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Journal articleRoyakkers J, Yang H, Gillett AJ, et al., 2024,
Synthesis of model heterojunction interfaces reveals molecular-configuration-dependent photoinduced charge transfer.
, Nat ChemControl of the molecular configuration at the interface of an organic heterojunction is key to the development of efficient optoelectronic devices. Due to the difficulty in characterizing these buried and (probably) disordered heterointerfaces, the interfacial structure in most systems remains a mystery. Here we demonstrate a synthetic strategy to design and control model interfaces, enabling their detailed study in isolation from the bulk material. This is achieved by the synthesis of a polymer in which a non-fullerene acceptor moiety is covalently bonded to a donor polymer backbone using dual alkyl chain links, constraining the acceptor and donor units in a through space co-facial arrangement. The constrained geometry of the acceptor relative to the electron-rich and -poor moieties in the polymer backbone can be tuned to control the kinetics of charge separation and the energy of the resultant charge-transfer state giving insight into factors that govern charge generation at organic heterojunctions.
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Journal articleReddick C, Sotelo-Vazquez C, Tam B, et al., 2024,
Photoelectrochemical disinfection efficiency of WO3-based photoanodes: development of multifunctional photoelectrocatalytic materials
, Catalysis Today, Vol: 437, ISSN: 0920-5861Access to safe water is a growing global concern, with millions lacking acceptable water sources. Photocatalysis offers eco-friendly water remediation, yet its combination with electrocatalysis for both water treatment and hydrogen production remain underexplored. This study investigates UVA LED photoelectrocatalysis using WO3-based photoanodes, alone or in heterojunction with BiVO4, to purify wastewater and co-produce hydrogen. Tests on polluted water streams containing 105 PFU mL−1 of MS2 bacteriophage virus and 106 CFU mL−1 of E. coli reveal that nanostructured WO3 achieves rapid MS2 disinfection within 5 min. (k= 0.80 min−1), with enhanced efficiency over flat counterparts. However, nanostructuring does not improve E. coli inactivation due to bacterium size constraints. These findings advance the design of tandem photoreactors for dual wastewater purification and energy generation.
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Journal articleWinchester B, Huang G, Beath H, et al., 2024,
Lifetime optimisation of integrated thermally and electrically driven solar desalination plants
, npj Clean Water, Vol: 7, ISSN: 2059-7037We compare the performance of photovoltaic (PV), flat-plate and evacuated-tube solar-thermal (ST), and hybrid photovoltaic-thermal (PV-T) collectors to meet the energy demands of multi-effect distillation (MED) desalination plants across four locations. We consider three scales: 1700 m3day-1, 120 m3day-1 and 3 m3day-1. We find a strongdependence of the capacity and configuration of the solar collectors on both the cost of sourcing electricity from the grid and the specific collector employed. We find specific costs as low as 7.8, 3.4 and 3.7 USDm-3 for the three plant capacities. We find that solar-driven systems optimised for the lowest specific cost result in CO2eq emissions equal to, or higher than, those from grid-driven reverse osmosis(RO) and in line with PV-RO. This highlights the need to consider the environmental footprint of these systems to ensure that desalination is in line with the United Nations’ Sustainable Development Goal 6.
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Journal articleBeath H, Mittal S, Few S, et al., 2024,
Carbon pricing and system reliability impacts on pathways to universal electricity access in Africa
, Nature Communications, Vol: 15, ISSN: 2041-1723Off-grid photovoltaic systems have been proposed as a panacea for economies with poor electricity access, offering a lower-cost “leapfrog” over grid infrastructure used in higher-income economies. Previous research examining pathways to electricity access may understate the role of off-grid photovoltaics as it has not considered reliability and carbon pricing impacts. We perform high-resolution geospatial analysis on universal household electricity access in Sub-Saharan Africa that includes these aspects via least-cost pathways at different electricity demand levels. Under our “Tier 3" demand reference scenario, 24% of our study’s 470 million people obtaining electricity access by 2030 do so via off-grid photovoltaics. Including a unit cost for unmet demand of 0.50 US dollars ($)/kWh, to penalise poor system reliability increases this share to 41%. Applying a carbon price (around $80/tonne CO2-eq) increases it to 38%. Our results indicate considerable diversity in the level of policy intervention needed between countries and suggest several regions where lower levels of policy intervention may be effective.
