Imperial College London

Dr Martyn A McLachlan

Faculty of EngineeringDepartment of Materials

Reader in Materials
 
 
 
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Contact

 

+44 (0)20 7594 9692martyn.mclachlan Website

 
 
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Location

 

RSM G03Royal School of MinesSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

105 results found

Macdonald TJ, Batmunkh M, Lin C-T, Kim J, Tune DD, Ambroz F, Li X, Xu S, Sol C, Papakonstantinou I, McLachlan MA, Parkin IP, Shapter JG, Durrant JRet al., 2019, Origin of performance enhancement in TiO2-carbon nanotube composite perovskite solar cells, Small Methods, Vol: 3, Pages: 1-10, ISSN: 2366-9608

Carbon nanotubes are shown to be beneficial additives to perovskite solar cells, and the inclusion of such nanomaterials will continue to play a crucial role in the push toward developing efficient and stable device architectures. Herein, titanium dioxide/carbon nanotube composite perovskite solar cells are fabricated, and device performance parameters are correlated with spectroscopic signatures of the materials to understand the origin of performance enhancement. By probing the charge carrier dynamics with photoluminescence and femtosecond transient absorption spectroscopy, the results indicate that charge transfer is not improved by the presence of the carbon nanotubes. Instead, carbon nanotubes are shown to passivate the electronic defect states within the titanium dioxide, which can lead to stronger radiative recombination in the titanium dioxide/carbon nanotube films. The defect passivation allows the perovskite solar cells made using an optimized titanium dioxide/carbon nanotube composite to achieve a peak power conversion efficiency of 20.4% (19% stabilized), which is one of the highest values reported for perovskite solar cells not incorporating a mixed cation light absorbing layer. The results discuss new fundamental understandings for the role of carbon nanomaterials in perovskite solar cells and present a significant step forward in advancing the field of high‐performance photovoltaics.

Journal article

Panidi J, Kainth J, Paterson AF, Wang S, Tsetseris L, Emwas AH, McLachlan MA, Heeney M, Anthopoulos TDet al., 2019, Introducing a nonvolatile N-type dopant drastically improves electron transport in polymer and small-molecule organic transistors, Advanced Functional Materials, Vol: 29, Pages: 1-10, ISSN: 1616-301X

KGaA, Weinheim Molecular doping is a powerful yet challenging technique for enhancing charge transport in organic semiconductors (OSCs). While there is a wealth of research on p-type dopants, work on their n-type counterparts is comparatively limited. Here, reported is the previously unexplored n-dopant (12a,18a)-5,6,12,12a,13,18,18a,19-octahydro-5,6-dimethyl- 13,18[1′,2′]-benzenobisbenzimidazo [1,2-b:2′,1′-d]benzo[i][2.5]benzodiazo-cine potassium triflate adduct (DMBI-BDZC) and its application in organic thin-film transistors (OTFTs). Two different high electron mobility OSCs, namely, the polymer poly[[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8- bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2′-bithiophene)] and a small-molecule naphthalene diimides fused with 2-(1,3-dithiol-2-ylidene)malononitrile groups (NDI-DTYM2) are used to study the effectiveness of DMBI-BDZC as a n-dopant. N-doping of both semiconductors results in OTFTs with improved electron mobility (up to 1.1 cm2 V−1 s−1), reduced threshold voltage and lower contact resistance. The impact of DMBI-BDZC incorporation is particularly evident in the temperature dependence of the electron transport, where a significant reduction in the activation energy due to trap deactivation is observed. Electron paramagnetic resonance measurements support the n-doping activity of DMBI-BDZC in both semiconductors. This finding is corroborated by density functional theory calculations, which highlights ground-state electron transfer as the main doping mechanism. The work highlights DMBI-BDZC as a promising n-type molecular dopant for OSCs and its application in OTFTs, solar cells, photodetectors, and thermoelectrics.

