† Corresponding author. E-mail:
Project supported by the National Natural Science Foundation of China (Grant No. 61675041) and the National Science Funds for Creative Research Groups of China (Grant No. 61421002).
Organic optoelectronic integrated devices (OIDs) with ultraviolet (UV) photodetectivity and different color emitting were constructed by using a thermally activated delayed fluorescence (TADF) material 4, 5-bis(carbazol-9-yl)-1, 2-dicyanobenzene (2CzPN) as host. The OIDs doping with typical red phosphorescent dye [tris(1-phenylisoquinoline)iridium(III), Ir(piq)3], orange phosphorescent dye {bis[2-(4-tertbutylphenyl)benzothiazolato-N, C2′]iridium (acetylacetonate), (tbt)2Ir(acac)}, and blue phosphorescent dye [bis(2, 4-di-fluorophenylpyridinato)-tetrakis(1-pyrazolyl)borate iridium(III), FIr6] were investigated and compared. The (tbt)2Ir(acac)-doped orange device showed better performance than those of red and blue devices, which was ascribed to more effective energy transfer. Meanwhile, at a low dopant concentration of 3 wt.%, the (tbt)2Ir(acac)-doped OIDs showed the maximum luminance, current efficiency, power efficiency of 70786 cd/m2, 39.55 cd/A, and 23.92 lm/W, respectively, and a decent detectivity of 1.07 × 1011 Jones at a bias of −2 V under the UV-350 nm illumination. This work may arouse widespread interest in constructing high efficiency and luminance OIDs based on doping phosphorescent dye.
The research of organic photoelectronic devices has made significant progress in recent years. Common organic photoelectronic devices include organic light-emitting diodes (OLEDs),[1,2] organic photovoltaic devices (OPVs),[3] organic photodetectors (OPDs),[4,5] and organic thin film transistors (OTFTs). Among them, OLEDs have been widely used in solid-state lighting and full-color flat-panel displays, which are considered as the most ideal and potential display technology in the 21st century.[6–8] Meanwhile, OPDs have drawn wide interests due to their potential application in fields of biological sensing, solar astronomy, smoke/fire monitoring, and missile plume detection.[9,10] Nowadays, organic optoelectronic integrated devices (OIDs) with OLED and ultraviolet OPD dual functions become an important part of organic optoelectronics, since the integration of the two functions is helpful to reduce the device size and simplify the fabrication process.[11–13]
OIDs have two working modes, as shown in Fig.
Nowadays, the thermally activated delayed fluorescence (TADF) materials are not only widely adopted to fabricate highly efficient OLEDs, but also used as the active layer to realize high performance OIDs.[15,18–21] Wang et al. reported a TADF-based integrated device which gave a detectivity of 1.4 × 1012 Jones and a brightness of 26370 cd/m2 by inserting an exciton adjusting layer (EAL).[15] In our previous works, the importance of EAL- to TADF-based OIDs was analyzed. Great improvements in UV-detectivity (from 109 Jones to 1012 Jones) and luminance (from 5000 cd/m2 to 25000 cd/m2) were achieved by changing the varieties and thickness of the EAL materials.[22] However, the research on the utilization of the TADF-featured material as the host of phosphorescent dye has not been carried out so far. As is known, the strong spin–orbit coupling of electronic states induced by the heavy metal-atom effect of phosphorescent dyes can realize radiative decay of both singlet and triplet excitons for phosphorescence, leading to the theoretical internal quantum efficiency (IQE) of 100%.[23–25] However, phosphorescent dyes suffer from serious concentration quenching effect, which is required to be doped as a guest into the host material.[26] It should be pointed out that the singlet-triplet splits (ΔEST) of a TADF material is usually under 100 meV, lower than the conventional host materials, it is a kind of suitable host material for high-efficiency and low roll-off phosphorescent OLEDs.[27,28] Furthermore, the conversion of triplet excitons on TADF materials to singlet excitons on phosphors can be realized via reverse inter-system crossing (RISC) followed by long-radius Förster resonance energy transfer (FRET), so the OLEDs using TADF materials as the host can achieve efficient host-guest energy transfer in a low doping concentration.[29] Although it is an effective strategy to overcome exciton quenching and improve the EL performance of OLED by utilizing the model of energy transfer between the host and dopant materials, there are few works that apply this method for realizing high performance OIDs. Thus, a study of doping system for the OIDs, correlating with the energy transfer between the host and the dopant, is imperative.
In this work, the effect of a novel TADF-featured host material of 2CzPN on the performance of several typical phosphorescent dyes (including tris(1-phenylisoquinoline)iridium(III) [Ir(piq)3], bis[2-(4-tertbutylphenyl)benzothiazolato-N,C2′]iridium (acetylacetonate) [(tbt)2Ir(acac)], bis(2, 4-di-fluorophenylpyridinato)-tetrakis(1-pyrazolyl)borate iridium(III) [FIr6]) doped OIDs is studied in detail. By changing the doping concentration, we report high efficiency and luminance OIDs with decent detectivity. The (tbt)2Ir(acac)-doped OID with a dopant concentration of 3 wt.% achieves a maximum luminance of 70786 cd/m2, which is three times brighter than pure 2CzPN-based device (26454 cd/m2). To analyze the effect of the phosphorescent doping concentration on the device, the mechanisms of spectral alterations, energy transfer, and UV-light absorption of the doping system are also discussed.