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Journal articleYu H, Nelson J, 2024,
Slow on, fast off.
, Nat Mater, Vol: 23, Pages: 585-586 -
Journal articleStojanović L, Coker J, Giannini S, et al., 2024,
Disorder-Induced Transition from Transient Quantum Delocalization to Charge Carrier Hopping Conduction in a Nonfullerene Acceptor Material
, Physical Review X, Vol: 14 -
Journal articleTam B, Babacan O, Kafizas A, et al., 2024,
Comparing the net-energy balance of standalone photovoltaic-coupled electrolysis and photoelectrochemical hydrogen production
, ENERGY & ENVIRONMENTAL SCIENCE, Vol: 17, Pages: 1677-1694, ISSN: 1754-5692 -
Journal articleHou X, Coker JF, Yan J, et al., 2024,
Structure–Property Relationships for the Electronic Applications of Bis-Adduct Isomers of Phenyl-C₆₁ Butyric Acid Methyl Ester
, Chemistry of Materials, Vol: 36, Pages: 425-438, ISSN: 0897-4756Higher adducts of a fullerene, such as the bis-adduct of PCBM (bis-PCBM), can be used to achieve shallower molecular orbital energy levels than, for example, PCBM or C60. Substituting the bis-adduct for the parent fullerene is useful to increase the open-circuit voltage of organic solar cells or achieve better energy alignment as electron transport layers in, for example, perovskite solar cells. However, bis-PCBM is usually synthesized as a mixture of structural isomers, which can lead to both energetic and morphological disorder, negatively affecting device performance. Here, we present a comprehensive study on the molecular properties of 19 pure bis-isomers of PCBM using a variety of characterization methods, including ultraviolet photoelectron spectroscopy, thermal gravimetric analysis, differential scanning calorimetry, single crystal structure, and (time-dependent) density functional theory calculation. We find that the lowest unoccupied molecular orbital of such bis-isomers can be tuned to be up to 170 meV shallower than PCBM and up to 100 meV shallower than the mixture of unseparated isomers. The isolated bis-isomers also show an electron mobility in organic field-effect transistors of up to 4.5 × 10⁻² cm²/(Vs), which is an order of magnitude higher than that of the mixture of bis-isomers. These properties enable the fabrication of the highest performing bis-PCBM organic solar cell to date, with the best device showing a power conversion efficiency of 7.2%. Interestingly, we find that the crystallinity of bis-isomers correlates negatively with electron mobility and organic solar cell device performance, which we relate to their molecular symmetry, with a lower symmetry leading to more amorphous bis-isomers, less energetic disorder, and higher dimensional electron transport. This work demonstrates the potential of side chain engineering for optimizing the performance of fullerene-based organic electronic devices.
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Journal articleSiemons N, Siemons N, 2023,
Controlling swelling in mixed transport polymers through alkyl side-chain physical cross-linking
, Proceedings of the National Academy of Sciences of USA, Vol: 120, ISSN: 0027-8424Semiconducting conjugated polymers bearing glycol side chains can simultaneously transport both electronic and ionic charges with high charge mobilities, making them ideal electrode materials for a range of bioelectronic devices. However, heavily glycolated conjugated polymer films have been observed to swell irreversibly when subjected to an electrochemical bias in an aqueous electrolyte. The excessive swelling can lead to the degradation of their microstructure, and subsequently reduced device performance. An effective strategy to control polymer film swelling is to copolymerize glycolated repeat units with a fraction of monomers bearing alkyl side chains, although the microscopic mechanism that constrains swelling is unknown. Here we investigate, experimentally and computationally, a series of archetypal mixed transporting copolymers with varying ratios of glycolated and alkylated repeat units. Experimentally we observe that exchanging 10% of the glycol side chains for alkyl leads to significantly reduced film swelling and an increase in electrochemical stability. Through molecular dynamics simulation of the amorphous phase of the materials, we observe the formation of polymer networks mediated by alkyl side-chain interactions. When in the presence of water, the network becomes increasingly connected, counteracting the volumetric expansion of the polymer film.
