Imperial College London

ProfessorJi-SeonKim

Faculty of Natural SciencesDepartment of Physics

Professor of Solid State Physics
 
 
 
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Contact

 

+44 (0)20 7594 7597ji-seon.kim

 
 
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Location

 

B909Blackett LaboratorySouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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154 results found

Wu J, Lee J, Chin Y-C, Yao H, Cha H, Luke J, Hou J, Kim J-S, Durrant Jet al., 2020, Exceptionally low charge trapping enables highly efficient organic bulk heterojunction solar cells, Energy & Environmental Science, ISSN: 1754-5692

In this study, we investigate the underlying origin of the high performance of PM6:Y6 organic solar cells. Employing transient optoelectronic and photoemission spectroscopies, we find that this blend exhibits greatly suppressed charge trapping into electronic intra-bandgap tail states compared to other polymer/non-fullerene acceptor solar cells, attributed to lower energetic disorder. The presence of tail states is a key source of energetic loss in most organic solar cells, as charge carriers relax into these states, reducing the quasi-Fermi level splitting and therefore device VOC. DFT and Raman analyses indicate this suppression of tail state energetics disorder could be associated with a higher degree of conformational rigidity and uniformity for the Y6 acceptor. We attribute the origin of such conformational rigidity and uniformity of Y6 to the presence of the two alkyl side chains on the outer core that restricts end-group rotation by acting as a conformation locker. The resultant enhanced carrier dynamics and suppressed charge carrier trapping are proposed to be a key factor behind the high performance of this blend. Low energetic disorder is suggested to be a key factor enabling reasonably efficient charge generation in this low energy offset system. In the absence of either energetic disorder or a significant electronic energy offset, it is argued that charge separation in this system is primarily entropy driven. Nevertheless, photocurrent generation is still limited by slow hole transfer from Y6 to PM6, suggesting pathways for further efficiency improvement.

Journal article

Daboczi M, Kim J, Lee J, Kang H, Hamilton I, Lin C-T, Dimitrov SD, McLachlan MA, Lee K, Durrant JR, Kim J-Set al., 2020, Towards efficient integrated perovskite/organic bulk heterojunction solar cells: interfacial energetic requirement to reduce charge carrier recombination losses, Advanced Functional Materials, Vol: 30, Pages: 1-8, ISSN: 1616-301X

Integrated perovskite/organic bulk heterojunction (BHJ) solar cells have the potential to enhance the efficiency of perovskite solar cells by a simple one‐step deposition of an organic BHJ blend photoactive layer on top of the perovskite absorber. It is found that inverted structure integrated solar cells show significantly increased short‐circuit current (J sc) gained from the complementary absorption of the organic BHJ layer compared to the reference perovskite‐only devices. However, this increase in J sc is not directly reflected as an increase in power conversion efficiency of the devices due to a loss of fill factor. Herein, the origin of this efficiency loss is investigated. It is found that a significant energetic barrier (≈250 meV) exists at the perovskite/organic BHJ interface. This interfacial barrier prevents efficient transport of photogenerated charge carriers (holes) from the BHJ layer to the perovskite layer, leading to charge accumulation at the perovskite/BHJ interface. Such accumulation is found to cause undesirable recombination of charge carriers, lowering surface photovoltage of the photoactive layers and device efficiency via fill factor loss. The results highlight a critical role of the interfacial energetics in such integrated cells and provide useful guidelines for photoactive materials (both perovskite and organic semiconductors) required for high‐performance devices.

Journal article

Nightingale J, Pitsalidis C, Pappa A-M, Tan E, Stewart K, Owens RM, Kim J-Set al., 2020, Small molecule additive for low-power accumulation mode organic electrochemical transistors, Journal of Materials Chemistry C, Vol: 8, Pages: 8846-8855, ISSN: 2050-7526

A small molecule additive, dodecylbenzenesulfonate (DBSA), is added to the electrolyte in OECTs to improve the device performance.

Journal article

Luo H, Dimitrov S, Daboczi M, Kim J-S, Guo Q, Fang Y, Stoeckel M-A, Samori P, Fenwick O, Sobrido ABJ, Wang X, Titirici M-Met al., 2020, Nitrogen-Doped Carbon Dots/TiO2 Nanoparticle Composites for Photoelectrochemical Water Oxidation, ACS APPLIED NANO MATERIALS, Vol: 3, Pages: 3371-3381, ISSN: 2574-0970

Journal article

Iandolo D, Sheard J, Levy GK, Pitsalidis C, Tan E, Dennis A, Kim J-S, Markaki AE, Widera D, Owens RMet al., 2020, Biomimetic and electroactive 3D scaffolds for human neural crest-derived stem cell expansion and osteogenic differentiation, MRS COMMUNICATIONS, Vol: 10, Pages: 179-187, ISSN: 2159-6859

