Ian G. Hill

Organic thin-film transistor-driven polymer-dispersed liquid crystal displays on flexible polymeric substrates

We have fabricated organic thin-film transistor (OTFT)-driven active matrix liquid crystal displays on flexible polymeric substrates. These small displays have 16×16 pixel polymer-dispersed liquid crystal arrays addressed by pentacene active layer OTFTs. The displays were fabricated using a low-temperature process (<110 °C) on flexible polyethylene naphthalate film and are operated as reflective active matrix displays.

Perovskite-fullerene hybrid materials suppress hysteresis in planar diodes

Solution-processed planar perovskite devices are highly desirable in a wide variety of optoelectronic applications; however, they are prone to hysteresis and current instabilities. Here we report the first perovskite–PCBM hybrid solid with significantly reduced hysteresis and recombination loss achieved in a single step. This new material displays an efficient electrically coupled microstructure: PCBM is homogeneously distributed throughout the film at perovskite grain boundaries. The PCBM passivates the key PbI3− antisite defects during the perovskite self-assembly, as revealed by theory and experiment. Photoluminescence transient spectroscopy proves that the PCBM phase promotes electron extraction. We showcase this mixed material in planar solar cells that feature low hysteresis and enhanced photovoltage. Using conductive AFM studies, we reveal the memristive properties of perovskite films. We close by positing that PCBM, by tying up both halide-rich antisites and unincorporated halides, reduces electric field-induced anion migration that may give rise to hysteresis and unstable diode behaviour.

Molecular level alignment at organic semiconductor-metal interfaces

In order to clarify the electronic structure of metal-molecular semiconductor contacts, we use photoemission spectroscopy to investigate the energetics of interfaces formed by vacuum deposition of four different molecular thin films on various metals. We find that the interface electron and hole barriers are not simply defined by the difference between the work functions of the metals and organic solids. The range of interface Fermi level positions is material dependent and dipole barriers are present at all these interfaces. The results demonstrate the breakdown of the vacuum level alignment rule at interfaces between these organic molecular solids and metals.

Air-stable n-type colloidal quantum dot solids.

Colloidal quantum dots (CQDs) offer promise in flexible electronics, light sensing and energy conversion. These applications rely on rectifying junctions that require the creation of high-quality CQD solids that are controllably n-type (electron-rich) or p-type (hole-rich). Unfortunately, n-type semiconductors made using soft matter are notoriously prone to oxidation within minutes of air exposure. Here we report high-performance, air-stable n-type CQD solids. Using density functional theory we identify inorganic passivants that bind strongly to the CQD surface and repel oxidative attack. A materials processing strategy that wards off strong protic attack by polar solvents enabled the synthesis of an air-stable n-type PbS CQD solid. This material was used to build an air-processed inverted quantum junction device, which shows the highest current density from any CQD solar cell and a solar power conversion efficiency as high as 8%. We also feature the n-type CQD solid in the rapid, sensitive, and specific detection of atmospheric NO2. This work paves the way for new families of electronic devices that leverage air-stable quantum-tuned materials.

Recent advances of non-fullerene, small molecular acceptors for solution processed bulk heterojunction solar cells

Organic, planar, and electron deficient small molecules were utilized as acceptors in the first reported bilayer heterojunction solar cells, however, current state-of-the-art organic photovoltaic (OPV) cells utilize fullerene derivatives as acceptor molecules. Recently, intensive efforts have been directed towards the development and understanding of soluble, non-fullerene, organic small molecules to fabricate bulk heterojunction (BHJ) solar cells. These efforts have been aimed at overcoming the inherent limitations of fullerene compounds such as the limited spectral breadth, air instability, and the typically higher production costs of fullerenes. In this focused review, we have highlighted the most recent progress over the last couple of years towards developing n-type organic small molecules utilized in BHJ devices in order to provide insight towards improving the overall performance of OPVs.

Surface oxidation activates indium tin oxide for hole injection

Oxygen plasma treatment of indium tin oxide (ITO) results in a change in work function and electron affinity by ∼0.5 eV. This change correlates with the measured increase in injected current in simple “hole-only” organic devices with O-plasma treated ITO electrodes. Neither addition nor removal of surface hydroxyl functionality accounts for the observed work function and electron affinity changes. X-ray and ultraviolet photoelectron spectroscopies show a new type of oxygen species is formed. Oxidation of surface Sn-OH to surface Sn-O• units is proposed to account for the observed changes in O-plasma treated ITO; this proposal can explain a wide variety of previously described ITO surface activation results.

Organic semiconductor interfaces: electronic structure and transport properties

Ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) have been used to investigate a wide range of metal/organic and organic/organic semiconductor interfaces. UPS was used to determine the binding energies of the highest occupied molecular orbitals and vacuum level positions, while XPS was used to find evidence of chemical interactions at these heterointerfaces. It was found that, with a few exceptions, the vacuum levels align at most organic/organic interfaces, while strong interface dipoles, which abruptly offset the vacuum level, exist at virtually all metal/organic semiconductor interfaces. Furthermore, strong dipoles exist at metal/organic semiconductor interfaces at which the Fermi level is completely unpinned within the semiconductor gap implying that the dipoles are not the result of populating or emptying Fermi level-pinning gap states.

Improved organic thin-film transistor performance using novel self-assembled monolayers

Pentacene-based organic thin-film transistors have been fabricated using a phosphonate-linked anthracene self-assembled monolayer as a buffer between the silicon dioxide gate dielectric and the active pentacene channel region. Vast improvements in the subthreshold slope and threshold voltage are observed compared to control devices fabricated without the buffer. Both observations are consistent with a greatly reduced density of charge trapping states at the semiconductor-dielectric interface effected by introduction of the self-assembled monolayer.

Combined photoemission/in vacuo transport study of the indium tin oxide/copper phthalocyanine/N,N′-diphenyl-N,N′-bis(l-naphthyl)-1,1′biphenyl-4,4″diamine molecular organic semiconductor system

Ultraviolet photoemission spectroscopy (UPS) was used to study the indium tin oxide/copper phthalocyanine (CuPc) and CuPc/N,N′-diphenyl-N,N′-bis(l-naphthyl)-1,1′biphenyl-1-4,4″diamine interfaces, which are commonly used as an anode/hole injection layer/hole transport layer combination in organic light emitting devices. In order to assess the validity of the transport barriers measured using UPS, in vacuo I–V measurements have been performed on simple devices grown and measured in the same system as the samples studied using UPS. I–V characteristics were modeled using numerical simulations. The parameters used in the simulated curves which best fit the measured I–V characteristics agree quantitatively with the UPS measured barriers.

Organic semiconductor heterointerfaces containing bathocuproine

The four organic–organic heterointerfaces formed by depositing the organic semiconductor bathocuproine on tris(8-hydroxy-quinoline)aluminum (Alq3), N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′ biphenyl-4,4″ diamine (α-NPD), 4,4′-N,N′-dicarbazolyl-biphenyl (CBP), and copper phthalocyanine (CuPc) have been studied using ultraviolet photoelectron spectroscopy. The relative positions of the vacuum levels and highest occupied molecular orbital levels of the organics were measured at each interface. Within our experimental uncertainty of 100 meV, the vacuum levels were found to align at all four interfaces.