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Journal articleZhao W, Wang J, Tam B, et al., 2023,
Structural Water in Amorphous Tungsten Oxide Hydrate Enables Fast and Ultrastable Regulation of Near-Infrared Light Transmittance
, ADVANCED OPTICAL MATERIALS, Vol: 11, ISSN: 2195-1071 -
Journal articleWinkler L, Pearce D, Nelson J, et al., 2023,
The effect of sustainable mobility transition policies on cumulative urban transport emissions and energy demand
, NATURE COMMUNICATIONS, Vol: 14- Author Web Link
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- Citations: 3
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Journal articleBeath H, Alonso JB, Mori R, et al., 2023,
Maximising the benefits of renewable energy infrastructure in displacement settings: optimising the operation of a solar-hybrid mini-grid for institutional and business users in Mahama Refugee Camp, Rwanda
, Renewable and Sustainable Energy Reviews, Vol: 176, ISSN: 1364-0321Humanitarian organisations typically rely on expensive, polluting diesel generators to provide power for services in refugee camps, whilst camp residents often have no access to electricity. Integrating solar and battery storage capacity into existing diesel-based systems can provide significant cost and emissions savings and offer an opportunity to provide power to displaced communities. By analysing monitored demand data and using computational energy system modelling, we assess the savings made possible by the integration of solar (18.4 kWp) and battery (78 kWh) capacity into the existing diesel-powered mini-grid in Mahama Refugee Camp, Rwanda. We find that the renewables infrastructure reduces fuel expenditure by $41,500 and emissions by 44 tCO2eq (both 74%) over five years under the generator’s current operational strategy. An alternative strategy, with deeper battery cycling, unlocks further savings of $4100 and 12.4 tCO2eq, using 33% of battery lifetime versus 15% under the original strategy. This reduces the cost of electricity by 33% versus diesel generation alone, whilst more aggressive cycling strategies could prove economical if moderate battery price decreases are realised. Extending the system to businesses in the camp marketplace can completely offset the system fuel costs if the mini-grid company charges customers the same tariff as the one it uses in the host community, but not the national grid tariff. Humanitarian organisations and the private sector should explore opportunities to integrate renewables into existing diesel-based infrastructure, and optimise its performance once installed, to reduce costs and emissions and provide meaningful livelihood opportunities to displaced communities.
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Journal articleSandwell P, Winchester B, Beath H, et al., 2023,
CLOVER: A modelling framework for sustainablecommunity-scale energy systems
, The Journal of Open Source Software, Vol: 8, Pages: 1-5, ISSN: 2475-9066Sustainable Development Goal 7 aims to provide sustainable, affordable, reliable and modernenergy access to all by 2030 (United Nations, 2015). In order for this goal to be achieved,sustainable energy interventions in developing countries must be supported with design toolswhich can evaluate the technical performance of energy systems as well as their economic andclimate impacts.CLOVER (Continuous Lifetime Optimisation of Variable Electricity Resources) is a softwaretool for simulating and optimising community-scale energy systems, typically minigrids, tosupport energy access in developing countries (Winchester et al., 2022). CLOVER can be usedto model electricity demand and supply at an hourly resolution, for example allowing users toinvestigate how an electricity system might perform at a given location. CLOVER can alsoidentify an optimally-sized energy system to meet the needs of the community under specifiedconstraints. For example, a user could define an optimum system as one which provides adesired level of reliability at the lowest cost of electricity. CLOVER can provide an insightinto the technical performance, costs, and climate change impact of a system, and allow theuser to evaluate many different scenarios to decide on the best way to provide sustainable,affordable and reliable electricity to a community.CLOVER can be used on both personal computers and high-performance computing facilities.Its design separates its general framework (code, contained in a source src directory) fromuser inputs (data, contained in a directory entitled locations) which are specific to theirinvestigations. The user inputs these data via a combination of .csv and .yaml files. CLOVER’sstraightforward command-line interface provides simple operation for both experienced Pythonusers and those with little prior exposure to coding. An installable package, clover-energy, isavailable for users to download without needing to become familiar with GitHub’s interface.Informat
