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Journal articleAzzouzi M, Nelson J, Eisner F, et al., 2022,
Reconciling models of interfacial state kinetics and device performance in organic solar cells: Impact of the energy offsets on the power conversion efficiency
, Energy and Environmental Science, Vol: 15, Pages: 156-1270, ISSN: 1754-5692Achieving the simultaneous increases in the open circuit voltage (Voc), short circuit current (Jsc) and fill factor (FF) necessary to further increase the power conversion efficiency (PCE) of organic photovoltaics (OPV) requires a unified understanding of how molecular and device parameters affect all three characteristics. In this contribution, we introduce a framework that for the first time combines different models that have been used separately to describe the different steps of the charge generation and collection processes in OPV devices: a semi-classical rate model for charge recombination processes in OPV devices, zero-dimensional kinetic models for the photogeneration process and exciton dissociation and one-dimensional semiconductor device models. Using this unified multi-scale model in conjunction with experimental techniques (time-resolved absorption spectroscopy, steady-state and transient optoelectronic measurements) that probe the various steps involved in charge generation we can shed light on how the energy offsets in a series of polymer: non-fullerene devices affect the charge carrier generation, collection, and recombination properties of the devices. We find that changing the energy levels of the donor significantly affects not only the transition rates between local-exciton (LE) and charge-transfer (CT) states, but also significantly changes the transition rates between CT and charge-separated (CS) states, challenging the commonly accepted picture of charge generation and recombination. These results show that in order to obtain an accurate picture of charge generation in OPV devices, a variety of different experimental techniques under different conditions in conjunction with a comprehensive model of processes occurring at different time-scales are required.
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Journal articleFew S, Barton J, Sandwell P, et al., 2022,
Electricity demand in populations gaining access: Impact of rurality and climatic conditions, and implications for microgrid design
, Energy for Sustainable Development, Vol: 66, Pages: 151-164, ISSN: 0973-0826Almost 800 million people currently lack access to reliable electricity, for many of whom solar microgrid systems are expected to be the most cost-effective solution. Quantifying current and future electricity demand is crucial for cost-effective design of reliable microgrids. However, electricity usage is connected to a wide range of social and economic factors alongside climatic conditions, making estimation of demand challenging. This paper presents a framework facilitating each stage of solar microgrid design from demand estimation through to cost-optimal sizing of the microgrid and its economic and environmental characterisation. Household demand is simulated based upon (1) climatic conditions, (2) appliance ratings and usage patterns, and (3) rates of growth in appliance ownership based upon the Multi-Tier Framework for measuring household electricity access. Microgrid demands are simulated based on the combination of these with (4) nondomestic demand based upon locally available data. The framework is demonstrated across four rates of domestic demand growth and two climatic conditions (‘tropical savanna’ and ‘humid subtropical’), alongside nondomestic demand based upon two operational microgrids (one rural and one peri-urban). When growth rates are high, newly introduced appliances tend to dominate, with differing impacts on the demand profile depending on power and usage times. Cooling represents a modest contribution to demand in the tropical savanna climate. However, in the hotter and more seasonally varying humid subtropical climate, cooling becomes the dominant driver of demand, increasing seasonality and proportion of demand at night. Nondomestic demand in the rural microgrid is primarily agricultural, and exhibits more seasonality and better alignment with daylight hours than demand in the peri-urban setting, which is more service-based. Across cases, increased seasonality and proportion of demand at night lead to poorer alignme
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Journal articleWard MD, Wade J, Shi X, et al., 2022,
Highly selective high-speed circularly polarized photodiodes based on π-conjugated polymers
, Advanced Optical Materials, Vol: 10, ISSN: 2195-1071Chiral π-conjugated molecular systems that are intrinsically sensitive to the handedness of circularly polarized (CP) light potentially allow for miniaturized, low-cost CP detection devices. Such devices promise to transform several technologies, including biosensing, quantum optics, and communication of data encrypted by exploiting the spin angular momentum of light. Here a simple, bilayer organic photodiode (CP OPD) comprising an achiral π-conjugated polymer–chiral additive blend as the electron donor layer and an achiral C60 electron acceptor layer is realized. These devices exhibit considerable photocurrent dissymmetry gph, with absolute values as high as 0.85 and dark currents as low as 10 pA. Impressively, they showcase a linear dynamic range of 80 dB, and rise and fall times of ≈7 µs, which significantly outperforms all previously reported CP selective photodetectors. Mechanistically, it is shown that the gph is sensitive to the thickness of both the chiral donor and achiral acceptor layers and that a trade-off exists between the external quantum efficiency and gph. The fast-switching speeds of these devices, coupled with their large dynamic range and highly selective response to CP light, opens up the possibility of their direct application in CP sensing and optical communications.