Journal article

Lin C-T, Lee J, Kim J, Macdonald TJ, Ngiam J, Xu B, Daboczi M, Xu W, Pont S, Park B, Kang H, Kim J-S, Payne DJ, Lee K, Durrant JR, McLachlan MAet al., 2020, Origin of open-circuit voltage enhancements in planar Perovskite solar cells induced by addition of bulky organic cations, Advanced Functional Materials, Vol: 30, ISSN: 1616-301X

The origin of performance enhancements in p‐i‐n perovskite solar cells (PSCs) when incorporating low concentrations of the bulky cation 1‐naphthylmethylamine (NMA) are discussed. A 0.25 vol % addition of NMA increases the open circuit voltage (Voc) of methylammonium lead iodide (MAPbI3) PSCs from 1.06 to 1.16 V and their power conversion efficiency (PCE) from 18.7% to 20.1%. X‐ray photoelectron spectroscopy and low energy ion scattering data show NMA is located at grain surfaces, not the bulk. Scanning electron microscopy shows combining NMA addition with solvent assisted annealing creates large grains that span the active layer. Steady state and transient photoluminescence data show NMA suppresses non‐radiative recombination resulting from charge trapping, consistent with passivation of grain surfaces. Increasing the NMA concentration reduces device short‐circuit current density and PCE, also suppressing photoluminescence quenching at charge transport layers. Both Voc and PCE enhancements are observed when bulky cations (phenyl(ethyl/methyl)ammonium) are incorporated, but not smaller cations (Cs/MA)—indicating size is a key parameter. Finally, it demonstrates that NMA also enhances mixed iodide/bromide wide bandgap PSCs (Voc of 1.22 V with a 1.68 eV bandgap). The results demonstrate a facile approach to maximizing Voc and provide insights into morphological control and charge carrier dynamics induced by bulky cations in PSCs.

Journal article

Hamilton I, Suh M, Kim K, Jeon DY, Bradley DDC, Kim J-Set al., 2020, Organic-inorganic hybrid composites as an electron injection layer in highly efficient inverted green-emitting polymer LEDs, Organic Electronics, Vol: 77, Pages: 1-8, ISSN: 1566-1199

Organic-inorganic hybrid light emitting diodes (HyLEDs) consist of an organic emission layer in combination with at least one metal oxide charge injection layer in an inverted structure. Low temperature, solution processing of metal oxide charge injection layers is one of the key factors in reducing the manufacture cost of HyLEDs. Herein, we report the use of composite materials, comprising conjugated polyelectrolytes (CPE) and zinc oxide nanoparticles (ZnO NPs), as the electron injection layer (EIL) in highly-efficient, green-light-emitting poly (9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) polymer LEDs that are carefully optimised for use in an inverted HyLED architecture for the first time. The composite CPE:ZnO EILs are processed via a room temperature, one-step, solution deposition and enable superior device performance relative to ZnO NPs on their own. We find that specifically, they (i) improve EIL morphology, reducing surface roughness as well as pin-hole size and density, (ii) induce a favourable vacuum level shift for electron injection by coordinate bonding between the CPE and ZnO constituents, and (iii) reduce interfacial quenching by passivation of ZnO chemical defects caused by oxygen vacancies. This work is also the first demonstration that blending ZnO NPs and CPE supports much faster electroluminescence turn-on times (∼7.12 μs) than for traditional ZnO/CPE bilayer devices (∼0.4 s) via ‘locking’ of the CPE mobile ions, as well as higher device performance. This demonstrates good suitability for display applications. After optimisation of the EIL composition and the thickness of the F8BT emissive layer, we achieve promising device efficiencies of 16.5 cd/A and 5.41 lm/W for devices with a 1.1 μm thick F8BT layer, which is particularly relevant for potential roll-to-roll fabrication. These results clearly demonstrate the potential that this organic-inorganic composite EIL material has for the realisation of cheap, scalable

Journal article

Tan E, Pappa A-M, Pitsalidis C, Nightingale J, Wood S, Castro FA, Owens RM, Kim J-Set al., 2020, A highly sensitive molecular structural probe applied to in-situ biosensing of metabolites using PEDOT:PSS, Biotechnology and Bioengineering, Vol: 117, Pages: 291-299, ISSN: 0006-3592