Indium tin oxide (ITO) coated glass substrates were cleaned by detergent, deionized water, acetone, and ethanol for 15 min at each ultrasonic step. Afterwards, the substrates were dried with nitrogen gas flow and then they were treated with O2 plasma in a vacuum chamber under a pressure of 25 Pa for 5 min to clean the surface and increase the work function of ITO. The organic functional layers and metallic cathode were thermally evaporated in a vacuum under the pressures of Pa and Pa, respectively, with a deposition rate of 0.5 Å/s. During the deposition process, the film thickness and deposition rate were monitored by an oscillating quartz crystal monitor. Phosphorescent dye doped OIDs were fabricated with a structure of ITO/MoO3 (15 nm)/NPB (20 nm)/mCP (6 nm)/2CzPN: phosphorescent dye (10 nm, 5 wt.%)/Bphen (30 nm)/Mg: Ag (100 nm), as shown in Fig.
The current density–voltage–luminance (J–V–L) characteristics and current density–voltage (J–V) characteristics in the dark and under illumination were measured using Keithley 4200 semiconductor characterization system, and 350 nm UV light source with a power of 1.0 mW/cm2 was used as the illumination source. The EL spectra of the devices were recorded with an OPT-2000 spectrophotometer. UV–visible (UV-vis) absorption spectra were obtained with a Shimadzu UV-1700 spectrophotometer. All the measurements were performed in air at room temperature without encapsulation.
In order to study the influence of phosphorescent dye varieties on the OID performance, three typical emitting phosphorescent dyes are chosen as the dopant, which are red phosphorescent dye Ir(piq)3, orange phosphorescent dye (tbt)2Ir(acac), and blue phosphorescent dye FIr6. When the OIDs operate in the OLED mode, holes travel from the ITO/MoO3 into the active layer, while electrons transport from the Mg: Ag cathode into the active layer, and finally the holes and electrons combine in the active layer of 2CzPN.[15] The EL spectra of these devices are displayed in Fig.
Figure
Figure
When a reverse bias is applied, the device functions as a UV-PD. Excitons are generated initially in the 2CzPN layer under 350 nm UV light illumination with an incident power of 1 mW/cm2. When transporting to nearby mCP/2CzPN interface, the excitons are dissociated into holes and electrons, which are collected by the electrodes under the reverse voltage bias.[14] The current density–voltage (J–V) characteristics of the four devices in the dark and under 1 mA/cm2 UV light are depicted in Fig.
Detectivity D* is introduced as the figure-of-merit for exhibiting the performance of UV-OPDs, which is widely employed to characterize the sensitivity of a photodetector and can be calculated as follows:
As the (tbt)2Ir(acac) doped device shows the highest OLED performances among the three phosphorescent dyes, four different doping concentrations of 1 wt.%, 3 wt.%, 5 wt.%, and 7 wt.% are chosen to analyze the influence of the (tbt)2Ir(acac) doping concentration on the OLED and UV-OPD performance. The notations of devices B1 (1 wt.%), B2 (3 wt.%), B3 (5 wt.%), and B4 (7 wt.%) are used to denote the corresponding devices. Figure
The J–V–L characteristics of the devices with stepped dopant concentrations of 1–7 wt.% are shown in Fig.
Figure
According to the independence of the J–V characteristics on the (tbt)2Ir(acac) doping concentration, it is obvious that host–guest energy transfer dominates the EL process. Therefore, the creation and evolution of excited states in orange device B2 can be described by Fig.
In a reverse bias, 2CzPN is an intramolecular charge transfer complex so as to dissociate photo-induced excitons directly. Moreover, as Fig.
The EL properties and UV detectivity of 2CzPN-based OIDs are systematically investigated via material varieties and concentration analysis of doped phosphorescent. Ir(piq)3, (tbt)2Ir(acac), and FIr6, which hold different energy levels and emitting color, are chosen as the doped materials. It is proved that a deeper HOMO of phosphorescent contributes to a lower turn-on voltage under OLED mode, and through doping (tbt)2Ir(acac) into the TADF host, the OLED mode achieves the maximum current efficiency, power efficiency, and luminance of 42.18 cd/A, 25.16 lm/W, and 88774 cd/m2, respectively, significantly improving the performance of OLED. In contrast, all devices doped with phosphorescent have similar lower detectivity than the pure 2CzPN-based device. Moreover, the device with 3 wt.% doped (tbt)2Ir(acac) shows the maximum current efficiency, power efficiency, and luminance of 39.55 cd/A, 23.92 lm/W, and 70786 cd/m2, respectively. Under a reverse bias, the device achieves a detectivity of 1.07 × 1011 Jones. It is obvious that the device not only shows high luminance and efficiency, but also has decent OPD performance. According to the schematic diagram of the emission process, the efficient utilization of triplet excitons in 2CzPN for phosphorescence is responsible for the superior performance. The results suggest the high potential of phosphorescent materials as guests to realize OIDs with high efficiency.
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