Spectra stable deep-blue light-emitting diodes based on cryolite-like cerium(III) halides with nanosecond d-f emission.

Next-generation wide color gamut displays require the development of efficient and toxic-free light-emitting materials meeting the crucial Rec. 2020 standard. With the rapid progress of green and red perovskite light-emitting diodes (PeLEDs), blue PeLEDs remain a central challenge because of the undesirable color coordinates and poor spectra stability. Here, we report Cs3CeBrxI6-x (x = 0 to 6) with the cryolite-like structure and stable and tunable color coordinates from (0.17, 0.02) to (0.15, 0.04). Further encouraged by the short exciton lifetime (26.1 ns) and high photoluminescence quantum yield (~76%), we construct Cs3CeBrxI6-x-based rare-earth LEDs via thermal evaporation. A seed layer strategy is conducted to improve the device's performance. The optimal Cs3CeI6 device achieves a maximum external quantum efficiency of 3.5% and a luminance of 470 cd m-2 with stable deep-blue color coordinates of (0.15, 0.04). Our work opens another avenue to achieving efficient and spectrally stable deep-blue LEDs.

Thermal and Humidity Stability of Mixed Spacer Cations 2D Perovskite Solar Cells.

In this article, two different types of spacer cations, 1,4-butanediamonium (BDA2+) and 2-phenylethylammonium (PEA+) are co-used to prepare the perovskite precursor solutions with the formula of (BDA)1- a (PEA2) a MA4Pb5X16. By simply mixing the two spacer cations, the self-assembled polycrystalline films of (BDA)0.8(PEA2)0.2MA4Pb5X16 are obtained, and BDA2+ is located in the crystal grains and PEA+ is distributed on the surface. The films display a small exciton binding energy, uniformly distributed quantum wells and improved carrier transport. Besides, utilizing mixed spacer cations also induces better crystallinity and vertical orientation of 2D perovskite (BDA)0.8(PEA2)0.2MA4Pb5X16 films. Thus, a power conversion efficiency (PCE) of 17.21% is achieved in the optimized perovskite solar cells with the device structure of ITO/PEDOT:PSS/Perovskite/PCBM/BCP/Ag. In addition, the complementary humidity and thermal stability are obtained, which are ascribed to the enhanced interlayer interaction by BDA2+ and improved moisture resistance by the hydrophobic group of PEA+. The encapsulated devices are retained over 95% or 75% of the initial efficiency after storing 500 h in ambient air under 40 ± 5% relative humidity or 100 h in nitrogen at 60 °C.

Enhancing Device Performance in Quasi-2D Perovskite ((BA)2(MA)3Pb4I13) Solar Cells Using PbCl2 Additives.

Quasi-2D Ruddlesden-Popper perovskites exhibit excellent photostability/environmental stability. However, the main drawback is their relatively low photovoltaic properties compared with three-dimensional perovskites. Herein, we demonstrated that chlorine-based additives via adjusting the proportion of PbI2 and PbCl2 in the precursor (BA)2(MA)3Pb4I13 (n = 4) solutions show an optimized device performance of over 15%, and the devices exhibit much improved humidity stability. Upon PbCl2 addition, the quasi-2D perovskites have larger and more compact grains, which result in high quality of films. The photoluminescence gives rise to a much prolonged lifetime under the PbCl2 additive, indicating fewer trap states to reduce the nonradiative recombination. The capacitance characteristics confirm that the PbCl2 additive can largely decrease the trap states in quasi-2D perovskite films. The capacitance-voltage characteristics indicate that using the PbCl2 additive decreases the charge accumulation toward increasing the charge collection in quasi-2D perovskite solar cells. Our work indicates that the addition of PbCl2 is an effective method to improve the device performance by reducing trap states and increasing charge collection toward developing high-performance quasi-2D perovskite devices.

Revealing the Cooperative Relationship between Spin, Energy, and Polarization Parameters toward Developing High-Efficiency Exciplex Light-Emitting Diodes.

