Shining a light on the photochemistry of methylammonium lead iodide perovskite for solar cell application

Since their first use in solar cells in 2009, lead halide perovskites have been employed in a wide variety of solar cell architectures leading to an impressive 25.5 % power conversion efficiency in 2020. However, the complex properties of perovskite materials as well as their instability in ambient atmosphere is impeding their development. In particular, trap states, through their formation and passivation, are believed to be responsible for the instability and subsequent degradation of the perovskite, as well as decreasing the device performances. In this work, 3 aspects of the methylammonium lead iodide (MAPI) perovskite are investigated. First, the steady-state photoluminescence and degradation of MAPI were investigated using fluorescence spectroscopy, microscopy, and X-ray diffractometry. Photobrightening, due to the passivation of trap states, and photodarkening, due to the degradation of the perovskite by moisture and oxygen, are found to be competing processes, making the onset of degradation difficult to accurately determine. Second, two models used to simulate charge carrier recombination and time-resolved photoluminescence (TRPL) in perovskite materials, and extract information related to trap states are investigated. The importance of the excitation fluence and repetition period used is highlighted. The models are then used to fit TRPL decays of a MAPI thin-film and a bimolecular rate constant of ∼ 50 × 10−20 cm3/ns is obtained. Finally, the optical, morphological, photoluminescence and crystallographic properties, and stability of MAPI and 5-aminovaleric acid iodide-modified MAPI (AVA-MAPI) infiltrated in mesoporous carbon perovskite solar cells are investigated. Superoxide formation was measured using the dihydroethidium fluorescence probe and AVA-MAPI is shown to generate less superoxide than MAPI when infiltrated in mesoporous layers (ZrO2, TiO2/ZrO2 and TiO2/ZrO2/C). This is partially explained by the presence of a uniform capping layer in AVA-MAPI, hindering oxygen diffusion in the film hence reducing superoxide formation

PEARS: A Web Tool for Fitting Time-Resolved Photoluminescence Decays of Perovskite Materials

Time-resolved photoluminescence (TRPL) is a powerful tool to investigate charge carrier recombination processes in emissive materials. Perovskite materials are extremely promising for applications in solar cells; however, the interpretation of their TRPL is arduous due to the complicated nature of the recombination processes occurring in these materials. We present here the PErovskite cArrier Recombination Simulator (PEARS) web tool for effortlessly and quickly fitting TRPL of perovskite materials using advanced charge carrier recombination models, allowing for the extraction of recombination rate constants and trap state concentration. PEARS is flexible and can adapt to different situations, by ignoring recombination processes or fixing known parameters (e.g., the doping concentration). The tool is publicly available at https://pears-tool.herokuapp.com

Visible-light induced photocatalytic degradation of estrone (E1) with hexagonal copper selenide nanoflakes in water

Steroid hormones, being potent endocrine-disruptors, are a menace to human health and aquatic life. Herein, visible-light induced photocatalytic degradation of estrone (E1) by hexagonal copper selenide (CuSe) nanoflakes has been reported. CuSe was synthesised by a facile and low-temperature (100 oC) co-precipitation method and was characterised. The nanocrystals were of stoichiometric Cu:Se ratio with Se2- and Cu in the + 1/+ 2 mixed-valence state and exhibited laminar, flake-like morphology with a preferred hexagonal close-packed structure (P63/mmc) having average particle size and thickness of 0.229 ± 0.146 µm and 0.05 ± 0.02 µm, respectively. The adsorption isotherms of E1 were linear and the adsorption process was exothermic. The reactivity of E1 under aqueous suspensions of CuSe exposed to visible light exhibited pseudo-first-order kinetics with a rate constant, k, that varied with initial E1 concentration, light power, catalyst dose, and pH. Particularly, k was almost constant over the range pH5–9 but substantially increased as pH rose to 11, while light power and catalyst dose increased k up to a maximum, and the initial concentration reduced k. Surprisingly, CuSe oxidised E1, even in the absence of light, and leached species that were identified and their time-dependency was determined. We concluded that the disappearance of E1 by CuSe is attributed to synergetic effects of adsorption, oxidation by CuSe, and photocatalytic degradation. Supported by liquid-mass spectrometry analysis and molecular chemistry calculations, we also suggested a possible mechanism for E1 degradation. Thus, hexagonal CuSe nanocrystals can be a promising candidate for the treatment of endocrine-disrupting chemicals (EDC)-contaminated wastewaters

