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Journal articleHuang J, Xu B, Tian L, et al., 2019,
A heavy metal-free CuInS<sub>2</sub> quantum dot sensitized NiO photocathode with a Re molecular catalyst for photoelectrochemical CO<sub>2</sub> reduction
, Chemical Communications, Vol: 55, Pages: 7918-7921, ISSN: 1359-7345<p>Heavy metal-free CuInS<sub>2</sub> QDs as a photosensitizer and a Re molecular catalyst have been successfully employed to co-sensitize a NiO photocathode for CO<sub>2</sub> reduction into CO.</p>
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Journal articleDu T, Burgess C, Lin C-T, et al., 2018,
Probing and controlling intra-grain crystallinity for improved low-temperature processed perovskite solar cells
, Advanced Functional Materials, Vol: 28, ISSN: 1616-301XHere, previously unobserved nanoscale defects residing within individual grains of solution‐processed methylammonium lead tri‐iodide (CH3NH3PbI3, MAPI) thin films are identified. Using scanning transmission electron microscopy (STEM), the defects inherently associated with the established solution‐processing methodology are identified, and a facile processing modification to eliminate these defects is introduced. Specifically, defect elimination is achieved by coannealing the as‐deposited MAPI layer with the electron transport layer (phenyl‐C61‐butyric acid methyl, PCBM) resulting in devices that significantly outperform devices prepared using the established methodology—with power conversion efficiencies increasing from 13.6% to 17.4%. The use of transmission electron microscopy allows the correlation of performance enhancements to improved intragrain crystallinity and shows that highly coherent crystallographic orientation results within individual grains when processing is modified. Detailed optoelectronic characterization reveals that the improved intragrain crystallinity drives an improvement of charge collection and a reduction of PEDOT:PSS/perovskite interfacial recombination. The study suggests that the microstructural defects in MAPI, owing to a lack of structural coherence throughout the thickness of thin film, are a significant cause of interfacial recombination.
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Journal articleKim J, Godin R, Dimitrov SD, et al., 2018,
Excitation density dependent photoluminescence quenching and charge transfer efficiencies in hybrid perovskite/organic semiconductor bilayers
, Advanced Energy Materials, Vol: 8, ISSN: 1614-6832This study addresses the dependence of charge transfer efficiency between bilayers of methylammonium lead iodide (MAPI3) with PC61BM or poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) charge transfer layers on excitation intensity. It analyzes the kinetic competition between interfacial electron/hole transfer and charge trapping and recombination within MAPI3 by employing a range of optical measurements including steady-state (SS) photoluminescence quenching (PLQ), and transient photoluminescence and absorption over a broad range of excitation densities. The results indicate that PLQ measurements with a typical photoluminescence spectrometer can yield significantly different transfer efficiencies to those measured under 1 Sun irradiation. Steady-state and pulsed measurements indicate low transfer efficiencies at low excitation conditions (<5E + 15 cm−3) due to rapid charge trapping and low transfer efficiencies at high excitation conditions (>5E + 17 cm−3) due to fast bimolecular recombination. Efficient transfer to PC61BM or PEDOT:PSS is only observed under intermediate excitation conditions (≈1 Sun irradiation) where electron and hole transfer times are determined to be 36 and 11 ns, respectively. The results are discussed in terms of their relevance to the excitation density dependence of device photocurrent generation, impact of charge trapping on this dependence, and appropriate methodologies to determine charge transfer efficiencies relevant to device performance.
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Journal articleKosco J, Sachs M, Godin R, et al., 2018,
The effect of residual palladium catalyst contamination on the photocatalytic hydrogen evolution activity of conjugated polymers
, Advanced Energy Materials, Vol: 8, ISSN: 1614-6832The effect of residual Pd on hydrogen evolution activity in conjugated polymer photocatalytic systems is systematically investigated using colloidal poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) nanoparticles as a model system. Residual Pd, originating from the synthesis of F8BT via Pd catalyzed polycondensation polymerization, is observed in the form of homogeneously distributed Pd nanoparticles within the polymer. Residual Pd is essential for any hydrogen evolution to be observed from this polymer, and very low Pd concentrations (<40 ppm) are sufficient to have a significant effect on the hydrogen evolution reaction (HER) rate. The HER rate increases linearly with increasing Pd concentration from <1 ppm to approximately 100 ppm, at which point the rate begins to saturate. Transient absorption spectroscopy experiments support these conclusions, and suggest that residual Pd mediates electron transfer from the F8BT nanoparticles to protons in the aqueous medium.
