Three thermally activated delayed fluorescence cationic cuprous complexes [Cu(POP) (ECAF)]PF6 (1, POP = bis(2-diphenylphosphinophenyl)ether, ECAF = 9,9-bis(9-ethylcarbazol-3-yl)-4,5-diazafluorene), [Cu(POP) (EHCAF)]PF6 (2, EHCAF = 9,9-bis(9-ethylhexylcarbazol-3-yl)-4,5-diazafluorene), and [Cu(POP) (PCAF)]PF6 (3, PCAF = 9,9-bis(9-phenylcarbazaol-3-yl)-4,5-diazafluorene) with bipolar 4,5-diazafluorene ligand substituted by bis-carbazole have been successfully prepared, and their UV absorption, photoluminescent properties, and electrochemical behaviors were investigated. At room temperature, complexes 1, 2, and 3 exhibit efficient yellowish-green emission with peak maxima of 550, 549, and 556 nm, respectively, and lifetimes of 5.7 μs. In powder states, the quantum yields (ϕPL) of 22.4% for 1, 18.5% for 2, and 20.0% for 3, respectively, are found. These metal phosphors can be vacuum-evaporated and applied in the organic light-emitting diodes (OLEDs) of indium tin oxide/poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (40 nm)/4,4′,4″-tri(9-carbazoyl)triphenylamine (15 nm)/cuprous complexes (10 wt %): 1,3-bis(9-carbazolyl)benzene (30 nm)/1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (50 nm)/LiF (0.5 nm)/Al (100 nm). Complex 1-based device D1 achieved a maximum luminance of 11 010 cd m–2, a current efficiency of 47.03 cd A–1, and an external quantum efficiency of 14.81%. The high electroluminescence efficiencies of these complexes are assumed to be due to their good thermal stabilities and capture of both singlet and triplet excitons. The research presented here provides a powerful tool toward highly efficient and cheap OLED devices.

5,6,11,12-Tetraphenylnaphthacene (rub), an organic small molecular semiconductor widely used in organic field-effect transistors and organic light-emitting diodes, was introduced into MAPbI3-based inverse-architecture perovskite solar cells as a hole transport layer due to its high hole mobility, good hydrophobic properties, favourable highest occupied molecular orbital (HOMO), low-cost, and low-temperature treatment process of rub. A high open-circuit voltage of 0.96 V, short-circuit current of 22 mA cm−2, and power conversion efficiency of 14.3% were achieved in the inverted planar heterostructure perovskite solar cells based on the rub hole-transport layer due to the HOMO energy level matching between rub and MAPbI3, large hole conductivity of rub, and large crystalline grain size of MAPbI3 formed on rub.

5,6,11,12-Tetraphenylnaphthacene (rub), an organic small molecular semiconductor widely used in organic field-effect transistors and organic light-emitting diodes, was introduced into MAPbI3-based inverse-architecture perovskite solar cells as a hole transport layer due to its high hole mobility, good hydrophobic properties, favourable highest occupied molecular orbital (HOMO), low-cost, and low-temperature treatment process of rub. A high open-circuit voltage of 0.96 V, short-circuit current of 22 mA cm−2, and power conversion efficiency of 14.3% were achieved in the inverted planar heterostructure perovskite solar cells based on the rub hole-transport layer due to the HOMO energy level matching between rub and MAPbI3, large hole conductivity of rub, and large crystalline grain size of MAPbI3 formed on rub.

•Biomimetic nanostructures like butterfly wing's scale are fabricated.•The bio-inspiration nanostructures are fabricated with AAO templates.•The bio-inspiration nanostructures exhibit hydrophobic properties.•The nanostructures are applied as a flexible encapsulation layer in OLEDs.•The nanostructures improve OLED's lifetime.Biomimetic nanostructures like butterfly wing's scale were fabricated with an anodic aluminum oxide (AAO) nanoimprint lithography technique. This bio-inspiration nanostructure exhibits an intrinsic hydrophobic property and thus was applied as a flexible encapsulation layer in organic light-emitting diodes (OLEDs) to improve device's lifetime. A ∼80% enhancement on lifetime was obtained with simply imprinting the biomimetic nanostructures onto the flexible substrates. Our work provides a simple encapsulation approach for OLEDs, especially for flexible OLEDs.

