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Subjects:
Chemistry, Organic
Organic/polymer light-emitting diodes (OLEDs/PLEDs) have attracted a rising number of investigations due to their promising applications for high-resolution fullcolor displays and energy-saving solid-state lightings. Near-infrared (NIR) emitting dyes have gained increasing attention for their potential applications in electroluminescence and optical imaging in optical tele-communication platforms, sensing and medical diagnosis in recent decades. And a growing number of people focus on the “heavy metal-free” NIR electroluminescent materials to gain more design freedom with cost advantage.
- near-infrared emitting materials
- organic compounds
- organic/polymer light emitting diode
1. Introduction
In 1987, the thin-film organic light-emitting diodes (OLEDs) which have high brightness and high electroluminescent efficiency at low driving voltage was launched by Tang and Vanslyke [1]. Since then, the applications of organic/polymer light emitting diodes (OLEDs/PLEDs) in solid-state lighting source and flat-panel displays have entered a new period [2,3,4]. After years of hard works, OLEDs/PLEDs have been used to achieve great breakthroughs, especially in the visible region (400–700 nm) [5,6,7,8]. Up to now, Near-infrared (NIR)-emitting organic materials have aroused growing interest on account of promising applications in some fields such as infrared signaling and displays, bio-sensing, and telecommunications. A rising number of investigations have been made to develop NIR luminescent materials with an emission wavelength longer than 700 nm due to its potential applications in bio-imaging [9,10,11,12], chemical sensors [13,14,15,16], light emitting electrochemical cells (LECs) [17,18,19,20], OLEDs [21,22,23,24] and photovoltaic cells [25,26,27,28] etc.
To date, the materials with NIR-emitting are mainly grouped into two categories: inorganic luminescent materials including rare earth metals [29,30,31] and alkaline earth metal luminescent materials [32], and organic luminescent materials covering transition metal complexes [33,34,35,36], small molecules [37,38,39] and polymers [40,41,42]. According to the mechanism of luminescence, these emitters are divided into two types, fluorescent [43] and phosphorescent materials [44,45,46]. The luminescence is generally defined as the radiation emitted by the atoms or molecule return to the lower energy state after the material absorbing energy and jumping to the excited state of the higher energy level. The main types of luminescence include fluorescence and phosphorescence. As shown in Figure 1, the emission originated from the transition that from the lowest excited singlet state (S1) to the lowest excited singlet state to the singlet ground state (S0) is called fluorescence, while the emission from the lowest excited triplet state (T1) to S0 is called phosphorescence.
Figure 1. Brief mechanisms for (a) TTA, (b) doublet, (c) HLCT, (d) fluorescence, phosphorescence, and TADF.
2. Tuning the Emission of Materials into NIR Region
The energy gap of the organic molecules is the decisive factor in the optical and electronic properties, which means the energy separation between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), also called the HOMO-LUMO gap (HLG) [61]. There are several approaches to construct molecules with NIR emission. Diminution of the HLG is the key to result the absorption and emission spectra. Generally speaking, when tuning the HLG of molecules, several factors should be considered, including conjugation length, bond length alternation, and donor-acceptor (D-A) charge transfer [62,63,64]. For the π-conjugated systems, extension of its conjugation length can lead to a decrease in the HLG for certain. Mullen et al. [65] tuned the energy gap over a quite large range from probably 1.29 eV (960 nm) to 2.15 eV (577 nm) by adjusting the number of naphthalene units of the rylenediimide dyes. And the energy gap has a gradual downtrend as the size of polycyclic aromatic hydrocarbons grow, starting from benzene to the one containing 222 carbons by the number of benzene units or sextet carbons[66]. In the polycyclic aromatic hydrocarbons systems, the energy gap is generated by alternating single and double bonds. The smaller bond length alternation is, the lower energy gap of a conjugated compound will be. As a result, reducing the bond length alternation is a significant step toward the reduction of the energy gap in the conjugated systems. In addition to these, introduction of an intramolecular D-A system in organic polymers is a popular strategy of lowering the energy gap [67]. The hybridization of energy level after donor and acceptor bonding could make the energy level of HOMO higher than that of donor, while the energy level of LUMO is lower than that of acceptor, resulting in abnormally small HOMO-LUMO separation [68,69]. For example, Skene et al. [70] synthesized azomethines compound with the maximal absorption and emission are 440 and 534 nm. In addition, on this basis, they got the push-push (D-D), pull-pull (A-A) and push-pull (D-A) azomethines by substituting the donor and acceptor groups at both ends. The D-A azomethines has the most obvious red-shifts in absorption (148 nm) and emission (126 nm).
