Ross A. Hatton

Interpenetrating multiwall carbon nanotube electrodes for organic solar cells

Low concentrations of multiwall carbon nanotubes (MWCNTs) (∼1wt%) uniformly distributed within the donor layer of a heterojunction organic solar cell are shown to be an effective means of greatly reducing cell series resistance without compromising cell shunt resistance, resulting in an increase in cell fill factor of 50–60%. By chemically functionalizing MWCNTs with polar surface moieties, it is also possible to manipulate device open circuit voltage. The results of this study demonstrate that MWCNTs have excellent potential as a versatile interpenetrating electrode material for organic solar cells.

Carbon nanotubes: a multi-functional material for organic optoelectronics

In recent years carbon nanotubes have emerged as a versatile material suitable for hybrid macro-electronic devices employing organic semiconductors, with the potential to address many of the barriers to the widespread exploitation of organic semiconductors as the basis for low cost, large area, flexible optoelectronics. This paper focuses on the applications of carbon nanotubes in organic light-emitting diodes and photovoltaics. These related devices are currently being pursued with much anticipation as energy efficient solid state lighting and a scalable means of harvesting electrical energy from sunlight. In this paper particular emphasis is placed on the utility of multi-wall carbon nanotubes which have received much less attention than single wall carbon nanotubes for these applications despite offering a number of key advantages.

Organic electroluminescent devices : enhanced carrier injection using an organosilane self assembled monolayer (SAM) derivatized ITO electrode

A protocol for the reproducible silylation of indium–tin oxide coated glass (ITO) using small molecule chlorosilanes is reported, and shown to be a convenient means of dramatically improving the performance of the ITO anode invariable used in organic electroluminescent devices. Using the model system: ITO|TPD|Alq3|Al (where TPD is N,N′-bis(3-methylphenyl)-N,N′diphenyl-1,1′biphenyl-4,4′-diamine and Alq3 is tris(quinolin-8-olato)aluminium) luminance-voltage, current–voltage, quantum efficiency and luminance efficiency data illustrate the superior performance of the silane modified device over a reference. Static contact angle measurements confirming the successful silylation of the ITO electrode surface, are supported by direct measurement of the effect the dipolar monolayer has on the work function of the underlying ITO, using a scanning Kelvin probe. This research builds on our earlier work with dipolar phosphonic acids. However, unlike phosphonic acids, chlorosilanes are known to adhere to oxide surfaces via a covalent bond, and so the silylated ITO electrodes are expected to exhibit improved durability. Such electrode modification provides a method of tuning the work function of the ITO electrode to the HOMO of the hole-transporting layer and thus, improving device performance.

Structural and optoelectronic properties of C60 rods obtained via a rapid synthesis route

High purity single crystal C60 rods with uniform dimensions are synthesized by a rapid and facile approach which can be completed over a timescale of typically a few minutes. The morphology of the fullerene product has been characterized in detail by scanning electron microscopy, scanning transmission electron microscopy, and atomic force microscopy, demonstrating that the resulting materials are solid, hexagonal cross-sectioned rods with novel faceted tips. High resolution transmission electron microscopy investigations reveal that the rods are face-centered cubic packed single crystals. Vibrational and electronic spectroscopy studies provide compelling evidence that the rods are a van der Waals solid since the electronic structure of the component C60 molecules is largely preserved. The structures obtained are found to possess novel optoelectronic properties exhibiting low energy absorption not reported in related structures and materials to date. Furthermore significant room temperature photoluminescence is obtained from the C60 rods accompanied by a small blue shift of the spectra which is also observed for the first ‘allowed’ absorption transitions. Given their rapid synthesis, excellent purity, optical and charge transport properties these fullerene structures are expected to be a promising materials for nanoelectronic devices including thin film organic solar cells and photodetectors.

Water-soluble multiwall-carbon-nanotube-polythiophene composite for bilayer photovoltaics

A water-soluble acid oxidized multiwall carbon nanotube (o-MWCNTs)-polythiophene composite for bilayer photovoltaics is reported. Discrete heterojunction photovoltaic cells utilizing this nanocomposite material as the donor layer exhibit a ∼20% increase in fill factor and commensurate increase in power conversion efficiency as compared to cells without o-MWCNTs. Crucially o-MWCNTs are incorporated into the cell structure using an environmentally compatible solvent without complicating the process of device fabrication.