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

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

Ambroz F, Sathasivam S, Lee R, Gadipelli S, Lin C, Xu S, Poduval RK, Mclachlan MA, Papakonstantinou I, Parkin IP, Macdonald TJet al., 2019, Influence of Lithium and Lanthanum Treatment on TiO 2 Nanofibers and Their Application in n‐i‐p Solar Cells, ChemElectroChem, Vol: 6, Pages: 3529-3529, ISSN: 2196-0216

Journal article

Ambroz F, Sathasivam S, Lee R, Gadipelli S, Lin C, Xu S, Poduval RK, Mclachlan MA, Papakonstantinou I, Parkin IP, Macdonald TJet al., 2019, Influence of lithium and lanthanum treatment on TiO2 nanofibers and their application in n‐i‐p solar cells, ChemElectroChem, Vol: 6, Pages: 3590-3598, ISSN: 2196-0216

The addition of cations to TiO2 photoelectrodes is routinely accepted as a route to enhance the performance of conventional n‐i‐p solar cells. However, this is typically achieved in multiple steps or by the incorporation of expensive and hydroscopic cationic precursors such as lithium bis(trifluoromethanesulfonyl)imide. In addition, it is often unclear as to whether the incorporation of such cation sources is inducing “doping” or simply transformed into cationic oxides on the surface of the photoelectrodes. In this study, TiO2 nanofibers were produced through a simple electrospinning technique and modified by introducing lithium and lanthanum precursors in one step. Our results show that the addition of both cations caused minimal substitutional or interstitial doping of TiO2. Brunauer‐Emmett‐Teller measurements showed that lanthanum‐treated TiO2 nanofibers had an increase in surface area, which even exceeded that of TiO2 P25 nanoparticles. Finally, treated and untreated TiO2 nanofibers were used in n‐i‐p solar cells. Photovoltaic characteristics revealed that lanthanum treatment was beneficial, whereas lithium treatment was found to be detrimental to the device performance for both dye‐sensitized and perovskite solar cells. The results discuss new fundamental understandings for two of the commonly incorporated cationic dopants in TiO2 photoelectrodes, lithium and lanthanum, and present a significant step forward in advancing the field of materials chemistry for photovoltaics.

Journal article

Georgiadou DG, Lin Y, Lim J, Ratnasingham S, McLachlan MA, Snaith HJ, Anthopoulos TDet al., 2019, High Responsivity and Response Speed Single‐Layer Mixed‐Cation Lead Mixed‐Halide Perovskite Photodetectors Based on Nanogap Electrodes Manufactured on Large‐Area Rigid and Flexible Substrates, Advanced Functional Materials, Vol: 29, Pages: 1901371-1901371, ISSN: 1616-301X

Journal article

Mehmood U, Harrabi K, Hussein IA, Shanmugam N, Mekki A, Mekki M, McLachlan MAet al., Correction to: A study on stability of active layer of polymer solar cells: effect of UV–visible light with different conditions, Polymer Bulletin, ISSN: 0170-0839

Journal article

Sun G, Shahid M, Fei Z, Xu S, Eisner FD, Anthopoulos TD, McLachlan MA, Heeney Met al., 2019, Highly-efficient semi-transparent organic solar cells utilising non-fullerene acceptors with optimised multilayer MoO3/Ag/MoO3 electrodes (vol 3, pg 450, 2019), MATERIALS CHEMISTRY FRONTIERS, Vol: 3, Pages: 955-955

Journal article

Sun G, Shahid M, Fei Z, Xu S, Eisner FD, Anthopolous TD, McLachlan MA, Heeney Met al., 2019, Highly-efficient semi-transparent organic solar cells utilising non-fullerene acceptors with optimised multilayer MoO3/Ag/MoO3 electrodes, Materials Chemistry Frontiers, Vol: 3, Pages: 450-455, ISSN: 2052-1537

We report the optimisation of a semi-transparent solar cell based on a blend of a recently reported high performance donor polymer (PFBDB-T) with a non-fullerene acceptor derivative (C8-ITIC). The performance is shown to strongly depend on the nature of the semi-transparent electrode, and we report the optimal fabrication conditions for a multilayer MoO3/Ag/MoO3 electrode. The effect of deposition rate and layer thickness of both the Ag and the outer MoO3 on transparency and sheet resistance is investigated, and is shown to have a significant impact on the overall device performance. The optimised PFBDB-T:C8-ITIC based devices exhibit an average power conversion efficiency (PCE) of 9.2% with an average visible transmittance (AVT) of 22%.