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Journal articleWang J, Zhao W, Tam B, et al., 2023,
Pseudocapacitive porous amorphous vanadium pentoxide with enhanced multicolored electrochromism
, CHEMICAL ENGINEERING JOURNAL, Vol: 452, ISSN: 1385-8947- Author Web Link
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- Citations: 2
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Journal articleSayani R, Ortega-Arriaga P, Sandwell P, et al., 2022,
Sizing solar-based mini-grids for growing electricity demand: Insights from rural India
, The Journal of High Energy Physics, Vol: 5, Pages: 1-26, ISSN: 1029-8479Mini-grids are a critical way to meet electricity access goals according to current and projected electricity demand of communities and so appropriately sizing them is essential to ensure their financial viability. However, estimation of demand for communities awaiting electricity access is uncertain and growth in demand along with the associated cost implications is rarely considered during estimation of mini-grid sizing. Using a case study of two rural communities in India, we assess the implications of demand growth on financial costs and performance of a mini-grid system consisting of solar photovoltaic (PV) panels and battery storage using two different system sizing approaches. We show a cost-saving potential of up to 12% when mini-grids are sized using a multi-stage approach where mini-grids gradually expand in several stages, rather than a single-stage optimisation approach. We perform a sensitivity analysis of the cost of the two sizing approaches by varying six key parameters: demand growth rate, logistics cost, system re-sizing frequency, likelihood of blackouts, solar PV and battery cost, and degradation rate. Of these, we find that system costs are most sensitive to variations in demand growth rates and cost decreases in solar PV and batteries. Our study shows that demand growth scenarios and choice of mini-grid sizing approaches have important financial and operational implications for the design of systems for rural electrification.
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Journal articleHillman SAJ, Sprick RS, Pearce D, et al., 2022,
Why do sulfone-containing polymer photocatalysts work so well for sacrificial hydrogen evolution from water?
, Journal of the American Chemical Society, Vol: 144, Pages: 19382-19395, ISSN: 0002-7863Many of the highest-performing polymer photocatalysts for sacrificial hydrogen evolution from water have contained dibenzo[b,d]thiophene sulfone units in their polymer backbones. However, the reasons behind the dominance of this building block are not well understood. We study films, dispersions, and solutions of a new set of solution-processable materials, where the sulfone content is systematically controlled, to understand how the sulfone unit affects the three key processes involved in photocatalytic hydrogen generation in this system: light absorption; transfer of the photogenerated hole to the hole scavenger triethylamine (TEA); and transfer of the photogenerated electron to the palladium metal co-catalyst that remains in the polymer from synthesis. Transient absorption spectroscopy and electrochemical measurements, combined with molecular dynamics and density functional theory simulations, show that the sulfone unit has two primary effects. On the picosecond timescale, it dictates the thermodynamics of hole transfer out of the polymer. The sulfone unit attracts water molecules such that the average permittivity experienced by the solvated polymer is increased. We show that TEA oxidation is only thermodynamically favorable above a certain permittivity threshold. On the microsecond timescale, we present experimental evidence that the sulfone unit acts as the electron transfer site out of the polymer, with the kinetics of electron extraction to palladium dictated by the ratio of photogenerated electrons to the number of sulfone units. For the highest-performing, sulfone-rich material, hydrogen evolution seems to be limited by the photogeneration rate of electrons rather than their extraction from the polymer.