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Journal articleAlmora O, Baran D, Bazan GC, et al., 2021,
Device performance of emerging photovoltaic materials (version 2)
, Advanced Energy Materials, Vol: 11, Pages: 1-41, ISSN: 1614-6832Following the 1st release of the “Emerging photovoltaic (PV) reports”, the best achievements in the performance of emerging photovoltaic devices in diverse emerging photovoltaic research subjects are summarized, as reported in peer-reviewed articles in academic journals since August 2020. Updated graphs, tables, and analyses are provided with several performance parameters, e.g., power conversion efficiency, open-circuit voltage, short-circuit current density, fill factor, light utilization efficiency, and stability test energy yield. These parameters are presented as a function of the photovoltaic bandgap energy and the average visible transmittance for each technology and application and are put into perspective using, e.g., the detailed balance efficiency limit. The 2nd instalment of the “Emerging PV reports” extends the scope toward tandem solar cells and presents the current state-of-the-art in tandem solar cell performance for various material combinations.
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Journal articleBozal-Ginesta C, Rao RR, Mesa CA, et al., 2021,
Redox-state kinetics in water-oxidation IrOx electrocatalysts measured by operando spectroelectrochemistry
, ACS Catalysis, Vol: 11, Pages: 15013-15025, ISSN: 2155-5435Hydrous iridium oxides (IrOx) are the best oxygen evolution electrocatalysts available for operation in acidic environments. In this study, we employ time-resolved operando spectroelectrochemistry to investigate the redox-state kinetics of IrOx electrocatalyst films for both water and hydrogen peroxide oxidation. Three different redox species involving Ir3+, Ir3.x+, Ir4+, and Ir4.y+ are identified spectroscopically, and their concentrations are quantified as a function of applied potential. The generation of Ir4.y+ states is found to be the potential-determining step for catalytic water oxidation, while H2O2 oxidation is observed to be driven by the generation of Ir4+ states. The reaction kinetics for water oxidation, determined from the optical signal decays at open circuit, accelerates from ∼20 to <0.5 s with increasing applied potential above 1.3 V versus reversible hydrogen electrode [i.e., turnover frequencies (TOFs) per active Ir state increasing from 0.05 to 2 s–1]. In contrast, the reaction kinetics for H2O2 is found to be almost independent of the applied potential (increasing from 0.1 to 0.3 s–1 over a wider potential window), indicative of a first-order reaction mechanism. These spectroelectrochemical data quantify the increase of both the density of active Ir4.y+ states and the TOFs of these states with applied positive potential, resulting in the observed sharp turn on of catalytic water oxidation current. We reconcile these data with the broader literature while providing a unique kinetic insight into IrOx electrocatalytic reaction mechanisms, indicating a first-order reaction mechanism for H2O2 oxidation driven by Ir4+ states and a higher-order reaction mechanism involving the cooperative interaction of multiple Ir4.y+ states for water oxidation.
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Journal articleEisner F, Tam B, Belova V, et al., 2021,
Color-tunable hybrid heterojunctions as semi-transparent photovoltaic windows for photoelectrochemical water splitting
, Cell Reports Physical Science, Vol: 2, Pages: 1-16, ISSN: 2666-3864The strong but narrow-bandwidth absorption spectra of organic semiconductors make them excellent candidates for semi-transparent solar cell applications in which color specificity is important. In this study, using a hybrid heterojunction combining the transparent inorganic semiconductor copper thiocyanate (CuSCN) with organic semiconductors (C70, PC70BM, C60, ITIC, IT-4F, or Y6), we show that simple color-tunable solar cells can be fabricated in which the transmission spectrum is determined solely by choice of the organic semiconductor. Using a joint electrical-optical model, we show that it is possible to combine the unique attributes of high photovoltage and color tunability to use these heterojunctions as photovoltaic windows in tandem photoelectrochemical (PEC)-photovoltaic (PV) cells. We demonstrate that this configuration can lead to a reduction in the parasitic absorption losses in the PEC-PV cells and, thus, to solar-to-hydrogen efficiencies (>3%) that are higher than that predicted using the traditionally used architecture in which the PV is placed behind the PEC.
