Highly luminescent perovskite–aluminum oxide composites

In this communication we report on the preparation of CH3NH3PbBr3 perovskite/Al2O3 nanoparticle composites in a thin film configuration and demonstrate their high photoluminescence quantum yield. The composite material is solution-processed at low temperature, using stable alumina nanoparticle dispersions. There is a large influence of the alumina nanoparticle concentration on the perovskite morphology and on its photoluminescence

Highly efficient light-emitting electrochemical cells

El consumo eléctrico destinado a iluminación supone actualmente cerca del 20% de la producción energética a nivel mundial, de modo que es posible alcanzar un importante ahorro energético mediante el uso de sistemas de iluminación más eficientes. El desarrollo tecnológico permite hacer uso de aplicaciones más eficientes y novedosos en dicho campo. Los diodos orgánicos emisores de luz (OLEDs) suponen una alternativa de futuro a los dispositivos emisores de luz actuales. El procesado de materiales orgánicos se adapta más fácilmente a nuevos diseños de dispositivos emisores de luz y es, a día de hoy, una realidad en pequeñas pantallas para dispositivos móviles o similares. Sin embargo, los OLEDs presentan serias limitaciones que aumentan el coste de fabricación del producto y los hacen no aptos para ser aplicados en sistemas de iluminación. Otros tipos de dispositivos basados en materiales orgánicos son las células electroquímicas emisoras de luz (LECs). Los LECs, presentan una estructura más simple compatible con procesos de fabricación de bajo coste. El peculiar mecanismo de operación de este tipo específico de dispositivo requiere la presencia de iones en su capa emisora y permite operar los dispositivos a bajos voltajes, lo que se traduce en un bajo consumo. Desde su descubrimiento, los materiales más empleados y eficientes han sido los complejos de metales de transición iónicos (iTMC-LECs). El trabajo científico ha permitido, a lo largo de varias décadas de investigación obtener prometedores resultados respecto a eficiencia, valores de luminancia y sobretodo estabilidad de los dispositivos. Actualmente, resulta imprescindible mejorar las prestaciones de los dispositivos LEC a fin de hacer la tecnología competitiva a nivel del mercado actual. Todo el trabajo presentado en esta tesis doctoral se centra en la mejorar la eficiencia de los dispositivos LEC mediante el aumento del rendimiento cuántico de emisión en estado sólido. Para ello, se adoptaron tres estrategias. En primer lugar, la funcionalización remota del ligando auxiliar de iTMC, permite modificar las características fotofísicas del complejo emisor y mejorar las prestaciones del dispositivo. Con esta idea en mente, partiendo de iTMCs previamente utilizados en LECs, se ha estudiado una serie de complejos que iridio (Ir-iTMCs) donde la presencia de distintos grupos sustituyentes afecta significativamente a las propiedades fotoluminiscentes y electroluminiscentes. Por un lado, se ha establecido un efecto negativo con la presencia de bromuros en la estructura del iTMC. Por otro lado, la funcionalización del ligando auxiliar con un grupo naftilo ha permitido mejorar la eficiencia del dispositivo. En segundo lugar, las mejores prestaciones en LECs se han obtenido empleando complejos de iridio catiónicos con hexafluorofosfato como anión. Debido a la típica ruta de síntesis establecida para dichos materiales, es habitual encontrar la presencia de iones cloruro como impureza. En esta tesis doctoral, se ha demostrado que la presencia de aniones cloruro a nivel de trazas afecta negativamente al funcionamiento del dispositivo basado en el complejo [Ir(ppy)2(bpy)][PF6], comprometiendo la eficiencia del mismo. Dicho efecto es consecuencia de una interacción especifica entre el anión cloruro y los protones externos del ligando bipiridina que se traduce en una pérdida del rendimiento cuántico de la capa delgada y por tanto en las prestaciones del dispositivo. En tercer y último lugar, se ha demostrado la posibilidad de usar sistemas host-guest en LECs. Esta estrategía ha sido ampliamente utilizada