A near‐infrared photodetector with optimized performance is reported using varied thickness (20, 40, 60, and 80 nm) of the active layer comprising chloroaluminium phthalocyanine (ClAlPc) and fullerene (C70) at the ratio of 1:3, and TAPC:10% MoO3 and BPhen as electron and hole blocking layers, respectively. The experimental results reveal that the photodetector with 80 nm thick active layer provides the best performance at the wavelength of 730 nm achieving a very low dark current density of 1.15 × 10−9 A cm−2 and an external quantum efficiency of 74.6% with a responsivity of 0.439 A W−1 at −2 V bias. Additionally, the device exhibits a dramatic high detectivity of 4.14 × 1013 cm Hz1/2 W−1 at 0 V bias. The device exhibits not only a large linear response over a wide optical power range (LDR of 173.0 dB), but also a broad frequency response (778.7 kHz) and rise/fall time of 2.13/0.77 µs (based on trigger pulses at a frequency of 10 kHz) at the applied bias of −2 V. Based on the impedance spectroscopic study and the conventional characterization of electro‐optical properties, the results demonstrate the superiority of this device over other small molecule‐based near‐infrared photodetectors.
We report a new efficient exciplex-forming system consisting of a biscarbazole donor and a triazine-based acceptor. The new exciplex was characterized with a high photoluminescence quantum yield up to 68% and effective thermally activated delayed fluorescence behavior. The BCzPh:3P-T2T (2:1, 30 nm) blend was examined not only as an emitting layer (device D1) but also a reliable co-host of fluorescent and phosphorescent emitters for giving highly efficient exciplex-based organic light-emitting diodes (OLEDs) with a high maximum external quantum efficiency of 15.5 and 29.7% for devices doped with 1 wt % C545T (device D2) and 8 wt % Ir(ppy)(acac) (device D4), respectively. More strikingly, a strongly enhanced lifetime ( T = 16 927 min.) of the C545T-doped device was obtained. The transient electroluminescence measurement as well as capacitance-voltage and impedance-voltage correlations were utilized to explore the factors governing the high efficiency and stability. The obtained results clearly show that the energy transfer and charge transport is highly efficient; they also show the photoelectric semiconducting characteristics of exciplex-based OLEDs, which are significantly different from those of unipolar host-based reference devices D3 (Alq: 1 wt % C545T) and D5 (CBP: 8 wt % Ir(ppy)(acac)). Our works have established a systematic protocol to shed light on the mechanisms behind exciplex-based devices. The combined results also confirm the bright prospect of the exciplex-forming system as the co-host for highly efficient and stable OLEDs.
An efficient organic light-emitting diode based on the BCzPh:CN-T2T exciplex as an emitting layer (EML) has been fabricated by exploiting charge balance and favorable molecular orientations. To further understand the details of the exciplex-forming mechanism, time-resolved photoluminescence (TRPL), capacitance-voltage (CV), impedance spectroscopy (IS), and transient electroluminescence (EL) measurements were used to probe the photophysical and electrical characteristics of EL devices by incorporating interfacial (BCzPh/CN-T2T) and bulk (BCzPh:CN-T2T) exciplexes as the emitting layer. Interfacial-and bulk-exciplex devices exhibit a maximum external quantum efficiency (EQE) of 7.7 and 26.4%, respectively. The reason for different device performances was rationalized by comparing the accumulated amount of charge density at the EML's interface responsible for exciplex emission. In addition, the TRPL measurement monitored from short to long wavelengths was used to explore the harvest of nonradiative triplets back to singlets via reverse intersystem crossing and to examine the efficiency of delayed fluorescence. The bulk-exciplex system showed a distinct delayed fluorescence as compared to the interfacial one, which was also corroborated by the observation in the transient EL. The result indicates that the bulk exciplex can reduce the accumulated charge in the EML rapidly, resulting in improvement of EL efficiency. This assumption was further verified by CV and IS measurements. Our results reveal that the accumulated charge density and the bulk resistance of the bulk-exciplex device are much lower as compared to those of the interfacial counterpart device.
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