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ReportPearce D, Pearce A, Gambhir A, et al., 2021,
Research pathways for net-zero transport
The future is uncertain and there are many different pathways ahead for technology and society. Some, but not all, ofthese pathways will deliver the required amount of decarbonisation to comply with the Paris Agreement. Whilst wecannot know for certain which of these pathways will be chosen, research allows us both to identify the patterns andtrends that can help shape the pathways, and ultimately our future, and to develop the technologies and approachesthat can assist decarbonisation
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Journal articleSzumska AA, Maria IP, Flagg LQ, et al., 2021,
Reversible Electrochemical Charging of n-Type Conjugated Polymer Electrodes in Aqueous Electrolytes
, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, Vol: 143, Pages: 14795-14805, ISSN: 0002-7863- Author Web Link
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- Citations: 40
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Journal articleSchmidt J, Weatherby J, Sugden I, et al., 2021,
Computational screening of organic semiconductors: exploring side-group functionalisation and assembly to optimise charge transport in chiral molecules
, Crystal Growth and Design, Vol: 21, Pages: 5036-5049, ISSN: 1528-7483Molecular materials are challenging to design as their packing arrangement and hence properties are subject to subtle variations in the interplay of soft intermolecular interactions that are difficult to predict. The rational design of new molecular materials with tailored properties is currently hampered by the lack of knowledge of how a candidate molecule will pack in space and how we can control the polymorphs we can experimentally obtain. Here, we develop a simplified approach to aid the material design process, by the development of a screening process that is used to test 1344 helicene molecules that have potential as organic electronic materials. Our approach bridges the gap between single molecule design, molecular assembly, and the resulting charge-carrier mobilities. We find that fluorination significantly improves electron transport in the molecular material by up to 200%; the reference [6]helicene packing showed a mobility of 0.30 cm2 V-1 s-1, fluorination increased the mobility to up to 0.96 and 0.97 (13-fluoro[6]H and 4,13-difluoro[6]H), assuming an outer reorganisation energy of 0.30 eV. Side groups containing triple bonds largely lead to improved transfer integrals. We validate our screening approach through the use of crystal structure prediction to confirm the presence of favourable packing motifs to maximize charge mobility.
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Journal articleJux N, Reger D, Haines P, et al., 2021,
A family of superhelicenes - easily tunable, chiral nanographenes by merging helicity with planar π-systems
, Angewandte Chemie International Edition, Vol: 60, Pages: 18073-18081, ISSN: 1433-7851Incorporating helicity into large polycyclic aromatic hydrocarbons (PAHs) constitutes a new field of research at the interface between chemistry and material sciences. Lately, interest in the design of π-extended helicenes has surged. This new class of twisted, chiral nanographenes not only reveals unique characteristics but also finds its way into emerging applications such as spintronics. Insights into their structure-property relationships and on-demand tuning are scarce. To close these knowledge gaps, we designed a straightforward synthetic route towards a full-fledged family of π-extended helicenes: superhelicenes. Common are two hexa-peri-hexabenzocoronenes (HBCs) connected via a central 5-membered ring. By means of structurally altering this 5-membered ring, we realized a versatile library of molecular building blocks. Not only the superhelicene structure, but also their features are tuned with ease. In-depth physico-chemical characterizations served as a proof of concept thereof. The superhelicene enantiomers were separated, their circular dichroism was measured in preliminary studies and concluded with an enantiomeric assignment. Our work was rounded-off by crystal structure analyses. Mixed stacks of M- and P-isomers led to twisted molecular wires. Using such stacks, charge-carrier mobilities were calculated, giving reason to expect outstanding hole transporting properties.