A large amount of research within organic biosensors is dominated by organic electrochemical transistors (OECTs) that use conducting polymers such as poly(3,4-ethylene dioxythiophene doped with poly(styrenesulfonate) (PEDOT:PSS). Despite the recent advances in OECT-based biosensors, the sensing is solely reliant on the amperometric detection of the bioanalytes. This is typically accompanied by large undesirable parasitic electrical signals from the electroactive components in the electrolyte. Herein, we present the use of in-situ resonance Raman spectroscopy to probe subtle molecular structural changes of PEDOT:PSS associated with its doping level. We demonstrate how such doping level changes of PEDOT:PSS can be used, for the first time, on operational OECTs for sensitive and selective metabolite sensing whilst simultaneously performing amperometric detection of the analyte. We test the sensitivity by molecularly sensing a lowest glucose concentration of 0.02 mM in phosphate buffered saline (PBS) solution. By changing the electrolyte to cell culture media, the selectivity of in-situ resonance Raman spectroscopy is emphasized as it remains unaffected by other electroactive components in the electrolyte. The application of this molecular structural probe highlights the importance of developing biosensing probes that benefit from high sensitivity of the material's structural and electrical properties whilst being complimentary with the electronic methods of detection.

Journal article

Stewart K, Limbu S, Nightingale J, Pagano K, Park B, Hong S, Lee K, Kwon S, Kim J-Set al., 2020, Molecular understanding of π-conjugated polymer/solid-state ionic liquid complex as a highly sensitive and selective gas sensor, Journal of Materials Chemistry C, ISSN: 2050-7526

<p>An electric-field driven chemical doping modulation in a blend of solution-processed organic semiconductors (OSC) and solid-state ionic liquids (SSIL) in response to volatile organic compounds (VOC) provides a new exciting...</p>

Journal article

Kasimatis M, Nunez-Bajo E, Grell M, Cotur Y, Barandun G, Kim J-S, Guder Fet al., 2019, Monolithic solder-on nanoporous Si-Cu contacts for stretchable silicone composite sensors, ACS Applied Materials and Interfaces, Vol: 11, Pages: 47577-47586, ISSN: 1944-8244

We report a method of creating solderable, mechanically robust, electrical contacts to interface (soft) silicone-based strain sensors with conventional (hard) solid-state electronics using a nanoporous Si-Cu composite. The Si-based solder-on electrical contact consists of a copper-plated nanoporous Si top surface formed through metal-assisted chemical etching and electroplating, and a smooth Si bottom surface which can be covalently bonded onto silicone-based strain sensors through plasma bonding. We investigated the mechanical and electrical properties of the contacts proposed under relevant ranges of mechanical stress for applications in physiological monitoring and rehabilitation. We also produced a series of proof-of-concept devices, including a wearable respiration monitor, leg band for exercise monitoring and Squeeze-ball for monitoring rehabilitation of patients with hand injuries or neurological disorders, to demonstrate the mechanical robustness and versatility of the technology developed, in real-world applications.

Journal article

Decataldo F, Druet V, Pappa A-M, Tan E, Savva A, Pitsalidis C, Inal S, Kim J-S, Fraboni B, Owens RM, Iandolo Det al., 2019, BMP-2 functionalized PEDOT:PSS-based OECTs for stem cell osteogenic differentiation monitoring, FLEXIBLE AND PRINTED ELECTRONICS, Vol: 4, ISSN: 2058-8585

Journal article

Daboczi M, Hamilton I, Xu S, Luke J, Limbu S, Lee J, McLachlan MA, Lee K, Durrant JR, Baikie I, Kim J-Set al., 2019, The origin of open-circuit voltage losses in perovskite solar cells investigated by surface photovoltage measurement, ACS Applied Materials & Interfaces, Vol: 11, Pages: 46808-46817, ISSN: 1944-8244

Increasing the open circuit voltage (Voc) is one of the key strategies for further improvement of the efficiency of perovskite solar cells. It requires fundamental understanding of the complex optoelectronic processes related to charge carrier generation, transport, extraction and their loss mechanisms inside a device upon illumination. Herein we report the important origin of Voc losses in methylammonium lead iodide perovskite (MAPI) based solar cells, which results from undesirable positive charge (hole) accumulation at the interface between the perovskite photoactive layer and the PEDOT:PSS hole transport layer. We show strong correlation between the thickness-dependent surface photovoltage and device performance, unraveling that the interfacial charge accumulation leads to charge carrier recombination and results in a large decrease in Voc for the PEDOT:PSS/MAPI inverted devices (180 mV reduction in 50-nm-thick device compared to 230-nm-thick one). In contrast, accumulated positive charges at the TiO2/MAPI interface modify interfacial energy band bending, which leads to an increase in Voc for the TiO2/MAPI conventional devices (70 mV increase in 50-nm-thick device compared to 230-nm-thick one). Our results provide an important guideline for better control of interfaces in perovskite solar cells to improve device performance further.

