Imperial College London

Professor Jenny Nelson

Faculty of Natural SciencesDepartment of Physics

Professor of Physics
 
 
 
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Contact

 

+44 (0)20 7594 7581jenny.nelson

 
 
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Location

 

1007Huxley BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

412 results found

Zhu L, Zhang M, Xu J, Li C, Yan J, Zhou G, Zhong W, Hao T, Song J, Xue X, Zhou Z, Zeng R, Zhu H, Chen C-C, MacKenzie RCI, Zou Y, Nelson J, Zhang Y, Sun Y, Liu Fet al., 2022, Single-junction organic solar cells with over 19% efficiency enabled by a refined double-fibril network morphology., Nat Mater, ISSN: 1476-1122

In 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.

Journal article

Tan E, Kim J, Stewart K, Pitsalidis C, Kwon S, Siemons N, Kim J, Jiang Y, Frost JM, Pearce D, Tyrrell JE, Nelson J, Owens RM, Kim Y-H, Kim J-Set al., 2022, The Role of Long-Alkyl-Group Spacers in Glycolated Copolymers for High Performance Organic Electrochemical Transistors., Adv Mater

Semiconducting 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.

Journal article

Azzouzi M, Nelson J, Eisner F, Gallop N, Yan J, Zheng X, Cha H, He Q, Fei Z, Heeney M, Bakulin Aet 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: 1754-5692

Achieving 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.

Journal article

Few S, Barton J, Sandwell P, Mori R, Kulkarni P, Thomson M, Nelson J, Candelise Cet 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: 0973-0826

Almost 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

Journal article

Ward MD, Wade J, Shi X, Nelson J, Campbell AJ, Fuchter MJet al., 2022, Highly Selective High-Speed Circularly Polarized Photodiodes Based on π-Conjugated Polymers, Advanced Optical Materials, Vol: 10, ISSN: 2195-1071

Journal article

Hou X, Clarke AJ, Azzouzi M, Yan J, Eisner F, Shi X, Wyatt MF, Dennis TJS, Li Z, Nelson Jet al., 2022, Relationship between molecular properties and degradation mechanisms of organic solar cells based on bis-adducts of phenyl-C<sub>61</sub> butyric acid methyl ester, Journal of Materials Chemistry C, ISSN: 2050-7526

<jats:p>Change of the side chain position of bis-PCBM can modulate the molecular parameters and further serve as a strategy to control the stability of organic solar cells.</jats:p>

Journal article

Eisner F, Tam B, Belova V, Ow W, Yan J, Azzouzi M, Kafizas A, Campoy-Quiles M, Hankin A, Nelson Jet 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: 2666-3864

The 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.

Journal article

Almora O, Baran D, Bazan GC, Berger C, Cabrera C, Catchpole KR, Erten-Ela S, Guo F, Hauch J, Ho-Baillie AWY, Jacobsson TJ, Janssen RAJ, Kirchartz T, Kopidakis N, Li Y, Loi MA, Lunt RR, Mathew X, McGehee MD, Min J, Mitzi DB, Nazeeruddin MK, Nelson J, Nogueira AF, Paetzold UW, Park N-G, Rand BP, Rau U, Snaith HJ, Unger E, Vaillant-Roca L, Yip H-L, Brabec CJet al., 2021, Device Performance of Emerging Photovoltaic Materials (Version 2), ADVANCED ENERGY MATERIALS, Vol: 11, ISSN: 1614-6832

Journal article

Eisner F, Foot G, Yan J, Azzouzi M, Georgiadou DG, Sit WY, Firdaus Y, Zhang G, Lin Y-H, Yip H-L, Anthopoulos TD, Nelson Jet al., 2021, Emissive charge-transfer states at hybrid inorganic/organic heterojunctions enable low non-radiative recombination and high-performance photodetectors, Advanced Materials, ISSN: 0935-9648

Hybrid 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.

Journal article

Schmidt J, Weatherby J, Sugden I, Santana-Bonilla A, Salerno F, Fuchter M, Johnson E, Nelson J, Jelfs Ket 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: 1528-7483

Molecular 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.