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Journal articlePulignani C, Mesa C, Hillman S, et al., 2022,
Rational design of carbon nitride photoelectrodes with high activity toward organic oxidations
, Angewandte Chemie International Edition, ISSN: 1433-7851Carbon nitride (CNx) is a scalable polymeric light-absorber with excellent performance in photocatalytic suspension systems, but the activity of CNx photoelectrodes has remained low. Here, cyanamide-functionalized CNx (NCNCNx) has been co-deposited with ITO nanoparticles on a 1.8 Å thick alumina-coated FTO-glass electrode. Transient spectroscopy and impedance measurements support that ITO acts as conductive binder and improves the electron extraction from the NCNCNx, whilst the alumina underlayer reduces the electrical resistance between the ITO and the FTO-coated electrode. The Al2O3|ITO:NCNCNx electrode displays a new benchmark performance for CNx-based photoanodes with a remarkably low onset of –0.4 V vs the reversible hydrogen electrode (RHE) and an outstanding 1.4 ± 0.2 mA cm–2 at 1.23 V vs RHE for the selective oxidation of 4-methylbenzyl alcohol to the corresponding aldehyde. This facile assembly will enable the exploration of CNx in fundamental and applied PEC studies, paving the way for the development of high-performance photoelectrodes using other semiconductor powders
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Journal articleSiemons N, Pearce D, Cendra C, et al., 2022,
Impact of side chain hydrophilicity on packing, swelling and ion interactions in oxy-bithiophene semiconductors.
, Advanced Materials, Vol: 34, ISSN: 0935-9648Exchanging hydrophobic alkyl-based side chains to hydrophilic glycol-based side chains is a widely adopted method for improving mixed-transport device performance, despite the impact on solid state packing and polymer-electrolyte interactions being poorly understood. Presented here is a Molecular Dynamics (MD) force field for modelling alkoxylated and glycolated polythiophenes. The force field is validated against known packing motifs for their monomer crystals. MD simulations, coupled with X-ray Diffraction (XRD), show that alkoxylated polythiophenes will pack with a 'tilted stack' and straight interdigitating side chains, whilst their glycolated counterpart will pack with a 'deflected stack' and an s-bend side chain configuration. MD simulations reveal water penetration pathways into the alkoxylated and glycolated crystals - through the π-stack and through the lamellar stack respectively. Finally, the two distinct ways tri-ethylene glycol polymers can bind to cations are revealed, showing the formation of a meta-stable single bound state, or an energetically deep double bound state, both with a strong side chain length dependance. The minimum energy pathways for the formation of the chelates are identified, showing the physical process through which cations can bind to one or two side chains of a glycolated polythiophene, with consequences for ion transport in bithiophene semiconductors. This article is protected by copyright. All rights reserved.
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Journal articleCalado P, Gelmetti I, Hilton B, et al., 2022,
Driftfusion: an open source code for simulating ordered semiconductor devices with mixed ionic-electronic conducting materials in one dimension
, JOURNAL OF COMPUTATIONAL ELECTRONICS, Vol: 21, Pages: 960-991, ISSN: 1569-8025- Author Web Link
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- Citations: 14
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Journal articleWard MD, Shi W, Gasparini N, et al., 2022,
Best practices in the measurement of circularly polarised photodetectors
, JOURNAL OF MATERIALS CHEMISTRY C, Vol: 10, Pages: 10452-10463, ISSN: 2050-7526- Author Web Link
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- Citations: 6
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Journal articleTan E, Kim J, Stewart K, et al., 2022,
The role of long-alkyl-group spacers in glycolated copolymers for high performance organic electrochemical transistors
, Advanced Materials, Vol: 34, ISSN: 0935-9648Semiconducting polymers with oligoethylene glycol sidechains have attracted strong research interest for organic electrochemical transistor (OECT) applications. However, key molecular design rules for high-performance OECTs via efficient mixed electronic/ionic charge transport are still unclear. Herein, we synthesize and characterize new glycolated copolymers (gDPP-TTT and gDPP-TTVTT) with diketopyrrolopyrrole (DPP) acceptor and thiophene-based (TTT or TTVTT) donor units for accumulation mode OECTs, where a long-alkyl-group (C12 ) attached to DPP unit acts as a spacer distancing the oligoethylene glycol from the polymer backbone. gDPP-TTVTT shows the highest OECT transconductance (61.9 S cm-1 ) and high operational stability, compared to gDPP-TTT and their alkylated counterparts. Surprisingly, gDPP-TTVTT also shows high electronic charge mobility in field-effect transistor, suggesting efficient ion injection/diffusion without hindering its efficient electronic charge transport. The elongated donor unit (TTVTT) facilitates the hole polaron formation more localized to the donor unit, leading to faster and easier polaron formation with less impact on polymer structure during OECT operation, as opposed to the TTT unit. This is supported by molecular dynamics (MD) simulation. We conclude that these simultaneously high electronic and ionic charge transport properties are achieved due to the long-alkyl-group spacer in amphipathic sidechains, providing an important molecular design rule for glycolated copolymers. This article is protected by copyright. All rights reserved.