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Journal articleReger D, Haines P, Amsharov KY, et al., 2021,
A Family of Superhelicenes: Easily Tunable, Chiral Nanographenes by Merging Helicity with Planar π Systems
, Angewandte Chemie, Vol: 133, Pages: 18221-18229, ISSN: 0044-8249<jats:title>Abstract</jats:title><jats:p>We designed a straightforward synthetic route towards a full‐fledged family of π‐extended helicenes: superhelicenes. They have two hexa‐peri‐hexabenzocoronenes (HBCs) in common that are connected via a central five‐membered ring. By means of structurally altering this 5‐membered ring, we realized a versatile library of molecular building blocks. Not only the superhelicene structure, but also their features are tuned with ease. In‐depth physico‐chemical characterizations served as a proof of concept thereof. The superhelicene enantiomers were separated, their circular dichroism was measured in preliminary studies and concluded with an enantiomeric assignment. Our work was rounded‐off by crystal structure analyses. Mixed stacks of M‐ and P‐isomers led to twisted molecular wires. Using such stacks, charge‐carrier mobilities were calculated, giving reason to expect outstanding hole transporting properties.</jats:p>
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Journal articleCilio L, Babacan O, 2021,
Allocation optimisation of rapid charging stations in large urban areas to support fully electric taxi fleets
, APPLIED ENERGY, Vol: 295, ISSN: 0306-2619- Author Web Link
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- Citations: 14
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Journal articleBeath H, Hauser M, Sandwell P, et al., 2021,
The cost and emissions advantages of incorporating anchor loads into solar mini-grids in India
, Renewable and Sustainable Energy Transition, Vol: 1, Pages: 1-14, ISSN: 2667-095XRenewables-based mini-grids have the potential to improve electricity access with lower emissions and better reliability than national grids. However, these systems have a challenging cost to revenue ratio, hindering their implementation. Combining residential loads with an anchor load, a relatively large non-domestic user, can help to improve mini-grid economics. Using measured electricity demand data from India and energy modelling, we assess the cost and emissions advantages of integrating health clinics as anchor loads within domestic solar mini-grids. For comparison, we also assess the ability of the national grid to meet our demand scenarios using monitored grid data. We apply a scenario-based approach, using separate domestic and anchor load demand profiles, and both in combination; we test meeting two levels of energy demand, 95% and 100%; and compare systems using PV and batteries, diesel, and hybrid generation. We find that the national grid has poor availability, at just over 50% at the most comparable monitoring site; and that it would meet a lower fraction of energy demand for our anchor load scenarios than the domestic only ones. For the off-grid systems, we find substantial cost and emissions reductions with anchor loads relative to demand scenarios without anchor loads. At 95% of demand met, we find PV and battery systems are 14-22% cheaper than diesel-only systems, with 10 times lower carbon intensity. Our findings illustrate the role off-grid systems can play in the provision of reliable low-carbon electricity and highlight the advantages of incorporating anchor loads like health centres into such systems.
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Journal articleEisner F, Nelson J, 2021,
Barrierless charge generation at non-fullerene organic heterojunctions comes at a cost
, Joule, Vol: 5, Pages: 1319-1322, ISSN: 2542-4351The advent of non-fullerene acceptors has enabled organic solar cells to reach power conversion efficiencies that were previously thought unreachable. However, in a recent Nature Materials article, Karuthedath and colleagues show that the electrostatic environment at the interface might put a limit to how much further they can be improved.
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Journal articleYan J, Rezasoltani E, Azzouzi M, et al., 2021,
Influence of static disorder of charge transfer state on voltage loss in organic photovoltaics
, Nature Communications, Vol: 12, ISSN: 2041-1723Spectroscopic measurements of charge transfer (CT) states provide valuable insight into the voltage losses in organic photovoltaics (OPVs). Correct interpretation of CT-state spectra depends on knowledge of the underlying broadening mechanisms, and the relative importance of molecular vibrational broadening and variations in the CT-state energy (static disorder). Here, we present a physical model, that obeys the principle of detailed balance between photon absorption and emission, of the impact of CT-state static disorder on voltage losses in OPVs. We demonstrate that neglect of CT-state disorder in the analysis of spectra may lead to incorrect estimation of voltage losses in OPV devices. We show, using measurements of polymer:non-fullerene blends of different composition, how our model can be used to infer variations in CT-state energy distribution that result from variations in film microstructure. This work highlights the potential impact of static disorder on the characteristics of disordered organic blend devices.
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Journal articleOrtega-Arriaga P, Babacan O, Nelson J, et al., 2021,
Grid versus off-grid electricity access options: A review on the economic and environmental impacts
, Renewable and Sustainable Energy Reviews, Vol: 143, Pages: 1-17, ISSN: 1364-0321This research reviews the economic and environmental impacts of grid-extension and off-grid systems, to inform the appropriate electrification strategy for the current population without electricity access. The principal technologies reviewed are centralised conventional fossil-fuel grid-extension and off-grid systems mainly based on solar PV and batteries. It finds that relatively few studies explicitly compare grid-extension electricity costs against off-grid systems costs and that there is a lack of consistency in the methodologies used to determine the least-cost solution. Nevertheless, the studies reviewed show a range of around $0.2–1.4/kWh for off-grid electricity access, compared to a range of below $0.1/kWh to more than $8/kWh for grid access, pointing to a number of cases in which off-grid access may already be the more cost-effective option. Existing literature on the environmental impacts primarily focuses on greenhouse gas emissions from electricity generation, with off-grid (solar PV and storage) systems’ emissions in the range of 50–130 gCO2-eq/kWh and grid generation from close to 0 gCO2-eq/kWh (for renewables and nuclear sources) to over 1,000 gCO2-eq/kWh (for coal). Emissions impacts stemming from transmission and distribution grids suggest a range of 0–30 gCO2-eq/kWh. Assessments of other environmental impacts such as water use, land use, biodiversity and e-waste are often absent in studies, whilst few studies explicitly compare the environmental impacts of grid versus off-grid systems. Further research should focus on comparing the costs of electricity access options using consistent metrics, expanding the scope of environmental impacts analysis, and integrating environmental and economic impacts into a comprehensive sustainability assessment of different options.
