Optimization of performance and long-term stability of p-type dye-sensitized solar cells with a cycloruthenated dye through electrolyte solvent tuning

p-Type dye sensitized solar cells (DSCs) have been assembled using the structurally unrefined, zwitterionic cyclometallated ruthenium dye [Ru(bpy) 2 (H 1 )] (H 3 1 = (4-(2-phenylpyridin-4-yl)phenyl)phosphonic acid) in combination with FTO/NiO photocathodes and an I 3 – /I – /acetonitrile (AN) electrolyte. Values of the short-circuit current density ( J SC ) = 4.06 mA cm –2 , open-circuit voltage ( V OC ) = 95 mV and photoconversion efficiency ( η ) = 0.139% are the highest achieved for state-of-the-art cyclometallated ruthenium dyes in p-type DSCs; data are confirmed using duplicate devices. J SC values are higher than those observed for the standard dye P1, and electrochemical impedance spectroscopy (EIS) shows that DSCs with [Ru(bpy) 2 (H 1 )] exhibit a both a lower transport resistance ( R t ) and recombination resistance ( R rec ) than DSCs with P1. Changing the electrolyte solvent from AN to propionitrile (PN), valeronitrile (VN), 3-methoxypropionitrile (MPN) or N -methylpyrrolidone (NMP) confirms a dependence of J SC on solvent in the order AN > PN > MPN > VN > NMP, whilst V OC follows the trend VN > PN > MPN > AN > NMP. The opposing trends in J SC and V OC lead to only a small drop in the overall η values for PN versus AN. EIS measurements revealed that although PN has a higher resistance to recombination reactions than AN, it also exhibits an increased amount of trapped charge carriers, leading to worsened DSC performance. DSCs based on AN do not remain stable over a 1-2 month period; both J SC and V OC decrease significantly. However, DSCs with the less volatile and more viscous PN show enhanced performance upon ageing with a gain in J SC over the first 33 days. Electrolytes with mixed solvents were investigated; addition of PN, VN or MPN to AN leads to lower J SC and this is most pronounced for VN and least marked for PN. The optimal solvent is an AN:PN mixture with volume ratios of 3:1 or 1:1; this mixed solvent results in enhanced long-term stability with respect to DSCs with pure AN and this is at the expense of a only small decrease in photoconversion efficienc

Anchoring of a dye precursor on NiO(001) studied by non-contact atomic force microscopy

The properties of metal oxides, such as charge transport mechanisms or optoelectronic characteristics, can be modified by functionalization with organic molecules. This kind of organic/inorganic surface is nowadays highly regarded, in particular, for the design of hybrid devices such as dye sensitized solar cells. However, a key parameter for optimized interfaces is not only the choice of the  compounds but also the properties of adsorption. Here, we investigated the deposition of an organic dye precursor molecule on a NiO(001) single crystal surface by means of non-contact atomic force microscopy at room temperature. Depending on the coverage, single molecules, groups of  adsorbates with random or recognizable shapes, or islands of closely packed molecules were identified. Single molecules and self assemblies are resolved with sub-molecular resolution showing that they are lying flat on the surface in a trans-conformation. Within the limits of our Kelvin probe microscopy setup a charge transfer from NiO to the molecular layer of 0.3 electrons per molecules was observed only in the areas, where the molecules are closed packed

Refining the anchor: optimizing the performance of cyclometallated ruthenium(II) dyes in p-type dye sensitized solar cells

A comparison of the performances of p-type dye sensitized solar cells (DSCs) sensitized by three cyclometallated ruthenium(II) dyes differing in their anchoring domains is presented. The dyes are [Ru(bpy) 2 (H 1 )] (H 3 1 = (4-(2-phenylpyridin-4-yl)phenyl)phosphonic acid) and the salts [ n Bu 4 N][Ru(bpy) 2 ( 1 )] and [Ru(bpy) 2 (H 2 )][PF 6 ] (H 2 2 = (4-(2-phenylpyridin-4-yl)phenyl)carboxylic acid). DSCs were fabricated with FTO/NiO working electrodes and either an I 2 /I 3 – /MeCN or I 2 /I 3 – /MeCN:EtCN (3:1 by volume) electrolyte. The results confirm the higher performance of dyes with a phosphonate versus carboxylic acid anchor, and reveal that [Ru(bpy) 2 (H 1 )] ( J SC = 3.24 mA cm –2 and η = 0.116%) performs better than [Ru(bpy) 2 ( 1 )] – . Electrochemical impedance spectroscopy (EIS) shows that a DSC with [Ru(bpy) 2 (H 1 )] offers the lowest transport and recombination resistances and the shortest hole lifetime and diffusion lengt

Modular synthesis of simple cycloruthenated complexes with state-of-the-art performance in p-type DSCs