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Journal articleSowe J, Varela Barreras J, Schimpe M, et al., 2022,
Model-informed battery current derating strategies: Simple methods to extend battery lifetime in islanded mini-grids
, Journal of Energy Storage, Vol: 51, Pages: 1-9, ISSN: 2352-152XIslanded mini-grids with batteries are crucial to enable universal access to energy. However, batteries are still costly, and how to select and operate them in an optimal manner is often unclear. The combination of variable climates with simple and low-cost passive thermal management also poses a challenge. Many techno-economic sizing tools usually consider simple battery degradation models, which disregard the impact of climatic conditions and operating strategy on battery performance. This study uses a semi-empirical Li-ion battery degradation model alongside an open-source techno-economic model to capture key insights. These are used to inform simple state of charge and temperature-based current derating strategies to increase lifetime. We demonstrate that such strategies can increase battery lifetime by 45% or 5–7 years in commercial systems already operational. It was found that, irrespective of climatic conditions, 80–90% of capacity fade can be attributed to calendar aging, due to low C-rates. SOC-based derating was found to be the most effective strategy, with temperature-based derating being less effective at extending lifetime and also leading to increased blackout periods. These results highlight the importance of accurate degradation modelling to achieve lifetime extension through improved operational strategies.
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Journal articleZhao W, Wang J, Tam B, et al., 2022,
Macroporous Vanadium Oxide Ion Storage Films Enable Fast Switching Speed and High Cycling Stability of Electrochromic Devices
, ACS APPLIED MATERIALS & INTERFACES, ISSN: 1944-8244- Author Web Link
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- Citations: 7
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Journal articleEisner F, Foot G, Yan J, et al., 2022,
Emissive charge-transfer states at hybrid inorganic/organic heterojunctions enable low non-radiative recombination and high-performance photodetectors
, Advanced Materials, Vol: 34, ISSN: 0935-9648Hybrid devices based on a heterojunction between inorganic and organic semiconductors have offered a means to combine the advantages of both classes of materials in optoelectronic devices, but, in practice, the performance of such devices has often been disappointing. Here, it is demonstrated that charge generation in hybrid inorganic–organic heterojunctions consisting of copper thiocyanate (CuSCN) and a variety of molecular acceptors (ITIC, IT-4F, Y6, PC70BM, C70, C60) proceeds via emissive charge-transfer (CT) states analogous to those found at all-organic heterojunctions. Importantly, contrary to what has been observed at previous organic–inorganic heterojunctions, the dissociation of the CT-exciton and subsequent charge separation is efficient, allowing the fabrication of planar photovoltaic devices with very low non-radiative voltage losses (0.21 ± 0.02 V). It is shown that such low non-radiative recombination enables the fabrication of simple and cost-effective near-IR (NIR) detectors with extremely low dark current (4 pA cm−2) and noise spectral density (3 fA Hz−1/2) at no external bias, leading to specific detectivities at NIR wavelengths of just under 1013 Jones, close to the performance of commercial silicon photodetectors. It is believed that this work demonstrates the possibility for hybrid heterojunctions to exploit the unique properties of both inorganic and organic semiconductors for high-performance opto-electronic devices.