Journal article

Lin C-T, Rossi F, Kim J, Baker J, Ngiam J, Xu S, Pont S, Aristidou N, Haque S, Watson T, McLachlan M, Durrant Jet al., 2019, Evidence for surface defect passivation as the origin of the remarkable photostability of unencapsulated perovskite solar cells employing aminovaleric acid as a processing additive, Journal of Materials Chemistry A, Vol: 7, ISSN: 2050-7496

This study addresses the cause of enhanced stability of methyl ammonium lead iodide when processed with aminovaleric acid additives (AVA-MAPbI3) in screen printed, hole transport layer free perovskite solar cells with carbon top electrodes (c-PSC). Employing AVA as an additive in the active layer caused a 40-fold increase in device lifetime measured under full sun illumination in ambient air (RH ~15%). This stability improvement with AVA was also observed in optical photobleaching studies of planar films on glass, indicating this improvement is intrinsic to the perovskite film. Employing low-energy ion scattering spectroscopy, photoluminescence studies as a function of AVA and oxygen exposure, and a molecular probe for superoxide generation, we conclude that even though superoxide is generated in both AVA-MAPbI3 and MAPbI3 films, AVA located at grain boundaries is able to passivate surface defect sites, resulting in enhanced resistivity to oxygen induced degradation. These results are discussed in terms of their implications for the design of environmentally stable perovskite solar cells.

Journal article

Mehmood U, Harrabi K, Hussein IA, Shanmugam N, Mekki A, Mekki M, McLachlan MAet al., 2019, A study on stability of active layer of polymer solar cells: effect of UV–visible light with different conditions, Polymer Bulletin, Vol: 76, Pages: 525-537, ISSN: 0170-0839

© 2018, Springer-Verlag GmbH Germany, part of Springer Nature. Abstract: The objective of this study is to investigate the stability of the active layer of polymer solar cells from the effect of UV–visible light irradiation using different conditions with respect to time. The active layers were composed of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM), deposited on conductive glass substrates through spin coating. These samples are placed in a UV–visible light exposure chamber using different conditions (heat and water) over the specific periods of time. The samples are analyzed by UV–visible absorption spectroscopy, X-ray photoelectron spectroscopy and Fourier transforms infrared spectroscopy (FTIR) measurements. The results indicate that after continuous exposure to UV irradiation for 72 and 120 h, the sample shows a significant decrease in absorption of the main peak. The sample shows around 25% loss in absorption (main peak) after 72 h of irradiation. The FTIR results illustrate a progressive decrease in intensities of all typical absorption peaks owing to P3HT ring scission, side chain oxidation as well as degradation of the side groups of PCBM. Graphical abstract: [Figure not available: see fulltext.].

Journal article

Kafizas A, Xing X, Selim S, Mesa CA, Ma Y, Burgess C, McLachlan MA, Durrant JRet al., 2019, Ultra-thin Al<inf>2</inf>O<inf>3</inf>coatings on BiVO<inf>4</inf>photoanodes: Impact on performance and charge carrier dynamics, Catalysis Today, Vol: 321-322, Pages: 59-66, ISSN: 0920-5861

Bismuth vanadate (BiVO 4 ) has emerged as one of the most promising photoanode materials for oxidising water due to its visible light activity and low cost. Recent studies have shown that the performance of BiVO 4 photoanodes can be remarkably improved when coated with ultra-thin passivation layers. In this article we investigate the use of ultra-thin Al 2 O 3 layers grown using atomic layer deposition (ALD). At an optimum thickness (~0.33nm, 3 ALD cycles), the Al 2 O 3 layer favourably shifted the onset potential by ~200mV and increased photocatalytic currents for the water oxidation reaction. When held at 1.23V RHE , we observe a remarkable increase in the theoretical solar photocurrent; from ~0.47mAcm -2 in uncoated BiVO 4 to ~3.0mAcm -2 in Al 2 O 3 -coated BiVO 4 . Using transient photocurrent (TPC) and transient absorption spectroscopy (TAS) the charge carrier dynamics in Al 2 O 3 -coated BiVO 4 photoanodes were examined for the first time. TPC showed that photogenerated electrons in the BiVO 4 layer were extracted within ~1ms. TAS showed that the remaining holes oxidised water from ~100ms to 1s. Ultra-thin Al 2 O 3 coatings did not improve the reaction kinetics towards water oxidation, but rather, suppressed bi-molecular recombination on the μs-ms timescale in BiVO 4 , and increased the yield of long-lived holes on the ms-s timescale required to oxidise water. This is attributed to an inhibition of surface recombination on BiVO 4 by Al 2 O 3 , which inhibited the early timescale recombination of charge carriers formed within the space charge layer.