A modular approach based on Suzuki-Miyaura cross coupling and Miyaura borylation has been used to prepare two cyclometallated [Ru(N^N) 2 (C^N)] + complexes which possess either a carboxylic or phosphonic acid group attached via a phenylene spacer to the 4-position of the pyridine ring in the C^N ligand. The key intermediate in the synthetic pathway is [Ru(bpy) 2 ( 1 )] + where bpy = 2,2'-bipyridine and H 1 is 4-chloro-2-phenylpyridine. The crystal structure of [Ru(bpy) 2 ( 1 )][PF 6 ] is presented. Reaction of [Ru(bpy) 2 ( 1 )][PF 6 ] with 4-carboxyphenylboronic acid leads to [Ru(bpy) 2 (H 6 )][PF 6 ], while the phosphonic acid analogue is isolated as the zwitterion [Ru(bpy) 2 (H 5 )]. The cyclometallated complexes have been characterized by mass spectrometry, multinuclear NMR spectroscopy, absorption spectroscopy and electrochemistry. [Ru(bpy) 2 ( 5 )] adsorbs onto NiO FTO/NiO electrodes (confirmed by solid-state absorption spectroscopy) and its performance in p-type dye-sensitized solar cells (DSCs) has been compared to that of the standard dye P1; two-screen printed layers of NiO give better DSC performances than one layer. Duplicate DSCs containing [Ru(bpy) 2 (H 5 )] achieve short-circuit current densities ( J SC ) of 3.38 and 3.34 mA cm –2 and photoconversion efficiencies ( η ) of 0.116 and 0.109%, respectively, compared to values of J SC = 1.84 and 1.96 mA cm –2 and η = 0.057 and 0.051% for P1. Despite its simple dye structure, the performance of [Ru(bpy) 2 (H 5 )] parallels the best-performing cyclometallated ruthenium(II) dye in p-type DSCs reported previously (He et al , J. Phys. Chem. C , 2014, 118 , 16518) and confirms the effectiveness of a phosphonic acid anchor in the dye and the attachment of the anchoring unit to the pyridine (rather than phenyl) ring of the cyclometallating ligan

Comparing a porphyrin- and a coumarin-based dye adsorbed on NiO(001)

Properties of metal oxides, such as optical absorption, can be influenced through the sensitization with molecular species that absorb visible light. Molecular/solid interfaces of this kind are particularly suited for the development and design of emerging hybrid technologies such as dye-sensitized solar cells. A key optimization parameter for such devices is the choice of the compounds in order to control the direction and the intensity of charge transfer across the interface. Here, the deposition of two different molecular dyes, porphyrin and coumarin, as single-layered islands on a NiO(001) single crystal surface have been studied by means of non-contact atomic force microscopy at room temperature. Comparison of both island types reveals different adsorption and packing of each dye, as well as an opposite charge-transfer direction, which has been quantified by Kelvin probe force microscopy measurements

Dye Sensitized Solar Cells – An Investigation of p-type DSSCs

Two of the major problems that our world is facing is the energy crisis and the rising of the global temperature. Scientists are called to investigate and assess the problem and the results are very alarming. The fossil fuels used until now for the majority of our energy demands would face a shortage in the foreseeable future. In addition, the use of fossil fuels has a major contribution to the rising of the global warming. The solution may be hidden in the sky during a cloudy day, the sun. The sun can produce in 1.5 hours the energy needed to power the global activities for a year. Among the renewable energies, solar energy is the most abundant, and the technology to harvest this energy already exists in the form of solar cells. There are many types of solar cells, and their assessment is done when taking under consideration their cost of production and the efficiency with which they can produce electricity. Dye sensitized solar cells (DSSCs), fulfil the requirements concerning low cost fabrication and green disposal but on the other hand their efficiency lies below the commonly used silicon solar cells, even though their theoretical efficiency could overcome the efficiency of the silicon cells used today. Moreover, dye sensitized solar cells have a good performance in indoor application under low light intensities, in contrast to the silicon solar cells. All facts considered DSSCs are excellent candidates for research since they answer the environmental problems of our society. In this thesis p-type DSSCs were investigated. These could replace the platinum electrode in n-type DSSCs used today a produce tandem DSSCs

A Phosphonic Acid Anchoring Analogue of the Sensitizer P1 for p-Type Dye-Sensitized Solar Cells

We report the synthesis and characterization of the first example of an organic dye, PP1, for p-type dye-sensitized solar cells (DSCs) bearing a phosphonic acid anchoring group. PP1 is structurally related to the benchmarking dye, P1, which possesses a carboxylic acid anchor. The solution absorption spectra of PP1 and P1 are similar (PP1 has λmax = 478 nm and εmax = 62,800 dm3 mol−1 cm−1), as are the solid-state absorption spectra of the dyes adsorbed on FTO/NiO electrodes. p-Type DSCs with NiO as semiconductor and sensitized with P1 or PP1 perform comparably. For PP1, short-circuit current densities (JSC) and open-circuit voltages (VOC) for five DSCs lie between 1.11 and 1.45 mA cm−2, and 119 and 143 mV, respectively, compared to ranges of 1.55–1.80 mA cm−2 and 117–130 mV for P1. Photoconversion efficiencies with PP1 are in the range 0.054–0.069%, compared to 0.065–0.079% for P1. Electrochemical impedance spectroscopy, open-circuit photovoltage decay and intensity-modulated photocurrent spectroscopy have been used to compare DSCs with P1 and PP1 in detail