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Journal articleArmin A, Li W, Sandberg OJ, et al., 2021,
A history and perspective of non-fullerene electron acceptors for organic solar cells
, Advanced Energy Materials, Vol: 11, ISSN: 1614-6832Organic solar cells are composed of electron donating and accepting organic semiconductors. Whilst a significant palette of donors has been developed over three decades, until recently only a small number of acceptors have proven capable of delivering high power conversion efficiencies. In particular the fullerenes have dominated the landscape. In this perspective, the emergence of a family of materials–the non-fullerene acceptors (NFAs) is described. These have delivered a discontinuous advance in cell efficiencies, with the significant milestone of 20% now in sight. Intensive international efforts in synthetic chemistry have established clear design rules for molecular engineering enabling an ever-expanding number of high efficiency candidates. However, these materials challenge the accepted wisdom of how organic solar cells work and force new thinking in areas such as morphology, charge generation and recombination. This perspective provides a historical context for the development of NFAs, and also addresses current thinking in these areas plus considers important manufacturability criteria. There is no doubt that the NFAs have propelled organic solar cell technology to the efficiencies necessary for a viable commercial technology–but how far can they be pushed, and will they also deliver on equally important metrics such as stability?
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Journal articleMoss B, Babacan O, Kafizas A, et al., 2021,
A review of inorganic photoelectrode developments and reactor scale-up challenges for solar hydrogen production
, Advanced Energy Materials, Vol: 11, Pages: 1-43, ISSN: 1614-6832Green hydrogen, produced using solar energy, is a promising means of reducing greenhouse gas emissions. Photoelectrochemical (PEC) water splitting devices can produce hydrogen using sunlight and integrate the distinct functions of photovoltaics and electrolyzers in a single device. There is flexibility in the degree of integration between these electrical and chemical energy generating components, and so a plethora of archetypal PEC device designs has emerged. Although some materials have effectively been ruled out for use in commercial PEC devices, many principles of material design and synthesis have been learned. Here, the fundamental requirements of PEC materials, the top performances of the most widely studied inorganic photoelectrode materials, and reactor structures reported for unassisted solar water splitting are revisited. The main phenomena limiting the performance of up‐scaled PEC devices are discussed, showing that engineering must be considered in parallel with material development for the future piloting of PEC water splitting systems. To establish the future commercial viability of this technology, more accurate techno‐economic analyses should be carried out using data from larger scale demonstrations, and hence more durable and efficient PEC systems need to be developed that meet the challenges imposed from both material and engineering perspectives.
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Journal articleBaranda Alonso J, Sandwell P, Nelson J, 2021,
The potential for solar-diesel hybrid mini-grids in refugee camps: A case study of Nyabiheke camp, Rwanda
, Sustainable Energy Technologies and Assessments, Vol: 44, Pages: 1-18, ISSN: 2213-1388Electricity access in refugee camps is often limited to critical operations for humanitarian agencies and typically powered by diesel generators. We study the economic and environmental benefits that optimised fully renewable and diesel-hybrid mini-grid designs can provide in humanitarian settings by displacing diesel use. Considering the case study of Nyabiheke camp in Rwanda we found that these benefits are substantial, with total cost and emissions reductions of up to 32% and 83% respectively, and cost payback times ranging from 0.9 to 6.2 years. Despite their different cost structures, we find that all levels of hybridisation provide cost and emission savings compared to the incumbent diesel system, with hybrid systems being able to offset emissions more cost-effectively than fully renewable systems. We highlight how modelling tools can facilitate the introduction and progressive expansion of systems, improving asset utilisation and reducing lifetime costs compared to one-off installations, and can inform operational considerations on the ground. These benefits are enhanced when connecting productive users for whom demand matches the solar generation profile. Multiple energy needs and objectives can be met simultaneously but financial resources, environmental considerations and operational timeframes will influence the most appropriate system design for humanitarian actors on a case-by-case basis.