Investigating the Superoxide Formation and Stability in Mesoporous Carbon Perovskite Solar Cells with an Aminovaleric Acid Additive

Perovskite solar cells have attracted a great deal of attention thanks to their high efficiency, ease of manufacturing, and potential low cost. However, the stability of these devices is considered their main drawback and needs to be addressed. Mesoporous carbon perovskite solar cells (m-CPSC), consisting of three mesoporous layers (TiO2/ZrO2/C) infiltrated with CH3NH3PbI3 (MAPI) perovskite, have presented excellent lifetimes of more than 10 000 h when the additive NH2(CH2)4CO2HI (5- aminovaleric acid iodide; 5-AVAI) is used to modify the perovskite structure. Yet, the role of 5-AVAI in enhancing the stability has yet to be determined. Here, superoxide-mediated degradation of MAPI m-CPSC with and without the 5-AVAI additive is studied using the fluorescence probe dihydroeth-idium for superoxide detection. In situ X-ray diffractometry shows that amino valeric acid methylammonium lead iodide (AVA-MAPI) perovskite infiltrated in mesoporous layers presents higher stability in an ambient environment under illumination, evidenced by a slower decrease of the MAPI/PbI2 peak ratio. Superoxide yield measurements demonstrate that AVA-MAPI generates more superoxide than regular MAPI when deposited on glass but generates significantly less when infiltrated in mesoporous layers. It is believed that superoxide formation in m-CPSC is dependent on a combination of competitive factors including oxygen diffusion, sample morphology, grain size, and defect concentration

Sustainable solvent selection for the manufacture of methylammonium lead triiodide (MAPbI3) perovskite solar cells

Perovskite solar cells have emerged as a promising and highly efficient solar technology. Despite efficiencies continuing to climb, the prospect of industrial manufacture is hampered by concerns regarding the safety and sustainability of the solvents used in lab scale manufacture. In this paper, we aim to present a methodology for green solvent selection informed by EHS considerations from the Chem-21 guide for succesful methylammonium lead triiodide (MAPbI3) precursor dissolution. Through the use of this methodology we present a N,N-dimethylformamide (DMF)-free alternative solvent system for deposition of MAPbI3 precursors (MAI and PbI2) consisting of dimethylsulfoxide (DMSO), dimethylpropyleneurea (DMPU), 2-methyltetrahydrofuran (2-MeTHF) and ethanol (EtOH). We have investigated 3 candidate solutions with slightly different compositions of these four solvents, all of which produce dense, uniform and pinhole-free perovskite films via spin coating. All three candidate solutions (A-C) match the average device efficiencies of the DMF/DMSO contol devices (12.4%) with Candidate A, which consists of 40% DMSO, 30 % DMPU, 20% 2-MeTHF and 10% EtOH (vol%), producing a champion PCE of 16.1% compared to 16.2% for DMF/DMSO (80/20 vol%). Perovskite films cast from the three candidate solutions show improved crystallinity, higher flourescence emission, and improved crystal size uniformity than those cast from DMF/DMSO. This work aims to: highlight the key solvent parameters which determine effective MAPbI3 precursor dissolution; provide a set of criteria for appropriate alternative solvent selection; and demonstrate the application of green chemistry principles to solvent selection for perovskite photovoltaic manufacturing

Role of the GB-France electricity interconnectors in integration of variable renewable generation

10.1136/injuryprev-2015-04188

Des modèles pour les sciences du végétal et à vocation agronomique

La compréhension des mécanismes génétiques, moléculaires et physiologiques sous-tendant les grandes fonctions biologiques et les caractères d’intérêt agronomique chez les plantes à fleur représente un enjeu considérable. Les efforts se concentrent sur un nombre réduit de modèles présentant des caractéristiques favorables et réunissant une masse critique de chercheurs et de moyens. Arabidopsis thaliana, le riz et Medicago truncatula sont ces modèles

Models for vegetal sciences and agronomic vocation

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