Journal article

Szumska AA, Maria IP, Flagg LQ, Savva A, Surgailis J, Paulsen BD, Moia D, Chen X, Griggs S, Mefford JT, Rashid RB, Marks A, Inal S, Ginger DS, Giovannitti A, Nelson Jet 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

Journal article

Jux N, Reger D, Haines P, Amsharov KY, Schmidt JA, Ullrich T, Bönisch S, Hampel F, Görling A, Nelson J, Jelfs KE, Guldi DMet 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: 1433-7851

Incorporating 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.

Journal article

Reger D, Haines P, Amsharov KY, Schmidt JA, Ullrich T, Bönisch S, Hampel F, Görling A, Nelson J, Jelfs KE, Guldi DM, Jux Net 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

Journal article

Eisner F, Nelson J, 2021, Barrierless charge generation at non-fullerene organic heterojunctions comes at a cost, Joule, Vol: 5, Pages: 1319-1322, ISSN: 2542-4351

The 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.

Journal article

Yan J, Rezasoltani E, Azzouzi M, Eisner F, Nelson Jet al., 2021, Influence of static disorder of charge transfer state on voltage loss in organic photovoltaics, Nature Communications, Vol: 12, ISSN: 2041-1723

Spectroscopic 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.

Journal article

Ortega-Arriaga P, Babacan O, Nelson J, Gambhir Aet 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: 1364-0321

This 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.

Journal article

Baranda 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: 2213-1388

Electricity 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.

Journal article

Yiwen W, Jinho L, Xueyan H, Labanti C, Jun Y, Amber P, Eva M, Jenny N, Ji-Seon K, Zhe L, Yiwen W, Jinho L, Xueyan H, Chiara L, Yan J, Eva M, Amber P, Jenny N, Ji-Seon K, Zhe Let al., 2021, Recent progress and Challenges toward highly stable nonfullerene acceptor‐based organic solar cells, Advanced Energy Materials, Vol: 11, ISSN: 1614-6832

Organic 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.

Journal article

Armin A, Li W, Sandberg OJ, Xiao Z, Ding L, Nelson J, Neher D, Vandewal K, Shoaee S, Wang T, Ade H, Heumueller T, Brabec C, Meredith Pet al., 2021, A History and Perspective of Non-Fullerene Electron Acceptors for Organic Solar Cells, ADVANCED ENERGY MATERIALS, Vol: 11, ISSN: 1614-6832

Journal article

Almora O, Baran D, Bazan GC, Berger C, Cabrera CI, Catchpole KR, Erten-Ela S, Guo F, Hauch J, Ho-Baillie AWY, Jacobsson TJ, Janssen RAJ, Kirchartz T, Kopidakis N, Li Y, Loi MA, Lunt RR, Mathew X, McGehee MD, Min J, Mitzi DB, Nazeeruddin MK, Nelson J, Nogueira AF, Paetzold UW, Park N-G, Rand BP, Rau U, Snaith HJ, Unger E, Vaillant-Roca L, Yip H-L, Brabec CJet al., 2020, Device Performance of Emerging Photovoltaic Materials (Version 1), ADVANCED ENERGY MATERIALS, Vol: 11, ISSN: 1614-6832

Journal article

Few S, Djapic P, Strbac G, Nelson J, Candelise Cet 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: 0960-1481

Highly 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.

Journal article

Lubert-Perquel D, Szumska AA, Azzouzi M, Salvadori E, Ruloff S, Kay CMW, Nelson J, Heutz Set al., 2020, Structure dependence of kinetic and thermodynamic parameters in singlet fission processes., Journal of Physical Chemistry Letters, Vol: 11, Pages: 9557-9565, ISSN: 1948-7185

Singlet 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.

Journal article

He Q, Eisner FD, Pearce D, Hodsden T, Rezasoltani E, Medranda D, Fei Z, Nelson J, Heeney Met 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: 2050-7526

In 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.

Journal article

Cong S, Creamer A, Fei Z, Hillman SAJ, Rapley C, Nelson J, Heeney Met 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: 1616-5187

A 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.