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Journal articleAlmora O, Baran D, Bazan GC, et al., 2021,
Device Performance of Emerging Photovoltaic Materials (Version 1)
, ADVANCED ENERGY MATERIALS, Vol: 11, ISSN: 1614-6832- Cite
- Citations: 90
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Journal articleYiwen W, Jinho L, Xueyan H, et al., 2021,
Recent progress and Challenges toward highly stable nonfullerene acceptor‐based organic solar cells
, Advanced Energy Materials, Vol: 11, ISSN: 1614-6832Organic solar cells (OSCs) based on nonfullerene acceptors (NFAs) have made significant breakthrough in their device performance, now achieving a power conversion efficiency of ≈18% for single junction devices, driven by the rapid development in their molecular design and device engineering in recent years. However, achieving long‐term stability remains a major challenge to overcome for their commercialization, due in large part to the current lack of understanding of their degradation mechanisms as well as the design rules for enhancing their stability. In this review, the recent progress in understanding the degradation mechanisms and enhancing the stability of high performance NFA‐based OSCs is a specific focus. First, an overview of the recent advances in the molecular design and device engineering of several classes of high performance NFA‐based OSCs for various targeted applications is provided, before presenting a critical review of the different degradation mechanisms identified through photochemical‐, photo‐, and morphological degradation pathways. Potential strategies to address these degradation mechanisms for further stability enhancement, from molecular design, interfacial engineering, and morphology control perspectives, are also discussed. Finally, an outlook is given highlighting the remaining key challenges toward achieving the long‐term stability of NFA‐OSCs.
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Journal articleAbdulla A, Hanna R, Schell KR, et al., 2021,
Explaining successful and failed investments in US carbon capture and storage using empirical and expert assessments
, ENVIRONMENTAL RESEARCH LETTERS, Vol: 16, ISSN: 1748-9326- Author Web Link
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- Citations: 24
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Journal articleFew S, Djapic P, Strbac G, et al., 2020,
Assessing local costs and impacts of distributed solar PV using high resolution data from across Great Britain
, Renewable Energy, Vol: 162, Pages: 1140-1150, ISSN: 0960-1481Highly spatially resolved data from across Great Britain (GB) are combined with a distribution network modelling tool to assess impacts of distributed photovoltaic (PV) deployment up to 2050 on local networks, the costs of avoiding these impacts, and how these depend upon context. Present-day deployment of distributed PV, meter density, and network infrastructure across GB are found to be highly dependent on rurality, and data on these are used to build up three representative contexts: cities, towns, and villages. For each context, distribution networks are simulated, and impacts on these networks associated with PV deployment and growth in peak load up to 2050 calculated. Present-day higher levels of PV deployment in rural areas are maintained in future scenarios, necessitating upgrades in ambitious PV scenarios in towns and villages from around 2040, but not before 2050 in cities. Impacts of load growth are more severe than those of PV deployment, potentially necessitating upgrades in cities, towns, and villages from 2030. These are most extensive in cities and towns, where long feeders connect more customers, making networks particularly susceptible to impacts. Storage and demand side response are effective in reducing upgrade costs, particularly in cities and towns.
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Journal articleLubert-Perquel D, Szumska AA, Azzouzi M, et al., 2020,
Structure dependence of kinetic and thermodynamic parameters in singlet fission processes.
, Journal of Physical Chemistry Letters, Vol: 11, Pages: 9557-9565, ISSN: 1948-7185Singlet fission-whereby one absorbed photon generates two coupled triplet excitons-is a key process for increasing the efficiency of optoelectronic devices by overcoming the Shockley-Queisser limit. A crucial parameter is the rate of dissociation of the coupled triplets, as this limits the number of free triplets subsequently available for harvesting and ultimately the overall efficiency of the device. Here we present an analysis of the thermodynamic and kinetic parameters for this process in parallel and herringbone dimers measured by electron paramagnetic resonance spectroscopy in coevaporated films of pentacene in p-terphenyl. The rate of dissociation is higher for parallel dimers than for their herringbone counterparts, as is the rate of recombination to the ground state. DFT calculations, which provide the magnitude of the electronic coupling as well as the distribution of molecular orbitals for each geometry, suggest that weaker triplet coupling in the parallel dimer is the driving force for faster dissociation. Conversely, localization of the molecular orbitals and a stronger triplet-triplet interaction result in slower dissociation and recombination. The identification and understanding of how the intermolecular geometry promotes efficient triplet dissociation provide the basis for control of triplet coupling and thereby the optimization of one important parameter of device performance.
