Unconventional materials for light-emitting and photovoltaic applications

La motivación de este trabajo es el problema energético y éste ha sido abordado de dos maneras en esta tesis doctoral: promover un aumento de producción de energía mediante energías renovables junto con una reducción del consumo energético mediante el uso de sistemas más eficientes. Para este propósito, en esta tesis se han desarrollado dispositivos electroluminiscentes y fotovoltaicos novedosos de bajo coste y eficientes. En concreto, el trabajo se ha focalizado en el desarrollo de células electroquímicas emisoras de luz (LECs) emisoras en el rojo e infrarrojo cercano y en la mejora de su estabilidad. Así como en la fabricación de células solares de perovskita de alta eficiencia. Los dispositivos LECs han sido fabricados con complejos de metales de transición iónicos basados en el complejo [Ir(ppy)2(btzpy)][PF6], siendo (ppy = 2-fenilpiridinato y btzpy = 2-(piridin-2-yl)benzo[d]tiazol), manteniendo el ligando ppy constante y variando la funcionalización del ligando btzpy o el propio ligando btzpy. Los dispositivos preparados con la serie de complejos presentaron emisión electroluminiscente desde el rojo al infrarrojo cercano. A pesar de los moderados valores de luminancia máxima obtenidos, los dispositivos presentaron una extremada alta estabilidad con tiempos de vida medio entre 1000 y 6000 h, siendo los dispositivos LECs emisores en el rojo más estables publicados hasta la fecha. Los valores de EQEs obtenidos fueron moderados, pero considerables a pesar de los bajos valores de PLQY y las altas densidades de corriente pulsada empleadas. Además, se ha demostrado en este trabajo la modificación de los niveles de luminancia junto con una reducción del tiempo de encendido y sin pérdida de estabilidad cuando a los dispositivos fueron aplicados densidades de corriente muy altas. Para el desarrollo de células solares basadas en perovskita de alta eficiencia se realizaron dos acciones: estudio del efecto del espesor de la capa absorbente de luz en el rendimiento de la célula solar y la influencia de las capas transportadoras de carga en células solares tipo p-i-n y n-i-p. Estos trabajos pudieron ser llevados a cabo debido al preciso control del espesor de las capas mediante su deposición por la técnica de evaporación a vacío. En primer lugar se fabricaron una serie de capas de MAPbI3 a vacío con espesores entre 210 y 900 nm y posteriormente se implementaron en dispositivos tipo p-i-n. La fotocorriente generada se vio mejorada al aumentar el espesor del material absorbente de luz, pero la extracción de la carga se vio afectada, probablemente debido a la baja conductividad de la capa transportadora de huecos. La oxidación parcial de esta capa en una célula solar con capa absorbente de luz muy gruesa (900 nm) mejoró la extracción de las cargas viéndose el rendimiento de la célula solar recuperado, obteniendo valores similares al dispositivo más eficiente de la serie con capa absorbente de luz más fina. En este trabajo se demostró que con capas orgánicas no limitantes, el rendimiento fotovoltaico de células solares basadas en perovskita depositadas a vacío es independiente del espesor de la capa absorbente MAPbI3. Este trabajo también demostró que la longitud de difusión de carga en células solares cuya capa MAPbI3 ha sido evaporada a vacío es larga. Para mejorar la extracción de la carga en los dispositivos, se fabricaron células solares basadas en perovskita tipo p-i-n y n-i-p, cuya capa absorbente de luz (MAPbI3) fue depositada entre capas intrínsecas y dopadas. En este estudio, se realizó una optimización de la concentración de dopante en las capas dopadas y se investigó la función e influencia de las capas intrínsecas y dopadas en el rendimiento de las células solares para la obtención de muy alta eficiencia. Se observó que la presencia de ambas capas es necesaria para tan altas eficiencias. Una vez optimizado el dispositivo, las células solares presentaron eficiencias de 16.5% (p-i-n) y 20% (n-i-p) sin histéresis. Estos valores son los más altos publicados en células solares basadas en perovskita depositada por evaporación en vacío y cuyo material absorbente de luz es MAPbI3.The motivation of this thesis is to reduce the energy consumption to generate illumination and the amount of fuel fossil employed in the generation of energy. For this purpose, novel, efficient and low-cost electroluminescent and photovoltaic devices need to be developed. The work was focused on the development of red- and near-infrared LECs and the improvement of the device stability. The iTMCs studied were based on the [Ir(ppy)2(btzpy)][PF6] complex (ppy = 2-phenylpyridinate and btzpy = 2-(pyridin-2-yl)benzo[d]thiazole) and all of the complexes showed red- and near-infrared photoluminescence in the solid-state with moderate PLQYs values (<18%). The LECs prepared with the complexes also exhibit red to near-infrared electroluminescence. Although the maximum luminance values were moderate, they exhibited extremely high device stability with lifetimes in the range of 1000–6000 h, being the most stable red-emitting LECs reported up to date. The EQEs obtained were moderate (EQE<2%), however, these values were impressive in view of its low PLQY values and the high current density applied. Moreover, the possibility of tuning the luminance levels was demonstrated, having a fast response with almost no loss in device stability, maintaining its impressive characteristics by increasing the average current density. An in-depth study of the photovoltaic efficiency by increasing the photocurrent obtained through modification of the perovskite thickness was performed. A series of vacuum-deposited MAPbI3 layers with layer thicknesses ranging from 210 to 900 nm were fabricated and implemented into p-i-n devices. The JSC was enhanced when the perovskite thickness was increased but the FF and hence the PCE was reduced due to the low mobility of the polyTPD layer. The partial oxidation of the polyTPD layer increases its conductivity and the device recovers the FF reaching a PCE of 12.7%, being the same high efficiency than the most efficient device of the series with thinner perovskite films (12%). In this work was demonstrated that with non-limiting organic layers, the PV performance of vacuum-deposited perovskite solar cells is independent on the perovskite layer thickness and that the charge carrier diffusion length is not limiting in the devices. Fully vacuum-deposited p-i-n and n-i-p perovskite solar cells, employing a MAPbI3 perovskite layer deposited between an intrinsic and doped n- or p- type organic charge transport layers, respectively, were fabricated. The optimization of the dopant concentration was carried out and it was found that the presence of the undoped and doped charge transport layers were required for highly efficient solar cells. The optimized solar cells lead to hysteresis-free and very high efficiencies exceeding 16.5% (p-i-n) and 20% (n-i-p), the highest efficiencies reported for vacuum-deposited perovskite and for MAPbI3-based solar cells

