Efficient Green Spin Light-Emitting Diodes Enabled by Ultrafast Energy- and Spin-Funneling in Chiral Perovskites.

Introducing molecular chirality into perovskite crystal structures has enabled the control of carrier spin states, giving rise to circularly polarized luminescence (CPL) in thin films and circularly polarized electroluminescence (CPEL) in LEDs. Spin-LEDs can be fabricated either through a spin-filtering layer enabled by chiral-induced spin selectivity or a chiral emissive layer. The former requires a high degree of spin polarization and a compatible spinterface for efficient spin injection, which might not be easily integrated into LEDs. Alternatively, a chiral emissive layer can also generate circularly polarized electroluminescence, but the efficiency remains low and the fundamental mechanism is elusive. In this work, we report an efficient green LED based on quasi-two-dimensional (quasi-2D) chiral perovskites as the emitting layer (EML), where CPEL is directly produced without separate carrier spin injection. The optimized chiral perovskite thin films exhibited strong CPL at 535 nm with a photoluminescence quantum yield (PLQY) of 91% and a photoluminescence dissymmetry factor (glum) of 8.6 × 10-2. Efficient green spin-LEDs were successfully demonstrated, with a large EL dissymmetry factor (gEL) of 7.8 × 10-2 and a maximum external quantum efficiency (EQE) of 13.5% at room temperature. Ultrafast transient absorption (TA) spectroscopic study shows that the CPEL is generated from a rapid energy transfer accompanied by spin transfer from 2D to 3D perovskites. Our study not only demonstrates a reliable approach to achieve high performance spin-LEDs but also reveals the fundamental mechanism of CPEL with an emissive layer of chiral perovskites.

Lifetime Enhancement and Degradation Study of Blue OLEDs Using Deuterated Materials.

Significant lifetime enhancement, up to an eight-fold increase in T90, has been demonstrated in blue organic light-emitting diode (OLED) devices through the deuteration of host and hole transport materials. We observed a progressive increase in T90 using a series of anthracene-based hydrocarbon hosts with incremental deuteration in the emitting layer. In addition, we realized further lifetime improvement using a deuterated hole-transport layer along with the deuterated emitting layer. To elucidate the deuteration effects, we utilized laser desorption/ionization-time-of-flight (LDI-TOF) mass spectrometry for in situ UV irradiation to induce photodegradation and immediate chemical analysis of the resultant photodegradation species. Adducts between the host and moieties from transport materials were identified in UV-degraded films comprising a mixture of host and transport materials, indicating that similar species could be produced in OLED devices using these materials. Deuteration, in effect, mediated the formation of these adduct species, presumably electroluminescence quenchers, and thus improved the device lifetime. An approximate agreement was obtained between the kinetic isotope effect of the photodegradation reactions and the enhancement in device lifetime with deuteration.

Lattice strain modulation toward efficient blue perovskite light-emitting diodes.

The successful implementation of perovskite light-emitting diodes (PeLEDs) in advanced displays and lighting has proven to be challenging because of the inferior performance of blue devices. Here, we point out that a strained system would lead to the quasi-degenerate energy state to enhance the excited-state transition due to the formation of double-polarized transition channel. The tensile strained structure also brings about a synergetic control of the carrier dynamics in virtue of lattice structure deformation and reduced dimensional phase regulation to promote carrier population in large bandgap domains and to realize near-unit energy transfer from the large bandgap phases to the emitter phases. Accordingly, high external quantum efficiencies of 14.71 and 10.11% are achieved for the 488- and 483-nanometer PeLEDs. This work represents a versatile strategy using a strained system to achieve enhanced radiative emission for the development of efficient PeLEDs.

Silicon-based material with spiro-annulated fluorene/triphenylamine as host and exciton-blocking layer for blue electrophosphorescent devices.

A novel silicon-based compound, 10-phenyl-2'-(triphenylsilyl)-10H-spiro[acridine-9,9'-fluorene] (SSTF), with spiro structure has been designed, synthesized, and characterized. Its thermal, electronic absorption, and photoluminescence properties were studied. Its energy levels make it suitable as a host material or exciton-blocking material in blue phosphorescent organic light-emitting diodes (PhOLEDs). Accordingly, blue-emitting devices with iridium(III) bis[(4,6-difluorophenyl)-pyridinato-N,C(2)']picolinate (FIrpic) as phosphorescent dopant have been fabricated and show high efficiency with low roll-off. In particular, 44.0 cd A(-1) (41.3 lm W(-1)) at 100 cd m(-2) and 41.9 cd A(-1) (32.9 lm W(-1)) at 1000 cd m(-2) were achieved when SSTF was used as host material; 28.1 lm W(-1) at 100 cd m(-2) and 20.6 lm W(-1) at 1000 cd m(-2) were achieved when SSTF was used as exciton-blocking layer. All of the results are superior to those of the reference devices and show the potential applicability and versatility of SSTF in blue PhOLEDs.

[Spectra characteristics and electroluminescence performance of novel fluorene derivatives with aggregation fluorescence enhancement].

The photoluminescence (PL) spectra and UV-Visible absorption spectra of three novel fluorene derivatives solution containing different triphenylamine (TPA) and tetraphenyl-benzene (TPB) groups were systematically investigated. The PL spectra of the acetone/water solution were tested to analyze the capability of suppression concentration quenching (SCQ). The results showed that when water fraction ranged from 50% to 90%, the spectral irradiance of the mixture was obviously increased. Meanwhile, the PL spectra had blue shift due to the blue crystalline aggregation of novel fluorene derivatives, and the blue shift is proportional to the order of crystalline aggregation. Moreover, since the tetraphenyl-benzene and triphenylamine functional groups were tailored to fluorene backbone to suppress the concentration quenching of the fluorene dye and improve the charge carrier transporting ability, the non-doped organic light-emitting devices (OLEDs) were achieved.

New dibenzofuran/spirobifluorene hybrids as thermally stable host materials for efficient phosphorescent organic light-emitting diodes with low efficiency roll-off.

A new class of host materials DBFSF (DBFSF2 and DBFSF4) is facilely synthesized through a Suzuki coupling reaction between dibenzofuran and spirobifluorene. Their thermal, electrochemical, electronic absorption and photoluminescent properties are fully investigated. High glass transition temperatures (T(g)) of 115 °C and 124 °C are observed for DBFSF2 and DBFSF4, respectively, due to the introduction of bulky spirobifluorene groups. As expected, the DBFSF4 with a twisted-linkage exhibits higher triplet energy than DBFSF2 and can be used in blue and green phosphorescent OLEDs. Electrophosphorescent devices with DBFSF2 and DBFSF4 as hosts were fabricated. Besides the good current efficiencies of 22.2 cd A(-1) for blue and 64.4 cd A(-1) for green, low efficiency roll-off has also been achieved for both devices.