As-synthesized Au nanoparticles (NPs) composed of bone-like and rod shapes and a minority of cube and irregular spheres, generating three localized surface plasmon resonance (LSPR) peaks of 525, 575, and 775 nm, were doped into poly(3, 4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) layer and realized a power conversion efficiency of as high as 9.26% in our polymer solar cells. Optical, electrical, and morphology changes induced by Au NPs were analyzed and results demonstrate that the outstanding device performance is mainly attributed to the LSPR- and scattering-induced absorption enhancement in the active layer. Besides, mixed Au NPs also decreased the bulk resistance of PEDOT:PSS, which is found to facilitate hole transport and collection.

Bone-like Au nanoparticles (NPs) along with a small number of by-products of nanorods, nanocubes and other irregular shapes were synthesized using a seed-mediated growth approach. The mixed Au NPs generate a very wide absorption spectra of 300–1000 nm with three main absorption peaks at 520, 600, and 770 nm, extending to the main absorption, cut-off and transparence region of the poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methylester (PCBM) active layer. The mixed Au NPs were attached onto the ITO anode via a self-assembly method, and then P3HT:PCBM-based polymer photovoltaic cells (OPVs) were fabricated. The short-circuit current density and power conversion efficiency are significantly enhanced by 18.6% and 24.2% respectively, accompanied by the optimization of NPs distribution density. Optical, electrical, and morphological changes with the incorporation of Au NPs in the cells were thoroughly analyzed, and the results demonstrated that the cell performance improvement is mainly attributed to a synergistic reaction, including both the localized surface plasmon resonance- and scattering-induced absorption enhancement of the active layer, Au NPs-induced hole extraction ability enhancement, and large interface roughness-induced efficient exciton dissociation and hole collection.

In top-emitting white organic light-emitting diodes (TWOLEDs), it is usually difficult to realize a good chromaticity due to the strong suppression on the blue emission induced by the microcavity effect. In our work, the blue emission layer (EML) is located near the hole transport layer and the reflective anode to strengthen the wide-angle interference on the blue emission and enhance the output of light. Then we utilize the dual blue EMLs based on an electron-rich heterojunction to constrain most of the excitons in the blue EMLs. With the above two strategies, the intensity of the blue emission is significantly enhanced accompanying the chromaticity improvement in white emission. Some key factors including exciton distribution, energy transfer, and carrier trapping are analyzed to design the structure of the EMLs to acquire the pure and stable white emission. The excellent color stability with a Commission International de L’Eclairage (CIE) coordinate drift of only (0.009, 0.001) in the luminance range of 10–104 cd/m2 is obtained in our optimized TWOLED. The TWOLED also shows the high performances with a maximum luminance of 15360 cd/m2, the CIE coordinates of (0.33, 0.41), and a current efficiency of 13.3 cd/A.Keywords: dual blue emission layer; heterojunction; organic light-emitting diode; stability; top-emitting; white; wide-angle interference;