Carbon nanotubes grown on In2O3:Sn glass as large area electrodes for organic photovoltaics

The authors report the growth of multiwall carbon nanotubes directly onto indium tin oxide glass via chemical vapor deposition as large area semitransparent electrodes for organic solar cell applications. The rate of nanotube growth on this ternary oxide is greatly reduced as compared to that of silicon dioxide and glass substrates enabling a high degree of control over nanotube height. The strong potential of this nanostructured semitransparent substrate as an interpenetrating hole-extracting electrode in bulk-heterojunction organic solar cells is also demonstrated.

Tin perovskite/fullerene planar layer photovoltaics: improving the efficiency and stability of lead-free devices

We report the first demonstration of orthorhombic CsSnI3 films prepared from solution at room temperature that have defect densities low enough for use as the light harvesting semiconductor in photovoltaic devices even without using excess Sn in the preparative method, and demonstrate their utility in a model p–i–n photovoltaic device based on a CuI | CsSnI3 | fullerene planar layer architecture. We also report an effective strategy for simultaneously improving both the efficiency and stability of these devices towards air exposure based on the use of excess of SnI2 during CsSnI3 synthesis from CsI and SnI2. A combination of photoelectron spectroscopy, contact potential measurements and device based studies are used to elucidate the basis for this improvement and role of the excess SnI2. The open-circuit voltage in these lead-free photovoltaic devices is shown to be strongly dependent on the degree of alignment between the perovskite conduction band edge and the lowest occupied molecular orbital (LUMO) in the fullerene electron transport layer. Furthermore, the energetics at the perovskite–fullerene interface are shown to be a function both of the LUMO energy of the fullerene and the nature of the interaction at the heterojunction which can give rise to a large abrupt vacuum level shift across the interface. A champion open-circuit voltage of ∼0.55 V is achieved using indene-C60 bis-adduct as the electron extraction layer, which is twice that previously reported for a CsSnI3 based PPV.

Increased efficiency of small molecule photovoltaic cells by insertion of a MoO3 hole-extracting layer

We report a ∼60% increase in open circuit voltage (Voc) and power conversion efficiency in a chloroaluminium phthalocyanine (ClAlPc)/fullerene (C60) planar heterojunction photovoltaic device after insertion of a MoO3 hole-extracting layer at the interface between the indium tin oxide (ITO) electrode and the ClAlPc donor layer, with an associated improvement in device stability. A similar improvement was observed in heterojunction devices based on mixed ClAlPc/C60 layers. We propose that the improvements in device performance are due to the pinning of the ITO Fermi level to the valance band of the MoO3 interlayer, where the latter is closely aligned with the highest occupied molecular orbital of ClAlPc.

Increased efficiency in small molecule organic photovoltaic cells through electrode modification with self-assembled monolayers

We report that through the incorporation of polar self-assembled monolayers (SAMs) at the indium tin oxide (ITO)/donor interface in chloroaluminium phthalocyanine (ClAlPc)/C60 discrete heterojunction organic photovoltaic (OPV) cells, the power conversion efficiency can be dramatically increased. This enhanced performance is due to better alignment between the hole-extracting electrode Fermi level and the highest occupied molecular orbital (HOMO) of the ClAlPc donor, as well as an improved surface compatibility which provides a more optimised electrode/donor interface. Optimised cells demonstrate an increase of ∼85% in open circuit voltage which results in a near three-fold increase in power conversion efficiency from 1.3% to 3.3% under 1 sun illumination. Comparative studies are made using cells based on two other organic donor materials, copper phthalocyanine (CuPc) and boron subphthalocyanine (SubPc).

Operation of a reversed pentacene-fullerene discrete heterojunction photovoltaic device

The photoresponse of reversed bilayer organic photovoltaic device based on pentacene and C60 is examined, and the mechanism of photocurrent generation is shown to be different to that in conventional heterojunction devices, with free charge carriers generated at the electrode-organic interfaces rather than the organic heterojunction. This hypothesis is tested with silver nanoclusters incorporated at the organic heterojunction to quench excitons and facilitate recombination of free charge carriers, which shows a predicted increase in Jsc. The large Voc in this reversed cell structure is also rationalized in the context of the model proposed.