en OLEDs para evitar los procesos de quenching que limitan las eficiencias en procesos de electroluminiscencia. A pesar de que la estrategía ha sido aplicada anteriormente en LECs, el trabajo presentado aquí ha sido aplicado con éxito para la fabricación de LECs con emisión en luz azul, amarilla e infraroja con altos niveles de luminancia y eficiencia. Además de estos prometedores resultados, mediante esta estrategia se ha demostrado, por primera vez, la posibilidad utilizar moléculas orgánicas iónicas como único material activo en la preparación de LECs. Este hito, permite eliminar la dependencia de usar materiales basados en átomos pesados de baja abundancia (iridio) de elevado precio y ampliar el rango de materiales disponibles que pueden ser utilizados para aplicaciones en iluminación de bajo coste.Artificial lighting is one of the primary need for modern society. Lighting represent 20% of the electrical consumption. In this context, new efficient lighting systems are required to cope the ever-growing demand of energy. Moreover, the evolution of technology needs for innovative lighting and display products. Organic light-emitting diodes (OLEDs) are a promising type of emitting devices. Currently, most mobile phones screens use OLED-based displays. The main strategy to reach this state has consisted in increasing the number of organic layers in the device, in order to separate the injection, transport and recombination processes. In this way, OLED technology leads to expensive prototypes for lighting applications. Light-emitting electrochemical cells (LECs) are other interesting type of light emitting devices based on organic materials. LECs have a much simpler architecture than OLEDs. The presence of ions in LECs plays a critical role in their operation mode, which allow them to be operated under low voltage. Up to now, the best performing devices have been reported using ionic transition metal complexes (iTMCs). High efficiencies and luminance as well as high stable devices have been reported during the last decades. However, in order to make the technology competitive to reach the market, further efficient device are required. The work presented in this thesis is focused to improve the efficiency of LEC devices by increasing the photoluminescence quantum yield in thin film. To achieve this goal, three different strategies were adopted: First, remote functionalization of the ancillary ligand of iridium complexes (Ir-iTMCs) affects the photophysic characteristics of the emitter. Hence, a series of Ir-iTMCs with different substituents attached to the ancillary ligand were studied in LEC devices. On the one hand, a detrimental effect on the LEC performance was found when bromide is incorporated to the complex chemical structure. On the other hand, by incorporating a naphthyl group to the ancillary ligand, the device performance was improved with respect to the archetype iridium complex. Second, the best performing Ir-iTMC have been reported using hexafluorophosphate salts. In general, due to the synthetic route, chloride is a typical impurity for Ir-iTMCs. In this thesis, it has been demonstrated that the presence of chloride affects strongly the device performance of LECs based on [Ir(ppy)2(bpy)][PF6] complex. An interaction between the chloride anion with the external protons of the bypiridine leads to a decrease of the photoluminescence quantum yield of the material. Hence, lower device efficiencies were achieved when chloride is present in the iTMC. Finally, three different host-guest systems has been employed to increase the photoluminescence quantum yield of thin films. This strategy have been used in OLEDs to reduce the quenching processes in organic layer during the electroluminescence process. Using host-guest sytems, three different blue, yellow and infrared LECs have been reported with high luminance and efficiency. Moreover, using the host-guest approach, ionic small molecules as active components for LECs has been demonstrated. This type of materials are cheaper than Ir-iTMCs and increase the range of materials that can be used for LEC production with low cost