3. NIR Fluorescent Materials Based on Polymers
Conjugated polymers with fluorescence units have attracted a multitude of attention due to the academic and commercial value when used as the active materials in PLEDs [76]. The turn-on voltage, color purity, and stability of the devices should be optimized to accommodate PLEDs. Some of the principal advantages of conjugated polymers are easy manufacture, solution processability, low-cost, flexibility and suitability to form large area surfaces [77]. The synthetic organic flexibility is the most obvious feature of the conjugated polymers. Through the manipulation of the structures of the monomer and polymer, the physical, thermal, optical, and electrochemical properties could be adjusted for specific applications.
For conjugated polymer, the value of the energy level band gap is the key to their performance in PLEDs. The most common and effective approach is introducing D-A units consisting of various donors and acceptors to tune the HOMO-LUMO levels. Another approach to lower the band gap is utilizing the polar effect caused by the heavy atoms. The notable cases of decreasing the energy gap involve the replacement of oxygen and sulfur atoms with heavier ones like selenium and tellurium in a conjugated system [71].
4. NIR Fluorescent Materials Based on Small Molecules
Due to the parity-forbidden radiative 4f-4f transitions of the rare earth ions, the corresponding LEDs usually have a nonmeasurable or very low EQE and low power output. In contrast, the luminescence of organic molecules originates from their allowed S1–S0 transitions and thus free from the luminescence efficiency limitation. By using phosphorescent heavy metal complexes that can effectively harvest both the singlet and triplet excitons. Unfortunately, the EL quantum efficiency drops rapidly at high current densities.
Initially, attempts were made to construct NIR luminescent materials using molecules with a large area of conjugated systems. In 2006, Kageyama et al. [95] investigated that OLED using tris(8-quinolinolato)aluminum (Alq3) highly doped with N,N′ -bis(neopentyl)-3,4:9,10-perylenebis(dicarboximide) as an emitting layer exhibit near-infrared EL with a peak at 805 nm originating from N,N′ -bis(neopentyl)-3,4:9,10-perylenebis(dicarboximide) aggregates. Phthalocyanines are known to be organic semiconductors and have attracted much attention because of their high chemical stability, various synthetic modifications, epitaxial growth of thin films by organic molecular beam epitaxy and unique absorption bands extending from the ultraviolet region to infrared region [96,97]. Cheng et al. [98] reported the OLED device used purple phthalocyanine single crystal as an active light-emitting layer with the emission of 936 nm.
5. NIR Phosphorescent Materials Based on Small Molecules
In general, holes and electrons injected from electrodes to emitters generate excitons, and the excitons are classified into singlet and triplet excitons that are formed at a ratio of 1:3. In the case of fluorescent emitting materials, only singlet excitons can be transformed into photons, and so only 25% internal quantum efficiency (QE) is theoretically possible, where the remaining 75% of non-radiation energy is lost. Therefore, breaking spin statistics to utilize the other 75% triplet energy is the key factor to improving OLED efficiency.