Journal article

Wu J, Luke J, Lee HKH, Tuladhar PS, Cha H, Jang S-Y, Tsoi WC, Heeney M, Kang H, Lee K, Kirchartz T, Kim J-S, Durrant JRet al., 2019, Tail state limited photocurrent collection of thick photoactive layers in organic solar cells, Nature Communications, Vol: 10, ISSN: 2041-1723

Weanalyseorganic solar cells with four differentphotoactive blends exhibiting differing dependencies ofshort-circuit current upon photoactive layer thickness.These blends and devices are analysedbytransient optoelectronic techniques ofcarrier kinetics and densities, airphotoemission spectroscopyof material energetics, Kelvin probe measurements of work function, Mott-Schottky analyses of apparent doping density and by device modelling. We concludethat,for the device series studied, the photocurrent losswith thick active layersis primarilyassociatedwith the accumulation of photo-generated charge carriers in intra-bandgap tail states.This charge accumulation screens the device internal electricalfield, preventing efficient charge collection. Purification of one studied donor polymer is observed to reduce tail statedistribution anddensity and increase the maximal photoactive thickness forefficient operation. Ourwork suggests that selectingorganic photoactive layerswith a narrow distribution of tail states isa keyrequirement for the fabrication of efficient, high photocurrent, thick organic solar cells.

Journal article

Yan H, Limbu S, Wang X, Nightingale J, Hamilton I, Wade J, Kwon S, Lee K, Kim J-Set al., 2019, Efficient charge carrier injection and balance achieved by low electrochemical doping in solution-processed polymer light-emitting diodes, Advanced Functional Materials, Vol: 29, Pages: 1-9, ISSN: 1616-301X

Charge carrier injection and transport in polymer light‐emitting diodes (PLEDs) is strongly limited by the energy level offset at organic/(in)organic interfaces and the mismatch in electron and hole mobilities. Herein, these limitations are overcome via electrochemical doping of a light‐emitting polymer. Less than 1 wt% of doping agent is enough to effectively tune charge injection and balance and hence significantly improve PLED performance. For thick single‐layer (1.2 µm) PLEDs, dramatic reductions in current and luminance turn‐on voltages (VJ = 11.6 V from 20.0 V and VL = 12.7 V from 19.8 V with/without doping) accompanied by reduced efficiency roll‐off are observed. For thinner (<100 nm) PLEDs, electrochemical doping removes a thickness dependence on VJ and VL, enabling homogeneous electroluminescence emission in large‐area doped devices. Such efficient charge injection and balance properties achieved in doped PLEDs are attributed to a strong electrochemical interaction between the polymer and the doping agents, which is probed by in situ electric‐field‐dependent Raman spectroscopy combined with further electrical and energetic analysis. This approach to control charge injection and balance in solution‐processed PLEDs by low electrochemical doping provides a simple yet feasible strategy for developing high‐quality and efficient lighting applications that are fully compatible with printing technologies.

Journal article

Wade J, Pugh H, Nightingale J, Kim J, Williams STet al., 2019, Colour in bivalve shells: Using resonance Raman spectroscopy to compare pigments at different phylogenetic levels, Journal of Raman Spectroscopy, Vol: 50, Pages: 1527-1536, ISSN: 0377-0486

Several studies have suggested that shell colour may be phylogenetically distributed within the phylum Mollusca, but this pattern is confounded by our ignorance of the homology of colour and lack of understanding about the identity of most molluscan pigments. We use resonance Raman spectroscopy to address this problem by examining bivalve pigments producing a range of colours and compare spectra from taxa at different phylogenetic levels. The spectra of most shell pigments exhibited a skeletal signature typical of partially methylated polyenes, possibly modified carotenoids, with the strongest peaks occurring between 1,501–1,540 cm−1 and 1,117–1,144 cm−1 due to the C═C (ν1) and C–C (ν2) stretching modes, respectively. Neither pigment class nor mineral structure differentiated Imparidentia and Pteriomorphia. Spectral acquisitions for purple pigments for two species of Asaphis suggest that identical or nearly identical pigments are shared within this genus, and some red pigments from distantly related species have similar spectra. Conversely, two species with brown shells have distinctly different pigments, highlighting the difficulty in determining the homology of colour even within a single class of pigments. Curiously, we were unable to detect any Raman activity for green‐coloured shell or pigment peaks for the yellow area of Codakia paytenorum, suggesting that these colours are due to structural elements or a pigment that is quite different from those observed in other taxa examined to date. Our results are consistent with the idea that classes of pigments are evolutionarily ancient but heritable modifications may be specific to clades.