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Journal articleCorby S, Francàs L, Selim S, et al., 2018,
Water oxidation and electron extraction kinetics in nanostructured tungsten trioxide photoanodes
, Journal of the American Chemical Society, Vol: 140, Pages: 16168-16177, ISSN: 1520-5126A thorough understanding of the kinetic competition between desired water oxidation/electron extraction processes and any detrimental surface recombination is required to achieve high water oxidation efficiencies in transition-metal oxide systems. The kinetics of these processes in high Faradaic efficiency tungsten trioxide (WO3) photoanodes (>85%) are monitored herein by transient diffuse reflectance spectroscopy and correlated with transient photocurrent data for electron extraction. Under anodic bias, efficient hole transfer to the aqueous electrolyte is observed within a millisecond. In contrast, electron extraction is found to be comparatively slow (∼10 ms), increasing in duration with nanoneedle length. The relative rates of these water oxidation and electron extraction kinetics are shown to be reversed in comparison to other commonly examined metal oxides (e.g., TiO2, α-Fe2O3, and BiVO4). Studies conducted as a function of applied bias and film processing to modulate oxygen vacancy density indicate that slow electron extraction kinetics result from electron trapping in shallow WO3 trap states associated with oxygen vacancies. Despite these slow electron extraction kinetics, charge recombination losses on the microsecond to second time scales are observed to be modest compared to other oxides studied. We propose that the relative absence of such recombination losses, and the observation of a photocurrent onset potential close to flat-band, result directly from the faster water oxidation kinetics of WO3. We attribute these fast water oxidation kinetics to the highly oxidizing valence band position of WO3, thus highlighting the potential importance of thermodynamic driving force for catalysis in outcompeting detrimental surface recombination processes.
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Journal articleCollado L, Reynal A, Fresno F, et al., 2018,
Unravelling the effect of charge dynamics at the plasmonic metal/semiconductor interface for CO2 photoreduction
, Nature Communications, Vol: 9, ISSN: 2041-1723Sunlight plays a critical role in the development of emerging sustainable energy conversion and storage technologies. Light-induced CO2 reduction by artificial photosynthesis is one of the cornerstones to produce renewable fuels and environmentally friendly chemicals. Interface interactions between plasmonic metal nanoparticles and semiconductors exhibit improved photoactivities under a wide range of the solar spectrum. However, the photo-induced charge transfer processes and their influence on photocatalysis with these materials are still under debate, mainly due to the complexity of the involved routes occurring at different timescales. Here, we use a combination of advanced in situ and time-resolved spectroscopies covering different timescales, combined with theoretical calculations, to unravel the overall mechanism of photocatalytic CO2 reduction by Ag/TiO2 catalysts. Our findings provide evidence of the key factors determining the enhancement of photoactivity under ultraviolet and visible irradiation, which have important implications for the design of solar energy conversion materials.
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Journal articleSachs M, Sprick RS, Pearce D, et al., 2018,
Understanding structure-activity relationships in linear polymer photocatalysts for hydrogen evolution
, Nature Communications, Vol: 9, ISSN: 2041-1723Conjugated polymers have sparked much interest as photocatalysts for hydrogen production. However, beyond basic considerations such as spectral absorption, the factors that dictate their photocatalytic activity are poorly understood. Here we investigate a series of linear conjugated polymers with external quantum efficiencies for hydrogen production between 0.4 and 11.6%. We monitor the generation of the photoactive species from femtoseconds to seconds after light absorption using transient spectroscopy and correlate their yield with the measured photocatalytic activity. Experiments coupled with modeling suggest that the localization of water around the polymer chain due to the incorporation of sulfone groups into an otherwise hydrophobic backbone is crucial for charge generation. Calculations of solution redox potentials and charge transfer free energies demonstrate that electron transfer from the sacrificial donor becomes thermodynamically favored as a result of the more polar local environment, leading to the production of long-lived electrons in these amphiphilic polymers.