As discussed previously, there are several approaches to design high efficiency materials with NIR emitting through utilizing triplet energy. One of these is to harness the triplet excitons of organic fluorescent materials involves triplet fusion (TF) [133,134,135]. The theoretical maximum singlet exciton production yield through TF is 50%, which would contribute a maximum radiative exciton ratio of up to 62.5%. To enable highly efficient NIR-OLEDs through TF, Qiao et al. [136] used the more feasible approach of efficient TF via the host rather than direct TF from the emitter, since the triplet excitons of the NIR emitter may decay dominantly via non-radiative transition according with the energy gap law. They realized high performance NIR-OLEDs via the high-efficiency TF of a bipolar host doped with a special naphthoselenadiazole emitter 4,9-bis(4-(2,2-diphenylvinyl)phenyl)naphtho[2,3-c][1,2,5]selenadiazole. Unlike typical NIR organic D-A chromophores, 4,9-bis(4-(2,2-diphenylvinyl)phenyl)naphtho[2,3-c][1,2,5]selenadiazole features a non-D-A structure and a very large HOMO/LUMO overlap, displaying strong deep-red to NIR emitting and unique ambipolar character. The corresponding photoluminescence quantum efficiency of NSeD reached 52% in solution and retained 17% in the blend film. The optimized NIR-OLEDs demonstrated a strong emission at 700 nm with a high EQEmax of 2.1% and the EQE remained at around 2% over a wide range of current densities from 18 to 200 mA cm−2. However, this method would lose some of the energy of triplet excitons, so the quantum efficiency in theory is not 100%.For standard closed-shell organic semiconductors, holes and electrons occupy the HOMO and LUMO respectively, and recombine to form singlet or triplet excitons. The radical emitter has a SOMO in the ground state, giving an overall spin 1/2 dipole. In the high energy ground state, where both electrons and holes occupy the SOMO level, recombination returns the system to the ground state and does not emit light. However, in 2015, Li et al. [49] achieved selective hole injection into HOMO and electron injection into SOMO to form a fluorescent two-photon excited state with near unit internal quantum efficiency and proposed an open-shell organic molecule 9-(4-(bis(2,4,6-trichlorophenyl)methyl)-3,5-dichlorophenyl)-9H-carbazole as an NIR-emitter of OLEDs.
The hybridized local and charge-transfer excited state (HLCT) possesses two combined and compatible characteristics with a large transition moment from a local excited (LE) state and a weakly bound exciton from a charge transfer (CT) state [137,138,139]. The former contributes to a high-efficiency radiation of fluorescence, while the latter is responsible for the generation of a high fraction of singlet excitons. The twisting D-A molecule may be an ideal carrier to realize this strategy that may possess two combined and compatible characteristics with large transition moment from LE state and weakly bound exciton from CT state.
6. Conclusions
Shifting the spectral range of OLEDs/PLEDs from the visible to the NIR region of the electromagnetic spectrum is of great interest. To date, much efforts have been made to develop NIR phosphorescent OLEDs/PLEDs using transition metal complexes. However, high costs, limited resources of phosphorescent materials, and efficiency roll-offs at high current densities remain challenges for their applications in long-term mass production. To reduce cost and improve environmental sustainability, the development of highly efficient OLEDs/PLEDs that does not rely on heavy metal-containing compounds remains an important need. As an alternative material system, the “heavy metal-free” NIR fluorophores have been widely investigated for their cost advantage and versatility in tuning molecules. However, the EQE of traditional organic near-infrared fluorescent OLEDs is generally about 0.1% or even lower due to low exciton utilization rate and low fluorescence quantum yield in solid state, which has become an almost insurmountable obstacle for their further development. Therefore, several strategies have been proposed to realize high quantum efficiency in pure organic dyes by utilizing triplet energy. Nevertheless, this research field is still in its infancy, and while many examples harvesting triplet excitons are reported, only a few studies have focused on their NIR emission, particularly in terms of OLEDs/PLEDs applications.
This entry is adapted from the peer-reviewed paper 10.3390/polym15010098
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