Journal article

Lee HKH, Barbe J, Meroni SMP, Du T, Lin C-T, Pockett A, Troughton J, Jain SM, De Rossi F, Baker J, Carnie MJ, McLachlan MA, Watson TM, Durrant JR, Tsoi WCet al., 2019, Outstanding Indoor Performance of Perovskite Photovoltaic Cells - Effect of Device Architectures and Interlayers, SOLAR RRL, Vol: 3, ISSN: 2367-198X

Journal article

Du T, Burgess C, Lin C-T, Eisner F, Kim J, Xu S, Kang H, Durrant J, McLachlan Met al., 2018, Probing and controlling intra-grain crystallinity for improved low-temperature processed perovskite solar cells, Advanced Functional Materials, Vol: 28, ISSN: 1616-301X

Here, 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.

Journal article

Kim J, Godin R, Dimitrov SD, Du T, Bryant D, McLachlan MA, Durrant JRet al., 2018, Excitation density dependent photoluminescence quenching and charge transfer efficiencies in hybrid perovskite/organic semiconductor bilayers, Advanced Energy Materials, Vol: 8, ISSN: 1614-6832

This 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.

Journal article

Loh AYY, Burgess CH, Tanase DA, Ferrari G, McLachlan MA, Cass AEG, Albrecht Tet al., 2018, Electric single-molecule hybridization detector for short DNA fragments, Analytical Chemistry, Vol: 90, Pages: 14063-14071, ISSN: 0003-2700

By combining DNA nanotechnology and high-bandwidth single-molecule detection in nanopipets, we demonstrate an electric, label-free hybridization sensor for short DNA sequences (<100 nucleotides). Such short fragments are known to occur as circulating cell-free DNA in various bodily fluids, such as blood plasma and saliva, and have been identified as disease markers for cancer and infectious diseases. To this end, we use as a model system an 88-mer target from the RV1910c gene in Mycobacterium tuberculosis, which is associated with antibiotic (isoniazid) resistance in TB. Upon binding to short probes attached to long carrier DNA, we show that resistive-pulse sensing in nanopipets is capable of identifying rather subtle structural differences, such as the hybridization state of the probes, in a statistically robust manner. With significant potential toward multiplexing and high-throughput analysis, our study points toward a new, single-molecule DNA-assay technology that is fast, easy to use, and compatible with point-of-care environments.

Journal article

Heeney MJ, Creamer A, Wood C, Howes P, Casey A, Cong S, Marsh A, Godin R, Panidi J, Anthopoulos T, Burgess C, Wu T, Fei Z, McLachlan M, Stevens Met al., 2018, Post-polymerisation functionalisation of conjugated polymer backbones and its application in multi-functional emissive nanoparticles, Nature Communications, Vol: 9, ISSN: 2041-1723

Backbone functionalisation of conjugated polymers is crucial to their performance in many applications, from electronic displays to nanoparticle biosensors, yet there are limited approaches to introduce functionality. To address this challenge we have developed a method for the direct modification of the aromatic backbone of a conjugated polymer, post-polymerisation. This is achieved via a quantitative nucleophilic aromatic substitution (SNAr) reaction on a range of fluorinated electron deficient comonomers. The method allows for facile tuning of the physical and optoelectronic properties within a batch of consistent molecular weight and dispersity. It also enables the introduction of multiple different functional groups onto the polymer backbone in a controlled manner. To demonstrate the versatility of this reaction, we designed and synthesised a range of emissive poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) based polymers for the creation of mono and multifunctional semiconducting polymer nanoparticles (SPNs) capable of two orthogonal bioconjugation reactions on the same surface.

Journal article

Du T, Kim J, Ngiam J, Xu S, Barnes PRF, Durrant JR, McLachlan MAet 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, ISSN: 1616-301X

Recombination 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.