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Journal articleCha H, Tan C-H, Wu J, et al., 2018,
An analysis of the factors determining the efficiency of photocurrent generation in polymer:nonfullerene acceptor solar cells
, Advanced Energy Materials, Vol: 8, ISSN: 1614-6832Herein, a meta‐analysis of the device performance and transient spectroscopic results are undertaken for various donor:acceptor blends, employing three different donor polymers and seven different acceptors including nonfullerene acceptors (NFAs). From this analysis, it is found that the primary determinant of device external quantum efficiency (EQE) is the energy offset driving interfacial charge separation, ΔECS. For devices employing the donor polymer PffBT4T blended with NFA and fullerene acceptors, an energy offset ΔECS = 0.30 eV is required to achieve near unity charge separation, which increases for blends with PBDTTT‐EFT and P3HT to 0.36 and ≈1.2 eV, respectively. For blends with PffBT4T and PBDTTT‐EFT, a 100 meV decrease in the LUMO of the acceptor is observed to result in an approximately twofold increase in EQE. Steady state and transient optical data determine that this energy offset requirement is not associated with the need to overcome the polymer exciton binding energy and thereby drive exciton separation, with all blends studied showing efficient exciton separation. Rather, the increase in EQE with larger energy offset is shown to result from suppression of geminate recombination losses. These results are discussed in terms of their implications for the design of donor/NFA interfaces in organic solar cells, and strategies to achieve further advances in device performance.
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Journal articleSorcar S, Thompson J, Hwang Y, et al., 2018,
High-rate solar-light photoconversion of CO2 to fuel: controllable transformation from C-1 to C-2 products
, ENERGY & ENVIRONMENTAL SCIENCE, Vol: 11, Pages: 3183-3193, ISSN: 1754-5692 -
Journal articleTian L, Németh B, Berggren G, et al., 2018,
Hydrogen evolution by a photoelectrochemical cell based on a Cu2O-ZnO-[FeFe] hydrogenase electrode
, Journal of Photochemistry and Photobiology A: Chemistry, Vol: 366, Pages: 27-33, ISSN: 1010-6030 -
Journal articleGodin R, Hisatomi T, Domen K, et al., 2018,
Understanding the visible-light photocatalytic activity of GaN:ZnO solid solution: the role of Rh2-yCryO3 cocatalyst and charge carrier lifetimes over tens of seconds
, Chemical Science, Vol: 9, Pages: 7546-7555, ISSN: 2041-6520A persistent challenge for the widespread deployment of solar fuels is the development of high efficiency photocatalysts combined with a low-cost preparation and implementation route. Since its discovery in 2005, GaN:ZnO solid solution has been a benchmark overall water splitting photocatalyst. Notably, GaN:ZnO functionalised with an appropriate proton reduction cocatalyst is one of the few particulate photocatalyst systems that can generate hydrogen and oxygen directly from water using visible light. However, the reasons underlying the remarkable visible light activity of GaN:ZnO are not well understood and photophysical studies of GaN:ZnO have been limited to date. Using time-resolved optical spectroscopies, we investigated the charge carrier dynamics of GaN:ZnO and the effect of Rh2-y Cr y O3 proton reduction cocatalyst. Here we show that charge trapping and trap state filling play an important role in controlling the photophysics of GaN:ZnO. We also find that electrons transfer to Rh2-y Cr y O3 on sub-microsecond timescales, important to reduce the electron concentration within GaN:ZnO and promote hole accumulation. Operando measurements showed that the water oxidation process is the rate determining process, and that the dependence of the rate of water oxidation on the accumulated hole density is similar to common metal oxides photoanodes such as TiO2, α-Fe2O3, and BiVO4. Remarkably, we show that the recombination timescale of holes accumulated on the surface of GaN:ZnO is on the order of 30 s, distinctly longer than for metal oxides photoanodes. We conclude that the unusual visible light activity of GaN:ZnO is a result of large electron-hole spatial separation due to the preferential flow of holes to the GaN-rich surface and efficient electron extraction by the cocatalyst. Our studies demonstrate that in depth spectroscopic investigations of the charge carrier dynamic
