Showing papers by "Martyn A. McLachlan published in 2018"

Elucidating the Origins of Subgap Tail States and Open‐Circuit Voltage in Methylammonium Lead Triiodide Perovskite Solar Cells

Abstract: 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 opencircuit 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.

Post-polymerisation functionalisation of conjugated polymer backbones and its application in multi-functional emissive nanoparticles.

Abstract: 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.

Large-area plastic nanogap electronics enabled by adhesion lithography

Abstract: 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. A collaborative team led by Thomas Anthopoulos from King Abdullah University of Science and Technology demonstrated a novel method for patterning coplanar electrodes with inter-electrode distances of less than 15 nm and arbitrarily large aspect ratios. This facile manufacturing method, called adhesion lithography, was used to fabricate a series of functional nanoscale electronic devices on various substrates. These flexible electronic devices, which are technologically relevant, included self-aligned-gate transistors, radio frequency diodes and rectifying circuits, multi-colour organic light-emitting diodes and multilevel non-volatile memory devices. Given its versatility in the fabrication of functional electronic devices and its solution-processing compatibility with a range of semiconductors and substrate materials, adhesion lithography is an attractive processing method for the high-throughput manufacture of large-scale flexible electronics at the nanoscale.

ZnO-PCBM bilayers as electron transport layers in low-temperature processed perovskite solar cells

Abstract: 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-C61-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.

Passivation against oxygen and light induced degradation by the PCBM electron transport layer in planar perovskite solar cells

Abstract: Herein, we investigate the causes of a 20 fold improved stability of inverted, planar structure devices (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 the fullerene acceptor, attributed to the low LUMO level of PCBM energetically inhibiting superoxide generation. We also find that planar structure devices show 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.

Copper (I) Selenocyanate (CuSeCN) as a Novel Hole-Transport Layer for Transistors, Organic Solar Cells, and Light-Emitting Diodes

Abstract: 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 cm2 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.

High-Efficiency Fullerene Solar Cells Enabled by a Spontaneously Formed Mesostructured CuSCN-Nanowire Heterointerface.

Abstract: 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.

Electric Single-Molecule Hybridization Detector for Short DNA Fragments.

Abstract: 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 ...

Environmentally friendly, aqueous processed ZnO as an efficient electron transport layer for low temperature processed metal–halide perovskite photovoltaics

Abstract: 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.

An All-Electric Single-Molecule Hybridisation Detector for short DNA Fragments

Abstract: In combining DNA nanotechnology and high-bandwidth single-molecule detection in nanopipettes, we demonstrate an all-electric, label-free hybridisation sensor for short DNA sequences (< 100 nt). 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 a 88-mer target from the RV1910c gene in Mycobacterium tuberculosis that 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 nanopipettes is capable of identifying rather subtle structural differences, such as the hybridisation state of the probes, in a statistically robust manner. With significant potential towards multiplexing and high-throughput analysis, our study points towards a new, single-molecule DNA assay technology that is fast, easy to use and compatible with point of care environments.