Publications

Journal article

Beath H, Hauser M, Sandwell P, Gambhir A, Few S, Chambon CL, Nelson Jet 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: 2667-095X

Renewables-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.

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.

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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

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

Organic 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?

Journal article

Moss B, Babacan O, Kafizas A, Hankin Aet 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: 1614-6832

Green 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.

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

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., 2021,

Device Performance of Emerging Photovoltaic Materials (Version 1)

, ADVANCED ENERGY MATERIALS, Vol: 11, ISSN: 1614-6832

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Citations: 90

Journal article

Yiwen W, Jinho L, Xueyan H, Labanti C, Jun Y, Amber P, Eva M, 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

Abdulla A, Hanna R, Schell KR, Babacan O, Victor DGet 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

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Citations: 56

Journal article

Wade J, Hilfiker J, Brandt J, Liirò-Peluso L, Wan L, Shi X, Salerno F, Ryan S, Schöche S, Arteaga O, Jávorfi T, Siligardi G, Wang C, Amabilino D, Beton P, Campbell A, Fuchter Met al., 2020,

Natural optical activity as the origin of the large chiroptical properties in π-conjugated polymer thin films

, Nature Communications, Vol: 11, ISSN: 2041-1723

Polymer thin films that emit and absorb circularly polarised light have been demonstrated with the promise of achieving important technological advances; from efficient, high-performance displays, to 3D imaging and all-organic spintronic devices. However, the origin of the large chiroptical effects in such films has, until now, remained elusive. We investigate the emergence of such phenomena in achiral polymers blended with a chiral small-molecule additive (1-aza[6]helicene) and intrinsically chiral-sidechain polymers using a combination of spectroscopic methods and structural probes. We show that – under conditions relevant for device fabrication – the large chiroptical effects are caused by magneto-electric coupling (natural optical activity), not structural chirality as previously assumed, and may occur because of local order in a cylinder blue phase-type organisation. This disruptive mechanistic insight into chiral polymer thin films will offer new approaches towards chiroptical materials development after almost three decades of research in this area.

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.

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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

Giovannitti A, Rashid RB, Thiburce Q, Paulsen BD, Cendra C, Thorley K, Moia D, Mefford JT, Hanifi D, Weiyuan D, Moser M, Salleo A, Nelson J, McCulloch I, Rivnay Jet al., 2020,

Energetic control of redox-active polymers toward safe organic Bioelectronic materials

, Advanced Materials, Vol: 32, ISSN: 0935-9648

Avoiding faradaic side reactions during the operation of electrochemical devices is important to enhance the device stability, to achieve low power consumption, and to prevent the formation of reactive side‐products. This is particularly important for bioelectronic devices, which are designed to operate in biological systems. While redox‐active materials based on conducting and semiconducting polymers represent an exciting class of materials for bioelectronic devices, they are susceptible to electrochemical side‐reactions with molecular oxygen during device operation. Here, electrochemical side reactions with molecular oxygen are shown to occur during organic electrochemical transistor (OECT) operation using high‐performance, state‐of‐the‐art OECT materials. Depending on the choice of the active material, such reactions yield hydrogen peroxide (H2O2), a reactive side‐product, which may be harmful to the local biological environment and may also accelerate device degradation. A design strategy is reported for the development of redox‐active organic semiconductors based on donor–acceptor copolymers that prevents the formation of H2O2 during device operation. This study elucidates the previously overlooked side‐reactions between redox‐active conjugated polymers and molecular oxygen in electrochemical devices for bioelectronics, which is critical for the operation of electrolyte‐gated devices in application‐relevant environments.