Journal article

Way A, Luke J, Evans AD, Li Z, Kim J-S, Durrant JR, Hin Lee HK, Tsoi WCet al., 2019, Fluorine doped tin oxide as an alternative of indium tin oxide for bottom electrode of semi-transparent organic photovoltaic devices, AIP Advances, Vol: 9, Pages: 085220-1-085220-5, ISSN: 2158-3226

Indium tin oxide (ITO) is commonly used as the transparent bottom electrode for organic solar cells. However, it is known that the cost ofthe ITO is quite high due to the indium element, and in some studies ITO coated glass substrate is found to be the most expensive componentof device fabrication. Moreover, indium migration from ITO can cause stability issues in organic solar cells. Nevertheless, the use of ITO asthe bottom electrode is still dominating in the field. Here, we explore the possibility of using fluorine doped tin oxide (FTO) as an alternativeto ITO for the bottom electrode of organic solar cells particularly on semi-transparent cells. We present side-by-side comparisons on theiroptical, morphological and device properties and suggest that FTO could be more suitable than ITO as the bottom electrode for glass substratebased organic photovoltaic devices.

Journal article

Speller EM, Clarke AJ, Luke J, Lee HKH, Durrant JR, Li N, Wang T, Wong HC, Kim J-S, Tsoi WC, Li Zet al., 2019, From fullerene acceptors to non-fullerene acceptors: prospects and challenges in the stability of organic solar cells, Journal of Materials Chemistry A, ISSN: 2050-7488

<p>This review highlights the opportunities and challenges in stability of organic solar cells arising from the emergence of non-fullerene acceptors.</p>

Journal article

Cha H, Fish G, Luke J, Alraddadi A, Lee HH, Zhang W, Dong Y, Limbu S, Wadsworth A, Maria IP, Francas L, Sou HL, Du T, Kim J-S, McLachlan MA, McCulloch I, Durrant JRet al., 2019, Suppression of Recombination Losses in Polymer:Nonfullerene Acceptor Organic Solar Cells due to Aggregation Dependence of Acceptor Electron Affinity, ADVANCED ENERGY MATERIALS, Vol: 9, ISSN: 1614-6832

Journal article

Du T, Xu W, Daboczi M, Kim J, Xu S, Lin C-T, Kang H, Lee K, Heeney MJ, Kim J-S, Durrant JR, McLachlan MAet al., 2019, p-Doping of organic hole transport layers in p–i–n perovskite solar cells: correlating open-circuit voltage and photoluminescence quenching, Journal of Materials Chemistry A, Vol: 7, Pages: 18971-18979, ISSN: 2050-7488

Doping is a widely implemented strategy for enhancing the inherent electronic properties of charge transport layers in photovoltaic (PV) devices. Here, in direct contrast to existing understanding, we find that a reduction in p-doping of the organic hole transport layer (HTL) leads to substantial improvements in PV performance in planar p–i–n perovskite solar cells (PSCs), driven by improvements in open circuit voltage (VOC). Employing a range of transient and steady state characterisation tools, we find that the improvements of VOC correlate with reduced surface recombination losses in less p-doped HTLs. A simple device model including screening of bulk electric fields in the perovskite layer is used to explain this observation. In particular, photoluminescence (PL) emission of complete solar cells shows that efficient performance is correlated to a high PL intensity at open circuit and a low PL intensity at short circuit. We conclude that desirable transport layers for p–i–n PSCs should be charge selective contacts with low doping densities.

Journal article

He Q, Shahid M, Panidi J, Marsh AV, Huang W, Daboczi M, Kim J-S, Fei Z, Anthopoulos TD, Heeney Met al., 2019, A versatile star-shaped organic semiconductor based on benzodithiophene and diketopyrrolopyrrole, Journal of Materials Chemistry C, Vol: 7, Pages: 6622-6629, ISSN: 2050-7526

We report the synthesis of a new star-shaped π-conjugated oligomer, BDT(DPP)4, containing a benzodithiophene core and four diketopyrrolopyrrole arms. The thermal, electrochemical and optical properties are characterized and the results complemented by computational studies. The utility of the molecule is demonstrated in both solar cell and field-effect transistor devices. In the former, BDT(DPP)4 displays low efficiency when used as an acceptor in blends with poly(3-hexylthiophene) but exhibits promising performance as a donor, in blends with either a fullerene or a non-fullerene acceptor. In field-effect transistors BDT(DPP)4 exhibits typical p-type transistor behavior, which is in accordance with its better donor performance in solar cell devices.