Journal article

Du T, Kim J, Ngiam J, Xu S, Barnes P, Durrant J, McLachlan MAet 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: 1616-301X

Recombination 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.

Journal article

Lin C, Pont S, Kim J, Du T, Xu S, Li X, Bryant D, McLachlan MA, Durrant Jet 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: 2398-4902

Herein, 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.

Journal article

Semple J, Georgiadou DG, Wyatt-Moon G, Yoon M, Seitkhan A, Yengel E, Rossbauer S, Bottacchi F, McLachlan MA, Bradley DDC, Anthopoulos TDet al., 2018, Large-area plastic nanogap electronics enabled by adhesion lithography, npj Flexible Electronics, Vol: 2, ISSN: 2397-4621

Large-area manufacturing of flexible nanoscale electronics has long been sought by the printed electronics industry. However, the lack of a robust, reliable, high throughput and low-cost technique that is capable of delivering high-performance functional devices has hitherto hindered commercial exploitation. Herein we report on the extensive range of capabilities presented by adhesion lithography (a-Lith), an innovative patterning technique for the fabrication of coplanar nanogap electrodes with arbitrarily large aspect ratio. We use this technique to fabricate a plethora of nanoscale electronic devices based on symmetric and asymmetric coplanar electrodes separated by a nanogap < 15 nm. We show that functional devices including self-aligned-gate transistors, radio frequency diodes and rectifying circuits, multi-colour organic light-emitting nanodiodes and multilevel non-volatile memory devices, can be fabricated in a facile manner with minimum process complexity on a range of substrates. The compatibility of the formed nanogap electrodes with a wide range of solution processable semiconductors and substrate materials renders a-Lith highly attractive for the manufacturing of large-area nanoscale opto/electronics on arbitrary size and shape substrates.

Journal article

Sit WY, Eisner FD, Lin YH, Firdaus Y, Seitkhan A, Balawi AH, Laquai F, Burgess CH, Mclachlan MA, Volonakis G, Giustino F, Anthopoulos TDet al., 2018, High-efficiency fullerene solar cells enabled by a spontaneously formed mesostructured CuSCN-nanowire heterointerface, Advanced Science, Vol: 5, ISSN: 2198-3844

Fullerenes and their derivatives are widely used as electron acceptors in bulk-heterojunction organic solar cells as they combine high electron mobility with good solubility and miscibility with relevant semiconducting polymers. However, studies on the use of fullerenes as the sole photogeneration and charge-carrier material are scarce. Here, a new type of solution-processed small-molecule solar cell based on the two most commonly used methanofullerenes, namely [6,6]-phenyl-C61-butyric acid methyl ester (PC 60 BM) and [6,6]-phenyl-C71-butyric acid methyl ester (PC 70 BM), as the light absorbing materials, is reported. First, it is shown that both fullerene derivatives exhibit excellent ambipolar charge transport with balanced hole and electron mobilities. When the two derivatives are spin-coated over the wide bandgap p-type semiconductor copper (I) thiocyanate (CuSCN), cells with power conversion efficiency (PCE) of ≈1%, are obtained. Blending the CuSCN with PC 70 BM is shown to increase the performance further yielding cells with an open-circuit voltage of ≈0.93 V and a PCE of 5.4%. Microstructural analysis reveals that the key to this success is the spontaneous formation of a unique mesostructured p-n-like heterointerface between CuSCN and PC 70 BM. The findings pave the way to an exciting new class of single photoactive material based solar cells.

Journal article

Wijeyasinghe N, Tsetseris L, Regoutz A, Sit WY, Fei Z, Du T, Wang X, Mclachlan MA, Vourlias G, Patsalas PA, Payne DJ, Heeney M, Anthopoulos TDet al., 2018, Copper (I) selenocyanate (CuSeCN) as a novel hole-transport layer for transistors, organic solar cells, and light-emitting diodes, Advanced Functional Materials, Vol: 28, ISSN: 1616-301X