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Journal articlePont S, Foglia F, Higgins A, et al., 2018,
Stability of polymer:PCBM thin films under competitive illumination and thermal stress
, Advanced Functional Materials, Vol: 28, ISSN: 1616-301XThe combined effects of illumination and thermal annealing on the morphological stability and photodimerization in polymer/fullerene thin films are examined. While illumination is known to cause fullerene dimerization and thermal stress their dedimerization, the operation of solar cells involves exposure to both. The competitive outcome of these factors with blends of phenyl‐C61‐butyric acid methyl ester (PCBM) and polystyrene (PS), supported on PEDOT:PSS is quantified. UV–vis spectroscopy is employed to quantify dimerization, time‐resolved neutron reflectivity to resolve the vertical composition stratification, and atomic force microscopy for demixing and coarsening in thin films. At the conventional thermal stress test temperature of 85 °C (and even up to the PS glass transition), photodimerization dominates, resulting in relative morphological stability. Prior illumination is found to result in improved stability upon high temperature annealing, compatible with the need for dedimerization to occur prior to structural relaxation. Modeling of the PCBM surface segregation data suggests that only PCBM monomers are able to diffuse and that illumination provides an effective means to control dimer population, and thus immobile fullerene fraction, in the timescales probed. The results provide a framework for understanding of the stability of organic solar cells under operating conditions.
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Journal articleHong J, Sung MJ, Cha H, et al., 2018,
Understanding Structure-Property Relationships in All-Small-Molecule Solar Cells Incorporating a Fullerene or Nonfullerene Acceptor
, ACS APPLIED MATERIALS & INTERFACES, Vol: 10, Pages: 36037-36046, ISSN: 1944-8244 -
Journal articleJin MH, Shin E, Jin S, et al., 2018,
Solvothermal Synthesis of Ferroelectric BaTiO3 Nanoparticles and Their Application to Dye-sensitized Solar Cells
, JOURNAL OF THE KOREAN PHYSICAL SOCIETY, Vol: 73, Pages: 627-631, ISSN: 0374-4884 -
Journal articleLi M, Zhao C, Wang Z-K, et al., 2018,
Interface Modification by Ionic Liquid: A Promising Candidate for Indoor Light Harvesting and Stability Improvement of Planar Perovskite Solar Cells
, ADVANCED ENERGY MATERIALS, Vol: 8, ISSN: 1614-6832 -
Journal articleShoaee S, Durrant JR, 2018,
Oxygen diffusion dynamics in organic semiconductor films (vol 3, pg 10079, 2015)
, JOURNAL OF MATERIALS CHEMISTRY C, Vol: 6, Pages: 8553-8554, ISSN: 2050-7526 -
Journal articleDu T, Kim J, Ngiam J, et al., 2018,
Elucidating the origins of subgap tail states and open-circuit voltage in methylammonium lead triiodide perovskite solar cells
, Advanced Functional Materials, Vol: 28, Pages: 1-11, ISSN: 1616-301XRecombination via subgap trap states is considered a limiting factor in the development of organometal halide perovskite solar cells. Here, the impact of active layer crystallinity on the accumulated charge and open‐circuit voltage (Voc) in solar cells based on methylammonium lead triiodide (CH3NH3PbI3, MAPI) is demonstrated. It is shown that MAPI crystallinity can be systematically tailored by modulating the stoichiometry of the precursor mix, where small quantities of excess methylammonium iodide (MAI) improve crystallinity, increasing device Voc by ≈200 mV. Using in situ differential charging and transient photovoltage measurements, charge density and charge carrier recombination lifetime are determined under operational conditions. Increased Voc is correlated to improved active layer crystallinity and a reduction in the density of trap states in MAPI. Photoluminescence spectroscopy shows that an increase in trap state density correlates with faster carrier trapping and more nonradiative recombination pathways. Fundamental insights into the origin of Voc in perovskite photovoltaics are provided and it is demonstrated why highly crystalline perovskite films are paramount for high‐performance devices.