•Our flexible top-emitting WOLEDs are among the best performances reported so far.•We achieve a stable warm-white illumination over luminance range of four orders.•The stability, chromaticity and performances of WOLEDs are optimized step by step.•Energy transfer, electrical balance, and microcavity are systematically analyzed.The flexible top-emitting white organic light-emitting diode (FTEWOLED) with a very high efficiency but a significant color alteration is achieved with a blue/red/blue sandwiched tri-emission-layer. The voltage-dependent recombination region alternation and the emission mechanism are systematically investigated through a delta-doping method and the time-resolved transient photoluminescence lifetime measurement. By locating the main exciton recombination region at the 4,4′,4″-Tris(carbazol-9-yl)-triphenylamine (TCTA) and 9,9-spirobifluoren-2-yl-diphenyl-phosphine oxide (SPPO1) interface, replacing the carrier-trapping red dopant guest with an orange guest that utilizes energy transfer mechanism, and using a P–I–N structure together with the FIrpic blue guest dopant to balance the electron and hole carriers, an extremely color stable and a very high efficient FTEWOLED is fabricated, with the resulting high current and power efficiencies of 22.7 cd/A and 14.27 lm/W, and a warm white illumination with a small chromaticity variation of (−0.0087, +0.0015) over a broad luminance range of more than four orders of magnitude. In addition, the performances can be further improved to 23,340 cd/m2, 24.49 cd/A and 15.39 lm/W with a slight concentration alteration of the orange emitter.Graphical abstract

The commonly used donor material poly(3-hexylthiophene) (P3HT) confines the power conversion efficiency (PCE) in P3HT-based polymer solar cells due to its relatively large bandgap of ∼1.9 eV and the resultant limited absorption wavelength region of less than 650 nm. In this communication, the highly efficient up-conversion (UC) material NaYF4:2% Er3+, 18% Yb3+, converting near-infrared radiation into green and red emissions, is introduced into a P3HT/P3HT:[6,6] phenyl C61 butyric acid methyl ester (PC61BM) bulk heterojunction solar cell, referred to as a “bilayer cell”, to compensate for the non-absorbable wavelength region of P3HT. With an optimal UC doping concentration of 11.7% (weight ratio of UC to P3HT) in the P3HT matrix, the short-circuit current density and PCE for UC-doped bilayer cell are as high as 10.89 mA cm−2 and 3.62%, about 16.6% and 10.7% higher than the P3HT/P3HT:PC61BM bilayer cell and 22.4% and 16.4% higher than the standard P3HT:PC61BM bulk heterojunction one, respectively, although the fill factor in the UC-doped bilayer cell shows a slight decrease. The research result demonstrates that both the emission and the scattering of UC nanoparticles are beneficial to the enhancement of the solar cell's electrical performances.

White organic light-emitting diodes (WOLEDs) have attracted more and more attention in recent years because of their potential applications on flat-panel displays, solid-state lighting, and liquid-crystal display backlighting sources. With the goal towards practical applications, it requires WOLEDs possess not only high brightness and large electroluminescent (EL) efficiency, but also excellent stability. Here, good device stability includes two aspects, these are long operation lifetime and good color stability over a wide EL range. In this review, we explored all possible factors rendering a shift in color in both single- and multiple-emitting layer WOLEDs and summarized some typical design strategies for preventing shift in color of white emission. We hope the present paper can provide valuable clues to academic researchers and industrial designers in developing highly efficient WOLEDs with extremely stable chromaticity.

•Inserting a Ir(ppz)3 thin film between emitting layers improved efficiency and color stability.•The origin on the color-stable mechanism was explored.•The mechanical bending lifetime with PMMA or MoOx-modified PETs was explored.Flexible white top-emitting organic light-emitting diodes (WTEOLEDs) with red and blue phosphorescent dual-emitting layers were fabricated onto polyethylene terephthalate (PET) substrates. By inserting a 2-nm thin tris(phenypyrazole)iridium between the red and the blue emitters as an electron/exciton blocking layer, significant improvements on luminous efficiency and color stability were observed, reaching 9.9 cd/A (3.74 lm/W) and a small chromaticity change of (0.019, 0.011) in a wide luminance range of 80–5160 cd/m2. The origin on color stability was explored by analyzing the electroluminescent spectra, the time-resolved transient photoluminescence decay lifetimes of phosphors, and the tunneling phenomenon. In addition, mechanical bending lifetimes in WTEOLEDs with spin-coatedpolymethylmethacrylate (PMMA) and thermally evaporated MoOx onto the PETs were respectively measured, where PMMA or MoOx is used as a surface planarization layer. Analysis indicates that the poorer lifetime of PMMA-modified WTEOLED than the MoOx-modified ones is mainly due to the low surface energy of PMMA.Graphical abstract