The synthesis and characterization of copper (I) selenocyanate (CuSeCN) and its application as a solution-processable hole-transport layer (HTL) material in transistors, organic light-emitting diodes, and solar cells are reported. Density-functional theory calculations combined with X-ray photoelectron spectroscopy are used to elucidate the electronic band structure, density of states, and microstructure of CuSeCN. Solution-processed layers are found to be nanocrystalline and optically transparent ( > 94%), due to the large bandgap of ≥3.1 eV, with a valence band maximum located at -5.1 eV. Hole-transport analysis performed using field-effect measurements confirms the p-type character of CuSeCN yielding a hole mobility of 0.002 cm 2 V -1 s -1 . When CuSeCN is incorporated as the HTL material in organic light-emitting diodes and organic solar cells, the resulting devices exhibit comparable or improved performance to control devices based on commercially available poly(3,4-ethylenedioxythiophene):polystyrene sulfonate as the HTL. This is the first report on the semiconducting character of CuSeCN and it highlights the tremendous potential for further developments in the area of metal pseudohalides.

Journal article

Zhang J, Tan CH, Du T, Morbidoni M, Lin CT, Xu S, Durrant JR, McLachlan MAet al., 2018, ZnO-PCBM bilayers as electron transport layers in low-temperature processed perovskite solar cells, Science Bulletin, Vol: 63, Pages: 343-348, ISSN: 2095-9273

We 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.

Journal article

Godin R, Ma X, González-Carrero S, Du T, Li X, Lin CT, Mclachlan MA, Galian RE, Pérez-Prieto J, Durrant JRet al., 2018, Tuning charge carrier dynamics and surface passivation in organolead halide perovskites with capping ligands and metal oxide interfaces, Advanced Optical Materials, Vol: 6, ISSN: 2195-1071

Organolead halide perovskites have emerged as exciting optoelectronic materials but a complete understanding of their photophysical properties is still lacking. Here, a morphological series of methylammonium lead bromide (MAPbBr 3 ) perovskites are studied by transient optical spectroscopies over eight orders of magnitude in timescale to investigate the effect of nanostructuring and surface states on the charge carrier dynamics. The sample preparation route and corresponding morphology changes influence the position of the optical features, recombination dynamics, excitation fluence dependence, and dramatically impact surface trap passivation. Growth of the perovskite layer in the presence of capping ligands or within mesoporous alumina increases the photoluminescence efficiency by multiple orders of magnitude, indicating that interfacing with metal oxides can lead to the passivation of surface nonradiative recombination centers. Nanoparticles (NPs) dispersed in solution show mixed behavior since they consist of NPs on nanoplatelets, while isolated NPs could be grown within mesoporous alumina with the addition of capping ligands. Investigation on the microsecond timescale suggests that an exponential distribution of states below the band edges results in long-lived charges. The investigations of the relationship between sample architecture and charge carrier dynamics will help in the appropriate choice of perovskite morphology for use in optoelectronic devices.

Journal article

Zhou FC, Yuan L, Feng SH, McLachlan MA, Zhang JQet al., 2018, Seed-layer effect on highly oriented ZnO nanorod array fabrication, Chinese Journal of Inorganic Chemistry, Vol: 34, Pages: 1655-1662, ISSN: 1001-4861

© 2018, Chinese Chemical Society. All Rights Reserved. Vertically aligned ZnO nanorod arrays have been extensively applied to the fabrication of various optoelectronic devices owing to their versatile properties. However, to control and investigate the morphology of nanorods is directly related to device performances. A seeded substrate is favourable for the hydrothermal synthesis of nanorods by lowering the nucleation barrier and restricting the migration of the disordered nucleation. Therefore, varying the seed layer properties is a feasible and effective way to control the nanorod growth. In this paper, three different methods sol-gel, spray pyrolysis and pulse laser deposition (PLD) were employed to the ZnO seed layer preparation. For each preparation method, the crystal structure, morphologies, surface roughness and properties of ZnO seed layers were characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM), respectively. ZnO nanorod arrays with different structures and morphological properties were synthesised on the prepared seed layers by hydrothermal treatment. The ZnO nanorods exhibit strong substrate-dependent properties. Combined with the characteristics of ZnO seed layers, the difference of growth mechanisms for each method is proposed by analyzing the nanorod properties on the seed layers.