Efficient photovoltaic and electroluminescent perovskite devices

Planar diode structures employing hybrid organic-inorganic methylammonium lead iodide perovskites lead to multifunctional devices exhibiting both a high photovoltaic efficiency and good electroluminescence. The electroluminescence strongly improves at higher current density applied using a pulsed driving method

Regioisomerism in cationic sulfonyl-substituted [Ir(C^N)2(N^N)]+ complexes: its influence on photophysical properties and LEC performance

In a series of regioisomeric [Ir(C^N) 2 (bpy)] + complexes containing methylsulfonyl groups on the cyclometallating ligands, the influence of the substitution position on photophysical, electrochemical and LEC device properties is investigated

Chiral iridium(III) complexes in light-emitting electrochemical cells : exploring the impact of stereochemistry on the photophysical properties and device performances

Despite hundreds of cationic bis-cyclometalated iridium(III) complexes having been explored as emitters for light-emitting electrochemical cells (LEECs), uniformly their composition has been in the form of a racemic mixture of Λ and Δ enantiomers. The investigation of LEECs using enantiopure iridium(III) emitters, however, remains unprecedented. Herein, we report the preparation, the crystal structures and the optoelectronic properties of two families of cyclometalated iridium(III) complexes of the form of [(C^N)2Ir(dtBubpy)]PF6 (where dtBubpy is 4,4'-di-tert-butyl-2,2'-bipyridine) in both their racemic and enantiopure configurations. LEEC devices using Λ and Δ enantiomers as well as the racemic mixture of both families have been prepared and the device performances were tested. Importantly, different solid-state photophysical properties exist between enantiopure and racemic emitters, which are also reflected in the device performances.Publisher PDFPeer reviewe

Highly stable and efficient light-emitting electrochemical cells based on cationic iridium complexes bearing arylazole ancillary ligands

A series of bis-cyclometalated iridium(III) complexes of general formula [Ir(ppy)2(N∧N)][PF6] (ppy− = 2-phenylpyridinate; N∧N = 2-(1H-imidazol-2-yl)pyridine (1), 2-(2-pyridyl)benzimidazole (2), 1-methyl-2-pyridin-2-yl- 1H-benzimidazole (3), 2-(4′-thiazolyl)benzimidazole (4), 1- methyl-2-(4′-thiazolyl)benzimidazole (5)) is reported, and their use as electroluminescent materials in light-emitting electrochemical cell (LEC) devices is investigated. [2][PF6] and [3][PF6] are orange emitters with intense unstructured emission around 590 nm in acetonitrile solution. [1][PF6], [4][PF6], and [5][PF6] are green weak emitters with structured emission bands peaking around 500 nm. The different photophysical properties are due to the effect that the chemical structure of the ancillary ligand has on the nature of the emitting triplet state. Whereas the benzimidazole unit stabilizes the LUMO and gives rise to a 3MLCT/3LLCT emitting triplet in [2][PF6] and [3][PF6], the presence of the thiazolyl ring produces the opposite effect in [4][PF6] and [5][PF6] and the emitting state has a predominant 3LC character. Complexes with 3MLCT/3LLCT emitting triplets give rise to LEC devices with luminance values 1 order higher than those of complexes with 3LC emitting states. Protecting the imidazole N−H bond with a methyl group, as in complexes [3][PF6] and [5][PF6], shows that the emissive properties become more stable. [3][PF6] leads to outstanding LECs with simultaneously high luminance (904 cd m−2), efficiency (9.15 cd A−1), and stability (lifetime over 2500 h).Spanish Ministry of Economy and Competitiveness (MINECO) of Spain (projects CTQ2014- 58812-C2-1-R, MAT2014-55200, CTQ2014-55583-R, CTQ2014-61914-EXP, CTQ2015-71955-REDT, CTQ2015- 70371-REDT, CTQ2015-71154-P, and Unidad de Excelencia Marıá de Maeztu MDM-2015-0538), European Feder funds (CTQ2015-71154-P), Obra Social “la Caixa” (OSLC-2012- 007), Junta de Castilla y León (BU033-U16), and Generalitat Valenciana (Prometeo2016/135

Peripheral halo-functionalization in [Cu(N^N)(P^P)]+ emitters: influence on the performances of light-emitting electrochemical cells