High efficiency single-junction semitransparent perovskite solar cells

Semitransparent perovskite solar cells with a high power conversion efficiency (PCE) above 6% and 30% full device transparency have been achieved by implementing a thin perovskite layer and a simple foil compatible layout

Vacuum Deposited Triple-Cation Mixed-Halide Perovskite Solar Cells

Hybrid lead halide perovskites are promising materials for future photovoltaics applications. Their spectral response can be readily tuned by controlling the halide composition, while their stability is strongly dependent on the film morphology and on the type of organic cation used. Mixed cation and mixed halide systems have led to the most efficient and stable perovskite solar cells reported, so far they are prepared exclusively by solution-processing. This might be due to the technical difficulties associated with the vacuum deposition from multiple thermal sources, requiring a high level of control over the deposition rate of each precursor during the film formation. In this report, thermal vacuum deposition with multiple sources (3 and 4) is used to prepare for the first time, multications/anions perovskite compounds. These thin-film absorbers are implemented into fully vacuum deposited solar cells using doped organic semiconductors. A maximum power conversion efficiency of 16% is obtained, with promising device stability. The importance of the control over the film morphology is highlighted, which differs substantially when these compounds are vacuum processed. Avenues to improve the morphology and hence the performance of fully vacuum processed multications/anions perovskite solar cells are proposed