Based on a modified electromagnetic theory, a bilayer metal cathode consisting of an electron injection layer and a silver (Ag) layer is designed to improve the color chromaticity in blue top-emitting organic light-emitting diodes (TEOLEDs). The effects of the complex refractive index of the electron injection material on the reflectivity and transmittivity of the bilayer cathode are investigated in detail, and then samarium (Sm) is selected as the electron injection material due to its proper refractive index of ∼1.22 + 1.12i and work function of ∼2.7 eV. Then, the emission peak wavelength, the full width at half maximum, and the Commission International de L’Eclairage coordinates of the blue TEOLEDs with different Sm/Ag bilayer cathodes are calculated and discussed. According to the theoretical results, a blue TEOLED with the optimized bilayer cathode of Sm (15 nm)/Ag (5 nm) is fabricated. The measurement results indicate that the blue TEOLED possesses an excellent chromaticity which is even better than that of a bottom-emitting organic light-emitting diode. Besides, the excellent angle stability is observed in the blue TEOLED even with a large viewing angle change of 0–75°.Graphical abstractHighlights► Chromaticity drift in blue top-emitting organic light-emitting diodes (TEOLEDs) is investigated. ► A bilayer metal cathode based on Sm and Ag is designed to improve the chromaticity in blue TEOLEDs. ► Detailed theoretical analysis on the Sm/Ag bilayer cathode is given to guide the device design. ► A blue TEOLED based on the optimized Sm/Ag cathode possesses excellent saturated color and angle stability.

A top-emitting organic light-emitting diode (TEOLED) with a very low luminous reflectance of 1.66% and very high contrast ratios of 405.9:1 and 1350.6:1 under on-state luminance of 300 and 1000 cd/m2 is fabricated by using a Ni/ZnS/MgF2/Ni contrast-enhancing stack (CES) and a copper-phthalocyanine (CuPc)/C60 anti-reflection (AR) bilayer. Although the introduction of the CES structure, it almost does not influence electrical performances of devices and a high luminance is obtained in TEOLED by using light output CuPc/C60 bilayer. The working mechanism on the reduced reflectance in high contrast TEOLED is further explored. Research shows that very low reflectance is attributed to both the effective absorption of C60 and CuPc and a destructive interference to the ambient light.Graphical abstractHighlights► A top-emitting organic light-emitting diode with a very high contrast ratio of 1350.6:1 is fabricated. ► A Ni/ZnS/MgF2/Ni contrast-enhancing stack and a CuPc/C60 anti-reflection bilayer is used to realize a low reflectance. ► The low reflectance is due to both destructive interference and absorption of CuPc and C60.

Blue top-emitting organic light-emitting devices (TEOLEDs) based on blue phosphor iridium (III) bis[(4,6-difluorophenyl)-pyridinato-N,C2′] picolinate (FIrpic) are demonstrated. Instead of using microcavity effects to obtain the blue top emission in conventional reports, the blue emission with a good chromaticity is achieved in this paper through the suppression of the multiple-beam interference by introducing a 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline light outcoupling layer onto the semitransparent Sm/Ag cathode. The electroluminescent performances of the blue emission is further improved based on the effective carrier transport and the consideration of the wide-angle interference at the emission peak of FIrpic with a maximum luminance of 8029 cd/m2 and a luminous efficiency of 4.02 cd/A achieved at 14 V and 10 V, respectively. With a polarizer on the top cathode, a high pixel contrast ratio of 113:1 is realized under an ambient illumination of 140 lx and a pixel brightness of 1000 cd/m2.Graphical abstractThe EL spectra of devices A, B, and C under the current density of 1 mA/cm2. Inset: normalized EL spectra of devices A, B, and C at the same current density of 1 mA/cm2, and the measured PL spectra of the blue phosphor FIrpic in a solid film state.Research highlights► The blue top-emitting organic light-emitting devices are obtained by using a BCP layer as a light outcoupling layer onto the semitransparent top metal cathode. ► With an optimized BCP thickness of 35 nm, a low reflectivity of the top Sm/Ag cathode is achieved with desirable suppression of multiple-beam interference in devices. ► We can improve the blue electroluminescent performance by satisfying wide-angle interference at the emission peak of FIrpic. ► The pixel contrast ratio is obviously improved by using the BCP light outcoupling layer together with a polarizer.