Journal article

Moia D, Abe M, Wagner P, Saguchi H, Koumura N, Nelson J, Barnes PRF, Mori Set al., 2020,

The effect of the dielectric environment on electron transfer reactions at the interfaces of molecular sensitized semiconductors in electrolytes

, The Journal of Physical Chemistry C: Energy Conversion and Storage, Optical and Electronic Devices, Interfaces, Nanomaterials, and Hard Matter, Vol: 124, Pages: 6979-6992, ISSN: 1932-7447

Electron transfer theories predict that rates of charge transfer vary with the dielectric properties of the environment where the reaction occurs. An appropriate description of this relation for molecular sensitized semiconductors in electrolytes must account for the restricted geometry of these systems compared to “free” molecules in solution. Here, we explore the extent to which dielectric properties of the surrounding medium can explain the rates of charge transfer processes, measured using transient absorption spectroscopy, involving photo-oxidized thiophene–carbazole-based molecules on oxide semiconductors in inert or redox-active electrolytes. We observe no clear correlation between the activation energy of hole hopping between molecules on oxide surfaces or the recombination rate between photogenerated electrons in the oxide and holes on the adsorbed molecules and the dielectric properties of the surrounding solvent. The activation energy of hole hopping tends to increase with time following initial photogeneration of the holes, which we attribute to energetic disorder in the molecular monolayer. The recombination rate in different solvents scales with the hole hopping rate. It can also be varied by adding inert salts in the electrolyte and by controlling the access of cations in solution to the oxide surface. Finally, we show that fast electron transfer from cobalt complexes to photo-oxidized molecules in solvents with low polarity is verified, but the kinetics are limited by the ionic dissociation. Our study highlights the importance of electronic coupling between the redox-active components and their solvation, besides the reorganization energy and the driving force, in the determination of electron transfer rates at molecular sensitized interfaces in electrolytes.

Journal article

Hamid Z, Wadsworth A, Rezasoltani E, Holliday S, Azzouzi M, Neophytou M, Guilbert AAY, Dong Y, Little MS, Mukherjee S, Herzing AA, Bristow H, Kline RJ, DeLongchamp DM, Bakulin AA, Durrant JR, Nelson J, McCulloch Iet al., 2020,

Influence of Polymer Aggregation and Liquid Immiscibility on Morphology Tuning by Varying Composition in PffBT4T-2DT/Nonfullerene Organic Solar Cells

, ADVANCED ENERGY MATERIALS, Vol: 10, ISSN: 1614-6832

Journal article

Guilbert AAY, Zbiri M, Finn PA, Jenart M, Fouquet P, Cristiglio V, Frick B, Nelson J, Nielsen CBet al., 2019,

Mapping Microstructural Dynamics up to the Nanosecond of the Conjugated Polymer P3HT in the Solid State

, CHEMISTRY OF MATERIALS, Vol: 31, Pages: 9635-9651, ISSN: 0897-4756

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Citations: 13

Journal article

Warren R, Privitera A, Kaienburg P, Lauritzen AE, Thimm O, Nelson J, Riede MKet al., 2019,

Controlling energy levels and Fermi level en route to fully tailored energetics in organic semiconductors

, NATURE COMMUNICATIONS, Vol: 10, ISSN: 2041-1723

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Citations: 34

Journal article

Szumska AA, Sirringhaus H, Nelson J, 2019,

Symmetry based molecular design for triplet excitation and optical spin injection

, Physical Chemistry Chemical Physics, Vol: 21, Pages: 19521-19528, ISSN: 1463-9076

Spintronics, as a relatively new scientific field, is developing rapidly together with our understanding of spin related phenomena and spin manipulation. One of the challenges in the field is spin injection, which has been achieved optically in inorganic crystalline semiconductors, but not yet in organic semiconductors. Here, we introduce an approach whereby we apply group theory and computational methods to design molecular materials in which spin can be injected optically via circularly polarized light (CPL). Our approach is based on the use of group theory and double group theory to identify families of molecules whose symmetry satisfies design rules for optical excitation of triplets of particular properties. Employing such screening prior to detailed calculation can accelerate design by first identifying any structures that fail some criterion on grounds of symmetry. Here, we show using group theory and computational methods that particular families of molecules possess a low lying triplet state that can be excited with circularly polarized light causing spin polarization of an excited electron. Such structures are of potential interest for organic or molecular spintronics. We present an efficient procedure to identify candidate point groups and determine the excited state symmetry using group theory, before full calculation of excited states using relativistic quantum chemistry.