A series of heteroleptic [Cu(N^N)(P^P)][PF6] complexes is described in which P^P = bis(2-(diphenylphosphino)phenyl)ether (POP) or 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (xantphos) and N^N = 4,4′-diphenyl-6,6′-dimethyl-2,2′-bipyridine substituted in the 4-position of the phenyl groups with atom X (N^N = 1 has X = F, 2 has X = Cl, 3 has X = Br, 4 has X = I; the benchmark N^N ligand with X = H is 5). These complexes have been characterized by multinuclear NMR spectroscopy, mass spectrometry, elemental analyses and cyclic voltammetry; representative single crystal structures are also reported. The solution absorption spectra are characterized by high energy bands (arising from ligand-centred transitions) which are red-shifted on going from X = H to X = I, and a broad metal-to-ligand charge transfer band with λmax in the range 387–395 nm. The ten complexes are yellow emitters in solution and yellow or yellow-orange emitters in the solid-state. For a given N^N ligand, the solution photoluminescence (PL) spectra show no significant change on going from [Cu(N^N)(POP)]+ to [Cu(N^N)(xantphos)]+; introducing the iodo-functionality into the N^N domain leads to a red-shift in λmaxem compared to the complexes with the benchmark N^N ligand 5. In the solid state, [Cu(1)(POP)][PF6] and [Cu(1)(xantphos)][PF6] (fluoro-substituent) exhibit the highest PL quantum yields (74 and 25%, respectively) with values of τ1/2 = 11.1 and 5.8 μs, respectively. Light-emitting electrochemical cells (LECs) with [Cu(N^N)(P^P)][PF6] complexes in the emissive layer have been tested. Using a block-wave pulsed current driving mode, the best performing device employed [Cu(1)(xantphos)]+ and this showed a maximum luminance (Lummax) of 129 cd m−2 and a device lifetime (t1/2) of 54 h; however, the turn-on time (time to reach Lummax) was 4.1 h. Trends in performance data reveal that the introduction of fluoro-groups is beneficial, but that the incorporation of heavier halo-substituents leads to poor devices, probably due to a detrimental effect on charge transport; LECs with the iodo-functionalized N^N ligand 4 failed to show any electroluminescence after 50 h

CF3 Substitution of [Cu(P^P)(bpy)][PF6] complexes: Effects on Photophysical Properties and Light-emitting Electrochemical Cell Performance

We report [Cu(P^P)(N^N)][PF 6 ] complexes with P^P = bis(2-(diphenylphosphino)phenyl)ether (POP) or 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (xantphos), N^N = CF 3 -substituted 2,2'-bipyridines (6,6'-(CF 3 ) 2 bpy, 6-CF 3 bpy, 5,5'-(CF 3 ) 2 bpy, 4,4'-(CF 3 ) 2 bpy, 6,6'-Me 2 -4,4'-(CF 3 ) 2 bpy). We present the effects of CF 3 substitution on structures, and electrochemical and photophysical properties. The HOMO–LUMO gap is tuned by the N^N ligand; the largest redshift in the MLCT band is for [Cu(P^P)(5,5'-(CF 3 ) 2 bpy)][PF 6 ]. In solution, the compounds are weak yellow to red emitters. The emission properties depend on the substitution pattern but this cannot be explained by simple electronic arguments. For powders, [Cu(xantphos)(4,4'-(CF 3 ) 2 bpy)][PF 6 ] has the highest PLQY (50.3%) with an emission lifetime of 12 µs. Compared to 298 K solution behaviour, excited state lifetimes lengthen in frozen Me-THF (77 K) indicating thermally activated delayed fluorescence (TADF). TD-DFT calculations show that the energy gap between the lowest-energy singlet and triplet excited states (0.12–0.20 eV) permits TADF. LECs with [Cu(POP)(6-CF 3 bpy)][PF 6 ], [Cu(xantphos)(6-CF 3 bpy)][PF 6 ] or [Cu(xantphos)(6,6'-Me 2 -4,4'-(CF 3 ) 2 bpy)][PF 6 ] emit yellow electroluminescence. A LEC with [Cu(xantphos)(6,6'-Me 2 -4,4'-(CF 3 ) 2 bpy)][PF 6 ] had the fastest turn-on time (8 min); the LEC with the longest lifetime ( t 1/2 = 31 h) contained [Cu(xantphos)(6-CF 3 bpy)][PF 6 ]; these LECs reached maximum luminances of 131 and 109 cd m –2

Dipole reorientation and local density of optical states influence the emission of light-emitting electrochemical cells

Herein, we analyze the temporal evolution of the electroluminescence of light-emitting electrochemical cells (LECs), a thin-film light-emitting device, in order to maximize the luminous power radiated by these devices. A careful analysis of the spectral and angular distribution of the emission of LECs fabricated under the same experimental conditions allows describing the dynamics of the spatial region from which LECs emit, i.e. the generation zone, as bias is applied. This effect is mediated by dipole reorientation within such an emissive region and its optical environment, since its spatial drift yields a different interplay between the intrinsic emission of the emitters and the local density of optical states of the system. Our results demonstrate that engineering the optical environment in thin-film light-emitting devices is key to maximize their brightness