Journal article

Jayaram AK, Pitsalidis C, Tan E, Moysidou C-M, De Voider MFL, Kim J-S, Owens RMet al., 2019, 3D Hybrid Scaffolds Based on PEDOT:PSS/MWCNT Composites, FRONTIERS IN CHEMISTRY, Vol: 7, ISSN: 2296-2646

Journal article

Wang Y, Daboczi M, Mesa CA, Ratnasingham SR, Kim JS, Durrant JR, Dunn S, Yan H, Briscoe J, Wang Y, Daboczi M, Mesa CA, Ratnasingham SR, Kim J-S, Durrant JR, Dunn S, Yan H, Briscoe Jet al., 2019, Bi₂Fe₄O₉ thin films as novel visible-light-active photoanodes for solar water splitting, Journal of Materials Chemistry A, Vol: 7, Pages: 9537-9541, ISSN: 2050-7496

We report the chemical solution deposition (CSD) of a phase-pure Bi2Fe4O9 thin film for use as a photoanode in photoelectrochemical (PEC) water splitting. The energy levels of Bi2Fe4O9 films have been measured and n-type characteristics have been confirmed. With band gaps determined as 2.05 eV (indirect) and 2.80 eV (direct) and valence and conduction bands straddling the water oxidation and reduction potentials, this material is highly promising as a photocatalyst for solar water splitting. The photocurrent of a planar photoanode reached 0.1 mA cm−2 at 1.23 VNHE under AM1.5G illumination. The addition of H2O2 as a hole scavenger increased the photocurrent to 0.25 mA cm−2, indicating hole injection is one limiting factor to the performance. The performance was enhanced by nearly 5-fold when the Bi2Fe4O9 photoanode is coupled to a Co–Pi surface co-catalyst. The photoanode also shows excellent stability with no change in photocurrent over three hours of continuous illumination. These results indicate that this material represents a promising addition to the growing selection of low-cost, stable photocatalysts for use in solar water splitting.

Journal article

Luke J, Speller EM, Wadsworth A, Wyatt MF, Dmiitrov S, Lee HKH, Li Z, Tsoi WC, McCulloch I, Bagnis D, Durrant JR, Kim J-Set al., 2019, Twist and degrade – Impact of molecular structure on the photostability of non-fullerene acceptors and their photovoltaic blends, Advanced Energy Materials, Vol: 9, Pages: 1-14, ISSN: 1614-6832

Non-fullerene acceptors (NFAs) dominate organic photovoltaic (OPV) research due to their promising efficiencies and stabilities. However, there is very little investigation into the molecular processes of degradation, which is critical to guiding design of novel NFAs for long-lived, commercially viable OPVs. Here we investigate the important role of molecular structure and conformation on NFA photostability in air by comparing structurally similar but conformationally different promising NFAs; planar O-IDTBR and non-planar O-IDFBR. We identify a three-phase degradation process: (i) initial photo-induced conformational change (i.e. torsion about the Core-BT dihedral), induced by non-covalent interactions with environmental molecules, (ii) followed by photo-oxidation and fragmentation, leading to chromophore bleaching and degradation product formation, and (iii) finally complete chromophore bleaching.Initial conformational change is a critical prerequisite for further degradation, providing fundamental understanding of the relative stability of IDTBR and IDFBR, where the alreadytwisted IDFBR is more prone to degradation. When blended with the donor polymer P3HT, both NFAs exhibit improved photostability whilst the photostability of the polymer itself is significantly reduced by the more miscible twisted NFA. Our findings elucidate the important role of NFA molecular structure on photostability of OPV systems, and provide vital insights into molecular design rules for intrinsically photostable NFAs.

Journal article

Speller EM, Clarke AJ, Aristidou N, Wyatt MF, Francàs L, Fish G, Cha H, Lee HKH, Luke J, Wadsworth A, Evans AD, McCulloch I, Kim JS, Haque SA, Durrant JR, Dimitrov SD, Tsoi WC, Li Zet al., 2019, Toward improved environmental stability of polymer:fullerene and polymer:non-fullerene organic solar cells: a common energetic origin of light and oxygen induced degradation, ACS Energy Letters, Vol: 4, Pages: 846-852, ISSN: 2380-8195

With the emergence of nonfullerene electron acceptors resulting in further breakthroughs in the performance of organic solar cells, there is now an urgent need to understand their degradation mechanisms in order to improve their intrinsic stability through better material design. In this study, we present quantitative evidence for a common root cause of light-induced degradation of polymer:nonfullerene and polymer:fullerene organic solar cells in air, namely, a fast photo-oxidation process of the photoactive materials mediated by the formation of superoxide radical ions, whose yield is found to be strongly controlled by the lowest unoccupied molecular orbital (LUMO) levels of the electron acceptors used. Our results elucidate the general relevance of this degradation mechanism to both polymer:fullerene and polymer:nonfullerene blends and highlight the necessity of designing electron acceptor materials with sufficient electron affinities to overcome this challenge, thereby paving the way toward achieving long-term solar cell stability with minimal device encapsulation.