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Journal articleDu T, Kim J, Ngiam J, et al., 2018,
Elucidating the origins of sub-gap tail states and open-circuit voltage in methylammonium lead triiodide perovskite solar cells
, Advanced Functional Materials, Vol: 28, ISSN: 1616-301XRecombination via sub-gap trap states is considered a limiting factor in the development of organometal halide perovskite solar cells. Here, we demonstrate the impact of active layer crystallinity on the accumulated charge and open-circuit voltage (Voc) in solar cells based on methylammonium lead triiodide (CH3NH3PbI3, MAPI). We show MAPI crystallinity can be systematically tailored by modulating the stoichiometry of the precursor mix, where small quantities of excess methylammonium iodide (MAI) improves crystallinity increasing device Voc by ~200 mV. Using in-situ differential charging and transient photovoltage measurements, charge density and charge carrier recombination lifetime are determined under operational conditions. Increased Voc is correlated to improved active layer crystallinity and a reduction in the density of trap states in MAPI. Photoluminescence spectroscopy shows that an increase in trap states correlates with faster carrier trapping and more non-radiative recombination pathways. We provide fundamental insights into the origin of Voc in perovskite photovoltaics and demonstrate why highly crystalline perovskite films are paramount for high-performance devices.
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Journal articleKasap H, Godin R, Jeay-Bizot C, et al., 2018,
Interfacial Engineering of a Carbon Nitride-Graphene Oxide-Molecular Ni Catalyst Hybrid for Enhanced Photocatalytic Activity
, ACS CATALYSIS, Vol: 8, Pages: 6914-6926, ISSN: 2155-5435 -
Journal articleChristoforidis KC, Syrgiannis Z, La Parola V, et al., 2018,
Metal-free dual-phase full organic carbon nanotubes/g-C3N4 heteroarchitectures for photocatalytic hydrogen production
, NANO ENERGY, Vol: 50, Pages: 468-478, ISSN: 2211-2855 -
Journal articleLin C, Pont S, Kim J, et al., 2018,
Passivation of oxygen and light induced degradation by the PCBM electron transport layer in planar perovskite solar cells
, Sustainable Energy and Fuels, Vol: 2, Pages: 1686-1692, ISSN: 2398-4902Herein, we investigate the causes of a 20 fold improved stability of inverted, planar structure (ITO/PTAA/CH3NH3PbI3/PCBM/BCP/Cu) compared to conventional structure devices (FTO/compact-TiO2/meso-TiO2/CH3NH3PbI3/spiro-OMeTAD/Au) under oxygen and light stress. The PCBM layer is shown to function as an oxygen diffusion barrier and passivation layer against superoxide mediated degradation. The passivation properties of the PCBM layer are shown to depend on the electron affinity of fullerene acceptor, attributed to low LUMO level of PCBM energetically inhibiting superoxide generation. We also find the planar structure devices shows slower lateral oxygen diffusion rates than mesoporous scaffold devices, with these slower diffusion rates (days per 100 μm) also being a key factor in enhancing stability. Faster degradation is observed under voltage cycling, attributed to oxygen diffusion kinetics being ion motion dependent. We conclude by discussing the implications of these results for the design of perovskite solar cells with improved resistance to oxygen and light induced degradation.
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Journal articleLee HKH, Durrant JR, Li Z, et al., 2018,
Stability study of thermal cycling on organic solar cells
, JOURNAL OF MATERIALS RESEARCH, Vol: 33, Pages: 1902-1908, ISSN: 0884-2914 -
Journal articleJain SM, Phuyal D, Davies ML, et al., 2018,
An effective approach of vapour assisted morphological tailoring for reducing metal defect sites in lead-free, (CH3NH3)(3)Bi2I9 bismuth-based perovskite solar cells for improved performance and long-term stability
, NANO ENERGY, Vol: 49, Pages: 614-624, ISSN: 2211-2855 -
Journal articleHong DP, Thu TD, Kim J, et al., 2018,
Molecular Engineering Using an Anthanthrone Dye for Low-Cost Hole Transport Materials: A Strategy for Dopant-Free, High-Efficiency, and Stable Perovskite Solar Cells
, ADVANCED ENERGY MATERIALS, Vol: 8, ISSN: 1614-6832 -
Journal articleBaran D, Gasparini N, Wadsworth A, et al., 2018,
Robust nonfullerene solar cells approaching unity external quantum efficiency enabled by suppression of geminate recombination
, Nature Communications, Vol: 9, ISSN: 2041-1723Nonfullerene solar cells have increased their efficiencies up to 13%, yet quantum efficiencies are still limited to 80%. Here we report efficient nonfullerene solar cells with quantum efficiencies approaching unity. This is achieved with overlapping absorption bands of donor and acceptor that increases the photon absorption strength in the range from about 570 to 700 nm, thus, almost all incident photons are absorbed in the active layer. The charges generated are found to dissociate with negligible geminate recombination losses resulting in a short-circuit current density of 20 mA cm−2 along with open-circuit voltages >1 V, which is remarkable for a 1.6 eV bandgap system. Most importantly, the unique nano-morphology of the donor:acceptor blend results in a substantially improved stability under illumination. Understanding the efficient charge separation in nonfullerene acceptors can pave the way to robust and recombination-free organic solar cells.