Journal article

Vezie MS, Azzouzi M, Telford AM, Hopper TR, Sieval AB, Hummelen JC, Fallon K, Bronstein H, Kirchartz T, Bakulin AA, Clarke TM, Nelson Jet al., 2019,

Impact of marginal exciton–charge-transfer state offset on charge generation and recombination in polymer:fullerene solar cells

, ACS Energy Letters, Vol: 4, Pages: 2096-2103, ISSN: 2380-8195

The energetic offset between the initial photoexcited state and charge-transfer (CT) state in organic heterojunction solar cells influences both charge generation and open-circuit voltage (Voc). Here, we use time-resolved spectroscopy and voltage loss measurements to analyze the effect of the exciton–CT state offset on charge transfer, separation, and recombination processes in blends of a low-band-gap polymer (INDT-S) with fullerene derivatives of different electron affinity (PCBM and KL). For the lower exciton–CT state offset blend (INDT-S:PCBM), both photocurrent generation and nonradiative voltage losses are lower. The INDT-S:PCBM blend shows different excited-state dynamics depending on whether the donor or acceptor is photoexcited. Surprisingly, the charge recombination dynamics in INDT-S:PCBM are distinctly faster than those in INDT-S:KL upon excitation of the donor. We reconcile these observations using a kinetic model and by considering hybridization between the lowest excitonic and CT states. The modeling results show that this hybridization can significantly reduce Voc losses while still allowing reasonable charge generation efficiency.

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Cheetham NJ, Ortiz M, Pereyedentsev A, Dion-Bertrand L-I, Greetham GM, Sazanoyich I, Towrie M, Parker AW, Nelson J, Silva C, Bradley DDC, Hayes SC, Stavrinou PNet al., 2019,

The Importance of Microstructure in Determining Polaron Generation Yield in Poly(9,9-dioctylfluorene)

, CHEMISTRY OF MATERIALS, Vol: 31, Pages: 6787-6797, ISSN: 0897-4756

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Citations: 19

Journal article

Yang W, Godin R, Kasap H, Moss B, Dong Y, Hillman SAJ, Steier L, Reisner E, Durrant JRet al., 2019,

Electron accumulation induces efficiency bottleneck for hydrogen production in carbon nitride photocatalysts

, Journal of the American Chemical Society, Vol: 141, Pages: 11219-11229, ISSN: 1520-5126

This study addresses the light intensity dependence of charge accumulation in a photocatalyst suspension, and its impact on both charge recombination kinetics and steady-state H2 evolution efficiency. Cyanamide surface functionalized melon-type carbon nitride (NCNCNx) has been selected as an example of emerging carbon nitrides photocatalysts because of its excellent charge storage ability. Transient spectroscopic studies (from ps to s) show that the bimolecular recombination of photogenerated electrons and holes in NCNCNx can be well described by a random walk model. Remarkably, the addition of hole scavengers such as 4-methylbenzyl alcohol can lead to ∼400-fold faster recombination kinetics (lifetime shortening to ∼10 ps). We show that this acceleration is not the direct result of ultrafast hole extraction by the scavenger, but is rather caused by long-lived electron accumulation in NCNCNx after hole extraction. The dispersive pseudo-first order recombination kinetics become controlled by the density of accumulated electrons. H2 production and steady-state spectroscopic measurements indicate that the accelerated recombination caused by electron accumulation limits the H2 generation efficiency. The addition of a reversible electron acceptor and mediator, methyl viologen (MV2+), accelerates the extraction of electrons from the NCNCNx and increases the H2 production efficiency under one sun irradiation by more than 30%. These results demonstrate quantitatively that while long-lived electrons are essential to drive photoinduced H2 generation in many photocatalysts, excessive electron accumulation may result in accelerated recombination losses and lower performance, and thus highlight the importance of efficient electron and hole extraction in enabling efficient water splitting photocatalysts.

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