Modulating the electron transporting properties of Subphthalocyanines for inverted perovskite solar cells

The lack of organic non-fullerene ETMs with good electron transport and device stability is an important problem for the further development and commercialization of perovskite solar cells. Herein, the use of SubPcs as ETMs in PSCs is explored. To this end, we analyze the influence of SubPc peripheral functionalization on the efficiency and stability of p-i-n PSCs. Specifically, ETMs based on three SubPcs (with either six or twelve peripheral fluorine and chlorine atoms) have been incorporated into PSCs with the perovskite layer deposited by solution processing (CsFAMAPbIBr). The device performance and morphology of these devices are deeply analyzed using several techniques, and the interfacial effects induced by the SubPcs are studied using photoluminescence and TR-PL. It is observed that the device stability is significantly improved upon insertion the SubPc layer. Moreover, the impact of the SubPc layer-thickness is assessed. Thus, a maximum power conversion efficiency of 13.6% was achieved with the champion devic

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

Efficient vacuum deposited p-i-n and n-i-p perovskite solar cells employing doped charge transport layers

Methylammonium lead halide perovskites have emerged as high performance photovoltaic materials. Most of these solar cells are prepared via solution-processing and record efficiencies (>20%) have been obtained employing perovskites with mixed halides and organic cations on (mesoscopic) metal oxides. Here, we demonstrate fully vacuum deposited planar perovskite solar cells by depositing methylammonium lead iodide in between intrinsic and doped organic charge transport molecules. Two configurations, one inverted with respect to the other, p-i-n and n-i-p, are prepared and optimized leading to planar solar cells without hysteresis and very high efficiencies, 16.5% and 20%, respectively. It is the first time that a direct comparison between these two opposite device configurations has been reported. These fully vacuum deposited solar cells, employing doped organic charge transport layers, validate for the first time vacuum based processing as a real alternative for perovskite solar cell preparation

Synthesis, properties and Light-Emitting Electrochemical Cell (LEEC) device fabrication of cationic Ir(III) complexes bearing electron-withdrawing groups on the cyclometallating ligands

The authors are grateful to the European Research Council (grant 321305), the EPSRC (EP/M02105X/1) and the University of St Andrews for financial support. C.M. thanks Ministry of Economy and Competitiveness (MINECO, Spain) for her predoctoral contract.The structure-property relationship study of a series of cationic Ir(III) complexes in the form of [Ir(C^N)2(dtBubpy)]PF6 [where dtBubpy = 4,4′-ditert-butyl-2,2′- bipyridine and C^N = cyclometallating ligand bearing an electron-withdrawing group (EWG) at C4 of the phenyl substituent, i.e. -CF3 ( 1 ), -OCF3 ( 2 ), -SCF3 ( 3 ), -SO2CF3 ( 4 )] have been investigated. The physical and optoelectronic properties of the four complexes were comprehensively characterized, including by X-ray diffraction analysis. All the complexes exhibit quasi-reversible dtBubpy-based reductions from -1.29 V to -1.34 V (vs. SCE). The oxidation processes are likewise quasi-reversible (metal+C^N ligand) and are between 1.54- 1.72 V (vs. SCE). The relative oxidation potentials follow a general trend associated with the Hammett parameter (σ) of the EWGs. Surprisingly, complex 4 bearing the strongest EWG does not adhere to the expected Hammett behavior and was found to exhibit red-shifted absorption and emission maxima. Nevertheless, the concept of introducing EWGs was found to be generally useful in blue-shifting the emission maxima of the complexes (λem = 484-545 nm) compared to that of the prototype complex [Ir(ppy)2(dtBubpy)]PF6 (where ppy = 2- phenylpyridinato) (λem = 591 nm). The complexes were found to be bright emitters in solution at room temperature (ΦPL = 45-66%) with long excited-state lifetimes (τe = 1.14-4.28 μs). The photophysical properties along with Density Functional Theory (DFT) calculations suggest that the emission of these complexes originates from mixed contributions from ligand-centered (LC) transitions and mixed metal-to-ligand and ligand-to-ligand charge transfer (LLCT/MLCT) transitions, depending on the EWG. In complexes 1 , 3 and 4 the 3LC character is prominent over the mixed 3CT character while in complex 2 , the mixed 3CT character is much more pronounced, as demonstrated by DFT calculations and the observed positive solvatochromism effect. Due to the quasi-reversible nature of the oxidation and reduction waves, fabrication of light emitting electrochemical cells (LEECs) using these complexes as emitters was possible with the LEECs showing moderate efficiencies.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