Journal article

Rezasoltani E, Guilbert AAY, Yan J, Rodríguez-Martínez X, Azzouzi M, Eisner F, Tuladhar SM, Hamid Z, Wadsworth A, McCulloch I, Campoy-Quiles M, Nelson Jet 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: 0897-4756

The 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.

Journal article

Babacan O, De Causmaecker S, Gambhir A, Fajardy M, Rutherford AW, Fantuzzi A, Nelson Jet al., 2020, Assessing the feasibility of carbon dioxide mitigation options in terms of energy usage, Nature Energy, Vol: 5, Pages: 720-728, ISSN: 2058-7546

Measures 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.

Journal article

Xiao B, Calado P, Mackenzie R, Kirchartz T, Yan J, Nelson Jet 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: 2331-7019

The 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.

Journal article

Woods DJ, Hillman S, Pearce D, Wilbraham L, Flagg L, Duffy W, Mcculloch I, Durrant J, Guilbert A, Zwijnenburg M, Sprick RS, Nelson J, Cooper Aet al., 2020, Side-chain tuning in conjugated polymer photocatalysts for improved hydrogen production from water, Energy & Environmental Science, Vol: 13, Pages: 1843-1855, ISSN: 1754-5692

Structure–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.

Journal article

Azzouzi M, Calado P, Telford A, Eisner F, Hou X, Kirchartz T, Barnes P, Nelson Jet al., 2020, Overcoming the limitations of transient photovoltage measurements for studying recombination in organic solar cells, Solar Rrl, Vol: 4, ISSN: 2367-198X

Transient photovoltage (TPV) measurements are frequently used to study recombination processes in thin-film solar cells by probing the decay of a small optically-induced voltage perturbation to infer the charge carrier dynamics of devices at open circuit. However, the validity of this method to probe organic semiconductors has recently come into doubt due to large discrepancies in reported carrier lifetime values for the same systems, and the reporting of unrealistic reaction order values. In this work, we explore the validity of TPV to extract reliable charge carrier lifetimes in thin-film solar cells through the use of time-dependent drift diffusion simulations and measurements. We find that in low mobility materials, TPV serves primarily as a probe of charge carrier redistribution in the bulk rather than bulk recombination dynamics, and that the extracted time constant is highly mobility dependent. To address this shortcoming, we develop Transient Photo-Charge (TPQ) a new technique to measure the charge carrier density during the photo-voltage decay and apply it to study the recombination dynamics in a series of (fullerene and non-fullerene) organic solar cell systems. We show that using this technique the charge carrier recombination lifetime in the active layer can be more accurately determined.

Journal article

Kaienburg P, Krückemeier L, Lübke D, Nelson J, Rau U, Kirchartz Tet al., 2020, How solar cell efficiency is governed by the αμτ product, Physical Review Research, Vol: 2, Pages: 023109 – 1-023109 – 15, ISSN: 2643-1564

The interplay of light absorption, charge-carrier transport, and charge-carrier recombination determines the performance of a photovoltaic absorber material. Here we analyze the influence on the solar-cell efficiency of the absorber material properties absorption coefficient α, charge-carrier mobility μ, and charge-carrier lifetime τ, for different scenarios. We combine analytical calculations with numerical drift-diffusion simulations to understand the relative importance of these three quantities. Whenever charge collection is a limiting factor, the αμτ product is a good figure of merit (FOM) to predict solar-cell efficiency, while for sufficiently high mobilities, the relevant FOM is reduced to the ατ product. We find no fundamental difference between simulations based on monomolecular or bimolecular recombination, but strong surface-recombination affects the maximum efficiency in the high-mobility limit. In the limiting case of high μ and high surface-recombination velocity S, the α/S ratio is the relevant FOM. Subsequently, we apply our findings to organic solar cells which tend to suffer from inefficient charge-carrier collection and whose absorptivity is influenced by interference effects. We estimate that a modest increase in absorption strength by a factor of 1.5 leads to a relative efficiency increase of more than 10% for state-of-the-art organic solar cells.

Journal article

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