Bone-like Au nanoparticles (NPs) along with a small number of by-products of nanorods, nanocubes and other irregular shapes were synthesized using a seed-mediated growth approach. The mixed Au NPs generate a very wide absorption spectra of 300–1000 nm with three main absorption peaks at 520, 600, and 770 nm, extending to the main absorption, cut-off and transparence region of the poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methylester (PCBM) active layer. The mixed Au NPs were attached onto the ITO anode via a self-assembly method, and then P3HT:PCBM-based polymer photovoltaic cells (OPVs) were fabricated. The short-circuit current density and power conversion efficiency are significantly enhanced by 18.6% and 24.2% respectively, accompanied by the optimization of NPs distribution density. Optical, electrical, and morphological changes with the incorporation of Au NPs in the cells were thoroughly analyzed, and the results demonstrated that the cell performance improvement is mainly attributed to a synergistic reaction, including both the localized surface plasmon resonance- and scattering-induced absorption enhancement of the active layer, Au NPs-induced hole extraction ability enhancement, and large interface roughness-induced efficient exciton dissociation and hole collection.

The commonly used donor material poly(3-hexylthiophene) (P3HT) confines the power conversion efficiency (PCE) in P3HT-based polymer solar cells due to its relatively large bandgap of ∼1.9 eV and the resultant limited absorption wavelength region of less than 650 nm. In this communication, the highly efficient up-conversion (UC) material NaYF4:2% Er3+, 18% Yb3+, converting near-infrared radiation into green and red emissions, is introduced into a P3HT/P3HT:[6,6] phenyl C61 butyric acid methyl ester (PC61BM) bulk heterojunction solar cell, referred to as a “bilayer cell”, to compensate for the non-absorbable wavelength region of P3HT. With an optimal UC doping concentration of 11.7% (weight ratio of UC to P3HT) in the P3HT matrix, the short-circuit current density and PCE for UC-doped bilayer cell are as high as 10.89 mA cm−2 and 3.62%, about 16.6% and 10.7% higher than the P3HT/P3HT:PC61BM bilayer cell and 22.4% and 16.4% higher than the standard P3HT:PC61BM bulk heterojunction one, respectively, although the fill factor in the UC-doped bilayer cell shows a slight decrease. The research result demonstrates that both the emission and the scattering of UC nanoparticles are beneficial to the enhancement of the solar cell's electrical performances.

White organic light-emitting diodes (WOLEDs) have attracted more and more attention in recent years because of their potential applications on flat-panel displays, solid-state lighting, and liquid-crystal display backlighting sources. With the goal towards practical applications, it requires WOLEDs possess not only high brightness and large electroluminescent (EL) efficiency, but also excellent stability. Here, good device stability includes two aspects, these are long operation lifetime and good color stability over a wide EL range. In this review, we explored all possible factors rendering a shift in color in both single- and multiple-emitting layer WOLEDs and summarized some typical design strategies for preventing shift in color of white emission. We hope the present paper can provide valuable clues to academic researchers and industrial designers in developing highly efficient WOLEDs with extremely stable chromaticity.