Recombination in Perovskite Solar Cells:Significance of Grain Boundaries, Interface Traps, and Defect Ions

Trap-assisted recombination, despite being lower as compared with traditional inorganic solar cells, is still the dominant recombination mechanism in perovskite solar cells (PSCs) and limits their efficiency. We investigate the attributes of the primary trap-assisted recombination channels (grain boundaries and interfaces) and their correlation to defect ions in PSCs. We achieve this by using a validated device model to fit the simulations to the experimental data of efficient vacuum-deposited p–i–n and n–i–p CH₃NH₃PbI₃ solar cells, including the light intensity dependence of the open-circuit voltage and fill factor. We find that, despite the presence of traps at interfaces and grain boundaries (GBs), their neutral (when filled with photogenerated charges) disposition along with the long-lived nature of holes leads to the high performance of PSCs. The sign of the traps (when filled) is of little importance in efficient solar cells with compact morphologies (fused GBs, low trap density). On the other hand, solar cells with noncompact morphologies (open GBs, high trap density) are sensitive to the sign of the traps and hence to the cell preparation methods. Even in the presence of traps at GBs, trap-assisted recombination at interfaces (between the transport layers and the perovskite) is the dominant loss mechanism. We find a direct correlation between the density of traps, the density of mobile ionic defects, and the degree of hysteresis observed in the current–voltage (<i>J</i>–<i>V</i>) characteristics. The presence of defect states or mobile ions not only limits the device performance but also plays a role in the <i>J</i>–<i>V</i> hysteresis

Effectiveness of a new one-hour blood pressure monitoring method to diagnose hypertension: a diagnostic accuracy clinical trial protocol

INTRODUCTION: 24-hour ambulatory blood pressure monitoring (ABPM) is the gold standard diagnostic method for hypertension, but has some shortcomings in clinical practice while clinical settings often lack sufficient devices to accommodate all patients with suspected hypertension. Home blood pressure monitoring (HBPM) and office blood pressure monitoring (OBPM) also have shortcomings, such as the white coat effect or a lack of accuracy. This study aims to study the validity of a new method of diagnosing hypertension consisting of monitoring blood pressure (BP) for 1 hour and comparing it with OBPM and HBPM and examining the sensitivity and specificity of this method compared with 24-hour ABPM. The patient experience will be examined in each method. METHODS AND ANALYSIS: A minimum sample of 214 patients requiring a diagnostic test for hypertension from three urban primary healthcare centres will be included. Participants will undergo 24-hour ABPM, 1-hour BP measurement (1-BPM), OBPM for three consecutive weeks and HBPM. Patients will follow a random sequence to first receive 24-hour ABPM or 1-hour ABPM. Daytime 24-hour ABPM records will be compared with the other monitoring methods using the correlation coefficient and Bland Altman plots. The kappa concordance index and the sensitivity and specificity of the methods will be calculated. The patient's experience will be studied, with selected indicators of efficiency and satisfaction calculated using parametric tests. ETHICS AND DISSEMINATION: The protocol has been authorised by the research ethics committee of the Hospital Clinic of Barcelona (Ref. HCB/2014/0615): protocol details and amendments will be recorded and reported to ClinicalTrials.com. The results will be disseminated in peer-reviewed literature, and to policy makers and healthcare partners. TRIAL REGISTRATION: NCT03147573; Pre-results