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Journal articleYan J, Rodríguez-Martínez X, Pearce D, et al., 2022,
Identifying structure-absorption relationships and predicting absorption strength of non-fullerene acceptors for organic photovoltaics
, Energy & Environmental Science, Vol: 15, Pages: 2958-2973, ISSN: 1754-5692Non-fullerene acceptors (NFAs) are excellent light harvesters, yet the origin of their high optical extinction is not well understood. In this work, we investigate the absorption strength of NFAs by building a database of time-dependent density functional theory (TDDFT) calculations of ∼500 π-conjugated molecules. The calculations are first validated by comparison with experimental measurements in solution and solid state using common fullerene and non-fullerene acceptors. We find that the molar extinction coefficient (εd,max) shows reasonable agreement between calculation in vacuum and experiment for molecules in solution, highlighting the effectiveness of TDDFT for predicting optical properties of organic π-conjugated molecules. We then perform a statistical analysis based on molecular descriptors to identify which features are important in defining the absorption strength. This allows us to identify structural features that are correlated with high absorption strength in NFAs and could be used to guide molecular design: highly absorbing NFAs should possess a planar, linear, and fully conjugated molecular backbone with highly polarisable heteroatoms. We then exploit a random decision forest algorithm to draw predictions for εd,max using a computational framework based on extended tight-binding Hamiltonians, which shows reasonable predicting accuracy with lower computational cost than TDDFT. This work provides a general understanding of the relationship between molecular structure and absorption strength in π-conjugated organic molecules, including NFAs, while introducing predictive machine-learning models of low computational cost.
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Journal articleZhu L, Zhang M, Xu J, et al., 2022,
Single-junction organic solar cells with over 19% efficiency enabled by a refined double-fibril network morphology
, Nature Materials, Vol: 21, ISSN: 1476-1122In organic photovoltaics, morphological control of donor and acceptor domains on the nanoscale is the key for enabling efficient exciton diffusion and dissociation, carrier transport and suppression of recombination losses. To realize this, here, we demonstrated a double-fibril network based on a ternary donor–acceptor morphology with multi-length scales constructed by combining ancillary conjugated polymer crystallizers and a non-fullerene acceptor filament assembly. Using this approach, we achieved an average power conversion efficiency of 19.3% (certified 19.2%). The success lies in the good match between the photoelectric parameters and the morphological characteristic lengths, which utilizes the excitons and free charges efficiently. This strategy leads to an enhanced exciton diffusion length and a reduced recombination rate, hence minimizing photon-to-electron losses in the ternary devices as compared to their binary counterparts. The double-fibril network morphology strategy minimizes losses and maximizes the power output, offering the possibility of 20% power conversion efficiencies in single-junction organic photovoltaics.
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Journal articleHou X, Clarke AJ, Azzouzi M, et al., 2022,
Relationship between molecular properties and degradation mechanisms of organic solar cells based on bis-adducts of phenyl-C₆₁ butyric acid methyl ester
, Journal of Materials Chemistry C, Vol: 10, Pages: 7875-7885, ISSN: 2050-7526Environmental stability remains a major challenge for the commercialisation of organic solar cells and degradation pathways remain poorly understood. Designing materials for improved device stability requires an understanding of the relationship between the properties of the donor or acceptor molecule and different degradation mechanisms. Here we study the correlations between various molecular parameters of the fullerene derivative bis-PCBM and the degradation rate of polymer:bis-PCBM organic solar cells, based on the same carbazole-alt-benzothiadiazole polymer, in aerobic and anaerobic conditions. We compare eight high purity bis-PCBM isomers with different electronic, chemical and packing properties along with PCBM and the mixture of bis isomers. In the case of aerobic photodegradation, we find that device degradation rate is positively correlated to the LUMO energy of the bis-PCBM isomer and to the degree of crystallinity of the isomer, while the correlation of degradation with driving force for epoxide formation is unclear. These results support the idea that in these samples, aerobic photodegradation proceeds via superoxide formation by the photogenerated polaron on the fullerene, followed by further chemical reaction. In the absence of air, photodegradation rate is correlated with molecular structure, supporting the mechanism of microstructural degradation via fullerene dimerization. The approach and findings presented here show how control of specific molecular parameters through chemical design can serve as a strategy to enhance stability of organic solar cells.
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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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