the binding energy of nitrogen atoms in p-Amn-phen film before and after Ag doping by XPS. As shown in Figure 1b, for p-Amn-phen, two-types of nitrogen atoms existed, with one being the ring sp2-N, while the other existed as an exocyclic sp3-N. Nearly identical binding energies of sp2-N were observed, indicating that no coordinating interaction occurred with the Ag atom. On the contrary, the binding energy of sp3-N after Ag doping decreased by 0.5 eV; meanwhile, the value of Ag increased by 0.5 eV, suggesting the occurrence of a coordinating process between Ag and sp3-N, as illustrated in Figure 1c.31,32 This competitive reaction hindered the release of free electrons from the coordinating process between sp2-N and Ag, disabling the n-doping effect. Figure 1d displays the energy levels of both films, also measured by UPS, validating that no change in the energy levels of both highest occupied molecular orbitals (HOMOs) and Fermi levels occurred, referring to vacuum levels. Figure 1 | (a) The current density–voltage curves of EODs with 20 wt % Ag-doped phen-type ligands. (b) XPS analysis of N 1s core level for pristine p-Amn-phen and Ag-doped p-Amn-phen films, Ag 3d core level for pristine Ag and Ag-doped p-Amn-phen films. (c) Coordination illustration between phen-type ligands and Ag. (d) UPS analysis of before/after Ag-doped. Download figure Download PowerPoint To fully unlock the strong donating ability of the amino groups suppressing the coordinating ability of sp3-N while promoting that of sp2-N is highly desired but challenging. The electrostatic surface potential (ESP) of both p-Amn-phen and p-Meo-phen was calculated, as shown in Supporting Information Figure S1, which reflected the molecular nucleophilicity. Though the ESP of ring nitrogen atoms in p-Amn-phen improved slightly (0.092), compared with that of p-Meo-phen (0.091), it was still not enough to suppress the competitive coordination reactions. Suppose one could enhance the disperse of electrons from sp3-N significantly to phen 1,10-phenanthroline ring to enhance the coordinating ability of sp2-N, the coordination reaction between sp2-N and Ag might be greatly promoted to suppress the that between sp3-N and Ag. To achieve this goal, the conjugation effect should be enhanced, which required the exocyclic nitrogen p orbital to be coplanar with the phen-ring to delocalize the lone-pair electrons across the phen-ring. Therefore, cyclic aliphatic alkanes, including piperidine and pyrrolidine, were introduced as the amnio substituents, developing 4,7-di(piperidin-1-yl)-1,10-phenanthroline (p-Pip-phen) and 4,7-di(pyrrolidin-1-yl)-1,10-phenanthroline (p-Pyr-phen), respectively as shown in Supporting information Scheme S1. The configuration structures of p-Amn-phen, p-Pip-phen, and p-Pyr-phen were calculated employing the density functional theory with B3LYP/6-31g (d,p) basis set, as shown in Figure 2a and Supporting Information Figure S2. Undoubtedly, we observed that the dihedral angles between the metal-coordinated phen-ring and the lone-pair electrons plane for p-Pip-phen and p-Pyr-phen were 42.5° and 26.5°, respectively, both smaller than that of p-Amn-phen (48.3°), indicating that the exocyclic nitrogen p orbitals of p-Pip-phen and p-Pyr-phen assumed a more coplanar geometry with the phen-ring than that of p-Amn-phen. In this scenario, the lone-pair nitrogen electrons would delocalize more feasibly in the phen-ring, enhancing the conjugation effect between them. This scenario could also be proven by the average bond angles of the tertiary amino substituents were 116.9°, 118.2°, and 119.7° for p-Amn-phen, p-Pip-phen, and p-Pyr-phen, respectively, which progressively approached that of the traditional sp2 hybridization orbital (120.0°).33 In contrast, the bond distances between the exocyclic nitrogen and attached carbon atoms on the phen-ring also decreased from 1.412, 1.403, to 1.388 Å for p-Amn-phen, p-Pip-phen, and p-Pyr-phen, respectively, gradually approaching the bond distance of "–C=N–" double bond.34 Collectively, these results proved that, compared with that of p-Amn-phen, the exocyclic nitrogen p orbitals of p-Pip-phen and p-Pyr-phen