Experimental studies to reveal the cooperative relationship between spin, energy, and polarization through intermolecular charge-transfer dipoles to harvest nonradiative triplets into radiative singlets in exciplex light-emitting diodes are reported. Magneto-photoluminescence studies reveal that the triplet-to-singlet conversion in exciplexes involves an artificially generated spin-orbital coupling (SOC). The photoinduced electron parametric resonance measurements indicate that the intermolecular charge-transfer occurs with forming electric dipoles (D+• →A-• ), providing the ionic polarization to generate SOC in exciplexes. By having different singlet-triplet energy differences (ΔEST ) in 9,9'-diphenyl-9H,9'H-3,3'-bicarbazole (BCzPh):3',3'″,3'″″-(1,3,5-triazine-2,4,6-triyl)tris(([1,1'-biphenyl]-3-carbonitrile)) (CN-T2T) (ΔEST = 30 meV) and BCzPh:bis-4,6-(3,5-di-3-pyridylphenyl)-2-methyl-pyrimidine (B3PYMPM) (ΔEST = 130 meV) exciplexes, the SOC generated by the intermolecular charge-transfer states shows large and small values (reflected by different internal magnetic parameters: 274 vs 17 mT) with high and low external quantum efficiency maximum, EQEmax (21.05% vs 4.89%), respectively. To further explore the cooperative relationship of spin, energy, and polarization parameters, different photoluminescence wavelengths are selected to concurrently change SOC, ΔEST , and polarization while monitoring delayed fluorescence. When the electron clouds become more deformed at a longer emitting wavelength due to reduced dipole (D+• →A-• ) size, enhanced SOC, increased orbital polarization, and decreased ΔEST can simultaneously occur to cooperatively operate the triplet-to-singlet conversion.

Enhancing Photovoltaic Performance of Inverted Planar Perovskite Solar Cells by Cobalt-Doped Nickel Oxide Hole Transport Layer.

Electron and hole transport layers have critical impacts on the overall performance of perovskite solar cells (PSCs). Herein, for the first time, a solution-processed cobalt (Co)-doped NiO X film was fabricated as the hole transport layer in inverted planar PSCs, and the solar cells exhibit 18.6% power conversion efficiency. It has been found that an appropriate Co-doping can significantly adjust the work function and enhance electrical conductivity of the NiO X film. Capacitance-voltage ( C- V) spectra and time-resolved photoluminescence spectra indicate clearly that the charge accumulation becomes more pronounced in the Co-doped NiO X-based photovoltaic devices; it, as a consequence, prevents the nonradiative recombination at the interface between the Co-doped NiO X and the photoactive perovskite layers. Moreover, field-dependent photoluminescence measurements indicate that Co-doped NiO X-based devices can also effectively inhibit the radiative recombination process in the perovskite layer and finally facilitate the generation of photocurrent. Our work indicates that Co-doped NiO X film is an excellent candidate for high-performance inverted planar PSCs.

Fine-Tuning the Quasi-3D Geometry: Enabling Efficient Nonfullerene Organic Solar Cells Based on Perylene Diimides.

The geometries of acceptors based on perylene diimides (PDIs) are important for improving the phase separation and charge transport in organic solar cells. To fine-tune the geometry, biphenyl, spiro-bifluorene, and benzene were used as the core moiety to construct quasi-three-dimensional nonfullerene acceptors based on PDI building blocks. The molecular geometries, energy levels, optical properties, photovoltaic properties, and exciton kinetics were systematically studied. The structure-performance relationship was discussed as well. Owing to the finest phase separation, the highest charge mobility and smallest nongeminate recombination, the power conversion efficiency of nonfullerene solar cells using PDI derivatives with biphenyl core (BP-PDI4) as acceptor reached 7.3% when high-performance wide band gap donor material poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b']dithiophene))-alt-(5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl)benzo[1',2'-c:4',5'-c']dithiophene-4,8-dione))] was blended.

Spin-dependent deprotonation induced giant magnetocurrent in electrochemical cells.

A giant magnetocurrent (>100%) is observed in the electrochemical system based on tertiary amines at room temperature. This giant magnetocurrent is ascribed to spin-dependent deprotonation during the oxidation of tertiary amines. This presents a new approach of using spin-dependent deprotonation to generate giant magnetocurrent in electrochemical reactions.