Transoid-to-Cisoid Conformation Changes of Single Molecules on Surfaces Triggered by Metal Coordination

Conformational isomers are stereoisomers that can interconvert over low potential barriers by rotation around a single bond. However, such bond rotation is hampered by geometrical constraints when molecules are adsorbed on surfaces. Here we show that the adsorption of 4,40-bis(4-carboxyphenyl)-6,60-dimethyl-2,20-bipyridine molecules on surfaces leads to the appearance of pro-chiral single-molecules on NiO(001) and to enantiopure supramolecular domains on Au(111) surfaces containing the transoid molecule conformation. Upon additional Fe adatom deposition, molecules undergo a controlled interconversion from a transoid to cisoid conformation as a result of coordination of the Fe atoms to the 2,20-bipyridine moieties. As confirmed by atomic force microscopy 1 images and X-ray photoelectron spectroscopy measurements, the resulting molecular structures become irreversibly achiral

The impact of frailty on ICU and 30-day mortality and the level of care in very elderly patients (≥ 80 years)

Purpose: Very old critical ill patients are a rapid expanding group in the ICU. Indications for admission, triage criteria and level of care are frequently discussed for such patients. However, most relevant outcome studies in this group frequently find an increased mortality and a reduced quality of life in survivors. The main objective was to study the impact of frailty compared with other variables with regards to short-term outcome in the very old ICU population. Methods: A transnational prospective cohort study from October 2016 to May 2017 with 30 days follow-up was set up by the European Society of Intensive Care Medicine. In total 311 ICUs from 21 European countries participated. The ICUs included the first consecutive 20 very old (≥ 80 years) patients admitted to the ICU within a 3-month inclusion period. Frailty, SOFA score and therapeutic procedures were registered, in addition to limitations of care. For measurement of frailty the Clinical Frailty Scale was used at ICU admission. The main outcomes were ICU and 30-day mortality and survival at 30 days. Results: A total of 5021 patients with a median age of 84 years (IQR 81–86 years) were included in the final analysis, 2404 (47.9%) were women. Admission was classified as acute in 4215 (83.9%) of the patients. Overall ICU and 30-day mortality rates were 22.1% and 32.6%. During ICU stay 23.8% of the patients did not receive specific ICU procedures: ventilation, vasoactive drugs or renal replacement therapy. Frailty (values ≥ 5) was found in 43.1% and was independently related to 30-day survival (HR 1.54; 95% CI 1.38–1.73) for frail versus non-frail. Conclusions: Among very old patients (≥ 80 years) admitted to the ICU, the consecutive classes in Clinical Frailty Scale were inversely associated with short-term survival. The scale had a very low number of missing data. These findings provide support to add frailty to the clinical assessment in this patient group. Trial registration: ClinicalTrials.gov (ID: NCT03134807)

Sepsis at ICU admission does not decrease 30-day survival in very old patients: a post-hoc analysis of the VIP1 multinational cohort study

Background: The number of intensive care patients aged ≥ 80 years (Very old Intensive Care Patients; VIPs) is growing. VIPs have high mortality and morbidity and the benefits of ICU admission are frequently questioned. Sepsis incidence has risen in recent years and identification of outcomes is of considerable public importance. We aimed to determine whether VIPs admitted for sepsis had different outcomes than those admitted for other acute reasons and identify potential prognostic factors for 30-day survival. Results: This prospective study included VIPs with Sequential Organ Failure Assessment (SOFA) scores ≥ 2 acutely admitted to 307 ICUs in 21 European countries. Of 3869 acutely admitted VIPs, 493 (12.7%) [53.8% male, median age 83 (81-86) years] were admitted for sepsis. Sepsis was defined according to clinical criteria; suspected or demonstrated focus of infection and SOFA score ≥ 2 points. Compared to VIPs admitted for other acute reasons, VIPs admitted for sepsis were younger, had a higher SOFA score (9 vs. 7, p < 0.0001), required more vasoactive drugs [82.2% vs. 55.1%, p < 0.0001] and renal replacement therapies [17.4% vs. 9.9%; p < 0.0001], and had more life-sustaining treatment limitations [37.3% vs. 32.1%; p = 0.02]. Frailty was similar in both groups. Unadjusted 30-day survival was not significantly different between the two groups. After adjustment for age, gender, frailty, and SOFA score, sepsis had no impact on 30-day survival [HR 0.99 (95% CI 0.86-1.15), p = 0.917]. Inverse-probability weight (IPW)-adjusted survival curves for the first 30 days after ICU admission were similar for acute septic and non-septic patients [HR: 1.00 (95% CI 0.87-1.17), p = 0.95]. A matched-pair analysis in which patients with sepsis were matched with two control patients of the same gender with the same age, SOFA score, and level of frailty was also performed. A Cox proportional hazard regression model stratified on the matched pairs showed that 30-day survival was similar in both groups [57.2% (95% CI 52.7-60.7) vs. 57.1% (95% CI 53.7-60.1), p = 0.85]. Conclusions: After adjusting for organ dysfunction, sepsis at admission was not independently associated with decreased 30-day survival in this multinational study of 3869 VIPs. Age, frailty, and SOFA score were independently associated with survival