participated in the conjugation system of the phen-ring, which markedly enhanced the delocalization of the lone-pair nitrogen electrons into the phen-ring, as illustrated in Figure 2b. Figure 2 | (a) The optimized molecule structure of phen-type ligands. (b) Delocalization illustration of p-Amn-phen and p-Pyr-phen. (c) 1H NMR for the aromatic region of BPhen derivatives. (d) IE of Ag with phen-type ligands. (e) Electrostatic potential surfaces of different phen-type ligands and the maximum electrostatic potential measured around the molecules. Download figure Download PowerPoint The direct evidence about the enhanced electron-donating ability of exocyclic nitrogen in p-Pip-phen and p-Pyr-phen could be reflected by the chemical shift of hydrogens on the 3,8-carbon atoms (H3, H8) obtained from nuclear magnetic resonance (NMR) measurement, given that the substituents on 4,7-carbon atoms would affect the electron densities of both ring nitrogen and 3,8-carbon atoms. As shown in Figure 2c, compared with that of p-MeO-phen (7.29 ppm), all H3 chemical shifts of p-Amn-phen, p-Pip-phen, and p-Pyr-phen were up-field shifted to 7.17, 7.15, 6.74 ppm, respectively, suggesting the improved electron densities of their 3,8-carbon atoms. These pieces of evidence demonstrated that electrons on the amino substituents of the 4,7-carbon atoms were donated into the phen-ring, as mentioned earlier. Unprecedently, a significant up-field shift of 0.55 ppm was observed for p-Pyr-phen, compared with that of p-MeO-phen, indicating the strongest electron-donating ability of Pyr groups since the more coplanar structures facilitated the participation of lone-pair electrons of the exocyclic nitrogen into the conjugation system of the phen-ring. As shown in Figure 2d, The IEs of Ag-doped in p-Amn-phen, p-Pip-phen, and p-Pyr-phen were calculated to be 3.86, 3.80, and 3.72 eV, respectively, for the enthalpy changes during the process of Ag(phen) → [Ag(phen)]+ + e−. Such results aligned with the ESP of ring nitrogen in p-Pip-phen and p-Pyr-phen, which were also calculated to be −0.095 and −0.100, respectively, and were both larger than that of p-Amn-phen or p-Meo-phen. These results also evidenced the enhanced conjugation effect of the exocyclic nitrogen to phen-ring in p-Pip-phen and p-Pyr-phen, as illustrated in Figure 2e. The large ESP values indicated the strong electron-donating ability of the ring nitrogen in p-Pip-phen and p-Pyr-phen, thus, facilitating electron donation in the coordination reaction to lower IEs of Ag. Clearly, the stronger the electron-donating ability of the ligands' substituents, the lower the anticipation of the IEs of Ag. Given their viable potentials, EODs were constructed and illustrated in Figure 3a, with structures of ITO/Bphen (100 nm)/phen-type ligands: 5 and 20 wt % Ag (5 nm)/Al (150 nm). As expected, the device current densities were boosted with p-Pip-phen and p-Pyr-phen as a ligand, compared with that based on p-Meo-phen. a current density of p-Pip-phen and p-Pyr-phen ligands required much lower cathode of and respectively, while p-Meo-phen a voltage of This doping efficiency should from the enhanced electron-donating ability of ring nitrogen of p-Pip-phen and p-Pyr-phen, a promising ligand engineering strategy in this As mentioned the relatively low doping concentration of Ag is desired for The of EODs with 5 wt % Ag were thereof For the voltage of current density could the energy level of the cathode and n-doping as shown in Supporting Information Figure Interestingly, for the of EODs with 5 and 20 wt % Ag were This indicated that almost the energy levels of the devices existed, suggesting that owing to the strong coordinating ability of p-Pyr-phen, even a small of Ag could realize efficient charge EODs with p-Pip-phen or p-Meo-phen as ligand showed significantly larger with 5 wt % Ag than those with 20 wt % Ag, from the relatively coordinating ability of the ligand, thus, a high Ag EOD with 5 wt % Ag as injection layer showed both lower and slightly high current density than those with 20 wt % Ag. Given that 20 wt % Ag layer even than that with as it could be that 5 wt % Ag could provide enough electrons