Understanding morphology and photo- stability of organic solar cells via advanced structural probes

The development of organic solar cells as a novel form of renewable energy has been driven by their potential for low-cost, large-scale fabrication though the solution-processing of semiconducting polymers and small molecules. Certified power conversion efficiencies have reached 13% as of 2016 thanks to the development of new donor-acceptor molecules, but the efficiency of any given device is still highly sensitive to the morphology that these materials adopt during deposition. It is essential that morphology is characterized thoroughly in order to establish the relationships between molecular structure, morphological properties and device performance; in order to maximise efficiency and make organic solar cells an economically competitive source of renewable energy. In this thesis, several spectroscopic techniques are used to probe the impact of various processing parameters on the molecular order, crystallinity and phase separation of polymer:fullerene blends. For the model blend system P3HT:PCBM, P3HT molecular order can be measured by resonant Raman spectroscopy, and PCBM is found to dissolve in the amorphous domains of the semi-crystalline polymer up to a miscibility limit of 25 %wt, above which it can only be accommodated by increased disorder. In situ annealing demonstrates that when heated above a glass transition temperature of ~50C, disordered blends separate into purer domains of high molecular order that correlate well to improved charge transport and efficiency for thermally annealed devices. Raman spectroscopy is also used to probe the stability of the high-efficiency PTB7:PC70BM blend. Photo-oxidation of PTB7 was found to induce specific vibrational changes that correlate to formation of a hydroxyl group on the benzodithiophene unit. In situ experiments reveal that hydroxylation precedes the loss of chromophores that results in deterioration of device performance, and is accelerated by blending with PC70BM. Understanding the impact of morphology on charge extraction from the active layer requires the selective probing of interfacial properties at the top and bottom of the organic film, which we demonstrate using SERS. For both a polymer:fullerene blend (PTB7:PC70BM) and a polymer:polymer blend (P3HT:F8TBT), spin-coated films exhibit interfacial compositions different from that of the bulk film and favourable to charge extraction from inverted device architectures, but can be modified by pre- or post-annealing treatments. Finally, we investigated the morphology of a novel low band-gap polymer, a tellurium analogue of polythiophene, in order to understand the impact of the heavy atom on chain planarity and polymer crystallinity. The Raman spectrum of P3ATe exhibited a much stronger sensitivity to molecular order, which was highly dependent on the length and linearity of the alkyl side chain, but there was no clear morphological reason why P3ATe reportedly performs poorly compared to P3HT, despite the superior absorption of its smaller band-gap.Open Acces

Detection and Degradation of Adenosine Monophosphate in Perchlorate-Spiked Martian Regolith Analogue, by Deep-Ultraviolet Spectroscopy

The search for organic biosignatures on Mars will depend on finding material protected from the destructive ambient radiation. Solar ultraviolet can induce photochemical degradation of organic compounds, but certain clays have been shown to preserve organic material. We examine how the SHERLOC instrument on the upcoming Mars 2020 mission will use deep-ultraviolet (UV) (248.6 nm) Raman and fluorescence spectroscopy to detect a plausible biosignature of adenosine 5′-monophosphate (AMP) adsorbed onto Ca-montmorillonite clay. We found that the spectral signature of AMP is not altered by adsorption in the clay matrix but does change with prolonged exposure to the UV laser over dosages equivalent to 0.2–6 sols of ambient martian UV. For pure AMP, UV exposure leads to breaking of the aromatic adenine unit, but in the presence of clay the degradation is limited to minor alteration with new Raman peaks and increased fluorescence consistent with formation of 2-hydroxyadenosine, while 1 wt % Mg perchlorate increases the rate of degradation. Our results confirm that clays are effective preservers of organic material and should be considered high-value targets, but that pristine biosignatures may be altered within 1 sol of martian UV exposure, with implications for Mars 2020 science operations and sample caching

The Cell and the Sum of Its Parts: Patterns of Complexity in Biosignatures as Revealed by Deep UV Raman Spectroscopy

The next NASA-led Mars mission (Mars 2020) will carry a suite of instrumentation dedicated to investigating Martian history and the in situ detection of potential biosignatures. SHERLOC, a deep UV Raman/Fluorescence spectrometer has the ability to detect and map the distribution of many organic compounds, including the aromatic molecules that are fundamental building blocks of life on Earth, at concentrations down to 1 ppm. The mere presence of organic compounds is not a biosignature: there is widespread distribution of reduced organic molecules in the Solar System. Life utilizes a select few of these molecules creating conspicuous enrichments of specific molecules that deviate from the distribution expected from purely abiotic processes. The detection of far from equilibrium concentrations of a specific subset of organic molecules, such as those uniquely enriched by biological processes, would comprise a universal biosignature independent of specific terrestrial biochemistry. The detectability and suitability of a small subset of organic molecules to adequately describe a living system is explored using the bacterium Escherichia coli as a model organism. The DUV Raman spectra of E. coli cells are dominated by the vibrational modes of the nucleobases adenine, guanine, cytosine, and thymine, and the aromatic amino acids tyrosine, tryptophan, and phenylalanine. We demonstrate that not only does the deep ultraviolet (DUV) Raman spectrum of E. coli reflect a distinct concentration of specific organic molecules, but that a sufficient molecular complexity is required to deconvolute the cellular spectrum. Furthermore, a linear combination of the DUV resonant compounds is insufficient to fully describe the cellular spectrum. The residual in the cellular spectrum indicates that DUV Raman spectroscopy enables differentiating between the presence of biomolecules and the complex uniquely biological organization and arrangements of these molecules in living systems. This study demonstrates the ability of DUV Raman spectroscopy to interrogate a complex biological system represented in a living cell, and differentiate between organic detection and a series of Raman features that derive from the molecular complexity inherent to life constituting a biosignature