Journal article

Goudarzi H, Limbu S, Cabanillas-González J, Zenonos VM, Kim J-S, Keivanidis PEet al., 2019, Impact of molecular conformation on triplet-fusion induced photon energy up-conversion in the absence of exothermic triplet energy transfer, Journal of Materials Chemistry C, Vol: 7, Pages: 3634-3643, ISSN: 2050-7526

The use of photon energy up-converted luminescence driven by triplet-exciton annihilation reactions (TTA-UC) is increasingly gaining attention for developing next-generation light-management, and wavelength-shifting technologies. Here we present a spectroscopic study for elucidating the photophysical mechanism that operates in an unusual TTA-UC model system comprising the blue-light emitting poly(fluorene-2-octyl) (PFO) activator mixed with the green-light absorbing (2,3,7,8,12,13,17,18-octaethyl-porphyrinato) PtII (PtOEP) metalo-organic complex. The unconventional character of the PFO:PtOEP composite manifests in the fact that no exothermic triplet energy transfer (TET) is possible between triplet-excited PtOEP and PFO. Yet green-to-blue TTA-UC luminescence of PFO is obtained even when PtOEP is selectively photoexcited by pulsed laser intensities as low as 2.5 mW cm−2. Continuous-wave photo-induced absorption spectroscopy verifies that no energy transfer from triplet-excited PtOEP to the triplet level of PFO takes place, pointing to triplet–triplet annihilation (TTA) events in the PtOEP phase as the origin of the observed TTA-UC PL signal. In the PFO:PtOEP composite, the PtOEP component holds a dual role of annihilator/sensitizer; photon energy storage in PtOEP is enabled via TTA when triplet exciton diffusion coefficient values of DPtOEP = 4.1 × 10−9 cm2 s−1 are reached. With a simple yet powerful solution processing protocol, and by combining Raman and time-gate photoluminescence (PL) spectroscopy we demonstrate that the brightness of the produced TTA-UC luminescence depends on the molecular conformation of the PFO activator. A four-fold increase in the TTA-UC luminescence intensity is registered in the time-integrated and time-gated PL spectra, when the PFO matrix is arrested in its planar β-phase molecular conformation. Further enhancement of the TTA-UC PL signal is achieved when temperature lowers from 290 K down to 100 K. Th

Journal article

Kim J-S, 2019, Impact of initial bulk-heterojunction morphology on operational stability of polymer:fullerene photovoltaic cells, Advanced Materials Interfaces, Vol: 6, ISSN: 2196-7350

Controlling initial bulk-heterojunction (BHJ) morphology is critical for device performance of organic photovoltaic (OPV) cells. However, its impact on performance, specifically long-term operational stability is still poorly understood. This is mainly due to limitations in direct measurements enabling in-situ monitoring of devices at a molecular level. Here, we utilize thermal annealing preconditioning step to tune initial morphology of model polymer:fullerene BHJ OPV devices and molecular resonant vibrational spectroscopy to identify in-situ degradation pathways. We report direct spectroscopic evidence for molecular-scale phase segregation temperature (TPS) which critically determines a boundary in high efficiency and long operational stability. Under operation, initially well-mixed blend morphology (no annealing) shows interface instability related to the hole-extracting PEDOT:PSS layer via de-doping. Likewise, initially phase-segregatedmorphology at a molecular level (annealed above TPS) shows instability in the photoactive layer via continuous phase segregation between polymer and fullerenes in macroscales, coupled with further fullerene photodegradation. Our results confirm that a thermal annealing preconditioning step is essential to stabilize the BHJ morphology; in particular annealing below TPS is critical for improved operational stability whilst maintaining high efficiency.

Journal article

Dash BP, Hamilton I, Tate DJ, Crossley D, Kim J-S, Ingleson MJ, Turner Met al., 2019, Benzoselenadiazole and benzotriazole directed electrophilic C-H borylation of conjugated donor-acceptor materials, Journal of Materials Chemistry C, Vol: 7, Pages: 718-724, ISSN: 2050-7526

Benzoselenadiazole and benzotriazole directed electrophilic borylation using BCl3 results in the C–H functionalization of an adjacent aromatic unit and produces fused boracycles. Subsequent arylation at boron afforded air and moisture stable products displaying large bathochromic shifts and significantly reduced LUMO energy levels. OLEDs fabricated containing borylated benzoselenadiazole derivatives showed emission centered at 723 nm in the near infra-red region of the spectrum.