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Journal articleHe Q, Eisner FD, Pearce D, et al., 2020,
Ring fusion in tetrathienylethene cored perylene diimide tetramers affords acceptors with strong and broad absorption in the near-UV to visible region
, Journal of Materials Chemistry C, Vol: 8, Pages: 17237-17244, ISSN: 2050-7526In this work, we designed and synthesized two novel perylene diimide (PDI) tetramers based on a tetrathienylethene core, named TTE-PDI4 and FTTE-PDI4, and investigated their application as non-fullerene acceptors for organic photovoltaics. The free rotation of PDIs and adjacent thiophene units renders TTE-PDI4 with a highly twisted molecular geometry. The ring fusion of TTE-PDI4 yields FTTE-PDI4, a more rigid molecule with increased intramolecular stacking. Interestingly, TTE-PDI4 and FTTE-PDI4 possess similar energy levels but very different UV-Vis absorptions, with the latter showing strong broad-band absorption with multiple sharp peaks in the 300–600 nm region. Through time-dependent density functional theory (TD-DFT) calculations, we show that this broad absorption spectrum in FTTE-PDI4 arises from the combination of multiple bright transitions in the visible region with a strong vibronic progression, tentatively assigned to the dominant C[double bond, length as m-dash]C stretching mode. TTE-PDI4, despite having a lower energy absorption onset, shows weaker absorption at long wavelengths. Due to its higher absorption as well as its increased rigidity, FTTE-PDI4 shows a higher photocurrent and hence a higher power conversion efficiency (PCE), of 6.6%, when blended with the polymer donor PFBDB-T than TTE-PDI4 based blends (PCE of 3.8%). The greater rigidity of FTTE-PDI4 is likely to contribute to the good fill factor of the blend devices. Potential for further improvement through reducing voltage losses is identified.
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Journal articleCong S, Creamer A, Fei Z, et al., 2020,
Tunable control of the hydrophilicity and wettability of conjugated polymers by a postpolymerization modification approach.
, Macromolecular Bioscience, Vol: 20, Pages: 1-8, ISSN: 1616-5187A facile method to prepare hydrophilic polymers by a postpolymerization nucleophillic aromatic substitution reaction of fluoride on an emissive conjugated polymer (CP) backbone is reported. Quantitative functionalization by a series of monofunctionalized ethylene glycol oligomers, from dimer to hexamer, as well as with high molecular weight polyethylene glycol is demonstrated. The length of the ethylene glycol sidechains is shown to have a direct impact on the surface wettability of the polymer, as well as its solubility in polar solvents. However, the energetics and band gap of the CPs remain essentially constant. This method therefore allows an easy way to modulate the wettability and solubility of CP materials for a diverse series of applications.
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Journal articleRezasoltani E, Guilbert AAY, Yan J, et al., 2020,
Correlating the phase behavior with the device performance in binary poly-3-hexylthiophene: nonfullerene acceptor blend using optical probes of the microstructure
, Chemistry of Materials, Vol: 32, Pages: 8294-8305, ISSN: 0897-4756The performance of photovoltaic devices based on blends of conjugated polymers with nonfullerene acceptors depends on the phase behavior and microstructure of the binary, which in turn depends on the chemical structures of the molecular components and the blend composition. We investigate the correlation between the molecular structure, composition, phase behavior, and device performance of a model system consisting of semicrystalline poly-3-hexylthiophene (P3HT) as the donor polymer and three nonfullerene acceptors, two of which (O-IDTBR/EH-IDTBR) have a planar core with different side chains and one (O-IDFBR) of which has a twisted core. We combine differential scanning calorimetry with optical measurements including UV–Vis spectroscopy, photoluminescence, spectroscopic ellipsometry, and Raman spectroscopy and photovoltaic device performance measurements, all at varying blend composition. For P3HT:IDTBR blends, the crystallinity of polymer and acceptor is preserved over a wide composition range and the blend displays a eutectic phase behavior, with the optimum solar cell composition lying close to the eutectic composition. For P3HT:IDFBR blends, increasing acceptor content disrupts the polymer crystallinity, and the optimum device composition appears to be limited by polymer connectivity rather than being linked to the eutectic composition. The optical probes allow us to probe both the crystalline and amorphous phases, clearly revealing the compositions at which component mixing disrupts crystallinity.