Aqueous alteration processes in Jezero crater, Mars—implications for organic geochemistry

The Perseverance rover landed in Jezero crater, Mars, in February 2021. We used the Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals (SHERLOC) instrument to perform deep-ultraviolet Raman and fluorescence spectroscopy of three rocks within the crater. We identify evidence for two distinct ancient aqueous environments at different times. Reactions with liquid water formed carbonates in an olivine-rich igneous rock. A sulfate-perchlorate mixture is present in the rocks, which probably formed by later modifications of the rocks by brine. Fluorescence signatures consistent with aromatic organic compounds occur throughout these rocks and are preserved in minerals related to both aqueous environments

Quantitative photography for rapid, reliable measurement of marine macro‐plastic pollution

Abstract Plastics are now ubiquitous in the environment and have been studied in wildlife and in ecosystems for more than 50 years. Measurement of size, shape and colour data for individual fragments of plastic is labour‐intensive, unreliable and prone to observer bias, particularly when it comes to assessment of colour, which relies on arbitrary and inconsistently defined colour categorisations. There is a clear need for a standard method for data collection on plastic pollution, particularly one that can be readily automated given the number of samples involved. This study describes a new method for standardised photography of marine plastics in the 1–100 mm size range (meso‐ and macro‐plastics), including colour correction to account for any image‐to‐image variation in lighting that may impact colour reproduction or apparent brightness. Automated image analysis is then applied to detect individual fragments of plastic for quantitative measurement of size, shape, and colour. The method was tested on 3793 fragments of debris ingested by Flesh‐footed Shearwaters (Ardenna carneipes) on Lord Howe Island, Australia, and compare results from photos taken in two separate locations using different equipment. Photos were acquired of up to 250 fragments at a time with a spatial resolution of 70 μm/pixel and were colour‐corrected using a reference chart to ensure accurate reproduction of colour. The automated image analysis pipeline was found to have a 98% success rate at detecting fragments, and the different size and shape parameters that can be outputted by the pipeline were compared in terms of usefulness. The evidence shown in this study should strongly encourage the uptake of this method for cataloguing macro‐scale plastic pollution, as it provides substantially higher quality data with accurate, reliable measurements of size, shape and colour for individual plastics that can be readily compared between disparate datasets.Marie Skłodowska-Curie Action PLASTISCAN MSCA Grant Number: 101030480Copyright© 2023 The Authors. Methods in Ecology and Evolution published by John Wiley & Sons Ltd on behalf of British Ecological Society. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.NHM Repositor

In-situ monitoring of molecular vibrations of two organic semiconductors in photovoltaic blends and their impact on thin film morphology

We report in-situ simultaneous monitoring of molecular vibrations of two components in organic photovoltaic blends using resonant Raman spectroscopy. Blend films were composed of a low bandgap copolymer thieno[3,2-b]thiophene-diketopyrrolopyrrole (DPPTTT) and (6,6)-phenyl-C71-butyric acid ester (PC 70BM). Changes in Raman spectra associated with crystallization processes of each component and their impact on thin film morphology were studied during thermal annealing and cooling processes. Transition temperatures to crystalline phases in blends were measured at ∼150 °C and ∼170 °C for DPPTTT and PC 70BM, respectively. Such phase changes lead to modifications in local chemical composition reducing relative Raman peak intensities (IPC70BM/IDPPTTT) from ∼0.4 in PC 70BM-rich domains to ∼0.15 in homogeneous areas

Effects of Side-Chain Length and Shape on Polytellurophene Molecular Order and Blend Morphology

We investigate the molecular order and thin film morphology of the conjugated polymer polytellurophene, in order to understand how the tellurium atom and the choice of side-chain influence the conjugated polymer’s backbone planarity and performance in organic transistors. We find that poly­(3hexyltellurophene) (P3HTe) continues the trend from polythiophene (P3HT) to polyselenophene (P3HS): substitution with Tellurium leads to a more planar backbone, evident from the shifts of the CC vibrational peak to lower wavenumbers (∼1389 cm^–1) and a smaller optical band gap (∼1.4 eV). Resonant Raman spectroscopy revealed that molecular order was highly dependent on the structure of the P3ATe alkyl side-chain: a longer chains introduces kinetic hindrance, reducing the fraction of ordered phase obtained at room temperature, while a branched side-chain introduces steric hindrance, with intrinsic disorder present even when deposited at higher temperatures. When blended with the insulator HDPE, all three polymers exhibit little additional disorder and instead form phase-separated networks of high molecular order that are beneficial to percolated charge transport in transistors. We find that molecular order, as measured by Raman, correlates well with reported transistor mobilities and provides a greater understanding of the structure–property relationships that determine the performance of these novel organometallic polymers in electronic devices