Journal article

Steiger P, Zhang J, Harrabi K, Hussein I, Downing J, McLachlan MAet al., 2017, Hydrothermally grown ZnO electrodes for improved organic photovoltaic devices, Thin Solid Films, Vol: 645, Pages: 417-423, ISSN: 0040-6090

Here we report a simple, solution based processing route for the formation of large surface area electrodes resulting in improved organic photovoltaic devices when compared with conventional planar electrodes. The nanostructured electrode arrays are formed using hydrothermally grown ZnO nanorods, subsequently infiltrated with blends of poly(3-hexylthiophene-2,5-diyl) (P3HT) and indene-C60 bisadduct (IC60BA) as photoactive materials. This well studied organic photoactive blend allows the composition/processing/performance relationships to be elucidated. Using simple solution based processing the resultant nanostructured devices exhibited a maximum power conversion efficiency (PCE) of 2.5% compared with the best planar analogues having a PCE of around 1%. We provide detailed structural, optical and electrical characterization of the nanorod arrays, active layers and completed devices giving an insight into the influence of composition and processing on performance. Devices were fabricated in the desirable inverse geometry, allowing oxidation resistant high work-function top electrodes to be used and importantly to support the hydrothermal growth of nanorods on the bottom electrode — all processing was carried out under ambient conditions and without the insertion of a hole transport layer below the anode. The nanorods were successfully filled with the active layer materials by carrying out a brief melt processing of a spin-cast top layer followed by a subsequent thermal anneal which was identified as an essential step for the fabrication of operational devices. The growth method used for nanorod fabrication and the active layer processing are both inherently scalable, thus we present a complete and facile route for the formation of nanostructured electron acceptor layers that are suitable for high performance organic active layers.

Journal article

Zhang J, Morbidoni M, Huang K, Feng S, McLachlan MAet al., 2017, Environmentally friendly, aqueous processed ZnO as an efficient electron transport layer for low temperature processed metal-halide perovskite photovoltaics, Inorganic chemistry frontiers, Vol: 5, Pages: 84-89, ISSN: 2052-1553

Here we report the incorporation of ZnO electron transport layers (ETLs), deposited using a remarkably simple water-based processing route, for use in methylammonium lead iodide (MAPI, CH3NH3PbI3) perovskite solar cells. The influence of ZnO processing temperature on the thermal stability and surface morphology of the perovskite films is studied in detail. We find that operational devices are achieved over the entire ZnO processing temperatures range investigated (100–450 °C) – however those prepared at 100 °C are significantly affected by current–voltage hysteresis. We find that the insertion of a thin phenyl-C61-butyric acid methyl ester (PCBM) layer between the ZnO and the MAPI significantly reduces current–voltage (J–V) hysteresis. Additionally we determine that the thermal stability of the MAPI improves when PCBM is inserted as an interface modifier. The fabrication of the PCBM modified ZnO at 100 °C enables the formation of low-temperature processed, thermally stable normal architecture cells with negligible hysteresis.

Journal article

McLachlan MA, Morbidoni M, Burgess CH, Wu J, Harrabi K, Payne DJ, Durrant Jet al., 2017, Nanoscale structure-property relationships in low temperature solution-processed electron transport layers for organic photovoltaics, Crystal Growth and Design, Vol: 17, Pages: 6559-6564, ISSN: 1528-7483

Here we elucidate the nanostructure–property relationships in low-temperature, solution-processed ZnO based thin films employed as novel electron transport layers (ETLs) in organic photovoltaic (OPV) devices. Using a low-cost zinc precursor (zinc acetate) in a simple amine–alcohol solvent mix, high-quality ETL thin films are prepared. We show that at a processing temperature of 110 °C the films are composed of nanoparticles embedded in a continuous organic matrix consisting of ZnO precursor species and stabilizers. Using a combination of transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), we study the thermally induced morphological and compositional changes in the ETLs. Transient optoelectronic probes reveal that the mixed nanocrystalline/amorphous nature of the films does not contribute to recombination losses in devices. We propose that charge transport in our low-temperature processed ETLs is facilitated by the network of ZnO nanoparticles, with the organic matrix serving to tune the work function of the ETL and to provide excellent resistance to current leakage. To demonstrate the performance of our ETLs we prepare inverted architecture OPVs utilizing Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7): [6,6]-Phenyl-C71-butyric acid methyl ester (PC71BM) as active layer materials. The low-temperature ETL devices showed typical power conversion efficiencies (PCEs) of >7% with the champion devices achieving a PCE > 8%.

Journal article

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