Journal article

Newman MJ, Speller EM, Barbe J, Luke J, Li M, Li Z, Wang Z-K, Jain SM, Kim J-S, Lee HKH, Tsoi WCet al., 2018, Photo-stability study of a solution-processed small molecule solar cell system: correlation between molecular conformation and degradation, Science and Technology of Advanced Materials, Vol: 19, Pages: 194-202, ISSN: 1468-6996

Solution-processed organic small molecule solar cells (SMSCs) have achieved efficiency over 11%. However, very few studies have focused on their stability under illumination and the origin of the degradation during the so-called burn-in period. Here, we studied the burn-in period of a solution-processed SMSC using benzodithiophene terthiophene rhodamine:[6,6]-phenyl C71 butyric acid methyl ester (BTR:PC71BM) with increasing solvent vapour annealing time applied to the active layer, controlling the crystallisation of the BTR phase. We find that the burn-in behaviour is strongly correlated to the crystallinity of BTR. To look at the possible degradation mechanisms, we studied the fresh and photo-aged blend films with grazing incidence X-ray diffraction, UV–vis absorbance, Raman spectroscopy and photoluminescence (PL) spectroscopy. Although the crystallinity of BTR affects the performance drop during the burn-in period, the degradation is found not to originate from the crystallinity changes of the BTR phase, but correlates with changes in molecular conformation – rotation of the thiophene side chains, as resolved by Raman spectroscopy which could be correlated to slight photobleaching and changes in PL spectra.

Journal article

Keivanidis PE, Khan JI, Katzenmeier L, Kan Z, Limbu S, Constantinou MK, Lariou E, Constantinides G, Hayes SC, Kim J-S, Laquai Fet al., 2018, Impact of structural polymorphs on charge collection and non-geminate recombination in organic photovoltaic devices, The Journal of Physical Chemistry C, Vol: 122, Pages: 29141-29149, ISSN: 1932-7447

The formation of different types of structural polymorphs of poly(3-hexyl-thiophene) (P3HT) affects the performance of organic photovoltaic (OPV) devices that use thermally-annealed P3HT:PCBM[60] blend films as photoactive layer. Here it is demonstrated that, when densely-packed and non-densely packed P3HT polymorphs co-exist in the P3HT:PCBM[60] layer, non-geminate charge recombination is fast; however, in a device non-geminate recombination is effectively overruled by efficient and fast charge carrier extraction. In stark contrast, when only a less-densely packed P3HT polymorph is present in the blend, non-geminate charge recombination losses are less pronounced, and the charge carrier extraction efficiency is lower. The antagonistic non-geminate charge recombination and charge carrier extraction processes in these systems are monitored by time-delayed-collection field (TDCF) and ultrafast transient absorption (TA) experiments. Furthermore, resonance Raman spectroscopy reveals that in the absence of the densely-packed P3HT polymorph, the energetic disorder present in the P3HT:PCBM[60] blend is higher. High-resolution atomic force microscopy imaging further identifies pronounced differences in the layer morphology when the polymorph distribution varies between unimodal and bimodal. These results indicate that less-densely packed P3HT polymorphs increase disorder and impede charge collection, leading to a reduction of the device fill factor.

Journal article

Nightingale J, Wade J, Moia D, Nelson J, Kim J-Set al., 2018, Impact of molecular order on polaron formation in conjugated polymers, The Journal of Physical Chemistry C, Vol: 122, Pages: 29129-29140, ISSN: 1932-7447

The nature of polaron formation has profound implications on the transport of charge carriers in conjugated polymers, but still remains poorly understood. Here we develop in situ electrochemical resonant Raman spectroscopy, a powerful structural probe that allows direct observation of polaron formation. We report that polaron formation in ordered poly(3-hexyl)thiophene (P3HT) polymer domains (crystalline phase) results in less pronounced changes in molecular conformation, indicating smaller lattice relaxation, compared to polarons generated in disordered polymer domains (amorphous phase) for which we observe large molecular conformational changes. These conformational changes are directly related to the effective conjugation length of the polymer. Furthermore, we elucidate how blending the P3HT polymer with phenyl C-61 butyric acid methyl ester (PCBM) affects polaron formation in the polymer. We find that blending disturbs polymer crystallinity, reducing the density of polarons that can form upon charge injection at the same potential, whilst the lost capacity is partly restored during post-deposition thermal annealing. Our study provides direct spectroscopic evidence for a lower degree of lattice reorganisation in crystalline (and therefore more planarised) polymers than in conformationally disordered polymers. This observation is consistent with higher charge carrier mobility and better device performance commonly found in crystalline polymer materials.

Journal article

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