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Journal articleBabacan O, De Causmaecker S, Gambhir A, et al., 2020,
Assessing the feasibility of carbon dioxide mitigation options in terms of energy usage
, Nature Energy, Vol: 5, Pages: 720-728, ISSN: 2058-7546Measures to mitigate the emissions of carbon dioxide (CO2) can vary substantially in terms of the energy required. Some proposed CO2 mitigation options involve energy-intensive processes that compromise their viability as routes to mitigation, especially if deployed at a global scale. Here we provide an assessment of different mitigation options in terms of their energy usage. We assess the relative effectiveness of several CO2 mitigation routes by calculating the energy cost of carbon abatement (kilowatt-hour spent per kilogram CO2-equivalent, or kWh kgCO2e–1) mitigated. We consider energy efficiency measures, decarbonizing electricity, heat, chemicals and fuels, and also capturing CO2 from air. Among the routes considered, switching to renewable energy technologies (0.05–0.53 kWh kgCO2e–1 mitigated) offer more energy-effective mitigation than carbon embedding or carbon removal approaches, which are more energy intensive (0.99–10.03 kWh kgCO2e–1 and 0.78–2.93 kWh kgCO2e–1 mitigated, respectively), whereas energy efficiency measures, such as improving building lighting, can offer the most energy-effective mitigation.
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Journal articleXiao B, Calado P, Mackenzie R, et al., 2020,
Relationship between fill factor and light intensity in solar cells based on organic disordered semiconductors: The role of tail states
, Physical Review Applied, Vol: 14, Pages: 024034 – 1-024034 – 17, ISSN: 2331-7019The origin of the relationship between fill factor (FF) and light intensity (I) in organic disordered-semiconductor-based solar cells is studied. An analytical model describing the balance between transport and recombination of charge carriers, parameterized with a factor, Γm, is introduced to understand the FF-I relation, where higher values of Γm correlate to larger FFs. Comparing the effects of direct and tail-state-mediated recombination on the FF-I plot, we find that, for low-mobility systems, direct recombination with constant transport mobility can deliver only a negative dependence of Γm,dir on light intensity. By contrast, tail-state-mediated recombination with trapping and detrapping processes can produce a positive Γm,t versus sun dependency. The analytical model is validated by numerical drift-diffusion simulations. To further validate our model, two material systems that show opposite FF-I behavior are studied: poly{4,8-bis[5-(2-ethylhexyl)thiophen-2-yl]benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-[4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene)-2-carboxylate-2-6-diyl]} (PTB7-Th):[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) devices show a negative FF-I relation, while PTB7-Th:(5Z,5′Z)-5,5′-{[7,7′ -(4,4,9,9-tetraoctyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl)bis(benzo[c][1,2,5]thiadiazole-7,4-diyl)]bis(methanylylidene)}bis(3-ethyl-2-thioxothiazolidin-4-one) (O-IDTBR) devices show a positive correlation. Optoelectronic measurements show that the O-IDTBR device presents a higher ideality factor, stronger trapping and detrapping behavior, and a higher density of trap states, relative to the PC71BM device, supporting the theoretical model. This work provides a comprehensive understanding of the correlation between FF and light intensity for disordered-semiconductor-based solar cells.
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Journal articleWoods DJ, Hillman S, Pearce D, et al., 2020,
Side-chain tuning in conjugated polymer photocatalysts for improved hydrogen production from water
, Energy & Environmental Science, Vol: 13, Pages: 1843-1855, ISSN: 1754-5692Structure–property–activity relationships in solution processable polymer photocatalysts for hydrogen production from water were probed by varying the chemical structure of both the polymer side-chains and the polymer backbone. In both cases, the photocatalytic performance depends strongly on the inclusion of more polar groups, such as dibenzo[b,d]thiophene sulfone backbone units or oligo(ethylene glycol) side-chains. We used optical, spectroscopic, and structural characterisation techniques to understand the different catalytic activities of these systems. We find that although polar groups improve the wettability of the material with water in all cases, backbone and side-chain modifications affect photocatalytic performance in different ways: the inclusion of dibenzo[b,d]thiophene sulfone backbone units improves the thermodynamic driving force for hole transfer to the sacrificial donor, while the inclusion of oligo ethylene glycol side-chains aids the degree of polymer swelling and also extends the electron polaron lifetime. The best performing material, FS-TEG, exhibits a HER of 72.5 μmol h−1 for 25 mg photocatalyst (2.9 mmol g−1 h−1) when dispersed in the presence of a sacrificial donor and illuminated with λ > 420 nm light, corresponding to a hydrogen evolution EQE of 10% at 420 nm. When cast as a thin film, this HER was further boosted to 13.9 mmol g−1 h−1 (3.0 mmol m−2 h−1), which is among the highest rates in this field.
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