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Journal articleFrancas Forcada L, Burns E, Steier L, et al., 2018,
Rational design of a neutral pH functional and stable organic photocathode.
, Chemical Communications, Vol: 2018, ISSN: 1359-7345In this work we lay out design guidelines for catalytically more efficient organic photocathodes achieving stable hydrogen production in neutral pH. We propose an organic photocathode architecture employing a NiO hole selective layer, a PCDTBT:PCBM bulk heterojunction, a compact TiO2 electron selective contact and a RuO2 nanoparticle catalyst. The role of each layer is discussed in terms of durability and function. With this strategically designed organic photocathode we obtain stable photocurrent densities for over 5 h and discuss routes for further performance improvement.
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Journal articleLee HKH, Wu J, Barbe J, et al., 2018,
Organic photovoltaic cells - promising indoor light harvesters for self-sustainable electronics
, JOURNAL OF MATERIALS CHEMISTRY A, Vol: 6, Pages: 5618-5626, ISSN: 2050-7488 -
Journal articleHong DP, Hayasake K, Kim J, et al., 2018,
One step facile synthesis of a novel anthanthrone dye-based, dopant-free hole transporting material for efficient and stable perovskite solar cells
, JOURNAL OF MATERIALS CHEMISTRY C, Vol: 6, Pages: 3699-3708, ISSN: 2050-7526 -
Journal articleZhang J, Tan CH, Du T, et al., 2018,
ZnO-PCBM bilayers as electron transport layers in low-temperature processed perovskite solar cells
, Science Bulletin, Vol: 63, Pages: 343-348, ISSN: 2095-9273We investigate an electron transport bilayer fabricated at < 110 °C to form all low-temperature processed, thermally stable, efficient perovskite solar cells with negligible hysteresis. The components of the bilayer create a symbiosis that results in improved devices compared with either of the components being used in isolation. A sol-gel derived ZnO layer facilitates improved energy level alignment and enhanced charge carrier extraction and a [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) layer to reduce hysteresis and enhance perovskite thermal stability. The creation of a bilayer structure allows materials that are inherently unsuitable to be in contact with the perovskite active layer to be used in efficient devices through simple surface modification strategies.
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Journal articleLee HKH, Telford AM, Rohr JA, et al., 2018,
The role of fullerenes in the environmental stability of polymer: fullerene solar cells
, Energy and Environmental Science, Vol: 11, Pages: 417-428, ISSN: 1754-5692Environmental stability is a common challenge for the commercialisation of low cost, encapsulation-free organic opto-electronic devices. Understanding the role of materials degradation is the key to address this challenge, but most such studies have been limited to conjugated polymers. Here we quantitatively study the role of the common fullerene derivative PCBM in limiting the stability of benchmark organic solar cells, showing that a minor fraction (<1%) of photo-oxidised PCBM, induced by short exposure to either solar or ambient laboratory lighting conditions in air, consistent with typical processing and operating conditions, is sufficient to compromise device performance severely. We identify the effects of photo-oxidation of PCBM on its chemical structure, and connect this to specific changes in its electronic structure, which significantly alter the electron transport and recombination kinetics. The effect of photo-oxidation on device current–voltage characteristics, electron mobility and density of states could all be explained with the same model of photoinduced defects acting as trap states. Our results demonstrate that the photochemical instability of PCBM and chemically similar fullerenes remains a barrier for the commercialisation of organic opto-electronic devices.
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