for electronic Also, the relatively current density of EOD with 20 wt % Ag should to the increased electrical conductivity due to the relatively high Ag p-Pyr-phen the of the current density of electrons by the doping concentration of Ag for in the This was for top-emitting OLEDs since high a doping concentration of Ag usually plasmonic and thus, device To this the transparency of of and 20 wt % Ag-doped p-Pyr-phen were as illustrated in Supporting Information Figure Compared with the pristine and 5 wt % Ag-doped the transparency of the 20 wt % Ag-doped was greatly which is for device Figure | (a) The current density–voltage curves of the EODs with 5 and 20 wt % Ag-doped phen-type ligands. (b) XPS analysis of before/after Ag-doped p-Pyr-phen. (c) UPS analysis of p-Pyr-phen with different (d) The of pristine and p-Pyr-phen for Download figure Download PowerPoint The XPS of pristine p-Pyr-phen film and 5 wt % Ag-doped p-Pyr-phen film was measured to the n-doping We observed that with and without Ag the core level of sp3-N almost the without significant binding energy On the contrary, the core level of sp2-N shifts toward binding energy by 0.5 eV after Ag while that of Ag shifted toward lower binding energy by 0.5 eV, to the coordination bond formed between Ag and sp2-N by effect from sp2-N to Ag, as shown in Figure and Supporting Information Figure referring to the results of p-Amn-phen, we that enhancing the disperse of the electrons on the exocyclic nitrogen the coordination reaction between sp3-N and Ag, while that between sp2-N and Ag could be enhanced, the of our molecular The energy level shifts of the after Ag doping were also by UPS, as illustrated in Figure and Supporting Information Figure which revealed that the work function of the Ag cathode was from to and eV doped in and p-Pyr-phen, respectively, with the being eV lower than the reactive Besides, for p-MeO-phen and p-Amn-phen, a much larger work function of and eV was The of p-Pip-phen and p-Pyr-phen measured from UPS were and eV, As shown in Supporting Information Figure in of the optical energy gap obtained from the of UV–vis the LUMO energy levels obtained were and eV, With these results, the shifted energy level after the Ag doping could be as illustrated in Figure 3d and Supporting Information Figure The of for pristine p-Pip-phen and the p-Pyr-phen film was at and eV, above the Fermi level and decreased by and eV, respectively, Ag with both ligands in their electron injection in the of p-Pyr-phen, even under a low concentration of Ag, ohmic contact was In the of ohmic contact and free of the injection current on the charge in the organic layers, as the current which can be ( 2 and for in a of and three for organic respectively, while represents the represents the of and is the of the With the current obtained the electron-injection efficiency of the EOD with 5 wt % Ag-doped p-Pyr-phen was calculated, using the injection injection Here, the values of and which were × 10−5 and × respectively, were obtained from measurements reported in the The for electron-injection efficiency voltage is shown in the Supporting Information Figure even in the low voltage of of was observed, also evidence of ohmic In this scenario, for ligand with large the ohmic contact could be feasibly a of Ag, allowing to tune the doping concentration for the desired current For p-Meo-phen or p-Pip-phen ligand, under low dopant no ohmic contact was and thus, high Ag concentration was The in Ag concentration of and p-Pyr-phen were attributed to the in their coordinating In this doping the coordination interaction between and the equilibrium between Ag and to the forward free Also, the strong binding of p-Pyr-phen should pushing the equilibrium between Ag and to the forward direction with the formation of more which might be with p-Meo-phen as the ligand, as it much binding sites. Therefore, even with a low concentration of 5 wt % Ag, a of free electrons could be released for ohmic On the contrary, high Ag doping concentration was required to achieve ohmic contact for p-Meo-phen due to its low of ring nitrogen. The

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